Summary of recently selected studies using biochars and their influence on retention process of herbicides.
\r\n\tOne of the most important elements that contributes to fatigue failures are the welded joints due to different factors that act over the joint and are producing early failure. Fatigue cracks usually initiate from surface of the component and propagate across the transverse section, perpendicular to the stress direction. Consequently, the crack propagation and fracture mechanics theories should be studied if we want to understand and to find the causes that produce the component failures and to calculate the residual life. Another factor that affects the crack propagation and fatigue is the environment. A comprehensive understanding of the influence of environment over the crack propagation and fatigue has been blocked by the complexity of the problem. The difficulties in understanding the various micromechanisms governing crack initiation and crack propagation are caused by a lack of a truly interdisciplinary approach to the problem.
\r\n\tFrom another side, currently, numerical models and simulation have become a good alternative to solve problems related to fatigue and crack propagation due the relatively low cost and short time expended in this labor, so the book will try to give an overview of the topic. Generally, the fatigue has been studied through the materials more used in engineering (steel, aluminum, bronze), but in recent years, the use of the non-conventional materials like naturals (bamboo, wood, natural fibers), composite materials and others has taken great relevance in engineering applications.
Oxidative phosphorylation is the center of energy metabolism in plants, animals and several microbial life forms . In eukaryotes, this process occurs in mitochondria. The mitochondria is a cytoplasmic organelle surrounded by two membranes, outer and inner membrane, which main function is the production of most of the phosphate compounds necessary for the energetic balance of the cell. In addition, other functions such as the regulation of the body’s heat generation [2, 3, 4] programmed cell death [5, 6, 7], reactive oxygen species (ROS) generation and cell signaling  is also associated with mitochondria. Cellular vitality is directly related to mitochondria, and mitochondrial dysfunctions are frequent causes of accidental cell death [5, 9, 10, 11], cancer [12, 13], diabetes [14, 15, 16] and neurodegenerative diseases [17, 18, 19], among others.
The characterization of the respiratory electron chain could be performed in studies using the fractionation of its components by certain detergents that at low concentrations break the interactions between proteins and lipids in the membranes, leaving associations between proteins intact . In electron transport chain, through this process, four protein complexes were found. They were named complex I (or NADH-Ubiquinone oxidoreductase), complex II (succinate dehydrogenase), complex III (Ubiquinol -cytochrome c oxidoreductase, or complex bc1) e complex IV (cytochrome c oxidase). The complex V is also known as ATP synthase. Despite glycerophosphate dehydrogenase (glycerol-3-phosphate dehydrogenase) and ETF–ubiquinone oxidoreductase have not complex nomenclature, they are connect to the electron transport chain, as complex I and complex II, i.e., delivering electron to ubiquinone .
The redox carriers within the respiratory chain consists of flavoprotein containing tightly bound FAD or FMN as prosthetic groups, protein-bound couper, ironsulphur (nonhaem iron) proteins and cytochromes, with haem prosthetic groups. The ubiquinone also participated in electron transport chain as a free and diffusible cofactor . While electron transport occurs through the mitochondrial complexes, complexes I, II, and III pump protons from mitochondrial matrix to the intermembrane space. The energy associated to this process is used to the production of ATP by ATP synthase (Figure 1) .
The ROS comprise a variety of molecules derived from molecular oxygen, including oxygen radicals and non-radical oxygen derivate. The major intracellular site of ROS formation in most tissues is mitochondria [23, 24]. Within mitochondria, the electron transport chain continuously generates water from O2 through the electronic reduction at the cytochrome c oxidase level (Figure 1). These electrons reach cytochrome c oxidase by sequential transfer from the reduction of other components, and are initially removed from NADH and FADH2. During this transfer, a small amount of electrons are lost at intermediate stages in the electron transport chain, mainly in the complex I and complex III [25, 26, 27] in mammals, leading to a monoelectronic reduction of O2 .
This monoelectronic reduction of O2 results in the formation of anion superoxide radical. While complex I releases superoxide only in the mitochondrial matrix, complex III releases superoxide in both sides of inner mitochondrial membrane . Complex II could theoretically generate superoxide, due presence of flavoprotein in its structure. However, the redox centers are arranged in a manner that aids the prevention of ROS by avoiding the access of O2 to the flavoprotein. This may explain the reason why this complex does not show a ROS formation by itself , but only due reverse electron transfer, i.e., when electrons flow from succinate to ubiquinone and back to complex I .
In addition to the electron transport chain, recent studies in mammalian tissues have shown that proteins belonging to the α-ketoglutarate dehydrogenase complex located in the mitochondrial matrix are also a source of ROS in a mechanism stimulated by the low concentration of NAD+ [32, 33]. In Saccharomyces cerevisiae, the deletion of the LPD1 gene, which leads to the inactivation of the enzyme dihydrolipoyl dehydrogenase, E3 subunit of the pyruvate dehydrogenase complex, also leads to a decrease in ROS production. This finding shows the importance of other mitochondrial proteins, other than those associated with the electron transport chain, in the regulation of redox balance .
The term reactive specie is not restricted to oxygen, but is also include others, as reactive nitrogen (RNS). Nitric oxide is a membrane permeable free radical that participates in a multiple process in the cells as signaling molecule, but also can contribute in cell oxidative damage. Its effect depends on NO levels and localization in the cell microenvironment [35, 36]. When nitric oxide is present in environment, as in mitochondrial matrix, the reaction of this free radical with superoxide can form others RNS, as peroxynitrite.
Besides mitochondria electron chain and enzyme linked to mitochondrial dehydrogenase complexes, other sources of ROS in cells include enzymes, as NADPH oxidases, cytochrome P450, cyclooxygenases, and the system xanthine/xanthine oxidase. Autoxidation is another example of source of ROS that in cells occurs when a biochemical compound is exposure to O2, as it occurs in FADH2, L-DOPA and in nitric oxide synthase with generation of superoxide. The auto oxidation can be catalyzed by metallic ions, finally, harm proteins, in which O2 bind Fe2+ could lead to superoxide, as in hemoglobin .
ROS and RNS are normally produced in metabolism and have an important role as signaling molecules regulating diverse physiological cell events, as cell signaling, metabolism and regulation of transcription factors [35, 38, 39, 40, 41, 42].
The steady state of reactive species will depend on their generation, reactivity and removal by antioxidant defenses. When the level of reactive species generation is much larger than their removal it is said that there is a condition called oxidative stress, i.e., an imbalance between reactive species and antioxidants in favor of reactive species. The maintenance of cell redox state is important to cell viability . The increased level of reactive species can lead to oxidative damage to a vast number of biological molecules, as DNA [44, 45, 46], proteins , lipids , including membranes  leading to a range of pathologies, as cancer , neurological disease , cardiac disease [50, 51], inflammation process  and aging.
There is a grand amount of theories about aging process, at least 300 theories according Medvedev . In 1956, Harman proposed in his “free radical theory of aging” that the damage of biomolecules that occurs during aging is due oxidative stress, ROS increments . Mitochondria, as the major site of ROS production, have been associated with aging process [55, 56]. Moreover, studies with caloric restriction in yeast and mammals have shown that the mitochondria, ROS, and RNS have an important role in the aging process [34, 56, 57, 58, 59, 60, 61].
During oxidative stress, ROS can attack molecules at electron-dense sites or abstract protons, producing secondary radical species, which undergo conformational change generating more stable products. The molecules that are vulnerable to these deleterious modifications include the lipids, proteins and nucleic acids. In other words, when the generation of reactive species exceeds antioxidant capacity, the cellular macromolecules also become targets of oxidation by these species. The possible consequences originated from this extensive oxidation, including an increased risk for cardiovascular disease, cancer and neurodegenerative disease (as detailed in Section 4).
Under oxidative stress conditions, proteins suffer extensive modification [62, 63, 64, 65]. Basically, ROS can oxidize amino acids cysteine and methionine, resulting in the production of dithiol and methionine sulfoxide crosslinks, respectively . Moreover, reactive species also can cause protein modification by nitration of tyrosine and by nitrosation of amino acids with thiol group. These changes often result in the alteration of function or inhibition of enzyme activities. The protein adducts have been observed in several pathologic conditions [67, 68], suggesting their deleterious effects. However, whether these endogenous modifications are produced in a controlled manner, they may also control physiological responses [69, 70].
It is important to stress that the presence of proteins containing nitrotyrosine residues, for example, has been a biomarker of damage by reactive species [67, 68]. The tyrosine nitration occurs by addition of NO2 to the ortho position of the phenolic ring of this amino acid. In fact, this NO2 group is obtained from peroxynitrite (ONOO−), a very strong oxidant . During oxidative stress conditions, especially in inflammatory processes, a proportion of O2•− reacts with NO to form ONOO−. This last is a much more powerful oxidant than O2•− and, beyond the tyrosine residues, can damage several classes of molecules. ONOO−, its protonated form peroxynitrous acid (ONOOH), and its secondary radical product, react with electron-rich groups, such as sulfhydryls, ironsulphur centers, zinc-thiolates and active site sulfhydryl in tyrosine phosphatases [67, 68, 72, 73].
The thiol group (-SH) of cysteine, for example, it is another relevant protein targets of ROS. Disulfide bond is important in protein structure and function , and recently its role in redox signaling has also been evidenced . The reaction of H2O2 with the deprotonated thiol group of cysteine produces a sulfenic acid (R-SOH). This last may be oxidized again producing a sulfinic acid (R-SO2H). With high levels of stress oxidative, cysteines can further be oxidized to a sulfonic acid (R-SO3H) [70, 76]. While sulfenic and sulfinic acids can be enzymatically reversible by the glutathione and thioredoxin enzyme systems  (Details about antioxidant mechanisms in next section), the sulfonic acid in cysteine residues seems to represent an irreversible protein damage.
The reactive species react directly with nucleic acids producing oxidative damage. Since oxidative DNA damage is a major threat to genetic integrity, causing mutations and modifications in gene expression pattern, it has been implicated in a wide variety of diseases, including cancer, cardiovascular and neurodegeneration disease, as well as aging process [46, 73].
The nitrogenous bases as well as the sugar suffer radical attacks, causing several base alterations and strand breaks . In fact, around 80 different bases have been observed in DNA exposed to oxidants . In this context, •OH is the most important reactive species that attacks DNA, since it reacts with the four bases and sugar moiety of the DNA backbone [78, 80] with a reaction rate limited by diffusion (4.5 × 109 to 9 × 109 M−1s−1) . •OH attacks carbo-carbon double bonds of bases due to the high electron density. These attacks produce the hydroxylation at C5 and C6 of pyrimidines and C4, C5 and C8 of purines [78, 80]. These secondary radicals are subjected to other oxidation and reduction reactions, producing a wide DNA lesions, including the well characterized derivatives, 7,8-dihydro-8-oxodeoxyguanine (8-oxoG) and 2,6-diamino-4-hydroxy-5-formamido-pyrimidine (FapyGua) . 8-oxo-G is the most stable of these altered bases and can give rise to mutations due to insert Adenine (A) opposite 8-oxo-G during DNA replication, instead of the Cytosine (C) [46, 71].
Another mutation produced by oxidative damage is C to thymine (T) transition, mainly due to the cytosine-derived products uracil glycol and 5-hydroxyuracil mispairing with A, instead of the G . Although other pathways also induce this mutation, it is important to stress that C to T transition is the most frequent mutations found in cancers and in the tumor suppressor gene p53 [81, 82].
Under conditions of oxidative stress occur an oxidative process termed lipid peroxidation that affects lipids containing multiple double bonds, such as fatty acids, phospholipids, glycolipids and cholesterol, modifying properties of cellular membranes [73, 83]. This degenerative process is believed to contribute to aging and several diseases, such as atherosclerosis, Alzheimer’s disease, peptic ulcer disease, and cancer [84, 85].
Cellular membranes are especially vulnerable to lipid peroxidation not only because of their high levels of unsaturated fatty acids, but also because of their connection with molecules capable of producing reactive species. They attack mainly the unsaturated fatty acids which contain carbon-carbon double bonds and CH2 groups with particularly reactive hydrogen, and start radical peroxidation chain reactions . These chain reactions are going to terminate when primary or secondary radicals directly react. Lipid peroxidation is accelerated by the presence of Fe2+ and Cu2+ ions [87, 88]. It is important to stress that lipid peroxides are unstable derivatives from the oxidation of unsaturated fatty acids and decompose to form reactive carbonyl molecules, such as malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE) [85, 89]. These two products are abundant biomarkers of lipid peroxidation [85, 90].
Membrane-bound proteins are also involved in the process of lipid peroxidation. Aldehyde products, such as MDA and 4-HNE, react with amine and thiol groups of membrane protein, causing several damages, including inactivation of enzymes. Conformational changes of membrane molecules also include lipid–lipid cross-links and lipid–protein cross-links [91, 92].
Moreover, lipid peroxidation modifies the global biophysical properties of the membranes. This process affects the packing of lipids and the permeability to solutes, which in turn, changes its function, including the membrane potential. Furthermore, the process of peroxidation can inhibit the activity of protein transporters and ion channels [89, 91]. The increase of the permeability also seems to occur in internal mitochondrial membrane, uncoupling respiratory-chain phosphorylation . Finally, the lipid peroxidation leads the severe damages: modification of membrane permeability, enzymatic inhibitions, inactivation of transporters [37, 92].
The exposure cells and tissues to the harmful effects of free radicals cause a cascade of reactions and induces activation of some strategies to damage prevent, repair mechanism to alleviate the oxidative damages, physical protection mechanism against damage, and the final most important is the antioxidant defense mechanisms [94, 95].
The antioxidant defenses are the first line of choice to take care of the stress. Endogenous antioxidant defenses include antioxidant enzymes and non-enzymatic molecules that are usually distributed within the cytoplasm and various cell organelles . The exogenous antioxidants are present in consumed fruits, vegetables, juice, tea, coffee, nuts and cereal products .
The concept of biological antioxidant refers to any compound present at a lower concentration which is able to either delay or prevent the oxidation of the substrate. Antioxidant functions imply lowering oxidative stress, DNA mutations, malignant transformations, as well as other parameters of cell damage . Antioxidants reactions can deplete molecular oxygen or decreasing its local concentration, removing pro oxidative metal ions, trapping aggressive ROS such as superoxide anion radical or hydrogen peroxide, scavenging chain initiating radicals like hydroxyl OH·, alkoxyl RO· or peroxyl ROO·, breaking the chain of a radical sequence or quenching singlet oxygen (1O2) .
The antioxidants include some high molecular weight (SOD, GPx, catalase, albumin, transferrin, and metallothionein) and some low molecular weight substances (uric acid, ascorbic acid, lipoic acid, glutathione, ubiquinol, tocopherol/vitamin E, flavonoids). Natural food-derived components have received great attention in the last 2 decades, and several biological activities showing promising anti-inflammatory, antioxidant, and anti-apoptotic-modulatory potential have been identified. These enzymatic and nonenzymatic antioxidant systems are necessary for sustaining life by maintaining a delicate intracellular redox balance and minimizing undesirable cellular damage caused by ROS [94, 97, 98].
Antioxidant enzymes catalyze ROS conversion directly via an active-site metal ion or through pathways involving the donation of an electron from the moiety-conserved redox couples thioredoxin and glutathione, which require continuous regeneration of the reduced species . Superoxide and H2O2 metabolizing enzymes are generally considered to be the primary antioxidant enzyme defense system in the body .
The SOD is a family of enzymes catalyzing dismutation of superoxide into oxygen and H2O2. Three types of superoxide dismutases can be encountered in mammalian tissues: copper-zinc containing superoxide dismutase (SOD1) present in the cytosol, manganese containing superoxide dismutase (SOD2) found in the mitochondrial matrix and extracellular superoxide dismutase (SOD3). All three are highly expressed, mainly in the renal tubules of healthy kidneys [15, 98, 100]. The final product of the SOD activity - H2O2, is then converted into water and oxygen by the catalase (CAT). This enzyme is a homotetrameric protein containing four iron heme and largely located in the peroxisomes [15, 100].
Other important enzymatic antioxidants in the first line of defense include glutathione peroxidase (GPX) and myeloperoxidase (MPO) enzymes. The GPX is a selenium-containing enzyme, catalyzes both the reduction of H2O2, and organic hydroperoxides to water or corresponding alcohols. Reduced glutathione functions as effective electron donor in the process, as free thiol groups are oxidized to disulfide bonds: H2O2 + 2GSH → GS-SG + 2H2O . The MPO, a heme peroxidase, abundant in granules of human inflammatory cells, catalyzes the conversion of H2O2 to HClO with the production of ROS. The ROS production is associated with cardiovascular disease, chronic obstructive pulmonary disease, and Alzheimer’s disease. Oxidant species derived from MPO lead to the production of specific oxidation products, such as 3-Cl-Tyr. This can be used as biomarker in several diseases, as above described, and its levels correlate with MPO .
Other enzymes could be cited by our antioxidant activity, such as Peroxiredoxin Family (PRX). These enzymes are a family of abundantly present 20–30 kDa peroxidases that are excessively reactive with H2O2. So, they are likely to be critical for both oxidative stress protection as well as redox signaling . The antioxidant enzymes may possibly offer novel treatment options for redox-related diseases, provided that the molecular mechanisms are known and can be specifically targeted. Besides that, inhibiting a given antioxidant enzyme or specifically silencing its gene expression may help treat disorders related to a gain of enzymatic function  and this fact can will help the researchers to explore future options in enzymatic antioxidant system and diseases.
Among the nonenzymatic antioxidant compounds, the principals are obtained from dietary as the class of phenolic compounds, vitamins C and E, and carotenoids . Phenolic compounds represent a large group of secondary metabolites , among them flavonoids, phenolic acids, tannins and tocopherols as the most common natural source phenolic antioxidants .
The phenolic compounds are composed of one or more aromatic rings with varying degrees of hydroxylation, methoxylation and glycosylation, and various studies have associated the structure of phenolic compounds with their antioxidant properties [102, 104]. The antioxidant activity generally increases with the degree of hydroxylation in aromatic rings and decreases with C-3 methoxylation [105, 106]. The antioxidant activity is based on the availability of electrons to neutralize the free radicals; in addition, it is related to the number and nature of the hydroxylation pattern in the aromatic ring and the ability to act as a hydrogen donor .
The flavonoid group is the most diverse within phenolic compounds, with two aromatic rings associated via C-C bonds by a 3C oxygenated heterocycle. Flavonoids have antioxidant and chelating properties, inactivate ROS, acting against the oxidation of low density lipoproteins (LDL) and improving inflammation of the blood vessels. They also reduce the activity of the xanthine oxidase enzymes and the nicotinamide adenine dinucleotide phosphate oxidase, enzymes that stimulate the production of ROS .
In cellular compartments, flavonoids function as antioxidants inactivating free radicals both in hydrophilic and lipophilic compartments. For example, the antioxidant activity of phenolic compounds present in spices (cinnamon, sweet weed and mustard) differs between aqueous and lipid systems .
Vitamins C and E act together to inhibit lipid peroxidation and protect the cell against oxidative damage, as DNA damage. The antioxidant activity of vitamin C involves the transfer of an electron to the free radical and the consequent formation of the radical ascorbate . In addition, vitamin C acts synergistically with vitamin E, which regenerate the vitamin C has better antioxidant activity in hydrophilic media, and in aqueous phase of extracellular fluids, it is able to neutralize ROS in the aqueous phase before they can attack lipids. Vitamin E is an important fat soluble antioxidant, acting as the chain breaking antioxidant within the cell membrane and playing an important role in the protection of membrane fatty acids against lipid peroxidation .
Vitamins C and E inhibit lipid peroxidation and protect against oxidative damage by their scavenging actions of ROS, as well as by modulateing numerous enzymatic complexes involved in the production of ROS, endothelial function and aggregation of platelets. These vitamins can also regulate NADPH oxidase, the most important source of O2•− in the cardiovascular system. It has been reported that ascorbic acid and α-tocopherol, derivated from vitamin C and E respectively, may involved in the transcriptional modulation of NADPH oxidase .
The most common carotenoids are xanthophylls and carotenes. Carotenoids can neutralize singlet oxygen by quenching it or can break the chain reaction of free radicals, or scavenging it, not so effective action (scavenging). The structure of the free radical is the main factor that determines if the carotenoid will have quenching or scavenging action. It also depends on the region where the radical is in heterogeneous biological tissue, aqueous or lipid region (plasma, blood, heart, liver, brain etc.), and the structure of the carotenoids (number of conjugated, cyclic or acyclic double bonds), polar or nonpolar groups, redox properties [112, 113, 114].
The physical quenching is the transfer of excitation energy from the singlet oxygen to the carotenoid. The oxygen returns to ground state and the carotenoid is in the excited triplet state, the energy is dissipated producing stable carotenoid and thermal energy and the carotenoid can undergo other cycles of singlet oxygen quenching [112, 115].
The chemical quenching the carotenoid combines with oxygen or is oxidized, leading to its destruction and producing a variety of oxidized products. Carotenoids can also extinguish the triplet-excited state of chlorophyll or other excited sensitizers, thus preventing the formation of singlet oxygen . The free radical scavenging can occur in three ways, by electron transfer, by hydrogen abstraction, and by addition [112, 116].
All living cells have molecular tools to perceive and respond properly to environmental cues. All the cascades of intracellular reactions involved in promoting a biochemical response are denoted as signal transduction. There are well known receptor types or systems of signal transduction such as the G protein-coupled receptors (GPCR), tyrosine kinase receptors (TKR), ion channels, cell adhesion receptors, nuclear receptors and guanylyl-cyclases. Since cells often need to deal with many signals at the same time, the final biochemical response is a result of the integrations of many simultaneous cascades produced by one or more systems.
Before we move on exploring the targets of ROS in health and disease, an important question is raised: “Which are the main sources of cellular ROS?” Enzymes such as NADPH oxidases (Nox), xanthine oxidase (XO), lipoxygenase, MPO and uncoupled nitric oxide synthase are involved in the production of the anion radical superoxide (O2∙−). Furthermore, the mitochondrial aerobic respiration contributes with a huge amount of O2∙−. Peroxynitrite (ONOO−) is formed by the reaction of nitric oxide and superoxide and is thought to contribute to eNOS uncoupling . The majority of O2∙− generated within the mitochondrial matrix or the cytosol is dismutated to H2O2 by the SOD antioxidant enzyme. Moreover, metal exposure can mediate the generation of H2O2, O2∙−, and even the hydroxyl radical (OH∙), mainly via the Fenton or the Haber-Weiss reactions .
Some ROS such as O2∙− and HO∙ are highly reactive and have a brief half life. For this reason they are not considered signaling molecules, but intermediates of nonselective nature. On the other hand, H2O2 is relatively stable and can both mediate intracellular signaling and also serve to paracrine signaling (i.e., cell-to-cell communication involving nearby cells), since it can cross biological membranes .
Up to date, several proteins have been recognized as downstream targets of ROS, such as kinases, phosphatases, mitogen-activated protein kinases (MAPK), small G proteins, transcription factors, microRNAs, and phospholipases. In this section, we do not intend to deeply review the literature, but to show an overview of important targets and exemplify their involvement in the signal transduction by ROS in health and disease.
ROS can induce alterations in the intracellular and extracellular processes, for example, in the PI3K/AKT signaling. The lipid phosphatidylinositol 3,4,5-triphosphate (PIP3) has a function as a second messenger and is not present in the quiescent cells, but it rises within seconds to minutes when there is a stimuli. PIP3 is produced by the phosphorylation of the phosphatidylinositol 4,5-bisphosphate (PIP2) catalyzed by the phosphatidylinositol 3-kinase (PI3K). This enzyme is activated by ROS through two different pathways, or directly, throught amplications of downstream PI3K pathway, or indirectly by inhibition of the phosphatase and tensing homolog deleted on chromosome 10 (PTEN). PTEN is responsible for the degradation of PIP3 signaling, since it catalyzes the hydrolysis of phosphate in the 3′ position on PIP3 to produce PIP2 . ROS, mainly, H2O2, can oxidize and inhibit PTEN, which culminates in an increase in the PIP3 production, that acts in cell signaling, through activation of proteins, as serine/threonine protein kinase, AKT/PKB, among others [120, 121]. The AKT activation provides the transcription of several targets, such as GSK3, BAD, FOXO, p53, NF-kB, mTOR/p70S6K1 and HIF-1 [122, 123]. In this way, ROS increase the final cascade response in cell, i.e., cell cycle progression, proliferation, anti-apoptosis, invasion, autophagy and angiogenesis . The PI3K/AKT pathway hyper activated by ROS might favor carcinogenesis in the end of the process.
An important class of redox regulated proteins is the Src family of nonreceptor tyrosine kinases (SFKs), a group of structurally related kinases that catalyze the phosphorylation of tyrosine in downstream targets to regulate cellular functions coupling receptors such as the TKR, the cell adhesion molecules (CAMs), and the GPCR to the cellular signaling machinery . For example, during focal adhesion while the extracellular matrix (ECM) contact triggers a slight or partial activation of SFKs, the ROS production is associated with a strong oxidative-dependent activation and recruitment of Src kinases to cell membranes. The redox-activation of SFKs can induce sustained PI3K, protein kinase C (PKC), and extracellular regulated kinase (ERK) activation and, thereby, create conditions for tumor cell growth, invasion, angiogenesis, and resistance to apoptosis . In a variety of human cancers an increased activity of Src kinases have been described, as well as activation of important Src downstream targets such as PI3K/Akt, focal adhesion kinase (FAK), paxillin, p130Cas, signal transducer and activator of transcription 3 (STAT3) and stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK) [127, 128, 129, 130].
Carcinogenesis is also related with activator protein-1 (AP-1) transcription factor activation. Among other systems, ROS are recognized as activators of AP-1; however, the signaling transduction events involved are not totally understood. Chromium, cobalt, cadmium and vanadium are metals involved in the activation of AP-1 through signaling cascades involving the production of ROS and comprised of proteins and enzymes such as thioredoxin (Trx), redox factor-1 (Ref-1), ERK/MAPK, NADPH oxidase, I kappa B kinase (IKK), p38, JNK/c-jun [117, 131, 132].
ROS are important for the regulation of vascular tone, however an excess of reactive species might be associated with pathological dysfunction. Endothelial nitric oxide synthase (eNOS) regulates smooth muscle cells (SMC) relaxation through the production of the second messenger nitric oxide (NO) from L-arginine, which activates guanylyl cyclases to initiate the conversion of GTP to cyclic guanosine monophosphate (cGMP), which allosterically activates the cGMP-dependent protein kinases (PKGs). The enzyme eNOS can be uncoupled and, consequently, change its profile from NO synthesis to O2∙− production instead. Two major events are involved in eNOS uncoupling. First, an increase of ROS might generate the peroxynitrite (ONOO−) through the reaction of NO and O2∙−. The anion ONOO− reacts with and oxidizes tetrahydrobiopterin (THB/BH4), a cofactor of eNOS . Second, an increased ratio of oxidized glutathione (GSSG)/reduced glutathione (GSH) cause reversible S-glutathionylation and uncoupling of eNOS . Paradoxically, H2O2 produced by NADPH oxidase increases eNOS expression and NO production, but this effect is not believed to counteract the effects of oxidative stress .
Interestingly, in a scenario of reduced NO levels, in which it would be expected a lack of input signals to PKG activation (e.g., cGMP), the H2O2 can cause vasodilation through PKG oxidation . Another target of ROS is the small GTPase RhoA, which when oxidized activates its downstream partner Rho kinase (ROCK), leading to inhibitory phosphorylation of myosin light chain (MLC) phosphatase and, ultimately, to SMC contraction [137, 138]. For a more explored involvement of ROS in the regulation of signal transduction in the cardiovascular system, check the review of Brown and Griendling .
The activating or deactivating switch, in which a group of kinases is active or a group of phosphatases is active, provokes different downstream cascades with consequences in the cellular response. As we described above, several kinases are susceptible to ROS reactions, but also phosphatases are vulnerable to ROS, since they react with a group of amino acids presents in different enzymes. The reaction between ROS and phosphatases causes the oxidation and inhibition of those enzymes, increasing the kinases signaling . Another phosphatase inhibited by ROS is PTEN, which increases the PIP3 signaling, as described above.
A vascular injury promotes an increase in the expression of platelet derived growth factor (PDGF) and PDGF receptor, which in turn cause stimulation for the vascular smooth muscle cells to migrate . The activation of the PDGF receptor is controlled by the action of low molecular weight protein tyrosine phosphatase (LMW-PTP). The Cys12 and Cys17 in LMW-PTP is susceptible to a reaction with ROS resulting in a disulfide bond, and so its inactivation . Therefore, without the LMW-PTP deactivation upon PDGF receptor, its signal is amplified, which generates migration. Oxidized LMW-PTP also increases the Rho family signal, since PDGF receptor is stimulated, and it binds to phospholipase C, Src, and PI3K. As described before, PI3K catalyzes the reaction and formation of PIP3. The Rho-guanine nucleotide exchange factors are activated by PIP3, which triggers Rho-GTPase family members’ activation (Rho, Rac, and cdc42). As Nox family is activated by Rac, it produces ROS. Therefore, this process is kept by a positive feedback: generated ROS oxide Rho in a redox sensitive motif and restrain the LMW-PTP action [118, 138].
Phospholipases are enzymes that hydrolyze phospholipids and generate second messengers involved in the regulation of many physiological functions. Phospholipase A2 (PLA2) cleaves the fatty acyl group at the sn-2 position of the glycerol backbone, releasing arachidonic acid (AA) and lysophospholipid. It was attributed a role for the Ca2+-independent PLA2 (iPLA2) isoform in the excessive production of O2∙− by primed neutrophils of patients with poorly controlled diabetes. This study suggested that hyperglycemia is related to the activation of iPLA2 and AA formation which, in part, regulate NADPH oxidase activity (i.e., generation of O2∙−) .
PLA2 activation has also been related to alterations implicated in the pathogenesis of neurodegenerative diseases, such as neuronal excitation, cognitive and behavioral function, oxidative and nitrosative stress . Phospholipase C (PLC) is a well-known enzyme especially involved in the signaling transduction of GPCR coupled to Gq/11 protein and some G protein βγ subunits (PLC-β), but also in RTK (PLC-γ and PLC-ε), Ras and Rho small GTPases (PLC-ε) and Ca2+ (PLC-δ) signaling pathways, which involves the generation of the phosphate-containing head group inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG) through the hydrolysis of the membrane phospholipid PIP2 . The activation of PLC-γ1 was shown to have an important protective function during mouse embryonic fibroblasts (MEF) response to oxidative stress (H2O2) treatment . A further study suggested that this function of PLC-γ1 involved the PKC-dependent phosphorylation of Bcl-2 and inhibition of caspase-3 . Phospholipase D (PLD) cleaves a phosphodiester bold in membrane-bound lipids, similarly to PLC. However, its activity generates phosphatidic acid (PA) and an alcohol, usually choline or ethanolamine . A link between oxidative stress and PLD has been proposed by Kim et al. , in a study that suggests that H2O2 induces rat vascular smooth muscle cells tyrosine kinase activity, and PLD1-dependent PKC-α activation.
In the innate immune system, mononuclear monocytes/macrophages eliminate pathogen, antigen and cellular components through generation of ROS/RNS . When there is an imbalance in the equilibrium between oxidative/nitrosative stress and cellular requirements, the stress can generates pathological complications. Among others, rheumatoid arthritis is an autoimmune disease that has oxidative/nitrosative stress as one of the causes. The cellular immune system is vulnerable to reactions caused by ROS, which in turn can affect the regular physiological process and activates inflammatory signaling pathways that produce pro-inflammatory cytokines, chemokines and prostaglandins. The inflammatory mechanism involves synovial cellular infiltrate and peripheral blood inflammatory cells following by polymorphonuclear neutrophils and lymphocytes culminating in the joint damage [150, 151]. The signaling cascade occurs via activation of NFkB for synthesizing pro-inflammatory cytokines and chemokines . The Th1 cytokines are one of the most important because can provide the development of autoimmune disorders. These cytokines can directly or indirectly promote oxidative stress in the cells, intensifying the rheumatoid arthritis.
Prostaglandins have a pivotal role in the formation of the inflammatory response, since they mediate pathogenic mechanisms and provide the development of the cardinal signs of acute inflammation. Their biosynthesis involves the initial enzyme, phospholipase A2 (PLA2). PLA2 catalyzes the conversion of membrane phospholipids in AA. Then, cyclooxygenases convert AA into prostaglandins. Prostaglandin E2, in particular, rises vasoactive components (histamine, bradykinin, and nitric oxide), hence generating edema, pain and hyperalgesia at the local inflammatory sites, and so the inflammation . ROS stimulate this process through the activation of cyclooxygenases. Prostaglandins, also, activate NADPH oxidase, which produces superoxide anion radical . Therefore, this system becomes cyclic, ROS activate cyclooxygenases and so the prostaglandins biosynthesis, further prostaglandins trigger NAPH oxidases, increasing ROS.
The microRNA (miRNA) is a small noncoding endogenous RNA, that has an important role, since it regulates gene expression. Its function can be modified depending on epigenetic changes, chromosomal abnormalities and oxidative stress. It has been found that miRNA can respond to ROS, implying in its ability to activate certain genes transcription during stress, and this is prominent in cancer cells, which was correlated to the adaptation of these cells to unfavorable and/or hypoxic environment [130, 154, 155]. However, studies showed that some types of miRNAs can regulate gene expression of protective proteins and antioxidant enzymes [156, 157]. Some ROS dependent miRNAs play a role as oncogenic (miR21 and miR155), but interesting miR21 also targets SOD, which can be interpreted that this miRNA regulate the ROS levels in the cell. When miR21 is stimulated, it also affects the immune system through the chemokine CXCL10. CXCL10 adjusts innate and adaptive immune response by activating T lymphocytes, macrophages and inflammatory dendritic cells. The miR155 also has opposite actions, it can be oncogenic (the targets are BCL2, FOXO3a, RhoA) or tumor suppressor (the targets are TGF-beta/SMAD) . The literature about miR155 is vast, and we suggest the articles by Higgs and Slack  and Mattiske et al.  for a deep reading. Besides these two miRNAs cited above, others miRNAs are upregulated by ROS, such as miR23, miR200, miR210, etc., affecting migration, invasion; tumor growth, angiogenesis; cell cycle, DNA damage (among others), respectively .
In addition to the miRNAs that are ROS upregulated as cited above, there are ROS downregulated miRNAs important in the carcinogenic process, such as miR34 family. Some miR34 members regulate p53 causing a cell cycle arrest in G1 and apoptosis when DNA is impaired. The miR34a, for example, induce tumor suppression and metastasis inhibition. Another miRNA, miR124, has been shown to be affected by H2O2 . This miRNA is correlated to the regulation of tumor cell proliferation, migration and drug resistance through its action upon R-Ras, PI3KCA, AKT2, ROCK1, Src, DNA methyltransferases and others. The miR199a is also downregulated by ROS, some of its targets are ERBB2, ERBB3, IKKB, HIF-1alfa, ApoE, CCR7, having an effect upon cell proliferation, invasion, metabolism and metastasis [126, 161]. This is just a summary of some important miRNAs and their responses in carcinogenesis, for more information check the review Mu and Liu .
As previously discussed in this chapter, cells have a repertoire of antioxidant molecules and enzymes as a defense mechanism to an increase in ROS production. However, oxidative stress takes place when the antioxidant capacity is overwhelmed by reactive species production. In this scenario, to maintain cell homeostasis and/or terminate the ROS signal transduction there are some stress sensors that regulate the translation of antioxidant proteins. The antioxidant responsive element (ARE) is a region of non-coding DNA (short consensus sequence) which is localized upstream and regulates the transcription of many antioxidant neighboring genes such as glutathione S-transferases (GST), NAD(P)H:quinone oxidoreductase (NQO1) , heme oxygenase 1 (HO-1), γ-glutamylcysteine synthetase (γ-GCS) , metallothionein-1 and -2 (MT-1 and MT-2) , and SOD .
It was shown that ARE induction protected against oxidative stress mediated by 6- hydroxydopamine in vitro, a mitochondrial inhibitor used to model Parkinson’s disease . The nuclear-factor erythroid-2 related factor (Nrf2) is a central transcription factor involved in the upregulation of ARE-containing genes and, consequently, synthesis of proteins with antioxidant function. However, there are also nuclear factors that negatively regulate ARE-mediated gene expression, such as Mafs (MafG and MAfK), large Maf (c-Maf), c-Fos, and Fra1 .
Finally, in this section, we showed an overview of processes regulated by fluctuating levels of ROS and their molecular sensors. Furthermore, we showed that in response to oxidative stress and to maintain homeostasis, cells can upregulate the synthesis of antioxidant defenses (Figure 2).
H.U.’s research is supported by the São Paulo Research Foundation (FAPESP proj. No. 2012/50880-4).
Improved understanding of herbicide fate, effects, and environmental risks through worldwide studies is crucial to minimize impacts to nontarget organisms, especially in tropical regions rich in biodiversity . In recent years, according to Yavari et al. , there has been widespread international concern about the toxic effects of herbicides on humans, faunas, and native floras. Therefore, the adoption of agricultural practices that minimize the environmental effects of herbicides has been frequently studied, for example, the addition of biochar (charcoal or black carbon) to agricultural soils.
Biochar can be defined as the by-product of a thermal process conducted under low oxygen or oxygen-free conditions (pyrolysis) to convert plant biomass to biofuels [3, 4], where the biochar is the solid product of pyrolysis . Pyrolysis or carbonization is a process that involves the application of heat to the biomass in order to concentrate the heat and collect the by-products. It is an interesting alternative, mainly for the treatment of residues, in general of the biomass, aiming its direct application in the soil or composing the compost. The main difference between pyrolysis and incineration is that by pyrolysis it is possible to recover condensable gases (pyroligneous or bio-oil) and those that do not condense, but with high combustion power providing additional energy for the processing unit. Added to this, its contribution is notable for the fact that it minimizes the emissions of greenhouse gases (GHGs) inherent in current agriculture.
Initially, biochar, described as “Terra-Preta de Índio” by Sombroek , found in Amazonian soils, and formed by the anthropogenic addition of ceramics and artifacts, has pyrogenic carbon molecules that undergo partial carbonization and are more stable than other forms [7, 8]. Studies of the material provided the basis for agricultural use because of its properties and its benefits to the soil. Since then, biochar has been used to mitigate agronomic problems . Biochar has also been studied as an alternative in the remediation of chemical contaminants in the soil; its use has implicated in the behavior and efficacy of herbicides in the control of weeds and in the environmental impact of these products.
In addition to being found in nature, biochar can also be produced artificially, as previously described and the factors related to the pyrolytic process, such as temperature, heating rate, and pressure, can alter the recovery amounts of each final product, the values of energy of the bio-oils and the physicochemical properties of biochar , as well as the types of materials used in the firing can present different answers regarding these characteristics.
Biochar has been used in agricultural fields with positive effects on the soil microbiota , changes in soil properties, increasing surface area, pH, C/N ratio , nutrient cycling in the soil, increase of available water in the soil for the plants, soil organic matter (SOM) construction, reduction of soil bulk density, C sequestration, and reduction of herbicide transport to surface and subsurface waters . This carbonaceous material can also stabilize heavy metals and decrease their release at levels toxic to the soil , being an important alternative in soils with water deficit and nutrient deficiency. However, biochar production by pyrolysis with an incomplete combustion process can be considered as a producer of pollution, containing inorganic molecules, heavy metals, and others that can be harmful to the environment .
The use of biochar in agricultural soils as fertilizer and soil conditioner has been more exploited; however, little is known of the effect of this material on soil contaminated with herbicides and also its effect on weed control. The fate and environmental behavior of herbicides as well as their effectiveness when applied directly to the soil, such as preemergent herbicides with residual action, are strongly influenced by the retention binding with soil colloidal particles and the organic carbon (OC) content . Therefore, the addition of biochar to the soil can easily potentiate the herbicide retention process, which, in addition to contributing positively to the reduction of chemical contaminants in the environment, may exert negative effects on herbicide behavior and the effectiveness of these products in weed control. Thus, this chapter will present the general characteristics of biochar, as well as the impact of this material on sorption-desorption of herbicides in the soil.
In an approach on the issue, Brito  presents several factors that can act on the nature and yields of pyrolysis products. These factors are as follows:
Factors related to the nature of the raw material: elemental composition, density, grain size, mineral content, composition of the three main polymers (cellulose, hemicelluloses, and lignin), calorific value, and mechanical strength.
Process factors: final temperature, pressure, residence time in the heating zone, heating rate, thermal fluxes and the heat transfer coefficients resulting from the heating rate, chemical or thermal pretreatments in the biomass.
However, here are described the aspects related to the use and efficiency of biochar in the soil, aiming to increase the properties of the set.
In general, the action of heat on biomass via pyrolysis (>500°C) results in a more graphitized (aromatic) material—biochar, with greater specific surface area and reduced abundance of surface functional groups. In contrast, lower pyrolysis temperatures (<400°C) produce biochars with low specific surface area, which implies partitioning and specific interactions with functional groups on the surface of the biochar . Porosity is another property affected. Higher temperatures result in a more porous biochar when compared to those obtained under lower pyrolysis temperatures [17, 18]. According to the authors, the increase in porosity is accompanied by an increase in the proportion of micropores that contribute significantly to the sorption power of these carbonaceous materials. For example, Figure 1 shows an image of scanning electron microscope (SEM) of the biochar derived of Eucalyptus spp. The biochar exhibits a rough and irregular surface that is characteristic of the material.
In addition, it is well known that desorption is affected by the extent of herbicide absorption in the biochar, controlling its bioavailability in agricultural soils. In general, higher pyrolysis temperatures lead to higher hysteretic desorption processes . One of the intrinsic properties of biochar is its sorption capacity. It may influence other properties, such as mechanical resistance. In India, Japan, and some European countries, its high sorption capacity has been used to capture radioactive elements in the soil, such as Radon (Rn), for example .
According to Andrade and Della Lucia , this sorption is related to the high porosity of biochar, given by the difference between its actual specific mass and its apparent specific mass. The authors also stated that the porosity of the biochar is linked to the final temperature of the applied pyrolysis, but also add to the existence of the effects of the density of the raw material that gave rise to it.
According to Maia et al. , the chemical composition of the biomass is highly variable according to the botanical species and the biomass part (leaves, branches, wood, bagasse, residues from the extraction of vegetable oil, among others) and thus significantly influences the products obtained after pyrolysis. Added to this, they provide anatomically distinct biochar (porosity, grain size, among others).
Yu et al.  observed higher sorption of diuron in the biochar produced at 850°C than in another produced at 450°C. According to Sharma et al. , the elevation of the pyrolysis temperature in the biochar production can raise the retention potential of organic contaminants when applied to the soil; however, the pyrolysis at 400°C decreased the surface area of the biochar particle. Chen et al.  indicated that at the temperature of 700°C, the biochar presents half of the specific surface area. The temperature of the process determines the type of carbon present in the biochar, with the reaction time being less decisive .
Thus, porosity constitutes one of the most relevant characteristics for biochar aiming at its application in agricultural soils. Yu et al.  studied the sorption capacity of herbicidal agents in soils with and without the incorporation of biochar produced from different species and pyrolysis temperatures. The results obtained by the authors showed better results for the soils that received the biochar obtained at the highest temperature, regardless of the biomass used.
The density of feedstock is of great importance in obtaining the biochar, since for the same mass, one desires good yields in conversion of the biochar, provided that the density of the feedstock is high. In practical terms, the higher the density of the source material, the higher the density of the biochar after the pyrolysis. Some studies report that the increase in density is associated with the increase of the lignin content, which implies higher yield in biochar. According to Pétroff and Doat , lignin is rich in carbon and thus favors conversion.
The true density is another variable that makes up the biochar, that is, the apparent specific mass discounting the volume of the internal porosity. When the true density is related to the apparent density, the porosity is measured. The porosity as mentioned above (Section 2.1) is the measurement of empty space, constituting an intrinsic characteristic of biochar with direct influence on its hygroscopicity, reactivity, combustion performance, and sorption capacity. The true density is dependent on the temperature of the pyrolysis used, where the higher the temperature, the higher the true density and, consequently, the greater the porosity of the biochar produced. In order to determine the porosity of the porous surface, the porous surface of the porous surface of the porous surface of the porous surface of the porous surface of the porous surface of the porous surface of the porous porous surface of the porous porous porous surface of the porous porous porous pores. Thus, calculated as a function of apparent density and true density, the porosity will increase as a function of the apparent specific mass of the biochar .
The addition of biochar to the soil, in granulometry similar to the fractions that make up the sand, silt, and clay, can alter the limits of consistency, improve water retention capacity, increase pH, and contribute to soil structure improvement . This fact is attributed to the great presence of positive charges at the ends of the carbon chains of the biochar. The subsidized hypothesis for this is that the higher the pyrolysis temperature, the higher the fixed carbon content, and consequently the greater the presence of positive charges in the structure obtained from the biochar.
Clay and Malo  found that the biochars of corn and Panicum virgatum, produced at high temperatures (>650°C), regardless of processing time, were very alkaline (pH > 9). In processes with lower temperatures (<550°C), the materials had pH < 5.
However, Yang et al.  considered the pyrolysis temperature more relevant in the production of biochar for the treatment efficiency of contaminated soils than the material used. In studies comparing the pyrolysis temperature of the wood biochar of Pinus radiata, the herbicide terbuthylazine was more sorbed in the biochar obtained at 700° C than those obtained at 350°C . In contrast, Li et al. , studying the leaching of 2,4-D and acetochlor, found a reduction in leaching and amplification in herbicide efficacy with biochars produced at low-temperature (350°C) pyrolysis.
In practice, the biochars obtained by higher pyrolysis temperatures have a higher capacity to raise the pH of the soils that were initially incorporated.
The biochar obtained from the pyrolysis biomass generally presents low content of nitrogen and hydrogen, which results in a high C:N and H:C ratio . Oxygen is the second most abundant element of the material and its content is inversely related to the final pyrolysis temperature applied. There are also ashes, which come from the mineral elements mainly from bark. In the ashes, potassium, calcium, phosphorus, and sodium predominate. The composition of the ashes is strongly related to the chemistry of the soils where the original biomass was developed. These properties give the biochar a great capacity of persistence in the soil. In studies of dating of biochar fragments (coal) found in soils, it is common to observe samples with thousands of years . Some studies also suggest the positive effect of the coals on physical water properties of soils, increasing their retention capacity and humidity . In general, the pyrolysis gives rise to the C:N and H:C ratio in relation to the raw material that gave rise to the biochar.
The immediate chemical composition is formed by the contents of volatile materials, ashes, and fixed carbon. The denominated fraction of volatile material is emitted during the heating of the biochar constituted of molecules of CO, CO2, and hydrocarbons. Another amount of carbon remains relatively intact, and as it is not eliminated along with the volatile material, it is called the fixed carbon. In practice, the content of volatile material and fixed carbon is determined by heating the biochar at a temperature of around 900°C. Ash is the residue of mineral oxides obtained by the complete combustion of the biochar. The oxidized residue obtained is calculated as the biochar ash content.
Biomass, when subjected to the action of heat, at high temperatures undergoes a process of transformation, in which all its components are extensively modified . Better biochar immediate properties of biochar—higher fixed carbon content and lower volatile and ash content—are associated with high lignin feedstocks for certain pyrolysis conditions. Each temperature range generates a different product, and the final temperature has a great influence on the final characteristics of the biochar.
Singh et al.  indicated a difference in the values of the total carbon fixed in biochars of different materials, so that this content was higher in those coming from eucalyptus wood and eucalypt leaves when compared to cattle manure. The higher carbon content influences the sorption capacity of the herbicides. Cabrera et al.  verified the effect of different biochars on sorption of bentazone so that the sorption increased according to the organic carbon contents dissolved by the materials. For aminocyclopyrachlor, Cabrera et al.  found higher sorption in biochars that contained larger surface area and humification index. Exemplifying the hypothesis that each herbicide interacts with the biochar properties govern its behavior in the environment.
In conclusion, the properties of biochar are attributed to the conditions of pyrolysis, temperature, and time, as well as the raw material used . In Brazil, due to the high availability of wood and its derivatives, besides the agricultural base residues, these materials are the basis of the biomass used as a raw material for conversion into biochar. These materials contain a high proportion of cellulose, hemicelluloses, and lignin, the main compositions being converted into carbon matrix in the formation of the biochar, so that these aggregates and their proportional abundance determine the properties of the biochar produced .
The environmental performance and herbicide efficacy applied to the soil are strongly influenced by the retention with the soil particles, mainly the OC content , which can be potentiated with the addition of biochar. The herbicide-soil interaction is directly related to the physical-chemical properties of the products and the soil, in addition to the environmental conditions. The impacts of biochar on sorption and desorption of herbicides are presented in Table 1, according to the surveys of recent years on this subject.
|Feedstock||Herbicide||Retention of herbicide||Effect||Source|
|Wheat ash (1%)||Diuron||Sorption||Increased sorption (fourfold) in amended soils||Yang and Sheng |
|Biochar derived from wheat (0.05, 0.5, and 1%)||Diuron||Sorption||Increased sorption (7- to 80-fold) with 1% of biochar||Yang et al. |
|Biochar derived from Eucalyptus spp. (450°C) at 0.1, 0.5, 1.0, 2.0, and 5.0% application rate||Diuron||Sorption||Increased sorption (7- to 80-fold) with 1% of biochar||Yu et al. |
|Biochar derived from Eucalyptus spp. (50°C) at 0.1, 0.2, 0.5, 0.8, and 1.0%||Diuron||Sorption||Increased sorption (5- to 125-fold) in amended soils||Yu et al. |
|Biochar derived from sawdust||Atrazine and acetochlor||Sorption||Increase of 1.5-fold the Kd for acetochlor. Sorption of atrazine was also increased||Spokas et al. |
|Sewage of dairy products, 200°C for 4 h, 350°C for 4 h||Atrazine||Sorption||Increase of the sorption in the biochar 200°C in amended soil||Cao et al. |
|Biochar derived from charcoal (350°C)||Terbuthylazine||Sorption||Increased sorption (2.7-fold) in amended soils||Wang et al. |
|Biochar derived from sawdust (700°C)||Terbuthylazine||Sorption||Increased sorption (63-fold) in amended soils||Wang et al. |
|Hardwood sawing (500°C), hardwood (540°C), and wooden pallets (>500°C) at 2%||MCPA and fluometuron||Sorption||Sorption increased by 240–5200%||Cabrera et al. |
|Biochar produced from chicken bed and wheat straw (400°C)||Fluridone and norflurazon||Sorption||Sorption increased by 24- and 36-fold, respectively||Sun et al. |
|Paper mill slurry (500°C) at 1–5% and chicken bed (500°C) at 1%||Diuron and atrazine||Sorption||Increase of sorption of diuron in 220–448% and atrazine in 270–515%||Martin et al. |
|Eucalyptus sp. (0.1–1%)||Isoproturon||Sorption||Sorption and hysteresis increased||Sopeña et al. |
|Pallets of wood (>500°C), macadamia nuts (850°C), and hardwood (540°C) at 10%||Aminocyclopyrachlor and bentazone||Sorption||Increased sorption of 18–240% for aminocyclopyrachlor and 13–35% for bentazone||Cabrera et al. |
|Beechwood (550°C) at 1.5%||Imazamox||Sorption||Increased sorption of <5%||Dechene et al. |
|Biochar produced from sugarcane bagasse, soybean meal, wood chips, among others||MCPA, nicosulfuron, terbuthylazine, and indaziflam||Sorption||Increased sorption for all herbicides||Trigo et al. |
|Biochar derived from corn silage (750°C) at 0.5%||Isoproturon||Sorption||The amount of bioavailable herbicide was reduced 10- to 2283-fold in treatment with biochar||Eibisch et al. |
|Biochar of wood pallets (650°C), wood chips (500°C), and corn bran (490°C)||Aminocyclopyrachlor, picloram, metsulfuron-methyl, oxyfluorfen, and alachlor||Sorption||Aminocyclopyrachlor, metsulfuron-methyl, and picloram showed relatively low sorption; alachlor intermediate sorption and oxyfluorfen heavily sorbed||Hall et al. |
|Biochar of pecan, cherry, and apple flakes (350, 500, 700, and 900°C) and wooden pallets (350, 500, and 700°C)||Glyphosate||Sorption||Sorption increased according to the pyrolysis temperature (higher at 900°C), depending on concentration||Hall et al. |
|Biochar derived from sawdust (700°C)||Terbuthylazine||Desorption||Reduced desorption||Wang et al. |
|Sawing of hardwood (500°C), hardwood (540°C), and wooden pallets (>500°C) at 2%||MCPA and fluometuron||Desorption||Reduced desorption in 85- to 3000-fold||Cabrera et al. |
|Eucalyptus sp. (0.1–1%)||Isoproturon||Desorption||Reduced desorption in amended soils||Sopeña et al. |
According to Khorram et al. , sorption is the first process that occurs after the addition of the herbicide in the soil. Thus, retention is an important factor that directly affects other processes, such as herbicide transport via leaching, surface runoff, and volatilization, as well as bioavailability and impacts on nontarget organisms . High OC content, higher surface area, and more porous structures result in higher herbicide sorption capacities . In soils amended with biochar, sorption of herbicides can be increased, reducing the risks of contamination and exposure in the ecosystem and human health . Martin et al.  found an increase in the Freundlich sorption coefficient (Kf) of atrazine using chicken litter biochar at the dose of 10 t ha−1, when compared to soil without biochar. Tatarková et al.  indicated that sorption of MCPA (4-chloro-2-methylphenoxyacetic acid) by biochar without soil and by soil amended with biochar (1.0% m m−1) was 82 and 2.53 times higher than in unamended soil, respectively. For one of the most used herbicides in the world, atrazine, a study demonstrated an increase in the Kf value of 5 for sandy soil and 4.3 times for clayey soil with the 1% amended of wheat biochar . Xu et al.  reported increased sorption and Kf values of 1.5 and 3 times in soil, when there was amended soil in 0.1 and 0.5% with rice straw biochar.
The pyrolysis temperature, as previously reported, alters the properties of the biochar, thus altering its relationship with herbicide retention. Using the wheat straw biochar, with a firing temperature of 300°C, Spokas et al.  reported the high sorption capacity of the sawdust biochar (5% m m−1) in sandy soil for atrazine and acetochlor, attributed to the high OC content (69%) and specific surface area (1.6 m2 g−1) of biochar. Hall et al.  in glyphosate study obtained higher values of sorption of the herbicide according to the increase in the temperature of burning of the materials, where the highest retention was reported in biochar produced at 900°C.
Another important feature affected by the pyrolysis temperature is the chemical composition of the biochar surface, so that the types of molecules present directly influence the retention of the herbicides. Sun et al. , analyzing the composition of wood and grass biochars under different thermal treatments (200–600°C), found the presence of cellulose, hemicellulose, and lignin groups (more present in wood biochar). At temperatures between 400 and 600°C, the noncarbonized aromatic carbon signals from the remaining lignin were identified and disappeared at temperatures above 500°C. The aliphatic carbon appeared between temperatures of 300 and 600°C, but above 600°C, was not pronounced. These variations interfere with the sorption capacity of the biochars, which in this experiment were exemplified by the sorption of fluridone in portions of aromatic biochar and values of sorption coefficient normalized as a function of the OC content (Koc) higher than in the others, which were present in low temperatures.
Biochars that contain high specific surface area contribute to the increase of soil sorption of herbicides, as shown by Cabrera et al. , where they found almost complete sorption for aminocyclopyrachlor and bentazone in soils amended with biochar from wooden pallets (500°C). Sopeña et al.  indicated that biochar of Eucalyptus dunni with high specific surface area, sorption of isoproturon was five times higher than in unamended soils with biochar. When using wood of Pinus radiata as raw material for the production of biochar (350 and 700°C), Wang et al.  found increased terbuthylazine sorption in areas that had increased OC content of the soil and specific surface area with the addition of biochar. The total volume of pores in the soil indicates the greater or lesser specific surface area for the sorption of a herbicide. Biochar increases this amount of pores in the soil, as identified by Sandhu and Kumar . The total pore volume is also related to the pyrolysis temperature, which, according to Wei et al. , is reduced at temperatures higher than 500°C for rice bark biochar, but there is increase in specific surface area (greater amount of micropores). In this same study, the pyrolyzed biochar at 750°C with a smaller pore diameter (9.23 nm) provided stronger sorption capacity for metolachlor due to intraparticle diffusion mechanisms and pore filling.
Besides the effect that the physical properties exert under the sorption of the different herbicides, the chemical properties also demonstrate strong influence in this process. The pH variation confers different sorption behaviors of ionic and nonionic molecules to the biochar as adsorbent in the soil. Sun et al.  observed that for norflurazon, the pH change had no effect on the sorption of this, because the nonionic molecules of the herbicide bind to the sorption sites and remain unaffected. For fluridone, sorption in both biochars (wood and grass) decreased with the pH increase because it ionizes at pH values lower than 4 (acid ionization constant—pKa = 1.7); however, in regions in which the pH remained higher (pH 4–14), there was no ionization, and that with the increase of the proton concentration, there was the conversion of negative ionic functions to neutral sorption sites that fluridone probably binds.
Yang et al.  observed a decrease in sorption of diuron with increasing pH in the wheat biochar due to the alteration of the surface charge properties by deprotonation of the functional groups over the pH range. The study also evaluated pH variation in bromoxynil sorption, as well as Sheng et al. , both of which verified sorption of the herbicide in soil amended with wheat biochar was higher at low pH than at high pH. This fact can be justified because its pKa is 4.06, and at pH < 4.06, this herbicide is in its molecular form, and already at pH > 4.06, it will be in the anionic form, which leads to the repulsion of loads on the soil and lower sorption of the product. In the case of ametryn, when in soil without biochar, the sorption of the herbicide occurred at lower pH and, when in addition to the biochar, the higher sorption occurred at higher pH, showing the increase of the affinity of the herbicide with the biochar due to protonation .
The herbicide sorbed by the soil particles usually returns to the soil solution, that is, it is desorbed to be available again to the transport by absorption and degradation; however, this process in soils with biochar was less studied when compared to the sorption process .
In contrast to sorption, desorption decreases when biochar is added to the soil and the herbicide eventually returns to the soil solution, and sorption can often become irreversible. Irreversible sorption of herbicides was reported by Yu et al. , Wang et al. , and Sopeña et al. , which included sorption of the herbicides in the specific surface area, trapped in micropores, and partitioned into condensed structures of the biochar particles. In a study with wheat straw at 1% (m m−1), Tatarková et al.  found the reduction of MCPA sorption from 64.2% in unamended soil to 55.1% in biochar amended soil. For Loganathan et al. , the amount of atrazine remaining in sandy and clayey soils amended with biochar was higher than in unamended soils. Cabrera et al.  found the almost negligible desorption for aminocyclopyrachlor in biochar amended soil from wood pallets (>500°C). Wang et al.  also reported slower and lower desorption rates for terbuthylazine in soil amended with sawdust biochar produced at 700°C, followed by biochar produced at 350°C. On the other hand, Khorram et al.  have reported easier desorption of fomesafen molecules weakly bound to rice bark biochar because the specific surface areas of the material are relatively low.
The reduction in herbicide desorption and consequently the lower concentration of these in the soil solution is more evident in altered soil with biochar in relation to the soil amended with the material. In this sense, there is, in general, a lower leaching potential of the herbicides and lower bioavailabilities of these, both for the degradation and the control of weeds.
Another important factor in the properties of biochar is the action of time, which can alter the interaction of biochar with herbicides. Some physical processes such as breaking of the structure of biochar by the action of the climate can increase the specific surface area of the biochars  and the sorptive capacity of these materials. Kumari et al.  obtained higher specific surface area and cation exchange capacity (CEC) in soils amended with wood biochar (500°C) after 7–19 months, resulting in increased sorption of glyphosate in all soils of the study. Trigo et al.  observed an increase in the sorption of metolachlor in soils amended with distinct biochars over the years (macadamia: fresh = 2.4 times, 1 year = 2.5 times, 4 years = 1.9 times, wood: fresh = 2 times, and 5 years = 14 times). Martin et al.  evaluated sorption of atrazine and diuron in soil aged with biochar (10 t ha−1) for 32 months. In this study, with fresh soil amended with biochar, there was an increase of twofold to fivefold in sorption of the herbicides in relation to the soil without biochar. With 5 years of aging, the biochar presented, in experiment with biochar of residues of the production of mushrooms and rice husk (70%) and peels of cotton seeds (30%) (400°C), increases in specific surface area of 98–114.3% according to Dong et al. . However, the average pore diameter decreased and the surface was more propitious for material leaching. Structurally, there was no difference in fresh material.
On the other hand, in some studies, the aging of the biochar particles in the soil resulted in the reduction in the sorption capacity of the herbicides by reducing the specific surface area and the porosity of the biochar as the material aging, blocking the pores and the sorption sites, especially for high molecular weight molecules . Martin et al. , in the same experiment mentioned previously, obtained for the diuron 47–68% reduction in the sorption capacity in relation to the control soil, which may be due to the clogging of the pores of the soil particles over time. Cao et al. , when aging rice husk biochar (500°C for 30 minutes) in soil, for a period of 13 months, found reduction in carbon and nitrogen content, reduction in pH values close to neutralized, and reduction in porosity and specific surface area of the biochar.
The degradation of the biochar particles throughout the weathering process alters their mass and their effect on herbicide remediation. Dong et al.  found a loss of biochar mass of ~40% over 5 years, regardless of the amount applied (30, 60, or 90 t ha−1). Other authors have also observed loss of mass over time [68, 69]. Aging of the biochar can also entail structural changes in the material. Trigo et al. , analyzing the surface of aged biochars for 1 and 2 years, found clay minerals adhered by the addition of biochar to the soil, carboxylic acids covering the structure, and the elimination of fatty acids throughout the incubation periods, thus altering the types of possible connections to be made with this surface and the herbicide retention capacity. In the same study, after 2 years of incubation, the specific surface area on the biochar particles was even larger and the pores filled with mineral material. During the incubation period, there was a reduction in the OC content, from 80.4 mg L−1 of fresh biochar to 31.6 mg L−1 of biochar with 1 year of incubation, which may be due to natural elimination or degradation. However, these structural and chemical changes that alter the sorptive capacity of the biochar vary according to the nature of each material, as well as the pyrolysis temperature and the conditions and time at which the material will be incubated in the soil.
The application of biochar in the remediation of soil contaminant herbicides is an interesting management alternative due to high sorption and low desorption with these chemicals. Although the use is well explored worldwide, there is still a need for further research because of contradictory results regarding the benefits of using biochar and its effects on production, plant protection, and environmental contaminants. In addition to verifying the different interactions of the biochar and its properties in agricultural soils, it is worth emphasizing the importance of the possibility of using different materials in the production of biochars derived from different agricultural and industrial activities, which can promote the rational use of resources and destine them to agricultural production and minimization of the environmental impacts caused by these activities.
The authors would like to thank the São Paulo Research Foundation (FAPESP) process 2016/17683-1, for the financial support.