Biochar: The Black Diamond for Soil Sustainability, Contamination Control and Agricultural Production

Production of biochars from agricultural wastes reduces signiicantly the volume and weight of the wastes, and hence, it can be considered as a promising means for managing the agricultural wastes. Biochar has received great interest during the last few years, due to its beneicial role to mitigate CO 2 emission through enhancing the long-term carbon sequestration. The efects of biochar on soil properties vary widely, depending on the characteristics of soil and the biochar. Most types of biochars are of alkaline nature and of high C content. Addition of biochar to the soil can improve the cation exchange capacity enrich soil with the nutrients and enhance the microbial growth, and improve some soil physical properties such as water retention and aggregation. For contamination control, biochars have proven to be a suitable tool for controlling the contaminants in the environment. The high surface area, porous structure, alkaline nature, and the presence of functional groups characterized the biochar as alternative option for the remediation of heavy metal contaminated waters and soils. However, there is a lack of knowledge regarding the efects of biochar in the presence of mineral and/or organic fertilizers on the plant growth and nutrient transformation in soils. In addition, biochar is successfully used for treating the acid soils; therefore, future studies are needed to investigate the neutralization of alkaline performance of biochar to be used safely in alkaline soils.


Introduction
The increasing demand for food and the fertilizers (inorganic and organic) is a day fact. Mineral fertilizers are of great importance for food production. Accordingly, the world demand for mineral fertilizers has increased during the last few decades to meet the increased demand

Production and characterization of biochars derived from diferent wastes 2.1. Historical view of the biochar
Biochar production is an ancient practice over that past 70 centuries in the Egyptian societies. It seems that the production of biochar was not the main target, the Egyptian societies used the liquid wood tars to embalm the bodies of their dead, and the liquid preserving agent was produced from charring processes [11]. Similarly, the use of biochar as soil amendment irst began over the past 2,500 years in South America (terra preta), the place which named "the black earth." Biochar is created both naturally by forest ires and by human through burning bits for diferent practices, that is, cooking and manufacturing. Terra preta is a famous soil located in the Amazon Basin. The acidic condition of terra preta in the past due to the toxic levels of exchangeable aluminum hindered the agricultural production; however, the continuous accumulation of biochar in the soils led to enrich the soil in calcium and phosphate and elevated pH level in comparison with the surrounding soils.
In addition, terra preta soil contains about 50 Mg ha −1 carbon in a form of biochar within approximately 1 m depth [12]. Consequently, aluminum toxicity in this soil was neutralized, and soil status in terms of physical, biological, and chemical features has been modiied that made it one of the most fertile soils over the world. The promising beneits of biochar have alerted the sign for researchers in the past to determine the positive performance of biochar, for example, the role of biochar for improving vegetative growth and enhancing soil fertility has been studied by Trimble [13] and Retan [14]. Due to the several beneits of biochar, many researches and extension initiatives of biochar have been established all over the world in order to spread the knowledge and cooperation of biochar and its applications, for example, the Australia New Zealand Biochar Research Network (www.anzbiochar.org/project.html), the US Biochar Initiative (htp://biochar-us.org/biochar-research), the European Biochar Research Network (htp://cost.european-biochar.org/en), the UK Biochar Research Center (htp://www.biochar.ac.uk/), the China Biochar Network (htp://www.biochar-international.org/chinanetwork), the Japan Biochar Association (htp://www.geocities.jp/yasizato/ JBA.htm), the New Zealand Biochar Research Centre (htp://www.massey.ac.nz/massey/ learning/colleges/college-of-sciences/research/agriculture-environment-research/soil-earthsciences/biochar-research-centre/biochar-research-centre_home.cfm) and the Biochar India (www.biocharindia.com).

Biomasses for biochar production
The rapid population growth led to subsequent increases in food production, and consequently, large amounts of organic residues are produced annually [8]. Therefore, it is essential to recycle their organic residues efectively. Various types of biomass have been used for biochar production, including: (i) agricultural and forestry by-products, that is, wood chips, straw, nut shells, rice hulls, tree bark, wood pellets, and switch grass, (ii) industrial by-products, that is, sugar cane bagasse, paper sludge, and pulp, (iii) animal wastes such as chicken liter, dairy and swine manure, and (iv) sewage sludge. Producing the biochar from biomass, especially wastes ofer an excellent way for the recycling of wastes into beneicial materials. Pyrolysis treatment reduces the volume of biomasses by 44-90 and 75-80% and weight by 44-93 and 71-77% [8,15].

Production technologies of biochar
Biochar is produced through the pyrolysis process, in which the biomasses are burned in the absence of oxygen. As mentioned above, the main objective of biochar production is to use it as a soil amendment or for usage in other aspects such as remediation and industrial technologies. The process is closely similar to those of gasiication; however, in case of gasiication, the process is performed in two steps, irstly, the biomass is heated to around 600°C, and hydrocarbon gases and tar are evaporated; secondly, char is gasiied by reaction with oxygen, hydrogen, and steam under high temperature. However, in case of pyrolysis, the biomass is burned in the absence of oxygen along the production time. There are many important secondary products upon producing the biochar, including a synthetic gas that can be used to generate electricity and bio-oil, which can be used as diesel fuel. As shown in Table 1, biochar can be produced through fast and slow pyrolysis techniques; the main diference between them is the heating rate and the amount of the produced bio-oil. Figure 1 shows the development of biochar production. The people used to simply gather piles of agricultural wastes and cover them and burn them slowly with limited air. They have used several ways to exclude air penetration into burning places, such like covering with soil particles. This traditional method is still used today in developing countries; however, considerable amounts of smokes and almost half amount of carbon dioxide in the original biomass are released into the atmosphere. Briely, biomasses were put together tightly and covered with a layer of soil in a large pit kiln then a small part of the biomass was burned up. To achieve a successful pyrolysis process, people used to make small holes in the soil surface to provide amount of air uniformly in order to maintain a productive balance between burning and pyrolysis. The pit kiln has some disadvantages, that is, the release of almost 50% of C into the atmosphere and the high ash content of the produced biochar. To overcome these problems, brick kilns were developed to achieve more control for aeration. These kilns were beter insulated and allowed a beter airlow control, which allowed higher biochar

Parameter
Biochar production
Engineering Applications of Biochar yields and lower ash contents of the produced biochar. The above-mentioned techniques are in situ biochar production units, where the biochar was made at places where suitable raw material was abundant. By beginning of the 1930s, transportable, cylindrical metal kilns were developed in Europe and became popular in the 1960s, in developing countries. They are often made out of oil drums and are more easily to handle than traditional pits. The sealed container allows a high control of airlow, and the biochar can easier be recovered [17]. The portable kilns are still used in developing countries in the small farms and have been used experimentally by Abdelhafez et al. [8,9] in China and Egypt, respectively. However, the traditional methods may contaminate the environment due to the emited syngas and bio-oils. Therefore, advanced instruments have been developed successfully to eliminate the emited syngas and bio-oil and to use them as by products by using speciic condensers for gas and bio-oil collection.

Diferences between biochar and charcoal
Man used to create charcoal instinctively for heating, industry and production beginning from the creation. Both biochar and charcoal contain high carbon materials; however, there are some major diferences as follows [4]: (i) Charcoal is produced primarily as a source of energy, while biochar is manufactured as a soil amendment for improving soil fertility, carbon (C) sink, or water iltration.
(ii) Wood is the major source of charcoal production; however, biochar can be produced from any biomass.
(iii) The carbonization trend of biochar is not complete as in charcoal; consequently, charcoal contains much ash content compared to the biochar.
Carbon is present in the biochar in a form of six C atoms linked together. The formation of graphite is more likely to occur when the C atoms arranged together without O or H ions. However, in case of biochar, graphite does not form because the arranged atoms of carbon are corrupted by O and H ions; as a result, C atoms are arranged irregularly according to the type of biomass used for biochar production and temperature of pyrolysis [4].

Physicochemical characteristics of biochar
All biochars are black but are not created equal and are not of the same physicochemical characteristics. Both the types of biomass and pyrolysis conditions play important roles for identifying the characteristics of the produced biochars [5,18]. The produced material of biochar is a solid, structured, carbonaceous material and exhibits a high surface area [19], low oxygen and hydrogen contents [20], and litle amount of nutrients [21,22]. The physical characteristics of the produced biochar depend mainly on the type of biomass and the pyrolysis conditions, in terms of, heating rate, highest temperature of burning, pressure, burning time and the characteristics of burning vessel. It is well known that organic materials start to decompose after 120°C; hemicellulose compounds decompose at 200-260°C, and lignins decompose at 240-350°C [23]. Biochar has proven to be a suitable tool for the removal of heavy metals from aqueous solutions [10] due to the presence of macrospores with an average pore size of 51-138 m 2 g −1 [24],. The presence of functional groups on the surface of biochar candidate it for the removal of organic and inorganic contaminants from aqueous solutions. Abdelhafez and Li [10] demonstrated that the spectrums of sugar cane and orange peel biochars are quite similar; both biochars exhibited absorption bands on 3448. 13 [25,26]. Therefore, biochar contains much amount of alkali metal ions causing its liming performance when it is applied to the soils [8,9]. As shown in Table 2, more than 80% of the produced biochars is C, while nitrogen contents are relatively low because most of nitrogen in the feedstock starts to be volatile at temperature above 200°C. Therefore, the nitrogen contents of biochars derived from agricultural wastes are quite low. However, the nitrogen content of sewage sludge biochar seems to be higher than the agricultural wastes biochars [15]. Furthermore, most of the stated biochars characterized by its high pH values, and this could be atributed to the presence of alkaline metal ions, that is, Ca, Mg, and K, which are stable and does not volatile in the biomass during the production of biochars. The previous studies demonstrated that increasing the pyrolysis time and temperature led to increase the surface area and pours structure of the produced biochar [27,28]. Similarly, the pH of the produced biochar depends on the pyrolysis temperature and time; by increasing the pyrolysis temperature, the pH of the produced biochars increased to reach 11.5 in some studies [29]. A point to note that, biochar has a liming efect when it is applied to the soil; therefore, possible increment in soil acidity (pH) might occur [8]. In addition, adsorption of macronutrients (N, P and K) on the surfaces of biochar might hinder its uptake by the growing plants. Applying biochar to the soils has been found to increase the bioavailability and plant uptake of phosphorus (P), alkaline metals and some trace metals [30], but the mechanisms for these increases are still a mater of speculation. Moreover, the beneits of biochar for the removal of organic and inorganic contaminants from water are well documented [31,32]. However, to date, only limited studies are available on biochar efects combined with diferent mineral and organic fertilization levels on soil properties and plant growth. The behavior of biochar is not equal for all elements; some studies have reported that biochar has the potential for the stabilization of Pb in shooting range and metal smelter contaminated soils [7,10]. Abdelhafez et al. [7,8]. illustrated the beneicial efect of biochar for soil improvement and Pb remediation in a military shooting range and metal smelter contaminated soils. Moreover, it was found that biochar increased the bioavailability of Cu (shooting range soil) and As (metal smelter soil). Therefore, the chemical behavior of biochar with heavy metal ions is not constant and needs to be investigated.

Fresh organic mater versus biochar as soil amendment
Soil organic carbon is originated by photosynthesis under highly reduced conditions (estimated by 600 mV) which are presented in leaf chloroplast [42]. Such fresh materials are probably the most reduced fraction when added to soils, acting as electron pumps to more oxidized species [43]. Generally, organic residues are used as amendments to improve soil quality and productivity [44].
The organic amendments that persist longer in soil might exert high impacts on soil physicochemical reactivity [45]. In deep soil layers, organic materials are relatively more stable than in the surface ones due to the absence of fresh organic carbon, an essential source of energy for soil microbes [46]. Probably, compounds that contain less oxygen (lower electron richness) are less easily decomposed than do compounds having comparable size, solubility, and molecular complexity [47].
Speciic mechanisms might guarantee stabilization of organic C in soil, for example, biotic exclusion which might take place through adsorption of organics and aggregation with soil minerals forming mineral-bound OM [48]. Also, preservation of recalcitrant (stable) compounds might stabilize organic C [49]. It is thought that the recalcitrant compounds are present in organic materials in much higher proportions than those classiied as labile [50].
Pyrolysis is the converting of unstable organic mater into more stable forms (biochar) that can be applied to soils [51]. This can be atained by heating carbon bearing solids in the absence of oxygen [52] to produce porous materials of low density [53] and more stable forms of carbon [54] which are more resistant to biodegradation as compared to fresh organic materials.

Efect of biochar on soil properties
Biochar is used as an amendment to improve soil properties. It improves soil-water holding capacity [56,57], saturated hydraulic conductivity [58], increases cation exchange capacity (CEC) [8,59], decreases bulk density [60], and minimizes the loss of nutrients and other agricultural chemicals in soil run-of [4]. It also decreases soil penetration resistance and increases aggregation and iniltration [61]. On the other hand, biochar does not show any signiicant efect on soil porosity either directly through pore contribution, or indirectly through improving aggregate stability [62]. Besides, applications of biochar increase soil electrical conductivity (EC) in addition to its high contents of phosphorus and nitrogen [63].

Efect of biochar on the plant growth and soil biota
The main roles of biochar for enhancing plant growth are directly through its nutrients contents, and indirectly through its efects on nutrients use eiciency. Biochar serves as energy provider [64] for wheat [65], rice [36], maize [6], soy bean [66], and spring barley [67]; thus, it improves Engineering Applications of Biochar root density, crop growth, and productivity [68]. It was found that chicken manure-derived biochar increased the dry weights of the shoot and root of the Indian mustard by 353 and 572% upon its application to soil at a rate of only 1% [69]. Even biochar produced from wastewater sludge increased the productivity of cherry tomatoes by 64% as compared to the control [63].
Thus, such amendment is recommended for low-fertile and degraded soils [4] as well as highly weathered soil [70]. The zone of plant rhizosphere becomes larger with application of biochar [71]. Moreover, biochar increases plant resistance toward biotic stresses [72]. Some types of biochar amendments are rich in nutrients [73], and on the other hand, it minimizes the leaching of nutrients from soil, i.e., nitrate [74], ammonium, and probably phosphate [75]. However, the majority of biochars produce ethylene which is an inhibitor for soil microbes [68], beside of the released organic molecules which might suppress activities of some beneicial soil biota [76].

Biochar: alternative option for soil sustainability
Using biochar as a soil amendment can fulill three main targets, that is, increasing plant productivity, thus achieving food security [4], improving soil properties, and disputing land degradation [77] beside of minimizing the change of climate [78]. Moreover, biochar changes organic wastes into value-added biochar which acts as sorbents for eliminating contaminants in wastewater [79]. As mentioned above, the transformation of terra preta soil into a high fertile soil due to biochar addition is a great evidence of the role of biochar for soil sustainability. The recycling of agricultural wastes into beneit materials guarantees the sustainability of agricultural lands.

Efect of biochar on CO 2 emissions
Soils can store more carbon than do plants or atmosphere [47]. Globally, soil organic mater (SOM) contains about three times as much carbon as either the atmosphere or terrestrial vegetation [80]. In soils of low N content, CO 2 is the dominant greenhouse gases (GHGs) component [81]. Accordingly, strategies that migrate excess CO 2 from atmospheric air might be more important than reducing equivalent emissions of CO 2 to air [64]. The promising approach in lowering CO 2 from air is biochar [78]. Thus, biochar could be considered as the geo-engineering solution to control climate change [82] probably by means of carbon sequestration [83], thus minimizing the emissions of the greenhouse gases [84] while supplying energy and improving the productivity of the cultivated crops [64]. Roberts et al. [84] found that 62−66% of CO 2 emissions could be sequestered within biochar. Accordingly, adopting biochar technologies can ofer inancial incentive in emission trading markets [82].

Efect of biochar on CH 4 and N 2 O emissions
Pyrolysis process serves also in reducing emissions of the other GHGs such as methane (CH 4 ) and nitrous oxide (N 2 O) when amended to agricultural soils and pastures [64]. Biochar decreases the emissions of CH 4 and, therefore, increases the stock of soil organic carbon [85]. This probably takes place through suppressing the oxidation of ambient CH 4 [51]. On the other hand, the emissions of CH 4 might increase in rice paddy soil amended with biochar [36].
The efect of biochar on the transformation processes of nitrogen (N) in soil is not well deined [86]. Probably, biochar reduced GHGs emissions only in neutral to acidic soils with high N content [87]. In this concern, emissions of N 2 O as well as leaching ammonium from soil could be reduced when using biochar rather than fresh organic material as soil amendments [86]. Generally, biochar suppresses production of N 2 O [54]. It is found that 10.7-41.8% of the total emissions of N 2 O decreased with application of biochar at rates of 20 and 40 Mg ha −1 , respectively [87]. Similar results show that soil N 2 O luxes decreased up to 79% in soils amended with biochar as compared to the control [88]. In an experiment conducted by Mukherjee et al. [89], it was found that 92% of the cumulative N 2 O emissions reduced when amending soils with biochar. Even under the reduced conditions of the rice paddy soil, biochar can also minimize the emission of N 2 O [36]. Such reductions might be atributed to the oxidative reactions that take place on the surfaces of biochar with ageing [86]. Accordingly, reductions of the emissions of N 2 O owing to application of biochar to soils improve the GHGs-to-yield ratio conditions [90].
Others found no signiicant diferences in emissions of both CO 2 and N 2 O from soils owing to application of biochar as compared to nonamended soils [56]. Likewise, Mukherjee et al. [89] found that the total cumulative emissions of CH 4 and CO 2 emissions were not afected signiicantly by amending soils with biochar. It is worthy to mention that biochar production itself can increase, to some extent, the greenhouse gases emited to the atmosphere; however, more studies are needed to fulill this point of study and to lessen GHGs emited during production process.

Biochar as means for decontaminating soils from heavy metals and pesticides
Biochars produced at relatively high temperature pyrolysis are more eicient in sorption of organic contaminants, whereas those produced at low temperatures are more eicient for removing heavy metals [102]. At low temperature, the produced biochar is of acidic nature, whereas those produced at high temperature were of alkaline nature [91]. This approach ofers a new safe solution for decontaminating soil pollution [92]. Generally, biochars are eicient in reducing the phytoavailability of many organic pollutants in soil, that is, (1) herbicides, for example, atrazine and acetochlor [51], Fluometuron and 4-chloro-2-methylphenoxyacetic acid [93], (2) pesticides, for example, pyrimethanil [94], atrazine [95], simazine [96], azoxystrobin [97], (3) fungicides, that is, tricyclazole in alluvial paddy soil [98] in addition to (4) phenols [99], thus controlling their toxicity and transfer in soil [100]. Immobilization of these organic residues might be take place because of the high ainity and ability of biochar to sequester such organics [101]. High temperature pyrolysis biochar is characterized by its high Engineering Applications of Biochar surface area, high micro-porosity, and hydrophobicity [102], and thus, combined adsorption and partition mechanisms might take place with the herbicide, pesticides, and the fungicide on carbonized and noncarbonized fractions [96]. In case of phenols, its sorption might take place on the microspores surface area of the biochar in addition to sorption on the carboxylic and lactonic groups [99]. Sorption ainity with the organic contaminants is found irreversible [94] and can increase with decreasing solid/solution ratio [96].
Biochars can also immobilize the phytotoxcity of heavy metals in soil forming less bioavailable organic bound fraction [69]. Biochar is of an alkaline nature, thus applying biochar to soils is associated with increases in soil pH [103]. The mechanism of immobilization might be a result of precipitation due to the rise in soil pH due to the application of the basic biochar or even by the electrostatic interaction on the carboxyl groups of the biochar [104] or through coordination by π electrons (C═C) of carbon [105].
Many experiments revealed the successfulness of biochar treatments on partitioning of heavy metals in soil, for example, Cd, Cu, and Pb [69]. Surprisingly, using biochar for decontaminating soils decreased the leachable fractions of Cd and Zn by 300 and 45-folds in compared to the untreated treatments [106]. In another experiment, it was found that treating soils with biochar removed Pb, Zn, and Cd by 97.4, 53.4, and 54.5%, respectively [107]. It is worthy to mention that the oxidized biochars, rich in carboxyl groups, showed higher ainity to immobilize Pb, Cu, and Zn than did the un-oxidized ones [104].

Biochar as a means of decontaminating heavy metals and organic residues from wastewater
Biochars act as sorbents for decontaminating wastewaters from heavy metals [79]. This might take place mainly through sorption on the surface functional groups of biochar [108], for example, oxygen-containing carboxyl, hydroxyl, and phenolic surface functional groups [109]. The kinetics of adsorption followed pseudo second order [10,110]. The stability of heavy metals by biochar correlated signiicantly with the oxygen-containing functional groups of the biochar [108] with maximum adsorption atained within the pH range 5.0-6.0 [110]. Digested dairy waste biochar and digested whole sugar beet biochar were found to be eicient in removing Pb 2+ , Cu 2+ , Ni 2+ , and Cd 2+ from wastewater [111]. Also, biochars can eiciently remove organic contaminants from wastewaters. It was found that the fast pyrolysis pine wood biochar could remove salicylic acid and ibuprofen from solutions [112].
Biochar can also efectively remove phosphate from wastewater [113]. This probably takes place on the colloidal and nano-sized MgO particles on its surface [114]. Most of the sorbed phosphate is bioavailable and can be added to soils as slow release P-fertilizers [115]. Moreover, 60% of the sorbed phosphate can be desorbed within 24 h [116].
Treating biochar hydrothermally with H 2 O 2 increased its ainity to remove heavy metals from aqueous solutions because this treatment increased the oxygen-containing functional groups [117]. Another type of biochar is chitosan-modiied one which is a low-cost synthesized biochar eicient for immobilizing heavy metal in the environment [118]. Also, a graphene/biochar composite is a safe economic adsorbent that can decontaminate heavy metals through surface complexation with C─O, C C, ─OH, and O C─ groups [119].

Conclusion and future challenges of biochar
The previous demonstration showed that biochar plays an important role in environmental management and soil sustainability. Several beneicial roles of biochar have been observed. Biochar improves soil fertility and plant growth, mitigates the greenhouse gasses, and could be used successfully for the remediation of soils and waters from contaminants. However, several research questions are still unknown and need intensive researches, that is, the efect of biochar on minerals and/or organic fertilizers use eiciency and the neutralization of alkaline performance of biochar to be used safely in alkaline soils. In addition, the stability of biochar in the amended soils needs a sustainable experiment to determine exactly the degradation rate of diferent types of biochars.