Differences between gasification and pyrolysis processes.
Production of biochars from agricultural wastes reduces significantly 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 beneficial role to mitigate CO2 emission through enhancing the long-term carbon sequestration. The effects 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 effects 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.
- mineral fertilizers
- environmental management
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 for agricultural products in response to the growing population . However, several studies indicated that mineral fertilizers contain various amounts of heavy metals as impurities [1, 2]. Consequently, the addition of large amounts of mineral fertilizers to the soil for long periods may result in the accumulation of heavy metals in soils and plant grown thereon, as confirmed by previous studies [1, 3].
Intensive researches have been performed to overcome the infertility problems of the agricultural soils. The addition of organic residues in forms of compost, manure, and other organic forms has proven to be suitable alternative options for mineral fertilization. However, these materials need to be applied intensively due to their low nutritive contents and rapid degradation rate. Biochar is a charcoal produced from the pyrolysis of biomass at relatively low temperature (<700°C) . Biochar has received great interest during the last few years, due to its beneficial roles in environmental management. Several beneficial objectives could be achieved through biochar application for environment, that is, waste management, energy production, soil improvement, maximizing agricultural production, contamination control (soils and waters), and greenhouse gases (GHGs) mitigation [5–10].
2. Production and characterization of biochars derived from different 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 . Similarly, the use of biochar as soil amendment first 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 fires and by human through burning bits for different 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 . Consequently, aluminum toxicity in this soil was neutralized, and soil status in terms of physical, biological, and chemical features has been modified that made it one of the most fertile soils over the world. The promising benefits 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  and Retan . Due to the several benefits 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 (http://biochar-us.org/biochar-research), the European Biochar Research Network (http://cost.european-biochar.org/en), the UK Biochar Research Center (http://www.biochar.ac.uk/), the China Biochar Network (http://www.biochar-international.org/chinanetwork), the Japan Biochar Association (http://www.geocities.jp/yasizato/JBA.htm), the New Zealand Biochar Research Centre (http://www.massey.ac.nz/massey/learning/colleges/college-of-sciences/research/agriculture-environment-research/soil-earth-sciences/biochar-research-centre/biochar-research-centre_home.cfm) and the Biochar India (www.biocharindia.com).
2.2. Production of biochar
2.2.1. 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 . Therefore, it is essential to recycle their organic residues effectively. 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 litter, dairy and swine manure, and (iv) sewage sludge. Producing the biochar from biomass, especially wastes offer an excellent way for the recycling of wastes into beneficial materials. Pyrolysis treatment reduces the volume of biomasses by 44–90 and 75–80% and weight by 44–93 and 71–77% [8, 15].
2.2.2. 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 gasification; however, in case of gasification, the process is performed in two steps, firstly, the biomass is heated to around 600°C, and hydrocarbon gases and tar are evaporated; secondly, char is gasified 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 difference between them is the heating rate and the amount of the produced bio-oil.
|Fast pyrolysis||Slow pyrolysis||Gasification|
|Heating rate||up to 1000°C min−1||Slow 5–30°C min−1||–|
|Time||Few seconds||Hours ~ days||–|
|Aeration||Oxygen free||Oxygen free or limited||Oxygen limited|
2.2.3. Development of biochar production
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. Briefly, 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 better insulated and allowed a better airflow control, which allowed higher 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 airflow, and the biochar can easier be recovered . 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 emitted syngas and bio-oils. Therefore, advanced instruments have been developed successfully to eliminate the emitted syngas and bio-oil and to use them as by products by using specific condensers for gas and bio-oil collection.
2.3. Differences 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 differences as follows :
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 filtration.
Wood is the major source of charcoal production; however, biochar can be produced from any biomass.
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 .
2.4. 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 , low oxygen and hydrogen contents , and little 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 . Biochar has proven to be a suitable tool for the removal of heavy metals from aqueous solutions  due to the presence of macrospores with an average pore size of 51–138 m2 g−1 ,. 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  demonstrated that the spectrums of sugar cane and orange peel biochars are quite similar; both biochars exhibited absorption bands on 3448.13 and 3429.4 cm−1 corresponding to C─OH functional groups; around 1637.27 and 1384.85 cm−1, there were C═O and C─C bands and the adsorption bands on 1101.43 cm−1 present the C─O, C─C, and C─OH bands. Therefore, both biochars had the ability to adsorb Pb(II) ions from aqueous solutions. During the pyrolysis of biomass, heating causes some nutrients to be volatilized, especially at the surface of the material, while other nutrients become concentrated in the remaining biochar. In case of wood-rich materials, carbon (C) begins to volatilize around 100°C, N above 200°C, S above 375°C, and potassium (K) and P between 700°C and 800°C. The volatilization of magnesium (Mg), calcium (Ca), and manganese (Mn) occurs at temperatures above 1000°C [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 . Furthermore, most of the stated biochars characterized by its high pH values, and this could be attributed 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 . A point to note that, biochar has a liming effect when it is applied to the soil; therefore, possible increment in soil acidity (pH) might occur . 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 , but the mechanisms for these increases are still a matter of speculation. Moreover, the benefits 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 effects combined with different 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 beneficial effect 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.
|Feedstock||Temperature||pH||%||CEC, cmolc kg−1||C/N ratio||%||H/C ratio||O/C ratio||SSA, m2 g−1||Reference|
|Sugar cane bagasse||<500||8.63||74.02||1.00||0.24||–||0.17||0.32||2.00||69.62||74.02||87.80||12.21||0.42||0.23||92.30|||
|Hard wood saw dust||500.00||–||63.80||0.22||–||0.01||–||–||–||290.00||–||22.80||0.60||0.14||1.00|||
3. The beneficial effect of biochars derived from different wastes on soil fertility
3.1. Fresh organic matter 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 . Such fresh materials are probably the most reduced fraction when added to soils, acting as electron pumps to more oxidized species . Generally, organic residues are used as amendments to improve soil quality and productivity .
The organic amendments that persist longer in soil might exert high impacts on soil physicochemical reactivity . 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 . Probably, compounds that contain less oxygen (lower electron richness) are less easily decomposed than do compounds having comparable size, solubility, and molecular complexity .
Specific 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 . Also, preservation of recalcitrant (stable) compounds might stabilize organic C . It is thought that the recalcitrant compounds are present in organic materials in much higher proportions than those classified as labile .
Pyrolysis is the converting of unstable organic matter into more stable forms (biochar) that can be applied to soils . This can be attained by heating carbon bearing solids in the absence of oxygen  to produce porous materials of low density  and more stable forms of carbon  which are more resistant to biodegradation as compared to fresh organic materials. Biochars’ half-life in soil is relatively long .
3.2. Effect 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 , increases cation exchange capacity (CEC) [8, 59], decreases bulk density , and minimizes the loss of nutrients and other agricultural chemicals in soil run-off . It also decreases soil penetration resistance and increases aggregation and infiltration . On the other hand, biochar does not show any significant effect on soil porosity either directly through pore contribution, or indirectly through improving aggregate stability . Besides, applications of biochar increase soil electrical conductivity (EC) in addition to its high contents of phosphorus and nitrogen .
3.3. Effect 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 effects on nutrients use efficiency. Biochar serves as energy provider  for wheat , rice , maize , soy bean , and spring barley ; thus, it improves root density, crop growth, and productivity . 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% . Even biochar produced from wastewater sludge increased the productivity of cherry tomatoes by 64% as compared to the control . Thus, such amendment is recommended for low-fertile and degraded soils  as well as highly weathered soil . The zone of plant rhizosphere becomes larger with application of biochar . Moreover, biochar increases plant resistance toward biotic stresses . Some types of biochar amendments are rich in nutrients , and on the other hand, it minimizes the leaching of nutrients from soil, i.e., nitrate , ammonium, and probably phosphate . However, the majority of biochars produce ethylene which is an inhibitor for soil microbes , beside of the released organic molecules which might suppress activities of some beneficial soil biota .
4. Biochar: alternative option for soil sustainability
Using biochar as a soil amendment can fulfill three main targets, that is, increasing plant productivity, thus achieving food security , improving soil properties, and disputing land degradation  beside of minimizing the change of climate . Moreover, biochar changes organic wastes into value-added biochar which acts as sorbents for eliminating contaminants in wastewater . 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 benefit materials guarantees the sustainability of agricultural lands.
5. Biochar and the environmental change
5.1. Effect of biochar on CO2 emissions
Soils can store more carbon than do plants or atmosphere . Globally, soil organic matter (SOM) contains about three times as much carbon as either the atmosphere or terrestrial vegetation . In soils of low N content, CO2 is the dominant greenhouse gases (GHGs) component . Accordingly, strategies that migrate excess CO2 from atmospheric air might be more important than reducing equivalent emissions of CO2 to air . The promising approach in lowering CO2 from air is biochar . Thus, biochar could be considered as the geo-engineering solution to control climate change  probably by means of carbon sequestration , thus minimizing the emissions of the greenhouse gases  while supplying energy and improving the productivity of the cultivated crops . Roberts et al.  found that 62−66% of CO2 emissions could be sequestered within biochar. Accordingly, adopting biochar technologies can offer financial incentive in emission trading markets .
5.2. Effect of biochar on CH4 and N2O emissions
Pyrolysis process serves also in reducing emissions of the other GHGs such as methane (CH4) and nitrous oxide (N2O) when amended to agricultural soils and pastures . Biochar decreases the emissions of CH4 and, therefore, increases the stock of soil organic carbon . This probably takes place through suppressing the oxidation of ambient CH4 . On the other hand, the emissions of CH4 might increase in rice paddy soil amended with biochar .
The effect of biochar on the transformation processes of nitrogen (N) in soil is not well defined . Probably, biochar reduced GHGs emissions only in neutral to acidic soils with high N content . In this concern, emissions of N2O as well as leaching ammonium from soil could be reduced when using biochar rather than fresh organic material as soil amendments . Generally, biochar suppresses production of N2O . It is found that 10.7–41.8% of the total emissions of N2O decreased with application of biochar at rates of 20 and 40 Mg ha−1, respectively . Similar results show that soil N2O fluxes decreased up to 79% in soils amended with biochar as compared to the control . In an experiment conducted by Mukherjee et al. , it was found that 92% of the cumulative N2O emissions reduced when amending soils with biochar. Even under the reduced conditions of the rice paddy soil, biochar can also minimize the emission of N2O . Such reductions might be attributed to the oxidative reactions that take place on the surfaces of biochar with ageing . Accordingly, reductions of the emissions of N2O owing to application of biochar to soils improve the GHGs-to-yield ratio conditions .
Others found no significant differences in emissions of both CO2 and N2O from soils owing to application of biochar as compared to nonamended soils . Likewise, Mukherjee et al.  found that the total cumulative emissions of CH4 and CO2 emissions were not affected significantly by amending soils with biochar. It is worthy to mention that biochar production itself can increase, to some extent, the greenhouse gases emitted to the atmosphere; however, more studies are needed to fulfill this point of study and to lessen GHGs emitted during production process.
6. The beneficial role of biochar for contamination control of soils and waters
6.1. Biochar as means for decontaminating soils from heavy metals and pesticides
Biochars produced at relatively high temperature pyrolysis are more efficient in sorption of organic contaminants, whereas those produced at low temperatures are more efficient for removing heavy metals . At low temperature, the produced biochar is of acidic nature, whereas those produced at high temperature were of alkaline nature . This approach offers a new safe solution for decontaminating soil pollution . Generally, biochars are efficient in reducing the phytoavailability of many organic pollutants in soil, that is, (1) herbicides, for example, atrazine and acetochlor , Fluometuron and 4-chloro-2-methylphenoxyacetic acid , (2) pesticides, for example, pyrimethanil , atrazine , simazine , azoxystrobin , (3) fungicides, that is, tricyclazole in alluvial paddy soil  in addition to (4) phenols , thus controlling their toxicity and transfer in soil . Immobilization of these organic residues might be take place because of the high affinity and ability of biochar to sequester such organics . High temperature pyrolysis biochar is characterized by its high surface area, high micro-porosity, and hydrophobicity , and thus, combined adsorption and partition mechanisms might take place with the herbicide, pesticides, and the fungicide on carbonized and noncarbonized fractions . 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 . Sorption affinity with the organic contaminants is found irreversible  and can increase with decreasing solid/solution ratio .
Biochars can also immobilize the phytotoxcity of heavy metals in soil forming less bioavailable organic bound fraction . Biochar is of an alkaline nature, thus applying biochar to soils is associated with increases in soil pH . 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  or through coordination by π electrons (C═C) of carbon .
Many experiments revealed the successfulness of biochar treatments on partitioning of heavy metals in soil, for example, Cd, Cu, and Pb . 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 . 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 . It is worthy to mention that the oxidized biochars, rich in carboxyl groups, showed higher affinity to immobilize Pb, Cu, and Zn than did the un-oxidized ones .
6.2. Biochar as a means of decontaminating heavy metals and organic residues from wastewater
Biochars act as sorbents for decontaminating wastewaters from heavy metals . This might take place mainly through sorption on the surface functional groups of biochar , for example, oxygen-containing carboxyl, hydroxyl, and phenolic surface functional groups . The kinetics of adsorption followed pseudo second order [10, 110]. The stability of heavy metals by biochar correlated significantly with the oxygen-containing functional groups of the biochar  with maximum adsorption attained within the pH range 5.0–6.0 . Digested dairy waste biochar and digested whole sugar beet biochar were found to be efficient in removing Pb2+, Cu2+, Ni2+, and Cd2+ from wastewater . Also, biochars can efficiently remove organic contaminants from wastewaters. It was found that the fast pyrolysis pine wood biochar could remove salicylic acid and ibuprofen from solutions .
Biochar can also effectively remove phosphate from wastewater . This probably takes place on the colloidal and nano-sized MgO particles on its surface . Most of the sorbed phosphate is bioavailable and can be added to soils as slow release P-fertilizers . Moreover, 60% of the sorbed phosphate can be desorbed within 24 h .
Treating biochar hydrothermally with H2O2 increased its affinity to remove heavy metals from aqueous solutions because this treatment increased the oxygen-containing functional groups . Another type of biochar is chitosan-modified one which is a low-cost synthesized biochar efficient for immobilizing heavy metal in the environment . 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 .
7. Conclusion and future challenges of biochar
The previous demonstration showed that biochar plays an important role in environmental management and soil sustainability. Several beneficial 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 effect of biochar on minerals and/or organic fertilizers use efficiency 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 different types of biochars.