Pedosphere 25(5): 654–665, 2015 ISSN 1002-0160/CN 32-1315/P c 2015 Soil Science Society of China ⃝ Published by Elsevier B.V. and Science Press
Biochar: An Emerging Panacea for Remediation of Soil Contaminants from Mining, Industry and Sewage Wastes Hossain M. ANAWAR1,∗ , Farjana AKTER2 , Zakaria M. SOLAIMAN1 and Vladimir STREZOV3 1 School
of Earth and Environment (M087) and UWA Institute of Agriculture, The University of Western Australia, Crawley WA 6009 (Australia) 2 Sher-e-Bangla Agricultural University, Sher-e-Bangla Nagar Dhaka-1207 (Bangladesh) 3 Department of Environmental Sciences, Faculty of Science and Engineering, Macquarie University, Sydney NSW 2109 (Australia) (Received May 13, 2015; revised July 19, 2015)
ABSTRACT Mine tailings, waste rock piles, acid mine drainage, industrial wastewater, and sewage sludge have contaminated a vast area of cultivable and fallow lands, with a consequence of deterioration of soil and water quality and watercourses due to the erosion of contaminated soils for absence of vegetative cover. High concentrations of toxic elements, organic contaminants, acidic soils, and harsh climatic conditions have made it diﬃcult to re-establish vegetation and produce crops there. Recently, a signiﬁcant body of work has focussed on the suitability and potentiality of biochar as a soil remediation tool that increases seed emergence, soil and crop productivity, above ground biomass, and vegetation cover on mine tailings, waste rock piles, and industrial and sewage wastecontaminated soils by increasing soil nutrients and water-holding capacity, amelioration of soil acidity, and stimulation of microbial diversity and functions. This review addresses: i) the functional properties of biochar, and microbial cycling of nutrients in soil; ii) bioremediation, especially phytoremediation of mine tailings, industrial waste, sewage sludge, and contaminated soil using biochar; iii) impact of biochar on reduction of acid production, acid mine drainage treatment, and geochemical dynamics in mine tailings; and iv) treatment of metal and organic contaminants in soils using biochar, and restoration of degraded land. Key Words: revegetation
acid mine drainage, contaminated soil, interaction, mine tailings, mining waste, phytoremediation, phytostabilization,
Citation: Anawar H M, Akter F, Solaiman Z M, Strezov V. 2015. Biochar: An emerging panacea for remediation of soil contaminants from mining, industry and sewage wastes. Pedosphere. 25(5): 654–665.
INTRODUCTION Soil and water quality degradation by current and abandoned mine tailings, waste rock piles, industrial wastewater, and sewage sludge are common environmental problems that contribute highly to mineralized soil and water acidiﬁcation to local watercourses through the erosion of contaminated soils which are primarily due to lack of vegetative cover. A major weakness of reducing the amount of pollution from these wastes is due to the diﬃculty of re-establishing vegetation and active soil processes on sites that are devoid of vegetation and have adverse conditions (e.g. toxic elements, organic contaminants, acidic soils, harsh climatic conditions) hostile to plant establishment and crop production. As for example, in China, more than 2 × 107 ha of farmland have been contaminated with heavy metals (Wei and Yang, 2010), which has led to a sharp decrease in crop production and food quality in recent decades (Gu et al., 2003; Zhong and Wu, 2007). ∗ Corresponding
author. E-mail: [email protected]
Therefore, remediation of contaminated soils to reduce contamination and minimize downstream damage is essential (Powlson et al., 2011). Diﬀerent strategies and approaches were employed to address soil contamination including soil washing, soil vapor extraction, farming, soil ﬂushing, and ion exchanges (Zhou and Song, 2004; Prasad and Nakbanpote, 2015). However, these traditional methods, when applied in situ, are usually expensive and further it might create new problems, such as fertility loss and soil erosion (Khan et al., 2004; Kumpiene et al., 2008). Therefore, new approaches like phytoremediation, bioremediation, and ecological remediation are being sought (Sun et al., 2001; Kong et al., 2014). To address the challenge of revegetation at mining, industrial, and sewage waste sites, the suitability of biochar as a soil remediation tool is receiving increasing attention. When compared to seeding alone, the addition of biochar in the highly weathered acidic soil has inﬂuenced the seed germination, plant growth, vegeta-
BIOCHAR FOR REMEDIATION OF SOIL CONTAMINANTS
tion cover, as well as N and P use eﬃciency (Zhu et al., 2014). Biochar could potentially enhance soil and crop productivity by increasing nutrient and soil moisture availability, ameliorating acidic soils, and stimulating soil microbial activity. Due to its excellent adsorption properties, biochar, alone or in combination with organic compost, have a promising prospect in industrial applications and environmental remediation, including water and wastewater treatment, and restoration and revegetation of mine tailings (Gwenzi et al., 2015). Recently, a volume of research was devoted to addressing the physical and chemical properties of biochar, its usefulness for improving soil quality, soil fertility, and crop production in harsh environments, as well as environmental applications and mine rehabilitation. However, a comprehensive review is lacking, particularly on the remediation and rehabilitation of contaminated lands using biochar. Therefore, the present review aimed at synthesising the previous works and providing an overview in relation to: i) physical and chemical properties of biochar and microbial cycling of nutrients in soils; ii) remediation of mine tailings, waste rock piles, acid mine drainage, and contaminated soil with
biochar; and iii) treatment of metal, metalloid, and organics-contaminated soil and restoration of degraded and contaminated land using biochar. Functional properties of biochar Biochar is the solid product of pyrolsysis produced through heating of biomass at 300 to 500 ◦ C in absence of oxygen, that contains stable aromatic organic matter with carbon concentrations of about 70% to 80% (Lehmann et al., 2002) and mineral matter, including nutrients. Biochar has high surface area, higher porosity, variable charge, and functional groups (Fig. 1a, b) that can increase soil water-holding capacity, pH, cation exchange capacity (CEC), surface sorption capacity, base saturation, and crop resistance to disease when added to soil (Glaser et al., 2002; B´elanger et al., 2004; Keech et al., 2005; Liang et al., 2006; Tang et al., 2013). These properties vary with the pyrolysis temperature and the properties of feedstocks (Gundale and DeLuca, 2006; Bornermann et al., 2007; Chan and Xu, 2009; Singh et al., 2010). The nutrient content and availability in biochar is inﬂuenced by the biomass type, processing conditions (Singh et al., 2010),
Electron micrograph (a) (Leng et al., 2012) and molecular structure (b) of biochar (Bourke et al., 2007).
H. M. ANAWAR et al.
and type of bonds associated with the elements involved (DeLuca et al., 2009; Yao et al., 2010). Biochar application to soil is widely advocated for a variety of reasons related to sustainability (Jeﬀery et al., 2015). The most often claimed beneﬁts of biochar include: 1) carbon sequestration; 2) soil fertility improvement; 3) pollutant immobilization; and 4) waste management. Biochar additions to soil can alter soil microbial diversity, shift functional groups (Pietik¨ainen et al., 2000), and reduce soil bulk density (Gundale and DeLuca, 2006). Biochar strongly sorbs salts and ameliorates salt stress eﬀects on plants in agricultural, urban, and contaminated soils (Thomas et al., 2013). Additions of biochar to soil have increased the availability of P and Zn, and the total N concentrations (Glaser et al., 2002; Lehmann et al., 2003). The biochar should be applied with fertilisers to maximise the beneﬁt on plant growth and nutrition as well as soil biology improvement (Solaiman et al., 2010). Biochar additions can increase crop yields at lower rates of fertiliser use (Blackwell et al., 2010). Banding of biochar in soil can minimise wind erosion risk and place biochar close to crop roots. Microbial cycling of nutrients in soils Biochar amendment to soil could change microbial community composition, increase microbial species richness, and enhance microbial diversity involved in N, P or S nutrient transformations (Pietik¨ainen et al., 2000; Thies and Suzuki, 2003). It has the capacity to support the presence of bacteria (Pietik¨ainen et al., 2000; Rivera-Utrilla et al., 2001), with which the organisms may inﬂuence soil processes. The fungi, identiﬁed to colonize the soil which was amended with biochar, were both saprophytic and mycorrhizal (Saito and Marumoto, 2002). Mycorrhizal colonisation increased in wheat when biochar was added to soil (1.5–6 t ha−1 ) (Solaiman et al., 2010) and was inoculated with spores of Glomus etunicatum that improved the yields of onion (Matsubara et al., 1995). Interactions of biochar with soil microorganisms are complex (Lehmann and Rondon, 2006) and depend on the amount and type of biochar present in or added to soil. Soil microbial diversity and population size as well as population composition and activity that have signiﬁcant inﬂuences on nutrient cycles and nutrient availability to plants, are aﬀected by biochar application. Due to higher surface area and surface hydrophobicity of both the microorganisms and the biochar, the soil amended with biochar results in better retention of the microorganisms and higher microbial activity and diversity (Rivera-Utrilla et al., 2001; Mills, 2003).
Interactions of biochar in soil It has been reported that the interactions between biochar, soil, microbes, and plant roots might occur within a short period after application to the soil (Lehmann and Joseph, 2009). The extent, rates, and implications of these interactions are yet to be explored. Joseph et al. (2010) reviewed the properties of biochar and suggested possible reactions that may occur after the addition of biochar to soil (e.g., adsorption-desorption, precipitation-dissolution, acidbase and redox reactions). Other studies (Steiner et al., 2007; Bruun et al., 2008; Singh and Cowie, 2008; Kuzyakov et al., 2009) suggested that the types and rates of interactions that take place in the soil depended on: 1) feedstock composition, particularly mineral fraction; 2) pyrolysis conditions; 3) biochar particle size and delivery system; and 4) soil properties and local environmental conditions. Low-temperature biochar, which has a less-condensed ‘C’ structure and higher nutrients content, is expected to have a greater reactivity in soils than higher-temperature biochar and a better contribution to soil fertility (Steinbeiss et al., 2009). The aging of biochar, after incorporation into soil, is partly governed by conditions of moisture and temperature prevailing in soil (Nguyen and Lehmann, 2009). Water has a major role in processes such as dissolution, hydrolysis, carbonation and decarbonation, hydration, and redox reactions, aﬀecting biochar decomposition in soil, as well as interactions with soil biota. The rates at which these reactions occur depend on the nature of the reactions, type of biochar, and pedoclimatic conditions. REMEDIATION OF MINE TAILINGS, WASTES AND CONTAMINATED SOILS Mining activities remove the top soil, disturb soil structure and change soil biology and vegetation, resulting in extensive soil degradation. The contaminated soil after mining, tailings, and the waste rock piles become devoid of vegetation due to metal toxicity and high acidity (Kelly et al., 2014). Remediation and rehabilitation of these contaminated soil and hazardous waste can be achieved by phytostabilization, a longterm and cost-eﬀective rehabilitation strategy, through promoting the revegetation to reduce the risk of pollutant transfer and ecological restoration (Fellet et al., 2011), although these are diﬃcult without proper soil amendments (Reverchon et al., 2015). The biochar addition to contaminated soil and waste rock piles may increase soil pH, water holding capacity, and soil fertility, reduce the mobility of plant-available pollutant,
BIOCHAR FOR REMEDIATION OF SOIL CONTAMINANTS
and promote revegetation (Fellet et al., 2014; Kelly et al., 2014). However, fertilisation (NPK fertiliser) is required along with biochar amendment to signiﬁcantly increase plant biomass production (Beesley et al., 2013). Phytostabilization of mine tailings with biochar The addition of biochar prepared from orchard prune residues and manure pellets at four doses (0%, 1%, 5%, and 10% biochar in the mine tailings) showed the signiﬁcant beneﬁts of biochar use on mining wastes to revegetate the plant species of Anthyllis vulneraria subsp. polyphylla (Dc.) Nyman, Noccaea rotundifolium (L.) Moench subsp. cepaeifolium, and Poa alpine L. subsp. alpina in phytostabilization of mining waste (Fellet et al., 2011, 2014). Eﬀects of biochar application at diﬀerent doses on phytostabilization of mine tailings are summarised in Table I. The pH, nutrient retention, cation exchange capacity, and water-holding capacity of mine tailings increased, and the bioavailability of Cd, Pb, and Zn decreased proportionally with the increase of biochar content (Fellet et al., 2011, 2014). Biochar, alone or in combination with compost, incorporated into acidic mining waste increased soil pH from 3.33 to 3.63 and 4.07 to 4.77, respectively, as well as organic matter content, base cations, and nitrate availability, but decreased bulk density, extractable metal content (Al, Cd, Cu, Pb, Ni, and Zn), and extreme soil acidity (Beesley et al., 2014; Kelly et al., 2014; Reverchon et al., 2015; Rodr´ıguez-Vila et al., 2014, 2015). However, the dissolved organic carbon initially released from biochar and compost applied to soil may increase the phytotoxicity to plants, bioavailable Pb, Cu, and As, as well as the electrical conductivity (Beesley et al., 2014; Kim et al., 2014). The addition of biochar compost prepared from Rhododendron ponticum and poultry litter biochar to large colliery spoil-impacted areas (high in As and Cu) and other
mines improve the growth of grass, lettuce (Lactuca sativa), rye (Secale cereal), and birdsfoot trefoil (Lotus corniculatus), compared with unamended colliery spoil and mine soil (McDonald et al., 2014; Ryan et al., 2014). The technosol and biochar mixture increased the shoot biomass from 0.74 to 2.95 g pot−1 and generally reduced the metal concentrations in plant species of Brassica juncea L. when grown for phytostabilization of mine soils (Rodr´ıguez-Vila et al., 2015). The green waste compost amendment reduced the pore water and ryegrass (Lolium perenne L. var. Cadix) shoot Pb concentrations, while biochar application reduced Cu content in pore water and ryegrass shoot, indicating the metal speciﬁc suitability of these amendments for treating a heavily Cu- and Pb-contaminated soil from a former copper mine (Karami et al., 2011). Reverchon et al. (2015) reported that jarrah biochar soil amendment (37 and 74 t ha−1 ) increased soil C/N ratio, photosynthetic N use eﬃciency, and biological N ﬁxation rates of a native legume (Acacia tetragonophylla) grown in a mixture of topsoil and mine rejects, indicating positive eﬀects on soil properties and plant nutritional status. However, Kelly et al. (2014) could not ﬁnd any changes in microbial population or activity in mining waste upon biochar application. Therefore, the characteristics of the substrate to be treated are crucial for the biochar selection. Biochar has turned a wasteland enriched with As, Cd, Pb, and Zn on a steep mountainside into a haven for natural grasses and wildﬂowers that have stabilized the slope and almost diminished the risk of heavy metal leaching into the city’s main water supply around Hope Mine (Fig. 2). Impact of biochar on acid production and acid mine drainage treatment Jain et al. (2014) revealed that incorporation of biochar into high sulphur mining wastes inhibited the
Fig. 2 Abandoned Hope silver mine landscape at Aspen, Colorado, USA before (a) and after (b) biochar application to topsoil in July 2010 and August 2011 (ACES, 2011), respectively.
Mining waste rock
0%, 10%, 20%, and 30%
Enhanced activities of soil microbial C, phosphatise and dehydrogenase; lowered mobile Cd, Cu and Zn concentrations
Increased soil pH from 2.83 to 6.18 and shoot biomass from 0.74 to 2.95 g; reduced metal concentration in plant Increased soil pH, C content, C/N ratio, and biological N ﬁxation rates; decreased soil δ 13 C Increased pH, C and TN concentration in soil; decreased extractable Co, Cu, Ni and soil acidity Salt eﬀects on seed germination at lowest biochar treatment (0.5% weight/weight); increased productivity of vegetation and forage yield (40%) Increased vegetation cover
Proportional increase of pH, CEC, and waterholding capacity; decreased bioavailability of Cd, Pb, and Zn Change in pH, EC, and CEC; reduced shoot Cd and Pb concentration Increase in soil pH, OM, and NO− 3 concentration; low bulk density and extractable Al, Cd, Cu, Pb, and Zn content in mining waste Reduced pore water Cu and Pb concentration
= not available. = cation exchange capacity; EC = electrical conductivity; OM = organic matter; TN = total nitrogen.
Poultry litter biochar
Abandoned mining site Mining sites
Cu mine settling pond soil
20%, 40%, 80%, and 100%
Spent mine sites
20%, 40%, 80%, and 100%
37 and 74 t ha−1
Biochar mixed with compost
Biochar derived from British Oak, Ash, Sycamore, and Birch Technosol and biochar
0%, 1.5%, and 3%
Biochar from pruning residues manure Biochar from pine wood
0%, 1%, 5%, and 10%
Biochar from orchard prune residues
Eﬀects of biochar application at diﬀerent doses on phytostabilization of mine tailings
Karami et al., 2011
Rodr´ıguez-Vila et al., 2015
Phytostabilization by Brassica juncea L.
Revegetation by various plants
Phytostabilization by lettuce, rye, and birdsfoot trefoil
Hanauer et al., 2012
McDonald et al., 2014
Reverchon et al., 2015 Rodr´ıguez-Vila et al., 2014
Kelly et al., 2014
Phytostabilization by Spinacia oleracea, Brassica napus and Triticum aestivum Phytostabilization by ryegrass
Phytostabilization by Acacia tetragonophylla Phytostabilization by Brassica juncea L.
Fellet et al., 2014
Fellet et al., 2011
658 H. M. ANAWAR et al.
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acid production rate from 10.4 to 3.8 kg Mt−1 h−1 and enhanced the alkali consumption from 9.7 to 13.9 kg Mt−1 h−1 , resulting in neutralization of all the acid produced (Table II). The eﬃciency of biochar to reduce the rate of acid production may be due to either the reduction in Fe3+ concentration or production of reduced S compounds in reducing conditions of biochar or reduced availability of oxygen due to competition between biochar and oxygen that accelerate the sulphide oxidation and acid generation. The spent coffee grounds biochar, sewage sludge biochar, and aromatic spent biochar applied to acid mine drainage decreased the heavy metal (Cd, Cr, Cu, and Pb) concentrations and phytotoxicity to bok choy (Brassica campestris L. ssp. chinensis Jusl.) by increasing the pH (Lu et al., 2012; Khare et al., 2013; Kim et al., 2014). The Pb sorption primarily involved the coordination with organic hydroxyl and carboxyl functional groups as well as the co-precipitation on mineral surfaces. The Gracilaria modiﬁed biochar, generated from the waste after the commercial extraction of agar from cultivated seaweeds with ferric chloride (FeCl3 ), immediately removed 98% of the selenate (SeO2− 4 , hereafter SeVI ) from the prepared Se solution, but higher concentrations of SO2− reduced the uptake of SeVI 4 from acid mine eﬄuents from coal mines and coal-ﬁred power stations (Johansson et al., 2015). The biocharimpregnated sediment showed nearly 3 times more attenuation capacity for cyanide than non-amended sediment, thus indicating possibility of using biochar
to cleanse cyanide from eﬄuent discharges from gold mining and processing activities (Sawaraba and Rajashekhar Rao, 2015). Impact of biochar on geochemical dynamics in mine tailings The addition of fresh biomass, such as woodchips, caused dynamic hydrogeochemical changes in Cu-Au tailings leachate, increased the load of salts and metals in tailing pore water, and aﬀected seepage water quality (Table II). In contrast, biochar with highly stable C may help alleviate geochemical environment in the tailings (Li et al., 2013). At medium pH (7–8), addition of greenwaste biochar increased adsorption capacity, enhanced adsorption of NH+ 4 -N/NH3 -N and lowered NH3 volatilization in bauxite-processing residue amended with di-ammonium phosphate, while at high pH (9), the majority of NH+ 4 -N/NH3 -N pools was lost via NH3 volatilization due to the strong acid-base reaction at this pH (Chen et al., 2013). Impact of biochar on arsenic mobility and uptake from mining wastes An orchard prune residue biochar signiﬁcantly increased As concentrations in pore water (500–2 000 µg L−1 ), whilst root and shoot As concentrations of tomato (Solanum lycopersicum L.) plantlets were signiﬁcantly reduced compared to the control without biochar (Beesley et al., 2013), and toxicity-transfer risk was negligible (< 3 µg kg−1 ). However, Gregory et al.
TABLE II Impact of biochar on acid mine drainage and geochemical dynamics in mine tailings Biochar
Biochar prepared from lemongrass (Cymbopogon flexuosus) Spent coﬀee grounds biochar Sewage sludge biochar
Inhibited acid production rate; enhanced alkali consumption; neutralized acidity
Jain et al., 2014
Acid mine drainage, mine soil Acid mine drainage
Kim et al., 2014
Fe-treated seaweed biochar
Coal mine eﬄuents
Decreased heavy metal concentrations and phytotoxicity to bok choy; increased pH Removed Pb from acidic solution with capacities of 16–31 mg g−1 at pH 2–5 Removed 98% of the SeVI from the prepared solution, but only 3% from mine eﬄuent; reduced uptake of SeVI by high concentrations of SO2− 4
Gold mining eﬄuent
Cu-Au tailing leachate
Sawaraba and Rajashekhar Rao, 2015 Li et al., 2013
Rehabilitated bauxite residue sand
3 times more attenuation capacity for cyanide than non-amended sediment, indicating biochar use to cleanse cyanide from spills No substantial changes in most examined properties of leachate except for reduction in DOC and NO− 2 with biochar amendment but with reduced leachate pH under the tree woodchip treatment Losses of NH+ 4 -N/NH3 -N pools at low (5) and high pH (9) from residue via NH3 volatilization; lowered NH3 volatilization at medium pH (7, 8)
Lu et al., 2012 Johansson et al., 2015
Chen et al., 2013
(2014) reported that at increasing dose rates of biochar from willow feedstock (Salix sp.) amendment from 30 to 60 t ha−1 , shoot tissue of ryegrass extracted signiﬁcantly higher (P < 0.05) concentrations of As. The Al(III)-modiﬁed biochar, prepared from the straws of rice, soybean, and peanut, had greater sorption capacity for As(V) under acidic conditions compared with corresponding unmodiﬁed biochars, increased with decreasing suspension pH, and could substitute Fe/Al oxides used for water puriﬁcation under acidic conditions at pH > 4.0 (Qian et al., 2013). Biochar treatment of metal, metalloid and organicscontaminated soils It is diﬃcult to remediate sites contaminated with mixtures of metals, metalloids, and organics, because each contaminant type may require a diﬀerent treatment (Sneath et al., 2013). Biochar can remediate soil with both heavy metal and organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and terbuthylazine (herbicides for vegetation management) (Wang et al., 2010; Chen and Yuan, 2011; Tang et al., 2013) through electrostatic interaction and precipitation of heavy metals, and the surface adsorption, partition and sequestration of organic contaminants (Zhang X et al., 2013). Neither biochar treatment (1%, weight/weight) nor iron treatment could successfully reduce both Cu and As leaching, but iron treatment negatively impacted soil structure and sunﬂower plant mortality. In contrast, the mixture of biochar and iron reduced both Cu and As leaching, increased phenanthrene degradation, and enabled sunﬂower growth, suggesting this as a useful approach for treating co-contaminated mining sites (Sneath et al., 2013). Biochar was more eﬀective than greenwaste compost at reducing bioavailable fractions of phytotoxic Cd and Zn as well as the heavier, more toxicologically relevant PAHs (Beesley et al., 2010). Biochar could promote bioremediation of PAHs contaminated soil as microbial carriers of immobilized-microorganism technique. However, it is vital to select an appropriate biochar as an immobilized carrier to stimulate biodegradation (Chen et al., 2012). The removal efﬁciencies of two- to four-ring PAHs were higher than those of ﬁve- and six-ring PAHs in contaminated soil amended with biochar (Liu et al., 2015). Biochar addition to soil could stimulate PAH-metabolizing bacterial activity by enhancing the number of gene copies related to PAH degradation and changing the structure of soil microbial community. However, in the case of agriculture, it is reported that application of biochar decreased eﬃcacy of pesti-
H. M. ANAWAR et al.
cides, which indicates a trade-oﬀ between the potentially promising eﬀect of biochar on pesticide remediation and its negative eﬀects on pesticide eﬃcacy (Tang et al., 2013; Evangelou et al., 2015). Therefore, further research is needed before biochar application is widely implemented in crop farming. Restoration of degraded and contaminated land using biochar The adequate restoration of the degraded and contaminated environment requires cooperation, integration, and assimilation of diﬀerent biotechnological advances along with traditional and ethical sense to unravel the science of the emerging ﬁeld of bioremediation, especially the mechanisms of phytoremediation of heavy metals (Mani and Kumar, 2014). Success in restoration and reclamation is dependent on the physico-chemical soil characteristics and soil community complexity. Soil amendments by compost, biochar, and arbuscular mycorrhizal fungi may facilitate grassland recovery in severely degraded habitats and the promotion of grassland ecosystem sustainability (Ohsowski et al., 2012). Biochar has potential impacts on restoration of degraded and contaminated land (Table III). Biochar amendment signiﬁcantly improved root traits, particularly root mass density and root length density, enhanced root establishment in contaminated soils, and reduced Cu uptake to plants compared to the control soil (Brennan et al., 2014). Although biochar addition increased extractable Ca, K, P, Cu, Zn, and Mn, CEC, mesoporosity, and water-holding capacity in ﬂy ash (an inorganic waste of coal-ﬁred power generation), it had a little or no stimulatory eﬀect on the size of the soil microbial community, N fertility, or plant growth during revegetation on ﬂy ash. This might be attributable to the lack of metabolizable C and an insigniﬁcant N-supplying capacity (Belyaeva and Haynes, 2012). Biochars immobilized soil Cd in industrial wastewater treatment, but did not improve growth of the emerging wetland plant species (Juncus subsecundus) at the early growth stage, probably due to the interaction between biochars and waterlogged environment (Zhang Z et al., 2013). Further study is needed to elucidate the underlying mechanisms. BIOCHAR PRODUCTION FROM MUNICIPAL SOLID WASTES Biochar (maize-derived) increased biomass production of oat (Avena sativa L.) and reduced Zn and Cd uptake by plants grown on wastewater-irrigated soil or sewage-ﬁeld soil (Wagner and Kaupenjohann, 2014). In
BIOCHAR FOR REMEDIATION OF SOIL CONTAMINANTS
TABLE III Impact of biochar on restoration of degraded and contaminated land Biochar
Heavy metal-contaminated ecosystems
Mani and Kumar, 2014
PAH and toxic element-contaminated soil
Degraded and contaminated land Wasteland, mine soil, landﬁlls
Viable, sustainable and eco-friendly bioremediation technologies, especially phytoremediation of heavy metals Decreased PAHs bioavailability, with less eﬀects on reducing metal mobility by biochar, in contrast to increased PAHs and metal bioavailability by Eisenia fetida Grassland recovery in severely degraded habitats
Metal and PAH-polluted soils
Heavy metals and organic pollutants PAH-contaminated soils
Biochar from dairy manure and rice hull Biochar
Biochar of pine needle
Herbicides in forest soils
Contaminated coal colliery sites
Enhanced nutrient availability, organic matter addition, microbial stimulation and pH buﬀering; decreased bioavailability of toxic metals Phytostabilization by maize; improved root mass and root length density; decreased Cu and As uptake; enhanced root establishment in soil More eﬀective ability of biochar at reducing bioavailable fractions of Cd and Zn as well as PAHs than greenwaste compost Increased soil pH and contribution to stabilization of heavy metals; remediation of contaminated soils Promotion of bioremediation of contaminated soil as microbial carriers of immobilized-microorganism technique Higher removal eﬃciencies of two-ring to four-ring PAHs than ﬁve- and six-ring PAHs in soil Reduced Cd accumulation; no improvement of the growth of Juncus subsecundus plant Enhanced sorption of PAHs to soil mitigating PAHscontaminated soils Enhanced soil sorption of terbuthylazine and reduced possibility of hydrophobic herbicide leaching to groundwater Improved grass growth
Gomez-Eyles et al., 2011
Ohsowski et al., 2012 Ram and Masto, 2014 Brennan et al., 2014 Beesley et al., 2010 Zhang et al., 2013 Chen et al., 2012 Liu et al., 2015 Zhang et al., 2013 Chen and Yuan, 2011 Wang et al., 2010 Ryan et al., 2014
= polycyclic aromatic hydrocarbons.
contrast, metal concentrations in soil leachate increased, possibly due to colloidal transport of Zn precipitates and Cu transport in the dissolved fraction. However, hydrochar (poplar-derived) is not suitable for metal immobilization. Conversion of sewage sludge into biochar is a potential way to manage these wastes, because the use of sewage sludge biochar and sewage sludge may have diﬀerent eﬀects on soil biochemical properties as indicators of soil quality (Hossain et al., 2010). Microbial biomass C, soil respiration, net N mineralization, and enzyme activities showed a diﬀerent response to the treatments of sewage sludge biochar and unpyrolyzed sewage sludge with higher quality after sewage sludge biochar treatment (PazFerreiro et al., 2012). ENERGY AND BIOCHAR FROM PLANTS GROWN ON CONTAMINATED SOIL Pyrolysis or combustion of waste wood can pro-
vide a renewable source of energy and produce biochar which can be used to land amelioration. Jones and Quilliam (2014) concluded that low levels of contamination from Cu-treated wood (preservative-treated timber) should pose minimal environmental risk to biochar and ash destined for land application. The root biomass of ryegrass (Lolium perenne var. Calibra) treated with biochar from birch (Betula pendula) wood produced on trace element-contaminated soil was lower than that of the non-amended plants, while that of the shoot was higher (Evangelou et al., 2014). The biochar addition to metal-contaminated soils can cultivate bioenergy crops, rapeseed (Brassica napus L.), without encroaching on agricultural lands that in turn be used as feedstock for pyrolysis to produce both bioenergy and new biochar (Houben et al., 2013). CONCLUSIONS AND RECOMMENDATIONS Biochar amendment can increase soil water-holding
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capacity, pH, CEC, and surface sorption capacity due to its high surface area, porosity, variable charge, and functional groups, which are controlled by the temperature of biochar formation and various source materials. Addition of biochar to soil enhances microbial diversity and species richness that are involved in N, P or S nutrient transformations. Biochar undergoes multiple reactions including adsorptiondesorption, precipitation-dissolution, acid-base, and redox reactions when it is added to soils. The conditions of soil moisture and temperature control the ageing, dissolution, hydrolysis, carbonation, decarbonation, hydration, and redox reactions aﬀecting biochar weathering in soils. The addition of biochar shows the signiﬁcant beneﬁts to revegetate and rehabilitate mining waste, tailings, and waste rock pile. It increases the shoot biomass and generally reduces the concentrations of toxic metals in plant species when grown for phytostabilization of mining wastes. Addition of biochar to sulphidic mining waste, tailings, and acid mine drainage decreases the acid production rate and enhances the alkalinity, resulting in neutralization of produced acid and decrease in heavy metal concentrations and phytotoxicity to plant species. Biochar can remediate contamination of heavy metals through electrostatic interaction and precipitation, and organic contaminants (PAHs, herbicides, etc.) through surface adsorption, partition, and sequestration. Soil amendments by biochar and compost may facilitate grassland recovery and revegetation in severely degraded habitats, mining waste, waste rock, and contaminated soil. The production of biochar from sewage sludge has greater eﬀect on soil quality than direct sewage sludge application. The biochar addition to mining, industrial, and sewage-contaminated soils can cultivate bioenergy crops that in turn be used to produce both bioenergy and new biochar. Biochar decreases organic pollutants signiﬁcantly; however, it is less eﬀective to reduce mobility of potentially toxic elements and shows diﬀerential eﬀects depending on the type of metals. Therefore, further research is needed to understand the eﬀects of biochar on dynamics of heavy metals in mining waste and contaminated soil. The eﬀects of biochar additions on the accumulation of C in mine tailings and waste over the long term should be studied in details. There is a need to overcome multiple risks and constraints such as lack of ﬁnance, socio-economic constraints including negative perceptions and attitudes of both researchers and consumers, and environmental and public health risks.
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