Using biochar for remediation of soils contaminated

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Using biochar for remediation of soils contaminated with heavy metals and organic pollutants Article in Environmental Science and Pollution Research · April 2013 DOI: 10.1007/s11356-013-1659-0 · Source: PubMed

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Environ Sci Pollut Res DOI 10.1007/s11356-013-1659-0 CONTAMINATED LAND, ECOLOGICAL ASSESSMENT AND REMEDIATION CONFERENCE SERIES (CLEAR 2012) : ENVIRONMENTAL POLLUTION AND RISK ASSESSMENTS

Using biochar for remediation of soils contaminated with heavy metals and organic pollutants Xiaokai Zhang & Hailong Wang & Lizhi He & Kouping Lu & Ajit Sarmah & Jianwu Li & Nanthi S. Bolan & Jianchuan Pei & Huagang Huang

Received: 5 January 2013 / Accepted: 18 March 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract Soil contamination with heavy metals and organic pollutants has increasingly become a serious global environmental issue in recent years. Considerable efforts have been made to remediate contaminated soils. Biochar has a large surface area, and high capacity to adsorb heavy metals and organic pollutants. Biochar can potentially be used to reduce the bioavailability and leachability of heavy metals and organic pollutants in soils through adsorption and other physicochemical reactions. Biochar is typically an alkaline material which can increase soil pH and contribute to stabilization of heavy metals. Application of biochar for remediation of contaminated soils may provide a new solution to the soil pollution problem. This paper provides an overview Responsible editor: Zhihong Xu X. Zhang : H. Wang : L. He : K. Lu : J. Li Zhejiang Provincial Key Laboratory of Carbon Cycling in Forest Ecosystems and Carbon Sequestration, Zhejiang A & F University, Lin’an, Hangzhou, Zhejiang 311300, China X. Zhang : H. Wang (*) : L. He : K. Lu : J. Li : J. Pei : H. Huang School of Environmental and Resource Sciences, Zhejiang A & F University, Lin’an, Hangzhou, Zhejiang 311300, China e-mail: [email protected] A. Sarmah Department of Civil & Environmental Engineering, Faculty of Engineering, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand N. S. Bolan Centre for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes, South Australia, Australia e-mail: [email protected] H. Huang (*) Yancao Production Technology Center, Bijie Yancao Company of Guizhou Province, Bijie 551700, China e-mail: [email protected]

on the impact of biochar on the environmental fate and mobility of heavy metals and organic pollutants in contaminated soils and its implication for remediation of contaminated soils. Further research directions are identified to ensure a safe and sustainable use of biochar as a soil amendment for remediation of contaminated soils. Keywords Biochar . Black carbon . Heavy metals . Organic pollutants . Remediation . Soil contamination

Introduction In recent years, increasingly more soils are found to be contaminated with organic and inorganic toxins globally due to waste emissions from industrial production, mining activities, waste (i.e., biosolids and manures) application, wastewater irrigation, and inadequate management of pesticides and chemicals in agricultural production (Bolan et al. 2004; Mench et al. 2010). More environmentally acceptable alternatives to unsustainable waste management technologies have been sought to minimize further soil contamination (Beesley et al. 2011). Pollutants in soils are not only harmful to ecosystems and agricultural production but also a serious threat to human wellbeing. For example, it has been estimated that 3.5 million sites in industrial and mine sites, landfills, energy production plants, and agricultural land are potentially contaminated in Europe (Petruzzelli 2012), and therefore, soil contamination has been identified as an important issue for action in the European Community strategy for soil protection. In China, economy has been developing rapidly in the last few decades, which has also brought some environmental problems. For example, arable land subjected to heavy metal contamination is close to 20 million hectares, accounting for 20 % of the total agricultural land area in China (Xi et al. 2011). Considerable efforts have been made

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to remediate polluted soils, as shown in the increasing literature information on the amendment of soil pollutions (Bolan and Duraisamy 2003; Naidu et al. 2008). Physical/chemical remediation, bioremediation, and integrated remediation were used to manage contaminated soils (Mullainathan et al. 2007; Lee et al. 2008; Mendez and Maier 2008). Biochar is the solid product from pyrolysis of waste biomass residues from agricultural and forestry production (Wang et al. 2010; Liu et al. 2011; Xu et al. 2013). Application of biochar to soil has been considered as to having great potential to enhance long-term carbon sequestration because most carbon in biochar has an aromatic structure and is very recalcitrant in the environment (Lehmann 2007). Typically, biochar has a high pH value and cation exchange capacity, and can enhance soil productivity (Jeffery et al. 2011; Kookana et al. 2011). A number of studies have also demonstrated that biochar has a high capacity to adsorb pollutants in soils (Beesley et al. 2011; Yuan and Xu 2011). In this paper, we aim to provide an overview of the current practices in remediation of contaminated soils and the effects of biochar on the mobility and bioavailability of soil contaminants, and explore the potential of using biochar for remediation of contaminated soils. We will also identify future research directions associated with using biochar for remediation of contaminated soils.

Biochar for remediation of soils contaminated with heavy metals Heavy metals are not biodegradable, and persist for a long time in contaminated soils. It is expensive and time consuming to remove heavy metals from contaminated soils (Cui and Zhang 2004). Stabilization of heavy metals in situ by adding soil amendments such as lime and compost is commonly employed to reduce the bioavailability of metals and minimize plant uptake (Bolan and Duraisamy 2003; Bolan et al. 2004; Kumpiene 2010; Komárek et al. 2013). Biochar can stabilize heavy metals in the contaminated soils, improve the quality of the contaminated soil (Ippolito et al. 2012) and has a significant reduction in crop uptake of heavy metals. Therefore, application of biochar can potentially provide a new solution for remediation of the soils contaminated by heavy metals. Stabilization of heavy metals in soils with application of biochar could involve a number of possible mechanisms, as illustrated in Fig. 1 (Lu et al. 2012). Taking Pb2+ as an example, the authors proposed various mechanisms for Pb2+ sorption by sludge-derived biochar that could include (1) heavy metal exchange with Ca2+, Mg2+, and other cations associated with biochar, attributing to co-precipitation

Fig. 1 Conceptual illustration of the possible mechanisms of Pb adsorption on biochar (from Lu et al. 2012)

and innersphere complexation with complexed humic matter and mineral oxides of biochar; (2) the surface complexation of heavy metals with different functional groups, and innersphere complexation with the free hydroxyl of mineral oxides and other surface precipitation; and (3) the physical adsorption and surface precipitation that contribute to the stabilization of Pb2+ (Lu et al. 2012). In case of acidic contaminated soils, depending on the type of biochars and exchangeable cations (Na, Mg, K, and Ca) present in it could hold the key for the release of some of the these cations during sorption process with the heavy metal, and thus may enrich the stabilization process. Lu et al. (2012) further demonstrated that the heavy metal exchange with Ca2+, Mg2+, and other cations (Na+ and K+) associated with sludge-derived biochar was the main mechanism responsible in their study; however, contribution of monovalent (Na+ and K+) cations for heavy metal exchange was found to be negligible. Therefore, it is conceivable that under realistic field situation, sorption mechanisms for metalcontaminated soils by biochar could be dependent on the type of soils and the cations present in both soils and biochar, and thus implications for metal remediation in contaminated soils could vary. The mineral components such as phosphates and carbonates in biochar play an important role in stabilization of heavy metals in soils because these salts can precipitate with heavy metals and reduce their bioavailability (Cao et al. 2009). Cao and Harris (2010) propose that the main mechanism for dairy

Environ Sci Pollut Res

manure biochar to be effective to retain Pb was the precipitation of insoluble Pb phosphates. Generally, during the manufacture of biochar, water-soluble P, Ca, and Mg increased when heated to 200 °C but decreased at higher temperatures probably due to increased crystallization of Ca–Mg–P, as evidenced by the formation of whitlockite (Ca, Mg)3(PO4)2 when pyrolysis temperature increased to 500 °C, thereby facilitating the precipitation of Pb (Cao and Harris 2010). Alkalinity of biochar can also promote heavy metal precipitation in soils. Chan and Xu (2009) reviewed biochar pH values from a range of feedstocks in the literature and obtained a mean value of pH 8.1. With the same feedstock material, biochar pH value increases with pyrolysis temperature because of increased ash content in biochar (Wu et al. 2012). Therefore, most biochars are alkaline material and have a liming effect, which contributes to the reduction of the mobility of the heavy metals in contaminated soils (Sheng et al. 2005). However, the adsorption ability of the same type of biochar varies with different types of heavy metals. Effect of biochar on heavy metal mobility Biochar application can reduce the mobility of heavy metals in contaminated soils (Table 1), which renders a reduced risk of taking up by plants. Studies have shown that biochar derived from bamboo can adsorb Cu, Hg, Ni, and Cr from both soils and water, and Cd in polluted soils (Skjemstad et al. 2002; Cheng et al. 2006). Cao et al. (2009) reported that dairy manure-derived biochar pyrolyzed at 200 °C was more effective in sorbing Pb than biochar produced at 350 °C because the 200 °C biochar had the higher concentration of soluble phosphate. Given that biochar characteristics are a function of feedstock and pyrolytic conditions, not one type of biochar could be universally used to remediate soils contaminated with various types of heavy metals. Additionally, not one type of mechanism, or a particular feedstock, or pyrolytic condition could hold true for heavy metal remediation of soil using biochar as an adsorbent. Therefore, when biochar is to be utilized as an amendment

for the remediation of soils contaminated with heavy metals, one should take into account the types of heavy metals present in the contaminated soil, and the biochar production temperature as the biochar characteristics are dependent on pyrolysis conditions such as highest treatment temperature, moisture content of the feedstock, residence time, and the type of feedstock used. The effect of biochar on metal bioavailability varies with the types of biochar products as well as types of heavy metals. A soil contaminated with Cd and Zn was amended with a hardwood-derived biochar and the concentration of both metals in pore water reduced (Beesley et al. 2010). Using the same soil in a column leaching experiment, biochar addition immobilized both Cd and Zn, and consequently, the pore water Cd and Zn concentrations were reduced 300- and 45-folds, respectively (Beesley and Marmiroli 2011). Namgay et al. (2010) reported that the concentrations of extractable As and Zn in soil increased with biochar application rate, whereas the concentration of extractable Pb decreased, Cu did not change, and Cd showed an inconsistent trend. They also found that sorption of trace elements on biochar with initial loadings up to 200 μmol at pH 7 occurred in the order: Pb > Cu > Cd > Zn > As. Biochar application can also reduce the leaching of metals through its effect of redox reactions of metals. For example, Choppala et al. (2012) showed that the application of biochar derived from chicken manure to chromate (CrVI)-contaminated soils enhanced the reduction of mobile Cr(VI) to less mobile Cr(III), thereby decreasing the leaching of Cr. The decrease in the leaching of Cr(III) is attributed to the adsorption of Cr(III) onto cation exchange sites and also to the precipitation as Cr(OH)3 resulting from the release of OH − ions during the Cr(VI) reduction process (Fig. 2; Bolan et al. 2013). Effect of biochar on the bioavailability of heavy metals The bioavailability of heavy metals determines the toxicity in the soil and potential risk in entering human food chain. The bioavailability of pollutants governs their ecotoxicology and

Table 1 Effect of biochar application on the mobility of heavy metals in soils Feedstock

Production temperature

Contaminant

Effect

Reference

Bamboo

Cd As, Cd, Cu, Zn

Combined effect of electrokinetics, removal of extractable Cd by 79.6 % within 12 days Reduction in Cd in soil pore water by 10-folds; Zn concentrations reduced 300- and 45-folds, respectively, in column leaching tests

Ma et al. (2007)

Hardwood

Not available 450 °C

Hardwood

450 °C

Wood

200 °C and 400 °C

As, Cd, Cu, Pb, Zn Cd, Zn

Biochar surface mulch enhanced As and Cu mobility in the soil profile; little effect on Cd and Pb Reduction in Zn and Cd leaching loss by >90 %

Beesley et al. (2010); Beesley and Marmiroli (2011) Beesley and Dickinson (2011) Debela et al. (2012)

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Fig. 2 Concomitant reduction and immobilization of chromium in biochar carbon-amended soils (from Bolan et al. 2013)

degradation in contaminated soils. Environmental microbiologist defines bioavailability as the contaminant fraction which represents the accessibility of a chemical to a living organism for assimilation, degradation, and ecotoxicology expression (Naidu et al. 2008). A number of studies have shown that biochar application is effective in heavy metal immobilization, thereby reducing the bioavailability and phytotoxicity of heavy metals (Table 2). Fellet et al. (2011) evaluated the potential of

application of biochar to ameliorate the heavy metal toxicity in the mine tailings. They applied biochar derived from orchard prune residues at four rates (0 %, 1 %, 5 %, and 10 % biochar in the mine tailings). The pH, cation exchange capacity, and the water-holding capacity increased as the biochar rates increase and the bioavailability of Cd, Pb, and Zn of the mine tailings decreased, with Cd having the greatest reduction. Zhou et al. (2008) used cotton stalkderived biochar to amend Cd-contaminated soil and studied the uptake of Cd by the cabbage. They found that the cotton stalk-derived biochar can reduce the bioavailability of soil Cd through adsorption or co-precipitation. Méndez et al. (2012) evaluated the effects of biochar derived from sewage sludge on heavy metals solubility and bioavailability in a Mediterranean agricultural soil and compared with those of sewage sludge, which was not charred. The biochar treatments reduced plant availability of Ni, Zn, Cd, and Pb when compared to sewage sludge treatments. Table 2 summarizes the effect of different biochar types on the bioavailability and uptake of range of contaminants. Park et al. (2011) reported that both chicken manure- and green waste-derived biochars significantly reduced Cd, Cu, and Pb uptake by Indian mustard. The study also found that the reduction of the plant metal concentrations increased with biochar application rates except for Cu concentration. Elsewhere, a study conducted by Jiang et al. (2012) demonstrated that the rice straw biochar was more efficient in the immobilization of Cu and Pb than Cd. Therefore, when the

Table 2 Effect of biochar application on the bioavailability of heavy metals in soils Feedstock

Production temperature

Contaminant

Effect

Reference

Cotton stalks

450 °C

Cd

Hardwoodderived biochar Eucalyptus

400 °C

As

Reduction of the bioavailability of Cd in soil by adsorption or co-precipitation Significant reduction of As in the foliage of Miscanthus

550 °C

As, Cd, Cu, Pb, Zn Cd, Cr, Cu, Ni, Pb, Zn

Zhou et al. (2008) Hartley et al. (2009) Namgay et al. (2010) Fellet et al. (2011)

Decrease in As, Cd, Cu, and Pb in maize shoots

Orchard prune residue

500 °C

Chicken manure and green waste Chicken manure

550 °C

Cd, Cu, Pb

Significant reduction of the bioavailable Cd, Pb, and Zn, with Cd showing the greatest reduction; an increase in the pH, CEC, and water-holding capacity Significant reduction of Cd, Cu, and Pb accumulation by Indian mustard

550 °C

Cr

Enhanced soil Cr(VI) reduction to Cr(III)

Sewage sludge

500 °C

Rice straw

Not clear

Cu, Ni, Zn, Cd, Pb Cu, Pb, Cd

Quail litter

500 °C

Cd

Oak wood

400 °C

Pb

Significant reduction in plant availability of the metals studied Significant reduction in concentrations of free Cu, Pb, and Cd in contaminated soils; identification of functional groups on biochar with high adsorption affinity to Cu Reduction of the concentration of Cd in physic nut; greater reduction with the higher application rates Bioavailability reduction by 75.8 %; bioaccessibility reduction by 12.5 %

Park et al. (2011) Choppala et al. (2012) Méndez et al. (2012) Jiang et al. (2012) Suppadit et al. (2012) Ahmad et al. (2012)

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purpose of utilization of biochar is to immobilize heavy metals, particular attention should be paid to the selection of feedstock, as biochar properties are dependent on the feedstock's inherent properties as well as the pyrolysis conditions under which the biochar is prepared. In a pot experiment, Namgay et al. (2010) applied an activated wood biochar to a soil spiked with heavy metals in order to investigate the impact of biochar on the availability of As, Cd, Cu, Pb, and Zn to maize. Biochar treatment decreased the concentration of As, Cd, and Cu in maize shoots. However, the effects of adding biochar were inconsistent on Pb and Zn concentrations in the shoots. Soil pH is closely related to the bioavailability of heavy metals in soils. Uchimiya et al. (2010b) suggested that biochar application can increase the soil pH and cation exchange capacity, and subsequently enhance the immobilization of heavy metals in soil. Ahmad et al. (2012) used mussel shell, cow bone, and biochar to reduce Pb toxicity in the highly contaminated military shooting range soil in Korea. Bioavailability of Pb in the soils was found to decrease by 75.8 % with biochar treatment. Increases in soil pH and the adsorption capacity were considered as the mechanisms of remediation effect of the biochar. For example, the bioavailability of Pb in the soils was decreased by up to 92.5 % with mussel shell, a liming material (Ahmad et al. 2012). Although many studies showed that biochar can reduce heavy metal mobility and its bioavailability, majority of these studies were conducted under controlled laboratory and greenhouse experiments and in small plot trials. It is only when large-scale field trials are conducted, the practical usefulness of biochar as a remediation material can be best appreciated; however, to date, this has not been attempted anywhere.

Biochar for remediation of soils contaminated with organic pollutants Soil contamination with organic pollutants is usually caused by a wide range of industrial activities, farming practices, and inadequate application of wastes. Some of the organic pollutants are recalcitrant to degradation, and some are carcinogenic or mutagenic (Fabietti et al. 2010). Organic pollutants include persistent organic pollutants (POPs) and emerging organic pollutants. Many organic pollutants are currently or were in the past used as pesticides. Others are used in industrial processes and in the production of a range of products such as solvents, additives, and pharmaceuticals. For example, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/DFs), polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons (PAHs) are some of the well-known persistent organic pollutants (POPs) (WHO 2010). Typically, POPs would accumulate in soil horizons

rich in organic matter (OM) where they may be retained for years (Masih and Taneja 2006). Emerging pollutants are suspected of causing adverse effects in humans and wildlife. For example, phthalate acid esters [PAEs, e.g., dibutyl phthalate and di(2-ethylhexyl)phthalate], naturally released estrogenic steroid hormone and its metabolites (e.g., estradiol and estrone), pharmaceutical and personal care products (PPCPs, e.g., trimethoprim and triclosan), etc. are considered as emerging organic pollutants (Petrović et al. 2001). Biochar has been reported to be very effective in adsorption of many natural and anthropogenic organic compounds (Accardi-dey and Gschwend 2003; Lohmann et al. 2005; Cao et al. 2009; Cui et al. 2009; Sarmah et al. 2010). Many past studies have demonstrated that given the highly aromatic nature, high surface area, micropore volume, and the presence of abundance of polar functional groups in biochar, the material has been found to be effective in the uptake of a variety of organic chemicals including pesticides, PAHs, and emerging contaminants such as steroid hormones (Kookana et al. 2011). Though the highly porous nature and large surface area of biochar are important for effective removal of pollutants, the nature and type of organic carbon and the degree of aromaticity also play an important role (Sarmah et al. 2010). Biochar can reduce the bioavailability of the organic pollutants through sorption, and reduce the risk of the pollutants entering human food chain or leaching to groundwater. However, the long-term environmental fate of the sequestered contaminants is still unknown, and further research is warranted to bridge this gap especially under realistic field conditions through biochar-mediated remediation trials. Effect of biochar on adsorption of organic pollutants Behavior of contaminant sorption to biochar is closely related to the process that regulates the concentration of organic pollutants in contaminated soils. It can consequently affect other processes such as bioavailability, degradation, leaching, and volatilization of contaminants (Table 3). Biochar's high specific surface area governs most soil– biochar interactions. This property is affected by the nature of the feedstock biomass material and the conditions under which the biochar is produced (Downie et al. 2009; Wu et al. 2012). Biochar adsorption and desorption of organic pollutants in the soil is greatly influenced by pyrolysis temperature. James et al. (2005) determined the phenanthrene uptake isotherms with the wood biochars from the species Pinus sylvestris and Betula pendula. The isotherm data clearly demonstrate that phenanthrene sorption increases for materials exposed to higher temperatures. These sorption increases also coincide with increases in surface area of biochars produced at higher temperatures. Eucalyptus wood-derived biochar pyrolyzed at 850 °C (BC850) contained mainly micropores, whereas that pyrolyzed at

Environ Sci Pollut Res Table 3 Effect of biochar application on sorption of organic pollutants in soils Feedstock

Production temperature

Contaminant

Effect

Reference

Eucalyptus wood

450 °C and 850 °C 500 °C

Dairy manure

200 °C and 350 °C 350 °C and 700 °C 450 °C 350 °C and 700 °C

Higher pyrolysis temperature and higher rates of biochar applied to soils result in stronger adsorption and weaker desorption of pesticides Acetochlor adsorption increased 1.5 times; atrazine adsorption also increased At 200 °C, partitioning of atrazine is positively related to biochar carbon content Soil sorption increased 2.7- and 63-folds in the BC350 and BC700 treatments, respectively Biochar enhanced adsorption of pesticide Biochar produced at 700 °C showed a greater ability at enhancing a soil's sorption ability than that prepared at 350 °C Sorption capacity increased with pyrolysis temperature

Yu et al. (2006)

Woodchip

Diuron chlorpyrifos and carbofuran Atrazine and acetochlor Atrazine

Pine wood Green wastes Pine wood

Pine needles

Eucalyptus wood chips Poultry litter, wheat straw, and swine manure Swine manure

Terbuthylazine Atrazine Phenanthrene

100 °C, 300 °C, 400 °C, and 700 °C 850 °C

PAHs

250 °C and 400 °C

Herbicides

350 °C and 700 °C

Carbaryl

Diuron

Spokas et al. (2009) Cao et al. (2009) Wang et al. (2010) Zheng et al. (2010) Zhang et al. (2010)

Chen and Yuan (2011)

Pesticide absorption increases with the biochar contact time with soil and application rate Biochars showed high sorption ability for two herbicides, fluridone and norflurazon

Yu et al. (2011b)

At low carbaryl concentrations, the sorption capacity BC700 > BC350; similar sorption capacity at high carbaryl concentrations

Zhang et al. (2013)

450 °C (BC450) was essentially not a microporous material (Yu et al. 2006). As a result, BC850 had a much higher capacity to adsorb diuron in a soil than that the BC450 did. Similarly, the biochar pyrolyzed at 700 °C had a much higher adsorption capacity but weaker desorption capacity of terbuthylazine in soils than the biochar produced at 350 °C (Fig. 3). As discussed above, the high specific surface areas and microporosity make biochar very efficient sorbents for a range of organic compounds. However, such behaviors may change with time after biochars are applied to soils. This process is commonly referred to as “aging” (Kookana 2010). The interactions between biochars and other soil constituents such as natural organic molecules and clay minerals contribute to the aging of biochars (Uchimiya et al. 2010a). It has been suggested that natural organic matter can block the micropores of biochars and suppress sorption of organic contaminants (Pignatello et al. 2006). Wang et al. (2010) observed that biochar-enhanced soil adsorption of herbicide terbuthylazine is much greater in a soil with low organic matter than that in a soil with higher organic matter content (Fig. 3). It is stipulated that the higher concentration of dissolved organic molecules that exists in the latter soil may compete with terbuthylazine for sorption sites on biochar (Wang et al. 2010). Zhang et al. (2010) found that the adsorption capacity of the pine-derived biochars was consistently reduced after the biochars were incubated with

Sun et al. (2012)

soil for 4 weeks (Fig. 4). Martin et al. (2012) studied the sorption–desorption behavior of herbicides in a soil either amended with freshly prepared biochars or with biochars aged under field conditions for 32 months. The sorption capacity of the aged biochars was reduced at least by 47 % for herbicide diuron. All these studies showed that the aging of biochar can affect its properties, and consequently, this can lead to lower the capacity for the biochar to absorb contaminants of interest. So, more understanding of the aging process is essential for determination of the biochar application rate and frequency to improve the remediation efficiency. Effect of biochar on the bioavailability of organic pollutants Many studies have demonstrated that biochar-amended soil can help absorb a variety of organic contaminants, thereby reducing their uptake by plants. Application of a small amount of biochar to soil can significantly reduce the accumulation of pesticides and other organic pollutants in plants (Hilber et al. 2009; Kookana 2010; Table 4). Yang et al. (2006) reported that increasing biochar content in a soil can reduce the bioavailability of the herbicides. They found that even at low application rate (0.1 %), biochar in soil would appreciably reduce the bioavailability of diuron. Similarly, Graber et al. (2012) tested the influence of two biochars on phytoavailability of two herbicides S-metolachlor and

Environ Sci Pollut Res Fig. 3 Effect of biochar treatments on the adsorption and desorption of terbuthylazine in soils. Lines are sorption and single-step desorption isotherms fitted to Freundlich equation. a Landing site soil (1.2 % organic C) only. b Topsoil (5.1 % organic C) only. c Landing site soil treated with biochar produced at 700 °C. d Topsoil treated with biochar produced at 700 °C. e Landing site soil treated with biochar produced at 350 °C. f Topsoil treated with biochar produced at 350 °C (modified from Wang et al. 2010)

18

18

Adsorption Desorption

B

A 12

12

Landing site soil (LS) Control (No Biochar)

6

6 0

0 0.0

TA in soil phases, mg kg -1

Topsoil (TS) Control (No Biochar)

0.5

1.0

1.5

2.0

2.5

0.0

3.0

0.5

1.0

1.5

2.0

2.5

18

18

D

C 12

12

LS+1%BC700

3.0

TS+1%BC700

6

6

0

0 0.0

0.5

1.0

1.5

2.0

18

2.5

0.0

3.0

0.5

1.0

1.5

2.0

2.5

18

E

3.0

F

12

12

LS+1%BC350

6

TS+1%BC350

6 0

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

TA in aqueous phases, mg L -1

No biochar amendment 0.1% 350BC no aging 0.1% 350BC with aging 0.5% 350BC no aging 0.5% 350BC with aging

sulfentrazone. They found that biochars, particularly the biochar with a high specific surface area, can significantly reduce the bioavailability and efficacy of herbicide for weed control. Shi et al. (2011) added rice straw-derived biochar to phenanthrene-contaminated soil and noticed a significant reduction of phenanthrene uptake by maize seedlings. Beesley et al. (2010) examined biochar-amended soil and showed a reduction in soil pore water concentration of PAH by 50 %. Song et al. (2012a) investigated wheat straw biochar on the sorption, dissipation, and bioavailability of hexachlorobenzene. They observed that hexachlorobenzene sorption by biochar was 42 times higher than that by the control soil, thereby reducing the volatilization and earthworm (Eisenia foetida) uptake of hexachlorobenzene from the soil. Where pest control by chemicals is necessary, biochar application should be carefully planned to avoid unintended consequence of offsetting pesticide efficacy (Graber et al. 2012).

0.1% 700BC no aging 0.1% 700BC with aging 0.5% 700BC no aging 0.5% 700BC with aging

5.0

logCs, ug/kg

4.5

4.0

3.5

3.0

2.5

Biochars for enhanced remediation of soils contaminated with organic pollutants 2.0 -2

-1

0

1

2

3

logCe, ug/L Fig. 4 Sorption of phenanthrene in a sandy soil (0.16 % organic C) with or without biochar amendment. Solid lines and dashed lines are sorption and single-step desorption isotherms fitted to Freundlich equation, respectively. BC350 and BC700 are biochars pyrolyzed at 350 °C and 700 °C, respectively (from Zhang et al. 2010)

While biochar application can increase sorption of organic contaminants by reducing their bioavailability and leachability, at the same time, biological degradation of organic pollutants in soil can be also substantially slowed down because of reduced microbial accessibility to the organic pollutants (Kookana 2010; Sopeña et al. 2012). For instance, using

Environ Sci Pollut Res Table 4 Effect of biochar application on bioavailability of organic pollutants in soils Feedstock

Production temperature

Contaminant

Effect

Reference

Eucalyptus

450 °C and 850 °C 450 °C

Cotton straw

450 °C and 850 °C

Chlorpyrifos and fipronil

Bamboo Hardwood

600 °C 600 °C

Pentachlorophenol PAHs

Wheat straw Wheat straw

500 °C

Chlorobenzenes (CBs)

250 °C, 300 °C, and 500 °C

Hexachlorobenzene (HCB)

Reductions of chlorpyrifos and carbofuran in total plant residues, respectively Pore water concentrations of PAHs were reduced by biochar, with greater than 50 % decrease of the heavier, more toxicologically relevant PAHs Chinese chive uptake of fipronil and chlorpyrifos reduced by 52 % and 81 %, respectively, with 1 % of 850 °C biochar addition Biochar reduced PCP bioavailability in soil Biochar application reduced concentration and biological activity of PAHs in soil Biochar amendment significantly reduced the bioavailability of CBs Biochar amendment of soil resulted in a rapid reduction in the bioavailability of HCB, even at 0.1 % biochar application rate

Yu et al. (2009)

Hardwood

Diuron chlorpyrifos and carbofuran PAHs

biochars derived from cotton straw and pyrolyzed at 850 °C, demonstrated the effect of biochar application on pesticide dissipation rates. The authors reported that the half-life for chlorpyrifos increased from 21.3 days in the untreated soil to 55.5 days in the soil amended with 1 % biochar, and the half-life for fipronil increased from 27.3 to 60.3 days. Similarly, in a laboratory study, Song et al. (2012a) found that application of wheat straw biochar significantly reduced the dissipation of hexachlorobenzene in soil because of the strong adsorption capacity of the biochar. Ideally, soil organic pollutants need to be degraded for sustainable remediation of contaminated soils. In recent years, some attempts have been made to investigate the feasibility of using biochar to accelerate the degradation of organic pollutants in soils (Kemper et al. 2008; Oh et al. 2012). For instance, one of the findings is that the special structure of biochar was regarded as both sorption sites and electron conductors, which could catalyze the reduction of some organic contaminants, e.g., nitroaromatic compounds, thereby enhancing their degradation (Kemper et al. 2008). Using pine wood-derived biochar as a catalyst, Yu et al. (2011a) investigated the reduction of nitrobenzenes to anilines by sulfides at room temperature, and demonstrated that biochar could serve not only as an adsorbent but also as a platform to accelerate the reduction of nitrobenzenes. However, more attention should be directed towards the utilization of biochar in the transformation of toxic nitrobenzenes to non-toxic anilines in the aquatic system and sediments. This would enable the development of in- situ remediation technique using biochar as a catalyst for the degradation of nitrobenzenes in sediments. Recently, biochar-mediated degradation of pesticides through hydrolysis has been explored (Zhang et al. 2013). Hydrolysis is one of the important mechanisms for abiotic

Beesley et al. (2010)

Yang et al. (2010)

Xu et al. (2011) Gomez-Eyles et al. (2011) Song et al. (2012b) Song et al. (2012a)

degradation of contemporary pesticides (Sarmah and Sabadie 2002). Factors, such as pH, dissolved ions, clay, and metal oxides can catalyze the hydrolysis reaction and subsequently influence pesticide degradation. Manure-derived biochar contains a high ash content, which is expected to influence the hydrolysis of pesticides. Zhang et al. (2013) investigated the effect of pig manure-derived biochars on hydrolysis of two pesticides, carbaryl and atrazine. The authors reported that carbaryl was hydrolyzed rapidly in the suspension of the untreated biochars. The 7-day hydrolysis achieved up to 90 % degradation of carbaryl in the presence of biochar acquired at 700 °C pyrolysis temperature. However, the hydrolysis of atrazine was much slower than that of carbaryl. When washed biochar was used, the effect of biochar on carbaryl hydrolysis is substantially reduced. It was concluded that the ash constituents, including the alkalinity, released dissolved metal ions, and the mineral surface, played the catalytic role in carbaryl hydrolysis. However, hydrolysis of atrazine was mainly enhanced by high pH and mineral surface (Zhang et al. 2013). Bioremediation is one of the commonly practiced technologies for cleaning up soils contaminated with organic pollutants through employment of plants or microorganisms (Smith et al. 2009; Chen and Yuan 2012). To enhance the efficiency of bioremediation, bioaugmentation that involves the addition of concentrated microorganisms capable of decomposing certain types of organic pollutants (e.g., high molecular weight PAHs) have been proposed (Chen et al. 2012). It requires well-designed immobilized carriers that are intended to offer a protective space for inoculated microorganisms and minimize competition from indigenous soil microbes. The access of organic pollutants in contaminated soil to immobilized cells is dependent on the concentration of pollutants in carriers

Environ Sci Pollut Res

(Dzul-Puc et al. 2005). With high sorption capability to organic pollutants in soils and resistance to degradation, biochars can pre-concentrate pollutants in contaminated soil then feed to the immobilized microbial decomposers (Su et al. 2006). Chen et al. (2012) conducted a study on the dissipation of PAHs in a contaminated soil amended with immobilized bacteria using biochar as a carrier (Fig. 5). The process is known as immobilized microorganism technique (IMT). They found that the IMT is an effective bioaugmentation approach for enhancing bioremediation of PAH-contaminated soil. The immobilized bacteria could directly degrade the biocharsorbed PAHs. It is important to select an appropriate biochar as an immobilized carrier to stimulate biodegradation of organic pollutants (Chen et al. 2012). Many literatures have showed that biochars are effective in the immobilization of a wide range of organic and inorganic chemicals (Skjemstad et al. 2002; Cheng et al. 2006; Beesley et al. 2010; Hilber et al. 2009; Kookana 2010; Yang et al. 2009; Zhang et al. 2013). Thus, it is conceivable that biochar application to soil could influence the plant uptake of a range of organic compounds thereby impacting plant growth, but this aspect has not received much attention in the literature so far. Chagger et al. (1998) noted that the presence of combustiondriven toxic organic compounds, such as polynuclear aromatic hydrocarbons (PAHs), chlorinated hydrocarbons, and dioxins, is often suspected in biochar products, and their concentrations are dependent on carbonization temperature and feedstock source (Brown et al. 2006). Some biochars are often found to be rich in heavy metal contents. For example, Singh et al. (2010) observed that Zn contents of Eucalyptus saligna wood and poultry litter biochars were found to range from 1,312 to 1,661 mg kg−1, and from 1,449 to 1,642 mg kg−1, respectively. Therefore, when biochars are used to remediate contaminated soils, the carbonization temperature and feedstocks should be carefully chosen to avoid the high concentrations of metals that may be present in the biochar.

efficacy of both heavy metal and organic pollutants in soil. Biochars produced from different biomass materials and with different pyrolysis conditions (e.g., temperatures) present highly heterogeneous physicochemical properties, which can affect the efficacy in the remediation of contaminated soils. As a potential technology for remediation of contaminated agricultural soils, many aspects are still yet to be developed. Several knowledge gaps have been identified, and further research is required to close these gaps. Some key research needs are outlined below: &

&

&

&

&

Conclusions and future research directions Biochar has the potential to be developed as a viable technology for remediation of contaminated soils. Obviously, biochar can conceivably reduce the bioavailability and Fig. 5 Conceptual illustration for enhancing bioremediation of soil organic pollutants with immobilized microorganism technique (IMT) using biochar as a strong adsorption carrier (modified from Chen et al. 2012)

&

So far, the studies about using biochar for remediation of contaminated soils mainly focus on the laboratory and greenhouse experiments and small plot trials. Largescale field trials are essential before operational scale remediation projects are implemented. The biochar characteristics vary with different biomass materials and pyrolysis conditions. It is important to optimize production systems to produce designer biochar products to be used effectively for specific remediation work. The strong sorption and weak desorption of pollutants in biochar indicate that biochar sequesters pollutants in itself. Biochar application can lead to accumulation of contaminant residues in the amended soils. However, the long-term environmental fate of the sequestered contaminants is still not well understood. The capacity of biochar to adsorb or sequester pollutants decreases with time due to aging process. More understanding of the aging process is warranted for further research. This information is essential for determination of the biochar application rate and frequency to improve remediation efficiency. Limited studies have demonstrated that biochar not only can reduce the bioavailability and leachability of the pollutants in soils through the process of sorption but also may facilitate accelerated dissipation of some organic pollutants in soil. Further research is required to explore the practical feasibility of biochar-assisted dissipation of organic pollutants. The immobilized microorganism technique (IMT) with biochar as microbial carrier shows promising feasibility for cleaning up soils contaminated with organic

Biochar + Specialist microbial decomposer Immobilized microbial organisms (IMO)

Apply IMO to contaminated soils

Contaminants are concentrated on biochar via sorption

Adsorbed contaminantsare degraded by IMO

Environ Sci Pollut Res

pollutants. Development of biochar for production of optimum carrier should be conducted. Acknowledgments This study was funded by the National Natural Science Foundation of China (41271337), Research Funds of the Department of Education of Zhejiang Province (Y201225755) and Zhejiang A & F University Research and Development Fund (2010FR097, 2012FR063).

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