using biochar for remediation of soils contaminated with heavy metals and organic pollutants

CONTAMINATED LAND, ECOLOGICAL ASSESSMENT AND REMEDIATION CONFERENCE SERIES (CLEAR 2012) : ENVIRONMENTAL POLLUTION AND RISK ASSESSMENTS

Using biochar for remediation of soils contaminatedwith 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 organ-ic pollutants has increasingly become a serious global envi-ronmental issue in recent years. Considerable efforts havebeen made to remediate contaminated soils. Biochar has alarge surface area, and high capacity to adsorb heavy metalsand organic pollutants. Biochar can potentially be used toreduce the bioavailability and leachability of heavy metalsand organic pollutants in soils through adsorption and otherphysicochemical reactions. Biochar is typically an alkalinematerial which can increase soil pH and contribute to stabi-lization of heavy metals. Application of biochar for remedi-ation of contaminated soils may provide a new solution tothe soil pollution problem. This paper provides an overview

on the impact of biochar on the environmental fate andmobility of heavy metals and organic pollutants in contam-inated soils and its implication for remediation of contami-nated soils. Further research directions are identified toensure a safe and sustainable use of biochar as a soil amend-ment for remediation of contaminated soils.

Keywords Biochar . Black carbon . Heavymetals . Organicpollutants . Remediation . Soil contamination

Introduction

In recent years, increasingly more soils are found to becontaminated with organic and inorganic toxins globallydue to waste emissions from industrial production, miningactivities, waste (i.e., biosolids and manures) application,wastewater irrigation, and inadequate management of pesti-cides and chemicals in agricultural production (Bolan et al.2004; Mench et al. 2010). More environmentally acceptablealternatives to unsustainable waste management technolo-gies have been sought to minimize further soil contamina-tion (Beesley et al. 2011). Pollutants in soils are not onlyharmful to ecosystems and agricultural production but also aserious threat to human wellbeing. For example, it has beenestimated that 3.5 million sites in industrial and mine sites,landfills, energy production plants, and agricultural land arepotentially contaminated in Europe (Petruzzelli 2012), andtherefore, soil contamination has been identified as an im-portant issue for action in the European Community strategyfor soil protection. In China, economy has been developingrapidly in the last few decades, which has also brought someenvironmental problems. For example, arable land subjectedto heavy metal contamination is close to 20 million hectares,accounting for 20 % of the total agricultural land area inChina (Xi et al. 2011). Considerable efforts have been made

Responsible editor: Zhihong Xu

X. Zhang :H. Wang : L. He :K. Lu : J. LiZhejiang Provincial Key Laboratory of Carbon Cycling in ForestEcosystems 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. HuangSchool of Environmental and Resource Sciences, Zhejiang A & FUniversity, Lin’an, Hangzhou, Zhejiang 311300, Chinae-mail: [email protected]

A. SarmahDepartment of Civil & Environmental Engineering, Faculty ofEngineering, University of Auckland, Private Bag 92019,Auckland 1142, New Zealand

N. S. BolanCentre for Environmental Risk Assessment and Remediation,University of South Australia, Mawson Lakes, South Australia,Australiae-mail: [email protected]

H. Huang (*)Yancao Production Technology Center, Bijie Yancao Company ofGuizhou Province, Bijie 551700, Chinae-mail: [email protected]

Environ Sci Pollut ResDOI 10.1007/s11356-013-1659-0

to remediate polluted soils, as shown in the increasingliterature information on the amendment of soil pollutions(Bolan and Duraisamy 2003; Naidu et al. 2008).Physical/chemical remediation, bioremediation, and inte-grated remediation were used to manage contaminated soils(Mullainathan et al. 2007; Lee et al. 2008; Mendez andMaier 2008).

Biochar is the solid product from pyrolysis of wastebiomass 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 tohaving great potential to enhance long-term carbon seques-tration because most carbon in biochar has an aromaticstructure and is very recalcitrant in the environment(Lehmann 2007). Typically, biochar has a high pH valueand cation exchange capacity, and can enhance soil produc-tivity (Jeffery et al. 2011; Kookana et al. 2011). A number ofstudies have also demonstrated that biochar has a highcapacity to adsorb pollutants in soils (Beesley et al. 2011;Yuan and Xu 2011).

In this paper, we aim to provide an overview of thecurrent practices in remediation of contaminated soils andthe effects of biochar on the mobility and bioavailability ofsoil contaminants, and explore the potential of using biocharfor remediation of contaminated soils. We will also identifyfuture research directions associated with using biochar forremediation of contaminated soils.

Biochar for remediation of soils contaminatedwith heavy metals

Heavy metals are not biodegradable, and persist for a longtime in contaminated soils. It is expensive and time con-suming to remove heavy metals from contaminated soils(Cui and Zhang 2004). Stabilization of heavy metals in situby adding soil amendments such as lime and compost iscommonly employed to reduce the bioavailability of metalsand minimize plant uptake (Bolan and Duraisamy 2003;Bolan et al. 2004; Kumpiene 2010; Komárek et al. 2013).Biochar can stabilize heavy metals in the contaminatedsoils, improve the quality of the contaminated soil(Ippolito et al. 2012) and has a significant reduction in cropuptake of heavy metals. Therefore, application of biocharcan potentially provide a new solution for remediation of thesoils contaminated by heavy metals.

Stabilization of heavy metals in soils with application ofbiochar could involve a number of possible mechanisms, asillustrated in Fig. 1 (Lu et al. 2012). Taking Pb2+ as anexample, the authors proposed various mechanisms forPb2+ sorption by sludge-derived biochar that could include(1) heavy metal exchange with Ca2+, Mg2+, and other cat-ions associated with biochar, attributing to co-precipitation

and innersphere complexation with complexed humic mat-ter and mineral oxides of biochar; (2) the surface complex-ation of heavy metals with different functional groups, andinnersphere complexation with the free hydroxyl of mineraloxides and other surface precipitation; and (3) the physicaladsorption and surface precipitation that contribute to thestabilization of Pb2+ (Lu et al. 2012). In case of acidiccontaminated soils, depending on the type of biochars andexchangeable cations (Na, Mg, K, and Ca) present in itcould hold the key for the release of some of the thesecations during sorption process with the heavy metal, andthus may enrich the stabilization process. Lu et al. (2012)further demonstrated that the heavy metal exchange withCa2+, Mg2+, and other cations (Na+ and K+) associated withsludge-derived biochar was the main mechanism responsi-ble in their study; however, contribution of monovalent(Na+ and K+) cations for heavy metal exchange was foundto be negligible. Therefore, it is conceivable that underrealistic field situation, sorption mechanisms for metal-contaminated soils by biochar could be dependent on thetype of soils and the cations present in both soils andbiochar, and thus implications for metal remediation incontaminated soils could vary.

The mineral components such as phosphates and carbon-ates in biochar play an important role in stabilization of heavymetals in soils because these salts can precipitate with heavymetals and reduce their bioavailability (Cao et al. 2009). Caoand Harris (2010) propose that the main mechanism for dairy

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

Environ Sci Pollut Res

manure biochar to be effective to retain Pb was the precipita-tion of insoluble Pb phosphates. Generally, during the manu-facture of biochar, water-soluble P, Ca, and Mg increasedwhen heated to 200 °C but decreased at higher temperaturesprobably due to increased crystallization of Ca–Mg–P, asevidenced by the formation of whitlockite (Ca, Mg)3(PO4)2when pyrolysis temperature increased to 500 °C, therebyfacilitating the precipitation of Pb (Cao and Harris 2010).Alkalinity of biochar can also promote heavy metal precipita-tion in soils. Chan and Xu (2009) reviewed biochar pH valuesfrom a range of feedstocks in the literature and obtained amean value of pH 8.1. With the same feedstock material,biochar pH value increases with pyrolysis temperature be-cause of increased ash content in biochar (Wu et al. 2012).Therefore, most biochars are alkaline material and have aliming effect, which contributes to the reduction of the mobil-ity of the heavy metals in contaminated soils (Sheng et al.2005). However, the adsorption ability of the same type ofbiochar varies with different types of heavy metals.

Effect of biochar on heavy metal mobility

Biochar application can reduce the mobility of heavy metalsin contaminated soils (Table 1), which renders a reducedrisk of taking up by plants. Studies have shown that biocharderived from bamboo can adsorb Cu, Hg, Ni, and Cr fromboth soils and water, and Cd in polluted soils (Skjemstad etal. 2002; Cheng et al. 2006). Cao et al. (2009) reported thatdairy manure-derived biochar pyrolyzed at 200 °C was moreeffective in sorbing Pb than biochar produced at 350 °Cbecause the 200 °C biochar had the higher concentration ofsoluble phosphate. Given that biochar characteristics are afunction of feedstock and pyrolytic conditions, not one typeof biochar could be universally used to remediate soilscontaminated with various types of heavy metals.Additionally, not one type of mechanism, or a particularfeedstock, or pyrolytic condition could hold true for heavymetal 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 metalspresent in the contaminated soil, and the biochar productiontemperature as the biochar characteristics are dependent onpyrolysis conditions such as highest treatment temperature,moisture content of the feedstock, residence time, and thetype of feedstock used.

The effect of biochar on metal bioavailability varies with thetypes of biochar products as well as types of heavy metals. Asoil contaminated with Cd and Zn was amended with ahardwood-derived biochar and the concentration of bothmetalsin pore water reduced (Beesley et al. 2010). Using the same soilin a column leaching experiment, biochar addition immobilizedboth Cd and Zn, and consequently, the pore water Cd and Znconcentrations were reduced 300- and 45-folds, respectively(Beesley and Marmiroli 2011). Namgay et al. (2010) reportedthat the concentrations of extractable As and Zn in soil in-creased with biochar application rate, whereas the concentra-tion of extractable Pb decreased, Cu did not change, and Cdshowed an inconsistent trend. They also found that sorption oftrace elements on biochar with initial loadings up to 200 μmolat pH 7 occurred in the order: Pb > Cu > Cd > Zn > As.

Biochar application can also reduce the leaching ofmetals through its effect of redox reactions of metals. Forexample, Choppala et al. (2012) showed that the applicationof biochar derived from chicken manure to chromate(CrVI)-contaminated soils enhanced the reduction of mobileCr(VI) to less mobile Cr(III), thereby decreasing theleaching of Cr. The decrease in the leaching of Cr(III) isattributed to the adsorption of Cr(III) onto cation exchangesites and also to the precipitation as Cr(OH)3 resulting fromthe 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 inthe soil and potential risk in entering human food chain. Thebioavailability of pollutants governs their ecotoxicology and

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

Feedstock Productiontemperature

Contaminant Effect Reference

Bamboo Notavailable

Cd Combined effect of electrokinetics, removal of extractable Cd by 79.6 %within 12 days

Ma et al. (2007)

Hardwood 450 °C As, Cd, Cu,Zn

Reduction in Cd in soil pore water by 10-folds; Zn concentrations reduced300- and 45-folds, respectively, in column leaching tests

Beesley et al. (2010);Beesley andMarmiroli (2011)

Hardwood 450 °C As, Cd, Cu,Pb, Zn

Biochar surface mulch enhanced As and Cu mobility in the soil profile;little effect on Cd and Pb

Beesley andDickinson (2011)

Wood 200 °C and400 °C

Cd, Zn Reduction in Zn and Cd leaching loss by >90 % Debela et al. (2012)


degradation in contaminated soils. Environmental microbiol-ogist defines bioavailability as the contaminant fraction whichrepresents the accessibility of a chemical to a living organismfor assimilation, degradation, and ecotoxicology expression(Naidu et al. 2008).

A number of studies have shown that biochar applicationis effective in heavy metal immobilization, thereby reducingthe bioavailability and phytotoxicity of heavy metals(Table 2). Fellet et al. (2011) evaluated the potential of

application of biochar to ameliorate the heavy metal toxicityin the mine tailings. They applied biochar derived fromorchard prune residues at four rates (0 %, 1 %, 5 %, and10 % biochar in the mine tailings). The pH, cation exchangecapacity, and the water-holding capacity increased as thebiochar rates increase and the bioavailability of Cd, Pb, andZn of the mine tailings decreased, with Cd having thegreatest reduction. Zhou et al. (2008) used cotton stalk-derived biochar to amend Cd-contaminated soil and studiedthe uptake of Cd by the cabbage. They found that the cottonstalk-derived biochar can reduce the bioavailability of soilCd through adsorption or co-precipitation. Méndez et al.(2012) evaluated the effects of biochar derived from sewagesludge on heavy metals solubility and bioavailability in aMediterranean agricultural soil and compared with those ofsewage sludge, which was not charred. The biochar treat-ments reduced plant availability of Ni, Zn, Cd, and Pb whencompared to sewage sludge treatments.

Table 2 summarizes the effect of different biochar typeson the bioavailability and uptake of range of contaminants.Park et al. (2011) reported that both chicken manure- andgreen waste-derived biochars significantly reduced Cd, Cu,and Pb uptake by Indian mustard. The study also found thatthe reduction of the plant metal concentrations increasedwith biochar application rates except for Cu concentration.Elsewhere, a study conducted by Jiang et al. (2012) demon-strated that the rice straw biochar was more efficient in theimmobilization of Cu and Pb than Cd. Therefore, when the

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

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



Cotton stalks 450 °C Cd Reduction of the bioavailability of Cd in soil byadsorption or co-precipitation

Zhou et al.(2008)

Hardwood-derived biochar

400 °C As Significant reduction of As in the foliage of Miscanthus Hartley etal. (2009)

Eucalyptus 550 °C As, Cd, Cu,Pb, Zn

Decrease in As, Cd, Cu, and Pb in maize shoots Namgay etal. (2010)

Orchard pruneresidue

500 °C Cd, Cr, Cu,Ni, Pb, Zn

Significant reduction of the bioavailable Cd, Pb, and Zn,with Cd showing the greatest reduction; an increase in the pH, CEC, andwater-holding capacity

Fellet et al.(2011)

Chicken manureand green waste

550 °C Cd, Cu, Pb Significant reduction of Cd, Cu, and Pb accumulation by Indian mustard Park et al.(2011)

Chicken manure 550 °C Cr Enhanced soil Cr(VI) reduction to Cr(III) Choppala etal. (2012)

Sewage sludge 500 °C Cu, Ni, Zn,Cd, Pb

Significant reduction in plant availabilityof the metals studied

Méndez etal. (2012)

Rice straw Not clear Cu, Pb, Cd Significant reduction in concentrations of free Cu, Pb, and Cd incontaminated soils; identification of functional groups on biochar with highadsorption affinity to Cu

Jiang et al.(2012)

Quail litter 500 °C Cd Reduction of the concentration of Cd in physic nut; greaterreduction with the higher application rates

Suppadit etal. (2012)

Oak wood 400 °C Pb Bioavailability reduction by 75.8 %; bioaccessibility reduction by 12.5 % Ahmad etal. (2012)


purpose of utilization of biochar is to immobilize heavymetals, particular attention should be paid to the selectionof feedstock, as biochar properties are dependent on thefeedstock's inherent properties as well as the pyrolysis con-ditions under which the biochar is prepared.

In a pot experiment, Namgay et al. (2010) applied anactivated wood biochar to a soil spiked with heavy metals inorder to investigate the impact of biochar on the availabilityof As, Cd, Cu, Pb, and Zn to maize. Biochar treatmentdecreased the concentration of As, Cd, and Cu in maizeshoots. However, the effects of adding biochar were incon-sistent on Pb and Zn concentrations in the shoots.

Soil pH is closely related to the bioavailability of heavymetals in soils. Uchimiya et al. (2010b) suggested thatbiochar application can increase the soil pH and cationexchange capacity, and subsequently enhance the immobi-lization of heavy metals in soil. Ahmad et al. (2012) usedmussel shell, cow bone, and biochar to reduce Pb toxicity inthe highly contaminated military shooting range soil inKorea. Bioavailability of Pb in the soils was found todecrease by 75.8 % with biochar treatment. Increases in soilpH and the adsorption capacity were considered as themechanisms of remediation effect of the biochar. For exam-ple, the bioavailability of Pb in the soils was decreased byup to 92.5 % with mussel shell, a liming material (Ahmad etal. 2012).

Although many studies showed that biochar can reduceheavy metal mobility and its bioavailability, majority of thesestudies were conducted under controlled laboratory and green-house experiments and in small plot trials. It is only whenlarge-scale field trials are conducted, the practical usefulnessof biochar as a remediation material can be best appreciated;however, to date, this has not been attempted anywhere.

Biochar for remediation of soils contaminatedwith organic pollutants

Soil contamination with organic pollutants is usually causedby a wide range of industrial activities, farming practices,and inadequate application of wastes. Some of the organicpollutants are recalcitrant to degradation, and some arecarcinogenic or mutagenic (Fabietti et al. 2010). Organicpollutants include persistent organic pollutants (POPs) andemerging organic pollutants. Many organic pollutants arecurrently or were in the past used as pesticides. Others areused in industrial processes and in the production of a rangeof products such as solvents, additives, and pharmaceuticals.For example, polychlorinated dibenzo-p-dioxins and diben-zofurans (PCDD/DFs), polychlorinated biphenyls (PCBs),and polycyclic aromatic hydrocarbons (PAHs) are some ofthe well-known persistent organic pollutants (POPs) (WHO2010). Typically, POPs would accumulate in soil horizons

rich in organic matter (OM) where they may be retained foryears (Masih and Taneja 2006). Emerging pollutants aresuspected of causing adverse effects in humans and wildlife.For example, phthalate acid esters [PAEs, e.g., dibutylphthalate and di(2-ethylhexyl)phthalate], naturally releasedestrogenic steroid hormone and its metabolites (e.g., estra-diol and estrone), pharmaceutical and personal care products(PPCPs, e.g., trimethoprim and triclosan), etc. are consid-ered as emerging organic pollutants (Petrović et al. 2001).

Biochar has been reported to be very effective in adsorptionof 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). Manypast studies have demonstrated that given the highly aromaticnature, high surface area, micropore volume, and the presenceof abundance of polar functional groups in biochar, the mate-rial has been found to be effective in the uptake of a variety oforganic chemicals including pesticides, PAHs, and emergingcontaminants such as steroid hormones (Kookana et al. 2011).Though the highly porous nature and large surface area ofbiochar are important for effective removal of pollutants, thenature and type of organic carbon and the degree of aromatic-ity also play an important role (Sarmah et al. 2010). Biocharcan reduce the bioavailability of the organic pollutantsthrough sorption, and reduce the risk of the pollutants enteringhuman food chain or leaching to groundwater. However, thelong-term environmental fate of the sequestered contaminantsis still unknown, and further research is warranted to bridgethis gap especially under realistic field conditions throughbiochar-mediated remediation trials.

Effect of biochar on adsorption of organic pollutants

Behavior of contaminant sorption to biochar is closely re-lated to the process that regulates the concentration of or-ganic pollutants in contaminated soils. It can consequentlyaffect 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 natureof the feedstock biomass material and the conditions underwhich the biochar is produced (Downie et al. 2009; Wu et al.2012). Biochar adsorption and desorption of organic pollut-ants in the soil is greatly influenced by pyrolysis tempera-ture. James et al. (2005) determined the phenanthreneuptake isotherms with the wood biochars from the speciesPinus sylvestris and Betula pendula. The isotherm dataclearly demonstrate that phenanthrene sorption increasesfor materials exposed to higher temperatures. These sorptionincreases also coincide with increases in surface area ofbiochars produced at higher temperatures. Eucalyptuswood-derived biochar pyrolyzed at 850 °C (BC850)contained mainly micropores, whereas that pyrolyzed at


450 °C (BC450) was essentially not a microporous material(Yu et al. 2006). As a result, BC850 had a much highercapacity to adsorb diuron in a soil than that the BC450 did.Similarly, the biochar pyrolyzed at 700 °C had a muchhigher adsorption capacity but weaker desorption capacityof terbuthylazine in soils than the biochar produced at350 °C (Fig. 3).

As discussed above, the high specific surface areas andmicroporosity make biochar very efficient sorbents for arange of organic compounds. However, such behaviorsmay change with time after biochars are applied to soils.This process is commonly referred to as “aging” (Kookana2010). The interactions between biochars and other soilconstituents such as natural organic molecules and clayminerals contribute to the aging of biochars (Uchimiya etal. 2010a). It has been suggested that natural organic mattercan block the micropores of biochars and suppress sorptionof organic contaminants (Pignatello et al. 2006). Wang et al.(2010) observed that biochar-enhanced soil adsorption ofherbicide terbuthylazine is much greater in a soil with loworganic matter than that in a soil with higher organic mattercontent (Fig. 3). It is stipulated that the higher concentrationof dissolved organic molecules that exists in the latter soilmay compete with terbuthylazine for sorption sites onbiochar (Wang et al. 2010). Zhang et al. (2010) found thatthe adsorption capacity of the pine-derived biochars wasconsistently reduced after the biochars were incubated with

soil for 4 weeks (Fig. 4). Martin et al. (2012) studied thesorption–desorption behavior of herbicides in a soil eitheramended with freshly prepared biochars or with biocharsaged under field conditions for 32 months. The sorptioncapacity of the aged biochars was reduced at least by47 % for herbicide diuron. All these studies showed thatthe aging of biochar can affect its properties, and conse-quently, this can lead to lower the capacity for the biochar toabsorb contaminants of interest. So, more understanding ofthe aging process is essential for determination of thebiochar application rate and frequency to improve the reme-diation efficiency.

Effect of biochar on the bioavailability of organic pollutants

Many studies have demonstrated that biochar-amended soilcan help absorb a variety of organic contaminants, therebyreducing their uptake by plants. Application of a smallamount of biochar to soil can significantly reduce the accu-mulation 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 canreduce the bioavailability of the herbicides. They found thateven at low application rate (0.1 %), biochar in soil wouldappreciably reduce the bioavailability of diuron. Similarly,Graber et al. (2012) tested the influence of two biochars onphytoavailability of two herbicides S-metolachlor and

Table 3 Effect of biochar application on sorption of organic pollutants in soils



Eucalyptuswood

450 °C and850 °C

Diuronchlorpyrifosand carbofuran

Higher pyrolysis temperature and higher ratesof biochar applied to soils result in strongeradsorption and weaker desorption of pesticides

Yu et al. (2006)

Woodchip 500 °C Atrazine andacetochlor

Acetochlor adsorption increased 1.5 times;atrazine adsorption also increased

Spokas et al. (2009)

Dairy manure 200 °C and350 °C

Atrazine At 200 °C, partitioning of atrazine is positivelyrelated to biochar carbon content

Cao et al. (2009)

Pine wood 350 °C and700 °C

Terbuthylazine Soil sorption increased 2.7- and 63-folds in theBC350 and BC700 treatments, respectively

Wang et al. (2010)

Green wastes 450 °C Atrazine Biochar enhanced adsorption of pesticide Zheng et al. (2010)

Pine wood 350 °C and700 °C

Phenanthrene Biochar produced at 700 °C showed a greaterability at enhancing a soil's sorption abilitythan that prepared at 350 °C

Zhang et al. (2010)

Pine needles 100 °C, 300 °C,400 °C, and700 °C

PAHs Sorption capacity increased with pyrolysistemperature

Chen and Yuan (2011)

Eucalyptuswood chips

850 °C Diuron Pesticide absorption increases with the biocharcontact time with soil and application rate

Yu et al. (2011b)

Poultry litter,wheat straw,and swinemanure

250 °C and400 °C

Herbicides Biochars showed high sorption ability for twoherbicides, fluridone and norflurazon

Sun et al. (2012)

Swine manure 350 °C and700 °C

Carbaryl At low carbaryl concentrations, the sorptioncapacity BC700 > BC350; similar sorptioncapacity at high carbaryl concentrations

Zhang et al. (2013)


sulfentrazone. They found that biochars, particularly thebiochar with a high specific surface area, can significantlyreduce the bioavailability and efficacy of herbicide for weedcontrol. Shi et al. (2011) added rice straw-derived biochar tophenanthrene-contaminated soil and noticed a significant re-duction of phenanthrene uptake bymaize seedlings. Beesley etal. (2010) examined biochar-amended soil and showed a re-duction in soil pore water concentration of PAH by 50%. Songet al. (2012a) investigated wheat straw biochar on the sorption,dissipation, and bioavailability of hexachlorobenzene. Theyobserved that hexachlorobenzene sorption by biochar was 42times higher than that by the control soil, thereby reducing thevolatilization and earthworm (Eisenia foetida) uptake ofhexachlorobenzene from the soil. Where pest control bychemicals is necessary, biochar application should be carefullyplanned to avoid unintended consequence of offsetting pesti-cide efficacy (Graber et al. 2012).

Biochars for enhanced remediation of soils contaminatedwith organic pollutants

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

logCe, ug/L-2 -1 0 1 2 3

log C

s, ug

/kg

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0.1% 350BC no aging0.1% 350BC with aging0.5% 350BC no aging0.5% 350BC with aging

0.1% 700BC no aging0.1% 700BC with aging

No biochar amendment

0.5% 700BC no aging0.5% 700BC with aging

Fig. 4 Sorption of phenanthrene in a sandy soil (0.16 % organic C)with or without biochar amendment. Solid lines and dashed lines aresorption and single-step desorption isotherms fitted to Freundlichequation, respectively. BC350 and BC700 are biochars pyrolyzed at350 °C and 700 °C, respectively (from Zhang et al. 2010)

0.0 0.5 1.0 1.5 2.0 2.5 3.00

6

12

18

B

0.0 0.5 1.0 1.5 2.0 2.5 3.00

6

12

18

0.0 0.5 1.0 1.5 2.0 2.5 3.00

6

12

18F

D

Topsoil (TS) Control (No Biochar)

TS+1%BC350

TS+1%BC700

0.0 0.5 1.0 1.5 2.0 2.5 3.00

6

12

18AdsorptionDesorption A

Landing site soil (LS) Control (No Biochar)

TA in

so

il p

has

es, m

g kg

-1

0.0 0.5 1.0 1.5 2.0 2.5 3.00

6

12

18E

LS+1%BC350

0.0 0.5 1.0 1.5 2.0 2.5 3.00

6

12

18C

LS+1%BC700

TA in aqueous phases, mg L -1

Fig. 3 Effect of biochartreatments on the adsorption anddesorption of terbuthylazine insoils. Lines are sorption andsingle-step desorption isothermsfitted to Freundlich equation.a Landing site soil (1.2 %organic C) only. b Topsoil (5.1%organic C) only. c Landing sitesoil treated with biocharproduced at 700 °C. d Topsoiltreated with biochar produced at700 °C. e Landing site soiltreated with biochar produced at350 °C. f Topsoil treated withbiochar produced at 350 °C(modified from Wang et al.2010)


biochars derived from cotton straw and pyrolyzed at 850 °C,demonstrated the effect of biochar application on pesticidedissipation rates. The authors reported that the half-life forchlorpyrifos increased from 21.3 days in the untreated soilto 55.5 days in the soil amended with 1 % biochar, andthe half-life for fipronil increased from 27.3 to 60.3 days.Similarly, in a laboratory study, Song et al. (2012a) foundthat application of wheat straw biochar significantly re-duced the dissipation of hexachlorobenzene in soil becauseof the strong adsorption capacity of the biochar. Ideally,soil organic pollutants need to be degraded for sustainableremediation of contaminated soils.

In recent years, some attempts have been made to investi-gate the feasibility of using biochar to accelerate the degrada-tion of organic pollutants in soils (Kemper et al. 2008; Oh et al.2012). For instance, one of the findings is that the specialstructure of biochar was regarded as both sorption sites andelectron conductors, which could catalyze the reduction ofsome 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 anilinesby sulfides at room temperature, and demonstrated that biocharcould serve not only as an adsorbent but also as a platform toaccelerate the reduction of nitrobenzenes. However, more at-tention should be directed towards the utilization of biochar inthe transformation of toxic nitrobenzenes to non-toxic anilinesin the aquatic system and sediments. This would enable thedevelopment of in- situ remediation technique using biochar asa catalyst for the degradation of nitrobenzenes in sediments.

Recently, biochar-mediated degradation of pesticidesthrough hydrolysis has been explored (Zhang et al. 2013).Hydrolysis is one of the important mechanisms for abiotic

degradation of contemporary pesticides (Sarmah and Sabadie2002). Factors, such as pH, dissolved ions, clay, and metaloxides can catalyze the hydrolysis reaction and subsequentlyinfluence pesticide degradation. Manure-derived biochar con-tains a high ash content, which is expected to influence thehydrolysis of pesticides. Zhang et al. (2013) investigated theeffect of pig manure-derived biochars on hydrolysis of twopesticides, carbaryl and atrazine. The authors reported thatcarbaryl was hydrolyzed rapidly in the suspension of theuntreated biochars. The 7-day hydrolysis achieved up to90 % degradation of carbaryl in the presence of biocharacquired at 700 °C pyrolysis temperature. However, the hy-drolysis of atrazine was much slower than that of carbaryl.When washed biochar was used, the effect of biochar oncarbaryl hydrolysis is substantially reduced. It was concludedthat the ash constituents, including the alkalinity, releaseddissolved metal ions, and the mineral surface, played thecatalytic role in carbaryl hydrolysis. However, hydrolysis ofatrazine was mainly enhanced by high pH and mineral surface(Zhang et al. 2013).

Bioremediation is one of the commonly practiced technol-ogies for cleaning up soils contaminated with organic pollut-ants through employment of plants or microorganisms (Smithet al. 2009; Chen and Yuan 2012). To enhance the efficiencyof bioremediation, bioaugmentation that involves the additionof concentrated microorganisms capable of decomposing cer-tain types of organic pollutants (e.g., high molecular weightPAHs) have been proposed (Chen et al. 2012). It requireswell-designed immobilized carriers that are intended to offera protective space for inoculated microorganisms and mini-mize competition from indigenous soil microbes. The accessof organic pollutants in contaminated soil to immobilized cellsis dependent on the concentration of pollutants in carriers

Table 4 Effect of biochar application on bioavailability of organic pollutants in soils



Eucalyptus 450 °C and 850 °C Diuron chlorpyrifosand carbofuran

Reductions of chlorpyrifos and carbofuranin total plant residues, respectively

Yu et al. (2009)

Hardwood 450 °C PAHs Pore water concentrations of PAHs werereduced by biochar, with greater than 50 %decrease of the heavier, more toxicologicallyrelevant PAHs

Beesley et al. (2010)

Cottonstraw

450 °C and 850 °C Chlorpyrifos andfipronil

Chinese chive uptake of fipronil and chlorpyrifosreduced by 52 % and 81 %, respectively, with1 % of 850 °C biochar addition

Yang et al. (2010)

Bamboo 600 °C Pentachlorophenol Biochar reduced PCP bioavailability in soil Xu et al. (2011)

Hardwood 600 °C PAHs Biochar application reduced concentrationand biological activity of PAHs in soil

Gomez-Eyles et al.(2011)

Wheatstraw

500 °C Chlorobenzenes (CBs) Biochar amendment significantly reducedthe bioavailability of CBs

Song et al. (2012b)

Wheatstraw

250 °C, 300 °C,and 500 °C

Hexachlorobenzene(HCB)

Biochar amendment of soil resulted in a rapidreduction in the bioavailability of HCB, evenat 0.1 % biochar application rate

Song et al. (2012a)


(Dzul-Puc et al. 2005). With high sorption capability to or-ganic pollutants in soils and resistance to degradation,biochars can pre-concentrate pollutants in contaminated soilthen feed to the immobilized microbial decomposers (Su et al.2006). Chen et al. (2012) conducted a study on the dissipationof PAHs in a contaminated soil amended with immobilizedbacteria using biochar as a carrier (Fig. 5). The process isknown as immobilized microorganism technique (IMT). Theyfound that the IMT is an effective bioaugmentation approachfor enhancing bioremediation of PAH-contaminated soil. Theimmobilized bacteria could directly degrade the biochar-sorbed PAHs. It is important to select an appropriate biocharas an immobilized carrier to stimulate biodegradation of or-ganic pollutants (Chen et al. 2012).

Many literatures have showed that biochars are effectivein the immobilization of a wide range of organic and inor-ganic chemicals (Skjemstad et al. 2002; Cheng et al. 2006;Beesley et al. 2010; Hilber et al. 2009; Kookana 2010; Yanget al. 2009; Zhang et al. 2013). Thus, it is conceivable thatbiochar application to soil could influence the plant uptakeof a range of organic compounds thereby impacting plantgrowth, but this aspect has not received much attention inthe literature so far.

Chagger et al. (1998) noted that the presence of combustion-driven toxic organic compounds, such as polynuclear aromatichydrocarbons (PAHs), chlorinated hydrocarbons, and dioxins,is often suspected in biochar products, and their concentrationsare dependent on carbonization temperature and feedstocksource (Brown et al. 2006). Some biochars are often found tobe rich in heavy metal contents. For example, Singh et al.(2010) observed that Zn contents of Eucalyptus saligna woodand poultry litter biochars were found to range from 1,312 to1,661 mg kg−1, and from 1,449 to 1,642 mg kg−1, respectively.Therefore, when biochars are used to remediate contaminatedsoils, the carbonization temperature and feedstocks should becarefully chosen to avoid the high concentrations of metals thatmay be present in the biochar.

Conclusions and future research directions

Biochar has the potential to be developed as a viable tech-nology for remediation of contaminated soils. Obviously,biochar can conceivably reduce the bioavailability and

efficacy of both heavy metal and organic pollutants in soil.Biochars produced from different biomass materials andwith different pyrolysis conditions (e.g., temperatures) pres-ent highly heterogeneous physicochemical properties,which can affect the efficacy in the remediation of contam-inated soils.

As a potential technology for remediation of contaminat-ed agricultural soils, many aspects are still yet to be devel-oped. Several knowledge gaps have been identified, andfurther research is required to close these gaps. Some keyresearch needs are outlined below:

& So far, the studies about using biochar for remediation ofcontaminated soils mainly focus on the laboratory andgreenhouse experiments and small plot trials. Large-scale field trials are essential before operational scaleremediation projects are implemented.

& The biochar characteristics vary with different biomassmaterials and pyrolysis conditions. It is important tooptimize production systems to produce designerbiochar products to be used effectively for specific re-mediation work.

& The strong sorption and weak desorption of pollutants inbiochar indicate that biochar sequesters pollutants initself. Biochar application can lead to accumulation ofcontaminant residues in the amended soils. However, thelong-term environmental fate of the sequestered contam-inants is still not well understood.

& The capacity of biochar to adsorb or sequester pollutantsdecreases with time due to aging process. More under-standing of the aging process is warranted for furtherresearch. This information is essential for determinationof the biochar application rate and frequency to improveremediation efficiency.

& Limited studies have demonstrated that biochar not onlycan reduce the bioavailability and leachability of thepollutants in soils through the process of sorption butalso may facilitate accelerated dissipation of some or-ganic pollutants in soil. Further research is required toexplore the practical feasibility of biochar-assisted dis-sipation of organic pollutants.

& The immobilized microorganism technique (IMT) withbiochar as microbial carrier shows promising feasibilityfor 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 contaminants are degraded by IMO

Fig. 5 Conceptual illustrationfor enhancing bioremediationof soil organic pollutants withimmobilized microorganismtechnique (IMT) using biocharas a strong adsorption carrier(modified from Chen et al.2012)


pollutants. Development of biochar for production ofoptimum carrier should be conducted.

Acknowledgments This study was funded by the National NaturalScience Foundation of China (41271337), Research Funds of the Depart-ment of Education of Zhejiang Province (Y201225755) and ZhejiangA&FUniversity Research and Development Fund (2010FR097, 2012FR063).

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using biochar for remediation of soils contaminated with heavy metals and organic pollutants

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