Immobilization of Heavy Metals by Co-pyrolysisof Contaminated Soil with Woody Biomass

F. Debela & R. W. Thring & J. M. Arocena

Received: 3 February 2011 /Accepted: 11 August 2011 /Published online: 26 August 2011# Springer Science+Business Media B.V. 2011

Abstract We investigated the potential application ofpyrolysis treatment to a mixture of woody biomassand a metal-contaminated soil as an alternative eco-friendly option to stabilize metals in soils. Ourspecific objective was to test the optimum combina-tion of high heating temperature (HHT) and heatingtime to effectively encapsulate metals in a contami-nated soil into a biochar. For this purpose, we used alaboratory bench batch reactor to react a mixture ofmulti-element metal contaminated soil with 0%(control) 5%, 10%, and 15% (w/w) sawdust. Eachmixture was reacted at 200°C and 400°C HHT for 1and 2 h heating times. Physicochemical and morpho-logical characterization along with standard EPASynthetic Precipitation Leaching Procedure (SPLP)test were conducted to assess the effectiveness of theheat treatment to immobilize the metals in thecontaminated soil. Compared to controls, we recordedup to 93% reduction in Cd and Zn leachability after1 h heat treatment at 400°C, with the addition of 5–10% biomass. Pb leaching was reduced by 43% bythe same treatment but without the addition of

biomass. At lower pyrolysis temperature (200°C),however, there was a substantial increase in both Asand Zn leaching compared to the untreated controls.Our study suggests that several factors such as thetype of metal, heating temperature, heating period,and the addition of biomass influence the efficiency ofpyrolysis to immobilize metals in the contaminatedsoil.

Keywords Metal leaching . Biochar . SPLP.

Sequential extraction

1 Introduction

Biochar is the product of a partial-oxidation (pyroly-sis) process, which is the decomposition of organiccarbon bearing compounds at elevated temperature inthe absence of oxygen (Brown 2009). Biochar is amulti-use soil amendment material with demonstratedenvironmental, economic, and social benefits (Cao etal. 2009; Lehmann and Rondon 2006; Steinbeiss et al.2009; Warnock et al. 2007; Yu et al. 2009). In additionto the production of biochar, pyrolytic processes havebeen used to reduce metal leaching from severalorganic wastes such as municipal sewage sludge(Kistler et al. 1987), fly ash (Jakob et al. 1996), andbiomass (Stal et al. 2010). Similar reductions in metalleaching from dredged sediments upon pyrolysis havebeen reported by Rienks (1998), Wijesekara et al.(2007) and Zhang et al. (2009).

Water Air Soil Pollut (2012) 223:1161–1170DOI 10.1007/s11270-011-0934-2

F. Debela (*)Natural Resources and Environmental Studies,University of Northern British Columbia,Prince George, BC, Canada V2N 4Z9e-mail: debela@unbc.ca

R. W. Thring : J. M. ArocenaEnvironmental Science and Engineering,University of Northern British Columbia,Prince George, BC, Canada V2N 4Z9

Excess levels of heavy metals in soils persist overlong periods of time and pose significant human- andecosystem-health risks, if left untreated. The in situstabilization of Pb and Zn using inorganic P amend-ments is currently one of the most recommendedpractices as a cost-effective and ecologically friendlyremediation technology (Adriano 2001; Hettiarachchiet al. 2001; Chrysochoou et al. 2007). However,studies have shown that soil organic matter (SOM)can form complexes with metals in soils interferingwith the efficient formation of the target metal-phosphate compounds (Hashimoto et al. 2009; Langand Kaupenjohann 2003; Scheckel and Ryan 2004).In our previous in situ immobilization study, we foundthat Pb and Zn have a very high affinity to formcomplexes with SOM resulting in a poor efficiency ofthe formation of Pb- and Zn–P after amendment(Debela et al., under review).

Taking aside the negative effect of SOM to theefficiency of the in situ P stabilization method, theintimate association between organic and mineral matterin soils is desirable to achieve an ideal soil aggregationand overall improved soil quality. In nature, this iscommonly achieved by soil biota particularly earth-worms. Earthworms ingest organic and mineral matterand comminute the mixture inside the intestine; theresulting fecal materials are granular in shape and exhibitthe best soil structure (Brown et al. 2000; Pulleman et al.2005; Six et al. 2004). Organic materials in theexcrements of soil biota are known to contain variousfunctional groups such as carboxyl and amide charac-teristics of partially oxidized SOM (Arocena et al.1995). We hypothesize that the natural process ofintimate mixing of organic matter and minerals in soilcan be partly abiotically mimicked by controlledpyrolysis process of subjecting a known mixture ofbiomass and contaminated soil to various heat applica-tions to attain a partially oxidized organic matter similarto fecal materials. The concept presents an excellentopportunity to enhance the encapsulation of metalswithin biochar produced from co-pyrolysis of mixturesof biomass and contaminated soil to reduce thebioavailability of metals, while at the same timeimproving the physical quality of a contaminated soil.Biochar is considered to be highly resilient in soilenvironments (Lehmann et al. 2009). Therefore, thepotential immobilization of metals within biochar canbe considered a long-term and eco-friendly remediationapproach for metal-contaminated soils.

The purpose of this study was the assessment ofthe potential encapsulation of metals (Pb, Zn, As, Cd,and Cu) in a contaminated soil into biochar by co-pyrolysis of the contaminated soil with biomass (sawdust) to reduce the leaching of metals from the soil.To the best of our knowledge, there are no publisheddata to date on the direct application of pyrolysis toimmobilize metals in contaminated soils either aloneor by co-pyrolysis with deliberately added biomass.

2 Materials and Methods

2.1 Sample Preparation

A soil sample (SAD4) highly contaminated with Pband Zn with additional moderate levels of As, Cd andCu contamination was obtained from an unidentifiedmine disposal site no longer in existence. Uponreceipt, the soil sample was air-dried and passedthrough a 2-mm nylon sieve. The sample was thenthoroughly mixed, quartered into four equal portions,and each portion was mixed with 0%, 5%, 10% and15% (w/w) of sawdust obtained from the forest andwood research laboratory at the University of North-ern British Columbia. To keep the uniformity in thesample preparation as much as possible, we used onebatch of sawdust material.

2.2 Pyrolysis Experiment

The pyrolysis experiment was conducted using a Parr4575 500-ml batch reactor with a standard externalheater and a custom Watlow Firerod 9243 internalcartridge heater (Fig. 1). During a reaction, a

Fig. 1 Schematic diagram of Parr 4575 batch reactor

1162 Water Air Soil Pollut (2012) 223:1161–1170

continuous flow of air stream was used to protect theshaft of the mixer from overheating and a coolingwater loop was used to quench the reaction at the endof each run. Trial experiments revealed that a workingvolume of 150 ml of the sawdust–soil mixture wasoptimum for ensuring uniform heating without stir-ring. Thus, for each run, 150 ml of the preparedsample was added to the reactor vessel and the vesselwas pressurized to 103.4 kPa with N2. The pyrolysisexperiment consisted of the combinations of two highheating temperatures (HTT) and times, specifically,200°C (Pr200) and 400°C (Pr400) for the duration of1 h (Pr200-1) and 2 h (Pr200-2) of heating time ateach HTT. Each pyrolysis experiment was conductedin duplicates. The target HTT was reached after 30–80 min and maintained for the duration of theexperiment. At the end of the reaction period, thevessel was allowed to cool to room temperature andany residual pressure was vented before the solidproduct was collected.

2.3 Physicochemical Characterization

We compared some selected physicochemical character-istics of the SAD4 soil before and after pyrolysistreatment to investigate any physical, chemical, andmorphological alterations after the heat treatment.Standard soil analytical techniques were followed forthe characterization of all samples (Carter 1993; Klute1986). Total elemental Pb, Zn, As, Cd and Cu weredetermined by inductively coupled plasma mass spec-troscopy (ICP-MS) after microwave acid digestion. SoilpH was determined in H2O (soil/H2O ratio, 1:2). Cationexchange capacity (CEC) was estimated using theammonium acetate method while organic carbon (OC)content was determined by dry heat combustion. Thesurface chemistry and morphology of the samples werestudied using a Perklin Elmer 2000 Fourier-transforminfrared spectrometer (FTIR) and a Philips XLS 30scanning electron microscope (SEM), respectively.

2.4 Leaching Test

A batch SPLP was carried out on the control (notreated) and pyrolysed samples to determine theextent of metal encapsulation with the reactedbiomass (biochar). Standard SPLP Method 1312 withminor modifications was followed for this test (USEPA 1994). Briefly, we applied 100 ml of two

leaching fluids (a 40:60 mix of concentrated HNO3

and H2SO4 diluted with distilled H2O to pH 4.2 and5.0) to a 5.0-g sample in a glass jar. The mixture wasthen agitated gently on a horizontal shaker for 18 h.At the end of the extraction period, the samples wereallowed to settle for 30 min, the pH of the supernatantwas determined and a small portion was taken andfiltered through a 0.45-μm membrane syringe filter.The filtrate was acidified with concentrated HNO3 topH<2.0 and the concentrations of Pb, Zn, As, Cd, andCu were determined by ICP-MS.

2.5 Chemical Sequential Extraction

A chemical sequential extraction (SE) proceduremodified from the methods by Scheckel et al.(2005) and Santos et al. (2010) was conducted todetermine the distribution of Pb, Zn, Cd, As, and Cuin SAD4 soil before and after pyrolysis treatment intofive operationally defined fractions. These fractionsare as follows: F1, exchangeable; F2, associated withcarbonates; F3, associated with iron and manganeseoxides; F4, associated with organic matter; F5,residual fraction. In brief, 1 M MgCl2, pH 7.0 (F1),1 M NaOAc adjusted to pH 5.0 with CH3COOH (F2),0.04 M NH2OH⋅HCl in 25% (v/v) HOAc (F3), and30% H2O2 (F4) were sequentially added to a 0.5-gsample in 25-ml centrifuge tubes. At the end of eachextraction period, the sample was centrifuged(5,000 rev/min for 5 min), and the supernatant wascarefully transferred to a 50-ml beaker. The remainingsolid was then washed once with distilled-deionizedH2O before continuing with the next extraction stepand the washing was added to the beaker. An aliquotwas taken from the beaker, filtered through a 0.45-μmmembrane syringe filter, acidified with concentratedHNO3 to pH<2 and stored at 4°C for analysis.Concentrations of Pb, Zn, As, Cd, and Cu in F1 toF4 fractions were determined by ICP-MS. Theresidual fraction (F5) for each element was deter-mined by subtracting the sum of F1 to F4 fractionsfrom the total acid digest in SAD4.

2.6 Statistical Analysis

Analysis of variance (ANOVA) and least significantdifference (LSD) comparison of the means wereconducted to determine a statistical mean differencebetween (p<0.05) the results of the SPLP test.

Water Air Soil Pollut (2012) 223:1161–1170 1163

3 Results

3.1 Physicochemical Characterization

Products collected at the end of the pyrolysis experi-ments were physically distinct from the controlsamples (no treatment). The products from Pr400-1and Pr400-2 treatments were odorless and noticeablydarker in color than the products from Pr200 treatedsamples. Pr200 treated samples had a very strongaroma and were only slightly darker than theuntreated samples.

The total elemental analysis revealed no signif-icant losses of Pb, Zn, As, Cd, and Cu due to thepyrolysis treatments (Table 1). There was a slightincrease in pH (0.5–0.8 pH units) of Pr400 treatedsamples with added biomass. There was no changein pH due to the Pr200 treatments regardless ofheating time and biomass addition (Table 1). Com-pared to the control samples, CEC was reduced by

more than 50% in all samples upon heat treatment.However, the Pr400 treatments resulted in higherreduction of CEC values than the correspondingPr200 treatments (Table 1).

With increased additions of biomass to the SAD4soil, total OC content after pyrolysis treatment wasnotably increased from 1.9% (0% biomass) to amaximum of 7.8% (upon the addition of 15%biomass) (Table 1). FTIR analysis did not show anynoticeable difference in the surface chemistry ofSAD4 after any of the pyrolysis treatments (data notshown). However, SEM analysis revealed that thesurface morphology of the samples after pyrolysiswere distinctly different. The sharp angular edges ofthe soil materials and fiber structure in the biomasswere evident before the heat treatment (Fig. 2a, b).Upon pyrolysis treatment, partially charred biomasswas observed as surface coatings on particles of thesoil materials as well as in some parts of the biomass(Fig. 2c, d).

pH OC (%) CEC (cmolc kg−1 soil) Cu (%) Zn (%) As (%) Cd (%) Pb (%)

SAD4+0% sawdust

Control 6.0 1.88 3.26 0.40 20.85 0.07 0.08 3.92

Pr200-1 6.1 1.61 2.43 0.39 19.45 0.06 0.07 4.18

Pr200-2 6.1 1.74 3.00 0.40 19.49 0.06 0.08 4.02

Pr400-1 6.3 1.75 1.12 0.40 21.24 0.07 0.08 4.08

Pr400-2 6.1 1.75 1.94 0.40 22.43 0.08 0.09 4.62

SAD4+5% sawdust

Control 6.1 3.83 6.13 0.41 19.37 0.06 0.08 4.02

Pr200-1 6.3 3.76 4.38 0.41 18.81 0.07 0.07 3.89

Pr200-2 6.3 4.19 5.00 0.35 20.53 0.06 0.08 3.92

Pr400-1 6.6 3.34 2.00 0.39 20.11 0.06 0.08 4.34

Pr400-2 6.7 3.01 2.32 0.39 21.00 0.07 0.08 4.56

SAD4+10% sawdust

Control 6.1 6.46 5.65 0.34 18.69 0.06 0.07 3.69

Pr200-1 6.0 5.35 5.08 0.34 18.58 0.05 0.07 3.85

Pr200-2 6.1 7.85 4.10 0.37 19.93 0.08 0.08 4.33

Pr400-1 6.6 4.41 2.42 0.41 21.00 0.06 0.08 4.62

Pr400-2 6.8 5.19 2.19 0.44 21.51 0.06 0.08 4.65

SAD4+15% sawdust

Control 6.1 8.13 9.32 0.36 17.82 0.06 0.07 3.65

Pr200-1 5.9 7.79 5.88 0.35 18.35 0.06 0.07 3.65

Pr200-2 5.9 7.43 5.86 0.39 18.70 0.07 0.07 3.98

Pr400-1 6.6 6.81 2.15 0.39 19.13 0.06 0.07 4.06

Pr400-2 6.5 6.59 2.26 0.44 21.03 0.06 0.08 4.12

Table 1 Physicochemicalcharacteristics of SAD4 soilbefore (control) and afterpyrolysis treatments (meanvalues reported, n=2)

1164 Water Air Soil Pollut (2012) 223:1161–1170

3.2 Leaching Test

The amounts of metals measured from the SPLPusing the two extraction fluids (at pH 4.2 and 5.0)were similar for all treatments. The results reportedhere are from the first extraction fluid (pH 4.2)(Fig. 3).

All pyrolysis treatments without biomass additiondid not reduce the Zn concentrations in leachates.Pr200 treatments with 5% biomass addition also didnot result in any significant change in Zn leaching.Pr200 treatments with 10% and 15% biomass addi-tion, however, resulted in significant increases in Znleaching. Zn in the leaching solution was significantlyreduced by all Pr400 treatments with the addition of5%, 10%, and 15% biomass (Fig. 3a).

There was a significant decline in levels of Pb inleachates upon Pr400 treatments without the additionof biomass. With the addition of 15% biomass, Pr200-2 treatment and both Pr400 treatments significantlyincreased Pb leaching (Fig. 3b).

All pyrolysis treatments, with or without the additionof biomass, were effective in reducing Cd in leachingsolution. The addition of 5%, 10% and 15% biomasswere more effective to further reduce Cd leaching ascompared to treatments without any biomass addition.With the addition of 5% and 15% biomass, Cd leachingin Pr400 treated samples were significantly lower thanPr200 treated samples (Fig. 3c).

We did not detect any significant Cu (abovebackground) levels in the leachates including thecontrol samples (data not shown). The same was truefor As leachability in control and Pr400 treatedsamples. However, As contents in leachates wasincreased by up to 10 times above the backgroundlevels by Pr200 treatments with and without biomassaddition (data not shown).

3.3 Sequential Extraction

Pyrolysis treatment with or without biomass additionhad very little effect on the organic fractions of (F4)

a

c

b

d

Fig. 2 SEM micrographsof before (a and b) and after400°C pyrolysis treatmentfor 2 h (c and d) with theaddition of 10% sawdust

Water Air Soil Pollut (2012) 223:1161–1170 1165

0

200

400

600

800

1000

1200

1400

Control Pr200-1 Pr200-2 Pr400-1 Pr400-2

Zn

(m

g L

-1)

0% OM 5% OM 10% OM 15% OM

*

**** *

**

**

**

* *

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Control Pr200-1 Pr200-2 Pr400-1 Pr400-2

Pb

(m

g L

-1)

0% OM 5% OM 10% OM 15% OM

**

* * *

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

Control Pr200-1 Pr200-2 Pr400-1 Pr400-2

Cd

(m

g L

-1)

0% OM 5% OM 10% OM 15% OM

*

*

**

** **** * * * *

* *** **

a

b

c

Fig. 3 SPLP test results forZn (a), Pb (b), and Cd (c).*Statiscally significantmean difference (p<0.05) ofmetal leachability at eachlevel of sawdust addition.OM = % of organic matteraddition in forms of woodybiomass

1166 Water Air Soil Pollut (2012) 223:1161–1170

Pb and As (Table 2). However, both Pr200 and Pr400treatments resulted in a substantial increase of Cd, Cu,and Zn in the F4 factions (Table 2). For these threemetals, observed increase in the F4 was higher uponPr400 treatments. The increase in F4 for Cd, Cu and Znwas slightly influenced by heating time and the additionof biomass. The increased Cd, Cu, and Zn in theorganic fractions were associated with a notable declinein their corresponding carbonate (F2) and residualfractions (F5). There was a significant shift in the Asfractionation from F5 (residual) to F3 (Fe/Mn oxideassociated fraction) and carbonate (F2) fractions upon

pyrolysis treatments. Pyrolysis-induced shift for Pb wasmainly a decline in F5 with corresponding substantialincreases in Pb carbonate (F2) fractions (Table 2).

4 Discussion

The degree of biomass charring was directly related tothe HTT than the duration of the heating time. At200°C, the added biomass was only partially decom-posed with some physically distinct intact organicresidue remaining at the end of each reaction. The

F1 (%) F2 (%) F3 (%) F4 (%) F5 (%)

aa b c d a b c d a b c d a b c d a b c d

Arsenic

Control <1 <1 <1 <1 2 2 1 2 23 21 23 25 12 11 8 6 63 65 67 66

Pr200-1 <1 <1 <1 <1 2 8 9 8 19 47 54 47 12 12 9 5 67 33 27 39

Pr200-2 <1 <1 <1 <1 2 7 8 10 17 39 42 51 11 10 7 6 70 43 43 33

Pr400-1 <1 <1 <1 <1 15 20 8 9 60 66 54 45 7 6 5 6 17 8 33 40

Pr400-2 <1 <1 <1 <1 16 19 9 10 62 69 52 46 7 5 5 5 16 6 34 39

Cadmium

Control 16 14 13 9 15 9 7 9 5 3 3 3 35 38 36 35 30 36 41 44

Pr200-1 9 5 5 4 7 6 6 7 9 10 9 8 49 47 41 35 26 32 39 46

Pr200-2 10 4 4 5 7 5 7 5 6 9 8 7 43 47 43 37 33 35 37 47

Pr400-1 5 3 3 2 5 2 4 3 5 3 3 3 63 65 60 61 22 26 29 31

Pr400-2 5 3 3 3 4 3 2 3 4 4 3 3 61 65 65 62 26 25 26 29

Copper

Control <1 <1 <1 <1 4 2 3 2 <1 <1 <1 <1 28 51 41 38 67 47 56 60

Pr200-1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 41 41 38 36 58 58 62 63

Pr200-2 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 40 40 36 34 60 60 64 65

Pr400-1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 73 63 53 49 26 37 46 51

Pr400-2 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 <1 69 57 68 57 31 43 32 43

Lead

Control 3 4 6 5 30 23 21 16 10 11 9 8 6 5 3 5 49 57 61 65

Pr200-1 3 7 9 9 32 23 nd nd 11 10 15 11 4 6 7 7 50 54 nd nd

Pr200-2 3 8 10 9 38 30 nd nd 9 12 12 12 3 6 5 6 47 44 nd nd

Pr400-1 5 10 7 8 19 25 37 37 11 10 9 7 5 5 4 4 60 50 42 44

Pr400-2 6 12 7 9 21 33 58 41 11 9 8 8 5 5 4 4 57 41 23 38

Zinc

Control 9 7 7 5 31 23 19 19 7 6 5 6 32 37 35 34 23 27 35 37

Pr200-1 10 9 9 8 20 22 18 19 8 8 6 7 42 41 35 30 20 20 32 35

Pr200-2 9 8 9 9 18 18 18 15 6 7 7 7 39 41 36 31 27 25 30 37

Pr400-1 9 3 2 2 16 10 10 9 8 6 6 6 55 61 59 58 13 20 23 25

Pr400-2 9 2 2 2 11 11 6 9 7 7 6 6 53 61 63 59 20 18 24 24

Table 2 Sequentialchemical fractionationof As, Cd, Cu, Pb and Znbefore and after pyrolysistreatments

F1 exchangeable, F2associated with carbonates,F3 associated with iron andmanganese oxides, F4associated with organicmatter, F5 residual fraction,nd not determineda a=0%, b=5%, c=10%,d=15% biomass addition

Water Air Soil Pollut (2012) 223:1161–1170 1167

slight discoloration and strong aroma in Pr200 treatedproducts may suggest the impartial decomposition of thebiomass at 200°C. Upon doubling the HTT to 400°C,more charring of the added biomass was achieved asrevealed by the darker color of the product and theabsence of any visible intact organic residues and aroma.Our data indicates 1 and 2 h heating times at both HTTwere equally effective to yield similar products (Table 1,Fig. 3). The observed slight increase in the pH of theproducts upon the Pr400 treatments can be associatedwith the decomposition of metal carbonates to oxides(Kistler et al. 1987) and the decomposition of OC andthe release alkali metals above 300°C (Maiti et al.2006; Yuan et al. 2011). The successive increasedaddition of biomass was directly related to the increaseof the total OC content of the pyrolysis products.Despite the substantial increase in the OC content,there was no apparent change in the surface chemistryof the samples as revealed by our FTIR analysis. Thismay suggest that the added biomass was only partiallyoxidized with no distinct and resolved functionalgroups observed in highly oxidized organic materialssuch as soil humus (Brady and Weil 2002). Similarly,regardless of the increase in OC content, there was nosubstantial increase in CEC of all pyrolysed samples.This also indicates that charred biomass is initiallychemical inert. Similar results are previously reportedfor an initially very low CEC value of fresh biocharwith substantial increase in CEC and development ofmore functional groups upon ageing (Cheng et al.2006, 2008). CEC is the measure of exchangeablecations adsorbed onto the negatively charged surfacesin soils (e.g., clays, organic matter, and Fe/Al oxides)(Brady and Weil 2002). The consistent decline in theCEC upon all pyrolysis treatments can be attributed toseveral processes including the loss of some functionalgroups from the inherent soil organic matter upon heattreatment, the physical blocking of charged surfaces bythe charred biomass or the destruction of the negativecharges in 2:1 expanding clays. Arocena and Opio(2003) attributed the 20–28% reduction in CEC afterprescribed burning to the disappearance of X-rayreflection at 1.7 nm characteristics for high-chargedclays. The total elemental analysis conducted beforeand after pyrolysis treatment (Table 1) indicated novapor loss of metals due to pyrolysis. Similar no vaporloss of Cu, Zn and Pb from a contaminated sedimenttreated at temperatures up to 800°C was reported byZhang et al. (2009).

Previously, the SPLP test was regarded as anapplicable test to evaluate metal leaching frominorganic wastes such as contaminated soils (Caoand Dermatas 2008). Our SPLP data show that thesuccess of metal encapsulation with biochar isdependent on several contributing factors includingthe type of metal, heating temperature and biomassaddition. We attributed the significant decreases in Cdand Zn leaching due to encapsulation of metals withbiochar (Fig. 3a and c). While the decline in Cdleaching was enhanced by the addition of biomass,the inclusion of biomass was critical in the reductionof Zn in leachates. On the other hand, reduction in Pbleaching was only possible by heat treatment withoutbiomass addition. This suggests that different metalsbehave differently upon heat treatment. The data fromour SE analysis showing the notable increases in theorganic factions (F4) of Zn and Cd without biomassaddition suggests that pyrolysis treatment can be usedto enhance metal complexation with the inherentSOM. We suggest that the additional biomass en-hanced further decline in Zn and Cd leaching byphysically coating the mineral grains with the charredbiomass residue (Fig. 2c). The lack of remarkablechemical changes (e.g., pH, CEC, and surfacechemistry) after all pyrolysis treatments furthersuggests that changes in the leachability of metalsare likely due to physical rather than chemicalmechanisms.

We found pyrolysis treatment at 400°C for 1 hheating time with the addition of 5–10% biomass wasthe most effective treatment to reduce Zn and Cdleaching by up to 90% and 93%, respectively.Wijesekara et al. (2007) also reported similar resultswhere Pb and Zn leaching was significantly reducedfrom a dredged sediment treated at 400°C. Pr400treatment with no biomass addition was also the mosteffective treatment to reduce Pb leaching by 43%.Increasing heating time to 2 h had very little effect tofurther reduce metal leaching. Similarly, there waslittle, if any, added benefits of increasing biomassaddition above 10% to reduce Cd and Zn leaching.On the other hand, our result also suggests that somepyrolysis treatments could have undesirable effectsfor some metals. For example, the amounts of As, Pb,and Zn in leachates increased upon pyrolysis treat-ments at 200°C. From our SE data, we attributedthese occasional increases in As, Pb and Zn leachingupon pyrolysis treatment to increases in the relatively

1168 Water Air Soil Pollut (2012) 223:1161–1170

easily soluble exchangeable (F1) and carbonate (F2)fractions of the metals (Table 2). This shows that it iscritical to understand the characteristic of differentmetals in contaminated soils and how they wouldbehave to various heat treatments under differentenvironments.

5 Conclusion

From our study, we conclude that there is a goodpotential to exploit pyrolytic processes to encapsulateor immobilize heavy metals such as Zn and Cd incontaminated soils with highly resilient biochar. Ourassessment also indicates that heat treatment by itselfcan be effective to immobilize heavy metals such asPb in contaminated soils. With further evaluation andrefinement, we believe the immobilization of metalswith biochar produced from co-pyrolysis of biomassand contaminated soil can be developed into a novelsoil remediation approach with added economic,social, and environmental benefits.

Acknowledgements This research was supported by theCanada Research Chair Program and the Natural Sciences andEngineering Research Council of Canada. We would like tothank Stacey Shantz, Allen Esler and Dr. Quanji Wu for theirassistance in the pyrolysis experiment and laboratory analysis.

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