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Environmental Pollution 159 (2011) 474e480

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Environmental Pollution

journal homepage: www.elsevier .com/locate/envpol

The immobilisation and retention of soluble arsenic, cadmiumand zinc by biochar

Luke Beesley a,*, Marta Marmiroli b

a Faculty of Science, Liverpool John Moores University, Liverpool L3 3AF, UKbDepartment of Environmental Sciences, Section of Genetics and Biotechnologies, University of Parma, 43100 Parma, Italy

Biochar retains readily soluble cadmium and zinc from a contaminatedon arsenic.

soil, significantly reducing their potential mobility, but has limited effects

a r t i c l e i n f o

Article history:Received 17 August 2010Received in revised form12 October 2010Accepted 14 October 2010

Keywords:BiocharCadmiumZincArsenicHeavy MetalsSorption

* Corresponding author at: The Macaulay Land Usbuckler, Aberdeen AB15 8QH, UK.

E-mail address: l.beesley@macaulay.ac.uk (L. Bees

0269-7491/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.envpol.2010.10.016

a b s t r a c t

Water-soluble inorganic pollutants may constitute an environmental toxicity problem if their movementthrough soils and potential transfer to plants or groundwater is not arrested. The capability of biochar toimmobilise and retain arsenic (As), cadmium (Cd) and zinc (Zn) from a multi-element contaminatedsediment-derived soil was explored by a column leaching experiment and scanning electronmicroanalysis(SEM/EDX). Sorption of Cd and Zn to biochar’s surfaces assisted a 300 and 45-fold reduction in theirleachate concentrations, respectively. Retention of both metals was not affected by considerable leachingof water-soluble carbon from biochar, and could not be reversed following subsequent leaching of thesorbant biochar with water at pH 5.5. Weakly water-soluble As was also retained on biochar’s surface butleachate concentrations did not duly decline. It is concluded that biochar can rapidly reduce themobility ofselected contaminants in this polluted soil system, with especially encouraging results for Cd.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Biochar is a low-density charred material produced by burningbiomass under conditions of low temperatures and minimaloxygen. Experimental application of this material to soils tosequester carbon (Lehmann, 2007a,b) has been encouraged by itsvery high organic carbon content (Yin Chan and Xu, 2009).Furthermore, unlike fresh organic residues, biochar has been foundnot to significantly increase CO2 productionwhen it is added to soils(Spokas et al., 2009; Novak et al., 2010) and can adsorb dissolvedorganic carbon (DOC) (Pietikainen et al., 2000). There are alsowideragro-environmental benefits of adding biochar to soils, for example,increasing soil pH and reducing leaching of soluble macronutrients(Novak et al., 2009; Laird et al., 2010). The microscopic physicalstructure of biochar is one of the key determinants in its soilconditioning properties because, during the charring process, thesurface area of the pre-charred source material can be increasedseveral thousand fold, greatly enhancing its water holding capacity(Thies and Rillig, 2009). Although the capability of carbonaceousmaterials, such as biochar, to adsorb organic compounds (Oen et al.,

e Research Institute, Craigie-

ley).

All rights reserved.

2006; Brandli et al., 2008) and remove heavy metals and As fromwaste-waters (Mohan et al., 2007) has been explored, applicationto real world contaminated soil systems has received surprisinglylittle systematic investigation to date, although initial results areencouraging regarding the retention of both inorganic and organicpollutants (Beesley et al., 2010b).

In polluted soil systems, water-soluble contaminants, rather thanthe total concentrations in soil, determine ecological and toxicologicalrelevance because they have the greatest immediate environmentalimpact (Temminghoff et al., 1998). One factor influencing elementalsolubility in soils is pH. Arsenic is more soluble in alkaline soils withinthe range pH 3e8 (Fitz and Wenzel, 2002) whereas the opposite istrue of some metals, such as Zn (Kiekens, 1990). Given that somebiochars have been reported to have apHgreater than8 (Yin Chan andXu, 2009), there could be opposing impacts on themobility of specifictrace elements if such alkaline materials are added to contaminatedsoils. Furthermore, co-mobilisation of soluble carbon and some tracemetals has previously been found in soil pore water collected in fieldexperiments in both contaminated (Clemente et al., 2008) and non-contaminated soils (BeesleyandDickinson, 2010),which raises furtherconcerns regarding the addition of carbon-rich materials to soils.

The present study evaluates the efficacy and permanence ofelement retention by biochar, examining concentrations of As, Cdand Zn in successive leachate samples from a column leaching test

L. Beesley, M. Marmiroli / Environmental Pollution 159 (2011) 474e480 475

and determining relative abundance of these elements on soiland biochar surfaces before and after the leaching test, by scanningelectron microscope X-ray emission (SEM/EDX). Additional influ-ences of pH and soluble carbon are also considered in the context oftheir effects on element mobility.

2. Materials and methods

2.1. Study site and soil analysis

Soil was sourced from a canal embankment in Kidsgrove, Staffordshire, UK(53�05023.000 N, 02�15005.700 W) which has been previously studied in relation totransfer of Cd and Zn to vegetation (Lepp andMadejon, 2007), mobility of As, Cd andZn in the soil profile (Beesley et al., 2010a), and the effects of greenwaste compostand biochar on themobility of As, Cd, Cu, Pb and Zn (Beesley et al., 2010b). The originof the excess levels of pollutants in this soil are believed to be due to the discharge orspillage of contaminated effluent from neighbouring manufacturing industries intothe canal during the early decades of the last century. Some years hence the canalis thought to have been dredged and the sediments piled onto the banks withoutfurther treatment. Over subsequent decades the area has become spontaneouslyvegetated and presently supports a variety of shrubs (mainly Salix caprea andS. viminalis) and perennial herbs (Lepp and Madejon, 2007).

For the present study, bulk soil samples were taken from three faces of a shallow,30 cmdeep soil trenchexcavatedbyhand ina secludedareaof the studysitewhichhadnot been sampled by previous studies. Triplicate 500 g soil samples collected in thefieldwere bulked andmixed in the laboratory to create a single, representative sampleof the soil for use in the column leaching test. Large debris and biological remnantswere removed and soil was crushed and sieved to <2 mm prior to the followinganalysis. Particle size distributionwas determined using a Beckman Coulter LS 13 laserdiffraction particle size analyser, with samples having been carefully pre-treated withhydrogen peroxide (H2O2, 30%) to remove organic matter. Soil pH was determined ina 1:10 suspension of soil in de-ionised water, shaken for 3 h and allowed to settle for45 min at room temperature (22 �C), measured by a digital pH meter (Jenway, Staf-fordshire, UK). Total organic carbon (TOC) in solid samples was determined using anSSM-5000A/TOC-VE analyser (Shimadzu, Tokyo, Japan). Aliquots of soil (0.2 g) weremicrowave digested in concentrated 14 M, general purpose reagent (GPR) grade HNO3

Fig. 1. a) Three of the six leaching columns, showing a soil column (left) connected to an(indicated by a white arrow) was used for collecting eluate samples from the soil column bethe experiment and the passage of water into the soil upwards from the base of the column ccolumn leaching test fractions where ‘EC’ denotes eluate collection points. Note that biocindependently of the soil columns, in accordance with the biochar control (marked *), for f

and analysed for pseudo-total As, Cd and Zn by ICP-MS (XSERIES 2 ICP-MS; ThermoScientific, MA, USA). Certified reference material (CMI 7004) was used to verifyaccuracy, with recoveries of >80% for all elements.

Biochar was produced by pyrolysis of hardwoods including Oak (Quercus spp.),Common Ash (Fraxinus excelsior), Sycamore (Acer spp.), Birch (Betula spp.) andCherry (Prunus spp.) at 400 �C in steel ring furnaces (Bodfari Charcoal, Denbigh,North Wales, UK). Biochar’s pH and TOC were determined as above.

2.2. Leaching column test

The leaching column test used six identical glass columns (XK50, PharmaciaBiotech, UK), 20 cm in length and 5 cm in internal diameter, of which two werepackedwith 400 g of air-dried soil and the remaining four were packed to 3/4 of theirlength with biochar. Biochar had a low density (0.3 gml�1) compared to soil(1.8 gml�1; Beesley et al., 2010b) so biochar was packed into the columns notby weight, but to the equivalent length of 400 g of air-dried soil in the columns.Columns were leached upwards from their base continuously with de-ionised water(pH 5.5) at 0.1 mlmin�1

flow rate, maintained by a peristaltic pump followingBeesley et al. (2010a). Two of the columns containing biochar were linked in parallelto the preceding two columns containing soil (shown in Fig. 1a) to allow biochar tointercept contaminated eluate directly leached from the soil columns. Eluatesamples (10 ml) from soil were collected prior to passing through the biocharcolumns, by means of a small outlet at the joint between columns which remainedclosed at all times except during sample collection (see Fig. 1a). The remainingtwo biochar filled columns were leached by the same method, but independently ofthe soil columns, to provide a biochar control. Columns were leached for 8 weeksin total. For the first 5 weeks (fractions 1e5) eluate was collected from all of thecolumns. In fractions 6, 7 and 8 (weeks 6, 7 and 8) leaching of soil columns wasterminated, but both the intercepting biochar and biochar control columnscontinued to be leached as previously described (see Fig. 1b). This method wasdesigned to test the reversibility of any element retention by the intercepted biocharafter the source of contaminated eluate had been removed. The pH of the eluatesamples from each fraction was determined using the same equipment describedearlier, whilst water-soluble total (WSTC) and water-soluble organic carbon (WSOC)were analysed by a TOC-VE water analyser (Shimadzu, Tokyo, Japan). Arsenic, Cd,and Zn concentrations in the eluate samples were determined by ICP-MS followingBeesley et al. (2010a).

intercepting biochar column (middle). The small outlet at the joint of the columnsfore it passed through biochar. Note that this photograph was taken at the beginning ofan clearly be seen by the darker colouration of the soil. b) Schematic explanation of thehar columns intercepted contaminated soil eluate in fractions 1e5, but were leachedractions 6, 7 and 8 of the test.

Table 1Characteristics of soil and biochar before the column leaching test commenced(mean n¼ 3; �s.e.m.).

Soil Biochar

pH 6.2 (0.1) 9.9 (0.0)TOC (%) 11 (0.2) 53 (2.4)Sand (2 mme63 mm) % 43 (2.6) e

Silt (62.9e2 mm) % 48 (2.3) e

Clay (<2 mm) % 8.4 (0.3) e

Pseudo-total (mg kg�1)As 96 (6.6) ndCd 119 (1.8) ndZn 249 (41) ndFe (%) 9.7 (0.4) ndAl (%) 12.9 (0.9) nd

nd ¼ Not determined.

L. Beesley, M. Marmiroli / Environmental Pollution 159 (2011) 474e480476

2.3. Scanning electron microanalysis (SEM/EDX)

The purpose of using the following assay was to evaluate, by visual comparativemeans, the relative abundance of As, Cd and Zn on soil and biochar surfaces beforeand after the leaching test to establish the extent to which sorption and de-sorptionhad occurred. Following the collection of the last fraction of eluate from the leachingcolumns (for soils this was fraction 5 and for biochar this was fraction 8; Fig.1b), 25 gof soil and biochar were immediately removed from their respective columns andair-dried as previously described. Small sub-samples of the leached soil and biochar,along with soil and biochar that had not been leached were treated for scanningelectron microscope analysis as follows. Each sample was ground to fine powderand deposited into a 1.8� 1.8 cm2 polyethylene mould with a depth of 3 cm andoven-dried at 35 �C for 24 h. The samples were impregnated with epossidic resin,which is a thermosetting polymer of epoxide and polyamine, to eliminate air withinthe powder micro-holes, then kept under vacuum at 50 cm Hg for 12 h after whichthey were vulcanised. Each sample was taken out of its mould, horizontally cut witha diamond blade and polished on a cast-iron plate with SiC powder (12 mm) for 12 h.The samples were sonicated to eliminate residual SiC powder from their surface,dried for 12 h, then polished with a Texmet pad (Buehler, Illinois, USA) before beingimpregnatedwith ethylene glycol and Al2O3 powder (0.05 mm) for 48 h. The sampleswere sonicated for 10 min to eliminate residues of Al2O3 powder and dried at roomtemperature for 24 h. Each replicate sample had a surface area of 1.8� 1.8 cm2 anda thickness of 0.5 cm. Samples were placed onto slides, each of which had beencoated, under vacuum, with colloidal graphite to ensure conductivity. Samples wereanalysed with a Jeol 6400 Scanning Electron Microscope (SEM; Osaka, Japan)equipped with an Oxford X-ray detector (EDX; Oxford Instruments, Oxford, UK).Images and element dot maps were visualised by LINK-ISIS 4.0 software (OxfordInstruments, Oxford, UK). Soil and biochar’s microscopic surfaces were compactedand smoothed by the process described above to allow a full and even coverage bythe electron beam across the whole sample. Without smoothing and polishing,highly anisotropic electron beam scatter can occur and hence lead to erroneousresults. For each analysis the voltagewas set at 20 keV (Marmiroli et al., 2004), whilsttheworking distancewas 9 mmand the dead time for X-ray acquisition between 20%and 25%. A colour code was assigned for the elements; blue for arsenic, green for

Soil &

Biochar

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1 2 3 4 5

lg

metaule

ninoitartnecno

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Fig. 2. Water-soluble total carbon concentration (WSTC), water-soluble organic carbon conbiochar (b.) and biochar control (c.). Dotted, vertical line indicates where the leaching ofcontinued (mean n¼ 3 and error bars represent� s.e.m.).

cadmium and red for zinc. In the dot maps, lighter shades of colour correspond tothe higher intensity of the signal, which is directly proportional to the elementconcentration. Therefore these dot maps are a relative indicator of concentrations ofelements before and after the leaching test, but do not indicate actual concentrations.Dot maps were acquired at 180� magnification and 600 frames each to optimizedelectron detection from the surface of the soil and biochar samples.

2.4. Statistical analysis

ANOVA analysis of trace metals, arsenic, pH and water-soluble total and organiccarbon in eluate was used to determine the effects of both leaching time and biocharon these parameters, whilst Pearson correlations were used to check for relation-ships between parameters. All analyses were carried out using SPSS v.14 forWindows.

3. Results and discussion

3.1. Influences on trace element concentrations inleaching column eluate

Given that biochar had a higher pH than soil (Table 1) and effec-tively buffered the mildly acidic pH of eluate from the soil (Fig. 2a) atmildly alkaline levels (wpH 8) throughout the column test (Fig. 2b), itmight have been expected that any soluble As from soil would not beretained by biochar; an increase in arsenic solubility with morealkaline pH at soil pH> 7 has previously been reported (O’Neill, 1990;Fitz and Wenzel, 2002). However, arsenic concentrations were byfar the lowest of all elements measured in the eluate from the soil(Fig. 3a; <10 mg l�1), but did increase slightly when soil eluate waspassed through biochar (Fig. 3b), merely suggesting a cumulativeeffect given that concentrations were also very low from the biocharalone (Fig. 3c). Indeed no significant correlation was found betweenpH of the eluate and As concentrations in the eluate (data not shown).Soluble organic substances can also enhance trace element leachingin soils through complexation (Bernal et al., 2009) and hence co-mobilisation. In the present study however, no positive correlationbetweenmetals or As concentrations and soluble carbon in eluatewasfound, with the exception of Cd with WSOC (R2¼ 0.93; p< 0.001),a relationship that was only found in the biochar control, whereconcentrations of this metal were consistently very low (Fig. 3c, f andi). Indeed, concentration of water-soluble total and organic carbon ineluate from the soil were stable throughout the test (15e20 mg l�1 forWSTC and 7e15 mg l�1 for WSOC; Fig. 2a) whereas, in the biocharcolumns, both water-soluble total and organic carbon were signifi-cantly reduced by successive leaching from initially far higherconcentrations (Fig. 2b and c; Table 2). Given the configuration of this

6 7 8

tion -----------------------------------------------------------

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1 2 3 4 5 6 7 8

Hpet

aulE

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centration (WSOC) and pH of eluate from soil (a.), soil leached through interceptingsoil was terminated and the leaching of the intercepting biochar and biochar control

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−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−

Fig. 3. Concentration of arsenic (a, b and c), cadmium (d, e and f) and zinc (g, h and i) in eluate from soil, soil leached through intercepting biochar and biochar control. Note thediffering scales between soil and biochar for Cd and Zn. Dotted, vertical line indicates where the leaching of soil was terminated and the leaching of the intercepting biochar andbiochar control continued (mean n¼ 3 and error bars represent� s.e.m.).

Table 2Changes in heavy metal and arsenic concentrations, pH and water-soluble total/organic carbon concentrations in eluate during the first 5 fractions of the columnleaching test. ‘[’ denotes a significant increase, ‘Y’ denotes a significant decrease at*p< 0.05, **p< 0.01, ***p< 0.001. n.s.¼ not statistically significant (ANOVA).

As Cd Zn pH WSTC/WSOC

Soil n.s. Y** Y* n.s. n.s.Soilþ Biochar n.s. n.s. n.s. [** Y***Biochar control n.s. Y** n.s. n.s. Y***

L. Beesley, M. Marmiroli / Environmental Pollution 159 (2011) 474e480 477

experiment, in which the acidic soil eluate, with low water-solublecarbon concentrations flowed into the biochar columns, soil itself wasnot subject to an increase in alkalinity or flushingwith soluble organicsubstances (soluble carbon) whereas the effects could be different ifthe two media were mixed together. Similarly time did not influencearsenic concentration in eluate in the respect that concentrationswere not significantly reduced or increased during the leaching test(Table 2), probably because Aswas soweaklywater-soluble in this soildespite high pseudo-total concentrations (Table 1).

Unlike arsenic, whose solubility was low and whose concentra-tions in eluate were negligible, cadmium and zinc concentrations ineluate from soil were high, especially in the first 2 leaching fractions(200e500 mg l�1 in each fraction; Fig. 3d and g) which resulted incumulative removals ofw4 mgCdper kg�1 andw2.5 mgZnper kg�1

of soil over the first 5 fractions of the column test. Both metals hada similar trend to decrease successively and significantly (p¼ 0.009and 0.017 for Cd and Zn respectively) after the second fraction of

the leaching test (Fig. 3d and g; Table 2) suggesting strong initialde-sorption from soil which reduced as the water-soluble pool wasdepleted. In this particular soil significant concentrations of solubleCd and Zn have been associated with a relatively high water-soluble/readily exchangeable fraction whereas a very rapid depletion of thesmall water-soluble pool of As was observed because the labile

Fig. 4. Scanning Electron Microscope (SEM) image of the surface of biochar at 190�magnification showing variously sized pores (A.) and longitudinal, vertically cut cross-section ofthose pores at 550� magnification (B.). Note both the vast surface area and interior pore structure.

L. Beesley, M. Marmiroli / Environmental Pollution 159 (2011) 474e480478

fractions of this metalloid form only a small proportion of its totalconcentration (Beesley et al., 2010a).

3.2. Reductions in soluble cadmium and zinc by biochar

Biochar rapidly and significantly (p< 0.05) reduced concentra-tions of both Cd and Zn as it intercepted contaminated eluate fromthe soil columns (Fig. 3e and h). There was an especially stark effecton Cd, with consistent reductions in eluate concentrations of thismetal in every fraction of the column test (Fig. 3e) indicating a strongtendency of biochar to retain this metal. A similar, albeitless pronounced effect was also found in a field study measuringpore water concentrations of Cd and Zn after mixing biochar withsoil (Beesley et al., 2010b). In that study it was speculated thatthe mechanisms of individual element mobility were different. Forexample Cu was influenced primarily by dissolved organic carbon(DOC), arsenic by an increase in pH but the mechanisms controllingthe large decrease in Cd and Zn were more difficult to elucidate.The present study found a significant negative correlation between

Fig. 5. Scanning Electron Microscope (SEM) image and colour coded SEM/EDX dot maps (acfor cadmium and red for zinc) of soil before (A.) and after a 5 week column leaching test (B.)shown by darker colour shades.

pH and Cd (p< 0.01) in eluate from soil when it had passed throughbiochar, indicating that raising the pH could be one mechanism bywhich Cd mobility was reduced by biochar, considering the highsoluble Cd concentrations in this soil. The rapid increase in alkalinityof eluate from soil after it had passed through biochar (p< 0.05;Fig. 2b) would therefore have assisted Cd retention. Other workershave previously noted that increases in soil pH increased the reten-tion of Cd in solution by adsorption (Azeez et al., 2010).

The reversibility of the retention of Cd and Zn was tested bydisconnecting the intercepting biochar from the contaminated soilcolumns and continuing to leach this material independently ofsoil (Fig. 1b). In these final 3 fractions of the column test (leachingfractions 6, 7 and 8, collected, 1, 2 and 3 weeks after leaching of soilwas terminated) there was a small, but none significant increase inCd in eluate from the intercepting biochar (Fig. 3e), indicating thatthe removal of the high concentrations of this metal by this materialwere not readily reversible. The permanence of the immobilisationeffect at least in the immediacy suggested strong sorption of thesoluble metals to the surface of biochar. Given biochar’s pore

quired at 600 frames, for 3 h and 30 min at 180� magnification; blue for arsenic, green. Note the reduction in relative concentrations of elements, especially Cd, after leaching,

Fig. 6. Scanning Electron Microscope (SEM) image and colour coded SEM/EDX dot maps (acquired at 600 frames, for 3 h and 30 min at 180� magnification; blue for arsenic, greenfor cadmium and red for zinc) of biochar not leached (A.) and after leaching with contaminated soil eluate in a 5 week column leaching test (B.). Note the increase in relativeconcentrations of elements after leaching, shown by lighter colour shades.

L. Beesley, M. Marmiroli / Environmental Pollution 159 (2011) 474e480 479

structure, large surface area (Fig. 4) and thus high water retentioncapability (Thies and Rillig, 2009) it was also important to establishthat biochar alone was free from significant amounts of As, Cd andZn inwater-soluble form andwould be safe to use as a soil treatmentwithout itself contributing to any environmental toxicity effects.Very low concentrations of all elements weremeasured in the eluateof the biochar control columns (Fig. 3c, f and i).

3.3. Retention of trace elements by biochar

Cadmium and, to a lesser proportion, Zn and As were de-sorbedfrom the soil surface during the column leaching test, indicated bythe SEM/EDX element dot maps (Fig. 5A and B), explaining theelevated concentrations of Cd and Zn in eluate from soil during thecolumn test (Fig. 3d and g). In turn both of thesemetals were sorbedto the intercepting biochar, indicated by relative increases in theirsurface concentrations after the leaching test (Fig. 6B) compared tobefore (Fig. 6A). In the case of arsenic, pseudo-total concentrationwas high in the soil (Table 1) but solubility was low (Fig. 3a) becausethemajority of As is considered to be in a more stable form (residual>60%), compared to Cd and Zn (residual<5% and<25% respectivelyBeesley et al., 2010a). Despite its lack of retention by the biocharobserved from the leaching test, arsenic was also shown to beretained by biochar (Fig. 6A and B). There could be several possibleexplanations for this. Laird et al. (2010) examined macro-nutrientimmobilisation by biochar in a column experiment, finding thathardwood derived biochar could act as a sink for dissolved P addedto soil by supplementary manure amendment. Phosphorous andarsenic are chemically analogous, with the possibility that Ascould be sorbed to biochar’s surfaces given low concentrations ofcompeting soluble P. Previous increases in As mobility in biochartreated soils have been explained by out-competition of P with Asfor binding sites in alkaline pH conditions, and the subsequentrelease of As (Hartley et al., 2009). Furthermore the soil used in thisexperimentwas relatively Fe rich, towhich a high proportion of As isbound in this soil (>30%), compared to Cd and Zn (<10%; Beesleyet al., 2010a). Nguyen et al. (2008) found that ageing black carbon,often synonymously referred to with biochar, retained Fe on particlesurfaces after exposure to soils. It is possible that, in the presentstudy that there was a cumulative retention of As onto biochar’s

surfaces as the leaching test progressed associated with re-distribution from the iron-rich soil to the iron-retentive biochar.

Biochar’s structure might also explain some of the effectsobserved. It is thought that oxidation of biochars occursmost rapidlyon their outer surfaces, followed by interior pores (Nguyen et al.,2008), which might result in differential element retentionbetween surface and interior sites as a result of shifting CEC forexample (Cheng et al., 2008). In the present study, arsenic sorptionmight only have occurred on the outer surfaces of the biochar, wherethe SEM/EDX detected relatively high amounts after the leachingtest. Cadmium and Zn, which were far more abundantly water-soluble and present in far higher concentrations may have beenretained both to outer surfaces and, when those retention sites wereeffectively saturated, further adsorbed to the network of pores andfissures that form biochar’s complex inner micro-structure (Fig. 4B).Such differential nutrient retention and leaching effects related topore sizes, outer and inner surfaces of biochar are discussed furtherby Major et al. (2009) but in the present study it is only possible tospeculate on such effects relating to As, Cd and Zn until further workis carried out. For the purposes of this study the SEM/EDX methodprovided a useful mechanistic illustration of the transfer of solubleand readily exchangeable elements from soil to biochar, confirmingsurface retention.

4. Conclusions

Biochar has proven to be effective at reducing high concentra-tions of soluble Cd and Zn originating from a contaminated soil andwe can now more affirmatively say that sorption is one of themechanism by which those metals are retained. It is even furtherencouraging that the retention of these pollutants was not imme-diately reversible. For arsenic more investigation is required on soilswith high water-soluble concentrations of this metalloid. However,in wider environmental terms the results of this study allow us tovisualise biochar assisting the interception and arresting themigration of toxic elements to groundwater or its deployment inshallow, immature polluted soils to reduce soil-plant transfer ofmetals in the rhizosphere. There are also implications of our resultsbeyond soils, to the treatment of contaminated waters. The focus offuture work should now determine the repeatability of our results

L. Beesley, M. Marmiroli / Environmental Pollution 159 (2011) 474e480480

on a variety of contaminated soils and themechanisms of individualelement retention by a variety of biochars in the light of dynamic soilphysical, microbial and chemical conditions found in the field.

Acknowledgements

The authors are grateful to Eduardo Moreno for his assistancewith statistical analysis.

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