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Environmental Pollution 150 (2007) 347e354www.elsevier.com/locate/envpol

Remediation of metal contaminated soil with mineral-amended composts

Rene van Herwijnen a,b,c,d, Tony R. Hutchings b, Abir Al-Tabbaa c, Andy J. Moffat b,Mike L. Johns d, Sabeha K. Ouki a,*

a University of Surrey, School of Engineering, Guildford, Surrey GU2 7XH, UKb Forest Research, Land Regeneration and Urban Greening Group, Alice Holt Lodge, Farnham, Surrey GU10 4LH, UK

c University of Cambridge, Department of Engineering, Trumpington Street, Cambridge CB2 1PZ, UKd University of Cambridge, Department of Chemical Engineering, Pembroke Street, Cambridge CB2 3RA, UK

Received 13 September 2006; received in revised form 23 January 2007; accepted 27 January 2007

Composts can increase or decrease the bioavailability of metals in soil.

Abstract

This study examined the use of two composts derived from green waste and sewage sludge, amended with minerals (clinoptilolite or ben-tonite), for the remediation of metal-contaminated brownfield sites to transform them into greenspace. Soils contaminated with high or low levelsof metals were mixed with the mineral-enhanced composts at different ratios and assessed by leaching tests, biomass production and metalaccumulation of ryegrass (Lolium perenne L.). The results showed that the green waste compost reduced the leaching of Cd and Zn up to48% whereas the composted sewage sludge doubled the leachate concentration of Zn. However, the same soil amended with composted sewagesludge showed an efficient reduction in plant concentrations of Cd, Cu, Pb or Zn by up to 80%. The results suggest that metal immobilisation andbioavailability are governed by the formation of complexes between the metals and organic matter. The amendment with minerals had onlylimited effects.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Leaching test; Bentonite; Zeolite; Heavy metals; Trace elements; Ryegrass

1. Introduction

Most large cities in the world have to remediate brownfieldsites within their boundaries to reduce the need for greenfielddevelopment. The UK Environment Agency has estimated thataround 20,000 contaminated sites in England and Wales mayrequire treatment (Environment Agency, 2004). The costs oftreatment of these sites would become more manageable if re-mediation is combined with redevelopment. However, devel-opers generally prefer to choose greenfield over brownfieldland due mainly to the high costs associated with remediation(Environment Agency, 2004). The option to remove and

* Corresponding author. Tel.: þ44 1483 686 633; fax: þ44 1483 450 984.

E-mail address: s.ouki@surrey.ac.uk (S.K. Ouki).

0269-7491/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.envpol.2007.01.023

dispose of contaminated soils also has become expensivedue to recent increases in landfill costs, adding further to theneed for alternative low-cost remediation techniques. One pos-sible remediation solution for brownfield sites in the urban en-vironment is transformation into green space. Anotherpossibility is the use of the treated material for landscapingaround housing or business development sites. Both optionsare fully compatible with the increased need for the incorpora-tion of green space into the urban environment (De Sousa,2003; Jim, 2004).

Many urban brownfield sites have concentrations of metalsin soils, significantly exceeding the levels that are generallystated as environmentally acceptable and therefore form a po-tential health risk for humans, animals, and plants (De Sousa,2003; Environment Agency, 2004). The threat of these metalsto the environment can be reduced by fixation in the soil itself,

348 R. van Herwijnen et al. / Environmental Pollution 150 (2007) 347e354

thereby lowering the bioavailability and reducing furthermobility. A considerable amount of research has been carriedout amending soils with clay minerals, zeolites, and compostto immobilise metals through cation exchange, sorption, com-plexation and precipitation (Mule and Melis, 2000; Simon,2001; Geebelen et al., 2002; Roman et al., 2003; Castaldiet al., 2005). Amendment with compost alone may initially in-crease plant growth by improving the nutrient status of the soiland by immobilisation of metals, but after degradation of theorganic matter of the compost, the adsorbed metals might bereleased and become available again to plants and animals.In contrast to the wide publication of many studies which re-port the immobilisation of metals with composts, a limitednumber report the possibility that compost amendments in-crease the leaching or extractability of metals. For exampleit has been shown that certain composts in combination withspecific soils increase arsenic leaching (Cao et al., 2003;Mench et al., 2003). There are also references on the mobili-sation effects of composts on other metals of concern.Clemente et al. (2006) reported increased bioavailability ofcopper after amendment of a soil from a lead-zinc mine areawith a compost made from olive leaves and the solid fractionof olive-mill waste water. Interestingly, both zinc andlead were effectively immobilised by the compost. Dissolvedorganic matter is known to mobilise Cu and Pb throughcomplexation but not Cd and Zn (Bradl, 2004). Naturally oc-curring minerals such as clays and zeolites also interact withmetals to form a matrix in which the bioavailability of themetals is decreased (Ouki and Kavannagh, 1999; Simon,2001). Although zeolites on their own are recommended as ag-ricultural soil amendment (Rehakova et al., 2004) it can alsohave an adverse effect on the soil structure and the amountto be added to the soil should therefore be limited (Geebelenet al., 2002). Enrichment of compost with bentonite or clinop-tilolite could provide long-term immobilisation of the metalsbecause these minerals will not be degraded and so will bindthe metals for much longer. Altogether an improved biologicalquality of the soil will be obtained by improved nutrient levelsthough the compost as well as a reduced toxicity of the soilthrough immobilisation of the metals. A method usingwaste-produced materials (waste-derived composts) and inex-pensive minerals (zeolite and bentonite) may offer a low cost,sustainable solution for the remediation and establishment ofgreenspace on contaminated brownfield sites. In this paper,we report on short term experiments undertaken to assessthe viability of mineral-enhanced composts for the remedia-tion of predominantly zinc and cadmium contaminated soils.Tests were carried out on soils collected from a site in theAvonmouth region, Avon, UK, that were contaminated withhigh and low concentrations of both metals. The contaminatedsoils were amended with two types of composts, derived fromgreen waste (GWC) and sewage sludge (SC). Both compostswere used in their unamended form and after the addition ofzeolite or bentonite clay. The effects of all soil amendmentson metal leaching and bioavailabilty as well as biomass pro-duction of ryegrass (Lolium perenne L. ‘Kent Indigenous’)were investigated.

2. Materials and methods

2.1. Soils and amendments

Contaminated soil of a silt clay texture was obtained from a dismantled

zinc smelter (Britannia Zinc) near Avonmouth, UK (N51:30:32, W2:40:47).

The soil was sampled at two distances, 250 m and 800 m, from the former

chimney stack at a dept of 0e0.02 m below the organic layer if present. Char-

acteristics of the two soils are given in Table 1. The closest sample to the chim-

ney, referred to as H-soil, was highly contaminated, mainly with Cd, Cu, Pb,

and Zn, whereas the second sample (L-soil) had lower contaminant levels. The

greenwaste compost (GWC) was produced from green, garden waste and ob-

tained from Onyx Environmental Group plc. London, UK. Composted sewage

sludge (SC), obtained from Terra Eco Systems, Bracknell, UK, was produced

from sewage sludge mixed with wood chips and after initial composting of two

months was matured for at least 6 months. It should be noted that the batch of

SC we used exceeded the PAS100 levels for Cd, Cu, Hg and Zn in compost

(BSI, 2005) and its use is therefore not recommended. The zeolite (clinoptilo-

lite) was of Australian origin and was supplied by Euremica Environmental

Ltd., Cleveland, UK. The bentonite clay was obtained from Colin Stewart

Minchem Ltd, Winsford, UK. The chemical and physical characteristics of

the composts, bentonite and clinoptilolite are given in Table 1.

The sampled soils were dried and sieved to soil lump sizes smaller than

2.5 cm. Soil mixtures were prepared by mixing the high (H-soil) or low

(L-soil) contaminated soil with compost at 7%, 14%, or 20% (w/w compost/

soil). The composts were either used as received or amended with zeolite or

bentonite at proportions of 7%, 14%, or 20% (w/w). In total, there were 86

different soil mixtures including controls of unamended soils as shown in

Table 2. The plant experiments were started one week after mixing the soils,

the leaching tests were performed half a year after mixing and the soil mix-

tures were stored at 10 �C.

2.2. Batch leaching experiments

Batch leaching tests were performed according to a standard European

leaching test for materials with a particle size below 4 mm (BSI, 2002). The

tests were performed in duplicates on a subset of 15 H-soil mixtures shown

in Table 2. One-hundred-gram samples of each soil mixture and a control of

pure soil were mixed with 1 litre of carbonated de-ionised water at a pH of

approximately 5.6. The carbonated water was prepared by flushing the water

with CO2 until a pH of 5.6 was achieved. Each water-soil mixture was put

in a 1-litre polypropylene bottle and rotated on a bottle roller for 24 � 1 h.

After rotation, the mixture was stood for 7 days at room temperature after

which the supernatant (leachate) was filtered over a 0.45 mm nitrocellulose

membrane filter (VWR international Ltd, Poole, UK) for chemical analysis.

2.3. Nursery trials

Ryegrass seeds were sown into pots containing 1 kg of each soil mixture;

each treatment was replicated three times in randomised complete blocks. The

ryegrass seeds were provided by Holliwell Seed & Grain, Ashford, UK. The

grass was grown in a greenhouse from 15 June to 9 August 2004 at a minimum

night temperature of 10 �C and day temperature varying between 20 and 45 �C.

During the growing period, the water content of the soil in all pots was main-

tained between 0.35 and 0.5 m3 m�3 by manual watering with tap water. After

59 days the ryegrass leaves produced on all L- and H-soil treatments, and roots

grown on L-soil treatments, were harvested for chemical analysis, and the dry

weight of the shoots was measured. The shoots were sampled by cutting off

all shoots at 1 cm above the soil, roots were sampled by separating them from

the soil mixtures and washing them three times with deionised water.

2.4. Analysis

Analysis of Zn and Cd in the batch leachate samples was performed with

a UNICAM 929 Atomic Absorption Spectrophotometer (AAS) according to

the manufacturer instructions (Thermo Fisher Scientific, Inc., Waltham, MA,

USA). Only Zn and Cd were analysed because Cu and Pb concentrations in

349R. van Herwijnen et al. / Environmental Pollution 150 (2007) 347e354

Table 1

Chemical and physical conditions of soils, composts, clay and zeolite used in the experiment

Units L-soil H-soil GWC SC Bentonite clay Zeolite Soil intervention

valuesa

Nutrients

Available phosphorus mg/kg 11.4 7.0 294.2 356.2 1.3 1.8

Available potassium mg/kg 245 229 7471 1167 503 206

Available magnesium mg/kg 271 161 783 526 6057 554

Nitrate nitrogen mg/kg 17.9 2.6 470 6797 0.95 0.06

Ammonium nitrogen mg/kg 6.5 2.4 16.8 13.6 1.6 0.45

Total nitrogen %w/w 0.48 0.25 1.52 2.43 0.009 0.02

Total phosphorus mg/kg 811 638 2490 40,760 42 190

Total sulphate mg/kg 991 777 602 13,560 691 89

Soil structure

pH 6.8 7 8.6 4.6 9.1 9.2

Organic matter %w/w 8.7 3.8 28.5 29.1 0.5 0.7

Sand 2.00e0.063 mm %w/w 6 2.5 2 88.5

Silt 0.063e0.002 mm %w/w 56 57 53 7

Clay <0.002 mm %w/w 38 40.5 45 4.5

Soil texture Silty clay Silty clay Silty clay Sand

Metals

Total copper mg/kg 46.7 218 78.75 672.5 0.9 2.55 190

Total zinc mg/kg 1074.5 9580.5 178.5 1217.5 34.4 28 720

Total iron mg/kg 33,230 35,410 11,315 71,220 3708 6814

Total lead mg/kg 315 1688 110 175.5 38.6 27.9 530

Total arsenic mg/kg 19.7 30.6 4.9 5.7 10.4 4.4 55

Total cadmium mg/kg 10.9 128 0.63 5.7 0.27 0.065 12

Total mercury mg/kg 0.43 1.28 0.14 2.495 <0.02 <0.02 10

Total nickel mg/kg 27.0 31.5 9.85 20.7 2.0 1.5 210

Total chromium mg/kg 36.7 41.1 14.8 28.1 0.7 0.7 380

a (VROM, 2000).

the leachate samples were below detection limit of the AAS (Cu <0.5 mg/l;

Pb <0.5 mg/l), not enough sample was left for analysis with the same equip-

ment as used for the plant samples below. Plant samples were dry-ashed at

450 �C for 18 h and then wet digested (Chapman, 1967). Wet digestion was

performed by adding 0.75 cm3 concentrated HNO3 to each sample and incu-

bating for 1 h at 60 �C, followed by adding 2.25 cm3 concentrated HCl and in-

cubating for 14 h, and then heating for 2 h at 110 �C. The samples were then

cooled and 0.15 cm3 of 30% H2O2 was added after which they were heated for

30 min at 110 �C. The H2O2 treatment was performed twice to ensure that all

organic matter was completely oxidised. The digested samples were analysed

for Cd, Cu, Pb and Zn with a Spectro Flame Inductively Coupled Plasma eOptical Emission Spectrometer (ICP-OES) (Kilbride et al., 2006).

2.5. Statistical analysis

Statistical analysis was performed using Analysis of Variance (ANOVA) in

GenStat (Genstat, 2005). A natural logarithm transformation of the data was

Table 2

Overview of tested soil/compost mixtures as used in the nursery trial (N) and leaching tests (L)

Soils Compost Soil/compost ratio (w/w) Mineral/compost ratio (w/w) Grand Total

None Bentonite (%) Zeolite (%)

7 14 20 7 14 20

H-soil No compost 0% N þ L 1

GWC 7% N þ L N N N þ L N N N þ L 7

14% N þ L N N N N N N 7

20% N þ L N N N þ L N N N þ L 7

SC 7% N þ L N N N þ L N N N þ L 7

14% N þ L N N N N N N 7

20% N þ L N N N þ L N N N þ L 7

L-soil No compost 0% N 1

GWC 7% N N N N N N N 7

14% N N N N N N N 7

20% N N N N N N N 7

SC 7% N N N N N N N 7

14% N N N N N N N 7

20% N N N N N N N 7

86

350 R. van Herwijnen et al. / Environmental Pollution 150 (2007) 347e354

used for some datasets to satisfy the assumption of normality and variance

homogeneity, and means and standard error were back-transformed for presen-

tation. Specific treatment pairs were compared using a t-test. When means are

given in the text a 95% confidence interval is given between brackets. Statis-

tical procedures were performed according to Miller and Miller (2000).

3. Results

3.1. Leaching test

Concentrations of Zn and Cd leached from H-soil mixturesare presented in Fig. 1. The two composts had a different ef-fect on leaching. On average, Zn and Cd concentrations inthe leachates of H-soil amended with SC were respectively2.7 and 1.9 times higher than in the leachates of the H-soilamended with GWC (Cd: P < 0.001; Zn: P < 0.001). SCamendment to H-soil almost doubled the leachability of Zncompared to the control (P < 0.001). To test if increased Znconcentration in the leachates of the SC amended soil wasdue to leaching from the compost, SC was subjected to thesame leaching conditions for comparison purposes. The resultsshowed that the level of Zn leaching from the SC was 0.1 mg/lwhich was less than 2% of Zn leachate concentration in any ofthe SC treated soil mixtures. This was a surprising result con-sidering the high concentration of Zn in the SC and a low pHof 5.5 of the leachate, for which we do not have a feasibleexplanation. In contracts to SC, addition of 14% and 20%GWC to the H-soil caused a significant reduction in the

Fig. 1. Concentrations of Zinc (A) and Cadmium (B) in the leachates of H-soil

amended with different levels of greenwaste compost (GWC) or composted

sewage sludge (SC). The composts were used unamended or amended with

7% or 20% of bentonite (B7, B20) or zeolite (Z7, Z20). Error bars indicate

the standard error of the means.

concentration of Zn in the leachate (P ¼ 0.02, P < 0.001 re-spectively) with the 20% GWC addition causing the greatestreduction in Zn leaching (48.8% compared to the control). Ad-dition of 7%, 14% or 20% GWC also significantly reducedleaching of Cd (P ¼ 0.004, P < 0.001, P < 0.001 respec-tively) with the highest reduction of about 47% obtained at20% GWC addition, but there was no significant difference be-tween the effects of 14% or 20% GWC addition. The reduc-tion in leaching of Cd or Zn by the amendment of GWCwas in all significant cases higher than what could havebeen expected from the dilution of the soil through compostamendment, which was at maximum 20%. The pH of theleachates was only slightly influenced by the composts. Themean pH of soil mixtures containing SC was 6.50 (6.45,6.56), which was significantly lower than the mean pH of6.90 (6.89, 6.98) for GWC amended mixtures (P < 0.001).However the mean pH of SC- or GWC-amended mixtureswere not significantly different from the mean pH of 6.72(6.58, 6.86) for the control (P ¼ 0.16 for both cases). ThepH of leachates from soils used in the nursery trials werealso determined and were not significantly different from thesoils used for the leaching trials (P ¼ 0.09). Addition of zeo-lite or bentonite to the composts had no significant effectson the leaching of Zn and Cd from the amended soils.

3.2. Shoot dry weight

Ryegrass grown on unamended L-soil produced 54 timesmore dry matter (the mass of the dry matter was 6.34 (4.46,9.02) g/pot on L-soil) than when grown on H-soil (the massof the dry matter was 0.12 (0.08, 0.17) g/pot on H-soil)(P < 0.001) (Fig. 2). In H-soil, addition of increasing quanti-ties of either composts successively increased dry matter pro-duction (Fig. 2). However, the rate of increase was higher forSC, resulting in 13 times more dry weight at the 20% levelthan the control whereas the same amount of GWC producedonly 3.2 times more dry weight than the control (SC:P < 0.001, GWC: P ¼ 0.001). The addition of SC to L-soil(Fig. 2) had no significant effect on growth compared to thecontrol and addition of 20% SC reduced growth comparedto 14% SC addition (P ¼ 0.002). GWC amended to L-soil at14% and 20% significantly reduced growth by more thanone-half compared to the unamended control (P ¼ 0.02,P < 0.001 respectively).

In most cases, the addition of minerals to the composts didnot affect dry matter production; however, in the zeolite/SC mix-tures, the addition of 7% or 20% zeolite produced 3.11 (2.70,3.60) and 4.18 (3.62, 4.83) g/pot of dry matter compared withonly 1.96 (1.70, 2.26) g/pot in soil amended with SC only(P ¼ 0.024, P < 0.001 respectively). The amendment of SCmixed with 7% or 14% zeolite translates to 63% more ryegrassdry weight produced compared to the amendment with SC only.

3.3. Metal accumulation into the ryegrass leaves

The leaf concentrations of Cd, Cu, Pb, and Zn of plantsgrown on the two soils and two composts are shown in

351R. van Herwijnen et al. / Environmental Pollution 150 (2007) 347e354

Fig. 3. H-soil amended with SC caused 52% to 80% lower leafconcentrations for Cd, Cu, Pb, and Zn than the control, andthese were in all cases lower than the GWC amendmentswhich caused maximum reduction in the metal leaf concentra-tions of 36% to 54%. L-soil amended with SC or GWC did notgive significantly different leaf concentrations than the control.But 20% GWC amendment caused significantly higher leafconcentration for Cd and Pb than 7% GWC (P ¼ 0.014,P ¼ 0.042 respectively) and similarly 20% SC amendmentcaused significantly higher leaf concentrations than 7% SCamendment (P ¼ 0.005).

To account for the total amount of metal that is transportedabove ground by the accumulation by the ryegrass, an over-view of the total accumulation per pot is given in Fig. 4.From this figure can be observed that on H-soil SC amendmentcaused a higher total accumulation of all four metals comparedto GWC and the control while GWC is never significantly dif-ferent from the control. On L-soil GWC gave a significantlylower accumulation of all four metals compared to the controland SC (P < 0.05 in all cases) while SC amendment was neversignificantly different from the control.

3.4. Metal accumulation into the ryegrass roots

Concentrations of metals in ryegrass roots were determinedonly for L-soil because the trials on H-soil did not produceenough roots for analysis. GWC amendments applied to L-soil decreased the root concentrations of Cd, Pb and Zn upto 47% compared to the control (Fig. 5). In the case of SCamendments, a similar significant decrease of about 24%was observed in the root concentration of Cd between at 7%and 20% SC addition (P ¼ 0.006). Conversely, concentrationsof Cu in the roots increased to in excess of 120% for the 20%

Fig. 2. Biomass production of ryegrass growing on H-soil (A) and L-soil (B)

amended with different levels of green waste compost (GWC) or composted

sewage sludge (SC). Error bars indicate the standard error of the means.

SC amendment compared to the control (P < 0.001). Simi-larly, application of 20% SC gave a 16% increase in the con-centrations of Zn in the roots compared to the 7% SC addition(P ¼ 0.016). Root concentrations of Cd, Cu, Pb and Zn forplants grown on L-soil amended with GWC were in mostcases lower than for plants grown on L-soil amended withSC. There was no clear trend observed with the addition ofbentonite or clinoptilolite to either composts with respect tometal accumulation by the roots.

4. Discussion

The objective of this study was to investigate the potentialfor the remediation of metal contaminated soils by the additionof composts in an attempt to reduce metal leaching and bio-availability to plants. Other researchers have demonstratedthe potential of compost for this purpose (Geebelen et al.,2002; Roman et al., 2003; Castaldi et al., 2005; Simon,2005). The results from this study demonstrated similar effectsbut importantly showed that the containment of the metals alsocould be affected. The affection of metal containment is par-ticularly true in the case of Zn where leachability increasedin SC amended H-soil.

The increased leachability of Zn from SC amended soilcould be caused by the formation of complexes of Zn with or-ganic matter compounds from the SC, such as humic and ful-vic acids (da Silva and Oliveira, 2002) or by changes in pHwith the addition of SC. Effects for organic matter havebeen reported for Cu where a mixture of organic householdwaste compost and wood chips reduced the concentration offree Cu2þ in the soil water but increased the concentrationof complexed Cu2þ (Kiikkila et al., 2001; Clemente et al.,2003). However, the final effect was a reduced toxicity tosoil bacteria (Kiikkila et al., 2002). Apart from Cu it is alsoknown that leaching of As and Pb can be increased by com-plexation with dissolved organic matter (Bradl, 2004; Menchet al., 2003) and these metals are therefore likely to haveraised concentrations in the leachates of compost amendedsoils in our experiments. We have observed increased concen-trations of Cu in the plant roots after SC addition, confirmingthat SC probably also caused increased Cu concentration inthe leachate. Cd and Zn are not known to form complexeswith organic matter but do chelate with EDTA (Bradl,2004). Potentially the composted sewage sludge contains che-lating compounds which enhance the leaching of Cd and Zn.SC addition did have a slight effect on the leachate pH com-pared to GWC addition but at the pH range of the leachates(6.5 to 7.0) no large effect on leaching of metals from thesoil can be expected by addition of these composts (Martinezand Motto, 2000). The little effect of the composts on theleachate pH can best be explained by a high buffer capacityof the soil. A similar study with composted sewage sludgeamended to mine spoils (Simon, 2005) did not report increasedmetal leaching.

Addition of either composts to H-soil substantially im-proved ryegrass growth but it should be kept in mind thatthis improvement was not nearly as high as the biomass

352 R. van Herwijnen et al. / Environmental Pollution 150 (2007) 347e354

Fig. 3. Concentration of Cd, Cu, Pb, and Zn in leaves of ryegrass grown on H-soil (A) and L-soil (B) amended with green waste compost (GWC) or composted

sewage sludge (SC). Error bars indicate the standard error of the means.

production of the L-soil control. For a full remediation of theH-soil, more amendments or even other remediation tech-niques are required. The improved plant growth also couldbe due to immobilisation of the metals, but could also have

been caused by either the addition of nitrate to the soil, orby dilution of the contaminants through compost and zeoliteamendment. The dilution of the contaminants was probablyonly a minor factor as can be seen from the leaching results

Fig. 4. Mean accumulation of Cd, Cu, Pb, and Zn by ryegrass per pot grown on H-soil (A) and L-soil (B) amended with green waste compost (GWC) or composted

sewage sludge (SC). Error bars indicate the standard error of the means.

353R. van Herwijnen et al. / Environmental Pollution 150 (2007) 347e354

Fig. 5. Concentration of Cd, Cu, Pb, and Zn in the roots of ryegrass grown on L-soil amended with green waste compost (GWC) or composted sewage sludge (SC).

Error bars indicate the standard error of the means.

where the effect of the amendments was more than what couldbe expected from dilution alone. The effect of nitrate can beobserved from the improved growth after the addition of SCwhich did not immobilise the metals but had a very high ni-trate level. The lower leaf concentrations of the metals afteraddition of SC to the soil resulted from the higher biomassproduction, caused by nitrogen from SC, diluting the foliarconcentration of metals even though the total accumulationper pot was increased by SC addition. The lowest leaf concen-trations of all metals on amended H-soil and those of Cd, Pb,and Zn on amended L-soil were still higher than those ob-served on clean soils of 0.1e0.3 mg kg�1 for Cd, 0.8e2.4 mg kg�1 for Pb and 40e60 mg kg�1 for Zn (Aten andGupta, 1996; Antoniadis and Alloway, 2003). Nevertheless,H-soil supported very little vegetation, and the advantage isthat by vegetation establishment, wind and water erosionwill be reduced and so stop spreading of the contaminationover a larger area (Tordoff et al., 2000). From the latter pointof view, GWC could be a useful addition to the H-soil.

Addition of SC to L-soil did not improve growth and actu-ally suppressed growth at the highest rate (20%), possibly be-cause of toxic effects by the high metal and nutrient levelsfrom the compost. In contrast, addition of GWC suppressedgrowth at all rates, possibly because of nitrogen lock-upcaused by continued composting in the mixtures (Chaveset al., 2005). The absence of improved and even suppressedgrowth by GWC or SC amendment to L-soil indicates thatlevels of nutrients and organic matter in this soil were nota limiting factor for plant growth. Nevertheless, low levelsof GWC did not inhibit plant growth on this soil, and some re-striction of metal accumulation by the leaves, roots and totalaccumulation per pot was achieved. Therefore low levelamendments by GWC could have potential for the remediationof moderately contaminated soils.

No noticeable effect on metal mobility was achieved by theaddition of bentonite or clinoptilolite to the composts. How-ever, the levels added were relatively low and consequentlytheir effects could be masked by the effects of the compost it-self. Alternatively the organic matter could have blocked ac-cess to the metal exchange sites of the minerals. The effectof the minerals addition could become important in the longer

term when organic matter is degraded, levels of organic matterin the soil declines and consequently metal bioavailability mayincrease (Mule and Melis, 2000). Additional research withhigher levels of minerals will have to be undertaken in orderto establish the longer term effects of metal immobilisationand behaviour of compost when applied to contaminated soil.

5. Conclusions

Overall, a reduction in the leaf concentration of metals andincreased biomass production by ryegrass was achieved by theaddition of composts to the highly contaminated soil (H-soil),but the actual accumulation per pot also was increased by ei-ther composts. No such effects were observed on the less con-taminated soil (L-soil). Zn leaching was reduced by theaddition of green waste compost (GWC) but increased bythe addition of composted sewage sludge (SC) to the soils.The increased growth by addition of compost to H-soil was as-cribed to immobilisation of the toxic metals (GWC) but also tothe introduction of nutrients like nitrate in the compost. Fromour results, we conclude that addition of GWC to either L- orH-soil is a suitable remediation measure to contain metalswithin the soil. The use of SC appears to have no beneficialeffects on the L-soil and could even worsen the situation onthe H-soil.

The results indicate that the effect of compost amendmentdepends on the type of compost, the soil type, and related con-taminant levels. It is therefore recommended that plant andleaching tests be performed before composts are applied a fieldscale for the reclamation or remediation of metal-contaminatedsoils on brownfield sites. The increased metal leaching was as-cribed to the formation of metal-organic complexes or chelatingcompounds from the SC, but changes in soil pH could also havean effect. More research into the formation of complexesbetween metals and organic matter, and into the occurrence ofchelating compounds in composts is required to achieve a betterunderstanding of the mechanisms involved with the use ofcompost for the remediation of metal-contaminated soils. Addi-tional research to the effect of the tested composts on the leach-ing of As, Cu and Pb is also recommended. Addition of mineralsto composts at the examined levels did not appear to affect the

354 R. van Herwijnen et al. / Environmental Pollution 150 (2007) 347e354

leaching or bioavailability of metals in the soils, and higher ratesof amendment may be necessary to obtain effective remediationof brownfield sites.

Acknowledgements

The authors are grateful to the following people who havegenerously supplied materials for the research work or assistedwith the practical work: Adrian Nash from Halcrow who pro-vided the contaminated soil and also arranged for the transport;Andy Johnson and Andrew Weeks from Thames Water e TerraEco Systems for supplying the composted sewage sludge;Allan Holliwell from Holliwell Seed & Grain for supplyingthe ryegrass seeds; and Wohid Noman for his assistance withthe running of the nursery trial. The financial contributionfrom the EPSRC, who funded this research work as a part ofthe SUBR:IM consortium under Grant GR/S148809/01, isgratefully acknowledged.

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