Remediation of heavy metal contaminated soil washing residues with amino polycarboxylic acids
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Journal of Hazardous Materials 173 (2010) 697704
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Journal of Hazardous Materials
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Remediation of heavy metal contaminated soil wpolycar
Zandra A KroBert Allaa SAKAB AB, SEb Man-Technol , SE-7c Solventic AB,
a r t i c l
Article history:Received 20 AReceived in reAccepted 28 AAvailable onlin
Keywords:Heavy metalsAmino polycarboxylic acidsSoilRemediationEDDSMGDA
he aS) anhad perimestrucafter
signicant differences in extraction efciencies. Extraction with citric acid was generally less efcient,however equal for Zn (mainly) after 10d. Metal removal was similar in batch and WTC-mixer systems,which indicates that a dynamic mixer system could be used in full-scale. Use of biodegradable aminopolycarboxylic acids for metal removal, as a second step after soil washing, would release most remain-ing metals (Cu, Pb and Zn) from the present soils, however only after long leaching time. Thus, a full-scaleprocedure, based on enhanced metal leaching by amino polycarboxylic acids from soil of the present
The relemade soil ctal problemunlike orgathe soil or bquantities ooption, dueOne commois soil washne soil co(sand and ginated soilsmall soil pinorganic coheavy meta
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0304-3894/$ doi:10.1016/j.kind, would require a pre-leaching step lasting several days in order to be efcient. 2009 Elsevier B.V. All rights reserved.
ase of heavy metals and other pollutants in society hasontamination one of the most important environmen-s. Heavy metals cannot be mineralized or decomposed,nic contaminants, and must therefore be removed frome physically and/or chemically contained . The largef polluted soils make landll disposal an unrealisticto environmental constraints and space limitations.nly used method for remediation of contaminated soiling, which is a technique that uses water to separatemponents (clay and silt) from the coarse componentsravel) in a mechanical process. The volume of contam-is reduced, since the contaminants tend to bind to thearticles. Soil washing can be used on both organic andntaminants, but the main target contaminant group isls [2,3]. In the design of a washing process, treatment
ding author at: SAKAB AB, SE-692 85 Kumla, Sweden.2872300.ress: email@example.com (Z. Arwidsson).dress: Eurons Environment Sweden AB, P.O. Box 737, SE-531 17eden.
duration is of major concern. The time required to arrive at a givenclean-up goal is depending on the nature of the metal retentionmechanism and the inuence of factors such as pH, soil type (com-position, exchange properties, etc.), the presence of natural organicmatter, age of the contamination, and the presence of other inor-ganic contaminants [2,4]. Synthetic complexing agents, chelatorsin particular, are sometimes used in combination with soil wash-ing to further enhance the removal of metals. However, many ofthese chelators have long degradation times in soil, resulting intoxicity and stress to the community of soil-living microbes andeco systems .
There are many factors to consider when deciding whether anarticial chelating agent can be used in full-scale remediation ornot. The chelating agents need to be added in excess in relationto the contaminant concentration, since they generally are non-selective and form complexes with most of the major polyvalentcations present in the soil (i.e., Fe, Mn, as well as Ca, Mg, etc.). Thechemical state of the metals in the soil also affects the efciency ofthe chelators, sincemetals in recently contaminated soils, aswell asin articially contaminated laboratory system, are more labile andaccessible than metals in soils that are historically contaminated[2,6]. Furthermore, the chosen agent must be biodegradable but atthe same time has a high metal binding capacity. It is also desirablethat the chelator can be recovered and used several times, consid-
see front matter 2009 Elsevier B.V. All rights reserved.jhazmat.2009.08.141boxylic acids
rwidssona,b,, Kristin Elgh-Dalgrenb, Thomas vonrdb, Patrick van Heesb,1
-692 85 Kumla, Swedenogy-Environment Research Centre, School of Science and Technology, rebro UniversitySE-591 35 Motala, Sweden
e i n f o
pril 2009vised form 28 August 2009ugust 2009e 4 September 2009
a b s t r a c t
Removal of Cu, Pb, and Zn by tethylenediaminedisuccinic acid (EDDwas tested. Three soil samples, whichwereutilized in the leaching tests. Expmixer system (Water Treatment Conefcient in removing Cu, Pb, and Zn/ locate / jhazmat
ashing residues with amino
nhelma, Ragnar Sjbergc,
01 82 rebro, Sweden
ction of the two biodegradable chelating agents [S,S]-d methylglycinediacetic acid (MGDA), as well as citric acid,reviously been treated by conventional soil washing (water),ntswereperformed inbatches (0.3 kg-scale) andwith aWTC-
tion, 10kg-scale). EDDS and MGDA were most often equally1060min. Nonetheless, after 10d, there were occasionally
698 Z. Arwidsson et al. / Journal of Hazardous Materials 173 (2010) 697704
ering the economy of a full-scale process using chemicals that areexpensive.
In the past, EDTA (ethylenediamineacetic acid) has been themost widely used chelating agent for extracting metals fromcontaminatcomplexatiability in thand can thwith the mEDDS ([S,S]an environm(methylglycronmentallcould be usof adsorbedin a techniction couldpH have neof corrosionstituents.
The aimable complsoil after asolutions (nMGDA, andformed. EDbinding nitstructure. Mposition inCitric acid cgroup (nottors were tsoil washincan be useventional smetals remwash that hdue to theboxylic acidin terms offull-scale rea subsequeing agents?capacity ofnor to comporigin or coformance othe metal frfrom sites wwith EDDS/times, can bdure.
2.1. Soil cha
The soilmetal conta
A: A sandysite with
B: A sandymatter;
C: A clay-dfrom a st
Table 1Total acid leachablea metal concentrations (mgkg1), pHb, and ignition residuec, LOI(%) in soils A, B and C. N=3, mean SD. MKM=Limits for soil usage on areas that areless sensitive, established by the Swedish Protection Agency .
owaver to sC, 2hexceeding
threposifractan 5r thing esep
al byrrieses wromf there
ith pammin si
ashinxchaed tn oftionwou
thanitatiophapresent leaching experiments were performed on the
es that still carried considerable levels of heavy metals, i.e.,t reach the desired remediation levels (MKMlimits for soilon areas that are less sensitive, c.f. Table 1), that are estab-by the Swedish Environmental Protection Agency (SEPA)he total metal concentrations of the soil residues (acid diges-M HNO3) are given in Table 1, and the distribution of metals,ned by sequential leaching, is given in Fig. 1, based on theprocedure , slightly modied.
Water-soluble (water; 20 C)Cation exchangeable (1.0M NH4Ac, pH 7; 20 C)Carbonates and hydroxides (1.0M NH4Ac, pH 5.0; 85 C)Hydrous oxides (0.043M NH2OHHCl in 25% acetic acid (v/v); 85 C)Labile organics and amorphous metal suldes (0.02M HNO3 and 30%H2O2 (3:5, v/v), pH 2.0; 85 C); after completed leaching 3.2M NH4Acin 20% HNO3 +water was appliedConsolidated organics, metal suldes and residual fraction (microwavedigestion in 7M HNO3)ed soils . EDTA has a high capacity for metalon and extraction but a comparably low biodegrad-e environment . EDTA is also quite mobile in soilsereby be transported to the groundwater, togetherobilized metals [7,8]. In several previous studies,
-ethylenediaminedisuccinic acid) has been proposed asentally friendly and safe chelant [14,1723]. MGDA
inediacetic acid) is another example of a more envi-y friendly chelator [14,17]. Mineral acids like nitric acided to produce a low pH that would lead to a releasehydrolyzed (cationic) metals from the soil. However,al scale it would be an advantage if a metal mobiliza-be achieved at a near neutral pH. Solutions with lowgative effects on the soil washing equipment in terms, and also lead to irreversible damage to the soil con-
of this studywas to test the capacity of somebiodegrad-exing agents for removal of metals remaining in theconventional soil wash procedure. A batch study usingeutral pH) with the two chelating agents EDDS andcitric acid as a reference for comparison, was per-
DS, with four carboxylic groups and two potentiallyrogen positions in the molecule, is similar to EDTA inGDA, with three carboxylic groups and one nitrogenthe molecule, is similar to NTA (nitrilotriacetic acid).ontains three carboxylic groups, as well as a hydroxy
complexing at low or near neutral pH). The same chela-ested in a 10kg-scale using equipment designed forg (Water Treatment Construction), WTC-mixer, whichd in a full-scale process in combination with a con-oil washing technique. The key issues were: (1) Canaining in contaminated soil, after conventional soil-as removed themetal carryingne-fraction, be releasedaction of the selected biodegradable amino polycar-s? (2) How do these agents compare with citric acidmetal removing capacity? (3) Is it feasible to design amediation process based on soil-wash, combined withnt leaching step using biodegradable strong complex-Thus, the aim is neither to establish the complexingEDDS and MGDA, which is already well-documented,are results from unique contaminated soils of variousmpile data from different cases in a review of the per-f EDDS and MGDA. The main purpose is to nd out ifaction remaining after soil-wash, in some selected soilsith old and aged metal contamination, can be removed
MGDA, and if the WTC-procedure, giving short leachinge utilized in a one-step soil-wash remediation proce-
ls and methods
residues after conventional soil washing of three heavyminated soils (AC) were used:
soil, low in natural organic matter; from an industrialmixed contaminants (metals as well as hydrocarbons).
coarse-grained moraine with minor amounts of organicfrom a shooting-range.ominated soil with minor amounts of organic matter;eel works site.
a Micrb Watc 550d Note Inclu
Thein comminor(less th
Afte(includmerelymaterithat caParticlrated fmost oAC) wtionwup to 52mmtests.
Thesured.with msoil wated eexpectpositioadsorpas Fe)ratherprecipcarrier
Theresidudid nousagelished. Ttion, 7as deTessierF1F2F3F4F5
F6oil A Soil B Soil C MKM
4600 820 7930 380 6160 19056 14 67 14 370 57 150d
230 9.0 400 75 84 60 2003100 480 23500 3270 55200 5480300 37 370 74 680 5938 8.0 39 4.0 250 57 120
2370 260 2520 1040 480 27 400200 5.0 790 270 170 80 5006.9 0.1 7.0 0.2 5.8 0.1
12.0 0.1e 1.5 0.3 1.1 0.2e digestion with 7M HNO3.olid 1:5 (v:v)..ding 1% Cr(VI).some 10mgkg1 hydrocarbons.
e soils come from different locations, but all are similartion: moraine, rich in feldspars and quartz (sand) withions of natural organics, and usually low levels of clays%, except for soil C).e conventional soil washing, most of the ne-fractionssentially all of the clay) was removed. Soil washing isaration of the ne-fraction of the soil from the coursesuspension in water and removal of the water phasethe suspended particulate and colloidal matter .
ith diameters below 0.1mm are to a large extent sepa-the rest of the soil. This size fraction generally carriese metal contamination. The remaining residues (soilshighly heterogeneous with respect to the size distribu-rticle sizes ranging from0.1 to 2mm,with larger grains, andwith visiblemetallic residues (soil C). Grains aboveze were removed from the soils prior to the leaching
ion exchange capacities of the soils were not mea-ever, they are all sandy and fairly coarse-grained
r amounts of clayish materials remaining (after theg that removed the clay fraction with its associ-
ngeable metals). The exchange capacity would not beo exceed 25meqkg1, considering the general com-the soils . However, the major mechanism forof the metals in this study (Cu, Zn, Pb, as wellld be surface complexation of hydrolyzed species,electrostatic interaction (ion-exchange), as well as co-n with Fe (and possibly Al) constituting a hydroxidese.
Z. Arwidsson et al. / Journal of Hazardous Materials 173 (2010) 697704 699
Fig. 1. Sequensoils A, B andF2= cation exF5= labile orgametal suldes
2.2. Batch e
The soilcomplexing0.18M citriThe chosenof magnitudZn in the so(200 rpm) tSamples (trand 60min,(4000 rpm)stored at 4tained throuNaOH).tial extraction of metals (Al, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) fromC: Six extraction steps, modied from . F1 =Water-soluble;
changeable; F3= carbonates and hydroxides; F4=hydrous oxides;nics and amorphous metal suldes; F6= consolidated organics and(residual). The fractions represent mean values, N=3.
residues (A, B, and C) were exposed to solutions withagents: 0.18M EDDS (pH 7), 0.18M MGDA (pH 7), and
c acid (pH 7). Tap water (pH 8.2) was used as a control.molarity of the chelating agents was about one ordere above the total levels of the metals Cr, Cu, Ni, Pb andils, c.f. Table 1. 300g of soil was continuously agitatedogether with 300mL solution in 2.5 L plastic containers.iplicates) of the slurries were collected after 10, 20, 30,24h, and 10d. The samples were centrifuged for 6minand the supernatants were acidied with 1% HNO3 andC until metal analysis. A constant pH of 7 was main-ghout the experiment (adjustments with HNO3 and/or
Fig. 2. Schema die (1), desidroplets that h
The WTuid phasethe right drates highly(grains) tharial on thePressure, dcan be variexperimentiments.
The two(0.18MEDDa control, wwith 10 orwater soluttinuously aushing. Wthan a minuof the slurri10d (witho(4000 rpm)stored at 4
withHNO3,the 10-d ex
Metal coOES (Plasmquantitativatic illustration of the WTC-mixer system. The uid phase comes fromgned to create the right drop size. The pressure of the uid createsit the soil material (grains) that is placed on a net (2).
ic washingthe WTC-systemC-mixer  is schematically illustrated in Fig. 2. The(water) comes from a die, specially designed to createop size (point 1 in Fig. 2). The pressure of the uid cre-energetic droplets and these droplets hit the materialt is placed on the net (point 2 in Fig. 2). The solid mate-net is exposed to a bombardment of distinct droplets.roplet size, ows and temperature in the mixing zoneed. These parameters were xed throughout all of thes to allow for a direct comparison with the batch exper-
amino polycarboxylic acids used in the batch studiesS andMGDA, pH 7), togetherwith tapwater (pH 8.2) asere used also in the WTC-system (10kg of soil ushed20L solution). The applied pressure was 2.5MPa. Theions were collected in 50 L containers and were con-gitated by a metal blade rotor (100 rpm) during thehen no soil was left inside the WTC-mixer (after lesste), the agitation was terminated. Samples (triplicates)es were collected after 10, 20, 30, and 60min, 24h, andut agitation). The samples were centrifuged for 6minand the supernatants were acidied with 1% HNO3 andC prior tometal analysis. The slurrieswere pH-adjustedwhenneeded, inorder tomaint...