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Chelant soil-washing technology for metal-contaminated soilDavid Voglara & Domen Lestanab

a Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101,Ljubljana 1000, Sloveniab Envit Ltd., Vojkova 63, Ljubljana 1000, SloveniaPublished online: 06 Jan 2014.

To cite this article: David Voglar & Domen Lestan (2014) Chelant soil-washing technology for metal-contaminated soil,Environmental Technology, 35:11, 1389-1400, DOI: 10.1080/09593330.2013.869265

To link to this article: http://dx.doi.org/10.1080/09593330.2013.869265

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Environmental Technology, 2014Vol. 35, No. 11, 1389–1400, http://dx.doi.org/10.1080/09593330.2013.869265

Chelant soil-washing technology for metal-contaminated soil

David Voglara and Domen Lestana,b∗

aAgronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, Ljubljana 1000, Slovenia;bEnvit Ltd., Vojkova 63, Ljubljana 1000, Slovenia

(Received 29 July 2013; accepted 18 November 2013 )

We demonstrate here, in a pilot-scale experiment, the feasibility of ethylenediaminetetraacetate (EDTA)based washingtechnology for soils contaminated with potentially toxic metals. Acid precipitation coupled to initial alkaline toxic metalremoval and an electrochemical advanced oxidation process were used for average recovery of 76 ± 2% of EDTA per batchand total recycle of water in a closed process loop. No waste water was generated; solid wastes were efficiently bitumen-stabilized before disposal. The technology embodiment, using conventional process equipment, such as a mixer for soilextraction, screen for soil/gravel separation, filter chamber presses for soil/liquid and recycled EDTA separation and soilrinsing, continuous centrifuge separator for removal of precipitated metals and electrolytic cells for process water cleansing,removed up to 72%, 25% and 66% of Pb, Zn and Cd from garden soil contaminated with up to 6960, 3797 and 32.6 mg kg−1

of Pb, Zn and Cd, respectively, in nine 60 kg soil batches. Concentrations of Pb and Zn remaining in the remediated soiland bioaccessible from the simulated human intestinal phase soil were reduced by 97% and 96% and were brought underthe level of determination for Cd. In the most cost-effective operation mode, the material and energy costs of remediationamounted to 50.5¤ ton−1 soil and the total cost to 299¤ ton−1.

Keywords: toxic metals; EDTA; soil washing; pilot scale; cost analysis

1. IntroductionPotentially toxic metals (PTMs), commonly referred toas ‘heavy metals’, are ubiquitous soil contaminants.[1,2]There are more than 1.8 million contaminated sites in west-ern central and south-eastern Europe, of which 240,000 arein need of remedial treatment. In almost 40% of these sites,PTMs are the most important contaminants.[3] In the USA,PTMs are present in 77% of the Superfund Sites (NationalPriority List), in 72% of the Department of Defense Sitesand in 55% of the Department of Energy Sites.[4]

The proper remediation and management of PTM-contaminated soil is a generally widespread and costlyissue. The selection of appropriate remediation technol-ogy depends on the contamination and soil type and finaluse of the reclaimed land. In this pilot-scale experiment,we used the chelating agent ethylenediaminetetraacetate(EDTA) for ex situ extraction of Pb, Zn and Cd fromcontaminated garden soil. Soil washing with an aqueoussolution of EDTA is considered to be a remedial option witha potentially low impact on soil quality.[5] Divalent andtrivalent PTMs can be EDTA-extracted from soil becausethe reported ordering of EDTA complex stability con-stants – Na+ < Mg2+ < Ca2+ < Fe2+ < Al3+ < Zn2+ <

Cd2+ < Pb2+ < Ni2+ < Cu2+ < Hg2+ < Fe3+,[6] –favours the removal of metal contaminants over natural

∗Corresponding author. Email: [email protected]

hardness ions.[7] Mobilization of PTMs in soil by theformation of water-soluble toxic EDTA chelate [8] posesa threat to the environment and EDTA-washed soil mustbe thoroughly rinsed to remove all mobilized PTM speciesbefore remediated soil is returned to the site of excavation.Although EDTA is not a particularly expensive chemi-cal, the cost of chelant use can be significant, since lowEDTA concentrations often do not extract PTMs fromsoil effectively.[9,10] Demonstrated on a laboratory level,but not available commercially, there have been severalproposals of how to recycle spent EDTA from the usedwashing solution. PTMs can be separated from EDTA withNa2S under alkaline conditions, resulting in almost a com-plete recovery of metals through precipitation in the formof insoluble metal sulphides.[11] Zero-valent bimetallicmixtures (Mg0–Pd0, Mg0–Ag0) can also be used to pre-cipitate PTMs from the solution, while liberating EDTAin alkaline pH.[12] Electrolytic recovery of toxic metalsand EDTA from washing solution in a two-chamber elec-trolytic cell separated with a cation exchange membrane toprevent EDTA anodic oxidation has been reported.[13] Inanother electrochemical process, PTMs and EDTA are sep-arated in an electrolytic cell under alkaline conditions usinga sacrificial Al anode. Al substitutes the PTMs in com-plex with EDTA and the released metals are removed by

© 2013 Taylor & Francis

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electro-precipitation/coagulation.[14] EDTA can also berecycled by substituting toxic metals with Fe3+ under acidicconditions, followed by the precipitation of the releasedmetals with phosphate at near neutral pH. Fe3+ ions arethen precipitated as hydroxides at high pH using NaOH,thus liberating the EDTA.[15,16]

In addition to EDTA recycling, the generation of largeamounts of wastewater after soil washing, which needstreatment before safe disposal, has long remained anunsolved problem. We recently proposed and laboratorytested a method for both EDTA and clean process waterrecycling from waste soil-washing solution. EDTA wasrecovered by acid precipitation coupled with initial alkalinePTMs trans-complexation and precipitation. The remainingEDTA and metals in the waste solution were removed usingan electrochemical advanced oxidation process (EAOP) toyield clean process water.[17,18] The initial technologyembodiment [19] identified time- and material-consumingbottle-neck procedures. In the present study, the same chem-ical principles (Supplemental Material) were used in thedevelopment of improved embodiment of soil remediationtechnology with novel process water-management scheme.The novel technology was evaluated in a pilot-scale experi-ment for remediation efficiency by measuring PTM removaland PTM bioavailability before and after soil remediation.Different modes of operation: pH of soil extraction, anodematerials for EAOP and process solid-waste stabilizationwith thermoplastic material before disposal, were evalu-ated. The capital and operational costs of the technologywere also determined.

2. Materials and methods2.1. Site location and soil propertiesSoil was collected from the 0–30 cm surface layer of a veg-etable garden in the Meza Valley, Slovenia, contaminatedwith Pb, Zn and Cd by emissions from a nearby smelt-ing plant. The pH in the soils was measured in a 1/2.5(w/v) ratio of soil and 0.01 M CaCl2 water solution sus-pension. Soil samples were analysed for organic matter bymodified Walkley–Black titrations,[20] soil texture by thepipette method,[21] and cation exchange capacity (CEC)as the sum of base cations measured after soil extractionwith ammonium acetate (pH = 7) and extractable aciditydetermined by the BaCl2 method.[22] Saturated hydraulicconductivity was measured on soil core samples (100 cm3)by a laboratory permeameter, steady-state head method.[23]The following values were obtained for the original soil: pH6.9, organic matter 7.0%, the soil texture was sandy loam(51% sand, 42% silt, 7% clay) and CEC 31 mmolc 100 g−1.Saturated hydraulic conductivity was 11.46 ± 2.76 m day−1

(n = 5), soil porosity 52.9% and bulk density 1.25 cm g−3.This result shows that soil has medium permeability forwater.

2.2. Small-scale extractionsSmall-scale soil extractions were performed to determinethe required addition of fresh EDTA to the recycled washingsolution. We placed 100 g of air-dried soil and 100 mL ofrecycled washing solution, amended with various amountsof Na2EDTA (from 1.12 to 6.7 g) in 250 mL flasks. Thesoil was extracted on a rotating shaker (3040 GFL, Ger-many) for 2 h at 16 RPM, the used soil-washing solutionseparated by centrifugation at 2880 g for 10 min and PTMconcentration in the solution measured using X-ray fluores-cence spectrophotometer (XRF), as explained below. Thepercentage of PTMs removed from the soil was calculatedaccordingly; the amount of fresh EDTA required to be addedto the recycled washing solution for the same Pb extractionefficiency as fresh 120 mmol EDTA kg−1 washing solutionwas determined and recalculated for the pilot scale.

2.3. Solid-waste stabilizationPTMs in solid waste from the process were stabilized aftermixing with 0.7–0.9 g g−1 of bitumen. The waste sample(10 g) was mixed with bitumen on a hotplate at 120◦Cfor 5 min and then cooled to obtain a solidified monolith.The monolith was crushed to reduce the particle size toless than 2.0 mm. Leaching of PTMs from the stabilizedwaste particles was determined using the Toxicity Charac-teristic Leaching Procedure (TCLP), as defined by UnitedStates Environmental Protection Agency (US-EPA) [24]and by using deionized water extraction test.[25] For theTCLP analysis, the soil sample specimens were crushedand ground to reduce the particle size to less than 2.0 mmand agitated in 20 mL of 0.0992 M acetic acid and 0.0643 MNaOH extraction solution (1:20 ratio) with a pH of 4.93 ±0.05 for 18 h at 300 rpm. The leachate was filtered through a0.45- μ m membrane filter to remove suspended solids andstored in the cold for determination of PTMs present in theleachate. For deionized water extraction test, the soil sam-ples were air-dried, ground and sieved through a 2 mm meshagain. One hundred mL of deionized water was applied toeach soil sample (10 g) and agitated for 24 h at room tem-perature. Elutriates were filtered through a Whatman No. 4filter and concentrations of PTMs determined.

2.4. Orally bioaccessible PTMsFor human hazard assay, unified bioaccessibility method(UBM) was performed according to Wragg et al.[26] Soilsamples were sieved through a 2 mm mesh and ground to250 μ m with an agate mill. Approximately 0.6 g of soilwas weighed directly into polycarbonate tubes, to which9 mL of simulated saliva (pH 6.5 ± 0.5) was added. Thesuspension was manually shaken and adjusted with HClor NaOH for a pH of 1.20 ± 0.05 before simulated gastricsolution (13.5 mL) of pH 0.9–1.0 was added. The extrac-tion vessel was then placed in an end-over-end shakerin a thermostatically controlled water bath at 37◦C, thus

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simulating the stomach (pH 1.2–1.7). After 1 h, pH waschecked to measure pH < 1.50 (if not, the procedure wasrestarted from the beginning with a special insistence on thepH stability 1.20 ± 0.05 for every 15 min) and centrifugedat 4500 g for 20 min. To simulate the intestinal phase (pH6.3 ± 0.5), the latter procedure was repeated and after thepH of the stomach phase had been checked, 27 mL of duode-nal (pH 7.4 ± 0.2) and 9 mL of bile (pH 8.0 ± 0.2) solutionswere added and the tubes were returned to the water bathfor a further 4 h. Vessels were then centrifuged for 20 minat 4500 g and supernatant was collected by careful pipettingand stored at 4◦C until analysis.

2.5. Analysis of metalsAir-dried soil samples (1 g) were ground in an agate mill,digested in aqua regia (28 mL), diluted with deionized waterup to 100 mL, and PTMs, Fe, Ca and Mn analysed byflame atomic absorption spectrophotometer (AAS) with adeuterium background correction (Varian, AA240FS). Themetals in the solution were determined by ASS directly. Astandard reference material (Wepal 2004.3/4, WageningenUniversity, The Netherlands) was used in the digestion andanalysis. The limits of quantification (LQ) were 0.1, 0.01,0.02, 0.06, 0.01 and 0.02 mg L−1 for Pb, Zn, Cd, Fe, Ca andMn, respectively.

During the remediation process, we assessed PTMsand Fe concentrations in process solutions directly, using(XRF, Delta DS-4000, Olympus Innov-x, USA). The fol-lowing factors relate XRF to ASS measurements: Pb(mg L−1)XRF × 0.779, ZnXRF × 0.789 and FeXRF × 0.878.The high regression coefficients (R2), 0.998 for Pb, 0.996for Zn and 0.997 for Fe, respectively, indicate a comparableaccuracy of AAS and XRF measurements, although XRFhad a higher LQ of all PTMs. XRF was not used to measureCd, since the LQ of this element (10 mg L−1) was above orclose to the actual concentration in the samples.

2.6. EDTA determinationThe concentration of EDTA was determined spectrophoto-metrically according to the procedure of Hamano et al.[27]The method involves the reaction of EDTA in wash-ing solution with Fe3+ under acidic conditions to pro-duce Fe-EDTA chelate (trans-complexation), followed bythe removal of excess of Fe3+ by chelate extraction inthe aqueous phase, using chloroform and N -benzoyl-N -phenylhydroxylamine and the formation of a chromophorewith 4,7-diphenyl-1,10-phenanthroline-disulphonic acid.Using a spectrophotometer, absorbance was measured at535 nm against a blank solution with the 4,7-diphenyl-1,10-phenanthroline-disulphonic acid replaced with an equalvolume of distilled water. The LQ of EDTA determinationwas 20 mg L−1.

2.7. Process equipment used in the pilot-scaleremediation plant

A concrete mixer (350 L, 20 RPM) was used as a mix-ing vessel for soil washing (Supplemental Material). Soilslurry was separated and soil rinsed in a chamber filterpress (Lotos Ltd., Slovenia) with the following workingcapacity: 12 plates (50 × 50 cm), working V 21.3 L, work-ing P 190–260 bar, filter cloth MKI-3290 with 22 dm3/dm2

min air permeability (Ecofil Ltd., Slovenia). Precipitated Feand PTMs hydroxides were removed from the alkalinizedprocess water by a continuous centrifuge separator (AlfaLaval, LAPX 404) under the following operating condi-tions: separator frequency 50 Hz, solution flow rate 1 L s−1.EDTA precipitated from the acidic solution was removedby a small chamber filter press (Lotos Ltd., Slovenia): 6plates (5.5 × 5.5 cm), V 0.27 L, P 0.5–4 bar, filter clothmaterial MKI-3290). PTMs precipitated from the processwater after EAOP cleansing and electro-corroded graphitewere removed by the press. Process water circulated fromthe mixing reactor (200 L, 54 RPM) through the electrolyticcells (flow rate 9 L min−1) using gravity flow and waspumped back to the reactor. Graphite anodes (2920 cm2,Graphtek LLC, PA, USA) were placed in three electrolyticcells (V 8.5 L) between two stainless steel cathodes (dis-tance 10 mm). The current density on the electrodes waskept at 51.4 mA cm−2 using a DC power supply (Envit,Ljubljana, Slovenia), based on optimal current density asdefined in lab-scale experiments. The voltage between theelectrodes ranged from 7.66 to 10.74 V, time of electroly-sis 6 h. Boron-doped diamond anodes (BDDA) (Condias,Itzehoe, Germany) were placed in the electrolytic tank cell(V 5 L) containing 10 anodes and 12 stainless steel cath-odes (distance = 2 mm) arranged in mono-polar mode. Theoverall anode surface area was 4920 cm2; the anode:cathodesurface area ratio was 1:1. The current density on theelectrodes was kept at 40.6 mA cm−2 using the same DCpower supply (based on optimal current density as definedin lab-scale experiments). The voltage between the elec-trodes ranged from 4.87 to 7.23 V with an average time ofelectrolysis of 16.8 h.

2.8. Pilot-scale soil remediationThe pilot-scale soil remediation (Supplemental Material)is a batch process comprising eight steps similar to ini-tial technology embodiment [19] but with more effectivepathway of process water recycle (Figure 1). Soil waswashed at three different pHs and using two different anodematerials for EAOP. Operational parameters were deter-mined based on processes developed and optimized in smalllaboratory-scale experiments.[17,18]

Step 1. Soil extractionBatches of 60 kg of air-dried contaminated soil were

extracted in a mixing vessel using 60 L of soil-washing solu-tion. In batches i to i + 9 (Figure 1), a recycled washing

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Washing MeEDTA

Substitution:CaEDTA + Me2+

(pH 12)

Me(OH)n

(~1.4 kg)

Protonation:H4EDTA + Ca2+

(pH 2)

Dissolution of recycledH4EDTA

Me0, Me(OH)n, graphite(~0.29 kg, n = 3)

Soild/liquidphase

separation,soil rinsing

Degradation:residual EDTA

Batchi–1 Batchi+1

Cleansed process water i(~40 L)

Batchi

Grinding,mixing,

formulation

Contaminated soili (60 kg)

Remediated soili

(~59 kg)

Cleansed process wateri–1

Fresh water(30 L)

Fresh Na2EDTA(~0.6 kg)

Stone/sand rinsing

Recycled washing solution i(~60 L)

Recycled washing solutioni–1

Fresh Stones water (5 L) (~1 kg)

1

3

2

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5

6

8

Water surplus:batch i–n to i

Process wateri–1Process water i(~70 L)

7

1st

2nd

3rd

Figure 1. Process scheme and mass balance of the novel soil-washing technology.

solution with final pH 10.7 ± 0.4 after mixing with soil(batches i to i + 3), pH 8.1 ± 0.2 (batches i + 4 to i + 6) andpH 7.2 ± 0.3 (batches i + 7 to i + 9), was used. Two dif-ferent anode materials for EAOP were also tested; graphite(batches i to i + 3) and BDDA (batches i + 4 to i + 9). Forthe initial soil-washing solution, 120 mmol of fresh EDTA(disodium salt) per kg of soil was dissolved in tap water(batch i − 1) before testing commenced. For the extractionof subsequent soil batches, the washing solution with recy-cled EDTA prepared in a previous batch was used after pHadjustment. The final volume of soil suspension in the mixerwas 85 L. Soil was extracted for 2 h. The volumes, elemen-tal composition and properties of the washing solution andother process solutions defined below are given in Table 1.

Step 2. Separation of the oversized materialAfter soil washing, the process oversized material was

separated from the soil slurry by wet screening through a2 mm sieve. Oversized material retained on the sieve wasfirst washed with 40 L of the first process water exiting thechamber filter press (Figure 1) in the solid/liquid separationphase of the process (Step 3) and afterwards also rinsedwith 5 L of fresh tap water. The used process suspensionwas fed back into the chamber filter press within the samebatch as shown in Figure 1. Cleansed larger stones wereremoved from the process and disposed of and the cleansedsand fraction was mixed with remediated soil (Step 4). Inan average batch, 18 ± 3% of initial bulk soil weight wasseparated as stones and sand.

Step 3. Phase separation and soil rinsingThe combined soil slurry from Step 1 and process

suspension from Step 2 (average volume 89 ± 2 L) were

pumped (membrane pressure pump) into a (larger) chamberfilter press for phase separation. The solid phase within thepress was then rinsed. We used 140 L of tap water for rinsingsoil from the initial batch (i − 1). Batches (i to i + 3) wererinsed first with 70 ± 2 L of the third, less-contaminatedprocess water (Table 1) exiting the press (Figure 1) fromthe previous batches (i − 1 to i + 2) and afterwards with42 ± 1 L of fresh tap water. Batches (i + 4 to i + 9) wererinsed first with 70 L of the third process water exiting thepress and then with 40 L of the cleansed process water fromthe previous batches (i + 3 to i + 8) and afterwards with30 L of tap water, until all the EDTA-mobilized PTMs hadbeen removed from the soil. Figure 2 shows the concen-tration of PTMs in the process water exiting the press, asdetermined using XRF. The rinsed soil was removed fromthe press as remediated soil and led into Step 4 for final for-mulation. Poorly soluble CaSO4, which precipitated withthe EDTA in acidic Step 6, was removed from the process.

Step 4. Preparation of remediated soilBlocks of remediated soil from the chamber filter

press, containing an average 10.2 ± 1.3% of moisture, wereground by rubbing through a 5 mm sieve and mixed withthe sand fraction from Step 2.

Step 5. Alkaline substitution and metal precipitationThe second process water exiting the press (Figure 1) in

Step 3 (average 120 ± 1 L) contained a high concentration(and larger part) of the EDTA and toxic metal concentration(Table 1). The second process water in batches i to i + 3 wasalkalinized to a pH value of 12, with an average 347 ± 43 gof hydrated lime (with min. 92% of Ca(OH)2, obtained froma local building materials store), in batches i + 4 to i + 6

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Table 1. Concentrations of Pb, Zn, Cd, Fe and EDTA, and pH of process waters occurring during soil washing.

Volumemin, max Pbmin, max Znmin, max Cdmin, max Femin, max EDTAmin, maxProcess waters (L) (mg L−1) (mg L−1) (mg L−1) (mg L−1) (mmol kg−1) pHmin, max

pH 10.7Washing solution 59, 60 177, 192 16, 19 3.9, 4.3 0, 0 120, 134.5 11.96, 12.08Second process water 118, 121 1815, 1926 246, 278 5.5, 6.7 214, 388 50, 62.2 7.54, 7.84Third process water 69, 71 22, 30 0, 0 0.01, 0.02 0, 0 0.01, 0.01 7.32, 7.52Tap watera 70 0 0 0 0 0 6.95Alkaline process waterb 118, 122 177, 192 16, 19 3.9, 4.3 0, 0 49.7, 61.3 12.01, 12.05Acidified process waterc 155 180 17 4.2 0 4.3 2.07

pH 8.1Washing solution 59, 61 237, 291 27, 33 3.1, 4.3 0, 0 121.6, 132.8 8.97, 9.09Second process water 119, 121 2291, 2413 451, 454 8.7, 9.6 694, 818 51.4, 62.5 6.91, 7.18Third process water 68, 70 26, 29 0, 0 0.01, 0.02 0, 0 0.01, 0.01 6.72, 7.27Cleansed process water 40 0, 0 0, 0 0, 0 0, 0 0, 0.01 6.70Alkaline process waterb 119, 121 237, 291 27, 33 3.2, 4.1 0, 0 50.7, 60.9 12.01, 12.08Acidified process waterc 154 266 29 3.8 0 4.1 2.1

pH 7.2Washing solution 59, 61 222, 289 28, 32 3.2, 4.3 0, 0 119.8, 134.1 6.47, 6.53Second process water 118, 120 2234, 2372 417, 472 8.5, 10.1 1009, 1042 51.1, 61.9 7.04, 7.67Third process water 69, 71 24, 31 0, 0 0.01, 0.02 0, 0 0.01, 0.01 6.63, 6.81Cleansed process water 40 0, 0 0, 0 0, 0 0, 0 0, 0.01 6.65Alkaline process waterb 118, 120 222, 289 28, 32 3.5, 4.2 0, 0 50.5, 61.2 12.02, 12.09Acidified process waterc 154 258 31 3.9 0 4 2.09

Notes: Minimum and maximum values are shown for three consecutive batches of soil remediation at pH 10.7 ± 0.4 (batches i to i + 3),8.1 ± 0.2 (batches i + 4 to i + 6) and 7.2 ± 0.3 (batches i + 7 to i + 9).aAfter initial batches recycled process water was used.bAfter metal precipitation.cAfter EDTA precipitation.

with 558 ± 53 g and in batches i + 7 to i + 9 with 714 ±36 g of lime. Precipitated metal hydroxides were removedfrom the alkalinized process water by centrifugation. Onaverage, 86 ± 2.2, 91 ± 1.9, 100 and 76 ± 1.6% of Pb, Zn,Fe and Cd, respectively, were removed. A volume of 60 Lof process water was led into Step 8 for the preparationof the recycled washing solution, as described below, andthe remaining 50 L was collected over three consecutivebatches as process water surplus and then led into Steps 6and 7 for EDTA and process water recovery. Process waterfor the preparation of recycled washing solution (60 L) inbatches i to i + 3 was left non-regulated at pH 12, in batchesi + 4 to i + 6 was adjusted to pH 9 using H2SO4 and inbatches i + 7 to i + 9 to pH 6.5.

Wastes discharged from the centrifuge (average 4 L perbatch with 40% of solids) were dried at 60◦C in a dehy-drator oven with a fan and vents for air circulation. Afterdrying, the wastes were combined with solid waste from theelectrochemical process, bitumen stabilized and disposed.

Step 6. Acidic EDTA precipitationThe collected surplus of the process water from Step 5

(average 150 L) was acidified to pH 2.20 ± 0.02 after everythird batch, using 660, 710 and 710 mL of 96% H2SO4 inbatches i + 3, i + 6 and i + 9, respectively. PrecipitatedEDTA was removed from the process water with filtra-tion (smaller press), dried at 60◦C to a constant weight and

in this way recycled. Protonation and acidic precipitation(H4EDTA) recycled on average 93 ± 1.8% of the chelant.

Step 7. Electrochemical cleansing of the processsolution

After the recovery of the major part of the EDTA fromthe process water in the acidic phase of the process (Step 6),the remaining, smaller part of the chelating agents stillpresent in the process solution up to this phase of the process(acidified process water, Table 1) was oxidatively degradedusing EAOP (Figure 3(d)). The pH of the process water(average 155 L) was adjusted in the mixing reactor to avalue of 5 by the addition of 124, 131 and 167 g of hydratedlime in batches i + 3, i + 6 and i + 9, respectively, and wasleft unregulated during the electrochemical process to riseto an average final pH of 6.51, 6.65 and 6.61, respectively.EAOP with a graphite anode was used in batch i + 3 andBDDA in batches i + 6 and i + 9. The EAOP was termi-nated when the concentration of Pb in the process waterdecreased below 5 mg L−1, assessed by XRF (Figure 3(a)).Graphite (only in batch i + 3) and precipitated PTMs wereseparated from the cleansed process water by filtration(smaller press), dried in a dehydrator oven and bitumen sta-bilized. The cleansed process water (Table 1, SupplementalMaterial) was split into three equal volumes and used forsoil rinsing in the next three batches, as described above(Step 3).

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0

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Pb

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)pH 10.7

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Rinsing solution (L)

(a)

(b)

(c)

Figure 2. Concentrations of Pb (a), Zn (b) and Cd (c) in vol-umes of process water (soils were extracted at pH 10.7 ± 0.4,pH 8.1 ± 0.2 and pH 7.2 ± 0.3, three consecutive batches wereextracted at each pH) obtained from the chamber filter press afterthe separation of the soil/sediment solid phase and washing solu-tion, and rinsing of the solid phase with raw and cleansed processwater from the previous batches and with fresh water. Error barsrepresent standard deviation from the mean value (n = 3).

At the end of the electrochemical treatment, the cath-odes were etched with 600 mL of 65% HNO3 and rinsed in5400 mL solution to dissolve and later measure the concen-tration of electro-deposited PTMs. An average 99.3 ± 0.1,98.8 ± 0.1 and 98.7 ± 0.1% of Pb, Zn and Cd, respec-tively, were separated by electro-deposition. The graphiteanodes were weighed before and after the treatment of threesoil batches to determine the amount of electro-corrodedmaterial. After drying, the amount of solid waste from theelectrochemical process was 290 g in each of batches i + 3,i + 6 and i + 9.

Step 8. Preparation of the recycled washing solutionApproximately one-third of the amount of H4EDTA

recycled (Step 6) each time in batches i + 3, i + 6 and i + 9(from the process solution collected from three consecutivebatches, as described above) was dissolved in batches i + 4

to i + 9 in the process water (60 L) obtained after centrifu-gation in Step 5 and with pH values set to 12, 9 and 6.5,as described above. Losses of EDTA during the process(absorption into the soil in Step 1 and oxidative degrada-tion by EAOP in Step 7) were replaced in this step by freshNa2EDTA to maintain the PTM extraction efficiency of therecycled washing solution close to the efficiency of fresh120 mmol kg−1 EDTA washing solution (Section 2.1). Inbatches i to i + 3, an average 1079 ± 124 g of fresh EDTAwas added, in batches i + 4 to i + 6 an average 898 ± 355 gof recycled and 601 g of fresh EDTA were added and inbatches i + 7 to i + 9 an average 861 ± 419 g of recycledand 601 g of fresh EDTA were added. After the additionof recycled and fresh EDTA and mixing with soil (Step 1),the pH of the washing solutions was established at finalaverages of 10.7, 8.1 and 7.2, as reported in Step 1.

3. Results and discussion3.1. Extraction of PTMs and major soil cationsSoil washing with 120 mmol EDTA kg−1 soil removed upto 72% and 66% of Pb and Cd, respectively, and up to25% of Zn (Table 2). The low extractability of Zn wasdue to the specific Zn association with non-labile soilfractions.[28,29] This is evident from the concentration oforally bioaccessible Zn which was much lower than the totalZn soil concentration (Tables 2 and 3). Other factors affect-ing extraction are metal mineralogy, desorption/dissolutionkinetics and soil conditions (pH, redox potential).

In order to evaluate soil remediation technologies mon-itoring tools indicative of actual contaminant toxicity areessential. An important form of exposure to PTMs is theaccidental ingestion of contaminated soil and inhalationof soil-dust particles. Oral bioaccessibility, measured bythe UBM method, represents the fraction of a contaminantthat is released from the soil matrix into solution by humandigestive fluids. Oral-bioaccessibility in the stomach phasewas reduced with remediation by 77%, 54% and 76% forPb, Zn and Cd, respectively, and by 97% and 96% for Pband Zn, respectively, in the intestinal phase. Bioccessibilityof Cd in the intestinal phase was brought under the level ofdetermination in the remediated soil (Table 3). The absorp-tion of PTMs into the blood takes place mainly in the smallintestine,[30] so PTMs available from the intestinal phasegive a better evaluation of potential risks of soil ingestionand inhalation. The remaining contaminants in remedi-ated soil were, therefore, unlikely to represent a significantexposure hazard for humans through the gastro-intestinaltract.

EDTA solubilized a significant amount of Fe from thesoil (Table 2), probably from poorly crystalline Fe oxy-hydroxides. Iron forms strong complexes with EDTA; thestability constant (log Ks) of EDTA complex formation withFe2+is 14.3 (at 25◦C and ionic strength (μ = 0.1) and withFe3+ even 25.1.[6] For comparison, the log Ks values of

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(a) (b)

(c) (d)

Figure 3. Concentrations of Pb (a), Zn (b), Cd (c) and EDTA (d) in the collected surplus of the process waters (from three consecutivebatches) during electrochemical treatment (expressed as increasing electric energy consumption) using a graphite anode (results from asingle treatment) and BDDA (average of measurements from two treatments).

Table 2. Initial concentrations of PTMs, Fe, Ca and Mn(minimum and maximum values from three consecutive soilbatches extracted at pH 10.7 ± 0.4, pH 8.1 ± 0.2 and pH7.2 ± 0.3) in original soil and residual concentrations ofmetals in remediated soil are shown.

Initialmin, max Residualmin, max Removedavrg(mg kg−1) (mg kg−1) (%)

pH 10.7Pb 5035, 5800 1418, 1587 72Zn 3150, 3444 2323, 2650 24Cd 19.6, 20.2 6.2, 7.9 66Fe 39,073, 42,285 38,453, 41,655 2Ca 81,204, 92,101 80,331, 90,188 2Mn 926, 1065 428, 484 54

pH 8.1Pb 6309, 6960 1778, 2014 72Zn 3345, 3797 2460, 2885 25Cd 30.1, 32.6 11, 12.3 63Fe 36,328, 38,932 34,806, 37,420 4Ca 82,855, 93,572 80,290, 89,851 3Mn 888, 964 466, 510 47

pH 7.2Pb 6443, 6896 1990, 2186 69Zn 2949, 3336 2472, 2694 22Cd 29.5, 31.9 9.3, 11.2 66Fe 38,230, 41,417 36,192, 39,392 5Ca 84,184, 94,575 80,763, 87,304 5Mn 926, 981 488, 588 44

Note: Average percentages of metal removal are calculated.

other cationic PTM complexes with EDTA are 18.8, 16.5and 16.4 for Pb, Zn and Cd, respectively.[6] Recycled wash-ing solution with an extraction pH of 10.7 contained up to388 mg L−1 of Fe (Table 1) and removed 2.5 times less Fethan a solution with pH 7.2 (max. 1042 mg L−1 Fe). Thiscan be explained by a sharp decline of Fe–EDTA com-plex stability in alkaline pH [31] and consequently lowerFe extraction.

In the scientific literature, mildly acidic and close to pH-neutral EDTA-washing solutions have often been reportedto be most effective for Pb and other PTM removal.[32] Thisis attributed to the high stability of PTM–EDTA complexesin this pH region [31] and the alkaline pH optimum of theircompetitive precipitation reactions.[33] Our results, how-ever, indicated that PTM extractions at pH 10.7, 8.1 and 7.2were almost equally effective (Table 2) and similar to PTMextraction efficiency at close to pH neutral reported in ourprevious study.[32] Due to the lower stability of FeEDTAcomplexes at higher pH [31] the soil-washing solution afterextraction at pH 7.2 contained up to 4.9 times more Fe thanafter extraction at pH 10.7 (Table 1). Since Fe competeswith PTMs for complexation with EDTA, this could neg-atively impact on PTM extraction at lower pH. EDTA isalso known to bind more strongly to the soil solid phase(especially to soil Fe-oxides) in acidic and neutral pH con-ditions [34] and this part of the EDTA was not available forextraction (described below). Probably, however, the high

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EDTA concentration (120 mmol EDTA kg−1 soil) that weused to assure robust operation minimized the effect of pHand other PTM extraction efficiency factors. At this highchelant concentration, Pb, Zn and Cd extraction reached aplateau (data not shown). Since most of the EDTA is recy-cled, the use of a high chelant concentration is possiblewithout a prohibitive increase in the soil remediation cost.

Papassiopi et al. [35] reported that in calcareous soils(such as that used in our study), the dissolution of calciteconsumes most of the available EDTA, resulting in a con-siderable loss of soil Ca. However, as shown in Table 2,up to 5% of initial soil Ca was removed after extraction atpH 7.2, while Ca removal at pH 10.7 was 2.5 times lower.This is the same trend as described above for Fe. The logKs of Ca–EDTA (10.7) is several magnitudes lower thanthat of PTM–EDTA and Fe–EDTA (data given above) andthese competitive complexation reactions presumably pre-vented extensive Ca complexation and extraction by EDTA.Soil extraction at pH 10.7 was above and at pH 7.2 and 8.1below pH 8.3 – the pH of the carbonate (calcite) dissolutionequilibrium [36] leading to proteolytic dissolution of car-bonates, which explains the higher Ca removal (Table 2).Liming of the process water (Step 5) could somewhat helpto preserve some soil Ca, although Ca removal from the soilfrom the same contaminated site was also fairly low in ourprevious study where the percentage of soil Ca removedafter four consecutive extractions with 40 mmol kg−1 was0.5%.[37] In contrast to Fe and Ca, the loss of Mn from soilwas substantial (Table 2). The log Ks of Mn–EDTA (13.6)is higher than that of Ca and lower than those of PTMsand Fe.[6] More Mn (19%) was extracted in alkaline (pH10.7) conditions than at pH 7.2. This is the opposite trendto that observed for Fe and Ca and can be explained by thehigh stability of Mn–EDTA complexes at alkaline pH com-pared to competitive cations, particularly of Pb.[31] Mn isan important essential element for plants and soil organ-isms and the loss should therefore be compensated to thesoil after remediation.

The amount of recycled (Step 6) and fresh EDTA thatwas added to the recycled washing solution (Step 8) tocompensate EDTA loss due to soil absorption in Step 1(described below) and oxidative degradation (Step 7) was

determined by small-scale soil extractions (Section 2.1).On average, the molar concentration of EDTA in recycledwashing solutions was 6% higher than that in fresh solution(127.3, 127.2 and 126.9 mmol EDTA kg−1 for extractionsat pH 10.7, 8.1 and 7.2, respectively). A slightly higherrecycled EDTA concentration was required since part ofthe EDTA (up to 1.2%, calculated from data in Table 2)was already complexed with PTMs and since the extrac-tion efficiency of Ca–EDTA is apparently somewhat lowerthan that of Na2–EDTA, although experimental evidence iscontradictory.[11]

3.2. Operational aspects of remediation technologyThe bottle-neck procedures are separation of Fe and PTMsfrom alkaline process water (Step 5) and cleansing of pro-cess water using EAOP (Step 7). Fe precipitated from thealkaline solution as an amorphous material, which wasimpossible to separate by conventional settling or filtration.More stringent conditions of settling by imposing gravita-tional force using a continuous separator centrifuge had tobe used. However, extensive Fe extraction by EDTA is notexpected to occur in all soil types. Fe species available forcomplexation with chelant are hindered in many soils bythe low aqueous solubility of Fe-bearing soil minerals.[38]Borggaard [39] reported that EDTA extraction of Fe presentas soil oxyhydroxides (i.e. goethite) was very slow andElliot and Shastri [40] reported that EDTA did not extractFe, due to the coordination of several surface Ca ions byhexadentate EDTA, resulting in passivation of the Fe oxidesurface.

Our previous study identified the electrolysis as the mosttime-consuming processes: almost 12 h (operational time)of EAOP were required to treat 115 L of acidified pro-cess water.[19] In the novel technology embodiment, wesolved this problem by applying different process water-management schemes (Steps 5 and 8). We collected 155 Lof process water surplus (Table 1) over three consecutivebatches i − 3 to i (Figure 1). This reduced the volume ofEAOP-treated water and the time of electrolysis for 55%per soil batch. In EAOP, the anode material is the mostimportant efficiency parameter and two different materials

Table 3. Concentrations of Pb, Zn and Cd (minimum and maximum values from consecutive soil batches extracted atpH 10.7 ± 0.4, pH 8.1 ± 0.2 and pH 7.2 ± 0.3) bioaccessible from original and remediated soil into the simulated stomachand intestinal phase are shown.

Original Remediated Reduction

Stomachmin, max Intestinemin, max Stomachmin, max Intestinemin, max Stomachavrg IntestineavrgPTMs (mg kg−1) (mg kg−1) (mg kg−1) (mg kg−1) (%) (%)

Pb 3497, 3877 315, 452 693, 859 9.8, 14.2 77 97Zn 119, 133 9.2, 20.4 52.2, 56.8 0.36, 0.65 54 96Cd 2.4, 2.6 0.33, 0.43 0.52, 0.76 LQ, LQ 76 /

Notes: Average percentages of PTMs bioaccessibility reduction are calculated.LQ, limit of quantification.

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were tested. BDDA is inert and provides extreme oxygenoverpotential (1.3 V) as a voltage window for hydroxyl radi-cal production before O2 evolution.[41] BDDA, however, isstill fairly expensive. Graphite is inexpensive, non-toxic anddoes not release metal ions back into the solution. The twoanode materials seemed almost equally effective in processwater cleansing (Figure 3). Although the initial concentra-tions of Pb and Zn in process waters were not the same,their removal rate for energy input was similar for bothtested anodes. Corrosion and loss of graphite from the anodesurface was extensive (Table 4), presumably boosting thegraphite efficiency by creating a new active surface for OHproduction. Various other anode materials, Pt and metaloxides (IrO2, RuO2, SnO2) on a titanium substrate, are alsoregularly used in EAOP and will be tested in further opti-mization. Another AOP could also be applied in Step 7. Acombination of ozone and UV has been demonstrated toprovide effective oxidative decomposition of EDTA.[42]

3.3. Materials and energy balance and costThe material and energy balance of the pilot-scale pro-cess is shown in Table 4. We measured the electric energyconsumption (and cost) of the process; the energy con-sumption of the apparatus and EAOP are shown sepa-rately. Since the water recycling process is complete, nowaste water was generated. The amount of fresh waterrequired was controlled by the difference between the

original and remediated soil humidity (av. 5.1 ± 0.3 and10.4 ± 0.2%, respectively) and water evaporation duringthe process.

Over nine soil remediation batches, an average of76 ± 2% of used EDTA was recycled. This is better than71% yield of recycled EDTA obtained in the preceding tech-nology embodiment.[19] A minor part of the EDTA waslost during electrolytic degradation in Step 7 (Figure 3(d))and most due to soil absorption. During soil washing,EDTA absorbs via re-absorption of metal complexes on soilminerals, especially crystalline iron oxide, mainly throughouter-sphere surface complexation.[43] We observed that20.4 ± 1.0%, 21.68 ± 0.44% and 22.3 ± 0.4% of EDTA inthe recycled washing solution was absorbed into the soilsolid phase after extraction (Step 1) at pH 10.7, 8.1 and7.2, respectively, and soil rinsing (Step 3). Lower EDTAabsorbance from more alkaline washing solutions can beexplained by the negative charge of soil iron oxide at a pHabove its point of zero charge at pH 7.7–9.3,[44] which hin-dered the absorption of equally negatively charged metal–EDTA complex.[45] Minor initial PTM–EDTA leachinginto the lysimeters was observed after the remediated soilswere examined in experimental plots (paper in preparation).

The combined weight of solid wastes from alkaline(Step 5) and electrochemical processes (Step 7) amountto 23–27 kg per ton of remediated soil (Table 4). Thedisposal cost of solid hazardous waste transportation anddisposal was assessed to be approximately 200¤ per ton.[46]

Table 4. Material and energy consumption of the soil remediation process using graphite and BDDA electrolysis and extractionat pH 10.7 ± 0.4 (soil batches i to i + 3), 8.1 ± 0.2 (batches i + 4 to i + 6) and 7.2 ± 0.3 (batches i + 7 to i + 9) and cost per tonof processed soil; solid and liquid waste generated and cost of their disposal.

Total Consumption/generation Costs per tonConsumables consumption/generation per ton of soil Cost of soil

EnergyApparatus 46.8a, 49.1b, 51.7c kWh 260.1a, 272.8b, 287.2c kWh 0.0545¤ kWh−1 14.2a, 14.9b, 15.7c¤Electrolysis 20.7d, 21.9e kWh 120.7d, 127.7e kWh 0.0545¤ kWh−1 6.6d, 7.0e¤

MaterialsGraphite 79 g 439 g 1.8¤ kg−1 0.8¤EDTA 2.7 kg 14.7 kg 1.5¤ kg−1f 22.1¤Water 165 L 765 L 0.5347¤ m−3 0.41¤H2SO4 1.21a, 1.38b, 1.46c kg 6.71a, 7.67b, 8.12c kg 0.187¤ kg−1f 1.26a, 1.43b, 1.51c¤Lime 1.04a, 1.67b, 2.14c kg 5.78a, 9.29b, 11.91c kg 0.095¤ kg−1f 0.55a, 0.88b, 1.13c¤Bitumen 3.70a, 4.05b, 4.32c kg 20.56a, 22.51b, 24.01c kg 0.411¤ kg−1f 8.45a, 9.25b, 9.87c¤

WasteToxic solid waste 4.1a, 4.5b, 4.8c kg 22.8a, 25.0b, 26.7c kg 0.2¤ kg−1h 4.6a, 5b, 5.3c¤Stabilized solid waste 7.8a, 8.6b, 9.1c kg 43.1a, 47.5b, 50.6c kg 0.04¤ kg−1h 1.7a, 1.9b, 2.0c¤g

aSoil extracted at pH 10.7.bSoil extracted at pH 8.1.cSoil extracted at pH 7.2.dGraphite electrode.eBDDA electrode.f Internet source (http://www.alibaba.com) average price of five sellers.gSolid waste was disposed as stabilized solid waste.hLocal provider.

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Table 5. Concentrations of Pb, Zn, Cd in the leachate from theprocess solid waste before and after stabilization with differentamounts of bitumen, assessed using TCLP and deionized waterextraction test.

Pbavrg Znavrg Cdavrg(mg L−1) (mg L−1) (mg L−1)

TCLPWaste 579 ± 37 129 ± 12 3.8 ± 0.5Waste + 0.7 g g−1 bitumen 386 ± 32 94 ± 11 1.9 ± 0.9Waste + 0.8 g g−1 bitumen 291 ± 35 70 ± 8 0.6 ± 0.1Waste + 0.9 g g−1 bitumen 0 ± 0 0 ± 0 0 ± 0H2OWaste 110 ± 16 6 ± 0.2 0.5 ± 0.1Waste + 0.7 g g−1 bitumen 83 ± 7 5 ± 0.3 0.3 ± 0.1Waste + 0.8 g g−1 bitumen 55 ± 10 3 ± 0.4 0.1 ± 0.01Waste + 0.9 g g−1 bitumen 0 ± 0 0 ± 0 0 ± 0

Note: Average concentrations and standard deviation of PTMs inTCLP and water extracts from wastes from nine soil remediationbatches are shown.

Alternatively, the combined solid wastes can be treatedand stabilized with bitumen as a thermoplast (Supplemen-tal Material). The mobility of the PTMs in the stabilizedmaterial was assessed by TCLP and a deionized water test.As shown in Table 5, thermal mixing of 0.9 kg of bitumenwith 1 kg of solid waste completely prevents PTM leach-ing, in both the TCLP solution and water, and thus madestabilized solid process wastes suitable for disposal as anon-hazardous material.

Direct use of the second process water (Figure 1) forrecycled washing solution (after Fe and PTMs separationin Step 5) and soil extraction at pH 10.7 (as shown above,the pH of the soil washing did not effect PTM removalefficiency) was the most cost-effective. Direct use with-out pH adjustment saved the operation in Step 8 and alsosome energy and materials, especially lime (Table 4). Thetotal costs of material and energy for this mode of oper-ation amount to 50.5 and 56.1¤ ton−1 for direct disposalof toxic solid wastes and for waste stabilization with bitu-men and disposal as a non-hazardous material, respectively.This is significantly less compared to 66.2¤ ton−1 for mate-rial and energy cost of remediation using initial technologyembodiment.[19] The total cost for soil remediation usingthe novel technology, which includes also a fixed cost forplant erection, site infrastructure, equipment, regulatoryrequirements, etc., was evaluated for a small, 6-ton of soilper day, remediation plant for the Meza Valley contami-nated site. Elements of fixed costs for novel soil-washingtechnology are given in Table 6 and elements of variablecosts including labour in Table 7. The total cost amountedto 299¤ ton−1 of soil (Table 8). The cost of technology isfavourable compared to 400¤ ton−1 for traditional soil ‘digand dump’.[47] In addition, the novel technology preservesthe soil as a productive natural substrate (SupplementalMaterial).

Table 6. Fixed costs for remediation plant with capacity6 ton soil day−1 (1500 ton year−1).

Fixed costs Cost (in 1.000 ¤)

Permitting, safety and regulatory 50Site characterization 25Characterization studies 30Bench-scale treatability tests 10Vendor selection/contracting 6Process design and optimization 25Site infrastructure requirements and

preparation50

Transport of equipment to the site 12Plant erectiona 985Plant decommissioning

(decontamination and transport)22

Total 1215

aIncludes: land purchase, building permits, processequipment, support process equipment, hall constructionand electrification, central control system, cost of financing,construction labour costs, start-up and commissioningprocedures, acceptance test run, insurance and property sell.

Table 7. Elements of variable costs for novel soil-washingtechnology.

Cost per year Cost per tonVariable costs (in 1.000 ¤) (¤)

Site excavation and soiltransport

25 16.7

Equipment lease 10 6.7Labour (three employees) 100 66.7Personnel protective

equipment2 1.3

Soil/process water treatmentFuel/electricity 31 20.8Water 1 0.4Chemical agents and material 38 25.2MaintenanceEquipment 34 22.7Property 17 11.4Sampling and chemical

analysis15 10.0

Disposal cost of treatmentprocess wastes

12 8.0

Total 285 189.8

Table 8. Total remediation costs for novel soil-washing tech-nology.

Total remediation cost Cost per ton (¤)

Variable costs 189.8Depreciation 40.5Profit 69.1Total 299.4

4. ConclusionThe goal of soil remediation is to reduce the risk thatpolluted soil poses to the environment and human health.

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Pilot-scale EDTA soil washing removed most of Pb and Cdfrom contaminated soil and to a great extent reduced oralbioavailability of all three metallic contaminants. Technol-ogy enables efficient EDTA and complete process waterrecycling. The efficiency of the recycled EDTA washingsolution to extract metals was pH independent over a widerange of neutral to alkaline pH and direct use (without pHadjustment) of recycled washing solution further simplifiedtechnology and saved some energy and materials. Inexpen-sive graphite and state-of-the-art BDDA anode were foundequally effective in the electrolytic removal of PTMs fromrecycled process water. The single process waste was solidmetallic material from alkaline and electrochemical steps.Stabilization with bitumen efficiently prevented leaching ofPTMs and enabled solid-waste disposal as non-hazardousmaterial.

The novel embodiment of remediation technology wascost-effective and treated soil preserved the function ofplant substrate. However, the results of this study arecase specific. The remediation and cost efficiency of thenovel soil-washing technology will be, therefore, furtherevaluated for soils with different properties and differentconcentrations and types of PTM contamination.

FundingThis work was supported by the Slovenian Research Agency[Grant J4-3609].

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