Extraction, Recovery, and Biostability of EDTA for Remediation of Heavy Metal-Contaminated Soil

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Extraction, Recovery, and Biostability of EDTA forRemediation of Heavy Metal-Contaminated SoilP. K. Andrew Hong a , Chelsea Li a , Shankha K. Banerji b & Tulsi Regmi ba Department of Civil and Environmental Engineering, University of Utah, Salt Lake City, UT84112b Department of Civil Engineering, University of Missouri, Columbia, MO 65211Published online: 24 Jun 2010.

To cite this article: P. K. Andrew Hong , Chelsea Li , Shankha K. Banerji & Tulsi Regmi (1999) Extraction, Recovery, andBiostability of EDTA for Remediation of Heavy Metal-Contaminated Soil, Journal of Soil Contamination, 8:1, 81-103

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81

Journal of Soil Contamination, 8(1):81–103 (1999)

Extraction, Recovery, andBiostability of EDTA forRemediation of Heavy

Metal-Contaminated Soil

P. K. Andrew Hong,1 Chelsea Li,1

Shankha K. Banerji,2 and TulsiRegmi2

1Department of Civil and EnvironmentalEngineering, University of Utah, Salt Lake City,UT 84112; 2Department of Civil Engineering,University of Missouri, Columbia, MO 65211

Chelation removal of heavy metals fromcontaminated soil is seen as a viableremediation technique. A useful chelatingagent should be strong, reusable, andbiostable during metal extraction and re-covery operations. This work tested theextraction, recovery, and biostability ofEDTA as a potential remediating agent.Parameters, including EDTA concentration,soil type, soil content, washing cycle, pre-cipitant concentration and type, and pH,were varied and tested during metal ex-traction and recovery operations. Factors,including EDTA concentration, aqueous and5% soil slurry, presence of Pb, acclimatedand unacclimated activated sludges, alongwith abiotic control, were varied and stud-ied in the biodegradation of EDTA. Theresults showed that EDTA was able toextract lead completely from the tested soils,amenable to recovery by addition of cat-ionic and anionic precipitants in the alka-line pH range, relatively biostable evenunder conditions very favorable toward bio-degradation. Thus, EDTA is a strong, re-coverable, and relatively biostable chelat-ing agent that has potential for soilremediation application.

KEY WORDS: chelation removal, heavy metals, contaminated soil, chelating agent.

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INTRODUCTION

EAVY metal contamination of soil is a common problem encountered atmany hazardous waste sites throughout the U.S. and in other industrialized

countries. In several National Priority List (NPL) sites, elevated concentrations oflead, chromium, cadmium, zinc, and mercury have been observed. The sources forheavy metal contamination of soils at these sites may include the followingactivities: mining, smelting, metal plating/finishing, battery production/recycling,agriculture/industrial chemical applications, incineration processes, and vehicleemissions. In addition, at a large number of army installations heavy metal con-tamination is also very common (Bricka and Jones, 1994).

Heavy metals are toxic to human beings and animals and pose a threat togroundwater supplies if they are not disposed of properly. In soil matrix, heavymetals are often strongly retained, with their adverse impact on the environmentand human health persisting for substantial periods. Unlike organic contaminantsthat can be destroyed by biodegradation, chemical oxidation, or incineration, metalcontaminants can remain on site and threaten environmental quality for a long timeor until they are removed. Physical removal of the metal contaminated soil to aResources Conservation and Recovery Act (RCRA) landfill is often practiced,which may be quite an expensive alternative.

Chelating extraction offers permanent removal of heavy metals from the con-taminated soil. The use of this process for soil remediation is relatively new,especially the component dealing with the recovery and reuse of the chelatingagent (Allen and Chen, 1993; Hong et al., 1995a, b; MaCauley and Hong, 1995;Hong and Chen, 1996).

Strong chelating agents such as EDTA extract heavy metals from contaminatedsoils more completely; however, an increasing challenge remains to recover boththe extracted metals and the chelating agents for reuse. Reuse of the chelator atleast three to four times is necessary for the process to be economical. Thus,chelation remediation with focus on recovery and reuse must be demonstrated.Laboratory tests thus far have shown that chelation extraction is viable for soilremediation and that only moderate strength chelators are recoverable (Chen et al.,1995; Chen and Hong, 1995). The work presented here intends to address aremaining challenge of using a strong, biostable, and yet recoverable chelator forextraction of heavy metals from contaminated soil. As the presence of numeroussoil microorganisms at the site may cause some biodegradation of the organicchelator compound, the biostability of the chelator must be addressed if recoveryand reuse are required.

MATERIALS AND METHODS

Extraction and Recovery Experiments

Distilled-deionized water (18 MΩ-cm) was obtained from a heat distiller (BarnsteadGlass Bi-Distiller, Barnstead) in connection with a 4-stage Milli-Q Plus system

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(Millipore) and was used throughout. Chelating agent EDTA (Sigma) was used asreceived. Soils used in the metal extraction part of the study were taken fromcontaminated sites in Salt Lake City, air-dried over 1 month, then passed througha 2-mm sieve. Soil properties were characterized and shown in Table 1A. Typicalexperiments were conducted in 125-ml glass Erlenmeyer flasks using a batchsolution volume (V) of 100 ml. All flasks were sealed with stoppers to reduce gasexchange with the atmosphere during experiments. All pH adjustments wereperformed by addition of either a 5 M HNO3 or 5 M NaOH solution. pH measure-ments were with an Orion model SA 720 pH meter. Stock metal solutions (1000mg/l) were prepared for instrument calibrations according to ASTM Methods(ASTM Annual Standards D3559, D1688, D3557, and D1691 for Pb, Cu, Cd, andZn, respectively). A gyratory shaker table (New Brunswick Scientific Co., ModelG-2) provided agitation during extraction procedures. The soil was kept in suspen-sion by operating the shaker table at 260 rpm. All experiments were conducted atthe room temperature of 23 ± 1°C. Total dissolved metal concentrations (MeT)were measured in aliquots withdrawn from the reaction mixtures and filteredthrough a 0.45-µm filter (Gelman Sciences sterile aerodisc), then acidified withnitric acid. Metal analyses were by atomic absorption (AA) spectrometry (PerkinElmer Model 280) using ASTM methods. Whenever available, standard proce-dures were followed for other laboratory procedures (ASTM Annual Standards;APHA, 1995; U.S.EPA, 1979; 1986).

Metal extraction and recovery experiments were conducted varying the soil typeand contamination level (i.e., sandy loam, 15,500 mg Pb/kg soil; and loamy sand,2370 mg Pb/kg soil), EDTA concentration (3 to 50 mM), soil content (5 to 40%slurry), washing cycle (1 to 6), precipitant concentration (3 to 10 mM Na2S withand without Ca(OH)2), and pH (recovery at 6.5 to 11). For extraction, a measuredamount of soil was added to an EDTA solution of which the pH was initiallyadjusted to between 7.0 and 7.5. The slurry was continuously maintained insuspension by a shaker table. After an extraction period of 4 h, the slurry wascentrifuged, decanted, filtered, and the resulting aqueous solution analyzed forvarious metal contents (e.g., Pb, Cu, Zn, Fe, Ca). For recovery, that is, separationof metals from EDTA after extraction, the complex solution was added with ameasured amount of Na2S solid and the solution pH adjusted to pH 9 as necessary.The solution was continually agitated and allowed to precipitate for 1 h. It was thencentrifuged, decanted, filtered, and analyzed for various metal contents remainingin the aqueous phase. At this point, the aqueous phase contained EDTA that couldbe recycled. For experiments using reclaimed EDTA, the pH of the recoveredsolution was adjusted to about 7 and was added with soil to start another extractionand recovery cycle.

Results are presented by plotting the extracted metal concentration in theaqueous solution or the remaining metal concentration in the aqueous solution afterthe recovery process. Extraction efficiencies were calculated by taking the ratio ofthe extracted metal amount in solution to the total available (as characterized by

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10% HNO3, 60°C, 8 h) in the amount of soil used. Separation efficiencies werecalculated using the metal amount in the extraction solution before and the recov-ery process.

Biodegradation Experiments

Materials. EDTA (99% pure) was purchased from Fisher Scientific, St. Louis,MO. Ingredients for PTYG (peptone, tryptone, yeast extract, and glucose-reagentgrade) media were also purchased from Fisher Scientific, St. Louis, MO. The basalsalt media consisted of a mixture of the following chemicals: (NH4)2NO3, K2HPO4,KH2PO4, MgSO4.7H2O, MnCl2. 4H2O, CaCl2.2H2O, and FeSO4. These salts wereof reagent grade and procured from Fisher Scientific. The exact recipe for thePTYG media was as follows: peptone, 0.25 g; tryptone, 0.25 g; yeast extract, 0.5 g;glucose, 0.5 g; MgSO4.7 H2O, 0.6 g; and CaCl.2H2O, 0.6 g in 1-L distilled water.To provide needed salts for the growth of aerobic microorganisms, a basal saltmedia recipe was used that included: (NH4) NO3, 0.4 g; K2HPO4, 0.1 g; KH2PO4,0.05 g; MgSO4.7H2O, 0.005 g; MnCl2.4H2O, 0.02 g; CaCl2.2H2O, 0.2 g; FeSO4,0.005 g in 1 L distilled water. The ingredients were kept the same to maintainidentical growth conditions. Lead was used as a typical heavy metal for theexperiments. Lead nitrate [Pb(NO3)2] was the source of lead. It was purchased fromFisher Scientific.

Microorganisms. The seed microorganisms were obtained from the WastewaterTreatment Plant, Columbia, MO. The sludge was withdrawn from the recycle lineof the secondary activated sludge treatment unit. As soon as the sludge was broughtto the laboratory, the enrichment of the microbial population was initiated in a 5-Lbatch reactor in the presence of PTYG and basal salt media. The initial COD of themedia was about 500 mg/l, with COD:N:P = 100:8:1. The pH level in the reactorwas kept between 7.0 to 7.5 by the addition of 5N HCL or 5N NaOH. The unitswere aerated using filtered compressed air through air diffusers. The aeration wasstopped daily for 1 h, the cells (sludge) were allowed to settle down, about 2 to2.5 L of supernatant was discarded, and fresh media was added. After a few daysa fixed amount of cells (sludge) was wasted from the unit in order to maintain aconstant initial mixed liquor suspended solids. The wasted sludge from this unitwas used for experimentation or for developing cultures acclimated to EDTA.

The acclimation process for EDTA was initiated in a separate 4-l batch reactor.It was accomplished by feeding progressively higher amounts of EDTA to thereactor in place of PTYG media. Initially, the COD loading was 500 mg/l (50 mg/las EDTA and 450 mg/l as PTYG media) with basal salts added to supplement thenutrient needs. This loading was continued on a daily basis with sludge andsupernatant wastage, and feed addition for a 2 week period. The suspended solids,pH, COD, and EDTA concentrations were monitored daily. After that the EDTA

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concentration was raised to 100 mg/l with 400 mg/l PTYG media (on a CODbasis). This daily loading (and wastage) continued for 2 weeks. The biomassproduction, COD reduction, and EDTA concentration was monitored periodically.Subsequently, the EDTA loading was increased in steps to 500 mg/l as COD withno PTYG addition. This sludge was acclimated to the EDTA molecule over aperiod of several weeks and was used for biodegradation experiments.

Soil in Biodegradation Study. In the biodegradation part of the study, the soilused for experiments simulating metal contamination was Menfro series silty loam.This soil consists of deep, well-drained soils on uplands. The soil was sievedthrough 2-mm screen prior to use. The physical and chemical properties of the soilare given in the Table 1B (Regmi, 1996).

Analytical Methods. Biomass determinations in the batch reactors were made bydetermining total suspended solids (SS) using the procedures outlined in theStandard Methods (APHA et al., 1995). Chemical oxygen demand (COD) test wasconducted using Hach’s procedure (Hach Co., Loveland, CO). Their method isvery similar to the Method 5220 D in the Standard Methods (APHA, 1995).

Concentrations of complexed and uncomplexed EDTA were determined by aspectrophotometric method, as described by Flaschka (1959). The method dependson the fact that EDTA forms a stable complex with chromium ion and after heatingproduces a distinctive color that is proportional to the concentration of EDTA.EDTA forms a more stable complex with chromium than with lead ions. Thesamples for EDTA determination were prepared by filtering mixed liquor througha 0.45-µm filter. Four ml of the filtrate was added to a test tube containing 1 mlof 0.1 M Cr(NO3)3 solution. The pH of the solution was maintained between 3 and4. The test tube was tightly closed and digested in an oven at 100°C for 15 min. Ablank sample containing 4 ml of deionized water was treated in the same manner.After cooling, the digested sample was placed in a spectrophotometer to measurethe absorbance at 555 nm wavelength against the blank.

In all experiments where EDTA removals were being measured by the loss ofEDTA using the compleximetric method, simultaneous COD analyses were alsoconducted to examine the loss of the substrate.

Biodegradation Test Procedure.Known amounts of biomass from acclimatedand nonacclimated batch cultures grown on PTYG media were placed in a 125-mlshake flasks to which different concentrations of EDTA solutions were added. TheCOD: N:P ratio in the flasks were adjusted to 100:8:1, as mentioned earlier. Allexperiments were carried out at room temperature (20 to 25°C). The pH of themedia was adjusted to 7.0 to 7.5 by adding HCl or NaOH solution. Duplicate flasksat one EDTA concentration were placed on a shaker and shaken at 200 rpm. Ablank and abiotic flask with 5 g/l Hg2Cl2 were also included for all biodegradationexperiments. The incubation of the flasks was carried out in the dark to avoid

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photodegradation. Ten ml of aliquots were withdrawn from the flasks at differenttimes and filtered through 0.45-µm filter. The filtrate COD and EDTA weredetermined for each sample. In addition, the amounts of biomass in the flasks weremeasured by determining the SS in the system.

Some biodegradation experiments were also conducted in 4-l batch reactorswhere the acclimated biomass, EDTA, basal salts, and tap water were aerated bypassing compressed air through air stones. The incubation, sampling, and other testprocedures were similar to that of the shake flasks described earlier.

A shake flask experiment was conducted where the chelator was first complexedwith lead (EDTA : lead = 2:1 mole ratio) for 24 h at a pH between 3.5 to 5.0. ThepH of the flask was then adjusted to 7.0 and a biodegradation test was conductedusing the procedures described earlier. Another set of shake flask experimentswere conducted where EDTA biodegradation was determined in the presence of5% soil slurry with or without lead-EDTA complexation. In the case of complexedchelator system, 5 g of soil was added to a flask containing 50 ml of a knownconcentration of lead nitrate solution. It was equilibrated for 24 h on a shaker. ThepH was decreased to 3.5 and an appropriate amount of EDTA was added. The flaskcontents were shaken for 24 h to ensure complexation of the metal. The pH wasincreased to 7.0, basal salts and acclimated biomass cultures were added to initiatethe biodegradation experiments, as described earlier. A blank and abiotic controlwas also included.

RESULTS AND DISCUSSION

Chelation Extraction and Recovery

The effects of soil type, contamination level, EDTA concentration, soil slurrycontent, washing cycle, precipitant concentrations, and pH on chelation extractionand recovery processes were experimentally studied. Figure 1 shows extraction ofPb, Zn, and Cu metals from a contaminated sandy loam soil using 3 to 20 mMEDTA solutions. The results showed that after one cycle of washing up to 91, 60,and 56% of Pb, Zn, and Cu, respectively, were extracted from the soil. Theextraction efficiency of Pb, for example, was calculated by the ratio of extractedPb in solution to the total amount of Pb in the 5 g of soil used in 100 ml (i.e., 3.4mM/3.74 mM = 91%). It should be noted that 5 g of the sandy loam (15,500 mgPb/kg soil, Table 1A) amounted to 3.74 mM Pb in 100 ml. The extracted amountof Pb, the major contaminant, was proportional to the employed EDTA concentra-tion.

Figure 2 shows extraction of Pb, Zn, and Cu metals from a less contaminatedloamy sand using 10 to 50 mM EDTA solutions. Also shown were extractedambient cations Fe and Ca. The removals after one cycle of washing with 50 mM

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FIGURE 1

Extraction of metals from a sandy loam soil using different EDTA concentrations. (Metalconcentrations in this and all other figures indicated metal contents in the aqueous phase.)

EDTA were 88, 13, and 36% for Pb, Zn, and Cu, respectively (based on soilcontamination levels for loamy sand as shown in Table 1A).

Figure 3 shows extraction and separation results of metals using 20 mMEDTA for slurries of different soil suspension contents. Although increasingextracted amounts were observed with increasing soil contents, the removal ratio(i.e., extracted/total available) was lower with higher suspension content. The opensymbols show metal concentrations remaining in the solution after the extractsolution was treated with 50 mM Na2S. The separation of metals from EDTA waspromoted by the presence of Na2S, resulting in > 99%, 70 to 74%, and 93 to 98%recovery of extracted Pb, Zn, and Cu, respectively. For example, the recoveryefficiency of Zn at 40% soil slurry was computed to be 72% using the aqueous Znconcentration before (0.3 mM) and after (0.083 mM) Na2S treatment, that is, 1 to0.083 mM/0.30 mM = 72%.

Figure 4a shows extraction performance of EDTA (50 mM) on removal ofcontaminants Pb, Zn, and Cu from soil during successive washings using in eachcycle reclaimed EDTA solution from its preceding cycle. As expected, the amount

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FIGURE 2

Extraction of metals from a loamy sand using different EDTA concentrations.

of extracted metal decreased with each successive washing cycle, as the availableamount in the soil decreased. The cumulative removal over six washing cyclesranged from 100% for lead, 14% for Zn, and 48% for Cu, which indicatedsignificant increases in metal removal efficiencies when compared with one wash-ing cycle. The figure also shows metal recovery of the first three cycles, rangingfrom 73 to 99%, based on calculations described previously.

Figure 4b shows similar extraction results using a weaker EDTA solution(20 mM). Cumulative amounts of 100, 11, and 33% of Pb, Zn, and Cu, respec-tively, were removed from soil over seven washing cycles.

The extraction of lead from this authentic contaminated soil was apparentlymuch more complete than of zinc or copper. Chen and Hong (1995) studied thechelation extraction of Pb and Cu from soil using two different chelators andobserved similarly high extraction effectiveness for these metals. Chen et al.(1995) performed equilibrium modeling and computation and found the techniqueuseful in determining the extraction potential of different chelators toward differ-ent metals. In the present case, chemical solubility data alone cannot account for

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FIGURE 3

Extraction of metals using EDTA and recovery using Na2S as a function of soil content inthe slurry. Solid symbols with solid lines indicate extracted metal amounts in the aqueousphase, and open symbols with dash lines indicate metal amounts remaining in the aqueousphase following precipitation.

the discrepancy in extraction. Equilibrium modeling cannot predict the extractiondifferences without a sufficient understanding of the complex solution-soil-chela-tor processes that require mineralogical details of the soil as well as mechanisticdetails regarding the metal-soil, complex-soil, chelator-soil, and metal-chelatorinteractions. The metals appear to be bound to different degrees that may haveresulted from age, minerals, and other physical factors. However, with repeatedwashings EDTA seems to be capable of rather completely releasing Pb, the majorcontaminant metal in this authentic contaminated soil.

Experiments were conducted to demonstrate the effectiveness of recoveringmixed metals and chelator from their complex solution. Figure 5 shows results ofrecovering mixed metals (Pb, Cu, and Cd) and EDTA at different pH conditions(6.5 to 11) and Na2S dosages (3 to 10 mM). The mixed complex solutions wereprepared by mixing 1 mM each of Pb, Cd, and Cu, and 5 mM EDTA (i.e.,[metal]:[EDTA] = 1:5 for each metal in each solution). The solutions were added

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FIGURE 4A

FIGURE 4B

Extraction of metals during consecutive washings using (a) 50 mM, and (b) 20 mM of EDTA.Where shown, solid symbols with solid lines indicate extracted metal amounts in theaqueous phase, and open symbols with dash lines indicate metal amounts remaining in theaqueous phase following precipitation.

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FIGURE 5

Recovery of metals using different pH and Na2S concentrations. Metal concentrationsindicated those remaining in the aqueous phase after the recovery process.

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with different amounts of Na2S at pH 6.5 to 11. The results were plotted asremaining metal concentrations in the aqueous solution after the Na2S treatment atdifferent pH. Thus, a lower metal content in the supernatant indicated better metalrecovery efficiency. The metals were most readily recovered in this decreasingorder: Cu, Cd, and Pb, and they all were recoverable under appropriate Na2S andpH conditions. It appeared that a total sulfide concentration (i.e., ST = [H2S] +[HS–] + [S2–] ∼ 5 mM) at a dosage ratio of 1 : 1 = ST : EDTA would be sufficientto recover chelated Cu and Cd almost completely over a wide pH range (6.5 to 11),whereas the same amount of sulfide would require pH > 10 to achieve 97% (e.g.,at pH 10.5) recovery of Pb. It appears possible to use lower concentration of sulfide(e.g., Cd) to promote acceptable recovery provided it is carried out at an alkalinepH.

Figure 6 shows results of extracting and recovering Pb, Zn, and Cu contaminantsfrom the sandy loam using reclaimed EDTA solution (3 to 10 mM initial EDTAconcentrations) in successive washing cycles (using a fresh soil sample in eachcycle). The extraction was performed at pH 7 for 4 h and recovery performed atpH 12 for 1 h. To aid in recovery, precipitating agents Na2S and Ca(OH)2 wereadded at 5 mM each. Ca(OH)2 was used as a cationic precipitant that provided Ca2+

ions to compete for EDTA replacing the chelated contaminant metal and encourageits release from the chelator, while Na2S was used as an anionic precipitant thatprovided HS–/S2– anions to compete with EDTA for the precipitation of thecontaminant metal. The contaminant metals were readily recovered in this decreas-ing order: Cu, Zn, and Pb, and they all yield good recovery results (typically over90%) under appropriate chelator-to-precipitants ratios (e.g., [precipitant] : [EDTA]∼ 1 : 1). All extraction and recovery efficiencies were determined based on aqueousconcentrations before and after an extraction or recovery step. There appeared tobe a small to modest loss of removal efficiency during the successive applicationof reclaimed EDTA. For example, using a 10-mM EDTA solution, the removalswere 42%, 47%, and 30% for Pb, 45%, 62%, and 38% for Zn, and 40%, 42%, and22% for Cu in the first, second, and third applications of reclaimed EDTA. Therecovery of Zn and Cu typically exceeded 95% in each cycle under most testconditions, and ranged from 80 to 100% when a concentration ratio of [precipitant]: [EDTA] ∼ 1 : 1 was used.

The results show that:

1. Better extraction performance can be achieved with higher EDTA concen-tration (e.g., 50 mM) and with more washing cycles

2. Metal extraction can be performed with a wide soil slurry content, forexample, 5 to 40% soil slurry, with a diminishing efficiency with increasingsolid content

3. EDTA can be reclaimed using a slight excess (mole basis) of Na2S precipi-tant at moderately alkaline condition (e.g., pH 10), and reused over severalcycles

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FIGURE 6

Recovery of metals and reuse of reclaimed EDTA during consecutive extractions usingdifferent EDTA concentrations. Metal concentrations indicated those remaining in theaqueous phase after the recovery process. (1st EX = after the first extraction process; 1stSP = after the first separation process, similarly for the 2nd EX, 2nd EP, etc.)

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Biodegradation Experiments

EDTA Biodegradation by Unacclimated Microorganisms. Figure 7a shows theCOD removal with time and Figure 7b shows the EDTA removal with time in theshake flask experiments using unacclimated microorgainsims that were grown onPTYG media. The concentration of EDTA in the shake flasks varied from 50 mg/lto 500 mg/l (0.15 to1.75 mM). It can be seen that there was no decrease in theconcentration of EDTA in the flasks for up to 60 h. The lack of availability of easilydegradable organic matter caused the cells to utilize endogenously cellular materials,which caused a decrease in the biomass concentration (SS) with time. In addition,the soluble COD values increased with time, which was also indicative ofsolubilization of biomass materials.

EDTA Biodegradation by Acclimated Microorganisms. In this experiment, thebiomass used were acclimated to the EDTA molecule over several weeks. TheEDTA concentrations tested varied from 100 mg/l to 1000 mg/l (0.3 to 3.5 mM).Figure 8 and Table 2 show the results of this experiment. At 100 mg/l EDTAconcentration about 52% was biodegraded in 10 days. There was an initial lag inthe biodegradation rate, but after 3 to 4 days there was a consistent EDTA removal.The removal of EDTA was due to biodegradation and it was evident because of thefollowing observations: the abiotic flask showed no sorption or removal of EDTAby abiotic means; photodegradation of the compound was eliminated because theflasks were incubated in the dark; the consistent decrease of soluble organic matteras measured by COD followed the same pattern as the removal of EDTA andEDTA was the only organic substrate added to the solution; and the assimilationof EDTA was indicated by a growth of the biomass present.

The biodegradation of EDTA at 300 mg/l was also quite evident and followeda similar pattern as the system with 100 mg/l EDTA. However, the overall removalwas only about 32% in 10 days. As the concentration of EDTA was increased to500 and 1000 mg/l, the biodegradation rate decreased significantly. There was stillsome amount of EDTA biodegradation occurring at 500 mg/l EDTA, but at 1000mg/l EDTA concentration biological activities were inhibited. The SS and CODdata supported these conclusions also. A decrease in biomass at 1000 mg/l EDTAconcentration and increase in soluble COD values with time were indicative of cellsolubilization (and decay) under these conditions.

EDTA Biodegradation by Acclimated Microorganisms in 5% Soil Slurry. Inthis experiment EDTA biodegradation was measured in the presence of 5% soilslurry in shake flasks. The results were quite similar to that observed under theaqueous conditions. Figure 9 and Table 3 show these results. EDTA biodegradationat concentration 100 mg/l was about 60% in 10 days with some initial lag. Theremoval of EDTA at concentrations of 300 and 500 mg/l was lower (31 and 21%,respectively). The biodegradation of EDTA at a concentration of 1000 mg/l was

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FIGURE 7B

Degradation of EDTA with unacclimated microbial populations.

FIGURE 7A

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FIGURE 8

Biodegradation of EDTA by acclimated mixed culture in aqueous solution.

inhibited as was observed in the aqueous system. The cell solubilization was alsoobserved in this experiment at EDTA concentration of 1000 mg/l. The solubleCOD of the system increased with time with no decrease in EDTA concentration.The biomass for the system was difficult to determine as the presence of soilnegated the use of SS as an indicator of the biomass.

Biodegradation of Pb-EDTA Complex (1:2) in 5% Soil Slurry by AcclimatedMicroorganisms. In this experiment Pb-EDTA complex (1:2 molar ratio) wasadded to a 5% soil slurry containing acclimated microorganisms and basal salts.The concentration of EDTA was about 100 mg/l (0.3 mM) and 300 mg/l (0.9 mM).Figure 10 and Table 4 show the results of this experiment. The EDTA at aconcentration of 100 mg/l was biodegraded by about 61% in 10 days with someinitial lag for the first 2 days. The COD decrease during the test period in thesystem was about 42%. The removal of EDTA at the concentration of 300 mg/l wasless than (∼ 34%) the system with 100 mg/l EDTA. The corresponding CODremoval was about 21%. The complexed lead present in the system did not causeinhibition to the biodegradation process. The microorganism growth during EDTA

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FIGURE 9

Biodegradation of EDTA in soil slurry by acclimated mixed culture.

metabolism could not be evaluated by measuring the increase in the SS concentrationbecause of the presence of soil.

The results presented here show that in aqueous cultures EDTA up to a concen-tration of 500 mg/l could not be degraded by unacclimated cultures. Only under themost favorable conditions with acclimated cultures some EDTA biodegradation(3 to 50%) was observed. Madsen and Alexander (1985) also found that wastewa-ter microorganisms could not degrade EDTA in low concentrations (< 1 mg/l) in72 days even with supplementary sodium acetate as a carbon source. The resultsreported by Tiedje (1975) showed that over a 5 to 8 week period, EDTA wasbiodegraded under aerobic conditions by soil microorganisms up to 28% in thepresence of organic amendments. These results were somewhat similar to ourfindings that EDTA is fairly resistant to biodegradation. He also found that EDTAchelates of Cu, Cd, Zn, Mn, Ca, and Fe were degraded equally under the testconditions, which also matches our results with lead chelate using acclimatedcultures.

Kari and Giger (1996) found that very little EDTA was being removed bybiological or chemical means in wastewater plants. Fe(III)-EDTA complex was

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FIGURE 10

Biodegradation of Pb-EDTA complex (1:2) in aqueous solution by acclimated mixedculture.

quickly degraded by photolysis, while other EDTA species were very slowlytransformed.

The results of Nortemann (1992) are interesting because he found certain mixedcultures isolated from sewage that were capable of utilizing EDTA as a sole carbonand nitrogen source. The growth of the culture with EDTA was slightly reducedin the presence of high MgSO4 in the media, but FeCl3, MnCl2, or CaCl2 did notcause any decrease in growth rate. These results indicate that under certain condi-tions there are species that can biodegrade EDTA completely. However, in oursystem complete biodegradation was not observed.

From the point of view of reusing the reclaimed EDTA for soil metal removal,the biostability of the EDTA molecule is quite helpful as most soil microbialspecies would not be able to degrade it as they would not be acclimated to thismolecule. Thus, it would be possible to reuse EDTA for several cycles of soilremediation.

CONCLUSIONS

The extraction, recovery, and biostability of EDTA relevant to soil remediationwere studied. EDTA extracted lead effectively from soil and was amenable to

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TABLE 4Biodegradation of Pb-EDTA complex (1:2) in 5% SoilSlurry with EDTA Concentration of 100 and 300 mg/l

Initial EDTA conc.- 100 mg/l Initial EDTA conc.- 300 mg/l

EDTA EDTATime, day SS mg/l COD mg/l mg/l SS mg/l COD mg/l mg/l

0 2305 113 110 2305 325 3101 — — 115 — — 3122 — — 100 — — 3103 — — 98 — — 3054 — — 95 — — 2855 — — 88 — — 2786 — — 85 — — 2577 — — 75 — — 2458 — — 68 — — 2359 — — 54 — — 220

10 — 65 43 — 258 205

recovery with the use of cationic and anionic precipitants such as calcium sulfideions. It is capable of being reused several times with some loss of extractionactivity. EDTA is relatively biostable and is only degraded partially under special-ized condition, that is, via acclimated culture. At the actual site, the soil microor-ganisms are unacclimated to EDTA and have very little capability to biodegradeit, which would be good from the point of view of reuse. Thus, the reuse potentialof EDTA for metal extraction from soil is high and this make its use economicallyfavorable. As a strong, reusable, yet relatively biostable chelating agent, EDTA ispotentially valuable for remediation of soils contaminated with heavy metals.

An apparent application of chelation technology is in the remediation of metal-contaminated soils, in which the soil is brought into contact with a chelator solutionunder controlled conditions, including contact time, pH, soil suspension content,chelator, and concentration. The extraction process is followed by the recoveryprocess in which the complex solution is separated from the soil and treated furtherunder appropriate conditions, including contact time, pH, and precipitant concen-tration. The recovered, relatively biostable chelator can then be reused in the nextextraction-recovery cycle.

ACKNOWLEDGMENTS

We thank the U.S. EPA Regions 7 and 8 Great Plains/Rocky Mountain HazardousSubstance Research Center for funding of this collaborative work.

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REFERENCES

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APHA. 1995. Standard Methods for the Examination of Water and Wastewater, American PublicHealth Assoc., Washington, D.C.

ASTM Standards (Annual), Philadelphia, PA, American Society for Testing and Materials.Bricka, R., Williford, C., and Jones, L. 1994. Heavy metal soil contamination at U.S. Army

installations: Proposed research and strategy for technology Development, Technical ReportIRRP-94-1, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.

Chen, T.-C. and Hong, A. 1995. Chelating extraction of lead and copper from an authentic contami-nated soil using N-(2-acetamido)iminodiacetic acid and S-carboxymethyl-L-cysteine. J. Haz.Mater., 41(1–2), 147–160.

Chen, T.C. Macauley, E., and Hong, A. 1995. Selection and test of effective chelators for removalof heavy metals from contaminated soils. Canad J. Civil Eng., 22, 1185–1197.

Flaschka. 1959. EDTA Titration — An Introduction to the Theory and Practice, New York, PergamonPress.

Hong, A., Chen, T. C., and Okey, R.1995a. Chelating extraction of copper from soil withS-carboxymethylcysteine. Water Environ. Res., 67, 971–978.

Hong, A., Chen, T. C., and Okey, R. 1995b. Chelating extraction of zinc from soil with N-(2-acetamido)iminodiacetic acid. ACS Symposium Series 607, Chap. 17, 207–223.

Hong, A., and Chen, T.-C. 1996. Extractive recovery of cadmium from soil usingpyridine2,6dicarboxylic acid. Water, Air, Soil Pollut., 86, 335–346.

Kari, F. G. and Giger, W. 1996. Speciation and fate of ethylenediametetaacetate (EDTA) in municipalwastewater treatment. Water Res., 30(1), 122–134.

Macauley, E. and Hong, A. 1995. Chelation Extraction of Lead from Soil Using Pyridine-2,6-dicarboxylic Acid. J. Haz. Mater., 40(3), 257–270.

Madsen, E. L., and Alexander, M. 1985. Effects of chemical speciation on the mineralization oforganic compounds by microorganisms. Appl. Environ. Microbiol., 50(2), 342–349.

Nortemann, B. 1992. Total degradtion of EDTA by mixed cultures and a bacterial isolate. Appl.Environ. Micrbiol., 58 (2), 671–676.

Regmi, T. 1996. Biodegradation of Chelating Agents for Removal of Metals from ContaminatedSoils. M.S. thesis, Department of Civil Engineering, University of Missouri, Columbia, MO.

Tiedje, J. Em., 1975. Microbial degradtion of etylenediaminotetraacetate in soils and sedimants.Appl. Environ. Microbiol., 30(2), 327–329.

U.S.EPA. 1979. Methods for the Chemical Analysis of Water and Wastes. EPA600/4–79-020.U.S.EPA. 1986. Testing Methods for Evaluating Solid Waste (SW-846). Vol. IA, IB, IC, and II, 3rd

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Documents

Extraction, Recovery, and Biostability of EDTA for Remediation of Heavy Metal-Contaminated Soil