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UV treatment of EDTA extractants
Chemosphere 63 (2006) 1736–1743 www.elsevier.com/locate/chemosphere
Heap leaching of Pb and Zn contaminated soil using ozone/UV treatment of EDTA extractants Nezˇa Finzˇgar, Domen Lesˇtan
*
Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia Received 26 May 2005; received in revised form 19 September 2005; accepted 22 September 2005 Available online 8 November 2005
Abstract The feasibility of a novel EDTA-based soil heap leaching method with treatment and reuse of extractants in a closed process loop was evaluated on a laboratory scale. Ozone and UV irradiation were used for oxidative decomposition of EDTA-metal complexes in extractants from Pb (1243 mg kg1) and Zn (1190 mg kg1) contaminated soil. Released metals were absorbed in a commercial metal absorbent Slovakite. Six-consecutive additions of 2.5 mmol kg1 EDTA (total 15 mmol kg1 EDTA) removed 49.6 ± 0.6% and 19.7 ± 1.7% of initial total Pb and Zn from soil (4.6 kg) packed in 22 cm high columns. The efficiency of extraction was similar to small-scale simulations of heap leaching (150 g of soil), where EDTA used in the same manner removed 49.7 ± 1.0% and 13.7 ± 0.4% of Pb and Zn. The new heap leaching method produced discharge extractant with fairly low final concentrations of Pb, Zn and EDTA (1.98 ± 2.17 mg l1, 4.55 ± 2.36 mg l1, and 0.05 ± 0.04 mM, respectively), which could presumably be reduced even further with continuation of treatment. The results of our study indicate that for soils contaminated primarily with Pb, treating the EDTA extractants with ozone/UV and reuse of extractants enables efficient soil heap leaching with very little or no wastewater generation, easy control over emissions, and lowers the requirements for process water. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Pb and Zn contamination; Soil heap leaching; EDTA; Ozone; UV
1. Introduction Lead contamination of soils is a common problem throughout the world. Soil contamination is seldom mono-metallic. In soils contaminated primarily with Pb, Zn and other heavy metals are usually also present in elevated concentrations. Current remediation activi* Corresponding author. Address: Center for Soil and Environmental Science, Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia. Tel.: +386 01 423 1161; fax: +386 01 423 1088. E-mail address: [email protected] (D. Lesˇtan).
ties have involved excavation and removal of the contaminated soil, immobilization and containment of heavy metals in the soil by mixing or injecting agents such as cement, lime and different phosphates, electrokinetic mobilization and removal of heavy metals from the soil by precipitation on the electrodes, phytoextraction of metals using metal-accumulating plants, and soil washing and flushing. Soil washing involves the separation of contaminants from soil solids by solubilizing them in a washing solution. This can be done in a reactor (extraction of soil slurry) or preferably as soil heap leaching. The heap leaching process is operationally simple. Heavy metal contaminated soil is excavated, screened and mounded on a pad. Heavy metals are
0045-6535/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.09.015
N. Finzˇgar, D. Lesˇtan / Chemosphere 63 (2006) 1736–1743
removed by passing washing solution through the soil using some type of liquid distribution system. The extractant is collected in a pregnant solution pit and processed to remove heavy metals (Hanson et al., 1992). In practice, acid washing and chelator soil washing are the two most prevalent heavy metal removal methods. A number of chelators have been tested. For soils contaminated primarily with Pb, EDTA was in most cases the most effective (Peters, 1999). One of the main problems of current EDTA-based soil washing technologies is the separation of chelator–heavy metals complexes from the waste extractant. EDTA is toxic (Dirilgen, 1998) and its complexes are poorly chemo- and bio-degradable (No¨rtemann, 1999) and must therefore be removed before the extractant can be safely discharged. For Pb–EDTA extractant Kim and Ong (1999) proposed trans-complexation: Pb in the EDTA complex was substituted with Fe3+ at low pH, followed by precipitation of Pb ions with phosphate or sulfate ions. Fe3+ was then separated from the EDTA with precipitation at high pH. The method allows chelator recycling and reuse. Di Palma et al. (2003a) proposed EDTA recovery after soil washing of Pb or Cu ‘‘artificially’’ contaminated soils in two steps: an initial evaporation treatment that lead to reduction of the extractant volume by 75%, followed by acidification, which precipitated more than 90% of EDTA complexes. The same research team (Di Palma et al., 2003b) also proposed reverse osmosis for the reduction of the extractant volume. Tejowulan and Hendershot (1998) have developed a simple procedure to remove heavy metal–EDTA complexes from soil extractants using an anion exchange resin. Juang and Wang (2000) proposed electrolytic recovery of Pb and Cu from a solution containing EDTA in a two-chamber cell, separated by a cation exchange membrane. They used simulated soil extractant (water solution with an equimolar solution of EDTA and total heavy metals). To treat decontamination wastewaters from the nuclear industry and other aqueous effluents contaminated with EDTA, the chemical destruction of EDTA and its complexes using advanced oxidation processes (AOP) has been proposed (Gilbert and HoffmannGlewe, 1990; Korhonen et al., 2000; Munoz and von Sonntag, 2000; Chitra et al., 2003). AOP involves the use of ozone, H2O2, ultrasonic waves, UV irradiation, FentonÕs reagent (Fe2+ and H2O2), alone or in combination, to generate free hydroxyl radicals; powerful, effective, non-specific oxidizing agents. Ozone is most often used in AOPs. It oxidizes organic compounds in two ways: by direct oxidation with ozone molecules and by the generation of free hydroxyl radicals. Gilbert and Hoffmann-Glewe (1990) observed complete degradation of EDTA in an aqueous solution treated with ozone alone. They identified several degradation prod-
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ucts, including ethylenediamine diacetic acid, iminodiacietic acid, nitrilotriacetic acid, oxalic acid etc. The concentration of hydroxyl radicals under normal ozonation conditions is, however, small. Under conditions favoring hydroxyl radical production, such as exposure to UV light, hydroxyl oxidation starts to predominate over oxidation with molecular ozone (Hoigne and Bader, 1976). Removal of EDTA–heavy metal complexes from soil extractants using AOP has not yet been tested. However, ozone has been successfully applied for chemical oxidation of certain recalcitrant organic pollutants in soil, such as polyaromatic hydrocarbons, chlorinated organic compounds and petroleum products, to overcome the limitations imposed by their low aqueous solubility and biodegradation rate (Masten and Davies, 1997). The aim of this study was to assess the feasibility of a novel soil heap leaching method for remediation of Pb and other heavy metal contaminated soil, using ozone and UV for advanced oxidation of EDTA complexes in soil extractants, removal of released metals from extractant by absorption, and reuse of extractant in a closed process loop. The presence of residual Pb, Zn, Fe, Ca and EDTA in the process wastewaters was examined. The efficiency of the novel soil heap leaching method for Pb and Zn removal was compared to the results of a small-scale simulation of soil heap leaching using different modes of EDTA soil addition.
2. Materials and methods 2.1. Soil samples and analysis Soil was collected from the 0–30 cm surface layer at an industrial site of a former Pb smelter in the Mezˇica Valley in Slovenia. The Mezˇica Valley has been exposed to more than three hundred years of active lead mining and smelting. Soils in the valley, including 6600 ha of agricultural land, are polluted especially with Pb, but also with Zn. In 1990, lead ore mining and smelting stopped and recycling old car batteries started. Soil pH was measured in a 1:2.5 (v/v) ratio of soil and 0.01 M CaCl2 water solution suspension. Soil samples were analyzed for organic matter by Walkley–Black titrations, cation exchange capacity by ammonium acetate and the Mehlich method, soil texture by the pipette method, easily extractable P was determined colorimetrically according to the Egner–Doming method, and carbonates manometrically after soil reaction with HCl (Kalra and Maynard, 1991). Sequential extractions (Lestan et al., 2003) were used to determine fractionation of heavy metals into six soil fractions. Selected soil characteristics and heavy metal contents are summarized in Table 1.
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Table 1 Physical and chemical characteristics and fractionation of Pb and Zn in the used soila
EDTA
pH (CaCl2) Organic matter (%) P (mg kg1) 1 CO2 3 ðmg kg Þ CEC (mmol C+ 100 g1) Sand (%) Silt (%) Clay (%) Texture
7.1 9.3 61.8 15.4 23.3 56.3 32.6 11.1 Sandy loam–sandy
1
Total Pb (mg kg ) Total Zn (mg kg1)
1243 ± 68 1190 ± 116
Fractionation
Pb (%)
Zn (%)
In soil solution Exchangeable Bound to carbonate Bound to Fe and Mn oxides Bound to organic matter Residual fraction
0.07 ± 0.04 0.27 ± 0.02 24.1 ± 0.3 0.28 ± 0.04 56.2 ± 1.4 8.04 ± 0.4
0.05 ± 0.01 0.20 ± 0.20 7.60 ± 0.25 2.17 ± 0.02 15.0 ± 0.7 61.0 ± 2.57
Recovery a
89
86
Means and standard deviations (n = 3) are presented.
2.2. Small-scale simulation of soil heap leaching Soil (150 g) was placed in perforated 250 ml polypropylene flasks with 0.5 mm plastic mesh at the bottom to retain the soil. Soil was extracted in triplicates with 100 ml of 2.5–40 mmol kg1 EDTA disodium salt (unbuffered). Extractant was circulated through the soil for 24 h using a peristaltic pump (flow rate 1.2– 1.5 ml min1). In experiments where the effect of reaction time on Pb removal was studied with 20 mmol kg1 EDTA, the extractant was circulated from 8 to 96 h. The soils were subsequently washed with 1.9 l of tap water (flow rate 1.2–1.5 ml min1) to remove all chelator– heavy metal complexes. The washing solution was collected, filtrated, sampled and stored (20 ml) in the cold for further analysis. Pb and Zn removal from soil was determined by measuring Pb and Zn content in the washing solution. 2.3. Advanced oxidation of EDTA complexes with ozone/UV and metal recovery An ozone/UV treatment unit for EDTA soil extractants consisted of an ozone generator, ozonation flask (oxygen/ozone flow rate 0.15 l min1), UV-light in a continuous flow housing, and absorption column. A peristaltic pump was used to force the solution or extractant (flow rate 14 ml min1) through the unit (Fig. 1).
Heavy-metal H contaminated contaminated soil
Absorbent
Soil properties
UV
Flowmeter
O2
Ozone generator O3/O2 sparger
Fig. 1. Flowsheet of the novel EDTA soil heap leaching method with ozone/UV treatment and reuse of extractant in a closed process loop.
Ozone was produced in an ozone generator (V-4, Crystal Air, Surrey, British Columbia, Canada) from pure commercial oxygen (flow rate 0.45 l min1). The ozone concentration was determined by the indigo colorimetric method (APHA, 1995). Ozonation with a porous oxygen/ozone sparger allowed a concentration of ozone in tap water up to 14.2 ± 1.2 mg l1. A 320 mm long 8 W UV-light bulb (MK-8, Lenntech, Delft, The Netherlands) was installed in a quartz glass and stainless steel continuous flow housing. Metals, released after advanced oxidation of EDTA complexes, were removed from soil extractants by passing them through the absorption column with 50 g of commercial sorbent Slovakite (IPRES, Bratislava, Slovak Republic) mixed with 50 g of vermiculite as a carrier material. Slovakite is a mixture of natural raw materials: dolomite, diatomite, smectite basaltic tuff, bentonite, alginite and zeolite. Preliminary experiments (data not shown) indicated that Slovakite is more efficient absorbent for Pb, Zn, Cd and Cu than commonly used apatite. 2.4. Heap leaching using ozone/UV treatment of EDTA extractants Soil heap leaching was simulated in triplicate in 15 cm diameter soil columns at room temperature. Airdied soil (4.6 kg) was sieved through a 5 mm mesh sieve and placed in a column 22 cm high. Plastic mesh (D = 0.2 mm) at the bottom of the column retained the soil. The soil was treated with six-consecutive EDTA additions with 2.5 mmol kg1 EDTA in 3100 ml unbuffered tap water being added first (the amount was equal to 145% of the field water capacity of the soil). Extractant was first circulated through the soil column for 48 h using a peristaltic pump (flow rate 14 ml min1) and then through an ozone/UV unit and soil column
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2.5. Heavy metal determination Soils (3 g) were ground in an agate mill, digested in aqua regia (100 ml) and Pb and Zn was analyzed by AAS (Perkin–Elmer 1100-B, Norwalk, USA). Pb, Zn, Fe and Ca concentrations in extractants were determined by AAS directly. Controls of the analytical procedure were performed using blanks and reference materials (BCR 141R, Community Bureau of Reference, Brussels, Belgium; for soil) treated in the same way as the experimental samples. 2.6. EDTA determination EDTA in extractants was determined spectrophotometrically according to procedure of Hamano et al. (1993). 2.7. Statistical analysis The Duncan multiple range test was used to determine the statistical significance (P < 0.05) between different soil treatments, using the computer program Statgraphic 4.0 for Windows.
3. Results and discussion 3.1. Small-scale heap leaching—the effect of the mode of EDTA addition Small-scale heap leaching simulation experiments (150 g of soil in perforated flasks) were used to provide process parameters for the heap leaching in soil columns. The effectiveness of heavy metal soil removal depends, among other factors, on the time allowed for reaction between the chelator in extractant and metals bound to the soil solid phase (Heil et al., 1998). Pb extraction with 20 mmol kg1 EDTA increased with the reaction time (Fig. 2). In further small-scale heap
120 pH 9.1±0.0 Pb removed (mg kg-1)
(Fig. 1). Each pass of extractant through the ozone/UV unit was considered to be a treatment cycle. After 3–4.5 treatment cycles, the extractant was supplemented with tap water to compensate for the water lost during the process (approx. 10%), and re-used as a medium for new 2.5 mmol kg1 EDTA soil addition. Before each EDTA addition, the ozone/UV unit was dismantled. The quartz tube, embodying the UV-light bulb, was cleaned of brown-yellowish deposits (probably goethite), and the absorbent (Slovakite mixed with vermiculite) was discharged and replaced with a fresh one. After heap leaching was completed, samples were taken from different soil layers in the columns and from the bulk of the soil. Samples were stored at 4 °C before analysis.
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100 a
80
b, c
b
a
d
pH 9.0±0.1 c, d
pH
pH 9.2±0.2
9.0±0.1
60 pH 9.3±0.1 40 pH 9.7±0.0 20 0 0
12
24
36 48 60 72 Extraction time (h)
84
96
Fig. 2. Removal of soil Pb after heap leaching (small-scale simulation) with 20 mmol kg1 EDTA using different reaction times, and pH of extractants. Error bars represent standard deviation from the mean value (n = 3). Means followed by the same letters are not significantly different, according to the Duncan test (P < 0.05).
leaching experiments, we allowed 24 h reaction time before we started soil rinsing. As expected, increasing the EDTA concentration from 2.5 to 40 mmol kg1 (molar ratios 0.42–6.67:1 for EDTA:Pb; and 0.14–2.20:1 for EDTA:Zn) increased Pb and Zn soil removal (Fig. 3). However, the increase was not proportional but rather small, with the EDTA concentration doubled. These results are consistent with the findings of Elliot and Brown (1989) and Steele and Pichtel (1998). Consecutive extractions using low concentrations of EDTA were more effective than single soil extraction with concentrated EDTA (Fig. 3). For example: four consecutive treatments with 2.5 mmol kg1 EDTA (total 10 mmol kg1 EDTA) removed significantly more Pb than a single extraction with 10 mmol kg1 EDTA, and more Zn than a single extraction with 40 mmol kg1 EDTA. Six treatments with 2.5 mmol kg1 EDTA (total 15 mmol kg1 EDTA) removed Pb equally effectively than a single treatment with 40 mmol kg1 EDTA, and Zn equally effectively than a double treatment with 40 mmol kg1 EDTA. Sun et al. (2001) reported that a considerable mobilisation of Fe occurred during leaching of heavy metals contaminated soil with EDTA. When treating Pb contaminated calcareous soil with EDTA, Theodoratos et al. (2000) observed complexation and mobilisation of Ca, which caused considerable dissolution of calcite from the soil. For the soil used in this study, the results of a preliminary experiment indicated that the ratio of Pb extracted to EDTA used increased with increasing the number of consecutive extractions and the lowering EDTA doses. The total amount of EDTA was the same. The ratio of Ca extracted to EDTA used decreased with increasing the number of consecutive extractions and the lowering EDTA doses (data not shown). This presumably explains why consecutive
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The proposed set up of the novel heap leaching method is shown in Fig. 1. In the novel method, we allowed a 48 h reaction time before we started to treat the extractant in the ozone/UV unit. After passing the ozone/UV treatment unit, where the released Pb and Zn were recovered by absorption, the extractant (with lower Pb and Zn content) was immediately used for soil rinsing (treatment cycle). After 3–4.5 treatment cycles (12–18 h), approximately 95% and 80% of Pb and Zn, respectively, was removed from the extractant (Fig. 4). The extractant was then used as a medium for the next addition of 2.5 mmol kg1 EDTA. As expected, the concentrations of Pb and Zn in the extractant (before treatment in the ozone/UV unit) decreased with the consecutive soil treatments with EDTA (Fig. 4). Calculated from the initial Pb and Zn concentrations in all six extractants and the volume of extractant in the system, 72% and 18% of Pb and Zn was removed from the soil. However, soil analysis after heap leaching revealed that in fact 49.6 ± 0.6% and 19.9 ± 1.7% of Pb and Zn was actually removed. The difference can be attributed to EDTA residues, capable
Pb removed (%)
80
60
40
20
0 20
Zn removed (%)
15
10
5
0
400 2.5
5
10
20
I
40 II
EDTA (mmol kg-1)
EDTA extractions were more effective than a single or double extraction (Fig. 3). The mode of extraction with EDTA did not significantly effects the ratio of Fe extracted to EDTA used. EDTA extracted more Pb than Zn (Fig. 3). Speciation in a heavy metal–chelator system is controlled by reaction kinetics and many soil parameters (Nowack, 2002). Theoretically, however, a higher efficiency of Pb removal could be explained by the lower stability of Zn–EDTA complexes compared to Pb–EDTA complexes (log K 16.5 and log K 18.0, respectively, at 25 °C and ionic strength l = 0.1; Martell and Smith, 2003). Another possible explanation is the high portion of non-extractable Zn bound to the residual soil fraction, whereas most of the Pb was found in the more labile soil organic matter fraction (Table 1).
Pb (mg l-1)
300
III IV
200
V VI
100
0 80
60 Zn (mg l-1)
Fig. 3. The removal of soil Pb and Zn after repetitive heap leaching (small-scale simulation) using various EDTA concentrations. Error bars represent standard deviation from the mean value (n = 3).
40
20
0 1.5
3.2. Heap leaching with ozone/UV treatment and reuse of extractant in a closed process loop The feasibility of the novel heap leaching method was evaluated in soil columns. Based on the results of smallscale heap leaching experiments (Fig. 3), we chose to use 2.5 mmol kg1 EDTA in consecutive additions.
3.0
4.5
6.0
Cycles
Fig. 4. Concentration of Pb and Zn in extractants during soil heap leaching with six-consecutive additions (I–VI) of 2.5 mmol kg1 EDTA (soil column simulation). Extractants were treated in the ozone/UV unit in 3.5–6 treatment cycles. Error bars represent standard deviation from the mean value (n = 3).
N. Finzˇgar, D. Lesˇtan / Chemosphere 63 (2006) 1736–1743
a
EDTA (mM l-1)
1.6
1.2
0.8
0.4
0.0 15.0
b
Fe (mg l-1)
12.0 9.0 6.0 3.0 0.0
c
300 250 Ca (mg l-1)
of Pb and Zn chelation, which were presumably biodegraded in the soil and caused some accumulation of Pb and Zn in the soil column. This part of the Pb and Zn was not absorbed by Slovakite (which was discharged before each EDTA addition) and permanently removed from the soil, and was re-extracted after each EDTA addition. The bio-degradability of EDTA oxidation products obtained after ozonation and mentioned earlier in this text has been reported by Gilbert and Hoffmann-Glewe (1990). After the final, 6th addition of EDTA, the extractant was treated in 4.5 cycles. The final concentration of Pb and Zn was 1.98 ± 2.17 and 4.55 ± 2.36 mg l1. The extractants obtained from heap leaching had pH values ranging between 8.01 ± 0.09 at the beginning and 7.45 ± 0.04 in the final extractant. The concentration of EDTA in the extractant after the final, 6th addition of 2.5 mM EDTA and 48 h reaction time in soil, and before treatment in the ozone/UV unit, was approximately two-times lower than the initial concentration (Fig. 5(a); 1.21 ± 0.15 mM EDTA at cycle 0). The probable reason is adsorption of EDTA complexes in the soil organic matter, as was reported by Di Palma et al. (2005). Whether this residual EDTA was completely removed during the heap leaching process by rinsing the soil with treated extractant or remained in the soil needs to be further studied. Similar to Pb and Zn (Fig. 4) the concentration EDTA in the extractant decreased during treatment in the ozone/UV unit (Fig. 5(a)). The EDTA concentration in the final extractant (after 4.5 treatment cycles) was thus very low, 0.05 ± 0.04 mM, and could probably be brought below the detection limit of the analytical method (0.0013 mM EDTA; Hamano et al., 1993) with additional treatments. The concentrations of Fe and Ca, the two metals readily chelated by EDTA (Theodoratos et al., 2000; Sun et al., 2001), were also measured in the final extractant. The Fe concentration decreased with treatment cycles (Fig. 5(b)) while the concentration of Ca remained higher than 100 mg l1 (Fig. 5(c)). The background concentration of Ca in the tap water, however, was also quite high, 85.3 mg l1. Heap leaching in a soil column reduced the concentrations of Pb and Zn in the soil from the initial 1243 ± 68 and 1190 ± 116 mg kg1 to 627 ± 29 and 953 ± 23 mg kg1, respectively. As shown in Fig. 6, Pb and Zn were removed quite uniformly throughout the soil profile. Heap leaching in soil columns was similarly effective as small-scale heap leaching simulation, where 49.7 ± 1.0% and 13.7 ± 0.4% of Pb and Zn were removed after six-consecutive treatments with 2.5 mmol kg1 EDTA (Fig. 3). The cost of EDTA is an important issue in soil heap leaching. Methods that recycle not only the process water, but also the chelator may therefore be economically more feasible. However, at the current stage
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200 150 100 50 0 0
1.5
3
4.5
Cycles
Fig. 5. Concentration of EDTA (a), Fe (b) and Ca (c) in extractants after the final, 6th addition of EDTA. Extractants were treated in the ozone/UV unit in 4.5 treatment cycles. Error bars represent standard deviation from the mean value (n = 3).
of development, the proposed EDTA recycling methods either require the use of expensive materials or are technically demanding. For example, trans-complexation (Kim and Ong, 1999) is operationally complicated and could prove even more difficult if the EDTA is complexed with more than one other heavy metal (in multi-contaminated soils). Although transcomplexation allows partial EDTA recycling, it also involves consumption of other chemicals and does not provide a solution for the final treatment of EDTA contaminated extractants. The feasibility of evaporation of the extractant (Di Palma et al., 2003a) is probably constrained by the high cost of water evaporation, a high energy consuming operation. In reverse osmosis of the soil extractant (Di Palma et al., 2003b), colloidal particles (clays and humic materials) and bacteria could clog the membranes and thus diminish the
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References
before after 0
Soil depth (cm)
-5.5
-11
-16.5
-22 0
500
1000
Pb (mg kg-1)
1500
0
500
1000
1500
Zn (mg kg-1)
Fig. 6. Pb and Zn concentration through the soil profile in soil column before (dotted line) and after (solid line) heap leaching with six-consecutive additions of 2.5 mmol kg1 EDTA. Error bars represent standard deviation from the mean value (n = 3).
performance and shorten the life-time of the membranes. The high price of anion exchange resins (more than $100 for 500 g), proposed for removal of metal– EDTA complexes from soil extractants (Tejowulan and Hendershot, 1998), is a constraint that could be overcome by effective resin recycling, which, however, still needs to be developed. 4. Conclusions The results of our laboratory scale study indicated that the use of ozone/UV and absorption of released heavy metals is viable for treatment of EDTA soil extractants, obtained during heap leaching of soils principally contaminated with Pb. Final treated extractants were almost free of heavy metals and EDTA, and safe for discharge. During the process treated extractants were successfully re-used as a medium for consecutive EDTA additions and for rinsing the soil to remove residual EDTA. Ozone/UV treatment of extractant could therefore also reduce the requirements for process water. The use of ozone/UV treatment of EDTA soil extractant could lead to environmentally safe, efficient new heap leaching remediation technologies. Acknowledgement This work was supported by the Slovenian Ministry for Education, Science and Sport, Grant J4-6134-048104/4.03.
APHA, 1995. Chitra, S., Paramasivan, K., Sinha, P.K., Lal, K.B.J., 2003. Ultrasonic treatment of liquid waste containing EDTA. J. Clean. Prod. 12, 429–435. Dirilgen, N., 1998. Effect of pH and chelator EDTA on Cr toxicity and accumulation in Lemma minor. Chemosphere 37, 771–783. Di Palma, L., Ferrantelli, P., Merli, C., Biancifiori, F.J., 2003a. Recovery of EDTA and metal precipitation from soil flushing solutions. J. Hazard. Mater. B 103, 153–168. Di Palma, L., Ferrantelli, P., Merli, C., Petrucci, E., 2003b. Treatment of the solution extracted from metal contaminated soils by reverse osmosis and chemical precipitation. Annli Chim. 93, 1005–1011. Di Palma, L., Ferrantelli, P., Pitzolu, I., Verdone, N., 2005. EDTA leaching of copper from contaminated soils: Experimental study and transport model application for parameters estimation. Environ. Technol. 26, 189–198. Elliot, H.A., Brown, G.A., 1989. Comparative evaluation of NTA and EDTA for extractive decontamination of Pbpolluted soils. Water Air Soil Pollut. 45, 361–369. Gilbert, E., Hoffmann-Glewe, S., 1990. Ozonation of ethylenediaminetetraacetic acid (EDTA) in aqueous solution, influence of pH value and metal ions. Water Res. 24, 39–44. Hamano, T., Mitsuhashi, Y., Kojma, N., Aoki, N., 1993. Sensitive spectrophotometric method for the determination of ethylenediaminetetraacetic acid in foods. Analyst 118, 909–912. Hanson, A.T., Samani, Z., Dwyer, B., Jacquez, R., 1992. Heap leaching as a solvent-extraction technique for remediation of metals-contaminates soils. In: Sabatini, D.A., Knox, R.C. (Eds.), Proceedings of the ACS Symposium Series No. 491. American Chemical Society, Transport and Remediation of Subsurface Contaminants, Washington, USA, 108–121. Heil, D.M., Samani, A., Hanson, T., Rudd, B., 1998. Remediation of lead contaminated soil by EDTA. I. Batch and column studies. Water Air Soil Pollut. 113, 77–95. Hoigne, J., Bader, H., 1976. Role of hydroxyl radical reactions in ozonation processes in aqueous solutions. Water Res. 10, 377–386. Juang, R.S., Wang, S.W., 2000. Electrolytic recovery of binary metals and EDTA from strong complexed solutions. Water Res. 34, 3179–3185. Kalra, Y.P., Maynard, D.G., 1991. Methods Manual for Forest Soil and Plant Analysis. Canadian Forest Service, Northern Forestry Centre, Edmonton. Kim, C., Ong, S.-K., 1999. Recycling of lead-contaminated EDTA wastewater. J. Hazard. Mater. B 69, 273–286. Korhonen, M.S., Metsarinne, S.E., Tuhkanen, T.A., 2000. Removal of ethylenediaminetetraacetic acid (EDTA) from pulp mill effluents by ozonation. Ozone Sci. Eng. 22, 279– 286. Lestan, D., Grcman, H., Zupan, M., Bacac, N., 2003. Relationship of soil properties to fractionation of Pb and Zn in soil and their uptake into Plantago lanceolata. Soil Sediment Contam. 12, 507–522. Martell, A.E., Smith, R.M., 2003. NIST Critically Selected Stability Constants of Metal Complexes; Version 7.0, NIST, Gaithersburg.
N. Finzˇgar, D. Lesˇtan / Chemosphere 63 (2006) 1736–1743 Masten, J.S., Davies, S.H.R., 1997. Efficacy of in-situ ozonation for the remediation of PAH contaminated soils. J. Contam. Hydrol. 28, 327–335. Munoz, F., von Sonntag, C.J., 2000. The reaction of ozone with tertiary amines including the complexing agents nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) in aqueous solution. Chem. Soc., Perkin Trans. 2, 2029–2033. No¨rtemann, B., 1999. Biodegradation of EDTA. Appl. Microbiol. Biotechnol. 51, 751–759. Nowack, B., 2002. Environmental chemistry of aminopolycarboxylate chelating agents. Environ. Sci. Technol. 36, 4009– 4016. Peters, R.W., 1999. Chelant extraction of heavy metals from contaminated soils. J. Hazard. Mater. 66, 151–210.
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Steele, M.C., Pichtel, J., 1998. Ex-situ remediation of a metalcontaminated superfund soil using selective extractants. J. Environ. Eng. 124, 639–645. Sun, B., Zhao, F.J., Lombi, E., McGrath, S.P., 2001. Leaching of heavy metals from contaminated soils using EDTA. Environ. Pollut. 113, 111–120. Tejowulan, R.S., Hendershot, W.H., 1998. Removal of trace metals from contaminated soils using EDTA incorporating resin trapping techniques. Environ. Pollut. 103, 135– 142. Theodoratos, P., Papassiopi, N., Georgoudis, T., Kontopoulos, A., 2000. Selective removal of lead from calcareous polluted soils using the Ca–EDTA salt. Water Air Soil Pollut. 122, 351–368.
Heap leaching of Pb and Zn contaminated soil using ozone/UV treatment of EDTA extractants Nezˇa Finzˇgar, Domen Lesˇtan
*
Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia Received 26 May 2005; received in revised form 19 September 2005; accepted 22 September 2005 Available online 8 November 2005
Abstract The feasibility of a novel EDTA-based soil heap leaching method with treatment and reuse of extractants in a closed process loop was evaluated on a laboratory scale. Ozone and UV irradiation were used for oxidative decomposition of EDTA-metal complexes in extractants from Pb (1243 mg kg1) and Zn (1190 mg kg1) contaminated soil. Released metals were absorbed in a commercial metal absorbent Slovakite. Six-consecutive additions of 2.5 mmol kg1 EDTA (total 15 mmol kg1 EDTA) removed 49.6 ± 0.6% and 19.7 ± 1.7% of initial total Pb and Zn from soil (4.6 kg) packed in 22 cm high columns. The efficiency of extraction was similar to small-scale simulations of heap leaching (150 g of soil), where EDTA used in the same manner removed 49.7 ± 1.0% and 13.7 ± 0.4% of Pb and Zn. The new heap leaching method produced discharge extractant with fairly low final concentrations of Pb, Zn and EDTA (1.98 ± 2.17 mg l1, 4.55 ± 2.36 mg l1, and 0.05 ± 0.04 mM, respectively), which could presumably be reduced even further with continuation of treatment. The results of our study indicate that for soils contaminated primarily with Pb, treating the EDTA extractants with ozone/UV and reuse of extractants enables efficient soil heap leaching with very little or no wastewater generation, easy control over emissions, and lowers the requirements for process water. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Pb and Zn contamination; Soil heap leaching; EDTA; Ozone; UV
1. Introduction Lead contamination of soils is a common problem throughout the world. Soil contamination is seldom mono-metallic. In soils contaminated primarily with Pb, Zn and other heavy metals are usually also present in elevated concentrations. Current remediation activi* Corresponding author. Address: Center for Soil and Environmental Science, Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia. Tel.: +386 01 423 1161; fax: +386 01 423 1088. E-mail address: [email protected] (D. Lesˇtan).
ties have involved excavation and removal of the contaminated soil, immobilization and containment of heavy metals in the soil by mixing or injecting agents such as cement, lime and different phosphates, electrokinetic mobilization and removal of heavy metals from the soil by precipitation on the electrodes, phytoextraction of metals using metal-accumulating plants, and soil washing and flushing. Soil washing involves the separation of contaminants from soil solids by solubilizing them in a washing solution. This can be done in a reactor (extraction of soil slurry) or preferably as soil heap leaching. The heap leaching process is operationally simple. Heavy metal contaminated soil is excavated, screened and mounded on a pad. Heavy metals are
0045-6535/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2005.09.015
N. Finzˇgar, D. Lesˇtan / Chemosphere 63 (2006) 1736–1743
removed by passing washing solution through the soil using some type of liquid distribution system. The extractant is collected in a pregnant solution pit and processed to remove heavy metals (Hanson et al., 1992). In practice, acid washing and chelator soil washing are the two most prevalent heavy metal removal methods. A number of chelators have been tested. For soils contaminated primarily with Pb, EDTA was in most cases the most effective (Peters, 1999). One of the main problems of current EDTA-based soil washing technologies is the separation of chelator–heavy metals complexes from the waste extractant. EDTA is toxic (Dirilgen, 1998) and its complexes are poorly chemo- and bio-degradable (No¨rtemann, 1999) and must therefore be removed before the extractant can be safely discharged. For Pb–EDTA extractant Kim and Ong (1999) proposed trans-complexation: Pb in the EDTA complex was substituted with Fe3+ at low pH, followed by precipitation of Pb ions with phosphate or sulfate ions. Fe3+ was then separated from the EDTA with precipitation at high pH. The method allows chelator recycling and reuse. Di Palma et al. (2003a) proposed EDTA recovery after soil washing of Pb or Cu ‘‘artificially’’ contaminated soils in two steps: an initial evaporation treatment that lead to reduction of the extractant volume by 75%, followed by acidification, which precipitated more than 90% of EDTA complexes. The same research team (Di Palma et al., 2003b) also proposed reverse osmosis for the reduction of the extractant volume. Tejowulan and Hendershot (1998) have developed a simple procedure to remove heavy metal–EDTA complexes from soil extractants using an anion exchange resin. Juang and Wang (2000) proposed electrolytic recovery of Pb and Cu from a solution containing EDTA in a two-chamber cell, separated by a cation exchange membrane. They used simulated soil extractant (water solution with an equimolar solution of EDTA and total heavy metals). To treat decontamination wastewaters from the nuclear industry and other aqueous effluents contaminated with EDTA, the chemical destruction of EDTA and its complexes using advanced oxidation processes (AOP) has been proposed (Gilbert and HoffmannGlewe, 1990; Korhonen et al., 2000; Munoz and von Sonntag, 2000; Chitra et al., 2003). AOP involves the use of ozone, H2O2, ultrasonic waves, UV irradiation, FentonÕs reagent (Fe2+ and H2O2), alone or in combination, to generate free hydroxyl radicals; powerful, effective, non-specific oxidizing agents. Ozone is most often used in AOPs. It oxidizes organic compounds in two ways: by direct oxidation with ozone molecules and by the generation of free hydroxyl radicals. Gilbert and Hoffmann-Glewe (1990) observed complete degradation of EDTA in an aqueous solution treated with ozone alone. They identified several degradation prod-
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ucts, including ethylenediamine diacetic acid, iminodiacietic acid, nitrilotriacetic acid, oxalic acid etc. The concentration of hydroxyl radicals under normal ozonation conditions is, however, small. Under conditions favoring hydroxyl radical production, such as exposure to UV light, hydroxyl oxidation starts to predominate over oxidation with molecular ozone (Hoigne and Bader, 1976). Removal of EDTA–heavy metal complexes from soil extractants using AOP has not yet been tested. However, ozone has been successfully applied for chemical oxidation of certain recalcitrant organic pollutants in soil, such as polyaromatic hydrocarbons, chlorinated organic compounds and petroleum products, to overcome the limitations imposed by their low aqueous solubility and biodegradation rate (Masten and Davies, 1997). The aim of this study was to assess the feasibility of a novel soil heap leaching method for remediation of Pb and other heavy metal contaminated soil, using ozone and UV for advanced oxidation of EDTA complexes in soil extractants, removal of released metals from extractant by absorption, and reuse of extractant in a closed process loop. The presence of residual Pb, Zn, Fe, Ca and EDTA in the process wastewaters was examined. The efficiency of the novel soil heap leaching method for Pb and Zn removal was compared to the results of a small-scale simulation of soil heap leaching using different modes of EDTA soil addition.
2. Materials and methods 2.1. Soil samples and analysis Soil was collected from the 0–30 cm surface layer at an industrial site of a former Pb smelter in the Mezˇica Valley in Slovenia. The Mezˇica Valley has been exposed to more than three hundred years of active lead mining and smelting. Soils in the valley, including 6600 ha of agricultural land, are polluted especially with Pb, but also with Zn. In 1990, lead ore mining and smelting stopped and recycling old car batteries started. Soil pH was measured in a 1:2.5 (v/v) ratio of soil and 0.01 M CaCl2 water solution suspension. Soil samples were analyzed for organic matter by Walkley–Black titrations, cation exchange capacity by ammonium acetate and the Mehlich method, soil texture by the pipette method, easily extractable P was determined colorimetrically according to the Egner–Doming method, and carbonates manometrically after soil reaction with HCl (Kalra and Maynard, 1991). Sequential extractions (Lestan et al., 2003) were used to determine fractionation of heavy metals into six soil fractions. Selected soil characteristics and heavy metal contents are summarized in Table 1.
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Table 1 Physical and chemical characteristics and fractionation of Pb and Zn in the used soila
EDTA
pH (CaCl2) Organic matter (%) P (mg kg1) 1 CO2 3 ðmg kg Þ CEC (mmol C+ 100 g1) Sand (%) Silt (%) Clay (%) Texture
7.1 9.3 61.8 15.4 23.3 56.3 32.6 11.1 Sandy loam–sandy
1
Total Pb (mg kg ) Total Zn (mg kg1)
1243 ± 68 1190 ± 116
Fractionation
Pb (%)
Zn (%)
In soil solution Exchangeable Bound to carbonate Bound to Fe and Mn oxides Bound to organic matter Residual fraction
0.07 ± 0.04 0.27 ± 0.02 24.1 ± 0.3 0.28 ± 0.04 56.2 ± 1.4 8.04 ± 0.4
0.05 ± 0.01 0.20 ± 0.20 7.60 ± 0.25 2.17 ± 0.02 15.0 ± 0.7 61.0 ± 2.57
Recovery a
89
86
Means and standard deviations (n = 3) are presented.
2.2. Small-scale simulation of soil heap leaching Soil (150 g) was placed in perforated 250 ml polypropylene flasks with 0.5 mm plastic mesh at the bottom to retain the soil. Soil was extracted in triplicates with 100 ml of 2.5–40 mmol kg1 EDTA disodium salt (unbuffered). Extractant was circulated through the soil for 24 h using a peristaltic pump (flow rate 1.2– 1.5 ml min1). In experiments where the effect of reaction time on Pb removal was studied with 20 mmol kg1 EDTA, the extractant was circulated from 8 to 96 h. The soils were subsequently washed with 1.9 l of tap water (flow rate 1.2–1.5 ml min1) to remove all chelator– heavy metal complexes. The washing solution was collected, filtrated, sampled and stored (20 ml) in the cold for further analysis. Pb and Zn removal from soil was determined by measuring Pb and Zn content in the washing solution. 2.3. Advanced oxidation of EDTA complexes with ozone/UV and metal recovery An ozone/UV treatment unit for EDTA soil extractants consisted of an ozone generator, ozonation flask (oxygen/ozone flow rate 0.15 l min1), UV-light in a continuous flow housing, and absorption column. A peristaltic pump was used to force the solution or extractant (flow rate 14 ml min1) through the unit (Fig. 1).
Heavy-metal H contaminated contaminated soil
Absorbent
Soil properties
UV
Flowmeter
O2
Ozone generator O3/O2 sparger
Fig. 1. Flowsheet of the novel EDTA soil heap leaching method with ozone/UV treatment and reuse of extractant in a closed process loop.
Ozone was produced in an ozone generator (V-4, Crystal Air, Surrey, British Columbia, Canada) from pure commercial oxygen (flow rate 0.45 l min1). The ozone concentration was determined by the indigo colorimetric method (APHA, 1995). Ozonation with a porous oxygen/ozone sparger allowed a concentration of ozone in tap water up to 14.2 ± 1.2 mg l1. A 320 mm long 8 W UV-light bulb (MK-8, Lenntech, Delft, The Netherlands) was installed in a quartz glass and stainless steel continuous flow housing. Metals, released after advanced oxidation of EDTA complexes, were removed from soil extractants by passing them through the absorption column with 50 g of commercial sorbent Slovakite (IPRES, Bratislava, Slovak Republic) mixed with 50 g of vermiculite as a carrier material. Slovakite is a mixture of natural raw materials: dolomite, diatomite, smectite basaltic tuff, bentonite, alginite and zeolite. Preliminary experiments (data not shown) indicated that Slovakite is more efficient absorbent for Pb, Zn, Cd and Cu than commonly used apatite. 2.4. Heap leaching using ozone/UV treatment of EDTA extractants Soil heap leaching was simulated in triplicate in 15 cm diameter soil columns at room temperature. Airdied soil (4.6 kg) was sieved through a 5 mm mesh sieve and placed in a column 22 cm high. Plastic mesh (D = 0.2 mm) at the bottom of the column retained the soil. The soil was treated with six-consecutive EDTA additions with 2.5 mmol kg1 EDTA in 3100 ml unbuffered tap water being added first (the amount was equal to 145% of the field water capacity of the soil). Extractant was first circulated through the soil column for 48 h using a peristaltic pump (flow rate 14 ml min1) and then through an ozone/UV unit and soil column
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2.5. Heavy metal determination Soils (3 g) were ground in an agate mill, digested in aqua regia (100 ml) and Pb and Zn was analyzed by AAS (Perkin–Elmer 1100-B, Norwalk, USA). Pb, Zn, Fe and Ca concentrations in extractants were determined by AAS directly. Controls of the analytical procedure were performed using blanks and reference materials (BCR 141R, Community Bureau of Reference, Brussels, Belgium; for soil) treated in the same way as the experimental samples. 2.6. EDTA determination EDTA in extractants was determined spectrophotometrically according to procedure of Hamano et al. (1993). 2.7. Statistical analysis The Duncan multiple range test was used to determine the statistical significance (P < 0.05) between different soil treatments, using the computer program Statgraphic 4.0 for Windows.
3. Results and discussion 3.1. Small-scale heap leaching—the effect of the mode of EDTA addition Small-scale heap leaching simulation experiments (150 g of soil in perforated flasks) were used to provide process parameters for the heap leaching in soil columns. The effectiveness of heavy metal soil removal depends, among other factors, on the time allowed for reaction between the chelator in extractant and metals bound to the soil solid phase (Heil et al., 1998). Pb extraction with 20 mmol kg1 EDTA increased with the reaction time (Fig. 2). In further small-scale heap
120 pH 9.1±0.0 Pb removed (mg kg-1)
(Fig. 1). Each pass of extractant through the ozone/UV unit was considered to be a treatment cycle. After 3–4.5 treatment cycles, the extractant was supplemented with tap water to compensate for the water lost during the process (approx. 10%), and re-used as a medium for new 2.5 mmol kg1 EDTA soil addition. Before each EDTA addition, the ozone/UV unit was dismantled. The quartz tube, embodying the UV-light bulb, was cleaned of brown-yellowish deposits (probably goethite), and the absorbent (Slovakite mixed with vermiculite) was discharged and replaced with a fresh one. After heap leaching was completed, samples were taken from different soil layers in the columns and from the bulk of the soil. Samples were stored at 4 °C before analysis.
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100 a
80
b, c
b
a
d
pH 9.0±0.1 c, d
pH
pH 9.2±0.2
9.0±0.1
60 pH 9.3±0.1 40 pH 9.7±0.0 20 0 0
12
24
36 48 60 72 Extraction time (h)
84
96
Fig. 2. Removal of soil Pb after heap leaching (small-scale simulation) with 20 mmol kg1 EDTA using different reaction times, and pH of extractants. Error bars represent standard deviation from the mean value (n = 3). Means followed by the same letters are not significantly different, according to the Duncan test (P < 0.05).
leaching experiments, we allowed 24 h reaction time before we started soil rinsing. As expected, increasing the EDTA concentration from 2.5 to 40 mmol kg1 (molar ratios 0.42–6.67:1 for EDTA:Pb; and 0.14–2.20:1 for EDTA:Zn) increased Pb and Zn soil removal (Fig. 3). However, the increase was not proportional but rather small, with the EDTA concentration doubled. These results are consistent with the findings of Elliot and Brown (1989) and Steele and Pichtel (1998). Consecutive extractions using low concentrations of EDTA were more effective than single soil extraction with concentrated EDTA (Fig. 3). For example: four consecutive treatments with 2.5 mmol kg1 EDTA (total 10 mmol kg1 EDTA) removed significantly more Pb than a single extraction with 10 mmol kg1 EDTA, and more Zn than a single extraction with 40 mmol kg1 EDTA. Six treatments with 2.5 mmol kg1 EDTA (total 15 mmol kg1 EDTA) removed Pb equally effectively than a single treatment with 40 mmol kg1 EDTA, and Zn equally effectively than a double treatment with 40 mmol kg1 EDTA. Sun et al. (2001) reported that a considerable mobilisation of Fe occurred during leaching of heavy metals contaminated soil with EDTA. When treating Pb contaminated calcareous soil with EDTA, Theodoratos et al. (2000) observed complexation and mobilisation of Ca, which caused considerable dissolution of calcite from the soil. For the soil used in this study, the results of a preliminary experiment indicated that the ratio of Pb extracted to EDTA used increased with increasing the number of consecutive extractions and the lowering EDTA doses. The total amount of EDTA was the same. The ratio of Ca extracted to EDTA used decreased with increasing the number of consecutive extractions and the lowering EDTA doses (data not shown). This presumably explains why consecutive
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The proposed set up of the novel heap leaching method is shown in Fig. 1. In the novel method, we allowed a 48 h reaction time before we started to treat the extractant in the ozone/UV unit. After passing the ozone/UV treatment unit, where the released Pb and Zn were recovered by absorption, the extractant (with lower Pb and Zn content) was immediately used for soil rinsing (treatment cycle). After 3–4.5 treatment cycles (12–18 h), approximately 95% and 80% of Pb and Zn, respectively, was removed from the extractant (Fig. 4). The extractant was then used as a medium for the next addition of 2.5 mmol kg1 EDTA. As expected, the concentrations of Pb and Zn in the extractant (before treatment in the ozone/UV unit) decreased with the consecutive soil treatments with EDTA (Fig. 4). Calculated from the initial Pb and Zn concentrations in all six extractants and the volume of extractant in the system, 72% and 18% of Pb and Zn was removed from the soil. However, soil analysis after heap leaching revealed that in fact 49.6 ± 0.6% and 19.9 ± 1.7% of Pb and Zn was actually removed. The difference can be attributed to EDTA residues, capable
Pb removed (%)
80
60
40
20
0 20
Zn removed (%)
15
10
5
0
400 2.5
5
10
20
I
40 II
EDTA (mmol kg-1)
EDTA extractions were more effective than a single or double extraction (Fig. 3). The mode of extraction with EDTA did not significantly effects the ratio of Fe extracted to EDTA used. EDTA extracted more Pb than Zn (Fig. 3). Speciation in a heavy metal–chelator system is controlled by reaction kinetics and many soil parameters (Nowack, 2002). Theoretically, however, a higher efficiency of Pb removal could be explained by the lower stability of Zn–EDTA complexes compared to Pb–EDTA complexes (log K 16.5 and log K 18.0, respectively, at 25 °C and ionic strength l = 0.1; Martell and Smith, 2003). Another possible explanation is the high portion of non-extractable Zn bound to the residual soil fraction, whereas most of the Pb was found in the more labile soil organic matter fraction (Table 1).
Pb (mg l-1)
300
III IV
200
V VI
100
0 80
60 Zn (mg l-1)
Fig. 3. The removal of soil Pb and Zn after repetitive heap leaching (small-scale simulation) using various EDTA concentrations. Error bars represent standard deviation from the mean value (n = 3).
40
20
0 1.5
3.2. Heap leaching with ozone/UV treatment and reuse of extractant in a closed process loop The feasibility of the novel heap leaching method was evaluated in soil columns. Based on the results of smallscale heap leaching experiments (Fig. 3), we chose to use 2.5 mmol kg1 EDTA in consecutive additions.
3.0
4.5
6.0
Cycles
Fig. 4. Concentration of Pb and Zn in extractants during soil heap leaching with six-consecutive additions (I–VI) of 2.5 mmol kg1 EDTA (soil column simulation). Extractants were treated in the ozone/UV unit in 3.5–6 treatment cycles. Error bars represent standard deviation from the mean value (n = 3).
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a
EDTA (mM l-1)
1.6
1.2
0.8
0.4
0.0 15.0
b
Fe (mg l-1)
12.0 9.0 6.0 3.0 0.0
c
300 250 Ca (mg l-1)
of Pb and Zn chelation, which were presumably biodegraded in the soil and caused some accumulation of Pb and Zn in the soil column. This part of the Pb and Zn was not absorbed by Slovakite (which was discharged before each EDTA addition) and permanently removed from the soil, and was re-extracted after each EDTA addition. The bio-degradability of EDTA oxidation products obtained after ozonation and mentioned earlier in this text has been reported by Gilbert and Hoffmann-Glewe (1990). After the final, 6th addition of EDTA, the extractant was treated in 4.5 cycles. The final concentration of Pb and Zn was 1.98 ± 2.17 and 4.55 ± 2.36 mg l1. The extractants obtained from heap leaching had pH values ranging between 8.01 ± 0.09 at the beginning and 7.45 ± 0.04 in the final extractant. The concentration of EDTA in the extractant after the final, 6th addition of 2.5 mM EDTA and 48 h reaction time in soil, and before treatment in the ozone/UV unit, was approximately two-times lower than the initial concentration (Fig. 5(a); 1.21 ± 0.15 mM EDTA at cycle 0). The probable reason is adsorption of EDTA complexes in the soil organic matter, as was reported by Di Palma et al. (2005). Whether this residual EDTA was completely removed during the heap leaching process by rinsing the soil with treated extractant or remained in the soil needs to be further studied. Similar to Pb and Zn (Fig. 4) the concentration EDTA in the extractant decreased during treatment in the ozone/UV unit (Fig. 5(a)). The EDTA concentration in the final extractant (after 4.5 treatment cycles) was thus very low, 0.05 ± 0.04 mM, and could probably be brought below the detection limit of the analytical method (0.0013 mM EDTA; Hamano et al., 1993) with additional treatments. The concentrations of Fe and Ca, the two metals readily chelated by EDTA (Theodoratos et al., 2000; Sun et al., 2001), were also measured in the final extractant. The Fe concentration decreased with treatment cycles (Fig. 5(b)) while the concentration of Ca remained higher than 100 mg l1 (Fig. 5(c)). The background concentration of Ca in the tap water, however, was also quite high, 85.3 mg l1. Heap leaching in a soil column reduced the concentrations of Pb and Zn in the soil from the initial 1243 ± 68 and 1190 ± 116 mg kg1 to 627 ± 29 and 953 ± 23 mg kg1, respectively. As shown in Fig. 6, Pb and Zn were removed quite uniformly throughout the soil profile. Heap leaching in soil columns was similarly effective as small-scale heap leaching simulation, where 49.7 ± 1.0% and 13.7 ± 0.4% of Pb and Zn were removed after six-consecutive treatments with 2.5 mmol kg1 EDTA (Fig. 3). The cost of EDTA is an important issue in soil heap leaching. Methods that recycle not only the process water, but also the chelator may therefore be economically more feasible. However, at the current stage
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200 150 100 50 0 0
1.5
3
4.5
Cycles
Fig. 5. Concentration of EDTA (a), Fe (b) and Ca (c) in extractants after the final, 6th addition of EDTA. Extractants were treated in the ozone/UV unit in 4.5 treatment cycles. Error bars represent standard deviation from the mean value (n = 3).
of development, the proposed EDTA recycling methods either require the use of expensive materials or are technically demanding. For example, trans-complexation (Kim and Ong, 1999) is operationally complicated and could prove even more difficult if the EDTA is complexed with more than one other heavy metal (in multi-contaminated soils). Although transcomplexation allows partial EDTA recycling, it also involves consumption of other chemicals and does not provide a solution for the final treatment of EDTA contaminated extractants. The feasibility of evaporation of the extractant (Di Palma et al., 2003a) is probably constrained by the high cost of water evaporation, a high energy consuming operation. In reverse osmosis of the soil extractant (Di Palma et al., 2003b), colloidal particles (clays and humic materials) and bacteria could clog the membranes and thus diminish the
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References
before after 0
Soil depth (cm)
-5.5
-11
-16.5
-22 0
500
1000
Pb (mg kg-1)
1500
0
500
1000
1500
Zn (mg kg-1)
Fig. 6. Pb and Zn concentration through the soil profile in soil column before (dotted line) and after (solid line) heap leaching with six-consecutive additions of 2.5 mmol kg1 EDTA. Error bars represent standard deviation from the mean value (n = 3).
performance and shorten the life-time of the membranes. The high price of anion exchange resins (more than $100 for 500 g), proposed for removal of metal– EDTA complexes from soil extractants (Tejowulan and Hendershot, 1998), is a constraint that could be overcome by effective resin recycling, which, however, still needs to be developed. 4. Conclusions The results of our laboratory scale study indicated that the use of ozone/UV and absorption of released heavy metals is viable for treatment of EDTA soil extractants, obtained during heap leaching of soils principally contaminated with Pb. Final treated extractants were almost free of heavy metals and EDTA, and safe for discharge. During the process treated extractants were successfully re-used as a medium for consecutive EDTA additions and for rinsing the soil to remove residual EDTA. Ozone/UV treatment of extractant could therefore also reduce the requirements for process water. The use of ozone/UV treatment of EDTA soil extractant could lead to environmentally safe, efficient new heap leaching remediation technologies. Acknowledgement This work was supported by the Slovenian Ministry for Education, Science and Sport, Grant J4-6134-048104/4.03.
APHA, 1995. Chitra, S., Paramasivan, K., Sinha, P.K., Lal, K.B.J., 2003. Ultrasonic treatment of liquid waste containing EDTA. J. Clean. Prod. 12, 429–435. Dirilgen, N., 1998. Effect of pH and chelator EDTA on Cr toxicity and accumulation in Lemma minor. Chemosphere 37, 771–783. Di Palma, L., Ferrantelli, P., Merli, C., Biancifiori, F.J., 2003a. Recovery of EDTA and metal precipitation from soil flushing solutions. J. Hazard. Mater. B 103, 153–168. Di Palma, L., Ferrantelli, P., Merli, C., Petrucci, E., 2003b. Treatment of the solution extracted from metal contaminated soils by reverse osmosis and chemical precipitation. Annli Chim. 93, 1005–1011. Di Palma, L., Ferrantelli, P., Pitzolu, I., Verdone, N., 2005. EDTA leaching of copper from contaminated soils: Experimental study and transport model application for parameters estimation. Environ. Technol. 26, 189–198. Elliot, H.A., Brown, G.A., 1989. Comparative evaluation of NTA and EDTA for extractive decontamination of Pbpolluted soils. Water Air Soil Pollut. 45, 361–369. Gilbert, E., Hoffmann-Glewe, S., 1990. Ozonation of ethylenediaminetetraacetic acid (EDTA) in aqueous solution, influence of pH value and metal ions. Water Res. 24, 39–44. Hamano, T., Mitsuhashi, Y., Kojma, N., Aoki, N., 1993. Sensitive spectrophotometric method for the determination of ethylenediaminetetraacetic acid in foods. Analyst 118, 909–912. Hanson, A.T., Samani, Z., Dwyer, B., Jacquez, R., 1992. Heap leaching as a solvent-extraction technique for remediation of metals-contaminates soils. In: Sabatini, D.A., Knox, R.C. (Eds.), Proceedings of the ACS Symposium Series No. 491. American Chemical Society, Transport and Remediation of Subsurface Contaminants, Washington, USA, 108–121. Heil, D.M., Samani, A., Hanson, T., Rudd, B., 1998. Remediation of lead contaminated soil by EDTA. I. Batch and column studies. Water Air Soil Pollut. 113, 77–95. Hoigne, J., Bader, H., 1976. Role of hydroxyl radical reactions in ozonation processes in aqueous solutions. Water Res. 10, 377–386. Juang, R.S., Wang, S.W., 2000. Electrolytic recovery of binary metals and EDTA from strong complexed solutions. Water Res. 34, 3179–3185. Kalra, Y.P., Maynard, D.G., 1991. Methods Manual for Forest Soil and Plant Analysis. Canadian Forest Service, Northern Forestry Centre, Edmonton. Kim, C., Ong, S.-K., 1999. Recycling of lead-contaminated EDTA wastewater. J. Hazard. Mater. B 69, 273–286. Korhonen, M.S., Metsarinne, S.E., Tuhkanen, T.A., 2000. Removal of ethylenediaminetetraacetic acid (EDTA) from pulp mill effluents by ozonation. Ozone Sci. Eng. 22, 279– 286. Lestan, D., Grcman, H., Zupan, M., Bacac, N., 2003. Relationship of soil properties to fractionation of Pb and Zn in soil and their uptake into Plantago lanceolata. Soil Sediment Contam. 12, 507–522. Martell, A.E., Smith, R.M., 2003. NIST Critically Selected Stability Constants of Metal Complexes; Version 7.0, NIST, Gaithersburg.
N. Finzˇgar, D. Lesˇtan / Chemosphere 63 (2006) 1736–1743 Masten, J.S., Davies, S.H.R., 1997. Efficacy of in-situ ozonation for the remediation of PAH contaminated soils. J. Contam. Hydrol. 28, 327–335. Munoz, F., von Sonntag, C.J., 2000. The reaction of ozone with tertiary amines including the complexing agents nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) in aqueous solution. Chem. Soc., Perkin Trans. 2, 2029–2033. No¨rtemann, B., 1999. Biodegradation of EDTA. Appl. Microbiol. Biotechnol. 51, 751–759. Nowack, B., 2002. Environmental chemistry of aminopolycarboxylate chelating agents. Environ. Sci. Technol. 36, 4009– 4016. Peters, R.W., 1999. Chelant extraction of heavy metals from contaminated soils. J. Hazard. Mater. 66, 151–210.
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Steele, M.C., Pichtel, J., 1998. Ex-situ remediation of a metalcontaminated superfund soil using selective extractants. J. Environ. Eng. 124, 639–645. Sun, B., Zhao, F.J., Lombi, E., McGrath, S.P., 2001. Leaching of heavy metals from contaminated soils using EDTA. Environ. Pollut. 113, 111–120. Tejowulan, R.S., Hendershot, W.H., 1998. Removal of trace metals from contaminated soils using EDTA incorporating resin trapping techniques. Environ. Pollut. 103, 135– 142. Theodoratos, P., Papassiopi, N., Georgoudis, T., Kontopoulos, A., 2000. Selective removal of lead from calcareous polluted soils using the Ca–EDTA salt. Water Air Soil Pollut. 122, 351–368.