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Electrochemical EDTA recycling with sacrificial Al anode for remediation of Pb contaminated soil
Environmental Pollution 158 (2010) 2710e2715
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Electrochemical EDTA recycling with sacrificial Al anode for remediation of Pb contaminated soil Maja Pociecha, Domen Lestan* Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia
Aluminium anode at alkaline pH in conventional electrolytic cell enables efficient recycling of EDTA as a part of soil washing remediation technology.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2010 Received in revised form 19 April 2010 Accepted 23 April 2010
Recycling chelant is a precondition for cost-effective EDTA-based soil remediation. Extraction with EDTA removed 67.5% of Pb from the contaminated soil and yielded washing solution with 1535 mg L1 Pb and 33.4 mM EDTA. Electrochemical treatment of the washing solution using Al anode, current density 96 mA cm2 and pH 10 removed 90% of Pb from the solution (by electrodeposition on the stainless steel cathode) while the concentration of EDTA in the treated solution remained the same. The obtained data indicate that the Pb in the EDTA complex was replaced by electro-corroded Al after electro-reduction of the EDTA and subsequently removed from the solution. Additional soil extraction with the treated washing solution resulted in total removal of 87% of Pb from the contaminated soil. The recycled EDTA retained the Pb extraction potential through several steps of soil extraction and washing solution treatment, although part of the EDTA was lost by soil absorption. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Lead Contaminated soil Remediation EDTA Electrochemical treatment
1. Introduction Recent decades have witnessed the rapid development of various technologies for the remediation of heavy metals contaminated soils. One of the most promising methods is soil washing with aqueous solutions of chelants. The advantages of chelants include a high efficiency of metal extraction, good solubility of the metal complexes formed and minor impact on the soil’s physical, chemical and biological properties compared to acid soil extraction (Lim et al., 2004). For Pb contaminated soils, ethylenediamine tetraacetic acid (EDTA) has often been shown to be the most effective chelant (Lestan et al., 2008). In practice, however, the use of EDTA in full-scale is prohibited by a large volumes of waste washing solution generated, which must be treated before disposal. Effective treatment methods for waste washing solution, particularly recycling and reuse of EDTA, are needed. Although several EDTA recycling procedures have been demonstrated on a laboratory scale, there is currently no practical and commercially available method. Ager and Marshall (2001) used zero-valent bimetallic mixtures (Mg0ePd0, Mg0eAg0) to precipitate Pb from the solution while liberating EDTA in alkaline pH. Metals liberated from the EDTA complex were cemented to the surfaces of
* Corresponding author. E-mail address: [email protected] (D. Lestan). 0269-7491/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2010.04.014
the excess magnesium or removed from the solution as insoluble hydroxides. The method is efficient but could be economically prohibitive. Hong et al. (1999) separated Pb from EDTA with Na2S, resulting in almost complete recovery of metals through precipitation in the form of insoluble metal sulphides. This method has found limited application due to the hazardous nature of the reagents and the sludge produced. Kim and Ong (1999) recycled chelant from Pb-EDTA solution by substituting Pb with Fe3þ in acidic conditions, followed by precipitation of the released Pb with phosphate near neutral pH. Fe3þ ions were then precipitated as hydroxides at high pH using NaOH, thus liberating EDTA. The process does not use expensive or hazardous reagents but it is complicated, with several operations involved and a slow kinetics of some reactions. Electrochemical technologies are simple and efficient methods for the treatment of many wastewaters, characterised by a compact size of the equipment, simplicity of operation, and low capital and operating costs (Chen, 2004). Johnson et al. (1972) reported that using a Pt anode in a conventional electrolytic cell oxidized EDTA into CO2, formaldehyde and ethylendiamine, and could thus potentially be used for treating waste soil washing solutions. However, simultaneous recovery of metals and chelant was not possible with this system. A two chamber cell separated by a cation-selective membrane was therefore proposed to allow liberation of metals from the complex and to prevent oxidation of EDTA at the anode. Metals, including Pb, were reduced and deposited onto the cathode and the EDTA was simultaneously
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recycled (Allen and Chen, 1993; Juang and Wang, 2000). This method, however, is prone to operational problems such as membrane fouling and degradation. We recently proposed electrocoagulation for the removal of metals and EDTA from a washing solution obtained after extraction of primarily Pb contaminated soil (Pociecha and Lestan, 2009). In electrocoagulation, Al (or Fe) ions are generated from the sacrificial anode. The reactions for the electrochemical system at the Al anode are as follows (Eqs. 1 and 2):
Al/Al3þ þ 3e
(1)
Al3þ þ 3H2 O / AlðOHÞ3
(2)
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The resulting solution with some finely suspended precipitate of EDTA, which is insoluble in acidic media, was stored in the cold for Pb determination. At the end of the electrochemical treatment, the cathodes were etched with 30 mL of 65% HNO3 to dissolve and later measure the concentration of electro-deposited Pb. The Al anode was weighed before and after treatment of the washing solution to determine the amount of electro-corroded Al. During the electrolysis, the surface of the Al anode was passivised by an oxide/ hydroxide layer, which increased the potential between the electrodes (Mouedhen et al., 2008). In order to break down this passive layer and reduce the power consumption, we applied small amounts of Cl (as NaCl) when the voltage increased above 8 V (Chen, 2004). To prepare the recycled EDTA solution for soil extractions, we electrochemically treated the washing solution at pH 10 for 24 min (contact time) and separated the recycled EDTA solution from the Al hydroxide precipitate by centrifugation at 2880 g for 30 min. 2.4. Treatment of the soil washing solution with dosing Al-salt
3þ
The nascent Al ions are very effective coagulants and form large networks of Al-O-Al-OH flocks, with a large surface area and considerable absorption capacity (Shen et al., 2003). In our study Pb was almost entirely removed from the soil washing solution while, to our surprise, some EDTA remained in the washing solution. This early result indicated separation of Pb from EDTA. In the current work, the feasibility of electrochemical separation of EDTA and Pb in waste soil washing solution using an Al anode and a single-chamber electrolytic cell was studied.
A weight of 4110 mg of AlCl3 was dosed into 100 mL of the soil washing solution with pH 10 and gently stirred for 22.68, 45.36 and 113.4 min, which corresponds to 1, 2 and 3-times the total contact time of the electrochemical treatment, respectively. The amount of chemically dosed Al was the same as the molar amount of Al electrocorroded from the anode during electrochemical treatment. During the coagulation treatment with Al dosing, the pH of the washing solution was kept at pH 10, using 5 M NaOH. The precipitate was removed from the treated solution by centrifugation at 2880 g for 30 min, and the concentrations of Pb and EDTA in the supernatant measured. Afterwards, the pH of the chemically treated washing solution was adjusted to 4.3 and the solution reused for soil Pb extraction.
2. Materials and methods
2.5. EDTA determination
2.1. Soil properties
The concentration of EDTA was determined spectrophotometrically according to the procedure of Hamano et al. (1993). The method involves the reaction of EDTA in washing solution with Fe3þ under acidic conditions to produce the Fe-EDTA chelate (trans-complexation), followed by the removal of excess of Fe3þ by chelate extraction in the aqueous phase using chloroform and N-benzoyl-N-phenylhydroxylamine and the formation of a chromophore with 4,7-diphenyl-1,10-phenanthrolinedisulfonic acid. Using a spectrophotometer, absorbance was measured at 535 nm against a blank solution with the 4,7-diphenyl-1,10-phenanthroline-disulfonic acid replaced with an equal volume of distilled water. The limit of EDTA quantification was 20 mg L1.
Soil contaminated with 3980 60 mg kg1 of Pb was collected from the 0e30 cm surface layer of a vegetable garden in the Me zica Valley, Slovenia. The Me zica Valley has been exposed to more than three hundred years of active lead mining and smelting. For standard pedological analysis, the pH in soils was measured in a 1/2.5 (w/v) ratio of soil and 0.01 M CaCl2 water solution suspension. Soil samples were analyzed for organic matter (as C content) by modified WalkleyeBlack titrations (ISO 14235, 1998), cation exchange capacity (CEC) by the ammonium acetate method (Rhoades, 1982) and soil texture by the pipette method (Fiedler et al., 1964). The following values were obtained: pH 6.57, C content 8.2%, CEC 20.7 mval 100 g1 of soil, sand 51.0%, silt 42.5%, clay 6.5%. The soil texture was sandy loam. 2.2. Soil washing The extraction of soil with EDTA solutions was performed in two scales. To obtain the washing solution for the electrochemical treatment, we placed 0.5 kg of air-dried soil and 875 mL of aqueous solution (1:1.75 soil: washing solution ratio) of 75 mmol of EDTA (disodium salt) per kg of soil (43 mM EDTA), pH 4.3, in 1.5 L flasks. Soil was extracted on a rotating shaker (3040 GFL, Germany) for 72 h at 16 RPM. Approximately 400 mL of the washing solution was decanted from each flask after the soil was allowed to settle for 24 h. The decanted washing solution was filtered (filter paper was wide-pored, grade 388, density was 80 g m2). The same procedure was used to extract the soil with the recycled EDTA solution, except that centrifugation at 2880 g for 5 min and not decantation was used to separate the soil from the washing solution, to minimise solution loss. Fine particles were removed from the solution by filtration as described above.
2.6. Pb determination Air-dried soil samples (1 g) were ground in an agate mill, sieved through a 160 mm mesh and digested in a glass beaker on a hotplate with 28 mL of aqua regia solution (HCl and HNO3 in a 3:1 ratio (v/v)) for 2 h at 110 C. Condensation of evaporating fumes was achieved via circulation of cool tap water through the glass tubes placed on top of the glass beakers. After cooling, digested samples were filtered through Whatman no. 4 filter paper and diluted with deionised water up to 100 mL. The pseudo-total concentration of Pb was determined by flame (acetylene/ air) AAS with a deuterium background correction (Varian, AA240FS). The Pb in the solutions was determined by AAS directly. A standard reference material used in inter-laboratory comparisons (Wepal 2004.3/4, Wageningen University, Wageningen, Netherlands) was used in the digestion and analysis as part of the QA/QC protocol. The limit of quantification for Pb was 0.01 mg L1. Reagent blank and analytical duplicates were also used where appropriate in order to ensure accuracy and precision in the analysis. 2.7. Statistics
2.3. Electrochemical treatment of the soil washing solution The electrolytic cell consisted of an Al anode placed between two stainless steel cathodes (distance ¼ 10 mm), the overall anode surface 63 cm2 and the surface area ratio between the cathodes and anode 1:1. The electrodes were placed in 500 mL of magnetically stirred soil washing solution in a 1.0 L flask. Current density was kept at 96 mA cm2, and the cell voltage measured with a DC power supply (Elektronik Invent, Ljubljana, Slovenia). The electrode cell was cooled using a cooling mantle and tap water (flow rate 250 mL min1) to keep the temperature of the treated washing solution below 35 C. The contact time of the electrochemical treatment was calculated as the ratio of the electrode cell volume to the volume of the washing solution and multiplied by the operation time (initially 30 min of operation time equalled 3.78 min of contact time). During the electrochemical treatment, the pH of the washing solution was regulated to pH 6, 8 and 10 by drop-vice addition of 5 M NaOH and HCl. Samples (20 mL) of washing solution were collected periodically and the pH and EC measured immediately. Samples were afterwards centrifuged at 2880 g for 10 min and the supernatant stored in the cold for further analysis of Pb and EDTA. The pellet (mainly Al hydroxide precipitate) was suspended in 200 mL of deionised water acidified with 37% HCl to pH 1.5.
The Duncan multiple range test (Statgraphics 4.0 for Windows) was used to determine the statistical significance (P < 0.05) between different treatments.
3. Results and discussion 3.1. Soil washing Soil extraction with 75 mM EDTA per kg of dry soil removed 67.5% of Pb. The molar ratio between the Pb initially present in the soil and the EDTA in the washing solution was 1:3.9. It is known that even strong chelants such as EDTA cannot remove heavy metals from the soil entirely, even at high molar ratios of EDTA vs. heavy metal concentration applied (Nowack et al., 2006). However, the Pb residual in soil after stringent soil washing with EDTA is encapsulated in soil minerals or strongly bound to the non-labile
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soil fractions and therefore essentially non-leachable and nonbioavailable (Udovic et al., 2009). The concentrations of Pb and EDTA in the soil washing solution before treatment in the electrolytic cell were 1535 and 12 444 mg L1 (33.4 mM), respectively. The pH of the washing solution before treatment was 7.91.
Pb (mg L-1)
A
3.2. Electrochemical treatment of soil washing solution
1800 1600
pH 6
1400
pH 8
1200
pH 10
1000 800 600 400
M EDTA2
Cell voltage
!
M2þ þ EDTA4
M2þ þ 2e / MðsÞ
(3) (4)
Metals liberated from the EDTA complex could then be removed from the solution by direct electrodeposition on the cathode, precipitation as insoluble hydroxides, or absorption and co-precipitation on Al hydroxide flocks according to the following reaction (Eq. 5):
AlðOHÞ3 þ M2þ / AlðOHÞO2 M þ 2Hþ
200 0
B
450 400 350
Pb (mg L-1)
Soil washing solutions were treated at various pH (6, 8 and 10). The pH of the solution tended to increase with treatment time, since the electrochemical system generated enough OH at the electrode to counteract the Hþ released by the formation of Al hydroxides as a net final product (Canizares et al., 2006). Solution treated at pH 10 consequently required very little pH adjustment. The voltage between the electrodes also tended to increase with treatment time, regardless of the pH of the washing solution. The main reason for the voltage increase was the passivisation of the Al anode surface by formation of an insulating film of Al oxide (Mouedhen et al., 2008). In order to break down the passive film and thus to reduce the cell voltage surge and increase of power consumption, small amounts of Cl (as NaCl) were applied (Chen, 2004), to keep the voltage close to initial 8 V. The amount of Al consumed from the Al anode was 14.6 2.5, 12.0 0.4 and 9.4 0.4 g L1 of solutions treated at pH 6, 8 and 10, respectively. The amount of electro-corroded Al decreases with increasing pH. A higher aluminium current efficiency at higher alkaline conditions than at neutral is generally found in electrochemical systems (Chen, 2004). The electro-conductivity of the washing solution increased from an initial 6.1 to up to 10.0 mS cm1 (solution with pH 6). This increase followed the increasing concentration of charged Al hydroxide (electrolyte) (data not shown) during the electrochemical process. pH is an important operating factor influencing the performance of electrochemical processes (Chen, 2004). The effect of different pH of electrochemical treatment on the dynamics of Pb removal and precipitation from the washing solution and on the mass balance of Pb is shown in Fig. 1 and Table 1. During electrochemical treatment, metals complexed to EDTA could be removed from the soil washing solution by absorption on Al hydroxide flocks (electrocoagulation). Metals (M) could also be released from the EDTA complex after reduction reactions at the cathode (Juang and Wang, 2000), Eqs. 3 and 4.
300 250 200 150 100 50 0 0
5
10
15
20
25
30
35
Contact time (min) Fig. 1. Removal of Pb from the washing solution (A) and accumulation of Pb in the precipitate (B). Washing solutions were electrochemically treated at pH 6, 8 and 10. Error bars represent standard deviation from the mean value (n ¼ 3).
After treatment at pH 10, the EDTA remained almost entirely preserved quantitatively in the washing solution, while at pH 6 and 8, approximately one half of the initial EDTA was removed (Fig. 2). EDTA was presumably removed from the washing solution by electrocoagulation adsorption on Al hydroxide flocks, although some EDTA degradation by anodic oxidation (Johnson et al., 1972) might also occur. At pH 6 and 8, negatively charged EDTA complexes (M-EDTA2-) are probably partly absorbed on various monomeric and polymeric Al hydroxides species formed during the electrocoagulation process (Nowack and Sigg, 1996). These species, 4þ 3þ 4þ such as Al(OH)2þ, Al(OH)þ 2 , Al2(OH)2 , Al6(OH)15 , Al7(OH)17 , 4þ 5þ , Al13O4(OH)7þ or Al (OH) , have a long lasting positive Al8(OH)20 24 13 34 charge before they finally transform into amorphous Al(OH)3 according to complex precipitation kinetics (Mouedhen et al., 2008; Rebhun and Lurie, 1993). Amorphous Al(OH)3 is a typical amphoteric metal hydroxide, which leads in alkaline conditions to the reaction shown in Eq. (6) (Chen, 2004).
AlðOHÞ3 þ OH / AlðOHÞ 4
(6)
The formation of negatively charged Al hydroxides explains
(5)
Table 1 indicates that, in all electrochemical treatments, the majority of Pb was removed from solution by electrodeposition on the cathode. However, at pH 8 and 10, a significantly higher amount of Pb was removed this way than at pH 6. This could again be explained by a higher current efficiency of Al anode electrochemical systems at higher pH (Chen, 2004). The phenomenon presumably also explains the faster rate of Pb removal from the washing solution treated at pH 10 (Fig. 1).
Table 1 Balance of Pb after electrochemical treatment of the soil washing solution at different pH. Standard deviation from the mean value (n ¼ 3) was calculated. Treated washing solution Pb balance (%) In solution Precipitated Electrodeposited S pH 6 pH 8 pH 10
30 6 13 2 11 3
20 7 71 15 2
33 5 68 5 62 5
83 10 88 7 88 7
M. Pociecha, D. Lestan / Environmental Pollution 158 (2010) 2710e2715
14000
70 pH 6 pH 8
10000
pH 10
8000 6000 4000
20
0 15
20
25
30
35
Contact time (min) Fig. 2. Concentration of EDTA in the washing solutions during electrochemical treatment at pH 6, 8 and 10. Error bars represent standard deviation from the mean value (n ¼ 3).
why, at pH 10, negatively charged EDTA was also not be removed from the washing solution by electrocoagulation (Fig. 2). Electrochemical treatment at pH 10 efficiently removed the majority of the Pb from the washing solution (Fig. 1), while the EDTA remained almost completely preserved in the washing solution (Fig. 2). As explained below, these data indicate the replacement of Pb from the complex with EDTA, removal of liberated Pb from the solution, and formation of Al-EDTA complex (trans-complexation). EDTA is a hexaprotic system. The degree of EDTA protonation and complexation with metals depends on the pH of the washing solution and the nature of the metal ions present. Al has a lower complex formation stability constant (Ks) than Pb (log Ks are 16.3 and 18.0 (at 20 C and ionic strength m ¼ 0.1) for Pb and Al, respectively, Martell and Smith, 2003). However, Al ions formed in abundant concentrations during electro-corrosion of the Al anode and Treacy et al. (2000) showed that the stability of Al-EDTA complex was higher in a solution with pH 9 than in solutions with pH 7 and 4. On the other hand, the stability of Pb-EDTA complex decreases in solutions with pH > 8 (Chang et al., 2007). The trans-complexation hypothesis, however, still needs to be proven. Our data indicate that using an Al anode enables electrochemical treatment in a conventional, simple, one-compartment electrolytic cell, without significant EDTA degradation. Presumably Al is oxidized at the anode (Eq. (1)) preferentially to EDTA oxidation, due to the high Al reactivity (electro-positivity). The standard electrode (oxidation) potential of the Al/Al3þ couple is 1.66 V (Evangelou, 1998). This is the first report on using this type of electrochemical system for EDTA recycling from Pb soil washing solution. The significance of pH for effective treatment was demonstrated.
c
30
0 10
Treated solution Non-treated solution
40
10
5
Fresh solution
b
50
2000 0
a
60 Removed Pb (%)
EDTA (mg L-1)
12000
2713
d
d e
Original soil
Extracted soil
Fig. 3. Removal of Pb from the original and previously extracted soil using fresh EDTA solution, EDTA solution after soil extraction but not treated, and electrochemically treated EDTA solution. Soil washing solutions were treated at pH 10 and, for soil extraction, the solutions’ pH was adjusted to 4.3. EDTA concentration was equal in all solutions (30 mM). Error bars represent standard deviation from the mean value (n ¼ 3); letters (a, b, c) denote statistically different Pb removal potential within the categories of original and extracted soil, according to the Duncan test (p < 0.05).
Interestingly, the potential of the treated washing solution to extract Pb from previously (once) extracted soil was even higher than that of fresh EDTA solution, although the difference was not statistically significant (P < 0.05), Fig. 3. Since we used a high molar ratio of EDTA against soil Pb (as usual in soil washing; Nowack et al., 2006) only part of the EDTA in the washing solution was complexed to Pb (approximately 22% calculated from data on Pb and EDTA concentration in the washing solution, section 3.1.), some EDTA was presumably left in the original form or in various stages of protonation. Spent non-treated washing solution was therefore expected to retain some Pb extraction potential, as indeed shown in Fig. 3. Table 2 shows the potential of recycled EDTA for Pb extraction from the original soil through several steps of soil extraction and washing solution treatment. First, fresh EDTA solution was used (in the 1st ext./treat. step) following by the use of electrochemically treated washing solutions (2nd and 3rd ext./treat. step) for soil extraction. Washing solution treated once retained 86% and solution treated twice 69% of the Pb extraction efficiency of fresh EDTA solution (calculated from data on percentages of Pb removed from the soil presented in Table 2). The decrease of Pb extraction potential can be explained by the loss of EDTA from the solution (Table 2), mainly due to EDTA absorption into the soil solid phase (Nowack and Sigg, 1996). Some EDTA (<10%) was also lost during the solution treatment phase, in which EDTA was either precipitated or anodically oxidized. Alternatively, EDTA could also be degraded by chlorine (Cl2) and hypochlorite (HOCl), which are strong oxidants and could be generated anodically following additions of NaCl into the electrolytic chamber to break down the anodic passive film (Chen, 2004).
3.3. Recycling and reuse of the treated EDTA soil washing solution 3.4. Chemical dosing of Al The efficiency of EDTA recycled from a washing solution electrochemically treated at pH 10 to extract Pb from the soil is shown in Fig. 3. After adjustment to pH 4.3 (pH of the fresh EDTA washing solution), the treated washing solution retained almost 90% of the Pb extraction potential (from original soil) compared to freshly prepared EDTA solution of the same molarity and pH. In this experiment (soil was extracted in two separate batches), fresh EDTA solution removed 63% of the Pb from the original soil and further extraction using the treated solution on the once extracted soil removed an additional 24% of Pb (Fig. 3). In total, using EDTA recycling, almost 90% of Pb was removed from the soil.
Electrochemical treatment of wastewaters with a sacrificial Al anode is an alternative to more commonly used chemical coagulation of pollutants by dosing Al-salts. To compare these two methods, the same amount of Al (as AlCl3) was dosed in the washing solution (pH 10) as the amount of Al electro-corroded during the corresponding electrochemical treatment. Chemical dosing (Table 3) removed very little Pb from the washing solution, much less than electrochemical treatment (Fig. 1, Table 1), but did remove some EDTA. The reason is that chemical coagulation with AlCl3 removed Pb with EDTA complexes, while electrochemical
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Table 2 Potential of fresh and electrochemically treated EDTA solutions for Pb extraction from original (previously not-extracted) soil during three consecutive steps of soil extraction and washing solution treatment. Washing solutions were electrochemically treated at pH 10 and, for soil extraction, the pH was adjusted to 4.3. The percentage of lost EDTA after soil extraction (Lost EDTA e extraction) and after electrochemical treatment of the solution (Lost EDTA e treatment) was calculated. Soil extraction/Solution treatment
Pb removed Initial EDTA (%) concentration (mM)
EDTA conc. after extraction (mM)
EDTA conc. after treatment (mM)
Lost EDTA e extraction (mM) (%)
Lost EDTA e treatment (%)
1. ext./treat. 2. ext./treat. 3. ext./treat.
71 61 49
32 23 15
29 22 /
26 19 32
9.9 6.4
43 29 22
treatment liberated Pb and separated the EDTA. Chemical dosing did not lead to EDTA recycling; the Pb extraction potential of the chemically treated solution (with pH adjusted to 4.3) was even lower than the extraction efficiency of non-treated washing solution with the same pH (27 3% removed Pb, Fig. 3). 3.5. Cost and safety considerations An accurate evaluation of the costs associated with soil remediation would require a pilot-scale experiment. However, the cost of Al and electricity consumption, Al-hydroxide sludge disposal and treatment of the final spent washing solution (which represent the major part of the material costs) can be extrapolated from the obtained data. If we assume two re-cycles of EDTA washing solution treatment and reuse, then (including compensation for lost EDTA, Table 2) extraction of 1 ton of soil (with 75 mmol kg1 of fresh and recycled EDTA) would require 13.9 kg of EDTA. At a price of 1.3 V per kg1 EDTA (information obtained from a major European EDTA producer) this translates into 18.1 V. Treatment of the washing solution and EDTA recycling at a constant current density of 96 mA cm2 and an average voltage of 8 V would require approximately 115 kW h and, at an approximate cost of 0.1 V per kW h, this translates into 11.5 V. During the treatment/EDTA recycling, approximately 5 kg of Al would be expected to electro-corrode from the anode. The current prices of Al in the open market are below 1.6 V kg1, which equals 8 V for spent Al. During the process, approximately 20 kg of liquid Al hydroxide sludge was formed, calculated per ton of treated soil. The sludge can be deposited after treatment, i.e., after solidification (and stabilisation of metals) with cement. The disposal cost of solid hazardous waste transportation and disposal was assessed to be approximately 200 V per ton (Meunier et al., 2006), which adds an additional 4 V to the total cost. Since Al hydroxide is the major component of aluminium ore bauxite (together with AlO(OH) and Al2O3), it could perhaps be reused in the Hall-Héroult process to recycle aluminium and avoid disposal costs. The spent washing solution contains EDTA and Pb, which need to be completely removed before safe discharge. In a previous paper, we proposed an electrochemical advanced oxidation process using a boron-doped diamond anode for EDTA degradation and removal of Pb from treated solution by (electro) precipitation (Finzgar and Lestan, 2008). The electricity cost for this operation was estimated to be 10.3 V ton1 of soil. The total Table 3 Pb and EDTA removal from the washing solution after chemical dosing of AlCl3 at pH 10, and Pb extraction efficiency of chemically treated soil washing solution (after pH was adjusted to 4.3). The treatment time of chemical dosing was selected as a 1e3 multiple of the electrochemical treatment time (tel). Standard deviation from the mean value (n ¼ 3) was calculated. Treatment time (min)
Pb removed (%)
EDTA removed (%)
Pb extraction potential (%)
23 (1 tel) 45 (2 tel) 113 (3 tel)
35 45 93
34 9 27 4 26 3
26 2 23 6 21 0
estimated cost of major material expenses for the treatment of 1 ton of contaminated soil thus amounts to approximately 52 V. This cost does not include capital investment in the equipment, which, in terms of electrochemical technologies, is characterised as relatively low (Chen, 2004). The cost seems favourable compared to the current cost of soil washing, which can go up to 350 V per ton (Summergill and Scott, 2005). During the proposed remediation technique, some deposition of Al from the EDTA complex into the soil is expected. Aluminium is known to reduce plant growth on acid soils, in which Al3þ cations disturb root growth and function. However, Al constitutes aluminosilicate minerals and sesquioxides and is naturally present in the soil. It is harmless to plants, immobile and non-toxic in pH-neutral soils (Andersson, 1988).
4. Conclusions The following conclusions can be drawn from our study: Electrochemical treatment of soil washing solution obtained after EDTA extraction of Pb contaminated soil, using an Al anode in a conventional electrolytic cell at pH 10, efficiently separated EDTA and Pb. Pb was relatively efficiently removed from the treated washing solution (>85%, Fig. 1), mostly by electrodeposition on the cathode. Electrochemical treatment separated EDTA in an active form. We demonstrated that, after treatment, the EDTA solution retains almost all its Pb extraction potential. Less than 10% of EDTA was lost during electrochemical treatment. More EDTA was lost from the solution due to absorption onto the soil solid phases during soil extraction. Chemical dosing of Al was not effective in separating Pb and EDTA in the washing solution. We conclude, therefore, that electro-reduction of EDTA on a cathode (Eq. (3)) is essential for the exchange of Pb from the EDTA complex. Electrochemical treatment of the washing solution with an Al anode at alkaline pH has potential for cost-effective recycling and reuse of EDTA as a part of soil washing technologies.
Acknowledgement This work was supported by the Slovenian Research Agency, Grant J4-9277.
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M. Pociecha, D. Lestan / Environmental Pollution 158 (2010) 2710e2715 Canizares, P., Martinez, F., Jimenez, C., Lobato, J., Rodrigo, M.A., 2006. Coagulation and electrocoagulation of wastes polluted with dyes. Environ. Sci. Technol. 40, 6418e6424. Chang, F.-C., Lo, S.-L., Ko, C.-H., 2007. Recovery of copper and chelating agents from sludge extracting solutions. Sep. Pur. Technol. 53, 49e56. Chen, G., 2004. Electrochemical technologies in wastewater treatment. Sep. Pur. Technol. 38, 11e41. Evangelou, V.P., 1998. Environmental Soil and Water Chemistry. John Wiley & Sons Inc., New York. Fiedler, H.J., Hoffmann, Fr, Schmiedel, H., 1964. Die Untersuchung der Boden, Band 1, first ed. Theodor Steinkopff, Dresden und Leipzig. Finzgar, N., Lestan, D., 2008. The two-phase leaching of Pb, Zn and Cd contaminated soil using EDTA and electrochemical treatment of the washing solution. Chemosphere 73, 1484e1491. Hamano, T., Mitsuhashi, Y., Kojma, N., Aoki, N., Shibata, M., Ito, Y., Oji, Y., 1993. Sensitive spectrophotometric method for the determination of ethylenediaminetetraacetic acid in foods. Analyst 118, 909e912. Hong, A.P.K., Li, C., Banerji, S.K., Regmi, T., 1999. Extraction, recovery, and biostability of EDTA for remediation of heavy metal-contaminated soil. J. Soil Contamin. 8, 81e103. ISO 14235, 1998. Soil quality e Determination of Organic Carbon by Sulfochromic Oxidation. International Organisation for Standardization, Genève, Switzerland. Johnson, J.W., Jiang, H.W., Hanna, S.B., James, W.J., 1972. Anodic oxidation of ethylenediaminetetraacetic acid on Pt in acid sulphate solution. J. Electrochem. Soc. 119, 574e580. Juang, R.-S., Wang, S.-W., 2000. Electrolytic recovery of binary metals and EDTA from strong complexed solutions. Water Res. 34, 3179e3185. Kim, C.S., Ong, S.K., 1999. Recycling of lead-contaminated EDTA wastewater. J. Hazard. Mater. 69, 273e286. Lestan, D., Luo, C., Li, X., 2008. The use of chelating agents in the remediation of metal-contaminated soils: a review. Environ. Pollut. 153, 3e13.
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Lim, T.T., Tay, J.H., Wang, J.Y., 2004. Chelating agent-enhanced heavy metal extraction from a contaminated acidic soil. J. Environ. Eng. 130, 59e66. Martell, A.E., Smith, R.M., 2003. NIST Critically Selected Stability Constants of Metal Complexes; Version 7.0. NIST, Gaithersburg. Meunier, N., Drogui, P., Montane, C., Hausler, R., Mercier, G., Blais, J.F., 2006. Comparison between electrocoagulation and chemical precipitation for metals removal from acidic soil leachate. J. Hazard. Mater. 137, 581e590. Mouedhen, G., Feki, M., De Petris Wery, M., Ayedi, H.F., 2008. Behavior of aluminium electrodes in electrocoagulation process. J. Hazard. Mater. 150, 124e135. Nowack, B., Schulin, R., Robinson, B.H., 2006. Critical assessment of chelantenhanced metal phytoextraction. Environ. Sci. Technol. 40, 5225e5232. Nowack, B., Sigg, L., 1996. Adsorption of EDTA and metal-EDTA complexes to goethite. J. Colloid Interface Sci. 177, 106e121. Pociecha, M., Lestan, D., 2009. Using electrocoagulation for metal and chelant separation from washing solution after EDTA leaching of Pb, Zn and Cd contaminated soil. J. Hazard. Mater. 165, 533e539. Rebhun, M., Lurie, M., 1993. Control of organic matter by coagulation and floc separation. Water Sci. Technol. 27, 1e20. Rhoades, J.D., 1982. Cation exchange capacity. In: Page, A.L., Miller, R.H., Keeney, D.R. (Eds.), Methods of Soils Analysis. Part 2: Chemical and Microbiological Properties, Serie Agronomy N 9. ASA e SSSA, Madison, pp. 149e157. Shen, F., Gao, P., Chen, X., Chen, G., 2003. Electrochemical removal of fluoride ions from industrial watewaters. Chem. Eng. Sci. 58, 987e993. Summergill, I.M., Scott, D.W., 2005. Remediation Technology Costs in the UK & Europe. In: Proceedings of the 9th International FZK/TNO Conference on SoilWater Systems, Bordeaux, France. Treacy, G.M., Rudd, A.L., Breslin, C.B., 2000. Electrochemical behaviour of aluminium in the presence of EDTA-containing chloride solutions. J. Appl. Electrochem. 30, 675e683. Udovic, M., Drobne, D., Lestan, D., 2009. Bioaccumulation in Porcellio scaber (Crustacea, Isopoda) as a measure of the EDTA remediation efficiency of metalpolluted soil. Environ. Pollut. 157, 2822e2829.
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Electrochemical EDTA recycling with sacrificial Al anode for remediation of Pb contaminated soil Maja Pociecha, Domen Lestan* Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia
Aluminium anode at alkaline pH in conventional electrolytic cell enables efficient recycling of EDTA as a part of soil washing remediation technology.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2010 Received in revised form 19 April 2010 Accepted 23 April 2010
Recycling chelant is a precondition for cost-effective EDTA-based soil remediation. Extraction with EDTA removed 67.5% of Pb from the contaminated soil and yielded washing solution with 1535 mg L1 Pb and 33.4 mM EDTA. Electrochemical treatment of the washing solution using Al anode, current density 96 mA cm2 and pH 10 removed 90% of Pb from the solution (by electrodeposition on the stainless steel cathode) while the concentration of EDTA in the treated solution remained the same. The obtained data indicate that the Pb in the EDTA complex was replaced by electro-corroded Al after electro-reduction of the EDTA and subsequently removed from the solution. Additional soil extraction with the treated washing solution resulted in total removal of 87% of Pb from the contaminated soil. The recycled EDTA retained the Pb extraction potential through several steps of soil extraction and washing solution treatment, although part of the EDTA was lost by soil absorption. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Lead Contaminated soil Remediation EDTA Electrochemical treatment
1. Introduction Recent decades have witnessed the rapid development of various technologies for the remediation of heavy metals contaminated soils. One of the most promising methods is soil washing with aqueous solutions of chelants. The advantages of chelants include a high efficiency of metal extraction, good solubility of the metal complexes formed and minor impact on the soil’s physical, chemical and biological properties compared to acid soil extraction (Lim et al., 2004). For Pb contaminated soils, ethylenediamine tetraacetic acid (EDTA) has often been shown to be the most effective chelant (Lestan et al., 2008). In practice, however, the use of EDTA in full-scale is prohibited by a large volumes of waste washing solution generated, which must be treated before disposal. Effective treatment methods for waste washing solution, particularly recycling and reuse of EDTA, are needed. Although several EDTA recycling procedures have been demonstrated on a laboratory scale, there is currently no practical and commercially available method. Ager and Marshall (2001) used zero-valent bimetallic mixtures (Mg0ePd0, Mg0eAg0) to precipitate Pb from the solution while liberating EDTA in alkaline pH. Metals liberated from the EDTA complex were cemented to the surfaces of
* Corresponding author. E-mail address: [email protected] (D. Lestan). 0269-7491/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2010.04.014
the excess magnesium or removed from the solution as insoluble hydroxides. The method is efficient but could be economically prohibitive. Hong et al. (1999) separated Pb from EDTA with Na2S, resulting in almost complete recovery of metals through precipitation in the form of insoluble metal sulphides. This method has found limited application due to the hazardous nature of the reagents and the sludge produced. Kim and Ong (1999) recycled chelant from Pb-EDTA solution by substituting Pb with Fe3þ in acidic conditions, followed by precipitation of the released Pb with phosphate near neutral pH. Fe3þ ions were then precipitated as hydroxides at high pH using NaOH, thus liberating EDTA. The process does not use expensive or hazardous reagents but it is complicated, with several operations involved and a slow kinetics of some reactions. Electrochemical technologies are simple and efficient methods for the treatment of many wastewaters, characterised by a compact size of the equipment, simplicity of operation, and low capital and operating costs (Chen, 2004). Johnson et al. (1972) reported that using a Pt anode in a conventional electrolytic cell oxidized EDTA into CO2, formaldehyde and ethylendiamine, and could thus potentially be used for treating waste soil washing solutions. However, simultaneous recovery of metals and chelant was not possible with this system. A two chamber cell separated by a cation-selective membrane was therefore proposed to allow liberation of metals from the complex and to prevent oxidation of EDTA at the anode. Metals, including Pb, were reduced and deposited onto the cathode and the EDTA was simultaneously
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recycled (Allen and Chen, 1993; Juang and Wang, 2000). This method, however, is prone to operational problems such as membrane fouling and degradation. We recently proposed electrocoagulation for the removal of metals and EDTA from a washing solution obtained after extraction of primarily Pb contaminated soil (Pociecha and Lestan, 2009). In electrocoagulation, Al (or Fe) ions are generated from the sacrificial anode. The reactions for the electrochemical system at the Al anode are as follows (Eqs. 1 and 2):
Al/Al3þ þ 3e
(1)
Al3þ þ 3H2 O / AlðOHÞ3
(2)
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The resulting solution with some finely suspended precipitate of EDTA, which is insoluble in acidic media, was stored in the cold for Pb determination. At the end of the electrochemical treatment, the cathodes were etched with 30 mL of 65% HNO3 to dissolve and later measure the concentration of electro-deposited Pb. The Al anode was weighed before and after treatment of the washing solution to determine the amount of electro-corroded Al. During the electrolysis, the surface of the Al anode was passivised by an oxide/ hydroxide layer, which increased the potential between the electrodes (Mouedhen et al., 2008). In order to break down this passive layer and reduce the power consumption, we applied small amounts of Cl (as NaCl) when the voltage increased above 8 V (Chen, 2004). To prepare the recycled EDTA solution for soil extractions, we electrochemically treated the washing solution at pH 10 for 24 min (contact time) and separated the recycled EDTA solution from the Al hydroxide precipitate by centrifugation at 2880 g for 30 min. 2.4. Treatment of the soil washing solution with dosing Al-salt
3þ
The nascent Al ions are very effective coagulants and form large networks of Al-O-Al-OH flocks, with a large surface area and considerable absorption capacity (Shen et al., 2003). In our study Pb was almost entirely removed from the soil washing solution while, to our surprise, some EDTA remained in the washing solution. This early result indicated separation of Pb from EDTA. In the current work, the feasibility of electrochemical separation of EDTA and Pb in waste soil washing solution using an Al anode and a single-chamber electrolytic cell was studied.
A weight of 4110 mg of AlCl3 was dosed into 100 mL of the soil washing solution with pH 10 and gently stirred for 22.68, 45.36 and 113.4 min, which corresponds to 1, 2 and 3-times the total contact time of the electrochemical treatment, respectively. The amount of chemically dosed Al was the same as the molar amount of Al electrocorroded from the anode during electrochemical treatment. During the coagulation treatment with Al dosing, the pH of the washing solution was kept at pH 10, using 5 M NaOH. The precipitate was removed from the treated solution by centrifugation at 2880 g for 30 min, and the concentrations of Pb and EDTA in the supernatant measured. Afterwards, the pH of the chemically treated washing solution was adjusted to 4.3 and the solution reused for soil Pb extraction.
2. Materials and methods
2.5. EDTA determination
2.1. Soil properties
The concentration of EDTA was determined spectrophotometrically according to the procedure of Hamano et al. (1993). The method involves the reaction of EDTA in washing solution with Fe3þ under acidic conditions to produce the Fe-EDTA chelate (trans-complexation), followed by the removal of excess of Fe3þ by chelate extraction in the aqueous phase using chloroform and N-benzoyl-N-phenylhydroxylamine and the formation of a chromophore with 4,7-diphenyl-1,10-phenanthrolinedisulfonic acid. Using a spectrophotometer, absorbance was measured at 535 nm against a blank solution with the 4,7-diphenyl-1,10-phenanthroline-disulfonic acid replaced with an equal volume of distilled water. The limit of EDTA quantification was 20 mg L1.
Soil contaminated with 3980 60 mg kg1 of Pb was collected from the 0e30 cm surface layer of a vegetable garden in the Me zica Valley, Slovenia. The Me zica Valley has been exposed to more than three hundred years of active lead mining and smelting. For standard pedological analysis, the pH in soils was measured in a 1/2.5 (w/v) ratio of soil and 0.01 M CaCl2 water solution suspension. Soil samples were analyzed for organic matter (as C content) by modified WalkleyeBlack titrations (ISO 14235, 1998), cation exchange capacity (CEC) by the ammonium acetate method (Rhoades, 1982) and soil texture by the pipette method (Fiedler et al., 1964). The following values were obtained: pH 6.57, C content 8.2%, CEC 20.7 mval 100 g1 of soil, sand 51.0%, silt 42.5%, clay 6.5%. The soil texture was sandy loam. 2.2. Soil washing The extraction of soil with EDTA solutions was performed in two scales. To obtain the washing solution for the electrochemical treatment, we placed 0.5 kg of air-dried soil and 875 mL of aqueous solution (1:1.75 soil: washing solution ratio) of 75 mmol of EDTA (disodium salt) per kg of soil (43 mM EDTA), pH 4.3, in 1.5 L flasks. Soil was extracted on a rotating shaker (3040 GFL, Germany) for 72 h at 16 RPM. Approximately 400 mL of the washing solution was decanted from each flask after the soil was allowed to settle for 24 h. The decanted washing solution was filtered (filter paper was wide-pored, grade 388, density was 80 g m2). The same procedure was used to extract the soil with the recycled EDTA solution, except that centrifugation at 2880 g for 5 min and not decantation was used to separate the soil from the washing solution, to minimise solution loss. Fine particles were removed from the solution by filtration as described above.
2.6. Pb determination Air-dried soil samples (1 g) were ground in an agate mill, sieved through a 160 mm mesh and digested in a glass beaker on a hotplate with 28 mL of aqua regia solution (HCl and HNO3 in a 3:1 ratio (v/v)) for 2 h at 110 C. Condensation of evaporating fumes was achieved via circulation of cool tap water through the glass tubes placed on top of the glass beakers. After cooling, digested samples were filtered through Whatman no. 4 filter paper and diluted with deionised water up to 100 mL. The pseudo-total concentration of Pb was determined by flame (acetylene/ air) AAS with a deuterium background correction (Varian, AA240FS). The Pb in the solutions was determined by AAS directly. A standard reference material used in inter-laboratory comparisons (Wepal 2004.3/4, Wageningen University, Wageningen, Netherlands) was used in the digestion and analysis as part of the QA/QC protocol. The limit of quantification for Pb was 0.01 mg L1. Reagent blank and analytical duplicates were also used where appropriate in order to ensure accuracy and precision in the analysis. 2.7. Statistics
2.3. Electrochemical treatment of the soil washing solution The electrolytic cell consisted of an Al anode placed between two stainless steel cathodes (distance ¼ 10 mm), the overall anode surface 63 cm2 and the surface area ratio between the cathodes and anode 1:1. The electrodes were placed in 500 mL of magnetically stirred soil washing solution in a 1.0 L flask. Current density was kept at 96 mA cm2, and the cell voltage measured with a DC power supply (Elektronik Invent, Ljubljana, Slovenia). The electrode cell was cooled using a cooling mantle and tap water (flow rate 250 mL min1) to keep the temperature of the treated washing solution below 35 C. The contact time of the electrochemical treatment was calculated as the ratio of the electrode cell volume to the volume of the washing solution and multiplied by the operation time (initially 30 min of operation time equalled 3.78 min of contact time). During the electrochemical treatment, the pH of the washing solution was regulated to pH 6, 8 and 10 by drop-vice addition of 5 M NaOH and HCl. Samples (20 mL) of washing solution were collected periodically and the pH and EC measured immediately. Samples were afterwards centrifuged at 2880 g for 10 min and the supernatant stored in the cold for further analysis of Pb and EDTA. The pellet (mainly Al hydroxide precipitate) was suspended in 200 mL of deionised water acidified with 37% HCl to pH 1.5.
The Duncan multiple range test (Statgraphics 4.0 for Windows) was used to determine the statistical significance (P < 0.05) between different treatments.
3. Results and discussion 3.1. Soil washing Soil extraction with 75 mM EDTA per kg of dry soil removed 67.5% of Pb. The molar ratio between the Pb initially present in the soil and the EDTA in the washing solution was 1:3.9. It is known that even strong chelants such as EDTA cannot remove heavy metals from the soil entirely, even at high molar ratios of EDTA vs. heavy metal concentration applied (Nowack et al., 2006). However, the Pb residual in soil after stringent soil washing with EDTA is encapsulated in soil minerals or strongly bound to the non-labile
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soil fractions and therefore essentially non-leachable and nonbioavailable (Udovic et al., 2009). The concentrations of Pb and EDTA in the soil washing solution before treatment in the electrolytic cell were 1535 and 12 444 mg L1 (33.4 mM), respectively. The pH of the washing solution before treatment was 7.91.
Pb (mg L-1)
A
3.2. Electrochemical treatment of soil washing solution
1800 1600
pH 6
1400
pH 8
1200
pH 10
1000 800 600 400
M EDTA2
Cell voltage
!
M2þ þ EDTA4
M2þ þ 2e / MðsÞ
(3) (4)
Metals liberated from the EDTA complex could then be removed from the solution by direct electrodeposition on the cathode, precipitation as insoluble hydroxides, or absorption and co-precipitation on Al hydroxide flocks according to the following reaction (Eq. 5):
AlðOHÞ3 þ M2þ / AlðOHÞO2 M þ 2Hþ
200 0
B
450 400 350
Pb (mg L-1)
Soil washing solutions were treated at various pH (6, 8 and 10). The pH of the solution tended to increase with treatment time, since the electrochemical system generated enough OH at the electrode to counteract the Hþ released by the formation of Al hydroxides as a net final product (Canizares et al., 2006). Solution treated at pH 10 consequently required very little pH adjustment. The voltage between the electrodes also tended to increase with treatment time, regardless of the pH of the washing solution. The main reason for the voltage increase was the passivisation of the Al anode surface by formation of an insulating film of Al oxide (Mouedhen et al., 2008). In order to break down the passive film and thus to reduce the cell voltage surge and increase of power consumption, small amounts of Cl (as NaCl) were applied (Chen, 2004), to keep the voltage close to initial 8 V. The amount of Al consumed from the Al anode was 14.6 2.5, 12.0 0.4 and 9.4 0.4 g L1 of solutions treated at pH 6, 8 and 10, respectively. The amount of electro-corroded Al decreases with increasing pH. A higher aluminium current efficiency at higher alkaline conditions than at neutral is generally found in electrochemical systems (Chen, 2004). The electro-conductivity of the washing solution increased from an initial 6.1 to up to 10.0 mS cm1 (solution with pH 6). This increase followed the increasing concentration of charged Al hydroxide (electrolyte) (data not shown) during the electrochemical process. pH is an important operating factor influencing the performance of electrochemical processes (Chen, 2004). The effect of different pH of electrochemical treatment on the dynamics of Pb removal and precipitation from the washing solution and on the mass balance of Pb is shown in Fig. 1 and Table 1. During electrochemical treatment, metals complexed to EDTA could be removed from the soil washing solution by absorption on Al hydroxide flocks (electrocoagulation). Metals (M) could also be released from the EDTA complex after reduction reactions at the cathode (Juang and Wang, 2000), Eqs. 3 and 4.
300 250 200 150 100 50 0 0
5
10
15
20
25
30
35
Contact time (min) Fig. 1. Removal of Pb from the washing solution (A) and accumulation of Pb in the precipitate (B). Washing solutions were electrochemically treated at pH 6, 8 and 10. Error bars represent standard deviation from the mean value (n ¼ 3).
After treatment at pH 10, the EDTA remained almost entirely preserved quantitatively in the washing solution, while at pH 6 and 8, approximately one half of the initial EDTA was removed (Fig. 2). EDTA was presumably removed from the washing solution by electrocoagulation adsorption on Al hydroxide flocks, although some EDTA degradation by anodic oxidation (Johnson et al., 1972) might also occur. At pH 6 and 8, negatively charged EDTA complexes (M-EDTA2-) are probably partly absorbed on various monomeric and polymeric Al hydroxides species formed during the electrocoagulation process (Nowack and Sigg, 1996). These species, 4þ 3þ 4þ such as Al(OH)2þ, Al(OH)þ 2 , Al2(OH)2 , Al6(OH)15 , Al7(OH)17 , 4þ 5þ , Al13O4(OH)7þ or Al (OH) , have a long lasting positive Al8(OH)20 24 13 34 charge before they finally transform into amorphous Al(OH)3 according to complex precipitation kinetics (Mouedhen et al., 2008; Rebhun and Lurie, 1993). Amorphous Al(OH)3 is a typical amphoteric metal hydroxide, which leads in alkaline conditions to the reaction shown in Eq. (6) (Chen, 2004).
AlðOHÞ3 þ OH / AlðOHÞ 4
(6)
The formation of negatively charged Al hydroxides explains
(5)
Table 1 indicates that, in all electrochemical treatments, the majority of Pb was removed from solution by electrodeposition on the cathode. However, at pH 8 and 10, a significantly higher amount of Pb was removed this way than at pH 6. This could again be explained by a higher current efficiency of Al anode electrochemical systems at higher pH (Chen, 2004). The phenomenon presumably also explains the faster rate of Pb removal from the washing solution treated at pH 10 (Fig. 1).
Table 1 Balance of Pb after electrochemical treatment of the soil washing solution at different pH. Standard deviation from the mean value (n ¼ 3) was calculated. Treated washing solution Pb balance (%) In solution Precipitated Electrodeposited S pH 6 pH 8 pH 10
30 6 13 2 11 3
20 7 71 15 2
33 5 68 5 62 5
83 10 88 7 88 7
M. Pociecha, D. Lestan / Environmental Pollution 158 (2010) 2710e2715
14000
70 pH 6 pH 8
10000
pH 10
8000 6000 4000
20
0 15
20
25
30
35
Contact time (min) Fig. 2. Concentration of EDTA in the washing solutions during electrochemical treatment at pH 6, 8 and 10. Error bars represent standard deviation from the mean value (n ¼ 3).
why, at pH 10, negatively charged EDTA was also not be removed from the washing solution by electrocoagulation (Fig. 2). Electrochemical treatment at pH 10 efficiently removed the majority of the Pb from the washing solution (Fig. 1), while the EDTA remained almost completely preserved in the washing solution (Fig. 2). As explained below, these data indicate the replacement of Pb from the complex with EDTA, removal of liberated Pb from the solution, and formation of Al-EDTA complex (trans-complexation). EDTA is a hexaprotic system. The degree of EDTA protonation and complexation with metals depends on the pH of the washing solution and the nature of the metal ions present. Al has a lower complex formation stability constant (Ks) than Pb (log Ks are 16.3 and 18.0 (at 20 C and ionic strength m ¼ 0.1) for Pb and Al, respectively, Martell and Smith, 2003). However, Al ions formed in abundant concentrations during electro-corrosion of the Al anode and Treacy et al. (2000) showed that the stability of Al-EDTA complex was higher in a solution with pH 9 than in solutions with pH 7 and 4. On the other hand, the stability of Pb-EDTA complex decreases in solutions with pH > 8 (Chang et al., 2007). The trans-complexation hypothesis, however, still needs to be proven. Our data indicate that using an Al anode enables electrochemical treatment in a conventional, simple, one-compartment electrolytic cell, without significant EDTA degradation. Presumably Al is oxidized at the anode (Eq. (1)) preferentially to EDTA oxidation, due to the high Al reactivity (electro-positivity). The standard electrode (oxidation) potential of the Al/Al3þ couple is 1.66 V (Evangelou, 1998). This is the first report on using this type of electrochemical system for EDTA recycling from Pb soil washing solution. The significance of pH for effective treatment was demonstrated.
c
30
0 10
Treated solution Non-treated solution
40
10
5
Fresh solution
b
50
2000 0
a
60 Removed Pb (%)
EDTA (mg L-1)
12000
2713
d
d e
Original soil
Extracted soil
Fig. 3. Removal of Pb from the original and previously extracted soil using fresh EDTA solution, EDTA solution after soil extraction but not treated, and electrochemically treated EDTA solution. Soil washing solutions were treated at pH 10 and, for soil extraction, the solutions’ pH was adjusted to 4.3. EDTA concentration was equal in all solutions (30 mM). Error bars represent standard deviation from the mean value (n ¼ 3); letters (a, b, c) denote statistically different Pb removal potential within the categories of original and extracted soil, according to the Duncan test (p < 0.05).
Interestingly, the potential of the treated washing solution to extract Pb from previously (once) extracted soil was even higher than that of fresh EDTA solution, although the difference was not statistically significant (P < 0.05), Fig. 3. Since we used a high molar ratio of EDTA against soil Pb (as usual in soil washing; Nowack et al., 2006) only part of the EDTA in the washing solution was complexed to Pb (approximately 22% calculated from data on Pb and EDTA concentration in the washing solution, section 3.1.), some EDTA was presumably left in the original form or in various stages of protonation. Spent non-treated washing solution was therefore expected to retain some Pb extraction potential, as indeed shown in Fig. 3. Table 2 shows the potential of recycled EDTA for Pb extraction from the original soil through several steps of soil extraction and washing solution treatment. First, fresh EDTA solution was used (in the 1st ext./treat. step) following by the use of electrochemically treated washing solutions (2nd and 3rd ext./treat. step) for soil extraction. Washing solution treated once retained 86% and solution treated twice 69% of the Pb extraction efficiency of fresh EDTA solution (calculated from data on percentages of Pb removed from the soil presented in Table 2). The decrease of Pb extraction potential can be explained by the loss of EDTA from the solution (Table 2), mainly due to EDTA absorption into the soil solid phase (Nowack and Sigg, 1996). Some EDTA (<10%) was also lost during the solution treatment phase, in which EDTA was either precipitated or anodically oxidized. Alternatively, EDTA could also be degraded by chlorine (Cl2) and hypochlorite (HOCl), which are strong oxidants and could be generated anodically following additions of NaCl into the electrolytic chamber to break down the anodic passive film (Chen, 2004).
3.3. Recycling and reuse of the treated EDTA soil washing solution 3.4. Chemical dosing of Al The efficiency of EDTA recycled from a washing solution electrochemically treated at pH 10 to extract Pb from the soil is shown in Fig. 3. After adjustment to pH 4.3 (pH of the fresh EDTA washing solution), the treated washing solution retained almost 90% of the Pb extraction potential (from original soil) compared to freshly prepared EDTA solution of the same molarity and pH. In this experiment (soil was extracted in two separate batches), fresh EDTA solution removed 63% of the Pb from the original soil and further extraction using the treated solution on the once extracted soil removed an additional 24% of Pb (Fig. 3). In total, using EDTA recycling, almost 90% of Pb was removed from the soil.
Electrochemical treatment of wastewaters with a sacrificial Al anode is an alternative to more commonly used chemical coagulation of pollutants by dosing Al-salts. To compare these two methods, the same amount of Al (as AlCl3) was dosed in the washing solution (pH 10) as the amount of Al electro-corroded during the corresponding electrochemical treatment. Chemical dosing (Table 3) removed very little Pb from the washing solution, much less than electrochemical treatment (Fig. 1, Table 1), but did remove some EDTA. The reason is that chemical coagulation with AlCl3 removed Pb with EDTA complexes, while electrochemical
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Table 2 Potential of fresh and electrochemically treated EDTA solutions for Pb extraction from original (previously not-extracted) soil during three consecutive steps of soil extraction and washing solution treatment. Washing solutions were electrochemically treated at pH 10 and, for soil extraction, the pH was adjusted to 4.3. The percentage of lost EDTA after soil extraction (Lost EDTA e extraction) and after electrochemical treatment of the solution (Lost EDTA e treatment) was calculated. Soil extraction/Solution treatment
Pb removed Initial EDTA (%) concentration (mM)
EDTA conc. after extraction (mM)
EDTA conc. after treatment (mM)
Lost EDTA e extraction (mM) (%)
Lost EDTA e treatment (%)
1. ext./treat. 2. ext./treat. 3. ext./treat.
71 61 49
32 23 15
29 22 /
26 19 32
9.9 6.4
43 29 22
treatment liberated Pb and separated the EDTA. Chemical dosing did not lead to EDTA recycling; the Pb extraction potential of the chemically treated solution (with pH adjusted to 4.3) was even lower than the extraction efficiency of non-treated washing solution with the same pH (27 3% removed Pb, Fig. 3). 3.5. Cost and safety considerations An accurate evaluation of the costs associated with soil remediation would require a pilot-scale experiment. However, the cost of Al and electricity consumption, Al-hydroxide sludge disposal and treatment of the final spent washing solution (which represent the major part of the material costs) can be extrapolated from the obtained data. If we assume two re-cycles of EDTA washing solution treatment and reuse, then (including compensation for lost EDTA, Table 2) extraction of 1 ton of soil (with 75 mmol kg1 of fresh and recycled EDTA) would require 13.9 kg of EDTA. At a price of 1.3 V per kg1 EDTA (information obtained from a major European EDTA producer) this translates into 18.1 V. Treatment of the washing solution and EDTA recycling at a constant current density of 96 mA cm2 and an average voltage of 8 V would require approximately 115 kW h and, at an approximate cost of 0.1 V per kW h, this translates into 11.5 V. During the treatment/EDTA recycling, approximately 5 kg of Al would be expected to electro-corrode from the anode. The current prices of Al in the open market are below 1.6 V kg1, which equals 8 V for spent Al. During the process, approximately 20 kg of liquid Al hydroxide sludge was formed, calculated per ton of treated soil. The sludge can be deposited after treatment, i.e., after solidification (and stabilisation of metals) with cement. The disposal cost of solid hazardous waste transportation and disposal was assessed to be approximately 200 V per ton (Meunier et al., 2006), which adds an additional 4 V to the total cost. Since Al hydroxide is the major component of aluminium ore bauxite (together with AlO(OH) and Al2O3), it could perhaps be reused in the Hall-Héroult process to recycle aluminium and avoid disposal costs. The spent washing solution contains EDTA and Pb, which need to be completely removed before safe discharge. In a previous paper, we proposed an electrochemical advanced oxidation process using a boron-doped diamond anode for EDTA degradation and removal of Pb from treated solution by (electro) precipitation (Finzgar and Lestan, 2008). The electricity cost for this operation was estimated to be 10.3 V ton1 of soil. The total Table 3 Pb and EDTA removal from the washing solution after chemical dosing of AlCl3 at pH 10, and Pb extraction efficiency of chemically treated soil washing solution (after pH was adjusted to 4.3). The treatment time of chemical dosing was selected as a 1e3 multiple of the electrochemical treatment time (tel). Standard deviation from the mean value (n ¼ 3) was calculated. Treatment time (min)
Pb removed (%)
EDTA removed (%)
Pb extraction potential (%)
23 (1 tel) 45 (2 tel) 113 (3 tel)
35 45 93
34 9 27 4 26 3
26 2 23 6 21 0
estimated cost of major material expenses for the treatment of 1 ton of contaminated soil thus amounts to approximately 52 V. This cost does not include capital investment in the equipment, which, in terms of electrochemical technologies, is characterised as relatively low (Chen, 2004). The cost seems favourable compared to the current cost of soil washing, which can go up to 350 V per ton (Summergill and Scott, 2005). During the proposed remediation technique, some deposition of Al from the EDTA complex into the soil is expected. Aluminium is known to reduce plant growth on acid soils, in which Al3þ cations disturb root growth and function. However, Al constitutes aluminosilicate minerals and sesquioxides and is naturally present in the soil. It is harmless to plants, immobile and non-toxic in pH-neutral soils (Andersson, 1988).
4. Conclusions The following conclusions can be drawn from our study: Electrochemical treatment of soil washing solution obtained after EDTA extraction of Pb contaminated soil, using an Al anode in a conventional electrolytic cell at pH 10, efficiently separated EDTA and Pb. Pb was relatively efficiently removed from the treated washing solution (>85%, Fig. 1), mostly by electrodeposition on the cathode. Electrochemical treatment separated EDTA in an active form. We demonstrated that, after treatment, the EDTA solution retains almost all its Pb extraction potential. Less than 10% of EDTA was lost during electrochemical treatment. More EDTA was lost from the solution due to absorption onto the soil solid phases during soil extraction. Chemical dosing of Al was not effective in separating Pb and EDTA in the washing solution. We conclude, therefore, that electro-reduction of EDTA on a cathode (Eq. (3)) is essential for the exchange of Pb from the EDTA complex. Electrochemical treatment of the washing solution with an Al anode at alkaline pH has potential for cost-effective recycling and reuse of EDTA as a part of soil washing technologies.
Acknowledgement This work was supported by the Slovenian Research Agency, Grant J4-9277.
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