Study on Optimal Conditions for Recovery of EDTA from Soil Washing Effluents

Transcription

1 Study on Optimal Conditions for Recovery of EDTA from Soil Washing Effluents Irene M. C. Lo, M.ASCE, 1 and Weihua Zhang 2 Abstract: The recovery of ethylenediaminetetraacetic acid EDTA from washing effluents is essential to reduce the cost of EDTAenhanced soil washing and the production of wastewater. This study evaluated a recovery method, in which Pb or Zn was first dissociated from Pb or Zn EDTA complex through the replacement reaction by adding FeCl 3 and then removed as phosphate precipitates through adding Na 2 HPO 4. Finally, Fe III was removed as Fe OH 3 precipitates through adding Ca OH 2. As a result, EDTA was recovered as Ca EDTA for further use in soil washing process. The optimal conditions for EDTA recovery, including the molar ratios of FeCl 3 and Na 2 HPO 4 to EDTA as well as the ph after adding Na 2 HPO 4 and adding Ca OH 2, were well investigated. Under the optimal conditions, 96% of Pb or 83% of Zn was removed from the Pb or Zn EDTA, respectively. The four-cycle recovery and reuse of EDTA experiments indicated the recovered EDTA from soil washing effluents did not lose much chelating capacity for Pb removal. However, there is a loss of 15% of its chelating capacity in the first cycle reuse for Zn-contaminated soil washing due to substantial Zn residual in the recovered EDTA solution. DOI: / ASCE : CE Database subject headings: Heavy metals; Soils; Washing; Precipitation; Recycling. Introduction Chemical-enhanced soil washing is a promising technology to remediate the contaminated soils with potential applicability and economical feasibility Tobia 1993; Paff et al. 1994; Roy et al. 1994; Davis and Singh 1995; Griffiths 1995; Robin and Krishna 1996; Abumaizar and Smith 1999; Saponaro et al. 2002; Chu 2003; Tandy et al Chelating agents such as ethylenediaminetetraacetic acid EDTA have a chelating effect on heavy metals in contaminated soils, resulting in the formation of various complexes, which are appreciably soluble and highly stable in solution. This chelating effect greatly increases the solubility of heavy metals in the washing solution and is conducive to remove heavy metals from soils Pichtel and Pichtel Experimental studies show EDTA-enhanced soil washing is appreciably effective to remove heavy metals in contaminated soils Elliott et al. 1989; Peters and Shem 1992; Davis and Singh 1995; Peters In addition, Lo and Yang 1999 developed an empirical equation based on the sum of the released heavy metals from the individual components to check if there were interactions among the different soil components when mixed, and the estimated 1 Associate Professor, Dept. of Civil Engineering, The Hong Kong Univ. of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. cemclo@ust.hk 2 Research Assistant, Dept. of Civil Engineering, The Hong Kong Univ. of Science and Technology, Clear Water Bay, Kowloon, Hong Kong. zhangwh@ust.hk Note. Discussion open until April 1, Separate discussions must be submitted for individual papers. To extend the closing date by one month, a written request must be filed with the ASCE Managing Editor. The manuscript for this paper was submitted for review and possible publication on July 20, 2004; approved on February 28, This paper is part of the Journal of Environmental Engineering, Vol. 131, No. 11, November 1, ASCE, ISSN /2005/ / $ values agreed favorably well with the experimental results for the 0.01 mol/l M EDTA system. This study laid a sound foundation on the wide application of EDTA-enhanced soil washing. However, the high cost of EDTA has prevented its large-scale application in the remediation of metal contaminated soils. Additionally, EDTA is not ready to be biodegraded when it is chelated with heavy metals Davis and Green All these problems make this study on the recovery and reuse of EDTA necessary. The recovery of EDTA can be achieved by acidifying the metal EDTA solution or using the precipitation of metals reacted with a hydroxide, sulfide, or oxalate. Palma et al conducted a two-step experiment to recover the EDTA. An initial evaporation process led to concentrate the diluted Pb EDTA through reducing the solution volume by about 75%. This was followed by the acidification of the residual solution, which precipitated more than 93% of the used EDTA. This method is practically feasible for a concentrated EDTA solution rather than a diluted EDTA solution, as it requires a costly concentrating process. Under an alkaline condition, the metal EDTA complex can be dissociated by precipitating metals as a hydroxide based on the stability of metal EDTA complexes and the hydroxide products. However, this method is only effective for few metal complexes due to the high stability of metal EDTA complexes. The cations of Fe 3+, for example, can be completely dissociated from the metal EDTA complex using the Fe OH 3 precipitation under a moderate alkaline ph condition, such as ph For the most of metal complexes, the cations of metals can be completely dissociated only under an extreme ph condition. Based on theoretical calculations, only 12% of Zn, for example, can form Zn OH 2 precipitates in a M Zn EDTA solution at ph Rudd et al found sulfide precipitation could reduce the lead concentration to an acceptable level. However, this method is only effective for some cases such as Cu EDTA and Pb EDTA solutions, but ineffective for the Zn EDTA solution according to the theoretical equilibrium JOURNAL OF ENVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2005 / 1507

2 Cd contaminations. However, the possible factors that could affect the effectiveness of this method have not been investigated in detail yet. This paper is aimed to 1 develop a technique for recovery and reuse of EDTA from heavy metal-contaminated soil washing effluents; 2 investigate the potential effects of added FeCl 3 and Na 2 HPO 4 as well as ph for different precipitation; and 3 optimize the conditions for this recovering process. Materials and Methods Uncontaminated Soil Fig. 1. Conditional stability constants of metal EDTA complexes data from Tingborn and Wannirien 1979 calculations. Huang et al and Juang and Wang 2000 applied the electrolysis process in conjunction with a cationexchange membrane for the EDTA recovery from metal EDTA complexes. Their results showed the recovery of metal and EDTA was approximate 99 and 91%, respectively. However, the ph in the cathode compartment was found to increase with time as a result of hydroxyl ion production, which caused the degradation of the membrane Allen and Chen Based on the relationship of conditional stability constants of the metal EDTA to ph, Kim and Ong 1999 developed a method to recycle Pb EDTA wastewater. Chelated Pb or Zn were initially replaced by Fe III at ph 2 4, where the conditional stability constant of Fe III EDTA is larger than that of Pb EDTA, and the dissociated Pb were then precipitated with PO 4 3 or SO 4 2. Finally, Fe III EDTA complexes were dissociated at a high ph and Fe OH 3 precipitates were formed, since the formation of Fe OH 3 K sp_fe OH 3 =10 38 Snoeyink and Jenkins 1980 is more stable than Fe III EDTA K Fe-EDTA = Allen and Chen The experimental results indicated this method had a potential application to recover EDTA from soil washing effluents. In addition, according to conditional stability conditions of metal EDTA complexes as show in Fig. 1, most heavy metals, such as Cu, Zn, Ni, Co, and Cd, have similar properties to Pb, of which the conditional stability constants chelated with EDTA are smaller than that of Fe EDTA under a certain ph. These characteristics may be adopted for the recovery of EDTA from the effluents of EDTA-enhanced soil washing of Cu, Zn, Ni, Co, and The uncontaminated soil used in this study is completely decomposed granite CDG, which is the most commonly found soils in Hong Kong. The soils were air dried at room temperature and pre-sieved with a U.S. standard No. 10 sieve to remove coarses and stones. Table 1 summarized the properties of the studied soils and the measurement methods as well. The soil has relatively high sand, but relatively low clay and silt as well as organic matter content, so it is suitable for the soil washing treatment USEPA 1988; 1989; 1990; Willichowski Synthetic Pb- or Zn-Contaminated Soil The synthetic Pb-contaminated soils were prepared by completely mixing the 6.00 g air-dried soils with ml of 90 mg/l Pb NO 3 2 solution in a 250 ml polyester bottle using a rotator at 60 rpm for 6 h. The soils and solution in the mixture were then separated using a Bechman Auegra 6 Centrifuge at 3,000 rpm for 10 min. The Pb concentration in the resulting supernatant was analyzed using atomic absorption spectrophotometer AAS. The resulting soils were air dried at room temperature for a week and stored in a desiccator. The adsorbed Pb on soils was about 1,400 mg/kg soil. The synthetic Zn-contaminated soils were prepared following a similar procedure to the Pb-contaminated soils, but 200 mg/l Zn NO 3 2 solution was used, and the resulting soils adsorbed Zn about 750 mg/kg soil herein. These synthetic contaminated soils were used in the study of the reuse of the recovered EDTA solution. Synthetic Pb or Zn EDTA Solutions The EDTA used in this study was reagent-grade tetrasodium salt Invitrogen, purity 99%. The synthetic Pb or Zn EDTA solution was prepared to simulate the EDTA washing effluents of Table 1. Properties of Uncontaminated Soils Soil properties Values Method used Equipment used ph 6.2 Method 9045 in USEPA SW Corning ph/ion analyzer Cation exchangeable capacity mmol + /kg Method 9081 in USEPA SW-846 Z-8200 Polarized Zeeman atomic absorption Spectrophotometer AAS Total carbons 0.079% 5000A Shimadzu Inorganic carbons 0.056% Total organic Total organic carbons 0.023% Carbon analyzer Particle size distribution ASTM Soil Classification Endecotts ISO laboratory Test sieves Silt and clay 75 M 7.24% Fine sand 425 m 34.33% Medium sand 2mm 58.43% 1508 / JOURNAL OF ENVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2005

3 Recovery of EDTA from the Synthetic Pb or Zn EDTA Solution Fig. 2. Procedure of the EDTA recovery from the synthetic Pb or Zn EDTA solution. Note: Me represents Pb or Zn. the contaminated soils. The Pb or Zn and EDTA concentrations in the synthetic solutions were based on the effluents of soil washing, where 1 g soils containing 1,400 mg/kg Pb or 750 mg/kg Zn were washed by 25 ml of M EDTA solution the molar ratio of Pb to EDTA, i.e., Pb : EDTA =1:2 atph7or25ml of M EDTA solution Zn : EDTA =1:1 at ph 9, respectively. Therefore, the synthetic Pb EDTA solution was prepared by mixing 21.6 ml of M EDTA and 56.0 ml of 1.60 g/l Pb NO 3 2 solution Pb : EDTA =1:2 in a 1,000-mL volumetric flask and diluting the mixture to 1,000 ml, and the ph was then adjusted to 7.0 by adding NaOH. The synthetic Zn EDTA solution was prepared by mixing 18.4 ml of M EDTA and 30.0 ml of 2.90 g/l Zn NO 3 2 solution Zn : EDTA =1:1 in a 1,000-mL volumetric flask and diluting the mixture to 1,000-mL, and the ph was then adjusted to 9.0. Pb or Zn and EDTA concentrations in these solutions were consistent with those in the effluent of soil washing, where 1 g soils containing 1,400 mg/kg Pb or 750 mg/kg Zn were washed by 25 ml of M or M EDTA solution, respectively. The optimal conditions for the EDTA recovery from the synthetic Pb EDTA or Zn EDTA solution were obtained using four series of batch experiments following the procedure shown in Fig. 2. A 0.05 M FeCl 3 solution was added into the synthetic Pb or Zn EDTA solution. As FeCl 3 decreased the ph of the solution, a ph adjustment was not necessary at this stage. The resulting solution was shaken for 2 h using a magnetic stirrer. A M Na 2 HPO 4 solution was then added into the mixture, and the ph was adjusted to facilitate the precipitation. The resulting precipitates were separated using GC-50 glass fiber filters. The ph of the filtrate was then adjusted to facilitate the Fe OH 3 precipitation by adding a 0.05 M Ca OH 2 mixed suspending solution. The formed precipitates were then separated using GC-50 glass fiber filters. The concentration of residual Pb or Zn in the filtrate was monitored using the AAS. The optimal conditions for the metal removal were obtained by comparing the performance under various combinations of experimental parameters, including the molar ratio of FeCl 3 to EDTA, the molar ratio of Na 2 HPO 4 to EDTA, ph after adding Na 2 HPO 4, and ph after adding Ca OH 2. The experimental parameters were listed in Table 2. Experiments 1 4 were sequentially carried out to probe the optimal FeCl 3 : EDTA, Na 2 HPO 4 : EDTA, ph after adding Na 2 HPO 4, and ph after adding Ca OH 2, respectively. The parameters with asterisks in Table 2 were the optimal conditions obtained from the preceding experiments. The remaining parameters were selected based on either trial experiments or theoretical predictions. Reuse of the Recovered EDTA Solutions The chelating capacity of recovered EDTA was studied by applying it on the synthetic Pb- or Zn-contaminated soils. 1 g Pb-contaminated soils were mixed with 25 ml of M fresh EDTA solution at ph 7.0 at speed of 60 rpm for 2 h. Similarly, 1 g Zn-contaminated soils were completely mixed with 25 ml of M fresh EDTA solution at Table 2. Parameters of Batch Experiments for the Synthetic Pb EDTA or Zn EDTA Solution Index Optimal parameter to be probed EDTA solution to recover FeCl 3 : EDTA Na 2 HPO 4 : EDTA Parameter ph after adding Na 2 HPO 4 ph after adding Ca OH 2 Experiment 1 FeCl 3 : EDTA Pb EDTA 1:2, 3:4, 1:1, 3:2, 2:1 2: Zn EDTA 1:2, 3:4, 1:1, 3:2, 2:1 2: Experiment 2 Na 2 HPO 4 : EDTA Pb EDTA 1:1 * 1:3, 1:2, 2:3, 1: Zn EDTA 3:2 * 2:3, 1:1, 4: Experiment 3 ph after adding Pb EDTA 1:1 * 2:3 * 2.59, 3.0, 3.5, 4.07, 4.71, 4.91, 5.71 Na 2 HPO 4 Zn EDTA 3:2 * 4:3 * 5.0, 6.0, 7.0, 7.5, Experiment 4 ph after adding Pb EDTA 1:1 * 2:3 * 3.5 * 9.0, 10.0, 10.5, 11.0, 11.5 Ca OH 2 Zn EDTA 3:2 * 4:3 * 7.5 * 9.0, 10.0, 10.5, 11.0, 11.5 Note: Initial EDTA for Pb EDTA is M; for Zn EDTA is M. and * optimal operating conditions obtained from preceding experiments. JOURNAL OF ENVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2005 / 1509

4 FeCl 3 : EDTA =1:1. Taking into account of the amount of FeCl 3 used, the optimal FeCl 3 : EDTA was selected as 1:1. The Pb removal after adding Ca OH 2 was primarily due to adsorption of Fe OH 3. Fe OH 3 precipitates were observed occurring at ph 10.5, and the resulting precipitates adsorbed the residual Pb, leading to more Pb removal. Similar phenomena were also reported by Ridge and Sedlak 2004 and Deliyanni et al As also shown in Fig. 3, the highest total Zn removal efficiency and that after adding Na 2 HPO 4 were achieved at FeCl 3 : EDTA =3:2. At ph 7.5 substantial red flocs were observed in the solution due to precipitation of FePO 4, and thus less free Fe 3+ was available to replace Zn to chelate with EDTA as shown in Fig. 3. Pb or Zn removal as a function of the molar ratio of FeCl 3 to EDTA for Experiment 1 ph 9.0. The mixture was then centrifuged at 3,000 rpm for 10 min, and the resulting supernatant was recovered under the optimal conditions obtained by the preceding experiments. Meanwhile, Pb or Zn concentration was monitored using the AAS. Finally, the recovered EDTA solution was reused to wash synthetic Pb- or Zn-contaminated soils following the similar process for the fresh EDTA solution. The recovery-reuse procedure was conducted over four cycles. Results and Discussion Optimal Molar Ratio of FeCl 3 to EDTA Experiment 1 was conducted to probe the optimal molar ratio of FeCl 3 to EDTA. The results were presented in Fig. 3. It was found that the highest Pb removal after adding Na 2 HPO 4 was achieved at FeCl 3 : EDTA =1:1 because of the integrated effect of the dissociation of Pb 2+ from Pb EDTA complex and the availability of PO 3 4 in the solution. Since Pb : EDTA =1:2 in the initial synthetic Pb EDTA solution, a half of EDTA was not chelated with Pb, thus the added Fe 3+ reacted first with the excess EDTA. At FeCl 3 : EDTA 1, Fe 3+ was not sufficient to replace all Pb 2+ following this replacement reaction. Pb EDTA 2 +Fe 3+ Fe EDTA +Pb 2+ 1 As a result, less released Pb 2+ reacted with added phosphate to form Pb 3 PO 4 2 as in the following reaction. 3Pb HPO 4 2 ph=3.5 2H + +Pb 3 Pb 4 2 At FeCl 3 : EDTA 1:1, excess Fe III existed as free Fe 3+, which competed against Pb 2+ to react with Na 2 HPO 4 following the reaction to form FePO 4 HPO 2 4 +Fe 3+ FePO 4 +H + 3 Thus, the Pb removal decreased as shown in Fig. 3. This is also confirmed by previous study by Palma et al In addition, the Pb removal increased after adding Ca OH 2 as shown in Fig. 3, and at FeCl 3 : EDTA =3:2 the total Pb removal efficiency reached 91.4%, slightly higher than that at 2 Zn EDTA 2 +Fe 3+ Fe EDTA +Zn 2+ 4 As a result, an excess of FeCl 3 was necessary to obtain high removal efficiency. On the other hand, the resulting FePO 4 precipitates may make a favorable contribution to Zn removal through adsorbing a portion of Zn, so the competition of Fe 3+ against Zn 2+ to react with Na 2 HPO 4 could be partially depressed. In addition, the removal efficiency dropped when increasing FeCl 3 : EDTA from 3:2 to 2:1, because the competition of excess Fe 3+ against Zn 2+ to react with PO 3 4 dominated the process compared with its positive contribution. Therefore, the optimal FeCl 3 : EDTA was selected as 3:2 for the Zn EDTA solution. Of interest, the Zn removal was significantly less than that of Pb at every FeCl 3 : EDTA investigated in this study. It was evident that the solubility of Pb 3 PO 4 2 or Zn 3 PO 4 2 as well as the difference of conditional stability constants between Pb or Zn EDTA and Fe III EDTA at the ph after adding Na 2 HPO 4 might affect the metal removal. In addition, Zn removal after adding Ca OH 2 was much less than that of Pb removal because of the difference of the ph after adding Na 2 HPO 4. The ph after adding Na 2 HPO 4 was 7.5 for the Zn removal and 3.5 for the Pb removal, respectively. At ph 7.5, the adsorption of resulting FePO 4 precipitates had already removed part of Zn; whereas such precipitates were negligible at ph 3.5, and thus only at ph 11 after adding Ca OH 2 they can contribute to remove Pb. As a result, the Pb removal after adding Ca OH 2 was higher than the Zn removal. Optimal Molar Ratio of Na 2 HPO 4 to EDTA After identifying the optimal molar ratio of added FeCl 3 to EDTA, the optimal molar ratio of Na 2 HPO 4 to EDTA was subsequently investigated using Experiment 2. The results were shown in Fig. 4. The Pb and Zn removal monotonically increased with respect to Na 2 HPO 4 : EDTA. However, the increment became less at the higher molar ratio of Na 2 HPO 4 to EDTA. The total Pb removal increased only 0.8% from 94.3 to 95.1% when changing Na 2 HPO 4 : EDTA from 2:3 to 1:1. In consideration of the balance of the cost and the efficiency, the optimal Na 2 HPO 4 : EDTA was chosen as 2:3. The added Na 2 HPO 4 reacted with Pb 2+ following the Reaction 2, of which the stoichiometric equivalencies required Na 2 HPO 4 : Pb =2:3. Since Pb : EDTA =1:2 in the initial Pb EDTA solution, the molar ratio of Na 2 HPO 4 to EDTA should be 1:3 to satisfy the stoichiometric equivalency of Reaction 2. However, an excess of Na 2 HPO 4 was needed to maintain the concentration of PO 3 4,as the majority of phosphatic species were H 2 PO 4 and H 3 PO 4, and PO 3 4 only accounted for 1/ of the total phosphatic species at ph / JOURNAL OF ENVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2005

5 Fig. 4. Pb or Zn removal as a function of the molar ratio of Na 2 HPO 4 to EDTA for Experiment 2 Fig. 5. Pb or Zn removal as a function of the ph after adding Na 2 HPO 4 for Experiment 3 The highest Zn removal was achieved at the highest Na 2 HPO 4 : EDTA, which is equal to 4:3, so this ratio was chosen as the optimal Na 2 HPO 4 : EDTA. Under this condition, the precipitation occurred at ph 7.5, where the PO 3 4 accounted for about 1/ of the total phosphatic species. In addition, the conditional stability constant of Fe EDTA was comparable with that of Zn EDTA at this ph as shown in Fig. 1, i.e., the equilibrium between Fe EDTA and Zn EDTA determined that a considerable amount of Zn was still chelated with EDTA. Therefore, a considerable excess of Na 2 HPO 4 was needed to move the following reaction forward to form Zn 3 PO Zn HPO 4 2 ph=7.5 2H + +Zn 3 PO4 2 Moreover, the existing Fe 3+ reacted with Na 2 HPO 4 following Reaction 3, and thus a higher Na 2 HPO 4 : EDTA was necessary. It was also observed that the FePO 4 precipitates, accompanied with the Zn 3 PO 4 2 precipitates were significant, resulting in part of Zn was removed by the adsorption of FePO 4 precipitates at this stage. Therefore, the mechanism of Zn removal after adding Na 2 HPO 4 was the cooperation of the adsorption of resulting FePO 4 precipitates and the precipitation of Zn 3 PO conducive to produce more PO 4 3 available to form Zn 3 PO 4 2 and FePO 4 precipitates for the adsorption. As mentioned before, the coupled effect of the adsorption of the FePO 4 precipitates and the formation of Zn 3 PO 4 2 precipitates was the primary mechanism of Zn removal, resulting in the maximum Zn removal occurred at ph 7.5. Optimal ph after Adding Ca OH 2 Solution The last step to investigate the optimal conditions was to probe the optimal ph after adding Ca OH 2 using Experiment 4. It was immediately noted that the Pb or Zn remove efficiency kept almost constant at ph 11.0 as shown in Fig. 6, so the ph 11.0 was chosen as the optimal ph after adding Ca OH 2. As the primary mechanism of Pb or Zn removal at this phase is the adsorption of Fe OH 3, the general trend of the Pb or Zn removal correlated reasonably well with the Fe 3+ removal. Under the optimal conditions, the maximum 96% of Pb removal and 83% of Zn removal from the synthetic Pb or Zn EDTA solution was achieved, respectively. Optimal ph after Adding Na 2 HPO 4 Solution It is well known that the ph plays a key role in phosphate precipitations, so Experiment 3 was subsequently conducted to probe the optimal ph after adding Na 2 HPO 4. The results were presented in Fig. 5. The maximum Pb removal after adding Na 2 HPO 4 was achieved at ph 3.5, which were selected as the optimal ph after adding Na 2 HPO 4. This value could be traced to the difference of conditional stability. The maximum difference of conditional stability constants between Pb EDTA and Fe EDTA coincidently occurs at ph around 3.5 as shown in Fig. 1, resulting in Fe 3+ have the highest competency against Pb 2+ to complex with EDTA at this optimal ph. As also shown in Fig. 5, the maximum Zn removal after adding Na 2 HPO 4 occurred at ph 7.5. The conditional stability constant of Fe EDTA was higher than that of Zn EDTA at ph 7.7 in Fig. 1. Therefore, a ph 7.7 was favorable for the replacement of Zn 2+ by Fe 3+. On the other hand, a higher ph was Fig. 6. Fe, Pb, and Zn removal as a function of the ph after adding Ca OH 2 for Experiment 4 JOURNAL OF ENVIRONMENTAL ENGINEERING ASCE / NOVEMBER 2005 / 1511

6 Fig. 7. Reuse of the recovered ethylenediaminetetraacetic acid EDTA solution. Conditons for Pb-contaminated soil washing: 1,383 mg Pb/kg soil, Pb : EDTA 2:1, ph 7.0, mix for 2 h; for effluent recovery: FeCl 3 : EDTA 1:1, Na 2 HPO 4 : EDTA 2:3, ph 3.5 after adding Na 2 HPO 4, ph 11.0 after adding Ca OH 2. Conditions for Zn-contaminated soil washing: 750 mg Zn/kg soil, Zn : EDTA 1:1, ph 9.0, mix for 2 h; for effluent recovery: FeCl 3 : EDTA 3:2, Na 2 HPO 4 : EDTA 4:3, ph 7.5 after adding Na 2 HPO 4, ph 11.0 after adding Ca OH 2. Reuse of the Recovered EDTA Solution The chelating capacity of the recovered EDTA was evaluated by applying it on the synthetic Pb- or Zn-contaminated soils. The experimental results were reported in Fig. 7. It was found that the recovered solution from synthetic Pb-contaminated washing effluents did not lose much of its chelating capacity after being reused four cycles, whereas the loss of 15% of its chelating capacity of the recovered EDTA from synthetic Zn-contaminated washing effluents was observed in its first-cycle reuse because of the significant amount of residual Zn. As a contrast, less than 10% of Pb remained in the recovered solution, whereas more than 20% of Zn remained. The substantial Zn residual maybe reduce the chelating capacity of the recovered EDTA. In addition, Pb removal efficiency from the real soil washing effluent in the first-cycle EDTA recovery, equal to 96.5% as shown in Fig. 7, approximated that from the synthetic soil washing effluent, equal to 96.0% as shown in Fig. 6 at ph The similar case occurred on the Zn removal efficiency during EDTA recovery processes. These results indicated the effect of ions introduced from soils during soil washing processes on Pb or Zn removal during EDTA recovery processes was negligible. Therefore, the optimal conditions obtained from the synthetic soil washing effluents also could be applied on the real soil washing effluents. Conclusions To assess the feasibility of EDTA recovery from the sandy soil washing effluents, a chemical precipitation method and associated operating conditions were evaluated using four series of batch experiments. Based on the results, it is concluded the optimal conditions for the EDTA recovery were: FeCl 3 : EDTA =1:1, Na 2 HPO 4 : EDTA =2:3, ph=3.5 after adding Na 2 HPO 4, and ph=11.0 after adding Ca OH 2 for the Pb EDTA solution; FeCl 3 : EDTA =3:2, Na 2 HPO 4 : EDTA =4:3, ph=7.5 after adding Na 2 HPO 4 ; and ph=11.0 after adding Ca OH 2 for the Zn EDTA solution, respectively. Under such optimal conditions, 96% of Pb or 83% of Zn was removed from the synthetic Pb or Zn EDTA solution, respectively. The results also revealed the mechanism of the Pb or Zn removal from EDTA solution was phosphate precipitation and the adsorption of the resulting FePO 4 or Fe OH 3 precipitates. The recovered EDTA was reused to wash the synthetic sandy Pb- or Zn-contaminated soils to evaluate its chelating capacity. The four-cycle recovery-reuse experiments evidenced the recovered EDTA from sandy soil washing effluents maintained much of its chelating capacity except the loss of 15% of its chelating capacity in the first cycle reuse for Zn-contaminated soil washing because of substantial residual Zn in such recovered EDTA solution. This recycle-reuse process is promising for the sandy soil washing as it can substantially reduce the amount of EDTA washing solution while keeping the initial removal efficiency. 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