Background, aim, and scope Various metals such as cationic metals (Cu, Pb, Zn) and anionic metals (As, Cr) often coexist in real soils, and normal soil washing techniques for the removal of cationic metals with a single-washing reagent make it rather difficult to simultaneously remove all of them. Oxalate could effectively remove anionic As and EDTA could effectively remove the cationic metals, so it was possible to remove all coexisting cationic and anionic metals by washing with the combination of Na 2 EDTA and oxalate. The objective of this study was to (1) discuss the possibility of removing five metals, As, Cd, Cu, Pb, and Zn, effectively from the soil by washing with Na 2 EDTA-combined oxalate; (2) optimized through the consecutive washing. Materials and methods The soil sample was collected, airdried, and sieved. The typical extraction tests were conducted in 50-mL-capacity polyethylene tubes. Tubes containing 1 g samples and a measured volume of extractant (L/S=20 mL g −1 ) were stirred for a given time. The soil was first treated with 0.01 M Na 2 EDTA and 0.1 M oxalate individually to compare the removal efficiencies. Then, consecutive extractions were performed so that 1 g of soil was treated by four cycles of washing with 2 h for each washing. To optimize the combination of Na 2 EDTA and oxalate, three methods of consecutive washing were designed: washing with Na 2 EDTA followed by oxalate, for two rounds (A); washing with oxalate followed by Na 2 EDTA, for two rounds (B); and washing with the admixture of oxalate and Na 2 EDTA for four cycles (C). Results After 24 h washing, Na 2 EDTA removed only 2.3% of As from the soil, while oxalate removed 59.9%. In contrast, the removal of Pb for oxalate was 1.5% while it was 27.4% for Na 2 EDTA. A large amount of Ca (379.72 mg l −1 ) was released when washing with Na 2 EDTA; while the released Ca was far lower (15.86 mg l −1 ) with oxalate. In contrast, washing with oxalate resulted in a large amount of Al (123.34 mg l −1 ) and Fe (305.9 mg l −1 ) in the solution, far higher than washing with Na 2 EDTA (14.03 mg l −1 for Al and 38.40 mg l −1 for Fe). The consecutive experiments indicated that after four cycles of washing, three methods could effectively remove all of both cationic and anionic metals, and the accumulative removals of five metals were 54.7-65.6% for As, 28.6-33.8% for Cd, 80.3-86.6% for Cu, 15.8-42.9% for Pb, and 43.2-45.2% for Zn, respectively. In the first cycle of method A, the total molar concentration of major elements was 11.53 mmol l −1 , higher than the molar concentration of Na 2 EDTA. Arsenic released was linearly correlated with the concentrations of Fe and Al in the eluate to a significant degree and the correlation coefficients (r) were 0.942 and 0.920, respectively. Discussion The reason for the low As removal for Na 2 EDTA was that the anionic form of As does not allow for the formation of a stable complex with Na 2 EDTA. The reason for the low removal of Pb by oxalate was due to the formation of low-solubility lead oxalate precipi...
