Pyrophosphate coupling with chelant-enhanced soil flushing of field contaminated soils for heavy metal extraction.

This study investigated the influence of flushing duration, [S,S]-ethylenediaminedisuccinic acid (EDDS) dosage, humic acid and various combinations of ethylenediaminetetraacetic acid (EDTA), EDDS and tetrasodium pyrophosphate (Na(4)P(2)O(7)) on metal extraction during soil flushing, through column experiments. A lesser extent of enhancement in metal extraction efficiencies was found when the flushing duration and the dosage of EDDS was doubled, compared to their efficiencies measured at pore volume 100. Metal extraction efficiency was mainly influenced by the initial metal distribution in the soils rather than the flushing duration and the EDDS-to-metal molar ratio. Humic acid of less than 10mg/L as dissolved organic carbon (DOC) posed an insignificant effect on metal extraction during EDDS enhanced soil flushing. The extraction rate of Ni by EDTA and EDDS was time dependent, and was initially fast in the case of EDDS, whereas it was slow for EDTA. However, the overall Ni extraction efficiency by EDTA was higher when the flushing time was longer. Na(4)P(2)O(7) promoted the mineral dissolution which enhanced the metal extraction as a result of soil disruption. The order of metal extraction by Na(4)P(2)O(7) was Ni>Cr>Cu, probably be due to the different affinities between metals and P(2)O(7)(4-).