Influence of EDDS-to-metal molar ratio, solution pH, and soil-to-solution ratio on metal extraction under EDDS deficiency

Abstract

In situ biodegradable EDDS ([S,S]-stereoisomer of ethylenediaminedisuccinic acid) applications at low concentration may present conditions where applied EDDS is insufficient relative to sorbed metals in soils. This study investigated the influence of EDDS-to-metal molar ratios (MR), solution pH and soil-to-solution ratio on metal extraction under EDDS deficiency (i.e., MR<1). Batch kinetics experiments showed that Pb and Zn extraction exhibited different kinetic behaviors at MR 0.35-0.75, while Cu extraction was comparable. At MR 0.75 or below, newly extracted Pb was re-adsorbed onto the soil surfaces. Similar re-adsorption phenomenon, to a lower extent, was observed for newly extracted Zn at MR 0.5 or below, whereas this appeared to be marginal at MR 0.75, reflecting Zn extraction was less affected by EDDS deficiency than Pb extraction. Moreover, Pb extraction at an alkaline condition was preferable under EDDS deficiency because at MR 0.5 it was 30% higher at pH 8 and 9 than pH 5.5 and 7. The influence of varying soil-to-solution ratios (1:50-1:5) at MR 0.5 was marginal compared with that of MR and solution pH. These findings indicated that Pb extraction by deficient EDDS would be more difficult to accomplish compared to Cu and Zn extraction.