Heavy metal extraction from an artificially contaminated sandy soil under EDDS deficiency: significance of humic acid and chelant mixture.

Biodegradable EDDS ([S,S]-ethylenediaminedisuccinic acid) has been suggested for enhancing heavy metal extraction from contaminated soils. Recent studies showed that Zn and Pb are less effectively extracted due to metal exchange and re-adsorption onto the soil surfaces, especially for EDDS-deficiency conditions. This study therefore investigated the influence of dissolved organic matter and the co-presence of EDTA (ethylene-diamine-tetraacetic acid) on metal extraction from an artificially contaminated sandy soil under deficient amount of chelants in batch kinetics experiments. The addition of 10 and 20mgL(-1) of humic acid as dissolved organic matter (DOC) suppressed metal extraction by EDDS, probably resulting from the competition of adsorbed humic acid for heavy metals and adsorption of metal-humate complexes onto the soil surfaces. The effects were most significant for Pb because of greater extent of metal exchange of PbEDDS and high affinity towards organic matter. Thus, one should be cautious when there is a high content of organic matter in soils or groundwater. On the other hand, compared to individual additions of EDDS or EDTA, the equimolar EDDS and EDTA mixture exhibited significantly higher Pb extraction without notable Pb re-adsorption. The synergistic performance of the EDDS and EDTA mixture probably resulted from the change of chemical speciation and thus less competition among Cu, Zn and Pb for each chelant. These findings suggest further investigation into an optimum chemistry of the chelant mixture taking into account the effectiveness and associated environmental impact.

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

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.