Release of metal elements from clays during elution with EDTA: Insights into the side effects of washing contaminated sites

Ethylenediaminetetraacetic acid (EDTA) is frequently used to wash heavy metal contaminated fields, especially low-permeability zones which are rich in clays and have strong adsorption. However, the release kinetics of cations from clayey sediments during EDTA elution and the potential mechanisms controlling the cation release remain open questions. This study implemented heavy metal invasion and EDTA flushing experiments in a sandbox with clay-containing low-permeability layers, and batch kinetic experiments of cation release from clays in the presence of EDTA. Relatively small amounts of interlayer cations (i.e., Ca, Na, and K) and almost no structural cations (i.e., Al, Mg, Fe) were released due to ion exchange during the invasion of heavy metals. EDTA elution greatly enhanced the release of cations from clays; more interlayer cations were brought out via the complexation, and structural cations were released from clays because metal-oxygen bonds in octahedral sheets were broken by EDTA. The first-order release rate constants of structural cations were around 0.008 min-1 and almost one order of magnitude smaller than those of interlayer cations (around 0.05 min-1), that is, mineral dissolution was slower. Additionally, when EDTA was injected into groundwater systems together with heavy metals from pollutant source zones, EDTA was mostly chelated with heavy metals and few cations were released from sediments. By adjusting the timing of EDTA injection, the complexation between EDTA and heavy metals became preferred, and little free EDTA remained and affected clays. This provides new insights into the side effects of washing heavy metal contaminated sites with chelating agents.