Influence of Organo-Metal Interactions on Regeneration of Exhausted Clay Mineral Sorbents in Soil Columns Loaded with Heavy Metals

Natural clay minerals can play an important role in crude remediation of wastewater polluted with the heavy metals (HMs) Cu, Zn and Ni. The presence and timing of addition of natural dissolved organic matter (DOM) have a significant effect on the HM removal by clay mineral sorbents. However, the influence of the presence of DOM on the remediation of the used clay mineral sorbents once saturated with HMs is largely unknown. To resolve this, clay mineral-rich soil column of varying composition, loaded (i) with Cu, Zn and Ni only, (ii) first with DOM followed by Cu, Zn and Ni, or (iii) with DOM, Cu, Zn and Ni simultaneously, was used in a set of desorption experiments. The soil columns were leached with 0.001 mol L⁻¹ CaCl₂ dissolved in water as control eluent and 0.001 mol L⁻¹ CaCl₂ dissolved in DOM as treatment eluent. During the preceding loading phase of the sorbent, the timing of DOM addition (sequential or concurrent with HMs) was found to have a significant influence on the subsequent removal of the HMs. In particular when the column was loaded with DOM and HMs simultaneously, largely irreversible co-precipitation took place. Our results indicate that the regeneration potential of clay mineral sorbents in wastewater treatment will be significantly reduced when the treated water is rich in DOM. In contrast, in manured agricultural fields (where HMs enter together with DOM), HM mobility will be lower than expected from interaction dynamics of HMs and clay minerals.