The role of dissolved organic matter in adsorbing heavy metals in clay-rich soils

Adsorption of tested heavy metals on Egyptian soils was large in all situations tested and follows the order: Cu >> Ni ≈ Zn. Copper was influenced by the timing of dissolved organic matter addition more than Ni and Zn. Specific binding mechanisms (inner-sphere complexes) dominated the affinity of Cu for the solid phase, whereas nonspecific electrostatic interactions were the dominant mechanism for Ni and Zn.
Heavy metals (HMs) are toxic to human life and the environment when present in excessive concentrations. Therefore, determining the interactions of HMs with soils and dissolved organic matter (DOM) is essential to predict their fate. To find out the effect of DOM and soil properties (clay minerals, oxides, and bulk organic matter [OM]) on the uptake of Cu, Ni, and Zn, batch adsorption experiments were conducted using five soils sampled from Egypt. The sorption isotherms were well described by the initial mass (IM) isotherm model. The amount and timing of DOM addition was found to play a pivotal role in determining the affinity of the HMs for soil. When DOM and HMs were added simultaneously, the affinity of Cu decreased in Fe-(hydr)oxide-rich soils (by 7%) and increased in soils poor in Fe-(hydr)oxide (by 6-10%). When DOM was added first, followed by HMs, the affinity of Cu strongly increased. In contrast, affinity of both Ni and Zn was enhanced (3-18%) in the presence of DOM, regardless of the timing of DOM addition. The difference is explained by Cu binding to the solid phase and DOM through strong inner-sphere complexes, whereas Ni and Zn adsorbed predominantly through weaker electrostatic interactions. As a result, Cu was able to bind more strongly to previously adsorbed DOM on the solid phase in the case of smectite, while this effect was counteracted by the coating of available specific binding sites on Fe-(hydr)oxides. The study has revealed that Egyptian soils hold great potential to remove HMs from aqueous solutions.