Competitive adsorption behavior and mechanisms of coexisting EDTA-Cu(Ⅱ) and EDTA-Cr(Ⅲ) on ferrihydrite

Abstract

EDTA-complexed heavy metals (EDTA-CHMs), one of the typical contaminants in industrial effluents, present challenges for wastewater purification because of their stability and high solubility over a wide pH range. Selective adsorption of various EDTA-CHMs coexisting in industrial wastewater is crucial for the recovery of heavy metal resources. Hence, investigating the competitive behavior and selective adsorption mechanisms of coexisting EDTA-CHMs during the effluent removal process is worthwhile. In this study, we chose Ferrihydrite (Fh) as the adsorbent and focused on the competitive adsorption behavior of two common EDTA-CHMs (EDTA-Cu(Ⅱ) and EDTA-Cr(Ⅲ)) on the Fh surface, as well as their removal mechanisms. For binary systems (EDTA-Cu(Ⅱ) and EDTA-Cr(Ⅲ)), the maximum adsorption capacity of Fh for EDTA-Cu(Ⅱ) was 17.32 mg/g, but because of the coexisting EDTA-Cu(Ⅱ), adsorption of EDTA-Cr(Ⅲ) by Fh was severely inhibited. In a competition experiment with binary contaminants of different EDTA complexes, Fh consistently exhibited stronger adsorption performance on EDTA-Cu(Ⅱ) when the ratio and addition order of EDTA-Cu(Ⅱ) and EDTA-Cr(Ⅲ) in the coexisting system was changed, and the valence state of the EDTA-CHMs was the key factor. The adsorption mechanism indicated that Fh could directly adsorb EDTA-CHMs through electrostatic interaction and surface complexation. The recycling rate of EDTA-Cu(Ⅱ) was still approximately 70.63% after five cycles of treatment with Na2SO4 solution. This study provides new insights into the adsorption behavior of coexisting EDTA-CHMs on adsorbents and the selective removal of related pollutants.