Mechanistic insight into pH-dependent adsorption and coprecipitation of chelated heavy metals by in-situ formed iron (oxy)hydroxides

Fe(III) coagulation-precipitation process has been widely used to remove heavy metals from industrial effluents; however, the influences of organic ligands on the sequestration of different heavy metals in the Fe(III)/metal-EDTA ternary system are not well understood. In this study, the pH-dependent mechanisms of CuII-EDTA and NiII-EDTA removed by in-situ formed iron (oxy)hydroxides were studied using surface complex modeling and a suite of characterization techniques. Results of surface complex model indicated that there should be minimal difference between removal of CuII-EDTA and NiII-EDTA by iron (oxy)hydroxides if adsorption was the dominant mechanism. However, through the speciation analysis and characterization of the precipitates generated after coagulation and precipitation, we have demonstrated that at neutral pH the complexation of Fe(III) and EDTA influenced the surface properties of iron (oxy)hydroxides formed, with the higher removal of Cu2+ (compared to Ni2+) contributed by its coprecipitation with Fe(III). Moreover, at basic pH, decomplexation of CuII-EDTA occurred on the iron (oxy)hydroxides surface with the released copper ions involved in the formation of (oxy)hydroxides. The low removal of nickel (from NiII-EDTA) was ascribed to the higher conditional stability constant of NiII-EDTA. Results of this study have advanced our understanding of the complicated interactions among Fe(III), organic ligands and heavy metals in the industrial effluents, and provide insight to optimization of the process efficiency.