Role of iron colloids in copper speciation during neutralization in a coastal acid mine drainage, South Korea: Insight from voltammetric analyses and surface complexation modeling

A coastal acid mine drainage (AMD) of ~. 800. m length, Chonam-ri Creek flows through a neutralizing pond mid-stream before discharging to the Kwangyang Bay of South Korea. The creek water displays a low dissolved organic carbon (DOC) concentration of ~. 1. mg/L, with a wide range of pH from ~. 3 to ~. 7 and Fe concentration from 0.005 to 106. mg/L. Our previous field study showed that the fraction of labile Cu and Pb in the creek water, determined by anodic stripping voltammetry (ASV), decreases rapidly with increasing pH, but no such pH dependence was observed for Cd or Zn. To further understand geochemical processes governing metal speciation during neutralization in AMD, we conducted two sets of laboratory experiments using Chonam-ri Creek water samples and model solutions, as well as Visual MINTEQ modeling. When the natural pH of the creek water samples was adjusted to a range between 2 and 8 by adding acid or base, the pH dependence of metal speciation or lack thereof was reproduced. A model solution simulating the water of Chonam-ri Creek also demonstrated the pH dependence of Cu speciation, determined by competitive ligand exchange-cathodic stripping voltammetry (CLE-CSV) in the presence of Fe and strong organic ligand. Based on VMINTEQ modeling indicating that 0.45 μm filterable Fe in the downstream sites of Chonam-ri Creek is most likely in the form of Fe colloid, surface complexation modeling of Cu adsorption onto Fe colloid using 1-pK Three Plane Model (TPM) was successful (RMSE = 0.185) in predicting the labile Cu concentrations. VMINTEQ modeling suggests that Cu speciation is dominated by interaction with Fe colloids over dissolved organic matter (DOM) during neutralization in a typical AMD with high Fe and low DOM, and that Cu is discharged to the bay in the form of colloids. The coupled application of voltammetric technique and geochemical modeling provides useful information on metal speciation in AMD.