The effect of pH, ion strength and reactant content on the complexation of Cu 2+ by various natural organic ligands from water and soil in Hong Kong

Abstract

The complexation constants for copper associated with different natural organic ligands, including dissolved organic carbon (DOC) extracted from water, water soluble organic carbon (WSOC), fulvic acid (FA) and humic acid (HA) from soil, were determined and then compared based on discrete single site model. Both ion-selective electrode (ISE) and anodic stripping voltammetry were used to determine the content of free copper ions, while the relative number of complexation sites was estimated using a fluorescence quenching (FQ) at the same time. ISE proved to be the most applicable technique when the concentration of copper was above 10 −7 mol l −1. The log k values for two WSOC sample sites (Song Ziyuan and Xin Niangtan) were 4.64 and 4.66; higher than both the DOC and HA values yet lower than the FA values, which were unusually high due to unavoidable pollution from the cation exchange resin used during the purification process. Binding affinities between the copper ions and the organic ligands obtained from streams in Yong Wei (DOC) and Song Ziyuan (WSOC) were influenced by pH, ion strength, and reactant concentration. Values for log k increased with increases in pH (ion strength of 0.1 N). At pH 4, the log k values decreased with increases in the supporting electrolyte concentration and total copper ion additions.