The Application of a Simple Chemical Model of Natural Waters to Metal Fixation in Particulate Matter

Abstract

A simple chemical model was used to investigate the mechanisms controlling the distribution of metals between soluble and particulate fractions in natural waters. The model particulates used were potassium bentonite, hydrous MnO₂, and solid humic acid. The soluble species in natural waters were modelled by soluble humic acid, tannic acid, and bicarbonate. The sorption curves for Cu(II), Cd(II), and Zn(II) onto humic acid and MnO₂ obeyed Langmuir adsorption isotherms whereas the sorption of the above ions onto bentonite followed a Freundlich isotherm. Chemical analysis of the total model using atomic absorption spectroscopy and differential pulse anodic stripping voltammetry indicated that copper distribution depends on the pH of the suspension: above pH 6.0, 50 % of the copper is sorbed onto the particulates whereas the copper in solution is in a complexed form; between pH 6 and 3.8 the soluble copper is distributed between organic complexes and "free" copper ion; between pH 4.2 to 2.5 copper is being desorbed from the particulates; and below pH 2.5 all the copper is present in solution as "free" copper ion.