A study of the sorptive properties of surface sediments in a landfill project region

Abstract

Surface sediment samples were collected from 3 regions of a river island (a landfill area, a foreshore zone and a swamp segment) and the adsorptive properties of the - 180 μm and “clay” fractions were examined by studying the effects of concentration, pH and complex formation on the uptake of copper or zinc ions. The fine sand fraction contained basic material which raised the pH of aqueous suspensions to ∼8, at which value near total precipitation of added metal ions occurred. Uptake by the clay fractions (at pH 5) conformed to a Langmuir type adsorption isotherm, yielding parameters of similar order to values reported for soil clays and clay minerals. Using constant initial concentrations the amount of Cu or Zn adsorbed increased with pH until the threshold for precipitation ( ∼7) was exceeded. Both uptake and precipitation were inhibited in the presence of ligands which formed stable anionic complexes (e.g. NTA, EDTA, citric acid). The foreshore and swamp samples contained organic matter which retained part of the added metal ion as a solute in alkaline media. Treatment of these clay fractions with hydrogen peroxide did not destroy all the organic content, but it did change adsorptive behaviour. Extraction studies with 0.01 M nitric acid, 0.1 M oxalic acid, 0.1 M ammonium acetate, and 0.01 M EDTA showed that both the clay fractions and fine sands contained measurable amounts (0.05–40 mmol kg −1) of the metal ions Pb, Cd, Cu and Zn, but because of the basic nature of the sediments, adjacent waters had solution levels which remain well within acceptable limits. The overall results have been used to predict factors likely to promote formation of high leachate levels.