Critical coagulation in sulfidic sediments from an east-coast Australian acid sulfate landscape

Sulfidic clays are agriculturally and environmentally important. In this work we examine the impacts of electrolytes and pH on the behaviour of colloidal clay mineral particles extracted from such sediments. The distribution of ferrous iron released by pyrite oxidation, aluminium by acidic weathering and cations in soils and pore waters in the field are reported. The behaviour of open-structured sulfidic colloidal clay mineral particles in response to changes in solution ionic composition were studied; (i) to evaluate the effects of natural oxidation of iron sulfide material in pedogenic development, and (ii) to investigate the response of these sediments to changes in pore water ionic composition as an option for soft sediment engineered dewatering. Photon correlation spectroscopy (PCS) was used to quantify these effects. As expected, Mg²⁺ and Ca²⁺ were more effective in inducing coagulation of the colloidal clay mineral particles than Na⁺; however, the effect was more pronounced than theoretically expected according to DLVO theory. Comparing the presence/absence of protons in cation saturated experiments showed new evidence for the formation of H-colloidal clay mineral particle complexes that resist competitive cation exchange. The critical concentrations of acidic cations required for mass rapid aggregation in these experiments is comparable to the pore water composition within the soil profile where structural collapse has already occurred.