The application of sediment capping agents on phosphorus speciation and mobility in a sub-tropical dunal lake

Experimental sediment cores from Lake Ainsworth, Australia, were exposed to an induced 46-day, anoxic/oxic cycle in the laboratory, mimicking the seasonal thermal stratification cycle commonly observed in the lake’s waters every summer. Under oxic conditions the supply of phosphorus (P) and iron (Fe) to the overlying water was slow, however, induced anoxia led to an enhanced release of P and Fe from the sediments to the water column. An inverse relationship between total P, Fe and redox potential suggests that Lake Ainsworth sediments are redox sensitive. Phosphorus speciation analysis of Lake Ainsworth sediments revealed the presence of a large pool of organic P, reactive Fe-bound P, and CaCO₃-bound P, the latter fraction decreasing during anoxic conditions. Two sediment-capping agents, a lanthanum modified bentonite clay and Bauxsol (a waste product from the aluminium smelting industry) were assessed for their ability to reduce the levels of P released from Lake Ainsworth sediments during the 46-day, anoxic/oxic cycle. The bentonite clay was highly effective at reducing plant available P in anoxic/oxic conditions, but levels of dissolved Fe were enhanced with its use. Although the use of Bauxsol to remove plant available P is not recommended in anoxic waters, its use in suspension in oxic waters warrants further study.