Influence of amendments on acidification and leaching of Na from bauxite processing sand

The effects of addition of acidifying agents (phosphogypsum, iron sulphate, elemental S, ammonium sulphate and biosolids) on pH of bauxite processing residue sand, and on subsequent leaching of Na and other ions was investigated in 6- and 10-week incubation/leaching experiments. Addition of phosphogypsum and iron sulphate caused a reduction in pH and ESP both before and after leaching. Addition of biosolids reduced pH prior to leaching and reduced ESP and increased ECEC before and after leaching. By contrast, addition of elemental S or ammonium sulphate had no effect on pH or ESP either before or after leaching. Addition of soil and/or biosolids to the residue sand did not significantly promote S oxidation or nitrification. In all treatments leaching caused a substantial reduction in pH as well as exchangeable Na, EC and ESP. In control treatments pH was reduced by about two units following leaching. Addition of gypsum and iron sulphate resulted in precipitation of alkalinity and leaching of Na+ with added SO42− as the major balancing anion. By contrast, where there was no initial decrease in pH (e.g. control), Na+ leached with HCO3−/CO32 (i.e. soluble alkalinity) as the main balancing anions and, as a result, residue pH was decreased. This occurred because the mobile Na+ ion was present in residue at concentrations greatly in excess of the CEC of the residue sand. It was concluded that leaving residue sand for a sufficient period of time under leaching conditions is a very important management strategy since it will result ina substantial reduction in pH, EC exchangeable Na and ESP making conditions much more conducive for revegetation.