Properties of seawater neutralized bauxite residues and changes in chemical, physical and microbial properties induced by additions of gypsum and organic matter

Seawater neutralization is a technique that can be used to treat bauxite residue prior to its storage but, as yet, no attempts have been made to revegetate it. Seawater neutralized bauxite residue was found to have a pH of 9.3 and to be highly saline (EC 16.5 dS m). After leaching pH rose to 9.7 and the residue was still highly sodic (ESP = 64-69%). Addition of 1% gypsum, prior to leaching, arrested this increase in pH while with 5% gypsum addition the pH was lowered to 8.9. Addition of 5% gypsum also reduced ESP to 38% and increased watercress germination in the residue from 58% in control treatments to 88%. The major ions in leachates were Na and Cl and gypsum application increased the quantities of Na, Ca and SO leached. Addition of 6% biosolids or 6% poultry manure added exchangeable cations to the mud and lowered ESP by 5-11%.The EC was 2.8-3.7 (mean 3.1) times higher and pH 0.2-0.7 (mean 0.43) units lower in saturation paste compared with 1:5 soil:water extracts. Addition of amendments had only small effects on physical properties. While organic C content was increased more by biosolids than poultry manure addition the reverse was the case for soluble organic C, microbial biomass C and basal respiration. It was concluded that although seawater neutralization initially lowers the pH of bauxite residues it is unlikely to increase the ease with which they can be revegetated