Phosphorus movement through red mud amended soils

Field and laboratory column experiments show that red mud (waste from bauxite refining) neutralised with gypsum, increases Phosphorus (P) sorption capacity of sandy soils. some l.65g P/kg RMG (red mud neutralised by 5% gypsum) was sorbed from 9. 5 mg/L P solution during a continuous flow of 50 cm/d for 750 pore volumes when breakthrough started. Batch tests indicate almost no desorption of sorbed P, however during continuous flow leaching after 58 pore volumes of leaching a total of 14% of the sorbed P, 0. 23g P/kg RMG, was desorbed. During flooding - drying cycles of laboratory columns with secondary effluent, removal efficiencies were proportional to the hydraulic conductivities of the columns, 91% P removal occurred through 30% RMG (30% RMG, 70% Bassendean sand mixture) and 64% removal through 20% RMG, both with some more capacity to sorb P, whereas 10% RMG continued to sorb P at 45% efficiency even after the calculated sorption capacity was exhausted. In terms of unit P retained per unit P input and per unit RMG amount in the columns, i.e. kgP/kgP-kg RMG, Column III had a value of 0.36 which was greater than that for Column I and II which were 0.23 and 0.24 respectively. This also indicates that even though its efficiency was lower, Column III retained more P, probably also due to higher P input. Initially, previously sorbed P in soil would be leached out by the alkaline leachate from red mud incorporated above it. In general the potential for P removal by red mud renovated soils seems high, thus red mud in neutralised form such as RMG can be used as a soil amender for agricultural as well as secondary sewage effluent recharge purposes. The only technical problem remaining is the ultimate disposal of RMG layers in the field when their sorption capacities are exhausted and/or their gypsum is leached out