Phosphorus retention and leachates from sandy soil amended with bauxite residue (red mud)

Abstract

Bauxite residue (red mud) is a finely crushed, alkaline by-product of the alumina industry. The application of red mud to soil has the potential to reduce eutrophication of rivers and waterways by retaining nutrients on infertile sandy soils. The areas which may benefit from amendment with red mud are often groundwater recharge areas for drinking water and those near environmentally sensitive waterways, because of this, the off-site effects of red mud must be assessed before its widespread use. This research aimed to assess the length of time that phosphorus continued to be taken up by red mud and the best application rate of red mud to retain applied phosphorus. The effect of gypsum-amended red mud on phosphorus retention was examined. The composition of leachates from the red mud was compared with drinking water standards for humans and an untreated control. Monthly rainfall was simulated and leachate was collected from lysimeters filled with bleached grey sand amended with 5–80 t/ha of red mud, with and without gypsum. Leachates from over 12 months of simulated rainfall were tested for potential pollutants (Cd, Al, Fe, As, F, SO²₄^-), electrical conductivity, pH, and P. The rainfall simulation was continued for the equivalent of 5 years and P levels were monitored during this time. The ionic concentrations of the leachates from columns treated with red mud were similar to the concentrations in the controls or fell to similar levels after the equivalent of 3 months of rainfall. The concentrations of these leachates were below the maximum recommended limits for drinking water, except in the case of fluoride which only occurred when gypsum was applied. The concentration of fluoride that leached from the gypsum-amended red mud dropped to drinking water standards within the equivalent of 7 months of rainfall. The best application rates of red mud which will reduce phosphorus leaching are 10–20 t/ha, without gypsum. The improved nutrient retention from red mud continues for the equivalent of at least 5 years of fertiliser application.