No 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. SO:-), 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.

Red mud is a finely crushed, iron-rich, alkaline residue, obtained by digesting bauxite with caustic soda to remove the alumina. The remnant alkalinity of red mud is equivalent to 11% pure calcium carbonate. Phosphorus leaching from infertile sandy soils has resulted in eutrophication of estuaries and has caused algal blooms. Red mud has been shown to reduce leaching of phosphorus from sandy soil. This research was undertaken to determine the effect of red mud on pasture growth and uptake of heavy metals. Red mud, either untreated or treated with gypsum, was applied at rates of 0, 10, 20, 40, and 80 t/ha to a subterranean clover and ryegrass hay paddock. There were 3 replicates of each treatment and a completely randomised design was used. The experimental design was 5 rates of red mud x 2 untreated and treated with gypsum x 3 replicates, resulting in 30 plots. Plant growth, and nutrient and heavy metal composition of the plant tops, were measured. An application of 40 t/ha of red mud increased hay (mainly subterranean clover and ryegrass) production by 24% and increased soil pH in the top 10 cm by 1.0 unit from 3.5 (1 : 5 soil : 0.1 M CaCl2). The increase in production was probably because of the liming effect of the remnant alkali in the red mud, which may have potential as a replacement for crushed limestone. Sodium carbonate, the predominant alkali in red mud, is more soluble than calcium carbonate from crushed limestone and has the potential to change the pH of the soil more rapidly. The soil was top-dressed with red mud, without disturbing the existing pasture, resulting in changes to the pasture production and nutrient composition consistent with a change in soil pH throughout the rooting depth. If crushed limestone is not mixed into the soil it may take many years to increase the pH of the soil; however, this mixing results in extra cost from re-seeding and an initial depression in yield. Although much more red mud is needed than lime, the cost is comparable when the haulage distance is less than about 30 km. In previous trials, at red mud application rates > 500 t/ha, gypsum was mixed into the red mud to reduce salinity and pH. At these rates, the red mud had overwhelmed the buffering capacity of the soil. The gypsum reduced the pH by changing the sodium carbonate in the red mud to calcium carbonate, thus changing the pH from > 10 to about 8.5. However, amendment of the red mud with gypsum when applied at rates < 80 t/ha proved unnecessary in this experiment, probably because the proportion of soil was sufficient to alter the pH of the red mud. When red mud is applied to acidic infertile sands, manganese application as manganese sulfate may be necessary, because the rise in pH may rapidly induce manganese deficiency in plants. Care should be taken to monitor other nutrients which have their availability for plants affected by pH (e.g. copper, zinc, and molybdenum). Red mud did not elevate the concentrations of heavy metals in the soil, hay, or fresh plant tissue.