Revegetation strategies for seawater neutralized bauxite residue at Rio Tinto Alcan using various amendments

Abstract

In northern Queensland, two alumina refineries process bauxite from a deposit which is located in far north Queensland. At both refineries the bauxite residue mud they produce is seawater-neutralized prior to its deposition in storage lagoons. This study was initiated to residue. There are, as yet, no published studies on the revegetation of such material although it is presumed, by some, that it will be less challenging than revegetation of non-neutralized residue (because it has a lower pH and ESP). Four experiments were carried out (two laboratory incubation studies, a greenhouse pot experiment and a leaching column experiment in the greenhouse). In the first experiment, it was found that compared to non-neutralized bauxite residue from Western Australia (used in previous studies), seawater neutralized residue from north Queensland had a lower Si and quartz content but due to seawater neutralization it had a greater EC and exchangeable Ca, Mg, K and Na content and a lower pH (9.3 compared to 11.0-11.3) and ESP. Before revegetation, the accumulated salts would need to be leached out. However, during leaching the pH of both leachate and residue from north Queensland residue increased due to the dissolution of solid alkalinity while the opposite was the case for Western Australian residues. In the second experiment, seawater neutralized bauxite residue was amended with residue sand (25% v/v), gypsum at 1% or 5%, poultry manure or biosolids (6% w/v) and incubated for 4 weeks and then leached with 6 pore volumes of water. After seawater neutralization the pH of unamended residue was 9.3 but during leaching it rose to 9.6. Addition of amendments had very small effects on physical properties of the residues while organic amendments increased the size and activity of the soil microbial community. After leaching, exchangeable Na, ESP and SAR were reduced by gypsum application and to a lesser extent by addition of poultry manure and biosolids. It was shown that even after seawater neutralization, exchangeable and soluble Na, ESP and SAR in residue were very high and applications of gypsum at about 5% and subsequent leaching would be required prior to revegetation. In the third study greenhouse and laboratory experiments were carried out to evaluate the effects of addition of organic amendments, gypsum and subsequent leaching on improving the properties of residue as a growth medium for Rhodes grass and the effects of drying and rewetting on the physical properties of residue mud. In amended treatments, yields of Rhodes grass were greatly promoted by leaching (due to reductions in soluble Na, SAR and EC) and were greatest in the leached biosolids plus gypsum treatment. It was also shown that when bauxite residue is dried, it loses considerable volume and forms a massive structure which when crushed, forms water-stable aggregates. In the leaching column experiment, carried out over a 6-month period, all amendments were incorporated into the surface layer (0-10 cm). The main cation leached was Na+ and the main balancing anions were Cl- and SO42-. During leaching, the pH of leachates rose from 7-8 up to 9-10 and concentrations of Al in leachates also rose. At the end of the study, gypsum plus organic amendments had the greatest effect in lowering exchangeable Na and pH in both the surface layer and the subsurface (10-30 cm) layers. Rhodes grass dry matter followed the order: control < gypsum < cattle manure < gypsum plus cattle manure < biosolids < gypsum plus cattle manure. When organic amendments were applied, root growth into the subsurface layer was greatly promoted. It was concluded that although seawater neutralization of bauxite residue lowers the pH from 11-13 down to 9, subsequent leaching (which is necessary to leach out the excess salts accumulated during seawater neutralization) results in a rise in pH up to about 10 due to dissolution of residual alkalinity in the residue. Addition of 5% gypsum into the surface horizon can arrest this pH rise in that layer as well as promoting leaching of Na. To provide for a reduction in pH and more particularly a leaching of Na from both the topsoil and subsoil layers (with a reduction in exchangeable Na/ESP), a combination of gypsum plus organic amendments is highly effective. This allows root growth into the subsoil layers which is essential for plant survival during dry periods of the year. Prior to revegetation, the residue needs to be allowed to dry and solidify and the surface horizon can then be tilled to form a seed bed of water stable aggregates and fertilizers can then be added. In the future, field studies need to be initiated in north Queensland using such a strategy to field-test its effectiveness.