The remediation of bauxite residue is a major cost for the alumina industry. Alcoa alumina refineries separate the residue into sand- and mud-sized fractions on-site. The residue sand is used to form the outer perimeter embankments and the residue mud is deposited within them. Revegetation (using plants native to a sand dune ecosystem) is on the residue sand but growth of plants can be disparate and sometimes poor and there is little regeneration of vegetation. This study investigated whether, in addition to the application of phosphogypsum and inorganic fertilizers (the current practice), addition of organic wastes and/or bauxite residue mud to the sand would improve its properties as a growth medium for plants. It also examined whether lack of reaction of phosphogypsum with subsurface layers of sand could be a factor limiting revegetation success. The research was carried out in four experiments: (i) a laboratory incubation in which five organic amendments were added to gypsum-treated residue sand, (ii) a laboratory incubation in which residue mud or a combination of residue mud and two organic wastes was added (iii) a laboratory/glasshouse experiment in which carbonated versus seawater neutralized mud either alone or in combination with green waste compost was added and growth of two plant species was measured and (iv) a 26-week leaching column experiment in which the surface layer of sand was amended with poultry manure and/or gypsum-treated residue mud and the subsurface layer was left untreated or amended with phosphogypsum.
The addition of phosphogypsum, and subsequent leaching, was found to lower the pH of the residue sand, displace Na+ and result in Ca2+ becoming the dominant cation. The organic wastes used were biosolids, poultry manure, green waste compost, mushroom compost and green waste derived biochar. All organic amendments increased the available waterholding capacity and the amount of water held at field capacity. With the exception of biochar, organic amendments increased the microbial biomass C, basal respiration and activities of key enzymes (β-glucosidase, L-asparagine, alkali phosphatase and arylsulphatase) involved in the oxidation of organic C, N, P and S respectively. The addition of organic amendments (other than biochar) also tended to increase nutrient status. For biosolids excessive amounts of P and NO3- accumulated while green waste compost generally had the least affect on nutrient status.
The addition of residue mud to the sand increased mesoporosity at the expense of macroporosity and increased available water holding capacity and the quantity of water held at field capacity. However, its addition increased the EC, alkalinity and Na levels in the sand and reduced the germination index of watercress in the carbonated mud-amended treatments. Seawater neutralized mud had a lower pH, EC, soluble HCO3-/CO32- and Na content than carbonated mud and therefore had a less negative effect on plant germination and plant growth. Where leaching occurred, amending the mud with 5% gypsum prior to its addition to the sand, in order to cause further neutralization of alkalinity and to provide excess Ca2+ to displace and promote leaching of the Na+ was found to be a successful strategy in substantially lowering EC, pH and ESP so its addition caused no depression in subsequent plant growth.
The addition of residue mud and poultry manure together induced the formation of macro-aggregates and these aggregates were relatively water stable. It was suggested that this was due to micro-aggregation caused by formation of organic matter-clay associations followed by macro-aggregation induced by the binding actions of microbially-produced polysaccharides. Green waste compost and biosolids, in association with residue mud, were much less effective at forming stable aggregates and this was attributed to the much lower microbial biomass supported compared to that in poultry manure-amended treatments.
In the leaching column experiment, where both the surface and subsurface layers were phosphogypsum-treated, the major cation leached was Na+ and the major balancing anion was SO42-. Where phosphogypsum was not applied to the subsurface, the main cation leached was still Na+ but the main balancing anions were a combination of HCO3-/CO32- and SO42-. At the end of the experiment, pH and concentrations of exchangeable Na in subsurface layers were broadly similar regardless of whether gypsum had been applied to that layer or not. Lack of reaction of gypsum with the subsurface layer is therefore unlikely to be a major factor limiting revegetation of residue sand since the excess Na+ leaches with the residual alkalinity (HCO3-/CO32-)
It was concluded that additions of organic wastes and residue mud improve the physical properties of residue sand (particularly available water holding capacity) while organic wastes also improve nutrient status and cause large increases in the size and activity of the microbial community. It was recommended that an in-situ field trial or large lysimeter study be established at Alcoa to investigate the practicality and viability of adding these amendments to the surface layer of residue sand before revegetation commences.