An investigation into the general characterisation of bauxite residue  and its potential for use as a low-cost adsorbent for the removal and re-use of phosphorus from forest run-off and an agricultural

Alumina production by the Bayer process generates bauxite residue at a global rate of approximately 150 Mt per annum. An estimated 3 Bt has been produced worldwide to date, which is either landfilled or collected in land storage facilities. Depending on the refinery, bauxite residue may undergo a separation technique in which the coarse fraction is separated from the fine fraction (<100 µm). The coarse fraction is often used in the construction of roadways around the bauxite residue disposal area, in which the fine fraction is stored. European Union policy advocates the re-use of materials in order to establish a more ‘circular economy’ and concurrent to this, there is emphasis on Europe maintaining a steady supply of critical raw materials (CRMs) such as gallium, phosphate rock and phosphorus, which are of high economic value due to their supply risk. Consequently, there is a focus on finding suitable secondary sources of CRMs. Bauxite residue has been identified as one possible source. In addition, due to its high abundance of iron and aluminium oxides, which are desirable for phosphorus (P) retention, it may be re-used as a low-cost adsorbent of P in the treatment of forest run-off and agricultural wastewater, which are two key attributors to poor water quality in Ireland. Therefore, the aim of this study was to investigate, for the first time, fine fraction (<100 µm) bauxite residue as a low-cost adsorbent in the removal and re-use of P from wastewater. 

Bauxite residue samples representing twelve years of disposal were characterised for their mineralogical, elemental and physico-chemical properties. Changes in these properties may affect the potential for the re-use of bauxite, but no study has examined this to date. The general composition did not vary greatly, with the exception of pH and electrical conductivity, which ranged from 10±0.1 to 12.0±0.02 and from 0.4±0.01 to 3.3±0.2 mS cm-1, respectively. The main mineralogical composition comprised iron and aluminium oxides, which were estimated to range from 40.1±1.4 to 47.5±2% and from 14.8±1.5 to 17.8±0.73%, respectively. One notable high value CRM found in the bauxite residue was gallium (107±7.3 mg kg-1).

To determine the P adsorbent potential of bauxite residue, batch studies were performed using synthetic P water. Fresh samples of bauxite residue were obtained from two European alumina refineries and then treated with either gypsum or seawater, to determine if the treatments enhance its P adsorption capacity. Untreated and treated samples were placed in 50 mL capacity containers and overlain with water made up to concentrations ranging from 0 to 150 mg P L-1. They were then placed on a reciprocating shaker for 24 hr, before being allowed to settle and the supernatant water tested for its P concentration. Langmuir adsorption isotherms were then used to model the data. The study found that bauxite residue has an adsorption capacity varying between 0.345 to 2.73 mg P g-1, and that seawater and gypsum enhances the P adsorption capacity. Following the batch studies, a column study was used in order to assess the true P adsorbent capacity of the bauxite residue using real wastewater. Fine fraction bauxite residue was placed in small-scale columns and loaded with forest run-off and dairy soiled water (DSW) over a period of 24 to 36 hr. Data obtained were modelled to predict breakthrough curves and longevity of the bauxite.The longevity of the bauxite residue was estimated to be 1.08 min g-1 for the forest run-off and 0.28 min g-1 for DSW. 

The efficacy of P-saturated bauxite residue from the column study as a fertiliser replacement was compared to a conventional superphosphate fertiliser in the growth of Lolium perenne L. in a P-deficient soil. The bauxite residue was comparable to the superphosphate fertiliser in terms of the biomass yield and there was no evidence to suggest phytotoxic effects on the growth of ryegrass, nor any effects on the E. fetida L. present in the soil. 

The findings of this study highlight the potential re-use of bauxite residue as both a low-cost adsorbent and also as a potential resource/nutrient source for P supply.