Re-use of waste red mud: Production of a functional iron oxide adsorbent for removal of phosphorous

An iron oxide sorbent derived from red mud waste (termed red mud akaganeite (RMA)) was investigated for the treatment of waters containing excess phosphorus (P) based nutrients such as phosphate. The hypothesis was that this innovative sorbent could outperform commercial granular ferric hydroxide (GFH). Sorbent performance was tested by a combination of sorption kinetics and equilibrium studies. Key factors evaluated included influence of phosphorous concentration, solution pH, and presence of competing anions. The kinetics of phosphorous sorption on both RMA and GFH were best described by the pseudo first order model, albeit the RMA material showed a higher rate of sorption. The equilibrium capacity as modelled by the Langmuir equation was 12.9 mg P/g for RMA, which was over 140% greater than that of GFH. X-ray photoelectron spectroscopy suggested that surface hydroxyl species were part of the mechanism for phosphorus uptake. The efficiency of phosphorous uptake by RMA was relatively unaffected by solution pH. In contrast with GFH, a significant loss in performance above pH 8 was found. It was postulated that point of zero charge on the sorbent and ability to release protons and chloride into the water may explain the pH data. Nitrate, sulfate, and chloride had minimal effect on phosphorus uptake by RMA or GFH, but both materials were impacted by the presence of carbonate species. Infrared spectroscopy indicated the presence of carbonate on the sorbent surface. Treatment of stormwater revealed that RMA was potentially suitable as a sorbent for phosphorus from real water samples.