Activated red mud as a permeable reactive barrier material for fluoride removal from groundwater: parameter optimisation and physico-chemical characterisation.

The main aim of this work is to test the performance of red mud as a permeable reactive barrier (PRB) material for fluoride removal from water. Batch experiments were carried out to optimise the fluoride removal efficiency (RE) of activated red mud (ARM) based on four selected parameters, namely, the initial fluoride concentration (3-40 mg/L), adsorbent dose (0.5-5 g/L), pH (3.0-11.0) and ionic strength (0.001-0.5 M). Statistical analysis of the results revealed the optimum conditions as initial fluoride concentration -21.46 mg/L, adsorbent dose -2.77 g/L, pH 7.01 and ionic strength -0.24 M, respectively. Under the optimum conditions, fluoride RE of 87.3% was achieved. The individual effects due to initial fluoride concentration, adsorbent dose and ionic strength on fluoride removal were highly significant (F = 59.69; P < 0.005); whereas adsorbent dose, pH and ionic strength showed the greatest squared effects (F = 26.05; P < 0.001). The interaction effect due to initial fluoride concentration and adsorbent dose was also found to be significant (F = 12.52; P = 0.002) for fluoride removal. Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) analyses were performed to identify the change in functional group and surface topography following red mud activation.