Use of Mixotrophic Bacteria as Flocculating Agents to Separate Iron from Red Mud (Alumina Refinery Residue)

Red mud is a term used to describe the waste from the bauxite converting process into alumina, which is alkaline and has colloidal particles, high moisture contents, and high metal contents in oxide minerals form. One widely used method proven to be effective for the sedimentation of fine particles is bioflocculation. Bioflocculation uses biosurfactants generated by mixotrophic bacteria that can modify the mineral surfaces as flocculating agents; thus, fine minerals can be settled and separated from clean water (supernatant). This work investigated the possibility of bioflocculation of red mud by using mixotrophic bacteria as flocculating agents. Sixty-two mixotrophic bacterial strains of the genera Alicyclobacillus sp., Bacillus spp., Paenibacillus spp., Morganella sp., Pseudomonas spp., Dietzia sp., Serratia sp., Lysinibacillus spp., Comamonas sp., Pseudoclavibacter spp., Diaphorobacter sp., Neisseria sp., Alcaligenes sp., Micrococcus sp., Stenotrophomonas sp., Aeromonas sp., and Citrobacter spp. were examined for their potential to be bioflocculants. After one hour, all bacterial strains showed that above 70% weight of red mud was settled. The best result was 93.59% w/w that could be recovered with the bioflocculant Paenibacillus pasadenensis. The minimal settling rate using bioflocculants was 8.33 × 10–4 cm/s, 10 times greater than settling without flocculants. Furthermore, another advantage of bacterial use was decreasing the pH of supernatant from alkaline (10.83) to neutral (about 6.8). These results encourage further research on the beneficiation process that is low cost and environmentally friendly. In addition, six bacterial strains exhibiting the best flocculation results (settling rate, flocculating rate, and weight recovered) were further characterized to determine the number of biosurfactants produced by the emulsification index (E24) method. E24 values of all the bacterial strains were above 50% after 72 h of cultivation. The attachment of those six bacterial strains to red mud was observed through organic groups based on the Fourier transform infrared (FTIR) analysis. To our knowledge, this is the first report on using bacteria and their metabolites as bioflocculants in the treatment of red mud through a selective flocculation process for the water and wastewater treatment of mining and metallurgy industries.