Studies on microbial remediation of bauxite residue sites

Abstract

Red mud (bauxite residue) is waste produced during aluminum extraction from bauxite ore with concentrated sodium hydroxide at elevated temperature in the Bayer process (Evans, 1993). The major components in the red mud are iron oxide, silica, un-reacted alumina and residual NaOH as Na2CO3 as well as alkali bound in the form of sodalite, ferrite etc. The residual alkali content makes the red mud alkaline with a pH range from 9 to 13, and disposed off by putting them into red mud ponds. Reclamation of red mud ponds are difficult because of high pH, high concentration of soluble ions such as sodium and carbonate which are toxic and competitively inhibit the uptake of nutrients in plants and microbes. In microbiological analysis, it was found that the bacterial count was very less, which varied from 5 to 100 cfu per gram of red mud. Ten bacterial isolates, which were studied, had different Rep-PCR profile and 16S rDNA sequences. 16S rDNA sequences showed that these isolated closely related to Planococcus sp., Bacillus sp., Psedomonas sp.,Kocuria sp., Micrococcus sp., Agromyces sp. and Salinococcus sp. These isolates mainly belong to Phylym- Firmicutes, Actinobacteria and Proteobacteria. Multiple alignment of closely related bacteria and red mud bacterial isolates showed that similarity varied from 97 to 99%, but none of the isolates showed 100% sequence similarity with the bacteria of existing database. The clones that had sequence identities of over 98% to a known organism may represent the same species. These sequences that share an identity 88% to 98% are usually considered to be part of the same genus (Sadowsky et al., 1996). On this basis, seven red mud isolates described here probably represent new members of the known genera. Furthermore, none of the isolates showed homology with the Biolog database when grown on Biolog plates, which depicts that these strains are not characterized previously. Screening of the clone library resulted in distinct 16S rDNA sequences as compared to isolated cultures.