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. These were mainly belonging to Proteobacteria and Bacteriodetes. These bacterial clones were affiliated to Beta- (34%) and Gamma- (56%) Proteobacteria while 4.8% clones represent Bacteriodetes. Interestingly, all sequences of clone library had 99% or more similarity with the uncultured and cultivable isolates. But, these clones showed no homology with the cultivable isolates of red mud. These results showed that the bacterial diversity is complex in this environment. The present study gives the first insight about the microbiological (bacterial diversity) analysis of red mud. The indigenous bacterial isolates and exogenous Aspergillus tubingensis were used in nursery trial for the remediation and revegetation of red mud in combination with fly ash, topsoil, gypsum and sewage sludge. Although, gypsum amended red mud showed maximum reduction of alkalinity (pH) and attained pH about 8.0, but the level of soluble ion in this treatment was relatively high as revealed by the measurement of electrical conductivity. Red mud amended sewage sludge and bacterial treatment had highest level of organic carbon, organic matter and available phosphorus. Red mud amended sewage sludge and bacterial treatment showed maximum metabolic diversity of bacteria on Biolog plates. Curiously, red mud inoculated with bacteria also showed growth of grass, but red mud control and red mud inoculated with A. tubingensis did not support the grass growth. It showed that indigenous bacterial consortia played an important role in the growth promotion of Bermuda grass without any chemical amendment. A. tubingensis did not promote the growth of Bermuda grass when used alone (without any chemical amendment). It promoted grass growth, which was highest when used in combination with sludge. Overall, the present study showed that A. tubingensis and indigenous bacteria play an important role in reducing the pH of the red mud and also promote the plant growth, though bauxite residue is a poor substrate for plant growth because of very high pH, salinity and sodicity. To best of our knowledge it the first report of amelioration of red mud using biological means in nursery trial. Alkali tolerance mechanism were studied on two red mud isolates RM9P and RM8, which had shown that the buffering capacities are influenced by the pH of the medium and culture conditions. RM8 showed more buffering capacity than the RM9P. Alkali tolerance is strongly dependent on the cellular metabolism, as it was evident from the glycolysis reactions and organic acid production of RM9P and RM8 at different pH.