ASPERGILLUS TUBINGENSIS REDUCES THE pH OF THE BAUXITE RESIDUE (RED MUD) AMENDED SOILS PANKAJ KRISHNA 1 , M. SUDHAKARA REDDY 1,∗ and S. K. PATNAIK 2 1 Thapar Institute of Engineering & Technology, Department of Biotechnology, Patiala 147 004, India; 2 National Aluminium Company, M & R Complex, Damanjodi, Orissa, India ( ∗ author for correspondence, e-mail: vasu70@yahoo.com, Tel.: +91-175-2393043, Fax: +91-175-2393738) (Received 23 April 2004; accepted 8 June 2005) Abstract. Bauxite residue (red mud), a poor substrate for plant growth because of very high pH, salinity and sodicity, is required to be revegetated. The possibility of using Aspergillus tubingensis (AT1), a phosphate solubilizer in red mud amended soils to reduce the alkalinity of the red mud was studied in the present study. A. tubingensis was tested for its ability to grow at high pH and in different concentrations of aluminium (Al), iron (Fe) and sodium (Na). The results showed that A. tubingensis was able to grow at high pH and reduce the alkalinity of the nutrient medium and also to accumulate different metals in the mycelium. The pH values of the red mud amended soils were also significantly reduced (by 2–3 units) by A. tubingensis, which resulted in maize growth improvement. These results suggest that A. tubingensis plays an important role in reducing the pH of the red mud and also helps in promoting the plant growth in it. Keywords: Aspergillus tubingensis, bauxite residue, bioremediation, red mud Introduction Bauxite residue (red mud) is formed during heat treatment of bauxite ore in sodium hydroxide solution using the Bayer process leading to soluble alumina as Al(OH) 3 . The remaining insoluble materials (principally iron oxide) are known as red mud or bauxite residues (Li and Rutherford, 1996). The bauxite residue is highly alkaline mixture of fine particle-sized metal oxides and the most common method for its disposal is storage in impounded dike deposits adjacent to the alumina processing plants. Millions of tones of bauxite residues are generated every year in India. The pH of the bauxite residues varies from 10 to12 because of the incomplete removal of NaOH used in the ore processing. The high alkali content of bauxite residue deposits from alumina production plants in industrial nation poses a challenge to reestablish flora and fauna at the deposit sites. These deposits also pose a potential risk of groundwater contamination and also to human health. The potential use of bauxite residue as a lime substitute for treating acid soils as well as acid mine drainage sites and to enhance minerals deficient soil was suggested by Edward et al. (1999). Attempts were made to rehabilitate the residue deposits and bauxite mines with and without neutralization by establishing vegetation cover using indigenous Water, Air, and Soil Pollution (2005) 167: 201–209 C  Springer 2005