Date Published: June 7, 2016
Publisher: Springer Berlin Heidelberg
Author(s): Sumaira Aslam, Ali Hussain, Javed Iqbal Qazi.
We conducted a study for enhanced biological rehabilitation of chromium-contaminated soils using a chromium-reducing and nitrogen-fixing bacterial species (Bacillus megaterium-ASNF3). The bacterial species was isolated from a chromium-rich land area, characterized, and employed under optimum conditions for the treatment of artificially prepared chromium-rich soil. The bacterium reduced Cr(VI) up to 86 % in a 60-day trial of incubation in the soil bioreactor. The nitrogenase activity of the bacterium yielded up to 486 nmol of ethylene/mL/h after an incubation period of 40 days when it was optimally cultured in growth medium at neutral pH and 30 °C. Although the nitrogen-fixing ability of the bacterium reduced significantly in the presence of 1000 ppm of Cr(VI), yet, the bacterium was proved to be a potential bio-fertilizer for enhancing nitrogen contents of the contaminated soil even under the higher chromium stress, together with the metal reduction. In the biologically treated soil, higher values of wheat growth variables were achieved. Application of metal-resistant B. megaterium-ASNF3 in selected situations rendered chromium-laden soils arable with significant increment in crop-yield parameters.
Industrial activities and sewage sludge depositions have largely contributed to the spread of toxic metals in terrestrial and aquatic environments. The long-term depositions of metal-loaded effluents have transformed fertile land areas into non-arable lands. Agricultural activities on such lands would result in bioaccumulation of toxic metals in food chain (Lu et al. 2011; Liu et al. 2013). Wide industrial applications of Cr in textile, leather tanning, metal finishing, and inorganic chemicals’ manufacturing result into discharges of Cr-loaded effluents into the environment. The environmental and public health concerns have made the metal contaminations a highly attention seeking problem (Zayed and Terry 2003).
The present study aimed at using an N-fixing bacterial species indigenous to Cr-loaded waste disposal sites which thus may have evolved metal detoxification mechanisms for biotransformation of Cr(VI) to non-toxic Cr(III) together with its potential to fix N. The gram-positive Bacillus megaterium-ASNF3 worked efficiently for detoxification of Cr-contaminated soil in the present investigation and reduced up to 86 % of 1000 mg/L of Cr(VI) in the soils within 60 days of incubation at 30 °C. Reduction mechanism was more active in liquid phase of the bioreactor as compared with the solid. The bacterium also retained its N-fixation potential up to 21.8 C2H4/mL/h while passing through exposure to toxic Cr and later bioremediation process. A soil rehabilitation success was achieved where wheat growths not only recovered in the treated soil but with even higher values of some growth parameters when compared with intact soils. These findings elucidate importance of the bacterium in agricultural as well as bioremedial technologies concomitantly.