Abstract
The indiscriminate disposal of chrome shavings, splits and trimmings on land and close to water bodies leads to high concentration of chromium within such environments. Therefore, a treatment process which was able to reduce 99% of the chromium concentration in the waste was adopted, and the fiber left after the de-chroming process was investigated as a potential carrier for nitrogen-fixing bacteria. The product of the bacteria and the carrier is used as rhizobium bio-fertilizer. De-chromed tannery solid wastes with a pH of 4.88 and eggshell (a natural source of calcium carbonate) with a pH of 9.36 were mixed at ratio 3:2 to attain a desired pH of 6.71. This pH value was found to be suitable for the growth of Bradyrhizobium japonicum strain (USDA 110), nitrogen-fixing bacteria often used as bio-fertilizer inoculant. The mixture was sterilized using gamma irradiation and autoclaving. The strain was obtained from the Soil Microbiology Laboratory of the Institute for Agricultural Research (IAR), Ahmadu Bello University, Zaria. A fresh culture of the strain was prepared on Yeast Mannitol Agar (YMA) plates. The sterile carrier was inoculated with the cultured inoculum at 50% and 55% moisture content and cured for 21days at a temperature of 28oC for maturation in an orbital incubator. The survival and growth of the strains after the maturation period confirmed the suitability of the developed carrier for the growth of rhizobium bio-fertilizer. The strain was re-isolated from the developed carrier using serial dilution at 10-10 dilution factor on YMA plates. Gram stained isolates from serial dilution were observed under microscope at 100x magnification and found to be gram negative short rods which is a typical morphology of rhizobium cells.

Key words: Tannery wastes, sterilization, Bradyrhizobium japonicum, bio-fertilizer, carrier.