Detoxification and accumulation of chromium from tannery effluent and spent chrome effluent by Paecilomyces lilacinus fungi

The tannery industry process involves chromium (Cr) salts as a main constituent of the process. The Cr recovery is a part of the process where other salts are used to achieve separation and recovery for using Cr back in the process. The process steps may contain both forms of Cr Cr(VI): hexavalent and Cr(III): trivalent]. The recovery of Cr from tannery industry effluent through biological systems is much needed. The diverse physicochemical characteristics of these effluents may limit the growth of microorganisms and hence the limitation towards possible practical application of microorganisms in real industrial effluent conditions. The present study attempted the ability of the Cr-resistant fungus Paecilomyces lilacinus isolated through an enrichment culture technique at 25 000 mg l⁻¹ of Cr(III)] to grow and remove Cr Cr(VI) and Cr(III)] from two physicochemically different undiluted tannery industry effluents (tannery effluent and spent chrome effluent) in the presence of cane sugar as a carbon source. Such attempts are made keeping in view the potential integration of biological processes in the overall Cr removal and recovery processes to improve its efficiency and environmental sustainability. The fungus has broad pH tolerance range and can reduce Cr(VI) both in acidic (pH 5.5) and alkaline (pH 8.0) conditions. The fungus showed the ability to remove Cr(VI) (1.24 mg l⁻¹) and total Cr (7.91 mg l⁻¹) from tannery effluent below the detection level within 18 h and 36 h of incubation, respectively, and ability to accumulate 189.13 mg Cr g⁻¹ of dry biomass within 600 h of incubation from spent chrome effluent containing 3731.4 mg l⁻¹ of initial Cr(III) concentration].At 200 mg l⁻¹ of Cr(VI) in growth media, with 100% detoxification and with only 10.54% of total Cr accumulation in the biomass, P. lilacinus showed Cr(VI) reduction as a major mechanism of Cr(VI) detoxification. The time-course study revealed the log phase of the growth for the maximum specific reduction of Cr(VI) and stationary phase of the growth for its maximum specific accumulation of both the forms of Cr Cr(III) and Cr(VI)] in its biomass. In growth media at 50 mg l⁻¹ and 200 mg l⁻¹ of Cr(VI), P. lilacinus showed 100% reduction within 36 h and 120 h of incubation, respectively. The high degree of positive correlation and statistically high degree of relationship (r² = 0.941) between the fungal growth and % Cr(VI) reduction by the fungus support the role of metabolically active cellular growth in Cr(VI) reduction by the fungus. Results indicate that expanded solid (sludge) retention times (SRTs) (stationary phase) can be recommended for the removal of Cr(III) through accumulation. In case of Cr(VI), reduction needs a priority; therefore, a non-expanded SRT is recommended for designing a continuous-flow completely stirred bioreactor so that a log phase of cellular growth can be maintained during the reduction process. This study reveals the strong potential of P. lilacinus fungi for the removal of Cr from tannery effluent and spent chrome effluent.