Microbial degradation of reactive red-35 dye: Upgraded progression through Box–Behnken design modeling and cyclic acclimatization

Abstract

Indigenous halo-alkaliphilic bacterium Nesterenkonia lacusekhoensis EMLA3 isolated from alkaline textile effluent was used to degrade the widely used recalcitrant reactive red-35 dye, at high pH (11.5). The bacterium decolorized more than 90 % dye in nutrient broth (NB) within 9−48 h up to 1000 mg L−1 dye concentration. Optimized multi-parameter combinations using Box-Behnken Design (BBD) model upgraded the degradation response, particularly at high dye concentrations, which was much higher than that obtained by optimizing one parameter at a time. The microbe when subjected to repeated dye exposure (9 h cycles), advanced the decolorization process, reducing the degradation time to 1 h, which is attributable to short term acclimatization. Spectral scans (using UV–vis spectrophotometer) of microbe-treated dye gave evidence for breakdown of the complex aromatic dye structure into simpler low molecular weight metabolites. Kinetics of the reaction was analyzed and based on LCMS analyses of degrading dye metabolites, pathway of biotransformation has been proposed. Role of azoreductase was indicated in microbial degradation of the dye, and the final degraded products showed no phytotoxicity when tested using Vigna radiata seeds. Nesterenkonia was not only effective in removal of the dye in presence of heavy metals and salt that occur as co-pollutants in textile effluent, but it also removed the toxic Cr (VI) and COD from wastewater. A small supplement of starch as carbon source was found essential to trigger microbial action leading to degradation of the reactive red-35 (RR-35) dye from simulated and real textile wastewater.