BIOREMOVAL OF TOXIC CHROMIUM(VI) VIA DARK HYDROGEN FERMENTATION OF MULTICOMPONENT ORGANIC WASTE

Chromium(VI) is one of the most hazardous contaminant of the water and soil environment. Thermodynamic calculations enable to determine the optimal metabolic pathways for microbial extraction of toxic soluble hexavalent chromium compounds from contaminated sewage. The purpose of this research was to predict theoretically and confirm experimentally the possibility of hazardous chromium(VI) removal by hydrogen producing microbiome with simultaneous destruction of multicomponent organic waste and hydrogen synthesis. Gas composition was determined by the standard gas chromatography method. The redox potential (Eh) and рН of the medium were measured potentiometrically by EZODO MP-103 universal ionomer. The Cr(VI) concentration was determined by a photocolorimetric method with 1,5-diphenylcarbazide (DFK). Multicomponent organic waste was effectively destroyed by hydrogen producing microbiome in control conditions (at the absence of chromium). Maximum concentration of H2 was 36% and biohydrogen yield was 81 l/kg of absolutely dry weight of waste. The weight of organic waste was 92 times decreased (Kd=92) during only 82 hours. The hydrogen fermentation cycle was not significantly inhibited by addition of Cr(VI). Thus, the efficiency of waste destruction and biohydrogen yield decreased consequently on 10% (Kd=83) and 7% (75 l/kg of waste) after addition of 50 ppm Cr(VI) to the culture liquid at the beginning of the final phase (50 hours) of the fermentation. As it was expected, the addition of 100 ppm Cr(VI) into the liquid phase during the final fermentation phase (80 hours) caused rapid increase of the redox potential and short-term inhibition of the fermentation process. However, after complete reduction of soluble CrO42- to insoluble Cr(OH)3∙nH2O↓ by hydrogen producing microorganisms, H2 synthesis resumed and metabolic parameters returned to initial values (the Eh was -185 mV and -99 mV after complete extraction of 50 ppm and 100 ppm Cr(VI) consequently). The optimal pathway of microbial detoxification of toxic chromium(VI) compounds was thermodynamically predicted and experimentally confirmed. High efficiency of CrO42- removal by strict anaerobic hydrogen producing microbiome via dark hydrogen fermentation of multicomponent organic waste was demonstrated. The obtained results could be used as the basis for development of novel environmental biotechnology of chromium-containing sewage purification and simultaneous destruction of environmentally hazardous organic waste as well as obtaining of eco-friendly energy carrier – biohydrogen.