The reduction of hexavalent chromium and nitrates by Desulfuromonas YSDS-3, isolated from the soil of Yasiv sulfur mine
Authors: Сhayka O. M. , Peretyatko T. B. 
Published: March 2019

Such compounds as compounds of hexavalent chromium, nitrates and nitrites are one of the most distributed pollutants of the environment. The compounds of Cr (VI) are found in the soil, water and territories adjacent to mining enterprises in elevated concentrations. The hexavalent chromium has toxic, mutagenic and carcinogenic effect on living forms. Some sulfur-reducing bacteria are attractable for scientists attention like potential purification agentsto clear the wastewater from organic and inorganic pollutants. In this way,the purpose of our work is to study the patterns of the reduction of compounds of hexavalent chromium and nitrate by Desulfuromonas sp.YSDS-3, isolated from the soils of theYasiv sulfur mine. Bacteria were cultivated in modified Postgate C medium. To study the in fluence of Сr (VI) and NO3 on the sulfidogenic activity and the growth of bacteria they were grown in Postgate C medium containing sulfur (32 mM) and different concentrations of K2Cr2O7 (0,1–1 мМ) and КNO3 (5–15 мМ), as well as without sulfur for 10 days. The biomass of bacteria, Cr (VI), Cr (IІІ) and nitrites content was measured spectrophotometrically. Agar was used for the immobilization of the cells. Th ability of Desulfuromonas sp. YSDS-3 to utilize different electron acceptors, has been studied. It has been established that bacteria can growin media with presence of ferrum (III) citrate (at concentrations of 0.5 mM, 1 mM and 5 mM), MnO2 (at concentrations of 1 mM, 2 mM and 3 mM), KNO3 (at concentrations of 5 mM, 10 mM, 15mM) and cysteine (at a concentration of 3 mM). In a medium with potassium bichromate bacteria grow well at concentration of 0.1 mM. Perhaps, increase of K2Cr2O7 concentration from 0.5 to 1 mM leads to the suppression of growth and sulfidogenic activity of Desulfuromonas sp. YSDS-3. The quantitative measurement of Cr (VI) and Cr (III) ions in the cell-free medium had been shown that hexavalent chromium was completely reduced to Cr (III) in the presence of less than 0.5 mM of K2Cr2O7. At the same time, the increase of K2Cr2O7 caused a decreaseof reduction effectivity. It has been found that culture of Desulfuromonas sp.YSDS-3 is able to reduce nitrates and nitrites completely. At the same time, the process of sulfur reduction was oppressed in conditions of simultaneous presence of KNO3 and sulfur in the medium. There are various methods of wastewater purification from Cr (VI): the addition of aerobic or anaerobic consortia of microorganisms; treatment of sewage with various sorbents, such as natural glauconites, activated carbon, brown coal, etc. The most promising methods of the environment purification from pollutants are considered to be biological methods with exertion of microorganisms. For this purpose, in modern biotechnological approaches different methods of immobilization of microorganisms with organic and inorganic carriers are widely used. It has been found some patterns of chromium reduction by the cells of Desulfuromonas sp. YSDS-3, immobilized in agar. The model solution contained 0.5 mM Cr (VI) as an electron acceptor, and sodium lactate as a donor of electrons. It has been shown that the mood of Cr (VI) usage by immobilized Desulfuromonas sp. YSDS-3 cells was similar to the one by the liquid culture. The immobilized microbiota has a number of advantages, compared to the traditional method of artificial biological treatment of sewage in aerotanks: the immobilized cells can bee asily removed from the reaction medium; all the carriers are adapted for repeateduse; the immobilization of microbial enzymatic biomass enables the increase of the catalytic activity of some enzymes. It has been shown that it is possible to use the immobilized Desulfuromonas sp. YSDS-3 cells time after time to purify the environment from Cr (VI) pollutant.The cells of Desulfuromonas sp. YSDS-3 (the amount of cells was 1 g/L) completely reduced Cr (VI) to Cr (III) after 72 hours in the conditions of model medium addition containing 0.5 mM K2Cr2O7. Afterre-injection of 0.5 mM K2Cr2O7 in 72 hours, the reduction efficiency was about 70 %. Subsequent addition of 0.5 mM Cr (VI) led to lower reduction rate of Cr (VI): nearby 57 %. The rate of reduction of Cr (VI) depends on the concentration of the immobilized cells. With the increase of the amount of immobilized cells (2 g/L, 3 g/L, 5 g/L), it was observed more intense reduction of Cr (VI).The working concentration of the cells of Desulfuromonas sp. YSDS-3 about 5 g/L is the fit amount of bacteria for the most efficient reduction of Cr (VI). After the third introduction of the model solution, the reduction efficiency was 67 % after 72 hours of incubation. The duration and intensity of chromium reduction were directly dependent on the concentration of immobilized cells: if the amount of fixed cells was higher, the recovery process was more effective. The immobilized Desulfuromonas sp. YSDS-3 cells can completely detoxicate Cr (VI) in amount of 0.5 mM in 72 hours. The increase of the concentration of the cells accelerates the process of chromate utilization in the model solution. So, Desulfuromonas sp. YSDS-3 is promising for an aerobic treatment of the sewage from chromium-containing compounds.