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Indigenous isolates from the waters of chromite mining sites at Sukinda Valley, Orissa, India, showed a considerable enhancement in Cr (VI) bioreduction rate through adaptation and consortia development. On the basis of 16S-rRNA sequencing, these isolates were identified as Bacillus subtilis VITSUKMW1, Acinetobacter junii VITSUKMW2, and Escherichia coli VITSUKMW3. The native isolates showed a high tolerance at 500−1000 mg L −1 of Cr (VI). An increase in the reduction rate from 0.199−0.477 mg L −1 h −1 to 0.5−1.16 mg L −1 h −1 at 5−20 mg L −1 of initial Cr (VI) concentration was achieved by the adapted isolates. An increase in the growth rate and Cr (VI) reduction rate [0.86−2.6813 mg L −1 h −1 at 5−100 mg L −1 of initial Cr (VI) concentration] was observed in the ternary consortium of adapted isolates. The FT-IR spectra revealed the active participation of the bacterial surface groups in the reduction. The development of sequential processes (native → adapted → consortia) employing Cr (VI) tolerant isolates, proves to be a potential bioremediation strategy for specific chromite mine sites.

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The current study deals with Cr(VI) removal using unmodified alumina nanoparticles of two different sizes (NA1: mean hydrodynamic diameter = 75.3 ± 2.8 nm; NA2: mean hydrodynamic diameter = 229.8 ± 3.3 nm). The equilibrium adsorption capacities, 73.2 and 59.4 mg of Cr(VI)/g of adsorbent, were noted for NA1 and NA2, respectively, under optimized conditions (pH 7.0, temperature = 27 °C, initial Cr(VI) concentration = 20 mg L −1 , adsorbent dosage = 0.1g L −1 ) but different contact times (120 min for NA1, 180 min for NA2). For both sorbents, the equilibrium adsorption data fitted well with the Langmuir isotherm model. The adsorbent NA1 followed a pseudo-second-order kinetics, whereas NA2 followed pseudo-firstorder kinetics. Surface characterization studies (zeta potential measurement, scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDX), and Fourier transform infrared (FTIR) spectroscopy) substantiated oxyanionic binding on the sorbent surface. The EPR and XRD spectroscopy confirmed the existence of reduced Cr(III) on the adsorbent surface. The applicability of the sorbent in Cr(VI)-contaminated water was studied.

In the current study, indigenous bacterial isolates Bacillus subtilis VITSUKMW1 and Escherichia coli VITSUKMW3 from a chromite mine were adapted to 100 mg L(-1) of Cr(VI). The phase contrast and scanning electron microscopic images showed increase in the length of adapted E. coli cells and chain formation in case of adapted B. subtilis. The presence of chromium on the surface of the bacteria was confirmed by energy dispersive X-ray spectroscopy (EDX), which was also supported by the conspicuous Cr-O peaks in FTIR spectra. The transmission electron microscopic (TEM) images of adapted E. coli and B. subtilis showed the presence of intact cells with Cr accumulated inside the bacteria. The TEM-EDX confirmed the internalization of Cr(VI) in the adapted cells. The specific growth rate and Cr(VI) reduction capacity was significantly higher in adapted B. subtilis compared to that of adapted E. coli. To study the possible role of Cr(VI) toxicity affecting the Cr(VI) reduction capacity, the definite assays for the released reactive oxygen species (ROS) and ROS scavenging enzymes (SOD and GSH) were carried out. The decreased ROS production as well as SOD and GSH release observed in adapted B. subtilis compared to the adapted E. coli corroborated well with its higher specific growth rate and increased Cr(VI) reduction capacity.

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The present work deals with the photocatalytic removal of Cr(VI) by zinc oxide particles. Cr(VI) removal capacity of 494.4 mg/g, 369.27 mg/g, and 355.59 mg/g was noted in sunlight, fluorescent, and dark conditions respectively at optimized parameters (pH: 5.0, initial Cr(VI) concentration: 50 mg/L, particle dosage: 10 mg/L, contact time: 60 min). A Langmuir isotherm model and pseudo first order kinetics were observed. The aggregation of Cr(VI) interacted ZnO particles was observed by dynamic light scattering (DLS), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The results suggest the photon‐assisted reduction of Cr(VI) to Cr(III) on the ZnO particle. The EDX, FTIR, and EELS indicated Cr(VI) removal by an adsorption‐coupled reduction mechanism. Lower aggregation observed in the sunlight condition favoured enhanced Cr(VI) sorption compared to that in fluorescent light. Fourier transform infrared spectroscopy (FTIR) and energy‐dispersive X‐ray spectroscopy (EDX) confirmed the interaction of Cr(VI) on the particle surface and the electron energy loss spectroscopy (EELS) confirmed its subsequent reduction to Cr(III). The applicability of the process was studied with Cr(VI)–Cr(III) binary mixture and the multi–metal solutions.