Biswaranjan Dhal

Chemical and microbial remediation of hexavalent chromium from contaminated soil and mining/metallurgical solid waste: A review

Chromium is a highly toxic non-essential metal for microorganisms and plants, and its occurrence is rare in nature. Lower to higher chromium containing effluents and solid wastes released by activities such as mining, metal plating, wood preservation, ink manufacture, dyes, pigments, glass and ceramics, tanning and textile industries, and corrosion inhibitors in cooling water, induce pollution and may cause major health hazards. Besides, natural processes (weathering and biochemical) also contribute to the mobility of chromium which enters in to the soil affecting the plant growth and metabolic functions of the living species. Generally, chemical processes are used for Cr- remediation. However, with the inference derived from the diverse Cr-resistance mechanism displayed by microorganisms and the plants including biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate efflux, bioremediation is emerging as a potential tool to address the problem of Cr(VI) pollution. This review focuses on the chemistry of chromium, its use, and toxicity and mobility in soil, while assessing its concentration in effluents/wastes which becomes the source of pollution. In order to conserve the environment and resources, the chemical/biological remediation processes for Cr(VI) and their efficiency have been summarised in some detail. The interaction of chromium with various microbial/bacterial strains isolated and their reduction capacity towards Cr(VI) are also discussed.

Optimization and characterization of chromium(VI) reduction in saline condition by moderately halophilic Vigribacillus sp isolated from mangrove soil of Bhitarkanika, India

A Gram-positive moderately halophilic Cr(VI) tolerant bacterial strain H4, isolated from saline mangrove soil, was identified as Vigribacillus sp. by biochemical characterization and 16S rRNA analysis. In LB medium, the strain could tolerate up to 1000 mg L(-1) Cr(VI) concentration and reduced 90.2 and 99.2% of 100 mg L(-1) Cr(VI) under optimized set of condition within 70 h in absence and presence of 6 wt.% NaCl, respectively. The fitting of time course reduction data to an exponential rate equation yielded the Cr(VI) reduction rate constants in the range (0.69-5.56)×10(-2)h(-1). Analyses of total chromium and bacterial cell associated with reduced product by AAS, SEM/EDS, TEM/SAED, FT-IR and UV-vis-DRS indicated the formation of about 35% of insoluble Cr(III) either as Cr(OH)(3) precipitate in nanometric size or immobilized on the bacterial cell surface while the remaining 65% of reduced chromium was present as soluble Cr(III) in the growth medium. Powder XRD analysis revealed the amorphous nature of the precipitated Cr(OH)(3). The high Cr(VI) reducing ability of the strain under saline condition suggests the Vigribacillus sp. as a new and efficient strain capable of remediating highly saline Cr(VI) polluted industrial effluents.

Characterizing toxic Cr(VI) contamination in chromite mine overburden dump and its bacterial remediation

Cr(VI) generated due to natural oxidation of chromite mineral present in chromite mine overburden (COB) dumps of Sukinda, India, has been characterized by different physico-chemical methods. The Cr(VI) was found to be associated with goethite matrix at a contamination level of 500 mg Cr(VI)kg(-1) of COB. Bacillus sp. isolated from the overburden sample exhibiting high tolerance to the hexavalent chromium, was used for the remediation of Cr(VI) in the overburden. The process was optimized while varying the parameters such as pH (2-9), pulp density (10-60%) and temperature (25-40 °C). Optimal reduction of more than 98% of Cr(VI) in the COB sample was achieved in 16 h at pH∼7.0 and 60% pulp density with the Bacillus sp. (4.05 × 10(7)cells mL(-1)) in absence of media. The exponential rate equation yielded rate constant value of 2.14 × 10(-1)h(-1) at 60% pulp density. The mode of bio-reduction of Cr(VI) in the overburden sample was established by FT-IR, XRD, EPMA and SEM-EDS studies.

Bioreduction of Hexavalent Chromium by Bacillus cereus Isolated from Chromite Mine Overburden Soil

The presence of soluble Cr(VI) particularly in the overburden soil samples of the chromite mines area is about 300-500mg Cr(VI)/kg. The level of Cr(VI) in final effluents needs to be reduced to the permissible limit <0.05mg/L (USEPA) using appropriate technology before it is discharged into the soil. Out of 12 bacterial isolates from the mine samples, CSB-9 was proven effective in reducing hexavalent chromium to its trivalent form with its inherent ability to survive proficiently in 200ppm Cr(VI). The isolate, confirmed to be Bacillus cereus, was characterised as gram-positive and capsule forming with the optimum growth at pH 7.0 and 35°C. The process of bioreduction of Cr(VI) using B. cereus was optimized with various parameters, viz., pH, initial concentration, dosage of adsorbent, temperature. The bacterium gave 90% reduction from 100ppm Cr(VI) aqueous feed in 120h at pH 7.0, 35°C using 1% (v/v) cells/mL.

Microbial sorption studies for removal of trivalent chromium from model tanning bath

Out of the industrial waste streams/effluents entering in the aquatic system containing metallic species, tanneries release high amounts of chromium, an anthropogenic pollutant because of use of basic chromium sulphate in the tanning processes. Trivalent chromium, Cr (III) is the targeted ionic species for removal by biosorption on a fungal species in this work, as the technique has inherent merit of easy adsorbent regeneration and lower capital costs. The study involves the use of Aspergillus niger (A. niger), to remediate chromium from a model tanning bath with Cr (III) concentration of 500 mg L-1. The fungal species was grown in Czapek Dox media at pH 2.5 and 35°C temperature and its biomass was used in various forms such as live, autoclaved and alkali treated. With 1% (w/v) alkali treated biomass, the biosorption of chromium reached a maximum of 91% for a feed concentration of 500 mg L-1 in 2 h time at pH 2.5, temp 35°C and A/R (adsorbent : solution volume) ratio of 1/100. The lower biosorption of metal (42 - 44%) was observed with live and autoclaved biomass. The biosorption of chromium (III) on the fungal biomass was explained with various isotherms and fitted to the kinetic model involving first order expression. The study focuses on establishing the mechanism of bioremediation of chromium on A. niger.