Heavy metal contamination from anthropogenic activities has an adverse effect on the environment due to its cytotoxicity, carcinogenicity and mutagenicity. Environment harbours microorganisms, some of which have been found to modify physico-chemical conditions of their surrounding environment through certain processes such as detoxification, metal homeostasis, precipitation, redox transformations etc. Investigations in the past have shown that short term contact with metals of certain bacteria causes the selection of resistant bacteria within weeks, while a prolonged exposure showed selected strains able to thrive better. Hence biotic methods could assist removal of heavy metals based on biosorption or bioaccumulation by microorganisms, which are cost-effective and environmental friendly in the long run. Microbial remediation is influenced by biotic and environmental factors as also the contamination site characteristics. The aim of this paper is to highlight and review some of the mechanisms of microbial remediation through techniques such as biostimulation, bioaugmentation etc.

The removal of fluoride is essential for water contaminated with fluoride before being utilized since the unsafe concentration of fluoride with respect to the permissible limits. In the present study, there are 61 bacterial strains belonging to fluoride tolerance were isolated from the contaminated soil of Dharmapuri District, Tamil Nadu, India and they were evaluated for different characterization. Among the strains isolated, the strain PN4 showed a high tolerance to fluoride ranging from 500 to 2500 ppm under different stress conditions. The strain PN4 was selected as a possible organism for the degradation and removal of fluoride in an aqueous medium. Based on the morphology, biochemical characteristics and the 16S rRNA sequencing, the bacterium PN4 was identified as Bacillus flexus. In batch mode studies, the glucose was showed the maximum removal of fluoride (86%) followed by beef extract (82%) and a significant level of defluoridation was observed at pH 7.0 and the temperature at 35°C. In the antibiotic-resistance pattern, the strain Bacillus flexus PN4 was shown sensitive to three different antibiotics. Intracellular accumulation of fluoride by the bacterial cell was characterized by SEM- EDAX, TEM and FTIR analysis. GRAPHICAL ABSTRACT

A new approach was carried out with the spent mushroom substrate (SMS) of Pleurotus florida on ferrous iron (Fe²⁺) removal using live, dead and pretreated substrate. In this study, the various dosage levels of SMS namely, 0.25, 0.50, 1.0 and 1.50 g/50 mL were used for the removal of Fe²⁺ at different time intervals for 90 min. The effect of various temperatures and pH on Fe²⁺ removal was studied with optimized dosages and time intervals. The biosorption potential of P. florida SMS was checked against the iron-contaminated groundwater collected from in and around Salem, Namakkal and Dharmapuri districts of Tamil Nadu. The biosorption data were obtained and analyzed in terms of their kinetic behavior. Among the SMS of P. florida, the live SMS showed potential Fe²⁺ removal (100%) from aqueous metal solution in all the tested concentrations. SMS of P. florida showed high potential removal of Fe²⁺ in neutral pH, at room temperature and explored an efficient sorption ability (100%) in the tested water sample (SW10). The adsorption kinetic values fitted very well with pseudo-second-order when comparing with pseudo-first-order reaction. FTIR, SEM and EDX analysis proved the accumulation of Fe²⁺ by the SMS. The present study confirmed that the live SMS of P. florida may serve as a potential and eco-friendly biosorbent for removal of Fe²⁺ from the iron-contaminated water. Graphic