Characterization of Arsenic (As) Transforming Bacteria and Red Mud to Construct a two-step Bio-filter Column for total as Removal from Groundwater

Abstract

Groundwater arsenic (As) contamination inherently affects millions of people worldwide through drinking water, food materials, irrigation water, and soil. Arsenic is a toxic metalloid, designated as category – I human carcinogen causing potential threat to individuals exposed to concentrations of 10 ppb from ingested water for a prolonged period of time. Severe carcinogenic and non-carcinogenic illnesses can occur over prolonged As exposure such as keratosis, melanosis, neurological disorders, skin lesions, respiratory complications, hepatic damage, and various form of cancers. The two major countries extensively affected by groundwater As-contamination are India and Bangladesh. In India, the middle Gangetic plains, covering particularly 89% geographical area in the state of Bihar is severely affected which holds prospective alluvial aquifers. The Bhojpur district in Bihar, located in the middle Gangetic plains, amidst the flood-prone belt of the Sone-Ganga interfluvial region is one of the worst affected areas from geogenic As. The present investigation begins with a microcosm based bio-stimulation study and substrate amendments over 45 days to analyze the bacterial community structure and distribution to indicate the possible in-situ bioremediation strategy in six severely As contaminated groundwater sites of the middle Gangetic plains in Bhojpur district of Bihar, India. Proteobacteria was primarily the dominant bacterial phylum in all the samples, followed by Actinobacteria, Bacteroidetes, Firmicutes, and Cyanobacteria. The major groups in the genus level were Delftia, Acinetobacter, Lysobacter, Bacillus, and Pseudomonas in the As-rich aquifer system. The bio-stimulated samples showed a change in the community structure with the dominance of Planctomycetes, with a minute portion of Proteobacteria. The species richness was determined using the Alpha diversity and Chaol curve with an As tolerant capacity of 152.28 ppb. The supremacy of γ- proteobacteria and α- proteobacterial members in the high and low As-containing water samples indicated their role in As mobilization and detoxification. The extensive role of arsenite [As (III)] oxidizing microbial communities within different levels of As-contaminated areas in Bihar was studied due to complete change in microbial community structure within the bio-stimulated conditions, deciphering the considerable role of these microbial communities in the As-biogeochemical cycle.
From the previous study, it was learned that several As (III) oxidizing groups dominated the predominant aquifers in the region. Several superior As (III) oxidizing strains were isolated by enrichment technique from six shallow aquifers of Bhojpur district. The isolates were screened using silver nitrate assay for As (III), and arsenate [As (V)] tolerance up to 100 mM of As (III) and 1000 mM of As (V). The isolates were also tested for their multi-metal resistance and utilization of various carbon sources. The molecular identification of the four highly efficient As (III) oxidizing strains showed their relatedness with different Delftia sp. Finally, a gram-negative rod-shaped strain of Delftia sp. BAs29 was characterized thoroughly based on its superior As (III) oxidation potential for its growth and effective As (III) oxidation ability. A mixed growth associated, facultative chemolithotrophic As (III) oxidation process was revealed by the strain, with a Km and Vmax value of 21.97 μM and 0.657μM/min. Further, testing of natural and cost-effective bio-sorbents showed efficient As (V) removal from the contaminated water. Moringa oleifera showed the maximum As (V) removal capacity of 57.89% among the tested bio-sorbents, followed by sawdust and riverside red mud. The process of As (III) biooxidation was optimized using Response Surface Methodology by considering four factors such as temperature (30 °C – 37 °C), inoculum percentage (1% – 5%), initial As (III) concentration (80 μM –120 μM), and time (10 h –16 h). Hence, this study deciphered a proficient process of As removal by combining an indigenous As (III) oxidizing strain and a natural bio-sorbent. The next study aimed at utilizing powdered neutralized red mud as a natural adsorbent for As removal as it is a waste from the alumina industry. Its enormous volume production and difficulty of disposal makes it a cost-effective adsorbent. The mineralogical composition of red mud consists of iron, aluminium, silicon, and titanium oxides which cause elemental species transformation and precipitation during bio-oxidation of complexes such as arsenopyrites by inherent microbial organisms. The iron bearing minerals significantly affect the solution chemistry of As, making it a valuable adsorbent for effective As removal. The previously used Moringa oleifera is a seasonal adsorbent and its production may be expensive. Hence microbial transformation of As (III) seems to be a favorable approach, coupled with several adsorption techniques such as powdered neutralized red mud without producing lethal by-products or demanding chemical addition. This study highlighted the potential contribution of the previously isolated highly efficient As (III) transforming bacteria Delftia sp. BAs29 and the adsorption of transformed As (V) using powdered neutralized red mud under suitable treatment conditions. The rate and oxidation efficiency elucidated diverse experimental conditions for the process. The neutralized red mud was characterized using X-Ray diffraction (XRD) microanalysis, Scanning electron microscopy – Energy dispersive X-Ray spectroscopy (SEM-EDX), and Fourier – Transform infrared spectroscopy (FTIR). The adsorption of As (V) using powdered neutralized red mud was also studied as a function of time and pH, initial As (V) concentration and adsorbent dosage. The decrease in solution pH significantly increased the adsorption efficiency. The maximum monolayer capacity for adsorption of 274.1 mg/g As (V) was found at optimum conditions of pH 4.0, a contact time of 30 min at a temperature of 30 °C respectively. Hence, various strategic methods of As bioremediation could be developed using this process to remove the inherent groundwater As. The final objective aimed at developing a two-step approach to combat the toxic metalloid As by combining the previously isolated, highly efficient As (III) oxidizing bacteria; Delftia sp. BAs29 and porous red mud pellets to remove the total As from groundwater including both As (III) and As (V) ions. For the first step, the maximum capacity of As (III) oxidation by Delftia sp. BAs29 was seen to be 95.65% for 500 ml of As contaminated groundwater using an optimized As (III) concentration of 300 ppb and 6.5 g of bacterial cell mass for 7 days. The second step indicated the maximum As (V) adsorption capacity by the stacked red mud pellets to be 97.91% for 500 ml of As contaminated groundwater using an optimized pore size of 106 μ - 125 μ for 7 days. The total As removal efficiency increased to 98.76% at a flow rate of 50 ml/h on combination of both the steps. Further, the chemical composition, morphological properties, and crystal structure of the As (V) adsorbed red mud pellets were characterized by Fourier – Transform infrared spectroscopy (FTIR), Scanning electron microscopy – Energy dispersive X-Ray spectroscopy (SEM-EDX), and X-Ray diffraction (XRD) microanalysis. The techno economic feasibility of this entire unit was studied using SuperPro 10 software to estimate its optimal demand and potential. Hence, in the near future the scaling up of this two-step bio-filter column can serve as a cost-effective and efficient filtration unit to eradicate the total As from drinking water, both at household and industrial levels.