Biological approaches to tackle heavy metal pollution: A survey of literature

Arsenic contamination of drinking water is a major global problem, with contamination in Bangladesh deemed most serious. Although the current World Health Organisation (WHO) maximum contamination limit (MCL) for arsenic in drinking water is 10 μg L−1, due to practical and economic constraints, the standard limit in Bangladesh and many other developing nations is 50 μg L−1. We propose that an ideal arsenic sensor, designed for routine monitoring, will have five essential qualities: sensitivity and selectivity for arsenic; speed and reliability; portability and robustness; reduced health and environmental risks; and affordability and ease of use for local technicians. It is our opinion that many of these characteristics can be accentuated by microfluidic systems. We describe candidate colorimetric, electrochemical, biological, electrophoretic, surface-sensing, and spectroscopic methods for arsenic detection; and comment on their potential for portable microfluidic adaptation. We also describe existing developments in the literature towards the ultimate creation of microfluidic total analysis systems (μTASs) for arsenic detection. The fundamental purpose of this review is to highlight the need for better portable arsenic contamination detection, and describe how microfluidic technology may be developed to address this need.

Detection of trace arsenic in drinking water: challenges and opportunities for microfluidics

A paper based microfluidic device is fabricated that can rapidly detect very low concentrations of As3+ ions using a gold nanosensor, Au–TA–TG. This simple but efficient system develops a visible bluish-black colour precipitate due to the formation of nanoparticle aggregates through transverse diffusive mixing of Au–TA–TG with As3+ ions on a paper substrate. The approach is extremely selective for arsenic with a detection limit of 1.0 ppb, which is lower than the WHO's reference standard for drinking water.

A paper based microfluidic device for the detection of arsenic using a gold nanosensor

Present work demonstrates Cr (VI) detoxification and resistance mechanism of a newly isolated strain (B9) of Acinetobacter sp. Bioremediation potential of the strain B9 is shown by simultaneous removal of major heavy metals including chromium from heavy-metals-rich metal finishing industrial wastewater. Strain B9 tolerate up to 350 mg L−1 of Cr (VI) and also shows level of tolerance to Ni (II), Zn (II), Pb (II), and Cd (II). The strain was capable of reducing 67 % of initial 7.0 mg L−1 of Cr (VI) within 24 h of incubation, while in presence of Cu ions 100 % removal of initial 7.0 and 10 mg L−1 of Cr (VI) was observed with in 24 h. pH in the range of 6.0–8.0 and inoculum size of 2 % (v/v) were determined to be optimum for dichromate reduction. Fourier transform infrared spectroscopy and transmission electron microscopy studies suggested absorption or intracellular accumulation and that might be one of the major mechanisms behind the chromium resistance by strain B9. Scanning electron microscopy showed morphological changes in the strain due to chromium stress. Relevance of the strain for treatment of heavy-metals-rich industrial wastewater resulted in 93.7, 55.4, and 68.94 % removal of initial 30 mg L−1 Cr (VI), 246 mg L−1 total Cr, and 51 mg L−1 Ni, respectively, after 144 h of treatment in a batch mode.

Evaluation of Acinetobacter sp. B9 for Cr (VI) resistance and detoxification with potential application in bioremediation of heavy-metals-rich industrial wastewater.

Cr(VI) represents a serious threat to human health, living resources and ecological system as it is persistent, carcinogenic and toxic, whereas, Cr(III), another stable oxidation state of Cr, is less toxic and can be readily precipitated out of solution. The conventional methods of Cr(VI) removal from wastewaters comprise of chemical reduction followed by chemical precipitation. However, these methods utilize large amounts of chemicals and generate toxic sludge. This necessitates the need for devising an eco-technological strategy that would use the untapped potential of the biological world for remediation of Cr(VI) containing wastewaters. Among several viable approaches, biotransformation of Cr(VI) to relatively non-toxic Cr(III) by chromium resistant bacteria offers an economical- and environment-friendly option for its detoxification. Various studies on use of Cr(VI) tolerant viable bacterial isolates for treatment of Cr(VI) containing solutions and wastewater have been reported. Therefore, a ...

Bioremediation of chromium solutions and chromium containing wastewaters.

This study had an objective to identify the most potent chromium-resistant bacteria isolated from tannery effluent and apply them for bioremediation of chromium in tannery effluents. Two such strains (previously characterized and identified by us)—Enterobacter aerogenes (NCBI GenBank USA Accession no. GU265554) and Acinetobacter sp. PD 12 (NCBI GenBank USA Accession no. GU084179)—showed powerful chromium resistivity and bioremediation capabilities among many stains isolated from tannery waste. Parameters such as pH, concentration of hexavalent chromium or Cr (VI), and inoculum volume were varied to observe optimum bioconversion and bioaccumulation of Cr (VI) when the said strains were grown in M9 minimal salt media. E. aerogenes was used to remediate chromium from tannery effluents in a laboratory level experiment. Observation by Scanning Electron Microscope and chromium peak in Energy Dispersive X-ray Spectroscopic microanalysis revealed that E. aerogenes helped remediate a moderate amount of Cr (VI) (8–16 mg L−1) over a wide range of pH values at 35–37°C (within 26.05 h). High inoculum percentage of Acinetobacter sp. PD 12 also enabled bioremediation of 8–16 mg L−1 of Cr (VI) over a wide range of temperature (25–37°C), mainly at pH 7 (within 63.28 h). The experiment with real tannery effluent gave very encouraging results. The strain E. aerogenes can be used in bioremediation of Cr (VI) since it could work in actual environmental conditions with extraordinarily high capacity.

Bioremediation of chromium by novel strains Enterobacter aerogenes T2 and Acinetobacter sp. PD 12 S2

Arsenic, a common contaminant of groundwater, affects human health adversely. According to the World Health Organization (WHO), the maximum recommended contamination level of arsenic in drinking water is 10 μg/L. The purpose of this research was to develop user-friendly kits for detection of arsenic to measure at least up to 10 μg/L in drinking water, so that a preventive measure could be taken. Two different kits for detection of total arsenic in water are reported here. First, the arsenic in drinking water was converted to arsine gas by a strong reducing agent. The arsine produced was then detected by paper strips via generation of color due to reaction with either mercuric bromide (KIT-1) or silver nitrate (KIT-2). These were previously immobilized on the detector strip. The first one gave a yellow color and the second one grey. Both of these kits could detect arsenic contamination within a range of 10 μg/L-250 μg/L. The detection time for both the kits was only 7 min. The kits exhibited excellent performance compared to other kits available in the market with respect to detection time, ease of operation, cost and could be easily handled by a layman. The field trials with these kits gave very satisfactory results. A study on interference revealed that these kits could be used in the presence of 24 common ions present in the arsenic contaminated water. Though the kits were meant for qualitative assay, the results with unknown concentrations of real samples, when compared with atomic absorption spectrophotometer (AAS) were in good agreement as revealed by the t-test.

Low-cost field test kits for arsenic detection in water

The purpose of this study was to isolate chromium resistant bacteria and characterize in depth before being applied for bioremediation or construction of chromium biosesnors. One of the major objectives was to identify the most useful strains with respect to chromium removal efficiency. Chromium resistant bacteria were isolated from tannery waste samples. These were characterized through biochemical tests and antibiotic assays. Cross metal reactivity, plasmid curing and cysteine quantifications were also performed. Gene sequencing was done for all the isolates and accession numbers received after submission to National Center for Biotechnology Information (NCBI). The removal capacity was also studied in broth cultures. The results were very encouraging since six new bacteria were recognised to withstand high concentrations of chromium. Out of these six bacteria, three i.e. Enterobacter aerogenes, Aeromonas sp., Acinetobacter sp. PD 12 performed very well by removing about 99% (average) from initial 19.8 ppm of Cr(VI) from a synthetic culture media. Enterobacter aerogenes, being the most efficient strain, was used to remediate hexavalent chromium from soil of potted plants- very encouraging results were obtained. These bacteria may be exploited for bioremediation of Cr (VI) and construction of microbial sensor in future.

Isolation and identification of chromium-resistant bacteria: Test application for prevention of chromium toxicity in plant.

Hexavalent chromium or Cr(VI) enters the environment through several anthropogenic activities and it is highly toxic and carcinogenic. Hence it is required to be detected and remediated from the environment. In this study, low-cost and environment-friendly methods of biosensing and bioremediation of Cr(VI) have been proposed. Crude cell free extract (CFE) of previously isolated Enterobacter aerogenes T2 (GU265554; NII 1111) was prepared and exploited to develop a stable biosensor for direct estimation of Cr(VI) in waste water, by using three electrodes via cyclic voltammetry. For bioremediation studies, a homogeneous solution of commercially available sodium alginate and CFE was added dropwise in a continuously stirred calcium chloride solution. Biologically modified calcium alginate beads were produced and these were further utilized for bioremediation studies. The proposed sensor showed linear response in the range of 10-40 μg L(-1) Cr(VI) and the limit of detection was found to be 6.6 μg L(-1) Cr(VI). No interference was observed in presence of metal ions, e.g., lead, cadmium, arsenic, tin etc., except for insignificant interference with molybdenum and manganese. In bioremediation studies, modified calcium alginate beads showed encouraging removal rate 900 mg Cr(VI)/m(3) water per day with a removal efficiency of 90%, much above than reported in literature. The proposed sensing system could be a viable alternative to costly measurement procedures. Calcium alginate beads, modified with CFE of E. aerogenes, could be used in bioremediation of Cr(VI) since it could work in real conditions with extraordinarily high capacity.

Biosensing and bioremediation of Cr(VI) by cell free extract of Enterobacter aerogenes T2.

Carcinogenic heavy metal chromium, in its hexavalent form Cr(VI) is spuriously used in various industrial operations because of its hardness and stability. Due to lack of proper remediation processes, effluents contain Cr(VI) in large excess of WHO’s statutory limit of 50 μg l−1. In this study, a biosorption-based remediation strategy is proposed for the remediation of Cr(VI) from contaminated samples. The novel bacterial strain Enterobacter aerogenes T2 (GU265554, NII 1111) was isolated from Cr(VI)-contaminated tannery effluent and used for bioremediation studies. The cell-free extract (CFE) was prepared and encapsulated in spherical calcium alginate biopolymeric beads. A semi-batch plug-flow-packed bed made of these beads was used for biosorption of Cr(VI). A remediation rate of 932 mg Cr(VI) m−3 water per day with a sorption efficiency of 93.2 % was achieved. The method exhibited the best results when compared to those reported in the literature. Various isotherm models of adsorption such as Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) were studied. The results predicted adsorption in a multi-layered fashion via physisorption. The developed technology could be considered as green since no synthetic chemical was used for sorption of Cr(VI).

Jigisha Panda

Papers

Bioremediation of chromium by novel strains Enterobacter aerogenes T2 and Acinetobacter sp. PD 12 S2

Low-cost field test kits for arsenic detection in water

Isolation and identification of chromium-resistant bacteria: Test application for prevention of chromium toxicity in plant.

Biosensing and bioremediation of Cr(VI) by cell free extract of Enterobacter aerogenes T2.

Biosorption of Cr(VI) by Calcium Alginate-Encapsulated Enterobacter aerogenes T2, in a Semi-Batch Plug Flow Process