Bacterial reduction of arsenic contamination

Following the tragic discovery ten years ago that tens of millions of Bangladeshis drink groundwater that is naturally contaminated by arsenic, the lessons learnt there have been applied on a global scale.  That has resulted in further cases with similar causes coming to light.  Remediation is chemically quite simple, and since the US reduced the maximum permissible arsenic level in public water supplies from 50 to 10 parts per billion in 2001 research into methods of removal have increased rapidly.  There are a number of methods that are based on adsorption of arsenic by iron and aluminium hydroxides and are low-cost.  But it seems that biological activity in aquifers can be equally effective (Kirk, M.F. et al. 2004.  Bacterial sulphate reduction limits natural arsenic contamination in groundwater.  Geology, v. 32, p. 953-956).  In the anaerobic conditions that favour the dissolution of iron hydroxide, which is often the most important source of arsenic in sediments, the conditions are also suitable for chemotrophic bacteria.  Among these are species that obtain metabolic energy from the reduction of sulphate ions to sulphide.  Where metal ions are also present, they combine with the sulphur to precipitate sulphide minerals.  In turn, sulphides readily accept arsenic from solution, thereby helping decontaminate potentially dangerous groundwater.  Arsenic-bearing groundwater is also found to have high methane levels, which suggests that methanogenic bacteria dominate its micro-ecosystem when sulphate ions are at low concentrations.  Perhaps it will prove possible to encourage sulphate-reducers to thrive in such waters, by the addition of some sulphate by injection.  That would a cheap remedy to what seems to be a growing risk in areas that extract groundwater from aquifers that are full of organic matter that creates the oxygen-free conditions that release arsenic into solution.

Bacteria in groundwater seem to have another benefit.  Where landfill contaminates subsurface waters with a cocktail of pollutants, the nutrients encourage bacterial colonisation, often in the form of biofilms in pore spaces.  It seems that their metabolism generates electrical currents (Gosline, A. 2004.  Bug “batteries” send out pollution alert.  New Scientist !8 December 2004, p. 17).  These create electrical potentials of several hundred millivolts that are easily detected by passive electrical monitoring.  The voltage highs occur at the margins of pollutant plumes in the groundwater, and can therefore be used to monitor spread of contamination and to indicate safe supplies.

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