Sustainable remediation of heavy metals in groundwater
S. Himmelstein | July 15, 2024
Technologies are available to extract hazardous heavy metals from potable water supplies, but then what is the fate of these produced pollutants? The metal-laden sludge and other residues generated are typically landfilled, posing a continued threat to water quality. Such disposal is eliminated in a new three-step treatment scheme that provides a more environmentally sustainable solution.
The method engineered by researchers from the Centre for Sustainable Technologies (CST), Indian Institute of Science (IISc) was tested with arsenic-contaminated water, as this heavy metal poses a pervasive water quality issue throughout India. Polluted water is first sent through a bed of biodegradable adsorbent composed of crustacean-derived chitosan doped with iron and aluminum hydroxide/oxyhydroxide. The adsorbent bed grabs the toxic inorganic arsenic through electrostatic forces and complex formation between arsenic and the adsorbent. The alkaline wash is recycled within the scheme to repeatedly regenerate the adsorbent bed after its saturation.
The sodium hydroxide and arsenic in the wash solution are then separated in a membrane system, after which the sodium hydroxide solution returns to regenerate the bed. The concentrated arsenic is subjected to bioremediation in the last step where it is converted to low-toxicity organic arsenic via methylation by microbes found in cow dung. The toxic inorganic arsenic is reduced to below the maximum permissible limit specified by WHO standards within a span of eight days. The remaining cow dung sludge can be safely disposed of in landfills, since the arsenic is locked in it in an organic form.
“On average, these organic species are approximately 50 times less toxic than the inorganic form present in groundwater,” explained the researchers.
The developers are now tailoring the system for the removal of other heavy metals.