IIT Guwahati Reactor Strips Arsenic and Fluoride From Water Together in Lab Tests

Researchers at the Indian Institute of Technology Guwahati have built a water-treatment reactor that removes arsenic and fluoride, two major groundwater contaminants, at the same time and within minutes. In their experiments the system achieved up to 98.2 percent removal of arsenate and 91.8 percent removal of fluoride. Groundwater supplies drinking water to millions of people in India, and naturally occurring arsenic and fluoride can make it unsafe at elevated levels.

Treating both together is difficult because the two behave differently in conventional purification and can compete for the same removal sites, according to Prof Mihir Kumar Purkait of the institute's Department of Chemical Engineering. The team therefore designed a rotating-anode electrocoagulation (RA-EC) reactor. Electrocoagulation uses electrodes to generate chemicals inside the water that trap contaminants; here the usual stationary aluminium electrode was replaced with a rotating aluminium anode.

When current passes through the reactor, aluminium ions and hydroxide ions are generated and combine into microscopic flocs — clumps that contaminants stick to. Arsenic and fluoride are then separated from the treated water by adsorption, coagulation and precipitation. The spinning electrode improves mixing, moves contaminants faster towards the reacting surfaces and renews the electrode surface, which the researchers say helps address electrode passivation, a build-up that reduces the efficiency of conventional electrocoagulation systems.

The team examined how rotational speed, current density, electrode spacing and treatment time affected performance, and tested the reactor under realistic conditions with naturally occurring ions such as calcium, magnesium, bicarbonate, sulphate and phosphate present. It was also evaluated on real groundwater samples collected from Assam. The study was led by Prof Purkait with research scholar Mukesh Bharti.

The estimated operating cost was Rs 18 to Rs 58 for treating 1,000 litres, depending on contaminant concentration — a figure from the initial demonstration, not the cost of a commercial plant. The researchers plan a pilot-scale continuous-flow version of the reactor.