The compound synthesised by IISER Kolkata researchers can reduce copper, mercury, cadmium and iron content in drinking water to below 2 ppb, which is far below the WHO limit for these metals. After 10 minutes of treatment, the fluoride concentration dropped from 10 ppm to 10 ppb. The compound can also absorb a wide variety of oils and organic solvents from water. The researchers are already working with a company based in Chennai to commercialise it.

A chemical compound (Meisenheimer complex) synthesised through a simple, single-step process of mixing two chemicals (picric acid and dicyclohexylcarbodiimide) at room temperature has been found to be highly effective in removing fluoride and metal ions such as lead, mercury, cadmium, copper, and iron from drinking water.

The compound is water-repelling (hydrophiobic) in nature. Since a water-repelling material can absorb oil, a two-member team led by Prof. Debasish Haldar from the Department of Chemical Sciences at the Indian Institute of Science Education and Research (IISER) Kolkata tested its efficiency in removing oil-spills. A polystyrene sponge that is water absorbing in nature became a water-repelling material (water contact angle of 131 degree) when coated with the compound. The sponge coated with the compound was able to absorb a wide variety of oils and organic solvents from water.

“The compound has both negative and positive charged parts and this helps it to absorb metal ion pollutants [which are positively charged] and fluoride [which are negatively charged] from water,” says Prof. Haldar.

“One gram of the compound was able to remove large amount of lead and mercury (817 mg and 830 mg respectively) from water and nearly half its weight of copper and iron (451 mg and 511 mg respectively),” says Tanmay Das from the Department of Chemical Sciences at IISER Kolkata and first author of a paper published in the journal ACS Omega. “The compound can absorb lesser amount of different metals at the same time or a higher amount of a single metal.”

The compound can either be added to water or the compound can added to silica gel and water allowed to pass through the column. Filtering water by passing it through a column is more effective in removing the hazardous contaminants.

Making drinking water safe

The researchers tested the efficiency of the compound present in a column to remove metal ion contaminants. They passed water containing 50-100 parts per million (ppm) of metal solutions through 2 grams of the compound mixed in 60 grams of silica gel. The absorption capacity was extremely high at about 99%.

To test the compound’s ability to remove metal ions below the World Health Organisation’s limit, the researchers used the compound to treat water containing 5 ppm of copper, mercury, cadmium and iron. After 10 minutes of treatment, the concentration levels of the metals in water dropped to below 2 parts per billion (ppb), which is far below the WHO limit for these metals.

In the case of fluoride, water with a high concentration of fluoride (10 ppm) was treated with the compound. After 10 minutes of treatment, the fluoride concentration dropped to 10 ppb. One gram of the compound was found to absorb up to 712 mg of fluoride.

Recycling the compound

The silica gel filter containing the compound showed equal efficiency for three cycles and has to be recycled after the third cycle. For recycling, ethyl acetate is run through the column containing silica gel and the compound with adsorbed metals ions. The compound dissolves in ethyl acetate and comes out from the silica gel. Ethyl acetate solution becomes metal-free when washed thoroughly with water. As ethyl acetate evaporates it leaves behind the compound which can then be added to silica gel. “This is only for industrial use. We are trying to figure out using the compound for domestic use,” says Prof. Haldar.

“We are trying to commercialise it. We are already working with a company based in Chennai, which is testing the material. A water filter using our compound may become commercially available in a year’s time,” Prof. Haldar says.

Removing oil-spills

Sponge coated with the compound was able to remove oil droplets efficiently. It was used for removing oil spill and solvents from the water surface. The researchers were able to use it for removing heavier organic solvents such as chloroform that settle at the bottom. “The sponge can absorb any oil immaterial of its density. It can be used for removing crude oil too,” Prof. Haldar says. “The absorbed oil can be removed by squeezing the sponge and the sponge reused. In the case of crude oil, 5-10% of crude oil will still remain in the sponge and cannot be removed.”

Published in The Hindu on November 27, 2017