In a paper that addresses the increasing levels of water pollution, Prof. Sujit Ghosh’s group has developed a  hybrid material that can purify water by removing a wide range of toxic ions from water.

“Inorganic oxoanions, classified among the primary substances present in polluted water, end up in aquatic environments and are often difficult to filter out even by traditional commercially available energy-efficient technologies,” says Prof. Sujit K. Ghosh, professor in chemistry at IISER Pune.

Prof. Sujit K. Ghosh and team of PhD and undergraduate students work on organic as well as organic-inorganic frameworks, which at a molecular scale are large chemical structures built in a manner that within their custom-defined pores and crevices they can capture other smaller molecules. When these smaller molecules happen to be pollutants, one would have created a means to capture the molecules and purify the original content. The trick however is to define the pore such that the pollutants fit snugly within the pores. But not too comfortably as well, because one would want to reuse the purifying material. This leads one into the realm of understanding molecular architectures, affinities, rates of association and disassociation of interacting molecules and many other foundational concepts in inorganic chemistry.  

In this work, the team has developed a sponge-like anion exchangeable hybrid material by covalent linking between two individual materials: a cationic metal-organic polyhedra (MOP) and a covalent organic framework (COF) that exhibits a high removal efficiency of a wide range of toxic oxoanions.

(Left) Schematic illustration of selective sequestration of toxic oxoanions by hybrid material; (Right) A simple set-up such as this with the hybrid material can selectively­ capture arsenic ions (As(V)) by from a drinking water reservoir containing 1000-fold excess of other competing anions. Image Courtesy: Prof. Sujit K. Ghosh; Image on the left used with permission from American Central Science; further use requires permission from the journal. Ref: ACS Cent. Sci. 2020 6(9):1534-1541

The MOP acts as a nanotrap scavenger for toxic oxoanions by virtue of its exchangeable chloride ions, Zr (IV) SBU and free -NH₂ functional groups, while the porous COF matrix tightly anchors the MOP molecules. The covalent linkage between the MOP and COF mitigates the leaching problem and facilitates exposure of most of the active sites of MOP molecules for targeted capture studies which is hard to achieve in solitary MOPs.

The hybrid material the team prepared entrapped high (50 ppm) as well as very low (1 ppm)  concentration of a wide range of toxic oxoanions of arsenic, selenium, chromium, etc. Since both industrial wastewater and natural groundwater contain several different anions, which severely affects the adsorption process, the capture study was carried out in the presence of 1000-fold excess of other interfering anions, such as, chloride, nitrate, bromide, sulphate, chlorate, etc. The team found that the compound displayed superior capture kinetics and selectivity compared to known adsorbent materials reported thus far.

With a high adsorption performance, the hybrid material showed great promise. The team then explored the possibility of treating polluted natural water using this material. They selected arsenic-contaminated groundwater as a prototype oxoanion for application purposes. Even trace levels arsenic in drinking water can cause severe cardiovascular and neurological disorders, skin cancers, stillbirth, etc.

To test the practical utility of the material, the team gathered drinking water samples from two locationsm in West Bengal’s Malda district, that have experienced water pollution with arsenic for long. They found that the hybrid material rapidly reduced the arsenic levels (c.a. >10 ppb) from highly contaminated samples to well below the WHO permitted level of drinking water.

“We believe that the versatile attributes of superior selectivity, very fast kinetics, decent recyclability, and low cost could render this newly developed anion exchangeable hybrid material as a potential water purifier in the future,”  says Prof. Ghosh speaking on the possibilities for the material in providing better quality drinking water.

Nanotrap grafted anion exchangeable hybrid materials for efficient removal of toxic oxoanions from water. Samraj Mollick, Sahel Fajal, Satyam Saurabh, Debanjan Mahato, and Sujit K. Ghosh. ACS Central Science (2020) ACS Cent. Sci. 2020 6(9):1534-1541.

In the same issue, an accompanying "First Reaction' article offers a review of this work. 

- With inputs from Dr. Sujit K. Ghosh, Samraj Mollick