IIT Gandhinagar-led team develops MOF filter to strip toxic metals from wastewater and recover rare earths

A single gram of powder, barely a pinch, can hold inside it a surface area comparable to a football field. That is the property at the center of new work from IIT Gandhinagar, the University of Cambridge and the University of Birmingham, where researchers have built a material meant to pull toxic heavy metals out of wastewater while recovering valuable rare-earth elements at the same time.

The material is a metal-organic framework, or MOF. Researchers describe it as a molecular fishing net. Its countless pores create an enormous catching surface, and specially designed chemical sites work like hooks that latch onto particular metals. The team tuned the net to favor the metals they wanted. They compared it to picking only the red and green marbles out of a bucket holding hundreds of colors. Some MOFs have internal surface areas of up to 7,000 square metres per gram, which is why they make strong adsorbents.

The problem is a two-sided one. Industrial activity can put toxic metals such as lead, cadmium, nickel and manganese into water. Rare-earth elements, meanwhile, are valuable and worth recapturing. The challenge, as the researchers frame it, is removing the harmful metals and reclaiming the useful ones from the same stream.

Prof Misra of IITGN said conventional treatments such as precipitation, coagulation and flocculation can be useful but are generally sensitive to pH, with the best removal confined to narrow ranges. Precipitation lets heavy impurities settle out, coagulation makes impurities clump together, and flocculation makes small clumps collide and grow into larger clusters that can be separated.

The team tested its protocol beyond the clean conditions of an idealized experiment. Industrial effluents from manufacturing, mining and chemical processing carry many metals along with contaminants such as pesticides, detergents and organic pollutants. The researchers tried the method on wastewater that was alkaline (pH above 8.5), turbid and high in dissolved solids. They also used artificial seawater and samples derived from e-waste. They say the workflow can be applied to similar adsorbents and other contaminants.

The capacity figures come from the team's own experiments. Dhruv Menon, an IITGN BTech graduate now a doctoral student in chemical engineering and biotechnology at Cambridge, said one gram of the copper-based MOF captured 490 mg of lead, 264 mg of cadmium and 226 mg of manganese. For rare earths, the numbers were 351 mg/g of neodymium, 343 mg/g of yttrium, 335 mg/g of dysprosium, 337 mg/g of terbium and 345 mg/g of europium.

Dr Misra was careful about what those numbers can and cannot say. Controlled batch experiments, he noted, cannot fully predict behaviour in complex wastewater. Before real-world use, the work needs to address large-scale fabrication, cost analysis, MOF life-cycle assessment and regulatory testing. He also stressed the need for approaches that solve several environmental challenges at once.

Other team members include Prathmesh Bhadane (PhD, IITGN; postdoctoral fellow, IIT Bombay), Priya Mahato (doctoral student, IITGN), Prateek Goyal (PhD, IITGN), Dr Iseult Lynch (Professor, School of Geography, Earth and Environmental Sciences, University of Birmingham) and Dr Swaroop Chakraborty (NERC Fellow, University of Birmingham). The next questions are about scale, cost and regulation, not chemistry in the lab.