From Toxic to Valuable: Cleaning Wastewater, Finding Treasure
We generally throw away a broken camera lens, earphones, or an old phone that has stopped working and is beyond repair, as they feel useless when non-functional. However, these objects hide within them rare earth elements (REEs) that have applications in magnets, superconductivity, optics, and batteries, among others. When such objects become part of water, recovering REEs becomes difficult.
On the other hand, industrial activities can introduce toxic metals such as lead, cadmium, nickel and manganese into the same water. The challenge is no longer simply how to remove these unwanted, harmful metal pollutants from water, but also efficiently recapturing the valuable REEs.
What if a single strategy could help address both issues? As an effort in this direction, researchers from the Indian Institute of Technology Gandhinagar (IITGN), the University of Cambridge, and the University of Birmingham have collaboratively developed a protocol using a class of highly porous materials called metal-organic frameworks (MOFs). They can efficiently capture toxic metals from water as well as recover valuable REEs from waste streams. Published in Nature Protocols, the study provides a framework for designing, characterising and deploying MOFs in water remediation as well as circular resource applications with respect to real-world conditions.
An Intelligent Filter
Think of the MOF as a molecular fishing net. Its countless pores spread out a vast catching surface, while specially designed chemical sites act like hooks that can latch on to particular metals. The researchers tuned this net such that it preferentially traps the metals they are looking for. It is like identifying and specifically picking red and green coloured marbles from a bucket containing hundreds of marbles in different colours.
Some MOFs can have internal surface areas of up to 7,000 square metres per gram. To put that number in perspective, one gram may look like a tiny pinch of powder, but inside it is a surface area comparable to an entire football field!
This huge internal surface makes MOFs attractive as adsorbents. In simple terms, they function as materials that arrest substances on their surfaces. While conventional methods such as precipitation, coagulation and flocculation can be useful for water treatment, they are generally sensitive to pH, with optimal removal confined to narrow ranges. Broadly speaking, precipitation lets heavy impurities settle out, coagulation makes impurities clump together, and flocculation makes small impurity clumps collide and grow into larger clusters that can be separated.
These approaches also require large infrastructure and generate large amounts of sludge, a thick, muddy waste that then has to be handled and disposed of. Other processes, such as the electrochemical treatment, have high economic and environmental costs.
Adsorption offers an effective alternative that is simple to operate, with broad applicability and effectiveness at low contaminant concentrations. Since it focuses on the extent of affinity of the target metals to the tailored surface, it has the potential to remove 90 – 99% of contaminants like a highly selective filter! Using adsorption-based approaches also avoids generating secondary contaminants and can be regenerated and reused. Within this space, MOFs stand out for their exceptional design flexibility in capturing and recovering metals.
Beyond the Laboratory
In the real world, industrial effluents, which are waste products discharged from manufacturing, mining, and chemical processing facilities into water bodies, contain not just one but many metals, along with other contaminants like pesticides, detergents, and organic pollutants.
The protocol details a methodology which can be used to employ MOFs as an adsorbent for the recovery and removal of metals under controlled conditions. The protocol features testing in complex real-world scenarios. For example, removal and recovery were evaluated using wastewater that is alkaline (pH ~ 8.5), turbid, and high in dissolved solid impurities. The method was also tested in artificial seawater and e-waste-derived samples. The workflow developed in this study can be applied to similar adsorbents and other contaminants.
“For me, the most interesting aspect was observing the MOF in our protocol demonstrate substantial adsorption capacities,” said Dhruv Menon (BTech, IITGN). He added, “The experiments showed that one gram of the copper-based MOF could capture nearly half a gram of lead (490 mg/g), and roughly a quarter of a gram each of cadmium (264 mg/g) and manganese (226 mg/g). For rare-earth elements, it could capture around one-third of a gram per gram of material (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).” Mr Menon is currently a doctoral student in the Department of Chemical Engineering and Biotechnology at the University of Cambridge.
Towards Sustainability
As explained by Prof Superb Misra, “There is a need for approaches that can solve multiple environmental challenges simultaneously. Our focus was on engineering the performance of MOFs to employ them for environmental remediation applications. That said, it is crucial to understand that controlled batch experiments cannot fully predict behaviours in complex wastewater. There is a need to look at factors like large-scale fabrication, cost analyses, MOF life-cycle assessment and regulatory testing,” Dr Misra is the Jibaben Patel Chair Professor in the Department of Materials Engineering, IITGN, and the Principal Investigator at the Bio Nano Materials Group.
Other team members involved in this study included 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).
This research aligns with the Government of India’s Critical Mineral Recycling Incentive Scheme, as part of the National Critical Mineral Mission, to develop domestic recycling capacity for separating and producing critical minerals from secondary sources, including growing volumes of e-waste and battery waste. IIT Gandhinagar is a partner in the Centre of Excellence on National Critical Minerals Mission hosted at IIT (ISM) Dhanbad. The findings are also in alignment with the UK-India Technology Security Initiative, which involves collaboration on sustainable extraction and recovery of critical minerals from end-of-life waste streams. They also coincide with the United Nations Sustainable Development Goals 6, 9 and 12, focused on clean water and sanitation, industry and innovation, and responsible consumption and production.
