How Bio-Waste Materials Can Capture Heavy Metals in Water

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How Bio-Waste Materials Can Capture Heavy Metals in Water
Be it fruit peels, used tea bags or spent coffee grounds — such organic materials are more than just waste. In fact, they can capture toxic heavy metal pollutants in water — a capability which could help to clean contaminated drinking water, especially in developing countries. Now scientists from France and Switzerland performed molecular simulations using PRACE supercomputing resources to gain insight into the mechanisms at play, and to lay the groundwork for a systematic employment of these important processes.

Sustaining life requires fresh, clean water. Although this fact seems self-evident, it is not a given in many regions of the world. Especially in developing countries, numerous water sources and reservoirs are contaminated by pollutants, particularly heavy metals such as lead, cadmium or mercury. These materials are common by-products of industrial processes such as those in the mining, steel, or car industries, and are harmful to humans even in very low concentrations. Lead, for instance, can accumulate in bone, teeth and brain tissue, and cause chronic poisoning that, amongst other things, results in defects of the nervous system and muscle tissue. Cadmium can damage our genetic material and cause cancer, as well as anaemia, bone fractures, and diseases of the digestive tract or the bone marrow.

“There is overwhelming evidence that natural waste materials can efficiently capture heavy metals from water. Nevertheless, related research efforts have been modest.”

Wanda Andreoni
Professor emeritus of physics at EPFL, Switzerland

Several technologies to remove heavy metals from water do exist, like membrane filtration, activated carbon adsorption, and electrocoagulation, which uses electrical charges to capture the particles. However, these processes are complex and expensive, and thus scarcely used in developing countries. As the World Health Organization (WHO) estimates, 844 million people still do not have access to basic, clean drinking water. “Heavy metal contamination in water has long been a pressing concern,” says Wanda Andreoni, a professor emeritus of physics at the Swiss Federal Institute of Technology in Lausanne (EPFL), Switzerland.

A cost-efficient alternative

The physicist has recently explored the potential of a surprisingly cheap and low-tech alternative that has emerged in the last decades: Certain types of organic waste like fruit peelings, spent coffee grounds, and tea leaves possess the ability to capture heavy metals in water. “There is overwhelming evidence that natural waste materials can efficiently capture heavy metals from water. Nevertheless, related research efforts have been modest,” Andreoni points out. For example, in 2018, a study from India showed that spent coffee grounds can remove lead from contaminated water with an efficiency of 90 percent. And in 2020, Turkish scientists used brewed tea waste to successfully remove four different heavy metals from a water sample.

To investigate how exactly such organic waste materials manage to capture heavy metals in water, Andreoni and her colleagues performed simulations using PRACE supercomputing resources. “We wanted to begin to understand how these inexpensive and abundant natural materials do the job, and look to them for inspiration,” explains Andreoni. Together with Fabio Pietrucci, a professor at Sorbonne University in Paris, and Mauro Boero, a professor at the University of Strasbourg, Andreoni performed both ab initio and classical molecular dynamics simulations. The ab initio approach is based on density-functional theory and calculates the interatomic interactions from the electronic structure; on the other hand, classical molecular dynamics relies on a-priori assumptions for the interatomic interactions, but in return, it allows the scientists to explore larger molecular systems and longer time scales.

Naturally efficient biomaterial

The molecular system the team chose for the simulations is hemicellulose, which consists of chains of different sugar entities. While biomaterials like fruit peels or spent coffee and tea waste contain thousands of different chemical components, hemicellulose is one of the most frequent and common ones. The simulated hemicellulose models consisted of four chains comprising more than 840 atoms for the ab initio calculations. For the classical molecular dynamics simulations, a much larger oligomer model was used, consisting of more than 200 monosaccharides and therefore of many thousands of atoms including over 30 000 water molecules.

Snapshots from the classical molecular dynamics simulation show several lead ions bound to hemicellulose along the unbinding path — an energetically strongly unfavourable process.

These snapshots from the classical molecular dynamics simulation show several lead ions (blue) strongly bound to hemicellulose (green) in image A. Along the unbinding path (images B, C, and D) one of the heavy metals is re-solvated in water — an energetically strongly unfavourable process, as the results have shown, which makes heavy atom binding by hemicellulose virtually irreversible.
Image source F. Pietrucci et al. Chemical Science (2021)

The simulations shed new light on the energetic hurdles for heavy metal uptake from water using the example of lead. Specifically, the results indicated that the free energy barriers for hemicellulose to adsorb lead ions from water are negligible, whereas it is energetically very expensive to release them back into water. This means that this polysaccharide can, by itself, easily and firmly adsorb lead ions from water, and that it indeed is an efficient agent for removing heavy metals from water.

Wanted: collaborative effort

The scientists’ results also identified which of the hemicellulose’s chemical groups play a role in this efficient heavy metal binding. Responsible are mostly the sugars’ carboxylate groups (R-COO-) and, to a little lesser extent, their hydroxy groups (R-OH). “These insights can now be extended to more complex scenarios and promote further and targeted development,” Andreoni points out.

The professor stipulates that, going forward, lab experiments should be combined with dedicated molecular simulations in collaborative efforts to further investigate these complex biological systems. “Our conviction that this can be a successful strategy lies in the fact that such collaborative efforts have been successfully applied in a similarly challenging field: drug discovery.” According to Andreoni, the goal must be to use the output of fundamental research to guide a more efficient and systematic employment of these biomaterials, to rationally design new bio-inspired materials and processes, and ultimately to improve and increase the clean water supply in developing countries.