My colleagues and I, led by physicists of Heidelberg University and Kyoto University and polymer chemists of Osaka University, have developed a new water treatment system by hyper-confinement of polymer materials inspired by plants. Learning from the molecular design of phytochelatin that selectively captures heavy metal ions in plants, we designed and synthesized a polymer possessing similar side chain functionalities.
Water shares about 60 % of our body by weight. Access to safe water, sanitation, and hygiene is a fundamental requirement for the well-being of our society, which is enlisted in the SDGs of the United Nations. For example, the contamination of water in river and ground water by heavy metal ions are known to cause many health risks.
Currently, various materials are used to remove heavy metal ions, such as synthetic/natural zeolites, reverse osmosis membranes, and ion exchange resins. Although these materials offer an efficient and cost-effective solution to treat large amounts of wastewater, they capture the ions possessing similar sizes and charges.
Looking into biological systems, plants and some bacteria have a highly abundant protein, phytochelatin, oligomers of glutathione that selectively capture heavy metal ions. Phytochelatin proteins form complexes with heavy metal ions and sequester them in vacuoles, while the interaction with essential ions is negligible. Environmental remediation using plants, called “phytoremediation”, has been put into practical use in recent years increasingly.
Getting a hint from plants, we designed synthetic polymers carrying similar side chain functionalities to phytochelatin, achieving the binding affinity to cadmium ions that is two to five orders of magnitude higher than not only previously reported materials but also phytochelatin itself.
Notably, the affinity to calcium ions, possessing similar sizes and charge as cadmium, was four orders of magnitude weaker. This allows the selective capture of heavy metal ions even in the presence of excess amount of abundant ions, realizing a high loading capacity. The molecules confined in 3 mL volume removed cadmium ions from 0.3 L of simulated industrial wastewater down to the WHO’s drinking water level.
This material opened a potential application for the highly selective removal of harmful contaminants from the environmental water. We foresee that our approach, by “learning from biological systems to achieve higher functions”, will be applied for the design of new materials with high selectivity and specificity.
New water treatment materials inspired by plants
