Due to various factors, it is becoming increasingly difficult to find clean water sources globally, making the issue of access to safe drinking water more severe. Scientists have now drawn inspiration from the ability of plants to purify land and have developed a biomimetic water purifier that can selectively filter harmful heavy metal ions.
Access to clean drinking water is a fundamental requirement for survival in today’s society. People often resort to using synthetic or natural zeolites, reverse osmosis membranes, and ion exchange resins as efficient and cost-effective solutions for treating large amounts of wastewater. However, these methods have inherent flaws. Zeolites and ion exchange resins typically have limited capacity for treating wastewater and struggle to increase their processing capability. Once the wastewater exceeds the capacity, the effectiveness of filtering out pollutants diminishes significantly, rendering them non-reusable.
Moreover, these water treatment materials lack high selectivity and tend to filter substances that meet the filtration criteria equally. The pores of these purifiers tend to capture metal ions or other substances with similar sizes, resulting in the unintentional removal of beneficial trace metal elements or substances essential for the human body.
A research team from Kyoto University and Osaka University in Japan, along with Heidelberg University in Germany, drew inspiration from plants’ ability to remediate land and discovered that plant proteins have the capability to filter heavy metal ions selectively. They developed a water purifier that can selectively filter out cadmium ions. Their findings were published on July 11th in the journal “Nature”.
Plants contain abundant proteins, phytochelatins, and glutathione oligomers that can selectively capture metal ions present in soil, groundwater, and air, detoxifying them. For instance, chelating proteins can form complexes with cadmium ions (Cd2+), isolating them in vacuoles without interacting with essential metal ions needed by the plant.
Researchers synthesized pAA-Cys5, a biomimetic copolymer mimicking plant substances, using acrylic acid (AA) and thiolated cysteine acrylamide (S-Tri-Cys-Aam), to simulate plants’ water purification capability. Techniques such as isothermal titration calorimetry (ITC) and ion spectroscopy were then used to evaluate the affinity of pAA-Cys5 for cadmium ions and calcium ions (Ca2+).
The results revealed that this biomimetic copolymer has high loading capacity and can differentiate between cadmium ions and calcium ions due to the carboxylic acid groups and thiol groups of pAA-Cys5, which mostly interact with cadmium ions but have minimal interaction with calcium ions, showing a significant difference in affinity between the two.
Subsequently, solutions containing cadmium ions (industrial wastewater level) were filtered through silica particles (10 µm) and cellulose membranes with and without pAA-Cys5 added.
The results showed that both silica particles and cellulose membranes containing pAA-Cys5 effectively filtered large amounts of cadmium ions, while those without pAA-Cys5 were unable to do so. In addition, the water purifier containing pAA-Cys5 could filter out 0.3 liters of potable water from wastewater in one hour.
This water purifier containing pAA-Cys5 can be regenerated by capturing metal ions with the chelating agent ethylenediaminetetraacetic acid (EDTA) with a recovery rate of up to 83% or through high-temperature recovery at 600°C. This highlights the recyclable and reusable features of the water purifier containing pAA-Cys5.
Furthermore, the pAA-Cys5 water purifier exhibits high affinity for mercury ions, indicating its effectiveness in removing other heavy metals.
The choice to target cadmium ions is primarily due to their propensity to accumulate in the human body, gradually causing toxic effects that are nearly irreversible. Chronic cadmium poisoning can lead to kidney damage, osteoporosis, lung damage, among other health issues.
The researchers suggest that synthetic biomimetic polymer materials offer a promising alternative to efficiently and selectively remove heavy metal ions from aquatic environments, which can enhance the efficiency of water treatment.