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The substances behind the slimy strings from okra and the gel from fenugreek seeds could trap microplastics better than a commonly used synthetic polymer. Previously, researchers proposed using these sticky natural polymers to clean up water. Now,  that okra and/or fenugreek extracts attracted and removed up to 90% of microplastics in ocean water, freshwater and groundwater.

Rajani Srinivasan and colleagues have been exploring nontoxic, plant-based approaches to attract and remove contaminants from water. In one set of lab experiments, they found that polymers from okra, fenugreek and tamarind stick to microplastics, clumping together and sinking for easy separation from water. Srinivasan spoke about  at ACS Spring 2022, a meeting of the American Chemical Society. In this next stage of the research, they have optimized the process for okra and fenugreek extracts in various types of water.

To extract the sticky plant polymers, the team soaked sliced okra pods and blended fenugreek seeds in separate containers of water overnight. Then, researchers removed the dissolved extracts from each solution and dried them into powders. Analyses showed that the powdered extracts contained polysaccharides, which are natural polymers. Initial tests in pure water spiked with microplastics showed that:

• One gram of either powder in a quart (one liter) of water trapped microplastics the most effectively.
• Dried okra and fenugreek extracts removed 67% and 93%, respectively, of the plastic in an hour.
• A mixture of equal parts okra and fenugreek powder reached maximum removal efficiency (70%) within 30 minutes.
• The natural polymers performed significantly better than the synthetic, commercially available polyacrylamide polymer used in wastewater treatment.

Then the researchers tested the plant extracts on real microplastic-polluted water. They collected samples from waterbodies around Texas and brought them to the lab. The plant extract removal efficiency changed depending on the original water source: Okra worked best in ocean water (80%), fenugreek in groundwater (80-90%), and the 1:1 combination of okra and fenugreek in freshwater (77%). The researchers hypothesize that the natural polymers had different efficiencies because each water sample had different types, sizes and shapes of microplastics.

Polyacrylamide is currently used to remove contaminants during wastewater treatment, but the researchers say that okra and fenugreek extracts could serve as biodegradable and nontoxic alternatives.   

“Utilizing these plant-based extracts in water treatment will remove microplastics and other pollutants without introducing additional toxic substances to the treated water,” says Srinivasan, “thus reducing long-term health risks to the population.”

The authors acknowledge funding from the U.S. Department of Energy; Tarleton State University; High Plains Water District located in Lubbock, Texas; the Burnaby Munson Endowed Research Professorship; the National Science Foundation Research Experiences for Undergraduates program; and the Welch Foundation.

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