Arsenic baiting in rice paddy fields

 Arsenic contamination in flooded paddy fields endangers the health of more than half the world's population, which relies on rice as their primary source of nutrition. Existing methods for removing arsenic are costly, require additional chemicals, and can degrade soil quality. Researchers at Xi'an Jiaotong-Liverpool University in China, led by Zheng Chen, have pioneered a simple, inexpensive, and sustainable alternative that involves inserting plastic tubes coated with naturally forming iron oxides into the soil. In the Journal of Hazardous Materials, they describe their method.

"Iron oxides can absorb large amounts of arsenic, so we wondered if it would be possible to use them as a trap for this poisonous element. "In our first attempt, we discovered that the arsenic naturally took the bait," Zheng says.

Iron oxides occur naturally in high concentrations in specific areas of soil, such as at the soil-water interface and on the root surfaces of wetland plants. However, because these small regions are dispersed throughout the soil, it is difficult to separate the complexes of iron oxides and arsenic for removal. Based on previous research findings from the 1980s, the team knew that plastics in flooded soil can naturally induce iron oxides to form on their surface.

This prior knowledge prompted them to investigate the factors influencing the formation of plastic-induced iron oxides, such as differences in plastic types, structures, soil properties, and soil exposure to light, nitrate, and oxygen. Experiments revealed that low-density porous polyethylene tubes that allow oxygen to pass through the tube walls can cause large amounts of iron oxides to form. They also discovered a link between the amount of iron oxides formed and the capture of arsenic by the iron oxide "hooks," as Zheng refers to them.

The regular-interval tube system reduced the arsenic content of rice grown in trial plots by more than 10%. This enhancement will hopefully be enhanced further as the procedure is refined.

Another improvement to consider is replacing polyethylene with biodegradable plastics such as polylactic acid to avoid the risk of long-lasting polluting microplastics particles contaminating the soil.

"The tubes are cheap, they can be reused after removing a small amount of arsenic-containing solid waste, and automating the process of tube insertion and removal could further reduce costs and increase the effectiveness of this novel soil remediation technology," Zheng concludes.