Nanoscale zerovalent iron particles for magnet-assisted soil washing of cadmium-contaminated paddy soil: proof of concept

Abstract

Environmental contextCadmium contamination in paddy soil can lead to elevated cadmium concentrations in rice, potentially affecting millions of rice consumers worldwide. This study used nanoscale zerovalent iron to sequester cadmium from soil before using magnetic separation to retrieve cadmium-sorbed particles from the soil slurry. This approach hypothetically resulted in a 97% reduction of cadmium levels in rice, rendering the rice safe for human consumption. AbstractCadmium (Cd) exposure causes serious health effects, including osteopenia, itai-itai disease, kidney disease and cancer. Millions of people are at risk of Cd-contaminated rice consumption resulting from Cd-contaminated paddy soil. While several soil restoration techniques, including phytoremediation (time-consuming) and soil washing using calcium chloride (generating Cd-contaminated wastewater requiring further treatment), face technical challenges, there is room for nanotechnology to offer a rapid and low-cost restoration technique. Here, we propose novel magnet-assisted (ex situ) soil washing using nanoscale zerovalent iron (NZVI) to remove Cd from paddy soil. Conceptually, Cd-contaminated paddy soil is mixed with water and NZVI to create a soil slurry. The NZVI promotes a reduction condition, which accelerates the Cd desorption from the paddy soil to the aqueous phase in the soil slurry. Subsequently, desorbed Cd in water is resorbed onto the NZVI surface, which is retrieved from the soil slurry through magnetic separation, leaving behind treated paddy soil and treated washing water. In our laboratory feasibility study with actual Cd-contaminated paddy soil (191.51±5.54mgkg−1), we found that, although magnet-assisted soil washing using NZVI cannot remove all the Cd from the soil to meet the EU and Thai soil standards (78% removal of total Cd), it effectively removes mobile Cd (exchangeable and carbonate fractions) from the soil (93%), which potentially results in a 97% reduction of cadmium in rice, which is safe for human consumption. The proposed technique has no unacceptable effects on the decline of macro- and micro-nutrients or the germination of rice seed.