Reduction of cadmium bioavailability in paddy soil and its accumulation in brown rice by FeCl washing combined with biochar: A field study.

Cadmium (Cd) removal from paddy soil to reduce Cd accumulation in brown rice is essential for agroecology, food safety, and human health. In this study, we demonstrate that ferric chloride (FeCl) washing combined with biochar treatment efficiently remediates Cd-contaminated paddy soil in field trials. Our results showed that 30.9 % of total Cd and 41.6 % of bioavailable Cd were removed by the addition of 0.03 M FeCl at a liquid/soil ratio of 1.5:1. The subsequent addition of 1 % biochar further reduced bioavailable Cd by 36.5 and 41.5 %, compared with FeCl washing or biochar treatment alone. The principal component regression analysis showed that the Cd content in brown rice was primarily affected by the bioavailable Cd in soil. The combined remediation contributed to the decreased Cd contents in brown rice by 45.5-62.5 %, as well as a 2.7-11.8 % increase in rice yield. The Cd contents in brown rice decreased to 0.12 and 0.04 mg kg in two cultivars of rice (Zhuliangyou189 and Zhuliangyou929), lower than the national food safety standard limit value of China (0.2 mg kg). Meanwhile, the combined remediation promoted the restoration of soil pH and organic matter as well as the improvement of available nutrients. This finding suggests that the combination of FeCl washing and biochar is an effective remediation strategy to minimize Cd bioavailability in paddy soil, and improves soil quality, thus contributing to food safety.