In-situ Immobilization of Cd in Acidic Paddy Soil of Hunan Province, China by Layered Double Hydroxides

Hunan Province has abundant mineral resources in China. Due to unreasonable mining, serious Cd contamination is highlighted in paddy soil. In addition, the long-term use of acidic chemical fertilizers causes that the soil is seriously acidified, and Cd possesses the high bioavailability under acidic conditions, which is easily absorbed by rice, resulting in excess Cd accumulation in rice grains. Layered double hydroxides (LDHs) have been reported as an efficient in-situ stabilizer for Cd remediation. In this work, two kinds of LDHs (MgAl-LDH/Ca(OH)2 (102A) and CaAl-LDH (112A)) are used for the first time in the remediation of Cd-contaminated acidic paddy soil in Hunan Province. The location of experimental field is in a village, Heshan District, Yiyang City, Hunan Province, China. The basic characteristics of the soil are as follows: clay, silt, sand, pH, total nitrogen, available phosphorus, available potassium, cation exchange capacity, soil organic matter, and total Cd are 49.3%, 48.0%, 2.7%, 5.20, 2.63 g·kg−1, 16.90 mg·kg−1, 96.00 mg·kg−1, 11.60 cmol·kg−1, 42.31 g·kg−1 and 0.530 mg·kg−1, respectively. Herein, the applications of 102A, 112A and mixture of 102A and 112A have increased the soil pH value and decreased the available Cd content of the soil effectively. They show excellent performance in reducing the Cd content of rice grains. Group A4 (3 t/ha 102A) demonstrates the most effective Cd passivation efficiency, the available Cd content in the soil has reduced by 66.8%. The Cd content of early rice grains and late rice grains are 0.093 and 0.113 mg·kg−1, respectively, which is below the standard value of Cd in rice grain (0.2 mg·kg−1, GB 2762–2017), while the Cd content is 0.403 and 0.547 mg·kg−1 in the control group field, respectively. This work provides an alternative approach to remediate the Cd contamination in practical paddy soil.