Characteristics and its assessment of heavy metal content in soil and rice with different repair methods

Abstract: Soil is the basis of agricultural production. Heavy metals accumulate in the soil and enter the food chain through crops, causing serious problems to the residents' health. In order to explore the effects of different repair methods on physicochemical properties of heavy metal contaminated soil and the contents of total Pb, total Cd and inorganic As in soil and rice, and find a new way to fix heavy metal in soils and rice, the experiment was carried out in the contract field of a cooperative in Huanghua Town, Changsha County, Hunan Province from March 2016 to November 2017, in which Zhuliangyou-819 was used as the experimental material. In this experiment, 6 different repair methods named T1-T6 were designed. In T1-T6, T1 didn't have any treatment; quicklime was applied in T2; in T3 quicklime was applied and the optimal water management was conducted; in T4 quicklime and Chuge soil heavy metal conditioner were applied and the optimal water management was conducted; quicklime and foliar spray inhibitor were applied and the optimal water management was conducted in T5; in T6 quicklime and Chuge soil heavy metal conditioner and foliar spray inhibitor were applied and the optimal water management was conducted. Assessment of heavy metal contamination in unrepaired soil and repaired soil was carried out by using the cumulative index and potential ecological risk index, and the hazard index was used to evaluate the rice grown on the repaired soils. The results showed that the average pH value, soil organic matter and cation exchange capacity of unrepaired soil were 6.508, 52.881 g/kg and 10.598 cmol/k, and the potential ecological risk index of background content of inorganic As, total Pb and total Cd in unrepaired soil was 78.235, indicating that there are some heavy metal pollutions in the local paddy soils. Amount of cation exchange, organic matter and pH value in soil were changed after repair. Besides, an increase of 0.033-1.017 was observed for pH value of soil. The content of inorganic As in soil got an increase of 0.072-1.481 mg/kg after repair. However, a decrease of 0.481-3.133 mg/kg and a decrease of 0.038-0.113 mg/kg were observed for total Pb and total Cd, respectively. The maximum removal of total Pb and total Cd was achieved by the repair of T4, which declined by 0.120 and 3.133 mg/kg, respectively. The content of total Cd in rice with the repair of T6 was slightly below the national secondary standard. The total Cd contents in rice with the repair of T1-T5 exceeded the national secondary standard, and the content of total Cd in rice with the repair of T1 was 4.5 times beyond the national secondary standard. Through the comparison of repair specific operating procedures in T1-T6, it was found that T4 repair operating procedure, which used the Chuge soil heavy metal conditioner, was useful to reduce soil Pb and Cd content. The order of hazard index of rice under T1-T6 repair was T1 > T2 > T3 > T5 > T4 > T6. Among them, the hazard index of T6 repair was the smallest. Through the comparison of various repair methods, it was found T6 repair method was more comprehensive, which indicates that the combination of multiple repair methods is more effective and more beneficial to the rice security in study area and human health.