Characteristics of mercury form in soil-rice system and food security assessment in wastewater-irrigated paddy fields of Tianjin

Three wastewater-irrigated paddy fields in Beitang, Dagu and Beijng drainage river basin and a control paddy field in Haihe basin were chosen to study the effect of mercury (Hg) form on the concentration and distribution of total mercury (THg) and methylmercury (MeHg) in rice organisms and assess the risk of Hg exposure for the population with rice consumption. Eight sets of whole rice plants and corresponding soils samples were collected in each polluted area and 4 sets of samples were collected in control area. For soil samples, we also analyzed 5 Hg forms by sequential extraction, which were soluble and exchangeable form, specifically sorbed form, oxide-bound form, organic-bound form, and residual form in order. Safety threshold of soluble and exchangeable Hg in wastewater-irrigated paddy soil was determined preliminarily. The results showed that the THg contents in paddy soil of Beitang, Dagu and Beijng drainage river were (1215±135), (806±255) and (378±162) ng/g, respectively, which were significantly higher than that in the control site of Haihe area ((154±43) ng/g). The corresponding soil MeHg concentrations were (4.15±0.34), (2.99±0.37), (1.046±0.28) and (0.34±0.22) ng/g, accounting for 0.42%, 0.37%, 0.28% and 0.22% of THg, respectively. The relative partitioning of various Hg forms according to the sequence was as follows: Residual form > organic-bound form > oxide-bound form > specifically sorbed form > soluble and exchangeable form in paddy soil of Dagu and Beijing drainage river and Haihe River, whereas organic-bound form in paddy soils of Beitang was the highest and the order was organic-bound form > residual form > oxide-bound form > specifically sorbed form > soluble and exchangeable form. The THg in rice tissues was distributed differently in different areas due to the THg in ground tissues influenced by Hg in the atmosphere. In the control site and less contaminated area of Beijing drainage river, the THg distribution was as follows: leaf > root > stem > grain, while in the more serious sites of Dagu and Beitang drainage river, the order was root > leaf > stem > grain. Higher organic-bound Hg and MeHg were found in the paddy soil of Beitang drainage river, suggesting that the higher MeHg in the paddy soils may be related to the higher ratio of organic-bound Hg which may promote MeHg to produce. The highest MeHg concentration was in grain, followed by root, stem and leaf, which showed the same patterns in all study areas and significantly correlated with MeHg in soils. Rice grain had a strong ability on the bioaccumulation of MeHg, and the bioaccumulation factor was between 1.63 and 3.70. Based on the calculation of security value for daily MeHg intake and the maximum permissible limit of 20 ng/g issued by Chinese National Standard Agency, the soluble and exchangeable Hg in soil should not exceed 9.19 and 6.25 ng/g, respectively. The probable daily intake (PDI) of MeHg for an adult population in studied wastewater-irrigated area was 0.0016-0.24 µg/kg, which showed that there was a great concern of MeHg exposure risk for rice consumption in wastewater-irrigated area. The exceeding standard rate of MeHg PDI in wastewater-irrigated area was up to 20.83%. We may remedy the polluted soils or change crops planted in the soil. © 2016, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.