天津污灌区水稻土壤汞形态特征及其食品安全评估

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

选择天津污水灌溉区作为研究区域,该文的主要研究目的是分析土壤不同形态汞对水稻不同组织中总（THg）和甲基汞MeHg富集的影响以及评估污灌区稻米食用汞暴露风险,并对污灌区土壤溶解与可交换态汞安全阀值进行了初步分析。结果表明,北塘、大沽和北京污灌区稻田土壤THg和MeHg浓度显著高于对照区海河土壤THg浓度。大沽、北京河污灌区和海河土壤汞的形态为残渣态浓度最高,而北塘污灌区为有机结合态浓度最高。由于水稻地上组织中THg浓度受到大气汞浓度影响,各组织中汞浓度在不同污灌区分布规律不同。水稻籽粒的MeHg富集系数为1.63～3.70,通过人体每天摄入MeHg量的安全值计算得出土壤溶解与可交换态汞质量分数应低于9.19 ng/g。污灌区食用稻米MeHg暴露对居民健康存在较大风险,人体MeHg每天摄入量超标率达到20.83%。因此,可以对超标土壤进行治理或改变耕作方式来降低土壤污染对人体MeHg暴露造成的影响。

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（MeH g） 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 MeH g 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 MeH g were found in the paddy soil of Beitang drainage river, suggesting that the higher Me Hg in the paddy soils may be related to the higher ratio of organic-bound Hg which may promote MeH g to produce. The highest Me Hg concentration was in grain, followed by root, stem and leaf, which showed the same patterns in all study areas and significantly correlated with MeH g in soils. Rice grain had a strong ability on the bioaccumulation of Me Hg, and the bioaccumulation factor was between 1.63 and 3.70. Based on the calculation of security value for daily Me Hg 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 MeH g 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 MeH g exposure risk for rice consumption in wastewater-irrigated area. The exceeding standard rate of Me Hg PDI in wastewater-irrigated area was up to 20.83%. We may remedy the polluted soils or change crops planted in the soil.