The Distributive Of The THg And MeHg In The Rice Paddy Ecosystem

The Minamata disease erupted in Minamata bay at1950s’,making people realizethe harm of mercury,especially methylmercury to the human body。In the past for along time, scientists believed that eating fish and seashell is the main way absorbingmethylmercury into body. But in recent years, Researchers have found that inorganicmercury can be converted to methylmercury in the rice paddy ecosystem in highmercury zones，and the rice seed, in which the MeHg concentration is several dozentimes than in soils, has a high capacity to accumulate MeHg. Rice is the source andcollect of the methylmercury. However，The domestic and foreign researches for totalmercury＆methylmercury and methylation mechanism in rice paddy ecosystem in lowmercury zones, which account for the most area, are less. So the research of total Hgand MeHg in the rice has very important realistic and theoretic significance forestimating mercury exposure risk and grain production safety.In this paper, the main base for rice production of Henan province——Xinyangwere selected as the researching object. Rice samples and non-irrigated samples weredirectly collected from field at the stage of the harvest period in the nine samplingsites(Gushi、Luoshan、Xinxian、Shangcheng、Huaibin、Xixian、Huangchuan、Guangshan、Pingqiao).The rice plants were divided into six parts：root、stem、leaf、stalk、hull and grain. The concentration of THg and MeHg in soil, rice plants and watersamples were analyzed. Hg in samples was determined using dual-stage goldamalgamation method and cold vapor atomic fluorescence spectrometry (CVAFS)detection. The analysis results contrast to National food safety standard and rice inother domestic and foreign zones. Local residents were also estimated to assesshuman’s mercury exposure risk; the main results are shown as follows:(1)The mean of THg concentration in rice paddy soil was30.75±18.67(range:9.32~83.25)ng/g, being lower than300ng/g, which is regulated in Soil EnvironmentalQuality Standard (GB15618—2008).This showed that the area in Xinyang didn’t sufferfrom severe mercury pollution. The mean of THg is21.75±11.34ng/g in root,9.92±4.81ng/g in stem,50.38±46.89ng/g in leaf,29.01±36.03ng/g in stalk,6.92±3.59ng/g in hull and10.04±9.91ng/g in grain. The THg concentrations in rice ranges from3.81~40.47ng/g,with samples in Panxinzhen、 Shangshiqiao andHefengqiao sites exceeding Chinese national standard limit for rice （0.02mg/kg,GB2762-2005).The mean MeHg concentration in rice paddy soil was0.090±0.053(range:0.021~0.203) ng/g, and the mean of MeHg was0.220±0.129ng/g in root,0.087±0.081ng/g in stalk,0.075±0.055ng/g in leaf,0.099±0.101ng/g inpeduncle,0.130±0.107ng/g in hull and0.326±0.121ng/g in grain). THg concentrationsin rice ranges from0.169~0.748ng/g,which were much lower than that in Guizhou Hgmining area.（16ng/g）.THg concentrations in rice were close to previously reportedvalues（0.08~2.40ng/g）for rice samples collected from other production areas.(2)In Xixian Changling villag（e11°12′35″E，32°38′20″N）,a special sampling sitesabout60000square meters were selected，which has a rice paddy ecosystem adjacentto non-irrigated ecosystem and pond ecosystem.This sampling site is an ideal area toresearch the ability of methylation. The study suggested that: the mean THgconcentration(54.45ng/g)in paddy field soil was1.6times larger than that innon-irrigated(33.06ng/g); the MeHg concentration(0.055ng/g)in paddy field soil wassignificantly larger6.5times than MeHg concentration in soil ofnon-irrigated(0.024ng/g).The mean THg concentration(7.68ng/g)in grain was1.9timeshigher than that in dry crop(4.02ng/g),but the MeHg concentration(0.351ng/g) was12timeshigher than that in dry crop(0.030ng/g),the bioaccumulation factors(BAFs)was5.2times and methylation ability was6.3times of that in dry crop. Studies have shownthat the paddy ecosystem has high methylation ability at high and low mercury area. Itis one of the sources and collects of the methylmercury.(4)In correlative result, significant and positive correlations were observedbetween in paddy soil and that in rice（R=0.691，P<0.01，n=24）,hull（R=0.669，P<0.01，n=24）,stalk（R=0.479，P<0.01，n=24）and roo（tR=0.775，P<0.01，n=24）.Therewas no obvious correlation between MeHg concentration in soil and MeHgconcentration in stalk（R=0.032，P<0.01，n=24）and leaf（R=0.057，P<0.01，n=24）,which implied that MeHg in the tissues(rice,hull,stalk and root)of rice plant controlledby MeHg in paddy soil.(5)The weekly intake of THg for Xinyang residents was0.0219±0.0216μg kg-1week-1per capita through rice., and the maximum was only0.0883μg kg-1week-1;itwas much lower than the suggested level3.99μg kg-1week-1by World HealthOrganization, and the maximal risk factors was0.0221(HI<1,risk-free).The weeklyintake of MeHg was0.00966±0.00359μg kg-1week-1per capita, and the maximum was0.02213μg kg-1week-1,lower than the suggested level1.61μg kg-1week-1by WorldHealth Organization, the maximal risk factors was only0.01375.In all, we come to the following important conclusions:(1)rice paddy ecosystem isthe source and collect of MeHg.(2)In general, the rice in Xinyang didn’t suffer seriousmercury pollution, and THg and MeHg concentrations are at lower levels, only didtotal mercury few samples exceed the national standard.