Influencing Factors Of Mercury Of Rice And Preliminary Study On Remediation Technology In Wanshan Mercury-mined Areas

The regularities of distribution and the influencing factors of total mercury(THg)and methylmercury(Me Hg) in Rice and paddy soil were studied from a regional dimension in Hg-mined areas,Wanshan,Guizhou Province. Amendments or agronomic measures also were used to repair the pollution of THg and Me Hg in rice with a situ experiment.The results show that,1.The distribution characteristics and influentive factors of THg and Me Hg in paddy soilThe concentrations of THg in soil range from 0.49 μg.g-1to 188.00 μg.g-1with an average of 31.62 μg.g-1.The THg concentrations of whole samples exceed the national standards(THg=0.3μg.g-1).The contents of Me Hg range from 0.72 ng.g-1to 6.70 ng.g-1with a average of 2.82 ng.g-1.Furthermore,the methylation rates range from 0.002% to0.237% with a average of 0.041%.THg and Me Hg concentrations in soil tend to gradually decrease away from the key Hg-mined areas to the less polluted areas on the whole.Simultaneously,Si O2,Al2O3,TFe2O3,TS,TP,TN,organic matter,p H and Se in soil were analyzed to understand the variables influencing facts to THg and Me Hg in soil.Our results found that THg concentrations in soil are only negatively linearly correlated with Si O2(P<0.01),and positively correlated with Se concentrations in soil(P<0.01).At the same time,Me Hg in soil is positively correlated with THg in soil,TS,TP,Se(P<0.01),and TN,organic matter(P<0.05),but showing a negative correlation with Si O2(P<0.01).The result shows that Me Hg in paddy soil is not only related with THg, but also some nutrition factors in the soil.2.The distribution characteristics and influentive factors of THg and Me Hg in riceThe concentrations of THg in Rice in major basins of Hg-mined areas range from2.84 ng.g-1to 295.53 ng.g-1with an average of 50.82 ng.g-1.About 60% of rice samples have exceeded the national standards(THg=20ng.g-1).The contents of Me Hg in rice range from 2.30 ng.g-1to 43.18 ng.g-1with the average content being 13.33 ng.g-1.The Me Hg rates of rice ranges from 7.12% to 68.37 % with an average of 34.04%,and the enrichment ability of rice to Me Hg in soil was 12.27 times and the average was 5.The contents of THg and Me Hg in rice show a different decreasing trend in the spatial distribution with the direction away from Hg-mined areas.Simultaneously,Si O2,Al2O3,TFe2O3,TS,TP,TN,organic matter,p H,Se,THg and Me Hg in soil including the proportion of THg(Me Hg) in rice accounted for THg insoil were analyzed to understand the relations.Our results found that THg concentrations in rice are significantly linearly correlated with TS(P<0.05),and TN,organic matter,p H(P<0.01),but negatively correlated with Si O2(P<0.05),but no positively correlated with THg and Se in soil.we found that the contents of Me Hg in rice shows a significant positive correlation with TN(P<0.05),and TP,organic matter,p H,THg,Me Hg in soil,THg in rice(P<0.01),but a significant negative correlation with Si O2(P<0.01).At the same time,the proportion of THg(Me Hg) in rice accounted for THg in soil show a significant negative correlation with Se in soil(P<0.01),indicating that Se in soil significantly affects the biological accumulation of THg and Me Hg in paddy soil system.3.Mercury exposure through rice consumption and its health risk assessment for the residentsThe daily intake and idex of health risk(HRI) from THg through the consumption of rice by the residents respectively range from 0.02μg/(kg.d) to 1.77μg/(kg.d) and0.03 to 2.49(JECFA-recommended safety standards).The daily intake and HRI by Me Hg respectively range from 0.01μg/(kg.d) to 0.26 μg/(kg.d) and 0.09 to 1.13(JECFA-recommended safety standards),0.14 to 2.59(USEPA-recommended safety standards).The concentrations of Me Hg in the hairs of residents range from 0.17μg.g-1to 3.11μg.g-1,and only 5% of the monitoring points exceed the JECFA-recommended value,but a percentage of 38.33 for the USEPA-recommended value.The health risk from THg and Me Hg through the consumption of rice by the residents both show varying degrees of decrease away from the key Hg-mined areas to the less polluted areas.Further more,the exposure levels are associated closely with the location of cultivation.Generally speaking,the risk of exposure is currently still in a controllable status, and we need take timely appropriate measures to avoid the expanding risk.4.The passivation study of THg and Me Hg in RiceAmendments can generally inhibit the accumulation of THg in rice from soil in both Mei Zixi(MZX) and Ao Zhai(AZ),and the reducing rates reach respectively from10.86% to 40.95% and 13.54% to 35.8% with reducting contents from 23.0ng.g-1 to90.5ng.g-1 and 34.54 ng.g-1 to 68.78 ng.g-1.The Farmyard manure receive the highest average reductive rate and amount about 30.55 % and 70.94 ng.g-1 in both places,and followed about 30.18 % and 70.59 ng.g-1 by Fly ash.In a word,the passivation effect of calcium magnesium phosphate fertilizer,farmyard manure,medical stone powder and activated carbon to THg in rice can achieve significant difference in experimental groups comparing with the control groups(P<0.05).Amendments can also generally inhibit the accumulation of Me Hg in rice in both high and low contents,and the reducing rates reach from 27.86% to 71.61% and 29.48% to 64.96% with a average of51.55% and 49.41%,respectively,in MZX and AZ.The ratio of Me Hg/THg in rice respectively decreases from 17.58% to 65.94% and 12.64% to 59.49% in theexperimental groups in MZX and AZ.In general,Phosphorite powder,Medical stone powder,Farm manure,Activated carbon,Activated clay,Sodium sulfide and Calcium magnesium phosphate fertilizer in different test areas own a significant Me Hg passivation effect(P<0.05) with a reduction over 50%,and Calcium magnesium phosphate fertilizer,activated clay and activated carbon achieve a relatively stable effect of passivation to the ratio of Me Hg/THg in rice.No matter in MZX or AZ and THg and Me Hg in rice,Calcium magnesium phosphate fertilizer,Farm manure,Medical stone powder and activated carbon achieve a significant passivation effect(P<0.05),and the effect of modifiers on Me Hg is much better than THg.The various clay minerals produce different effects,and the variance of agronomic measures including irrigation and fertilization will have an important impact on the enriched ability of rice to THg and Me Hg.5.Effect of amendments on the contents of THg and Me Hg in Rice by repeated testIn the repeated verification test,the contents of THg and Me Hg of rice in the control group are respectively 74.39 ng.g-1and 32.24 ng.g-1,and the modified agents produce respectively a decrease for the contents of THg and Me Hg from 23.71% to 42.67%and 28.69% to 75.25% with the average are 36.50% and 55.77%.The effect is still relatively stable,while the passivation effect of Me Hg is always better than the THg in rice,which had been found in preliminary test. In both experiences,no matter in MZX or AZ and THg and Me Hg in rice,Calcium magnesium phosphate fertilizer,Farm manure and Medical stone powder achieve a significant passivation effect(P<0.05),and the effect of Medical stone powder to THg and Calcium magnesium phosphate to Me Hg are both more stable.