Study On Arsenic Volatilization In Rice-soil System By Atmospheric Oxidative Sedimentation

Rice is an important food crop in the world which is more likely to absorb and enrich arsenic than other crops.Rice has become the most important human arse-nic exposure pathway in the world.Hydrogen peroxide and perchlorate exist in the atmosphere and in various water bodies.Their sources and concentrations are different,which are affected by the com-plicated factors.In the atmosphere,H₂O₂ is mainly produced by photochemical reac-tions,and exists in both polluted environments and clean environments,which plays an important role.Hydrogen peroxide has a closely relationship with the formation of O₃.The concentration of hydrogen peroxide can be characterized to some extent by the strength of atmospheric oxidation,which accounts for 70%to 90%.The concen-tration of perchlorate in the atmosphere is directly or indirectly affected by humans.Perchlorate as a strong oxidizing properties and high stability substance,which is widely used in aerospace,military and industrial fields,and emissions from aircraft exhaust and industrial exhaust gas directly.In this paper,we studied the oxidizing substances（H₂O₂,Cl O₄^-）in atmospheric deposition in Guangzhou.We tried to find the law of annual rainfall and the changes of oxidizing substances.Based on the study,different oxidants were applied in differ-ent arsenic-contaminated rice-soil systems to study the effects of oxidants on systemic arsenic volatilization and soil and rice arsenic morphology.We have the following results:（1）Monitoring the results of 81 rainfalls from 2016 to 2017 showed that the concentration of H₂O₂ in rainwater showed high spring and summer and low in au-tumn and winter.The annual fluctuation ranged from 0.1μmol/L to 118.7μmol/L,and the annual average concentration was 20.1μmol/L.Spring temperature and relative humidity favor the formation of H₂O₂ in the atmosphere,resulting in a higher concen-tration of H₂O₂.The annual variation of Cl O₄^-in rainwater is 1.4μg/L to 110.3μg/L,and the an-nual average concentration is 13.9μg/L,which is characterized by low in summer and high in winter.There are two reasons.Firstly,the rainfall in the winter is small,the purification effect of rainwater on the atmosphere is weakened.Secondly major win-ter festivals are more,and the number of fireworks and firecrackers is increased,re-sulting in an increase in atmospheric Cl O₄^-.The annual average flux of arsenic in rainfall is 6.4 mg/（m²·year）.The arsenic resource comes from the discharge of exhaust gas from the printing and dyeing facto-ries and hardware processing plants,and from the exhaust gas generated by busy traf-fic conditions.The seasonal variation of arsenic is winter>spring>autumn>sum-mer change,which can be judged to have a close relationship with rainfall.（2）To investigate changes in the composition of rainwater during complete rain-fall,we monitored changes in rainwater chemical composition during the seven full rainfalls in 2017.During the entire rainfall process,H₂O₂,Cl O₄^-,As and other ions are continuously removed.During the monitoring process,the highest removal rate reached 93.4%.Moreover,the longer the rainfall time,the more significant the effect of rainwater removal.Pearson correlation was used to analyze the correlation between different oxida-tive sedimentation and ion,rain and meteorological factors in the rain.It was found that the concentration of H₂O₂ was significantly positively correlated with Mg,Ca,NH₄⁺and SO₄^2-.We conclude that the concentration of H₂O₂ decreases with increas-ing SO₂ concentration,and the concentration of H₂O₂ also increases with the increase of SO₄^2-concentration,which is positively correlated.There was a significant nega-tive correlation between Cl O₄^-and temperature（T）（r²>0.5,p<0.01）.A significant positive correlation showed between Cl O₄^-and K,p H,Eh and EC（r²>0.5,p<0.01）,Cl O₄^-.Significantly negatively correlated with rainfall.Cl O₄^-is significantly posi-tively correlated with K,probably due to the use of potassium perchlorate（KCl O₄）.There are some factories near the monitoring site.The factory may use perchlorate or perchlorate precursor（Cl_xO_y）in the production process,which is discharged into the atmosphere during the production process,resulting in atmospheric perchlorate pollu-tion.（3）In order to investigate the effects of oxidants on the arsenic volatilization of rice-soil systems under different types of arsenic contamination,we set up three sets of experiments（soil group,rice-soil group and rice group）.The results show that the oxidant has an inhibitory effect on the arsenic volatilization in the system.It shows the same effect in different types of arsenic-contaminated soil and different oxidant types.The inhibition rate of oxidant on arsenic volatilization is between 2%and 38%.50μmol/L H₂O₂ had the least inhibitory effect on arsenic volatilization,and 5μg/L Cl O₄^-had the greatest inhibitory effect on arsenic volatilization,and the inhibitory effect of Cl O₄^-on arsenic volatilization was sustained.Since hydrogen peroxide is easily decomposed,its inhibitory effect on arsenic volatilization is first suppressed,and then the inhibitory effect is weakened with time.Perchlorate,as a new type of persistent pollutant,can be stably present in the environment,and thus maintains a longer period of inhibition of arsenic volatilization.In the correlation between the ox-idant concentration and the arsenic volatilization amount in each group,the determi-nation coefficient（r²）was less than-0.65,indicating that the oxidant concentration was strongly negatively correlated with the arsenic volatilization amount.In addition,rice has weaker methylation and volatilization ability of arsenic in the whole system.The amount of arsenic volatilization involved in rice is twice as high as that in the absence of rice.（4）During the different growth stages of rice,the root exudates produced by the different roots,which causes the difference in the species and quantity of rhizosphere microorganisms and leads to the volatility of arsenic.In general,the arsenic volati-lization during rice filling was significantly greater than that at maturity.The effect of arsenic volatilization during grain filling was greater than that at maturity during dif-ferent treatments.In the first season of rice filling period,the arsenic volatilization in the natural precipitation group was significantly higher than that in the other groups,and the perchlorate addition group had the lowest volatilization.In this study,the arsenic volatilization fluxes in the rice-soil system during grain filling and maturity were 21.6μg/（m³·d）and 20.1μg/（m³·d）,respectively.Due to the different growth degree of rice,the whole system volatilization in rice filling period is22%to 127%higher than that in mature stage.According to the toxicity threshold of arsenic exposure,the lowest toxic concentration of arsenic inhaled by humans is 325μg/m³,and the arsenic volatilization in the rice-soil system of arsenic has not reached the level of harm to human health.However,for the US to impose a maximum al-lowable concentration of arsenic in the air of residential areas of 3μg/m3,the arsenic volatilization of rice-soil is not optimistic.In outdoor environments,the inhibitory effect of oxidants on arsenic volatiliza-tion is not obvious,which may be related to complex environmental factors in the field.In addition,the application of oxidants（H₂O₂ and Cl O₄^-）increases the formation of iron film on the root surface of rice,thereby reducing the absorption of arsenic in rice.Soil organic matter and dry weight of rice were not significantly different be-tween the experimental group and the control group.In the rice yield,the yield of the test group to which the oxidant was added was significantly higher than that of the control group.In summary,oxidants are common in typical rainfall components in Guangzhou,and their contents are closely related to single-field rainfall and seasons.Atmospheric oxidant deposition has a certain inhibitory effect on the bio-volatilization of arsenic in rice-soil system.Rice has a certain inhibitory effect on arsenic absorption.In view of the inhibitory effect of oxidant on the absorption of arsenic in rice,in practical appli-cations,the oxidant can be placed on the arsenic-contaminated farmland to reduce the absorption of arsenic in rice,thereby reducing the risk of arsenic absorption.