Effect Of Micromolar Hydrogen Peroxide Mediated Fenton-driven On Environmental Behavior Of Several Environmental Pollutants

Elevated concentrations of arsenic（As） in groundwater continue to threaten the health of millions of people worldwide. Rice is the dietary staple for about half of the world’s population and unfortunately, rice consumption has been one of the main arsenic exposure route in recent years. Animal wastes from concentrated animal feeding operations can cause agr-environment arsenic pollution due to the widespread use of organoarsenic feed additives. In addition, the widespread of new chemical herbicides and herbicide-resistant crops have been evolved resistance to control weeds. Unfortunately, this can lead to a major environmental concern of herbicide pollution. Elevated solar ultraviolet（UV） radiation and air pollution have caused to increase concentration of hydrogen peroxide（H₂O₂） and reactive oxygen species（ROS）. H₂O₂ is an aggressive oxidant and it is commonly present in rainwater. Due to relatively anoxic and organic-rich conditions, iron compounds in soil are subject to release of ferrous iron（Fe2+） into water-soil ecosysytem.During rainfall events, the Fe2+ in water-soil ecosysytem can react with rainwater-borne H₂O₂ to trigger Fenton reaction. Fenton reaction produces hydroxyl radical（?OH）, which is an even stronger oxidant, as compared to H₂O₂. Hydroxyl radical has the capacity to redox reactions with environmental pollutants and leads to influence of chemical pollutants transport, transformation, and fate end. Furthermore, hydroxyl radical（?OH） may have a role to play in affecting the activity of microbial communities in water-solil environments and the pollutants of ecological effects. In this study,（a） the laboratory experiment,greenhouse pot and pool experiment were conducted to examine the effects of Fenton reagent on removaling the water-borne arsenic and reducing As uptake by rice plants. The impacts of Fenton reagent on plant growth are also evaluated. In addition, the major biogeochemical mechanisms responsible for the observed phenomena are explored.Moreover,（b） three widely used agricultural herbicides in simplified aqueous systems are presented to demonstrate the link between their degradation and the exposure to a variety of Fenton reaction or humic acid（HA） combinations that are likely to be encountered in natural water systems. Finally,（c） various microcosm experiments were conducted to examine the effects of oxidation of ammonia and nitrite in aquatic ecosystem, observe simultaneous changes in ammonia, nitrite and nitrate in the soils inundated by water containing hydrogen peroxide at a concentration range frequently encountered in rainwater.The objective was to evaluate the effects of rainwater-borne hydrogen peroxide on nitrification and nitrogen biogeochemical processes in the soils that are prone to intermittent water inundation during rainy seasons.The main results are as follows:（1） Effect of micromolar H₂O₂ induce Fenton reaction-driven on inorganic arsenic environmental behavior in paddy soilMicrocosm experiments were conducted to examine Fenton reaction-driven removal of arsenate and arsenite exposed to a variety of Fe2+-H₂O₂ combinations that are likely to be encountered in natural water environments. The results show that these combinations had significant（p< 0.05） effects on removing the water-borne arsenic. However, the high concentration of HA has affected with the Fenton reaction-driven removal of arsenic.Hydroponic, greenhouse pot and pool experiments were conducted to examine the effects of Fenton reagent on paddy rice plant growing in inorganic arsenic-contaminated soils. Fenton reagent significantly reduced arsenic phytotoxicity, uptake by the plants and accumulation in rice grain. This is attributed to oxidation of As3+ to As5+ by hydroxyl radicals and immobilization of arsenate by reacting with precipitating Fe3+to form practically insoluble compounds. Although this process enhanced the formation of Fe-enriched coatings on root surface, it appears that root plaque had limited effects on inhibiting As uptake since most of the young roots were not covered by iron plaque. It is more likely that As immobilization in the bulk soils play a major role in reducing As flux towards rhizophere. The findings have implications for understanding As behavior in paddy field receiving rainwater-borne hydrogen peroxide and developing cost-effective techniques for reducing As level in rice grain produced from As-contaminated soils. Furthermore, the soil microbial community alpha diversity was significantly higher in Fenton reagent（T2）and rainfull（T3） treatments than in the treatment T1. The beta diversity of data also clearly show that the bulk and rhizosphere soil microbial community difference between the control（T1） and the treatment（T2, T3） was statistically significant for pool experiment.Comparison shows that treatment of Fenton reagent（T2） resulted in a significant（p< 0.05）increase in bulk and rhizosphere soil of microbial community diversity and relation fuction with arsenic or iron of bacteria. This may have an important influenced the inorganic arsenic environmental behavior in soil- microbial- rice system.（2） Effect of micromolar H₂O₂ induce Fenton reaction-driven on organic arsenic environmental behavior in paddy soilBatch, greenhouse pot and pool experiments were conducted to examine the effects of Fenton process on transformation of roxarsone in soils and its resulting impacts on the growth of and As uptake by a rice plant cultivar. The results show that addition of Fenton reagent markedly accelerated the degradation of roxarsone and produced arsenite, which was otherwise absent in the soil without added Fenton reagent. Methylation of arsenate was also enhanced by Fenton process in the earlier part of the experiment due to abundant supply of arsenate from Roxarsone degradation. Overall, addition of Fenton reagent resulted in the predominant presence of arsenate in the soils. Fenton process significantly improved the growth of rice in the Maturing stage of the first season. The concentration of methylated As species in the rice plant tissues among the different growth stages was highly variable. Addition of Fenton reagent into the soils led to reduce uptake of soil-borne As by the rice plants and this had a significant effect on reducing the accumulation of As in rice grains. The findings have implications for understanding As geochemistry in paddy rice field receiving rainwater-borne H₂O₂ and for development of mitigation strategies to reduce accumulation of As in rice grains. Furthermore, the soil microbial community diversity was significantly higher in Fenton reagent（T2） treatment than in the treatment T1. The beta diversity of data also clearly show that the difference between the bulk and rhizosphere soil microbial community. Comparison shows that treatment of Fenton reagent（T2） resulted in a significant（p <0.05） decrease in bulk and rhizosphere soil of relation fuction with arsenic or iron of bacteria. This may have an important influenced the organic arsenic environmental behavior and bioavailablity change of roxarsone in soil- rice system.However, there was significant difference in the environmental behavior between inorganic-As and organic-As treatment in paddy soil system. During the inorganic-As exposure, addition of Fenton reagent have attributed to oxidation of As3+to As5+by hydroxyl radicals and immobilization of arsenate by reacting with precipitating Fe3+ to form practically insoluble compounds. But for the organic-As exposure, Fenton process markedly accelerated the degradation of roxarsone and methylation of arsenate through hydroxyl radicals, which was decreased the degradation intermediate product of arsenite and reduced phytotoxicity to the rice plants.（3） Micromolar H₂O₂ induce Fenton reaction-driven degradation of agricultural herbicidesMicrocosm experiments were conducted to examine Fenton reaction-driven degradation of three common herbicides（diuron, butachlor and glyphosate） exposed to a variety of Fe2+-H₂O₂ combinations that are likely to be encountered in natural water environments. The results show that micromolar H₂O₂ induce Fenton-driven combinations had significant（p< 0.05） effects on removing the water-borne herbicides. Moreover,another observation in the present study was shown that the hydroxyl radical（?OH） is a selective degradation of three common herbicides, especially in destroying to glyphosate.Unfortunately, the high concentration of HA has inhibited with the Fenton reaction-driven degradation of diuron and butachlor in natural waters. However, the low concentratons of HA（5-20 mg/L） has promoted with the Fenton reaction-driven degradation of glyphosate in natural waters. This might the powerful oxidant of hydroxyl radical（?OH） preferentially attack high concentration of HA and reduced its oxidative capacity.（4） Effect of micromolar H₂O₂ induce Fenton reaction-driven on nitrogen biogeochemical processesLaboratory experiments with natural river water and differsnt soil type are conducted to examine Fenton reaction-driven transformation of nitrogen biogeochemical processes in water-soil environment system. The results show that micromolar H₂O₂ induce Fenton reaction-driven combinations had no significant difference in the concentration N species of NH4+-N and NO3--N between the control and the Fenton process treatment for the pure chemical NH3.H2 O or NH4 Cl system. However, the concentration N species of NO3--N and NO2--N was significant smaller in the Fenton process treatment than in the control for the pure chemical Na NO2 system. By comparison, the concentration of Total N was significantly（p< 0.05） less in the Fenton process treatment than in the control for the pure chemical Na NO2 system. In addition, there was a clear trend showing that micromolar H₂O₂ induce Fenton reaction inhibition of the NH4+-N and NO2--N transformation under the microbially mediated by NH4 Cl or Na NO2 system. Like the microbially mediated by NH4 Cl or Na NO2 system, the concentration N species of NH4+-N and NO3--N was consistently greater（significantly at p< 0.05） in Fenton process treatment than in the control for the four different soil type system with adding NH4Cl（sum of 50 mg N/kg soil）. The results obtained from the the pure chemical Na NO2 system experiment suggests that, under the set experimental conditions, addition of Fenton reagent markedly accelerated the oxidation damage of relation fuction with nitrogen biogeochemical processes of bacteria, resulting in inhibition of the NH4+-N and NO2--N transformation.In summary, micromolar H₂O₂ induce Fenton reaction-driven on influence the inorganic and organic biogeochemical in paddy soil ecosysytem. Fenton reagent significantly reduced arsenic phytotoxicity, uptake by the plants and accumulation in rice grain. Moreover, the abundance diversity of soil microbes and microbial genera related to iron and arsenic in the As-contaminated paddy rice soils tended to be significantly enhanced in the presence of Fenton reagent. The findings have implications for understanding As behavior in paddy field receiving rainwater-borne hydrogen peroxide and developing cost-effective techniques for reducing As level in rice grain produced from As-contaminated soils. Rainwater-borne H₂O₂ induce Fenton-driven significantly degradation of herbicides（diuron, butachlor and glyphosate） in natural waters. This discovery obtained from this preliminary work provide a rationale for undertaking further study to confirm the presence of an overlooked naturally-occurring process that may lead to rapid dissipation of many herbicides and other organic pollutants in open water environments. In the abiotic systems, H₂O₂ and hydroxyl radical was capable of chemically oxidizing nitrite but not ammonia. In the biotic system, microbially mediated nitrification was impeded in the presence of H₂O₂, which reacted with water-borne iron to trigger Fenton reaction to produce hydroxyl radical. The resulting inhibition of ammonia-oxidizing microbes reduced the removal rate of ammonium ion but also reduced the emission of gaseous nitrogen species. And the research findings obtained from this study have implications for understanding the complication of soil nitrification by rainwater-borne H₂O₂ during flood events.