Study On The Biogeochemical Behavior Of Iron,Manganese And Sulfur At The Sediment-porewater Interface And Associated Impacts On Arsenic Migration

The "source-to-sink" study is crucial to improve our understanding on interactions between land and sea.Sediment is an important part in the study of"source-to-sink".Substances exchange between sediment and overlying water depends on their migration ability,which is affected by geochemical process on the sediment-pore water interfaceAs a highly toxic carcinogen,arsenic can degrade the quality of environment.Iron and manganese oxides,with large specific surface area and strong adsorption capacity,are powerful adsorbents on arsenic.The geochemical behavior of Fe and Mn in sediments is influenced by the dissimilatory reduction of sulfate,and we should consider the impact of sulfate reduction on iron and manganese oxides reduction.It's important to calculate the exchange flux between sediment and overlying water and to understand biogeochemical behavior of iron,manganese,sulfur and arsenic on the sediment-pore water interface for environment management.This study is supported by National Science Foundation of China(NSFC,project number:41372242,41672226).To calculate the exchange flux between sediment and overlying water and to understand biogeochemical behavior of iron,manganese,sulfur and arsenic on the sediment-pore water interface,we choose downstream,estuary and coastal zone of Jiuxi River as our sampling site,combined the ZrO-Chelex DGT/AgI DGT and HR-Peeper technology to gain difference species of Fe\Mn\As and sulfide in pore water.Sediment column were also collected in the same time.Based on the results,we drew these conclusions:(1)Affected by the content of organic matter and easily reduced iron and manganese,the reduction of Fe\Mn\S is strongest in costal zone and weakest in downstream.The reduction rate is much weaker in winter than summer.The redox interface is deeper in intertidal zone than subtidal zone because of the inhibition of dissolved oxygen.(2)The reduction of manganese is controlled by the content of easily reducible manganese in sediments and restricted by the content of organic matter in summer,while the limiting factor in winter is mainly the content of organic matter.After manganese oxide is dissolved and Mn? released into pore water,it is easy to form organic complexed manganese in pore water.The concentration of organic complexed manganese is highest in costal zone and lowest in downstream,and the concentration of organic complexed manganese in the pore water in winter is less than that in summer.The oxidation of organic complexed manganese make it no longer limited on the surface of sediment particles that the reduction of manganese.(3)The reduction of iron is restricted by the content of active organic matter and easily reducible iron in the same time in summer,while it is mainly restricted by the content of active organic matter in sediments in winter.However,iron is more likely to be precipitated by S?,the content of dissolved organic complexed iron is lower than dissolved organic complexed manganese.(4)Sulfate reduction is restricted by the content of active organic matter and may be affected by temperature.In summer,the intensity of dissimilatory reduction and chemical reduction of iron and manganese are strong,while in winter,the degree of dissimilatory reduction of iron and manganese is lower and the chemical reduction is dominant.As content of organic matter in sediment decreased,The reduction of Fe and Mn in winter is weaker than that in summer.(5)Inorganic arsenic(As?,As?)is the main component of total dissolved arsenic in pore water,and the reduction caused dissolution of particulate iron and manganese oxides is the dominant factor controlling the release of inorganic arsenic into pore water.Dissolved complexed manganese can strongly absorb inorganic arsenic,and affect the distribution of total dissolved arsenic.S? can reduce the content of inorganic arsenic by co-precipitation with arsenic,or increase the total arsenic concentration in pore water by reducing iron and manganese oxides or reducing As?to form arsenite.(6)Due to the rise of water level,manganese,iron and arsenic in sediment in the downstream of Jiuxi supply by the overlying water in summer,and release to the overlying water in winter.The decrease of concentration of total dissolved manganese in the overlying water is the dominant factor causing the decrease of Mn flux from sediment to o-verlying water in winter.The different exchange flux of iron and arsenic between sediment-overlying water interface is mainly affected by the change of pore water concentration.