The Effect Of Hydrodynamic Conditions On Sulfidization Of Arsenic-bearing Ferrihydrite And The Fate Of Arsenic

Geogenic arsenic?As?contamination in groundwater has become a geoenvironmental problem across the globe.The exact pathway by which As mobilization takes place in groundwater is quite complex.The partition of arsenic in groundwater is largely controlled by sulfide-induced Fe-S-As interactions.The influence of chemical factors such as pH and coexisting ions on Fe-S-As interaction has been widely concerned,but the role of physical factors such as hydrodynamic conditions in the above process has been an underappreciated force.In this study,hydrodynamic intensity was set as experimental variable to carry out the column experimental study on the non-biological sulfidation of As-containing ferrihydrite.According to the monitoring results of solid and liquid phases during the reaction,the mechanism of hydrodynamic conditions on the sulfidation and As repartition was discussed.The main conclusions of this study are as follows:The adsorption of arsenite on the reduces the reactivity of ferrihydrite.Part of the adsorbed arsenic exists as outer sphere complex,which can be rapidly desorbed by advection,thus eliminating the blocking of active sites on the surface of ferrihydrite.The stronger hydrodynamic intensity and longer duration will enhance the desorption of the outer sphere complex,which will result in a higher Fe/S ratio for the sulfidation of ferrihydrite.The rapid consumption of sulfide inhibits the production of iron mono-sulphide,while the large amount of Fe²⁺produced by the reduction of ferrihydrite catalyzes the transformation of residual ferrihydrite into hematite and other crystalline Fe oxides.Relatively weak hydrodynamic intensity or shorter duration will make Fe/S lower,so the sulfide can also form iron mono-sulphide with the reduction product Fe2+when reducing the ferrihydrite.In addition,the hydrodynamic intensity also affects the transformation path of the initial iron mono-sulphide.Under slow advection conditions,the amorphous iron mono-sulphide is transformed into sulfite,while the rapid advection lead to the formation of low-temperature stable hexagonal pyrrhotite.At both flow rates,strong sulfidation resulted in the release of arsenic from the upstream solid phase.However,in the fast flow system,this part of arsenic is re-fixed in the downstream,while the downstream of the slow flow system fails to achieve the re-fixation of this part of arsenic.As was released from the upstream solid phase due to strong sulfidation,as was the case at both hydrodynamic intensities,and was retained downstream of the fast advection system,but not downstream of the slow advection system.The above experiments show that the hydrodynamic intensity have a significant influence on the Fe-S-As interaction in groundwater.The study provides a new perspective for the exploration of the causes of geogenic high As groundwater and a guidance for the hydrodynamic condition control during in-situ remediation of high As groundwater.