Performance Evaluation And Mechanism Of Modified Iron Oxyhydroxide For Remediation Of Arsenic Contaminated Sites

Arsenic pollution in arsenic chemical toxicant contaminated sites in China is deserving close attention because of its huge environmental risks.Most of the chemical toxicant "red agent" has been degraded into arsenate and phenylarsonic acid over time.Water and soil around the sites have widely detected them.The iron oxyhydroxide(FeOOH)adsorbent has been extensively studied due to its high efficiency and low cost.However,most of the current applications of FeOOH are focusing on inorganic arsenic or organic arsenic-containing feeds in water.Few reports concerned about organic and inorganic arsenic pollution in the aqueous and soil of arsenic contaminated chemical toxicant sites simultaneously.And FeOOH prone to agglomerate which leads to decay performance due to its nanometer size.Therefore,we used cetyltrimethylammonium bromide(CTAB)as a functional agent to modify δ-FeOOH in the liquid phase by using coprecipitation method in this study.Further,we studied its adsorption performance of arsenate and phenylarsonic acid in water and soil influenced by various environmental factors.Finally,we verified the adsorption mechanism of arsenic species by the material through characterization and theoretical calculations.The main conclusions summarized are as follows:(1)CTAB addition experiment showed the best adsorption performance appeared in the ratio of CTAB to precursor ferric salt was 1:50.Through Fourier transform infrared spectrometer,X-ray photoelectron spectrometer,zeta potential and contact angle,etc.CTA+function in CTAB was linked to theδ-FeOOH surface.The dispersing ability of surfactant and electrostatic interaction could reduce the risk of agglomeration.And the specific surface area of the modified material increased and the modified material had new functional groups.(2)The maximum adsorption capacity of CTAB modified δ-FeOOH for arsenate and phenylarsonic acid in a neutral aqueous phase is 45.7mg g-1 and 85.3mg g-1,respectively.The pseudo-second-order kinetics and Freundlich isothermal adsorption model could describe the adsorption fittingly,and the process is spontaneous and endothermic.And the material would preferentially adsorb arsenate in water contaminated by the mixture of arsenate and phenylarsonic acid.Solution pH could affect arsenate adsorption significantly,while phenylarsonic acid was less.The experiment of coexisting substances showed that CO32-,PO43-and SiO32-had significant interference on adsorption,while humic acid in natural water had little effect on the material propertiese Characterization and calculations indicated the adsorption mechanism of arsenate was bidentate binuclear inner-sphere complexation and electrostatic attraction.And the mechanism of phenylarsonic acid adsorption was monodentate mononuclear,bidentate binuclear inner-sphere complexation,hydrogen bonding,electrostatic attraction,and π-hydrophobic interaction.(3)In vitro gastrointestinal simulation(IVG)was used to evaluate the stabilization efficiency of arsenic in soil before and after remediation,CTAB modified 8-FeOOH could reduce the mammalian bioavailable arsenate in the soil from 78.2 and 81.6mg kg-1(Gastric phase,intestinal phase)to 7.05 and 4.55mg kg-1 when the material accounts for 5%and the water holding capacity was 40%,while the mammalian bioavailable phenylarsonic acid in the soil reduced from 28.8 and 25.0mg kg-1(Gastric phase,intestinal phase)to 6.9 and 4.75 mg kg-1 in the short period.The speciation experiment of arsenic in soil showed the CTAB modified δ-FeOOH mainly transformed the non and specifically bound arsenic with strong mobility into the crystalline hydrous oxide bound arsenic with weak mobility.The characterization showed that inner-sphere complexation and coprecipitation are the two main mechanisms of arsenic stabilization.Besides,the diversity of soil bacterial community recovered by 25.4%after stabilized by CTAB modified δ-FeOOH.