Study On Environmental Behavior Of Dimethylarsenic(DMA) At Ferrihydrite-water Interface Acid Interaction Interface

Ferrihydrite,as one of the most common iron(hydr)oxides,is widely found in soils and sediments.Because of its small particles,large specific surface areas and thus high reactivity,ferrihydrite significantly affects the distribution,migration and transformation of organic pollutants in the environment.Dimethylarsenic(DMA)is a common organic arsenic pollutant in the environment.It has a weak binding force with soil and is therefore more mobile in the environment.Although some studies have been carried out on the adsorption behaviors of DMA at the iron(hydr)oxide-water interfaces,natural iron(hydr)oxide minerals often contain high contents of aluminum,and thus have different physical and chemical properties from those of minerals without the substituent,and then different reactivities towards various pollutants.In addition,DMA can coprecipitate with iron(hydr)oxides during the mineral crystallization and transformation.However,few studies have been conducted on the environmental behaviors of DMA in the non-biological transformation processes of such coprecipitation.Therefore,in this work,powder X-ray diffraction(XRD),field emission scanning electron microscopy(SEM),Fourier transform infrared spectroscopy(FTIR)and X-ray absorption fine structure spectroscopy(EXAFS)were used to study the adsorption mechanism of DMA on the surfaces of ferrihydrite with different aluminum substitution levels and the environmental behaviors of DMA during the transformation of DMA-ferrihydrite coprecipitates induced by Fe(II)or at relatively high temperatures.The main conclusions are listed as following:1.When the initial molar ratios of Al/(Al+Fe)are 0,0.05,0.1,0.2 and 0.3,the final aluminum substitution amounts in the samples are 0 mol.%,3.02 ± 0.01 mol.%,6.71 ±0.11 mol.%,12.89 ± 0.34 mol.% and 20.50 ± 0.76 mol.%,respectively.DMA adsorption kinetics on ferrihydrite surfaces show that,adsorption approaches equilibrium after 24 h.The p H adsorption edge experiments show that,when the p H<6,the DMA adsorption density on ferrihydrite slightly increases,while when the p H>6,the DMA adsorption density decreases significantly.With the increase of ionic strength from 0.00 M to 0.72 M,at p H 6,the adsorption densities of DMA by pure phase ferrihydrite and aluminum-doped 0.1 ferrihydrite increase slightly,while at p H 9,the adsorption densities of DMA on the minerals increase significantly.Isothermal adsorption experiments show that DMA adsorption on ferrihydrite surfaces conforms to the Langmuir isothermal adsorption model,and aluminum isomorphous substitution significantly improves the adsorption capacity of DMA on ferrihydrite.FTIR and EXAFS analysis show that,DMA mainly forms bidentate binuclear(BB)inner-sphere complexes on the ferrihydrite surfaces.2.The DMA-ferrihydrite coprecipitate samples with As/Fe molar ratios of 0,0.0096 ± 0.0007,0.0473 ± 0.0036 and 0.0876 ± 0.0036 were successfully synthesized and named as Fh,Fh DMA0.01,Fh DMA0.05 and Fh DMA0.1,respectively.With the increase of DMA content,the mineral structure and morphology do not change significantly,the mineral particle size and the content of As-O-Fe bond in the mineral increase slightly,while the specific surface area and pore size decrease slightly.Fe(II)-catalyzed mineral transformation experiments show that,at p H 7,DMA in Fh DMA0.01 inhibits the further conversion of lepidocrocite to goethite,while DMA in Fh DMA0.05 and Fh DMA0.1 completely inhibits the ferrihydrite transformation.Experiments at different p Hs show that,no mineral phase conversion occurs in both the Fh and Fh DMA0.1 systems after reaction for 336 h at p H 4.At p H 10,both Fh and Fh DMA0.1 are partly converted to goethite,and the proportions of goethite are 30.00± 1.10 % and 20.66 ± 0.63 %,respectively.This indicates that the coprecipitated DMA inhibits the ferrihydrite mineral phase transformation at p H 7 and p H 10,and the inhibition effect was stronger at p H 7 than at p H 10.During the transformation of all coprecipitated samples,As is continuously released from the solid phase into the solution.At p H 7,with the increase of DMA content,the proportion of As released is also increased from 17.16% of Fh DMA0.01 to 75.54% of Fh DMA0.1,respectively,while that of Fh DMA0.05 is higher than that in the control experiment without Fe(II).As release from all samples can be divided into two stages: a rapid release within the first 24 h and then a slow release in the subsequent stage.3.Mineral transformation experiments at 70 °C show that all samples are completely converted to hematite and goethite at p H 7.The proportions of hematite in the final products for DMA-coprecipitated ferrihydrite samples decrese in the order of Fh(82.01 ± 0.70%)> Fh DMA0.01(79.19 ± 0.51%)> Fh DMA0.05(45.72 ± 0.46%)>Fh DMA0.1(33.00 ± 0.85%),while the proportions of goethite are 17.99 ± 0.70%,20.81± 0.51%,14.93 ± 0.15% and 17.79 ± 0.45%,respectively.This indicates that DMA inhibits the transformation of ferrihydrite to hematite,and the inhibition is proportional to the content of DMA.Transformation experiments at different p Hs show that ferrihydrite are converted to hematite and goethite.Compared with Fh in the absence of DMA,DMA inhibits the transformation of ferrihydrite to goethite at p H 4,while has almost no effect on the mineral transformation at p H 10.Continuous release of As from the solid phase to the solution is also observed in these systems.Compared with the release of As in the presence of Fe(II),the release rate of As at high temperature is greater than that of Fe(II).In addition,the As release can be divided into two stages,that is,a rapid release within the first 24 h and a slow release stage till to the end.