The Arsenic Stabilization Mechanism Of Iron-based Oxides And Its Application In Arsenic Contaminated Soils

Arsenic(As) is a non-metallic element which is widely distributed in nature. It is often used in pesticides, herbicides, and alloy production. China has become the largest producer of As products, accounting for more than 56% of global market since 2013. Due to the high As background value in soil, disorderly exploitation of As mineral resources and excessive use of arsenical agricultural formulations, some areas are suffering soil As contamination problems in Hunan, Guangxi, Jiangxi and so on. It has been a serious threat to the health of residents and the development of economy and society. Stabilization is an effective, economical, and fast remediation technology for contaminated soils. Iron-based oxides have caused widespread attention in the remediation of As contaminated soils due to its high adsorption capacity, fast adsorption rate and other advantages. The purposes of this study were to embed rare metal in iron-based oxide to increase its As adsorption and environmental stability, and further improve its As adsorption capacity by surfactant modification. Micromodel was used as a simulation system of soil pore which could revealed the bottleneck problem in the actual application. In addition, a trial test was carried out in the As contaminated soil of Wenshan to verify its application effect. The specific results as follows:1) FC was the best stabilizer the stabilization of As-contaminated soils from Dalian, Chenzhou and Wenshan. It could transform non-specially sorbed and specially sorbed As to hydroxides of Fe, Al and Mn combined. In the FC treatments, TCLP and available As concentrations of soils decreased by 84.07%-98.26% and 12.30%-31.78%, respectively. The results also suggested that FC showed small impact on soil p H and significantly reduced the soil available P concentration( still higher than the high yield fertility level standard 25 mg/kg). The SEM-EDS, XRD and BET analysis demonstrated that FC was an irregular porous amorphous material, with a range of particle size from 500 nm to 20 ?m, and its specific surface area was about 117.06 m2/g. The FTIR and XPS analysis revealed that Fe-OH groups were the main binding sites for As adsorption, and imbedding of Ce made the surface hydroxyl groups rise to 45.86%.2) Four kinds of typical surfactants were used to further improve the As adsorption capacity of FC, such as sodium dodecyl benzene sulfonate(SDBS), rhamnolipid, twain(Tween 20) and starch. The result showed that 1% of starch modified FC could reduce the average particle size of modified Fe-Ce(SFC) to 0.25?m. Pseudo-second order kinetic model fitting equation's R2 is higher, and the largest initial adsorption rate could be up to 293.46 mg·g-1·h-1. SFC's Langmuir model fitting equation's R2 is higher, and its saturated adsorption capacity could be up to 200.32 mg·g-1. The FTIR spectra showed that the C-O bond vibration peak moved to a longer wave direction, indicating the chemical adsorption between starch and FC oxide. After As adsorbed on SFC, structure peak strength decreases significantly at 1020cm-1, while the 817cm-1 appeared an obvious peak of wave(As-O-Fe group), suggesting that the Fe-OH is the main function group for As adsorption on the surface. The XPS analysis revealed that the particle size of modified material decreased significantly, thus generated more-OH. After As adsorbed on SFC, the adsorption product is likely to be the compound of monodentate mononuclear ligand Fe-O-As O-(OH)2 and bidentate dinuclear ligand Fe-O-As O(OH)-O-Fe.3) The present study introduced a micromodel system(include preparation, injection, reaction, and analysis modules) into the research of soil contamination stabilization. The visualization image showed that pixel of SFC decreased with the reduction of pore size(Macropore 250 ?m, Mesopore 75 ?m, Micropore 30?m). Macropore was full of SFC, while some enclosure space appeared in the mesopore and micropore. It meant that the main area of SFC transport was macropore, and the sizes of pore throat were the key limiting factor. The LIBS analysis showed that 2-D distribution of Fe and As were similar with the visualization images. By contrasting elements distribution between Fe and As, we found that deposition and As adsorption of SFC decreased with transport distance increasing or pore scale reducing. In addition, we also studied the characteristic of SFC transport by single-collector theory. The results revealed that C/C0 reached its balance faster in micropore, and then the pore throat of micropore would be blocked earlier. The ?G of macropore was much higher than the other pores, therefore most of SFC deposited in its pore area.4) To increase the transport and As adsorption of SFC in mesopore and micropore of soil, we also investigated the effects of particle size, ionic strength on SFC transport and revealed the effects of p H, competitive ions and other factors on As re-release in stabilized products. The results of single-collector theory revealed that reduction of particle size and ionic strength could significantly improve SFC diffusion adsorption in the pores(?D), so as to effectively enhance the transport and removal efficiency(?). An empirical model of contaminant rate of release was also introduced into the study. The results suggested that the As desorption of the labile fraction constant(Kfast) and slowly released(non-labile) fraction constant(Kslow) increased with the pore scales rising. In the same pore scale, the Kfast reduced by 30.20-73.25% when Ca2+ concentration increased to 5 mmol/L, and the Kfast and Kslow decreased by 47.39-60.29% and 34.29-46.60%, respectively when the p H decreased to 4.5) To further verify the actual application of SFC, a trial test was carried out in the As contaminated area in Wenshan autonomous prefecture. The result suggested that SFC could adsorbed and combined As to the solid phase, significantly reduce environmental risk of soil. After six months, applying phosphate fertilizer including calcium magnesium phosphate and potassium dihydrogen phosphate led to soil As reactivation, while SFC could keep the soil's F1 + F2 concentrations at a low level. After adding SFC, As mainly accumulated at the iron plaque of root, and the As(III) proportions of P. Notoginseng significantly increased in root. 0.5% was the best dosage of SFC, and it increased the biomass of root(the main medicinal active constituents) by 41.6% and reduced the root As concentration by 55.8%. In addition, the soil p H changed in a smaller range after using the SFC, showed a strong buffer capacity.6) The stabilization of As contaminated soil by iron-based oxide was systematically studied. The newly-developed stabilizer(SFC) really had high As adsorption and steady environmental suitability. The micromodel could provide visual and effective evidences in soil pore to solve the bottleneck problem in its actual application. All the results would provide important information and effective methods on As contaminated soils, and even other heavy metal polluted areas.