Application Research On Arsenic Immobilization Through Natroalunite Phase

As one of the most predominant arsenic pollutant, arsenic-containing wastewater has gained more and more attention. Many different techniques, such as precipitation method, ion-exchange method, membrane separation technology, coagulation process had been developed for arsenic removal from waste water. In recent years, precipitation process for arsenic removal has gained considerable interest. By comparing the arsenic leaching characteristics of different arsenic-containing precipitations, for example, calcium arsenate, co-precipitation of arsenic with ferric ions, scorodite and arsenical-natroalunite solid solutions, the arsenical-natroalunite crystallization technology showed the most promise for arsenic stabilization/immobilization.In this thesis, much research has been carried out on the formation mechanism and dissolution characteristics of arsenical-natroalunite solid solutions. Meanwhile, the acidic arsenic-containing waste water, discharged from the procedure of sulfuric acid preparation, was treated by the crystallization of arsenical-natroalunite to verify the feasibility of the arsenical-natroalunite crystallization technology on the arsenic removal of waste water. Some important results were acquired as follows:(1) The factors influenced on the synthesis and structure of arsenical-natroalunite were systematically studied, including the temperature(T), reaction time(t), the initial pH value of the reaction solution, sodium sulfate concentration c(Na2SO4) and aluminum sulfate concentration c(Al2(SO4)3). It was showed that all above list factors had remarkable impacts on the synthesis of arsenical-natroalunite, and the optimum synthesis conditions were as follows:T=200℃, t=3.0 h, pH=3.0, c(Na2SO4)=0.050 mol/L and c(Al2(SO4)3)=0.12 mol/L, in which condition, the pure arsenical-natroalunite with high crystallization were prepared, and arsenic and aluminum precipitation rate could reach 12% and 84%, respectively.(2) A series of arsenical-natroalunite solid solutions with different arsenic content were hydrothermally synthesized by changing the (As/(As+S))aq molar ratio in the initial solution, and the products were characterized by means of XRD, ICP, SEM-EDS, FT-IR and TG-DSC to research the reaction mechanism and replacement law. The results showed that when (As/(As+S))aq was 0～0.178, arsenic precipitated almost exclusively as arsenical-natroalunite and the amount of AsO43- substitution observed in the natroalunite structure was up to ～8% molar, and when (As/(As+S))aq=0.185, a mixture of arsenical-natroalunite, mansfieldite and amorphous phase was formed and the amount of ASO43- substitution was ～11% molar. Moreover, the substitution of AsO43--for-SO42- in natroalunite followed the replacement law of (As/(As+S))na(?)0.45(As/(As+S))aq. However, a slight deficiency of Na+(6%-14%) and Al3+(3%-7%) in arsenical-natroalunite structure was inevitable due to the replacement of Na+ by HaO+, while charge imbalance caused by the deficiency of Al3+ could be maintained steadily by protonation of hydroxy (OH-). The charge imbalance caused by the difference of charge number between the AsO43- and SO42-would be balanced through the protonation of hydroxy(OH-). The general chemical formula of prepared arsenical-natroalunite solid solutions colud be defined as:Na1-x(H3O)xAl3-y (SO4)2-z(AsO4)z(OH)6-3y-z(H2O)3y+z。(3) Solubility and stability of synthesized arsenical-natroalunite solid solutions were studied. The dissolution of natroalunite was congruent at pH 1.5 and temperature of 25℃, 35℃ and 45℃, which followed the first-order reaction model. The dissolution could reach an equilibrium after 30 days at 25℃, after 60 days at 35℃ and 45℃. The final concentrations of the elements of Na, S, Al and As were detected, and the solubility characteristics were calculated by using PHREEQC modeling program according to above-detected data of final concentrations. The results showed that the logKsp value of the arsenical-natroalunite solid solutions were in the range of -11.41～-11.96, standard Gibbs free energy changes (AGr,298°) of the dissolution reactions were from 64.89 KJ/mol to 68.26 KJ/mol, and the standard Gibbs free energy of formation (ΔGf,298°) of the arsenical-natroalunite solid solutions were in the range of -4556.56～-4568.27 KJ/mol.(4)The acidic arsenic-containing waste water, which was discharged from the procedure of sulfuric acid preparation and contained many toxic elements such as As, S, F, Zn, Cu, was treated by the crystallization of arsenical-natroalunite solid solutions. The resulted product was compound arsenical-natroalunite solid solutions and the general formula of compound arsenical-natroalunite solid solutions could be deduced as: (Na5Cu,Ca,H3O)(Al,Fe)3(SO4,AsO4)2(OH,F)6. It could be found that there were other substitutions between Ca2+, Cu2+ and Na+, Fe3+ and Al3+ and F- and OH-, besides the replacement rate of 0.09-0.16 of SO42- by AsO43-, and the amounts could reach the range of 0.03～0.05,0.01～0.02,0.05～0.07 and 0.80～0.88, respectively.The results of arsenic leaching experiments indicated that the arsenical-natroalunite solid solutions showed higher stability under the liquid surrounding with pH 3.0 to 10.0 and the leaching concentration of arsenic was from 0.10 to 0.58 mg/L, far lower than the limits value of standard (5 mg/L). So the arsenical-natroalunite solid solutions are appropriate for safe long-term storage.