Preparation Of Sodium Arsenate Sodium Hydroxyphosphate Solid Solution And Study On Its Toxic Leaching Characteristics

As carcinogenic substances,arsenic and its compounds are highly poisonous to humans and other organisms.Arsenic-containing wastewater is the major source of arsenic pollution,which is usually treated by chemical precipitation method.Arsenic precipitation yield and the stability of arsenic-containing slag are the main indicators for assessing the processing technology,and it is the important research direction for arsenic-containing wastewater to improve the stability of arsenic-containing slag on the basis of high arsenic precipitation yield.In this study,to form a more stable inorganic arsenic compound by the substitution of similar AsO43-for PO43-was proposed.In this paper,Arsenate-substituted Sodium Iron Phosphate Hydroxide solid solution(Na2Fe(PO4)1-x(AsO4)xOH)with high crystallinity was synthesized by using ferrous sulfate heptahydrate(FeSO4·7H2O),phosphoric acid(H3PO4),arsenic acid(H3ASO4)and sodium hydroxide(NaOH)based on hydrothermal synthesis of Sodium Iron Phosphate Hydroxide(Na2FePO4OH).The effects of the factors such as the arsenic content n(As/P),the reaction time t and reaction temperature T on the synthesis,structure and performance of Arsenate-substituted Sodium Iron Phosphate Hydroxide solid solution were researched in order to get the optimal synthesis conditions.Moreover,the stability of products was studied by regular arsenic toxicity leaching experiments.The main results were obtained as follows:(1)The factors influenced on the hydrothermal synthesis of Sodium Iron Phosphate Hydroxide were systematically studied,including the reaction time t,the reaction temperature T and initial pH of the reaction solution.The products were characterized by means of XRD,SEM-EDS and FT-IR.The results showed that the optimum synthesis conditions for Sodium Iron Phosphate Hydroxide were as follows:t=12h,Tr=200℃ and pHinitiai=8.0.At the condition,the product had good crystal microstructure and high precipitation yield Y,which was about 75.60%.(2)Under the premise of pHinitiai=8.0,adding arsenic acid(H3ASO4)into the synthetic system of Sodium Iron Phosphate Hydroxide,the factors influenced on the hydrothermal synthesis of Arsenate-substituted Sodium Iron Phosphate Hydroxide solid solution were systematically studied,including the arsenic content n(As/P),the reaction time t and reaction temperature T.The results showed the optimum synthesis conditions for Arsenate-substituted Sodium Iron Phosphate Hydroxide solid solution were n(As/P)=0.35,t=12 h and T=200℃.At this time,the atomic content of arsenic in the product was about 2.54%,the As/P atomic ratio was about 0.345,and the arsenic precipitation yield was about 19.39%.(3)By means of XPS,XRD,SEM-EDS,FT-IR,the crystal phase,morphology,groups and chemical composition of samples were studied.Arsenate-substituted Sodium Iron Phosphate Hydroxide solid solution synthesized under the best experimental conditions had the highest crystal crystallinity.On the other hand,its arsenic leaching concentration was lower,which might be related to the degree of chemical bonds between atoms within the crystal.(4)The arsenic toxicity leaching characteristics of Arsenate-substituted Sodium Iron Phosphate Hydroxide solid solution synthesized under different conditions were investigated.The arsenic leaching concentration was increased firstly,then decreased,and remained stable when the leaching time arrived 30 d.In the neutral environment,the arsenic leaching concentration and arsenic leaching rate μ were 3.56 mg·L-1 and 0.42%,respectively.In the weak acid and weak alkaline environments,the arsenic leaching concentrations were 4.12 mg/L and 3.95 mg/L,respectively.Meanwhile,the arsenic leaching rate μ were 0.49%and 0.46%,respectively.The arsenic leaching concentration in the neutral environment was lower than those in the acid or alkaline environment,which indicated that Arsenate-substituted Sodium Iron Phosphate Hydroxide was the most stable in the neutral environment.(5)The phase composition,morphology and chemical composition of samples before and after dissolution were compared and analyzed by XRD and SEM-EDS.The results showed that the phase and elementary composition had no change,and crystal surface characteristics just had a little change.