Arsenic Precipitation In Arsenic-containing Wastewater Based On Scorodite Formation

Arsenic is one of the major associated elements in non-ferrous metallic ores; the amount of arsenic pollutants yielded by smelting is up to about50,000tons a year. Arsenic compounds are highly toxic, carcinogenic, and very harmful to human health. Therefore, it is very necessary to convert arsenic to harmless and stable compounds.In this study, scorodite, an arsenic compound (FeAsO4-2H2O) that has a high arsenic rate, a crystal structure and a low leaching toxicity, can be precipitated from arsenic-containing wastewater by air oxidation using ferrous sulfate. Thermodynamic analysis of Fe-As-H2O system at high temperature, optimization of precipitation condition, characterization of scorodite, leaching toxicity of scordite and the scorodite crystal growth kinetics were investigated in this paper. The main results were showed as follows:1) Based on the thermodynamic analysis and electrochemical principles, the potential-pH diagram of Fe-As-H2O system at high temperature (t=95℃, a=1) was drawed. This diagram showed that the theoretical stable range of FeAsO4was0.03<pH<5.17. Experimental results showed that under weak acidic conditions (pH=5-6), the arsenic compounds with crystal structure could be obtained when reaction time was above5h. If the reaction time was too short, precipitates contained both scorodite FeAsO4-2H2O and amorphous ferric arsenate FeAsO4-(2+x) H2O. Amorphous ferric arsenate would be gradually transformed into the crystal scorodite if extending reaction time.2) The effects of initial pH value of arsenic, reaction time, reaction temperature, air flow rate, iron-arsenic molar ratio and initial arsenic concentration on precipitation of scorodite were investigated. The leaching toxicity analysis of these precipitates was carried out. The results showed that the optimum conditions for preparation of crystal scorodite were as follows:initial pH4, temperature95℃, reaction time7h, air flow above120L/h, iron-arsenic molar ratio1.5, initial arsenic concentration lOg/L. 3) The precipitation of scorodite follows "oxidation-crystal precipitation" mechanism, and can be analyzed by Avrami-Erofeev equation. The whole crystallization process can be divided into nucleation period and crystal growth period.4) Initial pH value and reaction temperature have remarkable influences on the growth of scorodite crystal. The apparent rate constant of nucleation process and crystal growth process increased along with the improvement of initial pH value. Increasing the temperature could promote the precipitation reaction because the apparent rate constant of nucleation process and crystal growth process increased while the temperature increased.5) At nucleation stage, slightly excess of Fe can promote the formation of new nuclei because the apparent reaction rate constant of this period increased along with the increase of the iron-arsenic molar ratio. But the apparent reaction rate constant reduced as the increase of the iron-arsenic molar ratio at the crystal growth period.