| The treatment of alkaline arsenic-containing wastewater is a significant environmental problem in nonferrous smelter which should be resolved emergently nowadays. In this study, a novel technology named as arsenic removal from aqueous system by ferrite coagulation-waste activated sludge adsorption is developed to solve the problems of arsenic-containing wastewater with the characteristics of high pH, high concentration of arsenic and low concentration of other heavy metal ions discharged from a certain nonferrous smelter in western part of Hunan province. The experiments conditions and mechanism of this technology are investigated. The conclusions can be drawn as below:1) When FeSO4 is used as the arsenic removal reagent, the removal rate of arsenic could achieve 55.6% and the optimum reaction conditions at 30 centigrade are pH=5.5, Fe/As=4 and reaction time=4 hours. When FeCl3 is used as the arsenic removal reagent, the removal rate of arsenic could achieve 93% and the optimum reaction conditions at 30 centigrade are pH=5, Fe/As=2 and reaction time=3 hours. When Fe2(SO4)3 is used as the arsenic removal reagent, the removal rate of arsenic could achieve 99% above and the optimum reaction conditions at 30 centigrade are pH=5, Fe/As=2, and reaction time=2 hours. According to removal efficiency, Fe2(SO4)3 is the most effective reagent. So, Fe2(SO4)3 is used as a reagent to treat the arsenic contained wastewater. The proper dose PAM can dramatically improve the performance of precipitate settlement and quality of effluence. The optimum dose is 2mg/L.When Fe2(SO4)3 is used as the arsenic removal reagent, We find that the treatment effectiveness of H2O2 oxidized arsenic-containing wastewater is better than no H2O2 oxidized arsenic-ontaining wastewater. Because of the pureness and effectiveness of H2O2, We use H2O2 as oxidant to oxidize arsenite. The optimum reaction conditions at 30 centigrade are H2O2/As(Ⅲ)=2, reaction time=30minutes. The oxidation rate of arsenite could achieve 93% above.When Fe2(SO4)3 is used as the arsenic removal reagent to treat the H2O2 oxidized arsenic-containing wastewater, the results show that at the conditions of initial arsenic concentration=1000mg/L, pH=5, Fe/As=2, reaction time=2hours and temperature=30℃, the removal rate of arsenic could achieve 99% above and the effluence arsenic concentration is 10mg/L below. If Fe/As≥10, the effluence concentration is 0.5mg/L below, it is such a big diseconomy.2)At the conditions of temperature=30℃, the Fe2(SO4)3 treated wastewater arsenic concentration = 10mg/L, reaction time= 1 hour, pH=7 and the dose of waste activated sludge is 10g/L, The effluence can satisfy the integrated wastewater discharge standard (GB8978-1996). If Fe3+, Cu2+,Pb2+,PO43- are in the arsenic contained aqueous system, the effectiveness of arsenic removal would be better. The arsenic would be desorbed from the arsenic-absorbed sludge at the strong acidic condition and the arsenic- desorbed sludge would be reused. The desorption liquor can be used as neutralization reagent of the former coagulation precipitation process.3)The precipitate of ferrite coagulation is analyzed by XRD, EDX and FTIR. The results show that the precipitate is a noncrystal which contains As-O bond, O-As-O bond, Fe-O-Fe bond, SO42-and -OH.The Fe2(SO4)3 coagulation reaction kinetics is followed by formula as follow:Y=-1.667+0.023X-1.05X2+1.525X3. (Y: reaction velocity, mg/min,X: reaction time, min)The disciplinarian of arsenic adsorption by waste activated sludge can be depicted by Freundlich adsorption model as follow: lgV=-0.32591+0.617691gC.From the above formula, we know that chemisorption plays a key role in arsenic adsorption by waste activated sludge.To sum up, this technology can effectively remove arsenic from the wastewater. The effluence can satisfy the integrated wastewater discharge standard (GB8978-1996). This technology can conquer the shortage of technology of lime neutralization, and the arsenic containing precipitate is very steady which can not cause a second pollution. |
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