| The wastewater from sulfuric acid processing industry is highly acidic,containing high concentrations of arsenic and cadmium, and is named “dirty acidwastewater”. The wastewater if discharged untreated could destroy the ecosystem andendanger human health through food chain and drinking water. Therefore, varioustreatment processes have been proposed for this acid wastewater to eliminate thecontamination. Among these, combination of lime neutralization with ferric ironcoagulation has been recognized as the best choice. However, it has becomeincreasingly unsustainable for the non-ferrous metal metallurgy industry, because ofthe expensive costs for the disposal of large amounts of As-rich sludge and theensuing secondary pollution.Thus, the combination of sulphide precipitation with lime-ferric precipitationwas proposed for the treatment of acid wastewaters by a metal smelter company ofHubei province, China. After sulphide precipitation, the concentration of As wasdecreased from3000~9000mg/L to20~50mg/L. To satisfy the current environmentalregulations, the arsenic and cadmium removal efficiency from the acid wastewaterafter sulphide precipitation was evaluated by lime neutralization and the combinationof lime with ferric iron was investigated in the present study. The optimal process wasevaluated on an industrial scale. Further, the mechanism of As and Cd from the acidwastewater with ferric precipitation was also investigated by Zeta, FTIR and a surfacecomplexation model CD-MUSIC.From the experiment for treatment technology, it was found that the residuallevels of arsenic, copper, lead and zinc leached out of the arsenic-containing solidwaste from the combination of lime with ferric iron treatment process could satisfythe limits specified by the hazardous waste discharge standard (GB5085.3-2007)Thus, the combined treatment process is a preferred process because of its higherefficiency and harmlessness compared with single lime neutralization. Thecorresponding parameters were also evaluated to achieve the optimized arsenic removal efficiency. The results indicate that the acid solutions were neutralized withlime in two hours to a final pH of2.0~3.0in the first stage; the second-stage involvedCa(ClO)2oxidation with a Ca(ClO)2/As molar ratio of6, ferric flocculation with aFe/As molar ratio of8, and lime neutralization to a final pH of8, within an hour. Afterthe two stages, the residual concentration of arsenic could be below the tolerance level(0.3mg/L) without hazardous precipitates. It was noted that the main content of thedried sludge from the first neutralization process is CaSO4with high concentration of84.1%, which is more than the regulation values (≥75%) of the identificationstandards for by-product gypsum used in cement (GB/T21371-2008) of the Ministryof Land and Resources of PRC. Thus, the produced sludge can be used as additivesfor producing concrete.The mechanism of arsenate (As(Ⅴ)) and cadmium (Cd(Ⅱ)) in the process ofiron coagulation was investigated, it was found that the effect of As and Cd wasdetermined by pH of the solution. At the range of pH at3.0~7.0, the existence of Assurface complexes were≡FeOAsO3H-1.5and FeOAsO(H2O)2-0.5, and≡FeOHCd+1.5for Cd, therefore, there was a interference from each other by the competing theacting site on the iron oxide. Accordingly, above7.0pH levels,≡FeOAsO3H-1.5waspredominance and Cd(OH)2gradually produced, which could increase the As removalby co-precipitation. |
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