Efficiency And Mechanism Of Phosphate Removal By Combined Use Of KMnO₄ And FeSO₄

With the increasing human activities, the amount of industrial wastewater and domestic sewage was gradually increased. However, the sewage treatment rate had not been significantly increased, leading to deterioration of water quality and destruction of aquatic ecosystems. Eutrophication is one of the most important water pollution problems in the world. Phosphate is the most important factor for eutrophication. Control of phosphate concentration is the key way to solve the eutrophication. In this study, based on the hot issue of the problem of excessive phosphate concentration from secondary effluent, a new chemicsl process for phosphate removal (KMnO4-FeSO4 process) was developed. The KMnO4-FeSO4 process overcomes the poor performance of FeSO4. In addition, compared with traditional iron coagulants, the KMnO4-FeSO4 significantly could improve the phosphate removal in secondary effluent. It provided an effective technical support for water quality and had important practical significance. In this paper, efficiency of phosphate removal, factors and reaction mechanism in KMnO4-FeSO4 process were studied. In addition, the phosphate and multi-component pollutant removal from secondary treatment effluent were investigated. It provided the necessary theoretical foundation and technical reserves for the application of technology.Results showed that the effect of phosphate removal in Oxidant-FeSO4 process (including H2O2-FeSO4 process, NaClO-FeSO4 process, KMnO4-FeSO4 process, O3-FeSO4 process and O2-FeSO4 process) decreased with the increase of the pH and increased with the increase of the amount of iron dosages. In H2O2-FeSO4 process, NaClO-FeSO4 process and KMnO4-FeSO4 process, the phosphate removal gradually increased with the increasing of molar ratio of oxidant and FeSO4. In addition, the removal of phosphate by iron formed aging was significantly lower than that by iron formed in situ in Oxidant-FeSO4 process. It was considered that the removal of phosphate by oxidant-FeSO4 process was due to the complex role of the co-precipitation and adsorption.On the basis, the removal of phosphate in KMnO4-FeSO4 process was investigated. Results showed that compared with Fe2(SO4)3 and FeSO4, KMnO4-FeSO4 process significantly increased the effectiveness of phosphate removal in the solution. Stirring speed had little effect on phosphate removal in KMnO4-FeSO4 process. The efficiency of phosphate removal by KMnO4-FeSO4 process increased with the increase of iron dosage and decreased with the increase of initial phosphate concentration. The phosphate removal in KMnO4-FeSO4 process decreased with the increase of pH value. At lower pH, the turbidity in the solution had a little effect on phosphate removal in KMnO4-FeSO4 process. At higher pH, phosphate removal significantly increased. The presence of NO3-and Cl- had no influence on phosphate removal in KMnO4-FeSO4 process. The presence of SO42-and HCO3- inhibited the phosphate removal in KMnO4-FeSO4 process, but inhibition was limited. At lower pH, the presence of SiO32- promoted the efficiency of phosphate removal in KMnO4-FeSO4 process. At higher pH, the phosphate removal could be significantly decreased. In addition, the presence of SiO32- reduced the surfaceζpotential of floc particles formed in situ and increased the amount of residual iron in the solution. In contrast, Ca2+ and Mg2+ significantly promoted the effectiveness of phosphate removal in KMnO4-FeSO4 process. Similarly, Ca2+ and Mg2+ increased the surfaceζpotential of floc particles formed in situ and decreased the amount of residual iron in the solution. In addition, SiO32-, Ca2+ and Mg2+ had a certain influence on surface features for floc particles formed the KMnO4-FeSO4 process. Complexing agent such as EDTA, sodium oxalate and sodium citrate, there was a certain inhibition for reaction of coagulation and phosphate, and increased the amount of iron remaining in solution. At higher pH, humic acid significantly decreased the effectiveness of phosphate removal in KMnO4-FeSO4 process. Moreover, the presence of humic acid could decrease the surfaceζpotential of floc particles formed in situ and increase the amount of iron residual in the solution.KMnO4 and MnO2 formed in situ had no effect on phosphate removal. FeSO4 had a little effect on phosphate removal in KMnO4-FeSO4. Fe/Mn oxide formed in situ had the dominant factor on the removal of phosphate. Phosphate removal by KMnO4-FeSO4 process was a complex process of precipitation and adsorption. In addition, compared with Fe2(SO4)3, KMnO4-FeSO4 process could improve the removal of phosphate. At higher pH values, the KMnO4-FeSO4 process could reduce the amount of residual iron in the solution.The floc particles structure formed in situ in KMnO4-FeSO4 process was amorphous and had a large specific surface area. In the presence of phosphate, surface of floc particles had a certain change. XRF analysis showed that composed elements of the floc particles surface were mainly O, Fe, P and Mn. FTIR analysis indicated phosphate and Fe/Mn oxide formed in situ by KMnO4-FeSO4 process was the inner compound, not surface adsorption alone. XPS analysis showed that Fe and Mn elements in sediment by KMnO4-FeSO4 process existed mainly in the form of FeOOH and MnO2.In the sewage treatment plant for high phosphate concentration, phosphate removal by KMnO4-FeSO4 process was investigated. Compared with traditional FeCl3 and Fe2(SO4)3, KMnO4-FeSO4 process could significantly improve the phosphate removal in secondary effluent. The removal of TOC, COD and UV254 by KMnO4-FeSO4 process were higher than the traditional ferric coagulant FeCl3 and Fe2(SO4)3. The economy comparative analysis showed that KMnO4-FeSO4 process had economic advantages compared with traditional FeCl3 and Fe2(SO4)3.Conclusively, the integrated technology of KMnO4-FeSO4, with several mechanisms as oxidation, adsorption and precipitation, exhibits good efficiency of removing pollutants from secondary effluent. On the other hand, this technology, being highly effective, economical and convenient to be employed, is promising to treat polluted water and enhances pollutants removal from secondary effluent, and is especially suitable to be employed in China.