Causes And Mechanisms Study On The Stability Of Polymeric Ferric Sulfate And Polymeric Ferric Chloride As Coagulants

Polymeric ferric sulfate（PFS）and polymeric ferric chloride（PFC）are very important products in inorganic coagulants.In the coagulation-flocculation process,they can remove the turbidity,SS,COD_Cr,phosphorus and heavy metals.For instance,lower effluent turbidity could be obtained when using PFS or PFC to treat water and wastewater and low temperature has less adverse effect on its flocculation.Industrial production often used sodium chlorate direct oxidation or sodium nitrite catalytic oxidation,and its products are often prone to precipitation during the placement process,especially for high Basicity of the products,which to some extent affected the performance of PFS or PFC.In this paper,the PFS and PFC prepared by the direct oxidation of chlorate were studied.The following results were obtained in this study:（1）The stability of PFS by chlorate method was studied.The relationship between the Basicity and the stability of PFS was investigated,and the Ferron-Timed Spectrophotometry was carried out on PFS with different Basicity.Furthermore,the morphology of the precipitates in PFS were evaluated using FTIR,XRD,TG-DTG and FESEM.The results showed that,the stability of PFS prepared by direct oxidation of chlorate decreased with the increase of Basicity.The Fe_a content in PFS was negatively correlated with Basicity,and the content of Fe_b was not obvious with the change of Basicity,yet the content of Fe_c increased gradually with the increase of Basicity.The Basicity has no effect on the structure and morphology of the precipitates.The precipitates in PFS prepared by NaClO₃ were sodium jarosite,and the precipitates in PFS prepared by KClO₃ were potassium jarosite.（2）Considering the existence of sodium ions in PFS prepared by NaClO₃,the effects of Na₂SO₄,NaCl and NaNO₃ on the stability of PFS were investigated,and the composition and formation mechanism of the precipitates were also probed.The results indicated that the presence of sodium ions can significantly reduce the stability of the products,resulting in a large number of ocher yellow precipitates,especially the largest number of precipitates is occurred in the PFS added Na₂SO₄.The results also indicated that the precipitates in PFS prepared by H₂O₂was an amorphous structure containing Fe³⁺,H⁺,SO₄^2-and OH^-.However,the precipitates in PFS prepared by H₂O₂ added sodium salt were rhombohedra（pseudo cubic）crystals sodium jarosite,which was consisted with the precipitates in PFS prepared by NaClO₃.Above all,we concluded that the existence of sodium ions was an important cause for the instability of PFS.（3）The stability of PFC by sodium chlorate method was studied.The relationship between Basicity and the stability of PFC was investigated,and the precipitates in PFC were characterized by FTIR,XRD and FESEM.The results show that the stability of PFC is negatively correlated with the Basicity.The precipitates in PFC is not a real sense of precipitates,but a colloidal substance that can be dissolved in water.After centrifugation and freeze-drying treatment,the precipitates in PFC with low Basicity are amorphous.But the precipitates in PFC with high Basicity are FeOOH,which may be due to the ferrous hydroxide colloid in PFC is dehydrated during the freeze-drying process.（4）The coagulation experiment of kaolin wastewater was carried out on the dilution of precipitates in PFC.The results indicated that,whether it is a high Basicity or a low Basicity,the dilution of precipitates in PFC still has a good coagulation effect.The turbidity removal rate of wastewater increased with the increase of dosing dose,and the efficiency of water treatment was improved.The pH of wastewater has a great influence on the coagulation effect.Under the acidic condition with pH less than 6,the coagulation effect is the worst,and with the increase of pH,the turbidity removal rate increases rapidly and then tend to be gentle.In the pH range of 7.0 to10.0 can achieve a satisfactory turbidity removel effect.（5）Coagulation-flocculation experiment was used to treat actual river water with the dilution of precipitates in PFC.To minimize turbidity,the interaction between the dosage of diluent,the PAM dosage and the pH on the coagulation effect was investigated by response surface methodology（RSM）.The results indicated that,the quadratic regression model established by RSM can accurately reflect the trend of residual turbidity,and the optimal conditions for the responses of residual turbidity were the diluent dosage of 168 mg/L,PAM dosage of 6.4 mg/L and pH 8.3.