Study On The Preparation Of Polysilicate Ferrous Aluminum And Its Chromium Removal Performance

In this paper, polysilicate ferrous aluminum（PSFA） was prepared by sodiumsilicate、 ferrous sulfate and aluminum sulfate, meanwhile effects of differentpreparation factors on the performance of removing chromium was investigated.Moreover, this paper had evaluated the major factors affecting coagulationperformance of the self-made flocculant, subsequently discussed the performance ofPSFA and the mechanism of removing chromium preliminarily.In the experiments, it was found that using Ammonium persulfate method inreferences to measure total chromium in the water with ferri ion led to a lower result.Therefore the method was modified by simplifying operation steps and only usingphosphoric acid to adjust acidity, which was effective to avoid the interference offerri ion. The modified method was not only effective to avoid the interference offerri ion, but also have simpler operation and more accurate results.Through investigating effects of different preparation factors of the self-madeflocculant on the removal of chromium and turbidity, the optimal preparationconditions were achieved:（Fe2++Al3+）/SiO₂mole ratio1:1,（Al3+/Fe2+） mole ratio1:2,aging pH value3.0³.1, activation time of silicic acid2h. During storing, PSFAflocculant was gradually polymerized and ferrous ion in the flocculant was slowlyoxidized. However, the results of experiments showed that when the flocculant wasdiluted, its preservation time would be greatly prolonged. Furthermore, the flocculantcould be kept a long time without bad effects on the coagulation performance in asealed condition.PSFA flocculant was effective in removing chromium. For the simulated waterwith initial hexavalent chromium concentration of0.8mg/L, when the dosage of PSFAwas40mg/L, the concentration of residual hexavalent chromium in the effluent wasbelow0.05mg/L, which met the drinking water standards （GB5749-2006）. Comparedwith PFS and PAC, the self-made flocculant performance of removing turbidity wasbetween PFS and PAC, while the performance of removing chromium was muchbetter. Proved by experiments, during the coagulation most of hexavalent chromiumwas reduced to trivalent chromium by ferrous ions in the self-made flocculant, thenboth the action of precipitation and flocculation were effective in removing trivalentchromium. Consequently, PSFA flocculant could remove most of the chromium in the simulated water.The results of researching the major factors affecting the coagulationperformance show that: the optimum pH value was7¹0, and in this case the removalrate of total chromium would exceed90%, due to the strong action of precipitationand flocculation; the optimum water temperature was15³0℃, too high or too lowwere disadvantageous to coagulation; PSFA flocculant had a wide adaptation range ofwater turbidity and a range of turbidity from10to100NTU had little effect oncoagulation performance.In conclusion, PSFA flocculant which was effective in removing chromium, wasapplicable to conventional treatment process in waterorks and could be used as a newmethod for the emergency treatment of raw water polluted by chromium.