Study On Fluoride And Calcium Removal From Wastewater By Modified Iron-bioflocculant

High concentration of fluoride in industrial wastewater not only causes equipment to corrode, but also pollutes the environment, and damages human health, so removal of fluoride from wastewater is necessary. Moreover, the effluent containing high calcium causes scaling when reused, which influences water recycling. Consequently, it is of importance to study on calcium removal to realize zero discharge. Flocculation is widely used in wastewater treatment, and seeking non-toxic, high efficient, cheap and composite flocculants is the development trend for the wastewater treatment.Iron-bioflocculant modified by carboxymethyl chitosan and phosphoric acid were investigated respectively, synthesizing BPFS-CMC and PBPFS. The optimum flocculation effect of BPFS-CMC and PBPFS were obtained respectively under CMC/Fe mass ratio of1and P/Fe molar ratio of0.1. In addition, BPFS-CMC and PBPFS had better flocculate performance than BPFS. IR, SEM and UV analyses showed that the structure and morphology of BPFS had changed after modification.Removing fluoride from wastewater with modified iron-bioflocculant was studied. The results indicated that the best conditions of BPFS-CMC and PBPFS for treating wastewater containing20mg/L fluoride were as follows:CMC/Fe mass ratio0.05, dosage3mL/100mL, pH value5, reaction time10minutes; P/Fe molar ratio0.2, dosage2mL/100mL, pH value5, and reaction time30minutes. The residual concentration was less than the emission standards of pollutants for lead and zinc industry. After comparing, it was found that PBPFS had the best removal efficiency of fluoride, BPFS-CMC was next, and BPFS was the third. BPFS-CMC adsorbed fluorine on the floc by hydrogen bonding or other ways, then made the adsorption precipitation. Meanwhile, complexation of iron and fluorine was another way for BPFS-CMC to remove fluorine. When PBPFS removed fluorine, a certain quantity of fluorine was introduced into PBPFS, and caused changes of Fe-PO4polymer, forming some chemical structures, such as-Fe-O-Fe-,-P-O-P-,-Fe-F-Fe-and-Fe-P-Fe-By studying calcium removal from wastewater containing calcium of520mg/L with modified iron-bioflocculant, the results suggested that when pH value was10, and reaction time was20minutes, BPFS-CMC and PBPFS got better removal efficiency under the conditions of CMC/Fe mass ratio2and P/Fe molar ratio2respectively. BPFS-CMC could improve the ability of removing calcium to some extent, but dose was big. The mechanism was that BPFS-CMC removed calcium by means of combining calcium with-C=O,-OH and-NH2. When dose of PBPFS was1mL/100mL, the residual concentration of calcium was156mg/L. which was lower than180mg/L and achieved the water quality standard for drinking water sources. The mechanism of PBPFS to remove calcium was that PO43-and-OH interacted with calcium. It could be concluded that PBPFS had the best removal efficiency, and BPFS-CMC was better than BPFS.Based on the wastewater quality characteristics of a smelter, we adopted the technology that neutralized the water first and then added PBPFS to remove fluoride, calcium and heavy metals. When we first adjusted pH to9, controlled P/Fe molar ratio for2, added1mL/100mL of flocculant, rose the second pH to10, and reacted for5minutes, the residual concentration of zinc, lead, copper, cadmium, arsenic, fluoride and phosphorus were0.008,0.133,0.011,0.003,0.06,6.07and0.5mg/L which were less than the emission standards of pollutants for lead and zinc industry. Calcium concentration was92mg/L, achieving the water quality standard for drinking water sources.