Phosphorus release from coprecipitants formed during orthophosphate removal with Fe(III) salt coagulation: Effects of pH, Eh, temperature and aging time

Abstract

Although Fe(III) salts coagulation has been widely used for phosphate removal, knowledge about the phosphorus release from the formed Fe-P coprecipitants is limited. This paper investigated the effects of Fe/P molar ratio, aging time, pH, Eh, and temperature on the release of phosphorus and iron in coprecipitants using batch tests. At neutral pH, three kinds of coprecipitants were prepared: unaged FP-1 (Fe/P=3), FP-2 (Fe/P=6) and aged FP-2 for 3 months (Fe/P=6). Results showed that solution pH (4–10) affected the release of phosphorus obviously while its effect on Fe release was slight. Alkaline condition (pH 10) resulted in the greatest release rate of phosphorus due to the desorption of adsorbed phosphorus mainly via ligand exchange, which finally reached about 10% after 48h. Under near-neutral pH and oxidizing conditions (Eh>200mV), P release rates were below 10% and proper aging was favorable for inhibiting P release. Under reducing conditions, the release rates of iron and phosphorus were higher than those under oxidizing conditions. As the Eh ranged from −261mV to −194mV, remarkable P release rates (>15%) began to occur after 48h. Higher Fe/P molar ratio benefited to control the P release. With the rising of water temperature from 10°C to 40°C, P release rates increased. Under reducing conditions and at different temperatures, good linear correlation between Fe and P release hinted the predominant role of Fe for P release. Surface property analyses of coprecipitants including XRD, zeta potentials and FTIR were further conducted.