Effects of calcined oyster shell powder composited with iron salt on the simultaneous removal of phosphorus and arsenic during algal bloom control process

Abstract

Calcined oyster shell powder (COS) has the potential to serve as an efficient coagulant aid in water treatment for iron salt (IS) due to its potentially high aid capacity in alkalinity recovery and the simultaneous removal effects on algae, arsenic, phosphorus, and organic substances. However, little information is available on the specific effects and environmental impacts of COS composited with IS in coagulation for algal bloom removal. Hence, the COS was used as a coagulant aid to investigate the simultaneous removal effects on phosphorus and arsenic as well as the water environment changes during the bloom control of Microcystis aeruginosa using ferric chloride as a coagulant impacted by different hydraulic disturbances. Results showed that compared to the single IS coagulation with a pH value of about 3.1, the pH value significantly increased and stabilized at 7.5 from the 3rd day with the COS as the coagulant aid condition. Meanwhile, the removal rates of arsenic and phosphorus also improved by 14.4 % and 50.6 %, respectively, within 12 h, achieving near complete removal with little re-release risk, demonstrating excellent alkalinity adjustment and persistently simultaneous removal effects on phosphorus and arsenic. Furthermore, the coagulation aid of COS effectively inhibited the release of fulvic acid-like substances from algal cells on the 7th day. Meanwhile, it promoted the secretion of humic acid-like substances and microcystins production, as well as the release of fluorescent substances under hydraulic disturbance conditions. From the structural information presented by X-ray photoelectron spectroscopy (XPS), the precipitates mainly consisted of proteins and lipids derived from algal cells, calcium carbonate, ferric hydroxide, and trace amounts of arsenite, further confirming their precipitated mechanism of algal cell combined through COS and IS. The results provide scientific support for the practical application of COS in algal blooms and its subsequent environmental risk control.