Lysine surface modified Fe3O4@SiO2@TiO2 microspheres-based preconcentration and photocatalysis for in situ selective determination of nanomolar dissolved organic and inorganic phosphorus in seawater

A new, sensitive, and rapid spectrophotometric method for selective determination of nanomolar levels of dissolved organic phosphorus (DOP) and inorganic phosphorus (DIP) in seawater was proposed. The microsphere of Fe3O4@SiO2@TiO2 (FST) was surface modified with lysine (FST-Lys) and used as solid phase adsorbent, photocatalyst, and sensor. After surface modification with lysine, the zeta potential vs pH curve for FST could be reversed and then 98.1% of DIP anions in the seawater could be specifically adsorbed by electric attraction onto FST-Lys (1.2 mg/mL) at the pH value of natural seawater (8.1–8.3) after only stirring for 30 min. The adsorbed DIP could be eluted with hydrochloric acid solution (pH 4.0) for electric repulsion. Therefore, acid hydrolysis of DOP could be inhibited and its influence on the determination of DIP could be avoided. DIP could be determined spectrophotometrically and selectively by phosphomolybdenum blue method. After UV irradiation, DOP was photodegraded by FST-Lys into DIP, the total concentration of dissolved phosphorus (i.e., DIP and DOP) could be determined, and then the concentration of DOP could be calculated. FST-Lys could be easily recycled by applying an external magnetic field while maintaining the adsorption and catalytic activity without significant decrease even after running 10 times. The proposed method with high sensitivity (detection limit of 1.42 nmol/L), wide linear range (3.0–1200 nmol/L), and selective determination was successfully applied for in situ measure of DIP and DOP in South China Sea and the Taiwan Channel.

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