Navigating the complexity of colloidal phosphorus in water: Sources, occurrence, transformations, and eco-friendly removal strategies

Colloidal phosphorus (P), defined as P associated with particles within the size range from 1 to 1000 nm, is an often-neglected yet significant component of waterborne P, posing global water quality concerns. This comprehensive review elucidates the various sources and concentrations of colloidal P across diverse water environments, highlighting its prominence in agricultural wastewater, with mean concentrations of 122.6 μg/L (n = 437), compared to groundwater (52.3 μg/L, n = 26), surface freshwater (22.3 μg/L, n = 246), and seawater (5.4 μg/L, n = 36). Correlations between physicochemical water parameters, elemental composition and colloidal P concentrations are obtained, and the dominant subfractions and chemical species of colloidal P in water are identified. Notably from chemical speciation, colloidal organic P exhibits dominance in surface freshwater with higher abundance relative to colloidal inorganic P. Meanwhile, statistical analyses on physical fractions of colloidal P indicate that fine subfractions prevail in agricultural wastewater. Accordingly, this review investigates potential transformation mechanisms of colloidal P due to physical and chemical behaviors such as aggregation, desorption, and photodegradation, influenced by factors such as pH, salinity, coexisting elements and UV irradiation. Drawing on these insights, we propose eco-friendly mitigation strategies, including constructed wetlands or ecological drainage ditches and UV irradiation, aimed at treating water (especially agricultural wastewater) with elevated colloidal P levels. This review not only enhances our understanding of the occurrence and transformations of colloidal P, but also offers practical guidance for its effective environmental management.