Mechanistic study of P retention by dewatered waterworks sludges

Eutrophication caused by excess phosphorus (P) loading poses serious environmental risk to freshwater bodies around the world. Advancing our fundamental understanding towards practical reduction of this risk using novel industrial by-products as P adsorbents is the focus of this study. The study examined the combined effect of solution chemistry and the inherent properties of a novel adsorbent (dewatered waterworks sludges) on their P retention. The overall aim was to contribute to a mechanistic understanding of P retention by the sludges; and to better understand what properties regulate their P retention. Results confirm a strong but variable affinity for P by the sludges. Aluminium (Al)-based sludges generally had higher total specific surface areas; and tended to have higher P sorption capacities (6.09–26.95 mg-P/g) than iron (Fe)-based sludges (5.83–23.75 mg-P/g). In most cases, adsorption data was well fitted with the Freundlich model. However, data for two of the Al-based sludges was best described by the Langmuir model with very minimal leaching of Al, Calcium (Ca) and sulphate (SO4²⁻) ions observed; indicating surface complexation via P binding into the Al hydr(oxide) as the main mechanism for these Al-based sludges.Principal component and multiple linear regression analyses revealed that the metal content (Al, Fe, Aloxalate and Feoxalate) and total specific surface area components had the most significant explanation for the variance of: (i) P-uptake at different initial P concentrations; (ii) the adsorption maxima; and (iii) the Freundlich constant (Kf); (p<0.001). Total carbon (TC), organic carbon, Ca content and exchangeable Ca components explained a significant reasonable variance in P-uptake and Kf. This explanation was demonstrated for the role of Ca content in chemical P precipitation mechanism; and also for exchanging TC sites on the surface of the sludges with phosphate ions via ligand exchange mechanism. Overall, giving the combined effect of intrinsic sludge properties and solution chemistry; dewatered waterworks sludges with high reactive metal content (Al and Fe), Ca and SO4²⁻ ions, and total specific surface area would be the best choice for P retention in practical applications.