Fly ash as reactive sorbent for phosphate removal from treated waste water as a potential slow release fertilizer

•Fly ash from different coal power plants were evaluated as phosphate sorbents.•Sorption proceeds as a combination of dissolution of CaO(s) and brushite formation.•Formation of insoluble Ca-phosphates with limited fertilizing properties is avoided.•Higher P-loadings were obtained at pH values of 8 for both fly ash samples.•P-PO4 availability test confirms the suitability of fly ash as potential fertilizer.

There is interest in recovering phosphate (P(V)) from secondary sources, such as waste water streams for potential use as fertilizers reducing the environmental impacts of P(V) discharges and providing alternative phosphorus sources. The goal of this work was to provide an understanding of P(V) removal by fly ash (FA) from coal power plants. Phosphate removal using Ca(II) rich FA was evaluated in terms of i) sorption equilibrium, ii) sorption kinetics under the expected pH values and P(V) concentrations in wastewaters effluents, and iii) P(V) availability of the FAs in agricultural applications. At the pH values (6-9) expected for wastewater effluents, P(V) removal proceeds as a combination of CaO(s) dissolution and brushite (CaHPO4(s)) formation on the FA particles. This process avoids the formation of relatively insoluble Ca-phosphates, such as, hydroxyapatite (Hap) with limited fertilizing properties. High P-loadings were achieved (up to 50 mgP-PO4/g FA (5% P(V) by weight)) at a pH of 8. The removal kinetics data were well described as a diffusion-based process of phosphate ions (H2PO4− and HPO42−) on FA particles, and the CaO(s) dissolution process was discarded as the rate controlling step. The P(V) availability from loaded samples was determined via an agronomical assay with NaHCO3 solutions with P(V) release ratios of 10-30 mgP-PO4/g in FA, confirming the appropriateness of this material as a potential fertilizer, even in calcareous soils.