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BACKGROUND A powdered zeolitic material synthesised from fly ash (FA) (NaP1‐FA) and its calcium modified form (CaP1‐NA) were studied as sorbent materials for the recovery of phosphate from treated waste‐water effluents. Phosphate‐sorption equilibrium experiments were performed by varying the experimental conditions, including solution pH, phosphate concentration, and the presence of competing ions. RESULTS The maximum phosphate‐sorption capacities were 57 ± 5 and 203 ± 11 mgP–PO4 g−1 for NaP1‐FA and CaP1‐NA, respectively. The sorption capacities of both zeolites in the pH range expected for waste‐water effluents (pH 7 to 9) were slightly dependent on pH, exhibiting maxima at pH 8. Phosphate removal proceeds through two main mechanisms: (a) surface complexation with ≅AlOH and ≅FeOH groups of the zeolitic structure or unreacted minerals from the FA; and (b) the formation of Ca–phosphate phases, mainly brushite. CONCLUSIONS The removal mechanisms were confirmed by XRD analyses and P speciation. The stabilities of the phosphate‐loaded zeolite samples evaluated by the extraction experiments confirmed their potential availabilities in soil applications. Finally, the higher solubility of brushite compared with that of Hap makes this zeolitic material promising as a novel inorganic zeolite/CaP1‐NA/brushite fertiliser. © 2015 Society of Chemical Industry

16In this work, phosphorous recovery as hydroxyapatite (Ca 5 (PO 4 higher, and also the mean size and degree of crystallinity of the prepared particles increased. 31

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The use of low-cost inorganic sorbents as a new sustainable strategy to enhance the valorization of nutrients (N-P-K), from the urban water cycle (e.g., side streams from sewage sludge anaerobic digestion), in agriculture applications is presented. The simultaneous recovery and stabilization of ammonium and phosphate by using a mixture of two reactive sorbents (Na and K zeolites and magnesium oxide) was evaluated. The nutrients stabilization process, favoured at alkaline pH values, is carried out by a) the precipitation of phosphate ions with magnesium and/or ammonium ions and b) the sorption of ammonium by Na- and K-zeolites. MgO(s) promoted the stabilization of phosphate as bobierrite (Mg(PO)(s)) or struvite (MgNHPO(s)) depending on the applied dose. Doses with the stoichiometric molar ratio of Mg/P promote the formation of bobierrite, while molar ratios higher than 3 favour the formation of struvite. Na zeolites (NaP1-NA, NaP1-IQE) demonstrated efficiency on ammonium stabilization between 60±2 (for 15gZ/L) to 90±3% (for 50gZ/L). The ammonium recovery efficiency is limited by the zeolite sorption capacity. If the target of the fertilizing criteria should include K, then the use of a K-zeolite (e.g., 5AH-IQE) provides a good solution. The optimum pH for the precipitation of struvite and bobierrite is 9.5 and the optimum pH for ammonium removal is between 4 and 8.5. N is present in higher concentrations (up 0.7-1gNH/L) when pH is ranged between 8.2 and 8.6. The ammonium recovery ratios were better than those previously reported using only magnesium oxide or even a more expensive reagent as newberrite (MgHPO(s)). The recovery mechanisms described generate low-solubility stabilized nutrients forms that potentially can be applied as slow-release fertilizers in agriculture. Thus, the use in agriculture of blends of digested sludge with low-solubility stabilized nutrients forms will improve soils quality properties in terms of organic matter and nutrients availability.

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