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Adsorption represents a well-documented, effective and reliable means for phosphorus (P) recovery from waste streams, and is often followed by chemical precipitation for attaining value-added fertilizers or feedstocks. To assess the feasibility of recovering P from water by combining batch adsorption enrichment with struvite crystallization, we prepared four ternary layered double hydroxides (LDHs) with P-preferring elements (i.e., zirconium (Zr) or lanthanum (La)) via a facile coprecipitation method, and then evaluated their performance in capturing P from water, particularly in enriching P from a low-level P solution. We find that P adsorption on all ternary LDHs is pH-dependent and ionic strength-independent, showing a maximum adsorption efficiency at pH ~5 regardless of the ionic strength. Besides, all ternary LDHs demonstrate remarkably high P adsorption capacities, i.e., 842.2, 958.8, 499.6 and 1029.3 mg P g−1 under a certain condition for ZnFeZr, ZnFeLa, ZnAlZr and ZnAlLa, respectively, outperforming other LDHs reported so far. Microstructural analyses show that all ternary LDHs have high stability against the acidic or basic solution, and that the P uptake mechanisms are attributable to anion exchange between P and intercalated nitrate ions, complexation at both the edge of LDHs and the surface of metal (hydr)oxides co-occurred, and electrostatic attraction. Results of recycling tests indicate that all ternary LDHs present good enrichment for P, with enrichment factors above 2.6 after only five adsorption-desorption cycles. In addition, more than 96% of the phosphorus in the P-enriched eluates can be efficiently reclaimed via struvite crystallization in a fluidized bed reactor at an Mg:N:P ratio of 2:5:1 in the feed solution. These findings demonstrate the feasibility of combining adsorption enrichment with struvite crystallization for P recovery.

Fly ash-based geopolymer is a crucial application for tackling climate change, limiting the production of greenhouse emissions. The main aim of this study was to determine the effect of the fundamental parameters on compressive strength to ensure a feasible and effective solidification of mine tailings. Three sodium hydroxide (NaOH) molarities (5, 8, and 10 M) and sodium silicate (Na 2 SiO 3 ) were combined to form the alkaline solution. Four fly ash (FA) proportions (28%, 44%, 54, and 61%) of the dry mix and mine tailings (MT) were utilized as raw materials. This study demonstrated that both 25 °C and 65 °C have a considerable effect on the mechanical properties of geopolymers. The UCS value increased with an increase in NaOH molarity. In addition, the highest Unconfined Compressive Strength (UCS) value was achieved (36.04 MPa) at 10 M. Furthermore, the results also showed that UCS values kept decreasing with the increase of SiO 2 /Al 2 O 3 and Na 2 O/ SiO 2 ratios. The optimal UCS values found were in the range of 0.28-0.38 liquid/solid ratio. It has been concluded that the previously mentioned parameters have a strong influence on the mechanical strength of fly ash-based geopolymer with the new proposed FA proportion.

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