Removal and recovery of phosphorus from municipal wastewater by adsorption coupled with crystallization

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Phosphorus is both critical and detrimental so it is desirable to develop a process that can not only remove but also recovery phosphorus. As the global phosphate reserve will be exhausted in 50-100 years, there is a need to explore alternatives to phosphate ores. Municipal wastewater is a significant source of phosphorus for recovery due to extremely high volumes and low levels of hazardous substances. This study investigates the feasibility of removing and recovering phosphorus from municipal wastewater by adsorption coupled with crystallization. Adsorbents were prepared from soybean by-product (okara) using metal loading method. The results indicated that zirconium loaded okara (ZLO) was the best among three developed adsorbents. The maximum adsorption capacity of ZLO was 58.93 mg PO₄/g adsorbent. The rapid adsorption was observed with 95% of the removal efficiency in 30 min. Isotherm data was best fitted by Freundlich model while kinetic data was satisfactorily described by Pseudo-second order model. Thermodynamic results revealed that the adsorption was feasible, spontaneous, and endothermic. The solution pH did not affect PO₄³⁻ uptake in a wide range of 2-11. CO₃²⁻ had a profound effect on PO₄³⁻retention, whereas SO₄²⁻, NO₃⁻ and Cl⁻ were trivial inhibitors. Successful desorption and regeneration were achieved with 0.2 M NaOH and 0.1 M HCl, respectively. The ligand exchange was an important pathway for PO₄³⁻ capture by ZLO. The column results showed that the highest dynamic adsorption capacity of ZLO was 50.35 mg PO₄/g adsorbent. Thomas and Bed depth service time (BDST) models were most suitable for the description of the column adsorption behavior. ZLO column could be recycled at least three cycles with a reduction of 18.64% and 8.7% of adsorption capacity and adsorbent weight, respectively. A semi-pilot scale column packed with 100 g ZLO was capable of treating 132.5 L of municipal wastewater to meet the recommended discharge standard (1 mg P/L). The Zr⁴⁺ detachment from ZLO during its operation was negligible. The struvite (MAP - Magnesium ammonium phosphate hexahydrate - MgNH₄PO₄.6H₂O) recovery from desorption solution was most favored at pH 9, Mg: N: P molar ratio of 2:2:1, room temperature, using a combination of MgCl₂.6H₂O and NH₄Cl. The harvested MAP was characterized by 93% MAP and 89% by mass P-bioavailability. Overall, the removal and recovery of phosphorus from municipal wastewater can be achieved by means of adsorption and crystallization. However, further study is necessary to make the process more economically viable.
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