Green synthesis of a novel hybrid sorbent of zeolite/lanthanum hydroxide and its application in the removal and recovery of phosphate from water

• Hybrid sorbent of zeolite/lanthanum hydroxide has been prepared from coal fly ash.
• The waste alkaline solution was neutralized in the green synthesis method.
• The maximum sorption capacity for phosphate reached 71.94 mg/g.
• Ligand exchange mechanism dominated the sorption process.
• Sorbed phosphate could be recovered by treatment with 3 M NaOH at 250 °C.

The traditional process of zeolite synthesis from coal fly ash produces large amount of waste alkaline solution, besides zeolite product. A novel hybrid sorbent, which was composed of zeolite and lanthanum hydroxide (La-ZFA), was produced by using soluble lanthanum chloride to react with waste alkaline solution after the traditional process. This study investigated the capability of phosphate removal by La-ZFA. The sorbent has a high phosphate removal capacity, with a sorption maximum of 71.94 mg/g, according to the Langmuir model. The removal of phosphate by La-ZFA performs well at a wide pH range, reaching > 95% from pH 2.5 to pH 10.5 when initial P concentration < 100 mg/L. Release of OH− during the interaction of phosphate with La-ZFA was observed. The formation of the monoclinic LaPO4 phase was identified by XRD analysis. The uptake of phosphate by La-ZFA was explained on the basis of the adsorption mechanism of the ligand exchange process. The sorbed phosphate could be recovered by hydrothermal treatment in 3 M NaOH at 250 °C, with a simultaneous regeneration of La-ZFA. Repeated sorption–desorption tests indicated that regenerated La-ZFA could be reused for phosphate removal. Therefore, synthesis of La-ZFA could impart additional function of phosphate removal to original zeolite, with the effective utilization of waste alkaline solution.

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