Preparation of Mixed Metal Hydroxide Ash-Derived Adsorbent from Coal Fly Ash and Quicklime for Removal of Pb²⁺, NH₄⁺, and PO₄³⁻ from Aqueous Solution

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Preparation of a novel adsorbent, mainly composed of Si–(Al, Fe)–Ca mixed hydrous oxides, from a mixture of coal fly ash and quicklime (CaO) was attempted. Coal fly ash was mixed with NaOH powder and heated to 600°C for 6 h in an electric furnace, and after cooling to room temperature, the produced fused coal ash was mixed with quicklime. The mixture was then stirred in distilled water at room temperature for one day to prepare the adsorbent. With increase in CaO addition to the fused fly ash, the adsorbent changed from an amorphous material to a mixture of calcite [CaCO₃] and amorphous phases, then a mixture of calcite and hydrocalumite [Ca₄Al₂O₈(CO₃)·11H₂O], and finally that of calcite, hydrocalumite, and portlandite [Ca(OH)₂]. The removal ability of the adsorbent for NH₄⁺ is almost the same and that for Pb²⁺ and PO₄³⁻ increases with increasing CaO addition. The removal of NH₄⁺, PO₄³⁻, and Pb²⁺ depends on the pH of the solution. The adsorbent containing hydrocalumite without portlandite indicates good ability for multifunctional removal in neutral solution and fixation. Fly ash contents, Si and Al, in the adsorbent contribute to NH₄⁺ removal by ion exchange of Si–Al amorphous gels, PO₄³⁻ removal by synthesis of hydrocalumite to form hydroxyapatite, and Pb²⁺ removal by formation of PbSi₂O₅·1.6H₂O. The kinetics of the removal of NH₄⁺, PO₄³⁻, and Pb²⁺ using this adsorbent follows the pseudo-second-order kinetics model rather than the pseudo-first-order kinetics model, and the removal of Pb²⁺ is faster than that of NH₄⁺ and PO₄³⁻. These results indicate that a novel adsorbent with removal abilities for NH₄⁺, Pb²⁺, and PO₄³⁻ can be prepared from coal fly ash and quicklime, and suggests a new recycling method for industrial wastes.