Mg²⁺- Modified geopolymer induced struvite crystallization for nitrogen and phosphorus co-recovery and its synergistic lead immobilization

Abstract

Nitrogen and phosphorus pollution-induced eutrophication and soil heavy metal contamination are two critical global environmental challenges. Struvite precipitation can not only simultaneously removes ammonia nitrogen (NH₄⁺-N) and phosphate (PO₄^3--P) from water but also generates struvite for heavy metal immobilization. However, the small crystalline particles generated causes the difficult collection of struvite. In this study, a MgCl₂ modified metakaolin-based porous geopolymers (Mg²⁺-MKG) were prepared via a simple impregnation method and used as both magnesium source and crystal seed to induce the crystallization of struvite. The effects of MgCl₂ content, adsorbent dosage, pH, nitrogen/phosphorus (N/P) ratio, and initial NH₄⁺-N/PO₄^3--P concentrations on adsorption capacity were systematically evaluated. The maximum adsorption capacities of 105.91 mg·g⁻¹ (7.565 mmol·g⁻¹) for NH₄⁺-N and 168.05 mg·g⁻¹ (5.421 mmol·g⁻¹) for PO₄^3--P were achieved under the optimal conditions: 0.4 g·L⁻¹ 20Mg²⁺-MKG dosage, pH= 8, N/P molar ratio 2.2:1, and initial NH₄⁺-N/PO₄^3--P concentrations of 100 mg·L⁻¹. Kinetic and isothermal adsorption data for NH₄⁺-N and PO₄^3--P were well described by the pseudo-second-order and Langmuir models, respectively, indicating that this adsorption process was chemisorptive and spontaneous characteristics with exothermic behavior. NH₄⁺-N adsorption mechanisms included physical adsorption, cation exchange between NH₄⁺ and Mg²⁺, electrostatic interactions, and struvite formation, while PO₄^3--P removal occurred predominantly via struvite precipitation. Additionally, the geopolymer/struvite composite generated as a byproduct during NH₄⁺-N and PO₄^3--P co-adsorption process exhibited superior Pb immobilization performance in contaminated soils.