Synthesis of sodium silicate-activated metakaolin geopolymer microspheres (SS@MGMs) for efficient F− removal from acidic wastewater

To address the challenges of inefficient fluoride (F ⁻) removal from acidic wastewater and the limitations of existing adsorbents (e.g., high cost, complex synthesis and structural instability in acidic conditions), sodium silicate-activated metakaolin geopolymer microspheres (SS@MGMs) were successfully prepared with excellent sphericity, uniform particle size (117.52 μm) and high compressive strength (43.76 MPa) for rapid and sustainable F ⁻ removal. The analysis results showed that when SS@MGMs removed F ⁻ (30 mg/L) at dosage of 3 g/L and pH = 1, F ⁻ concentration was <1.5 mg/L within 15 min meeting discharge standard. The adsorption process followed pseudo-first-order and the Langmuir model, indicating chemisorption and monolayer homogeneous adsorption, with maximum adsorption capacity of 211.77 mg/g at 298 K. SS@MGMs exhibited an excellent selectivity towards F ⁻ under conditions of co-existing ions, the removal rate of F ⁻ by SS@MGMs could still be maintained at above 94.73 % even though the concentration of PO ₄ ³⁻ at 14 times of the concentration of F ⁻. Meanwhile, SS@MGMs exhibited excellent adsorption performance in real water matrices with the following order: surface water (94.89 %) > tap water (95.20 %) > seawater (96.13 %). SS@MGMs had a good immobilization effect on F ⁻ (< 1.5 % leaching), and retained 36.71 % of removal rate after four cycles, highlighting certain recyclable performance while effectively avoided the secondary pollution of the environment caused by the adsorbent after the completion of use. Adsorption mechanism revealed F ⁻ removal via ion exchange and chemisorption. This work provides a cost-effective, eco-friendly synthesis way for acidic F ⁻-contaminated wastewater treatment, offering significant potential for sustainable industrial applications.