Physicochemical properties and mechanism study of clinoptilolite modified by NaOH

•The appropriate concentration of NaOH can create mesoporous and microporous.•NaOH treatment would dissolve amorphous or poor crystallinity Si in clinoptilolite.•Introducing of Na+ would enhance the NH4+-N removal ability of clinoptilolite.

The change of surface area and pore volume distributions, cation exchange capacity, ammonia nitrogen removal ability of clinoptilolite modified by NaOH was investigated. The mechanism of modification was characterized through XRD, FTIR, NMR and XPS. The results indicated that alkalis modification would increase the proportion of mesoporous and the cation exchange capacity of the clinoptilolite. When the concentration of NaOH was 1.50 mol/L, the proportion of mesoporous and the cation exchange capacity were increased from 63.82%, 169.67 mmol·(100 g)−1 to 74.51%, 186.25 mmol·(100 g)−1, while the removal rate of ammonia nitrogen in water increased to 79.66% from original 53.45%. It found that ammonia nitrogen removal ability was positively correlated with the cation exchange capacity. The ICP-OES results showed that the solution emerged K+, Al3+ and Si4+ after modified, but the XRD, FTIR, NMR and XPS solid results showed that the crystal structure, molecular groups, the binding energy of atoms, and Si, Al ligand state of the clinoptilolite had not changed. Therefore, it was considered that the modification mechanism of NaOH was mainly through removal of the silicon which combined weaker with the clinoptilolite framework, thus dissolving out channel impurities and increasing the proportion of mesoporous, meanwhile enhanced the removal ability of ammonia nitrogen in water by the addition of Na+.

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