Binder-free clinoptilolite monoliths prepared by cold consolidation: structure and ammonium uptake

Natural clinoptilolite is widely studied for ammonium removal due to its high cation-exchange capacity, low cost, and chemical stability. However, its large-scale application in powdered or packed-bed forms remains limited due to poor mechanical robustness, post-treatment separation, and attrition losses. In this work, we demonstrate a binder-free, room-temperature, pressure-assisted consolidation route that converts clinoptilolite powder into mechanically robust, self-supported porous monoliths, preserving its intrinsic ammonium-exchange functionality. Unlike classical cold-sintering approaches, which rely on dissolution–precipitation and densification mechanisms, the present method employs a moderate uniaxial pressure of ∼200 MPa and a low water content of ∼3 wt% to induce particle rearrangement and interlocking without chemical activation or framework reconstruction. The resulting monoliths exhibit bending strengths of up to ∼10 MPa, smooth surfaces (with an average roughness of 30–40 nm), stable water fluxes of 20–50 L m⁻² h⁻¹, and a bimodal nanometric pore structure that supports continuous adsorption. Dynamic adsorption experiments reveal that the cumulative ammonium uptake of cold-consolidated monoliths is 45–50 mg g⁻¹, comparable to that of the parent powder under sufficiently long residence times, indicating that consolidation does not compromise ion-exchange performance. Long-term adsorption (up to 200 h) further demonstrates stable hydraulic behavior and pH-triggered adsorption/stripping. By decoupling mechanical shaping from high-temperature sintering and chemical binding, this study provides a practical pathway for transforming natural clinoptilolite from an unmanageable powder into a deployable, monolithic sorbent for continuous, reliable, and affordable ammonium removal in water and wastewater treatment applications.