Continuous-flow treatment of acid mine drainage by cold-consolidated clinoptilolite monoliths: Cu2⁺ uptake, structural resilience, and suppression by trivalent cations

Acid mine drainage (AMD) treatment remains difficult because of highly acidic conditions, high dissolved-metal loads, and intense multi-ion competition, which limit selective removal and recovery. While natural zeolites are attractive, low-cost sorbents, their use in the continuous treatment of mine waters is constrained by the hydraulic limitations of powders and by uncertainty regarding their behavior in strongly acidic, multi-metal systems. In this study, binder-free clinoptilolite monoliths prepared by cold consolidation were investigated as structured continuous-flow sorbents for Cu 2+ removal from AMD-like solutions containing Al 3+ and Fe 3+ as representative trivalent gangue cations. Single-pass adsorption experiments were carried out at pH 3 and 4, with a hydraulic flux of about 60 L m −2 h −1 . Breakthrough curves were interpreted in conjunction with cumulative equivalent uptake/release balances, and post-treatment stability was evaluated by aluminum leaching, Si/Al ratios, X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectra, and scanning electron microscopy (SEM). The monoliths maintained stable permeability throughout operation and resisted bulk acid-induced degradation: aluminum release remained very low, the Si/Al ratio stayed within the range expected for intact clinoptilolite, and the heulandite-type (HEU) framework was preserved. The uptake mechanism was strongly pH dependent. At pH 4, dissolved-metal uptake was approximately balanced by release of native cations, indicating predominantly stoichiometric ion exchange. At pH 3, native-cation release exceeded metal uptake, revealing a substantial proton-buffering contribution. Under these more aggressive conditions, Al 3+ and Fe 3+ strongly suppressed Cu 2+ uptake. At pH 3, the presence of 10–100 mg L −1 trivalent competitors caused a near-complete loss of dynamic Cu 2+ breakthrough capacity relative to the single-metal baseline. FTIR and SEM indicate that this suppression is mainly associated with localized interfacial reorganization and the competitive occupation of exchange-accessible domains, rather than bulk framework collapse or severe pore blockage. These results demonstrate that structured clinoptilolite monoliths can couple hydraulic operability with chemical resilience in acidic mine-water environments, and that Cu 2+ retention in AMD is governed by pH-dependent exchange-buffering behavior together with strong trivalent competition at the fluid–solid interface.