Interface Evolution and Multi-Process Coupling Mechanisms of Yttrium Recovery from Acid Mine Drainage by Limestone Cores Mediated by Iron and Manganese

Rare earth elements (REEs) are indispensable strategic resources widely used in advanced technologies, including renewable energy systems, electronic devices, catalysts, and high-performance magnetic materials. As global demand for REEs continues to increase, the development of alternative and sustainable resource recovery technologies has become increasingly important. Acid mine drainage (AMD), a common by-product of mining activities, often contains elevated concentrations of dissolved rare earth elements and therefore represents a potentially valuable secondary resource. Among REEs, yttrium (Y) is commonly regarded as a representative heavy rare earth element because of its similar ionic radius, coordination behavior, and geochemical characteristics. Efficient recovery of yttrium from AMD can simultaneously reduce environmental risks and enhance resource utilization efficiency. Although a variety of technologies, including chemical precipitation, solvent extraction, ion exchange, membrane separation, and biosorption, have been applied to recover REEs from aqueous systems, these methods often suffer from high operational costs, complex procedures, or limited applicability under acidic conditions. Limestone, which mainly consists of calcite (CaCO₃), is an inexpensive and widely available material capable of neutralizing acidity and interacting with dissolved metal ions. Previous studies have demonstrated that rare earth elements can be adsorbed onto carbonate minerals; however, the influence of coexisting metals such as iron (Fe) and manganese (Mn) on interfacial evolution and yttrium immobilization mechanisms remains poorly understood. In particular, the coupled effects of mineral dissolution, metal precipitation, aqueous complexation, and surface complexation have not been systematically investigated.In this study, natural limestone cores were employed as reactive media to investigate the recovery behavior of Y(III) from simulated AMD systems. Dynamic flow-through adsorption experiments were conducted to evaluate yttrium removal performance under continuous-flow conditions. Surface and mineralogical changes were characterized using Brunauer-Emmett-Teller (BET) surface area analysis, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Furthermore, PHREEQC geochemical simulations combined with a surface complexation model (SCM) were used to quantitatively analyze yttrium speciation, mineral-water interactions, and adsorption mechanisms. The results demonstrated that neither Fe nor Mn altered the crystal structure of calcite. XRD analyses showed that all samples remained dominated by calcite after treatment. However, significant differences in surface properties were observed. BET measurements revealed that Fe treatment increased the specific surface area of limestone from 13.99 to 24.36 m²·g⁻¹, representing an increase of approximately 74%, whereas Mn treatment produced only a modest increase. XPS spectra confirmed the successful incorporation of Fe and Mn onto the limestone surface and indicated the formation of additional oxygen-containing functional groups. These findings suggest that Fe and Mn were primarily deposited as amorphous or poorly crystalline hydroxide phases rather than forming new crystalline minerals. Dynamic adsorption experiments showed that Fe substantially enhanced yttrium recovery performance. Under Fe-mediated conditions, the concentration of yttrium in the effluent remained nearly zero during the first 100h of operation, indicating extremely strong initial retention and delayed breakthrough behavior. The recovery efficiency exceeded 99% under optimized conditions. In contrast, Mn also improved yttrium removal but exhibited significantly weaker enhancement compared with Fe. These observations indicate that iron plays a dominant role in regulating yttrium immobilization at the mineral-water interface. PHREEQC simulations revealed that calcite dissolution strongly influenced system geochemistry. The initial acidic solution (pH 4.0) evolved toward alkaline conditions (approximately pH 9.2) as calcite dissolved and consumed hydrogen ions. This process released carbonate species into solution and promoted the formation of stable yttrium-carbonate complexes. Speciation calculations indicated that carbonate complexation became the dominant control on dissolved yttrium distribution after reaction. Nevertheless, surface adsorption remained an important mechanism for yttrium immobilization. When Fe was incorporated into the system, additional Fe-OH surface sites were introduced through the formation of hydrous ferric oxide-like coatings. Surface complexation modeling demonstrated that yttrium could form stable inner-sphere complexes with these Fe-OH groups. Compared with adsorption on calcite surfaces, Fe-mediated adsorption exhibited stronger binding strength and greater resistance to competitive effects from Ca²⁺ and carbonate ligands. Consequently, the presence of iron significantly enhanced yttrium fixation stability and reduced fluctuations in effluent concentration during long-term operation. A key finding of this study is that the iron-rich coating traditionally regarded as a passivating layer does not merely inhibit calcite dissolution. Instead, it evolves into a highly reactive interfacial layer that provides abundant adsorption sites for yttrium. This transformation fundamentally changes the role of iron precipitates in AMD treatment systems. The recovery process is controlled by a multi-process coupling mechanism involving calcite dissolution, pH regulation, iron hydroxide precipitation, aqueous carbonate complexation, and surface complexation reactions. These processes collectively determine the migration, enrichment, and stabilization of yttrium within the limestone system. Overall, this study provides new insights into the interfacial evolution and recovery mechanisms of yttrium in limestone-based AMD treatment systems. The results demonstrate that iron-mediated limestone possesses considerable potential for rare earth resource recovery and highlight the importance of reactive iron oxide interfaces in controlling yttrium behavior. The integrated experimental-modeling framework developed in this work offers a useful approach for understanding complex mineral-water interactions and provides a theoretical basis for the sustainable recovery of rare earth elements from acid mine drainage.