Functionalized and hybrid silica-based sorbents for Cu(II) removal from water: a review

Silica (SiO₂)-based sorbents, with their high surface area, porosity, chemical stability, and ease of surface modification, have emerged as promising materials for the selective and efficient removal of Cu(II) from aqueous environments. This comprehensive review examines recent advances in the synthesis, functionalization, and application of silica (SiO₂)-based adsorbents for copper ion sequestration. The specific surface areas of the reviewed silica-based sorbents generally range from approximately 50 to over 700 m²/g; however, no simple correlation exists between surface area and Cu(II) adsorption capacity, as adsorption performance is primarily determined by surface functionalization, active-site density, and pore accessibility. The reported Cu(II) adsorption capacities of the reviewed silica-based sorbents vary widely, from approximately 19 to 870 mg/g, reflecting differences in surface chemistry, functional-group density, pore accessibility, and experimental conditions. The highest capacities are generally associated with polymer-silica hybrids and highly functionalized materials containing dense and accessible amino, polyamine, thiol, EDTA, or Schiff-base binding sites, whereas unmodified silica and materials with low active-site densities typically exhibit substantially lower capacities. Most sorbents show optimal Cu(II) uptake within pH 4-7, with the most frequently reported optimum range being pH 5.0-6.5. Adsorption decreases at lower pH because of functional-group protonation and competition with H⁺ ions, while at higher pH, copper hydrolysis and possible Cu(OH)₂ precipitation may contribute to the apparent removal. The influences of critical operational parameters (e.g., pH and competing ions) on adsorption performance are systematically analyzed, alongside discussions of adsorption isotherms, kinetics, desorption processes, and potential for column applications.