Sustainable Magnetic Nanosilica Composites Derived from Biosilica for Efficient Brackish Water Treatment: Adsorption Performance, Mechanisms, and Regeneration

Sustainable technologies for brackish water treatment are essential for addressing global freshwater scarcity. In
this study, bio-derived magnetic nanosilica composites were synthesized from rice husk ash and integrated with Fe₃O₄
nanoparticles to enable rapid magnetic recovery. Surface functionalization using amine, carboxyl, and chitosan groups was
introduced to enhance divalent-ion adsorption. Structural and physicochemical characterizations confirmed the formation of
mesoporous nanocomposites with a preserved amorphous silica structure, well-dispersed magnetite nanoparticles, and
tunable surface chemistry. Adsorption experiments demonstrated significantly enhanced Ca²⁺ removal by the functionalized
composites compared to unmodified nanosilica. The chitosan-functionalized material exhibited the highest adsorption
capacity and removal efficiency under neutral pH conditions at a salinity of 1000 mg/L while maintaining strong performance
at elevated salinities. Adsorption followed pseudo-second-order kinetics and was well described by the Langmuir isotherm
model, indicating monolayer adsorption on homogeneous active sites. Thermodynamic analysis revealed spontaneous and
endothermic adsorption behavior. The composites demonstrated excellent regeneration stability by retaining high
performance after ten continuous cycles, with magnetic separation efficiency exceeding 98%. Continuous-flow column
experiments further confirmed prolonged breakthrough times for the functionalized materials. The combined advantages of
bio-derived silica, tailored surface functionalization, and magnetic recoverability provide a sustainable platform for selective hardness removal in complex brackish-water systems