Silica–Lignin Composites—Insight into Physicochemical Properties and Adsorption Mechanisms

Two silica–lignin composites with varying biopolymer content were synthesized using the sol–gel method as effective adsorbents for the removal of organic dyes from aqueous solutions. The textural, structural, and surface properties of the synthesized materials were determined using nitrogen adsorption–desorption techniques, XRD, XPS, and FTIR. XPS and XRD analysis confirmed the effective modification of the silica surface with lignin. Increasing the lignin content in the composite altered its physicochemical properties. Textural analysis revealed a decrease in the porosity of the composites with increasing the amount of biopolymer incorporated into the silica. Thermal analysis with the identification of gaseous thermal destruction products allowed for the determination of the thermal stability of the composites. Dye adsorption experiments, including equilibrium and kinetic measurements, allowed the determination of adsorption capacity, adsorption rate and adsorption mechanisms. FTIR mapping enabled the identification of differences in the interactions of selected dyes with the synthesized composites and revealed preferential adsorption on surface areas of the adsorbents rich in lignin or silica. The obtained results confirmed the potential of silica–lignin materials in environmental applications as cheap and biodegradable adsorbents.