Acid-base properties of the surface of silicon oxycarbon composites based on activated carbon and silica gel and their effect on dye adsorption

A mechanochemical synthesis of silicon-oxygen-carbon composites was carried out using mixtures of activated carbon and silica gel in various mass ratios. The resulting materials were found to possess a uniform amorphous structure and well-developed porosity. It was shown that the acid-base surface properties are governed by phenolic, carboxyl, and silanol groups of different configurations, with the dominant contribution coming from groups with pK values of 8.5- 12. An increase in the concentration of these groups accompanied by a decrease in the concentration of groups with pK 0-5 leads to a higher pH at the point of zero charge. It was revealed that, due to electrostatic interactions, the synthesized composites adsorb methyl orange more effectively than methylene blue. The highest adsorption capacity toward methyl orange (286.2 mg/g) was observed for the sample obtained from the mixture with an activated carbon-to-silica gel mass ratio of 1:2.