A Method to Increase the Adsorption Properties of Silica at Acidic pH Using Iron Oxide Nanoparticles

Silica is a frequently used material for removing organic contaminants, dyes, and metal ions from wastewater due to its large specific surface area and ease of preparation at various porosities. In a wide range of pH values, a negative charge is created on its surface as a result of the deprotonation of silanols, which causes its strong adsorption properties in alkaline media. It is found that a composite material consisting of mesoporous silica with iron oxide nanoparticles, which have a point of zero charge (PZC) much larger than silica, can exhibit strong adsorption properties in acidic media as well. This is confirmed by the analysis of the Zeta potential and a significant increase in the adsorption of methylene blue by modified silica at pH values of 4 and above. It is also showed that the microscopic mechanism underlying this behavior relies on the coexistence of two length scales: the Debye shielding range for Coulombic interactions and the average pore size where the iron oxide nanoparticles are synthesized.