Adsorption of organic pollutants by amine functionalized mesoporous silica in aqueous solution. Effects of pH, ionic strength and some consequences of APTES stability

A mesoporous silica with MCM-41 type pore array was synthesized by hydrothermal method and then was functionalized with known amounts of 3-aminopropyltriethoxysilane (APTES) in order to study the selective adsorption towards several species/pollutants. Antibiotics doxycycline hydrochloride (DC) and norfloxacin (NFX), the dye methyl orange(MO) and the pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) were used as adsorbates. The adsorption has been studied under different batch experimental conditions such as contact time, pH and ionic strength. The modification with APTES produced a decrease in the surface area of the solid and reversed the surface charge although did not change appreciably the mosoporosity and shape of the particles. This change in the charge strongly increased the adsorption of species that behave as anions in aqueous solution such as 2,4-D and MO. On the contrary, for species with several polar groups and charge-development the effect was very low and only detectable at specific pH. Electrostatic attractions between the protonated amine-end groups of APTES-SiO2 and the negative functional groups of 2,4-D and MO were the main responsible of the adsorption process. Partial hydrolysis of the supported coupling agent to regenerate silanol groups played an important role in decreasing the adsorption capacity of polar species and in the reuse capacity for adsorbing anionic species, i.e., the performance decreased up to 75% after four cycles of successive adsorption-desorption. Finding new methodology or functionalization agents to improve the silica surface reactivity should be the main goal of the researchers in the future. This work perhaps contributes with such perspectives.