A study of modified mesoporous silica for the adsorption of selected organic pollutants.

Abstract

Water is an essential natural resource the world over. Only one-tenth of the world’s water is pol-lution free and hence there is a need for remediating polluted water. These pollutants, some of which are organic (dyes and pharmaceuticals), are often difficult to remediate from wastewater. Of all the available remediation techniques, adsorption is a proven technique that combines ease of use and cost effectiveness in removal of these organic pollutants. This study is aimed at the synthesis and characterisation of mesoporous silica/ordered mesoporous silica from agrowastes and commercial precursors, and their utilization in adsorption of organic pollutants. Silica was modified with citric acid, graphene oxide and graphene because of silica’s ready availability, low cost and its environmentally benign nature. The synthesized materials were charactersied by means of various standard analytical techniques. In the first study, silica nanoparticles (SNP) and silica nanotubes (SNT) were synthesized from elephant grass. Parameters such as surfactant con-centration and temperature were investigated on the morphology of mesoporous silica which yielded SNTs and SNPs. These parameters, especially the surfactant concentration had a pro-found effect on the morphology of silica. SNT and SNP were applied in the adsorption of 2 dyes; methylene blue (MB) and methylene red (MR). The adsorption of MB and MR on SNT were 109.97 mg/g and 40.6 mg/g, respectively. For SNP, MB and MR adsorption was 104.85 mg/g and 40.98 mg/g, respectively. The adsorption of MB and MR increased with an increase in pH with pH 6 and 8 being the optimal pH for MR and MB, respectively. This slight difference in sorption capacities can be attributed to the surface area. Pseudo-second order and the Langmuir isotherm model best fitted the data obtained. In the second study, mobil catalytic material (MCM-41) was synthesised using tetraethylorthosilicate and elephant grass as precursors. Citric acid was used a modifier and applied in the adsorption of methylene blue. The citric acid had a significant effect on the adsorption capacity, which increased at basic pH. The mechanism for adsorption was electrostatic ion interaction. The maximum adsorption capacity of the modified material improved considerably with a qm of 204.08 mg/g at pH 10 and a temperature of 25 °C. The adsorption kinetics favoured the pseudo-second-order model and the best fit model for the equilibrium data was the Freundlich model. In the third study, MCM-41 was synthesized from millet straw (agrowaste). A citric acid-MCM-41 (SCA-MCM-41) composite was prepared by grafting citric acid (CA) onto MCM-41 for improvement of its adsorptive capacity for methylene blue (MB). The highest adsorption capacity was achieved at pH 8. The Temkin isotherm model was the best fit in analyzing the equilibrium data. Kinetics favoured the pseudo-second order and the optimum temperature for adsorption was 25 oC. In the fourth study, SNT was synthesized using elephant grass as a precursor and was encapsulated with graphene oxide and graphene and applied in the remediation of sulfamethoxazole from aqueous media. The highly hydrophobic materials had high adsorptive capacities for sulfamethoxazole. In this study, ordered mesoporous silica-graphene oxide (MCM-41-GO) and graphene (MCM-41-G) composites and as-synthesised MCM-41 were synthesized and applied in the adsorption of acetaminophen and aspirin from aqueous solution. Elemental analysis showed that the percentage carbon was MCM-41-G > MCM-41-GO > AS-MCM-41 > MCM-41. This conferred various degrees of hydrophobicity on these materials and hydrophobic interaction was the main mechanism of adsorption. The pseudo-second order model best fitted the adsorption kinetics and the Freundlich isotherm best described the equilibrium. These materials had higher sorption capacities as compared to unmodified MCM-41. In the last study, MCM-48 encapsulated with graphene and graphene oxide, and as-synthesised MCM-48 were synthesized and applied in the remediation of caffeine and phenacetin from wastewater. The results obtained were similar to that of the previous study and hydrophobic interaction was the main mechanism of adsorption. Overall the modification of mesopo-rous/ordered mesoporous materials proved very useful in the remediation of organic pollutants from aqueous media.