Design and synthesis of polymer and organic functionalized porous silica gels extracted from diatomaceous earth and their applications in the treatment of oil impacted water

Abstract

This study aims to synthesize silica gel-based adsorbents for organic contaminated wastewater treatment using diatomaceous earth as the silica precursor. First, the extraction of silica from diatomaceous earth was optimized with the help of response surface methodology (RSM) and then the extracted silica was used to produce the silica gels using non – templated methods. The synthesized porous gels then were surface grafted with selected organic materials such as polypyrrole, acetyl groups and a novel benzimidazolone based polymer. The grafted gels were characterized using techniques such as scanning electron microscopy, Fourier Transform – Infra Red (FT – IR), X – ray powder diffraction (XRD) and 1H and 13C NMR to ascertain the physicochemical properties and morphological arrangement of the synthesized gels. The gels were then tested for the treatment of synthetic oily wastewaters. The first part of the work presented the optimization of silica from diatomaceous earth (DE) using response surface methodology. Three methods were used in the optimization namely conventional solvent extraction (CSE), ultrasound assisted extraction using alkali (Alkali UAE) and ultrasound assisted extraction using acids (Acid UAE). The CSE and alkali UAE methods produced silica of 87% and 79% purity and 13% and 16% yield respectively which was less than the amount of silica in DE while the acid UAE method produced silica of 95% purity and 82% yield with optimum parameters: 1.99 h for sonication time, 2.82 M for concentration of HCl, 220 mL for volume, 0.524 for cycle and 72.6% amplitude. In the second part, the successful solvent free synthesis and characterization of benzimidazolone and analogues is reported as well as their functionalization on silica. The benzimidazolone functionalized silica were then applied in the removal of oil from laboratory simulated oily wastewater as well as the optimization of oil removal using response surface methodology. The results show a percentage oil adsorption of 98% using 0.004 g of adsorbent, 6650 mg/L oil concentration, and a contact time of 16 h. Part three reports the synthesis and characterization of acetylated silica and the application of this in the removal of oil from synthetic oily wastewater (SOWW) and the optimization of oil removal using response surface methodology. The optimum conditions for oil removal were as follows 9250 mg/L oil concentration, with adsorbent dosage of 0.003 g and a duration of 16 h. The percentage oil adsorption was found to be 99.3%. The final section looks at the behaviour of synthesized polypyrrole-silica complex in the removal of oil from SOWW as well as the optimization of oil removal from SOWW using response surface methodology with and without shaking. The adsorbent showed excellent oil uptake when compared with similar materials with optimum conditions being 7250 mg/L oil concentration, 0.003 g adsorbent dosage and contact time of 38.2 h in experiments without shaking giving a percentage oil adsorption of 99.3%. The study successfully created an optimized method for the extraction of silica from diatomaceous earth and created new polymer and organic functionalized porous silica gels for the removal of oil from synthetic oily wastewater. Also, the physicochemical and morphological properties of diatomaceous earth, extracted silica and all synthesized gels were studied. A relatively cheap and conservative method for the recycling of organic wastewaters and oily wastewater has thus been found and this is expected to help ease the water stress in South Africa and other countries. For the first time, three silica-based adsorbents have been synthesized with all three having over 98% oil removal. Future work will entail further characterization of the adsorbents and regeneration studies.