Synthesis and evaluation of novel porous materials for environmental remediation

AI-Mashaykhi, Zahraa Hashim Athab

Abstract

Porous materials have been widely used as adsorbents for water treatment due to their unique properties such as high surface area, excellent mechanical properties and good chemical stability. The work in this thesis aimed to develop novel porous materials for pollution remediation, with the focus being on materials that can be produced economically and environmentally friendly. The first part of this thesis covers two types of mesoporous carbon materials including mesoporous and magnetic mesoporous carbon materials which were fabricated through a soft templating method (Chapter 3). It has been shown that these porous carbon materials with monolithic form have high surface area, which is envisaged excellent adsorbent capacity. But there are some drawback which limit their use for water treatment as the preparation of these materials is time consuming, there are high operation cost and difficulties in regeneration and operation. Cellulose was considered as an attractive alternative material for preparation of porous materials for pollution remediation because it is naturally abundant, renewable, non-toxic and a lowcost biopolymer.

In the second part of this work a cellulose-based hydrogel was successfully synthesized using hydroxypropyl cellulose (HPC) with divinyl sulfone (DVS) as a chemical crosslinker via a modified temperature induced phase separation (TIPS) method (Chapter 4). The HPC hydrogel obtained was characterised and the results showed that the properties of this hydrogel depended on the gelation temperature. The FTIR results confirmed the chemical cross-linking between HPC and DVS. HPC hydrogel demonstrated a flexible behaviour without breakage under compression tests. In addition, there were good shape recovery properties upon adsorption of water. The morphology of the cross-linked HPC hydrogel showed an interconnected macroporous network structure, which allowed application for water purification.

Further work was then carried to develop a new and simple method to prepare a novel thermoresponsive HPC hydrogel with a graded pore size (Chapter 5). This method combined two approaches, varying the temperature between the upper and lower part of the hydrogel utilising the lower critical solution temperature (LCST) via the temperature induced phase separation (TIPS) method, which achieved a gradual change in pore geometry and pore size. The added inclusion of cryogenic treatment of the sample ensured a gradient porous HPC hydrogel was obtained with high permeability.

Double network (DN) hydrogels have a structure that can effectively improve the adsorption capacity as the second network can introduce more functional groups into hydrogel structures, which is of great importance in the adsorption process due to improve the adsorption capacities. The DN hydrogels can also improve the mechanical strength of hydrogel materials, which makes it easier to regenerate. To this purpose, novel hybrid
double networks hydrogel was prepared in this work via mixing two types of crosslinked polymers, these were covalently crosslinked HPC with DVS and ionically crosslinked alginate with calcium ions (Chapter 6). Alginate was selected to be the second network polymer as alginate has carboxylate functional groups that can be used to remove cationic pollutant by electrostatic interactions, thus improving the adsorption capacity of the HPC single network (HPC SN) hydrogel. SEM images of the double network produced s confirmed that the hydrogel was composed of two independently cross-linked networks with a homogeneous interconnecting porous structure. The mechanical tests on the DN hydrogel showed that it was much stronger compared with HPC SN hydrogel. The adsorption and filtration of organic pollutants by HPC hydrogel were investigated through dye adsorption experiments (Chapter 7). The results were showed a great ability of HPC hydrogel for selective adsorption towards MB dye. In order to evaluate the possibility of reuse of HPC hydrogel, the recyclability of these materials was examined. The obtained results indicated that the reusability of the HPC hydrogel was at some cycles without any loss in its sorption behaviour. Therefore, the HPC hydrogel can be a good reusable and economical adsorbent to remove the cationic species. It is important to note that HPC hydrogel column was further used for the first time for selective separation of dye mixtures by simple gravity filtration and the hydrogel can be re-used multiple times. Despite being one of the most promising types of porous materials for environmental applications, their low adsorption capacity is a significant disadvantage for their use in
these applications. Adsorption of methylene blue dye (MB) on HPC/CA DN hydrogel was investigated through batch and column adsorption experiments and compared with HPC SN hydrogel (Chapter 7). The adsorption isotherms for both HPC SN and HPC/CA DN hydrogels fitted well with the Langmuir adsorption model and the maximum adsorption capacity of HPC/CA DN hydrogel was found to be 169.49 mg g-1, which is larger than for the HPC SN hydrogel (112.35 mg g-1). The results showed a significant pH-dependent equilibrium for the adsorption capacity of MB dye for both hydrogels in this study, which decreased dramatically with decreasing the pH of the MB dye solution. This meant that the MB-loaded HPC hydrogel could be easily regenerated under acidic conditions. The thermodynamic analysis of MB dye adsorption onto both HPC SN and
HPC/CA DN hydrogels were also studied and the process was shown to be an exothermic and spontaneous process. An adsorption kinetic study was also carried out and the results obtained showed that the adsorption of MB dye adsorption on both hydrogels was well described by the pseudo-second-order kinetic model. In the column study, the adsorbent reuse was investigated and the selective separation of a dye mixture was also studied through ten cycles. Both hydrogels columns showed efficient selective adsorbent for cationic dyes, with the removal of MB dye being very efficient, whilst extremely low removal of FL dye. However, the HPC/CA DN hydrogels column exhibited a higher adsorption capacity than HPC SN hydrogel due to the dual functional groups (hydroxyl and carboxyl groups) in HPC/CA DN hydrogel. Based on the selective adsorption
towards cationic methylene blue over anionic sodium fluorescein dye, HPC SN and HPC/CA DN hydrogels columns could easily separate two dyes from aqueous solutions of dye mixtures by simple gravity filtration. Both HPC SN and HPC/CA DN hydrogel column showed high separation efficiency of more than 99%. It was also found that separation efficiency of the HPC SN decreased to 86% by the 10th cycle for this column, while no significant losses in the separation efficiency were detected even after ten cycles for the HPC/CA DN hydrogel column. These results show that the HPC/CA double network polymer hydrogels have great potential for improving the adsorption capacity with good reusability and would be a promising eco-friendly adsorbent for the treatment of dye wastewaters.

Citation

AI-Mashaykhi, Z. H. A. (2019). Synthesis and evaluation of novel porous materials for environmental remediation. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4224341

• Thesis Type: Thesis
• Deposit Date: Oct 6, 2022
• Publicly Available Date: Feb 24, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4224341
• Additional Information: Department of Chemistry and Biochemistry, The University of Hull
• Award Date: Jun 1, 2019