Mesoporous ACF and its complex materials, and degradation of polystyrene

Abstract

As increased liquid and solid wastes are posing serious problems to environment all over the world, simple and efficient techniques are required to treat these pollutants. This study focuses on water treatment material activated carbon fiber (ACF) adsorbent and the degradation of polystyrene (PS). In order to make ACF more appropriate for large-molecular-size pollutants treatment, polyvinyl alcohol (PVA) based activated carbon fibers with a high mesoporosity have been fabricated. The surface area and pore size distribution were fine-tuned by the careful control of activation temperature and time. A mesopore volume fraction of 66 % was obtained by activation at 900 °C for 120 min or 1000 °C for 90 min. Raman spectroscopy revealed that more defective sites in the graphitic structure was found in samples with higher mesoporosity. Adsorption experiments demonstrated that the PVA-ACF possessed good adsorption capacity for both small-molecular-size iodine and the large-molecular-size methylene blue with adsorption capacities up to 1934 and 709 mg g-1, respectively. Adsorption kinetics of methylene blue onto the PVA-ACF was simulated by both pseudo-second-order kinetic model and intraparticle diffusion model. With an increase in mesoporosity, rate limitation of adsorption due to intraparticle diffusion was dramatically reduced. This resulted in an up to 7-folds increase in adsorption rate compared to microporous PVA-based ACF. The potential practical application of PVA-ACF was also carried out by using rapid small-scale column tests (RSSCT) and employing MB as target pollutant. The effects of bed density, bed depth, initial MB concentration and solution flow rate on the performance of microcolumn were studied. The results show that the static air activated PVA-ACF presented high sorption capacity for MB and performed well in a broad range of bed density. Column adsorption data were analyzed by the bed depth service time (BDST) model. The modeling results confirmed the results that the kinetics involved external mass transfer and intraparticle diffusion. The results from batch and dynamic tests suggest the static air activated PVA-ACF is capable to remove pollutants in wastewater in terms of column. For the view of prolonging ACF lifetime, composite material TiO2/ACF was prepared by combining preparation methods of TiO2 and PVA-ACF. SEM images indicate that the loaded TiO2 was not uniform in size and shape. And the size was in the range of 200-500 nm. XRD patterns of TiO2/ACF and neat gel-based TiO2 suggested that by loading of TiO2 sol to dehydrated fiber, the transformation of anatase type to rutile type can be inhibited, even fiber was activated at 1050 °C, anatase type TiO2 can still be detected. However, no anatase was detected for TiO2 calcined at 800 °C from neat gel. TG analysis suggested that loaded gel have no significant negative effect on dehydrated fiber thermo behavior. Optical absorption spectra proved that UV rays can irradiate anatase surface and excite the semiconductor, though their intensity might be weakened. Different adsorption capacity for MB and RhB implied that development of TiO2 was helpful for meso- or macro-pores development, and endow TiO2/ACF with higher adsorption capacity for larger-molecular-size pollutants. RhB saturated TiO2/ACF can further remove RhB under UV irradiation, this confirmed the catalytic ability of the composite material. Fe/ACF was also prepared to improve the lifetime of ACF. The effects of solution initial pH, peroxide concentration and preparation condition on its degradation activity of MB were investigated. The prepared Fe/ACF possessed activity and its catalytic ability had no significant difference in pH range of 4.37-5.43. Peroxide concentration in the range of 0.03-0.35 mol L-1 affected degradation rate significantly, and 0.35 mol L-1 provided the best performance of MB degradation. The re-usability of Fe/ACF was stable. Even reaction was cycled up to 9 times, the degradation efficiency kept higher than 90%, and this endowed ACF with a longer lifetime. The aim of degradation of polystyrene (PS), which is a main contributor to white pollution, was achieved by a two-step technique. By sulfonic modification reaction as first step, PS was proved to be degraded by followed homogeneous photo-Fenton reaction which was denoted as second step. Solid degradation ratio achieved 98 % in 60 min at condition of Fe 0.40 mM, hydrogen peroxide 141 mM and pH 2.0. By further adding hydrogen peroxide each hour and reacted for 300 min, only 570 mg L-1 TOC remained. Sulfonic groups were a key role in PS degradation. It adsorbed iron ions and advantaged hydroxyl radicals to attack polymer chains. SEM detected oxidation induced chasms on PS spherical particles. FTIR spectra of solid samples indicated that polymer chain scission occurred without generation other detectable functional groups on remained solid phase. MS and FTIR spectra of dissolvable by products confirmed the presence of further degradable organic segments which basically consist of phenyl, sulfonic, carboxyl and hydroxyl groups. By this two-step approach, commercial foam PS products can be easily degraded.

Date of Award	15 Feb 2011
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hon Wah Michael LAM (Supervisor)