Development of multifunctional nanomaterials and adsorption - photocatalysis hybrid system for wastewater reclamation.

Abstract

This thesis study aimed to develop multi-functional nano-catalyst and porous adsorbents from low-cost and locally available materials, and then implement this into an Adsorption-Photocatalysis hybrid system for wastewater reclamation. The project involves two major technological practices for wastewater treatment: adsorption and photocatalysis. For each technology, a specific functional nanomaterial has been developed and investigated regarding their removal capability for a pilot-scale water treatment process. The experimental studies include: 1) evaluation and characterisation of the natural clay minerals that deliver the most suitable properties for adsorption performance and immobilisation of titanium dioxide (TiO₂); 2) synthesis, modification, and characterisation of the clay mixtures as alternative adsorbents, and titania immobilised onto modified porous kaolin as the photocatalyst; 3) evaluation and optimisation of their removal capability, kinetics and mechanisms toward different surrogate indicators of both nanomaterials via the batch and continuous water treatment system; and 4) integration of the adsorption-photocatalysis hybrid system as a major technical outcome for the treatment and reclaimantion of wastewater. Three Australian natural clay minerals, bentonite, kaolins, and zeolite, were investigated to gain understanding of their physiochemical properties as well as their adsorption capabilities towards Congo red (CR) dye as a chemical surrogate indicator. Microscopic characterisations revealed the variation of the layered structures among clays, resulting in the differences in their adsorbent-adsorbate interaction profiles. The removal capacities of the clays were evaluated through the adsorption isotherms and kinetic studies, where it was found that Na-bentonite showed the best removal performance, followed by kaolin and zeolite. Thermodynamic and pH effect studies indicated that dye adsorption by the studied clays was a spontaneous and exothermic reaction, while pH conditions appeared insignificant. Further investigation has been emphasised on using different natural kaolins, in which the recyclability of these clay minerals was also taken into account. These results depicted a very high thermal stability of the kaolin structure. Repetitive recycled kaolin trials revealed good recovery of dye removal efficiency even after five experimental tests. This study demonstrated the potential employment of these natural clays as alternative adsorbents for wastewater treatment. To improve the removal efficiency of these natural clays as an economically viable adsorbent for wastewater treatment, a physical modification of the clay minerals was adopted in this present work. A feasible technical approach of combination and calcination of these natural clay materials to improve dye removal efficiency was developed to compromise the indigenous weakness of individual clays. The application of a mixture of clay minerals would be able to compromise the indigenous constraints of the individual clays. An optimisation study using calcium hydroxide or slaked lime as an additional calcium source for the clay mixture was included. Different characterisation methods, i.e. differential temperature analysis (DTA) coupled with thermogravimetric analyser (TG), scanning electron microscopy (SEM), and x-ray diffraction (XRD), were applied to comprehend the changed properties of the adsorbents during calcination treatment. The clay mixture and lime showed superior decolourisation, over 10–20 times to those of bentonite, kaolin and zeolite, at the optimum thermal condition at 300°C for 1.5 h. The great enhancement in dye removal efficiency was the contribution of the combination of an adsorption/precipitation mechanism. The instant precipitation of dissolved Ca ions with dye molecules illustrated the major contributor to dye removal, followed by the constant adsorption. The adsorbent mixture possessed the potential for recovery by heat treatment, of which their removal capacity was found comparable to the fresh materials even after the 5th cycle. The application of the adsorbent mixture was investigated in a pilot scale implementation, in which the laboratory scale fluidised-bed reactor (FBR) was developed in our research group. Optimisation of the operating parameters influencing pollutant removal performance of the FBR system, i.e. adsorbent loading, aeration rate, reaction time etc. was undertaken to facilitate the continuous operating scheme. The removal performance of oxyanion phosphate and nitrate in wastewater effluent, as well as their interference effect on dye elimination was also determined. The results revealed that the very effective elimination of CR and phosphate as complete removal can be achieved, while the reduction of nitrate became less extensive due to the difference in their removal mechanisms, i.e. adsorption and precipitation etc. The feasibility of using the FBR system in the wastewater treatment was also investigated. Several municipal primary effluent samples were treated using the FBR system in continuous operation mode. The results showed an average 10-15% and 20-40% reduction of the nitrate and chemical oxygen demand (COD), respectively, while 100% phosphate removal was obtained over the experimental period. This study demonstrated that the FBR system with the formulated clay-lime mixture can be a cost-effective alternative treatment process for large-scale application in the wastewater industry. Another advanced technology, heterogeneous photocatalysis, was used in this study to improve the quality of treated wastewater. A modified two-step sol-gel method was developed to synthesise a titanium dioxide impregnated kaolin (TiO₂-K) nanophotocatalyst, in which various parameters affecting the sol-gel formation and photocatalyst preparation were optimised. Further detailed investigation was carried out to improve the clay surface function prior to the impregnation. The natural kaolin was subjected to a series of acidic-alkali treatments to delaminate the clay structure, followed by thermal treatment. This clay pre-modification was designed to increase the specific surface area available for heterocoagulation with the microporous titania particles. Characterisation and photocatalytic activity of the TiO₂-K catalyst were performed by different microscopic techniques, i.e. XRD, SEM, TEM, UV-diffuse reflectance etc., and CR degradation, respectively. We examined thermal regeneration cycles of the catalyst lifespan, where the improvement of the photocatalytic activity was observed as a result of the change in average titania nanocrystal size and their porosity. This TiO₂-K exhibited a superior removal capability over the commercial TiO₂ in terms of initial adsorption and catalyst recovery. The self-settling capability of this catalyst can facilitate its separation after photooxidation treatment. Finally, the integration of adsorption and photocatalysis techniques was investigated as an alternative hybrid system for municipal wastewater treatment. The primary and secondary biological effluents were preliminary treated by the FBR system with the synthesised clay-lime mixture before being subjected to an annular slurry photoreactor (ASP) using the TiO₂-K catalysts. The formulated clay-FBR system demonstrated a prevailing removal efficiency towards PO₄³⁻ , NO₃⁻ and suspended solids; whereas the TiO₂-K-ASP showed superior degradation of dissolved organic content. This hybrid treatment approach demonstrated a synergetic enhancement for the chemical removal efficiency, and might be able to be employed as a feasible alternative treatment process for wastewater reclamation.