Development of Effective Bentonite Adsorbents for Colour Removal

Adsorption techniques have been widely used to remove certain classes of pollutants from wastewaters, especially those that are not easily biodegradable. Desired adsorbent should have sufficient adsorption capacity, high thermal and chemical stability, low cost and low environmental impact. The successful implementation of solid-liquid phase dye adsorption requires stable and effective adsorbents. This thesis focuses on the development of effective bentonite adsorbent for dye removal. Bentonite is a layered clay having the potential to be an alternative low-cost adsorbent because it is abundantly available and possesses unique physiochemical properties, such as cation exchange ability. However, it is this property that makes bentonite unlikely to adsorb anionic species. In order to remove contaminants in effluents that normally contain both cationic and anionic materials, it is necessary to develop a composite adsorbent which is capable of removing both cationic and anionic dyes. The screening of various bentonite based adsorbents was first conducted for the purpose of exploring the possibility of anionic dye removal. From the literature, there are a number of bentonite modification techniques effective for dye removal. Considering the previous work conducted in our group, the methods of acid activation and aluminium pillared clay were preliminary tested. However, the dye removal results showed that there was no improvement following these modification processes. We found that it is necessary to investigate the adsorption mechanism before proceeding with any further modification attempts. A systematic cationic dye adsorption study was then carried out between bentonite and a cationic dye under various experimental conditions, such as the changes of solution temperature and pH, and the addition of salt. The results indicated that it is the cation exchange property dominating the removal of dye from aqueous solution. In this study, a simple and effective technique to modify the natural unpurified bentonite clay has been developed and the process parameters have been optimised. The modification was achieved by introducing layered double hydroxide (LDH) into bentonite interlamellar region, allowing for anionic materials to be adsorbed. LDHs are anionic clays, consisting of positively charged metal hydroxide layers with balancing interlayer anions and water molecules. These anions together with water presented in the interlayer space or on the surface of LDHs can be exchanged with other anionic species. The modification of bentonite with LDH is expected to take the advantage of LDH to adsorb anionic dyes. To obtain the optimum modification result, the influence of a number of process parameters has been investigated, such as LDH loading and particle size, as well as the calcination temperature of the modified clay. The surface properties of the novel composite adsorbent was characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen sorption techniques. The adsorption ability of the LDH modified bentonite adsorbent was examined in the single dye adsorption, binary component dye adsorption and the column systems. Five commercial dyes were selected to conduct the experiments, they are Acid Red 18 (AR18), Acid Yellow 23 (AY23), Basic Red 2 (BR2), Reactive Yellow 2 (RY2) and Direct Blue 1 (DB1). The single component adsorption equilibrium data were analysed using the Langmuir, Freundlich, and Redlich-Peterson isotherm models. The adsorption capacity estimated from the linear Langmuir equation was found to be in the order of AY23 > AR18 > RY2 > DB1 > BR2. The binary component dye adsorption involved two pairs, one was composed of AR18 and AY23, and another one was RY2 and BD1. The adsorption parameters obtained from single component were adopted to predict the binary results, but the fitting results were poor because of the significant competition and interaction between the dyes. The bed depth service time (BDST) model was applied to the column adsorption system and the results could be used as basis for the design of industrial scale-up operation