Removal of cationic and anionic dyes from aqueous solution using a clay-based nanocomposite.

Abstract

Some industries such as textiles, ceramics, paper and printing are known to use significant amounts of dye to colour their products and during the colouring process, certain quantities of dyes are absorbed by the products, and some of them end up in wastewater. Depending on their application, some synthetic dyes are designed to be chemically or biologically resistant and their presence in the environment can cause severe environmental problems because of their colour impartation to water bodies. Therefore, proper treatment is required to remove these pollutants from wastewater before discharge into the environment. In this thesis, the potential of dye removal from wastewater by calcined magnesite, halloysite nanoclay and calcined magnesite - halloysite nanoclay composite was evaluated. To this end, the study was subdivided to four segments. The first segment of the study focused on evaluating the efficiency of using calcined magnesite to remove Methylene Blue (MB), Direct Red 81 (DR81), Methyl Orange (MO) and Crystal Violet (CV) dyes from aqueous systems using a batch study. To achieve that, several operational factors like residence time, adsorbent dosage, dye concentration and temperature were appraised. The adsorbent was subjected to different kinds of physicochemical characterization to determine the various characteristics that would assist in the dye uptake process. Characterization results showed that the adsorbent material was highly crystalline with magnesite, periclase, dolomite, and quartz as some of the crystalline phases. The batch study proved that calcined magnesite is effective in the treatment of dye contaminated water and moreover it performed well in terms of colour removal, though exceptional results were recorded for CV removal with complete decolourisation occurring in first few minutes of contact. In terms of experimental adsorption capacity, the performance of calcined magnesite was in the order CV (14.99 mg/g) > DR81 (12.56 mg/g) > MO (0.64 mg/g) > MB (0.39 mg/g). Mechanisms of adsorption where explained by fitting the experimental data into adsorption isotherms, kinetics, and thermodynamic parameters. Neither, the Langmuir, nor the Freundlich nor the Dubinin Radushkevich, nor the Temkin model could perfectly describe the adsorption of the four dyes onto calcined magnesite, however adsorption kinetics obeyed the pseudo second order model, implying that, the dye removal process was primarily a chemical process. In accordance with the results of this study, it can be concluded that calcined magnesite can be used effectively for the removal of dyes in aqueous solution and thus can be applied to treat wastewater containing dyes. The second segment of the study focused on the removal of MB, DR81, MO and CV dyes by halloysite nanoclay. Physicochemical characterisation revealed that the nanoclay has a surface area of 42 m²/g and its ABSTRACT iv morphology is predominated by tubular structures, which exhibit some hollow rod like structures. Various important parameters namely contact time, initial concentration of dyes, dosage, solution temperature and solution pH were optimized to achieve maximum adsorption capacity and it was observed that the effect of initial pH and temperature of the aqueous solution was neglibible on removal of the four dyes. The experimental adsorption capacities towards 40 mg/L of MB, DR81, MO and CV dyes were 17.51, 14.11, 0.38, and 4.75 mg/g respectively. The results indicate that natural halloysite nanoclay is an efficient material for the removal of the selected dyes. Due to its low cost and non-toxicity, halloysite nanoclay can be considered a good replacement option of other high cost materials used to treat coloured wastewater especially in developing countries like South Africa. Having observed the performance of calcined magnesite and halloysite nanoclay individually in the removal of selected dyes, a third study was designed with the aim of preparing a nanocomposite adsorbent from the aforementioned adsorbent materials and then evaluating the synergistic influence of the mechanochemical modification by a ball miller on the removal of MB, DR81, MO and CV dyes. Physicochemical characterization was carried out to get an insight of pre- and -post adsorption characteristics of the nanocomposite material and results showed major changes which could be an indication of dye uptake by the nanocomposite material. According to the results, the nanocomposite material outcompeted its component individual constituent materials i.e (calcined magnesite and halloysite nanoclay material) in the removal of DR81, which in turn was the highest removal efficiency observed for the whole batch adsorption study recording a maximum adsorption capacity and percentage removal of 19.89 mg/g and 99.40% respectively. Experimental results fitted the Langmuir and pseudo-second order models perfectly hence demonstrating that adsorption took place on a homogenous adsorbent layer via chemisorption. In overall, the results suggested that the nanocomposite is a suitable adsorbent for decolourising industrial wastewater. Based on the overall performance of the adsorbents in removing the four dyes, it was observed that the nanocomposite material had a high affinity for DR81 dye hence it was chosen as the model dye for further application in column studies. The effect of flow rate, bed height and initial dye concentration on the removal of DR81 was investigated. Maximum bed capacity and equilibrium dye uptake were determined and break through curves were plotted. Percentage dye removal increased with decrease in flow rate and increase in bed height. The maximum capacity of column was found to be about 51.73 mg DR81 per gram of the nanocomposite adsorbent for a flow rate of 3 mL/min, initial concentration of 10 mg/L and 4 cm bed height. Data from column studies was fitted to the Thomas model and Adams-Bohart models. The comparison of the R2 values obtained from both models showed a better fit for the nanocomposite material than the individual halloysite nanoclay and calcined magnesite materials. The study revealed the applicability of calcined magnesite- halloysite nanoclay composite in fixed bed column for the removal of DR81 dye from aqueous solution. v The reuse of an adsorbent is essential in order to make the adsorption process economic and environmentally friendly. To recover the adsorbate and renew the adsorbent for further use, a chemical method of regeneration was applied by using 0.1 M NaOH as the desorbent. Regeneration with 0.1 M NaOH proved very efficient for some dyes and less efficient for others depending on the adsorbent material used at the time. The general observation was that the adsorption capacity of the adsorbent materials decreased with successive adsorption – desorption cycles. Furthermore, regeneration with NaOH, favoured the acidic dyes (DR81 and MO) more than the basic dyes (MB and CV) possibly due to electrostatic interactions between oppositely charged molecules allowing for reversible reactions to take place. The three tested adsorbents namely calcined magnesite, halloysite nanoclay and their nanocomposite thereof were applied for the treatment of real wastewater effluent from a printing and ink industry. The adsorbents performed very well in terms of colour removal as recommended by the South African standards of wastewater discharge, However, in terms of pH, calcined magnesite and the nanocomposite produced a highly alkaline solution hence wastewater neutralisation by an acid is recommended before discharge. These findings show that the two natural clay-based materials (calcined magnesite and halloysite nanoclay) and their nanocomposite thereof have a great potential for application in dye wastewater remediation since the materials used in the process are inexpensive, abundant and require minimal modifications.