Evaluation of physicochemical methods in enhancing the adsorption performance of natural zeolite as low-cost adsorbent of methylene blue dye from wastewater

The main aim of this study was to evaluate different facile and cost-effective physicochemical methods in enhancing the adsorption capacity of natural zeolite and subsequently demonstrate the removal of methylene blue (MB). Four different physicochemical methods, namely: acid treatment (AT), base treatment (BT), combined acid-thermal (ATT) and base-thermal treatments (BTT) were investigated. This was followed by understanding the impact of the physicochemical methods on the surface characteristics and properties of modified zeolite through advanced characterisations using field emission-scanning electron microscopy, Fourier-transformed infrared spectroscopy and Brunauer-Emmett-Teller specific surface area and porosity measurements. Batch adsorption studies were carried out using the modified zeolite adsorbents, in order to evaluate the highest removal efficiency of MB under varying adsorption conditions of: pH, initial MB concentration and modified zeolite loading. Results showed that both the physicochemical methods of AT (using 0.4 M HCl) and BT (using 4.0 M NaOH) are able to enhance the adsorption capacity of natural zeolite from 41% to 98.8% and 52.2%, respectively. Surprisingly, the combined physicochemical methods of ATT and BTT showed a reduction in adsorption performance when benchmarked to AT and BT alone. The adsorption data were analysed and modelled using Langmuir, Freundlich, Redlich-Peterson and Koble-Corrigan isotherm models, also the adsorption kinetics were evaluated using pseudo-first and pseudo-second order models. It was found that the adsorption data and kinetics were best represented using the Koble-Corrigan and pseudo-second order models. The improvements yielded through both the AT and BT modified zeolites were found to be promising and have the potential to be used as low-cost adsorbents for wastewater treatment.