Mineralogical and chemical characterisation of terracotta clays and their application for dyes removal from aqueous solution

Abstract

The continuous discharge of untreated or semi-treated dye effluents is one of the global leading causes of surface water quality degradation. The present research aimed at investigating the potential of terracotta clay, herein termed (RTC) and its composite with zinc oxide nanoparticles (RTC@ZnONPs) to effectively sequester cationic (Methylene Blue) and anionic (Congo Red) dyes from aqueous solution. The mineralogical and chemical characteristics of the clay prior to sorption were investigated using Fourier Transform Infrared (FT-IR), X-ray fluorescence (XRF), X-ray diffraction (XRD), Scanning Electron Microscope coupled with Energy Dispersive X-ray Spectroscopy (EDS), and Brunauer Emmett Teller (BET). The adsorbents’ point of zero charge was also determined. The clays’ characterisation revealed that it was crystalline and mesoporous. Effects of variations in sorption time, adsorbent dosage, initial adsorbate concentration, pH, temperature and water chemistry were investigated in batch experiments. The optimum pH for MB adsorption was identified as 10, and 2 for that of CR. The pseudo second order model provided the best fit for the kinetic data, while Langmuir and Freundlich isotherms justified the state of equilibrium data. Both the Langmuir and Freundlich isotherm models provided explanations for the adsorption processes. Thermodynamics investigations revealed that the reactions were viable, spontaneous and exothermic. After five cycles of adsorption/desorption for the sequestration of both MB and CR dyes from aqueous solutions, the composite RTC@ZnONPs demonstrated outstanding reusability (88.75%) and (61.43%), respectively. The current study showed that (RTC@ZnONPs) can be employed for the removal of other dyes such as Crystal Violet, Indigo Carmine and Methyl Orange from wastewater. Moreover, both RTC and its composite (RTC@ZnONPs) are ideal for the economical and environmentally acceptable removal of MB and CR from aqueous solution.