A Comparative Study on the Efficacy of Modified Clay Soil for the Decolouration of Acidic and Basic Textile Dyes: A Case Study

Master of Earth Science in Hydrology and Water Resources

Department of Earth Sciences

Dyes play a crucial role in the production of various products, including paper, plastics, and leather. However, their release into the environment leads to pollution and poses health risks to both humans and animals. Therefore, it is essential to treat dye-containing effluents before they are discharged into the environment. This study employed a cost-effective method to remove dyes from wastewater by utilizing natural clay as an adsorbent. The efficacy of Mn²⁺/Al³⁺ oxide-modified Mukondeni black clay soil (smectite) in removing acidic (Methyl Orange, MO) and basic (Crystal Violet, CV) textile dyes was evaluated. A 1:3 modified smectite ratio was identified as the most effective adsorbent. Characterization using X-ray Fluorescence (XRF), Brunauer-Emmet-Teller (BET), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Fourier Transform Infrared (FTIR), and Point of Zero Charge (PZC) revealed that Modified-SMT contained 18.42% Al₂O₃ and 61.08% SiO₂, had a surface area of 35.30 m²/g, and featured functional groups such as C–O, C–O–C, and O–H, with a PZC at pH 4.30. Maximum dye removal efficiencies were recorded at 91.95% for MO after 90 min under the conditions (initial dye concentration = 20 mg/L, temperature = 30 °C, contact time = 90 minutes, shaking speed = 250 rpm, solution volume = 30 mL, and an adsorbent mass = 2 g). For CV, an efficiency of 96.35% was achieved after 30 minutes under conditions (initial dye concentration = 20 mg/L, temperature = 30 °C, contact time = 30 minutes, shaking speed = 250 rpm, solution volume = 30 mL, and an adsorbent mass = 0.1 g). Adsorption isotherm analysis indicated that MO followed a multilayer mechanism on heterogeneous surfaces (Freundlich model), while CV exhibited monolayer adsorption on homogeneous surfaces (Langmuir model). Kinetic studies confirmed pseudo-second-order behaviour. Thermodynamic parameters Δ𝐻 = - 47.72 KJ/mol and Δ𝑆 = -0.17 J/mol/K showed that MO adsorption was non-spontaneous and exothermic (ΔG = 12. 64 KJ/mol for 80 °C), whereas Δ𝐻 = 22.64 KJ/mol and Δ𝑆 = 0.09 J/mol/K for CV adsorption was spontaneous and endothermic (ΔG = -9.55 KJ/mol for 80 °C). The adsorbent retained its efficiency over four regeneration cycles, demonstrating its suitability as a sustainable and effective solution for dye removal in wastewater treatment. It can be concluded that Mn²⁺/Al³⁺ oxide-modified Mukondeni black clay soil (smectite) can remove both MO and CV dye from aqueous solution.