Application of surface-modified clay in a custom reactor for efficient dye removal from water: adsorption behavior and mechanistic insights

Abstract The study investigates the efficacy of surface-modified (thermally) clay as an inexpensive adsorbent for removing Malachite green (MG) dye, utilizing a customized annular reactor to improve adsorption efficiency. Natural clay collected from the banks of the Ganga River was subjected to calcination at temperatures ranging from 200 °C to 500 °C, with 500 °C determined to be best for enhancing porosity, surface area (100.50 nm crystallite size), and stability, as validated by XRD, FTIR, TEM, TGA, and BET analyses. The annular reactor, which includes concentric glass chambers with a 3 cm annular gap and 1.8 cm vertical clearance, was chosen for its enhanced fluid dynamics, uniform adsorbate distribution, and reduced dead zones, optimizing contact between dye molecules and adsorbent. Batch investigations conducted under optimum conditions (1 g l −1 adsorbent, 50 mg l −1 dye, 200 rpm, 30 °C, 90 min) achieved a 98% removal efficiency, corroborated by response surface methodology (R 2 > 0.97). Adsorption agreed with the Langmuir isotherm (R 2 = 0.99) and exhibited pseudo-second-order kinetics, signifying chemisorption-induced monolayer development. The Shrinking Core Model determined that pore-film diffusion is the rate-limiting factor (Biot number: 0.00039–0.0019). Thermodynamic analysis verified exothermic (ΔH = −12.45 kJ mol −1 ), and spontaneous (ΔG = −8.34 kJ mol −1 ) adsorption. The adsorbent maintained 88.56% efficiency across five cycles, and ecotoxicity assessments verified the safety of the treated water. The design of the annular reactor, which integrates regulated flow dynamics and scalable modularity, was crucial in attaining improved adsorption performance. This study emphasizes thermally modified clay as a sustainable and economical approach for dye wastewater treatment, with the annular reactor providing an excellent basis for enhancing adsorption technology in environmental remediation.