Comprehensive Study on Congo Red Removal Using Natural Adsorbent: Characterization, Box-Behnken Design, Isotherm Modeling, and Cost Analysis

F. Benyahia, A. Boudaoud, M. Benalia, M. Djedid and I. Nouacer

Abstract
In the present paper, the potential of a locally sourced plant-based adsorbent, Ziziphus lotus stone, for the removal of Congo Red (CR) dye from aqueous media was evaluated. The adsorbent was characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Brunauer-Emmet-Teller (BET) surface area analysis. These combined characterization methods provided a comprehensive profile of the material, confirming the presence of active surface functional groups, a porous structure and high thermal stability. The BET surface area was estimated at 4.7758 m² g–1, with a pore size of 4.3578 nm. About 10 % enhancement in the adsorption efficiency of Congo Red dye was achieved using response surface methodology (RSM) based on the Box-Behnken Design (BBD), considering three key factors: adsorbent dose, pH, and initial CR dye concentration. As a result, the overall adsorption efficiency reached up to 95 %. The isotherm study, based on non-linear isotherm models, indicated that the adsorption data were best described by the Freundlich model, with maximum adsorption capacity Qmax of 11 mg g–1. Thermodynamic analysis revealed negative values of ∆G0, ∆H0 (–11.0959 kJ mol–1), and ∆S0 (–29.587 J K–1 mol–1), indicating that the adsorption process was exothermic, spontaneous, and accompanied by decreased randomness at the solid-liquid interface. Furthermore, a techno-economic assessment of the based-plant adsorbent and the overall adsorption process was conducted. The estimated production cost of the Ziziphus lotus stone adsorbent was estimated at around 0.11304 USD/kg, while the overall adsorption cost was approximately 0.112 USD/m3. These findings confirm the viability and high efficiency of Ziziphus lotus stone as a low-cost adsorbent for large scale and low-cost environmental applications.

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Keywords
agricultural waste, adsorption, Congo Red, BBD design, process optimization, cost estimation