From batch to fixed-bed biosorption of Congo red using surfactant-modified wood chips: Mechanistic insights, regeneration, real wastewater validation and machine-learning modelling.

This study examines the biosorption capacity of raw wood chip (WC) and cetyltrimethylammonium bromide-modified wood chips (WC-CTAB) in removal of Congo Red (CR) dye from aqueous solutions using integrated batch-column approach combined with artificial neural network (ANN) modelling. The lignocellulosic biosorbent was characterized using FTIR, SEM and pHPZC analysis, confirming the successful introduction of cationic functional groups after CTAB treatment. Batch biosorption experiment showed that the maximum Langmuir biosorption capacity increased from 15.3 to 25.8 mg/g after modification, and the biosorption obeyed pseudo-first order kinetics and Langmuir-Hill isotherms, showing a mainly physisorption based monolayer type biosorption mechanism with a cooperative pattern. Thermodynamic parameters (ΔG° = -33.9 to -48.3 kJ/mol; ΔH° < 0) confirmed that biosorption was spontaneous and exothermic. In order to assess the scalability of the process, fixed-bed column experiments were carried out at different bed heights (5-10 cm), inlet concentrations (20-50 mg/L) and flow rates (5-10 mL/min). The dynamic biosorption capacity increased from 12 to 15 mg/g for WC to 19-26 mg/g for WC-CTAB and breakthrough time almost tripled with bed depth (46 vs. 138 min at 20 mg/L and 5 mL/min). Breakthrough data were well fitted by the Thomas and Yoon-Nelson models (R2 > 0.97) while BDST revealed linear depth-service time behavior suitable for scale-up. Regeneration studies showed 70 % capacity retention in batch mode after three cycles and 85 % capacity retention in column mode after three cycles. ANN modelling accurately predicted biosorption performance and indicates potential usefulness as a support tool for process optimization. The findings show the possibility of transformation of low-value wood waste into an inexpensive, recyclable and scalable adsorbent for continuous industrial dye removal.