Extraction of pectin from orange peel and adsorption of azo dye direct violet 35 (DV 35) by orange peel waste (OPW)

Abstract

This study introduces a seminal dual-phase valorization strategy for OPW, where sequential pectin extraction (6.3 % yield) and subsequent application as a biosorbent achieves 96.03 % removal efficiency of DV 35 dye, a previously understudied azo pollutant. Through Response Surface Methodology (RSM) with Central Composite Design, optimal conditions were identified as pH 2.28, 85 mg L⁻¹ initial dye concentration, and 15.84 g L⁻¹ OPW dose, validated by high model accuracy (R² = 0.98, adjusted R² = 0.96). The adsorbent demonstrated exceptional Freundlich isotherm compliance (R² = 0.99), revealing heterogeneous multilayer adsorption with a favorable dimensionless separation factor (0.019–0.006), and achieved a monolayer capacity of 43.84 mg g⁻¹ at 303 K, surpassing prior agricultural waste-based adsorbents. BET (30.276 m²/g surface area), SEM-EDX, and FTIR identified critical functional groups (−OH, −COOH) enabling electrostatic binding with DV 35's sulfonate groups, while pseudo-second-order kinetics (R² = 0.99). Thermodynamic profiling revealed spontaneous (ΔG° = −25.02 kJ mol⁻¹) and endothermic (ΔH° = 59.48 kJ mol⁻¹) adsorption, driven by entropy (ΔS° = 0.29 kJ mol-1 K⁻¹). The process exhibited dual-phase diffusion dynamics: rapid film diffusion (67 % removal within 4 min) followed by pore diffusion, with external film diffusion as the initial rate-limiting step. Present work aligned with circular economy principles by transforming waste into a high-value resource—first extracting pectin, then repurposing residues for dye remediation. This work establishes OPW as a cost-effective, eco-friendly solution for textile wastewater challenges, addressing a critical gap in azo dye adsorption research.