Eichhornia crassipes-derived biochar via slow pyrolysis for the removal of Reactive Yellow 176 dye from aqueous media: adsorption isotherm, kinetic and thermodynamic studies

Abstract

Biochars (300BC, 500BC, and 700BC) were produced from water hyacinth at pyrolysis temperatures of 300 °C, 500 °C, and 700 °C. The adsorption capacities were evaluated using Reactive Yellow 176 dye. Characterization of the resultant biochar was conducted using proximate analysis, ultimate analysis, SEM, EDX, XRD, FTIR, BET surface area, and TGA-DTG analysis. Increasing pyrolysis temperature enhanced pore formation and surface heterogeneity, facilitating the mass transfer of dye molecules. FTIR analysis indicated hierarchical aromaticity and graphitization with increasing temperature, complemented by the confirmation of carbonaceous structures in XRD analysis. The specific surface area of the biochar pyrolyzed at 700 °C (211.3 m² g⁻¹) increased nearly 59-fold and 7-fold compared to the 300 °C (3.6 m² g⁻¹) and 500 °C (30.9 m² g⁻¹) counterparts, promoting pore filling, electrostatic interactions and π–π interactions. Adsorption kinetics followed the pseudo-second-order model, while the equilibrium isotherm analysis revealed a perfect agreement with the Langmuir isotherm model (R² = 0.99 for each biochar species, low RMSE values from 0.23 to 0.39, and reduced χ² values ranging from 0.06 to 0.35), indicating monolayer adsorption on a homogeneous surface. The intraparticle diffusion model confirmed that the initial film diffusion was limited by the subsequent intraparticle diffusion. 700BC demonstrated the best dye adsorption capacity (19.68 mg g⁻¹) attributed to its microporous structure, elevated specific surface area (211.3 m² g⁻¹), highest point of zero charge (10.3), and highest aromaticity. The spontaneity and exothermic nature of the adsorption mechanism were validated using ΔG (−24.71 kJ mol⁻¹ to −30.42 kJ mol⁻¹) and ΔH (−27.59 to −45.73 kJ mol⁻¹) values. The proposed adsorption pathway aligns with existing literature, suggesting that pore filling, electrostatic attractions, hydrophobic interactions, H–H bonding, ion exchange, and π–π interactions drive the adsorption mechanism. The study demonstrated that the water hyacinth-derived biochar can emerge as a sustainable and efficient adsorbent for reactive anionic dyes.