Eichhornia crassipes biochar using slow pyrolysis technique for the removal of Reactive Yellow 176 dye from aqueous medium: Isotherm, kinetics and thermodynamics study

Abstract

Biochars (300BC, 500BC, and 700BC) were produced from water hyacinth at 300°C, 500°C, and 700°C pyrolysis temperatures. 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. The ultimate and proximate analyses confirmed increased aromatization, thermal enrichment of inorganics, hydrophobicity, and decreased polarity index with increasing pyrolysis temperatures. Morphological analysis using SEM revealed enhanced pore formation and surface heterogeneity in 500BC and 700BC, 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 o C (211.3 m² g⁻¹) increased nearly 59fold and 7-fold compared to the 300 °C (3.6 m² g⁻¹) and 500 o C (30.9 m² g⁻¹) counterparts, respectively. Each biochar sample 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). 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 zero charge (10.3), and highest aromaticity. The gradual increase of point zero charge (pH pzc ) signifies that 500BC and 700BC offered more favourable surfaces for electrostatic interactions with anionic dyes, correlating with their superior adsorptive performances. The spontaneity and exothermic nature of the adsorption mechanism were validated by ΔG (-24.71 kJ mol-1 to -30.75 kJ mol-1) and ΔH (-27.59 to -45.73 KJ mol-1) values. The proposed adsorption pathway aligns with existing literature, suggesting pore filling, electrostatic attractions, hydrophobic interactions, H-H bonding, ion exchange, and π-π interactions drive the adsorption mechanism. The study establishes the water hyacinth-derived biochar as an efficient adsorbent for reactive anionic dyes.