Preparation of activated carbon from Ziziphus jujuba stones by H3PO4-assisted thermo-chemical activation: application in the removal of anionic diazo dye from synthetic water

Abstract

Agricultural waste serves as a valuable source of organic matter for producing activated carbon, which can be used for the removal of the anionic dye Direct Blue 227 from aqueous solutions. In this study, Ziziphus jujuba stones (ZJS) were utilized as a precursor to synthesize activated carbon (ACZJS) via H₃PO₄ activation, with an impregnation ratio (H₃PO₄ mass/precursor mass) of 2. The activation process was conducted at 600 °C for 2 h. The structural and surface properties of ACZJS were characterized using pH_PZC, ATR-FTIR, SEM–EDX, and BET analysis. Batch experiments were performed to investigate the influence of key parameters, including contact time (0–240 min), pH (2 − 10), ACZJS dose (0.2–1.2 g/L), initial dye concentration (80–500 mg/L), and temperature (20–50 °C), on the adsorption performance. The dominant adsorption mechanism at pH < pH_PZC was identified as electrostatic attraction and cation exchange between DR227 and ACZJS. Thermodynamic analysis confirmed that the adsorption process was spontaneous and endothermic. Various isotherm models were examined, and based on APE and R² values, the Sips and Redlich–Peterson models suggest a hybrid adsorption mechanism on the ACZJS surface, combining homogeneous and heterogeneous adsorption sites, with minor contributions from surface heterogeneity as indicated by β = 0.95. The non-linear Langmuir model estimated a maximum monolayer adsorption capacity of 177 mg/g at pH 2. Moreover, the adsorption kinetics of DR227 dye at different pH levels (2–10) were best described by the pseudo-second-order model (R² > 0.99). The intraparticle diffusion analysis indicated that adsorption was not solely governed by intraparticle diffusion, as evidenced by R² > 0.95 and the presence of a boundary layer effect in the liquid film. Thermodynamic parameters further confirmed that the adsorption process was spontaneous (ΔG < 0) and endothermic (ΔH > 0). The regeneration of DR227-loaded ACZJS was evaluated over four consecutive adsorption/desorption cycles. Overall, the findings suggest that this low-cost, eco-friendly activated carbon derived from Ziziphus jujuba stones holds promise for wastewater treatment applications involving anionic dye removal.