High-performance activated carbons from Canarium schweinfurthii and Ricinodendron heudelotii shells for the efficient removal of indigo carmine from water

Abstract

In this study, two biomass wastes, namely, Canarium schweinfurthii shells (CSS) and Ricinodendron heudelotii shells (RHS), were converted into activated carbons by chemical activation using phosphoric acid. The two prepared adsorbents were designated as AC-CSS-H₃P and AC-RHS-H₃P, representing the activated carbons derived from CSS and RHS, respectively, through H₃PO₄ activation. The samples were characterized using N₂ adsorption (BET), scanning electron microscopy (SEM), and Fourier-transform infrared (FTIR) spectroscopy analyses. To achieve the removal of indigo carmine (IC) in the batch mode, four adsorption parameters, namely, contact time (5–30 min), pH (2–11), adsorbent dose (10–110 mg), and initial dye concentration (5–25 mg L⁻¹), were thoroughly investigated. Four adsorption isotherms and kinetic models were also evaluated. The specific surface area and pore volume were determined to be 1612 m² g⁻¹ and 0.78 cm³ g⁻¹ for AC-CSS-H₃P and 1696 m² g⁻¹ and 0.90 cm³ g⁻¹ for AC-RHS-H₃P, respectively, with a predominance of acidic functional groups for both samples. After 10 minutes of the contact time with 10 mg of the adsorbent in 100 mL of the IC solution (10 mg L⁻¹), the IC removal efficiency reached approximately 95%. The adsorption of IC was best described by the Langmuir isotherm model, with maximum adsorption capacities of 588 mg g⁻¹ and 1250 mg g⁻¹ for AC-CSS-H₃P and AC-RHS-H₃P, respectively. The pseudo-second-order kinetic model provided the best fit, with correlation coefficients (R²) close to unity, indicating a chemisorption mechanism. The adsorption process was predominantly facilitated by hydrogen bonding between the acidic functional groups on the adsorbent surfaces and the anionic form of IC under acidic conditions. Conversely, IC removal decreased in alkaline media due to strong electrostatic repulsion between hydroxide ions and the IC anion. After six regeneration cycles, both adsorbents retained most of their adsorption capacity, demonstrating good reusability. Overall, these results indicate that the activated carbons derived from CSS and RHS are effective adsorbents for the elimination of organic dyes from aqueous solutions.