Methylene blue removal using spent tyre-derived carbon black

Introduction: Spent tyre-derived carbon black is a potentially valuable secondary carbon material, but its hydrophobic surface may limit its direct application in aqueous treatment. This study investigated the removal of methylene blue (MB) using unmodified tyre-derived carbon black (UCB) and carbon black modified by nitric acid, sodium hydroxide, ethanol washing and copper oxide deposition.

Methods: The adsorbents were characterised using elemental analysis, Fourier-transform infrared spectroscopy, X-ray diffraction, Brunauer–Emmett–Teller surface-area analysis, contact-angle measurement and thermal analysis. Batch adsorption experiments examined the effects of adsorbent dosage, solution pH, contact time, initial dye concentration and ionic strength. Adsorption kinetics were analysed using pseudo-first-order, pseudo-second-order and Weber–Morris intraparticle-diffusion models. Regeneration and continuous-flow adsorption were also investigated using repeated adsorption cycles and a packed-bed microcolumn.

Results: Among the modification methods examined, treatment with 6 M HNO₃ produced the highest screening performance, increasing MB removal from 12.10% for UCB to 38.15%. Adsorption generally followed the pseudo-second-order kinetic model, with calculated equilibrium capacities closely matching the experimental values. The highest experimentally observed adsorption capacity was 33.10 mg g^-1. Dye removal increased with adsorbent dosage and was enhanced in the presence of sodium chloride. Continuous-flow adsorption was demonstrated using a microcolumn containing 0.10 g of carbon black with a 2 cm bed height and a flow rate of 1.4 mL min⁻¹. The effluent-to-influent concentration ratio remained close to zero during the initial 8 min, exceeded the 10% breakthrough criterion between 8 and 12 min, reached approximately 0.52 at 20 min and increased to approximately 0.88 after 36 min, approaching the 90% exhaustion criterion.

Conclusions: Nitric-acid modification improves the aqueous compatibility and adsorption performance of spent tyre-derived carbon black. The results support the potential reuse of this waste-derived carbon material as an adsorbent for dye-contaminated water.