A Proposed Model for Breakthrough Curves of Methylene Blue Adsorption on Biochar

Dye pollutants, mainly discharged from the textile industry, have caused severe risks to human health and the ecosystem because of their toxicity, non-biodegradability, and carcinogenicity. This study investigated the use of commercial biochar derived from melaleuca wood as an adsorbent for the removal of methylene blue (MB) using a packed-bed column. The selected biochar was characterized by nitrogen adsorption-desorption isotherms, Fourier transform infrared spectroscopy, and scanning electron microscopy. The experiments were performed to determine breakthrough curves (BTCs) with varying pH (3–9), inflow rate (5–20 mL min–1), bed height (16–65 cm), and initial MB concentration (0.75–9 mg L–1). The biochar (particle size of 1–2 mm) exhibited a low adsorption capacity for MB (~21 mg kg–1), resulting in a short breakthrough time. The Thomas, Bohart-Adams, Yoon-Nelson, and Bed Depth Service Time models were quite suitable for describing the experimental BTCs, with R2-values ranging from 0.92 to 0.98. The obtained BTCs were not in the typical S-shape, which characterizes diffusion-controlled adsorption. Therefore, a serial logistic-exponential model, which accounts for both the mass transfer and interaction contributions, was proposed. The experimental data effectively fit this proposed model, as indicated by high R2-values (>0.998). The dominant influence of mass transfer compared to interaction in controlling the adsorption rate of MB was highlighted.