Title : Activated carbon from Sargassum: Adsorption of diazepam and midazolam in wastewater
Abstract:
Since 2011, massive influxes of Sargassum on Caribbean coasts have become a serious environmental problem, generating ecological damage and significant management challenges. At the same time, the presence of pharmaceutical compounds and their metabolites in aquatic environments have emerged as a major concern due to their persistence, toxicity, and potential for bioaccumulation. Among these contaminants, benzodiazepines such as diazepam (DZP) and midazolam (MDZ) are frequently detected in wastewater, where conventional treatment processes are largely ineffective. In this context, this work aims to valorize Sargassum biomass through its transformation into activated carbon and to evaluate its performance for the removal of DZP and MDZ from water via adsorption processes. In addition, the adsorption behavior under single and competitive conditions, the adsorbate–adsorbent interactions, and the potential for adsorbent reuse are investigated.
Activated carbon was produced from Sargassum by thermochemical activation using phosphoric acid as the activating agent. The physicochemical properties of the resulting material were characterized using nitrogen adsorption–desorption isotherms, determination of the point of zero charge (pHPZC), Boehm titration, Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The activated carbon exhibited a well-developed porous structure, composed mainly of micropores and mesopores (39.6% and 60.4%, respectively), and a high specific surface area of 1695 m²/g, indicating its suitability as an adsorbent material.
Adsorption experiments were conducted in batch systems to obtain adsorption kinetics and equilibrium isotherms for DZP and MDZ under single and competitive conditions. The influence of pH, initial drug concentration, and contact time was systematically evaluated. Drug concentrations were quantified by UV spectrophotometry and high-performance liquid chromatography (HPLC). In addition, dynamic adsorption studies were carried out in fixed-bed columns to obtain individual and competitive breakthrough curves, which were modeled using the Thomas model.
The results demonstrated that the Sargassum-based activated carbon exhibits a high adsorption capacity for both pharmaceuticals. In single-component systems, maximum adsorption capacities of 659.4 mg/gAC for diazepam at pH 7 and 583.3 mg/gAC for midazolam at pH 9 were obtained at 25 °C. The Langmuir model provided the best fit for equilibrium data, suggesting monolayer adsorption on a relatively homogeneous surface. In competitive adsorption systems, reduced adsorption capacities were observed due to competition for active sites, and the experimental data were satisfactorily described by the Jovanovic model with a random distribution of settling times.
Breakthrough experiments showed breakthrough times of 61 min for DZP and 15 min for MDZ under single-component conditions, which decreased to 40 and 10 min, respectively, in competitive systems. The Thomas model accurately described the dynamic behavior, with average relative errors below 1.5%. Theoretical analysis indicated that adsorption is dominated by dispersive π–π interactions, with secondary hydrogen bonding, pointing to a physisorption mechanism. Overall, the results highlight the potential of Sargassum-derived activated carbon as an effective and sustainable material for the removal of pharmaceutical contaminants from water.
