Hydrochar from Sotol bagasse for groundwater remediation of arsenic and fluoride contaminants

Abstract

Hydrothermal carbonization (HTC) has emerged as a promising method for converting biomass waste into functional carbonaceous materials (hydrochar) capable of adsorbing hazardous contaminants from groundwater. This study optimized the HTC process parameters (temperature and time) to produce an efficient adsorbent from Sotol bagasse (Dasylirion leiophyllum), targeting the simultaneous removal of arsenic (As) and fluoride (F). A 3^K full factorial design was employed to synthesize hydrochars, which were evaluated based on yield, severity factor, and adsorption capacity. Characterization via scanning electron microscopy (SEM), infrared spectroscopy (FTIR), and thermal analysis (TGA) revealed distinct physicochemical properties among the materials. Textural parameters were acquired, attaining surface areas ranging from 2.9 to 8.9 m² g⁻¹. Adsorption tests in real groundwater demonstrated simultaneous removal capacities of 0.98 μg g⁻¹ for As and 91.95 μg g⁻¹ for F. Pareto statistical analysis identified the optimal hydrochar, synthesized at 300 °C for 80 min, as the most effective adsorbent. The effect of initial contaminant concentration was evaluated for both arsenic and fluoride, within the typical groundwater range. Adsorption isotherms were fitted into the Langmuir and the Freundlich models, providing essential parameters such as adsorption capacity and affinity constants. The hydrochar also effectively removed nickel alongside As and F. In addition, adsorption-desorption cycles were performed to demonstrate the feasibility of achieving the contamination limits set by the World Health Organization (WHO) for drinking water. Compliance was attained in two adsorption cycles for As (0.5 g hydrochar dose). For F, increasing the adsorbent dosage attained the WHO fluoride limits without needing pH adjustment.