Optimization of one-pot H3PO4-activated hydrochar synthesis by Doehlert design: Characterization and application

In this work, the hydrochar process optimization was carried out from Brewer’s spent grains (BSG) using wet pyrolysis (WP) with one-pot acid activation. The Doehlert Matrix was used as an experimental design to evaluate the influence of temperature (100, 125, 150, 175, and 200 °C) and residence time (4, 14, and 24 h), totaling nine experiments. The responses to evaluate the synthesis performance were mass yield and methylene blue (MB) dye removal capacity in an aqueous solution. The physicochemical properties of each material were determined. The optimized hydrochar (150 °C, 14 h) presented a specific surface area of 9 m² g−1. The pseudo-second-order kinetic model and the Freundlich isotherm model best fit the experimental data of MB removal by hydrochar. In the isotherm study, the amount of MB adsorbed at equilibrium varied from 39.39 to 107.66 mg g−1 for initial concentrations of 50–250 mg L−1, with a maximum adsorption capacity of 134.78 mg g−1, estimated by the Langmuir model. These results were obtained using 1.00 g L−1 of biochar at 30 °C and pH = 6. Finally, it can be concluded that WP with activation in a single step proved to be a suitable way to treat BSG, whose obtained material has a high potential to be used as a low-cost adsorbent for remediating organic pollutants in water.