Treatment of Diesel Oil-Contaminated Water Using Hydrochar Derived from the Hydrothermal Carbonization of Aloe Vera Leaves

Abstract The cultivation of aloe vera and the exploitation of the valuable gel is an emerging agro-industry in various parts of the world, including the Mediterranean region and south-east Asia. Following the separation of the gel from the aloe vera leaf, the latter is considered waste and a sustainable valorization pathway has not been established so far. Due to its high moisture content, it is well-suited for hydrothermal carbonization (HTC). In this study, HTC of aloe vera leaves was carried out at temperatures of 180—220 °C and residence times of 2- 8 h. After an initial screening of the hydrochars for diesel oil (DO) adsorption, the optimum sample was prepared at 180 °C and 2 h treatment time (AV-180–2). This sample was characterized using elemental and surface analysis methods and was used without modification in the adsorption DO from aqueous solutions. The maximum achievable adsorption capacity of hydrochar AV-180–2 was 12 g g −1 at 25 °C, with an initial DO content of 10 g L −1 , rotating speed of 100 rpm, hydrochar quantity of 1.0 g L −1 and pH ~ 7.0, adsorption equilibrium was achieved after 15 min of contact time. The adsorption process followed the pseudo-second-order kinetic model with a correlation coefficient R 2 = 0.995. The experimental data fitted best to the Freundlich isotherm (R 2 = 0.998) and the thermodynamic study indicated a spontaneous and endothermic adsorption process. Overall, hydrothermal carbonization offers a new and sustainable exploitation pathway for aloe vera waste and the hydrochars showed a promising behaviour towards DO-contaminated aqueous phases.