Valorization of aloe vera waste for the production of Ca and P-rich hydrochars

The production of gel from Aloe vera plants is an emerging industry, however the residual leaves are considered waste and are not utilized. The purpose of this work was to prepare hydrochars from Aloe vera leaves under different conditions, and determine the fate of macro- and micronutrients in the solid products, in view of developing a novel biofertilizer. In this framework, distilled water, citric acid and potassium hydroxide solutions were used as feedwaters and the experiments were performed at the temperatures of 180 and 220 °C and treatment times of 1 – 8 h. All obtained hydrochars had increased Ca and P contents compared to the feedstock, whereas the highest Ca and P concentration of 10.4% and 7382 mg kg−1, respectively, were achieved under alkaline conditions after 8 h of treatment. Citric acid promoted carbonization, achieving H/C = 1.11, O/C = 0.34 of the hydrochar obtained at 220 °C after 8 h of treatment. The use of KOH counterbalanced the production of acids from the degradation of the biomass, thus restricting the carbonization progress. The metals K, Fe, Cu and Mg showed high solubilization tendencies, however their concentrations on the hydrochars increased beyond the 4 h treatments due to precipitation and re-sorption phenomena. The N content in the hydrochars ranged between 0.56 – 1.12%, increasing with temperature and time due to the formation and precipitation of pyridine, pyrroline and pyrrolidine derivatives. Overall, due to the high ash content, low pH and increased Ca and P concentrations, the hydrochars may be suitable as substrates for the development of bio-fertilizers purposed for alkaline soils.