Natural organic acid hydrophobic modified alkaline hydroxylated charcoal based on supercritical CO2 medium as well as preparation method and application of biochar

The invention discloses natural organic acid hydrophobic modified alkaline hydroxylated biochar based on a supercritical CO2 medium as well as a preparation method and application of the biochar. The method comprises the following steps: (1) adding acetone into biochar, performing mixing, and carrying out ultrasonic treatment, filtering and washing to obtain clean biochar; (2) adding hydrochloric acid with stirring, and performing filtering, washing, and drying in an inert gas atmosphere to obtain dried biochar; (3) adding the dried biochar into an alkali solution, performing heating and stirring, performing filtering, and performing drying in an inert gas atmosphere to obtain hydroxylated biochar; (4) adding the hydroxylated biochar into a solution in which natural macromolecular organic acid, absolute ethyl alcohol and a dehydrating agent are dissolved, and performing heating reaction in a supercritical CO2 medium to obtain a reacted mixed solution; and (5) washing the filtered precipitate, and performing d