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Hydrochars produced via hydrothermal carbonization of rice husk were post-treated using three different acids and two bases. The post-treated hydrochars were characterized using different methods and evaluated for adsorption of dyes and antibiotics from aqueous solutions. The posttreated hydrochars had carbon and oxygen contents in the range of 37.4% to 44.7% and 29.9% to 38.6%, respectively, and their surfaces were rich in oxygen-containing functional groups and negative surface charges. Post-treatment remarkably influenced the physico-chemical properties of the hydrochars’ surfaces. All post-treated hydrochars could adsorb the model organics from aqueous solutions with the highest adsorption amount of 327, 353, 336, and 309 mg/g for Congo red, berberine hydrochloride, tetracycline, and acridine orange, respectively, at 0.3 mg/mL and 25 °C. The combined effects of functional groups, surface area, and negative surface charge of the hydrochars could assist in the adsorption of organics. The Langmuir-isotherm model provided a good fit for the adsorption of acridine orange, tetracycline, and berberine hydrochloride on the selected hydrochars, while the adsorption of Congo red generated linear isotherms indicating a partition-involved mechanism. The adsorption of the organics onto the hydrochars is a spontaneous and mainly physical adsorption process with a quite fast adsorption rate. [ABSTRACT FROM AUTHOR]