In this work, surface-functionalization agents and various salts as auxiliaries were used to improve the adsorption capacity of biochars produced via hydrothermal carbonization of glucose, cellulose, and hazelnut shell. The effect of surface functionalization with different acids and bases as well as neutral salts on the adsorption of methylene blue and methyl orange was investigated using Boehm titration, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy techniques. Surface functionalization of biochars leads to an increase in the number of oxygen -containing functional groups on the surface, thus increasing the adsorption capacity. To probe the influence of ionic strength on the adsorption process, three auxiliaries, namely potassium chloride, cetyltrimethylammonium bromide, and sodium bicarbonate, were examined. The increase in total electrolyte concentration significantly affected the pH of the solution and as a result electrostatic interactions between dyes and biochars. Dye adsorption capacities reached 234.57 mg/g for methylene blue and 306.13 mg/g for methyl orange in the presence of cellulose-based and NaOH-functionalized biochar and glucose-based H2SO4-functionalized biochar, respectively.