The removal of Cr(VI) from wastewater is a crucial task due to its high toxicity. In this study, slumgum-originated biochar materials were obtained by three different methods: high-temperature pyrolysis with H3PO4 or CO2 and the high-temperature treatment of CO2-activated slumgum-originated biochar in an Ar atmosphere. The obtained materials were subjected to physicochemical characterization (nitrogen adsorption/desorption isotherms, CHN elemental analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy) and tested for their adsorption properties towards Cr(VI) ions. The solution pH, contact time, and effects of the Cr(VI) concentration on Cr(VI) adsorption onto biochar materials were studied. The kinetics and isotherm experimental data were best fitted to the Elovich (R2 = 0.848) and Freundlich (R2 = 0.965) theoretical models for H3PO4-modified biochar. The highest adsorption capacity (45.0 mg g−1) for Cr(VI) was obtained for biochar modified with H3PO4. It was stated that the relatively fast rate of Cr(VI) adsorption onto this biochar (equilibrium reached within 120 min) is related to its mesoporous structure. The mechanism of Cr(VI) adsorption onto H3PO4-modified biochar was studied in detail. The obtained biochar was successfully applied for efficient Cr(VI) removal from wastewater originating from spent bricks with a low biochar dosage (4.0 g L−1).