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Heavy metal Cr(VI) in water body has attracted extensive attention due to its carcinogenicity, teratogenicity and mutagenicity. In this study, magnetic walnut shell biochar (Fe 3 O 4 @WSB-n) were prepared from walnut shell (WS) by impregnation (to destroy the wooden structure of walnut shell), alkali activation and hydrothermal synthesis. Not only solved the problem of difficult separation of adsorbent, but also realized the reuse of environmental waste. Results manifest that Fe 3 O 4 @WSB-1 has a hierarchical honeycomb structure with a specific surface area of 914.01 m2 g−1. And the adsorption process of Cr(VI) on Fe 3 O 4 @WSB-1 belongs to monomolecular chemisorption, and the maximum adsorption capacity is reached to 230.63 mg g−1 (0.01 g, 30 mL pH = 2.0, 25 °C). Adsorption capacity of Fe 3 O 4 @WSB-1 for Cr(VI) can still keep 66.85 mg g−1 after 5 cycles. The XPS and FT-IR results show that the oxygen-containing functional groups play a significant role in the adsorption process, and the magnetic centers further promote the reduction of Cr(VI) through reducibility of Fe2+. Due to efficient conversion of hardwood structures, high stability in solution and easy recovery, magnetic biochar is suitable for practical wastewater treatment. [Display omitted] • An efficient method for converting walnut shell hardwood structure into biochar by pyrolysis co-activation. • The composite Fe 3 O 4 @WSB-1 has a hierarchical honeycomb structure with a specific surface area of 914.01 m2 g−1. • Fe 3 O 4 @WSB-1 has excellent adsorption performance (230.63 mg g−1) for hexavalent chromium in wastewater. • Fe 3 O 4 @WSB-1 has strong magnetism and stability, which is beneficial to recovery and reuse of adsorbent. [ABSTRACT FROM AUTHOR]