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Graphical abstract Abstract Potassium humate possesses a wide variety of oxygen-containing functional groups and has good adsorption and complexation properties for many kinds of transition metal ions. In this work, a simple and separable route had been developed to synthesize Fe 3 O 4 /graphene nanocomposite, derived from potassium humate, were employed for the effective adsorption of Cr(VI) in aqueous phase. In addition, the Fe 3 O 4 /graphene nanocomposite prepared at 600 °C (S-600) showed a maximum adsorption capacity of 280.6 mg/g for Cr(VI) and good cycling performance, which was much higher than those reported graphene-based adsorbents and other conventional adsorbents. The adsorptive behavior was well fitted to the Langmuir isotherm model. The adsorption kinetics for removal of Cr(VI) agreed with pseudo-second-order equation, which indicates a kind of chemical adsorption. Its superior adsorption performance might be due to the effective adsorption sites at the surface and formation of FeOH2+ on the surface of Fe 3 O 4 /graphene. This work provided a promising approach for the removal of Cr(VI) from wastewater using a Fe 3 O 4 /graphene nanocomposite, which shows a huge number of application prospects. [ABSTRACT FROM AUTHOR]

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]

Highlights: [•] The iron oxide is gained via a facile, green, and economical synthesis process. [•] The promising adsorptive applications in water treatment are investigated. [•] Congo red removal capacity is 253.8mgg−1, and commercial α-Fe2O3 is 42.4mgg−1. [•] The maximum adsorption capacity for Cr(VI) is 17.0mgg−1. [ABSTRACT FROM AUTHOR]