ZIF-67 derived magnetic nanoporous carbon coated by poly(m-phenylenediamine) for hexavalent chromium removal

•The new magnetic nanocomposites (MNC@PmPD) was synthesized in situ polymerization.•The maximum adsorption capacity of MNC@PmPD2 for Cr(VI) was 240.44 mg·g-1.•The possible adsorption and reduction mechanism of Cr(VI) was proposed. A novel core–shell magnetic nanomaterial is synthesized to adsorb highly toxic chromium (Cr(VI)) from water. Cobalt-based MOF (ZIF-67) is used as precursor to prepare magnetic nanoporous carbon (MNC) by one-step carbonization and etching process. Poly(m-phenylenediamine) (PmPD) is assembled on the surface of MNC by in-situ polymerization at low temperature. The magnetic MNC@PmPDs with definite core-shell structure are successfully fabricated. The adsorption capacity of MNC@PmPD2 for Cr(VI) can reach 240.44 mg·g-1, much higher than the bare MNC (126.27 mg·g-1). The core–shell structure improves the adsorption efficiency of the PmPD shell to 445.64 mg·g-1, compared with that of bare PmPD (332 mg·g-1). The MNC@PmPD2 can be easily separated from the solution by the advantage of magnetic-core. Moreover, the prepared MNC@PmPD2 exhibits high stability during adsorption with negligible Co leaching and structural decomposition. The mechanism of Cr(VI) removal is considered to be based on the synergistic effect of adsorption and reduction. The removal rate of Cr(VI) follows the pseudo-second-order model and the adsorption isotherm data fits well to the modified Langmuir isothermal model. The excellent adsorption capacity, easy magnetic separation and high structure stability indicate a promising application of MNC@PmPD for Cr(VI) remediation in wastewater.