Simultaneously photocatalytic removal of Cr(VI) and metronidazole by asynchronous cross-linked modified sodium alginate

•Cr (VI) and metronidazole can be simultaneously removed by asynchronous cross-linked modified sodium alginate.•Amino groups and cross-linked Fe3+ can promote the rapid separation and transfer of electrons.•Cr (VI) is removed by the synergistic effect of electrostatic adsorption and photocatalytic reduction, and metronidazole was removed via photocatalytic oxidation.