Enhanced Cr(VI) reduction and As(III) oxidation in ice phase: Important role of dissolved organic matter from biochar

- Biochar-derived dissolved organic matter (DOM) effectively reduced Cr(VI) and oxidized As(III).
- Cr(VI) and As(III) could serve as a redox couple.
- Cr(VI) and As(III) redox conversion was more effective in the ice phase than aqueous phase.
- FTIR and ESR showed that biochar DOM served as both electron donor and acceptor.