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A cellulose-based composite adsorbent (PEI-PAM-CF) was fabricated via a facile cross-linking of polyethyleneimine/polyacrylamide (PEI/PAM) and cellulose using epichlorohydrin as the cross-linker. It possessed a double-network structure and emitted blue fluorescence. Compared with the control samples (PEI-CF and PAM-CF), the PEI-PAM-CF showed a synergy effect for the Cr(VI) removal. It can effectively remove Cr(VI) at low concentration with no structure destroyed for the adsorbent. Other coexisting anions (NO 3 -, Cl-, H 2 PO 4 - and CH 3 COO-) and cations (Na+, K+, Mg2+, Ca2+ and Cr3+) had no significant effect on Cr(VI) removal except SO 4 2-. After five adsorption/desorption cycles, the Cr(VI) removal only decreased with a loss of 7%, suggesting that the PEI-PAM-CF had good regeneration performance. Most importantly, the electrostatic adsorption mechanism between Cr(VI) and the PEI-PAM-CF was revealed by a reversible fluorescence "on-off-on" phenomenon and the DFT calculation. The adsorption sites on the PEI-PAM-CF were attributed to the amino groups of the PEI and PAM. Hence, the PEI-PAM-CF was potential for the treatment of Cr(VI)-containing wastewater at low concentration. [Display omitted] • A cellulose-based composite adsorbent with a dual-network structure was prepared. • The adsorbent PEI-PAM-CF can effectively uptake the low concentration Cr(VI). • A synergy effect was observed after adding PEI and PAM together. • The proposed electrostatic adsorption mechanism was clarified. • The DFT calculation was performed to study the adsorption sites. [ABSTRACT FROM AUTHOR]