Ultra-high adsorptive removal of dyeing wastewater by salt-tolerant MOGs via green fabrication strategy and mechanism investigations

High-concentration salts in dyeing wastewater restrict the applications of adsorption technology. To address this challenge, this study aimed to enhance the salt tolerance of adsorbents by focusing on coordination interactions between adsorbents and adsorbates. Accordingly, two metal ions with distinct electronic structures (Cu: nearly saturated, Co: unsaturated), possessed varying coordination abilities and were employed to fabricate metal-organic gels (Cu-MOG and Co-MOG) via the room-temperature fabrication strategy for exploring the adsorptive removal efficiencies towards congo red (CR). Specifically, Cu-MOG and Co-MOG had hierarchical pore structures, abundant functional groups and relatively big surface areas (191.68 and 241.02 m²/g), and the adsorption processes followed pseudo-second-order kinetics ( R 2 > 0.99) and the Freundlich isotherm model ( R 2 > 0.98), achieving maximum CR adsorption capacities of 4635.5 mg/g and 3603.2 mg/g with adsorption rate of 0.0042 g/mg·min and 0.118 g/mg·min, respectively. Notably, Co-MOG exhibited the strong salt-tolerance and remained the satisfied CR adsorption capacities (> 75 mg/g) even under ions with high concentrations (0.3 M). The reasons could be the stable coordination interactions of Co-MOG with SO 3 – group of CR validated by “Hard Soft Acid Base (HSAB)” theory, as well as the “ionic atmosphere”, salting-out effect and spatial hindrance in the system. The results of DFT, comprising the low energy gap (E gap ) of Co-MOG (1.94 eV), the big absolute value of E ads (Co-MOG@CR:-1.95 eV) and RDG analysis, also interpreted the superiority of Co-MOG for CR removal. Moreover, adsorption mechanisms included pore filling, water affinity, electrostatic interactions, hydrogen bonding, coordination interactions and π-π interactions. Besides, MOGs possessed the excellent recyclability (relatively CR removal efficiency of 74.5 % and 79.8 % for Cu-MOG and Co-MOG, respectively) and treated CR solution expressed no harm to the wheat growth from the phytotoxicity evaluation after 11 days cultivation, reflecting the broad application potentials in actual dyeing wastewater treatment. This work highlights the novel strategy for adsorbent fabrication, and enriches the choice of adsorbents for efficient dye removal under saline conditions. • The novel Co/Cu-MOG was successfully fabricated under ambient conditions. • Co-MOG exhibited superb CR adsorption capacities ( q m =3261.1 mg/g at 298 K). • Co-MOG showed strong salt tolerance (ion concentrations: 0–300 mM). • Coordination of unsaturated Co 2+ with CR plays a crucial role in CR adsorption. • Multiple mechanisms dominated by coordination interactions enhanced salt tolerance.