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Highly efficient removal of both cationic and anionic dyes from wastewater with a water-stable and eco-friendly Fe-MOF via host-guest encapsulation

Shuanghong Tian,Shuang Xu,Jinting Liu,Chun He,Ya Xiong,Pingyun Feng

A water-stable cationic Fe based metal organic framework, CPM-97-Fe, was successfully synthesized in a large scale by a modified method (CPM: Crystalline Porous Materials). The adsorption behaviors , including removal efficiency, adsorption kinetics and isotherms of nine hazardous dyes in wastewater were investigated with different pH and organic solvents. The pseudo-second-order adsorption rate constant follows the order of Reactive Brilliant Red X-3B (X-3B, 0.0205 g mg −1 min −1 ) > Acid orange 7 (AO7, 0.0112 g mg −1 min −1 ) > Congo Red (CR, 0.0080 g mg −1 min −1 ) >  Methylene Blue (MB, 0.0061 g mg −1 min −1 ) >  Rhodamine B (RhB, 0.0049 g mg −1 min −1 ) > Croscein Scarlet 3B (CS3B, 0.0041 g mg −1 min −1 ) > Methyl Blue (MB, 0.0017 g mg −1 min −1 ) = New Coccine (NC, 0.0017 g mg −1 min −1 ) > Acid Black 1 (AB1, 0.0014 g mg −1 min −1 ). It is found that the adsorption quasi-equilibrium can reach quickly within 30 min, but the dyes with proper structure (i.e. proper distance between two sulfonic groups, less azo benzene hindrance effect, more reactive Cl groups, and more benzene rings), smaller molecular size, more negative charges, lower molecular mass favor the faster adsorption. The adsorption capacity obeys another sequence of CR (831 mg g −1 ) > X-3B (648 mg g −1 ) > AO7 (502 mg g −1 ) > MLB (380 mg g −1 ) > CS3B (356 mg g −1 ) > AB1 (325 mg g −1 ) > RhB (306 mg g −1 ) > MB (304 mg g −1 ) > NC (157 mg g −1 ). It is disclosed that CPM-97-Fe can adsorb more dyes with larger molecular mass , less negative charges, and a proper structure. Especially for Congo red, the adsorption capacity on CPM-97-Fe is 831 mgg −1 , 8.1 times that on activated carbon (103 mgg −1 ). Even the adsorption capacities of very bulky Rhodamine B and Methyl Blue reach as high as 306 and 304 mgg −1 , respectively, ranking the highest level among MOF materials. The special structure of CPM-97-Fe with big open windows and high specific surface area of 1397.0 m 2  g -1 contributes to the fast adsorption and high capacity towards all investigated dyes via the host-guest encapsulation mechanism including electrostatic interaction, ion exchange, π-π stacking interaction, as well as pore filling.