Adsorption mechanism of Rhodamine wastewater in the MOFs materials: Effect of metal center and organic ligand

Dye wastewaters are discharged mainly from the textile industry, the dyeing industry, the paper and pulp industry, the tannery and paint industry and the dye manufacturing industry which will cause serious environmental pollution and pose serious threats to humans and aquatic creatures [1]. Rhodamine B (RhB) as a typical synthetic dye, is widely applied in the production of dye lasers, stamp pad inks, fireworks, carbon sheets, and ballpoint pens [2] and is frequently used as a model dye pollutant in the wastewater treatment study due to its high stability in the solution [3]. Rhodamine 6G (Rh6G) as the geometric isomer of RhB, is a synthetic dye used for the dyeing of wool, cotton, silk, and papers with fluorescent effects, which is more stable than RhB and more difficult to be degraded [4]. Current technologies for the treatment of Rhodamine wastewater mainly focus on the biodegradation, chemical degradation, and physical removal method. The biodegradation method using microorganisms under specified conditions can achieve partial RhB decomposition but the concentration of wastewater is limited and the intermediates and by-products will produce in the degradation process, which requires further deep retreatment [5]. The chemical degradation of Rhodamine wastewater has been extensively studied in recent years, including photocatalytic degradation [6], catalytic wet oxidation degradation [7], and other advanced oxidation processes [8] that can ensure deep degradation of the RhB wastewater. However, the energy consumption and catalytic activity are the main concerns in chemical degradation. Physical removal technologies involving adsorption, ion exchange, coagulation or flocculation, membrane filtration, nanofiltration or ultrafiltration and reverse osmosis have the advantage of energy savings and easy secondary disposal which can be used for the wide range of dye wastewater treatment [9].

Physical adsorption is frequently used in the treatment of dye wastewater using proper porous adsorbents such as activated carbons, molecular sieves, metal–organic frameworks (MOFs), metal oxides, bio-adsorbents, and polymer-based materials [10], [11]. MOFs materials as novel adsorbents have attracted much attention in recent years due to its high specific areas, adjustable metal centers and controllable pore size distribution, which enable the selectivity of the adsorption process [12], [13], [14]. Md Jamal Uddin et al. [15] systematically reviewed the adsorptive removal of dye wastewater using metal–organic frameworks materials and pointed out that the dye adsorption mechanisms on MOFs were controlled by electrostatic attraction, functional groups, ligands, pH and so on. Semanur Sağlam et al. [16] reviewed the metal–organic frameworks used for the adsorption removal of dye, and the isotherms and kinetics of adsorption were discussed in the adsorption of dye wastewater by MOFs adsorbents. The MOFs adsorbents showed high adsorption capacity with adjustable pore diameters and surface morphology, which exhibited superior properties compared with those of conventional porous materials. Recent studies on MOFs adsorption in dye wastewater treatment involved Co/Zn MOFs for the removal of Reactive Black 5 and Methyl Orange [17], Zn-MOF nanocubes for the removal of malachite green dye [18], Ti-MOFs for the adsorption of methylene blue, methyl orange and indigo [19]. Additionally, UiO-66, MIL-101 [20], MIL-53, MOF-818, ZIF-8, et al. were used intensively as adsorbents or catalyst supports in wastewater treatment [21], [22]. Among them, UiO-66 is a typical microporous MOFs material employing Zr⁴⁺ as the metal center and p-phthalic acid as the organic ligand that is frequently applied in adsorption and catalysts [23], [24], [25]. In the previous study, UiO-66 was prepared and granulated for continuous adsorption of three type organic wastewaters that showed distinguishing adsorption properties [26]. The adsorption mechanism of the MOFs materials for dye wastewater needs to be further deep studied.

The study of the adsorption mechanism is essential for understanding the adsorption process thus further providing enhancement strategies of adsorption process and the important guidance of optimizing the synthesis procedures and ingredients of novel adsorbents. MOFs materials can provide a new way to study the adsorption mechanism because of their changeable metal center and organic ligand and thus realise the adjustable of pore size distribution and adsorption site. Furthermore, the interaction mechanism of the adsorbents and adsorbates can be established and illuminated in depth. For example, UiO-67 [27], [28] and MOF-808 [29], [30] have the same Zr⁴⁺ center as UiO-66 but different organic ligand which can make the pore size of MOFs materials controllable. In addition, MIL-101(Fe) [31] and MIL-101 (Cr) [32], [33] have the same organic ligand as UiO-66 but different metal center. These MOFs materials showed good hydrothermal stability and can be used as adsorbents or catalysts in organic wastewater [34], [35]. However, in-depth research on the interaction of MOFs materials with the adsorption process is rarely reported. Exploring a new method to evaluate the effect of material properties on the adsorption process is a significant work in wastewater treatment.

The aim of this study is to explore the adsorption mechanism of Rhodamine wastewater on MOFs materials (using UiO-66, UiO-67, MOF-808, MIL-101(Fe) and MIL-101(Cr) as the adsorbents) by studying the effects of metal center and organic ligand on the adsorption process of RhB and Rh6G wastewater. The pseudo-first-order kinetic model, pseudo-second-order kinetic model, and Weber-Morris model were used for the adsorption kinetic fittings. The Langmuir and Freundlich models were employed to study the adsorption isotherms. FT-IR, XRD, N₂ adsorption–desorption and TGA characterization of the MOFs materials were analyzed in the adsorption process.