Green synthesis of Sn(II)-BDC MOF: Preferential and efficient adsorption of anionic dyes

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Highlights

  • An Sn(II)-BDC metal-organic framework was synthesized via a green synthesis route.

  • The as-synthesized material displayed high aqueous and thermal stability.

  • Sn(II)-BDC adsorbent was applied for preferential adsorption of toxic anionic dyes.

  • The adsorbent exhibited excellent dye removal efficiency.

  • Sn(II)-BDC could be easily regenerated with promising multicyclic reusability.

Abstract

Tin metal-organic framework [Sn(II)-BDC MOF] was synthesized following the hydrothermal synthesis route in environmentally benign conditions. The material was applied for preferential and efficient removal of toxic anionic dyes viz. CR, EBT, and EY from aqueous solution. The thermal and aqueous stability of the material was confirmed using PXRD and FTIR analysis. Adsorption kinetic study and the thermodynamic parameters were also investigated. A better fit with the second-order kinetic model indicated that the process is predominantly chemisorption. The Langmuir and Freundlich isotherm models were used for validating the experimental data, where the Langmuir model provided a good fit. The maximum adsorption capacity (qm) for three anionic dyes (CR, EBT, and EY) was 95.2 mg g−1, 125 mg g−1, and 208.3 mg g−1, respectively. The actual evaluation of the adsorption performance was based on the partition coefficient (PC) values. At an initial concentration of 100 mg L−1, the PC values for CR, EBT, and EY were 33.79, 30.29, and 40.44 mg g−1 μM−1, respectively. Based on high PC values and maximum adsorption capacity, Sn(II)-BDC MOF turned out to be an excellent material for the removal of anionic dyes. Further, retention of >86% of its adsorption efficiency after three adsorption-desorption cycles indicated the potential reusability of the synthesized material. This study provides valuable insight into the eco-friendly synthesis of a novel water-stable Sn(II)-BDC MOF and its application as a promising adsorbent in the removal of toxic anionic dyes from aqueous solutions.

Graphical abstract

A water stable Sn(II)-benzedicarboxylate MOF has been synthesized. The MOF material produced preferential removal of anionic dyes with high removal efficiency compared to cationic dyes.

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Introduction

Synthetic dyes from various industrial sources such as paper, printing, textile cause pollution of the water bodies. Organic dyes, due to their complex structure, are resistant to biodegradation [1,2]. Owing to their carcinogenic, toxic and mutagenic nature, the organic dyes possess a severe threat to living beings and the environment [3]. In addition to its health effects, the presence of dyes in wastewater streams is highly visible, which reduces the penetration of sunlight and lowers oxygen solubility, thereby affecting the aquatic ecosystem [4]. Thus it is of prime concern to remove such toxic dyes from our environment.

Physico-chemical treatment processes such as flocculation, coagulation, oxidation, membrane separation, chemical precipitation, and adsorption are primarily employed for dye removal from the aqueous environment [[5], [6], [7], [8], [9]]. However, among these methods, adsorption has been widely adopted owing to its simplicity, low cost, ease of operation, process efficiency, and environment-friendly nature [10]. Various adsorbents such as activated carbon, carbon nanotubes, resin, and activated diatomite have been explored for the adsorptive removal of dyes from aqueous solution [11,12]. However, these adsorbents were accompanied by certain limitations such as low adsorption capacity, poor selectivity, complex preparation techniques, and difficulty in regeneration [13]. Hence, it is a prerequisite for an adsorbent to possess promising adsorption capacity along with potential recyclability. Metal-organic framework (MOF) which are a class of porous crystalline inorganic-organic hybrid material based on inorganic metal ion center coordinated to bridging organic ligand via coordination bond, has received extensive attention due to their potential applications in catalysis [14], gas adsorption/separation [15], drug delivery [16] and sensing [17]. In the past few years, MOFs have been explored for the removal of dyes from aqueous solutions due to their diverse chemical compositions, tunable pore size, a vast array of structures, high surface area, and coordinately unsaturated metal sites providing multiple functionalities regulating its adsorption capability.

Moreover, the breathing effect, presence of open metal sites, and framework of metal ions in MOFs are reported as potential mechanisms of dye removal [[18], [19], [20]]. In comparison with the conventional porous materials such as zeolite and activated carbon, which require high temperature during synthesis, MOFs require less processing temperature. The pore size of the activated carbon and zeolites being limited to 0.3–1 nm, it potentially hinders the adsorption of large dye molecules and deters the desorption process [21].

In contrast, due to the presence of abundant larger pores (micro to mesopores), adsorption of hazardous organic dyes is enhanced in MOFs providing, a promising removal efficiency for these harmful dyes. All of the aforementioned advantages establish MOFs as a superior material for adsorption purposes [22]. However, the drawback of MOFs has been its stability in aqueous solution due to the hydrophilic nature of the metal nodes where the metal-linker coordination bond gets hydrolyzed, thereby disrupting the framework structure [23]. To date, very few MOFs are reported to be stable under aqueous environment for a more extended period [24]. The stability of the MOF can be explained based on Pearson's HSAB principle. Sn(II), being a hard Lewis acid, can form a relatively stronger coordination bond with hard Lewis base, i.e., carboxylate, resulting in the formation of a rigid interconnected framework structure [25]. Besides, the stability of MOF increases with the increasing inertness of the metal ion [26].

Sn(II)-dicarboxylate coordination polymers have been reported to be hydrothermally or solvothermally synthesized under alkaline conditions producing known homoleptic 1D and 2D architectures or 3D frameworks consisting of tin-oxide as secondary building units (SBUs) [27,28]. Recently, air and moisture stable Sn(II) coordination polymers have been reported with polytropic carboxylate based organic linkers [29]. Tin being an environmentally benign material has been used widely in food packaging. Tin salts possess low toxicity to the human body as it is poorly absorbed and rapidly excreted in the form of feces [30]. From the commercial point of view, Sn(II) salt act as a low-cost precursor for material synthesis [31]. To the best of our knowledge, tin-based MOF has not been explored for wastewater applications, and there is a scarcity in studies with coordination polymers of Sn(II). The present research focuses on the adsorptive removal of different anionic dyes from aqueous solution using Sn(II)-BDC MOF. The material was synthesized using hydrothermal DMF free synthesis route using Sn(II) as inorganic metal ion and BDC as a source of the organic linker. The aqueous stability of the synthesized material was confirmed. The adsorption efficiency of different anionic dyes, like Congo red (CR), Erichrome black T (EBT), Eosin yellow (EY), and Methyl red (MR) were compared with cationic dyes viz. Methylene blue (MB), Crystal violet (CV), Rhodamine B (RhB) and Bismark brown (BB). Batch adsorption experiments were carried out to study the effect of dose, initial dye concentration, contact time, and temperature on the removal efficiency. The experimental data were fitted into three linear forms of kinetic models, and the plausible mechanism for adsorption was proposed. Finally, the multi-cyclic efficiency and reusability of the synthesized material were also tested. The Sn(II)-BDC MOF proved to be a promising adsorbent with excellent aqueous stability for preferential removal of anionic dyes.

Section snippets

Materials and reagents

Stannous chloride dihydrate [SnCl2. 2H2O] and terephthalic acid (Benzene-1,4-dicarboxylic acid, BDC) were used as the precursor salt and polycarboxylic acid linker, respectively, for the MOF synthesis. Sodium hydroxide (NaOH) and different organic dyes (Fig. 1) used in this study were all purchased from Merck India Ltd and Himedia India, respectively. The reagents in this study were of an analytical grade and used without further purification.

Synthesis of Sn(II)-BDC MOF

1.0 mmol SnCl2.2H2O was dissolved in 5.0 ml

Characterization of Sn(II)-BDC MOF

The FT-IR spectrum of Sn(II)-BDC MOF, as depicted in Fig. 2a reveals the characteristic peak at ~580 cm−1, which relates to Sn–O stretching vibration [32,33]. Further, the peaks at 1539 cm−1, 834 cm−1, 735 cm−1 can be attributed to the Cdouble bondO and C–H stretching vibrations, respectively [34,35]. The surface area of the Sn(II)-BDC MOF was found to be much less with pores predominantly in the microporous range. The surface area was calculated via the BET method in the relative pressure P/Po range of

Desorption and recyclability

Desorption experiments were carried out to determine the feasibility of regenerating the dye-saturated Sn(II)-BDC MOF for reuse. The desorption study was performed at a pH range of 8–12 using 0.1 M NaOH for the dyes adsorbed at pH ~6. It was observed that the desorption efficiency was directly proportional to an increase in solution pH (Fig. 5c). At pH > 11, the desorption efficiency was maximum, amounting to ~50%, ~80%, and ~99% for CR, EY, and EBT, respectively (Fig. S3). At higher pH, the

Conclusion

In this study, novel Sn(II)-BDC MOF was successfully synthesized, and its application in the removal of anionic pollutant dye was studied in detail. The results revealed that the adsorption followed the pseudo-second-order kinetic model, and the equilibrium adsorption data fitted well with the Langmuir isotherm model. The proposed mechanism for the adsorption is primarily electrostatic interaction between the positively charged MOF surface and negatively charged anionic dyes. π-π stacking

CRediT authorship contribution statement

Arnab Ghosh: Formal analysis, Writing - original draft, Data curation. Gopal Das: Formal analysis, Writing - original draft.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The financial support by DBT New Delhi, India through grant BT/COE/34/SP28408/2018 is highly acknowledged. The author also thanks to the Central Instrument Facility (CIF), IIT Guwahati for providing required instrumental facilities. AG thanks IIT Guwahati for a research fellowship.

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