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DOI: 10.1039/D2EN01035F (Paper) Environ. Sci.: Nano, 2023, 10, 672-682

Adsorption and photocatalytic desorption toward Cr(VI) over defect-induced hierarchically porous UiO-66-(OH)2: a sustainable approach†

Yu-Hang Li ab, Meng-Yuan Liu ab, Yu-Wei Wei ab, Chong-Chen Wang *ab and Peng Wang ab
aBeijing Key Laboratory of Functional Materials for Building Structure and Environment Remediation, School of Environment and Energy Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China. E-mail: chongchenwang@126.com
bBeijing Energy Conservation & Sustainable Urban and Rural Development Provincial and Ministry Co-construction Collaboration Innovation Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, China

Received 16th November 2022 , Accepted 8th January 2023

First published on 10th January 2023

Abstract

A facile modulation strategy was adopted to fabricate hierarchically porous HP-UOH-X (HP, UOH and X represented the hierarchical pores, UiO-66-(OH)2 and the dosage of benzoic acid, respectively) via introducing benzoic acid with different dosages into the precursor solution of UiO-66-(OH)2. The formation of hierarchical pores boosted the exposure of –OH groups and the Cr(VI) mass transfer in HP-UOH-X, which vastly enhanced its sorption capacities and sorption rates. The optimal adsorbent (HP-UOH-80) displayed better sorption capacity (266.74 mg g−1) toward Cr(VI) (T = 308 K, pH = 2.0) and faster diffusion rate (k1 = 14.21 mg g−1 min0.5, k2 = 6.25 mg g−1 min0.5) than those of the pristine UiO-66-(OH)2 and other HP-UOH-X adsorbents. Interestingly, HP-UOH-80 exhibited good selective uptake ability toward Cr(VI) in different simulated water samples containing various competing anions. The corresponding mechanism was proposed that the –OH groups and the defect sites played the dominant contribution to Cr(VI) adsorption, which could be affirmed by Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, the strategy of photocatalytic Cr(VI) reduction for desorption was introduced to replace traditional chemical desorption. In all, this work presented an effective adsorbent and a sustainable approach for Cr(VI) elimination, which can regenerate the adsorbent via a photocatalytic process rather than chemical washing.


Environmental significance

Hexavalent chromium (Cr(VI)), as a highly toxic heavy metal ion, is generally detected in wastewater of various industries like paint-making, chromate leather tanning and so on. Therefore, it is essential to exploit effective techniques for purifying Cr(VI)-containing wastewater due to its huge threat to human health. In this work, a series of hierarchically porous UiO-66-(OH)2 (HP-UOH-X) was successfully fabricated via defect engineering for achieving highly effective Cr(VI) removal. The hierarchically porous structure not only exposed hydroxyl groups inside the UiO-66-(OH)2 framework but also boosted the mass transfer of Cr(VI) in the HP-UOH-X interior. The optimal HP-UOH-80 adsorbent exhibited an outstanding adsorption capacity (266.74 mg g−1) toward Cr(VI) and rapid diffusion rate. In addition, HP-UOH-80 exhibited good selectivity and reusability in Cr(VI) removal. A facile and green strategy of photocatalytic Cr(VI) reduction for desorption was presented to replace traditional chemical desorption. In all, this work presented an efficient adsorbent and a sustainable approach for Cr(VI) removal, which could regenerate the adsorbent via a photocatalytic process rather than chemical washing.

Introduction

Recently, water pollution problems induced by heavy metal ions had been widely concerned due to their huge threat to human health.1–3 Therein, hexavalent chromium (Cr(VI)), as an extremely toxic heavy metal ion, was generally detected in wastewater of various industries like paint-making, chromate leather tanning etc.4–6 Therefore, it was essential to exploit effective techniques for purifying Cr(VI)-containing wastewater. Adsorption was deemed as a prospective technique among many treatment techniques, considering that it was low-cost, green, effective and free of secondary pollution.7–9

Metal–organic frameworks (MOFs), as typical porous crystalline materials, gained increasing interest in water remediation due to their special structures and chemical features.10–12 As a typical MOF, UiO-66 and its composites were widely utilized in the adsorption field due to their outstanding water/chemical stability, superior specific surface areas, and highly adjustable structure.13,14 However, pristine UiO-66 was restricted by deficient sorption sites and a limited window (UiO-66: ca. 6 Å),15 inhibiting the maximum sorption capacity and the diffusion rate of contaminant into the frameworks. Modulating the UiO-66 framework via defect engineering could obtain larger pore sizes and more sorption sites than perfect UiO-66,16,17 which would overcome these disadvantages to further boost the adsorption performance. For instance, our group firstly utilized seignette salt to coordinate Zr(IV) on the [Zr6O4(OH)4] cluster of NH2-UiO-66 to achieve partial missing of Zr atoms and linkers, leading to both the exposure of rich unsaturated coordination oxygen and the formation of hierarchical pores.18 It was reported that the optimal defective NH2-UiO-66 displayed outstanding sorption capacity (186.14 mg g−1) toward Pb(II) and rapid diffusion rate (32.1 mg g−1 min0.5), in which the maximum sorption capacity and diffusion rate were about 34.2 and 66.9 times higher than those of pristine NH2-UiO-66, respectively. To boost the sorption capacity of UiO-66 toward Cr(VI), UiO-66-(OH)2, a derivative of UiO-66, was selected as the adsorbent due to the strong affinity between hydroxyl groups and Cr(VI).19,20 Zhan and co-workers adopted density functional theory (DFT) calculations to confirm that the (001) surface exposed BiOCl displayed good adsorption performance toward Cr(VI), which was attributed to the content of surface –OH groups and oxygen vacancies (OVs).21 Meanwhile, modulation could effectively enlarge the pore diameter of UiO-66-(OH)2 to further increase the amount of exposure of hydroxyl groups. The produced oxygen vacancies during defect construction could be as the active sorption sites toward Cr(VI). Therefore, selecting suitable modulators to modulate the structure of UiO-66-(OH)2 was a significant point to enhance the sorption capacity of UiO-66-(OH)2 toward Cr(VI). In addition, developing an effective, facile and green desorption method to replace chemical desorption also deserves more attention.

In this work, a simple and facile modulation strategy by adding benzoic acid (BA) with different dosages into the precursor solution of UiO-66-(OH)2 was presented to obtain hierarchically porous UiO-66-(OH)2 (HP-UOH-X; HP and UOH represented the hierarchical pores and UiO-66-(OH)2, respectively). The involvement of benzoic acid could compete with 2,5-dihydroxyterephthalic acid (H2BDC-(OH)2) linkers to combine with Zr–O clusters to generate unsaturated coordination sites. After the activation process, hierarchical pores will be formed within the framework of UiO-66-(OH)2 due to the loss of benzoic acid, which was favorable to increasing the mass transfer rates of Cr(VI). Meanwhile, hydroxyl groups on both ligand and defect sites inside the framework would be exposed, which could be regarded as active sorption sites due to the affinity between hydroxyl groups and Cr(VI). The optimal HP-UOH-80 exhibited outstanding sorption capacity, in which various influence factor experiments were performed to evaluate the influences of the initial pH values, natural water and coexisting anions on the sorption performance of Cr(VI). The possible sorption mechanisms were explored by various experiments and characterization methods. Additionally, we first proposed that Cr(VI) adsorbed on HP-UOH-X was reduced to Cr(III) by photocatalysis to achieve the regeneration of adsorbent, considering the good photocatalytic ability of UiO-66-(OH)2. This work offered an effective adsorbent for treating Cr(VI)-containing wastewater and provided a green strategy to achieve effective desorption.

Materials and methods

All the chemicals and characterization methods are listed in the ESI† (Sections S1 and S2).

Syntheses of hierarchically porous UiO-66-(OH)2

The fabrication method of UiO-66-(OH)2 followed a previous report with some modifications22 listed in the ESI,† Section S3. The synthetic process of hierarchically porous UiO-66-(OH)2 (HP-UOH-X, where X was the molar ratio of BA/H2BDC-(OH)2) was similar to that of UiO-66-(OH)2. Differently, benzoic acid (BA) with different dosages (the molar ratio of BA/H2BDC-(OH)2 was 0.2, 0.4, 0.8 and 1.6) was introduced into the precursor solution of UiO-66-(OH)2. To remove the coordinated benzoic acid, the obtained solid was activated by a mixed solution of 10% HCl and 90% DMF. Then, the activated powder was washed with methanol three times and dried at 60 °C for 12 h.

Batch adsorption experiments

The Cr(VI) adsorption experiments were performed by using K2Cr2O7 as the pollution model, and the remanent Cr(VI) concentration of samples was tested by using an Auto Analyzer 3 (AA3) Flow Injection Analyzer. The related content of adsorption capacity (qe) of the adsorbent, sorption kinetic experiments, sorption isotherms and thermodynamic parameters are listed in the ESI† (Sections S4 and S5).

Cyclic experiments and fixed-bed breakthrough column studies

Fresh adsorbent (20.0 mg) was mixed into 150 mL of 5.0 mg L−1 Cr(VI) solution at 298 K and pH 2.0, and the mixture suspension was shaken in a constant temperature shaker at the same speed for 60.0 min. Then, the mixture solution continued to be shaken up to 60.0 min under visible light (the spectrum of visible light is displayed in Fig. S1†) irradiation, ensuring that the Cr(VI) adsorbed on the adsorbent could be photocatalytically reduced into Cr(III). Then, the adsorbent was separated and dried for the next cycle.

The column study was carried out in a solid-phase extraction (SPE) column with a volume of 12.0 mL. In detail, 50.0 mg of fresh adsorbent (UiO-66-(OH)2 and HP-UOH-80) were packed into an empty SPE column. With the help of a vacuum pump, 5.0 mg L−1 Cr(VI) solution flowed through the SPE column at a flow rate of about 5.0 mL min−1 (a photograph of the device is displayed in Fig. S2†). 3.0 mL aqueous solution was filtered and collected at regular intervals to measure the residual Cr(VI) concentration using AA3.

Results and discussion

Characterizations of hierarchically porous UiO-66-(OH)2

The powder X-ray diffraction (PXRD) patterns (Fig. 1a) exhibited that all peaks of the as-prepared UiO-66-(OH)2 and various HP-UOH-X were similar to those simulated from the single-crystal data of UiO-66 (CCDC: 837796), implying that not only both UiO-66-(OH)2 and various HP-UOH-X were successfully fabricated, but also the structures of all defective UiO-66-(OH)2 were maintained well after defect construction. Similarly, as depicted in Fig. 1b–f, the scanning electron microscope (SEM) images showed that the as-prepared UiO-66-(OH)2 and series HP-UOH-X exhibited analogous octahedral morphology with diameters of about 200–300 nm. In addition, the Fourier-transform infrared (FTIR) spectra demonstrated that all peaks of the as-prepared HP-UOH-X were still retained compared to those of pristine UiO-66-(OH)2 (Fig. S3†), further illustrating that the structures of the series HP-UOH-X were kept well. However, the formed defect sites could be preliminarily observed via high-resolution transmission electron microscopy (HR-TEM).23,24 The HR-TEM images displayed the global and local morphologies of UiO-66-(OH)2 (Fig. 1g and h) and HP-UOH-80 (Fig. 1i and j). Different from the local morphology of pristine UiO-66-(OH)2 (Fig. 1h), some voids could be observed in the local magnification of HP-UOH-80 (Fig. 1j). The observed voids might result from the enlarged pore size due to the lost linkers, in which the position of the missing linkers could be regarded as the defect sites in the HP-UOH-80 framework.
image file: d2en01035f-f1.tif
Fig. 1 (a) The PXRD patterns and (b–f) SEM images of both pristine UiO-66-(OH)2 and various HP-UOH-X. HR-TEM images of (g and h) UiO-66-(OH)2 and (i and j) HP-UOH-80.

Previous studies reported that the formation of hierarchically porous structures could be ascribed to missing ligands and/or metal clusters.25,26 N2 adsorption–desorption isotherms and the corresponding pore size distribution were determined and thermogravimetric analyses (TGA), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were performed to affirm the influences of benzoic acid with different concentrations on the structural features of the HP-UOH-X series. As demonstrated in Fig. 2a, pristine UiO-66-(OH)2 exhibited a type I isotherm, displaying a typical microporous structure (Fig. 2b). However, the as-prepared HP-UOH-80 displayed a weak adsorption–desorption hysteresis in the range of 0.8 < P/P0 < 1.0, indicating that extra mesopores existed in the framework (Fig. 2b).16 Moreover, the specific surface area of HP-UOH-80 (889.02 m2 g−1) was slightly larger than that of UiO-66-(OH)2 (860.62 m2 g−1). More significantly, HP-UOH-80 displayed additional mesopores (11.8–50.6 nm), different from that of UiO-66-(OH)2 (1.09–1.48 nm), indicating the successful construction of hierarchically porous structures. TGA was performed to affirm the thermal decomposition properties of the adsorbents.27,28 The TGA curves of both UiO-66-(OH)2 (Fig. 2c) and HP-UOH-80 (Fig. 2d) displayed three weight loss regions with increase of temperature. The first weight loss region (40–160 °C) was ascribed to both the adsorbed and the lattice water molecules. Subsequently, the lattice water existing on the Zr–O cluster would be eliminated, in which the Zr–O cluster would be changed from [Zr6(μ3-O)4(μ-OH)4] to [Zr6(μ3-O)6] when the temperature increased to 160–300 °C.29 Finally, the BDC-(OH)2 linker was decomposed when the temperature was increased from 300 to 730 °C. The numbers of coordinated BDC-(OH)2 linkers in UiO-66-(OH)2 and HP-UOH-80 were calculated to be 5.60 and 5.18, respectively (ESI,† Section S6). Theoretically, the Zr–O cluster in UiO-66-(OH)2 needed to coordinate more than six BDC-(OH)2 linkers to keep the stability of the framework,16 affirming that only some linkers of HP-UOH-80 were missing.


image file: d2en01035f-f2.tif
Fig. 2 (a) The N2 adsorption–desorption isotherms, (b) the corresponding pore size distribution, (c and d) TGA curves and (e) the spectra of O 1s of UiO-66-(OH)2 and HP-UOH-80. (f) Fluorescence emission spectra of UiO-66-(OH)2 and HP-UOH-X series.

As demonstrated in Fig. 2e, the XPS O 1s signal showed that the binding energy of the Zr–O–Zr peak of HP-UOH-80 had a higher shift of 0.03 eV than that of UiO-66-(OH)2, which was due to the missing linkers in the framework.30 The areas of –OH peak displayed a distinct increase from 45.89% of UiO-66-(OH)2 to 46.61% of HP-UOH-80 due to the missing linkers, leading to the presence of rich unsaturated coordination sites in the Zr–O cluster, and the –OH groups would coordinate with those unsaturated coordination sites.16 Moreover, missing ligands might cause a change of fluorescence property of the adsorbents.31 The intensities of the fluorescence signals (Fig. 2f) increased with increasing benzoic acid dosage, following the order HP-UOH-160 > HP-UOH-80 > HP-UOH-40 > HP-UOH-20 > UiO-66-(OH)2. The change of intensities was attributed to the decrease of coordinated ligand numbers in the HP-UOH-X framework.31 Based on the above discussion, the Zr–O clusters in the pristine UiO-66-(OH)2 could be coordinately saturated with BDC-(OH)2 linkers to generate micropore structures (Fig. 3a). When benzoic acid is introduced into the precursor solution of UiO-66-(OH)2, it would compete with the ligand (BDC-(OH)2) to coordinate with Zr–O clusters. Then, most of the benzoic acid could be dislodged after activation by acid solution due to its instability, leading to the formation of both hierarchical pores and rich unsaturated sites in the framework (Fig. 3b). Thus, all the aforesaid characterization results verified that a series of hierarchically porous UiO-66-(OH)2 were fabricated successfully.


image file: d2en01035f-f3.tif
Fig. 3 The schematic diagram of formation of (a) perfect UiO-66-(OH)2 and (b) HP-UOH-X.

Cr(VI) adsorption kinetics over HP-UOH-X

30.0 mg L−1 Cr(VI) solution was used to estimate the sorption capacities and rates of the adsorbents. As displayed in Fig. 4a, all the as-prepared HP-UOH-X displayed better sorption performances and quicker sorption rates toward Cr(VI) than pristine UiO-66 and UiO-66-(OH)2. The sorption kinetic curves of all adsorbents fitted better with the pseudo-second-order model (R2 > 0.99) compared to the pseudo-first-order model, demonstrating that chemical adsorption was dominant during the Cr(VI) sorption process (Table S1†).32,33 In addition, the sorption kinetic curves of UiO-66-(OH)2 and the HP-UOH-X series fitted well with the pseudo-first-order model (R2 > 0.98), indicating that physical adsorption might also play a significant role in Cr(VI) sorption due to the positive zeta potential (34.89 mV) at pH 2.0 (Fig. 5b). Among all the as-prepared HP-UOH-X adsorbents, HP-UOH-80 could be regarded as the optimal adsorbent because of the best sorption capacity and rate. Based on previous studies, –OH groups displayed a strong affinity interaction toward Cr(VI), in which the BDC-(OH)2 ligand provided sorption sites for Cr(VI) adsorption. Unfortunately, UiO-66-(OH)2 with the micropore structure would inhibit the mass transfer rate of Cr(VI), further decreasing the sorption capacity of the adsorbent toward Cr(VI). Therefore, partial BDC-(OH)2 ligand loss could enlarge the pore size and expose more sorption sites inside the framework than pristine UiO-66-(OH)2. However, excessive loss of ligand signified the excessive reduction of sorption sites, which could decrease the sorption capacity of the adsorbent toward Cr(VI). Therefore, HP-UOH-80 displayed better sorption capacity toward Cr(VI) than those of UiO-66-(OH)2 and other HP-UOH-X adsorbents. The Weber–Morris model was adopted to investigate the intraparticle diffusion process.34 Generally, there are three linear regions in the entire diffusion process: (i) the film diffusion and surface adsorption (k1), (ii) the intraparticle diffusion (k2) and (iii) the adsorption equilibrium (k3).35,36Fig. 4b and Table S2† showed that the optimal HP-UOH-80 adsorbent displayed a faster liquid film diffusion rate (k1, 14.21 mg g−1 min0.5) and intraparticle diffusion (k2, 6.25 mg g−1 min0.5) than UiO-66-(OH)2 and other HP-UOH-X adsorbents because of the positive surface zeta potential (Fig. 5b) and the exposure of sorption sites.
image file: d2en01035f-f4.tif
Fig. 4 (a) The pseudo-first-order and pseudo-second-order kinetic curves and (b) the intraparticle diffusion curves of Cr(VI) sorption over UiO-66-(OH)2 and HP-UOH-X. Experimental conditions: sorbent dosage = 20.0 mg, volume = 150.0 mL, Cr(VI) concentration = 30.0 mg L−1, pH = 2.0, T = 298 K. (c) The Cr(VI) sorption isotherms of HP-UOH-80 under different temperatures. Experimental conditions: sorbent dosage = 20.0 mg, volume = 150.0 mL, pH = 2.0. (d) Breakthrough curves of UiO-66-(OH)2 and HP-UOH-80.

image file: d2en01035f-f5.tif
Fig. 5 (a) The existing species of Cr(VI) in different pH solutions. (b) The effects of initial pH values and corresponding zeta potential on the Cr(VI) sorption efficiencies of HP-UOH-80. Experimental conditions: sorbent dosage = 20.0 mg, volume = 150.0 mL, Cr(VI) concentration = 30.0 mg L−1, T = 298 K. (c) The Cr(VI) adsorption performances of HP-UOH-80 in different simulated Cr(VI)-containing wastewater samples formulated with deionized water, tap water and lake water; the excitation–emission matrix spectra of (d) tap water and (e) lake water used in this study. (f) The influence of competitive anions for Cr(VI) sorption over HP-UOH-80 in simulated wastewater. (g) Reusability experiments of the HP-UOH-80. Experimental conditions: sorbent dosage = 20.0 mg, volume = 150.0 mL, Cr(VI) concentration = 10.0 mg L−1, pH = 2.0, T = 298 K. (h) The SEM image of HP-UOH-80 after the cyclic experiments. (i) The Cr 2p spectra of HP-UOH-80 after adsorption and photocatalysis.

Cr(VI) adsorption isotherms by HP-UOH-80

Fig. 4c displayed the Cr(VI) elimination performances of HP-UOH-80 as a function of Cr(VI) solution with different initial concentrations under different temperatures (288 K, 298 K and 308 K). Three sorption isotherm models involving Langmuir, Freundlich and D–R were utilized to explore the interaction between HP-UOH-80 and Cr(VI). Combined with Table S3,† the isotherms fitted better with the Freundlich model (R2 >0.99) than the Langmuir and D–R models, indicating that the sorption process was multilayer and chemical sorption.37,38 Moreover, isotherm experiments revealed that the maximum adsorption capacity (qmax) of HP-UOH-80 could reach 266.74 mg g−1 at 308 K after 240 min, in which the qmax of HP-UOH-80 was better than those of the counterpart adsorbents (Table 1).
Table 1 The adsorption capacities toward Cr(VI) of some counterpart adsorbents
Adsorbents q max (mg g−1) pH Temperature Time Ref.
P–Fe2O3 175.5 5.0 328 K — 39
PANI@NC-600 198.04 1.0 298 K 480 min 40
nZVI@C 206 3.0 25 °C 1440 min 41
NZVI@ZD 226.5 5.0 25 °C 180 min 42
BUC-17 121 4.0 — 8 h 43
UiO-66 36.4 3.0 25 °C 1200 min 44
(Ce)–UiO-66 30 — 298 K 30 min 45
Form-UiO-66 243.9 2.0 298 K 1400 min 46
Nano UiO-66-NH2 32.36 6.5 298 K 24 h 47
Chitosan–UiO-66 93.6 2.0 40 °C 8 h 48
HP-UOH-80 266.74 2.0 308 K 240 min This work

Thermodynamic parameters

To further expose the adsorption behaviors, the enthalpy change (ΔH°), standard free energy change (ΔG°) and entropy change (ΔS°) could be calculated by eqn (S9)–(S11).† The values of ΔH° and ΔS° were obtained from the slope and intercept of the straight line via fitting the plot of ln[thin space (1/6-em)]Kdversus 1/T (ref. 49) (Fig. S4†). As listed in Table S4,† the positive ΔH° value (38.85 kJ mol−1) signified that the sorption process was endothermic, demonstrating that the sorption capacity of HP-UOH-80 could be enhanced with increasing temperature.50 The value of ΔG° was negative, implying that the adsorptive interaction between HP-UOH-80 and Cr(VI) was spontaneous. The negative value of ΔG° from −20 to −80 kJ mol−1 could be attributed to the effect of both chemical and physical adsorption.51 The value of ΔS° of the sorption between HP-UOH-80 and Cr(VI) was 201.96 J mol−1 K−1, demonstrating that the interface was inclined to be disordered during the Cr(VI) sorption process.52

Fixed-bed column studies

Fixed-bed breakthrough column tests were carried out to evaluate the application prospect in industrial wastewater treatment. The integrated wastewater discharge standard (GB 8978-1996) stipulated that the discharge standard of Cr(VI) was 0.5 mg L−1, which was selected as the limiting value of the breakthrough curves. Fig. 4d showed that the as-prepared HP-UOH-80 (50.0 mg) could purify 700 mL Cr(VI)-containing wastewater with an initial concentration of 5.0 mg L−1. Fig. 4d showed that 100.0% Cr(VI) removal efficiencies could be achieved over HP-UOH-80 up to that 450 mL Cr(VI)-containing wastewater flowed through the column. In addition, HP-UOH-80 displayed better purification capacity under identical conditions than that of pristine UiO-66-(OH)2 (50.0 mg, 250 mL).

Environmental applications

Influences of initial pH value and co-existing matter

It was well known that the initial pH value of the solution could affect the surface charge (zeta potential) of materials53 and the existing form of Cr(VI) (like Cr2O72−, HCrO4−, CrO42− and H2CrO4),54 which further altered the adsorptive interactions between the adsorbent and Cr(VI) as well as the corresponding adsorption mechanism. Thus, Cr(VI) adsorption experiments were conducted at pH 2.0, 3.0, 4.0, 6.0 and 8.0. As shown in Fig. 5a, the chemical species of Cr(VI) were Cr2O72− and HCrO4− when the pH value of the aqueous solution was between 2.0 and 4.0. CrO42− would occur at pH >4.0.55 As indicated in Fig. 5b, HP-UOH-80 displayed the best sorption performance toward Cr(VI) at pH 2.0, contributing to the electrostatic attraction that promoted the sorption interactions between Cr(VI) and adsorbent. To evaluate the water stability of the adsorbent, HP-UOH-80 was soaked in acidic solution at pH 2.0 for 24 h. The PXRD pattern (Fig. S5†) showed that all characteristic peaks of HP-UOH-80 after soaking were still identical to those of pristine HP-UOH-80 and the simulated one from the single-crystal data of UiO-66 (CCDC: 837796). In addition, the Zr dissolution (Fig. S6†) of HP-UOH-80 in aqueous solution with different pH values was all less than 0.85 mg L−1. These results all affirmed the outstanding water stability of HP-UOH-80.

The presence of co-existing ions and organic matter might affect the adsorption process. Therefore, five common cations like Cl−, NO3−, HCO3−, SO42− and H2PO4− as well as humic acid (HA) were selected to investigate the influences of competing cations and organic matter on the Cr(VI) removal efficiencies of HP-UOH-80. As shown in Table S5 and Fig. S7,† all inorganic ions and organic matter exhibited no influence on Cr(VI) removal, indicating its good practical application prospects. Furthermore, various simulated Cr(VI)-containing wastewater samples were prepared using tap water and lake water (Minghu lake in the Daxing campus, BUCEA) to verify the sorption capacity of HP-UOH-80. The quality parameters of the tap and lake water samples are shown in Table S6.† As indicated in Fig. 5d and e, the excitation–emission matrix (EEM) spectra displayed that some dissolved organic matter (DOM) existed in tap water and lake water.56 As illustrated in Fig. 5c, the co-existing ions from tap water and lake water had no effects on adsorption performance, in which 100.0% Cr(VI) could be eliminated within 30.0 min.

Selective adsorption of HP-UOH-80

The selective adsorption experiment was performed to evaluate the practical application of HP-UOH-80. 20.0 mg of HP-UOH-80 and 100.0 mL of simulated wastewater containing Cr(VI), As(III), V(V) and Re(VII) were mixed to react at 298 K for 60.0 min. The concentrations of As(III), V(V) and Re(VII) were determined by ICP-OES. As shown in Fig. 5f, although the concentrations of other coexisting toxic anions were much higher than that of Cr(VI), HP-UOH-80 still exhibited superior selective adsorption capacity (97.59%), which was higher than those of As(III) (9.53%), V(V) (1.34%) and Re(VII) (3.11%).

The selectivity towards Cr(VI) was determined through calculation of the distribution coefficients (KQ, mL g−1, eqn (1)). A larger KQ value indicated a stronger interaction between the target and the adsorbent along with better adsorption effect.57

 
image file: d2en01035f-t1.tif(1)

The results displayed that the KQ value (202.97 mL g−1) of Cr(VI) was much larger than those of As(III) (0.53 mL g−1), V(V) (0.07 mL g−1) and Re(VII) (0.16 mL g−1), indicating that HP-UOH-80 exhibited the highest removal efficiency and the best adsorption interaction toward Cr(VI).

Reusability of HP-UOH-80

Cyclic experiments were adopted to investigate the practical water treatment applications of HP-UOH-80 for Cr(VI) removal. Meanwhile, exploiting an effective Cr(VI) desorption approach was significant, which could not only achieve the regeneration of adsorbent but also promote the enrichment and recycling of Cr element. Considering that UiO-66-(OH)2 possessed good photocatalytic Cr(VI) reduction ability, photocatalytic desorption could be deemed as a green technique compared to the chemical method. UV-vis DRS results display the light sorption properties of pristine UiO-66-(OH)2 and various HP-UOH-X adsorbents. Fig. S8† illustrates that UiO-66-(OH)2 and the HP-UOH-X series could be excited by visible light. Electrochemical impedance spectroscopy (EIS) was used to confirm that HP-UOH-80 exhibited faster charge transfer rate under visible light irradiation (Fig. S9a†). Mott–Schottky plots of HP-UOH-80 (Fig. S9b†) were tested to determine the energy of the lowest unoccupied molecular orbital (LUMO) of HP-UOH-80, which was calculated as −0.49 eV vs. NHE. The ELUMO of HP-UOH-80 was more negative than the standard redox potential of O2/O2˙− (−0.33 eV vs. NHE), indicating that O2˙− radicals could be generated to further promote the photocatalytic Cr(VI) reduction process. As shown in Fig. 5g, more than 80% Cr(VI) uptake efficiency could still be achieved over HP-UOH-80 after three cyclic experiments. Under visible light irradiation, Cr(VI) that was not adsorbed in aqueous solution could be further unceasingly adsorbed and reduced, in which over 96% Cr(VI) could be removed. In addition, more than 90% of the Cr adsorbed on the HP-UOH-80 could be released in the form of Cr(III) via theoretical calculations. Therefore, there were two application scenarios in practical Cr(VI)-containing wastewater treatment: (i) HP-UOH-80 could achieve almost 100% Cr(VI) removal efficiency via adsorption and in situ photocatalysis. (ii) If we recycled Cr(III) to prepare valuable commodities like Cr(NO3)3, Cr2O3 and so on, HP-UOH-80 after the adsorption process could be placed into fresh acidic aqueous solution to photocatalytically desorb and recycle Cr(III) under visible light irradiation. Furthermore, the SEM image (Fig. 5h) and PXRD (Fig. S10a†) and FITR (Fig. S10b†) results of HP-UOH-80 after cyclic experiments displayed that HP-UOH-80 possessed good stability in aqueous solution, signifying that HP-UOH-80 could be used for long-term operation. Additionally, the XPS Cr 2p spectra of HP-UOH-80 after both adsorption and photocatalysis were determined to confirm the valence and distribution of the Cr element. As indicated in Fig. 5i, the peaks at 587.5 eV and 578.7 eV could be attributed to Cr(VI).58,59 The Cr(III) content calculated from the area of the corresponding peak (577.44 eV (ref. 60)) in the Cr 2p signal after adsorption reached 27.15%, which could be contributed to the catalytic reduction ability of HP-UOH-80. The strong oxidizing property of K2Cr2O7 induced indirect electron transfer due to the transformation of Zr(IV)/Zr(III), which led the reduction process from Cr(VI) to Cr(III). After photocatalysis, a new peak (587.3 eV (ref. 61)) appeared, and the areas of the Cr(VI) peaks exhibited a significant decrease from 72.85% to 35.14%, affirming the successful reduction of Cr(VI).53,62 However, some Cr including Cr(VI) and Cr(III) after the photocatalysis process still remained on the active sites of the adsorbent, which might also affect the cyclic performance of HP-UOH-80 to some extent.

Cr(VI) removal mechanism

The corresponding Cr(VI) elimination mechanism over HP-UOH-80 could be affirmed by element mapping, FTIR and XPS. After the Cr(VI) sorption, the element mapping (Fig. 6a) displayed the uniform distribution of Cr(VI) throughout the used HP-UOH-80. Fig. 5b displayed that the positive zeta potential (34.89 mV) at pH 2.0 indicated that electrostatic interaction between Cr(VI) and the adsorbent provided some contribution during the sorption process. As illustrated in Fig. 6b, the FTIR spectra of HP-UOH-80 before and after adsorption displayed that the –OH vibration peak of fresh HP-UOH-80 shifted from 3426 cm−1 to 3394 cm−1 after Cr(VI) sorption, which could be ascribed to the complexation between –OH and Cr(VI).34 Moreover, the –OH bending peak was located at 1654 cm−1,63 and the peak with decreasing intensity indicated the formation of coordination between –OH and Cr(VI). Also, the results of XPS (Fig. 6c and S11†) showed that –OH groups could coordinate with Cr(VI). A red shift of 0.14 eV occurred to the binding energy of –OH after adsorption (Fig. 6c), which could be ascribed to the formation of the O–Cr bond.64,65 Then, the binding energy of Zr–O–Zr shifted from 530.82 eV before adsorption to 530.93 eV after adsorption, indicating that –OH on the Zr–O cluster also provided partial contribution to Cr(VI) adsorption. Based on the above discussion, there were three Cr(VI) removal pathways (Fig. 6d): (i) the open Zr sites provided a positive potential to attract Cr(VI) by electrostatic interaction; (ii) the –OH from the ligand and defect sites could coordinate with Cr(VI); (iii) some Cr(VI) could be catalytically reduced to Cr(III) according to the results of the Cr 2p spectra of HP-UOH-80 after adsorption.
image file: d2en01035f-f6.tif
Fig. 6 (a) The HR-TEM image and corresponding element mapping of HP-UOH-80 after adsorption. (b) The FTIR spectra of pristine HP-UOH-80 and after adsorption. (c) The O 1s spectrum of HP-UOH-80 and after Cr(VI) adsorption. (d) The corresponding mechanism of HP-UOH-80 for Cr(VI) adsorption.

Conclusions

A series of defect-induced hierarchically porous UiO-66-(OH)2 were successfully fabricated via defect engineering. Various characterization methods like HR-TEM, TGA, XPS and PL confirmed that the formation of hierarchical pores contributed to the missing linker. Due to the missing linker, abundant hydroxyl groups inside the framework could be exposed because of the expansion of pore size. Also, extra hydroxyl groups would be generated in the defect sites because of the compensation model, which not only provided more sorption sites but also strengthened the mass transfer of Cr(VI) in the framework interior. Among the as-prepared HP-UOH-X adsorbents, HP-UOH-80 displayed outstanding sorption capacity (266.74 mg g−1) toward Cr(VI) and rapid diffusion rate (k1 = 14.21 mg g−1 min0.5, k2 = 6.25 mg g−1 min0.5) at 308 K and pH 2.0, which were better than those of UiO-66-(OH)2, other HP-UOH-X and most other types of adsorbents. In addition, HP-UOH-80 displayed outstanding Cr(VI) selectivity uptake in the different simulated water samples containing various competing anions. FTIR and XPS analyses demonstrated that –OH on both the ligand and the defect sites provided a dominant contribution for boosting the Cr(VI) adsorption. Moreover, the good photocatalytic ability of HP-UOH-80 was used to reduce Cr(VI) adsorbed on HP-UOH-80 to Cr(III), achieving effective desorption. This work provided useful guidance for fabricating defect-induced hierarchically porous MOF adsorbents for wastewater purification and offered a new and green desorption strategy for improving the cyclicity of the adsorbent.

Author contributions

Yu-Hang Li: data curation, investigation, visualization, writing – original draft. Meng-Yuan Liu: investigation, methodology. Yu-Wei Wei: software, methodology. Chong-Chen Wang: conceptualization, funding acquisition, supervision, project administration, writing – review & editing. Peng Wang: resources, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51878023, 22176012), the Beijing Natural Science Foundation (8202016), the Science and Technology General Project of Beijing Municipal Education Commission (KM202110016010), and the BUCEA Post Graduate Innovation Project (PG2022052).

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Footnote

† Electronic supplementary information (ESI) available: Extra experimental details, data analyses, FTIR, UV-vis DRS and XPS survey scan. See DOI: https://doi.org/10.1039/d2en01035f
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