Fast adsorption and removal of 2-methyl-4-chlorophenoxy acetic acid from aqueous solution with amine functionalized zirconium metal–organic framework

Abstract

An amino functionalized zirconium-based MOF named UiO-66-NH₂ was synthesized and explored as a novel adsorbent for the fast removal of 2-methyl-4-chlorophenoxy acetic acid (MCPA) in aqueous solution. The adsorption process of kinetics, adsorption isotherms, thermodynamics, and adsorbent regeneration were investigated and the effects of key parameters such as adsorbent dosage, pH value and ionic strength on the adsorption of MCPA were also studied. The results showed that the adsorption of MCPA on UiO-66-NH₂ was very fast, and most of MCPA were adsorbed in the first 3 min. A pseudo-second-order rate equation effectively described the adsorption kinetics. The adsorption process fits the Langmuir adsorption model well, and the maximum adsorption capacity is 300.3 mg g⁻¹ at 25 °C. The analysis of the adsorption mechanism showed that the hydrogen bond interaction, electrostatic interaction and π–π stacking interaction between the MCPA and UiO-66-NH₂ were responsible for the efficient adsorption. Regeneration experiments indicated that the used UiO-66-NH₂ was recycled at least six times without significant loss of adsorption capacity. The fast adsorption kinetics, high adsorption capacity, excellent reusability and good chemical stability make UiO-66-NH₂ attractive for removal MCPA from aqueous solution.

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1. Introduction

Every year about two million tonnes of pesticides are applied to intensive agriculture in the world.¹ The wide utilization of pesticides in agricultural activity has already contributed to the increasing of contamination in surface and ground waters. 4-Chloro-2-methylphenoxyacetic acid (MCPA, Scheme S1†), as a postemergence phenoxyacetic acid herbicide, is widely used to control the spread of annual and perennial broadleaved weeds in agricultural fields.² However, because of the character of highly mobile, relatively soluble in water (825 mg L⁻¹ at 293 K) and leaching from soil into water, MCPA has potentially toxicity to human beings.³ Moreover, MCPA can damage the nervous system irreversibly upon adsorption through the skin and cause abdominal pain, cough and dizziness after prolonged inhalation.⁴ It has been classified by the U.S. Environmental Protection Agency as a potential groundwater contaminant.⁵ The removal of MCPA from aqueous solutions is thus critical required.

To date, various strategies have been developed to efficient remove MCPA from aqueous solution, including oxidation,³ photocatalysis,⁶ and adsorption.^2,7,8 Although each technique has its own merits, the adsorption has attracted more attentions owing to its relatively low initial cost and simple operation. Some adsorbents have been used for the adsorption of MCPA such as activated carbon, but the adsorption capacities of these absorbents are quite limited.⁸ Therefore, it is still of great significance and necessary to explore novel adsorbents for efficient adsorption and removal of MCPA with high capacity.

Metal–organic frameworks (MOFs) have received tremendous attention in various fields, such as sensing, catalysis and separation because of their extraordinary surface area, accessible functionalized tunnels, tunable pore size and good chemical resistance and thermal stability.^9–20 Moreover, over the past several years, MOFs have been shown the excellent properties in the field of adsorption.^21–29 Owing to MOFs' promising potential, they have been found to work effectively to remove hazardous materials in aqueous environments, including dyes,^22,23 metal ions,²⁴ pesticides,^25,26 antibiotics,²⁷ environmental endocrine disrupters,²⁸ and phenols.²⁹

UiO-66 is a cubic rigid 3D porous zirconium-based MOF, which is built up from 1,4-benzene-dicarboxylate ligands and zirconium clusters Zr₆O₄(OH)₄ as 12-connected nodes. It possesses the octahedral (11 Å) and tetrahedral (8 Å) cavities with narrow triangle windows (5–7 Å) and offers large surface area and thermal and chemical stability.³⁰ Additionally, the ligand in the UiO-66 can be modified with amine group to form UiO-66-NH₂ which can further improve the adsorption capacity via electrostatic interaction, hydrogen bond interaction and so on.^31–33 Thus, UiO-66 and UiO-66-NH₂ have been used to adsorb environmental pollutants in water, such as fluoride,³¹ phosphate,³² cationic dyes,³³ etc.

In this work, UiO-66-NH₂ was prepared for the fast and efficient adsorption of MCPA (Scheme 1). Due to the high surface area and appropriate cubic rigid 3D pores, UiO-66-NH₂ will be a good adsorbent for MCPA, and amine group is also benefit for the acid group in MCPA. To show the performance of the material, the adsorption isotherm, kinetics, thermodynamics and regeneration for MCPA on UiO-66-NH₂ will be also investigated. Furthermore, river water will be chosen as real water sample for the practical applications. MCPA were removed effectively indicating UiO-66-NH₂ has great potential for practical application.

Scheme 1 A plausible mechanism for the adsorptive removal of MCPA using UiO-66-NH₂.

2. Experimental

2.1 Materials and chemicals

All reagents used were at least of analytical grade. Zirconium tetrachloride (ZrCl₄) was supported by J&K Scientific Ltd. (Beijing, China). 2-Aminoterephthalic acid (2-NH₂-BDC) and 4-chloro-2-methylphenoxyacetic acid (MCPA) were purchased from TCI (Shanghai, China). Terephthalic acid was provided by Macklin. N,N-Dimethylformamide (DMF), activated carbon and ethanol were obtained from Tianjin Concord Technology Co., Ltd. (Tianjin, China). Water used in the experiment was deionized water, and was purchased from Tianjin Physical and Chemical Analysis Center (Tianjin, China). River water samples were collected from Weijin River (Tianjin, China).

2.2 Instrumentation

The X-ray diffraction (XRD) patterns were recorded with a D/max-2500 diffractometer (Rigaku, Japan) using Cu Kα radiation (λ = 1.5418 Å) over the angular range from 2° to 50°. Transmission electron microscopy (TEM) was conducted on a JEM-2010HR microscope (JEOL, Japan) at an accelerating voltage of 200 kV, the sample was dispersed in ethanol and dropped on a 200 mesh copper grid. A MAGNA-IR 560 spectrometer (Nicolet, USA) was used to measure FT-IR spectra of the samples with KBr tablet method and the scanning wavenumber range from 2500 cm⁻¹ to 400 cm⁻¹. Zeta potentials of UiO-66-NH₂ in pure water were measured on a Zetasizer Nano ZS instrument (Malvern, UK) and the concentration of samples is 5 mg L⁻¹. N₂ adsorption–desorption isotherms were performed on automated surface area and pore size analyzer (3Flex, Micromeritics Instrument Corporation). All adsorption experiments were conducted at 180 rpm in a thermostatic shaker (ATS-032R, China).

2.3 Preparation of adsorbents

UiO-66-NH₂ was synthesized according to the work of Kandiah et al. with a little modification.³⁰ Briefly, ZrCl₄ (1.50 g, 6.4 mmol) and 2-NH₂-BDC (1.56 g, 6.4 mmol) were well mixed with 180 mL DMF at room temperature. The obtained mixture was transferred to a Teflon lined steel autoclave. The autoclave was placed in a pre-heated oven at 80 °C for 12 h and then held at 100 °C for 24 h. After that, the resulting solid was collected by centrifugation at 8000 rpm for 5 min and washed with ethanol for three times. Then, the obtained UiO-66-NH₂ was repeatedly washed with absolute ethanol for 24 h while heated at 80 °C in a water bath. Finally, the solid was dried at 120 °C under vacuum overnight and kept in a desiccator. The synthesis of UiO-66 is described in the ESI (Text S1, ESI†).

2.4 Adsorption experiments

In the experiments, an aqueous stock solution of MCPA (600 mg L⁻¹) was prepared by dissolving MCPA in deionized water, which was further diluted to the required concentration before use. The concentration of MCPA was determined using a double beam model UV 3600 spectrophotometer (Shimadzu, Japan) at 278 nm and the scanning wavelength range was from 400 nm to 200 nm.

2.4.1 Effects of adsorbent dosage, pH value and ion strength. The effect of the dose of UiO-66-NH₂ on the MCPA removal was evaluated by adding the different amounts of adsorbent (varying from 0.25 g L⁻¹ to 4.0 g L⁻¹) into 300 mg L⁻¹ MCPA solution. The effect of solution pH on the adsorption of MCPA on UiO-66-NH₂ was investigated by varying the pH from 1.5 to 10.0, with a MCPA concentration of 300 mg L⁻¹. The pH of the MCPA solution was adjusted with negligible volumes 0.1 mol L⁻¹ HCl or 0.1 mol L⁻¹ NaOH aqueous solution. The effect of ionic strength on MCPA adsorption was studied at the MCPA solution (300 mg L⁻¹) including different concentrations of NaCl and Na₂SO₄.

2.4.2 Adsorption kinetics of MCPA on UiO-66-NH₂. In the kinetic adsorption experiment, 5 mg UiO-66-NH₂ was added to 5 mL of a fixed pH solution of MCPA (300 mg L⁻¹). The MCPA solutions and the adsorbents were mixed thoroughly in a thermostatic shaker at 25 °C. After adsorption for a predetermined time (from 3 to 180 min), the mixture was filtered with a membrane filter (PES, hydrophobic, 0.45 μm). The residual MCPA concentration in supernatant was measured and the amount of absorbed MCPA was calculated by the following equation:where C₀ (mg L⁻¹) is the initial MCPA concentration, C_t (mg L⁻¹) is the concentration of MCPA left at time t, q_t (mg g⁻¹) is the amount of MCPA adsorbed at equilibrium at time t, V (L) is the volume of the MCPA solution and m (g) is the weight of adsorbent used.

2.4.3 Adsorption isotherm and thermodynamics for the adsorption of MCPA on UiO-66-NH₂. To study the adsorption isotherm and thermodynamics, 5 mg UiO-66-NH₂ was added to 5 mL of a fixed pH solution of MCPA (150–600 mg L⁻¹). The adsorption proceeded at a certain temperature (25 to 50 °C) for 3 h, the mixture was filtered with a membrane filter (PES, hydrophobic, 0.45 μm). The residual MCPA concentration in supernatant was measured and the amount of absorbed MCPA was calculated and expressed as q_e (mg g⁻¹) according to eqn (2).where C₀ and C_e (mg L⁻¹) are the initial and equilibrium concentrations of the MCPA, respectively, V (L) is the volume of the MCPA solution and m (g) is the mass of adsorbent used.

2.4.4 The regeneration of adsorbent. To investigate the regeneration of UiO-66-NH₂ for the adsorption of MCPA, 100 mg of UiO-66-NH₂ was mixed 100 mL of 300 mg L⁻¹ MCPA solution for 3 h. The mixture was separated by centrifugation. The residual MCPA concentration in supernatant was measured. Then, the adsorbent was regenerated with ethanol, washed with deionized water and reused in next round of MCPA adsorption. The cycle measurements were recorded at least six times to calculate the adsorption capacity.

2.4.5 Comparison of UiO-66-NH₂ with other adsorbents for the adsorption of MCPA. To study the adsorption capacities of MCPA on different adsorbents, 5 mg adsorbents were added to 5 mL of a fixed pH solution of MCPA (50–450 mg L⁻¹). The adsorption proceeded at 25 °C for 12 h, the mixture was filtered with a membrane filter (PES, hydrophobic, 0.45 μm). The residual MCPA concentration in supernatant was measured.

2.4.6 Performance in river water. The adsorption kinetics of MCPA in river water on UiO-66-NH₂ was evaluated to demonstrate the performance of UiO-66-NH₂ for practical adsorption of MCPA in natural water. A solution of MCPA (300 mg L⁻¹) was prepared by dissolving 300 mg MCPA in 1 L river water. 5 mg UiO-66-NH₂ was added to 5 mL of MCPA solution (300 mg L⁻¹) in river water. After adsorption for a predetermined time (from 3 to 240 min), the mixture was filtered with a membrane filter (PES, hydrophobic, 0.45 μm). The residual MCPA concentration in supernatant was measured.

3. Results and discussion

3.1 Material characterization

Fig. 1A illustrates the X-ray diffraction pattern of the synthesized UiO-66-NH₂. It is clear that the UiO-66-NH₂ possesses a similar structure to UiO-66 and the XRD pattern of UiO-66-NH₂ was consistent with those reported in literatures.^31–33 The crystalline form of MCPA adsorbed UiO-66-NH₂ is similar to UiO-66-NH₂ indicating the stability of UiO-66-NH₂. To further prove the successful functionalization of amine groups, FTIR analysis of the synthesized UiO-66 and UiO-66-NH₂ has been performed and the results are presented in Fig. 1B. Compared with the FTIR spectrum of UiO-66, the peaks at 1624 cm⁻¹ and 1257 cm⁻¹ are clearly observed in the spectrogram of UiO-66-NH₂, which are ascribed to the N–H bending vibration and the characteristic C–N stretching of aromatic amines, respectively. The peak at 768 cm⁻¹ correspond to the wagging vibrations of N–H. The peaks at 1653 cm⁻¹ and 1496 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibrations of C=O, respectively. The TEM image of the synthesized UiO-66-NH₂ is shown in Fig. 2. The particles have uniform sizes and their diameter is in the range of 100–150 nm, which favor their dispersion in an aqueous solution. The zeta potential results show that the UiO-66-NH₂ is positive charged in the pH range from 2 to 5.1 (Fig. S1†). The N₂ adsorption result shows the BET surface area of UiO-66-NH₂ is 694 m² g⁻¹ (Fig. S2†).

Fig. 1 Characteristics of the as-synthesized UiO-66-NH₂: (A) XRD patterns of (a) the as-synthesized UiO-66, (b) as-synthesized UiO-66-NH₂, (c) MCPA adsorbed UiO-66-NH₂; (B) FT-IR spectra of UiO-66-NH₂ and UiO-66.

Fig. 2 The TEM image of UiO-66-NH₂.

3.2 Effect of adsorbent dosage on the adsorption of MCPA on UiO-66-NH₂

The adsorbent dosage is an important parameter in the adsorption process, which determines the capacity of sorbent for a given initial concentration of adsorption solution. As shown in Fig. S3,† the adsorption removal percentage of MCPA increased along with UiO-66-NH₂ dosage due to the increasing adsorbent surface area and adsorption sites. However, the adsorption capacity for MCPA had a dramatic decreased with increasing UiO-66-NH₂ dosage. This might because the adsorption sites overlap increasing the difficulty of diffusion. We considering the above factors, 1 g L⁻¹ of adsorbent was used for the subsequent experiments. Furthermore, we scaled up quantities while keeping the same ratio and the results were showed in Fig. S4.† It can be seen that the adsorption capacities for MCPA has no significant loss, which would be critical for further scale-up applications.

3.3 Effect of pH on the adsorption of MCPA on UiO-66-NH₂

The solution pH affects the surface charge of UiO-66-NH₂ and the degree of ionization of MCPA. The effect of pH on the adsorption of MCPA on UiO-66-NH₂ was estimated in the pH range from 1.5 to 10.0 (Fig. 3). The capacity of MCPA on UiO-66-NH₂ increased as the pH increased from 1.5 to 3.1, then leveled off in the pH range of 3.1 to 7.0, and decreased with further increase of pH from 7.0 to 10.0. The zeta potential results show that the surface charge of UiO-66-NH₂ is positive at pH < 5.1, while it is negative at pH > 5.1 (Fig. S1†). MCPA exists mainly in neutral and anionic forms at pH < 3.1 and at pH > 3.1, respectively, because the pK_a value of MCPA is around 3.1.³⁴ Therefore, the favorable adsorption of MCPA over UiO-66-NH₂, especially at pH 3.1–5.1, may be explained by electrostatic interaction between MCPA anions and the positively charged surface of UiO-66-NH₂. When pH was in the range of 5.1–7.0, electrostatic repulsion occurred between the surface of the UiO-66-NH₂ and MCPA. However, the capacity of MCPA remained at high level. The fact suggested that the factor controlling the adsorption of MCPA was not just electrostatic interaction, and there should be other modes of action that controlled the adsorption process (such as π–π stacking interaction and hydrogen bond interaction). When the pH of the MCPA solution is higher than 7.0, the capacity of MCPA on UiO-66-NH₂ decreased. This may be explained as follows: with the increase of the alkaline, repulsion between UiO-66-NH₂ and MCPA anions enhanced. While, when the pH decreased from 3.1 to 1.5 the adsorption capacity for MCPA reduced. This might because in this pH range the negative charge of MCPA decreased, causing the electrostatic interaction between positive charged UiO-66-NH₂ and MCPA decreased. Considering the effect of pH on stability of MCPA and UiO-66-NH₂, the solution pH was adjusted to 6.0 ± 0.1 in the subsequent experiments. Furthermore, we studied the variation of pH before and after adsorption with the initial pH 6.0. After adsorption, the pH decreased to 4.1. This may be explained that UiO-66-NH₂ adsorbed anion of MCPA resulting in the increased concentration of hydrogen ion in the solution, so the pH decreased.

Fig. 3 Effect of pH on the adsorption of MCPA on UiO-66-NH₂.

3.4 Adsorption kinetics

The adsorption kinetics of MCPA on UiO-66-NH₂ was investigated at three initial concentrations (100, 200 and 300 mg L⁻¹) at 25 °C. The results were shown in Fig. 4A and S5.† The adsorption capacity for MCPA reached equilibrium within the first 5 min, with an initial MCPA concentration of 100 mg L⁻¹. With the increase of the initial concentration of MCPA, more time was needed to reach adsorption equilibrium, but all the adsorptions can reach equilibrium within 15 min at the initial concentration below 300 mg L⁻¹. It is worth noting that nearly all MCPA was adsorbed from aqueous solution at the initial concentration below 200 mg L⁻¹. Moreover, as the initial MCPA concentration increased, the adsorption capacity significantly increased, showing the favourable adsorption at high concentration of MCPA.

Fig. 4 (A) The adsorption kinetics of MCPA at different initial concentrations on UiO-66-NH₂ at 25 °C and pH 6.0. (B) Corresponding plots of pseudo-second-order kinetics for the adsorption of MCPA on UiO-66-NH₂.

To further analyze the adsorption kinetics, the pseudo-first-order (Fig. S6†) and pseudo-second-order kinetic models (Fig. 4B) were applied to study the kinetics of MCPA adsorption on UiO-66-NH₂. The pseudo-first-order model and pseudo-second-order model are generally expressed as eqn (3) and (4), respectively.

ln(q_e − q_t) = ln q_e − k₁t (3)

where q_e and q_t are the adsorption capacity (mg g⁻¹) at equilibrium and at time t (min), respectively; t (min) is adsorption time, k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) are the rate constant of the pseudo-first-order adsorption and pseudo-second-order adsorption, respectively.

The kinetic parameters obtained from linear regression for the two models were summarized in Table 1. Obviously, the time dependence for the adsorption of MCPA on UiO-66-NH₂ was much better described by a pseudo-second-order kinetics model than the pseudo-first-order kinetic model according to the values of correlation coefficient. As the initial concentration varied from 100 to 300 mg L⁻¹, the equilibrium adsorption capacity q_e increased from 99 to 270 mg g⁻¹. The value of k₂ decreased with the initial concentration of MCPA increased, indicating that chemisorption was significant in the rate-limiting step, involving valency forces through sharing or exchanging electrons between MCPA and UiO-66-NH₂.²²

Table 1 Kinetic parameters for the adsorption of MCPA on UiO-66-NH₂ at 25 °C^a

C0 (mg L^−1)	qe(exp) (mg g^−1)	Pseudo-second-order kinetic model			Pseudo-first-order kinetic model
		qe(cal) (mg g^−1)	k2 (g mg^−1 min^−1)	R^2	qe(cal) (mg g^−1)	k1 (min^−1)	R^2
a C0, initial concentration of MCPA; qe(cal), calculated adsorption capacity; qe(exp), experimental adsorption capacity.
100	99.89	99.70	8.031 × 10^−1	0.9999	1.283	2.721 × 10^−2	0.8403
200	199.6	199.2	1.810 × 10^−1	0.9999	3.127	2.289 × 10^−2	0.7051
300	273.6	273.2	1.437 × 10^−2	0.9999	12.68	3.028 × 10^−2	0.6522

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3.5 Adsorption isotherms

The adsorption isotherms of MCPA on UiO-66-NH₂ were conducted at three different temperatures (25, 40 and 55 °C) and pH 6.0 in the concentration range of 100–600 mg L⁻¹, and the results were showed in Fig. 5A. It can be seen that the adsorption capacities for MCPA increased and then reached a platform with the increasing initial concentration of MCPA. Subsequently, to evaluate the maximum adsorption capacity of MCPA on UiO-66-NH₂, the adsorption isotherms were investigated with the Langmuir (Fig. 5B) and Freundlich models (Fig. S7†). The linearized Langmuir isotherm and Freundlich isotherm are expressed as eqn (5) and (6), respectively.where q_e (mg g⁻¹) is the equilibrium adsorption capacity of UiO-66-NH₂, C_e (mg L⁻¹) is the equilibrium concentration of MCPA, Q₀ (mg g⁻¹) is the maximum adsorption capacity, and b (L mol⁻¹) is the Langmuir constant, K_F (mg g⁻¹) and n are the Freundlich constants.

Fig. 5 Adsorption isotherms for the adsorption of MCPA (A) on UiO-66-NH₂ at different temperatures and pH 6.0 and its corresponding Langmuir plots for MCPA (B).

C_e/q_e is plotted against C_e, the Q₀ and b constant can be determined from the slope and the intercept (Fig. 5B). The values of K_F and n are obtained from the intercept and slope of the linear plot of ln q_e against ln C_e (Fig. S7†). The Langmuir and Freundlich fitting parameters are summarized in Table 2.

Table 2 Langmuir and Freundlich adsorption isotherms fitting parameters of MCPA on UiO-66-NH₂

T (°C)	Langmuir parameters			Freundlich parameters
	Q0 (mg g^−1)	b (L mol^−1)	R^2	KF (mg g^−1)	n	R^2
25	300.3	4.327 × 10^4	0.9991	133.2	6.340	0.9402
40	326.8	6.348 × 10^4	0.9995	151.4	5.493	0.9533
55	349.7	7.496 × 10^4	0.9993	170.7	6.727	0.9342

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The correlation coefficient values of the fitting by the Freundlich was not as high as that obtained from Langmuir model (R² > 0.99). It was obvious that the adsorption isotherms at different temperatures were well fitted by Langmuir model. The maximum adsorption capacities at the three temperatures (25, 40 and 55 °C) were respectively 303.3, 326.8, 349.7 mg g⁻¹, indicating the adsorption of MCPA on UiO-66-NH₂ is favorable at high temperatures with an endothermic process.³⁵

3.6 Thermodynamics for the adsorption of MCPA on UiO-66-NH₂

To gain further insight into the adsorption mechanism, the thermodynamic parameters, free energy (ΔG, kJ mol⁻¹), enthalpy (ΔH, kJ mol⁻¹) and entropy (ΔS, J mol⁻¹ K⁻¹) changes were calculated from the adsorption isotherms using the following eqn (7)–(9).

ΔG = − RT ln K₀ (8)

where K₀ is the adsorption equilibrium constant, q_e (mg g⁻¹) is the equilibrium adsorption capacity of UiO-66-NH₂, C_e (mg L⁻¹) is the equilibrium concentration of MCPA, R is the gas constant (8.314 J mol⁻¹ K⁻¹), and T (K) is the temperature.

The values of ln K₀ were obtained by plotting ln(q_e/C_e) against q_e and extrapolating q_e to zero (Fig. S8†). The values of ΔH and ΔS were obtained from the slope and intercept of ln K₀ against 1/T (Fig. S9†). The determined values of ln K₀, ΔG, ΔH and ΔS are list in Table 3. The values of ΔG were negative at different temperatures, suggesting that the adsorption of MCPA on UiO-66-NH₂ was a spontaneous process. The value of ΔG became more negative with the temperature increased, indicating that the higher temperature could facilitate the adsorption. The positive value of ΔH confirmed that the adsorption of MCPA on UiO-66-NH₂ was endothermic. The positive ΔH is unfavorable for spontaneous adsorption of MCPA. Meanwhile, the positive value of ΔS is favorable for spontaneous adsorption of MCPA on UiO-66-NH₂, for the reason that the number of desorbed water molecules is larger than that of adsorbed MCPA molecules.³⁵ Therefore, the driving force for the adsorption of MCPA on UiO-66-NH₂ is controlled by the entropy change.

Table 3 Thermodynamic parameters for adsorption of MCPA on UiO-66-NH₂

T (°C)	ln K0	ΔG (kJ mol^−1)	ΔH (kJ mol^−1)	ΔS (J mol^−1 K^−1)
25	8.533	−21.15	39.22	202.5
40	9.277	−24.15
55	9.980	−27.23

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3.7 Effect of ionic strength on the adsorption of MCPA on UiO-66-NH₂

As there are various salts and metal ions in natural water and groundwater, to study the effect of ionic strength on the adsorption capacity of MCPA onto UiO-66-NH₂ was necessary. Different concentrations of NaCl and Na₂SO₄ were utilized in current investigation. The results are shown in Fig. S10,† the adsorption capacities gradually decrease with the concentration of salt increased. It can be seen that once 0.1 mol L⁻¹ NaCl or Na₂SO₄ was added, the adsorbed capacities of UiO-66-NH₂ for MCPA decreased by 12.21% and 43.77%, respectively. The divalent electrolyte (Na₂SO₄) had more negative effect on the adsorption of MCPA on the adsorbent than the univalent electrolyte (NaCl). The above results indicate that electrostatic interaction is one of the mechanisms for the adsorption of MCPA on UiO-66-NH₂.

3.8 Regeneration of UiO-66-NH₂

The regeneration ability of the adsorbent is an important indicator for practical applications. To demonstrate the reusability of UiO-66-NH₂, the adsorption–desorption cycle was repeated six times. The experimental results are shown in Fig. 6. As can be seen, there is no significant loss of the adsorption capacity on the regenerated UiO-66-NH₂ even after six cycles. Furthermore, UiO-66-NH₂ has not changed significantly according to the XRD pattern and FT-IR spectrum after the six cycles of adsorption (Fig. S11 and S12†). It revealed that UiO-66-NH₂ could be successfully regenerated and be potential for the removal of MCPA from aqueous solution in practical applications.

Fig. 6 Reusability of UiO-66-NH₂ for the adsorptive removal of MCPA (C₀: 300 mg L⁻¹).

3.9 Compared with other adsorbents

To evaluate the advantage of UiO-66-NH₂ for the adsorption of MCPA, the adsorption kinetics of MCPA on UiO-66 and activated carbon were studied for comparison. The results are shown in Fig. 7, UiO-66-NH₂ provided much larger adsorption capacity than UiO-66 and activated carbon. The adsorption capacity of MCPA on UiO-66-NH₂ is larger two times than that of UiO-66 under the same experimental conditions, indicated amine group played a very important role in the adsorption process. This can be explained that the amine groups on the surface of UiO-66-NH₂ could form hydrogen bond with MCPA.

Fig. 7 Adsorption isotherms for the adsorption of MCPA on different absorbents at 25 °C and pH 6.0.

The adsorption capacity of UiO-66-NH₂ for MCPA is much higher than other adsorbents (Table 4).

Table 4 Comparison of adsorption capacities of various adsorbents for MCPA

Adsorbent	Q0 (mg g^−1)	Reference
UiO-66-NH2	300.3	This work
UiO-66	132.8	This work
Commercial activated carbon	104.5	This work
SWCNT	25.70	7
Norit 0.8	133.6	8
Aquacarb 207C	117.7	8
Aquacarb 208A	106.2	8

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3.10 Performance in river water

In order to evaluate whether the adsorbent was suitable for practical adsorption of MCPA in natural water, the adsorption kinetics of MCPA (300 mg L⁻¹) in river water on UiO-66-NH₂ was studied (Fig. 8). The results showed that the adsorption capacity for MCPA decreased 10.33% and reached equilibrium within 30 min. The reason for this phenomenon may be account for the ionic strength in natural water is greater than that in deionized water. Although the performance for the adsorption of MCPA on UiO-66-NH₂ decreased, the adsorption capacity is still very higher than other adsorbents and the rate of adsorption is also fast. This indicated that UiO-66-NH₂ is potential for practical removal of MCPA from river water.

Fig. 8 The adsorption kinetics of MCPA on UiO-66-NH₂ in river water and deionized water at 25 °C and pH 6.0.

3.11 Adsorption mechanism

Based on these studies, a plausible mechanism of the adsorption for MCPA was proposed. According to analysis of pH, adsorption kinetics, isotherms and thermodynamic, it can be inferred that the adsorption of MCPA was related to the electrostatic interaction. The π–π stacking interaction was also involved in the adsorption process, because both the MCPA and the UiO-66-NH₂ have benzene rings. Except for the electrostatic interaction and π–π stacking interaction, hydrogen bond interaction also played an important role in the adsorption inferred from the experiment of comparison with UiO-66.

4. Conclusions

In summary, UiO-66-NH₂ has been successfully developed as novel adsorbents for fast adsorption and removal of MCPA from aqueous solution. We have made a detailed study on the adsorption of MCPA on UiO-66-NH₂ in view of the kinetics, adsorption isotherms, thermodynamics and adsorbent regeneration. The results showed that UiO-66-NH₂ had efficient performance for removing MCPA in a wide pH window. Moreover, the adsorption of MCPA on UiO-66-NH₂ was very fast, and the adsorption capacity was larger than UiO-66 and those of many other reported adsorbents. The adsorption behaviors of MCPA on UiO-66-NH₂ were mainly derived from hydrogen bond interaction, electrostatic interaction and π–π stacking interaction between the two materials. After desorption and regeneration, UiO-66-NH₂ still maintained at a high adsorption capacity for removing MCPA in aqueous solution. These advantages make UiO-66-NH₂ promising as novel adsorbent for the removal of MCPA from aqueous solution.

Acknowledgements

The financial support from Tianjin Research Program of Application Foundation and Advanced Technology (15JCYBJC23800) is gratefully acknowledged.

References

1. H. Ikeura, F. Kobayashi and M. Tamaki, J. Hazard. Mater., 2011, 186, 956.
2. M. Kersten, D. Tunega, I. Georgieva, N. Vlasova and R. Branscheid, Environ. Sci. Technol., 2014, 48, 11803.
3. O. Gimeno, A. Aguinaco, A. Rey, F. J. B. Novillo and J. R. Toledo, J. Chem. Technol. Biotechnol., 2014, 89, 1219.
4. X. H. Ren, Y. M. Sun, Z. F. Wu, F. L. Meng and Z. J. Cui, Chemosphere, 2012, 88, 39.
5. V. Addorisio, S. Esposito and F. Sannino, J. Agric. Food Chem., 2010, 58, 5011.
6. E. O. Oseghe, P. G. Ndungu and S. B. Jonnalagadda, J. Photochem. Photobiol., A, 2015, 312, 96.
7. A. D. Martino, M. Iorio, B. S. Xing and R. Capasso, RSC Adv., 2012, 2, 5693.
8. O. Gimeno, P. Plucinski, S. T. Kolaczkowski, F. J. Rivas and P. M. Alvarez, Ind. Eng. Chem. Res., 2003, 42, 1076.
9. G. Férey, Chem. Soc. Rev., 2008, 37, 191.
10. H. Li, M. Eddaoudi, M. O'Keeffe and O. M. Yaghi, Nature, 1999, 402, 276.
11. H. Furukawa, K. E. Cordova, M. O'Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444.
12. C. X. Yang, C. Liu, Y. M. Cao and X. P. Yan, RSC Adv., 2014, 4, 40824.
13. H. C. Zhou, J. R. Long and O. M. Yaghi, Chem. Rev., 2012, 112, 673.
14. Z. Z. Huang and H. K. Lee, J. Chromatogr. A, 2015, 1401, 9.
15. G. Lu and J. T. Hupp, J. Am. Chem. Soc., 2010, 132, 7832–7833.
16. B. Gole, A. K. Bar and P. S. Mukherjee, Chem. Commun., 2011, 47, 12137.
17. L. Ma, C. Abney and W. Lin, Chem. Soc. Rev., 2009, 38, 1248.
18. M. Yoon, R. Srirambalaji and K. Kim, Chem. Rev., 2011, 112, 1196.
19. Z. Y. Gu and X. P. Yan, Angew. Chem., Int. Ed., 2010, 49, 1477.
20. B. Chen, C. D. Liang, J. Yang, D. S. Contreras, Y. L. Clancy, E. B. Lobkovsky, O. M. Yaghi and S. Dai, Angew. Chem., Int. Ed., 2006, 45, 1390.
21. F. Yan, Z. Y. Liu, J. L. Chen, X. Y. Sun, X. J. Li, M. X. Su, B. Li and B. Di, RSC Adv., 2014, 4, 33047.
22. S. H. Huo and X. P. Yan, J. Mater. Chem. A, 2012, 22, 7449.
23. K. Y. A. Lin and H. A. Chang, Chemosphere, 2016, 139, 624.
24. F. Ke, L. G. Qiu, Y. P. Yuan, F. M. Peng, X. Jiang, A. J. Xie, Y. H. Shen and J. F. Zhu, J. Hazard. Mater., 2011, 196, 36.
25. B. K. Jung, Z. Hasan and S. H. Jhung, Chem. Eng. J., 2013, 234, 99.
26. Y. S. Seo, N. A. Khan and S. H. Jhung, Chem. Eng. J., 2015, 270, 22.
27. C. S. Wu, Z. H. Xiong, C. Li and J. M. Zhang, RSC Adv., 2015, 5, 82127.
28. M. M. Zhou, Y. N. Wu, J. L. Qiao, J. Zhang, A. McDonald, G. T. Li and F. T. Li, J. Colloid Interface Sci., 2013, 405, 157.
29. K. Y. A. Lin and Y. T. Hsieh, J. Taiwan Inst. Chem. Eng., 2015, 50, 223.
30. M. Kandiah, M. H. Nilsen, S. Usseglio, S. Jakobsen, U. Olsbye, M. Tilset, C. Larabi, E. A. Quadrelli, F. Bonino and K. P. Lillerud, Chem. Mater., 2010, 22, 6632.
31. K. Y. A. Lin, Y. T. Liu and S. Y. Chen, J. Colloid Interface Sci., 2016, 461, 79.
32. K. Y. A. Lin, S. Y. Chen and A. P. Jochems, Mater. Chem. Phys., 2015, 160, 168.
33. Q. Chen, Q. Q. He, M. M. Lv, Y. L. Xu, H. B. Yang, X. T. Liu and F. Y. Wei, Appl. Surf. Sci., 2015, 327, 77.
34. F. Bruna, R. Celis, I. Pavlovic, C. Barriga, J. Cornejo and M. A. Ulibarri, J. Hazard. Mater., 2009, 168, 1476.
35. J. Q. Jiang, C. X. Yang and X. P. Yan, ACS Appl. Mater. Interfaces, 2013, 5, 9837.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra18520g

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