# Enhanced Kinetic Removal of Ciprofloxacin onto Metal-Organic Frameworks by Sonication, Process Optimization and Metal Leaching Study

^{1}

^{2}

^{3}

^{4}

^{5}

^{*}

## Abstract

**:**

_{4}, with a rhombic crystalline morphology and 1375 m

^{2}/g BET surface area, has the highest CIP adsorption efficiency among the studied MOFs. The mathematical sorption model predicted that the highest CIP removal (99.2%) occurs when adsorbent dose, pH, and agitation time are adjusted to 6.82, 832.4 mg/L, and 39.95 min, respectively. Further studies revealed that the CIP adsorbed onto ZIF-67-SO

_{4}in monolayer (q

_{max}: 2537.5 mg/g) and chemisorption controlled the rate of the process. Mass transfer kinetic coefficients improved significantly by sonication at 35 KHz in comparison with mechanical agitation. Thermodynamic parameters (minus signs of ∆G° [7.8 to 14.2], positive signs of ∆H° (58.9 KJ/mol), and ∆S° (0.23 KJ/mol·K)) demonstrated the spontaneous, endothermic, and chemical sorption of CIP. The level of cobalt leached from ZIF-67-SO

_{4}structure varied 1.2–4.5 mg/L, depending on pH, mixing time, and agitation type. In conclusion, the excellent adsorption properties of ZIF-67-SO

_{4}for CIP, made it an outstanding candidate for environmental protection purposes.

## 1. Introduction

## 2. Experimental Design, Materials, and Methods

#### 2.1. Reagents and Chemicals

#### 2.2. Synthesis of Metal-Organic Frameworks (MOFs)

^{-1}and wavelength 270 nm.

#### 2.3. Experimental Design

^{4}= 16 factorial points, and six center points. To explain functional interactions between input parameters and the response, a second-order polynomial equation was applied to model the sorption process as following:

_{i}and X

_{j}are the independent variables, β

_{0}is a constant value, β

_{i}, β

_{ii}, and β

_{ij}are the regression coefficients for a linear, second-order, and interaction effects, respectively.$\text{}\mathsf{\epsilon}$ is the error of the model [17,18].

_{e}) at the equilibrium were calculated by using the following equations, respectively:

_{4}has the highest CIP removal. Thus, ZIF-67-SO

_{4}was selected to continue the CIP adsorption study. Response surface methodology (RSM) using R software was utilized to model the sorption process. Before run design, the operational variables were coded in the following order: X

_{1}, X

_{2}, X

_{3}, and X

_{4}representing contact time (min), adsorbent dosage (g/L), pH and CIP concentration (mg/L), respectively. The range of independent variables and their coded values are shown in Table 3.

_{i}is the actual value of input variables, X

_{0}is the actual value of input variables at the center point, and $\u2206\mathrm{X}$ is the step change value. After performing the experiments according to the design matrix (Table 4), the data were analyzed using the analysis of variance (ANOVA), coefficient of determination (R

^{2}), and lack of fit (LOF).

## 3. Results and Discussion

#### 3.1. Adsorbent Characterization

#### 3.2. Study of MOFs for Ciprofloxacin (CIP) Removal

_{4}, and the level of MOF affinity decreased by ZIF-67-Cl > ZIF-8-NO

_{3}> ZIF-8 leaf = ZIF-8 octahedron > ZIF-8-OAc > ZIF-8-cube > UIO-66. ZIF-67-SO

_{4}also shows the highest capacity for CIP in this stage.

#### 3.3. Model Development Using Response Surface Methodology (RSM)

^{2}(0.97) and adjusted R

^{2}(0.95) are close to one and within ± 0.2 range of each other. Moreover, the F-value (41.85), P-value (5.718 × 10

^{–9}), and non-significant value for lack of fit (0.1155) indicate that the model is statistically adequate.

#### 3.4. Effects of Model Variables and Their Interactions

#### 3.5. Model Optimization and Adequacy Checking

#### 3.6. Isotherm Modeling

^{2}

_{Adj}), and Chi-square (x

^{2}) tests were used to check and compare the validity of the models. The ability of the model to predict experimental data could be concluded from the lower values of SSE and x

^{2}and higher values of R

^{2}[25]. As a result, Langmuir isotherm was selected as the best model for fitting data due to the lower SSE and x

^{2}and higher R

^{2}values.

#### 3.7. Kinetics Modeling under Convectional Mixing and Sonication

_{t}) of ZIF against time (t) are shown in Figure 6. Pseudo-second-order model was found to be the best fitting model because it provided the highest determination coefficients (R

^{2}≥ 0.992) in comparison with the others in all of CIP initial concentration. In addition, the maximum adsorption capacity of ZIF predicted by the pseudo-second-order model (514.8 mg/g) is closer to experimental ones (509.06 mg/g) when compared with those calculated by other models. Rate constant values of pseudo-second-order model (k

_{2}) decreased with an increasing initial concentration of CIP, suggesting that the adsorption rate decreased as CIP concentration increased [21].

^{−1}and in an ultrasonic bath at an initial concentration of CIP 100 mg·L

^{−1}. As seen in Table 8, experimental q

_{e}and rate constant of the pseudo-second-order model in the ultrasonic bath were higher than those reported under mechanical mixing. The formation of micro jets and high-pressure zones during the sonication could promote the migration of CIP molecules into the nano-sized pores of ZIF-67-SO

_{4}. Similar findings were also obtained for the removal of blueberry anthocyanins [26], congo red [27], phosphate [28], and different dyes [29]. Studies also demonstrated that sonication can be used as a modifier to promote the structural properties of adsorbents to reach higher capacity for target contaminants [30,31].

#### 3.8. Thermodynamic of Adsorption

_{0}vs 1/T) according to the following:

#### 3.9. ZIF-67-SO_{4} Structural Stability

_{4}under environmental agitation. The stability test was conducted under different initial pH for 30 and 60 min under conventional mechanical agitation and sonication at 35 KHz. Figure 7 shows the concentration of Co ions in the final solution when ZIF-67-SO

_{4}was applied to the solutions. As expected, the concentration of Co ions increased dramatically by increasing the corrosiveness as a result of higher H

^{+}in low pH. Our previously published work showed that ZIFs were completely dissolved in strong acidic environments (with a pH of about 3) [28]. Moreover, the concentration of Co ions increases by mixing time (~30–60 min), which may be attributed to an increased opportunity for metal ions to leach from the ZIF-67-SO

_{4}structure. As Figure 7 shows, sonication could result in much higher Co ions when compared with conventional mixing. This could be attributed to higher shear forces during sonication which results in de-agglomeration of crystals and the probable break-down in the structure of ZIF-67-SO

_{4}[26,27].

## 4. Conclusions

_{4}˃ ZIF-67-Cl > ZIF-8-NO

_{3}> ZIF-8 leaf > ZIF-8 octahedron > ZIF-8-OAc > ZIF-8-cube > and UIO-66. A prediction model for CIP removal was developed by performing the experiments according to BBD. Optimization of the model yield the highest CIP removal (99.2%) to occur at 6.82, 832.4 mg/L, and 39.95 min for adsorbent dose, pH, and agitation time, respectively. Among nonlinear form of two-parameter and three-parameter isotherm models, the Langmuir model described the data well according to SSE, R

^{2}

_{Adj}, and x

^{2}. The maximum adsorption capacity of the adsorbent was 2.537 g/g according to the Langmuir model. Kinetic constants were improved in the sonication at 35 KHz when compared with mechanical agitation mode, and in both cases followed the pseudo-second-order model. The level of cobalt leached from ZIF-67-SO

_{4}was influenced by solution pH, mixing time, agitation type, and was observed to be in the range of 1.2–4.5 mg/L. Thermodynamic parameters (minus signs of ∆G° (7.8 to 14.2), positive signs of ∆H° (58.9 KJ/mol), and ∆S° (0.23 KJ/mol·K)) demonstrate the spontaneous, endothermic, and chemisorption nature of the process. In short, ZIF-67-SO

_{4}shows outstanding capacity and excellent affinity toward CIP and thus is a promising candidate for preventing CIP discharge into the aquatic environment.

## Supplementary Materials

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Characteristics of the as-synthesized MOFs: scanning electron microscope (SEM) image of MOFs; (

**a**) ZIF-67-OAC, (

**b**) ZIF-67-Cl, (

**c**) ZIF-67-NO

_{3}, (

**d**) ZIF-67-SO

_{4}, (

**e**) ZIF-8-Cuboid, (

**f**) ZIF-8-Leaf, (

**g**) ZIF-8-Cube, (

**h**) ZIF-8-Octahedron, (

**i**) UIO-66, (

**j**) x-ray diffraction (XRD) pattern.

**Figure 2.**Removal efficiency and adsorption capacity of as-synthesized MOFs for CIP (CIP: 62.5 mg/L, MOF: 0.5 g/L, time: 30 min).

**Figure 4.**Effect of sorption variables on CIP removal: (

**a**) the effect of MOF and CIP concentration, (

**b**) MOF and pH, (

**c**) MOF and agitation time.

**Figure 6.**Nonlinear plots of kinetic models used for adsorption of CIP (

**a**) 50 mg/L, (

**b**) 100 mg/ L, (

**c**) 100 mg/L sonication.

Ciprofloxacin Structure | Molecular Formula | pKa |
---|---|---|

C_{17}H_{18}FN_{3}O_{3} | pKa_{1} = 5.9 | |

pKa_{2} = 8.9 |

**Table 2.**Summary of synthesis condition and structural properties of as-synthesized metal-organic frameworks (MOFs).

MOFs | Ligand | Metal Source | Ligand/ Metal Mole Ratio | Structural Morphology | BET Surface Area (m^{2}/g) | Total Pore Volume (cm^{3}/g) |
---|---|---|---|---|---|---|

UIO-66 | Terephthalic acid | ZrCl_{4} | 1 | Plate | 765 | 0.44 |

ZIF-67 | 2-methylimidazole | Co(NO_{3})_{2} | 20 | Granular | 734 | 0.34 |

2-methylimidazole | Co(OAC)_{2} | 20 | Rhombic Dodecahedron | 1323 | 0.57 | |

2-methylimidazole | CoSO_{4} | 20 | Rhombic Dodecahedron | 1375 | 0.62 | |

2-methylimidazole | CoCl_{2} | 20 | Rhombic Dodecahedron | 1278 | 0.52 | |

ZIF-8 | 2-methylimidazole | Zn(NO_{3})_{2} | 29.4 | Octahedron | 1151.2 | 0.58 |

2-methylimidazole | Zn(OAc)_{2} | 7.9 | Leaf | 12.7 | 0.04 | |

2-methylimidazole | Zn(NO_{3})_{2} | 7.9 | Cuboid | 890.4 | 0.48 | |

2-methylimidazole | Zn(NO_{3})_{2} | 2 | Cube | 978 | 0.51 |

Factor | Variable Level | |||
---|---|---|---|---|

Code | −1 | 0 | +1 | |

contact time (min) | X_{1} | 2 | 16 | 30 |

MOF dosage (g/L) | X_{2} | 0.1 | 0.55 | 1 |

pH | X_{3} | 4 | 7.5 | 11 |

ciprofloxacin (mg/L) | X_{4} | 30 | 62.5 | 100 |

Run No. | Coded Variable | Response (% Removal) | Run No | Coded Variable | Response (% Removal) | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|

X_{1} | X_{2} | X_{3} | X_{4} | Observed | Predicted | X_{1} | X_{2} | X_{3} | X_{4} | Observed | Predicted | ||

1 | −1 | 1 | 0 | 0 | 80 | 81 | 16 | 1 | 0 | 0 | −1 | 97 | 100 |

2 | −1 | −1 | 0 | 0 | 73 | 68.2 | 17 | −1 | 0 | 0 | 1 | 70 | 68.9 |

3 | 0 | 1 | −1 | 0 | 79 | 79.7 | 18 | 0 | 0 | 0 | 0 | 94 | 96 |

4 | 1 | 0 | 1 | 0 | 75 | 73.6 | 19 | 0 | 1 | 0 | 1 | 79 | 79 |

5 | 0 | 1 | 1 | 0 | 65 | 62.7 | 20 | 1 | 0 | −1 | 0 | 77 | 77.5 |

6 | 0 | 0 | 1 | 1 | 48 | 50.4 | 21 | 1 | −1 | 0 | 0 | 96 | 92.5 |

7 | 0 | −1 | 0 | 1 | 74 | 76.6 | 22 | 0 | 0 | 0 | 0 | 95 | 96 |

8 | 0 | 1 | 0 | −1 | 98 | 96.2 | 23 | 0 | 0 | 1 | −1 | 65 | 65.1 |

9 | 0 | 0 | 0 | 0 | 95 | 96 | 24 | −1 | 0 | −1 | 0 | 71 | 73.3 |

10 | −1 | 0 | 1 | 0 | 41 | 41.1 | 25 | −1 | 0 | 0 | −1 | 76 | 78.4 |

11 | 1 | 1 | 0 | 0 | 91 | 93.4 | 26 | 0 | 0 | −1 | −1 | 86 | 81.2 |

12 | 0 | −1 | −1 | 0 | 70 | 73.9 | 27 | 0 | 0 | −1 | 1 | 73 | 70.4 |

13 | 0 | 0 | 0 | 0 | 97 | 96 | 28 | 1 | 0 | 0 | 1 | 85 | 84 |

14 | 0 | −1 | 1 | 0 | 54 | 54.8 | 29 | 0 | −1 | 0 | −1 | 84 | 84.9 |

15 | 0 | 0 | 0 | 0 | 99 | 96 | - | - | - | - | - | - | - |

**Table 5.**Estimated coefficients of the fitted polynomial model for CIP adsorption onto ZIF-67-SO

_{4}.

Model Term | Coefficient Estimate | Std. Error | t-Value | p-Value |
---|---|---|---|---|

Intercept | 96.02 | 1.47 | 65.30 | <0.0001 |

${\mathrm{X}}_{1}$ | 9.18 | 0.95 | 9.68 | 1.40 × 10^{−7} |

${\mathrm{X}}_{2}$ | 3.42 | 0.95 | 3.60 | 0.0028997 |

${\mathrm{X}}_{3}$ | −9.03 | 0.95 | −9.51 | 1.74 × 10^{−7} |

${\mathrm{X}}_{4}$ | −6.38 | 0.95 | −6.72 | 9.84 × 10^{−6} |

${\mathrm{X}}_{1}{\mathrm{X}}_{2}$ | −3.00 | 1.64 | −1.82 | 0.0894262 |

${\mathrm{X}}_{1}{\mathrm{X}}_{3}$ | 7.08 | 1.64 | 4.30 | 0.0007283 |

${\mathrm{X}}_{1}{\mathrm{X}}_{4}$ | −1.63 | 1.64 | −0.99 | 0.3396952 |

${\mathrm{X}}_{2}{\mathrm{X}}_{3}$ | 0.50 | 1.64 | 0.30 | 0.7654894 |

${\mathrm{X}}_{2}{\mathrm{X}}_{4}$ | −2.25 | 1.64 | −1.37 | 0.1926667 |

${\mathrm{X}}_{3}{\mathrm{X}}_{4}$ | −1.00 | 1.64 | −0.61 | 0.5527336 |

${\mathrm{X}}_{1}^{2}$ | −6.82 | 1.29 | −5.28 | 0.000116 |

${\mathrm{X}}_{2}^{2}$ | −5.42 | 1.29 | −4.20 | 0.0008955 |

${\mathrm{X}}_{3}^{2}$ | −22.83 | 1.29 | −17.69 | 5.66 × 10^{−11} |

${\mathrm{X}}_{4}^{2}$ | −6.41 | 1.29 | −4.96 | 0.0002087 |

Factor | Time (min) | MOF Dose (g/L) | pH | CIP (mg/L) | Removal (%) | |
---|---|---|---|---|---|---|

Predicted | Experimental | |||||

Value | 30 | 0.22 | 7.31 | 100 | 100 | 99.9 |

Isotherm | Parameters | Values |
---|---|---|

Langmuir | b (L/mg) | 1.89166 |

q_{e} (mg/g) | 2537.52777 | |

χ^{2} | 3024.69308 | |

SSE | 12,098.77233 | |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.99567 | |

Freundlich | K_{f} (mg/g)/(mg)^{1/n} | 1345.1812 |

n | 2.69053 | |

χ^{2} | 30,900.26595 | |

SSE | 123,601.06379 | |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.9558 | |

Jovanovic | q_{m} (mg·g^{−1}) | 2256.44168 |

K_{j} (L·mg^{−1}) | −1.64853 | |

χ^{2} | 4439.2249 | |

SSE | 17,756.89959 | |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.99365 | |

Temkin | A_{T} (L/mg) | 31.62846 |

b_{T} | 447.91592 | |

B (J/mol) | - | |

χ^{2} | 15,237.21971 | |

SSE | 60,948.87886 | |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.9782 | |

Sips | q_{ms} (mg/g) | 2593.86316 |

K_{S} (L/mg)^{ms} | 1.71831 | |

m_{s} | 0.94708 | |

χ^{2} | 3786.66401 | |

SSE | 11,359.99203 | |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.99458 | |

Toth | K_{T} | 2577.35956 |

A_{T} | 0.52871 | |

T_{T} | 0.94943 | |

χ^{2} | 3995.17545 | |

SSE | 11,985.52634 | |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.99428 | |

Khan | q_{s} (mg/g) | 2772.45949 |

b_{K} | 1.67977 | |

a_{K} | 1.03572 | |

χ^{2} | 3999.75536 | |

SSE | 11,999.26609 | |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.99428 |

Concentration (mg/L) | 50 | 100 | 100 |
---|---|---|---|

Agitation Type | Magnetic Stirrer | Sonication | |

q_{e}, exp (mg/g) | 256.4 | 509.06 | 560 |

Pseudo-First Order | |||

q_{e} (mg/g) | 6.13223 | 480.42467 | 527.8629 |

k_{1} (min^{−1}) | 2.51783 | 2.58553 | 3.0163 |

χ^{2} | 156.19152 | 918.66385 | 1109.7541 |

SSE | 780.95761 | 4593.31927 | 5548.7708 |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.93144 | 0.89479 | 0.86801 |

Pseudo-Second Order | |||

q_{e} (mg/g) | 258.37326 | 514.80069 | 562.129 |

k_{2} (g/mg·min) | 0.01483 | 0.0076 | 0.00849 |

χ^{2} | 16.21654 | 62.69378 | 65.9758 |

SSE | 81.08269 | 313.4689 | 329.8794 |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.99288 | 0.99282 | 0.99215 |

Intraparticle Diffusion | |||

k_{3} | 40.94423 | 81.66703 | 80.3091 |

C | 142.60978 | 285.45518 | 339.36581 |

χ^{2} | 618.10256 | 2061.48156 | 1950.1885 |

SSE | 3090.51278 | 10,307.40782 | 9750.9426 |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.72867 | 0.76391 | 0.7680 |

Elovich | |||

a | 10,708.00341 | 22,609.6118 | 56,005.9375 |

b | 0.03048 | 0.0154 | 0.0156 |

χ^{2} | 214.18997 | 605.792 | 558.3242 |

SSE | 1070.94984 | 3028.9600 | 2791.6209 |

${\mathrm{R}}_{\mathrm{Adj}}^{2}$ | 0.90598 | 0.9306 | 0.9335 |

Temperature K | Ce mg/L | −∆G° kJ/mol | ∆H° KJ/mol | ∆S° KJ/mol.K |
---|---|---|---|---|

293 | 3.65 | 7.88 | 58.9 | 0.23 |

303 | 2.55 | 9.12 | ||

313 | 0.7 | 12.88 | ||

323 | 0.5 | 14.2 |

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Dehghan, A.; Mohammadi, A.A.; Yousefi, M.; Najafpoor, A.A.; Shams, M.; Rezania, S.
Enhanced Kinetic Removal of Ciprofloxacin onto Metal-Organic Frameworks by Sonication, Process Optimization and Metal Leaching Study. *Nanomaterials* **2019**, *9*, 1422.
https://doi.org/10.3390/nano9101422

**AMA Style**

Dehghan A, Mohammadi AA, Yousefi M, Najafpoor AA, Shams M, Rezania S.
Enhanced Kinetic Removal of Ciprofloxacin onto Metal-Organic Frameworks by Sonication, Process Optimization and Metal Leaching Study. *Nanomaterials*. 2019; 9(10):1422.
https://doi.org/10.3390/nano9101422

**Chicago/Turabian Style**

Dehghan, Aliakbar, Ali Akbar Mohammadi, Mahmood Yousefi, Ali Asghar Najafpoor, Mahmoud Shams, and Shahabaldin Rezania.
2019. "Enhanced Kinetic Removal of Ciprofloxacin onto Metal-Organic Frameworks by Sonication, Process Optimization and Metal Leaching Study" *Nanomaterials* 9, no. 10: 1422.
https://doi.org/10.3390/nano9101422