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Significantly enhancing Cu (II) adsorption onto Zr-MOFs through Novel cross-flow disturbance of ceramic membrane Ke Wang, Zhaobin Tian, and Na Yin Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04850 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Industrial & Engineering Chemistry Research

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Significantly enhancing Cu (II) adsorption onto Zr-MOFs through Novel cross-flow disturbance of ceramic membrane

Ke Wang 1,2, Zhaobin Tian2, Na Yin2*

1. School of chemical engineering & technology, China University of Mining and Technology, Xuzhou 221116, China 2. College of Material & Chemical Engineering, Bengbu University, Bengbu 233030, China

* To whom all correspondence should be addressed. Tel.: +86-552-317-9528; E-mail: [email protected].

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Abstract Novel cross-flow disturbance of ceramic membrane was used for the first time to enhance the adsorption of highly toxic Cu (II) onto Zr-based metal-organic frameworks (Zr-MOFs). Effects of temperature and pH were investigated on the adsorption process of Cu (II) onto the MOFs using Jartest. The results showed that the MOFs can well adsorb Cu (II) (59.8 mg·g-1) at pH of 6 and temperature of 40 oC. Based on this, the MOFs were then added into a novel cross-flow ceramic membrane filtration system. Effects of the operation temperature and membrane pore size were also investigated on the adsorption process. And the results showed that Zr-MOFs adsorption of Cu (II) increased with temperature. The ceramic membrane with a pore size of 200 nm exhibited better in Cu (II) removal and membrane flux than those of the 50 nm membrane, as well as better kinetic data fit of the pseudo-second-order model. With the aid of cross-flow disturbance of ceramic membrane, the capacity was incredibly increased to 988.2 mg·g-1 at pH of 6, temperature of 40 oC, cross-flow velocity of 4.5m·s-1 and trans-membrane pressure of 0.05MPa. The principal advantages of this method are a significant enhancement of adsorption onto the Zr-MOFs in the removal of Cu (II) and convenient application in the continuous treatment of heavy metal wastewater. And this can be used as an effective method in continuous adsorptive removal of heavy metals from wastewater. Key words ： Enhanced adsorption; Zr-MOFs; cross-flow disturbance; Cu (II) removal; water pollution


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1. Introduction Water is widely regarded as one of the most essential natural resources. Yet water pollution due to highly toxic heavy metals remains a serious worldwide problem. Heavy metal of copper (Cu) is widely used and discharged to the environment in different industrial activities such as leather preserving, pulp, paper board, petroleum refining, plating, smelting, mining, brass manufacture and electroplating 1. Cu (II) is highly toxic in drinking water second to mercury. Although it is very important in the metabolism of human body, but excessive intake of Cu (II) may damage kidney and liver, and be responsible for immunotoxicity, anaemia and developmental toxicity 2. The co-carcinogenic character of Cu results in lung and stomach cancers, causing convulsions, cramps, vomiting, and even death. The treatments of the heavy metal ions contain lime softening, precipitation, ion exchange, adsorption, membrane technology and bio-treatment 3. The most widely used method is adsorption because of its easy operation, rapid response and cost-efficiency 4-6. Due to the increasingly stringent requirements of environmental treatment, the extensively used adsorbents such as activated carbons, biochar or biomass, clays, zeolites, graphene or its oxide, carbon nanotubes, and nanoparticles are constantly updated and improved 7-9. In the 1990s, Metal-Organic frameworks (MOFs) was put forward for the first time as a new concept, and the Co-MOFs was prepared. Up to now, MOFs has been widely used in such areas as gas storage, drug loading, catalysis, sensing, gas adsorption and separation

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. At present, MOFs as a solid

adsorbent, has been used for adsorptive removal of heavy metal ions, mainly including Ca-MOF for Cd (II) 11, Zr-MOF for Pb (II) 12 and Cr (VI) 13, Cu-MOF 14 or Ni-MOF 15 for Hg (II), Ag-MOF for Cr (VI) 16

, and etc. But MOFs is rare and ineffective in the removal of Cu (II). A zeolitic imidazolate

framework-8 (ZIF-8) MOF only had a capacity of 98%, 1.50 g) were added in. After fully dissolved, the solution was heated in a microwave oven (Glanz G70F20CN3LC2K (G4), Guangdong, China) for 5 min. After being cooled down, the obtained pale yellow samples were each cross-washed with DMF and CHCl3 for three times, and then subsequently dried at 70 °C to a constant weight. All reagents were of analytical grade without further purification. 2.2 Adsorption In batch Jar-test, Cu (II) solution (100mL, 10 mg·L-1) was heated on an integrated heating stirrer (120 r·min-1) at set temperature and pH, and then Zr-MOFs (0.01 g) were added in adsorption of Cu (II), the changes in Cu (II) concentration at a certain time interval were monitored in the adsorption process. pH adjustment was achieved by adding HNO3 of 0.1 mol·L-1. The ceramic membrane cross-flow disturbance setup was shown in Figure 1. Cu (II) solution (10L, 10 mg·L-1) was first added in the feed tank of the setup (Tianjin University, FLOM-UT-D1), and then 0.01 g·L-1 of the Zr-MOFs was also added in to adsorb Cu (II) in a continuous recycling mode. Both the retentate and the permeate streams were recycled back to the feed tank in 120 min with permeate valve open. Herein, MOFs were combined with the ceramic membrane cross-flow system by recirculating the MOFs in the membrane system. Viz. the ceramic membrane cross-flow disturbance was introduced into the MOFs adsorption process. Permeate flux was measured by a graduated cylinder and a stopwatch at set conditions. The ceramic membrane (Nanjing, Jiuwu High-Tech Co. Ltd., with average pore size of 50 and 200 nm) can reject 100% of the Zr-MOFs in the separation of heavy metal ion Cu (II). The temperature can be controlled by jacketed tap water, and samples at certain time intervals in the feed and permeate side were monitored to study the effect of temperature on the adsorption process.



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2.3 Analytical methods The XRD (BRUKER AXS, D8 ADVANCE) of the Zr-MOFs was characterized to determine the crystal structure. The morphology and particle size of the Zr-MOFs were also investigated by scanning electronic microscopy (SEM, JEOL, JSM-6490LV) and high resolution transmission electron microscopy (HRTEM, JEOL, JEM-2100). The rejection of the Zr-MOFs by ceramic membrane was monitored using a turbidity meter (Shanghai Shangtian, WGZ-1). The concentrations of Cu (II) before (C0, mg·L-1) and after adsorption in a certain time t (Ct, mg·L-1), were determined by atomic absorption spectrophotometer (Beijing Rayleigh analytical instruments, WFX-130A). The relative errors of all the experimental data were within 5%. It should be noted that all permeates showed 100% rejection of the Zr-MOFs in terms of turbidity removal. Hence, only other items will be discussed hereinafter. The removal efficiency of the heavy metal ion (R), the adsorption amount of Cu (II) adsorbed at time t (qt) and the permeate flux (L·m-2·h-1) (Flux) can be calculated using can be calculated using the following equations (1)-(3).

R=

(C0 − Ct ) × 100 % C0

qt =

(C0 − Ct ) × V W

Flux =

VP × 3.6 t× A

(1)

(2)

(3)

Wherein, V is the volume (mL), W is the amount of adsorbent (mg), VP is the permeated volume (mL), t is the permeated time (s), and A is the membrane filtration area (0.11m2). 3. Results and discussion 3.1 Characterization of the MOFs Figure 2A shows that the Zr-MOFs obtained by the microwave-promoted method had the same peaks’ position and crystal type as those of the traditional hydrothermal method, which directly


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matched the simulated UiO-66 pattern reported in reference

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, indicating similar crystal structures of

UiO-66-NH2 and confirming the successful syntheses of the MOFs. Because the intensity peaks of the UiO-66-NH2 at 7.2° and 8.3° in 2-theta are the strongest, and the peaks at 28° to 40° that assigned to UiO-66-NH2 can also be readily seen in the patterns

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. The good quality of the MOFs is consistent

with the Scherrer equation, which predicts increased broadening with smaller and non-spherical crystals 26. SEM image in Figure 2B shows the morphology of the Zr-MOFs particles, exhibiting a sphere-like appearance and nanoscale sizes. Related work by Lemaire et al. 27 also found the MOFs (UiO-66-NH2) as octahedral crystals around 150 nm. This can be further supported by the structure observed by HRTEM (Figure 2C). It shows that the Zr-MOFs has high porosities and multi-cage structures, due to the similar Zr6O4(OH)4 nodes and larger pores. The mean particle size was only around 100nm, and this was very close to the value reported by Lin et al. in literature 28. 3.2 Effect of temperature on the MOFs adsorption As shown in Figure 3A, the removal efficiency of Cu (II) was increased with increasing temperature from 25 to 40 oC. This illustrates that the temperature had a promoting effect on the adsorption of Cu (II) onto the Zr-MOFs, proving that the adsorption was an endothermic process. Within 0 to 3 h, the removal efficiency of Cu (II) was increased to nearly 90% with the temperature, and then reached the equilibrium in 12 h due to the homogeneous porous nature of the MOFs. Most of the active sites on the MOFs were available and this resulted in rapid adsorption of Cu (II) in the initial stages 29. After the rapid adsorption, the available active sites decreased gradually, it then took long time (more than 15 h) to achieve the equilibrium state of the adsorption. At 40 oC, qt was 81.9 mg·g-1 (>40 mg·g-1, reported in literature for a ZIF-8 adsorbent 17). Because Cu (II) has a strong attraction to the lone pairs of electrons on the amidogen nitrogen atoms of the MOF, thus can form a stable complex 30. The adsorption data were calculated using the pseudo-second-order model as the following equation



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(4):

1 t t = 2+ qt kqe qe

(4)

Where k (g·mg–1min–1) is the pseudo-second-order rate constant, and qe is the adsorption capacity at the equilibrium state. According to the results shown in Figure 3B, the data showed a good compliance with the pseudo-second-order mechanism due to the high correlation coefficient (R2>0.999). Hence, the overall rate of the adsorption process was governed by the chemical reaction 31. This verified that the chemisorption was the rate controlling step during the whole adsorption process

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. At 40 oC, the

calculated value of qe was 82.0 mg·g–1 (close to 81.9 mg·g–1 of qt). And this value was much higher than that of the newly reported Ca-MOF (maximum value of 68 mg·g–1) for Cu (II) removal

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. The

proper temperature was selected as 40 oC. 3.3 Effect of pH on the MOFs adsorption It is very important to study the effect of pH because it has a strong effect on the capacity of the adsorption process. Figure 4A shows there is no adsorption of Cu (II) onto the Zr-MOFs at pH of 2.3 and 4.5, as R=0%. Because the isoelectric point (IEP) of the MOFs was 4.3

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. At pHIEP, R was around 80%, with qt up to 74.3 mg·g-1. The MOFs was negatively charged, which was beneficial to the adsorption of Cu (II), due to the electrostatic attraction between the MOFs and Cu


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(II). Adsorption kinetic modeling is a very useful method for the better understanding of the Cu (II) adsorption mechanisms onto the MOFs, in order to fit the obtained adsorption experimental data, the most well-known pseudo-second-order model was used. The experimental data were then correlated with the linear form using the pseudo-second-order model. As shown in Figure 4B, the kinetic data displayed the good-fitting results of the pseudo-second-order model with the correlation coefficients R2 were higher than 0.997. The results show a good concordance between the experimental data and the pseudo-second-order kinetic model, showing the importance of the chemical reactions 35. The performance of Cu (II) removal was evaluated at pH of 7. As shown, the maximum adsorption of Cu (II) for the MOFs occurred at pH of 7. But this may be caused by the precipitation of Cu (OH) 2 at pH > 6.5 36. Hence, the proper pH was then selected at 6 hereinafter this study. 3.4 Effect of ceramic membrane cross-flow disturbance on the MOFs adsorption Figure 5A shows that the ceramic membrane with a pore size of 50 nm exhibited increasing steadystate fluxes (154 to 415 L·m-2·h-1) with increasing temperatures from 25 to 40 oC. Because increasing temperature decreases the feed viscosity, and this leads to an increase in flux 37. The ceramic membrane with a pore size of 50 nm showed a flux decline around 6 % to 22 % of the initial flux value, revealing no severe fouling caused by the MOFs to the membrane with a pore size of 50 nm. In Figure 5B, the ceramic membrane with a pore size of 200 nm also showed an increase in steadystate flux from 207 to 449 L·m-2·h-1 with increased temperature over the range of 25 to 40 oC. The ceramic membrane with a pore size of 200 nm showed a more insignificant flux decline, usually around 5 % and not exceeding 19 % of the initial flux value, revealing a less severe fouling caused by the MOFs to the membrane with a pore size of 200 nm than that of 50 nm. At the same conditions, the membrane with the pore size of 200 nm had a higher flux than that of the 50 nm due to a lager pore size. Figure 6A shows increasing removal efficiency in steady-state (R from 62 % to 73 %) of the ceramic membrane with a pore size of 50 nm with increasing temperatures from 25 to 40 oC. In Figure 6B, the



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ceramic membrane with a pore size of 200 nm can well remove the heavy metal Cu (II). The R of the 200 nm ceramic membrane in steady-state was increased with increasing temperature (25 to 40 oC) from 65 % to the highest value close to 90%. This was probably caused by the larger permeate flux of the membrane with lager pore size 38. As for Cu(II), the removal efficiency showed a simple increasing trend due to the endothermic nature of the adsorption. But the removal of Pb(II) corresponded to the combination of both endo- and exothermic processes. And it was also noted that the removal efficiency at the equilibrium state varied for different heavy metal ions, with Cu(II) 90% at 10 mg·L-1, and Pb(II) 60% at 30 mg·L-1 21. This gives a metal affinity trend of Cu(II)

adsorption onto Zr-MOFs through Novel cross-flow disturbance of

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