Separation and Sequential Recovery of Tetracycline and Cu(II) from

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Separation and sequential recovery of tetracycline and Cu(II) from water using reusable thermo-responsive chitosan-based flocculant Kexin Ren, Hongwei Du, Zhen Yang, Ziqi Tian, Xuntong Zhang, Weiben Yang, and Jianqiang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00828 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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ACS Applied Materials & Interfaces

Separation and sequential recovery of tetracycline and Cu(II) from water using reusable thermo-responsive chitosan-based flocculant

Kexin Ren†,#, Hongwei Du†,#, Zhen Yang†,*, Ziqi Tian‡, Xuntong Zhang†, Weiben Yang†,*, Jianqiang Chen§

†

School of Chemistry and Materials Science, Jiangsu Provincial Key Laboratory of

Materials Cycling and Pollution Control, Nanjing Normal University, Nanjing 210023, P. R. China ‡

Department of Chemistry, University of California, Riverside, CA 92521, United

States §

College of Biology and the Environment, Nanjing Forestry University, Nanjing

210037, P. R. China *

Corresponding authors. Tel.: +86-25-85891307. E-mail: [email protected]

(Z.Y.), [email protected] (W.Y.) #

Author contributions. K.R. and H.D. contributed equally to this work.

Keywords: Reusable thermos-responsive flocculant; Antibiotics; Heavy metals; Sequential recovery; Theoretical calculation

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Abstract: Coexistence of antibiotics and heavy metals is typically detected in water containing both organic and inorganic contaminants. In this work, flocculation method, using a reusable thermal-responsive chitosan-based flocculant (CS-g-PNNPAM), was applied for separation and sequential recovery of tetracycline (TC) and Cu(II) from water. High synergistic removal rates of both TC and Cu(II) from water (>90%) were reached. Interactive effects among targeted water’s temperature (T1), stock solution’s temperature (T2) and flocculant dosage on flocculation performance were assessed using response surface methodology. In order to optimize flocculation, operation strategies of adjusting T2 and dosage according to T1, based on the interactive effects, were given through mathematical analyses. Flocculation mechanism, as well as interfacial interactions among CS-g-PNNPAM, TC and Cu(II), was studied through experimental investigations (floc size monitoring, XPS and UV spectra) and theoretical calculations (density functional theory (DFT) and molecular dynamics (MD) simulations): Coordination of Cu(II) with TC and the flocculant promoted flocculation; switchable interactions (H-bonds and hydrophobic association) of TC-flocculant at different temperatures were key factors affecting operation strategies. By weaken these interactions step by step, TC and Cu(II) were sequentially recovered from flocs using certain solutions. Meanwhile, the flocculant in flocs was regenerated and found reusable with high flocculation efficiency.

1. INTRODUCTION Antibiotics and heavy metals often co-exist in water, for example in intensive livestock farming or centralized industrial wastewaters.1,2 Complex species of antibiotics-heavy metals are easily formed in these waters, making their environmental behaviors more complicated.3,4 Increasing evidences have suggested 2


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that these complex species would be more toxic than individual components.5,6 Consequently, removal of the combined contaminants from water are urgently needed. Among various efficient methods to achieve such a goal, flocculation using functionalized bio-polymers have been proven promising according to previous works:7-10 carboxyethyl and sulfhydryl groups introduced onto chitosan backbones can enhance interactions between flocculants and heavy metals, which then drag antibiotics into flocs through coordination of antibiotics-heavy metals;7,8 aromatic rings-functionalized chitosan can further produces direct interactions between flocculants and antibiotic molecules via π-π stacking, engendering higher removal efficiency;9 in addition, to overcome the flocculation difficulty that antibiotics possess switchable hydrophobic/hydrophilic characteristic, bio-polymer-based flocculants with switchable hydrophobicity/hydrophilicity,10 via grafting thermo-responsive polymer branches, are developed for strengthening flocculants’ affinity with different antibiotic species under various water conditions. Despite the above positive results on water purification efficiency, it should always be noted that transfer of the combined contaminants from water to flocs is not the end of environmental remediation. Reuse of valuable materials in contaminants, should be one of welcome alternative solutions.10,

11

For bettering such process,

separation and recovery of each component from flocs are required. Besides, if antibiotics and heavy metals are separated from flocs, the remaining flocculants are possible to be reused, which further decreases the cost of agents in water treatment. On the basis of results in authors’ previous study,10, 12 chitosan-based flocculants with dual switches are expected to realize the aforementioned proposal: one is the temperature switch of grafted thermo-responsive polymer branches, which gives rise to enhanced or weakened affinity with different contaminants’ species at different 3



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temperatures;13 the other is the pH switch, since original amino groups on chitosan wield either electrostatic attraction or repulsion to contaminants at different pHs.14 When antibiotics and heavy metals are designed to be removed from water, switches are controlled to enhance interaction between flocculants and contaminants; however, by further gradually regulating the two switches, one component in flocs is possible to produce repulsion with other components, and then separated and enriched into the recovery liquid. High removal rate of combined contaminants from water is the premise for subsequent recovery of valuable components from flocs. A number of factors affect the removal rate, among which flocculant dosage is an important one.15-18 Additionally, since there exists a hysteresis effect during the “hydrophilic coil-to-hydrophobic globule” transition of thermos-responsive polymers,19 both temperatures of flocculant’s stock solution and targeted water have been found to influence flocculation performance when thermo-responsive flocculants are applied.12 Generally, to reach desired performance, it is rarely possible to adjust the temperature of targeted water with a large volume due to high energy cost; whereas adjusting flocculant’s dosage and temperature of high concentrated stock solution with a small volume is feasible. Hence, it is necessary to select appropriate dosage and stock solutions’ temperature according to the temperature of targeted water. To facilitate this strategy, interactive effects among dosage, stock solution’s temperature and targeted water’s temperature on flocculation performance should be investigated; nevertheless, no such investigation has been taken under consideration in previous literatures. Given that response surface methodology (RSM) is a powerful means in evaluating interactive effects on the basis of statistical principles,18 RSM is to be applied here. Furthermore, in other to give an in-depth understanding of the interactive effects 4


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during flocculation process, flocculation mechanism in molecular scale,9 which also matters to the design of both flocculants’ structure and contaminants’ recovery process, is needed to be studied. Experimental investigations on flocculation mechanism, including characterization of solution properties and floc structures, are typical used methods. Besides, theoretical calculations, which could provide more detailed clarification on intermolecular interactions, have attract more attention in the research fields of environmental science and engineering.20, 21 Consequently, theoretical study on flocculation mechanism is also to be performed in this work. In the current study, a novel thermal-responsive bio-polymer-based flocculant (chitosan-graft-poly(N-n-propylacrylamide), marked as CS-g-PNNPAM, with a temperature switch near room temperature) was synthesized and applied for separation and recovery of antibiotics and heavy metals from water. Tetracycline (TC) and Cu(II) were employed as model antibiotic and heavy metal, respectively.11 Interactive effects among flocculant dosage, stock solution’s temperature and targeted water’s temperature on flocculation performance were assessed using RSM. Flocculation mechanism in molecular scale was investigated by both experimental analyses and theoretical calculation. Finally, separation and recovery of each component from flocs were carried out and optimized using RSM.

2.MATERIALS AND METHODS 2.1 Materials CS-g-PNNPAM was synthesized by a “graft-to” method using amidation reaction between chitosan and poly(N-n-propylacrylamide) with a carboxyl end group (PNNPAM-COOH) according to Fig. 1a. Detailed synthetic routes were provided in Supporting Information Text S1. TC (purity of 99.5%; chemical structure in 5



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Supporting Information Fig. S1) was purchased from Sigma-Aldrich Chemical Co. All other chemicals were purchased from Sinopharm Chemical Reagent Co. Ultrapure water was used in all experiments. -Figure 12.2 Characterization of CS-g-PNNPAM Characterization methodologies of CS-g-PNNPAM, including Fourier-transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1H NMR), X-ray diffraction (XRD), zeta potential (ZP), molecular size measurements, temperature switch (lower critical solution temperature, LCST) determination and scanning electron microscope (SEM), are available in Supporting Information Text S2. 2.3 Flocculation Experiments The flocculation experiments were conducted using standard jar tests, consisting an initial period of rapid mixing (200 rpm) for 2 min after adding flocculant stock solution, followed by 8 min of slow mixing (50 rpm), and finally a period for 1 h without stirring for flocs sedimentation. Stock solution of CS-g-PNNPAM with a concentration of 5 g/L, and synthetic wastewater containing TC and Cu(II) (100 mg/L of each contaminant) with a neutral pH (adjusted by adding Na2CO3 solution), were freshly prepared before each test. After sedimentation period, purified water samples (collected at a depth of 2 cm in the supernatant) and flocs (carefully withdrawn from the bottom) were kept for further analyses. TC and Cu(II) concentrations were measured by a Hitachi UH5300 UV−vis spectrophotometer and an GBC Savant AA atomic absorption spectrophotometer, respectively. Residual concentration percentage (RC, %) of contaminants was applied to evaluate flocculation performance of CS-g-PNNPAM. RC = C/C0 × 100%

(1) 6


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where C and C0 are concentrations of contaminants after and before flocculation. Each experiment was triplicated and an average value was obtained. Pearson correlation coefficient between RC(TC) and RC(Cu(II)) in bivariate correlation analysis was calculated using SPSS statistics 20 (IBM Co.).20 In this work, flocculant dosage (0-0.4 g/L), stock solution’s temperature (15-35 o

C) and targeted water’s temperature (15-35 oC) varied in different jar tests, to

investigate interactive effects among these parameters on flocculation performance. Since their units and variation limits were different, the three independent variables were firstly coded according to Supporting Information Text S3 and Table S1-S3 for better comparisons.18 RC(TC) and RC(Cu(II)) were selected as dependent response variables, which were fitted by second-order models in RSM:

= + ∑ + ∑ + ∑

∑

(2)

where Ym and Xi represented dependent and independent variables, respectively; a0 was the offset term; ai, aii and aij were linear, quadratic and interactive coefficients, respectively. The coefficients of the response equations and corresponding analyses on variations were evaluated using Design Expert 8.0 and MATLAB R2014b. 2.4 Flocculation mechanism investigations Experimental investigations on flocculation mechanism consisted of floc size monitoring, microscopic observation and spectral analyses. Floc size under corresponding optimal conditions at different initial targeted water’s temperature was monitored using an in-situ Malvern Mastersizer 3000 system.22, 23 Vaccum-dried flocs were analyzed by SEM and X-ray photoelectron spectroscopy (XPS). UV spectra of solutions with different concentrations of CS-g-PNNPAM and/or TC and/or Cu(II) were recorded for pairwise interaction investigations. Besides, theoretical calculations, including molecular dynamics (MD) and density 7



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functional theory (DFT) simulations were carried out using LAMMPS and Gaussian software,24-26 respectively, to get insight into the interactions between the flocculant and contaminants at different temperatures. Computational details are provided in Supporting Information Text S4. 2.5 Recovery of TC and Cu(II), and regeneration of CS-g-PNNPAM After flocculation procedure under selected optimal conditions, flocs were obtained by filtration. Sequential recovery of TC and Cu(II) was made up of two steps: (1) Predesigned alkali water-alcohol recovery solutions at certain temperatures (Solution I, 20 mL per 0.1 g wet flocs) were applied to desorb TC from flocs firstly. RSM was employed to obtain the optimal condition. (2) Then, the other type of predesigned acidic acetone solutions (Solution II, 20 mL per 0.1 g wet flocs) at certain temperatures were used to desorb Cu(II) from the rest solid in step 1. Finally, the remaining flocculant under optimal conditions, in solid form at the bottom of acetone solution, was filtrated and vacuum-dried for further flocculation experiments to test its reusability.

3. RESULTS AND DISCUSSION 3.1. Basic physical and chemical properties of CS-g-PNNPAM Previous literatures have demonstrated the merits of graft copolymers in flocculation.27, 28 In most of them, graft copolymers were synthesized by a “graft-from” method which started with the preparation of a polymer backbone with large numbers of initiation sites for subsequently polymerization of side chains. However, in this work, CS-g-PNNPAM was synthesized using a “graft-to” strategy as described in Supporting Information Text S1. Since the side chains were prepared independently prior to coupling, their structures are more facile to be controlled and characterized,29 8


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compared with those synthesized by the “graft-from” method. The average number of repeated units of PNNPAM-COOH branches in this work is ~17.0 (calculated by an acid-base titrimetric method), long enough to provide temperature-responsiveness. Structural analyses results (FTIR, 1H NMR, XRD and SEM images) in Fig. 1b-1d, Supporting Information Text S5 and Fig. S2 confirm the chemical structure of CS-g-PNNPAM. The grafting ratio of PNNPAM branches is 19.8% calculated from 1

H NMR result. Thermo-responsiveness is revealed by transmittance-temperature

profile in Fig.

1e, where the temperature-switch (LCST) is found to be ~25 oC. It

implies that PNNPAM branches in flocculant are more hydrophilic at temperature < 25 oC; whereas those branches are more hydrophobic at higher temperatures, exerting enhanced interaction with hydrophobic organic contaminants’ species. Hence, flocculation performance is possibly affected by temperature, and then, both stock solution’s temperature and targeted water’s temperature in the following jar tests varies from 15 to 35 oC. In addition to solution transmittance, both molecular size and ZP decrease with the increase of temperature in Fig. 1e, which is ascribed to (i) the collapsed structure of PNNPAM and (ii) the shielding of positively charged amino groups by the collapsed PNNPAM,13 respectively. Molecular size represents the stretching degree of flocculant molecules, and ZP demonstrates the possibility of electrostatic interaction of flocculant-contaminants, which will be taken into consideration in the following discussion.

3.2. Flocculation performance Jar tests, in which TC and Cu(II) were employed as model contaminants, were performed at various flocculant dosages, targeted water’s temperatures (T1) and stock 9



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solution’s temperatures (T2) as aforementioned. The results are depicted in Supporting Information Fig. S3. It is demonstrated by an overview of Fig. S3 that the dosing of CS-g-PNNPAM promotes the removal of both contaminants for any of the targeted water at any dosage. At each T1, the lowest RC of contaminants with optimal T2 and dosage can reach close to 90%. Correlation analysis of RC(TC)-RC(Cu(II)) shows a significant synergistic removal effect of TC and Cu(II), with a high Pearson correlation coefficient (R2=0.935) and a low significant level (α

Separation and Sequential Recovery of Tetracycline and Cu(II) from

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