graphene hydrogels through an enhanced Fenton reaction

Journal Pre-proof Tetracycline removal by double-metal-crosslinked alginate/ graphene hydrogels through an enhanced Fenton reaction Yan Kong, Yuan Zhuang, Baoyou Shi

PII:

S0304-3894(19)31014-3

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121060

Article Number:

121060

Reference:

HAZMAT 121060

To appear in:

Journal of Hazardous Materials

Received Date:

21 May 2019

Revised Date:

13 August 2019

Accepted Date:

20 August 2019

Please cite this article as: Kong Y, Zhuang Y, Shi B, Tetracycline removal by double-metal-crosslinked alginate/ graphene hydrogels through an enhanced Fenton reaction, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121060

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Tetracycline removal by double-metal-crosslinked alginate/ graphene hydrogels through an enhanced Fenton reaction Yan Kong1, Yuan Zhuang1,* [email protected], Baoyou Shi1,2

1. Key Laboratory of Drinking Water Science and Technology, Research Center for EcoEnvironmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China. Tel: 86-1062922155

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2. University of Chinese Academy of Sciences, Beijing 100049, China

Corresponding author. E-mail address: [email protected]. (Y. Zhuang).

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Graphical abstract

Highlights  Improved stability through double-metal-crosslinked alginate/GO hydrogel.  The graphene and double-metal-crosslinked significantly enhanced the catalytic

activity.  The highest performance of adding Ce3+ in AG-Fe-M hydrogels.

Abstract Polymer hydrogels usually has limited catalytic activity and stability in catalysis. Here, we presented for the first time the preparation of a novel double-metal-crosslinked alginate hydrogel using graphene oxide to facilitate the Fe(II)/Fe(III) redox cycles. Five multivalent metal cations were used

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as crosslinkers to prepare different alginate-GO-M (Fe(III), Fe(II), La(III), Ce(III), and Co(II)), and the effects of assisted metal cations (La(III), Ce(III), and Co(II)) on different Fe(II) bimetallic

alginate-GO-Fe-M(AG-Fe-M) complexes were investigated. Double-metal-crosslinked alginate-GO hydrogels can degrade tetracycline much faster during the initial 10 min than single-metal-

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crosslinked hydrogels. In addition, the release of iron from AG-Fe-Ce (10.59 ppm) is less than that from AG-Fe-Co (21.57 ppm) and AG-Fe-La (25.6 ppm) during the Fenton reaction. More

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importantly, the AG-Fe-Ce does not release TOC and maintains most of the catalytic activity after four reuse cycles, confirming its excellent stability. For the treatment of raw water containing a high

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proportion of proteinaceous matter and tetracycline, the AG-Fe-Ce significantly reduces the molecular weight of the dissolved organic matter. We deduced that the humic acid and protein show good complexation ability to tetracycline, thereby reducing its bioavailability. This study provides

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new insights into the synthesis of polymer catalysts for water treatment.

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Keywords: alginate; Fenton; hydrogel; tetracycline.

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1. Introduction

Antibiotics are potent medicines that have been used for therapeutic treatment of infectious

diseases in both humans and animals. Over the past few years, over 250 different antibiotic entities have been registered for use in human and animal medicine[1–3]. The use of antibiotics has rapidly increased worldwide and has received widespread attention[4,5]. At the same time, antibiotic residues continue to exist in aquatic ecosystems even at low concentrations; thus they are considered to be emerging environmental pollutants[6–8]. Among various antibiotics, tetracycline (TC) is the

second most widely used antibiotic in the world, because of its high quality, low price, and broadspectrum antimicrobial activity against a variety of diseases[9]. However, TC has a high aqueous solubility and long environmental half-life; therefore, TC contamination levels in the environment have been increasing[10,11]. It is very important to removal TC residues before discharging wastewater into the environment. As a result, advanced and effective treatment technologies for antibiotic removal are urgently needed. Numerous approaches have been employed for TC removal from wastewaters, including biodegradation[12], coagulation/flocculation[13], adsorption[14,15], chemical oxidation[16,17], membrane separation[18,19], and catalytic degradation[20–24].

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Advanced oxidation processes (AOPs) are effective and rapid methods to remove TC owing to the in situ generation of highly reactive radicals (·OH)[25–28]. In particular, Fenton processes are

successfully used in degradation of aromatic compounds that are resistant to conventional treatment technologies[29–31]. Thus, they are applicable as a pretreatment stage to reduce the effluent toxicity

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before biological treatment. Fenton processes with hydrogen peroxide and transitional metals,

particularly ferrous iron, typically occur near pH 3[32–36]. The effectiveness of Fenton processes

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decreases with increasing pH because of iron speciation and precipitation. At a circumneutral pH, Fe(III) precipitation as hydrous oxyhydroxides, which do not readily redissolve, inhibits both the

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regeneration of the active species Fe(II) and the formation of hydroxyl radicals. Moreover, it may be a difficult challenge to include an additional separation process at the end of the reaction to the

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catalytic metal in some cases. To avoid these limitations, researchers have focused on catalytic metal immobilization on inert materials to develop heterogeneous catalysts for Fenton reactions, such as activated carbon, alumina, and silica gel[37–40]. Currently, increasing attention is being paid to the

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use of polymers, which provide interesting affinities for supporting catalytic metals; the structure of these polymers can also induce specific properties such as enhanced stereo-selectivity (chirality).

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Alginate acid is a naturally occurring linear polysaccharide composed of (1–4)-linked-d-

mannuronic acid (M units) and -l-guluronic acid (G units) monomers, which vary in number and sequential distribution along the polymer chain depending on the alginate source[41,42]. Alginate chelates with multivalent metal cations to form hydrogels through the tightly held junctions formed between G-blocks and multivalent metal cations[43]. Cruz et al.[44] noted the effect of a pH decrease during the reaction process and its relation to Fe(III)/alginate heterogeneous catalyst stability. However, these Fe(III)/alginate heterogeneous catalysts degrade the pollutants via

photoFenton. This limitation involves the necessity of photoactive activity, as the heterogeneous catalyst is prone to self-decomposition and release of iron under illumination. To avoid this drawback, it is necessary to explore new means to maintain iron as an effective catalyst at a neutral pH. In general, Fe(II) can be oxidized to Fe(III) in an aqueous solution; however, the binding of Fe(II) ions to free carboxylate groups of alginate reduces or even prevents the oxidation to Fe(III)[45,46]. Furthermore, the application of alginate-based catalysts during the Fenton process is promising because the numerous functional groups distributed along the alginate chains can stabilize metal ions and prevent solid sludge formation. Graphene oxide (GO) is a popular carbon

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nanomaterial with many chemical functionalities and excellent electron conductivity[47]. More importantly, we have found that GO can be well integrated with the alginate matrix because of its sheet-like properties to provide a maximal surface area[48]. It is widely believed that graphene

addition into the catalyst is beneficial to electron transfer to facilitate the Fe(II)/Fe(III) redox cycles

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during the Fenton process [49]. Thus, GO addition into an alginate hydrogel is of great importance to increase catalytic activity.

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Therefore, in this study, five multivalent metal cations were chosen as crosslinkers to prepare different alginate-GO-M (Fe(III), Fe(II), La(III), Ce(III), and Co(II)) (AG-M), and the effects of

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assisted metal (La(III), Ce(III), Co(II)) prepared different Fe(II) bimetallic alginate-GO-Fe-M(AGFe-M) complexes are also studied. Then, we evaluated and compared the structural properties and

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performance of TC removal of single metal AG-M hydrogels and bimetallic AG-Fe-M hydrogels. More importantly, we found the addition of Ce(III) as the assisted metal in the AG-Fe-Ce hydrogel resulted in an obvious increase in the catalytic activity during TC degradation. At the same time, the

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effects of reaction conditions on TC degradation were studied to determine the optimal conditions for the removal of pollutants by AG-Fe-Ce hydrogel. Raw water with tetracycline containment was also

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used to further determine the application potential of the AG-Fe-Ce hydrogel. Based on the results, a proposed mechanism of an excellent tetracycline removal process is presented.

2. Experimental 2.1 Materials

All chemicals were purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) in an analytical purity and used during the experiments without any further purification. All solutions were prepared using deionized water. 2.2 Preparation of AG-M and AG-Fe-M hydrogels Graphite oxide was obtained using the modified Hummers’ method, dispersed in deionized water and sonicated in an ultrasound bath for 4 h to obtain a graphene oxide (GO) solution. A total of 4 g of sodium alginate was dissolved in 100 ml of deionized water at room temperature and stirred for 4 h to obtain homogeneous sodium alginate. Then, the sodium alginate was added to the GO

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solution and stirred for 1 h to obtain homogeneous solutions. These mixture solutions were pumped via a peristaltic pump at 10 mL/min into a gently stirred 0.10 mol L−1 different multivalent metal cation (Fe(III), Fe(II), La(III), Ce(III), and Co(II)) solution to form alginate-GO-M (Fe(III), Fe(II), La(III), Ce(III), and Co(II))(AG-M). To prepare the alginate-GO-Fe-M (La(III), Ce(III), and Co(II))

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(AG-Fe-M), the mixture solutions were pumped via a peristaltic pump at 10 mL/min into gently stirred mixed metal salt aqueous solutions of FeSO4 and LaCl3, CeCl3, and CoCl2; the concentrations

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of the total metal ions remained constant at 0.10 mol L−1. To obtain complete gelation inside the beads, the beads were immersed in the metal salt aqueous solutions for 24 h and then washed using

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deionized water. All details of characterization methods and batch degradation experiments were

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presented in Supplementary data (Text S1and Text S2).

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3. Results and discussion

3.1 Syntheses and characterization of the alginate-GO-metal hydrogel

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The surface morphology of the alginate-GO-metal (AG-M) hydrogel was determined using SEM analysis and the obtained results are shown in Fig. 1. By comparing alginate-GO-Fe(III) (AGFe(III)), alginate-GO-Fe(II) (AG-Fe(II)), alginate-GO-La (AG-La), alginate-GO-Ce(AG-Ce), and alginate-GO-Co (AG-Co), it can be seen that the surface of the alginate-GO-M (Fe(III), Fe(II), La(III)) has more bulges with fold structures and roughness (Fig. 1a, 1b and 1c) compared to that of the alginate-GO-M (Ce(III), Co(II)) (Fig. 1d and 1e). Each sample shows large differences in the network density and the roughness of the hydrogel. This distinction might be because of the GO

sheet introduced into the hydrogel and the intrinsic difference between metal cations interacting with alginates. Comparing single-metal crosslinking (AG-M) and double-metal crosslinking (AG-Fe-M), it can be concluded that the double-metal crosslinking (Fig. 1f, 1g, and 1h) shows a denser and porous network structure than that of the single-metal crosslinking (Fig. 1a, 1b, 1c, 1d, and 1e), particularly for AG-Fe-La and AG-Fe-Ce that show a uniformed network structure and high porosity. The SEM results indicate that the pore structure and network density are dependent on the metal ion performance and new environments around the carboxylate group for a given alginate solution. In the presence of metal ions, alginate acid forms different structure hydrogels as metals ionically

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interact with carboxylic groups on different units of the alginate chains. Additionally, because the GO has oxygen functional and carboxylate groups on the basal planes and edges, it can bond to metal ions. However, it is possible that the alginate chain and GO sheets are intertwined via strong

hydrogen‐ bonding interactions. The relative atomic contents of these elements are listed in Table

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S1. Among them, the AG-Fe-Ce hydrogel has the lowest metal content, though it has a more porous structure according to SEM. Fig. 2 shows a SEM photograph and the corresponding EDS elemental

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mapping of AG-Fe-Ce. The EDS elemental maps confirm that C, O, S, Cl, Fe, and Ce are highly dispersed in AG-Fe-Ce. As previously mentioned, we deduce that while the metal concentration

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increases, the point of junction to the carboxylic groups on the alginate hydrogel increases, decreasing the pore size and resulting in a higher pore density. Moreover, the metal concentration

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increases can also increase its interaction with the GO sheet to reduce the pore size. Sing et al.[50] reported metal ions fill a large space between the alginate molecules producing a tight arrangement of the film resulting in smaller voids and an increase in the extent of binding with increasing ionic

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radius. For the transition elements, the ionic size of the Fe(II) and Fe(III) are 76 and 64 pm, respectively, and compared to 74 pm for Co(II). On the other hand, the relative contents of Fe(II),

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Fe(III), and Co(II) in the AG-M hydrogels are 9.093, 90062, and 7.221, respectively. Therefore, the Fe(II) and Fe(III) ions fill the gap of the “egg-box” structure to form bulges with fold structures and roughness. For lanthanide metal ions, there is a preference for GG blocks residues over MM block residues, however, a different interaction between the metal ion and the alginate chain is occurring indicated by the relatively strong binding to MM blocks.

Lanthanide metal ions showed a

preference for GG blocks and an increase in the extent of binding with increasing ionic radius. It is worthy of remark that differences in density, radius and atomic weight of both cations create a new

environment around the carboxyl groups. Therefore, the influencing factors are more complicated.

To further study the interaction between GO, alginate chains and metal ions in the hydrogels, FTIR results of the AG-M and AG-Fe-M hydrogels are shown in Fig. 3. The distribution of some important functional group vibrations are summarized in Table S2. By comparing alginate, GO (Fig. S1), AG-M (Fig. 3a), and AG-Fe-M hydrogel (Fig. 3b), it can be seen that the AG-M have a similar band shift trend and have alkoxy, epoxy, and carboxyl groups characteristic of GO. One of the

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carboxylate oxygen interacts with the metal ion while the other participates in a hydrogen bond with an adjacent hydroxyl group (2-OH). Further, the carboxylate oxygen on the

polyguluronic (G) acid chain can form a hydrogen bond with the hydroxyl group on the GO. In the case of the β-d-mannuronate (M) metal complex, a threefold left-handed helix is formed by

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a weak intra-molecular hydrogen bonding between the 3-OH group and the ring oxygen of the adjacent residue of the nonreducing side[51]. In addition, previous studies have demonstrated

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strong hydrogen bonding between alginate chains and with oxygen-containing groups on GO[52,53]. This phenomenon in the FTIR spectra revealed that the vibrational changes at the

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peaks of 3389 cm-1 (-OH stretching vibration) and 1616 cm-1 (-COOH group) which caused by hydrogen bonding. The peaks for AG-M at 3389 cm-1 (-OH stretching vibration) and 1616 cm-1 (-

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COOH groups) shifted to low wave numbers, which was attributed to the interaction of alginate, metal ions, and GO through intermolecular hydrogen bonds. For the AG-Fe(II) hydrogel spectrum, the peaks at 3363, 2905, 1581, 1419, and 1027 cm-1 are because of the OH, C-H, and symmetric and

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asymmetric COO— stretching vibrations of the carboxylate salt group and symmetric C-O stretching in the C-O-C group, respectively. The peaks at 1739, 1080, and 1171 cm-1 are attributed to the C=O

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stretching vibration of the carboxylic group, C-O stretching vibration mode related to the presence of the alkoxy group, and C-O stretching vibration, respectively. This is the characteristic band in GO, indicating that GO was well embedded in the hydrogel. Since the metal cations is combined with the alginate polyglucuronic acid units to form a structure such as an “egg box”, limiting the C-H stretching, the peak of the C-H stretching vibration at 2905 cm-1 is remarkably weak. Compared to the other AG-M hydrogels, the broader bands at 3370 cm-1 and 1577 cm-1 are stronger in the AG-Ce and AG-La hydrogels with cerium ion and lanthanum ion-crosslinking, indicating that a stronger

hydrogen bond in the AG-Ce leads to a dense structure and a better mechanical property. These results agree well with the SEM results. However, for AG-Fe-Ce, the main peaks are observed and no new peaks appear in the spectrum compared to that of AG-Fe. However, the broad band at approximately 3341 cm-1 is stronger, and the peaks at 3355 and 1593 cm-1 shifted to the low wavenumbers 3331 and 1591 cm-1, which could be attributed to alginate and GO interaction through intermolecular hydrogen bonds.

The hydrogel consists of a three-dimensional cross-linked network of alginate and GO, which is

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capable of absorbing large amounts of water in a swollen state because of the presence of abundant oxygen-containing hydrophilic functional groups in the alginate and GO network. However, the high swelling properties of hydrogels generally have low mechanical strength limiting their application in water treatment. The swelling ratio results of the AG-M hydrogels are shown in Fig. 4. AG-Ce and

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AG-La have lower swelling ratios which is good for maintaining their original shape and having good pressure resistance, corresponding to the strong hydrogen bond from FTIR and the dense

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crosslinked structure from SEM. The AG-Fe(III) and AG-Fe(II) swelling ratios are higher than those of the AG-Fe-Ce hydrogel, AG-Fe-La hydrogel, and AG-Fe-Co hydrogel. It can be seen that the

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addition of cerium, lanthanum, and cobalt ions can improve the AG-Fe swelling performance. This is due to the AG-La and AG-Ce have a flat and dense structure rather than a three-dimensional

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highly porous structure, which provides less contact surface area and slows the penetration of water molecules into the hydrogel network, resulting in lower swelling rate. In addition, the three-dimensional interconnected porous structure of the AG-Fe-M hydrogel provides more

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pores for water absorption, thereby greatly improving the swelling properties of the hydrogel.

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3.2 Catalytic performance on TC degradation with different catalysts To investigate the catalytic activity of AG-M, TC was selected as a model contaminant. Fig. 5

shows the TC adsorption and catalytic removal curves for the AG-M hydrogels. By comparing different single metal cations to cross-linked alginate-GO hydrogels, it can be seen that the amount of TC absorbed by AG-Fe(Ⅲ) and AG-Fe(Ⅱ) is 51.49% and 41.13% higher than that by the AG-Co, AG-La, and AG-Ce, which are 10.30, 7.59, and 10.32%, respectively, confirming AG-Fe(Ⅲ) and AGFe(Ⅱ) have favorable adsorption. After H2O2 is added, it was observed that the TC removal rate

followed the order AG-Fe(Ⅱ) > AG-Fe(Ⅲ) > AG-Ce > AG-La > AG-Co (96.08% > 85.72% > 67.11% >56.68% > 53.55%). Among the AG-M hydrogels, AG-Fe(Ⅱ) has a rapid degradation rate (0.0087 min-1) of tetracycline that can reach 85.57% within 5 min. However, the alginate chain solubilizations in AG-Fe(Ⅱ) and AG-Fe(Ⅲ) are shown in Fig. 6; particularly, the AG-Fe(Ⅲ) hydrogel is completely solubilized. Therefore, it is important to investigate the monitoring of soluble metal ions and TOC, as shown in Fig. S3. An increase in TOC must originate from the dissolution of GO and alginate chains from AG-M hydrogels because the initial TC dosing contributes approximately 50 mg/L of TOC. It can be concluded that AG-Fe(Ⅱ) hydrogel and AG-Fe(Ⅲ) hydrogel have a very

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high iron release and soluble organic carbon rate. Among the AG-M hydrogels, AG-CO, AG-La, and AG-Ce have nearly no metal ion dissolution and TOC release showing excellent stability. However, these single-metal-crosslinked hydrogels have a lower tetracycline removal rate. By comparing a single-metal-crosslinked and a double-metal-crosslinked alginate-GO hydrogels under the same

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reaction conditions, it can be seen that AG-Fe-Ce, AG-Fe-Co, and AG-Fe-La more rapidly degraded than AG-Ce, AG-Co and AG-La during the initial 10 min, and they have a higher tetracycline

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degradation rate. More importantly, the higher degradation rate (0.00261 min-1) of AG-Fe-Ce than that (0.00147 and 0.00205 min-1) of AG-Fe-Co and AG-Fe-La. In addition, the release of iron from

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AG-Fe-Ce (10.59 ppm) is lower than that of AG-Fe-Co (21.57 ppm) and AG-Fe-La (25.6 ppm) during the reaction; the AG-Fe-Ce does not release TOC which indicates it has the highest stability

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further studies.

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among these hydrogels. Thus, the AG-Fe-Ce hydrogel was chosen as a catalyst for TC degradation in

3.3 Effect of parameters on the catalytic activity of AG-M for TC degradation Effect of the initial pH of the TC solution

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3.3.1

Because of the inert activity of iron-based catalysts at high pH during the Fenton or Fenton-like

oxidation process, the initial pH is generally considered to be among the most important factors. Meanwhile, Cruz et al.[44] proved that the solution pH is the most important reaction conditions for the stability of Fe(III)/alginate catalysts; the proposed mechanism is that the stability of catalysts is related to the pH of the sample and the pKa of the alginate carboxylic groups. To investigate the AGFe-Ce pH adaptability, TC degradation was conducted at pH values of 3.0, 3.7, 6.0 and 7.0, for

comparison (Fig. 7a); the AG-Fe(II) and AG-Ce samples were investigated. It can be seen that approximately 94.04, 96.54, 85.12, 84.59, and 32.17% of the TC is removed via AG-Fe-Ce after 30 min when conducted at a pH of 2.0, 4.0, 6.0, 8.0, and 11.0, respectively (Fig. S3). The removal rate gradually increased with the initial pH increasing from 2.0 to 8.0, whereas the removal rate decreased rapidly when increased to 11.0. The same trends are found in AG-Fe(II) hydrogel. In contrast, the TC removal efficiencies are approximately 22.42, 37.60, 41.96, 15.30, and 42.99% via the AG-Ce hydrogel at a pH of 2.0, 4.0, 6.0, 8.0, and 11.0, respectively. Apparently, the TC removal rate increased via the AG-Ce hydrogel with the pH increased from 2.0 to 6.0, and the maximum

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degradation rate was obtained at pH 6. Thereafter the removal rate rapidly decreases with the initial pH increasing from 6.0 to 11.0. According to a previous study, TC is an amphoteric compound. It exhibits four different forms at different pH values because of the protonation-deprotonation

reaction: TCH3+, TCH20, TCH−, and TC2-[54]. Furthermore, the zeta potentials of the AG-Fe-Ce

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hydrogel are negative and decrease with an increase in pH from 4.0 to 10.0 (Fig. 7b). The pH

evolution is examined along with the experiment at different initial pH values. It can be seen in the

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experiments after 5 minutes of reaction at different initial pH values (Fig. 7c), the pH tends to decrease to an equilibrium value (the pKa of the alginate ranges between 3.4 and 5.0); particularly

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when the initial pH is 8, the pH value tends to decrease to an equilibrium value of approximately 4.5. This is mainly because alginate and GO are gradually protonated under an acidic condition, and in

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addition, carboxylic acids are formed during oxidation without a buffer solution. When the pH is in the range of 2 to 8, tetracycline molecules is in a near-neutral form (TCH20) after 5 minutes of reaction, and the negative charge of the AG-Fe-Ce hydrogel is increased; thus, electrostatic attraction

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occurred. The TCH3+and TCH20 forms have strong electron-acceptor capabilities, promoting the π-π stacking, hydrogen bonding, and cation-π bonding with GO and alginate, which can be favorable for

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TC adsorption onto the AG-Fe-Ce hydrogel and can be efficiently degraded. When the pH was set to 11, the AG-Fe-Ce hydrogel surface was negative charge and the tetracycline molecules were in a TC2- form. It is possible to inhibit the adsorption of TC molecule on the carboxyl groups of AG-FeCe hydrogel surface due to the electrostatic repulsive force and suppressed the π-π stacking and cation-π bonding with GO and alginate, which the degradation rate is not high. Moreover, at pH ≥ 4, H2O2 can be decomposed into water (H2O) and oxygen (O2). For the AG-Ce hydrogel, the low tetracycline removal rate at a pH of 2.0 may have been because of the excess H+ ions competing with

TC for the adsorption sites. We suspect that no oxidation reaction occurs and only adsorption reactions occur. The aforementioned results show that the AG-Fe-Ce hydrogel has a high catalytic activity over a wide pH range.

3.3.2. Effect of H2O2 concentration and catalyst dosage on TC degradation In the heterogeneous Fenton reaction, the H2O2 concentration and catalytic dosage directly affects the concentration of active radicals produced via TC degradation. The impact of the H2O2 concentration on TC degradation of the catalyst was investigated within the range of 1-50 mM as

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shown in Fig. 8a. It is apparent that the rate of TC degradation increases to 97.0% with increasing amounts of H2O2 to 20 mM for AG-Fe-Ce hydrogel. More amount H2O2 were adsorbed by the

surface active site of the AG-Fe-Ce hydrogel with the increase of H2O2 concentration, the metal

Fe(II) in the hydrogel promoted the formation of more •OH radicals. However, the TC degradation is

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retarded with a further increase in the H2O2 concentration to 50 mM. This phenomenon occurred because H2O2 molecules can be decomposed to oxygen and water at high H2O2 concentrations

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(Eqs.(1)); in addition, H2O2 itself can scavenge •OH radicals to form hydroperoxyl radicals (•O2H). The •O2H exhibit lower activity and they can also scavenge the •OH radicals (Eqs.(2)-(3)). A similar

2H2O2 → 2H2O + O2

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H2O2 + ∙OH →∙O2H + H2O

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result was also observed by other authors. Therefore, 20 mM of H2O2 was the optimal concentration.

∙O2H +∙OH → H2O + O2

(1) (2) (3)

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The effects of catalyst dosage on TC degradation was tested using 0.2, 0.5, 1, and 2 g/L as shown in Fig. 8b. The TC degradation rate has been proven to obviously increase; when the AG-Fe-

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Ce hydrogel dosage ranged from 0.2 to 1 g/L, the TC degradation rate increased from 77.88 to 97.04%. These results imply that the active sites for H2O2 activation increase with increasing catalyst dosage, promoting hydroxyl radical formation. However, the TC degradation rate showed only a slight increase with a further increase in the catalyst dosage to 2 g/L. This may be attributed to the lack of hydrogen peroxide limiting hydroxyl radical formation. In addition, excessive AG-Fe-Ce hydrogel may result in hydroxyl radicals that are trapped in its surface and not effectively utilized, limiting the TC degradation process. Therefore, 1 g/L of AG-Fe-Ce hydrogel was the optimum

dosage.

3.3.3 Stability and recyclability of the AG-Fe-Ce hydrogels The deactivation and potential reusability of the AG-Fe-Ce hydrogel is an important characteristic for practical application. In the present work, the stability test of tetracycline degradation by AG-Fe-Ce hydrogel was carried out by recycling the catalyst four times. As shown in Fig. 9a, TC degraded by 97.1% within 90 min when fresh AG-Fe-Ce hydrogel was used; however, approximately 81.4% of the TC degraded in the 4th reuse of the AG-Fe-Ce hydrogel. It is observed

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that the reaction rate decreases with increasing catalyst uses, perhaps because of the mass loss of the AG-Fe-Ce hydrogel catalyst. In addition, the surface active sites of the AG-Fe-Ce hydrogel were covered by residual byproducts and reactants, which are difficult to completely remove from the active catalytic sites in the following regeneration and cleaning procedures. Although the efficiency

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of AG-Fe-Ce hydrogel removal of TC decrease slightly, the AG-Fe-Ce hydrogel after the fourth reuse still maintains high activity, indicating that the AG-Fe-Ce hydrogel catalyst has high stability. The

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catalytic stability still needs improvement in future studies. After the fourth degradation test, the exhausted catalysts were soaked in a 100 mL 0.1 mol L−1 FeSO4 solution for 6 h. The immersion

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treatment mostly recovered the degreed of TC removal, which means that the addition of Fe(II)

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increased the generation of free radicals in the Fenton-like reaction system.

3.4. Reaction mechanism

To better understand the mechanism for the catalytic activity of the AG-Fe-Ce hydrogel, we

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compared the XPS of the AG-Fe-Ce hydrogel before and after the stability test as shown in Fig. 9b, 9c, and 9d. The C1s spectra of the fresh AG-Fe-Ce hydrogel can be deconvoluted into four

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peaks, which correspond to the following functional groups: C C/C–C (284.3 eV) in the carbon sp3, C–O (286.1 eV) for epoxy and alkoxy, and the C O (287.7 eV) and O–C O (288.5 eV) groups. It is clear that the C C/C–C bond and C-O bond of the AG-Fe-Ce hydrogel are the dominant peaks, and the peaks of C O and O–C O are weak. However, after the 4th reuse, it is clear that the C-O bond of the AG-FeCe hydrogel becomes the dominant peak, and the intensity of the C C/C–C peak noticeably decreases further indicating the alginate chain solubilization. The Ce 3d spectra of the AG-Fe-Ce hydrogel before and after the 4th reaction consist of four main peaks (3d5/2 at 882.3 eV and 885.9 eV and 3d3/2 at

900.7 eV and 904.5 eV) attributed to the positive Ce(III); meanwhile, Ce4+ is not observed. The 2p3/2 and 2p1/2 peaks of the Fe ion can be observed at 710.7 eV and 724.1 eV, respectively. Additionally, the Fe 2p3/2 peak can be divided into two components, corresponding to Fe(II) and Fe(III) at 710.6 eV and 713.7 eV, respectively. After the 4th reaction, it is clear that the intensity of Fe(II) in the AG-Fe-Ce hydrogel slightly decreases compared to that of the fresh hydrogel, which may be because of the Fe(II) and Fe(III) cycle on the surface of the AG-Fe-Ce hydrogel. The high stability of the AG-Fe-Ce hydrogel can be attributed to its stable structure of the Ce(III) cross-linked alginate without participating in the reaction. The results show that the AG-Fe-

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Ce hydrogel can contribute to the Fe(III)/Fe(II) cycle and it has excellent stability. It is well known that a good heterogeneous catalyst should have the ability to absorb the reactant molecules and be sufficiently strong for them to react. The N2 adsorption-desorption isotherm of the AG-Fe-Ce

hydrogel was measured and proved to have a large surface area (47.34 m2 g−1) and high total pore

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volume (0.28 cm3 g−1) (Fig. S6). Therefore, the AG-Fe-Ce hydrogel can provide sufficient surface area for adsorption and efficient penetration of the reactants into the active catalytic sites.

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To ascertain the reaction paths, we used DMPO spin-trap ESR to monitor the reactive oxygen species (ROS) involved in the AG-Fe-Ce hydrogel and H2O2 system. As shown in Fig. 9e and 9f, no

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significant signals were attributed to •OH and HO2•/O2•- during the control experiments in the absence of AG-Ce hydrogel, while •OH and HO2•/O2•– signals were observed in the suspension of the

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AG-Fe hydrogel and AG-Fe-Ce hydrogel. The intensity in the AG-Fe-Ce hydrogel-H2O2 system is much higher than that in the AG-Fe hydrogel, indicating that more ROS are generated in the former system.

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Based on the experimental results and a review of the literature, a possible reaction mechanism of the H2O2 activation via the AG-Fe-Ce hydrogel is proposed as follows: because of the high porosity and

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specific surface area of the AG-Fe-Ce hydrogel, it is beneficial to promote molecular diffusion of TC into the interior materials. First, the TC molecules are adsorbed from the bulk solution on the surface of the catalyst, with the assistance of the π-π interaction and oxygen-containing functional groups interaction between the aromatic rings of the TC and the GO structure of the AG-Fe-Ce hydrogel. Once the H2O2 is added, Fe(II)/Fe(III) species in the AG-Fe-Ce hydrogel can efficiently activate H2O2 to produce the active •OH and HO2•/O2•- radicals through the intramolecular electron transfer process; ·OH is the main ROS for catalytic elimination of TC in the AG-Fe-Ce hydrogel /H2O2

system (Eqs. (4), (5), and (6)). Moreover, the GO structure of the AG-Fe-Ce hydrogel with high electrical conductivity could promote the free-electron transfer between the AG-Fe-Ce hydrogel and iron active centers during the Fe(II)/Fe(III) redox cycle, which might provide more reactive sites for H2O2 activation. Eventually, TC is readily degraded by •OH generated on the catalyst surface (Eq. (8)). S-Fe2+ + H2O2 → S-Fe3+ + •OH + OH−

(4)

S-Fe3+ + H2O2 → S–Fe2+ + HO2• + H+

(5)

•OH + H2O2 → HO2• + H+ + H2O

(6)

HO2• + S-Fe3+ → S-Fe2+ + O2 + H+

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(7)

TC + •OH/HO2• → Intermediates → CO2 + H2O where S represents the catalyst surface.

(8)

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In order to evaluate the removal performance of organic matter in raw water after degradation by the AG-Fe-Ce hydrogel, excitation-emission matrix (EEM) fluorescence spectroscopy profiles of

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the raw water before and after treatments via AG-Fe-Ce hydrogel+H2O2 are shown in Fig. 10a and 10b. The raw water contains high protein content, but the content of humic acid is relatively low. The

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content of proteinaceous substances is high and the content of humic acid is relatively low in the raw water. It is apparent that the low levels of humic acid, fulvic acid, and soluble microbial byproduct

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after the AG-Fe-Ce hydrogel+ H2O2 treatment. According to the molecular weight of dissolved organic matter in the raw water after the AG-Fe-Ce hydrogel+ H2O2 treatment as shown in Fig. 10e and 10f, it can be seen that the molecular weight of the dissolved organic matter in raw water

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significantly decreases in water after treatment in the range of 600 Da-10000 Da; the proportion of high molecular weight is more significantly than the decrease of the proportion of low molecular

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weight. Fig. 10c and 10d showed the 3D-EEM of additional tetracycline raw water and catalyzed water via the AG-Fe-Ce hydrogel+ H2O2. It has been suggested that the dissolved organic matter of the humic acid and protein has good complexation ability to TC, therefore reducing its bioavailability[55,56]. However, no significant fluorescence signals were observed after the degradation process via the AG-Fe-Ce hydrogel+H2O2, indicating the AG-Fe-Ce hydrogel shows excellent degradation for organic matter.

4. Conclusions In this study, we prepared a Fenton catalyst using alginate hydrogels using various metal cations as the assisted metal and graphene oxide as the electron transmission media to enhance the catalytic activity. The double-metal-crosslinked alginate-GO hydrogel alginate-GO-Fe-M could degrade TC much faster during the initial 10 min than the single-metal-crosslinked hydrogel alginate-GO-M (Fe(II), Fe(III), La(III), Ce(III), and Co(II)) (AG-M) with a low iron release during the Fenton reaction. For raw water treatment containing a high proportion of proteinaceous matter and TC, the

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AG-Fe-Ce hydrogel significantly reduces the molecular weight of the dissolved organic matter. With their low cost, high catalytic activity and stability, and easy separation, the alginate-based hydrogels have great potential for use as a Fenton catalyst in water treatment.

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Acknowledgements

This research was supported by the National Natural Science Foundation of China (No.

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51808538) and National Key Research and Development Program of China (2016YFA0203204).

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Fig. 1 SEM of the AG-M and AG-Fe-M hydrogels: (a)AG-Fe(III), (b) AG-Fe(II), (c) AG-

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La, (d) AG-Ce, (e) AG- Co, (f) AG-Fe-La, (g) AG-Fe-Co, and (h) AG-Fe-Ce

Fig. 2 SEM photograph and elemental mapping images of AG-Fe-Ce for C, O, S, Cl, Fe,

(b)

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AG-Fe(II) AG-Ce

AG-La

AG-Co

500 1000 1500 2000 2500 3000 3500 4000 -1

Wavenumber (cm )

AG-Fe-La

Transmittance

AG-Fe(III)

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Transmittance

(a)

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and Ce with color superposition

AG-Fe-Ce

AG-Fe-Co

500 1000 1500 2000 2500 3000 3500 4000

Wavenumber (cm-1)

Fig. 3 FTIR spectra of (a) AG-M and (b)AG- Fe -M hydrogels

Swelling ratio (%)

4000 3500 3000 2500 2000 1500 1000 500

-F e(3) e A (2) G -F -Co e( A 2) G -F -La e( 2) -C e

2)

G A

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Fig. 4 Swelling ratios of the AG-M and AG-Fe-M hydrogels

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-F G

G A

A

-F

e(

-C

a -L

G A

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G A

-C G A

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Fig. 5 Adsorption and catalytic degradation curves(a, b), degradation rate of different hydrogels(c, d)

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Fig. 6 Optical images of AG-M and AG-Fe-M hydrogels after the experiment

Fig. 7 (a) Influence of pH; (b) Zeta potential of AG-Fe, AG -Ce, and AG-Fe-Ce hydrogel; and (c) pH values at different reaction times

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Fig. 8 (a) Effects of initial H2O2 concentration, (b) effects of catalyst dosage, (c, d)degradation

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rate under different processes

(b) soak

0.8

C/C0

0.6

1st

2nd

4th

3rd

6th

5th

0.4 0.2 0.0

288.49 eV (O-C=O)

904.49 eV (Ce3+)

291

288

285

800.74 eV (Ce3+) 882.32 eV (Ce3+)

After

885.94 eV (Ce3+)

Binding Energy (eV)

Fe 2p1/2

Fe 2p

Fe 2p3/2

Before

722.8 eV (Fe2+)

Satellite peaks

725.7 eV (Fe3+)

713.7 eV (Fe3+)

710.6 eV (Fe2+)

After

740 735 730 725 720 715 710 705 700

Binding Energy (eV)

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915 910 905 900 895 890 885 880

(f)

(e)

AG-Fe-Ce

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AG-Fe-Ce

AG-Fe

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AG-Fe

3460

282

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Ce 3d

294

(d)

Before

Ce 3d5/2

Intensity (a.u.)

Intensity (a.u.)

Ce 3d3/2

After

Binding Energy (eV)

Time (min)

(c)

284.30 eV (C-C/C=C)

286.11 eV (C-O)

287.71 eV (C=O)

297

0 80 0 80 0 80 0 80 0 80 0 80

Before

C 1s

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(a)

Intensity (a.u.)

1.0

3480

3500

3520

AG-Ce

3540

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Magnetic Field (G)

AG-Ce

3460

3480

3500

3520

3540

Magnetic Field (G)

Fig. 9 (a) Stability of AG-Fe-Ce hydrogel for TC degradation; (b) C 1s, (c) Ce 3d, (d) Fe 2p

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XPS spectrum of the AG-Fe-Ce hydrogel before and after recycle runs; (e) DMPO spintrapping ESR spectra of ·OH radicals; (f) DMPO spin-trapping ESR spectra of HO2•/O2•radicals

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Fig. 10 3D-EEM fluorescence spectroscopy of organic matter in raw water (a) before and (b)

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after treatments using AG-Fe-Ce hydrogel+ H2O2 treatment; 3D-EEM fluorescence spectroscopy of tetracycline and organic matter in raw water (c) before and (d) after treatment

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using the AG-Fe-Ce hydrogel + H2O2 treatment; Molecular weight distribution for raw water

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(e) before and (f) after treatment