Fenton reaction induced in-situ redox and re-complexation of polyphenol-Cr complex and their products

Journal Pre-proof Fenton reaction induced in-situ redox and re-complexation of polyphenol-Cr complex and their products Hongrui Ma, Qing Wang, Yongyong Hao, Chao Zhu, Xiangping Chen, Chuanyi Wang, Yonglin Yang PII:

S0045-6535(20)30407-0

DOI:

https://doi.org/10.1016/j.chemosphere.2020.126214

Reference:

CHEM 126214

To appear in:

ECSN

Received Date: 18 September 2019 Revised Date:

29 January 2020

Accepted Date: 13 February 2020

Please cite this article as: Ma, H., Wang, Q., Hao, Y., Zhu, C., Chen, X., Wang, C., Yang, Y., Fenton reaction induced in-situ redox and re-complexation of polyphenol-Cr complex and their products, Chemosphere (2020), doi: https://doi.org/10.1016/j.chemosphere.2020.126214. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Credit author statement Hongrui Ma: Conceptualization, Methodology, Funding acquisition; Qing Wang: Data curation, Writing- Original draft preparation; Yongyong Hao: Writing- Reviewing and Editing; Chao Zhu: Writing- Reviewing and Editing; Xiangping Chen: Writing- Reviewing and Editing, Funding acquisition; Chuanyi Wang: Writing- Reviewing and Editing; Yonglin Yang: Writing- Reviewing and Editing;

Graphical abstract

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Fenton reaction induced in-situ redox and re-complexation of

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polyphenol-Cr complex and their products

3 4 5 6

Hongrui Ma†*, Qing Wang†, Yongyong Hao, Chao Zhu, Xiangping Chen, Chuanyi

7

Wang, Yonglin Yang

8 9 10 11

School of Environmental Science and Engineering, Shaanxi University of Science and

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Technology, Xi’an 710021, China.

13 14 15 16 17 18 19

*To whom correspondence should be addressed.

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Email: [email protected] (H. Ma)

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Tel: +86-13991376232

†

Dual first authorship

1

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Abstract

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In this study, in-situ Fenton oxidation was used for the de-complexation and degradation

24

of tannin-Cr(III) complexes. Cr(III) can be oxidized into free Cr(VI) under the effect of

25

·OH and oxidation products of tannin can be used as reductant for Cr(VI) to establish a

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redox cycle of Cr(III)-Cr(VI)-Cr(III). Thus, it is crucial to investigate the interactions of

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Cr(III) with tannin derived oxidation products due to negligible accumulation of Cr(VI)

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during Fenton oxidation treatment. Here, sequential filtration/ultrafiltration was applied

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to reveal the distribution characteristics of TOC and Cr fractions during the oxidation of

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tannin-Cr(III). As the increase of colloidal size of tannic acid products, residual TOC and

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Cr mainly distribute in larger size range after the oxidation of tannin-Cr(III) which can be

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ascribed to re-complexation between oxidation products and Cr(III). Besides, analytical

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results indicate that carboxyl group and hydroxyl group in oxidation products may cause

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the re-complexation of Cr(III), resulting in the formation of highly conjugated materials

35

containing Cr(III). It can be concluded that due attention should be paid to the efficient

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removal technology and mechanism for polymer-Cr complexes, as well as the oxidation

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intermediates in the role of conversion and removal of Cr species.

38 39

Keywords: Tannin-Cr; Oxidation products; De-complexation; Re-complexation; Cr

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redox

41 42 43

2

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1. Introduction

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Chromium (Cr), one of the seventeen chemicals posing a threat to humans as listed by

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the United States Environmental Protection Agency (US EPA), is widely used in leather,

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textile, electroplating and metallurgical industry etc., and it usually enters environmental

48

systems through the effluents. Cr exists in several oxidation states ranging from Cr(II) to

49

Cr(VI), but Cr(III) and Cr(VI) are the most common and stable, of which the former is

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considered less toxic and comparatively immobile (Carolin et al., 2013; Dhal et al., 2013;

51

Jobby et al., 2018; Shahid et al., 2017).

52

Up to now, growing evidence indicates that organic ligands could increase the mobility

53

and stability of Cr(III) (James and Bartlett, 1983; Li and Xue, 2001). Nakayama et al.

54

reported that the percentage of organic-Cr(III) complexes is 45-60% of the total Cr in

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natural waters (Nakayama et al., 1981). Humic substances, an important component of

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natural organic matter in soil and water, are proved to bind with Cr(III) to form a

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monomeric and dimeric complex (Gustafsson et al., 2014). Meanwhile, carboxyl groups

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of organic acids such as citrate, oxalate, EDTA also have strong complexation with

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Cr(III) (Liu et al., 2018; Malek et al., 2009). Besides, various organic-Cr(III) complexes

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also exist in industrial wastewater, and abatement of the recalcitrant Cr(III) fractions

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remains a challenge (Bartlett and James, 1979; Jaikumar et al., 2017; Kocaokutgen and

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Özkınalı, 2004; Puzon et al., 2005; Zhao et al., 2015). Advanced oxidation processes

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(AOPs) have received considerable attention for removal of organic-Cr(III) complexes. It

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is generally believed that generated ·OH radicals can oxidize Cr(III) into Cr(VI) with

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consequential release due to its weak complexation with organic ligands (Dai et al., 2010;

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Dai et al., 2011; Durante et al., 2011; Li et al., 2014). Therefore, the utilization of AOPs

3

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for Cr(III) complexes removal has been intensively hindered by the formation and

68

accumulation of much more toxic Cr(VI) species. Currently, several Fenton-like

69

processes show that the generated Cr(VI) is in situ reduced back to Cr(III) by aqueous

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Fe(II), and then it can be precipitated with the subsequent alkaline treatment (Ye et al.,

71

2017; Ye et al., 2018).

72

Polyphenolic compounds, the secondary metabolites by plants, are widely distributed in

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natural environment (Gharras 2009; Jakobek 2015). Their structures consist of aromatic

74

ring and one or more hydroxyl substituents. An oxygen anions will be generated once the

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phenolic group is deprotonated, which can react with metal ions to form stable five-

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membered rings (see Fig. S1) (Badhani et al., 2015; Jaikumar et al., 2017). Tannic acid,

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which is composed of a central glucose structure derivatized at its hydroxyl groups with

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10 galloyl moieties (see Fig. S2), has been extensively used in leather industry with

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Cr(III) compounds to achieve strength, color or softness for the leathers (Fu and Chen,

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2019; Jaikumar et al., 2017; Lina and Maria, 2018; Shirmohammadli et al., 2018).

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Unfortunately, they are not fixed completely, and result in a presence of huge amount of

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tannin-Cr(III) complexes in wastewater (Lofrano et al., 2013). In fact, they are difficult to

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be removed due to the high complexation affinity and high stability of complexes over a

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wide pH range (Wang et al., 2016; Wang et al., 2018). Moreover, recent studies indicate

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that polyphenols tend to be converted into less complex intermediate products rather than

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completely mineralized (Ayoub et al., 2010; Chen et al., 2014; Kang et al., 2019;

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Waggoner et al., 2015). However, the interactions between intermediate products and Cr

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have rarely been studied, and further investigation should be taken for the exploration of

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the detailed reaction mechanism.

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Herein, the objective of this study is to illustrate the interaction between Cr and Fenton-

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derived oxidation products. De-complexation and degradation of polyphenol-Cr, using

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tannin-Cr as model, was investigated with in-situ Fenton oxidation. Sequential filtration/

94

ultrafiltration was used to reveal the size distribution of the TOC and Cr before and after

95

oxidation, and combined spectral analysis was used to characterize the complex structure

96

of the products.

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2. Experimental

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2.1. Chemicals and Materials

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Chromium(III) sulfate hexahydrate was purchased from the Guangdong Guanghua Sci-

100

Tech Co., Ltd., China. Tannic acid, ferrous sulfate heptahydrate, sulphuric acid and

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sodium hydroxide were purchased from the Tianli Chemical Reagent Co.,Ltd., China.

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Hydrogen peroxide was purchased from the Kermel Analytical Reagent Co.,Ltd., China.

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According to the characteristics of tannery process and tannery wastewater, the initial

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Cr(III) concentration is 100 mg·L-1. Tannin-Cr(III) complexes were freshly prepared by

105

dissolving weighted amounts of Cr2(SO4)3·6H2O with tannic acid in ultrapure water at pH

106

3.5 and 37 °C for 24 h, then the solution was filtered through a 0.45 µm membrane.

107

Tannic acid solution was prepared under the same conditions.

108 109

2.2. Fenton induced in-situ redox for tannin-Cr complex

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Fenton experiments were performed in a beaker filled with 300 mL tannin-Cr solution

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(pH 3.5), which was kept stirring at 60 rpm. To initiate the Fenton reaction, a varying

112

amount of Fe2+ and H2O2 were added into the solution to generate ·OH radicals (Eq. (1)).

5

113

After 2 h, 2% NaOH was added to the solution to achieve pH 8, then, Cr(III) was

114

removed via precipitation (Eq. (2)). In batch experiments, effects of tannin dosage (250-

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750 mg·L-1), H2O2 dosage (11-44 mmol·L-1) and mole ratio of Fe2+/H2O2 (1:2-1:6) on the

116

removal of tannin-Cr were systematically investigated. Afterwards, the mixture was

117

filtered through 0.45 µm membrane, and the concentration of TOC, total Cr and Cr(VI) in

118

the solution was determined. Meanwhile, tannic acid solution (tannin dosage = 750 mg·L-

119

1

120

50 mL colorimetric which contained a certain amount of Cr(VI) (Cr(VI) concentration =

121

1.28 mg·L-1). Concentration of Cr(VI) was measured after 5 minutes, and concentration

122

of total Cr was measured after adjusting pH to 8 and passing through 0.45 µm membrane.

123

Then Cr(III) concentration of the liquid phase could be calculated from the concentration

124

difference between total Cr and Cr(VI).

125

Fe2+ + H2O2 → Fe3+ + ·OH + OH- (1)

126

Cr3+ + 3OH- → Cr(OH)3 ↓ (2)

) was oxidized under the same conditions. Oxidation products of 1 mL was transferred to

127 128

2.3. Re-complexation of Cr(III) and oxidation products.

129

The distribution characteristics of TOC and Cr(III) in different sizes was obtained by a

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typical sequential procedure consisting of microfiltration and ultrafiltration, which have

131

been described in detail elsewhere (Dulekgurgen et al., 2006; Karahan et al., 2008). In

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this experiment, a continuously stirred cell (MSC300, Mosu Co. Ltd., China) and the

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commercial polyethersulfone ultrafiltration membranes were used as the microfiltration/

134

ultrafiltration unit (Wang et al., 2016). In addition to 100 mL permeate through TOC and

135

total Cr measurements, while the retentate on membrane disc was freeze-drying for the

6

136

subsequent analyses. Thus, the filtrate was started with an initial volume of 2 L. In order

137

to verify re-complexation processes of Cr(III) and oxidation products. A certain amount

138

of Cr(III) (Cr concentration = 60 mg·L-1) was added to the oxidation products of tannic

139

acid at pH 3.5±0.2. Membrane separation experiments were carried out after 24 h under

140

the same conditions.

141 142

2.4. Analytical methods.

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The dissolved organic carbon (TOC) content was determined with a total organic carbon

144

analyzer (Liqui TOC II, Elementar, Germany). Total Cr was detected by the potassium

145

permanganate oxidation-1,5-diphenylcarbohydrazide colorimetric method, while that of

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Cr(VI) was detected by the 1,5-diphenylcarbohydrazide colorimetric method (Rice 2012).

147

Elementary composition of the >0.45 µm fraction was characterized by scanning electron

148

microscopyenergy dispersive X-ray spectroscopy (Vega 3 SBH, TESCAN, Czech). The

149

UV-vis spectra were collected with ultraviolet-visible spectrometer (UV2800A, UNICO,

150

China). Infrared spectroscopy analysis of freeze-dried <0.45 µm fraction was performed

151

on an FT-IR spectrometer (Broker, Vertex70, Germany) from 400 to 4000 cm-1 after KBr

152

pellet sample preparation. Chemical composition of <0.45 µm fractions were determined

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by X-ray photoelectron spectroscopy (XPS, AXIS SUPRA, England). All binding

154

energies (BE) were referred to the C1s peak at 284.6 eV. The mass spectrometry (MS)

155

was carried out by a high performance liquid chromatography (HPLC) coupled with an

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LTQ Orbitrap XL™ hybrid mass spectrometer (Thermo Fisher Scientific, Rochester,

157

NY). Chromatographic separation was performed on a ZORBAX SB-C18 column (2.1 ×

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150 mm with 5 µm, particles). Water containing 0.1% (v/v) formic acid and methanol

7

159

were named as mobile phase A and B, and a gradient elution was used with a flow rate of

160

0.2 mL·min-1. The typical gradient was showed in Table S1.

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3. Results and discussion

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3.1. Effect of tannin dosage

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Effect of initial tannin dosage on the removal of TOC and Cr by Fenton oxidation from

165

jar tests is plotted in Fig. 1. At different tannin dosage, the initial TOC concentration is

166

87.85 mg·L-1, 189.56 mg·L-1 and 254.94 mg·L-1, respectively, with an initial Cr

167

concentration of 63.48 mg·L-1 for all samples. Fig. 1a shows that the residual TOC

168

concentration gradually increases from 6.66 to 121.57 mg·L-1, while the residual Cr

169

concentration increases from 0.89 to 36.88 mg·L-1 (see Fig. 1b). The removal rate of Cr

170

decreases sharply to 41.90% at the tannin dosage of 750 mg·L-1, whereas the removal rate

171

is 98.60% at tannin dosage of 250 mg·L-1. The residual Cr(VI) concentration after Fenton

172

oxidation is illustrated in Fig. 1c. As can be seen, residual Cr(VI) concentration is 0.46

173

mg·L-1 at tannin dosage of 250 mg·L-1. Then, residual Cr(VI) concentration decreases to

174

0.05 mg·L-1 at tannin dosage of 500 mg·L-1. In particular, negligible Cr(VI) is detected at

175

tannin dosage of 750 mg·L-1, and the valence state of Cr in precipitation is further probed

176

by X-ray photoelectron spectroscopy (XPS). Generally, Cr(III) yields two XPS signals

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with binding energy (BE) peaking at 577.0-578.0 eV and 586.0-588.0 eV, while the

178

signals of Cr(VI) are usually peak at 580.0-580.5 eV and 589.0-590.0 eV (Guan et al.,

179

2016; Guan et al., 2017a; Guan et al., 2017b; Kera et al., 2017). As shown in Fig. 1d, two

180

significant BE peaks are found at 577.11and 586.76 eV, both of which are only attributed

181

to Cr(III), corresponding to the Cr 2p3/2 and Cr2p1/2 orbitals, respectively. The results

8

182

indicate lower removal efficiency of Cr and TOC at higher concentration of polyphenol,

183

meanwhile the formation and accumulation of Cr(VI) can be neglected. Therefore,

184

removal of tannin-Cr at higher concentration under stronger oxidation conditions is

185

further studied.

TOC (mg⋅L-1)

250 200

60

150

40

100

20

50

0

0 250

500

750

Tannin dosage (mg⋅L-1)

100 Before Fenton After Fenton 80

Cr(III) (mg⋅L-1)

(b)100 80 60

60

40

40

20

20

Removal rate (%)

186

0

0 250

187

Removal rate (%)

100 Before Fenton After Fenton 80

(a)350 300

500

750

Tannin dosage (mg⋅L-1)

9

Cr( ) (mg⋅L-1)

(c)0.6 0.4

0.2 Cr(

)<0.01

0.0 250

500

750

Tannin dosage (mg⋅L-1)

188

Intensity (a.u)

(d)

Cr2p3/2 Cr2p1/2

590 588 586 584 582 580 578 576 Binding Energy (eV)

189 190

Fig.1. Removal of tannin-Cr(III) complexes by Fenton oxidation at different initial tannin

191

dasage. (a) evolution of TOC; (b) evolution of total Cr (c) evolution of Cr(VI); (d) XPS

192

spectra of the precipitation after oxidation at the tannin dosage 750 mg·L-1. (H2O2 dosage

193

= 11 mmol·L-1 , Fe2+ /H2O2 = 1:6, initial pH = 3.5, reaction time = 2 h, precipitation pH =

194

8.0).

195 196

3.2. Effect of Fenton's reagent dosage.

197

Effect of H2O2 dosage on the removal of TOC and total Cr is explored to test stronger

198

oxidation conditions for tannin-Cr complexes control (tannin dosage=750 mg·L-1) (Fig.

199

2a). It can be discovered that the residual TOC declines from 89.92 mg·L-1 to 46.82

200

mg·L-1 and residual Cr decreases from 36.88 mg·L-1 to 23.04 mg·L-1, with the increase of

10

201

H2O2 dosage from 11 mmol·L-1 to 22 mmol·L-1. However, further increase in the dosage

202

of H2O2 up to 44 mmol·L-1 doesn’t result in a significantly enhanced removal of TOC

203

and Cr. It can be calculated that the removal efficiency of TOC and Cr at H2O2 dosage of

204

22 mmol·L-1 is accounted for 81.63% and 63.71%, respectively. Fig. 2b shows residual

205

TOC and Cr concentration at different mole ratio of Fe2+/H2O2. The residual TOC is

206

94.31 mg·L-1, 47.13 mg·L-1 and 46.82 mg·L-1, while the residual Cr is 31.74 mg·L-1,

207

23.77 mg·L-1, and 23.04 mg·L-1 at the ratio of 1:2, 1:4 and 1:6, respectively. Previous

208

study has shown that H2O2 will be decomposed instead of generating ·OH radicals at high

209

concentration (Dutta et al., 2003). It has also been reported that Fe2+ can promote the

210

formation of ·OH, while high concentration of Fe2+ will cause wasteful decomposition of

211

H2O2 (Neyens and Baeyens, 2003; Wongniramaikul et al., 2007). Thus, the optimal H2O2

212

dosage is 22 mmol·L-1 and the optimal mole ratio of Fe2+/H2O2 is 1:6 for tannin-Cr

213

complex. Under this condition, the residual TOC and Cr concentration of the solution is

214

still as high as 46.82 mg·L-1 and 23.04 mg·L-1, respectively. In addition, Cr(VI) cannot be

215

detected in above-mentioned experiments.

11

80 60

60

40

40

20

20

0

11

216

TOC/Total Cr (mg⋅L-1)

(b)100

44

0

100 TOC Total Cr 80

80 60

60

40

40

20

20

0

217

22 H2O2(mmol⋅L-1)

Removal rate (%)

100 TOC Total Cr 80

1:2

1:4 1:6 Fe2+/H2O2 ratio

Remove rate (%)

TOC/Total Cr (mg⋅L-1)

(a)100

0

218

Fig. 2. Removal of tannin-Cr(III) complexes by Fenton oxidation with different (a) H2O2

219

dosage (initial TOC=254.94 mg·L-1, initial Cr=63.48 mg·L-1, Fe2+ /H2O2 = 1:6, initial pH

220

= 3.5, reaction time = 2 h, precipitation pH = 8.0), and (b) Fe2+ /H2O2 ratio (initial

221

TOC=254.94 mg·L-1, initial Cr=63.48 mg·L-1, H2O2 dosage = 22 mmol·L-1 , initial pH =

222

3.5, reaction time = 2 h, precipitation pH = 8.0).

223 224

3.3. Degradation products and proposed pathways of tannic acid.

225

Degradation products of tannic acid were separated and identified using HPLC-MS. The

226

total ion chromatogram (TIC) of tannic acid and its products are showed in Fig. S3. It can

227

be observed four peaks of tannic acid at 21.63 min, 22.72 min, 24.61 min and 25.19 min,

228

respectively. After oxidation, its products show a distinct peak at 22.63 min and a weak

12

229

shoulder peak at 22.15 min. According to the results and literatures, a complex mixture of

230

di-, tri-, tetra-, penta-, hexa-, hepta- and octagalloyl glucose can be formed after cleavage

231

of galloyl group in tannic acid solution (Chávez-González et al., 2014). These hydrolysis

232

products as major components are observed in the mass spectra (see Fig. S4). Under ·OH

233

attacking, high molecular weight gallotannins and highly polymerized tannins are usually

234

degraded into more simple molecules with low degree of polymerization (Tuominen and

235

Sundman, 2013; Xiang et al., 2016). MS information of oxidation products are presented

236

in Fig. S5. Several products such as C2H2O4 (m/z 88.99), C7H4O5 (m/z 167.87), C7H6O7

237

(m/z 200.01), C11H10O7 (m/z 253.01) and C14H8O10 (m/z 325.18) are detected in the mass

238

spectra. It indicates that galloyl groups can be formed to quinones at first, which were

239

oxidized further to acids. At last, ·OH mineralization will lead to the cleavage of carboxyl

240

groups and formation of oxalic acid, formic acid, CO2, H2O and etc (Fig. 3). Meanwhile,

241

some complex oxidation products are also formed with m/z values of 1009.31, 1019.05,

242

1066.86, 1102.41, 1376.33, 1637.95, etc. Based on the identified products, the hydrolysis

243

and oxidation pathway of tannic acid is proposed as shown in Fig. S6. OH

O OH

Oxidation

OH O

O Oxidation

O

O

O Oxidation

O

O

O

O HO C HO C

CO2+H2O

244 245

O

OH OH

O O

O HO

OH

OH

C

O

H O

O

Fig. 3. Proposed pathways for the oxidation of galloyl groups.

246

13

247

3.4. Plausible Mechanism for tannin-Cr(III) Removal

248

Based on the above discussions, the possible mechanism for tannin-Cr(III) removal by

249

Fenton oxidation is proposed and schematically illustrated in Fig. 4. It’s well known that

250

·OH (E0(·OH/H2O) = +2.8 V) are generated for the removal of recalcitrant organic

251

compounds and transformation of Cr(III) to Cr(VI) due to the reaction between Fe2+ and

252

H2O2 (Eq. (1)) (Bokare and Choi, 2011; Bokare and Choi, 2014; Neyens and Baeyens,

253

2003). Recent advances shown that ·OH radicals are firstly involved with transformation

254

of Cr(III) into Cr(VI), rather than attacking the organic ligands (Dai et al., 2010; Dai et al.,

255

2011; Durante et al., 2011). Due to the weak complexation ability, the formation of

256

Cr(VI) is accompanied by the process of de-complexation, which dominates the overall

257

process during the presence of H2O2 and Fe2+(Ye et al., 2017; Ye et al., 2018). Cr(VI) is a

258

strong oxidant E0(HCrO4-/Cr3+ = 1.35 V), which can be transformed to Cr(III) rapidly by

259

reacting with reducing agents (Fe2+, H2O2, organic matter) (Bokare and Choi, 2011;

260

Chen et al., 2011; Guan et al., 2016; Guan et al., 2017a; Guan et al., 2017b). In this

261

system, the added Fe(II) can be rapidly oxidized to Fe(III) (k = 63-76 M-1 s-1), which

262

causes extremely low Fe(II) concentration in solution (Bokare and Choi, 2014; Neyens

263

and Baeyens, 2003). Additionally, the standard oxidation potential of Fe(III) E0(Fe3+/Fe2+

264

= 0.771 VNHE) is lower than Cr(VI), which indicates that Cr(VI) is preferentially

265

reduced. Thus, Fe(II) has very little contribution to the Cr(VI) reduction. Although H2O2

266

is a strong oxidant [E0(H2O2/H2O) = 1.77 V], it can also serve as a reductant [E0(O2/

267

H2O2) = -0.68 V] of the Cr(VI), which can be used as a Fenton-like system for the

268

oxidative mineralization of organic pollutants (Bokare and Choi, 2011; Bokare and Choi,

269

2014). Furhermore, the experimental data in Fig. S7 proves that the accumulated Cr(VI)

14

270

can be subsequently transformed to Cr(III) via reduction by tannic acid and its products

271

upon the depletion of H2O2. Due to incomplete oxidation, the residual Cr(III) and TOC

272

have become another major problem in the treatment of wastewater. Therefore, it is

273

imperative to investigate the speciation of the residual TOC and Cr before and after

274

Fenton oxidation, over the investigation of size fractions, by a way to visualize

275

correlations between TOC and Cr fractions. De-complexation

Redox of Cr H2O2

Cr(Ⅵ Ⅵ)

HO O HO O

Cr(? ) O

Free Cr(Ⅲ Ⅲ)

·OH

O O

O

OH

OH OH

O

Tannin-Cr(Ⅲ Ⅲ)

H2O2 +

Fe2+

·OH

OH O

O

OH OH

O

O HO

C

O HO

C O

Oxidation products

CO2+H2O

276 277

Fig. 4. The possible mechanism for tannin-Cr(III) removal by Fenton oxidation.

278 279

3.5. Distribution characteristics of TOC and Cr

280

A significant difference in TOC distribution between tannic acid and tannin-Cr complex

281

can be observed in Fig. 5a. For tannic acid, TOC is mainly concentrated in the range of <

282

1 KDa fractions (217.68 mg·L-1), which is caused by hydrolysis of tannic acid (Lina and

283

Maria, 2018; Shirmohammadli et al., 2018). After complexing with Cr(III), it can be

284

calculated that about 15.02% TOC precipitated with Cr(III). Besides, TOC fractions is

285

mainly located in 0.1 µm-100 KDa (136.08 mg·L-1) and 100 KDa-50 KDa (47.31 mg·L-

286

1

287

conditions, the residual TOC concentration of tannic acid is 87.8 mg·L-1, and located

288

mostly in 0.1 µm-100 KDa and 100 KDa-50 KDa regions. As is well known, unsaturated

289

aliphatic acids can undergo polymerization via radical processes and gradually transform

). TOC distribution after oxidation is presented in Fig. 5b. Under the same oxidation

15

290

into larger sized molecules (Kang et al., 2019; Waggoner et al., 2015). For tannin-Cr

291

complex, the residual TOC concentration is relatively low in those regions. And TOC is

292

mainly concentrated in the 0.45 µm-0.22 µm and < 1KDa fractions, which accounted for

293

31.25% and 24.99%, respectively.

294

Comparative presentation of the Cr fraction is provided in Fig. 5c. Similar to distribution

295

of TOC, the highest Cr concentration is 27.68 mg·L-1 at 0.1 µm-100 KDa fraction range

296

for tannin-Cr. Then Cr species is mainly located in the range of <10 KDa, which summed

297

up to 46.33% of total Cr. After oxidation, Cr contents are unevenly distributed among the

298

colloid sizes. The Cr species at the 0.1 µm-100 KDa fraction range drops dramatically,

299

with residual Cr concentration of 2.17 mg·L-1. On the contrary, the Cr concentration at

300

the range of 0.45 µm-0.22 µm increases to 8.7 mg·L-1, which occupies 37.76% of the

301

residual total Cr. These figures show that the residual TOC and Cr after oxidation is

302

mainly distributed among larger size range. As previously reported, Cr(III) is supposed to

303

coordinate with hydroxyl or oxy groups in tannic acid oxidation products so strongly. In

304

order to prove that, a certain amount of Cr(III) was added to oxidation products of tannic

305

acid at pH of 3.5±0.2. The elemental analysis from the energy dispersive X-ray spectrum

306

indicates that organic matter precipitated with Cr(III) after standing for 24 hours (see Fig.

307

S8a). Meanwhile, it is also obvious that the TOC concentration decreases in the regions

308

of 0.1 µm-100 KDa and 100 KDa-50 KDa, accompanied with an increase in the 0.45 µm-

309

0.22 µm and 0.22 µm-0.1 µm portions (see Fig. S8b).

16

(a) TOC (mg·L-1)

250 200

Tannic acid Tannin-Cr

150 100 50 0

310

TOC (mg·L-1)

(b)

50

Tannic acid

40

Tannin-Cr

30 20 10 0

(c)

Total Cr (mg·L-1)

311

30

Before Fenton

25

After Fenton

20 15 10 5 0

312

17

313

Fig. 5. Size distribution of TOC in tannic acid and tannin-Cr complex (a) before Fenton

314

oxidation (TOC concentration in tannic acid = 300 mg·L-1, TOC concentration in tannin-

315

Cr = 254.94 mg·L-1); (b) after Fenton oxidation (TOC concentration in tannic acid =

316

87.80 mg·L-1, TOC concentration in tannin-Cr = 46.82 mg·L-1). (c) size distribution of Cr

317

in tannin-Cr complex before (Cr concentration = 64.38 mg·L-1) and after Fenton

318

oxidation (Cr concentration = 23.04 mg·L-1).

319 320

3.6. Combined spectral analyses

321

The UV-vis spectra of the tannic acid and tannin-Cr (50-fold dilution) are displayed in

322

Fig. 6a. Tannic acid has two absorption bands at 200 nm-250 nm and 260 nm-300 nm

323

whose maximum absorbance (λmax) wavelength is 212 nm and 272 nm, respectively. A

324

slight red shift is observed in the absorption band of 260 nm-300 nm after complexing

325

with Cr(III). Noteworthy, it is found that tannic acid can be degraded by Fenton oxidation

326

that resulted in a total disappearance of the absorption bands after treatment. Fig. 6b

327

shows the visible light absorption spectra of tannic acid and tannin-Cr without dilution.

328

The absorbance increases at the region of 350-450 nm after oxidation, which suggests

329

new products might be formed. In analogy with other results, some of new transformed

330

products also show absorption in the higher region (e.g. 436 nm), which might indicate

331

highly conjugated product (Audenaert et al., 2013; Nöthe et al., 2009).

332

The FTIR spectra of several colloid sizes (0.45 µm-0.22 µm, 100 KDa-50 KDa and <1

333

KDa) are showed in Fig. 6c and Fig. 6d. The wide bands in the range of 3000 cm-1-3600

334

cm-1 show the groups of phenolic -OH, which is intensively presented in the raw tannin-

335

Cr complexes and its oxidation products. All fractions show wide peaks within the range

18

336

of 2900 cm-1 and 3000 cm-1 which indicate the aliphatic C-H groups. Tannic acid exhibits

337

four strong bands, two of them at 1615 cm-1-1606 cm-1 and 1452 cm-1-1446 cm-1 assigned

338

to aromatic ring stretch vibrations and the other two at 1211 cm-1-1196 cm-1 and 1043

339

cm-1-1030 cm-1 assigned to stretch vibrations of C-O bond27. All of them decrease after

340

oxidation, indicating the degradation of tannic-Cr. The peak position around 1684 cm-1 in

341

oxidation product might be attributed to the stretch vibration of conjugate C=O (Bu et al.,

342

2010). The peak at 1142 cm-1 is due to the presence of ether group (C-O-C) (Yurtsever

343

and Şengil, 2009), which shows an extra weak peak in 0.45 µm-0.22 µm fractions. The

344

results indicate that carboxyl groups are presented in the oxidation products.

345

The C1s and O 1s high resolution spectra for 100 KDa-50 KDa fractions are obtained by

346

XPS analyses. As shown in Fig. 6e, the C 1s spectral envelop reveals four components.

347

The C1s peak at 284.58 eV is assigned to aromatic C–C/C–H bonds, the peak at 285.88

348

eV to C-O bonds, the peak at 288.48 eV to C=O bonds, and the peak at 289.58 eV to O-

349

C=O bonds (Guan et al., 2016; Guan et al., 2017a; Guan et al., 2017b). Three O 1s peaks

350

can be observed in Fig. 6f. The first peak at 531.48 eV is mainly allocated to O-C=O

351

bonds, the second peak at 532.58 eV to O-C-O/C-O bonds, and the third peak at 535.98

352

eV, most probably assign to Cr-O bonds (Guan et al., 2016; Guan et al., 2017a; Guan et

353

al., 2017b).

19

(a) 2.0 1.5

Tannic acid Tannin-Cr Tannic acid oxidation products

Abs.

Tannin-Cr oxidation products

1.0 0.5 0.0 200

354

(b)1.6

Abs.

1.2

250 300 350 Wavelength (nm)

400

Tannic acid Tannin-Cr Tannic acid oxidation products Tannin-Cr oxidation products

0.8 0.4 0.0 400

355

500 600 700 Wavelength (nm)

800

20

(c) Transmittance(%)

1609 cm

-1

-1 1448 cm

1205 cm

-1 -1 1032 cm

<1 KDa 100 KDa-50 KDa 0.45 µm-0.22 µm

Before oxidation

4000 3500 3000 2500 2000 1500 1000

Wavenumber(cm-1)

356

500

Transmittance(%)

(d) 2919 cm

-1

1684 cm 3113 cm

-1

-1

1142 cm

After oxidation

4000 3500 3000 2500 2000 1500 1000

357

-1

<1 KDa 100 KDa-50 KDa 0.45 µm-0.22 µm

Wavenumber(cm-1)

500

21

Intensity (a.u)

(e) Aromatic C–C/C–H

C=O

C-O

O-C=O

292

290 288 286 284 Binding Energy (eV)

358

282

Intensity (a.u)

(f) O-C=O

O-C-O/C-O Cr-O

538

359

536 534 532 530 Binding Energy (eV)

528

360

Fig. 6. Combined spectral analyses of <0.45 µm fraction (a, b) UV-vis spectra; (c, d)

361

FTIR spectra; (e, f) XPS spectra.

362 363

3.7. Proposed complex structures of Cr(III) and oxidation products

364

As mentioned above, residual TOC and Cr after oxidation is mainly distributed among

365

larger size range, which is associated with re-complexation on Cr(III) and intermediates.

366

Unfortunately, there is no available reference reporting the specific chemical structure of

367

Cr(III) complexes through LC-MS method. Based on experimental data, mineralization

368

by ·OH and subsequent alkali precipitation can remove 81.63% of TOC and 64.21% of

369

total Cr, which leads to the lower TOC/Cr value in the solution. Meanwhile, the residual

370

Cr exists as free Cr(III) firstly after a redox cycle of Cr(III)-Cr(VI)-Cr(III). Numerous

22

371

literature reported that Cr(III) dissolved into water undergoes a process from hexaaqua-

372

Cr(III) cationic complex to various polynuclear chromium-olation compounds

373

(polychromiums) (Ding et al., 2015). Besides, it has also been demonstrated that very

374

stable chemical bonds can be formed between chromium atoms through oxygen bridges

375

with the increase of the alkalinity of solution according to the chromium-olation model

376

during the leather tanning process (Morera et al., 2011). Then, it can be deduced that

377

larger complexes with more atoms of chromium are formed in alkaline solution with

378

lower TOC/Cr ratio. Prior study demonstrated that Cr(III) could cross-link the carboxyl

379

groups. Thus, polychromiums can play an important role as the cross-linking agent in the

380

tannic acid oxidation products and lead to the formation of larger-sized molecular

381

species. This implies higher stability of the chromium complex with respect to an

382

increase of pH. Based on the above analyses, re-complexation process and proposed

383

structure of Cr(III) and intermediates is described in Fig. 7. R1COO

Free Cr(Ⅲ Ⅲ)

OH-

O Cr(? ) O

O Cr(? )

Intermediates

O n

384 385

Olation

OOCR3 O Cr(? ) Cr(? ) O O R2COO OOCR4 O

n

Re-complexation

Fig. 7. The re-complexation process and proposed structure of Cr(III) and intermediates.

386 387

4. Conclusions

388

Based on the above results and discussions, it can be confirmed that polyphenol-Cr

389

complexes cannot be completely oxidized during Fenton processing. And the potential

390

mechanism for Cr conversion and removal from tannin-Cr complexes during Fenton

391

oxidation is proposed. Firstly, ·OH radicals generated by Fenton's reagent transforms

23

392

Cr(III) into Cr(VI) which is then released from tannin-Cr complex. Then the transformed

393

products containing attached carboxyl and hydroxyl groups during this process present

394

significant effect on the conversion and removal of Cr. On one hand, it is involved with

395

the redox cycle of Cr as a reductant. On the other hand, reduced free Cr(III) can complex

396

with organic ligands again, accompanied with an increase of species size. Fraction with

397

>0.45 µm size can precipitate with free metal ions after subsequent alkaline treatment and

398

the fraction with < 0.45 µm is presented in solution stably. Meanwhile, the re-

399

complexation of Cr(III) with the intermediates also hinders the removal of Cr and leads to

400

the formation of larger-sized molecular species. Obviously, further study is still required

401

to achieve the precise molecular structures of the complex of intermediate products and

402

Cr. And present work unveils the essential role of oxidation intermediates for the first

403

time during the conversion and removal of polyphenol-Cr complex, which implies that

404

the control of oxidation products during AOPs is crucial to achieve zero emission of Cr.

405

In particular, this study is essential to understand Cr removal in concentrated organic

406

wastewater containing chromium (tannery wastewater, etc.) by AOPs.

407 408

Acknowledgment

409

This work was supported by the National Major Project on Water Pollution Control (No.

410

2017zx07602-001) and National Natural Science Foundation of China (No. 51704189).

411

412

Supplementary Information available

24

413

The structures of polyphenol-Cr complex (Fig. S1), structure of tannic acid (Fig. S2),

414

gradient conditions of mobile phase for HPLC (Table S1), total ion chromatogram of

415

tannic acid and its products (Fig. S3), MS information for the hydrolysis products of

416

tannic acid (Fig. S4), MS information for the oxidation products of tannic acid (Fig. S5),

417

proposed pathways for the hydrolysis and oxidation of tannic acid (Fig. S6),

418

concentration of Cr species after reduction by tannic acid and its oxidation products (Fig.

419

S7), the energy dispersive X-ray spectrum of the >0.45 µm fraction (Fig. S8a), size

420

distribution of TOC and Cr in the tannic acid oxidation products after complexing Cr

421

(Fig. S8b) are available in the Supplementary Information file.

422 423

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574 575 576

31

Highlights 1. De-complexation and degradation of tannin-Cr(III) can be achieved by in-situ Fenton oxidation; 2. Fenton process cannot sufficiently oxidize high concentrated polyphenol-Cr complexes; 3. Cr(III) reduced from Cr(VI) can be complexed with degraded products of tannic acid; 4. AOPs can be effective route for treatment of concentrated organic wastewater enriched with Cr.

Declaration of interests The authors declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.