The characteristic of novel extraction & photo-induced precipitation

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The characteristic of novel extraction & photo-induced precipitation approach by PEG/SA and fluorescence monitoring of Cr(VI) Wei Ma, Moyan Wang, Zhen Chen, Fanqing Meng, Zihong Cheng, Dazhi Tan, Yuzhen Pan, Shibo Duan, and Jian Gao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00025 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Sustainable Chemistry & Engineering

The characteristic of novel extraction & photo-induced precipitation approach by PEG/SA and fluorescence monitoring of Cr(VI) Wei Maa*, Moyan Wanga, Zhen Chena, Fanqing Menga, Zihong Chengb, Dazhi Tana, Yuzhen Pana, Shibo Duana, Jian Gaoa aDepartment

of Chemistry, Dalian University of Technology, Dalian 116023, P R China

bNational

Institute of Clean-and-Low-Carbon Energy, Beijing 102211, PR China Tel. +86(411) 8470 6303; Fax. +86(411) 8470 7416; E-mail: [email protected] (Wei Ma)

*Wei Ma: Tel: +86(411) 8470 6303; E-mail: [email protected]; Address: Lingshui road; Room 415; Department of Chemistry, Dalian University of Technology, Dalian 116023, China Moyan

Wang:

Lingshui

road;

Room

404;

Dalian

116023,

China

([email protected]) Zhen Chen: Lingshui road; Room 419; Dalian 116023, China ([email protected]) Fanqing

Meng:

Lingshui

road;

Room

404;

Dalian

116023,

China

([email protected]) Zihong Cheng: National Institute of Clean-and-Low-Carbon Energy, Beijing 102211, PR China ([email protected]) Dazhi Tan: Lingshui road; Room 419; Dalian 116023, China ([email protected]) Yuzhen Pan: Lingshui road; Room 302; Dalian 116023, China ([email protected]) Shibo Duan: Lingshui road; Room 404; Dalian 116023, China ([email protected]) 1

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Jian Gao: Lingshui road; Room 302; Dalian 116023, China ([email protected])

Abstract

A novel route based on bio-hydrogel sodium alginate (SA) for extraction by aqueous two-phase system (ATPS), recovery of chromium by photo-induced precipitation process and monitoring the process by fluorescence spectroscopy (F-S) were developed. The processes were explored by means of FTIR, Micro-FTIR, XPS, F-S and the role of polyethylene glycol and sodium alginate (PEG/SA) in the process was analyzed. Results showed that SA acted as a extractant to form complex with Cr(VI). The maximum extraction point appeared at pH=5.50 and its kinetic equation showed CCr(VI) was the key point which can control the reaction. The Cr(VI) was reduced to Cr(III) within 10 min of 300W solar irradiation in the presence of PEG. Then co-precipitation of water insoluble poly-nuclear complexes consist of SA, trivalent and hexavalent chromium was formed. The bio-hydrogel from SA underwent a static quenching process, the extraction process and phase diagram can be monitored by F-S. It belongs to the transform π-π structure from SA. The work may provide a promising and attractive approach to realize a high-rate and low-cost process for recovery of chromium.

2


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Keywords:

PEG/SA;

Extraction;

Fluorescence

spectroscopy;

Monitor;

Photo-induced precipitation

Introduction

Chromium is one of the most common pollutants introduced into natural waters due to the discharge of a variety of industrial wastewater, including leather tanning, electroplating, textile, and metal finishing industries.1,2 Chromium essentially exists in two forms namely Cr(VI) and Cr(III). Cr(VI) is 500-1000 times toxic to the environment compares to Cr(III), and even has been proven to be mutagenic and carcinogenic due to its strong oxidizing properties.3 So the enrichment and reduction of Cr(VI) to Cr(III) from wastewater has drawn considerable attention in recent years. Aqueous two-phase system has been introduced as a promising liquid–liquid extraction system for metal separation because it has the ability to quickly split into two phases and also mostly uses water, nontoxic and nonflammable constituents.4-6 In general, one of the phases is rich in polymer, whilst the other contains more electrolytes or another polymer.7,8 However, it is worth noting that the extraction of metal ions by ATPS generally requires the addition of organic extracts,9,10 otherwise the extraction efficiency is extremely low. But the system can be chemically toxic with the addition of them. Therefore, we use natural bio-hydrogel to replace traditional organic extractant. Polymeric hydrogels are the representative charming and versatile soft materials

3



with potential applications in biological engineering,11 releasing matrices for drug delivery,12 and tissue engineering,13-16 as well as the adsorption of heavy metals.17 Bhattarai et al. investigates the advanced development of chitosan hydrogels has led to new drug delivery systems that release their payloads under varying environmental stimuli. Ge et al. prepares the supramolecular gels underwent thermo- and pH-induced reversible gel-sol transitions for specific drug release. Above researches proved that hydrogels have many functional groups can coordinate with drugs and metal ions. Therefore, SA (sodium alginate) was dissolved in water to form a bio-hydrogels as extractant with wealth of functional groups that can form organized material18 and react with Cr(VI). Then, SA@Cr(VI) is extracted to the top phase by ATPS. In this process, chemical transfer takes place and a complex structure is generated,19 also the carbonyl functional group that comes from SA can emit fluorescence at a specific excitation wavelength, so this feature can applied for fluorescence detection. Fluorescence spectroscopy is a method to study the weak interaction between molecules.20,21 The fluorescence quenching phenomenon is widely used in organic chemical-metal complex22-24 and the interactions between fluorescent contaminants and dissolved organic matter (DOM) in aqueous phase.25-27 Although fluorescence excitation-emission matrix (EEM) measurements have been employed to monitor water quality, their application for detection of trace amounts of contaminants is limited to monitoring of dissolved organic matter, such as humic-like and protein-like matters.28 In contrast, fluorescent agent are usually used for detection of heavy metals in biochemistry. A fluorescent agent is an organic molecular indicator that exhibits 4


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quantifiable changes in fluorescence spectra upon binding a particular guest ion. The design and development of novel fluorescent agent remains an active area of research and various fluorescent agents for heavy metal ions have been reported.29 However, their application for environmental use can be a chemical toxic. Hence, we developed a natural hydrogel with carbonyl structure as a fluorescent agent for determination of Cr ions by fluorescence spectroscopy. When Cr(VI) is extracted into the top phase, reducing its toxicity becomes a key point. Various technologies including bioremediation, photocatalytic reduction and chemical treatment have been explored for conversion of Cr(VI) to Cr(III) to decrease the toxicity. Photo-reduction technology has attracted attention due to its natural occurrence and reduced energy consumption. This interest reflected in a huge numbers of research papers. However, few literatures systematically studied the enrichment-reduction process of Cr ions. Therefore，the article is mainly divided into the following aspects. (ⅰ). A novel aqueous two-phase system which formed by PEG@SA/Na2SO4/H2O is developed. Fluorescence spectroscopy was applied for fitting the phase diagram of the system due to the fluorescence characteristics of SA. Some extraction parameters such as pH, temperature and kinetic were investigated and the complexation of Cr(VI) with SA was testified by FT-IR. (ⅱ). The Cr(VI) enriched in top phase by ATPS were photo-reduced to form SA@Cr(VI)/(III) precipitates and effects of different media and pH were studied. The apparent characteristics of precipitation were explored by means of XPS and Micro-FTIR. (ⅲ). The whole processes were monitored by 5



fluorescence quenching (F-S), and the role of SA/PEG was analyzed. The present work highlights a systematic new approach for extraction, reduction and monitoring of Cr(VI) and it has potential application in recovering of chromium.

MATERIAL AND METHODS Materials and Chemicals The polyethylene glycol used in this study with an average molar mass of 2000 g/mol and sodium alginate were both obtained from Sinopharm (Shanghai, China). Na2CrO4·4H2O, Na2SO4, NaOH and H2SO4 were purchased from Kermel (Tianjin, China). All the reagents were of analytical grade and used as received without any further purification. Aqueous two-phase system composition To form ATPS, stock solutions of PEG–2000 (30% w/w), Na2SO4 (40% w/w) and SA (1.5% w/w) were prepared. The ATPS is formed by mixing 10 mL PEG– 2000, 6 ml Na2SO4, 2 ml SA and 2 ml H2O. Characterization Fourier transform infrared spectroscopy (FT-IR) spectra is obtained by using Bruker TENSOR 27 FTIR and the spectra is recorded in 400-4000 cm-1 range. The precipitate was divided into different regions by Micro-FTIR (AIM-9000, Shimadzu, Japan ) to determine whether the components at different locations stay the same and the spectra is recorded in 700-4000 cm-1 range. Fluorescence spectra is obtained by 6


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Perkinelmer Fluorescence LS55. The content of Cr ions in ATPS is analyzed by Atomic Absorption Spectrometry (AAS, Optima AA6100, China). The light intensity was provided by PLS-SXE300 (full spectrum, China). The surface electronic states was analyzed by X-ray photoelectron spectroscopy (XPS, Escalabs 250Xi). The pH of the solution is measured by an acidometer (PHS-2 Model, China). Quantitative analysis The salt concentration in each phase of the ATPSs was obtained by conductimetry (DDS-307, Leici, China). Salt solutions showed the same conductivity in water or diluted polymer solutions. The polymer concentration was measured by refractive index (2W, Shanghai, China). The refractive index (𝒏𝑫) can be related to mass fraction of Na2SO4 (𝒘𝟏) and PEG (𝒘𝟐) as follows:

(1)

𝒏𝑫 = 𝒏𝑶 + 𝒂𝟏𝒘𝟏 + 𝒂𝟐𝒘𝟐

Where 𝑎1 and 𝑎2 are constants of salt and polymer, respectively, and were obtained as 0.00132 and 0.00212 for this system. 𝑛𝑂 is refractive index of pure water and it was measured as 1.3330. The tie-line length (TLL) is an empirical measure of the compositions of two phases, which can be calculated using eq(2): 2

2

𝑏𝑜𝑡𝑡𝑜𝑚 TLL = (𝐶𝑡𝑜𝑝 ) ― (𝐶𝑏𝑜𝑡𝑡𝑜𝑚 ― 𝐶𝑡𝑜𝑝 𝑝 ― 𝐶𝑝 𝑠 𝑠 )

(2)

The slope of the tie-line (STL) is given by the ratio of the difference between the polymer(C𝑝) and salt(C𝑠) concentrations in the top and bottom phases, which can be calculated using eq(3): 7



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𝑏𝑜𝑡𝑡𝑜𝑚 𝐶𝑡𝑜𝑝 𝑝 ― 𝐶𝑝

(3)

STL = 𝐶𝑡𝑜𝑝 ― 𝐶𝑏𝑜𝑡𝑡𝑜𝑚 𝑠

𝑠

Extraction experiments The extract ability of Cr(VI) in ATPS is studied by batch experiments in an isothermal shaker (150 r/min). Unless otherwise stated the parameters: temperature at 298.15 K, solution volume 20 mL, contact time was 5 minutes, CCr

(VI)

=0.5×10-3

mol/L. The effect of initial pH is from 1.50 to 13.50 adjusted by H2SO4 and NaOH solution. Then the solution is centrifuged at 2000 rpm for 30s, and waited for centration analysis by AAS. The extraction efficiency (𝐸𝑞, mg/(g·min)) and extraction rate (𝐸) of Cr (VI) and other metal ions were calculated according to the follow equations: 𝐶𝑡·𝑉𝑡

(4)

𝐸𝑞 = 𝑚𝑠·𝑇 𝑛𝑡

𝐸％ = 𝑛𝑇 × 100

(5)

where 𝐶𝑡 denotes the Cr(VI) concentration in top phase; 𝑉𝑡 is the volume of top phase; 𝑚𝑠 represents the total mass of SA, 𝑇 represents extraction time; 𝑛𝑡 is the number of moles of metallic ion in the top phase and 𝑛𝑇 is the total number of moles of metallic ions added in the system, respectively. Fluorescence quenching Stern-Volmer quenching theoretical model： 𝐹0 𝐹 = 1 + 𝐾 𝜏 [𝑄] = 1 + 𝐾 [𝑄] 𝑛 𝑞 0 𝑠

(6)

Where 𝐹𝑛 and 𝐹0 correspond to the fluorescence intensity of the target solution with and without metal ions, respectively. 𝐾𝑞 and [𝑄] are the quenching constant of 8


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biomolecules and the concentration of the quencher, respectively. 𝜏0 is the fluorescence lifetime of a fluorescent substance in the absence of a quencher, approximately 10ns30 for biological macromolecules, and 𝐾𝑠 = 𝐾𝑞𝜏0 represents the Stern-Volmer quench constant.

RESULTS AND DISCUSSION

Extraction regulars and sodium alginate function Construction and stability of extraction system The basic system composed of PEG+Na2SO4+H2O was prepared and the binodal curves were determined by cloud–point method.31 The phase diagram and binodal curve data of the system under different pH value were presented in Figure S1 and Table S1, respectively. The results revealed that required Na2SO4 for phase separation slightly decreased as pH value increased. Besides, the components of two phases had been measured under different pH value. The data were presented in Table S2. Moreover, a comparable conduct had been illustrated in references.32,33 When the pH value was changed from 1.5 to 7.5, the concentration of PEG in the top phase slightly increased from 29.88% to 31.14%, with an increase of 4.00%, and the concentration of salt remained basically at 1.10%. The concentration of PEG in the bottom phase was about 4.60%, which is much lower than that in the top phase, while concentration of salt decreased from 12.80% to 11.07%. The concentration fluctuated very little in the range of pH 1.5-7.5, so the system can exist stably. Furthermore, the phase

9



diagram and binodal curve data of the system under different temperature at fixed pH=5.5 were presented in Figure 1(a) and Table S3, respectively. The figure indicated that the two-phase region expanded by temperature increases, which has a comparable conduct with references.34 Equilibrium data of PEG2000+Na2SO4+Water system at 298.15, 308.15 and 318.15 K were presented in Table S4. The data indicated that increasing the temperatures also increases the concentration of PEG in top phase, the increase rate was 2.50%, the concentration of salt decreased slightly from 1.11% to 0.99%. The system does not fluctuate greatly at different temperatures and can exist stably. A novel bio-hydrogel based aqueous two-phase system composed of biomass sodium alginate and poly ethylene glycol was prepared. SA used as a natural, non-toxic extractant in the extraction system and dissolved in water to form a hydrogel containing a large number of functional groups, wherein the carbonyl group can form complex with Cr(VI). The phase diagram of the system obtained by the fluorescence spectroscopy has a higher fitting degree compared with the conventional phase diagram (Figure S2 and Table S5) and the linear relationship proved this method was feasible (Figure S3). The effect of different pH values and temperatures on SA in ATPS was investigated, as can been seen in Figure S4 and Figure S5, respectively. Compare with Figure S4(a), Figure S4(b) shows characteristic peaks at 2926 cm-1 and 940 cm-1 under alkaline conditions which means SA converges in the bottom phase. Peak intensity decreases with the increase of pH value and there is a red shift occur at peak 10


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(A), which means that the content of SA in the top phase is decreasing (Figure S4(c) and S4(d)). As we can see from Figure S5(A), there is no characteristic peak appeared at 2926 cm-1 and 940 cm-1 in Figure S5(b), indicating that SA exists only in the top phase. The fluorescence intensity basically remains unchanged at 420 nm under the excitation wavelength of 320 nm (Figure S5(c)). There is no peak value appears at 420 nm in the bottom phase, indicating that temperature has no effect on SA under the condition of fixed pH value. In order to test the stability of the ATPS that contain SA as a extractant, F-S and FT-IR characterization were performed in Figure 1. Figure 1(b) shows the FT-IR spectrum of the SA, PEG:SA=300:1, 200:1 and 100:1. The absorption bands before SA mixed with PEG at 3321, 2935, 1630, 1100 cm−1 corresponding to -OH stretching vibrations, C-H stretching vibration, C=O stretching vibrations, and C-O-C bonds stretching vibrations,35,36 respectively. The infrared spectra (Figure 1(c)) has been measured when the minimum amount of Na2SO4 was added to the solution (PEG:SA=200:1) to form ATPS and when the salt crystallized, respectively. There is no new functional group appearing, the solution maintains a homogeneous state and no significant change in peak values after addition of PEG, so SA can be stably present in PEG. Figure 1(d) is obtained at an excitation wavelength of 320 nm, which is the optimum excitation wavelength of SA. With the SA concentration decreases, the peak at 424 nm decreases significantly, indicating that SA as a native fluorescent agent without toxicity could be detected by fluorescence spectroscopy. As seen in Figure 11



1(e), there is no change in intensity when Na2SO4 has been added to the solution (PEG:SA=200:1) to form ATPS compared to the salt crystallized. Meanwhile, the stability of bio-hydrogel based ATPS also has been demonstrated.

Figure 1. (a):Binodal curves for PEG-2000+Na2SO4+H2O system at 298.15, 308.15 and 318.15 K. (b); (c): FT-IR transmittance spectra, (d); (e): F-S spectra; Although PEG@SA1+Na2SO4+H2O is observed to have larger two-phase area than the other systems, but with the concentration of SA decreases, the extraction rate also decreased rapidly. PEG@SA2-based ATPS has been taken as the research object, cause this system maintains greater phase-separation ability and stability.37,38 12


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Effect of time on extraction As seen in Figure 2(a), the extraction amount of chromium ions increases with reaction time increasing and the process reaches equilibrium within 4 minutes. At the beginning of the extraction process, the extraction rate is extremely high cause there are large amounts of Cr(VI) present in the solution. Also, there are a large number of reaction sites on the hydrogel for reaction operation. With the increasing of time, the concentration of Cr(VI) in the solution gradually decreases, indicating the chemical capture sites, chemical capture rate and the chemical capture ability gradually reaches the equilibrium. Effect of initial concentration It can be seen from the Figure 2(b), the extraction amount of Cr(VI) in ATPS increases with the increased initial concentration and gradually reaches the equilibrium under the condition of fixed pH = 5.5. At low concentrations, the reaction sites of the sodium alginate hydrogel are not saturated and the amount of extraction is relatively low. In contrast, there is a large amount of metal ions in the high-concentration solution, the contact chances between the metal ions and the colloidal surface is much higher than the low-concentration solution, thus making extraction amount of ATPS is relatively high. With the increment of initial concentration, the reaction sites of hydrogels on chromium ions gradually saturate, so that the extraction amount of Cr ions in ATPS no longer increases.

13



Figure 2. (a) Effect of time on extraction of chromium ions (fixed-pH=5.5; initial volume=10 mL; initial concentration 0.42 mmol/L). (b)The chemical capture of Cr (VI) at different initial concentrations (fixed-pH=5.5; initial volume=20 mL; 2 minutes); (c) Effect of pH on extraction rate and extraction efficiency Effect of initial solution pH The extraction rate and chemical capture of SA are presented in Figure 2(c). The optimized pH value for the capture of Cr (VI) is around 5.5. The chemical capture capacity increases from pH 1.5 to 5.5, and the optimized chemical capture is 6.08 mg/(g·min). Such chemical capture behavior of the system can be explained by hydrophilic and hydrophobic ability of SA. After Cr(VI) has been captured, sodium alginate still contains a large amount of -COO-.39 Under acidic conditions, -COO- is converted into -COOH, ionization and hydrophilicity of SA are reduced, which made it easier to combine with PEG into the top phase. When the pH increases, the -COOH groups continue to dissociate and the hydrophilicity of SA is increased, prompted 14


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bio-hydrogel converges in the bottom phase, resulting in reduced chemical capture of Cr (VI). Extraction kinetics Several technologies can be used to remove Cr (VI) from wastewater, including ion exchange,40 electrodialysis,41 adsorption,42,43 bio-adsorption44 and solvent extraction.45 Compared with those methods, the phase separation time is short and the equilibrium process is fast in ATPS, so the study of kinetics becomes the most important and difficult. Arrhenius equation is used to describe the connection between rate coefficient and temperature: 𝑟0 = 𝑟𝑒𝑥𝑝( ― 𝐸𝑎 𝑅𝑇)

(7)

Where 𝐸𝑎 is the experimental activation energy (kJ/mol); 𝑟 refers to the former parameter, also known as the frequency factor. Considering Cr (VI) extraction mechanism, it is important to premeditate the Cr (VI) speciation in the actual solution. When pH value is greater than 6.8, only chromate (CrO42− ) is stable in solution, and at pH range from 0.5 to 6, there is a gradual conversion of CrO42− ions into HCrO4−,46 so the equation for extracting Cr(VI) by PEG@SA is showed as follows: 𝑃𝐸𝐺@𝑆𝐴 +HCr𝑂4― = 𝑃𝐸𝐺@𝑆𝐴@HCr𝑂4―

(8)

The concentration of the extract in the phase is low during the initial reaction time. The reaction is far from equilibrium, so the reverse reaction is negligible.47 The kinetics of the metal ions extraction depends on the concentration of the reactants in the reaction zone.48 When the extraction process is controlled by the chemical reaction, the extraction rate equation can be summarized as follows: 15



𝑟 = 𝑘[𝑃𝐸𝐺@𝑆𝐴]𝑥[HCr𝑂4― ]

Page 16 of 41

𝑦

(9)

where 𝑘 is the extraction rate constant. 𝑥 and 𝑦 are the reaction orders; [𝑃𝐸𝐺@𝑆𝐴] is the concentrations of SA in solutions (mol/L); [𝐻𝐶𝑟𝑂4― ] is the concentrations of Cr(VI) in solutions (mol/L); 𝑟 is the initial extraction rate (mol/L/s); the order of reaction can be obtained by the initial rate method. At beginning, a certain concentration of reactants is used to obtain a curve, which reflects the connection between 𝐶Cr(VI) and time. The tangent slope of the curve at t=0 is the initial extraction rate, which corresponding to the initial concentration. The initial extraction rate of Cr (VI) has been measured at CCr

(VI)

=0.42×10-3

mol/L and T= 298 K, while CSA = 5.7×10-3, 7.6×10-3, 9.5×10-3 and 1.14×10-2 mol/L. The connection between 𝑙𝑛 𝑟0 and 𝑙𝑛 𝑃𝐸𝐺@𝑆𝐴 is presented in Figure 3(a), the initial extraction rate of Cr(VI) increased with CSA increasing. The slope of the line after regression is 0.47 and the reaction order of SA is 0.47, x = 0.47.

16


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Figure 3. Some factor in top phase on extraction rate (a: CSA, R2=0.96; b: CCr(VI), R2=0.98;c: temperature, R2=0.99) The initial extraction rate of SA has been measured at CSA = 7.50×10-3 mol/L and T=298 K, while CCr(VI) = 0.21×10-3, 0.32×10-3, 0.42×10-3 and 0.53×10-3 mol/L. The connection between 𝑙𝑛 𝑟0 and 𝑙𝑛𝐻𝐶𝑟𝑂4― is obtained from the initial extraction rate at different concentrations of Cr(VI), Figure 3(b) shows the initial extraction rate of SA increased with CCr(VI) increasing. The slope of the line after regression is 1.14, so the reaction order of 𝐻𝐶𝑟𝑂4― is 1.14, y = 1.14. From the obtained reaction order values, under the condition of the temperature of 298K, the reaction kinetics rate equation for the extraction of Cr(VI) by ATPS is [HCr𝑂4― ]

1.14

𝑟 = 6.86·[𝑃𝐸𝐺@𝑆𝐴]0.47

. Depending on the reaction order, CCr(VI) is the key point which can 17



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control the reaction. The effect of Interface area reaction experiment is investigated (Figure S6) and the calculating methods about dynamics equations are also proposed in supporting information. The

initial

extraction

rate

of

Cr(VI)

by

ATPS

is

measured

at

CCr(VI)=0.42×10-3mol/L and CSA = 7.50×10-3 mol/L at T=298.15 K, 308.15 K and 318.15 K, respectively. As shown in Figure 3(c), the initial extraction rate of Cr(VI) increased with increasing temperature. The activation energy 𝐸𝑎=23.50 kJ/mol is 𝐸𝑎

obtained from 𝑙𝑛 𝑟0 = ― 𝑅𝑇 + 𝑙𝑛 𝑟. Because the effect of temperature on the diffusion-controlled extraction rate is not as pronounced as that of the chemical-controlled extraction rate, and the diffusion-controlled extraction process typically does not exceed 20.9 kJ/mol.49 Therefore, higher activation energy indicates that the extraction of Cr (VI) is controlled by chemical reactions. Previous experiments demonstrated that a chemical reaction takes place between SA and Cr(VI), resulting in a ground state complex. To confirm the extract process of the system is controlled by chemical reactions, FT-IR has been performed. As shown in Figure 4, the bands observed at around 3600-3400 cm−1 and 2900-2700 cm-1 are attributed to the O–H stretching vibrations and the C-H stretching vibrations, respectively,50 while at 1350-840 cm-1 are some peaks belongs to PEG barely unchanged both before and after extraction. After the addition of SA, the peak around 1615 cm-1 (C=O asymmetric vibration from –COO-) and 1413 cm-1 (C-O symmetric vibration from –COO-) of PEG@SA increased.51 After extraction of Cr(VI), the peak at 620 cm-1 enhanced compared to that of [email protected] The characteristic band 18


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changes from 1615 cm-1 to 1642 cm−1 and becomes weakened, meanwhile, characteristic band around 1720 cm-1 is appeared, indicating that carbonyl groups in SA can coordinates with Cr(VI) to form a carbonyl-like complex.

Figure 4. FT-IR analysis in the three different systems; The regular and mechanism of Photo-induced precipitation Effect of different solution media Experiments in Figure 5 demonstrate that the concentration of Cr(VI), here present as C, does not decrease at SA media. However, Cr(VI) concentration decreases obviously compare to initial Cr(VI) concentration (C0) with the addition of PEG and PEG@SA.

19



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Figure 5. Effect of different medias on photo-induced precipitation of Cr(VI) (The initial solution pH value is 3.5. the added dosage of PEG-2000 is 300g/L. The initial Cr(VI) concentration is 50mg/L. The light intensity is 300W.). There are many species of chromium at different pH values, resulting in difficulty to predict mechanism of reduction process. The oxygen atoms on PEG molecular chain are easy to be protonated and thus make PEG molecule positively charged to be [PEG ∙ nH]𝑛 +

[PEG ∙ nH]𝑛 +

cation in acidic medium.53 Subsequently, the

cation can associate with those anionic SA@Cr(VI) species to

formneutral ion-pair esters due to the electrostatic interaction. The formation of PEG-SA@Cr(VI) ester can be represented as the following equation: PEG@SA + n𝐻 + + [𝐶𝑟(𝑉𝐼)𝑠𝑝𝑒𝑐𝑖𝑒𝑠]𝑛 ― ↔[PEGnH]𝑛 + ·[𝑆𝐴@𝐶𝑟(𝑉𝐼)𝑠𝑝𝑒𝑐𝑖𝑒𝑠]𝑛 ― (10) When the PEG-SA@Cr(VI) ester is exposed to sunlight, it absorbs light quanta, passes into an excited state and decomposes with the transfer of electrons from the oxygen atoms of PEG molecular chain to the Cr(VI) anion that results in the reduction 20


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of Cr(VI) to Cr(III). A peak at 1250 cm-1 appears in the figure after photo-induced precipitation (Figure S7b), and it can be attributed to C-O-C stretching vibrations of ether. At the same time, the T% of peak at 1078 cm-1 strengthens due to C-O stretching vibrations of alcohols, we assumed that C-O-C segments of oxyethylene subunit (-CH2CH2O) on PEG molecular chain was oxidized to a kind of alcohols with the transfer of electrons from the oxygen atoms to the SA@Cr(VI), resulting in green precipitation. It can be described as the following equation: ℎ𝑣

[PEGnH]𝑛 + ·[𝑆𝐴@𝐶𝑟(𝑉𝐼)𝑠𝑝𝑒𝑐𝑖𝑒𝑠]𝑛 ― 𝑃𝐸𝐺/𝑆𝐴@𝐶𝑟(𝐼𝐼𝐼) + alcohols

(11)

Similar behavior has been reported in previous papers54 except one point which is co-precipitate formed by synergism between SA and Cr. Effect of initial solution pH Figure 6 shows the influence of different initial solution pH values on photo-induced precipitation behaviors of Cr(VI) in the presence of PEG-2000. The reduction rate of Cr(VI) increases obviously with the increase in solution acidity. In addition, the variation of Cr(VI) cannot be ignored. The species of Cr(VI) in aqueous solution changes at different pH values. When the solution pH values are in the range from 0.5 to 6, Cr(VI) predominantly exists in the from of HCrO4−. CrO42− becomes main species when pH value is above 7. Concentration of CrO42− increase when pH value increases as well. So less HCrO4− is available to from formation with PEG@SA, thus the reduction rate decreases.

21



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Figure 6. Effect of initial solution pH on photo-induced reduction of Cr(VI) The reduction process is more likely to occur at lower pH values, which is also consistent with changes in the apparent potentials for Cr(VI)/Cr(III) redox couple. In the case of dichromate, the reduction reaction is:55 𝐶𝑟2𝑂27 ― +14𝐻 + +6𝑒 ― →2𝐶𝑟3 + +7𝐻2𝑂 φ(V) = 1.33 ― 0.138pH

(12)

The redox potential of Cr(VI) increases with pH decreases, therefore, Cr(VI) is easy to be reduced in acid solution. And it is found that precipitation was formed without adjusting the pH of the solution. So SA may play a synergistic role in the reduction process. Kinetic analysis and mechanism of photo-induced precipitation process The linear dependence of rate constants of Cr(VI) reduction with pH values suggests the reduction is a pseudo-first-order reaction (Table S6). The reaction rate constant is 0.25 min-1 when the pH=1.5, and the reaction rate decrease when pH 22


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increase. The increase in hydrogen ion concentration promotes the protonation of the oxygen atoms on the PEG molecular chain, and electrons from the oxygen atoms of C-O-C segments on PEG molecular chain transfer to the SA@Cr(VI) anion, therefore, the photo-induced electron transfer could be effective enough to initiate the redox process.56

Figure 7. The XPS spectra of precipitate (a: High-resolution XPS spectra of Cr2p; b: High-resolution XPS spectra of O1s) The precipitate was generated and also subjected to XPS analysis. The results are shows in Figure 7. 63.5% of Cr (VI) is reduced to Cr(III) species (575.57 eV/585.02 eV),57,58 the remaining Cr(VI) may come from the complex which formed by SA-Cr(VI). 7.52% Cr-O (530.11 eV) is generated, and the remaining was attributable to C=O\C-OH (531.54 eV). The chemical structure of a specific part of the precipitate was measured directly by Micro-FTIR and a high-quality infrared spectrum of the tiny region was obtained. As present in Figure 8, the precipitate was divided into 100 nm × 100 nm regions, and each had been subjected to infrared measurement. Nine of them were selected and the spectra were given in Figure 8(1) (2) and (3). Different regions 23



have the same peak position, and the difference in absorbance indicates that the induced-precipitate is a compound rather than a mixture.

Figure 8. Micro-FTIR of precipitate Base on previous discussion, complex of SA-Cr(VI) enters top PEG-rich phase and the process of photo-induced precipitation was taken under solar irradiation. Then the C-O-C segments of oxyethylene subunit (-CH2CH2O) on PEG molecular chain was oxidized to a kind of alcohols with the transfer of electrons from the oxygen atoms to the SA@Cr(VI), result in formation of precipitation. Monitoring of Cr(VI) by Fluorescence Spectroscopy The EEM spectrums of PEG@SA shows in Figure 9. Peak (A) reflects the fluorescence intensity of PEG@SA. As shown in (b) and (c), with the CCr(VI) increases, the peak (A) to (C) decreases gradually. There is a red shift occur at peak (A), thus, Cr(VI) destroys the π-π structure of the carbonyl comes from SA. Ten different concentrations of Cr(VI) from 0.1×10-3 mol/L to 1×10-3 mol/L for extraction 24


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experiments were prepared. AAS was used to detect the ion concentration in the bottom phase, and the top phase was monitored by fluorescence. As shown in Figure 9(d), the fluorescence intensity values gradually decreases with the increase of CCr(VI) at different concentrations until completely quenching.

Figure 9. EEM analysis of three different systems (a: PEG@SA; b and c: PEG@SA with different concentrations of Cr(VI)); d: The quenching curves of the system at different concentrations of CCr(VI) (the drawings in the figure shows the linear relationship between fluorescence intensity values and CCr(VI), R2=0.97); e: Three different stages of top phase under Ex/Em=320 nm/ 420 nm) The above experimental results show that the fluorescence characteristics of SA can be used to make phase diagrams and monitor the extraction process of Cr(VI). However, the mechanism of interaction between SA and Cr (VI) remains unclear. Therefore, stock solutions of Cr(VI) (0.02 mol/L) and SA (1.5% w/w) were prepared for the quenching experiment. The fluorescence emission spectrum of SA (solution volume 4 mL) was measured both before and after the successive additions of an appropriate amount of quencher (Cr(VI) was added in a volume of 10 μL per time, and the concentration was calculated under the assumption that the total volume was 25



constant). The data of fluorescence quenching are shown in Table 1. The fluorescence intensity increases and the static quenching constant decreases with the temperature rises, so the slope decreases (Figure S8). It means with the temperature increases, the stability of the state complexes decreases, indicating that SA/Cr(VI) belongs to the type of static quenching. The analysis of peak intensity shows that the combination of Cr(VI) and -COO- from SA causes some conformational changes, forms a non-luminous ground state complex which can destroy the equilibrium of the system and result in static quenching. This is in accordance with the red shift in the EEM spectrum. The conclusion is consistent with red shift which occurred in EEM spectrum. Table 1 The experimental fluorescence intensity values Fn of the solution system while titrating gradually and respectively the Cr(VI) to target solution (F0) at 298.15 and 318.15 K. F0 1 2 3 4 5 6 201 189 175 153 143 132 115 25℃ 268 249 237 221 199 189 171 45℃ The Cr(VI) enriched in top phase could be reduced to Cr(III) precipitation by photo-induced precipitation system composed of PEG+SA+H2O. The supernatant was analyzed by F-S without corresponding fluorescence intensity at the optimal emission wavelength (Figure 9e). Besides, after adjusting the supernatant to the optimal extraction pH value, the Cr(VI) was extracted and found that extraction rate was extremely low. However, after adding SA, the Cr(VI) could be extracted again by ATPS. These experimental results proved that SA and Cr co-precipitate by photo-induced precipitation process. The schematic steps and mechanism of all processes are presented in Figure 10. 26


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There is no redox reaction cause the top phase does not contain Cr (III) which can be proved by chemical analysis. In the process of extraction of Cr(VI), PEG applies as a dispersant in the system. And SA is used as an extractant duo to carbonyl functional group that can coordinates with Cr(VI) to form metalloid carbonyls. The results can be proven by FT-IR spectroscopy and static quenching phenomenon. When pH range from 4 to 6, one pair of π bonds between the chromium ion and the oxygen ion opens, binding to H+, so there is a gradual conversion of CrO42− into HCrO4−. According to the fluorescence static quenching theory, the C=O in SA opens to form a ground state complex with HCrO4−. At pH=3.5, the photochemical reduction process is carried out. PEG acts as a reduce-agent to convert Cr(VI) to Cr(III), and SA was used as precipitant to form precipitation with Cr (III). We assume that the precipitation was a compound with complex components, and this conclusion requires more evidence, so we will work on this for further research.

27



Figure 10. The schematic steps of all processes (PEG: dispersant and reduce-agent; SA: extractant, fluorescer and precipitant)

CONCLUSIONS

In this study, a novel system of extraction and photo-induced precipitation was established. SA acts as an extractant to form a complex with Cr(VI) which enriched in top phase. Static quenching occurs during this process, and SA is used as a fluorescent agent for monitoring of Cr(VI). The solution containing Cr (VI) enriched in top phase was photo-induced to a precipitate without adjusting pH value. SA played a synergistic role in the system. The results show the maximum ability extraction of Cr(VI) is obtained at pH=5.50. Cr(VI) can combine with -COO- which provided by 28


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sodium alginate to form complexes, and the conclusion can be proved by FT-IR. In addition, the green precipitate was proved to be compound with complex components which formed by photo-induced precipitation of Cr(VI). At last, the extraction mechanism is revalidated by static quenching and the fluorescence intensity values gradually decreases with the increase of CCr(VI) at different concentrations until completely quenching. Furthermore, the supernatant after photo-induced precipitation was also analyzed by F-S without corresponding fluorescence intensity at the optimal emission wavelength, which means SA plays a synergistic role in the reduction process. The current work highlights a systematic new approach for extraction, monitoring and reduction of Cr(VI). Therefore, it has potential application in recovering of chromium. Acknowledgements Financial supports from National Natural Science Foundation of China (21677027), NSFC-Xinjiang Unite Funds (U1403194) and Fundamental Research Funds for the Central Universities (DUT16ZD208) were gratefully acknowledged. Thank SHIMADZU enterprise management (China) Co., Ltd. Shenyang branch for its instrument support (Micro-FTIR (AIM-9000) characterization) and thanks for Xiangyi Ma's ([email protected]) for testing some date in manuscripts, Department of life science, Tunghai University, Taiwan Conflict of interests The authors declare that they have no conflict of interests. Appendix A. Supplementary data 29



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Synopsis: The enrichment and detoxification of Cr(VI) in this study could be a new potential approach for dealing with Cr(VI)–containing wastewater in any field.

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The enrichment and detoxification of Cr(VI) in this study could be a new potential approach for dealing with Cr(VI)–containing wastewater in any field. 344x281mm (150 x 150 DPI)


The characteristic of novel extraction & photo-induced precipitation

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