Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process

JES-01419; No of Pages 10 J O U RN A L OF E N V I RO N ME N TA L S CI EN CE S X X (2 0 1 8 ) XX X–XXX

Available online at www.sciencedirect.com

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Haotian Hao1 , Yili Wang1,⁎, Baoyou Shi2,3 , Kun Han2,4 , Yuan Zhuang2 , Yan Kong2,4 , Xin Huang2

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Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process

1. College of Environmental Science and Engineering, Beijing Key Lab for Source Control Technology of Water Pollution, Beijing Forestry University, Beijing 100083, China 2. State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 3. University of Chinese Academy of Sciences, Beijing 100049, China 4. College of Petrochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, China

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Article history:

Dye wastewater containing heavy metal ions is a common industrial effluent with complex 20

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Received 27 October 2017

physicochemical properties. The treatment of metal–dye binary wastewater is difficult. In 21

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Revised 18 January 2018

this work, a novel in-situ ferrite process (IFP) was applied to treat Methylene Blue (MB)–Cu(II) 22

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Accepted 19 January 2018

binary wastewater, and the operational parameters were optimized for MB removal. Results 23

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Available online xxxx

showed that the optimum operating conditions were OH/M of 1.72, Cu2+/Fe2+ ratio of 1/2.5, 24

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Keywords:

40°C. Moreover, the presence of Ca2+ and Mg2+ moderately influenced the MB removal. 26

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Dye and metal removal

Physical characterization results indicated that the precipitates yielded in IFP presented 27

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Ferrite process

high surface area (232.50 m2/g) and a multi-porous structure. Based on the Langmuir model, 28

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Multi-porous

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Physisorption

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reaction time of 90 min, aeration intensity of 320 mL/min, and reaction temperature of 25

the maximum adsorption capacity toward MB was 347.82 mg/g for the precipitates 29

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produced in IFP, which outperformed most other adsorbents. Furthermore, IFP rapidly 30

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sequestered MB with removal efficiency 5 to 10 times greater than that by general ferrite 31 adsorption, which suggested a strong enhancement of MB removal by IFP. The MB removal 32 process by IFP showed two different high removal stages, each with a corresponding 33 was achieved by predominantly electrostatic interactions. Then the sweep effect and 35 © 2018 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. 37

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Introduction

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Every year, more than 10,000 types of commercial dyes, with production of 7 × 105 tons, are used in various industries, including textiles, paper, plastics, and leather tanning (Natarajan et al., 2017; W. Wang et al., 2017). Approximately 15%

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encapsulation were dominant in the second longer stage.

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removal mechanism. In the first brief stage (<5 min), the initial high MB removal (~95%) 34

Published by Elsevier B.V. 38

of the produced dyes are released as wastewater (Konicki et al., 2017), causing serious threats to public health and the environment (Daneshvar et al., 2017; Srivastava and Sillanpää, 2017). Moreover, hazardous heavy metals commonly coexist with dyes in some effluents (Stawiński et al., 2017; Zhao et al., 2015). The conventional removal techniques, such as ion-exchange and

⁎ Corresponding author. E-mail: [email protected] (Yili Wang).

https://doi.org/10.1016/j.jes.2018.01.019 1001-0742/© 2018 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019

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x M2þ þ ð3−xÞFe2þ þ 6OH− þ 1=2O2 →Mx Feð3−xÞ O4 þ 3H2 O

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All the chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) in analytically pure grade and were used without further purification. Stock solutions were prepared by dissolving appropriate amounts of metal salts or dye powder in deionized water.

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1.2. Batch experiment

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The experiment was performed in a series of 30 × 300 mm glass test tubes, in which the solution was mixed with aeration by an air pump. A rubber stopper with a condensing tube and an aerator pipe was used to plug each glass test tube. The reaction temperature was controlled by a thermostatic water bath. After transferring 80 mL of simulated dye wastewater containing Cu2+ to the tube, a desired amount of solid FeSO4 ∙7H2O was added. The solution was mixed well by aeration, and then various quantities of NaOH (5 mol/L) were added dropwise, upon which a fine precipitate formed immediately. The precipitate was maintained in suspension by continuous aeration. Blank experiments without the addition of NaOH were conducted to ensure that the decrease in concentration was not actually due to evaporation. Samples were withdrawn intermittently and filtered by a 0.45 μm membrane. The residual dye concentrations in the solution were determined using a UV–visible spectrophotometer (MAPADA UV-6100, Shanghai, China) at 662 nm for MB. The metal concentrations were analyzed by inductively coupled plasma optical atomic emission spectrometry (ICP-OES) ICPE9800 (Shimadzu, Japan). All the tests were conducted in duplicate. The calculation of adsorption capacity (mg/g) is shown in Eq. (1), and the removal efficiency was calculated using Eq. (2):

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Lou and Huang (2009) indicated that sewage containing metal ions treated by FP could meet effluent standards, and the resulting sludge also satisfied the toxicity characteristic leaching procedure (TCLP). Moreover, the ferrite precipitates generated from FP can be separated easily due to its magnetism (Tu et al., 2013) and can be recycled as catalysts (M.-Q. Cai et al., 2017; Lou and Huang, 2009). Furthermore, ferrites with magnetic and electrical properties have attracted extensive attention due to their applications in preparation of adsorbents, active catalysts, magnetic recording media, suspension materials in ferromagnetic liquids, refractory materials, magnetic seeds, super-hard materials and high temperature sensors (Cai et al., 2017; Ding et al., 2013; Manna et al., 2017). So far, application of magnetic ferrites to treat environmental pollution has received a great deal of interest (Almasian et al., 2016; Mahmoodi, 2013; Wu et al., 2016). Magnetic ferrites such as copper ferrite (CuFe2O4), manganese ferrite (MnFe2O4) and cobalt ferrite (CoFe2O4) not only possess photocatalytic and catalytic activity (López-Ramón et al., 2017; Ren et al., 2015; Stoia et al., 2017), but also exhibit good adsorption efficiency owing to electrostatic interaction and surface functional group complexation (Wang et al., 2012, 2015; Wu et al., 2016; Yavari et al., 2016; Zhao et al., 2014). In particular, some magnetic ferrites synthesized by sol–gel, co-precipitation or hydrothermal methods were reported to be effective in adsorbing various dyes such as azo–dyes (Chen et al., 2014; Wang et al., 2012; Wu et al., 2004), and heterocyclic dyes (K. Cai et al., 2017; Hashemian et al., 2013; Iram et al., 2010). Recently, it has been demonstrated that in-situ formed materials, such as Mn-(hydr) oxides (in-situ MnOx) (Lu et al., 2014; Zhang et al., 2008), present a smaller particle size and higher adsorption capacity than aged materials. Thus, it can be an interesting option to conduct an in-situ FP in metal-dye binary wastewater, in which co-precipitation will occur between non-ferrous metals and Fe, as well as dye adsorption by the in-situ formed ferrites. However, to our knowledge, no previous report exists on metal–dye binary wastewater treatment using in-situ FP. In this study, an in-situ FP (IFP) was developed to treat Cu (II)–Methylene Blue (MB) binary wastewater. The influence of operational parameters and solution conditions in IFP formation on MB uptake was studied for practical applications. Then, the MB removal performance was also evaluated through isothermal analysis. In addition, the MB removal mechanism by IFP was elucidated after detailed characterization using various techniques.

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adsorption on activated carbons, are time-consuming and arduous, with mediocre efficiency due to the complexity of metal–dye binary wastewater (Stawiński et al., 2017; Visa et al., 2010). Hence, developing an efficient and cost-effective method to treat such refractory wastewater is significant and urgent. The ferrite process (FP) is an effective method for removing various heavy metal ions from wastewater (Barrado et al., 2002; Erdem and Tumen, 2004; Tu et al., 2012, 2013). In FP, heavy metal ions can be incorporated into a spinel structure through co-precipitation to form ferrites. The principle is presented as shown below:

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qt ¼ ðC0 −Ct Þ  W¼

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C0 −Ct C0

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where C0 (mg/L) is the residual concentration of blank sample (mg/L) and Ct (mg/L) is the residual concentration of analyte; V (L) is the initial solution volume; m (g) is the total initial Cu2+ and Fe2+ dosage. The effects of OH/M, Cu2+/Fe2+ ratio, reaction temperature, aeration intensity, adsorption isotherms, and water hardness on MB removal were investigated. The influence of OH/M was investigated by changing the molar ratio of hydroxyl ions to the combined heavy metal (Cu2+ and Fe2+) concentration. When testing the effect of different experimental conditions, only one condition was varied at a time while all other factors remained constant. Unless otherwise specified, the initial MB concentration and Cu2+ concentration of the simulated dye wastewater were 50 mg/L and 0.02 mol/L, respectively.

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1.3. Characterization methods

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After the formation of IFP, the precipitates were collected, 169 washed with distilled water, and then freeze-dried in vacuum 170 prior to analysis unless otherwise specified. Scanning electron 171

Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019

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The MB removal efficiency and metal removal efficiency under different OH/M ratios as a function of reaction time is shown in Fig. 1. As indicated in Fig. 1a, all the adsorption processes were very fast within the first 30 min and then gradually reached equilibrium after 90 min. Therefore, 90 min was selected as the duration for the subsequent experiments. Second, MB removal slowly increased with increasing OH/M from 1.25 to 1.56, and then slightly decreased as the OH/M increased from 1.56 to 1.72. Afterward, MB removal sharply decreased when the ratio was greater than 1.72. This could be ascribed to the fact that sodium hydroxide could coprecipitate cupric and ferrous ions as hydroxides, forming an intermediate complex hydro sol often referred to as “green rust” (Ding and Zeng, 1992; Erdem and Tumen, 2004), which is followed by dehydration of the sol as shown below:

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Fig. 1 – Influence of OH/M on Methylene Blue removal efficiency (a) and iron and cupric ions removal efficiency (b) by in-situ ferrite process (0.02 mol/L Cu(NO3)2, 0.06 mol/L FeSO4, 50 mg/L MB, and 40°C). MB: Methylene Blue.

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2. Results and discussion

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microscopy (SEM) at different magnifications was performed with a JEOL JSM-7001F SEM (JEOL, Japan) (lower than 10,000 magnification) and Zeiss Merlin field-emission SEM (Zeiss, Germany) (higher than 10,000 magnification). The same instrument was used to carryout energy dispersive spectroscopy (EDX) for elemental analysis. The point of zero charge of precipitates was determined by isoelectric point titration as a function of pH using a Zetasizer Nano ZS90 (Malvern, UK). Particle size was determined by a Mastersizer 2000 (Malvern, U.K.). The BET isotherms were measured by an Accelerated Surface Area and Porosimetry system (Micromeritics, ASAP 2020) using N2 as the adsorbate. FT-IR spectra were recorded by a Bruker VERTEX 70 spectrometer. The X-ray diffraction (XRD) patterns were obtained using an XRD-7000 diffractometer (Shimadzu, Japan) with Cu Kα radiation; data were collected from 2θ = 10°–80° at a continuous scan rate of 2°/sec for crystal phase identification. Xray photoelectron spectroscopy (XPS) data were obtained using an ESCALab250 electron spectrometer from Thermo Scientific Corporation with monochromatic 150 W Al Kα radiation. The narrow-scan pass energy was 30 eV. The base pressure was approximately 6.5 × 10−10 mbar. The binding energies were referenced to the C1s line at 284.8 eV from alkyl or adventitious carbon.

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When OH/M increased from 1.25 to 1.56, the increasing OH/M resulted in an increase in amount of the target adsorbent (copper ferrite) generated, thus increasing the MB adsorption; while excess OH/M could result in more intermediate complex being formed, resulting in a sharp reduction in the target product.

As shown in Fig. 1b, the cupric and iron ion removal efficiency gradually increased with increasing OH/M. Cupric ion removal gradually decreased with the reaction time, while an opposite trend was observed for iron ion removal efficiency. The IFP could effectively treat wastewater containing MB and heavy metals. The MB removal efficiency was satisfactory (>95%) as the OH/M varied from 1.25 to 1.72 at a reaction time of 90 min. In particular, the highest OH/M ratio between 1.25 and 1.72 should be selected to obtain high metal removal efficiency. Thus, the optimum OH/M was 1.72.

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Fe2+ dosage is an important factor determining the performance of FP. The effect of Cu2+/Fe2+ ratio in the IFP on MB removal and metal removal is shown in Fig. 2. The results indicate that the Cu2+/Fe2+ ratio significantly affects the MB removal and metal removal. As shown in Fig. 2a, MB removal increased from 46.84% to 96.63% as the Cu2+/Fe2+ ratio declined from 2/1 to 1/2.5 and then remained stable accordingly as the Cu2+/Fe2+ ratio continually dropped to 1/4. Fig. 2b shows that when the Cu2+/Fe2+ ratio increased from 1/4 to 2/1, the cupric ion removal efficiency increased from

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Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019

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Fig. 3 – Influence of reaction temperature on the removal of MB by IFP (0.02 mol/L Cu(NO3)2, 0.05 mol/L FeSO4, 1.72 OH/M, 50 mg/L MB, and 90 min). MB: Methylene Blue.

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The effect of aeration intensity on the removal of MB by IFP was investigated, as shown in Fig. 4. When the aeration intensity increased from 160 mL/min to 320 mL/min, the MB removal efficiency increased over the entire 90 min. Afterward, as the aeration intensity was further increased from 480 mL/min to 800 mL/min, the MB removal efficiency was rapidly increased in the first 30 min and then remained stable accordingly at around 92%, 93%, and 93%. The maximum value of MB removal efficiency was 96.37% when the aeration intensity was 320 mL/min at 90 min. In IFP, higher aeration intensity leads to an increase in the bubble population in the reaction volume and the creation of greater gas/liquid interface area, which is favorable for oxygen transfer into the aqueous phase. The enhancement of the oxidation rate could speed up the oxidation–hydrolysis reactions (Eq. (3)) (Perales Perez and Umetsu, 2000). Accordingly, the enhanced mass–transfer rate under higher aeration intensity could also accelerate the formation of ferrites, as well as shorten the MB adsorption equilibrium time. Furthermore, an extremely fast oxidation reaction could destroy the incipient copper–ferrous framework by consuming ferrous ions and promoting the

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Perez and Umetsu, 2000). In IFP, dye adsorption takes place during the formation of ferrites, so higher reaction temperature is favorable. Generally, the smaller the particle size, the larger the specific surface area. By wet dispersion analysis with deionized water as dispersant, the particle sizes of untreated final precipitates were found to gradually decrease with increasing temperature (Appendix A Fig. S9). Hence, the increasing specific surface area of the final precipitates could also result in more MB adsorption. In consideration of MB removal and practicality, a heating temperature of 40°C was selected for the subsequent experiments. Note that the treatment of MB by IFP was accomplished under a relatively moderate reaction condition, compared with reaction temperatures of 50–200°C required with the general FP for removing metal ions (Erdem and Tumen, 2004; Kondo et al., 1982; Tu et al., 2012, 2013).

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Fig. 2 – Influence of Cu /Fe ratio on MB removal efficiency (a) and iron ions and cupric ions removal efficiency (b) by IFP (0.02 mol/L Cu(NO3)2, 1.72 OH/M, 50 mg/L MB, 40°C, and 90 min). MB: Methylene Blue.

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The effect of reaction temperature on MB removal in IFP is shown in Fig. 3. MB removal efficiency gradually increased from 78.15% to 95.32% as the reaction temperature increased from 21 to 40°C, then basically remained stable. In ferrite formation, dehydration is the rate-limiting step (Eq. (3)). Increasing temperature can promote the dehydration of the ferrite precursor and accelerate the formation of ferrites (Perales

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47.40% to 97.09% and iron ion removal efficiency showed only slight change (being more than 99% throughout). Two potential reasons for the observed trends of MB removal in the Cu2+/Fe2+ ratio test are: (i) the yields of the precipitates increased with the increase of Fe2+ dosage (Appendix A Fig. S8), resulting in more adsorption sites generated between MB and the adsorbent. (ii) More Cu2+ could be reduced to Cu+ (Appendix A Table S2) to generate Cu2O in the early stage of IFP (proved in Section 2.3) with increasing Fe2+ dosage, even if the proportion of Cu2+ in the final precipitate gradually decreased. Additionally, Cu2O was found to possess good adsorption capability toward cationic dyes due to its strong positive surface charge (Ho et al., 2017), which could be helpful for MB removal in IFP. To obtain high MB removal efficiency, the optimum Cu2+/Fe2+ ratio should be 1/2.5.

Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019

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those for copper ions. In this case, insufficient alkalinity could 338 be the main reason for the decrease in MB removal efficiency. 339 340

The detailed dynamic process of IFP is illustrated in Fig. 5a. Note that the changes in MB removal showed two distinct stages. First, the MB removal efficiency was extremely high (95.97%) in the first one minute and then sharply decreased to 46.26% at

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Compared with trace-level heavy metal ions, calcium and magnesium ions are frequently present at significant levels in the wastewater, which could have an adverse effect on the FP, reducing metal recovery as well as the magnetization and stability of ferrite (Wang et al., 1996). Appendix A Fig. S1 shows the removal performance of MB using IFP in the presence of different concentrations of Ca2+ and Mg2+. MB removal efficiency gradually decreased from 96.63% to 60.88% as the Ca2+ concentration increased from 0 mg/L to 400 mg/L. Similarly, increasing Mg2+ concentration decreased the MB removal efficiency from 96.63% to 56.19%. The phenomenon could be explained by two reasons. First, the presence of calcium and magnesium ions could consume alkalinity, reducing the production of the target copper ferrite. Second, the variations in ionic radius could affect the structure and composition of the final products. Calcium ions have a significantly larger radius (1.14 Å) than copper ions (0.72 Å) and would be difficult to incorporate into the ferrite lattice (Wang et al., 1996), which could interfere with the growth of crystals. The finding that the BET surface area (140.05 m2/g) of final precipitates in the presence of 50 mg/L Ca2+ is lower than that (232.50 m2/g) of precipitates without calcium supports this conclusion. In the case of magnesium ions, the ionic radius of Mg2+ (0.65 Å) and BET surface area of the precipitates for solutions containing 50 mg/L Mg2+ (198.05 m2/g) are similar to

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generation of ferric-rich precipitates with an amorphous, voluminous nature (Perales Perez and Umetsu, 2000), which could result in some inhibition of MB removal. Fig. S10 shows the particle sizes of untreated final precipitates as a function of aeration intensity in IFP. The growth in particle size of untreated final precipitates with increasing aeration intensity verified this deduction. Based on this study and previous literature, 320 mL/min was selected as the optimum aeration intensity.

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Fig. 4 – Influence of aeration intensity on the removal of MB by IFP (0.02 mol/L Cu(NO3)2, 0.05 mol/L FeSO4, 1.72 OH/M, 50 mg/L MB, 40°C, and 90 min). MB: Methylene Blue; IFP: insitu ferrite process.

Fig. 5 – (a) MB removal efficiency at different times in the IFP (0.02 mol/L Cu(NO3)2, 0.05 mol/L FeSO4, 1.72 OH/M, 50 mg/L MB, and 40°C). (b) The pHpzc of the in situ-formed precipitates and the pH of the reaction at different times. (c) Particle sizes of the precipitates at different reaction times. MB: Methylene Blue; IFP: in-situ ferrite process.

Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019

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The composition of the precipitates at different reaction times was analyzed by XPS (Fig. 6a) and XRD (Fig. 6b). The precipitates consist of Fe, Cu, S, C, O, and N, and the main component C (C-C bonds in nonoxygenated rings, 284.8 eV) (Dong and Wang, 2016) from MB gradually increased (from 68.59% to 79.62%) with reaction time from 1 min to 90 min. This phenomenon indicated the increasing MB adsorption as the reaction proceeded. From the XRD pattern and XPS spectra of Cu2p (Appendix A Fig. S2) and Fe2p (Appendix A Fig. S3) of the precipitates at different times, it can be seen that Cu2O (JCPDS No. 65-3288) was a major component in the first 5 min of this reaction, and then gradually disappeared. Meanwhile, CuFe2O4 (JCPDS No. 25-0283) appeared at 30 min and its amount gradually increased with reaction time. Other species including CuO (JCPDS No. 78-0428), FeOOH (JCPDS No. 896096), and Fe2O3 (JCPDS No. 47-1409) were also detected in IFP but in low amounts. The composition of the precipitates at different reaction times was proposed to be as follows:

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five minutes. The phenomenon could be explained by electrostatic interaction, as indicated by Fig. 5b. In the first minute, the pH of the reaction was higher than the isoelectric point (pHpzc) of the in-situ formed precipitates, leading to electrostatic attraction between the negatively charged precipitate and cationic MB molecules. Afterward, due to the significant decrease in the pH of the reaction in the next few minutes, the in-situ formed precipitates became positively charged and repelled the cationic MB molecules. The decrease in particle size in the first 5 min (Fig. 5c) can also reflect this brief process. Starting from the fifth minute of the reaction, the MB removal efficiency showed a gradual increase even though electrostatic repulsion exists between the adsorbent and the adsorbate. In addition, the particle size of the precipitates rapidly increased from 52.60 to 310.35 μm as the reaction proceeded from 5 to 90 min. During this period, the MB removal could mainly be due to the sweep effect and encapsulation during the crystal growth process of the precipitates. Appendix A Fig. S6 shows the SEM images of the precipitates at different reaction times. Appendix A Fig. S7 shows the morphologies of the final precipitates at different magnifications. Clearly, the precipitates exhibited amorphous and gel-like aggregates with multi-porous structures from the beginning of IFP, which is favorable for MB removal. The in-situ formed high-surface area support not only facilitated the fast diffusion and transport of MB and products (Qi et al., 2016), but also provided more contact sites for MB removal in IFP (Khan et al., 2013). The EDX spectrum result (Appendix A Table S1) confirmed the presence of MB in the precipitates. Moreover, the rapidly increased particle size and dense pore distribution could hinder the release of MB, eventually leading to MB adsorption equilibrium.

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Fig. 6 – XPS spectra of survey scan (a) and XRD patterns of the precipitates at different reaction times (b). XPS: X-ray photoelectron spectroscopy; XRD: X-ray diffraction.

The findings are also consistent with our previous results of metal removal rates (Fig. 1b). In IFP, ferrous irons reduced Cu (II) to Cu (I) and Cu2O was formed in the first few minutes, and the in-situ formed Cu2O with negative charge could possess good adsorption capability toward MB due to electrostatic attraction (Ho et al., 2017). Then Cu (I) was oxidized to Cu (II) by oxygen, and some ions were released back into the solution while others were gradually incorporated into the precipitates, then MB molecules were swept and encapsulated accordingly during the latter process. In addition, the crystal structure of the precipitates gradually changed from crystalline to amorphous as shown in Fig. 6b. This transformation could be the reason for the high surface area of the precipitates and superior performance regarding MB uptake (Goldberg and Johnston, 2001).

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The MB uptake by IFP versus initial concentration and the 411 adsorption isotherms are shown in Fig. 7a. Langmuir and 412 Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019

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According to the Langmuir model, the maximum MB adsorption capacity (qm) for the amorphous precipitates in IFP was 347.82 mg/g, which was superior to most other adsorbents previously reported (Table 2), such as groundnut shell carbon (164.9 mg/g) (Kannan and Sundaram, 2001), graphene oxide (144.92 mg/g) (Li et al., 2013), graphene–carbon nanotube (81.97 mg/g) (Ai and Jiang, 2012), PAA/MnFe2O4 (53.3 mg/g) (Wang et al., 2015), porous MnFe2O4 (20.67 mg/g) (Hou et al., 2011), and PA/CoFe2O4 (32.1 mg/g) (K. Cai et al., 2017). It is noteworthy that the surface of as-synthesized ferrite nanoparticles is usually positively charged, so that owing to electrostatic interactions, it shows better adsorption properties for anionic dyes; correspondingly, the adsorption capacity for cationic MB dye is lower. If ferrite is grafted with another material with negatively charged functional groups, its adsorption capacity for MB could be improved, but it is still lower than that of biobased materials and carbon adsorbents, which could be mainly due to its lower specific surface area. By contrast, the amorphous precipitates in IFP exhibited a high BET surface area (232.50 m2/g) and possessed a negatively charged surface at the initial stage of the reaction, causing superior adsorption ability for MB. In particular, the process of MB wastewater treatment could be very rapid (< 5 min), which is very advantageous in practical applications. For comparison, copper ferrite prepared under the same conditions was used to adsorb MB. A significant discrepancy was observed when comparing the MB removal performance of the general FP adsorption and that of IFP (Fig. 7b), suggesting the strong enhancement of MB removal by IFP. Macroscopic techniques were also employed to verify the MB removal mechanism in IFP. The adsorption thermodynamics was studied by investigating the effect of temperature on the sorption of MB in IFP at 291, 301, 311, 321, and 331 K with an initial MB concentration of 50 mg/L. The enthalpies were calculated using the Van't Hoff equation (D. Ding et al., 2013; Kumar and Jena, 2016; G. Wang et al., 2017)

414 415 416 417 418

t1:1 t1:3 t1:2

t1:4

Freundlich models were employed to fit the experimental data. The equations and parameters for both the Langmuir and Freundlich models are listed in Table 1. Results showed that the Freundlich model was a better fit for the adsorption isotherms, which might indicate the presence of heterogeneous active sites in this process (Yang et al., 2017).

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Fig. 7 – (a) Adsorption isotherms of MB by IFP (0.02 mol/L Cu (NO3)2, 0.05 mol/L FeSO4, 1.72 OH/M, 25–500 mg/L MB, and 40°C). (b) The MB removal performance comparison between IFP (0.02 mol/L Cu(NO3)2, 0.05 mol/L FeSO4, 1.72 OH/M, 50 mg/L MB, 40°C, and 90 min) and prefabrication copper ferrite adsorption method (preparation condition: 0.02 mol/L Cu(NO3)2, 0.05 mol/L FeSO4, 1.72 OH/M, 40°C, and 90 min; adsorption condition: 50 mg/L MB, pH 7.0 ± 0.2, 40°C, and 90 min). MB: Methylene Blue; IFP: in-situ ferrite process.

lnKC ¼ −

ΔG ΔS ΔH ¼ − RT R RT

where R is the gas constant (8.314 J mol− 1 K− 1), T is the solution temperature (K), and KC (qe/ce) is the adsorption equilibrium constant. The thermodynamic parameters were calculated from the plots of lnKC versus 1/T (Appendix A Fig. S4), and the results are summarized in Table 3. The observation of negative ΔG values in the range of 0 to − 20 kJ/mol in this study indicates that the MB removal in the IFP is a spontaneous physisorption process (Li et al., 2017; Wang et al., 2007). Furthermore, the ΔG values became more negative with the increase in temperature, indicating the

Langmuir qe ¼

t1:5

Freundlich 1

K L q m ce 1þK L ce

420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455

ð4Þ

Table 1 – Isotherm parameters of Langmuir and Freundlich models for metal and dye adsorption. Equations

419

qe ¼ K F cen

t1:6

qm (mg/g)

KL(L/mg)

R2

1/n

K1/n F (mg/g(mg/L))

R2

t1:7

347.82

3.88 × 10−3

0.958

0.72

3.54

0.975

Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019

457 456 458 459 460 461 462 463 464 465 466

8 t2:1

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Table 2 – Performance comparison between the IFP and various adsorbents for MB removal from water.

t2:3 t2:2

Adsorbent

Adsorption capacity (mg/g)

Reference

t2:5 t2:6 t2:7 t2:8 t2:9 t2:10 t2:11 Q1 t2:12 t2:13 t2:14 t2:15 Q2 t2:16 t2:17 t2:18 t2:19 t2:20

Bamboo activated carbon Coconut shell activated carbon Groundnut shell carbon Multi-wall carbon nanotube Graphene–carbon nanotube Graphene oxide/calcium alginate composites ᴋ-Carrageenan/graphene oxide gel beads Graphene oxide PAA/MnFe2O4 MnO–Fe2O3 Porous MnFe2O4 PA/CoFe2O4 Tea waste/CuFe2O4 Humic acid–Fe3O4 Graphene nanosheet/Fe3O4 IFP

454.2 200.01 164.9 48.06 81.97 181.81 658.4 144.92 53.3 40.97 20.67 32.1 32.25 93.08 43.82 347.82

(Hameed et al., 2007) (Islam et al., 2017) (Kannan and Sundaram, 2001) (Ai et al., 2011) (Ai and Jiang, 2012) (Li et al., 2013) (M. Yang et al., 2017; X. Yang et al., 2017) (Li et al., 2013) (Wang et al., 2015) (Hou et al., 2011) (Hou et al., 2010) (G. Cai et al., 2017; W. Cai et al., 2017) (Hashemian et al., 2013) (Zhang et al., 2013) (Ai et al., 2011) This study

t2:21 t2:22

MB: Methylene Blue; IFP: in-situ ferrite process.

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t2:4

2.5. FT-IR and UV analysis

479

The FT-IR spectra of the precipitates in the IFP with and without MB are compared in Appendix A Fig. S5a. No new peaks were observed, which indicated that no chemical bonds were formed between the adsorbents and adsorbates. Moreover, some peaks showed a slight shift after the reaction. These changes could be attributed to the electrostatic interaction between the MB molecules and adsorbent (Deng et al., 2009; Hou et al., 2010). The supernatant at different times during the reaction was also analyzed by UV spectroscopy (Fig. S5b). No new adsorption peak appeared in the full wavelength range during the whole reaction. The FTIR and UV analyses showed that MB removal by IFP was mainly predominated by comprehensive physical interactions.

480 481 482 483 484 485 486 487 488 489 490

t3:1

C

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475

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474

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473

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471

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470

492 491

A novel IFP based on conventional FP was successfully applied to simultaneously remove MB and copper from wastewater. The MB removal performance by IFP is strongly dependent on the OH/M and Cu2+/Fe2+ ratios, but weakly dependent on aeration intensity, reaction temperature, and water hardness. The superior adsorption capacity of MB (347.82 mg/g) calculated from the Langmuir model and comparison with general ferrite adsorption suggested a strong enhancement of MB removal by IFP. The MB removal process was divided into two distinct stages, both of which have good performance but different removal mechanisms. Electrostatic interactions should be the main reason for MB removal in the first stage, and then the sweep effect and encapsulation were dominant in the second stage. The results of this study demonstrated that IFP has potential as a technology in metal– dye binary wastewater treatment.

493

Acknowledgments

509 508

This work was supported by National Key Research and Development Program of China (No. 2016YFA0203204) and the National Natural Science Foundation of China (Nos. 51478041 and 51678053).

510

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469

3. Conclusions

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477

increased favorability of MB removal by IFP at higher temperatures. The positive value of ΔS suggests an increased randomness at the solid/liquid interface with some structural changes obtained in this system. The positive value of ΔH confirms the endothermic nature of the process, and that increasing temperature would enhance the MB removal process, which coincides with the experimental results regarding temperature dependency (Fig. 3). The ΔH value in the range of 0 to 42 kJ/mol also indicates that the process is physical adsorption, with no chemical bond formation (Yang et al., 2014).

468

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Table 3 – Thermodynamic parameters for MB removal in the IFP.

t3:3 t3:2

ΔG (kJ mol−1) at T

t3:4 t3:5

291 K

301 K

311 K

321 K

331 K

−1.672

−3.631

−5.153

−6.021

−8.153

t3:6

MB

t3:7 t3:8

MB: Methylene Blue; IFP: in-situ ferrite process.

ΔH (kJ mol−1 K−1)

ΔS (J mol−1 K−1)

41.901

153.809

Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019

494 495 496 497 498 499 500 501 502 503 504 505 506 507

511 512 513

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Please cite this article as: Hao, H., et al., Strong enhancement of methylene blue removal from binary wastewater by in-situ ferrite process, J. Environ. Sci. (2018), https://doi.org/10.1016/j.jes.2018.01.019