Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-2029-8
RESEARCH ARTICLE
High removal performance of a magnetic FPA90-Cl anion resin for bromate and coexisting precursors: kinetics, thermodynamics, and equilibrium studies Zhengming Xu 1 & Dexia Han 1 & Yuan Li 1 & Pingling Zhang 1 & Lijun You 1 & Zhengang Zhao 1,2,3 Received: 19 January 2018 / Accepted: 12 April 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract In this study, the FPA90-Cl resin was magnetized with supported Fe3O4 particles using a chemical co-precipitation method and its removal performance of bromate and coexisting precursors was explored. The magnetized FPA90-Cl resin was structurally characterized by SEM, FT-IR, and XRD. The effects of the initial concentrations, temperature, and resin dosage on bromate and bromide ion removal in drinking water were investigated using batch experiments. The magnetized FPA90-Cl resin exhibited a high removal efficiency for bromate and bromide ions at three initial concentrations, and the residual bromate concentrations were under the maximum contaminant level (MCL) of 10 μg L−1 after 80 min. The adsorption data of bromate and bromide ion could be well described by a pseudo-first-order kinetic model (R2 0.98). The bromate removal alone was further studied by varying the initial solution pH, temperature, and competitive anions. The results showed that the magnetized FPA90-Cl resin could be used over a wide pH range (4.0–9.0). The maximum sorption capacity of the magnetized FPA90-Cl resin for bromate reached 132.83 mg g−1 at 298 K. The Freundlich and Redlich-Peterson isotherm models fit the bromate adsorption equilibrium better (R2 0.99) than the Langmuir isotherm model (R2 0.98). The thermodynamic analysis showed that the bromate adsorption process was endothermic. The negative ΔG and positive ΔS indicated that the process was spontaneous and that randomness increased after adsorption, respectively. The competition of coexisting anions with bromate was in the order of SO42− > CO32− > Cl− > NO3− > HCO3− > PO43−. Additionally, the magnetized FPA90-Cl resin could maintain a high bromate and bromide ion adsorption capacity after five cycles of regeneration by a 0.1 M NaCl solution. Keywords FPA90-Cl resin . Fe3O4 . Adsorption . Bromate and bromide ions . Kinetics . Thermodynamics
Introduction Bromate is currently classified by the International Agency for Research on Cancer (IARC) as a Group 2B carcinogen. The Responsible editor: Guilherme L. Dotto * Zhengang Zhao
[email protected] 1
School of Food Science and Engineering, South China University of Technology, Guangzhou 510640, China
2
Guangdong Province Key Laboratory for Green Processing of Natural Products and Product Safety, South China University of Technology, Guangzhou 510640, China
3
Overseas Expertise Introduction Center for Discipline Innovation of Food Nutrition and Human Health (111 Center), Guangzhou 510640, China
main effects of bromate include cancer, abdominal pain, vomiting, nausea, diarrhea, hemolytic anemia, various degrees of central nervous system depression, and pulmonary edema (Moore and Chen 2006; Naushad et al. 2015). The US Environment Protection Agency (US EPA) and the American Water Works Association (AWWA) have set the maximum contaminant level (MCL) of bromate in drinking water at 10 μg/L due to its harmful effects on human health (Bhatnagar et al. 2009). Bromate is not naturally present in surface water, and its presence in rivers is likely a result of industrial activities (BUTLER et al. 2005). In addition, bromate is a disinfection byproduct originating from the reaction of ozone or OH radicals with naturally occurring bromide in drinking water (Li et al. 2016). Bromide is naturally present in surface and ground waters at concentrations varying from a few μg/L to approximately 800 μg/L (Wisniewski et al. 2014). During ozone treatment of water, which can destroy
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microorganisms and reduce the color and total organic carbon, the bromide present in the water is oxidized to hypobromite ions (OBr−) and bromate (BrO3−) (Wisniewski et al. 2011; Xu et al. 2014). The concentrations of bromate in drinking water after ozonation generally range from 2 to 293 μg/L, and its formation is influenced by many factors, such as the ozone dosage, bromide concentration, alkalinity, pH, temperature, contact time, and ammonia and natural organic matter (NOM) contents (Han et al. 2014; Li et al. 2016). According to a nationwide survey conducted in China, due to seawater intrusion, industrial and agricultural wastewater discharge, and special geological circumstances, the concentrations of bromide ions can reach 2 mg/L or higher (Ding et al. 2012a; Magazinovic et al. 2004). Bromide is not regulated in drinking water because it has not been reported to be detrimental to public health. However, bromate and bromide ions need to be removed from drinking water because they are the precursors of detrimental disinfection by-products (Ding et al. 2012a; Wisniewski et al. 2011). There are three main, reported approaches for controlling the bromate concentration in drinking water. One method is inhibiting the formation of bromate by removing bromate precursors, such as bromide and NOM, before water is treated with ozone (Johnson and Singer 2004; Marhaba and Bengraine 2003). Another method is to minimize bromate production by pH control in a low pH range, by addition of ammonia or hydrogen peroxide (Matos et al. 2008; Wisniewski et al. 2014). Additionally, hydroxyl radical scavengers can be added to prevent bromide from oxidizing (Pinkernell and Von 2001). A third approach is to use physical, chemical, or biological methods to remove the bromate produced after ozonation (Chen et al. 2014; Liu et al. 2012; Wu et al. 2013), including UV irradiation (Peldszus et al. 2004); high-energy beam (HEEB) irradiation (Bhatnagar et al. 2009); reduction by zerovalent iron (Fe0), Fe2+, and SO32− (Gordon et al. 2002; Xie and Shang 2006, 2007); electrocatalytic reduction (Ding et al. 2012b; Liang et al. 2010; Skunik and Kulesza 2009); catalytic hydrogenation (Chen et al. 2010); membrane filtration (Hatzistavros et al. 2004; Listiarini et al. 2010; Matos et al. 2008); biochemical reactions (Asami et al. 1999; Kirisits et al. 2001; Liu et al. 2012); and adsorption processes. Adsorbents such as activated carbon (Huang and Cheng 2008; Kirisits et al. 2000; Wang et al. 2010), clay (Chitrakar et al. 2011a), metal hydroxides (Bhatnagar et al. 2009; Chitrakar et al. 2011b; Yu et al. 2013), and a variety of ferricmodified materials (He et al. 2012; Li et al. 2016; Xu et al. 2004) have been widely studied, and the adsorption processes have been reported to efficiently remove bromate ions. Even though these methods have been shown to eliminate bromate in water, most of them are not applicable in industry fields due to the high material costs, high-energy consumption, secondary pollution, or complicated operational processes. For example, UV irradiation removes only 19% of bromate
ions (Peldszus et al. 2004). Reduction by zero-valent iron, Fe2+, and SO3 or by adding other chemicals to the source water causes secondary pollution, and zero-valent iron is easily oxidized when exposed to air. Adsorption and reduction of activated carbon can result in a promising efficiency of bromate removal, but high cost restricts its use, along with membrane filtration (Chellam 2000). In addition, electrocatalytic reduction is a high-energy consumption process, and metal hydroxide cannot be regenerated. Anion exchange resins are commercially available, cheap, and chemically stable, and they can be conveniently regenerated (Chen et al. 2014). Therefore, they are attractive materials for adsorbing inorganic anions in water (Awual and Jyo 2009; Chabani et al. 2006; Kim et al. 2012; Milmile et al. 2011; Tang et al. 2013; Yoon et al. 2009). However, traditional anion resin particles are poorly dispersed and slow settling in water. The FPA90-Cl resin is a strong-base resin with a macroporous, polystyrene matrix in the chloride form and quaternary amine functional groups. The FPA90-Cl resin can remove dissolved organic matter from raw drinking water and can be used to adsorb inorganic anions, such as bromate and bromide ions, using chloride as the exchange ion. In addition, the macroporous structure of FPA90-Cl resin is favorable for the entry of Fe3O4 particles, which increases the Fe3O4 loading capacity. Thus, this study aims to prepare a magnetic adsorbent using FPA90-Cl resin with Fe3O4 particles. Integrating iron oxide into the polymer matrix of the FPA90-Cl resin facilitates the aggregation and settling of resin particles, and the particles are better suspended under a certain water flow speed, resulting in properties superior to those of traditional resins. The bromate and bromide ions coexist in water after preozonation. If any bromide is left, it continues to be oxidized into bromate during the post-ozonation. In this study, the performance of a magnetic FPA90-Cl resin for the removal of trace bromate and bromide ions was investigated. Then, the removal of high bromate concentrations by the magnetized FPA90-Cl resin was further studied. The effects of different factors such as the temperature, initial solution pH, and other anions were investigated. The adsorption kinetics, equilibrium isotherms, and thermodynamics were investigated, and the thermodynamic parameters, such as ΔG, ΔH, and ΔS, were calculated. Finally, this study investigated the regeneration and reusability of this magnetic anion resin.
Materials and methods Materials and reagents The Amberlite FPA90-Cl resin was obtained from H&E Co. Ltd. (Beijing, China). This product is a macroporous, strongbase anion exchange resin with a polystyrene matrix and quaternary amine functional groups and uses chloride as the
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exchange ion. The virgin FPA90-Cl resin was stored in water. The average diameter of the FPA90-Cl resin is 660 μm. The resin was washed with Milli-Q ultrapure water and ethanol to remove the impurities before use, and it was pretreated with hydrochloric acid, ultrapure water, and sodium hydroxide to create the chloride type resin. All chemicals used in this study were reagent grade or better and were purchased from Aladdin Industrial Corporation (Shanghai, China). The 1000 mg L−1 single-compound stock solutions of bromate, bromide, sulfate, nitrate, carbonate, bicarbonate, chloride, and phosphate were prepared using their sodium salts. They were stored at 4 °C and were used by diluting the stock to various concentrations.
Analytical methods The pH of the solution was measured using an Accumet AB15 pH meter from Fisher Scientific. Prior to each use, the pH meter was calibrated. The concentrations of bromide, bromate, and other anions were analyzed by an ion chromatograph (ICS-900, Dionex) equipped with a suppressed conductivity detector, an IonPac guard column (AG23, 4 mm × 250 mm), an IonPac analytical column (AS23, 4 mm × 250 mm), and a 100-μL injection loop. The detection limits for BrO3− and Br− were 2.5 μg L−1. The correlation coefficient of the calibration curve (R2) was 0.9999. All of the experiments were determined in triplicate and averaged.
Magnetic anion resin preparation The virgin FPA90-Cl resin and analytical-grade reagents (FeCl 3 ·6H 2 O, FeCl 2 ·4H 2 O, NaOH, C 2 H 5 OH, and CH 3 COOH) were used as the raw materials. The magnetic iron oxide was integrated into the macroporous structure of the FPA90-Cl resin by a chemical co-precipitation method under a nitrogen atmosphere. Specifically, 25 mL of the virgin resin was first swollen in 50 mL of 2% acetic acid. Then, 100 mL of a FeCl 3 ·6H 2 O ethanol-deionized water (1:1) solution (1.25 mol L−1) and 100 mL of a FeCl2·4H2O ethanol-water (1:1) solution (0.625 mol L−1) were added and mixed with constant magnetic stirring for 4 h. The Fe3+ and Fe2+ were in molar proportion of 2:1 (Liu et al. 2011). The obtained solution was subjected to intense ultrasonic vibration for 30 min. After the solution was thoroughly mixed, 50 mL of a NaOH solution (10 mol L−1) was slowly added into the above solution dropwise within 30 min under vigorous stirring. In alkaline conditions, nitrogen atoms in the active group on the resin provide lone pairs of electrons to form a coordinate bond for iron and ferrous ions. Iron and ferrous ions trapped in resin pores react with hydroxide ions to form ferriferrous oxide in situ. After 1 h of reaction, the solution temperature was increased to 60 °C for 2 h. Finally, the solution naturally cooled to room temperature, and the separated resin was repeatedly washed with deionized water until it was neutral. The magnetic anion resin was then washed with ethanol 2–3 times and dried in a vacuum oven at 45 °C for 24 h (Tran et al. 2010).
Characterization The morphologies of the virgin FPA90-Cl resin and magnetized FPA90-Cl resin were characterized using a field emission scanning electron microscope (FE-SEM). The structure and chemical composition changes of the virgin and magnetized FPA90-Cl resin were analyzed by transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD).
Batch adsorption experiments Kinetic studies on the coexistence of bromate and bromide ions The kinetic studies were conducted using a batch experiment. The magnetized FPA90-Cl resin dosage was fixed at 0.5 g and was added to a set of 1000-mL sealed glass beakers containing 500 mL of the adsorbate with different initial concentrations of bromate and bromide ions at 298 K. The initial solution pH was 7.00 ± 0.5. The beakers were maintained on a digital display, stable temperature magnetic stirrer (IKA C-MAG HS 4) with a constant 300 rpm agitation. The reaction time was different based on the bromate removed below 10 μg L−1 or completely eliminated to undetectable levels by an ion chromatograph (ICS-900, Dionex). Small amounts of the samples were taken at specific time intervals and filtered through 0.22-μm disposable membrane filters. The filtered solutions were analyzed for the bromate and bromide ion concentrations by ion chromatography. The adsorption capacity of the magnetized resin for bromate and bromide ions at time t, qt, was calculated by Eq. (1). The solid-liquid distribution coefficients of the bromate and bromide ions on the magnetized resin and in the solution after sorption were determined by Eq. (2): ðC0 −Ct ÞV W ðC0 −Ct Þ V KD Ct W
qt ¼
ð1Þ ð2Þ
where C0 and Ct (μg L−1) are the concentrations of the bromate or bromide ions at the beginning and at time t, respectively. V (L) is the volume of the solution, W (g) is the mass of the magnetic resin, and KD is the solid-liquid distribution coefficient of bromate or bromide ions on magnetized FPA90-Cl resin and in solution after sorption.
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Independent variable studies A batch of independent variable experiments was carried out to investigate the impact of different variables on the removal of bromate and bromide ions. The effects of different parameters on the elimination of coexisting bromate and bromide ions were examined by varying magnetized FPA90-Cl resin dosages, initial concentrations, and temperatures. In addition, the impacts of initial solution pH and temperature on the adsorption equilibrium of bromate were also investigated. The initial pH values of the bromate solution were adjusted by adding hydrochloric acid or sodium hydroxide. The temperature was controlled by heating or adding ice when necessary. Regeneration and recyclability studies Regeneration is one of the most important properties of adsorbents. The regeneration studies were carried out in batch mode. The magnetized FPA90-Cl resin (0.2 g) was saturated with 500 mL of 400 μg L−1 BrO3− and 2 mg L−1 Br− solutions in 1000-mL sealed glass beakers at 300 rpm and 298 K for 24 h. Then, the exhausted resin was filtered and washed several times with ultrapure water to eliminate un-adsorbed ions. Because chloride was the mobile counter ion in the magnetic resin, the resin was regenerated by 500 mL of a 0.1 M sodium chloride solution under the same conditions to saturate the resin. Additionally, the regenerated resin was filtered and washed several times to remove the free ions. Then, the resin was reused to remove bromate and bromide ions. Five cycles were performed using the same process as above.
Results and discussion Surface morphology of the surface of FPA90-Cl resin before and after magnetization The SEM micrographs of the virgin resin and magnetic resin are presented in Fig. 1 and show the surface topography changes of the FPA90-Cl resin. Figure 1a, b, which are images of the virgin resin at × 300 and × 5000 magnification, respectively, show the spherical shape and smooth surface of the resin particles without any sediments. Figure 1d, e, which are images of the magnetized FPA90-Cl resin at the same magnifications, show that the resin surface is rough and covered with sediments. This is due to the formation of Fe3O4 and loading of the resin particles. In addition, the virgin resin at × 50,000 magnification is shown in Fig. 1c, and the pores on the resin surface are clearly visible. However, for the magnetized resin at × 50,000 magnification (Fig. 1f), the pores on the resin surface are not visible, and only some deposits can be seen. This may be ascribed to the generated Fe3O4 covering the
resin surface. At the same time, the specific surface area of the resin increased from 14.96 to 21.06 m 3/g after coprecipitation process according to the BET results.
Structural characterizations of the magnetic resin The FT-IR results for the virgin and magnetic resins are shown in Fig. 2. The spectroscopic analyses show that the virgin and magnetic resins have some similar transmission bands. The peak at approximately 3428 cm−1 may be due to the O–H stretching vibration from adsorbed water. The band at 1630 cm−1 is attributed to the N–H bending vibration. The band at 1486 cm−1 is associated with the C–H bending vibration of the quaternary ammonium groups in the resin. For the bare Fe3O4 materials loaded onto the resin, the band at 581 cm−1 could be attributed to the Fe–O stretching vibration (Li et al. 2016).
XRD analysis The XRD patterns of pure Fe3O4 and the magnetized resin are shown in Fig. 3. The pure Fe3O4 has six characteristic peaks (2θ = 30.80, 36.17, 43.84, 54.21, 57.72, and 63.26), which correspond to the (220), (311), (400), (422), (511), and (440) planes. The same characteristic peaks can be observed in the XRD pattern of the magnetized resin, which indicated that Fe3 O4 particles were successfully loaded onto the resin without a phase change in Fe3O4 during the magnetic process.
Comparison of adsorption abilities between virgin FPA90-Cl resin and magnetized resin The adsorption capacities for bromate and bromide ion between virgin FPA90-Cl resin and magnetized resin were compared by batch experiments. The initial concentrations of bromate and bromide ion were 100 and 300 μg L−1, respectively. The reaction time was 90 min. It could be observed from Fig. 4 that the adsorption capacities of the magnetized FPA90-Cl resin for bromate and bromide ions were lower than the virgin resin. Because to access the pores of the resin, the ions needed to permeate the layer of Fe3O4 covering the magnetized resin surface.
Kinetic studies on coexisting bromate and bromide ions The sorption of three different concentrations of coexisting bromate and bromide ions in aqueous solutions with the magnetized FPA90-Cl resin was investigated as a function of the contact time. The initial concentrations of the bromate and bromide ions in Fig. 5a–c were 50 and 200 μg L−1, 100 and 300 μg L−1, and 200 and 400 μg L−1, respectively, and the reaction times were 60, 120, and 145 min, respectively. The
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Fig. 1 SEM micrographs of the virgin FPA90-Cl resin (a–c) and magnetized FPA90-Cl resin (d–f)
magnetic FPA90-Cl resin exhibited good bromate and bromide ion adsorption. Figure 5a–c shows that the concentration of bromate ions was less than 10 μg L−1, and the concentration of bromide ions was less than 25 μg L−1 after 80 min. The bromate ion concentration was reduced to below 10 μg L−1 within 30 min under a relatively high bromide ion content in Fig. 5a. The bromate and bromide ion adsorption process can
be divided into three stages. The bromate and bromide ion concentrations dramatically declined in the first 20 or 30 min due to the large concentration gradient between the solution and the surface of the magnetic resin (Tong et al. 2011). The rate of the decrease in the bromate and bromide ion concentrations significantly slowed down during the next 20 or 30 min, because of the decline in the concentration
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Transmittance
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Fig. 2 FTIR spectra of the virgin FPA90-Cl resin (a) and magnetized FPA90-Cl resin (b)
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Fig. 3 XRD patterns of the pure Fe3O4 (a) and magnetized FPA90-Cl resin (b)
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The k values for the other two bromate and bromide ion concentrations are similar. This might occur because a higher concentration (200 and 400 μg L−1) results in a greater driving force for the ions being adsorbed (Chen et al. 2014). However, bromate and bromide ions are completely eliminated in a shorter time at low concentrations. In addition, the calculated qe values are close to the experimental values, which further verified the bromate and bromide ion removal follows the pseudo-first-order kinetic model.
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The effect of temperature on the bromate and bromide ion removal 0
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Fig. 4 Comparison of adsorption abilities for bromate (100 μg L ) and bromide ion and (300 μg L−1) between virgin resin (black line) and magnetized FPA90-Cl resin (red line) (adsorbent dosage, 1.0 g L−1; agitation speed, 300 rpm; pH, 7.0 ± 0.5)
difference between the liquid and solid phases. Subsequently, no significant change was observed in the bromate and bromide ion concentrations because the bromate and bromide ions were almost completely adsorbed. Comparing the three figures at equilibrium time shows that the bromate and bromide ion sorption capacities increased from 36.74 and 196.26 μg g−1 to 176.12 and 394.14 μg g−1, respectively, as the initial concentrations increased from 50 and 200 μg L−1 to 200 and 400 μg L−1, respectively. The time to reach equilibrium slightly increased. The resin dosage was adequate, and many vacant adsorption sites were still available in these cases. Though the adsorption sites were initially vacant, the driving force of the concentration difference played an important role in the first stage of adsorption. We determined that the bromate and bromide ion removal rates were the highest in the first 20 min when the initial concentrations were 200 and 400 μg L−1, respectively. To quantitatively evaluate the removal efficacy of the adsorbate on the magnetic resin, a pseudo-first-order kinetic model was applied to the bromate and bromide ion adsorption process. The kinetic rate equation can be expressed by Eq. (3): lnðqe −qt Þ ¼ lnqe −kt
ð3Þ
where qe and qt are the amounts adsorbed at equilibrium and at time t, respectively, and k is the pseudo-first-order rate constant. The pseudo-first-order kinetic model parameters are presented in Table 1. The pseudo-first-order model well fits the bromate and bromide ion removal when the ions coexist based on the high correlation coefficients (R2 > 0.98). The k values at the lowest concentrations of bromate and bromide ion concentrations (50 and 200 μg·g−1) are the highest because the contents were below the detection limit after 60 min of adsorption.
The solution temperature is an important parameter in the adsorption of bromate and bromide ions on the magnetized FPA90-Cl resin. Experiments varying the solution temperature from 288 to 328 K were carried out. The results are presented in Fig. 6. With the increase in the solution temperature from 288 to 328 K, the amount of bromate and bromide ions adsorbed on the magnetic resin increased from 40.09 and 168.16 μg g−1 to 47.07 and 194.09 μg g−1, respectively. This indicated that increasing the solution temperature was beneficial for the bromate and bromide adsorption. The reason for this might be that the higher temperature accelerated the ion thermal motion, which promoted the ion exchange reaction. In addition, we concluded that the adsorption of bromate and bromide ions on the magnetic resin was endothermic. Similar results have been reported for the removal of phosphate by MIEX resin (Tang et al. 2013).
The effect of the adsorbent dosage on the bromate and bromide ion removal efficiency A batch of experiments was performed to investigate the bromate and bromide ion removal efficiency with different magnetized FPA90-Cl resin dosages from 0.2 to 1.0 g L−1. The results presented in Fig. 7a show that the bromate and bromide ion removal rates rapidly increased from 76.25 and 83.96% to 96.62 and 98.34%, respectively, as the resin dosage increased from 0.2 to 0.6 g L−1. However, the removal rate was nearly constant as the magnetic resin dosage increased from 0.6 to 1.0 g L−1. The number of accessible adsorption sites was not sufficient for the removal of bromate and bromide ions at concentrations of 100 and 300 μg L−1, respectively, when the magnetic resin dosage was below 0.4 g L−1. Nevertheless, with a high magnetic resin dosage (> 0.6 g L−1), the number of adsorption sites was adequate, but the sites were not utilized effectively. This phenomenon agreed with the findings of a previous report (Ding et al. 2012a). Figure 7a shows that the amounts of bromate and bromide ions adsorbed on the magnetic resin decreased from 1271.08
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Fig. 5 Effect of the contact time on the bromate and bromide ion adsorption (a–c) and pseudo-first-order kinetic model plots for bromate and bromide ion adsorption on the magnetized FPA90-Cl resin (d, e)
(bromate and bromide ion concentrations: a 50 and 200 μg L−1; b 100 and 300 μg L−1; c 200 and 400 μg L−1, respectively; adsorbent dosage, 1.0 g·L−1; temperature, 298 K; pH, 7.0 ± 0.5)
and 406.26 μg g−1 to 296.65 and 97.38 μg g−1, respectively. The decrease in the amount of bromate and bromide ions adsorbed on the magnetic resin may also be a result of the lower utilization as the adsorbent dosage increased. This conclusion is consistent with the argument that the adsorbent
surface sites are heterogeneous (Das et al. 2003). According to the surface site heterogeneity model, the surface is composed of sites with a spectrum of binding energies. At a low adsorbent dosage, all types of sites are entirely exposed, and the surface adsorption saturates more quickly, showing a
Environ Sci Pollut Res Table. 1 Pseudo-first-order model parameters for bromate and bromide ion adsorption on the magnetized FPA90-Cl resin
C0(μg·L−1)
Experimental qe(μg·g−1)
Calculated qe(μg·g−1)
k(min−1)
R2
SD(%)
BrO3−
39.68
36.74
37.21
0.058
0.9983
0.14
Br−
81.54 178.36 199.20
79.30 176.12 196.26
74.56 164.99 196.48
0.044 0.045 0.078
0.9856 0.9933 0.9984
0.19 0.12 0.20
280.53 405.51
277.18 394.14
296.65 402.27
0.064 0.065
0.9923 0.9998
0.29 0.30
higher qt value. However, at a higher adsorbent dosage, the availability of higher energy sites decreases as a larger fraction of lower energy sites are occupied, resulting in a lower qt value (Liao and Shi 2005). In addition, the adsorbent particles at higher dosages are more prone to aggregation, which can lead to a decreasing surface area and increasing diffusion path length (Daifullah et al. 2007). Therefore, the decline in the adsorption amount is partly due to these reasons. The solid-liquid distribution coefficients (KD) of bromate and bromide ion on the magnetic resin and in solution after adsorption were calculated by Eq. (4): KD ¼
ðC0 −Ct Þ V Ct W
ð4Þ
As Fig. 7b shows, the distribution coefficients, KD, of the bromate and bromide ions increased from 21.75 and 27.78 L g−1 to 68.15 and 109.63 L g−1, respectively, as the resin dosage increased from 0.2 to 0.6 g L−1 and then decreased from 68.15 and 109.63 L g −1 to 37.35 and 88.49 L g−1 as the dosage increased from 0.6 to 1.0 g L−1. The same reason can explain the change in the law of KD, i.e., the adsorptive sites are effectively used for the dosages from
200
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45 40
0.2 to 0.6 g L−1, but they are not for the dosages from 0.6 to 1.0 g L−1. Previous reports have also obtained similar results (Ding et al. 2012a; Lv et al. 2008). To maximize the utilization of the adsorption sites on the magnetic resin, 0.4 g L−1 was selected for the following equilibrium adsorption studies.
The effect of the initial solution pH on the bromate adsorption equilibrium The results from the batch experiments varying the initial solution pH from 2 to 11 are shown in Fig. 8. The bromate uptake on the magnetized FPA90-Cl resin increased from 311.27 to 493.05 μg g−1, as the pH increased from 2 to 6, possibly due to the introduction of chloride ions from the hydrochloric acid that was used to lower the initial solution pH, which retrained the ion exchange reaction between the bromate and chloride ions that were used as the mobile counter ion in the magnetic resin. As the initial solution pH continued to increase from 6 to 11, the uptake of bromate ions decreased from 493.05 to 317.47 μg g−1. This was because the hydroxyl ions from sodium hydroxide competitively adsorbed on the magnetic resin with the bromate ions. Previous research reported that hydroxyl ions exchanged on the surface of the MIEX resin when it was used at pH 11.0 based on the Fourier translation infrared spectroscopy (Ding et al. 2012a). Although the sorption of bromate ions on the magnetic resin was strongly pH dependent, the percentage of bromate removals was above 90% (not given in Fig. 8), and the residual concentrations were below 10 μg L−1 when the initial solution pH varied in the ranges of 4.0 to 9.0. Therefore, the magnetized FPA90-Cl resin can be used for bromate ion removal in natural water that generally has pH values from 4.0 to 9.0.
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The effect of the temperature on the bromate adsorption equilibrium
Temperature/(K)
Fig. 6 Effect of the temperature on the adsorption of bromate (50 μg L−1) and bromide ion (100 μg L−1) on the magnetized FPA90-Cl resin (adsorbent dosage, 1.0 g L−1; agitation speed, 300 rpm; pH, 7.0 ± 0.5)
As previously mentioned, the adsorption amount of trace levels of bromate ion at time t (qt) slightly increased as the temperature increased. Adsorption equilibrium
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a
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-
-1
E(Br ) E(BrO3 )
85
E/(%)
Fig. 7 Effect of the adsorbent dosage on the adsorption of bromate (100 μg L−1) and bromide ions (300 μg L−1) on the magnetized FPA90-Cl resin (temperature, 298 K; agitation speed, 300 rpm; pH, 7.0 ± 0.5)
600 80
300
75
0 0.2
0.4
0.6
0.8
1.0
-1
FPA90Cl dosage/(g L ) 120
b
100
-
Br BrO3
-1
Kd/(L g )
80
60
40
20
0.2
0.4
0.6
0.8
1.0
-1
FPA90Cl dosage/(g L )
Fig. 8 Effect of the initial solution pH on the adsorption of bromate ions on the magnetized FPA90-Cl resin (adsorbent dosage, 0.4 g L−1; temperature, 298 K; agitation speed, 300 rpm; contact time, 12 h)
80
480
70 60 50
-1
qe/( g g )
400
qe 40
residual concentration
360
30 320 20
-1
Residual concentration( g L )
440
280
10
2
4
6
8
pH
10
12
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a 120
80 -1
qe/(mg g )
experiments with different initial concentrations of bromate were carried out at 298, 313, and 328 K. As shown in Fig. 9, the higher the initial bromate concentration, the more the adsorption capacity increased with increasing temperature. In fact, the equilibrium adsorption capacity increased from 132.83 to 156.67 mg g−1 as the temperature increased from 298 to 328 K when the initial bromate concentration was 100 mg L−1. This phenomenon can be explained by the solution viscosity decreasing at higher temperatures, which improves the bromate diffusion across the external boundary layer and in the internal pores of the resins (Bissen et al. 2010). Therefore, the bromate adsorption process on the magnetized FPA90-Cl resin was again proven to be endothermic.
Experimental point Langmuir Freundlich Redlich-Peterson
40
0
0
10
20
30
40
-1
Ce/(mg L )
Bromate adsorption isotherms
160
b 120
-1
qe/(mg g )
An adsorption isotherm is important in an adsorption system, and it can clearly show the interaction pathway of pollutants with remediation materials (Xu et al. 2012). As well, isotherms can usually provide some insight into the sorption mechanism, surface properties, and affinity of the adsorbent (Ding et al. 2010). The adsorption equilibrium results for bromate on the magnetized FPA90-Cl resin at 298, 313, and 328 K are given in Fig. 10. The Langmuir and Freundlich two-parameter adsorption isotherm models and three-parameter adsorption isotherm model were used to fit the experimental data. The Langmuir isotherm model refers to homogeneous adsorption, i.e., all the sites possess an equal affinity for the adsorbate, and it assumes monolayer adsorption. The Freundlich isotherm model is applied to multilayer adsorption with a non-uniform distribution of the adsorption
80
Experimental point langmuir Freundlich Redlich-Peterson
40
0
0
10
20
30
40
-1
Ce/(mg L ) 160
c
120 160
-1
qe/(mg g )
140 120
Experimental point Langmuir Freundlich Redlich-Peterson
40
-1
qe/(mg g )
100
80
80 60
298K 313K 328K
40 20
0
0
10
20
30
40
-1
Ce/(mg L ) 0 0
10
20
30
40
Fig. 10 Adsorption isotherms of bromate ions on the magnetized FPA90l resin at a 298 K, b 313 K and c 328 K
-1
Ce/(mg L )
Fig. 9 Effect of the initial concentration and solution temperature on the adsorption of bromate ions on the magnetized FPA90Cl resin (adsorbent dosage, 0.4 g L−1; agitation speed, 300 rpm; pH, 7.0 ± 0.5; contact time, 24 h)
heat and affinities over a heterogeneous surface (Foo and Hameed 2010). The Redlich-Peterson isotherm model is a hybrid isotherm featuring both the Langmuir and
Environ Sci Pollut Res
Freundlich isotherms. These isotherms can be expressed by Eqs. (5)–(7) (Ding et al. 2012a): Langmuir :
Ce Ce 1 ¼ þ qe qmax qmax b
ð5Þ
logCe þ logk f ð6Þ n Ce ln KRP −1 ¼ β ln Ce þ lnαRP ð7Þ qe
Freundlich : logqe ¼ Redlich−Peterson :
where qmax (mg g−1) is the theoretical maximum bromate adsorption capacity, b is the Langmuir model constant, n and Kf are the Freundlich model constants, KRP and αRP are the Redlich-Peterson constants, and β is the Redlich-Peterson exponent. The corresponding parameters of the fitting results at different temperatures are shown in Table 2. A high correlation coefficient (R2) and low standard deviation (SD) represent a good regression; thus, the Freundlich isotherm model, with correlation coefficients of 0.9957, 0.9987, and 0.9983, respectively, is the best fit of the data at all temperatures. This indicates that the bromate ion removal process is similar to a multilayer adsorption and that the surface of the magnetic resin is heterogeneous. The value of n relates to the surface heterogeneity of the adsorbent and indicates whether the adsorption process is favorable. If n > 1, the adsorption conditions are favorable, and new adsorption sites are generated; n = 1 indicates that the adsorption is homogeneous, and there is no interaction between the adsorbed species; and n < 1 indicates that the adsorption is unfavorable, and the adsorption capacity decreases (Rauf et al. 2008). In Table 2, the values of n are 2.0, 2.0, and 1.96 at 298, 313, and 328 K, respectively,
and the adsorption of bromate on the magnetized FPA90Cl resin is favorable. The correlation coefficients for the Redlich-Peterson isotherm model, 0.9926, 0.9967, and 0.9943 at the corresponding temperatures, respectively, are slightly lower than those for the Freundlich isotherm model. Accordingly, the Redlich-Peterson hybrid isotherm model also depicts the equilibrium adsorption well.
Bromate adsorption thermodynamics The energy change of the bromate adsorption process can be quantitatively analyzed through adsorption thermodynamics. The corresponding thermodynamic parameters such as the standard enthalpy change (ΔH0, J mol−1 K−1), standard entropy change (ΔS0, kJ mol−1), and standard Gibbs-free energy change (ΔG0, kJ mol−1) were calculated by Eqs (8)–(9): ΔS0 ΔH0 − R RT
ð8Þ
ΔG0 ¼ ΔH0 −TΔS0
ð9Þ
lnKD ¼
where KD (L g−1) is the distribution coefficient calculated from Eq.(4), R (8.314 J mol−1 K−1) is the universal gas constant, and T (K) is the temperature in Kelvin. The values of ΔH0 and ΔS0 for the bromate ions on the magnetic resin can be obtained from the slope and intercept of a plot of lnKD vs 1/T (presented in Fig. 11). In addition, ΔG0 can be calculated by Eq.(9). All the thermodynamic parameters are listed in Table 3. The negative values for the Gibbs-free energy change (ΔG0) at all the temperatures indicate that the sorption of bromate ions on the magnetic resin is spontaneous and thermodynamically feasible. In addition, ΔG0 is more negative at higher temperatures which indicates that the spontaneity
Table 2 Constants, correlation coefficients, and SD for the Langmuir, Freundlich, and Redlich-Peterson isotherm models
Langmuir qmax(mg·g−1) b R2 Freundlich Kf n R2 Redlich-Peterson KRP αRP β R2
298 K
313 K
1.5
328 K y=-613.71x+3.2838 2 R =0.999
1.4
176.86 0.07 0.9823
189.75 0.07 0.9847
216.64 0.06 0.9830
21.91 2.0 0.9957
23.45 2.0 0.9987
24.46 1.96 0.9983
39.18 0.98 0.65 0.9926
39.04 0.93 0.64 0.9967
39.64 0.90 0.63 0.9943
ln Kd
Isotherm model
1.3
1.2
1.1 0.0030
0.0032
0.0034
0.0036
-1
1/T/(K )
Fig. 11 The plot used to calculate the thermodynamic parameters of bromate ion adsorption on the magnetized FPA90-Cl resin
Environ Sci Pollut Res 400
Table 3 Thermodynamic parameters for bromate adsorbed on the magnetized FPA90-Cl resin ΔH0 (kJ mol−1)
ΔS0 (J mol−1 K−1)
288 298
3.15 3.40
− 2.76 − 3.03
5.10
27.30
313
3.74
− 3.44
323 328
4.00 4.10
− 3.72 − 3.85
increases with increasing temperature (Zhao et al. 2011). The enhancement of spontaneity may be favorable to the adsorption rate and equilibrium adsorption capacity of bromate, which can be observed increasing with increasing temperature in Figs.6 and 9, respectively. Therefore, we can conclude that the adsorption of bromate is negatively correlated with the ΔG0 value. The standard enthalpy change (ΔH0) is positive, verifying that the sorption process is endothermic. In addition, the standard entropy change (ΔS0) is 27.30 J mol−1 K−1, showing the randomness at the solid-liquid interface increases during the sorption of bromate ions on the magnetic resin (Khambhaty et al. 2009). This may be due to the lower affinity of bromate ions for the active adsorption sites of the resin than that of the chloride ions used as the exchange ions.
The effect of coexisting anions on the bromate adsorption equilibrium Because there are various anions that exist in natural water bodies, the effects of several anions (PO43−, HCO3−, Cl−, NO3−, CO32−, and SO42−) on the removal of bromate ions by the magnetized resin were investigated in this study. Figure 12 shows that the bromate ion adsorption capacity gradually declined as the initial concentrations of the competing anions increased from 10 to 200 mg L−1. This can be explained by the competition between the anions and bromate ions for the active adsorption sites on the magnetic resin surface. The bromate ion uptake decreased from 381.0 μg g−1 without other anions to 29.46, 47.48, 54.65, 63.57, 105.81, and 129.26 μg g−1 with 200 mg L−1 of SO42−, CO32−, Cl−, NO3−, HCO3−, or PO43−, respectively. Figure 12 also shows that the order of the anion inhibition effects on bromate removal is as follows: SO42− > CO32− > Cl− > NO3− > HCO3− > PO43−. The larger affinities of sulfate and carbonate for the adsorption sites may be due to their divalent charges. As the mobile exchange ion, chloride has the largest effect on the bromate ion removal among the monovalent ions. Some of the bicarbonate and carbonate ions tend to be hydrolyzed. For phosphate, the large ionic size influences the electrostatic interactions and some phosphate ions are hydrolyzed to monatomic and divalent ions (Chen et al. 2010).
-
HCO3 300
-
Cl NO3 2-
CO3
-1
ΔG0 (kJ mol−1)
qt/(mg g )
KD (L g−1)
2-
200
SO4
100
0 0
50
100
150
200 -1
Concentrations of coexisting anions/(mg L )
Fig. 12 Effect of coexisting anions on the bromate ion adsorption (200 μg L−1) on the magnetized FPA90Cl resin (adsorbent dosage, 0.4 g L−1; temperature, 298 K; pH without any adjustment; contact time, 12 h)
Regeneration and recyclability of the magnetic resin The results of the adsorption-desorption recyclability of the magnetized FPA90-Cl resin are shown in Fig. 13. When the number of cycles is B0,^ the resin is fresh, and the adsorption efficiencies for bromate and bromide ions are 99.16 and 96.34%, respectively. The bromate and bromide ion removal efficiencies gradually declined as the number of cycles increased. This indicated that some adsorption sites that were saturated by the bromate and bromide ions were difficult to regenerate by the ion exchange action of a chloride solution. However, the bromate and bromide ion removal efficiencies of the regenerated resin were still 98.58 and 93.63%, respectively, after 5 cycles. Therefore, the magnetized FPA90-Cl 120 -
Br BrO3
100
Removal efficiency/(%)
T (K)
3-
PO4
80
60
40
20
0 0
1
2
3
4
5
Number of cycles
Fig. 13 Reusability efficiency of the regenerated magnetized FPA90Cl resin for 5 cycles (adsorbent dosage, 0.4 g L−1; temperature, 298 K; contact time, 24 h)
Environ Sci Pollut Res
resin is a promising adsorbent with a high reusability and can be used for the removal of high concentrations of bromate and bromide ions.
Conclusions The results showed that chemical co-precipitation is a promising method for magnetizing FPA90-Cl resin. The magnetized FPA90-Cl resin exhibited extremely high adsorption capacities for bromate and bromide ions at both low and high concentrations. The magnetized resin can be efficiently utilized over a wide pH range (4.0–9.0). For the magnetized FPA90-Cl resin, the higher the water temperature, the better the adsorption of bromate and its precursors. Though coexisting anions had different negative effects on the bromate ion removal, large or small, the magnetized FPA90-Cl resin effectively removed bromate from aqueous solution. In addition, the recycling experiments showed great recyclability of the magnetized FPA-90Cl resin. Based on the above advantages, the magnetized FPA90-Cl resin is a promising material for the removal of bromate and its precursors from drinking water in industry. Acknowledgements This work was supported by the Water Resource Science and Technology Innovation Program of Guangdong Province (201624), the 111 Project (B17018). We are grateful for the technical support from the Analytical and Testing Center, South China University of Technology, Guangzhou, China.
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