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Bromate removal from aqueous solutions by ordered mesoporous carbon a
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Chunhua Xu , Xiaohong Wang , Xiaolei Shi , Sheng Lin , Liujia Zhu & Yaming Chen a
School of Environmental Science and Engineering, Shandong University, Shandong Key Laboratory of Water Pollution Control and Resource Reuse, Jinan 250100, People's Republic of China b
Environmental Simulation and Pollution Control State Key Joint Laboratory, School of Environment, Tsinghua University, Beijing 100084, People's Republic of China Published online: 22 Nov 2013.
To cite this article: Chunhua Xu, Xiaohong Wang, Xiaolei Shi, Sheng Lin, Liujia Zhu & Yaming Chen (2014) Bromate removal from aqueous solutions by ordered mesoporous carbon, Environmental Technology, 35:8, 984-992, DOI: 10.1080/09593330.2013.857725 To link to this article: http://dx.doi.org/10.1080/09593330.2013.857725
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Environmental Technology, 2014 Vol. 35, No. 8, 984–992, http://dx.doi.org/10.1080/09593330.2013.857725
Bromate removal from aqueous solutions by ordered mesoporous carbon Chunhua Xua∗ , Xiaohong Wanga , Xiaolei Shib , Sheng Lina , Liujia Zhua and Yaming Chena a School
of Environmental Science and Engineering, Shandong University, Shandong Key Laboratory of Water Pollution Control and Resource Reuse, Jinan 250100, People’s Republic of China; b Environmental Simulation and Pollution Control State Key Joint Laboratory, School of Environment, Tsinghua University, Beijing 100084, People’s Republic of China
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(Received 2 June 2013; final version received 14 October 2013 ) We investigated the feasibility of using ordered mesoporous carbon (OMC) for bromate removal from water. Batch experiments were performed to study the influence of various experimental parameters such as the effect of contact time, adsorbent dosage, initial bromate concentration, temperature, pH and effect of competing anions on bromate removal by OMC. The adsorption kinetics indicates that the uptake rate of bromate was rapid at the beginning: 85% adsorption was completed in 1 h and equilibrium was achieved within 3 h. The sorption process was well described with pseudo-second-order kinetics. The maximum adsorption capacity of OMC for bromate removal was 17.6 mg g−1 at 298 K. The adsorption data fit the Freundlich model well. The amount of bromate removed was found to be proportional to the influent bromate concentration. The effects of competing anions and solution pH (3–11) were negligible. These limited data suggest that OMC can be effectively utilized for bromate removal from drinking water. Keywords: bromate; adsorption; ordered mesoporous carbon; sorption isotherms; kinetics
1.
Introduction Bromate (BrO− 3 ) is a genotoxic chemical species and a possible carcinogen formed during ozonation of drinking water.[1–4] Ozonation is a disinfection method that can destroy microorganisms, reduce colour and total organic carbon. In the process, bromate can be formed through both a molecular ozone pathway and a free radical pathway depending on dissolved organic carbon, Br− content, alkalinity and pH of the source water.[5] Bromate usually exists in its ionic form (BrO− 3 ) in water. Based on provisional WHO guidelines, the maximum contaminant level (MCL) of bromate is 25 μg L−1 . In the EU, the USA and China, the MCL of bromate in drinking water must not exceed a level of 10 μg L−1 according to proposed EU directives, US regulations and Chinese water sanitary standards (GB 57492006; GB 8537-2008). The United States Environmental Protection Agency (US EPA) has classified bromate as a possibly carcinogenic substance to humans by the oral route of exposure.[6] Based on a linearized multistage model, the concentrations of BrO− 3 in drinking water associated with an excess lifetime cancer risk of 10−4 , 10−5 and 10−6 are 30, 3 and 0.3 μg L−1 , respectively.[7] Other toxic effects of bromate include vomiting, abdominal pain, nausea and diarrhea, varying degrees of central nervous system depression and hemolytic anemia.[8] Therefore, an efficient method is required for bromate removal from drinking water.
∗ Corresponding
Bromate control options have been predicated upon minimization and removal strategies. Until the last cen− tury, most BrO− 3 control methods involved inhibiting BrO3 formation through pH lowering, ammonia addition, hydrogen peroxide addition, and modified ozone contactor design and operation.[9] One approach is to remove the bromate precursors, such as bromide and natural organic matter, before the ozonation process.[4,10] Another is to control the bromate formation during ozonation through pH control by adding ammonia or hydrogen peroxide, and by modifying ozonation operation.[11,12] A third approach is using physical and chemical methods to remove bromate after ozonation. Most research has been focused on the use of activated carbons (granular-activated carbon (GAC) or powdered-activated carbon (PAC)) to remove bromate owing to the high removal efficiency of activated carbon.[5,13–17] It has been widely accepted that the removal of BrO− 3 by activated carbon occurs by the following steps: bromate is adsorbed first, then reduced to hypobromite (BrO− ) and finally reduced to bromide (Br− ) on the activated carbon surface.[5] However, Asami et al. [18] and Huang and Cheng [15] showed that the BrO− 3 removal rate substantially decreased with the transition from fresh GAC to biological-activated carbon. In addition, the adsorption capacity of GAC is carbon-specific and depends on the chemical condition of the source water.[16]
author. Email: [email protected]; [email protected]
© 2013 Taylor & Francis
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Environmental Technology Ordered mesoporous carbon (OMC) materials are also a promising material for BrO− 3 removal from drinking water because of their large active surface area and high bromate adsorption capacity.[19–21] Various production methods such as arc discharge, laser ablation, chemical vapour deposition and template synthesis techniques are used to obtain mesoporous carbons in the single-wall, multi-wall or disordered-wall form.[22,23] OMC can be an excellent adsorbent for the removal of bromate because of its high efficiency and ecofriendly nature.[23] The objectives of this study were to prepare OMC and to test its performance for bromate removal from drinking water. OMC was synthesized using the mesoporous SiO2 molecular sieves known as SBA-15 [24] as templates. The OMC products were characterized using transmission electron microscopy (TEM) and zeta potential measurement. The adsorption capacities of OMC and the adsorption kinetics of BrO− 3 on OMC were measured. We assessed the effects of various parameters such as adsorbent dosage, pH, temperature and initial bromate concentrations on bromate removal. The adsorption process is described by different isotherms and kinetic models. The results give us a better understanding of bromate removal by OMC.
2. Experimental 2.1. Materials OMC used in this study was amorphous and prepared under laboratory conditions and was evaluated with batch experiments for removing BrO− 3 . The chemical reagents used in the preparation of OMC, such as tetraethyl orthosilicate (TEOS), sulphuric acid (H2 SO4 ) and sucrose (C12 H22 O11 ), were reagent-grade compounds obtained from Sinopharm Chemical Reagent Co. and Pluronic P-123 (EO20 PO70 EO20 , MW = 5800) was from Sigma-Aldrich Chemical Co. Ltd., the USA.
2.2. Synthesis of OMC OMC was synthesized according to the procedure reported by Jun et al. [25] First, SBA-15 was synthesized using the amphiphilic triblock copolymer poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (EO20 PO70 EO20 ; average molecular weight, 5800; Aldrich). A typical synthesis [26,27] was as follows: 4.0 g of the amphiphilic triblock copolymer was dispersed in 30 g of water and 120 g of a 2M hydrochloric acid (HCl) solution and stirred for 5 h. Thereafter, 9.5 g of TEOS was added slowly to the homogeneous solution under stirring. The resulting gel was aged at 40◦ C for 20 h and finally heated at 100◦ C for 24 h. In a second set of experiments, the gels were prepared as described above and crystallized at 110◦ C. After synthesis, the obtained solids
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were calcined in flowing air at 550◦ C to decompose the triblock copolymer. To make OMC, 1 g of SBA-15 was added to a solution obtained by dissolving 1.25 g of sucrose and 0.8 mL H2 SO4 in 5 mL H2 O. The mixture was placed in a drying oven for 6 h at 373 K, and subsequently the oven temperature was increased to 433 K and maintained at the same temperature for 6 h. The sample turned dark brown or black during the treatment in the oven. The silica template, containing partially polymerized and carbonized sucrose at this stage, was treated again at 373 and 433 K using the same drying oven after the addition of 0.8 g of sucrose, 0.5 mL H2 SO4 in 5 mL H2 O. The carbonization was completed by pyrolysis with heating to about 1173 K under vacuum. The carbon-silica composite obtained after pyrolysis was washed with 5 wt% hydrofluoric acid at room temperature to remove the silica template. The template-free carbon product thus obtained was filtered, washed with ethanol and dried at 393 K. 2.3. Characterization of OMC Images of the synthesized OMC were obtained using a TEM (JEM-100CX II, Japan). The porosity of the particles was measured by the nitrogen sorption technique using a surface area and porosimetry analyzer (JW-BK 122 W, Beijing Jingweigaobo S&T Co. Ltd., China) at liquid nitrogen temperature (77 K). Zeta potential measurements were performed on a JS94H Zetamaster (Shanghai Powereach Instrument Co. Ltd., Shanghai). Ten sorbent samples (∼20 mg each) were placed in 50-mL glass conical flasks with 20 mL of 0.1 M NaCl. OMC was titrated to different pH from 3 to 11 with 0.1 M HCl and 0.1 M sodium hydroxide (NaOH). Zeta potential measurements were done immediately after the pH titration. Each sample was measured three times to provide an average reading. Between the samples, the cell was flushed with 20 mL of deionized water. Zeta potential was then plotted against pH. The point of zero charge (PZC), is physical chemistry, is a concept relating to the phenomenon of adsorption, and it describes the condition when the electrical charge density on a surface is zero. This method has been described in detail in the literature.[28] 2.4. Adsorption experiments The adsorption of bromate on OMC was studied at room temperature (20 ± 2◦ C) by batch experiments. The chemical reagents used in the adsorption experiments were reagent grade and are commercially available. NaBrO3 (AR Grade, Sinopharm Chemical Reagent Co., China) was used to represent bromate. All adsorption experiments of bromate to OMC were conducted with 100 mL of bromate solution in 250 mL sealed Erlenmeyer flasks with agitation on a thermostated rotary shaker (SHZ-88, Jintan Medical Instrument Factory, China). In each experiment, the solution prepared
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at a predetermined bromate concentration using deionized water was first filled into the flasks. The pH in each flask was adjusted using 0.1 M HCl (AR Grade, Fuyu Chemical Reagent Co., Tianjin, China) and 0.1 M NaOH (AR Grade, Jinan Reagent Factory, China) within ±0.1 of the predetermined pH. In the system, the temperature was controlled at 298 K, and the agitation speed was set at 150 rpm for all the experiments. The pH was measured before and after the experiments by a pH meter (pHS3C, Shanghai Precision & Scientific Instrument Co. Ltd., China). Preliminary experiments indicated the adsorption equilibrium can be established within 4 h for the bromate system. Single-column ion chromatography (IC1010, Shanghai Tianmei Science Instrument Co. Ltd., China) with a low-conductivity mobile phase was employed to determine the bromate concentration in the solution. Through a mass balance calculation, the adsorbed amount of bromate on the OMC was then calculated. All the samples were filtrated using 0.22 μm disposable membrane filters (Shanghai Mili Membrane Separation Technology Co. Ltd., China) before analysis. To assure that the interaction between the conical flasks and bromate solution is negligible and the sampling procedure is appropriate, blank experiments were also conducted. Bromate remained nearly constant (98–100% of original concentrations) during the time scale of this study, suggesting that the interaction of the bromate with the conical flasks can be neglected. The amount of bromate adsorbed (qe in μg g−1 ) was determined as follows: qe =
(C0 − C)V , m
(1)
where C0 and C (μg L−1 ) are the initial and equilibrium concentrations of bromate in solution, V (L) is the volume of solution and m (g) is mass of the adsorbent. The adsorption was studied as a function of contact time, initial bromate concentration, temperature, pH and the effect of competing anions. Concentrations of BrO− 3 in drinking water after ozonation of water containing background Br− ion have been reported as 0–0.2 mg L−1 .[29] Pilot and full-scale drinking water processes have also shown as 0.15 mg L−1 .[30] In this work, to achieve the maximum adsorption capacity of OMC, the initial bromate concentrations were set to 1 mg L−1 . The effect of contact time (5 min–12 h) with different absorbent dosages (0.01, 0.02, 0.03, 0.05, 0.06 and 0.07 g) was examined with 100 mL initial bromate concentrations of 1 mg L−1 at pH 6.0–7.0. The adsorption isotherm was studied by varying the initial bromate concentrations from 0.1 to 10 mg-total bromate L−1 with 0.6 g L−1 absorbent at pH 6.0–7.0. The effect of equilibrium pH was investigated by adjusting the solution pH from 3 to 11 using 0.1 M HCl and 0.1 M NaOH at an initial bromate concentration of 1 mg L−1 and 0.6 g L−1 dosage.
The effect of competing anions (chloride, nitrate, carbonate, sulphate and phosphate) on bromate adsorption was investigated by performing bromate adsorption at the same fixed bromate concentration (1 mg L−1 ), and adding initial competing anion concentrations of 1–20 mg L−1 at pH 6.0– 7.0 with an OMC dosage of 0.6 g L−1 . Anion concentrations were determined by ion chromatography. 2.4.1. Kinetics of adsorption The kinetics of adsorption was determined for OMC. Different dosage amounts of OMC (varying from 0.1 to 0.7 g L−1 ) were introduced into closed flasks containing 1 L of the bromate solutions and shaken for up to 12 h at 298 K to reach equilibrium, with a pH of 6 (pH of water). This set of experiments was done in triplicate. The reduction of bromate concentration in solution indicates that it was being adsorbed onto the carbon materials. By varying the dosage of OMC, a calibration curve can be obtained using the experimental data. The pH changed minimally with time. Selected experiments were repeated and the results were found to be consistent. Non-linear fitting for the kinetic models was carried out in the software package (OriginPro.v8.0). The pseudo-second-order kinetic model [31] is expressed by the following equation dq = k2 (qe − q)2 , dt
(2)
where q and qe (mg g−1 ) are the amounts of BrO− 3 at time t and at the equilibrium, k2 (g mg−1 min−1 ) is the rate constant for the second-order model. −1 The concentration of BrO− 3 in the solid, q (mg g ), can be obtained from Equation (1). Upon integration, Equation (2) may be written as 1 1 t = + . qt qe2 k2 qe2 2
(3)
2.4.2. Adsorption isotherms Isotherms were obtained by measuring the BrO− 3 concentration at equilibrium. The classic equations of Freundlich and Langmuir (Equations (4) and (5), respectively) were used to fit the experimental equilibrium adsorption data (through linear fitting to their linearized forms): 1 (4) ln Ce , ln Qe = ln K + n Ce Ce 1 = + , Qe Qm K L Qm
(5)
where Ce (mg L−1 ) and Qe (mg g−1 ) are the equilibrium concentrations of bromate in the liquid and solid phases. KF and 1/n are Freundlich constants, associated with the adsorption capacity and adsorption intensity, respectively. KL is the Langmuir constant and Qm (mg g−1 ) is the maximum adsorption capacity.
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Figure 1. (a) TEM images of SBA-15 (left) and OMC (right) and (b) N2 adsorption–desorption isotherms for OMC (inset: the BJH pore size distribution).
3. Results and discussion 3.1. Characterization of OMC TEM was used to investigate the morphology and size distribution of pristine OMC. TEM shows an intact longrange periodic order (Figure 1(a)). Figure 1(b) shows the N2 adsorption–desorption isotherms for OMC and the BarrettJoyner-Halenda (BJH) pore size distribution derived from the desorption branch of the isotherm. The type IV isotherm indicates an interconnected porous system. Surface area at 77 K was calculated on the basis of the Brunauer–Emmet– Teller (BET) method. The BET surface areas were found to be 1061 m2 g−1 and the corresponding BJH desorption pore size was determined to be 3.6 nm.
3.2. Kinetics of bromate adsorption to OMC Bromate (1 mg L−1 ) adsorption on OMC was investigated with different OMC dosages (0.1, 0.2, 0.3, 0.5, 0.6 and 0.7 g L−1 ), at initial pHs of 5.5–6.5 (Figure 2). As can be seen, at concentrations of OMC lower than 0.6 g L−1 , high
adsorption of bromate was not observed over a 4-h period. However, at dosages of 0.6 and 0.7 g L−1 OMC, equilibrium adsorption of BrO− 3 occurred after 240 and 120 min, respectively. A word to cover here: the probably reduction product bromide was not detected after all adsorption experiments. Thus, we did not discuss the bromide ion in the following section. In the case of PAC/GAC, this equilibrium time was significantly longer (a few days).[13,17,32,33] From the timeand dosage-dependent variations of equilibria, we conclude that 0.1–0.5 g L−1 of OMC does not have enough reactive −1 surfaces to adsorb BrO− of OMC provides 3 , but 0.6 g L enough amounts of reactive surface for adsorbing the full amount of BrO− 3 . The decrease in equilibrium time as OMC dosage increased may be due to the abundant adsorption sites available at higher dosages. The adsorption kinetics of BrO− 3 on OMC was evaluated (Figure 3). The concentration decreased dramatically in the first 1 h then a slow but gradual removal of bromate was observed until equilibrium was reached after 4 h. Usually, a longer period of time is needed for equilibrium;
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1.0 blank blank -1 –1 0.1 0.1ggLL -1 –1 0.5 0.5 g L
C/C0
0.8
–1
-1 0.2ggLL –1 0.6 g L
Pseudo-second-order model
–1
-1 0.3 0.3 g L -1 –1 0.7 0.7 g L
Dosage (g L−1 )
qe
(m g−1 )
k2 −1 (g mg min−1 )
qe,2 (mg g−1 )
R2
0.0134 0.0154 0.0259 0.0448 0.0619 0.0970
4.5282 3.6212 2.7453 1.9439 1.6066 1.2864
0.9931 0.9977 0.9870 0.9979 0.9944 0.9998
0.6
0.1 0.2 0.3 0.5 0.6 0.7
0.4
0.2
0.0
0
100
200
300
400
500
Time (min)
Figure 2. Effect of adsorbent dosage on bromate removal by −1 OMC. Reaction conditions: initial BrO− 3 1000 μg L , reacted for 12 h at 298 K.
120
0.1 g L–1 0.2 g L–1 0.3 g L–1 0.5 g L–1 0.6 g L–1 0.7 g L–1
100 80 60
t / qt
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Kinetic parameters for adsorption of bromate on OMC.
40 20 0 0
50
t
100
150
Figure 3. Kinetics of bromate adsorption onto different amounts of OMC fit to the pseudo-second-order kinetic model. Reaction −1 conditions: initial BrO− 3 1000 μg L , reacted for 12 h at 298 K.
however, small adsorbent particle sizes were used in the experiments to eliminate mass transfer limitations, which accelerates the adsorption process, since the rate of adsorption is inversely proportional to the square of the adsorbent particle diameter.[34] The pseudo-second-order adsorption kinetics was determined using Equation (3). The parameters found for the second-order model are listed in Table 1. Figure 3 shows that the second-order model fits the adsorption data very well, since the plot of t/qe2 vs. t is linear and consequently the R2 values (shown in Table 1) are high in the case of second-order kinetics. Since the second-order fitting curves were consistent with the experimental data very well, only the second-order model parameters will be discussed here. High values of k
4.4837 3.5170 2.6669 2.0032 1.6914 1.1729
were found for mOMC = 0.6 g L−1 . Generally, the rate constant associated with the adsorption kinetics decreases for lower adsorbent dosages due to limited adsorption sites on OMC. A constant value of k should be observed if lower concentrations of BrO− 3 were used. 3.3. Influence of pH on bromate adsorption One of the critical parameter in the treatment of bromate by the sorption medium is pH. Figure 4(a) presents the adsorption results of the initial 1 mg L−1 BrO− 3 onto OMC, with variations in the initial pH and final pH causing corresponding changes in the reaction solution. An obvious increase in bromate removal efficiency from initial pH 3.0 to 6.0 was observed, and a stable removal zone occurred between initial pH 6.0 and 10.0. However, the value decreases sharply at pH 11.0. The maximum adsorption of BrO− 3 is 1.312 mg L−1 from the experimental data at pH 6.0. The electrophoretic mobility of BrO− 3 -treated OMC at different initial pH (5.0–10.0) solutions was also measured to determine the PZC, at which net surface charge is zero. PZC of BrO− 3 -coated OMC was found to be in the range of 5.2–8.0 (Figure 4(b)). Therefore, we concluded that OMC becomes more positively charged as the pH decreases from its PZC. The positive charges on OMC are fully titrated by pH 11.0, producing a negatively charged surface which repels the negatively charged bromate. The relationship between the initial and final pH of BrO− 3 reacted OMC (Figure 4(a)) shows that regardless of the initial pH, the final pH ranged from 3.4 to 3.8 at an initial pH around 6.0–10.0, until an initial pH of 11.0 which only decreased to 10.4. Then a stable pH zone was formed because a very insignificant change of pH occurred in that region. This may be due to (1) the low protonation of OMC at higher pH lowering the final pH while OMC reacting with BrO− 3 ; (2) a buffer effect of OMC. In this case, the negatively charged BrO− 3 adsorbed on OMC and neutralized the positive charges. 3.4. Effects of temperature and adsorption isotherms In order to evaluate the adsorption potential of OMC for bromate removal, the equilibrium adsorption of bromate was
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(a) 90 11 80 10 70
Removal efficiency (%)
9 60 7 40
Removal efficiency Final pH
30
6
Final pH
8 50
5 20 4 10
(stable pH zone)
3
0 2 2
3
4
5
6
7
8
9
10
11
12
Initial pH
(b) 0
Zeta potential (mV)
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–10 –20 –30 –40 –50
OMC with BrO 3–
–60 5
6
7
8
9
10
11
Initial pH
Figure 4. (a) Effect of initial pH on BrO− 3 adsorption onto OMC and (b) electrophoretic mobility of bromate treated with OMC with respect to initial pH. Reaction conditions: initial BrO− 3 1000 μg L−1 , 0.6 g L−1 OMC, reacted for 12 h at 298 K.
studied as a function of bromate concentration at different temperatures (288, 298 and 308 K). In general they fit both the Langmuir and the Freundlich equations reasonably well. The plots of effect of temperature on bromate adsorption are shown in Figure 5(a). The equilibrium can be achieved as long as bromate concentration was increased. It is also clear from Figure 5 that initially the isotherm rises sharply indicating that plenty of readily accessible sites are available for adsorption. However, as the concentration increased, site saturation of OMC occurred and the plot reached a plateau indicating that no more sites were available for adsorption. Adsorption capacities of 18.472, 17.590 and 16.109 mg g−1 were observed for bromate on OMC at 288, 298 and 308 K, respectively. OMC shows higher chemical affinity for bromate removal compared with other conventional GAC adsorbents (2–7 mg g−1 ) for bromate.[35] The adsorption isotherms at different temperatures reveal that temperature does not show much influence on bromate adsorption by OMC. The adsorption equilibrium data were further analysed using the Langmuir and Freundlich models (Section 2.4). The Freundlich plots between log Qe and log Ce for the adsorption of bromate are shown in Figure 5(b) and 5(c).
Figure 5. Effect of temperature on bromate adsorption and sorption isotherms. (a) Plots of adsorption capacity vs. equilibrium concentration at different temperatures, (b) the Freundlich isotherm and (c) the Langmuir isotherm. Reaction conditions: 0.6 g L−1 OMC, pH 6.0–7.0, reacted for 12 h.
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Table 2. The Freundlich and Langmuir constants for the adsorption of bromate at different temperatures. Freundlich isotherm T (K)
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288 298 308
1/n
KF
R2
0.535 7.379 0.976 0.568 6.823 0.988 0.587 5.633 0.966
Langmuir isotherm Qm
(mg g−1 )
23.546 24.378 22.936
K
(L mg−1 ) 0.482 0.416 0.319
Thermodynamic parameters R2 0.890 0.986 0.952
The values of KF and 1/n were obtained from the slope and intercept of the linear Freundlich plots and are listed in Table 2. The R2 values from 0.96 to 0.98 at the different temperatures studied indicate that the Freundlich model was more applicable to the present study than the Langmuir model. Therefore, bromate is adsorbed in the form non-linear even far before the saturation of the OMC sites. 3.5. Thermodynamics of adsorption The thermodynamic constants – standard free energy change (G 0 ), enthalpy change (H 0 ) and entropy change (S 0 ) – were calculated to evaluate the thermodynamic feasibility of the process and to confirm the nature of the sorption process (Table 3). The thermodynamic parameters were calculated using Equations (6)–(8) and reported in Table 3[36] G = −RT ln K, H + Ce , RT G = H − T S, ln K =
Table 3. Thermodynamic parameters for adsorption of BrO− 3 at different temperatures.
(6) (7) (8)
where G is the Gibbs free energy change, R is the ideal gas constant (4.187 J mol−1 K−1 ), T is the absolute temperature (K), K is the Langmuir or Freundlich isotherm constant, H is the enthalpy change and S is the entropy change. The free energy change (G) was negative for all temperatures investigated. This suggests that the adsorption of BrO− 3 onto OMC is a spontaneous adsorption process. Meanwhile, the value of G decreased with decreasing temperature, indicating that the adsorption became more favourable at lower temperature, consistent with the results in the adsorption equilibrium study. The absolute value of G was < 20 kJ mol−1 , indicating the involvement of physical adsorption.[37] In addition, the measured decrease in bromate adsorption with increasing temperature is evidence of an exothermic adsorption reaction. 3.6. Effects of competing anions Drinking water and surface water contain several anions which could compete with bromate for adsorption on OMC. Therefore, it is important to study the effect of the competing anions which are generally present in the groundwater.
T (K)
ln KF
G (kJ mol−1 )
S (kJ K−1 mol−1 )
H (kJ mol−1 )
288 298 308
2.00 1.92 1.73
−2.41 −2.40 −2.23
−0.01
−5.14
The removal of bromate in the presence of individual competing anions, such as chloride (Cl− ), nitrate (NO− 3 ), 2− 2− phosphate (PO3− 4 ), sulphate (SO4 ) and carbonate (CO3 ), was investigated with the concentrations of anions at 1 and 20 mg L−1 (Figure S1, Supplementary information). The pH was controlled at 4.5. It is clear from the figure that the competing anions did not show great influence on bromate sorption at the lower concentration (1 mg L−1 ) of competing anions. The presence of 1 mg L−1 of competing anions such as Cl− , PO3− 4 − and SO2− 4 had some little effect on BrO3 uptake, lowering it to 77% of bromate alone, because of occupation of the reactive sites of OMC. However, when concentrations were further increased (up to 20 mg L−1 ), these anions started reducing the bromate adsorption onto OMC. The relative bromate adsorption dropped to 61.3%, 56.9% and 29.4% − − in the presence of 20 mg L−1 SO2− 4 , Cl and NO3 ions, respectively. Relative bromate adsorption was only 15.8% 2− and 6.89% in the presence of 20 mg L−1 PO3− 4 and CO3 , which may be due to the occupation of reactive surface sites of OMC to form outer-sphere complexation by CO2− 3 − 3− and PO3− 4 . Hence, it could be concluded that NO3 , PO4 − and CO2− 3 compete with BrO3 for sorption sites on OMC and reduce the adsorption capacity. Similar results were reported by Bhatnagar et al. [38] and Siddiqui et al. [5] 3− However, the concentration of NO− 3 and PO4 in daily drinking water is not usually high enough for these anions to influence bromate sorption.
4. Conclusions Results of the present study demonstrate the potential use of OMC for bromate removal from aqueous solutions. The adsorption isotherm fits the Freundlich model better than the Langmuir model. Kinetic results indicate that the sorption process can be defined favourably by the pseudo-secondorder kinetic model under the selected contact time range. Bromate removal was unaffected over a wide pH range (3–10) and competing anions with low level concentration almost did not hinder bromate sorption. However, anions 3− 2− especially NO− 3 , PO4 and CO3 with high concentration did affect bromate sorption. The thermodynamic parameters showed that the adsorption of bromate was a spontaneous
Environmental Technology and exothermic physical process. This experimental data can be further used to guide and optimize pilot-scale experiments that can enable the commercial exploitation of OMC for bromate removal from drinking water. Acknowledgements The authors thank Dr Pamela Holt for proofreading the manuscript.
Funding
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This research was supported by the National Natural Science Foundation of China [grant number 51102157], [grant number 51178255]; Jinan Science and Technology Bureau [grant number 201102042]; Shandong Key Scientific and Technological Projects of China [grant number 2010G0020606].
Supplementary data Supplemental data for this article accessed in online version at http://dx.doi.org/10.1080/09593330.2013.857725.
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[13] Bao ML, Griffini O, Santianni D, Barbieri K, Burrini D, Pantani F. Removal of bromate ions from water using granular activated carbon. Water Res. 1999;33:2959–2970. [14] Huang X, Gao NY, Lu PP. Bromate reduction by granular activated carbon. J Environ Sci. 2007;28:2264–2269. [15] Huang WJ, Cheng YL. Effect of characteristics of active carbon on removal of bromate. Sep Purif Technol. 2008;59: 101–107. [16] Kirisits MJ, Snoeyink VL, Kruithof JC. The reduction of bromate of granular activated carbon. Water Res. 2000;17:4250–4260. [17] Wang L, Zhang J, Liu JZ, He H, Yang M, Yu JW, Ma ZC, Jiang F. Removal of bromate ion using powdered activated carbon. J Environ Sci (China). 2010;22:1846–1853. [18] Asami M, Aizawa T, Morioka T, Nishijima W, Tabata A, Magara Y. Bromate removal during transition from new granular activated carbon (GAC) to biological activated carbon (BAC). Water Res. 1999;33:2797–2804. [19] Gadiou R, Saadallah SE, Piquero T, David P, Parmentier J, Vix-Guterl C. The influence of textural properties on the adsorption of hydrogen on ordered nanostructured carbons. Micropor Mesopor Mat. 2005;79:121–128. [20] Xia KS, Gao QM, Wu CD, Song SQ, Ruan ML. Activation, characterization and hydrogen storage properties of the mesoporous carbon CMK-3. Carbon. 2007;45: 1989–1996. [21] Zhou M, Guo LP, Lin FY, Liu HX. Electrochemistry and electrocatalysis of polyoxometalate-ordered mesoporous carbon modified electrode. Anal Chim Acta. 2007;587: 124–131. [22] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes: their properties and applications. 3rd ed. New York: Academic; 1996. [23] Kyotani T, Tsai LF, Tomita A. Formation of ultrafine carbon tubes by an anodic aluminum oxide film as a template. Chem Mater. 2000;7:1427–1428. [24] Kruk M, Jaroniec M, Ko CH, Ryoo R. Characterization of the porous structure of SBA-15. Chem Mater. 2000;12:1961– 1968. [25] Jun S, Joo SH, Ryoo R, Kruk M, Jaroniec M, Liu Z, Ohsuna T, Terasaki O. Synthesis of new, nanoporous carbon with hexagonally ordered mesostructure. J Am Chem Soc. 2000;122:10712–10713. [26] Hartmann M, Vinu A. Mechanical stability and porosity analysis of large-pore SBA-15 mesoporous molecular sieves by mercury porosimetry and organics adsorption. Langmuir. 2002;18:8010–8016. [27] Hu LH, Xiao SF, Guo CX, Wu PY, Nie PY. Preparation and characterization of tungsten carbide confined in the channels of SBA-15 mesoporous silica. J Phys Chem B. 2007;111;3599–3608. [28] Bjelopavlic M, Newcombe G, Hayes R. Adsorption of NOM onto activated carbon: effect of surface charge, ionic strength, and pore volume distribution. J Colloid Interf Sci. 1999;210:271–280. [29] von Gunten U, Hoigné J. Bromate formation during ozonation of bromide-containing waters: interaction of ozone and hydroxyl radical reactions. Environ Sci Technol. 1994;28;1234–1242. [30] Chitrakar R, Mizobuchi K, Sonoda A, Hirotsu T. Uptake of bromate ion on amorphous aluminum hydroxide. Ind Eng Chem Res. 2010;49:8726–8732. [31] Ho YS. Review of second-order models for adsorption systems. J Hazard Mater. 2006;136:681–689. [32] Mills A, Belghazi A, Rodman D, Hitchins P. The removal of bromate from potable water using granular activated carbon. Water Environ. 1996;10:215–217.
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[33] Chen WF, Zhang ZY, Li Q, Wang HY. Adsorption of bromate and competition from oxyanions on cationic surfactantmodified granular activated carbon (GAC). Chem Eng J. 2012;203:319–325. [34] Illies E, Tombácz E. The effect of humic acid adsorption on pH-dependent surface charging and aggregation of magnetite nanoparticles. J Colloid Interf Sci. 2006;295:15–123. [35] Huang WJ, Cheng YL. Effect of characteristics of activated carbon on removal of bromate. Sep Sci Technol. 2008;59:101–107.
[36] Ajmal M, Khan AH, Ahmad S, Ahmad A. Role of sawdust in the removal of copper (II) from industrial wastes. Water Res. 1998;32:3085–3091. [37] Ding L, Deng HP, Wu C, Han X. Affecting factors, equilibrium, kinetics and thermodynamics of bromide removal from aqueous solutions by MIEX resin. Chem Eng J. 2012;181:360–370. [38] Bhatnagar A, Choi YH, Yoon YJ, Shin YS, Jeon BH, Kang JW. Bromate removal from water by granular ferric hydroxide (GFH). J Hazard Mater. 2009;170:134–140.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Bromate removal from aqueous solutions by ordered mesoporous carbon a
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Chunhua Xu , Xiaohong Wang , Xiaolei Shi , Sheng Lin , Liujia Zhu & Yaming Chen a
School of Environmental Science and Engineering, Shandong University, Shandong Key Laboratory of Water Pollution Control and Resource Reuse, Jinan 250100, People's Republic of China b
Environmental Simulation and Pollution Control State Key Joint Laboratory, School of Environment, Tsinghua University, Beijing 100084, People's Republic of China Published online: 22 Nov 2013.
To cite this article: Chunhua Xu, Xiaohong Wang, Xiaolei Shi, Sheng Lin, Liujia Zhu & Yaming Chen (2014) Bromate removal from aqueous solutions by ordered mesoporous carbon, Environmental Technology, 35:8, 984-992, DOI: 10.1080/09593330.2013.857725 To link to this article: http://dx.doi.org/10.1080/09593330.2013.857725
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Environmental Technology, 2014 Vol. 35, No. 8, 984–992, http://dx.doi.org/10.1080/09593330.2013.857725
Bromate removal from aqueous solutions by ordered mesoporous carbon Chunhua Xua∗ , Xiaohong Wanga , Xiaolei Shib , Sheng Lina , Liujia Zhua and Yaming Chena a School
of Environmental Science and Engineering, Shandong University, Shandong Key Laboratory of Water Pollution Control and Resource Reuse, Jinan 250100, People’s Republic of China; b Environmental Simulation and Pollution Control State Key Joint Laboratory, School of Environment, Tsinghua University, Beijing 100084, People’s Republic of China
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(Received 2 June 2013; final version received 14 October 2013 ) We investigated the feasibility of using ordered mesoporous carbon (OMC) for bromate removal from water. Batch experiments were performed to study the influence of various experimental parameters such as the effect of contact time, adsorbent dosage, initial bromate concentration, temperature, pH and effect of competing anions on bromate removal by OMC. The adsorption kinetics indicates that the uptake rate of bromate was rapid at the beginning: 85% adsorption was completed in 1 h and equilibrium was achieved within 3 h. The sorption process was well described with pseudo-second-order kinetics. The maximum adsorption capacity of OMC for bromate removal was 17.6 mg g−1 at 298 K. The adsorption data fit the Freundlich model well. The amount of bromate removed was found to be proportional to the influent bromate concentration. The effects of competing anions and solution pH (3–11) were negligible. These limited data suggest that OMC can be effectively utilized for bromate removal from drinking water. Keywords: bromate; adsorption; ordered mesoporous carbon; sorption isotherms; kinetics
1.
Introduction Bromate (BrO− 3 ) is a genotoxic chemical species and a possible carcinogen formed during ozonation of drinking water.[1–4] Ozonation is a disinfection method that can destroy microorganisms, reduce colour and total organic carbon. In the process, bromate can be formed through both a molecular ozone pathway and a free radical pathway depending on dissolved organic carbon, Br− content, alkalinity and pH of the source water.[5] Bromate usually exists in its ionic form (BrO− 3 ) in water. Based on provisional WHO guidelines, the maximum contaminant level (MCL) of bromate is 25 μg L−1 . In the EU, the USA and China, the MCL of bromate in drinking water must not exceed a level of 10 μg L−1 according to proposed EU directives, US regulations and Chinese water sanitary standards (GB 57492006; GB 8537-2008). The United States Environmental Protection Agency (US EPA) has classified bromate as a possibly carcinogenic substance to humans by the oral route of exposure.[6] Based on a linearized multistage model, the concentrations of BrO− 3 in drinking water associated with an excess lifetime cancer risk of 10−4 , 10−5 and 10−6 are 30, 3 and 0.3 μg L−1 , respectively.[7] Other toxic effects of bromate include vomiting, abdominal pain, nausea and diarrhea, varying degrees of central nervous system depression and hemolytic anemia.[8] Therefore, an efficient method is required for bromate removal from drinking water.
∗ Corresponding
Bromate control options have been predicated upon minimization and removal strategies. Until the last cen− tury, most BrO− 3 control methods involved inhibiting BrO3 formation through pH lowering, ammonia addition, hydrogen peroxide addition, and modified ozone contactor design and operation.[9] One approach is to remove the bromate precursors, such as bromide and natural organic matter, before the ozonation process.[4,10] Another is to control the bromate formation during ozonation through pH control by adding ammonia or hydrogen peroxide, and by modifying ozonation operation.[11,12] A third approach is using physical and chemical methods to remove bromate after ozonation. Most research has been focused on the use of activated carbons (granular-activated carbon (GAC) or powdered-activated carbon (PAC)) to remove bromate owing to the high removal efficiency of activated carbon.[5,13–17] It has been widely accepted that the removal of BrO− 3 by activated carbon occurs by the following steps: bromate is adsorbed first, then reduced to hypobromite (BrO− ) and finally reduced to bromide (Br− ) on the activated carbon surface.[5] However, Asami et al. [18] and Huang and Cheng [15] showed that the BrO− 3 removal rate substantially decreased with the transition from fresh GAC to biological-activated carbon. In addition, the adsorption capacity of GAC is carbon-specific and depends on the chemical condition of the source water.[16]
author. Email: [email protected]; [email protected]
© 2013 Taylor & Francis
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Environmental Technology Ordered mesoporous carbon (OMC) materials are also a promising material for BrO− 3 removal from drinking water because of their large active surface area and high bromate adsorption capacity.[19–21] Various production methods such as arc discharge, laser ablation, chemical vapour deposition and template synthesis techniques are used to obtain mesoporous carbons in the single-wall, multi-wall or disordered-wall form.[22,23] OMC can be an excellent adsorbent for the removal of bromate because of its high efficiency and ecofriendly nature.[23] The objectives of this study were to prepare OMC and to test its performance for bromate removal from drinking water. OMC was synthesized using the mesoporous SiO2 molecular sieves known as SBA-15 [24] as templates. The OMC products were characterized using transmission electron microscopy (TEM) and zeta potential measurement. The adsorption capacities of OMC and the adsorption kinetics of BrO− 3 on OMC were measured. We assessed the effects of various parameters such as adsorbent dosage, pH, temperature and initial bromate concentrations on bromate removal. The adsorption process is described by different isotherms and kinetic models. The results give us a better understanding of bromate removal by OMC.
2. Experimental 2.1. Materials OMC used in this study was amorphous and prepared under laboratory conditions and was evaluated with batch experiments for removing BrO− 3 . The chemical reagents used in the preparation of OMC, such as tetraethyl orthosilicate (TEOS), sulphuric acid (H2 SO4 ) and sucrose (C12 H22 O11 ), were reagent-grade compounds obtained from Sinopharm Chemical Reagent Co. and Pluronic P-123 (EO20 PO70 EO20 , MW = 5800) was from Sigma-Aldrich Chemical Co. Ltd., the USA.
2.2. Synthesis of OMC OMC was synthesized according to the procedure reported by Jun et al. [25] First, SBA-15 was synthesized using the amphiphilic triblock copolymer poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (EO20 PO70 EO20 ; average molecular weight, 5800; Aldrich). A typical synthesis [26,27] was as follows: 4.0 g of the amphiphilic triblock copolymer was dispersed in 30 g of water and 120 g of a 2M hydrochloric acid (HCl) solution and stirred for 5 h. Thereafter, 9.5 g of TEOS was added slowly to the homogeneous solution under stirring. The resulting gel was aged at 40◦ C for 20 h and finally heated at 100◦ C for 24 h. In a second set of experiments, the gels were prepared as described above and crystallized at 110◦ C. After synthesis, the obtained solids
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were calcined in flowing air at 550◦ C to decompose the triblock copolymer. To make OMC, 1 g of SBA-15 was added to a solution obtained by dissolving 1.25 g of sucrose and 0.8 mL H2 SO4 in 5 mL H2 O. The mixture was placed in a drying oven for 6 h at 373 K, and subsequently the oven temperature was increased to 433 K and maintained at the same temperature for 6 h. The sample turned dark brown or black during the treatment in the oven. The silica template, containing partially polymerized and carbonized sucrose at this stage, was treated again at 373 and 433 K using the same drying oven after the addition of 0.8 g of sucrose, 0.5 mL H2 SO4 in 5 mL H2 O. The carbonization was completed by pyrolysis with heating to about 1173 K under vacuum. The carbon-silica composite obtained after pyrolysis was washed with 5 wt% hydrofluoric acid at room temperature to remove the silica template. The template-free carbon product thus obtained was filtered, washed with ethanol and dried at 393 K. 2.3. Characterization of OMC Images of the synthesized OMC were obtained using a TEM (JEM-100CX II, Japan). The porosity of the particles was measured by the nitrogen sorption technique using a surface area and porosimetry analyzer (JW-BK 122 W, Beijing Jingweigaobo S&T Co. Ltd., China) at liquid nitrogen temperature (77 K). Zeta potential measurements were performed on a JS94H Zetamaster (Shanghai Powereach Instrument Co. Ltd., Shanghai). Ten sorbent samples (∼20 mg each) were placed in 50-mL glass conical flasks with 20 mL of 0.1 M NaCl. OMC was titrated to different pH from 3 to 11 with 0.1 M HCl and 0.1 M sodium hydroxide (NaOH). Zeta potential measurements were done immediately after the pH titration. Each sample was measured three times to provide an average reading. Between the samples, the cell was flushed with 20 mL of deionized water. Zeta potential was then plotted against pH. The point of zero charge (PZC), is physical chemistry, is a concept relating to the phenomenon of adsorption, and it describes the condition when the electrical charge density on a surface is zero. This method has been described in detail in the literature.[28] 2.4. Adsorption experiments The adsorption of bromate on OMC was studied at room temperature (20 ± 2◦ C) by batch experiments. The chemical reagents used in the adsorption experiments were reagent grade and are commercially available. NaBrO3 (AR Grade, Sinopharm Chemical Reagent Co., China) was used to represent bromate. All adsorption experiments of bromate to OMC were conducted with 100 mL of bromate solution in 250 mL sealed Erlenmeyer flasks with agitation on a thermostated rotary shaker (SHZ-88, Jintan Medical Instrument Factory, China). In each experiment, the solution prepared
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at a predetermined bromate concentration using deionized water was first filled into the flasks. The pH in each flask was adjusted using 0.1 M HCl (AR Grade, Fuyu Chemical Reagent Co., Tianjin, China) and 0.1 M NaOH (AR Grade, Jinan Reagent Factory, China) within ±0.1 of the predetermined pH. In the system, the temperature was controlled at 298 K, and the agitation speed was set at 150 rpm for all the experiments. The pH was measured before and after the experiments by a pH meter (pHS3C, Shanghai Precision & Scientific Instrument Co. Ltd., China). Preliminary experiments indicated the adsorption equilibrium can be established within 4 h for the bromate system. Single-column ion chromatography (IC1010, Shanghai Tianmei Science Instrument Co. Ltd., China) with a low-conductivity mobile phase was employed to determine the bromate concentration in the solution. Through a mass balance calculation, the adsorbed amount of bromate on the OMC was then calculated. All the samples were filtrated using 0.22 μm disposable membrane filters (Shanghai Mili Membrane Separation Technology Co. Ltd., China) before analysis. To assure that the interaction between the conical flasks and bromate solution is negligible and the sampling procedure is appropriate, blank experiments were also conducted. Bromate remained nearly constant (98–100% of original concentrations) during the time scale of this study, suggesting that the interaction of the bromate with the conical flasks can be neglected. The amount of bromate adsorbed (qe in μg g−1 ) was determined as follows: qe =
(C0 − C)V , m
(1)
where C0 and C (μg L−1 ) are the initial and equilibrium concentrations of bromate in solution, V (L) is the volume of solution and m (g) is mass of the adsorbent. The adsorption was studied as a function of contact time, initial bromate concentration, temperature, pH and the effect of competing anions. Concentrations of BrO− 3 in drinking water after ozonation of water containing background Br− ion have been reported as 0–0.2 mg L−1 .[29] Pilot and full-scale drinking water processes have also shown as 0.15 mg L−1 .[30] In this work, to achieve the maximum adsorption capacity of OMC, the initial bromate concentrations were set to 1 mg L−1 . The effect of contact time (5 min–12 h) with different absorbent dosages (0.01, 0.02, 0.03, 0.05, 0.06 and 0.07 g) was examined with 100 mL initial bromate concentrations of 1 mg L−1 at pH 6.0–7.0. The adsorption isotherm was studied by varying the initial bromate concentrations from 0.1 to 10 mg-total bromate L−1 with 0.6 g L−1 absorbent at pH 6.0–7.0. The effect of equilibrium pH was investigated by adjusting the solution pH from 3 to 11 using 0.1 M HCl and 0.1 M NaOH at an initial bromate concentration of 1 mg L−1 and 0.6 g L−1 dosage.
The effect of competing anions (chloride, nitrate, carbonate, sulphate and phosphate) on bromate adsorption was investigated by performing bromate adsorption at the same fixed bromate concentration (1 mg L−1 ), and adding initial competing anion concentrations of 1–20 mg L−1 at pH 6.0– 7.0 with an OMC dosage of 0.6 g L−1 . Anion concentrations were determined by ion chromatography. 2.4.1. Kinetics of adsorption The kinetics of adsorption was determined for OMC. Different dosage amounts of OMC (varying from 0.1 to 0.7 g L−1 ) were introduced into closed flasks containing 1 L of the bromate solutions and shaken for up to 12 h at 298 K to reach equilibrium, with a pH of 6 (pH of water). This set of experiments was done in triplicate. The reduction of bromate concentration in solution indicates that it was being adsorbed onto the carbon materials. By varying the dosage of OMC, a calibration curve can be obtained using the experimental data. The pH changed minimally with time. Selected experiments were repeated and the results were found to be consistent. Non-linear fitting for the kinetic models was carried out in the software package (OriginPro.v8.0). The pseudo-second-order kinetic model [31] is expressed by the following equation dq = k2 (qe − q)2 , dt
(2)
where q and qe (mg g−1 ) are the amounts of BrO− 3 at time t and at the equilibrium, k2 (g mg−1 min−1 ) is the rate constant for the second-order model. −1 The concentration of BrO− 3 in the solid, q (mg g ), can be obtained from Equation (1). Upon integration, Equation (2) may be written as 1 1 t = + . qt qe2 k2 qe2 2
(3)
2.4.2. Adsorption isotherms Isotherms were obtained by measuring the BrO− 3 concentration at equilibrium. The classic equations of Freundlich and Langmuir (Equations (4) and (5), respectively) were used to fit the experimental equilibrium adsorption data (through linear fitting to their linearized forms): 1 (4) ln Ce , ln Qe = ln K + n Ce Ce 1 = + , Qe Qm K L Qm
(5)
where Ce (mg L−1 ) and Qe (mg g−1 ) are the equilibrium concentrations of bromate in the liquid and solid phases. KF and 1/n are Freundlich constants, associated with the adsorption capacity and adsorption intensity, respectively. KL is the Langmuir constant and Qm (mg g−1 ) is the maximum adsorption capacity.
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Figure 1. (a) TEM images of SBA-15 (left) and OMC (right) and (b) N2 adsorption–desorption isotherms for OMC (inset: the BJH pore size distribution).
3. Results and discussion 3.1. Characterization of OMC TEM was used to investigate the morphology and size distribution of pristine OMC. TEM shows an intact longrange periodic order (Figure 1(a)). Figure 1(b) shows the N2 adsorption–desorption isotherms for OMC and the BarrettJoyner-Halenda (BJH) pore size distribution derived from the desorption branch of the isotherm. The type IV isotherm indicates an interconnected porous system. Surface area at 77 K was calculated on the basis of the Brunauer–Emmet– Teller (BET) method. The BET surface areas were found to be 1061 m2 g−1 and the corresponding BJH desorption pore size was determined to be 3.6 nm.
3.2. Kinetics of bromate adsorption to OMC Bromate (1 mg L−1 ) adsorption on OMC was investigated with different OMC dosages (0.1, 0.2, 0.3, 0.5, 0.6 and 0.7 g L−1 ), at initial pHs of 5.5–6.5 (Figure 2). As can be seen, at concentrations of OMC lower than 0.6 g L−1 , high
adsorption of bromate was not observed over a 4-h period. However, at dosages of 0.6 and 0.7 g L−1 OMC, equilibrium adsorption of BrO− 3 occurred after 240 and 120 min, respectively. A word to cover here: the probably reduction product bromide was not detected after all adsorption experiments. Thus, we did not discuss the bromide ion in the following section. In the case of PAC/GAC, this equilibrium time was significantly longer (a few days).[13,17,32,33] From the timeand dosage-dependent variations of equilibria, we conclude that 0.1–0.5 g L−1 of OMC does not have enough reactive −1 surfaces to adsorb BrO− of OMC provides 3 , but 0.6 g L enough amounts of reactive surface for adsorbing the full amount of BrO− 3 . The decrease in equilibrium time as OMC dosage increased may be due to the abundant adsorption sites available at higher dosages. The adsorption kinetics of BrO− 3 on OMC was evaluated (Figure 3). The concentration decreased dramatically in the first 1 h then a slow but gradual removal of bromate was observed until equilibrium was reached after 4 h. Usually, a longer period of time is needed for equilibrium;
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1.0 blank blank -1 –1 0.1 0.1ggLL -1 –1 0.5 0.5 g L
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k2 −1 (g mg min−1 )
qe,2 (mg g−1 )
R2
0.0134 0.0154 0.0259 0.0448 0.0619 0.0970
4.5282 3.6212 2.7453 1.9439 1.6066 1.2864
0.9931 0.9977 0.9870 0.9979 0.9944 0.9998
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Figure 2. Effect of adsorbent dosage on bromate removal by −1 OMC. Reaction conditions: initial BrO− 3 1000 μg L , reacted for 12 h at 298 K.
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0.1 g L–1 0.2 g L–1 0.3 g L–1 0.5 g L–1 0.6 g L–1 0.7 g L–1
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t / qt
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Kinetic parameters for adsorption of bromate on OMC.
40 20 0 0
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t
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Figure 3. Kinetics of bromate adsorption onto different amounts of OMC fit to the pseudo-second-order kinetic model. Reaction −1 conditions: initial BrO− 3 1000 μg L , reacted for 12 h at 298 K.
however, small adsorbent particle sizes were used in the experiments to eliminate mass transfer limitations, which accelerates the adsorption process, since the rate of adsorption is inversely proportional to the square of the adsorbent particle diameter.[34] The pseudo-second-order adsorption kinetics was determined using Equation (3). The parameters found for the second-order model are listed in Table 1. Figure 3 shows that the second-order model fits the adsorption data very well, since the plot of t/qe2 vs. t is linear and consequently the R2 values (shown in Table 1) are high in the case of second-order kinetics. Since the second-order fitting curves were consistent with the experimental data very well, only the second-order model parameters will be discussed here. High values of k
4.4837 3.5170 2.6669 2.0032 1.6914 1.1729
were found for mOMC = 0.6 g L−1 . Generally, the rate constant associated with the adsorption kinetics decreases for lower adsorbent dosages due to limited adsorption sites on OMC. A constant value of k should be observed if lower concentrations of BrO− 3 were used. 3.3. Influence of pH on bromate adsorption One of the critical parameter in the treatment of bromate by the sorption medium is pH. Figure 4(a) presents the adsorption results of the initial 1 mg L−1 BrO− 3 onto OMC, with variations in the initial pH and final pH causing corresponding changes in the reaction solution. An obvious increase in bromate removal efficiency from initial pH 3.0 to 6.0 was observed, and a stable removal zone occurred between initial pH 6.0 and 10.0. However, the value decreases sharply at pH 11.0. The maximum adsorption of BrO− 3 is 1.312 mg L−1 from the experimental data at pH 6.0. The electrophoretic mobility of BrO− 3 -treated OMC at different initial pH (5.0–10.0) solutions was also measured to determine the PZC, at which net surface charge is zero. PZC of BrO− 3 -coated OMC was found to be in the range of 5.2–8.0 (Figure 4(b)). Therefore, we concluded that OMC becomes more positively charged as the pH decreases from its PZC. The positive charges on OMC are fully titrated by pH 11.0, producing a negatively charged surface which repels the negatively charged bromate. The relationship between the initial and final pH of BrO− 3 reacted OMC (Figure 4(a)) shows that regardless of the initial pH, the final pH ranged from 3.4 to 3.8 at an initial pH around 6.0–10.0, until an initial pH of 11.0 which only decreased to 10.4. Then a stable pH zone was formed because a very insignificant change of pH occurred in that region. This may be due to (1) the low protonation of OMC at higher pH lowering the final pH while OMC reacting with BrO− 3 ; (2) a buffer effect of OMC. In this case, the negatively charged BrO− 3 adsorbed on OMC and neutralized the positive charges. 3.4. Effects of temperature and adsorption isotherms In order to evaluate the adsorption potential of OMC for bromate removal, the equilibrium adsorption of bromate was
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–10 –20 –30 –40 –50
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Initial pH
Figure 4. (a) Effect of initial pH on BrO− 3 adsorption onto OMC and (b) electrophoretic mobility of bromate treated with OMC with respect to initial pH. Reaction conditions: initial BrO− 3 1000 μg L−1 , 0.6 g L−1 OMC, reacted for 12 h at 298 K.
studied as a function of bromate concentration at different temperatures (288, 298 and 308 K). In general they fit both the Langmuir and the Freundlich equations reasonably well. The plots of effect of temperature on bromate adsorption are shown in Figure 5(a). The equilibrium can be achieved as long as bromate concentration was increased. It is also clear from Figure 5 that initially the isotherm rises sharply indicating that plenty of readily accessible sites are available for adsorption. However, as the concentration increased, site saturation of OMC occurred and the plot reached a plateau indicating that no more sites were available for adsorption. Adsorption capacities of 18.472, 17.590 and 16.109 mg g−1 were observed for bromate on OMC at 288, 298 and 308 K, respectively. OMC shows higher chemical affinity for bromate removal compared with other conventional GAC adsorbents (2–7 mg g−1 ) for bromate.[35] The adsorption isotherms at different temperatures reveal that temperature does not show much influence on bromate adsorption by OMC. The adsorption equilibrium data were further analysed using the Langmuir and Freundlich models (Section 2.4). The Freundlich plots between log Qe and log Ce for the adsorption of bromate are shown in Figure 5(b) and 5(c).
Figure 5. Effect of temperature on bromate adsorption and sorption isotherms. (a) Plots of adsorption capacity vs. equilibrium concentration at different temperatures, (b) the Freundlich isotherm and (c) the Langmuir isotherm. Reaction conditions: 0.6 g L−1 OMC, pH 6.0–7.0, reacted for 12 h.
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Table 2. The Freundlich and Langmuir constants for the adsorption of bromate at different temperatures. Freundlich isotherm T (K)
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288 298 308
1/n
KF
R2
0.535 7.379 0.976 0.568 6.823 0.988 0.587 5.633 0.966
Langmuir isotherm Qm
(mg g−1 )
23.546 24.378 22.936
K
(L mg−1 ) 0.482 0.416 0.319
Thermodynamic parameters R2 0.890 0.986 0.952
The values of KF and 1/n were obtained from the slope and intercept of the linear Freundlich plots and are listed in Table 2. The R2 values from 0.96 to 0.98 at the different temperatures studied indicate that the Freundlich model was more applicable to the present study than the Langmuir model. Therefore, bromate is adsorbed in the form non-linear even far before the saturation of the OMC sites. 3.5. Thermodynamics of adsorption The thermodynamic constants – standard free energy change (G 0 ), enthalpy change (H 0 ) and entropy change (S 0 ) – were calculated to evaluate the thermodynamic feasibility of the process and to confirm the nature of the sorption process (Table 3). The thermodynamic parameters were calculated using Equations (6)–(8) and reported in Table 3[36] G = −RT ln K, H + Ce , RT G = H − T S, ln K =
Table 3. Thermodynamic parameters for adsorption of BrO− 3 at different temperatures.
(6) (7) (8)
where G is the Gibbs free energy change, R is the ideal gas constant (4.187 J mol−1 K−1 ), T is the absolute temperature (K), K is the Langmuir or Freundlich isotherm constant, H is the enthalpy change and S is the entropy change. The free energy change (G) was negative for all temperatures investigated. This suggests that the adsorption of BrO− 3 onto OMC is a spontaneous adsorption process. Meanwhile, the value of G decreased with decreasing temperature, indicating that the adsorption became more favourable at lower temperature, consistent with the results in the adsorption equilibrium study. The absolute value of G was < 20 kJ mol−1 , indicating the involvement of physical adsorption.[37] In addition, the measured decrease in bromate adsorption with increasing temperature is evidence of an exothermic adsorption reaction. 3.6. Effects of competing anions Drinking water and surface water contain several anions which could compete with bromate for adsorption on OMC. Therefore, it is important to study the effect of the competing anions which are generally present in the groundwater.
T (K)
ln KF
G (kJ mol−1 )
S (kJ K−1 mol−1 )
H (kJ mol−1 )
288 298 308
2.00 1.92 1.73
−2.41 −2.40 −2.23
−0.01
−5.14
The removal of bromate in the presence of individual competing anions, such as chloride (Cl− ), nitrate (NO− 3 ), 2− 2− phosphate (PO3− 4 ), sulphate (SO4 ) and carbonate (CO3 ), was investigated with the concentrations of anions at 1 and 20 mg L−1 (Figure S1, Supplementary information). The pH was controlled at 4.5. It is clear from the figure that the competing anions did not show great influence on bromate sorption at the lower concentration (1 mg L−1 ) of competing anions. The presence of 1 mg L−1 of competing anions such as Cl− , PO3− 4 − and SO2− 4 had some little effect on BrO3 uptake, lowering it to 77% of bromate alone, because of occupation of the reactive sites of OMC. However, when concentrations were further increased (up to 20 mg L−1 ), these anions started reducing the bromate adsorption onto OMC. The relative bromate adsorption dropped to 61.3%, 56.9% and 29.4% − − in the presence of 20 mg L−1 SO2− 4 , Cl and NO3 ions, respectively. Relative bromate adsorption was only 15.8% 2− and 6.89% in the presence of 20 mg L−1 PO3− 4 and CO3 , which may be due to the occupation of reactive surface sites of OMC to form outer-sphere complexation by CO2− 3 − 3− and PO3− 4 . Hence, it could be concluded that NO3 , PO4 − and CO2− 3 compete with BrO3 for sorption sites on OMC and reduce the adsorption capacity. Similar results were reported by Bhatnagar et al. [38] and Siddiqui et al. [5] 3− However, the concentration of NO− 3 and PO4 in daily drinking water is not usually high enough for these anions to influence bromate sorption.
4. Conclusions Results of the present study demonstrate the potential use of OMC for bromate removal from aqueous solutions. The adsorption isotherm fits the Freundlich model better than the Langmuir model. Kinetic results indicate that the sorption process can be defined favourably by the pseudo-secondorder kinetic model under the selected contact time range. Bromate removal was unaffected over a wide pH range (3–10) and competing anions with low level concentration almost did not hinder bromate sorption. However, anions 3− 2− especially NO− 3 , PO4 and CO3 with high concentration did affect bromate sorption. The thermodynamic parameters showed that the adsorption of bromate was a spontaneous
Environmental Technology and exothermic physical process. This experimental data can be further used to guide and optimize pilot-scale experiments that can enable the commercial exploitation of OMC for bromate removal from drinking water. Acknowledgements The authors thank Dr Pamela Holt for proofreading the manuscript.
Funding
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This research was supported by the National Natural Science Foundation of China [grant number 51102157], [grant number 51178255]; Jinan Science and Technology Bureau [grant number 201102042]; Shandong Key Scientific and Technological Projects of China [grant number 2010G0020606].
Supplementary data Supplemental data for this article accessed in online version at http://dx.doi.org/10.1080/09593330.2013.857725.
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