In this study, we investigated the removal of phosphate from aqueous solutions using (vinylbenzyl)-trimethylammonium chloride (VBTAC) grafted onto poly(ethylene terephthalate) (PET) fibers (PET-g-VBTAC). Batch-mode experiments were conducted using various contact times, initial phosphate concentrations, temperatures, pH values, and competing anions, to understand phosphate sorption onto PET-g-VBTAC. The phosphate sorption capacity of PET-g-VBTAC increased with increasing solution pH and was highest near pH 7. The equilibrium data fitted the Langmuir isotherm model well. The maximum sorption capacity (qm) of PET-g-VBTAC for phosphate was 55.6–56.0 mg/g at 25–45 °C. The sorption process followed a pseudo-second-order kinetic model. The obtained values of the mean free energy indicated that sorption of phosphate on PET-g-VBTAC occurs via ion exchange. Thermodynamic parameters, enthalpy change, entropy change, and Gibb's free energy, confirmed that phosphate sorption was spontaneous and endothermic. The adverse effects of competing anions on phosphate removal by PET-g-VBTAC were insignificant. These results demonstrate that PET-g-VBTAC effectively removes phosphate from aqueous solutions by ion exchange.

INTRODUCTION

Phosphorus is an essential nutrient for biomass growth, and is used globally in fertilizers (Schröder et al. 2011). However, discharge of excess phosphorus into water bodies such as rivers, lakes, and lagoons causes eutrophication, which has become a major environmental concern in recent decades (Barca et al. 2012). The discharge limits on phosphorus are becoming more stringent, to prevent eutrophication, and therefore, there is an increasing need for the development of advanced techniques for sewage and wastewater treatment. A number of methods for phosphorus removal have been reported, including biological treatment (Bassin et al. 2012), chemical precipitation (Moriyama et al. 2001), adsorption (Zhang et al. 2009), and ion exchange (Sendrowski & Boyer 2013). Among these, adsorption and ion exchange are the most promising techniques, because of their low initial costs, simple design, ease of operation, and insensitivity to coexisting pollutants (Rafatullah et al. 2010; Long et al. 2011; Xiong et al. 2011).

Although various sorbents for phosphorus removal from aqueous media have been investigated, including aluminum hydroxide (Tanada et al. 2003), magnetite (Daou et al. 2007), goethite (Chitrakar et al. 2006), iron oxide (Zeng et al. 2004), synthetic hydrotalcite (Kuzawa et al. 2006), and dolomite (Karaca et al. 2006) with varying degrees of success, the identification of more effective and less expensive sorbents continues to be a demand. Recently, graft polymerization sorbents have been widely developed with various functional groups, high efficiencies, and ion-selective properties because grafting can be restricted to the surface of the substrate polymer without affecting any bulk properties, and various shapes and qualities can be adopted for the substrate polymer.

In our previous paper (Na & Park 2014), (vinylbenzyl)trimethylammonium chloride (VBTAC) was successfully grafted onto polyester fibers coated with a surfactant, without any comonomer, by photoirradiation-induced graft polymerization using benzophenone (BP) as the photoinitiator. It was verified that a single-monomer grafting system, in which VBTAC alone is directly grafted onto poly(ethylene terephthalate) (PET) fibers coated with surfactant, is more practical and effective for the preparation of VBTAC-containing anion-exchangers, and the resulting PET-g-VBTAC materials are as effective as commercial anion-exchange resins. However, the characteristics of anion sorption by PET-g-VBTAC fabric have not previously been reported in detail.

The objective of this study was to investigate the removal of phosphate from aqueous solutions using PET-g-VBTAC. Batch-mode experiments were conducted using various contact times, initial phosphate concentrations, and temperatures, to understand sorption of phosphate on PET-g-VBTAC. The effects of pH and the presence of competing anions were also studied.

MATERIALS AND METHODS

Materials and reagents

The PET–surfactant (PETs) fabric, of weight 80 g/m2, used as the substrate polymer for grafting was obtained from the Samsung Non-woven Fabric Co., Korea. It was coated with a succinate salt to improve the water uptake ability of the PET non-woven fabric. VBTAC (Acros Organics) and BP (Yakuri Pure Chemical Co., Kyoto, Japan) were used as the monomer and photoinitiator, respectively. Methanol and deionized water were used as solvents. All reagents were of the highest grade available and used without further purification. Extra-pure NaH2PO4, NaNO3, Na2SO4, NaHCO3, and NaCl were used to prepare anion solutions for the sorption experiments.

Preparation of PET-g-VBTAC fabric

PET-g-VBTAC fabrics were prepared by grafting VBTAC onto PETs non-woven fabrics, using the procedure described in our previous paper (Na & Park 2014).

The PETs fabric was cut into pieces of area 1 × 10 cm, and dried at 60 °C before grafting. The monomer solution was prepared using 80/20 (v/v)% water/methanol as the solvent, 0.7 M VBTAC, and 0.2 (w/v)% BP. One piece of PETs fabric was placed in a Pyrex glass tube containing 20 mL of the monomer solution, and then exposed to ultraviolet (UV) light for 3 h at 60 °C. The light source used was a 400 W high-pressure mercury lamp (Miya Electric Co., Cheonan, Korea), and the glass tubes were rotated and simultaneously revolved around the light source at a distance of 10 cm. After the grafting reaction, the samples were removed from the glass tubes and washed three times with hot water, extracted with methanol for 3 h in a Soxhlet apparatus to remove the unreacted monomer and homopolymer, and dried at 60 °C to constant weight. PET-g-VBTAC fabrics with degrees of grafting in the range 86 ± 9% were prepared and used for sorption experiments.

Characterization of PET-g-VBTAC fabric

The anion-exchange capacity (AEC; meq/g) of PET-g-VBTAC was determined by the standard method using 0.2 M NH4Cl and 0.2 M KNO3 solution (SSSA 1996). The surface area, total pore volume, and pore size distribution of the PET-g-VBTAC fabric were analyzed by N2 adsorption at a relative pressure of 0.3 at 77.3 K, using a system for surface area and porosity measurements (Micrometrics TriStar II 3020). The surface charge potential of the PET-g-VBTAC fabric was measured by potentiometric titration, as described elsewhere (Schwarz et al. 1984).

Sorption experiments

Sorption experiments were carried out under batch conditions; sorbent (0.1–0.2 g) was added to 100 mL high-density polyethylene bottles containing aqueous solutions (50–100 mL) of known ion species at known concentrations. The bottles were settled in an incubator shaker and then continuously shaken at 25–45 °C and 120 rpm for a predetermined time. After the predetermined sorption time, the solid and liquid phases were separated, and the anion concentrations in the liquid phase were determined. The initial and final anion concentrations in the liquid solutions were determined using a UV spectrometer (Shimadzu UV-2401PC) and ion chromatography (Waters LC). The initial pH of the solution was adjusted to the desired value by adding HCl or NaOH solution. In sorption selectivity measurements, 0.01 M acetate buffer solution (pH 4.7) was used as the matrix solution to maintain a constant pH value. The sorption capacity, q (mg/g), was calculated using the following mass balance equation: 
formula
1
where C0 and C are the initial and equilibrium liquid-phase concentrations of anions (mg/L), respectively, V is the volume of solution (L), and W is the dry weight of adsorbent used (g).

Adsorption kinetics

Batch experiments were conducted at three initial phosphate concentrations, namely 20, 50, and 80 mg/L, and a constant sorbent dosage of 1.0 g/L, to determine the sorption kinetics. The samples were shaken for 10–240 min at 25 °C in an incubator shaker. The obtained results were analyzed using pseudo-first-order and pseudo-second-order kinetic equations to find the best-fit kinetic model.

Adsorption isotherm

The phosphate isotherms for PET-g-VBTAC were investigated as follows. VBTAC (0.1 g) was added to various phosphate solution samples (100 mL) with concentrations in the range 10–100 mg/L. Each sample was shaken for 24 h at 25, 35, and 45 °C. After equilibrium time the obtained phosphate adsorption data were fitted to the Langmuir and Freundlich isotherm models.

Effect of coexisting anions

The adsorption capacity of PET-g-VBTAC was examined in the presence of coexisting anions like Cl, , , and . The experiments were conducted using a binary system, with acetic acid buffer solution (pH 4.7) as the matrix solution; the phosphate ion concentration was 50 mg/L, and the concentrations of each competing ion were 0, 20, 50, and 100 mg/L.

RESULTS AND DISCUSSION

Characteristics of PET-g-VBTAC fabric

The AEC of the PET-g-VBTAC fabric used as the sorbent in this study was 2.04 ± 0.06 meq/g. The multipoint Brunauer–Emmett–Teller surface area, total pore volume, and average pore diameter of the PET-g-VBTAC fabric were 1.83 m2/g, 0.0012 cm3/g, and 4.36 nm, respectively. The Barrett–Joyner–Halenda adsorption cumulative surface area of pores between 1.7 and 300 nm was 1.13 m2/g. The cumulative volumes of micropores (d ≤ 2.0 nm), mesopores (2.0 < d < 50 nm), and macropores (d ≥ 50 nm) were 11.3%, 75.8%, and 12.9%, respectively. The PET-g-VBTAC fabric can therefore be considered to be a mesoporous material. The point of zero charge (pHpzc) of the PET-g-VBTAC fabric was approximately pH 8.6 ± 0.2. This means that at pH less than 8.6, the PET-g-VBTAC fabric surface is positively charged. Positively charged surface sites on the sorbent favor the sorption of negatively charged anionic phosphates, based on electrostatic attraction.

Effect of pH on phosphate sorption

The solution pH has a strong influence on sorbate sorption. The effect of pH on the sorption of phosphate onto PET-g-VBTAC was examined at an initial phosphate concentration of 50 mg/L and a constant sorbent dosage of 1.0 g/L. The solution pH was controlled after sorption equilibrium was reached at 3.0–9.0 by adding HCl or NaOH solution. Figure 1 shows the results for the equilibrium sorption of phosphate onto PET-g-VBTAC as a function of the equilibrium pH of the solution. As shown in Figure 1, the sorption equilibrium increased with increasing solution pH, but decreased slightly when the solution pH was almost 9. This is explained by changes in the phosphate speciation and the surface charge on the sorbent. As the solution pH increases, , , and become the major phosphate species, and these are more favorable for sorption by the PET-g-VBTAC fabric. Simultaneously, the number of positively charged surface sites decreases with increasing solution pH, which is unfavorable for sorption of negatively charged anions. This is one of the reasons for the decreased sorption equilibrium of phosphate near pH 9.0.

Figure 1

Effect of pH on phosphate sorption onto PET-g-VBTAC fabric.

Figure 1

Effect of pH on phosphate sorption onto PET-g-VBTAC fabric.

Sorption kinetics

The plots of the phosphate sorption kinetic data are shown in Figure 2.

Figure 2

Sorption rates of phosphate onto PET-g-VBTAC fabric.

Figure 2

Sorption rates of phosphate onto PET-g-VBTAC fabric.

To interpret the experimental data, the time-dependent sorption data were analyzed using the linear forms of the pseudo-first-order and pseudo-second-order kinetic equations.

The Lagergren pseudo-first-order kinetic model is expressed as (Lagergren 1898) 
formula
2
where qe and qt are the amounts of sorbate sorbed at equilibrium and at time t (mg/g), respectively, and k1 is the rate constant of pseudo-first-order sorption (min−1). The values of k1 and qe, and the evaluated regression coefficients (r2), are presented in Table 1. Although the r2 values are reasonably high, the calculated qe values obtained from this equation are too low compared with the experimental qe values. This indicates that the sorption process does not follow the pseudo-first-order kinetic model.
Table 1

First- and second-order kinetic parameters for phosphate sorption onto PET-g-VBTAC fabric

  1st kinetic (<50 min) 2nd kinetic 
Conc. (mg/L) Exp. qe k1 qe r2 k2 qe r2 
20 14.29 0.0146 1.05 0.9814 0.0567 14.29 1.0000 
50 29.41 0.0229 2.35 0.9828 0.0553 28.94 1.0000 
80 38.46 0.0289 5.83 0.9611 0.0143 37.89 1.0000 
  1st kinetic (<50 min) 2nd kinetic 
Conc. (mg/L) Exp. qe k1 qe r2 k2 qe r2 
20 14.29 0.0146 1.05 0.9814 0.0567 14.29 1.0000 
50 29.41 0.0229 2.35 0.9828 0.0553 28.94 1.0000 
80 38.46 0.0289 5.83 0.9611 0.0143 37.89 1.0000 
The pseudo-second-order kinetic equation is given as (Ho & McKay 1999) 
formula
3
where k2 is the rate constant of pseudo-second-order sorption (g/(mg min)). Figure 3 shows linear plots of the pseudo-second-order equation using the data from Figure 2. The plots were linear over the entire sorption period, with high regression coefficients (r2 = 1.0000), confirming the applicability of the pseudo-second-order kinetic model. The pseudo-second-order rate constants (k2) and the equilibrium capacities (qe), determined from the slopes and intercepts of the plots (Figure 3), are listed in Table 1. The theoretical qe values obtained using the pseudo-second-order kinetic equation were very close to the experimental qe values (Table 1). It is clear from the accuracy of the model that the sorption of phosphate onto PET-g-VBTAC is more appropriately described by the pseudo-second-order kinetic model. The results show that qe increases, but k2 decreases, with increasing initial phosphate concentration. This means that the rate of sorption decreases with increasing sorbate concentration; this is in agreement with previously reported results from other works for phosphate adsorption (Tian et al. 2009; Huang et al. 2014).
Figure 3

Pseudo-second-order kinetic plot for phosphate sorption onto PET-g-VBTAC fabric.

Figure 3

Pseudo-second-order kinetic plot for phosphate sorption onto PET-g-VBTAC fabric.

Sorption isotherms

The experimental data were plotted using the linear forms of the Langmuir (Langmuir 1918) and Freundlich (Freundlich 1906) isotherm models 
formula
4
 
formula
5
where Ce is the equilibrium concentration (mg/L), qe is the amount adsorbed at equilibrium (mg/g), qm is the Langmuir constant, related to the maximum adsorption capacity (mg/g), and b is an energy term (L/mg), which varies as a function of surface coverage caused by variations in the heat of adsorption. KF and n are the Freundlich constants, related to the adsorption capacity of the adsorbent and the magnitude of the adsorption driving force, respectively. Plots of the Langmuir and Freundlich isotherm models for phosphate sorption onto PET-g-VBTAC at different temperatures are shown in Figure 4. The calculated Langmuir and Freundlich parameters, and the regression coefficients, are listed in Table 2.
Table 2

Langmuir and Freundlich isotherm parameters

  Langmuir isotherm Freundlich isotherm 
Temp (°C) qm r2 KF n r2 
25 0.0462 54.95 0.9908 4.17 1.71 0.9815 
35 0.0542 55.56 0.9870 5.15 1.82 0.9851 
45 0.0652 55.87 0.9861 5.93 1.89 0.9842 
  Langmuir isotherm Freundlich isotherm 
Temp (°C) qm r2 KF n r2 
25 0.0462 54.95 0.9908 4.17 1.71 0.9815 
35 0.0542 55.56 0.9870 5.15 1.82 0.9851 
45 0.0652 55.87 0.9861 5.93 1.89 0.9842 
Figure 4

Langmuir (a) and Freundlich (b) plots of phosphate sorption on PET-g-VBTAC fabric.

Figure 4

Langmuir (a) and Freundlich (b) plots of phosphate sorption on PET-g-VBTAC fabric.

It was observed that for data obtained at higher temperatures, the maximum adsorption capacity (qm) of the PET-g-VBTAC for phosphate was slightly higher; the maximum adsorption capacities were 54.95 mg/g, 55.56 mg/g, and 55.87 mg/g at 25 °C, 35 °C, and 45 °C, respectively. This indicates that an increase in temperature favored phosphate sorption onto PET-g-VBTAC, but the effect was not large. The obtained qm values for our PET-g-VBTAC material compare favorably with those obtained using other adsorbents (Table 3). These results suggested that the PET-g-VBTAC was effective for phosphates removal.

Table 3

Comparison of phosphate adsorption capacity with some reported adsorbents

Adsorbent pH Concentration/range (mg/L) Temperature (°C) Model used to calculate adsorption capacity qm(mg/g) References 
Aluminum oxide hydroxide  Room temp.  24.55 Tanada et al. (2003)  
Iron oxide 6.6 0–20  Langmuir Zeng et al. (2004)  
Goethite 20–23 Langmuir 24 Chitrakar et al. (2006)  
Magnetite  Room temp.  5.2 Daou et al. (2007)  
Dolomite 3.5 0–100 20 Langmuir 48 Karaca et al. (2006)  
Synthetic hydrotalcite 6.9 0–200 25 Langmuir 47.3 Kuzawa et al. (2006)  
PET-g-VBTAC 4.7 10–100 25 Langmuir 54.95 Present study 
Adsorbent pH Concentration/range (mg/L) Temperature (°C) Model used to calculate adsorption capacity qm(mg/g) References 
Aluminum oxide hydroxide  Room temp.  24.55 Tanada et al. (2003)  
Iron oxide 6.6 0–20  Langmuir Zeng et al. (2004)  
Goethite 20–23 Langmuir 24 Chitrakar et al. (2006)  
Magnetite  Room temp.  5.2 Daou et al. (2007)  
Dolomite 3.5 0–100 20 Langmuir 48 Karaca et al. (2006)  
Synthetic hydrotalcite 6.9 0–200 25 Langmuir 47.3 Kuzawa et al. (2006)  
PET-g-VBTAC 4.7 10–100 25 Langmuir 54.95 Present study 

The lower r2 values show that the Freundlich isotherm model was less suitable for representation of the experimental data than the Langmuir isotherm model was. The magnitudes of n were at the level of moderately difficult (2 ≥ n ≥ 1) sorption. The increases in n and KF with increasing temperature show that sorption is favored by an increase in temperature.

The Langmuir and Freundlich isotherms do not explain the adsorption mechanism. The type of adsorption was determined using the Dubinin–Radushkevich (D–R) isotherm (Singh & Pant 2004; Kundu & Gupta 2006), which can be expressed as 
formula
6
 
formula
7
where E is the mean free energy of adsorption, qe is the amount of phosphate adsorbed (mg/g) at equilibrium per unit weight of adsorbent, qm is the maximum adsorption capacity (mg/g), Ce is the equilibrium concentration of phosphate in the solution (mg/L), and k is a constant related to the adsorption energy (mol2 kJ−2); ɛ is the Polanyi potential, which can be expressed as ɛ = RTln[(1 + (1/Ce)], where R is the universal gas constant (8.314 kJ/(mol K)) and T is the temperature (K). The D–R isotherm was drawn by plotting lnq against ɛ2 (Figure 5). The calculated isotherm parameters and the regression coefficients are given in Table 4. The values of E found in this study were 9.64 kJ/mol, 10.00 kJ/mol, and 10.25 kJ/mol at 25 °C, 35 °C, and 45 °C, respectively. This indicates that phosphate adsorption on PET-g-VBTAC occurs via ion exchange (8 < E < 18 kJ/mol) (Mahramanlioglu et al. 2002).
Table 4

D–R model parameters for phosphate sorption on PET-g-VBTAC fabric

Temp (°C) E (kJ/mol) qm (mol/g) r2 
25 9.64 0.0028 0.9917 
35 10.00 0.0026 0.9941 
45 10.25 0.0025 0.9936 
Temp (°C) E (kJ/mol) qm (mol/g) r2 
25 9.64 0.0028 0.9917 
35 10.00 0.0026 0.9941 
45 10.25 0.0025 0.9936 
Figure 5

D–R model for phosphate sorption on PET-g-VBTAC fabric.

Figure 5

D–R model for phosphate sorption on PET-g-VBTAC fabric.

Sorption thermodynamics

The experimental data were used to calculate various thermodynamic parameters, namely the enthalpy change (ΔH°), entropy change (ΔS°), and Gibb's free energy (ΔG°), at temperatures in the range 25–45 °C, using the following equations (Singh & Pant 2004) 
formula
8
 
formula
9
where bM is the Langmuir isotherm constant (L/mol) at temperature T and R is the universal gas constant [8.314 J/(mol K)]. The plots of lnbM versus 1/T are shown in Figure 6. The values of ΔH° and ΔS° for phosphate sorption were calculated from the slopes and intercepts, respectively, of the plots. The values of ΔG° were calculated using Equation (9). The thermodynamic parameters for phosphate sorption on PET-g-VBTAC are presented in Table 5. The distribution coefficient Kd was used as an alternative to the Langmuir constant bM to evaluate the thermodynamic parameters. Here, Kd is approximated as an empirical equilibrium constant at a particular initial concentration, as defined below 
formula
10
where CAe is the amount adsorbed on the solid at equilibrium (mg/kg). The plots of lnKd against 1/T are also shown in Figure 6, and the calculated thermodynamic parameters are presented in Table 5. Straight line regression analyses showed that the regression coefficients of all the plots were high (r2 > 0.99). These results indicate that the thermodynamic parameters were determined accurately using the slopes and intercepts of the regression lines. However, the thermodynamic parameters calculated using Kd systematically varied with the initial concentration, within a narrow range, and roughly corresponded to those calculated using bM, as shown in Table 5. This suggests that Kd values can be used in sorption thermodynamic studies. It is also worth noting that the values of ΔS° and ΔH° decreased logarithmically, whereas the values of ΔG° increased linearly, with increasing initial phosphate concentration. This may be because the number of active sorption sites on the sorbent decreases with increasing initial sorbate concentration.
Table 5

Thermodynamic parameters for phosphate sorption on PET-g-VBTAC fabric

 C0 (mg/L) H° (kJ/mol) S° (J/(mol K)) G° at 25 °C (kJ/mol) 
Langmuir constant (bM10–100 12.98 113.82 − 20.96 
Empirical equilibrium constant (Kd10 18.52 126.62 − 19.23 
30 11.74 101.26 − 18.45 
50 9.67 91.54 − 17.62 
70 8.62 85.80 − 16.96 
100 7.51 80.22 − 16.41 
 C0 (mg/L) H° (kJ/mol) S° (J/(mol K)) G° at 25 °C (kJ/mol) 
Langmuir constant (bM10–100 12.98 113.82 − 20.96 
Empirical equilibrium constant (Kd10 18.52 126.62 − 19.23 
30 11.74 101.26 − 18.45 
50 9.67 91.54 − 17.62 
70 8.62 85.80 − 16.96 
100 7.51 80.22 − 16.41 
Figure 6

Plots of lnbM and lnKd versus 1/T for phosphate sorption on PET-g-VBTAC fabric.

Figure 6

Plots of lnbM and lnKd versus 1/T for phosphate sorption on PET-g-VBTAC fabric.

The negative values of the free energy, Δ, confirm that the phosphate sorption process is spontaneous. The free energy values decreased with increasing temperature. This indicates that the reaction is easier at higher temperatures, which is in agreement with the results that the Langmuir constant, qm, and the Freundlich constant, KF, which are related to the adsorption capacity, increased with increasing temperature. The positive value of the enthalpy change (ΔH°) confirms that the process is endothermic. The positive value of the entropy change (ΔS°) suggests an increase in randomness at the solid–solution interface during phosphate sorption onto the PET-g-VBTAC fabric.

The Arrhenius equation was used to evaluate the activation energy of phosphate sorption (Smith 1981) 
formula
11
where A is a temperature-independent factor (g/(mg min)), k2 is the pseudo-second-order rate constant for ion sorption, and Ea is the activation energy (kJ/mol). The plot of lnk2 versus 1/T was linear, with good r2 values for all temperatures and an initial phosphate concentration of 50 mg/L (Figure 7). The temperature-independent factor and the activity energy were calculated to be 0.0316 kJ/mol and 1.17 kJ/mol, respectively. The low activation energy obtained suggests that phosphate sorption on PET-g-VBTAC is mainly physical (<4.2 kJ/mol) or non-activated chemical (near 0 kJ/mol) (Zou et al. 2006). The positive value of Ea indicates that an increase in temperature favors phosphate sorption onto PET-g-VBTAC, and the sorption process is endothermic.
Figure 7

Arrhenius plot for phosphate sorption on PET-g-VBTAC fabric.

Figure 7

Arrhenius plot for phosphate sorption on PET-g-VBTAC fabric.

Effects of competing anions

Various anions are generally present in phosphate-containing water, and these affect the sorption efficiency. In this study, the effects of competing anions, namely Cl, , , and , on the sorption of phosphate ions onto PET-g-VBTAC were examined; the results are shown in Figure 8. As shown in Figure 8, phosphate sorption onto PET-g-VBTAC decreased with increasing concentration of competing anions. However, it is clear that phosphate sorption is less sensitive to the presence of competing univalent anions such as Cl, , and (<25% decrease) than to the presence of divalent ions (30–80% decrease). Phosphate exists predominantly as a univalent species, , under our experimental condition (pH 4.7), and therefore it is clear that the reason for the strong sorption of competing ions is the increased negative valence, which is more favorable for anion exchange on the PET-g-VBTAC fabric. Figure 8 also shows the effect of competing ions on the sorption of phosphate ions when the solution pH was adjusted to 7.4 ± 0.1, after sorption equilibrium was reached, by adding NaOH solution (open symbols). At this pH, phosphate sorption onto PET-g-VBTAC was less affected by the presence of competing ions (<30% decrease), because the phosphate exists predominantly as a divalent species, . It can be concluded from these results that the PET-g-VBTAC fabric has a significantly high affinity for phosphate ions, and its sorption capacity is decreased by 25–80% in the presence of competing univalent anions, and even by competing divalent anions at high pH.

Figure 8

Effect of competing anions on phosphate sorption onto PET-g-VBTAC fabric.

Figure 8

Effect of competing anions on phosphate sorption onto PET-g-VBTAC fabric.

CONCLUSIONS

PET-g-VBTAC exhibited a high phosphate removal capacity. The phosphate adsorption capacity of PET-g-VBTAC was highest at pH 7, and decreased with increasing solution pH. The equilibrium data fitted the Langmuir isotherm model well. It was observed that at high temperatures, the maximum adsorption capacity of PET-g-VBTAC for phosphate increased; qm was 54.95–55.87 mg/g at 25–45 °C. The adsorption process followed a pseudo-second-order kinetic model. The obtained values of the mean free energy indicated that adsorption onto PET-g-VBTAC occurred via ion exchange. Thermodynamic parameters, namely ΔG°, ΔH°, and ΔS°, showed that the phosphate sorption process was spontaneous and endothermic. The activation energy for phosphate sorption onto PET-g-VBTAC was 1.17 kJ/mol, indicating that an increase in temperature favored phosphate sorption, and the sorption process was mainly physical or non-activated chemical. Competing univalent anions such as Cl, , and showed an insignificant adverse effect on phosphate sorption onto PET-g-VBTAC. The effect of competing divalent ions on phosphate sorption decreased greatly with increasing solution pH. These results demonstrate that PET-g-VBTAC fabric can be effectively used for removal of phosphate from aqueous solutions.

ACKNOWLEDGEMENT

This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20120008986).

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