Application of modified clay for removal of phenol and PO 4 3− ions from aqueous solutions

Abstract

Simultaneous adsorption of phenol and phosphate(V) ions on the surfactant-modified clay from the binary mixture was studied and compared with the single phenol or phosphate(V) sorption. The maximum adsorption capacity of hexadecyltrimethylammonium–bentonite was 18.8 mg/g for phenol and 38.5 mg/g for phosphate(V) at simultaneous adsorption of both of them. The optimal pH values of adsorption on hexadecyltrimethylammonium–bentonite was found to be 7.0 and >7 for phosphate(V) and phenol, respectively. The kinetic studies showed that the adsorption followed a pseudo-second-order reaction for both. The equilibrium data were analyzed using the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm models.

Introduction

Several methods are available for removing phenol and phosphate(V) from aqueous solutions. These are chemical precipitation (Aksu and Akpinar, 2001; Aksu and Gonen, 2006), solvent extraction, and adsorption (Chitrakar et al., 2005; Ciesielczyk et al., 2015). Among them, adsorption is an attractive method (Zhang et al., 2008), due to its high efficiency (Ozacar, 2003), ease of handling, and availability of different adsorbents (Bartczak et al, 2016; Ciesielczyk et al., 2016; Klapiszewski et al., 2015; Saha et al., 2009). Various kinds of new adsorbents for removal (Navarro et al., 2009) and recovery (Tanada et al., 2003) of phosphate have been reported (Geelhoed et al., 1997), of which natural clays and their composites are considered as particularly effective (Yang et al., 2016), low cost (Xue et al., 2009), and characterized by chemical stability (Wang and Xing, 2004; Tanada et al., 2003). In this paper sodium bentonite was modified by hexadecyltrimethylammonium bromide (HDTMA–Br) to obtain a more efficient sorbent. The paper deals with the subject of common sorption of phosphates and phenol which should be removed from wastewaters due to their unfavorable effects on the environment. So far the literature data on this subject have not been available. The effects of various experimental parameters including pH of the solution, contact time, initial ions concentration, dose of sorbent, and temperature as well as adsorption kinetics of isotherm models were investigated.

Materials and methods

Adsorbents

Preparation of HDTMA–bentonite

In order to obtain HDTMA–bentonite, 1 g of Na–bentonite was shaken for 4 h at 60℃ with 100 cm³ of 0.01 mol/dm³ hexadecyltrimethylammonium–bromide (HDTMA–Br) solutions. Then the time samples of the sorbent were decanted, washed with distilled water, and finally dried in air.

Adsorption experiments

Phosphate and phenol adsorption experiments were conducted using the batch technique under the following (conditions) terms: T 20 or 40℃, pH range 2–11, shaking time 6 h, 0.4 g of adsorbent, and 100 cm³ of phenol and phosphate solution with the initial concentration of 0.5 mmol/dm³. Sorption experiments were performed with the solutions containing phosphate and phenol and also compared with a solution containing only phosphate or phenol. The concentration of phosphate and phenol in the equilibrium solutions was analyzed spectrophotometrically by the molybdenum blue (Marczenko and Balcerzak, 1998) or the 4-Aminoantipyrine method (Marczenko and Balcerzak, 1998), respectively. The amounts of phosphate and phenol adsorbed onto the HDTMA–bentonite at equilibrium were calculated from the following equation

$${\text{q}}_{\text{e}}=({\text{c}}_{\text{in}}-{\text{c}}_{\text{eq}})\times 0.25$$

(1) where q_e is the amount adsorbed per unit gram of HDTMA–bentonite (mg/g), c_in is the initial concentration (mg/dm³), c_eq is the equilibrium concentration (mg/dm³), and 0.25 is the volume ratio of the aqueous phase to the solid phase. In the desorption experiment, 0.4 g of HDTMA–clay was equilibrated with 100 cm³ of 0.5 mmol/dm³ phosphate or phenol solution. The solid phase was dried in air, and phosphate or phenol was desorbed from it through equilibration with 100 cm³ of water. The pH of the leaching solution, controlled by NaOH or HCl, was designated as pH_desorption. The percentage of desorption was found from D=c_l/c_s, where c_l denotes the phosphate or phenol concentration in the leaching solution, and c_s is the phosphate or phenol concentration in HDTMA-clay after loading. In order to describe the adsorption of phosphate and phenol on HDTMA–bentonite, the following models were applied to examine the experimental data. The linear Langmuir and Freundlich adsorption isotherms were expressed as follows

$$\frac{{\text{C}}_{\text{eq}}}{{\text{q}}_{\text{e}}}=\frac{1}{{\text{K}}_{\text{L}}{\text{q}}_{\text{max}}}+\frac{{\text{c}}_{\text{eq}}}{{\text{q}}_{\text{max}}}$$

(2)

$${\text{lnq}}_{\text{e}}=\frac{1}{\text{n}}{\text{lnc}}_{\text{eq}}+{\text{lnK}}_{\text{F}}$$

(3)

The Temkin isotherm was represented by equation

$${\text{q}}_{\text{e}}=\frac{\text{RT}}{{\text{b}}_{\text{T}}}{\text{lnK}}_{\text{T}}{\text{c}}_{\text{eq}}$$

(4)

The Dubinin–Radushkevich isotherm was given by the following equation (Itodo and Itodo, 2010)

$${\text{q}}_{\text{e}}={\text{Q}}_{\text{m}}\times \text{exp}(-{\text{K}}_{\text{D}-\text{R}}\times [\text{RT}\times \text{exp}(1+\frac{1}{{\text{c}}_{\text{eq}}}){]}^2)$$

(5)

In the equations, q_e and c_eq denote the equilibrium concentrations of phenol or phosphate(V) in the clay (mol/g) and aqueous phase (mol/dm³); q_max, K, and n stand for adsorption maximum (mol/g), adsorption constants, heterogeneity parameter of the surface, respectively; Q_m is the theoretical maximum capacity (mol/g); and b_T is the Temkin constant (J/mol). The mean free energy of adsorption E (kJ/mol) from the Dubinin–Radushkevich isotherm was calculated using equation (6)

$$\text{E}=\frac{1}{(2{\text{K}}_{\text{D}-\text{R}})0.5}$$

(6)

Results and discussion

Adsorption study

As follows from the graphs (Figure 1(a)) the adsorption of PO₄³⁻ ions and phenol evidently increases with the contact time of the aqueous phase and the adsorbent. The adsorption equilibrium for both is determined after about 6 h. The pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model were tested for kinetic studies. The R² values obtained for the pseudo-first-order model (0.73 for phosphate and 0.68 for phenol adsorption) and for intraparticle diffusion model (0.56 for phosphate and 0.51 for phenol adsorption) were low. The experimental data fitted very well the pseudo-second-order kinetic model and high correlation coefficients (>0.97) were obtained. These suggest that the adsorption data are well represented by pseudo-second-order kinetics (Abdelwaha, 2007). The constant k and the regression coefficient were 45.26 g/mol min and 0.981, respectively for phosphate(V) and 12.4 g/mol min and 0.976 for phenol. The rate of phosphate adsorption by HDTMA–bentonite was higher than that of phenol adsorption. The increase of phosphate and phenol adsorption is observed also in proportion to the increasing mass of the adsorbent as shown in Figure 1(b). The adsorption of phosphate and phenol adsorbed onto HDTMA–bentonite at different pH values is shown in Figure 1(c). As follows from Figure 1(c) phosphate(V) ions exhibit the highest sorption in the pH range 7–9. At pH<7 and pH>9 there is observed drop in sorption and the lowest value is found at pH 2–3. These results indicate enormous effect of solutions pH on phosphate ions sorption on HDTMA–bentonite. Phosphate cannot be ion exchanged onto the adsorbent under alkali or acid conditions. This result indicates that the dominant mechanism of phosphate adsorption onto HDTMA–bentonite was ion exchange. The diagram for phenol shows that an increase in pH of the solution has a positive effect on adsorption of phenol HDTMA–bentonite. In the aqueous phase phenol is dissociated and can occur in the form of neutral undissociated molecules as well as anions (dissociated). The share of both forms in the total concentration changes with the increasing pH. In the solutions of pH>7 the share of phenolate anion increases in the total concentration of phenol so the sorption increase is observed with the increasing pH and reaches the maximal value at pH>8. The negatively charged phenolate anion is electrostatically attracted by the positively charged HDTMA–bentonite surface. The dissociation of phenol may also be the result of phenol interaction with the surfactant cation present in both the bentonite structure and aqueous phase. This interaction contributes to more ready phenol dissociation through polarization of the OH bond in the hydroxyl group of phenol (Majdan et al., 2009).

Figure 1.

Time (a), mass of adsorbent (b), and pH (c) influence on simultaneous adsorption of phosphate and phenol on HDTMA–bentonite. HDTMA: hexadecyltrimethylammonium.

Figure 2 presents the compared isotherms of phosphate and phenol adsorption at 20 and 40℃. The comparison of both isotherms confirms evidently that phosphate sorption on this adsorbent is much effective than phenol sorption. These diagrams along with those in Figure 2 show that the temperature increase of the process results in the increasing adsorption for phosphates and phenol. Figure 2 presents the adsorption of phosphate ions and phenol in their presence in the solution. As can be seen in the case of phosphates the presence of phenol deteriorates their sorption on HDTMA–bentonite though insignificantly. At the same time the addition of phosphate ions increases phenol adsorption significantly.

Figure 2.

Adsorption isotherms of phosphate(V) (a) and phenol (b) on HDTMA–bentonite. HDTMA: hexadecyltrimethylammonium.

The Freundlich, Langmuir, Dubinin–Radushkevich, and Temkin isotherm models were used to describe the adsorption equilibrium. The parameters of the isotherms for simultaneous sorption of phenol and phosphate(V) ions onto HDTMA–bentonite were listed in Table 1. The Langmuir model provides the maximum values of q_max where they could not be reached in the experiments. The high K_L values indicate high adsorption affinity. The monolayer saturation capacity, q_max, was calculated to be 18.8 mg/g (2·10⁻⁴ mol/g) and 38.5 mg/g (4.1·10⁻⁴ mol/g) for phenol and phosphate(V), respectively. The R² values suggest that the Freundlich isotherm provides a good model for the sorption system. The constant K_F is an approximate indicator of adsorption capacity while 1/n is a function of the adsorption strength in the adsorption process. The sorption constants, K_F for phosphate are higher than those for phenol and it indicates that phosphate ions are favorably adsorbed on HDTMA–bentonite. In addition, the value of the parameter n> 1 also confirms more effective sorption of phosphate(V) than that of phenol. Based on the Temkin and Dubinin–Radushkevich equation there were appointed heat of adsorption and the average energy of adsorption. They are listed in Table 2. The table shows that the heat of adsorption and the mean free sorption energy for phosphates are 234 J/mol and 12.7 kJ/mol, respectively. The calculated values are in the range characteristic for chemisorption process. In the case of phenol, this is rather a physical sorption mechanism as suggested by the value of heat (33.2 J/mol) and energy (4.59 kJ/mol) of adsorption (Majdan et al., 2009). Taking into account the fact that phenol in an aqueous solution is dissociated and exists as undissociated neutral molecules and dissociated phenolate anions. Therefore, adsorption of phenol is carried out with the participation of weaker van der Waals forces for neutral molecules and by stronger electrostatic interaction between the phenolate anion and positively charged surface of HDTMA–bentonite.

Table 1.

Parameters of isotherms for simultaneous adsorption of phenol and phosphate(V) ions onto HDTMA–bentonite.

Freundlich isotherm			Langmuir isotherm			Dubinin–Radushkevich isotherm				Temkin isotherm			Adsorbate
n	KF (dm3/mol)	R2	qmax (mol/g)	KL (dm3/mol)	R2	Qm (mol/g)	KD–R (mol2/kJ2)	E (kJ/mol)	R2	bt (J/mol)	KT (dm3/g)	R2
2	978	0.987	4.1×10−4	5781.3	0.978	4.4×10−4	3.62×10−8	12.72	0.972	234	407.3	0.967	PO43−
0.3	87.6	0.989	2×10−4	267.7	0.967	7.8×10−5	1.67×10−8	4.59	0.976	33.2	554.24	0.976	Phenol

Table 2.

Desorption of phosphate(V) and phenol from HDTMA–bentonite.

pH	Pdesphosph (%)	Pdesphen (%)
pH3	16.1	56.25
pH 7	8.1	38.82
pH 9	13.2	50.18

HDTMA: hexadecyltrimethylammonium.

The data collected in Table 2 prove that adsorption combination of HDTMA–bentonite–phosphates is more stable under any pH conditions than the phenol combination. In both cases the desorption in the acidic environment is the most intense and that in the neutral solutions is the least. As follows from the data presented in Table 2, desorption of phosphate ions is insignificant at pH 7 being about 8% whereas that of phenol is quite large, that is, about 40% at pH 7 and 50–60% at pH 3 or 9. Thus after earlier elution of phenol, it is possible to reuse HDTMA–bentonite as a sorbent for phosphate ions and phenol at pH close to 7 as under these conditions phosphate ions, which may be present on the sorbent, cannot desorb. However, this is the most favorable pH range for adsorption of both phosphate ions and phenol. Phosphate ions are strongly bonded with the adsorbate as the process is chemisorption. There desorption of these ions is insignificant even in alkaline or acid solutions. As phenol adsorption proceeds by means of weaker van der Waals binding, phenol can be washed away from the adsorbent surface using alkaline, acidic, or even of neutral pH solutions.

Conclusions

Simultaneous sorption of phosphate ions and phenol is possible on HDTMA–bentonite. The largest effectiveness of the process is observed at pH 7 for both ions. The values of the apparent free energy of adsorption from the Dubinin–Radushkevich isotherm and the heat of adsorption from the Temkin isotherm depict the physiosorption process for phenol and chemisorption in the case of phosphate ions.