Abstract

In this study, the removal of phosphate (PO4) from wastewater using glauconite was investigated. Glauconite was characterized by N2 adsorption–desorption isotherm, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and Fourier transform infrared (FTIR) spectroscopy. The effects of contact time, pH, initial phosphate concentrations, adsorbent dose, and temperature were investigated by batch experiments. The isotherms, kinetics and thermodynamics for phosphate removal were studied. The results showed that glauconite had a rough surface and abundant pores. The determined Brunauer–Emmett–Teller (BET) surface area was 55 m2/g with a pore radius of 1.99 nm and the pore volume was 0.032 cm3/g. FTIR analysis revealed that the abundance of various functional groups on the surface of glauconite may play an important role for the adsorption process. The optimum pH was 11 with complete removal of phosphate in a short time (nearly 1 min). The experimental data fitted very well with the Langmuir isotherm (R2 = 0.999) with a maximum adsorption capacity of 32.26 mg/g at 50 °C. Adsorption kinetic data were best fitted with the pseudo-second-order kinetic model (R2 = 0.999). Thermodynamic study confirmed the spontaneous, endothermic and irreversible adsorption process. Therefore, glauconite is a promising natural low-cost adsorbent for phosphate removal from wastewater.

INTRODUCTION

Phosphate is an important nutrient for plant and organism growth and the normal functioning of ecosystems (Liu & Zhang 2015). Phosphate is discharged to surface water from different sources such as agricultural applications, industries, wastewater, and other human activities (Huang et al. 2008; Mangwandi et al. 2014). The extensive discharge of phosphate into surface water can cause many pollution problems, notably eutrophication, which can lead to lower water quality, an increase in water treatment costs, loss of aquatic species, parasitic infection, algal bloom, and fish death (Karaca et al. 2004; Rodrigues & da Silva 2010; Lalley et al. 2016). Therefore, the removal of phosphates from wastewater is an important environmental task.

Various physical, chemical, and biological treatment methods have been applied for phosphate removal (Yeon et al. 2009; Wang et al. 2012). However, biological processes are unsteady because the quality of water has a great impact on phosphate removal ratio. Meanwhile, chemical methods often consume a large amount of chemicals and are still subject to costs and problems of sludge handling, disposal, and neutralization of the effluent (Özacar 2003; Liu et al. 2013). Some physical methods, as in the case of reverse osmosis and electrodialysis, have been proven to be either inefficient in removal or too expensive (Huang et al. 2017).

In comparison with the above-mentioned methods, the adsorption process is a promising method for phosphate removal (Yadav et al. 2015; Xiong et al. 2017). The adsorption process is an attractive method due to high removal efficiency, simple operation, low cost, and fast adsorption rate (Yan et al. 2010; Yao et al. 2011). During recent years, the application of easily available and low-cost materials has been widely studied for phosphate removal. From these materials, blast furnace slag (Kostura et al. 2005), red mud (Liu et al. 2007; Huang et al. 2008), fly ash (Li et al. 2006), alunite (Özacar 2003), dolomite (Karaca et al. 2004), calcite (Karageorgiou et al. 2007), aluminum hydroxide (Tanada et al. 2003), iron oxide tailings (Zeng et al. 2004), palygorskite (Ye et al. 2006), zirconium oxide (Su et al. 2013), eggshell waste (Mezenner & Bensmaili 2009; Köse & Kıvanç 2011), and modified wheat residue (Xu et al. 2009) have been investigated. In addition, different studies have reported on the use of ferric sludge (Song et al. 2011), coir-pith activated carbon (Kumar et al. 2010) and modified biochar (Jung et al. 2015; Cui et al. 2016; Jung et al. 2017; Li et al. 2017) for phosphate removal.

Glauconite, a clay mineral, is a natural three-layer sheet aluminosilicate. It contains multivalent iron, aluminum, and magnesium ions confined between two flat silicate layers that also contain OH groups; the triple layers alternating with a layer of water molecules and alkaline earth metals, most notably potassium. Glauconite has been investigated as a natural adsorbent for removal of metals, uranium and thorium (Smith et al. 1996; Ali et al. 2015).

The objective of this study is to investigate the sorption of phosphate (PO4) from wastewater onto local Egyptian glauconite as a function of various operational parameters. Moreover, isotherm, kinetic, and thermodynamic characteristics of the sorption process of phosphate onto glauconite were also investigated.

MATERIALS AND METHODS

Adsorbent preparation and characterization

Glauconite clay was obtained from mines in Bahariya Oases, Egypt. It was ground in a laboratory-type mill and sieved to give less than 75 μm particles. The received glauconite was placed in a glass bottle and was kept in a desiccator during the experiments for further use. In this study, glauconite has been tested for adsorption experiments without any pre-treatment.

The specific surface area, pore size and pore volume of glauconite powder were measured by Brunauer–Emmett–Teller (BET) surface area analyzer (Quantachrome, USA). The surface morphology of glauconite was observed by scanning electron microscopy (SEM) (JEOL; JSM-6510LV, Japan). In addition, the elemental composition was characterized using high-resolution transmission electron microscopy and energy-dispersive X-ray (EDX) analysis. Fourier transform infrared (FTIR) spectroscopy (Thermo Scientific Nicolet IS10, USA) was used to reveal the functional groups in the glauconite surface.

Adsorbate preparation

Phosphate (PO4) stock solution was prepared by dissolving the anhydrous potassium phosphate (K2HPO4) (obtained from Al-Gomhoriah Company for chemicals and medical supplies, Egypt) in distilled water to give 1,000 mg P/L concentration. Different phosphate concentrations were prepared by diluting the stock solution with distilled water and contained 0.01 M KCl to adjust ionic strength when necessary. All other reagents used in this study were of analytical grade.

Batch adsorption experiments

The adsorption experiments were conducted by a batch technique. In the experiments, 0.1 g of glauconite, except in the studies of adsorbent dose effect, was stirred with 100 mL of phosphate solution of desired concentrations until equilibrium was reached. The solution was stirred on a magnetic stirrer at 400 rpm with initial phosphate concentrations ranging from 5 to 75 mg/L. The effect of temperature on the adsorption of phosphate was investigated at different temperatures ranging from 15 to 50 °C.

The initial pH values of the solution in the range from 2 to 12 were adjusted with dilute 0.5 M HCl or NaOH using a pH meter. The solution after adsorption was centrifuged for 15 min at 3,000 rpm and supernatants were analyzed for the remaining phosphate concentration. The measurements of phosphate concentrations were performed by the molybdovanadate method described in Standard Methods (APHA 2005) using a spectrophotometer (Shimadzu UV-visible, 1601PC, Japan) at a wavelength of 690 nm by a standard calibration curve. The adsorption amount Qe (mg/g) and removal rate of phosphate were calculated using Equations (1) and (2):
formula
(1)
formula
(2)
where Qe (mg/g) is the adsorption capacity, Co and Ct (mg/L) are the initial phosphate concentration and phosphate concentration at time t, respectively, V (L) is the solution volume, and w (g) is the adsorbent mass.

RESULTS AND DISCUSSION

Adsorbent characterization

The N2 adsorption–desorption isotherm and pore size distribution for glauconite powder is shown in Figure 1. The glauconite exhibits a type IV isotherm with a hysteresis loop of the H4 type, indicating the presence of mesopore and micropore structure in this material (Thommes et al. 2015). The determined BET surface area of glauconite powder was found to be 54.96 m2/g. Moreover, the pore radius and pore volume were found to be 1.99 nm and 0.032 cm3/g, respectively.

Figure 1

N2 adsorption–desorption isotherm for glauconite. Inset is the pore size distribution of glauconite.

Figure 1

N2 adsorption–desorption isotherm for glauconite. Inset is the pore size distribution of glauconite.

Figure 2 shows the SEM micrographs of glauconite before and after the adsorption process. It is shown in Figure 2(a) and 2(b) that glauconite has a generally irregular structure and contains various bent and curled crystals that form numerous mesopores, which significantly raised the adsorption ability. Figure 2(c) and 2(d) shows that the phosphate ions were adsorbed onto the surface of the glauconite. Furthermore, the elements present in the glauconite were characterized with EDX analysis. The major elements found in the solid sample were 59.8% oxygen (O), 13.05% silicon (Si), 8.65% iron (Fe), 2.9% aluminum (Al), 4.22% calcium (Ca), 1.45% magnesium (Mg), 2.93% potassium (K), 0.56% carbon (C), and 4.93% sulfur (S).

Figure 2

SEM micrographs of glauconite (a) before adsorption at 4,000×, (b) before adsorption at 2,000×, (c) after adsorption at 2,000×, (d) after adsorption at 1,000×.

Figure 2

SEM micrographs of glauconite (a) before adsorption at 4,000×, (b) before adsorption at 2,000×, (c) after adsorption at 2,000×, (d) after adsorption at 1,000×.

The FTIR spectrum of glauconite is shown in Figure 3. The presence of wide bands at 1,004 cm−1 and 1,092 cm−1 corresponds to the Si–O stretch of the phyllosilicate clay structure. Bands at 3,420, 3,675, 3,693, and 3,752 cm−1 are due to the OH- stretching vibrations of the glauconite structure (Selim et al. 2014). The band at 672 cm−1 corresponds to the structural vibration of the OH group, while the band at 802 cm−1 corresponds to the deformation mode of the Al2OH group. The bands in the range 454 cm−1 to 596 cm−1 are associated with metal–oxygen and metal–hydroxyl vibrations, whereas the 1,624 and 1,684 cm−1 bands correspond to H–O–H bending in the water molecule (Shekhar et al. 2017; Selim et al. 2018). It is clear that after the adsorption process the various peaks are shifted or disappear, indicating that the functional groups on the surface of the glauconite were covered or interacted with phosphate ions.

Figure 3

FTIR spectra of glauconite before and after phosphate adsorption.

Figure 3

FTIR spectra of glauconite before and after phosphate adsorption.

Sorption factors

Effect of contact time and pH

The removal rate of phosphate from solution as a function of time is shown in Figure 4(a). It is obvious that the adsorption of phosphate by glauconite is very fast during the first minute (97.2% removal) for an initial phosphate concentration of 5 mg/L with 1 g/L glauconite dose at pH 11. This is because there are a large number of available surface adsorption sites of glauconite (Wang et al. 2016; Jung et al. 2017). Phosphate is totally adsorbed into glauconite and equilibrium is reached within 4 min only. This indicates that glauconite is an efficient, fast adsorbent for phosphate removal from aqueous solutions.

Figure 4

Effect of (a) contact time and (b) pH on the removal rate of phosphate (adsorbent dose = 1 g/L, initial concentration = 5 mg/L, temperature = 25 °C).

Figure 4

Effect of (a) contact time and (b) pH on the removal rate of phosphate (adsorbent dose = 1 g/L, initial concentration = 5 mg/L, temperature = 25 °C).

From Figure 4(b), it is clear that phosphate adsorption onto glauconite is strongly pH dependent. The results confirm that the sorption of phosphate onto glauconite increased sharply with increasing pH from 8 to 11 and it reached the maximum value at pH 11. With increasing pH the negative charge on the surface of the adsorbent increases, thus enhancing the phosphate adsorption, while at low pH values, low uptake occurs. This is due to the protonation of the active sites in glauconite, which inhibits their binding ability towards the phosphate. Moreover, at low pH, the adsorbent surface is positively charged and adsorption of phosphate ions is low due to the electrostatic repulsion (Siswoyo et al. 2014).

Effect of glauconite dose

In order to examine the effect of glauconite dose on phosphate removal rate, adsorption experiments were set up with different doses of glauconite (0.05, 0.1, 0.25, 0.5, 1.0, 1.5, and 2.0 g/L). The effect of glauconite dose on the removal rate of phosphate and adsorption capacity is shown in Figure 5. The removal rate increases from 14.6% to 98.1% with an increase in glauconite dose from 0.05 g/L to 0.50 g/L. This increase can be attributed to the increase in the surface area and adsorption functional sites (Köse & Kıvanç 2011). It is also observed that the adsorption capacity decreases from 18.07 to 9.80 mg/g as the glauconite dose increases from 0.15 to 0.50 g/L. This reduction can be attributed to the sites remaining unsaturated during the adsorption process for the higher dose (Rodrigues & da Silva 2010; Liu et al. 2016). Moreover, the glauconite dose of 1.0 g/L gives 100% phosphate removal and the doses of 1.5 and 2 g/L give the same results of 1.0 g/L with 100% removal, so the dose of 1.0 g/L is more sufficient for use at the initial phosphate concentration of 5 mg/L.

Figure 5

Effect of glauconite dose on the removal rate of phosphate (contact time = 5 min, pH = 11, initial concentration = 5 mg/L, temperature = 25 °C).

Figure 5

Effect of glauconite dose on the removal rate of phosphate (contact time = 5 min, pH = 11, initial concentration = 5 mg/L, temperature = 25 °C).

Effect of initial phosphate concentration

Figure 6 presents the effect of initial phosphate concentration on adsorption by glauconite. Initial phosphate concentrations from 5 to 20 mg/L are completely adsorbed with 1 g/L glauconite dose at pH = 11 after 5 min contact time, while the removal rate of phosphate gradually decreases from 100% to 20% with an increase in phosphate concentration from 20 to 75 mg/L. This indicates that there are reductions in immediate solute adsorption, due to the shortage of available active sites required for the high initial phosphate concentrations (Amin 2008). However, the adsorption capacity was found to increase from 5 to 26 mg/g as the concentration of phosphate increased from 5 to 75 mg/L. This increase may be attributed to the increase in the driving force provided by the concentration gradient of phosphate between the solid phase and aqueous (Liu et al. 2016).

Figure 6

Effect of initial phosphate concentration on the removal rate (contact time = 5 min, pH = 11, adsorbent dose = 1 g/L, temperature = 25 °C).

Figure 6

Effect of initial phosphate concentration on the removal rate (contact time = 5 min, pH = 11, adsorbent dose = 1 g/L, temperature = 25 °C).

Effect of temperature

The effect of temperature on phosphate removal rate at different concentrations is presented in Figure 7. It is observed that there is a slight change of the removal rate of phosphate at low concentrations, while at high concentrations, the removal rate of phosphate increases with an increase in temperature. At 50 mg/L phosphate concentration, the removal rate increases from 49.29% to 61.67% with an increase of temperature from 15 to 50 °C. This increase indicates that the adsorption of phosphate onto glauconite might involve not only physical but also chemical sorption (Ali et al. 2015). In addition, this effect may be as a result of the increase in the number of adsorption sites generated due to bond capture (Chen et al. 2010).

Figure 7

Effect of temperature on the removal rate at different concentrations (contact time = 5 min, pH = 11, adsorbent dose = 1 g/L).

Figure 7

Effect of temperature on the removal rate at different concentrations (contact time = 5 min, pH = 11, adsorbent dose = 1 g/L).

Adsorption isotherm

The adsorption isotherms of phosphate on glauconite at different temperatures were investigated using Langmuir, Freundlich, and Dubinin–Radushkevich models.

The Langmuir isotherm model is represented by the following equation (Yadav et al. 2015):
formula
(3)
where Qm (mg/g) is the theoretical maximum adsorption capacity and KL (L/mg) is the Langmuir adsorption constant. Ce is the equilibrium concentration of phosphate in the solution (mg/L) and qe is the adsorption capacity at equilibrium (mg/g). The Langmuir constant values can be obtained from the plot between Ce/qe and Ce. The correlation coefficients (R2) and Langmuir constants at different temperatures are calculated and presented in Table 1. From Table 1, higher R2 values (0.999) indicate that the Langmuir model fits the adsorption data very well. It is observed that the maximum adsorption capacity is increased from 27.03 to 32.26 mg/g by increasing the temperature from 25 °C to 50 °C, which indicates an endothermic nature of the adsorption (Huang et al. 2008).
Table 1

Adsorption isotherm parameters of phosphate on glauconite

Isotherm equationTemperature
25 °C35 °C50 °C
Langmuir 
Qm (mg/g) 27.03 29.41 32.26 
KL (L/mg) 12.32 11.33 4.42 
R2 0.999 0.999 0.999 
Freundlich 
1/n 0.046 0.057 0.06 
KF (L/mg) 23.01 24.1 25.35 
R2 0.961 0.969 0.994 
Dubinin–Radushkevich 
B (mol2/J23.0E − 8 6.0E − 9 4.0E − 9 
Qs (mg/g) 20.49 28.39 29.90 
E (KJ/mol) 4.082 9.128 11.180 
R2 0.708 0.950 0.904 
Isotherm equationTemperature
25 °C35 °C50 °C
Langmuir 
Qm (mg/g) 27.03 29.41 32.26 
KL (L/mg) 12.32 11.33 4.42 
R2 0.999 0.999 0.999 
Freundlich 
1/n 0.046 0.057 0.06 
KF (L/mg) 23.01 24.1 25.35 
R2 0.961 0.969 0.994 
Dubinin–Radushkevich 
B (mol2/J23.0E − 8 6.0E − 9 4.0E − 9 
Qs (mg/g) 20.49 28.39 29.90 
E (KJ/mol) 4.082 9.128 11.180 
R2 0.708 0.950 0.904 
The Freundlich isotherm model is represented by the following equation:
formula
(4)
where KF (L/mg) is the Freundlich capacity constant and n is the Freundlich intensity constant. The Freundlich constants were obtained from a linear plot between log Ce and log qe at different temperatures. Table 1 shows the Freundlich adsorption isotherm constants and correlation coefficients. The R2 values (0.961, 0.969, and 0.994) indicate a good fit between experimental and isotherm data. The value of 1/n was found to lie between 0 and 1, indicating that phosphate is favorably adsorbed by glauconite adsorbent (Wang et al. 2012).
The D–R isotherm model is represented by the following equation:
formula
(5)
where Qs is the theoretical saturation capacity (mg/g), B is the D–R model constant (mol2/J2), and ɛ is the Polanyi potential and is equal to:
formula
(6)
The mean free energy of sorption, E (kJ/mol), is related to B as:
formula
(7)
where R is the universal gas constant (8.314 J/mol.K), T is the temperature (K). The D–R constants were calculated from the linear plot of ɛ2 vs ln qe at different temperatures. The calculated D–R constants and mean free energy for adsorption are shown in Table 1. It is observed that at high temperatures (35 °C and 50 °C) with high R2 values, the adsorption process has a chemical character due to the E values (E values are between 8 and 16 KJ/mol). Thus, the adsorption process of phosphate onto glauconite was preceded chemically and it confirmed that the ion exchange reactions occurred onto glauconite at higher temperatures (Dai et al. 2011; Liu et al. 2016). From the results of the isotherm models, the adsorption of phosphate onto glauconite followed the different isotherm models, while the Langmuir equation had the best fit to the experimental data (R2 = 0.999) among the examined models.

The Qm obtained in this study could be compared with those other adsorbents for phosphate removal, as presented in Table 2. It can be seen that the phosphate adsorption capacity of glauconite is very high among the other adsorbents with the shortest contact time. Therefore, glauconite has a great potential capability for adsorption of phosphate from wastewater.

Table 2

Comparison of adsorption capacities of phosphate on different adsorbents

AdsorbentConditions
Qm (mg/g)Reference
Co (mg/L)Dose (g/L)pHTemp. (°C)
Glauconite 5–75 11 25–50 27.03–32.26 Present study 
Iron oxide tailings 5–150 3.2 20 8.21 Zeng et al. (2004)  
Ferric sludge 5–50 5.5 room 25.5 Song et al. (2011)  
Fly ash 0.31–3,100 25 20.16 Li et al. (2006)  
Lanthanum-doped AC 5–30 20–50 15.3 Liu et al. (2013)  
Palygorskite 5–150 4.2 25 3.73–8.31 Ye et al. (2006)  
Sugar beet tailings biochar 5–640 4.1 22 0.133 Yao et al. (2011)  
Coir-pith AC 10–40 35–60 7.74 Kumar et al. (2010)  
Modified bentonites 25–60 30–50 10.5–12.7 Yan et al. (2010)  
Activated fruit juice residue 10–30 3, 5 25–45 13.89 Yadav et al. (2015)  
Iron–zirconium modified AC 5–60 room 26.31 Xiong et al. (2017)  
Iron-doped AC 12–43 3.8 15–35 7.31–15.29 Wang et al. (2012)  
Hydrous niobium oxide 10, 50 25–65 15 Rodrigues & da Silva (2010)  
Iron hydroxide–eggshell 2.8–110 7.5 20–45 14.49 Mezenner & Bensmaili (2009)  
Zirconic chitosan beads 5–50 0.2 5–25 61.7 Liu & Zhang (2015)  
Bayoxide 140 20 25.52–38.8 Lalley et al. (2016)  
Calcined egg-shell 50–200 2–10 25–45 23.02 Köse & Kıvanç (2011)  
Blast furnace slags 50–50 11.7 20 8.5–18.94 Kostura et al. (2005)  
Red mud 0.4–8 30–40 0.271–1.01 Huang et al. (2008)  
Mg-alginate modified biochar 5–50 3–10 28 11.53–46.56 Cui et al. (2016)  
AdsorbentConditions
Qm (mg/g)Reference
Co (mg/L)Dose (g/L)pHTemp. (°C)
Glauconite 5–75 11 25–50 27.03–32.26 Present study 
Iron oxide tailings 5–150 3.2 20 8.21 Zeng et al. (2004)  
Ferric sludge 5–50 5.5 room 25.5 Song et al. (2011)  
Fly ash 0.31–3,100 25 20.16 Li et al. (2006)  
Lanthanum-doped AC 5–30 20–50 15.3 Liu et al. (2013)  
Palygorskite 5–150 4.2 25 3.73–8.31 Ye et al. (2006)  
Sugar beet tailings biochar 5–640 4.1 22 0.133 Yao et al. (2011)  
Coir-pith AC 10–40 35–60 7.74 Kumar et al. (2010)  
Modified bentonites 25–60 30–50 10.5–12.7 Yan et al. (2010)  
Activated fruit juice residue 10–30 3, 5 25–45 13.89 Yadav et al. (2015)  
Iron–zirconium modified AC 5–60 room 26.31 Xiong et al. (2017)  
Iron-doped AC 12–43 3.8 15–35 7.31–15.29 Wang et al. (2012)  
Hydrous niobium oxide 10, 50 25–65 15 Rodrigues & da Silva (2010)  
Iron hydroxide–eggshell 2.8–110 7.5 20–45 14.49 Mezenner & Bensmaili (2009)  
Zirconic chitosan beads 5–50 0.2 5–25 61.7 Liu & Zhang (2015)  
Bayoxide 140 20 25.52–38.8 Lalley et al. (2016)  
Calcined egg-shell 50–200 2–10 25–45 23.02 Köse & Kıvanç (2011)  
Blast furnace slags 50–50 11.7 20 8.5–18.94 Kostura et al. (2005)  
Red mud 0.4–8 30–40 0.271–1.01 Huang et al. (2008)  
Mg-alginate modified biochar 5–50 3–10 28 11.53–46.56 Cui et al. (2016)  

Adsorption kinetics

In order to study the mechanism of the adsorption of phosphate on glauconite, the following kinetic models are adopted to analyze the experimental data: (1) pseudo-first-order model, (2) pseudo-second-order model and (3) intraparticle diffusion model.

The pseudo-first-order model

The integrated form of the pseudo-first-order model is represented as the following equation (Karaca et al. 2004):
formula
(8)
where qe and qt are the amounts of phosphate adsorbed on the adsorbent at equilibrium and at time t, respectively (mg/g), and k1 is the rate constant of pseudo-first-order (min−1). The values of q and k1 can be calculated from the intercept and slope of the plot between log (qeqt) and t (Amin 2008). The calculated values of qe, k1 and the correlation coefficient (R2) are listed in Table 3. The concluded R2 is found to be 0.808 and the calculated value of qe (1.33 mg/g) is not close to the experimental one. This behavior indicates that the pseudo-first-order model is not the perfect model to explain the adsorption process of phosphate on glauconite.
Table 3

Kinetic parameters of phosphate on glauconite

First-order kinetic model
Second-order kinetic model
Intraparticle diffusion model
k1qR2k2qR2kdCR2
1.458 1.33 0.808 7.905 5.03 0.999 2.513 0.867 0.788 
First-order kinetic model
Second-order kinetic model
Intraparticle diffusion model
k1qR2k2qR2kdCR2
1.458 1.33 0.808 7.905 5.03 0.999 2.513 0.867 0.788 

The pseudo-second-order model

The linearized form of the pseudo-second-order model is represented as the following equation (Mezenner & Bensmaili 2009):
formula
(9)
where k2 is the rate constant of pseudo-second-order (min−1). The values of qe and k2 can be calculated from the intercept and slope of the linear plot between t/qt and t. The calculated values and correlation coefficient are listed in Table 3. It is evident from the table that a higher correlation coefficient (0.999) is identified. In addition, the calculated value of qe is closer to the experimental value. These results indicate that the kinetics of phosphate adsorption on glauconite are better fitted by the pseudo-second-order model, which is based on the assumption that the rate-limiting step may be chemical sorption or chemisorption involving valency forces through sharing or ion exchange between adsorbent (glauconite) and adsorbate (phosphate) (Chen et al. 2010).

The intraparticle diffusion model

The linearized form of the intraparticle diffusion model is represented as the following equation (Mangwandi et al. 2014):
formula
(10)
where kd is the intraparticle diffusion rate constant (mg/g.min−0.5) and C (mg/g) is the intercept which represents the boundary layer thickness. A larger C value confirms a greater effect of the boundary layer (Mahmoud et al. 2016). The values of C and kd can be calculated from the intercept and slope of the linear plot between qt and t0.5. The constants and R2 value are shown in Table 3 and these values confirm that the intraparticle diffusion process plays a role in the adsorption of phosphate onto glauconite adsorbent. But, the calculated value of R2 (0.788) indicates that the intraparticle diffusion model is also not the perfect model to explain the adsorption process of phosphate on glauconite.

Therefore, the collected results from the kinetic models of the adsorption of phosphate onto glauconite adsorbent are well and better fitted by the pseudo-second-order model. From the effect of contact time and by comparing the kinetic results in this paper to the results in the literature review, it can be argued that adsorption of phosphate onto glauconite is fast.

Adsorption thermodynamic

The thermodynamic parameters such as free energy change (ΔGo), enthalpy change (ΔHo) and entropy change (ΔSo) were determined for different phosphate concentrations at different temperatures. ΔSo and ΔHo could be estimated from the intercept and slope of the van 't Hoff plot between ln (qe/ce) and 1/T according to Equation (11) (Tang et al. 2014):
formula
(11)
Then ΔGo could be determined from the following equation:
formula
(12)

The obtained values of ΔHo, ΔSo and ΔGo are listed in Table 4. As shown in Table 4, ΔGo is negative at all investigated temperatures, which means that the adsorption of phosphate on glauconite is spontaneous in nature (Wang et al. 2012). In addition, ΔGo values at concentrations 20, 30 and 40 mg/L decrease when the temperature increases, suggesting more adsorbable phosphate with increasing temperature (Mezenner & Bensmaili 2009). The positive values of ΔHo indicate the endothermic nature of the adsorption process (Wang et al. 2016). The values of ΔSo are positive, confirming that there is an increase in randomness at the solid/solution interface for the adsorption of phosphate on glauconite (Rodrigues & da Silva 2010).

Table 4

Thermodynamic parameters of phosphate on glauconite

Co (mg/L)TemperatureΔG°(KJ/mol)ΔS°(J/mol.K)ΔH°(KJ/mol)
20 298.15 −14.021 163.43 37.687 
308.15 −15.755 
323.15 −18.356 
30 298.15 −4.522 68.68 15.954 
308.15 −5.209 
323.15 −6.239 
40 298.15 −1.645 40.46 10.417 
308.15 −2.050 
323.15 −2.657 
Co (mg/L)TemperatureΔG°(KJ/mol)ΔS°(J/mol.K)ΔH°(KJ/mol)
20 298.15 −14.021 163.43 37.687 
308.15 −15.755 
323.15 −18.356 
30 298.15 −4.522 68.68 15.954 
308.15 −5.209 
323.15 −6.239 
40 298.15 −1.645 40.46 10.417 
308.15 −2.050 
323.15 −2.657 

CONCLUSION

This study showed the efficiency of glauconite as a natural and low-cost material with acceptable capacity for the removal of phosphate from wastewater in a short time (nearly 1 min). The surface area (55 m2/g), the pore radius (1.99 nm), the pore volume (0.032 cm3/g) and the irregular structure are the important characteristics of glauconite. FTIR revealed that there are different functional groups on the surface of the glauconite, which may increase the adsorption of phosphate. Temperature, pH, adsorbent dose, and initial phosphate concentration affect the adsorption process. The optimum pH is 11 with complete removal of phosphate in a short time (nearly 1 min). The equilibrium data are better fitted using the Langmuir isotherm model (R2 = 0.999) with maximum adsorption capacity of 32.26 mg/g at 50 °C. Adsorption kinetics follow the pseudo-second-order model. Changes in free energy (ΔGo), enthalpy (ΔHo) and entropy (ΔSo) show the spontaneity, endothermic nature, and irreversibility of the adsorption process. Glauconite is a good natural adsorbent and displays promise as a new adsorbent to remove phosphate from wastewater in a short time.

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