Phosphate removal from aqueous solutions by using natural Jordanian zeolitic tuff

Abstract

In this study, a naturally occurring zeolitic tuff located in Jordan was investigated as a potential adsorbent for phosphate removal from aqueous solution. Adsorption kinetics and thermodynamics of phosphate adsorption under different temperatures were studied. The pseudo-second-order kinetic model adequately fitted the collected experimental data under different initial ion concentrations. The Langmuir model is found to be successfully fitting the experimental data. Thermodynamic parameters such as Gibbs's free energy change, standard enthalpy change, and entropy change were evaluated and the results indicated that the sorption process is spontaneous, exothermic with small degree of randomness during the sorption process.

Keywords

Phosphate wastewater sorption zeolitic tuff Jordan

Introduction

Waste generated from industrial, commercial, mining, and agricultural operations and from community activities may result in contamination in water recourses. Fertilizer industry, laundries, and household washings produce wastewater enriched in phosphate ions (Mahmood et al., 2015). The presence of phosphate ions in water and wastewater is often responsible for eutrophication, which leads to various environmental and aesthetic problems in lakes, coastal areas, and other confined water bodies (Ouz et al., 2003). In addition, there is human health problems associated with phosphates. Phosphate residues on objects that have been cleaned with phosphate-containing detergents have been known to cause nausea, diarrhea, and skin irritations (New York State Department of Environmental Conservation, 2010: Permit No. GP-0-10-002). To meet the environmental phosphate regulations, the wastewater containing phosphate must be treated before being discharged into aquatic bodies.

Several techniques have been adopted to remove phosphate ions from water and wastewater (Morse et al., 1998; Rittmann et al., 2011; Strom, 2006). These include chemical precipitation (Song et al., 2002; Valsami-Jones, 2001), crystallization (Donnert and Salecker, 1999), biological processes (De La Noue and De Pauw, 1988, De-Bashan and Bashan, 2004; Oehmen et al., 2007; Van Loosdrecht et al., 1997), reverse osmosis (Van Voorthuizen et al., 2005), flotation (Anastassakis et al., 2004) magnetically enhanced coagulation (Woodard, 2006), membrane separation (Reardon, 2006), and adsorption. Nevertheless, adsorption is most adequate to treat wastewater, especially at lower phosphate concentrations (Zongmin et al., 2012). Extensive research has been conducted to utilize naturally occurring or synthetic adsorbent materials for phosphate removal. Many of these adsorbents have proven to be potential materials for the removal of phosphate from water and wastewater (Ahmed et al., 2012; Huang et al., 2008; Karapinar, 2009; Kioussis et al., 2000; Montalvo et al., 2011; Shin et al., 2004; Wang et al., 2007; Xiong and Peng, 2008).

Jordanian zeolitic tuffs (JZT) are hydrated alumino-silicates of the alkaline and earth metals principally; Na, K, Ca, and Mg. JZT minerals were generated from alteration of volcanic tuff in northeast and central of Jordan. The zeolite content in these tuffs varies from 20% to 65%. The Jordanian zeolitic tuff is characterized by high degree of hydration, low density, and large void volume when hydrated, stability of the crystal structure when dehydrated, cation exchange properties, and uniform molecular-sized channels in dehydrated crystals.

The JZT has been investigated toward its potential use as adsorbent for heavy metal removal (Al-Anbar and Al-Anbar, 2008; Al Dwairi and Al-Rawajfeh, 2012; Al-Shaybe and Khalili, 2009; Almjadleh et al., 2014; El-Bishtawi and Al-Haj, 1979; Musleh et al., 2005; Salem et al., 2010; Taamneh and Al Dwairi, 2013), phenols and derivatives (Baker et al., 2012; Yousef and El-Eswed, 2007), and sulfur compounds removal from diesel fuels (Mustafa et al., 2012). No studies have been conducted to investigate JZT for phosphate removal. In this study, we investigate the use of JZT as a potential adsorbent material for phosphate removal from aqueous solution. This research involves studying the kinetics of sorption process under different initial ion concentrations. The study also involves characterizing the equilibrium adsorption process by analyzing the adsorption isotherm at different adsorption temperatures.

Theoretical background

Adsorption isotherms

To examine the mechanism of phosphate sorption on JZT, the experimental data were subjected to different sorption isotherm models, namely: Langmuir, Freundlich, Temkin, and Dubinin-Kaganer-Radushkevich (DKR) isotherm equations.

Langmuir isotherm

Langmuir isotherm is used to describe single layer adsorption. The linearized form of Langmuir isotherm model is given as (Langmuir, 1916)

\frac{1}{q_{e}} = \frac{1}{q_{m}} + \frac{1}{b q_{m} C_{e}}

(1)

where

q_{e}

is the amount adsorbed per unit weight of adsorbate (mg/g),

C_{e}

is the equilibrium concentration of adsorbate in the solution after sorption (mg/L),

q_{m}

and b are the Langmuir constants.

q_{m}

indicates the saturated monolayer sorption capacity (mg/g) and b represents the sorption equilibrium constant (L/mg). The Langmuir isotherm parameters can be obtained by plotting

\frac{1}{q_{e}}

versus

C_{e}

Freundlich isotherm

The Freundlich isotherm is used for heterogenous surface systems. The linearized form of Freundlich isotherm equation is given as (Freundlich, 1906)

\log q_{e} = \log K + \frac{1}{n} \log C_{e}

(2)

where

q_{e}

is the amount adsorbed per unit weight of adsorbate (mg/g),

C_{e}

is the equilibrium concentration of adsorbate in the solution after sorption (mg/L), K and n are the Freundlich constants. K indicates the sorption capacity and n represents the reaction energy. The Langmuir isotherm parameters can be obtained by plotting

\log q_{e}

versus

\log C_{e}

Temkin isotherm

The Temkin isotherm model is based on the assumption that heat of sorption is linearly decreasing rather than logarithmically. The linearized form of Temkin isotherm equation is given as (Foo and Hameed, 2010)

q_{e} = B \ln A + B \ln C_{e}

(3)

where

B = RT / b_{T}

q_{e}

is the amount adsorbed per unit weight of adsorbate (mg/g),

C_{e}

is the equilibrium concentration of adsorbate in the solution after sorption (mg/L), T is the absolute temperature (K), R is the universal gas constant (8.314 J/mol.K), and

b_{T}

is a constant related to the heat of adsorption (J/mol). The Temkin isotherm parameters can be obtained by plotting

q_{e}

versus

\ln C_{e}

DKR isotherm

The DKR isotherm model does not assume homogenous surface or constant sorption potential. The linearized form of the DKR isotherm equation is given by (Foo and Hameed, 2010)

\ln q_{e} = \ln X_{m} - β ɛ^{2}

(4)

where

q_{e}

is the amount adsorbed per unit weight of adsorbate (mg/g),

X_{m}

is the maximum amount of ions that can be sorbed onto unit weight of sorbate (mg/g), β is the activity coefficient related to the sorption energy and ɛ is the Polanyi potential which is equal to

ɛ = RT \ln (1 + \frac{1}{C_{e}})

(5)

where R is the gas constant (kJ/mol.K), T is the absolute temperature (K) and

C_{e}

is the equilibrium concentration of adsorbate in the solution after sorption (mol/L).

The mean free energy of sorption E can be calculated by using the following relationship

E = \frac{1}{\sqrt{- 2 β}}

(6)

Adsorption kinetics

Several kinetics models are used to study the kinetics of phosphate sorption. This is crucial in identifying the characteristics of the sorbent surface and its effect on the sorption rate. These models are pseudo-second-order model, intraparticle diffusion model, pseudo-second-order model, and Elovich model. In this article, we present the analysis of pseudo-second-order model and Elovich model; other models failed in satisfactory description the kinetics of sorption.

Pseudo-second-order model

The pseudo-second-order model is given as (Ho and McKay, 1999)

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{t}{q_{e}}

(7)

where

q_{t}

is the amount of solute sorbed (mg/g) at a given time t (min) and

k_{2}

is the rate constant of the second order kinetic equation (g/mg.min).

Elovich model

The rate parameter for Elovich model is calculated by using the following equation (Juang and Chen, 1999)

q_{t} = E_{1} \ln (E_{2}) + E_{1} \ln t

(8)

where

E_{1}

and

E_{2}

are the equilibrium rate constants for the Elovich model.

Thermodynamic parameters

The following equations are used to determine the Gibbs free energy of sorption $Δ G^{o}$ (kJ/mol), heat of sorption $Δ H^{o}$ (kJ/mol) and the standard entropy change $Δ S^{o}$ (kJ/mol.K)

Δ G^{o} = - RT \ln b

(9)

\ln b = \frac{Δ S^{o}}{R} - \frac{Δ H^{o}}{RT}

(10)

where b is the Langmuir isotherm constant in a unit of (L/mol).

Experimental

Preparation and characterization of the adsorbent

The JZT was obtained from Jabal Aritayn area located around 50 km to the east from Amman.

The JZT was crushed then sieved. Particle size range of (125–200 µm) was utilized in this study without any further treatment.

The JZT was characterized by X-ray diffraction (XRD) and X-ray fluorescence (XRF). The samples were ground further to suit XRD sample preparation requirements. A computer controlled Hilton brooks generator with a Philips® X'Pert Pro PW 3040/60 diffractometer with an automatic divergence slit and a Cu anode producing X-rays of wavelength of 1.54056 A^o was used. The diffractometer was operating at 40 kV and 20 mA, and automatic routines allowed scanning for values of 2Ɵ from 3 ° to 83 ° using a step size of 0.05 ° and scan speed of 2 °/min controlled by Phillips® High Scor software. The diffraction data was analyzed by diffraction Technology “Traces V.3®” X-ray analytical software. Identification of mineral contained in the sample was achieved by comparing the X-ray spectrum with a database (Joint Committee on Powder Diffraction Standards–International Centre for Diffraction Data (JCPDS-ICDD)).

The chemical analysis of the JZT sample was carried out utilizing a Wavelength Dispersive Sequential X-Ray Fluorescence Spectrometry (XRF), type Philips PW1404. The spectrometer is controlled by Philip X40 software.

Preparation of phosphate solution

Artificial phosphate solutions were used throughout the adsorption tests. Initially, a stock solution of 1000 ppm in phosphates was prepared by dissolving a certain amount of chemically pure K₂HPO₄.3H₂O in distilled water. An aliquot of the stock solution was mixed with a certain volume of distilled water so that a phosphate solution prepared at the desired experimental concentration.

Batch sorption studies

The sorption studies were carried out by conducting batch sorption studies in 500 mL volumetric flasks. The dosage of JZT was kept constant at a concentration of 10 g/L. Hundred milliliters of solution containing different initial phosphate ion concentrations (50, 100, 200, 300, and 500 mg/L) and constant JZT dosage (10 g/L) were mixed by a magnetic stirrer (900 r/min) at different temperatures (293, 326, and 334 K). Phosphate ions were determined by standard spectrophotometric methods. Spectrophotometric methods often require the initial conversion of phosphates to soluble orthophosphates which can then be determined colorimetrically. Orthophosphates was analyzed quantitatively by the stannous chloride method (APHA, AWWA, WPCF, 1985). Molybdophosphoric acid reduction by stannous chloride to molybdenum results in the development of a blue color. Twenty-five grams of ammonium molybdate reagent (NH₄)₆Mo₇O₂₄.4H₂O was dissolved in 175 mL distilled water. Two hundred eighty milliliters of conc H₂SO₄ was added to 400 mL distilled water. The molybdate was then added and diluted to 1000 mL. Stannous chloride reagent 1: 2.5 g of SnCl₂.2H₂O was dissolved in 100 mL glycerol. The samples, standards, and reagents were kept within 2 ℃ of one another and in the temperature range between 20 ℃ and 30 ℃. Forty milliliters of the sample was taken and diluted to 100 mL with distilled water after discharging the pink color with acid. Then, 50 mL of this solution was taken and diluted to 100 mL with distilled water. Four milliliters of molybdate reagent was then added with thorough mixing. Finally, 0.5 mL (10 drops) stannous chloride reagent was added. Color measurements were taken 11 min after the reagents were added. The color was measured at 690 nm using Schimadzu UV-1601 Spectrophotometer. Distilled water was used as the blank.

Results and discussion

Characterization of JZT

The XRD pattern of JZT is depicted in Figure 1.

Figure 1.

XRD pattern of the raw JZT.

The XRD analysis revealed the presence of phillipsite as the major zeolite mineral phase in JZT. Other mineral phases are also present in JZT such as hematite, augite, bellite, and aluminian. This is in agreement with data reported in literature (Al-Harahsheh et al., 2014).

The XRF analysis of JZT is shown in Table 1.

Table 1.

XRF analysis of the raw JZT.

Oxide	Al₂O₃	CaO	Fe₂O₃	K₂O	MgO	MnO	Na₂O	P₂O₅	SiO₂	TiO₂	L.O.I
wt %	12.008	8.214	10.268	1.426	8.600	0.121	2.211	0.750	41.414	2.549	14.45

As shown in Table 1, SiO₂ is the main chemical constituent of JZT. Other oxides such as Al₂O₃, CaO, Fe₂O₃, and MgO are also present in JZT.

Effect of contact time

The time required to achieve maximum sorption of ions when contacted with the sorbent must be critically examined. The efficiency of separation is evaluated based on the percentage of phosphate removal as follows

% Re moval = \frac{C_{o} - C_{f}}{C_{0}} 100

(11)

where

C_{o}

and

C_{f}

the initial and final sorbate concentration (mg/L), respectively. The percentage removal of phosphate ions (100 mg/L) from 100 mL aqueous solution by utilizing 1 g of JZT has been studied at different contact times and the results are shown in Figure 2.

Figure 2.

Effect of contact time on the sorption of phosphate ions (900 r/min mixing rate, 100 mg/L initial phosphate concentration, 125 µm particle size and 20 ℃).

Increasing the contact time resulted in an increase in the percentage removal of phosphate ions from the aqueous solution for the first 3 h. The percentage removal of phosphate ions was initially high due to the large surface area of the JZT. Information from literature indicates that the BET surface area of JZT having a particle size <250 µm is 98.55 m²/g (Al-Harahsheh et al., 2014). As adsorption process time goes, any further increase in the contact time will has no effect on the sorption process. This indicates that the available surface for the sorption process has reached it maximum capacity within the first 3 h of contact time and no further enhancement in the percentage removal could be achieved by increasing the contact time. In addition, the results show that the process of sorption is stable when subjected to longer contact times. After 3 h of contact time, the adsorbent reached its maximum capacity which is reflected in a stable value of percentage removal.

Adsorption isotherms

The sorption of phosphate ions from the aqueous solution have been modelled by different adsorption isotherm equations (The linearized Langmuir, Freundlich, Temkin, and DKR isotherms) and at three different temperatures (293, 316, and 334 K). The linearized Langmuir, Freundlich, Temkin, and DKR isotherm fitting plots are shown in Figures 3 –6. The calculated Langmuir, Freundlich, Temkin, and DKR isotherm constants are listed in Table 2.

Figure 3.

The linearized Langmuir isotherm model of phosphate sorption on JZT.

Figure 4.

The linearized Freundlich isotherm model of phosphate sorption on JZT.

Figure 5.

The linearized Temkin isotherm model of phosphate sorption on JZT.

Figure 6.

The linearized DKR isotherm model of phosphate sorption on JZT.

Table 2.

Isotherm parameters for phosphate sorption on JZT.

			Temperature
			293 K	316 K	334 K
Langmuir isotherm	$q_{m}$	mg/g	18.9	21.3	25.0
	b	L/mg	0.0050	0.0021	0.0011
	$R_{L}$	—	0.53	0.72	0.82
	R ²	—	0.970	0.994	0.999
Freundlich isotherm	K	mg/g	0.247	0.116	0.048
	$\frac{1}{n}$		0.685	0.733	0.996
	R ²	—	0.952	0.961	0.996
Temkin isotherm	A	mg/L	0.048	0.043	0.030
	B	L/g	4.53	2.83	2.79
	b _T	KJ/mol	0.538	0.928	0.995
	R ²	—	0.828	0.988	0.962
DKR isotherm	β	(mol/kJ)²	0.008	0.008	0.008
	$X_{m}$	mg/g	62.2	47.2	50.2
	R ²	—	0.947	0.978	1.000
	E	kJ/mol	7.91	7.91	7.91

The criteria employed for comparing the performance of the different isotherms models in fitting this research experimental data is in comparing the average of the coefficient of determination (R² value) values for different isotherms. A comparison of the average of the coefficient of determination (R² value) for the different isotherms has been made and listed in Table 2. Langmuir isotherm model was found to be the most suitable for the data, followed by DKR, Freundlich then Temkin isotherm model. The Langmuir isotherm model has the best fit for the adsorption of phosphate ions on JZT at various temperatures.

The agreement between the Langmuir model and the experimental data suggests the validity of the Langmuir assumptions in describing the sorption process, which indicates that the sorption process is reversible, single site and monolayer molecule adsorption. To investigate whether the sorption process of phosphate ions by JZT is favorable or unfavorable for the Langmuir type adsorption, the separation factor was calculated based on the following equation (Webber and Chakkravorti, 1974)

R_{L} = \frac{1}{1 + b C_{o}}

(12)

where

R_{L}

is the initial concentration. It is reported that if

R_{L}

is less than unity, then the adsorption process is favorable (Foo and Hameed, 2010). It is clear from the results shown in Table 2 that the separation factor is less than unity for all temperature ranges applied in this study. This indicates that the sorption process of phosphate ions on JZT is a favorable process.

According to the obtained Langmuir isotherm parameters, the adsorption capacity increased with increasing the solution temperature. The saturated monolayer adsorption capacity of JZT increased from 18.9 to 25 mg/g by increasing the solution temperature from 293 to 334 K. On the other hand, increasing the solution temperature from 293 to 334 K decreased the value of the Langmuir adsorption constant from 0.0050 to 0.0011 L/mg. This indicates that the equilibrium phosphate ion concentration in the solution increases as a result of increasing the solution temperature. This might be attributed to solubility enhancement in the solution by temperature increase. This can be explained by visualizing the dynamic competing process that is expected to occur. At high temperatures, the activity of both the adsorbent and the adsorbate increases which result in easier escape of adsorbate from the adsorption sites. On the same time, the adsorbate solution will have a higher solvation capacity to accommodate the returning species which is called solubility enhancement. This can be visualized as a competing effect between adsorption affinity to be accommodated on the adsorbent and solvation affinity to be accommodated in the solvent. At high temperature, increasing the solution temperature will increase its ability to induce desorption of phosphate ions from the surface of JZT into the aqueous solution, thereby increasing its solution equilibrium concentration.

Another possible explanation that the sorption of phosphate is exothermic process. Increasing the sorption temperature will decrease the bond strength between phosphate ions and JZT, thereby, releasing phosphate ions from the surface of JZT into the solution and increasing their equilibrium concentration. This behavior is in agreement with literature data for the sorption of mercury onto activated carbon (Erhayem et al., 2015). The values of the maximum monolayer adsorption capacity of JZT are comparable to those reported in literature for similar systems. Liu et al. (2008) have reported a capacity of 29.7 mg/L for the adsorption of phosphate on a mesoporous ZrO₂ system. Long et al. (2011) have reported a capacity of 13.6 mg/L for the adsorption of phosphate on a magnetic Fe–Zr binary oxide. Rodrigues and Silva (2009) have reported a capacity of 13 mg/L for the adsorption of phosphate onto a hydrous niobium oxide system. Zongmin et al. (2012) have reported a capacity of 33.4 mg/L for the adsorption of phosphate onto a Fe–Zr binary oxide system.

The DKR isotherm has also successfully described the experimental equilibrium sorption data especially at higher temperatures. The coefficients of determination were calculated to be 0.978 and 1.000 for adsorption temperatures of 316 and 334 K, respectively. The DKR isotherm model does not assume a homogeneous surface or a constant sorption potential (Rao and Kashifuddin, 2014). It explains the physical and chemical characterization of adsorption. The DRK isotherm parameter was found to be 0.008 mol²/kJ² for all adsorption temperatures employed in this study. The maximum sorption capacities as predicted by the DKR isotherm model were found to be 62.2, 47.2, and 50.2 mg/g for adsorption temperatures of 293, 316, and 334 K, respectively. The maximum sorption capacities as obtained by the DKR isotherm model are larger than those predicted by the Langmuir model and those obtained experimentally. The corresponding mean free energy of sorption was found to be 7.91 kJ/mol independent on the adsorption temperature. The mean free energy of sorption can be used to evaluate the nature of interaction between phosphate ions and the binding sites of JZT. It has been reported that values of mean free energy of sorption larger than 8 and less than 16 kJ/mol represent chemical ion exchange process, whereas values <8 kJ/mol represent a physical type adsorption process (Rao and Kashifuddin, 2014). Based on the calculated mean free energy of the sorption of phosphate ions by JZT, the process of sorption might be considered a physical adsorption process.

The experimental data also adequately fitted the Freundlich isotherm model especially at higher solution temperature. The coefficient of determination is 0.996 when the model is employed to fit the data obtained at a sorption temperature of 334 K. This suggests that JZT at higher sorption temperatures undergoes nonideal and reversible adsorption behavior. The values of the Freundlich sorption capacity (K) increased with increasing the temperature. The values of the slope ( $\frac{1}{n}$ ) for temperatures are below 1, which indicates a strong phosphate ion bonding on JNZ surface.

The experimental data was barely fitted the Temkin isotherm model. The coefficients of determination were calculated to be 0.828, 0.988, and 0.962 for adsorption temperatures of 293, 316, and 334 K, respectively. The Temkin isotherm constant b_T is considered an indicator of the maximum binding energy. The results show that increasing the adsorption temperature increased the equilibrium binding constant. Therefore, the adsorption of phosphate is favorable at lower adsorption temperature.

Kinetics of sorption

The kinetic of sorption was modeled by two sorption kinetic models, namely, psedo second order kinetic model and the Elovich model . The resluts are shown in Figures 7 and 8 and the coressponding constants are listed in Table 3.

Figure 7.

The linearized pseudo-second-order kinetics plot for phosphate sorption on JZT.

Figure 8.

The linearized Elovich kinetics plot for phosphate sorption on JZT.

Table 3.

Kinetic parameters for phosphate sorption on JZT.

		Initial phosphate concentration
		50 mg/L	100 mg/L	200 mg/L	300 mg/L
Pseudo-second-order kinetics model	q _e	2.27	4.72	6.33	8.70
	q _{e, exp}	2.18	4.45	6.16	8.27
	k ₂	0.977	0.226	0.125	0.066
	R ²	0.998	0.997	0.997	0.999
Elovich model	E ₁	1.35	0.91	0.79	0.35
	E ₂	83.0	230.6	57.1	73.5
	R ²	0.936	0.645	0.912	0.931

A comparison of the coefficient of determination (R² value) for the different kinetic models shows that pseudo-second-order kinetic model was most appropriate to fit the data. The pseudo-second-order kinetic model has best fitted for the adsorption of phosphate ions on JZT for different initial phosphate concentrations.

The pseudo-second-order kinetic model assumes that the rate of sorption is proportional to the square of the number of active sites available for the sorption process. Higher sorption rates are obtained at lower initial phosphate concentrations. This is reflected by the higher pseudo-second-order rate constants obtained at lower initial phosphate concentration. This might be explained by easier access of the phosphate ions into the active sites of JZT at lower concentration. Higher phosphate ion concentrations will induce ion interaction that might hinder the accessibility of ions into the active sites of JZT.

It is worth mentioning that the values of the equilibrium adsorption capacity q_e as predicted by the pseudo-second-order kinetic model are very close to those obtained experimentally. This confirms the adequacy of the pseudo-second-order kinetic model in describing the kinetics of phosphate ion sorption by JZT.

Adsorption thermodynamics

Adsorption thermodynamic analysis is a very important component in getting the full analysis picture of this research. Thermodynamics is concerned with energy changes that can predict the spontaneity direction of processes (destiny of processes). This can clearly evidenced the favorability in adsorption common terms.

The plot between $\ln b$ and $\frac{1}{T}$ was found to be linear (Figure 9). Thermodynamic parameters for the adsorption of phosphate ions on JZT are given in Table 4.

Figure 9.

Gibbs's free energy change plot.

Table 4.

Thermodynamic parameters for the sorption of phosphate ions on JZT.

	T = 293 K	T = 316 K	T = 334 K
$Δ G^{o}$ (kJ/mol)	−15.0	−13.8	−12.8
$Δ H^{o}$ (kJ/mol)	−35.5
$Δ S^{o}$ (J/mol.K)	−0.068

The calculated thermodynamic parameters showed a negative value for the Gibbs's free energy change $Δ G^{o}$ . This means that the sorption of phosphate ions by JZT is a spontaneous process. The results also show that increasing the adsorption temperature increases the Gibbs's free energy change. This suggests that sorption is preferred at lower temperatures. The negative value for the enthalpy change $Δ H^{o}$ indicates that the sorption of phosphate ions by JZT is an exothermic process and confirms the spontaneous nature of adsorption at lower temperatures. The negative value for the entropy change $Δ S^{o}$ indicates that the sorption of phosphate ions by JZT induces small degree of chaos of the system and indicates small structural changes occur at the JZT surface.

Conclusions

Jordanian zeolitic tuff has a high adsorption capacity for phosphate ion removal from aqueous solution. The adsorption process follows the Langmuir, Freundlich, and DKR isotherm models with Langmuir model being the best in describing the experimental data. The adsorption process is spontaneous and exothermic. The adsorption process is favorable at lower adsorption temperature. The sorption is attributed to physisorption with small degree of randomness occurring at the surface of adsorbent. The pseudo-second-order kinetic model is applicable for the whole range for contact time and applied successfully for different initial ion concentrations.

Jordanian zeolitic tuff is a potentially viable natural adsorbent material for phosphate ion removal from aqueous solution.

Footnotes

Acknowledgements

Lab engineer Ali Alzoubi of the Chemical Engineering Department at Mutah University is acknowledged for laboratory assistance. Also, the authors thank the anonymous reviewers for their insightful comments and feedback.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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