Investigation of Taguchi optimization,equilibrium isotherms,and kinetic modeling for phosphorus adsorption onto natural zeolite of clinoptilolite type

Abstract

Adsorption process plays an important role in the removal of phosphorus from aqueous solutions. A laboratory experiment was conducted to investigate the adsorption characteristics of phosphorus onto natural zeolite and to find out the relative importance of some controllable treatments in phosphorus adsorption process using the Taguchi optimization methodology. Results showed that the adsorption of phosphorus in the presence of Fe³⁺ and Al³⁺ was higher than that in the absence of these two cations probably due to the adsorption of phosphorus-bearing anions on opposite charges of these cations. Also, increase in contact time tended phosphorus adsorption to be increased. The addition of base and acid treatments caused an increase and a decrease, respectively, on phosphorus adsorption. The order of effectiveness of treatments on the values of phosphorus adsorption was as follows: acid/base treatment >sorbent to sorbate ratio > modification with aluminium (Al)/iron (Fe) >contact time >phosphorus concentration. Phosphorus adsorption data well fitted to the Freundlich isotherm model. The pseudo-second order was the best model describing phosphorus adsorption kinetics. According to the results reported herein, it is assumed that the main mechanism controlling phosphorus adsorption onto natural zeolite is chemisorption.

Keywords

Phosphate ions adsorption zeolite kinetics Taguchi

Introduction

The discharge of excess phosphorus (P) into freshwater ecosystems can cause eutrophication which is a concern from environmental standpoint demonstrating the importance of P removal from wastewater via physical, chemical, and/or biological techniques (Lürling et al., 2014; Wang et al., 2016). Adsorption plays an important role in the removal of nutrients including P (Zhang et al., 2014). Due to simplicity, being cost-effective and environmentally friendly (Googerdchian et al., 2018), and relatively high efficient nature, this technique is one of the most prevalent physical techniques for the removal of a wide range of pollutants (Callery et al., 2016; Oguz, 2004).

Zeolites, naturally occurring hydrated alumino-silicates, are kinds of plentiful, high chemical and mechanical stability, high specific surface area, abundant reserve, low cost, and environmentally friendly materials which have been widely used as adsorbents for environmental cleanup (Šljivić et al., 2009; Wang and Peng, 2010; Zhan et al., 2017). Natural zeolites especially clinoptilolite types are known as an efficient ion exchanger and ion selective materials and substrates for fixing aqueous ionic species through their wide range of channels present in their crystalline structure and also their strong surface reactivity (Altin et al., 1998; Mahabadi et al., 2007). Guo et al. (2017) studied P adsorption by parent Al-modified eggshell and they observed that the value of P adsorption in Al-modified eggshell was significantly higher than parent eggshell. Similar results for Fe³⁺ and Na⁺ in P removal by ferric-modified zeolite reported by Gao et al. (2018). Presence of Ca²⁺ and Na⁺ enhanced P adsorption by zirconium-modified zeolite (Zhan et al., 2017).

Adsorption can be affected by different factors including solute concentration, sorbate to sorbent ratio, contact time, temperature, sorbent type and modification, ionic strength, and pH (Wang et al., 2016; Zhang et al., 2015; Zolfaghari et al., 2011). Adsorption characteristics of P can be investigated using linear and nonlinear kinetic and isotherms studies (Zhang et al., 2015). Commonly used Langmuir and Freundlich models are useful in describing adsorption isotherms and providing effective factors in predicting adsorption characteristics of nutrients (Cheng et al., 2014).

Wang et al. (2016) found that the Langmuir isotherm well predicts the patterns of P adsorption by fly ash, red mud, and ferric-alum water treatment residues. They reported that the pseudo-second order was the best model describing the kinetics of P adsorption onto different solid wastes.

To achieve maximum removal of pollutant, adsorption factors should be optimized. Therefore, using an experimental design based on statistical techniques of optimization has been found to be an effective method for analysis and evaluation of the best conditions of different investigations (Googerdchian et al., 2018). The Taguchi approach is a statistical experimental method designed based on orthogonal arrays which are much efficient in comparison with full factorial experiments since the number of experiments and hence time and cost of the experiment can be drastically reduced by using this method (Madan and Wasewar, 2017). It has been proved to be successful over the last 15 years for the improvement of product quality and process performance (Roy, 2002). Taguchi optimization methods have been used in aqueous solution to investigate removal of different pollutants such as removal of nickel (Ni) from waste water by calcined ouster shell powders (Yen and Li, 2015), adsorption of P onto Al/Ca-impregnated granular clay material (Yu et al., 2015), Cd adsorption by spent coffee (Yen and Lin, 2016), removal of Th (IV) from aqueous solution by deoiled karanja seed cake (Varala et al., 2016), removal of copper and Ni by growing Aspergillus sp. (Pundir et al., 2016), removal of benzeneacetic acid from aqueous solution using CaO₂ nanoparticles (Madan and Wasewar, 2017) and lead and mercury by nanostructure and modified zinc-oxide (Zolfaghari et al., 2011).

Research on the P removal from aquatic systems using low-cost materials is of great interest for researchers. Limited research (if any) has been conducted on Taguchi optimization for P adsorption using modified zeolite. The main objectives of present study were to investigate the adsorption characteristics of P onto natural zeolite and to find out the relative importance of initial P concentration, sorbent to sorbate ratio, contact time, modification of sorbent and acid/base treatments on the retention of P onto natural zeolite using Taguchi optimization methodology.

Experimental analysis

Taguchi experimental method

According to the Taguchi method, experimental design is arranged based on the orthogonal arrays which consist of the controllable factors. Five controllable factors including sorbent to sorbate ratio (SR), P concentration (C), contact time (T), acid/base addition (AB) and sorbent modification using Al/Fe (AF) were considered in the present study (Table 1).

Table 1.

Controllable factors and associated levels.

Factors	Description	Level 1	Level 2	Level 3
SR	Sorbent to sorbate ratio (g mL⁻¹)	(1:100)	(1:150)	(1:250)
C	Phosphorus concentrations (mg L⁻¹)	60	75	90
T	Contact time (hours)	5	10	20
AB	Acid/base addition	—	NaOH	HCl
AF	Sorbent modification using Al/Fe	—	Al	Fe

The Taguchi experimental design (L27 orthogonal array) (Minitab, Inc., version 14, USA) was used to determine the optimal conditions for the adsorption of P from aqueous solution (Table 2). A total number of 3⁵ = 243 experiments would require for a full factorial experiment. By using the Taguchi method, the number of experiments was reduced to 27 experiments. Therefore, only about 10% of the total number of samples was considered by the orthogonal array.

Table 2.

Design of experiments.

Test identification	Factor
Test identification	T	C	SR	AB	AF
1	1	1	1	1	1
2	1	1	1	1	2
3	1	1	1	1	3
4	1	2	2	2	1
5	1	2	2	2	2
6	1	2	2	2	3
7	1	3	3	3	1
8	1	3	3	3	2
9	1	3	3	3	3
10	2	1	2	3	1
11	2	1	2	3	2
12	2	1	2	3	3
13	2	2	3	1	1
14	2	2	3	1	2
15	2	2	3	1	3
16	2	3	1	2	1
17	2	3	1	2	2
18	2	3	1	2	3
19	3	1	3	2	1
20	3	1	3	2	2
21	3	1	3	2	3
22	3	2	1	3	1
23	3	2	1	3	2
24	3	2	1	3	3
25	3	3	2	1	1
26	3	3	2	1	2
27	3	3	2	1	3

T: contact time; C: phosphorus concentrations; SR: sorbent to sorbate ratio; AB: acid/base addition; AF: sorbent modification using Al/Fe.

The signal to noise ratio (S:N) is needed for the evaluation of the experimental results and quality determination. Three types of S:N ratio analysis are embedded: (1) lower is better (LB), (2) nominal is best (NB), and (3) higher is better (HB). The S:N ratio of HB was considered since the target of this study was to maximize P removal efficiency (equation 1)

{S:N}_{(HB)} = - 10 log [\frac{1}{n} \sum {(\frac{1}{R M})}^{2}] (Rezaei et al ., 2017)

(1)

where the subscript HB represents the type of S:N ratio, higher is better, n is the number of repetitions under the same experimental conditions and RM represents the results of measurements. To achieve this objective, the results of S:N ratios were analyzed using Minitab software package (version 14; Minitab Inc., version 14).

As the goal of this study was to remove P by zeolite, the quality characteristic selected was HB of P removal defined by equation (2).

R e m o v a l p e r c e n t a g e = \frac{C o - C e}{C o} \times 100 (Idan et al ., 2018)

(2)

where c₀ and c_e (mg L⁻¹) are the initial and equilibrium concentration of P, respectively.

ANOVA statistical method is also used to have a better understanding of how the observed results are reliable (Googerdchian et al., 2018). ANOVA performs to investigate the influence of each controllable factor on the removal of P and to determine percentage contribution of each factor (Madan and Wasewar, 2017). One-way ANOVA using Duncan’s multiple range test was employed to determine the significant differences (α = 0.05) in relevant parameters during modification. The result rechecked by ANOVA variance statistical approach using SPSS 17.0. The percentage contribution of each factor, RF, was calculated using equation (3).

R F = \frac{S S R - (D O F)}{S S T} \times 100 (Zolfaghari et al ., 2011)

(3)

where SSR and SST represent total and treatment sum of squares, respectively, and DOF represents the degree of freedom for each controllable factor.

Three levels of initial P concentrations (60, 75, and 90 mg L⁻¹) as potassium dihydrogen phosphate in 0.01 M calcium chloride solution were used. Zeolite used in the experiment belongs to a natural zeolite mine located in Semnan province, Iran (Table 3). Three levels of SR including 1:100, 1:150, and 1:250 considered in the experiment. Three levels of acid/base treatments including 0 M hydrochloric acid, 0.5 M hydrochloric acid, or 0.5 M sodium hydroxide (1:10 zeolite mass) were also considered. Blank (nontreated zeolite) aluminum hydroxide (Merck, product number 1091) and iron metal (BDH, product number 28602) used as Al/Fe treatments for zeolite modification (1:5 zeolite mass) because of P-bearing anions have negative charges and could be easily adsorbed onto positively charged cations. Zeolite treated by Al/Fe or acid/base for 24 hours in an oven at 65°C. Acid/base and Al/Fe treatments add to the samples with abovementioned concentrations and shaken with an end over end shaker for periods of 5, 10 and 20 hours as T treatments. The selection of these periods was due to the fact that adsorption of P occurred mostly through the first shaking periods up to 20 hours (Figure 2). Similarly, Wang et al. (2016) observed that a considerable increase in P removal usually occurred during first shaking periods up to about 1 day. After each shaking period, samples centrifuge for 15 minutes and filtrated with Whatman no. 42 filter paper. The content of P was measured according to the Olsen method (1954) by spectrophotometer (Shimadzu AA-670G).

Table 3.

The values of zeolite characteristics.

Characteristics	Values	Characteristics	Values	Characteristics	Values
SiO₂	65%	P₂O₅	0.01%	CEC	54 cmol₍₊₎ kg⁻¹
Al₂O₃	12%	Na₂O	1.8%	Bulk density	1.8 g cm⁻³
Fe₂O₃	1.5%	CaO	2.3%	Color	Light gray

Table 4.

The values of phosphorus removal and S:N ratio for different treatments.

	Average removal				Average S:N
T	Zeolite	Zeolite + Al	Zeolite + Fe	T	Zeolite	Zeolite + Al	Zeolite + Fe
5	65.41742	59.75699	86.89963	5	33.94356	33.66498	38.72545
10	61.74762	62.02222	73.64109	10	34.75996	35.031	36.91042
20	72.80563	79.03683	89.95509	20	36.67395	37.814	39.05458
Mean	66.65689	66.93868	83.4986	Mean	35.12582	35.50333	38.23015

Kinetics experiment

To investigate the kinetics of P adsorption, zeolite equilibrates with five concentrations of P (30, 45, 60, 75, and 90 mgL⁻¹) for periods of 5, 10, 20, 30, and 40 hours. This experiment performs with three SR ratios of 1:100, 1:150, and 1:250. The samples were centrifuged immediately and then filtered to separate the liquid and solid phase. The amount of P in supernatant was measured by using spectrophotometer.

Kinetics equations used in the experiment were as follows (equations (4) to (7))

Pseudo-first-order kinetics model (\frac{d q t}{d t} = k 1 (q e - q t))) (Lin et al ., 2018)

(4)

Pseudo-second-order kinetics model (\frac{d q t}{d t} = k {(q e - q t)}^{2}) (Lin et al ., 2018)

(5)

Elovich equation (\frac{d q t}{d t} = α exp (- β q t)) (Ghasemi-Fasaei et al ., 2012)

(6)

Two constant rate (power function) (q t = a t^{b}) (Ghasemi-Fasaei et al ., 2012)

(7)

where qt is the sorption capacity at time t (mg g⁻¹), q_e is the amount of P adsorbed at equilibrium (mg g⁻¹), k is the rate constant of adsorption (L mg⁻¹ min⁻¹), k1 is the rate constant of pseudo-first-order sorption (mg g⁻¹), α is initial sorption rate of the Elovich (mg g⁻¹ min⁻¹), and β is the sorption constant for the Elovich model (g mg⁻¹). The parameter a is initial sorption constant (mg g⁻¹ min⁻¹), and b represents sorption rate coefficient (mg g⁻¹) (Yang et al., 2018).

The goodness of fits was determined by comparing the values of coefficients of determination (R²), and standard errors of estimate (SE). Kinetic models with relatively high R² and low SE values were considered as the best-fitted models.

Adsorption isotherms

To investigate the influence of different SR on P adsorption, 0.12, 0.2, and 0.3 g of zeolite equilibrated with 30 mL solution containing 30, 45, 60, 75, or 90 mg P L⁻¹ as potassium dihydrogen phosphate in 0.01 M CaCl₂ on an end over end shaker for 40 hours. The time 40 hours considered as equilibrium time in present study (Figure 1). Similarly, in previous studies, it has been observed that after 35 hours, the absorption of P is steady and without changes (Wang et al., 2016). Samples centrifuged and filtered and the amount of P in supernatants determined. The amount of P adsorption at equilibrium was calculated by equation (8)

q e = \frac{c 0 - c e}{w} \times v (Zhu and Yang, 2018)

(8)

where c₀ and c_e (mg L⁻¹) are the initial and equilibrium concentration of P, respectively. V is the volume of solution (L), and W is the mass of dry zeolite (g). The adsorption data of P was fitted to the Langmuir and Freundlich isotherms (equations (9) and (10), respectively).

q = \frac{q m . k l . c e}{1 + k l . c e} (Loganthan et al ., 2018)

(9)

q e = K f . C e^{1 / n} (Loganthan et al ., 2018)

(10)

where the q_e is the amount of P adsorbed per mass unit of zeolite (mg g⁻¹), C_e is equilibrium concentration of P in solution (mg L⁻¹), q_m represents maximum adsorption capacity on a monolayer of adsorbent (mg g⁻¹), and K_L (Langmuir constant) is a criterion of the adsorption intensity (mg L⁻¹).

Figure 1.

Time effect in amount of phosphorus (P) adsorbed by zeolite in different P concentrations.

The values of n and k_f, the Freundlich coefficients, calculated from the slope and intercept of the log q_e versus log C_e in the linear form of Freundlich isotherm. It has been shown that the adsorption mechanism is chemical, physical, and linear for n being less than, greater than and equal to unity, respectively (Bansal et al., 2005).

Result and discussion

Taguchi analysis

Taguchi method used to find the ideal conditions for removing P from aqueous solution. Different S:N ratios for the evaluation of the significance of treatments were shown in Figure 2. As seen in Figure 2(a), the values of S:N ratios increased by increasing in contact times as 20 hours treatment was the maximum S:N ratio resulted in higher P removal by zeolite. A similar result of P adsorption in zeolite was reported by Gan et al. (2016). They observed that greatest P removal from aqueous solution by modified zeolite at 20–25 hours. According to Figure 2(b), the value of S:N ratio increased as initial P concentration decreased. Yu et al. (2015) and Zhang et al. (2014) observed similar results for P removal percentage in aqueous solutions. Decreasing initial Pb concentration by increasing the values of S:N ratio reported by Googerdchian et al. (2018).

Figure 2.

Estimated individual factors effect on percentage of S:N by zeolite from aqueous solution.

The effect of SR on P adsorption is presented in Figure 2(c). As SR increased, S:N ratios decreased probably due to the reduction in adsorption sites. Similar results have been reported by Yen and Lin (2016). Maximum dosage for P adsorption in the bentonite-alum system was 0.1 g or SR (1:300) (Mahadevan et al., 2018) which was relatively close to the value of SR in the present study (1:250).

Increase in Cd and Hg adsorptions as a result of an increase in pH value has been reported by Zolfaghari et al. (2011) which is in agreement with results of a present study which demonstrated the significant role of NaOH in increasing P adsorption (Figure 1(d)). According to the results of Oliveira et al. (2015), zeolite was an efficient and promising adsorbent for P removal. They observed that maximum P removal occurred in higher pH. Al-Zboon (2017) showed that high P removal efficiency by activated carbon–silica nanoparticles composite, kaolin, and olive cake at high pH conditions. Gao et al. (2018) investigated phosphate removal by ferric-modified zeolite and reported modification treatment media such as Na salt and HCl increased phosphate sorption removal in order Na + Fe > Fe > HCl + Fe.

Effect of sorbent modification using Al/Fe treatments was presented in Figure 2(e). The sorbent modification of zeolite using Fe was an effective operation which considerably increased the value of P adsorption by sorbent from aqueous solution. It appears that Fe is also effective in the stabilization of cadmium (Saffari et al., 2015). Oliveira et al. (2015) observed high P removal ability in Fe-modified zeolite followed by Al-modified zeolite. Shanableh et al. (2016) studied P adsorption using Al/Fe modified bentonite. They observed that using Fe-modified bentonite increased both adsorption capacity and adsorption rate as compared to using Al-modified bentonite.

The highest significant factor influencing P adsorption onto zeolite was AB and the order of treatments importance was as follows AB > SR > AF > T > C. Yu et al. (2015) observed that the type of adsorbent was the most significant factor affected P removal. The results of ANOVA showed the same order for the influence of different treatments on P adsorption. Sum of square (SSF) and percentage contribution (PF) of each controllable factor are given in Table 5. The highest SSF and PF were related to AB treatment, followed by SR, AF, T, and C, respectively. The adsorption of P in the presence of Fe³⁺ and Al³⁺ was higher than that in the absence of these two cations, probably due to the fact that P-bearing anions have negative charges and could be easily adsorbed onto positively charged cations, resulting in increasing percentage of P removal from the solution (Zhou et al., 2011). Other researchers have reported similar results in their research on other adsorbents along with Fe and Al oxides (Liu et al., 2007; Oliveira et al., 2015; Pengthamkeerati et al., 2008; Xiong and Peng, 2008).

Table 5.

Response table of S:N ratio and contribution of each controllable factor.

	T	C	SR	AB	AF
Level 1	35.445	36.691	37.737	36.501	35.126
Level 2	35.567	36.660	36.766	39.960	35.503
Level 3	37.848	35.509	34.356	32.398	38.230
DOF_F	2	2	2	2	2
SSF	16.483	4.086	27.267	128.989	25.822
PF(%)	0.022	-0.012	0.052	0.334	0.048
Rank	4	5	2	1	3

DOF: degrees of freedom; SSF: sum of square of factor; PF: percentage contribution of factor.

Kinetics of P adsorption

The patterns of P adsorption at different periods evaluate by different models including pseudo-second-order, pseudo-first-order, elovich, and power function models (Table 6). The values of R² and SE for different models are given in Table 6. The kinetics of P adsorption from aqueous solution by zeolite well fitted to the models in order of pseudo-second order, pseudo-first order, power function, and elovich (Table 6). The fit of experimental data to elovich equation was not satisfied due to high values of SE for this model (Table 6). The values of P adsorption decreased as the values of SR increased (Figure 3). The conformity of the experimental data to the pseudo-second-order model indicates that the P adsorption process is chemically controlled (Ofomaja et al., 2010). Similarly, Aljbour et al. (2017) observed that phosphate removal from aqueous solutions by natural Jordanian zeolitic tuff well conformed to pseudo-second-order model. As shaking periods increased, P sorption on zeolite surfaces increased which is in agreement with other studies (Wang et al., 2016; Zhang et al., 2016). Lin et al. (2018) reported P adsorption onto chitosan fitted to pseudo-second-order model indicating the occurrence of chemical adsorption. Pseudo-second-order parameters (q_e and k) were calculated and shown in Table 7. The comparison of q_e and initial concentrations of P indicated that q_e increased as initial P concentration increased. The value of q_e derived from pseudo-second order was higher with 0.2 g zeolite (1:150 SR) which is in accordance with the results of isotherm study. Zhang et al. (2014) reported relatively similar results and observed that the optimum dosage for P removal by zeolite was 0.25 (g L⁻¹).

Table 6.

The values of R² and SE for different kinetic models.

		Pseudo-first order		Pseudo-second order		Elovich		Power function
C	SR	SE	R ²	SE	R ²	SE	R ²	SE	R ²
30	100	0.048944	0.926799	0.010997	0.999256	19.35553	0.998069	0.015057	0.993072
	150	0.082669	0.945902	0.035237	0.981134	88.45164	0.990866	0.033738	0.99099
	250	0.039338	0.925855	0.028622	0.986035	177.8219	0.918006	0.056229	0.848516
45	100	0.016748	0.960763	0.007885	0.999064	41.09864	0.988457	0.011854	0.980344
	150	0.083568	0.942776	0.057056	0.858671	447.5988	0.92612	0.011854	0.877219
	250	0.010543	0.979447	0.004788	0.999092	37.77507	0.995272	0.122411	0.993899
60	100	0.033945	0.948539	0.01233	0.994323	160.1708	0.971712	0.005744	0.952756
	150	0.004997	0.998825	0.011365	0.992385	185.8319	0.974688	0.032524	0.965074
	250	0.003935	0.988415	0.002933	0.999265	94.11642	0.950479	0.011051	0.908634
75	100	0.035001	0.909821	0.007875	0.994797	273.5557	0.94616	0.036327	0.902862
	150	0.022391	0.946232	0.003642	0.998676	137.6866	0.984419	0.0159	0.972885
	250	0.020013	0.958654	0.004663	0.996391	306.5427	0.954393	0.028307	0.917286
90	100	0.003248	0.999601	0.007731	0.990035	339.2684	0.974512	0.029368	0.967349
	150	0.033444	0.927877	0.006922	0.99217	443.8576	0.933543	0.043551	0.877696
	250	0.017906	0.980036	0.002562	0.997606	178.939	0.995324	0.00774	0.99627
Mean		0.030446	0.95597	0.01364	0.98526	195.4714	0.966801	0.030777	0.94299

C: concentration; SE: standard error; SR: sorbent to sorbate ratio.

Figure 3.

Phosphorus (P) adsorption at different sorbent to sorbate ratio using the nonlinear pseudo-second-order kinetic model, the P concentration in solution was 90 (mg L⁻¹).

Table 7.

Mean R² and kinetic parameters for pseudo-second-order model at different concentration (C) and sorbent to sorbate ratio (SR).

	Pseudo-second order
C	q_e	k	R ²
30	3055.556	7.65E-07	0.988808
45	6111.111	1.23E-06	0.952275
60	8333.333	6.89E-07	0.995324
75	10833.33	4.15E-07	0.996621
90	11666.67	1.71E-07	0.993271
SR	q_e	k	R ²
1:100	6666.667	8.52E-07	0.995495
1:150	9166.667	2.67E-07	0.964607
1:250	8166.667	8.46E-07	0.995677

Adsorption isotherms

The P adsorption data were fitted to the Langmuir and Freundlich adsorption isotherm models (Table 8). Both the Freundlich and Langmuir models well described the patterns of P adsorption onto zeolite at different SRs as evidenced by their relatively high R² and low SE values. Langmuir showed lower SE values and also lower R² values as compared to the Freundlich. According to Kim et al. (2018) n_f values between 1 and 10 showed that the adsorption was effective. They believed that as k_f value increased the affinity of sorbate toward the sorbent increased. The value of n_f derived from the Freundlich isotherm was in the following order: D2 (1:150) > D3 (1:200) > D1(1:100), indicating that 0.2 g zeolite had higher P adsorption efficiency (P adsorption per area unit). The largest k_f were in D1 (1:100) zeolite which was attributed to the presence of the highest sorption site under this condition. Mahadevan et al. (2018) reported the values of n_f ranged from 0.7 to 2.5 for P removal by the bentonite-alum system. Kim et al. (2018) reported the same results for P adsorption by fly ash. According to Novais et al. (2018), the values of n_f ranged from 1 to 3 for P adsorption onto different types of biochar. A similar relationship was observed in Langmuir model between P adsorption capacity (b_L) and zeolite dose. The zeolite dose of 0.2–0.3 (g ml⁻¹) was the best dose for Cd adsorption as reported by Selim et al. (2017). The values of maximum adsorption capacity criterion (b_L) ranged from about 40 to 50 mg g⁻¹. Similar ranges reported by Abdelhay et al. (2017) for the adsorption of phosphate onto natural reed.

Table 8.

Langmuir and Freundlich isotherm parameters for phosphorus adsorption.

	Freundlich
SR	SE	R ²	n _f	k _f
1:100	0.028191781	0.992797	0.8273	33.2353
1:150	0.00545885	0.999639	2.0559	20.9411
1:250	0.026528557	0.992281	1.7911	24.1434
	Langmuir
	SE	R ²	b_L (µg/g)	k _L
1:100	0.000545262	0.967273	40290.08864	8.73021E-06
1:150	0.000487376	0.905353	49407.11462	1.07717E-05
1:250	0.000170943	0.984838	49091.80167	1.83845E-05

SE: standard error; SR: sorbent to sorbate ratio.

Conclusion

Results of Taguchi method shown that acid/base treatment was the most effective treatment influencing the values of P adsorption by zeolite from aqueous solution. The order of treatments importance was as acid-base addition >sorbent to sorbate ratio >sorbent modification using Al/Fe >contact time > phosphorus concentration. The optimization conditions for P removal in the present study are contact time (20 hours), P initial concentration (60 mg L⁻¹), sorbent to sorbate ratio (1:250), and modification of zeolite with NaOH and Fe. The Freundlich model was better in describing the P adsorption on different sorbent to sorbate ratios as compared to the Langmuir. The kinetics of P adsorption from aqueous solution by zeolite well fitted to the pseudo-second-order model. Based on the results of kinetic studies, it appears that adsorption of P by zeolite is chemically controlled. Results reported herein demonstrated that the modification of zeolite with Fe along with base addition could be promising for efficient P removal from aqueous solutions.

Footnotes

Acknowledgements

Authors would like to appreciate Shiraz University for providing research facilities.

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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