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Water SA

versión On-line ISSN 1816-7950
versión impresa ISSN 0378-4738

Water SA vol.39 no.1 Pretoria ene. 2013


IICollege of Resources and Civil Engineering, Northeastern University, Shenyang, 110819, China
IIILaboratory of Nanomineralogy and Environmental Material, School of Resources and Environmental Engineering, Hefei University of Technology, Hefei, 230009, China




The objectives of this study were to modify Chinese natural zeolite by NaCl and to investigate its suitability as a low-cost clay adsorbent to remove ammonium from aqueous solution. The effect of pH on ammonium removal was investigated by batch experiments. The findings indicated that pH has a significant effect on the removal of ammonium by M-Zeo and maximum adsorption occured at pH 8. Ion exchange dominated the ammonium adsorption process at neutral pH, with the order of exchange selectivity being Na+ > Ca2+ > K+ > Mg2+. The Freundlich model provided a better description of the adsorption process than the Langmuir model. The maximum ammonium adsorption capacity was 17.83 mg/g for M-Zeo at 293K. Considering the adsorption isotherms and thermodynamic studies, the adsorption of ammonium by M-Zeo was endothermic and spontaneous chemisorption. Kinetic studies indicated that the adsorption of ammonium onto M-Zeo is well fitted by the pseudo-second-order kinetic model. Ea in the Arrhenius equation suggested the adsorption of ammonium on M-Zeo was a fast and diffusion-controlled process. The regeneration rate was 90.61% after 5 cycles. The removal of ammonium from real wastewater was carried out, and the removal efficiency was up to 99.13%. Thus, due to its cost-effectiveness and high adsorption capacity, M-Zeo has potential for use in ammonium removal from aqueous solutions.

Keywords: zeolite, sodium chloride modified, adsorbent, regeneration, wastewater




Nitrogen compounds are nutrients and are essential to all forms of life. In surface water, concentrations of ammonium nitrogen (NH4+-N) exceeding 0.3-0.5 mg/L (eutrophication) can promote the growth of algae and decrease the dissolved oxygen required for aquatic life (Hussain et al., 2007). With increased awareness and understanding of the deleterious effects of nitrogen, authorities have introduced stringent laws to restrict nitrogen discharges from both wastewater treatment facilities and other point-source contributors (Karapinar, 2009). Thus, efficient removal of ammonium has gained greater attention in water and wastewater treatment.

Traditionally, ammonium removal is achieved by a typical biological nitrogen removal (BNR) process, where NH4+ is transformed to nitrite/nitrate in the nitrification process, and then nitrite/nitrate is finally transformed to nitrogen gas through the denitrification process (Pan et al., 2015; Pan et al., 2017). However, biological systems confront great challenges in full-scale treatment plants and water bodies with low ammonium concentrations (< 5 mg/L). As an alternative to biological treatment, NH4+ removal by ion exchange and adsorption is of great importance for nutrient removal/recycling operations (Karapinar, 2009).

Considering the benefits of low-cost and high-safety ion exchange, zeolite has been shown to be an abundant cation exchange material and is economically used in water and wastewater treatment (Widiastuti et al., 2011; Kolakovic et al., 2014; Onyango et al., 2011). Zeolites are hydrated aluminosilicates with symmetrically stacked alumina- and silica-tetrahedrals, which results in an open and stable three-dimensional honeycomb structure possessing high cation exchange capacity, cation selectivity, higher void volume and great affinity for NH4+ (Huang et al., 2010). However, the NH4+ removal capacity of natural zeolite varies with the source of the zeolite, and the location within a particular deposit (Daramola et al., 2012; Zhao et al., 2010).

Malekian et al. (2011) reported the maximum NH4+ exchange capacity of natural Iranian zeolite to be 11.31 mg/g. Sarioglu (2005) found the maximum NH4+ exchange capacity of natural Turkish zeolite to be 25.93 mg/g. It seems that natural zeolite from different origins show different characteristics (Sarioglu, 2005). Therefore, each specific zeolite is required to be studied individually (Alshameri et al., 2014).

Additionally, the ion exchange selectivity of zeolite was reported to follow Cs+> Rb+> K+> NH4+> Ba2+> Sr2+> Na+> Ca2+> Fe3+> Al3+> Mg2+> Li+ (Lin et al., 2013). Natural zeolite generally has a high Si/Al ratio and contains quite a few impurities, which greatly reduce its cation exchange capacity (Wang, Lin, and Pang, 2008). In order to enhance the NH4+ adsorption capacity of zeolite, several modification methods have been applied, including microwave pre-treatment, NaOH, HCl, and NaCl solution treatment, magnetic material application, silicate-carbon solution treatment, among others (Lei, Li, and Zhang, 2008; Li et al., 2011; Liu et al., 2013). Many studies have proved that natural zeolite treated by NaOH solution could transform low-grade natural materials to high capacity cation exchangers (Wang, Lin, and Pang, 2008). However, in many of those studies, the long time of conversion, high temperature, complex operation process, and a significant amount of residual raw impurities have limited modified zeolite application in NH4+ adsorption. NaCl-modified zeolite is a common and cheap method and the adsorbent was easily obtained. NaCl modification effectively increased ammonium adsorption capacity by increasing the Na contents in zeolite and by modifying the surface morphology to enhance film mass transfer rate (Lin et al., 2013).

Although a large number of studies related to the removal of ammonium by using types of zeolites have been reported in the literature, zeolites from different locations with special physical and chemical properties require individual investigation (Alshameri et al., 2014; Huang et al., 2010; Malekian et al., 2011; Sarioglu, 2005). The mineral reserve of clinoptilolite in Xuancheng is abundant, and clinoptilolite has significant performance in adsorption of ammonium from aqueous solution. Zeolite modified by sodium chloride solution can have a greatly increased adsorption capacity for ammonium. Therefore, it is important to study the property of ammonium adsorption for natural zeolite (N-Zeo) and NaCl-modified zeolite (M-Zeo). The objectives of this study were: (i) to prepare M-Zeo and systematically investigate its application for NH4+ removal from aqueous solution; (ii) to elucidate the effects of environmental conditions, including pH, initial concentration of ammonium and temperature on the adsorption of ammonium to M-Zeo by batch experiments; (iii) to reveal the exchange selectivity of Na+, Ca2+, K+, Mg2+ contained in zeolite for ammonium; (iv) to study the adsorptive mechanism of NH4+ on M-Zeo through adsorption isotherms, thermodynamic and kinetic models; (v) to discuss the rate-controlled process of NH4+ adsorption onto M-Zeo according to the apparent activation energy; (vi) to investigate the treatment of real wastewater containing ammonium by M-Zeo. The aim of this paper is to evaluate the suitability of M-Zeo as an efficient and low-cost clay adsorbent for adsorption of ammonium from aqueous solution and wastewater in environmental clean-up, and the Arrhenius formula was employed to reveal the rate-controlled process of ammonium adsorption onto M-Zeo




Natural zeolite (N-Zeo) used in the experiments was obtained from Xuancheng in Anhui Province, China, which was ground and selected for particle sizes of 45-74 µm. Due to adsorbents with smaller particle size and larger specific surface area showing higher adsorption performance, zeolite powder in the particle size of 45-74µm was used in this study. Zeolite samples (25 g) were dispersed in 500 mL of 2 mol/L NaCl solution by magnetic stirring for 24 h; the concentration of NaCl used followed Lin et al. (2013). Then the mixtures were centrifuged, washed 5 times with deionized water, and dried at 105°C for 12 h. The obtained NaCl-modified zeolite (M-Zeo) was finally ground and screened though a 200 mesh sieve (74 µm).

Stock ammonium solution (10000 mg NH4+-N/L) was prepared by dissolving 38.207 g NH4Cl into 1 L deionized water. All working solutions were prepared by diluting this stock solution with deionized water.

Batch adsorption experiments

To investigate the impact of pH values on the adsorption capacity of ammonium, natural and modified zeolite were tested. Ammonium solutions (25 mL, 1000 mg/L) were added into 150 mL conical flasks with stoppers, and the pH of solutions was adjusted to 5, 6, 7, 8, and 9 by adding 0.1-1 M NaOH solution and 0.1-1 M HCl solution. After adding 0.5 g of adsorbent, the flasks were stirred at 200 r/min in thermostatic shakers for 24 h at 293 K. After the mixture was centrifuged, the supernatant was filtered through a 0.45 µm membrane filter prior to the determination of ammonium concentrations. The equilibrium adsorptive capacity was calculated by Eq. 1:

where qt is the adsorptive capacity at time t, mg/g; C0 is the initial concentration of ammonium in the solution, mg/L; Ct is the concentration of ammonium in the solution at time t, mg/L; V is the volume of the solution, L; and W is the mass of the adsorbent, g.

Adsorption isotherms for ammonium were carried out in thermostatic shakers for 24 h at desired temperatures (293, 303, 313 K). Adsorbents (0.5 g) were mixed with ammonium solutions (25 mL) at different initial concentrations ranging from 5 to 1 000 mg/L (5, 10, 25, 50, 100, 200, 500, 800, 1 000 mg/L) at pH 8. The order of exchange selectivity was evaluated by examining the concentrations of cations.

Adsorption kinetics for ammonium were evaluated at pH 8 and at an ambient temperature of 293 K. Adsorbents (0.5 g) were added to ammonium solutions (25 mL) with an initial concentration of 1 000 mg/L. Samples withdrawn at different time intervals of 0.25, 0.5, 1, 2, 3, 4, 8 and 12 h were analysed for ammonium concentration.

The regeneration study was performed by evaluating the effect of regeneration cycles on the ammonium adsorptive capacity at pH 8 and at an ambient temperature of 293 K. Adsorbents (0.5 g) were added in 25 mL of 1 000 mg/L ammonium solutions. The adsorbents were collected after adsorption and regenerated by 250 mL of 2 mol/L NaCl solution; the concentration of NaCl was consistent with the concentration used in the absorbent preparation. Then, the zeolites were washed by deionized water and centrifuged several times. The regenerated zeolite was reused for adsorption of ammonium from aqueous solution.

Analysis methods

Ammonium concentrations in liquid samples were analysed by spectrophotometry with a spectrophotometer (V-1100D, Mapada Co., Shanghai, China). The concentrations of Na+, K+, Ca2+ and Mg2+ in solution were measured by atomic absorption spectroscopy (AAS-6300C, Shimadzu, Japan). Elemental compositions of M-Zeo were determined using X-ray fluorescence (XRF) (XRF-1800, Shimadzu, Japan). Mineral phases were identified by X-ray diffraction (XRD) using a D/max-RB powder diffraction meter (Rigaku, Japan), with a Cu-target operated at 40 kV, 100 mA. The XRD patterns were taken in the range of 4-70° at a scan rate of 4°/min, which were analysed using the software (Search-Match) by comparing the experimental data with those included in the Joint Committee of Powder Diffraction Standards (JCPDSs) database.




XRD patterns of N-Zeo and M-Zeo are illustrated in Fig. 1. Diffraction patterns at 2θ = 9.88, 11.22, 17.34, 22.74, 26.12, 29.06, and 32° are identified as clinoptilolite when compared with the standard database. The characteristic peaks of quartz can be found at 26.7 and 50.1°. The intensity of quartz became weaker after modification. The XRD spectra of M-Zeo showed no significant differences from N-Zeo, indicating that the main mineral phases of zeolite were not changed after modification.



XRF was applied to analyse the elemental compositions of N-Zeo and M-Zeo, presented as percentage of element in the highest oxidation state (Table 1). It can be clearly seen that the contents of the exchangeable cations such as K+, Ca2+, and Mg2+ in the M-Zeo were decreased, while the amount of Na+ was increased significantly. The result suggested that Ca2+, K+, and Mg2+ were replaced by Na+, which can be used to remove NH4+ in ion-exchange applications.

Effect of pH

Figure 2a shows the adsorption of ammonium onto N-Zeo and M-Zeo as a function of initial pH. In acidic condition, increasing pH favoured both N-Zeo and M-Zeo adsorption of ammonium. In basic conditions (pH 8 to 10), reduced adsorption capacity with increasing pH is seen in Fig. 2a, leading to the highest adsorption capacities of 11.39 and 17.77 mg/g on N-Zeo and M-Zeo, respectively, at pH 8. This clearly implies that ammonium adsorption onto both N-Zeo and M-Zeo was pH dependent. The dominant mechanism of ammonium adsorption onto N-Zeo and M-Zeo was assumed to be ion-exchange between cations (Na+, K+, Ca2+, Mg2+, et al.) on the adsorbent surface and ammonium in the solution. As shown in Fig. 2b, ammonium existed in the form of NH4+ in aqueous solution at pH 2-8, and as NH3 at pH 10-13. The species of ammonium were converted from NH4+ to NH3 when pH was between 8 and 10. At pH < 8, ammonium adsorption increased with increasing pH, principally being attributed to the decline in competing hydrogen ions, and with cation exchange being the dominant mechanism (Liu et al., 2013). At pH > 8, ammonium removal decreased with increasing pH, likely owing to the conversion of NH4+ into NH3 in alkaline solution. Thus, molecule adsorption was the main mechanism for ammonium removal, which resulted in the reduction of ion-exchange potential. This observation correlated with findings reported in the literature (He et al., 2016; Lin et al., 2013). Therefore, the optimum pH of M-Zeo for adsorption of ammonium is that of a neutral solution.



Ion-exchange adsorption

The ion exchange process between the zeolite frame and aqueous ammonium solution can be expressed by Eq. 2 (Lin et al., 2013):

where M represents the loosely held cations in zeolite and n is the number of electric charges. Assuming M in zeolite are Na+, K+, Ca2+ and Mg2+, the ion exchange capacity (IEC) can be calculated as the sum of exchange cations as follows:

As shown in Fig. 3, the adsorption capacity of ammonium onto M-Zeo increased significantly at different initial ammonium ion concentrations, while the equivalent concentrations of Mg2+, K+, Na+ and Ca2+ released into the solution increased significantly. The IEC for the sum of the four cations was a little lower than the ammonium adsorption capacity at equilibrium, indicating that ion exchange is predominant in the adsorption of ammonium by zeolite. The extra amount of ammonium adsorption on M-Zeo is ascribed to electrostatic attraction between negative charges on the adsorbent surface and NH4+ (Alshameri, Ibrahim, et al., 2014).



When the initial ammonium concentration rose to 25 mg/L, K+ started to be released from zeolite. Mg2+ appeared in the aqueous solution after the initial ammonium concentration rose to 50 mg/L. Na+ was the dominant cation exchanged with ammonia under an initial ammonia concentration of less than 100 mg/L, while Ca2+ was the dominant cation exchanged with ammonia under an initial ammonia concentration higher than 100 mg/L. Thus, the effect of the metal ions on ammonium adsorption to zeolite suggests an order of preference of Na+ > Ca2+ > K+ > Mg2+. A similar result was reported by other researchers (Lin et al., 2013). A slightl difference in the order was determined as Na+> K+> Ca2+ > Mg2+ by other researchers (Lei et al., 2008; Watanabe et al., 2007).

Adsorption isotherms

The adsorption isotherms of ammonium on zeolite were fitted by two typical models, Langmuir and Freundlich, as Eqs 4 and 5 (Liu et al., 2013; Langmuir, 1918):

where Ce is the equilibrium concentration (mg/L) in the solution; qe is the adsorption capacity on adsorbent (mg/g); qm refers to the maximum adsorption capacity at monolayer coverage (mg/g). The values of k (L/mg) and Kf (mg/g) are the Langmuir and Freundlich adsorption constants, respectively. 1/n is a constant relating to adsorption intensity or surface heterogeneity.



The relative parameters (qm, k, Kf and 1/n) were calculated from the slope and intercept of the linear plots based on the Langmuir and Freundlich adsorption isotherms. As the correlation coefficient of the Freundlich model (R2 > 0.9905) was higher than that of the Langmuir model (R2 < 0.9815), the Freundlich model was suggested to better fit ammonium sorption onto both N-Zeo and M-Zeo. This indicated that adsorption occurred on a structurally heterogeneous adsorbent (Pan et al., 2017). The maximum adsorption capacity on a monomolecular layer of M-Zeo was estimated to be 17.83 mg/g at 293 K, which is higher than that found by other researchers. For example, Mazloomi and Jalali (2016) found the maximum adsorption of ammonium by Iranian zeolite to be 10.08 mg/g. Saltalı et al. (2007) reported that the adsorption capacity of ammonium by natural Turkish zeolite was 9.64 mg/g at 294 K (Saltali et al., 2007). It has also been reported that the maximum adsorption of ammonium using a salt-activated Chinese (Hulaodu) zeolite was 9.52 mg/g (Alshameri et al., 2014). Meanwhile the ammonium exchange capacity for natural and modified Yemeni zeolites were 11.18 mg/g and 8.29 mg/g, respectively (Alshameri et al., 2014). The constant of 1/n for the Freundlich model is related to the adsorption intensity, which varies with the heterogeneity of materials. The values of 1/n were lower than 0.52 in this study, which suggests that the adsorption of ammonium on N-Zeo and M-Zeo was highly favourable (Table 2).

The Dubinin-Redushckevich (D-R) isotherm was also employed to reveal the type of adsorption (physical adsorption or chemical adsorption) (Mazloomi and Jalali, 2016). The D-R equation has the linear form:

where qm is the D-R adsorption capacity (mol/g); β is the constant of the adsorption energy (mol2/J2), related to the average energy of adsorption per mole of the sorbate as it is transferred to the surface of the solid from infinite distance in the solution; ε is Polanyi potential, which is described as:

where T is the absolute temperature (K) and R is the gas constant (8.314 J/mol·K).

Moreover, the mean energy of adsorption E (kJ/mol) can be calculated from the D-R parameter β using the following formula:

As seen in Table 2, the correlation coefficients of the D-R model for ammonium sorption on N-Zeo and M-Zeo were higher than 0.977, suggesting the D-R model was acceptably applied to fit the experimental data in this study. The relative parameters (β and qm) were calculated from the slope and intercept of Eq. 6. The value of mean energy of adsorption E is in the range of 1-8 kJ/mol and 8-16 kJ/mol for physical and chemical adsorption, respectively. In this study, the E values of ammonium adsorption on N-Zeo and M-Zeo were in the range of 8-16 kJ/mol, indicating that the adsorption process was essentially chemisorption (Table 2).

Thermodynamic parameters

The thermodynamic parameters can be calculated from the temperature-dependent adsorption isotherms based on Eqs 9-11:

where Kd is the distribution coefficient, mL/g; ΔG0 is the change of Gibbs energy, kJ/mol. The values of enthalpy (ΔH0) and entropy (ΔS0) can be obtained by the slope and intercept of the plot of lnKd versus 1/T (Fig. 5). The values of Kd, ΔG0, ΔH0 and ΔS0 are summarized in Table 3. Negative values of ΔG0 and positive values of ΔH0 were found, which reveals that the processes of ammonium adsorption on N-Zeo and M-Zeo were endothermic, feasible and spontaneous. The change of entropy (ΔS0) was 0.032 and 0.038 kJ/(mol·K) for the adsorption of ammonium on N-Zeo and M-Zeo, respectively. The positive values of ΔS0 suggested that the randomness increased during the removal of ammonium ions from aqueous solution onto N-Zeo and M-Zeo.



Adsorption kinetics

The adsorption kinetics of ammonium on M-Zeo and N-Zeo was simulated by four typical kinetic models. The kinetic equations, including pseudo first-order model, pseudo second-order model, Elovich model and intraparticle diffusion model, are described as follows (Huang et al., 2015; Malekian et al., 2011; Yang et al., 2015):

where qt is the adsorbed amount at time t, mg/g; qe is the adsorption amount at equilibrium, mg/g; k1 is the rate constant of pseudo first-order adsorption, g/(mg·h); k2 is the rate constant of pseudo second-order adsorption, g/(mg·h); the parameter ae is the initial adsorption rate, mg/(g·h), and be is related to extent of surface coverage and activation energy for chemisorption, g/mg; k3 is the intraparticle diffusion rate constant, mg/(g·h0.5).

Table 2 tabulates the relative parameters calculated from these four kinetic models. Constants k1 and k2 were respectively determined from the slope of the line obtained by plotting ln(qe-qt) versus t in the pseudo-first-order model and the intercept of the line by plotting t/qt versus t in the pseudo-second-order model, while the initial adsorption rate ae was determined from the intercept of the line obtained by plotting qt versus ln t in the Elovich equation. The intraparticle diffusion rate constant k3 was calculated from the slope of the line obtained by plotting qt and t0.5. The correlation coefficient for the pseudo-second-order model is the highest among the four kinetic models, revealing that the pseudo-second-order model best describes the adsorption kinetics of ammonium onto M-Zeo, and that chemisorption dominates in the adsorption process (Huang et al., 2017; Liao et al., 2012). This conclusion matched the fitting results from the D-R isotherm. Moreover, , the theoretically adsorbed amount at equilibrium (16.34 mg/g) obtained from the pseudo-second-order model was much closer to the adsorbed amount at equilibrium obtained from experiment (17.83 mg/g) than that obtained from the other models. Due to the high correlation coefficient (>0.99), the Elovich equation was also found to be suitable to describe the second-order kinetic, assuming that the actual solid surfaces are energetically heterogeneous (Mezenner and Bensmaili, 2009). The initial adsorption rate ae was 49 166 mg/(g·h) for ammonium adsorption onto M-Zeo, which was much higher than 80.27 mg/(g·h) for ammonium adsorption onto N-Zeo. The intraparticle diffusion model is assumed to be the sole rate-controlling step if the regression of qt versus t0.5 is linear and the plots pass through the origin (Huang et al., 2010). The fitting results show that the regression was linear, but the plot did not pass through the origin. As seen in Fig. 6d, the ammonium adsorption onto N-Zeo and M-Zeo involved two steps and presented a multilinearity. Therefore, the adsorption processes of ammonium onto N-Zeo and M-Zeo can be divided into two steps. The first, fast step was mainly contributed by boundary layer diffusion or macro-pore diffusion. The second, gradual step was attributed to intraparticle diffusion or micro-pore diffusion (Pan et al., 2017; Widiastuti et al., 2011).

The apparent activation energy

The linear form of the Arrhenius equation can be expressed as the following formula:

where k is the rate constant of pseudo second-order adsorption; Ea is the apparent activation energy, J/mol; R is the gas constant, 8.314 J/mol·K; T is the absolute temperature (K).

Comparison of the outputs of adsorption kinetics models suggested that the adsorption process of ammonium onto N-Zeo and M-Zeo was in accordance with the pseudo-second order model. Thus, the apparent activation energy can be determined by the slope of the line plotting ln k (k2 in Table 4) versus 1/T according to the Arrhenius equation. Chen (2016) stated that the reaction rate would be fast if Ea is less than 40 kJ/mol at room temperature, and rather slow if Ea is greater than 120 kJ/mol (Chen et al., 2016). Moreover, adsorption would be a diffusion-controlled process if the Ea is less than 25-30 kJ/mol (Lazaridis and Asouhidou, 2003; Mezenner and Bensmaili, 2009). In the present study, Ea was 13.37 kJ/mol for ammonium adsorption onto N-Zeo and ٩.٨٤ kJ/mol for ammonium adsorption onto M-Zeo, which was less than 25 kJ/mol. Thus, the adsorption of ammonium on M-Zeo was a fast and diffusion-controlled process and its reaction rate was much higher than the adsorption of ammonium on N-Zeo.



Regeneration and treatment of real wastewater

Fig. 8 shows the regeneration of M-Zeo after adsorbing ammonium. After being regenerated after the first cycle, the adsorption capacity of ammonium onto M-Zeo was slightly decreased from 17.36 mg/g to 16.77 mg/g. After 5 cycles in regeneration, the adsorption capacity had dropped to 15.73 mg/g and the regeneration ratio was up to 90.61%. A similar result was reported by Ji et al. (2007). Treatment of real wastewater by adsorbents could evaluate if the adsorbents can be used for environmental purification (Kostic et al., 2017; Kostic et al., 2018). In this study, sewage, which was collected from a domestic wastewater pipe at Xiamen University of Technology campus, with an initial ammonium concentration of 14.92 mg/L was employed to estimate the possibility of M-Zeo being used for real wastewater treatment. Under the condition of a pH value of 8.5 for the campus sewage, the final concentration of ammonium was 0.13 mg/L, indicating a removal efficiency for ammonium of 99.13%. Given the high removal efficiency and regeneration ratio of M-Zeo, it can be considered as a promising adsorbent in the preconcentration and removal of ammonium from aqueous solutions in environmental clean-ups.




The synthetic M-Zeo was successfully prepared from N-Zeo by dispersing into NaCl solution and drying at 105°C. The adsorption isotherms and kinetics of ammonium by M-Zeo could be satisfactorily simulated by the Freundlich model and the pseudo-second-order model, respectively. Thermodynamic parameters indicated that the adsorption process of ammonium onto M-Zeo was endothermic and spontaneous. The fitting results of the D-R isotherm determined that ammonium removal by M-Zeo was chemisorption. According to the intraparticle diffusion model, ammonium adsorption onto M-Zeo involved two adsorption steps: (i) boundary layer diffusion or macro-pore diffusion, and (ii) intraparticle diffusion or micro-pore diffusion. The Ea in the Arrhenius equation suggested that the adsorption of ammonium on M-Zeo was a fast and diffusion-controlled process. The findings of this study suggest that the low cost, high adsorption capacity and good regeneration performance of M-Zeo indicate that it is a promising adsorbent to be widely utilized for ammonium removal from aqueous solution.



The authors would like to express their gratitude for the financial support provided by the Natural Science Foundation of Fujian Province, China (2016J05140), the Open Research Fund Program from Key Laboratory of Environmental Biotechnology (XMUT), Fujian Province University (EBL2018004), the Scientific Research Project of Xiamen Overseas Talents (201631402), Scientific Climbing Program of Xiamen University of Technology (XPDKQ18031) and the Science and Technology Project of Longyan City (2017LY63).



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Received 16 September 2018
Accepted in revised form 27 September 2019



* Corresponding author, email:




Comparison of refined and non-refined wastewater effect on wheat seed germination and growth under drought



Hassan Heidari*; Saman Moradi

Department of Agronomy and Plant Breeding, Faculty of Agricultural Science and Engineering, Razi University, Kermanshah, Iran




Wastewater has attracted special attention as a possible source of irrigation. The present study aimed to compare the effect of refined and non-refined wastewater on wheat seed germination and growth under induced drought conditions in laboratory and pot experiments. The laboratory experiment included the iso-osmotic potentials of 0.275, 0.4, and 0.47 MPa of polyethylene glycol (PEG, as a drought factor) and wastewater. In addition, the pot experiment included a wastewater factor (i.e., tap water, 100% refined wastewater, 50% refined wastewater + 50% non-refined wastewater, and 100% non-refined wastewater) and a drought factor (i.e., an irrigation interval of two and three days as normal and drought conditions, respectively). The results demonstrated that the drought related to PEG did not reduce seed germination while wastewater decreased seed germination. Further, an osmotic potential of 0.47 MPa resulted in the highest and lowest radicle length in both wastewater and PEG, respectively. The results also revealed that caulicle length and seed vigour were decreased by PEG as the osmotic potential increased while no significant difference was observed between wastewater treatments and distilled water (control). Based on the results, an irrigation interval of 3 days with 100% non-refined wastewater produced the highest chlorophyll content and 100% refined and 100% non-refined wastewater produced a larger leaf area compared to the control. Furthermore, drought with wastewater application increased specific leaf weight whereas it reduced the total biomass compared to control (i.e., tap water with an irrigation interval of 2 days), except for 100% non-refined wastewater. Therefore, wastewater application compensates for the adverse effect of drought due to nutrient addition.

Keywords: chlorophyll, drought, refined wastewater, seed vigour




Drought is considered to be one of the most important factors which limits crop production across the globe. Agricultural production should be increased to meet global population demands (Tilman et al., 2011). Irrigation is regarded as one of the methods which effectively increases crop production while water use efficiency typically remains stable under different water amounts (Curt et al., 1995). According to Mustafa Tahir et al. (2014), shortening the irrigation interval at the critical periods of growth can lead to an increase in plant height and forage yield for oats.

Chlorophyll concentration is a key factor in photosynthesis rate (Ghosh et al., 2004), which increases under drought since leaf area reduces and the leaf becomes thicker, leading to an increase in chlorophyll concentration (Barraclough and Kate, 2001).

Wastewater (e.g., domestic and industrial types) can be used for crop irrigation. In addition, it has been found to increase plant height and biomass in wheat (Pandey and Singh, 2015). According to Ashraf and Ali (2007), seed germination is one of the sensitive stages in plant growth and, as an index of plant sensitivity to contamination, has attracted the attention of different studies. Wastewater contains salts and heavy metals. Li et al. (2005) reported that an increase in heavy metal concentration caused a decline in seed germination percentage. The highest wheat seed germination was recorded at 25% effluent concentration when compared to a variety of other concentrations (e.g., 0%, 25%, 50%, 75%, and 100%) of effluents from a textile and sugar factory (Nandal et al., 2017). Seedling root and shoot growth decreased relative to a control when applying effluents from a pharmaceutical and battery industry at various irrigation intervals (Raju et al., 2015).

However, various studies have reported different adverse effects of salinity and drought. Both salinity and drought reduce coleoptile and root length, as well as the fresh and dry weight of the root and coleoptiles in wheat (Jovovic et al., 2018). The drought and salinity in these studies resulted from polyethylene glycol and NaCl, respectively, and had no significant effect on seed germination percentage, germination rate, or seedling shoot and root weight in wheat, compared to the control (Mohammadi and Dargahi, 2015). Farmers may use wastewater for irrigation, in refined or non-refined form, from different sources including a domestic source. Further, physical and chemical processes may be utilized for treating the wastewater (Mlakar et al., 2017). The application of non-refined wastewater for irrigation can inhibit plant growth through cell division, due to sticky and lagging chromosomes (Sik et al., 2009), and irrigating plants with non-refined wastewater may cause disease if directly applied by a person. Conversely, several other studies have indicated that non-refined wastewater can promote plant growth (Moradi et al., 2016; Khaleel et al., 2013).

It is not clear whether refined wastewater can have the same promoting effect as non-refined wastewater. Furthermore, wastewater refinement is a costly process and its efficiency needs evaluation. Therefore, the current study sought to compare physical, chemical, and biological traits of refined and non-refined wastewater and their effects on seed germination and early plant growth in wheat under drought.



Laboratory experiment

A laboratory experiment was conducted at Physiology Laboratory, the College of Agricultural Science and Engineering, Razi University, during 2014. Based on the aim of the study, the treatments encompassed the iso-osmotic potentials of 0.275, 0.4, and 0.47 MPa (equal to 100% refined wastewater, 50% refined wastewater + 50% non-refined wastewater, and 100% non-refined wastewater, respectively), of polyethylene glycol (PEG, as a drought factor) and refined or non-refined wastewater. Table 1 demonstrates some of the parameters of refined and non-refined wastewater. PEG solutions were prepared according to Michel and Kaufmann's formula (1973):




where: Ys, C, and T demonstrate the osmotic potential, the concentration of PEG in gkg1 H2O, and the temperature in degrees Celsius, respectively. The osmotic potentials of wastewater were measured using an osmometer and distilled water was utilized as a control. Therefore, the experiment comprised 7 treatments (i.e., one control and 0.275, 0.4, and 0.47 MPa of polyethylene glycol and wastewater). The study was conducted as a completely randomized design with 3 replications. Ghareso is a river in Kermanshah and the wastewaters from industrial and domestic sources are discharged into this river. Additionally, a wastewater treatment plant exists in Kermanshah to purify part of the effluents. In general, the purification process occurs in 3 steps: physical purification, which encompasses the construction of overflow, screens, the initial sedimentation ponds, the secondary sedimentation ponds, and the pump house of the sludge; chemical purification, which involves activated sludge and chlorination pools; and, finally, biological treatment, which involves biological reactors (Iran's Environmental Health, 2019). Wastewater samples were collected from Kermanshah wastewater treatment plant with an output of 60 000 m3day1 (Ghamarnia et al., 2014), which serves 400 000 persons.

To assess the effect of wastewater and drought on seed germination, the seeds of wheat (Triticum aestivum cv. Sirwan) were placed on filter paper in Petri dishes and then 6 mL of the prepared solution was added to each Petri dish. Next, these dishes were kept in a germinator for a week, after which several parameters were measured: seed germination percentage, caulicle length, radicle length, radicle to caulicle ratio, and seed vigour. Seed vigour was calculated by Heidari's (2013) equation.

Pot experiment

An outdoor pot experiment was conducted at the College of Agricultural Science and Engineering, Razi University, in 2014. The experiment was conducted as a factorial arrangement based on a randomized complete block design with 3 replicates. One factor was wastewater (i.e., tap water, 100% refined wastewater, 50% refined wastewater + 50% non-refined wastewater, and 100% non-refined wastewater). The other factor included irrigation intervals (2 and 3 days) which were determined by a pre-experiment. In the pre-experiment, the irrigation intervals of 2 and 3 days were determined as well-watered and drought treatments, respectively, based on soil factors and plant symptoms. At each irrigation event, the soil surface was gradually watered to ensure the soil was totally wet. Then, watering was stopped when the pot soil started to drain and the seeds of the wheat (Triticum aestivum cv. Sirwan) were sown in pots (7 cm in diameter and 7.5 cm in height) filled by field soil. The experiment lasted 21 days, after which related parameters were estimated: leaf chlorophyll content, plant height, leaf area, stem fresh and dry weight, leaf fresh and dry weight, leaf to stem ratio, total biomass, and specific leaf weight. Specific leaf weight was calculated by dividing leaf dry weight by leaf area. A SPAD (soil plant analytical development) device was used to determine the index of leaf chlorophyll (Bail et al., 2005). Finally, leaf and stem samples were dried in an oven at 70°C for 24 h in order to calculate their dry weight.

Data analysis

Data were analysed by SAS software and the means were compared by applying Duncan's multiple range test at a probability level of 5%.



Water quality

The effluent quality was improved by the wastewater treatment plant (Table 1). For example, heavy metals, as well as some essential nutrients for plant growth, such as nitrogen and phosphorus, were reduced by wastewater treatment. Fe increase could be attributed to FeCl3 for coagulation. Mn increase could be due to the lack of subsurface oxygen. pH increase could be attributed to liming.

Laboratory experiment

Figure 1 illustrates the influence of polyethylene glycol (PEG) iso-osmotic potentials and wastewater on the seed germination traits of wheat. Drought related to PEG fails to reduce seed germination while wastewater decreases it (Fig. 1a). Further, germination reduction by wastewater is initiated by the lowest osmotic potential. The osmotic potential of 0.47 MPa for the wastewater and PEG resulted in the highest and lowest radicle length, respectively (Fig. 1b). An increase in osmotic potential leads to a decrease in the caulicle length with PEG, whereas no significant difference is observed between wastewater treatments and distilled water as control (Fig. 1c). With regard to the radicle to caulicle ratio, there is also no significant difference between wastewater treatments and distilled water (Fig. 1d). Furthermore, seed vigour decreases with PEG through increasing the osmotic potential, while wastewater treatments and distilled water demonstrate no significant difference in terms of seed vigour (Fig. 1e).



Pot experiment

The effect of irrigation interval and wastewater on early growth traits in wheat is displayed in Fig. 2. The irrigation interval of 3 days with 100% non-refined wastewater produces the highest chlorophyll content. More precisely, drought reduces leaf area while increasing leaf thickness (specific leaf weight), leading to an increase in chlorophyll concentration which is confirmed by the results of the current experiment (Fig. 2a). Under the irrigation interval of 2 days, leaf fresh weight increases for all wastewater treatments whereas under the irrigation interval of 3 days, only 100% non-refined wastewater results in an increase in leaf fresh weight (Fig. 2b). Drought reduces leaf dry weight (Fig. 2c). Additionally, 100% refined wastewater produces the highest stem dry weight. A remarkable reduction of some nutrients while improving some physiochemical properties of wastewater (e.g., electrical conductivity) in 100% refined wastewater results in increasing the stem dry weight (Fig. 2d). In addition, 100% refined wastewater with an irrigation interval of 2 days reveals higher plant height compared to the control, namely, tap water with an irrigation interval of 2 days. By increasing the irrigation interval, the height of the plant irrigated by 100% non-refined wastewater shows no reduction (Fig. 2e), which is likely due to the positive effect of wastewater on plant growth. Further, 100% refined and 100% non-refined wastewater produce a higher leaf area compared to the control (Fig. 2f). The lowest and highest leaf to stem ratio, among the treatments, is observed for 100% refined wastewater with an irrigation interval of 2 and 3 days, respectively (Fig. 2g). On the other hand, drought increases leaf to stem ratio under 100% refined wastewater. This suggests that stem elongation initiation is postponed by drought. Additionally, drought with wastewater application leads to an increase in specific leaf weight compared to the control (Fig. 2h), and finally, drought reduces total biomass compared to the control, except in the case of 100% wastewater (Fig. 2i).



Laboratory experiment

The reduction in seed germination by wastewater can be attributed to salinity and heavy metal stresses, which is confirmed by the data in Table 1. Further, higher electrical conductivity and some inhibiting elements, including Na, in the wastewater are the main reasons for such a reduction in seed germination. Apparently, salinity stress had a negative effect on germination while some nutrients promoted radicle and caulicle length after germination in the present study. However, these nutrients demonstrated their effect only at high concentration. The adverse effect of osmotic potential was compensated for by the positive effect of wastewater due to nutrients (Li et al., 2005). Raju et al. (2015) also reported a decline in wheat seed germination due to effluent from a pharmaceutical and battery industry, which is in line with the results of the current study. Furthermore, some other studies found that seed priming by sodium compounds such as sodium silicate (Hameed et al., 2013) and sodium nitroprusside (Ali et al., 2017) enhance seedling root and shoot growth in wheat due to a reduction in the oxidative stress of salinity. This corroborates the results of the present study. Contrary to the results of the current study, Sayar et al. (2010) concluded that drought stress resulting from polyethylene glycol (PEG) had a higher adverse effect on germination percentage in wheat compared to NaCl-related salinity. Additionally, PEG-induced drought stress had an even more negative effect on radicle to caulicle ratio compared to wastewater (Fig. 1d). Essential nutrients available in wastewater reduced the negative effects of drought. Saidi et al. (2010) reported that root weight increases in wheat seedlings under increasing drought. Seed treated with wastewater represented higher seed vigour under the osmotic potential of 0.4 and 0.47 MPa, compared to the seed treated with PEG (Fig. 1e). The results of another study indicated that NaCl had a more adverse effect on germination compared to drought (Al-taisan, 2010) while other studies have reported that drought had a more adverse effect on germination compared to NaCl (e.g., Okcu et al., 2005; Rahimi et al., 2006).

Pot experiment

Heidari et al. (2011) found an increase in the chlorophyll content of water-stressed plants while Abdalla and El-khoshiban (2007) reported that drought reduced the chlorophyll content. This contradiction can be attributed to stress severity. In other words, chlorophyll is destroyed under severe drought which leads to a decrease in its content. In this respect, the effect of wastewater should be considered as well. Wastewater contains essential nutrients, including nitrogen, for plant growth and chlorophyll formation, and chlorophyll content, as one of source strength components, is affected by wastewater (Ghosh et al., 2004). In addition, 100% non-refined wastewater contains more nutrients than tap water and refined wastewater (Table 1). Leaf fresh weight mainly relies on leaf moisture and its trend indicates that wastewater salts may be absorbed by plant tissues. Further, salts accumulate water, thus the leaf becomes succulent (Sen and Rajpurohit, 2012) and drought reduces leaf dry weight. In the present study, the comparison of leaf dry and fresh weight revealed that wastewater increased leaf moisture (Fig. 2). According to Alyemeny (1998), a reduction in leaf biomass helps the plant to tolerate water deficit, and plant height reduction as a result of drought is considered as an adaptation response of plants in order to reduce transpiration (Karam et al., 2003). Furthermore, drought decreases cell size and internode length (Ludlow et al., 1990). However, based on the results of the current study, drought did not reduce leaf area for the 100% non-refined wastewater treatment (Fig. 2). A larger amount of nutrients such as nitrogen and phosphorus in 100% non-refined wastewater plays a role in chlorophyll production and increasing leaf area, compared to refined wastewater. Additionally, drought diminishes cell elongation and division, and thus reduces leaf area. Other studies, including Karam et al. (2003), have reported leaf area reduction due to drought as well. In addition, the results of the present study confirm that drought reduces the leaf area and increases the leaf thickness, leading to an increase in specific leaf weight (Fig. 2). The results further indicate that wastewater can promote plant growth by increasing the leaf thickness in order to capture more radiant energy. Water deficit leads to various morpho-physiological changes in the plants. For example, leaf growth is inhibited first, before photosynthesis and respiration, when there is a reduction in water potential, and soil water deficit reduces xylem sap movement toward the leaves (Liptay et al., 1998).



In general, an osmotic potential of 0.47 MPa of wastewater, resulted in the lowest seed germination percentage. Further, wastewater at low water potential failed to reduce seed germination traits such as seed vigour, while using polyethylene glycol to create a low osmotic potential decreased these traits. It is likely that the positive physiochemical properties of wastewater, including nitrogen content, acted to promote plant growth. Furthermore, plants irrigated with 100% refined wastewater under the well-watered condition produced the highest total biomass, since 100% refined wastewater contained growth-promoting ingredients such as nitrogen and phosphorus. Based on the results, drought reduced different growth parameters such as leaf and stem dry weight whereas wastewater application compensated for the negative effect of drought. Finally, drought decreased total biomass compared to control, except in the case of the 100% wastewater treatments. Therefore, irrigation of wheat with wastewater is recommended after germination instead of at the germination stage, and irrigation with refined wastewater can promote plant growth.



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