Removal of ammonia-nitrogen in wastewater using a novel poly ligand exchanger-Zn(II)-loaded chelating resin

The ammonium ion is the most prevalent inorganic form of nitrogen, which is present in surface water and is part of a variety of biochemical and physicochemical processes. Toxic NH₃ molecules appear only in water with high pH, temperature and in the presence of ammonium. NH₃-N originates from industrial wastewater, municipal sewage and agricultural wastes or is decomposed from organic nitrogen compounds in those wastewaters and wastes. High concentration NH₃-N could cause damage to aqueous ecosystems and even humans (Sareer et al. 2015). Therefore, effective removal of NH₃-N from wastewater is significantly important, and tremendous efforts have been spent on developing techniques to remove NH₃-N from aqueous solution.

Current methods for NH₃-N removal include precipitation (Chen et al. 2013), biological processes (Tang et al. 2013, 2017), ion exchange (Wirthensohn et al. 2009), adsorption (Babou and Hamoudi 2014), electrochemical technology (Mook et al. 2012), and stripping (Yuan et al. 2016). Biological treatment requires long retention time and has low removal efficiency in winter-time. Electrochemical technologies such as electrochemical reduction are less efficient in removing ammonium. Compared with other techniques, the adsorption method is considered as a promising technology for the treatment of NH₃-N wastewater because of its simple and effective process, lower operating cost, and economical and practical properties. Conventional adsorbents, including activated carbon, various zeolites, bentonite, and fly ash are inert for NH₃-N removal (Halim et al. 2010). In recent years, NH₃-N removal by adding struvite (MgNH₄PO₄·6H₂O, magnesium ammonium phosphate, MAP) (Peng et al. 2017) has received extensive attention because of excellent solid-liquid separation performance. However, the key problem that needs to be resolved is how to reduce the treatment cost of MAP precipitation for the application of this process in the engineering treatment of NH₃-N-containing solutions. For example, if treatment conditions are not controlled properly, it can result in the formation of byproducts such as Mg₃(PO₄)₂ and Mg(OH)₂. In the past decade, the field of adsorption has seen the emergence of a new class of materials called metal-organic frameworks (MOFs) (Doonan et al. 2010). These compounds are formed by the self-assembly of metal ions, acting as coordination centers, and polyatomic organic bridging ligands (James 2003). This results in the formation of highly porous crystalline solids (James 2003). Owing to the diversity of their metallic centers and organic functionalities, one can tailor materials with specific pore shape, size, volume and chemistry (James 2003). Despite the large enthusiasm surrounding the MOFs, only a few research groups started to investigate their use for NH₃-N removal (Doonan et al. 2010; Saha & Deng 2010). Moreover, the potential instability of MOFs on NH₃-N adsorption at ambient conditions is scarcely addressed in the literature.

The anion exchange resin used in this work was purchased from Shanghai Huizhu Resin Company, China. Because of its weak iminodiacetate functional group, small size and microporous structure, the original resin was readily modified by a zinc solution prepared using analytical-grade ZnSO₄·7H₂O. After loading, the resin exhibited high removal efficiency for NH₃-N. The physical properties of the resin are shown in Table S1 (Supplementary Material, SM, available with the online version of this paper). The virgin resin has a specific surface area of 13.60 m²/g and an average diameter of pores of 513 nm. All chemicals were of analytical grade and were purchased from Xilong Chemical Co., Ltd, China. Two NH₄Cl stock solutions were prepared by dissolving NH₄Cl solids into 1.0 g cations/L and 10 g cations/L, respectively. Deionized water was used throughout the experiments.

The virgin resin was washed extensively with distilled water to remove non-adhesive impurities and small particles, and then dried at 398 K for 24 hr to remove moisture. 500 mL of dried resin was filled into an exchange column, then washed slowly with saturated ZnSO₄ solution until the effluent had the same zinc concentration as the initial solution to ensure that the Zn(II)-loaded resin was saturated. Afterwards, the resin was washed rapidly by deionized water until the effluent zinc concentration was below 0.5 mg/L. After washing, the resin was dried at 398 K for 24 hr.

The sorption of NH₃-N onto Zn(II)-loaded resin was investigated in 120 mL batch reactors. Each batch reactor was filled with 1 g of Zn(II)-loaded resin and NH₃-N solution with concentrations ranging from 50 to 4,000 mg/L. The mixture was placed in a SHZ-82A temperature-controlled water-bath oscillator at 200 rpm for 10 hr. In addition, the effect of solution pH on the adsorption capacity of the Zn(II)-loaded resin was also investigated at pH values 3.10, 3.68, 5.49, 6.32, 7.00, 8.02, 9.03, 9.54, 10.48, 11.31, and 11.96 at 298 K. The pH of the solution was adjusted by 0.1 mol/L HCl and 1.0 mol/L NaOH.

Equilibrium experiments for NH₃-N removal were carried out in the batch system at 298 K, 313 K, 333 K with an accurately known amount of 1 g of Zn(II)-loaded resin sample equilibrated with 100 mL of ammonium solution in the reactor.

The pH of the solutions was measured using a Mettler Toledo 320 pH meter. The resin was dried in a 101 electric blast-drying oven. The NH₃-N and Zn²⁺ concentrations were determined by using Nessler reagent and zinc reagent with a VIS-7220 spectrophotometer. The resins that were immersed in different solutions were all shaken in a SHZ-82A water-batch constant temperature oscillator. The surface morphology and chemical composition of the resin were characterized using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) (Hitachi Zeeman Z-8200, Japan). Chemical structure of the resin was analyzed using a Fourier transform infrared (FTIR) spectrometer (Thermo Nicolet Company 380, USA) equipped with a DTGS detector with a spectral resolution of 4 cm⁻¹. Each FTIR spectrum was an average of 64 scans.

The Zn(II)-loaded resin was successfully prepared, and the scheme of preparation and adsorption is shown in Figure 1. First, Zn(II)-loaded resin was prepared by replacing Na⁺ ions on the original resin with Zn²⁺. Then NH₃-N in the solution was adsorbed onto the Zn(II)-loaded resin surface and reacted with Zn to form the complexes. The surface morphologies of the virgin and Zn(II)-loaded resin were characterized by SEM-EDS analysis (Figure 2,), and EDS results (Figure 2(b) and 2(d),) demonstrated that Zn²⁺ was loaded onto the resin successfully. As shown in Figure 2(c), the surfaces of the Zn(II)-loaded resin were rough and nonporous. In addition, the Brunauer-Emmett-Teller (BET) results suggested that the zinc load doubled the specific surface area of the resin (Table S2, available online).

Figure 3(a) shows the FTIR spectra of virgin, Zn-loaded, and NH₃-N adsorbed resins. There was a peak shift for the C = O bond in the infrared spectrum after zinc loading as well as after NH₃-N adsorption (from 1,591 cm⁻¹ to 1,605 cm⁻¹ and 1,600 cm⁻¹) and the band corresponding to the C–O stretching also shifted from 984 cm⁻¹ to 1,013 cm⁻¹ (Wang et al. 2014; Guan et al. 2018) (Figure 3(a),). The intensity of C = O was weakened after loading with zinc, suggesting the formation of coordinated covalent bond between Zn and carboxylic groups in the resin. In addition, after Zn modification, peak intensity at 1,326 cm⁻¹ decreased, while that at 1,500 cm⁻¹ showed a slight increase, which could be attributed to the transformation of monodentate to bidentate and bridging (Mustafa et al. 1997) (Figure 3(b)). After NH₃-N adsorption, the peak of the C=O bond further shifted from 1,605 cm⁻¹ to 1,600 cm⁻¹ and two new bands appeared at 533 cm⁻¹ and 475 cm⁻¹, assigned to the stretching vibration of N–Zn and O–Zn, suggesting the involvement of –NH₂ and –COO– groups during adsorption (Sun & Wang 2006).

pH is one of the most critical controlling parameters in ammonium removal by Zn(II)-loaded resin, since it can influence both the characters of the exchanging ions and the resin itself. In order to investigate the effect of pH on the adsorption of NH₃-N by Zn(II)-loaded resin, 1 g of modified resin was added into 100 mL of NH₃-N solution (100 mg/L). Experiments were performed in the pH range of 3.00–12.00 and the solutions were shaken for 10 hr at 200 rpm speed. All possible reactions and species involved in the system are presented in Table S4 (available online), which could be used to calculate the concentration distribution of each species under various pH values. Using the Academic Software (UK, http://www.acadsoft.co.uk), the content distributions of Zn²⁺ and NH₃-N species with pH are calculated and presented in Figure 4. The effect of solution pH on the sorption process of ammonium is shown in Figure 5.

NH₃-N in aqueous solution exists in two forms, un-ionized (NH₃) and ionized (NH₄⁺). The fraction of these two forms of NH₃-N changes with the change of solution pH value. As shown in Figure 4, at pH values lower than 6.50, stably ionized NH₄⁺ exists predominantly in aqueous solution. At pH values from 6.50 to 12.00, un-ionized (NH₃) increased sharply with the increase of solution pH. Moreover, in this pH range, NH₃-N could easily react with Zn²⁺ to form Zn(NH₃)_n²⁺. At pH values higher than 8.00, the formation of Zn(NH₃)₄²⁺ became prominent.

As shown in Figure 5, at low pH values (ranging from 3.00 to 7.00), the NH₃-N removal efficiency hardly changed with solution pH, and the overall removal efficiency was below 7.0%. In the pH range between 7.00 and 9.54, the removal efficiency increased sharply from 6.6% to 35.4% with the increase of solution pH. According to the result in Figure 4, in this pH range Zn²⁺ could react with NH₃ to form Zn(NH₃)²⁺_n. Above pH 9.54, the NH₃-N removal efficiency decreased with increasing solution pH. Simultaneously, a sharp increase in the OH⁻ concentration may result in forming the unexpected precipitate Zn(OH)₂ and lead to the decrease of Zn(II) loaded on the resin. Hence, the NH₃-N removal efficiency was the highest at a pH value around 9.54.

The salient features of adsorption isotherms are necessary to optimize the design of an adsorption system for the adsorption of adsorbate. The dynamic adsorptive separation of the solute from the solution onto the adsorbent depends upon a good description of the equilibrium separation between the two phases. Various models have been proposed and applied to explain the equilibrium characteristics of adsorption. However, the most important factor is to have the applicability over the entire adsorption process. The most widely used isotherm models for solid-liquid adsorption are the Langmuir, Freundlich, Temkin, and Redlich-Peterson isotherms. The parameters of all reaction-based models were calculated using nonlinear regression analysis.

Figure 6(a) shows the plots of Q_e against C_e at three temperatures for the fitting of the Langmuir isotherm model. The Langmuir constants were calculated from the plots and are summarized in Table 1. The maximum monolayer adsorption capacities obtained from the Langmuir plots at 298 K, 313 K and 333 K were 38.55, 33.66 and 24.98 mg/g, respectively. The NH₃-N adsorption capacity on Zn-loaded resin increased with the increase of initial NH₃-N concentrations in solutions and decreased with the increase of solution temperature, suggesting that the NH₃-N adsorption process might be an exothermic reaction.

The R_L value indicates the shape of the isotherm to be either unfavorable (R_L > 1), linear (R_L = 1), favorable (0< R_L <1), or irreversible (R_L = 0). The calculated values of R_L at different initial concentrations of NH₃-N were all below 1.0, confirming that the adsorption of ammonium was a favorable process.

As shown in Table 1, the superior regression coefficients for the Langmuir model compared to the Freundlich model suggested that the adsorption process was monolayer. The Langmuir values of R_L below 1.0 and Freundlich values of 1/n between 0.446 and 0.645 (0 < 1/n< 1) revealed the favorability of the adsorption of NH₄⁺ onto Zn(II)-loaded resin.

The equilibrium studies revealed that Zn(II)-loaded resin exhibited a much higher uptake concentration of ammonium at equilibrium at 298 K compared with those at 313 K or 333 K. According to the thermodynamic calculation (detailed in SM, Table S3, available online), the adsorption process was an exothermic reaction. Therefore, increasing temperature could result in a decrease in ammonium adsorption. Furthermore, as shown in Table 1, the maximum adsorption capacity reached 38.55 mg/g at 298 K, which was significantly higher than those of the natural zeolite (Saltalı et al. 2007), natural Turkish clinoptilolite (Karadag et al. 2006), clinoptilolite (Zhu & Ni 2009) and Sardinian natural zeolites (Cincotti et al. 2001).

Adsorption kinetics analysis, an important method for describing the adsorption process, is used to interpret the transport of adsorbates inside adsorbents for real applications and plays a key role in elucidating the dominant adsorption mechanism. To describe the changes in solution of NH₃-N with time, three kinetic models of adsorption; that is, pseudo-first-order equation, pseudo-second-order equation, and intra-particle diffusion, were fitted for the NH₃-N adsorption onto Zn(II)-loaded resin. The pseudo-first-order model is one of the most widely used kinetic models for depicting the adsorption process in solution. The pseudo-second-order model was used to describe the chemisorption process (Habibul & Chen 2018). The intra-particle diffusion model assumes that intra-particle diffusion is the only rate limiting step (Zhou et al. 2015). Selecting the best model is significantly important to understand the behavior of the adsorbent, with different systems relating to different models. The parameters of all reaction-based models were calculated using nonlinear regression analysis.

The rate constants, k₁, were found to be in the range of 0.0475–0.0722 min⁻¹ at initial NH₃-N concentrations of 50–200 mg/L (Table 2). The equilibrium sorption capacity, Q_e, increased with initial concentration. However, as shown in Figure 7(a) and Table 2, although the correlation coefficients of the plots have values higher than 0.9, Q_m values obtained from the model were not consistent with experimental Q_e values. Therefore, the sorption of NH₃-N onto Zn(II)-loaded resin did not follow a pseudo-first-order mechanism.

The same trend, compared to the Lagergren pseudo-first-order kinetic, was observed (Table 2,); that is, Q_e increased with increase in initial NH₃-N concentration, but the values were higher. The correlation coefficients R² were higher than that of the pseudo-first-order model (>0.983), and the calculated equilibrium sorption capacity (Q_m) was more consistent with the experimental data (Figure 7(b) ). These results indicated that the adsorption of ammonium onto Zn(II)-loaded resin could be pseudo-second-order sorption. In the ammonium adsorption onto the Zn(II)-loaded resin, the ammonium was likely to form complexes with Zn on the resin surface, which was confirmed by the FTIR analyses. These results indicated that the NH₃-N adsorption on Zn-loaded resin was mostly controlled by chemisorption behavior (Zhou et al. 2014).

The fitting result is shown in Figure 7(c), and the calculated parameters are listed in Table 2. The Weber-Morris diffusion model can be divided into three steps. The first stage could be attributed to the diffusion of NH₃-N through the solution to the external surface of Zn(II)-loaded resin and diffusion of NH₃-N through the boundary layer to the surface of Zn(II)-loaded resin. The second stage described the gradual sorption, and intraparticle diffusion was the rate-limiting factor. The third stage was attributed to the final equilibrium, for which the intraparticle diffusion started to slow down due to the extremely low concentration of NH₃-N left in solution. In general, the diffusion rates decreased with increasing contact time due to the smaller pores for diffusion, since the NH₃-N diffused into the inner structure of the Zn(II)-loaded resin. Judged from the values of R², the Weber-Morris model did not fit well with the experimental data, indicating that particle diffusion was not the only rate-limited step in the adsorption process. The NH₃-N adsorption process might be affected by a variety of mechanisms.

In the present work, a novel poly ligand exchanger-Zn(II)-loaded resin was designed for the adsorption removal of NH₃-N from wastewater. The surfaces of the Zn(II)-loaded resin were rough and nonporous, and zinc load doubled the specific surface area of the resin. After Zn(II) loading, the resin structure changed from monodentate to bidentate/bridging as suggested by the FTIR results. When compared with literature, the Zn(II)-loaded resin showed excellent performance for NH₃-N adsorption and the maximum adsorption capacity can be up to 38.8 mg/g. The adsorption of NH₃-N onto the Zn(II)-loaded resin was affected by the temperature, initial concentration and pH, while the impact of Zn(II)-loaded resin particle size was negligible. The correlation coefficients of Langmuir were higher than those of other models, indicating that the Langmuir model best describes the adsorption of NH₃-N onto Zn(II)-loaded resin. Furthermore, the adsorption of NH₃-N onto Zn(II)-loaded resin appeared to follow a pseudo-second-order kinetic process and was predominantly controlled by chemisorption behavior. The poly ligand exchanger-Zn(II)-loaded resin may provide an alternative for the adsorptive removal of NH₃-N nitrogen from wastewater.

The authors acknowledge the financial support from Fundamental Research Funds for the Central Universities of Central South University.