Enhanced ammonium removal from aquatic systems by a novel copolymer

Egypt is dependent on the Nile River as the major source of its water supply for all economic and service activities. Water quality is considered a very critical factor with regard to the intended uses of water, including drinking water production, irrigation, and other industrial activities based mainly on the use of water of high-quality specifications. From this perspective, water quality in Egypt faces many problems, especially in governorates located at the end of the Rosetta branch of the Nile River; downstream governorates include Kafr El-Sheikh and Beheira governorates. Pollution is considered as one of the most serious problems affecting water quality in Egypt, as raw water resources in Kafr El-Sheikh Governorate are affected by many contaminants, particularly during the winter season and low demand period of the Nile River, in which water level decreases and organic matter increases resulting in decreased dissolved oxygen, and increased ammonia, nitrite and bacterial load in the water (Gad 2017). The problem of increased ammonium ion concentration in Nile River water was and still represents a very serious dilemma and a major challenge for governmental water-purification facilities, especially those located in the downstream governorates, especially during the low demand period of the Nile River. Many studies were conducted for the removal of ammonium from water, the conventional removal techniques include biological treatment, bio-filtration, air stripping, wet oxidation, break-point chlorination, chemical precipitation, ion exchange/adsorption (Jorgensen & Weatherley 2003; Adam et al. 2019) other studies used zeolites and clays for ammoniacal nitrogen removal from water (Rožić et al. 2000). In common, these conventional techniques failed to present such a practical, efficient, time, and cost-effective solution.

When the pH is low, the reaction is driven to ⁠, and when the pH is high, the reaction is driven to NH₃. At a pH, less than 6.0, the proportion of NH₃ is decreased and the proportion of is increased; with increasing pH (around 8.0), the proportion of NH₃ is 10% or less, and at a pH slightly above 9.0, the proportion is about 50%. In surface water, the pH value ranges between 7 and 8 and the ammonium ion form represents 90% or more. Ammonium ion has a positive charge, which makes it very convenient for electrostatic attraction phenomena to remove the ammonium ion from water by using negatively charged adsorbents (Sawyer 2008).

Polymers achieve promising results in removing ammonia from an aqueous solution. Recently, a few studies used different polymers to remove ammonia from polluted water. Helminen & Paatero (2006) used sulfonated polystyrene grafted silica gel sorbent for ammonium removal from aqueous solutions. Cruz et al. (2018) used polymer hydrogels for the rapid sorption of ammonium from domestic wastewater coupled with efficient regeneration by mild acid washing, the sorption capacity of the hydrogel was 8.8–32.2 mg NH₄-N/g, which corresponds to removal efficiencies ranging from 68% to 80% NH₄-N. Adsorbents should be solid material as stationary phase; so that they could be easily removed from the solution after ammonium adsorption. The adsorbents consist of a crossed-linked copolymer produced from acrylic acid and styrene; to this synthetic matrix, functional groups are added, which act as ion exchange sites, the functional groups are sulfonic (–SO₃H) or carboxylic (–COOH) acids for resins capable of exchanging cations. Cation exchange resins that are based on crossed-linked copolymers are available in the form of small beads. The size and porosity of the beads, which are important properties governing the ion exchange process, are controlled by the conditions during polymerization and the subsequent application of uniform particle size technology. The chains produced through the polymerization of styrene or acrylic acid are cross-linked to each other through DVB groups. A higher percentage of DVB used during polymerization leads to a higher degree of cross-linkage between the chains. Higher cross-linked resins have the advantage that they swell less in aqueous solution (swelling in water is a common feature of these ion exchangers) and that they exhibit an improved mechanical strength. At the same time, however, the porosity of the resin beads decreases. This entails that the solute less easily reaches the functional groups (sites of ion exchange), and this leads to a less-efficient ion exchange process. Moreover, the number of functional groups that can be attached to the matrix of higher cross-linked resins is reduced, and thus, less functional groups are available to exchange ions. The process of ion exchange, therefore, is less efficient for highly cross-linked resins. To balance the different effects, most commercially available resins have cross-linkages of not more than 10% (Korkisch & Worsfold 1990).

Styrene (S) and acrylic acid (AA) monomers and divinylbenzene cross-linked (DVB) were supplied from Sigma-Aldrich. Ammonium persulfate (APS) was purchased from Fisher Scientific Company, UK. Ammonium chloride standard solution and sulfuric acid were purchased from Merck Company, Germany.

In general, styrene–acrylic acid [P(AA/S)] copolymers are prepared by conventional free radical polymerization, The steps could be summarized as follows: 2 g of polyvinyl alcohol (PVA) as a dispersing agent was dissolved in 200 mL of distilled water, followed by the addition of different mass ratio from AA/S (25, 50, and 75%) as monomers and divinylbenzene as a cross-linked agent, then the mixture was stirred at 600 rpm and bubbled with nitrogen for 10 min. The content was heated to 80 °C, and 0.5 g of APS as an initiator for free radical polymerization. The copolymerization was carried out for 6 h with stirring under an inert gas atmosphere (nitrogen gas [N₂] atmosphere). The five white beaded copolymer products [PAA, P(AA75/S25), P(AA50/S50), P(AA25/S75), and PS] were collected by filtration then washed with warm water and dried at 40 °C overnight (Sánchez et al. 2007).

Sulfonated copolymers [P(AA75/S25), P(AA50/S50), P(AA25/S75), and PS] were prepared according to the procedure described by Castagna et al. (2010) and Martins et al. (2003). Acetyl sulfate was synthesized by combining concentrated sulfuric acid with a solution of acetic anhydride in dichloromethane with acetic anhydride in excess. Freshly prepared acetyl sulfate was added slowly into a gently agitated solution of copolymer in dichloromethane at 40 °C. The sulfonation reaction was stirred for 4 h and was then terminated by the addition of methanol. The copolymers were isolated by precipitation into methanol and then were washed several times with deionized water. The sulfonated products [P(AA/SS25), P(AA/SS50), P(AA/SSt75), and PSS] were used as adsorbents for ammonium uptake.

After the adsorbent reached the equilibrium on solution, the adsorbent was separated by direct filtration. The analytical result was recorded as the first removal percent; the separated loaded adsorbent was stirred in 20 mL 0.1 mol/L of different acids (sulfuric acid, nitric acid, hydrochloric acid, methanesulfonic acid, etc.) solution for 10 min. After separation, ammonium concentration in the acidic eluent was measured for determining the desorption efficiency of different acids. On the other hand, the adsorbent was washed with 20 mL of distilled water several times. The regenerated adsorbent was contacted again with 50 mL solution to obtain the second removal percent; after the detection of the suitable acid for regeneration process, a similar procedure was repeated and the removal percent for multicycle adsorption–desorption process was then achieved (Zheng et al. 2009).

A JASCO FT/IR 6800 instrument (4 cm⁻¹ resolution) equipped with a DTGS detector was used for the FTIR studies. Solid samples were dried in an oven at 60 °C overnight. Then, they were ground with KBr powder at 1% wt/wt and pressed to form a thin disk. FTIR was used for the characterization of polymers synthesis.

Analysis of variance (ANOVA), Pearson correlation, and the least significant difference (LSD) of multiple means comparison were conducted using Microsoft^® Excel^® 2016 MSO (16.0.12527.22045) 64-bit software. Statistical analyses included one-way ANOVA and two-way ANOVA, data were calculated as means ± standard deviations (SD) and analyzed using analysis of variance, where a probability of 0.05 or less was considered significant (Girden 1992). The Pearson correlation coefficient, R, is a measure to determine the relationship (instead of difference) between two quantitative variables and the degree to which the two variables coincide with one another. Correlation coefficients whose magnitude lies between (0.9) and (1.0) indicate variables which can be considered very highly correlated, while correlation coefficients with a magnitude between (0.5) and (0.7) indicate variables which can be considered moderately correlated (Freedman et al. 2007). Multiple means comparisons were conducted using the LSD test; (α = 0.05), according to Williams & Abdi (2010). Three adsorption isotherm models, Langmuir, Freundlich, and Temkin, were applied to represent the relationship between adsorbent and adsorbate at equilibrium according to Dada et al. (2012), data were analyzed and graphs were plotted by OriginPro 2021 (64-bit), version: 9.8.0.200, Copyright © 1991-2020 OriginLab Corporation.

The adsorption of onto P(AA25/PSS75) is based on the electrostatic attraction between positively charged and negatively charged –COO⁻ and – groups. As the adsorbing material immersed in water, –COOH and –SO₃H groups are dissociated to –COO⁻ and – groups, respectively, causing an increase of the adsorbing material dimensions because of increasing the repulsion force between the negatively charged carboxylate and sulfonate groups, which leads to the diffusion and trapping of ions within the polymeric networks due to the ionic attraction, such an explanation was showed by Zheng et al. (2012).

In order to compare the adsorption capacity of P(AA25/PSS75) with other adsorbents including zeolite and activated carbon (used in this study) and other adsorbents reported in the literature, the monolayer adsorption capacity (i.e., q_m) calculated from Langmuir isotherm equation was applied, whereas the q_m is an important element with regard to predicting the adsorption capacity of the adsorbent surface at equilibrium (as all the adsorption sites are saturated). Table 2 lists different adsorbents used for ammonium removal and related monolayer adsorption capacity, results showed a distinct adsorption capacity of polymer P(AA25/PSS75) when compared with other adsorbents; moreover, it also presented many advantages among other adsorbents such as the rapid adsorption rate, the high adsorption capacity, and the fast trapping of ions within the polymeric networks due to the strong ionic attraction.

Table 2

The adsorption capacity for with different adsorbents

ADS .	Adsorption conditions .		qm .	Reference .
	concentration .	Contact time .
CTS-g-PAA/REC (10 wt%)	10–1000	30 min	123.8	Zheng et al. (2009)
Activated carbon	35–280	2 h	11.57	Vassileva et al. (2009)
Sepiolite	150–7000	6 h	25.49	Balci (2004)
Volcanic tuff	20–300	3 h	13.64	Maranon et al. (2006)
Zeolite 13X	5–400	30 min	8.61	Zheng et al. (2008)
Mordenite	10–200	3 days	9.47	Weatherley & Miladinovic (2004)
New Zealand clinoptilolite	10–200	3 days	6.58	Weatherley & Miladinovic (2004)
Natural Turkish clinoptilolite	25–150	40 min	8.12	Karadag et al. (2006)
Natural zeolite	5–120	8 h	6.3	Widiastuti et al. (2011)
Zeolite	0–200	4 h	23.83	Lei et al. (2008)
Clinoptilolite	11–115	1.5–2.5 h	1.74	Wang et al. (2006)
PAA/BTb	10–2000	30 min	344.47	Zheng & Wang (2010)
CTS-g-PAA/UVMT	25–1000	30 min	78.32	Zheng et al. (2012)
Zeolite	5–100	30 min	20.99	This work
Activated carbon	5–100	30 min	6.3	This work
P(AA25/PSS75)	5–100	30 min	61.89	This work

ADS .	Adsorption conditions .		qm .	Reference .
	concentration .	Contact time .
CTS-g-PAA/REC (10 wt%)	10–1000	30 min	123.8	Zheng et al. (2009)
Activated carbon	35–280	2 h	11.57	Vassileva et al. (2009)
Sepiolite	150–7000	6 h	25.49	Balci (2004)
Volcanic tuff	20–300	3 h	13.64	Maranon et al. (2006)
Zeolite 13X	5–400	30 min	8.61	Zheng et al. (2008)
Mordenite	10–200	3 days	9.47	Weatherley & Miladinovic (2004)
New Zealand clinoptilolite	10–200	3 days	6.58	Weatherley & Miladinovic (2004)
Natural Turkish clinoptilolite	25–150	40 min	8.12	Karadag et al. (2006)
Natural zeolite	5–120	8 h	6.3	Widiastuti et al. (2011)
Zeolite	0–200	4 h	23.83	Lei et al. (2008)
Clinoptilolite	11–115	1.5–2.5 h	1.74	Wang et al. (2006)
PAA/BTb	10–2000	30 min	344.47	Zheng & Wang (2010)
CTS-g-PAA/UVMT	25–1000	30 min	78.32	Zheng et al. (2012)
Zeolite	5–100	30 min	20.99	This work
Activated carbon	5–100	30 min	6.3	This work
P(AA25/PSS75)	5–100	30 min	61.89	This work

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Increased ammonia concentration in Nile River water represents a serious problem. A novel copolymer was used as an adsorbent for enhanced ammonium removal; four adsorbents of styrene–acrylic acid copolymers were prepared by conventional free radical polymerization followed by sulfonation of benzene group, the copolymer P(AA25/PSS75) achieved the highest ammonium adsorption capacity (55.8 mg/g) due to the electrostatic attraction between positively charged and negatively charged –COO⁻ and – groups which is increased with raising sulfonated styrene ratio alongside maintaining a ratio of carboxylate groups which increase the adsorbing material dimensions. Adsorption studies were performed for increasing the adsorption capacity of P(AA25/PSS75); the contact time experiment proved that the ammonium removal percentage reached 95.2 after 30 min, and the effect of pH showed that pH 7 achieved the highest adsorption capacity of P(AA25/PSS75) adsorbent and when pH is lowered than 5 and above 9, the adsorption capacity is extremely decreased. Three adsorption isotherm equations were applied; Langmuir, Freundlich, and Temkin models to study the adsorption mechanism, results indicated that Freundlich isotherm was the most suitable for fitting the experimental data which might expose the heterogeneity of the adsorbent surface and predicted the existence of multilayer coverage at the surface of the adsorbent. Reusability study of P(AA25/PSS75) adsorbent revealed that sulfuric acid was that prober eluent for regeneration which maintained polymer adsorption capability over five cycles.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.