Trung Thanh Nguyen,Vu Anh Khoa Tran3,Le Ba Tran3,Phuoc Toan Phan,Minh Tan Nguyen,Long Giang Bach,Surapol Padungthon,Cong Khiem TaNhat Huy Nguyen

1 Nanomaterial Laboratory,An Giang University,18 Ung Van Khiem St.,Dong Xuyen Dist,Long Xuyen City,An Giang Province,Viet Nam

2 Vietnam National University Ho Chi Minh City,Linh Trung Ward,Thu Duc District,Ho Chi Minh City,Viet Nam

3 Faculty of Natural Resources and Environment,Ho Chi Minh City University of Technology (HCMUT),268 Ly Thuong Kiet St.,Dist.10,Ho Chi Minh City,Viet Nam

4 Faculty of Engineering– Technology– Environment,An Giang University,18 Ung Van Khiem St,Dong Xuyen Dist,Long Xuyen City,An Giang Province,Viet Nam

5 Department of Scientific Research and International Relation,Bac Lieu University,178 Vo Thi Sau St.,Ward 8,Bac Lieu City,Bac Lieu Province,Viet Nam

6 NTT Institute of High Technology,Nguyen Tat Thanh University,298–300A Nguyen Tat Thanh,Ho Chi Minh City,Viet Nam

7 Department of Environmental Engineering,Khon Kaen University,123 Moo 16 Mittraphap Rd.,Nai-Muang,Muang District,Khon Kaen 40002,Thailand

Keywords:Iron oxide/hydroxide Magnesium oxide/hydroxide Cation exchange resin Adsorption Environment Nanomaterials

ABSTRACT In this study,we reported on the concept and practical use of cation exchange resin (CER) for removing anions in water via pretreating the CER with metal salts.The cation exchange resin-supported iron and magnesium oxides/hydroxides composite (Fe-Mg/CER) was synthesized and introduced as a new and potential adsorbent for selective removal of nitrate ion in the water environment.Characteristics of Fe-Mg/CER were determined by techniques such as Fourier-transform infrared spectroscopy,scanning electron microscopy,and X-ray diffraction.The results showed that Fe-Mg/CER material had a high nitrate adsorption capacity of 200 mg NO3-﹒g-1 with a fast equilibrium adsorption time of 30 min at pH 5.In addition,it had good durability of at least 10 times of regeneration,which could be applied to practical water and wastewater treatment.

1.Introduction

Water pollution due to the presence of nitrogen compounds is one of the most frequently encountered environmental problems worldwide.This may be due to the inefficient wastewater treatment processes or the excessive use of nitrogen-containing fertilizers in agricultural production.The high concentration of nitrate is considered to be the main cause of eutrophication,leading to excessive phytoplankton growth,lack of oxygen,and deterioration of quality water,endangering aquatic life [1].In addition,a high concentration of nitrate is also a risk to human health,which can lead to problems such as ‘‘Blue baby syndrome”– methemoglobinemia.On the other hand,nitrate can be converted into toxic nitrite (e.g.,nitrosamine) which is the cause of increasing the risk of cancer for humans [2].

Many biological,chemical,and physical processes have been applied to eliminate nitrate in water and wastewater treatment[3–6].In particular,the adsorption process by the ion exchange mechanism is considered to be an effective,simple,and low-cost method,in which the adsorbent has the ability to be reused many times [7,8].Therefore,ion exchange resins have recently been studied extensively[9–11].For instance,mesoporous silica materials(e.g.,SBA-15 and MCM-48)have been synthesized and modified by amine groups to remove nitrate effectively [12–14].However,the application of these materials in practical terms is very limited because of the complicated synthesis procedure which resulted in a small-scale and high-cost production.Moreover,the study of anion exchange resins (AER) in combination with metal components (e.g.Fe-Mn/AER) in environmental treatment has attracted attention to the purpose of efficiency improvement to traditional processes in recent years [15].Basically,pristine AER has its own affinity to removal anion such as nitrate in the water while cation exchange resins(CER)should not have the ability of nitrate adsorption.However,CER,in the role of a carrier,has a large surface area and the strong binding force between its metal nanoparticles.Furthermore,the concept and practical application of a metals/CER have not been found in any publication of an anion removal in water up till now.Therefore,we came to the idea of using CER as a carrier in combination with metal (via ion exchange) and applying the synthesized metals/CER material for removal of nitrate in water.

In this study,we presented a proof-of-concept for preparing cation exchange resin-supported iron and magnesium oxides/hydroxides composite (Fe-Mg/CER).The synthesized material was then characterized to explore its properties and the nitrate adsorption experiment was conducted for evaluation of its applicability in environmental treatment.

2.Materials and Methods

2.1.Chemical

All chemicals used in this study were provided by Merck Co.(Germany)with at least 99%purity.Iron(III)chloride(FeCl3﹒6H2O),magnesium sulfate (MgSO4﹒7H2O),and deionized (DI) water were used to synthesize materials.Potassium bromide (KBr) was used in the FTIR analysis of the material.Water samples and the standard solution containing nitrate were prepared from potassium nitrate (KNO3).

2.2.Synthesis and characterization of Fe-Mg/CER

The support of CER was prepared using a commercial cation exchange resin of 220Na from Ion Exchange Co.(India)with a size of 0.3–1.2 mm.It was washed five times with DI water and dried at room temperature for 24 h.Nanoparticles of Fe and Mg compounds with a mass ratio of 4% (in terms of Fe3O4and MgO) were then added onto the surface of CER by the ion-exchange method using the solutions of FeCl3and MgSO4.Typically,5.02 g FeCl3﹒6H2O and 13.12 g MgSO4﹒7H2O were dissolved in 200 ml of DI water.The solution was then added with 30 g CER resin and stirred at speed of 150 r﹒min-1in 3 h.The material was subsequently washed with distilled water for 30 min,dried at room temperature (i.e.30 °C) for 8 h.The collected material (denoted as Fe-Mg/CER)was finally stored in a desiccator for further use.Characteristics of Fe-Mg/CER were determined by techniques such as Fourier transform infrared spectroscopy (FTIR,Alpha,Bruker,Germany),scanning electron microscopy (SEM,JSM-7401F,JEOL,Japan),and X-ray diffraction(XRD,D2 Phaser,Bruker,Germany).The determination of zero point charge(pHzpc)followed the method developed by Faria et al.[16].Briefly,six beakers containing 1000 ml of 0.01 mol﹒L-1KCl solution was used as the tested solutions.0.1 mol﹒L-1HCl and NaOH solutions were used to adjust the initial pH value(pHi)of the KCl solution from 2 to 12.After that,5 mg of Fe-Mg/CER was added into the solutions and let stable for 48 h before measurement the final pH (pHf).The pH value that does not change after adding material (i.e.,pHi– pHf=0) is the isoelectric point (pHzpc) of the material.

Fig.1.Pictures of (a) CER and (b) Fe-Mg/CER and SEM images of Fe-Mg/CER (c and d).

2.3.Adsorption experimental

Nitrate removal tests were carried out in batch adsorption experiments using 50 ml of nitrate solution under ambient conditions.Each test was replicated 3 times and the average results were reported in this work with the maximum standard deviation of 1.66% of the reported values.The adsorption of nitrate using Fe-Mg/CER material was investigated at different adsorbent amounts(0.05–1.6 g),nitrate concentrations(10–500 mg﹒L-1),solution pH (3–10),and temperatures (20–40 °C) during 5–120 min.After each test,the adsorbent was separated by filtration,which was followed by centrifuging at a speed of 10,000 r﹒min-1for 10 min.The nitrate concentration in the supernatant was then analyzed by UV–Vis spectroscopy using sodium salicylate (on SPECORD 210 Plus,Analytik Jena,Germany) described in Method for the determination of nitrate content in wastewater (Standard Method 4500-Nitrogen (Nitrate)).In the cycle test,the adsorbent was regenerated by adding 100 mg of adsorbent after adsorption into 200 ml of DI water and stirring for 60 min.The material was then reused for adsorption test with a total number of cycles up to 10 to assess its durability.

Nitrate adsorption capacity q (mg﹒g-1) of the material is calculated based on the following equation:

where Co(mg﹒L-1) and Ce(mg﹒L-1) are initial and equilibrium adsorption nitrate concentrations,respectively.V(50 ml)is the volume of nitrate solution and m (mg) is the adsorbent amount [17].

3.Results and Discussion

3.1.Material characterization

As presented in Fig.1,CER (Fig.1(a)) and Fe-Mg/CER (Fig.1(b))have a distinct color difference.In Fe-Mg/CER material,the pristine yellow color of CER was replaced with brown-yellow resulted from the combination of Fe(III)and Mg(II)with CER.Fig.1(c)and(d)are SEM images of Fe-Mg/CER material which had an unchanged spherical structure of CER resin.Besides,it can be said that iron and magnesium oxides/hydroxides were attached simultaneously to the resin surface,which is further evidenced by the mapping result of the material.The mapping results showed the simultaneous presence of iron (1.44%,w/w) and magnesium (2.97%) in the material(Fig.2),along with the presence of several elements such as O (38.02%),Na (1.14%),C (43.8%) and S (12.63%).

Fig.2.EDX and mapping results of Fe-Mg/CER material.

Fig.3.XRD patterns of (1) CER and (2) Fe-Mg/CER.

XRD results of CER and Fe-Mg/CER materials are depicted in Fig.3.In CER material,the XRD pattern showed obtuse peaks with relatively low intensity.Similar peaks were found in Fe-Mg/CER material at low 2θ of 25°-30°,proving that the addition of Fe and Mg did not change the crystalline structure of the CER material.This could be due to the low contents of Mg and Fe in the materials or iron and magnesium present in the form of amorphous oxides/hydroxides.

Infrared spectroscopy plays an important role in determining the presence of functional groups or chemical bonds on the surface of materials [18].FTIR spectra of CER and Fe-Mg/CER materials before and after nitrate adsorption are plotted in Fig.4.In CER,characteristic peaks for styrene–divinylbenzene bonds in the resin structure were noted.In particular,C—H bonds in benzene ring and—CH2in cross-linked polystyrene matrix were found at 3063 cm-1and 2923 cm-1,respectively [7].Bands of stretching vibrations at 3424 cm-1and 3385 cm-1were representative for O—H bonds while the peak at 1632 cm-1was characteristic of C—C bonds in styrene ring [19].In addition,the peak band from 843 cm-1to 1177 cm-1might be assigned to the stretching vibrations of the benzene ring due to the styrene–divinylbenzene bonding matrix of the resin[20].In Fe-Mg/CER sample,in addition to typical vibrations similar to the CER carrier sample,vibration peak at 674 cm-1could be attributed to the Fe-O bond [21].In addition,there were vibrations that can characterize the Mg-O bond at positions of 468 cm-1and 445 cm-1[22].Particularly,the FTIR spectrum of Fe-Mg/CER after adsorption of nitrate appeared a new vibration peak at the position of 1413 cm-1,proving that Fe-Mg/CER material has the ability to adsorb nitrate ion [23].

3.2.Nitrate adsorption ability of Fe-Mg/CER

One of the important factors affecting the adsorption capacity of a material is the contact time.The result in Fig.5(a) showed that nitrate absorption capacity of Fe-Mg/CER increased quickly and the maximum nitrate adsorption capacity reached around 160 mg﹒g-1after 30 min.Subsequently,it decreased slightly before experiencing a gradual increase until reaching adsorption equilibrium.This result suggests that the material could give fast equilibrium adsorption after 30 min.The slight decrease and then increase after its peak adsorption are still unexplainable.It might be due to the desorption of the adsorbed nitrate with weak bond,which still needs a more thorough examination to be explained in the future.

Fig.4.FTIR spectra of CER (A),Fe-Mg/CER before (B) and after (C) nitrate adsorption,and (D) Fe/CER.

Fig.5.Effects of (a) adsorption time,(b) pH,(c) adsorbent amount,and (d) temperature and nitrate concentration on the nitrate adsorption of Fe-Mg/CER.

pH is one of the factors that have a strong effect on the adsorption efficiency.It was apparent from Fig.5(b) that Fe-Mg/CER material gave good nitrate adsorption (nitrate concentration of 50 mg﹒L-1) at pH in a range of 4–6,in which the highest efficiency reached at pH 5 with an adsorption capacity of 200 mg﹒g-1.With pH <4,the decrease in adsorption efficiency may be due to the low activity of iron and magnesium oxides/hydroxides on the surface of CER under strongly acidic conditions.When the pH increased from 6 to 10,the rapid decrease in adsorption capacity may be due to the adsorption competition between nitrate ions and OH-ions in the solution.In addition,the point of zero charge (pHpzc) of the material was around 7.Therefore,at pH <7,the surface of Fe-Mg/CER material was positively charged,so the adsorption of anion nitrate took place by the electrostatic attraction mechanism.On the contrary,at pH >7,the surface of the material was negatively charged,which was not favorable for the adsorption of nitrate anion.Moreover,the adsorption capacity decreased because of competition of surface positions by OH-ions inhibiting the adsorption of anions such as nitrate [24].

The adsorbent amount could have a significant effect on the nitrate adsorption capacity of the material.Under the same volume of 50 ml of 50 mg﹒L-1at pH 5,the adsorption capacity continuously decreased when the adsorbent amount increased from 0.05 to 1.6 g (Fig.5(c)).This can be explained by the greater adsorbent amount resulting in the greater density of Fe-Mg/CER particles which led to the adsorption competition between Fe-Mg/CER particles.The nitrate adsorption capacity was inversely proportional to the material mass,thus decreased with the increase in adsorbent mass.The increase in the adsorbent amount provided a larger contact area;however,the same concentration of nitrate in the solution limited the quantity of nitrate adsorbed.Conversely,the removal efficiency increased with the increase in adsorbent amount,demonstrating that the output concentration of nitrate decreased.

Fig.6.Mechanism of nitrate adsorption on Fe-Mg/CER.

Fig.7.(a) Effect of coexisting ions and (b) adsorption ability of Fe-Mg/CER after 10 cycles.

The next experiment is that the adsorption process was carried out at different nitrate concentrations(10–500 mg,50 ml,pH 5)with the same amount of Fe-Mg/CER under different temperatures of 20,30,and 40 °C,which was controlled by a digital general purpose water bath (Model WB-22,DAIHAN Scientific Co.,Ltd).Results in Fig.5(d) showed that the adsorption capacity was proportional to the initial nitrate concentration and the solution temperature.High concentration condition facilitated the interaction between nitrate ions and the material,promoting the contact of the material with nitrate as well as its nitrate capture.Similarly,nitrate ions in the solution became more active at higher temperatures,which increases the frequency of the collision between the material and the nitrate ions,hence inevitable increase in more chance of nitrate ions being captured,which can be clearly found in the solutions with nitrate concentration ≥100 mgat temperatures in a range of 20–40°C.This result is consistent with those reported by Ogata et al.[25],where the adsorption of nitrate is proven as chemical absorption and also increased with the temperature when using Fe-Mg-type hydrotalcites.The mechanism of the nitrate adsorption is then proposed in Fig.6.

3.3.Effect of coexisting ions and durability test

One of the important factors in the treatment of pollutants is ion selectivity,which focuses on the target ion pollutants.Fig.7(a) illustrates how much effect that coexisting ions including sulfate,phosphate,chloride,and bicarbonate had on nitrate adsorption of Fe-Mg/CER.The experiments were conducted at room temperature,pH 5,and adsorbent amount of 100 mg with 50 mg﹒L-1of nitrate (0.8064 mmol﹒L-1) and 50 mg﹒L-1of each ion (i.e.,(0.5205 mmol﹒L-1or 1.041 meq﹒L-1),(0.5265 mmol﹒L-1or 1.5795 meq﹒L-1),Cl-(1.4103 mmol﹒L-1or meq﹒L-1),and(0.8196 mmol﹒L-1or meq﹒L-1)).Accordingly,nitrate adsorption efficiency was not so significantly reduced under the presence of these ions in the solution,where the adsorption capacity decreased from around 200 to 96.3–130.9 mg.This indicates that the ability of Fe-Mg/CER for the adsorption of nitrate is still higher than those of phosphate,chloride,and sulfate.This could be an advantage of this material for nitrate removal in the practical application of water treatment where many other anions exist,but the nitrate is the more targeted ion.

The used Fe-Mg/CER(i.e.after adsorption)was then regenerated with DI water and reused for 10 cycles to evaluate its durability.As plotted in Fig.7(b),the adsorption capacity after 10 times of adsorption–desorption was closed to that of the original material after the nitrate adsorption capacity of the material declined slowly to 152.90 mg.The Fe and Mg contents were undetected in the treated water after adsorption using atomic absorption spectroscopy with the limit of detection were 0.05 mg﹒m-3for Fe and 0.02 mg﹒m-3for Mg.These results proved that Fe-Mg/CER is a durable material and can be reused many times (at least 10 times).This is one of the important factors when commercializing the material for practical applications.

4.Conclusions

In this study,high nitrate adsorption efficiency was witnessed after Fe-Mg/CER material was successfully synthesized and applied.The adsorption capacity reached around 200 mgat pH 5 after 30 min of adsorption and can be reused at least 10 times.This research contributed to a new material that has good nitrate adsorption with relatively high selectivity in the field of environmental treatment and a high potential for future commercialization.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research is funded by Vietnam National University-Ho Chi Minh City under grant number A2020-16-01.