Fabrication of a novel bifunctional nanocomposite with improved

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Applications of Polymer, Composite, and Coating Materials

Fabrication of a novel bifunctional nanocomposite with improved selectivity for simultaneous nitrate and phosphate removal from water Wenlan Yang, Xinxing Shi, Jicheng Wang, Wenjing Chen, Lili Zhang, Weiming Zhang, Xiaolin Zhang, and Jilai Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08826 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Fabrication of a novel bifunctional nanocomposite with improved selectivity for simultaneous nitrate and phosphate removal from water Wenlan Yang a*, Xinxing Shi a, Jicheng Wang a, Wenjing Chen a, Lili Zhang b*, Weiming Zhang c, Xiaolin Zhang c, Jilai Lu d a

School of the Environmental Science and Engineering, Yangzhou University, Yangzhou 225000, P. R. China

b

Jiangsu Engineering Laboratory for Environment Functional Materials, Huaiyin Normal University, Huaian 223300, P. R. China

c

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, P. R. China

d Jiangsu

Provincial Key Laboratory of Environmental Engineering, Nanjing 210036,

P. R. China

Corresponding authors * E-mail: [email protected]. Tel.: +86-514-8797-9480. * E-mail: [email protected]. Tel.: +86-517-8352-5790.

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ABSTRACT: Phosphorus and nitrogen compounds are both the main sources of eutrophication coexist in some municipal effluents or eutrophic waters, elimination of phosphorus and nitrogen from wastewater is becoming an imperative but also hard task. Herein, an innovative bifunctional nanocomposite HFO@TPR was developed for synchronous nitrate/phosphate elimination from water. A macroporous polystyrene microspheres modified with triethylamine functional groups was synthesized as the host of HFO@TPR for selective nitrate removal, and Fe(III) hydroxide (HFO) nanoparticles was implanted inside as the active species for specific phosphate removal. Compared to other commercial adsorbents, HFO@TPR exhibited outstanding selectivity and preference toward nitrate and phosphate, and the coexisting anions exert insignificant effect on adsorption performance. Such exceptional bifuntionality of HFO@TPR was achieved through two pathways, that is, nitrate was preferentially adsorbed by the fixed triethylamine groups through the electrostatic attraction, and phosphate was preferentially captured by the encapsulated HFO nanoparticles through the inner-sphere complexation. The exhausted HFO@TPR could be effectively regenerated by using a NaOH-NaCl mixed reagent for cyclic use with a relative constant efficiency. In addition, column adsorption experiments demonstrated that HFO@TPR could eliminate nitrate from 18 to ＜10 mg N/L with the treatment capacity of ~600 bed volume (BV), and meanwhile remove phosphate from 2.5 to ＜0.2 mg P/L with the treatment capacity of ~750 BV. We believe what we found in this study could advance the method on how to develop bifunctional adsorbents for synchronous removal of coexisting contaminants from water.

Keywords: Bifunctional nanocomposite; Fe(III) hydroxide; Triethylamine groups; Nitrate and phosphate; Simultaneous removal; Selectivity.


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1. Introduction Excessive emission of nitrogen and phosphorus triggers eutrophication of water bodies, and the adverse effects induced by eutrophication have been attracted worldwide attention.1 Nitrate and phosphate are widely used in agriculture as the major nutrient for crop growth.2 But meanwhile, they are also considered to be nutrient contaminants which can increase eutrophication level in surface waters, 1 leading to lots of negative impacts including damage of the self-purification capacity of receiving water and degeneration of the water ecosystem.3-5 Nitrate and phosphate commonly coexist in some effluents, and it is widely accepted that phosphate is the key factor responsible for water eutrophication.6, 7 In addition, nitrate can transform into nitrite that elevates toxicity level, leading to multiple diseases including methemoglobinemia, cancer, deformity, and also adversely affect the aquatic plants, animals and microorganism.8-11 Hence, simultaneous elimination of nitrate and phosphate prior to wastewater discharge is imperative for de-eutrophication as well as ensuring water safety. Currently, several techniques have been used to eliminate superfluous phosphate from water, including biological process,12-14 chemical precipitation,15-17 and adsorption.18-21 The most widely used techniques (e.g., biological process and chemical precipitation) are difficult to effectively reduce the concentration of phosphate to trace levels, and meanwhile, a large amount of sludge derived from these processes needs further disposal.20 Comparatively, adsorption is becoming a competitive technique for phosphate removal owing to its effectiveness, practicability, and restorability.19, 22 As to nitrate, various methods have been employed for excessive nitrate elimination from water, such as biological or chemical de-nitrification,23-27 electrodialysis,28, 29 reverse osmosis,30, 31 and adsorption/ion exchange etc.32, 33 Among the available methods, adsorption/ion exchange has been proven to be a promising method with stable efficiency to solve the nitrate issues.11 However, traditional adsorption/ion exchange technologies are hard to remove nitrate selectively due to the high stability and solubility of nitrate.10 For the past few years, a variety of adsorbents including carbon-based materials, agriculture wastes, metal (hydro) oxides (e.g., Fe, Zr, Cu), and synthetic polymer have been developed for nitrate or phosphate removal.10, 11, 34, 35 However, in review of the existing adsorbents, few can co-remove nitrate and phosphate efficiently in a single 3 / 30 ACS Paragon Plus Environment


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reactor. In theory, nitrate and phosphate should be able to be co-removed by anion-exchanger since they are both present as anionic species in aqueous systems. But in fact, nitrate and phosphate in the same form of anion will compete with each other for limited anionic exchange sites, which would negatively affect their respective removal performance. On the other hand, there are also some common competing anions (e.g., sulfate, chloridion and bicarbonate etc.) coexist in wastewater in practical cases, which would compete for limited active sites in anion-exchanger, resulting in a dramatically decrease in removal efficiency for both nitrate and phosphate. Therefore, it is an imperative but challenging task to develop new adsorption/ion exchange materials capable of synchronous capture nitrate and phosphate from water selectively. In recent years, Fe(III) hydroxide (HFO) of high specific surface area and strong surfactivity has been shown to exhibit specific affinity for adsorbing phosphate from wastewater.21, 36, 37 However, they could not remove nitrate effectively. Previous studies have shown that nitrate could be preferentially captured by the anion-exchanger modified with amino-group of long alkyl chains (e.g., trimethylamine groups).38, 39 The affinity is possibly ascribe to the long alkyl chains on the amino-groups, which making the polymer more hydrophobic and thus exerting a stronger preference for nitrate with lower hydration energy.38 However, such anion-exchanger could not remove phosphate selectively. In our recent research, we developed a polymer-based nanocomposite for simultaneous phosphate/p-nitrophenol removal from wastewater.40 The newly fabricated adsorbent possessed two different active sites, and thus could take different paths to sequestrate different classes of contaminants selectively. Enlightened by the research above, we can anticipate that the encapsulation of Fe(III) hydroxide nanoparticles within a polystyrene anion exchange resin functionalized with triethylamine would most likely obtain an adequatea bifunctional adsorbent for the synchronous sequestration of nitrate and phosphate. In this study, we aimed to fabricate an innovative bifunctional nanocomposite adsorbent of high selectivity and reusability for synchronous nitrate/phosphate elimination from contaminated water. The expectant nanocomposite HFO@TPR was fabricated through immobilizing HFO nanoparticles inside a triethylamine functionalized polystyrene resin (TPR), which was believed to possess two distinct 4 / 30 ACS Paragon Plus Environment

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active sites and thus have specific adsorption capacity toward both nitrate and phosphate. Batch adsorption experiments were performed to evaluate the performance of HFO@TPR for nitrate and phosphate retention, as well as to explicate their co-removal mechanism. Column adsorption was also conducted by using a synthetic feeding solution containing both pollutants and other co-existing ions to further assess the feasibility of HFO@TPR in practical application.

2. Materials and methods 2.1. Materials All chemicals used involved in this work were all of analytical grade and used as received. The Styrene-divinylbenzene (St-DVB) copolymer in spherical beads was kindly provided by NJU international research institute for environmental industries & technology (Jiangsu, China). To provide a comparison, a granular activated carbon (GAC) (Chaoyang activated carbon factory, Jiangsu, China) and a Poly(St-DVB) adsorbent XAD-4 (Sigma-Aldrich, USA) were also employed in this study. 2.2. Preparation of TPR and HFO@TPR As the host of the nanocomposite adsorbent, TPR was synthesized as shown below and the procedure of synthesis was depicted in Scheme S1 (Supporting Information). Firstly, chloromethyl methyl ester was used for chloromethylating the macroporous Styrene-divinylbenzene copolymers by adopting anhydrous zinc chloride as catalyst. And then triethylamine was stepwise added for amination of the chloromethylated polystyrene particle, where the introduced triethylamine groups will substitute the chloromethyl groups bonding to the polystyrene skeleton. Finally, the triethylamine functionalized macroporous polystyrene resin (TPR) was obtained. The preparation method for HFO@TPR fabrication is depicted in Supporting Information (SI Section S1), and the prime characteristics of TPR and HFO@TPR are summarized in Table 1. 2.3. Batch Adsorption Unless otherwise specified, the dosage of all adsorbents used in the batch adsorptions was set as 0.50 g/L, the solution pH was adjusted to 7.2 ± 0.1 by using 0.10 M HCl or NaOH reagent, the initial nitrate and phosphate concentration was 50 mg N/L and 10 mg P/L, respectively, with the experiment temperature of 298 K. In detail, the given adsorbents were put into a series of conical flasks containing 100 mL 5 / 30 ACS Paragon Plus Environment


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synthetic solution, followed by vibrating under 180 rpm for 12 h to ensure equilibrium. Adsorption kinetic studies were performed by soaking 0.3 g HFO@TPR into a flask containing 600 mL solution. At various time intervals, 0.5 mL supernatant was sampled out for the determination of nitrate and phosphate concentrations, the volume of the sampled solution could be negligible. Additional parameters about adsorption and regeneration could be obtained in the relevant figures or tables. 2.4. Column Adsorption Fixed-bed adsorption-desorption tests were carried out in glass columns with the length and diameter of 230 mm and 12 mm respectively, which outfitted with water bath to maintain a stable temperature. 5 mL (a bed volume) of wet TPR and HFO@TPR were packed into two columns separately. The feeding solution comprised of nitrate, phosphate and other coexisting ions was pumped down-flow by a Lange-100 peristaltic pump (China), and the hydraulic retention time (HRT) was adjusted to 10 min (flow rate 6 BV/h). When the breakthrough occurred, an in situ desorption of the saturated adsorbent was carried out by using a NaOH-NaCl binary solution (both 5 wt %) at 323K with the elution velocity of 1 BV/h. 2.5. Characterization and analysis The content of iron element in HFO@TPR was determined by digesting the solid adsorbent with HNO3-HClO4 acid for further analysis of iron concentration. The nitrate and phosphate concentration in solution were analyzed according to the National Standard Methods in China.41 The details for material characterization in this study are presented in Supporting Information (SI Section S2).

3. Results and discussion 3.1 Salient Properties of HFO@TPR The TPR precursors were presented as beige microsphere with a diameter of 0.5-0.8 mm, and the newly fabricated nanocomposites HFO@TPR exhibited the same form and dimensions as their host TPR but showed a reddish brown color (Fig. S1). Some prime physicochemical characteristics of HFO@TPR are aggregated in Table 1. The BJH pore size distribution of TPR and HFO@TPR were depicted in Fig. 1a. As shown, the pore volume of HFO@TPR declined noticeably in the range from 20 to 80 nm owing to partial pore blockage induced by the embedding of HFO, whereas the newly formed micropore leading to an increase in pore volume below 20 nm. The 6 / 30 ACS Paragon Plus Environment

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encapsulation of HFO also elevated the specific surface area from 14.3 m2/g of TPR to 17.8 m2/g of HFO@TPR, while the average pore diameter decreased from 15.4 nm of TPR to 10.1 nm of HFO@TPR. SEM image (Fig. 1b) revealed the porosity of HFO@TPR. SEM-EDS image demonstrated that most of Fe was distributed in the outer region of TPR (Fig. 1c), TEM micrograph of HFO@TPR (Fig. 1d) manifested that HFO had been successfully deposited in the pore channels of host TPR and were presented as nanoclusters or nanoparticles with size of 10-30 nm. The loading content of Fe was determined as 15.6 % in mass, and the encapsulated HFO nanoparticle was amorphous in nature based on the X-ray diffraction pattern of HFO@TPR (Fig. 1e). 3.2 Adsorption performance of different adsorbents Two commercial adsorbents including GAC, XAD-4 as well as the host TPR were employed to compare with the newly fabricated nanocomposite HFO@TPR. The removal performances of these four adsorbents for nitrate and phosphate in sole or binary solutions are presented in Fig. 2a and b, respectively. As Fig. 2a displayed, the adsorption capacities for nitrate were in the sequence of TPR > HFO@TPR > XAD-4 > GAC. TPR exhibited the highest adsorption capacity because of its functionalized trimethylamine groups have specific affinity for nitrate uptake.38 HFO@TPR got the same functional groups exhibited a little drop in nitrate uptake because it has a higher density than the host TPR after HFO loading. XAD-4 of hydrophobic property exhibited the lowest adsorption capacity because it cannot be positively charged under the trial conditions, and thus had no absorbability for anionic nitrates. Phosphate adsorption showed a distinct order from nitrate, where the corresponding sequence was HFO@TPR > TPR > XAD-4 > GAC. The best adsorption performance belonged to HFO@TPR, who could sequestrate phosphate specifically by the embedded HFO via inner-sphere complexation.37 In order to gain deeper insights into the interaction of nitrate and phosphate in adsorption, the batch adsorption studies were launched in binary systems. As can be noted in Fig. 2b, the binary system exhibited a consistent adsorption trend as the sole system, and the nitrate adsorption of HFO@TPR and TPR kept relative stabilization with phosphate coexistence. Additionally, HFO@TPR displayed a negligible decline in phosphate adsorption in presence of nitrate, whereas phosphate uptake onto TPR was significantly suppressed by coexisting nitrate. The results above indicated that TPR reacts preferentially with nitrate rather than phosphate, and the coexisting nitrate 7 / 30 ACS Paragon Plus Environment


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significantly suppresses phosphate uptake by competing for the triethylamine groups of TPR. Whereas HFO@TPR sequestrate nitrate and phosphate through two distinct pathways, i.e., triethylamine groups fixed on host for selective nitrate removal, and HFO nanoparticles immobilized inside for specific phosphate removal, as illustrated in Fig. 3. In summary, HFO@TPR exhibited the best performance for synchronous nitrate and phosphate elimination. The FTIR spectra of HFO@TPR before and after nitrate/phosphate uptake were illustrated in Fig. 4. As can be seen, the newly emerged peak at 1384.6 cm−1 after adsorption demonstrated that nitrate have been adsorbed onto HFO@TPR because the characteristic absorption peak appeared at 1384 cm-1 was in the charge of nitrate.8, 42 It is also noticeable that the peak at 1330 cm-1 (O-H bending vibration) disappeared after adsorption, while a new peak appeared at 530 cm-1, which corresponding to the stretching vibration of P-O or O-P-O.43 The results above demonstrated that the hydroxyl groups on HFO@TPR have been substituted by phosphate and meanwhile the new Fe-O-P coordinate bonds have been formed. As shown in Fig. S2, the SEM-EDX elemental images of a cross section from the nitrate/phosphate loaded sample indicated that the N and P elemental distributions are in accordance with the cross section shape of HFO@TPR, which further confirmed that the nitrate and phosphate have been captured by HFO@TPR synchronously. 3.3 Effect of solution pH The solution pH has a significant effect on the performance of a given adsorbent.44 Fig. 5a and b depicted the effects of solution pH on nitrate and phosphate uptake onto HFO@TPR. During the studied pH range, both adsorptions exhibited the highest nitrate and phosphate uptake at neutral pH. Specifically, the uptake of nitrate by HFO@TPR increased slowly with pH increasing from 3.2 to 6.5, and then sharply declined with the pH further ascended to alkaline range (pH 7.8 to 11). It is comprehensible that nitrates are mainly present as anions in solution, anion exchange through electrostatic interaction plays a dominant role in nitrate retention, and thus the nitrate sequestration is mainly affected by the competition between the hydroxide and nitrate, accordingly high concentrations of hydroxide at alkaline pH will inevitably antagonize the nitrate uptake onto HFO@TPR. As for phosphate, the effect of solution pH on phosphate uptake was more complicated. During the studied pH range, phosphate adsorption was firstly enhanced 8 / 30 ACS Paragon Plus Environment

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with the pH increase from 3.0 to 7.0, and then gradually declined with pH dropping to alkaline region. It is reasonable because two distinct active sites of HFO@TPR were responsible for phosphate retention, the –N+(C2H5)3 groups of the host TPR and the surface -OH groups of the immobilized HFO. The interaction between the active sites and phosphate are described as follows (the overbar represents the solid phase): xR - NH   C2 H 5 3 Cl   H n PO4 (3 n )  (aq )   R - NH   C2 H 5 3   H n PO4 (3 n )    xCl  (aq ) x

(1)

Fe   OH m  yH n PO4 (3 n )  (aq )  Fe   OH m  y  H n PO4 (3 n )    yOH  (aq )

(2)

y

According to the pH-dependent speciation of phosphoric acid (Fig. S3),45 one can see that the proportion of phosphate with multiple charges enhanced accompany with the raise of pH level in the studied pH range, which was beneficial to phosphate retention by the trimethylamine groups of TPR through electrostatic interaction (Eq. 1). Meanwhile, phosphate could also be preferentially sequestrated by the immobilized HFO nanoparticles through inner-sphere complexation (Eq. 2), but the reaction would be restrained by the raise of hydroxide concentration at alkaline pH, which was adverse to phosphate retention by HFO. Thus, the influence of pH on the retention of phosphate by HFO@TPR was determined by the combined action of TPR and HFO on phosphate sequestration. At high alkaline pH, the increase of OH- could cause a strong competitive effect on phosphate uptake by TPR, and meanwhile the HFO was deprotonated and negatively charged, resulting in a Donnan co-ion exclusion effect, which was unfavorable for phosphate removal by HFO@TPR. As can be seen in Fig. S4, the isoelectric point (IEP) of HFO@TPR showed a slight decrease (from 9.39 to 8.05) after nitrate and phosphate ingestion. The surface of HFO@TPR got more negatively charged on account of the uptake of nitrate and phosphate anions, leading to a translation of IEP of HFO@TPR to the range of lower pH value.11 3.4 Influence of competing anions In consideration of some common anions (e.g., SO42−, HCO3−, and Cl−) generally involved in natural water or wastewater, it is essential to systematically investigate the influences of ionic strengths on the performance of HFO@TPR, and the host polystyrene anion exchanger TPR was also employed for comparison. Compared with HCO3− and Cl−, SO42− exhibits a stronger competitive advantage for target anions to be removed,21, 39, 46 thus the SO42− was reasonably selected as the representative competing anion in this study. As can be noted in Fig. 6a, with the elevation of the 9 / 30 ACS Paragon Plus Environment


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SO42− concentration from 0 to 200 mg/L, the adsorption of phosphate by TPR dropped sharply to almost zero, whereas HFO@TPR just showed a limited decrease whether in the sole or binary solution. Even if further raised the SO42− concentration to 800 mg/L (80 times of the initial phosphate), HFO@TPR could still remain approximately 50% adsorption of phosphate, while TPR was almost entirely paralyzed. Such different tendency demonstrated that HFO@TPR exhibited a much more marked preference for phosphate retention than the host anion exchanger TPR. As discussed above, the removal of phosphate by HFO@TPR attributed to two diverse adsorption mechanisms: anion exchange through the -N+(C2H5)3 groups of host TPR (nonspecific adsorption) and inner-sphere complexation generated by the immobilized HFO nanoparticles (specific adsorption). The competing anion at high levels would greatly restrain phosphate uptake by TPR, where the introduced triethylamine groups sequestrate phosphate only through nonspecific Columbic attraction. As for HFO@TPR, the embedded HFO nanoparticles exhibit specific affinity for phosphate retention via a specific Lewis acid-base interaction, where the phosphate anions serve as Lewis base offering electron pairs to form the inner-sphere complexes with the Fe atoms.47 As a result, further increases in competing anion concentration pose slight or even negligible effects on phosphate uptake by HFO@TPR. STEM-HAADF examines were carried out to further investigate the specific sequestration of phosphate by HFO@TPR, the STEM-HAADF and corresponding EDS elemental maps of phosphate-loaded HFO@TPR are displayed in Fig. 7. Explicitly, for the phosphate-loaded HFO@TPR sampled from the background of sulfate, the elemental distribution of phosphorus was highly consistent with that of iron, indicating that HFO nanoparticles embedded in HFO@TPR provided specific adsorption toward phosphate. As can be noticed in Fig. 6b, both the nitrate uptake by TPR and HFO@TPR exhibited a slight drop with the raise of the SO42− concentration from 0 to 200 mg/L, either in sole or in binary solution. With further addition of sulfate from 200 to 800 mg/L (4 to 16 times of the initial nitrate), both the nitrate adsorption reached a plateau and still exhibited approximately 50% removal efficiency. Such performance indicated that both TPR and HFO@TPR exhibit strong priority for nitrate retention in existence of high concentrations of coexisting anions. Compared with traditional anion exchanger, TPR and HFO@TPR were all modified by triethylamine groups 10 / 30 ACS Paragon Plus Environment

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with longer alkyl chains, which render the adsorbent more hydrophobic, and thus exhibited more preferential adsorption of anions with lower hydration energies. In this case, nitrate possesses a much lower hydration energy (ΔG°, -314 kJ/mol) than sulfate (ΔG°, -1103 kJ/mol) does.38,48 Therefore, it is reasonable that both TPR and HFO@TPR can maintain a decent removal performance for nitrate even in presence of high concentrations of coexisting anions. The effects of chloride and bicarbonate ions on adsorption of nitrate and phosphate were also evaluated in the sole and binary solutions, and the tendencies were found to be the similar with the sulfate ion (SI Fig. S5 and S6). 3.5 Adsorption kinetics Adsorption kinetics of HFO@TPR toward nitrate and phosphate was performed in binary solutions to evaluate the adsorption rate, and the results were described in Fig. 8. It is worth noting that both the adsorptive rates of nitrate and phosphate were fast during the inception phase and then gradually reduced until adsorption equilibrium. The equilibration time of phosphate was approximately 450 min while that of nitrate was within 60 min. The adsorption kinetic data were then fitted by using two representative kinetic models, pseudo-first and pseudo-second order mode.49 As can be noted in Table S1, the derived parameters demonstrated that the pseudo-first order mode could better describe the adsorption of nitrate by HFO@TPR, while the phosphate adsorption could be better represented by the pseudo- second order model. It is understandable that uptake of nitrate and phosphate onto HFO@TPR occur via diverse pathways and take reaction with distinct active sites. Specifically, ion exchange through Columbic attraction with the grafted triethylamine groups generally serves as the basic interaction for nitrate adsorption, whereas inner-sphere complexation between phosphate and HFO plays a significant role in phosphate retention in binary solutions. 3.6 Regeneration and reuse Considering that reusability is of great significant to the potential application of a given adsorbent, cyclic adsorption-regeneration tests based on a simply desorption method were carried out to examine the reusability of HFO@TPR. The effects of solution pH demonstrated that alkaline pH was unfavorable for nitrate and phosphate retention onto HFO@TPR, implying that alkaline solution is favorable for regeneration of exhausted HFO@TPR, and the NaOH-NaCl mixed reagent was 11 / 30 ACS Paragon Plus Environment


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reasonably employed as the desorbent. Here a successive 10-cycle batch adsorption-desorption tests were conducted to examine the reusability of HFO@TPR, and the results and detailed procedure of regeneration were provided in Fig. 9. Before the next batch of adsorption, the regenerated HFO@TPR was rinsed by a 5% NaCl solution to flush out the residual OH- inside HFO@TPR, which is unfavorable for nitrate and phosphate retention. As can be observed in Fig. 9, the nitrate adsorption capacity remained stable throughout the cycle process, while the phosphate retention dropped slightly as compared with the initial and 4-10 cycles, but there was no accumulated capacity loss from cycle 4 to 10. The results above proved that HFO@TPR could be reused for simultaneous nitrate and phosphate removal with no significant capacity loss, and meanwhile no iron leaching from HFO@TPR was detected. 3.7 Column experiments Column adsorption and desorption tests were performed to further estimate the practical application potential of HFO@TPR, and a synthetic solution comprising nitrate, phosphate and other commonly coexisting anions was fed up-to-down through the column by a peristaltic pump. For comparison, the host polymer TPR was also employed to purify the same synthetic feeding solution under identical conditions. Fig. 10 presented the ingredient of the feeding solution. The breakthrough point of nitrate and phosphate in column adsorption were set as 10 and 0.2 mg/L respectively by referring to the environmental quality standards for surface water in China.50 The breakthrough profiles of nitrate and phosphate were depicted in Fig. 10. The results indicated that HFO@TPR showed an approximate nitrate removal performance to TPR with treatment capacity of ~600 BV (bed volume), a slight capacity loss for HFO@TPR possibly ascribed to the occultation of partial exchange sites by the embedded HFO nanoparticles. As for phosphate, the HFO@TPR generated ~750 BV effluent before the breakthrough occurred, which was significantly superior to that of TPR (~150 BV), indicating that the immobilization of HFO nanoparticles significantly facilitated the phosphate removal. Upon the adsorption process finish, the spent HFO@TPR was subjected to in situ regeneration by using 10 BV NaOH– NaCl mixed reagent (both 5 wt %), followed by 10 BV NaCl reagent (5 wt %) and 10 BV DI water, the cumulative desorption efficiency of nitrate and phosphate could reach up to ~99% and ~94%, respectively. All the above results showed that 12 / 30 ACS Paragon Plus Environment

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HFO@TPR could be employed for long-term fix-bed application.

4. Conclusions A novel bifunctional nanocomposite adsorbent (HFO@TPR) was fabricated for synchronous elimination of nitrate and phosphate from wastewater, with triethylamine functionalized spherical polystyrene host for preferential nitrate uptake, and encapsulated HFO nanoparticles for specific phosphate retention. HFO@TPR exhibited the best efficiency for the simultaneous nitrate/phosphate removal than other commercial adsorbents, including GAC, XAD-4 and its host TPR. The outstanding performance benefit from its unique structure, in which the triethylamine functionalized polystyrene host (TPR) showed preferential sorption of nitrate via selective ion-exchange, and the encapsulated Fe(III) hydroxide nanoparticles exhibited specific affinity for phosphate retention through inner-sphere complexation. More attractively, the spent HFO@TPR could be sufficiently regenerated by using a NaOH-NaCl mixed reagent for cyclic use with an enduring and stable adsorption performance. Compared with several other adsorbents prepared recently for synchronous nitrate/phosphate removal,4, 42, 51-55 HFO@TPR showed strong competiveness in aspect of selectivity, reusability, and availability. This study is believed to advance the method on how to develop bifunctional adsorbents for co-removal of coexisting contaminants from water. Further research is required for investigating the performance of HFO@TPR in pilot and even field application to extend knowledge on HFO@TPR properties. Acknowledgements We greatly acknowledge the financial support from Open Research Fund of Jiangsu Provincial Key Laboratory of Environmental Engineering (NO. KF2018-004), Jiangsu Engineering Laboratory for Environment Functional Materials Foundation (NO. JSEFM201804), State Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRF18032), Natural Science Foundation of China (NO. 51508221) and Graduate Student Scientific Practical Innovation Projects in Jiangsu Province (NO. SJCX18_0801). Supporting Information 13 / 30 ACS Paragon Plus Environment


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Additional adsorption kinetics and isotherms data, morphology images, SEM-EDX elemental maps, zeta potential data, influence of chloride and bicarbonate ions data, synthetic procedure of TPR. References (1) Conley, D. J.; Paerl, H. W.; Howarth, R. W.; Boesch, D. F.; Seitzinger, S. P.; Havens, K. E.; Lancelot, C.; Likens, G. E., Ecology controlling eutrophication: nitrogen and phosphorus. Science 2009, 323, 1014-1015. (2) Hua, G. H.; Salo, M. W.; Schmit, C. G.; Hay, C. H., Nitrate and phosphate removal from agricultural subsurface drainage using, laboratory woodchip bioreactors and recycled steel byproduct filters. Water Res. 2016, 102, 180-189. (3) Gao, Q.; Wang, C. Z.; Liu, S.; Hanigan, D.; Liu, S. T.; Zhao, H. Z., Ultrafiltration membrane microreactor (MMR) for simultaneous removal of nitrate and phosphate from water. Chem. Eng. J. 2019, 355, 238-246. (4) Yin, Q. Q.; Wang, R. K.; Zhao, Z. H., Application of Mg-Al-modified biochar for simultaneous removal of ammonium, nitrate, and phosphate from eutrophic water. J Clean Prod 2018, 176, 230-240. (5) Li, R. H.; Morrison, L.; Collins, G.; Li, A. M.; Zhan, X. M., Simultaneous nitrate and phosphate removal from wastewater lacking organic matter through microbial oxidation of pyrrhotite coupled to nitrate reduction. Water Res. 2016, 96, 32-41. (6) Shi, W. M.; Fu, Y. W.; Jiang, W.; Ye, Y. Y.; Kang, J. X.; Liu, D. Q.; Ren, Y. Z.; Li, D. S.; Luo, C. G.; Xu, Z., Enhanced phosphate removal by zeolite loaded with Mg-Al-La ternary (hydr)oxides from aqueous solutions: Performance and mechanism. Chem. Eng. J. 2019, 357, 33-44. (7) Bui, T. H.; Hong, S. P.; Yoon, J., Development of nanoscale zirconium molybdate, embedded anion exchange resin for selective removal of phosphate. Water Res. 2018, 134, 22-31. (8) Wu, Y.; Wang, Y.; Wang, J. N.; Xu, S. Q.; Yu, L.; Philippe, C.; Wintgens, T., Nitrate removal from water by new polymeric adsorbent modified with amino and quaternary ammonium groups: Batch and column adsorption study. J Taiwan Inst Chem E 2016, 66, 191-199. (9) Song, H. O.; Yao, Z. J.; Shuang, C. D.; Li, A. M., Accelerated removal of nitrate from aqueous solution by utilizing polyacrylic anion exchange resin with magnetic separation performance. J Ind Eng Chem 2014, 20, (5), 2888-2894. (10) Loganathan, P.; Vigneswaran, S.; Kandasamy, J., Enhanced removal of nitrate from water using surface modification of adsorbents - A review. J. Environ. Manage. 2013, 131, 363-374. (11) Xu, X.; Gao, B. Y.; Zhao, Y. Q.; Chen, S. H.; Tan, X.; Yue, Q. Y.; Lin, J. Y.; Wang, Y., Nitrate removal from aqueous solution by Arundo donax L. reed based anion exchange resin. J. Hazard. Mater. 2012, 203, 86-92. (12) Ye, Y. Y.; Ngo, H. H.; Guo, W. S.; Liu, Y. W.; Zhang, X. B.; Guo, J. B.; Ni, B. J.; Chang, S. W.; Nguyen, D. D., Insight into biological phosphate recovery from sewage. Bioresour. Technol. 2016, 218, 874-881. (13) Wang, X. X.; Zhao, J.; Yu, D. S.; Chen, G. H.; Du, S. M.; Zhen, J. Y.; Yuan, M. F., Stable nitrite accumulation and phosphorous removal from nitrate and municipal wastewaters in a 14 / 30 ACS Paragon Plus Environment

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Table 1 Prime physicochemical characteristics of the GAC, XAD-4, TPR and HFO@TPR GAC

a

XAD-4

TPR

HFO@TPR

Grain shape

Granule

Spherical beads

Appearance

Black

White

White

Reddish brown

Matrix structure

Carbon

St-DVBa

St-DVBa

St-DVBa

Functional sites

/

/

-N(C2H5)3

-N(C2H5)3; HFO

Amount of the groups (mmol/g)

/

/

3.8

2.8 HFO

Particle size (mm)

0.5-1

0.5-0.8

0.5-0.8

0.5-0.8

BET Surface area (m2/g)

920

760

14.3

17.8

Average pore diameter (nm)

2.9

5.84

15.4

10.1

Packing density (g/mL)

/

/

0.516

0.683

HFO content (Fe mass %)

/

/

/

15.6

Styrene-divinylbenzene.


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Figure Captions Fig. 1 Characterizations of the nanocomposite HFO@TPR (a)The BJH pore size distribution of TPR and HFO@TPR (b) SEM image of the cross section, (c) elemental radial distribution, (d) TEM image and (e) XRD diffraction pattern of HFO@TPR. Fig. 2 Adsorption performance of different adsorbents for nitrate and phosphate in (a) sole and (b) binary solutions. Fig. 3 Schematic illustration of HFO@TPR and its possible mechanism for synchronous nitrate and phosphate sequestration. Fig. 4 Comparison of FTIR spectra before and after adsorption of nitrate and phosphate onto HFO@TPR. Fig. 5 Effect of pH on the retention of (a) nitrate and (b) phosphate by HFO@TPR. Fig. 6 Effect of coexisting sulfate on the adsorption performance of HFO@TPR and TPR for (a) phosphate and (b) nitrate in sole and binary solutions. Fig. 7 (a) STEM-HAADF image of phosphate-adsorbed HFO@TPR, and (b) the corresponding elemental maps of Fe, O, P and N in the yellow frame in panel a. Fig. 8 Adsorption kinetics of HFO@TPR for (a) nitrate and (b) phosphate in binary solutions. Fig. 9 Cyclic batch adsorption-desorption tests of HFO@TPR for synchronous nitrate and phosphate elimination (Desorption: 5 mL 5% NaOH–NaCl binary solution + 5 mL 5% NaCl solution + 20 mL DI water in turn). Fig. 10 Breakthrough curves of TPR and HFO@TPR for (a) nitrate and (b) phosphate co-removal from a synthetic wastewater (HRT, 10 min; pH, 7.2 ± 0.1; T, 298 K; 19 / 30 ACS Paragon Plus Environment


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nitrate, 18 mg N/L; phosphate, 2.5 mg P/L; SO42-, 40 mg/L; Cl-, 65 mg/L; Mg2+, 10 mg/L; Ca2+, 36 mg/L).The inserts are the elution profiles of HFO@TPR for (a) nitrate and (b) phosphate.


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Fig. 1 Characterizations of the nanocomposite HFO@TPR (a)The BJH pore size distribution of TPR and HFO@TPR (b) SEM image of the cross section, (c) elemental radial distribution, (d) TEM image and (e) XRD diffraction pattern of HFO@TPR.



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Fig. 2 Adsorption performance of different adsorbents for nitrate and phosphate in (a) sole and (b) binary solutions.


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Fig. 3 Schematic illustration of HFO@TPR and its possible mechanism for synchronous nitrate and phosphate sequestration.



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Fig. 4 Comparison of FTIR spectra before and after adsorption of nitrate and phosphate onto HFO@TPR.


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Fig. 5 Effect of pH on the retention of (a) nitrate and (b) phosphate by HFO@TPR.



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Fig. 6 Effect of coexisting sulfate on the adsorption performance of HFO@TPR and TPR for (a) phosphate and (b) nitrate in sole and binary solutions.


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Fig. 7 (a) STEM-HAADF image of phosphate-adsorbed HFO@TPR, and (b) the corresponding elemental maps of Fe, O, P and N in the yellow frame in panel a.



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Fig. 8 Adsorption kinetics of HFO@TPR for (a) nitrate and (b) phosphate in binary solutions.


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Fig. 9 Cyclic batch adsorption-desorption tests of HFO@TPR for synchronous nitrate and phosphate elimination (Desorption: 5 mL 5% NaOH–NaCl binary solution + 5 mL 5% NaCl solution + 20 mL DI water in turn).



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Fig. 10 Breakthrough curves of TPR and HFO@TPR for (a) nitrate and (b) phosphate co-removal from a synthetic wastewater (HRT, 10 min; pH, 7.2 ± 0.1; T, 298 K; nitrate, 18 mg N/L; phosphate, 2.5 mg P/L; SO42-, 40 mg/L; Cl-, 65 mg/L; Mg2+, 10 mg/L; Ca2+, 36 mg/L).The inserts are the elution profiles of HFO@TPR for (a) nitrate and (b) phosphate.


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Fabrication of a novel bifunctional nanocomposite with improved

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