Rational Design of Antifouling Polymeric Nanocomposite for

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Rational Design of Antifouling Polymeric Nanocomposite for Sustainable Fluoride Removal from NOM-rich Water Xiaolin Zhang, Lu Zhang, Zhixian Li, Zhao Jiang, Qi Zheng, Bin Lin, and Bing-Cai Pan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04164 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Environmental Science & Technology

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Rational Design of Antifouling Polymeric Nanocomposite for Sustainable

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Fluoride Removal from NOM-rich Water

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Xiaolin Zhang,† ‡ Lu Zhang,† Zhixian Li,† Zhao Jiang,† Qi Zheng,† Bin Lin,† Bingcai

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Pan*†‡

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†

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Environment, Nanjing University, Nanjing 210023, China

State Key Laboratory of Pollution Control and Resource Reuse, School of the

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‡

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University, Nanjing 210023, China

Research Center for Environmental Nanotechnology (ReCENT), Nanjing

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* To whom correspondence should be addressed

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E-mail: [email protected] (B.C.P)

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Tel: +86-25-8968-0390

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ACS Paragon Plus Environment


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ABSTRACT

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The presence of natural organic matters (NOM) exerts adverse effect on adsorptive

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removal of various pollutants including fluoride from water. Herein, we designed a

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novel nanocomposite adsorbent for preferable and sustainable defluoridation from

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NOM-rich water. The nanocomposite (HZO@HCA) is obtained by encapsulating

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hydrous zirconium oxide nanoparticles (HZO NPs) inside hypercrosslinked

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polystyrene anion exchanger (HCA) binding tertiary amine groups. Another

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commercially available nanocomposite HZO@D201, with the host of a crosslinked

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polystyrene anion exchanger (D201) binding ammonium groups, was involved for

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comparison.

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meso/macropores of HZO@D201, resulting in the inaccessible sites for NOM due to

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the size exclusion. Also, the tertiary amine groups of HCA favor an efficient

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desorption of the slightly loaded NOM from HZO@HCA. As expected,

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Sigma-Aldrich humic acid even at 20 mg DOC/L did not exert any observable effect

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on fluoride sequestration by HZO@HCA, whereas a significant inhibition was

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observed for HZO@D201. Cyclic adsorption runs further verified the superior

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reusability of HZO@HCA for defluoridation from NOM-rich water. In addition, the

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HZO@HCA column could generate ~80 bed volume (BV) effluent from a synthetic

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fluoride-containing groundwater to meet the drinking water standard (10 nm. Such remarkable drop of

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meso-/macropore is consistent with the size of the encapsulated NPs (Figure 2a),

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implying that the in situ growth of HZO NPs occupied the meso-/macroporous region

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of HCA. Accordingly, the microporous volume increased significantly, possibly

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arising from the new micropores generated among or inside the formed HZO NPs.

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As elucidated below, the unique pore structure of HZO@HCA, i.e., abundant

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micropores and negligible meso-/macropores, plays a dominant role in sustainable

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fluoride removal from NOM-rich water. Comparatively, HZO@D201 exhibits a broad

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pore size distribution from several to dozens of nanometers (Figure 2c).

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Adsorption isotherm. The adsorption isotherm of HZO@HCA toward fluoride is

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described in Figure S4 and is well fitted by Langmuir model:

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𝑄𝑄𝑒𝑒 =

𝑄𝑄𝑚𝑚 𝐾𝐾𝐿𝐿 𝐶𝐶𝑒𝑒 1+𝐾𝐾𝐿𝐿 𝐶𝐶𝑒𝑒

(1)

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Where Qe is the F capacity at equilibrium (mg/g), KL is the binding constant (L/mg),

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and Qm is the maximum adsorption capacity (mg/g). The Qm of HZO@HCA was

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calculated as 20.9±0.3 mg/g, comparable to other superior adsorbents for fluoride.42-46 10 / 32


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For instance, Qm value of a sulfate-doped Fe3O4/Al2O3 NPs was 48.5 mg/g toward

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fluoride at pH 7.0.42 At pH 6.0, four bauxite ores from various regions exhibited Qm of

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~8.5 mg/g.43 Activated alumina and a granular zirconium-iron oxide were utilized for

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defluoridation of drinking water, generating Qm values of 7.7 mg/g and 22.8 mg/g

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respectively at pH 7.0.45 Note that the Qm value of HZO@D201 toward fluoride was

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~20 mg/g12, which is very close to HZO@HCA. It is not beyond our expectation as

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HZO@HCA is similar to HZO@D201 in structure except for pore structure and host

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functional groups. We recently observed that the adsorption kinetics of target

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pollutants (e.g., arsenic and fluoride) onto polymer nanocomposites was controlled by

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the intraparticle diffusion inside the embedded NPs instead of inside the polymer

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host.40,

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negligible effect on fluoride uptake. In addition, as illustrated in Figure S1, both hosts,

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i.e., D201 and HCA, are positively charged during adsorption, thereby posing similar

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impact on fluoride adsorption.

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Effect of solution chemistry. As depicted in Figure 3a, the presence of HA even at 20

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mg DOC/L, higher than most of the real concentration in natural groundwater and

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surface water,47 exerted negligible effect on fluoride removal. However, more than 90%

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adsorption of HZO@D201 toward fluoride was suppressed under similar conditions.29

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Effect of HA on fluoride adsorption by pristine HZO was depicted in Figure S5, from

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which one can see that around 50% capacity loss of the pristine HZO occurred in the

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presence of HA (20 mg DOC/L), indicating an intense inhibition on defluoridation in

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the presence of NOM. Besides, three humic substances from International Humic

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Thus, the abundant microporous structure of HZO@HCA would exert

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Substance Society (IHSS) were utilized to further verify the anti-fouling property of

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HZO@HCA. Similarly, all the NOM samples exerted negligible effect on the

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adsorption of HZO@HCA toward fluoride (Figure S6), suggesting that the

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anti-fouling property of HZO@HCA may be universally valid for humic substances

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from various regions. The kinetic data shown in Figure 3b indicate that fluoride

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adsorption onto HZO@HCA reached equilibrium within 24 h, and the presence of HA

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slightly reduced the adsorption kinetics but did not change the final fluoride

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adsorption capacity. The effect of pH on fluoride removal is elucidated in Figure 3c,

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and increasing pH played an unfavorable role in defluoridation. It is expected because

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surface deprotonation of the embedded HZO NPs would be enhanced as pH increased,

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thereby suppressing fluoride uptake due to the electrostatic repulsion. Also, the

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presence of HA exerted negligible effect on defluoridation at pH of 5-9.

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In addition, the effects of other ubiquitous anions, including chloride, sulfate,

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nitrate and silicate on fluoride removal by the resultant nanocomposite were examined

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in the background of HA or HA free, respectively. As observed in Figure S7(a-c), the

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removal of fluoride by HZO@HCA is independent on the presence of chloride, sulfate

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and nitrate at the concentration levels of interest (0-500 mg/L). It is mainly due to the

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specific affinity of HZO toward fluoride, which is clearly elucidated elsewhere.12, 48

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Nevertheless, increasing silicate concentration from 0 to 500 mg/L would inhibit

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fluoride removal remarkably (Figure S7d), possibly arising from the formation of

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inner-sphere complex between HZO and silicate.49 Note that no observable difference

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occurrs for the defluoridation efficiency of HZO@HCA in HA-rich and HA-free 12 / 32


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water.

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Reusability of the nanocomposite. The presence of NOM will not only compromise

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the defluoridation efficiency of a given adsorbent, but also cause adverse effects on

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their sustainable applications of nanocomposite adsorbents because it is challenging to

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desorb the loaded NOM from the adsorbent surface.31,

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reusability of HZO@HCA, HZO@D201 and HCA by successive adsorption-

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regeneration runs, where the adsorbents were first utilized to decontaminate

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NOM-rich water from fluoride, followed by alkaline rinsing to desorb the loaded

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fluoride for the next cyclic run.12 As suggested in Figure 4a, the capacity of

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HZO@HCA remained constant after five cyclic runs. On the contrary, HZO@D201

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and HCA exhibited continuous capacity loss during cyclic runs, e.g., ~50% and ~80%

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capacity were lost for the latter two adsorbents. The results in Figure 4b depicted the

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excellent stability of HZO@HCA in a wide pH range (2-12), i.e., negligible

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dissolution of HZO occurring at pH≥2, and the presence of HA did not exert any

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adverse effect on its stability properties.

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The antifouling mechanism. To elucidate the size exclusion effect on the preferable

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removal of fluoride on HZO@HCA, we employed EEM fluorescence spectroscopy to

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characterize the variation of aqueous organic matters before and after adsorption onto

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three solid adsorbents.50-52 As shown in Figure 5a, the EEM of HA can be divided into

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five regions, representing aromatic protein (region I and II), fulvic acid-like substance

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(region III), humic acid-like substance (region IV) and soluble microbial

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by-product-like substance (region V), respectively.50, 52 HA mainly consisted of the

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Thus, we evaluated the


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region III and IV. The HCA host and HZO@D201 exhibited similar removal of HA,

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e.g., ~40% and ~50% removal of region III and IV respectively, which is

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quantitatively determined based on a fluorescence regional integration technique that

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integrates the volume beneath an EEM.50 It is not beyond expectation because the

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surface amine/ammonium groups as well as their meso-/macroporous structure of

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both solids favored HA adsorption via electrostatic attraction and pore filling

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respectively. However, it is not the case for HZO@HCA, and its removal toward

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region III and IV was only ~15% due to its insufficient meso-/macro pores to allow

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HA diffusion inside (Figure 2b). We further obtained the FT-IR spectra of

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HZO@HCA and HZO@D201 preloaded with fluoride from HA solution. Negligible

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change was observed for the fluoride-preloaded HZO@HCA as compared with the

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fresh one (data not shown), whereas a new peak at ~1354 cm-1 occurred in the

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fluoride-preloaded HZO@D201, which could be assigned to the carboxyl groups

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(Figure S8) from the adsorbed HA.

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The HA adsorption onto the solid adsorbent were achieved mainly via two

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pathways, i.e., electrostatic attraction with the positively charged groups and pore

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filling. To distinguish different roles of both pathways, we carried out batch HA

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adsorption in the background of sulfate or sulfate free solution. Pre-experimental

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results indicated that sulfate at 1000 mg/L could fully screen the role of electrostatic

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interaction in HA adsorption due to the competitive effect. As depicted in Figure 5b,

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the presence of sulfate at 1000 mg/L enhanced the removal of HA by HCA and

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HZO@D201, resulting in >70% removal for region III and IV. Such favorable role of 14 / 32


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sulfate may be ascribed to the salting-out effect on the adsorption of organic matters

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occurring via pore filling.48, 49 On the contrary, the added sulfate ions compromised

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the HA adsorption onto HZO@HCA to nearly zero and ~9.0% for region III and IV

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respectively, further suggesting that the electrostatic attraction instead of pore filling

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is responsible for the low HA removal from the sulfate free solution by HZO@HCA

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(Figure 5a). In other words, the size exclusion effect works expectedly to inhibit HA

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adsorption onto HZO@HCA. Aldrich HA is mainly in the form of aggregates of ~2.9

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nm in hydrodynamic diameter in water26, and it increased up to ~6 nm and even ~10

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nm in the presence of other minerals. Obviously, HZO@HCA possessing abundant

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micropore (

Rational Design of Antifouling Polymeric Nanocomposite for

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