Subscriber access provided by Gothenburg University Library
Article
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 33
Environmental Science & Technology
1 2
Rational Design of Antifouling Polymeric Nanocomposite for Sustainable
3
Fluoride Removal from NOM-rich Water
4 5
Xiaolin Zhang,† ‡ Lu Zhang,† Zhixian Li,† Zhao Jiang,† Qi Zheng,† Bin Lin,† Bingcai
6
Pan*†‡
7 8
†
9
Environment, Nanjing University, Nanjing 210023, China
State Key Laboratory of Pollution Control and Resource Reuse, School of the
10
‡
11
University, Nanjing 210023, China
Research Center for Environmental Nanotechnology (ReCENT), Nanjing
12 13
* To whom correspondence should be addressed
14
E-mail:
[email protected] (B.C.P)
15
Tel: +86-25-8968-0390
16 17 18 19 20
1 / 32
ACS Paragon Plus Environment
Environmental Science & Technology
21
Page 2 of 33
ABSTRACT
22
The presence of natural organic matters (NOM) exerts adverse effect on adsorptive
23
removal of various pollutants including fluoride from water. Herein, we designed a
24
novel nanocomposite adsorbent for preferable and sustainable defluoridation from
25
NOM-rich water. The nanocomposite (HZO@HCA) is obtained by encapsulating
26
hydrous zirconium oxide nanoparticles (HZO NPs) inside hypercrosslinked
27
polystyrene anion exchanger (HCA) binding tertiary amine groups. Another
28
commercially available nanocomposite HZO@D201, with the host of a crosslinked
29
polystyrene anion exchanger (D201) binding ammonium groups, was involved for
30
comparison.
31
meso/macropores of HZO@D201, resulting in the inaccessible sites for NOM due to
32
the size exclusion. Also, the tertiary amine groups of HCA favor an efficient
33
desorption of the slightly loaded NOM from HZO@HCA. As expected,
34
Sigma-Aldrich humic acid even at 20 mg DOC/L did not exert any observable effect
35
on fluoride sequestration by HZO@HCA, whereas a significant inhibition was
36
observed for HZO@D201. Cyclic adsorption runs further verified the superior
37
reusability of HZO@HCA for defluoridation from NOM-rich water. In addition, the
38
HZO@HCA column could generate ~80 bed volume (BV) effluent from a synthetic
39
fluoride-containing groundwater to meet the drinking water standard (10 nm. Such remarkable drop of
206
meso-/macropore is consistent with the size of the encapsulated NPs (Figure 2a),
207
implying that the in situ growth of HZO NPs occupied the meso-/macroporous region
208
of HCA. Accordingly, the microporous volume increased significantly, possibly
209
arising from the new micropores generated among or inside the formed HZO NPs.
210 211
As elucidated below, the unique pore structure of HZO@HCA, i.e., abundant
212
micropores and negligible meso-/macropores, plays a dominant role in sustainable
213
fluoride removal from NOM-rich water. Comparatively, HZO@D201 exhibits a broad
214
pore size distribution from several to dozens of nanometers (Figure 2c).
215
Adsorption isotherm. The adsorption isotherm of HZO@HCA toward fluoride is
216
described in Figure S4 and is well fitted by Langmuir model:
217
𝑄𝑄𝑒𝑒 =
𝑄𝑄𝑚𝑚 𝐾𝐾𝐿𝐿 𝐶𝐶𝑒𝑒 1+𝐾𝐾𝐿𝐿 𝐶𝐶𝑒𝑒
(1)
218
Where Qe is the F capacity at equilibrium (mg/g), KL is the binding constant (L/mg),
219
and Qm is the maximum adsorption capacity (mg/g). The Qm of HZO@HCA was
220
calculated as 20.9±0.3 mg/g, comparable to other superior adsorbents for fluoride.42-46 10 / 32
ACS Paragon Plus Environment
Page 11 of 33
Environmental Science & Technology
221
For instance, Qm value of a sulfate-doped Fe3O4/Al2O3 NPs was 48.5 mg/g toward
222
fluoride at pH 7.0.42 At pH 6.0, four bauxite ores from various regions exhibited Qm of
223
~8.5 mg/g.43 Activated alumina and a granular zirconium-iron oxide were utilized for
224
defluoridation of drinking water, generating Qm values of 7.7 mg/g and 22.8 mg/g
225
respectively at pH 7.0.45 Note that the Qm value of HZO@D201 toward fluoride was
226
~20 mg/g12, which is very close to HZO@HCA. It is not beyond our expectation as
227
HZO@HCA is similar to HZO@D201 in structure except for pore structure and host
228
functional groups. We recently observed that the adsorption kinetics of target
229
pollutants (e.g., arsenic and fluoride) onto polymer nanocomposites was controlled by
230
the intraparticle diffusion inside the embedded NPs instead of inside the polymer
231
host.40,
232
negligible effect on fluoride uptake. In addition, as illustrated in Figure S1, both hosts,
233
i.e., D201 and HCA, are positively charged during adsorption, thereby posing similar
234
impact on fluoride adsorption.
235
Effect of solution chemistry. As depicted in Figure 3a, the presence of HA even at 20
236
mg DOC/L, higher than most of the real concentration in natural groundwater and
237
surface water,47 exerted negligible effect on fluoride removal. However, more than 90%
238
adsorption of HZO@D201 toward fluoride was suppressed under similar conditions.29
239
Effect of HA on fluoride adsorption by pristine HZO was depicted in Figure S5, from
240
which one can see that around 50% capacity loss of the pristine HZO occurred in the
241
presence of HA (20 mg DOC/L), indicating an intense inhibition on defluoridation in
242
the presence of NOM. Besides, three humic substances from International Humic
41
Thus, the abundant microporous structure of HZO@HCA would exert
11 / 32
ACS Paragon Plus Environment
Environmental Science & Technology
243
Substance Society (IHSS) were utilized to further verify the anti-fouling property of
244
HZO@HCA. Similarly, all the NOM samples exerted negligible effect on the
245
adsorption of HZO@HCA toward fluoride (Figure S6), suggesting that the
246
anti-fouling property of HZO@HCA may be universally valid for humic substances
247
from various regions. The kinetic data shown in Figure 3b indicate that fluoride
248
adsorption onto HZO@HCA reached equilibrium within 24 h, and the presence of HA
249
slightly reduced the adsorption kinetics but did not change the final fluoride
250
adsorption capacity. The effect of pH on fluoride removal is elucidated in Figure 3c,
251
and increasing pH played an unfavorable role in defluoridation. It is expected because
252
surface deprotonation of the embedded HZO NPs would be enhanced as pH increased,
253
thereby suppressing fluoride uptake due to the electrostatic repulsion. Also, the
254
presence of HA exerted negligible effect on defluoridation at pH of 5-9.
255
In addition, the effects of other ubiquitous anions, including chloride, sulfate,
256
nitrate and silicate on fluoride removal by the resultant nanocomposite were examined
257
in the background of HA or HA free, respectively. As observed in Figure S7(a-c), the
258
removal of fluoride by HZO@HCA is independent on the presence of chloride, sulfate
259
and nitrate at the concentration levels of interest (0-500 mg/L). It is mainly due to the
260
specific affinity of HZO toward fluoride, which is clearly elucidated elsewhere.12, 48
261
Nevertheless, increasing silicate concentration from 0 to 500 mg/L would inhibit
262
fluoride removal remarkably (Figure S7d), possibly arising from the formation of
263
inner-sphere complex between HZO and silicate.49 Note that no observable difference
264
occurrs for the defluoridation efficiency of HZO@HCA in HA-rich and HA-free 12 / 32
ACS Paragon Plus Environment
Page 12 of 33
Page 13 of 33
Environmental Science & Technology
265
water.
266
Reusability of the nanocomposite. The presence of NOM will not only compromise
267
the defluoridation efficiency of a given adsorbent, but also cause adverse effects on
268
their sustainable applications of nanocomposite adsorbents because it is challenging to
269
desorb the loaded NOM from the adsorbent surface.31,
270
reusability of HZO@HCA, HZO@D201 and HCA by successive adsorption-
271
regeneration runs, where the adsorbents were first utilized to decontaminate
272
NOM-rich water from fluoride, followed by alkaline rinsing to desorb the loaded
273
fluoride for the next cyclic run.12 As suggested in Figure 4a, the capacity of
274
HZO@HCA remained constant after five cyclic runs. On the contrary, HZO@D201
275
and HCA exhibited continuous capacity loss during cyclic runs, e.g., ~50% and ~80%
276
capacity were lost for the latter two adsorbents. The results in Figure 4b depicted the
277
excellent stability of HZO@HCA in a wide pH range (2-12), i.e., negligible
278
dissolution of HZO occurring at pH≥2, and the presence of HA did not exert any
279
adverse effect on its stability properties.
280
The antifouling mechanism. To elucidate the size exclusion effect on the preferable
281
removal of fluoride on HZO@HCA, we employed EEM fluorescence spectroscopy to
282
characterize the variation of aqueous organic matters before and after adsorption onto
283
three solid adsorbents.50-52 As shown in Figure 5a, the EEM of HA can be divided into
284
five regions, representing aromatic protein (region I and II), fulvic acid-like substance
285
(region III), humic acid-like substance (region IV) and soluble microbial
286
by-product-like substance (region V), respectively.50, 52 HA mainly consisted of the
32
13 / 32
ACS Paragon Plus Environment
Thus, we evaluated the
Environmental Science & Technology
287
region III and IV. The HCA host and HZO@D201 exhibited similar removal of HA,
288
e.g., ~40% and ~50% removal of region III and IV respectively, which is
289
quantitatively determined based on a fluorescence regional integration technique that
290
integrates the volume beneath an EEM.50 It is not beyond expectation because the
291
surface amine/ammonium groups as well as their meso-/macroporous structure of
292
both solids favored HA adsorption via electrostatic attraction and pore filling
293
respectively. However, it is not the case for HZO@HCA, and its removal toward
294
region III and IV was only ~15% due to its insufficient meso-/macro pores to allow
295
HA diffusion inside (Figure 2b). We further obtained the FT-IR spectra of
296
HZO@HCA and HZO@D201 preloaded with fluoride from HA solution. Negligible
297
change was observed for the fluoride-preloaded HZO@HCA as compared with the
298
fresh one (data not shown), whereas a new peak at ~1354 cm-1 occurred in the
299
fluoride-preloaded HZO@D201, which could be assigned to the carboxyl groups
300
(Figure S8) from the adsorbed HA.
301
The HA adsorption onto the solid adsorbent were achieved mainly via two
302
pathways, i.e., electrostatic attraction with the positively charged groups and pore
303
filling. To distinguish different roles of both pathways, we carried out batch HA
304
adsorption in the background of sulfate or sulfate free solution. Pre-experimental
305
results indicated that sulfate at 1000 mg/L could fully screen the role of electrostatic
306
interaction in HA adsorption due to the competitive effect. As depicted in Figure 5b,
307
the presence of sulfate at 1000 mg/L enhanced the removal of HA by HCA and
308
HZO@D201, resulting in >70% removal for region III and IV. Such favorable role of 14 / 32
ACS Paragon Plus Environment
Page 14 of 33
Page 15 of 33
Environmental Science & Technology
309
sulfate may be ascribed to the salting-out effect on the adsorption of organic matters
310
occurring via pore filling.48, 49 On the contrary, the added sulfate ions compromised
311
the HA adsorption onto HZO@HCA to nearly zero and ~9.0% for region III and IV
312
respectively, further suggesting that the electrostatic attraction instead of pore filling
313
is responsible for the low HA removal from the sulfate free solution by HZO@HCA
314
(Figure 5a). In other words, the size exclusion effect works expectedly to inhibit HA
315
adsorption onto HZO@HCA. Aldrich HA is mainly in the form of aggregates of ~2.9
316
nm in hydrodynamic diameter in water26, and it increased up to ~6 nm and even ~10
317
nm in the presence of other minerals. Obviously, HZO@HCA possessing abundant
318
micropore (
