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In-situ characterization of dissolved organic matter removal by coagulation using differential UV–Visible absorbance spectroscopy
Journal Pre-proof In-situ characterization of dissolved organic matter removal by coagulation using differential UV–Visible absorbance spectroscopy Yuxuan Zhou, Yaping Xie, Min Wang, Fang Zou, Chenyang Zhang, Zengfu Guan, Mingquan Yan PII:
S0045-6535(19)32301-X
DOI:
https://doi.org/10.1016/j.chemosphere.2019.125062
Reference:
CHEM 125062
To appear in:
ECSN
Received Date: 11 July 2019 Revised Date:
2 October 2019
Accepted Date: 4 October 2019
Please cite this article as: Zhou, Y., Xie, Y., Wang, M., Zou, F., Zhang, C., Guan, Z., Yan, M., In-situ characterization of dissolved organic matter removal by coagulation using differential UV–Visible absorbance spectroscopy, Chemosphere (2019), doi: https://doi.org/10.1016/ j.chemosphere.2019.125062. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
In-situ Characterization of Dissolved Organic Matter Removal by
2
Coagulation Using Differential UV-Visible Absorbance Spectroscopy
3
Yuxuan Zhou a, Yaping Xie b, Min Wang c, Fang Zou c, Chenyang Zhang a, Zengfu
4
Guan a, Mingquan Yan a, * a
5
6
of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China b
7
8
Department of Environmental Engineering, Chang’An University, Xi’an 710064,
Shanxi, China c
9
Department of Environmental Engineering, Peking University, The Key Laboratory
Technology Institute of Beijing Waterworks Group Co., Ltd., Beijing 100085, China
10
11
* Corresponding author. Address: Department of Environmental Engineering, College
12
of Environmental Sciences and Engineering, Peking University, Beijing 100871,
13
China; Tel: +86-10-62758501. E-mail: [email protected]
14
1
15
Abstract
16
Removing dissolved organic matter (DOM) is of great concern due to its adverse
17
effects on water supplies. Great effort has been given to studying DOM removal by
18
coagulation, while the mechanism of DOM removal and the changes in its properties
19
during coagulation have not been clearly illustrated due to the limitations of detection
20
methods under practical environmental conditions. In this paper, the changes in DOM
21
during coagulation were quantified using differential UV-Visible absorbance
22
spectroscopy, and the differential spectra of DOM in the wavelength range of 200-600
23
nm could be deconvoluted into six Gaussian bands with maxima at approximately 200,
24
240, 276, 316, 385, and 457 nm after coagulation, respectively. The intensity of these
25
maxima decreased with the type and dosage of coagulants. These observations should
26
reflect the difference in the removability of DOM by coagulation, and this perspective
27
was further confirmed by examining the deprotonation-protonation properties of
28
DOM before and after coagulation. The affinity sites of DOM in coagulated waters,
29
quantified by spectra parameter DlnA400 (differential log-transformed spectra at
30
wavelength 400 nm) in combination with the revised NICA model, increased as the
31
coagulant dosage, which indicates that coagulation is inclined to remove the DOM
32
fraction with fewer functional groups. Polyaluminum chloride (PAC) and
33
Al-aggregate (Al13) were more efficient than Alum for removing DOM due to their
34
high efficiency for removing DOM fractions with fewer functional groups. The
35
residual dissolved Al concentration depended on the total amount of reactive binding
36
sites in DOM, and there was a strong linear correlation between residual dissolved Al
2
37
and the total amount of reactive binding sites in DOM for Alum, while a weaker
38
correlation was presented for PAC and Al13. This indicates that Ala was the dominant
39
species to bind with the affinity sites in DOM to form residual dissolved Al.
40
Keywords: coagulation, dissolved organic matter, polyaluminum chloride, residual
41
aluminum, UV-Visible absorbance spectroscopy
3
42
1. Introduction
43
Dissolved organic matter (DOM) is a complex matrix of organic substances, and is
44
ubiquitous in surface and ground waters as a result of different hydrological,
45
biological, and geological interactions (Leenheer and Croué 2003, Mayorga et al.
46
2005). The removal of DOM from source water has attracted great attention due to its
47
adverse effects in drinking water supplies, including it causing issues (such as bad
48
odors or color), its potential to contribute to algal blooms or corrosion in drinking
49
water distribution, and its potential to transport toxic heavy metals (Jacangelo et al.
50
1995, Weng et al. 2006). DOM is also a precursor of disinfection by-products (DBPs)
51
that have been demonstrated to be extremely cytotoxic and genotoxic (Hua and
52
Reckhow 2007, Nieuwenhuijsen et al. 2000, Plewa et al. 2004). Among the different
53
DOM removal treatments (such as coagulation, membrane filtration, pre-oxidation,
54
and activated carbon adsorption), coagulation is deemed to be the most economical
55
and effective technique (Sillanpaa et al. 2018, Yan et al. 2009).
56
The efficiency of DOM removal by coagulation is not only determined by the
57
coagulation operation parameters (such as pH, alkalinity, and coagulants types and
58
dosage), but also the properties of the DOM itself (Korshin et al. 2009, Lin and Lee
59
2013, Yan et al. 2008b, Yan et al. 2008c). Coagulation preferentially removes the high
60
molecular weight and/or hydrophobic fractions of DOM over the low molecular
61
weight
62
chromatography (HPSEC) (Nissinen et al. 2001, Sillanpaa et al. 2018), ultrafiltration
63
(Yan et al. 2007), and resin absorption fraction (Sillanpaa et al. 2018, Volk et al. 2000).
and/or
hydrophilic
fractions
by
4
high-performance
size-exclusion
64
Conventional optical DOM indices, which are sensitive and non-destructive, are also
65
widely used for characterizing DOM in natural water and water treatment. As an
66
example, specific ultraviolet absorbance at 254 nm (UV254 and SUVA254) is expected
67
to describe the hydrophobic and hydrophilic performance of DOM and estimate its
68
quality and proportion (Liang and Singer 2003, Weishaar et al. 2003). Despite the
69
extent of these studies, the changes in DOM properties during coagulation are yet to
70
be quantified in more detail.
71
Recent studies have demonstrated the sensitivity of DOM chromophores to
72
interactions with protons, metal cations, and oxidants, and the presence of prominent
73
features in differential spectra can be resolved into Gaussian bands. These bands are
74
manifestations of the structures contained in DOM and their behaviors in water
75
chemistry processes (Yan et al. 2013a, Yan and Korshin 2014, Yan et al. 2013c, Yan et
76
al. 2013d). This study employed differential UV-visible absorbance spectroscopy to
77
characterize the changes in DOM properties during coagulation. The spectral
78
parameter was introduced to quantify the carboxylic and/or phenolic moieties of
79
DOM before and after treatment by coagulation. The effect of coagulant type and
80
dosage on the coagulation performance and generation of residual dissolved Al is
81
partially elucidated and specific to the aspect of DOM properties. This study
82
demonstrates a simple and powerful tool to study the behavior of DOM in coagulation
83
and other relevant water treatment processes.
5
84
2. Materials and Methods
85
2.1 Reagents and chemicals
86
Unless stated otherwise, all chemicals were of reagent-grade. All solutions were
87
prepared using Milli-Q water (18.2 MΩ cm-1, Millipore Corp., MA, USA). Stock
88
Al2(SO4)3 (simplified as alum) and FeCl3 solutions were prepared using Al2(SO4)3 and
89
FeCl3 salts purchased from the Aldrich Chemical Company (Milwaukee, WI). The
90
commercial polyaluminum chloride (simplified as PAC) product was provided by
91
Wanshui
92
AlO4Al12(OH)247+ polymer (simplified as Al13) was prepared following methods
93
provided in previous research (Bottero et al. 1987, Shi et al. 2007). All stock
94
coagulant solutions were prepared with 0.4 mol L-1 of Al or Fe, excluding the Al13
95
stock solution, which is prepared with 0.1 mol L-1 of Al. The species distribution of
96
Al-based coagulants was determined by Ferron-complexation timed spectrometry, as
97
mentioned in previous studies (Wang et al. 2004, Yan et al. 2008a), and the results are
98
shown in Table S1 in the Supporting Information (SI).
99
2.2 Water sample
water
purification-reagent
CO.
(Beijing,
China).
A
high-purity
100
Source water for the Beijing Mega-city was used in this study. The samples were
101
collected in 2017 and stored at 4 °C in pre-cleaned polypropylene containers. Some of
102
the water was filtered through a 0.45-µm hydrophilic polypropylene membrane
103
(Tianjin, China) for further analysis and characterization.
104
2.3 Coagulation experiment
105
A 400-mL water sample was transferred into an 800-mL beaker with a sampling port
6
106
3 cm below the water’s surface. A programmable jar testing apparatus (ZR4-6,
107
Zhongrun Water Industry Technology Development Co. Ltd., China) was used with a
108
standard procedure at a room temperature of 25±1 °C: addition coagulant followed by
109
30 seconds of rapid mixing at 250 rpm, rapid mixing at 250 rpm for 2 minutes, 15
110
minutes of flocculation at 40 rpm and 30 minutes of settling. Some subsamples were
111
collected directly after settling to measure the turbidity and pH using turbidity (Hach
112
turbidimeter 2100P) and pH meters (Mettler Toledo S220 Seven Compact),
113
respectively. After settling, some subsamples were filtered through a 0.45-mm filter
114
for residual dissolved Al (ICP-MS, Element X Series, Thermo Scientific), DOC
115
(Shimadzu TOC-Vcsh carbon analyzer) and absorbance spectra analyses (Hitachi
116
U-3900 UV/Vis spectrophotometer, quartz cell length 5 cm).
117
2.4 Spectrophotometric titration
118
Prior to spectrophotometric titration, the samples were pumped over a Na
119
ion-exchange resin after filtration (Lewatit S1468F, Lanxess, Leverkusen, Germany),
120
which could remove metal cations (e.g., Ca2+, Mg2+, Al3+, and Fe3+) from samples to
121
prevent the cations and DOM from precipitating during pH titration.
122
Spectrophotometric titrations were conducted following similar methods published
123
previously (Yan et al. 2013b, Yan et al. 2014). The pH of the solutions was controlled
124
during titration by adding HClO4 or NaOH solutions at ca. 0.3 pH intervals between
125
pH levels of 3 and 11. The containers were continuously stirred during acid and base
126
addition, and equilibrated for 30 min before 15-mL aliquots were removed for
127
absorbance spectra analysis. The aliquots were then returned to the solutions before
7
128
the next addition of HClO4 or NaOH during titration. Absorbance spectra were
129
recorded at wavelengths from 200 to 600 nm. Dilution effects due to the addition of
130
the acid and base were considered in the final data.
131
2.5 Data interpretation
132
The differential and differential log-transformed absorbance spectra were calculated
133
using equations (1) and (2):
134
DAλ = Aλ, i- Aλ, ref
(1)
135
DLnAi(λ)= LnAi (λ)- LnAref (λ)
(2)
136
where Aλ, i and Aλ, ref are the DOM absorbance measured at the wavelength λ for any
137
selected condition (i) and reference, respectively (such as at pH 3.0 or before
138
coagulation).
139
The differential spectra were deconvoluted to determine the presence and
140
contributions of distinct bands using Peakfit (version 4.12), as described in previous
141
studies (Yan et al. 2013b, Yan and Korshin 2014). All bands should exhibit a Gaussian
142
shape when wavelength λ presents as the photon energy (measured in eV), calculated
143
as: Eev =
1240 3
144
Each Gaussian band (Ai, cm-1) was characterized by the locations of its maximum (E0i,
145
ev) width (Wi, ev), and amplitude (A0i, cm-1). The generated differential spectra
146
(△A(E)) were computed as: 1 − △ AE = △ = △ − 4 2 8
147
The spectral DlnA400 parameter of all the samples examined was calculated using
148
equation (2). Revised non-ideal competitive adsorption (NICA) models were
149
introduced to interpret the evolution of the DlnA400 parameter due to
150
protonation-deprotonation using the following equation (Yan et al. 2016b, Yan et al.
151
2013b):
152
DlnA$% λ = &./0
'()*+,- λ 5 7 123 4% 6 123
'()*923 λ : 5 7 923 4% 6 923
+ ./0
'()*923 λ
A 7 ./<0923 4% 5 6=>? @ 923
−;
'()*+,- λ
7 ./<0123 4% 5 6=>? @ 123
+
153
154
DlnALAS and DlnAHAS correspond to the maximum change in absorbance associated
155
with the deprotonation of carboxylic- and phenolic-type functional groups,
156
respectively, which are defined artificially as low- (LAS) and high-affinity
157
protonation-active sites (HAS). Previous studies demonstrated that DlnALAS and
158
DlnAHAS are dimensionless parameters independent of the DOC concentration, and
159
they reflect the number of protonation-active groups per mg L-1 DOM. pKHAS and
160
pKLAS are the median values of the proton affinity distributions for these groups, and
161
mLAS and mHAS define the width of these distributions and are measures of the
162
heterogeneity of DOM, respectively (Benedetti et al. 1996, Kinniburgh et al. 1996).
(5)
163
164
3. Results and Discussion
165
3.1 In-situ monitoring coagulation process
166
The UV-Visible spectra of water samples before and after coagulation with Alum,
167
PAC, Al13, and FeCl3 coagulants at varying dosages were recorded. The shapes of the
168
differential spectra induced by Al-based coagulants are similar, although their
9
169
intensities differ, therefore, the data for Alum were selected for comparison with those
170
for FeCl3, and the results are shown in Figure S1. The intensity of the absorbance
171
spectra decreased almost exponentially with wavelength and decreased slightly as the
172
coagulant dosage increased. To amplify subtle changes, differential absorbance
173
spectra (DA) were calculated using equation (1), referring to the spectra before
174
coagulation. The results are shown in Figure 1.
175
Figure 1
176
The differential absorbance spectra induced by the addition of the coagulant increased
177
with increasing coagulant dosage. This is mainly because the DOM removal
178
efficiency is higher at a higher coagulant dosage. Moreover, the shape of differential
179
absorbance differs greatly with coagulant type and dosage, for example, the peak at
180
275 nm in the differential spectra before and after coagulation with FeCl3 is more
181
notable than that with the Al-based coagulant. These observations could demonstrate
182
that the DOM removed by coagulation is chemically heterogeneous with the
183
coagulant type and dosage.
184
To ascertain this point, the differential spectra of DOM induced by coagulation were
185
deconvoluted to determine the presence and contributions of distinct bands. Selected
186
data are shown in Figures 2 and S2. All the differential spectra could be well fitted
187
between the experimental and calculated data (R2 ≧ 0.99). The deconvolution of the
188
differential spectra exhibited six Gaussian bands (Table S2) with fixed positions but
189
different intensities or widths. As described in previous studies (He et al. 2015, Huang
190
et al. 2018, Yan et al. 2013b, Yan and Korshin 2014), the six Gaussian bands denoted
10
191
as A5, A4, A3, A2, A1, and A0 had maxima located at 451 (2.74 ± 0.02 eV), 383 (3.24 ±
192
0.01 eV), 316 (3.93 ± 0.01 eV), 276 (4.50 ± 0.01 eV), 239 (5.18 ± 0.01 eV), and 201
193
nm (6.16 ± 0.03 eV), respectively.
194
As shown in Figure 2, the significant differences in the shapes of the differential
195
spectra between the Al- and Fe-based coagulants are because A2 is more remarkable
196
among the six Gaussian bands when the FeCl3 coagulant was applied. Therefore, the
197
Fe-based coagulant is more inclined to remove A2-DOM than Al-based coagulants.
198
Figure 2
199
The deconvolution of differential spectra at various Alum dosages is shown in Figure
200
S2. Notably, the intensity of each band increases as the coagulant dosage increases,
201
but the relative change is not linear. As the spectroscopic interference and matrix
202
influence (such as absorbance from inorganic ions) reduce the reliability of A0 and A1
203
to characterize the properties of DOM, the increase ratio of intensities of A2, A3 and
204
A4 at various coagulant dose to those at condition of coagulant as 0.04 mmol L-1 was
205
calculated according to Figure S2 and shown in Figure 3. Both A3 and A4 change
206
more slightly than A2 with the increasing Alum dose, but they change asynchronously,
207
indicating that the properties of DOM removal are also inconsistently related to the
208
increasing coagulant dosage.
209
Previous studies have consistently used UV254 as a parameter to quantify the removal
210
of DOM by coagulation, this finding demonstrates that the removal of DOM by
211
coagulation is chemically heterogeneous, and the spectral parameter, UV254, is
212
circumscriptus to quantifying DOM removal by coagulation.
11
213
Figure 3
214
3.2 Characterizing the properties of DOMs before and after coagulation
215
To further quantify the changes in DOM properties after coagulation, acid-base
216
spectrophotometric titrations of DOM before and after coagulation with different
217
coagulant types and dosages were conducted. The zero-order UV-Visible spectra of
218
DOM before and after coagulation (data for PAC are shown in Figure S3 as an
219
example) exhibit similar features, as demonstrated in previous studies (Yan et al.
220
2016b, Yan et al. 2013b). The intensity of absorbance decreased almost exponentially
221
with wavelength, and the changes caused by protonation-deprotonation were
222
inconspicuous. The subtle changes could be amplified and some clear signals could be
223
identified after the data were processed using equation (2). The results for PAC at
224
dosages of 0.02, 0.06, and 0.12 mmol L-1 are shown in Figure 4.
225
Figure 4
226
The differential absorbance of DOM induced by deprotonation/protonation increased
227
clearly as the pH, especially for wavelengths below 320 nm. The shape and intensity
228
of the differential spectra are strongly affected by the coagulant dosage and type. The
229
differential spectra of DOM in raw water exhibit a dominant peak at 275 nm. After
230
coagulation with PAC, the intensity of this peak increased significantly when the
231
dosage was 0.02 mmol L-1. However, at a higher PAC dosage (0.06 or 0.12 mmol L-1),
232
the intensity of the dominant peak decreased. The pH-differential spectra for the four
233
examined coagulants at 0.06 mmol L-1 are compared in Figure S4. The features of the
234
differential spectra of DOM before and after dosing with Al2(SO4)2 and Al13 were
12
235
similar to those of PAC, although their intensities were different. However, for FeCl3,
236
prominent peaks at 272 and 385 nm appear, which are deemed as a contributor to the
237
A2 band.
238
To further quantify the protonation/deprotonation capacity of DOM in the coagulation
239
process, the spectral parameter DlnA400 (differential log-transformed spectra at
240
wavelength 400 nm) was introduced using equation (2), which reflects the responses
241
of the carboxylic and phenolic groups in DOM to protonation-deprotonation (Yan et
242
al. 2016b, Yan et al. 2013b). The results of this are shown in Figure 5. The intensity
243
and shape of the DlnA400 curves against pH before and after coagulation vary greatly.
244
Figure 5
245
As described in previous studies (Yan et al. 2016b, Yan et al. 2013b), revised NICA
246
models were introduced to interpret the evolution of DlnA400 caused by
247
protonation-deprotonation using equation (5). Selected data are shown in Figure 5,
248
and the parameters applied in the revised NICA model for each set are shown in Table
249
S3. There was an extraordinary agreement between the experimental data of DlnA400
250
and the data predicted by the revised NICA model for all examined samples (R2＞
251
0.99). The total amount of carboxylic- (DlnALAS) and phenolic-type (DlnAHAS) groups
252
in all examined samples are shown in Figure 6. The results demonstrate that the total
253
amount of carboxylic- and phenolic-type groups per mg L-1 of DOC increases
254
gradually as the coagulant dosage increases, while the opposite trend was found for
255
the DOC of the examined samples. Taking Alum as an example, at a coagulant dosage
256
of 0.02 mM, the total amount of carboxylic- and phenolic-type groups per mg L-1
13
257
DOC increased from 1.04 to 1.36. With higher Alum dosages, this value reached 1.37
258
and 1.63 for 0.06 and 0.12 mM of Alum, respectively. When compared to the other
259
coagulants, it is clear that the total amount of carboxylic- and phenolic-type groups
260
per mg L-1 of DOC after coagulation with Alum is lower than those with PAC, FeCl3,
261
and Al13.
262
The amount of individual carboxylic-type groups per mg L-1 DOC in water samples
263
increased slowly after coagulation with PAC/Al13, while that of phenolic-type groups
264
per mg L-1 DOC increased significantly. Coagulation with Alum is more inclined to
265
elevate the amount of carboxylic-type groups per mg L-1 DOC. Coagulation with
266
FeCl3 exhibited an opposing phenomenon as it is more inclined to elevate the amount
267
of phenolic-type groups per mg L-1 DOC.
268
Figure 6
269
These results indicate that DOM with fewer functional groups is more easily removed
270
by coagulation; this is deemed to be the macromolecular and hydrophobic fraction of
271
DOM. Furthermore, PAC and Al13 are more likely to remove non-functional-group
272
DOM than Alum. The DOC removal by coagulation with PAC and Al13 at a low
273
dosage is higher than that with Alum, which may be because the macromolecular and
274
hydrophobic fraction of DOM is more likely to be removed at a low coagulant dosage
275
than the small-molecule and hydrophilic fraction.
276
3.3 Correlation between residual dissolved Al and the properties of DOM.
277
Residual aluminum (Al) in drinking water is becoming a great concern due to its high
278
potential risk to human health, such as causing Alzheimer’s disease. DOM strongly
14
279
affects the speciation and amount of residual Al after coagulation (Zhou et al. 2017),
280
and organic Al has been found to be the predominant fraction of the total residual Al
281
in treated drinking water (Yang et al. 2010). While there is an inherent correlation
282
between Al and DOM, it and its properties have not been elucidated in detail.
283
The residual dissolved Al after coagulation with Al-based coagulants was measured,
284
and the results are shown in Figure S5 in the SI. The residual dissolved Al increased
285
at a low coagulant dosage and then decreased as the coagulant dosage increased. The
286
residual Al of the water samples after coagulation with Al13 and PAC is lower than
287
that with Alum.
288
To show the effect of the DOM properties on the presence of residual dissolved Al in
289
coagulated waters, the total amount of affinity sites in the DOM was calculated as
290
(DlnALAS + DlnAHAS)*DOC. As the data in Figure 7 show, the residual dissolved Al
291
increased with as the total amount of binding sites in the DOM increased. The
292
residual dissolved Al and the total amount of reactive binding sites exhibit strong
293
linearity (R2=1.00) when Alum is the coagulant. However, the linearities for PAC and
294
Al13 are lower.
295
Figure 7
296
According to Ferron-complexation timed spectrometry methods (Wang et al. 2004,
297
Yan et al. 2008a), the species of Al in the coagulant could be divided into monomeric,
298
polymeric, and colloidal species based on particle size, denoted as Ala, Alb, and Alc,
299
respectively. As shown in Table S1, the speciation of the Al used in the coagulants of
300
this study differ greatly. The Ala fraction in Alum is much higher than that in the PAC,
15
301
and even higher than that in Al13. The residual dissolved Al and the fraction of Ala
302
were compared when the coagulant dosage was 0.06 mmol L-1, which was selected
303
because the binding sites could be satisfied at this dosage for all examined Al-based
304
coagulants, as shown in Figure S5. The result presented in Figure 8 shows that the
305
data of the residual dissolved Al and Ala fraction of coagulants exhibited good
306
linearity, further indicating that the fraction of Ala is the mainly Al species to form
307
dissolved states of Al-DOM, while Alb and Alc were more likely to form colloidal and
308
particular flocs in coagulation. This is consistent with our previous studies (Yan et al.
309
2008a). The data provided in Figure S5 and Figure 6 indicate that the residual
310
dissolved Al can be determined by the available reactive affinity sites in DOM when
311
the monomeric Al species is sufficiently abundant. The residual Al is lower for PAC
312
and the Al13 is at a low dosage as the monomeric Al species is not abundant and DOM
313
is unsatisfied. Figure 8
314
315
4. Conclusion
316
The results presented above support the following conclusions:
317
(1) The changes in DOM during coagulation can be reflected by tracking the
318
differential spectra before and after coagulation. The differential spectra after
319
coagulation were well fitted by six Gaussian bands with maxima at approximately 200,
320
240, 276, 316, 385 and 457 nm. The intensity of these increased inconsistently with
321
increasing coagulant dosage and is strongly affected by the type of coagulant.
322
(2) The affinity sites in the DOM of the examined water samples increased with
16
323
increasing coagulant dosage, indicating that coagulation is inclined to remove the
324
non-functional-group hydrophobic macromolecular DOM fraction. This fraction is
325
more efficiently removed by PAC and Al13 than Alum.
326
(3) The residual dissolved Al after coagulation and the total amount of affinity sites in
327
DOM exhibits strong linearity for Alum, and slightly less linearity for PAC and Al13.
328
The residual dissolved Al can be determined from the available binding sites in DOM,
329
given that the monomeric fraction of Al is abundant.
330
This study demonstrates that the changes in DOM properties during its removal by
331
coagulation could be well quantified by UV-Visible absorbance spectroscopy. This is
332
promising for developing simple and online monitoring of the amount of DOM
333
removed and the properties of residual DOM during coagulation. With further
334
understanding of the inherent meanings of the spectra signals, this could provide
335
useful information for optimizing coagulation and other water treatment processes in
336
the future.
337
338
Acknowledgements
339
This study was partially supported by the China NSF (No. 51578007) and The
340
National Key Research and Development Program of China (No. 2017YFD0801503).
341
The views represented in this publication do not necessarily represent those of the
342
funding agencies.
343 344
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21
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natural organic matter removal by polyaluminum chloride: Effect of coagulant
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particle size and hydrolysis kinetics. Water Research 42(13), 3361-3370.
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matter (NOM) removal in a typical North-China water plant by enhanced coagulation:
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Targets and techniques. Separation and Purification Technology 68(3), 320-327.
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Yan, M., Wang, D., Yu, J., Ni, J., Edwards, M. and Qu, J. (2008c) Enhanced
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coagulation with polyaluminum chlorides: Role of pH/Alkalinity and speciation.
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Chemosphere 71(9), 1665-1673.
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459
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Yan, M., Ma, J. and Ji, G. (2016c) Examination of effects of Cu(II) and Cr(III) on
461
Al(III) binding by dissolved organic matter using absorbance spectroscopy. Water
462
Research 93, 84-90.
463
Yang, Z., Gao, B., Yue, Q. and Wang, Y. (2010) Effect of pH on the coagulation
464
performance of Al-based coagulants and residual aluminum speciation during the
465
treatment of humic acid–kaolin synthetic water. Journal of Hazardous Materials
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178(1), 596-603.
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Zhou, Y., Yan, M., Liu, R., Wang, D. and Qu, J. (2017) Investigating the effect of
468
hardness
469
Al(III)-dissolved organic matter (DOM) binding. Water Research 115, 22-28.
cations
on
coagulation:
The
23
aspect
of
neutralisation
through
470
Captions of Figures
471
(a)
472
(b)
473 474
Figure 1. Differential absorbance spectra of water after coagulation with (a) Alum and
475
(b) FeCl3 at different dosages.
476
24
Differential absorbance (cm-1)
0.14
0.08 mM Alum
0.12 0.10 R2=0.99
0.08 0.06 0.04
experimental model A0 A1 A2 A3 A4 A5
(a)
0.02 0.00 225
275
325 375 425 475 Wavelength (nm)
525
575
477
Differential absorbance (cm-1)
0.08 0.08 mM FeCl3 0.06 R2=0.99 0.04
0.02
experimental model A0 A1 A2 A3 A4 A5
(b) 0.00 225
275
325 375 425 475 Wavelength (nm)
525
575
478 479
Figure 2. Gaussian band fitting of the differential spectra after coagulation with (a)
480
Alum; (b) FeCl3.
481
25
482 483
Figure 3. Comparison of the increase ratio of the intensity of individual Gaussian
484
bands with increasing Alum coagulant dosage, referenced to condition of coagulant as
485
0.04 mmol L-1.
486
26
(a)
(b)
(c)
(d)
487
488 489
Figure 4. DOC-normalized pH-differential absorbance spectra of water before (a) and
490
after coagulation with PAC at dosages of 0.02 (b), 0.06 (c), and 0.12 mmol L-1 (d).
491
Reference pH values are approximately 3.0.
492
27
1.8
Experiment Raw Water 0.02 mM Alum 0.06 mM Alum 0.12 mM Alum
1.6 1.4
1.4 1.2
1.0 0.8 0.6 0.4
Model Raw Water 0.02 mM PAC 0.06 mM PAC 0.12 mM PAC
1.0 0.8 0.6 0.4
(a)
0.2
(b)
0.2
0.0
0.0 3
4
5
6
493 1.8
Experiment Raw Water 0.02 mM Al13 0.06 mM Al13 0.12 mM Al13
1.6 1.4 1.2
DLnA400
Experiment Raw Water 0.02 mM PAC 0.06 mM PAC 0.12 mM PAC
1.6
DLnA400
DLnA400
1.2
1.8
Model Raw Water 0.02 mM Alum 0.06 mM Alum 0.12 mM Alum
7 pH
8
9
10
11
3
4
5
6
7 pH
8
9
10
11
Model Raw Water 0.02 mM Al13 0.06 mM Al13 0.12 mM Al13
1.0 0.8 0.6 0.4
(c)
0.2
(d)
0.0 3
494
4
5
6
7 pH
8
9
10
11
495
Figure 5. Comparison of the effects of coagulation on the spectral parameter DlnA400
496
against pH before and after coagulation with (a) Alum; (b) PAC; (c) Al13; and (d)
497
FeCl3. Line: modeled data, Dot: experimental data.
498
28
499 500
Figure 6. Comparison of the amount of affinity sites per mg L-1 DOM of water after
501
coagulation with PAC, Al13, Alum, and FeCl3
502
29
503 504
Figure 7. Correlation of residual dissolved Al and the total number of affinity sites
505
((DlnALAS + DlnAHAS )*DOC ) in the coagulated water samples.
30
506 507
Figure 8. Correlation of residual dissolved Al in coagulated water with the fraction of
508
Ala (%) in the coagulants. The dosage of Al-based coagulants is 0.06 mmol Al L-1.
31
Highlights
DOM properties during coagulation could be quantified by differential spectra
Property of DOM changes inconsistently with different type and dosage of coagulants
Coagulation is inclined to remove non-functional-group hydrophobic DOM fraction
The residual dissolved Al is determined by the available binding sites in DOM
S0045-6535(19)32301-X
DOI:
https://doi.org/10.1016/j.chemosphere.2019.125062
Reference:
CHEM 125062
To appear in:
ECSN
Received Date: 11 July 2019 Revised Date:
2 October 2019
Accepted Date: 4 October 2019
Please cite this article as: Zhou, Y., Xie, Y., Wang, M., Zou, F., Zhang, C., Guan, Z., Yan, M., In-situ characterization of dissolved organic matter removal by coagulation using differential UV–Visible absorbance spectroscopy, Chemosphere (2019), doi: https://doi.org/10.1016/ j.chemosphere.2019.125062. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
1
In-situ Characterization of Dissolved Organic Matter Removal by
2
Coagulation Using Differential UV-Visible Absorbance Spectroscopy
3
Yuxuan Zhou a, Yaping Xie b, Min Wang c, Fang Zou c, Chenyang Zhang a, Zengfu
4
Guan a, Mingquan Yan a, * a
5
6
of Water and Sediment Sciences, Ministry of Education, Beijing 100871, China b
7
8
Department of Environmental Engineering, Chang’An University, Xi’an 710064,
Shanxi, China c
9
Department of Environmental Engineering, Peking University, The Key Laboratory
Technology Institute of Beijing Waterworks Group Co., Ltd., Beijing 100085, China
10
11
* Corresponding author. Address: Department of Environmental Engineering, College
12
of Environmental Sciences and Engineering, Peking University, Beijing 100871,
13
China; Tel: +86-10-62758501. E-mail: [email protected]
14
1
15
Abstract
16
Removing dissolved organic matter (DOM) is of great concern due to its adverse
17
effects on water supplies. Great effort has been given to studying DOM removal by
18
coagulation, while the mechanism of DOM removal and the changes in its properties
19
during coagulation have not been clearly illustrated due to the limitations of detection
20
methods under practical environmental conditions. In this paper, the changes in DOM
21
during coagulation were quantified using differential UV-Visible absorbance
22
spectroscopy, and the differential spectra of DOM in the wavelength range of 200-600
23
nm could be deconvoluted into six Gaussian bands with maxima at approximately 200,
24
240, 276, 316, 385, and 457 nm after coagulation, respectively. The intensity of these
25
maxima decreased with the type and dosage of coagulants. These observations should
26
reflect the difference in the removability of DOM by coagulation, and this perspective
27
was further confirmed by examining the deprotonation-protonation properties of
28
DOM before and after coagulation. The affinity sites of DOM in coagulated waters,
29
quantified by spectra parameter DlnA400 (differential log-transformed spectra at
30
wavelength 400 nm) in combination with the revised NICA model, increased as the
31
coagulant dosage, which indicates that coagulation is inclined to remove the DOM
32
fraction with fewer functional groups. Polyaluminum chloride (PAC) and
33
Al-aggregate (Al13) were more efficient than Alum for removing DOM due to their
34
high efficiency for removing DOM fractions with fewer functional groups. The
35
residual dissolved Al concentration depended on the total amount of reactive binding
36
sites in DOM, and there was a strong linear correlation between residual dissolved Al
2
37
and the total amount of reactive binding sites in DOM for Alum, while a weaker
38
correlation was presented for PAC and Al13. This indicates that Ala was the dominant
39
species to bind with the affinity sites in DOM to form residual dissolved Al.
40
Keywords: coagulation, dissolved organic matter, polyaluminum chloride, residual
41
aluminum, UV-Visible absorbance spectroscopy
3
42
1. Introduction
43
Dissolved organic matter (DOM) is a complex matrix of organic substances, and is
44
ubiquitous in surface and ground waters as a result of different hydrological,
45
biological, and geological interactions (Leenheer and Croué 2003, Mayorga et al.
46
2005). The removal of DOM from source water has attracted great attention due to its
47
adverse effects in drinking water supplies, including it causing issues (such as bad
48
odors or color), its potential to contribute to algal blooms or corrosion in drinking
49
water distribution, and its potential to transport toxic heavy metals (Jacangelo et al.
50
1995, Weng et al. 2006). DOM is also a precursor of disinfection by-products (DBPs)
51
that have been demonstrated to be extremely cytotoxic and genotoxic (Hua and
52
Reckhow 2007, Nieuwenhuijsen et al. 2000, Plewa et al. 2004). Among the different
53
DOM removal treatments (such as coagulation, membrane filtration, pre-oxidation,
54
and activated carbon adsorption), coagulation is deemed to be the most economical
55
and effective technique (Sillanpaa et al. 2018, Yan et al. 2009).
56
The efficiency of DOM removal by coagulation is not only determined by the
57
coagulation operation parameters (such as pH, alkalinity, and coagulants types and
58
dosage), but also the properties of the DOM itself (Korshin et al. 2009, Lin and Lee
59
2013, Yan et al. 2008b, Yan et al. 2008c). Coagulation preferentially removes the high
60
molecular weight and/or hydrophobic fractions of DOM over the low molecular
61
weight
62
chromatography (HPSEC) (Nissinen et al. 2001, Sillanpaa et al. 2018), ultrafiltration
63
(Yan et al. 2007), and resin absorption fraction (Sillanpaa et al. 2018, Volk et al. 2000).
and/or
hydrophilic
fractions
by
4
high-performance
size-exclusion
64
Conventional optical DOM indices, which are sensitive and non-destructive, are also
65
widely used for characterizing DOM in natural water and water treatment. As an
66
example, specific ultraviolet absorbance at 254 nm (UV254 and SUVA254) is expected
67
to describe the hydrophobic and hydrophilic performance of DOM and estimate its
68
quality and proportion (Liang and Singer 2003, Weishaar et al. 2003). Despite the
69
extent of these studies, the changes in DOM properties during coagulation are yet to
70
be quantified in more detail.
71
Recent studies have demonstrated the sensitivity of DOM chromophores to
72
interactions with protons, metal cations, and oxidants, and the presence of prominent
73
features in differential spectra can be resolved into Gaussian bands. These bands are
74
manifestations of the structures contained in DOM and their behaviors in water
75
chemistry processes (Yan et al. 2013a, Yan and Korshin 2014, Yan et al. 2013c, Yan et
76
al. 2013d). This study employed differential UV-visible absorbance spectroscopy to
77
characterize the changes in DOM properties during coagulation. The spectral
78
parameter was introduced to quantify the carboxylic and/or phenolic moieties of
79
DOM before and after treatment by coagulation. The effect of coagulant type and
80
dosage on the coagulation performance and generation of residual dissolved Al is
81
partially elucidated and specific to the aspect of DOM properties. This study
82
demonstrates a simple and powerful tool to study the behavior of DOM in coagulation
83
and other relevant water treatment processes.
5
84
2. Materials and Methods
85
2.1 Reagents and chemicals
86
Unless stated otherwise, all chemicals were of reagent-grade. All solutions were
87
prepared using Milli-Q water (18.2 MΩ cm-1, Millipore Corp., MA, USA). Stock
88
Al2(SO4)3 (simplified as alum) and FeCl3 solutions were prepared using Al2(SO4)3 and
89
FeCl3 salts purchased from the Aldrich Chemical Company (Milwaukee, WI). The
90
commercial polyaluminum chloride (simplified as PAC) product was provided by
91
Wanshui
92
AlO4Al12(OH)247+ polymer (simplified as Al13) was prepared following methods
93
provided in previous research (Bottero et al. 1987, Shi et al. 2007). All stock
94
coagulant solutions were prepared with 0.4 mol L-1 of Al or Fe, excluding the Al13
95
stock solution, which is prepared with 0.1 mol L-1 of Al. The species distribution of
96
Al-based coagulants was determined by Ferron-complexation timed spectrometry, as
97
mentioned in previous studies (Wang et al. 2004, Yan et al. 2008a), and the results are
98
shown in Table S1 in the Supporting Information (SI).
99
2.2 Water sample
water
purification-reagent
CO.
(Beijing,
China).
A
high-purity
100
Source water for the Beijing Mega-city was used in this study. The samples were
101
collected in 2017 and stored at 4 °C in pre-cleaned polypropylene containers. Some of
102
the water was filtered through a 0.45-µm hydrophilic polypropylene membrane
103
(Tianjin, China) for further analysis and characterization.
104
2.3 Coagulation experiment
105
A 400-mL water sample was transferred into an 800-mL beaker with a sampling port
6
106
3 cm below the water’s surface. A programmable jar testing apparatus (ZR4-6,
107
Zhongrun Water Industry Technology Development Co. Ltd., China) was used with a
108
standard procedure at a room temperature of 25±1 °C: addition coagulant followed by
109
30 seconds of rapid mixing at 250 rpm, rapid mixing at 250 rpm for 2 minutes, 15
110
minutes of flocculation at 40 rpm and 30 minutes of settling. Some subsamples were
111
collected directly after settling to measure the turbidity and pH using turbidity (Hach
112
turbidimeter 2100P) and pH meters (Mettler Toledo S220 Seven Compact),
113
respectively. After settling, some subsamples were filtered through a 0.45-mm filter
114
for residual dissolved Al (ICP-MS, Element X Series, Thermo Scientific), DOC
115
(Shimadzu TOC-Vcsh carbon analyzer) and absorbance spectra analyses (Hitachi
116
U-3900 UV/Vis spectrophotometer, quartz cell length 5 cm).
117
2.4 Spectrophotometric titration
118
Prior to spectrophotometric titration, the samples were pumped over a Na
119
ion-exchange resin after filtration (Lewatit S1468F, Lanxess, Leverkusen, Germany),
120
which could remove metal cations (e.g., Ca2+, Mg2+, Al3+, and Fe3+) from samples to
121
prevent the cations and DOM from precipitating during pH titration.
122
Spectrophotometric titrations were conducted following similar methods published
123
previously (Yan et al. 2013b, Yan et al. 2014). The pH of the solutions was controlled
124
during titration by adding HClO4 or NaOH solutions at ca. 0.3 pH intervals between
125
pH levels of 3 and 11. The containers were continuously stirred during acid and base
126
addition, and equilibrated for 30 min before 15-mL aliquots were removed for
127
absorbance spectra analysis. The aliquots were then returned to the solutions before
7
128
the next addition of HClO4 or NaOH during titration. Absorbance spectra were
129
recorded at wavelengths from 200 to 600 nm. Dilution effects due to the addition of
130
the acid and base were considered in the final data.
131
2.5 Data interpretation
132
The differential and differential log-transformed absorbance spectra were calculated
133
using equations (1) and (2):
134
DAλ = Aλ, i- Aλ, ref
(1)
135
DLnAi(λ)= LnAi (λ)- LnAref (λ)
(2)
136
where Aλ, i and Aλ, ref are the DOM absorbance measured at the wavelength λ for any
137
selected condition (i) and reference, respectively (such as at pH 3.0 or before
138
coagulation).
139
The differential spectra were deconvoluted to determine the presence and
140
contributions of distinct bands using Peakfit (version 4.12), as described in previous
141
studies (Yan et al. 2013b, Yan and Korshin 2014). All bands should exhibit a Gaussian
142
shape when wavelength λ presents as the photon energy (measured in eV), calculated
143
as: Eev =
1240 3
144
Each Gaussian band (Ai, cm-1) was characterized by the locations of its maximum (E0i,
145
ev) width (Wi, ev), and amplitude (A0i, cm-1). The generated differential spectra
146
(△A(E)) were computed as: 1 − △ AE = △ = △ − 4 2 8
147
The spectral DlnA400 parameter of all the samples examined was calculated using
148
equation (2). Revised non-ideal competitive adsorption (NICA) models were
149
introduced to interpret the evolution of the DlnA400 parameter due to
150
protonation-deprotonation using the following equation (Yan et al. 2016b, Yan et al.
151
2013b):
152
DlnA$% λ = &./0
'()*+,- λ 5 7 123 4% 6 123
'()*923 λ : 5 7 923 4% 6 923
+ ./0
'()*923 λ
A 7 ./<0923 4% 5 6=>? @ 923
−;
'()*+,- λ
7 ./<0123 4% 5 6=>? @ 123
+
153
154
DlnALAS and DlnAHAS correspond to the maximum change in absorbance associated
155
with the deprotonation of carboxylic- and phenolic-type functional groups,
156
respectively, which are defined artificially as low- (LAS) and high-affinity
157
protonation-active sites (HAS). Previous studies demonstrated that DlnALAS and
158
DlnAHAS are dimensionless parameters independent of the DOC concentration, and
159
they reflect the number of protonation-active groups per mg L-1 DOM. pKHAS and
160
pKLAS are the median values of the proton affinity distributions for these groups, and
161
mLAS and mHAS define the width of these distributions and are measures of the
162
heterogeneity of DOM, respectively (Benedetti et al. 1996, Kinniburgh et al. 1996).
(5)
163
164
3. Results and Discussion
165
3.1 In-situ monitoring coagulation process
166
The UV-Visible spectra of water samples before and after coagulation with Alum,
167
PAC, Al13, and FeCl3 coagulants at varying dosages were recorded. The shapes of the
168
differential spectra induced by Al-based coagulants are similar, although their
9
169
intensities differ, therefore, the data for Alum were selected for comparison with those
170
for FeCl3, and the results are shown in Figure S1. The intensity of the absorbance
171
spectra decreased almost exponentially with wavelength and decreased slightly as the
172
coagulant dosage increased. To amplify subtle changes, differential absorbance
173
spectra (DA) were calculated using equation (1), referring to the spectra before
174
coagulation. The results are shown in Figure 1.
175
Figure 1
176
The differential absorbance spectra induced by the addition of the coagulant increased
177
with increasing coagulant dosage. This is mainly because the DOM removal
178
efficiency is higher at a higher coagulant dosage. Moreover, the shape of differential
179
absorbance differs greatly with coagulant type and dosage, for example, the peak at
180
275 nm in the differential spectra before and after coagulation with FeCl3 is more
181
notable than that with the Al-based coagulant. These observations could demonstrate
182
that the DOM removed by coagulation is chemically heterogeneous with the
183
coagulant type and dosage.
184
To ascertain this point, the differential spectra of DOM induced by coagulation were
185
deconvoluted to determine the presence and contributions of distinct bands. Selected
186
data are shown in Figures 2 and S2. All the differential spectra could be well fitted
187
between the experimental and calculated data (R2 ≧ 0.99). The deconvolution of the
188
differential spectra exhibited six Gaussian bands (Table S2) with fixed positions but
189
different intensities or widths. As described in previous studies (He et al. 2015, Huang
190
et al. 2018, Yan et al. 2013b, Yan and Korshin 2014), the six Gaussian bands denoted
10
191
as A5, A4, A3, A2, A1, and A0 had maxima located at 451 (2.74 ± 0.02 eV), 383 (3.24 ±
192
0.01 eV), 316 (3.93 ± 0.01 eV), 276 (4.50 ± 0.01 eV), 239 (5.18 ± 0.01 eV), and 201
193
nm (6.16 ± 0.03 eV), respectively.
194
As shown in Figure 2, the significant differences in the shapes of the differential
195
spectra between the Al- and Fe-based coagulants are because A2 is more remarkable
196
among the six Gaussian bands when the FeCl3 coagulant was applied. Therefore, the
197
Fe-based coagulant is more inclined to remove A2-DOM than Al-based coagulants.
198
Figure 2
199
The deconvolution of differential spectra at various Alum dosages is shown in Figure
200
S2. Notably, the intensity of each band increases as the coagulant dosage increases,
201
but the relative change is not linear. As the spectroscopic interference and matrix
202
influence (such as absorbance from inorganic ions) reduce the reliability of A0 and A1
203
to characterize the properties of DOM, the increase ratio of intensities of A2, A3 and
204
A4 at various coagulant dose to those at condition of coagulant as 0.04 mmol L-1 was
205
calculated according to Figure S2 and shown in Figure 3. Both A3 and A4 change
206
more slightly than A2 with the increasing Alum dose, but they change asynchronously,
207
indicating that the properties of DOM removal are also inconsistently related to the
208
increasing coagulant dosage.
209
Previous studies have consistently used UV254 as a parameter to quantify the removal
210
of DOM by coagulation, this finding demonstrates that the removal of DOM by
211
coagulation is chemically heterogeneous, and the spectral parameter, UV254, is
212
circumscriptus to quantifying DOM removal by coagulation.
11
213
Figure 3
214
3.2 Characterizing the properties of DOMs before and after coagulation
215
To further quantify the changes in DOM properties after coagulation, acid-base
216
spectrophotometric titrations of DOM before and after coagulation with different
217
coagulant types and dosages were conducted. The zero-order UV-Visible spectra of
218
DOM before and after coagulation (data for PAC are shown in Figure S3 as an
219
example) exhibit similar features, as demonstrated in previous studies (Yan et al.
220
2016b, Yan et al. 2013b). The intensity of absorbance decreased almost exponentially
221
with wavelength, and the changes caused by protonation-deprotonation were
222
inconspicuous. The subtle changes could be amplified and some clear signals could be
223
identified after the data were processed using equation (2). The results for PAC at
224
dosages of 0.02, 0.06, and 0.12 mmol L-1 are shown in Figure 4.
225
Figure 4
226
The differential absorbance of DOM induced by deprotonation/protonation increased
227
clearly as the pH, especially for wavelengths below 320 nm. The shape and intensity
228
of the differential spectra are strongly affected by the coagulant dosage and type. The
229
differential spectra of DOM in raw water exhibit a dominant peak at 275 nm. After
230
coagulation with PAC, the intensity of this peak increased significantly when the
231
dosage was 0.02 mmol L-1. However, at a higher PAC dosage (0.06 or 0.12 mmol L-1),
232
the intensity of the dominant peak decreased. The pH-differential spectra for the four
233
examined coagulants at 0.06 mmol L-1 are compared in Figure S4. The features of the
234
differential spectra of DOM before and after dosing with Al2(SO4)2 and Al13 were
12
235
similar to those of PAC, although their intensities were different. However, for FeCl3,
236
prominent peaks at 272 and 385 nm appear, which are deemed as a contributor to the
237
A2 band.
238
To further quantify the protonation/deprotonation capacity of DOM in the coagulation
239
process, the spectral parameter DlnA400 (differential log-transformed spectra at
240
wavelength 400 nm) was introduced using equation (2), which reflects the responses
241
of the carboxylic and phenolic groups in DOM to protonation-deprotonation (Yan et
242
al. 2016b, Yan et al. 2013b). The results of this are shown in Figure 5. The intensity
243
and shape of the DlnA400 curves against pH before and after coagulation vary greatly.
244
Figure 5
245
As described in previous studies (Yan et al. 2016b, Yan et al. 2013b), revised NICA
246
models were introduced to interpret the evolution of DlnA400 caused by
247
protonation-deprotonation using equation (5). Selected data are shown in Figure 5,
248
and the parameters applied in the revised NICA model for each set are shown in Table
249
S3. There was an extraordinary agreement between the experimental data of DlnA400
250
and the data predicted by the revised NICA model for all examined samples (R2＞
251
0.99). The total amount of carboxylic- (DlnALAS) and phenolic-type (DlnAHAS) groups
252
in all examined samples are shown in Figure 6. The results demonstrate that the total
253
amount of carboxylic- and phenolic-type groups per mg L-1 of DOC increases
254
gradually as the coagulant dosage increases, while the opposite trend was found for
255
the DOC of the examined samples. Taking Alum as an example, at a coagulant dosage
256
of 0.02 mM, the total amount of carboxylic- and phenolic-type groups per mg L-1
13
257
DOC increased from 1.04 to 1.36. With higher Alum dosages, this value reached 1.37
258
and 1.63 for 0.06 and 0.12 mM of Alum, respectively. When compared to the other
259
coagulants, it is clear that the total amount of carboxylic- and phenolic-type groups
260
per mg L-1 of DOC after coagulation with Alum is lower than those with PAC, FeCl3,
261
and Al13.
262
The amount of individual carboxylic-type groups per mg L-1 DOC in water samples
263
increased slowly after coagulation with PAC/Al13, while that of phenolic-type groups
264
per mg L-1 DOC increased significantly. Coagulation with Alum is more inclined to
265
elevate the amount of carboxylic-type groups per mg L-1 DOC. Coagulation with
266
FeCl3 exhibited an opposing phenomenon as it is more inclined to elevate the amount
267
of phenolic-type groups per mg L-1 DOC.
268
Figure 6
269
These results indicate that DOM with fewer functional groups is more easily removed
270
by coagulation; this is deemed to be the macromolecular and hydrophobic fraction of
271
DOM. Furthermore, PAC and Al13 are more likely to remove non-functional-group
272
DOM than Alum. The DOC removal by coagulation with PAC and Al13 at a low
273
dosage is higher than that with Alum, which may be because the macromolecular and
274
hydrophobic fraction of DOM is more likely to be removed at a low coagulant dosage
275
than the small-molecule and hydrophilic fraction.
276
3.3 Correlation between residual dissolved Al and the properties of DOM.
277
Residual aluminum (Al) in drinking water is becoming a great concern due to its high
278
potential risk to human health, such as causing Alzheimer’s disease. DOM strongly
14
279
affects the speciation and amount of residual Al after coagulation (Zhou et al. 2017),
280
and organic Al has been found to be the predominant fraction of the total residual Al
281
in treated drinking water (Yang et al. 2010). While there is an inherent correlation
282
between Al and DOM, it and its properties have not been elucidated in detail.
283
The residual dissolved Al after coagulation with Al-based coagulants was measured,
284
and the results are shown in Figure S5 in the SI. The residual dissolved Al increased
285
at a low coagulant dosage and then decreased as the coagulant dosage increased. The
286
residual Al of the water samples after coagulation with Al13 and PAC is lower than
287
that with Alum.
288
To show the effect of the DOM properties on the presence of residual dissolved Al in
289
coagulated waters, the total amount of affinity sites in the DOM was calculated as
290
(DlnALAS + DlnAHAS)*DOC. As the data in Figure 7 show, the residual dissolved Al
291
increased with as the total amount of binding sites in the DOM increased. The
292
residual dissolved Al and the total amount of reactive binding sites exhibit strong
293
linearity (R2=1.00) when Alum is the coagulant. However, the linearities for PAC and
294
Al13 are lower.
295
Figure 7
296
According to Ferron-complexation timed spectrometry methods (Wang et al. 2004,
297
Yan et al. 2008a), the species of Al in the coagulant could be divided into monomeric,
298
polymeric, and colloidal species based on particle size, denoted as Ala, Alb, and Alc,
299
respectively. As shown in Table S1, the speciation of the Al used in the coagulants of
300
this study differ greatly. The Ala fraction in Alum is much higher than that in the PAC,
15
301
and even higher than that in Al13. The residual dissolved Al and the fraction of Ala
302
were compared when the coagulant dosage was 0.06 mmol L-1, which was selected
303
because the binding sites could be satisfied at this dosage for all examined Al-based
304
coagulants, as shown in Figure S5. The result presented in Figure 8 shows that the
305
data of the residual dissolved Al and Ala fraction of coagulants exhibited good
306
linearity, further indicating that the fraction of Ala is the mainly Al species to form
307
dissolved states of Al-DOM, while Alb and Alc were more likely to form colloidal and
308
particular flocs in coagulation. This is consistent with our previous studies (Yan et al.
309
2008a). The data provided in Figure S5 and Figure 6 indicate that the residual
310
dissolved Al can be determined by the available reactive affinity sites in DOM when
311
the monomeric Al species is sufficiently abundant. The residual Al is lower for PAC
312
and the Al13 is at a low dosage as the monomeric Al species is not abundant and DOM
313
is unsatisfied. Figure 8
314
315
4. Conclusion
316
The results presented above support the following conclusions:
317
(1) The changes in DOM during coagulation can be reflected by tracking the
318
differential spectra before and after coagulation. The differential spectra after
319
coagulation were well fitted by six Gaussian bands with maxima at approximately 200,
320
240, 276, 316, 385 and 457 nm. The intensity of these increased inconsistently with
321
increasing coagulant dosage and is strongly affected by the type of coagulant.
322
(2) The affinity sites in the DOM of the examined water samples increased with
16
323
increasing coagulant dosage, indicating that coagulation is inclined to remove the
324
non-functional-group hydrophobic macromolecular DOM fraction. This fraction is
325
more efficiently removed by PAC and Al13 than Alum.
326
(3) The residual dissolved Al after coagulation and the total amount of affinity sites in
327
DOM exhibits strong linearity for Alum, and slightly less linearity for PAC and Al13.
328
The residual dissolved Al can be determined from the available binding sites in DOM,
329
given that the monomeric fraction of Al is abundant.
330
This study demonstrates that the changes in DOM properties during its removal by
331
coagulation could be well quantified by UV-Visible absorbance spectroscopy. This is
332
promising for developing simple and online monitoring of the amount of DOM
333
removed and the properties of residual DOM during coagulation. With further
334
understanding of the inherent meanings of the spectra signals, this could provide
335
useful information for optimizing coagulation and other water treatment processes in
336
the future.
337
338
Acknowledgements
339
This study was partially supported by the China NSF (No. 51578007) and The
340
National Key Research and Development Program of China (No. 2017YFD0801503).
341
The views represented in this publication do not necessarily represent those of the
342
funding agencies.
343 344
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cations
on
coagulation:
The
23
aspect
of
neutralisation
through
470
Captions of Figures
471
(a)
472
(b)
473 474
Figure 1. Differential absorbance spectra of water after coagulation with (a) Alum and
475
(b) FeCl3 at different dosages.
476
24
Differential absorbance (cm-1)
0.14
0.08 mM Alum
0.12 0.10 R2=0.99
0.08 0.06 0.04
experimental model A0 A1 A2 A3 A4 A5
(a)
0.02 0.00 225
275
325 375 425 475 Wavelength (nm)
525
575
477
Differential absorbance (cm-1)
0.08 0.08 mM FeCl3 0.06 R2=0.99 0.04
0.02
experimental model A0 A1 A2 A3 A4 A5
(b) 0.00 225
275
325 375 425 475 Wavelength (nm)
525
575
478 479
Figure 2. Gaussian band fitting of the differential spectra after coagulation with (a)
480
Alum; (b) FeCl3.
481
25
482 483
Figure 3. Comparison of the increase ratio of the intensity of individual Gaussian
484
bands with increasing Alum coagulant dosage, referenced to condition of coagulant as
485
0.04 mmol L-1.
486
26
(a)
(b)
(c)
(d)
487
488 489
Figure 4. DOC-normalized pH-differential absorbance spectra of water before (a) and
490
after coagulation with PAC at dosages of 0.02 (b), 0.06 (c), and 0.12 mmol L-1 (d).
491
Reference pH values are approximately 3.0.
492
27
1.8
Experiment Raw Water 0.02 mM Alum 0.06 mM Alum 0.12 mM Alum
1.6 1.4
1.4 1.2
1.0 0.8 0.6 0.4
Model Raw Water 0.02 mM PAC 0.06 mM PAC 0.12 mM PAC
1.0 0.8 0.6 0.4
(a)
0.2
(b)
0.2
0.0
0.0 3
4
5
6
493 1.8
Experiment Raw Water 0.02 mM Al13 0.06 mM Al13 0.12 mM Al13
1.6 1.4 1.2
DLnA400
Experiment Raw Water 0.02 mM PAC 0.06 mM PAC 0.12 mM PAC
1.6
DLnA400
DLnA400
1.2
1.8
Model Raw Water 0.02 mM Alum 0.06 mM Alum 0.12 mM Alum
7 pH
8
9
10
11
3
4
5
6
7 pH
8
9
10
11
Model Raw Water 0.02 mM Al13 0.06 mM Al13 0.12 mM Al13
1.0 0.8 0.6 0.4
(c)
0.2
(d)
0.0 3
494
4
5
6
7 pH
8
9
10
11
495
Figure 5. Comparison of the effects of coagulation on the spectral parameter DlnA400
496
against pH before and after coagulation with (a) Alum; (b) PAC; (c) Al13; and (d)
497
FeCl3. Line: modeled data, Dot: experimental data.
498
28
499 500
Figure 6. Comparison of the amount of affinity sites per mg L-1 DOM of water after
501
coagulation with PAC, Al13, Alum, and FeCl3
502
29
503 504
Figure 7. Correlation of residual dissolved Al and the total number of affinity sites
505
((DlnALAS + DlnAHAS )*DOC ) in the coagulated water samples.
30
506 507
Figure 8. Correlation of residual dissolved Al in coagulated water with the fraction of
508
Ala (%) in the coagulants. The dosage of Al-based coagulants is 0.06 mmol Al L-1.
31
Highlights
DOM properties during coagulation could be quantified by differential spectra
Property of DOM changes inconsistently with different type and dosage of coagulants
Coagulation is inclined to remove non-functional-group hydrophobic DOM fraction
The residual dissolved Al is determined by the available binding sites in DOM