Introduction
Salt lake brines are unique
natural waters that widespread over the Earth’s surface. They often
contain significant amounts of mineral resources such as lithium, boron,
and halite(Zheng, 2011). Therefore, salt lake brines are considered a
resource of mineral salts. In addition to inorganic components, salt
lake brines contain dissolved organic matter (DOM). Now that the scale
of development and exploitation of salt lakes is expanding rapidly in
the last decade(Peng-Sheng et al., 2011), the influence of DOM on the
exploitation of mineral resources (e.g. smells and colors)(Shalev,
Lazar, Köbberich, Halicz, & Gavrieli, 2018) and its role on the
geochemical cycle of nutrients, carbon and trace metals are
investigated(Hosen, Armstrong, & Palmer, 2018; Jun Wu, Zhang, Yao,
Shao, & He, 2012; Xiping, 2008). Nevertheless, contrary to DOM in other
aquatic environments such as freshwater and marine water bodies and
their sediments(C. H. Fan, Chang, & Zhang, 2016; Y. Li et al., 2017; Z.
Li et al., 2020; Singh, Inamdar, & Mitchell, 2015), little research has
been performed on DOM from salt lake brines. Therefore, it is essential
to study the chemical properties of DOM in salt lake brine and their
interactions with inorganic phases, and develop guidelines for resource
development and environmental assessment in the future.
The DOM isolation and concentration procedure are important first steps
in DOM research in general(Xingjun Fan, Song, & Peng, 2012; H. Li &
Minor, 2015; E. C. Minor, Swenson, Mattson, & Oyler, 2014; Sandron et
al., 2015), and even more so for hypersaline environments. Isolation
methods applied in desalination of saline waters fall mainly into two
categories, i.e. solid phase extraction (SPE) and ultrafiltration
(UF)(Dittmar, Koch, Hertkorn, & Kattner, 2008; Sandron et al., 2015; X.
Wang, Goual, & Colberg, 2012). Different isolation approaches usually
yield DOM with different structural properties. For instance, the SPE
approach tends to isolate hydrophobic DOM fractions(X. Fan, Song, &
Peng, 2013; Perminova et al., 2014), whereas UF more efficiently retains
oxygen-functionalized compounds(Abdulla, Minor, Dias, & Hatcher, 2010;
Hussain A. N. Abdulla, 2010). This is related to the different
adsorption and retention mechanism between SPE (mainly based on
hydrophobic or hydrophilic interactions between DOM and sorbents) and UF
(based on molecular size and shape)(Kruger, Dalzell, & Minor, 2011).
Comparative studies on chemical composition of DOM fractions obtained by
different methods is required to obtain unbiased conclusions on
structures and properties of DOM from hypersaline water.
In addition to isolation approaches, the MW distribution of DOM is an
important factor. Xu et. al. (H. Xu, Zou, Guan, Li, & Jiang, 2019)
observed that fulvic- and humic-like DOM from sediments were mainly in
the low MW fraction (LMW), while tyrosine- and tryptophan-like proteins
are main part of high MW (HMW) components. Wu et. al. (F. C. Wu &
Tanoue, 2001) found that amino acids in lacustrine DOM were relatively
more abundant in the HMW (>5 kDa) than in the LMW fraction
(<5 kDa). Regarding binding behavior with trace metals,
different MW fractions usually exhibit different binding potentials. For
example, HMW (>1 kDa) DOM generally had higher logK and
binding fluorophore abundance with Cu2+ or
Ca2+ than bulk (unfractionated) and LMW (<1
kDa) DOM for freshwater and sediment DOM(W. Chen, Smith, & Guéguen,
2013); The opposite result, however, was obtained for vinasse, where the
fractions with LMW (< 500 Da) showed the greatest
Cu2+ binding capacity (de Zarruk, Scholer, & Dudal,
2007). Evidence of DOM MW effect on metal binding affinity was also
reported for soil fulvic acid and humic acids, for both
Al3+ and Pb2+(Christl, Milne,
Kinniburgh, & Kretzschmar, 2001; Lakshman, Mills, Patterson, & Cronan,
1993; Shin, Hong, Lee, Cho, & Lee, 2001). However, studies on the
structure- and MW-dependent interactions between DOM and metals in brine
waters are scarce.
Nowadays, spectroscopic techniques such as UV-visible absorption and
fluorescence are widely employed for DOM characterization in different
aquatic environments(Carstea, Baker, Pavelescu, & Boomer, 2009; Gueguen
& Cuss, 2011; Sandron et al., 2015). Moreover, fluorescence quenching
experiments coupled with excitation-emission matrix (EEM) spectra and
parallel factor analysis (PARAFAC) can reveal information on
interactions between DOM and trace metals, which will contributable to
predict the mobility, fate and toxicity of trace heavy metal ions(Jun Wu
et al., 2012; Zhang et al., 2010).
In this study, DOM from salt lake brine with different sources, i.e.,
Qarhan Lake, Da Qaidam Salt Lake and West Ginair Salt Lake, were
fractionated using different isolation procedures (SPE, UF, and bulk
DOM). Comparative studies on the spectral properties and composition in
different fractions were investigated using dissolved organic carbon
(DOC) recovery, UV-visible
absorbance, EEM spectra and carbohydrate analysis. Moreover, Pb(II) was
applied as the representative trace heavy metal due to the existence of
lead-zinc deposits in the sampling region, whose binding properties with
different MW-fractionated samples were investigated using fluorescence
quenching titration combined with parallel factor
(PARAFAC) analysis. The objectives
of this study are (1) to investigate the effects of extraction methods
on chemical composition of DOM extracted from hypersaline water, and (2)
to explore the MW-dependent heterogeneity of Pb(Ⅱ) binding affinities
within the DOM fractions using PARAFAC analysis combined with the
fluorescence spectra.
Materials and methods
Sampling sites and description
The salt lake brine was collected in December 2019 from
three
the most representative hypersaline lakes, i.e. Lake Qarhan (36°57′54″N
and 95°14′23″E, chloride type), the largest salt lake in China; Da
Qaidam Salt Lake (37°51′32″N and 95°15′22″E, sulfate type), and West
Ginair Salt Lake (37°40′53″N and 93°24′30″E, sulfate type), all located
in Qaidam Basin of the Qinghai-Tibet plateau. Metallic deposits,
including lead-zinc deposits, are widespread in the region. In the
following analysis, DOM from Lake Qardam, Da Qaidam Salt Lake and West
Ginair are referred to as LQDOM, DQDOM and WGDOM, respectively
Samples were collected from the seepage of brine from pools which were
dug with pre-treated equipment. Two 2.5 L bottles were sealed in sterile
(acid-rinsed) plastic buckets. The collected samples were filtered
immediately (on site) through 0.7 μm pore size glass fiber filters
(Whatman GF/F, combusted at 450 ℃ for 5 h before use), and kept on ice
and in the dark during transported to the laboratory.
DOM fractionation
The procedure for DOM isolation by means of SPE has been described
previously (K. Yang, Zhang, Dong, & Li, 2017; K. Yang, Zhang, Dong,
Nie, & Li, 2017). Briefly, each 200 mL acidified filtrate was loaded on
the preconditioned PPL cartridge, and then isolated under reduced
pressure. Two cartridge volumes Milli-Q water were used to remove salts,
and methanol:water(v/v:9:1) was used to eluted the DOM. The elution was
dried in a vacuum oven (removing methanol) and diluted to initial volume
with Milli-Q water for
spectroscopic and DOC analysis.
The filtered brine samples were further fractionated into LMW
(<1 kDa) and HMW (1 kDa–0.7 μm) DOM using UF. Briefly,
aliquots of each sample (200 mL) was loaded on a 1 kDa pre-cleaned
membrane disc operated on the stirred cell ultrafiltration system
(Amicon, USA), with a N2 pressure of 345 kPa and
concentration factor of 40(Guo, Wen, Tang, & Santschi, 2000). The
permeate solution is LWM DOM and the retentates are HMW DOM. Salts were
further removed from the retentate via diafiltration with Milli-Q
water). HMW fractions were further diluted to the 200 mL volume for
spectroscopic and DOC analysis.
DOC analysis
The concentrations of DOC for the bulk and fractionated DOM samples were
measured using a high temperature catalytic oxidation method performed
on a total organic carbon analyzer (Analytik Jena N/C 3100, Germany).
Due to high salinity, it was necessary to dilute the samples (1:10) with
MilliQ-water before analysis. Moreover, all samples (including following
sections) were measured in duplicate. one-way ANOVA tests were conducted
to examine the significance of differences in data between different
sample sets.
Measurements of optical properties
The UV–Vis absorption data were obtained at the wavelength range from
190 to 800 nm at 1 nm increments using a spectrophotometer (TU-1901,
Puxi). Parameters such as absorption coefficient
(a(λ)= 2.303 A(λ)/ L), spectral
slope for the interval of 275~295 nm (S275~295 )
as well as specific UV absorbance at 254 nm (SUVA254)
were calculated as absorption coefficient a(λ) normalized to DOC
concentration.
Fluorescence excitation–emission matrix (EEM) spectra were obtained
using a Hitachi F-7000 spectrofluorometer with an excitation (Ex) range
set from 200 to 500 nm in 10 nm increments and an emission (Em) range
set from 250 to 550 nm in 2 nm increments. Instrument parameters were
excitation and emission slits, 5 nm; response time, auto; scan speed,
2400 nm min-1, and several preprocessing steps were
used to minimize interference scatter lines and inner-filtering effects
accordingly(Murphy, Stedmon, Graeber, & Bro, 2013; Ohno, Amirbahman, &
Bro, 2007; Stedmon & Bro, 2008).
On the basis of EEM data, some fluorescence indices, i.e., fluorescence
index (FIX, the ratio of fluorescence intensities between Em 450~500 nm
at Ex 370 nm), biological index (BIX, the ratio of Em intensity at 380
nm divided by the Em intensity maximum in 420~435 nm, obtained at Ex
310 nm), and humification index (HIX, the area under the Em435–480 nm
divided by the peak area under the Em 300~345 + 435~480 nm, at Ex 254
nm) were obtained herein(DeVilbiss, Zhou, Klump, & Guo, 2016; McKnight
et al., 2001).
Determination of total carbohydrate
The determination of carbohydrate concentration was based on a
spectrophotometric method described earlier (Hung, Tang, Warnken, &
Santschi, 2001; Myklestad, Skånøy, & Hestmann, 1997). Briefly, an
aliquot of sample was placed into a 5 mL ampoule, to which 1 N HCl was
added for hydrolysis. The ampoule was flame-sealed and held at 100 °C
for 24 h, followed by addition of 1 M NaOH. The neutralized solution was
mixed with a 0.7 M potassium ferricyanide and held at 100 °C for 10 min,
following addition of 1 mL of 2 M ferric chloride solution and 2 mL of
2.5 M 2,4,6-tripyridyl-s-triazine (TPTZ), and mixed on a Vortex. After
standing for 30 min, the absorbance was measured at a wavelength of 595
nm. The concentration of total carbohydrates (TCHO) was measured using
an external standard method (standard curve of glucose solutions). The
concentration of TCHO is expressed as mg-c/L.
PARAFAC analysis
PARAFAC modeling of EEMs has been described in detail elsewhere
(Andersen & Bro, 2003; Stedmon, Markager, & Bro, 2003). In this study,
the PARAFAC modeling was conducted in MATLAB 9.5.0 (Mathworks, Natick,
MA). After several post-acquisition steps for the original EEM spectra,
the PARAFAC model provides an estimate of the number of fluorophores as
well as the excitation and emission spectrum of these fluorophores.
Fluorescence titration and complexation modeling
The complexing properties of bulk and MW-fractionated DOM samples for
Pb(II) were determined by the titration experiment(Ohno et al., 2007;
Jun Wu et al., 2012; H. Xu, Yan, Li, Jiang, & Guo, 2018). Briefly, the
samples were stepwise diluted to [DOC] < 10 mg/L using
Milli-Q water to minimize inner filtering effects(Ohno, 2002). Then,
aliquots of 20 mL diluted solution were titrated with a Pb(II) using an
automatic syringe in 60 mL sealed vials. The concentrations of Pb(II) in
the final solutions ranged from 0 to 1000 μM via addition of
<20 μL of titrant. The pH values of the titrated solutions
were maintained at 6.0 to ensure that no precipitate formed in the
process. To ensure complexation equilibrium, all titrated solutions were
shaken in an incubator in the dark for 24 h at 25 ℃. The modified
Stern-Volmer model was used to determine the binding parameters for
Pb(II) with the PARAFAC-derived components(Hays, Ryan, & Pennell,
2004).
Results and Discussion
Abundances and
performance comparison in different isolation approaches based on
DOC analysis andcarbohydrates
DOC analysis (Table 1) showed that the bulk DOM solutions contained
41.70, 19.89 and 28.74 mg/L, for LQDOM, DQDOM and WGDOM, respectively.
These values are higher than estimates of the world average of DOC in
surface waters (6.48 mg/L) or open ocean surface waters (0.744~1.44
mg/L)(Dittmar et al., 2008; E. Minor & Stephens, 2008; Retelletti Brogi
et al., 2018), and are in the same range as DOC content of the Great
Salt Lake (~40 mg/L)(Leenheer, Noyes, Rostad, & Davisson, 2004), but
these values were lower than the DOC value reported in brines from Lake
Vida (> 580 mg/L), in the McMurdo Dry Valleys(Cawley et
al., 2016). These results indicated that many brine waters or
hypersaline lakes have higher DOC concentrations than other aquatic
systems.
For the SPE-DOM, the DOC concentration was 13.89, 7.21 and 10.86 mg/L
for LQDOM, DQDOM and WGDOM respectively, while DOC was 11.68, 5.15 and
9.1 mg/L for UF-DOM (HMW). Hence, SPE-DOM accounted for 33.31%, 36.25%
and 37.79% of DOC in the bulk solutions for the three sites, whereas
UF-DOM accounted for 28.01%, 29.56%, and 31.66%, respectively. The
results indicated that SPE exhibited higher DOC recovery than UF for the
hypersaline water DOM isolation. These values were significantly lower
than previously reported values for the use of SPE on the freshwater
(river or lake) and seawater samples worldwide(Dittmar et al., 2008; Y.
Li et al., 2016; Y. Li et al., 2017), indicating that the presence of
excess inorganic ions in the brines may negatively affect DOM isolation
efficiency for SPE and UF. Regarding the MW fractions, a large portion
of total DOC abundance was located in the LMW fraction (about 2/3 of the
DOM; Table 1). The HMW fraction in brine water DOM was generally lower
than that in sediment DOM (~50%) and river water (40.3%), but
comparable with lake water (30.2 ± 0.4%), seawater (23.2 ± 1.1%), and
estuary (33 ± 6%)(H. Xu & Guo, 2017; H. Xu et al., 2019).The
relatively low concentration of HMW-DOM in the hypersaline waters, or at
least of the DOM that is recovered using UF, may be related to the minor
terrestrial DOM component in these systems(Helms et al., 2008).
Spectral properties of chromophoric and fluorescent DOM
UV-Vis spectra analysis
Spectral paraments for UV-Vis are
shown in Table 1. The SUVA254 data of bulk DOM were
1.53±0.11, 1.67±0.15and 2.09±0.09 for LQDOM, DQDOM and WGDOM,
respectively. Our SUVA254 values were significantly
lower than that in the other natural waters(Helms et al., 2008; H. Xu &
Guo, 2017), indicating that the salt lake brine water DOM herein contain
a relatively low proportion of aromatic groups, perhaps due to the
absence of terrestrial organic material inputs such as lignin or humic
acids(H. Li & Minor, 2015).
As for the fractions based on different isolation approaches, the
SUVA254 value of SPE-DOM were higher than that of
UF-DOM, indicating that SPE tended to retain aromatics or hydrophobic
materials, which is consistent with previous studies(Y. Li et al., 2017;
Perminova et al., 2014). For the MW size fractions, the HMW had higher
SUVA254 than the LMW, for LQDOM and DQDOM, while the
reverse results were observed for WGDOM samples. The results indicated
that HMW-DOM were characterized with higher aromaticity than the LMW
counterparts for LQDOM and DQDOM. Thus, the spectral properties were
different between the isolation approaches, as well as the MW fractions
and DOM sampling locations.
TheS275–295 is inversely related to the MW of the
DOM. No systematic trend was found for S275–295 values between SPE-DOM and UF-HMW fractions in all samples. The
results enhanced the conclusion that the mechanism of SPE retained DOM
depending on hydrophilic–lipophilic interactions rather than MW(H. Li
& Minor, 2015). Moreover, the higher S275–295 values (Table 1) in LMW fraction for the samples in this study
confirmed an overall small molecular size for LMW-DOM and the efficiency
of ultrafiltration procedure applied in this study for MW fractionation.
Fluorescence EEM spectra of bulk DOMs and isolated fractions
Fig. 1 shows the EEM contours of bulk DOM samples and isolated fractions
with different isolation procedures. Although multiple peaks overlapped
in the EEM contours, the bulk DOM was characterized by three similar
main peaks, i.e., peak A (Ex/Em: 320/400 nm), peak B (Ex/Em: 260/400 nm)
and peak C (Ex/Em: 260/410 nm). Peak A was ascribed as
UVC humic-like, whereas peaks B
and C can be assigned as UVA
humic-like(Coble, 1996; Leenheer & Croué, 2003; Stedmon et al., 2003).
Interestingly, the fluorophores were all ascribed to humic-like
substances, and the protein-like (tryptophan or tyrosine)
fractions(Coble, Green, Blough, & Gagosian, 1990; Ohno et al., 2007),
were not detectable. The may be explained by the low microbial activity
in hypersaline brines(Waiser & Robarts, 2000). Comparison of the EEM
contours (Fig. 1) between bulk DOM and their isolated fractions, showed
that SPE isolates DOM that is markedly different from bulk DOM, probably
due to selective retention, pH changes or solvent usages leading to
irreversible chemical alterations of the resulting isolates. To the
contrary, both HMW and LMW fractions isolated by UF shared similar
fluorophores as bulk DOM.
Despite the similar EEM contours between bulk DOM and UF-DOM, some
differences are apparent from the fluorescence intensities, such as that
of peak A, which was higher than peak B and C (the two peaks very
closed) in bulk and LMW DOM (except for DQDOM), while the opposite was
observed for the HMW-DOM, irrespective of the sample origin. The
fluorescence intensity of peak A was lower than that of peaks B and C
for the DQDOM regardless of the MW, indicating that even the same
component extracted from various origin may also possess various
spectral properties.
MW-dependent fluorescence indices of salt lake brine DOMs from
different sources
The FIX (Table 1) served as a simple index to distinguish sources with
higher values (~1.9) indicating microbial DOM and lower
values (~1.4) related to terrestrial DOM sources. In our
study, the FIX of all samples ranged from 2.05 to 2.68, significantly
higher than that in other reported aquatic samples, such as river
samples (1.4~1.5), groundwater samples (1.9), lakes (1.3~1.9) and
Green Bay estuary (1.1~1.3)(DeVilbiss et al., 2016; McKnight et al.,
2001), and also higher than sediment-derived DOM (1.3~1.6)(Huacheng Xu,
Guo, & Jiang, 2016). For the bulk and MW fractionated samples, the
HMW-DOM had consistently lower FIX (2.05~2.09) than the LMW-DOM
(2.37~2.68), indicating that microbial source moieties were mainly
distributed in the LMW fraction. The BIX, which indicates the
contribution of autochthonous or freshly produced DOM, was significantly
higher in LMW-DOM irrespective of the sample types, indicating in situ
produced DOM mainly in LMW-DOM. The HIX, an indicator of the degree of
DOM humification(Ohno, 2002), was no significant systemic tend for the
bulk and MW-fractionated samples, indicating a comparable humification
degree.
Distribution of carbohydrates in different fractions based on
isolation approach
The concentration of TCHO for SPE-DOM and UF-HMW (Table 1) ranged from
1.68 mg C/L in the Da Qaidam Salt Lake to 2.13 mg C/L in the Lake Qardam
for UF-HMW DOM, and these values are higher than those previously
reported in both freshwater (river and lake waters) and seawater(H. Xu
& Guo, 2017). In terms of UF-HMW DOC, the percentages of TCHO varied
from 18.24% to 28.54%, which were lower than those reported previously
in both Jourdan River and Gulf of Mexico (55%-72%). The results agreed
well with previous observations that the percentage of HMW TCHO
generally decreased with increasing salinity(Xuri Wang, Cai, & Guo,
2010).
Compared with UF-HMW DOMs, TCHO had a relatively low percentage in the
SPE-DOM, ranging from 13.40% to 14.42%, and suggests that UF retains
more carbohydrates than SPE. This is consistent with previous results
obtained by 13C NMR analysis of marine DOM(Koprivnjak
et al., 2009). Thus, caution is demanded when different isolation
approaches are compared.
PARAFAC analysis
Fluorescent components
derived from PARAFAC modeling
PARAFAC models with three components were successfully identified for
the bulk and MW fractions based on the results of the residual and split
half analysis. As depicted in Fig. 2, component 1 (C1) exhibited two
peaks with Ex/Em of 250/400 and 310/400 nm, ascribed to humic-like
component(X. Yang, Meng, & Meng, 2019). Component 2 (C2, Ex/Em=260/480
nm) was identified as fulvic acid substance(Leenheer & Croué, 2003; X.
Yang et al., 2019). Component 3 ( C3) had a peak at Ex/Em = 280/480 nm
and can be defined as humic acid-like organics(W. Chen, Westerhoff, P.,
Leenheer, J. A., & Booksh, K, 2003). Compared to the peaks derived from
EEM contours (section 3.2), the PARAFAC-derived components showed a
red-shifting of the peak from Ex260/Em400 to Ex280/Em480. This
red-shifting may relate to metal addition after acidification, as
reported for iron–DOM complexes (Cawley et al., 2016; Weber, Allard, &
Benedetti, 2006). Overall, the PARAFAC-derived components were all
identified as humic substance, demonstrating the absence of protein-like
materials in the brine, corresponding with the results of EEM contours
analysis. This suggests that the microbial-derived DOM, which may be
expected to be rich in protein or other N-containing macromolecules, had
been subjected to considerable degradation.
The quantitative distribution of
PARAFAC-derived components and
isolated fractions in the brine DOM (Fig. 3) show similar proportions of
the MW fractions (Fig. 3a). Moreover, the humic-like C1 (Fig. 3b) had
the highest relative abundance in all samples, followed by C2 and C3,
irrespective of the sample types.
Behavior ofPARAFAC-derived components binding with Pb(II)
The variations in fluorescent intensity scores of the
PARAFAC-derived
components during metal (Pb) addition are illustrated in Fig. 4.
Entirely different quenching effects were observed for Pb(II) by the
three components. When metal addition was applied, C1 exhibited an
initially decrease followed by a gradual decline or stabilization
regardless of the sample types, and the trends of quenching curves were
generally in accordance with the previous studies for humic-like
fractions derived from various sources using fluorescence quenching
titration with different trace metals(Ohno et al., 2007; J. Wu, Zhang,
He, & Shao, 2011; Yamashita & Jaffé, 2008). The decline of fluorescent
intensities suggested an effective binding of C1 with metals(Ohno et
al., 2007; Stedmon et al., 2003), and suggests that C1 played a key role
in the complexation between heavy metals and DOM in brine waters.
All C2 fluorescent components (except the C2 of HMW-DOM) increased
sharply at low metal concentrations (e.g., <200 μM), followed
by a rapid decline upon further additions, indicating different binding
mechanisms between the C1 and C2 fluorophores. The increases in
fluorescence intensity at the initial stages of the titration may
reflect the quenching of the fluorescence of C2 due to interactions with
inorganic components in the initial brine. The fluorescence intensity
could be enhanced due to competition between inorganics and Pb(II)
resulting in the release of C2 components from the DOM-inorganic
complexes. With the addition of Pb(Ⅱ), the fluorescence of C2 component
may be quenched due to the replacement of the original quencher in the
DOM-inorganic complexes through the competition with Pb(II) whereby more
stable DOM-Pb(II)
complexes
are formed at later stages. The fluorescence titrating curves of C2 with
Pb(II) for HMW DOM were different from the bulk- and LMW-DOM
irrespective of sample origin, namely a sharp increase at low metal
concentrations (e.g., <200 μM) followed by a gradual
stabilization for further addition. This differences in fluorescent
intensity at the later stage can be ascribed to the changes in the
molecular environments of HMW DOM caused by increasing metal
concentrations(Yamashita & Jaffé,
2008).
Compared with fluorescent quenching of C1, the reverse trend was
observed for C3, which was also reported in previous studies(Ohno et
al., 2007; J. Wu et al., 2011), and confirmed the different binding
mechanisms for the three components. Based on pioneering papers on the
use of fluorescence spectrometry to determine the behavior of
metal–ligand, this phenomenon can be ascribed to multiple factors
(Yamashita & Jaffé, 2008), including changes in quantum yields of
fluorophores and the replacement of the other quencher or C3 during
Pb(II) addition.
MW-dependent binding properties of PARAFAC-derived components
Further analysis showed that the quenching extents of fluorescent
components exhibited clear differences related to sampling location and
MW. With the addition of Pb(Ⅱ), the intensities of fluorescent C1 in
bulk LQDOM decreased by 36.7% (from 53.9 to 34.1 A·U/mg-C/L), which was
lower than the decline observed for bulk DQDOM and WGDOM (44.0% and
43.3%, from 116.5 and 92.8 to 65.2 and 52.7 A·U/mg-C/L, respectively).
The results indicated that the humic-like component in DQDOM had higher
binding potential than the same component in the LQDOM and WGDOM
samples, which may be attributed to the higher humification degree of
the former (level of decay reflected by HIX). As for the MW fractions,
the quenching degrees of C1 in HMW fraction were generally higher than
those in LMW fraction for LQDOM and DQDOM (47.4–56.6 vs. 42.7–42.8%),
while the inverse was true for fractions of WGDOM.
The stability constants (Log K M) and fvalues based on the modified Stern-Volmer model were calculated (Table
2) to reveal the MW-dependent metal binding behaviors of humic-like
fluorescent component (C1). The Log K M values
ranged from 1.81 to 2.38, which were significantly lower than those
reported for commercial humic acids (4.28~5.15) (H. Xu et al., 2018) as
well as those found for the binding of other metals (i.e., Cu(Ⅱ), Fe(Ⅲ),
Al(Ⅲ) and Hg(Ⅱ)) from various sources(Hur & Lee, 2011; Ohno et al.,
2007; J. Wu et al., 2011; Yamashita & Jaffé, 2008; Yu et al., 2012).
This discrepancy may be partly due to the abundance of inorganic ion in
the brine, which can compete with lead complexation. Further analysis
showed that the Log K M values of C1 for bulk
LQDOM were slightly lower than for the bulk DQDOM and WGDOM samples.
This indicated that humic-like components in DQDOM and WGDOM had a
stronger binding affinity than LQDOM. As for the MW fractions, the logK M values of C1 in HMW fraction were higher than
those in LMW fraction for the LQDOM and DQDOM samples (Table 2). In
summary, the HMW-DOM exhibited higher binding affinities with Pb (Ⅱ)
than LOW-DOM, which was in agreement with their quenching extents.
The f values of the fluorescent components in bulk DOM and MWs
samples ranged from 0.30 to 0.63, which were in the same ranges as
compared to other natural DOM(W. B. Chen, Smith, & Gueguen, 2013; H. Xu
et al., 2018). Certainly, the difference of the bulk- and MW-fractions
were also observed for f values. For instance, the fvalues of C1 in the LMW fraction exceeded those in the HMW for LQDOM and
WGDOM, while the obviously reverse trend was observed for DQDOM
fractions.
Implication for the utilization of salt lake brine resource
The results may have important implications for the evaluation of DOM
treatment and their ecological impacts and consequences. For example,
spectral titration experiments showed that, among these PARAFAC
derived-components, C2 and C3 may be the preferential organic
constitutes binding with minerals, and then affected the quality of
products (color and grade). The modified Stern-Volmer model further
showed the stability constants Log K M and binding
capacities (f ) of organic ligand (C1) with trace metal,
indicating C1 could be capable of migrating and transporting trace
metals by formation of organometallic complexes in the Qaidam Basin
plateau region, although it exhibited a relatively lower metal binding
potential than that in the other natural waters, probably due to
competition with salts. Certainly, further research is needed to
characterize the DOM at the molecular scale, including non-fluorescent
compositional features such as abundances of lignin and other
polyphenolic DOM derivatives, as well as their behaviors in the
biogeochemical cycling of trace metals and the mobility of colloidal
particles of minerals in salt lake environments.
Conclusions
This study showed that salt lake brine DOM generally contains higher DOC
and FIX, but lower SUVA254 than that for other natural
waters, and the humic-like substances were the only observed
fluorophores. Isolation of DOM by
SPE recovered a larger proportion of the DOM, probably more aromatics
(and less carbohydrates), than UF. Moreover, SPE may induce
compositional changes of bulk DOM. Pb(Ⅱ) titration experiment showed
that PARAFAC-derived components
exhibited different quenching behavior. Further analysis indicated that
PARAFAC-derived C1 component exhibited higher Pb(II) binding affinities
for bulk DQDOM than for LQDOM and WGDOM. The HMW-DOM was had higher
Pb(II) binding potential than the LMW counterparts for LQDOM, while the
reverse results was true for DQDOM and WGDOM. This paper extended our
knowledge on the composition and properties of DOM in salt lake brine
environment.