3.5.1. Identification of pyrolysis products and their relative proportions of salt lake DOM
The DOM samples were prolific of compounds with numerous isomers and side-chain configurations, including alkane/alkene pairs, alkyl-substituted monoaromatic and polyaromatic hydrocarbons (MAHs/PAHs) and alicyclic compounds (Table S2). The products were grouped into 11 subclasses based on chemical structures (Table 4). Remaining compounds, including unidentified ones, were labelled as “other compounds”.
Table 4
Pyrolysis product groups and relative proportions (%) of the DOM samples analyzed.
Categories
|
SIP
|
SSP
|
SRP
|
SCP
|
SBP
|
ALICYCL
|
25.81
|
24.99
|
27.39
|
28.08
|
25.62
|
CARB
|
26.88
|
24.87
|
27.59
|
21.60
|
19.92
|
MAH
|
16.79
|
17.52
|
17.61
|
20.05
|
22.54
|
MCC
|
11.57
|
15.69
|
11.21
|
11.51
|
11.22
|
NCOMP
|
1.66
|
1.50
|
1.43
|
1.60
|
1.84
|
OTHER
|
5.52
|
4.89
|
4.96
|
5.57
|
6.20
|
PAH
|
3.68
|
3.85
|
3.65
|
4.94
|
5.95
|
PHEN
|
3.65
|
3.49
|
3.28
|
3.50
|
3.88
|
SCOMP
|
2.19
|
0.76
|
1.18
|
0.70
|
0.66
|
HALOGENCOMP
|
1.65
|
0.68
|
0.47
|
1.26
|
1.13
|
PENT
|
0.32
|
0.77
|
0.48
|
0.40
|
0.32
|
LIG
|
0.30
|
0.40
|
0.36
|
0.43
|
0.32
|
Abbreviations: ALICYCL = alicyclic compounds, CARB = carbohydrate, MAH = monocyclic aromatic hydrocarbon, MCC = methylene chain compound, NCOMP = nitrogen-containing compound, PAH = polycyclic aromatic hydrocarbon, PHEN = phenols, SCOMP = sulphur-containing compound, PENT = Pentanedioic acid, LIG = products of lignin and lignin-like phenolics. |
On the whole, the most abundant group (24.99 − 28.08%) was 1-ring unsaturated alicyclic compounds (ALICYCL) based on cyclopentene and cyclohex(adi)enes groups such as cyclohexadiene, C1-cyclohexene, C1-cyclohexadienes (4 isomers), C2-cyclohexadienes (10 isomers), C3-cyclohexadienes (7 isomers) and C4-cyclohexene (limonene). They represented a cyclic aliphatic component of the DOM. They could be terpenoid-derived, and shared similar features to the DOM from the Great Salt Lake (Leenheer et al., 2004). It is argued that the terpenoid-like DOM was probably degradation products of algal and bacterial precursors (pigments and steroids therein). Poorly-identified terpenoid-like structures were often found in the DOM sample, sometimes they contained abundant carboxylic groups. Such carboxylic groups could be present in the source of the alicyclic compounds, and have been eliminated during analytical pyrolysis (decarboxylation).
The second most abundant group (19.92 − 27.59 %) had a structure that is typical of the pyrolysis products of carbohydrates (CARB), such as acetic acid, hydropropanone, hydroxypropanal, cyclopentenones, furans and furaldehydes. These compounds probably originated from microbial polysaccharides, or other kinds of carbohydrates after significant influences of (photo-) degradation. Pyrans and anhydrosugars such as levoglucosan were not detected, suggesting that vascular plant-derived carbohydrates (holocellulose derivatives) were negligible. This is in agreement with the negligible fluxes of rivers and streams in the xeric environments of the Qaidam Basin (Yang et al., 2017a). The carbohydrate products probably reflected a labile component of algae and bacteria. Several salt lake DOM samples from North America also showed a significant carbohydrate component (Domagalski et al., 1989).
Monocyclic aromatic hydrocarbons (MAHs) included benzene and alkylbenzenes with 1 (toluene) to 5 carbon atoms in alkyl groups. This series included mono-unsaturated MAHs such as styrene, and accounted for 16.79 − 22.54%. The MAHs have been detected in most of the DOM studies, by Py − GC − MS or GC − MS, from (hyper) saline environments. Benzene, toluene, xylenes and other MAHs were also detected in oilfield-produced brine studies using GC − MS of dichloromethane-soluble DOM, indicating that the MAHs were not secondary pyrolysis products of non-aromatic DOM (Yang et al., 2015).
The (linear) n-alkanes, n-alkenes, isoprenoid alkanes and alkenes with a chain length ranging from C6 to C23 accounted for 11.21 − 15.69%. They were grouped as methylene chain compounds (MCC). Other MCC such as fatty acids, methylketones, alkylnitriles or alkylamides were not detected. These patterns were probably related to aliphatic material in microbial sources. Lack of phytadienes excluded a significant source in fresh phytoplankton, but intense photo-oxidation could efficiently eliminate such moieties. The isoprenoid compounds included C9-alkadiene (diemethylheptadiene compound), a C12-isoprenoid alkanone and the other unidentified isoprenoid ketones. These were uncommon pyrolysis products and highlighted the idiosyncratic nature of the DOM in hypersaline water. They might originate either from a specific kind of diatom or bacteria, or represent oxidation products of any isoprenoid MCC precursors.
The detected phenols (PHEN) including phenol, methylphenols, C2-alkylphenols (4 isomers) and a C3-alkylphenol accounted for 3.28 − 3.88%. These compounds could originate from many sources such as microbial ones. A partial confusion with C9-isoprenoid alkatriene groups (m/z 107 and 122) for the dimethylphenols could not be excluded.
The sum of compounds with a halogen atom (HALOGENCOMP) in its structure (0.47 − 1.65 %) was detected in our study. The compounds included bromomethane (MeBr) and iodomethane (MeI), but other organohalogens could be among the unidentified products. Polycyclic aromatic hydrocarbons (PAHs) were mostly indenes (C0-C3-alkylindenes) and naphthalenes (C0-C2). The extensive methylation was probably indicative of a terpenoid-like or asphaltene source, even though a contamination from sample treatment could not be excluded.
Compounds with nitrogen (NCOMP) in their structures were scarce (1.43 − 1.84%) and dominated by pyrroles (N-methylpyrrole, 2-methylpyrrole and 3-methylpyrrole), with traces of benzonitrile and C3/C4-alkylanilines. These compounds should probably be ascribed to microbial DOM. The lack of indoles, acetamides, cyanobenzenes and diketopiperazines indicated that the N-rich DOM was strongly affected by decay processes, as intact proteins, chitins or peptidoglycans would generate a more diverse N-product fingerprint upon Py − GC − MS. This decay process could also explain the rather low abundance of organic N in spite of a prevailing microbial source of the DOM: N-rich biopolymers tended to be relatively labile components. The only sulphur-containing products (SCOMP) were benzothiazole, S2 and/or SO2 (0.66 − 2.19%). Identification of thiophene was tentative and was added to the “other compounds”.
A peak with m/z 81, 109 and 124 at the expected retention time of guaiacol was the only possible sign of lignin (LIG), but a source in an isoprenoid alkadiene or trimethylcyclopentenone could not be excluded. Finally, the other compounds (4.89 − 6.20%) included numerous unidentified peaks, peaks from plasticizers (phthalic anhydride), C0-C3 alkylbenzofurans and a dioxane.
In summary, there were several classes of compounds with an aromatic character (MAH, PAH, PHEN, benzofurans), others with a cyclic aliphatic character (alicyclic compounds, carbohydrate products) and compounds with an acyclic structure (linear alkanes and alkenes, isoprenoid alkanes and alkenes, and pentanoic acid derivatives). Remaining compounds contained atoms such halogen, sulphur or nitrogen.
3.5.2. Comparison of pyrolysis products of DOM with different origins
In our previous studies, the DOM samples isolated from solar ponds of oilfield-produced brine (OSDOM) had been investigated by Py − GC − MS analysis (Table S1, S2). Both types of DOM samples produced complex Py − GC − MS fingerprints and it seems clear that they facilitate information on the differences in DOM from the salt lake (SSDOM) and the oilfield-produced brine, and also reflect the effect of prolonged sunlight exposure.
Both sample sets contained many products with linear and isoprenoid polymethylene structures (MCC, i.e. acyclic aliphatic), indicating that: 1) the DOM composition was significantly affected by degradation (MCC are relatively recalcitrant), and 2) a significant portion of the DOM originated from aquatic organisms was prolific of isoprenoids (bacteria and diatoms). This was in agreement with the almost complete absence of alkanes and alkenes with chain lengths over C23, and previous observations that alkanes and alkenes occurred with chain lengths up to C18 (Yang et al., 2017a; Yang et al., 2017b). This implies that vascular plant-derived aliphatic DOM was scarce. The MCC were slightly more abundant in the OSDOM samples and they increased slightly during sunlight exposure. Their abundance might be associated with photo-oxidation of more labile organic constituents.
By contrast, the other main O-lacking aliphatic components (alicyclic products) were more abundant in the pyrolyzates of the salt lake brine, and decreased in abundance in the oilfield-produced brine due to sunshine exposure. The presence in both sample sets suggest that their precursor was of microbial source (algae and bacteria), possibly relating to terpenoid-like bacterial structures. This was consistent with the results of NMR. These structures would be preferentially degraded in the solar ponds containing the oilfield-produced brine, as they were photochemically and/or biologically labile (Leenheer et al., 2004). Leenheer et al. (2004) performed a radiocarbon dating, showing that virtually all DOM in the Great Salt Lake, similar salt and DOM concentration as the Da Qaidam Salt Lake, was of modern age (kerogen could be a source of the defunctionalized terpenoid structures, but this was not the case). We believe the DOM studied here was also of recent, but nevertheless strongly degraded source.
The same phenomenon was observed for the polysaccharides: they were slightly more abundant in the salt lake brine and were preferentially degraded in the solar pond process. The difference with the alicyclic aliphatic compounds was that, for the carbohydrate products, a decrease due to exposure was more pronounced in the salt lake brine. Hence, the degradation occurring in the salt lake brine eliminated polysaccharides, whereas in the oilfield-produced brine the terpenoid-like component of the DOM was more severely affected. The results were in agreement with those of NMR and FTIR analyses.
Oxygen lacking aromatic DOM products (MAHs and PAHs) were more abundant in the salt lake brine than in the oilfield-produced brine, and the opposite was observed for the O-containing aromatic compounds (in particular, phenols). Their abundance did not change significantly with sunshine exposure time, suggesting that these compounds reflected DOM types of intermediate degradability. Nevertheless, they appeared to correspond to DOM of relatively recalcitrant nature in the salt lake brines (increased in final exposure phases). If the oilfield-produced brine contained significant amounts of petroleum-derived DOM, we would expect higher PAH levels than in the salt lake brine. Hence, it seems that the oilfield-produced brine DOM did not contain oil-derived DOM. Again, a source in microbial terpenoid-like structures was one of the more plausible explanations of the presence of the poly-alkyl substituted PAHs.
The value of the MAHs and PAHs as indications of aromatic DOM in the brine samples was compromised by their possible release from the exchange resins (Daignault et al., 1988). On the other hand, they have often been identified in DOM from (hyper) saline lakes, and are ubiquitous products of pyrogenic and geological OM (kerogen) as well (Witter and Jones, 1999). Many kinds of sufficiently degraded sources of organic sources, including humic acids, could produce aromatic groups lacking oxygen after pyrolysis (Song and Peng, 2010).
Chitins (e.g. from brine shrimps) and peptidoglycans (bacterial cell walls) were not recognized. If the DOM were indeed largely bacterial/planktonic, the relatively low nitrogen content (or low abundance of N-containing pyrolysis products), in comparison with fresh aquatic microbial OM (C/N < 10), could be explained by preferential decay of proteinaceous groups.
The organohalogen MeI (methyliodide) was much more abundant in the OSDOM than SSDOM and vice versa for MeBr. It also seemed to confirm the high halogen content in the oilfield-produced brine (Zhang, 1987; Tan H et al., 2007).