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Humic Acids and Fulvic Acids: Characteristics, Sorption of Hydrophobic Organic Contaminants, and Formation of Disinfection by-Products during Chlorination

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Hang Vo-Minh Nguyen, Jin Hur and Hyun-Sang Shin

Submitted: 04 May 2022 Reviewed: 23 May 2022 Published: 17 June 2022

DOI: 10.5772/intechopen.105518

Humus and Humic Substances IntechOpen
Humus and Humic Substances Recent Advances Edited by Abdelhadi Makan

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Humus and Humic Substances - Recent Advances

Edited by Abdelhadi Makan

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Abstract

Humic and fulvic acids, which can be extracted from soils, are abundant in surface water because of their high discharges from runoff during torrential rainfall, storm events, and summer monsoon. Both humic and fulvic acids adversely affect water supply as they produce disinfection by-products (DBPs) during chlorination and serve as the sorbent for the binding of hydrophobic organic contaminants. In the present study, we conducted chlorination and phenanthrene sorption for humic and fulvic acids that were extracted from nine soil samples. We also analyzed and compared their characteristics by using elemental 13C NMR analysis, spectroscopy analysis, and size exclusion chromatography. Our results showed that the changes in their structural characteristic, their DBP formation, and phenanthrene sorption behavior differed critically between humic and fulvic acids. For chlorinated humic acids, high SUVA, low molecular weight, low N/C, and low O groups of aromatic C were associated with high trihalomethane (THM) formation. In comparison, low O groups of aliphatic C in fulvic acids were associated with both oxidation and incorporation in terms of THM formation. Humic acids exhibited higher sorption ability than fulvic acids due to their higher MWw, SUVA, and %THLF. These findings provide key information for monitoring water quality in rivers and lakes.

Keywords

  • humic acids
  • fulvic acids
  • oxidation reaction
  • incorporation reaction
  • trihalomethane
  • sorption isotherm
  • hydrophobic organic contaminants

1. Introduction

Humic substances are principal compounds that account for 80–90% of soil organic matter [1]. Owing to their complexes of bioactive substances, humic substances can control the stability and ecosystem in soil [2]. Humic substances are organic macromolecules with multiple properties and contain a wide variety of structural functional groups [3], arising from physical, chemical, and microbiological processes [4]. In aquatic system, humic substances account for 40–60% of natural organic matter [5]; thus, they can have significant impact on water quality. In natural water resources, humic substances are formed from the degradation of plants, animal residues, and soil surface runoff [6]. In the events of torrential rainfall, storm events, and summer monsoon season, humic substances are abundantly formed from upstream land use and soil surface runoff [7, 8, 9]. Thus, it is essential to study the humic substances extracted from upland soils for water quality management.

Humic substances are mainly divided into humic acids, fulvic acids, and humin [10]. Humic acids are soluble in water only at pH >2, fulvic acids are soluble in water in all pH conditions, whereas humin is insoluble in water [11]. Thus, because of their solubility, humic and fulvic acids play indispensable roles in dissolving organic matter in aquatic system. Humic and fulvic acids majorly comprise carboxylic, phenolic, carbonyl, hydroxyl, amine, amide, and aliphatic moieties [12]. In surface water, fulvic acids account for the majority of humic substances, whereas humic acids account for only 10% of humic substances [6]. For the disinfection of water for drinking purpose, humic and fulvic acids present in water can react with disinfectant chemicals (i.e., chlorine and ozone) to form disinfection by-products (DBPs) such as trihalomethanes (THMs), haloacetic acids (HAAs), haloketones, and haloacetonitriles [13, 14]. Among these four DBPs, THMs and HAAs are the two most abundant halogenated DBPs [15, 16]. DBPs are considered to be dangerous to human health because of the presence of potential carcinogens [17] that particularly cause urinary bladder cancer [18, 19]. Humic and fulvic acids are the primary sorbent, which can impact the fate, mobility, and bioavailability of hydrophobic contaminants, especially the presence of polycyclic aromatic hydrocarbons (PAHs) in water system [20, 21]. PAHs contain more than two benzene rings [22] and are the most persistent and toxic organic micropollutants in surface water. Low concentrations of these hydrocarbons can have adverse effects on human health and aquatic systems because they contain carcinogenic, mutagenic compounds and potent immune suppressants [23, 24]. PAHs can be formed from biological process, industrial wastes, petroleum spills, incomplete combustion from nature sources (forest and brush fires), and/or human combustion sources (engine emissions) [24]. Recently, PAHs from urban runoff were reported to be a serious contaminant in rivers and lakes [9, 25, 26]. In addition, PAHs have been widely detected in surface water and drinking water at higher concentrations compared with other persistent organic pollutants [27, 28]. Of the 16 PAHs monitored by the US Environment Protection Agency, phenanthrene (PHE) was reported to be the most abundant PAH in surface water. Similar to humic and fulvic acids, PAHs can produce chlorinated PAHs during chlorination process for drinking water treatment. This is because PAHs contain an electronic-rich system that can be readily attacked electrophilically by hypochlorous acid [29]. The hypobromous acid might also be formed in the presence of Br− during chlorination because Br− ion is ubiquitous in both surface water and chlorine solution [30]. The hypobromous acid reacts with PAHs to form brominated PAHs. In comparison with PAHs, the chlorinated PAHs and brominated PAHs exhibit AhR activity, DNA damaging effects, and mutagenicity, and thus, they present a larger threat to human health [31, 32]. Hence, it is essential to investigate the characteristics of humic and fulvic acids and their PAHs sorption behavior in order to control the formation of DBPs from humic and fulvic acids as well as the formation of halogenated PAHs.

Many methods to identify the structure of humic and fulvic acids have been reported in the literature. Among them, 13C NMR is the most common method to identify functional groups and molecular structures such as aromatic and aliphatic C groups. In contrast, elemental analysis, which is a faster method than the 13C NMR method, reflects the atomic ratios that relate to aromatic C such as H/C and the derived sources of humic substance such as N/C [33]. The humic and fulvic acids can also be compared on the basis of molecular weight (MW) [34]. In addition, spectroscopic techniques such as UV–visible (Vis) absorbance and fluorescence are widely employed because of their simplicity, rapid process, and non-requirement of pretreatment of samples. Specific UV absorbance (SUVA) at 254 nm is an indicator for aromatic C, while their ratio E4/E6, S275–295, and S350–400 confirmed its humification and aromaticity [35]. Moreover, the sources of humic substance and its relationship with MW distribution humic substance can be determined using fluorescence properties and their index [36]. Although many studies have examined the characteristics of humic substances and their binding with PAHs, there is still a lack of sufficient information on the characteristics of humic and fulvic acids and their differences, formation of their DPBs, and binding behavior of their PAHs.

In this present study, humic and fulvic acids were extracted from soils and characterized by using 13C NMR, elemental analysis, MW, UV, and fluorescence methods. The study also reported the different formation of THMs and HAAs and the PHE behavior of humic and fulvic acids due to their different characteristics.

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2. Study sites and characterization methods

2.1 Soil sampling sites and extraction method

For this study, soil samples were collected from six different locations in Korea, and a minimum of 20 km distance was maintained between the sampling sites. Four of these samples were representative of granite soils and were named as Gori (KR), Wolseong (WS), Uljin (UJ), and Yeonggwang (YK). Two soil samples were collected from the foot of Mt. Seorak (Goseong (KS)) and from Mt. Hallan (Jeju Island, volcanic ash soil) (Halla (HL)). Three soil samples, namely Elliott Silt Loam Soil (Cat No. 1BS102M), Canadian peat moss soil sample (sphagnum peat moss), and Aldrich HA (Sigma-Aldrich, CAS no. 1415-93-6), were purchased and named as IHSS, Peat, and AL, respectively.

Humic and fulvic acids were extracted from six sampling soils (KR, WS, UJ, YK, KS, and HL) and Peat, according to the IHSS method [37] and ISO 12782-4:2012 [38]. The extracted fulvic acids were purified using XAD resin concentration and then passed through a Dowex-50X8(H+) column. The AL sample was purified using an acid–base precipitation method [10]. Figure 1 presents the extraction and purification process of humic and fulvic acids.

Figure 1.

The extraction and purification process of humic and fulvic acids.

2.2 Characterization methods

The UV–Vis absorbance of humic and fulvic acids in the 200–800 nm range was investigated using a UV–Vis spectrometer (Shimadzu, UV-1601PC). To measure the dissolves organic matter (DOC), samples were prepared at pH 7.0 and then filtered using a 0.45-μm membrane filter (cellulose acetate, Advantec). The ratio of UV absorbance at 254 nm to the DOC concentrations of the samples was calculated to determine the SUVA values. The UV–Vis absorbance ratio at 465 and 665 nm (E4/E6) and spectral slope were applied to characterize the humic material as well as the aromaticity. The spectra slope was calculated using log-transform linear regression at intervals of 275–295 nm (S275–295) and 350–400 nm (S350–400). These two narrow bands were chosen for spectral slope calculation because they present the greatest variations from a variety of sources (i.e., marsh, riverine, estuarine, coastal, and open ocean).

A fluorescence spectrometer (Perkin Elmer LS50B) was used to obtain synchronous fluorescence spectra. It is known that fluorescence intensity can alternate with measurement time depending on external conditions such as humidity. Thus, the measured fluorescence intensities were normalized as units of quinine sulfate (QSE) equivalents based on the fluorescence of a diluted series of quinine sulfate dehydrate in 0.05 M sulfuric acid at an excitation/emission wavelength of 350/450 nm. Both the excitation and emission slits were fixed at 10. The difference between the emission wavelength and the excitation wavelength (Δλ) was fixed at 30 nm and then measured from 250 to 600 nml to determine synchronous fluorescence spectrum. The relative fluorescence regions were classified into four groups: protein-like (%PLF) fluorescence, fulvic-like (%FLF) fluorescence, humic-like (%HLF) fluorescence, and terrestrial humic-like (%THLF) fluorescence. These groups of fluorescence regions corresponded to the relative percentage of fluorescence intensity at wavelengths of 250–300, 300–380, 380–420, and 420–600 nm, respectively.

The apparent weight-average molecular weight (MWw) values were determined using size exclusion chromatography. The polydispersity of samples with the relative precision of MWw and MWn were less than 5% and 7%, respectively. The elemental composition ratios of humic and fulvic acids (C/H, N/C, and (N + O)/C) were determined using CHNS-932 and VTF-900 (LECO Co.).

The cross-polarization magic-angle spinning method was used to determine carbon structure via 13C NMR spectroscopy (Bruker Avance II, 500 MHz). The spectrum was measured at 300 K with a 90-pulse width of 4.5 s, 1.5 ms contact time, 3 ms pulse delay time, and 6.0 kHz spinning speed. A qualitatively good signal-to-noise ratio was obtained by using a total of 3 × 104 scan signal free induction decays and a line broadening function of 40 Hz. Then, the C functional groups were determined by integrating the area of the spectra in the chemical shift area: 0–50 ppm (alkyl C), 50–110 ppm (O-alkyl C), 110–145 ppm (C,H-aryl), 145–165 ppm (O-aryl phenol), and 165–190 ppm (carboxyl).

2.3 Chlorination of humic and fulvic acids and THMs/HAAs measurement

Humic and fulvic acids extracted from six sampling soils (KR, WS, UJ, YK, KS, and HL), Peat, AL, and IHSS were diluted to 1 mg C/L. Then, 1 ml phosphate buffer was added to 50 ml of diluted humic and fulvic acids to adjust their pH value to 7.0 ± 0.2. Then, the humic and fulvic acids were incubated for 2–3 h before chlorination. Chlorination of the humic and fulvic acids was conducted using the Aldrich’s sodium hypochlorite solution (available chlorine >4%) (NaOCl). The glassware required for the experiment was washed with acetone and then baked at 400°C for 1 h to remove any remaining organic matter. A constant dose of 5 mg Cl2/L was added to each sample for chlorination. The final solutions were sealed and stored in the dark at 25°C for 24 h. Then, a 10% sodium sulfite solution was injected into the solutions to suppress the formation of additional by-products by residual chlorine.

THMs and HAAs were analyzed using USEPA Method 551.1 and Method 552.3. A micro-electron capture detector (Agilent 6890 GC-ECD) was used to conduct gas chromatography of the liquid–liquid extracts in order to quantify the different THMs and HAAs. This was followed by diazomethane derivatization. Four species of THM, i.e., such as chloroform (CF), dichlorobromomethane (DCBM), dibromochloromethane (DBCM), and bromoform, were measured. HAAs were analyzed using the Drinking Water Quality Process Test Method (ES 05552.2.). Three substances, i.e., dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), and dibromoacetic acid (DBAA), were used in this analysis.

2.4 PHE adsorption experiment and analysis

PHE (purity >97%) was purchased from Aldrich and used without further purification. The stock solution (1.0 mg/L) was prepared by dissolving an excess in methanol to make a saturated solution. This solution was filtered through a 0.45-μm cellulose acetate membrane filter (Advantec). In this filter, the sorption of PHE is negligible. The solution was sterilized by adding 5 mM CaCl2 and 0.01 mM HgCl2, and its pH was adjusted to 6 by adding either 0.1 M NaOH or 0.1 M HCl. Then, 100, 90, 80, 70, 60, 40, and 30% stock solution was used with 20 mg of humin added in a 10-mL glass vial in order to perform the PHE adsorption experiments. The headspace was kept minimal to reduce the solute vapor loss and minimize the effect of surface adsorption. In addition, separately manufactured vial caps were used. The experiment was conducted using a rotator (at 30 rpm). Based on the preliminary tests for apparent equilibrium, the reaction time was set to 5 d. After the reaction, a centrifuge (5000 rpm, 15 min) was used to separate the supernatant and precipitate from each sample. The HPLC (YoungLin, UV730D) was used to measure PHE concentration in the supernatant. The mobile phase for HPLC was prepared using acetonitrile and ultrapure water (80:20 v/v) with a flow rate of 1.8 mL/min. A C18 4.6 × 150 mm reverse-phase column (Supelcosil LC-18DB) was used to perform separation analysis using a UV detector (at 254 nm).

A modified Freundlich adsorption isotherm Eq. (1) was used to analyze the adsorption results. The Freundlich equation is related to multi-layer and heterogeneous adsorption and is, thus, commonly applied to organic matter and hydrophobic pollutant adsorption [39]. The Freundlich adsorption constant (KFOC) and isotherm linearity constant (n) were derived from the slope and y-intercept, respectively, as per Eq. (1):

SOC=KFOC×Ce/CsclnE1

where SOC is the concentration of the PHE adsorbed on the humin (μg/kg C), Ce is the freely dissolved PHE concentration (μg/L), Cscl is the supercooled solubility of PHE at 25°C (5970 μg/L) in supercooled aqueous solution, and KFOC (μg/kg C) and n are the Freundlich adsorption model parameters (adsorption isotherm linearity increases as n increases). The single-point sorption is as follows:

KOC=SOCCi=KFOC×Cin−1/CsclnE2

From Freundlich sorption coefficient, the Gibbs energy change (△G) can be calculated as follows [40]:

ΔG°=−RTlnKE3

where T is the absolute temperature in kelvins, R is the gas constant (8.314 J.mol−1.K−1), and K is the Freundlich adsorption coefficient (KOC). The K value is recalculated as a dimensionless coefficient by multiplying it by 55.5 (number of moles of water per liter of solution) to correct the △G° values [41]:

ΔG°=−RTln55.5KE4
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3. Characteristics of humic and fulvic acids

The humic acids exhibited higher values of elemental composition of H/C and N/C ratios compared with those of fulvic acids for almost samples (Figure 2). Fulvic acids exhibited higher values of (N + O)/C and, thus, presented higher polarity than those values for humic acids (Figure 2). Similar to previous research works, aromatic C (110–165 ppm) in humic acids was higher than that in fulvic acids, whereas the aliphatic C (0–110 ppm) exhibited the opposite trend (Figure 3). In addition, only O-alkyl C proportions in fulvic acids presented higher values compared with those in humic acids for all soil samples, except for Peat and HL soil. In contrast, O-alkyl C, C,H-aryl C, and O-aryl C exhibited the opposite trend. Moreover, the carboxyl and carbonyl groups (165–210 ppm) in fulvic acids were higher compared with the values in humic acids for all soil samples. Higher aromatic C compounds (C,H-aryl and O-aryl phenol) in humic acids indicated higher amount of lignin and polyphenol from various plants [42]. In addition, the presence of higher O-alkyl and carboxyl groups such as peptides and organic acids indicated higher solubility of humic acids compared with fulvic acids [43]. Fulvic acids contain high carboxyl groups, and hence, the appearance of both COOH and –HC=CH– structure in these acids might affect the result of H/C [44]. Thus, fulvic acids exhibited lower H/C ratios as compared to the values of these ratios in humic acids. The MWw values of humic acids ranged from 2545 to 4411 Da and were higher than the values of fulvic acids (from 1751 to 2584 Da). In comparison with fulvic acid, humic acids presented higher polydispersity index (MWw/MWn), revealing a larger distribution of MW for humic acids. Thus, humic acids extracted from soils revealed higher H/C (affected by COOH and –HC=CH– structure), N/C, MWw, and MWw/MWn and lower polarity and O-alkyl C compared with fulvic acids. In particular, fulvic acids contained higher aliphatic C owing to their extremely higher values of O-alkyl C.

Figure 2.

Atomic ratios of humic acids (HA) and fulvic acids (FA) extracted from soils.

Figure 3.

13C NMR spectra of (a) humic acids (HA); and (b) fulvic acids (FA) extracted from soils.

SUVA, E4/E6, S275–295, and S350–400 were employed to identify the difference in spectroscopic spectra between humic and fulvic acids. Humic acids presented higher SUVA values than those of fulvic acids, whereas E4/E6 exhibited the opposite trend. The negative relationship between E4/E6 and the degree of condensation of the aromatic carbon network and/or the MW has facilitated the increased usage of the E4/E6 ratio in the identification of humification and aromaticity of soil organic matter [35, 45, 46]. The lower values of E4/E6 for humic acids are associated with the higher values of SUVA, aromatic carbon (110–165 ppm), and MWw. Of all soil samples, S350–400 presented higher values of SUVA, aromatic carbon (110–165 ppm), and MWw for fulvic acids than for humic acids, whereas the soil sample S275–295 did not present any trend. Thus, spectra slope at longer wavelengths could be used as an effective index to distinguish the dissolved organic matter between humic acids and fulvic acids.

Figure 4 shows the synchronous fluorescence spectra of soil humic acids and fulvic acids. As shown in this figure, soil humic acids presented higher peaks at THLF regions, whereas fulvic acids exhibited lower peaks at FLF and HLF regions. With respect to fluorescence relative distribution, compared with fulvic acids, humic acids presented lower %FLF and %HLF, but higher %THLF values. The humification index (HIX) also presented higher values for humic acids than for fulvic acids. Based on these findings, it can be said that soil humic acids were more condense with polymerized humic-like structure (higher SUVA, aromatic C, MWw, and %THLF), whereas fulvic acids contained high levels of carbonyl and quinone, aliphatic groups, and oxygen functional groups related to fulvic- and humic-like fluorescence materials (higher (N + O)/C, E4/E6, S350–400, %FLF, %HLF O-alkyl, and carboxyl groups). These specific different molecular characteristics between humic and fulvic acids are important as they can aid in investigating the structural changes, generation of DBPs under chlorination, and PHE sorption behavior.

Figure 4.

Fluorescence spectra of (a) humic acids*; and (b) fulvic acids extracted from soils. *Figure 4a was presented in research [20].

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4. DPBs formation and the structural changes of humic and fulvic acids extracted from soils

4.1 DBPs formation of humic and fulvic acids extracted from soils

For the comparison of DBPs formation between humic and fulvic acids extracted from soils, the concentrations of THM, HAA, and their species (μg/L) were normalized to DOC (mg/L) and named as specific THM/HAA formation potential (STHMFP/SHAAFP) and CF, DCBM, DBCM, DCAA, TCAA, and DBAA. It was found that the formation of STHMFP and SHAAFP significantly differed between humic acids and fulvic acids. Humic acids exhibited higher STHMFP values that ranged from 141.9 to 194.6 μg/mg compared with the lower values in fulvic acids (98.7 to 100.6 μg/mg). In both humic acids and fulvic acids, chloroform was the most dominant compound of STHMFP, accounting for over 95% of STHMFP. Similar to the trend of the values of STHMFP, SHAAFP values for humic acids were found to be 5.5 times higher (ranging from 259.0 to 390.0 μg/mg) compared with the SHAAFP values for fulvic acids (ranging from 19.8 to 54.9 μg/mg). TCAA was the most abundant (83%) SHAAFP species in humic acids, whereas it accounted for only 17% in fulvic acids. In humic acids, SHAAFP (presenting as TCAA) presented higher values than STHMFP (presenting as CF). In contrast, STHMFP presented much higher values than SHAAFP for fulvic acids. During chlorination, humic molecules having higher aromatic content, such as humic acids, first reacted with chlorine to form TCAA and then form CF. This is the reason why TCAA production was higher than CF production [47]. In addition, fulvic acids contained alkyl, carbohydrate, and carboxyl groups that were less capable of producing CF and TCAA under chlorination than the aromatic carbon components [48]. Thus, the difference in DPB formation between humic and fulvic acids might be explained by the differences in their molecular structure.

4.2 Changes in spectroscopic characteristics and TOC and their relationship after chlorination

After chlorination, both humic and fulvic acids exhibited lower SUVA and TOC values. TOC removal presented higher values for fulvic acids. This demonstrated the presence of more organic carbon components in fulvic acids that can be easily mineralized to CO2 during chlorination as compared to humic acids. During chlorination, 50–80% of chlorine oxidated humic substances into CO2, whereas only 5–10% of chlorine participated in the incorporation reaction to form DBPs [49]. SUVA presented higher reduction values for humic acids than for fulvic acids. This indicated that high aromatic C (presented by high SUVA values) were transformed into lower ones (lower SUVA) by splitting the aromatic rings and decomposing the unsaturated carbon rather than converting it into CO2. After chlorination, humic and fulvic acids showed different changes in terms of fluorescence relative distribution. Moreover, after chlorination, only %THLF values decreased for humic acids, whereas %FLF and %THLF reduced for fulvic acids. The values of Δ%FLF, Δ%HLF, and Δ%THLF were calculated based on the differences in each relative distribution before and after chlorination (FLbefore − FLafter). These values were used to further examine the mechanisms of chlorination for humic and fulvic acids.

The relationship between the changes in humic/fulvic acid characteristics (TOC removal, SUVA removal, Δ%FLF, Δ%HLF, and Δ%THLF) due to chlorination and their original structural characteristics (SUVA, MW, C,H-aryl, and O-alkyl) can clarify the chlorination-induced structural changes in humic and fulvic acid in more depth. SUVA values were found to be strongly positively correlated with SUVA removal for both humic and fulvic acids (p < 0.05). Moreover, SUVA values were strongly negatively correlated with TOC removal values for humic acids (p < 0.01) and non-significantly correlated with TOC removal values for fulvic acids (p > 0.1). The higher SUVA values and aromatic carbon contents of humic acids than those of fulvic acids resulted in the reaction of more aromatic carbon compounds in humic acids with chlorine. This led to the production of higher DBP. In addition, for humic acids, higher MWw materials with richer N groups and a higher proportion of O-alkyl C reacted with chlorine to yield higher reductions of TOC and FLF components. In comparison, lower MWw materials with a higher percentage of C,H-aryl and O-aryl phenol and fewer N groups were associated with a higher reduction in SUVA and THLF values. In other words, high-MWw humic acids with aliphatic properties, high nitrogen content, and a low degree of unsaturation mostly reacted with chlorine via an oxidation reaction. In comparison, low-MWw humic acids with high aromatic C and low nitrogen content mainly reacted with chlorine via incorporation.

4.3 Specific relationship between DPBs and humic and fulvic acid characteristics

The correlations between STHMFP and humic/fulvic acid characteristics determined the effects of the molecular structures of humic/fulvic acids on DBP production. For humic acids, STHMFP was positively correlated with C,H-aryl, O-aryl phenol, and alkyl C (p < 0.05) but negatively correlated with N/C ratio, MWw, TOC removal, Δ%FLF, O-alkyl C, and carboxyl (p < 0.05). In summary, low-MW aromatic C such as C,H-aryl, O-aryl phenol, and aliphatic C such as alkyl C in humic acids were considered as DBP precursors as they generated STHMFP during the chlorination reaction. On the contrary, high N groups of O-alkyl C in humic acids participated in oxidation reactions during chlorination. For fulvic acids, STHMFP presented a strong positive correlation with alkyl C and negative correlations with C,H-aryl and O-aryl phenol (p < 0.05). The chlorination mechanism of fulvic acids shown in our study differed from those reported in previous studies that demonstrated that aromatic C, especially phenol components, were associated with the highest value of STHMFP [50, 51]. The reasons behind these notable findings can be explained as follows. First, unlike those in the humic acids, the aromatic contents in fulvic acids might not play a major role in the generation of STHMFP via incorporation. Second, the presence of N groups combined with aromatic molecules in the fulvic acids resulted in the formation of N-DBPs, and not in the generation of STHMFP and SHAAFP [52]. In previous studies, amino acids such as aspartic acids and asparagine acids in humic substances were also reported to yield N-DBPs (i.e., dichloroacetonitrile and halonitromethanes) during chlorination [16, 53]. In the present study, there was no relationship between the SHAAFP and HAA species and the humic/fulvic acid characteristics. In summary, the low-MW and low-N/C components of aromatic C and alkyl C in the humic acids might form STHMFP via incorporation reactions. In comparison, only low-N/C aliphatic compounds such as alkyl generated STHMFP in the fulvic acids. This detail will be helpful in elucidating the formation of aromatic/aliphatic N-DBPs for humic and fulvic acids in future. MWw and Δ%THLF were the appropriate factors for predicting the values of STHMFP in humic acids. In comparison, Δ%FLF and Alkyl C were applied for fulvic acids.

In particular, this study presented a new interpretation of differences between humic/fulvic acids in terms of molecular structure characteristics and chlorine reaction, including oxidation and incorporation reaction, and successfully provided sufficient factors to predict THM generation.

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5. Phenanthrene adsorption isotherm of humic and fulvic acids

5.1 Comparison of phenanthrene adsorption isotherm for humic and fulvic acids

All the experimental data of humic and fulvic acids fit well with the Freundlich model, presented by the high degree of correlations (R2 > 0.99). Also, for both humic and fulvic acids, all the sorption isotherms were non-linear (n < 1). For humic acids, PHE adsorption coefficient (KOC) ranged from 3.7 × 104 to 7.0 × 104 mL/g, while for fulvic acids, it ranged from 1.4 × 104 to 2.2 × 104 mL/g. The higher sorption affinity of humic acids might be explained by the higher aromatic and condense humic structure. High isotherm linearity (n) values were associated with a low degree of natural organic matter maturation and less heterogeneous sorption-site energy distribution [54]. The n values of humic acids were lower than those of fulvic acids. In humic acids, the hydrophobic nature enhanced more coiled or aggregated structures to provide specific non-ideal binding sites [55]. In addition, high polarity might reduce the sorption affinity [56]. The Gibbs free energy values were negative for both humic and fulvic acids at 20°C, revealing the thermodynamically favorable and spontaneous adsorption process [57].

5.2 Correlations between humic and fulvic acid structural characteristics and phenanthrene adsorption isotherm

For humic acids, sorption coefficient (logKOC) was positively related to HIX and negatively correlated with S350–400. In comparison, logKOC of fulvic acids presented positive correlation with %THLF and negative relationship with %FLF and E4/E6. No relationship was observed between logKOC and humic/fulvic acid relative carbon distribution. High sorption affinity was found to be positively related to the condense structure with high aromatic C and humification, presented by the high values of HIX and %THLF and low values of %FLF and E4/E6. For humic acids, n presented positive correlation with H/C, %FLF, and %HLF and negative relationship with SUVA and %THLF. However, no relationship was observed between n values and their structural characteristics for fulvic acids. The UV spectroscopic and fluorescence characteristics of humic and fulvic acids were primarily related to their sorption ability. In a previous study, PAH sorption ability was reported to be closely related to 3D fluorescence [58].

The principal component analysis was applied for 18 selected parameters in order to interpret the specific PHE binding behavior of humic and fulvic acids. The first two principal components (PCs) explained approximately 59.32% for PC1 and 14.70% for PC2 (Figure 5). PC1 was interpreted as a factor associated with the PHE sorption behavior, whereas PC2 presented for C,H-alkyl, H/C, and free energy. Thus, logKoc was found to be positively related to MWw, SUVA, HIX, %THLF, and Mw/Mn. Moreover, n values were positively correlated with %FLF, %HLF, E4/E6, and O-alkyl (Figure 5a). From the factor score plot presented in Figure 5b, the PHE sorption behavior of humic and fulvic acids was found to be completely different. Humic acids presented higher MWw, Mw/Mn, SUVA, and %THLF, which were related to sorption affinity (logKoc). In comparison, fulvic acids were related to higher n and fulvic−/humic-like components, lower degree of condensation of the aromatic carbon, and the MW (higher E4/E6 and O-alkyl). In summary, UV and fluorescence characteristics are powerful techniques to determine the PHE sorption behavior of humic and fulvic acids extracted from soils.

Figure 5.

(a) Factor loading plot for selected structural characteristics and PHE sorption of humic and fulvic acids as the first two principal components; and (b) factor score plot for humic and fulvic acids as the first two principal components.

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6. Conclusion

In this chapter, the structure characteristics of humic and fulvic acids extracted from soils were clarified. Soil humic acids were found to be more condensed and have a polymerized humic-like structure (higher SUVA, aromatic C, MWw, and %THLF). In comparison, fulvic acids contained high levels of carbonyl and quinone, more aliphatic groups, and more oxygen functional groups related to fulvic- and humic-like fluorescence materials (higher (N + O)/C, E4/E6, S350–400, %FLF, %HLF O-alkyl, and carboxyl groups). The differences in the molecular characteristics between humic and fulvic acids resulted in the differences in the generation of DBPs under chlorination and the PHE sorption behavior. For chlorination, the low-MW and low-N/C components of aromatic C and alkyl C in the humic acids might form STHMFP via incorporation reactions. However, only low-N/C aliphatic compounds such as alkyl generated STHMFP in the fulvic acids. Humic acids presented higher sorption coefficient (logKoc) because of the presence of higher MWw, Mw/Mn, SUVA, and %THLF. In comparison, fulvic acids are related to higher n and fulvic−/humic-like components and lower degree of condensation of the aromatic carbon and the MW. UV and fluorescence characteristics are powerful techniques to indicate the PHE sorption behavior of humic and fulvic acids extracted from soils. Compared with atomic ratio and relative C distribution, the UV and fluorescence characteristics approach provides the key information for water system managers to better predict and mitigate the formation of DBPs in chlorine-treated water and the behavior of hydrophobic organic contaminants in aquatic environment.

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Acknowledgments

This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. NRF-2020R1A6A1A03042742), and by the Korea Environment Industry & Technology Institute (KEITI) through the program for the management of aquatic ecosystem health, funded by the Korea Ministry of Environment (MOE) (Grant No. 2020003030005).

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Conflict of interest

The authors declare no conflict of interest.

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Written By

Hang Vo-Minh Nguyen, Jin Hur and Hyun-Sang Shin

Submitted: 04 May 2022 Reviewed: 23 May 2022 Published: 17 June 2022

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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