Q. Wang
Please Note
3 records found
1
It was widely acknowledged that dissolved organic matter (DOM) in natural water has ubiquitous competitiveness against organic micropollutants (OMPs) during adsorption onto activated carbon. However, some (model) low molecular weight organics have been reported to adsorb onto activated carbon, but were not competitive against co-adsorbates. The objective of this study is to identify which adsorbable DOM fractions in natural water contribute to the DOM competitiveness, and what is the impact of the OMP adsorbability and initial OMP concentration on this competitiveness. We, therefore, disassociated the adsorption of DOM fractions and OMPs (carbamazepine, caffeine and sulfamethoxazole) using a two-stage adsorption procedure, removing various adsorbable DOM fractions with powdered activated carbon pretreatment and then unraveling the competitiveness against OMPs of the remaining DOM. Our results demonstrated that DOM competition was not ubiquitous for all adsorbable fractions in natural water, and ∼ 25% of the adsorbable DOM was not competitive. The poorly adsorbable DOM was shown to be a non-competitive co-adsorbate, and its complexation even elevated the adsorption capacity of one of the OMPs (carbamazepine). The amount of DOM competitors increased for weaker adsorbable OMPs, and at higher initial OMP concentrations. The variability in DOM competition, differentiated by DOM adsorbability, has advanced the understanding of DOM competition, from ubiquitous competition to variable roles (varying competitiveness/complementary adsorption) of differently adsorbable DOM fractions during OMP adsorption.
Model DOM compounds (mDOMs) could be described individually and more accurately with molecular properties than a complex, real DOM matrix in water. To elucidate the impact of LMW DOM characteristics (hydrophobicity/polarity and aromaticity) on DOM competitiveness, fifteen model compounds (mDOM), differed in functional groups (hydroxyl, phenol, carboxyl groups, etc.), were used to represent several elemental structures of LMW DOM. By temporarily occupying adsorption sites prior to OMPs, LMW mDOM was found to be more competitive to inhibit OMP adsorption kinetics than OMP adsorption equilibrium. Although OMPs were more preferentially adsorbed onto activated carbon than mDOM, the large concentration asymmetry (~500 μg DOC/μg OMP) made a few mDOM compounds exert strong competition against OMPs. The mDOM competitiveness increased when compounds were more hydrophobic and more aromatic, whereas π-π interactions were more important to determine mDOM competitiveness than hydrophobic interaction for LMW mDOM compounds. As an integrated indicator, mDOM adsorbability, defined by mDOM adsorption capacity, was considered better to associate with mDOM competitiveness than hydrophobicity and aromaticity individually. The competition was found to be strong between strongly adsorbable mDOM and weakly adsorbable OMPs, where weakly adsorbable mDOM could even co-adsorb with strongly adsorbable OMPs with little to no competition.
To relate DOM adsorbability to competitiveness in natural waters, a two-stage adsorption procedure was designed to differentiate the adsorption of DOM fractions and OMPs by removing variously adsorbable DOM fractions with activated carbon pretreatment and analyzing the competitiveness of the remaining DOM fractions. Our results demonstrated that adsorbable (LMW) DOM was not necessarily competitive against OMPs. In addition, an increasing amount of DOM competitors was observed against the weaker adsorbable OMPs, compared to their stronger adsorbable counterparts. Similarly, more DOM competitors were identified at high initial OMP concentrations, due to the increased loading of OMPs on activated carbon, highlighting the variable roles (varying competitiveness/ complementary adsorption) of differently adsorbable DOM fractions in competition.
To elucidate the role of molecular weight (MW), polarity and aromaticity in DOM competition from a natural DOM with a complex molecular composition, activated carbon and anion exchange resin (AER) pretreatment served for differentiating competitive DOM from natural DOM. Ultrahigh-resolution Fourier transform mass spectrometry was employed for the DOM analysis at the molecular level. A large percentage of molecular formulas in untreated DOM was PAC-absorbable (97.8% for 40 mg PAC/L), while ~75% of PAC-absorbable formulas were considered poorly competitive, since these molecular formulas were not detected in DOM remaining after AER pretreatment that was highly competitive. The semi-quantitative analysis revealed that aromaticity was the dominant factor for LMW DOM adsorbability and competitiveness. In contrast, with higher MW, the competitiveness of an increasing number of aromatic DOM compounds was diminished due to strong dissociation induced by relatively high polarity.
Finally, the interference of ozone-modified NOM with the adsorption of 2-methylisoborneol (MIB, an odorous OMP) was studied in three natural waters and one standard humics solution in order to study how ozonation influences the competitiveness of DOM with different MW distributions. In the three natural waters, it was found that reducing NOM competition against MIB was found to coincide with increasing ozone consumption. The cleavage of the macromolecules in a standard humics solution, with larger molecular weight and higher aromaticity than the humics in natural waters, only induced a slightly stronger competition under low/moderate ozone consumptions. Overall, the declined aromaticity outweighed the produced LMW DOM in the competitiveness of DOM against MIB in ozonated natural waters. The UV absorbance of the LMW DOM was better correlated with the competitiveness of ozonated/non-ozonated waters than the LMW DOM concentration itself, underlining the role of LMW hydrophobic aromatics in competitive adsorption prediction.
From this thesis, it could thus be concluded that the DOM competition against OMPs is not ubiquitous for all (adsorbable, LMW) DOM fractions. The amount of DOM competitors, as well as their competitiveness, strongly varies with to OMP adsorbability and the initial OMP concentrations (i.e., the concentration asymmetry). For LMW DOM, aromaticity was a key characteristic to promote DOM competitiveness, while the high polarity reduced the DOM competitiveness by DOM dissociation (and thus high hydrophilicity/polarity). To project the competitiveness of ozonated DOM of which the hydrophobicity and aromaticity were simultaneously diminished, LMWUV can then be a handy DOM surrogate instead of LMW DOM concentration itself.
...
Model DOM compounds (mDOMs) could be described individually and more accurately with molecular properties than a complex, real DOM matrix in water. To elucidate the impact of LMW DOM characteristics (hydrophobicity/polarity and aromaticity) on DOM competitiveness, fifteen model compounds (mDOM), differed in functional groups (hydroxyl, phenol, carboxyl groups, etc.), were used to represent several elemental structures of LMW DOM. By temporarily occupying adsorption sites prior to OMPs, LMW mDOM was found to be more competitive to inhibit OMP adsorption kinetics than OMP adsorption equilibrium. Although OMPs were more preferentially adsorbed onto activated carbon than mDOM, the large concentration asymmetry (~500 μg DOC/μg OMP) made a few mDOM compounds exert strong competition against OMPs. The mDOM competitiveness increased when compounds were more hydrophobic and more aromatic, whereas π-π interactions were more important to determine mDOM competitiveness than hydrophobic interaction for LMW mDOM compounds. As an integrated indicator, mDOM adsorbability, defined by mDOM adsorption capacity, was considered better to associate with mDOM competitiveness than hydrophobicity and aromaticity individually. The competition was found to be strong between strongly adsorbable mDOM and weakly adsorbable OMPs, where weakly adsorbable mDOM could even co-adsorb with strongly adsorbable OMPs with little to no competition.
To relate DOM adsorbability to competitiveness in natural waters, a two-stage adsorption procedure was designed to differentiate the adsorption of DOM fractions and OMPs by removing variously adsorbable DOM fractions with activated carbon pretreatment and analyzing the competitiveness of the remaining DOM fractions. Our results demonstrated that adsorbable (LMW) DOM was not necessarily competitive against OMPs. In addition, an increasing amount of DOM competitors was observed against the weaker adsorbable OMPs, compared to their stronger adsorbable counterparts. Similarly, more DOM competitors were identified at high initial OMP concentrations, due to the increased loading of OMPs on activated carbon, highlighting the variable roles (varying competitiveness/ complementary adsorption) of differently adsorbable DOM fractions in competition.
To elucidate the role of molecular weight (MW), polarity and aromaticity in DOM competition from a natural DOM with a complex molecular composition, activated carbon and anion exchange resin (AER) pretreatment served for differentiating competitive DOM from natural DOM. Ultrahigh-resolution Fourier transform mass spectrometry was employed for the DOM analysis at the molecular level. A large percentage of molecular formulas in untreated DOM was PAC-absorbable (97.8% for 40 mg PAC/L), while ~75% of PAC-absorbable formulas were considered poorly competitive, since these molecular formulas were not detected in DOM remaining after AER pretreatment that was highly competitive. The semi-quantitative analysis revealed that aromaticity was the dominant factor for LMW DOM adsorbability and competitiveness. In contrast, with higher MW, the competitiveness of an increasing number of aromatic DOM compounds was diminished due to strong dissociation induced by relatively high polarity.
Finally, the interference of ozone-modified NOM with the adsorption of 2-methylisoborneol (MIB, an odorous OMP) was studied in three natural waters and one standard humics solution in order to study how ozonation influences the competitiveness of DOM with different MW distributions. In the three natural waters, it was found that reducing NOM competition against MIB was found to coincide with increasing ozone consumption. The cleavage of the macromolecules in a standard humics solution, with larger molecular weight and higher aromaticity than the humics in natural waters, only induced a slightly stronger competition under low/moderate ozone consumptions. Overall, the declined aromaticity outweighed the produced LMW DOM in the competitiveness of DOM against MIB in ozonated natural waters. The UV absorbance of the LMW DOM was better correlated with the competitiveness of ozonated/non-ozonated waters than the LMW DOM concentration itself, underlining the role of LMW hydrophobic aromatics in competitive adsorption prediction.
From this thesis, it could thus be concluded that the DOM competition against OMPs is not ubiquitous for all (adsorbable, LMW) DOM fractions. The amount of DOM competitors, as well as their competitiveness, strongly varies with to OMP adsorbability and the initial OMP concentrations (i.e., the concentration asymmetry). For LMW DOM, aromaticity was a key characteristic to promote DOM competitiveness, while the high polarity reduced the DOM competitiveness by DOM dissociation (and thus high hydrophilicity/polarity). To project the competitiveness of ozonated DOM of which the hydrophobicity and aromaticity were simultaneously diminished, LMWUV can then be a handy DOM surrogate instead of LMW DOM concentration itself.
Though the ozone-activated carbon process has been widely applied for drinking water purification, little is known about how ozone-modified natural organic matter (NOM) competes with micropollutants in activated carbon adsorption. In this study, three natural waters and one synthetic water (standard humics solution) with highly heterogeneous NOM compositions were employed to investigate the interference of ozonated NOM with the adsorption of 2-methylisoborneol (MIB). Analysis using liquid chromatography with online carbon and UV254 detection (LC-OCD-UVD) revealed that ozonation led to various disintegration patterns of macromolecules in NOM, and UV absorbance was reduced markedly for nearly all NOM fractions. Powdered activated carbon (PAC) adsorption experiments showed that increasing ozone consumption coincided with reducing NOM competition against MIB in the three natural waters, as expressed by the fitted initial concentrations of the equivalent background compound (c0,EBC). In the synthetic water, in contrast, competition increased under low/moderate specific ozone consumptions and then decreased with further elevation of ozone consumptions. Regarding the significance on affecting ozonated NOM interference, aromaticity reduction outweighed formation of low molecular weight (LMW) organics in most cases, enhancing MIB adsorption capacity. However, disintegration of the humics fraction with larger molecular weight (1,103 g/mol, as compared to 546–697 g/mol in three natural waters) into smaller, more competitive fractions caused the observed initial deteriorated MIB adsorption in synthetic water. A superior correlation between c0,EBC and the UV absorbance of LMW organics (R2 = 0.93) over concentrations of LMW organics underlined the importance of the aromatic properties in competitive adsorption projection for ozone pretreated natural waters. Furthermore, the change of relative concentration of UV absorbing compounds during ozonation could help estimate the decrease of c0,EBC, which could be a promising tool for waterworks to adjust PAC doses for MIB removal in ozonated waters.