Evaluating Activated Carbon Adsorption of Dissolved Organic Matter

Jan 24, 2017 - ... and Architectural Engineering, 428 UCB, University of Colorado, .... Current Opinion in Environmental Science & Health 2018 2, 12-1...
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Evaluating activated carbon adsorption of DOM and micropollutants using fluorescence spectroscopy Kyle Koyu Shimabuku, Anthony M. Kennedy, Riley E. Mulhern, and R. Scott Summers Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04911 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 29, 2017

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Evaluating activated carbon adsorption of DOM and micropollutants using fluorescence spectroscopy , Kyle K. Shimabuku*,†, Anthony M. Kennedy† ‡, Riley E. Mulhern†, and R. Scott Summers†



Department of Civil, Environmental and Architectural Engineering, 428 UCB, University of Colorado, Boulder, Boulder, CO 80309, USA



Technical Service Center, US Bureau of Reclamation, Denver Federal Center Building 67, Denver, CO, 80225, USA * Corresponding author: [email protected]

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Abstract Dissolved organic matter (DOM) negatively impacts granular activated carbon

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(GAC) adsorption of micropollutants and is a disinfection by-product precursor. DOM from

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surface waters, wastewater effluent, and 1 kDa size fractions was adsorbed by GAC and

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characterized

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chromatography (SEC). Fluorescing DOM was preferentially adsorbed relative to UV-absorbing

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DOM. Humic-like fluorescence (peaks A and C) was selectively adsorbed relative to

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polyphenol-like fluorescence (peaks T and B) potentially due to size exclusion effects. In the

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surface waters and size fractions, peak C was preferentially removed relative to peak A, whereas

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the reverse was found in wastewater effluent, indicating that humic-like fluorescence is

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associated with different compounds depending on DOM source. Based on specific UV-

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absorption (SUVA), aromatic DOM was preferentially adsorbed. The fluorescence index (FI), if

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interpreted as an indicator of aromaticity, indicated the opposite, but exhibited a strong

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relationship with average molecular weight suggesting FI might be a better indicator of DOM

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size than aromaticity. The influence of DOM intermolecular interactions on adsorption were

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minimal based on SEC analysis. Fluorescence parameters captured the impact of DOM size on

using

fluorescence

spectroscopy,

UV-absorption,

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size

exclusion

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the fouling of 2-methylisoborneol and warfarin adsorption, and correlated with direct

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competition and pore blockage indicators.

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Introduction

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Dissolved organic matter (DOM) is ubiquitous in natural and anthropogenically-impacted

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waters such as surface water and wastewater and plays an important role in environmental and

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engineered aquatic systems. For instance, it can serve as an energy source for bacteria, attenuate

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light, and influence the fate and transport of contaminants. DOM can also adversely impact water

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and wastewater treatment processes (e.g., by consuming disinfectants and fouling membranes)

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and is a disinfection by-product precursor.1-3

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Granular activated carbon (GAC) is used in water and wastewater treatment plants to

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remove DOM, as well as specific organic chemicals present at sub part per billion concentrations

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defined herein as micropollutants. Both DOM concentration, typically measured as dissolved

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organic carbon (DOC), and character influence its adsorbability as well as the extent to which it

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decreases micropollutant adsorption through direct competition and pore blockage, collectively

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termed fouling. The dependence of both DOC and micropollutant breakthrough on initial DOC

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concentration (DOC0) has been successfully modeled in previous studies.4,5 However, it is more

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difficult to model how DOM quality influences its adsorption behavior (i.e., adsorbability and

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micropollutant fouling) because DOM is a heterogeneous mixture of compounds that vary in

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physical and chemical properties. In addition, DOM indicators that capture the impact of DOM

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character on its adsorption behavior, particularly in dynamic flow-through systems, are still

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needed. Several studies have shown that DOM molecular size can determine its adsorption

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behavior.6-9 However, aromaticity and polarity can also be important,10 and it remains unclear

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which characteristic(s) governs DOM adsorption behavior. Also, the extent to which DOM

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interactions (e.g., supramolecular association) affect DOM adsorption remains unclear.

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Fluorescence spectroscopy is one technique that is growing in popularity to characterize

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DOM as it is simple and provides a large amount of information. Molecular size, aromaticity,

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and the presence of specific classes of DOM components have been related to different

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fluorescence parameters such as the peak C intensity to UV-absorption ratio (see Table S1),

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fluorescence index (FI), and fluorescence peaks, respectively.11-13 However, the physicochemical

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properties associated with many fluorescence indices are still unknown. GAC is mainly

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composed of graphite-like sheets that form a heterogeneous, microporous network, and it serves

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as a largely hydrophobic, highly conjugated sieve that simultaneously separates DOM

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components by molecular size and chemical characteristics such as aromaticity.14 However,

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GAC can exhibit variable pore size distributions and surface functional groups and could, e.g.,

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separate DOM as it passes through a column by selectively adsorbing DOM that does not

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experience electrostatic repulsion. Thus, fractionating DOM with GAC provides a unique

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method to characterize the physicochemical properties of fluorescence parameters compared to

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studies that have employed size exclusion chromatography (SEC) and/or high performance

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liquid chromatography (HPLC) where DOM is separated solely by molecular size or

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hydrophobicity, respectively.

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There is growing interest in using fluorescence to characterize DOM adsorption behavior.

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Recent studies have found that fluorescence metrics can be used to predict DOM fouling,15-17

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fluorescing DOM can be more adsorbable than non-fluorescing DOM,15,18 and PAC can

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selectively remove DOM that fluoresces at longer wavelengths.19 Previously, we showed that

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fluorescence parameters correlated with the competitive nature of DOM in batch-powdered

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activated carbon (PAC) systems.17 However, whether or not fluorescence indicators can also be

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used in flow-through systems to capture the influence of DOM character on micropollutant

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fouling is unknown considering that in batch-PAC systems micropollutants and DOM are

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simultaneously exposed to PAC, while in GAC adsorbers DOM can preload adsorption sites.

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The objectives of this study were to: (i) assess the preferential adsorption of fluorescing

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DOM components by GAC, (ii) relate physicochemical and fluorescence properties of DOM,

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(iii) examine if DOM intermolecular interactions influence DOM adsorption by GAC, (iv)

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develop relationships between fluorescence indices with DOC and micropollutant breakthrough,

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and (v) compare the significance of DOM molecular size and aromaticity on DOM adsorption

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and micropollutant fouling. The findings presented here provide new interpretations of

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fluorescence metrics and DOM adsorption phenomena. They also demonstrate that fluorescence

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metrics, which can be determined with on-line sensors or conventional analyzers that

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characterize discrete samples, could be used in the future to predict and monitor the control of

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micropollutants and DOM by GAC. This study focused on the adsorption behavior of surface

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water DOM fractioned with a 1 kDa ultrafilter and the unfractionated DOM, which were also

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examined in a companion study by Kennedy and Summers.20 These waters were chosen because

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they vary distinctly in molecular size and aromaticity. Supplementary experiments were

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conducted with additional surface waters and a wastewater effluent, and in batch-mode with

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three different PACs.

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Materials and Methods

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Waters. Water collected from a non-wastewater impacted mountain reservoir at Big Elk

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Meadows (BEM), CO, which has high DOC (~15 mg/L), low alkalinity (~21 mg/L as CaCO3),

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and low conductivity (~119 µS/cm), was concentrated using reverse-osmosis. If a surface water

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is wastewater impacted is distinguished here because wastewater effluent has a significant

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influence on fluorescence signatures.21,22 The reverse-osmosis concentrate was fractionated using

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a tangential flow ultrafilter (Amicon regenerated cellulose 1 kDa cartridge) into 1

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kDa nominal molecular size fractions. Nominal should be emphasized because DOM that is 1 kDa can be detected both in the 1 kDa ultrafilter retentate and permeate (Figure S1).

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The reverse-osmosis concentrate was also coagulated and used to create two matrices, one only

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spiked with the micropollutants 2-methylisoborneol (MIB) and warfarin (described below) and

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the other was spiked with 27 different micropollutants described in Table S2. The total mass

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concentration of the micropollutants was 3.0 ± 0.9 µg/L. The 27 micropollutants were not

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directly monitored in this study, but were added to make a representative anthropogenic-

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impacted water.23,24 Details describing the isolation, coagulation, and ultrafiltration conditions

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are provided in the Supporting Information (SI).

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The unfractionated concentrate, the 1 kDa fraction, the coagulated

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concentrate, and the micropollutant-spiked coagulated concentrate were each diluted into

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reverse-osmosis treated tap water to target a DOC of 4 mg/L and are designated herein as BEM,

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1K BEM, CB, and CBMP, respectively. Tertiary wastewater effluent collected prior

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to UV disinfection is designated herein as WWef. Water was also collected after alum

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coagulation from the Betasso Water Treatment Plant in Boulder, Colorado, which treats a non-

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wastewater impacted source water and is designated herein as Betasso. Influent water quality

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characteristics are provided in Table 1.

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Target adsorbates. Influent concentrations for micropollutants MIB and warfarin were

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107±3 and 100±6 ng/L, respectively. MIB and warfarin (pKa = 4.5) were predominately neutral

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and anionic, respectively. 14C MIB and 3H warfarin were obtained from American Radiolabeled

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Chemicals, Inc. (St. Louis, MO). They were analyzed with liquid scintillation counting, which

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yielded method detection limits of 19 ng/L for MIB and 8 ng/L for warfarin as detailed in the SI.

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GAC column tests. Rapid small-scale column tests were used to evaluate GAC

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adsorption, and details are provided in the SI. >1K BEM, 1K BEM, 1K BEM fraction, 1K BEM, 1K BEM, 1K BEM, 380 nm in an HPLC.37 Korak et al. showed that Suwanee River NOM,

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Suwanee River Fulvic Acid, and Suwanee River Humic Acid fluoresced in the peak A and C

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regions, though the relative intensities of peak A and C varied in each water.38 Therefore, it is

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more likely that there are different fluorescing substances that exhibit unique peak A/C ratios and

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adsorbabilities, and compounds with smaller A/C ratios are more adsorbable in the surface

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waters and size fractions, while the opposite is true for the WWef.

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The preferential removal of the compounds with different peak A/C ratios in GAC

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columns can be attributed to differences in their molecular size or chemical properties (e.g.,

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aromaticity and/or hydrophobicity). Li et al. showed that peaks A and C simultaneously eluted in

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an HPLC for a surface water and a wastewater effluent,36 which suggests that compounds with

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different A/C ratios exhibit similar polarities. Thus, the compounds in the surface waters that had

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higher peak A/C ratios might be larger and experience more size exclusion than compounds with

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smaller A/C ratios, and the reverse could be true for the WWef. However, Li et al. also found

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that fluorescence intensity in regions near peaks A and C simultaneously eluted using SEC. The

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reason that GAC could fractionate DOM components with different A/C ratios and Li et al. was

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unable to do so with SEC and HPLC could be that GAC can separate DOM through both size

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exclusion and adsorption. For instance, it is conceivable that when GAC adsorbs DOM,

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intermolecular bonds in DOM supramolecular structures are disrupted, which SEC is unable to

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provoke.36 These separated DOM molecules that have different A/C ratios might exhibit

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different MWs and experience varying degrees of size exclusion, which could cause these

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compounds to breakthrough at different rates. Such separations would not happen in an HPLC if

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these compounds had similar polarities.

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DOM breakthrough for the seven waters as assessed by the ratio of the sum of peaks T+B

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to the sum of peaks A+C is shown in Figure 2 and Figure S6. For each water, the peaks

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T+B/peaks A+C ratio values initially broke through above the influent peaks T+B/peaks A+C

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ratio values indicating that peaks T+B are less adsorbable by GAC than peaks A+C in waters

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that vary in MW and source. Several studies have found that the peaks in the T+B regions exhibit

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greater hydrophobicities than peaks in the A+C regions in both surface waters and

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wastewater.36,37,39 According to Li et al., peaks A+C could be more hydrophilic because they are

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dominated by humic substances that are anionic and, therefore, hydrophilic at circumneutral pH

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whereas compounds associated with peaks B+T (e.g., tannins) can be neutral and

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hydrophobic.36,40 However, because peaks A, B, C, and T can be associated with different classes

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of compounds (see below), their hydrophobicities could vary depending on the water source.

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Thus, further research is needed to confirm if peaks T+B are more hydrophobic than A+C

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regardless of the surface water or wastewater effluent. If peaks B+T are more hydrophobic than

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A+C here, the faster peak T+B breakthrough suggests that they are associated with larger DOM

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compounds that experience more size exclusion because GAC preferentially adsorbs small,

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hydrophobic compounds. Studies using SEC with fluorescence detection have found that peaks

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in the T+B regions exhibit greater MWs than peaks in the A+C regions.39,41

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While peaks A+C and peaks T+B are often referred to as humic-like and protein-like,

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respectively, DOM components that do not fit into these classifications can fluoresce in these

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regions.42 Although the hypothesis that peaks T+B are greater in MW than peaks A+C is in

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agreement with peaks T+B being associated with proteins, as proteins typically exhibit greater

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MWs (>10 kDa) than humic substances (average MWs < 5~ kDa), high MW polyphenolic

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compounds other than proteins (e.g., tannins MWs 0.5-20 kDa) can also exhibit “protein-like”

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fluorescence in surface waters and WWef.41-44 In addition, peaks T+B were concentrated in