Special Issue Introduction: Dissolved Organic Matter from the Suwannee River (GA,USA)

Abstract

The Suwannee River (GA, USA) has been the de facto “type locality” for aquatic humic substances ever since the International Humic Substances Society (IHSS) began to make freeze-dried standard samples from this river available to the scientific community. In 1983, the newly formed IHSS sponsored a sampling of dissolved organic matter (DOM) at the Suwannee River to collect humic and fulvic acids for characterization and for use by future researchers (Malcolm et al., 1986). The Suwannee River drains the Okefenokee Swamp in southeastern Georgia and is a black water river whose DOM appears to derive primarily from the degradation of swamp vegetation rather than from direct microbial (algal) sources (Malcolm et al., 1986). The relatively high dissolved organic carbon (DOC) concentration (>25 mgC/L) and the lack of strong anthropogenic inputs were highlighted as additional reasons why the Suwannee River was chosen for sampling (Malcolm et al., 1986). The XAD-8 resin technology was used to isolate the dissolved humic substances or so-called “hydrophobic acid” (HPOA) fraction, and humic acids were separated from fulvic acids by precipitation at low pH.

Suwannee River standard and reference samples collected in 1983 and/or in the subsequent years have proven essential for research on aquatic humic substances. As outlined in Green et al. (2015, this issue), in 1999, a different type of reference sample was collected by reverse osmosis (RO) and cation exchange (CEX); such a sample is referred to by the IHSS as Suwannee River natural organic matter (SR NOM). The IHSS standard and reference samples of Suwannee River humic and fulvic acid and reference samples of NOM have been used by dozens of researchers from around the world and have been essential to understanding the diverse roles of DOM in aquatic ecosystems. This material is in such high demand that it can be difficult to keep in adequate supply; hence, the community must continue to collect new samples for use in future research.

To this end, Dr. E. Michael Perdue (Ball State University) led an IHSS sponsored research team to collect new RO concentrate from the Suwannee River in May 2012. The 21-day sampling effort is described in detail by Green et al. (2015, this issue). The high [DOC] (>80 mgC/L) at the time of sampling allowed for collection of a large amount of IHSS sample, which has been designated 2R101N and is described as “SR NOM” herein. In addition to assisting with the RO sample collection, Daniel McInnis, a graduate student working with Dr. P.A. Maurice (University of Notre Dame [UND]), collected filtered surface water samples at various times during the 21-day sampling period and shipped them in ice-filled coolers to the U.S. Geological Survey laboratory of Dr. George Aiken (Boulder, CO). There, DOM was fractionated using XAD-8 (yielding HPOA) and XAD-4 (yielding transphilic acids [TPIA]) technologies by a UND graduate student Keshia Kuhn under the direction of Dr. Aiken and Kenna Butler (Kuhn et al., 2015, this issue). Although these XAD samples were not intended to become a part of the IHSS collection, they were disseminated to various laboratories for analysis of properties and reactivity, with many of the results presented in this special issue. This was a unique opportunity for direct comparison of RO (NOM), XAD-8 (HPOA), and XAD-4 (TPIA) samples collected from the Suwannee River during the same time interval (May, 2012). It should be noted that because of the small sample volume, the XAD-8 isolate (HPOA) was not separated into humic versus fulvic acids.

The article by Green et al. (2015) on “Suwannee River Natural Organic Matter: Isolation of the 2R101N Reference Sample by Reverse Osmosis” provides details of the sampling location and puts the May 2012 effort in the context of previous IHSS samplings. The authors describe how the flow of the river has changed over time as channelization structures have decayed. Moreover, the river's [DOC] in May 2012 was considerably higher than during previous IHSS samplings. This article also describes in detail the preparation of the RO sample; the XAD-8 and XAD-4 isolation procedures are described by Kuhn et al. (2015).

Kuhn et al. (2015) describe the composition (by weight C) of the filtered surface water as determined using a modified fractionation scheme of Aiken et al. (1992): HPOA, 60%; TPIA, 18%; hydrophilic acids, 13%; hydrophobic organic neutrals, 4%; and transphilic and hydrophilic neutrals, 5%. Based on these fractionation results and considering that RO concentrates a broad range of organic matter, one might expect the SR NOM sample to have properties that are a combination of HPOA and TPIA, but most closely aligned with the former. However, other organic components might be present in SR NOM that are not present in either HPOA or TPIA. It is also possible that the various methods alter at least some physicochemical properties of the DOM in different ways.

Much can be learned about the physicochemical properties of the three DOM samples from the various articles published in this special issue. Although DOM tends to be polydisperse, a key property (Cabaniss et al., 2000) is the average molecular weight (M_n). Pavlik and Perdue (2015) measured molecular weights by vapor pressure osmometry (VPO). They observed that the SR NOM, HPOA, and TPIA samples have M_n of 634, 583, and 498 g/mol, respectively. Hence, the average molecular weight of the SR NOM sample is considerably closer to that of the dominant river water HPOA fraction than the less substantial TPIA fraction, although the SR NOM molecular weight is higher than either XAD sample. Kuhn et al. (2015) observed using asymmetrical flow field flow fluid fractionation (AsFlFFF) with ultraviolet/visible light (UV/Vis) detection that nominal or apparent molecular weight for these samples decreased as SR NOM>RFSW>HPOA>TPIA, where RFSW is DOM in water that was filtered but not otherwise processed. Cawley et al. (2015) presented size exclusion chromatography (SEC) data using detection at 254 nm. Although average molecular weights were not calculated from the data, the chromatographs were also consistent with molecular weight decreasing as follows: SR NOM>HPOA>TPIA. Hence, the molecular weight trends were the same using VPO, AsFlFFF, and SEC.

The molecular weight trends agree with the results of previous sample comparisons. Maurice et al. (2002) applied SEC to samples from a black water stream in the New Jersey Pinelands (Atlantic Coastal Plain, USA) and observed that average molecular weight also decreased in the order NOM>RFSW>HPOA>TPIA. Maurice et al. (2002) suggested that condensation during the RO process might lead to some anomalously high molecular components in the NOM sample. It is important to note that while HPSEC and AsFlFFF with UV/Vis detection (generally at λ=254 nm) can provide useful information for comparing average molecular weights, the actual values calculated using these methods should not be considered definitive, especially because nonchromophoric molecules are not detected by absorbance measurements at 254 nm.

¹³C NMR is a key technique for analyzing the structures of organic compounds. Analysis by Nwosu and Cook (2014, this issue) revealed that the HPOA was higher in alkyl and aromatic moieties, while the TPIA isolate was higher in O-alkyl moieties. These findings are consistent with the HPOA being relatively more hydrophobic and the TPIA more hydrophilic (more properly transphilic). The SR NOM was found to be intermediate between the other two samples in terms of alkyl, aromatic, and O-alkyl moieties. Electron paramagnetic resonance spectrometry (EPR) analysis indicated that radical concentrations decreased in the following order: TPIA>HPOA>RO. Nwosu and Cook (2014) suggested that the higher radical concentrations of the TPIA and HPOA samples might be due to high pH conditions experienced during NaOH exposure during the XAD isolation procedures.

Driver and Perdue (2015, this issue) applied potentiometric titrations with data analysis by a variety of models and found that the carboxyl contents decreased in the following order: TPIA>RO>HPOA. Phenolic concentrations were similar to one another, appearing to decrease in the order HPOA>RO>TPIA but perhaps within experimental error. The authors noted the presence of highly acidic compounds in the RO sample that did not appear in the samples isolated by XAD procedures. Hence, the RO method appears to concentrate some highly acidic components that are not isolated by XAD methods.

Fluorescence indices (FI) calculated from emission values of λ=450 nm/500 nm at excitation at λ=370 nm have been used to infer the source of DOM. Lower FI values indicate more plant-derived, terrestrial DOM origins, whereas higher FI values indicate more microbial/algal origin (McKnight et al., 2001; Cory and McKnight, 2005).

FI values were measured by Kuhn et al. (2015, this issue) and decreased in the following order: TPIA (1.49)>RFSW (1.32) ∼SR NOM (1.30)>HPOA (1.25). Cawley et al. (2015, this issue) also observed that the TPIA had considerably greater FI than either SR NOM or HPOA.

Metal and rare earth element (REE) concentrations of the various DOM samples were measured by Kuhn et al. (2015) using inductively coupled plasma emission mass spectrometry (ICP-MS). Distributions of metals were determined as a function of apparent molecular weight using AsFlFFF with in-line UV/Vis, fluorescence, and ICP-MS detectors. Despite application of CEX during sample processing, all of the samples contained substantial metal concentrations. In Suwannee River filtered (but otherwise unprocessed) surface water, Fe and Al were associated primarily with intermediate to higher-than-average molecular weight DOM components. SR NOM, HPOA, and TPIA samples resulting from the May 2012 sampling effort showed similar molecular weight trends for Fe and Al. However, Cu tended to associate more with lower molecular weight DOM components. None of the fractograms showed obvious evidence for mineral nanoparticles, although some very small mineral nanoparticles might have been present at trace concentrations. The most important take-home message from this article is that Suwannee River DOM samples, including those available from the IHSS, contain metals at concentrations that need to be taken into consideration in a variety of studies of organic matter properties and reactivity.

A number of articles in this special issue deal explicitly with DOM reactivity. The photochemical behavior of DOM is important for many biogeochemical processes in environments that are open to sunlight. Cawley et al. (2015, this issue) measured the apparent quantum yields for the three DOM samples using a solar simulator. They found that the formation of hydroxyl radical HO*, singlet oxygen ¹O₂, and excited triplet states ³DOM* ranged from 0.95–1.65×10⁻⁵, 2.74–6.54×10⁻², and 2.61–4.20×10⁻⁴, respectively. In all cases, values for HPOA were the lowest, TPIA the highest, and SR NOM intermediate.

McInnis et al. (2015, this issue) investigated the transport of HPOA, TPIA, and SR NOM in columns packed with naturally Fe oxide-coated and Al oxide-coated sands. They found that the breakthrough curves (BTCs) of the HPOA and SR NOM samples were similar, but the TPIA BTCs were distinct. Although the median breakthrough times of all three samples were similar, BTC asymmetry and dispersivity were greater for TPIA. Moreover, the TPIA BTCs showed greater tailing. Thus, although TPIA tends to be a relatively small component (on a % basis) of Suwannee River DOM, it may affect any associated pollutant transport differently than the dominant HPOA fraction.

Kreller et al. (2015, this issue) studied the sorption and mobility of Suwannee River HPOA in sand-packed columns using a novel liquid chromatography-based approach. In agreement with the past batch and column experiments, they observed greater retention of higher molecular weight and more aromatic and less fluorescent components. This new approach needs to be applied to TPIA and SR NOM samples for comparison purposes.

Given that DOM samples from the Suwannee River are widely used for a broad range of environmental research, hopefully this special issue will prove useful for future researchers. Some key concepts include the following:

(1) The Suwannee River has changed hydrogeochemically over time and, although not explicitly addressed in this special issue, it is likely that DOM properties have changed over time as well.

(2) Suwannee River TPIA tends to have lower average molecular weight and to be less hydrophobic with higher FI and greater carboxyl content than HPOA. TPIA also tends to form more hydroxyl radical HO*, singlet oxygen ¹O₂, and excited triplet states ³DOM* upon photoirradiation. Transport of TPIA is distinctly different from that of HPOA in sand-packed columns; although the median breakthrough times are similar, TPIA BTCs show greater asymmetry and dispersivity as well as stronger tailing.

(3) Although SR NOM often shows properties and/or reactivity intermediate between those of HPOA and TPIA, this is not always the case. For example, average molecular weight tends to be higher for SR NOM than for either HPOA or TPIA, and SR NOM appears to contain some particularly acidic components that are not present in the XAD isolates.

(4) Despite processing, all of the DOM samples from the Suwannee River contain a broad range of metals and REEs that should be taken into consideration in any study of fundamental properties and/or reactivity.

Footnotes

Acknowledgments

The authors of this special issue thank the IHSS for funding sample collection at the Suwannee River (GA, USA) in May 2012. Special issue editor P. Maurice thanks Dawn Densmore of the EES editorial office for her patient guidance throughout this project.

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