Ionic liquid extraction unveils previously occluded humic-bound iron in peat soil pore water

Globally, peatland ecosystems store tremendous amounts of C relative to their extent on the landscape, largely owing to saturated soils which limit decomposition. While there is still considerable uncertainty regarding CO2 production potential below the water table in peatland ecosystems, extracellular Fe reduction has been suggested as a dominant pathway for anaerobic metabolism. However, colorimetric methods commonly used to quantitate Fe and partition between redox species are known to be unreliable in the presence of complex humic substances, which are common in peatland pore water. We evaluated both the standard o-phenanthroline (o-P) Method and an ionic liquid extraction (ILE) Method followed by quantitation with inductively coupled plasma optical emission spectrometry (ICP–OES) to compare total Fe recovery and Fe2+/Fe3+ ratios in four distinct peatland ecosystems, ranging from rich fen to bog. While total Fe concentrations measured with ILE and o-P were correlated, the ILE method proved to be superior in both total Fe quantitation and in separately quantifying ferric (Fe3+) and ferrous (Fe2+) iron. In peat pore water, the o-P Method underestimated Fe3+ by as much as 100%. A multivariate approach utilizing fluorescence- and ultraviolet (UV)–visable (Vis) spectroscopy identified indices of dissolved organic matter (DOM) humification and redox status that correlated with poor performance of the o-P Method in peat pore water. Where these interferences are present, we suggest that site-specific empirical correction factors for quantitation of total Fe by o-P can be created from ILE of Fe, but recommend ILE for accurate appraisals of iron speciation and redox processes

A potentiometric and 113Cd NMR study of cadmium complexation by natural organic matter at two different magnetic field strengths.

The binding of cadmium to Suwannee River natural organic matter (NOM) has been investigated across a broad range of Cd/C ratios (0.00056−0.0056) and pH values (3.5−11) by 113Cd NMR spectroscopy at two magnetic field strengths (B0 = 9.4 and 11.7 T). Caused by the very peculiar and highly complex nature of the Cd−NOM exchanging system, these 113Cd NMR spectra are characterized by a pH- and concentration-dependent superposition of slow, intermediate, and fast chemical exchange. The complex interplay of solution chemistry and chemical exchange requires a thorough mapping of this Cd−NOM chemically exchanging system through NMR acquisition at two magnetic field strengths and a systematic variation of Cd/C ratios and pH values. The interpretation of 113Cd NMR spectra is greatly facilitated and constrained by simultaneous measurements of pH and pCd, which allows a model-independent calculation of organically bound Cd2+ under all experimental conditions. Within the range of chemical conditions applied in this study, 113Cd NMR spectrometric evidence is consistent with coordination of cadmium by oxygen, nitrogen, and sulfur ligands in NOM. Under all experimental conditions, cadmium is primarily coordinated to oxygen; however, several lines of evidence point to the participation of nitrogen ligands, even in acidic solutions where nitrogen ligands are primarily bound to protons. Under alkaline conditions, up to one-third of cadmium may be coordinated to nitrogen, and a small, but unquantifiable, percentage of cadmium is coordinated to sulfur ligands, as evidenced by far-low-field 113Cd NMR resonances

The application of 113Cd NMR spectrometry to the study of cadmium complexation by natural organic matter.

The results of a comprehensive 113Cd NMR study of complexation of Cd2+ by Suwannee River natural organic matter (NOM) are presented and compared with the more familiar behavior of simple systems containing Cd2+ and a single ligand (e.g., ethylenediamine). pH-dependent trends in the chemical speciation of Cd(II), chemical shift of 113Cd, and line width of resonance peaks lead to the conclusion that the contribution of N donor atoms to the primary coordination sphere of Cd2+ increases steadily with increasing pH, so much so that relatively strong downfield resonances can be observed at alkaline pH. Those observations also indicate that there is no accessible, unique combination of magnetic field strength and solution chemistry for which the exchange rates of all forms of Cd are simultaneously fast, intermediate, or slow

Substitution patterns in aromatic rings by increment analysis: Model development and application to natural organic matter.

The aromatic region of two-dimensional heteronuclear 1H, 13C NMR spectra of natural organic matter and related materials (e.g., 1H and 13C chemical shifts ranging from approximately 5 to 10 and 80 to 140 ppm, respectively) is highly complex and difficult to interpret using conventional approaches. In principle, this region of the NMR spectrum should be amenable to detailed analysis, because the effects of many common substituents on the chemical shifts of aromatic carbon and hydrogen are well documented. This paper describes the development of a model for prediction of substitution patterns in aromatic rings by increment analysis (SPARIA). In the forward mode, SPARIA is used to predict the chemical shifts of 1H and 13C on aromatic moieties containing every possible combination of eight common substituents that are likely to be representative of substituents on aromatic moieties in natural organic matter. The accuracy of SPARIA in the forward mode is evaluated for 29 aromatic compounds (100 peaks) by comparison of predicted chemical shifts for 1H and 13C with experimental values and with predictions of commercially available software for prediction of NMR spectra. The most important development in this paper is the inverse mode that is built into SPARIA. Given chemical shifts for 1H and 13C (such as may be obtained from a two-dimensional, heteronuclear NMR spectrum), the inverse mode of SPARIA calculates all possible combinations of the eight selected substituents that yield chemical shifts within a specified window of chemical shift for both 1H and 13C. Both the distribution of possible substitution patterns and simple descriptive statistics of the distribution are thus obtained. The inverse mode of SPARIA has been tested on the 29 aromatic compounds (100 peaks) that were used to evaluate its forward mode, and the dependence of the inverse process on the size of the chemical shift window has been evaluated. Finally, the inverse mode of SPARIA has been applied to selected peaks from the two-dimensional heteronuclear HSQC spectrum of a sample of natural organic matter that was isolated by reverse osmosis from the Suwannee River in southeastern Georgia

Depicting molecular dissimilarity in complex materials.

Natural organic matter (NOM) occurs in soils, freshwater and marine environments, in the atmosphere and in the form of prebiotic organic matter and represents an exceedingly complex mixture of organic compounds that collectively exhibits a nearly continuous range of properties (size-reactivity continuum).   The fate NOM in the bio- and geosphere is governed according to the rather fundamental restraints of thermodynamics and kinetics. In these intricate materials, the “classical” signatures of the (geogenic or ultimately biogenic) precursor molecules, like lipids, glycans, proteins and natural products have been attenuated, often beyond recognition, during a succession of biotic and abiotic (e.g. photo- and redox chemistry) reactions. Because of this loss of biochemical signature, these materials can be designated non-repetitive complex systems.   The most informative, “bottom-up” approach to molecularly characterize these complex materials necessarily relies upon spectroscopic methods which translate high-precision frequency measurements into very significant molecular-level information. Frequencies can be measured with an accuracy of 15 digits. This extent of accuracy in frequency measurements translates directly into high resolution, itself a very useful and even indispensable feature to produce information-rich data with sufficient resolution to overcome the otherwise common and detrimental effects of intrinsic averaging, which deteriorate spectral resolution to the degree of a bulk-type characterization rather than to a molecular resolution analysis.   High-precision frequency measurements, which can be translated into isotope-specific molecular resolution detail of unprecedented significance and richness, define the core of the two most influential methods of organic structural spectroscopy for the investigation of complex materials, namely NMR spectroscopy (provide unsurpassed insight into close-range molecular order to assess the structural space) and FTICR mass spectrometry (provide unsurpassed resolution to explore the compositional space).   The quality of this stand-alone de novo molecular-level resolution data is of unparalleled mechanistic relevance and sufficient to fundamentally advance our understanding of structure and function of NOM, which at present are poorly amenable to meaningful target analysis. The currently available discrete analytical volumetric pixel space to describe complex systems (which is defined by NMR, FT mass spectrometry and separation technology) is in the range of 108-14 volumetric pixels and therefore capable to provide the necessary detail for a meaningful molecular level analysis of any complex mixture of organic molecules.   This presentation will provide an evaluation of state-of-the-art concepts and applications of molecular level structure elucidation to NOM materials of various origin. According to these findings, NOM is a rather active participant of the global carbon cycle, and the current perception of NOM being considered refractory can be regarded as a consequence of insufficient resolution of methods commonly used in its characterization

Suwannee River natural organic matter: Isolation of the 2R101N reference sample by reverse osmosis.

Suwannee River natural organic matter (SRNOM) is a well-known end member of NOM from an aquatic system, and is a reference material of the International Humic Substances Society (IHSS). In May 2012, an expedition to the Suwannee River to replenish this reference material yielded over 6kg of freeze-dried NOM. The quantity of isolated NOM was unprecedented, easily exceeding the combined recoveries of the standard and reference samples that were collected by the IHSS from the Suwannee River in 1983, 1999, and 2003. The NOM was acquired from 36,890L of filtered river water, which was concentrated 40-fold on-site using two portable reverse osmosis (RO) systems. After RO, the concentrated sample was desalted by cation exchange (CEX), freeze dried, and homogenized. Overall yield of dissolved organic carbon (DOC) was 84.2%, which is slightly lower than the yield of 88% in 1999 when RO and CEX were used to isolate the first sample of SRNOM, which is designated 1R101N. The final NOM sample supplied to the IHSS, which is designated 2R101N, contains only 3.89% inorganic ash, which reasonably allows most chemical analyses. Average river DOC concentration of 82.7mg/L was higher than during prior sampling trips, which contributed to the his- torically high recovery of NOM. Increased DOC concentration may be related to the removal of water control structures from the river. This article describes the methods of isolation used in collecting the 2R101N reference sample as background for other articles in this special issue of Environmental Engineering Science and for future researchers who will use this IHSS sample

Utilization and transformation of aquatic humic substances by autochthonous microorganisms.

Aquatic humic substances (HS) from a bog lake water, a river water, and a groundwater were isolated after enrichment on XAD 8 columns and added to a Czapek-Dox nutrient broth which was used either in full strength or without glucose and/or NaNO3. The individual flasks were inoculated with natural microbial populations of corresponding water samples or with a Pseudomonas fluorescens strain isolated from groundwater. The presence of HS resulted in an increase of bacterial numbers in nearly all cultures incubated for 3 weeks at 25 °C on a shaker. HS reisolated from cultures without glucose or NaNO3 showed no or only minor quantitative differences as compared to those from sterile controls. In full strength nutrient broth up to 27% of HS were utilized. Data obtained by spectroscopic methods (UV/vis/FTIR) and elemental analysis indicated a decrease in particle size and a loss in aromaticity and aliphatic carbon in HS reisolated from the microbial cultures. Simultaneously an increase in the N content of HS was observed, which probably originated from some constituents of microbial biomass such as proteins and amino sugars. The NMR data also documented that significant transformations of HS occurred in the individual microbial cultures. After incubation, increased amounts of aromatic acids were detected in some liquid media and residual HS by GC/MS or capillary electrophoresis. 1H NMR spectroscopy was less effective in indicating structural differences in the HS than 13C NMR but revealed considerable detail of the microbial degradation of riverine HS, when limited sample was available. The newly developed NMR increment analysis provided substantial detail of aromatic structures in a microbially altered HS. The microbial degradation of HS strongly depended on the composition of the HS, the species selection of the microorganisms, and to a lesser extent on the culture conditions. For any series of identical inoculum and HS, full broth media initiated the most extensive alteration of HS