Composition-Dependent Sorptive Fractionation of Anthropogenic Dissolved Organic Matter by Fe(III)-Montmorillonite

Emerging investigator series: preferential adsorption and coprecipitation of permafrost organic matter with poorly crystalline iron minerals

Future permafrost thaw will likely lead to substantial release of greenhouse gases due to thawing of previously unavailable organic carbon (OC). Accurate predictions of this release are limited by poor knowledge of the bioavailability of mobilized OC during thaw. Organic carbon bioavailability decreases due to adsorption to, or coprecipitation with, poorly crystalline ferric iron (Fe(III)) (oxyhydr)oxide minerals but the maximum binding extent and binding selectivity of permafrost OC to these minerals is unknown. We therefore utilized water-extractable organic matter (WEOM) from soils across a permafrost thaw gradient to quantify adsorption and coprecipitation processes with poorly crystalline Fe(III) (oxyhydr)oxides. We found that the maximum adsorption capacity of WEOM from intact and partly thawed permafrost soils was similar (204 and 226 mg C g-1 ferrihydrite, respectively) but decreased to 81 mg C g-1 ferrihydrite for WEOM from the fully thawed site. In comparison, coprecipitation of WEOM from intact and partly thawed soils with Fe immobilized up to 925 and 1532 mg C g-1 Fe respectively due to formation of precipitated Fe(III)-OC phases. Analysis of the OC composition before and after adsorption/coprecipitation revealed that high molecular weight, oxygen-rich, carboxylic- and aromatic-rich OC was preferentially bound to Fe(III) minerals relative to low molecular weight, aliphatic-rich compounds which may be more bioavailable. This selective binding effect was stronger after adsorption than coprecipitation. Our results suggest that OC binding by Fe(III) (oxyhydr)oxides sharply decreases under fully thawed conditions and that small, aliphatic OC molecules that may be readily bioavailable are less protected across all thaw stages.

Controls of Mineral Solubility on Adsorption-Induced Molecular Fractionation of Dissolved Organic Matter Revealed by 21 T FT-ICR MS

Mineral adsorption-induced molecular fractionation of dissolved organic matter (DOM) affects the composition of both DOM and OM adsorbed and thus stabilized by minerals. However, it remains unclear what mineral properties control the magnitude of DOM fractionation. Using a combined technique approach that leverages the molecular composition identified by ultrahigh resolution 21 T Fourier transform ion cyclotron resonance mass spectrometry and adsorption isotherms, we catalogue the compositional differences that occur at the molecular level that results in fractionation due to adsorption of Suwannee River fulvic acid on aluminum (Al) and iron (Fe) oxides and a phyllosilicate (allophane) species of contrasting properties. The minerals of high solubility (i.e., amorphous Al oxide, boehmite, and allophane) exhibited much stronger DOM fractionation capabilities than the minerals of low solubility (i.e., gibbsite and Fe oxides). Specifically, the former released Al3+ to solution (0.05-0.35 mM) that formed complexes with OM and likely reduced the surface hydrophobicity of the mineral-OM assemblage, thus increasing the preference for adsorbing polar DOM molecules. The impacts of mineral solubility are exacerbated by the fact that interactions with DOM also enhance metal release from minerals. For sparsely soluble minerals, the mineral surface hydrophobicity, instead of solubility, appeared to be the primary control of their DOM fractionation power. Other chemical properties seemed less directly relevant than surface hydrophobicity and solubility in fractionating DOM.

Probing Molecular-Level Dynamic Interactions of Dissolved Organic Matter with Iron Oxyhydroxide via a Coupled Microfluidic Reactor and an Online High-Resolution Mass Spectrometry System

The interactions between dissolved organic matter (DOM) and iron (Fe) oxyhydroxide are crucial in regulating the biogeochemical cycling of nutrients and elements, including the preservation of carbon in soils. The mechanisms of DOM molecular assembly on mineral surfaces have been extensively studied at the mesoscale with equilibrium experiments, yet the molecular-level evolution of the DOM-mineral interface under dynamic interaction conditions is not fully understood. Here, we designed a microfluidic reactor coupled with an online solid phase extraction (SPE)-LC-QTOF MS system to continually monitor the changes in DOM composition during flowing contact with Fe oxyhydroxide at circumneutral pH, which simulates soil minerals interacting with constant DOM input. Time-series UV-visible absorption spectra and mass spectrometry data showed that after aromatic DOM moieties were first preferentially sequestered by the pristine Fe oxyhydroxide surface, the adsorption of nonaromatic DOM molecules with greater hydrophobicity, lower acidity, and lower molecular weights (<400) from new DOM solutions was favored. This is accompanied by a transition from mineral surface chemistry-dominated adsorption to organic-organic interaction-dominated adsorption. These findings provide direct molecular-level evidence to the zonal model of DOM assembly on mineral surfaces by taking the dynamics of interfacial interactions into consideration. This study also shows that coupled microfluidics and online high-resolution mass spectrometry (HRMS) system is a promising experimental platform for probing microscale environmental carbon dynamics by integrating in situ reactions, sample pretreatment, and automatic analysis.

Fractionation, molecular composition, and biological effects of organic matter in bio-stabilization sludge with implication to land utilization

Bioactive organic compounds (BOCs) contained in bio-stabilized products of waste activated sludge (WAS) have attracted considerable attention, as they can enhance the fertilizing effect of WAS in land applications. This study investigated the molecular composition and plant-growth-promoting mechanisms of various BOCs in the bio-stabilized products of WAS. After stepwise fractionation, aerobic composting sludge (ACS) and anaerobic digestion sludge (ADS) were chemically fractioned into five subcomponents, namely dissolved organic matter (DOM) (C1), weakly interacted organic matter (OM) (C2), metal-bonded OM (C3), NaOH-extracted OM (C4), and strongly interacted OM (C5), in sequence. The results showed that fatty acids and carboxylic acid (CAs) present in ACS C2 promoted plant growth and enhanced the ability of plants against stresses by upregulating pathways related to "carbohydrate metabolism," "lipid metabolism," "amino acid metabolism," and "phenylpropanoid biosynthesis." However, in ACS C4, plenty of amino acids could promote plant growth via upregulating "carbohydrate metabolism" and "amino acid metabolism" pathways. As an important precursor, aromatic amino acids inside ACS C4 also stimulated the production of indoleacetic acids. In ADS C1, amino sugar and phytohormone were the major BOCs causing the up-regulation of "carbohydrate metabolism" and AAA catabolism in "amino acid metabolism" pathways. CAs enriched in ADS C2 stimulated plant growth through "amino acid metabolism" pathway. In summary, alkali extraction can recycle a large proportion of BOCs with low environmental risk from the bio-stabilization products of WAS. The results from this study provide scientific guidance for safe and value-added resource utilization of bio-stabilization products of WAS in land applications.

Nitrogen Enrichment during Soil Organic Matter Burning and Molecular Evidence of Maillard Reactions

Wildfires in forested watersheds dramatically alter stored and labile soil organic matter (SOM) pools and the export of dissolved organic matter (DOM). Ecosystem recovery after wildfires depends on soil microbial communities and revegetation and therefore is limited by the availability of nutrients, such as nitrogen-containing and labile, water-soluble compounds. However, SOM byproducts produced at different wildfire intensities are poorly understood, leading to difficulties in assessing wildfire severity and predicting ecosystem recovery. In this work, water-extractable organic matter (WEOM) from laboratory microcosms of soil burned at discrete temperatures was characterized by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry to study the impacts of fire temperature on SOM and DOM composition. The molecular composition derived from different burn temperatures indicated that nitrogen-containing byproducts were enriched with heating and composed of a wide range of aromatic features and oxidation states. Mass difference-based analysis also suggested that products formed during heating could be modeled using transformations along the Maillard reaction pathway. The enrichment of N-containing SOM and DOM at different soil burning intensities has important implications for ecosystem recovery and downstream water quality.

PFAS Analysis with Ultrahigh Resolution 21T FT-ICR MS: Suspect and Nontargeted Screening with Unrivaled Mass Resolving Power and Accuracy

Per- and polyfluoroalkyl substances (PFASs) are a large family of thousands of chemicals, many of which have been identified using nontargeted time-of-flight and Orbitrap mass spectrometry methods. Comprehensive characterization of complex PFAS mixtures is critical to assess their environmental transport, transformation, exposure, and uptake. Because 21 tesla (T) Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR MS) offers the highest available mass resolving power and sub-ppm mass errors across a wide molecular weight range, we developed a nontargeted 21 T FT-ICR MS method to screen for PFASs in an aqueous film-forming foam (AFFF) using suspect screening, a targeted formula database (C, H, Cl, F, N, O, P, S; ≤865 Da), isotopologues, and Kendrick-analogous mass difference networks (KAMDNs). False-positive PFAS identifications in a natural organic matter (NOM) sample, which served as the negative control, suggested that a minimum length of 3 should be imposed when annotating CF₂-homologous series with positive mass defects. We putatively identified 163 known PFASs during suspect screening, as well as 134 novel PFASs during nontargeted screening, including a suspected polyethoxylated perfluoroalkane sulfonamide series. This study shows that 21 T FT-ICR MS analysis can provide unique insights into complex PFAS composition and expand our understanding of PFAS chemistries in impacted matrices.

Enhanced Speciation of Pyrogenic Organic Matter from Wildfires Enabled by 21 T FT-ICR Mass Spectrometry

Wildfires affect soils through the formation of pyrogenic organic matter (pyOM) (e.g., char and soot). While many studies examine the connection between pyOM persistence and carbon (C) composition, nitrogen (N) transformation in wildfire-impacted systems remains poorly understood. Thermal reactions in wildfires transform biomass into a highly complex, polyfunctional, and polydisperse organic mixture that challenges most mass analyzers. High-field Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) is the only mass analyzer that achieves resolving powers sufficient to separate species that differ in mass by the mass of an electron across a wide molecular weight range (m/z 150-1500). We report enhanced speciation of organic N by positive-ion electrospray ionization (ESI) that leverages ultrahigh resolving power (m/Δm_50% = 1 800 000 at m/z 400) and mass accuracy (<10-100 ppb) achieved by FT-ICR MS at 21 T. Isobaric overlaps, roughly the mass of an electron (M_e^- = 548 μDa), are resolved across a wide molecular weight range and are more prevalent in positive ESI than negative ESI. The custom-built 21 T FT-ICR MS instrument identifies previously unresolved mass differences in C_cH_hN_nO_oS_s formulas and assigns more than 30 000 peaks in a pyOM sample. This is the first molecular catalogue of pyOM by positive-ion ESI 21 T FT-ICR MS and presents a method to provide new insight into terrestrial cycling of organic carbon and nitrogen in wildfire impacted ecosystems.

Soil Organic Matter Characterization by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR MS): A Critical Review of Sample Preparation, Analysis, and Data Interpretation

The biogeochemical cycling of soil organic matter (SOM) plays a central role in regulating soil health, water quality, carbon storage, and greenhouse gas emissions. Thus, many studies have been conducted to reveal how anthropogenic and climate variables affect carbon sequestration and nutrient cycling. Among the analytical techniques used to better understand the speciation and transformation of SOM, Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS) is the only technique that has sufficient mass resolving power to separate and accurately assign elemental compositions to individual SOM molecules. The global increase in the application of FTICR MS to address SOM complexity has highlighted the many challenges and opportunities associated with SOM sample preparation, FTICR MS analysis, and mass spectral interpretation. Here, we provide a critical review of recent strategies for SOM characterization by FTICR MS with emphasis on SOM sample collection, preparation, analysis, and data interpretation. Data processing and visualization methods are presented with suggested workflows that detail the considerations needed for the application of molecular information derived from FTICR MS. Finally, we highlight current research gaps, biases, and future directions needed to improve our understanding of organic matter chemistry and cycling within terrestrial ecosystems.

Molecular Determination of Organic Adsorption Sites on Smectite during Fe Redox Processes Using ToF-SIMS Analysis

Turnover of soil organic carbon (SOC) is strongly affected by a balance between mineral protection and microbial degradation. However, the mechanisms controlling the heterogeneous and preferential adsorption of different types of SOC remain elusive. In this work, the heterogeneous adsorption of humic substances (HSs) and microbial carbon (MC) on a clay mineral (nontronite NAu-2) during microbial-mediated Fe redox cycling was determined using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results revealed that HSs pre-adsorbed on NAu-2 would partially inhibit structural modification of NAu-2 by microbial Fe(III) reduction, thus retarding the subsequent adsorption of MC. In contrast, NAu-2 without precoated HSs adsorbed a significant amount of MC from microbial polysaccharides as a result of Fe(III) reduction. This was attributed to the deposition of a thin Al-rich layer on the clay surface, which provided active sites for MC adsorption. This study provides direct and detailed molecular evidence for the first time to explain the preferential adsorption of MC over HSs on the surface of clay minerals in iron redox processes, which could be critical for the preservation of MC in soil. The results also indicate that ToF-SIMS is a unique tool for understanding complex organic-mineral-microbe interactions.

Application of Fourier transform ion cyclotron resonance mass spectrometry in deciphering molecular composition of soil organic matter: A review

Swiftly deciphering soil organic matter (SOM) composition is critical for research on soil degradation and restoration. Recent advances in analytical techniques (e.g., optical methods and mass spectrometry) have expanded our understanding of the composition, origin, and evolution of SOM. In particular, the use of Fourier transform ion cyclotron resonance mass spectrometers (FTICR-MS) makes it possible to interpret SOM compositions at the molecular level. In this review, we discuss extraction, enrichment, and purification methods for SOM using FTICR-MS analysis; summarize ionization techniques, FTICR-MS mechanisms, data analysis methods, and molecular compositions of SOM in different environments (providing new insights into its origin and evolution); and discuss factors affecting its molecular diversity. Our results show that digenesis, combustion, pyrolysis, and biological metabolisms jointly contribute to the molecular diversity of SOM molecules. The SOM thus formed can further undergo photodegradation during transportation from land to fresh water (and subsequently oceans), resulting in the formation of dissolved organic matter (DOM). Better understanding the molecular features of DOM therefore accelerates our understanding of SOM evolution. In addition, we assess the degradation potential of SOM in different environments to better inform soil remediation methods. Finally, we discuss the merits and drawbacks of applying FTICR-MS on the analysis of SOM molecules, along with existing gaps in knowledge, challenges, and new opportunities for research in FTICR-MS applications and SOM identification.

Linkage Between Dissolved Organic Matter Transformation, Bacterial Carbon Production, and Diversity in a Shallow Oligotrophic Aquifer: Results From Flow-Through Sediment Microcosm Experiments

Aquifers are important reservoirs for organic carbon. A fundamental understanding of the role of groundwater ecosystems in carbon cycling, however, is still missing. Using sediment flow-through microcosms, long-term (171d) experiments were conducted to test two scenarios. First, aquifer sediment microbial communities received dissolved organic matter (DOM) at low concentration and typical to groundwater in terms of composition (DOM-1x). Second, sediments received an elevated concentration of DOM originating from soil (DOM-5x). Changes in DOM composition were analyzed via NMR and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Carbon production, physiological adaptations and biodiversity of groundwater, and sediment prokaryotic communities were monitored by total cell counts, substrate use arrays, and deep amplicon sequencing. The experiments showed that groundwater microbial communities do not react very fast to the sudden availability of labile organic carbon from soil in terms of carbon degradation and biomass production. It took days to weeks for incoming DOM being efficiently degraded and pronounced cell production occurred. Once conditioned, the DOM-1x supplied sediments mineralized 294(±230) μgC L⁻¹_sed d⁻¹, 10-times less than the DOM-5x fed sediment communities [2.9(±1.1) mgC L⁻¹_sed d⁻¹]. However, the overall biomass carbon production was hardly different in the two treatments with 13.7(±4.8) μgC L⁻¹_sed d⁻¹ and 14.3(±3.5) μgC L⁻¹_sed d⁻¹, respectively, hinting at a significantly lower carbon use efficiency with higher DOM availability. However, the molecularly more diverse DOM from soil fostered a higher bacterial diversity. Taking the irregular inputs of labile DOM into account, shallow aquifers are assumed to have a low resilience. Lacking a highly active and responsive microbial community, oligotrophic aquifers are at high risk of contamination with organic chemicals.

Effects of digestate DOM on chemical behavior of soil heavy metals in an abandoned copper mining areas

The influence of digestate dissolved organic matter (DOM) on chemical behavior of soil heavy metals (HMs) in an abandoned copper mining areas was explored by fluorescence quenching titration and heavy metal extracting experiment. Five fluorescent components were obtained from digestate DOM by PARAFAC model combined with the EEM data. The stability constant (log K_M) values were in the range of 4.95-5.53, 5.05-5.29, 5.21-6.00, and 4.12-4.75 for DOM-Cr(III), DOM-Cu(II), DOM-Fe(III) and DOM-Pb(II) complexes, respectively. Alcohols, ethers and esters in digestate DOM were preferentially combined with Fe(III), Cu(II) and Zn(II). However, phenolic hydroxyl groups were more likely to combine with Cr(III) and Pb(II). The speciation distribution of HMs indicated that mining resulted in a higher concentration of Cu(II) in the grassland soil (GS) than those in the agricultural soil (AS) and forest land soil (FS). Fe-Mn oxides and organic forms of Pb(II) increased dramatically due to mining. Digestate DOM extraction can increase the content of Cr(III), Fe(III) and Pb(II), and decrease the content of Cu(II) and Zn(II) in the AS, GS, and FS. However, the contents of HMs in the mining soil (MS) and slag soil (SS) decreased due to the application of digestate DOM, except for Cu(II) in the SS.

Quantification and molecular characterization of organo-mineral associations as influenced by redox oscillations

Organo-mineral association is one of the most important stabilization mechanisms of soil organic matter. However, few studies have been conducted to assess the retention, transformation, and transportation of colloids (1-1000 nm) and associated organic carbon (OC) in soil. Given the particularly significant role that wetland soils play in carbon storage and cycling, we quantified the dynamics of organo-mineral association within colloidal size range by conducting three consecutive 35-day redox (reduction-oxidation) oscillation experiments using a wetland soil. Molecular compositions of natural nanoparticle (NNP, 2.3-100 nm), fine colloid (100-450 nm), and particulate (450-1000 nm) fractions were measured using isotope ratio mass spectrometry (IRMS) and x-ray photoelectron spectroscopy (XPS). Results showed that NNP and fine colloids constituted up to 8.94 ± 0.50% and 22.19 ± 7.52% of bulk C concentration (2.3-1000 nm), respectively; indicating substantial contributions of these two fractions to the operationally defined "dissolved" (<450 nm) fraction. There was significant enrichment in heavier δ¹³C isotopes (p < 0.001) with size: NNP (-29.64 ± 0.32‰) < fine colloid (-28.81 ± 0.31‰) < particulate (-28.34 ± 0.25‰) fractions. NNP had the highest percentages of carbonyl/carboxyl C (C=O); while fine colloid and particulate fractions contained more reduced aromatic or aliphatic C (C-C, C=C, C-H). OC became more enriched (‰) in microbial-derived C (higher δ¹³C) with increasing particle size as well as with repeated redox oscillations. Our findings clearly demonstrate limitations of using the operationally defined "dissolved" fraction (<450 nm) to assess C cycling in ecosystems such as wetlands. Increase in colloid and OC concentrations and presence of more microbial-derived C in larger size fractions additionally imply that redox oscillations promote the formation of molecularly diverse sub-colloid sized organo-mineral associations. Being a composite unit of soil microaggregates, organic-mineral associations can thus influence the overall stability of OC in wetland soils that undergo frequent redox oscillations.

Preferential Sorption of Tannins at Aluminum Oxide Affects the Electron Exchange Capacities of Dissolved and Sorbed Humic Acid Fractions

Natural organic matter and humic substances (HS) in soils and sediments participate in numerous biogeochemical processes. Sorption to redox-inert aluminum oxide (Al₂O₃) was recently found to affect the redox properties of HS both in sorbed and dissolved state. With this study, we aim to decipher the molecular basis for these observations by applying Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS) and mediated electrochemical analysis to Elliott soil, Pahokee peat, and Suwannee river humic acid (HA) samples before and after sorption to polar Al₂O₃ and a nonpolar sorbent (DAX-8 resin). The FT-ICRMS data provided evidence of preferential sorption of specific HA fractions, primarily tannin-like compounds, to Al₂O₃. These oxygen-rich compounds bear a high density of redox-active functional groups, and their adsorption leads to a depletion of electron exchange capacity in dissolved HAs and enrichment of HAs adsorbed at Al₂O₃. Sorption of HAs to DAX-8 was less selective and caused only slight changes in electron exchange capacities of dissolved and sorbed HA fractions. By combining FT-ICRMS and electrochemical approaches, our findings suggest that a selective sorption of oxygen-rich compounds in HA fractions to mineral oxides is a decisive factor for the different redox properties of dissolved and sorbed HA fractions.

Effects of Sorption on Redox Properties of Natural Organic Matter

Natural organic matter (NOM) is an important redox-active component of natural porous media and predominantly occurs in the sorbed state. Nevertheless, the effects of NOM sorption at minerals on its redox properties are unknown and thus are the major objective of this study. We report how adsorption of three different humic acids (HAs) to redox-inert sorbents (polar Al₂O₃ and nonpolar DAX-8 resin) affects their electron-exchange capacities (EEC) and redox states. The electron-donating capacity of HAs sorbed at Al₂O₃ increased by up to 200%, whereas the EEC of the remaining dissolved HA fractions decreased compared with their initial properties. Sorption at DAX-8, however, did not affect significantly the EEC of HAs. We rationalize these results by (i) preferential sorption of NOM components rich in redox-active groups (e.g., quinone, polyphenols) and (ii) surface-catalyzed polymerization of polyphenolic compounds. Our results demonstrate that even in the absence of electron exchange with the sorbent, adsorption to polar mineral surfaces considerably affects the redox properties of NOM. Quantification of the redox state and EEC of adsorbed NOM is thus crucial for assessing electron-transfer processes as well as organic carbon stabilization and sequestration in soils and sediments.

Direct Evidence for Temporal Molecular Fractionation of Dissolved Organic Matter at the Iron Oxyhydroxide Interface

While the importance of organic matter adsorption onto reactive iron-bearing mineral surfaces to carbon stabilization in soils and sediments has been well-established, fundamental understanding of how compounds assemble at the mineral interface remains elusive. Organic matter is thought to layer sequentially onto the mineral surface, forming molecular architecture stratified by bond strength and compound polarity. However, prominent complexation models lack experimental backing, despite the role of such architecture in fractionated, compound-dependent persistence of organic matter and modulating future perturbations in mineral stabilization capacity. Here, we use kinetic assays and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry under high temporal frequency to directly detect the molecular partitioning of organic matter onto an iron oxyhydroxide during adsorption. We observed three sequential intervals of discrete molecular composition throughout the adsorption reaction, in which rapid primary adsorption of aromatic compounds was followed by secondary lignin-like and tertiary aliphatic compounds. These findings, paired with observed differential fractionation along formulas nitrogen and oxygen content and decreasing selective sorption with reaction time, support "zonal" assembly models. This work presents direct detection of sequential molecular assembly of organic matter at the mineral interface, an important yet abstruse regulator of carbon stabilization and composition across temporal and spatial scales.