Elucidating differences in the sorption properties of 10 humic and fulvic acids for polar and nonpolar organic chemicals

In this work we present a dataset of more than 1000 natural organic matter (NOM)/air partition coefficients covering polar and nonpolar organic compounds measured in 10 different humic and fulvic acids (HAs/FAs) from terrestrial and aquatic origins. Differences of more than 1 order of magnitude in the sorption coefficients of a given compound measured in HAs and FAs from different origins were found. The terrestrial HAs exhibited substantially higher sorption coefficients compared to aquatic HAs and FAs. The difference between any two types of NOM is mainly reflected by a constant shift in the partition coefficients that applies to all compounds in the same way. This indicates that it is the number of available sorption sites per mass of sorbent rather than the types of intermolecular interactions between the sorbate and the sorbent that governs the major differences between the sorption properties of various types of NOM. The experimental partition coefficients measured in all HAs and FAs were successfully described by polyparameter linear free energy relationships (pp-LFERs) that explicitly account for van der Waals as well as H-donor/acceptor interactions between the sorbate and the sorbent. These pp-LFER equations provide for the first time a tool that allows including the variability of the sorption properties of NOM in environmental fate models.     

Sorption of very hydrophobic organic compounds (VHOCs) on dissolved humic organic matter (DOM). 2. Measurement of sorption and application of a Flory-Huggins concept to interpret the data

Sorption phenomena of very hydrophobic compounds (VHOCs, log K(OW) > 5) on dissolved humic organic matter (DOM) are overwhelmingly based on partitioning processes. In this respect, DOM is very similar to "rubbery" soil/sediment OM. To exclude system adsorption effects, the DOM sorption coefficients (K(DOM)) of VHOCs were determined using a dynamic approach based on the VHOCs' aqueous solubility enhancement in the presence of DOM. Partition coefficients are strongly correlated to the analytes' Kow across the alkane, PAH, and PCB groups under study. These three "families" are regarded to be good models of hydrophobic partitioning. On the basis of a uniform one-parameter concept characterizing sorption on amorphous polymers, Hildebrand solubility parameters of amorphous polymeric sorbents, including DOM, and of sorbates can be calculated on the basis of partition coefficients. Likewise, partition coefficients can be estimated using Hildebrand solubility parameters. Literature-based partition coefficients on DOM fit very well in this universal one-parameter concept. On using our own sorption data of PAHs, PCBs, and alkanes on DOM, an almost identical solubility parameter for the DOM polymer under study is obtained. The concept is also very useful in understanding both waterborne and airborne bioconcentration processes, which are considered to be partitioning phenomena.     

Insights into the sorption properties of cutin and cutan biopolymers

Plant cuticles have been reported as highly efficient sorbents for organic compounds. The objective of this study was to elucidate the sorption and desorption behavior of polar and nonpolar organic compounds with the major structural components of the plant cuticle: the biopolymers cutin and cutan. The sorption affinity values of the studied compounds followed the order: phenanthrene > atrazine > chlorotoluron > carbamazepine. A higher sorption affinity of phenanthrene and atrazine to cutin was probably due to the higher level of amorphous paraffinic carbon in this biopolymer. Phenanthrene exhibited reversible sorption behavior and a high ratio of organic-carbon-normalized distribution coefficient (Koc) to carbon-normalized octanol-water partitioning coefficients (Kowc) with both biopolymers. This suggests that both biopolymers provide phenanthrene with a partition medium for hydrophobic interactions with the flexible long alkyl-chain moieties of the biopolymers. The low Koc/Kowc ratios obtained for the polar sorbates suggest that the polar sites in the biopolymers are not accessible for sorption interactions. Atrazine and carbamazepine exhibited sorption-desorption hysteresis with both sorbents, indicating that both sorbates interact with cutin and cutan via both hydrophobic and specific interactions. In general, the sorptive properties of the studied biopolymers were similar, signifying that the active sorption sites are similar even though the biopolymers exhibit different properties.     

Absence of mobile carbohydrate domains in dry humic substances proven by NMR, and implications for organic-contaminant sorption models

The mobility and domain structure of various standard humic substances have been investigated by 1H and 1H-13C solid-state nuclear magnetic resonance (NMR) experiments. In four dry humic acids, a fulvic acid, a natural organic matter sample, and a whole peat sample, segments that undergo fast, large-amplitude motions account for <9% of the sample. This disproves a previous suggestion, based on 1H NMR data, that flexible domains, presumably carbohydrates, make up >40% of various humic acids; these putative mobile domains were also linked to dual-mode sorption. In particular, neither the polar alkyl (carbohydrate) nor the aromatic components show any fast, large-amplitude mobility. A small fraction of mobile nonpolar alkyl segments identified by us before is the only component undergoing large-amplitude motions, apart from absorbed water that we observe in humic acids exposed to ambient air. 1H-13C wide-line separation NMR shows that, contrary to previous suggestions, the dipolar couplings in the aromatic regions are smaller than in the polar alkyl segments, most likely due to differences in local 1H densities. Series of 1H-13C heteronuclear correlation experiments with 1H spin diffusion reveal close proximity of aromatic and polar alkyl segments in several humic acids, precluding carbohydrate domains on a scale of > 1-nm diameter. In the standard peat humic acid, nonpolar aromatic segments also do not form sorption domains of significant size, while nonpolar aliphatic domains, which we had previously shown to correlate with sorption capacity, have been confirmed.     

Polyparameter linear free energy relationships for estimating the equilibrium partition of organic compounds between water and the natural organic matter in soils and sediments

Values of the organic-carbon-based partition coefficient (Koc) have often been estimated using one-parameter linear free energy relationships (op-LFERs), which include both correlations between log Koc and log Kow, where Kow is the octanol-water partition coefficient, and op-LFERs that are based on first-order molecular connectivity indices. For chemicals with tendencies toward strong hydrogen-bonding or other specific interactions with the organic phase, however,these methods are notsufficientlyaccurate. Polyparameter LFERs (pp-LFERs) address these shortcomings by explicitly considering contributions toward free energy change from multiple kinds of molecular interactions with both water and bulk organic phases. This paper reviews pp-LFER theory and presents the development of a new pp-LFER for organic chemical partitioning with soil/sediment organic matter (SOM) using a data set of 356 carefully selected experimental values of log Koc for 75 chemicals, including apolar, monopolar, and bipolar compounds. The paradigm of representing SOM by a single pp-LFER is qualitatively supported by our results, and the set of coefficients for the regression log Koc= (1.10+/-0.10)E - (0.72+/-0.14)S + (0.15+/-0.15)A - (1.98+/-0.14)B + (2.28+/-0.14)V + (0.14+/-0.10) represents a proposed set of water-SOM-specific properties for estimating log Koc. The developed correlation outperformed other currently recommended approaches with the given Koc data set and also compared favorably against the use of new multiple class-specific op-LFER regressions. Overall, the pp-LFER approach is recommended over other current methods for the purpose of Koc estimation and especially for polar chemicals.     

Ambient gas/particle partitioning. 2: The influence of particle source and temperature on sorption to dry terrestrial aerosols

In a companion paper we reported that, for apolar and most polar compounds, the dominating sorption mechanism governing ambient gas/particle partitioning under dry conditions is absorption into a water-insoluble organic matter (WIOM) phase, whereas under moist conditions, polar and ionized compounds can partition additionally into a mixed-aqueous phase. In order to understand how sorption into the WIOM varies for particles from diverse terrestrial locations, we looked at over 500 equilibrium gas/particle partitioning constants, Kip, measured at a specific temperature and relative humidity (15 degrees C, 50% RH), covering aerosol samples from all seasons and various locations. The data indicate that for every sample the WIOM exhibits similar intermolecular interactions with gas-phase organic compounds. For a given compound, the Kip values usually vary within a factor 3 for different aerosol samples, though they could vary by up to an order of magnitude. This is most likely due to variations in the WIOM weight fraction. Fitted poly parameter linear free energy relationships (PP-LFERs) were validated by giving good predictions of Kip values for many SVOCs in the literature, including n-alkanes, organochlorines, PCBs, though not PAHs as much of the particle-bound PAHs are likely nonexchangeable with the air phase. This study also investigated the influence of temperature on partitioning to WIOM, and found thatthe temperature dependence of Kip values can be reasonably predicted using the pure compound's enthalpy of vaporization.     

Sorption mechanisms of phenanthrene, lindane, and atrazine with various humic acid fractions from a single soil sample

The sorption behavior of organic compounds (phenanthrene, lindane, and atrazine) to sequentially extracted humic acids and humin from a peat soil was examined. The elemental composition, XPS and (13)C NMR data of sorbents combined with sorption isotherm data of the tested compounds show that nonspecific interactions govern sorption of phenanthrene and lindane by humic substances. Their sorption is dependent on surface and bulk alkyl carbon contents of the sorbents, rather than aromatic carbon. Sorption of atrazine by these sorbents, however, is regulated by polar interactions (e.g., hydrogen bonding). Carboxylic and phenolic moieties are key components for H-bonding formation. Thermal analysis reveals that sorption of apolar (i.e., phenanthrene and lindane) and polar (i.e., atrazine) compounds by humic substances exhibit dissimilar relationships with condensation and thermal stability of sorption domains, emphasizing the major influence of domain spatial arrangement on sorption of organic compounds with distinct polarity. Results of pH-dependent sorption indicate that reduction in sorption of atrazine by the tested sorbents is more evident than phenanthrene with increasing pH, supporting the dependence of organic compound sorption on its polarity and structure. This study highlights the different interaction mechanisms of apolar and polar organic compounds with humic substances.     

Ambient gas/particle partitioning. 1. Sorption mechanisms of apolar, polar, and ionizable organic compounds

There remain several ambiguities in the literature regarding the dominating sorption mechanisms involved in gas/particle partitioning, particularly for polar and ionizable compounds. The various hypothetical mechanisms would depend differently on relative humidity (RH) and the presence of various aerosol components. Thus, in order to resolve these ambiguities, here we measured the RH-dependency of gas/particle partitioning constants, K(ip), for four diverse aerosol samples and a large set of chemicals covering apolar, polar, and ionizable organic compounds. In addition, we also removed the water-soluble components from two ambient particle samples to study how their presence influences sorption behavior. The measured K(ip) values collectively indicate that a dual-phase sorption mechanism is occurring, in which organic compounds partition into a RH-independent water-insoluble organic matter phase and additionally into a RH-dependent mixed-aqueous phase. All K(ip) values could be successfully fitted to a RH-dependent dual-phase sorption model. The trends in K(ip) data further support findings that the sorption behavior of ambient aerosol samples is different from raw mineral surfaces and soot.     

Estimating partition coefficients for fuel-water systems: developing linear solvation energy relationships using linear solvent strength theory to handle mixtures

In many environmental transport problems, organic solutes partition between immiscible phases that consist of liquid mixtures. To estimate the corresponding partition coefficients, we evaluated the efficacy of combining linear solvation energy relationships (LSERs) developed for pure 1:1 systems via application of linear solvent strength theory. In this way, existing LSERs could be extended to treat solute partitioning from gasoline, diesel fuel, and similar mixtures into contacting aqueous mixtures. Unlike other approaches, this method allowed prediction of liquid-liquid partition coefficients in a variety of fuel-water systems for a broad range of dilute solutes. When applied to 37 polar and nonpolar solutes partitioning between an aqueous mixture and 12 different fuel-like mixtures (many including oxygenates), the root-mean-squared error was a factor of approximately 2.5 in the partition coefficient. This was considerably more accurate than application of Raoult's law for the same set of systems. Regulators and scientists could use this method to estimate fuel-water partition coefficients of novel additives in future fuel formulations and thereby provide key inputs for environmental transport assessments of these compounds.     

Environmental influences on the partitioning and diffusion of hydrophobic organic contaminants in microbial biofilms

A biofilm reactor was used to investigate kinetic and thermodynamic aspects of the sorption of polycyclic aromatic hydrocarbons (PAH) as model compounds for hydrophobic organic contaminants (HOC) to intact microbial biofilms. Effective diffusion coefficients are in the range of 10(-10) cm2 x s(-1) resulting in equilibration times of more than 3 days for a biofilm of 100 microm thickness. Diffusion in the biofilm was strongly temperature-dependent and increased by a factor of 3 (phenanthrene) to 6 (fluoranthene, pyrene) between 5 and 35 degrees C. Drying and rewetting of the biofilm as well as the inclusion of Ca2+ ions and of humic acids all strengthened the biofilm rigidity and slowed down the diffusion of PAH. The later two factors also influenced the thermodynamics of the process as they supported the partitioning of PAH into the biofilm. Humic acid inclusion from solution into the biofilm illustrates that a microbial biofilm can act as a primer allowing for the buildup of a particulate organic phase from dissolved organic matter. PAH metabolites (3-hydroxy-phenanthrene and 1-hydroxy-2-naphthoic acid) showed lower partition coefficients as compared to their parent compounds and 3-hydroxy-phenanthrene also showed a higher diffusion constant, indicating that these transformation products would be easily released into the water phase upon formation during PAH biodegradation in a biofilm. These results allow the quantification of the influence of environmental conditions on a biofilm's function as a sink or as a diffusion barrier for PAH from aqueous solution, and they indicate the importance of kinetic aspects of this partitioning process.     

Thermal analytical investigation of biopolymers and humic- and carbonaceous-based soil and sediment organic matter

Improved understanding of the physical, chemical, and thermodynamic properties of soil and sediment organic matter (SOM) is crucial in elucidating sorption mechanisms of hydrophobic organic compounds (HOCs) in soils and sediments. In this study, several thermoanalytical techniques, including thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), temperature-modulated differential scanning calorimetry (TMDSC), and thermal mechanical analysis (TMA) were applied to 13 different organic materials (three woods, two humic acids, three kerogens, and five black carbons) representing a spectrum of diagenetic and/or thermal histories. Second-order thermal transition temperatures (T(t)) were identified in most materials, where the highest observed T(t) values (typically characterized as glass transition temperatures (T(g were shown to closely relate to chemical characteristics of the organic samples as influenced by diagenetic or thermal alteration. Results further suggest a positive correlation between glass transition temperature and a defined diagenetic/thermal index, where humic-based SOM (e.g., humic and fulvic acids) possess lowertransition temperatures than more "mature" carbonaceous-based SOM (i.e., kerogens and black carbons). The observed higher thermal transition temperature of aliphatic-rich Green River shale kerogen (approximately 120 degrees C) relative to that of aromatic-rich humic acids suggests that a sole correlation of aromaticity to thermal transition temperature may be inappropriate.     

Role of the extractable lipids and polymeric lipids in sorption of organic contaminants onto plant cuticles

The distinct role of extractable and polymeric lipids in plant cuticle, precursors of SOM, has received scarce attention to elucidate plant uptake and soil affinity with organic contaminants. Sorption of naphthalene and 1-naphthol to fruit cuticular fractions isolated from two species were investigated. The polarity index, physical conformation, and glass transition temperature (Tg) of these cuticular fractions were characterized by elemental analysis, Fourier transform infrared spectroscopy, and differential scanning calorimetry, respectively. Cutin, a polymeric lipid, is the major sorption medium of the cuticle due to its large mass fraction and liquid-like nature (Tg approximately -30 degrees C). Sorption of cutin is suppressed by the extractable lipids (wax, Tg approximately 44 degrees C) acting as an antiplasticizer (enhance cutin's Tg) over nonpolar contributor. Whereas polysaccharide, as a plasticizer (lower Tg value) and polar contributor, regulates affinity of polymeric lipids (cutin and cutan). The contribution of cutin to sorption by bulk cuticle overshadows the role of waxes, and the sorption capability (K(oc)) of cutin overwhelms the octanol-water partition coefficient (K(ow)). Therefore, uptake of organic contaminants by these plants would be seriously under-predicted by their extractable lipid content and compound's K(ow) values. Along with the observed linear relationships of K(oc) with cutin content in these cuticular fractions, we suggest for the first time that the depolymerizable lipid fraction (cutin) is required to accurately predict plant accumulation of organic contaminants.     

Variation in phenanthrene sorption coefficients with soil organic matter fractionation: the result of structure or conformation?

Sorption of phenanthrene to varying soil types was investigated to better understand sorption processes. Humic acid and humin fractions were isolated from each soil sample, and sorption coefficients were measured by batch equilibration. Samples were characterized by carbon analysis and 13C cross polarization magic angle spinning (CP/ MAS) nuclear magnetic resonance (NMR) spectroscopy. Measured organic carbon-normalized sorption coefficients (Koc) of the fractions were greater in all cases when compared to the soils. The humin fractions exhibited greater Koc values than did source samples, suggesting that fractionation may reorganize organic matter in humin resulting in an increased availability of and/or more favorable sorption domains. Mass balance calculations revealed that the sum of sorption to the fractions is greater than sorption to the whole sample. The greatest difference between sorption values was found to occur with the mineral soils, suggesting that clay minerals influence the physical conformation of soil organic matter (SOM) and availability of sorption domains. The mass balance, sorption data, and a lack of consistent trends between observed Kco values and solid-state 13C NMR data suggest that the physical conformation of SOM and chemical characteristics both play important roles in sorption processes.     

Immobilizing humic acid in a sol-gel matrix: a new tool to study humic-contaminants sorption interactions

Humic substances originated from aquatic, soil, or sediment environments are mixtures of humic compounds with various characteristics. Sorption interactions with isolated, well defined humic fractions can be studied either in an aqueous phase ("dissolved humic substances"), or in a solid-phase, by coating mineral particles with the humic materials, or simply by working with humic acid particles (powder) at low pH to minimize dissolution. Each attitude, by definition, can be studied by different experimental techniques and has a different meaning for understanding natural environmental processes. In this study, a new tool for studying sorption interactions is presented. Sol-gel was used as an inert matrix to immobilize (entrap) various humic acids (HAs), and then used to study the interactions of several polycyclic aromatic hydrocarbons (PAHs) with the entrapped HA. Linear and nonlinear sorption coefficients were highly correlated with contaminant hydrophobicity. Sorption of pyrene to immobilized HA was in the order of soil HA > Aldrich HA approximately = peat HA. It was concluded that the entrapped HAs retained their original properties in the gel matrix and were accessible to the external contaminant through the pore network. Additionally, binding coefficients of pyreneto dissolved humic substances and to dissolved organic matter (DOM) were determined from the reduction in pyrene sorption to immobilized HA in the presence of dissolved humic material or DOM in solution. Binding coefficients of pyrene were in the order of the following: dissolved Aldrich HA > dissolved peat fulvic acid (FA) > DOM derived from mature compost > DOM derived from fresh compost.     

Linear free energy relationships used to evaluate equilibrium partitioning of organic compounds

In environmental chemistry, one often wants to interpret or predict equilibrium partitioning of organic compounds between any two phases. Hence, one needs to understand the partition variability that stems from using different types of compounds and the variability that arises from looking at different natural phases, e.g. different soil organic matter. It is current practice in environmental chemistry to evaluate equilibrium partitioning with the help of double logarithmic correlations between the unknown partition constant and a well-known partition constant of the compounds, e.g., partitioning between natural organic matter and water or air is correlated with the octanol/water or octanol/air partition constant, respectively. However, these relationships (in the following called one-parameter LFERs) can only predict the compound variability within a single substance class. They supply no means to understand the variability between substance classes or the variability between different natural organic phases. The reasons for these limitations are that (a) the complete compound variability cannot be described by a single parameter because partitioning results from different kinds of interactions that vary independently from each other and (b) the specific properties of the studied phase are represented in the slope and intercept of the double logarithmic correlation and not in a variable parameter. In contrastto one-parameter LFERs, polyparameter LFERs are based on a concept that considers all interactions involved in partitioning by separate parameters. They allow for predicting the complete compound variability by a single equation, and they also provide the possibility to evaluate and predict the variability in the sorption characteristics of different natural phases. Thus future research in the field of environmental partition processes should focus on adapting and improving the more comprehensive polyparameter LFERs rather than trying to refine existing one-parameter LFERs.