Prediction of the sorption of organic compounds into soil organic matter from molecular structure

A new model to estimate the soil-water partition coefficient of non-ionic organic compounds normalized to soil organic carbon, Koc, from the two-dimensional molecular structure is presented. Literature data of log Koc for 571 organic chemicals were fitted to 29 parameters with a squared correlation coefficient r2 of 0.852 and a standard error of 0.469 log units. The application domain includes the atom types C, H, N, O, P, S, F, Cl, and Br in various important compound classes. The multilinear model contains the variables molecular weight, bond connectivity, molecular E-state, an indicator for nonpolar and weakly polar compounds, and 24 fragment corrections representing polar groups. The prediction capability is evaluated through an initial two-step development using an 80%:20% split of the data into training and prediction, cross-validation, permutation, and application to three external data sets. The discussion includes separate analyses for subsets of H-bond donors and acceptors as well as for nonpolar and weakly polar compounds. Comparison with existing models including linear solvation energy relationships illustrates the superiority of the new model.     

An isotope exchange technique to assess mechanisms of sorption hysteresis applied to naphthalene in kerogenous organic matter

The sorption of organic compounds to natural sorbents is often found to show hysteresis. The objective of this study was to develop an experimental technique based on the use of 14C isotopes to distinguish hysteresis due to experimental artifacts from true hysteresis due to thermodynamically irreversible processes. The study was also designed to investigate causation of true hysteresis (irreversible sorption). The technique determines the rates and the degree of isotope exchange (IE) on equilibrated sorption and desorption points at different constant bulk chemical concentrations. The technique was applied to the sorption of naphthalene (NAPH) on Beulah-Zap lignite, a low rank reference coal composed mainly of kerogen. Sorption of bulk was found to be reversible below 10(-5) g L(-1), but irreversible above 10(-4) g L(-1). Complete isotope exchange on sorption and desorption points that defined an irreversible cycle demonstrated that hysteresis was true. A comparison of normalized uptake and release kinetics of labeled and bulk NAPH at different concentrations revealed slow structural deformation processes of the sorbent during bulk sorption and desorption. This is taken as corroborating evidence for the pore deformation hypothesis of hysteresis in which incoming sorbate molecules induce quasi-reversible changes in the organic matter that lead to different pathways for sorption and desorption. Although unable to rule it out completely, the data demonstrate that physical entrapment of sorbate molecules plays a minor, if any, role to the observed hysteresis in this system.     

Kinetics of Ni sorption in soils: roles of soil organic matter and Ni precipitation

The kinetics of Ni sorption to two Delaware agricultural soils were studied to quantitatively assess the relative importance of Ni adsorption on soil organic matter (SOM) and the formation of Ni layered double hydroxide (Ni-LDH) precipitates using both experimental studies and kinetic modeling. Batch sorption kinetic experiments were conducted with both soils at pH 6.0, 7.0, and 7.5 from 24 h up to 1 month. Time-resolved Ni speciation in soils was determined by X-ray absorption spectroscopy (XAS) during the kinetic experiments. A kinetics model was developed to describe Ni kinetic reactions under various reaction conditions and time scales, which integrated Ni adsorption on SOM with Ni-LDH precipitation in soils. The soil Ni speciation (adsorbed phases and Ni-LDH) calculated using the kinetics model was consistent with that obtained through XAS analysis during the sorption processes. Under our experimental conditions, both modeling and XAS results demonstrated that Ni adsorption on SOM was dominant in the short term and the formation of Ni-LDH precipitates accounted for the long-term Ni sequestration in soils, and, more interestingly, that the adsorbed Ni may slowly transfer to Ni-LDH phases with longer reaction times.     

Identification and characterization of sorption domains in soil organic matter using strucuturally modified humic acids

The sorption of phenanthrene was examined in humic acids (HAs) from different sources: a compost, a peat soil, and a mineral soil. Sub-samples of each HA were subjected to bleaching or hydrolysis to remove predetermined chemical groups from their structures. Bleaching successfully removed a large percentage of rigid, aromatic moieties, whereas hydrolysis removed the mobile, carbohydrate components. Phenanthrene sorption by all HAs was nonlinear (N < 1). However, the phenanthrene isotherms of the bleached HAs were more linear than those of the untreated HAs, whereas the removal of the carbohydrate components by hydrolysis produced more nonlinear isotherms. The introduction of pyrene to the phenanthrene sorption system yielded more linear isotherms for all the HAs, indicative of competitive sorption. Proton spin-spin (1H T2) relaxation determined by nuclear magnetic resonance (NMR) was used to identify separate rigid (condensed) and flexible (expanded) 1H populations and to determine their distribution. These 1H domains were highly sensitive to temperature and correlated well with reported glass transition temperatures for HAs. In combination with the chemical treatments, sorption, and spectroscopic data, we were able to observe some significant relationships among chemical groups, sorption behavior, and structural characteristics.     

History-dependent sorption in humic acids and a lignite in the context of a polymer model for natural organic matter

We examined sorption of two apolar compounds in three samples of macromolecular natural organic matter (NOM) in order to test whether history-dependent ("irreversible") behaviors, including sorption hysteresis and the conditioning effect, agree with a pore deformation/creation hypothesis applicable to the glassy organic solid state as proposed in the polymer literature. The compounds are 1,2,4-trichlorobenzene (TCB) and naphthalene (Naph). The NOM samples are a soil humic acid (H-HA), an Al3+-exchanged form of the same humic acid (Al-HA), and a low-rank coal (Beulah-Zap lignite, BZL). The HAs, at least, are believed free of environmental black carbon. The degree of nonlinearity in the isotherm and the ratio of hole-filling to solid-phase dissolution increased in the order of hardness (stiffness) of the solid: H-HA < Al-HA < BZL. Independent of solid, solutes show a 14-18 kJ/mol preference for hole "sites" as compared to dissolution "sites", which we attribute to the free energy needed in the dissolution domain to create a cavity to accommodate the solute. All solids exhibited hysteresis and the conditioning effect, which refers to enhanced re-sorption after pretreatment with a conditioning agent (in this case, chlorobenzene). Conditioning the sample results in increased sorption and increased contribution of hole-filling relative to dissolution. The effects of original hole population, matrix stiffness, and solute concentration on the hysteresis index and on the magnitude of the conditioning effect are consistent with a pore-deformation mechanism as the underlying cause of sorption irreversibility. This mechanism involves concurrent processes of irreversible hole expansion and the creation of new holes by the incoming sorbate (or conditioning agent). The results show that nonlinear and irreversible behavior may be expected for macromolecular forms of NOM that are in a glassy state and emphasize the case that NOM is not a passive sorbent but may be physically altered by the sorbate.     

Effects of competitor and natural organic matter characteristics on the equilibrium sorption of 1,2-dichlorobenzene in soil and shale

The competitive sorption behaviors of 1,2-DCB in binary solute systems in four natural sorbents having natural organic matter (NOM) matrixes of different physicochemical characters were investigated in batch reactors. Specifically, the study focused on investigating how the extent of 1,2-DCB competitive sorption depends on (i) the rigidity of NOM matrixes as assessed by the efficiency of chemical oxidation and (ii) the closeness of competitor structure to that of the primary solute. The chemical oxidation and elemental composition results suggest that the shale NOM is the most reduced and condensed, the peat was the most oxidized and amorphous, and two surface soils had intermediate NOM structures. Four chlorinated benzenes and phenanthrene were used as competing solutes. All five chemicals exhibited competition against 1,2-DCB in all sorbents, including the peat, but the extent of competition varied significantly. Little difference in the extent of competition with 1,2-DCB was observed for the various chlorinated benzenes even though some were liquids and some were solids at the experimental temperature. All of the chlorobenzenes were more effective competitors than phenanthrene. The shale showed markedly different competition features from the other sorbents, with a much smaller competitive effect at a given sorbed volume of competitor. However, normalizing sorbed competitor volumes by the capacity of the adsorption domain in the Polanyi-Manes single-solute partition-adsorption model (V0) produced qualitatively similar competitive behavior for each solute; displacement of 1,2-DCB increased with increasing sorbed competitor volumes up to V0, and little additional competition occurred beyond that point. The extent of competition was positively correlated with the maximum adsorption capacity and the fraction of "hard" and "soot" carbon contents as assessed by chemical and thermal oxidation methods. These findings indicate that competition is associated with voids in the NOM structure, that these voids are likely present within the condensed ("hard" plus "soot") carbon domain, and therefore that diagenetic alteration of NOM plays a central role in determining competitive sorption characteristics for hydrophobic contaminants.     

Investigation of the role of structural domains identified in sedimentary organic matter in the sorption of hydrophobic organic compounds

The role of composition and structure of sedimentary organic matter (SOM) in the sorption of hydrophobic organic compounds (HOCs) was investigated by spiking 13C-labeled phenanthrene onto six estuarine sediments known to vary in SOM content and character. After equilibration and HF treatment, 13C NMR cross polarization and stable carbon isotope analyses indicated that the amount of desorption-resistant phenanthrene was related to aromatic carbon content. Application of the 13C NMR spectral editing technique proton spin relaxation editing (PSRE) demonstrated that all samples consisted of a rapidly relaxing and a slowly relaxing component, further evidence that SOM can be described as a structurally heterogeneous sorbent. Further, comparison of corresponding control and spiked PSRE subspectra revealed that, for each of the six sediments, desorption-resistant phenanthrene had become associated almost exclusively with the rapidly relaxing component. In only two of the sediments were there even small amounts of phenanthrene discernible in the slowly relaxing component, which is signficant as it was not always true that aromatic carbon was concentrated exclusively in the rapidly relaxing phase. The implication of these findings is that not all aromatic fractions have the same affinity for phenanthrene and that some fractions may indeed have little affinity at all. These results were interpreted as indicative that rapidly relaxing aromatic carbon associated with either sediment-associated charcoal or diagenetic organic matter plays a controlling role in the sorption of HOCs. However, the exact manner in which this rapidly relaxing aromatic phase relates to models presented elsewhere remains unclear.     

Strong sorption of phenanthrene by condensed organic matter in soils and sediments

The nonhydrolyzable carbon (NHC) and black carbon (BC) in three contaminated soils and seven sediments from the Pearl River Delta and Estuary, China, were isolated upon treatments with an acid hydrolysis method and with a combustion method at 375 degrees C, respectively, and their sorption isotherms for phenanthrene (Phen) were established. It was found that NHC is chemically and structurally different from the biopolymer and humic substances and consists mainly of aliphatic and aromatic carbon using elemental analysis, 13C nuclear magnetic resonance spectroscopy (13C NMR), and Fourier transformed infrared spectroscopy (FTIR). All the sorption isotherms are nonlinear and are well fitted by the Freundlich model. The single-point organic carbon-normalized distribution coefficient (K(oc)) measured for the isolated NHC is 1.3-7.7 times higher than that for the bulk samples at the same aqueous concentration of Phen. The NHC fractions play a dominant role to the overall sorption in the bulk samples. The bulk soils and their NHC fractions have lower sorption capacity than the bulk sediments and their NHC fractions, relating to the different source of organic matter between soils and sediments. The Phen sorption capacity in the NHC samples is related significantlyto H/C ratios and aliphatic carbon, but negatively to aromatic carbon, demonstrating the important role of aliphatic carbon to the Phen sorption and the fate in the investigated soils and sediments.     

Evidence for pi-pi electron donor-acceptor interactions between pi-donor aromatic compounds and pi-acceptor sites in soil organic matter through pH effects on sorption

Elucidation of molecular-level interactions controlling the sorption of organic compounds in soils is of major theoretical and practical interest. Sorption of pi-electron donor compounds, pentamethylbenzene (PMB), naphthalene (NAPH), and phenanthrene (PHEN), in a number of soils was found to increase with decreasing pH in the range of approximately pH 2.5-7. This behavior could not be attributed to pH-dependent alteration of the hydrophobic character of humic substances, pi-H-bonding, interaction with mineral surfaces, interaction with black carbons, solute coplanarity, or pH effects on solute activity coefficient. No significant effect of pH was observed for non-pi-donor hydrophobic compounds, whether planar or not: trans-1,2-dichlorocyclohexane (DCCH), hexachloro-1,3-butadiene (HCBD), 1,2,4-trichlorobenzene (TCB), 2,2',5,5'-tetrachlorobiphenyl, and 3,3',4,4'-tetrachlorobiphenyl. The opposite pH effect was observed for 2-nonanol and 2-nonanone, which are non-pi-donors, but capable of H-bonding. Also, no pH-dependent sorption was observed between the pi-donor PHEN and alumina, a model inorganic surface. We propose that the pi-donor solutes interact with pi-acceptor sites in soil organic matter (SOM), including aromatic rings with multiple carboxyl groups, aromatic amines, or heteroaromatic amines. The pi-acceptor ability of such aromatic moieties would increase with protonation. pi-pi Interactions between PMB, NAPH, and PHEN, and model SOM acceptors, 1,3,5-benzenetricarboxylic acid (BTA), 1,4,5,8-naphthalenetetracarboxylic acid (NTA), and pyridine (PY) in methanol and methanol-water, were verified by the appearance of pH-dependent upfield 1H NMR chemical shifts induced by ring current effects. UV/vis spectra showed pH-dependent charge-transfer bands for various donors with NTA. No NMR shifts or charge-transfer bands were found for nondonor compounds paired with the model acceptors.     

Binding of organic ligands with Al(III) in dissolved organic matter from soil: implications for soil organic carbon storage

The binding characteristics of organic ligands with Al(III) in soil dissolved organic matter (DOM) is essential to understand soil organic carbon (SOC) storage. In this study, two-dimensional (2D) FTIR correlation spectroscopy was developed as a novel tool to explore the binding of organic ligands with Al(III) in DOM present in soils as part of a long-term (21-year) fertilization experiment. The results showed that while it is a popular method for characterizing the binding of organic ligands and metals, fluorescence excitation-emission matrix-parallel factor analysis can only characterize the binding characteristics of fluorescent substances (i.e., protein-, humic-, and fulvic-like substances) with Al(III). However, 2D FTIR correlation spectroscopy can characterize the binding characteristics of both fluorescent and nonfluorescent (i.e., polysaccharides, lipids, and lignin) substances with Al(III). Meanwhile, 2D FTIR correlation spectroscopy demonstrated that the sequencing/ordering of organics binding with Al(III) could be modified by the use of long-term fertilization strategies. Furthermore, 2D FTIR correlation spectroscopy revealed that the high SOC content in the chemical plus manure (NPKM) treatment in the long term fertilization experiment can be attributed to the formation of noncrystalline microparticles (i.e., allophane and imogolite). In summary, 2D FTIR correlation spectroscopy is a promising approach for the characterization of metal-organic complexes.     

Hydration of natural organic matter: effect on sorption of organic compounds by humin and humic acid fractions vs original peat material

Natural organic matter (NOM) hydration is found to change activity-based sorption of test organic compounds by as much as 2-3 orders of magnitude, depending on the compound and the specific NOM sorbent. This is demonstrated for sorption on humin, humic acid, and the NOM source material. Hydration assistance in organic compound sorption correlates with the ability of the sorbate to interact strongly with hydrated sorbents, demonstrating the important role of noncovalent polar links in organizing the sorbent structure. Differences in hydration effect between the sorbents are caused mainly by differences in compound-sorbent interactions in the dry state. For a given compound, hydration of the sorbent tends to equalize the sorption capability of the three sorbents. No correlation was found between the strength of sorbate-sorbent interactions or the type of sorbate functional groups and the extent of sorption nonlinearity. Sorption nonlinearity compared over the same sorbed concentration range is greater on the original NOM than on either of the two extracted fractions. In elucidating sorption mechanisms on hydrated NOM, it is important to explicitly consider the participation of water molecules in organic compound interactions in the NOM phase.     

Interactive priming of biochar and labile organic matter mineralization in a smectite-rich soil

Biochar is considered as an attractive tool for long-term carbon (C) storage in soil. However, there is limited knowledge about the effect of labile organic matter (LOM) on biochar-C mineralization in soil or the vice versa. An incubation experiment (20 °C) was conducted for 120 days to quantify the interactive priming effects of biochar-C and LOM-C mineralization in a smectitic clayey soil. Sugar cane residue (source of LOM) at a rate of 0, 1, 2, and 4% (w/w) in combination with two wood biochars (450 and 550 °C) at a rate of 2% (w/w) were applied to the soil. The use of biochars (~ -36‰) and LOM (-12.7‰) or soil (-14.3‰) with isotopically distinct δ(13)C values allowed the quantification of C mineralized from biochar and LOM/soil. A small fraction (0.4-1.1%) of the applied biochar-C was mineralized, and the mineralization of biochar-C increased significantly with increasing application rates of LOM, especially during the early stages of incubation. Concurrently, biochar application reduced the mineralization of LOM-C, and the magnitude of this effect increased with increasing rate of LOM addition. Over time, the interactive priming of biochar-C and LOM-C mineralization was stabilized. Biochar application possesses a considerable merit for long-term soil C-sequestration, and it has a stabilizing effect on LOM in soil.     

Effect of sodium and calcium cations on the ion-exchange affinity of organic cations for soil organic matter

Sorption of organic cations to soil organic matter was studied using dynamic column experiments with different compositions of electrolytes in aqueous eluents. The sorption affinity of the tested variety of charged compounds, including primary, secondary, and tertiary amines and quaternary ammonium compounds, all showed the same response to different medium compositions. The sorption affinity to Pahokee peat (i) strongly decreased with increasing electrolyte concentration, up to a factor 250 due to tested electrolyte compositions alone, (ii) was higher in NaCl solutions than in CaCl(2) solutions of similar ionic strength, and (iii) was more sensitive to a decrease in NaCl than to a decrease in CaCl(2), though the selectivity coefficients were not significantly different. For a weak base that was tested in eluent pH either above or below its pK(a), we demonstrated that the sorption affinity of (iv) the neutral base was hardly affected by different electrolyte compositions, comparable to a neutral reference compound, (v) the protonated weak base was strongly affected by different electrolyte compositions, and (vi) the protonated base was in the same range, or stronger, compared to the neutral base. Mass action law equations for ion-exchange reactions predicted similar trends in a qualitative but not in a quantitative way. More complex models are required to fully account for the contributions of ionic interactions to the sorption of organic cations. These results imply that risk assessment models for organic bases should take ion-exchange processes into account when estimating soil sorption coefficients and bioavailability.     

Organic matter chlorination rates in different boreal soils: the role of soil organic matter content

Transformation of chloride (Cl(-)) to organic chlorine (Cl(org)) occurs naturally in soil but it is poorly understood how and why transformation rates vary among environments. There are still few measurements of chlorination rates in soils, even though formation of Cl(org) has been known for two decades. In the present study, we compare organic matter (OM) chlorination rates, measured by (36)Cl tracer experiments, in soils from eleven different locations (coniferous forest soils, pasture soils and agricultural soils) and discuss how various environmental factors effect chlorination. Chlorination rates were highest in the forest soils and strong correlations were seen with environmental variables such as soil OM content and Cl(-) concentration. Data presented support the hypothesis that OM levels give the framework for the soil chlorine cycling and that chlorination in more organic soils over time leads to a larger Cl(org) pool and in turn to a high internal supply of Cl(-) upon dechlorination. This provides unexpected indications that pore water Cl(-) levels may be controlled by supply from dechlorination processes and can explain why soil Cl(-) locally can be more closely related to soil OM content and the amount organically bound chlorine than to Cl(-) deposition.     

Carbon sequestration in soil by in situ catalyzed photo-oxidative polymerization of soil organic matter

Here we describe an innovative mechanism for carbon sequestration in soil by in situ photopolymerization of soil organic matter under biomimetic catalysis. Three different Mediterranean soils were added with a synthetic water-soluble iron-porphyrin, irradiated by solar light, and subjected first to 5 days incubation and, then, 15, and 30 wetting and drying (w/d) cycles. The in situ catalyst-assisted photopolymerization of soil organic carbon (SOC) increased water stability of soil aggregates both after 5 days incubation and 15 w/d cycles, but not after 30 w/d cycles. Particle-size distribution of all treated soils confirmed the induced soil physical improvement, by showing a concomitant lower yield of the clay-sized fraction and larger yields of either coarse sand- or fine sand-size fractions, depending on soil texture, though only after 5 days incubation. The gain in soil physical quality was reflected by the shift of OC content from small to large soil aggregates, thereby suggesting that photopolymerization stabilized OC by both chemical and physical processes. A further evidence of the carbon sequestration capacity of the photocatalytic treatment was provided by the significant reduction of CO(2) respired by all soils after both incubation and w/d cycles. Our findings suggest that "green" catalytic technologies may potentially be the bases for future practices to increase soil carbon stabilization and mitigate CO(2) emissions from arable soils.