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.

Effect of biochar amendment on compost organic matter composition following aerobic composting of manure.

Biochar, a material defined as charred organic matter applied in agriculture, is suggested as a beneficial additive and bulking agent in composting. Biochar addition to the composting feedstock was shown to reduce greenhouse gas emissions and nutrient leaching during the composting process, and to result in a fertilizer and plant growth medium that is superior to non-amended composts. However, the impact of biochar on the quality and carbon speciation of the organic matter in bulk compost has so far not been the focus of systematic analyses, although these parameters are key to determine the long-term stability and carbon sequestration potential of biochar-amended composts in soil. In this study, we used different spectroscopic techniques to compare the organic carbon speciation of manure compost amended with three different biochars. A non-biochar-amended compost served as control. Based on Fourier-transformed infrared (FTIR) and 13C nuclear magnetic resonance (NMR) spectroscopy we did not observe any differences in carbon speciation of the bulk compost independent of biochar type, despite a change in the FTIR absorbance ratio 2925cm-1/1034cm-1, that is suggested as an indicator for compost maturity. Specific UV absorbance (SUVA) and emission-excitation matrixes (EEM) revealed minor differences in the extractable carbon fractions, which only accounted for ~2-3% of total organic carbon. Increased total organic carbon content of biochar-amended composts was only due to the addition of biochar-C and not enhanced preservation of compost feedstock-C. Our results suggest that biochars do not alter the carbon speciation in compost organic matter under conditions optimized for aerobic decomposition of compost feedstock. Considering the effects of biochar on compost nutrient retention, mitigation of greenhouse gas emissions and carbon sequestration, biochar addition during aerobic composting of manure might be an attractive strategy to produce a sustainable, slow release fertilizer.

Organic coating on biochar explains its nutrient retention and stimulation of soil fertility.

Amending soil with biochar (pyrolized biomass) is suggested as a globally applicable approach to address climate change and soil degradation by carbon sequestration, reducing soil-borne greenhouse-gas emissions and increasing soil nutrient retention. Biochar was shown to promote plant growth, especially when combined with nutrient-rich organic matter, e.g., co-composted biochar. Plant growth promotion was explained by slow release of nutrients, although a mechanistic understanding of nutrient storage in biochar is missing. Here we identify a complex, nutrient-rich organic coating on co-composted biochar that covers the outer and inner (pore) surfaces of biochar particles using high-resolution spectro(micro)scopy and mass spectrometry. Fast field cycling nuclear magnetic resonance, electrochemical analysis and gas adsorption demonstrated that this coating adds hydrophilicity, redox-active moieties, and additional mesoporosity, which strengthens biochar-water interactions and thus enhances nutrient retention. This implies that the functioning of biochar in soil is determined by the formation of an organic coating, rather than biochar surface oxidation, as previously suggested.

Electron transfer between iron minerals and quinones: estimating the reduction potential of the Fe(II)-goethite surface from AQDS speciation.

Redox reactions at iron mineral surfaces play an important role in controlling biogeochemical processes of natural porous media such as sediments, soils and aquifers, especially in the presence of recurrent variations in redox conditions. Ferrous iron associated with iron mineral phases forms highly reactive species and is regarded as a key factor in determining pathways, rates, and extent of chemically and microbially driven electron transfer processes across the iron mineral-water interface. Due to their transient nature and heterogeneity a detailed characterization of such surface bound Fe(II) species in terms of redox potential is still missing. To this end, we used the nonsorbing anthraquinone-2,6-disulfonate (AQDS) as a redox probe and studied the thermodynamics of its redox reactions in heterogeneous iron systems, namely goethite-Fe(II). Our results provide a thermodynamic basis for and are consistent with earlier observations on the ability of AQDS to "shuttle" electrons between microbes and iron oxide minerals. On the basis of equilibrium AQDS speciation we reported for the first time robust reduction potential measurements of reactive iron species present at goethite in aqueous systems (EH,Fe-GT ≈ -170 mV). Due to the high redox buffer intensity of heterogeneous mixed valent iron systems, this value might be characteristic for many iron-reducing environments in the subsurface at circumneutral pH. Our results corroborate the picture of a dynamic remodelling of Fe(II)/Fe(III) surface sites at goethite in response to oxidation/reduction events. As quinones play an essential role in the electron transport systems of microbes, the proposed method can be considered as a biomimetic approach to determine "effective" biogeochemical reduction potentials in heterogeneous iron systems.

Pb(II) binding to humic substances: an equilibrium and spectroscopic study.

The binding of Pb(II) to humic acids is studied through an approach combining equilibrium and spectroscopic measurements. The methods employed are potentiometric and fluorometric titrations, fluorescence excitation-emission matrices (EEM) and IR spectroscopy. Potentiometric titration curves are analyzed using the NICA equations and an electrostatic model treating the humic particles as an elastic polyelectrolyte network. EEMs are analyzed using parallel factor analysis, decomposing the signal in its independent components and finding their dependence on Pb(II) activity. Potentiometric results are consistent with bimodal affinity distributions for Pb(II) binding, whereas fluorometric titrations are explained by monomodal distributions. EEM analysis is consistent with three independent components in the humic fluorescence response, which are assigned to moieties with different degree of aromaticity. All three components show a similar quenching behavior upon Pb(II) binding, saturating at relatively low Pb(II) concentrations. This is attributed to metal ion induced aggregation of humic molecules, resulting in the interaction between the aromatic groups responsible for fluorescence; this is also consistent with IR spectroscopy results. The observed behavior is interpreted considering that initial metal binding (observed as strongly binding sites), correspond to bi- or multidentate complexation to carboxylate groups, including binding between groups of different humic molecules, promoting aggregation; further metal ions (observed as weakly binding sites) bind to single ligand groups.

Paradoxical cytotoxicity of tert-butylhydroquinone in vitro: What kills the untreated cells?

At high concentrations, tert-butylhydroquinone (tBHQ), a phenolic antioxidant frequently used as a food preservative, exerts cytotoxic effects, which are closely linked to its ability to form reactive oxygen species as a consequence of redox cycling processes. Here we describe that treatment of murine 3T3 cells with 300 µg/ml of tBHQ in 96-well culture plates induces the death of untreated cells in neighboring wells on the same plate. The mechanisms underlying that effect were investigated. Death of the seemingly untreated neighboring cells was caused by the more toxic and volatile tBHQ oxidation product tert-butyl-p-benzoquinone (tBQ) present at up to 3 µg/ml in the untreated neighboring wells. tBQ was formed from tBHQ in a non-enzymatic process involving copper ions and oxygen. The unexpected perturbation of cytotoxicity testing following treatment with tBHQ by its volatile metabolite tBQ shows that not only metabolic processes but also non-enzymatic mechanisms have to be considered as important parameters for in vitro assays. Furthermore, our data show that even cells several wells away from the treated wells do not necessarily constitute proper "untreated" controls when cells are treated with the frequently used compound tBHQ. This might lead to an underestimation of the effects observed on the Nrf2 signaling pathway, where tBHQ is frequently used as an inductor in vitro.

Modeling ion binding to humic substances: elastic polyelectrolyte network model.

A new model for the electrostatic contribution to ion binding to humic substances is proposed and applied to published data for proton binding to fulvic and humic acids. The elastic polyelectrolyte network model treats humic substance particles as composed by two parts, an external one directly in contact with the solution, and an internal part or gel fraction which is considered, from a statistical point of view, as a charged polymer network swelled by the electrolyte solution, in the framework of the Flory polymer network theory. The electrostatic effect is given by a Donnan-like potential, which can be regarded as an average value over the gel fraction of the humic particle. The gel fraction expands as the pH and humic charge are increased, determining the Donnan potential and consequently the ion activity inside the gel. The model was fitted to published experimental data with good agreement. The model predictions are discussed, and the behavior suggests, for some cases, the presence of a transition between closed and open structures attributed to the presence, at low pH, of intramolecular hydrogen bonds which are removed as the carboxylic sites become deprotonated.

Application of a constrained regularization method to extraction of affinity distributions: proton and metal binding to humic substances.

The binding of proton and metal cations to humic substances has been analyzed with a regularized fitting procedure (using the CONTIN software package) to extract conditional affinity distributions, valid at a given ionic strength, from binding (titration) curves. The procedure was previously tested with simulated titration curves using a simple bi-Gaussian model, the NICA-Donnan model, and the Stockholm humic model. Application to literature data for proton binding shows that in several cases the affinity distribution found is bimodal (carboxylic and phenolic sites) as usually assumed; however in other cases, specially for fulvic acids, a trimodal distribution is clearly discerned, with a smaller peak between the two noted above attributed to the presence of vicinal carboxylic groups. The analysis of metal binding curves has been performed in a few cases where the available data could be reliably processed, separating the proton affinity distribution and obtaining the conditional affinity spectra. For Cd(II) and Pb(II) a bimodal distribution is found, attributed in principle to mono- and bidentate binding, based on spectroscopic data. In the case of Cu(II), a more complex affinity distribution is found showing 3-4 peaks; this is consistent with spectroscopic studies, where different binding modes, up to tetradentate, have been observed.

Binding of Pb(II) in the system humic acid/goethite at acidic pH.

The study of Pb(II) binding to the system humic acid/goethite in acidic medium is reported in the present paper. From a macroscopic point of view, we have constructed the experimental sorption isotherms (using atomic absorption spectroscopy) and compared them with the prediction of the additivity rule. It is found that this system presents positive deviations, that is, the amount sorbed is about an order of magnitude higher than predicted. Apart from this, microscopic and structural aspects have also been studied using in situ and ex situ infrared spectroscopy. These results suggest that the presence of Pb(II) increases the amount of humic acid bound to the oxide. It is proposed that proton displacement due to the interaction between humic substances and the oxide, along with the formation of ternary complexes with the Pb(II) cation bridging the oxide and the humic substances (Type A complexes), cause the deviation from additivity.