Influence of Pb(II) Ions on the EPR Properties of the Semiquinone Radicals of Humic Acids and Model Compounds: High Field EPR and Relativistic DFT Studies

Quantification of Quinones in Environmental Media by Chemical Tagging with Cysteine-Containing Peptides Coupled to Size Exclusionary Separation

Quinones are one of the most important redox-reactive organic compounds in natural environments, such as soil, water, and sediment, playing an important role in regulating the environmental processes and biogeochemical cycles of critical elements under climate change, including the influences of extreme events such as wildfires. However, to date, no existing methods can quantify quinones in complex environmental media. To overcome this challenge, a quantification method was developed by coupling chemical tagging of quinones by cysteine-containing nonaromatic peptides (Cpep) through a Michael addition reaction with size exclusionary chromatography (SEC) separation and ultraviolet (UV) analysis─leveraging on the characteristic absorbance of aromatic rings at 254 nm and molecular size of peptide. The method was demonstrated using model quinones, including 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), and 1,4-anthraquinone (AQ), with a detection limit of 3.3, 0.7, and 0.2 μM, respectively. Concentrations of quinones in water extractions of biochars, soils, and wildfire-derived ashes were determined to range from 0.8 to 14 μM and were positively correlated with their redox reactivity determined by a chemical assay. This method provides a novel rapid quantification of quinones in complex environmental media as well as a quick assessment for redox reactivity and opens up new avenues for studying environmental transformation and remediation of contaminants.

Understanding the Exchange Interaction between Paramagnetic Metal Ions and Radical Ligands: DFT and Ab Initio Study on Semiquinonato Cu(II) Complexes

The exchange coupling, represented by the J parameter, is of tremendous importance in understanding the reactivity and magnetic behavior of open-shell molecular systems. In the past, it was the subject of theoretical investigations, but these studies are mostly limited to the interaction between metallic centers. The exchange coupling between paramagnetic metal ions and radical ligands has hitherto received scant attention in theoretical studies, and thus the understanding of the factors governing this interaction is lacking. In this paper, we use DFT, CASSCF, CASSCF/NEVPT2, and DDCI3 methods to provide insight into exchange interaction in semiquinonato copper(II) complexes. Our primary objective is to identify structural features that affect this magnetic interaction. We demonstrate that the magnetic character of Cu(II)-semiquinone complexes are mainly determined by the relative position of the semiquinone ligand to the Cu(II) ion. The results can support the experimental interpretation of magnetic data for similar systems and can be used for the in-silico design of magnetic complexes with radical ligands.

Preferential Destruction of Micropollutants in Water through a Self-Purification Process with Dissolved Organic Carbon Polar Complexation

Removing micropollutants in real water is a scientific challenge due to primary dissolved organic carbon (DOC) and high energy consumption of current technologies. Herein, we develop a self-purification process for the preferential destruction of various micropollutants in municipal wastewater, raw drinking water, and ultrapure water with humic acid (HA) driven by the surface microelectronic field of Fe⁰-Fe_yC_z/Fe_x-GZIF-8-rGO without any additional input. It was verified that a strongly polar complex consisting of an electron-rich HA/DOC area and an electron-poor micropollutant area was formed between HA/DOC and micropollutants, promoting more electrons of micropollutants in the adsorbed complex to delocalizing to electron-rich Fe species area and be trapped by O₂, which resulted in their surface cleavage and hydrolyzation preferentially. The higher micropollutant degradation efficiency observed in real wastewaters was due to the greater complex polarity of DOC. Moreover, the electron transfer process ensured the stability of the surface microelectronic field and continuous water purification. Our findings provide a new insight into low-energy combined-micropollution water treatment.

o-Semiquinone radical anion isolated as an amorphous porous solid

The use of metal cations is a commonly applied strategy to create S > 1/2 stable molecular systems containing semiquinone radicals. Persistent mono-semiquinonato complexes of diamagnetic metal ions (S = 1/2) have been hitherto less common and mostly limited to the complexes of heavy metal ions. In this work, a mono-semiquinonato complex of aluminum, derived from 1,2-dihydroxybenzene, is obtained using a surprisingly short and uncomplicated procedure. The isolated product is an amorphous and porous solid that exhibits very good stability under ambient conditions. To characterise its molecular and electronic structure, 9.7, 34 and 406 GHz EPR spectroscopy was used in concert with computational techniques (DFT and DLPNO-CCSD). It was revealed that the radical complex is composed of two chemically equivalent aluminum cations and two catechol-like ligands with the unpaired electron uniformly distributed between the two organic molecules. The good stability and porous structure make this complex applicable in heterogeneous aerobic reactions.

Effect of selected organic materials on soil humic acids chemical properties

Environmental friendly agricultural management has an urgent need for finding a sustainable strategy for the usage of different by-products from bioenergy production. These are either used as soil amendments or fertilizers. This study is aimed at evaluating if and how soil organic matter changes after the application of biochar, compost, and digestate. A pot experiment was conducted with Haplic Cambisol (low range arable soil) in Phytotron CLF PlantMaster (Wertingen, Germany). The chemical composition of isolated humic acids (HA) was determined by an inductively coupled plasma-mass spectrometer (ICP-MS). FT-IR spectroscopy and CHNS analysis were used for detailed chemical and optical characterization. Soil magnetic properties - radical concentration, g-parameters of radicals, and iron ions were evaluated by EPR spectroscopy. The results showed that amending arable soil with biochar, digestate and compost results in chemical and structural changes of humic substances. The radicals originated in biochar and digestate are built-in to the structure of the humic acid, which was confirmed by EPR g-parameter values. Despite a relatively high concentration of paramagnetic metal ions Fe and Mn the effect of semiquinone radical quenching was not observed. That suggests a conclusion that metal ions of studied amendments are binding in HA structure and did not disturb natural radical processes in the soil. It was also concluded that the effect of applied material depends mainly on its chemical properties and the soil type.

Can Carbamates Undergo Radical Oxidation in the Soil Environment? A Case Study on Carbaryl and Carbofuran

Radical oxidation of carbamate insecticides, namely carbaryl and carbofuran, was investigated with spectroscopic (electron paramagnetic resonance [EPR] and UV-vis) and theoretical (density functional theory [DFT] and ab initio orbital-optimized spin-component scaled MP2 [OO-SCS-MP2]) methods. The two carbamates were subjected to reaction with ^•OH, persistent DPPH^• and galvinoxyl radical, as well as indigenous radicals of humic acids. The influence of fulvic acids on carbamate oxidation was also tested. The results obtained with EPR and UV-vis spectroscopy indicate that carbamates can undergo direct reactions with various radical species, oxidizing themselves into radicals in the process. Hence, they are prone to participate in the prolongation step of the radical chain reactions occurring in the soil environment. Theoretical calculations revealed that from the thermodynamic point of view hydrogen atom transfer is the preferred mechanism in the reactions of the two carbamates with the radicals. The activity of carbofuran was determined experimentally (using pseudo-first-order kinetics) and theoretically to be noticeably higher in comparison with carbaryl and comparable with gallic acid. The findings of this study suggest that the radicals present in soil can play an important role in natural remediation mechanisms of carbamates.

pH-dependent formation of Hg(II)-semiquinone complexes from natural phenols

The ability of various natural phenols to form Hg(II)-semiquinone complexes was tested in the pH range of 2.8-12. EPR experiments performed at 9.6 and 34 GHz (the X- and Q-band, respectively) revealed that the complexes formed at low and high pH values exhibit a significant dissimilarity between their g-matrices (g-tensors), strongly suggesting that the complexes differ structurally. Our previous investigation on the low pH complex (Chemosphere 2015, 119, 479-484) had shown the Hg(II) ion to be tetracoordinated by two ligands, one of the ligands being monoprotonated with the unpaired electron mainly located on it. In order to reveal the molecular structure of the high pH form a DFT-based theoretical analysis was carried out in this work. For all the optimized model structures the g-matrices were computed and compared with their experimental counterparts. Good agreement was observed only if the geometry of the model Hg(II) complex was planar and the coordination sphere was composed of one fully deprotonated radical ligand and hydroxyl anions.

Pb(II), Cr(VI) and atrazine sorption behavior on sludge-derived biochar: role of humic acids

Pyrolyzing municipal wastewater treatment sludge into biochar can be a promising sludge disposal approach, especially as the produced sludge-derived biochar (SDBC) is found to be an excellent sorbent for heavy metals and atrazine. The aim of this study was to investigate how and why the coexisting humic acids influence the sorption capacity, kinetic, and binding of these contaminants on SDBC surface. Results showed humic acids enhanced Pb(II)/Cr(VI) sorption binding, and increased the corresponding Pb(II) Langmuir sorption capacity at pH 5.0 from 197 to 233 μmol g(-1), and from 688 to 738 μmol g(-1) for Cr(VI) at pH 2.0. It can be mainly attributed to the sorbed humic acids, whose active functional groups can offer the additional sites to form stronger inner-sphere complexes with Pb(2+), and supply more reducing agent to facilitate the transformation of Cr(VI) to Cr(III). However, humic acids reduced the atrazine adsorption Freundlich constant from 1.085 to 0.616 μmol g(-1). The pore blockage, confirmed by the decreased BET-specific surface area, as well as the more hydrophilic surface with more sorbed water molecules may be the main reasons for that suppression. Therefore, the coexisting humic acids may affect heavy metal stabilization or pesticide immobilization during SDBC application to contaminated water or soils, and its role thus should be considered especially when organic residues are also added significantly to increase the humic acid content there.

Understanding natural semiquinone radicals--multifrequency EPR and relativistic DFT studies of the structure of Hg(II) complexes

Multifrequency EPR spectroscopy and DFT calculations were used to investigate Hg(II) complexes with semiquinone radical ligands formed in a direct reaction between the metal ions and tannic acid (a polyphenol closely related to tannins). Because of the intricate structure of tannic acid a vast array of substituted phenolic compounds were tested to find a structural model mimicking its ability to react with Hg(II) ions. The components of the g matrix (the g tensor) determined from the high field (208 GHz) EPR spectra of the Hg(II) complexes with the radical ligands derived from tannic acid and from the model compounds were analogous, indicating a similar coordination mode in all the studied Hg(II) complexes. Since catechol (1,2-dihydroxybenzene) was the simplest compound undergoing the reaction with Hg(II) it was selected for DFT studies which were aimed at providing an insight into the structural properties of the investigated complexes. Various coordination numbers and different conformations and protonation states of the ligands were included in the theoretical analyses. g Matrices were computed for all the DFT optimized geometries. A good agreement between the theoretical and experimental values was observed only for the model with the Hg(II) ion tetracoordinated by two ligands, one of the ligands being monoprotonated with the unpaired electron mainly localized on it.

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.

The effect of Hg(II) ions on the free radicals of humic substances and their model compounds

Humic acids (HAs) and humin (HU) complexes with Hg(II) ions were studied using EPR, FTIR and CP MAS (13)C NMR spectroscopies. The analysis of the EPR spectra showed, especially for HA, a significant decrease in the g-factor value for the resulting Hg(II) radical species, as compared with that observed for the parent radicals. The concentration of the formed radical complexes was very low, precluding their detailed characterization. For this reason hydroxybenzenes and their benzoic acids were used as models of the humic substances' radical centres able to form radical ligands towards Hg(II) ions. The resulting radicals, characterized by a g-factor lower than that of the free electron (2.0006-2.00017), were efficiently produced upon Hg(II) complexation only for the hydroxybenzenes with their OH groups in the ortho position and, what is more important, the presence of carboxylic substituents was not necessary. It is most likely that formation of the radicals characterised by low g-factor proceeded in two steps. The first is oxidation process of the hydroxybenzenes and their derivatives resulting in a typical phenolic/semiquinone type radical while in the second step the radical is complexed by the excess of Hg(II), as the result spin density of the radical is shifted towards aromatic ring leading to the observed lowering of the g parameter value. The redox character of the process starting from Hg(II) was confirmed by the formation of elemental mercury. The g-factor of the semiquinone radical obtained from 2,4,6-trihydroxybenzoic acid was not changed by Hg(II) ions coordination, as indicated by g =2.0034.

Experimental and theoretical approach to aggregation behavior of new di-N-oxide surfactants in an aquatic environment

A homologous series of new dicephalic type surfactants (N,N-bis3,3'-(dimethylamino)propyl]alkylamide di-N-oxides) were synthesized and their aggregation phenomena were extensively studied. First, the pH-sensitivity of the investigated surfactants was tested in potentiometric titrations. Then, the adsorption isotherms were measured and interpreted using the Gibbs adsorption equation to determine physicochemical properties. The spin probe EPR technique was employed to monitor the micellization behavior of the surfactants, depending on temperature and surfactant concentration. Critical micelle concentrations (CMC) were determined through an analysis of the calculated spin probe rotational correlation times. A greater insight into the local microenvironment of the formed aggregates was gained by analyzing the properties of the immobilized spin probes. In addition, the CMC values were compared with the ones obtained from tensiometry measurements (taking into account the contributions of the various ionic and nonionic surfactant forms). The approximate size of the micellar aggregates was estimated by the dynamic light scattering (DLS) method. Good agreement between the experimental hydrodynamic radii and those predicted using density functional theory (DFT) guaranteed that the subsequently calculated aggregation numbers, representing the number of molecules in a micelle, were close to the real values. Moreover, the theoretical QSAR methods were used to determine the shape of the micelles via the prediction of the critical packing parameter (CPP).

Colorimetric assay for determination of lead (II) based on its incorporation into gold nanoparticles during their synthesis

In this report, we present a new method for visual detection of Pb2+. Gold nanoparticles (Au-NPs) were synthesized in one step at room temperature, using gallic acid (GA) as reducer and stabilizer. Pb2+ is added during the gold nanoparticle formation. Analysis of Pb2+ is conducted by a dual strategy, namely, colorimetry and spectrometry. During Au-NPs synthesis, addition of Pb2+ would lead to formation of Pb-GA complex, which can induce the aggregation of newly-formed small unstable gold nanoclusters. Consequently, colorimetric detection of trace Pb2+ can be realized. As the Pb2+ concentration increases, the color turns from red-wine to purple, and finally blue. This method offers a sensitive linear correlation between the shift of the absorption band (Δλ) and logarithm of Pb2+ concentration ranging from 5.0×10(-8) to 1.0×10(-6) M with a linear fit coefficient of 0.998, and a high selectivity for Pb2+ detection with a low detection limit down to 2.5×10(-8) M.

Effect of metal ions on the indigenous radicals of humic acids: high field electron paramagnetic resonance study

The interaction of indigenous radicals of humic acid (HA) with metal cations has been studied using high magnetic field (10.5T-285 GHz) electron paramagnetic resonance (HFEPR) spectroscopy. Strong [HA]-[metal] interaction was observed in the case of heavy metals, Cd(2+), Pb(2+), and Sr(2+), leading to formation of covalent bonds with the radicals of HA. On the contrary, alkaline earth metal ions, such as Mg(2+), generate only electrostatic interaction. The two types of indigenous radicals that exist in all HAs are influenced by the metal cations in a unified manner. This provides evidence that the two types of indigenous radicals in HAs originate from a unique, phenolic, moiety in HA. Mg(2+) ions dramatically changed the pH profile of the two radical types of HA, downshifting their interconversion pK(a) by ca. 3 pH units. This is the first experimental observation of the effect of metals on the H-dissociation of the radical centers in HAs.