Electrostatic potential of specific mineral faces

Discovery of rosin-based acylhydrazone derivatives as potential antifungal agents against rice Rhizoctonia solani for sustainable crop protection

BACKGROUND

The use of fungicides to protect crops from diseases is an effective method, and novel environmentally friendly plant-derived fungicides with enhanced performance and low toxicity are urgent requirements for sustainable agriculture.

RESULTS

Two kinds of rosin-based acylhydrazone compounds were designed and prepared. Based on the antifungal activity assessment against Rhizoctonia solani, Fusarium oxysporum, Phytophthora capsici, Sclerotinia sclerotiorum, and Botrytis cinerea, acylhydrazone derivatives containing a thiophene ring were screened and showed an inhibitory effect on rice R. solani. Among them, Compound 4n, with an electron-withdrawing group on the benzene ring structure attached to the thiophene ring, showed optimal activity, and the EC₅₀ value was 0.981 mg L^-1 , which was lower than that of carbendazim. Furthermore, it was indicated that 4n could affect the mycelial morphology, cell membrane permeability and microstructure, cause the generation of reactive oxygen species in fungal cells, and damage the nucleus and mitochondrial physiological function, resulting in the cell death of R. solani. Meanwhile, Compound 4n exhibited a better therapeutic effect on in vivo rice plants. However, the induction activity of 4n on the defense enzyme in rice leaf sheaths showed that 4n stimulates the initial resistance of rice plants by removing active oxygen, thereby protecting the cell membrane or enhancing the strength of the cell wall. Through the quantitative structure-activity relationship study, the quantitative chemical and electrostatic descriptors significantly affect the binding of 4n with the receptor, which improves its antifungal activity.

CONCLUSION

This study provides a basis for exploiting potential rosin-based fungicides in promoting sustainable crop protection. © 2022 Society of Chemical Industry.

Electrochemical Perspective on Hematite–Malonate Interactions

Organic matter (OM) interactions with minerals are essential in OM preservation against decomposition in the environment. Here, by combining potentiometric and electrophoretic measurements, we probed the mode of coordination and the role of pH-dependent electrostatic interactions between organic acids and an iron oxide surface. Specifically, we show that malonate ions adsorbed to a hematite surface in a wide pH window between 3 and 8.7 (point of zero charge). The mode of interactions varied with this pH range and depended on the acid and surface acidity constants. In the acidic environment, hematite surface potential was highly positive (+47 mV, pH 3). At pH < 4 malonate adsorption reduced the surface potential (+30 mV at pH 3) but had a negligible effect on the diffuse layer potential, consistent with the inner-sphere malonate complexation. Here, the specific and electrostatic interactions were responsible for the malonate partial dehydration and surface accumulation. These interactions weakened with an increasing pH and near PZC, the hematite surface charge was neutral on average. Adsorbed malonates started to desorb from the surface with less pronounced accumulation in the diffuse layer, which was reflected in zeta potential values. The transition between specific and non-specific sorption regimes was smooth, suggesting the coexistence of the inner- and outer-sphere complexes with a relative ratio that varied with pH.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

X-ray Analyses of Lead Adsorption on the (001), (110), and (012) Hematite Surfaces

Predicting the environmental fate of lead relies on a detailed understanding of its coordination to mineral surfaces, which in turn reflects the innate reactivity of the mineral surface. In this research, we investigated fundamental dependencies in lead adsorption to hematite by coupling extended X-ray absorption fine structure (EXAFS) spectroscopy on hematite particles (10 and 50 nm) with resonant anomalous X-ray reflectivity (RAXR) to single crystals expressing the (001), (012), or (110) crystallographic face. The EXAFS showed that lead adsorbed in a bidentate inner-sphere manner in both edge and corner sharing arrangements on the FeO₆ octahedra for both particle sizes. The RAXR measurements confirmed these inner-sphere adsorption modes for all three hematite surfaces and additionally revealed outer-sphere adsorption modes not seen in the EXAFS. Lead uptake was larger and pH dependence was greater for the (012) and (110) surfaces, than the (001) surface, due to their expressing singly- and triply coordinated oxygen atoms the (001) surface lacks. In coupling these two techniques we provide a more detailed and nuanced picture of the coordination of lead to hematite while also providing fundamental insight into the reactivity of hematite.

Mapping Electrochemical Heterogeneity at Iron Oxide Surfaces: A Local Electrochemical Impedance Study

Alternating current scanning electrochemical microscopy (AC-SECM) was used for the first time to map key electrochemical attributes of oriented hematite (α-Fe2O3) single crystal surfaces at the micron-scale. Localized electrochemical impedance spectra (LEIS) of the (001) and (012) faces provided insight into the spatial variations of local double layer capacitance (C(dl)) and charge transfer resistance (R(ad)). These parameters were extracted by LEIS measurements in the 0.4-8000 Hz range to probe the impedance response generated by the redistribution of water molecules and charge carriers (ions) under an applied AC. These were attributed to local variations in the local conductivity of the sample surfaces. Comparison with global EIS measurements on the same samples uncovered highly comparable frequency-resolved processes, that were broken down into contributions from the bulk hematite, the interface as well as the microelectrode/tip assembly. This work paves the way for new studies aimed at mapping electrochemical processes at the mesoscale on this environmentally and technologically important material.

Electrolyte ion adsorption and charge blocking effect at the hematite/aqueous solution interface: an electrochemical impedance study using multivariate data analysis

A model-free multivariate analysis using singular value decomposition is employed to refine an equivalent electrical circuit model in order to probe the electrochemical properties of the hematite/water interface in dilute NaCl and NH4Cl solutions using electrochemical impedance spectroscopy. The result shows that the surface protonation is directly related to the mobility and trapping of charge carriers at the mineral surface. Moreover, the point of zero charge can be found at pH where the charge transfer resistance is the highest, in addition to the minimum double layer capacitance. The inner-sphere interaction of the NH4(+) ion with the surface is indicated by an increase of capacitance for charge carrier trapping from the protonated surface as well as lower double layer capacitance and open circuit potential. It is clear that the intrinsic electrochemical activity of hematite depends on the degree of surface (de)protonation and other inner-sphere adsorption, as these processes affect the charge carrier density in the surface state. This work also highlights an important synergistic effect of the two spectral analyses that enables EIS to be utilized in an in-depth investigation of mineral/water interfaces.

Spontaneous water oxidation at hematite (α-Fe2O3) crystal faces

Hematite (α-Fe2O3) persists as a promising candidate for photoelectrochemical water splitting, but a slow oxygen evolution reaction (OER) at its surfaces remains a limitation. Here we extend a series of studies that examine pH-dependent surface potentials and electron-transfer properties of effectively perfect low-index crystal faces of hematite in contact with simple electrolyte. Zero-resistance amperometry (ZRA) was performed in a two electrode configuration to quantify spontaneous dark current between hematite crystal face pairs (001)/(012), (001)/(113), and (012)/(113) at pH 3. Exponentially decaying currents initially of up to 200 nA were reported between faces over 4 min experiments. Fourth-order ZRA kinetics indicated rate limitation by the OER for current that flows between (001)/(012) and (001)/(113) face pairs, with the (012) and (113) faces serving as the anodes when paired with (001). The cathodic partner reaction is reductive dissolution of the (001) face, converting surface Fe(3+) to solubilized aqueous Fe(2+), at a rate maintained by the OER at the anode. In contrast, OER rate limitation does not manifest for the (012)/(113) pair. The uniqueness of the (001) face is established in terms of a faster intrinsic ability to accept the protons required for the reductive dissolution reaction. OER rate limitation inversely may thus arise from sluggish kinetics of hematite surfaces to dispense with the protons that accompany the four-electron OER. The results are explained in terms of semiquantitative energy band diagrams. The finding may be useful as a consideration for tailoring the design of polycrystalline hematite photoanodes that present multiple terminations to the interface with electrolyte.

Potentiometric and electrokinetic signatures of iron(ii) interactions with (α,γ)-Fe2O3

The interactions of Fe(ii) with iron(iii) oxides give rise to the electrochemical signatures consistent with the iron solubility–activity curve.

Electrochemical properties and relaxation times of the hematite/water interface

Electric double layer properties and protonation rates at the surface of a mechanically and chemically polished (001) surface of hematite (α-Fe2O3) contacted with aqueous solutions of NaCl were extracted by electrochemical impedance spectroscopy (EIS). Effects of pH (4-12) and ionic strength (10-1000 mM) on the EIS response of the electrode were predicted using an electrical equivalent circuit model accounting for hematite bulk and interfacial processes. These efforts generated diffuse layer as well as compact layer capacitances and resistance values pertaining to interfacial processes. Diffuse layer capacitance values lie in the 0.5-0.6 μF cm(-2) region and are about 1.5 times smaller than those obtained on a roughened hematite surface. Compact layer capacitances are strongly pH dependent as they pertain to the transfer of ions (charge carriers) from the diffuse layer onto surface (hydr)oxo groups. These values, alongside those of resistance adsorption, pointed a 50% decrease in proton adsorption/desorption resistance under acidic and alkaline conditions relative to that of the point of zero charge (pH 8-9). Increasing ionic strength generally induces larger diffuse layer capacitances, larger adsorption capacitances, and lower resistance values. Such a response is in line with the concept for thinner electric double layers and facilitated proton adsorption reactions by solutions of high ionic strengths. Relaxation times pertaining to the transfer of charge carriers across the compact plane induced by the EIS experiments lie in the 0.7-4.2 s range and become larger under acidic conditions. Decreases in site availability and increases in electrostatic repulsion are two possible contributing factors impeding reaction rates below the point of zero charge. Collectively, these finding are underpinning important relationships between classical views on mineral surface complexation reactions and electrochemical views of semiconductor/water interfaces.

Constraints to the flat band potential of hematite photo-electrodes

Reasons are discussed for the widely disparate, reported flat band potentials for semiconducting hematite (photo-) electrodes in aqueous solutions.

Electrochemical impedance study of the hematite/water interface

Reactions taking place on hematite (α-Fe(2)O(3)) surfaces in contact with aqueous solutions are of paramount importance to environmental and technological processes. The electrochemical properties of the hematite/water interface are central to these processes and can be probed by open circuit potentials and cyclic voltammetric measurements of semiconducting electrodes. In this study, electrochemical impedance spectroscopy (EIS) was used to extract resistive and capacitive attributes of this interface on millimeter-sized single-body hematite electrodes. This was carried out by developing equivalent circuit models for impedance data collected on a semiconducting hematite specimen equilibrated in solutions of 0.1 M NaCl and NH(4)Cl at various pH values. These efforts produced distinct sets of capacitance values for the diffuse and compact layers of the interface. Diffuse layer capacitances shift in the pH 3-11 range from 2.32 to 2.50 μF·cm(-2) in NaCl and from 1.43 to 1.99 μF·cm(-2) in NH(4)Cl. Furthermore, these values reach a minimum capacitance at pH 9, near a probable point of zero charge for an undefined hematite surface exposing a variety of (hydr)oxo functional groups. Compact layer capacitances pertain to the transfer of ions (charge carriers) from the diffuse layer to surface hydroxyls and are independent of pH in NaCl, with values of 32.57 ± 0.49 μF·cm(-2)·s(-φ). However, they decrease with pH in NH(4)Cl from 33.77 at pH 3.5 to 21.02 μF·cm(-2)·s(-φ) at pH 10.6 because of the interactions of ammonium species with surface (hydr)oxo groups. Values of φ (0.71-0.73 in NaCl and 0.56-0.67 in NH(4)Cl) denote the nonideal behavior of this capacitor, which is treated here as a constant phase element. Because electrode-based techniques are generally not applicable to the commonly insulating metal (oxyhydr)oxides found in the environment, this study presents opportunities for exploring mineral/water interface chemistry by EIS studies of single-body hematite specimens.