Preparation of graphene from chemical reduction of graphene oxide (GO) is recognized as one of the most promising methods for large-scale and low-cost production of graphene-based materials This study reports a new, green and efficient reducing agent (caffeic acid/CA) for GO reduction The CA-reduced GO (CA-rGO) shows a high C/O ratio (715) that is among the best rGOs prepared with green reducing reagents Electronic gas sensors and supercapacitors have been fabricated with the CA-rGO and show good performance, which demonstrates the potential of CA-rGO for sensing and energy storage applications

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Green preparation of reduced graphene oxide for sensing and energy storage applications

Green preparation of reduced graphene oxide for sensing and energy storage

Plants release a multitude of organic compounds into the rhizosphere, some of which are flavonoids. These products of secondary metabolism are mainly studied for their antioxidant properties and for their role in the establishment of rhizobium-legume symbiosis; however, it has been recently demonstrated that flavonoids can also affect nutrient availability through soil chemical changes. This review will give an overview of the types and amounts of flavonoids released by roots of different plant species, as well as summarize the available knowledge on root exudation mechanisms. Subsequently, factors influencing their release will be reported, and the methodological approaches used in the literature will be critically described. Finally, the direct contribution of plant-borne flavonoids on the nitrogen, phosphorous and iron availability into the rhizosphere will be discussed.

Release of plant-borne flavonoids into the rhizosphere and their role in plant nutrition

Chromium reaches the soil environment through waste disposal emanating from a number of industrial activities, including coal-fired power production, electroplating, leather tanning, timber treatment, pulp production, and mineral ore and petroleum refining. Of the heavy metals, chromium (Cr) is a major pollutant, poses a great threat to flora and fauna and persists for long time. The most abundant species of Cr—Cr(III) and Cr(VI)—have very different properties. The toxicity, mobility, and bioavailability of Cr mainly depend on its speciation. In the natural environment, Cr(III) is most immobile, less soluble and stable, whereas Cr(VI) is highly mobile, soluble and bioavailable. Redox reactions play an important role in the interconversion of Cr(VI) and Cr(III). As our awareness of the rising toxicity of Cr increases, it is necessary to develop new and advanced strategies to mitigate this toxicity in the environment. Several physicochemical methods have been developed but these techniques are expensive and are not readily applicable to large contaminated zones. This chapter provides an overview of the concepts of Cr biogeochemistry, bioavailability and integrated risk management. The physicochemical factors, speciation and toxicity have been discussed with special emphasis on the remediation methods due to the complex reactions associated with Cr toxicity mitigation. Furthermore, this study identified systematically the future needs for understanding Cr biogeochemistry and low-cost remediation methods.

Chromium contamination and its risk management in complex environmental settings

Although iron (Fe) is one of the most abundant elements in the earth’s crust, its low solubility in soils restricts Fe uptake by plants. Most plant species acquire Fe by acidifying the rhizosphere and reducing ferric to ferrous Fe prior to membrane transport. However, it is unclear how these plants access Fe in the rhizosphere and cope with high soil pH. In a mutant screening, we identified 2-oxoglutarate-dependent dioxygenase Feruloyl-CoA 6′-Hydroxylase1 (F6′H1) to be essential for tolerance of Arabidopsis (Arabidopsis thaliana) to high pH-induced Fe deficiency. Under Fe deficiency, F6′H1 is required for the biosynthesis of fluorescent coumarins that are released into the rhizosphere, some of which possess Fe(III)-mobilizing capacity and prevent f6′h1 mutant plants from Fe deficiency-induced chlorosis. Scopoletin was the most prominent coumarin found in Fe-deficient root exudates but failed to mobilize Fe(III), while esculetin, i.e. 6,7-dihydroxycoumarin, occurred in lower amounts but was effective in Fe(III) mobilization. Our results indicate that Fe-deficient Arabidopsis plants release Fe(III)-chelating coumarins as part of the strategy I-type Fe acquisition machinery.

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Feruloyl-CoA 6′-Hydroxylase1-Dependent Coumarins Mediate Iron Acquisition from Alkaline Substrates in Arabidopsis

The transfer of several metal ions from the soil to the plant absorbing cells is mediated principally by organic molecules of low molecular weight with complexing and reducing activity, among which caffeic acid (CAF) is particularly important. Here we report the results of a survey which deals with the oxidation of CAF by the Fe(III) ions bound to a polygalacturonate network (Fe(III)-PGA network). The interaction between Fe(III) and CAF was studied by using Fe(III)-PGA networks equilibrated in the 2.4-7.0 pH range by means of kinetic and spectroscopic methods. The reducing power was found to depend on the nature of the Fe(III)-PGA network complexes: when the ferric ion was complexed only by the PGA carboxylic groups, a high redox activity was observed, whereas the Fe(III) reduction was found to be lower when a hydroxylic group was inserted in the Fe(III) coordination sphere. The iron complexed in the network was protected from hydrolysis reactions, as shown by the high pH values at which its reduction occurred. Two different fractions of Fe(II) produced were identified, one diffusible and another exchangeable with CaCl2 6.0 mM. The existence of the exchangeable form was attributed to the electrostatic interaction of the Fe(II) ions with the carboxylate groups of the fibrils and with the degradation products of CAF. The arrangement of the fibrils was altered following the substitution of Ca(II) by Fe(III) ions and was restored following the reduction of Fe (III) by CAF.

Redox activity of caffeic acid towards iron(III) complexed in a polygalacturonate network

Reduction of Cr(VI) by caffeic acid

Zinc(II) adsorption on aluminium hydroxide

Influence of organic acids exuded by plants on the interaction of copper with the polysaccharidic components of the root mucilages

The interaction between polygalacturonic acid and Fe(III) was studied in the presence and in the absence of pyruvic, malic, and citric acids. Kinetical data and FT‐IR analyses show that the polysaccharidic matrix acts as an accumulator of Fe(III) and that the metal ion interacts electrostatically with both the carboxylic and other functional groups of the polysaccharidic matrix. Copper(II) ions, which have a high affinity towards the carboxylic groups of the polysaccharide, do not influence markedly the Fe(III) absorption indicating that the carboxylic groups are not determining in the Fe(III) accumulation process. Furthermore, the results suggest that iron inside the fibrils is under an hydrolyzed form or as Fe(III) hydroxy polymer. In the presence of malic and citric acids the amount of Fe(III) accumulated at pH 4.7 and 6.0 is markedly lower than that found in the presence of pyruvic acid what was attributed to the higher affinity of citric and malic acid towards the metal ion.

Alessandra Maria Premoli

Papers

Redox activity of caffeic acid towards iron(III) complexed in a polygalacturonate network

Reduction of Cr(VI) by caffeic acid

Zinc(II) adsorption on aluminium hydroxide

Influence of organic acids exuded by plants on the interaction of copper with the polysaccharidic components of the root mucilages

Role of Uronic Acid Polymers on the Availability of Iron to Plants