Panagiota Stathi

Humic acid-inspired hybrid materials as heavy metal absorbents

Three SiO(2)-based materials were prepared via covalent immobilization of carboxyl groups (COOH), phenolic groups (GA), or humic acid on an SiO(2) surface. Their sorbing properties were evaluated for removal of heavy metals (Pb(2+), Cd(2+), Cu(2+), Zn(2+), and Mg(2+)) from aqueous solution. The data show a significant improvement for metal uptake, compared to unmodified silica that can be attributed to the adsorption of metals to the deprotonated form of functional groups (COOH, GA, HA). The metal-uptake capacity of the SiO(2)-HA material was 10 times higher that those of the other two materials. The present data provide direct experimental proof that HA can be viewed and modeled as a combination of -COO and R-OH functional groups.

Effects of acetate on cation exchange capacity of a Zn-containing montmorillonite: physicochemical significance and metal uptake.

Fundamental properties such as cation exchange capacity (CEC), permanent charge, pH(PZC), and metal uptake of a Zn-containing montmorillonite are modified, in a predictable manner, by a mild chemical treatment using acetate. Acetate treatment allows a controllable increase of the CEC of montmorillonite up to 180 mequiv/100 g. The CEC of the clay is increasing for decreasing Zn content, with a slope of Delta[Zn]/Delta[CEC] approximately -2. X-ray powder diffraction analysis shows that the lamellar structure of the clay remains unaltered by the acetate treatment, while XPS substantiates the removal of Zn. H(+) uptake data show that the intrinsic protonation pK values and concentration of the variable charge sites ( identical with SOH) are not modified by the acetate treatment. In contrast, the concentration of the permanent charge sites ( identical with X(-)) increased linearly with Zn removal by acetate, leading to a significant H(+) and Cd(2+) uptake enhancement. A physical model is suggested where acetate removes Zn ions strongly bound in the clay, and this in turn modulates the permanent charge and the CEC of the clay.

Stabilization of Phenolic Radicals on Graphene Oxide: An XPS and EPR Study.

A graphene oxide-gallic acid hybrid material was synthesized by the immobilization of gallic acid (3,4,5-trihydroxobenzoic acid) on graphene oxide. The grafting was achieved via the formation of amide bonds between the amine groups on the organofunctionalized graphite oxide surface and the carboxyl groups of the gallic acid molecules. The EPR signal of the gallic acid radicals in this hybrid material remained almost unaltered over at least 500 days, with less than 3% signal decay over that period, pointing to the truly remarkable stability of these radicals. The produced material was characterized by Fourier transform infrared, X-ray photoelectron, and electron paramagnetic resonance spectroscopies as well as by thermogravimetric analysis and the Kaiser test. The stability of the radicals in the material was studied in powder form and in aqueous solution vs pH. We demonstrate that in the graphene oxide-gallic acid hybrid material a radical is favorably stabilized on the ring-O while the oxidation of the second OH is precluded, and this results in long-term stabilization of the gallic acid radicals in solid hybrid material. Thus, in applications where it will be used under O2-free and humidity-free conditions, the graphene oxide-gallic acid hybrid material is a reliable spintronics scaffold.

Mechanism of heavy metal uptake by a hybrid MCM-41 material: surface complexation and EPR spectroscopic study.

A novel hybrid MCM-41-based material was synthesized by incorporation of AEDTC [N-(2-aminoethyl)dithiocarbamate] in the MCM-41 pores. The derived MCM-41 x AEDTC material possesses high AEDTC loading 35% [w:w], and a well-defined array of regular mesopores with a specific surface area of 632 m(2)/g. Heavy metal, Cd, Pb, Cu, and Zn, uptake was studied in detail at physiological pH values 6-8, by a combination of analytical and electron paramagnetic resonance (EPR) spectroscopic techniques. The analytical data show a significant improvement, i.e., 200-500%, for Pb, Cu, and Zn uptake by the MCM-41 x AEDTC hybrid vs the unmodified MCM-41. In contrast, Cd shows an exceptional behavior: (a) Cd uptake by MCM-41 x AEDTC is very low. (b) Competitive metal uptake experiments reveal that Cd ions cause a characteristic inhibition of Cu or Pb uptake by the MCM-41 x AEDTC while Cd binding itself always remained low. The present findings are analyzed by a combination of surface complexation modeling and EPR spectroscopy. Accordingly, in the MCM-41 x AEDTC the sulfur atoms of AEDTC provide strong binding sites for metal binding, with a stoichiometry [S(AEDTC)]:[Metal] = 1:1. Cd inhibits accessibility of Cu or Pb ions in the AEDTC sites.

Physicochemical study of amino-functionalized organosilicon cubes intercalated in montmorillonite clay: H-binding and metal uptake

Two organic-modified montmorillonite clays were prepared by embedding organosilanes bearing different chelating amino-functional groups [Apteos] (3-amino-propyltriethoxysilane), and [Edaptmos] (3-(2-aminoethylamino)propyltrimethoxysilane), in the interlayer space of a Zenith montmorillonite. XRD and FTIR spectroscopic data show that the amino organosilanes are intercalated into the interlamelar space forming cube-like structures bearing one polymanino tail at each cube apex. The intercalated cubes cause an increase of the interlayer spacing of the clay sheets by 6.6 A in [Zenith-Apteos] and by 7.1 A in [Zenith-Edaptmos]. The H-binding properties of the intercalated polyamino organosilanes were studied by potentiometric titration. The Cu-, Cd-, and Pb-binding capacity of [Zenith-Apteos] and [Zenith-Edaptmos] were evaluated in aqueous solution as a function of the pH. Both [Zenith-Apteos] and [Zenith-Edaptmos] showed improvement vs Zenith for metal binding in the order Cu > Pb > Cd. [Zenith-Edaptmos] showed the most important results vs Zenith. Theoretical analysis of the pH edge, achieved by a surface complexation model, shows that (a) the amino-functionalized cube-like structures constitute high affinity metal-binding sites; and (b) the metal ions are bound in a monodendate mode with the amino group of the cube, thus resulting in a maximization of metal-binding efficiency.

Isothermal decomposition of hydroxylamine and hydroxylamine nitrate in aqueous solutions in the temperature range 80–160 °C

Hydroxylamine (HA) and hydroxylamine nitrate (HAN) have been involved independently in several tragic accidents, which incurred numerous fatalities and injuries. Following these incidents, adiabatic calorimetry and computational chemistry research was conducted on those compounds, suggesting potential reaction pathways of their decomposition, but the mechanism of their unstable behavior, still have not been completely understood. In the present work, isothermal decomposition tests were performed accompanied with HPLC, ion chromatography and UV analyses in the temperature range 80-160 degrees C. Condition-dependent autocatalytic decompositions were demonstrated for HA and HAN, and an intermediate formation has been observed that is most likely responsible for their autocatalytic behavior. These findings corroborate previously reported computational chemistry results.

Back-clocking of Fe2+/Fe1+ spin states in a H2-producing catalyst by advanced EPR

A mononuclear Fe-(P(PPh2)3) ((P(PPh2)3) = tris[2-diphenylphospino)ethyl]phosphine) catalyst was studied in situ under catalytic conditions using advanced electron paramagnetic resonance (EPR) techniques. Fe-(P(PPh2)3) efficiently catalyses H2 production using HCOOH as substrate. Dual-mode continuous-wave (CW) EPR, used to study the initial Fe2+(S = 2) state, shows that the complex is characterised by a – rather small – zero field splitting parameter Δ = 0.45 cm−1 and geff = 8.0. In the presence of HCOOH substrate the complex evolves and a unique Fe1+(S = 1/2) state is trapped. The Fe1+ atom is coordinated by four 31P nuclei in a pseudo-C3 symmetry. Only a small fraction of the Fe1+ spin density is delocalised onto the 31P atoms. Four-pulse electron spin echo envelope modulation (ESEEM) and two-dimensional hyperfine sublevel correlation spectroscopy (2D-HYSCORE) data reveal the existence of two types of 1H couplings. One corresponds to weak, purely dipolar coupling, tentatively assigned to phenyl protons. The most important is a – rather unusual – 1H coupling with negative Aiso (−2.75 MHz) and strong dipolar part (T = 5.5 MHz). This 1H is located on the pseudo-C3 symmetry axis of the Fe1+-(P(PPh2)3-HCOO− complex where one substrate molecule, formate anion, is coordinated on the Fe1+ atom.

Interfacial Hydrogen Atom Transfer by nanohybrids based on Humic Acid Like Polycondensates.

Novel nanohybrid materials were prepared by covalent grafting of a polyphenolic polymer [Humic Acid Like Polycondensate (HALP)] on SiO2 nanoparticles. Four nanohybrids were so-produced, using four different types of SiO2 i.e. three Aerosil flame-made nanoparticles with nominal specific surface area of 50, 90 and 300 m(2)/g, herein codenamed OX50, A90, A300 respectively, plus a colloidal SiO2[S300] with SSA=300 m(2)/g. The antioxidant activity of the SiO2-HALP nanohybrids was evaluated by assessing their kinetics for Hydrogen Atom Transfer [HAT] to DPPH radicals. When normalized per same HALP concentration, bigger NPs SiO2[OX50]-HALP NPs can scavenge 280 μmoles of DPPH radicals per gram of HALP, while [A90]-HALP and [A300]-HALP NPs can scavenge 514 and 832 μmoles of DPPH radicals per gram of HALP, respectively. The colloidal SiO2[S300]-HALP can scavenge fewer DPPH radicals (252 μmoles) per gram of HALP. Based on detailed kinetic data it is shown that (i) surface grafted HALPs perform 300% better HAT than non-grafted HALP in solution. (ii) By controlling the particle type and grafting-loading, we can control/optimize the HAT performance: when grafted on the appropriate SiO2 surface the HALP macromolecules are able to quench up to 0.8 mmoles of DPPH-radical per gram of HALP.