Consuelo Escolástico

Soluble organic carbon and pH of organic amendments affect metal mobility and chemical speciation in mine soils

We evaluated the effects of pH and soluble organic carbon affected by organic amendments on metal mobility to find out the optimal conditions for their application in the stabilization of metals in mine soils. Soil samples (pH 5.5-6.2) were mixed with 0, 30 and 60 th a(-1) of sheep-horse manure (pH 9.4) and pine bark compost (pH 5.7). A single-step extraction procedure was performed using 0.005 M CaCl2 adjusted to pH 4.0-7.0 and metal speciation in soil solution was simulated using NICA-Donnan model. Sheep-horse manure reduced exchangeable metal concentrations (up to 71% Cu, 75% Zn) due to its high pH and degree of maturity, whereas pine bark increased them (32% Cu, 33% Zn). However, at increasing dose and hence pH, sheep-horse manure increased soluble Cu because of higher soluble organic carbon, whereas soluble Cu and organic carbon increased at increasing dose and correspondingly decreasing pH in pine bark and non-amended treatments. Near the native pH of these soils (at pH 5.8-6.3), with small doses of amendments, there was minimum soluble Cu and organic carbon. Pine bark also increased Zn solubility, whereas sheep-horse manure reduced it as soluble Zn always decreased with increasing pH. Sheep-horse manure also reduced the proportion of free metals in soil solution (from 41% to 4% Cu, from 97% to 94% Zn), which are considered to be more bioavailable than organic species. Sheep-horse manure amendment could be efficiently used for the stabilization of metals with low risk of leaching to groundwater at low doses and at relatively low pH, such as the native pH of mine soils.

Chemical speciation and mobilization of copper and zinc in naturally contaminated mine soils with citric and tartaric acids.

A one-step extraction procedure and a leaching column experiment were performed to assess the effects of citric and tartaric acids on Cu and Zn mobilization in naturally contaminated mine soils to facilitate assisted phytoextraction. A speciation modeling of the soil solution and the metal fractionation of soils were performed to elucidate the chemical processes that affected metal desorption by organic acids. Different extracting solutions were prepared, all of which contained 0.01 M KNO(3) and different concentrations of organic acids: control without organic acids, 0.5 mM citric, 0.5 mM tartaric, 10 mM citric, 10 mM tartaric, and 5 mM citric +5 mM tartaric. The results of the extraction procedure showed that higher concentrations of organic acids increased metal desorption, and citric acid was more effective at facilitating metal desorption than tartaric acid. Metal desorption was mainly influenced by the decreasing pH and the dissolution of Fe and Mn oxides, not by the formation of soluble metal-organic complexes as was predicted by the speciation modeling. The results of the column study reported that low concentrations of organic acids did not significantly increase metal mobilization and that higher doses were also not able to mobilize Zn. However, 5-10 mM citric acid significantly promoted Cu mobilization (from 1 mg kg(-1) in the control to 42 mg kg(-1) with 10 mM citric acid) and reduced the exchangeable (from 21 to 3 mg kg(-1)) and the Fe and Mn oxides (from 443 to 277 mg kg(-1)) fractions. Citric acid could efficiently facilitate assisted phytoextraction techniques.

Complexes of rhodium and iridium derived from 2,5-bis(pyrazol-1'-yl)-1,4-dihydroxybenzene

Abstract The behaviour of the ligand 2,5-bis(pyrazol-1′-yl)-1,4-dihydroxybenzene (H 2 LL) towards Rh I , Ir I , Rh III and Ir III complexes is reported. This compound with two OH groups might act as a neutral ligand (H 2 LL), as a monoanionic ligand (HLL − ) or as a dianionic ligand (LL 2− ). Complexes of all the three kinds have been isolated. In the case of H 2 LL, the compounds are not organometallic complexes but clathrates. The crystal and molecular structure of the host-guest complex [{(η 5 -C 5 Me 5 )RhCl} 2 -(μ-Cl) 2 ]-H 2 LL ( 6a ) is reported. Both the host and the guest have crystallographic C i symmetry. No metal-H 2 LL chemical bonds are present, and van der Waals interactions between host and guest molecules govern the crystal packing. An heterobimetallic derivative [IrRh(η 5 -C 5 Me 5 )Cl 2 (LL)] ( 7c ) has been isolated.

Variety in the Coordination Modes of the Ligand Hexakis(pyrazol-1-yl)benzene (Hpzb) to PdII, PtII and CuI Centres

The structure and conformational analysis of eight complexes derived from hexakis(pyrazol-1-yl)benzene (hpzb) have been carried out by means of X-ray crystallography and NMR spectroscopy. The metallic fragments [Pd(C6F5)2, [Pd(η3-2-Me-C3H4)]+, PtCl2 and Cu(PMe3)+] coordinate with two adjacent pyrazole residues yielding complexes with one, two or three metallic centres. The crystal and molecular structures of [{Pd(C6F5)2}2(hpzb)], [{Pd(η3-C4H7)}2(hpzb)](TfO)2, [{Cu(PMe3)}2(hpzb)](BF4)2 and [{Pd(η3-C4H7)}3(hpzb)](TfO)3 have been determined. The structure of each of the complexes depends on the fact that the metal atoms, when two or three are present, prefer to be situated on opposite faces of the benzene ring. Evidence for restricted rotation of the uncoordinated pyrazolyl groups has been found for the free ligand and monometallic derivatives, while free rotation of these groups at room temperature has been observed for the dimetallic complexes. Furthermore, the derivative {[Cu(PMe3)]2(hpzb)}(BF4)2 and probably [{Pd(η3-C4H7)}2(hpzb)](TfO)2 exhibit metallotropy on the NMR time scale. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Structures of NH-pyrazoles bearing only C-methyl substituents: 4-methylpyrazole is a hydrogen-bonded trimer in the solid (100 K)

The X-ray molecular structure of 4-methylpyrazole (3) at 100 K has been determined. The compound crystallizes as a trimer with the N–H protons localized. 4-Methylpyrazole belongs to the series of eight NH-pyrazoles bearing only H and CH3 substituents on the carbon atoms. There are two pairs of tautomers so the number of pyrazoles existing in solution is only six. The thermal behavior of the six pyrazoles was investigated by differential scanning calorimetry and their melting points analyzed on the basis of their solid state structures and number and position of C-methyl substituents.

Imidazole and benzimidazole addition to quinones. Formation of meso and d,l isomers and crystal structure of the d,l isomer of 2,3-Bis(benzimidazol-1'-yl)-1,4-dihydroxybenzene

Abstract The hydroquinones obtained by addition of imidazole, 2-methylimidazole and benzimidazole to 1,4-benzoquinone and 1,4-naphthoquinone have been isolated and identified. In the case of 1,4-benzoquinone they are monosubstituted hydroquinones 1a -1e, o-substituted hydroquinones 2a - 2c, and p-disubstituted derivatives 3a - 3c while in the case of 1,4-naphthoquinone, only disubstituted derivatives 5a and 5c have been isolated. In solution, 2,3-bis(2′-methylimidazol-1′-yl)-1,4-dydroxybenzone (2b), 2,3-bis (benzimidazol-1′-yl)-1,4-dihydroxybenzene (2c) and 2,3-bis(benzimidazol-1′-yl)-1,4-dihydroxynaphthalene (5c) exist as mixtures of meso and d,l isomers. NMR spectroscopy (n.O.e. experiments in 1H NMR and solid-state 13C CPMAS spectra) and AM1 semiempirical calculations have been used to establish the structure of the isomers both in the solid state and in solution as well as their interconversion pathways. Compound 2c-d,l crystallizes with two methanol solvate molecules as guests and it has a crystallographic twofold axis through the middle. The host molecules are linked together by means of the methanol molecules through O-H⋯O/N hydrogen bonds giving rise to chains along the c axis centrosymmetrically related.

Aromatic propellenes. Synthesis, X-Ray structures and conformational study of polypyrazolylpyridines

Abstract The crystal and molecular structures of three pentapyrazolylpyridines pz5py, dmpz5py and pz4dmpzpy (pz = pyrazol-1′-yl, dmpz = 3,5-dimethylpyrazol-1′-yl) have been determined by X-Ray crystallography. The two pyrazole rings close to the pyridine nitrogen tend to be as coplanar as possible with the pyridine plane. Two different conformations with the nitrogen lone pairs of pyrazole N2 directed towards or opposite to the N1 nitrogen of pyridine, are present in the crystals. Semiempirical AM1 calculations have been performed to evaluate the stability of these conformations which appear to be present also in solution ( 1 H and 13 C NMR and dipole moments).

Multinuclear NMR solution studies on complexes of hexakis(pyrazol-1-yl)benzene (hpzb) with Ag(I)

Abstract The reaction of the polydentate N-donor ligand hexakis(pyrazol-1-yl)benzene (hpzb) with AgSbF6/PPh3 or AgClO4PPh3 in a 1:1 ratio leads to the complexes [Ag(PPh3)(hpzb)]SbF6 (1) or [Ag(PPh3)(hpzb)]ClO4 (3). When two or three equivalents of the silver derivatives are used the species [Ag(PPh3)2]X are formed in addition to 1 or 3. In the case of SbF6− the remaining Ag+ competes with AgPPh3+ for coordination to hpzb and the derivative [Ag(hpzb)]SbF6 (2) is also obtained, while in the case of ClO4− the insoluble salt AgClO4 is formed. The same reactions performed with hpzb–15N12 confirmed the presence of the N-donor ligand in 1–3. At room temperature a Ag–P dissociation process is observed for all the derivatives containing Ag–PPh3 fragments. Complexes 1–3 show in solution an intramolecular argentotropic shift that makes the six pyrazolyl rings of the hpzb ligand equivalent on the 1H NMR time scale. When the amount of counteranion present in solution is sufficiently high, an intermediate of lower coordination number (stabilized with the anion) is observed for complexes 1 and 3 in the low temperature 31P NMR spectra in addition to the three-coordinated species. This intermediate is more stable in the case of ClO4− due to the higher coordinating ability of this anion.

Some Considerations about the Structure of 3(5)-Methylpyrazole

The behavior of neat 3(5)-methylpyrazole has been studied at low temperatures by DSC and by NMR (13C and 15N). The main conclusion is that the supercooled liquid is a mixture of four trimers formed by 3-methyl and 5-methyl tautomers.

Aromatic propellenes. Part 3. NMR, X-ray crystallography and semi-empirical calculations on the conformational isomerism of 1,2,4,5-tetrakis (pyrazol-1′-yl)-3,6-bis(3″,5″-dimethylpyrazol-1′-yl) benzene

The molecular and crystal structures of two crystalline forms of 1,2,4,5-tetrakis(pyrazol-1′-yl)-3,6-bis(3″,5″-dimethylpyrazol-1″-yl) benzene and one inclusion complex with two molecules of acetic acid were determined by x-ray analysis. The acetic acid forms dimers through symmetry centers and the only interactions in the structures are mainly due to weak C(SINGLE BOND)H…N interactions. All 14 possible conformations of the pyrazole with regard to the benzene ring were explored by means of AM1 semi-empirical calculations. The observed conformation in the crystal structures agrees fairly well with the most stable conformation which presents the pyrazole rings with the N(2) alternating between both sides of the phenyl plane. These calculations allow one to identify the minor isomer present in solution together with the major isomer corresponding to the crystal structure.