Josef Friedl

THE EFFECT OF Al ON Fe OXIDES. XIX. FORMATION OF Al-SUBSTITUTED HEMATITE FROM FERRIHYDRITE AT 25°C AND pH 4 TO 7

Iron oxides in surface environments generally form at temperatures of 25 ± 10°C, but synthesis experiments are usually done at higher temperatures to increase the rate of crystallization. To more closely simulate natural environments, the transformation of 2-line ferrihydrite to hematite and goethite at 25°C in the presence of different Al concentrations and at pH values from 4 to 7 was studied in a long-term (16–20 y) experiment. Aluminum affects the hydrolysis and charging behavior of 2-line ferrihydrite and retards crystallization. Al also promotes the formation of hematite over goethite and leads to multidomainic discoidal and framboidal crystals instead of rhombohedral crystals. The strong hematite-promoting effect of Al appears to be the result of a lower solubility of the Al-containing ferrihydrite precursor relative to pure ferrihydrite. Hematite incorporates Al into its structure, as is shown by a decrease in the a and c-cell lengths and a decrease in magnetic hyperfine fields (Mössbauer spectroscopy). With hematite formed at low-temperature, these decreases were, however, smaller for the cell length and greater for the magnetic field than for hematite produced at higher temperatures. Both phenomena are removed by heating the hematite at 200°C. They are attributed to structural OH and/or structural defects. The relative content of Al in the structure is lower for hematite formed at 25°C than for hematites synthesized at higher temperatures (80 and 500°C). The maximum possible substitution of one sixth of the Fe positions was not achieved, similar to soil hematites. These results show that properties of widely distributed soil Al-containing hematites can reflect formation environment.

FORMATION AND PROPERTIES OF A CONTINUOUS CRYSTALLINITY SERIES OF SYNTHETIC FERRIHYDRITES (2- TO 6-LINE) AND THEIR RELATION TO FeOOH FORMS

Ferrihydrite is a poorly crystalline Fe oxide of which 2-(XRD)line and 6-line varieties are commonly used in experiments, although species with intermediate numbers of peaks have been found in nature. To simulate nature, we synthesized two continuous series of ferrihydrites with between 2 and 6–7 peaks at room temperature in two different ways: (1) by varying the rate of hydrolysis of an Fe(NO3)3 solution (HR series); and (2) by oxidizing an FeCl2 solution containing up to 73 mmol Si/L (Si series), both at pH 7. Mössbauer spectra at 4.2 K showed that the ferrihydrites of the HR series had a constant magnetic hyperfine field (Bhf) at 4.2 K of 48.8 T whereas Bhf in the Si series dropped from 49.4 to 46.7 T as the Si content of the ferrihydrites increased from 0 to 74.7 g/kg of Si. Temperature scans between 4 and 170 K illustrate that the magnetic order breaks down at a temperature which is lower the higher the hydrolysis rate and the Si concentration in the ferrihydrite.

197 Au Mössbauer study of the gold species adsorbed on carbon from cyanide solutions

The gold species present on activated carbon after adsorption from solutions of Au(CN)2− have been studied by197Au Mössbauer spectroscopy as a function of the pH value of the solution, the loading of the carbon, the coadsorption of polyvalent cations, and the treatment of the samples after adsorption. The gold was found to be adsorbed mainly as Au(CN)2−. Coadsorbed polyvalent cations (Ca²+, Gd³+) have no influence on the Mössbauer parameters of the adsorbed gold complex. After adsorption from acidic solutions (pH ≲ 4), one finds a substantial amount of adsorbed gold with Mössbauer parameters similar to those of crystalline AuCN. Presumably, this gold is bound in Aux(CN)x+1 oligomers which form during drying. An additional product with Mössbauer parameters close to those of KAu(CN)2Cl2 was observed on dried samples after adsorption at pH 1. A minor gold species with an uncommonly small electric quadrupole splitting was found on wet carbons but disappeared on drying.

197AU,57Fe and121Sb Mössbauer study of gold minerals and ores

A survey of the197Au Mössbauer spectra of naturally occurring gold species is given. Gold minerals have been studied as natural specimens or as synthetic analogues. Gold impurities have been identified in pyrites and arsenopyrites. An example of the use of121Sb, and57Fe Mössbauer spectroscopy in characterizing gold-bearing ore minerals is given.

197Au Mössbauer study of the deactivation and reactivation of a carbon-supported AuCl 4 − hydrochlorination catalyst

Acetylene hydrochlorination catalysts consisting of activated carbon impregnated with a solution of HAuCl4·xH2O in aqua regia have been studied by197Au Mössbauer spectroscopy. The relative amounts of AuCl4−, of Au(0), and of an Au(J) species formed under certain process conditions were determined quantitatively. Deactivation of the catalyst at low and high temperatures was shown to be due to different mechanisms, and the reactivation of the catalyst by Cl2 gas was studied.

The mechanism of adsorption of Au(CN)4− onto activated carbon

The Au(CN)4− species adsorbs onto activated carbon as a chemically unchanged ion pair of the type Mn+[Au(CN)4−]n, in a manner similar to that postulated for Au(CN)2−. The affinity of carbon for Au(CN)4− is greater than that for Au(CN)2−. A Mossbauer spectroscopic investigation has confirmed that Au(CN)4− is the species adsorbed from both acidic and neutral solutions. The Mossbauer spectrum of carbon that is loaded with Au(CN)4− and subsequently boiled in hydrochloric acid is interpreted in terms of the presence of Au(CN)2Cl2−, Au(CN)2− and AuCN as products of the decomposition of Au(CN)4−.

A197Au Mössbauer study of the adsorption of aurocyanide onto activated carbon

The chemical state of gold adsorbed on activated carbon from solutions of KAu(CN)2 has been studied by197Au Mössbauer spectroscopy as a function of the pH and concentration of the solution, the loading of the carbon, and the drying conditions of the samples. On freshly prepared specimens Au(CN)2− is always the dominant species. Drying leads to the formation of other forms of gold, mainly in samples loaded from strongly acidic media. An oligomeric form of AuCN is, however, also formed when samples loaded at high pH values are dried at temperatures around 100°C.

197Au Mössbauer study of a gold-doped YBa2Cu3O7-σ high-temperature superconductor

A high temperature superconductor with the composition YBa2Cu2.9Au0.1O7−δ was studied by Mössbauer spectroscopy with the 77 keV gamma rays of197Au, by X-ray diffraction, and by magnetization measurements. The main component in the Mössbauer spectrum, a quadrupole doublet with an isomer shift of +3.29 mm/s with respect to metallic gold and a quadrupole splitting of 3.61 mm/s, can be attributed to Au3+ in a square planar oxygen coordination, as is expected for gold replacing Cu(1). A further, weak doublet becomes dominant upon reduction of the specimen, and has an isomer shift of +3.46 mm/s and a quadrupole splitting of 7.12 mm/s. This component may be Au+ or Au3+ in an unusual coordination. Only a small fraction of the gold was found to be metallic in both the oxidized and the reduced state.