Wayde N. Martens

Raman and infrared spectroscopic study of the vivianite-group phosphates vivianite, baricite and bobierrite

Abstract The molecular structure of the three vivianite-structure, compositionally related phosphate minerals vivianite, baricite and bobierrite of formula M32+(PO4)2‧8H2O where M is Fe or Mg, has been assessed using a combination of Raman and infrared (IR) spectroscopy. The Raman spectra of the hydroxyl-stretching region are complex with overlapping broad bands. Hydroxyl stretching vibrations are identified at 3460, 3281, 3104 and 3012 cm-1 for vivianite. The high wavenumber band is attributed to the presence of FeOH groups. This complexity is reflected in the water HOH-bending modes where a strong IR band centred around 1660 cm-1 is found. Such a band reflects the strong hydrogen bonding of the water molecules to the phosphate anions in adjacent layers. Spectra show three distinct OH-bending bands from strongly hydrogen-bonded, weakly hydrogen bonded water and non-hydrogen bonded water. The Raman phosphate PO-stretching region shows strong similarity between the three minerals. In the IR spectra, complexity exists with multiple antisymmetric stretching vibrations observed, due to the reduced tetrahedral symmetry. This loss of degeneracy is also reflected in the bending modes. Strong IR bands around 800 cm-1 are attributed to water librational modes. The spectra of the three minerals display similarities due to their compositions and crystal structures, but sufficient subtle differences exist for the spectra to be useful in distinguishing the species.

Microstructure of HDTMA+-Modified Montmorillonite and its Influence on Sorption Characteristics

A series of organoclays with monolayers, bilayers, pseudotrilayers, paraffin monolayers and paraffin bilayers were prepared from montmorillonite by ion exchange with hexadecyltrimethylammonium bromide (HDTMAB). The HDTMAB concentrations used for preparing the organoclays were 0.5, 0.7, 1.0, 1.5, 2.0 and 2.5 times the montmorillonite cation exchange capacity (CEC). The microstructural parameters, including the BET-N2 surface area, pore volume, pore size, and surfactant loading and distribution, were determined by X-ray diffraction, N2 adsorption-desorption and high-resolution thermogravimetric analysis (HRTG). The BET-N2 surface area decreased from 55 to 1 m2/g and the pore volume decreased from 0.11 to 0.01 cm3/g as surfactant loading was increased from Na-Mt to 2.5CEC-Mt. The average pore diameter increased from 6.8 to 16.3 nm as surfactant loading was increased. After modifying montmorillonite with HDTMAB, two basic organoclay models were proposed on the basis of HRTG results: (1) the surfactant mainly occupied the clay interlayer space (0.5CEC-Mt, 0.7CEC-Mt, 1.0CEC-Mt); and (2) both the clay interlayer space and external surface (1.5CEC-Mt, 2.0CEC-Mt, 2.5CEC-Mt) were modified by surfactant. In model 1, the sorption mechanism of p-nitrophenol to the organoclay at a relatively low concentration involved both surface adsorption and partitioning, whereas, in model 2 it mainly involved only partitioning. This study demonstrates that the distribution of adsorbed surfactant and the arrangement of adsorbed HDTMA+ within the clay interlayer space control the efficiency and mechanism of sorption by the organoclay rather than BET-N2 surface area, pore volume, and pore diameter.

DSC AND HIGH-RESOLUTION TG OF SYNTHESIZED HYDROTALCITES OF Mg AND Zn

A combination of DSC and high resolution DTG coupled to a gas evolution mass spectrometer has been used to study the thermal properties of a series of Mg/Zn hydrotalcites of formulae MgxZn6-xAl2(OH)16(CO3) ·4H2O where x varied from 6 to 0. The effect of increased Zn composition results in the decrease of the endotherms and mass loss steps to lower temperatures. Evolved gas mass spectrometry shows that water is lost in a number of steps. The interlayer carbonate anion is lost simultaneously with hydroxyl units.

Vibrational spectroscopy of the basic manganese and ferric phosphate minerals : strunzite, ferrostrunzite and ferristrunzite

The Raman spectra of strunzite, ferristrunzite and ferrostrunzite have been obtained at 298 and 77K using a combination of a thermal stage and Raman microscopy. These spectra are compared with their infrared spectra. The vibrational spectra of the two minerals are different, in line with differences in crystal structure and composition. Some similarity in the Raman spectra of the hydroxyl-stretching region exists, particularly at 298K, but characteristic differences in the OH deformation regions are observed. Significant shifts in the position of the Raman bands are observed by obtaining the spectra at 77K. Differences are also observed in the phosphate stretching and deformation regions.

Raman spectroscopy of basic copper(II) and some complex copper(II) sulfate minerals: Implications for hydrogen bonding

Abstract Raman spectroscopy has been applied to the study of basic Cu sulfates including antlerite, brochiantite, posnjakite, langite, and wroewolfeite and selected complex Cu sulfate minerals. Published X-ray diffraction data were used to estimate possible hydrogen bond distances for the basic Cu sulfate minerals. A Libowitzky empirical expression was used to predict hydroxyl-stretching frequencies and agreement with the observed values was excellent. This type of study was then extended to complex basic Cu sulfates: cyanotrichite, devilline, glaucocerinite, serpierite, and ktenasite. The position of the hydroxyl-stretching vibration was used to estimate the hydrogen bond distances between the OH and the SO4 units. The variation in bandwidth of the OH-stretching bands provided an estimate of the variation in these hydrogen bond distances. By plotting the hydrogen bond O…O distance as a function of the position of the SO4 symmetric stretching vibration, the position of the SO4 symmetric stretching band was found to be dependent upon the hydrogen bond distance for both the basic Cu sulfates and the complex Cu sulfates.

Absorption of the selenite anion from aqueous solutions by thermally activated layered double hydroxide

The presence of selenite or selenate in potable water is a health hazard especially when consumed over a long period of time. Its removal from potable water is of importance. This paper reports technology for the removal of selenite from water through the use of thermally activated layered double hydroxides. Mg/Al hydrotalcites with selenite in the interlayer were prepared at different times from 0.5 to 20 h through ion exchange. X-ray diffraction of the MgAlSeO3 hydrotalcites indicates that the selenite anion entered the interlayer spacing of Mg/Al hydrotalcite and MgAlSeO3 hydrotalcite was formed. Raman spectra proved the presence of selenite anion in the hydrotalcite interlayer as the counter anion. The band intensity and width of MgAlSeO3 hydrotalcite in the region of 3800-3000 cm(-1) increase with the adsorption of selenite by the Mg/Al hydrotalcite. The characteristic bands of free selenite anions in the MgAlSeO3 hydrotalcites are located between the region between 850 and 800 cm(-1). The Raman spectra of the lower wave number region of 550-500 cm(-1) show a shift toward higher wave numbers with adsorption of the selenite. An estimation of the amount of selenite anion removed by the thermally activated layered double hydroxide was obtained through the measurement of the intensity of the selenite Raman bands at 814 and 835 cm(-1) resulting from the amount of selenite anion remaining in solution. Thermally activated LDHs provide a mechanism for removing selenite anions from aqueous solutions.

Vibrational spectroscopy of the basic copper phosphate minerals: pseudomalachite, ludjibaite and reichenbachite

The vibrational spectra of pseudomalachite, reichenbachite and ludjibaite have been obtained at 298 K using a combination of FTIR and Raman microscopy. The vibrational spectra of the minerals are different, in line with differences in crystal structure and composition. Some similarity in the Raman spectra of the three polymorphs pseudomalachite, reichenbachite and ludjibaite exists, particularly in the OH stretching region, but characteristic differences in the OH deformation regions are observed. Differences are also observed in the phosphate stretching and deformation regions.

Synthesis, characterization of mono, di and tri alkyl surfactant intercalated Wyoming montmorillonite for the removal of phenol from aqueous systems.

Organoclays were synthesized by the ion exchange of cationic surfactants containing single, double and triple alkyl chains for sodium ions in an aqueous suspension of Wyoming Na-montmorillonite. The characterization of organoclays with and without adsorbed phenol was determined by X-ray diffraction, TEM and thermal analysis. Differences in the surfaces and in the interlayer of the mono, di and tri alkyl chain organoclays resulted in differences in the adsorption efficiency for phenol with tri > di > mono >> Na-Mt. The results prove that organoclays can be effective for the removal of phenol from an aqueous solution and this removal is a function of the surfactant molecule and its concentration. In general, the higher the concentration as measured by the CEC value and the greater the number of alkyl chains in the surfactant molecule, the greater the percentage of the phenol that is removed.

Raman spectroscopy of the minerals boleite, cumengeite, diaboleite and phosgenite - implications for the analysis of cosmetics of antiquity

Abstract The application of Raman spectroscopy to the study of the mixed cationic Pb-Cu and Pb-Cu-Ag minerals: boléite, cumengéite and diaboléite has enabled their molecular structures to be compared. Each of these three minerals shows different hydroxyl-stretching vibrational patterns, but some similarity exists in the Raman spectra of the hydroxyl-deformation modes. The low-wavenumber region is characterized by the bands assigned to the cation-chloride stretching and bending modes. Phosgenite is also a mixed chloride-carbonate mineral and a comparison is made with the molecular structure of the aforementioned minerals. Raman spectroscopy lends itself to the study of these types of minerals in complex mineral systems of secondary mineral formation.

An infrared spectroscopic study of the basic copper phosphate minerals: Cornetite, libethenite, and pseudomalachite

Abstract The molecular structures of the basic copper phosphate minerals pseudomalachite, libethenite, and cornetite were studied using a combination of infrared emission spectroscopy, infrared absorption, and Raman spectroscopy. Infrared emission spectra of these minerals were obtained over the temperature range 100 to 1000°C. The infrared spectra of the three minerals are different, in line with differences in crystal structure and composition. The absorption spectra are similar, particularly in the OH stretching region, but characteristic differences in the bending regions are observed. Differences are also observed in the phosphate stretching and bending regions. The IR emission of the basic copper phosphates studied shows that the minerals are completely dehydroxylated by 550°C.