Ronald Fuge

The geochemistry of iodine - a review.

Iodine has long been recognised as an important element environmentally. Despite this there are many gaps in our knowledge of its geochemistry and even where information is available much of this is based on old data which, in the light of recent data, are suspect.Iodine forms few independent minerals and is unlikely to enter most rock-forming minerals. In igneous rocks its concentration is fairly uniform and averages 0.24 mg/kg. Sedimentary rocks tend to have higher concentrations with average iodine contents of:-recent sediments 5–200 mg/kg, carbonates 2.7 mg/kg, shales 2.3 mg/kg and sandstones 0.8 mg/kg. Organic-rich sediments are particularly enriched in iodine.Soils, generally, are much richer in iodine than the parent rocks with the actual level being decided mainly by soil type and locality. Little soil iodine is water-soluble and much iodine is thought to be associated with organic matter, clays and aluminium and iron oxides. Most iodine in soils is derived from the atmosphere where, in turn, it has been derived from the oceans. Seawater has a mean iodine content of 58 μg/L, while non-saline surface waters have lower and very variable levels. Subsurface brines and mineral waters are generally strongly enriched in iodine.Marine plants are frequently enriched in iodine while terrestrial plants have generally low contents. Iodine is essential for all mammals.Consideration of the geochemical cycle of iodine reveals that its transfer from the oceans to the atmosphere is probably the most important process in its geochemistry.

Quantitative analysis of trace elements in carbonates using laser ablation inductively coupled plasma mass spectrometry

Laser ablation inductively coupled plasma mass spectrometry has been applied to the analysis of carbonate materials. Multi-element synthetic standards, prepared both as pressed powders and fused glass discs, were used for calibration. The elements Mg, Mn, Sr, Ba and Pb were added to the pressed powder standards and these elements, together with U, were added to the fused glass standards. Calibration graphs, which are linear over at least three orders of magnitude, were produced using both types of standard but the fused glass discs gave better precision. The accuracy of the technique was evaluated using reference materials. Acceptable values were obtained using the pressed powders [e.g. BCS 393 (limestone) certified values of 905 ppm (Mg), 77 ppm (Mn) and 160 ppm (Sr)], but better accuracy was achieved with fused glass discs [e.g., BCS 393; 957 ppm (Mg), 79.6 ppm (Mn) and 167 ppm (Sr)]. The technique is applied to the analysis of carbonate shell material and demonstrates its potential in environmental monitoring.

Communication. Mineral microanalysis by laser ablation inductively coupled plasma mass spectrometry

The use of a modified Nd:YAG laser coupled to an inductively coupled plasma mass spectrometer is described for the direct analysis of carbonates, zircon, olivine and feldspars to give a variety of major and trace element data. The modification to the laser produces Q-switched ablation craters with a diameter between 20 and 30 µm compared with 150–200 µm in the un-modified version. This is suitable for direct mineral analysis in rocks. Calibration is achieved by ratioing analyte peak intensities to an internal standard and concentrations are produced by comparison with reference materials. The selection of internal standards is discussed for each mineral type and is dependant upon the assumption that the element chosen is present at a fixed molecular proportion in the mineral, despite other compositional changes. Spatial resolution of micro-analysis by laser ablation inductively coupled plasma mass spectrometry (ICP-MS) at (20–30 µm) is approximately ten times that of an electron probe micro-analysis (EPMA, 2–5 µm) but has the advantage of much lower detection limits and wider dynamic range (from ppm to 10–100% m/m) than routine EPMA analysis. Laser ablation ICP-MS can also offer isotopic information.

Sources of halogens in the environment, influences on human and animal health

Of the halogens, fluorine has the highest crustal abundance (544 mg/kg) while iodine has the lowest (0.25 mg/kg), however, chlorine is by far the most abundant halogen in the cosmos. The geochemistries of the four naturally occurring halogens have some similarities with fluorine, chlorine and bromine being classified as lithophile elements while iodine is more chalcophile in nature. Bromine and iodine behave in a similar fashion in the secondary environment and could be classified as biophile elements being concentrated in organic matter. Chlorine, bromine and iodine are strongly enriched in the sea while iodine and to a lesser extent bromine are further concentrated in the marine algae.Apart from the occurrence of fluorine in fluorite (CaF2) there are few commonly occurring minerals which contain the halogens as essential constituents. In the igneous environment fluorine and chlorine tend to occupy hydroxyl lattice sites in micas, amphiboles, apatites etc., while in sediments clays can contain appreciable quantities of these elements. Bromine and iodine, however, would be unlikely to fit into the lattice sites of common rock-forming minerals.Bromine, like iodine, is probably volatilised from the marine environment and is carried on to land surfaces. This behaviour of iodine and bromine is reflected in the increased I/CI and Br/CI ratios of surface run-off in continental compared with near coastal environments.Limited information on the soil geochemistry of the halogens suggests that the soil contents of chlorine, bromine and iodine are influenced by proximity to the sea. Soil fluorine, however, is generally dependent on its content in the parent material. In some areas pollutant sources of the halogens contribute appreciably to their concentration in the environment.Iodine and chlorine are essential elements for mammals and fluorine has been shown to have beneficial effects on bone and tooth formation. However, excess quantities of dietary fluorine can be harmful. It is possible, in view of its ubiquitous occurrence in the biosphere, that bromine has a hitherto unknown function in human and animal health.

Geochemistry of iodine in relation to iodine deficiency diseases

Abstract Seawater is the most important reservoir for terrestrial iodine (mean concentration 58 µgl−1 I); this is a major influence on iodine distribution in the secondary environment. Volatilization of iodine from the oceans, possibly as elemental iodine or as an organically-bound species, is the main source of the element in the environment. The distribution of iodine in the secondary environment is, therefore, largely controlled by proximity to the oceans, with rainwater and surface run-off relatively enriched in iodine in near-coastal regions. Soil iodine content is also strongly influenced with coastal soils being much enriched and central continental soils being depleted. Topography has a marked influence with soils in rain shadow areas being relatively depleted in iodine. While iodine input is a major controlling influence on its geographical distribution in soils, the soil’s ability to retain iodine is also an important factor. Organic matter together with iron and aluminium oxides and clays are the important sinks of soil iodine. An additional factor in the distribution of iodine in the secondary environment, and its subsequent availability to the biosphere, relates to its speciation in soils. In acid oxidizing conditions iodine is likely to be present as the I− ion and as such liable to be volatilized as I2. In near-neutral or alkaline conditions iodine is likely to be present as the IO3− ion which is not volatilized. Soils in limestone areas, with pH values of 7 and above, have thus been found to be much enriched in iodine compared to acidic soils in neighbouring areas. It is suggested that iodine from oceanic sources migrates in a series of ‘steps’ across landmasses by deposition followed by revolatilization. High pH soils and organic-rich soils then act as a migration barrier for iodine. In addition, it is suggested that volatilized iodine is bioavailable and any geochemical barrier to such volatilization deprives the biosphere of a major source. In this context it is interesting to note that several goitre endemias occurred in areas with limestone bedrock.

Trace-element analysis of volcanic glass shards by laser ablation inductively coupled plasma mass spectrometry: application to tephrochronological studies

Abstract The trace-element composition of fine-grained (0.25-0.125 mm) volcanic glass shards in distal tephra beds has been determined using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Accuracy is within 15% for most elements and the precision, as defined by the mean relative standard deviation, is better than 20%. This is very acceptable for tephrochronological studies. The very small amount of glass ablated (∼80 μg) and the short time involved in a single analysis (∼2 min) means that contamination can be easily recognized in the presence of replicate analyses and a detailed definition of trace-element compositional variability of the glass can be obtained. The need to use another trace-element analytical method to obtain the internal standard, Ce in this study, is a disadvantage but preliminary work in progress suggests that 57Fe can be used for this purpose. This would obviate the need to use another trace-element method because the major-element composition of glass shards, as determined by electron microprobe analysis, constitutes a routine part of tephra characterization studies.

Soils and Iodine Deficiency

Iodine has long been known as an essential element for humans, and mammals in general, where it is concentrated in the thyroid gland. It is a component of the thyroid hormone thyroxine. Deprivation of iodine results in a series of iodine deficiency disorders (IDD), the most obvious of which is endemic goiter, a condition where the thyroid gland becomes enlarged in an attempt to be more efficient. Iodine deficiency during fetal development and in the first year of life can result in endemic cretinism, a disease which causes stunted growth and general development along with brain damage. However, while these two diseases are easily recognizable, perhaps the more insidious problem is that iodine deficiency impairs brain development in children even when there is no obvious physical effect. Many researchers have suggested that a relatively low degree of iodine deficiency during fetal development can result in a significant reduction of IQ in children. Indeed it has been suggested that iodine deficiency is the most common preventable cause of mental retardation (see Chaps. 8 and 9, this volume). For a detailed discussion of the problems resulting from iodine deficiency in humans the reader is referred to the many publications on the topic by Basil Hetzel (e.g. Hetzel 1987, 2001; Hetzel et al. 1990)

Developments in the quantitative and semiquantitative determination of trace elements in carbonates by laser ablation inductively coupled plasma mass spectrometry

Standards for fully quantitative laser ablation inductively coupled plasma mass spectrometric analysis of carbonates have been produced by adding elemental standard solutions to a carbonate powder base, making possible the production of standards of low concentration (as low as 10 ppm of addition) with high internal precision reflecting homogeneity. Such low concentration standards would be impossible to produce by additions of elemental oxide powders to a carbonate matrix. A single internal standard analytical technique (so called ‘semiquantitative’ analysis) has been developed based on one multi-element standard which is used to determine the instrument response across the mass range 6–240 u. Results for this technique are generally within ±10% of accepted values for geological reference carbonate materials [e.g., Bureau of Analysed Samples, British Chemical Standard, Reference Material 393 (Limestone): Mg certified 900 ppm, analysed 892 ppm; Sr certified 160 ppm, analysed 186 ppm]. Semiquantitative analysis has the advantages of requiring less time to standardize the instrument and less data processing.

Minor and trace element chemistry of modern shells: a laser ablation inductively coupled plasma mass spectrometry study

Laser ablation inductively coupled plasma mass spectrometry was used in a study of minor and trace metal contents of the hard parts of modern shelly organisms from the west coast of Wales. This technique achieves sub-parts per million sensitivities with a spatial resolution of 25 μm. Seasonal variations in Mg are observed in Patella vulgata, Patella aspera and Mytilus edulis which relate to variations in water temperature. Lead, Zn and Cu are unevenly distributed in the shells. Large, long-lived bivalves such as Arctica islandica have the potential to record single, major pollution events from which chronologies may be erected, as well as reflecting regional geochemical differences. The maximum pollutant metal contents of Patella sp., a grazer and Mytilus edulis, a filter feeder, reflect both the regional seawater chemistry of Cardigan Bay and the feeding mode. Bulk shell compositions will not necessarily reflect these features. The soft tissues concentrate metals over a long period and show lower-magnitude regional variations than the hard parts.

The automated colorimetric determination of fluorine and chlorine in geological samples

Abstract Fluorine and chlorine are determined in geological samples following a sodium carbonate fusion and precipitation of interfering elements in the leach with ammonium carbonate. Fluorine is determined in the acidified leach utilising its fading action on the zirconium-xylenol orange complex. Chlorine is determined using the ferric thiocyanate method. The determinations are performed using the Technicon Auto Analyser.