Volker Karius

pH and grain-size variation in leaching tests with bricks made of harbour sediments compared to commercial bricks

Bricks made of 50% wt. harbour sediments from Bremen, Germany, harbour sediment and nine commercial bricks made of common raw materials were leached in various experiments. The harbour sediment is polluted with heavy metals, e.g. Zn, Cd, Pb and organic compounds, e.g. tributyltin. To assess the environmental impact in the potential use of sediment bricks we consider the influence of pH (4-11) and grain size (50-30 000 microm), the two prime variables in the life-cycle of the bricks. Leachability of trace contaminants increased at acidic pH values and remained low at neutral and alkaline pH values. Leachability increased for smaller grain sizes in relation to the increasing specific surface areas. Grain sizes below 63 microm showed reverse effects for V, Cr, Ni, As, Sr, Mo and Pb due to sorption on the sample material or freshly precipitated phases such as barite, anhydrite or cuprous ferrite. A grain-size fraction of 125-1000 microm was selected in the leaching tests in order to compare different brick types. In general, the leachability of heavy metals from the sediment brick was in the upper range of the commercial bricks. At a temperature of 1050 degrees C thermal treatment of harbour sediments led to an immobilization of most trace contaminants. Chromium, V, As and Mo became even more mobile after thermal treatment, but the enhanced mobilization of V, As and Mo differed strongly among the bricks compared.

Relationship between micrometer to submicrometer surface roughness and topography variations of natural iron oxides and trace element concentrations.

The surface area and roughness of natural iron oxide precipitations were quantified by 3D optical microscopy in order to get information about fluid-rock interface topography in high-permeability zones. Converged surface roughness data of microscale to submicroscale topography show the predominance of macroporous half-pores (>500 nm) and the occurrence of smaller half-pores (<500 nm) that dominate the BET surface area of iron oxides. A relationship was found between the occurrence of macroporous surface structures (micrometer range) and the uranium content of iron oxide encrustations. Iron-normalized uranium concentrations of an X-ray amorphous iron oxide encrustation correlate linearly with maximum topography heights of 1 to 2 mum on hand specimen subsamples. Our study shows the potential importance of micrometer- to submicrometer-size surface features, whose environmental impact is often ignored.

Constructing modal mineralogy from geochemical composition: A geometric-Bayesian approach

Modal mineralogical composition is known to carry more information than major element geochemistry, though the latter is far easier to determine in the lab. Constructing mineral compositions from geochemistry can be seen as a typical end-member problem, where one assumes that some multivariate observations are generated by a convex linear mixture of a few pure end-members: these end-member characteristics as well as the coefficients of the linear mixture for the observations can be then estimated from geochemical data. We propose a mixed geometric-probabilistic solution to this problem. First, we assume known end-members, in number and properties, and study the set of solutions from a purely geometric perspective. Second, we discuss how to select representative solutions from this space, in particular under some distributional assumptions. Third, we allow the end-member properties to randomly vary in a controlled, interpretable fashion. Finally we build a Bayesian model, with a parsimonious parametrization characterizing each of these three steps, that can be treated by conventional Markov-Chain Monte Carlo techniques. In the illustration case study, we apply the method to reconstruct the mineralogy of a set of fluvio-glacial monomictic sediments from an Alpine granitoid massif. Results suggest a trend of enrichment in chlorite, muscovite and Ti-bearing minerals, along with depletion in quartz and feldspar. This is tentatively interpreted as an effect of comminution combined with differential mechanical properties. Moreover, mineral chemistry is estimated to exhibit very low Na in muscovite, Fe-rich garnet, Na-rich plagioclase, K-feldspar with up to 10% Na-component (albite), and biotite with Mg>Fe. The reconstructed modal mineralogy stays in a reasonable agreement with quantitative XRD phase analyses.

Correlation between sub-micron surface roughness of iron oxide encrustations and trace element concentrations.

Iron oxide encrustations are formed on black slate surfaces during oxidative weathering of iron sulfide and phosphate bearing, organic matter-rich slates. Synchronously, trace elements are released during ongoing weathering. Laser ablation ICP-MS analyses of a weathered and encrusted slate showed that major portions of the V, Cu, As, Mo, Pb, Th, and U reside in the encrustation.Recently a potential relationship between several micrometer to 500 nm surface topography roughness of such encrustations and its uranium concentration was shown. Based on laser scanning microscopy measurements, the present study shows that this interrelation must be expanded to small submicron-sized half-pores with diameters between 100 nm and 500 nm. We demonstrate that the relationship is not limited to topography variations of a single encrustation in the hand-specimen scale. Surface topography and geochemical analyses of iron oxide encrustations from several locations but from the same geochemical environment and with similar weathering history showed that the concentrations of U, P, Cu, and Zn correlate inversely with the surface roughness parameter F. This parameter represents the total surface area and is - in this case - a proxy for the root-mean square surface roughness Rq.This study substantiates the environmental importance that micrometer- to submicrometer topography variations of fluid-rock interfaces govern the trapping of trace elements.