Applied Geochemistry (v.17, #12)

Carbonation processes in municipal solid waste incinerator bottom ash and their effect on the leaching of copper and molybdenum by Jeannet A Meima; Renata D van der Weijden; T.Taylor Eighmy; Rob N.J Comans (1503-1513).
The interaction of CO2 with municipal solid waste incinerator (MSWI) bottom ash was studied in order to investigate the resulting changes in pH and bottom ash mineralogy and the impact that these changes have on the mobility of Cu and Mo. Carefully controlled carbonation experiments were performed on bottom ash suspensions and on filtered bottom ash leachates. Changes in leachate composition were interpreted with the geochemical model MINTEQA2, and neoformed minerals were investigated by means of chemical and spectroscopic analysis. The leaching of Cu and Mo during artificial carbonation is compared to the leachability of Cu and Mo from a sample of naturally carbonated bottom ash from the same incinerator. During carbonation in the laboratory, a precipitate was formed that consisted mainly of Al-rich amorphous material, calcite, and possibly gibbsite. Carbonation to pH ≈8.3 resulted in a reduction of more than 50% in Cu leaching, and a reduction of less than 3% in Mo leaching. The reduction in Cu leaching is attributed to sorption to the neoformed amorphous Al-minerals. During natural weathering/carbonation of bottom ash, additional sorption sites are formed which further reduce the leaching of Cu and Mo on a time scale of months to years.

S- and O-isotopic character of dissolved sulphate in the cover rock aquifers of a Zechstein salt dome by Zsolt A Berner; Doris Stüben; Markus A Leosson; Hans Klinge (1515-1528).
Sulfur and O isotope analyses of dissolved SO4 were used to constrain a hydrogeological model for the area overlying the Gorleben–Rambow Salt Structure, Northern Germany. Samples were collected from 80 wells screened at different depth-intervals. The study area consists of a set of two vertically stacked aquifer systems. Generally, the isotope data show a good spatial correlation, outlining well-defined groundwater zones containing SO4 of characteristic isotopic composition. Highly saline waters from deeper parts of the lower aquifer system are characterized by rather constant SO4 isotopic compositions, which are typical of Permian Zechstein evaporites (δ34 S=9.6–11.9‰; δ18 O=9.5–12.1‰). Above this is a transition zone containing ground waters of intermediate salinity and slightly higher isotopic values (average δ34 S=16.6‰; δ18 O=15.3‰). The confined groundwater horizon on the top of the lower aquifer system below the low permeable Hamburg Clays is low in total dissolved solids and is characterized by an extreme 34S enrichment (average δ34 S=39.1‰; δ18 O=18.4‰), suggesting that bacterially mediated SO4 reduction is a dominant geochemical process in this zone. Two areas of distinct isotopic composition can be identified in the shallow ground water horizons of the upper hydrogeological system. Sulfate in groundwaters adjacent to the river Elbe and Löcknitz has a typical meteoric isotopic signature (δ34 S=5.2‰; δ18 O=8.2‰), whereas the central part of the area is characterized by more elevated isotopic ratios (δ34 S=12.7‰; δ18 O=15.6‰). The two major SO4 pools in the area are represented by Permian seawater SO4 and a SO4 of meteoric origin that has been mixed with SO4 resulting from the oxidation of pyrite. It is suggested that the S-isotope compositions observed reflect the nature of the SO4 source that have been modified to various extent by bacterial SO4 reduction. Groundwaters with transitional salinity have resulted from mixing between brines and low-mineralized waters affected by bacterial SO4 reduction.

Pyrolysis kinetics are determined for a series of asphaltene samples associated with different types of kerogens. The activation energy distributions obtained for asphaltenes related to type I kerogens cover a wide range, with significant hydrocarbon generation with activation energies above 350 kJ/mol. The ranges of activation energy distributions are relatively narrow for asphaltenes associated with type II and III kerogens, with little hydrocarbons generated with activation energies above 350 kJ/mol. If the type of associated kerogen can be reasonably constrained, the pyrolysis kinetics of the asphaltenes is potentially a very useful tool for assessing the maturity levels of reservoired oils. The results of pyrolysis kinetics indicate that there is still significant potential for asphaltenes to regenerate hydrocarbons.

A recent discovery of Pliocene tree trunks (Taxodium, Glyptostrobus or Sequoia) perfectly preserved in the fluvial lacustrine sediments deposited in the old Tiberin Lake (near the town of Dunarobba, Central Italy) has led to the study of the conditions responsible for the trunks preservation. The present work has applied Sr isotope techniques to examine and confirm fluid migration in the different sediment types which cross the entire fluvial-lacustrine sequence. Based on the 87Sr/86Sr ratios measured in carbonate and in silicate fractions of samples collected from a bore hole (from 0 to 120 m) it is concluded that each sedimentary level has limited communication between zones and is characterised by different isotope values. This suggests that the intrinsic properties of these clays make them a good material for isolating waste, both for direct use and for a better buffer in other formations, possibly less secure, such as salt or granite. The results obtained in the present work have demonstrated some of the potential uses of the 87Sr/86Sr isotopic ratio as an environmental tracer for confirming results from some of the geotechnical (such as permeability) and geochemical parameters of these host sediments.

Retention behaviour of humic substances onto mineral surfaces and consequences upon thorium (IV) mobility: case of iron oxides by Pascal Reiller; Valérie Moulin; Florence Casanova; Christian Dautel (1551-1562).
The influence of humic acids (HA) on the retention of Th(IV) onto the surface of Fe oxides has been investigated. Thorium (IV) is here considered as an analogue of tetravalent radionuclides (Zr, Hf, U, Np, Pu). Hematite (α–Fe2O3) and gœthite (α–FeOOH) have been chosen as model phases because of their widespread presence in the earth's crust. Retention experiments were performed by batch procedure at constant ionic strength and varying pH in the range 2–10; the HA concentration varied in the range 1–100 mg l−1. Compared with the former data obtained on the retention of Th(IV) with the same hematite sample free of HA, the study of a ternary system (i.e. hematite–HA–Th), clearly shows the influence of HA on the retention mechanisms of Th(IV) even in neutral to slightly alkaline medium. In particular, it is illustrated that the retention of Th(IV) onto Fe oxides closely depends upon the ratio between humic and surface (Fe oxide) sites. This ratio has to be greater than a critical value in order to impede the sorption of Th(IV).

Contrasting the geochemistry of oxic sediments across ecosystems: a synthesis by Leah I Bendell-Young; Christine A Thomas; J.R.Pierre Stecko (1563-1582).
The geochemistry of oxic sediments was contrasted across a range of Canadian aquatic ecosystems; 7 freshwater lakes, (3 circumneutral and 4 acidic), 15 peatlands (5 mineral-rich, 5 moderately-poor and 5 mineral-poor), 9 wetlands (3 oligosaline, 3 mesosaline and 3 euryhaline), an estuary (deposited and suspended sediments) and an intertidal region. Sediments were characterized by a simultaneous extraction that separated sediments into easily reducible (ER) metal (oxyhydroxides of Mn and easily reducible amorphous oxyhydroxides of Fe) and reducible (R) metal (more crystalline forms of oxyhydroxides of Fe), organic matter, and, the concentrations and partitioning of Zn, Cu and Cd associated with these 3 sediment components. Ecosystems were distinct with respect to ER Fe and organic matter [canonical variate analysis (CVA)] with 53% of among system variation in geochemistry attributed to these 2 components. Sediments of peatlands and wetlands contained greater amounts of organic matter whereas sediments of lakes, intertidal and estuarine deposited and suspended sediments were characterized by greater amounts of ER Fe. A further 21% of among system variation could be ascribed to organically bound Fe that was greater in acidic lakes and mineral-rich peatlands as compared to other systems. Concentration and partitioning of Cd within sediments was regionally dependent with 41% of among system variation (CVA) attributed to differences in ER Cd and R Cd. Cadmium from peatlands and lakes located in Ontario was recovered from all 3 sediment components whereas sediment from wetlands, the estuary and the intertidal regions of British Columbia (BC) contained no organically bound Cd with amounts recovered occurring mostly as ER Cd. Lakes and peatlands located in Ontario contained 3–5-fold total Cd as compared to ecosystems located in BC. A further 21% of among ecosystem variation was attributed to Zn partitioning. Zinc in peatland and wetland sediments occurred as R Zn as compared to lake and estuarine deposited sediments where Zn was recovered both as ER and R Zn. Total Zn was also 3–5-fold greater in sediments of systems in Ontario as compared to those sampled in BC. Concentration and partitioning of Cu was similar across all systems with Cu recovered from the organic component of sediment. The geochemistry of surficial oxic sediments with respect to ER Mn, R Fe and organic matter and the geochemical associations among these sediment components is ecosystem and region dependent. For assessing impacts of metals on sediment dwelling biota the geochemical characteristics of the system under study should first be defined.