Keith Nicholson

Geochemistry of manganese oxides: metal adsorption in freshwater and marine environments

Abstract The effect of pH and metal ion concentration on the adsorption of Cu, Co, Cd, Mn, Ni and Zn onto manganese oxides was investigated. The oxides used in the experiments were deposited around individual sand grains in water treatment filter beds in NE Scotland and show reproducible adsorption profiles regardless of location or age. The oxides remained amorphous throughout the four-year study period, and show an inferred pHZPC of 1.5. Metal adsorption increases towards more alkaline pH. The adsorption series Co > Cu > Cd > Zn > Mn > Ni is shown at pH 3 and Cu > Co > Cd > Mn > Zn > Ni at pH 6. Changing the concentration of the metal ions in solution leads to an increase in the mass of metal adsorbed, though the relationship is not linear as the proportion of metal taken up falls as concentration rises. Adsorption from an artificial seawater solution shows a significant reduction in the amount of metal taken up in all cases except Cu which shows an adsorption profile comparable with that in deionized water. The linear Freundlich isotherm accurately and reproducibly models the adsorption profile of all metals at each pH and concentration examined. The data do not, however, replicate the enrichment sequence observed in manganese nodules. The study demonstrates that for the enrichments observed in manganese nodules to be attained, the active oxide surface must be replenished by regular precipitation of fresh oxide material to enable continued adsorption to take place.

Investigations of decayed sandstone colonised by a species ofTrentepohlia

The occurrence of a sub-aerial microbial community dominated by a species ofTrentepohlia, a red-orange pigmented filamentous green alga, growing on a 13th century sandstone building in the Scottish borders appears to be associated with unusual stone decay. Decay, in the form of extensive surface spalling and grain dissagregation over large areas of external stonework, are coincident with the distribution of the algal community on the north facade of the building. Comparison of S.E.M. images taken of fresh intact stone and spalled stone from decayed zones suggests that the mechanisms of decay may, in part, be a physical process which results in quartz crystals becoming separated. The investigations carried out revealed little evidence of chemical processes which may be involved in the decay. Large filaments ofTrentepohlia are clearly visible growing vertically between separate grains of stone and the amount of highly absorbent mucilage present makes the friable dry spall become soft and gelatinous on wetting. It is possible that the observed decay may have been created through the mechanical action of wet-dry cycling of mucilage and expansive stress from the growth of algal filaments.

Correlation of the methylating capacity of river and marine sediments to their organic sediment index

Methylation experiments of the metals Sn, Pb and Hg were carried out using representative terrestrial and marine sediment samples from the Axios river and Thermaikos Gulf in northern Greece. GC-FID, GC-TCD and GC-MS were used. The experiments were carried out on sterilised and bioactive samples by adding pure metals and metal salts (chloride, nitrate, oxalate, acetic, penicillaminic, methioninic and cysteinic). Except for sterilised HgCl2, methylated derivatives of Sn, Pb and Hg were produced only from bioactive sediments, and therefore higher yields were measured when nutrients were added to the sediments. Volatile products (CH4, CO2, H2S) of biological activity range between 35 and 250 mg l-. The correlation of methylation yield with organic sediment index (OSI), determined as the wt% product of [organic carbon] × [organic nitrogen], is positive for all the metals and metal salts added in the sediments. Methylation yields for Hg are found to be four orders of magnitude higher than those of Pb and Sn. In low OSI (terrestrial) sediments, the rate of Hg-methylation is higher than those of Pb and Sn. In high OSI (marine) sediments, where methylation of most of the contained Hg has taken place, methylation of Pb is slightly faster than Sn.

Chemistry and adsorption-desorption properties of manganese oxides deposited in Forehill Water Treatment Plant, Grampian, Scotland

Manganese oxide coatings on sand particles within filtration beds from a water treatment plant in Grampian, Scotland were examined to determine their control on metal mobility. This study first sought to characterise the oxides, notably their mineralogy and metal content, to provide a foundation for studies on the adsorption of dissolved metals from the treated water by the oxides. The oxides were examined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and analysed by flame atomic absorption spectrophotometry (AAS). These techniques showed the oxide coatings were amorphous but uniformly distributed over each sand grain. The oxides were selectively removed from the sand grains prior to analysis by AAS using a hydroxylamine hydrochloride selective leaching method. The compositional range of the extracts was 100–150 mg L−1 Mn; 30–55 mg L−1 Fe; 17–56 mg L−1 Ca; 4.6–7.0 mg L−1 Ni; 4.6–6.8 mg L−1 Zn and 1.3–5.7 mg L−1 Mg. When these results are expressed as mg of metal per mg of Mn, the metal content of the oxides is remarkably uniform: 0.25–0.37 mg Fe; 0.14–0.35 mg Ca; 0.035–0.042 mg Ni; 0.035–0.040 mg Zn; 0.01–0.04 mg Mg. The greatest metal concentrations were consistently found in the upper 3 cm of the filtration bed, and these decrease with increasing depth. After the beds are cleaned a more uniform distribution of metals occurs throughout the bed. The metals taken up by the manganese coating are retained over a wide pH range with the exception of Ca and Mg which desorb to a significant extent. The percentage of calcium and magnesium lost from the coating ranges from 30–94%, the amount being dependent on the final pH of the solution. The presence of manganese oxide in the filtration beds appears to be advantageous in terms of removal of transition metals from the treated water.

An EPMA and SEM study of the Mn-oxide mineralization of Kato Nevrokopi, Macedonia, northern Greece: Controls on formation of the Mn4+ oxides

Abstract EPMA and SEM studies of tunnel-structured todorokite and nsutite and layer-structured chalcophanite and birnessite from the Kato Nevrokopi battery grade Mn-mineralization were performed. The chemistry of todorokite, formed at hypogene and supergene environments in an increasing order of weathering, reflects different pH and oxidation conditions. The data also point to a clear dependence of the development of late stage hypogene and early supergene Mn4+-oxide paragenesis on the host rock and protore composition and the mobilities of base metals (mainly Zn), alkalis and alkaline earths (mainly Ca). The chemistry and development of (large cation-bearing) nsutite as in situ oxidations and banded poorly crystalline (with structural defects) aggregates in veins and associated karstic cavities, together with birnessite, indicates the influence of climate, oxidation and water table fluctuations. The data on chalcophanite revealed a deposition, under low fO2 conditions, from acid meteoric fluids produced by weathering of vein sulphides. Birnessite formation reflects a prevalence of higher fO2 in weathered veins and karstic cavities. The evolution of well developed layer structures through poorly crystalline to amorphous tunnel-structured nsutite is probably controlled by a Zn-saturation of the ore fluids.

Boron determination in water by ion-selective electrode

The fluoroborate electrode shows a linear Nernstian response down to 0.35 mg/L of boron (B) and can be reproducibly calibrated to 0.001 mg/L B. Reproducibility at 0.1 and 1.0 mg/L of B is better than 0.5%. Electrode sensitivity declines significantly below 0.035 mg/L B, and this is the practical detection limit. Minimum equilibrium times increase with decreasing concentration, from 1 min at 5.0 mg/L to 10 min at <0.05 mg/L of B. A pH hysteresis effect was observed between pH 2–12. Effects of pH are minimised if analyses are performed between pH 3.5 and 5.0. Prolonged storage of fluoroborate solutions results in a loss of the ion from the solution. In both glass and plastic containers, a decrease of up to 16% in concentration was observed for concentrations of 0.1, 1.0, and 10.0 mg/L of B over a two-week period. Acid washing containers did not prevent boron loss. Fluoroborate ion formation, following addition of hydrofluoric acid to the sample, requires about 2 h. Samples should not be analysed before this complexation time has elapsed.

Precambrian to modern manganese mineralization: Changes in ore type and depositional environment

Manganese mineralization is diverse in occurrence, origin, mineralogy and geochemistry. These variations reflect differences in the processes of formation and depositional environments, which in turn are a response to changes in the land-ocean-atmosphere system over geological time. As such, manganese deposits can act as markers of major events in the dynamic evolution of the Earth’s surface. Modern manganese accumulations provide insights into key factors controlling manganese deposition that cannot readily be determined from examination of ancient ores. A knowledge of oceanic currents, ocean chemistry or small-scale variations in physicochemical patterns of recent basins, for example, may extend our understanding of depositional processes in the past. Equally, the study of Precambrian deposits not only elucidates ancient mechanisms of manganese metallogenesis, but also helps to unravel the impact of comprehensive environmental changes on metal deposition on a scale not realized in younger geological times. The papers collected in this volume provide insights into this changing nature of manganese mineralization from Precambrian sedimentary ores to crusts and nodules on the Cenozoic sea bed. The volume is introduced by Supriya Roy with a review of the range of terrestrial manganese deposits and their relative abundance through geological time. The manner in which manganese mineralization reflects changes in planetary environmental chemistry that was noted above and mentioned by Roy is further developed by papers

Soil and Soil-Gas Surveys

Water and gas sampling of natural discharges are the most common types of survey over known geothermal areas. However, where discharge features are few and where the extent of the field is not known, soil and soil-gas surveys can prove helpful. These surveys can identify permeable regions in a field and possible upflow or boiling zones. They can also delineate the margins of a geothermal system, and therefore often complement geophysical surveys particularly where interpretation of geophysical data is difficult, eg. due to topographic effects.

Sedimentary geochemistry of Lake Daviumbu, part of the Fly River system, Papua New Guinea: impact of upstream copper-gold mining.

Sequential extraction method: a review and