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Aquatic Geochemistry (v.14, #1)
Interaction of Freshly Precipitated Silica Gel with Aqueous Silicic Acid Solutions under Ambient and Near Neutral pH-conditions: A Detailed Analysis of Linear Rate Law by Jiří Faimon; Martin Blecha (pp. 1-40).
Interaction of freshly precipitated silica gel with aqueous solutions was studied at laboratory batch experiments under ambient and near neutral pH-conditions. The overall process showed excellent reversibility: gel growth could be considered as an opposite process to dissolution and a linear rate law could be applied to experimental data. Depending on the used rate law form, the resulting rate constants were sensitive to errors in parameters/variables such as gel surface area, equilibrium constants, Si-fluxes, and reaction quotients. The application of an Integrated Exponential Model appeared to be the best approach for dissolution data evaluation. It yielded the rate constants k dissol ∼ (4.50 ± 0.68) × 10−12 and k growth ∼ (2.58 ± 0.39) × 10−9 mol m−2 s−1 for zero ionic strength. In contrast, a Differential Model gave best results for growth data modeling. It yielded the rate constants k dissol ∼ (1.14 ± 0.44) × 10−11 and k growth ∼ (6.08 ± 2.37) × 10−9 mol m−2 s−1 for higher ionic strength (I ∼ 0.04 to 0.11 mol L−1). The found silica gel solubility at zero ionic strength was somewhat lower than the generally accepted value. Based on the $${}^{298}K_{
m SS}^{(I=0)} = 10^{-2.754pm 0.017}$$ and $$^{298}K_{
m DBP}^{(I=0)} = 10^{-2.728pm 0.003},$$ standard Gibbs free energy of silica gel formation was calculated as $$Updelta{G}_{
m f}^0(298,{
m K}) sim -hbox{850,463} pm 98hbox{ J mol}^{-1}$$ and −850,318 ± 20 J mol−1, respectively. Activation energies for silica gel dissolution and growth were determined as $$E_{
m A}^{
m dissol} sim 62.0pm 3.2,hbox{kJ mol}^{-1}$$ and $$E_{
m A}^{
m growth} sim 48.8pm 4.4,hbox{ kJ mol}^{-1},$$ respectively. An universal value for growth of any silica polymorph, $$E_{
m A}^{
m growth} sim 37.4 pm 9.4,hbox{kJ mol}^{-1},$$ is not consistent with the value for silica gel growth, which questions the hypothesis about one unique activated complex controlling the silica polymorph growth.
Keywords: Dissolution; Growth; Dynamics; Error; Experiment; Flux; Model; Reversibility; Silica gel
Geochemical Eccentricity of Ground Water Allied to Weathering of Basalts from the Deccan Volcanic Province, India: Insinuation on CO2 Consumption by N. J. Pawar; J. B. Pawar; Suyash Kumar; Ashwini Supekar (pp. 41-71).
Analyses of 72 samples from Upper Panjhara basin in the northern part of Deccan Plateau, India, indicate that geochemical incongruity of groundwater is largely a function of mineral composition of the basaltic lithology. Higher proportion of alkaline earth elements to total cations and HCO3>Cl + SO4 reflect weathering of primary silicates as chief source of ions. Inputs of Cl, SO4, and NO3 are related to rainfall and localized anthropogenic factors. Groundwater from recharge area representing Ca + Mg–HCO3 type progressively evolves to Ca + Na–HCO3 and Na–Ca–HCO3 class along flow direction replicates the role of cation exchange and precipitation processes. While the post-monsoon chemistry is controlled by silicate mineral dissolution + cation exchange reactions, pre-monsoon variability is attributable chiefly to precipitation reactions + anthropogenic factors. Positive correlations between Mg vs HCO3 and Ca + Mg vs HCO3 supports selective dissolution of olivine and pyroxene as dominant process in post-monsoon followed by dissolution of plagioclase feldspar and secondary carbonates. The pre-monsoon data however, points toward the dissolution of plagioclase and precipitation of CaCO3 supported by improved correlation coefficients between Na + Ca vs HCO3 and negative correlation of Ca vs HCO3, respectively. It is proposed that the eccentricity in the composition of groundwater from the Panjhara basin is a function of selective dissolution of olivine > pyroxene followed by plagioclase feldspar.The data suggest siallitization (L < R and R k) as dominant mechanism of chemical weathering of basalts, stimulating monosiallitic (kaolinite) and bisiallitic (montmorillonite) products. The chemical denudation rates for Panjhara basin worked out separately for the ground and surface water component range from 6.98 to 36.65 tons/km2/yr, respectively. The values of the CO2 consumption rates range between 0.18 × 106 mol//km2/yr (groundwater) and 0.9 × 106 mol/km2/yr (surface water), which indicates that the groundwater forms a considerable fraction of CO2 consumption, an inference, that is, not taken into contemplation in most of the studies.
Keywords: Weathering; Groundwater composition; Chemical denudation; CO2 consumption; Deccan Plateau; India
Global Radiation and Onset of Stratification as Forcing Factors of Seasonal Carbonate and Organic Matter Flux Dynamics in a Hypertrophic Hardwater Lake (Sacrower See, Northeastern Germany) by Philipp Bluszcz; Emiliya Kirilova; André F. Lotter; Christian Ohlendorf; Bernd Zolitschka (pp. 73-98).
A 2-year (October 2003–October 2005) high-resolution sediment trap study was conducted in Sacrower See, a dimictic hardwater lake in northeastern Germany. Geochemical and diatom data from sediment trap samples were compared with a broad range of limnological and meteorological parameters to quantify the impact of single parameters on biochemical calcite precipitation and organic matter production. Our goals were to disentangle how carbonaceous varves and their sublaminae form during the annual cycle to better understand the palaeorecords and to detect influences of dissolution, resuspension as well as of global radiation and stratification on lake internal particle formation. Total particle fluxes in both investigated years were highest during spring and summer. Sedimentation was dominated by autochthonous organic matter and biochemically precipitated calcite. Main calcite precipitation occurred between April and July and was preceded and followed by smaller flux peaks caused by resuspension during winter and blooms of the calcified green algae Phacotus lenticularis during summer. In some of the trap intervals during summer up to 100% of the precipitated calcite was dissolved in the hypolimnion. High primary production due to stable insolation conditions in epilimnic waters began with stratification of the water column. Start and development of stratification is closely related to air and water surface temperatures. It is assumed that global radiation influences the onset and stability of water column stratification and thereby determining the intensity of primary production and consequently of timing and amount of calcite precipitation which is triggered by phytoplanktonic CO2 consumption. Sediment fluxes of organic matter and calcite are also related to the winter NAO-Index. Therefore these fluxes will be used as a proxy for ongoing reconstruction of Holocene climate conditions.
Keywords: Lake sediments; Sediment traps; Sediment flux; Water column stratification; Global radiation; Calcite saturation; Phosphate; Phacotus ; Diatoms