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Aquatic Geochemistry (v.18, #4)

Sorption of Cesium on Clay Colloids: Kinetic and Thermodynamic Studies by R. Deepthi Rani; P. Sasidhar (pp. 281-296).

Sorption of radionuclides onto stable colloids can significantly enhance their transport in groundwater. Batch adsorption studies were performed to evaluate the influence of various experimental parameters like initial pH, contact time, temperature and concentration of Na⁺ and Ca²⁺ ions on the sorption of Cs on clay. The sorption process is dependent on pH of the solution with distribution coefficient (K _d) found to increase with increase in pH. The kinetic experiments were carried out at different temperatures, and the results have shown that the sorption process fits well into a pseudo-second-order mechanism with apparent activation energy of 45.7 kJ/mol. The rate constant was found to decrease with increase in temperature. The thermodynamic parameters such as ∆G ⁰, ∆H ⁰ and ∆S ⁰ were calculated. The negative value of ∆H ⁰ indicates that the reaction is exothermic. The negative values obtained for ∆G ⁰ indicated that the sorption of cesium on clay was spontaneous at all studied concentrations. The distribution coefficient was found to decrease with increasing concentration of Na⁺ and Ca²⁺ ions. The cesium sorption data were fitted to Freundlich, Langmuir, Temkin and Dubinin–Radushkevich (D–R) isotherms. The values of Langmuir separation factor (R _L) indicate a favorable Cs adsorption. The values of mean free energy of sorption (E) at various temperatures ranged from 10.5 to 11.1 kJ/mol, which indicates that the sorption process follows chemisorption.

Keywords: Cesium; Sorption; Exothermic; Clay colloids; Kinetics

Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury by Brad A. Bessinger; Dimitri Vlassopoulos; Susana Serrano; Peggy A. O’Day (pp. 297-326).

A 1-D biogeochemical reactive transport model with a full set of equilibrium and kinetic biogeochemical reactions was developed to simulate the fate and transport of arsenic and mercury in subaqueous sediment caps. Model simulations (50 years) were performed for freshwater and estuarine scenarios with an anaerobic porewater and either a diffusion-only or a diffusion plus 0.1-m/year upward advective flux through the cap. A biological habitat layer in the top 0.15 m of the cap was simulated with the addition of organic carbon. For arsenic, the generation of sulfate-reducing conditions limits the formation of iron oxide phases available for adsorption. As a result, subaqueous sediment caps may be relatively ineffective for mitigating contaminant arsenic migration when influent concentrations are high and sorption capacity is insufficient. For mercury, sulfate reduction promotes the precipitation of metacinnabar (HgS) below the habitat layer, and associated fluxes across the sediment–water interface are low. As such, cap thickness is a key design parameter that can be adjusted to control the depth below the sediment–water interface at which mercury sulfide precipitates. The highest dissolved methylmercury concentrations occur in the habitat layer in estuarine environments under conditions of advecting porewater, but the highest sediment concentrations are predicted to occur in freshwater environments due to sorption on sediment organic matter. Site-specific reactive transport simulations are a powerful tool for identifying the major controls on sediment- and porewater-contaminant arsenic and mercury concentrations that result from coupling between physical conditions and biologically mediated chemical reactions.

Keywords: Sediment cap; Remediation; Reactive transport; Biogeochemical kinetics

Phosphorus Mobilization at the Sediment–Water Interface in Softwater Shield Lakes: the Role of Organic Carbon and Metal Oxyhydroxides by Michel Lavoie; Jean-Christian Auclair (pp. 327-341).

The adsorption of phosphorus on natural diagenetic iron (Feox) and manganese (Mnox) oxyhydroxides was studied in deep and littoral zone sediments of mesotrophic Lac Saint-Charles (46°56 N, 71°23 W), using a Teflon sheet technique for collecting diagenetically produced metal oxyhydroxides. Collected metal oxide amounts were greater at the deep-water station, relative to littoral zone stations reflecting sediment and local diagenetic differences. Two-layer surface complexation modeling on iron oxyhydroxide was consistent with the measured total P/Fe molar ratios except for the upper mixed Mn–Fe oxide layer from the littoral stations, where measured phosphorus exceeded the modeled phosphorus by more than fivefold. Soluble reactive phosphorus (SRP) exchange between oxyhydroxide samples and natural lake water in the laboratory revealed a labile phosphorus pool. Phosphorus determined on the Teflon sheets from the littoral zone stations appears to be related to a distinct non-humic organic carbon pool that readily exchanges SRP, while little exchange was observed from material collected from the deep-water station. We suggest that the enhanced SRP release from littoral zone sediments is due to an organic carbon and/or metal oxide-impoverished sediment matrix, limiting microbial oxide reduction and allowing phosphorus to be rapidly recycled at the sediment–water interface, instead of being slowly incorporated into humic material. The SRP fluxes revealed in our study, which originate from the solid phase at the sediment–water interface, would be difficult to resolve using interstitial pore-water samplers and might be a quantitatively important source of inorganic phosphorus in Shield lakes.

Keywords: Iron oxyhydroxides; Phosphorus; Lake; Eutrophication; Organic carbon

The 273.15-K-Isothermal Evaporation Experiment of Lithium Brine from the Zhabei Salt Lake, Tibet, and its Geochemical Significance by Feng Gao; Mianping Zheng; Pengsheng Song; Lingzhong Bu; Yunsheng Wang (pp. 343-356).

The Zhabei Salt Lake is located in the hinterland of the northern Qinghai-Tibet Plateau, China. The salt lake is of the sodium sulfate subtype and rich in lithium. In order to exploit lithium in the brine of the salt lake, a 273.15 K-isothermal evaporation experiment of the brine from the Zhabei Salt Lake was carried out for the first time. Based on the 273.15 K-isothermal phase diagram of the quaternary system Na⁺, Mg²⁺//Cl⁻, SO₄ ²⁻–H₂O, the evaporation-crystallization path of the brine was drawn and the sequence of salt precipitation was confirmed. The experimental results show that due to the large amount of SO₄ ²⁻ precipitated as mirabilite, the concentration of SO₄ ²⁻ in the brine was stabilized at about 0.25 mol L⁻¹, thus avoiding formation of Li₂SO₄·3Na₂SO₄·12H₂O in the early stages of the low-temperature evaporation process. Lithium was effectively concentrated to 1.09 mol L⁻¹ in residual mother liquor as the evaporation rate reached 96.76 %, and the molar ratio of Mg/Li decreased from 1.25 to 0.23. The enrichment behavior of lithium is described as a power function of Y = k(1 − X)⁻¹, where Y = the concentration of lithium in the residual brine, X = the extent of evaporation, and k = the initial value of lithium concentration of the brine. In addition, the experiment suggests that the salts precipitated from the brine of the Zhabai Salt Lake contain not only the mineral assemblage with the characteristics of carbonate-type salt lakes, such as borax and lansfordite, but also the mineral assemblage with the characteristics of sulfate-type salt lakes, such as carnallite and lithium sulfate monohydrate. This finding, the data from the Zhabei Salt Lake’s brine in 1978, and the distribution characteristics of the hydrochemical zones of salt lakes on the Qinghai-Tibet Plateau, indicate that this salt lake is in a special stage of transition from a sulfate-type salt lake to a carbonate-type.

Keywords: Zhabei Salt Lake; Isothermal evaporation; Lithium enrichment; Hydrochemistry; Brine evolution

Multiple Factors driving Variability of CO₂ Exchange Between the Ocean and Atmosphere in a Tropical Coral Reef Environment by Rachel F. S. Massaro; Eric Heinen De Carlo; Patrick S. Drupp; Fred T. Mackenzie; Stacy Maenner Jones; Katie E. Shamberger; Christopher L. Sabine; Richard A. Feely (pp. 357-386).

In this paper, we present the results of the first automated continuous multi-year high temporal frequency study of CO₂ dynamics in a coastal coral reef ecosystem. The data cover 2.5 years of nearly continuous operation of the CRIMP-CO₂ buoy spanning particularly wet and dry seasons in southern Kaneohe Bay, a semi-enclosed tropical coral reef ecosystem in Hawaii. We interpret our observational results in the context of how rapidly changing physical and biogeochemical conditions affect the pCO₂ of surface waters and the magnitude and direction of air–sea exchange of CO₂. Local climatic forcing strongly affects the biogeochemistry, water column properties, and gas exchange between the ocean and atmosphere in Kaneohe Bay. Rainfall driven by trade winds and other localized storms generates pulses of nutrient-rich water, which exert a strong control on primary productivity and impact carbon cycling in the water column of the bay. The “La Niña” winter of 2005–2006 was one of the wettest winters in Hawaii in 30 years and contrasted sharply with preceding and subsequent drier winter seasons. In addition, short-term variability in physical forcing adds complexity and helps drive the response of the CO₂–carbonic acid system of the bay. Freshwater pulses to Kaneohe Bay provide nutrient subsidies to bay waters, relieving the normal nitrogen limitation of this system and driving phytoplankton productivity. Seawater pCO₂ responds to the blooms as well as to physical forcing mechanisms, leading to a relatively wide range of pCO₂ in seawater from about 250 to 650 μatm, depending on conditions. Large drawdowns in pCO₂ following storms occasionally cause bay waters to switch from being a source of CO₂ to the atmosphere to being a sink. Yet, during our study period, the southern sector of Kaneohe Bay remained a net source of CO₂ to the atmosphere on an annualized basis. The integrated net annual flux of CO₂ from the bay to the atmosphere varied between years by a factor of more than two and was lower during the wet “La Niña” year, than during the following year. Over the study period, the net annualized flux was 1.80 mol C m⁻² year⁻¹. Our CO₂ flux estimates are consistent with prior synoptic work in Kaneohe Bay and with estimates in other tropical coral reef ecosystems studied to date. The high degree of climatological, physical, and biogeochemical variability observed in this study suggests that automated high-frequency observations are needed to capture the short-, intermediate-, and long-term variability of CO₂ and other properties of these highly dynamic coastal coral reef ecosystems.

Keywords: Carbon dioxide; Calcification; Coral reef; Gas exchange; Primary productivity; Tropical

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Aquatic Geochemistry (v.18, #4)

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Aquatic Geochemistry (v.18, #4)