Chemical evolution of the Mt. Hekla,Iceland, groundwaters: A natural analogue for CO2 sequestration in basaltic rocks

A detailed study of the chemical composition of the groundwater surrounding the Mt. Hekla volcano in south Iceland was performed to assess fluid evolution and toxic metal mobility during CO₂-rich fluid basalt interaction. These fluids provide a natural analogue for evaluating the consequences of CO₂ sequestration in basalt. The concentration of dissolved inorganic C in these groundwaters decreases from 3.88 to 0.746 mmol/kg with increasing basalt dissolution while the pH increases from 6.9 to 9.2. This observation provides direct evidence of the potential for basalt dissolution to sequester CO₂. Reaction path calculations suggest that dolomite and calcite precipitation is largely responsible for this drop in groundwater dissolved C concentration. The concentrations of toxic metal(loid)s in the waters are low, for example the maximum measured concentrations of Cd, As and Pb were 0.09, 22.8 and 0.06 nmol/kg, respectively. Reaction path modelling indicates that although many toxic metals may be initially liberated by the dissolution of basalt by acidic CO₂-rich solutions, these metals are reincorporated into solid phases as the groundwaters are neutralized by continued basalt dissolution. The identity of the secondary toxic metal bearing phases depends on the metal. For example, calculations suggest that Sr and Ba are incorporated into carbonates, while Pb, Zn and Cd are incorporated into Fe (oxy)hydroxide phases.     

Fluorapatite surface composition in aqueous solution deduced from potentiometric, electrokinetic, and solubility measurements, and spectroscopic observations

The surface chemistry of fluorapatite in aqueous solution was investigated using electrokinetic techniques, potentiometric titrations, solubility measurements, and attenuated total reflection infrared spectroscopy. All methods indicate the formation of Ca/F depleted, P enriched altered layer via exchange reactions between H⁺ and Ca²⁺, and OH⁻ and F⁻ at the fluorapatite (FAP) surface. Observations suggest that this leached layer has a di-calcium phosphate (CaHPO₄) composition and that it controls the apparent solubility of FAP. Electrokinetic measurements yield an iso-electric point value of 1 ± 0.5 consistent with a negatively charged FAP surface at pH > 1. In contrast, surface titrations give an apparent pH of point of zero charge of ∼7.7, consistent with a positively charged surface at pH < 7.7. These differences are shown to stem from proton consumption by both proton exchange and dissolution reactions at the FAP surface. After taking account for these effects, FAP surface charge is shown to be negative to at least pH 4 by surface titration analysis.     

Dissolution of basalts and peridotite in seawater, in the presence of ligands, and CO2: Implications for mineral sequestration of carbon dioxide

Steady-state silica release rates (r_Si) from basaltic glass and crystalline basalt of similar chemical composition as well as dunitic peridotite have been determined in far-from-equilibrium dissolution experiments at 25 °C and pH 3.6 in (a) artificial seawater solutions under 4 bar pCO₂, (b) varying ionic strength solutions, including acidified natural seawater, (c) acidified natural seawater of varying fluoride concentrations, and (d) acidified natural seawater of varying dissolved organic carbon concentrations. Glassy and crystalline basalts exhibit similar r_Si in solutions of varying ionic strength and cation concentrations. Rates of all solids are found to increase by 0.3-0.5 log units in the presence of a pCO₂ of 4 bar compared to CO₂ pressure of the atmosphere. At atmospheric CO₂ pressure, basaltic glass dissolution rates were most increased by the addition of fluoride to solution whereas crystalline basalt rates were most enhanced by the addition of organic ligands. In contrast, peridotite does not display any significant ligand-promoting effect, either in the presence of fluoride or organic acids. Most significantly, Si release rates from the basalts are found to be not more than 0.6 log units slower than corresponding rates of the peridotite at all conditions considered in this study. This difference becomes negligible in seawater suggesting that for the purposes of in-situ mineral sequestration, CO₂-charged seawater injected into basalt might be nearly as efficient as injection into peridotite.     

An experimental study of crystalline basalt dissolution from 2 ? pH ? 11 and temperatures from 5 to 75 °C

Steady-state element release rates from crystalline basalt dissolution at far-from-equilibrium were measured at pH from 2 to 11 and temperatures from 5 to 75 °C in mixed-flow reactors. Steady-state Si and Ca release rates exhibit a U-shaped variation with pH where rates decrease with increasing pH at acid condition but increase with increasing pH at alkaline conditions. Silicon release rates from crystalline basalt are comparable to Si release rates from basaltic glass of the same chemical composition at low pH and temperatures ?25 °C but slower at alkaline pH and temperatures ?50 °C. In contrast, Mg and Fe release rates decrease continuously with increasing pH at all temperatures. This behaviour is interpreted to stem from the contrasting dissolution behaviours of the three major minerals comprising the basalt: plagioclase, pyroxene, and olivine. Calcium is primarily present in plagioclase, which exhibits a U-shaped dissolution rate dependence on pH. In contrast, Mg and Fe are contained in pyroxene and olivine, minerals whose dissolution rates decrease monotonically with pH. As a result, crystalline basalt preferentially releases Mg and Fe relative to Ca at acidic conditions. The injection of acidic CO₂-charged fluids into crystalline basaltic terrain may, therefore, favour the formation of Mg and Fe carbonates rather than calcite. Element release rates estimated from the sum of the volume fraction normalized dissolution rates of plagioclase, pyroxene, and olivine are within one order of magnitude of those measured in this study.     

Is silt the most influential soil grain size fraction?

The contribution of individual grain size fractions (2000–500, 500–250, 250–63, 63–2 and < 2 μm) to bulk soil surface area and reactivity is discussed with reference to mineralogical and oxalate and dithionite extractions data. The 63–2 μm fraction contributed up to 56% and 67% of bulk soil volume and BET surface area, respectively. Consideration of these observations and the mineralogy of this fraction suggest that the 63–2 μm fraction may be the most influential for the release of elements via mineral dissolution in the bulk soil.     

Geochemistry and Behavior of Trace Elements During the Complete Evaporation of the Merouane Chott Ephemeral Lake: Southeast Algeria

The Merouane Chott, located in arid southeastern Algeria, experiences annual cycles of filling from September through February followed by its complete evaporation from February through June. The concentration of 15 trace elements (Li, B, Ti; V, Cr, Mn, Co, Cu, Ni, Zn, As, Sr, Ba, Pb, Bi, and U) were measured in chott water samples collected from January through June 2003 during the complete evaporation of the lake. The corresponding concentrations of these trace elements in the major external inputs to this closed basin chott were also obtained. The trace metals show two distinct behaviors. Li, B, Cr, Co, and U tend to be conserved in the chott waters throughout its evaporation. Much of Cr, Co, and U originated from external sources. It is likely, therefore, that the concentration of these elements will increase in the chott waters in future years. In contrast, Ti, Sr, Ba, Zn, Ni, and Pb precipitate continuously during chott evaporation. Of these elements, most of the Sr, Ba, and Zn originated from outside the chott, and thus it is likely these elements will become increasingly concentrated in the chott bottom salts with time. V, As, and Cu exhibit intermediate behaviors. These contrasting behaviors are confirmed by analysis of chott bottom solids.     

再论各向异性介质转换波地震学,第一部分:理论

我们业已研发了计算各向异性、非均质介质中P- SV转换波(C-波)的转换点和旅行时的新理论。据此 可以利用诸如相似性分析、迪克斯模型建模、克契 霍夫求和等常规方法来完成各向异性的处理和各向 异性处理,并使各向异性的处理成为可能。这里将 我们的新发展分作两部分来介绍。第一部分为理 论,第二部分为对速度分析和参数计算的应用。第 一部分理论包括转换点的计算和动校正的分析。     

An experimental study of lake water-sediment interaction rates

Dissolution rates of sediments obtained from the Oued Cherf reservoir were measured in closed-system batch reactors at 25 °C in fluids sampled concurrently from the same locations as the sediments. The BET surface areas of the sediments ranged from 16 to 45 m²/g and consisted primarily of quartz, calcite, and clay minerals. After a brief initial period, release rates of Si, Mg, Ca, Cl, SO₄, and NO₃ from these sediments are approximately linear with time over the course of the experiments, which lasted from 3 to 5 months. BET surface area normalized Si release rates ranged from 10^–17.4 to 10^–18.4 mol/cm²/s. These release rates match closely Si release rates from quartz and clay minerals determined from laboratory dissolution rates reported in the literature. This coherence suggests that laboratory measured silicate dissolution rates can be used with confidence to predict the dissolution behavior of sediments in natural surface waters.     

Magnesite growth rates as a function of temperature and saturation state

Magnesite growth rates and step velocities have been measured systematically as a function of temperature from 80 to 105 °C and saturation state in 0.1 M NaCl solutions using hydrothermal atomic force microscopy (HAFM). The observations indicate that at these conditions magnesite precipitation is dominated by the coupling of step generation via spiral growth at screw dislocations and step advancement away from these dislocations. As these two processes occur in series the slowest of these dominates precipitation rates. At 100 °C magnesite growth rates (r) determined by HAFM are consistent with

r=k(Ω-1)²,