Geochemical Speciation Model Research Articles

The Use of Geochemical Speciation Modelling to Predict the Impact of Uranium to Freshwater Biota

Uranium is the prime potential contaminant in mine waste waters that may be released from the Ranger Uranium Mine (RUM) into the receiving waters of the Magela Creek, Alligator Rivers Region, Northern Australia. The potential ecological impact of the migration of uranium, that would result from an elevation in its concentration above background, in the Magela Creek downstream of the RUM, has been experimentally investigated by integrating biomonitoring with geochemical speciation modelling. The freshwater bivalve Velesunio angasi, abundant throughout the Magela Creek catchment, was exposed to a variety of uranium concentrations in a synthetic Magela Creek water, at four pH levels (5.0, 5.3, 5.5 and 6.0), in the presence (3.05 and 7.50 mg l{sup -1}) and absence of a model fulvic acid (FA), and its behavioural response was measured. Speciation modelling, using the HARPHRQ code, provided evidence that UO{sup 2+}{sub 2} and UO{sub 2}OH{sup +} are the uranium species most responsible (ca. 96%) for eliciting an adverse behavioural response when UO{sup 2+}{sub 2} is assigned twice the toxic effect of UO{sub 2}OH{sup +}. This finding rejects the notion that biota respond specifically to the sum total of inorganic uranyl species. (orig.)

Coprecipitation of Sodium, Magnesium, and Silicon with Calcium Carbonate

AbstractCalcite oversaturation in soil solutions is often attributed to overestimations of Ca2+ (aq) and CO2−3 (aq) activities, organic matter mineralization, and the presence of metastable forms of CaCO3. Earlier research, however, has indicated that these explanations are often insufficient to explain the levels of calcite oversaturation observed in soil suspensions. This study was conducted to evaluate the solubilities of precipitates formed during evapoconcentration of synthetic irrigation waters. Solids precipitated from waters of various Mg and Si concentrations were redissolved in deionized water under controlled CO2 conditions. Ion activities were calculated from the solution composition using the geochemical speciation model MINTEQA2. X‐ray diffraction (XRD) and infrared absorption spectroscopy provided evidence for precipitation of poorly crystalline Mg silicates and hydrous Na‐Mg silicates (sepiolite [Mg2Si3O7.5(OH)·3H2O], loughlinite [Na2Mg3Si6 (OH)16·8H2O], stevensite [Na0.33Mg2.87Si4O10(OH)2], or magadiite [Na Si7O13(OH)3·3H2O]). Calcite, aragonite, gypsum, and amorphous SiO2 were also identified. The solubility of the solid phase was proportional to the amount of Si and Mg in the solid, and dissolution of the solids in deionized water under atmospheric CO2 yielded an average CaCO3 ion activity product value of 10−8.0. The maximum mole fraction of Mg (XMg) in the calcite determined by XRD was <0.03, while XMg values calculated from mass balance were up to 0.53. The high solubility of the solid phases and the high Ca2+ (aq) and CO2−3 (aq) activities are attributed to poorly crystalline Mg silicates and Na silicates similar in composition to sepiolite, loughlinite, stevensite, and magadiite.

Trace-metal behaviour in oxic and anoxic CaCl brines of the Wright Valley drainage, Antarctica

Geochemical speciation and reaction path models have been used to identify the major factors controlling Mn, Fe, Cu, Pb, Zn, Ni and Co concentrations in meltwaters and CaCl brines of the closed-basin Wright Valley drainage system in Antarctica. The major aqueous features of this drainage system are: (a) the Onyx River, a meltwater stream; (b) Lake Vanda, an ice-covered stratified lake in which CaCl brines become more concentrated and ultimately anoxic with depth; and (c) Don Juan Pond, a small, highly saline CaCl brine pond. Trace-metal concentrations in the groundwater have been estimated and used to derive hypothetical concentrations for trace metals in the ancient brine pool from which Lake Vanda was formed. The results of this calculation support the premise that the trace-metal distribution profiles observed in Lake Vanda are attributable to the influx of trace metals from the Onyx River and to modern, ongoing lake processes. In the Onyx River and in the oxic CaCl brines of Lake Vanda and Don Juan Pond, trace-metal concentrations appear to be consistently controlled by adsorption and desorption from the surface of particulate Fe-oxides or Fe-hydroxides. Variations in the trace-metal concentrations measured in the oxic brines can be directly related to the effect of changes in pH and major-ion chemistry on the adsorption of each metal at the Fe-oxide surface. For the Onyx River and Lake Vanda, this hypothesis is presented as modification to the currently accepted model (Green et al., 1989) which postulates Cu and Ni adsorption onto particulate Mn-oxide. In the anoxic, H 2S-bearing CaCl brines at the base of Lake Vanda, Cu, Pb, Zn, Ni, Co and Fe concentrations appear to be limited by precipitation of metal-sulphide mineral phases and by the stability of metal-bisulphide and -polysulphide complex species. Mn is relatively mobile in the anoxic brine, but Mn concentrations may be limited by precipitation of a Mn-carbonate phase.

Soluble rare earth and actinide species in seawater

To test the value of geochemical speciation models, the solubilities of Nd(III), Th(IV), and U(VI) (as uranyl) in seawater have been calculated. The calculations do not include sorption processes on suspended particulate matter. The calculated values agree well with observed concentrations of Nd(III), underestimate the concentration of Th(IV) and overestimate that of uranyl. The possible implications of these observations are discussed.

An evaluation of chemical speciation in the MEXAMS metal transport model

The Environmental Protection Agency computer model MEXAMS (Metal Exposure Analysis Modeling System) was tested for realistic speciation of trace metals in aquatic systems. The model was compared to nine other geochemical speciation models using an order-of-magnitude comparison statistic. Comparisons were generally favorable with output from most models falling within 1 order of magnitude of MEXAMS output. Model performance was also evaluated. Documentation of model operation was found to be unclear in places and poorly indexed.

The Role of Groundwater in Delaying Lake Acidification

Models used to study lake acidification generally divide a watershed into several zones or compartments (e.g., soil and groundwater zones) through which water and chemicals are routed. To date most emphasis has been placed on the buffering of acid inputs by chemical reactions in the soil zone. Although buffering in the soil zone undoubtedly is important, the focus of this paper is the role of the groundwater system in delaying lake acidification. In this study, a deterministic approach was selected over the standard parametric modeling used in watershed models. Specifically, an advection‐dispersion model of the groundwater system was linked to a simple groundwater‐rain‐lake mixing model, which in turn was linked to a geochemical speciation and reaction path model. Simulations of hypothetical groundwater‐lake systems utilized chemical data from small lake systems in northern Wisconsin, an area which is currently experiencing acid rain (pH = 4.6) and is potentially sensitive to acid rain impacts owing to the absence of carbonate minerals in the underlying glacial tills and bedrock. Results of the modeling studies demonstrated that even when the effects of chemical reactions are ignored, groundwater inflow can mitigate the effects of acid deposition on lakes. When chemical reactions in the groundwater system were included in the model using a retardation factor, the acidification rate of the lake was significantly decreased. When speciation reactions between lake water and net acid additions were simulated using the geochemical model PHREEQE (Parkhurst et al., 1980), the depression of lake water pH was markedly reduced, indicating that low alkalinity lakes, like the ones in northern Wisconsin, may have appreciable buffering capacity.

Intracellular control of Cd concentrations in marine mussels

Cadmium transfer from M. edulis L. blood plasma to tissues was investigated in relation to its chemical speciation in the blood. 109Cd was injected into the posterior adductor muscle along with synthetic chelators oxine, ethylenediaminetetraacetic acid (EDTA) or cyclohexanediaminetetraacetic acid (CDTA). Chelator concentrations were chosen to bind up from 0 to 98.7% of the total blood Cd, based on speciation calculations using the geochemical speciation model MINTEQA2. Increases in synthetically chelated Cd were accompanied by linear decreases in the calculated percentages of Histidine-rich Glycoprotein (HRG)-bound Cd, and nonlinear decreases in Cd2+ and Cd chloro-complexes. Cadmium uptake by the kidneys decreased with increasing percentage synthetic chelation, while uptake by other tissues was not affected by chelation. Results indicate that there is at least one mechanism of Cd uptake common to all M. edulis tissues, and an additional, more rapid uptake mechanism in the kidneys that is mediated by CdHRG.

Copper speciation in soft, acid, humic waters: Effects on copper bioaccumulation by and toxicity to simocephalus serrulatus (Daphnidae)

The stoichiometry and affinity (conditional stability constants) of Cu chelation by naturally-occurring organic ligands were determined by numerical solutions of Scatchard relationships. These values were then used in the geochemical speciation model, GEOCHEM, to determine the relative importance of each Cu species in dynamic equilibrium with competing metals and ligands. The accumulation of Cu by and toxicity of Cu to the soft water cladoceran Simocephalus serrulatus (Daphnidae) were measured and related to Cu speciation. When 10 μg Cu · L −1 were added to pond water in the fall, the distribution of Cu was: Cu ++, 36.0%; Cuorganic, 63%; CuOH −, 0.5% and CuSO 4, 0.5%. The organics in pond water reduced Cu accumulation and toxicity. However, when the 24 h-LC 50 values were reported as Cu ++, this species of copper was more toxic in pond water than well water.