Total Analyte Concentration Research Articles

A compartmentalized solute transport model for redox zones in contaminated aquifers: 1. Theory and development

This paper, the first of two parts [see Abrams and Loague, this issue], takes the compartmentalized approach for the geochemical evolution of redox zones presented by Abrams et al. [1998] and embeds it within a solute transport framework. In this paper the compartmentalized approach is generalized to facilitate the description of its incorporation into a solute transport simulator. An equivalent formulation is developed which removes any discontinuities that may occur when switching compartments. Rate‐limited redox reactions are modeled with a modified Monod relationship that allows either the organic substrate or the electron acceptor to be the rate‐limiting reactant. Thermodynamic constraints are used to inhibit lower‐energy redox reactions from occurring under infeasible geochemical conditions without imposing equilibrium on the lower‐energy reactions. The procedure used allows any redox reaction to be simulated as being kinetically limited or thermodynamically limited, depending on local geochemical conditions. Empirical reaction inhibition methods are not needed. The sequential iteration approach (SIA), a technique which allows the number of solute transport equations to be reduced, is adopted to solve the coupled geochemical/solute transport problem. When the compartmentalized approach is embedded within the SIA, with the total analytical concentration of each component as the dependent variable in the transport equation, it is possible to reduce the number of transport equations even further than with the unmodified SIA. A one‐dimensional, coupled geochemical/solute transport simulation is presented in which redox zones evolve dynamically in time and space. The compartmentalized solute transport (COMPTRAN) model described in this paper enables the development of redox zones to be simulated under both kinetic and thermodynamic constraints. The modular design of COMPTRAN facilitates the use of many different, preexisting solute transport and geochemical codes. The companion paper [Abrams and Loague, this issue] presents examples of the application of COMPTRAN to field‐scale problems.

Applications in natural resource management

Facilitated by advances in analytical chemistry, soil, plant and water analysis now play a key role in the qualitative and quantitative characterisation of the environment. Ecological and health risk assessments can predict probabilities and risks of nominated environmental outcomes from the analytical data. The accuracy of these predictions, however, is dependent on data interpretability, which is constrained by our ability to determine that fraction of the contaminant that is active in biological systems. Soil analyses, for example, are used to characterise chemically contaminated sites and determine remediation thresholds. Such decisions, though, typically are based on total analyte concentrations that often are unrelated to environmental risk. While combined ecotoxicity and chemistry tests enable quantification of soil toxicity and identification of the analytes responsible, further progress may depend on development of sequential extraction and speciation tests calibrated against bioavailability/toxicity. Similarly, while we can measure most contaminants in plant tissues and understand how they affect plant metabolism, we remain unable to identify/quantify the physiologically active fraction. There is scope to improve interpretation by linking sub‐cellular chemical forms to toxicity. Substantial gains, too, have been made in water testing with the development of ambient water quality criteria related to organism‐level responses. We have much to learn, though, about effects of speciation, complexation, sediment/water column interactions, and ecosystem‐level responses. Solving such challenges should expand opportunities for environmental managers to use soil, plant and water analysis in the 21st Century.

Spectroscopic and Potentiometric Investigation of a Diprotic Acid: An Experimental Approach to Understanding Alpha Functions

A laboratory experiment suitable for second- or higher-year chemistry students uses thymol blue, an organic dye, for investigating the fraction of each colored species present as a function of pH. Visible color changes in the vivid, highly absorbing dye permit students to “see” relative amounts of acid, base, and ampholyte species during the titration, while potentiometric measurement of pH and spectroscopic determination of concentrations permit quantitation of the species present. A standard Beer’s law calibration curve is constructed for known concentrations of the pure forms of each species at a pH where that species is present as nearly 100% of the total analytical concentration of the dye. Using the molar absorptivities of the three species computed from the calibration curves, simultaneous determination of concentration by species is performed on a set of solutions having constant total concentration of dye such that each solution represents one point in the range pH 0 to pH 13. A mathematical software application is used to solve the simultaneous equations and determine the fraction of each species present. Students then use a spreadsheet application to plot the fractions as functions of pH (the alpha functions). The significance of the isosbestic points in the spectra is also explored.

Sensitivity Analysis of Uranium Solubility Under Strongly Oxidizing Conditions

AbstractTo evaluate the effect of geochemical conditions in the repository on the solubility of uranium under strongly oxidizing conditions, a mathematical model has been developed to determine the solubility, by utilizing a set of non linear algebraic equations to describe the chemical equilibria in the groundwater environment. The model takes into account the predominant precipitation-dissolution reactions, hydrolysis reactions and complexation reactions that may occur under strongly oxidizing conditions. The model also includes the solubilitylimiting solids induced by the presence of carbonate, phosphate, silicate, calcium, and sodium in the groundwater. The thermodynamic equilibrium constants used in the solubility calculations are essentially taken from the NEA Thermochemical Data Base of Uranium, with some modification and some uranium minerals added, such as soddyite, rutherfordite, uranophane, uranyl orthophosphate, and becquerelite. By applying this model, the sensitivities of uranium solubility to variations in the concentrations of various groundwater component species are systematically investigated. The results show that the total analytical concentrations of carbonate, phosphate, silicate, and calcium in deep groundwater play the most important role in determining the solubility of uranium under strongly oxidizing conditions.

ICP-MS for elemental speciation studies

The use of inductively coupled plasma mass spectrometry (ICP-MS) coupled with separation techniques for the purpose of elemental speciation has recently gained a lot of attention. Much of this is due to ever improving separation capabilities of Chromatographic techniques, the high sensitivity of ICP-MS, and the continuing development of better interface techniques. Additionally, there is a growing awareness of the need to monitor various species of an analyte, rather than just total analyte concentrations, due to their often varying natures. For the sake of learning from different elemental speciation approaches, this review brings together some selected types of elemental speciation which have been recently seen in literature. These include separations using various forms of liquid chromatography, such as reversed phase, reversed phase ion pairing, micelle, ion exchange, and size exclusion. Elemental speciation employing gas Chromatographie separations and supercritical fluid separations are discussed as well as elemental speciation using capillary electrophoresis.

Solid-phase microextraction for determining the binding state of organic pollutants in contaminated water rich in humic organic matter

Solid-phase microextraction (SPME) in both conventional and headspace mode was used for determining the freely available and reversibly bound fraction of phenols and polycyclic aromatic hydrocarbons in contaminated water rich in dissolved polymeric organic matter (DOM). The SPME results obtained by using internal calibration with deuterated surrogates gave the total analyte concentration based on the identical partitioning behaviour of both, native pollutants and deuterated spikes, and agreed well with data from exhaustive LLE. Data obtained with direct and headspace SPME are very similar. Investigated matrices include both contaminated water and artificial DOM solutions. The DOM compounds were isolated from different aquatic and terrestrial sources and cover a wide range of hydrophobicity. A new approach based on progressive dilution of samples rich in DOM was developed for determining both the freely-dissolved fraction of an unknown analyte and its partition coefficients. Theoretical considerations provide strong evidence that the commonly used term `concentration' in SPME analysis should be replaced by the more appropriate term `activity' of the solute.

Repeatability and Reproducibility Estimates from Collaborative Studies Based on Total Concentration of Trace Analytes

In the process of validating a given analytical method for the total concentration of a trace analyte, the precision indicators, repeatability and reproducibility, are obtained from a collaborative study of the method based on a standard one-way completely randomized model. This report discusses the shortcomings of the statistical models used in such studies, defines the component makeup for estimates of the repeatability and reproducibility variances based on these models, and considers suggestions offered as new policy regarding method performance based on total concentration.

Extraction of85Sr by the nitrobenzene solution of bis-1,2-dicarbollylcobaltate in the presence of dibenzo-18-crown-6

Extraction of strontium by the nitrobenzene solution of bis-1,2-dicarbollylcobaltate in the presence of dibenzo-18-crown-6 (DB18C6, L) has been investigated. the equilibrium data and the typical maxima concerning the dependencies of the Sr distribution ratios on the total analytical concentration of DB18C6 in the system under study can be explained assuming that the particles Sr2+, SrL2+, SrL22+, SrHL3+ and SrHL23+ are extracted into the nitrobenzene phase. The values of the corresponding extraction and stability constants of the extracted species in nitrobenzene saturated with water have been determined.

On simulating reactive chemical transport with dominating precipitated species controlling chemical equilibrium

We present an approach developed to compute chemical equilibrium and its corresponding reactive chemical transport when dominating precipitated species (DPS) exist. In computing chemical equilibrium, most models take the concentrations or activities of component species and precipitated species as the master variables. However, when the amount of a precipitated species is much larger than those of other species, small computational errors on this DPS concentration might introduce large errors on the concentrations of other species and would cause non‐mass‐conserved numerical results. To deal with the existence of DPS, we pick as master variables the concentration change, rather than the concentration, of DPS to compute chemical equilibrium. Since the concentration changes of DPS will no longer be much larger than the concentrations of other species in determining equilibrium, our approach is able to provide correct numerical results. We also employ the modified total analytical concentrations, rather than the total analytical concentrations, of aqueous components as the dependent variables in presenting and solving corresponding transport equations. Several examples are given to reveal the numerical problems associated with DPS and to demonstrate the success of our approach.

Analytical Techniques for the in situ Measurement and Speciation of Trace Compounds in Natural Waters

A major research component of the analytical activities of the Analytical and Biophysical Environmental Chemistry group of the University of Geneva (CABE) is focused on the development of chemical sensors and mini or microanalytical systems for in situ measurements of trace compounds in aquatic environmental systems, including surface waters, sediments or water treatment plants. In this field, new concepts are required in order to determine not only the total concentration of environmental analytes but also the concentrations and physicochemical properties of their environmentally relevant chemical forms (chemical speciation). New selective analytical systems integrating reliable and rugged sensors with simple separation principles must be imagined to perform in situ (at depth), real-time, automatic measurements. Microtechnology is a key factor in such developments. New analytical methods must also be developed to characterize the nature and properties of the major natural, often colloidal or polymeric, complexing agents. In this context, the scientific approach of CABE is explained.

Modeling Multicomponent Reactive Chemical Transport in Nonisothermal Unsaturated/Saturated Soils - Part 2. Numerical Simulations

The set of partial differential equations developed for the simultaneous transfer of moisture, heat, and reactive chemicals under saturated/unsaturated soil conditions (Wu and Chieng, 1995) has been solved by the Galerkin finite element method. The nonlinear algebraic equations of the chemical reactions are solved by the iterative Newton-Raphson method. The chemical transport equations are formulated in terms of the total analytical concentration of each component species, and can be solved sequentially. Three methods of solving the resulting systems of linear equations are implementedthe banded matrix method for one-dimensional and small two-dimensional problems, the iterative method and the sparse matrix method for large two-dimensional and three-dimensional problems. Two efficient and reliable iterative methods are providedthe conjugate gradient method for symmetric matrix equations and the biconjugate gradient squared method for general nonsymmetric matrix equations. A typical subsurface drainage problem was solved using the developed computer program. Two simulations were run to investigate the behavior of the program. Both simulations produced reasonable and satisfactory results.

A critical evaluation of recent developments in hydrogeochemical transport models of reactive multichemical components

Many hydrogeochemical models have appeared in recent years for simulating subsurface solute transport. The hydrological transport of solutes can be described by a set of linear partial differential equations, and the chemical equilibria are described by a set of nonlinear algebraic equations. Three approaches are currently used to formulate the problem: (1) the mixed differential and algebraic equation (DAE) approach, (2) the direct substitution approach (DSA), and (3) the sequential iteration approach (SIA). An extremely important consideration in any approach is the choice of primary dependent variables (PDVs). Six types of PDVs have been employed in the existing models: (1) concentrations of all species, (2) concentrations of all component species and precipitated species, (3) total analytical concentrations of aqueous components, (4) total dissolved concentrations of aqueous components, (5) concentrations of aqueous component species, and (6) hybrid concentrations. Because of many possible combinations of PDVs and approaches, many hydrogeochemical transport models for multicomponent systems have been developed. This paper critically evaluates and discusses these models. The discussion and evaluation are conducted in terms of (1) how severe can the constraints be that a model imposes on computer resources, (2) which equilibrium geochemical processes can a model include, and (3) how easily can a model be modified to deal with mixed kinetic and equilibrium reactions. The use of SIA models leads to the fewest constraints on computer resources in terms of central processing unit (CPU) memory and CPU time; both DAE and DSA models require excessive CPU memory and CPU time for realistic two‐ and three‐dimensional problems. Only those models that use the first three types of PDVs can treat the full complement of equilibrium reactions simultaneously. DAE and SIA models can be modified with reasonable ease to handle mixed chemical kinetics and equilibria. DSA models require strenuous efforts to modify for treating mixed chemical kinetics and equilibria. Therefore SIA models using the third type of PDVs are recommended for their practicality and flexibility. DSA and DAE models should remain research tools for one‐dimensional investigations.

Europium and cerium extraction by the nitrobenzene solution of dicarbolide in the presence of polyethylene glycols

Extraction of Eu3+ and Ce3+ microamounts from 0.1-0.4M perchloric acid by the nitrobenzene solution of dicarbolide H+[Co(C2B9H11)2]- in the presence of polyethylene glycols (Mr = 200, 300, 400) has been studied. The equilibrium data and the typical maxima on the dependence of the metal distribution ratio on the total analytical concentration of polyethylene glycol in the system can be explained assuming that the species ML3+org, ML3+2org, ML3+3org, MLH2+-1org, and HL+org (where M3+ = Eu3+, Ce3+; L = polyethylene glycol) are extracted into the organic phase. The values of extraction and equilibrium constants in the organic phase were determined and the effect of the polyethylene glycol molecular weight on the equilibrium constants and on the abundances of individual species in the organic phase is discussed. It has been found that the addition of polyethylene glycol to the acid - nitrobezene - dicarbolide system increases the values of the separation factors αCe/Eu.

Chemical composition of aqueous extracts of fly ash: Ionic speciation as a controlling factor

Abstract Concentrations of Na, K, Ca, Mg, SO4, and Si in eight successive extracts of a fly ash sample were determined. Calculation of elemental speciation in solution phase using the chemical equilibrium computer program: GEOCHEM indicated that Na and K were mainly in the free ionic forms whereas, concentrations of Ca2+ and Mg2+ constituted 58–92% of the total analytical concentrations. Concentrations of Ca and Mg ion pairs with OH‐ and SO2‐ 4 ions ranged from 8–42% of total concentrations. Computed saturation indices suggested that the precipitation‐dissolution reactions of ettringite, brucite, sepiolite, laumontite, and phillipsite may be controlling various ion activities in the extracts.

Necessary and sufficient conditions for the appearance of extrema on concentration distribution curves in complex equilibrium systems

It is shown that species distribution curves with at least one minimum may be found in most equilibrium systems with three or more components. Whether such concentration minima are actually observed then depends on the values of the equilibrium constants and on the total (analytical) concentrations of the components. A general algorithm is given for the necessary and sufficient conditions for the appearance of extrema in multicomponent systems. Three-component systems are studied in more detail and special attention is given to the limiting case of a horizontal inflection, i.e., the point where the concentration minimum just disappears. Two well-studied chemical examples, the Cu 2+-diethylenetriamine-OH − and Hg 2+-Cl −-OH − systems are discussed, along with a simple model system showing as many as five extrema on a single distribution curve.

Kinetics of the molybdate catalyzed oxidation of iodide by hydrogen peroxide

AbstractThe kinetics of the oxidation of iodide by hydrogen peroxide catalyzed by acidic molybdate have been studied by a spectrophotometric stopped‐flow method. The results are interpreted in terms of the mechanism equation image and the implied rate law where [mol] is total analytical concentration of molybdate. The values obtained for the rate and equilibrium constants are k4 = (3.3 ± 1) × 102 1./mole · s, K1 = (1.2 ± 0.6) × 104 1./mole, K2 = (1.3 ± 0.7) × 103 1./mole, and K3 = (4 ± 3) × 102 1./mole at 298°K.

Calculation of equilibrium distributions of chemical species in aqueous solutions by means of monotone sequences

A numerical method for the calculation of equilibrium distributions of chemical species in aqueous solutions of electrolytes is presented. This method is constructed by transforming the problem of determining the set of unknown concentrations satisfying the mass balance and mass action equations into the equivalent problem of finding the limits of certain well-behaved mathematical sequences in a multivariable direct iteration scheme. The total (analytical) concentrations are taken as the starting estimates, and the recursive equations are constructed from the starting equations. It is shown that the sequences so constructed are both monotonic and bounded, hence convergent in an orderly fashion by a mathematical axiom. Each unknown free ion concentration (that is, each limit) is approached simultaneously from above and from below, being effectively “sandwiched” in an ever tightening manner. Strict error bounds therefore are easily constructed. The method has been found to be exceedingly efficient in practice, with the error bound (maximum fractional error in taking the upper estimate as the final answer) decreasing in an approximately exponential fashion with respect to iteration number, commonly 0.5–1.5 orders of magnitude per iteration. Correction for nonideality is presented, and the possibility of this giving rise to more than one solution is discussed in connection with three examples of natural waters of low-, medium-, and high-ionic strengths.

Kinetics of oxidation of thiocyanate by aqueous iodine

The kinetics of oxidation of thiocyanate to sulphate by aqueous iodine in the pH range 9.2-12.5 have been studied using a spectrophotometric stopped flow technique. The reaction is general base-catalysed, having the rate law ��������������������� -d[I2]a/dt = ([SCN-][I3-]/[I-]2)Σ kB[B] where [I2]a is the total analytical concentration of iodine, [B] is the concentration of base, and where the summation is taken over all bases present. Rate constants, kB, and activation energies have been measured for the bases, OH-, PO43- and CO32-. ��� A mechanism involving the initial steps ����������������� I2+SCN- ↔ ISCN+I- �����������������(rapid equilibrium) ������������� ISCN+H2O+B → HOSCN+I- + HB+ �����������(rate determining) followed by rapid reactions of HOSCN with itself or with iodine is proposed.

Potential-sweep chronoamperometry theory of kinetic currents in the case of a first order chemical reaction preceding the electron-transfer process

A theory is presented for the calculation of the current/time curves at linearly varying potential, for the following reaction scheme: ▪Theoretical curves have been calculated for different values of the equilibrium constant K = kf/kb and of the parameterλ=kf + kbv .RTnF, which is a function of the sum of the chemical rate constants (kf + kb) and of the potential sweep rate (v).The conditions for the appearance of kinetic currents have been deduced, as well as the conditions for the existence of diffusion currents corresponding to the total analytical concentration of the depolarizer, [I] + [A], or to the actual concentration of the species A, at the equilibrium.The possibility of measuring the equilibrium constant as well as the rate constants, from the current/time curves, is discussed.