Solute Concentration Measurements Research Articles

Estimating spatially-variable first-order rate constants in groundwater reactive transport systems

Numerical reactive transport models are often used as tools to assess aquifers contaminated with reactive groundwater solutes as well as investigating mitigation scenarios. The ability to accurately simulate the fate and transport of solutes, however, is often impeded by a lack of information regarding the parameters that define chemical reactions. In this study, we employ a steady-state Ensemble Kalman Filter (EnKF), a data assimilation algorithm, to provide improved estimates of a spatially-variable first-order rate constant λ through assimilation of solute concentration measurement data into reactive transport simulation results. The methodology is applied in a steady-state, synthetic aquifer system in which a contaminant is leached to the saturated zone and undergoes first-order decay. Multiple sources of uncertainty are investigated, including hydraulic conductivity of the aquifer and the statistical parameters that define the spatial structure of the parameter field. For the latter scenario, an iterative method is employed to identify the statistical mean of λ of the reference system. Results from all simulations show that the filter scheme is successful in conditioning the λ ensemble to the reference λ field. Sensitivity analyses demonstrate that the estimation of the λ values is dependent on the number of concentration measurements assimilated, the locations from which the measurement data are collected, the error assigned to the measurement values, and the correlation length of the λ fields.

Rapid online calibration for ATR-FTIR spectroscopy during batch crystallization of ammonium sulphate in a semi-industrial scale crystallizer

The knowledge of solute concentration throughout a batch crystallization process is essential from process control perspective. Despite the progress in process analytical technology (PAT), there still exist several challenges for online measurement of solute concentration at industrial scale. In this study, concentration monitoring was realized using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy at lab as well as semi-industrial scale. Applications of the calibration model developed at lab scale for the measurements at the semi-industrial scale however resulted into strongly biased concentration predictions, caused by the differences in the curvature of fiber optics and the uneven thermal expansion of the probe. Therefore an alternative rapid online calibration method was developed during the start-up phase of the process. With this method, the time required for developing a working calibration model for concentration monitoring during crystallization of ammonium sulphate in a semi-industrial scale draft tube crystallizer has been reduced approximately by 90%. With the help of simultaneous concentration and crystal size distribution measurements at semi-industrial scale, the descriptive capability of the model was improved due to better kinetic parameters.

Semiautomated Identification of the Phase Diagram for Enantiotropic Crystallizations using ATR-FTIR Spectroscopy and Laser Backscattering

A semiautomated procedure for measuring the phase diagram for enantiotropes in a dimorphic system was developed using Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy and laser backscattering (Focused Beam Reflectance Measurement, FBRM) for in situ measurement of solute concentration and the particle counts, respectively. The approach is demonstrated using l-phenylalanine, an enantiotropic pseudodimorph. The procedure involves the determination of the anhydrate-form solubility from in situ infrared spectroscopy of an equilibrated slurry, followed by the dissolution of the anhydrate form by heating. The monohydrate form is then recrystallized, and its solubility determined from slow heating until complete dissolution is detected by FBRM. The cycle is repeated for higher temperatures after addition of anhydrate crystals to create a slurry. The solubility of the monohydrate form was determined differently from the anhydrate form due to the interference on the infrared measurements from small needle-like monohydrate crystals. This single-experiment approach is expected to be applicable to other enantiotropic dimorph systems for the measurement of the phase diagram in a more efficient manner.

Change in macroscopic concentration at the interface between different materials: Continuous or discontinuous

There have been conjectures that a spatial average could result in a volume‐average concentration which is no longer continuous at sharp interfaces between different materials. However, convincing experimental evidence showing the existence of such a discontinuity is not available because of the difficulty associated with measuring solute concentration in the void space within a porous medium. In this paper we used pore‐scale simulations to explore the change in macroscopic concentration when solute moves from one material into another. Water flow through the void space was assumed to be laminar, and solute transport consisted of molecular diffusion and advection; both were simulated using the lattice Boltzmann equation methods. To accurately represent the fluid‐solid interface, the multiple‐relaxation‐time lattice Boltzmann equation method was used to simulate fluid flow. We first simulated solute transport in a 3D column with one half packed with fine glass beads and the other half with coarse glass beads. The simulated solute concentration and solute flux at pore scale were then spatially averaged to produce volume‐average and flux‐average concentration profiles, respectively, in attempts to understand if solute accumulates at the media interface when moving from one medium into another. The results revealed that, when solute migrated from the coarse medium into the fine medium, it did accumulate at the media interface; we also found mass accumulation at the reservoir‐column interface. Such accumulations made solute take more time to break through the column when flowing from the coarse medium to the fine medium than from the fine medium to the coarse medium. We also simulated solute movement in an idealized 2D column packed with different rectangular solids and with high porosity; the results indicated that, although the dispersive properties of the two media differed considerably, there was no mass accumulation and the macroscopic concentration was found to be continuous at the media interface. These simulated results suggest that a sharp change in material properties with moderate porosity will likely lead to a mass accumulation, but knowing the transport properties of the two materials alone is not sufficient to determine if a mass accumulation could develop. What causes mass accumulations appears to be some microstructures in the vicinity of the interface, which cannot be accounted for by the macroscopic transport parameters of each of the two media.

Discrepancies in Thorium Oxide Solubility Values: Study of Attachment/Detachment Processes at the Solid/Solution Interface

The solubility of thorium under oxide and/or hydroxide forms has been extensively studied for many years. Nevertheless, a large discrepancy in the solubility values is noticed in the literature. We study Th atom exchange between thorium oxide surfaces and various aqueous solutions (0.01 mol·L(-1) NaCl for 0.0 < pH < 5.2) to address this issue. By solid-state characterization [X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, and atomic force microscopy], we determined that 80% of the XPS accessible near the surface region of sintered thorium oxide is represented by the less reactive ThO(2)(cr) grains. The remaining 20% corresponds to ThO(x)(OH)(y)(H(2)O)(z), which is largely associated with grain boundaries. Only the latter fraction is involved in solid/solution exchange mechanisms. Local conditions (thorium concentrations, pH values, etc.) in grain boundaries lead to an adjustment of the "local solubility constraints" and explain the thorium concentration measured in our experiments. For pH <5.2, the thorium concentration and pH gradient between the bulk solution and grain-boundary regions imply that the solubility values mainly depend on the availability and accessibility of ThO(x)(OH)(y)(H(2)O)(z). We have performed two solubility experiments with a (232)ThO(2)(cr) solid in a 0.01 mol·L(-1) NaCl solution for 300 days. In a first experiment, we measured (232)Th concentrations in dissolution experiments in order to determine the detachment rates of Th atoms from the solid surface. In a subsequent step, we added (229)Th to the solution in order to measure the surface attachment rate for dissolved Th atoms. This allowed an assessment of the net balance of Th atom exchange at the solid/solution interface. The empirical solubility data do not correspond to the thermodynamic bulk phase/solution equilibrium because measured solution concentrations are controlled by site-specific exchange mechanisms at the solid/solution interface. Therefore, for sparingly soluble solids, one needs to quantify site-specific surface attachment and detachment rates if one wants to assess solubility constraints.

A Comparative Study of ATR-FTIR and FT-NIR Spectroscopy for In-Situ Concentration Monitoring during Batch Cooling Crystallization Processes

Crystallization is a widely used separation and purification process in the chemical industry. The final product properties such as the crystal size distribution and the morphology are largely defined during this process. In order to achieve proper control over the crystallization process, accurate in situ measurements are necessary. This paper presents the assessment of in situ infrared spectroscopic techniques for solute concentration measurements. It presents a comparative study of attenuated total reflectance (ATR), Fourier transform infrared (FTIR), and Fourier transform (FT) near infrared (NIR) spectroscopy for batch cooling crystallization of four different crystallization systems. These two techniques were compared based on the partial least square models developed by extensive calibration in the laboratory, the independent data sets generated while determining saturation concentrations in the presence of crystals and monitoring the batch cooling crystallization processes. The ATR-FTIR spectroscop...

Non-contact method for measuring solution concentration using surface plasmon resonance apparatus and heterodyne interferometry

A simple non-contact method is proposed for measuring the concentration of solutions. Using the significant phase difference between p- and s-polarizations of the reflected light of a surface plasmon resonance apparatus, the variation in the phase difference, which is caused by a variation in the concentration of a test solution, can be accurately measured by common path heterodyne interferometry. Then, by substituting the corresponding variation in the incident angle of light at the base of the SPR prism in a specially derived equation, the concentration of the test solution can be determined. The validity of this method was demonstrated experimentally. This method is characterized by the advantages of the device having a simple non-contact structure; it being easy to operate; and its high accuracy, stability, and resolution.

Understanding and Predicting the Effect of Cocrystal Components and pH on Cocrystal Solubility

Understanding how cocrystal solubility−pH dependence is affected by cocrystal components is important to engineer cocrystals with customized solubility behavior. Equations that describe cocrystal solubility in terms of solubility product, cocrystal component ionization constants, and solution pH are derived for cocrystals with acidic, basic, amphoteric, and zwitterionic components. Studies with carbamazepine-salicylic acid and carbamazepine-4-aminobenzoic acid show that cocrystals of a nonionizable drug achieve pH-dependent solubility when cocrystallized with ionizable coformers. These findings are in good agreement with predicted behavior and provide insight on the ability of coformer to determine the shape of the pH−solubility curve. It is shown that measurement of solution concentrations and pH at the eutectic point, Ctr, is valuable to (a) estimate cocrystal solubility−pH dependence, (b) evaluate the effectiveness of coformer in stabilizing or precipitating cocrystal, and (c) guide cocrystal selection without the time and material consuming determination of full phase solubility diagrams.

Evidence of infaunal effects on porewater advection and biogeochemistry in permeable sediments: A proposed infaunal functional group framework

Bioturbating infauna significantly modify reaction and transport processes in permeable sediments, though most studies to date are limited in the scope of species examined. We conducted a comparative field study measuring density-dependent effects of six common bioturbating species on porewater advection and biogeochemistry, across three intertidal permeable sediment habitats. The species in this study are; head-down like deposit feeders (Abarenicola pacifica and Balanoglossus aurantiacus), surface deposit feeders (Diopatra cuprea and Onuphis jenneri) and gallery diffusers (Upogebia pugettensis and Neotrypaea californiensis). Tracer loss from gel diffusers was used to assess relative differences in porewater advection among sites, and porewater peepers were used to measure solute concentrations of carbon, nitrogen, phosphate, and silicate in experimental plots. Characteristic surface features of different infauna were counted and used as a proxy for infaunal density. Density of surface features was then used in regression analyses as an explanatory variable affecting porewater transport and chemistry. Significant infaunal density effects on porewater transport or biogeochemistry were found in all but one species, D. cuprea. The species-specific attributes and mechanisms by which these infauna affect permeable sediment processes are explored. A process based functional group framework is presented for permeable sediments. Bulk granulometric properties also were assessed. There were little to no within-site effects of porosity, hydraulic conductivity, or organic matter on porewater transport and biogeochemistry. However, significant across-site differences in granulometry and site properties were found and these are addressed in relation to infaunal effects on porewater transport and chemistry.

Selective Crystallization of the Metastable α-Form of l-Glutamic Acid using Concentration Feedback Control

A systematic methodology is presented for the selective crystallization of the metastable form of a monotropic dimorph, l-glutamic acid, for batch cooling crystallization. Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy coupled with chemometrics was used to determine the solute concentration and solubility curves of both α- and β-forms of l-glutamic acid in aqueous solution. The metastable limit associated with secondary nucleation for a seeded system was determined using laser backscattering (focused beam reflectance measurement, FBRM). Batch crystallizations seeded with the metastable α-form crystals following various preset supersaturation profiles were implemented using concentration feedback control which regulated the cooling rate based on the in situ measurement of solute concentration. Batch crystallizations operated at constant relative supersaturation in an appropriate temperature range prevented secondary nucleation of both polymorph types and were successful in s...

Effect of Passive Surface Water Flux Meter Design on Water and Solute Mass Flux Estimates

Standard methods for determining pollutant loads in streams typically require the collection of separate instantaneous measurements of water velocities and solute concentrations at discrete points in space and time. A recently developed device, the passive surface water flux meter (PSFM), has been introduced as an alternate method for the measurement of time-integrated surface water flux (velocity) and solute mass flux. This paper extends PSFM development by evaluating and comparing two PSFM designs in laboratory flumes, as well as reporting on initial steady-state field testing. The shape of the PSFM body determines the velocity with which water passes through the device, and different designs may thus be preferred for different applications. Experiments compared the accuracy of flux measurement by the previously introduced hydrofoil-shaped PSFM and nitrate-sorbing cartridge with that of a newly designed cylindrical-shaped PSFM and phosphate-sorbing cartridges. Testing was performed in a laboratory flume...

Gradual conditioning of non-Gaussian transmissivity fields to flow and mass transport data: 1. Theory

The paper presents a new stochastic inverse method for the simulation of transmissivity ( T ) fields conditional to T measurements, secondary information obtained from expert judgement and geophysical surveys, transient piezometric and solute concentration measurements, and travel time data. The formulation of the method is simple and derived from the gradual deformation method. It basically consists of an iterative optimization procedure in which successive combinations of T fields, that honour T measurements and soft data (secondary data obtained from expert judgement and/or geophysical surveys), gradually lead to a simulated T field conditional to flow and mass transport data. Every combination of fields requires minimizing a penalty function that penalizes the difference between computed and measured conditioning data. This penalty function depends on only one parameter. Travel time conditioning data are considered by means of a backward-in-time probabilistic model, which extends the potential applications of the method to the characterization of groundwater contamination sources. In order to solve the mass transport equation, the method implements a Lagrangian approach that allows avoiding numerical problems usually found in Eulerian methods. Besides, to deal with highly heterogeneous and non-Gaussian media, being able to reproduce anomalous breakthrough curves, a dual-domain approach is implemented with a first-order mass transfer approach. To determine the particle distribution between the mobile domain and the immobile domain the method uses a Bernoulli trial on the appropriate phase transition probabilities, derived using the normalized zeroth spatial moments of the multirate transport equations. The presented method does not require assuming the classical multiGaussian hypothesis thus easing the reproduction of T spatial patterns where extreme values of T show high connectivity. This feature allows the reproduction of a property found in real formations, which is often crucial to obtain safe estimations of mass transport predictions. Furthermore, very few existing methods can afford with this stochastic property. In fact, this new approach gathers a set of capabilities so far not included in any existing method.

Gradual conditioning of non-Gaussian transmissivity fields to flow and mass transport data: 3. Application to the Macrodispersion Experiment (MADE-2) site, on Columbus Air Force Base in Mississippi (USA)

A large-scale natural-gradient tracer experiment conducted in a highly heterogeneous aquifer at the Macrodispersion Experiment (MADE-2) site on Columbus Air Force Base in Mississippi (USA) is simulated using the gradual conditioning (GC) method. This methodology allows the stochastic inversion of hydraulic conductivity data (K), and transient piezometric (h) and solute concentration (c) measurements in a non-Gaussian framework, including soft and secondary data. Results show (i) that the GC method allows the reproduction of the heavy tailing of the tracer plume as observed in the field by using a dual-domain mass transfer approach together with conditioning to K, h and c data, in a non-Gaussian framework, (ii) a good agreement between data and simulated mass distribution at time 328 days, including the non-Gaussian plume behaviour, (iii) the necessity of using a dual-domain mass transfer approach – or other transport equation different to the advection–dispersion equation (ADE) – when treating with upscaled models regardless of what random function is used to generate the K distribution, (iv) the reduction of uncertainty results when conditioning to all available information and not only to K data, and (v) the importance of preferential flow paths on the anomalous tracer plume spreading at the MADE site. Besides, the viability of the GC method in a highly heterogeneous 3D aquifer is proven, and also its contribution to the state-of-the-art in stochastic inverse modelling.

An improved technique for concentration measurement of galactomannan solutions by differential refractive index

Galactomannans such as guar gum and locust bean gum are naturally occurring polysaccharides which have been widely used in various industries. One of the issues when dealing with these galactomannans is that they do not dissolve completely into water; therefore the actual dissolved concentration of the solution will always be ambiguous. In this article, we present an easy and robust method for determining the concentration of galactomannan solutions by utilizing a Differential Refractive Index (DRI) detector. Calibration charts for guar gum and locust bean gum were constructed for the DRI method, with the accuracy of results being compared to that of the traditional phenol–sulfuric acid method. The results of this investigation showed that the DRI method is a more accurate and reliable technique for determining the concentration of galactomannan solutions, having an average error of ±0.5% compared to ±5.0% for the phenol–sulfuric acid method.

Interlaboratory Validation of the Mehlich 3 Method for Extraction of Plant-Available Phosphorus

The Mehlich 3 (M3) method is widely used for extraction of plant-available phosphorus (P) from soil over a wide range of pH values. The method is also used by many laboratories to determine multiple plant-available nutrients simultaneously. However, this method has not been statistically validated within and among laboratories. The objective of this study was to determine the repeatability (within-laboratory performance) and reproducibility (among-laboratories performance) of the M3 method by using a wide variety of soils. An in-house homogeneity test was conducted for 10 soils. Three replicates of each of the 10 soils were sent to 26 domestic and international laboratories primarily for P analysis. Samples were scooped, weighed, or both scooped and weighed for extraction. The P in extracts was quantified by the participating laboratories by using inductively coupled plasma-atomic emission spectrometry (ICP-AES) or colorimetrically. For the scooped samples analyzed colorimetrically, the repeatability relative standard deviation (RSDr) ranged from 2.07 to 12.1%; the RSDr ranged from 2.2 to 21.4% for the scooped samples analyzed by ICP-AES. For the weighed samples analyzed colormetrically, the RSDr values were 1.09-9.34%, and for the weighed samples analyzed by ICP-AES, they were 1.70-5.76%. For the reproducibility data, the RSDR values ranged from 6.85 to 50.8% for the scooped-colorimetry category, from 6.95 to 73.9% for the scooped-ICP-AES category, from 7.19 to 42.6% for the weighed-colorimetry category, and from 5.29 to 35.9% for the weighed-ICP-AES category. The greatest RSD values were associated with the Susitna soil, which had the smallest concentration of extractable P. Because of the relatively small concentration of P in this soil, the laboratories were attempting to measure solution concentrations that were close to the detection limits. The Horwitz ratios (HorRat) were also used to evaluate the repeatability, HorRat(r), and reproducibility, HorRat(R). Overall, the M3 P method appears to be both repeatable and reproducible across the 4 categories, and the vast majority of the HorRat values for both repeatability and reproducibility were within the acceptable range. The results of this study indicate that the M3 P method for the determination of plant-available P in soil is both accurate and precise when standardized procedures are used. The method has been shown to be suitable for use as a reference method for testing soil materials for extractable P.

In Situ Measurement of Solution Concentration during the Batch Cooling Crystallization of l-Glutamic Acid using ATR-FTIR Spectroscopy Coupled with Chemometrics

The in situ measurement of solution supersaturation associated with the batch cooling crystallization of l-glutamic acid (LGA) at 500 mL and 20 L scale sizes is assessed via ATR-FTIR spectroscopy. A partial least squares chemometric calibration model was developed for the online prediction of LGA concentration from measured FTIR absorbance spectra overcoming some significant challenges related to the low sensitivity of LGA in the mid-IR frequency range, its low solubility in water, and its complex speciation chemistry. The solubility data of LGA in water over the temperature range from 40 to 80 °C, using ATR-FTIR, reveals excellent agreement with those obtained both from using a gravimetric method and literature data. The metastable zone width determined using the turbidimetric methods as a function of heating/cooling rates and solute concentration is found to increase with increasing cooling rate while it decreases with increasing solution concentration. Monitoring online crystallization via both spontaneous and seeded in 500 mL and 20 L crystallizers reveals good concentration predictions for seeded crystallization, while fouling of the ATR crystal prevents its routine use for unseeded crystallization studies. Higher supersaturation levels are found for the larger crystallizer scale-size consistent with enhancement of secondary nucleation at the smaller scale-size.

Using data assimilation method to calibrate a heterogeneous conductivity field and improve solute transport prediction with an unknown contamination source

Hydraulic conductivity distribution and plume initial source condition are two important factors affecting solute transport in heterogeneous media. Since hydraulic conductivity can only be measured at limited locations in a field, its spatial distribution in a complex heterogeneous medium is generally uncertain. In many groundwater contamination sites, transport initial conditions are generally unknown, as plume distributions are available only after the contaminations occurred. In this study, a data assimilation method is developed for calibrating a hydraulic conductivity field and improving solute transport prediction with unknown initial solute source condition. Ensemble Kalman filter (EnKF) is used to update the model parameter (i.e., hydraulic conductivity) and state variables (hydraulic head and solute concentration), when data are available. Two-dimensional numerical experiments are designed to assess the performance of the EnKF method on data assimilation for solute transport prediction. The study results indicate that the EnKF method can significantly improve the estimation of the hydraulic conductivity distribution and solute transport prediction by assimilating hydraulic head measurements with a known solute initial condition. When solute source is unknown, solute prediction by assimilating continuous measurements of solute concentration at a few points in the plume well captures the plume evolution downstream of the measurement points.

Transverse Mixing in a Trapezoidal Compound Open Channel

Transverse mixing characteristics of solute in the open channel flow can provide useful information for river environmental management. The lateral mixing coefficient is a crucial parameter for reproducing the transverse mixing either by numerical simulation or by analytical prediction. Since the solute mixing can be greatly affected by the lateral variations in water depth, mixing coefficient should be determined in each sub-section (i.e., the main channel, side slope and flood plain) separately. In this article, the transverse mixing in a symmetric trapezoidal compound channel was studied based on laboratory measurement of longitudinal and transverse velocity components and lateral distribution of solute concentration. The lateral mixing coefficient was estimated by adopting different Schmidt numbers in different sub-sections divided according to the developing trend of the eddy viscosity, and finally a piecewise linear profile of mixing coefficient was adopted to analytically predict the transverse solute concentration. The comparison between the analytically predicted data and the measuring solute concentration proved that this is an effective way to estimate the lateral mixing in the open channel flow with lateral variations in water depth.

Reconciliation of Solute Concentration Data with Water Contents and Densities of Multi-Component Electrolyte Solutions

Density and chemical masses are two of the most important parameters tracked in chemical plant flowsheets. Unfortunately, chemical plant laboratories commonly avoid density and solvent concentration measurements. Without these data, it is difficult to reconcile solute concentrations reported by the laboratories with the total mass and volume tracked in flowsheets. In this paper, the Laliberte-Cooper density model is used in conjunction with a numerical algorithm to simultaneously estimate both density and water content from measured solute concentrations for aqueous electrolyte solutions. The algorithm numerically optimizes the water content until the sum of the water and solute concentrations (in mass per volume units) equals the density predicted by the Laliberte-Cooper model for that composition. The algorithm was tested against an experimental dataset of simulated nuclear waste supernatant solutions containing mixtures of ten different electrolytes with total ionic strengths up to 8 mol⋅L−1. The algorithm was able to predict the measured densities with an R2 of 0.9912 and an average relative percent error of just 0.05%. The model error was not correlated to the estimated water content or any of the electrolyte concentrations. Thus, the algorithm can be successfully used to simultaneously predict density and water content of aqueous electrolyte solutions containing many electrolytes at high concentrations from analytical data reported in moles or mass of solute per volume.

On the measurement of solute concentrations in 2-D flow tank experiments

Abstract. In this study we describe and compare photometric and resistivity measurement methodologies to determine solute concentrations in porous media flow tank experiments. The first method is the photometric method, which directly relates digitally measured intensities of a tracer dye to solute concentrations, without first converting the intensities to optical densities. This enables an effective processing of a large number of images in order to compute concentration time series at various points of the flow tank and concentration contour lines. This paper investigates perturbations of the measurements; it was found both lens flare effects and image resolution were a major source of error. Attaching a mask minimizes the lens flare. The second method for in situ measurement of salt concentrations in porous media experiments is the resistivity method. The resistivity measurement system uses two different input voltages at gilded electrode sticks to enable the measurement of salt concentrations from 0 to 300 g/l. The method is highly precise and the major perturbations are caused by temperature changes, which can be controlled in the laboratory. The two measurement approaches are compared with regard to their usefulness in providing data for benchmark experiments aimed at improving process understanding and testing numerical codes. Due to the unknown measurement volume of the electrodes, we consider the image analysis method more appropriate for intermediate scale 2D laboratory benchmark experiments for the purpose of evaluating numerical codes.