Aqueous Complexation Reactions Research Articles

Reactive Transport Modelling of the Long-Term Interaction between Carbon Steel and MX-80 Bentonite at 25 °C

The geological disposal in deep bedrock repositories is the preferred option for the management of high-level radioactive waste (HLW). In some of these concepts, carbon steel is considered as a potential canister material and bentonites are planned as backfill material to protect metallic waste containers. Therefore, a 1D radial reactive transport model has been developed in order to better understand the processes occurring during the long-term iron-bentonite interaction. The numerical model accounts for diffusion, aqueous complexation reactions, mineral dissolution/precipitation and cation exchange at a constant temperature of 25 °C under anoxic conditions. Our results suggest that Fe is sorbed at the montmorillonite surface via cation exchange in the short-term, and it is consumed by formation of the secondary phases in the long-term. The numerical model predicts precipitation of nontronite, magnetite and greenalite as corrosion products. Calcite precipitates due to cation exchange in the short-term and due to montmorillonite dissolution in the long-term. Results further reveal a significant increase in pH in the long-term, while dissolution/precipitation reactions result in limited variations of the porosity. A sensitivity analysis has also been performed to test the effect of selected parameters, such as corrosion rate, diffusion coefficient and composition of the bentonite porewater, on the corrosion processes. Overall, outcomes suggest that the predicted main corrosion products in the long-term are Fe-silicate minerals, such phases thus should deserve further attention as a chemical barrier in the diffusion of radionuclides to the repository far field.

Reactive transport modelling of long-term interactions between iron and MX-80 bentonite

Abstract. The geological disposal in deep bedrock repositories is the preferred option for the management of high-level radioactive waste. In some of these concepts, carbon steel is considered as potential canister material and bentonites are planned as backfill material to protect metal waste containers. Therefore, a 1D radial reactive transport model has been developed in order to better understand the processes occurring during the long-term iron–bentonite interaction. The conceptual model accounts for diffusion, chemistry of the porewater and aqueous complexation reactions, mineral dissolution/precipitation and absorption, at a constant temperature of 25 ∘C under anoxic conditions. The geometry of the axisymmetric model reflects the canister–bentonite interface and the bentonite. The primary phases considered are montmorillonitic smectite, quartz, muscovite, albite, illite, pyrite and calcite. We assume that carbon steel is composed only of iron. The potential secondary phases considered are from reported experiments, such as magnetite, nontronitic smectite, greenalite, cronstedtite and siderite. The numerical model results suggest that at the iron–bentonite interface, Fe is adsorbed at the smectite surface via ion exchange in the short term and it is consumed by formation of the secondary phases in the long term. Furthermore, calcite precipitates are due to cation exchange in the short term and due to montmorillonitic smectite dissolution in the long term. The numerical model predicts the precipitation of nontronitic smectite, magnetite and greenalite as corrosion products. Results further reveal a significant increase in pH in the long term, whereas dissolution/precipitation reactions result in limited variations of the porosity. Progressing bentonite dissolution owing to the rising pH and concomitantly increasing silicate concentrations in the porewater induce formation of Fe-silicates as corrosion products at the expense of magnetite. A sensitivity analysis has also been performed to study the effect of selected parameters, such as corrosion rate, diffusion coefficient and composition of the porewater, on the corrosion products. Overall, outcomes suggest that pH and concentration of dissolved Si play an important role in corrosion mechanisms. The predicted main secondary phases in the long term are Fe-silicate minerals. Thus, such phases deserve further attention as possible chemical barriers for radionuclide migration in the repository near-field.

Mobilization of co-occurring trace elements (CTEs) in arsenic contaminated aquifers in the Bengal basin

The mobilization of co-occurring trace elements (CTEs) in aquifers with known metal(loid) contamination may exacerbate potential human health threats where release mechanisms are not geochemically limited. This study investigates the co-mobilization of the potentially toxic CTEs antimony (Sb), molybdenum (Mo), vanadium (V), uranium (U), and selenium (Se) with arsenic (As) and iron (Fe) in contrasting high and low As aquifers surrounding the river Bhagirathi in Murshidabad, West Bengal, India. We sought to determine the dominant release mechanisms and fate of each CTEs as they relate to redox conditions, major element dissolution pathways, and characteristics of dissolved organic matter (DOM). Groundwater samples collected in 2013 and 2015 field campaigns across six villages in the Murshidabad district were analyzed for major and trace elements, as well as the quantity and optical properties of DOM. East of the river Bhagirathi, the Holocene aquifer contained groundwater with compositions reflective of carbonate mineral dissolution and silicate mineral weathering, along with clear signs of Fe reduction and associated mobilization of As, Sb, Mo, V, and U. Relatively oxic groundwater in the Pleistocene aquifer underlying the study sites on the west side of the river showed minimal Fe reduction and signatures of carbonate mineral dissolution and silicate mineral weathering (i.e., Nabagram), or evaporite mineral dissolution (i.e., Kandi). In groundwater samples from the more oxic Pleistocene aquifer, concentrations of As, Sb, Mo, and V were substantially lower than in the Holocene aquifer, however, U concentrations were relatively higher where carbonate mineral dissolution predominated (i.e., Nabagram). Compelling positive correlations between Fe and CTEs in the Holocene but not in the Pleistocene aquifer implicated Fe reductive processes as the probable pathway for CTEs release. The presence of humic-like DOM ligands and inorganic ions (i.e., Ca2+, CO32−) may also enhance CTEs mobility via aqueous complexation reactions, or possibly by inhibiting CTEs adsorption. We further hypothesize that carbonate mineral dissolution mobilizes U in the absence of Fe reduction, and that U solubility in the reducing Holocene aquifer may be facilitated by carbonate mineral dissolution products (e.g., Ca2+, HCO3−, CO32−) that promote U complexation and the inhibition of (bio)immobilization pathways. Conclusively, potentially toxic CTEs are being mobilized concomitantly with As in the Murshidabad district of West Bengal, probably by microbial Fe reduction. Because these aquifers still contain requisite organic matter and bulk sediment Fe required for Fe reductive processes, it follows that concentrations of CTEs may increase in the future. We therefore argue that despite moderately low concentrations of CTEs in the current study, mechanistic modeling efforts and more extensive monitoring of trace metal(loid)s should be performed to ensure that drinking water supplies do not pose additional health threats in the coming years.

Generating isocoulombic reactions as a tool for systematic evaluation of temperature trends of thermodynamic properties: Application to aquocomplexes of lanthanides and actinides

Most of the available thermodynamic data concerning radioactive waste disposal are restricted to values of reaction equilibrium constants (logK298°) at 25 °C and 1 bar. Simple estimation methods such as isocoulombic reactions can be used for extrapolating the properties of reactions involving aqueous species and minerals to elevated temperatures. The aim of this study was to validate the applicability of various alternative isocoulombic reactions to estimate logKT° values of aqueous complexation reactions for lanthanides and actinides to elevated temperatures while taking advantage of new additional literature data, and to identify criteria for choosing the “best” reactions. For each chemical species of interest, a systematic approach using dedicated software and database allowed us to identify the isocoulombic reactions and types of extrapolation that yield the best estimates of standard thermodynamic properties at elevated temperatures, when very limited or no experimental data are available. We have tested aqueous complexation reactions for selected lanthanides and actinides of different valences with chloride, fluoride, sulfate, carbonate, nitrate, phosphate and silicate ligands. “Model” complexation reactions, having known temperature trends, were systematically combined with complex formation reactions of interest whose temperature trends are unknown, into many alternative isocoulombic reactions. For each ion, we investigated which of the generated isocoulombic reactions provide the best estimates for logeKT° of the reaction of interest at elevated temperatures in order to compile the guidelines for choosing the optimal ones, then applying these guidelines to “prediction” subsets. In most cases, knowing only logeKT° at 25 °C (for the reaction of interest), it was possible to obtain rather accurate estimates of logeKT° values at elevated temperatures using isocoulombic reactions that exchange ions with similar charge and hydration properties (hydrated ionic radius and structure of the hydration shell) and known logmKT° of model reactions. These ions and their complexes interact with the solvent in comparable ways, so that their similar heat capacity and entropy effects largely cancel out on both sides of an “optimal” isocoulombic reaction.

Modelling the non-biogenic steps of arsenic retention in horizontal subsurface flow constructed wetlands

A technically viable and alternative treatment option for the removal of arsenic from water is the constructed wetland (CW) technology. Modelling has emerged as a powerful tool for increasing understanding of water treatment systems such as CWs. The present contribution presents the first and second step implementation of the two most significant arsenic retention processes that take place in CWs (aqueous complexation followed by arsenic precipitation, and arsenic sorption in the granular media) in RetrasoCodeBright (RCB) (giving the RCB–ARSENIC model), showing the first simulation results (on the physical and chemical processes). The aqueous complexation reactions included in the RCB–ARSENIC model were considered equilibrium reactions. One sorption reaction and six mineral precipitation–dissolution reactions were included in the model. Five theoretical case studies (total arsenic concentrations from 23.5 to 300μg/L) were simulated in order to test the model resulting from the first step implementation. Once the second step had been implemented, the model was checked and then tested in a CW prototype operating without plants, considering different total arsenic concentrations at the inlet (from 50 to 300μg/L). In terms of total As concentrations in effluent, the simulated data closely matched the data measured in most evaluated cases. The iron and arsenic species relationships, as well as the arsenic retention percentages obtained from simulations, were in agreement with the literature. According to the quality of the obtained results, at this stage it can be considered that the first and second step implementation of the RCB–ARSENIC model has provided reasonably good response values.

Multicomponent reactive transport modeling of uranium bioremediation field experiments

A reaction network integrating abiotic and microbially mediated reactions has been developed to simulate biostimulation field experiments at a former Uranium Mill Tailings Remedial Action (UMTRA) site in Rifle, Colorado. The reaction network was calibrated using data from the 2002 field experiment, after which it was applied without additional calibration to field experiments performed in 2003 and 2007. The robustness of the model specification is significant in that (1) the 2003 biostimulation field experiment was performed with 3 times higher acetate concentrations than the previous biostimulation in the same field plot (i.e., the 2002 experiment), and (2) the 2007 field experiment was performed in a new unperturbed plot on the same site. The biogeochemical reactive transport simulations accounted for four terminal electron-accepting processes (TEAPs), two distinct functional microbial populations, two pools of bioavailable Fe(III) minerals (iron oxides and phyllosilicate iron), uranium aqueous and surface complexation, mineral precipitation and dissolution. The conceptual model for bioavailable iron reflects recent laboratory studies with sediments from the UMTRA site that demonstrated that the bulk (∼90%) of initial Fe(III) bioreduction is associated with phyllosilicate rather than oxide forms of iron. The uranium reaction network includes a U(VI) surface complexation model based on laboratory studies with Rifle site sediments and aqueous complexation reactions that include ternary complexes (e.g., calcium–uranyl–carbonate). The bioreduced U(IV), Fe(II), and sulfide components produced during the experiments are strongly associated with the solid phases and may play an important role in long-term uranium immobilization.

The influence of natural organic matter on the speciation and solubility of Eu in Boom Clay porewater

AbstractThe influence of natural organic matter (NOM) on the speciation and solubility of europium (Eu) was studied under geochemical conditions representative for the Boom Clay. Different organic matter types were used, and analysis was performed both after 0.45 μm microfiltration and after 30000 MWCO ultrafiltration to distinguish between larger colloids (assumed to be immobile underin situconditions) and small dissolved species.Equilibrium was approached from undersaturation starting from synthesised Eu(OH)3(s), which, during the experiment, transformed into EuOHCO3(s), in agreement with thermodynamic considerations. In the absence of NOM, the Eu solution concentrations after 0.45 μm filtration exceeded the thermodynamic solubility of EuOHCO3(s) by several orders of magnitude, indicating the presence of inorganic Eu colloids. In the presence of NOM, the Eu solubility increased with increasing NOM concentration as was expected, but, surprisingly, was dependent on the operational size cut-off: at an identical NOM concentration in the filtrate, the Eu solution concentration after 0.45 μm filtration was consistently higher compared to the Eu concentration after 30000 MWCO filtration. This latter observation necessitates detailed knowledge concerning the pore size cut-off of Boom Clay underin situconditions in order to use the correct Eu-NOM complexation constant and/or maximum solubility in transport calculations. At higher NOM concentrations (TOC>30 mg/L) the Eu solubility after 0.45 μm filtration was seemingly independent of the NOM concentration.In contrast, after 30000 MWCO ultrafiltration, the Eu solution increased linearly with increasing DOC, from the expected thermodynamic solubility (∼5×10−7 mol L−1) at 0 mg L−1DOC to ∼3×10−5 mol L−1at 80 mg L−1DOC. All of the data sets were modelled using the Nagra/PSI database [8] for solubility, hydrolysis and inorganic aqueous complexation reactions, and fitted organic complexation reactions between Eu3+and NOM functional groups. Both a free ligand approach (with electrostatic correction) and the humic ion-binding model VI [23], which was for the first time successfully introduced into Phreeqc geochemical code, were tested and provided equally good fits to the data.

Reactive transport modeling of column experiments on the evolution of saline–alkaline waste solutions

Leakage of saline–alkaline tank waste solutions often creates a serious environmental contamination problem. To better understand the mechanisms controlling the fate of such waste solutions in the Hanford vadose zone, we simulated reactive transport in columns designed to represent local site conditions. The Pitzer ion interaction module was used, with principal geochemical processes considered in the simulation including quartz dissolution, precipitation of brucite, calcite, and portlandite, multi-component cation exchange, and aqueous complexation reactions. Good matches were observed between the simulated and measured column data at ambient temperature (∼ 21 °C). Relatively good agreement was also obtained at high temperature (∼ 70 °C). The decrease of pH at the plume front is examined through formation of secondary mineral phases and/or quartz dissolution. Substantial formation of secondary mineral phases resulting from multi-component cation exchange suggests that these phases are responsible for a decrease in pH within the plume front. In addition, a sensitivity analysis was conducted with respect to cation exchange capacity, selectivity coefficient, mineral assemblage, temperature, and ionic strength. This study could serve as a useful guide to subsequent experimental work, to thermodynamic models developed for the concentrated solutions at high ionic strength and to other types of waste plume studies.

Column removal of Ni(II) from synthetic electroplating waste water using a strong-acid resin

Ion exchange is widely used for the recovery and removal of metals from waste streams in chemical process industries. The removal of Ni(II) from synthetic electroplating waste water using a strong-acid resin in fixed beds was investigated. Besides NiSO 4, the water contained NH 4Cl and anionic ligands NaH 2PO 4 and citrate. Batch studies showed that the exchange isotherms were well fitted by the Langmuir equation. Column experiments were performed under different pH values (0.5–5.0), metal concentrations (5.1–11.9 mol/m 3), volumetric flow rates (5–12 cm 3/min), and bed volumes (3.7–9.8 cm 3). The Thomas model that consists of two parameters was applied to describe the breakthrough curves. It was shown that the calculated breakthrough curves agreed well with the measured ones (standard deviation 6%), except the curves before break point obtained at pH > 3.0. The non-zero Ni(II) concentration in the effluent at the early stage of the whole process was ascribable to the presence of anionic ligands. A chemical model that considers possible aqueous complexation reactions was proposed to explain such breakthrough characteristics.

Ion exchange recovery of Ni(II) from simulated electroplating waste solutions containing anionic ligands

Ion exchange is widely used for the recovery and removal of metals from process and waste streams in chemical process industries. The Na-form of strong-acid Purolite NRW-100 resin was used to recover Ni(II) from a simulated electroplating waste solution containing NiSO 4, NH 4Cl, NaH 2PO 4, and citrate. A set of mass balance equations that take into account possible aqueous complexation reactions was used to establish the pH diagram of Ni(II) species in the presence of anionic ligand citrate or phosphate. Experiments were performed as a function of initial solution pH (0.5–6.0), initial concentration of Ni(II) (0.85–11.9 mol/m 3), and temperature (15–45 °C). It was shown that the amount of Ni(II) exchanged leveled off when the equilibrium pH was higher than around 2.5. The exchange isotherms obtained at various equilibrium pH values were well fitted by the Langmuir equation. The enthalpy of Ni(II) exchange was also evaluated based on the Langmuir constant. Finally, the kinetics of the present ion exchange process was analyzed.

Solution properties of trivalent lanthanide trinuclear complexes with ligand 1,3,5-triamino-1,3,5-trideoxy- cis-inositol

The aqueous complexation reactions of trivalent lanthanide cations with the ligand 1,3,5-triamino-1,3,5-trideoxy- cis-inositol (TACI) have been characterised using potentiometry, electrospray ionisation mass spectrometry and solution NMR. Isostructural trinuclear complexes, [Ln 3(TACIH −3) 2] 3+, are formed all along the series and their stability constants are extremely dependent on the cation radius.

Microbial reduction of Fe(III) and sorption/precipitation of Fe(II) on Shewanella putrefaciens strain CN32.

The influence of Fe(II) on the dissimilatory bacterial reduction of an Fe(III) aqueous complex (Fe(III)-citrate(aq)) was investigated using Shewanella putrefaciens strain CN32. The sorption of Fe(II) on CN32 followed a Langmuir isotherm. Least-squares fitting gave a maximum sorption capacity of Qmax = 4.19 x 10(-3) mol/10(12) cells (1.19 mmol/m2 of cell surface area) and an affinity coefficient of log K = 3.29. The growth yield of CN32 with respect to Fe(III)aq reduction showed a linear trend with an average value of 5.24 (+/-0.12) x 10(9) cells/mmol of Fe(III). The reduction of Fe(III)aq by CN32 was described by Monod kinetics with respect to the electron acceptor concentration, Fe(III)aq, with a half-saturation constant (Ks) of 29 (+/-3) mM and maximum growth rate (micromax) of 0.32 (+/-0.02) h(-1). However, the pretreatment of CN32 with Fe(II)aq significantly inhibited the reduction of Fe(III)aq, resulting in a lag phase of about 3-30 h depending on initial cell concentrations. Lower initial cell concentration led to longer lag phase duration, and higher cell concentration led to a shorter one. Transmission electron microscopy and energy dispersive spectroscopy revealed that many cells carried surface precipitates of Fe mineral phases (valence unspecified) during the lag phase. These precipitates disappeared after the cells recovered from the lag phase. The cell inhibition and recovery mechanisms from Fe(II)-induced mineral precipitation were not identified by this study, but several alternatives were discussed. A modified Monod model incorporating a lag phase, Fe(II) adsorption, and aqueous complexation reactions was able to describe the experimental results of microbial Fe(III)aq reduction and cell growth when cells were pretreated with Fe(II)aq.

Aqueous complexation of trivalent lanthanide and actinide cations by N, N, N′, N′-tetrakis(2-pyridylmethyl)ethylenediamine

The aqueous complexation reactions of trivalent lanthanide and actinide cations with the hexadentate ligand N, N, N′, N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) have been characterized using potentiometric and spectroscopic techniques in 0.1 M NaClO 4. At 25°C, the stability constant of Am(TPEN) 3+ is two orders of magnitude larger than that of Sm(TPEN) 3+, reflecting the stronger interactions of the trivalent actinide cations with softer ligands as compared to lanthanide cations.

Numerical Simulation of Alteration of Sodium Bentonite by Diffusion of Ionic Groundwater Components

AbstractBecause of its ability to retard the movement of radionuclides, sodium bentonite is being considered as a packing and backfilling material in a number of nuclear waste isolation programs. Few studies, however, have considered how the chemical properties of the bentonite backfill will vary with distance from the canister and change in time as groundwater constituents diffuse through the bentonite. Of particular importance is the conversion of sodium bentonite to calcium bentonite by ion exchange, which will in turn affect the migration of cationic radionuclides, such as Cs+and Sr2+, leaking from a waste canister.Experiments measuring the movement of trace amounts of radionuclides through compacted bentonite have typically used unaltered bentonite. Models based on experiments such as these may not lead to accurate predictions of the migration through altered or partially altered bentonite of radionuclides that undergo ion exchange. To address this problem, we have modified an existing transport code to include ion exchange and aqueous complexation reactions. The code is thus able to simulate the diffusion of major ionic groundwater components through bentonite and reactions between the bentonite and groundwater. Numerical simulations have been made to investigate the conversion of sodium bentonite to calcium bentonite for a reference groundwater characteristic of deep granitic formations.