Presence Of Carbonate Ions Research Articles

Comparison of adsorption and desorption of phosphate on synthesized Zn-Al LDH by two methods in a simulated soil solution

Abstract Zinc aluminum layered double hydroxides (LDH) with Zn/Al molar ratios of 2 and 3 were synthesized by general (Zn 2G -Al, Zn 3G -Al) and modified (Zn 2M -Al, Zn 3M -Al) urea hydrolysis methods. The effects of time, pH, selectivity and initial phosphorous concentration were investigated on phosphate uptake by synthesized LDH in a simulated soil solution. Moreover, phosphate desorption from studied LDH was evaluated. Based on the results, nitrate containing Zn-Al LDH were synthesized by both methods. However, the presence of carbonate ion in the interlayer space of Zn 3G -Al was recorded. The kinetics experiments indicated that the pseudo-second order model well described phosphate uptake on all LDH. The lowest uptake (1.09 mmol g − 1 ) was observed for Zn 3G -Al likely due to Zn/Al molar ratio, the presence of carbonate in the interlayer space and incomplete phosphate intercalation mechanism. Phosphate adsorption isotherms of LDH were in agreement with Freundlich model. Although phosphate uptake decreased in the presence of other anions, the highest selectivity especially in the pure nitrate containing LDH were measured for phosphate anion. Phosphate uptake declined with increasing the pH from 6 to 8. The amounts of phosphate desorption from prepared LDH were between 23.2 and 36.3% suggesting that synthesized LDH may have potential to be used as a slow release phosphate fertilizer in soils.

Removal of 2,4-dichlorophenoxyacetic acid in aqueous solution by pulsed corona discharge treatment: Effect of different water constituents, degradation pathway and toxicity assay

A multiple pin-plane corona discharge reactor was used to generate plasma for the degradation of 2,4 dichlorophenoxyacetic acid (2,4-D) from the aqueous solution. The 2,4-D of concentration 1 mg/L was completely removed within 6 min of plasma treatment. Almost complete mineralization was achieved after the treatment time of 14 min for a 2,4-D concentration of 10 mg/L. Effects of different water constituents such as carbonates, nitrate, sulphate, chloride ions, natural organic matter (humic acids) and pH on 2,4-D degradation was studied. A significant antagonistic effect of carbonate and humic acid was observed, whereas, the effects of other ions were insignificant. A higher first order rate constant of 1.73 min−1 was observed, which was significantly decreased in the presence of carbonate ions and humic acids. Also, a higher degradation of 2,4-D was observed in acidic pH conditions. Different 2,4-D intermediates were detected and the degradation pathway of 2,4-D in plasma treatment process was suggested. The toxicity of 10 mg/L 2,4-D was completely eradicated after 10 min of plasma treatment.

Calcite and barite precipitation in CaCO3-BaSO4-NaCl and BaSO4-NaCl-CaCl2 aqueous systems: kinetic and microstructural study

During the production of hydrocarbons from subterranean reservoirs, scaling with calcium carbonate and barium sulfate causes flux decline and dangerous problems in production facilities. This work is intended to study the effect of calcium ions on the precipitation of barium sulfate (barite); then, the effect of the formed barite on calcium carbonate crystallization. The conductometric and pH methods were used to follow the progress of the precipitation reaction in aqueous medium. The obtained precipitates were characterized by FTIR, RAMAN, SEM, and XRD. It was shown that Ca2+ in the reaction media does not affect the microstructure of barite even for higher calcium–barium molar ratio. It influences the precipitation kinetics and the solubility of barite by the formation of CaSO4° ion pairing as a predominant role of complex formation (CaSO4) and the increase of the ionic strength. In Ca(HCO3)2-BaSO4-NaCl aqueous system, experiments have showed that added or formed barite in the reaction media accelerates calcite precipitation. No effect on the microstructure of heterogeneous formed calcite which remain calcite shape. However the presence of carbonate ions affects slightly the microstructure of barite.

Photocatalytic treatment of aqueous solutions at high dye concentration using praseodymium-doped ZnO catalysts

In this work the photocatalytic activity of Pr-doped ZnO (Pr-ZnO) photocatalysts has been addressed for the first time in the treatment of aqueous solutions at high concentration of organic dyes under UV or visible light irradiation. Pr-ZnO photocatalysts were prepared by a precipitation method. The catalysts have been characterized by different techniques such as X-ray diffraction (XRD), UV–Vis diffuse reflectance (UV–Vis DRS) and Raman spectroscopy. XRD results showed that Pr3+ ions were successfully incorporated into the ZnO lattice. UV–Vis DRS spectra evidenced that Pr-ZnO samples present band-gap values of about 3.0eV, lower than undoped ZnO (3.3eV). The efficiency of photocatalysts has been tested in the photocatalytic removal of the azo dye Eriochrome Black T (EBT) under UV or visible light irradiation. The experimental results showed that the values of discoloration and mineralization rate are correlated to the Pr doping level, evidencing that the optimal loading of Pr in the ZnO structure is 0.46mol%. The influence of process parameters (catalyst dosage, initial dye concentration and presence of carbonate ions) on the extent of photocatalytic performances has been investigated. The efficiency of the optimized photocatalyst was also evaluated in the photocatalytic treatment of aqueous solutions containing the triphenylmethane Patent Blue V (PB) dye and in the treatment of a solution containing simultaneously the two selected dyes (EBT and PB). Photocatalytic activity tests in presence of ion scavengers (carbonate ions) showed that the formulated Pr-ZnO photocatalyst is not subjected to deactivation phenomena. At last, a very interesting result was observed for the treatment of a real wastewater containing Basic Red 51 dye, showing that the complete discoloration and mineralization was achieved.

The effect of carbonate and sulfate ions on chloride threshold level of reinforcement corrosion in mortar with/without fly ash

The aim of the study is to evaluate the effect of carbonate and sulfate ions on chloride threshold level of reinforcement corrosion in mortar with/without fly ash. The initiation of active corrosion for the reinforcement was detected by half-cell potential and electrochemical impedance spectra methods. The free chloride or total chloride by mass of binder was used to express the chloride threshold level. The results indicate that the presence of carbonate ions in the chloride solution shortened the time to initiation of reinforcement corrosion and decreased chloride threshold level in both the Portland cement mortar and fly ash mortar specimens. And compared with the single chloride solution, the concomitant presence of chloride ions and sulfate ions decreased the chloride threshold level, but did not influence the time to initiation of chloride-induced reinforcement corrosion. Besides, although the presence of fly ash decreased the chloride threshold level, it delayed the time to initiation of reinforcement corrosion.

Enhanced interfacial proton migration on BaZr(Y)O3 by molten carbonate: A first principles study

Enhanced proton conductivity of Y-doped BaZrO3 (BZY) by molten carbonate (MC) has recently been reported. To understand the phenomenon, the present work investigates the pathways and energetics of proton migration along ZrO2-terminated (100) surface of BaZr(Y)O3 without and with the presence of MC from a density functional theory (DFT) perspective. The calculations explicitly show that the proton migration on BaZrO3 prefers a curved pathway with an energy barrier of 0.567eV. In the presence of carbonate-ion, the proton migration barrier is remarkably reduced to 0.332eV, which is in excellent agreement with 0.33eV of the experimental activation energy for conductivity in the BZY-MC electrolyte. Overall, the enhanced proton conductivity in the BZY-MC composite electrolyte is originated from facile interfacial proton migration between BZY and MC phase.

Infrared Study of (Poly)tungstate Ions in Solution and Sorbed into Layered Double Hydroxides: Vibrational Calculations and In Situ Analysis

In situ attenuated total reflection infrared spectroscopy (ATR-FTIR) was used to study the speciation of (poly)tungstate ions in solution and the sorption process into layered double hydroxides (LDHs). In the first section, the W(VI) speciation in solution was performed by comparison of thermodynamical calculations and infrared spectroscopy of solutions as a function of pH. Then, the exchange mechanism within LDHs was followed in real time with the in situ ATR technique and compared with batch experiments characterized by powder X-ray diffraction (PXRD), elemental analysis, and FTIR. Decomposition of the (W–O) stretching vibration bands, between 800 and 1000 cm–1, has allowed the identification of the high affinity of LDHs for the polytungstate species, W7O246–, despite the presence of carbonate ions and higher charged polytungstate ions in solution. This was confirmed by the DFT methods used to investigate the structure and the vibrational modes of this anion, and very good agreement with experimental sp...

Degradation of anatoxin-a by UV-C LED and UV-C LED/H2O2 advanced oxidation processes

In the present study UV-C LED photolysis and UV-C LED/H2O2 treatment processes were investigated for the degradation of anatoxin-a. The initial anatoxin-a concentration in the aqueous solution was kept at 1μM. During the treatment by UV-C LED, 50% degradation of anatoxin-a was achieved at UV fluence 4032J/m2. The optimum reaction conditions were: λ=260nm, initial pH=6.4, temperature=24°C and distance from water surface=5mm. The degradation of anatoxin-a followed the pseudo first order kinetics (k=2×10−4m2/J). The degradation of anatoxin-a was reduced to 28% in presence of carbonate ions (50mg/L) and at higher concentrations of carbonate ions the degradation was completely inhibited. The concentration of anatoxin-a was significantly reduced (62%) in the presence of dissolved organic matter (DOC) depicting the photosensitization effect of humic acid on the direct photolysis of anatoxin-a. The pseudo first order kinetic constant k at 0.03mg/L concentration of DOC was found to be 3×10−4m2/J. It was also observed that UV-C/H2O2 process enhanced the oxidation rate of anatoxin-a by 4.5 times in comparison to direct photolysis by UV-C LED leading to 97% degradation of anatoxin-a. In Pitkäjäärvi lake water, 96% removal of anatoxin-a was achieved by photolysis using UV-C LED and 79% removal was achieved with UV-C/H2O2 process. The acute toxicity tests were conducted using Vibrio fischeri. The results indicated a decrease in the acute toxicity of the treated samples, both after the UV-C LED photolysis and UV-C/H2O2 process.

Influence of carbonate ion in the crystallization medium on the formation and chemical composition of CaHA–SrHA solid solutions

The study of the influence of carbonate ions in a solution to Sr-distribution in system «solution–crystal» and to ion substitutions and the non-stoichiometry of formed CaHA–SrHA solid solutions was carried out. The CaHA–SrHA solid solutions were synthesized by precipitation from aqueous solutions with the atomic C/P ratio equal to 0, 0.05 and 0.1 at T=90°C. Resulting precipitates were studied using various methods including X-ray powder diffraction, infrared spectroscopy and different chemical analyses.The results of the study have shown that in the range of values of (Ca+Sr)/P in the water solution from 40% to 85%, the presence of carbonate ions (C/P=0.05–0.1) promotes the incorporation of strontium in the apatite. Crystalline apatite solid solutions formed from water solutions of such composition are more defective compared to apatites that are mainly calcium or strontium. They are characterized by a smaller size coherence scattering domain length along [001] direction and a greater number of carbonate ions, water molecules and vacancies at the Ca-sites.

The effect of basic pH and carbonate ion on the mechanism of photocatalytic destruction of cylindrospermopsin

This study investigated the mechanistic effects of basic pH and the presence of high carbonate concentration on the TiO2 photocatalytic degradation of the cyanobacterial toxin cylindrospermopsin (CYN). High-performance liquid chromatography combined with quadrupole time-of-flight electrospray ionization tandem mass spectrometry (LC/Q-TOF-ESI-MS) was employed for the identification of reaction byproducts. The reaction pathways were proposed based on the identified degradation byproducts and radical chemistry. In high pH system (pH = 10.5) similar reaction byproducts as those in neutral pH system were identified. However, high pH appeared to inhibit sulfate elimination with less sulfate elimination byproducts detected. In the presence of carbonate in the photocatalytic process, hydroxyl radical reaction would be largely inhibited since carbonate ion would react with hydroxyl radical to form carbonate radical. The second order rate constant of carbonate radical with CYN was estimated to be 1.4 × 108 M−1s−1, which is much smaller than that of hydroxyl radical. However, the more significant abundance of carbonate radical in the reaction solution strongly contributed to the transformation of CYN. Carbonate radical has higher reaction selectivity than hydroxyl radical and hence, played a different role in the photocatalytic reaction. It would promote the formation of byproduct m/z 420.12 which has not been identified in the other two studied photocatalytic systems. Besides, the presence of carbonate ion may hinder the removal of toxicity originated from uracil moiety due to the low reaction activity of carbonate radical with uracil moiety in CYN molecule. This work would further support the application of photocatalytic technologies for CYN treatment and provide fundamental information for the complete assessment of CYN removal by using TiO2 photocatalysis process.

Antimicrobial silver: an unprecedented anion effect.

Silver is an indispensable metal but its use has to be minimised for sustainable growth. Much of the silver lost during use is unrecoverable; an example being its use as an antimicrobial agent, a property known since ages. While developing methods to create an affordable drinking water purifier especially for the developing world, we discovered that 50 parts per billion (ppb) of Ag+ released continuously from silver nanoparticles confined in nanoscale cages is enough to cause antimicrobial activity in conditions of normal water. Here we show that the antibacterial and antiviral activities of Ag+ can be enhanced ~1,000 fold, selectively, in presence of carbonate ions whose concentration was maintained below the drinking water norms. The protective layers of the organisms were affected during the carbonate-assisted antimicrobial activity. It is estimated that ~1,300 tons of silver can be saved annually using this new way to enhance its antimicrobial activity.

Obtainment of Spherical-Shaped Calcite Crystals Induced by Intramineral Proteins Isolated from Eggshells of Ostrich and Emu

In this work, we have performed a set of experiments to study how intramineral proteins affected the growth of calcite crystals and induced spherical growth. Intramineral proteins were first isolated from ostrich and emu eggshells and then purified by liquid chromatography. Samples of these proteins were analyzed by means of cyclic voltammetry to check their selectivity for carbonate ions. Then they underwent assays in solution through dynamic light scattering (DLS) to help us establish their homogeneity and stability (whether individually or in pairs) in the presence of carbonate ions. This was followed by in vitro crystallization experiments to determine how they affected the morphology of calcite crystals interacting with the intramineral proteins. Finally, scanning electron microscopy (SEM) showed that the morphology of the calcite crystals was strongly modified in the presence of these intramineral proteins.

Preparation and properties of calcium-silicate filled resins for dental restoration. Part I: chemical-physical characterization and apatite-forming ability

Objective. The aim of this study was to measure dimensional changes due to hygroscopic expansion and the bioactivity of two experimental methacrylate-based dental adhesives either incorporating Bioglass® 45S5 (3-E&RA/BG) or MTA (3-E&RA/WMTA). Materials and methods. 3-E&RA/BG, 3-E&RA/WMTA and a control filler-free resin blend (3-E&RA) were formulated from commercially available monomers. Water sorption (WS) and solubility (SL) behaviour were evaluated by weighing material disks at noted intervals; the relationship between degree of hydration and the glass transition temperature (Tg) was investigated by using differential scanning calorimetry (DSC). In vitro apatite-forming ability as a function of soaking time in phosphate-containing solutions was also determined. Kruskal-Wallis analysis of variance (ANOVA) was used to evaluate differences between groups for maximum WS, SL, net water uptake and the percentage change in Tg values. Post-ANOVA pair-wise comparisons were conducted using Mann-Whitney-U tests. Results. 3-E&RA/BG and 3-E&RA/WMTA exhibited values of maximum WS and net water uptake that were significantly higher when compared to 3-E&RA. However, no statistically significant differences were observed in terms of SL between all the adhesives. The addition of the Bioglass® 45S5 and MTA to the 3-E&RA showed no reduction of the Tg after 60 days of storage in deionized water. ATR Fourier Transform Infrared Spectroscopy (ATR-FTIR) of the filled resin disks soaked in DPBS for 60 days showed the presence of carbonate ions in different chemical phases. Conclusion. Dentine bonding agents comprising calcium-silicates are not inert materials in a simulated oral environment and apatite formation may occur in the intra-oral conditions. Clinical significance. A bioactive dental material which forms apatite on the surface would have several benefits including closure of gaps forming at the resin–dentine interface and potentially better bond strength over time (less degradation of bond).

Degradation of methyl t-butyl ether (MTBE) by photochemical process in nanocrystalline TiO2 slurry: Mechanism, by-products and carbonate ion effect

Methyl tert-butyl ether (MTBE) was removed from aqueous solution by photochemical reaction catalyzed by nanocrystalline TiO2 and UV radiation as a treatment strategy. The experiments were conducted at high concentration of MTBE (10,570μL−1) in a recirculating batch reactor. The optimum loading of TiO2 was found experimentally. The concentrations of MTBE and its by products were measured using a purge-and-trap gas chromatography and ion chromatography methods. Additionally, in order to explore the degradation mechanism of MTBE, by-products of MTBE oxidation were identified by using a Head space gas chromatograph–mass spectrometer (HS/GC/MS) as tert-butyl formate (TBF), tert-butyl alcohol (TBA), acetone, 2-methyl-1-propene and isopropanoic acid, all are readily degraded photocatalytically. Nearly complete removal of MTBE (>99.9%) was achieved after 2h of reaction and a pseudo first order rate constant for the degradation of MTBE was estimated to be 6.4×10−2min−1. Moreover TBF degradation rate was slower and proceeded with a first-order rate constant of 2.8×10−2min−1. Finally, the percent of photocatalytic degradation of MTBE was studied in the presence of carbonate ion. Mass balance calculations have shown that MTBE is virtually completely mineralized. This study offers a potential remediation technique for treating MTBE-contaminated groundwater.

Thiamine pyrophosphate intercalation in layered double hydroxides (LDHs): An active bio-hybrid catalyst for pyruvate decarboxylation

The intercalation of thiamine pyrophosphate (TPP) into Zn–Al and Mg–Al layered double hydroxides has been investigated using two different methods: the co-precipitation procedure (at different pH values) and the ion-exchange reaction. The structure of the hybrid materials was studied by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy verifying that the TPP molecules prefer creating a bilayer arrangement at pH=7.5, 7.75 and 8, while in the case of pH=7 a monolayer of TPP molecules coexists in the interlayer space of LDHs. In addition, the thermal decomposition of the bio-hybrid materials was examined by thermogravimetry and differential thermal analysis (TG–DTA). The total amount of the deintercalated TPP molecules from LDHs was calculated after de-intercalation reactions in the presence of carbonate ions by Ultraviolet (UV) spectroscopy. Furthermore, the hybrid materials (TPP/LDH) have been tested as biomimetic catalysts for decarboxylation of pyruvic acid showing a comparable activity to the homogenous system (TPP). Our combined data confirm that thiamine pyrophosphate can be safely loaded and stored into LDH's layers forming a new bio-active hybrid material.

Determination of trace level carbonate ion in Mg–Al layered double hydroxide: Its significance on the anion exchange behaviour

The presence of trace level (<1ppm) carbonate ion in the interlayer space of MgAl-LDH has been estimated by ion chromatography technique using a novel inverse chemical suppression method using 0.5mM H2SO4 as mobile phase and 25mM LiCl as regenerating solution. The presence of carbonate ion in the LDH structure was found to affect the anion exchange capacity of MgAl-LDH. This has been illustrated by carrying out anion exchange of nitrate ion in the MgAl-LDH structure with chloride ion. The importance of this simple but accurate technique for the carbonate ion estimation is demonstrated and reported in this communication.

Enhanced Degradation of Phenol by Carbonate Ions With Dielectric Barrier Discharge

The influence of carbonate ions on plasma degradation of phenol in water treated by dielectric barrier discharge was tested. The plasma degradation pathway of phenol was also investigated through detecting the main degradation products by HPLC and HPLC/MS. Our results indicate that the degradation efficiency of phenol is significantly intensified in the presence of carbonate ions, particularly at lower phenol concentration, and is gradually increased with increasing carbonate concentration up to a maximum at 1.2 g · L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . As a consequence of this process, carbonate radicals are generated from the scavenging of the hydroxyl radical by carbonate ions and are highly reactive to electron-rich compounds such as phenol. Moreover, the results clearly demonstrate that the plasma degradation pathway of phenol is dominated by a carbonate radical derived from the reaction of phenoxyl radical and peroxyl radical.

Adsorption of Uranyl Species onto the Rutile (110) Surface: A Periodic DFT Study

To model the structures of dissolved uranium contaminants adsorbed on mineral surfaces and further understand their interaction with geological surfaces in nature, we have performed periodic density funtional theory (DFT) calculations on the sorption of uranyl species onto the TiO(2) rutile (110) surface. Two kinds of surfaces, an ideal dry surface and a partially hydrated surface, were considered in this study. The uranyl dication was simulated as penta- or hexa-coordinated in the equatorial plane. Two bonds are contributed by surface bridging oxygen atoms and the remaining equatorial coordination is satisfied by H(2)O, OH(-), and CO(3)(2-) ligands; this is known to be the most stable sorption structure. Experimental structural parameters of the surface-[UO(2)(H(2)O)(3)](2+) system were well reproduced by our calculations. With respect to adsorbates, [UO(2)(L1)(x)(L2)(y)(L3)(z)](n) (L1=H(2)O, L2=OH(-), L3=CO(3)(2-), x≤3, y≤3, z≤2, x+y+2z≤4), on the ideal surface, the variation of ligands from H(2)O to OH(-) and CO(3)(2-) lengthens the U-O(surf) and U-Ti distances. As a result, the uranyl-surface interaction decreases, as is evident from the calculated sorption energies. Our calculations support the experimental observation that the sorptive capacity of TiO(2) decreases in the presence of carbonate ions. The stronger equatorial hydroxide and carbonate ligands around uranyl also result in U=O distances that are longer than those of aquouranyl species by 0.1-0.3 Å. Compared with the ideal surface, the hydrated surface introduces greater hydrogen bonding. This results in longer U=O bond lengths, shorter uranyl-surface separations in most cases, and stronger sorption interactions.

Turning sunlight into stone: the oxalate-carbonate pathway in a tropical tree ecosystem

Abstract. An African oxalogenic tree, the iroko tree (Milicia excelsa), has the property to enhance carbonate precipitation in tropical oxisols, where such accumulations are not expected due to the acidic conditions in these types of soils. This uncommon process is linked to the oxalate-carbonate pathway, which increases soil pH through oxalate oxidation. In order to investigate the oxalate-carbonate pathway in the iroko system, fluxes of matter have been identified, described, and evaluated from field to microscopic scales. In the first centimeters of the soil profile, decaying of the organic matter allows the release of whewellite crystals, mainly due to the action of termites and saprophytic fungi. In addition, a concomitant flux of carbonate formed in wood tissues contributes to the carbonate flux and is identified as a direct consequence of wood feeding by termites. Nevertheless, calcite biomineralization of the tree is not a consequence of in situ oxalate consumption, but rather related to the oxalate oxidation inside the upper part of the soil. The consequence of this oxidation is the presence of carbonate ions in the soil solution pumped through the roots, leading to preferential mineralization of the roots and the trunk base. An ideal scenario for the iroko biomineralization and soil carbonate accumulation starts with oxalatization: as the iroko tree grows, the organic matter flux to the soil constitutes the litter, and an oxalate pool is formed on the forest ground. Then, wood rotting agents (mainly termites, saprophytic fungi, and bacteria) release significant amounts of oxalate crystals from decaying plant tissues. In addition, some of these agents are themselves producers of oxalate (e.g. fungi). Both processes contribute to a soil pool of "available" oxalate crystals. Oxalate consumption by oxalotrophic bacteria can then start. Carbonate and calcium ions present in the soil solution represent the end products of the oxalate-carbonate pathway. The solution is pumped through the roots, leading to carbonate precipitation. The main pools of carbon are clearly identified as the organic matter (the tree and its organic products), the oxalate crystals, and the various carbonate features. A functional model based on field observations and diagenetic investigations with δ13C signatures of the various compartments involved in the local carbon cycle is proposed. It suggests that the iroko ecosystem can act as a long-term carbon sink, as long as the calcium source is related to non-carbonate rocks. Consequently, this carbon sink, driven by the oxalate carbonate pathway around an iroko tree, constitutes a true carbon trapping ecosystem as defined by ecological theory.

Preparation of silver nanoparticles in aqueous solutions in the presence of carbonate ions as stabilizers

Silver nanoparticles are prepared by reducing Ag+ ions with sodium borohydride in aqueous solutions containing carbonate ions (5 × 10−5−1 × 10−2 mol l−1). It is established that carbonate ions represent an efficient stabilizer that provides nanoparticles with electrostatic protection via the formation of an electrical double layer. The maximum stability of a silver dispersion is observed at a carbonate ion concentration of 1 × 10−3 mol l−1. The average size of silver nanoparticles is 10.0 ± 2.5 nm. The formation kinetics of silver nanoparticles is described by an equation for a first-order reaction with a rate constant of 2.3 × 10−3 s−1 (±20%).