Bulk Dissolution Rates of Cadmium and Bismuth Tellurides As a

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Bulk aqueous dissolution kinetics of cadmium and bismuth tellurides under environmental conditions: effects of pH, temperature and dissolved oxygen Marc Biver, and Montserrat Filella Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05920 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 7, 2016

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Environmental Science & Technology

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Bulk dissolution rates of cadmium and bismuth tellurides as a function of- pH,

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temperature and dissolved oxygen

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Marc Bivera, Montserrat Filellab,*

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a

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1115 Luxembourg

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b

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Switzerland

Bibliothèque nationale de Luxembourg, Annexe Kirchberg, 31, Boulevard Konrad Adenauer, L-

Institute F.-A. Forel, University of Geneva, 66 Boulevard Carl-Vogt, CH-1205 Geneva,

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ABSTRACT: The toxicity of Cd being well established and that of Te suspected, the bulk,

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surface-normalized steady-state dissolution rates of two industrially important binary tellurides –

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polycrystalline cadmium and bismuth tellurides– were studied over the pH range 3 to 11, at

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various temperatures (25-70°C) and dissolved oxygen concentrations (0 to 100% O2 in the gas

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phase). The behavior of both tellurides is strikingly different. The dissolution rates of CdTe

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monotonically decreased with increasing pH, the trend becoming more pronounced with

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increasing temperature. Activation energies were of the order of magnitude associated with

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surface controlled processes; they decreased with decreasing acidity. At pH 7, the CdTe

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dissolution rate increased linearly with dissolved oxygen. In anoxic solution, CdTe dissolved at a

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finite rate. In contrast, the dissolution rate of Bi2Te3 passed through a minimum at pH 5.3. The

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activation energy had a maximum in the rate minimum at pH 5.3 and fell below the threshold for

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diffusion control at pH 11. No oxygen dependence was detected. Bi2Te3 dissolves much more

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slowly than CdTe; from one to more than 3.5 orders of magnitude in the Bi2Te3 rate minimum.

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Both will readily dissolve under long-term landfill deposition conditions but comparatively

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slowly.

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INTRODUCTION

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Tellurium is an extremely rare element in the earth´s crust with an estimated upper continental

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crust abundance of 0.027 ppm1 and its distribution and fate in the environmental compartments

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have been comparatively little studied. There is considerable uncertainty as to its likely

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concentration in natural waters.2 Some 123 tellurium bearing phases are known to occur as

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natural minerals.3 None of these are abundant enough for economical exploitation. Tellurium, a

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chalcophile element, also occurs as an impurity in sulfidic copper and nickel ores, thus the

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element is extracted from mining and refining residues of the copper and nickel industries. It is

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only over the last two decades that economically significant technological applications of

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tellurium compounds have emerged. It has recently been identified as one of the key

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technologically critical elements.4 Metal tellurides are semiconducting materials, and CdTe is

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gaining importance as an absorber in thin-film photovoltaic conversion modules, owing to its

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band gap of 1.49 eV,5,6 which exactly coincides with the mean energy of solar light. The principal

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advantage of CdTe photovoltaic cells lies in the combination of low cost and high conversion

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efficiency. CdTe is also used in photoconductors, specifically in the manufacture of gamma and

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IR radiation detectors.6,7 Novel nanotechnological applications of CdTe as ‘quantum dots’ in

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analytical chemistry are becoming increasingly popular6,7 and have been reviewed.9 Bismuth

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telluride and its alloys are currently the most widely used thermoelectric materials5,10 e.g. in

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Peltier-type cooling elements in equipment as diverse as microcomputers and household cooling

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devices. Natural occurrences of CdTe are not known; Bi2Te3 exists as the rare mineral

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tellurobismuthite.3

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Cadmium is known to be extremely toxic to all forms of life,11 the toxicity of tellurium is

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still little understood,11 even though clear adverse health effects have been known for nearly a

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century12 while bismuth –used for more than a century in medicine in the treatment of disorders 3 ACS Paragon Plus Environment


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of the digestive tract and in dermatology13– is generally considered as environmentally

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benign11,14 although there are reports of genotoxicity of Bi2O3 nanoparticles,15 inhibition of

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growth of soil microorganisms16 and the neurotoxic potential of some bismuth compounds has

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been confirmed.17,18

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Thin films of CdTe, as they are employed in solar modules, are believed to be

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environmentally quite safe, because the compound is thermally extremely stable (melting point

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1042°C)19 and it is sealed within glass plates and contained in between thin layers of other

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compounds. Fortunately, CdTe can be recycled from used modules in excellent yield.20

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Nevertheless, the rate of release of problematic elements such as cadmium and tellurium from

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these important industrial materials ought to be known, as accidental release (due to inadequate

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handling or disposal of the finished products or the compounds themselves) can never be ruled

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out. We therefore studied the aqueous dissolution kinetics of bulk, polycrystalline CdTe and

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Bi2Te3 under environmentally relevant conditions.

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MATERIALS AND METHODS

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Reagents. Cadmium, bismuth and sodium tellurides were purchased from Alfa-Aesar,

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Germany (99.999%, 99.98% and 99.9% respectively) and used without pretreatment. Perchloric

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(70%), hydrochloric (35%) and acetic (100%) acids and ammonia solution (35%) were of

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suprapure quality (Roth, Germany). Sodium hydroxide, sodium perchlorate and the Good’s

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buffers MES, TRIS, MOPS, CHES and CAPS were p.a. grade from Roth, (Germany) and/or

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Fisher (US). Oxygen and nitrogen (>99.999%) were obtained from L’Air Liquide (Luxembourg).

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Atmospheric air for gas mixtures was supplied by a membrane pump (Rena® Air 300, France).

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Conductance (‘Milli-Q’) water (0.055 µScm-1), obtained from a reverse osmosis unit (TKA

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Genpure, Thermo Scientific, Germany) was used to prepare the solutions. 4 ACS Paragon Plus Environment

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Mixed Flow Reactor Experiments. The experimental setup is identical to the one used

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previously in the study of the dissolution kinetics of stibnite21. Briefly, the experiments were

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carried out in an Amicon Model 3200 ultrafiltration cell (Merck-Millipore, US) of 200 mL

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nominal volume, fitted with binder-free quartz fiber filter pads (pore size 0.6 µm). These were

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covered by an additional cellulose ashless filter disc of finest porosity (M&N greenpack) as the

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quartz fiber discs were found to disintegrate somewhat under the torque of the rapidly stirred

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liquid. The cell contents were stirred magnetically at 280 rpm using a suspended stirrer bar to

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avoid any grinding of the solid. Masses between 50 and 250 mg of metal telluride were

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accurately weighed into the reactor. The cell was completely submerged in a thermostated water

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bath (Thermo Scientific C10, US), controlling the temperature to within ±0.04 K. Influent

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solutions were stored in polyethylene tanks (capacity 30 L) which were open to the atmosphere

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via a guard tube filled with soda-lime in order to avoid absorption of atmospheric CO2 by the

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solutions. The desired pH was obtained by the addition of Good’s buffers (MES (pKa=6.15),

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MOPS (pKa=7.2), TRIS (pKa=8.07), CHES (pKa=9.3), CAPS (pKa=10.4)) and the necessary

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amounts of sodium hydroxide or perchloric acid. No buffer was added to make solutions from pH

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3 to 5. The total concentrations of buffer species were 1.00×10-3 mol L-1 and the ionic strength of

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the solutions was adjusted to 1.00×10-2 mol L-1 by addition of the calculated amount of sodium

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perchlorate. Good’s buffers, perchloric acid and sodium perchlorate were chosen because of their

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weakly coordinating nature.22,23 Different concentrations of dissolved oxygen (DO) in the

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influent were achieved by protracted sparging of the solutions with oxygen-air or nitrogen-air

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mixtures, from which carbon dioxide had been removed by passage through concentrated lye and

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scrubbing with distilled water. The gases were premixed (before entering the reagent container)

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using two rotameters (Omega® FL-2010 and FL-2012, The Netherlands) with a total flow rate of

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1-2 L min-1. From the flow rates of both gases, the volume percentage of oxygen in the mixture 5 ACS Paragon Plus Environment


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was computed. The concentration of DO was obtained by Tromans’s equation24 as a function of

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the temperature and the current atmospheric pressure (measured with a Torricelli (mercury)

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barometer). Additionally, the DO concentrations were checked with a digital oximeter and the

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readings were found to correspond perfectly with the theoretical prediction. At the ionic strength

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(I=0.01 mol L-1 by sodium perchlorate) used in these experiments, a correction to the solubility of

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O2 proved unnecessary, as predicted by the empirical relations of Schumpe et al.25 The solutions

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were pumped through the reactor by means of a variable speed peristaltic pump (Type PR4 from

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Seko, Italy); the flow rate was measured with a graduate and a digital stopwatch. A flow rate of

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about 1.4×10-4 L s-1 (= 8.4 mL min-1) was found adequate in most experiments. Connections to

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the cell were made by semi-rigid FEP or flexible silicone tubing (Roth, Germany). The

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concentrations of cadmium, bismuth and tellurium were found to decrease from initially high to

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constant values, indicating that the dissolution kinetics had attained a steady state. We chose to

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monitor the dissolution kinetics by measuring the concentrations of the metal ions rather than the

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concentration of the released tellurium, because only Te(IV) can be detected voltammetrically

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and the oxidation of tellurium, initially present in the solids in the –II oxidation state, is

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kinetically somewhat sluggish, as shown by experiments described below.

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From the constant concentrations at steady state (generally within 5%, frequently better), the

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dissolution rates were computed (as the rates of destruction of the solids) according to the

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formulae:26 [] [ ] [ ] =− =− 1

[ ] 1 [ ] 1 [ ] =− =− 2

2

2

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where Q stands for the flow rate, m for the mass of the solid in the reactor, and σ for its specific

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(i.e. BET) surface area. Rates quoted in the following will consequently be expressed in mol m-2 6 ACS Paragon Plus Environment

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s-1. Steady state conditions were in general achieved after 12 to 30 h of reactor operation; from

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then on, the effluent was sampled in hourly intervals and 8 to 12 measurements were collected

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from which to compute the dissolution rate and the error associated with it (at the 95%

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confidence limit). At each sampling time, the flow rate, pH (to 0.01 unit) and oxygen

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concentration were measured. A slight drift in pH was taken into account by averaging the

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hydrogen ion activity over the samples retained for the calculation and converting to an ‘average’

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pH.

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Analytical Methods. Cadmium, bismuth and tellurium concentrations in the effluent from

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the reactor were measured by inverse voltammetry using a Computrace VA797 instrument from

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Metrohm (Herisau, Switzerland) equipped with a hanging mercury drop electrode (HMDE), a Pt

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auxiliary electrode and an Ag/AgCl double junction reference electrode filled with 3 mol L-1 KCl

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as the bridge electrolyte. Cd(II) was determined by anodic stripping voltammetry (ASV) using

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the method provided by the instrument manufacturer; Bi(III) was similarly determined by ASV

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according to a published method.27 Tellurium was determined as Te(IV) by a method initially

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devised for the analysis of natural waters.2 This was adapted to make it less sensitive by

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shortening the deposition time from several minutes to 10 s. The instrumental parameters for the

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voltammetric methods are summarized in Table SI1. For standard additions, 1 mg L-1 (10 µg L-1

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for Te) standards were prepared daily by dilution of commercial AAS standards (Roth, Alfa

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Aesar, Germany). The pH of the effluent solution was measured with a Metrohm pH-Meter

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(Model 632, Switzerland) calibrated against NIST-traceable standards (pH 4, 7 and 10 from

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Hanna, Romania) and taking the temperature coefficient of the buffers into account. Dissolved

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oxygen was measured polarographically in the influent solution using a digital oximeter with a

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Clark cell (Hanna MI 9146, Romania), calibrated against ambient air. The specific surface areas

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of the solids were determined by recording multipoint N2 adsorption isotherms according to the 7 ACS Paragon Plus Environment


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BET method28, using a Gemini VII instrument (Micromeritics, US), as (4.63±0.06)×10-2 and

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(2.822±0.014)×10-1 m2g-1 for CdTe and Bi2Te3, respectively.

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Preliminary Investigations on the Oxidation Kinetics of Te(-II) in Solution. An aqueous

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stock solution containing approximately 1 g L-1 Te(-II) was prepared by degassing 20 mL of

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Milli-Q water (by sparging with N2 for 20 min.) and adding the calculated amount of Na2Te

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under a blanket of N2 and without shutting off the gas supply. The resulting deep violet solution

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was transferred to a disposable 25 ml PP syringe (Braun, Germany) via a stainless steel cannula

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(Braun, Germany). The syringe was stoppered immediately. The stock solution was standardized

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as described in the Supporting Information, by forming the sparingly soluble Cu2Te29 and

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complexometric back-titration of the excess copper. To investigate the kinetics of Te(-II)

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oxidation under ambient conditions, 20 µL of the stock solution was added to 20 mL of a 0.01

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mol L-1 MES buffer (pH=7.0) in a conical flask (producing a nominal starting concentration of

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Te(-II) of 1 mg L-1), mixed by swirling and the flask left open to the atmosphere; samples were

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taken over a duration of about 3 days and the Te(IV) concentration determined.

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Scanning Electron Microscope (SEM) Observations. Crystals were mounted on a

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conductive support (i.e., aluminium stub) with double-sided conductive carbon tape. An ultra-thin

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coating (ca 10 nm) of gold was then deposited on the samples by low vacuum sputter coating.

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Imaging and EDXS (Energy-dispersive X-ray spectroscopy) measurements were performed with

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a JEOL JSM 7001F Scanning Electron Microscope (Department of Earth Sciences, University of

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Geneva, Switzerland), with an integrated EDXS detector (model EX-94300S4L1Q; JEOL).

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RESULTS AND DISCUSSION

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SEM Microscopy and Microprobe Findings. SEM micrographs show that the CdTe

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powder mainly consists of irregular, partially aggregated particles with planar surfaces and 8 ACS Paragon Plus Environment

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rounded edges some 10 to 80 µm across, with very rare ultrafine particles of the order of several

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µm across adhering to them. At higher magnification, the presence of some fine holes (10-100

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nm across) in the surface becomes apparent (Fig. 1). The Bi2Te3 sample consists of mostly flat,

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foliated-looking particles, several tens to more than 100 µm across and several µm thick, with

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sharp edges and a large number of adhering ultrafine particles (Fig. 3). Higher magnifications

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revealed an extremely heterogeneous surface morphology. Euhydral crystals are clearly absent in

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both samples. EDXS analysis demonstrated the identity of the substances and showed that the

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stoichiometry corresponded exactly to the theoretical composition.

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Oxidation of Te(-II) Species. The time dependence of the concentration of Te(IV) in a

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buffer solution, spiked with approximately 1 ppm Te(-II) and left open to the atmosphere was

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monitored over about 70 h by the voltammetric method2. The experimental details can be found

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in the electronic supplemental material. The rate at which Te(IV) appeared in solution was

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indicative of first order or pseudo first order kinetics (Fig. 4). Since upon addition of the Te(-II)

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stock solution to the oxic buffer solution at the start of the experiment, a well discernible brown

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colloid formed instantly, it appears likely that the Te(-II) underwent very rapid oxidation to

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Te(0), which then slowly oxidized to Te(IV) following a first order rate equation (Fig. 4) with a

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rate constant k=8.02×10-4 min-1 (R2>0.9723) equivalent to a surprisingly long half-life of 864

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min. (=14.4 h). This experiment clearly showed that if one intended to take measurements of

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tellurium in order to monitor the dissolution kinetics of CdTe or Bi2Te3, the samples would have

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to be left for a considerable length of time before the concentration of Te(IV) realistically

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reflected the concentration of released tellurium.

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Dissolution Congruency. Preliminary experiments suggested that at the beginning of the

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experiments, the released tellurium concentrations were much higher than the metal ion

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concentrations and as the stationary state was approached, the released concentrations of all 9 ACS Paragon Plus Environment


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elements reached their stoichiometrically expected ratios, suggesting that dissolution became

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congruent after a certain induction period. This is exemplified in Fig. SI1, which shows the

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(molar) cadmium and tellurite ion concentrations in a dissolution experiment with CdTe at pH 10.

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This is consistent with the findings of Okkenhaug,30 who found that in her batch reactor

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experiments, the amount of mobilized tellurium always exceeded the amount of cadmium

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(because of the initial preferential leaching of tellurium).

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Cadmium Telluride – Dependence on pH. The dissolution kinetics of CdTe at different pH

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values from 2.97 to 11.68 at 25.0°C and 55.0°C and oxygen saturation corresponding to ambient

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air (at the specified temperatures and an atmospheric pressure of 740 mmHg on average) was

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studied. At 25.0°C, there was a very weak dependence of the rate r on hydrogen ion activity:

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r=10-8.24±0.02a(H+)0.021±0.002 (R2>0.9465) (3)

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where a(H+) denotes hydrogen ion activity. This rather weak pH dependence was the reason why

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the experiments were repeated at a higher temperature (to confirm that the trend was genuine): at

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55.0°C, a stronger dependence on a(H+) was indeed observed (Fig. 5):

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r=10-6.2±0.1a(H+)0.16±0.02 (R2>0.9196) (4). Given the overall weak pH dependence, the reaction is reminiscent of pyrite oxidation.31 Cadmium Telluride – Dependence on Temperature. Additional experiments were carried

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out at 40, 55 and 70°C and pH 3, 7 and 11. Apparent activation energies EA for the dissolution

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reaction were determined by fitting the measured rates to the Arrhenius equation (Fig. 6). At pH

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7, the Arrhenius equation was obeyed from 25 to 70°C with EA=41±10 kJ mol-1. At pH 3 and 11,

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substantially worse correlations were observed (R2>0.9403 and R2>0.5924), which improved if

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the rates at 70°C were excluded (R2>0.9906 and R2>0.9983). We interpret this as evidence that

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the Arrhenius equation is not obeyed at temperatures above 55°C, which may be indicative of

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changes in the reaction mechanisms; in the range 25-55°C, the apparent activation energies were 10 ACS Paragon Plus Environment

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89±9 and 33±1 kJ mol-1 respectively, i.e. the apparent activation energy decreases with

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decreasing acidity. No activation energies over temperature intervals exceeding 55°C were

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obtained as they are unlikely to be of practical relevance for this particular material. The energies

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are always in excess of the minimum required (about 21 kJ mol-1) for surface-controlled

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dissolution processes32. Note that the validity of this activation energy threshold as a criterion for

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mechanistic inferences has recently been questioned by Rimstidt.33 In Fig. 2, samples of CdTe

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after leaching experiments are shown. At low magnification (top images), there appears to be

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little difference between the leached and the unreacted material; in the latter, ultrafine particles

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are still visible. At higher magnification, ripple-mark-like ledges (bottom left image) and also a

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clear deepening of the existing holes (bottom right image) are visible, which suggest a surface

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controlled dissolution mechanism32.

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Alternatively, an overall activation energy can be calculated by combining pH dependent

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rates (under ambient air) at 25°C and the rates obtained at higher temperatures in a single

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multiple linear regression. This approach gives r=10(1.59±1.18)-(2716±375)/Ta(H+)0.11±0.02 (5)

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which leads to an overall activation energy of 52±7 kJ mol-1 for 3 0.9918. The reason why the

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standard parameter errors (particularly on the rate constants k1 and k2) obtained from the diagonal

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of the covariance matrix, appear large despite the very good correlation lies in the low number of

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degrees of freedom. This rate law confirmed the exactly linear dependence on oxygen activity,

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which is noteworthy, since most oxidative dissolution reactions are of fractional order in oxygen

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activity34. Because of the non-zero rate in the absence of oxygen, the data correlate only very

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poorly (particularly at very low oxygen activities) with a Langmuir isotherm model, which is

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frequently successful in describing the dependence of dissolution rates on the activities of

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dissolved gases.

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Bismuth Telluride – Dependence on pH. A plot of the decimal logarithm of the steady

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state rates vs. pH yielded a V-shaped curve, to which two straight lines, intersecting in an

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extrapolated minimum at pH 5.27, were fitted. Subsequently, an experiment was run in a MES

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buffered solution at this pH which confirmed the extrapolated rate very closely: rmin=8.91×10-12

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mol m-2 s-1 (extrapolated) vs. 1.00×10-11 mol m-2 s-1 (experimental). The empirical rates at the pH

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extrema (pH 2.96 and pH 10.53) were (1.01±0.08)×10-9 and (2.17±0.26)×10-9 mol m-2 s-1,

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respectively.

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The data are best represented by the following rate laws (Fig. 5): r=10-6.4±0.1a(H+)0.88±0.02 for 3≤pH≤5.27 (R2>0.9991) (8). 12 ACS Paragon Plus Environment

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At 5.27≤pH≤11, r=10-13.4±0.2a(H+)-0.45±0.02 (R2>0.9877) (9).

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Bismuth Telluride – Dependence on Temperature. The temperature dependence of the

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rates was investigated at pH 3, 5.3, and 11 at temperatures 25°C, 40°C, 55°C and 70°C. No

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anomalous behavior above 55°C (as in the case of CdTe) was detected (Fig. 7). The apparent

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activation energy EA was smallest at pH 11 (EA=4.8±0.4 kJ mol-1), intermediate at pH 3

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(EA=34±3 kJ mol-1) and highest pH 5.3 (EA=71±5 kJ mol-1), which is consistent with the rate

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minimum observed at this pH. These large differences suggest shifts in mechanism, i.e. a

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transition from surface controlled kinetics to diffusion control at high pH (EA drops below 20

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kJ/mol). SEM micrographs of reacted bismuth telluride showed a complex surface morphology,

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just as did the unreacted material and obviously not created by the dissolution process. No clear

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signs of diffusion controlled dissolution at high pH (such as rounded edges or corners32) could be

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observed, presumably because the experiments did not last long enough for significant amounts

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of material to become eroded. They did not give certain clues to surface control either, since the

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unreacted particles had a lamellar appearance that could erroneously be interpreted as dissolution

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terraces. Alternatively, the combined data (pH and temperature dependent) give

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r=10(5.1±2.36)-(3800±738)/Ta(H+)0.592±0.120 (pH5.3) (11),

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which yields activation energies of 73±14 kJ mol-1 (pH5.3). The

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latter (negative) activation energy and its comparatively large standard error should be contrasted

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with the vastly different values for EA (71 kJ mol-1 vs. 4.8 kJ mol-1) found when the data at pH

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5.3 and 11 are analysed separately. 13 ACS Paragon Plus Environment


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Bismuth Telluride – Dependence on Dissolved Oxygen. Experiments were conducted at

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pH 3, 5.3 and 11, and oxygen saturations corresponding to 0 to 100% (v/v) O2 in the sparging gas

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mixtures. Stationary states were not attained with certainty over the duration of all the

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experiments, but it became obvious that the rates obtained under vastly different O2

292

concentrations converged and therefore, oxygen does not intervene in the rate determining step of

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the dissolution process. Known instances of such ‘non-oxidative’ dissolution of chalcogenides

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include that of pyrrhotite35.

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Compensation Laws. We are aware that compensation laws are controversial and

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considered by some authors to be artifacts arising from regression analysis36,37. For the sake of

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completeness, compensation law plots (Fig. SI3) were constructed for CdTe and Bi2Te3

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individually. They reveal excellent correlations (R2>0.9959 and R2>0.9999) and suggest

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isokinetic temperatures of 22.5°C and 87.2°C, respectively (note that the observed pH

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dependence of the dissolution rate of CdTe is extremely weak at 25°C, close to the predicted

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isokinetic temperature). A compensation law plot for the combined data is still well correlated

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with R2>0.9775.

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ENVIRONMENTAL IMPLICATIONS

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The dissolution behavior of both tellurides is strikingly different in many respects. Bi2Te3

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dissolves on the whole much more slowly than the cadmium compound; the difference varies

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from one to more than 3.5 orders of magnitude in the rate minimum of Bi2Te3. This can be

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rationalized by the much larger degree of ionicity in the cadmium compound (the Pauling

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electronegativities for bismuth, tellurium and cadmium are 1.9, 2.1 and 1.69, respectively19).

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Formally, dissolution processes may be described as

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2 CdTe + 3 O2 + 2 H3O+ → 2 Cd2+ + 2 [TeO2(OH)]- + H2O 14 ACS Paragon Plus Environment

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and Bi2Te3 + 5 H2O → 2 Bi(OH)2+ + 3 HTe- + H3O+, followed by the non-rate determining oxidation of Te(-II) to Te(IV). We are aware of the recent, well justified criticism of the approach taken in this work (i.e.

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the use of bulk, surface-normalized rates), namely by Fischer et al.38 who argue that it ignores the

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probabilistic nature of the relationship between rate and surface area. While we do not expect that

317

the rate laws presented here will hold everywhere in the field exactly as they do under laboratory

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conditions, we believe that the general trends that they highlight, such as the profiles of pH,

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dissolved oxygen and temperature dependence, will retain some predictive value, depending on

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how closely the surface physical properties (crystallinity, grain size and its distribution) of the

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material under investigation match those of the materials used in this study. The proposed rate

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equations can help to assess the likely amounts of problematic substances that may be leached

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from these materials in the natural environment or in the context of their disposal in landfills, e.g.

324

through their incorporation into source terms designed for use in geochemical aquifer modeling).

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Following the derivation given by Schreiber and Rimstidt,3 the corresponding source terms for Te

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from CdTe and Bi2Te3 become: 1;

327

%&',)*&' = 5.83 × 101 23 )*&' 456789:

328

%&',A9B &'C = 1.68 × 101E 23 A9B &'C 456789:

;

1; ;

7

×>? @ (12) 7

H @ (13)

329

where Ff, C, ρmatrix, Φ and t stand for the the so-called field factor, the concentration of mineral in

330

the aquifer matrix (in ppm), the density of matrix solids (in g cm-3), the porosity and time (in s),

331

respectively. Clearly, the predictive power of such source terms critically depends on our

332

knowledge of these additional factors that have not been studied in the present work. A mean 15 ACS Paragon Plus Environment


333

(spherical) particle diameter of 50 µm, a roughness factor of 7, pH of 7 and temperature 298 K

334

have been assumed.

335

Page 16 of 29

As neither of the two substances showed any signs of passivation during dissolution, one

336

must assume that they would dissolve completely under environmental conditions, but as the

337

measured rates are slow on an absolute scale, the accidental release of these compounds into the

338

natural environment would hardly pose a threat and would in any case leave ample time to be

339

tackled. From this perspective, and given their good thermal stability, it appears that both CdTe

340

and Bi2Te3 are indeed environmentally safe materials. However, both will readily dissolve under

341

long-term deposition conditions such as in landfills or uncontrolled open dumps, with the

342

corresponding complete release of their potentially noxious elements, i.e. cadmium and tellurium.

343

ASSOCIATED CONTENT

344

Supporting information

345

Complementary information on analytical methods, numerical data and two additional figures are

346

reported in the Supporting Information. This material is available free of charge via the Internet

347

at http://pubs.acs.org.

348

AUTHOR INFORMATION

349

Corresponding author

350

*Email: [email protected]

351

ACKNOWLEDGEMENT

352

We thank Agathe Martignier (Department of Earth Sciences, University of Geneva) for her help

353

in SEM imaging. We are grateful to Prof. J.D. Rimstidt, whose constructive review contributed to

354

improving the manuscript significantly. In particular, the combined data (pH and temperature) 16 ACS Paragon Plus Environment

Page 17 of 29


355

treatment follow his calculations. Another (anonymous) reviewer´s helpful comments are also

356

gratefully acknowledged.

357

REFERENCES

358

(1)

359

Geol. 2008, 253, 205–221.

360

(2)

361

speciation at the low nanogram level in natural waters by catalytic cathodic stripping

362

voltammetry. Talanta 2015, 144, 1007–1013.

363

(3)

Strunz, H.; Nickel, E. H. Strunz Mineralogical Tables; 9th ed., Schweizerbarth: Stuttgart.

364

(4)

Cobelo-García, A.; Filella, M.; Croot, P.; Frazzoli, C.; Du Laing, G.; Ospina-Alvarez, N.;

365

Rauch, S.; Salaun, P;. Schäfer, J.; Zimmermann, S. Environ. Sci. Poll. Res. 2015, 22, 15188–

366

15194.

367

(5)

368

and references cited therein.

369

(6)

370

2010, 328, 699–701.

371

(7)

372

Korn, M. Enhancing reactive species generation upon photo-activation of CdTe quantum dots for

373

the chemiluminometric determination of unreacted reagent in UV/S2O82- drug degradation

374

process. Talanta 2015, 135, 27–33.

375

(8)

376

novel strategy for simultaneous determination of dopamine and uric acid using a carbon paste

377

electrode modified with CdTe quantum dots. Electroanalysis 2014, 27(2), 524–533.

Hu, Z.; Gao, S. Upper crustal abundances of trace elements: A revision and update. Chem.

Biver, M.; Quentel, F.; Filella, M. Direct determination of tellurium and its redox

Bouroushian, M. Electrochemistry of Metal Chalcogenides; Springer: Heidelberg, 2010,

Zweibel, K. The impact of tellurium supply on cadmium telluride photovoltaics. Science

Santana, R. M. M.; Oliveira, T.D.; Rodrigues, S. S. M.; Frigerio, C; Santos, J. L. M.;

Beitollahi H.; Hamzavi, M.; Torkzadeh-Mahani, M.; Shanesaz, M.; Karimi Maleh, H. A



Page 18 of 29

378

(9)

Amelia M.; Lincheneau, C.; Silvi, S.; Credi, A. Electrochemical properties of CdSe and

379

CdTe quantum dots. Chem. Soc. Rev. 2012, 41(17), 5728–5743.

380

(10)

381

Materials 2014, 7, 2577–2592.

382

(11)

383

of Metals; Academic Press: London, U.K., 2008.

384

(12)

385

preliminary report. Public Health Reports 1920, 35(16 ), 939–954.

386

(13)

387

Chichester, 2011

388

(14)

389

bismuth concentrations in surface waters. J. Environ. Monit. 2010, 12, 90–109.

390

(15)

391

assay. Chemosphere 2013, 93(2), 269–273.

392

(16)

393

microorganisms using thiols as model compounds. Journal of Environmental Science and Health

394

Part A 2006, 41, 161–172.

395

(17)

396

neurons of mice after single oral doses of bismuth compounds. Neurotoxicol. Teratol. 2000, 4,

397

559-563.

398

(18)

399

neuronal cell death in rat dorsal root ganglion: a stereological study. Acta Neuropathol. 2003,

400

105, 351-357.

Goldsmid, H. J. Bismuth telluride and its alloys as materials for thermoelectric generation.

Nordberg, G. F.; Fowler, B. A.; Nordberg, M.; Friberg, L. T. Handbook on the Toxicology

Shie, M. D.; Deeds, F. E. The importance of tellurium as a health hazard in industry. A

Hongzhe, Sun (ed.) Biological chemistry of arsenic, antimony and bismuth, Wiley,

Filella, M. How reliable are environmental data on ‘orphan’ elements? The case of

Liman, R. Genotoxic effects of bismuth (III) oxide nanoparticles by Allium and Comet

Murata, T. Effects of bismuth contamination on the growth and activity of soil

Pamphlett, R., Stoltenberg, M., Rungby, J., Danscher G. Uptake of bismuth in motor

Stoltenberg, M., Schonning, J.D., West, M.J., Danscher, G. (2003) Bismuth induced


Page 19 of 29


Lide, R.D., Ed. Handbook of Chemistry and Physics, 77th ed.; CRC Press: Boca Raton,

401

(19)

402

1996.

403

(20)

404

tellurium. In Proceedings of the 21st European photovoltaic solar energy conference. Dresden,

405

Germany 2006; pp 4–8.

406

(21)

407

Geochim. Cosmochim. Acta 2012, 79, 127–139.

408

(22)

409

1974, 12, 241–261.

410

(23)

411

Hydrogen ion buffers for biological research. Biochemistry 1966, 5, 467–477.

412

(24)

413

thermodynamic analysis. Hydrometallurgy 1998, 48, 327–342.

414

(25)

415

Biotechnology and Bioengineering 1978, 20, 145–150.

416

(26)

417

at 80°C and pH 3: the dependence on solution saturation state. Am. J. Sci. 1991, 291, 649–686.

418

(27)

419

and water samples by stripping voltammetry. Electroanalysis 1991, 3, 793–797.

420

(28)

421

Am. Chem. Soc. 1938, 60(2), 309–319.

422

(29)

423

copper(I) telluride. Talanta 1969, 16, 452-455.

Fthenakis, V.; et al. Recycling of CdTe photovoltaic modules: recovery of cadmium and

Biver, M.; Shotyk, W. Stibnite (Sb2S3) oxidative dissolution kinetics from pH 1 to 11.

Johansson, L. The role of the perchlorate ion as ligand in solution. Coord. Chem. Rev.

Good, N.E.; Winget, G.D.; Winter, W.; Connolly, T.N.; Izawa, S.; Singh, R.M.M.

Tromans, D. Temperature and pressure dependent solubility of oxygen in water: a

Schumpe, A.; Adler, I.; Deckwer, W.-D. Solubility of oxygen in electrolyte solutions.

Nagy, K. L.; Blum, A. E.; Lasaga, A. C. Dissolution and precipitation kinetics of kaolinite

Postupolski, A.; Golimowski, J. Trace determination of antimony and bismuth in snow

Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of gases in multimolecular layers. J.

White, W. W.; Sabo, J. G. EDTA titration of tellurium, based on the formation of



Page 20 of 29

424

(30)

Okkenhaug, G. Environmental risks regarding the use and final disposal of CdTe PV

425

modules; Document Nr 20092155-00-5-R; Norwegian Geotechnical Institute (NGI): Oslo, 2010.

426

(31)

427

reaction mechanism. Geochim. Cosmochim. Acta 2003, 67, 873-880.

428

(32)

429

Sci. 1978, 278, 1235–1252.

430

(33)

431

2015, 61, 99–108.

432

(34)

Rimstidt, J.D. personal communication 2016.

433

(35)

Chiriţă, P.; Rimstidt, J.D. Pyrrhotite dissolution in acidic media. Appl. Geochem. 2014,

434

41, 1-10.

435

(36)

436

Calorimetry 2007, 88(3), 751-755.

437

(37)

438

1998.

439

(38)

440

material ? Geochim. Cosmochim. Acta 2012, 98, 177–185.

441

(39)

442

Geochem. 2013, 37, 94-101.

Rimstidt, J.D.; Vaughan, D.J. Pyrite oxidation. A state-of-the-art assessment of the

Berner, R. A. Rate control of mineral dissolution under Earth surface conditions. Am. J.

Rimstidt, J.D. Diffusion control of quartz and forsterite dissolution rates. Appl. Geochem.

J. Norwisz, J.; Musielak, T. Compensation law again. Journal of Thermal Analysis and

Lasaga, A. Kinetic Theory in the Earth Sciences; Princeton University Press: Princeton,

Fischer, C.; Arvidson, R.S.; Lüttge, A. How predictable are dissolution rates of crystalline

Schreiber, M.E.; Rimstidt, J.D. Trace element source terms for mineral dissolution, Appl.


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Figure captions

444 445

Figure 1. SEM micrographs of unreacted Au-coated CdTe sample: ×190 (top left), ×3000 (top

446

right), ×20000 (bottom left), ×50000 (bottom right).

447

Figure 2. SEM micrographs of Au-coated CdTe after dissolution experiments: ×190 (top left),

448

×2200 (top right), ×50000 (bottom left), ×20000 (bottom right).

449

Figure 3. SEM micrographs of uncoated Au-coated Bi2Te3: ×190 (left), ×3000 (right).

450

Figure 4. 1st order / pseudo-1st order oxidation kinetics of Te(II/0) in aqueous solution at pH 7 at

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ambient temperature. Filled circles represent the measured concentrations of Te(IV); diamonds

452

stand for the natural logarithm of the ratio of the calculated residual concentration of Te in a

453

lower oxidation state (-II or 0) to the initial (reduced) Te concentration.

454

Figure 5. pH dependence of the CdTe and Bi2Te3 dissolution rates at 25°C under atmospheric

455

air.

456

Figure 6. Arrhenius plots for CdTe dissolution at pH 3, 7, and 11 between 25 and 70°C. At pH 3

457

and pH 5, the inclusion of the data at 70°C would lead to a substantial loss of correlation (from

458

R2>0.9906 to R2>0.9403 and R2>0.9983 to R2>0.5924). This is further discussed in the text.

459

Figure 7. Arrhenius plots for Bi2Te3 dissolution at pH 3, 5.3 and 11 between 25 and 70°C.



Figure 1. SEM micrographs of unreacted Au-coated CdTe sample: ×190 (top left), ×3000 (top right), ×20000 (bottom left), ×50000 (bottom right). 683x550mm (96 x 96 DPI)

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Page 23 of 29


Figure 2. SEM micrographs of Au-coated CdTe after dissolution experiments: ×190 (top left), ×2200 (top right), ×50000 (bottom left), ×20000 (bottom right). 338x280mm (96 x 96 DPI)



Figure 3. SEM micrographs of uncoated Au-coated Bi2Te3: ×190 (left), ×3000 (right). 683x274mm (96 x 96 DPI)


Page 24 of 29

Page 25 of 29


0

900

-0.5

800

-1

700

-1.5

600

-2

500

-2.5

400

-3

300

-3.5

200

-4

100

-4.5

0 0

500

1000

1500

2000 2500 time / min

3000


3500

4000

-5 4500

ln[Te(-II/0)]/[Te(-II/0)]0

[Te(IV)] / µg L-1

1000


Page 26 of 29

-6.0 CdTe 55°C

log10(r / mol m-2 s-1)

-7.0 -8.0

CdTe 25°C

-9.0 Bi2Te3 25°C

-10.0 -11.0 -12.0 1

2

3

4

5

6 pH

7


8

9

10

11

Page 27 of 29


-15 55°C pH 3 70°C

ln(r / mol m-2 s-1)

-16

pH 3

-17

40°C pH 7

-18

-19

-20 0.00285

pH 11

pH 11 70°C

0.00295

0.00305

0.00315 T-1 / K-1


25°C

0.00325

0.00335


Page 28 of 29

-18 -19 pH 11

ln(r / mol m-2 s-1 )

-20

pH 3

-21 -22 -23 -24

pH 5.3

-25 -26 0.00285

55°C

70°C 0.00295

0.00305

40°C T-1

0.00315 / K-1


25°C 0.00325

0.00335

Page 29 of 29



Bulk Dissolution Rates of Cadmium and Bismuth Tellurides As a

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