Specific adsorption of arsenic and humic acid on Pt and PtO films

Electrochimica Acta 55 (2010) 4942–4951

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Specific adsorption of arsenic and humic acid on Pt and PtO films Hebert A. Menezes, Gilberto Maia ∗,1 Department of Chemistry, Universidade Federal de Mato Grosso do Sul, Av. Filinto Muller, 1555, Cidade Universitária s/n, C.P. 549, Campo Grande, MS 79070-900, Brazil

a r t i c l e

i n f o

Article history: Received 20 November 2009 Received in revised form 29 March 2010 Accepted 30 March 2010 Available online 8 April 2010 Keywords: Cyclic voltammetry Cyclic massogram As Humic acid Specific adsorption

a b s t r a c t A study of specific adsorption of arsenic (As) and humic acid (HM) onto Pt and PtO films using cyclic voltammetry and cyclic massogram in 0.5-M H2 SO4 is presented, which may serve as an alternative to studies involving specific adsorption of these species on soil minerals. Adsorption of As is normally evaluated by conducting batch adsorption experiments, followed by analysis using hydride-generation atomic absorption spectrophotometry (HGAA) or inductively coupled plasma-optical emission spectrometry (ICP-OES). We found that specific adsorption of As and HM depends both on the surface and on these species present in the adsorption solution. HM does not desorb previously adsorbed As at the HM concentrations used in the present study, but it does co-adsorb with As from a 1 × 10−6 -M aqueous solution of As2 O3 containing 1 mg of carbon L−1 HM. Arsenic adsorbs strongly on Pt in the presence of HM or during sequential specific adsorption with HM. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Arsenic, a common element in nature, is a naturally occurring contaminant of drinking water and can be found in the Earth’s crust in soil and seawater, as well as in the organic world [1]. Occurring in natural waters in a variety of manners, including soluble, particulate, and organic-bound forms, but mainly in inorganic trivalent As(III) and pentavalent As(V) oxidation states [2], arsenic is a toxic element with detrimental effects on humans, plants and animals [1]. Diseases such as gastroenteritis and lung, skin, and bladder cancer can be caused by contact with As in aqueous or other media [3]. Wastewaters from activities such as metallurgy, mining, chemical pharmacy, chemical and pesticide production, and leather tanning are often polluted with As. Despite its four stable oxidation states (+3, +5, −3, and 0), it occurs in aqueous solutions as As(III) and As(V) in the form of arsenites (AsO3 3− ) and arsenates (AsO4 3− ), respectively [3]. In near-neutral pH waters, arsenite is present primarily as uncharged arsenous acid (As(OH)3 ), whereas arsenate occurs predominantly in anionic form [4]. Humic substances (HS), which are major components of natural organic matter (NOM), are some of the most abundant materials on Earth. They are formed during the decomposition of plant and animal biomass in natural systems and usually comprise a skeleton of alkyl and aromatic units with functional groups such as carboxylic acid, phenolic hydroxyl, and quinone groups attached

∗ Corresponding author. Tel.: +55 67 3345 3551; fax: +55 67 3345 3552. E-mail address: [email protected] (G. Maia). 1 ISE member. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.03.099

[5,6]. The presence of HS in natural waters may compete with target pollutants (including As) and diminish their removal efficiency by reducing their adsorption rates and equilibrium capacities [5]. In natural waters, HS occur in the range of a few mg of carbon L−1 to a few hundred mg of carbon L−1 [6]. Dissolved humic substances (DHS) are composed of a mixture of humic and fulvic acids (HM and FV) of different molecular weights [7]. HM can complex metals and metalloids by its oxygen-containing functional groups, and further may adsorb onto mineral particle surfaces. These modified surface sites often control the fate and transport of trace contaminants in both aquatic and terrestrial environments [8]. Many studies have reported the adsorption of As on different oxides in the presence or absence of HM. Bauer and Blodau [9], for instance, found that sorption of dissolved organic matter (DOM) has the potential to mobilize As from iron oxides, soils and sediments. Liu et al. [10], investigating the effect of NOM on As adsorption onto commercial TiO2 in a range of simulated As(III)-contaminated raw waters, reported that NOM decreased As adsorption within the tested pH range of 4.0–9.4 and suggested that NOM is an important factor controlling As speciation and adsorption onto TiO2 surfaces. Ko et al. [8], in studies evaluating the influence of contact order on speciation during As adsorption in equilibrated ternary systems consisting of As, HM, and hematite, found that overall As adsorption decreased in the presence of soil HM, unlike the behavior noted for cationic metal adsorption. The low affinity of HM for complexation with As leads to decreased adsorption [8]. Habuda-Stanic´ et al. [11] investigated As removal from drinking water using prepared adsorbents to coat the surface of two polymeric materials (natural alum silicate exchanger zeolite–clinoptilolite and an ionic exchanger resin modified with hydrous ferric oxide). They found

H.A. Menezes, G. Maia / Electrochimica Acta 55 (2010) 4942–4951

that adsorption of As(III) ions increased with higher pH and contact time when the modified ionic exchanger resin was used as an adsorbent. Increasing the initial concentration, however, had no significant effect on As(III) ion adsorption [11]. The procedure commonly employed to study adsorption and desorption of As by oxides involves carrying out batch experiments with bulk samples and quantifying the amounts of As by hydride-generation atomic absorption spectrophotometry (HGAA) [5] or inductively coupled plasma-optical emission spectrometry (ICP-OES) [10]. Previous studies have described interactions of As with different metallic oxides as well as the influence of NOM, DOM, or HM on its adsorption and desorption. To the best of our knowledge, however, no reports have been published describing the influence of HM on the adsorption of As onto Pt or PtO films. The present paper describes our investigation of these aspects using cyclic voltammetric and cyclic massogram (CM) experiments in 0.5-M H2 SO4 . This type of study serves as an alternative model to experiments involving oxide samples and can enhance understanding of factors that influence As adsorption/desorption.

2. Experimental The experimental conditions and apparatuses are the same as in our previously published study [12], with few additional modifications, as described below. Electrochemical and microgravimetric studies were carried out in a GC-15 three-electrode glass cell that included a CHC-15 crystal holder, clamp, and stopper (Maxtek). A 5-MHz AT-cut quartz crystal (25.4-mm diameter) vertically positioned in front of the counterelectrode acted as the working electrode (polycrystalline Pt), both sides of which were coated with Pt sputtered on a Ti layer in a keyhole pattern (geometric area in contact with solution = 1.37 cm2 ) (Maxtek). A reversible hydrogen electrode (RHE) was employed as the reference electrode. The ratio-of-roughness factor of the working electrode in the present paper (see [12] for details of this estimate) was found to be 2.9 ± 0.5 using the relation 2.10 C m−2 for the formation of a H monolayer on a Pt surface [13]. The Sauerbrey equation (m = –f/Cf [14]) was applied to relate mass change and resonance frequency shift, as described in [12]. As2 O3 was obtained from Aldrich (primary standard), HM from Aldrich (technical grade), and H2 SO4 from Merck (p.a.). The solutions were prepared with Milli-Q water and purged for 20 min with ultrapure nitrogen (White Martins) before each experiment. A 1 × 10−2 -M As2 O3 stock solution was prepared by dissolving the corresponding mass in 1 mL of 0.1-M NaOH and adding 5 mL of water, sonicating for 1 min, and completing the volume with water until the final stock solution concentration was attained. The specific adsorption procedure consisted in immersing a cleaned Pt or PtO film electrode for 20 min in eight different freshly prepared aqueous solutions (1 mg of carbon L−1 HM (S1); 1 × 10−4 -M As2 O3 (S2); 1 mg of carbon L−1 HM (S1)—copious washing with water—1 × 10−4 -M As2 O3 (S2); 1 × 10−4 -M As2 O3 (S2)—copious washing with water—1 mg of carbon L−1 HM (S1); 1 × 10−6 -M As2 O3 and 1 mg of carbon L−1 HM (S3); 5 × 10−6 -M As2 O3 and 1 mg of carbon L−1 HM (S4); 1 × 10−5 -M As2 O3 and 1 mg of carbon L−1 HM (S5); 5 × 10−5 -M As2 O3 and 1 mg of carbon L−1 HM (S6); 1 × 10−4 -M As2 O3 and 1 mg of carbon L−1 HM (S7); 5 × 10−4 -M As2 O3 and 1 mg of carbon L−1 HM (S8)). All solutions had pH values around 6.4 at 25 ◦ C. The solution was continuously stirred during specific adsorption to facilitate mass transfer. The open-circuit potential (Eoc ) of the different aqueous solutions in contact with the Pt or PtO film was found to be approximately 0.9 V. After immersion, the electrode was copiously washed with water and then

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Fig. 1. 1st CVs for (a) pristine Pt (), (b) Pt/HM (), and (c) Pt/As/HM () in 0.5-M H2 SO4 . The modified electrode surfaces were washed with water (5 times) before CV acquisition. Scans start at 0.05 V vs. RHE in the positive potential direction.  = 100 mV s−1 .

transferred to the electrochemical cell containing 0.5-M H2 SO4 . In accordance with the above sequence of immersion solutions, the modified Pt (or PtO) electrodes will be designated as: Pt/HM, Pt/As, Pt/HM/As, Pt/As/HM, Pt/M1, Pt/M2, Pt/M3, Pt/M4, Pt/M5, and Pt/M6 (or, in the case of their PtO counterparts, PtO/HM, PtO/As, PtO/HM/As, etc.). All experiments (immersion, electrochemical, and microgravimetric) were conducted in a nitrogen atmosphere. In order to maximize reproducibility, the immersion, electrochemical, and microgravimetric experiments were conducted at least in duplicate. The PtO film was produced as described in [12] and the ratio-ofroughness factor for the PtO film electrode was assumed to be the same as that calculated for the Pt deposited on the quartz crystal electrode. 3. Results and discussion 3.1. Cyclic voltammetry of HM and As specifically adsorbed on Pt Figs. 1 and 2 show the behavior of the first voltammetric cycle in 0.5-M H2 SO4 for pristine Pt and the most representative of 10 Pt electrodes modified by specific adsorption from eight different aqueous solutions (see Section 2). These three CV regions were previously described in [12] (Fig. 1), and their differences in the present study occurred in region II, additionally involving oxidation of adsorbed As atoms, and in region III, also involving additional oxi-

Fig. 2. 1st CVs for (a) Pt/M1 (), (b) Pt/M2 (), and (c) Pt/M4 () in 0.5-M H2 SO4 . The modified electrode surfaces were washed with water (5 times) before CV acquisition. Scans start at 0.05 V vs. RHE in the positive potential direction.  = 100 mV s−1 .

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dation of adsorbed As atoms and subsequent reduction of desorbed As atoms in the solution, in the negative potential scan direction. The decrease in current densities observed in region I relative to pristine Pt (Figs. 1 and 2) suggests specific adsorption of HM and/or As on active Pt sites for hydrogen desorption/adsorption. The increase in current densities observed in region II relative to pristine Pt results in an oxidation peak at 0.69 V, suggesting that the atoms adsorbed onto the Pt surface are oxidized in this region and that this oxidation peak is mainly affected in the Pt/M1–Pt/M6 electrodes (some of these electrodes are not shown in Fig. 2). No increase in current densities is observed for the Pt/HM electrode (curve b, Fig. 1). In region III, akin to region II, an oxidation peak is visible at 1.15 V (not present for pristine Pt), although not greatly altered for the Pt/As (not shown), Pt/HM/As (not shown), Pt/As/HM, and Pt/M2–Pt/M6 electrodes (Figs. 1 and 2). This oxidation peak is not detected for the Pt/HM and Pt/M1 electrodes, but increased current densities were detected after 1.09 V, compared to those for pristine Pt (curve b, Fig. 1, and curve a, Fig. 2). This suggests the oxidation of adsorbed As and/or HM at the Pt surface in this region. In the negative potential scan direction, a decrease is observed in current densities for the reduction peak at 0.77 V for the Pt/HM and Pt/M1 electrodes (curve b, Fig. 1, and curve a, Fig. 2) and the presence of a small reduction peak at 0.57 V for the Pt/As (not shown), Pt/HM/As (not shown), Pt/As/HM, and Pt/M2–Pt/M6 electrodes (see curve c, Fig. 1, and curves b and c, Fig. 2). The first peak is attributed to reduction of Pt oxide (PtOxads ) and the second to reduction of desorbed As atoms. In general terms, the values of current densities involving UPD hydrogen desorption/adsorption in region I follow the patterns pristine Pt > Pt/HM > Pt/HM/As > Pt/As > Pt/As/HM and pristine Pt > Pt/M1 > Pt/M2 > Pt/M5 = Pt/M3 > Pt/M6 > Pt/M4. In region II, the current densities involving oxidation of adsorbed As follow the patterns pristine Pt = Pt/HM < Pt/HM/As < Pt/As < Pt/As/HM and pristine Pt < Pt/M1 < Pt/M2 < Pt/M3 < Pt/M5 < Pt/M6 < Pt/M4. In region III, the current densities involving oxidation at 1.15 V follow the patterns pristine Pt < Pt/HM < Pt/HM/As = Pt/As < Pt/As/HM and pristine Pt < Pt/M1 < Pt/M2 < Pt/M5 < Pt/M3 < Pt/M6 < Pt/M4, while for those involving reduction at 0.57 V the patterns are pristine Pt = Pt/HM < Pt/HM/As = Pt/As < Pt/As/HM and pristine Pt < Pt/M1 < Pt/M2 < Pt/M3 = Pt/M5 = Pt/M6 < Pt/M4. These behaviors of current densities clearly reveal co-adsorption of As and HM in the specific adsorption experiments (see Section 3.3). We have recently proposed a mechanism describing the adsorption and oxidation of HM on Pt [12] and it should be stressed that the oxidation products of HM may be adsorbed again onto the Pt surface. In the case of As, we hypothesize that adsorption initially involves arsenous acid (As(OH)3 ), also referred to as arsenite [1,4]—whose oxidation state in the samples was not confirmed in the laboratory—which is formed from As2 O3 solubilized in aqueous solutions of near-neutral pH that were used for specific adsorption of As onto the Pt surface. Arsenite is thought to undergo reduction at Eoc , based on the measured amounts of deposited As atoms as a function of deposition potential, as calculated by Furuya and Motoo (Fig. 7 of their paper) [15], and based on the diagnosis by Cabelka et al. [16] that peaks A and B (of their paper) are not obtained when the cathodic (i.e., negative) scan limit (Ec ) is ≥0.0 V and peak B is concluded to correspond to the oxidation of As(0) ad-atoms. It should be noted that our peak at 0.69 V (oxidation peak) is coincident with the potential of peak B in the study by Cabelka et al. [16]. Arsenite is also thought to undergo reduction at Eoc , according to Shibata et al. [17], who obtained voltammograms after holding at a potential of 0.6 V in a sulfuric acid solution containing As ions. In their study [17], the anodic currents occurring at the potentials of the doublelayer charging region and at the Pt oxide region on the Pt electrode having an arsenic ad-atom coverage of 0.48 exhibited behaviors

similar to those seen in our Figs. 1 and 2, suggesting (also in our case) that As atoms (As ad-atoms) are deposited as follows: As(OH)3 + 3e + S · Pt(subs) + 3H+ → As–PtS (subs) + 3H2 O

(1)

where As–PtS (subs) designates Pt having arsenic ad-atoms and S is the number of Pt sites occupied by these ad-atoms [15]. Furuya and Motoo found the S value to be 2.5 [15]. These As ad-atoms are most effectively oxidized to As(III) [16] at 0.69 V (oxidation peak) (Figs. 1 and 2), according to the reaction: AsPtS (subs) + 3HSO4 − (or 1.5SO4 2− )

→ (HSO4 )3 Asads Pt(subs) + 3e or → (SO4 )3 (2As)ads Pt(subs) + 6e

(2)

In this equation (which is based on the finding by Shibata et al. [17] that deposited arsenic is oxidized to As(III) as (As2n )ads O3n complexes on Pt and Au surfaces), Asads are adsorbed arsenic atoms, or ad-atoms [18]. In the present study we are assuming the formation of (Asn )ads (HSO4 )3n and/or (As2n )ads (SO4 )3n complexes with n ≥ 1. This proposition will be explained in Section 3.2. The resulting (Asn )ads (HSO4 )3n and/or (As2n )ads (SO4 )3n complexes adsorbed onto the Pt surface are oxidized to As(V) [16] at 1.15 V (oxidation peak): (As)ads (HSO4 )3 → (HSO4 )As(SO4 )2 + 2e + 2H+

(3)

Furuya and Motoo [15] proposed the dissolution of arsenic atoms deposited on a Pt surface to occur as: As–PtS (subs) + 3H2 O → HAsO3 + 5H+ + S · Pt(subs) + 5e

(4)

Another possibility, suggested by Shibata et al. [17], is that the subsequent oxidation/hydrolysis/desorption of (As2n )ads O3n complexes will produce AsO4 3− [17,18]. Cabelka et al. [16] proposed that As(V) is deposited as As(0) at underpotential in the same region where we observed a reduction peak at 0.57 V (Figs. 1 and 2). The cyclic voltammetric results shown in Figs. 1 and 2 were quantitatively analyzed (Table 1) as done in our previous study [12]. The charge densities for oxidative desorption of hydrogen (QH des ) were calculated by integrating the anodic current densities vs. time for region I. From the values of QH des , the fraction  H I of active sites occupied by hydrogen was calculated using the equation: I

H =

QH des

(5)

QH blank

where QH blank = 161 ␮C cm−2 is the value obtained for pristine Pt. The fraction of hydrogen adsorption sites blocked by specifically adsorbed HM and As was calculated using the equation: I

ads comp/atom =

QH blank − QH des

(6)

QH blank

Table 1 Interfacial fractional coverage estimated from the cyclic voltammograms in Figs. 1 and 2. Interface

H I

 ads comp/atom I

 ads As

Pristine Pt Pt/HM Pt/As Pt/HM/As Pt/As/HM Pt/M1 Pt/M2 Pt/M3 Pt/M4 Pt/M5 Pt/M6

1.00 0.70 0.43 0.48 0.42 0.56 0.50 0.39 0.25 0.37 0.31

0 0.30 0.57 0.52 0.58 0.44 0.50 0.61 0.75 0.63 0.69

0 a

0.38 0.42 0.53 0.12a 0.43 0.62 0.88 0.57 0.63

a  ads comp III for HM was calculated as previously described [12], yielding the values 0.14 and 0.24 for Pt/HM and Pt/M1, respectively.

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Arsenic coverage ( ads As ) on pristine Pt was determined by the method described previously [15,17]. QAs ox was calculated by integrating the anodic current densities vs. time for regions II and III for electrodes modified by spontaneous adsorption involving As, subtracted from the integration of anodic current densities vs. time for region III in pristine Pt: ads As =

QAs ox 2QH blank

(7)

In this equation, factor 2 reflects the fact that 2.5 active Pt sites are occupied by one As ad-atom [15]. If each active Pt site is occupied by one Hads whose desorption involves one electron and if the oxidation of one As ad-atom (As(0)) to yield As(V) involves five electrons (Eq. (4)), then QAs ox /2QH blank will result in coverage by As ad-atoms bonded to every 2.5 active Pt sites.  ads comp/atom I varies with the compound and atom specifically adsorbed (Table 1), rising overall for the Pt/M1–Pt/M4 electrodes. At lower positive potentials where UPD hydrogen desorption occurs (see data in Table 1), we calculated, based on interfacial fractional coverage ( ads comp/atom I ) values, that 30% and 57% of the sites for hydrogen adsorption are blocked by specifically adsorbed HM and As, respectively, in the case of Pt/HM and Pt/As electrodes. For the Pt/HM/As and Pt/As/HM electrodes, we assumed that specifically adsorbed As predominates (52% and 58%, respectively), since the  ads comp/atom I values are similar to those obtained for the Pt/As electrode, HM has a lower value for the Pt/HM electrode, and no evidence is available of blockage by HM, as shown by the absence of decreased current densities for Pt oxide reduction at 0.77 V [12] (compare curve c with b, Fig. 1). In the case of the Pt/M1 electrode, 30% and 14% of the sites were blocked, respectively, by the specific adsorptions (co-adsorption) of HM and As. We assumed that the value for HM (30% of interfacial fractional coverage,  ads comp/atom I ) in this instance should be similar to that obtained for the Pt/HM electrode, with the difference in  ads comp/atom I being attributed to interfacial fractional coverage of As. Clearly, we assumed that  ads comp/atom I values are added for the Pt/M1 electrode. In this case, there is evidence of blockage by HM, as shown by the decrease in current densities for Pt oxide reduction at 0.77 V [12] (compare curve a, Fig. 2, with curve a, Fig. 1). For the Pt/M2–Pt/M6 electrodes, co-adsorption of HM could be considered negligible. This behavior was observed during co-adsorption from aqueous solutions of As2 O3 containing HM (see Section 3.3). The highest blockage of sites for hydrogen adsorption by specifically adsorbed As (75% of interfacial fractional coverage,  ads comp/atom I ) occurred on a Pt/M4 electrode. Blockage of active sites by specifically adsorbed HM at high positive potentials where oxide formation occurs (PtOx formation in region III) was similar to that found in our previous study [12], reaching 14% and 24%, respectively, for the Pt/HM and Pt/M1 electrodes (Table 1). A noteworthy feature is that the coverage of As specifically adsorbed on Pt ( ads As ) is comparable in magnitude to the blockage of sites for hydrogen adsorption ( ads comp/atom I ) (see Table 1) and does not block PtOx formation in region III, since the current densities are not modified during Pt oxide reduction at 0.77 V, relative to pristine Pt (see Figs. 1 and 2)—with the obvious exception of the Pt/HM and Pt/M1 electrodes. The description below is based on the interfacial fractional coverage of As ( ads As ) and follows the same reasoning expounded in the previous paragraph. On the Pt/As/HM electrode, specifically adsorbed As covers 53% of the Pt surface—much more than the coverage achieved on the Pt/As electrode (38%). In the case of the Pt/M1 electrode, specifically adsorbed As covers as little as 12% (co-adsorbed with 24% of HM) of the Pt surface, but on the Pt/M4 electrode As coverage reaches as much as 88% (without co-adsorption with HM) of the Pt surface.

Fig. 3. 1st CMs for (a) pristine Pt (), (b) Pt/HM (), and (c) Pt/As () in 0.5-M H2 SO4 . The modified electrode surfaces were washed with water (5 times) before CM acquisition. Scans start at 0.05 V vs. RHE in the positive potential direction.  = 100 mV s−1 .

3.2. Cyclic massogram study of HM and As specifically adsorbed on Pt Figs. 3 and 4 illustrate the behavior of the first CM in 0.5-M H2 SO4 for pristine Pt and the most representative of 10 Pt electrodes modified by specific adsorption from eight different aqueous solutions (see Section 2). Mass changes were taken as zero at the initial potentials. These three CM regions (Fig. 3) were previously described in [12], with the following differences in the present study. For pristine Pt, mass change approaches 18 ng cm−2 at 0.36 V (positive potential scan direction) and it approaches 22 and 6 ng cm−2 at 0.35 and 0.05 V (negative potential scan direction), respectively, in region I. Mass change approaches 15 ng cm−2 (mass difference between the potentials of 0.9 and 0.36 V, positive potential scan direction) and −9 ng cm−2 (mass difference between the potentials of 0.35 and 0.66 V, negative potential scan direction) in region II. In region III, it approaches 30 ng cm−2 between 0.89 and 1.4 V (positive potential scan direction). On the Pt/HM electrode, mass changes are overall decreased, compared with pristine Pt (curve b, Fig. 3). On the Pt/As electrode, mass changes are overall increased relative to pristine Pt (curve c, Fig. 3). For the other situations (Pt/HM/As and Pt/As/HM electrodes, not shown), mass changes are overall increased to 1.05 V (on average) in the positive potential scan direction, but decrease throughout the rest of the potential scan for CM, relative to pristine

Fig. 4. 1st CMs for (a) Pt/M4 (), (b) Pt/M5 (), and (c) Pt/M6 () in 0.5-M H2 SO4 . The modified electrode surfaces were washed with water (5 times) before CM acquisition. Scans start at 0.05 V vs. RHE in the positive potential direction.  = 100 mV s−1 .

17.10 31.15 11.82 18.60 20.54 21.30 20.34 19.93 21.05 19.52 20.00 35.40 31.48 10.53 15.32 16.65 11.02 11.72 17.35 11.46 14.02 15.8 9.77 −1.49 −1.01 −1.05 −1.63 −0.20 −0.91 −1.63 −1.82 −0.58 −1.63 8.96 9.78 7.14 7.96 4.50 8.20 6.19 6.52 4.26 4.88 4.25 20.03 13.06 30.52 29.1 31.76 26.82 34.23 39.83 24.27 24.67 19.02 7.58 6.09 10.80 10.64 14.64 4.78 10.62 11.47 – 14.13 – Pristine Pt Pt/HM Pt/As Pt/HM/As Pt/As/HM Pt/M1 Pt/M2 Pt/M3 Pt/M4 Pt/M5 Pt/M6

Region II (loss of HSO4 − ions or HSO4 − and/or SO4 ions) Region I (detection of desorption of specifically adsorbed HM and/or subsequently desorbed As; exception: pristine Pt) Region III (adsorption of oxygen forming PtOx) Region II (adsorption of HSO4 − ions or HSO4 − and/or SO4 ions) Region I (substitution of adsorbed UPD hydrogen by water molecules)

m/q × (Fn) = M (g mol−1 ), negative potential scan direction (m/q) × (Fn) = M (g mol−1 ), positive potential scan direction Interface

Pt. In the negative potential scan direction and in regions I and II, decreased mass changes are evident in relation to those occurring in the positive potential scan direction (see curve b, Fig. 3). Most probably, irreversible processes occur for the species adsorbed in the positive potential scan direction (fast species desorption), or these processes can be attributed to the presence of a specifically adsorbed compound and/or atom that are desorbed—desorptions that are more easily detected during CMs. (Note some degree of negative mass change at the end of CM on curve b, Fig. 3.) The behavior of mass change (not shown) on the Pt/M1 electrode is similar to that occurring on the Pt/HM electrode (curve b, Fig. 3). Regarding the Pt/M2 and Pt/M3 electrodes, mass change behaviors (not shown) are similar, for example, to the CM behavior of the Pt/HM/As electrode in 0.5-M H2 SO4 . On the Pt/M4 or Pt/M5 electrodes (curves a and b, Fig. 4) an increase in mass changes (positive potential scan direction) is observed for regions I and II, relative to the mass changes on pristine Pt (Fig. 3). On the Pt/M6 electrode (curve c, Fig. 4), a decrease occurs in mass changes in regions I–III in the positive and negative potential scan directions, relative to the situation occurring on the Pt/M5 electrode (curve b, Fig. 4). We performed quantitative identification of anions and solvated anions adsorbed onto the Pt surface, as done previously [12], in which case M (Table 2) was determined from the linear relation (slope) m vs. q in regions I–III depicted in Figs. 1 and 3. In region I, for pristine Pt, we calculated an M value of 7.58 g mol−1 that corresponds to a 0.42 monolayer of water. The decrease in M corresponds to the change from a 0.42 monolayer of H2 O on pristine Pt to a 0.34 monolayer of H2 O on the Pt/HM electrode and to a 0.27 monolayer of H2 O on the Pt/M1 electrode (Table 2). In the other cases (Table 2), an increase in M is detected, reaching values as high as 0.81 monolayer of H2 O on the Pt/As/HM electrode. (The increase in M is also noticeable for the Pt/M1–Pt/M5 electrodes.) Adsorption of HSO4 − ions, in region II is assumed to be associated with mass change increases on the pristine Pt and Pt/HM electrodes [12]. The results reported in Table 2 correspond to values of 0.21 HSO4 − (M = 20.03 g mol−1 ) and 0.13 HSO4 − (M = 13.06 g mol−1 ), respectively. In the other cases (Table 2), however, we assumed the mass change increase to be associated with adsorption of HSO4 − and/or SO4 ions on the As atoms adsorbed onto the Pt surface (in the case of SO4 , M in Table 2 should be multiplied by 2, whereas n = 2). We are not assuming these ions as solvated, because the M values found are smaller than the M values of HSO4 − or SO4 . Also, we are not assuming oxygen-species adsorption (OH or O) on the Pt electrode modified with As ad-atoms, as proposed by Shibata et al. [17], because the M values found are higher than 17 or 16 g mol−1 (n = 2), respectively. The value for the Pt/As electrode corresponds to 0.31 HSO4 − (M = 30.52 g mol−1 ) (Table 2). Similar values, of 0.30 HSO4 − (M = 29.10 g mol−1 ) and 0.33 SO4 (M = 31.76 g mol−1 ) (Table 2) were obtained for the Pt/HM/As and Pt/As/HM electrodes, respectively. M reaches a value of 39.83 g mol−1 (0.41 HSO4 − ) for the Pt/M3 electrode (Table 2). For the Pt/M5 and Pt/M6 electrodes, M values were decreased, reaching 19.02 g mol−1 (0.20 HSO4 − ) for the Pt/M6 electrode. The M value of 8.96 g mol−1 (n = 2) (Table 2), obtained for pristine Pt in region III, is attributed to the oxidation of Pt as Pt + H2 O ↔ PtOxads + 2H+ + 2e− in the positive potential scan direction [12]. M values were lower for atoms specifically adsorbed on Pt and a lower M value, of 4.25 g mol−1 (Table 2), was found for the Pt/M6 electrode. These results suggest smaller formation of PtOx or loss of adsorbed As by oxidation when this element is present in the aqueous solutions used during specific adsorption M values were significantly lower in the negative potential scan direction in region II—with the obvious exception of the pristine Pt and Pt/HM electrodes—and, more perceptibly, in region I (negative M), compared with the M values obtained in the same regions in the positive potential scan direction. M values in region III exhibited

Region III (loss of oxygen after PtOx reduction and loss of HSO4 − and/or SO4 ions)

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Table 2 Interfacial molar weight changes in regions I–III, calculated from the cyclic voltammograms in Figs. 1 and 2 and the cyclic massograms in Figs. 3 and 4.

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Table 3 Surface coverage estimated from the cyclic voltammograms (potential scan from 0.05 to 0.38 V vs. RHE) in Figs. 1 and 2 and mass changes corrected according to the mass changes at 0.35 V in the positive potential scan direction in Figs. 3 and 4. Interface

m0.35 V * (ng cm−2 )

Pristine Pt Pt/HM Pt/As Pt/HM/As Pt/As/HM Pt/M1 Pt/M2 Pt/M3 Pt/M4 Pt/M5 Pt/M6

– 1.27 17.93 14.53 14.1 7.35 12.61 11.18 22.8 24.81 18.66

NHM or As 0.35 V (×10−12 molecules or atoms cm−2 ) HM

As

– 0.08 – – – 0.08 – – – – –

– – 144.07 116.75 113.30 48.85 101.32 89.83 183.20 199.35 149.94

the opposite behavior in the negative potential scan direction, as they increased in relation to those obtained in the positive potential scan direction. The occurrence of an irreversible overall process, during which CVs were combined with CMs, may account for these results. In region III, in the negative potential scan, loss of O after PtOx reduction and loss of HSO4 − and/or SO4 ions influence region II (resulting in lower M values). For HM and As specifically adsorbed (except for Pt/As electrode) on Pt, M values were higher in region III relative to pristine Pt in the negative potential scan direction (see Table 2), suggesting additional loss of HSO4 − and/or SO4 ions that occupied the space left by the desorption of specifically adsorbed HM and/or As oxidized in the positive potential scan direction, as proposed in Section 3.1. In region I, the difference can be attributed to desorption of specifically adsorbed HM and/or subsequently desorbed As. These desorptions were more easily detected when CV was combined with CM (negative M values in region I; see Table 2). In a previous study [12] we combined CV and CM measurements to estimate the amounts of compounds specifically adsorbed onto ˜ et al. [19]. We took the Pt surface, as suggested by González-Pena into account the mass changes at the end of region I (near 0.35 V), along with the variety of situations depicted in Figs. 3 and 4, to correct the mass change (m0.35 V * ) (Table 3) related to the change in water level associated with Hads replacement, which is achieved by subtracting the mass change value corresponding to the end I I of region I from the multiplication of H2 O = H by m0.35 V blank −2 (18 ng cm ): m0.35

V

∗

I

= m0.3 V − (H2 O × m0.35 V blank )

(8)

The number of molecules and/or atoms adsorbed per unit area at this potential region, NHM or As 0.35 V , can be estimated from the relation [12]: NHM or As 0.35 V = (m0.3 V ∗ )

NA MWcomp

(9)

where NA is the Avogadro number and MWcomp is the molecular weight of the specifically adsorbed HM (∼ =10,000 g mol−1 [20]) or the atomic weight of As (74.92 g mol−1 ). The results of this calculation are provided in Table 3. The molecular weight of HM was used for the Pt/HM and Pt/M1 electrodes, whereas the atomic weight of As was used for the other Pt modified electrodes. We assumed the occurrence of co-adsorption, thus adding the m0.35 V * values for HM and As shown in Table 3, according to the same reasoning described in the previous section for  ads comp/atom I . The number of As atoms specifically adsorbed onto the Pt surface increased overall for the Pt/M1–Pt/M6 electrodes, according to the calculated values shown in Table 3, the highest of which occurred on a Pt/M5 electrode. Assuming the hydrodynamic radius of the HM

Fig. 5. (A) Mass changes vs. time on a Pt electrode during specific adsorption from solutions (a) S1 (); (b) S2 (䊉); (c) S2 () after specific adsorption as per (a), followed by copious washing with water; and (d) S1 () after specific adsorption as per (b), followed by copious washing with water. (B) Mass changes vs. time on a Pt electrode during specific adsorption from solutions (a) S3 (), (b) S4 (䊉), (c) S5 (), (d) S6 (), (e) S7 (), and (f) S8 ().

molecule to be 7 nm [21] and the atomic radius of As to be 0.115 nm, it is possible to calculate that 0.08 × 1012 HM molecules (Table 3) will occupy 0.12 cm2 and that 199.35 × 1012 As atoms (Table 3) will occupy 0.083 cm2 , suggesting low coverage on the Pt surface both by HM and As. 3.3. Study of mass changes during specific adsorption of HM and As on Pt Fig. 5 shows the behavior of mass changes taking place on Pt during specific adsorption from eight different aqueous solutions. Although an asymptotic increase in mass with time was initially evident in all the specific adsorption situations, mass increase became less pronounced (Fig. 5A) or even decreased over time (some curves, Fig. 5B). The observed mass increase was similar to that previously obtained by our group for solution S1 [12] and was not much higher than for solution S2 during specific adsorption onto the Pt surface (compare curves a and b, Fig. 5A). The number of HM molecules specifically adsorbed onto the Pt surface was therefore smaller than that of As atoms, since the molecular weight of HM is much higher than the atomic weight of As. The experiments conducted in sequences of adsorption (curves c and d, Fig. 5A) revealed stronger adsorption of As after the first specific adsorption in solution S1, because copious washing of the electrode with water before the second specific adsorption allowed the final mass change to be recovered comparably to that of the first specific adsorption in solution S2 (curve b, Fig. 5A). These experiments also revealed stronger adsorption of As after the first specific adsorption in solution S2, since the final mass change of HM specifically adsorbed during the second specific adsorption in solution S1 was smaller than that obtained in solution S1 alone (curve a, Fig. 5A). Stronger competition involving As and HM specific adsorptions (co-adsorption) was evident when solutions S3–S8 were used (see Fig. 5B and particularly the larger oscillations in mass with time in curves b–d, Fig. 5B). The final mass during the specific adsorptions (co-adsorption) follow the pattern S7 < S8 < S3 < S4 = S5 < S6 (242 ng cm−2 ). The number of adsorbed As atoms must have been greater than that of adsorbed HM molecules, particularly when solutions S3–S6 were used. The small final mass changes occurred when solutions S7 and S8 were used (Fig. 5B), changes that can be

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attributed to adsorption of a greater number of As atoms than of HM molecules. The number of adsorbed HM molecules was considered constant for solutions S3–S6, whereas for S7 and S8 the number of adsorbed HM molecules was considered to decrease drastically (see curve d, Fig. 5A, where the final mass of HM specifically adsorbed is sufficiently decreased after the first adsorption in solution S2). The higher final mass on curve f in relation to curve e, Fig. 5B, lends credence to the notion of fewer HM molecules being adsorbed in case f, since the final mass is similar to that occurring when As is separately adsorbed (curve b, Fig. 5A). It is also useful to compare the number of adsorbed molecules per cm2 during specific adsorption on Pt (Fig. 5A) with the number of adsorbed molecules per cm2 obtained after specific adsorption onto this surface (Table 3). Considering the lowest values of mass change (51.4 ng cm−2 ) during specific adsorption from solution S1 (Fig. 5A), the number of adsorbed molecules per cm2 was found to be 3.09 × 1012 —i.e., 39 times as many molecules per cm2 as those remaining on the Pt surface after washing with water (Table 3). Also considering the lowest value of mass change (89 ng cm−2 ) during specific adsorption from solution S2 (Fig. 5A), the number of adsorbed As atoms per cm2 was found to be 7.15 × 1014 , or 3.6 times as many atoms per cm2 as those remaining on the Pt surface after washing with water (Table 3). These comparisons corroborate the fact that the number of adsorbed molecules per cm2 remaining on Pt after washing is low for HM. Only strongly adsorbed HM molecules remain on the Pt surface. Arsenic atoms are strongly adsorbed (deposited) onto the Pt surface during specific adsorption from aqueous solutions containing As2 O3 . 3.4. Cyclic voltammetry of HM and As specifically adsorbed onto PtO film Figs. 6 and 7 show the behavior of the first voltammetric cycle in 0.5-M H2 SO4 for pristine PtO film and the most representative of 10 PtO films modified by specific adsorption from eight different aqueous solutions (see Section 2). Lower current densities were initially detected, as well as differing curves relative to Pt, owing to the presence of a PtO film (compare Figs. 1 and 6). For the PtO/HM and PtO/M1 electrodes, the differences in current densities (not shown) relative to pristine PtO film (curve a, Fig. 6) were not significant. However, for the PtO/As, PtO/As/HM, PtO/HM/As, and PtO/M2–PtO/M6 electrodes, there were significant differences in current densities relative to pristine PtO film. The differences in the positive potential scan direction were: (a) lower current densities in the potential region

Fig. 7. 1st CVs for (a) PtO/M3 (), (b) PtO/M4 (), and (c) PtO/M6 () in 0.5-M H2 SO4 . The modified electrode surfaces were washed with water (5 times) before CV acquisition. Scans start at 0.75 V vs. RHE in the positive potential direction.  = 100 mV s−1 .

from 0.9 to 1.05 V (curves b–d, Fig. 6) combined with increased current densities in the potential region from 0.9 to 1.1 V (curves a–c, Fig. 7); (b) higher current densities in the potential region from 1.05 to 1.4 V (curves b–d, Fig. 6) combined with sufficiently increased current densities in the potential region from 1.0 to 1.4 V (curves a–c, Fig. 7). The highest current densities observed in the latter region were obtained with the PtO/M6 electrode. This comparison was made against the pristine PtO film.  ads As PtO oxd (calculated by integrating the difference between anodic current densities vs. time from 0.9 to 1.4 V in PtO electrodes modified in solutions containing As, in relation to pristine PtO, and dividing this difference by the integration of anodic current densities vs. time in the positive potential scan direction in pristine PtO; Table 4) varies with the compound and/or atom specifically adsorbed, rising for the PtO/M1–PtO/M6 electrodes. Higher degrees of PtO surface coverage by specifically adsorbed As, of up to 110%, were found for the PtO/M6 electrodes. This suggests the formation of a second monolayer of adsorbed As onto the PtO surface, as proposed by Furuya and Motoo for Pt surfaces [15]. PtO surface coverage by specifically adsorbed As in the positive potential scan direction was also higher for the PtO/As electrode. In this case (positive potential scan direction), using the interfacial fractional coverage ( ads As PtO oxd ) values and following the same reasoning adopted in Section 3.1 for  ads As , 6% and 2% of the sites for oxide formation on the PtO/HM and PtO/M1 electrodes, respectively, were found to be blocked by specifically adsorbed HM. For the other cases, we assumed specifically adsorbed As to predominate on the Table 4 Interfacial fractional coverage estimated from the cyclic voltammograms in Figs. 6 and 7.

Fig. 6. 1st CVs for (a) pristine PtO film (), (b) PtO/As (), (c) PtO/HM/As (), and (d) PtO/As/HM () in 0.5-M H2 SO4 . The modified electrode surfaces were washed with water (5 times) before CV acquisition. Scans start at 0.75 V vs. RHE in the positive potential direction.  = 100 mV s−1 .

Interface

 ads As PtO oxd

Pristine PtO PtO/HM PtO/As PtO/HM/As PtO/As/HM PtO/M1 PtO/M2 PtO/M3 PtO/M4 PtO/M5 PtO/M6

0 a

0.72 0.40 0.53 a

0.13 0.17 0.58 0.50 1.10

a  comp PtO oxd for HM was calculated as previously described [12], yielding the values 0.06 and 0.02 for PtO/HM and PtO/M1, respectively.

H.A. Menezes, G. Maia / Electrochimica Acta 55 (2010) 4942–4951

Fig. 8. 1st CMs for (a) pristine PtO film (), (b) PtO/HM (), and (c) PtO/As () in 0.5-M H2 SO4 . The modified electrode surfaces were washed with water (5 times) before CM acquisition. Scans start at 0.75 V vs. RHE in the positive potential direction.  = 100 mV s−1 .

PtO film, with a coverage increase to 72% for the PtO/As electrode or to 110% for PtO/M6. 3.5. Cyclic massogram study of HM and As specifically adsorbed onto PtO film Figs. 8 and 9 illustrate the behavior of the first CM in 0.5-M H2 SO4 for pristine PtO film and the most representative of 10 PtO films modified by specific adsorption from eight different aqueous solutions (see Section 2). Mass changes were taken as zero at the initial potentials. An increase in mass change occurs after 0.98 V for pristine PtO film alone (positive potential scan direction). In the negative potential scan direction, a pronounced decrease in mass change is observed thereafter. The loss of mass (negative mass change) at the pristine PtO film can be attributed to PtOx film reduction. In the other cases (Fig. 8), steady increases in mass change take place near 1.0 V in the positive potential scan direction. This can be attributed to HM and/or As oxidation, which frees the PtO film surface, enabling it to produce additional PtOx film. The most effective mass gain was observed on the PtO/As electrode (curve c, Fig. 8). In the negative potential scan direction, mass change proved fairly steady, while above a given potential a decrease was observed (with negative mass change in several instances; see curves b and c,

4949

Fig. 8). These negative mass changes are likely caused by reduction of additional PtOx film. On the PtO/M1–PtO/M6 electrodes (some of these electrodes are not shown in Fig. 9), mass changes were more pronounced than those occurring on pristine PtO film. An overall increase in mass changes in the positive potential scan direction was detected. The most effective mass gain was observed on the PtO/M3 electrode (curve b, Fig. 9). However, a decrease in mass change was detected at the end of the positive potential scan direction, primarily on the PtO/M1 and PtO/M6 electrodes. This finding reinforces the idea of oxidation of adsorbed HM and/or As taking place in this potential region (curves a and c, Fig. 9). In the negative potential scan direction, the mass change was seen to increase (curve a, Fig. 9), and even remain constant (curve b, Fig. 9), with a decrease below a certain potential (curves a and b, Fig. 9). The increase in mass change in the positive potential scan direction is likely due to production of additional PtOx film, whereas the increase in mass change in the negative potential scan direction is likely caused by re-adsorption of molecules or atoms desorbed in the positive potential scan direction. On the other hand, the decrease in mass change in the negative potential scan direction is likely caused by reduction of additional PtOx film. On the PtO/M6 electrode we detected negative mass changes for almost the entire CM, most likely owing to desorption of As oxidized in the positive potential scan direction. The molar weights obtained for the positive potential scan direction correspond approximately to the formation of a PtOx film, thus being strongly suggestive of this occurrence. The molar weights obtained for the negative potential scan direction were lower than those calculated for the positive potential scan direction, a finding that corroborates the irreversibility of the reduction of the new PtOx film formed in this potential region. We are assuming that HM is electro-oxidized as previously proposed by us [12]. The electro-oxidation of As follows the mechanism proposed by Cabelka et al. [16] for an oxide-covered surface: PtO + As(OH)3 → PtOAs(OH)3

(fast)

(10)

PtOAs(OH)3 → Pt + OAs(OH)3

(veryslowr.d.s.)

(11)

+

Pt + H2 O → PtOH + H + e PtOH → PtO + H+ + e

(fast)

(12)

(fast)

(13)

The mass changes corrected for HM and/or As(III) desorption at the end of the positive potential scan direction are reported in Table 5. An overall increase was observed in the mass changes corrected for the modified PtO electrodes. The highest corrected mass change was obtained for the PtO/M3 electrode (Table 5). Table 5 Surface coverage estimated from the cyclic voltammograms (potential scan from 0.75 to 1.4 V vs. RHE) in Figs. 6 and 7 and mass changes corrected according to the mass changes obtained at the end of the positive potential scan direction in Figs. 8 and 9. Interface

Fig. 9. 1st CMs for (a) PtO/M1 (), (b) PtO/M3 (), and (c) PtO/M6 () in 0.5-M H2 SO4 . The modified electrode surfaces were washed with water (5 times) before CM acquisition. Scans start at 0.75 V vs. RHE in the positive potential direction.  = 100 mV s−1 .

Pristine PtO PtO/HM PtO/As PtO/HM/As PtO/As/HM PtO/M1 PtO/M2 PtO/M3 PtO/M4 PtO/M5 PtO/M6

m1.4 V * (ng cm−2 )

– 6.50 0.90 2.83 4.23 7.43 3.91 13.52 5.45 7.00 7.84

NHM or As 1.4 V (×10−12 molecules or atoms cm−2 ) HM

As

– 0.39 – – – 0.39 – – – – –

– – 7.21 22.77 34.00 7.46 31.44 108.65 43.76 56.25 63.00

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For the PtO/HM and PtO/M1 electrodes, calculation of the number of adsorbed molecules and/or atoms per unit surface area (Eq. (9)) was based on the molecular weight of HM. The atomic weight of As was used instead for the other PtO modified electrodes. The behavior of the PtO/M1 electrode was considered an instance of co-adsorption, for which we added the m0.30 V * values of HM and As2 O3 shown in Table 5, as done in Section 3.2. The number of specifically adsorbed As atoms on PtO film is increased overall on the PtO/M1–PtO/M6 electrodes, according to the calculated values shown in Table 5. The greatest number of specifically adsorbed As atoms was obtained on the PtO/M3 electrode. This can be attributed to a predominance of As adsorption on the PtO/M1–PtO/M6 electrodes. 3.6. Study of mass changes during the specific adsorption of HM and As onto a PtO film Fig. 10 shows the behavior of mass changes taking place on PtO film during specific adsorption from eight different aqueous solutions. An asymptotic average increase with time was evident in all the specific adsorption situations during the first 5 min, after which point the mass increase became less pronounced. Final mass increase was high (137 ng cm−2 ) for solution S2 used in the second step during the sequential specific adsorption of solution S1 onto PtO film (curve c, Fig. 10A). The number of HM molecules specifically adsorbed onto PtO film was smaller than that of As atoms, since the molecular weight of HM is much higher than the atomic weight of As, but their final mass changes were equal to or lower than those of As during specific adsorption (compare curves a and b and curves c and d, Fig. 10A). In the case of specific adsorption from solutions S3–S8, the highest final mass change (267 ng cm−2 ) was obtained using solution S3 (curve a, Fig. 10B). For this solution we assumed effective co-adsorption of HM molecules and As atoms onto the PtO film, given the decreased final mass changes when solutions S4 and S5 were used (curves b and c, Fig. 10B). We attributed this decreased final mass to the predominance of As adsorption onto PtO film in these two cases. When solutions S6–S8 were used, the final mass changes increased (see curves d–f, Fig. 10B), but these changes were smaller in comparison with the

specific adsorption from solution S3 (curve a, Fig. 10B), lending support to the view that a greater number of As atoms are adsorbed, in contrast to a negligible number of HM molecules. Comparing the number of molecules and/or atoms specifically adsorbed per cm2 of PtO film (Fig. 10A) with the number of molecules and/or atoms specifically adsorbed per cm2 of this film (Table 5) and considering the lowest value of mass change (57 ng cm−2 ) during specific adsorption from solution S1 (Fig. 10A), the number of adsorbed molecules per cm2 was found to be 3.44 × 1012 , or 8.8 times as many as those remaining on the film after washing with water (Table 5). Likewise, considering the lowest value of mass change (56 ng cm−2 ) during specific adsorption from solutions S2 (Fig. 10A), the number of As atoms adsorbed per cm2 was found to be 4.5 × 1014 , or 62.4 times as many as those remaining on PtO film after washing with water (Table 5). Given the magnitude of these values, compared to those occurring on a Pt surface (62.4 and 3.6 times, respectively), these differences reinforce the proposition by Cabelka et al. [16] regarding As adsorption onto oxide-covered surfaces (Eqs. (10)–(13)). These comparisons demonstrate that the number of adsorbed HM molecules or As atoms per cm2 remaining on the PtO film after washing with water is low. Only strongly adsorbed molecules and atoms remain on the PtO film surface. 4. Conclusions The number of adsorbed HM molecules per cm2 was lower on Pt than on PtO film, whereas the number of As atoms per cm2 was higher on Pt than on PtO film after these surfaces were washed with water. This suggests that specific adsorption depends both on the surface and on the species present in the adsorption solution. HM does not desorb previously adsorbed As at the HM concentrations used in the present study. However, it does co-adsorb with As from solution S3. Arsenic adsorbs strongly (deposition) on Pt in the presence of HM or during sequential specific adsorption with HM. Adsorption is more pronounced for solutions S3–S8. In this situation, the fraction of blocked active sites for hydrogen adsorption and the number of adsorbed atoms per cm2 are increased. In the potential region from 0.75 to 1.4 V, PtO film successfully served to detect that the number of As atoms adsorbed per cm2 onto PtO film was highest when the PtO/M3 electrode was utilized. Acknowledgments The authors thank PROPP-UFMS and CAPES for their financial support. References

Fig. 10. (A) Mass changes vs. time for a PtO film electrode during specific adsorption from solutions (a) S1 (); (b) S2 (䊉); (c) S2 () after specific adsorption as per (a), followed by copious washing with water; and (d) S1 () after specific adsorption as per (b), followed by copious washing with water. (B) Mass changes vs. time for a PtO film electrode during specific adsorption from solutions (a) S3 (), (b) S4 (䊉), (c) S5 (), (d) S6 (), (e) S7 (), and (f) S8 ().

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