Insigths of sphalerite weathering under simulated acidic, circumneutral and alkaline conditions to account for mineral activity and potential Zn release

The present study combines surface analyses of pristine and leached low Fe-bearing and Pb- 23 bearing sphalerite (PbS-ZnS) samples including XPS, GDOES, Raman spectroscopy, SEM and 24 AFM, along with chemical evolution of leachates after 24 h of contact with 0.1 M NaOH, 0.1 M 25 NaNO 3 , 0.1 M H 2 SO 4 or 0.1 M HClO 4 solution. A comprehensive electrochemical analysis using 26 Cyclic voltammetry, Chronopotentiometry, Chronoamperometry, Linear sweep voltammetry and 27 Tafel plots of PbS-ZnS and marmatite-like sphalerite (FeZnS) are conducted to compare its 28 oxidation activities. Mineral characterizations reveal sluggish weathering linked to 29 inhomogeneous and minor secondary polysulfides ( S n2- ) surface compounds distribution, thus 30 defining bare modifications of surface-activity relationships. The occurrence of secondary Zn- 31 bearing compounds was not identified on altered samples, which suggests that this heavy metal 32 diffuses into the bulk-solution. Electrochemical assessments confirm sluggish sphalerite 33 oxidation mainly composed by two subsequent stages, where the highest mineral activity was 34 obtained in NaOH conditions. It was found that the sphalerite oxidation is more active in the 35 presence of Pb, while the activity of sphalerite gradually decreases when it is a mineral rich in Fe 36 probably associated with progressive accumulation of S n 2- /S 0 compounds. We suggest general 37 oxidation mechanisms for sphalerite, and their environmental implications are discussed. scan AFM 2 system using contact mode. AFM images (topographic and deflection modes) of at least 10 regions per surface were obtained. The silicon cantilever showed a free resonance frequency between 13 and 15 kHz and a constant of 0.2 N m -1 during these experiments. Average roughness (Ra, nm) and root mean square roughness (Rq, nm) of


Introduction
Low-Fe bearing (~4.5 %) sphalerite samples present high-electrical resistivity (~10 10 Ω) (Ahlberg and Ásbjörnsson 1994; Crundwell 2021), which hamper the analysis of their redox behavior 70 using electrochemical methods, whereby its study is typically limited to chemical and 71 mineralogical analysis. An indirect way of studying this type of sphalerite has been through the 72 study of samples that presents reactive impurities such as galena or high Fe content in structure 73 (Giudici et al. 2002;Da Silva et al. 2003; Urbano et al. 2007). The study of environmental 74 sphalerite oxidation has been focused mainly towards understanding the weathering process when  The aim of the present study is to describe interfacial insigths and activity regarding sphalerite 93 oxidation to elucidate the main weathering mechanisms under typical environmental conditions 94 (i.e., H2SO4, NaNO3, NaOH, HClO4), the role of secondary products (i.e., ZnO2, Sn 2-, S 0 ), and 95 surface-activity relationships, among others. In order to achieve this goal, a combination of  After careful selection, sphalerite crystals were reduced to 3-5 mm size. Grains were then rinsed 117 with an acidic solution (HCl, 3 % v/v), washed with deionized water, and dried with direct N2 118 current. The samples were also stored in a N2 atmosphere until they were grinded using an agate 119 mortar and sieved to fraction size <106 μm under inert conditions for mineralogical and 120 electrochemical studies. X-ray diffraction patterns (Rigaku DMAX 2200, 2θ=0.02, from 10° to 121 90°, using Cu-Kα radiation) and elemental chemical analysis using atomic emission spectroscopy    Cyclic voltammetry (CV) was then initiated in the positive and negative direction at 5 mV·s -1 . 143 Additionally, a study of the variation of the positive potential limit (E±) was carried out to 144 evaluate the stages of the corresponding oxidation process. Chronoamperometry (CA) and/or 145 Chronopotentiometry (CP) were carried out using most significant anodic potentials or currents 146 in agreement with the CV; in order to apply potential (Ea) and/or current (Ia) pulses during 600 s.

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Tafel plots (TP) were obtained using linear sweep voltammetry scans (LSV) using RDE Pine  The total Zn, Fe and Cd were analyzed using atomic absorption spectroscopy with graphite 177 furnace (AAS-GF, Perkin Elmer 3100 atomic absorption spectrometer) with a 178 detection/quantification limit of 0.1/0.15, 0.1/0.15 and 0.05/0.1 mg·L -1 , respectively. As a quality 179 control, SRM 1643D (NIST, USA) trace element in water standard reference material was used.

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The percentage recovery values were better than 95% in all cases. Calibration of instruments, 181 blank correction, matrix effects, sample manipulation, and operating conditions met national and 182 international criteria (Eaton et al. 1995;DOF 2007 Hanna HI-2211 pH-meter.

Surface analyses
Pristine and leached PbS-ZnS samples were characterized using microscopic (SEM, AFM) and 188 spectroscopic techniques (XPS, GDOES, XRD and Raman). Scanning electron microscopy was 189 conducted using a JEOL JSM-6610LV with an energy dispersive detector Oxford X-MAX for 190 chemical analysis (SEM-EDS). XRD was carried out using a Rigaku DMAX 2200, (2θ=0.02, 191 from 10° to 90°) using Cu-Kα radiation. Raman spectra were recorded with a Horiba XploRA™ 192 PLUS spectrometer coupled with a SWIFT™ v2 confocal imaging module which used a solid-193 state laser beam (λ= 532 nm). A Si wafer disc (i.e., 521 cm -1 ) was used for calibration purposes. functions were used for peak decomposition. Charge correction was carried out using the C 1s 205 core line, setting the adventitious carbon signal (H/C signal) to 284.6 eV. AFM study was carried 206 out using a Nanosurf easy scan AFM 2 system using contact mode. AFM images (topographic 207 and deflection modes) of at least 10 regions per surface were obtained. The silicon cantilever 208 showed a free resonance frequency between 13 and 15 kHz and a constant of 0.2 N m -1 during 209 these experiments. Average roughness (Ra, nm) and root mean square roughness (Rq, nm) of each altered surface were also evaluated to obtain topographic information associated with secondary compounds (Table 1). GDOES analyses were performed using a glow discharge  This diagram was calculated using the chemical equilibrium software package Medusa®, which 224 is based on free energy minimization algorithms reported by Eriksson (1979). The reactions and 225 equilibrium constants required by the software to calculate these diagrams are available in the 226 HYDRA database within the Medusa® software package (Puigdomenech 2015). Note that the 227 experimental data (i.e., equilibrium constants) used to calculate this diagram have not been 228 validated (i.e., speciation, solubility), since this is a very demanding work which is beyond the 229 scope of the present study. Therefore, this thermodynamic information is herein used as a rapid 230 and predictive tool to evaluate the stability of released and/or oxidized species when the pH is 231 varied as a function of potential (Fig. S14).   Table 1). The formation and evolution of sub-micro sized surface  (Table 1). These behaviors confirm a sluggish sphalerite weathering process in agreement with 288 Fig. 1; however, there surface processes develop with minor accumulation of sub-micro sized 289 secondary products (Fig. 2e´ Table S1, and they can be used for secondary compounds identification in the present 315 study. Raman peak obtained at 521 cm -1 for Si-wafer disc was included to illustrate optimal 316 conditions during collection of Raman peaks (Fig. 4b). Raman spectra obtained for leached  (Table S1). 320 Additionally, the absence of Raman peaks around 980 cm -1 was identified for leached samples, 321 indicating the minor or null formation of Zn-bearing compounds for all tested conditions (i.e., 322 Zn(OH)2, ZnSO4, ZnCO3, Table S1). The findings presented in Fig. 4 suggested the occurrence 323 of polysulfides (i.e., Sn 2-) as main secondary surface products under all tested conditions.  Table S1). corresponding fitting parameters are given in Table 2, and a summary of typical binding energies 335 reported in the literature for main sulfur and zinc species are also given in Table S2. The Zn2p 336 spectrum of pristine sample presents one strong peak at 1021.28 eV that typically corresponds to  Table S1). Note that S2p spectrum in Fig.   346 S5a clearly indicated a more advanced oxidation state for sulfur species since this spectrum was  These results confirm what was observed by XPS (Fig. 5). It was observed that the 371 lowest intensity of Fe corresponds to the mineral treated with 0.1 M NaOH solution (Fig. 6c).  (Fig. 6d). Note that GDOES analysis has been typically used to characterize the variation in   Fig. 7 shows cyclic voltammograms conducted on CPE-PbS-ZnS (curves identified as i) and 402 CPE-FeZnS samples (curves identified as ii) in 0.1 M NaOH (Fig. 7a), 0.1 M NaNO3 (Fig. 7b), 403 0.1 M HClO4 (Fig. 7c) or 0.1 M H2SO4 (Fig. 7d) solutions. The CPE with 100 % graphite, which 404 is inert to sphalerite and electrolytes composed of conductive carbon phase, was also included for   H2SO4. This result confirms the evidence above described in the Cyclic Voltammetry analysis 502 (Fig. 7), Chronoamperometry (Figs. S9 and S10) and Chronopotentiometry (Figs. S8). The fact 503 that PbS-ZnS and FeZnS present a higher activity in NaOH is also corroborated in the rate 504 constants for oxidation (Table 3) which are three and one order of magnitude, respectively, 505 higher compared to the other solutions. Likewise, the * value in general reveals that FeZnS 506 oxidation is slightly more prone to occur than the PbS-ZnS sample; although both processes are 507 slow as previously described (i.e., Tafel slopes located between 100 and 150 mV dec -1 ).

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Accordingly, it was also observed that the oxidation of the PbS-ZnS sample proceeded first than  describing at least two major stages for the weathering processes (Figs. S6 to S12, Table 3).

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The chemical analysis of leachates generated during simulated sphalerite (i.e., PbS-ZnS) 545 weathering processes indicated limited Zn, Fe, Cd and sulfates ions dissolution (Fig. 1). 546 Additionally, mineralogical analysis of these mineral samples confirmed that the polysulfides 547 (i.e., Sn 2-) are the main sulfur compounds during weathering processes for all tested conditions.

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The progressive enrichment of altered samples with Sn 2-/S 0 compounds can also occur for a more 549 extend weathering process (Fig. S5a). It was not identified Zn-bearing secondary compounds  (Fig. 8). This fact was 631 associated with the enhanced release of Zn to bulk-system (Fig. 1d), avoiding more accumulation 632 of sub-micro sized secondary compounds regarding the other tested systems (Figs. 2 and 3, Table   633 1).

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The pollution process for Zn release in soils has been studied under different climate and      Zn2p3/2 and S2p3/2 * spectra of pristine and leached sphalerite samples (i.e., PbS-ZnS). 972 Table 3. Kinetic oxidation parameters of the Tafel plots analysis regarding sphalerite oxidation         Ambient conditions. Quiescent systems.