English

• Acid mine drainage (AMD) originates from oxidation of sulfide-containing minerals (e.g., pyrite) and includes a large amount of ferrous, sulfate, and heavy metal contaminants. Schwertmannite (Sch, Fe₈O₈(OH)_8–2x(SO₄)_x·nH₂O, where 1 < x < 1.75) is the considered as the most common oxidation product in sulfate-rich environments [1,2]. Because of its high reactivity and large surface area, the Sch potentially adsorbs and immobilize trace elements and heavy metals [3]. Notably, Sch is metastable and likely transforms to more thermodynamically stable Fe(Ⅲ) minerals [4,5]. This phase transformation process of Sch can significantly affects acidity dynamics, the cycling of Fe(Ⅲ)/Fe(Ⅱ), and the fate and transport of heavy metals [6]. Collectively, understanding the transformation of Sch is important for evaluating the Fe cycling and changes in water quality of AMD under natural conditions.

  Numerous studies have found that the production of Fe(Ⅱ) due to reductive dissolution of Sch accelerated the phase transformation of Sch through the interfacial electron transfer (IET) between Fe(Ⅱ) and surface Fe(Ⅲ) [7-9]. The reductive production of Fe(Ⅱ) from Sch is facilitated by biotic processes, dependent of dissimilatory iron-reducing bacteria (DIRB) including Shewanella and Geobacter [5,10]. More importantly, the role of photochemical processes relying on the abiotic reactions of microbially secreted low-molecular-weight organic acids (LMWOAs) cannot be overlooked [11,12]. This is predominantly because the complexation of LMWOAs with Sch strongly enhances the reductive dissolution and production of Fe(Ⅱ) through ligand-to-metal charge transfer (LMCT) [13-15]. Tartaric acid (TA), one of the most ubiquitous LMWOAs in the AMD, stems from the decomposition of macromolecules and the secretion of algae and bacteria [16]. It has been reported the presence of TA significantly enhances photochemical reduction of Fe(Ⅲ) along with the release of Fe(Ⅱ) from Sch [17,18]. However, little is known about if the presence of TA can significantly impact the photochemical transformation of Sch.

  The divalent heavy metals such as Cu(Ⅱ), which commonly coexisting with Sch with a potentially high concentration, may exhibit the significant impacts on the photochemical transformation of Sch [19,20]. Different forms of Cu(Ⅱ), e.g., those adsorbed on the surface or incorporated into the crystal structure, determined the photochemical transformation rate of Sch to varying extents [21]. In addition, the fates and sequestration of Cu(Ⅱ) in the AMD, including its mobility and bioavailability, are closely related to the forms of Fe minerals and may be regulated by the phase transformation of Sch [22]. In the presence of LMWOAs, the impacts of Cu(Ⅱ) on the photochemical transformation of Sch possibly become more complicated [23]. The objectives of this study were: (1) to understand the mechanisms of impact of Cu(Ⅱ) on photoinduced dissolution and phase transformation of Sch in the presence of TA and (2) to characterize the changes in speciation and mobility of Cu(Ⅱ). The transformation products of Sch were analyzed using X-ray diffractometer (XRD), Fourier transform infrared spectra (FTIR), and scanning electron microscope-Energy dispersive spectrometer (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). Photoelectrochemical analysis and electron paramagnetic resonance (EPR) confirmed the reactive species and photochemical mechanisms contributing to the reductive dissolution and transformation of Sch. Additionally, the speciation redistribution of Cu(Ⅱ) during the photochemical transformation of Sch was continuously monitored. The details of experimental procedures and characterizations were shown in Texts S1-S4 (Supporting information).

  XRD results showed Sch (PDF #47–1775) was successfully synthesized [24,25], and its crystalline phase did not change significantly as light exposure to Sch alone for 5 h (Fig. S1 in Supporting information). From Fig. 1a, in the absence of Cu(Ⅱ), a new peak assigned to magnetite (Mt) (PDF #96–900–5840) [26] emerged under the anoxic condition, suggesting the crystal structure of Sch was transformed with addition of TA during 5-h light aging. In comparison, the new peak of Gt instead of Mt (PDF #00–29–0713) [27] emerged upon adding 10 mg/L Cu(Ⅱ) (Fig. 1a), indicating that the Cu(Ⅱ) inhibited the photochemical phase transformation of Sch/TA system. Such phenomenon was more dominant with the weaker XRD peak of Gt as increasing concentration of Cu(Ⅱ). In addition, this inhibition of Cu(Ⅱ) was more pronounced under the oxic conditions than anoxic conditions (Fig. 1b). For example, under the oxic conditions, light-exposed Sch with TA only was converted to Gt, while addition of Cu(Ⅱ) completely inhibited the transformation of Sch without emergence of any new peak. FTIR analysis showed the changes in functional structures of Sch during photochemical reactions. As the characteristic peaks of Sch, the absorption bands at 970, 1000 and 1080 cm⁻¹ are generally assigned to the S-O stretching vibration in SO₄²⁻, while the absorption bands at 1620 cm⁻¹ and 3200 cm⁻¹ correspond to the O—H stretching vibration [25]. From Fig. 1c, for light-exposed Sch/TA system in the absence of Cu(Ⅱ), the broad band of Mt at 570–590 cm⁻¹ [28] emerged, accompanied by dissociation of S-O stretching vibration under the anoxic conditions. In contrast, the Fe-O-H bending vibration at 840 cm⁻¹ attributed to Gt in the presence of 10 mg/L of Cu(Ⅱ) [29,30]. As the Cu(Ⅱ) concentration increased to 30 mg/L, the peak of S-O stretching vibration belonging to Sch was observed again. The results collectively implied the inhibition effect of Cu(Ⅱ) on the photochemical transformation of Sch. Under the oxic conditions, the Fe-O-H bending vibration belonging to Gt was only observed for Sch/TA system in the absence of Cu(Ⅱ) and was not detected with addition of Cu(Ⅱ) (Fig. 1d). The phenomenon is consistent with above proposals that the inhibition effect of Cu(Ⅱ) was more obvious under the oxic conditions than the anoxic conditions.

  Figure 1

  

  Figure 1.  XRD results of light-exposed Sch/TA system under (a) anoxic conditions and (b) oxic conditions. FTIR spectra with additions of different Cu(Ⅱ) concentrations under (c) anoxic conditions and (d) oxic conditions.

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  As shown in Fig. S2 (Supporting information), SEM images revealed the formation of Mt nanoparticles within the Sch/TA system after 5-hour light exposure in anoxic conditions [31]. However, in the presence of 10 mg/L Cu(Ⅱ), needle-like Gt minerals emerged on the Sch (Fig. S2) [32]. As increasing Cu(Ⅱ) concentration to 30 mg/L, the needle-like morphology of Gt was weaker and a significant flower-like morphology of Cu(Ⅱ) oxides were observed (Fig. S2). SEM-EDS mapping results also supported the presence of Cu(Ⅱ) on the mineral surface under anoxic conditions (Fig. S2). This phenomenon likely suggested Cu(Ⅱ) adsorbed or precipitated onto the Sch surface and subsequently inhibited the transformation of Sch to Gt. Under the oxic conditions, the morphology of Gt was observed only in the absence of Cu(Ⅱ), while only a hedge-hog-like structure of Sch could be observed with the addition of Cu(Ⅱ) (Fig. S2) [32]. Notably, a weaker morphology and less distribution of Cu(Ⅱ) on the Sch surface were observed under oxic conditions.

  It is believed that adsorbed Fe(Ⅱ) on the mineral surface accelerated the transformation of Sch to more stable minerals through the IET process [8,9,27]. The amount of adsorbed Fe(Ⅱ) directly determined the transformation rate and products of Sch. Especially, the formed secondary mineral from Sch was Gt [4,30], whereas the higher concentrations of adsorbed Fe(Ⅱ) altered this transformation to form mixed-valence Mt with 1:2 ratio of Fe(Ⅱ) and Fe(Ⅲ) [26,33-35]. Before the onset of the light reaction, a 0.5-h dark adsorption treatment was conducted. As shown in Fig. S3 (Supporting information), at pH 3.20, Cu(Ⅱ) hardly affected the adsorption of TA on the surface of Sch. Further simulation using MINTEQ (Fig. S4 in Supporting information) revealed that under initial pH conditions, the complexation between TA and Cu(Ⅱ) is extremely weak, with TA mainly existing in the form of acid anions in the solution. As shown in Fig. S5 (Supporting information), the aqueous amount of Fe(Ⅱ) and pH of light-exposed Sch/TA system both increased significantly. For example, the production of Fe(Ⅱ) reached > 1.2 mmol/L and pH increased by ~4 unit within 60-min reaction. It is worth noting that in the presence of 10 mg/L Cu(Ⅱ), the release of Fe(Ⅱ) in solution is slightly lower than in the absence of Cu(Ⅱ), suggesting that Cu(Ⅱ) may compete with TA for complexation with Sch, thereby reducing the adsorption and complexation of TA on the surface of Sch, and thus weakening the photochemical dissolution of Sch to some extent [36]. However, in the presence of 30 mg/L Cu(Ⅱ), the release of Fe(Ⅱ) in solution was slightly higher than in the absence of Cu(Ⅱ). Studies have shown that the adsorption affinity of Cu(Ⅱ) on the mineral surface is higher than that of Fe(Ⅱ) [21], so Cu(Ⅱ) may compete with Fe(Ⅱ) in solution for adsorption, occupying some adsorption sites of Fe(Ⅱ) on the surface of Sch, resulting in a decrease in adsorbed Fe(Ⅱ) and an increase in dissolved Fe(Ⅱ) in solution (Fig. S5a). Consistent with previous findings [1,37-39], photoreductive dissolution of Sch produced the Fe(Ⅱ) and inevitably released OH⁻ (Eq. 1), this may occur because TA act as electron donors, while ≡Fe(Ⅲ) in Sch acts as an electron acceptor, resulting in the consumption of protons in the solution during the redox process (Fig. S5b). Because the interaction between Fe(Ⅱ) and Sch surface was more thermodynamically favorable at pH > 5.5 [40], photoproduced Fe(Ⅱ) readily adsorbed on surface of Sch and the observed phase transformation of Sch can be expected. In contrast, under the oxic conditions, the amounts of Fe(Ⅱ) and pH first increased but decreased rapidly to undetected levels (Figs. S5c and d). This observation suggested that the photogenerated Fe(Ⅱ) from Sch were oxidized to Fe(Ⅲ) by dissolved oxygen or reactive oxygen species (ROS), while re-hydrolysis of Fe(Ⅲ) released H⁺ ions to lower pH [41-43]. The rapid decrease of Fe(Ⅱ) and pH significantly ceased the transformation of Sch. Notably, the presence of Cu(Ⅱ) could significantly impacted the released amounts of Fe(Ⅱ) and changes in pH to varying extents. Combined with above findings, it is indicated that the Cu(Ⅱ) might affect the photochemical transformation of Sch by either altering the photoreduction of Sch along with production of Fe(Ⅱ), or by influencing the interaction between Fe(Ⅱ) and the Sch surface.

  $ \begin{aligned} & 3 \mathrm{Fe}_8 \mathrm{O}_8(\mathrm{OH})_6 \mathrm{SO}_{4(\mathrm{s})}+\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+6 \mathrm{H}_2 \mathrm{O} \rightarrow 24 \mathrm{Fe}^{2+}+3 {\mathrm{SO}_4}^{2-}{ }_{(\mathrm{aq})} \\ & +6 \mathrm{CO}_2+42 {\mathrm{OH}^{-}}_{({\mathrm{aq})}} \end{aligned} $

  (1)

  To identify the role of Cu(Ⅱ), the processes controlling the photoreduction of Sch/TA systems were compared. Recent studies reported the photogenerated electrons, LMCT processes, and ROS regulated the photoreductive dissolution of Sch [24,44,45]. The photogenerated electrons of Sch were qualitatively investigated by a photoelectrochemical analysis [44]. From Fig. S6a (Supporting information), photocurrents of Sch were monitored during cycles of light on-off. For example, the photocurrents reached peaks of 0.40–0.45 µA/cm² upon light activation, whereas rapidly declined by 50%−55% when light radiation was ceased. It was also observed that the photocurrents were reproducible during three successive light-switching cycles, indicating that the Sch exhibited the stable photoelectric conversion efficiency enabling the efficient separation of charge carriers (i.e., electrons and holes) as exposed to light [46]. Based on results of Fig. S6b (Supporting information), the production of Fe(Ⅱ) from Sch alone reached 0.39 mmol/L after 5-h light exposure under the anoxic conditions. Except for photogenerated hole-electron pairs, the LMCT process along with the generated ROS also played the important roles [17,24,47,48]. From results of EPR spectroscopy (Figs. 2a and b and Fig. S7 in Supporting information), six-line signals with intensity ratio of 1:1:1:1:1:1 were observed for Sch/TA systems under both oxic and anoxic conditions and can be assigned to the DMPO adducts by ^•CO₂⁻ (α_H = 19.01 G, α_N = 15.80 G). It is noteworthy that in the UV/TA system, the intensity of the signal peaks associated with ^•CO₂⁻ was relatively weak. However, the addition of Sch or Cu(Ⅱ) resulted in a significant enhancement of the signal peaks of ^•CO₂⁻. The observations declared that Sch and Cu(Ⅱ) are both capable of undergoing LMCT reactions with TA. Consequently, Cu(Ⅱ) may impede the photoreductive dissolution process of Sch by competing with TA for complexation with Sch [36,49-51]. As shown in Fig. 2c, under anoxic conditions, low concentrations of ^•OH were observed, which may originate from adsorbed water oxidized by photogenerated holes on the Sch surface (Eq. 2), as well as the photolysis of Fe(Ⅲ)‑hydroxyl complexes (Eq. 3). While under aerobic conditions, the production of ^•OH increases significantly (Fig. 2d), which may be attributed to the Fenton reaction between dissolved Fe(Ⅱ) and H₂O₂ (Eqs. 4–8) [24]. It is worth noting that the presence of Cu(Ⅱ) suppressed the generation of ^•OH, which may be attributed to ^•OH mediating the re-oxidation of Cu(Ⅰ) to Cu(Ⅱ), thus consuming ^•OH (Eqs. 9 and 10) [36]. These ^•OH subsequently participate in the re-oxidation process of Fe(Ⅱ).

  $ \mathrm{h}^{+}+\mathrm{H}_2 \mathrm{O} \rightarrow {^\cdot \mathrm{OH}}+\mathrm{H}^{+} $

  (2)

  $ \mathrm{Fe}\left(\mathrm{H}_2 \mathrm{O}\right)_5{ }^{3+}-\mathrm{OH}+h v \rightarrow \mathrm{Fe}\left(\mathrm{H}_2 \mathrm{O}\right)_6{ }^{2+}+{^\cdot \mathrm{OH}} $

  (3)

  $ \mathrm{e}^{-}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2+{\mathrm{O}_2} ^{\cdot-} $

  (4)

  $ { }^{\cdot} {\mathrm{CO}_2}^{-}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2+{\mathrm{O}_2} ^{\cdot-} $

  (5)

  $ \mathrm{Fe}(\mathrm{II})+\mathrm{O}_2 \rightarrow \mathrm{Fe}(\mathrm{III})+{\mathrm{O}_2} ^{\cdot-} $

  (6)

  $ \mathrm{Fe}(\mathrm{II})+{\mathrm{O}_2} ^{\cdot-}+2 \mathrm{H}^{+} \rightarrow \mathrm{Fe}(\mathrm{III})+\mathrm{H}_2 \mathrm{O}_2 $

  (7)

  $ \mathrm{Fe}(\mathrm{II})+\mathrm{H}_2 \mathrm{O}_2+\mathrm{H}^{+} \rightarrow \mathrm{Fe}(\mathrm{III})+{^\cdot \mathrm{OH} } $

  (8)

  $ \mathrm{Cu}(\mathrm{II})-\mathrm{TA}+h v \rightarrow \mathrm{Cu}(\mathrm{I})+{^\cdot \mathrm{CO}_2}^{-} $

  (9)

  $ \mathrm{Cu}(\mathrm{I})+{^\cdot \mathrm{OH}} \rightarrow \mathrm{Cu}(\mathrm{II})+\mathrm{OH}^{-} $

  (10)

  Figure 2

  

  Figure 2.  EPR spectra of DMPO adducts under (a) anoxic conditions and (b) oxic conditions. Time courses of ^•OH of light-exposed Sch/TA system with different Cu(Ⅱ) concentrations under (c) anoxic conditions and (d) oxic conditions.

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  As discussed above, the adsorbed Fe(Ⅱ) produced from photoreductive dissolution of Sch likely controlled the phase transformation of Sch/TA systems. Cu(Ⅱ) is proposed to modulate the adsorption of Fe(Ⅱ), thereby initiating the phase transformation of Sch. To test this hypothesis, the concentrations of adsorbed Fe(Ⅱ) on the Sch surface were tested. As shown in Fig. 3, the concentrations of Cu(Ⅱ) along with changes in transformation extent of Sch positively correlated with amounts of adsorbed Fe(Ⅱ). For instance, under the anoxic conditions, the presence of 10 mg/L and 30 mg/L Cu(Ⅱ) lowered the amounts of adsorbed Fe(Ⅱ) by 29.4% and 45.6%, respectively (Fig. 3a). Under the oxic conditions, the amounts of adsorbed Fe(Ⅱ) decreased by 36% and 44%, respectively (Fig. 3b). In addition, adsorbed Fe(Ⅱ) decreased rapidly regardless of presence of Cu(Ⅱ) due to oxidation of Fe(Ⅱ) by dissolved oxygen and ^•OH [52-54]. The phenomena can be explained by the following reactions. On one hand, Cu(Ⅱ) can form complexes with TA and partially inhibited the LMCT processes between Sch and TA, thus lowering the adsorbed amounts of Fe(Ⅱ) (Fig. S7) [36,55]. On the other hand, the Cu(Ⅱ) owned a strong affinity towards Sch surface and could observably compete with Fe(Ⅱ) for adsorption sites [21,56].

  Figure 3

  

  Figure 3.  Time courses of adsorbed Fe(Ⅱ) of Sch with different Cu(Ⅱ) concentrations under (a) anoxic conditions and (b) oxic conditions.

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  To summarize, the photochemical transformation of Sch induced by TA is a Fe(Ⅱ)-catalyzed transformation mechanism involving two main steps: Firstly, Sch released Fe(Ⅱ) through LMCT reaction and photoinduced electron (Eqs. 11–13), and Fe(Ⅱ) re-adsorbed onto the surface of Sch. Subsequently, the adsorbed Fe(Ⅱ) undergone re-oxidation to Fe(Ⅲ), which further undergone hydrolysis and recrystallization, resulting in the formation of new mineral phases. It is worth noting that while interfacial electron transfer was commonly believed to control the re-oxidation of Fe(Ⅱ) on the mineral surface (Eqs. 14 and 15) [30], in this study, the contributions of h⁺ and ^•OH to the re-oxidation of Fe(Ⅱ) cannot be overlooked (Eqs. 16 and 17), especially the latter, which is thermodynamically favorable considering the high value of E(^•OH/H₂O) (2.73 V vs. SHE). However, when coexisting with Cu(Ⅱ), Cu(Ⅱ) competed with TA for complexation with Sch (Eq. 9) and with Fe(Ⅱ) for surface adsorption on Sch, simultaneously inhibiting the photoreductive dissolution of Sch and interfacial electron transfer. Consequently, it impeded the transformation of Sch into minerals with higher crystallinity.

  $ \mathrm{Sch}+h v \rightarrow \mathrm{~h}^{+}+\mathrm{e}^{-} $

  (11)

  $ \mathrm{e}^{-}+\equiv \mathrm{Fe}(\mathrm{III}) \rightarrow \mathrm{Fe}(\mathrm{II}) $

  (12)

  $ \equiv \mathrm{Fe}(\mathrm{III})-\mathrm{TA}+h v \rightarrow \mathrm{Fe}(\mathrm{II})+{ }{^{\cdot} \mathrm{CO}_2}^{-} $

  (13)

  $ \equiv \mathrm{Fe}(\mathrm{III}) \mathrm{OFe}(\mathrm{II})^{+} \rightarrow \equiv \mathrm{Fe}(\mathrm{II}) \mathrm{OFe}(\mathrm{III})^{+} $

  (14)

  $ \equiv \mathrm{Fe}(\mathrm{II}) \mathrm{OFe}(\mathrm{III})^{+}+\mathrm{H}^{+} \rightarrow \equiv \mathrm{Fe}(\mathrm{III})_{\text {new }}+\mathrm{Fe}(\mathrm{II}) $

  (15)

  $ \mathrm{Fe}(\mathrm{II})+\mathrm{h}^{+} \rightarrow \equiv \mathrm{Fe}(\mathrm{III})_{\text {new }} $

  (16)

  $ \mathrm{Fe}(\mathrm{II})+{ }^{\cdot} \mathrm{OH} \rightarrow \equiv \mathrm{Fe}(\mathrm{III})_{\text {new }}+\mathrm{OH}^{-} $

  (17)

  Since pH_pzc of Sch is around 7.2 [57], Cu(Ⅱ) barely interacted with Sch surface and is difficult to be sequestrated by minerals in the AMD without photochemical reactions (Fig. S8 in Supporting information). Notably, upon photochemical reactions of AMD, the LMWOAs (e.g., TA) possibly enhanced the immobilization of heavy metals through promoting the dissolution and phase transformation of Sch [58,59]. To investigate the redistributions of Cu(Ⅱ), the changes in contents of dissolved, adsorbed, and structural Cu(Ⅱ) during the photochemical transformation of Sch/TA systems were measured by extraction methods [21,60]. From Figs. 4a and b, the amounts of structural Cu(Ⅱ) of light-exposed Sch/TA systems were enhanced under the anoxic conditions. In the presence of 10 mg/L Cu(Ⅱ), the proportions of dissolved and adsorbed Cu(Ⅱ) of Sch/TA systems decreased to 26% and increased to 62.7%, respectively. Correspondingly, during the 5-hour transformation of Sch/TA system, the proportion of structural Cu(Ⅱ) significantly increased from 4.66% to 15.49%. With increasing concentration of Cu(Ⅱ) to 30 mg/L, the proportions of structural Cu(Ⅱ) were lowered by ~30%. As shown in Figs. 4c and d, under the oxic conditions, the proportions of adsorbed Cu(Ⅱ) observably increased though no structural Cu(Ⅱ) was detected. The results suggest that the sequestration of Cu(Ⅱ) can be significantly enhanced during the photochemical transformation of Sch/TA systems. In addition, it is indicated that the proportions of adsorbed and structural Cu(Ⅱ) positively correlated with the photochemical transformation rates and extents of Sch. This is because the Cu(Ⅱ) could strongly adsorb onto the Sch and compete with Fe(Ⅱ) due to increase of pH induced by photoreduction of Sch. More importantly, the formation process of structural Cu(Ⅱ) may be closely related to the recrystallization and nucleation growth of metastable minerals such as Gt [61,62]. It is concluded that the photochemical transformation of Sch in the AMD obviously enhanced the sequestration of Cu(Ⅱ) and potentially reduced bioavailability and potential risks of heavy metals.

  Figure 4

  

  Figure 4.  Temporal changes of Cu(Ⅱ) distribution of light-exposed Sch/TA systems containing (a) and (c) 10, (b) and (d) 30 mg/L Cu, (a, b) and (c, d) represent the anoxic and oxic conditions, respectively.

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  XPS analyses were shown to explore the surface elemental changes of light-exposed Sch/TA systems. From the full XPS spectra (Fig. 5a), binding peaks at 168.70 ± 1.73 eV, 530.82 ± 3.37 eV, 711.74 ± 5.3 eV, and 933.38 ± 4.57 eV corresponded to S 2p, O 1s, Fe 2p and Cu 2p, respectively. During the photochemical transformation of Sch/TA systems under anoxic conditions, S 2p peak disappeared and Cu 2p peak emerged, indicating a significant transformation of Sch to Gt and the sequestration of Cu(Ⅱ). In contrast, under oxic conditions, S 2p peak along with the minor peak of Cu 2p were observed, supporting that the transformation of Sch and sequestration of Cu(Ⅱ) were substantially limited. The observations of XPS were consistent with above conclusions. From Fig. 5b, under the anoxic conditions, the Cu 2p spectrum was deconvoluted to Cu 2p_3/2 and Cu 2p_1/2 peaks at 932.68 and 952.49 eV assigned to CuO and satellite peak at 941.92 eV of Cu²⁺ [36,63]. This result implied the valence state of Cu did not change during the photochemical reactions. In comparison, Fe 2p peak was deconvoluted to two peaks 723.77 eV and 713.90 eV (Fig. 5c) assigned to FeOOH species [64], indicating the Gt phases emerged from the photochemical transformation of Sch. Fitted Fe 2p spectra showed the peaks located at binding energy of 711.26 eV and 709.75 eV can be assigned to Fe-O and Fe-S, respectively [65]. Under the anoxic conditions, the percentage of FeOOH increased to 48.79% and the percentage of Fe-S decreased to 17.90%. Differently, the percentage of FeOOH was significantly lower under the oxic conditions, supporting above conclusions that the phase transformation of Sch was more favorable under the anoxic conditions than oxic conditions. From Fig. 5d, the peaks at 168.06 eV and 169.4 eV assigned to Sch-bound S(Ⅵ) and SO₄²⁻ [66] were also lower under the anoxic conditions.

  Figure 5

  

  Figure 5.  XPS analysis of light-exposed Sch/TA systems in the presence of Cu(Ⅱ). (a) Full XPS spectrum, (b) XPS spectra of Cu 2p, (c) XPS spectra of Fe 2p, and (d) XPS spectra of S 2p.

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  In summary, the photoreductive dissolution of Sch/TA systems promoted the phase transformation to more thermodynamically stable Fe(Ⅲ) minerals of Gt and Mt under the oxic and anoxic atmospheres, respectively. Especially, the presence of Cu(Ⅱ) significantly impeded this transformation process, i.e., only Gt occurred as product under anoxic conditions and even no phase transformation under oxic conditions. The inhibitory effect of Cu(Ⅱ) was manifested in two ways: through complexation with TA, thereby weakening the photoreductive dissolution of Sch, and decreased the amount of adsorbed Fe(Ⅱ) that governs phase transformation by competing with Fe(Ⅱ) for adsorption sites on the mineral surface. Due to the increase of pH and recrystallization/nucleation growth of newly formed Gt under anoxic conditions, 62.7%−75.88% of Cu(Ⅱ) was adsorbed on the mineral surface. Additionally, during the nucleation and growth of secondary mineral phases, 15.49%−17.01% of Cu(Ⅱ) was incorporated into their crystal structure. The photochemical transformation of Sch significantly enhanced the proportions of adsorbed and structural Cu(Ⅱ), facilitating Cu(Ⅱ) sequestration and reducing its bioavailability. The findings enhanced understanding of the role and immobilization of Cu(Ⅱ) during the phase transformation of Sch in euphotic zone of AMD.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Xiaokang Hou: Writing – original draft. Huanxin Ma: Formal analysis, Data curation. Mengmeng Zhao: Formal analysis. Chunhua Feng: Writing – review & editing. Shishu Zhu: Writing – review & editing, Supervision, Project administration, Funding acquisition.

  Acknowledgments

  This project was financially supported by the National Natural Science Foundation of China (No. U21A2034) and the Guangdong Special Support Plan for Innovation Teams (No. 2019BT02L218).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110332.

  
  
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  Figure 1  XRD results of light-exposed Sch/TA system under (a) anoxic conditions and (b) oxic conditions. FTIR spectra with additions of different Cu(Ⅱ) concentrations under (c) anoxic conditions and (d) oxic conditions.

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  Figure 2  EPR spectra of DMPO adducts under (a) anoxic conditions and (b) oxic conditions. Time courses of ^•OH of light-exposed Sch/TA system with different Cu(Ⅱ) concentrations under (c) anoxic conditions and (d) oxic conditions.

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  Figure 3  Time courses of adsorbed Fe(Ⅱ) of Sch with different Cu(Ⅱ) concentrations under (a) anoxic conditions and (b) oxic conditions.

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  Figure 4  Temporal changes of Cu(Ⅱ) distribution of light-exposed Sch/TA systems containing (a) and (c) 10, (b) and (d) 30 mg/L Cu, (a, b) and (c, d) represent the anoxic and oxic conditions, respectively.

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  Figure 5  XPS analysis of light-exposed Sch/TA systems in the presence of Cu(Ⅱ). (a) Full XPS spectrum, (b) XPS spectra of Cu 2p, (c) XPS spectra of Fe 2p, and (d) XPS spectra of S 2p.

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