3.1 Characterization and speciation distribution of each scale layer
SEM images (Fig. 1) have demonstrated the differences in morphology of each scale layers. More loose and porous structure could be observed in the porous core layer (Fig. 1 (c)), which may be beneficial for metal adsorption and accumulation. More significant characteristics can be seen from images magnified by 50,000 times (Fig. 1 (d), (e), and (f)). Combined with the XRD spectra result (Fig. S2), the main components defined in the shell-like layer of the corrosion scales were magnetite (Fe3O4) and hematite (Fe2O3), considered to be relatively stable. Goethite (a-FeOOH) and lepidocrocite (γ-FeOOH) were the main crystalline iron minerals in surface layer and porous core layer. Table S5 and S6 shows the distribution of metal content in different layers of the iron corrosion scales. Cr exhibited the highest content in the surface layer of 102.63 mg/kg. And the contents of Cr in the shell-like layer and the porous core layer were 37.67 mg/kg and 47.05 mg/kg, which showed little difference. The total content of Cr was 187.35 mg/kg, that was comparable to the 90th percentile level in Peng's study of 58 scale samples from 20 US drinking water utilities (Peng et al., 2012).
The speciation distribution of metal elements in each scale layer was investigated to analyze their mobility and release potential in corrosion scales (Figure 2). The fraction distribution of Fe showed little difference in each of the three layers. But beyond that, the residual fraction of all the other metal elements was significantly much more in the porous core layer, especially for Al (83.46%) and Cr (72.05%). This result demonstrates that metal elements in the porous core layer mainly present in stable matrices and were relatively immobile. Whereas metal elements in the surface layer and shell-like layer have stronger mobility and release potential. Therefore, the surface layer and shell-like layer of corrosion scale samples were combined to represent the outer layer in subsequent studies, in comparison to the inner porous core layer. In the surface layer and shell-like layer, the majority of Cr (67.34% and 41.78%) existed in reducible fraction. This corroborated that the reducible fraction dominated by heavy metals bound to Fe-Mn oxides exhibited a strong affinity for Cr (Rai et al., 1989).
3.2 Factors influencing chromium accumulation
3.2.1 Effect of pH
The effects of pH on chromium accumulation are shown in Fig. 3 (a). Data was measured at 1440 min when the accumulation process was approaching chemical equilibrium. Results demonstrated that the accumulated amount of Cr(VI) slightly increased under higher pH conditions, and the average value was about 1.6 mg/L. But it is of note that Cr(III) accumulation was much smaller than Cr(VI) and changed dramatically when pH increased to 10. Previous studies have shown that under acidic conditions, H+ ions and metal ions compete with each other for the available surface sites (Gu et al., 2018), resulting in less heavy metal accumulation in corrosion scales. In addition, pH affects chromium speciation, especially Cr(III). The simulation results by Visual MINTEQ 3.1 (Fig. S3 (b)) show that Cr3+ and Cr(OH)2+ were the dominant species under acidic conditions, while under neutral conditions, Cr(III) mainly existed in the form of Cr(OH)3 precipitation. At pH levels above 10, Cr(III) existed partially in the form of Cr(OH)4-. Therefore, although the initial concentration of Cr(III) was 2.5 mg/L, the actual concentrations present in solution were 1.83, 1.7, 0.23, 0.32, 1.66 mg/L at pH 4, pH 5.5, pH 7, pH 8.5, pH 10, respectively. Thus the accumulation ratio was calculated and illustrated in Fig.S4 (a). Similarly, Cr(VI) accumulation ratio increased slightly as pH rose. The accumulation ratio of Cr(III) showed a rising trend in general but increased rapidly around pH 7. This was mainly attributed to the high level of Cr(OH)3 precipitation under near-neutral pH conditions. The accumulation ratio of Cr(VI) was about 90 % under all experimental pH conditions, significantly higher than that of Cr(III).
However, the pH of water in DWDS is universally neutral and at pH 7 the average amount of Cr(VI) accumulated was 1.56 mg/g. While only 0.16 mg/g Cr(III) was found to accumulate on average, approximately ten-fold less than Cr(VI) under the same pH conditions. There was also a difference of about 20% in accumulation ratio. This indicated that in DWDS Cr(VI) release should be considered as a major concern, due to its high toxicity and potential for accumulation.
3.2.2 Effect of temperature
The effect of temperature on chromium accumulation was investigated at different temperature conditions of 5 ℃, 15 ℃, and 25 ℃. Temperature mainly exerts an influence by affecting the reaction rate, chemical equilibrium and microbiological processes. Fig. 3 (b) and Fig. S4 (b) shows the result. The mean accumulated amounts of Cr(VI) were 2.24, 2.25, and 2.10 mg/g at temperatures of 5 ℃, 15 ℃, and 25 ℃, respectively, indicating that temperature exerted no significant effect on Cr(VI) accumulation. For Cr(III), rising temperatures induced an increase in accumulation, with the ionic mobility of chromium increasing, resulting in a greater contact probability between chromium and available sites on the corrosion scale surface (Gupta et al., 2013). When the temperature was 25 ℃, the accumulated amount and ratio of Cr(III) both reached a maximum level of 0.016 mg/g and about 70 % on average. Overall, Cr(VI) accumulation in corrosion scales was much greater than Cr(III) accumulation.
On the other hand, chromium accumulated in the inner layer was always more than that in the outer layer in general. Moreover, the accumulation of Cr(VI) was significantly greater than that of Cr(III). Compared with Cr(III), results indicate that it is easier for Cr(VI) to migrate from the water phase to the corrosion scale phase and, thus Cr(VI) was more likely to be absorbed and accumulate in corrosion scales.
3.2.3 Cr (VI) accumulation kinetics
The effects of reaction time on Cr(VI) accumulation are shown in Fig. S5. Due to sufficient availability of surface sites, the accumulation rate of Cr(VI) increased rapidly within the initial 60 min. However, equilibrium was not reached until 2880 min. To further investigate the mechanisms of Cr(VI) accumulation, the pseudo-first-order kinetic model, pseudo-second-order kinetic model, Elovich model, and Webber Morris intra-particle diffusion model were used to analyze Cr(VI) accumulation kinetics data. The kinetic parameters of these four models are summarized in Table S7. The correlation coefficients (R2) of the pseudo-second-order kinetic model were greater than that of the pseudo-first-order kinetic model, indicating that the rate-limiting step was the chemisorption process. The Elovich model R2 values for outer layer and inner layer Cr (VI) accumulation were 0.889 and 0.910, respectively. The validity of the Elovich model implies that the initial Cr (VI) accumulation process was rapid, while the desorption process was slow and occasional (Labied et al., 2018). Webber-Morris intra-particle diffusion model data exhibited two unique linear plots, demonstrating that the accumulation procedure was not only controlled by intraparticle diffusion.
3.3 Factors influencing chromium release
3.3.1 Effect of pH
Similar to the accumulation process, pH affected chromium release by changing the surface charge and degree of ionization of corrosion scales, the speciation distribution of chromium. Under low pH conditions, Cr(VI) mainly exists in the forms of HCrO4−, while CrO42− is the dominant species under high pH conditions. CrO42− can easily bind with heavy metals in corrosion scales (Xu et al., 2011). The effect of pH on Cr(VI) release is shown in Fig. 4. The Cr(VI) equilibrium release amounts in the outer layer and the inner layer were both ranked in the descending order of pH 4 > pH 7 > pH 5.5 > pH 10 > pH 8.5. Overall, acidic conditions were beneficial to Cr(VI) release, while alkaline conditions impeded release. With a reduction in pH, ion-exchange processes occur between H+ ions and Cr(VI) ions (Jin et al., 2018). Meanwhile, H+ ions promote the dissolution of Fe-Mn oxides (Guo et al., 2019), increasing Cr(VI) release from corrosion scales. Furthermore, an increase in pH may convert aqueous chromium ions into hydrolyzed ion forms, resulting in lower solvation energies being required for the surface binding process (Moutsatsou et al., 2006). However, Cr(VI) release reached a minimum level at pH 8.5 rather than pH 10, which may partly be due to higher pH conditions resulting in excessive deprotonation of the corrosion scale surface, decreasing the affinity between CrO42- ions and corrosion scales (Rahman et al., 2013).
Under all pH conditions tested, Cr(VI) released from the outer layer of corrosion scales was consistently greater than from the inner layer, which could be explained by different physicochemical speciation distributions of chromium, as shown in Fig. 2. The results of sequential extraction show that chromium in the outer layer of corrosion scales possessed greater mobility than the inner layer. In addition, chromium mainly existed in a bound form with carbonate minerals and Fe-Mn oxides in the outer layer, exhibiting to be more sensitive to pH variations.
Compared with Cr(VI), the release of Cr(III) fluctuated considerably, as shown in Fig.4 (c) and (d). This was largely due to the release amount of Cr(III) being much smaller and therefore, more affected by instrument error. Results appear to show that Cr(III) has weaker mobility and was less easily released from the corrosion scale phase to the water phase.
3.3.2 Effect of temperature and release kinetics
In general, DWDS temperature conditions exhibit distinct seasonal and diurnal changes. Therefore, the effects of temperature on chromium release were studied and the results are shown in Fig. 5. The amount of Cr(VI) released at 25 ℃ (28.10 μg/L from the outer layer, 23.82 μg/L from the inner layer) was significantly greater than at 5 ℃ (10.35 μg/L from the outer layer, 10.22 μg/L from the inner layer) or 15 ℃ (13.81 μg/L from the outer layer, 12.76 μg/L from the inner layer). This may be attributed to the mobility of metal ions and the ion exchange rate being increased at higher temperatures, thereby chromium migrated more easily from the corrosion scale phase to the water phase. Moreover, the increase in temperature could accelerate the dissolution of precipitants such as carbonate bound fractions, resulting in an increase in the release of chromium.
Cr(VI) release kinetics tests in the temperature range of 5 ℃ to 25 ℃ were performed to further illustrate the effect of temperature, with the Elovich model, Double constant model, and pseudo-second-order kinetic model applied. Detailed information and parameters for these models are provided in Table S8. The kinetic models selected all fitted data well (Fig. 5 (a)), while the Elovich model fitted best on the whole, indicating that chromium release may be controlled by a variety of complex reaction mechanisms.
The values of the parameters for the two-constant model at different reaction temperatures (Table S8) showed that the values of the initial release rate constant ‘a2’ varied relatively widely. This is consistent with previous results on the increase of Cr(VI) release as temperature went higher. In Elovich model, value ‘a1’ represents the initial desorption rate and value ‘b1’ is the desorption constant (Rezaei Rashti et al., 2014). In the present study, the Elovich model rate constant ‘a1’ values increased widely in both the outer layer and the inner layer when temperature increased (Table S7). Moreover, the values of constant ‘b1’ were compared in the outer layer and the inner layer, showing that ‘b1’ values in the outer layer were consistently larger at all assessed temperatures. This result is can also be explained by the different physicochemical speciation distributions of chromium in the two iron corrosion scale layers.
The release of Cr(III) under all conditions was less than 8 μg/L, with data showing no obvious trend with varying temperature conditions (Fig. S6). However, combined with the data on the effects of temperature on Cr(VI) release, it can be concluded that the release potential of chromium is theoretically smaller at lower temperatures. Therefore, more attention should be paid to chromium release in high temperature environments and seasons.
3.3.3 Effect of sulfate and chloride
The concentration of chromium released into solution under three sulfate and chloride concentrations (50, 150, 250 mg/L) was determined over time. Fig. 6 (a) and (b) exhibit the effect of sulfate on Cr(VI) release. In the outer layer, the equilibrium concentrations of Cr(VI) in solution were 47.52, 69.13 and 31.64 μg/L at sulfate concentrations of 50, 150, and 250 mg/L, respectively. The data for Cr(VI) release from the inner layer showed a similar trend with 22.23, 27.52 and 18.16 μg/L, although with relatively less variation. This could be attributed to competitive adsorption and competitive electron transfer reactions occurring between chromium ions and sulfate (Meena and Arai, 2016). Furthermore, sulfate may affect the structure, solubility, or microbiological processes in corrosion scales (Lytle et al., 2020), resulting in sulfate being beneficial to chromium release. However, when the concentration of sulfate increased to 250 mg/L, Cr(VI) release decreased. It has previously been reported, that excess sulfate appears to increase the sorption of chromium (Zachara et al. 1988). A possible explanation for this may be that complexes are formed between sulfate and Cr(VI), resulting in re-absorption onto corrosion scales. It has previously been demonstrated that a new species CrO3SO42− was formed in the presence of sulfate (Han et al., 2008).
The effect of chloride on Cr(VI) release is presented in Fig. 6 (c) and (d). When the concentration of chloride increased from 50 mg/L to 150 mg/L, the equilibrium concentration of released Cr(VI) increased from 8.94 to 13.20 μg/L, respectively. Similarly to sulfate, chloride also competes with heavy metals for adsorption sites and electron transfer reactions (Peng et al., 2013), increasing Cr(VI) release. The presence of sulfate and chloride could also increase the acidity of the solution (Sarin et al., 2004), promoting the dissolution of iron corrosion scales. However, when the concentration of chloride was increased to 250 mg/L, chromium release decreased by about 4.24 μg/L compared with that at 150 mg/L chloride, which was in agreement with previously reported results (Lytle et al., 2020). This phenomenon may occur due to the existence of CrO3Cl− under excessive chloride concentrations (Han et al., 2008), indicating that chloride may complex with Cr(VI) and therefore inhibit Cr(VI) release.
Fig. S7 illustrates the effects of varying concentrations of sulfate and chloride on Cr(III) release. The effect of sulfate on Cr(III) release was similar to the effect on Cr(VI), while chloride exerted no significant effect. However, the level of Cr(III) release was less than 5 μg/L and therefore, values fluctuated greatly and were more affected by instrument measurement error. Overall, sulfate and chloride exerted a similar effect on chromium release. When the concentration of sulfate or chloride increased from 50 mg/L to 150 mg/L, an increase in chromium release was observed. In contrast, an excess of sulfate or chloride (250 mg/L) may result in the formation of complexes with chromium ions, resulting in the inhibition of chromium release.
3.4 Analysis of factors affecting Cr(VI) release according to the results of the orthogonal experiments
Fig. 7 shows the results of orthogonal experiments based on range analysis. Detailed data is presented in Table S9 and Table S10. Results demonstrate that Cr(VI) released from the outer and inner layer exhibited similar trends under the co-effect of multifactors. Whereas Cr(VI) released from the inner layer was always less than that from the outer layer, which also illustrates the significant difference in heavy metal release from different layers of corrosion scales. As for the outer layer, temperature, sulfate, pH, and chloride exerted an inhibitory effect on Cr(VI) release (in descending order), as shown by the range analysis calculation results. According to the results of variance analysis (Table S11), temperature, sulfate, and pH had significant effects (1% significance level) on Cr(VI) release. Meanwhile, chloride was found to have a significant effect (5% significance level). Based on the results of this study, during the stable running period of DWDS, lower temperatures, alkaline pH values, and lower concentrations of sulfate and chloride can have significant effects on controlling Cr(VI) release. However, temperature is typically an uncontrolled factor in DWDS, hence the monitoring of water quality in DWDS should be strengthened in high temperature seasons and locations, to prevent rapid and large-scale release of heavy metals in drinking water resources. Furthermore, only modest changes are observed in the pH values in DWDS. Subsequently, the concentrations of sulfate and chloride in DWDS should be the main focus in control strategies, especially sulfate.
As for the inner layer, according to the range analysis and variance analysis, pH exerted a significant effect on Cr(VI) release at the 5% significance level, while temperature exerted a significant effect at the 10% level. In contrast, sulfate and chloride exerted no significant effect on Cr(VI) release. Similar to the results for the outer layer, control strategies should focus on the occurrence of heavy metal release due to seasonal changes in water temperature.
3.5 Mechanism analysis
Combined all the results, it could be concluded that the behaviors of chromium differed in corrosion scales, spatially in different layers and temporally in different DWDS running periods. The complexity of corrosion scales and DWDS makes it difficult to determine the mechanism. Some studies have investigated that surface complexation, surface precipitation are the principal mechanisms of chromium retention by iron (oxyhydr)oxides (Johnston and Chrysochoou, 2014; Kumar et al., 2014; Shi et al., 2021). Surface precipitation of Cr(OH)3 is the main retention mechanism of Cr(III) at neutral pH conditions. For Cr(VI), the initial driving force might be the electrostatic attraction between positively charged iron (oxyhydr)oxides and negatively charged Cr(VI) species (Fischer et al., 2007). Then ion exchange between surface hydroxyl and chromium is the principal mechanism involved (Zhang et al., 2019). Inner-sphere complexation may form through ligand exchange, while electrostatic interaction or van der Waals forces count on outer-sphere complexation (Kumar et al., 2014). Mineral species, metal species, and reacting conditions all make a difference to specific process. Section 3.1 have analyzed the difference in mineral composition of each scale layer. The shell-like layer was mainly composed of more thermodynamically stable iron oxides (hematite (Fe2O3) and magnetite (Fe3O4)). Whereas surface layer and porous core layer were dominated by iron oxyhydroxides (goethite (a-FeOOH) and lepidocrocite (γ-FeOOH)), relatively instable. The differences in crystallographic face and bulk structure of iron (oxyhydr)oxides make the retention mechanism and metal capacity vary. The mechanism analysis diagram of chromium retention by iron (oxyhydr)oxides is shown in Fig.S8.
Goethite (a-FeOOH) and lepidocrocite (γ-FeOOH) are different crystal forms of iron oxyhydroxide. Lepidocrocite (γ-FeOOH) has layered structure and goethite (a-FeOOH) has tunnel structure. CrO42- has a strong affinity on goethite (a-FeOOH) surface, especially under acidic conditions. Similar to hematite (Fe2O3), the dominant species changed from diodentate complex to monulnar complex with the increase of pH (Marchi et al., 2015). Wu et al. shows that lepidocrocite (γ-FeOOH) exerted higher Cr(VI) removal efficiency than goethite (a-FeOOH) (Wu et al., 2016).
Hematite (Fe2O3) is rhombohedral hexagonal and isomorphic to corundum. Several studies have suggested that the retention mechanism and chromium capacities of hematite (Fe2O3) were independent of ionic strength, but closely related to pH values (Johnston and Chrysochoou, 2012, 2014; Huang et al., 2016). Johnston and Chrysochoou revealed that nonprotonated monodentate complexes were dominant at pH 6 and above while more bidentate complexes were found below pH 6 (Johnston and Chrysochoou, 2014).
The crystallographic form of magnetite (Fe3O4) is cubic inverse spinel (Shi et al., 2021), the existence of both Fe2+ and Fe3+ in its structure make it different from other iron (oxyhydr)oxides. Oxidation–reduction between Cr(VI) and magnetite (Fe3O4) provides another mechanism for Cr(VI) retention (Chowdhury et al., 2012).