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Article

Reductive Sequestration of Chromate with Pyrite-Loaded nZVI@biochar Composites

by
Min Sun
1,2,
Yuechuan Feng
3,
Yao Zhao
2,* and
Xingrun Wang
2,*
1
School of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China
2
State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China
3
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Water 2024, 16(20), 2883; https://doi.org/10.3390/w16202883
Submission received: 30 August 2024 / Revised: 30 September 2024 / Accepted: 4 October 2024 / Published: 10 October 2024
(This article belongs to the Section Wastewater Treatment and Reuse)
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Abstract

:
Various green materials like biochar and Fe0 (nano-scale zerovalent iron, nZVI) have been applied to remediate aqueous Cr(VI) contamination, but few studies have tried to further improve the performance of nZVI and/or biochar composites with different sulfidation methods. Here, we modified a hybrid material of nZVI@biochar with Na2S and pyrite (FeS2), applied it to remove aqueous Cr(VI) under different experimental conditions, and revealed key factors influencing Cr(VI) removal performance. The results show that pyrite loading is an effective sulfidation method to increase the Fe and S contents in composites. FeSx-nZVI@BC (1:1) had a Cr(VI) removal efficiency of ~95% with 5 mg/L Cr(VI) loaded, which was much higher than other hybrid composites. The Cr(VI) removal efficiency of FeSx-nZVI@BC showed a decreasing trend under pH conditions that increased from pH 3 to pH 9. The presence of dissolved oxygen and aqueous Cu2+ and Cd2+ could significantly suppress the removal of aqueous Cr(VI), while humic acids at different concentrations did not suppress Cr(VI) removal. After the reaction, it was observed with an energy-dispersive spectrometer (SEM-EDS) that most Cr in the solid phase was closely associated with pyrite minerals. X-ray photoelectron spectroscopy (XPS) spectra, together with the Fe2+-quenching method, confirmed that Fe (Fe2+ or Fe0) acted as the main electron donor, contributing to ~90% of the Cr(VI) reduction. Our study indicates that pyrite loading could further improve the performance of remediation materials and that the pyrite-loaded nZVI@BC composite is a green material with strong potential to be applied in the remediation of water contaminated by Cr(VI).

1. Introduction

Chromate (Cr(VI)), a typical heavy-metal contaminant, can be released into natural water and soils from various industrial sources, like electroplating, ore mining, leather tanning, pigment production, and mud drilling [1,2,3]. Compared with Cr(III), Cr(VI) is highly toxic, soluble, and bioavailable and thus is more biohazardous in aquatic and soil ecosystems [4,5,6]. As such, there is a compelling need to remediate water and soils contaminated by Cr(VI).
Among various remediation materials, nano-scale zerovalent iron (nZVI) has shown advantages in the in situ treatment of redox-sensitive heavy metals, especially Cr(VI). nZVI has a large surface area and strong reducibility [7], which can readily reduce mobile Cr(VI) to low-mobility Cr(III). Then, Cr(III) could coprecipitate with Fe (oxyhydr)oxides and become fixed in the solid phase [8]. nZVI tends to aggregate to form micro/millimeter-scale flocs [9] and then loses a considerable portion of its reductive activity, whilst Fe(III) and/or Cr(III) precipitation on the surface of nZVI as a thick surface oxide layer passivates the reactivity of core Fe0 [10,11]. As such, different strategies have been investigated to improve the reactivity of nZVI, such as stabilization with biochar to disperse nZVI nano-particles [9]. Biochar made from organic waste via pyrolysis tends to have a large surface area, a porous structure, abundant functional groups, and corrosion resistance [12]. Thus, biochar is often selected as a carbon-based material for preparation of nZVI-BC composites [13] to increase the lifetime and availability of nZVI, which has been applied to remediate various contaminants [14,15,16]. However, the Cr(VI) removal efficiencies of nZVI-BC composites need to be improved to further control the environmental risks of toxic aqueous Cr(VI).
In addition, sulfidation is also widely applied to improve the contaminant removal efficiency of nZVI [17,18] because it is a simple technology that is cost-effective and has environmentally acceptable advantages. Controlled sulfidation has been performed using various sulfidation chemicals (like sulfide (S2−) and dithionite (S2O42−)) and sulfidation processes (like aqueous–aqueous and aqueous–solid processes) [17]. During the sulfidation process, FeS is formed as a shell around a core of Fe0. Because of the larger surface area and stronger reductivity, the hybrid material made from FeS and nZVI has stronger removal efficiency for metals and metalloids than a single material [19,20]. For example, FeS@nZVI can reduce aqueous TcO4− to form solid TcO2 or TcS2, which lowers the mobility of heavy metals [19]. FeS@nZVI can also fix Cd via surface adsorption/complexation on nZVI or by forming an (Fe, Cd)S phase [20]. Considering the fact that most previous studies mainly focused on mixtures of two materials, it is still ambiguous whether a combination of three materials (FeS, nZVI, and biochar) could further improve the removal efficiency of aqueous Cr(VI).
Despite controlled sulfidation with various sulfidation agents, introducing natural pyrite (FeS2) is also an effective but low-cost way to improve the removal efficiency of organic contaminants and heavy metals because pyrite is highly abundant and has strong reduction potential for Fe(II) and S2−, as well as good performance when activating oxidants [21,22]. Pyrite–biochar composites have been applied to remediate various organic pollutants like norfloxain [23] and tetracycline [23]. To the best of our knowledge, however, few studies have compared the different influences of controlled S-loaded and natural pyrite-loaded hybrid materials in the removal of aqueous Cr(VI).
Hence, we synthesized FeS- and pyrite-loaded nZVI@biochar composites and applied them to remove aqueous Cr(VI) under different conditions. Our objectives were (1) to estimate the removal efficiency of Cr(VI) with S-loaded and pyrite-loaded nZVI@biochar; (2) to investigate different environmental factors influencing the Cr(VI) removal efficiency of the composites; (3) to reveal the main mechanisms for Cr(VI) removal with FeS- and pyrite-loaded nZVI@biochar; and (4) to provide fundamental parameters for the application of these green materials to remediate Cr(VI) contamination.

2. Materials and Methods

2.1. Chemicals

All chemicals used in the present study were of analytical grade. Rice husks were obtained from Luoyang City, Henan Province, China. Pyrite was obtained from Maanshang City, Anhui Province, China.

2.2. Preparation of nZVI@biochar

Biochar was synthesized through pyrolysis of rice husks at 500 °C. It was milled and sieved with 0.15 mm and 0.074 mm sieves to obtain a biochar with a size of 75–150 um. About 10 g of biochar was soaked in 300 mL of a 5 g/L FeCl3·6H2O solution for 8 h. After filtration and freeze-dying, the solid materials were pyrolyzed at 800 °C under nitrogen for 5 h to obtain nZVI@biochar.

2.3. Preparation of FeS- or Pyrite-Loaded nZVI@biochar

About 0.8 g nZVI@biochar was added into 200 mL 0.5 mM S2− solutions in the presence of 2.5 g/L acetate sodium with the addition of 0.2 mL acetate acid. The S-nZVI@BC was synthesized after stirring for 4 h and then washed with de-ionized water, freeze-dried and stored for remediation experiments.
The pyrite was milled at 400 rpm with a zirconia ball mill machine, which comprised zirconia balls with diameters of 1, 5, and 10 mm. The pyrite was milled for 10 h, and the rotation direction was altered every hour. Finally, the pyrite was sieved with a 0.074 mm sieve to obtain particles with a size lower than 0.075 mm. Then, the materials were washed with DI water several times until the conductivity of the supernatant water was less than 20 μS cm−1 and then dried at 80 °C. About 0.2/0.8 g pyrite and 0.8 g nZVI@BC were added into 200 mL 2.5 g/L acetate solution with the addition of 0.2 mL acetate acid, where the weight ratio of FeSx to nZVI@BC was 1:4 and 1:1, respectively. After stirring for 4 h, the synthesized FeSx-nZVI@BC (1:4 and 1:1) was washed with DI water, freeze-dried and stored for remediation experiments.

2.4. Remediation Experiments

Considering the fact that Cr(VI) is mainly present in water under an oxic environment, all our remediation experiments were carried out in the presence of dissolved O2 at room temperature. To test the performance of different materials, 0.2 g of each of the composites were mixed with 200 mL 5/10/15 mg/L Cr(VI) solutions for a 48 h reaction. The pH of the solutions was adjusted with dilute HCl or NaOH solutions. The 2 mL subsamples were collected and filtered through a 0.45 μm filter membrane for Cr(VI) measurement at 0.5, 1, 2, 4, 6, 8, 10, 12, 24, 36 and 48 h.
Remediation experiments in the absence or presence of O2, humic acids and different pH conditions (pH 3, 5, 7, and 9) were also conducted to evaluate the influences of different parameters. To test the role of ferrous ions in the reaction of Cr(VI), 5.0 mmol/L 1,10-phenanthroline was introduced in the reaction suspension to quench Fe(II) formed in systems.

2.5. Characterization

The structure surface morphology and surface elemental composition of composites were characterized by a FEI Quanta 250 scanning electron microscope and energy-dispersive spectrometer (SEM-EDS). The minerals in the composites before and after the reaction with Cr(VI) were identified by X-ray powder diffraction (XRD) obtained with a Bruker D8 ADVANCE diffractometer with Cu-Ka radiation. The changes in the elemental compositions of composites were observed with X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha, Beijing, China).

2.6. Analytical Methods

The amount of Cr(VI) in the filtrate was measured by the 1,5-diphenylcarbazide colorimetric method at a wavelength of 540 nm [24,25] on a UV-vis spectrometer. The number of Fe2+ ions was measured with the 1,10-phenanthroline colorimetric method at a wavelength 510 nm [8]. The total amounts of aqueous Cr and Fe were analyzed by inductively coupled plasma mass spectrometry (ICP-MS, Shimadzu ICPE-9800, Beijing, China).

3. Results and Discussion

3.1. Characterization of Composites

The morphology and composition of composites were characterized by the SEM equipped with EDS (Figure 1). As shown in Figure 1a, the morphology of S-nZVI@BC seemed unchanged after sulfidation with the Na2S solution, but for FeSx-nZVI@BC (Figure 1b,c), micro/nanomineral particles were clearly observed. The EDS mapping images also confirmed the presence of a large amount of Fe and S on the surface of the biochar for the FeSx-nZVI@BC composites, where Fe and S were distributed closely together, indicating the presence of pyrite particles. Regarding the molar fractions of Fe and S in the composites, S and Fe were only 0.1% and 0.08% for S-nZVI@BC, respectively, much lower than those in the FeSx-nZVI@BC composites (both S and Fe > 1%). The molar ratio of S:Fe for FeSx-nZVI@BC (1:4) was near 1:1 but was 2:1 for FeSx-nZVI@BC (1:1) (Table 1). The different molar ratio of S:Fe in FeSx-nZVI@BC (1:4) was probably derived from the additional presence of zerovalent ion in the composites. Then, the XRD was applied to characterize the minerals (Figure 2), where peaks at 28.4°, 33.0°, 37.2°, 40.8°, 47.4°, 56.3°, etc., were identified for pyrite in the FeSx-nZVI@BC composites (1:4, and 1:1), suggesting pyrite as a dominating mineral. Overall, sulfidation with the addition of pyrite is an effective way to increase the reductive components in the composites, which could further influence the fate of Cr(VI) in systems.

3.2. Aqueous Cr(VI) Removal

The results of Cr(VI) removal with different composites are shown in Figure 3. Compared with the different Cr(VI) loadings, the Cr(VI) removal efficiency of all materials showed a decreasing trend with increasing Cr(VI) concentrations from 5 mg/L to 15 mg/L. The lower removal efficiency of Cr(VI) at higher Cr(VI) concentrations could be attributed to the limited capacity of materials to remove aqueous Cr(VI) via adsorption, reduction and surface precipitation. Compared with different materials, the Cr(VI) removal efficiency of the FeSx-nZVI@BC composites was much higher than that for nZVI@BC and S-nZVI@BC but a little lower than or similar to that for pure pyrite at the different Cr(VI) loadings. This confirmed that the loading of pyrite could increase the capacity of materials to remove aqueous Cr(VI) and fix Cr in the solid phase. Here, it was observed that FeSx-nZVI@BC (1:1) had a higher removal efficiency than FeSx-nZVI@BC (1:4), suggesting that the amount of pyrite loading was the key factor influencing the capacity for Cr(VI) removal. This was probably because the loading of pyrite introduced more reductive components like Fe0, Fe2−, S2−, and Sx2− into the composites, which were reactive to Cr(VI). In addition, it was also observed that the removal efficiency of Cr(VI) for nZVI@BC was higher than that for S-nZVI@BC at 5 mg/L and 15 mg/L Cr(VI). A possible reason for this was the lower amount of reductive components in S-nZVI@BC than that in nZVI@BC because some Fe0 was released from the solid phase during sulfidation with the Na2S solution (Figure S1). In addition, because the biochar had a porous structure, better corrosion resistance, good penetration properties and lower density, FeSx-nZVI@BC composites also inherited these advantages and thus presented a high potential to be applied as an in situ barrier material.

3.3. Effect of Different Conditions on Cr(VI) Removal

The influence of the initial pH conditions on the Cr(VI) removal was also investigated (Figure 4). The removal efficiency of Cr(VI) generally decreased with increasing pH conditions from pH 3 to pH 9, which is consistent with previous studies about the reaction between Cr(VI) and FeS@nZVI [8,26]. The reaction between aqueous Fe2+ and Cr(VI) might consume H+, and thus the redox reaction was favored at pH 3 [27]. Under acidic conditions, reductive components like Fe2+ could be readily released from the positively charged mineral surface and then react with Cr(VI) to form Fe(III) and Cr(III). Then, Fe(III)/Cr(III) could hydrolyze and precipitate from aqueous solutions, but it probably did not precipitate on the mineral surface to form hydroxide layers. In contrast, as the pH increased to pH 9, the mineral surface became negatively charged and favored the adsorption of Fe2+, but this was not the case for Cr(VI) [28,29]. As such, the reaction between Fe2+ and Cr(VI) mainly occurred on the mineral surface, and the reaction rate decreased [8]. The surface-bound Fe2+ or Fe0 directly reacted with Cr(VI) to form Fe(III) and Cr(III), which could form thick hydroxide layers on the mineral surface and inhibit further redox reactions [30].
As all the removal experiments for Cr(VI) were conducted in the presence of dissolved oxygen, the role of oxygen was also investigated under anoxic conditions. It was observed that the Cr(VI) removal efficiency of FeSx-nZVI@BC (1:1) increased from ~80% to 100% at 10 mg/L Cr(VI) in the absence of oxygen (Figure 5a). A possible reason for this is that the dissolved oxygen could react with reductive components in the composites, which reduced the capacity of composites to reduce and fix aqueous Cr(VI) [8]. As such, the application of FeSx-nZVI@BC (1:1) to remediate the contaminated underground water with low dissolved oxygen levels could achieve better performance.
Considering the fact that anions and cations prevail in water, representative organic matter (humic acid) and cations (Cu2+ and Cd2+) were selected to investigate their influences on Cr(VI) removal. As shown in Figure 5b, the presence of humic acid at 2 and 20 mg/L increased the removal rate of Cr(VI) within 5 h, while humic acid at 0.2 mg/L had no discernible influences on Cr(VI) removal. Here, humic acid at different concentrations had almost no influence on the final removal efficiency of Cr(VI). It was also documented that oxalic acid could promote the removal of aqueous Cr(VI), which is probably because oxalic acid can act as electron donors, facilitating electron transfer between FeS2 and Cr(VI) [26]. Although humic acid is negatively charged and can impede the adsorption of Cr(VI) onto pyrite [31,32], it also contains many reductive components like phenol or quinone structures, which could reduce Cr(VI) to Cr(III) [33,34,35,36]. It was easier for Cr(III) to chelate with carboxylic groups in the humic acids through ligand exchange and/or to coprecipitate with Fe(III) [28,37,38,39,40], after which Cr is removed from the solution to reach a solid phase. As for the presence of cations like Cu2+ and Cd2+, the removal of Cr(VI) was significantly reduced to half of that in the absence of cations. The influence of cations on Cr(VI) removal has seldom been reported in previous studies. In fact, underground water was enriched in various cations like Ca2+ and Mg2+, which probably caused the same suppression on the removal of aqueous Cr(VI) in contaminated regions. One possible mechanism for this is that Cu2+ and Cd2+ could be adsorbed onto the surface of biochar and then mitigate the negative surface charge to facilitate the adsorption of Cr(VI) onto the biochar surface. As shown in the SEM images (Figure 1), the pyrite particles, as the main reductants, dominated the surface of the biochar. The adsorption of Cr(VI) onto biochar might decrease the chance of Cr(VI) coming into contact with reductive chemicals like FeS2 in pyrite or aqueous Fe(II). Another mechanism behind this might be that Cu2+ and Cd2+ could also be adsorbed onto pyrite particles and then suppress the dissolution of FeS2 and the release of aqueous Fe2+, as a result of which less Cr(VI) would be reduced and removed from solution. In addition, the adsorption of cations on the pyrite mineral surface also hindered the contact and reaction between Cr(VI) and reductive components in the pyrite.
After three cycles of removal experiments with FeSx-nZVI@BC (1:1), the Cr(VI) removal efficiency decreased to 50% and 20% gradually (Figure 5d). This gradual decrease in the removal efficiency was probably due to the consumption of electron donors and/or the Fe(III)-Cr(III) hydroxides layers surrounding the reactive minerals [10,11], which hindered the transfer of electrons from electron donors to Cr(VI). Nevertheless, the Cr(VI) removal efficiency after two cycle was still similar to that with FeSx-nZVI@BC (1:4), S-nZVI@BC and nZVI@BC.

3.4. Mechanisms of Cr(VI) Removal

SEM, XRD and XPS were further applied to evaluate the change in the morphology, Cr distribution, mineral composition and surface valence state of Fe and S before and after the reactions. After reaction, the Cr in the solid phase was mainly associated with the pyrite, as observed with SEM-EDS, where signals of Cr synchronously increased with those for S and Fe (Figure 6). Our XRD results showed that after each reaction, the peaks for pyrite decreased but still existed in the reaction systems of FeSx-nZVI@BC (1:1 and 1:4) (Figure 2), while Cr(VI) was not completely removed (Figure 3). This was probably because Cr(III) formed a thick surface oxide layer around pyrite, hindered the reaction between pyrite and Cr(VI), and finally decreased the removal efficiency of Cr(VI). A similar phenomenon has been observed in previous studies, where Fe(III) and/or Cr(III) precipitated on the surface of nZVI to form a thick surface oxide layer and passivated the reactivity of core Fe0 [10,11]. Here, we provided direct evidence for the close association between Cr and pyrite through in situ observation. The distribution of Cr demonstrated that pyrite minerals were the reactive components used to control the fixation of Cr(VI). This explained why higher pyrite loading in FeSx-nZVI@BC (1:1) facilitated the removal of aqueous Cr(VI). On the other hand, the presence of pyrite particles suggested that there were large amounts of reductive chemicals, which might further react with Cr(VI) in suitable conditions. As such, the pyrite in the composites still had great potential capacity to remove Cr(VI) from the aqueous solution.
The valence of elements on the surface also changed after the reaction with Cr(VI). In Figure 7a, the Fe 2p peaks at 710.6 eV and 724.3 eV corresponded to Fe(II), while the peaks at 712.4 eV, 719 eV and 726.0 eV were assigned to Fe(III) [8]. Additionally, the peak at 706.4 eV was also observed for Fe0 in FeSx-nZVI@BC (1:1), which was a strong reductant. After the reaction, the peaks and the corresponding proportion of Fe0 significantly reduced from 14% to 3%, whilst the proportion of Fe(III) increased from 65% to 82%, confirming the reaction between Fe0 and Cr(VI). As for the valence of S in the pristine composites, the peaks at 161.6 eV and 164.0 eV were assigned to S2−, whilst the peak at 168.8 eV corresponded to SOx2− [41]. After the reaction with Cr(VI), the intensity of the peak at 162 eV significantly decreased, but the peaks for SOx2− seemed unchanged, possibly because SOx2− was released into the aqueous solution during the reaction process. The XPS patterns for the composites after the reaction presented a pair of Cr(III) peaks at 576.8 eV and 586.4 eV and a pair of Cr(VI) peaks at 579.3 eV and 588.3 eV (Figure 7), validating the reduction of Cr(VI) to Cr(III). Interestingly, the presence of Cr(VI) in the solid phase was probably attributed to the coprecipitation of Cr(VI) and Fe(III)-Cr(III) hydroxides, which protected Cr(VI) from attack by electrons [28]. Alternatively, Cr(VI) might be adsorbed onto biochar, likely into its surface pores, which could shield Cr(VI). In summary, aqueous Cr(VI) could be reduced by electrons; meanwhile, electron donors like Fe(II), Fe0 and S(II) were oxidized in the reaction.
As both Fe and S could act as electron donors for Cr(VI) reduction, the role of Fe(II) was thus investigated by adding 1,10-phenanthroline as an Fe(II) quencher. As shown in Figure 8, the presence of 1,10-phenanthroline decreased the Cr(VI) removal efficiency from 100% to 14% compared with systems without Fe(II) quenchers, suggesting that Fe(II) might contribute to 86% of aqueous Cr(VI) removal. Previous studies suggested that dissolved Fe2+ from FeSx contributed to 30~50% of Cr(VI) removal from a solution [8,27], which was lower than that in our studies, probably because of the presence of Fe0 in our systems. As our batch experiments were conducted in an oxic environment, the exposure to O2 could have resulted in the formation of aqueous Fe(II) from the reaction between Fe0 and O2, which contributed to Cr(VI) removal. During the reaction, the concentration of aqueous Fe(II) also increased to 0.32 mmol/L, which was much higher than in SO42−, indicating that a small amount of S(II) was involved in the reduction of Cr(VI). The lower reaction between S(II) and Cr(VI) was probably because it was difficult for Cr(VI) to come into contact with the solid structure of S(II) directly. As such, aqueous Fe(II) from the dissolution of Fe0 or FeS2 might be more effective for reducing Cr(VI), which contributes significantly to Cr(VI) removal. The contribution of Fe0 to Cr(VI) reduction was supported by the significant decrease in the amount of Fe0 observed in the XPS spectra. Overall, Fe(II) played a key role in the reduction and fixation of aqueous Cr(VI) during the treatment of FeSx-nZVI@BC(1:1).

4. Conclusions

In this study, we compared the Cr(VI) removal performance of S-loaded and pyrite-loaded nZVI@biaochar composites. The results showed that pyrite loading was an effective sulfidation method to increase the Fe and S contents in composites. FeSx-nZVI@BC (1:1) presented a higher removal efficiency than other hybrid composites. Regarding the influences of different environmental factors, the removal efficiency of Cr(VI) by composites showed a decreasing trend with increasing pH conditions. The presence of dissolved oxygen and heavy metals like Cu2+ and Cd2+ could suppress the removal of aqueous Cr(VI), while humic acids seemed to have no influence on Cr(VI) removal efficiency. After the reaction, most Cr in the solid phase was associated with pyrite minerals. The XPS spectra together with the Fe2+ quenching method confirmed that Fe (Fe2+ or Fe0), as the main electron donor, contributed to ~90% of Cr(VI) reduction. Our study suggests that the pyrite-loaded nZVI@BC composite showed strong potential to be applied in the remediation of water or soils contaminated by heavy metals like Cr(VI).

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/w16202883/s1: Figure S1: XPS spectra of the Fe 2p region, the S 2p region and the Cr 2p region of S-nZVI@BC before (a and b) and after reaction (c, d and e).

Author Contributions

Conceptualization, M.S.; methodology, M.S. and Y.Z.; investigation, M.S.; writing—original draft preparation, M.S.; writing—review and editing, Y.F., Y.F., Y.Z. and X.W.; supervision, X.W. and Y.Z.; project administration, Y.Z.; funding acquisition, X.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (No. 42307316).

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of 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.

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Water 16 02883 g001
Figure 1. SEM images and EDS mapping.
Figure 1. SEM images and EDS mapping.
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Figure 2. XRD patterns of different composites before and after reaction with Cr(VI).
Figure 2. XRD patterns of different composites before and after reaction with Cr(VI).
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Water 16 02883 g003
Figure 3. Comparison of Cr(VI) removal with different composites at different Cr(VI) concentrations of 5 mg/L (a), 10 mg/L (b) and 15 mg/L (c) in the presence of dissolved oxygen.
Figure 3. Comparison of Cr(VI) removal with different composites at different Cr(VI) concentrations of 5 mg/L (a), 10 mg/L (b) and 15 mg/L (c) in the presence of dissolved oxygen.
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Figure 4. Effect of the initial pH conditions on the removal of Cr(VI) at 10 mg/L in the presence of dissolved oxygen.
Figure 4. Effect of the initial pH conditions on the removal of Cr(VI) at 10 mg/L in the presence of dissolved oxygen.
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Figure 5. Influences of oxygen (a), humic acids (b) and cations (c), as well as the reusability of composites (d), on Cr(VI) removal at 10 mg/L Cr(VI) in the presence of dissolved oxygen.
Figure 5. Influences of oxygen (a), humic acids (b) and cations (c), as well as the reusability of composites (d), on Cr(VI) removal at 10 mg/L Cr(VI) in the presence of dissolved oxygen.
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Figure 6. Distribution of Cr in the solid phase after reaction with FeSx-nZVI@BC composites at pH 5 in the presence of 10 mg/L Cr(VI).
Figure 6. Distribution of Cr in the solid phase after reaction with FeSx-nZVI@BC composites at pH 5 in the presence of 10 mg/L Cr(VI).
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Figure 7. XPS spectra of the Fe 2p region, the S 2p region and the Cr 2p region of FeS2-nZVI@BC(1:1) before (a,b) and after the reaction (c–e).
Figure 7. XPS spectra of the Fe 2p region, the S 2p region and the Cr 2p region of FeS2-nZVI@BC(1:1) before (a,b) and after the reaction (c–e).
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Figure 8. Removal efficiency of Cr(VI) by FeS2-nZVI@BC(1:1) in the absence of 1,10-phenanthroline at 10 mg/L Cr(VI) in the presence of dissolved oxygen.
Figure 8. Removal efficiency of Cr(VI) by FeS2-nZVI@BC(1:1) in the absence of 1,10-phenanthroline at 10 mg/L Cr(VI) in the presence of dissolved oxygen.
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Table 1. EDS elemental analysis of materials.
Table 1. EDS elemental analysis of materials.
SampleMass Fraction (%)Molar Fraction (%)
CNOSiPSFeSFe
S-nZVI@BC84.980.6311.242.440.090.240.370.100.08
FeSx-nZVI@BC (1:4)70.23017.83.290.012.985.681.281.40
FeSx-nZVI@BC (1:1)53.4309.822.010.0618.4916.199.614.83
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Sun, M.; Feng, Y.; Zhao, Y.; Wang, X. Reductive Sequestration of Chromate with Pyrite-Loaded nZVI@biochar Composites. Water 2024, 16, 2883. https://doi.org/10.3390/w16202883

AMA Style

Sun M, Feng Y, Zhao Y, Wang X. Reductive Sequestration of Chromate with Pyrite-Loaded nZVI@biochar Composites. Water. 2024; 16(20):2883. https://doi.org/10.3390/w16202883

Chicago/Turabian Style

Sun, Min, Yuechuan Feng, Yao Zhao, and Xingrun Wang. 2024. "Reductive Sequestration of Chromate with Pyrite-Loaded nZVI@biochar Composites" Water 16, no. 20: 2883. https://doi.org/10.3390/w16202883

APA Style

Sun, M., Feng, Y., Zhao, Y., & Wang, X. (2024). Reductive Sequestration of Chromate with Pyrite-Loaded nZVI@biochar Composites. Water, 16(20), 2883. https://doi.org/10.3390/w16202883

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