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Article

Rapid Microwave Irradiation-Enhanced Detoxification and Mineralization of Cr(VI) by FeS2/ZVI Composites

by
Xiaoming Zhang
1,
Haiying Wang
1,2,
Mengying Si
1,2,
Qi Liao
1,2,
Zhihui Yang
1,2,
Qi Li
1,* and
Weichun Yang
1,2,*
1
School of Metallurgy and Environment, Central South University, Changsha 410083, China
2
Chinese National Engineering Research Centre for Control & Treatment of Heavy Metal Pollution, Changsha 410083, China
*
Authors to whom correspondence should be addressed.
Metals 2025, 15(4), 395; https://doi.org/10.3390/met15040395
Submission received: 26 February 2025 / Revised: 25 March 2025 / Accepted: 27 March 2025 / Published: 1 April 2025
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Abstract

:
The rapid detoxification and mineralization of Cr(VI) in aqueous environments hold critical importance for emergency response and resource recovery yet remain technically challenging. Herein, we report the synthesis of FeS2/ZVI composites through ethanol-assisted wet ball-milling and their application in Cr(VI) removal under microwave (MW) irradiation. This study systematically investigates the effects of MW irradiation on the removal efficiency of Cr(VI) using FeS2/ZVI composites, with particular focus on key parameters including composite dosage, initial pH, MW temperature, and Cr(VI) concentration. Notably, 1 g/L FeS2/ZVI composites achieved near-complete removal (>99%) of 50 mg/L Cr(VI) within 7 min at a MW irradiation temperature of 333 K, which exhibited 5.9-fold and 13.1-fold superior performance compared to pure pyrite and ZVI, respectively. Additionally, there is a 96.1% reduction in reaction time in comparison to non-MW irradiation system. In real electroplating wastewater samples, Cr(VI) concentration was reduced from 38.93 to 0.42 mg L−1 by MW irradiation-assisted treatment, validating its potential for practical applications in industrial Cr(VI) pollution control. The activation energy determined by fitting the Arrhenius equation showed a 39.7% reduction for the MW-assisted FeS2/ZVI system (16.0 kJ mol−1) compared to conventional thermal heating (from 25.6 kJ mol−1), indicating that MW irradiation induced catalytic enhancement of FeS2/ZVI, thereby lowering the energy barrier for Cr(VI) reduction. Moreover, MW irradiation-assisted processes facilitated the mineralization of reduced Cr(III) to stable spinel FeCr2O4. These findings collectively establish a synergistic mechanism between MW activation and FeS2/ZVI composites, offering innovative pathways for efficient Cr(VI) detoxification and resource recovery from high-strength industrial wastewaters.

1. Introduction

Wastewater originating from electroplating, leather tanning, printing and dyeing, chromium chemicals production, municipal waste landfill, etc., usually contains hexavalent chromium (Cr(VI)) with the concentration ranging from several to hundreds of mg/L [1,2]. Cr(VI), including CrO42−, HCrO4− and Cr2O72−, are highly soluble and mobile in water, which has been classified as a top priority heavy metal pollutant since it is carcinogenic, mutagenic and teratogenic to humans and animals [3]. The World Health Organization (WHO) stipulates that the maximum permissible concentrations of Cr(VI) are 0.05 mg/L for drinking water and 0.10 mg/L for industrial wastewater [4]. Meanwhile, in China, the required discharge limits of Cr(VI) concentration are set at 0.05 and 0.5 mg/L for surface water and wastewater, respectively [5]. Consequently, devising effective strategies for the removal of Cr(VI) from aquatic environments presents a significant challenge.
Researchers have made substantial efforts to keep the concentration of Cr(VI) beneath the recommended threshold. Up to now, the methods proposed for treating Cr(VI) wastewater include physical remediation, such as adsorption [6,7,8], ion exchange [2], membrane filtration [9], and biological treatment [10], as well as chemical approaches, such as chemical reduction [11] and photocatalytic reduction [12]. Among them, chemical reduction is the most prevalently employed method for treating Cr(VI) wastewater by various reductive materials (e.g., Fe0, iron sulfides), due to its adaptability, cost-effectiveness and energy-saving features. For example, the removal efficacy of Cr(VI) by 1 g/L of ball-milled S-mZVI (sulfidated microscale zero valent iron) achieved 55.8% within 180 min (concentration of 10 mg/L, pH = 6.0) [13]. Gong et al. find that Fe/FeS with a dosage of 0.3 g/L removed 82.1% Cr(VI) within 72 h (concentration of 25 mg/L, pH = 5.0) [14]. The Cr(VI) sequestration by SZVI-Cu (copper-sulfidated ZVI) with a dosage of 0.2 g/L reached 98.4% after 30 min (concentration of 5 mg/L, pH = 5.0) [15]. Nevertheless, the current chemical reduction approach has several limitations, such as being suitable only for low-concentration Cr(VI) wastewater, struggling to meet environmental discharge standards, having a narrow pH application scope, and requiring long reaction times. Considering the frequent occurrence of high-concentration Cr(VI)-contaminated wastewater situations, an effective and quick solution for the detoxification of aqueous Cr(VI) is desperately needed.
Microwave(MW) irradiation, different from conventional thermal heating, is a conversion process of the collision and friction between polar molecules in the electric and magnetic fields that rapidly change direction to generate heat [16]. For many years, MW irradiation has been used as a potential cost-effective substitute for existing heating technologies in a number of environmental applications, such as sludge treatment, soil remediation and wastewater detoxification [17,18,19]. In the field of wastewater treatment, MW irradiation-catalyzed oxidation technology induced by the hot spot effect has been widely explored for degrading organic pollutants [20,21]. Recently, research on microwave-induced reduction of Cr(VI))-containing wastewater has also emerged. For instance, Pang et al. synthesized an MoS2-MnFe2O4 composite, which removed 85.8% of Cr(VI) at a dosage of 2 g/L within 16 min under MW irradiation (initial concentration of 10 mg/L) [22]. However, the detoxification capacity of MoS2-MnFe2O4 was insufficient. A sphere-like ZnFe2O4 reductant can completely remove Cr(VI) with a 50 mg/L concentration after 10 min of MW irradiation at a dosage of 2 g/L in Yuan et al.’s study [23]. Notably, with the intimal pH increase from 2 to 4, 6 and 8, Cr(VI) removal efficacy decreased significantly from 100% to 95%, 62% and 43%. Therefore, these studies collectively demonstrated that the MW-induced catalytic reduction holds promise as an approach for ultrafast remediation of Cr(VI). Nevertheless, further research is warranted to identify materials with adequate Cr(VI)-reducing capabilities, a broader pH adaptation range, and superior MW absorptivity. Meanwhile, FeS2/ZVI composites prepared in our previous study have demonstrated exceptional Cr(VI) removal efficiency in soil matrices, reducing Cr(VI) concentration from an initial 3900.8 to 2.38 mg/kg under MW irradiation [24]. However, it is important to note that the soil environment differs significantly from the solution. As far as we are aware, there is a paucity of work on the MW irradiation-assisted reduction of Cr(VI) by FeS2/ZVI composite in solution, and the underlying reaction mechanism remains unclear.
In this study, an FeS2/ZVI composite was fabricated through ethanol-assisted wet ball milling, and its performance in the reduction of aqueous Cr(VI) under MW irradiation was evaluated. The study comprehensively examined the influence of critical parameters, including initial Cr(VI) concentration, catalyst dosage, solution pH, and temperature, on the MW irradiation reduction process. Furthermore, the mechanism by which MW irradiation promoted the reduction of aqueous Cr(VI) by FeS2/ZVI was explored. This research proposes a straightforward, rapid, and efficient strategy for the reduction of Cr(VI) and the immobilization of Cr(III) in aqueous solutions.

2. Materials and Methods

2.1. Materials

Pyrite (FeS2), zero-valent iron (ZVI) powder, ethanol, and dibenzoyl di-hydrazide were procured from Sinopharm Chemical Reagent Co. (Shanghai, China). All chemical reagents utilized in this study were of analytical grade. Reducing materials for rapid Cr(VI) removal enhanced by MW irradiation were prepared in a one-step process using wet ball milling with ethanol. Based on the previous preparation method [24,25], pyrite with an FeS2 content greater than 98% and iron powder were selected, the two precursors were mixed thoroughly at a molar ratio of 9:1, and ethanol was employed as an organic solvent to inhibit rapid passivation of the materials during wet ball milling. The ratio of ethanol to the mixture was 4 mL: 5 g. The speed and time duration of the ball milling were 400 rpm and 4 h, respectively.

2.2. MW Irradiation-Assisted Cr(VI) Reduction

The MW experiment was setup as follows: 150 mg of ZVI, FeS2, and FeS2/ZVI were separately added to 150 mL of 50 mg/L Cr(VI) solution and then placed on the microwave-ultrasonic combined reactor (Beijing Xianghu, Beijing, China, XH-300A), the MW power of which can be automatically and continuously adjusted between 0 and 1000 W. To precisely control the temperature of the bulk solution, the MW power was set to a maximum of 500 W. The reaction temperature was set to 333 K with a 1-min heating process followed by a 15-min temperature maintenance period. The initial pH of the solution was 5.2, and the magnetic stirrer operated at 350 rpm during the reaction. Meanwhile, control experiments were conducted to determine the Cr(VI) removal of ZVI, and FeS2 and FeS2/ZVI under no MW irradiation. All the other parameters (such as pH, stirring speed and so on) in these controls were identical to those in the MW experiments.
Batch experiments were also carried out to investigate factors affecting Cr(VI) removal under MW irradiation, including FeS2/ZVI dosage (0.5, 1, 1.5 g/L), pH levels (3, 5.2, 7.3, 10.3), temperature (333 K, 343 K, 353 K) and initial Cr(VI) concentration (30, 50, 70 mg/L). In each factorial experiment, the other variables were kept at their optimized values (dosage: 1 g/L, pH: 5.2, temperature: 333 K, initial Cr(VI) concentration: 50 mg/L).
All experiments were performed in triplicate.

2.3. Kinetics and Thermodynamics Experiment

To investigate the mechanism underlying the accelerated reduction of Cr(VI) by MW irradiation, a series of Cr(VI) removal experiments were conducted under conventional heat treatment conditions, without the application of MW irradiation. These heat treatment experiments were performed using a constant temperature water bath (HCJ-8D, Changzhou Gaode, Changzhou, China). A 50 mL solution of 50 mg/L Cr(VI) was thoroughly mixed with 1 g/L FeS2/ZVI, with the initial pH of the solution maintained at approximately 5.2. The temperature of the water bath was set to 333 K, 343 K, and 353 K, and samples were collected at predetermined time intervals to measure the concentration of Cr(VI). Detailed experimental procedures and modeling methodologies pertinent to the dynamics and thermodynamics sections are provided in Text S1 of Supporting Information .

2.4. Characterization and Analytical Methods

Samples were collected after the reaction of FeS2/ZVI with Cr(VI) in the presence or absence of MW irradiation. Zeta potentials of pyrite, ZVI, and FeS2/ZVI were determined by a zeta potential analyzer (Malvern Instruments, Malvern, UK, Zetasizer Nano). XRD (Bruker D8-Advance diffractometer, Billerica, MA, USA), SEM (Zeiss Sigma HD, Jena, Germany) and XPS (ESCALAB250Xi, ThermoFisher-VG Scientific, Waltham, MA, USA) were used to characterize the mineral phase, morphology and surface elements of the samples before and after the reaction, respectively. The pH of the solution was determined using a pH meter (PHSJ-4F, Inesa Scientific instrument, Shanghai, China), and the concentration of Cr(VI) was determined by diphenyl carbazide spectrophotometry using an ultraviolet–visible spectrophotometer (UV-1780, Shimadzu, Kyoto, Japan) at 540 nm (Method 3060A, EPA, Washington, DC, USA). The total Cr concentrations were determined by ICP-OES (ICAP7400 Radial, ThermoFisher Scientific, Waltham, MA, USA) at 267 nm.

3. Results and Discussion

3.1. Characterization of Synthesized FeS2/ZVI

The physical phase of FeS2/ZVI prepared by ethanol-assisted ball milling was characterized by XRD (Figure 1a). The results revealed that FeS2 was the predominant phase in the FeS2/ZVI composite prepared by current wet ball milling, in contrast to our previously reported FeS2/ZVI synthesized by dry ball milling [25]. Notably, new diffraction peaks corresponding to FeSO4·H2O (18.2°, 25.8°, 34.7°, and 35.7°) and FeSO4·4H2O (13.0°, 16.2°, 20.1°, 22.1° and 23.7°) emerged in this composite. The presence of these soluble compounds could be that the sample was not washed prior to freeze-drying after preparation. They may be formed because the wet ball milling atmosphere promoted dispersion and sulfation, significantly altering the phase composition compared to dry milling [26]. Additionally, no peaks of ZVI were observed in this composite, which may be due to the preferred orientation.
The zeta potential results of FeS2/ZVI are plotted in Figure 1b, and the equipotential points of the precursors FeS2 and ZVI are 5.6 and 4.7, respectively, which are easily deprotonated under neutral and alkaline conditions, resulting in electrostatic repulsion of HCrO4− or CrO42−, and inhibiting the Cr(VI) adsorption and further reduction [25]. Notably, the zeta potentials of FeS2/ZVI were all positive in the pH range of 1~10, which has a potent electrostatic conduction effect on Cr(VI), indicating that FeS2/ZVI has excellent reducing properties even under neutral or alkaline conditions.
The morphology of wet ball-milled FeS2/ZVI was shown in Figure 1c. The particle size of the composite is around 1 μm. Notably, the surface of it is rough and contains numbers of non-uniform, irregularly shaped particles, which should be the ground ZVI and structural state Fe(II) generated during the wet ball-milling vulcanization process. As we previously reported, ball milling promoted the production of a large number of nanoscale ZVI particles from the microscale [25]. The EDS test and elemental distribution results show that the FeS2/ZVI obtained contains the elements Fe, S and O with a uniform distribution, further confirming the presence of the FeSO4.

3.2. Cr(VI) Reduction by FeS2/ZVI Composite Under MW Irradiation

Figure 2 shows the effect of ZVI, FeS2 and FeS2/ZVI on the removal of Cr(VI) without and with MW irradiation. It can be seen that ZVI, FeS2 and FeS2/ZVI removed 8.39%, 15.68% and 93.76% of Cr(VI), respectively, after 180 min of reaction time without MW irradiation. The enhanced removal efficiency of the wet ball-milled FeS2/ZVI composite is mainly due to the formation of labile Fe2+ which favors the reduction reaction, and the regenerative ability of the divalent iron resulting from the reaction of the trivalent and zerovalent iron [25]. Meanwhile, the Cr(VI) removal efficiencies of ZVI, FeS2 and FeS2/ZVI reached 10.56%, 18.34% and 100%, respectively, after 15 min of MW irradiation. That meant the MW irradiation enhanced Cr(VI) reduction and greatly reduced the reaction time. In particular, the residual Cr(VI) concentration was 3.12 mg/L for the FeS2/ZVI composite treatment after 180 min without MW irradiation, while it decreased to 0.37 mg/L after 7 min of MW irradiation, which meets the discharge limit of Cr(VI) concentration of wastewater. It is well known that ZVI and pyrite have an excellent microwave absorption capacity, which will have obvious selective heating characteristics when the MW directly acts on them [16,27]. Consequently, the Cr(VI) removal efficiency of ZVI, FeS2, FeS2/ZVI composite all improved after MW irritation. In conclusion, these results confirmed that FeS2/ZVI composite and MW irradiation had a synergistic effect of on the fast reduction Cr(VI) solution.
To exhibit the advantages of current MW irradiation-enhanced reduction of Cr(VI) by as-obtained FeS2/ZVI, many research studies on Cr(VI) removal are listed and compared in Table 1. Though a chemical reduction method could achieve the complete reduction of Cr(VI) (residual Cr(VI) concentration less than 0.5 mg/L in most cases), it is only suitable for the low Cr(VI) concentration (5~10 mg/L). Photocatalytic reduction is considered to be the current state-of-the-art remediation technology for urgent Cr(VI) pollution. The concentration of Cr(VI) and the reduction capacity in photocatalytic reduction system could be as high as 100 mg/L and 188 mg/g, respectively. However, the Cr(VI) reduction ability was much lower ((0.07–1.50) vs. 5.0) and the reaction time was much longer than the current work (120–240) vs. 10). The prepared ZnFe2O4 microspheres removed 100% of 50 mg/L Cr(VI) under MW irradiation within 10 min, but it has a very narrow pH range and is only effective at a pH less than 4 [23]. The above comparison proved that the MW-enhanced FeS2/ZVI reduction was an effective, broad pH-adapted (details in Section 3.2) and rapid method for the removal of Cr(VI) contained in wastewater.
In addition, the FeS2/ZVI composites were employed for the microwave (MW)-assisted treatment of real electroplating wastewater to validate the practical applicability of this advanced reduction technique. The test sample was collected from an electroplating plant in Changsha, Hunan Province, China, with comprehensive water quality parameters provided in Table S1. As illustrated in Figure 3, the Cr(VI) concentration decreased from 39.83 to 0.42 mg/L by the addition of 1.5 g/L FeS2/ZVI with 15 min of MW irradiation. According to previous research [28,29], we used electric energy per order (EEO), i.e., the electric energy (kW h) required to reduce a contaminant’s concentration by one order of magnitude in 1 m3 of wastewater to calculate electrical energy consumption. And it was calculated according to Equation (1).
E E O ( k W h / m 3 ) = P × t × 1000 V × 60 × l o g ( C 0 / C t )
where P is the MW power (kW), t is the reaction time (min), V is the volume of wastewater (L), C0 and Ct are the initial and final Cr(VI) concentrations (mg/L), respectively, and the constant 60 converts min to h. In this study, the MW power is constantly varied to maintain a constant temperature during the MW process. The heating time to the target temperature is 1 min (the process is calculated with the maximum power of 500 W), and the maintaining period is 15 min (the process is calculated with an average power of 100 W). The EEO calculated in this study is therefore 112.41 kW h/m3, which is much lower than that of MW radiation-activated persulfate combined with FeS for the treatment of dinitrodiazophenol effluent (1926.86 kW h/m3) [29]. This treatment resulted in residual Cr(VI) concentrations reaching the permissible limit of the Chinese national integrated wastewater discharge standard [30]. It is noteworthy that higher dosages and longer reaction times were required to achieve efficient removal of Cr(VI) in real wastewater. This is probably because the NO3−, Cl− and SO42− ions contained in this wastewater competed with HCrO4−/Cr2O72−/CrO42− ions for adsorption/reduction, lowering the reduction effect compared to the simulated Cr(VI) solution. However, FeS2/ZVI composite, as a reducing agent, was much less effective in removing Cr(VI) when recycled for Cr(VI) removal, mainly due to surface passivation and reduced material consumption. The formation of inert products (e.g., Cr(III) hydroxide/oxide) would coat the material surface, blocking active sites and reducing electron transfer. Depletion of the reducing agent further reduced the reducing capacity. It is therefore not suitable for recycling.
Table 1. Cr(VI) reduction performance comparison of the typical chemical, photocatalytic and MW irradiation systems.
Table 1. Cr(VI) reduction performance comparison of the typical chemical, photocatalytic and MW irradiation systems.
Removal SystemMaterialsCr(VI) Concentration
(mg/L)		Optimized pHRemoval EfficiencyTime (min)Dosage (g/L)Adsorption/Reduction
Capacity (mg/g)		Reduction Ability *
(mg/(g min))		Reference
Chemical reductionSZVI-Cu5.0597.9%200.224.481.22[15]
S-ZVINa2S2O35.0699.0%900.59.900.11[31]
S-ZVINa2S2O45.06100.0%900.510.000.11[31]
S-ZVI5.05100.0%1200.225.000.21[32]
mZVI/AC10.0394.01%12019.400.08[33]
S-ZVI4.0598.0%1200.219.600.16[34]
Fe/FeS25588%43200.373.330.02[14]
Photocatalytic reductionSrTiO3102100%120110.000.08[35]
ZnFe2O4/CdS100290%1200.5180.001.50[36]
CeO2-MoS255.9100%1200.316.670.14[37]
BiOI/RGO/Bi2S350370%2401350.15[38]
MW irradiation reductionMoS2-MnFe2O4103.385.8%1624.290.27[22]
ZnFe2O4502100%10225.002.50[23]
FeS2/ZVI505100%10 505This study
Note: Reduction ability * means unit mass reduction of hexavalent chromium (Cr(VI)) by a specific reducing agent per unit time, which can be calculated as the following equation: Reduction ability * (mg/(g min) = (Cr(VI) concentration (mg/L) × Reduction efficiency (%))/(Reaction time (min) × (Dosage (g/L)) [23].

3.3. Factors Influencing MW Irradiation-Assisted Cr(VI) Reduction

The effects of pH, FeS2/ZVI dosage, and Cr(VI) concentration on MW irradiation-assisted Cr(VI) reduction by FeS2/ZVI were investigated. The influence of initial solution pH on MW irradiation-assisted Cr(VI) removal by FeS2/ZVI is shown in Figure 4a. As the pH increased from 3.0 to 7.3, the reduction rate gradually decreased, but the Cr(Ⅵ) in solution can be completely removed within 7–12 min. The removal efficiency decreased to 80% when the pH increased to 10.3. However, it is still superior to that of MW-induced ZnFe2O4 which only reached 43% when the pH increased to 8.0 [23]. These results indicated that FeS2/ZVI had a wide pH adaptability range. Which could be explained by the following:
(i) At the acidic environment, Cr(VI) exists mainly as Cr2O72− and HCrO4− and is easily reduced to Cr(III) as it is a strong oxidant with a very high standard redox potential (E0 (Cr(VI)/Cr(III)) = 1.35 V) [39].
(ii) The zeta potentials of the as-prepared FeS2/ZVI were all positive in the range of pH 1–10 as mentioned above, showing that the surface was positively charged even under alkaline conditions, which still benefit for the adsorption of CrO42− anions.
(iii) Cr(III) and Fe(III) resulting from Cr(VI) reduction would react with OH− to form Cr and/or Fe precipitates, which would deposit onto the surface of FeS2/ZVI and occupy some active sites, thus decreasing the efficiency of Cr(VI) reduction.
Metals 15 00395 g004
Figure 4. Effect of (a) pH (1.0 g/L FeS2/ZVI, 333 K, 50 mg/L Cr((VI)), (b) FeS2/ZVI dosage (pH 5.2, 333 K, 50 mg/L Cr((VI), and (c) initial Cr(VI) concentration (1.0 g/L FeS2/ZVI, pH 5.2, 333 K) on the Cr(VI) removal by FeS2/ZVI under MW irradiation.
Figure 4. Effect of (a) pH (1.0 g/L FeS2/ZVI, 333 K, 50 mg/L Cr((VI)), (b) FeS2/ZVI dosage (pH 5.2, 333 K, 50 mg/L Cr((VI), and (c) initial Cr(VI) concentration (1.0 g/L FeS2/ZVI, pH 5.2, 333 K) on the Cr(VI) removal by FeS2/ZVI under MW irradiation.
Metals 15 00395 g004
The effect of dosage on MW irradiation-assisted Cr(VI) removal is shown in Figure 4b. Apparently, with the increase of FeS2/ZVI dosage, the reduction rate of Cr(Ⅵ) greatly increased. The removal efficiency reached 100% within 7 min when the amount of FeS2/ZVI increased to 1.5 g/L. Previous studies have also shown that a higher dosage means more Cr(VI) is reduced [15,34]. The effect of the initial concentration of Cr(Ⅵ) solution on the reduction efficiency is shown in Figure 4c. When the initial concentration was 30~50 mg/L, Cr(Ⅵ) could be completely removed within 3~7 min, whereas, when the initial concentration reached 70 mg/L, the removal efficiency achieved 70% in 10 min. Certainly, it can be further increased by extending the MW irradiation time. Considering that the Cr(VI) concentration in Cr-containing wastewater needing urgent treatment is usually not higher than 50 mg/L, this MW irradiation-assisted FeS2/ZVI treatment has an important application prospect.

3.4. Kinetics and Thermodynamics of MW Irradiation-Assisted Cr(VI) Reduction

To investigate the kinetics and thermodynamics of the reduction of Cr(VI) by MW irradiation, two experimental series were designed to isolate the effects of MW irradiation: (1) conventional thermal heating of FeS2/ZVI at 333–353 K (series 1), and (2) MW irradiation-assisted FeS2/ZVI at identical temperature ranges (series 2). The Langmuir–Hinshelwood first-order model (Equation (S2)) was used to described the reaction kinetics [40], while the Arrhenius equation (Equation (S5)) and thermodynamic equations (Equations (S6) and (S7)) were used to determine the associated activation energy (Ea) and thermodynamic parameters. All kinetic and thermodynamic values are summarized in Table S2. The reaction rate constants for series 1 at temperatures of 333 K, 343 K and 353 K were determined to be 0.211, 0.264 and 0.363 min−1, respectively (Figure 5a,b), which suggest the reaction rate constants increased with temperature, confirming a thermal effect on Cr(VI) reduction. Meanwhile, the reaction rate constants for series 2 were 0.478, 0.544 and 0.664 min−1, respectively, which were 2.3–1.8 times greater than those of series 1. The rate of acceleration observed in series 2 cannot be solely attributed to thermal heating, as identical temperature ranges were applied in both series, indicating a synergistic effect of MW irradiation. The calculated Ea for series 1 was 26.5 kJ /mol (Figure 6a), whereas series 2 exhibited an Ea value of 16.0 kJ mol-1 (Figure 6c), which decreased by 39.7% compared to series 1. The significantly lower Ea in series 2 highlighted the unique role of MW irradiation, indicating that MW irradiation effectively lowered the Cr(VI) reduction barrier and accelerated reduction of Cr(VI) by FeS2/ZVI. Therefore, the expedited Cr(VI) reduction in series 2 arose from both thermal energy input and MW-induced catalytic reduction enhancement of FeS2/ZVI, providing a promising strategy for high-efficiency Cr(VI) detoxification.
The thermodynamic parameters are presented in Table S2. The Gibbs free energy change (ΔG0) is negative for both systems, indicating that the reduction of Cr(VI) occurs spontaneously. Notably, the absolute value of ΔG0 increased with rising temperature, which may suggest enhanced thermodynamic favorability at higher temperatures, consistent with the kinetic findings. In the MW irradiation-assisted system, the variations in standard enthalpy (ΔH) and entropy (ΔS) were relatively small compared to the conventional thermal heating system; however, the significant decrease in ΔG0 (e.g., from −10 to −15 kJ mol−1) highlights the pronounced effect of MW irradiation on reaction spontaneity.

3.5. Mechanism of Accelerated Reduction of Cr(VI) by MW Irradiation

The morphology, phase and composition of the products after accelerated reduction of Cr(VI) by MW irradiation of FeS2/ZVI were investigated. In comparison to the product obtained from the reduction of Cr(VI) by FeS2/ZVI under conditions devoid of external field (Figure 7a), the surface of the reacted FeS2/ZVI under MW irradiation was uniformly coated with a substantial number of spherical nanoparticles (Figure 7c). This phenomenon is attributed to the increased number of active sites on the material surfaces in the MW irradiation-assisted reduction system, which facilitated a higher degree of reaction with Cr(VI). The elemental mapping plots indicate a significant overlap and uniform distribution of Fe, Cr, and O on the FeS2/ZVI surface.
To elucidate the phases of the products after Cr(VI) reduction, the reaction products on the FeS2/ZVI surface were isolated via ultrasonic stripping and subsequently analyzed by XRD (Figure 8). In the absence of an external field, diffraction peaks were observed at 28.5°, 33.0°, 36.9°, 40.7°, 47.3°, 56.3°, 58.9°, 61.7°, 64.2°, 76.3°, and 78.8°, all corresponding to FeS2. This may be explained by the following: (1) FeS2 is the dominant component in the composite and was not fully reacted; and (2) Cr(VI) reacts more easily with ground ZVI and FeSO4 produced during wet ball milling. Notably, no distinct Cr-containing phase was detected, which may be due to the resulting product being amorphous Cr(III) precipitates and/or Fe(III)-Cr(III) co-precipitates. Meanwhile, diffraction peaks observed at 30.15°, 35.5°, 43.1°, 57.1°, and 62.7° in the MW irradiation-assisted FeS2/ZVI system suggest the formation of FeCr2O4, a spinel mineral known for its natural stability. Additionally, its magnetic properties make it easy to separate and recover. This product can be attributed to the “hot spot” effect induced by MW irradiation. Previous studies have demonstrated that MW irradiation can rapidly generate localized “hot spots” on solid surfaces. These “hot spots” not only enhance reaction kinetics by concentrating energy but also promote the transformation of contaminants into thermodynamically stable phases. For instance, Chen et al. demonstrated that copper-loaded attapulgite, acting as a MW absorber, can stabilize Cd in soil through a series of MW-induced phase transformations, forming more stable Cd-bearing crystalline minerals [41]. Similarly, Chen et al. employed MW hydrothermal processing to rapidly fix Cr(VI) from electroplating and pickling wastewater into the ferrite lattice. Their study revealed that Cr(VI) was reduced to Cr(III) and subsequently incorporated into FeCr2O4 spinel, which exhibited enhanced thermal and chemical stability [42].
Furthermore, the alteration in surface element configuration of FeS2/ZVI pre- and post-Cr(VI) treatment under MW irradiation was examined by XPS to elucidate the mechanism underlying Cr(VI) immobilization. Figure 9a illustrates that a strong peak centered at around 577.0 eV appeared after the MW irradiation reaction, which can be assigned to the Cr(2p3/2), suggesting the deposition of insoluble Cr on the surface of the reacted composite. To further explore the surface adsorption and reduction of Cr(VI)/Cr(III), the surface Cr species after treatment were analyzed. As shown in Figure 9b, the peaks near 577.47 and 587.15 eV are attributed to Cr(III) [23], and no peaks of Cr(VI) are found on the surface of the reacted composite, which indicates that the removal of Cr(VI) by FeS2/ZVI is mainly a chemical reduction process. Moreover, the high-resolution spectra of Fe2p and S2p before and after Cr(VI) removal were also analyzed. The peaks near 707.66 eV are attributed to Fe0, the peaks near 711.73 and 725.44 eV are attributed to Fe(III), and the peaks near 714.94 and 729.56 eV are attributed to Fe(II). The calculated distribution of the corresponding iron phase is presented in Figure 9c. And the Fe(Ⅱ) content after MW irradiation reaction was virtually unchanged compared to the content of Fe(Ⅱ) before reaction, which can be attributed to the regeneration of Fe(II) through by the reaction between Fe0 and Fe(Ⅲ), thereby sustaining an efficient reductive environment within the system. Meanwhile, the content of Fe0 in reacted composite was 17.41%, which was 30.9% lower than that of unreacted FeS2/ZVI, and the Fe(Ⅲ) content increased to 65.62% after the reaction. These results indicated that Fe0 and Fe(Ⅱ), acting as the electron donors, account for the reduction of Cr(VI) during the reaction process, and the produced Fe(Ⅲ) was adsorbed onto and subsequently coated the surface of the material. Moreover, as shown in Figure 9d, the peaks near 170.02 and 168.81 eV are attributed to SO42−, and the peaks at 162.79, 164.02, and 165.01 are attributed to S22−, Sn2−, and S0, respectively [11]. The S2− and SO42− was the dominant species in fresh FeS2/ZVI, which is consistent with the aforementioned XRD results that the primary phase present in the wet ball-milled FeS2/ZVI were FeS2 and FeSO4. The relative fraction of S2− decreased by 11.6% after the reaction, whereas the fraction of Sn2−, S0 and SO42− increased 16.94%, 11.4% and 3.2%, respectively, indicating that part of S2− participated in the reduction of Cr(VI), and Sn2− and S0 are the main products of S2− oxidation. Based on the above analyses, it can be concluded that Cr(VI) is fully reducible to Cr(III) on the FeS2/ZVI surface, which is associated with the oxidation of Fe0, Fe(-II) and S(-II).

4. Conclusions

The rapid and efficient remediation of Cr(VI) in wastewater was achieved through the combined application of MW irradiation and the wet ball-milled FeS2/ZVI. Under optimized conditions (temperature: 333 K, FeS2/ZVI dosage: 1 g L−1, pH: 5), nearly 100% Cr(VI) removal at a concentration of 50 mg/L was achieved within 7 min, which was 5.9-fold and 13.1-fold times more efficient than of pyrite and ZVI, respectively, and represented a 96.1% reduction in reaction time compared to the non-MW irradiation-assisted treatments. Kinetic analysis revealed that the apparent activation energy (Ea) for the MW irradiation-assisted FeS2/ZVI treatment was 16.0 kJ/mol, which was 39.7% lower than that of the conventional thermal heating treatment (25.6 kJ/mol). This suggests that, in addition to the thermal effects, the catalytic activity of FeS2/ZVI under MW irradiation also significantly enhanced the Cr(VI) reduction. Furthermore, the characteristic analyses indicated that MW irradiation promoted the mineralization of reduced Cr(III) into the spinel phase FeCr2O4, which provides a pathway for Cr resource recovery. This work has elucidated the process and mechanism underlying Cr(VI) removal by MW irradiation-assisted FeS2/ZVI, providing valuable insights into an ultrafast and efficient treatment approach for Cr(VI)-polluted wastewater.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/met15040395/s1. Text S1: Kinetics and thermodynamics experiments; Table S1: Electroplating wastewater quality parameters; Table S2: Kinetics and thermodynamics parameters of conventional thermal heating and MW irradiation-assisted systems. Reference [43] is cited in the supplementary materials.

Author Contributions

Conceptualization, H.W. and Q.L. (Qi Li); Methodology, Q.L. (Qi Liao); Validation, Q.L. (Qi Liao), Q.L. (Qi Li) and W.Y.; Formal analysis, X.Z. and Q.L. (Qi Li); Investigation, X.Z. and M.S.; Resources, M.S.; Data curation, X.Z. and Q.L. (Qi Li); Writing—original draft, X.Z. and Q.L. (Qi Li); Writing—review & editing, Z.Y., Q.L. (Qi Li) and W.Y.; Visualization, X.Z. and M.S.; Supervision, H.W. and Z.Y.; Funding acquisition, W.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 52274170), the Natural Science Foundation of Hunan (2023JJ0065).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no competing financial interest.

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Figure 1. (a) XRD spectra, (b) Zeta potential, (c) SEM image, (d) electronic image in EDS mode and (e) elemental mapping images of wet ball milled FeS2/ZVI.
Figure 1. (a) XRD spectra, (b) Zeta potential, (c) SEM image, (d) electronic image in EDS mode and (e) elemental mapping images of wet ball milled FeS2/ZVI.
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Figure 2. Cr(VI) removal by ZVI, FeS2 and FeS2/ZVI (a) without and (b) with MW irradiation (C0 = initial Cr(VI) concentration; C = residual Cr(VI) concentration after t min).
Figure 2. Cr(VI) removal by ZVI, FeS2 and FeS2/ZVI (a) without and (b) with MW irradiation (C0 = initial Cr(VI) concentration; C = residual Cr(VI) concentration after t min).
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Figure 3. Cr(VI) removal in electroplating wastewater by MW irradiation-assisted FeS2/ZVI composite (C0 = 39.83 mg/L, temperature = 353 K, reaction time = 20 min).
Figure 3. Cr(VI) removal in electroplating wastewater by MW irradiation-assisted FeS2/ZVI composite (C0 = 39.83 mg/L, temperature = 353 K, reaction time = 20 min).
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Figure 5. Effect of temperature on the Cr(VI) reduction (a,c) and kinetics at different temperatures (b,d). (a,b): conventional thermal heating; (c,d): MW irradiation. 1.0 g/L FeS2/ZVI, pH 5.2, 50 mg/L Cr((VI)).
Figure 5. Effect of temperature on the Cr(VI) reduction (a,c) and kinetics at different temperatures (b,d). (a,b): conventional thermal heating; (c,d): MW irradiation. 1.0 g/L FeS2/ZVI, pH 5.2, 50 mg/L Cr((VI)).
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Figure 6. Activation energy and thermodynamic parameters calculated by fitting the Arrhenius equation during (a,b) conventional thermal heating and (c,d) MW irradiation.
Figure 6. Activation energy and thermodynamic parameters calculated by fitting the Arrhenius equation during (a,b) conventional thermal heating and (c,d) MW irradiation.
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Figure 7. SEM images and elemental distributions after Cr(VI) removal by FeS2/ZVI under (a,b) conventional thermal heating and (c,d) MW irradiation.
Figure 7. SEM images and elemental distributions after Cr(VI) removal by FeS2/ZVI under (a,b) conventional thermal heating and (c,d) MW irradiation.
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Figure 8. XRD patterns of FeS2/ZVI after Cr(VI) removal by conventional thermal heating and MW irradiation.
Figure 8. XRD patterns of FeS2/ZVI after Cr(VI) removal by conventional thermal heating and MW irradiation.
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Figure 9. (a) XPS survey spectrum, (b) Cr 2p, (c) Fe 2p, and (d) S 2p maps before and after Cr(VI) removal by MW irradiation-assisted FeS2/ZVI composite.
Figure 9. (a) XPS survey spectrum, (b) Cr 2p, (c) Fe 2p, and (d) S 2p maps before and after Cr(VI) removal by MW irradiation-assisted FeS2/ZVI composite.
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Zhang, X.; Wang, H.; Si, M.; Liao, Q.; Yang, Z.; Li, Q.; Yang, W. Rapid Microwave Irradiation-Enhanced Detoxification and Mineralization of Cr(VI) by FeS2/ZVI Composites. Metals 2025, 15, 395. https://doi.org/10.3390/met15040395

AMA Style

Zhang X, Wang H, Si M, Liao Q, Yang Z, Li Q, Yang W. Rapid Microwave Irradiation-Enhanced Detoxification and Mineralization of Cr(VI) by FeS2/ZVI Composites. Metals. 2025; 15(4):395. https://doi.org/10.3390/met15040395

Chicago/Turabian Style

Zhang, Xiaoming, Haiying Wang, Mengying Si, Qi Liao, Zhihui Yang, Qi Li, and Weichun Yang. 2025. "Rapid Microwave Irradiation-Enhanced Detoxification and Mineralization of Cr(VI) by FeS2/ZVI Composites" Metals 15, no. 4: 395. https://doi.org/10.3390/met15040395

APA Style

Zhang, X., Wang, H., Si, M., Liao, Q., Yang, Z., Li, Q., & Yang, W. (2025). Rapid Microwave Irradiation-Enhanced Detoxification and Mineralization of Cr(VI) by FeS2/ZVI Composites. Metals, 15(4), 395. https://doi.org/10.3390/met15040395

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