Study on the removal effect and mechanism of calcined pyrite powder on Cr(VI)

Abstract Pyrite exhibits considerable potential as an adsorbent in wastewater treatment. However, few pyrite adsorbents are directly obtained from natural pyrite, as most are composite materials that require a complex preparation process. To develop a pyrite-based adsorbent with a simple preparation process, pyrite was processed by calcination at 400, 600, and 800 °C for 4 h and ball-milled into a fine powder. The adsorption properties of the pyrite powder were systematically explored. The calcined pyrite powder was characterized by SEM-EDS and XRD. The results revealed that the pyrite calcined at 600 °C exhibited excellent adsorption properties and was primarily composed of Fe7S8. The optimum conditions for Cr(VI) removal were a temperature of 45 °C, an adsorbent dosage of 1 g, an equilibration time of 60 min, and an initial pH of 3. Moreover, the calcined pyrite powder exhibited excellent reusability, and the Cr(VI) removal rate exceeded 65% after three cycles. The Cr(VI) adsorption on pyrite can be well described by the Freundlich model and pseudo-second-order kinetic equation. The calcination temperature is the main factor affecting the adsorption performance of pyrite. Therefore, the calcined pyrite powder is expected to be an excellent adsorbent for Cr(VI) in the wastewater treatment industry. NOVELTY STATEMENT Pyrite has shown promising development prospects in the field of wastewater purification. However, the preparation of most pyrite-based adsorbents is complicated. Upon high-temperature calcination, pyrite is used in traditional Chinese medicine clinics to promote the healing of fractures. The efficiency and underlying mechanism of Cr(VI) adsorption from water using calcined pyrite was investigated. The adsorbent was prepared using a simple method and exhibited excellent adsorption performance, thus allowing its application in preparing ore-based adsorbents for water pollution treatment.


Introduction
The qualities of water should be determined to identify the causes of pollution and take necessary precautions to minimize it.As a result, there has been a growing number of studies on wastewater treatment and water quality worldwide (Karadavut et al. 2011(Karadavut et al. , 2012;;Kalipci 2019).Cr is a prevalent and hazardous heavy metal that causes environmental pollution.The main sources of Cr in the environment are the ore mining, electroplating, and dyeing industries (Singh et al. 2023;Yang et al. 2023).Cr is carcinogenic and mutagenic and primarily exists in water as Cr(III) and Cr(VI), with Cr(VI) being the more toxic form (Liu et al. 2023).The maximum permissible Cr(VI) content in drinking water is 0.05 mg/L (NHCPRC 2022).In addition, Cr causes serious plant toxicity and accumulates in crops, affecting their normal growth (Aftab et al. 2023).Cr stress leads inhibits tomato plant growth, increases malondialdehyde and superoxide levels, and inhibits ascorbate acid-glutathione cycle enzymes (Gupta and Seth 2021;Yadav et al. 2022).Cr(VI) stress reduces photosynthetic pigments, leaf gaseous exchange parameters, and PSII efficiency in a dose-dependent manner in Helianthus annuus L. and Brassica juncea L. var.Varuna (Singh et al. 2017;Kumar and Seth 2021;Kumar et al. 2023).Therefore, the removal of Cr(VI) contamination from the environment is essential for the growth of plants and the health of humans.
The rapid increase in industrial development and metal usage has led to heavy metal pollution becoming a significant environmental problem for water systems, including oceans, lakes, and rivers in areas with intensive industry (Aras et al. 2017;Kalipci and Namal 2018).Methods for removing heavy metals include adsorption, chemical precipitation, membrane separation, and electrochemical methods (Fu et al. 2023).Adsorption is one of the most predominant methods owing to its low cost, high removal efficiency, and environmental friendliness (Shahnaz et al. 2020;Suganya et al. 2020;Feng et al. 2021).The chemical precipitation method has the advantages of simple operation and a high degree of automation.However, this method produces a large amount of toxic sludge, which requires further treatment (Nayeri and Mousavi 2022).Membrane separation technology offers the benefits of energy savings, chemicaladditive-free operation, and environmental protection.However, it involves complex production processes, has a high cost, and causes serious membrane pollution (Maurya et al. 2023).Electrochemical technology has the characteristics of versatility, strong selectivity, and easy automation.However, it requires substantial capital investment, and an expensive power supply (Kabdas ¸lı and T€ unay 2023).In addition, microwave catalysis is an emerging water treatment technology that has received considerable attention for its advantages of rapidity, high efficiency, and minimal secondary pollution (Li et al. 2023;Yu et al. 2023).
Pyrite is a sulfide mineral primarily composed of FeS 2 that is readily found in nature.Recently, pyrite has found applicability in environmental pollution treatment because of its low cost and high efficiency (Song et al. 2022).It has also been utilized in the removal of Cr(VI) (Oral et al. 2022), adsorption of gaseous elemental Hg in air (Wang et al. 2022), treatment of pharmaceutical wastewater by acting as a Fenton reaction catalyst (Kantar et al. 2019), and autotrophic denitrification for nitrate removal in water (Li et al. 2022).Pyrite and limestone were used as the matrix to build a constructed wetland, which enhanced long-term total nitrogen and total phosphorus removal after three years of operation (Ge et al. 2019).Pyrite/sodium hypochlorite treatment removed arsenic from fractured bedrock groundwater via surface adsorption (Lee et al. 2023).Pyrite is a traditional Chinese medicine used to reduce pain from fractures and accelerate healing.The traditional Chinese medicine theory holds that pyrite must be calcined at high temperature before it can be used clinically.We found that calcination increased the saturation magnetization of pyrite, and its surface became sparse and porous (Liu et al. 2017).Therefore, we speculate that pyrite prepared by the traditional Chinese process has the potential to serve as an excellent adsorbent.
Fe-based composite materials show great potential in water metal pollution treatment owing to their large specific surface area, strong redox ability, and high removal capacity (Gong et al. 2022).A cellulose hydrogel composite improved the dispersibility and oxidation resistance of nanoscale Fe 0 and enhanced the removal of Cr(VI) from water (Wang et al. 2020).A Cu-Fe-embedded crosslinked cellulose hydrogel removed Cr(VI) from water through various mechanisms of action, such as adsorption, reduction, and co-precipitation (Wang et al. 2021).Although Fe-based composites have promising applications in water pollution treatment, they have the disadvantage of a high synthesis cost, which restricts their mass production.Pyrite is a natural Fe-based material that has the prospect of being developed into a material with superior Cr(VI) adsorption properties.
To enhance the adsorption capacity for the removal of Cr(VI) from water, a simple preparation technique was devised wherein raw pyrite was calcined at 400, 600, or 800 C. Subsequently, the ball-milling process was optimized using an orthogonal experimental method to obtain powders with smaller particle sizes.XRD was performed to evaluate the phase composition of different pyrite products.Adsorption experiments were conducted to compare the Cr(VI) removal rates of different pyrite powders.The effects of the particle size of the adsorbent, initial pH, adsorbent dosage, equilibration time, and temperature on the adsorption efficiency were investigated.Cycling and column experiments were performed to verify the applicability of the calcined pyrite powders for Cr(VI) removal.Finally, the mechanism of underlying Cr(VI) removal by the calcined pyrite powders was elucidated using adsorption isotherms and kinetics.

Chemicals and instruments
Raw pyrite was purchased from Nanjing Zhong Shan Pharmaceutical Co., Ltd.(Nanjing, China).Raw pyrite was sourced from Henan Province, China, and primarily contained FeS 2 and SiO 2 , constituting 55.9 and 44.1% of the sample, respectively.The box muffle furnace was provided by Jinan Jing Rui Analytical Instrument Co., Ltd.(Jinan, China).A planetary ball mill was provided by Changsha Tian Chuang Powder Technology Co., Ltd.(Changsha, China).

Preparation of pyrite powders
Table 1 outlines the preparation methodologies employed for the different pyrite samples.The specific production method was as follows: Raw pyrite weighing 100 g was placed in a crucible and then calcined at 400, 600, or 800 C for 4 h in a muffle furnace.After cooling to room temperature, the calcined and raw pyrite samples were ground into powder using a ball crusher.
An orthogonal experimental method was used to optimize the preparation process of pyrite powders using the ball-to-material ratio, ball-milling time, and amount of ballmilling solvent as factors.The factor levels for the orthogonal experiments are listed in Table S1 (in the Supplementary Material).

Characterization and analytical methods
XRD (Smart Lab) and EDS (FEI-Quanta 250) were conducted to analyze the composition of the pyrite products.XRD pattern phase analysis was conducted using Jade 6.0, and the obtained diffraction data were matched with the JCPDS standard card of the International Powder Diffraction Data Center to obtain the phase composition.A laser particle size analyzer was used to measure the particle sizes of the pyrite products before and after ball milling.SEM (Quanta FEG 250) was used to observe the morphology of the samples.The surface Fe and Cr elements of calcined pyrite powder before and after adsorption and the chemical forms of these elements were analyzed through Xray photoelectron spectroscopy (XPS).Inductively coupled plasma mass spectrometry (ICP-MS, Analytic Jena AG, Germany) was used to measure the Cr(VI) concentration of the aqueous solutions.(The XRD, SEM conditions, and the working parameters of ICP-MS are in the Supplementary Material S5).

Adsorption experiments
Optimization of pyrite samples Cr(VI) solutions were prepared by dissolving K 2 Cr 2 O 7 in distilled water.Aqueous solutions of Cr(VI) (100 mL) at a concentration of 100 mg/L were prepared in 250 mL polytetrafluoroethylene conical flasks, and their pH was adjusted to 3, followed by the addition of 200 mg of raw or calcined pyrite at different temperatures and sonication for 30 min.
Then, the mixtures were shaken using a horizontal shaker for 4 h at 45 C.After equilibration, the solution samples were centrifuged at 12,000 rpm for 15 min, and the supernatant was obtained and filtered through a 0.22 mm membrane.The removal rate of Cr was calculated according to the following equation (Equation 1): where C i is the initial concentration (mg/L), and C e is the equilibrium concentration (mg/L).

Statistical analysis
Each experiment was repeated three times.The data were statistically analyzed using GraphPad Prism 9 (GraphPad Software, CA, USA).Student's t-tests were performed to compare the data between two groups, p < 0.05 was considered statistically significant.

Ball milling process
The results of the orthogonal experiments are summarized in Table S2 (in the Supplementary Material).Ball milling is an effective and environmentally-friendly method for modifying adsorbents.This method has been shown to increase the specific surface area of the adsorbent and enhance its adsorption efficiency (Xu et al. 2023;Zhang et al. 2023).
The ball-to-material ratio is the most crucial factor affecting the efficiency of ball milling.A considerably large ball-tomaterial ratio leads to the idling of the apparatus, whereas a considerably small ball-to-material ratio decreases productivity.The ball milling time is also a significant factor affecting the efficiency of the process, and selecting an appropriate time is conducive to maximizing the efficiency.Ethanol is used as a lubricant in the ball-milling process, which reduces the surface energy of the powder and acts as a heat absorber.Based on the mean of each factor listed in Table S2, we can conclude that under the condition of A 4 B 2 C 3 , the smallest powder particle size was obtained.The specific parameters were as follows: ball-to-material ratio of 20:1, ball milling for 4 h, and lubricant amount of 50 mL (ethanol).Under these conditions, the particle size of the pyrite powder obtained was $360 nm.Based on the range of each factor listed in Table S2, the contribution order of the factors affecting ball-milling efficiency was C > A > B. The results indicate that the amount of ethanol lubricant is the most important factor in determining ball milling efficiency.
In addition, three validation experiments were conducted to verify the reliability of the optimal process obtained by the orthogonal process experiments.The results are shown in Table S3 (in the Supplementary Material).The results confirm that the particle size of the pyrite powder is between 350 and 375 nm under the optimal process conditions, indicating that the optimized ball-milling process is stable and feasible.

Optimization of the pyrite product
The Cr(VI) removal rates of different products are shown in Figure 1, and the Cr(VI) removal rates of the raw pyrite powder and that calcined at 400 C were below 60%.The Cr(VI) removal rates of the pyrite powder calcined at 600 and 800 C gradually increased to 100% with time.Compared to the raw pyrite at the same equilibrium time, pyrite calcined at 600 C significantly increased the Cr(VI) removal rates (p < 0.05).In general, the rate of Cr(VI) removal by the pyrite powder increased with the calcination temperature.Similar results have been reported, mainly attributed to increased material porosity (Alidoust et al. 2015).Considering the energy consumption and the Cr(VI) removal rate, we chose the sample obtained by calcination at 600 C for subsequent experiments.Calcination is a simple and effective way to improve the ability of the material to remove Cr contamination.Shi et al. (2022) prepared a Fe-C material exhibiting high Cr(VI) removal efficiency through the calcination of municipal sludge.After calcination at 700 C, the performance of rectorite for Cr immobilization in soil significantly increased (Fang et al. 2023).

Particle size
The particle sizes of the calcined pyrite powders before and after grinding are shown in Figure 2. The particle size of the calcined pyrite powder before grinding was $1980 nm, with a polymer dispersion index (PDI) value of 0.471.The particle size of the calcined pyrite powder after grinding was $338.3 nm, with a PDI value of 0.214.The results indicate that grinding reduces the particle size of the calcined pyrite powder and helps increase the homogeneity of the powder.The smaller particle size of the adsorbent provided a larger surface area, which was beneficial for improving the adsorption capacity (Yu et al. 2023).

SEM
The SEM images of the pyrite powders before and after grinding are shown in Figure 3. Before ball milling, the surface of the calcined pyrite powder was lumpy and rough.After ball milling, the calcined pyrite powder appeared granular with a loose and porous surface, providing more adsorption sites for heavy metal ions.Ball milling is used as an efficient and energy-saving method to increase the surface area and the porosity of the material surface (Li et al. 2020).
XRD XRD was conducted to analyze the elemental composition of the various pyrite products, the results are shown in Figure 4 and summarized in Table 2.The mineral phase compositions of the pyrite samples were mainly influenced by the calcination temperature.The main components in raw pyrite and pyrite calcined at 400 C were FeS 2 and SiO 2 .Calcination at 600 C changed the phase composition of raw pyrite, and the main components were Fe 7 S 8 , FeS 2 , and SiO 2 .In the pyrite calcined at 800 C, the main components changed to Fe 3 O 4 , Fe 7 S 8 , and SiO 2 .The preferred pyrite was mainly composed of Fe 7 S 8 .This preferred pyrite had superior redox properties, a larger specific surface area, and more active sites resulting in excellent Cr(VI) adsorption ability, which may be attributed to its spatial configuration and mixed Fe valence states (Li et al. 2022).

EDS
The EDS results for the calcined pyrite powder before and after Cr(VI) adsorption are shown in Figure 5a and summarized in Table 3. Cr was not detected before adsorption, whereas it was detected after adsorption, demonstrating that the calcined pyrite powder exhibits Cr adsorption capacity.In addition, the Fe content of the calcined pyrite powder decreased after adsorption.Moreover, the Cr content of the aqueous solution decreased from 100.5 mg/L to 2.4 mg/L, while the Fe content of the aqueous solution increased from undetected to 20.7 mg/L (Table S4 in the Supplementary Material), indicating that Fe ions may have been exchanged with Cr ions during adsorption.

XPS
The XPS results for the calcined pyrite powder before and after Cr(VI) adsorption are presented in Figures 5b,c. Figure 5b illustrates the absence of a Cr peak before adsorption, whereas a distinct peak appeared after adsorption.Figure 5c showed that the peak values at 578 and 588 eV were attributed to Cr(VI), while the peak values at 586 and 576 eV were attributed to Cr(III) (Wang et al. 2022), which showed the presence of mainly Cr(III) and minimal Cr(VI) on the material's surface after adsorption.It was demonstrated that Cr(VI) was reduced to Cr(III) during the adsorption process.As shown in Figure 5d, the peaks at 706 eV belonged to Fe 0 .The peak of 711 eV was contributed by Fe(II), and the peaks of 724, 715, and 713 eV were attributed to Fe(III) (Wang et al. 2022).It meant that the Fe(II) decreased while the Fe(III) increased on the surface after the adsorption.The above-mentioned XPS analysis confirmed that the adsorption mechanism of calcined pyrite powder for Cr(VI) involves redox reactions.

Effect of particle size of adsorbent
The rate of Cr(VI) removal by the adsorbents with different particle sizes is shown in Figure 6a.When the particle size of the adsorbent was 265 nm, the Cr(VI) removal rate was 93.9%.When the particle size of the adsorbent was 3,471 nm, the Cr(VI) removal rate decreased sharply to 19.0%.We concluded that using an adsorbent with a small particle size improves Cr(VI) removal.The ability of the adsorbent to remove Cr(VI) from the water was improved after ball milling (Ai et al. 2023).The improved removal rate due to the smaller adsorbent particle size was mainly attributed to the greater surface activity and reaction area of the mechanically activated calcined pyrite powder (Yin et al. 2010).

Effect of the initial pH
The pH of the solution is an important factor affecting the efficiency of the removal of target pollutants from water.The Cr(VI) removal rates of the adsorbent at different pH values are shown in Figure 6b.The Cr(VI) removal rate reached a maximum when the pH was 3.0.However, when the pH was <2.0, the Cr(VI) removal rate decreased rapidly.At a pH of >3.0, the Cr(VI) removal rate reached a stable value.Therefore, the optimal initial pH was determined to be 3.0.This parameter was used in the subsequent experiments.

Effect of temperature
The Cr(VI) removal rates of the adsorbents at different temperatures are shown in Figure 6c.When the temperature was < 45 C, the Cr(VI) removal rate increased with increasing temperature.When the temperature was > 45 C, the removal rate reached a stable value.The optimal temperature of the water solution was determined to be 45 C this parameter was used for further research.

Effect of the adsorbent dosage
The Cr(VI) removal rates of different adsorbent dosages are shown in Figure 6d.When the adsorbent dose was <1 g, the removal rate increased with increasing adsorbent dosage because the number of adsorption sites in water increased as the adsorbent dosage increased.At an adsorbent dose of 1 g, the Cr(VI) removal rate reached 98%.When the adsorbent dose exceeded 1 g, the Cr(VI) removal rate did not increase significantly.The optimal absorbent dosage was determined to be 1 g this parameter was used for further research.

Effect of equilibration time
The Cr(VI) equilibration time was investigated in the range of 5-300 min, the results are shown in Figure 6e.The Cr(VI) removal rate reaches a maximum of 90% after an equilibration time of 60 min.The optimal equilibration time was determined to be 60 min this parameter was used for further research.Therefore, the optimum conditions for Cr(VI) removal were as follows: temperature of 45 C, an adsorbent dosage of 1 g, equilibration time of 60 min, and initial pH of 3.

Cycling experiment
The reusability of adsorbents is an important indicator of their economic feasibility (Kumar et al. 2022).In this study, we performed a cycling experiment on the adsorbent.The adsorbent was separated via centrifugation, and treated with 100 mL of 5% HNO 3 for 4 h.Then, the adsorbent was washed with ultrapure water and dried at 70 C.The recovered adsorbent was subjected to five subsequent experiment cycles under the same conditions.The results are shown in Figure 7a.The Cr(VI) removal rate of the calcined pyrite powder decreased from 99.9 to 65.9% after three cycles.Therefore, the calcined pyrite powder has excellent reusability and can be used as a natural adsorbent.

Column experiment
Column experiments were performed to investigate the removal of Cr(VI) by the calcined pyrite powders under dynamic conditions.Cotton-quartz sand (3 g), cotton-calcined pyrite powder (4 g), cotton-quartz sand (3 g), and cotton were filled sequentially in a 30 mL syringe (inner diameter of 2.3 cm, barrel length of 10 cm).The 250 mg/L Cr(VI) aqueous solution was run off at a rate of 1 mL/min using a constant speed pump, and 2 mL of the runoff liquid was collected at different time points for analysis.The Cr(VI) removal rate results are shown in Figure 7b.The results show that the removal of Cr(VI) reaches a maximum in 15 min, and gradually decreases over time, indicating that the adsorption sites on the surface of the adsorbent are abundant initially and decrease with time.

Adsorption isotherm
The Freundlich model (Equation 2) and Langmuir model (Equation 3) were used to analyze the adsorption behavior of the adsorbents.
where q e is the equilibrium adsorption capacity (mg/g), C e is the equilibrium concentration (mg/L), and k and 1/n are the constants related to capacity and adsorption intensity, respectively.q m is the maximum adsorption capacity (mg/g), and b is the adsorption constant.The Cr(VI) adsorption data at 25, 35, and 45 C were substituted into the Freundlich and Langmuir model equations, and the adsorption isotherms of the calcined pyrite powder for Cr(VI) were determined using the Langmuir and Freundlich models.The results are shown in Figure 8 and summarized in Table 4.The data listed in Table 4 show that the correlation coefficient R 2 of the Freundlich model (!0.9931) was greater than that of the Langmuir model ( 0.7620), indicating that the Freundlich adsorption model is more suitable for describing the adsorption of Cr(VI) by the calcined pyrite powder.The Freundlich model assumes that the surface properties of an adsorbent are not homogeneous and can be used to describe multilayer materials containing several active sites for surface adsorption (Freundlich 1906).The data indicated that Cr(VI) adsorption by the calcined pyrite powder was a multilayer   adsorption process.This multilayer adsorption was likely caused by the rough surface of the calcined pyrite powder after ball-milling, which is consistent with the SEM results.

Adsorption kinetics
The pseudo-first order kinetic model (Equation 4) and the pseudo-second order kinetic models (Equation 5) were used to explore the adsorption mechanisms of the adsorbents.
ln q e À q t ð Þ ¼ ln q e À k 1 Ã t (4) where q e is the equilibrium adsorption capacity (mg/g), q t is the equilibrium adsorption concentration (mg/L), t is the equilibrium time (min), k 1 is the pseudo-first-order kinetic rate constant, and k 2 is the pseudo-second-order kinetic rate constant.
The fitted kinetic equation of Cr(VI) adsorption by the calcined pyrite powder is shown in Figure 9 and Table 5.The R 2 values of the pseudo-second order kinetic model (!0.9992) were superior to those of the pseudo-first order kinetic model ( 0.7432).When the concentration of Cr(VI) was 400 mg/L, the q e of the pseudo-second order kinetic model was 36.1 mg/g.These data indicate that electrons were shared or exchanged between the calcined pyrite powder and Cr(VI) and that the adsorption of Cr(VI) by the calcined pyrite powder was mainly a chemical adsorption process (Liang et al. 2023).
The reduction of Cr(VI) to Cr(III) is an important method of remediating Cr(VI) contaminated soil and water (Wang et al. 2019).The pH of water affects the existing form of Cr(VI).When pH is 2-6, HCrO 4 À is the dominant form (Ai et al. 2023).Based on the adsorption isotherm, kinetics, and XPS results, we concluded that the mechanism underlying Cr(VI) adsorption by calcined pyrite powder involved both multilayer physical adsorption and redox reactions.We speculate that the mechanism underlying Cr(VI) adsorption is as follows:

Conclusion
In conclusion, adsorbent materials based on the natural mineral pyrite with superior performance were prepared by a simple calcination and ball milling method.Calcination and ball milling modified the physicochemical properties of the pyrite contributing to its Cr(VI) removal capacity.The calcined pyrite powder underwent multilayer Cr(VI) adsorption and exhibited excellent reusability.In addition, it had a fast desorption rate and the equilibrium was achieved within 60 min.The mechanism underlying Cr(VI) adsorption by the calcined pyrite powder includes physical adsorption and redox effects.The calcined pyrite powder can be applied as a new adsorbent to remove Cr(VI) from water.This study provides the basis for the development and application of pyrite-based adsorbents for wastewater treatment.

Figure 1 .
Figure 1.Cr(VI) removal rates of different pyrite powders.The data are expressed as the means ± SD (n ¼ 3).Ã p < 0.05 calcined at 600 C compared to raw pyrite.

Figure 2 .
Figure 2. Particle size distribution of pyrite powders calcined at 600 C before and after ball milling (a: before ball milling, b: after ball milling).

Figure 4 .
Figure 4. XRD patterns of the different pyrite powders.

Figure 5 .
Figure 5. EDS and XPS results of the calcined pyrite powders before and after Cr(VI) adsorption (a) EDS results, (b) Full survey results, (c) Cr 2p results, and (d) Fe 2p results.

Figure 6 .
Figure 6.Effects of different factors on the Cr(VI) removal rate (a) the particle size of the adsorbent, (b) initial pH, (c) equilibration temperature, (d) adsorbent dosage, and (e) equilibration time.

Figure 7 .
Figure 7. Cr(VI) removal rate of the calcined pyrite powder (a) with varying cycles, and (b) with varying time.

Table 1 .
Preparation processes for different pyrite samples.

Table 2 .
Phase compositions of different pyrite products.

Table 3 .
EDS results of the calcined pyrite powders before and after Cr(VI)adsorption.

Table 4 .
Adsorption isotherm parameters for the adsorption of Cr(VI) by the calcined pyrite powder.

Table 5 .
Adsorption kinetic parameters for the adsorption of Cr(VI) by the calcined pyrite powder.Graduate Student Research and Practice Innovation Program Project [No.KYCX23_2052].