Efficient removal of malachite green from aqueous solution using flue gas desulfurization gypsum-loaded ferrous sulfide composites: Synthesis, mechanism, and behavior

Yifang Song; Na Li; Haocheng Lai; Hongxiang Wu; Xukun Ma; Haihua Zhan; Wei Wang

doi:10.25259/AJC_380_2025

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Original Article

2025

:18;

3802025

doi:

10.25259/AJC_380_2025

Efficient removal of malachite green from aqueous solution using flue gas desulfurization gypsum-loaded ferrous sulfide composites: Synthesis, mechanism, and behavior

Yifang Song^a, Na Li^a, Haocheng Lai^a, Hongxiang Wu^a, Xukun Ma^a,, Haihua Zhan^b, Wei Wang^a,*

aSchool of Civil Engineering, Shaoxing Key Laboratory of Interaction between Soft Soil Foundation and Building Structure, Shaoxing University, Shaoxing 312000, China

bSchool of textile science and engineering, Shaoxing University, Shaoxing 312000, China

*Corresponding author: E-mail address: wellswang@usx.edu.cn (W. Wang)

Received: 2025-04-13, Accepted: 2025-08-09, Epub ahead of print: 2025-09-22, Published: 2025-10-15

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Abstract

Malachite green (MG) is a cationic dye that has been widely used in the dyeing of leather, silk, paper, and textiles. However, MG is difficult to adsorb and separate from treated water, leading to environmental pollution and posing a significant threat to living organisms and the natural environment. Herein, a composite material (FeS@FGD) loaded with ferrous sulfide (FeS) was synthesized from flue gas desulfurization gypsum (FGD) to adsorb MG from aquatic environments. The adsorption mechanism was analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and zeta potential measurements. Furthermore, the effects of pH, adsorbent mass, initial MG concentration, temperature, and adsorption time on the MG adsorption capacity of FeS@FGD were investigated. The results indicate that:(1) The adsorption mechanism of FeS@FGD for MG involves electrostatic interactions and hydrogen bonding. (2) At pH=10, with an FeS@FGD mass of 30 mg, an initial MG concentration of 100 mg/L, and a temperature of 313 K, FeS@FGD achieves optimal adsorption, with a removal rate of 96.9% and an adsorption capacity of 153.6 mg/g. (3) Elevated temperatures promote the adsorption of MG by FeS@FGD, indicating an endothermal reaction, with the adsorption behavior more accurately described by the Freundlich isotherm model. Adsorption equilibrium is reached at 480 min and aligns more closely with pseudo-second-order kinetic equations. (4) FeS@FGD maintains 76.4% efficiency after four regeneration cycles, demonstrating excellent reusability. In conclusion, the composite material derived from solid waste FGD and loaded with FeS shows promise as a low-cost and efficient adsorbent for the removal of MG from aqueous solutions, offering a valuable reference for addressing cationic dye pollution in the natural environment.

Keywords

Adsorption mechanism

Cyclic utilization

Dynamics

Isotherm

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1. Introduction

Due to the relentless expansion of industrial and urban frontiers, aquatic ecosystems are facing a silent crisis, with synthetic contaminants arising from unrestricted technological proliferation. Currently, the prevalence of hydrological networks reduces natural remediation capacities, which jeopardizes biodiversity while affecting planetary health. In recent years, the widespread use of dyes and their intensive application in different industrial fields—such as cosmetics, textiles, and paper printing has been increasingly highlighted [1,2]. These industrial activities significantly contribute to the growing presence of dyes in the environment, particularly in aquatic systems [3]. Synthetic dye effluents are especially persistent pollutants due to their xenobiotic nature, teratogenic potential, and tendency for bioamplification along trophic chains [4-6]. Malachite green (MG), a typical triphenylmethane cationic dye, is extensively used in the textile, printing, dyeing, and aquaculture industries owing to its high color rendering efficiency and low cost [7,8]. Alarmingly, epidemiological studies have shown that MG can induce genomic instability through intercalative DNA damage, leading to its classification as a Category 1B carcinogen under EU REACH regulations [9]. In general, the discharged of untreated dyes into water bodies poses a serious environmental threat [10]. These pressing issues underscore the urgent need for transformative innovations in aqueous MG remediation technologies.

Current wastewater treatment technologies primarily include membrane filtration [11-13], biological treatment [14,15], photocatalytic degradation [16-18], and chemical oxidation [18,19], though each has its own limitations. However, these methods often suffer from limitations such as high energy consumption, risk of secondary pollution, and stringent operational requirements. Among these, adsorption technology has emerged as a leading remediation strategy due to its operational flexibility, cost-effectiveness, and low environmental impact [20]. Although traditional adsorbents such as activated carbon [21,22] and carbon nanotubes [23,24] exhibit high adsorption capacities, their large-scale application is constrained by high production costs and limited regeneration efficiency [25]. While natural adsorbents have been extensively investigated, recent research has increasingly focused on composites and modified adsorbents to overcome these challenges and enhance performance [26].

In recent years, solid waste recycling strategies based on the concept of ‘treating waste with waste’ have offered an innovative pathway for the development of novel adsorbents [27,28]. Among these, flue gas desulfurization gypsum (FGD), a by-product generated in quantities exceeding 300 million tons annually by coal-fired power plants [29,30], poses environmental risks due to land occupation. At the same time, FGD represents a valuable resource, as it is primarily composed of calcium sulfate dihydrate (CaSO₄·2H₂O) [31,32]. However, the native adsorption capacity of unmodified FGD for MG is inadequate. Preliminary experiments indicate that the adsorption performance of raw FGD reaches only 41.53 mg/g, highlighting the urgent to enhance its adsorption efficiency through modification.

Iron–sulfur compounds are cost-effective, easy to synthesize, and generally do not contribute to secondary pollution [33]. They have been effectively applied in the remediation of contaminated water bodies and soils. These compounds are rich in Fe²⁺ and S²⁻ ions, both of which act as potent electron donors and confer strong reducing properties. Ferrous sulfide (FeS), a mineral comprising iron and sulfur, is particularly attractive due to its environmental benignity and strong affinity for binding with pollutants [34-36]. In this study, FeS was innovatively loaded onto the surface of a porous desulfurized gypsum matrix to construct a FeS@FGD composite adsorbent. This work introduces a novel composite architecture through the integration of FeS with FGD substrates (FeS@FGD). A suite of microstructural characterization techniques was employed to investigate the inter-reaction mechanisms of FeS@FGD with MG in aqueous solution. The findings position FeS@FGD as a transformative material for cationic dye remediation, offering a dual solution for solid water valorization and water purification.

2. Materials and Methods

2.1. Raw materials

2.1.1. Adsorbate

MG, with the molecular formula C₂₃H₂₅ClN₂, is an artificial synthetic organic compound, which is a toxic triphenylmethane chemical, as shown in Figure 1.

The MG aqueous solution used in this study was sourced from Fei Jing Biotechnology Co. Ltd., with a concentration of 10 mg/mL. X-ray fluorescence analysis was conducted to determine the relative content of each component in the MG sample, with results reported as mass percentages (wt%). The powder compaction method was employed: 10 mg of dried MG powder was ground through a 200-mesh sieve, mixed with boric acid, and pressed into a disc with a diameter of 30 mm under a pressure of 40 MPa. The analysis was performed using an ARL PERFORM’X WDXRF spectrometer under vacuum conditions. Elemental analysis (Table 1) identified S and Na as the predominant elements, while chemical composition analysis (Table 2) indicated that these elements were primarily present in the forms of SO₃ and Na₂O species.

Table 1. Main elements of MG.

Element	S	Na	Si	Cl	Others
content (%)	59.47	34.10	1.88	1.41	3.14

Table 2. Main chemical components of MG.

Chemical composition	SO₃	Na₂O	SiO₂	Others
content (wt%)	62.0	33.32	1.96	2.72

2.1.2. Adsorbent

The FeS@FGD adsorbent used in this study is a composite material synthesized by loading ferrous sulfide (FeS) onto FGD. The FGD was supplied by Yantai Anda Environmental Protection Technology Co., Ltd., with a calcium sulfate content of 92%. FeS was prepared using FeSO₄·7H₂O and Na₂S·9H₂O. FeSO₄·7H₂O, with a purity of 99.7%, was provided by Shandong Keyuan Biochemical Co., Ltd., and the Na₂S·9H₂O, with a purity of 98%, was obtained from Shanghai Macklin Biochemical Technology Co., Ltd. To avoid spatial inhomogeneity of active sites—caused by an excess of either Fe²⁺ or FGD, which could result in surface blockage or heterogeneous distribution, a moderate concentration ratio of Fe²⁺ to FGD was selected at 1:1. Furthermore, the stoichiometric optimization method proposed by literatures [37,38] was adopted, designing the synthesis based on an equimolar sulfur-to-iron ratio (S^2-:Fe²⁺ = 1:1). The specific synthesis steps are illustrated in Figure 2. First, 300 mL of deionized water was measured into a reaction flask, and 1g of FGD was added, followed by magnetic stirring for 1h. Next, 8.89 g of FeSO₄·7H₂O was added and stirred until fully dissolved. Then, 8.577 g of Na₂S·9H₂O was dissolved in deionized water and slowly added to the reaction flask using a peristaltic pump over the course of 1h. These initial steps were carried out in an N₂ atmosphere at a temperature of 70°C. After the reaction was complete, the mixture was allowed to cool to room temperature. The supernatant was decanted, and the remaining mixture was centrifuged. The resulting solid was then freeze-dried for 24 h to obtain the final FeS@FGD adsorbent powder.

2.2. Experiment design

Empirical studies have demonstrated that adsorption efficacy is predominantly influenced by four key operational parameters: aqueous phase pH, adsorbent mass, initial pollutant concentration, and reaction temperature. In this experiment, FeS@FGD composites were evaluated under varying pH levels, different adsorbent quantities, and multiple initial MG concentrations, as detailed in Table 3. Experimental parameters were initially optimized based on preliminary tests. Subsequently, temperature-dependent adsorption behavior was investigated at 293 K, 303 K and 313 K through adsorption isotherm analyses and thermodynamic parameter calculations. Upon establishing the optimal adsorption conditions, kinetic studies were performed over a 24 hrs period with strategically timed sampling intervals to systematically monitor adsorption progress. Comparative analyses of samples before and after adsorption provided comprehensive insights into the underlying adsorption mechanisms.

Table 3. Test design scheme.

Sample code	pH	FeS@FGD mass (mg)	Initial MG Concentration (mg/L)	Temperature (K)	Reaction time (h)
1	3, 4, 5, 6, 7, 8, 9, 10	50	120	303	3
2	10	10, 20, 30, 40, 50	100	303	3
3	10	30	60,80,100,120,140	293	3
4	10	30	60,80,100,120,140	303	3
5	10	30	60,80,100,120,140	313	3
6	10	30	100	313	0∼24

2.3. Adsorption procedure

The adsorption procedure was carried out according to the literature [39], as illustrated in Figure 3. First, FeS@FGD particles were sieved through a 0.05-mm mesh and accurately weighed using an analytical balance. A 10-mg/mL aqueous MG solution was homogenized by ultrasonication for 30 min to ensure uniform dispersion. MG working solutions (50 mL) were prepared in deionized water (produced by a UPW-R15 purification system, Shanghai Electric Scientific Instrument Co., China) to achieve the desired initial concentrations. The solution pH was monitored using a calibrated pH meter and adjusted to target values by the incremental addition of 0.1 mol/L NaOH (analytical grade, Euro Hongda Industrial Co., China) or HCl (analytical grade, Guangzhou Hewei Pharmaceutical Technology Co., China), ensuring the total volume change remained below 1%. Adsorption experiments were conducted in temperature-controlled shakers at 240 rpm at temperatures of 293 K, 303 K, or 313 K. FeS@FGD was added to the MG solutions and agitated for 3 h to reach adsorption equilibrium. After adsorption, the mixtures were centrifuged and left to stand for 24 h to facilitate phase separation. The supernatant was then diluted to 25 mL with deionized water, and a 1-mL aliquot was taken to measure absorbance at 618 nm using a UV-Vis spectrophotometer (UV75N, Yoke Instrument, China).

All experiments were performed in triplicate, with mean values reported as final results. Any outliers deviating by more than 10% from the mean were excluded, and additional replicates were performed to ensure statistical reliability. Meanwhile, the error bar is calculated based on standard deviation, which was used to check and validate the accuracy of the experimental results.

2.4. Microscale characterization methods

To investigate the adsorption mechanism of FeS@FGD for MG, a series of microscopic and physicochemical analyses were performed on the raw materials (FeS@FGD and MG) and post-adsorption samples (FeS@FGD/MG). The microstructural and surface properties of these samples were systematically characterized using a range of advanced techniques. Surface charge behavior was assessed by measuring the zeta potential at varying pH levels using a ZS90 analyzer (Malvern Corporation, USA). Morphological features and surface topography were examined via scanning electron microscopy (SEM, JSM-6360LV, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). Specific surface area was determined by Brunauer-Emmett-Teller (BET) analysis using a Tristar II 3020 system (Micromeritics, USA). Crystalline phases were identified through X-ray diffraction (XRD, Empyrean, Malvern Corporation, UK), while atomic force microscopy (AFM, Dimension Icon, Bruker, USA) was employed to obtain surface roughness profiles. Functional groups were characterized by Fourier-transform infrared spectroscopy (FTIR, IRPrestige-21, Shimadzu, Japan), and surface chemical states were examined using X-ray photoelectron spectroscopy (XPS, THS-103) to elucidate electronic interactions involved in the adsorption process.

2.5. Data analysis

The adsorption performance was quantitatively evaluated using eleven key equations. The specific definitions are explained as follows (Eqs. 1-3):

(1)

Q_{e} = \frac{(C_{0} - C_{e}) \times V}{m}

(2)

R = (1 - \frac{C_{e}}{C_{0}}) \times 100 %

(3)

K_{d} = \frac{Q_{e}}{C_{e}}

where m (mg) denotes the mass of the FeS@FGD powder, and V (mL) represents the volume of the solution.

Quasi-first-order adsorption kinetic Eq. (4):

(4)

Q_{t} = Q_{e} [1 - \exp (- k_{1} t)]

Quasi-second-order adsorption kinetic Eq. (5):

(5)

Q_{t} = \frac{k_{2} Q_{e}^{2} t}{1 + k_{2} Q_{e} t}

Intra-particle diffusion modelling (Eq. 6):

(6)

Q_{t} = k_{d} t^{0.5} + C

where Q_e (mg/g) denotes the equilibrium adsorbed amount per unit mass of adsorbent; Q_t (mg/g) represents the adsorbed amount at time t; rate constants k₁ (g/mg min) and k₂ (g/mg·min) correspond to the quasi-first-order and quasi-second-order kinetic models, respectively; K_d (mg/g·min^1/2) is the intra-particle diffusion rate coefficient; and C (mg/g) is a constant related to the thickness of the boundary layer.

Kinetic model (Eqs 7-9):

(7)

Langmuir model: Q_{e} = \frac{K_{L} Q_{\max} C_{e}}{1 + K_{L} C_{e}}

(8)

Freundlich model: Q_{e} = K_{F} C_{e}^{\frac{1}{n}}

(9)

Temkin model: Q_{e} = D \ln K_{T} + C \ln C_{e}

where Q_e (mg/g) and C_e (mg/L) represent the equilibrium adsorption capacity and equilibrium adsorbate concentration, respectively; Q_max (mg/g) is the theoretical maximum adsorption capacity; K_L (L/mg) denotes the Langmuir affinity constant, reflecting the binding site energy; K_F (mg/g) is the Freundlich equilibrium constant, which reflects the adsorption capacity at unit concentration; and n is the surface heterogeneity; K_T (L/mg) is the Temkin equilibrium constant; and C is an infinitesimal constant related to temperature and the nature of the adsorption system.

Thermodynamic Eqs. (10) and (11):

(10)

Δ G = - R T \ln K_{d}

(11)

\ln K_{d} = \frac{Δ S}{R_{1}} - \frac{Δ H}{R T}

where T (K) is the absolute temperature in Kelvin; R is the universal gas constant (8.3145 J/mol K); and K_d is the partition coefficient.

3. Results and Discussion

3.1. Physicochemical properties of materials

The elemental composition and main chemical components of FeS@FGD have been presented in Tables 4 and 5. The primary elements of the adsorbent are iron (Fe) and sulfur (S), with its main chemical constituents identified as Fe₂O₃ and S.

Table 4. Main elements of FeS@FGD.

Element	S	Fe	Ca	Na	Others
Content (wt%)	43.94	40.95	9.47	4.75	0.89

Table 5. Main chemical components of FeS@FGD.

Chemical composition	Fe₂O₃	S	CaO	Na₂O	SiO₂	Others
Content (wt%)	45.63	36.83	10.69	5.56	0.51	0.89

To elucidate the sorption characteristics of the FeS@FGD composites, BET surface area analysis was systematically performed following degassing pre-treatment at 100°C under vacuum. As illustrated in Figure 4, nitrogen physisorption isotherms were quantitatively analyzed, revealing a type IV hysteresis loop characteristic. The calculated BET-specific surface area was 30 m²/g. Furthermore, the pore size distribution of FeS@FGD is between 0 and 120nm, and the main pore size is 39.04nm. According to the definition of IUPAC, FeS@FGD belongs to a mesoporous material [40-42].

The particle size distributions of FeS@FGD and MG were systematically characterized using laser diffraction granulometry (Mastersizer 3000, Malvern Panalytical, UK), as shown in Figure 5. Quantitative analysis revealed distinct size dispersion patterns: FeS@FGD exhibited a polydisperse distribution ranging from 0.405 to 66.9 µm, with a volumetric mean diameter (Dv50) of 24.1 µm. In comparison, MG displayed a broader granulometric range from 0.40 to 127 µm, with a Dv50 value of 31.1 µm. This differential analysis confirms that MG particulates possess statistically larger median dimensions relative to FeS@FGD composites, as evidenced by the rightward shift in their cumulative distribution curves.

Granularity distribution of MG and FeS@FGD.

3.2. Microscopic morphology

3.2.1. SEM and TEM analyses

To elucidate the structural evolution of FeS@FGD during MG adsorption, SEM/TEM were employed to characterize the pristine FeS@FGD, unadsorbed MG and post-adsorption FeS@FGD/MG composites (Figure 6).

The comparative microscopy analysis of (a-b) FeS@FGD, (c-d) MG, and (e-f) FeS@FGD/MG composite.

The pristine FeS@FGD exhibited a smooth surface morphology with visible porosity and irregular block-like structures (Figures 6a, b), featuring heterogeneous particle sizes and distinct cross-sectional interfaces. In contrast, unadsorbed MG particles displayed polydisperse, rod-shaped configurations with disordered spatial arrangements (Figures 6c,d) [43]. Post-adsorption analysis revealed significant MG accumulation on the FeS@FGD surface, characterized by dense agglomerates (Figures 6e, f). This pronounced surface coverage and cluster formation directly corroborate the adsorption affinity of FeS@FGD for MG.

3.2.2. XRD and AFM analyses

To investigate the adsorption mechanism, XRD analysis was systematically performed on MG, pristine FeS@FGD, and post-adsorption FeS@FGD/MG composites (Figure 7). The XRD pattern of pristine MG exhibited a prominent diffraction peak at 2θ=24.5°, which disappeared following interaction with FeS@FGD. This suggests effective adsorption of MG onto the composite’s surface and within its internal pore structure, where MG molecules permeate the FeS@FGD matrix and bind to active sites, ultimately reducing residual MG concentrations below the analytical detection limit [44].

The XRD profiles of MG, FeS@FGD, and FeS@FGD/MG.

Phase analysis identified FeS@FGD as a multicomponent system comprising gypsum (CaSO₄), ferrous sulfide (FeS), and calcite (CaCO₃). Key reflections were observed at 2θ = 11.6° (CaSO₄(020)), 19.7° (CaSO₄ (020)), 28.5° (CaCO₃ (104)), and 30° (FeS (101)), corresponding with ICSD reference standards (PDF card). After adsorption, FeS@FGD/MG exhibited attenuated peak intensities at 11.6°, 19.7° (CaSO₄), and 30° (FeS), while no new crystalline phases were detected. This attenuation suggests partial dissolution or structural reorganization of gypsum and FeS during adsorption, likely enhancing the availability of active binding sites for MG uptake.

To validate the interfacial interaction mechanism between FeS@FGD and MG, AFM topographic analysis was systematically conducted on both pristine MG and FeS@FGD-modified MG specimens. As shown in Figures 8(a-d), a comparative evaluation of vertical height profiles revealed a significant increase in surface thickness for the FeS@FGD/MG composite compared to the unmodified MG sample. This dimensional enhancement is particularly evident in the maximum height, which increased from 8.10 nm for bare MG to 11.30 nm for the composite system. This 28.3% increment in thickness provides direct morphological evidence for successful surface modification, strongly suggesting the formation of a composite interface via preferential adsorption of FeS@FGD clusters onto the MG substrate, consistent with the proposed interaction mechanism.

The AFM images of (a,b) MG and (c,d) FeS@FGD/MG.

3.2.3. XPS analysis

XPS was employed to investigate the chemical speciation and electronic state changes of FeS@FGD before and after MG adsorption. Comparative high-resolution spectra (Figure 9a) revealed distinct elemental signatures: pristine FeS@FGD exhibited characteristic Fe2p, O1s, C1s, and S2p orbitals, whereas the post-adsorption FeS@FGD/MG composite showed marked attenuation of the Fe2p and S2p peak intensities, indicative of displacement of iron and sulfur species during MG chemisorption.

(a) The XPS spectra of FeS@FGD and FeS@FGD/MG, (b) Fe2p deconvolution, (c) S2p deconvolution, (d) O1s deconvolution, (e) C1s deconvolution.

Deconvolution of Fe2p spectra (Figure 9b) revealed valence state modulation, with the peak area ratios of Fe²⁻ and Fe³⁻ structures shifting from 28.54%, 19.25%, 18.92% and 33.29% to 32.12%, 34.31%, and 33.57%, respectively, accompanied by binding energy shifts to 724.32eV, 715.12eV, and 709.83eV. This oxidation state transition implicates Fe²⁺-mediated redox coupling as a critical adsorption pathway. Concurrently, S2p spectral analysis (Figure 9c) demonstrated sulfur speciation changes: the peak area ratios of SO₄²⁻, S₂²⁻, and S²⁻ species were initially 20.13%, 37.64%, 29.53%, and 12.70%, shifting to 23.85%, 40.31%, 20.42% and 15.24%, respectively, with corresponding binding energy shifts consistent with sulfur–MG coordination. The depletion of S²⁻ moieties further supports the participation of sulfate groups in interfacial electron transfer.

As shown in Figure 9(d) and 9(e), there was no significant change in the spectral peak position and peak area of O1s and C1s before and after adsorption. This indicates that the oxygen-containing functional groups and carbon matrix on the FeS@FGD surface do not undergo obvious chemical state changes during adsorption.

Collectively, these findings demonstrate that MG adsorption on FeS@FGD is governed by synergistic Fe²⁺ oxidation and S^2- ligand exchange mechanisms, where surface-bound iron and sulfur centers act as cooperative active sites for pollutant immobilization.

3.2.4. FTIR analysis

Fourier-transform infrared (FTIR) spectroscopic facilitates real-time monitoring of molecular bond vibrations and changes in functional groups during adsorption, providing molecular-level insights into interfacial interactions for mechanistic elucidation and optimization in surface chemistry studies. As shown in the MG spectrum in Figure 10, pristine MG exhibited characteristic bands at 3470 cm^-1 (O-H stretching), 1640 cm^-1 (aromatic C=C vibration) and 1200 cm^-1 (N–H bending) [45]. The FeS@FGD/MG spectrum closely resembles that of FeS@FGD, with the notable emergence of a new C=C vibrational peak at 1670 cm^-1 post-adsorption. Compared with before adsorption, it has moved 30cm^-1, which indicates that MG is adsorbed on FeS@FGD. Following adsorption, the O-H peak in FeS@FGD/MG shifted to a lower wavenumber of 3430 cm^-1, representing a displacement of 40 cm^-1, accompanied by peak broadening. Additionally, the original N-H peak of MG disappeared, indicating the formation of hydrogen bonds between FeS@FGD and MG. The FeS@FGD/MG composite also retained characteristic S-O and Fe-S vibrational peaks at 1140 cm^-1 and 600 cm^-1.

The FTIR profiles of FeS@FGD, MG and FeS@FGD/MG.

3.2.5. Adsorption mechanism

The interfacial interaction mechanism between MG and FeS@FGD was systematically elucidated through a multimodal characterization approach, integrating TEM, SEM, AFM, XRD, BET, XPS, and FTIR analyses. Morphological changes observed via SEM, TEM, and AFM imaging, alongside crystallographic variations from XRD and XPS profiles, confirmed significant structural reorganization of FeS@FGD after adsorption, accompanied by evident MG aggregation on the composite surface. FTIR spectra revealed key interfacial interactions, including hydrogen bonding involving surface hydroxyl (O-H) groups of FeS@FGD, and electrostatic coupling between FeS@FGD and the aromatic (C=C) and amine (N-H) moieties of MG.

The adsorption mechanism of FeS@FGD for MG involves a combination of electrostatic interactions, hydrogen bonding, and π-π interaction, as shown in Figure 11. First, ferrous sulfide consists of positively charged iron ions (Fe²⁺) and negatively charged sulfur ions (S²⁻). In aqueous solution, the surface of ferrous sulfide may acquire a net negative charge due to the dissociation of sulfur ions (S²⁻) or interaction with other anions, thereby providing negatively charged sites. MG, being a cationic dye with a positively charged central carbon(C⁺), is therefore electrostatically attracted to these negatively charged surfaces. Second, hydrogen bonding can occur between nitrogen- or oxygen-containing functional groups of MG molecules and protons in the aqueous phase or surface moieties of FeS@FGD. Specifically, the nitrogen atoms in the amino groups of MG form hydrogen bonds with hydrogen atoms in water molecules, utilizing their lone pairs of electrons.

Schematic diagram of the adsorption mechanism.

3.3. Effect of pH on adsorption performance

Advanced interfacial chemistry studies have identified several key factors governing adsorption processes. In particular, the pH-dependent surface charge characteristics play a fundamental role by modulating electrostatic interactions between the adsorbent and target molecules via regulation of potential-determining ions. To investigate the pH-dependent adsorption behavior, batch experiments were conducted at 303 K using 50 mg of FeS@FGD and a C₀ of 120 mg/L. The Q_e, R, and K_d all exhibited a unimodal dependence on pH, reaching a maximum at pH=10 (Q_e=117.77 mg/g, R=96.14%), while showing a marked decline under acidic conditions (pH 3: Q_e=102.51 mg/g, R=85.43%), as shown in Figure 12. Therefore, pH 10 was determined to be the optimum condition for adsorption capacity.

Effect of pH value on the MG adsorption.

The observed phenomena can be explained by four interrelated mechanisms. First, under alkaline conditions (pH=10), spontaneous aggregation and partial precipitation of MG molecules occur due to their intrinsic self-adhesive properties. Second, under acidic conditions (pH<7), the reduced adsorption capacity of FeS@FGD mainly results from the competitive occupation of active sites. Protons (H⁺) preferentially interact with negatively charged surface groups, thereby blocking binding sites and hindering the adsorption of cationic MG moieties [46]. Third, protonation-induced charge repulsion further limits adsorption at low pH: both the FeS@FGD surface and protonated MG molecules become positively charged, leading to mutual electrostatic repulsion that creates an energy barrier to effective adsorption. Conversely, above pH=7, deprotonation generates abundant anionic sites on the adsorbent surface. This charge reversal fosters strong electrostatic attraction with the cationic MG molecules, resulting in optimal adsorption efficiency at pH=10, where the surface attains its maximum negative potential [47].

Collectively, these mechanisms support pH=10 as the optimal adsorption condition, corresponding to the highest adsorption capacity (Q_e=117.77 mg/g, R = 96.14%).

Surface charge characterization via electrophoretic mobility measurements (Figure 13) revealed pH-dependent zeta potential variations [48]. As the pH increases, the zeta potential of MG gradually decreases from 22.5 mV to 5.5 mV, remaining positively charged across the entire pH range, which confirms that the dye molecules retain their cationic nature. According to the literature [49,50], pHpzc is defined as the pH value of a solution where the positive and negative charges at the adsorbent surface are equal, resulting in zero net charge. When the pH is below pHpzc, the adsorbent exhibits a positive total surface charge, leading to an attraction for anions and a repulsion for cations. Conversely, if the pH is above pHzcp, the adsorbent’s surface becomes negatively charged, allowing it to attract and bind positively charged substances. Experimental results show that the pHpzc value of the adsorbent FeS@FGD is 5. This indicates that when the pH in the aqueous solution is 5, the surface charge of FeS@FGD is zero. Since MG showed positive Zeta potential in all tested pH ranges, this suggests that when pH is greater than 5, there is an electrostatic attraction between the positive charges on the MG surface and the negative charges on the FeS@FGD surface. However, when pH is less than 5, the adsorption capacity decreases due to electrostatic repulsion.

The Zeta potentials of MG and FeS@FGD/MG.

In practical scenarios, industrial wastewater from dyeing and textile industries is typically alkaline, with pH values ranging from 9 to 11 [51,52]. This alkalinity arises because alkaline auxiliaries are used during the dyeing process to improve dye solubility and fiber treatment efficiency, resulting in naturally alkaline wastewater. Therefore, selecting pH 10 for adsorption trials not only reflects the chemical characteristics of real-world wastewater but also bridges the gap between laboratory experiments and practical applications, thereby enhancing the engineering value of the research findings. Consequently, all subsequent experiments in this study will be conducted at pH=10.

3.4. Effect of FeS@FGD dosage on adsorption

To evaluate the influence of adsorbent dosage on MG removal efficiency, Q_e, R and K_d were systematically measured under optimized conditions (T=303 K, pH=10, C₀=100 mg/L). As shown in Figure 14, the adsorption performance exhibited a pronounced nonlinear dependence on FeS@FGD dosage. While Q_e decreased steadily with increasing adsorbent mass, both R and K_d reached maximum values at 30 mg of FeS@FGD, with a peak removal efficiency of 96.24% (Q_e=152.4 mg/g). This behavior reflects the saturation of active adsorption sites at higher dosages, where excessive adsorbent mass leads to particle aggregation and a reduction in available surface area per unit mass, thereby diminishing adsorption capacity.

Effect of FeS@FGD mass on the MG adsorption.

3.5. Effect of MG concentration on adsorption

The initial concentration of the adsorbate is also a critical factor influencing the adsorption efficiency of the adsorbent. To determine the optimal MG concentration for FeS@FGD adsorption, equilibrium batch experiments were performed under fixed conditions (T=303 K, FeS@FGD mass=30 mg, pH=10), with the results shown in Figure 15. The equilibrium adsorption capacity (Q_e) increased linearly with rising MG concentration, whereas the removal efficiency (R) and partition coefficient (K_d) exhibited a unimodal trend, both peaking at C₀=100 mg/L, with maximum R and K_d values of 96.24% and 28.82L/g, respectively. This concentration-dependent behavior can be explained by competing interfacial processes: First, at low C₀, abundant active sites on FeS@FGD relative to MG molecules facilitate monolayer adsorption, enhancing R and K_d through efficient site utilization. Second, at intermediate C₀ (100mg/L), saturation of high-affinity sites maximizes electrostatic and hydrogen-bonding interactions, achieving adsorption equilibrium. Third, at high C₀, an excess of negatively charged adsorption sites promotes competitive adsorption between cationic MG and co-existing ions, reducing site accessibility, while charge screening effects weaken electrostatic forces, collectively lowering R and K_d. Consequently, C₀=100 mg/L was identified as the threshold that balances site occupancy and intermolecular competition, ensuring optimal FeS@FGD adsorption performance.

Effect of the initial MG concentration on the MG adsorption.

3.6. Adsorption thermodynamics

The adsorption performance of FeS@FGD towards MG was systematically evaluated under varying initial MG concentrations (60, 80, 100, 120, and 140 mg/L) and temperatures (293 K, 303 K, and 313 K) while maintaining fixed conditions (pH=10, FeS@FGD mass=30 mg). As shown in Figure 16(a), Q_e exhibited a positive correlation with both temperature and initial MG concentration, with the influence of temperature becoming more pronounced at higher C₀ values. However, only minor variations in Q_e were observed across different temperatures at identical C₀, indicating limited thermal dependence of the adsorption process.

Thermodynamic characterization and equilibrium modeling of MG adsorption (a) Adsorption isotherm, (b) Langmuir isothermal fitting curve, (c) Freundlich isothermal fitting line, (d) Temkin isothermal fitting curve.

To further elucidate the thermodynamics of the adsorption process, this study employs three internationally recognized isotherm models: Langmuir (Figure 16b), Freundlich (Figure 16c), and Temkin (Figure 16d). These models provide a comprehensive framework for analyzing the adsorption mechanism. The Langmuir model is based on the assumption of monolayer adsorption, where adsorbent molecules form a single uniform layer on the adsorbate surface, with all adsorption sites possessing equal energy and no interaction between adsorbed molecules [53]. The Freundlich model, by contrast, describes multi-layer adsorption on heterogeneous surfaces; when 1/n<1, it indicates preferential adsorption of the adsorbate by the adsorbent [54,55]. The Temkin model, similar to Langmuir in assuming identical adsorption sites, differs by accounting for a linear decrease in adsorption energy of unoccupied sites due to interactions with adjacent adsorbed particles [56]. Given that FeS@FGD is a non-uniform material, the Freundlich model is more appropriate for this system, as it better reflects adsorption behavior on heterogeneous surfaces. The Freundlich parameters K_F and 1/n simultaneously quantify adsorption capacity and surface heterogeneity, respectively. A value of 1/n<1 implies that FeS@FGD exhibits preferential adsorption for MG. Compared to the other models, the Freundlich model yields a higher R² value, confirming a better fit to the experimental data. Here, K_F represents the adsorption capacity per unit concentration, while Q_max denotes the theoretical maximum adsorption capacity (mg/g), reflecting the adsorbent’s potential when fully saturated with a monolayer of adsorbate. Both K_F and Q_max increase with temperature, indicating that the adsorption process is endothermic. Furthermore, the Temkin model constants C, which relate to temperature and system properties, are positive across all fittings, further corroborating the endothermic nature of the adsorption [57].

To investigate the temperature-dependent adsorption behavior of FeS@FGD towards MG, systematic experiments were carried out at three different temperatures (293 K, 303 K, and 313 K) across varying initial MG concentrations. As shown in Figure 17, the partition coefficient (K_d) exhibited a characteristic unimodal distribution with increasing MG concentration. Furthermore, the linear relationship observed between lnK_d and 1/T indicated a significant dependence on temperature. Thermodynamic parameters were derived using the Van’t Hoff equations (Eqs. 10-12), with key results summarized in Table 5. The enthalpy change (ΔH) reflects whether the reaction is exothermic or endothermic, the entropy change (ΔS) indicates variations in system disorder, and the Gibbs free energy change (ΔG) combines these effects to predict the spontaneity of the process [58,59]. According to the data, ΔG was negative for MG adsorption onto FeS@FGD at all tested temperatures, confirming the spontaneous nature of the adsorption. Notably, the absolute value of ΔG increased with temperature, suggesting enhanced spontaneity at higher temperatures [60,61]. The positive ΔS values imply an increase in randomness or degrees of freedom at the solid-liquid interface during adsorption. Additionally, the positive ΔH across different MG concentrations confirms that the adsorption process is endothermic, which is consistent with the adsorption isotherm model fitting results (Tables 6 and 7).

Table 6. Isotherm model regression statistics: Langmuir/Freundlich/Temkin constant comparison.

Temperature (K)	Langmuir			Freundlich			Temkin
Temperature (K)	Q_max (mg/g)	K_L (L/mg)	R²	K_F (mg/g)	n	R²	K_T (L/mg)	C	R²
293	560.57	0.058	0.92	26.62	1.142	0.99	0.389	120.13	0.95
303	793.16	0.041	0.93	36.13	1.103	0.96	0.423	117.91	0.97
313	1340.64	0.028	0.90	39.54	1.074	0.97	0.731	112.72	0.95

Table 7. Thermodynamic fitting parameters.

C_o (mg/L)	ΔG (kJ/mol)			ΔH (kJ/mol)	ΔS (J/mol·K)
C_o (mg/L)	293K	303K	313K	ΔH (kJ/mol)	ΔS (J/mol·K)
40	-6.115	-6.428	-7.186	17.28	3000
60	-6.584	-6.927	-7.625	17.09	3050
80	-6.948	-7.306	-7.917	16.63	2900
100	-6.376	-6.802	-7.259	16.04	2650
120	-5.465	-6.046	-5.116	18.31	3250

3.7. Adsorption kinetics

To elucidate the temporal evolution of MG adsorption onto FeS@FGD, kinetic experiments were conducted under optimized conditions (T=303K, pH=10, C₀=100mg/L, FeS@FGD mass=30mg) over a 24-h period.

Several models suggest that the diffusion of adsorbates on the surface of the adsorbent controls the adsorption kinetics, with the most commonly used being the quasi-first-order kinetic model and the quasi-second-order kinetic model. This study evaluates the quasi-first-order kinetic model and the quasi-second-order kinetic model, as they help clarify whether the process is controlled by physical adsorption or chemical adsorption [62,63]. The adsorption capacity over time and corresponding regression analyses have been systematically presented in Figures 18(a, b) and Table 8. Q_e displayed a biphasic kinetic profile: an initial rapid uptake phase (0-50 min), during which Q_e reached 78.3% of its maximum value (153.6 mg/g), followed by a slower approach to equilibrium (50-480 min) as active sites became saturated. The quasi-first-order kinetic model (R² = 0.892) inadequately described the adsorption process, indicating its limitations in accounting for boundary layer diffusion effects. In contrast, the quasi-second-order kinetic model (R² = 0.927) showed a superior fit to the experimental data, suggesting that chemisorption mechanisms, such as covalent bonding or electron transfer at the FeS@FGD–MG interface, dominate the process. The predominance of the quasi-second-order model underscores that the adsorption is primarily driven by interfacial chemical interactions rather than simple physisorption. These kinetic findings corroborate earlier thermodynamic results (ΔH > 0, ΔG < 0), collectively validating a temperature-enhanced, entropy-driven adsorption mechanism intrinsic to FeS@FGD composites [64].

Adsorption kinetics of MG. (a) Experimental vs. modeled kinetic curves (quasi-first/second-order), (b) Temporal evolution of MG removal efficiency.

Table 8. Fitting parameters of quasi-first order and quasi-second-order kinetic models.

Pseudo-first-order model

Pseudo-second-order model

Q_e

(mg/g)

k₁

(g/mg·min)

R²

Q_e

(mg/g)

k₂

(g/mg·min)

R²

150.211

0.016

0.892

161.745

1.628×10^-4

0.927

The intra-particle diffusion model provides insights into the mass transfer processes occurring within the adsorbent particles. As illustrated in Figure 19, the adsorption dynamics can be divided into three stages: initially, adsorption occurs on the external surface of the particles. This is followed by a gradual adsorption phase, during which intra-particle diffusion becomes the rate-limiting step. Finally, the system approaches equilibrium, characterized by a very low concentration of adsorbate in the solution and limited availability of adsorption sites, resulting in a marked decrease in intra-particle diffusion [63]. Quantitative analysis presented in Table 9 shows a progressive decline in the diffusion rate coefficients (K_d) across these successive stages, reflecting the gradual saturation of FeS@FGD active sites by MG molecules. This site saturation underpins the transition from rapid initial adsorption to slower uptake rates, ultimately culminating in thermodynamic equilibrium. Additionally, the non-zero intercepts observed in the regression plots indicate that other rate-limiting processes, beyond intra-particle diffusion, influence the adsorption kinetics, consistent with findings reported in previous studies [65].

Intraparticle diffusion mechanism analysis.

Table 9. Key parameters derived from intraparticle transport modeling.

Stage Ⅰ

Stage Ⅱ

Stage Ⅲ

k_d

(mg/g·min^1/2)

(mg/g)

R²

k_d

(mg/g·min^1/2)

(mg/g)

R²

k_d

(mg/g·min^1/2)

(mg/g)

R²

6.29

27.28

0.94

5.89

44.05

0.91

0.65

131.81

0.92

3.8. Desorption efficiency and reusability

To evaluate the recyclability of FeS@FGD, four consecutive adsorption–desorption–readsorption cycles were performed under optimized conditions (30 mg FeS@FGD, C₀ = 100 mg/L, T = 303 K). In the desorption test, 0.1 mol/L NaOH was employed as the eluent. After 30 min of magnetic stirring, the precipitate was collected from the solution and rinsed three times with deionized water. The regenerated FeS@FGD solids were then obtained by centrifugation followed by drying. FeS@FGD showed strong initial performance in the first cycle with a removal rate of 93.4%. As can be seen from Figure 20(a), with the increase of the number of cycles, the efficiency gradually decreased, and the removal rate was 76.4% after the fourth cycle. The actual test changes are shown in Figure 20(b).The removal efficiencies across the four cycles fit well to a linear regression model, revealing a negative correlation with cycle number, as shown in Figure 20(c). The fitting equation R = −5.62n + 98.6 indicates an average efficiency loss of 5.62% per cycle.

Desorption and recyclable test of FeS@FGD. (a) Adsorption efficiency of desorption-adsorption, (b) Actual test variations, (c) Efficiency trend graph of cyclic desorption.

SEM comparison of FeS@FGD under different recycling times: (a) the first time, (b) the second time, (c) the third time, and (d) the fourth time.

The morphological changes of FeS@FGD after washing the precipitate were characterized using SEM. As shown in Figure 21(a), the MG coating on FeS@FGD significantly diminished after the first desorption compared to the pre-desorption stage, with the surface nearly restored to its original condition without adsorbed dye. However, after multiple regeneration cycles, MG solids gradually accumulated on the FeS@FGD surface, as illustrated in Figures 21(b-d). This progressive deposition reduced the number of available adsorption sites, contributing to the observed decline in removal efficiency.

The regeneration process only requires mild conditions and can be reused at least 4 times through simple alkali regulation, which is both economical and environmentally friendly. Although the efficiency decreases after multiple cycles, it shows potential application value in intermittent wastewater treatment and other scenarios, and is expected to become the preferred solution for environmentally friendly adsorbents.

3.9. Economic evaluation

FGD, a by-product of the desulfurization process in coal-fired power plants, is traditionally disposed of via landfill or stockpiling, practices that result in land occupation and potential secondary pollution. This study repurposes FGD as a carrier for adsorbents, thereby creating high-value applications and advancing a circular economy model centered on ‘treating waste with waste’. By modifying FGD with ferrous sulfide (FeS), the resulting FeS@FGD composite exhibits an enhanced adsorption capacity for MG of 153.6 mg/g, representing a 2.69-fold increase compared to the original FGD’s 41.5 mg/g. The adsorbent can be efficiently regenerated using a 0.1M NaOH solution, retaining 76.4% of its adsorption capacity after four cycles. This approach not only provides a high-value utilization pathway for industrial solid waste desulfurized gypsum but also reduces the consumption of new materials and mitigates secondary pollution through a closed loop of ‘adsorption–regeneration–reuse’, thereby aligning with the core principles of the circular economy: reduce, reuse, and recycle.

Cost issues may be a significant factor in attracting industrialists. The unit prices of each material are calculated based on the market price in China. The cost of industrial-grade sodium sulfide and ferrous sulfate is about 20 yuan/kg and 5 yuan/kg, respectively. Desulfurization gypsum, a waste product from coal-fired power plants, has negligible costs. Including the required electricity and equipment, a rough estimate of FeS@FGD is from 20∼26 yuan/kg. The price of activated carbon is 30∼40 yuan/kg, which fluctuates according to different types. Furthermore, the adsorption capacity of FeS@FGD was compared with those of previously reported adsorbents for MG removal, as shown in Table 10. Compared to other activated carbon adsorbents, FeS@FGD not only has a higher adsorption capacity but is also cheaper. Compared to other metal or metal-loaded adsorbents, FeS@FGD has a higher adsorption capacity for MG, significantly enhancing its practicality. The adsorbent is cost-effective because FGD, an abundant industrial by-product, can be transformed into a valuable resource through waste-to-value conversion.

Table 10. Comparison of the Q_e and Q_max of FeS@FGD with other adsorbents for MG Dye removal.

Absorbent	Type	Q_e (mg/g)	Q_max (mg/g)	Reference
Mangosteen peel (KM)	Activated carbon	19.345	29.67	[66]
Rice husk activated carbon (RHAC)	Activated carbon	/	49.62	[67]
Jackfruit-based activated charcoal (JPAC)	Activated carbon	66.8	376.33	[68]
Bamboo activated carbon (BAC)	Activated carbon	96.29	263.58	[69]
nano-γ-alumina	Metal	72.4	72.4	[70]
Metal oxide nanoparticles (ZnO)	Metal	/	310.50	[71]
PPAC-ZnO-NC	Metal load	31.34mg/g	77.51	[71]
FeS@FGD	Metal load	152.28mg/g	793.16	This research

4. Conclusions

This study utilizes solid waste FGD loaded with ferrous sulfide to synthesize FeS@FGD composite materials for the removal of MG dye from aqueous solutions. A series of micro-scale experiments characterized the adsorption of MG by FeS@FGD, examining the effects of adsorbent dosage, pH, initial dye concentration, temperature, and contact time on adsorption efficiency. The adsorption process was further analyzed through isotherm, thermodynamic, and kinetic models. Additionally, the desorption and regeneration capabilities of FeS@FGD were investigated. The main conclusions are as follows:

A novel FeS-modified adsorbent, FeS@FGD, was successfully prepared and tested for MG adsorption. FeS@FGD achieved an efficient removal rate of 96.9% for MG dye in aqueous solutions, demonstrating its potential as an effective adsorbent for environmental pollutant remediation. Compared to unmodified desulfurization gypsum, FeS@FGD increased the adsorption capacity of MG by 2.69 times, significantly outperforming traditional adsorbents. This approach also provides a valuable method for utilizing desulfurization gypsum, contributing to ecosystem improvement and sustainable resource development.

The adsorption efficiency of FeS@FGD on MG was influenced by pH, adsorbent dosage, initial dye concentration, and temperature. Optimal conditions were found at pH 10, FeS@FGD dosage of 30 mg, initial MG concentration of 100 mg/L, and temperature of 313 K, under which the adsorption capacity (Qe), removal efficiency (R), and distribution coefficient (K_d) reached 153.6 mg/g, 96.9% and 37.1, respectively.

Adsorption isotherms fitted best to the Freundlich model, with 1/n < 1, indicating preferential adsorption of MG by FeS@FGD. The adsorption process is endothermic, with increased temperature facilitating the reaction. Kinetic analysis revealed that the pseudo-second-order model best describes the process. Intra-particle diffusion was not the sole rate-limiting step. The adsorption mechanism includes electrostatic interaction and hydrogen bond interaction. Furthermore, FeS@FGD demonstrated excellent reusability, maintaining 76.4% adsorption efficiency after four consecutive cycles.

In conclusion, FeS@FGD effectively removes MG from aqueous solutions, thereby reducing the potential environmental toxicity of MG in water. This work contributes to advancing the research on cationic dye removal and offers promising prospects for mitigating water pollution in natural environments.

This study also has some limitations. First, this study mainly focuses on the synthesis of FeS@FGD and the optimization of its adsorption performance for MG, but does not involve ecological toxicological evaluation, which is indeed one of the limitations of this work. In the future, standardized biological tests (such as the algal growth inhibition test and the acute toxicity test of fish) should be used to systematically evaluate the ecological risks of treated water. Based on the experimental results, targeted control strategies should be proposed to provide scientific support for the optimization of the actual wastewater treatment process and environmental risk management.

CRediT authorship contribution statement

Yifang Song: Writing-original draft. Na Li: Writing-review & editing. Haocheng Lai: Data curation. Hongxiang Wu: Methodology. Xunkun Ma: Investigation. Haihua Zhan: Conceptualization. Wei Wang: Formal analysis.

Declaration of competing interest

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

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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Show Sections

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Efficient removal of malachite green from aqueous solution using flue gas desulfurization gypsum-loaded ferrous sulfide composites: Synthesis, mechanism, and behavior

Abstract

Keywords

Adsorption mechanism

Cyclic utilization

Dynamics

Isotherm

1. Introduction

2. Materials and Methods

2.1. Raw materials

2.1.1. Adsorbate

2.1.2. Adsorbent

2.2. Experiment design

2.3. Adsorption procedure

2.4. Microscale characterization methods

2.5. Data analysis

3. Results and Discussion

3.1. Physicochemical properties of materials

3.2. Microscopic morphology

3.2.1. SEM and TEM analyses

3.2.2. XRD and AFM analyses

3.2.3. XPS analysis

3.2.4. FTIR analysis

3.2.5. Adsorption mechanism

3.3. Effect of pH on adsorption performance

3.4. Effect of FeS@FGD dosage on adsorption

3.5. Effect of MG concentration on adsorption

3.6. Adsorption thermodynamics

3.7. Adsorption kinetics

3.8. Desorption efficiency and reusability

3.9. Economic evaluation

4. Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Declaration of Generative AI and AI-assisted technologies in the writing process

References

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