High-efficiency removal of Cr(VI) in wastewater via coal gasification slag-based porous composite material

Yonglin Yang; Jiangyong He; Shuaige Shi; Jian Li; Bi Chen; Yuxuan Liu; Long Yan; Xiaodong Chen; Guohui Dong; Bo Yang; Hongrui Ma

doi:10.25259/AJC_193_2025

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Original Article

ARTICLE IN PRESS

doi:

10.25259/AJC_193_2025

High-efficiency removal of Cr(VI) in wastewater via coal gasification slag-based porous composite material

Yonglin Yang^a,*, Jiangyong He^a, Shuaige Shi^a, Jian Li^a,, Bi Chen^a, Yuxuan Liu^a, Long Yan^a,*, Xiaodong Chen^a, Guohui Dong^b, Bo Yang^c,, Hongrui Ma^b,

aSchool of Chemistry and Chemical Engineering, Yulin University, Yulin City, Shaanxi Province, P.R. China, Yulin, 719000, China

bSchool of Environmental Science and Engineering, Shaanxi University of Science & Technology, Xi’an, Shaanxi Province, P.R. China, Xi’an, 710021, China

cAll Solid State Battery Materials and Devices Research Center, Yulin Innovation Institute of Clean Energy, Shaanxi Province, P.R. China., Yulin, 719000, China

* Corresponding authors: E-mail addresses: yangyonglin@yulinu.edu.cn (Y. Yang); ylyanlong@126.com (L. Yan)

Received: 2025-02-24, Accepted: 2025-07-23, Epub ahead of print: 2025-10-08,

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Abstract

The safe and efficient treatment of Cr(VI)-containing industrial wastewater has attracted increasing attention, owing to its substantial potential hazards to both the ecosystem and humanity. A modified carbon-silicon porous composite material (MGFS) was prepared for efficient adsorption of Cr(VI) in wastewater in this study, using original coal gasification fine slag (GFS) - a typical solid waste as precursor material. It is demonstrated that the prepared porous composite material exhibited the richer pore structure, larger specific surface area, and more abundant functional groups compared with that of GFS. Under optimal conditions (adsorption time:150 min, pH: 3, temperature: 25°C, adsorbent dosage: 0.20 g), MGFS exhibited the highest Cr(VI) adsorption capacity of 4.94 mg/g with a removal rate of 98.85%, representing a 40.01% enhancement compared to GFS. Kinetic studies revealed that the adsorption process conforms to both the pseudo-second-order model and Ho-McKay model, indicating a multi-step rate-controlling mechanism. Equilibrium data was well-described by Langmuir isotherm models, suggesting a monolayer adsorption behavior. Thermodynamic analysis confirmed that the adsorption was a spontaneous exothermic process. The adsorption mechanism of Cr(VI) onto MGFS was systematically characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and other advanced analytical techniques. These findings not only provide theoretical support for the high-value utilization of coal gasification slag but also offer a viable strategy for heavy metal wastewater treatment.

Keywords

Adsorption

Coal gasification fine slag

Cr(VI)

Porous material

Wastewater

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1. Introduction

Presently, the pollution problem of heavy metal wastewater has become increasingly prominent with the acceleration of industrialization and the improvement of environmental protection awareness [1,2]. Chromium-containing wastewater mainly derives from industrial wastewater discharges, such as chrome ore processing, electroplating, printing and dyeing, and leather tanning. Chromium exists in the environment, mainly as Cr(VI) and Cr(III) species. It is well known that the toxicity of chromium is closely related to its valence state. According to the reports, Cr(VI) is high toxicity, about 100 times higher than that of Cr(III). In addition, Cr(VI) possesses carcinogenic, teratogenic, and persistent characteristics, posing significantly potential hazards to human health and ecosystems [3,4]. As a result, the safety and harmless disposal of wastewater bearing Cr(VI) is of particular concern. High-efficiency removal of Cr(VI) present in wastewater will be a difficult and urgent task under current environmental management [5,6].

Currently, there are many treatment ways to wipe off Cr(VI) ions in wastewater, primarily including precipitation, adsorption, membrane filtration as well as cation-anion exchange [7-9]. The adsorption method is favored owing to the high efficiency, simple operation, and economy [10,11]. Regarding adsorbent choice, activated carbon, chitosan, pyrite, as well as biomass are commonly used to prepare adsorbents and triumphantly applied to adsorption treatment of Cr(VI) in the water [12-15]. However, there are still some issues about these adsorbent materials, like complicated preparation process, high cost, poor adsorption properties, restricted feedstock, as well as instability [16]. Therefore, optimization of adsorbents remains one of the important directions for improving Cr(VI) adsorption removal.

In China, approximately 300 million tons of standard coal are converted into coal gasification products every year, and the corresponding emissions of coal gasification slag are also considerable [17]. According to relevant statistics, the yield of coal gasification slag has exceeded 35 million tons every year in China alone, classified as the bulk industrial coal-based solid waste [18]. Coal gasification slag possesses strong corrosivity and can release the pungent gas, which also involves many inorganic substances and heavy metals. Nowadays, the dominating disposal means for coal gasification slag include stacking and landfilling [19]. Large-scale recycling treatment is not available currently [20]. However, long-term stacking of the slag will cause large-scale land occupation, surface water and groundwater pollution, soil contamination, and enormous harm to the ecological environment and human health. As a result, the safe disposal and recycling of coal gasification slag has triggered widespread concern in the coal chemical industry in recent years. Based on the different operating conditions, reaction histories, and particle sizes of the slag, coal gasification slag is usually classified as fine and coarse residue. The former is more suitable for adsorption material due to its smaller particle size, well-developed pore structure, and excellent hydrophilic property [21]. Previous literature proposed the successful synthesis of high-purity porous composites by employing fine slag as precursor original material through adjusting the Si/Al ratio, with the adsorption amount of simulated methylene blue wastewater up to 137.5 mg/g [22]. Na-type zeolite adsorption material was successfully prepared by employing coal gasification slag and was applied to Pb²⁺ solution of 200 mg/L. The adsorption amount of heavy metal Pb²⁺ reached 16.49 mg/g, and the removal percentage of Pb²⁺ reached 82.45% [23]. As a result, coal gasification slag, after some pre-treatment, can be utilized as an adsorbent material for heavy metals wastewater and possesses high efficiency adsorption properties.

Coal gasification fine slag (GFS), having high added value, low cost, and various source, can be used as a precursor material for adsorbents, and exhibits enormous application potential in terms of environmental pollution prevention and abatement [24]. However, there is limited research literature about utilizing the fine slag to adsorb Cr(VI) presented in the wastewater. The adsorption properties of the obtained gasification slag-based porous material still need to be further improved to increase adsorption capacity and removal percentage of the pollutants. Moreover, the related Cr(VI) adsorption mechanism needs to be further revealed. Coal gasification slag is enriched with carbon and silicon components. The carbon-silicon composite exhibits enhanced adsorption performance and excellent reusability for heavy metal pollutants. The aim of this research was to create‌‌ C-Si composite porous materials employing GFS by alkaline activation treatment, and then modify their surfaces with (NH₄)₂S₂O₈ for highly efficient removal of Cr(VI). The results of this study are expected to develop a highly efficient, low-cost, and sustainable adsorbing material for the harmless disposal of industrial wastewater bearing Cr(VI), which will contribute to solving the environmental pollution problem of gasification slag and realize its high-value re-utilization.

2. Materials and Methods

2.1. Materials

GFS that derives from the chemical company of Yulin City, Shaanxi Province of China, was utilized as precursor material of porous adsorbent, which was fully crushed and screened with a 100-mesh sieve before the experiment, and the screened slag was used in the subsequent adsorption experiments of Cr(VI), abbreviated as GFS. The reagents used in this study are all analytically pure reagents purchased from Sinopharm Group Chemical Reagent Co., Ltd.

2.2. Experimentation process

2.2.1. Preparation of MGFS

The quartz and mullite present in GFS are structurally stable crystal phases, which can be converted into aluminosilicates via alkali activation treatment at higher temperatures. The most commonly used alkalis include NaOH, Na₂CO₃, and K₂CO₃ [25,26]. In this study, Na₂CO₃ was selected as an activator, and GFS was mixed with Na₂CO₃ according to a mass ratio of 3:1, subsequently held in the tube heating furnace under N₂ atmosphere at 650°C for 1 h, then cooled to room temperature, heated to boiling in deionized water, and stirred continuously by magnetic force for 1.5 h. After filtration and washing, it was dried at 120°C for 6 h. Then the dried slag was carried out by adding 20% diluted HCl solution based on the solid-liquid ratio of 1 g/20mL, while stirring at 90°C for 35 min. It was subsequently filtered under vacuum conditions. The aluminum, calcium, and iron elements in GFS can be almost completely removed by the diluted HCl after alkali corrosion activation [27]. Then, the sample was cleaned repeatedly using deionized water until the supernatant was neutralized by pH detection, and then dried at 120°C for 6 h. The coal gasification slag-based porous material was acquired after these operation steps. It has been shown that ammonium persulfate ((NH₄)₂S₂O₈) can modify the surface of carbon material to improve the adsorption performance on heavy metals [28,29]. In this study, (NH₄)₂S₂O₈ was used to modify the prepared porous material as follows. Firstly, (NH₄)₂S₂O₈ of 45.6 g was weighed and dissolved in 1 mol/L sulfuric acid solution, which was subsequently transferred into a 100 mL volumetric flask, and the acidic mixed solution of 2 mol/L (NH₄)₂S₂O₈ was prepared by volume-determining up to the scale line with 1 mol/L sulfuric acid solution. The coal gasification slag-based porous material was mixed with an acidic mixed solution based on a solid-liquid ratio of 1g/20mL, subsequently stirred at 60°C for 3.5 h, then vacuum-filtered and cleaned with distilled water. It was then desiccated at 120°C for 6 h, the modified porous composite material was obtained, which is denoted as MGFS. The schematic diagram of preparing MGFS material based on GFS has been depicted in Figure 1.

2.2.2. Adsorption experiment of Cr(VI)

Firstly, K₂Cr₂O₇ reagent was dissolved and stirred in deionized pure water to obtain a simulated wastewater, in which the content of Cr(VI) is 20 mg/L. The influence of adsorption time, dosage of adsorbent, temperature, and incipient pH of wastewater on Cr(VI) adsorption was explored respectively. After adsorption of Cr(VI) was completed, it was separated via a centrifuge and filtered with an aqueous membrane of 0.2 μm. The key effective parameter range for the chromium ion adsorption process includes adsorption time from 0 to 240 min, adsorbent dosage from 0 to 0.5 g, temperature from 25 to 50°C, and solution pH fluctuating within the range of 1 to 13. Cr(VI) concentration in the filtrate was detected through the Diphenylcarbazide spectrophotometric method by employing the UV-vis at 540 nm. Finally, the average of the three parallel tests for each sample is taken as the result of Cr(VI) content. The adsorbed amount and removal percentage of Cr(VI) under adsorption equilibrium state are calculated based on the equations (Eqs. 1,2), respectively.

(1)

q_{e} = \frac{(C_{0} - C e) V}{m}

(2)

r_{e} = \frac{(C_{0} - C_{e}) \times V}{C_{0} \times V} \times 100 %

In Eqs. (1,2), q_e is the adsorbed amount of Cr(VI) under adsorption equilibrium state (mg/g); C₀ is the incipient content of Cr(VI) from simulated wastewater (mg/L); C_e is the content of Cr(VI) from simulated wastewater under equilibrium state (mg/L); γ _e is the removal percentage of Cr(VI) under adsorption equilibrium state (%); V is volume of simulated wastewater (L); m is the added dosages of MGFS and GFS (g).

2.3. Testing instruments

The primary element composition of GFS was characterized via X-ray fluorescence spectrometry apparatus (XRF, Panalytical Axios, Rigaku Zsx Priums, America). The scanning electron microscope-energy spectrometer (SEM-EDS, Zeiss, Model σ300, Germany) was utilized to characterize the microscopic morphology and elemental composition of materials. Specific surface area and pore structure of GFS and MGFS adsorbents were determined by a specific surface area instrument (Brunauer-Emmett-Teller, BET, Anton Paar, Nova 800, America). The change in functional groups of the materials was characterized via an infrared spectral analyzer (Fourier transform infrared, FTIR, Bruker Tensor 27, Germany). The crystalline phase compositions were detected via X-ray diffractometer (XRD, Bruker D8 Advance, Germany). The chemical valence states of C, O, and Cr elements were characterized via X-ray photoelectron spectroscopy. The contents of Cr(VI) in wastewater before and after adsorption were tested via a UV spectrophotometer (UV-2450, Shimadzu, Japan).

3. Results and Discussion

3.1. Properties of adsorbing materials

3.1.1. Chemical compositions

The characterization result of GFS by using XRF has been depicted in Table 1. According to Table 1, its main components are SiO₂, CaO, and Fe₂O₃ with contents of 29.35%, 20.03%, and 17.79%, respectively. In addition, a little of Al₂O₃, SO₃, and MgO were also detected. XRD analysis indicates that the main crystalline phases of GFS are SiO₂ and CaO, as described in Figure 2. The curve background of the XRD pattern implies the presence of amorphous phases in GFS.

Table 1. The contents of different components in GFS through XRF characterization.

Components	wt.%	Components	wt.%	Components	wt.%
Na₂O	1.26	Cl	0.07	NiO	0.02
MgO	4.40	K₂O	0.72	CuO	0.03
Al₂O₃	16.30	CaO	20.04	ZnO	0.02
SiO₂	29.35	TiO₂	0.88	SrO	0.11
P₂O₅	0.18	MnO	0.47	Y₂O₃	0.01
SO₃	7.82	Fe₂O₃	17.79	BaO	0.50

Note: Ash residue of GFS sintered at 900°C was analyzed by XRF.

3.1.2. BET analysis of adsorbing materials

The adsorption-desorption curves and pore size distributions of the different materials have been displayed in Figure 3 under N₂ atmosphere. According to Figure 3(a), isothermal adsorption-desorption curves of two porous materials about GFS and MGFS all conform to the characteristics of type IV isotherms, along with a faster rising at the lower-pressure stage (0<P/P₀<0.2). The adsorption is principally single-layer adsorption, and MGFS material displays the larger adsorption capacity, suggesting that a large number of micro-pores were developed and formed [30]. The H4 hysteresis ring in the range of 0.4 ∼ 1.0 corresponds to typical parallel slit-type mesoporous structures in GFS and MGFS. Besides, GFS and MGFS are identified as mesoporous based on the IUPAC classification [31,32]. The specific surface area for GFS and MGFS was 366.42 m²/g and 445.68 m²/g, respectively (Table S1). Compared with that of GFS, the specific surface area of MGFS increases prominently, with a smaller mean pore diameter, according to Figure 3(b) and Table S1. However, both pore volume and average pore size decrease, attributed to the fact that Na₂CO₃ may react with certain components presented in GFS to form gaseous substances under high temperatures, and a large quantity of micropores and mesopores are generated as the gas spills out [33]. Meanwhile, the oxidation modification of (NH₄)₂S₂O₈ can promote the formation of new pores and increase the specific surface area of MGFS porous adsorbent, which may be beneficial to the adsorption and removal of Cr(VI) ions present in wastewater.

Supplementary Table 1

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BET analysis of GFS and MGFS: (a) N2 isothermal adsorption-desorption curves; (b) pore sizes.

3.1.3. Surface morphology and functional groups

The SEM diagrams of GFS and MGFS have been exhibited in Figure 4, and all images are magnified 2000 times. According to Figure 4(a), the surface of GFS is mostly irregular flocculent particles, as well as a large number of spherical particles with small particle size, due to the fact that pulverized coal is rapidly heated at a short time, resulting in part of mineral constituents being melted to liquid phase, which is cooled and contracted to form spherical particles by the role of surface tension. Furthermore, the surface of GFS is porous and its pore size is large, which is basically consistent with relevantly published literature [34]. On the basis of Figure 4(b), the surface of MGFS obtained after Na₂CO₃ alkali solvent and (NH₄)₂S₂O₈ modification activation is covered with honeycomb pore structures. The pore space is relatively dense and the pore size becomes obviously smaller, resulting in a lot of micropores and mesopores, which greatly increase the specific surface area of the prepared adsorbent material. These phenomena indicate that a large amount of ash present in GFS has been removed during the preparation of MGFS porous material, which is in favor of the enhancement of adsorption performance on Cr(VI) [35].

SEM images of different materials: (a) GFS, (b) MGFS.

Figure 5 depicts the FTIR spectra of GFS and MGFS. There are many chemical groups on the surface of GFS and MGFS, which have an important influence on its adsorption. The positions of the main absorption peaks are essentially the same for both materials. Absorption peak at 472 cm^-1 is ascribed to Si-O functional group, where peak intensity of MGFS is obviously enhanced compared with that of GFS, which indicates that a large number of metal ions and other ash impurities in GFS are effectively leached out by employing acid solution, mainly leaving the skeleton structure of Si and C, as demonstrated by Table S2. Therefore, the pore structure of the prepared MGFS is more abundant. The peaks at about 765 cm^-1 and 583cm^-1 are O-H stretching vibration peaks [36]. The peak at 1211 cm^-1 is the -CH₃ and -CH₂ stretching vibration peak. The peak at 1398 cm^-1 is a stretching vibration peak of C-N, along with which is prominently enhanced, which means that nitrogen is successfully doped in MGFS after (NH₄)₂S₂O₈ modification. The peak at 1585 cm^-1 is attributed to the bending vibration of -NH₂ [37]. In addition, the peak at 1720 cm^-1 is an absorption peak of -C=O or -COOH groups [38,39]; then the absorption peak of MGFS at 1720 cm^-1 is significantly enhanced, suggesting an increase of -C=O or -COOH functional groups in MGFS material. The peak at about 3405 cm^-1 is caused by the stretching vibration of O-H groups [40]. -NH₂, -C=O, and -COOH functional groups significantly enhance pollutant adsorption through chelation, electrostatic interactions, and hydrogen bonding. In composite materials, these groups exhibit synergistic effects with other components (e.g., metal-organic frameworks, polymers), thereby improving adsorption capacity and selectivity for heavy metals and organic pollutants in water treatment applications [41,42].

Supplementary Table 2

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3.2. Adsorption of Cr(VI) under different conditions

3.2.1. Influence of adsorption time

Adsorption treatment of heavy metals can reach the adsorption equilibrium state after a certain adsorption time. Hence, adsorption time is usually one of the main factors affecting the adsorption ability of the adsorbent. Figure 6 describes the variations of Cr(VI) adsorption over time via GFS and MGFS adsorption materials. With the increase of adsorption time, adsorption quantity and removal efficiency of Cr(VI) in wastewater have both increased. The adsorption process gradually becomes stable at 150 min, at this time, the amounts of Cr(VI) adsorbed through the two different porous materials almost reach the maximum levels, where the removal percentages of Cr(VI) are respectively 66.1% and 88.0%. Compared with that of GFS, the removal efficiency of Cr(VI) via MGFS improved by 33.1%. It is proven that the modification of ammonium persulfate dramatically promotes the removal of Cr(VI). Continuing to extend adsorption time, the adsorption amount of Cr(VI) increases rather slowly until the adsorption equilibrium state, owing to adsorption active sites on the surface of MGFS and GFS being gradually occupied. Cr(VI) presented in wastewater will take place to migrate from the macropores of adsorbents to the mesopores and microporous, slowing down the diffusion behavior of heavy metal ions, until the adsorption reaches equilibrium. Therefore, the optimal adsorption time of Cr(VI) can be selected as 150 min by GFS and MGFS adsorbents.

Absorption of Cr(VI) plotted against time: (a) GFS, (b) MGFS.

3.2.2. Influence of dosage

Figure 7 describes the adsorption change of Cr(VI) upon adding different dosages of GFS and MGFS adsorption materials. As the amount of GFS and MGFS increased, the removal efficiency of Cr(VI) firstly increased progressively, which is attributed to the corresponding increase in specific surface area and adsorption sites provided by the adsorbent. While the dosage of adsorbent is 0.20 g, the amount of adsorbed Cr(VI) is respectively 3.06 mg/g and 3.50 mg/g, with an increase of 14.34%. However, the content of Cr(VI) in wastewater is constant. Continuing to increase the amounts of adsorbents, the efficiency of removing Cr(VI) gradually tends to be stable, since Cr(VI) is almost completely adsorbed and the adsorption tends to saturation [43]. As a result, increasing the dosages of adsorbents can not effectively improve adsorption of Cr(VI). Conversely, the amount of Cr(VI) adsorbed via the adsorbent per unit mass decreases gradually along with the dosage increases. When the dosage of the adsorbent increased to 0.5 g, the adsorption amount of Cr(VI) decreased to 0.99 and 1.83 mg/g, respectively. Moreover, compared with that of a dosage of 0.2 g, the adsorption amount of Cr(VI) decreases by 67.6% and 47.7%, respectively. In conclusion, the optimal dosage of added adsorbent was 0.2 g. At this moment, the removal percentage of Cr(VI) reached the maximum level of 70.7% and 92.6%, respectively. And removal efficiency of Cr(VI) via employing MGFS improved by 30.8%, compared with that of the GFS adsorbent, indicating that activation and modification of GFS is beneficial for adsorbing Cr(VI) from wastewater.

Variation of adsorbed Cr(VI) with the dosage: (a) GFS, (b) MGFS.

3.2.3. Influence of temperature

Figure 8 depicts the change of Cr(VI) adsorbed via GFS and MGFS adsorption materials under the different temperatures. At 20°C, adsorption amounts of Cr(VI) via GFS and MGFS are 2.90 g and 4.20 g, where removal percentages of Cr(VI) are 58.7% and 84.0%, respectively. Compared with that of GFS, the removal efficiency of Cr(VI) via MGFS increases by 43.1%. The adsorption amount of Cr(VI) decreases as temperature increasing, suggesting that increasing temperature is adverse to adsorption, as well as the adsorption reaction of Cr(VI) may be exothermic. Above 40°C, the surface structure and functional group of GFS and MGFS may undergo changes, leading to a sharp decrease in adsorption efficient, with the removal percentages of 23.3% and 32.7% respectively. Therefore, the optimal temperature of Cr(VI) adsorption is the temperature of 25°C in this research; meanwhile removal percentage of Cr(VI) via MGFS adsorbent can reach the level of 84.0%.

Adsorption of Cr(VI) plotted against temperature: (a) GFS, (b) MGFS.

3.2.4. Influence of initial pH

Figure 9 depicts the adsorption variation of Cr(VI) via GFS and MGFS adsorption materials at different pH levels. The existing species of Cr in aqueous solution will vary with changes in pH. Hence, there are various chromium species in the solution. For example, Cr(VI) exists in the form of ${Cr}_{2} O_{7}^{2-}$ and ${HCrO}_{4}^{-}$ ions in the acidic medium, while it primarily exists in the form of ${CrO}_{4}^{2 -}$ ion in the alkaline medium [44,45]. As indicated in Figures 9 (a,b), the adsorption amount of Cr(VI) through the two adsorbents increased slightly at first and then decreased gradually as the value of pH increased. At the pH of 3, both GFS and MGFS exhibit the strongest adsorption performances for Cr(VI), and the removal efficiency of Cr(VI) is 82.6% and 93.9% respectively, with an increase of 13.7%. Under acidic conditions, the adsorbents have better adsorption efficiency on Cr(VI). Particularly, when the pH value is less than 7, the content of H⁺ in the solution is higher, and H⁺ will combine with functional groups presented in MGFS and GFS, resulting in the generation of more positively charged adsorption sites. In acidic solution, ${Cr}_{2} O_{7}^{2-}$ and ${HCrO}_{4}^{-}$ of Cr(VI) species exhibit a negative charge. Hence, there is an electrostatic action between negatively charged Cr(VI) species and positively charged adsorption sites on the adsorbent, which will accelerate the adsorption of Cr(VI) from wastewater. Compared with GFS adsorbent, MGFS obtained through alkaline activation and surface modification may possess more adsorption sites, resulting in better capture of ${Cr}_{2} O_{7}^{2-}$ and ${HCrO}_{4}^{-}$ by electrostatic attraction. As the pH value increased, the number of -OH groups on the surface of the adsorbents decreased, which weakens the electrostatic adsorption of Cr(VI) species.

The adsorption of Cr(VI) plotted against pH: (a) GFS, (b) MGFS.

Besides, when the value of pH is higher than 7, the content of OH^- in alkaline solution progressively increases. The acidic functional groups of the adsorbents will bind to OH^-, bringing about a decrease in positively charged adsorption sites. Furthermore, there is a competitive adsorption between OH^- and ${CrO}_{4}^{2 -}$ ions of Cr(VI) species under alkaline conditions, which leads to a weakening of the ability to adsorb ${CrO}_{4}^{2 -}$ , as well as a decrease in the adsorption amount of Cr(VI). In consequence, the appropriate pH for Cr(VI) adsorption is 3. At this point, its adsorption amount by MGFS is up to 4.7 mg/g, accompanied by an increase of 14.6%. As a result, pH is another important factor affecting the removal efficiency of heavy metal Cr(Ⅵ).

3.3. Comparison of removal of Cr(VI) on optimal adsorption conditions

According to the experimental results about the effects of different factors on the adsorption of Cr(VI), the best performance of Cr(VI) adsorption by GFS and MGFS adsorption materials was achieved at an adsorption time of 150 min, pH of 3, adsorption temperature of 25°C, as well as an adsorbent dosage of 0.20 g. Moreover, the adsorption efficiency of two adsorbing materials on Cr(VI) in wastewater was compared at optimal adsorption conditions, as illustrated in Table 2. According to Table 2, the adsorption performance of MGFS on Cr(VI) is greatly improved. Adsorption capacity of Cr(VI) reaches a maximum level of 4.94 mg/g, compared with that of GFS. Compared with the recently reported adsorbents, Cr (VI) adsorption capacity was at a moderate level (Table S2). Although its performance was inferior to PAD resin, Fe(III)-chitosan microbeads, Acid-treated feather, petroleum coke, and coal gangue, it significantly outperformed coal waste, Dry biomass, and other materials (Table S2). Furthermore, the removal percentage of Cr(VI) is as high as 98.85%, with an increase of 40.0% to that of GFS. The EDS analysis result of MGFS adsorbed Cr (VI) has been represented in Table S3. According to Table S3, the content of Cr presented in MGFS was 4.98%. However, GFS itself does not contain Cr element according to Table 1, demonstrating that Cr(VI) is successfully adsorbed in the prepared MGFS adsorption material. Under this state, the content of Cr(VI) in simulated wastewater is only 0.23 mg/L, which satisfies the limits of the relevant industrial wastewater discharge standard. Moreover, the above results have confirmed that the vast majority of Cr(VI) has been removed from the wastewater by MGFS material. After the heating alkali melting treatment, the structure of the stable phases presented in GFS may be broken out, then many inorganic salts, such as Ca, Fe, and Al have been removed by the acid wash, leading to these metallic elements being undetected, as verified by the characterization result of Table S3. The remaining primary components in MGFS are C and Si elements. Furthermore, a great quantity of micropores and mesopores were produced in the carbon-silicon adsorption material, leading to the decrease of average pore size, which may be put down to the release of some gas generated by the reaction_. Then, it was modified with ammonium persulfate to open the blocked pores on the adsorption material surface, leading to more new pore structures, which further improved the specific surface area and enriched functional groups of the MGFS material. As a result, the obtained MGFS carbon-silicon adsorption material is more conducive to adsorbing Cr(VI) in wastewater. The findings of this research have revealed that the removal efficiency of Cr(VI) in wastewater is remarkably enhanced after the activation and surface modification of GFS. Based on the above experimental results, GFS, that regarded as a sustainable and low-cost potential precursor material for heavy metal adsorbent, can achieve high-value re-utilization of waste through “waste treatment with waste.”

Supplementary Table 3

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Table 2. Comparison of Cr(VI) adsorbed by different adsorption materials under optimal conditions.

Absorbents	C₀ (mg/L)	C_e (mg/L)	q_e (mg/g)	γ_e (%)
GFS	20	6.10	3.53	70.60%
MGFS	20	0.23	4.94	98.85%

3.4. Analysis of adsorption kinetics and thermodynamics

3.4.1. Adsorption kinetics

A conclusion can be drawn from the above experimental results that the prepared MGFS material represents an outstanding adsorption performance for Cr(VI). For the purpose of quantitatively illustrating the adsorption kinetic process of Cr(VI) via MGFS, three kinetic models, such as Lagergren (pseudo-first-order dynamics model (PFO), Ho-McKay (pseudo-second-order dynamics model (PSO), as well as Intra-Particle Diffusion model (IPD), are utilized to investigate its kinetic mechanism.

The related equations are expressed as follows:

(3)

\log (q_{e} - q_{t}) = \log q_{e} - \frac{k_{1}}{2.303} t

(4)

\frac{t}{q_{t}} = \frac{1}{K_{2} q_{e}^{2}} + \frac{1}{q_{e}} t

(5)

q_{t} = k_{3} t^{0.5} + C

In Eqs. (3-5), q_t represents adsorption amount of Cr(VI) at time t (mg·g^-1); q_e represents equilibrium adsorption capacity of Cr(VI) (mg·g^-1); t represents adsorption time (min); k₁ (min^-1) and k₂ (g·mg^-1·min^-1) represents adsorption rate constant of PFO and PSO kinetics, respectively; k₃ represents the rate constants of IPD kinetics; C represents a constant related to thickness of boundary layer (mg·g^-1).

According to experimental results of MGFS adsorbing Cr(VI), the kinetic fitted results and relevant parameters of such adsorption processes are depicted in Table S4 and Figure 10. Compared with PFO model, PSO model is more reliable, because it can interpret the kinetic data with a regression coefficient R² of 0.981. Therefore, adsorption process of Cr(VI) via MGFS accords with PSO model, and is mainly limited by chemical adsorption, which may involve electron transfer or ion exchange between MGFS and Cr(VI) [46]. IPD model fitting was employed to further explain the control steps during removal process of Cr(VI). Figure 10(c) depicts that the curve fitted by IPD model obviously does not get through origin of coordinates, suggesting the removal of Cr(VI) involves more than just one process, and IPD is not a rate-controlling factor during MGFS adsorption. The non-zero intercept that represents thickness of boundary layer also confirms this fact. The first stage involves outer surface adsorption or transient adsorption, and then slow adsorption state formed by the internal diffusion adsorption, until adsorption equilibrium state.

Supplementary Table 4

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The fitted curves of Cr(VI) adsorption via MGFS: (a) PFO, (b) PSO, (c) IPD, (d) Langmuir isotherm models, (e) Freundlich isotherm models, (f) Adsorption thermodynamics.

3.4.2. Adsorption thermodynamics

The experimental data were fitted to the Langmuir and Freundlich isotherm models. The Langmuir (Eq. 6) and Freundlich (Eq. 7) isotherm models can be represented as follows:

(6)

\frac{C_{e}}{q_{e}} = \frac{1}{q_{m} \times K_{L}} + \frac{C_{e}}{q_{m}}

(7)

ln q_{e} = ln K_{F} + \frac{1}{n} ln C_{e}

Where C_e represents the equilibrium concentration (mg·L^-1), q_e represents the equilibrium adsorption capacity of Cr(VI) (mg·g^-1), K_L the Langmuir sorption constant (L·mg^-1), and q_m represents the monolayer adsorption capacity of Cr(VI) (mg·g^-1), n and K_F are the Freundlich model coefficients. Compared with Freundlich isotherm models, the Langmuir isotherm models is more reliable, because it can interpret the kinetic data with a regression coefficient R² of 0.991 (Figures 10 (d,e) and Table S5). Therefore, adsorption process of Cr(VI) via MGFS accords with Langmuir isotherm models, indicating that the adsorption process of Cr(VI) by MGFS is a monolayer adsorption [47,48]. The monolayer adsorption capacity of Cr(VI) obtained from Langmuir isotherm models was 4.360 mg·g^-1, which aligns well with the experimental findings (4.94 mg·g^-1).

Supplementary Table 4

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The analysis of thermodynamic parameters helps to reveal adsorption mechanism. In this study, the corresponding thermodynamic curves at adsorption temperatures of 298 K, 308 K, and 318 K have been depicted in Figure 10(f) according to the experimental data. The thermodynamic parameters at different temperatures, including ΔG^°, ΔH°, and ΔS^°, can be obtained by Eqs. (8-10), and the relevant data have been presented in Table S6. Due to ΔG^°< 0 and ΔH^°< 0, MGFS

Supplementary Table 6

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adsorption of Cr(VI) is a controlled and spontaneous exothermic reaction. With temperature increasing, the change trend of ΔG⁰ value increases gradually, indicating that the driving force of MGFS adsorbing Cr(VI) also decreases gradually, which reveals the weakening of Cr(VI) adsorption spontaneity at higher temperature [49,50]. The negative value of ∆S⁰ indicates that the procedure of MGFS adsorbing Cr(VI) weakens the randomness of the system, and becomes more orderly after reaching adsorption equilibrium [51]. Therefore, the randomness of the interface between solution and MGFS is weakened during the adsorption process of MGFS, which can be proved by ∆S⁰ > 0.

(8)

\ln K_{d} = - \frac{Δ H^{0}}{RT} + \frac{Δ S^{0}}{R}

(9)

K_{d} = \frac{m q_{e}}{V C_{e}}

(10)

Δ G^{0} = Δ H^{0} - T Δ S^{0}

Where K_d represents the distribution coefficient, q_e represents the equilibrium adsorption capacity, C_e represents the content of Cr(VI) under adsorption equilibrium state, R represents the constant (8.314 J·mol^-1·K^-1), and T represents solution temperature (K).

3.5. Exploration of adsorption mechanism

To further elucidate the adsorption mechanism of heavy metals via MGFS material, the adsorption behavior of Cr(VI) was researched in terms of material characterization, influencing factors, adsorption kinetics, and adsorption thermodynamics. The analysis of pore distribution and BET indicates that Cr(VI) adsorption mainly occurs within the mesopores of MGFS, and the adsorption procedure is monolayer adsorption. Based on the analysis of the kinetic modes, the theoretical adsorption capacity stemming from the PSO model is in agreement with experimental results. And adsorption process of Cr(VI) is concerned with physisorption and chemisorption, primarily limited by chemical adsorption. Furthermore, the removal process of Cr(VI) involves multiple steps, and PID is not the only step that limits the rate. In addition, thermodynamic analysis has indicated that adsorption of Cr(VI) on MGFS material is a spontaneous exothermic process; therefore, the lower temperatures are beneficial to its adsorption. Additionally, the absolute value of ΔG⁰ progressively decreases as increase of temperature increases, indicating that the rising temperature will actually reduce the driving force of MGFS adsorbing Cr(VI), thereby reducing its removal efficiency. The synergistic effect of these mechanisms results in excellent adsorption properties of the MGFS adsorbent. At room temperature, MGFS can quickly and efficiently adsorb Cr(VI), which is highly toxic and carcinogenic, leading to a diminishing of the biological toxicity and environmental damage of heavy metal wastewater, with broad application prospects.

Figures S1(a-f) showed that there were no significant morphological changes before and after MGFS adsorption, such as pore filling and the presence of surface adsorbed species, which were consistent with the proposed adsorption sites (such as hydroxyl, amino, and other functional groups). Figures S1(g-h) illustrate the changes in element types, distribution, and content of MGFS before and after adsorption of Cr(VI). Initially, MGFS was composed of C(57.92%), N(10.81%), O(24.19%), Al(0.14%), Si(6.00%), S(0.28%), Ca(0.42%) and Fe(0.23%) (Table S3). After adsorption of Cr(VI), a significant alteration in elemental composition of MGFS was observed (Table S3). Notably, Cr was detected at a content of 6.93%, providing direct evidence of efficient Cr(VI) adsorption by MGFS (Table S3). Moreover, the zeta potential of MGFS was measured at different pH values. The results showed that the MGFS adsorbent was negatively charged across all tested pH values (Figure 11a). As the pH value increased from 3.0 to 13.0, the adsorption capacity of MGFS gradually decreased. This could be attributed to the rapid deprotonation of -NH₂ groups on MGFS, since -NH₂ protonation normally promotes the binding of Cr(VI) to MGFS. However, when the pH value was further decreased from 3.0 to 1.0, the adsorption capacity also began to decrease. This is because Cr (VI) is mainly adsorbed on the MGFS surface in the form of H₂CrO₄ (pH=1), and the binding force between H₂CrO₄ and surface protonated -NH₂ groups is weak [52].

Supplementary Figure 1

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(a) Zeta potential plot of MGFS at different pH values; XPS spectra of MGFS before and after Cr(VI) adsorption: (b) Cr 2p, (c) C 1s, (d) O 1s.

In addition, XPS analysis offers valuable insights into the structural and compositional changes of MGFS before and after Cr(VI) adsorption. As shown in Figure 11(b), distinct Cr 2p signals were clearly observed in MGFS after Cr(VI) adsorption, providing direct evidence for the successful adsorption of Cr(VI) ions [53]. The high-resolution Cr 2p spectrum was deconvoluted into two components: a peak at 578.48 eV assigned to Cr(VI) 2p_3/2 and another around 587.78 eV attributed to Cr(VI) 2p_1/2 [54,55]. In the C 1s spectrum (Figure 11c), three peaks were observed at 285.38 eV (C-C), 286.18 eV (C-OH), and 289.58 eV (O=C-O) before adsorption, respectively [56]. After Cr(VI) adsorption, notable shifts were observed: the C-C peak was shifted to 285.18 eV, C-OH to 285.98 eV, and O=C-O to 289.08 eV. These shifts suggest electron density redistribution within carbon species due to Cr(VI) interaction, potentially via electron donation from carbon functional groups to chromium. The decreased binding energies of C-OH and O=C-O groups imply reduced oxidation states or enhanced coordination with Cr(VI) ions. The O 1s spectrum (Figure 11d) exhibited three peaks before adsorption: 531.48 eV (O=C-O), 532.18 eV (-OH), and 532.88 eV (C-O) [57]. After adsorption, the peak at 535.08 eV disappeared, while the C-O and C=O peaks remained at 530.68 eV, 531.68 eV, and 532.28 eV, respectively. Collectively, these XPS results indicate that both carbonyl/carboxyl groups and surface hydroxyls play crucial roles in Cr(VI) adsorption. The shifts in C 1s and O 1s spectra suggest a chemisorption mechanism involving electrostatic attraction and coordination bonding between MFGS functional groups and Cr(VI) ions. Furthermore, after five cycles of adsorption-desorption using MGFS, the Cr(VI) adsorption performance of MGFS slightly decreased by only 10% compared to the initial value (Figure S2), demonstrating that MGFS could maintain most of its adsorption performance through regeneration. As shown in Figure S3, XRD patterns of MGFS after five cycles exhibited no significant shifts in diffraction peaks compared to the pristine sample. The main crystalline phases remained intact, indicating that the material’s crystallinity was preserved.

Supplementary Figure 2

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Supplementary Figure 3

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According to above-mentioned research results, a briefly graphic adsorption and removal process of Cr(VI) via MGFS adsorption material is represented in Figure 12. First of all, Cr(VI) is pulled to the surface of MGFS adsorbent by electrostatic action and physico-chemical adsorption. In addition to little of Cr(VI) accumulating on the MGFS material surface, most of Cr(VI) gradually enters the inner layer of MGFS through the surface mesopores, and then continues to migrate into the internal pore to complete the adsorption. Moreover, some functional groups on the surface and pores of MGFS material can supply plentiful adsorption sites for Cr(VI), leading to the formation of lower local Cr(VI) concentration around MGFS. Driven by such concentration difference, Cr(VI) ions will continue to migrate to the internal pores of MGFS until adsorption equilibrium is reached. Cr(VI) in wastewater can be efficiently removed through the combined action of chemical adsorption, surface adsorption, and electrostatic attraction, etc. The research results will supply theoretical guidance and new strategic choices for the efficient environmental management and high-value-added reuse of a large amount of coal-based solid wastes.

Possible adsorption mechanism graphic of Cr(VI) by MGFS.

4. Conclusions

In this work, a novel alternative method for highly efficient removal of Cr(VI) in wastewater was proposed by utilizing coal gasification slag-based porous composite material. After the activation and surface modification of GFS, the obtained MGFS adsorption material displays more abundant pore structures, which contributes to adsorption of Cr(VI) from wastewater and greatly enhances its adsorption capacity. Compared with that of GFS, MGFS material exhibits stronger adsorption and removal performance for Cr(VI) under the optimum conditions, and Cr(VI) content at adsorption equilibrium state is only 0.23mg/L, which meets the limit of the relevant industrial wastewater discharge standard. Adsorption process of Cr(VI) via MGFS is the monolayer adsorption and is more consistent with the PSO kinetic model, mainly limited by chemical adsorption, and IPD is not a rate-controlling factor during Cr(VI) adsorption. Moreover, adsorption thermodynamics indicates that adsorption of Cr(VI) is a spontaneously exothermic process; consequently, the lower temperature is beneficial to its adsorption. The findings of research have confirmed that it is feasible to prepare high-performance adsorbent material of Cr(VI) using GFS, which is classified as bulk industrial solid waste. This process is an economically and eco-friendly alternative to wastewater disposal, with advantages of less chemical expenditure as well as environmental pollution. The used adsorption materials can be further incinerated to recover heat energy and heavy metals. It will be expected to realize industrial applying about safe, highly efficient, and large-scale treatment of heavy metal wastewater, particularly low-concentration Cr(VI)-containing wastewater. This investigation, from pollutant to valuable wastewater treatment adsorbent, will create a more economically viable and environmentally sustainable disposal strategy for effective management of large-scale industrial coal-based solid waste.

Acknowledgment

This work is funded by National Natural Science Foundation of China (22168043); Shaanxi Province Key Research Plan Project (2024SF-YBXM-574); Yulin City Industry-University-Research Program (2023-CXY-160); Youth Innovation Team Project of Shaanxi Education Department (24JP219); Youth Innovation Team Project of Shaanxi Education Department (23JP204); Pre-research fund of Yulin University (22GK08); Enterprise Scientific Research Project (H2024060087).

CRediT authorship contribution statement

Yonglin Yang: Methodology, experiment, original writing and draft preparation. Jiangyong He: Data validation, formal analysis, writing and editing. Shuaige Shi: Experiment and data acquisition. Jian Li and Bi Chen: Review and editing. Yuxuan Liu: Review. Long Yan and Xiaodong Chen: Data validation, review and supervision. Guohui Dong and Bo Yang: Sample preparation and characterization. Hongrui Ma: Conceptualization, methodology, data curation and supervision.

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.

Supplementary data

Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_193_2025.

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1. Introduction

2. Materials and Methods

2.1. Materials
2.2. Experimentation process
2.3. Testing instruments

3. Results and Discussion

3.1. Properties of adsorbing materials
3.2. Adsorption of Cr(VI) under different conditions
3.3. Comparison of removal of Cr(VI) on optimal adsorption conditions
3.4. Analysis of adsorption kinetics and thermodynamics
3.5. Exploration of adsorption mechanism

4. Conclusions

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High-efficiency removal of Cr(VI) in wastewater via coal gasification slag-based porous composite material

Abstract

Keywords

Adsorption

Coal gasification fine slag

Cr(VI)

Porous material

Wastewater

1. Introduction

2. Materials and Methods

2.1. Materials

2.2. Experimentation process

2.2.1. Preparation of MGFS

2.2.2. Adsorption experiment of Cr(VI)

2.3. Testing instruments

3. Results and Discussion

3.1. Properties of adsorbing materials

3.1.1. Chemical compositions

3.1.2. BET analysis of adsorbing materials

3.1.3. Surface morphology and functional groups

3.2. Adsorption of Cr(VI) under different conditions

3.2.1. Influence of adsorption time

3.2.2. Influence of dosage

3.2.3. Influence of temperature

3.2.4. Influence of initial pH

3.3. Comparison of removal of Cr(VI) on optimal adsorption conditions

3.4. Analysis of adsorption kinetics and thermodynamics

3.4.1. Adsorption kinetics

3.4.2. Adsorption thermodynamics

3.5. Exploration of adsorption mechanism

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

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

Supplementary data

References

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