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Article

Isotherm, Kinetics, and Adsorption Mechanism Studies of Coal Gasification Coarse Slag as Highly Efficient Phosphate Adsorbents

by
Xuzhi Shi
1,
Baoguo Yang
1,2,*,
Dayi Qian
2,
Dong Cui
1,
Hongbin Li
1,
Chao Wang
1,
Yuhao Zhu
1 and
Tao Yu
1,2
1
School of Resources and Environment, Yili Normal University, Yining 835000, China
2
Xinjiang Key Laboratory of Clean Conversion and High Value Utilization of Biomass Resources, Yili Normal University, Yining 835000, China
*
Author to whom correspondence should be addressed.
Separations 2024, 11(6), 182; https://doi.org/10.3390/separations11060182
Submission received: 16 May 2024 / Revised: 5 June 2024 / Accepted: 7 June 2024 / Published: 11 June 2024
(This article belongs to the Section Environmental Separations)
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Abstract

:
This study investigates the efficacy of a novel low-cost phosphate adsorbent, denoted as SH-CGCS, derived from coal gasification coarse slag (CGCS) via an alkali activation method. SH-CGCS is a mesoporous material with a specific surface area (64 m2/g) approximately six times larger than CGCS (11 m2/g), which enhances its adsorption capacity compared with CGCS. Furthermore, SH-CGCS achieves a phosphate adsorption capacity of 38.5 mg/g in strongly acidic water (pH 3) and demonstrates robust acid resistance, which makes it particularly effective for phosphate removal from acidic wastewater. Results from coexisting anion experiments affirm the good adsorption selectivity of SH-CGCS for phosphate. Moreover, SH-CGCS exhibits proficiency in treating water containing low phosphate concentrations under flowing conditions. The maximum phosphate adsorption capacity of SH-CGCS calculated using the Langmuir model is 23.92 mg/g, surpassing that of other reported adsorbents. Importantly, saturated SH-CGCS can be regenerated and reused, which contributes to its practical applicability. The adsorption mechanisms of SH-CGCS for phosphate involve ligand exchange, inner-sphere complexation, surface precipitation, and electrostatic adsorption. Thus, this study not only enhances the overall utility of CGCS but also presents a simple and efficient method for removing phosphate. Our findings indicate that SH-CGCS holds considerable potential as a phosphate adsorbent, offering a promising solution for wastewater treatment.

1. Introduction

Coal gasification coarse slag (CGCS) and coal gasification fine slag are two types of coal gasification slag (CGS) generated as a solid waste during the coal gasification process. Approximately 30 million tons of CGS are produced annually in China, with CGCS accounting for approximately 60–80% of this waste [1]. Currently, CGCS disposal is primarily performed via stacking and landfilling; moreover, only a few studies have explored CGCS focusing on its resource utilization. In addition to being safe and reliable, CGCS possesses well-developed pores and a large specific surface area as well as containing calcium, aluminum, iron, and metal oxide components, which makes it a promising material for phosphate adsorption. However, only limited research has been conducted in this domain, with only a few studies exploring phosphate removal using CGCS. For example, Yang et al. (2023) used modified CGCS with acid, metal elements, and waste residue for phosphate removal [2,3,4], while Ma et al. (2022) used CGS sequentially subjected to acid leaching, alkali dissolution, and ferrous sulfate modification for phosphate removal [5]. However, these methods involve complex processes, the introduction of foreign substances, and the inefficient utilization of CGCS components. Hence, it is essential to develop a simple and efficient method that uses the intrinsic components of CGCS for phosphate removal.
Phosphate is a primary contributor of water body eutrophication, presenting considerable challenges to the ecological environment. Current methods for phosphate removal from water primarily include precipitation [6,7], crystallization [8,9], electrochemical processes [10,11], adsorption [12,13], biological process [14,15], and ion exchange [16]. Among these, adsorption is the most preferred method for phosphate removal owing to its simplicity and high efficiency. The selection of inexpensive and efficient adsorbents with various sources is gaining popularity. CGCS, as a substantial source of solid waste, is a potential environmental threat. However, if employed as an adsorption material, its large reserves, low cost, easy availability, and transformative potential from waste to resource can make it a competitive choice. However, there has been a paucity of studies that have used CGCS for phosphate removal, with most studies using CGCS as a carrier and introducing foreign components, considerably reducing its utilization efficiency.
Herein, sodium hydroxide is used to activate CGCS and the activated CGCS (SH-CGCS) is utilized for removing phosphate from water. Furthermore, the physicochemical properties of CGCS before and after its activation as well as changes before and after phosphate adsorption by SH-CGCS are analyzed in this study. The aforementioned approach is different from those used in previous studies in terms of exclusively using the intrinsic components of CGCS for phosphate removal, thereby offering a simple and efficient method for the same. This study not only presents a novel, low-cost phosphate adsorbent but also realizes CGCS recycling, which is crucial for the development of low-carbon circular economy.

2. Materials and Methods

2.1. Materials

CGCS was obtained from a coal chemical enterprise in Ningxia, China, ground through a 100-mesh sieve, and stored in a dryer for later use. The major chemical components of CGCS were O (40.3%), Si (16.31%), Fe (14.43%), Ca (13.53%), Al (10.62%), K (0.74%), Na (0.63%), Mg (0.61%), Ti (0.58%), and others (2.25%). Chemical reagents such as NaOH, HCl, and NaCl were purchased from Sinopharm Group, while other reagents such as (NH4)6Mo7O24·4H2O, C4H4KO7Sb·1/2H2O, and KH2PO4 were procured from Kemiou Reagent Co., Ltd., Tianjin, China. All reagents used in this study were of analytical grade.

2.2. Preparation of Materials

CGCS (2 g) and NaOH particles were mixed at a mass ratio of 1:1.5, ground, placed in a crucible, and roasted at 500 °C in a muffle furnace for 4.5 h, resulting in the reaction between CGCS and sodium hydroxide to generate activated CGCS (labeled SH-CGCS). Thus obtained SH-CGCS was then washed using deionized (DI) water to near neutrality, dried at 100 °C, ground, bagged, and stored in a dryer for further use.

2.3. Characterizations

The main chemical composition of the CGCS sample was determined using X-ray fluorescence (Panalytical Axios, PANalytical, B.V.; Almelo, The Netherlands) spectroscopy. The microstructure and surface elemental distributions of the SH-CGCS and CGCS samples were analyzed utilizing scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS; Zeiss Gemini 300, Carl Zeiss AG, Oberkochen, Germany). The functional groups of SH-CGCS and SH-CGCS after phosphate adsorption were detected via Fourier-transform infrared (FTIR; Thermo Scientific Nicolet 6700, Thermo Fisher Scientific, Waltham, MA, USA) spectroscopy. The phase composition of SH-CGCS and SH-CGCS after adsorption was obtained utilizing X-ray diffraction (XRD; LabX XRD-6000, Shimadzu, Kyoto, Japan). The elemental composition and binding energy of SH-CGCS before and after phosphate adsorption were detected utilizing X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha, Thermo Fisher Scientific, MA, USA). Additionally, the point of zero charge (pHPZC) in CGCS and SH-CGCS was calculated using the pH drift method [17].

2.4. Batch Experiments

The phosphate wastewater used in the batch experiment is a simulated wastewater prepared from potassium dihydrogen phosphate. Batch adsorption experiments were conducted as follows. First, SH-CGCS (0.05 g) was added to a phosphate solution (40 mg/L; 50 mL). The resulting mixture was then allowed to react in a water bath shaker at room temperature (25 °C) and pH 6 for 24 h. After centrifugation, the residual phosphate concentration was determined using the molybdenum–antimony antispectrophotometric method, and the phosphate adsorption capacity (q) of SH-CGCS was calculated according to the formula presented in Text S1 (Supplementary Materials).
Initial pH-influence tests were performed at pH 3–10 using 0.1 M HCl or NaOH as an acid–base regulator. Various concentrations of the NaHCO3, NaNO3, NaCl, and Na2SO4 solutions (1 and 10 mM) and humic acid (HA) solutions (40 and 80 mg/L) were used to investigate the selective adsorption of phosphate by SH-CGCS. Furthermore, various concentrations of phosphate wastewater (10–80 mg/L) were selected to analyze adsorption isotherms for determining the adsorption capacity of SH-CGCS. The adsorption kinetics of SH-CGCS were investigated by estimating the adsorption amounts at various time periods (1–72 h). All batch experiments were triplicated.

2.5. Leaching Test

The horizontal oscillation method (Chinese standard HJ 557-2010) was used to determine whether SH-CGCS caused any secondary harm to the water environment. The leaching of toxic elements from SH-CGCS was estimated using inductively coupled plasma mass spectrometry (ICP-MS, X Series2, Thermo Fisher Scientific, Massachusetts, USA). Leaching results for components that aid in phosphate removal, such as Ca, Al, Fe, and Mg in DI water and phosphate solutions, were determined via ICP–optical emission spectrometry (ICP–OES, ICAP 7400, Thermo Fisher Scientific, Massachusetts, USA).

2.6. Column Adsorption Experiments

Column adsorption experiments were employed to evaluate the actual phosphate wastewater treatment capacity of SH-CGCS. An organic glass tube with an inner diameter and a length of 1.2 and 13 cm, respectively, was utilized as the fixed bed. SH-CGCS (1.5 g) was placed in the middle of this organic glass tube and sealed with degreasing cotton at both ends to prevent any SH-CGCS loss. Further, the actual wastewater was fed to the bottom of the organic glass tube at a flow rate of 0.7 mL/min. After reacting with SH-CGCS with a bed height of 1 cm, the treated water exited from the top outlet of the column, and its residual phosphate concentration was measured.

2.7. Reusability Experiments

Four cycles of reusability tests were performed for SH-CGCS. In each cycle, 50 mg of SH-CGCS was added to 50 mL of a 40 mg/L phosphate solution and allowed to react at 25 °C for 24 h. After adsorption, SH-CGCS was mixed with 50 mL of 1 M NaOH and allowed to react at 28 °C for 24 h. Subsequently, SH-CGCS was washed with DI water until it becomes neutral and dried for the next adsorption–desorption cycle.
The conditions for repeated adsorption tests were as follows: adsorbent dose = 1 g/L; initial phosphate concentration = 40 mg/L; reaction time = 24 h; and reaction temperature = 25 °C. After adsorption, the saturated SH-CGCS was mixed with 50 mL of 1 M NaOH, allowed to react at 25 °C for 24 h, washed with DI water until it became neutral, and dried for the next adsorption cycle.

3. Results and Discussion

3.1. Characterization

To assess changes in the specific surface area and pore size of CGCS before and after alkali activation, nitrogen adsorption–desorption isotherms, and pore size distribution analysis were performed, as illustrated in Figure 1 and Table S1. Figure 1a reveals that according to IUPAC definitions [18], CGCS and SH-CGCS exhibit type-IV isotherms with H3 hysteresis loops, indicating slit-like pores in SH-CGCS [19]. The pore size distribution of CGCS (Figure 1b), though ranging from micropores to macropores, is predominantly mesoporous. Meanwhile, SH-CGCS is a mesoporous material, concentrated primarily around 3 nm, and its mesoporous content is more than six times that of CGCS. This abundance of mesopores in SH-CGCS enhances its specific surface area to 64 m2/g compared with 11 m2/g in CGCS (Table S1), considerably augmenting phosphate adsorption capacity.
Morphological and compositional changes in CGCS and SH-CGCS before and after phosphate adsorption were explored using SEM–EDS. As depicted in Figure 2, CGCS exhibits an irregular block shape with varying hole sizes. The morphology of SH-CGCS, obtained from the reaction of CGCS with alkali at high temperatures, undergoes a considerable transformation. The dense block observed in CGCS transforms into petal-like flakes in SH-CGCS, which are stacked together to form numerous pore structures. This transformation is primarily attributed to the chemical reaction between sodium hydroxide, silica, or silicate in CGCS, leading to the formation of amorphous sodium silicate or metal oxides/hydroxides in SH-CGCS, as confirmed using XRD spectra. After phosphate adsorption, the petal-like flake materials on the SH-CGCS surface are covered by a layer of cotton-like material and phosphorus, as shown in Figure 2f, indicating successful phosphate adsorption by SH-CGCS.

3.2. Phosphate Adsorption

3.2.1. Effects of pH on Phosphate Adsorption

The influence of pH on phosphate adsorption by SH-CGCS and potential reaction processes were investigated considering the effect of pH on phosphate species in water and interaction between adsorbents and adsorbates. Figure 3 shows that the phosphate adsorption capacity of SH-CGCS gradually decreases with increasing pH. Unlike certain metal-based materials such as lanthanum-based adsorbents, which often exhibit reduced adsorption capacity in strong acid environments, SH-CGCS displays good adsorption capacity (38.5 mg/L at pH 3) and acid resistance, which is of great significance for the removal of phosphate from acidic wastewater.
As presented in Text S2, phosphate exists in the form of H2PO4− and HPO4− at pH 3–10. Figure 4 shows that compared with CGCS, the isoelectric point of SH-CGCS increases from 6.2 to 7.6. This renders SH-CGCS positively charged over a broader pH range, facilitating the adsorption of negatively charged phosphates. Considering the isoelectric point of SH-CGCS, its surface is predominantly positively (or negatively) charged in wastewater at a pH of less than (or more than) 7.6 [20]. Therefore, under neutral and acidic conditions, positively charged SH-CGCS allows the electrostatic adsorption of negatively charged H2PO4− and HPO4−. Conversely, under alkaline conditions, SH-CGCS with negative surface charge repels negatively charged H2PO4− and HPO4− due to electrostatic interaction. Additionally, the concentration of OH− increases with increasing pH, leading to negative OH− competing with H2PO4− and HPO4− for adsorption sites on SH-CGCS. These factors possibly contribute to the observed decrease in the phosphate adsorption capacity of SH-CGCS with increasing pH.
Notably, after the reaction between SH-CGCS and phosphate at the initial pH range of 3–8, the final pH exhibited varying degrees of increase, while the final pH decreased after the reaction in the initial pH range of 3–8. This phenomenon is attributed to the following reaction mechanism: ligand exchange occurs between phosphate and hydroxyl groups on the SH-CGCS surface, releasing numerous hydroxyl groups into water. This increases the concentration of hydroxide ions in the water, consequently increasing the pH value. In an alkaline environment, the decrease in the final pH is likely attributed to competitive adsorption between hydroxide ions and phosphate ions, leading to a reduction in the hydroxide ion concentration and pH value in the liquid phase [2].

3.2.2. Effects of Coexisting Anions and HA

Phosphate wastewater typically contains various anions, such as NO3−, SO42−, Cl−, and HCO3−, alongside organic matter such as HA. These substances compete with phosphate ions for adsorption sites, potentially affecting the adsorption capacity of the adsorbent. The influence of these coexisting substances on phosphate adsorption by SH-CGCS was examined in this study. Figure 5 shows that these anions have a minor negative effect on phosphate adsorption by SH-CGCS. This suggests that the interaction between SH-CGCS and phosphate ions is primarily specific adsorption, i.e., inner-sphere complexes. This is because nonspecific adsorption is highly sensitive to coexisting anion concentrations in phosphate wastewater, while specific adsorption is insensitive to coexisting anion concentrations [21]. The negative effect of HA on phosphate adsorption by SH-CGCS is also minimal, indicating favorable adsorption selectivity for phosphate.

3.2.3. Adsorption Isotherms

The maximum adsorption capacity of SH-CGCS and possible adsorption reaction process between phosphate and SH-CGCS were analyzed using adsorption isotherms and two common adsorption models: Langmuir and Freundlich. The Langmuir and Freundlich models are detailed in Text S3. As depicted in Figure 6 and Table S2, the Langmuir model better captures the phosphate adsorption process by SH-CGCS, evident from its superior fitting for the data (R2 = 0.994) compared with the Freundlich model (R2 = 0.944). This indicates a monolayer adsorption process for phosphate on SH-CGCS [19]. Furthermore, the calculated maximum phosphate adsorption capacity of SH-CGCS using the Langmuir model is 23.92 mg/g, surpassing that of other reported adsorbents (Table S3). Thus, owing to its low cost and large adsorption capacity, SH-CGCS emerges as a promising adsorbent.

3.2.4. Adsorption Kinetic Studies

The adsorption kinetics for phosphate on SH-CGCS were analyzed using three common kinetic models; these kinetics are detailed in Text S4. As shown in Figure 7a, the pseudo-second-order model exhibits a better fit with the experimental data compared with the pseudo-first-order model, as evidenced by its higher fitting coefficient (R2 = 0.982) and a calculated adsorption capacity (qe,cal) closer to the experimental value (7.652 mg/g vs. 7.576 mg/g). This suggests a chemical adsorption process for phosphate on SH-CGCS [3,22].
To investigate the rate control steps in phosphate adsorption by SH-CGCS, an intraparticle diffusion model analysis was conducted, as shown in Figure 7b. In this figure, the three slope line segments represent different adsorption stages: membrane diffusion at the start, intraparticle diffusion in the middle, and a final equilibrium stage. Notably, C ≠ 0 indicates that the phosphate adsorption rate of SH-CGCS is governed by a multistage rate control step rather than intraparticle diffusion. Kid1 surpasses Kid2 (Table S4), implying that boundary layer diffusion is the primary rate control step, with intraparticle diffusion serving as a secondary rate control measure [4,23].

3.2.5. Metal Leaching

To assess the safety of SH-CGCS in water, toxic leaching experiments were conducted, and the results are presented in Table S5. The leaching of heavy metals from SH-CGCS is minimal, well below the concentration limit specified for Class I surface water in China’s environmental quality standards for surface water (GB 3838-2002). This highlights the safety and reliability of SH-CGCS, indicating its resistance to the secondary pollution of water bodies.
The leaching of essential components involved in phosphate adsorption on SH-CGCS, including calcium, aluminum, iron, and magnesium, was also examined. As outlined in Table S6, except for minor amounts of calcium and aluminum, the leaching of the remaining components is negligible. Notably, the concentrations of these components in phosphate wastewater are significantly lower than those in deionized water, suggesting possible precipitation with phosphate.

3.2.6. Column Adsorption Experiments

Column adsorption experiments were conducted to assess the performance of SH-CGCS in treating actual phosphate-containing water (lake water) under continuous flow conditions. Key parameters of the water body are detailed in Table S7, with low phosphate concentrations noticed in most cases. To better simulate the column experiment when analyzing SH-CGCS performance for the treatment of actual low-concentration water, the initial phosphate concentration was adjusted to 2 mg/L. The breakthrough point, set at 0.5 mg/L based on the China’s urban wastewater control standard (GB 18918-2002), is presented in Figure 8. Under the column adsorption conditions of a flow rate of 0.7 and an empty bed contact time of 1.61, 1.5 g of SH-CGCS effectively treated actual phosphate water, exceeding 619 bed volumes (approximately 700 mL). This demonstrates the SH-CGCS’s capability to treat water containing low phosphate concentrations under flowing conditions.

3.2.7. Reusability

Assessing reusability is crucial when evaluating adsorbents. For operational convenience and experimental reliability, this study focused on four cycles of adsorption–desorption for SH-CGCS to explore its reusability (Figure 9). As the number of cycles increases, the adsorption capacity of SH-CGCS is reduced to varying degrees. Nevertheless, even in the fourth cycle, the adsorption capacity remains notable at 13.1 mg/g, affirming the robust reusability of SH-CGCS.
The decline in the phosphate adsorption capacity of SH-CGCS with an increasing number of cycles can be primarily attributed to two factors. First, the components of SH-CGCS that aid in phosphate removal are somewhat lost with increasing washing cycles. Second, some phosphates may form strong bonds with binding sites on SH-CGCS, resisting desorption by the resolving agent. This phenomenon weakens the re-adsorption performance of SH-CGCS.

3.3. Adsorption Mechanism

3.3.1. XRD

The XRD spectra of ground CGCS, SH-CGCS, and SH-CGCS after phosphate adsorption are presented in Figure 10. In CGCS, the dominant phase is quartz. Post alkali activation, the quartz phase in SH-CGCS disappears, suggesting a reaction between the quartz phase and alkali. This results in the formation of amorphous substances. Following phosphate adsorption, SH-CGCS exhibits characteristics of an amorphous phase, indicating that the products resulting from the interaction between SH-CGCS and phosphate are predominantly amorphous.

3.3.2. FTIR

Figure 11 displays the FTIR spectra of SH-CGCS and SH-CGCS after phosphate adsorption. The bands observed at approximately 3445 and 1657 cm−1 correspond to the stretching and bending vibrations of the OH group, respectively [24,25]. Additionally, the bands observed at approximately 1009 and 432 cm−1 correspond to the asymmetric stretching and bending vibrations of the T–O (T is Al or Si) bond, respectively [2]. Following phosphate adsorption by SH-CGCS, the intensity of peaks observed at approximately 3445 and 1657 cm−1 is considerably reduced, suggesting ligand exchange between the hydroxyl group on SH-CGCS and phosphate. Meanwhile, the bands observed at approximately 1486 cm−1, likely attributed to Ca–O [3,26], disappear after phosphate adsorption, indicating the involvement of Ca–O in the reaction between SH-CGCS and phosphate, possibly forming an inner-sphere complex.
In summary, considering the analyses of initial and final pH changes during SH-CGCS adsorption, the impact of coexisting anions, metal leaching tests, and combined XRD and FTIR analyses, the phosphate adsorption mechanism of SH-CGCS encompasses electrostatic adsorption induced by surface charges, ligand exchange induced by surface hydroxyl groups, inner-sphere complexation induced by metal (hydr)oxides, and the surface precipitation of metal components (Ca, Al, Fe, and Mg) with phosphate.

4. Conclusions

This study employed a straightforward and efficient alkali activation method for CGCS, resulting in an enhanced overall utilization efficiency and the development of a novel phosphate adsorbent (SH-CGCS). SH-CGCS, a mesoporous material concentrated primarily around 3 nm, exhibits a substantial increase in the specific surface area to 64 m2/g compared with CGCS (11 m2/g). This mesoporous structure considerably augments its adsorption performance. Furthermore, it demonstrates good adsorption capacity (38.5 mg/L at pH 3) and acid resistance. Coexisting anions exhibit a minimal negative effect on phosphate adsorption by SH-CGCS, showing good adsorption selectivity for phosphate. The Langmuir and pseudo-second-order models offer superior reflections of the adsorption process, demonstrating a maximum phosphate adsorption capacity of 23.92 mg/g. Cumulatively treating actual phosphate water bodies exceeding 619 bed volumes (approximately 700 mL) with 1.5 g of SH-CGCS reveals its capability to treat water containing low phosphate concentrations under flowing conditions. In terms of reusability, four cycles of adsorption–desorption were conducted for SH-CGCS, with the adsorption capacity in the fourth cycle reaching 13.1 mg/g, showcasing its commendable reusability. A comprehensive analysis of the results obtained in this study indicates that the phosphate adsorption mechanism of SH-CGCS predominantly involves electrostatic adsorption induced by surface charges, ligand exchange facilitated by surface hydroxyl groups, inner-sphere complexation induced by metal (hydr)oxides, and the surface precipitation of metal components (Ca, Al, Fe, and Mg) with phosphate.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/separations11060182/s1, Table S1: BET specific surface area and BJH pore parameters of CGCS and SH-CGCS; Table S2: Adsorption isotherm parameters. Adsorption conditions: reaction time, 24 h; sorbent dose, 1 g/L; initial phosphate concentration, 10–80 mg/L, temperature, 25 °C; Table S3: Comparison of the adsorption capacity of SH-CGCS and adsorbents given in literatures for phosphate; Table S4: Adsorption kinetic parameters of SH-CGCS to phosphate; Table S5:Comparison of the toxic leaching results of SH-CGCS with the concentration limit of Class I surface water in China’s surface water environmental quality standards; Table S6: Comparison of leaching results of aluminum, calcium, iron and magnesium from SH-CGCS; Table S7: Main components of actual wastewater [20,27,28,29,30,31,32,33,34,35,36,37,38,39].

Author Contributions

X.S.: Investigation, Data curation, and Writing—Original Draft. B.Y.: Conceptualization, Methodology, Supervision, Writing—Review and editing, and Funding. D.Q.: Supervision, Resources, and Funding. D.C.: Supervision, Resources, and Validation. H.L., C.W., Y.Z. and T.Y.: Investigation and Data curation. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Open Project of the Institute of Resources and Ecology at Yili Normal University (YLNURE202304); the Special Key Project of Yili Normal University to Enhance the Comprehensive Strength of Disciplines of Natural Science (22XKZZ01); and Special Funds for Improving the Comprehensive Strength of Disciplines of Yili Normal University (22XKZZ13).

Data Availability Statement

All data generated or analyzed during this study are included in this published article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Separations 11 00182 g001
Figure 1. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of CGCS and SH-CGCS.
Figure 1. (a) N2 adsorption–desorption isotherms and (b) pore size distribution of CGCS and SH-CGCS.
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Figure 2. SEM and EDS spectra of CGCS (a,b), SH-CGCS (c,d), and SH-CGCS after phosphate adsorption (e,f).
Figure 2. SEM and EDS spectra of CGCS (a,b), SH-CGCS (c,d), and SH-CGCS after phosphate adsorption (e,f).
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Figure 3. Effect of wastewater pH on phosphate adsorption capacities of SH-CGCS. Adsorption conditions: initial phosphate concentration, 40 mg/L; sorbent dose, 1.0 g/L; reaction time, 24 h; temperature, 25 °C.
Figure 3. Effect of wastewater pH on phosphate adsorption capacities of SH-CGCS. Adsorption conditions: initial phosphate concentration, 40 mg/L; sorbent dose, 1.0 g/L; reaction time, 24 h; temperature, 25 °C.
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Figure 4. The pHPZC of CGCS and SH-CGCS.
Figure 4. The pHPZC of CGCS and SH-CGCS.
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Figure 5. Effects of competitive anions and humic acid on phosphate adsorption by SH-CGCS. Adsorption conditions: sorbent dose, 1.0 g/L; pH, 6; initial phosphate concentration, 40 mg/L; reaction time, 24 h; temperature, 28 °C.
Figure 5. Effects of competitive anions and humic acid on phosphate adsorption by SH-CGCS. Adsorption conditions: sorbent dose, 1.0 g/L; pH, 6; initial phosphate concentration, 40 mg/L; reaction time, 24 h; temperature, 28 °C.
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Figure 6. Phosphate adsorption isotherm on SH-CGCS. Adsorption conditions: temperature, 25 °C; pH, 6; initial phosphate concentration, 10–80 mg/L.
Figure 6. Phosphate adsorption isotherm on SH-CGCS. Adsorption conditions: temperature, 25 °C; pH, 6; initial phosphate concentration, 10–80 mg/L.
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Figure 7. Adsorption kinetics plots fitted by (a) pseudo-first-order, pseudo-second-order, and (b) intraparticle diffusion models. Adsorption conditions: adsorbent dose, 1.0 g/L; initial phosphate concentration, 10 mg/L; temperature, 25 °C; pH, 6.
Figure 7. Adsorption kinetics plots fitted by (a) pseudo-first-order, pseudo-second-order, and (b) intraparticle diffusion models. Adsorption conditions: adsorbent dose, 1.0 g/L; initial phosphate concentration, 10 mg/L; temperature, 25 °C; pH, 6.
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Figure 8. Column breakthrough curve when treating actual wastewater with the SH-CGCS adsorbent. Bed height, 1 cm; flow rate, 0.7 mL/min.
Figure 8. Column breakthrough curve when treating actual wastewater with the SH-CGCS adsorbent. Bed height, 1 cm; flow rate, 0.7 mL/min.
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Figure 9. The variation of the adsorption amount of phosphate by SH-CGCS with the number of cycles. Adsorption conditions: adsorbent dose, 1.0 g/L; eluant, 1 M NaOH; initial phosphate concentration, 40 mg/L; temperature, 25 °C; pH, 6.
Figure 9. The variation of the adsorption amount of phosphate by SH-CGCS with the number of cycles. Adsorption conditions: adsorbent dose, 1.0 g/L; eluant, 1 M NaOH; initial phosphate concentration, 40 mg/L; temperature, 25 °C; pH, 6.
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Figure 10. XRD spectrum of samples.
Figure 10. XRD spectrum of samples.
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Figure 11. FTIR spectra of (a) SH-CGCS and (b) SH-CGCS after phosphate adsorption.
Figure 11. FTIR spectra of (a) SH-CGCS and (b) SH-CGCS after phosphate adsorption.
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Shi, X.; Yang, B.; Qian, D.; Cui, D.; Li, H.; Wang, C.; Zhu, Y.; Yu, T. Isotherm, Kinetics, and Adsorption Mechanism Studies of Coal Gasification Coarse Slag as Highly Efficient Phosphate Adsorbents. Separations 2024, 11, 182. https://doi.org/10.3390/separations11060182

AMA Style

Shi X, Yang B, Qian D, Cui D, Li H, Wang C, Zhu Y, Yu T. Isotherm, Kinetics, and Adsorption Mechanism Studies of Coal Gasification Coarse Slag as Highly Efficient Phosphate Adsorbents. Separations. 2024; 11(6):182. https://doi.org/10.3390/separations11060182

Chicago/Turabian Style

Shi, Xuzhi, Baoguo Yang, Dayi Qian, Dong Cui, Hongbin Li, Chao Wang, Yuhao Zhu, and Tao Yu. 2024. "Isotherm, Kinetics, and Adsorption Mechanism Studies of Coal Gasification Coarse Slag as Highly Efficient Phosphate Adsorbents" Separations 11, no. 6: 182. https://doi.org/10.3390/separations11060182

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