Novel Material Obtained from the Gasification Residues of Candiota Mine’s Coal for Cationic Dye Adsorption

Abstract

This work aims to prepare and characterize the unburned carbon obtained from gasification residues and evaluate its application as an adsorbent for the removal of textile dye contaminants. The results of physical and chemical properties showed a specific mass of 2.05 g/cm³, surface area of 23.983 g/cm², and diameter and pore volume of 0.844 nm and 2.262 cm³/g, respectively. These properties, along with the point of zero charge and chemical bonds present on the surface, favored the adsorption of cationic dyes. The adsorption results showed great potential for the removal of methylene blue, crystal violet, and basic fuchsin if compared with bromocresol green, and indigo carmine. The maximum removal values obtained for methylene blue were up to 99% and the kinetic adsorption was faster at the beginning of the process, reaching the equilibrium in less than 5 min. The results obtained through the adsorption isotherms showed a maximum adsorption capacity of 333.33 and 476.19 mg/g, at the temperature of 291 and 328 K, respectively. The satisfactory results showed that the use of unburned carbon is a cost-effective and eco-friendly alternative to reusing the residue from gasification and also contributes to the decontamination of watercourses.

1. Introduction

Raw coal (RC) is a fossil fuel widely found in worldwide [1]. Brazil has an estimated coal reserve of 6.5996 billion tons and the largest mine is located in the south of Rio Grande do Sul, the Candiota mine [2,3]. The coal from this region is considered sub-bituminous, and is known for its high ash content, above 50% dry basis [4,5]. Although the main energy source in various countries is related to combustion, the use of coal has been decreasing to avoid the impact caused by greenhouse gas emissions [1], which is related to global warming. The climate change catastrophe requires the greatest attention and the use of clean technology in coal processing has been attracting significant investments.

Regarding the techniques that produce lower gas emissions, it is worth mentioning coal gasification [6]. Gasification is a thermochemical process that converts solid combustibles into liquid and gas products. The main product of coal gasification is syngas, which can be used to obtain other chemical products [1,7,8]. Some residues are produced by this process, such as ash, which can be classified as fly ash (FA) and bottom ash (BA). The FA are composed of small particles, presenting an average particle size between 3 to 34.57 μm, and are mainly composed of SiO₂, Al₂O₃, Fe₂O₃, TiO₂, CaO, and MgO [8,9,10,11,12,13]. The disposal of FA may be related to economic and environmental problems around the world, and thus researchers have been developing studies regarding its reuse, such as the synthesis of zeolite [12,14,15], development of a catalyst used in the decomposition of NO and NH₃ [16], recovery of gallium and vanadium [17], Zn²⁺, Pb²⁺, As (V), hexavalent chromium and dye adsorption [11,18], and additive for the manufacture of cement, concrete, and concrete-admixed products. However, according to some authors, these applications may be limited since the proportion of unburned carbon (UC) can represent a barrier to its reuse [19,20,21].

UC is a material obtained through the incomplete gasification of coal, which is dragged with fly ash during the thermochemical process [1,22,23]. The morphology of this material differs from raw coal, as UC presents high porosity due to the heterogeneous reactions that cause changes in the particles’ porous structure [8,24]. Thus, recent studies have been investigating some technologies used in the removal of UC from FA, which are classified as dry and wet separation methods. The wet methods basically consist of sieving, magnetic, gravitational, and electrostatic methods or a combination of them, while the dry methods are usually flotation and agglomeration [25,26,27,28,29,30]. The UC content may be related to some factors such as the nature of coal (particle size, mineral matter, calorific value, volatile matter, and others), the gasification conditions, and FA size [20,27].

In the last twenty years, researchers have been studying the application of UC in the treatment of aqueous and gaseous solutions, such as the adsorption of mercury [31], SO₂ [16], mordant orange [32], nitric oxide [33], methylene blue [24,32,34], Rhodamine B [24,32], crystal violet [24,34], acid black 1 [24], humic acid [35], and others. Adsorption has been extensively used for the removal of contaminants from water at different scales [36] which is widely used in the removal of contaminants from textile effluents [37,38]. The novelty of this study is the proposal of a new adsorbent material from an industrial residue and the evaluation of its application as an adsorbent for the removal of contaminants. Therefore, considering the potential applications of UC, the present study aims to recover the UC from FA, evaluate its physical and chemical properties, and its application in textile dye adsorption.

2. Materials and Methods

2.1. Production of Unburned Carbon (UC)

The UC was obtained from the fly ash produced by the gasification of coal from the Candiota-RS mine. The gasification process was carried out in a bench plant, with a capacity to process 10 kg of coal per hour, located in the Energy and Carbochemistry Laboratory (ECL) at the Federal University of Pampa, located 40 km from the Candiota mine. The process of separation of the UC contained in FA was executed in two steps. First, the samples were sieved (Bertel electromagnetic stirrer, Model VP-01, São Paulo, Brazil) to obtain a particle size between 2 and 0.053 mm. Subsequently, the particles of 0.250 mm were submitted to decantation using 250 mL of water and 10 g of sample. The top layer obtained was composed of fly ash, and the bottom layer was called unburned coal (UC). UC was dried at 105 °C for 24 h before the characterization and adsorption studies.

2.2. Characterization of UC

The characterization analyses of the UC are shown in Table 1.

The colorimetric analysis was performed according to the CIELAB system using a colorimeter (Konica Minolta, model CM-2500d, Tokyo, Japan), and the sample was analyzed through the hue, brightness, and saturation. The calorimetric analysis was carried out with a Parr 6200 oxygen bomb calorimeter. The determination of specific mass was obtained through gas pycnometry (Quantachrome Instruments, model Ultrapyc 1200e V4.01, Boynton Beach, FL, USA). The specific area (SA), average pore size (APS), and pore size diameter (PSD) were determined by the Brunauer-Emmet-Teller (BET) technique (Quantachrome Instruments, model 1000, Boynton Beach, FL, USA) through nitrogen adsorption isotherm at 77 K. The Fourier transform infrared (FTIR) spectroscopy was applied to analyze the functional groups present on the surface of the material; this analysis was performed using a spectrophotometer (Rayleigh, model WQF-510, Chelmsford, UK) between the range of 400 to 4000 cm⁻¹. The spectrometry X-ray fluorescence (XRF) (Bruker, Model AXS GmbH, Ettlingen, Germany) allows us to identify the chemical elements present in a sample, as well as establish the proportion in which each one is present. The point of zero charge (pH_CZ) experiment was performed according to the methodology of Ribeiro et al. (2019) [11].

2.3. Adsorption of Industrial Dyes

A preliminary study was conducted to compare the adsorption of methylene blue (MB), crystal violet (CV), indigo carmine (IC), bromocresol green (BG), and basic fuchsin (BF). The experiments were performed under the following conditions: 0.1 g adsorbent dosage, 25 mL dye solution with an initial dye concentration of 100 mg/L, at room temperature. The adsorption process was carried out in a rotatory shaker (Nova Ética, 109/1, São Paulo, Brazil) at 150 rpm for 1 h. Samples were centrifuged (QUÍMIS, Model Q222TM216, Caxias Do Sul, Brazil) at 4000 rpm for 10 min. The residual concentration of textile dyes was quantified using a UV–vis spectrophotometer (EQUILAM, UV 755B, Diadema, Brazil) at the maximum wavelength indicated in Table 2. The adsorption capacity and percentage removal of each dye were calculated using Equations (1) and (2), where C₀ is the initial dye concentration (mg/L), C_e is the equilibrium dye concentration (mg/L), m is the mass adsorbent (g), V is the volume of the dye solution (L), and q_e is the adsorption capacity at equilibrium (mg/g).

(1)$q_e=\frac{C_0-C_e}{C_0}V$

(2)$Dye removal\left(\%\right)=\frac{\left(C_0-C_e\right)}{m}V00$

2.4. Kinetics and Equilibrium Adsorption

The kinetic experiments were conducted considering an adsorbent dosage of 0.1 g, and 25 mL dye solution with an initial dye concentration of 100 mg/L, at room temperature. The adsorption kinetic was performed in a rotary shaker (Nova Ética, 109/1) at 150 rpm for 0 to 20 min. Samples were centrifuged and analyzed in a UV–vis spectrophotometer (λ = 560 nm). The kinetic models of pseudo-first-order (PFO) and pseudo-second-order (PSO) were fitted to the experimental data [43]. An equilibrium isotherms study was carried out using MB solution with an initial concentration from 25 to 225 mg/L, at 303 and 323 K. The results were fitted to the equilibrium models of Langmuir and Freundlich. The models are presented in Table 3.

3. Results and Discussion

3.1. Characterization of UC

The UC’s specific mass (SM) was equivalent to 2.05 g/cm³ (Table 4) in accordance with the literature [26,44]. This result confirms the great performance of the fraction separation process, based on the difference in specific mass between the UC and FA, which presents a specific mass in the range of 2.3 to 2.45 g/cm³ [11,31]. The value obtained for UC was expected since the employment of high temperature in the gasification process promotes a decrease in the specific mass, which may be related to an increase in porosity [8,45]. The surface area (SA) was 23.984 m²/g, which is in the range of 16.5 to 58 m²/g obtained by other authors for UC [26,44,46]. Carbons derived from class F fly ash, which are obtained from the burning of low-calcium bituminous coal, normally have low surface areas when compared to active carbons [26,47].

The measurement of pore diameter (APD) was 0.844 nm, which is similar to the range indicated by the literature and, according to IUPAC, is classified as a micropore (<2 nm) [48]. The low SA can also be explained by the number of pores as well as by the low APD and PV, as these properties are related to each other [44]. Based on the calorimetric analysis, it was possible to observe an increase in the CV when compared to the coal from the Candiota mine, which presented values in the range of 3847.7 to 4701.8 cal/g. Table 5 shows the colorimetric analysis.

The colorimetric analysis indicated a tendency of the UC to be black in color as the value obtained for the L * parameter was lower than 28.44. This result can be related to the success of FA separation since the colorimetric analysis can be used as a technique to identify the presence, or not, of UC [49]. Based on these authors, a lighter sample indicates higher content of UC. The authors of [12] presented a colorimetric analysis of FA, obtaining an L * value of 10.34, which, compared to the value obtained in the present work and according to other literature [12], indicates an increase in UC content after the decantation process. Analyzing the data obtained, it is possible to observe an increase in L * value in the order RC > UC > FA, which is in accordance with another study [49], as this order follows the same tendency of the gasification process, where the raw material (RC) is the lightest, followed by the other products (UC and FA).

Figure 1 shows the FTIR spectra, which present peaks similar to the ones obtained by the literature [50] when analyzing high-density coal (>1.8 g/cm³). First, is possible to observe at approximately 3600 cm⁻¹ (OH stretching region) four bands that are characteristic of the kaolinite compound [50,51,52,53]. The first one is located at 3620 cm⁻¹ representing the inner hydroxyl groups; then the peaks at 3653 and 3669 cm⁻¹ indicate the in-phase symmetric stretching vibration and 3695 cm⁻¹ the out-of-phase stretching vibration [52]. The peaks at 2929 and 2853 cm⁻¹ (Figure 2) indicate CH₂ asymmetric and symmetric stretching vibration, respectively [50,54]. The low intensity of the peak at 2929 cm⁻¹ may be related to the fact that the proportion of aliphatic -CH₃ hydrogen is substituted by hydroxyl groups [53]. The 1438 cm⁻¹ peak represents the CH₃ asymmetric deformation vibration [50,53,54]. The peaks at 934 and 915 cm⁻¹ may be related to the hydroxyl group in the carboxylic acid structure and phenolic hydroxyl group, respectively [50,55]. The peak located at 777 cm⁻¹ is related to Si-O-Si bending vibration [56], and the band in the region near 600 cm⁻¹ denotes the bending of Si-OH groups [57].

The fluorescence X-ray analysis (Table 6) indicated that the UC is mainly composed of mineral compounds, such as SiO₂, Al₂O₃, and Fe₂O₃, corroborating the information indicated by the FT-IR spectrum, and the results obtained for high-carbon FA [28]. When compared to coal from the same mine, it is possible to observe that after the separation there is a decrease in mineral content, which was expected due to the separation process from FA.

The pH_CZ was 5.1, as indicated in Figure 2, which means that, in solutions with pH lower than this value, the adsorbent surface presents a positive charge, and at a higher pH value it has a negative charge. This value was similar to the results obtained by other authors, with pH_CZ equivalent to values lower than 7 [24,58]. The UC presented a wide range of pH values above the pH_CZ, indicating that it has favorable characteristics for interaction with cationic dyes, such as MB, CV, and BF [11,24,58]. The industrial conditions of UC production lead to acid functional group formation, such as carboxylic, lactonic, phenolic, and acyls in acid anhydrides, and neutral groups, such as carbonyl functional groups on the UC surface, as indicated by the FTIR analysis, which confirms its ability to interact with positively charged molecules [16].

Figure 3 and Figure 4 present the adsorption capacity and removal of industrial dyes, which show results in the following order: IC (1.66 mg/g) < BG (5.84 mg/g) < BF (13.83 mg/g) < MB (16 mg/g) < CV (16.34 mg/g). It is possible to notice that the UC allowed the more efficient removal of cationic dyes, such as CV, MB, and BF in accordance with the information presented by the pH_CZ analysis. However, there is a difference between the values obtained for these dyes, and this behavior may be associated with molecule dye dimensions, as well as the charge effect indicated previously by the PCZ study [24]. As reported by the literature, CV adsorption occurs through a bond between dye and oxygen atoms present on SiO₂ and Al₂O₃ [59], which, as reported by FRX analysis, were the major mineral compounds present in the UC. Reported adsorption capacity values were in the range of 33.46 to 80.36 mg/g for CV when applying an adsorbent material similar to UC. The difference between these values and those observed in the present study may be related to the employment of higher temperatures in the literature [58]. With regard to the MB adsorption onto UC, the literature has reported values near 79.96 mg/g, and the difference between this value and the one obtained in the present study may be related to the gasification conditions, which play an important role in adsorbent properties, such as surface functional groups, pore volume and size, surface area, and others [60,61]. The literature has indicated adsorption capacity values of BF in the range of 7.16 to 34 mg/g by material with similar properties to UC [51]. The values obtained for BG and IC are in agreement with values reported by other authors and may be related to the anionic nature of these dyes [62].

Figure 5 presents the kinetic data, which indicates that the first step of the adsorption process is characterized by a fast increase in dye adsorption capacity within the first five minutes. According to the literature, this may be related to the occupation of available adsorption sites at the beginning, which leads to a higher difficulty for other dye molecules to be adsorbed due to repulsion forces between the adsorbent and the dye surface [63]. Considering the high adsorption rate, it is possible to assume that the process adsorptive was determined mostly by the mass transfer mechanism convective, which was confirmed by the kinetic model fitting (Table 7). It is possible to observe that the adsorption capacity at equilibrium was equal to almost 50 mg/g, which is lower than the value obtained by Wang et al. [58] and must be related to the adsorbent dosage since the adsorption capacity may be related to the availability of adsorption sites.

The two kinetic models showed a good fit to the experimental data with high correlation coefficients (R² > 0.999); however, the PSO model presented the best fit. This result is consistent with those observed by other authors in their studies on the adsorption kinetics of dyes on UC [24,35,51,64,65]. The PSO model describes an adsorption process that occurs on a heterogeneous surface which has a random distribution and, also, indicates that the interaction between dye and adsorbate is chemical, which involves the sharing or exchange of electrons [59,66,67]. The fact that the adsorption process is mainly governed by chemisorption is related to the electrostatic interaction between the dye and the UC, which occurs due to the cationic nature of MB and the UC surface composed of negative charges. Another important aspect is the difference between the UC pore diameter and the MB molecule size. According to the characterization analysis, the UC has smaller pores than the MB molecule, which is unfavorable for diffusion adsorption and suggests surface adsorption related to a convective adsorption mechanism [68,69].

The equilibrium value k₂ is related to the adsorption rate, and the low value observed by the fit is in agreement with the fact that the system reaches equilibrium quickly. This result may be related to the high active sites and the O-containing functional groups on the surface of UC, which improve the electrostatic relationship between the UC and MB molecules [70].

3.2. Adsorption Isotherms

The adsorption studies were conducted at two different temperatures, 303 and 328 K to evaluate the influence of temperature on the adsorption process; the experimental data are shown in Figure 6.

According to IUPAC classification, the isotherms shown below may be classified as Type I, which is considered reversible and is given to microporous solids with small external surfaces, information that is in accordance with the results obtained through the BET analysis [48]. It was possible to observe that the Freundlich model presented the best fit for the lowest temperature of 218 K, which was also observed by other authors in a study regarding the adsorption of Naphthol Green B on UC at a temperature of 273 K [46]. This result (Table 8) suggests that the surface and pores of the UC are heterogeneous at lower temperatures, have non-uniform heat distribution and adsorption affinities, and have a multilayer adsorption mechanism [24,70,71,72,73].

It is possible to observe a decrease in the value of k_F with increasing temperature. The value of k_F obtained at 291 and 328 K were, respectively, s 5.48 and 4.65, which indicates the higher the temperature, the more favorable is the adsorption process [74]. The value of n indicates heterogeneity, with n higher than one (n > 1) [75]. For the maximum temperature of 328 K, the n obtained was 1.91. Thus, the higher the temperature, the less favorable it is to multilayer adsorption, and at a higher temperature, the behavior of the adsorbent will be heterogeneous. For the maximum temperature of 328 K, the best model was the Langmuir model, with a maximum adsorption capacity of 476 mg/g, higher than those reported by other authors in their studies on the adsorption of MB onto FA and UC [24,32]. It is possible to observe an increase in maximum adsorption capacity with the temperature, which can be related to an increase in the rate of diffusion of the adsorbate, also observed by Wang and Li [24]. According to the R_L value, determined through the Langmuir parameters, it is possible to observe that the isothermal form is favorable at both temperatures presented in the present work, given that 0 < R_L < 1 [71]. The k_L is a constant that may be related to the adsorption energy. In a study conducted by another author on methylene adsorption, it was observed that the k_L values ranged from 0.21 to 0.48 [60]. In the present study, at the temperature of 328 K, the best fit was the Langmuir model, obtaining a value of 0.26 for k_L. The results obtained indicate that the MB adsorption mechanism onto UC is related to the temperature.

4. Conclusions

It was possible to recover UC from FA by a simple and easy methodology. UC was shown to be an effective low-cost adsorbent for removing textile dyes from aqueous solutions due to its properties, such as high SA, average pore diameter, and pore volume of 23.984 m²/g, 0.844 nm, and 2.262 cm³/g, respectively. The characterization also indicated the presence of hydroxyl and carbonyl functional groups on the UC surface, which leads to great performance in cationic dye adsorption, in accordance with the pH_CZ. The UC was effective in the adsorption of CV, MB, and BF, reaching almost 80% removal. The kinetic model with the best fit was the pseudo-second-order, indicating a chemisorption mechanism between the dye and UC. Regarding the adsorption isotherm, it was observed that the Freundlich and Langmuir models fitted better at 291 and 328 K, respectively, indicating that increases in temperature promote the homogeneity of heat distribution on the adsorbent surface, as well as leading to a monolayer adsorption mechanism. It can be concluded that UC is a material with interesting properties as an adsorbent material, indicating an effective alternative for its reuse. Therefore, further studies are required with the aim of optimizing the experimental conditions of adsorption onto UC.

Author Contributions

Conceptualization: G.L. and T.L.R.; methodology: G.L. and T.L.R.; formal analysis: G.L. and T.L.R.; investigation: G.L. and T.L.R.; resources: G.L., G.S.d.R., T.L.R. and A.R.C.M.; writing—original draft preparation: G.L. and T.L.R.; writing—review: G.S.d.R., A.R.C.M. and J.V.d.R.; editing: G.L. and T.L.R.; supervision: G.S.d.R. and A.R.C.M.; Project administration: A.R.C.M.; Funding acquisition: A.R.C.M. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data available upon request.

Acknowledgments

The authors would like to acknowledge the support of the Energy and Carbochemistry Laboratory, the Particulate Systems Research Group, and the Federal University of Pampa. The study also received support from Companhia Riograndense de Mineração—CRM (Mining Company of Rio Grande do Sul), which provided the coal needed for laboratory tests.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. FTIR spectra of the UC.

Figure 2. UC point of zero charge.

Figure 3. Comparison of experimental adsorption of different textile dyes (capacity).

Figure 4. Comparison of experimental adsorption of different textile dyes (removal).

Figure 5. Effect of contact time on MB 100 mg·L−1 adsorption rate (pH 7.0 at 298 K).

Figure 6. Effect of initial dye concentration on the removal of MB by UC at various temperatures (303 and 328 K).

Table 1

Physicochemical analysis of the UC.

Analysis	Type	Norm
Granulometric analysis	Physical	NBR 10850 [39]
Colorimetric analysis	Physical	NBR 10850 [39]
Calorimetric analysis	Physical	ASTM 1038.5 [40]
Gas pycnometry	Physical	NBR 12076 [41]
BET	Physical	-
FTIR	Chemical	ASTM E1252 [42]
XRF	Chemical	-

Table 2

Chemical structure, molar mass, and wavelength of industrial dyes.

Dye	Molar Mass (g/mol)	Nature	Wavelength (nm)
MB	319.85	Cationic	560
CV	407.98	Cationic	590
IC	466.36	Anionic	610
BG	698.01	Anionic	450
BF	337.86	Cationic	558

Table 3

Kinetic and equilibrium adsorption models.

Model	Equation	Parameters
PFO	$q_t=q_e\left(1-e^{-k_{1}t}\right)$	qe: equilibrium adsorption capacity (mg/g)
		qt: adsorption capacity (mg/g)
		t: time t (min)
		k1: adsorption rate constant of PFO (1/min)
PSO	$q_t=\frac{q_e^2\left(k_{2}t\right)}{\left(1+q_e{k}_{2}t\right)}$	qe: equilibrium adsorption capacity (mg/g)
		qt: adsorption capacity (mg/g)
		t: time t (min)
		k2 are the adsorption rate constant of PSO (g/mg·min−1)
Langmuir	$q_e=\frac{q_{max}{C}_e}{1+k_L{C}_e}$	qmax: maximum adsorption capacity (mg/g)
		KL: Langmuir constant (L/mg)
Freundlich	$q_e=k_F{C}_e^{\frac{1}{n_F}}$	qmax: maximum adsorption capacity (mg/g)
		KF: Freundlich constant (mg/g)/(mgL)−1/nF
		1/nF: equilibrium constant

Table 4

Physical properties of the UC.

Properties	SM (g/cm3)	SA (m2/g)	APD (nm)	PV (cm3/g)	CV (cal/g)
UC	2.054 ± 0.002	23.984	0.844	2.262	4691 ± 11

Table 5

Colorimetric analysis.

	UC	RC	FA 1
L	28.45	31.25	10.34
a *	0.41	0.48	1.87
b *	0.36	0.19	8.78

¹ Raupp et al. (2022) [45]. a *: axis green/red chromaticity, b *: axis blue/yellow chromaticity.

Table 6

FRX results of the chemical composition of UC.

Element	UC
SiO2	45.20
Al2O3	14.20
Fe2O3	3.03
CaO	2.03
K2O	1.64
MgO	1.02
TiO2	0.559
MnO	0.0657

Table 7

Kinetic parameters for the MB adsorption onto UC.

PFO	qe (mg/g)	k1 (min−1)	R 2
	49.0858	3.5609	0.9996
PSO	qe (mg/g)	k2 (g/mg·min)	R 2
	49.4071	0.3080	0.9999

Table 8

Equilibrium isotherm parameters for MB adsorption onto UC.

Isotherm	Parametric	291 K	328 K
Langmuir	qmax (mg/g)	333.3333	476.1905
	kL (L/mg)	0.1209	0.2692
	RL	0.0136	0.0061
	R 2	0.7705	0.9194
Freundlich	nF	0.9151	1.5949
	KF (mg/g)(mg/L)−1/nF	5.4843	4.6511
	R 2	0.93	0.9164

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