Comparison and Contrast of Calcite vs. Dolomite after Heat Treatment to Enhance Toluidine Blue Removal from Water

Abstract

Significant increase in use of color dyes in modern society exerted a great pressure on environmental and water qualities. As such, studies for the removal of color dyes from water have been conducted extensively. In this study, common Earth materials dolomite and calcite were evaluated and contrasted for their removal of toluidine blue (TB), a cationic dye, before and after heat treatment. An increase by a factor of 3 in TB removal capacity from 3.5 to 10 mmol/kg was achieved after dolomite (Dol) was heated to 800 °C (designated as HDol). In contrast, the TB removal capacity increased by more than 100 times from 2 to 220 mmol/kg after calcite (Cal) was heated to 1000 °C (designated as HCal). For Dol and HDol, the TB removal increased as the solution’s pH increased but decreased with increases in the solution’s ionic strength. For Cal and HCal, the influence of the equilibrium solution’s pH and ionic strength on TB removal was negligible. The free energy of TB sorption on Dol, HDol, and Cal were −7 to −15 kJ/mol. The results suggested different removal mechanism for TB by Dol vs. Cal. X-ray diffraction data for Dol showed a slight increase in calcite content after heat treatment. For Cal, CaO was produced after heating, which converted back to calcite after 24 h of mixing with TB solutions. The significant TB removal by HCal could be attributed to its reaction with CaO. Thus, the best solution is to use freshly treated Cal for the removal of TB from solution.

1. Introduction

With the modern industrial development, the use of color dyes has increased dramatically over the last few decades. It is estimated that the global textile and clothing industry is worth about $480 billion and expected to reach $700 billion soon [1]. Often, the textile industry releases highly polluted coloring wastewater [1]. With the consumption of color dyes annually as high as 7 × 10⁵ tons just in textile industry, about 10–15% of the color dyes directly discharged into water systems [2]. As such, strategies and technologies have been developed to mitigate the potential threat of directly discharging dye-containing wastewater to the environment.

There are several approaches for dye removal from water, including physical, chemical, and biological. For chemical removal, catalyzed degradation is a common approach. However, the high material costs of the catalysts may limit its application. While for physical removal, sorptive removal is a common method. If the sorbents are low in material costs, high in removal capacity, and fast in removal rate, the dye removal by sorption may have its advantage over other approaches. Thus, finding an effective sorbent is becoming a common practice in research related to dye removal.

Toluidine blue (TB) is a phenothiazine dye. It is commonly used in areas of medicine, textile, and biotechnology [3]. It was used to determine the concentrations of sodium dodecyl sulfate (SDS), but the mechanism for the interaction was not mentioned [4]. It was found that at premicellar concentrations for SDS, the electrostatic interaction resulted in the formation of a dye (D⁺)-surfactant (S⁻) ion-pair (D⁺–S⁻) [5]. In another study, it was found that (i) ion-pair formation, (ii) pre-micellar aggregation, and (iii) micellar aggregation were responsible for the interactions between sodium lauryl ether sulfates (SLES) and TB and the limited solubility of SDS-TB ion pairs lead to precipitation of ion-pair clusters [6].

For TB sorption, most sorbents evaluated were derived from organic materials. For example, TB removal by orange peel waste was fast, with a capacity of 313 mg/g, and TB removal increases as the solution’s pH increased [7]. Composite materials were also tested for TB removal via sorption. A composite made of bentonite and char of sweet sorghum bagasse resulted in a TB sorption capacity of 9.4 mg/g [2]. In contrast, a composite material made of graphene oxide and bentonite achieved a TB sorption capacity of 458.7 mg/g [8]. Such a big difference (about 50 times) indicated more studies are needed to account for the cause of the difference.

Most of the methods for the removal of TB from water were in the area of photocatalytical degradation. For example, under UV irradiation, TB could be photocatalytically degraded when magnesium oxide (MgO) was present as a photocatalyst [9]. TB degradation under fluorescence irradiation could also be achieved when ZnO was present as a photocatalyst [10]. Also, TB could be photo-degraded in the presence of calcium oxide (CaO) produced from egg shells as the catalyst [11].

CaO produced from eggshells also showed efficient photodegradation of methylene blue (MB) in several studies [12,13,14,15]. MB and TB degradation could be achieved in 15 and 10 min with an initial concentration of 20 mg/L in the presence of CaO [11]. In another study, a photocatalyst derived from natural clam shell were evaluated for adsorption and photodegradation of organic dyes, including MB and Congo Red (CR), with their sorption capacities of 123.45 mg/g and 679.91 mg/g, respectively, but TB was not tested [16]. Degradation of MB and ciprofloxacin by CaO was tested in batch conditions by measuring the decrease in absorbance value at 665 nm with time; however, no degradation products were identified [17]. CaO was also tested for effective degradation of rhodamine B (RB) with a photodegradation activity of 63% in the first 120 min [18].

Earth materials were also studied extensively for their removal of environmental contaminants via sorption, due to their low material cost and high removal efficiency [19]. Gypsum was used as an inexpensive sorbent for the removal of TB from water and the results fitted to the Langmuir model with a monolayer sorption capacity 28 mg/g and the pseudosecond order model with a rate constant of 0.326 g/(mg-min) [20]. However, a sorption mechanism was not discussed.

Sorption of TB on bentonite alone reached a capacity of 18.4 mg/g, and the mechanism of TB removal was attributed to chemisorption [2]. In contrast, TB sorption reached a capacity of 5.8 mmol/g, or 1773 mg/g on a montmorillonite [21]. A Turkish zeolite had a TB sorption capacity of 55 mg/g [3]. Sorptive removal of TB by 1:1 layered clay minerals kaolinite and halloysite showed TB sorption capacities were about 25% more than their cation exchange capacity (CEC) values, with TB dimer or bilayer formation on the mineral surfaces, and N⁺ was involved in the electrostatic interaction between TB and negatively charged Si–O tetrahedral surface [22]. However, studies for sorptive removal or cationic dyes by carbonate minerals after heat treatment were limited.

Carbonated minerals exist extensively on the surface of the Earth. Calcite and dolomite are the major minerals for limestone and dolostone. Limestone is extensively quarried and heat-treated to make the raw materials for cement. As such, the material costs would be extremely low, and they are environmentally friendly and, thus, can be used extensively without worrying about causing environmental deterioration, if they can be used for contaminant removal from water.

In this study, the carbonate minerals calcite (Cal) and dolomite (Dol) were heat-treated to increase their performance for dye removal. Previous studies showed that their removal of alizarin red S (ARS), an anionic dye, increased significantly after heat treatment [23,24]. Thus, it is critical to evaluate their performance for the removal of cationic dye. In this article, TB removal by Dol and Cal before and after heat treatment was systematically evaluated under different physical and chemical conditions. In addition, the material properties before and after TB sorption were characterized using X-ray diffraction (XRD), Fourier transform infrared (FTIR) analyses, and Scanning electron microscopic (SEM) observations. The novelty of this study is in its aim to significantly improve the TB removal performance of Dol and Cal with heat treatment. The mechanism of TB removal on the heat-treated carbonates are discussed, and the results may provide further insights for future experiments to scale up the study or to remove other types of emerging contaminants.

2. Materials and Methods

2.1. Materials

The TB used has a CAS number of 92-31-9, a molecular weight of 305.82 g/mol, with chloride as the counter-ion. It is a planner molecule of 1.1 nm length and 0.7 nm width [25] (Figure 1). Its solubility is 30 g/L (Acros), and it has two pKa values of 2.4 and 11.6 [9].

The calcite and dolomite used were from the standard mineral collection of the Geosciences Department, University of Wisconsin, Parkside. XRD analyses confirmed they are pure mineral phases. They were ground to less than 200 meshes (<0.064 mm) using a mortar and pestle. Then, 6 g of the samples was put into each ceramic crucible, and 4 crucibles were placed into a muffle furnace for heat treatment. For dolomite, it was heated to 800 °C for 3 h. In contrast, calcite was heated to 1000 °C and maintained for 3 h. The samples were cooled down in the oven overnight afterward. The heated samples are named as HDol and HCal in this study.

2.2. TB Sorption Study

In all experiments, the solid mass used was 0.25 g, and the liquid volume was 10 mL. The mixtures were placed in 50 mL centrifuge tubes and shaken on a reciprocal shaker at a speed of 150 rpm for 24 h at room temperature (23 °C) for all experiments except those for kinetic and temperature studies. After shaking, mixtures were centrifuged for 15 min at a speed of 3500 rpm. The supernatant was then filtered using 0.45 μm syringe filters. The filtered supernatants were analyzed for equilibrium TB concentrations using a UV–Vis method. The amount of TB removed was calculated from the difference between the initial and equilibrium TB concentrations.

For the kinetic study, mixtures were shaken for varying amounts of time up to 24 h. For the temperature study, the mixtures were shaken at 23, 33, 43, and 53 °C. For the isotherm study, the initial concentrations varied up to 0.2 mM for Dol, HDol, and Cal, and up to 5.0 mM for HCal. For all other experiments, the initial concentration was fixed at 0.04 mM for Dol, HDol, and Cal, and 4.0 mM for HCal. For the pH study, the pH of the mixtures was periodically checked and adjusted with concentrated HCl or NaOH to make the equilibrium pH varied between 5 and 13 with an interval close to 1. For the ionic strength study, the final solution ionic strengths were set to NaCl concentrations of 0.001, 0.01, 0.1, and 1.0 M.

2.3. Instrumental Analyses

A UV–Vis spectrophotometer was deployed to measure the equilibrium TB concentrations with a wavelength set at 625 nm [20]. A Shimadzu IRXross spectrometer equipped with a quartz attenuated total reflectance (ATR) device was used to obtain the Fourier transform infrared (FTIR) spectra. Samples were scanned in the range of 400 to 4000 cm⁻¹ with a resolution of 2 cm⁻¹. The XRD patterns for the samples were obtained using a Shimadzu 6100 X-ray Diffractometer with a Ni-filtered CuKα radiation with a voltage of 30 kV and current of 40 mA. Samples were scanned from 20 to 60° (2θ) with a scanning speed of 2°/min.

For SEM observation, the samples were loaded onto 13 mm aluminum stubs first. Then, they were coated with carbon using a vacuum coater (Edwards 306A Coating System). A Hitachi S-4800 scanning electron microscope (Hitachi, Chiyoda, Tokyo) was used for SEM observation. Images were taken using a 10.0 kV accelerating voltage at a 15.0 mm working distance. As S is the essential element of TB, and Cl is the counterion of TC, they were scanned with an energy-dispersive X-ray spectroscope (EDS) (Bruker Quantax Esprit System) for selected samples to see their surface concentration and distribution.

3. Results and Discussions

3.1. TB Removal in Isotherm Studies

The experimental results of TB removal by Dol, HDol, Cal, and HCal were fitted to several isotherm models. The Langmuir isotherm model fitted the data best (Figure 2a,b). It has the following mathematical formula:

$$C_s={\displaystyle \frac{K_L{S}_m{C}_L}{1+K_L{C}_L}}$$

(1)

where C_L (mmol/L) and C_S (mmol/kg) are the TB concentration in solution and on solid at equilibrium, and S_m (mmol/kg) and K_L (L/mmol) are the Langmuir parameters, with the former reflecting the TB sorption capacity on solid and the latter affinity for the solid. To determine S_m and K_L by a linear regression, Equation (1) can be converted into the following:

$${\displaystyle \frac{C_L}{C_s}}={\displaystyle \frac{1}{K_L{S}_m}}+{\displaystyle \frac{C_L}{S_m}}$$

(2)

The fitted results are S_m = 3.5 and 10.5 mmol/kg and K_L = 329 and 23 L/mmol for TB removal by Dol and HDol, respectively. In comparison, the S_m and K_L values are 2 and 220 mmol/kg and 74 and 113 L/mmol for TB removal by Cal and HCal, respectively (

Table 1. Parameters of TB removal by Cal and Dol before and after heat treatment from isotherm and kinetic studies.

Sorption Parameters	Cal	HCal	Dol	HDol
Sm (mmol/kg)	2	220	3.5	10.5
KL (L/mmol)	74	113	329	23
qe (mmol/kg)	2	160	3	6
Kqe2 (mmol/kg-h)	0.3	10,000	4	2
k (kg/mmol-h)	0.06	0.38	0.54	0.06
r2 for pseudo-second-order fitting	0.995	0.98	0.99	0.97

). The S_m value for HCal is much bigger than 7–40 mmol/kg for TB sorption on a polymer/clay composite [26], 92 mmol/kg on gypsum [20], 30 mmol/kg on bentonite [2], 126 mmol/kg on zeolite [27], and 47 and 149 mmol/kg on kaolinite and halloysite [22], suggesting that HCal is a good sorbent for the removal of TB from water just from the isotherm study data.

As MgO and Cal were produced after Dol was heated to 800 °C, and CaO was produced after Cal was heated to 1000 °C, the sorption of TB on MgO and CaO was also conducted for the isotherm study, and the results are plotted in Figure 2c,d. Removal of TB by MgO followed the Langmuir isotherm with a capacity of 63 mmol/kg. In contrast, the TB sorption on CaO followed a linear isotherm with the TB distribution coefficient K_d value of 7300 L/kg. The significant difference in TB sorption by MgO and CaO confirmed the difference in TB sorption between HDol and HCal. Thus, the products after heating played a significant role in contaminant removal.

3.2. Kinetic Study of TB Removal

The kinetics of TB removal by both minerals before and after heat treatment can be seen in Figure 2e,f. Several kinetic models were used to fit the experimental data. The pseudo-second-order kinetics, which as the following form, fit the data best.

$$q_t={\displaystyle \frac{{kq}_e^{2}t}{1+k{q}_{e}t}}$$

(3)

Equation (3) can be rearranged into a linear form:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k{q}_e^2}}+{\displaystyle \frac{1}{q_e}}t$$

(4)

Thus, the kinetic parameters can be determined with a linear fitting. The parameters in Equations (3) and (4) are the rate constant k (kg/mmol-h), initial rate kq_e² (mmol/kg-h), and q_t and q_e (mmol/kg) are the amounts of TB removed at time t and at equilibrium. The fitted q_e values are 3 and 6 mmol/kg for TB removal by Dol and HDol. In contrast, the fitted q_e values are 2 and 160 mmol/kg for TB removal by Cal and HCal (Table 1). The initial rates and the rate constants are 4 and 2 mmol/kg-h and 0.54 and 0.06 kg/mmol-h for TB removal by Dol and HDol, respectively. In comparison, they are 0.3 and 10,000 mmol/kg-h and 0.06 and 0.38 kg/mmol-h for TB removal by Cal and HCal, respectively. These results also showed significant increase in TB removal kinetically by HCal. Compared to other Earth materials, the q_e and k values were about 90 mmol/kg and 200 g/mol-s for TB sorption on a zeolite [3], 25 mg/g and 0.56 g/mg-min for TB sorption on bentonite [2], 96 mmol/kg and 0.1 kg/mmol-h on another zeolite [27], and were 35 and 143 mmol/kg and 0.3 and 0.7 kg/mmol-h for TB sorption on kaolinite and halloysite [22].

3.3. TB Removal as Influenced by Equilibrium Solution pH, Ionic Strength, and Temperature

Influence of the solution’s pH on TB removal by Dol and HDol is illustrated in Figure 3a. The difference between Dol and HDol is negligible. Below pH 8, the amount of TB removed was about 1–3 mmol/kg on both materials and above 8, the TB removal increased up to 8 mmol/kg. In comparison, the influence of solution pH on TB removal by Cal and HCal was minute, although the amount of TB removal contrast significantly, with about 150 mmol/kg by HCal, in comparison to 1 mmol/kg by Cal (Figure 3b). Thus, the TB speciation may play a negligible role in its removal by these minerals. In comparison, TB sorption on halloysite was about 146–150 mmol/kg as equilibrium solution pH increased from 4 to 11 [22]. TB sorption on gypsum increased slightly when pH increased from 3.5 to 6.5 and decreased significantly when the pH further increased to 9 [20]. On montmorillonite, TB sorption increased from 30 to 100% with an increase in initial solution pH from 2 to 12 [21]. The solution’s ionic strength also had a minimal influence on TB removal, except at very high ionic strength of 1 M NaCl, under which the TB removal decreased slightly for HDol (Figure 3c,d).

Equilibrium temperature on TB removal is illustrated in Figure 3e,f. The K_d is the TB distribution coefficient between the solid and the solution and is related to the thermodynamic parameters by the following:

$$\ln K_d=-{\displaystyle \frac{\Delta H}{RT}}+{\displaystyle \frac{\Delta S}{R}}$$

(5)

in which the changes in enthalpy and entropy after TB removal are represented by ∆H and ∆S, while the gas constant and the equilibrium temperature in K are represented by R and T. The relation between free energy of sorption ∆G and ∆H and ∆S is the following:

$$\Delta G=\Delta H-T\Delta S$$

(6)

The calculated values are listed in

Table 2. Thermodynamics of TB removal by Dol, HDol, Cal, and HCal.

Minerals	∆G° (kJ/mol)				∆H° (kJ/mol)	∆S° (kJ/mol-K)
	296 K	306 K	316 K	326 K
Dol	−6.98	−7.73	−8.47	−9.22	15.17	0.075
HDol	−10.47	−12.05	−13.63	−15.21	36.29	0.158
Cal	−9.35	−10.10	−10.84	−11.59	12.80	0.075
HCal	10.25	10.03	9.80	9.58	16.89	0.022

. Overall, the ∆G values were between −7 and −9 kJ/mol for Dol, −9 and −12 kJ/mol for Cal, −10 and −15kJ/mol for HDol, suggesting that TB removal was endothermic. In contrast, the ∆G value was about 10 kJ/mol for TB removal by HCal. The ∆S° values were 0.02 to 0.16 kJ/mol, indicating more random orientation of TB on the mineral surfaces. The ∆H° values were 13 to 36 kJ/mol, indicating endothermic TB removal, and temperature influenced TB removal by HDol more (Table 2).

3.4. XRD Analyses

The XRD patterns showed that dominant mineral was dolomite for Dol and HDol with or without TB sorption, although a slight increase in d₍₁₀₄₎ peak of calcite could be seen (Figure 4a,b). Similarly, the dominant mineral was calcite for Cal and HCal after equilibrated with DI water or with TB solution for 24 h (Figure 4c,d). These results suggest no mineral-phase changes after TB sorption. However, there was a significant phase difference for the HCal before and after it was in contact with DI water. For HCal, before it was in contact with DI water, the mineral phase was made almost entirely of calcium oxide (Figure 4d). However, after contact with DI water or TB solution for 24 h, the solid phase changed from CaO to calcite (Figure 4d). In a previous study, the XRD pattern of the TB was mainly amorphous, with some small peaks at 31.7, 26.9, 25.1° (2θ) [22]. The TB peaks are not observed in these XRD patterns.

3.5. FTIR Analyses

The FTIR spectra of Dol, HDol, Cal, and HCal after TB removal are illustrated in Figure 5. Overall, the FTIR spectra of Dol and Cal are similar, except that the locations of some bands are at different wavenumbers. The ν₃ and ν₄ bands, corresponding to asymmetric stretching and in-plane banding [28], or asymmetric stretching and symmetric deformation of CO₃²⁻ [29], for calcite were at 1432 and 712 cm⁻¹, in comparison to 1480 and 730 cm⁻¹ for dolomite [30]. And the ν₄ bands at 730 cm⁻¹ was due to CO₃²⁻ vibration and was used to determine the dolomite quantitatively [31]. In this study, they were at 1418 and 727 cm⁻¹ for Dol and HDol and at 1394 and 712 cm⁻¹ for Cal and HCal. Their locations did not change after in contact with TB solutions of different concentrations, suggesting that CO₃²⁻ may not participate in the interactions with TB molecules. Or maybe the amount of TB on mineral surfaces were quite low and the bands would not be visible.

The FTIR spectrum of TB was also illustrated in Figure 5. The most important band at 1595 cm⁻¹ was assigned to the aromatic ring [32]. In this study, this band did not show up after TB being sorbed on Dol, HDol, Cal, and HCal. For Dol, Cal, and HDol, the amount of TB removed was low, with the highest removal by HDol was only 0.3%. But for HCal, TB removal was 6.7% at its removal capacity. Still, the band is missing, suggesting that the TB removal might not be via a sorption on HCal surfaces. Instead, it could be due a chemical reaction (Figure 5), suggesting that further work is needed to elucidate the reaction pathways of TB removal by HCal.

3.6. SEM Observation and EDS Analyses

Morphology and particle-size-wise, they were about the same for Dol and HDol and Cal and HCal (Figure 6a–d). The particle size was in the range of 0.5–5 µm per SEM observation as these are hand ground samples (Figure 6a,c). Slight increases in grain size could be observed for HDol and HCal (Figure 6b,d). This difference could be due to different grinding batches. Overall, the specific surface area was 0.5–2.0 m²/g based on BET specific surface area analyses. Also, the {104} cleavages are well developed for all samples regardless of whether they are Dol or HDol or Cal or HCal, agreeing well with the phase identified via the XRD and FTIR analyses. The EDS scan of element Cl showed similar density (Figure 6e–h), suggesting that the participation of Cl in TB removal was minute. Similarly, the S density was about the same for all samples after TB sorption (Figure 6i–l). If sorption was the only TB removal mechanism, the S density for HCal would be 100 times more than that of Cal. The similarity in S density between Cal and HCal may also suggest that major removal of TB by HCal was via a reaction.

3.7. Discussion

TB may form monomers and dimers, depending on its concentration and components in the media. The dimerization constant K_D is expressed as follows:

$$K_D={\displaystyle \frac{\left[\mathrm{D}\right]}{{\left[\mathrm{M}\right]}^2}}$$

(7)

in which the concentrations of dimers and monomers are represented by [D] and [M]. The K_D in distilled water was reported as 10,000 M⁻¹ [33] or 10,500 M⁻¹ [34]. A much lower value of 3311 M⁻¹ was also reported [35]. Using the value of 10,000 M⁻¹ and initial TB concentration of 5 mM, the [M] and [D] are 0.6 and 2.2 mM. At the lower end, when initial concentration is 0.2 mM, the [M] and [D] are 0.1 and 0.05 mM. If the K_D value of 3311 M⁻¹ was used, these values changed to 1.0 and 2.0 mM when the initial concentration was 5 mM and 0.14 and 0.03 mM when initial concentration was 0.2 mM. Thus, under high initial TB concentrations, when HCal was used, the dominant TB was in dimer form, and the TB removed could also be in dimeric form if the removal was via sorption. For Dol, HDol, and Cal, if the removal of TB was via sorption, the TB removed would be mostly in monomer forms.

It was reported that when dolomite was heated to 800, 900, and 1000 °C, a weak chemical interaction via inner-sphere complexation intervened the interactions between nonbonding electrons of nitrogen related to the amino groups and the surface oxides of MgO and/or CaO [36].

Micro-sized flower-like MgO particles prepared by a facile precipitation method through the reaction between Mg²⁺ and CO₃²⁻ at 70 °C showed excellent photocatalytic degradation of various dyes, including MB, Congo red, and other dyes, and their mixture [37]. In another study, significant photocatalytic degradation of various dyes, such as brilliant green, MB, crystal violet, methyl orange (MO), and brilliant blue, could be achieved using MgO nanoparticles under visible irradiation, with a removal efficiency varying from 40 to 90% and a capacity of 63.9 mg/g for brilliant green sorption on MgO [38]. Also, MgO was studied for the removal of MB and methylene violet (MV) in single and binary solutions via photocatalytical degradation, with a removal efficiency of 97 and 88% for MB and MV from a binary solution [39]. However, an extensive literature survey resulted in only one article dealing with TB photodegradation using MgO as a photocatalyst, but under a dark condition, about 15% of TB sorbed on MgO [9]. In this study, no difference was found for TB removal by MgO under a lighted condition in comparison to a total dark condition (Figure 2c). Thus, the slight increase in TB removal by HDol in comparison to Dol could be attributed to minute production of MgO and calcite, as indicated by the XRD analyses.

CaO was studied extensively for the photodegradation of various contaminants. For the degradation of MB, 0.4 g of CaO and 0.02 mmol/L MB solution were tested under different physico-chemical conditions, and the free radical of ●OH was responsible for MB degradation under an irradiated condition [40]. However, the volume of the solution was not mentioned in the study. Also, ●OH was attributed to phytocatalytic degradation of indigo carmine (IC), an anionic dye, by CaO [41]. In a more recent study, the degradation of MB by CaO produced from coffee and eggshell wastes reached a removal efficacy of 88% [42]. However, the mass ratio of CaO to MB was 75 mg/0.5 mg (50 mL of 10 ppm solution) or 150:1 in their study. In a more recent study, CaO for phytocatalytic degradation of IC was tested under different physico-chemical conditions [43], but the degradation products were not identified, and the amount of CaO to dye used was about 20 times (30 mg and 20 mL of 80 mg/L solution). Again, only one study in an extensive literature survey was found for the degradation of TB by CaO with the optimized degradation conditions of pH 9, and a catalyst dosage of 50 mg for 100 mL TB solution at 20 mg/L [11]. At this TB concentration and CaO dosage, adsorption accounted for 35 to 60% of TB removal in 1 h, and the remaining by degradation [11]. In this study, 0.25 g of HCal and 10 mL of concentrations up to 5 mM, or 1500 mg/L, were used. The TB removal increased more than 150 times when HCal was used in comparison to Cal. In addition to CaO, calcium hydroxide (Ca(OH)₂) also show good MB and MO removal from solution via sorption [44]. In all these studies, no XRD was performed for the samples after the reaction. In this study, the ratio of CaO to TB was 250 mg to 15 mg, and the removal was also significant. The XRD showed that the CaO was the product after the calcite was heated to 1000 °C for 3 h. After being mixed with DI water or TB solution for 24 h, calcite was formed and CaO disappeared. This reversal reaction may be responsible for the significantly increased TB removal by HCal, a new approach for the advanced utilization of Earth materials for environmental application. Similarly, no difference in TB removal by CaO under lighted vs. dark conditions was noticed, suggesting that photocatalytical degradation of TB by CaO may not be a major pass way. Again, the results suggest further detailed study for the identification of other degradation products is needed.

4. Conclusions

This study evaluated the enhanced TB removal by dolomite and calcite after heat treatment. The results showed that TB removal increased drastically (by more than 100 times in TB removal capacity) after calcite was heat-treated. In contrast, heat treatment of dolomite increased TB removal slightly (by 3 times in TB removal capacity). This may be due to the formation of different products after heat treatment. For calcite, calcium oxide was produced, and when mixed with DI water or TB solution, calcium oxide converted back into calcite. Meanwhile, TB was removed from the solution. The FTIR results showed that no TB bands were found on the solid samples after its removal from aqueous solution, suggesting that TB may not be present on the surface of HCal or in the solution. The EDS scan of S and Cl showed essentially no difference among the four samples, suggesting no additional accumulation of TB on the surface of HCal. Thus, the TB disappearance from solution could be resulted from its reaction with calcium oxide. Thus, results from this study warrant a further in-depth investigation of TB degradation by CaO produced from the heat-treatment of calcite, which could expend the utilization of carbonated Earth materials for further environmental application.