1 Introduction 

Methylene blue (MB) is a phenothiazine cationic dye that has many uses. It is used in textile, paper and other dyeing and printing industries. It also acts as chemical indicator and is used in biological staining, and pharmaceutical and food industries (Oz et al., 2011; Su & Nan, 2016; Subramaniam & Ponnusamy, 2015). Reports elucidated that up to 15% of the dye gets lost in the drain during textile dyeing process (Kabra et al., 2012). The leakage of MB from industrial wastewaters may lead to serious environmental problems. The dye can expend the dissolved oxygen in water and thus increase chemical oxygen demand (COD), biological oxygen demand (BOD), total organic carbon (TOC), and toxic chemical compounds in wastewater (Bulc & Ojstršek, 2008). Also, MB threatens the life of marine organisms by preventing light permeation through water (Abdelrahman et al., 2019). Thus, the removal of MB from wastewater becomes environmentally significant.

Different techniques are investigated for elimination of MB from aqueous media (Santoso et al., 2020). The most used manners are enzymatic degradation (Kishor, 2021), photocatalytic degradation (Karuppusamy et al., 2021), electrochemical methods (Fadillah et al., 2019), ozonation (Valdés et al., 2009), and adsorption methods (Gemici et al., 2021; Ghosh et al., 2021; Tuli et al., 2020). The adsorption techniques are superior to the other methods as it shows higher efficiency for treatment and removal of organic contaminants in wastewater treatment. Adsorption has many advantages including simple design, low cost, and efficiency (Mortada et al., 2023). However, most effective adsorbents are financially expensive or consume time and effort for preparation and modification (Seera et al., 2021).

Recently, clays and minerals have been announced for adsorption of dyes. As examples kaolin (Mouni et al., 2018), zeolite (Oukil et al., 2020), diatomite (Mohamed et al., 2019), bentonite (Bergaoui et al., 2018), natural phosphate (Hafdi et al., 2020), and kaolinite clay (Anoop Krishnan et al., 2015) show good adsorption activity toward different types of dyes. These naturally occurring minerals are widely distributed in the world and can be easily obtained in large amounts. Therefore, attention should be focused on the use of these compounds as adsorbents. The natural clays were also used to remove other pollutants from different samples. For example, orange-clay modified by humic acid and 2-mercaptobenzoxazole was employed for removal of Fe(III) from water samples (Dev et al., 2022). The modification of natural clays improves the adsorption activity toward phenols (Alkaram et al., 2009; Xue et al., 2013), organic dyes (Anirudhan & Ramachandran, 2015), and metal ions (Abdel Ghafar et al., 2020).

Dolomite is a sedimentary rock which consists mainly of calcium magnesium carbonate (CaMg(CO3)2). The typical structure of dolomite lattice composed of CO3−2 layers in-between alternating layers of Ca2+ and Mg2+, with varied amounts of Al2O3, SiO2, and Fe2O3 as impurities (Warren, 2000). It is used in ceramics, refractory brick, glass and metallurgical industries (Sadik et al., 2016). The ore powder has good adsorption properties toward potentially toxic substances such as heavy metals (Diwan et al., 2020; Mohammadi et al., 2015), fluoride (Wongrueng et al., 2016), phosphate (Gao et al., 2013), surfactants (Sanati & Malayeri, 2021; Sun et al., 2020), gases (Tao et al., 2021), and dyes (Shirazi et al., 2019, 2020; Ziane et al., 2018).

To meet the needs of the modern world, we need to provide a creative prospective way for disposal of MB from wastewater by an eco-friendly, effective, and cheap procedure. Review of literature showed that there are no studies on removal of MB using dolomite and therefore the current work aimed to explore the use of Egyptian dolomite powder as a felicitous sorbent for disposal of MB from aqueous medium. The parameters that may impact the removal process (i.e., the sample solution pH, clay dose, stirring time, surfactant type and amount, and ionic strength) were investigated to attain the better efficiency.

2 Experimental

2.1 Materials

Dolomite was collected from the deep layer of Ain Sokhna red sea mountain (Gulf of Suez region) (29°36′0″N, 32°19′0.12″E) in the mid of October 2021. The pulverized sample used during this study was obtained through ball mailing of the bulk sample. According to Bosworth study, the Gulf of Suez area is approximately 30 million years old (Bosworth, 2015).

For solution preparation and dilution, double distilled water was employed. Analytical grade chemicals were obtained from Merck (Darmstadt, Germany) or Sigma-Aldrich (St. Louis, MO, USA) and utilized without further purification. MB stock solution (1000 mg L−1) was made by dissolving 1.0 g in double distilled water. Working solutions of various concentrations were prepared daily and utilized in the adsorption investigation as well as creation of the MB calibration curve. Acetate buffer (pH 3.0–6.0), phosphate buffer (pH 7.0–8.0), and borate buffer (pH 9.0–10.0) were used to control the pH of the solution during the adsorption process.

2.2 Instrumentation

X-ray diffractometer Philips PW 1390 (Eindhoven, Netherlands) was used to record the XRD diffractogram with Cu target and K line running at 30 kV-10 mA and 1.540 Å wavelength. Scanning electron microscope paired with energy-dispersive X-ray analysis (SEM/EDX) was proceeded on a scanning electron microscope (Quanta 250FEG, FEI, USA). Fourier-transform infrared spectrum (FT-IR) was obtained using KBr disc (4000–400 cm−1) on a Thermo-Nicolet IS10 FT-IR spectrometer (Nicolet Instrument Co, Madison, WI, USA). The concentration of MB was determined spectrophotometrically using 7300 Genway spectrophotometer (Cole-Parmer Ltd., Staffordshire, UK). Metrohm 632 digital pH meter (Metrohm Autolab, Herisau, Switzerland) was used to measure the pH.

2.3 Adsorption Experiments

The batch procedure was used for the adsorption investigations. The effect of pH was investigated by mixing 50 mg of dolomite with 100 mL of aqueous solution containing fixed amount of MB (Ci) at different pH (3.0–10.0). The contents were stirred for 90 min at ambient temperature and the supernatant was separated by filtration. The remained concentration of MB in the filtrate (Ce) was measured by spectrophotometry at 665 nm (Mohamed et al., 2019) and the adsorption capacity (qe) was calculated by Eq. (1):

$$q_e\;({mg\;g}^{-1})=\frac{\left({\mathrm C}_{\mathrm i}-{\mathrm C}_{\mathrm e}\right)V}m$$
(1)

where V is the initial volume of solution in liter and m is the mass of dolomite in grams.

To study the effects of other parameters on the adsorption process, the conditions were adjusted as follows: sorbent dose ranging from 0.1 to 2.5 g L−1, stirring time ranging from 10 to 180 min, surfactant concentration ranging from 0.1 to 0.5% w/v, and NaCl concentration ranging from 0.1 to 1.0 mol L−1. Each experiment was carried out in triplicate and by varying one parameter and the others are kept constant. Figure 1 presents the general schematic procedure for removal of MB.

Fig. 1
figure 1

Schematic diagram for the removal of MB by dolomite 

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The adsorption isotherm for aqueous solutions of MB was performed by shaking 50 mg of dolomite in bottles containing 100 mL of MB solutions at various concentrations (10–180 mg L−1).

Following the adsorption process, dolomite was filtered and washed with double distillated water to remove the excess of MB. Thereafter, the sorbent was contacted with 5.0 mL of desorbing agents: (ethanol, acetone, NH4OH or NaOH). The desorption percentage (D) can be determined using Eq. (2):

$$\mathrm D\;(\%)=\frac{{\mathrm C}_{\mathrm d}{\mathrm V}_{\mathrm d}}{{(\mathrm C}_{\mathrm i}{-\mathrm C}_{\mathrm e})\mathrm V}\mathrm x100$$
(2)

where Cd is the concentration of desorbed MB (mg L−1), Vd is the volume of the eluent (L), and V is the initial volume of the solution (L).

3 Results and Discussion

3.1 Characterization of the Studied Mineral

X-ray diffraction analysis of the studied dolomite ore powder in combination with the suggested crystal structure is shown in Fig. 2. Characteristic sharp bands attributed to the sedimentary calcium magnesium carbonate phase originally located at 23.2, 29.4, 30.1, 36, 39.5, 43.1, 44.8, 47.3, and 48.6° were perceived and assigned to their corresponding Miller indices as earlier stated in JCPDS ASTM card NOS. 5–0586 (Kaczmarek, et al., 2017; Zucchini et al., 2014).

Fig. 2
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a XRD pattern and b crystal structure of dolomite sample

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SEM micrograph of dolomite supported by EDX, and mapping is shown in Fig. 3. Image analysis implies the homogenous crystalline structure of the dolomite. The percentage composition of each component is shown in Table 1 in combination of their assignment and attribution. EDX analysis for the studied dolomite sample detects the presence of both carbon (0.2774 keV) and oxygen (0.525 keV) in addition to other lines characterizing the presence of both calcium and magnesium in different states.

Fig. 3
figure 3

a SEM micrograph supported by b EDX and c mapping

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Table 1 EDX energies of the studied sample components 
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Figure 4 represents the FTIR spectral absorption data of dolomite. FTIR of pure dolomite Mg(CaCO3)2 was previously reported and assigned (Abdullah, 2021; Ji et al., 2009). The carbonate groups in the ore were characterized through the strong bands that appeared at about 2870, 2512, 1795, 1425, and 710 cm−1. Characteristic peaks were assigned to their function groups as reported in Table 2.

Fig. 4
figure 4

FTIR spectral data for the dolomite sample

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Table 2 FTIR band position and assignment
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3.2 Factors Affecting Adsorption Process

3.2.1 Initial pH of the Solution

One of the most critical factors that strongly influence the dye adsorption process is the initial pH of the solution. MB is a cationic dye and its ionization in the solution is pH dependent. Moreover, the adsorption of the charged species is influenced by charge of the sorbent surface which is mostly affected by the solution pH. Figure 5 displays the impact of pH (3.0–10.0) on the adsorption capacity of MB onto dolomite. As is evident, the adsorption capacity increases as the solution pH increases and achieves a maximum value at pH 8.0. Below this value the adsorption capacity decreases owing to the probable competition between the positively charged dye and the hydronium ions on the active centers of the sorbent (Arabpour et al., 2021). Dolomite has an isoelectric point (pHpzc) of 6.3; therefore, its surface becomes more negative as the pH of the solution increases (Gence & Ozbay, 2006). This enhances the adsorption of the cationic dye through the electrostatic bonding (Mohamed et al., 2019). When the pH of the sample solution exceeds pHpzc, the surface of the clay becomes negatively charged, promoting the adsorption of MB. At lower pH, the surface of the clay is positively charged and thus apt to repel dye cations. Accordingly, the adsorption process is proceeded at pH 8.0 through the study.

Fig. 5
figure 5

Effect of pH on the removal of methylene blue by dolomite

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3.2.2 Dose of the Sorbent

Another key influencing factor in the removal of pollutants from water is the relative quantity of sorbent. The effect of dolomite dose (in g L−1) on the adsorption capacity toward MB was investigated using 100 mL model containing 50 mg L−1 of MB. As presented in Fig. 6, the adsorption capacity of MB obviously increases with increasing sorbent dose from 0.1 g L−1 to 1.0 g L−1. These findings may be attributed to the increase of the available active adsorption sites on dolomite by increasing the sorbent dose. Above 1.0 g L−1 of dolomite dose, the adsorption capacity did not significantly enhance indicating that the maximum adsorption of MB by dolomite attained an optimum value at 1.0 g L−1. At equilibrium, the available adsorption sites of dolomite were almost loaded by the dye molecules; thence, any additional increase of the sorbent dose did not improve the adsorption efficiency. As a result, the sorbent dose value of 1.0 g L−1 was selected to perform the other experiments.

Fig. 6
figure 6

Effect of dolomite dose on the removal of methylene blue

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3.2.3 Stirring Time and Kinetic Study

The impact of stirring time on the adsorption capacity of MB by dolomite was examined by mixing 100 mg of the sorbent with 100 mL of a solution containing 20 mg L−1 of MB and pH 8.0. The containers were set in a mechanical shaker (200 rpm) for 180 min. At predefined periods, samples were taken from the supernatant to be analyzed for MB. Figure 7 shows that the adsorption capacity of dolomite toward MB enhances by increasing the contact time. The curve shows a fast adsorption rate within the first 45 min followed by a comparatively slow process, and ultimately it reaches equilibrium. The adsorption rate exceeds 80% of the maximum value after 45 min and attains equilibrium after 90 min. An increase in stirring duration beyond 90 min had no improvement effect on adsorption capacity. The initial rapid adsorption efficiency can be assigned to the considerable concentration gradient and availability of more active sites (Mortada & Abdelghany, 2020). Accordingly, 90 min shaking time was selected as the optimum equilibrium time.

Fig. 7
figure 7

Effect of stirring time on the adsorption of methylene blue by dolomite

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3.2.4 Presence of Surfactant

Generally, the surfactant acts as an emulsifier and can promote the adsorption process by decreasing the interaction between solid particles and hence the available active sites are maximal (Yegya Raman et al., 2016). Therefore, we investigated the effect of Triton X-114 and sodium dodecyl sulfate (SDS) as non-ionic and anionic surfactants, respectively. The results in Fig. 8 display that the addition of Triton X-114 and SDS increases the adsorption capacity from 7.8 mg g−1 to 11.5 mg g−1 and 15.1 mg g−1, respectively. There was no significant impact of the concentration of the surfactant on the adsorption process. As expected, the anionic surfactant (SDS) improved the adsorption capacity compared to the non-ionic one. The anionic surfactant molecules can arrange themselves around the sorbent particles forming micelles. The hydrophobic tails of the surfactant interact with the hydrophobic regions on the sorbent surface. On the other hand, the hydrophilic heads direct themselves outward. As a result, the presence of SDS can consolidate the adsorption of MB by dolomite increasing the negative charges on the surface. According to these results, SDS was added at concentration of 0.1% (w/v) during the adsorption experiments.

Fig. 8
figure 8

Effect of surfactants on the adsorption of methylene blue by dolomite

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3.2.5 Ionic Strength

Dye-containing wastewater generally has a high salt concentration, and the ionic strength effect is of great significance in studying the adsorption of dyes to adsorbents (Benaïssa, 2010). The impact of ionic strength of the solution on the adsorption of MB by dolomite was investigated using different amount of NaCl (0.1–1.0 mol L−1). The findings indicated that the ionic strength had no considerable strength on the adsorption capacity in the studied concentration range (Fig. 9).

Fig. 9
figure 9

Effect of ionic strength on the adsorption of methylene blue by dolomite

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3.2.6 Adsorption Isotherm

In the current work, Langmuir and Freundlich models were used to explore the equilibrium features of MB adsorption. Based on the Langmuir equation (Mohamed et al., 2019), the adsorption of MB will only take place at certain homogeneous sites on the surface of dolomite, and the energy level distribution is constant. The Langmuir isotherm is mathematically expressed as:

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{max}}}+ \frac{1}{{\mathrm{q}}_{\mathrm{max}}{\mathrm{K}}_{\mathrm{L}}}$$
(3)

The model can be utilized to determine the maximum adsorption capacity (qmax) and the Langmuir constant (KL) by plotting Ce against Ce/qe. The results that were summarized in Fig. 10 and Table 3 show that qmax was decreased by increasing the temperature indicating an exothermic process. The estimated qmax values were 22.2, 20.4, 16.1, and 14.2 mg g−1 at 25, 35, 45, and 55 °C, respectively. Table 4 shows comparison between dolomite and other sorbents with respect to the adsorption capacity of MB.

Fig. 10
figure 10

a Adsorption isotherm. b Linear Langmuir plots of methylene blue onto dolomite

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Table 3 Langmuir isotherm parameters at different temperatures
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Table 4 Adsorption capacities of various adsorbents toward methylene blue
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The favorability of the adsorption process can be assessed by calculating the Langmuir isotherm equilibrium parameter (RL) as follows:

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{i}}}$$
(4)

Based on the value of RL, the isotherm is defined as favorable based on RL values, if (0 < RL < 1) favorable, (RL > 1) unfavorable, linear (RL = 1), or irreversible (RL = 0). The estimated RL values at 25, 35, 45, and 55 °C were 0.51, 0.19, 0.34, and 0.29, respectively, suggesting that the adsorption is favorable.

The Freundlich equation for heterogeneous surface energy systems is given by Eq. 5

$${\ln\;q}_e={\ln\;K}_F+\frac1n{\ln\;C}_e$$

where KF and n are Freundlich constants that were identified from the ln qe versus ln Ce plot. The sorption capacity and intensity of the system are related to the parameters KF and 1/n. The value of 1/n indicates the favorability of the sorption process (Foo & Hameed, 2012). The linear relation between log (Ce) versus log (qe) shows high fitting of the adsorption results with a high determination coefficient (R2 = 0.999) for both using Triton X-100 and without using Triton X-100 (Table 5). The degree of non-linearity between the dissolved ion concentrations and the adsorption was related to the heterogeneity factor (1/n). From the slope of the linear plotting, the heterogeneity factor 1/n value was (0.359) when using Triton X-100 (less than unity) which reflected the chemical adsorption of dye and refers to a heterogeneous surface with minimum interactions between the adsorbed ions; on the other hand, 1/n value was 2.16 when Triton X-100 was not used (more than unity) which refers to a heterogeneous surface with maximum interactions between the adsorbed ions. Thus, the adsorption of methylene blue represented well by multi-layer adsorption rather than by monolayer adsorption when using Triton X-100.

Table 5 Freundlich isotherm parameters for the adsorption of methylene blue onto dolomite
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3.3 Desorption of Methylene Blue from Dolomite

Desorption of MB from dolomite was studied using different agents. The sorbent after the adsorption process was separated and rinsed three times by double distilled water and stirred with 5.0 mL of the eluent for 15 min. MB is acidic dye so desorbing agents employed in this work were basic eluents as acetone, ethanol, NH4OH, and NaOH. Moreover, the use of acids may cause dissolution of the sorbent. The effectiveness of the solvent was evaluated by the desorption percentage of the dye (Eq. 2). The results clearly show that the desorption percentages of MB were 14.6 ± 3.3, 26.6 ± 5.2, 46.5 ± 4.2, and 96.5 ± 2.6% for acetone, ethanol, NH4OH (0.5–1.0 mol L−1), and NaOH (0.5–1.0 mol L−1). The use of higher concentrations of NH4OH or NaOH did not significantly improve the efficiency of the desorption. The poor desorption of acetone, ethanol, and NH4OH is owing to the strong interactions between the active sites of dolomite and MB. Results from the reusability study are shown in Table 6. As obviously presented, the adsorption capacity progressively reduces and reaches 67.7% of the initial value after the fourth cycle indicating that dolomite can be effectively employed as an adsorbent for MB under the prescribed condition for 3 cycles. Therefore, the use of strong basic eluent NaOH as desorbing agent allows the reusability of the sorbent for three times without significant reduction in the adsorption capacity.

Table 6 Adsorption capacity of dolomite toward methylene blue versus the number of adsorption–desorption cycles
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4 Conclusion

Dolomite has high ability to remove MB from aqueous media with high efficiency under the optimum conditions. The maximum adsorption of MB occurred at pH 8.0, 90 min shaking time, at ambient temperature and in the presence of 0.1% SDS. Adsorption of MB onto dolomite occurred with a maximum adsorption capacity of 22.2 mg g−1 at 25 °C which reflects higher adsorption capacity toward MB when compared with various adsorbents. The same amount of dolomite can be used for adsorption MB several times after elution with NaOH with nearly the same adsorption efficiency. Our findings suggest that the raw can be effectively utilized for the adsorption of MB from aqueous solutions.