Maghara coal was used as a raw material. It was obtained from the Maghara coal field, located in Sinai City, Egypt. Maghara coal was crushed and sieved to a uniform size range of 0.4–0.75 mm and stored in closed bottles. Maghara coal was successfully imperginated by sodium hydroxide (NaOH) as an activating agent for production of activated carbon (AC). 5 gm of Maghara coal was added to NaOH solution with the concentration in the range of (10–50%) (Mass bases). The mixing process was done at 85 °C and kept for 2 h at 80 rpm. The resulting homogeneous slurry was dried at 110 °C for 6 h. The produced samples were carbonized in a muffle furnace under nitrogen (flow rate 9 L/h) at various carbonization temperature within the range of 300–700 °C. The carbonization time was maintained for 90 min before cooling under N2. The produced carbon material was mixed with NaOH solution at different impregnation ratios (IR), defined as:

After cooling the carbonized products treated with NaOH, the samples were washed using 250 ml of 0.1 M HCl solution at 85 °C for 30 min, followed by filtration. The acid-washed samples were then leached by mixing with 250 ml of distilled water at 85 °C for several times until the pH value of the water-carbon mixture was about 6–7. Finally, they were dried at 110 °C for 2 h.

A known quantity of activated carbon sample was placed, in series, in 250 ml flasks containing 200 ml of methylene blue (MB) solution with different concentrations in the range of 10–50 mg/L. The flasks were then shaken at constant temperature (25 ± 2 °C) until the equilibrium state after 1 h. Subsequently, the carbon materials were removed by centrifugation at 10,000 rpm for 10 min. Adsorbate concentrations in the filtrates were determined using UV–vis Spectrophotometer (Libra S.11) at wavelength λmax = 650 nm, using a calibration curve. MB uptake at equilibrium, qe (mg/g), was calculated by

The optimum of the synthesizd AC sample was characterized by transmission electron microscopy (TEM) (JEOL JEM-100CX, Japan with an accelerating voltage of 80 kV), scanning electron microscope (SEM) (JEOL JSM 6360LA, Japan), Fourier transform infrared (FTIR) (Spectrum BX 11 spectrometer FTIR LX 18–5255 Perkin Elmer), Raman spectroscopy (SENTERRA, spectrometer-Bruker, Germany) with a 532 nm Ar laser, Thermo gravimetric analysis (TGA) (Shimadzu TGA-50 instrument), and Brunauer–Emmett–Teller (BET) surface area were conducted at −200 °C using BelsorbminiII, BEl inc., Japan.

A 0.15 g of prepared material was mixed with 50 ml of 0.1 M NaCl. The solution pH was adjusted to remain within the range between 1 and 12 using 0.01 M NaOH and/or 0.01 M HCl. Equilibration was realized by shaking in a thermostatic bath for 24 h at 25 °C. The dispersions were then filtered and the final pH of the solutions were measured using digital pH meter (Jenway Ltd, Dunmow, Essex CM6 3LB, U.K).

The economic feasibility of the dye adsorption process onto the prepared nano-AC was determined through the regeneration of MB contaminated AC. Various desorbing agents such as 0.1 M HCl, 0.1 M NaOH, 0.1 M NaCl and distilled water were tested to determine the most efficient eluent. The AC material loaded with dye was separated by centrifugation from the treatment media and washed with distilled water, dried over night at 60 °C and the concentration of the residual dye solution was determined using spectrophotometer. Specific weight from nano-AC contaminated with dye of 0.5 g was mixed with 50 ml of eluent solutions for 90 min at 400 rpm. After elution, the material was separated using centrifugation and the MB concentration in the analyte was determined using spectrophotometer. The amount of MB desorbed (% desorption) for each eluent type was estimated by using the following relationship;

Maghara coal represents a cheap resource for the production of valuable adsorbent material. Therefore, it can be used as a starting material for production of cheap and highly efficient AC by simple manner. The proximate analysis is to determine the moisture, ash, volatiles matter and fixed carbon of Maghara coal that makes it suitable precursor for obtaining AC. The high and low values of volatile matter (50.6%) and ash content (4.12%), respectively in Maghara coal indicated that it is suitable to be imperginated into AC.

The content of AC in the activation product is comparatively higher than that produced from carbonization products without any chemical treatment. This is due to the action of NaOH that destroys the bond C–H to form amorphous carbon easily to be activated [14]. This chemical activation process is followed by a carbonization process, which is to increase the contents of carbon and oxygen, then decrease hydrogen and nitrogen contents. However, after the carbonization process, which the partial decomposition of volatiles compounds and degradation of organic substances yielded high pure carbon material [14]. Fig. 1 shows the variation of material weight losses during the carbonization process of Maghara coal into AC at temperature up to 700 °C. The weight loss occurs at the temperature range 100–300 °C. The decrease of weight at 300 °C is mainly due to the evolution of water and hydrogen [15,16]. The hydrogen release may be due to the substitution of –H groups in coal structures with –ONa during the chemical impregnation process with NaOH [15]. One step of the substitution reactions was the oxidation of coal by NaOH to form Na2CO3 and Na2O can also result in the release of hydrogen. It can be seen from Fig. 1 that the weight of the sample was rarely affected by increasing the carbonization temperature from 300 to 600 °C. The suppression of coal evolution probably resulted from the rapid formation of cross-linked network triggered by the interactions between the –H and –ONa groups located on different polymer chains in coal [15]. At temperatures above 600 °C the reactions between carbon atoms and alkaline salt complexes (such as Na2CO3 and Na2O) become significant, leading to carbon gasification and thus the formation of carbon dioxides. Therefore, the optimum carbonization temperature for Maghara coal in the range the temperature range of 300–600 °C. This is in an agreement with literature which indicate that Na+ is moved forcefully to separate the graphene layers hence creating the material porosity at high carbonization temperatures [17].

Fig. 1.

Weight loss during carbonization of NaOH-treated Maghara coal.

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It was realized that, during the increase at the carbonization temperature, Na+ was moved forcefully to separate the graphene layers hence creating the material porosity. This reaction is a function of temperature; therefore at low temperature, it is not effective for the formation of ACs [17]. On the other hand, at high carbonization temperatures, the formed alkaline surface complexes (Na2CO3 and Na2O) through their interaction between NaOH and carbonaceous percursors represent active sites for material gasification. The carbon gasification by Na2CO3 and Na2O through these active sites leads to the loss in the carbon yield at high carbonization temperatures. Accordingly, the carbonization of NaOH impregnated carbon material either at low or high temperatures is not favored for production of high yield of AC material.

In order to attain effective AC from Maghara coal, various parameters have an influence on the modification process such as activation temperature (350–600 °C), percentage of sodium hydroxide (10–50%), activation time (30–120 min), impregnation ratio (20–35/1), and particle size (0.478, 0.569, 0.659, 0.75 mm) were investigated. Also, the optimum conditions for reaching the high efficient removal of dye the used adsorbent amount was equal to 0.2 g at initial MB dye concentration of 50 ppm (200 ml), solution pH = 7 and contact time = 1 h and adsorption temperature of 25 ± 2 °C. The study of each parameter was carried out at the selected optimized predetermined parameter.

3.3.1Effects of activation temperature

The effects of activation temperature on the adsorption performances of produced AC materials is presented in Fig. 2. It was found that the adsorption capability of the yielded ACs significantly increased with increasing activating temperature. This may be due to the increase in materials' surface area with increasing activation temperature. The high specific surface area predicates the presence of high amounts of −OH groups on the surface of the produced AC [18]. On the other hand, at high carbonization temperature, the surface oxygen groups tend to be removed completely which increases the surface basicity of produced ACs [19]. It is indicated from Fig. 2 that the most proper produced ACs that attain the highest dye adsorption capacity of 15 mg/g is produced at activation temperature of 550 °C. It also is indicated that the improvement of the activation temperature above 550 °C decreases the adsorption capacity of the produced ACs material. This can be attributed to the domination of pore widening effect than the pore-opening effect, which results in a decrease of specific surface areas of the materials and micropore volume, and further increase of mesopore and macropore volume [20].

Fig. 2.

Effect of activation temperature on the adsorption capacity of produced AC for MB dye (10% NaOH of coal, 25 ± 2 °C).

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3.3.2Effects of active agent concentration

The main cost of the AC production is the consumption of the activating agent. Activation by NaOH is cheaper compared with other chemicals utilized and more environmentally friendly than KOH-AC [21]. NaOH is an effective activating agent for disordered carbon materials through surface reactions, where highly defective sites are present [22].

Moreover, the concentration of the activating agents represents an important parameter for the adsorption capacity of the carbon material. Maghara coal itself has very poor dye adsorption capability that increased remarkably after the activation process. It is indicated from Fig. 3 (a) that the MB dye adsorption capacity of produced ACs is increased with increasing the concentration of sodium hydroxide in solution. This improvement of adsorption capacity might be due to the formation of carbonates and subesequently formation the large density of basic groups on the surface of AC through sodium hydroxide reaction on the surface of ACs that explained from (Eqs. 4–7)[22,23]. The formation of Na2CO3 in abundance during activation showed an indication of the effectiveness of the activation process. So, the optimum concentration of sodium hydroxide was 50%.

Fig. 3.

Influence of a) NaOH concentrations as activation agent; b) activation time; c) activation reagents ratio of NaOH/C; and d) coal particle sizes at temperature 550 °C onto the adsorption capacity profile of produced activated at various initial concentrations at 25 ± 2 °C.

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The high reactivity between the activating agent and carbon precursor was attributed to the high release of volatile components associated with the improvement in the textural properties of ACs [24]. More pores of carbon were created through the alkaline activation process that were associated with the redox reactions and oxidative modifications responsible for the degradation and/or separation of graphitic layers resulting in the development of micro and mesoporous AC materials [25].

Generally, the AC adsorbs different molecules via electrostatic, Van der Waals interaction, and/ or hydrogen bonding, depending on the functional groups of the AC and the adsorbed molecules [26]. Therefore, the higher adsorption capability of the AC obtained from sodium hydroxide activation can be attributed also to the abundance of hydroxyl (−OH) groups on its surface, these hydroxyl groups can catalyse the adsorption capability of ACs toward the cationic MB dyes by forming hydrogen bond between hydroxyl (−OH) groups of ACs and the dye molecules [18].

The morphological structure of both Maghara coal and the optimum prepared NAC impregnated with NaOH are shown in Fig. 4 (A-1) and (A-2), respctively. It is observed that the particle size of the raw Maghara coal is reduced to nano-size of the prepared activated carbon with spherical shape and average diameter of 38 nm (Fig.4 (A-2)). This result indicates that the activation process of Maghara coal at the optimum predetermined conditions is capable to produce nano-activated carbon (NAC) impregnated with NaOH. Also, optimized NAC is characterized by its high total pore volume of 0.183 cm3/g compared with 0.033 cm3/g for its parent Maghara coal. Moreover, the average pore diameter of the prepared AC is decreased to 14.7 nm compared with 10.7 cm for its parent Maghara coal. These changes at the pore structure of prepared AC may be attributed to the evaporation of impregnated NaOH derived compounds from the parent coal after the carbonization process, which leaves the space previously occupied by the reagents [30]. The suggested reaction between NaOH and carbon material at higher temperatures is presented in Eq. 4 (described in section 3.3.2). The sodium carbonate molecules are entrapped and create a wider hole inside the material structure. These entrapped molecules are responsible for microporosity of produced AC material due to the release of H2, CO and CO2, gases which are generated during the decomposition of Na2CO3, respectively.

Fig. 4.

A) TEM micrograph, B) FT-IR, C) Raman spectra, and D) TGA of both raw Maghara coal and the prepared NAC respectively.

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In spite of the alkaline treatment, it enhances the surface functionality of produced NAC, which is reflected on its adsorption affinity toward water pollutants. It is, however, detrimental to the physical aspects of AC such as the BET surface area. The alkaline treatment reduces the BET surface area of the pristine coal that is measured as 116 m2/g to be 49 m2/g of the prepared AC. It may be attributed to the destruction of the porous structure of coal [31]. It is expected under alkaline treatment that the hydroxide anion will react with the surface functional groups of AC. This was verified previously at the literature, where AC treated with NaOH showed a major increase in the concentration of phenolic functional groups on the surface [32]. This result was confirmed from the EDS analysis of the prepared AC. It is evident that the prepared material is composed mainly from 70.43% carbon with 18.6% oxygen, 8.24% sodium contents and traces from both nitrogen (0.91%) and sulphur (1.82%). This analysis confirms the NaOH impregnation of prepared AC.

The FT-IR spectra of both Maghara coal and the optimum prepared NaOH impregnated NAC are displayed at Fig. 4 (B-1) and (B-2), respectively. The broad absorption band of 3419 cm−1 at the two materials is ascribed to the OH stretching vibration due to the existence of inter- and intra-molecular hydrogen bond of polymeric compounds (macromolecular associations). These function groups are originated from alcohols, phenols and carboxylic acids, thus, showing the presence of "free" hydroxyl groups on the adsorbent surface [33]. The peaks at 2939 cm−1 for the two materials may be assigned to various C–H symmetric and the asymmetric stretching vibration of methyl groups, indicating that hydrogen is largely removed during the activation process to produce NaOH impregnated AC [31]. The signal at 2353 cm-1 at the two materials are identical to the vibration of −COOH structure [32]. There is new, specific and strong peak which emerged at 1635 cm−1 that was observed for prepared AC material. This peak reflects the CC stretching of aromatic rings whose intensity is enhanced by the presence of oxygen atoms as phenol or ether groups [27,28]. The peak at 1429 cm−1 in the two materials spectra is attributed to the presence of some carboxylates (CO) and asymmetric group. The peaks at 1057 and 873 cm−1 of the two materials indicated the presence of C–O (anhydrides) and C–H derivatives in the aromatic rings, respectively [30]. In addition, the band at 588 cm−1 is attributed to out-of-plane angular deformation of aromatic rings [34]. On the other hand, The specific structure nature of both Maghara coal and produced NAC can be further elucidated by the Raman spectroscopy. Fig. 4C indicates that the two materials have two characterstics peaks, one is sharp G-band at 1516 cm−1 (Fig. 4 (C-1)) for Maghara coal that shifted to 1582 cm−1 (Fig.4 (C-2)) for prepared AC. The other broad D-band is around 1302 cm−1 at Maghara coal that shifted to 1361 cm−1 for prepared AC. Accodingly, it was indicated that the characterstics peaks of prepared AC were shifted toward the higher wave numbers when compared with the raw Maghara coal. These two characterstics peaks are generally ascribed E2g and A1g in-plane vibration modes, respectively which is corresponding to the disordered carbon materials or defective graphitic structures [35]. In order to eludicate the thermal stability of the prepared NAC, TGA profiles of both Maghara coal and the prepred material were compared at Figures (4 (D1 and D2)). The prepared NAC has displayed typical Maghara coal behavior. The prepared NAC and its orgin coal that have lost their water content up to 100 °C exhibits no mass loss until 200 °C. After 250 °C, the mass loss rate had increased upon the removal of volatiles from the sample. After almost 420 °C, the degree of mass losses decereased and two materials were reached to a steady level about 600 °C. Also, it is evident from Figures (4D1 and 4D2) that the total mass losses of the prepared NAC had reached 7.6% compared to 29.2% of its origin Maghara coal that confirms its thermal stability. So, the prepared NAC has high thermal stability and thus preferred over the raw coal precursor as suitable adsorbent [36].

Mostly, both Langmuir and Freundlich models are used to describe the equilibrium data of adsorption and conclude the adsorption mechanism. The general linearized form of Langmuir equation [37] is

Fig. 5.

(a) Langmuir and (b) Freundlich isotherms of MB sorption onto AC prepared at the optimum condition of activation (NaOH concentration = 50%, carbonization temperature = 550 °C, NaOH/C ratio = 35/1, coal particle size =0.478 mm) at 25 ± 2 °C. 3.6. Adsorption mechanism.

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The general linear of the Freundlich isotherm is given by [38]

AC is an amphoteric material, which can be positive or negative charged depending on the solution pH. The adsorbent surface is negatively charged as a result of NaOH impregnation, which increases the electrostatic attraction between the positive adsorbate species and the adsorbent particles, which increases MB adsorption. According to the above discussion, the activation process and the adsorption mechanism can be described at Fig.6. MB is a cationic dye, which exists in aqueous solution in the form of positively charged ions. Cationic dye adsorption is favored at pH > pHpzc, due to the presence of functional groups such as OH− groups on the adsorbent agent. The point of zero charge (pHpzc) of prepared NAC is determined as 6.7. The dye solution pH being 7 represents the favored conditions for MB adsorption onto the prepared NAC. In this situation, the surface of prepared NAC material is rich with hydroxyl groups as a result of the activation process of Maghara coal with high amounts of NaOH at 550 °C under inert nitrogen atmosphere. The negative charge of the as-prepared AC can intensively attract the dye species with the positive charge [39,40]. Thus, MB dye is a cationic dye and strongly attractive through the electrostatic interaction of MB cations with negatively charged carbon material functionalized with OH groups.

Fig. 6.

Schematic diagram of optimized NAC activation process and its adsorption of MB dye.

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The adsorption interaction between the AC and MB dye molecules after dye adsorption process is confirmed using both SEM and FTIR of NAC adsorbed dye. SEM micrograph of NAC (Fig. 7 (A)) investigated the aggregation of the nano-activated due to the adsorption action of the dye molecules. In this situation, the previously separated nano-particles of AC illustrated at Fig. 4 (A-2) were agglomerated into larger scale due to the adsorption action of the dye molecules that bind the nano-particles together to produce large aggregates. These results confirm the adsorption of MB onto the prepared AC. Moreover, comparing the FTIR spectrums of NAC before (Fig. 4 (B-2)) and after (Fig. 7 (B)) dye adsorption process, a small shift was noticed for all characterstis peaks of NAC after the dye adsorption. This shift of peaks suggested the interactions of dye molecules with the functional groups of adsorbent material according to the previous described mechanism. Furthermore, a new characterstics peak was illustrated at the FTIR spectrum of NAC after the dye adsorption process (Fig. 7 (B)). This new peak was assigned at 1373 cm−1 that identical to NO, C–N corresponding to the MB dye molecules. This peak confirms the adsorption of dye molecules onto prepared AC [41]. So, from the SEM and FTIR results of NAC after the adsorption process, the interaction between MB dye molecules and prepare NAC was proofed to take place according to the suggested adsorption mechanism.

Fig. 7.

A) SEM micrograph, and B) FT-IR, of the prepared NAC after dye adsorption process respectively.

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Economically, the ability of an adsorbent material to be reused represents one of the paramount characteristics of efficient adsorbent material. Therefore, the reusability of the spent NAC after dye adsorption process was investigated using various eluents, and their ability for dye desorption are represented in Fig. 8. The results showed that HCl is the strongest potential eluent to desorb MB dye molecules from NAC compared with the other desorbing agents used. The percentage of MB desorption was 94% using HCl as desorbing agent, this was due to its ability to easily release the physisorbed dye molecules from the MB loaded adsorbent surface. Also, by using acidic desorbing agent, the surface of NAC will receive more H+ ions from the solution and hence the cationic exchange of MB ions occurs more rigorously. At lower pH values, while treating the material with HCl (as eluent), the surface of NAC is allowed to have frequent interactions with H+ present in the bulk, which triggers the surface protonation. On the other hand, both NaOH and NaCl had low desorption potential of NAC and the percentage dye desorption recorded were 14 and 18% respectively, this can be correlated to the poor reusability. However, when distilled water was used as eluent, the MB desorption from NAC was relatively high (up to 54%). These results are compatabile with findings of other researches [42]. Hence, it was concluded that MB dye is easily to be removed from the prepared NAC surface using HCl as an eluent agent.

Fig. 8.

Regeneration of nano-activated carbon (NAC) using various eluent solutions (V = 50 ml, material dosage=0.5 g, pH = 7, agitation speed = 400 rpm, solution temperature = 25 °C, desorption time = 90 min).

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In order to confirm that the regenerated active sites at the regenerated NAC have the ability to be reused and host dye molecules, the HCl regenerated NAC was tested for 10 cycles of MB dye adsorption. It was indicated from Fig. 9 that the dye adsorption efficiency showed negligibly small decrease from 98.7 (cycle 1) to 96.2% (cycle 10) for the regenerated NAC. This result may be due to incomplete MB desorption from the surface of NAC when repeatedly used after regenerated with HCl. Thus, this result confirms the efficient reusability and stability of prepared NAC for dye adsorption after its regeneration with HCl. Thus the prepared NAC showed excellent performance in the adsorption as well as in desorption processes. Consequently, the prepared NAC may be classified as an economical and efficient adsorbent agent, which is crucial for the industrial and large scale practical applications. Hence, the utilization of NAC as adsorbent agent will reduce the operational cost of the adsorption process as well as minimize the environmental hazard (secondary pollution) [43].

Fig. 9.

Reusability of nano-activated carbon (NAC) using HCl desorption solution (V = 50 ml, material dosage=0.5 g, pH = 7, agitation speed = 400 rpm, solution temperature = 25 °C, dye concentration = 50 mg/l, adsorption time = 90 min).

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In order to compare the efficiency of the prepared NAC with the other low-cost ACs, the maximum calculated monolayer adsorption capacity (qm) values were compared. Table 2 shows the comparable adsorption capacity of prepared NAC impregnated with NaOH for MB with respect to other low-cost ACs. Consequently, the prepared NAC from Maghara coal showed a suitable and promising result for MB removal from aqueous solutions since it has a relatively high adsorption capacity.