Conversion of Malaysian low-rank coal to mesoporous activated carbon:Structure characterization and adsorption properties

Ali H.Jawad ,Khudzir Ismail ,Mohd Azlan Mohd Ishak ,Lee D.Wilson

1 Faculty of Applied Sciences,Universiti Teknologi MARA,40450 Shah Alam,Selangor,Malaysia

2 Faculty of Applied Sciences,Universiti Teknologi MARA,02600 Arau,Perlis,Malaysia

3 Department of Chemistry,University of Saskatchewan,Saskatoon,Saskatchewan S7N 5C9,Canada

ABSTRACT Malaysian Selantik low-rank coal(SC)was used as a precursor to prepare a form of mesoporous activated carbon(SC-AC)with greater surface area(SA)via a microwave induced KOH-activation method.The characteristics of the SC and SC-AC were evaluated by the iodine number,ash content,bulk density,and moisture content.The structure and surface characterization was carried out using pore structure analysis(BET),scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX),X-ray diffraction(XRD),Fourier Transform Infrared(FTIR),elemental analysis(CHNS),thermogravimetric analysis (TGA),and determination of the point of zero charge (pHPZC).These results signify a mesoporous structure of SC-AC with an increase of ca.1160 times (BET SA=1094.3 m2·g-1) as compared with raw SC without activation(BET SA=1.23 m2·g-1).The adsorptive properties of the SC-AC with methylene blue(MB)was carried out at variable adsorbent dose(0.2-1.6 g·L-1),solution pH(2-12),initial MB concentrations (25-400 mg·L-1),and contact time (0-290 min) using batch mode operation.The kinetic profiles follow pseudo-second order kinetics and the equilibrium uptake of MB conforms to the Langmuir model with a maximum monolayer adsorption capacity of 491.7 mg·g-1 at 303 K.Thermodynamic functions revealed a spontaneous endothermic adsorption process.The mechanism of adsorption included mainly electrostatic attractions,hydrogen bonding interaction,and π-π stacking interaction.This work shows that Malaysian Selantik low-rank coal is a promising precursor for the production of low-cost and efficient mesoporous activated carbon with substantive surface area.

Keywords:Coal Activated carbon Mesoporous material KOH activation Microwave irradiation Methylene blue

1.Introduction

Activated carbons (ACs) possess favorable textural properties(surface area and pore structure) with various functional groups depending on the mode of preparation.AC has versatile utility in applications relevant to gas separation/storage,solvent recovery,super capacitor electrodes,catalyst supports,and adsorbents for organic and inorganic waterborne pollutants [1].The high cost of AC production limits its widespread application in various technologies.To address this limitation,low-cost carbonaceous precursors have been reported as alternatives to generate AC such as lignocellulosic materials [2],biopolymers [3],coal [4],char [5],and fruit peels [6].In fact,the textural properties and adsorption capacities of AC are strongly dependent on the nature of the starting martial,activation method,type of activator,and preparation conditions [7,8].

AC can be generated by two preparation methods involving physical or chemical activation.Physical activation involves carbonization of a carbonaceous precursor followed by activation of the resulting char in the presence of activating agents such as CO2or steam [9].Chemical activation involves impregnation of the raw precursor with activating agents such as metal salts [8],mineral acids[10-12],bases[13],and,nitride polymers[14],polymers with furfuryl alcohol [15],and metal-organic frameworks[16].The dehydrating effect of the chemical activators inhibit the formation of tar which leads to AC materials with greater carbonization and enhanced pore structure.Chemical activators also decrease the activation temperature and activation time in comparison to the physical activation method [17].Recent work indicates that microwave irradiation offers a rapid and alternative method to prepare,modify,and regenerate AC materials since heat is produced within the carbon particle and the energy is generated in situ at the molecular-level scale via dipole rotation and ionic conduction[18,19].By comparison with other activation methods,microwave radiation has rapid and uniform temperature rise,along with reduced time and energy demands [20].KOH is widely used in AC preparation from many types of carbonaceous precursors due to its many desirable properties such as a narrow pore size distribution with well-developed porosity,along with the eco-friendly properties of KOH [13].The use other low-cost and locally available coal as raw precursors from reliable sources has increased for the preparation of AC with large surface area (SA).

Coal formation results from physical and chemical processes(bacterial decay,compaction,heat and time)and involves agglomeration of complex mixtures of hydrocarbons[21].Coal consists of a wide variety of organic and mineral phases that contain H,S,O,and N that vary from coal deposit locations even within a common seam[22].In Malaysia,coal reserves are estimated ca.1712 million tons for various ranks from lignite to anthracite [23].The total known Malaysia coal reserve covers about 7324 km2[24].Malaysian Selantik coal(SC)is a low rank coal,mainly located in Sarawak state of east Malaysia.However,the high content of volatile matter,oxygen and moisture in this coal creates challenges for its wide industrial utilization.

Generally,low-rank coal is not a suitable energy source used for generating electricity since it is nonporous with low surface area(SA) which limits its practical application.Therefore,utilization of a low-rank coal as promising precursor for developing AC with large SA will aide in the development of potential applications such as super capacitor materials and/or a porous adsorbent for water and wastewater treatment processes.To the best of our knowledge,development of AC with large SA from low-rank Malaysian coal has not yet been investigated.Therefore,this work aims firstly to prepare a novel low-cost AC with a large SA from locally available Selantik low-rank coal (known as SC) by microwavesupported preparation and KOH activation.Secondly,this work aims to evaluate the adsorptive properties of SC-AC for removal of a model cationic dye (methylene blue;MB) from aqueous solution.Herein,MB was chosen as a dye probe due to its recalcitrant nature and the associated challenges for its removal from wastewater by conventional techniques such as biological treatment and chemical precipitation [25,26].Adsorption-based removal has advantages that relate to its facile design and operation,chemical selectivity,and versatile utility [27,28].The resulting SC-AC was characterized by use of BET,SEM-EDX,XRD,FTIR,CHN,TGA,and pHPZCmethods.

2.Materials and Methods

2.1.Chemicals

Methylene blue (MB) was obtained from R &M Chemicals,Malaysia,with a molar mass of 373.9 g·mol-1,molecular formula of C16H18ClN3S3·H2O and moderate water solubility of 40 g·L-1.HCl,KOH,NaOH,iodine,sodium chloride,sodium thiosulfate and potassium bromide were of analytical grade quality and purchased from reliable suppliers,where all chemicals were used without further purification.

2.2.Preparation of activated carbon

Malaysian Selantik low-rank coal (SC) sourced from Sarawak state of east Malaysia was selected as a precursor in this work.The samples were ground and sieved to 0.5 mm-0.85 mm.Raw SC was washed and rinsed with hot distilled water to eliminate adherents until the filtrate remained clear.SC was then dried in a vacuum oven for 24 h.The SC activated carbon(SC-AC)was prepared by mixing KOH with SC using an impregnation mass ratio of 1:3.This ratio was optimized based on the highest iodine number obtained with occasional stirring.The sample was activated by using a quartz glass reactor (7.5 cm diameter-8.5 cm length)sealed at the bottom and open from the top side to allow escape of pyrolysis gases.Pyrolysis was carried out in a modified microwave oven (model Panasonic NNJ-993) with a fixed power of 600 W for 20 min in the presence of nitrogen gas(99.995%)at a frequency of 2450 MHz,where the irradiation power and time of activation were pre-determined as the optimum activation conditions,where a description of the microwave-assisted pyrolysis system was reported elsewhere [29].The obtained AC was washed using 0.1 mol·L-1HCl,and rinsed repeatedly with distilled water until the filtrate reached a neutral pH.The resulting AC,referred to as Selantik coal AC (SC-AC) was oven dried at 100°C for 24 h.The SC-AC was stored in tightly closed bottles for further use.The SCAC yield (%) was calculated by Eq.(1):

2.3.Material characterization

2.3.1.Iodine test and bulk density

The iodine number indicates the porosity of the activated carbon and it is defined as the amount of iodine adsorbed by 1 g of carbon at the mg level,which was determined by a standard method[30].The bulk or apparent density was determined according to the Lubrizol Standard Test Method [31].

2.3.2.Ash content and moisture content

The ash content was determined by heating the samples inside an electrical muffle furnace using an ASTM standard method [32].The moisture content was determined by an oven drying method as described by Adekola and Adegoke [33].

2.3.3.Elemental analysis

Elemental analyses (C,H,N,and S) of the samples were measured by using a CHNS-O Analyzer(Flash 2000,Organic Elemental Analyzer,Thermo-scientific),where the oxygen content was calculated by difference.

2.3.4.X-ray diffraction (XRD) analysis

X-ray diffraction analysis(XRD)was performed on coal in order to determine the crystallinity or amorphous nature before and after activating process by X-ray diffraction (XRD) using Cu Kα radiation with a PANalytical,X’Pert Pro X-ray diffractometer.Scans were recorded with a scanning rate of 0.59(°)·s-1.The diffraction angle (2θ) was varied from 10° to 90°.

2.3.5.N2adsorption-desorption analysis

Nitrogen adsorption-desorption was carried out using a BET Sorptometer,BET-201A analyzer at 77 K.The specific surface area(SA)was determined by the BET method and the total pore volume was calculated by nitrogen adsorption at p/p0=0.995,where the pore size distribution was determined from the adsorption branch using the Barrett-Joyner-Halenda (BJH) theory.

2.3.6.FT-IR spectral analysis

Infrared spectra of the adsorbents were obtained using a Fourier transform infrared spectrophotometer at room temperature over a 4000-500 cm-1spectral range(Perkin Elmer,Spectrum One,FT-IR Spectrometer).Samples for FT-IR analysis were finely ground and mixed with KBr (Merck) in a mass ratio of 1:100 and pressed into a translucent pellet.

2.3.7.Thermogravimetric (TGA) analysis

Thermogravimetric experiments were performed using a thermobalance TGA/SETARAM,Instrumentation,model:SETSYS Evolution.The analysis was performed using a previously reported method [34].

2.3.8.SEM-EDX analysis

Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) analysis was performed to examine the surface morphology and elemental content of carbonaceous samples.The samples were placed on carbon tape and coated with a thin layer of gold-palladium in an argon atmosphere using a Agar Sputter Coater.

2.3.9.pHPZCanalysis

The pH at the point-of-zero charge(pHPZC)was estimated using a pH meter (Metrohm,Model 827 pH Lab,Switzerland),as described elsewhere [35].

2.4.Batch adsorption experiments

Batch mode experiments were performed using screw cap closed 250 ml Erlenmeyer flask with a water bath shaker (Memmert,water bath,model WNB7-45,Germany) at 90 strokes·min-1and 303 K until equilibrium was attained.The common variables of interest such as adsorbent dosage (0.2 to 1.6 g·L-1),pH (2 to 12),initial dye concentration (25 to 400 mg·L-1) and contact time(0 to 290 min)were used to investigate the optimum conditions for dye sorption.The required initial pH of MB solution was obtained by using HCl(0.10 mol·L-1)or NaOH(0.1 mol·L-1)with a pH meter(Metrohm,Model 827 pH Lab,Switzerland).The samples were taken at variable time intervals and the dye concentration was determined accordingly using a HACH DR 2800 Direct Reading Spectrophotometer at λmax661 nm.Similar procedure was adopted at the temperature of 313 and 323 K with the other reaction parameters kept constant to study the thermodynamic functions.The adsorptive uptake of MB at equilibrium,(mg·g-1)and the present of color removal (CR,%) was determined by Eqs.(2) and (3),respectively.

C0is the initial dye concentration (mg·L-1) prior to adsorption;Ceis the residual dye concentration at equilibrium (mg·L-1) after adsorption;V is the volume (L) of dye solution;and W is the dry mass of the adsorbent(g).Adsorption experiments were conducted in duplicate and are reported as average values.All kinetics and isotherm models were tested by performing fits against the experimental data to the nonlinear equations using a scientific data analysis and graphing software package,SigmaPlot 11.

3.Results and Discussion

3.1.Physical characteristics

The results of physical characterization (bulk density,iodine number and proximate analysis)of raw SC and SC-AC are outlined in Table 1.The key characteristic is the iodine number of SC-AC after activation (1019.8 mg·g-1) compared against raw SC(38.2 mg·g-1)without activation.The difference may be attributed to the ability of KOH for producing highly porous AC relative to the limited pore structure of raw SC.Table 1 also shows a relatively high product yield 48.80% of SC-AC that exceeds a value (46.99%)reported by Gao et al.[36]for AC prepared from coal by conventional pyrolysis with KOH activation.

Table 1 Physicochemical properties of the SC and SC-AC

3.2.Structural studies

3.2.1.Elemental analysis

The elemental compositions (CHN) of the raw SC and SC-AC materials are listed in Table 1.The use of KOH used as an activator has significant influence to produce a form of activated carbon(SCAC) with high ratios of C/H and C/N.In the simultaneous microwave pyrolysis and activation process,SC releases volatile species from the carbonaceous product.The ratio of C/H (25.7) of SC-AC was nearly twice as high (13.2) versus SC,as shown in Table 1.In fact,KOH plays a key role in coal matrix modification.Under microwave irradiation,metallic potassium is produced and is intercalated into the carbon matrix,and is attributed to further carbon gasification and release of gaseous products such as CO2,CO and H2[37].

3.2.2.Thermogravimetry analysis

The thermal stability of raw SC and SC-AC was examined using TGA-DTG as shown in Fig.1a-b.The thermal decomposition profile of SC (Fig.1a) has one major weight loss event ca.80% near 500°C and relates to the thermal release of organic and volatiles from the SC matrix,in agreement with results reported for Malaysian low rank coal [23].Moreover,the mass loss near 500°C is close to 80%,and confirms that pyrolysis of the raw coal precursor without an activator does not yield notable carbon production[1].On the other hand,the TGA profile of the activated coal (SC-AC)shows a key mass loss event ca.25% near 650°C (Fig.1b).Thus,SC-AC exhibits greater thermal stability over SC that relates to the catalytic hydrogen bond breaking reactions in the coal framework due to the presence of KOH,resulting in the evolution of light vapors and H2[37].The TGA data are in parallel agreement with the elemental analysis results.

3.2.3.FT-IR spectral analysis

Fig.1.TG-DTG curves for SC (a) and SC-AC (b) at heating rate of 10°C·min-1.

Fig.2.FT-IR spectra for SC (a),SA-AC (b),and SC-AC after MB adsorption (c).

Fig.2 shows the FT-IR spectra of several carbonaceous materials(SC,SC-AC,and SC-AC after MB adsorption).Fig.2a shows the IR spectrum of raw SC,the broad IR bands at 4000-3500 cm-1relate to the stretching vibration of hydrogen-bonded hydroxyl (-OH)groups,which support the presence of carboxyl,phenol or alcohol groups on the surface of SC[38].The IR band at 2300-2400 cm-1is associated with the CO2formation [39]and the signature at～1650 cm-1relates to C=O stretching vibration of an amide group(amide I)[40,41].The IR band at～1350-1300 cm-1(methyne C-H bending) [42],while that at 1000-1300 cm-1relates to C-O stretching in acids,alcohols,phenols,ethers,and/or ester groups[43].The bands at 750-600 cm-1are assigned to alcohol,OH outof-plane bend [42].The evidence obtained from Fig.2b (SC-AC)indicates that an obvious modification of SC occurs due to significant reduction in the band intensities of(-OH),(C=O),and(C≡C)which are ascribed to the catalytic bond breaking (O-H and C-H bonds) via an activation process with KOH.After MB adsorption on SC-AC surface (Fig.2c),new IR bands appear and many functional groups are either frequency shifted or attenuated.New bands assigned to anhydride (～1680) and -NO2(～1383 cm-1)are attributed to MB.The band intensities at～3400 cm-1are clearly reduced and clear shift of bands at～1300 cm-1can be also observed.Thus,the functional groups of SC-AC that likely contribute to interactions with MB cations are the O-H and -COOH Lewis base sites of the adsorbent.

3.3.Surface characteristics

3.3.1.BET analysis

The N2adsorption/desorption isotherms for SC and SC-AC are shown in Fig.3a and b,respectively.The textural parameters of raw SC and SC-AC in Table 1 reveal that SC-AC is a porous material with large BET SA (1094.3 m2·g-1).By contrast,raw SC is nonporous with a very low SA(1.23 m2·g-1),where the large difference in textural properties between each material is attributed to KOH activation.An increase (ca.890-fold) in SA and pore volume (PV)of SC-AC over the raw precursor (SC) is noted.According to the

Fig.3.Isotherms of N2 adsorption-desorption for SC (a),and SC-AC (b).

International Union of Pure and Applied Chemistry(IUPAC) classification,the isotherm profile in Fig.3b belongs to a Type I isotherm[44].The hysteresis loop of the coal after the activation process is shown in Fig.3b for SC-AC and is a type H4 loop based on the IUPAC classification.Type H4 loops have been reported for many types of ACs and other nanoporous adsorbents [45,46],where these isotherms are of a composite nature [47,48].The profile in the N2isotherm of SC-AC displays a hysteresis curve which relates to the difference in energy of desorption and capillary condensation within mesopores that was also reported by Hutson[49].Pore sizes are classified in accordance with the IUPAC classification system(USA),where pores may possess variable diameter(d):micropores (d ＜2.0 nm),mesopores (2.0 nm ＜d ＜50 nm),and macropores (d ＞50 nm) [44].The textural properties are listed in Table 1 and clearly indicate that raw CS is a macroporous material.After the activation process,the surface structure of SC-AC has mesoporous properties with an average pore size of ca.2.09 nm.Thus,the results given in Table 1 relate to the mesoporous character for SC-AC,where such mesoporosity is favorable for adsorptionbased applications,especially for dyes such as MB [50].Porosity formation in AC by KOH activation was associated with gasification according to the following reaction [51].

Thus,the conversion of SC to SC-AC with a relatively high yield of 48.8% (Table 1) occurs via the KOH activation process.Furthermore,the high iodine number of 1019.8 mg·g-1(Table 1) further confirms that SC-AC has a large SA (1094.3 m2·g-1) and a pronounced pore structure.These results concur with other ACs obtained by KOH activation at similar impregnation mass ratio 1:3 conditions using other lignocellulosic materials;karanj fruit hulls (SA=828.3 m2·g-1) [52],and corn cob (SA=1600 m2·g-1)[53].Therefore,SC is a promising precursor for the production of high-surface-area AC materials.

3.3.2.SEM-EDX analysis

The SEM and EDX results of raw SC,SC-AC,and SC-AC after adsorption are shown in Fig.4a-c.In Fig.4a,the raw SC surface is compact,homogenous,and regular,and has no apparent pore structure with some visible cracks on its surface.Additionally,the corresponding EDX spectrum (Fig.4a) indicates that the raw CS consists of mainly C and O.By comparison,the surface of SCAC(Fig.4b)is highly porous and heterogeneous,where pores with variable size and shape are clearly visible.Pore generation is the result of KOH activation after treatment of the raw precursor,where the greater pore structure and SA of SC-AC provides enhanced adsorption of MB within its mesopore and surface sites.The foregoing is supported by Fig.4c,where the SC-AC surface is altered after MB adsorption,as noted by more dense and less visible pores on the surface of SC-AC.It was also observed that all samples consist mainly of C and O.The slight and gradual increment in the C and O of SC-AC(Fig.4b)and SC-AC after MB adsorption(Fig.4c)is compared to SC(Fig.4a)which relates to the role of the KOH activation and the adsorption of MB onto the SC-AC surface.The presence of Au peaks in all spectra is noteworthy and arises due to the gold coating and serves to increase electric conduction and improve the SEM image quality.

3.3.3.XRD analysis

The XRD patterns of the raw SC and SC-AC are shown in Fig.5 and are indexed based on standard diffraction reference pattern(PCPDF No:898487).The appearance of a broad diffraction background at 25°and the absence of a sharp signature provide support of the amorphous carbonaceous structure [54].A sharp XRD line observed at 26.5° (denoted by *) is assigned to the minor crystalline phase of graphite (hexagonal phase) carbon present in the raw SC.A new carbon phase (denoted by#) can be seen from the XRD pattern of the SC-AC with (002) and (101) diffraction peaks shifted to lower angles (24° and 42.5°,respectively) that are evident after the activation process.Shifting of diffraction peaks to lower angles could be attributed to the lattice expansion of graphene layers due to the presence of residual ash in the carbon phase.The XRD pattern of SC-AC reveals a broad line at 25° that corresponds to an amorphous carbonaceous phase of the sample.

3.4.Batch adsorption studies

Based on the above-discussed results,SC-AC possesses favorable textural properties(greater SA and PV)along with polar functional groups on its surface.This structural information provides motivation to study the adsorptive properties of SC-AC by using MB as a model dye adsorbate.Variable experimental conditions(adsorbent dosage,initial pH,initial dye concentration,and contact time) in batch mode were evaluated as described in further detail below.

3.4.1.Effect of adsorbent dosage

The quantity of the available sorbent in the liquid phase(dosage) is an important parameter that strongly affects sorption capacity.Variable dosage levels of SC-AC(0.02-0.16 g)were evaluated in the 100 ml MB solution,where the effect of SC-AC dosage on MB removal is shown in Fig.6.It is apparent that an increase in the SC-AC dosage from 0.02 to 0.1 g results in greater MB removal(85.4%to 99.8%).Similar trends were noted in MB removal for an increased SC-AC dosage due to an increase in the available active surface sites for adsorption.Thus,a dosage of 0.10 g per 100 ml was selected for further adsorption studies.

3.4.2.Effect of pH

The pHPZCis the pH where the net surface charge of the adsorbent reaches a zero-point value.A determination of the pHPZCprovides insight on the role of electrostatic interactions for the adsorbent-adsorbate system.A pHPZCvalue (6.2) was obtained for SC-AC material as shown in Fig.7a,while the effect of pH is outlined in Fig.7b.The effect of pH (2-12) on MB removal by SC-AC was examined for an initial MB concentration (100 mg·L-1).At a pH below 6.2,the surface of SC-AS is positively charged,while SC-AC adopts a negative surface charge for pH ＞pHPZC.Above pH 6.2,greater adsorption of MB cations occurs via attractive electrostatic attractions.However,an electrostatically driven adsorption process does not adequately account for the slight decrease in MB uptake removal at pH 7-12.An additional mode of adsorption such as ion-exchange,chelation,or precipitation of MB may result in lower removal at higher pH [55],especially above pH 9.The optimum pH for the removal of MB by SC-AC is near the pHPZC.The pH value of the original MB solution was observed near pH 5.6,near the optimal pH range,and this condition at pH 5.6 of unadjusted MB solutions was used throughout in this study.

Fig.5.XRD patterns for SC (a),and SC-AC (b).

Fig.6.Effect of SC-AC dose on MB removal([MB]o=100 mg·L-1,V=100 ml,pH 5.6,shaking speed=90 strokes·min-1,Temp.=303 K,contact time=60 min).

3.4.3.Effect of agitation rate

The effect of agitation rate on MB adsorption on SC-AC was performed at 35-145 strokes·min-1.Agitation rate can assist in the even distribution of MB and SC-AC so that the adsorption process occurs faster and the equilibrium adsorption time can be reduced.Fig.8 shows the effect of agitation rate on the adsorption of MB on SC-AC.It was found that agitation rate plays a crucial role because as the agitation rate is increased,the uptake of MB also increased up to 90 strokes·min-1.Beyond this point,there is no significant influence on the amount of MB adsorption.Therefore,the agitation rate for SC-AC was fixed at 90 strokes·min-1as the adsorption of MB was at maximum.Agitation rate assures that all the active sites on SC-AC are made readily available for MB uptake.

3.4.4.Effect of initial dye concentration and contact time

Fig.7.(a)pHPZC for SC-AC suspensions,and(b)effect of pH on MB uptake(mg·g-1)(SC-AC dose=0.1 g,[MB]o=100 mg·L-1,V=100 ml,Temp.=303 K,shaking speed=90 strokes·min-1 and contact time=60 min).

Fig.8.Effect of agitation rate on the adsorption of MB onto SC-AC.

The influence of contact time and initial MB concentration (25 to 400 mg·L-1) on the adsorption capacity of SC-AC is shown in Fig.9,where an observed increase in the initial MB concentration(25 to 400 mg·L-1) led to an increase in the MB uptake by SC-AC at equilibrium which occurred from 29.3 to 390.5 mg·g-1.This was attributed to a greater collision rate between MB cations and the SC-AC surface by increasing the initial MB concentration.Furthermore,the time required to reach equilibrium increased with an increase in the initial MB concentration due to the tendency of MB to undergo intraparticle and pore diffusion from the surface of SC-AC to the micropore domains of the adsorbent phase [55].

Fig.9.Effect of initial MB concentrations,non-linear plots of the pseudo-first-and pseudo-second-order kinetic models for MB adsorption on SC-AC surface(V=100 ml,SC-AC dose=0.1 g,pH 5.6,shaking speed=90 strokes·min-1,and 303 K).

3.5.Modeling of kinetic adsorption isotherms

The rate and mechanism of the adsorption process for SC-AC with MB were evaluated using the non-linear pseudo-first-order(PFO) model and pseudo-second-order (PSO) model.The PFO model was initially proposed by Lagergren [56]and is expressed by Eq.(4):

qe(mg·g-1)and qt(mg·g-1)are the amounts of MB adsorbed by SCAC at equilibrium and time t,respectively;while k1(1·min-1)is the PFO rate constant.By comparison,the non-linear form of the PSO model [57]is described by Eq.(5):

The kinetic data was fit using the nonlinear form of the PFO and PSO models,where the best-fit between theory and experiment was evaluated by nonlinear coefficient of determination (R2) and average relative error (ARE) functions using Eqs.(6)-(7) [58].

qt.measand qt.cal(mg·g-1)are the experimental(measured)and calculated adsorption capacity at time t,and n is the number of observations.Fig.9 shows the predicted PFO and PSO models for MB adsorption on the SC-AC surface by use of non-linear fitting.The kinetic parameters of these two models at variable concentration alongwiththecorrespondingvaluesofR2andAREarelistedinTable2.At high R2values,the values of ARE were low for the PSO kinetic model.Moreover,the qt.measvalues agreed with the qt.expvalues.Better agreement using the PSO kinetic model suggests that chemisorption is the possiblerate-determining step that controls the adsorption process,where the adsorption rate of MB is proportional to the number of active sites available on the SC-AC surface[59].

3.6.Isotherm modeling

Adsorption isotherms are used to describe adsorbent-adsorbate interactions and the equilibrium distribution of adsorbate molecules between the solid-liquid phases[60].The adsorption equilibrium is shown by plotting the experimental level of adsorbed MB,qe(mg·g-1),against the dye concentration,Ce(mg·L-1) at equilibrium conditions as shown in Fig.9.The adsorption equilibrium was further evaluated using three isotherm models (Langmuir,Freundlich,and Temkin).The Langmuir isotherm model [61]describes the monolayer adsorption process onto uniform adsorption sites by Eq.(8).

qeis the equilibrium amount adsorbed in mg·g-1,Ceis the equilibrium concentration in mg·L-1,qmaxis the Langmuir maximum adsorption capacity,and Kais the Langmuir constant that relates to the adsorption energy of the system.

Conversely,the Freundlich model is an empirical equation proposed by Freundlich [62]to describe the sorption behavior onto heterogeneous surfaces.The nonlinear form of the Freundlich model can be expressed by Eq.(9).

Kfis the Freundlich coefficient that accounts for the adsorption capacity and 1/n is the Freundlich exponential coefficient that describes the intensity of the adsorption process.

The Temkin isotherm [63]relates the adsorbent-adsorbate interactions and their effect on linear decrease of the heat of adsorption with surface coverage.The nonlinear form is expressed by Eq.(10).

The term,B=RT/b,corresponds to enthalpy of adsorption,b is Temkin constant related to heat of sorption (J·mol-1),and A isbinding constant at equilibrium which corresponds to the maximum binding energy (L·g-1).

Table 2 Parameters of the PFO and PSO kinetic models for MB adsorption on SC-AC at different initial MB concentrations

Fig.10.Adsorption isotherm plots of the Langmuir,Freundlich and Temkin models for MB adsorption on SC-AC at 303 K.

Table 3 Parameters of the Langmuir,Freundlich and Temkin isotherm models for MB adsorption on SC-AC surface at 303 K

The nonlinear plots of the Langmuir,Freundlich and Temkin models relate to Eqs.(8)-(10) and are presented in Fig.10,where the corresponding isotherm parameters are listed in Table 3.To evaluate the best-fit isotherm model,the coefficient of determination (R2) and the residual root-mean squared error (RMSE) function were obtained by Eqs.(12) and (13) [64]:

qe.measand qe.cal(mg·g-1) are the measured and calculated adsorption capacity at equilibrium conditions.

Based on the isotherm parameters in Table 3,the Langmuir model is seen to provide a good fit of the experimental data based on high R2and low RMSE values.It can be inferred that adsorption takes place at homogeneous sites that are energetically equivalent[60].SC-AC has relatively high adsorption capacity for MB(qm=491.7 mg·g-1) that exceeds MB uptake values reported inTable 4 for other types of AC materials prepared using KOH activation with various precursors via the microwave activation method.Therefore,the SC-AC produced in this study is an effective adsorbent that can be derived from a viable precursor to yield an AC adsorbent material with enhanced adsorption properties over its precursor (SC).

Table 4 Comparison of adsorption capacities for MB onto different activated carbons developed from various precursors by microwave-induced KOH activation

3.7.Adsorption thermodynamics

Adsorption thermodynamics of MB on SC-AC were determined from the experimental data obtained at various temperatures of 303 K,313 K,323 K,and 333 K and the thermodynamic parameters,such as the change in Gibb’s energy (ΔG°),enthalpy (ΔH°)and entropy(ΔS°)of the adsorption process were calculated using the following Eqs.(13)-(15) [74]:

Fig.11.Plot of ln kd vs 1/T for determining of thermodynamic parameters for the adsorption of MB onto SC-AC.

Table 5 Thermodynamic parameters values for the adsorption of MB onto SC-AC

Fig.12.Illustration of the possible interaction between SC-AC and MB:(a)electrostatic attraction,(b) hydrogen bonding interaction,and (c) π-π stacking interactions.

The distribution coefficient is kd,concentration of MB adsorbed on SC-AC at equilibrium is qe(mg·L-1),equilibrium concentration of MB in the liquid phase is Ce(mg·L-1),universal gas constant is R(8.314 J·mol-1·K-1)and absolute temperature is T(K).The values of ΔH° and ΔS° were calculated from the slope and intercept of van’t Hoff plots of ln kdversus 1/T respectively as shown in Fig.11.The thermodynamic parameters are listed in Table 5.The negative values of ΔG°indicate the adsorption of MB on SC-AC was spontaneous and more favorable at high temperature.The positive value of the enthalpy change (ΔH°) indicates that the adsorption process is endothermic in nature.The positive entropy change(ΔS°)value corresponds to the increase in the randomness at solid-solution interface [75].

3.8.Adsorption mechanism

The adsorption of MB from aqueous solution by SC-AC is strongly dependent on the various functional groups available on the surface of SC-AC such as hydroxyl,carboxyl,and carbonyl which are testified by FTIR analysis.Therefore,the possible adsorption mechanism of MB on surface of SC-AC can be summarized in Fig.12.According to the available surface functional groups on the surface of SC-AC,the MB adsorption mechanism can be assigned to the various interactions:(i)electrostatic attractions between negatively charged functional groups on the surface of SC-AC and positively charged species of the hydrolyzed cationic MB+which strongly attracted from solution onto SC-AC surface as sketched in Fig.12a;(ii)hydrogen bonding interaction between the surface hydrogen bonds of the functional groups available on the SC-AC surface and nitrogen atoms of the MB as sketched in Fig.12b and(iii)π-π stacking interactions between aromatic rings of MB and hexagonal skeleton of SC-AC as sketched in Fig.12c.Based on the above-mentioned discussions,it can be concluded that these different types of interactions are responsible for enhanced MB adsorption on SC-AC surface.Similar observations were reported by other researchers for the adsorption on MB on the surface of chemically treated carbon microspheres[76],multi-wall carbon nanotubes[77],and wrapping carbon nanotubes[78].

4.Conclusions

Microwave-assisted KOH activation was used to prepare an AC adsorbent with markedly improved textural properties.Malaysian Selantik coal(SC)was used as a low-cost precursor for preparation of an activated carbon (SC-AC) with large pore volume,SA(1161.2 m2·g-1),and iodine number(1019.8 mg·g-1).The prepared SC-AC shows efficient removal of MB from aqueous solution that is well-described by the Langmuir isotherm model with a maximum adsorption capacity of 491.7 mg·g-1.The adsorption kinetics is described by the pseudo-second order model.The thermodynamic functions specify that the adsorption process is spontaneous and endothermic in nature.The mechanism of MB adsorption on SCAC included mainly electrostatic attractions,hydrogen bonding interaction,and π-π stacking interaction.This study reveals that formation of SC-AC via microwave radiation and KOH impregnation of low rank coal offers a low-cost route for the preparation of AC with favorable adsorptive removal of cationic dyes such as methylene blue.

Acknowledgments

The authors would like to thank the School of Chemistry and Environment,Faculty of Applied Sciences,Universiti Teknologi MARA for facilitating this work.The authors would also like to thank Dr.Mahesh Kumar Talari for his fruitful discussions on XRD analysis.