Alfarooq O.Basheer,Marlia M.Hanafiah2 ,Mohammed Abdulhakim Alsaadi ,Y.Al-Douri,Abbas A.Al-Raad

1 Department of Earth Sciences and Environment,Faculty of Science and Technology,Universiti Kebangsaan Malaysia,43600 Bangi,Selangor,Malaysia

2 Centre for Tropical Climate Change System,Institute of Climate Change,Universiti Kebangsaan Malaysia,Bangi 43600,Selangor,Malaysia

3 Nanotechnology and Catalysis Research Center (NANOCAT),University of Malaya,50603 Kuala Lumpur,Malaysia

4 National Chair of Materials Science and Metallurgy,University of Nizwa,611 Nizawa,Oman

5 University Research Center,Cihan University Sulaimaniya,Sulaymaniyah 46002,Iraq

6 Department of Mechatronics Engineering,Faculty of Engineering and Natural Sciences,Bahcesehir University,34349 Besiktas,Istanbul,Turkey

Keywords:Agricultural waste Nanocomposites Wastewater treatment Industrial applications

ABSTRACT Date palm fiber(DPF)derived from agrowaste was utilized as a new precursor for the optimized synthesis of a cost-effective,nanostructured,powder-activated carbon (nPAC) for aluminum (Al3+) removal from aqueous solutions using carbonization,KOH activation,response surface methodology(RSM)and central composite design (CCD).The optimum synthesis condition,activation temperature,time and impregnation ratio were found to be 650°C,1.09 hour and 1:1,respectively.Furthermore,the optimum conditions for removal were 99.5%and 9.958 mg﹒g-1 in regard to uptake capacity.The optimum conditions of nPAC was analyzed and characterized using XRD,FTIR,FESEM,BET,TGA and Zeta potential.Moreover,the adsorption of the Al3+ conditions was optimized with an integrated RSM-CCD experimental design.Regression results revealed that the adsorption kinetics data was well fitted by the pseudo-second order model,whereas the adsorption isotherm data was best represented by the Freundlich isotherm model.Optimum activated carbon indicated that DPF can serve as a cost-effective precursor adsorbent for Al3+removal.

1.Introduction

Various pollutants produced from the industrial activities such as fertilizer industries,mining operations,tanneries,metal plating facilities,batteries and heavy metals are directly and indirectly freed into aquatic environment,leading to increased global environmental concerns [1].Heavy metals are non-degradable and tend to accumulate within living organisms,potentially causing several disorders and illnesses.Among them,aluminum is highly reactive with both oxygen and carbon,thus toxic to human health as it has been identified as one of the major causative factors in autism spectrum disorders,Alzheimer’s disease,and neurotoxicity of the central nervous system [2].According to the Environmental Protection Agency (EPA) [3],acceptable levels for maximum aluminum concentrations in drinking water are 0.05–0.20 mg﹒L-1.Therefore,it is crucial to reduce the concentration of aluminum before it is discharged into aquatic environments.

Various treatment techniques for heavy metals ions removal from wastewater have been introduced.Conventional methods used for removing metal ions have included electrochemical treatment [4],chemical precipitation [5],reverse osmosis [6],flocculation[7],membrane filtration[8],ion exchange and adsorption[9],etc.However,there are several disadvantages of using such conventional methods,like high operational capital costs and high energy consumption.From all of these various methods applied for heavy metal removal,the adsorption technique is a wellrecognized for its equilibrium separation capability.Advantages of adsorption process include it being easily accessible,effective at pollutant removal,and simple to operate [10].Besides,the adsorption process is economical due to its low capital costs and abundance of adsorbent materials.Indeed,many natural and synthetic adsorbents have been developed previously,with powderactivated carbon as one of the popular adsorbents used in wastewater treatment [11].

However,commercial powder-activated carbon is commonly obtained with coal as a precursor,making it quite costly.Thus,exploring low cost and effective powder-activated carbon-based materials with high carbon content (mainly from agriculture waste) has gained much interest in recent years [12,13].Lignocellulose biomass,which is an abundant,renewable and low cost material,has been proposed as an alternative-powder activated carbon precursor.Previous studies have been conducted on removal of metals from wastewater and aqueous solutions by utilizing various agriculture waste-based adsorbents such as almond shells [14],various waste biomass [15],coconut shells [16],wood sawdust [17],tamarind wood [18],tea waste [19],palm oil fuel ash [20],sugarcane bagasse [21],newspaper pulp [22],grape industrial processing waste [23],rice husks [24] and palm oil kernel shells [22,25].

In the present study,Omani Date Palm Fiber (O-DPF) was chosen for Al3+removal.After annual trimming operations,large quantities of palm fiber waste are disposed.O-DPF,which is abundant,cost-effective,renewable,sustainable and contains high level of carbon,has been proposed as an alternative carbon precursor adsorbent for wastewater treatment.KOH is one of the most widely used chemicals for activating the carbonaceous materials in preparation of activated carbon.Particularly,KOH was effective in creating well-developed pores on the surfaces of the precursor,hence leading to the activated carbon with large surface area and porous structure which had high adsorption capacity on metals removal.Similar observations were reported by other researchers in their works of preparing activated carbons from jute and coconut fibers,apricot stones and pistachio-nut shells [26].

The aims of the present study were:i) to determine the optimum production conditions for changing O-DPF into Omani date palm fiber nanostructured powder-activated carbon (O-DPFnPAC) for a large,mesoporous surface area and ii) to optimize the adsorption of Al3+removal,and uptake capacity parameters,such as contact time,pH,O-DPF-nPAC dosage,through the optimization of O-DPF-nPAC.The effects of the main parameters on production,activation temperature,time and the KOH impregnation ratio-based for maximum adsorption process were determined with a central composite design (CCD).Moreover,O-DPFnPAC produced was characterized by various physicochemical techniques at optimized conditions,whereas the kinetic and isotherm models were investigated at optimized conditions.

2.Materials and Method

2.1.Materials

Omani date palm fiber (O-DPF) was utilized as a precursor for the synthesis of nanostructured powder-activated carbon was obtained from a local farm in Muscat,Oman.All Chemical reagents were purchased from Sigma-Aldrich,Malaysia.

2.2.Design of experiment (DoE)

The DoE is a technique used for identifying the common relationship between multiple procedure variables in order to evaluate the optimum operating conditions for the system [27,28].RSM is an efficient statistical method used in analysis and modelling for process optimization [29,30].

DoE with CCD was used to optimize the production parameters.The optimum conditions for O-DPF-nPAC fabrication were determined by examining the effect of the activation temperature of 650–850 °C (A),time of 1–3 hours (B),and the KOH impregnation ratio 1–3(C).The experimental run was designed at high(+1),central(0),and low(-1)levels to examination experimental variability (Table 1).Optimization based on percentage of removal and uptake capacity (mg﹒g-1) was conducted using DoE V7.0 software including ANOVA to maximize the response specified by O-DPFnPAC.ANOVA was carried out to clarify the ability of adapting the model for the examined responses.The R2coefficient was determined to test the certainty of the proposed model,while the interaction of various parameters was determine by RSM.

2.3.O-DPF-nPAC preparation

O-DPF was washed and cleaned to remove any surface-adhered particles by distilled water and then dried at 105 °C for 24 hours.The O-DPF dried sample was then crushed and sieved,subjected to the carbonization step under 99.995%purified nitrogen at a flow rate of 150 cm3﹒min-1for 2 hours in a horizontal tubular furnace at 700°C(the heating rate was fixed at 10°C﹒min-1).The carbonized samples were then scaled and soaked with KOH pallets at different Impregnation ratios as:

where IR is the impregnation ratio,WKOHis the dry weight(g)of the KOH pallets and WCarbonizedis the dry weight (g) of the carbonized.Distilled water was used and added to dissolve all KOH pallets.

The mixture was dried overnight at 105 °C [31].The activation step was carried out in a similar vertical tubular furnace but at a different activation time and temperature.The gas flow was converted from purified nitrogen to carbon dioxide following the activation of the temperature.The activated product was then cooled to room temperature under nitrogen flow.The sample was finally washed with hot deionized water and 0.1 mol﹒L-1hydrochloric acid (HCL) until the pH of the washing solution reached 6–7.

2.4.Screening

A fixed dosage was conducted at conical flasks of each adsorbent with 50 mg in 100 ml of a 5 mg﹒L-1concentration(Al3+)standard solution,and deionized water was used to prepare a pH 6.5.The flasks were agitated on a mechanical shaker system at 180 r﹒min-1and room temperature until equilibrium was attained.After operation,the solution was filtered via a syringe filter with a 0.45 μm polypropylene membrane.ICP-OES Perkin Elmer Optima 7000 DV was used to determine the final concentration of metal ion solution.Adsorbent production was optimized according to Al3+removal and capacity efficiency Table 2.The adsorbent with the maximum adsorption process (removal rate and capacity)was chosen for further analysis.The removal(%)and uptake capacity (mg﹒g-1) were calculated by Eqs.(2) and (3),respectively [32]:

Table 1 Levels of independent variables for synthesis of nPAC

where Coand Ceare the initial concentration of Al3+and the equilibrium concentration (mg﹒L-1),qcis the amount of Al3+adsorbed by nPAC (mg.g-1),V is the initial volume of the solution (L),and W is the mass of dry adsorbent used (mg).

2.5.Characterization of nPAC

The production of nPAC was characterized to be at optimum condition.The surface functional groups and chemical bonds were examined by a Fourier transform infrared (FTIR),(Perkin Elmer,USA).Powder X-ray diffraction (XRD) was used to analyze the structural phase by a Burker AXS D8 advance (Germany),while a thermogravimetric (TGA) was implemented to determine thermal oxidation using a STA-6000 thermal analyzer (Perkin Elmer,USA).Surface morphology was determined by a Field Emission Scanning Electron Microscope (FESEM),model ZEISS (Merlin,UK).Moreover,pore size and surface area were calculated by the Brunauer-Emmett-Teller (BET) method (TriStar Ⅱ 3020,USA).Finally,the surface charge was measured with a Zeta potential Malvern (Zeta Sizer,UK.)

2.6.Adsorption study

2.6.1.Optimization of Al3+adsorption

The optimization of Al3+removal was employed by the RSM method.The interaction and effect of the three parameters were explored in the present study,particularly on the adsorption nPAC dosage 5–20 mg,pH 3–11,and contact time 10–120 min.Table 3 displays the actual parameters of each run,in terms of DoE.Initial concentration of 5 mg﹒L-1was implemented for this optimization,and the flasks were agitated in a shaker at 180 r﹒min-1[33].

Table 2 Experimental design for the production of nPAC

Table 3 ANOVA for removal surface modified model

Table 4 Comparison surface area of nPAC with different agricultural waste-based activated carbon

2.6.2.Kinetic and isotherm adsorption

The ions transfer rate from the solution to the surface of the adsorbents and its correlated factors are pivotal aspects and can be specified through a kinetic study.The adsorbent efficiency is specified by the adsorption system kinetic rate to determine the possible adsorbent applications[34].The kinetic study was accomplished by fixing the adsorbent dosage and pH parameters,while the kinetic behavior was studied with different values of Al3+ion concentrations (3 and 5 mg﹒L-1) at different contact times until the equilibrium state was reached at 92 minutes.In the present study,three well-known kinetic models were applied,including the pseudo-first-order,pseudo-second-order and the intraparticle diffusion model.

The isotherm study was carried out by considering the optimum conditions of contact time,amount of nPAC dosage and pH,obtained from optimization study.In the isotherm study,two models were applied to investigate the Al3+ion adsorption into the nPAC surface including Freundlich and Langmuir,in which the initial concentration of Al3+varied from 3 to 40 mg﹒L-1[35].

3.Results and Discussion

3.1.Analysis and characterization

3.1.1.FTIR analysis

The qualitative information regarding the characteristics of the functional groups of the adsorbed materials on the surface of the porous carbons was analyzed and characterized by FTIR.Fig.1 shows the FTIR transmission spectra of raw O-DPF and nPAC under optimum conditions.For raw O-DPF,the broad and intense peaks at 3343.92 cm-1attributed to the O—H stretching vibrations due to the existence of water,phenol,alcohol,carboxylic and hemicellulose [36].The peak at 2917.40 cm-1can be attributed to either the stretching vibrations of the CCH bonds occurring in the alkanes or carbon bonding with hydrogen bonds [37].The band in region 1623.04 cm-1was attributed to the C=C stretching vibrations in the aromatic ring band.Moreover,the intense band at 1317.76 cm-1corresponds to the aromatic C—N stretches [38].The strongest peak was observed to be at 1031.73 cm-1and was attributed to C—O stretch in cellulose and hemicellulose presented in cell walls.

The obtained nPAC had fewer peaks compared with the raw O-DPF sample,and most of the group frequencies disappeared upon the O—H stretching vibrations of the functional groups in the fingerprint region when compared with the raw O-DPF sample.This difference implies that the presence of minor functional groups in the fingerprint region was mainly due to the existence of volatile compounds.Thus,the disappearance of certain peaksin the nPAC sample could be attributed to volatile compound loss at an activation temperature.The peak at 1981.24 cm-1was ascribed to C≡C,whereas the prevailing peaks at 1423.12 cm-1appeared due to stretching vibrations [39].Moreover,the band located at 996.09 cm-1was less intense and was related to the C—O stretching vibration of the bonds in the ester,ether,or phenol groups.Finally,the bands at 871.55 cm-1and 771.31 cm-1were related to the stretching vibrations of the aromatic bonds C—H[40].

3.1.2.X-ray diffraction (XRD) analysis

The XRD is a fundamental technique used for evaluating the carbon staking structure [27].Fig.2 shows the XRD patterns of raw O-DPF and nPAC under optimum conditions.The DPF amorphous structure corresponding with high contents of hemicellulose and lignin [41].The reduction of the amorphous structure was observed during the conversion of O-DPF to nPAC due to the hemicellulose and lignin,which were exposed to high temperatures[42].Moreover,the wide diffraction pattern exhibits a peak at 24.39°and 43.33°attributed to(002)and(101)of graphite,respectively.These peaks demonstrate a partially graphitic layer structure of nPAC [43].The hemicellulose and lignin also disappeared due to the degradation of activated O-DPF,which is in line with the FTIR results shown in Fig.3.

Fig.1.FTIR spectra of raw O-DPF and nPAC.

Fig.2.The X-ray diffraction of raw O-DPF and nPAC.

3.1.3.TGA analysis

Fig.3.TGA analysis of raw O-DPF and nPAC.

The behavior of the TGA curves with heating step in oxygen of O-DPF is shown in Fig.3.The first peak at 157°C can be explained by volatilization,hemicelluloses material in TFC,and moisture loss in the surface.The evaporation of these smaller volatile molecules was the result of the decomposition of the thermal treatment[44].Until the temperature reached 445°C,the massive sample loss was documented to be only 20%.The degradation of O-DPF began at 445°C and resulted in a rate of 30%.On the other hand,the degradation of nPAC began at 541.6 °C.The sharp mass degradation of amorphous carbon oxidized around 500 °C.This huge loss was caused by the CO2released from the nPAC samples.At a temperature of 570.39 °C,the immense losses took the value of 12.94%,which was mainly due to lignin decomposition,since it decomposes across a broad range of temperatures.

3.1.4.FESEM characterization

FESEM analysis was applied to examine the surface morphology of nPAC under optimum conditions.The FESEM images at different magnifications and the EDX analysis are illustrated in Fig.4.The nPAC produced from O-DPF demonstrated the homogeneous pore size distribution with a quite uniform pore arrangement.The pores available on the surface of nPAC were well-arranged and formed a honey-comb structure.Similar trends have been found in previous studies,with well-developed and clear pore structures obtained from a pistachio nut shell,empty fruit bunch,oil palm,and coconut shell-based AC [45–47].Micrographs indicated that nPAC has a high surface area and potential for adsorption application of water pollutants.Nasrullah et al.[48] discovered a direct relevance among the efficiency of adsorption and the surface area of the adsorbent.Furthermore,EDX analysis confirmed an nPAC element makeup of 83.6%(mass)carbon,10.9%(mass)oxygen,3.1%(mass)potassium,1.2% (mass) calcium,0.8% (mass) silicon,0.2% (mass)magnesium,and 0.2% (mass) aluminum.

Fig 5.N2 adsorption-desorption curves.

3.1.5.BET surface area

Fig.5.Shows the adsorption–desorption N2curves for nPAC.Moreover,nPAC displayed a type IV isotherm with a sudden increase in N2uptake within a distinct hysteresis loop and comparatively higher pressure ranges and,which indicates its totally mesopores nature.The present study also found that the,pore volume,pore diameter,and total surface area of nPAC were,0.598 cm3﹒g-1and 2 nm,1092.342 m2﹒g-1,respectively.The adsorbent pores are well categorized into three groups:micropore(diameter<2 nm),mesopore(2–50 nm)and macropore(>50 nm).

Table 4 displays a comparison between the surface area of nPAC and agriculture waste-based AC published by previous studies[49–53].It was found that DPF is an efficient precursor in the production of AC due to its high surface area.

Table 5 ANOVA for capacity surface modified model

3.1.6.Zeta potential

Fig.4.FESEM images under different magnification (a,b and c) and EDX analysis (d) of nPAC.

Zeta potential is an electrokinetic behavior of n-PAC occurring in an aqueous solution and exhibits a potential difference between the stationary layer of fluid attached and the dispersion medium to the dispersed particle.The difference of the zeta potential for different samples depends on their various pH levels.10 mg of n-PAC was dispersed in 20 ml and the results show zeta potential has an implied negative value from -10.4 to -0.0712 mV in the pH range of 3–11.Thus,negative zeta potential value indicates a decline in surface charge as pH increases and implies the surface of n-PAC is negatively charged.Li et al.reported that small particles in suspension tend to disperse homogeneously in the solution rather than aggregate.n-PAC can be obtained for enhancing the adsorption capacity due to homogeneous dispersion of the adsorbent.

3.2.Establishment model and analysis

The interaction effects of three variables (activation time,temperature,and KOH impregnation ratio)on CO2decomposition were investigated using RSM and CCD.A series of 16 runs were accomplished in addition to various variable analyses;specifically,ANOVA was applied to study interaction effects on removal (%)and uptake capacity (mg﹒g-1).The responses from the 16 runs are presented in Table 2.The quadratic model was found to be more significant,as was its probability.The quadratic model was also chosen for removal (%) and uptake capacity (mg﹒g-1) analysis due to higher R2values of 0.889 and 0.8737,respectively.

Tables 3 and 5 presents results for the coded quadratic models for the responses.The effect of parameters is defined as the change in response produced by a level change in the factor.This is due to the primary factors of interest involved in the experiment.Removal (%) from ANOVA results are shown in Table 3.The main effects of the activation time (B),impregnation ratio (C) and the second-order effects of activation time(B2)were found to be notable,while the Prob>F values were less than 0.05.Thus,it could be concluded that B,C and B2were the main parameters for removal(%).The residual activation parameters,included (A),interactions(AB),(AC),(BC),the second-order effect A2and C2were observed Prob >F values higher than 0.05.Table 5 presents the uptake capacity (mg﹒g-1) in which the ANOVA results were similar to removal(%).It could be conclude that these parameters had negligible effect on removal (%) and uptake capacity (mg.g-1) over the studied range.Nonetheless,to ensure that the chosen quadratic model remained hierarchical these model terms were not removed from the analysis.The regression equation for removal and uptake capacity are given by Eqs.(4) and (5):where A is activation temperature,B is activation time and C is impregnation ratio.

Table 6 The optimum condition to obtain the maximum removal and uptake capacity on DoE results

Fig.6.Predicted vs.actual data obtained from optimizing the synthesized of nPAC for (a) removal (%) and (b) uptake capacity (mg﹒g-1) using ANOVA.

A comparison among the model values and experimental results of removal(%)and uptake capacity(mg﹒g-1)predicted from the above Eqs.(4)and(5)is illustrated in Fig.6.A simulated model should be congruent with the results,indicated by a high R2value.The experimental and predicted values of removal (%) and uptake capacity (mg﹒g-1) are shown in Fig.6.with good agreement.

3.3.The effects of temperature,time and KOH impregnation ratio effects on synthesis nPAC

Fig.7.Three-dimensional surface plots of synthesized nPAC (a) effects of activation time and activation temperature,(b) effects of KOH impregnation ratio and activation temperature,(c) effect of KOH impregnation ratio and activation time on Al3+ removal and uptake capacity.

The interactions effects between activation temperature (A),activation time (B) and KOH impregnation ratio (C) on the Al3+adsorption process (removal,%) and uptake capacity (mg﹒g-1)throughout the creation of a three-dimensional response surface are displayed in Fig.2.Moreover,the effect of the activation time force is greater for adsorption than activation temperature for nPAC.As shown in Fig.7(a),Al3+adsorption increased with increase in activation time[54].Generally,a longer activation time is important to enhancing porosity and clear blocked pore entrances,thus leading to the highest Al3+adsorption process.Nonetheless,the decrease in the adsorption process leading to an increased activation temperature was due to a sintering effect occurring at a high temperature followed by shrinkage in nPAC[55].It can be concluded that the Increase in activation temperature and activation time caused more pore to enlarge resulting in enhanced adsorption capacity.However,if it goes beyond the threshold required it causes undesirable characteristics of activated carbon produced.The effect can be notice where at a very high or low activation temperature,the Al3+removal and uptake capacity were decreased.

As shown in Fig.7(b),the interaction between the KOH impregnation ratio and activation temperature was slightly significant.Besides,partial oxidation of some carbon in particles surfaces could also happened as well which caused in changes of these carbons into ashes,also as a result of weakening of macromolecular structure of O-DPF.Similarly,the increase in the KOH impregnation ratio caused an increase in the Al3+adsorption process;thus,KOH contributed to creating pores on the nPAC surface,which stimulates a high adsorption process.On the other hand,Fig.7(c)illustrates interaction effect between the activation time and KOH impregnation ratio on the adsorption process,which clarifies the significant influence of the interaction on the Al3+adsorption process due to the formation of well-developed porous structures,which can eventually promote the functionalization of the surface and generate additional active sites on which nPAC can bind to the surface of adsorbent.Which indicted the increasing of the KOH ratio increases the adsorption process.This is because the potassium ions fixed onto the carbon surface proceed as catalyst to accelerate the reaction between the carbon and KOH.

3.4.Synthesis optimization of nPAC

One of the main aims of this study was to find the optimum process parameters to maximize the adsorption of chromium from the developed mathematical model equations.The optimum preparation conditions at which nPAC was synthesized at a high removal rate (%) and uptake capacity (mg﹒g-1) were determined.However,the optimization of these two responses under the same conditions is difficult due to the different interest regions of their factors.Therefore,DoE software was applied to categorize the function of desirability for these two responses.Table 6 shows that the chosen optimum condition had a high value of desirability.The validation of the developed model was confirmed by conducting the experiment in triplicate and using the optimal processing condition.The experimental and predicted results of removal (%) and uptake capacity (mg﹒g-1) were found to be consistent.These results confirmed the prediction of the ANOVA model for both responses under the experimental conditions (see Table 7).

The optimum nPAC was attained by applying an activation temperature of 655.77 °C,an activation time of 1.09 hour,and a KOH impregnation ratio of 2.63.The results show that the predicted removal rate (99.58%) and an uptake capacity (9.96 mg﹒g-1) for Al3+adsorption were similar to the experimental values previously mentioned with only a slight error of 2.58%.

3.5.Adsorption study

3.5.1.Optimization study

In order to find the optimum variable parameters for Al3+removal,Table 2 shows the actual parameters of each run using RSM with CCD to conduct the experiments.Adsorption uptake capacity (mg﹒g-1) and removal (%) were adopted as a response function of the optimization study.Hence,the highest capacity and removal efficiency of nPAC on Al3+were 95.12 (mg﹒g-1) and 99.74 %,respectively,under optimum parameters of an nPAC dosage of 5 mg,a pH of 10.33 and a contact time of 120 minutes.

ANOVA modelling for the removal (%) and uptake capacity(mg﹒g-1) responses of nPAC presented the F-values 15.4 and 48.61,which indicates that both response models were significant.Tables 8 and 9 display the mean square,F-values and p-values for both responses removal and uptake capacity.Both response models were significant with a value of prob >F and less than 0.0500.For removal cases,B,C,BC and B2were considered as significantmodel terms.Nonetheless,for uptake capacity,the cases A,B,AB,A2and B2were also considered significant model terms.The coefficient correlation for removal R2(%)and uptake capacity(mg﹒g-1)were 0.9511 and 0.9843,respectively.Both response models are presented in the following Eqs.(6) and (7):

Table 7 List of DoE runs and the actual values obtained from each response

Table 8 ANOVA results for optimization Al3+ adsorption removal (%) by nPAC

Table 9 ANOVA results for optimization Al3+ adsorption capacity (%) by nPAC

where A,B and C represent nPAC dosage,pH and contact,respectively.

3.5.2.Effect of optimization variables

The interaction effect of pH and nPAC dosage shows that removal (%) was reached to a pH of 10.33.Upon an additional increase in pH,removal (%) began to decrease,as shown in Fig.8(a).The same phenomenon was observed for uptake capacity(mg﹒g-1).However,pH was observed to have a significant impact on both removal(%)and uptake capacity(mg﹒g-1)for Al3+adsorption process.Furthermore,pH might affect the surface charge of nPAC,the amount of pollutant ionization,and the separation of functional groups on nPAC as well as the structure of Al3+metal[28,56].In addition,nPAC dosage had a highly significant impact on the uptake capacity process (mg﹒g-1).

The interaction effect on the adsorption process of contact time and pH are illustrated in Fig.8(b).Contact time had a significant impact on the removal (%) and uptake capacity (mg﹒g-1) of Al3+,which increased with time until equilibrium was reached.This is in line with the results reported by Ibrahim et al.[57].Furthermore,the interaction effect taking place between nPAC dosage and contact were observed for adsorption removal(%)and uptake capacity(mg﹒g-1),as shown in Fig.8(c).nPAC dosage also increased adsorption due to the availability of the nPAC surface area,in consequence,more adsorption sites were able to capture Al3+from wastewater.

3.5.3.Kinetic behavior

Kinetic behavior was conducted to examine the reaction behavior of nPAC adsorbent.Three kinetic models were utilized for the experimental data,namely,pseudo-first-order,pseudo-secondorder and the intraparticle diffusion model [28].The kinetic study was carried out at different Al3+concentrations(3,5 and 20 mg﹒L-1)with fixed absorption dosage value of 5 mg and a pH of 10.33 at different time intervals until the equilibrium state was reached.A contact time of 12 h was utilized to certify that the reactions touched equilibrium.The results of the three well-known models detected that the was the pseudo-second-order better fitting at different concentrations of Al3+,which was expressed by a higher correlation coefficient R2.Pseudo-first-order and interparticle diffusion have lower values of R2and therefore is not suitable to explain the kinetics of Al3+adsorption on nPAC.The results of the pseudo-second-order are shown in Fig.9.Moreover,the three applied models for kinetic results are given in Table 10.Thus,the pseudo-second-order model is the most appropriate to describe the adsorption kinetics of nPAC,indicating the adsorption involve the interaction between Al(III)ions with all least two binding sites.

3.5.4.Isotherm study

Fig.8.Three-dimensional surface plots of Al3+ removal (%) and uptake capacity (mg﹒g-1) optimization,(a) interaction effect between pH and nPAC dosage,(b) interaction effect between pH and contact time,(c) interaction effect between nPAC dosage and contact time.

Fig.9.Plot of pseudo-second-order kinetic for Al3+ adsorption on nPAC.

Adsorption isotherms indicate how molecules subjected to adsorption distribute themselves between liquid and solid phases at equilibrium time.They provide some insight into the adsorption mechanism as well as the surface properties and affinities of the adsorbent.The Freundlich and Langmuir isotherm models were used in isotherm study to perform the Al3+ion adsorption into the nPAC adsorbent surface.Different Al3+concentrations were employed to study the adsorption isotherm (3,5,10,20,30 and 40 mg﹒L-1),using a pH of 10.33 and nPAC dosage of 5 mg,which were specified by the optimization study.The Freundlich and Langmuir isotherm results are presented in Fig.10 and detailed parameters are list in Table 11.Comparing these,the results reveal that the Freundlich displayed a better fit for Al3+adsorption into the nPAC adsorbent surface.The coefficient of correlation(R2)obtained from the Freundlich model was 0.9922,while the R2obtained from the Langmuir model was 0.8563,implying that the adsorption of Al3+occurs on a heterogeneous surface with an interaction between the molecules of the adsorbent [58].

Table 10 Adsorption kinetics and correlation coefficient

Table 11 Isotherm parameters and coefficient determination for Al3+ removal on nPAC

Table 12 Comparison of Al3+ adsorption capacities of the previous and present studies

Furthermore,the highest adsorption capacity of Al3+for nPAC and AC obtained from previous studies is reveals in Table 12.The adsorption capacity results showed that the synthesized nPAC was higher compared to other AC derived from different precursors[3,59–61].Overall,the high adsorption capacity of nPAC was due to its large surface as well as its mesoporous and amorphous structures.

4.Conclusions

Fig.10.Plots of isotherm models (a) Freundlich and (b) Langmuir.

It was found that the optimum synthesized conditions for activation temperature,time and KOH ratio were 650 °C,1.09 hours and 1:1,respectively.The optimum condition of nPAC was analyzed and characterized using FTIR,XRD,TGA,FESEM,BET and Zeta potential.The optimum Al3+adsorption process for parameters such as nPAC dosage,pH and contact time were 5 mg,10.33 and 120 minutes,respectively.The pseudo-second-order kinetic and freundlich isotherm best fitted to determine the experimental result.Consequently,O-DPF could be a new and cost-effective precursor for the synthesis of nPAC for Al3+removal.Based on the obtained results,nPAC was found to be a promising adsorbent material due to its high surface area,unique pore structure and high adsorption capacity in wastewater treatment.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors gratefully acknowledge Universiti Kebangsaan Malaysia and Universiti of Malaya for technical measurements support.