Adsorptive potential of Acacia nilotica based adsorbent for chromium(VI)from an aqueous phase

Raza Khalid ,Zaheer Aslam ,*,Aamir Abbas ,Waqar Ahmad ,Naveed Ramzan ,Reyad Shawabkeh

1 Department of Chemical Engineering,University of Engineering and Technology Lahore,54890,Pakistan

2 Department of Chemical Engineering,King Fahd University of Petroleum and Minerals,Dhahran 31261,Saudi Arabia

1.Introduction

Excessive amounts of wastewater containing heavy metals and other pollutants are discharged into an aquatic media resulting from rapid industrialization and urbanization.Heavy metals in the waste aqueous streams pose a serious threat to the aquatic environment due to their non-degradable nature and easy absorption into the living cells.By this way these can be bio-accumulated and biomagnified in higher trophic levels[1–3].The presence of chromium heavy metal in the industrial effluents is also of great worry.Chromium salts are used in various applications like the manufacturing of corrosive paints,as wood preservatives and in the treatment of hides.Other sources of chromium include metal plating,mining,preparation of photographic materials and steel production.Chromium exists in different oxidation states but Cr(III)(trivalent chromium)and Cr(VI)(hexavalent chromium)are more important in relevance to their environmental and toxicological properties.Trivalent chromium is 500 times less toxic than hexavalent chromium and the allowable limits in the industrial discharge stream are 5 mg·L?1for Cr(III)and 0.05 mg·L?1for Cr(VI)[4–7].Different techniques which are available and may be applied for the removal of chromium from industrial waste water are comprised of chemical precipitation[8–10],reverse osmosis[11],Nanofiltration,ultra filtration[12–14],ion exchange[15–17],electrochemical process[18],biological processes[19]and adsorption[20–22].

Among all,adsorption is considered as a cost effective,sustainable and ecofriendly technique for heavy metal removal from aqueous streams.This method also givespromising results even atlowerconcentrations of heavy metals.Commercial activated carbon has been widely used for the abatement of heavy metals because of its high surface area.But the increased price of activated carbons drives the research to look for low cost readily available adsorbents[23,24].Large numbers of potentially low cost adsorbents including agro-based material,microorganisms,seaweeds,industrial wastes,and natural residues have been reported in the literature[25–38].

Feedstock like peelings of citrus,sugarcane waste,coffee husk,lemon peelings,gram husk,and rice husk have been reported in the literature[39–44].Adsorbent based on waste tamarind hull was used to remove hexavalent chromium from aqueous solution,different adsorption parameters were studied and 99%removal of chromium was reported at optimum conditions[45].Azadirachta indica(neem)leaf powder effectively removes chromiumup to 96.4%using 160μm adsorbent size at an optimum dosage[46].Vikrant and Pant used Eucalyptus bark for chromium removal from aqueous solution and found optimum conditions as pH 2,concentration and time were 200 μg·g?1and 2 h respectively,and the efficiency of adsorbent was more than 99%[47].Acacia nilotica bark had been used as an effective adsorbent in batch process to remove chromium metal.Activated carbon derived from A.nilotica provided 93.1%of chromium metal removal,while in the case of untreated A.nilotica bark 68.1%of chromium was removed from synthetic solution[48].A.nilotica sawdust,which contains less tannin concentration,is an abundantly available waste by-product of the timber industry.Exploring its potentiality for pollutant abatement will notonly help in waste management but will also provide a cheap source of adsorbent.In the past,different types of saw dust had been used to remove hexavalent chromium Cr(VI)from waste water like Hevea brasiliensis sawdust,treated robust sawdust,Indian rose wood sawdust,and Beech sawdust.As referenced earlier,there is great potential for sawdust to remove heavy metals particularly Cr(VI)from industrial effluents.Sawdust is comprised of electron rich functional groups which provide active sites for heavy metals and one step acid or alkali activation further enhances the adsorption efficiency of raw sawdust.The focus of this research is to functionalize further the acid activated sawdust with aldehyde functional agent to improve the pollutant's removal efficiency.The research results presented herein are part of investigations conducted to turn the inefficient raw A.nilotica sawdust to an economical and effective adsorbent to remove hazardous hexavalent chromium.The effect of different process parameters like initial solution pH,initial chromium concentration,contact time,temperature,and adsorbent dose on adsorption capacity was evaluated.Moreover,sorption kinetics,isotherms and thermodynamics were studied to analyze the various aspects of chromium removal from aqueous solution.

2.Materials and Methods

2.1.Materials

Analytical grade Potassium dichromate(K2Cr2O7,294.2 g·mol?1)was obtained from Sigma Aldrich and it was used for Cr(VI)ion source in distilled water for experimentation.Hydrochloric acid(37%ACS Reagent grade,ACROS Organics)and formaldehyde(37%by mass,Fisher Scientific)were used for the modification of raw material.0.1 mol·L?1Sodium hydroxide(Merck Millipore,Germany)and 0.1 mol·L?1Nitric acid(65%,Merck Millipore,Germany)were used to adjust the pH of the adsorbate samples.1000 μg·g?1Cr(VI)atomic absorption spectrometry standard solution(Fluka Chemicals)was diluted to required concentrations with double distilled water(DDW)to run the calibration experiments.

2.2.Adsorbent preparation

The A.nilotica(keekar)sawdust(named as R-SD)was collected from a local saw mill situated in the vicinity.Raw sawdust was sun dried for 2 days and then sieved.The same sized particles were obtained by passing material through 20 mesh screens(US Taylor standard Sieve).Subsequently the sieved material was washed with distilled water several times to remove the dust from sample.Then washed sawdust was oven dried at 80°C for 12 h.The dried material was saved for subsequent chemical treatment.

500 g dried sawdust was treated with 1 mol·L?1HCl in ratio of 1:15(sawdust:HCl,W/V)in a beaker with continuous stirring on a hot plate magnetic stirrer for 3 h at 40°C.After stirring for a required time the sample was removed and kept soaked for 24 h at room temperature.The acid treated sawdust was filtered and washed with distilled water to remove the acid contents until the pH of filtrate was 6 to ensure that all the acid had been washed.Filtered material was dried in an oven at 60°C for 8 h,and then transferred to sealed ceramic bottles for further use.The HCl washed sawdust was divided into two portions;the first one was used in experimental work without further treatment(named as H-SD)while the second one was soaked and impregnated with 1%formaldehyde solution with ratio of 1:3(sawdust:formaldehyde,W/V)for 24 h.After one day of soaking,the material was washed repeatedly and filtered using What man no.42 filter paper to remove formaldehyde.The filtered cake was dried in an oven at 80 °C and sample was named as “HF-SD”.Both samples(H-SD and HF-SD)were tested for their potential efficiency of removing chromium pollutant from water by doing batch experimentation.

2.3.Adsorbent characterization

Various instruments were used to characterize the prepared adsorbents.Surface texture and morphology and spot analysis was done using Scanning Electron Microscopy(SEM)on an EDX analyzer equipped coupled with Energy Dispersive X-ray Spectroscopy(EDX).SEMmicrogram adsorbent specimen was obtained under15 kV accelerating voltage.Spot analyses were conducted in triplicate runs to determine the elemental composition.The FT-IR spectra of samples were recorded by using a Nicolet Avatar 360 FT-IR spectrometer,and sample pellet was made by mixing 1 mg of adsorbent to 100 mg of KBr powder and hydraulically pressed to 10 t·m?2.The thin pellet was oven dried at 102°C to prevent the interference of moisture.The FTIR machine was set to transmission mode.Thermal stability studies were carried out by using a Thermo-Gravimetric Analyzer(TGA)over the temperature range 20–900 °C at a heating rate of 10 °C·min?1under a nitrogen flow rate of 20 ml·min?1.The BET(Brunauer–Emmett–Teller)characterization of raw and modified sawdust was done using Micromeritics ASAP2020.Accurately weighted quantity of an adsorbent was degassed at 333 K for 1 h under 666.61 Pa(5 mm Hg)pressure to desorb the entrapped gas molecules.Subsequently N2adsorption–desorption measurements were recorded and specific surface area was calculated by the BET method and pore size distribution was calculated by the Barrett–Joyner–Halenda(BJH)method.The test to determine the point of zero charge(pHpzc)was determined by taking 100 ml of 0.01 mol·L?1KNO3solution in different Erlenmeyer flasks.In each flask,initial value of pH was maintained varying from 1 to 10 and 1.02 g of an adsorbent was added in each flask and placed on an orbital shaker at 120 rpm for 24 h and then put idly for 15 min to settle the adsorbent.Final pH of the solution(pHf)was recorded and difference between initialand final pH(pHi?pHf)was calculated and it was plotted against initial pH to get the pHpzc.

2.4.Preparation of adsorbate solution

The stock solution of hexavalent chromium of concentration 500 μg·g?1was prepared by dissolving 1.415 g of analytical grade Potassium dichromate(K2Cr2O7)in 1000 ml of deionized water.The stock solution was further diluted to obtain desired concentrations for succeeding batch experiments.

2.5.Batch adsorption studies

Batch adsorption studies were carried out to remove hexavalent chromium from synthetic solution by using raw and activated sawdust as an adsorbent.Erlenmeyer flasks of capacity 250 ml were used in experimentation,in each flask 100 ml of chromium solution was taken ata required dosage,various parameters were studied which had an effect on the adsorption process such as dosage in grams(0.25,0.7,1.5,2.5,3.5,4.5,6)with 100 mg·L?1of chromium concentration at 30 °C temperature.Kinetic studies were carried out at different time intervals ranging from 1 to 540 min,with optimum dose and 100 mg·L?1of chromium.Concentration of chromium in synthetic solution varied in mg·L?1(300,200,100,80,60,30,15,5)with optimum dose and time.Effect of pH of the solution was examined in the range of 1–10 with optimum dose,time,and concentration.The effect of temperature was observed by running the experiments at four different temperatures i.e.(20–50 °C with a gap of 10 °C).All the runs were carried out at fixed 130 r·min?1.Afterwards the solutions were filtered and filtrate was collected into sample bottles to find the remaining concentration of hexavalent chromium in synthetic solution.Chromium concentrations were analyzed by an Atomic Absorption Spectrophotometer(AA-6800 SHIMADZU Japan).Percentage removal of hexavalent chromium(R,%)from synthetic solution with raw and activated sawdust was calculated by using the following equation,

where Ciis the concentration of chromium(mg·L?1)before adsorption and Ctafter adsorption concentration when time t had been passed.To estimate the adsorption capacity of adsorbent the given expression was used,

whereas,qe(mg·g?1)is the adsorption capacity,Ciand Cf(mg·L?1)are concentrations of Cr(VI)initially and after equilibrium time,V(L)is the volume and M(g)is the mass of adsorbent used.

3.Results and Discussion

3.1.Adsorbent characterizations

3.1.1.Fourier transform infrared spectroscopy(FTIR)

The presence of functional groups affects the adsorption capacity.Functional group distribution on the surface of adsorbent was analyzed by Fourier transform infrared spectroscopy(FTIR)in the range of 400 to 4000 cm?1.FTIR spectra were shown in Fig.1.Adsorbents had distinct functional groups which appeared on spectra.All the analyzed samples had wide bands at wavenumber from 3000 to 3650 cm?1(with maximum at 3413 cm?1),the absorption bands from 2800 to 3000 cm?1(with maximum at2920 cm?1).The bands centered at 3413 cm?1represented the hydroxyl group stretching vibrations at the surface of adsorbents while the band at 2920 cm?1was assigned to C--H stretching vibrations.The bands around 1735 cm?1were attributed to carbonyl groups(C=O)and peaks at 1630 cm?1occurred at spectra due to stretching of carbonyl groups[49].In the case of HF-SD these bands were strong due to the attachment of carbonyl groups on the surface of sawdust and a kink had also occurred in the range 2300 to 2900 cm?1as compared to H-SD which confirmed that carbonyl groups were present due to aldehyde group attached on the surface of adsorbent after treating it with formaldehyde as compared to H-SD.But in R-SD,these bands also have some comparable intensities due to the presence of lignin which have carbonyl groups in it.The bands at lower wavenumber range 1508 cm?1,1463 cm?1,and 1428 cm?1were associated with lignin present in sawdust[44].The absorption bands at 1508 cm?1were assigned for aromatic skeleton,at 1463 cm?1and 1428 cm?1for C--H deformation.The bands centered at wavenumber 1380 cm?1were attributed to bending of C--H in methane group,while the stretching vibrations of C─O─C linkages were represented with the bands centered at 1163 cm?1and 1115 cm?1and C--O stretching vibrations at 1030 cm?1(polysaccharide peaks in cellulose)[50–52].

Fig.1.FTIR spectra of R-SD,H-SD and HF-SD.

3.1.2.Point of zero charge

In Fig.2 the point which intersects the horizontalline gave a value of pHpzc.The value ofpHpzcfor H-SDand HF-SDwas 4.5 and 5.5 respectively.These points indicated that before intersection with x-axis the adsorbent surface was positively charged while it was negatively charged after pHpzc.The value of pHpzcfor HF-SD was more than H-SD because the surface of adsorbent acquired additional positive charge due to carbonyl groups attached to the surface after the formaldehyde treatment.These additional carbonyl groups enhanced the adsorption capacity of adsorbent by enriching the positive charge over the surface of adsorbent,that developed more electrostatic interaction between active sites of adsorption and anions of chromium metal.The positively charged surface atlower pHwill favor the strong attraction ofadsorbate ions to the adsorbent surface.

Fig.2.pHpzc for H-SD and HF-SD.

3.1.3.SEM and EDX studies of an adsorbent

Fig.3 shows the surface morphology of R-SD and modified materials.Fig.3(a)indicated that the cross-section of R-SD has some distorted pores.Porosity can be seen but it is non-homogeneous and super ficial and bulk of the material seems to have no porosity resulting in less surface area.Filament like pattern of R-SD filled with material imparted anisotropic character.When raw sawdust was treated with HCl,the isotropy of R-SD was eliminated.There was a clear difference in surface morphology after treatment with acid as shown in Fig.3(b).After HCL treatment the surface of sawdust became rough and ruptured due to the acid reaction with components of sawdust,resulting in large cavities produced as this had also been confirmed by BET results for area measurement[53].Fig.3(c)depicts that when H-SD was further function alized with formaldehyde the surface morphology changes again and the cracked and busted surface of sawdust got healed.A glassy film type layer had covered the surface which decreases the surface area of HFSD as confirmed from BET analysis.This transformation indicated that the increase in adsorption of metal for adsorbents was probably due to the surface functional groups instead of porosity of an adsorbent.

Fig.3.Scanning electron micrographs of(a)R-SD,(b)H-SD and(c)HF-SD.

The spot analysis of raw sawdust contained major mass percent of carbon and oxygen,with traces of Calcium(Ca),Iron(Fe),Sodium(Na),and Silica(Si).The high percentage of carbon(52.3 wt%)and oxygen(47.4 wt%)in R-SD corresponds to cellulose,hemicellulose and lignin with small quantities of Na(0.02 wt%),Si(0.08 wt%),Ca(0.11 wt%),and Fe(0.06 wt%)which came from the type of soil during growth of plant.When R-SD was treated with acid it ruptured the surface of sawdust and devolved the cavities by removing organic material asa result of which carbon contents were increased and oxygen was decreased and trace elements were completely leached out.Acid treated sawdust had elemental composition as C(59 wt%),O(40.5 wt%),Ca(0.35 wt%),and Fe(0.13 wt%).As acid treated sawdust was further functionalized with formaldehyde carbon and oxygen contents again readjusted with small variations of trace element percentages,due to carbon and oxygen source provided by formaldehyde chemical.The detailed analysis is shown in Table 1.

Table 1 Chemical composition of raw and modified sawdust

3.1.4.Thermo-gravimetric analysis

Fig.4.Thermo-gravimetric(TG)and derivative thermo-gravimetric(DTG)analysis of R-SD,H-SD and HF-SD.

Thermal stability of raw and prepared adsorbents was studied using thermo-gravimetric analysis.Fig.4 compares the TGA and DTGA curves for R-SD,H-SD and HF-SD.Sawdust is composed of three main components hemicellulose,cellulose and lignin[54].The TGA curves show the initial mass loss of 3%–4%for R-SD,H-SD and HF-SD samples is attributed to the loss of surface adsorbed moisture[55].Thermal oxidation of R-SD,H-SD and HF-SD started at 250°C in the second phase and corresponds to decarboxylation reactions related to degradation of hemicellulose and cellulose up to 350°C,while lignin was burned between 350 °C and 450 °C[56,57].Residual mass of around 3.5%for RSD at the end of experimentation provides the indication of some impurities which were removed by HClsoaking which lead to complete burning of carbonaceous material with no residual mass.This observation is in agreement with EDX results.Further modification of H-SD with formaldehyde provided a protective layer which induces some resistance to thermal heating and avoided complete decomposition of material.

3.1.5.Surface porosity(BET)analysis

The surface area of R-SD was 0.04 m2·g?1which rises to 0.54 m2·g?1after soaking the R-SD in an acid solution.The subsequent treatment with formaldehyde solution reduces the surface area to 0.44 m2·g?1.Fig.5 summarizes the pore size distribution of sawdust and its modified forms.The raw material has few pores of meso-and macro-range which were affected by HCl treatment and decreases in numbers in addition to formation of some pores of micro-range.Later,the modification with formaldehyde creates few micropores due to the reaction with carbon structure of sawdust.The formation of new pores in the micro-range is compensated by a decrease in mesopores and resultant decrease in overall surface area of an adsorbent.

3.2.Adsorption studies

3.2.1.Effect of adsorbent dose

It is important to investigate the adsorption of Cr(VI)with different adsorbent doses,as it will illustrate the probability of adsorbent to adsorb chromium metal from solution having a particular initial concentration.As can be seen from Fig.6,with an increase of adsorbent dose the percentage removal of chromium increases for all three types of sawdust.At0.25 g of adsorbent dose,R-SD removes only 36%chromium while the other modified forms of sawdust give 53%and 75%for H-SD and HF-SD,respectively at the same dose as shown in Fig.6.Comparing the removal efficiency of modified sawdust shows that the HF-SD is superior to the H-SD.Probably,it is because of surface functional groups after the formaldehyde functionalization.The upturn of percentage removal was expected because with an increase of adsorbent dose the number of available active sites had been increased for adsorption of heavy metal ions[58].The analysis of experimental observations suggests 4.9 g can be considered as an optimum dose for the chromium removal for R-SD.On the other hand,after activation and functionalization of raw material the optimum dose goes much lower i.e.approximately 3.1 g for H-SD and 2.25 for HF-SD.The optimum dose required for HF-SD is 38%less than raw sawdust.As the adsorption potential of modified forms of sawdust is much higher than the raw sample,only activated and subsequently formaldehyde functionalized sawdust was taken for studying and optimizing the rest of the parameters.Comparing the increases in removal efficiency with small rise in BET surface area after modification of raws a wdust suggests the removal of metal ions is probably due to functional groups on the surface of modified sawdust instead of only pores on the surface.

Fig.5.BJH(Barrett–Joyner–Halenda)pore size distribution of R-SD,H-SD and HF-SD.

Fig.6.Effect of adsorbent dose on the percentage removal of chromium(temperature 30 °C,conc.100 mg·L?1,contact time 8 h.pH 6).

3.2.2.Effect of pH

pH of the aqueous solution of chromium is another most significant parameter which affects directly the adsorption of metal ions on the surface of an adsorbent.Fig.7 shows that the adsorption capacity decreases greatly with an increase in initial pH from 2 to 10.Chromium uptake was dropped down from 4.1 mg·g?1(at pH=2)to 2.8 mg·g?1for H-SD while it falls from 5.1 mg·g?1to 3.90 mg·g?1for the case of HF-SD,respectively.This may be due the factthatchromium anions behave differently at different pH[59].Hexavalent chromium exists in different forms like H2CrO4,CrO4?2,Cr2O7?2,and HCrO4?1in the aqueous solution.Available chromium exists in H2CrO4form below pH=2 and then chromium anions combined with hydrogen ions to form chromic acid instead of getting adsorbed on surface of adsorbent hence the percentage adsorption of chromium decreased.Above pH=2,chromiumtakes the formofHCrO4?1,Cr2O7?2and atneutral pH or higher it is present in the form of CrO4?2[60].At lower pH chromium adsorbed frequently in the form of HCrO4?1on surface of adsorbent,adsorbent was positively charged and electrostatic forces were developed between adsorbent surface and anions of chromium so,adsorption was high at lower pH.With the increase of pH,HCrO4?1ions of chromium start to deplete and surface charge also shifts from being positive to negative and as a result surface becomes repulsive towards chromium ions.At the same time the competition between OH?and CrO4?2would diminish the uptake to chromium ion onto the adsorbent surface.

Fig.7.Effect of pH on uptake capacity of chromium at(temperature 30°C,conc.100 mg·L?1,optimum doses,time(300 min)).

3.2.3.Kinetic studies of Cr(VI)adsorption

According to Fig.8,it can be observed that at the start,the rate of metal uptake on both(H-SDand HF-SD)was high because of availability of large number of vacant sites.As the time continues available active sites were going to decrease which diminishes the concentration potential and as a result,rate of adsorption slows down.Fig.8 depicts that formaldehyde functionalized adsorbentneeds a long time to reach equilibrium as compared to its counterpart i.e.H-SD.This is probably due to the formation of a thin adhesive layer on the wood fiber after the posttreatment of H-SD with formalin solution resulting in deceleration of water penetration into the structure of HF-SD.It was concluded that 80 min would be sufficient to achieve equilibrium for H-SD while HFSD requires 160 min to reach equilibrium where the maximum uptake was 4.8 mg·g?1.

The results of the effect of time were used to study the kinetics of adsorption for describing uptake rate of chromium on the modified sawdust.Different kinetic models were used to explore the mechanism involved and the rate controlling step of adsorption.Kinetic models like pseudo- first and second order,and intra-particle diffusion models were used.

Fig.8.Effect of contact time on uptake capacity of chromium at(temperature 30°C,conc.100 mg·L?1,optimum doses,pH=6).

Pseudo first order model was offered by Lagergren for a solid/liquid system[7].In most cases The pseudo- first order model does not satisfy the adsorption phenomenon for the entire range of time.The mathematical expression of the pseudo- first order model can be written as Eq.(3).where,qeand qtare the amount of chromiumions which were adsorbed on the unit mass of an adsorbent(mg·g?1)atequilibrium and atany instant of time t(minutes),respectively.k1is the rate constant(min?1)for the pseudo- first order adsorption model.The plot of ln(qe?qt)versus t in Fig.9(a),gives the values of rate constant(k1),and qefrom slope and intercept,respectively.The parameter values of pseudo first-order linear regression for each adsorbent are summarized in Table 2.The values of qe(experimental)were much higher than qe(calculated)for both H-SD and HF-SD and regression coefficient was around 90%for each adsorbent.

Pseudo-second order model can be expressed as given below in Eq.(4)[59].

where,k2is the rate constant(g·mg?1·min?1)for the pseudo second order model,qeis the adsorption capacity(mg·g?1)at equilibrium and qtis the instantaneous adsorption capacity(mg·g?1)at any time t(min).t/qtwas plotted against t and value of qewas determined from the slope of plot.The initial adsorption rate k2qe2(g·mg?1·min?1)had been used to determine the rate of adsorption,k2(g·mg?1·min?1)was determined from the intercept of the plot.The regression values are summarized in Table 2.Experimental values of qeare much closer to the calculated value and the regression coefficient is approximately equal to unity for both H-SD and HF-SD.It can be concluded that the best fitted pseudo second order model implies that the mechanism of the Cr(VI)sorption onto modified sawdust is the chemisorption process.

Fig.9.Kinetic models of adsorption for hexavalent chromium:(a)pseudo first order,(b)pseudo second order,(c)intra-particle diffusion and(d)Boyd mo del.

Table 2 Kinetic parameters for the adsorption of Cr(VI)onto H-SD and HF-SD

To get a better insight of the mechanism involved in adsorption phenomenon,it is necessary to determine the rate limiting step.Film diffusion and/or intra-particle diffusion step may limit such type of adsorption processes.The Weber model and Boyd model were tested to infer whether the process is diffusional or film controlling.The Weber model for intra-particle diffusion can be expressed as in the following equation

Here q is the amount of heavy metal adsorbed(mg·g?1)at time t(min),kidis intra-particle diffusion constant(g·mg?1·min1/2),and c is the intercept;t0.5is the square root of time(min0.5).The experimental data of q was plotted versus t0.5.It was observed that the plot was not a straight line.Instead,it presented a multi-linear behavior which directed about two steps involved in the adsorption phenomenon.Such type of results was obtained by R.Gottipati for adsorption of chromium on Marmelos fruit shell[60].First steep line in beginning of the plot(Fig.8)indicated sudden adsorption on external surfaces of adsorbent particles,while the line at later stage represents the intra-particle diffusion.Initially,abundant avail ability of free sites on the adsorbent surface with high concentration gradient of chromium resulted in the steep curve.At later stage,the slope of curve was small,because most of the external free sites on the adsorbent surface were occupied in the initial step,and metal ions now have to penetrate within the interstitial spaces to get adsorbed.For both,H-SD and HF-SD,initially the rate of adsorption was controlled by film diffusion and then in the second step rate was controlled by inter-and intra-particle diffusion after 10 min for H-SD with a value of q=2.45 mg·g?1and 5 min for HF-SD with a value of q=3.24 mg·g?1until equilibrium was achieved.After equilibrium,no further intra-particle diffusion controlled net adsorption occurred on adsorbent as shown in Fig.9(c).This describes the dual dependence of adsorption rate on film diffusion as well as intra-particle diffusion.Because of the double dependence of adsorption rate,the Boyd model was also applied for analysis of film diffusion kinetic data.The Boyd model for film diffusion can be expressed as,

F is the fraction of metal amount adsorbed at any time t(min)with respect to amount adsorbed that could be adsorbed at equilibrium,and Bt is a mathematical function of F.Above Eq.(6)had a rearranged form as,

While，F(xiàn)=q/qe

where q is the amount of chromium adsorbed(mg·g?1)at any time t(min)and qerepresents the amount of chromium adsorbed(mg·g?1)at equilibrium.

The Boyd model was plotted by using the values of[?0.4977 ?ln(1?F)]and t(min).It can be seen from Fig.9(d)that the plots were straight lines and did not passed through the origin having intercepts of?0.2446 and?0.6073 for H-SD and HF-SD.Straight lines with intercepts also described that initially the rate of adsorption was controlled by film diffusion;otherwise it was controlled by intra-particle diffusion.The value of diffusion coefficient Diwas calculated by using the given expression.

where Diis the effective diffusion coefficient(m2·g?1)and r is the radius of adsorbent particles.The values calculated are summarized in Table 2.The value of diffusional coefficient for HF-SDis 2.3 times the diffusional coefficient of H-SD due to the adhesive layer of formalin on the adsorbent.In first 5–10 min approximately 75%of metal had adsorbed for both H-SD and HF-SD here,inter-and intra-particle diffusion was the rate limiting step.

3.2.4.Adsorption isotherm

Adsorption isotherms are the most significant to understand interaction of adsorbate with adsorbents,and for the optimum design ofan adsorption system[14].Experimental data were fitted by the Langmuir and Freundlich models.These models can be expressed as follows.

Langmuir isotherm

Freundlich isotherm

Fig.10.Adsorption isotherm models fitting for removal of chromium.

Here qe(mg·g?1)represents the adsorption capacity and ce(mg·L?1)indicates the concentration of adsorbate in solution at equilibrium,qmis the maximum adsorption capacity,and KLis the Langmuir adsorption equilibrium constant(L·mg?1),KFand n are Freundlich constants related to the adsorption capacity and adsorption intensity of the adsorbents.Fig.10 represents the adsorption isotherms for both adsorbents.The nonlinear fitting parameters from the Langmuir and Freundlich models were listed in Table 3.It could be seen from Fig.10 that the Langmuir model fitted the data better than the Freundlich model,which was confirmed with high value of regression factor(R2)as compared to Freundlich as provided in Table 3.This elaborated that adsorption of chromium was monolayer adsorption.Maximum adsorption capacity(qm)for HF-SD was greater than that for H-SD probably due to the attached carbonyl groups on the surface of adsorbent which leads to enhanced electrostatic force between anions of chromium and adsorbent surface.And due to functional ization with formalin HF-SD needs fewer amounts as compared to H-SD for the same adsorption removal of chromium ions.

Table 3 Adsorption isotherms fitting parameters for H-SD and HF-SD

3.2.5.Estimation of thermodynamic parameters

The Gibbs free energy for the adsorption of Cr(VI)can be calculated by the Van't Hoff equation as provided here,

The adsorption heat at constant temperature and change in entropy can also be related with the Gibbs free energy by the relationship as,

By comparing and rearranging Eqs.(11)and(12),we get

Fig.11.Van't Hoff plot of adsorption equilibrium constant K.

In the above equations ΔG(J·mol?1)is the Gibbs free energy change,ΔS(J·mol?1·K?1)is the entropy change,ΔH(J·mol?1)is the change in enthalpy,R(8.314 J·mol?1·K?1)is universal gas constant,T(K)is the temperature,K is the constant of equilibrium,CAe(mg·L?1)is concentration of metal on the adsorbent at equilibrium,and Ce(mol·L?1)is the concentration of metal in solution at equilibrium.The plot(Fig.11)of ln K versus 1/T was used to calculate ΔH,ΔS.Negative values of ΔG describe that the adsorption process is spontaneous in nature and as a rule of thumb an increase in negative values of the Gibbs free energy with rise in temperature implies that the lower temperature favors the adsorption of chromium.A positive value of ΔH reflects that the adsorption process is endothermic and the adsorbate has to displace more than one water molecule for their adsorption and this results in the endothermicity of the adsorption process.The positive value ΔS may refer to the affinity of the adsorbent towards the adsorbate and suggests the increase in randomness at the solid–liquid interface with some structural changes in the adsorbate and the adsorbent.The adsorbed water molecules which were displaced by the chromium ions gain more translational entropy than is lost by the adsorbate chromium ions,thus gains more randomness in the system(Table 4).

Table 4 Thermodynamic parameters for the adsorption of hexavalent chromium onto,H-SD and HF-SD at different temperatures

In Table 5 comparison of adsorption capacity of A.nilotica sawdust with similar bio-sorbents illustrates that A.nilotica sawdust has almost similar or higher capability to adsorb the chromium metal from waste water.

Table 5 Comparison of adsorption capacity of Acacia nilotica sawdust with other bio-sorbents

4.Conclusions

The research results indicate that the keekar sawdust could serve as an inexpensive source to treat chromium(VI)bearing wastewater.The characterization of synthesized adsorbents by SEM images reveals that the acid treatment of raw sawdust creates few pores on the surface which were later covered by an adhesive layer of formaldehyde and as a consequence surface area decreases.The porosity results through BET analysis support the findings of SEM images.The thermal analysis with TGA reflects that the HCl soaking re fined the raw sawdust and impurities were removed which lead to complete burning of carbonaceous material with no residual mass and elemental analysis by EDX is in agreement with thermal analysis.The treatment of raw sawdust with HCl solution and subsequent modification with formaldehyde improves the adsorption characteristics and as a result the optimum amount required for HF-SD adsorbent is 0.72 times the optimum dose required for H-SD.The kinetic study verifies that the pseudo-second order model is better fitted to experimental observations.The Langmuir isotherm model was followed by both modified forms of sawdust with fairly high regression coefficient(R2=0.995 for H-SD,and R2=0.998 for HF-SD)as compared to the Freundlich model.Thermodynamic analysis depicts the spontaneous and endothermic nature of chromium adsorption onto sawdust based adsorbent.Considering the low price of A.nilotica sawdust,it could be an excellent material for water purification application.

Acknowledgments

Authors are thankful to Chemical Engineering Department,University of Engineering and Technology Lahore for providing the facilities for conducting this research study.

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