Mohamed Khitous *,Zineb Salem ,Djamila Halliche

1 Laboratory of Industrial Process and Engineering LSGPI,Faculty of Mechanical and Process Engineering,University of Sciences and Technology,Houari Boumediene,BP 32 El-Allia,Bab Ezzouar,Algiers 16111,Algeria

2 Laboratory of Natural Gas Chemistry,Institute of Chemistry,University of Sciences and Technology,Houari Boumediene,BP 32 El-Allia,Bab Ezzouar,Algiers 16111,Algeria

1.Introduction

Rapid industrialization and urbanization have resulted in elevated emission of toxic metals entering the biosphere.The release of heavy metals in biologically available forms may damage or alter both natural and man-made ecosystems[1].One of the most toxic heavy metal contaminants of concern is the hexavalent chromium,which is present in the effluents produced during the electroplating,leather tanning,cement,mining,dyeing and photography industries.It causes severe environmental and public health issues[2].Cr(VI)is highly mobile and is considered acutely as toxic and mutagenic for most organisms;in humans its main effects are on skin,liver,kidney and respiratory organs,resulting in a variety of diseases such as dermatitis,hepatic and renal tubular necrosis,bronchitis and bronchogenic carcinoma[3,4].The tolerance limit for Cr(VI)for discharge into surface water is 0.1 mg·L-1and in drinking wateris 0.05 mg·L-1[5].In orderto comply with this limit,it is essential that industries treat their effluents to reduce the concentration of Cr(VI)to acceptable levels.

In this respect,different techniques have been employed to remove Cr(VI)from wastewaters,like chemical precipitation,biological treatment,membrane filtration and sorption.The physicochemical treatments are commonly used because they are more economical than the electrochemical ones[6].Among these techniques,sorption is one of the most common and cost-effective physical processes that may be used to remove Cr(VI)from solution.It has been reported that zeolites[7],activated carbon[8],minerals[9],cationic and anionic clays have been used as sorbent materials for Cr(VI)[10,11].

Layered double hydroxides(LDHs),also known as hydrotalcite-like compounds,are a class of synthetic two-dimensional nanostructured anionic clays whose structure can be described as containing brucitelike layers,where a fraction of the divalent cations coordinated octahedrally by hydroxyl groups have been replaced isomorphously by trivalent cations,giving positively charged layers with charge-balancing anions between them.Some hydrogen bonded water molecules may occupy any remaining free space in the interlayer region[12,13].They may be represented by the general formula[M2+1-xM3+x(OH)2]x+(An-)x/nmH2O,where M2+and M3+are divalent and trivalent cations,respectively;x is the M2+/M3+molar ratio and An-is an anion.

Due to their intercalation ability of anionic species and other physicochemical properties,LDHs represent an inexpensive,versatile and potentially recyclable source of a variety of catalyst supports,catalyst precursors,sorbents,medicine stabilizers and ionic conductors[14-16].They have attracted attention as low-cost effective sorbents to remove negatively charged contaminants.Their high uptake levels of anionic species can be accounted for by their relatively large surface area,high exchange capacities[17],the flexible and large interlayer space containing a significant number of exchangeable anions,which is accessible to polar molecular species as well as harmful anions polluting water and soil.LDHs can take up anion species from solution by three different mechanisms:surface adsorption;interlayer anionexchange and reconstruction of a calcined LDH precursor by the“memory effect”.The last one takes place when LDHs are calcined to eliminate most of the interlayer anions followed by rehydration;anions are incorporated and the calcined LDHs recover their original layered structure[18].The memory effectof LDHs is one oftheir mostattractive features as sorbents for anionic species[19].Calcination allows the recycling and reuse of the sorbent with elimination of organic contaminants as CO2and water[20].Moreover,the calcined LDHs are more effective sorbents than uncalcined LDHs because interlayer carbonate ions of uncalcined LDHs are dificult to displace with other anions[21,22].However,the use of calcined LDHs presents two shortcomings:(1)a complicated regeneration stage because it needs a recalcination of the samples after sorption and(2)an increase in solution pH[23,24].

The anion-exchange capacity of LDHs is affected by the nature of the interlayer anions initially present and the layer charge density.When the layer charge density is very high,the exchange reaction may become dificult.LDHs have greater affinities for multivalent anions compared with monovalent anions[25,26].In particular,the favorable lattice stabilization enthalpy associated with carbonate results in these anions being dificult to displace in anion-exchange reaction.Therefore,the anion removal in LDHs by intercalation occurs when the interlayer are intercalated by weak electrostatic interactions with the layers,such as chloride or nitrate.

Previous studies have investigated the ability of LDHs to remove Cr(VI)from solution by both surface sorption and anion-exchange.Lazaridis et al.[22]studied the sorption of Cr(VI)over MgAl-CO3LDHs.They reported an increase of sorption capacity with agitation speed,at low pH and high Cr(VI)concentration.Detailed kinetic and equilibrium studies were done and they reported 17 mg·(Cr(VI)g)-1.Carriazo etal.[27]compared the chromate uptake capacity of MgAl-Cl LDH with its carbonate counterpart.The results show that MgAl-Cl samples are better sorbents than calcined MgAl-CO3samples.Recently,many studies have been reported on the effect of LDH structure for Cr(VI)removal.Wang et al.[28]studied the Cr(VI)uptake using MgAl,NiAl and ZnAl LDHs with molar ratio(M2+/M3+=3).The authors found that ZnAl-LDH exhibits excellent sorption capacity(68.07 mg·g-1)and good regeneration performance.Zhang et al.[29]reported the removal of Cr(VI)onto MgAlFe-NO3LDHs synthesized by a mechanohydrothermal method with varying Mg2+/Al3+molar ratio.The resulting LDHs have higher removal efficiency of Cr(VI)compared to that prepared by conventional mechanochemical method.

However,few studieshave reported the effectofthe interlayeranion on the sorption ability of LDHs to remove Cr(VI)[30,31].Compared to most of the studies focusing on sorption feasibility of LDHs,the sorption mechanism and its kinetics are not yet fully understood.The sorption mechanism was not explained accurately in most of the cases due to the scarcity of reliable methods[32].Most of the authors focused on the position of the basal reflections to determine the anion-exchange mechanism[30,32].Moreover,the explanation of the equilibrium isotherms,and the effects of operating and thermodynamic parameters to improve the overall efficiency of contaminants removal were not deeply discussed.Therefore,the objective of this study is to evaluate the sorption ability of MgAl LDHs to remove Cr(VI)from aqueous solution as a function of interlayer anion.The effect of operating conditions on Cr(VI)removal,including dosage of LDHs,solution pH,initial concentration,contact time and temperature,have been examined in batch tests.The explanation of the LDHs structure and sorption mechanism has been supported by X-ray diffraction and FTIR spectra.The sorption kinetic and isotherms are also investigated to determine the sorption mechanism.

2.Materials and Methods

2.1.Sorbent synthesis

Co-precipitation method was adapted to synthesis MgAl LDHs with NO3-,SO42-and Cl-as interlayer anion.All chemical reagents used were analytically pure reagents and obtained from Merck and Sigma-Aldrich.The solutions were prepared with deionized water.

MgAl-NO3and MgAl-SO4LDHs were synthesized by adding Mg(NO3)2·6H2O and Al(NO3)3·9H2O solution drop wise into a NaNO3or NaSO4solution,respectively.MgAl-Cl LDH was prepared by adding MgCl2and AlCl3solution into NaCl solution.For all the above syntheses,50 ml of Mg-Al precursor solution containing 0.025 mol of Mg2+and 0.0125 mol of Al3+to give Mg2+/Al3+molar ratio,R=2 and 50 ml of anion precursorsolution containing 0.025 molofanions were used.Aqueous solution of NaOH at3.4 mol·L-1was then added to the mixture drop wise,with vigorous stirring atroom temperature and the pHofprecipitation was adjusted to 10.The precipitate formed was aged at60°C in an oil bath shaker for 24 h,cooled and filtered.The obtained precipitate was washed repeatedly with distilled water to reach a neutral pH and then dried at80°C in an oven for24 h.Finally,the resulting solids were ground and sieved into powder of 50-100 μm in diameter,and kept in sample bottles for further use and characterization.The calcined MgAl-NO3LDH was prepared by heating its dried sample at 500°C in air for 4 h,with a heating rate of 4 °C·min-1.In this study,sulfate-containing LDH was selected because of the abundance and relatively low toxicity of sulfate ions.Moreover,as discussed by Y.Mitsuo[33],sulfate-containing LDH is effective for the removal of Cr(VI)via anion-exchange compared to CO32--type MgAl LDH for which the anion-exchange is very dificult to occur.On the other hand,sulfate ions in wastewater can be reduced during the feeding process of plants,algae and bacteria.

2.2.Sorbent characterization

Powder X-ray diffraction(XRD)analyses were carried outto identify the phase of Mg-Al LDHs using an XPERT-PR diffractometer with CuKαradiation(λ=0.154060 nm)at 45 kV and 40 mA.Scanning diffraction angle is set at the speed of 0.06(°)·s-1.The BET specific surface area and average pore diameter of the prepared MgAl-NO3LDH and its calcined product were measured from the N2adsorption/desorption isotherms at 77 K by using Tri-Star II 3020V1.03 Micromeretics surface area and pore size analyzer.The thermogravimetric analysis(TGA)was carried out using a thermal gravimetric analyzer(Perkin Elmer STA 6000).The samples were heated in air from 30 to 900°C with a ramping rate of 5 °C·min-1.The morphology of samples before and after sorption of Cr(VI)was viewed by scanning electron microscopy(SEM)using an XL30 ESEM at accelerating voltage of 20 kV.

Elemental chemical analysis was performed using an inductivity coupled plasma(ICP)emission spectrometer for metal ions in the samples to determine the Mg/Al molar ratio of the prepared materials.0.04 g of each sample was dissolved in solution containing 15 ml of concentrated HNO3and 5 ml of HCl.

FTIR spectra of LDHs materials and their samples after sorption of Cr(VI)were recorded using an Alpha Bruker FTIR spectrometer over the 4000-400 cm-1wave number range.

The pH at the point of zero charge(pHzpc)of the prepared MgAl-NO3LDH was determined by different concentrations of NaCl at 0.1 mol·L-1and 0.05 mol·L-1as inert electrolytes.The initial pH was adjusted from 2 to 13 by addition of 0.1 mol·L-1HNO3and NaOH solutions.The experiments were carried out in a thermostatic shaker at 200 r·min-1and 20°C for 2 h,by adding suspension of the same solid to solution ratio(1 g·(5 L)-1)for all experiments.Then the suspensions were filtered and the final pH values were measured.The pHzpcwas obtained through the plots of pHfinalversus pHinitial[34].

2.3.Sorption and kinetic experiments

All sorption experiments were carried out at room temperature using the batch equilibrium method in open bottles of 100 ml,by adding a given dose of sorbent to 50 ml of metal solution on a vertical rotary shaker.K2CrO4salt was used for the sorption experiments.The Cr(VI)stock solution was prepared by dissolving accurately weighed salt in distilled water to a concentration of 500 mg·L-1.The experimental solutions were obtained by diluting the Cr(VI)stock solution in accurate proportions of distilled water to different initial concentrations.Solution pH was adjusted to desired values by adding negligible volumes of HCl and NaOH at 0.1 mol·L-1.The sorbents were separated by centrifugation(1200 r·min-1).Then,chromium concentrations in the supernatant were determined by using 1,5-diphenyl carbazide(DPC)UV-visible spectrophotometry method at 540 nm[35].All assays were carried out in triplicate and only means values are presented.

The chromium uptake capacity of the hydrotalcite materials was determined from Eq.(1):

where C0and Ct(mg·L-1)are the concentrations initially and at time t,respectively.qt(mg·g-1)is the amount of Cr(VI)sorbed at time t,m(g)is the sorbent mass and V(L)is the solution volume.

The effect of initial Cr(VI)concentration,contact time,temperature,pH solution and sorbent dosage were studied by changing one of these parameters while keeping other constants.

2.4.Equilibrium sorption measurements and modeling

The equilibrium sorption isotherm is important for the design of sorption systems.Sorption isotherm data were generated by contacting 0.1 g of sorption media with Cr(VI)in aqueous solution.50 ml of Cr(VI)solution ranging from 5 to 200 mg·L-1were introduced in 100 ml bottle.The bottles were shaken for 2 h at room temperature.The shaking speed(W)was set at 200 r·min-1.

The Langmuir isotherm represents the equilibrium distribution of sorbate between the solid and liquid phases,and is often expressed as:

where Ce(mg·L-1)is the concentration of sorbate at equilibrium,qe(mg·g-1)is the amount sorbed per mass of sorbent at equilibrium,K(L·mg-1)is the equilibrium constant related to the sorption energy between the sorbate and sorbent,Q0(mg·g-1)is the limiting amount of sorbate that can be taken up per mass of sorbent.

The Langmuir isotherm assumes that metal ions are chemically sorbed at a fixed number of well-defined sites,that each site can hold only one ion,that all sites are energetically equivalent and there is no interaction between ions[36].

The linearized Langmuir equation is given as:

The linearform can be used forlinearization of experimentaldata by plotting Ce/qeversus Ce.

The efficiency of the sorption has been predicted by the dimensionless constant separation factor RL(equilibrium parameter),which is defined by the following equation:

where C0(mg·L-1)and K are the initial concentration and Langmuir constant,respectively.The values of RLin Table 1 indicate the nature of isotherm.

Table 1 R L values and type of isotherm

The Freundlich equation can be used to estimate the sorption intensity of the sorbent towards the sorbate and is expressed as:

where qe(mg·g-1)is the amount of sorbed metal,Ce(mg·L-1)is the equilibrium concentration and KFand n are constants incorporating all parameters affecting the sorption process,such as sorption capacity and intensity[37].

The Freundlich equation is conveniently used in the linear form by taking the logarithm of both sides as:

The values of KFand n were calculated from the intercepts and slopes of the Freundlich plots,respectively.These parameters describe the separation process of Cr(VI)from the solution.The n value is related to the distribution of bonded ions on the sorbent surface.

The Dubinin-Radushkevich isotherm was chosen to estimate the sorption energy[38].

The model is often expressed as:

where KDR(mol2·J-2)is the activity coefficient,ε is the Polanyi potential,R is the universal gas constant(8.314 J·mol·K-1)and T(K)is the temperature.

The plot of ln qeas a function of[RT ln(1+1/Ce)]2gives a straight line.The values of qmand KDRwere calculated from the intercepts and slopes of the Dubinin-Radushkevich plots,respectively.

The sorption energy was calculated using Eq.(9):

where E(J·mol-1)is the sorption energy.

2.5.Thermodynamic study

The thermodynamic parameters provide in depth information about internal energy changes that are associated with sorption.

The effect of temperature on Cr(VI)sorption was determined by adding 0.1 g of sorbent to 50 ml of Cr(VI)solution at 293,303 and 313 K,till equilibrium was reached.The thermodynamic parameters such as enthalpy change(ΔH0),entropy change(ΔS0)and Gibbs free energy change(ΔG0)were calculated using the following equations[39]:

where R is the ideal gas constant(8.314 J·K·mol-1),K(L·mol-1)is Langmuir constant and T(K)is the temperature.ΔH0(kJ·mol-1)and ΔS0(J·mol·K-1)are calculated from the slope and intercept of Van't Hoff plot of ln K versus 1/T.

3.Results and Discussion

3.1.Characterization of the sorbents

XRD analysis was carried out to study the formation of synthesized Mg-Al LDHs.The obtained XRD patterns are shown in Fig.1.It appears clearly that the diffraction patterns of the three LDHs have sharp and symmetric peaks at lower 2? as(003)and(006),which are characteristic ofthe hydrotalcite-like compounds with a high degree ofcrystallinity.The patterns are indexed to a hexagonal cell[40].No other phase was observed in the XRD patterns of materials.The basal reflections(001)at low angles(10-11°,2?)correspond to the interlayer distance of LDHs.The(110)reflections appear at high angles(60.561°,2?).The lattice parameters of the samples are estimated as c=l×d001;a=2×d110,and summarized in Table 2.These values agree well with the values reported in the literature[41].The non-uniform broadening of lines in the mid-2? region(30°-50°)is indicative of structural disorder.

As shown in Table 2,a slightdifference in the interlayer distance was observed when various anions were intercalated,due to their difference in dimension.When Cl-was inserted as a guest anion,the interlayer distance was≈0.772 nm;while when NO3-or SO42-were inserted,the distance was expanded to 0.837 nm and 0.879 nm,respectively.The results agree well with those reported by Srinivasa[41].They suggest that a distance of 0.78-0.80 nm is characteristic of a single atom thick interlayer,and the higher interlayer spacing of around 0.89 nm is evocative of high interlayer spacing with a thickness of 2-3 atoms.The order of basal spacing(d003)agrees with that of ionic size,Cl-(0.336 nm)＜NO3-(0.400 nm)＜SO42-(0.436 nm)[42].The d003is substantially determined by sum of the thickness of the basic layer and the size of intercalated anion[43].We can see also that the three samples have almost the same size d110values.

Fig.1.XRD patterns of synthesized(a)MgAl-SO4,(b)MgAl-NO3 and(c)MgAl-Cl LDHs samples.

Table 2 Characterization of the prepared LDHs as a function of interlayer anion

As presented in Table 2,the resultant Mg/Al molar ratio of MgAl-Cl,MgAl-NO3and MgAl-SO4LDHs were determined to be 1.95,1.97 and 1.94,respectively.This was in good agreement with the initial values of the starting solutions.Therefore,Mg-Al LDHs with various interlayer anions and Mg/Al molar ratio of 2 were successfully synthesized.

The thermogravimetric analysis(TGA)profiles of the prepared LDHs are shown in Fig.2.They are similar in shape with two distinct regions of weight loss.The initial weight decrease of approximately 13%as temperature increases from 30 to 180°C was due to the loss of interlayer water[44].The second weight decreases as temperature increases from 250 to 500°C was due to the dehydroxylation of the octahydral layers as well as the decomposition of the interlayer anion.

However,MgAl-SO4showed a relatively higher thermal stability,and its second weight loss ends at around 600°C.The total weight loss of all samples is shown in Table 2.They are around 37%-50%.

Fig.3(a)-(b)shows the XRD patterns of the calcined MgAl-NO3LDH before and after sorption of Cr(VI).It appears that the characteristic peaks of MgAl-NO3LDH disappeared,and two new broad peaks were observed at high 2? angles of 43.5°and 63°,corresponding to the(200)and(220)reflections,respectively,as given by Bakhti et al.[45].The result indicates also that the layered structure of LDH material was destroyed and transformed into amorphous phase when heated at 500°C.This is due to the decomposition of the original structure of LDH into mixed oxides of Mg and Al during the thermal treatment[46].

After sorption of Cr(VI),the layered structure of calcined MgAl-NO3was reconstructed,as shown in Fig.3(b).However,the peaks were broadened and their intensities decreased in comparison with those of the original LDH,indicating the reduction of crystallinity after rehydration.Therefore,the sorption of Cr(VI)by calcined LDH occurs via reconstruction.

BET analysis was also performed on both uncalcined and calcined MgAl-NO3LDHs,as shown in Table 3.The results showed that the surface area of MgAl-NO3LDH is very low(5.70 m2·g-1).This result is in good agreement with that reported by Wang et al.[47].They have reported a very low surface area(＜9 m2·g-1)for LDHs with NO3-,SO42-and Cl-as interlayer anions.

Moreover,the thermal treatment of the samples at 500°C led to the increase of surface area and adsorption average pore diameter,due to the decomposition of LDH into metal oxides,with removing of interlayer anions and water.

The morphology of LDHs was investigated by SEM,as shown in Fig.4(a)-(d).It seems that the LDHs formed “stones”morphology of varying size and shape,with a particle size ranging from 3 to 10 μm.High-resolution SEM analysis indicated also that these particles are nonporous(not shown here).Therefore,their surface area is very low.

This is in accordance with the results of BET surface area presented in Table 3.From Fig.4(d),we can see some modifications in the surface morphology of MgAl-NO3LDH after sorption of Cr(VI),which became rough after sorption.This indicates that there are some interactions between Cr(VI)ions and the sorbent,and Cr(VI)is successfully incorporated into MgAl-NO3LDH.This may be due to an exchange chemical reaction between Cr(VI)and interlayer nitrate of this material.

Fig.2.TGA analysis of(a)MgAl-SO4,(b)MgAl-Cl and(c)MgAl-NO3 LDHs.

To ensure that we have successfully synthesized the LDHs with aimed anions,FTIR analysis was also performed.The FTIR spectra of LDHs materials and their corresponding samples after sorption of Cr(VI)are shown in Fig.5.The characteristic peaks were observed for all synthesized materials.The intense broad bands observed at around 3500-3250 cm-1associated with stretching vibration of O-H bond in the brucite-like layers Mg(OH)2and Al(OH)3[48,49].The band at about 1642 cm-1can be assigned to the deformation vibration of water molecules in the interlayer domain[50].The bands at 540-650 cm-1can be due to the vibration of Al-O and Mg-O groups in the layers.Interlayer anions slight in fluenced the peak positions of Al-O and Mg-O vibrations[51].The characteristic sharp absorption band at about 1380 cm-1is associated with the antisymmetric stretching mode ofNO3-[12].For MgAl-SO4LDH,the sulfate species were detected by the appearance of a peak at 1095 cm-1;besides a small peak at 1358 cm-1suggested that certain amount of nitrate co-existed with sulfate in the sample[52].On the other side,the interlayer Cl-bands cannot be observed in the studied region of the spectrum.This result is similar to that reported by López et al.[51].

In addition,more informations about the interlayer anion were obtained from the FTIR analysis.The weak peaks at around 1362 cm-1for all samples are most probably due to the carbonate ions,which might have been formed by the adsorption of atmospheric CO2[53].

Fig.3.XRD patterns of calcined MgAl-NO3 LDH(a)before and(b)after sorption of Cr(VI).

Table 3 BET surface area and adsorption average pore diameter of uncalcined and calcined MgAl-NO3 LDH

By comparison between the FTIR spectra of Mg-Al LDHs and their samples after sorption of Cr(VI),we can see that the solids remain the same absorption characteristic bands,but their intensity was significantly reduced.This indicates thatthe solids maintain theirchemical structure and there is an ion exchange reaction between the solid and the solution in presence of interlayer anions and hydroxides(OH-)of the layers.The characteristic band of free chromate appeared at890 cm-1by Nakamoto[54],butitshifted towards the lowerfrequency slightly for all three LDHs compounds,which indicates that the Cr-O band for Cr-sorbent is weaker than that for free chromate[55].In the spectra of both solid samples after sorption,it appears that the characteristic band of Cr-sorbent remained almost constant and appeared at 878 cm-1,suggesting that the binding force of Cr-sorbent is independent on interlayer anions.

Fig.6 shows pHfinalversus pHinitialcurves at different electrolyte concentrations.The pHzpcof MgAl-NO3LDH is around 7.8 as revealed by crossing point of the curves,which shows an amphoteric surface.There is a reasonable prediction that positively charged sites with anions sorbed are predominant when pH is lower than 7.8,while negatively charged sites with cations sorbed are predominant when pH is higher than 7.8.Therefore,there are electrostatic forces during the sorption of CrO42-on MgAl-NO3LDH in addition to the main anion-exchange mechanism.

3.2.Sorption properties

3.2.1.Effect of pH

Earlier studies have indicated that the solution pH is an important parameter affecting the sorption of heavy metals at solid-liquid interfaces[56].The effect of solution pH on the removal of Cr(VI)by uncalcined and calcined MgAl-NO3LDH is shown in Fig.7.It was carried out at pH ranging from 2 to 8, fixed initial Cr(VI)concentration(Ci)of 100 mg·L-1,and sorbent dose of 2 g·L-1.The solutions are stirred for a period of 2 h to allow sufficient time for sorption equilibrium.

In an aqueous solution,chromium oxide species depend on pH solution and Cr(VI)concentration.The predominant species of Cr(VI)in solution are HCrO4-,CrO42-or Cr2O72-,depending on the value of pH,while Cr3+is in the form of Cr(OH)2+or CrO22-[57].In solution,Cr(VI)exists only as CrO42-and HCrO4-forms,whereas Cr2O72-exists only in more concentrated solutions and H2CrO4in more acidic solutions.There is an equilibrium between ions like bichromate HCrO4-,dichromate Cr2O72-and chromate CrO22-,when chromium salt dissociates in aqueous solution.The equilibrium is in favor of Cr2O72-ions(orange)at lower pH,but it shifts in favor of bivalent CrO42-ions(yellow)when the pH is more than 7.As a result,the bivalent CrO42-ions became the dominant species in basic medium[58].

Fig.4.SEM images of(a)MgAl-Cl,(b)MgAl-SO4 and(c)MgAl-NO3 LDHs before and(d)after sorption of Cr(VI).

Fig.5.FTIR spectra of(a)MgAl-Cl,(b)MgAl-NO3 and(c)MgAl-SO4 LDHs.a—before sorption;b—after sorption of Cr(VI).

Fig.7 shows that the removal of Cr(VI)by MgAl-NO3LDH is pH dependent.The sorption capacity increased as pH increased from 2 to 6,and decreased at pH ranging from 6 to 8.Hence,it decreased at much lower and higher pH,and the maximum sorption amount on uncalcined MgAl-NO3LDH was about 34 mg·g-1and 38 mg·g-1at pH values of 2 and 8,respectively.The highest sorption capacity was obtained at pH ranging from 5 to 6(48 mg·g-1).This suggests the existence of optimum values of pH for the maximum removal of Cr(VI).

Considering the properties of LDHs materials,they can develop surface charge due to the structural substitutions and anion-exchange reactions with anionic species in solution.Thus,they can form permanentand variable charges due to the sorption and desorption of protons and hydroxide ions.

In acid solution,the surfaces of MgAl-NO3LDH are protonated and therefore acquire positive charges.The degree of protonation reduces with the increase of pH,resulting less positive charges to combine Cr(VI)ions from aqueous solution.However,under strongly acidic conditions(below pH 3),low metal ion uptake is observed.This phenomenon can be explained by the dissolution of the sorbent at low solution pH.

Fig.6.pH fi nal as a function of pHinitial for MgAl-NO3 LDH.(m=0.2 g·L-1,T=20 °C and W=200 r·min-1).

In an alkaline pH condition(pH=8),the amount of sorbed Cr(IV)decreases compared to that obtained at pH ranging from 4 to 7.This is due to the charge reduction on the LDH surface,because of the interaction with hydroxyl ions from the solution.Hence,the optimum pH was selected as 6 for Cr(VI)removal by MgAl-NO3LDH.

On the other hand,although MgAl-NO3LDH has a surface positive charge at pH values under pHzpc,the hydroxide ions react with the positive charge in alkaline solution and even occurs as deprotonation.

In order to explain the effect of thermal treatment of material and the mechanism involved in sorption processes,we compared the sorption ability of Cr(VI)on MgAl-NO3LDH and its calcined product at 500°C at various pH values,as shown in Fig.7.

Fig.7.EffectofpHsolution for Cr(VI)removalonto MgAl-NO3 LDHand its calcined sample at 500 °C(Ci=100 mg·L-1,m=2 g·L-1,T=20 °C and W=200 r·min-1).

The results indicate much higher sorption capacity for MgAl-NO3LDH of about 50 mg·g-1at pH values of 5 and 6.This behavior is in disagreement with other results reported in the literature[59,60],which can be explained by the main anion-exchange mechanism involved in the sorption of Cr(VI)on this material.LDHs can remove anions from solution by three different mechanisms:(1)adsorption on externalsurface,(2)intercalation by anion exchange and(3)intercalation by reconstruction of calcined material.A combination of anionexchange and adsorption on external surface occurs for the removal of anions by uncalcined LDHs.The former taking place when the anions in the LDHs are intercalated by weak electrostatic interactions with the layers,such as Cl-or NO3-.The adsorption process on external surface is generally considered negligible.The second mechanism is the memory effect,which occurs when the material is calcined,undergoing dehydroxylation,removal of interlayer anion and increasing its surface area and pore volume(Table 3).The mixed oxides obtained after calcination can rehydrate to rebuild the initial LDH structure and incorporate anions in solution.Therefore,the significant difference between the sorption ability of these materials is mainly related to the contribution of the anion-exchange mechanism in the uncalcined LDHs.

3.2.2.Effect of sorbent dosage

Cr(VI)Solutions at 100 mg·L-1and pH of 6 were prepared and dosed with different sorbent quantities varying from 0.5 to 5 g·L-1,for a period of 2 h(Fig.8).

Fig.8.Effect of sorbent dosage for Cr(VI)removal onto MgAl-NO3 LDH.(pH=6,Ci=100 mg·L-1,T=20 °C and W=200 r·min-1).

The results show that the sorption capacity of MgAl-NO3LDH increased with increasing the sorbent dose from 0.5 g·L-1(30.84 mg·g-1)to 2.0 g·L-1(45.72 mg·g-1).This is due to the increase ofthe sorbentsurface area and the availability ofthe exchangeable sites at higher sorbent concentration,which offered more contact surface for Cr(VI)sorption.The increase of sorption capacity is due to the increase of the vacant sorption sites with more sorbent existing in solution[61].It appears also that the sorption capacity remains almost constant as the sorbent dose increased from 2 to 5 g·L-1.This suggests the existence of the optimum dose for the maximum Cr(VI)removal at 2 g·L-1.

3.2.3.Effect of initial concentration

The effect of initial concentration on the removal of Cr(VI)by Mg-Al-NO3LDH was investigated at various initial concentrations ranging from 5 to 200 mg·L-1.The results are shown in Fig.9.

Sorption was rapid in the first step,at the beginning up to 10 min,then slowed considerably till a saturation level was reached at 30 min.The initial rapid phase may be due to greater number of sorption sites available for the sorption of metal ions.The anion-exchange process is the major mechanism for the removal of oxyanions.In order to ensure a complete sorption equilibrium,120 min was chosen as the contact time in each experiment.

3.3.Equilibrium isotherms

The sorption isotherms of Cr(VI)on MgAl-NO3,MgAl-SO4and MgAl-Cl LDHs are shown in Fig.10.

The results showed a significant difference in the sorption ability of these materials,which was found to decrease in the following orderofinterlayer anions:NO3-＞Cl-＞SO42-.Hongo et al.[31]have found a similar tendency for the sorption of several harmful anions(F-,CrO42-,HAsO42-and HSeO3-)by nanosized Mg-Al layered double hydroxides with Cl-,NO3-or SO42-as interlayer anion.This can be explained as the result of divalent anions interacting more strongly with brucite-like sheets than monovalent anions.

According to the classification of Giles et al.[62],the isotherm on MgAl-SO4LDH is L-type,while that on MgAl-NO3and MgAl-Cl LDHs is H-type(Fig.10).The former reflects an increasing dificulty for Cr(VI)to find sorption sites on MgAl-SO4LDH because they become occupied.The latter indicates a high affinity between sorbate and sorbent so that chromium is almost completely sorbed from dilute solution.A similar tendency was also reported by Kameda et al.[63]for the removal of fluoride by Mg-Al LDHs intercalated with NO3-and Cl-.

As shown in Fig.11(a)-(c),the Langmuir,Dubinin-Radushkevich and Freundlich models were applied to describe the sorption behavior at equilibrium.The obtained parameters for all models are listed in Table 4.

The correlation coefficient values(R2)obtained from the three sorption models are shown in Table 4.From these R2values and the maximum capacities,it can be seen that the equilibrium data fitted the Langmuir isotherm with high correlation coefficient.The high coefficient values for the Langmuir isotherm indicate the monolayer sorption of Cr(VI)on Mg-Al LDHs.In addition,the values of RLare less than unity for MgAl-Cl and MgAl-NO3LDHs,showing a favorable sorption of Cr(VI),while that on MgAl-SO4LDH is close to 1,indicating a linear isotherm.

Freundlich and D-R isotherms are not as adequate as the Langmuir model(0.94＜R2＜0.99).The relationship between the sorbed amount and its equilibrium concentration in the solution is not adequately described.The maximum capacity qmcalculated by the Langmuir isotherm is close to that obtained at equilibrium(Fig.10),while the D-R isotherms give a maximum capacity not compatible with experimental qm.For Freundlich model,an increase in KFwith MgAl-NO3LDH is recorded.The 1/n values were between 0 and 1,indicating a favorable sorption of Cr(VI)on Mg-Al LDHs at the studied conditions,and the sorbent adsorbs better at low concentrations.According to Mckay et al.[64],the n values between 2 and 10 representa good sorption.The calculated n values for MgAl-Cl and MgAl-NO3LDHs of 2.47 and 2.59,respectively,indicate a good sorption of Cr(VI)on these materials.

The D-R isotherm modeldetermines also the nature ofsorption processes.The E(kJ·mol-1)value gives information about sorption mechanism(physical or chemical).If it lies between 8 and 16 kJ·mol-1,the sorption process takes place by chemical ion exchange,and while E ＜ 8 kJ·mol-1,the sorption process proceeds physically[39].We can see from Table 4 that E values were in the range of 8-16 kJ·mol-1.Thus,the Cr(VI)sorption processes on MgAl LDHs could take place by chemical ion-exchange mechanism.

According to Goh et al.[65],the common adsorption isotherms employed to describe equilibrium LDHs-oxyanion adsorption data follow the order Langmuir＞Freundlich＞modified Langmuir＞Redlich-Peterson～Sips.Liang et al.[66]have found that Aranovich-Donohue isotherm gave a better fit to the experimental data than Langmuir,Freundlich and Langmuir-Freundlich isotherms for the sorption of Pb(II)on Mg-Fe LDH.This difference in the sorption isotherm may be due to the sorption mechanisms involved in the process.As seen by Liang et al.[66],the sorption mechanism of Pb(II)on the LDH may be attributed to the surface-induced precipitation and the chemical binding adsorption.To the best knowledge of the authors,as yet no sorption system has reported on the incorporation of surface complexation models for describing the equilibrium oxyanion adsorption on LDHs[65].

Fig.9.Effect of the initial concentration for the Cr(VI)removal onto MgAl-NO3 LDH.(pH=6,m=2 g·L-1,T=20 °C and W=200 r·min-1).

3.4.Sorption dynamics

The dynamics of sorption process in terms of the order and the rate constant can be evaluated using the kinetic sorption data.The process of Cr(VI)removal from solution by sorbent can be explained by using kinetic models and examining the rate controlling mechanism of the sorption process such as chemical reaction,diffusion control and mass transfer.The kinetic parameters are useful in predicting the sorption rate,which can be used as important information in designing and modeling of sorption process.The kinetics of removal of metal ions are explicitly explained in the literature using the pseudo- first-order,pseudo-second-order and intraparticle diffusion models.

In order to investigate the mechanism involved in the sorption process,the linearized equations of these models were applied to the experimentaldata(Fig.12).The pseudo- first-order model[67]assumes that the binding is originated from physical sorption and the equation is given as:

Fig.10.Cr(VI)sorption isotherms on uncalcined MgAl-NO3,MgAl-SO4 and MgAl-ClLDHs.(pH=6,m=2 g·L-1,T=20 °C and W=200 r·min-1).

where qeand q(mg·g-1)are the amounts of Cr(VI)sorbed at equilibrium and at time t,respectively.k1(min-1)is the rate constant of the pseudo- first-order model.The values of qeand k1can be determined from the intercept and the slope of the linear plot of ln(qe-q)versus t.The pseudo-second-order model[68]is based on chemical sorption.It may be expressed as:

where qeand q(mg·g-1)follow the same definition as the pseudofirst-order model,and k2(g·mg-1·min-1)is the rate constant of the pseudo-second-order model.The slope and intercept of the linear plot of t/q versus t yielded the values of qeand k2.

The intraparticle diffusion model[69]describes sorption processes,where the rate of sorption depends on the speed at which sorbate diffuses towards sorbent,which is given as:

where C(mg·g-1)is the intercept and ki(mg·g·min-1/2)is the intraparticle diffusion rate constant,which can be evaluated from the slope of the linear plot of q versus t1/2.

The results of Fig.12(a)-(c)are fitted using pseudo- first-order and pseudo-second-order models,and intraparticle diffusion model.The fit of these models was checked by each linear plot of ln(qe-q)versus t,(t/qt)versus t and q versus t1/2,respectively.The kinetic coefficients obtained for each model are shown Table 5.

By comparing to the regression coefficients obtained for each expression,the pseudo- first-order and intraparticle diffusion models are not fully valid for the present system.Moreover,the experimental values of qedo not agree with the calculated ones obtained from the linear plots of models.

Contrariwise,a good agreement of the experimental data with the pseudo-second-order model was observed for all initial concentrations(Fig.12(b)).The correlation coefficients of the linear plots using the pseudo-second-order model are superior,and having values more than 0.99 in most cases.Theoretical and experimental values of qeshow also excellent agreement(Table 5).

Fig.11.Langmuir(a),Dubinin-Radushkevich(b)and Freundlich(c)plots for the sorption of Cr(VI)by uncalcined MgAl-NO3,MgAl-SO4 and MgAl-Cl LDHs.

Fig.12.Kinetic plots regression of MgAl-NO3 LDH:(a)pseudo- first-order model,(b)pseudo-second-order model and(c)intraparticle diffusion model.

Table 4 Isotherm models parameters for Cr(VI)sorption by MgAl-NO3,MgAl-SO4 and MgAl-Cl LDHs

Table 5 Coefficients of pseudo- first-order,pseudo-second-order and intraparticle diffusion models for Cr(VI)sorption by MgAl-NO3 LDH

Therefore,the sorption of Cr(VI)on MgAl-NO3LDH is kinetically controlled by a pseudo-second-order reaction rather than a pseudofirst-order process.Hence,chemical sorption is the rate-limiting step in the sorption process.Liang et al.[66]have also reported similar results during the sorption of Pb(II)by Mg-Fe LDH.

3.5.Thermodynamic parameters

With the purpose of understanding the effect of temperature on the sorption process,the thermodynamic parameters should be determined by the Langmuir isotherm(Fig.13).The thermodynamic parameters obtained at various temperatures are shown in Table 6.

Fig.13.Cr(VI)sorption isotherms on MgAl-NO3 LDH at various temperatures.(m=2 g·L-1,pH=6 and W=200 r·min-1).

As given in Table 6,the values of ΔS0and ΔH0are calculated to be 100.37 J·mol-1·K-1and 4.99 kJ·mol-1,respectively.The positive value of ΔH0shows the endothermic nature of sorption process.The positive value of ΔS0suggests the increased randomness at the solid-solution interface during the sorption of Cr(VI)on MgAl-NO3LDH.This implies some structural changes in sorbate and sorbent,which leads to an increase in the disorderness of the solid-solution system[70].The negative values of ΔG0indicate the feasibility and spontaneous nature of sorption process at the studied temperatures[67],and the values of ΔG0became more negative with the increase in temperature,showing that higher temperature facilitated the sorption of Cr(VI)on this material.This indicates that the spontaneity of process increases with the rise in temperature.At high temperature,anions are readily desolvated and hence its sorption becomes more favorable.

4.Conclusions

From the results obtained in this study,the following conclusions can be listed:

(1)The amount of sorption onto the LDHs differed between the starting interlayer anions,and decreased in the following order:NO3-＞Cl-＞SO42-.

(2)NO3-containing Mg-AlLDHs was an attractive sorbentto remove Cr(VI)from aqueous solution,and only 30 min was sufficient to reach the equilibrium state in Cr(VI)sorption.

(3)The study of sorption kinetic parameters(sorbent dosage and solution pH),permitted to establish the following optimum conditions:sorbent dose of 2 g·L-1and pH=6.

(4)Cr(VI)sorption through anion-exchange by MgAl-NO3is more efficient than that of the calcined one where the sorption occurs via reconstruction.

(5)Among the three tested models,Langmuir,Freundlich and Dubinin-Radushkevich models,the Langmuir model is the most appropriate to describe Cr(VI)sorption isotherm using MgAl LDHs with high correlation coefficient,indicating the monolayer sorption of Cr(VI)on these materials.From the D-R isotherm model,the sorption process takes place by chemical ion exchange mechanism.

(6)The kinetic study indicates that the Cr(VI)sorption mechanism on MgAl-NO3LDH follows a pseudo-second-order kinetic reaction.

(7)The thermodynamic study revealed that the Cr(VI)sorption nature on MgAl-NO3LDH is endothermic and spontaneous process.

Table 6 Thermodynamic parameters for Cr(VI)sorption by MgAl-NO3 LDH

Acknowledgments

The authors would like to thank Pr M.Azzaz for X-ray diffraction analysis.The work was financially supported by both the department of Process Engineering and Chemistry of USTHB(Algiers).

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