Removal of hexavalent chromium in aquatic solutions by pomelo peel

Qiong Wang*,Cong Zhou,Yin-jie Kuang,Zhao-hui Jiang,Min Yang

aHunan Provincial Key Laboratory of Comprehensive Utilization of Agricultural and Animal Husbandry Waste Resources,College of Urban and Environmental Sciences,Hunan University of Technology,Zhuzhou 412007,China

bHunan Provincial Engineering Laboratory of Key Technique of Non-metallic Packaging Waste Resources Utilization,Hunan University of Technology,Zhuzhou 421007,China

cPeople's Government of Luohong Town,Longhui County,Shaoyang 422211,China

dSchool of Chemistry and Food Engineering,Changsha University of Science and Technology,Changsha 410114,China

Abstract

Keywords:Adsorption;Hexavalent chromium removal;Biomass adsorbent;Pomelo peel;FeCl3-modified pomelo peel;Aquatic solution

1.Introduction

Among heavy metals,hexavalent chromium(Cr(VI))is well known for its toxicity and carcinogenicity,as well as its strong oxidation capability,which causes damage to animal and plant tissues(Tran et al.,2015).Cr(VI)-containing wastewater is mainly discharged from a variety of industries,including electroplating,pigment manufacturing,and leather tanning(Hameed et al.,2008;Argun et al.,2014).The removal of Cr(VI)from industrial wastewater has been a historical scientific and technological problem(Wu et al.,2017).

Numerous methods,including precipitation,electroplating,adsorption,biologicalmethods,ion exchange,membrane separation,photocatalytic reduction,and electrochemical condensation,have been used to treat Cr(VI)-containing wastewater(Nguyen et al.,2013).Among these methods,adsorption has become the most popular method for removing Cr(VI)due to its simplicity,economic ef ficiency,and environmental friendliness(Wang et al.,2018).The most critical factor of adsorption is the selection of adsorbents.The high cost of activated carbon,a preferred adsorbent,has limited its widespread use(Sun et al.,2016).Nowadays the research focus is to make use of natural resources through simple processing to produce low-cost,highefficiency adsorbents.

Various adsorbents obtained from biomass materials,such as banana peel(Memona et al.,2009),pomegranate peel(Rao and Rehman,2010),orange peel(L′opez-T′elleza et al.,2011),rice husk(Georgieva et al.,2015),Chitosan-iron(Li et al.,2016),corn cobs(Dimple and Dutta,2018),Auricularia auricula dregs(Li et al.,2018),polysaccharide beads(Gabriel et al.,2018),and biochar(Chen et al.,2018a),have been prepared to remove Cr(VI)from aqueous solutions.Pomelo peel(PP)mainly consists of lignin,cellulose,and hemicellulose,which are abundant in polar functional groups,such as hydroxyl,carbonyl,carboxyl,phenolic,and ether groups(Sud et al.,2008).These groups are electron-donating.Therefore,they can bind to heavy metal ions in solution through chelation(Saeeda et al.,2010;Selvakumar et al.,2008).However,the adsorption performance of PP on cationic heavy metals,such as Cu2+,Zn2+,Pb2+,Cd2+,and Ni2+,is better than that on anionic heavy metals such as Cr(VI).Therefore,it is hoped to improve the adsorption performance of PP on Cr(VI)-containing wastewater through modification(Wanna et al.,2009;Liu et al.,2012;Meisam et al.,2013).

The aim of this study was to improve the adsorption performance of PP by employing various modification methods to obtain a high removal efficiency of Cr(VI).The characteristics of the adsorbents before and after modification were investigated and compared through various methods,including surface area analysis,Fourier transform infrared spectroscopy(FTIR),scanning electron microscopy(SEM),elemental analysis,and zeta potentials analysis.The effects of pH,time,temperature,initial concentration,and adsorbent dose on removal of Cr(VI)were studied.The kinetic and thermodynamic mechanisms of adsorption were further examined and assessed using kinetic and thermodynamic equations,respectively.

2.Materials and methods

2.1.Adsorbate preparation

The wastewater solution was prepared by dissolving 0.2829 g of K2Cr2O7in distilled water to achieve a concentration of 100 mg/L.Experimental solutions with different initial concentrations of Cr(VI)were obtained by accurately diluting this wastewater solution(Foo and Hameed,2011).

2.2.Adsorbent preparation

Four kinds of effective modifying agents,i.e.,inorganic salts(FeCl3and Al2(SO4)3),polymers(polyacrylamide(PAM)and polyaluminium chloride(PAC)),alkalis(NaOH and CaO),and acids(HCl and H2SO4),were used to modify PP.PP used throughout the experiments was collected from local fruit merchants.The collected materials were washed with distilled water several times to eliminate the interference from surface particles.The washed materials were cut into small pieces and then dried in a blast oven at 105°C until they reached a constant weight.The dried material was crushed using a pulverizer,sieved to a maximum particle size of 420 μm(40 mesh),and preserved in an airtight container prior to further operation.

FPP was prepared by mixing PP and FeCl3,which were dissolved in water at a mass ratio of 5:1 for 1 h on the basis that better properties may result from a higher ratio of FeCl3(Fu et al.,2017).The specific operation was to immerse 50 g of PP in 100 mL of 0.37 mol/L FeCl3solution and stir well.Then the mixture was dried to a constant weight under the same conditions as PP.After cooling to room temperature,the material was crushed and sieved to a size lower than 420 μm for further use.

2.3.Characterization methods

The pore structures of PP and FPP were determined using a surface area analyzer(V-Sorb X800,Gold APP),with N2as the adsorbate at-196°C.Infrared spectra of the solid phase were obtained using an FTIR spectrometer(Avatar 360,Nicolet).FTIR analysis permits spectrophotometric observation of the adsorbent surface in the range 400-4000 cm-1and serves as a direct means for the identification of organic functional groups on the PP and FPP surfaces.Morphology and composition of the adsorbents were analyzed with a scanning electron microscope(Quanta 200,FEI)and an energy dispersive spectrometer(JSM-IT300LA,JEOL),respectively.Zeta potential was measured by a zeta-potentiometer(JS94HX,POWEREACH).

2.4.Adsorption experiments

Batch adsorption experiments were performed in 250-mL Erlenmeyer flasks by contacting 0.15 g of PP and 0.04 g of FPP,respectively,with 100 mL of Cr(VI)solution.To obtain adsorption isotherms,initial Cr(VI)concentrations in the ranges of 1-10 mg/L(PP)and 3-30 mg/L(FPP)at natural solution pH were used.The flasks were sealed and placed in an oscillator at a speed of 150 rpm with a temperature of 10-40°C until equilibrium was reached.To study the effect of adsorption time,samples with an initial concentration of 5 mg/L were withdrawn at a predefined time after the experiment and the residual Cr(VI)concentration was determined.The effect of pH on Cr(VI)adsorption was evaluated by adjusting the pH values of the solutions(at an initial concentration of 5 mg/L)to the designated values using concentrated HCl or NaOH.The solution was centrifuged at 5000 rpm for 5 min and then analyzed with an ultravioletvisible(UV-Vis)spectrophotometer(TU-1950,Persee)at a wavelength of 540 nm.

2.5.Data analysis

The Cr(VI)concentrations in the solutions were determined at the beginning and end of the shaking period.Eq.(1)and Eq.(2)are used to compute the adsorption capacity and removal efficiency,respectively,of the heavy metal:

where qeis the adsorption capacity at equilibrium(mg/g),C0is the initial concentration(mg/L),Ceis the equilibrium concentration(mg/L),V is the volume of the solution(L),m is the mass of the adsorbent(g),and R is the removal efficiency.

Adsorption kinetic models were used to determine the uptake rate of the adsorbate.The kinetic data were fitted to the intraparticle diffusion model(Eq.(3)),pseudo-first-order model(Eq.(4)),and pseudo-second-order model(Eq.(5)).The linear equations of the adsorption equilibrium kinetic models were expressed as follows(Wu et al.,2017;Wang et al.,2018):

where qtis the adsorption capacity at time t(mg/g),t is the incubation time(min),k is the rate constant of intraparticle diffusion(min-1),k1is the rate constant of pseudo-first-order adsorption(min-1),and k2is the rate constant of pseudosecond-order adsorption(g/(mg⋅min)).

Adsorption isotherm models were used to investigate the adsorption mechanisms.The models represented the equilibrium distribution of metal ions between the solid and liquid phases,defining the relationship between the amount of adsorption and the liquid phase concentration.Two models for adsorption isotherms were tested for fitting of the experimental data:the Langmuir(Eq.(6))and Freundlich(Eq.(7))models.The equations of these equilibrium isotherms are as follows:

where qmaxis the maximum adsorption capacity corresponding to complete mono-layer coverage(mg/g),KLis the Langmuir adsorption constant(L/mg),RLis the dimensionless constant separation parameter,KFis the Freundlich adsorption constant(L/mg),and 1/n is the heterogeneity factor.

2.6.Reusability

One hundred millilitres of Cr(VI)-containing wastewater(5 mg/L)were added to two clean 250-mL conical flasks.PP was added to one of the flasks and FPP was added to the other.After adsorption and filtration,20 mL of 6.25 mol/L NaOH solution were added to the filter residue,and then constant temperature oscillation was carried out.The analyte was separated,and the adsorbent was washed with distilled water until its pH reached 7.0.This adsorption process was repeated,and the absorbance measured.The adsorption rate was then calculated.

3.Results and discussion

3.1.Modification effect

Solid modifiers(FeCl3,Al2(SO4)3,PAM,PAC,NaOH,and CaO)were mixed with PP at mass ratios of 1/100,1/50,and 1/10,while liquid modifiers(HCl and H2SO4)were mixed with PP at mass fractions of 10%,20%,and 30%.The adsorption results of the adsorbents on Cr(VI)are shown in Fig.1.PP samples modified by acids and FeCl3show excellent adsorption performance for Cr(VI).Compared with acids,FeCl3is a common water purifying agent and has less impact on the environment and more advantages in application.Therefore,FeCl3-modified PP(FPP)was used for further exploration in this study.Based on the determination of FeCl3as the best modifier,the optimal mass ratio of FeCl3to PP was tested.The results in Fig.2 show that when the mass ratio of FeCl3to PP is 1/5,FPP performs best.

3.2.Surface and morphology characterization

Fig.1.Adsorption of Cr(VI)on each modified PP.

Fig.2.Comparison of removal percentage of Cr(VI)by FPP with different mass ratios of FeCl3.

The N2adsorption/desorption isotherms and pore size distributions of PP and FPP are shown in Fig.3,where P/P0is the relative pressure.In Fig.3(a),there are four curves.Two curves are the Brunauer-Emmett-Teller(BET)test results for the adsorption and desorption of FPP,and the curve with a smaller relative pressure is the desorption curve.The other two curves have similar meanings and are the BET test results for the adsorption and desorption of PP.The isotherm is an intermediate between type I and type II isotherms with an H3hysteresis loop as defined by the International Union of Pure and Applied Chemistry(IUPAC)classification,which indicates that PP and FPP are macroporous materials.The adsorption amount in the low-pressure zone is small without any inflection points appearing,indicating the rather weak interaction between the adsorbent and adsorbate.A higher relative pressure(P/P0)leads to the greater adsorption amount(Va)and the more frequent presence of pore filling.Adsorption molecules gather on the surface around the most attractive sites.Fig.3(b)shows that the pore diameter of PP is mainly distributed between 8 and 104 nm,while that of FPP is distributed between 10 and 74 nm.PP and FPP have average pore diameters of 77.82 and 92.16 nm(Table 1),respectively,which shows that a vast majority of the pores fall into the range of macropores.The BET surface area of FPP(10.40 m2/g)is higher than that of PP(3.85 m2/g).PP has a total pore volume of 0.08 cm3/g while FPP has a total pore volume of 0.24 cm3/g.Through the iron salt modification,it can be inferred that hydrolysis of iron ions can significantly damage the surface structure of PP,make the surface rough and uneven,and increase its surface pore size and surface area.

The SEM micrographs of PP(with a multiple of 500X)and FPP(with a multiple of 5000X)are shown in Fig.4.It is observed that PP has a fibrous structure(Fig.4(a))and FPP has a stereoscopic structure(Fig.4(b)).Comparison of the micrographs of PP and FPP show that the surface of FPP is rougher than that of PP.This may be due to a certain degree of oxidation and corrosion of FeCl3,which can react with the reductive substances in PP.As lignin has a condensation reaction under acidic conditions,FPP exhibits a certain degree of shrinkage.Compared with PP,FPP has smaller granules,greater specific surface area,and a larger average pore size.

3.3.Elemental characterization

The energy dispersive spectra of PP,FPP,and PP and FPP loaded with Cr(VI)(labeled as PP-Cr(VI)and FPP-Cr(VI),respectively)were investigated in relation to their adsorption processes(Table 2).It can be seen that the amount of chromium ion(5.76%)adsorbed by FPP is equivalent to nine times the amount of PP(0.64%)adsorption,demonstrating that FPP performs betterin adsorption ofchromium than PP.Comparing FPP with FPP-Cr(VI),the ratio of chlorine ion decreased and the ratio of iron ion changed little after adsorption,which indicated that the adsorption might be caused by the iron ions loaded on the adsorbents,which greatly reduced the zeta potential and facilitated the precipitation of chromate anions.More specifically,the reason is that the electrostatic attraction between chromate ions and iron ions is greater than the electrostatic attraction between chloride ions and iron ions,so that chromate ions replace the position of chloride ions to form a complex with iron ions coprecipitated in the filter residue.Then the chloride ions that are squeezed out remain in the solution.

Fig.3.Nitrogen isotherms and pore size distributions of PP and FPP(STP means standard temperature and pressure).

Table 1Porosity structure of PP and FPP.

Fig.4.SEM micrographs of PP and FPP.

Table 2Elemental analysis of PP,PP-Cr(VI),FPP,and FPP-Cr(VI).

The removal of heavy metal usually involves the electrostatic interaction with the adsorbent.We carried out zeta potential measurements on the PP and FPP samples before and after adsorption.The zeta potential values of PP,PP-Cr(VI),FPP,and FPP-Cr(VI)were(12.01±0.21),(29.14±0.32),(1.41±0.12),and(14.91±0.09)mV,respectively,at pH values of 5-7.The zeta potential of PP(12.01 mV)was higher than that of FPP(1.41 mV).The lower zeta potential of FPP was due to the addition of ferric ions,resulting in an enhancement of the specific Cr(VI)uptake.Comparatively,after the adsorption of Cr(VI),the potential values of PP and FPP adsorbents increased significantly.The amounts of surface groups and binding sites decreased with the progress of adsorption,leading to a decrease in electrostatic attraction.Therefore,increasing electrostatic attraction forces may have been responsible for the increasing adsorption capacity(Shen et al.,2017).For the FPP adsorbent,the surface-abundant ferric iron can form a coordination complex with the chromate anion under slightly acidic conditions(Wu et al.,2018).

Fig.5.FTIR spectra of PP and FPP.

Several characteristic FTIR peaks for PP and FPP are shown in Fig.5.The absorption peaks for PP were at wavenumbers of 3411,2935,2379,1739,1623,1437,1242,1068,831,and 628 cm-1.More specifically,a broad,strong band stretch was observed from 3000 to 3600 cm-1,indicating the presence of free or hydrogen-bonded O-H groups(alcohols,phenols,and carboxylic acids)as in pectin,cellulose,and lignin on the surface of the adsorbent.The peaks at 2935 cm-1showed the stretching of symmetric or asymmetric C-H vibration of aliphatic acids,and the peaks at 831 cm-1showed the out-of-plane deformation vibration of aromatics(Saeeda et al.,2010).The peaks at 2379 and 1623 cm-1correspond to the stretch of C═C in aromatic rings and C═O in carboxyl groups,respectively(Wang et al.,2011;Zu´~niga et al.,2015).In addition,the peak observed at 1739 cm-1is the stretching vibration of the C═O bond due to these functional groups(-COOH and-COOCH3)and may be assigned to carboxylic acids or their esters,while intensive peaks at 1437,1242,1068,and 628 cm-1are identical to in-plane O-H(carboxylic acids),asymmetric C-O-C(ester,ether,and phenol),C-O(anhydrides),and C-H(alkynes)derivatives(Foo and Hameed,2011).Compared with the spectrum of PP,an obvious decrease in the intensity of the absorption peak at 3411 cm-1can be observed in the spectrum of FPP,which is assigned to the stretching vibration of hydroxyl groups(-OH groups).Similarly,it can be observed from Fig.5 that most of the peaks still exist,but the intensity is reduced to some extent,maybe resulting from the strong hydrolysis and acidification of FeCl3.Some reducing substances such as lignin and hemicellulose may react,resulting in changes in functional groups,such as hydroxyl groups,aldehyde groups,carbonyl groups,and carboxyl groups.

3.4.Orthogonal experimental design

Fig.6.Effects of pH,time,temperature,initial concentration,and adsorbent dose on Cr(VI)adsorption.

The effect of pH on the removal of Cr(VI)is shown in Fig.6(a)under the conditions of 15 g/L PP and 0.4 g/L FPP.With the increase of the solution's pH,the removal rate of Cr(VI)in water declined continuously,indicating that acidic conditions favored the removal of Cr(VI).When FPP was added to the Cr(VI)-containing wastewater,a large number of H+ions were produced owing to the intense hydrolysis of Fe3+,resulting in a decrease in the pH of wastewater.Under strongly acidic conditions,Cr(VI)in the solution mainly existed in the forms of.The functional groups on the surface of FPP that include amino groups and the hydroxyl groups accepted H+to form positive adsorption sites of.With the increase of solution's pH,although Cr(VI)ions were still present in the forms ofand,the number of positively charged adsorption sites on the surface of FPP was reduced,resulting in the decreased adsorption of chromium anions.Therefore,the adsorption rate decreased.The optimum pH range for the treatment of Cr(VI)-containing wastewater by FPP was 1.5-10.

The rates of adsorption were determined by adding 15 g/L PP and 0.8 g/L FPP to Cr(VI)solution with a concentration of 5 mg/L over different time intervals.From Fig.6(b),it is evident that the removal rate of Cr(VI)by PP reached an equilibrium adsorption rate of 60% in 10 min,while the FPP adsorption rate was higher,reaching 93.7% within 2 min.This is probably due to the abundant active sites on the surface of FPP so that it can achieve the adsorption equilibrium more quickly and efficiently than PP.

The effect of temperature is shown in Fig.6(c)under the conditions of 15 g/L PP and 0.8 g/L FPP.It can be seen that the adsorption rate of FPP was higher than that of PP.Its fluctuation amplitude was not large at each separate curve,and adsorption experiments can be carried out within the range of feasibility.The temperature had little influence on the adsorption effect.

The Cr(VI)adsorption capacities of PP and FPP as a function of initial Cr(VI)concentration within the aqueous solution are shown in Fig.6(d)under the conditions of 15 g/L PP and 0.8 g/L FPP.Clearly,compared with PP,the concentration range of FPP treatment was large and the adsorption efficiency was high,which indicates that the surface area and adsorption sites of PP increased after modification.

One of the parameters that strongly affect adsorption capacity is the adsorbent dose.It can be clearly seen from Fig.6(e)under the conditions of 15 g/L PP and 0.8 g/L FPP that the adsorption effect of FPP and PP both increases with the increase of dose and eventually tended to balance.The adsorption dose of FPP was one order of magnitude less than that of PP.In other words,the amount of adsorbent was much smaller than that of PP,and the adsorption efficiency of FPP was higher than that of PP.

3.5.Adsorption kinetics

In order to explore the control mechanism of the adsorption process,thepseudo-first-order,pseudo-second-order,and intraparticle diffusion models were used to fit the data of adsorptio2n of Cr(VI)by PP and FPP.The correlation coef ficient(R)calculated from the linear plots and other kinetic parameters are listed in Table 3.It can be observed that thefitting results of the pseudo-second-order dynamic equation were better than the other two models since its correlation coefficient value was close to 1(Chai et al.,2015).In other words,the adsorption kinetics of Cr(VI)on PP and FPP both followed pseudo-second-order adsorption kinetics and the maximum adsorption capacities of Cr(VI)were 0.3168 and 12.0773 mg/g,respectively.This indicates that the adsorption rate can be controlled by chemisorption of electrons shared or exchanged between the adsorbent and the adsorbate(Saeeda et al.,2010).

Table 3Kinetic parameters for Cr(VI)adsorption on PP and FPP.

Table 4Langmuir and Freundlich isotherm constants and correlation coefficients(Cr(VI)concentrations are 3-30 mg/L for FPP and 1-10 mg/L for PP,agitation speed is 150 rpm,pH is 2,and equilibrium time is 2 min).

3.6.Adsorption isotherms of Cr(VI)onto adsorbents

In order to describe the adsorption equilibrium,two adsorption isotherms,the Langmuir and Freundlich,are usually used to fit the adsorption data.The Langmuir isotherm is a prediction of single-layer adsorption while the Freundlich isotherm is an assessment of the surface distribution and energy non-uniformity of the adsorbent(Langmuir,1918).The linear relation and isotherm fitting for the two isotherm models are shown in Table 4.It may be concluded from these observations that the adsorption of Cr(VI)by PP and FPP was better defined by the Langmuir equation than by the Freundlich equation,thereby indicating that the adsorption of Cr(VI)onto PP and FPP was governed by a chemical equilibrium and saturation mechanism(Saeeda et al.,2010).Furthermore,the results indicate that the adsorption of Cr(VI)onto the surface of PP and FPP may occur via the formation of a homogeneous mono-layer.The RLvalue indicates whether the isotherm is irreversible(RL=0),favorable(0＜RL＜1),linear(RL=1),or unfavorable(RL＞1).As shown in Table 4,the calculated RLvalues are in the range of 0-1,indicating favourable adsorption of Cr(VI)onto both PP and FPP.In addition,it can be seen that when the equilibrium time of the adsorption isotherm was 2 min,the temperature was 40°C and the pH value was 2,the maximum adsorption capacity of FPP based on Langmuir isotherm was 21.55 mg/g.It can be speculated that increasing temperature may cause a swelling effect on the porosity and pore volume of FPP,which accelerates the diffusion of Cr(VI)from the external boundary layer into the internal pores of the FPP particles(Wu et al.,2017).In addition,the saturated adsorption capacity of FPP is much larger than that of PP.Furthermore,compared with the traditional modified chitosan,which has an adsorption capacity of 15.784 mg/g for Cr(VI)(Liu et al.,2013),and other reported adsorption materials(Zhu et al.,2016,2017;2019;Chen et al.,2018b;Gao et al.,2016),FPP can effectively remove Cr(VI),which is a greater advantage in application.

Fig.7.Desorption rates of Cr(VI)from PP and FPP.

3.7.Limitation

The kinetics of Cr(VI)desorption from PP and FPP were also studied.The desorption of Cr(VI)from FPP reached a maximum desorption efficiency of 50% in 40 min(Fig.7).This desorption efficiency was higher than that of PP while was less than that of magnetic iron oxide nanoparticlemultiwalled carbon nanotube composites,which reached about 80% in the fifth adsorption-desorption cycle(Lu et al.,2017).A possible reason for this phenomenon is that the sodium hydroxide reacts with ferric iron in FPP to form precipitation,causing the reduction of the active absorption site on the adsorbent surface.When the adsorbent was reused for adsorption after it was immersed in hydrochloric acid solution,the adsorption efficiency could reach 80% because iron ions were activated again and thus more active adsorption sites were available,thereby demonstrating a potential possibility for reuse(Sheng et al.,2011;Adak et al.,2005).However,in terms of relatively long desorption time but low efficiency,the process needs to be further investigated and improved.

4.Conclusions

The removal of Cr(VI)from aqueous solutions by PP and FPP adsorbents was studied.The results show that FPP has a higher adsorption capacity because hydrolysis of FeCl3increases the specific surface area and the adsorption sites of FPP.They also show that the iron ions loaded on the adsorbents greatly reduce the zeta potential and facilitate the precipitation of chromate anions.Based on the orthogonal tests,the maximum adsorption amount of Cr(VI)by FPP was 21.55 mg/g when the temperature was 40°C and the pH value was 2,much higher than those of the commonly used biosorbents,such as chitosan.Therefore,FPP can serve as a potential adsorbent for Cr(VI)removal due to its high efficiency,economic efficiency,and environmental friendliness.