Lead removal from aqueous medium using fruit peels and polyaniline composites in aqueous and non-aqueous solvents in the presence of polyethylene glycol

Iman Farirzadeh,Majid Riahi Samani,*,Davood Toghraie

1 Department of Civil Engineering,Khomeinishahr Branch,Islamic Azad University,Khomeinishahr,Iran

2 Department of Mechanical Engineering,Khomeinishahr Branch,Islamic Azad University,Khomeinishahr,Iran

Keywords:Adsorption Banana peel Composites Pollution Polyaniline

ABSTRACT In the present study,composites of Polyaniline and some fruit peels were synthesized in various conditions and used for lead removal from aqueous solutions.Adsorption tests were conducted in batch mode using the most efficient adsorbent,and the effects of medium pH,initial lead concentration,contact time,adsorbent dosage,and adsorption isotherms were investigated.The results showed that all composites were more efficient in lead removal compared to the fruit peels alone.The highest removal percentage was related to the composite of banana peel and Polyaniline that synthesized in the aqueous solution in the presence of 2 g?L-1 Poly ethylene glycol.This composite showed 95.96% lead adsorption at pH=6,the contact time of 90 min,the initial lead concentration of 25 mg?L-1,and an adsorbent concentration of 8 g?L-1.Adsorption isotherm study showed that adsorption of lead by synthesized composite could be fitted by both Langmuir and Freundlich models,but the Langmuir model was more fitted than Freundlich.Besides,some characteristics of the composites,such as chemical composition(XRD analysis),structure,and morphology (SEM analysis) and functional groups (FTIR analysis),were studied.

1.Introduction

Water pollution due to heavy metals is the global and environmental issue[1].Water is the most abundant material in the world,which is found in solid,liquid and gas forms.Drinking and individual requirements were the first insight of human about water [2].Water is also the best solvent in which most gases and minerals are dissolved.Water resources limitations,rainfall shortage,water crisis and importance of water reuse along with surface and underground water pollution make it necessary to find environmental methods for water pollutant removal [3].Heavy metals are one of the important water pollutants,which come from industrial wastewater and threaten living organisms’ health.These metals are accumulated in ecological systems and their gradual increase causes negative effects in the systems[4].Accordingly,the concentration of these metals must be less than the standard criteria [5].Lead is one of the threating heavy metals,which enters wastewaters due to different activities such as mining,metal plating,printing,weaving,photographing,and producing acidic battery,ceramic,and glass,pipe and explosive materials [6,7].EPA criteria for lead level in drinking water is 0.05 mg?L-1showing that lead is very toxic at low water concentrations [8].Nowadays,there are several methods for heavy metal,including lead,separation from water,for example,chemical precipitation,reverse osmosis and surface adsorption [9].Each method has certain advantages and disadvantages.Chemical precipitation method produces a high volume of sludge,with no possibility of lead isolation;the sludge management and removal is very complicated,too [10].Reverse osmosis produces wastewater and needs much electricity and energy[11].Surface adsorption is a method gaining great attention today.In this method,no sludge is produced,less power is used and lead may be isolated and reused.The method is a physicochemical process at the interface of two phases;it is economic,highly efficient and flexible in designing and exploiting;therefore,it is a superior method in metal removal[12].Nowadays,scientists are searching for new adsorbent for heavy metal,including lead,removal.Due to economic justification,heavy metal adsorption with natural adsorbent has gained attention in recent decades Acharya [13].Tree peels,fruit peels,the hull of oilseeds and nonoil seeds,sawdust,etc.are examples of natural adsorbent [14].In this case,biological materials or useless biological materials,which are majorly composed of cellulose (C6H10O5),have been recommended.In aqueous media,the cellulosic surface is slightly negatively charged;thus,interact coulombically with anions in the media [15].Agricultural products and byproducts are useless materials that must be disposed;when they are burned for elimination,they produce CO2and other pollutants.Accordingly,there is a need for conversion of agricultural byproducts to useful and valuable products[16].A possible solution is the use of these materials as cheap adsorbent that are able to remove toxic metal ions from aqueous media.Polyaniline and its composites are of adsorbent gained attention[17,18].Polyaniline is an important conductive polymer,which gained great attention due to high electroconductivity,low price and simple method for synthesis [19].Polyaniline and its composites(e.g.composites with PEG,polyvinyl alcohol,and polyvinyl acetate)have been used in different studies for heavy metal removal [20–22].Previous studies have shown that various Polyaniline and its composites with different traits might be synthesized by manipulation of the synthesis conditions[23].The aim of the present study is to synthesize new composites of Polyaniline and to investigate their efficiency in lead removal.These composites might be superior in adsorption efficiency,contact time and reuse possibility compared to the present adsorbents for lead removal[24].Therefore,the aim of the present study is the investigation of new polyaniline composites for lead removal from water and wastewater.

2.Materials and Methods

All the chemicals were provided from Merck Company.All chemicals were used without purification,except of aniline that was distilled twice before use.Banana,Orange and kiwis peels were first washed with distilled water,then dried and pulverized before use.pH meter(P-M-T-Model 2002),atomic adsorption spectrophotometry (Perkin Elmer),scanning electron microscopy (LEO 440i),XRD(PHILIPS)and FTIR(Perkin Elmer;Spectrum RX1)apparatuses used in the present study.

2.1.Lead removal from aqueous medium

First of all,the composites were synthesized and prepared;then,the standard solution with 25 mg?L-1lead and pH=6 was prepared.In batch experiment,0.2 g of each adsorbent was added to 100 ml of lead solution and shook for 30 min.Finally,the solution was passed from the filter and lead concentration was determined in the filtered solution,and removal percentage was calculated according to Eq.(1),

In the equation,C0andCtare initial and final lead concentrations,respectively.

2.2.Synthesis of polyaniline-fruit peel composites under different conditions

To synthesize the composite,0.8 g potassium iodate,as oxidant,were shaken and dissolved in 500 ml of 1 mol?L-1sulfuric acid to obtain a homogeny solution.In another jar,0.2 g of dried fruit peels were mixed with 1 ml of polyaniline monomer and transferred to the first jar after fine wetting.This mixture was shaken 5 h at room temperature.Thereafter,the polymer was dried in the oven(50°C;48 h) and pulverized in a mortar and used for lead removal tests.

3.Results and Discussion

3.1.Lead removal using different polyaniline composites

Efficiency of polyaniline-fruit peel composites for lead removal is shown in Table 1.The results showed that all composites were more efficient than fruit peels alone in lead removal.The highest and lowest lead removal were related to polyaniline-banana peel composite(57.24%)and polyaniline-kiwis peel composite(16.36%).

Mechanism of lead removal by polyaniline-fruit peel composites might be surface adsorbance by nitrogenous sites (NH-) on polyaniline and cellulose molecules on fruit peels.In general,agricultural products,particularly those contain cellulose;have high potency to adsorb metals.Major constitutes of agricultural products are cellulose,lignin,lipids,proteins,sample sugars,water,carbohydrate and starch [3].Moreover,Polyaniline has nitrogenous sites that have electron on S2P3orbital;the electron may bond to positive charges of lead molecules and adsorb them [25].

3.2.Optimization of synthesis conditions of polyaniline-banana peel composite for lead removal

According to the higher efficiency of polyaniline-banana peel composite in the previous phase,the composite synthesis efficiency changed in the present phase.In the present phase,PEG was incorporated into the composite structure as a stabilizer.Moreover,aqueous and non-aqueous solvents were used for the composite synthesis.Table 2 shows the results of polyanilinebanana peel composite synthesis.

Table 1 Efficiency of different adsorbents for lead (II) removal from aqueous medium (pH:6,contact time:30 min,adsorbent amount:2 g?L-1)

Table 2 Lead removal using polyaniline-banana peel composites under different conditions

The results showed that the efficiency of polyaniline-banana peel composite in aqueous solvent was better.PEG incorporation led to increased efficiencies of all composites.The concentration of PEG was important and the highest efficiency of lead removal(87.92%) was obtained by polyaniline-banana peel composite in the presence of 2 g?L-1PEG.

Fig.1 shows SEM images of adsorbents in different alignments and magnification.Morphology of banana peel powder and polyaniline-banana peel composite are shown in Fig.1(a) and (b),respectively.As the figures have shown,the flaky polyaniline particles coated the banana peel particles and increased their efficiency.Fig.1(c) and (d) show photograph polyaniline-banana peel composite in the presence of 2 g?L-1PEG.The figures show that PEG caused more homogeny coating of the composite and increased removal efficiency.PEG is a surface-active compoundthat is adsorbed to growing polymer particles during polymerization;thus,affects shape,morphology,homogeny,side surface,size and other traits of ployaniline.SEM photographs,with different magnitudes,of the polymers supported that pores and heterogeneity increased in the adsorb surface compared to the other adsorbents [26].In the prepared composites,the aniline particles sit gently on the porous surface of the fruit and bond,creating a polymer-coated layer on the surface of the composites.Due to the different properties of fruit peels,the type of bonding has been different and the entry of the PEG in the structure has made the composite more homogeneous and SEM photos confirm that.

Fig.1.SEM Figure of (a) banana peel (×1000),(b) polyaniline-banana peel (×1000),(c) polyaniline-banana peel composites in the presence of PEG (2 g?L-1) (×1000),(d)polyaniline-banana peel composites in the presence of PEG (2 g?L-1) (×2000).

FTIR photographs of selected adsorbents for synthesis in the second phase are presented in Fig.2.Localization and shape of FTIR bands along with changes in the molecular environment might be suitable guidance of the molecular structure of a compound.Complete adaptation of two compounds’spectrums is indicator of similarity in the two compounds composition [27].As the figure shows,polyaniline coating on banana peel led to a change in the spectrum location and amount.In polyaniline-banana peel composite in the presence of PEG,spikes at 3400,2925,2110,1630,1193,1116 and 634 cm-1are related to functional groups of-OH or -NH,alkyl tension -CH,alkyne -C=C,aromatic -C=C and tension aromatic -CH,respectively.In general,functional groups of -OH and -NH are responsible for aqueous lead adsorbance to adsorbent.

Fig.3 and Table 3 show XRD analysis of polyaniline-banana peel composite in the presence of 2 g?L-1PEG.Accordingly,benzene compounds were found of the synthesized composite surface,indicating the synthesis of Polyaniline on the composite,and the composite was amorphous structurally.

Table 3 XRD results for polyaniline-banana peel composite in in the presence of PEG(2 g?L-1)

The BET analysis was used to measuring the total area of prepared composites.According to the BET analysis results,the specific surface area for polyaniline-banana peel composite in the presence of 2 g?L-1PEG was 7.152 m2?g-1.However,the specific surface area for the polyaniline-banana peel composite was 6.843 m2?g-1.These BET analysis results showed that polyanilinebanana peel composite in the presence of 2 g?L-1PEG has a larger specific surface area and more lead removal efficiency.

Fig.2.FTIR of (a) banana peel,(b) polyaniline-banana peel composite c) polyaniline-banana peel composite in in the presence of PEG (2 g?L-1).

3.3.Study on conditions of lead adsorption by selected composite(polyaniline-banana peel composite that synthesized in in aqueous medium with the presence of 2 g?L-1 PEG)

As the best lead removal percentage in the previous phase(87.92%) was related to polyaniline-banana peel composite in the presence of 2 g?L-1PEG in aqueous medium,in the present phase,it was tried to optimize other conditions such as the medium pH,contact time,adsorbent concentration,lead concentration and adsorption isotherms.

3.4.Effects of the medium pH on lead removal efficiency by the selected sorbent

Fig.3.XRD analysis graph for polyaniline-banana peel composite in in the presence of PEG (2 g?L-1).

Fig.4.Effects of pH on lead removal by the selected adsorbent (contact time:30 min,initial lead concentration:25 mg?L-1,adsorbent concentration:2 g?L-1).

Results of the medium pH on lead removal efficiency by the selected adsorbent are shown in Fig.4.Alkaline pH was avoided to inhibit lead precipitation.The results showed that the highest lead (II) removal (87.92%) was at pH 6 and the lowest level(41.88%) was at pH 2.The reason for lower adsorption in acidic media is that polyaniline particle protonate in the acidic media,leading to repulse positive-charged ions and lowered adsorption.Shaoet al.[28] used polyaniline-magnetic nano-tubes composite for lead removal by surface adsorption method,and found that an increase in pH from 3 to 7 led to increased removal percentage.It is due to that at acidic pH,the concentration of H+ions increases in the medium,which compete with the metal ions for adsorption the adsorbent surface;under this situation,H+ions occupy all active sites on the adsorbent surface [28].

Fig.5.Effect of contact time on lead (II) removal by the selected adsorbent (pH:6,initial lead concentration:25 mg?L-1,adsorbent concentration:2 g?L-1).

3.5.Effects of contact time on lead (II) removal by the selected adsorbent

Fig.5 shows the results of the effects of contact time at a medium pH of 6.Results showed that even after 90 min,the adsorption efficiency was almost constant.The lowest lead removal (59.6%)occurred after 5 min and the highest percentage(91.44%)occurred after 90 min.This shows that the lead removal increases along with time progression,because,the active sites on the adsorbent surface are occupied with time progression.

3.6.Effects of initial lead (II) concentration on removal efficiency by the selected adsorbent

Table 4 shows the results of the initial lead (II) concentration effects on removal efficiency by the selected adsorbent.The results showed that lead removal increased from 52%to 87.92%along with an increase in initial lead concentration from 1 to 25 mg?L-1.It was found in the next phase that lead removal increased from 52% to 87.92% along with an increase in initial lead concentration from 1 to 25 mg?L-1.This is due to increased ion density by an increase in the medium concentration,which leads the ions come closer to the adsorbent surface that increases adsorption.

Table 4 Effects of initial lead (II) concentration on removal efficiency by the selected adsorbent (pH:6,contact time:30 min,adsorbent concentration:2 g?L-1)

Table 5 Constants of the Langmuir and Freundlich isotherms for lead adsorption

3.7.Effects of the selected adsorbent concentration on lead(II)removal efficiency

Fig.6.Effect of the selected adsorbent concentration on lead(II)removal efficiency(pH:6,contact time:30 min).

Fig.6 shows the results of the concentration of the selected adsorbent on lead (II) removal efficiency.The results showed that an increase in adsorbent concentration led to increased lead removal efficiency.The selected composite was able to adsorb 95.96% of lead,when the lead initial concentration and the composite concentration were 25 mg?L-1and 8 g?L-1,respectively.Such an increase in the adsorption efficiency along with increase in adsorbent concentration is due to increase in adsorption sites in the medium [29].Karthik and Meenakshi [30] worked on lead and cadmium removal using chitosan-polyaniline composites and found increase in the adsorption efficiency along with an increase in adsorbent concentration.This might be due to increased number of adsorption sites for both metal ions.Moreover,when the adsorbent concentration increased from 0.5 to 0.15 g?L-1,lead (II)removal efficiency increased from 57.15% to 100%,and cadmium(II) removal efficiency increased from 47.45% to 96.95%.Overall,the results showed that the highest lead removal efficiency of the selected composite (polyaniline-banana peel in the presence of 2 g?L-1PEG) was achieved at the medium pH of 6,contact time of 90 min,initial lead concentration of 25 mg?kg-1and adsorbent concentration of 8 g?L-1.

3.8.Isotherms of lead adsorption by the selected adsorbent

To determine adsorption isotherms,different concentrations of the selected composite were added to the medium containing 25 mg?L-1lead and the metal concentrations were measured after 30 min.then,adsorption isotherm graphs were drawn for the different concentrations and Langmuir and Freundlich models were fitted [31].

Adsorption suitability and particular traits of Langmuir isotherm can be explained by a non-unit constant named isolation factor or equilibrium parameter,which was introduced by Hallet al[32].IfRLis zero,it is irreversible adsorption;if 01,it is non-suitable adsorption [33].

Freundlich model is an experimental equation of surface adsorption,which its general scheme is presented in the Eq.(4):lg（X）=lgk+1/nlg（Ce） （4）in which,(X)is the mass of adsorbed material(mg?g-1),Ceis the element concentration at equilibrium (mg?L-1),andkandnare the model’s constants.Freundlich model is not theoretical and its constants usually have no physical conceptions.However,kmay be considered as a quantitative coefficient related to adsorption capacity,because according to the equation,(X)=kif the adsorbed concentration is constant in equilibrium solution.Also,nis an indicator of the fitted equation curve and is known as indicator of adsorption rate.This constant shows homogeneity of the adsorbent surface,so that,if it intends to zero,heterogeneity and variation of adsorption sites increase;if it intends to 1,the adsorbent surface will be more homogeny [33].

Fig.7.Freundlich isotherm of the selected adsorbent.

Fig.8.Langmuir isotherm of the selected adsorbent.

It should be noted that adsorption isotherms,in fact,quantitatively describe adsorption of solvent on adsorbent surface,as constant temperature.Adsorption isotherms indicate adsorption and a function of adsorbed material’s concentration equilibrium.Figs.7 and 8 show fitted adsorptions of lead (II) based on Langmuir and Freundlich models.Parameters and constants of these models are presented in Table 5.According to the results,both isotherms had highR2for lead adsorption,but the Langmuir model had higherR2(0.9119).The main characteristic of this isotherm is defined by the equilibrium parameter (RL).RLwas calculated 0.01 for lead adsorption,which indicates suitable adsorption of lead by the adsorbent.In the Langmuir model,the highest adsorption for lead (b) was 192.3 mg?g-1adsorbent.In the Freundlich model,adsorption rate indicator (n) was calculated 0.86 for the lead.In this isotherm,with the increase in adsorbed material concentrations,the adsorption slope increases first,but,it decreases and stops over time by occupation of adsorption sites.This isotherm shows that adsorbent has a low affinity for lead adsorption,when the metal concentration is low;but the affinity increases along with the metal concentration elevation.As an indicator of adsorption efficiency,Kwas 2.59.Karthik and Meenakshi[30]worked on lead and cadmium removal using chitosan-polyaniline composites and found Langmuir model were more fitted for the adsorption data.As the metal ion adsorption follows the Langmuir model,the adsorption was single-layer.The highest capacity for singlelayer adsorption in the Langmuir model was 16.07 and 33.33 mg?g-1for lead and cadmium ions,respectively.Jiaoet al.[34] studied lead removal by CS/PVA/CuO,as a new adsorbent,and found that the highest lead adsorption was 116.84 mg?g-1at pH 5 and adsorbent concentration of 100 mg at 323 K.Siddiquiet al.[35] studied lead removal using carbon of pistachio shell and found that the highest lead (II) removal was 24 mg?g-1and 7.9 mg?g-1,which was at pH 6 and more fitted on Langmuir model.

4.Conclusions

In the present study,composites of polyaniline and some fruit peels,banana,orange and kiwis were synthesized in aqueous and non-aqueous solvents in the presence of polyethylene glycol(PEG),to be used for lead removal.Polyaniline-banana peel composite that synthesized in water with the presence of PEG (2 g?L-1) was chosen as the best adsorbent with 87.92% adsorption efficiency and used for the next trials.Monitoring of the effective parameters in lead adsorption by this composite showed that the composite had the highest efficiency at pH 6,contact time of 90 min,lead concentration of 25 mg?L-1and the composite concentration of 8 g?L-1.Both Langmuir and Freundlich models were well-fitted on the composite lead adsorption,but Langmuir pattern was more fitted than Freundlich.The SEM figures show that PEG caused more homogeny coating of the selected composite and increased removal efficiency.The XRD analysis of polyaniline-banana peel composite showed that the prepared composite has an amorphous structure.

Declaration of Competing Interest

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