Mohammad Saood Manzar,Shamsuddeen A.Haladu,Mukarram Zubair,Nuhu Dalhat Mu’azu,Aleem Qureshi,Nawaf I.Blaisi,Thomas F.Garrison,Othman Charles S.Al Hamouz,

1 Department of Environmental Engineering,Imam Abdulrahman Bin Faisal University,Dammam,Saudi Arabia

2 Department of Basic Engineering Sciences,Imam Abdulrahman Bin Faisal University,Dammam,Saudi Arabia

3 Department of Chemistry,King Fahd University of Petroleum and Minerals,Dhahran 31261,Saudi Arabia

Keywords:Erichrome Black T dye Cross-linked polymer Adsorption Regeneration Langmuir Thermodynamics

ABSTRACT A new series of polymers comprising four terpolymers was synthesized via Mannich polycondensation of benzene-1,4-diamine,formaldehyde and piperazine by varying the benzene-1,4-diamine and piperazine ratio.The new polyamines(labeled Dipip)were characterized using 13C solid-state NMR,FT-IR,TGA,DSC,XRD,SEM and EDX.The adsorptive performances of the synthesized polymers for Erichrome Black T(EBT)uptake from aqueous solution were investigated under batch process.Equilibrium,kinetic,and thermodynamic studies were conducted to determine the influence of different operational parameters of the adsorption process.The two most promising polymers among the series show an excellent EBT removal efficiency of～100%and～95%with high adsorption capacities of 775 mg﹒g-1 and 917 mg﹒g-1,respectively at a meager dosage of 5 mg.The sorption of EBT on the polymers was well described by Redlich-Peterson&Langmuir model while the kinetic studies indicate that pseudo-second order model was followed.For the thermodynamic studies,the negative ΔG and positive ΔH values obtained suggest a spontaneity of the sorption process which was endothermic in nature.The results of reusability test of the resins were promising even at the fourth cycle,showcasing the potentials of the new polymers in dyes contaminated water treatment.

1.Introduction

Water pollution is a global problem,which is triggered mainly by anthropogenic activities.Dyes,which are extensively used in various industries like paints,paper,dying,textile,medicine,agriculture,ceramics,rubber are well-known water pollutants due to their toxic and hazardous nature[1,2].The environmental damaging effects of dyes are linked to their chemical stability [3] nonbiodegradability [4],carcinogenic [5] and mutagenic tendencies[6,7].Furthermore,the existence of dyes in water decreases light penetration which hampers the photochemical activities of marine ecosystems [8].

Eriochrome Black T(EBT)is an anionic azo dye,which is a commonly used class of dyes found in industrial applications such as dying,laser printing and painting[9].In addition,EBT is employed as an indicator in the complexometric determination of calcium,magnesium,zinc ions[10]and polarographic determination of rare earth metals [11] from water.EBT is harzadous and toxins to human,and even its byproduct naphthoquinone is known to be carcinogenic [12].Therefore,it is imperative to eliminate this harmful dye from wastewater prior to release into the marine system.

Various treatment methods are available to treat dye contaminated waters including biological [13],coagulation [14],flocculation[15],chemical oxidation and reduction[16,17],photocatalysis [18,19],membrane processes [20],ozonation [21],and adsorption [12].Amongst them,adsorption stands out to be a viable technique owing to its ability to remove a variety of dyes and being accredited for its proficiency,reasonable with a trivial operational cost,and easiness of setup.The choice of a suitable adsorbent is driven by its ability to have high capacity,affinity for target contaminant and feasibility for regeneration and reuse[16].Moreover,adsorption is a preferred choice for wastewater decolorization since dyes are thermally stable at elevated temperatures,sun-light and chemically resistant to techniques like oxidation and biodegradation [22].

A number of cross-linked polymers bearing different monomers and functionalities were studied for dye removal.Among them,the cross-linking of γ-cyclodextrin with starch using epichlorohydrin as the cross-linking agent was reported and was further investigated in the adsorption of different dyes including congo red,methylene blue,and methyl purple [23].Moreover,the synthesis of cross-linked cationic polyamine folic acid composites was carried out,which efficiently removed congo red from water phase with an excellent sorption capacity of 256.4 mg﹒g-1[24].Similarly,another study revealed that polyethylene glycol cross-linked polymers were quiet efficient in removing methylene blue from aqueous medium with sorption capability of 15 mg﹒g-1[25].An efficient adsorption of methyl orange (MO) and eriochrome black T (EBT) was reported using a cross-linked polymer of N1,N1′-(eth ane-1,2-diyl)dipropane-1,3-diamine,N,N-diisopropylethylamine and epibromohydrin with adsorption capacities of 358.3 and 41.3 mg﹒g-1for MO and EBT,respectively [12].Furthermore,cyclotetrapolymerization method has been employed to prepare coross-linked polyzwitterionic polymers with dual ability to remove both methylene blue dye and Hg(II)ions[26].Other adsorbents used include activated carbon [27],magnetic carbon bubble[28],bentonite or clays [29],steel slag [30],cellulosic resins [31],MOF [32],red mud [33] and date palm ash LDH [34].However,many of these materials have many limitations such as limited capacity,low affinity for target contaminant(s)and poor regeneration ability.

The use of cross-linked polymers as adsorbents,as reported in this work,provides an avenue for designing excellent adsorbents with high adsorption capacities by facile tailoring of specific functionalities for target applications.Furthermore,adsorbents,like the polymer used in this study,should have high adsorption rates and high thermal stability for use in industrial applications [26].The better adsorption capability of cross-linked polymers is due to the strong interaction between their abundant active sites and the target dyes.

In this work,the synthesis and characterization of a new series of cross-linked terpolymers featuring benzene-1,4-diamine–formal dehyde–piperazine entities through Mannich polycondensation(Fig.1) are reported.The polymers were tested to assess their adsorption efficiency for removal of EBT dye from model wastewater coupled with evaluation of their equilibrium,kinetic and thermodynamic behaviors.

2.Experimental

2.1.Materials and characterization techniques

Benzene-1,4-diamine,piperazine,dimethylformamide (DMF),p-formaldehyde and Sodium hydroxide were purchased from Fluka and used as is.

FT-IR measurements were conducted using a PerkinElmer 16F PC FTIR (500–4000 cm-1region).A Bruker WB-400 spectrometer was used to conduct solid state13C NMR analysis.Thermogravimetric analysis (TGA) was performed using a TA Instruments SDT Q600 thermal analyzer,USA under inert nitrogen gas flow of 50.0 ml﹒min-1.Samples with an approximate mass of three milligrams were placed onto an alumina pan and then heated from 25 to 800°C at a heating rate of 10°C﹒min-1.Differential scanning calorimetery(DSC)was conducted using Netzsch DSC 204 F1,Germany.After adding approximately 3 mg of sample to an aluminum pan with a pierced lid,the samples were heated at a rate of 20°C﹒min-1from 25 to 110°C and held at 110 °C for two minutes to remove their thermal history,cooled to 25°C,and finally heated to 500 °C at a rate of 20 °C﹒min-1.BET measurement was performed using Quantachrome?Autosorb IQ instrument.The absolute concentration of EBT dye solution after each adsorption experiment cycle was assessed with a DR Hach 6000 UV visible spectrophotometer,USA.

2.2.Synthesis of Dipip series

Fig.1.Scheme for the synthesis of Dipip polymer series.

In a typical procedure,benzene-1,4-diamine (0.01 mol) and piperazine were dissolved in 25 ml DMF.P-formaldehyde(0.08 mol) was subsequently added to the reaction flask.The reaction mixture was then heated under a stream of nitrogen gas for two hours at 90°C,followed by heating at 120°C for two hours and finally heating at 140 °C for one hour to yield a solid yellow powder.The reaction mixture was allowed to cool down to room temperature and then filtered,washed with methanol and dried under vacuum at 60 °C until constant weight was achieved.A series of polymers (labeled Dipip) were synthesized by varying the ratios of benzene-1,4-diamine and piperazine.The results of the synthesized Dipip series are tabulated in Table 1.

Table 1 Polymerization reaction results of the Dipip series

2.3.EBT adsorption studies

An initial set of adsorption experiments were carried out to screen and select the most promising polymers for further testing.The best polymers were then subjected to further testing to elucidate the effect of key parameters including contact time,pH,initial dye concentration and adsorbent dosage.A mixture of 5 mg of each polymer with 40 ml of EBT solution(20–100 mg﹒L-1)in 50 ml plastic tubes was agitated for 10–480 min at 275 rpm and at temperature (25–45 °C).Adjustment of the initial pH was achieved using 0.1 mol﹒L-1solution of HNO3and NaOH.After agitation,the spent adsorbent was separated from the residual dye by centrifugation at 2000 r﹒min-1for 5 min.The concentration of the residual dye was determined based on absorbance measurements at 530 nm.

The equilibrium adsorption capacity,qe(mg﹒g-1),and removal efficacy were respectively calculated using Eqs.(1) and (2),

where,C0and Ceare the initial and equilibrium concentration(mg﹒L-1) of EBT in solution,respectively.M (g) and Vs(L) denote the mass of the polymer and the volume of the solution,respectively.

2.4.Regeneration experiment

In order to explore the reusability performance of the adsorbent,adsorption-regeneration tests were conducted.In an adsorption test,approximately 50 mg of a polymer adsorbent was placed into a vial containing 150 ml of 20 mg﹒L-1of EBT dye solution and then shaken for 3 h.To test the regeneration capacity of the polymers,the dye-treated adsorbent was subsequently immersed in 50 ml of 0.1 mol﹒L-1sodium hydroxide solution and shaken for 3 h.The regenerated adsorbent was centrifuged and carefully washed three times with deionized water (DI) and dried at 60 °C for 4 h.The regenerated adsorbent was re-immersed in EBT dye solution of 20 mg﹒L-1.This adsorption-regeneration cycle test was repeated for a total of four times.The final concentration of EBT dye solution after each adsorption test was estimated by the same method as indicated above.

2.5.Zero point charge (pHpzc)

The zero point charge states the pH at which the charge density on a sample surface is equivalent to zero,where,no activation of acidic or basic functional groups are sensed on the solution pH.Adsorbent surface charge may either to be positive or negative,which implies pH pHpzc,respectively.The pHpzc of the resins was calculated using the solid addition method.In this method,0.01 mol﹒L-1solution of sodium chloride was prepared in 100 ml of Erlenmeyer flasks within pH range of 2 to 12.Then,20 mg of the polymer sample was taken in each flask and the initial pH was recorded.After 24 h,the final pH of suspension was determined with a digital pH meter and plotted against initial pH.The pH at the point where the curve crosses the line of equality is referred to as pHpzc of samples.

3.Results and Discussion

3.1.Synthesis of polyamines

The series of polyamines were synthesized by Mannich polycondensation reaction of benzene-1,4-diamine,piperazine and pformaldehyde as a cross-linker.The choice of monomers was selected based on the structural features present in the EBT dye;aromatic and hydroxyl groups.In order to maximize the adsorption capacity of the polyamines;nitrogen and aromatic moieties were inserted in the polymer design by piperazine and benzene-1,4-diamine.

3.2.Characterization of terpolymers

The FT-IR spectra of the cross-linked polyamines are shown in Fig.2(a).All four polymers display a broad band at～3420 cm-1resulting from intermolecular hydrogen bonding —OH stretching vibrations.The bands at 2796–2928 cm-1are characteristic vibrations of aliphatic C—H stretching of the piperazine rings and their CH2linkage.The C—N absorption band appears at～1460 cm-1[35,36].The two bands at 1514 and 1635 cm-1are associated with—C=C— of the aromatic ring.

The solid state13C NMR spectra of the terpolymers are displayed in Fig.2(b).The spectral peaks at～145 and～120 are assigned to the aromatic=C—N and—C=C—carbons,respectively.The peak at～50 and～80 are assigned to the —CH2— groups of the piperazine rings and their CH2linkage,respectively.The peak at～70 is attributed to the —CH2groups of piperazine rings linkage at the ortho position of the aromatic ring.The NMR figure shows a decrease of aromatic peaks intensity from 100 to 150 and an increase in the intensity of the methylene peaks in the region of 20–65.These results confirm the successful incorporation of the monomeric units in the polymeric structure as designed.As the ration of piperazine units increases,the aliphatic methylene units increase in the polymeric structure,which can be shown by the higher intensity of aliphatic peaks and the decrease in the intensity of the aromatic peaks of the 1,4-benzenediamine.

The TGA thermograms the polymers and first derivatives are presented in Fig.3(a) and (b),respectively and the data for Tmax,T10and T50are summarized in Table 2.The figure shows that the synthesized polymers are thermally stable up to 200°C.At temperatures above 200°C,a major decomposition of the polymers starts which is attributed to the decomposition of the aliphatic portion of piperazine.The gradual weight loss from～300°C to 800°C is associated to the carbonization of the aromatic diamine [37,38].

Table 2 Summary of thermal degradation temperatures for Dipip series

The initial thermal stability of samples (up to～200 °C) was higher in samples containing higher ratios of the aliphatic piperazine,as shown by the differences in temperatures with 10%weight loss (T10) with Dipip 1,4 having the highest value for T10.The samples with higher piperazine content also had higher temperatures at peak degradation rate (Tmax) due to increased crosslinking density.The improved thermal stability of Dipip 1,4 for T10and Tmaxwere attributed to the higher crystallinity due to better packing.In contrast,Dipip 1,1 had the highest value for T50because the higher levels of residual aromatic content were more thermally stable at high temperatures.

The DSC thermograms (shown in Fig.3(c)) revealed strong exothermic peaks during the heating process above 250 °C,which corresponded to the TGA observations with Tmaxtemperatures above 250 °C.No glass temperatures,Tg,were observed below the degradation temperature for these highly cross-linked polymers.

Fig.4(a)shows the X-ray diffraction(XRD)patterns of the crosslinked polymers.The peak at 2θ=18.21°,associated to the piperazine crystals,clearly showed the excellent crystallinity of synthesized terpolymers.In addition,as the piperazine ratio increased there is substantial enhancement in terpolymer crystallinity indicating better packing of chains with longer piperazine chains.Previous studies have reported similar behavior [39].

The Brunauer-Emmet–Teller (BET) for the nitrogen isotherms shown in Fig.4(b) suggests that the Dipip series have low surface areas (Dipip 1,1:42 m2﹒g-1;Dipip 1,2:49 m2﹒g-1;Dipip 1,3:52 m2﹒g-1;Dipip 1,4:59 m2﹒g-1),the surface area values increase from Dipip 1,1 to Dipip 1,4.

SEM images and EDX showing elemental composition and distribution maps of raw Dipip 1,1 Dipip 1,4 are presented on Fig.5(a)and(b),respectively.The EDX shows homogeneous distribution of both C and N in the polymers.Furthermore,an increase in the percentage of N in Dipip 1,4 relative to Dipip 1,1 confirms the successful incorporation of additional piperazine moieties in the polymer back bone.

3.3.Effect of adsorption parameters on the removal efficiency of EBT dye

The adsorption properties of the Dipip polymer series were evaluated to determine their efficiency for adsorption of EBT.The results of the screening studies to determine the most effective polymers are displayed in Fig.S1.Based on the results,Dipip 1,1 and Dipip 1,4 exhibit superior efficiency to remove EBT,hence,the rest of the adsorption experiments were performed on these two polymers.

3.3.1.Effects of pH of the dye solution

The influence of solution pH on the removal efficiency of EBT dye on Dipip 1,1 and Dipip 1,4 is shown in Fig.6(a).pH effect is an important adsorption parameter that influences chemistry of adsorption sites on the surface of the polymer.As displayed in Fig.6(a),for both resins,the removal efficiency of EBT dye significantly reduced with increase in pH from 2 to 10.For instance,as the pH raises from 2 to 10 the percent removal of EBT by Dipip 1,1 and by Dipip 1,4 decreased from 99.06% and 95.45% to 24.53 and 42.68%,respectively.This behavior could be explained by the surface chemistry of the polymers (vide infra).Fig.S2 shows the pHPZCof Dipip 1,1 and Dipip 1,4 estimated using the pH drift method.The obtained values of pHPZCare 7.67 and 9.02 for Dipip 1,1 and Dipip 1,4,respectively.When the pH of the dye solution is less than pHPZC,the surface of the polymer is protonated and positively charged.This demonstrates that at lower pH,the amine moieties (R3N) on the polymers are protonated (R3NH+) leading to enhanced adsorption of the negatively charged EBT.Whereas at higher pH,the negative charge on the surface of the polymers increases causing repulsion with the EBT anions,hence,reduction in the removal performance.Therefore,based on these results investigating the effects of pH,succeeding tests were conducted at pH 2.

Fig.2.(a) FT-IR (b) solid-state 13C NMR of Dipip polymers.

Fig.3.(a) TGA-Thermogram (b) 1st derivative and (c) DSC-Thermogram of Dipip polymers.

The mechanism of adsorption of EBT on the polymers could be governed by both electrostatic including hydrogen bonding and non-electrostatic interactions involving the polymer surface and the dye.Such non-electrostatic interactions that might have played a role include π–π stacking,Vander Waals forces and hydrophobic interaction [12,40].Fig.6b depicts proposed interactions between the polymer surface and EBT.The electrostatic interaction may occur between the abundant protonated amines on the polymers and the sulfonate group on EBT.Hydrogen bonding may also take place between the hydroxyl groups on EBT and the various amines on the adsorbent.Furthermore,π–π stacking may occur between the aromatic rings on the polymer and the rings on EBT.

3.3.2.Effect of concentration

The effect of initial EBT concentration on the adsorption capacities of the resins was studied at varied concentration (20–100)mg﹒L-1.All the parameters including pH,temperature,dosage and contact time are kept constant at pH 2,25 °C,5 mg and 480 min,respectively.The obtained results are displayed in Fig.7(b).The adsorption capacity of Dipip 1,1 and Dipip 1,4 increased sharply with increase in concentration reaching to maximum adsorption capacity of 704.72 and 689.92 mg﹒g-1,respectively,at initial EBT concentration of 100 mg﹒L-1.The enhancement in adsorption performance of the polymers at higher dye concentrations can be attributed to the presence of more available dye molecules in the solution resulting to greater interaction with the polymer surface.

3.3.3.Effect of adsorbent dosage

Fig.4.(a) XRD pattern (c) BET surface of Dipip polymers.

Fig.5.SEM images and EDX showing elemental composition and distribution maps of raw (a) Dipip 1,1 (b) Dipip 1,4.

The effect of polymer dosage (0–25) mg on the removal efficiency of EBT dye is displayed in Fig.7(c).It was observed,that for Dipip 1,1 and Dipip 1,4,percent removal of EBT increased linearly from 60.55%to 98.45%and 58.86%to 94.61%when the dosage was increased from 2 to 5 mg,respectively.A further increase in polymer dosage to 25 mg showed no significant change.Increases in polymer dosage provides a higher number of sorption active sites leading to increased interactions with the anionic EBT molecules,thus,resulting in improved removal efficiency of EBT.This suggests that the synthesized polymeric resins are efficient for removing anionic dyes from aqueous solution.

3.3.4.Effect of contact time

To investigate the rate of adsorption of EBT and its equilibrium adsorption time on the polymers,experiments were conducted at three different concentrations(20,60 and 100)mg﹒L-1with varied contact time (0 to 480 min).The other experimental parameters were kept constant.From the results shown in Fig.7(d),for all dye concentrations,the rate of removal of EBT for both polymers was rapid within the first 60 min.The results indicate that at a lower concentration (20 mg﹒L-1),the equilibrium was reached at 360 min.However,for higher dye concentrations(60–100 mg﹒L-1),the polymers required greater contact times with dyes molecules and achieved equilibrium after 420 min.The fast rate of EBT removal followed by slower adsorption is attributed to the interactions of EBT molecules with active sites of the polymers.

3.4.Equilibrium isotherms

Adsorption isotherms models are usually applied to evaluate the interface behavior between sorbate particles (EBT) and the adsorbent surface (polymers) at the point of equilibrium.In order,to find out the sorption behavior of EBT molecules on the polymers,the equilibrium data was fitted to the Redlich-Peterson,Langmuir and Freundlich isotherm models

The Langmuir adsorption isotherm assumes a monolayer and finite site adsorption,implying non-interaction between the molecules adsorbed on adjoining sites.The mathematical form of Langmuir equation is expressed as:

Fig.6.The effect of initial pH on EBT removal by Dipip polymers,C0=20 mg﹒L-1,dosage=5 mg and t=480 min(b)Proposed mechanism of EBT adsorption by Dipip polymers.

where Ce(mg﹒L-1) and qe(mg﹒g-1) denote liquid phase concentration and solid phase concentration at equilibrium,respectively.KL(L﹒mg-1) and qmare Langmuir constants.

Freundlich isotherm model is an expression for multilayer adsorption that occurs on a heterogeneous surface and is typically given by:

where Ce(mg﹒L-1) and qe(mg﹒g-1) denote liquid phase concentration and solid phase concentration at equilibrium,respectively.Kfand 1/n denote Freundlich constant and heterogeneity factor,respectively.

Redlich–Peterson model (Eq.(5)) incorporates three variable parameters into an empirical isotherm and is used as a settlement between Freundlich and Langmuir systems.

where A,B and brpare the R–P constants.When brpis equal to 1,the equation reduces to the Langmuir isotherm,whereas at high liquidphase concentrations of the adsorbate it reduces to Freundlich isotherm.

The adsorption equilibrium data fittings to the linear forms of Langmuir,Freundlich &Redlich Peterson (not shown) isotherm models are illustrated in Fig.8(a) and (c).Table 3 summarizes the correlation coefficients and the values of the adsorption isotherm constants.As displayed in Table 3,the attained values of R2of Dipip 1,1 and Dipip 1,4 for Langmuir,Freundlich and Redlich Peterson isotherm models were (0.962 and 0.996),(0.974 and 0.970) and (0.989 and 0.996) respectively at 25 °C.These results clearly indicated that the sorption of EBT on Dipip 1,1 and Dipip 1,4 was well described by Redlich-Peterson &Langmuir model.

The theoretical maximum monolayer adsorption capacity of polymer Dipip 1,1 and Dipip 1,4 polymer calculated from Langmuir isotherm model were 775.19,and 917.43 mg﹒g-1respectively at 25°C.The brpvalue from R-P model for Dipip 1,1 is 0.58 which suggests that there are strong chances of multilayer adsorption on the adsorbent surface,whereas,brp(0.8856) value for Dipip 1,4 is closer to 1,which further confirms that the monolayer adsorption occurred on the surface of polymer.Moreover,The values of ‘1/n’(Table 3) of the Dipip 1,1 and Dipip 1,4 are (0.36 and 0.55),(0.32 and 0.40) at 25 °C K and 35 °C 308 K respectively are less than unity,suggesting that the adsorption of EBT is favorable[41].Likewise,the values of 1/n increased with increase in temperature,this shows that EBT adsorption process was unfavorable at elevated temperatures [34].

The comparative adsorption capacities of various absorbents for EBT as reported elsewhere [12,34,42–47] are listed on Table 4.It could be deduced from the results that the new polymers have outstanding capability to remove EBT from water.Hence,the Dipip polymers evaluated in this study have excellent potential for use in the treatment of industrial dye contaminated waters.

Table 3 Parameters of Langmuir,Freundlich and Redlich-Peterson isotherm models for adsorption of EBT onto polymer resins

3.5.Kinetic studies

To predict the EBT adsorption rate and main reaction mechanism involved,the kinetic data were fitted to the linearized forms of the pseudo-1st order (Eq.(6)) and pseudo-2nd order (Eq.(7))models.

Fig.7.(a)Adsorptive removal of EBT by Dipip polymers(b)The effect of initial concentration(c)dosage(d)contact time on the removal of EBT by Dipip polymers,Conditions;(b) C0=20 mg﹒L-1,dosage=5 mg,t=480 min,pH 2 at 25 °C (c) C0=20 mg﹒L-1,t=480 min,pH 2 at 25 °C (d) dosage=5 mg,pH 2 at 25 °C.

where k1(min-1) and k2(g﹒mg-1﹒min-1) are rate constants from pseudo-1st order and pseudo-2nd order models respectively.

The kinetic parameters of pseudo-first order,pseudo-second order estimated using their respective linear equations are listed in Table 5 and the linear plots presented in Fig.8(c)and(d),respectively.The results shown in Table 5 clearly deduced that the for all concentrations of EBT (20–100) mg﹒L-1,the correlation coefficient(R2)is greater for pseudo-2nd order model compared to pseudo-1st order model.The high R2(>0.99),indicates that the experimental kinetic data of EBT dye on both polymers satisfactorily fits with pseudo-2nd-order.Moreover,the experimental adsorption capacities obtained are (156.72,457.52,713.52 mg﹒g-1),and (152.4,444.88,691.62 mg﹒g-1) at concentrations (20,60,100 mg﹒L-1) for Dipip 1,1,and Dipip 1,4,respectively closely matches with adsorption capacities (158.73,476.19,714.28 mg﹒g-1) and(163.93,476.23,714.28 mg﹒g-1) obtained from pseudo-2nd order.As assumed by pseudo-2nd order model,it can be deduced that chemisorption mechanism is mainly the rate limiting step involved,comprising mainly electrostatic attractions between surface functionalities of the polymer with EBT anionic molecules.

Table 4 Comparative adsorption capacities of various absorbents for EBT

Table 5 Pseudo 1st order and pseudo 2nd order kinetic model parameters of polymers for EBT removal

Table 6 Thermodynamic parameters for removal of EBT dye

3.6.Thermodynamics

The major thermodynamic parameters (standard free energy(ΔG),entropy change (ΔS) and enthalpy change (ΔH)) of EBT adsorption on Dipip 1,1 &Dipip 1,4 were calculated at three temperatures using Eqs.(8) and (9):

where R is the universal gas constant,T is the absolute solution temperature,and Kdis the thermodynamic equilibrium constant calculated from the plot of ln(qe/Ce) vs.qe.Table 6 lists the values of ΔS and ΔH,as calculated from the plot of ln Kdvs.1/T (Fig.S3).

Since ΔG is negative the adsorption of EBT on both polymers was favorable.The positive values of ΔH demonstrated that the adsorption process of EBT is endothermic in nature.Likewise,the values of ΔG decreased from -16.41 to -21.33 for Dipip 1,1 and-12.52 to -16.47 for Dipip 1,4 as the temperature rises from 25°C to 45 °C,specifying an endothermic process.The values of ΔS for Dipip 1,1 (244.85) and Dipip 1,4 (197.04) J﹒mol-1﹒K-1are positive,which is attributed to the increased randomness at the adsorbent-adsorbate interface [49].

Fig.9.Adsorption/desorption with repeated cycles on Dipip 1,1 and Dipip 1,4 for EBT removal.

3.7.Reusability of the adsorbents

In order to make adsorption process economically feasible,an adsorbent should be reusable in several consecutive adsorptionsdesorption cycles.Hence,to evaluate the reusability of the synthesized polymers,the adsorption followed by desorption cycle was repeated 4 times and the results are illustrated on Fig.9.Both polymers show good recovery of at least 95% in the first two cycles,while the recovery in the fourth cycle was about 80%.Thus,these terpolymers could be reused multiple times,making these materials strong candidates as adsorbents for the treatment of dyecontaining wastewater.

4.Conclusions

A series of terpolymeric polyamines containing different ratios of benzene-1,2-diamine and piperazine was synthesized.The series was made by varying the ratios of benzene-1,2-diamine:piperazine(1:1,1:2,1:3 and 1:4).The polymers were characterized by a number of techniques including13C solid-state NMR,FT-IR,TGA,DSC,XRD,SEM and EDX,which helped to prove the proposed chemical structure of the polymers.The cross-linked polyamines exhibit excellent efficacy for removal of EBT dye from water.The best performing polymers are those having ratios 1:1 and 1:4 with removal efficiencies of～100% and～ 95% for EBT and adsorption capacities of 775 mg﹒g-1and 917 mg﹒g-1,respectively at 5 mg dose.Redlich-Peterson isotherm was the best fitting model and the kinetics of the process conforms to pseudo-second order model.With negative value of ΔG,the adsorption process is spontaneous in nature.The high efficiency and excellent reusability of the new polymers could potentially put them to use in treatment of dye contaminated waters.

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.

Acknowledgements

The authors thank Imam Abdulrahman Bin Faisal University(IAU),Dammam for providing research facilities.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.09.052.