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Removal of congo red dye from aqueous system using Phoenixdactylifera seeds
Deepak Pathania a,⁎, Arush Sharma a, Zia-Mahmood Siddiqi b
a School of Chemistry, Shoolini University, Solan 173212, Himachal Pradesh, Indiab Jubail University College, P.O. Box 10074, Jubail Industrial City 31961, Saudi Arabia
⁎ Corresponding author.E-mail address: [email protected] (D. Pathania
Article history:Received 6 January 2016Received in revised form 18 February 2016Accepted 6 March 2016Available online xxxx
In this study, Phoenix dactylifera seeds (PDS)was used as a biosorbent for the removal of congo red (CR) dye fromaqueous system. Biosorbent was characterized by some instrumental techniques such as Fourier transform infraredspectroscopy (FTIR), X-ray diffractometer (XRD), transmission electron microscopy (TEM). The effect of variousadsorption parameters such as initial dye concentration, sorbent dosage, contact time, pH, electrolyte, surfactantsand temperature was optimized for maximum sorption of dye. Langmuir, Freundlich and Tempkin isothermswere applied for the interpretation of experimental data. Langmuir model was found to be best fittedwith maximum adsorption capacity equal to 61.72 mg g−1. The kinetic study showed that the adsorptionprocess was described by pseudo-second order kinetics. The thermodynamic parameters such as energychange (ΔG°), enthalpy change (ΔH°) and entropy change (ΔS°) were found to be −3.51 kJ/mol,22.89 kJ/mol and 87.130 J/mol/K, respectively. Recycling efficiency of PDS was investigated for the sorptionof CR and after 5 cycles, the adsorption efficiency of PDS was reduced to 53.90% from 76.12%.The resultsindicated that Phoenix dactylifera seeds were used as a suitable sorbent for the adsorption of CR dye fromaqueous solution in a feasible, spontaneous and endothermic way.
Biomass has been broadly used for the removal of pollutants such asdyes, heavymetals, phenols and chlorophenols found in the effluents ofthe textile, leather, food processing, dyeing, cosmetics, paper and dyemanufacturing industries [1–2]. It was observed that 10–15% of dyeswere released into the environment during their manufacturing andusage [3]. Dyes and pigments present in wastewater of industriesleads to the generation of hazardous affects to the animal and humanhealth [4–5]. Dyes have complex aromatic molecular structures andstable towards heat and oxidizing agent. In addition, most of dyes aretoxic and harmful to somemicroorganisms [6]. Colored dyes are esthetic,carcinogenic and obstruct the light penetration into aquatic system.Several dyes are toxic to flora and fauna and therefore, caused healthhazard [7].
Congo red is a direct diazo dye and causes allergic reaction. It is arecalcitrant andmetabolized to benzidinewhich is a human carcinogen.Congo red is very difficult to remove because it possesses the thermal,physico-chemical and optical stability due to their aromatic structure[8].Thus the removal of dyes remains an important issue for researchersand environmentalists. Many conventional methods such as chemicalprecipitation, ion exchange, electro-dialysis, ultra-filtration membrane
).
separation, photo-degradation, electrochemical oxidation etc. have beenwidely used for the treatment of dyes bearing wastewater [9–13].However, due to various disadvantages associated with conventionalmethods, adsorption process has been explored by many researchers[14–18]. Adsorption is a well-known separation technique in terms ofinitial cost, simplicity of design, ease of operation, insensitive to toxicsubstances and ability to treat dyes in more concentrated form, besidesit provides sludge free cleaning operations [19–20].
Activated carbon has been used as a standard adsorbent for theremoval of colour and other effluents from water system. But the useof activated carbon is limited on large scale, mainly due to its highcost and connected hardship to regenerate [21–25].
Recently, attention has been paid on the development and use ofhighly effective and low cost adsorbents [26–28]. Thus renewablebiosorbents have been explored due to their low cost, easy availability,good modifiability and low toxicity. Various renewable materials, suchas rice husk [29], castor seed shell [30], banana peel [31], wheat shell[32], sugarcane bagasse [33], soy meal hull [34], orange peel [35], sawdust [36–37], waste wood-shaving bottom ash [38], Trichodermaharzianum mycelial waste [39], Ficus carica [40], luffa cylindrical fiber[41] etc. have been used for the removal of organic pollutants fromaqueous solution. These naturalmaterials are available in large quantityand can be discardedwithout expensive regeneration. Despite the avail-ability of large number of absorbents, newer adsorbent materials arebeing investigated for wastewater treatment.
360 D. Pathania et al. / Journal of Molecular Liquids 219 (2016) 359–367
Phoenix dactylifera seeds (PDS) a readily available agro-based mate-rial, may be used as an alternative for the removal of organic pollutantsfrom wastewater. Phoenix dactylifera, commonly known as dates is cul-tivated as an edible sweet-fruit and widely distributed in many trop-ical and subtropical regions [42]. Phoenix dactylifera seeds are the by-product of date stoning and used for the production of pitted dates ordate paste. These are traditionally used for animal feed. It can be usedas a source of oil which has antioxidant properties valuable in cosmeticsor as a coffee substitute.
In this paper, Phoenix dactylifera seeds (PDS) has been explored asnovel biosorbent for the removal of congo red inwastewater treatment.The biosorbent was characterized by Fourier transform infra-redspectroscopy (FTIR), X-ray diffractometer (XRD), transmission electronmicroscopy (TEM).The effect of some parameters such as initial dyeconcentration, contact time, temperature, adsorbent dosage, pH,electrolyte concentration and surfactants on the adsorption efficiencyhas been investigated. The adsorption isotherms, kinetics and thermo-dynamic study have been carried out to determine the mechanism ofadsorption process. The inputs obtained from this research have beenuseful for designing low-cost, easily available adsorbents based onagro-material for the removal of dye-containing effluents.
2. Materials and methods
2.1. Reagents
All chemicals such as congo red (C32H22N6Na2O6S2), hydrochloric acid(HCl), sodium hydroxide (NaOH), sodium chloride (NaCl), cetyltrimethylammonium bromide (CTAB), sodium dodecylbenzenesulfonate (SDS),polyoxyethyleneglycol t-octylphenyl ether (Triton X-100) used in thisresearch work were of analytical grade reagents. All dilutions andwashing were carried out using double distilled water.
2.2. Instrumentation
A double beamUV–visible spectrophotometer (Shimadzu UV-1601)was used for the determination of dye concentration. The pHmeasure-ments were made using a pH meter (ELICO model LI-127, India). Char-acterization of biosorbent was done using a Fourier transform infraredspectrophotometer (Perkin Elmer Spectrum-BX USA), transmissionelectronmicroscope (FEI Tecnai F 20) andX-raydiffraction (Philips1830diffractometer).
2.3. Preparation of biosorbent
The Phoenix dactylifera seeds (PDS) were collected from farmlandlocated in Saudi Arabia. The PDS were repeatedly washed with doubledistilled water until the water became colorless. Then biomaterialwere filtered and dried in oven at 60 °C for 24 h. The dried biomaterialsmashed into mesh size of 0.1 mm to 0.4 mm. The smashed particleswere stored in a desiccator for further study.
2.4. Characterization
2.4.1. Fourier transforms infrared absorption spectra (FTIR)FTIR spectra of PDS were recorded by Fourier transform infrared ab-
sorption spectrophotometer using KBr disc method. The bio-substancewas thoroughly mixed with KBr, powdered and disc was formed byapplying the pressure. The absorption spectra were recorded in therange from 4000 to 400 cm−1.
2.4.2. Transmission electron microscopy (TEM) studiesTEMmicrographs of PDSwas determinedbypreparing the suspension
of bio-substance in ethanol onto carbon copper grid and analyzed.
2.4.3. X-ray diffraction (XRD) studiesX-ray analysis was carried out by X-ray diffractometer (Philips 1830
diffractometer) equipped with the graphite monochromator using CuKα radiation. It was operated at 45 kV and 40 mA. The rotation occursbetween 10° to 90° at 2θ scales.
2.5. Adsorption experiment
The adsorptionof dye onto PDSwas performedusingbatch adsorptionexperiments. The batch adsorption experiments were conducted in100 mL Erlenmeyer flasks containing required amount of adsorbent and50 mL of CR solution with various initial concentrations. The pH of eachsolution was adjusted with 0.1 N HCl and 0.1 N NaOH solutions. Themixtures were agitated in an incubator shaker at 140 rpm and 30 °Cuntil the equilibrium reached. The resultant mixture was centrifuged at2500 rpm for 10 min. The equilibrium concentrations of dye in the solu-tion were measured at 498 nm using UV–visible spectrophotometer(ShimadzuUV-1601). The experiments have been conducted in duplicateand the negative controls (without sorbent) were simultaneously carriedout. No filter paper was used in the solid–liquid separation because thesurface acidic functional groups of filter paper have the potential toadsorb dye molecules. The amount of dye sorbed (qe mg g−1) andpercentage removal of CR were calculated using Eqs. (1) and (2) as:
qe ¼ Co−Ceð Þ VM
ð1Þ
%Removal ¼ C0−Ce
C0� 100 ð2Þ
where C0 and Ce is the initial and equilibrium concentration of CR (mg/L),V is the volume of solution (L),M is themass of biosorbent (g).The exper-imental conditions have been optimized at different dye concentrations(20–120 mg/L), adsorbent dosage (20–120 mg), pH (2−12), tempera-ture (30–55 °C) and contact time (20–140 min). The effect of electrolyteand surfactants has been investigated onto sorption process.
2.6. Desorption and regeneration
Thedye loaded biosorbentwasdesorbedwith 50mLof 0.05MNaOHsolution. It was stirred in an incubator shaker at 120 rpm for 1 h. Then itwas centrifuged at 2500 rpm for 10 min and biosorbent was washedwith double distilled water and regenerated. Subsequently, it wasused in five sequential cycles of desorption-adsorption.
2.7. Equilibrium isotherms
The adsorption isotherms indicate the amount of dye adsorbed ontoadsorbent and equilibrium concentration of the dye in solution at agiven temperature. The relationship between the concentration anddye uptake was determined by Langmuir, Freundlich and Tempkinisotherms.
2.7.1. Langmuir isothermThe Langmuir isotherm presumed that adsorption was observed at
homogenous surface containing a finite number of biosorption siteswithin uniform distribution of energy level and no transmigration ofadsorbate on the surface takes place. The linear form of Langmuirmodel is given as follows [43]:
Ce
qe¼ 1
bQmþ Ce
Qmð3Þ
where qe (mg g−1) is the amount of dye adsorbed per unit weight ofadsorbent, Ce (mg/L) is the unadsorbed dye concentration at equilibrium,b is the equilibrium constant or Langmuir constant related to the affinityof binding sites (L/mg) andQm represents the particle limiting adsorption
Fig. 2. (a) XRD, (b–c) transmission electron microphotographs of PDS at differentmagnification.
Fig. 1. Adsorbent dosage was 60 mg rather than 0.60 mg.
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capacity when the surface is fully covered with dye molecules andassist the comparison of adsorption performance. The values of Qm
and b were calculated from slope and intercept of a plot (Ce / qe vs.Ce), respectively.
The shape of Langmuir isotherm can be expressed in terms ofdimensionless separation factor (RL) which is presented as follows [44]:
RL ¼ 11þ bC0
ð4Þ
where RL indicates the shape of Langmuir isotherm and nature ofadsorption process, b is the Langmuir constant, C0 is the initial concen-tration of dye (mg/L).
2.7.2. Freundlich isothermFreundlich isotherm indicates heterogeneous adsorption with non-
uniform distribution of energy level. Freundlich isotherm describesthe reversible adsorption and not restricted to the formation of mono-layer. The linear form of Freundlich model is given as follows [45]:
lnqe ¼ lnkF þ 1n
lnCe ð5Þ
where KF and n are the Freundlich constant and determined from theplot of ln qe versus ln Ce. KF and 1 / n are related to sorption capacityand sorption intensity of the system, respectively. The magnitude ofthe term (1 / n) gives an indication about the favorability of thesorbent/adsorbate systems [46].
2.7.3. Tempkin isothermTempkin model assumes that heat of adsorption for decreased
linearly rather than logarithmic. The linearized form of Tempkinmodel is presented in following equation as [47].
qe ¼ Bln KTð Þ þ Bln Ceð Þ ð6Þ
where B (g mg−1 h−2) and KT (mg g−1 h−2) are the Tempkinconstant and related to heat of sorption andmaximum binding energy,respectively. The values of B and KT are calculated from the plot of qeagainst lnCe.
2.8. Adsorption kinetics
Adsorption kinetics has been proposed to elucidate the adsorptionmechanism. In order to investigate the mechanism of CR adsorption
onto PDS, pseudo first-order and pseudo second-order kinetic modelhas been examined [48–49].
The rate expression for pseudo-first order reaction can be describedas follows:
log qe−qtð Þ ¼ logqe−k1t
2:303ð7Þ
where qe and qt are the amount of dye adsorbed onto sorbent at equilib-rium and time t (min) respectively. k1 (min−1) is the rate constant ofpseudo-first order adsorption. The value of k1 and qe can be calculatedfrom the slope and intercept of log (qe − qt) versus t respectively.
Linearized formof pseudo-secondorder can be presented as follows:
tqe
¼ 1k2q2
eþ 1
qe
� �t ð8Þ
where qe and qt are the sorption capacity at equilibrium and at time t,respectively (mg g−1), k2 is the overall rate constant of pseudo-secondorder sorption (g mg−1 min−1). A plot of t / qt against t gives a linearrelationship, from which the value of qe and k2 can be determinedfrom the slope and intercept respectively.
2.9. Adsorption thermodynamics
Thermodynamic parameters such as free energy change (ΔG°),enthalpy change (ΔH°) and entropy change (ΔS°) has evaluated forCR sorption onto PDS. These parameters were calculated using thefollowing equation as [50]:
ΔGż −2:303RT logKd ð9Þ
Scheme 1. Chemical structure of congo red.
362 D. Pathania et al. / Journal of Molecular Liquids 219 (2016) 359–367
Kd ¼ qe
CeAlso;ΔG� ¼ ΔH�−TΔS�
ð10Þ
lnKd ¼ ΔS�
R−
ΔH�
RTð11Þ
where qe is the concentration of CR at equilibrium onto PDS (mg/L),R is universal gas constant (8.314 J/mol K), and Ce is the CR concen-tration at equilibrium in the solution (mg/L).The values of ΔH° andΔS° were determined from the slope and intercept of plot ln KD
versus 1 / T.
Fig. 3. Effect of (a) initial dye concentration (b) sorbent dosage (c) contact time (d) pH for CR s120 min, pH neutral, temperature 30 °C].
3. Results and discussion
3.1. Characterization
FTIR spectra of PDS before and after sorption of dye were shown inFig. 1a–b. The broad adsorption peak at 3363 cm−1 indicates the pres-ence of both free and hydrogen bonded\\OH group on the surface ofPDS. The peak at 2925 cm−1 corresponds to asymmetric and symmetricstretching of the C\\H bond of\\CH2 group. The peak at 1688 cm−1
may be due to the stretching vibration of non-ionic carboxyl group(COOH, COOH3) and carboxylic acids or their esters [51]. The peaks at1522 cm−1, 1380 cm−1, 1066 cm−1 may be due to CC, C\\N and C\\Ostretching vibrations, respectively.
Fig. 1b shows the FTIR spectrum of CR-loaded PDS. Due to the inter-action of the functional groups on biosorbent with CR, the peaks haveshifted to lower or higher wave numbers and new peaks belonging tothe adsorbate (or splitting of original bands) has appeared. The shiftingof bands to lower frequencies indicates bondweakening, while a shift tohigher frequencies indicates an increase in bond strength [52]. The peaksat 3363 cm−1 was shifted to 3389 cm−1, while peaks at 1522 cm−1,1444 cm−1, 1380 cm−1 shifted to 1521 cm−1, 1441 cm−1, 1378 cm−1,respectively after the sorption of CR onto PDS. In CR-loaded PDS, appear-ance of newpeaks at 2854 cm−1 and 1743 cm−1was due to introductionof new functionalities on the surface of biosorbent.
The XRD pattern for Phoenix dactylifera seeds mainly showsamorphous nature (Fig. 2a). The smashed seeds contains high amountof oils and proteins which constitute around 69% of total mass showsunresolved signals (primarily amorphous) [53]. The transmissionelectron micrographs of PDS were shown in the Fig. 2b–c. Theblack-darken spots present on biosorbent surface accounted for theporosity of the material.
363D. Pathania et al. / Journal of Molecular Liquids 219 (2016) 359–367
3.2. Application of Phoenix dactylifera seeds (PDS) for the removal of congored (CR) dye
The structure of congo red dye is shown in Scheme 1.The effect of different reaction parameters on the sorption of dye has
been described as follows.
3.2.1. Effect of initial dye concentrationInitial dye concentration has played a significant role in the sorption
process. The effect of dye concentration (20–120 mg/L) on the sorptionprocesswas shown in Fig. 3a. It was revealed that the removal efficiencydeclined from73.2 to 45.74%with increased in concentration from20 to120mg/L. Themaximum sorptionwas received at 20mg/L. This may beattributed to the fact that CR dye tend to aggregate at higher concentra-tion (120 mg/L) to form large sized micelles [54] which are difficult todiffuse through the micropores of biosorbent.
3.2.2. Effect of sorbent dosageThe sorbent dosages determine the capacity of sorbent for a given
initial concentration of the adsorbate. The effect of PDS dose on theadsorption of CR has been varied from 20 to120 mg at optimized dyeconcentration. Fig. 3b shows the effect of sorbent dose on the sorptionof CR. It was evident that CR removal increased proportionally withdose which is due to the availability of more active sites and greaterspecific surface area. After 60 mg/L of sorbent loading, no significantremoval was observed. It was due to conglomeration of adsorbentparticles, as there is no significant increase in effective surface area[55]. Therefore 60 mg/L was chosen as the optimum adsorbent dosefor further study.
Fig. 4. Effect of (a) ionic strength (b) surfactants (c) temperature for CR adsorption onto PDtemperature 30 °C].
3.2.3. Effect of contact timeThe effect of contact time on the sorption was investigated at
optimized condition of dye concentration (20 mg/L) and amount ofbiosorbent dosage (60 mg/L) from 20 to 140 min. Fig. 3c shows theeffect of contact time on the removal of CR dye. The adsorption efficiencyincreased from 23.5 to 76.6%, as the contact time varied from 20 to120 min. It became constant after 120 min, so based on results 120 minwas taken as equilibrium time in the sorption process. This occurs dueto the rapid adsorption of CR by the surface of biosorbent followed byslow diffusion of ions from the surface to the adsorption sites in themicropores which are less accessible [56].
3.2.4. Effect of pHThepHof dye solution is an important parameter for overall sorption
of CR onto biosorbent. The effect of pH on the sorption of CR by PDSwasstudied in between pH 2–12. Fig. 3d shows the effect of pH on thesorption of CR. The maximum removal (84%) was occurred at pH 2.0followed by significant decrease in % removal with increased in pH. AtpH 2.0 the H+ ion concentration in the system increased and surfaceof PDS acquires positive charge by absorbing H+ ions. Therefore atlow pH, high electrostatic attraction exists between the positivelycharged surface of PDS and CR (anionic dye molecule) leading tomaximum dye sorption. However, at higher pH, CR anions generallyexcluded away from the negatively charged surface of biosorbent,thus decreasing the dye sorption. A similar trend has been observedfor the sorption of anionic dyes as reported in literature [57–58].Themaximum sorption of CR is observed at 2 pH value so it was selectedas optimum pH for further studies.
S [dye concentration 20 mg/L, sorbent dosage 0.60 mg, pH 2.0, contact time 120 min,
364 D. Pathania et al. / Journal of Molecular Liquids 219 (2016) 359–367
3.2.5. Effect of electrolyteThewastewater emanating from thedye industry contains the various
impurities such as suspended and dissolved compounds, acids or alkalis,salts, metal ions and other toxic compounds. The presence of these saltsincreased the ionic strength of the solutionwhichmay affect the sorptioncapacity [59]. In present study, NaCl was selected as amodel salt to inves-tigate their influence on the sorption capacity of biosorbent. It wasrevealed that with increase in the ionic strength from 0 to 1.5 M, thepercentage removal of CR increased from 83.45 to 89.83% (Fig. 4a).Theincrease in dye removal at high concentration of NaCl was caused by anincrease in dimerization of dye molecules in solution [60]. Generally saltions forces the dye molecules to aggregate and enhancing the extent ofsorption onto PDS.
3.2.6. Effect of surfactantsThe effects of various surfactants such as cetyltrimethyl ammoni-
um bromide (CTAB), sodium dodecylbenzenesulfonate (SDS),polyoxyethyleneglycol t-octylphenyl ether (Triton X-100) were carriedonto sorption process at optimized conditions. All surfactants lower thesurface tension when added to water in small amount. It was revealedfrom Fig. 4b that non-ionic surfactant (TritonX-100) slightly increasedthe sorption capacity from 83.5 to 85.05%, while ionic surfactants CTAB
Table 1Isotherm models parameters for sorption of CR onto PDS.
and SDS reduce the sorption capacity to 54.6% and 47.59% respectively.Itwas due to the existence of barrier energy between the ionic surfactantsand dye molecule or sorbent [61].
3.2.7. Effect of temperatureTemperature has pronounced effect on the sorption of dye because
equilibrium capacity of adsorbent changes with temperature. The effectof temperature determines whether the ongoing sorption process isendothermic or exothermic. The effect of temperature on sorptionprocess was carried under the optimized condition of dye concentration(20 mg/L), sorbent dosage (60 mg), contact time (120 min) and pH ofsolution (2.0). Fig. 4c shows that the removal of CR increased with tem-perature and maximum adsorption was observed at 55 °C, suggestingthat sorption process is endothermic in nature. It was evident that per-centage removal increased from83.3% to 90.15%with increase in temper-ature. It was due to more diffusion of adsorbate molecules across theexternal boundary layer and internal pores of the adsorbent particle [62].
3.3. Sorption isotherms
Equilibrium relationships between sorbent and sorbate wasdescribed by sorption isotherms. It is usually the ratio between quantitysorbed and remaining in the solution at equilibrium. Langmuir,
reundlich isotherm Parameters Tempkin isotherm
.100 KT (mg g−1 h−2) 0.438
.954 B (g mg−1 h−2) 14.18
.858 R2 0.988
Table 2Kinetics and thermodynamic parameters for sorption of CR onto PDS.
Parameters Pseudo-first order Parameters Pseudo-second order
qe (mg g−1) 0.982 qe (mg g−1) 24.61R2 0.82 R2 0.97K1 (min−1) 0.040 K2 (g mg−1 min−1) 0.388
365D. Pathania et al. / Journal of Molecular Liquids 219 (2016) 359–367
Freundlich and Tempkin models give the information about mechanism,properties and tendency of the sorbent for adsorbate by the analysis ofexperimental equilibrium data [63–64].
3.3.1. Langmuir isothermThe Langmuir isotherm indicates that adsorption takes place at
specific homogeneous site with formation of monolayer. Fig. 5a showsLangmuir isotherm from which the Qm and b can be determined. Themaximum sorption capacity (Qm) was found to be 61.72 mg g−1. Thehigher value of correlation coefficient, R2 (0.994) confirms the applicabil-ity of Langmuir isotherm. It inferred the monolayer formation anduniform distribution of congo red on the surface of adsorbent. The valueb was further used to calculate the dimensionless separation factor (RL)and indicate the type of isotherm to be irreversible (RL = 0), favorable(0 b RL b 1), linear (RL = 1) or unfavorable (RL N 1) [65].Table 1, showsthe calculated value of RL. It was found to be in the range of 0–1,suggesting the favorable adsorption.
Fig. 6. (a) Pseudo-first order, (b) pseudo-second order kinetics, (c) Van't Hoff plot of CR adsorpfive successive cycles of desorption-adsorption.
3.3.2. Freundlich isothermThe equation is purely empirical and used for heterogeneous
sorption of energy system. The different parameters were obtainedfrom the linear correlations of lnqe versus lnCe (Fig. 5b). From Freundlichisotherms, kf, n and R2 were found to be 2.100, 1.858 and 0.954,respectively (Table 1). It is recorded that the value of n N 1 indicatesthe favorable adsorption [66]. The value of 1 / n is known as heteroge-neity factor which ranges between 0 and 1. For more heterogeneoussurfaces, the value of 1 / n is close to 0. The correlation coefficient (R2)value indicates that Langmuir model is better fitted as compared toFreundlich model.
3.3.3. Tempkin isothermTempkin model was found to be fairly good fitting of experimental
data showing higher coefficient of regression (0.988) (Fig. 5c). Thissuggests that there is a uniform distribution of binding energy over
tion onto PDS for evaluating thermodynamic parameters, (d) removal efficiency of PDS in
366 D. Pathania et al. / Journal of Molecular Liquids 219 (2016) 359–367
the population of surface binding adsorption sites, thus, supporting thehomogeneous mechanism of adsorption.
3.4. Sorption kinetics
Kinetics models are important for evaluating the time required forthe removal of dye and mechanisms involved in the sorption process.The rate of sorption was usually governed by initial transfer of CRfrom solution to the surface of sorbent. The pseudo-first order andpseudo-second order kinetic models were applied for the sorption ofCR onto the biosorbent and the results were shown in Table 2. Thevalues for qe and R2 obtained from the pseudo-second order modelwere found to be 24.61 mg g−1 and 0.97, respectively. The correlationcoefficient for pseudo-second order kinetic model (0.97) was higheras compared to pseudo-first order model (0.82) (Fig. 6a–b). So sorptionof dye onto the biosorbent, PDS follows the pseudo-second order kineticmodel. Hence chemisorption was the rate controlling step in theadsorption process [67].
3.5. Thermodynamic study
Thermodynamic parameters such as free energy change (ΔG0),enthalpy change (ΔH0) and entropy change (ΔS0) have been studiedto evaluate the thermodynamic feasibility and spontaneous nature ofadsorption process. The negative value of Gibbs free energy (ΔG0)indicated that the sorption process was spontaneous in nature anddegree of spontaneity increased with temperature (Fig. 6c). The positivevalue of ΔH0 (22.89 kJ/mol) indicated the endothermic sorption of CRonto biosorbent. Further, the positive value of ΔS0 (87.130 J mol−1 K−1)inferred the freedom of adsorbate ions and increase in randomness atthe solid–liquid interface during the sorption of CR onto the biosorbent[68].The thermodynamic parameters of sorption were summarized inTable 2.
3.6. Regeneration study
For real applications, the regeneration capacity of the adsorbent isvery important so that dye can be recovered and adsorbent should beregenerated for next applications. An excellent adsorbent should notonly possess high adsorption capacity, but also high desorption capabil-ity in order to reduce the overall cost of the adsorbent. For desorption,experiment has been conducted in NaOH solution (0.05 M) forCR-loaded PDS. After desorption, the regenerated sorbent was reusedand 5 cycles of desorption-adsorption were performed. Fig. 6d showsthe desorption-sorption cycles of CR with NaOH solution as desorbingagent. The regeneration results showed that the adsorption efficiencyof biosorbent was reduced to 53.90 from 76.12%. After every cycle,NaOH has been used as a desorption medium to remove adsorbed CRfrom the biosorbent surface.
4. Conclusion
Through this study Phoenix dactylifera seeds (PDS) were proved tobe a promising adsorbent for the removal of hazardous CR dye inwater system. The optimized operating parameters for maximumsorption were determined as follows: initial dye concentration(20 mg/L), sorbent dosage (60 mg), contact time (120 min), pH ofsolution (2.0) and temperature (55 °C). The biosorbent PDS wascharacterized and identified using different techniques such asFTIR, XRD and TEM. The equilibrium adsorption quantity of CR ontoPDS was appropriately described by the Langmuir model with amaximum monolayer capacity of 61.72 mg g−1. The rate of sorptionwas found to obey the pseudo-second order kinetics with goodcorrelation coefficient. Besides, the results of thermodynamic studyinferred that adsorption of CR onto PDS was highly feasible, spontaneousand endothermic. This study suggest that Phoenix dactylifera seeds, a
readily-available and environmental-friendly biomaterial, might be usedas a promising non-conventional biosorbent for removing dyes fromaqueous medium.
Acknowledgments
Weacknowledge Shoolini University, Solan, Himachal Pradesh, Indiafor providing research facilities and grant to carry out the presentresearch.
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