research article insight into equilibrium and kinetics of ... · adsorption process is considered...

Hindawi Publishing CorporationJournal of Applied ChemistryVolume 2013, Article ID 417180, 12 pageshttp://dx.doi.org/10.1155/2013/417180

Research ArticleInsight into Equilibrium and Kinetics of the Binding ofCadmium Ions on Radiation-Modified Straw from Oryza sativa

Sana Zulfiqar Ali, Makshoof Athar, Umar Farooq, and Muhammad Salman

Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistan

Correspondence should be addressed to Umar Farooq; [email protected]

Received 26 March 2013; Accepted 24 May 2013

Academic Editor: Luqman Chuah Abdullah

Copyright © 2013 Sana Zulfiqar Ali et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

The present study reports the chemical modification of agricultural waste (rice straw) with urea using microwave radiation andthe efficiency evaluation of this modified rice straw for the adsorption of a toxic heavy metal, cadmium. The elemental analysis ofurea modified rice straw affirmed urea grafting on rice straw, and FTIR spectra of chemically benign modified adsorbent showedthe presence of hydroxyl, carbonyl, and amino functional groups. Effects of process parameters (adsorbent dosage, contact time,agitation speed, pH, and temperature) were studied in batch mode. Parameters were optimized for the equilibrium study, andadsorptionmechanismwas elucidated using fivemathematical models (Langmuir, Freundlich, Temkin, Harkin-Jura, andDubinin-Radushkevich). Binding of Cd(II) ions on modified adsorbent followed Langmuir model, and the maximum uptake capacity wasfound to be 20.70mg g−1. Kinetic modeling was done using six different kinetic models. The process was considered physisorptionaccording to the obtained activation energy value. Thermodynamic parameters confirmed the process to be favorable and feasible.Exothermic nature of adsorption of Cd(II) ions on urea modified rice straw was confirmed by the negative value of ΔH∘.

1. Introduction

Rapid pace of industrialization has resulted in a number ofproblems among which water pollution is considered to beone of the serious problems. Industrial processes dischargehuge amounts of untreated wastewater daily into the sur-rounding environment, leading to detrimental effects on aqu-atic, plant, and human life. Heavy metal such as lead, cad-mium, chromium, and copper is regarded asmajor pollutantsin wastewater. These contaminants are of major concernbecause they do not degrade naturally [1].

Cadmium has attracted wide attention of environmentalchemists as one of the most toxic metals and has been cate-gorized as a human carcinogen by USEPA (United StatesEnvironment Protection Agency), WHO (World HealthOrganization), and NTP (National Toxicology Program) [2].It is a nonessential and nonbiodegradable metal which slowlyaccumulates in the human body, usually from food chain.The permissible limits for cadmium by WHO and USEPAare 3 𝜇g L−1 and 5 𝜇g L−1, respectively. It affects lungs, liver,and pancreas and disturbs the human DNA repair system.

Chronic cadmium poisoning causes characteristic yellowpigmentation of teeth (the yellow ring of cadmium) [2].

It is important to treat contaminated waters on a contin-uous basis due to need of hour. A number of technologiesare available with varying degree of success, and among themadsorption process is considered relatively better becauseof convenience, ease of operation, and simplicity of design[3]. Activated carbon has been utilized as the commercialadsorbent for the removal of pollutant from wastewaterbecause of its excellent adsorption ability. However, its use isrestricted due to high cost.

Recently, many nonconventional, low-cost adsorbentsincluding natural materials, (biosorbents) and waste materi-als have been proposed by several researchers. Cadmium hasbeen reported to be removed and recovered from aqueoussolutions by a number of biosorbents [4–8]. Modificationof the natural materials (biosorbents) has gained numerouscost-effective and efficient adsorbents for the uptake of pollu-tants from aqueous solution. Certain physical and chemicalmethods such as heating, freezing, drying, cross-linkingwith organic solvents, chemical reactions with a variety of

2 Journal of Applied Chemistry

RS

Microwave oven

RS

UMRS

12 minutesNHCONH2

NH2CONH+

2

Scheme 1

organic and inorganic compounds, and modification undermicrowave radiation in the absence of any solvent have beenutilized in this regard [9–13]. The modifications produceadsorbents which have greater exposed metal-binding sitesas compared to raw adsorbents. The chemically modifiedadsorbent offers a larger available surface area. The surfacechemistry is altered after modification due to the incorpo-ration of new functional groups by changing the alreadypresent functional groups on any adsorbent. These modifiedfunctional groups act as complexing or chelating agents formetals [14–16]. Modification with microwave radiation isadvantageous because it is a simple process and does notrequire any solvent. Also no harmful vapors are added to theenvironment.

Rice (Oryza sativa) is one of the major crops grownthroughout the world and is most important staple food fora large part of the world’s human population. Rice straw,obtained as a byproduct of rice industry, is an agriculturalwaste which is used as a cost-effective adsorbent for a numberof pollutants including metal ions. Rice straw is insoluble inwater, has good chemical stability, and has high mechanicalstrength [17], making it good adsorbent material for treatingheavy metals from wastewater.

The present study is based on the evaluation of effec-tiveness of a modified agricultural waste (rice straw) for theremoval of toxic Cd (II) ions from aqueous solution in a batchprocess. Detailed equilibrium, kinetic, and thermodynamicstudies elucidate the adsorption mechanism.

2. Materials and Methods

2.1. Collection and Urea Modification of Oryza sativa. Driedrice straw (Oryza sativa) was collected from the PunjabUniversity area of Lahore, Pakistan. It was washed with waterto remove the dust and particles and was air dried. Dried ricestraw was grinded and sieved to pass 40–60 mesh (ASTMstandard). It was again washed with water and dried in anoven at 105–110∘C till constant mass. The dried biomass wasmixed with urea (Merck, Germany) in 1 : 2 by mass andirradiated in amicrowave oven (D131, Dawlance) for a periodof 12 minutes. The procedure for modification is expressedin detail elsewhere [18] and shown in Scheme 1. This ureamodified rice straw was designated as UMRS and stored inairtight plastic bottles for further use.

2.2. Characterization of UMRS. The prepared materialUMRS was characterized using FTIR, elemental analysis,and surface area. In order to study the presence of potentialfunctional groups in UMRS, the FTIR spectrumwas scannedin 4000–400 cm−1 using standard method with the help of

FTIR spectrophotometer (Spectrum RX-1, Perkin Elmer).The elemental analysis was performed using elemental ana-lyzer (EL III, Elementar, Vario) using corn gluten as astandard. The surface area was determined using Langmuirequilibrium model.

2.3. Batch-Stirred Biosorption Experiments. A concentrationdifference method was used to study the effect of variousparameters on the biosorption of Cd(II) byUMRS. Cadmium(II) nitrate (Merck, Germany) was used to prepare theaqueous solution of Cd(II). Distilled water was used for alltypes of solution preparations and dilutions as per require-ment. In all experiments, measuring/conical flasks (100mL)were used containing Cd(II) solution (50mg/L, 50mL) ofa known concentration at a specific pH. A known amountof UMRS (0.2–1.4 g) was added to the solution and thenagitated on an orbital shaker (OSM-747, Vortex) at predefinedspeed (125 rpm). After a specific period of time, the contentswere filtered, and the filtrate was analyzed using atomicabsorption spectrophotometer (AAnalyst 100, Perkin Elmer)to determine the equilibrium Cd(II) ions concentrations.The difference of initial (𝐶

0

, mg L−1) and equilibrium (𝐶𝑒

,mg L−1) metal ion concentration was considered to be sorbedby UMRS. The removal (R%) of Cd(II) and/or the amountof Cd(II) sorbed per unit mass of UMRS at equilibrium (𝑞

𝑒

,mg g−1) were determined using the following formula:

𝑅% =𝐶0

− 𝐶𝑒

𝐶0

× 100, (1a)

𝑞𝑒

=𝐶0

− 𝐶𝑒

𝑚× V, (1b)

where “𝑚” (g) is the mass of UMRS and “V” (L) is the volumeof Cd(II) solution used for the experiment.

The effects of parameters like time of contact, pH, doseof UMRS, agitation speed, temperature, and concentrationof Cd(II) ions on the biosorption process were studied in asimilar way. Blank experiments were performed in order tostudy the adsorption of Cd(II) by the glassware.No detectableadsorption of Cd(II) was found by the glassware. All thegraphs were prepared using Microsoft Excel 2003 software.Regression analyses have also been performed by calculating𝑅2 to investigate the suitability of certain mathematical

model. Root mean square errors (RMSE) were calculated inorder to evaluate the error of the model predictions. Thesum of the squares of the difference between metal removalexperimental data (𝑞exp) and model predictions (𝑞cal) wasdivided by the number of data points (𝑁) for each data set,and the square root of this term was taken as follows:

RMSE = √∑(𝑞exp − 𝑞cal)

2

𝑁.

(1c)

3. Results and Discussion

3.1. Effect of pH. The pH of the solution is probably the mostimportant parameter as it affects the charges on biomass as

Journal of Applied Chemistry 3

C C C

OH

OH

OH

O

O

O

+H+H

−H −H

Scheme 2

well as metal speciation in the solution. The metal speciesare influenced by the solution pH. Cadmium ions are presentas free Cd2+ species along the whole acidic pH range. Asthe pH is increased above pH 7.5, it starts to precipitate asCd(OH)

2

, and thus it is no more “available” for biosorption.This narrows down the upper pH limit for the biosorption ofCd(II) by UMRS. So, during the study of the effect of pH, therange that should be scanned for the optimum pH is limitedto an upper value of 7.5 [16].

On the other hand, in highly acidic pH, there are a greaternumber of H+ ions present in the solution. These H+ ionsare readily sorbed on the sites of the biomass (UMRS) andthus protonate it before metal ions can attack these sites.This causes UMRS to behave as positive specie. Due to theelectrostatic repulsive force present between two positivespecies, a limited number of Cd(II) ions are sorbed onUMRS,and thus there should be low 𝑞

𝑒

value at low pHs. When thepH is raised from highly acidic pH, this positive characterdecreases. UMRS, being a modified lignocellulosic material,contains a variety of functional groups including carbonyl,amide, hydroxyl, and thiol. The behavior of each of thesefunctional groups changes with the change in solution pH.For example, carboxyl groups are protonated in highly acidicpHs (pH less than 3) acting as positively charged speciesand attracting negative charged ions [19]. On increasingthe pH, the deprotonation (ionization) of these functionalgroups causes them to act as negative moieties. At thisstage, they attract and attach positive cations like Cd(II) ionsmore readily. It may be represented as shown in Scheme 2[16].

The protonation and deprotonation of other availablefunctional groups can be explained on similar grounds. It canbe predicted that at highly acidic pHs, Cd(II) binding withUMRS is reduced, and the binding increases with increase inpH because UMRS is negatively charged. This effect of pHof solution on biomass, thus, decides the lower limit of pH.Based on the previous discussion, the effect of pHwas studiedin pH range of 2–7.

The pH profile study is shown in Figure 1. It is obviousthat the binding of Cd(II) by UMRS increased with increasein pH. This is in conformity with the previous discussion.No significant change in the 𝑞

𝑒

value is observed as the pHis increased above 5. This indicated that the pH had a vitalrole in the biosorption of Cd(II) by UMRS and themaximumbinding occurred at a pH of 5. A number of studies, in theliterature, on the biosorptive removal of Cd(II) by variousbiosorbents reported the optimum pH between 5-6 [18, 20–22]. Almost similar values of pH indicate that the biosorptionof Cd(II) ions seems not to be dependent on the biosorbentmaterial. The material only provides the lower pH limit forbiosorption of Cd(II) ions.

1 2 3 4 5 6 7 8

3.5

3

2.5

2

1.5

1

0.5

0

qe

(mg/

g)

pH

Figure 1: Effect of change in pH on biosorption of Cd(II) on UMRS(𝐶0

= 50mg/L, time = 10min).

0

0.5

1

1.5

2

2.5

3

3.5

0 10 20 30 40 50Time (min)

qe

(mg/

g)

Figure 2: Effect of contact time on Cd(II) biosorption by UMRS(𝐶0

= 50mg/L).

3.2. Effect of Contact Time-Biosorption kinetics. The contacttime studies are very critical as these endow with the min-imum time required to remove maximum amount of Cd(II)ions from the solution and thus help in scaling up the process.The optimum (equilibrium) time helps in studying the rate ofbiosorption process. With the help of kinetic data, the ratedetermining step of the transport mechanism and thus themodeling and design of the process can be described.

The effect of contact time on the removal of Cd(II) byUMRS is depicted in Figure 2. It was observed that by increas-ing the time of contact, the metal removal (𝑞

𝑒

) increasedrapidly. This continued till the 𝑞

𝑒

achieved a maximum valueat a time of contact of 10 minutes.The rapid increase in the 𝑞

𝑒

values in the initial 10 minutes can be attributed to a greaternumber of binding sites available during initial stages. Thisindicated the physical binding of Cd(II) ionswithUMRS [18].As time proceeded, the number of binding sites was reduceddue to accumulation of Cd(II) ions on the UMRS leading


Table 1: Kinetics of the biosorption of Cd(II) onto UMRS.

Model Linear equation 𝑞𝑒 exp (mg g

−1) Model parameters

Elovich model 𝑞𝑡

=ln (𝑎 × 𝑏)

𝑏+ln 𝑡𝑏

2.924

𝑎 (mg g−1 min−1)𝑏 (gmg−1)

𝑅2

4.62 × 1029

25.640.6931

First order 1𝑞𝑡

=1

𝑞𝑒

+𝑘1

𝑞𝑒

𝑡

𝑘1

(min−1)𝑞𝑒 calc (mg g

−1)𝑅2

0.04522.9140.5245

Pseudo-first order ln (𝑞𝑒

− 𝑞𝑡

) = ln 𝑞𝑒

− 𝑘1

𝑡

𝑘1

(min−1)𝑞𝑒 calc (mg g

−1)𝑅2

0.10740.15370.4734

Second order 1𝐶𝑒

−1

𝐶0

= 𝑘2

𝑡𝑘2

(gmg−1 min−1)𝑅2

0.00260.4644

Pseudo-second order 𝑡𝑞𝑡

=1

𝑘2

𝑞2𝑒

+𝑡

𝑞𝑒

𝑘2

(gmg−1 min−1)𝑞𝑒 calc (mg g

−1)𝑅2

4.68122.9290.999

Intraparticle diffusion 𝑞𝑡

= 𝑘WM√𝑡 + 𝐶𝑘WM (mg g

−1 min−1/2)𝐶

𝑅2

0.02452.79020.6166

to the decrease in the binding of metal ions. Thus the rateof biosorption decreases in the later stages. After a contacttime of 10 minutes, the graph becomes virtually parallel tothe time axis, indicating the establishment of the equilibrium.Thus 10 minutes were taken as the optimum time of contactfor the biosorption of Cd(II) onto UMRS. This equilibrium(optimum) time is found shorter than a number of studiesreported in the literature for the biosorption of Cd(II) ionsonto various biosorbents [18, 20–22].

The reaction kinetics was investigated by using a numberof different available kinetic models. The experimental dataobtained from the contact time studies was used for thepurpose.The linear forms of the Elovich, first-order, pseudo-first-order, second-order, and pseudo-second-order kineticmodels [23–27] are, respectively, given as

𝑞𝑡

=ln(𝑎 × 𝑏)𝑏

×ln 𝑡𝑏, (2a)

1

𝑞𝑡

=1

𝑞𝑒

+𝑘1

𝑞𝑒

𝑡, (2b)

ln (𝑞𝑒

− 𝑞𝑡

) = ln 𝑞𝑒

− 𝑘1

𝑡, (2c)

1

𝐶𝑒

−1

𝐶0

= 𝑘2

𝑡, (2d)

𝑡

𝑞𝑡

=1

𝑘2

𝑞2𝑒

+𝑡

𝑞𝑒

, (2e)

where 𝑎 (mg g−1min−1) gives the rate constant and 𝑏 (gmg−1)gives rate of adsorption at zero coverage in Elovich model.𝑘1

(min−1) is the first-order rate constant, 𝑞𝑒

and 𝑞𝑡

arethe amounts of metal ions sorbed per gram of biomass(mg g−1) at equilibrium and at time “𝑡,” respectively, and𝑘2

(mg g−1min−1) is the second-order rate constant. Theparameters for the kinetic models were determined fromrespective plots (Figure 3) and are given in Table 1.

The experimental data were used to study the kineticsof the process using Elovich model (Figure 3(a)). The valueof coefficient of determination (𝑅2 = 0.6931) is quite lessthan 0.98.This low value indicated that the kinetics of Cd(II)biosorption by UMRS could not be discussed based onthe Elovich model. In other way, Cd(II)-UMRS biosorptionsystem did not follow Elovich kinetic model. The literatureshows that Elovich model is the least applied kinetic modelto biosorption systems, and only a few examples showthe application of this model over the whole kinetic data[28, 29].

The plots for first-order and pseudo-first-order kineticmodels are shown in Figures 3(b) and 3(c).The comparison ofthe experimental and calculated 𝑞

𝑒

values provides a tool fordeciding the fitting of the model over the experimental data.As shown in Table 1, it can be observed that the calculated 𝑞

𝑒

(2.914mg g−1) value for the first-order model is comparablewith the experimental value (2.924mg g−1). This pointedto the possible fitting of the model for the process. Thevalue of the first-order rate constant 𝑘

1

is 0.0452 (min−1).However, 𝑅2 (0.5245) is quite less than 0.98. So, it can beinferred that first-order model cannot be applied to explainthe biosorption process under study. On the other hand,the calculated 𝑞

𝑒

(0.1537mg g−1) value for pseudo-first-ordermodel is significantly different from the experimental 𝑞

𝑒

(2.924mg g−1) value. The value of the pseudo-first-order rateconstant 𝑘

1

is 0.1074 (min−1). In addition, 𝑅2 value (0.4734)is quite less than 0.98. Hence it can be concluded that Cd(II)-UMRS biosorption system did not follow the pseudo-first-order kinetic model. This observation is in accordance withthe studies reported in the literature for the fitting of first-order and pseudo-first-order kinetic models (Table 2).

The plots for second-order (𝑡 versus 1/𝐶𝑒

− 1/𝐶0

) andpseudo-second-order (𝑡 versus 𝑡/𝑞

𝑡

) kinetic models are givenin Figures 3(d) and 3(e). The value of second-order rateconstant 𝑘

2

was quite low (0.0026 gmg−1min−1).The𝑅2 valuefor the second-order kineticmodel (0.4644) indicted that this


0

0.5

1

1.5

2

2.5

3

3.5

0 1 2 3 4

qt

ln t

y = 0.039x + 2.790

R2 = 0.693

(a)

0.34

0.344

0.348

0.352

0.356

0.36

0 0.5 1 1.5

1/q

t

1/t

y = 0.015x + 0.343

R2 = 0.524

(b)

−7

−6

−5

−4

−3

−2

−1

0

0 20 40 60t

y = −0.107x − 1.872

R2 = 0.473

ln(q

e−qt)

(c)

t

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60

y = 0.002x + 0.207

R2 = 0.464

1/Ce

(d)

t

0

2

4

6

8

10

12

14

16

0 20 40 60

t/qt

y = 0.341x + 0.024

R2 = 0.999

(e)

0

1

2

3

4

0 2 4 6 8

qt

y = 0.024x + 2.790

R2 = 0.616

t1/2

(f)

Figure 3: Kinetic models for the binding of Cd(II) onto UMRS (a) Elovich, (b) first-order, (c) pseudo-first-order, (d) second-order, (e)pseudo-second-order, and (f) intraparticle diffusion models.


Table 2: Comparison of capacity of UMRS with some other reported biosorbents.

Biomass Biosorption capacity (𝑞𝑚

, mg g−1) Equilibrium modela Kinetic modelb ReferenceSulfonated Juniperus monosperma wood 1.68 — PSO [42]Juniperus monosperma 2.80 — PSO [42]Zea mays 3.61 — [43]Coconut copra meal 4.92 L, RP — [44]Nauclea diderrichii 6.30 L PSO [45]Triticum aestivum (straw) 11.56 L — [21]Triticum aestivum (straw) 14.56 L PSO [40]Water Hyacinth 14.67 — PSO [46]Papaya wood 17.22 L PSO [47]Spent grain 17.30 L — [6]Urea modified Oryza sativa 20.70 L PSO Present workRhizopus cohnii 40.50 L — [48]aL: Langmuir model; RP: Redlich-Peterson model.bPSO: pseudo-second-order model.

model cannot be applied to investigate the kinetics of Cd(II)-UMRS biosorption system.

When the experimental data was used to draw the graphfor pseudo-second-order kinetic model, straight plot for thewhole set of data was observed.The comparison of calculated𝑞𝑒

(2.929mg g−1) and experimental 𝑞𝑒

(2.924mg g−1) valuesshowed that the difference between the two was too small tobe significant.This indicated the possible fitting of the kineticmodel for the process.The coefficient of determination (𝑅2 =0.9999) pointed to that the pseudo-second-order kineticmodel could be used for the investigation of kinetics of theCd(II) biosorption by UMRS.

It was found that Elovich, first-order, pseudo-first-order,and second-order models failed to explain the kinetics ofCd(II) biosorption by UMRS. In some cases, the experi-mental and calculated 𝑞

𝑒

values were in close proximity; yetthe coefficient of determination was opposing the fitting ofthe model (first-order model). Pseudo-second-order kineticmodel was the best model, showing fitting over the wholeset of experimental kinetic data. Hence, it can be concludedthat pseudo-second-order model was suitable to explain thekinetics of the system under study, that is, Cd(II)-UMRSbiosorption system. A number of authors have studied thebiosorption of divalent metal ions onto various biosorbentsand reported that such studies mostly followed pseudo-second-order kinetics (Table 2).

In order to have an insight into the rate determiningstep, Weber and Morris model, that is, intraparticle diffusion(IPD)model, was employed [30].The linear form is shown asfollows: (2f);

𝑞𝑡

= 𝑘WM√𝑡, (2f)

where 𝑘WM is the intraparticle diffusion rate constant(mg g−1min−1/2). A straight line passing through the originin 𝑞𝑡

versus 𝑡1/2 plot will indicate that the sorption processis governed by intraparticle diffusion; that is, intraparticlediffusion is the rate determining step; otherwise boundarylayer diffusion is the rate controlling step.

As shown in Figure 3(f), the varying extent of the bindingof Cd(II) during initial and final stages of the experimentled to a plot virtually parallel to time axis. As the curve didnot start from the origin (the intercept is not zero), so it canbe inferred that intraparticle diffusion did not play a role inthe rate determining step. So, boundary layer diffusion wasthe rate determining step, and the biosorption of Cd(II) byUMRS was governed by boundary layer diffusion. The valueof 𝑘WM is shown in Table 1. However, further studies arerequired to establish this observation.

3.3. Effect of Concentration-EquilibriumModeling. Therole ofadsorption in the biosorption of Cd(II) ions by UMRS can beexplained by the use of equilibriummodeling.The adsorptionmodels indicate how Cd(II) ions distribute between theliquid and solid phases at equilibrium. A number of differentmodels have been employed for the purpose in the presentstudy. These models, namely, Langmuir equation (3a) [31],Freundlich equation (3b) [32], Temkin equation (3c) [33],Harkin-Jura equation (3d) [34], and Dubinin-Radushkevichequation (3e) [35], are given as, respectively,

𝑞𝑒

=𝑞𝑚

𝐾𝐿

𝐶𝑒

1 + 𝐾𝐿

𝐶𝑒

, (3a)

𝑞𝑒

= 𝐾𝐹

𝐶1/𝑛

𝑒

, (3b)

𝑞𝑒

=𝑅𝑇

𝑏ln (𝐴𝑇

𝐶𝑒

) , (3c)

𝑞𝑒

= (𝐴

𝐵 − log𝐶𝑒

)

1/2

, (3d)

𝑞𝑒

= 𝑞𝑚

exp (−𝛽𝜀2) , (3e)

where 𝐾𝐿

, 𝐾𝐹

, 1/𝑛, 𝐴𝑇

, 𝑏, 𝐴, 𝐵, 𝛽, and 𝜀 are the constants ofthese models.

Experimental data for the biosorption of Cd(II) ions byUMRS is plotted as 𝐶

𝑒

versus 𝑞𝑒

graph (Figure 4). The fitting


Table 3: Equilibrium models studied at 303K.

Model Equation Parameters

Langmuir 𝑞𝑒

=𝑞𝑚

𝐾𝐿

𝐶𝑒

1 + 𝐾𝐿

𝐶𝑒

𝑞𝑚

(mg g−1)𝐾𝐿

(Lmg−1)𝑅𝐿

𝑅2

RMSE

20.700.0409

0.7576–0.23810.98550.65

Freundlich 𝑞𝑒

= 𝐾𝐹

𝐶1/𝑛

𝑒

𝐾𝐹

(mg g−1)𝑛

𝑅2

RMSE

1.20181.65750.91281.22

Temkin 𝑞𝑒

=𝑅𝑇

𝑏ln (𝐴

𝑇

𝐶𝑒

)

𝐾𝑇

(Lmg−1)𝐵 (RT/b, kJmol−1)

𝑅2

RMSE

1.90222.16120.9740.30

Harkin-Jura 𝑞𝑒

= (𝐴

𝐵 − log𝐶𝑒

)

1/2

𝐴

𝐵

𝑅2

RMSE

2.46061.17220.60021.14

D-R 𝑞𝑒

= 𝑞𝑚

exp (−𝛽𝜀2)

𝑞𝑚

(mg g−1)𝛽

𝐸 (kJmol−1)𝑅2

RMSE

5.85321 × 10

−6

0.7070.8140.97

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30

ExperimentalLangmuirFreundlich

Harkin-JuraD-RTemkin

Ce

qe

Figure 4: Equilibrium modeling of Cd(II) biosorption on UMRS.

of the equilibrium models is also represented on the sameplot. The values of different equilibrium parameters and 𝑅2values have been determined with the help of these plots andare shown in Table 3.

The Langmuir model is one of the most frequentlyused equilibrium models and is employed to determine themaximum capacity of the biosorbent to bind the metal ions.It assumes that the uptake/binding of Cd(II) ions occurs onthe homogenous surface by monolayer adsorption withoutany interaction between adsorbed ions. The nonlinear form

of Langmuir model is used to explain the behavior of theCd(II)-UMRS biosorption process. It can be observed thatthe Langmuir equilibrium curve significantly overlaps theexperimental data (Figure 4). The 𝑅2 value that is 0.9878(greater than 0.98) points to the inference that the Langmuirmodel can explain the equilibrium of the biosorption processunder study. The biosorption capacity of UMRS (𝑞

𝑚

) wasfound to be 20.7mg g−1, and the adsorption constant𝐾

𝐿

wasfound to be 0.0409 Lmg−1.

The 𝑞𝑚

value thus calculated was used to determine thespecific surface area (𝑆

𝐿

, m2 g−1) of the material (UMRS) byusing the following mathematical relationship:

𝑆𝐿

=𝑁𝐴

𝐴𝑞𝑚

𝑀, (3f)

where 𝑁𝐴

is Avogadro number (6.02 × 1023), 𝐴 refers tocross sectional area of metal ion (Å2), and 𝑀 is the atomicmass of the metal ion. The specific surface area is calculatedon the basis of the 𝑞

𝑚

value, and it shows the surface areaper gram of the material occupied by the metal ions. Theatomic mass of cadmium is 112, and the cross sectional area is3.73 Å2 (the radius of Cd(II) ions for close packed monolayeris 1.09 Å [18]). The specific surface area of UMRS for Cd(II)biosorption was found to be 4.15m2 g−1.

The feasibility of Langmuir model is usually expressedby a dimensionless constant separation factor or equilibriumparameter 𝑅

𝐿

, defined as

𝑅𝐿

=1

1 + 𝐾𝐿

𝐶0

, (3g)

where “𝐾𝐿

” is the Langmuir constant. The value of 𝑅𝐿

indicates the type of isotherm as unfavorable (𝑅𝐿

> 1), linear


(𝑅𝐿

= 1), irreversible (𝑅𝐿

= 0), or favorable (0 < 𝑅𝐿

< 1)[36].The 𝑅

𝐿

values were calculated to lie in 0.7576–0.2381. Asthese values lie between 0 and 1, it can be deduced that theadsorption of Cd(II) ions onto UMRS is favorable under thestudied conditions.

The Freundlich model assumes the nonzero interactionsbetween the adsorbate particles and is based on multilayeradsorption on heterogeneous surface. The nonlinear plot forthe model is shown in Figure 4. The comparison of the curvefor the Freundlich model with the experimental data showsthat this model can explain the biosorption of Cd(II) ions forthe initial set of data only. The coefficient of determination(𝑅2 = 0.9867) for initial range also supports the observation.The 1/𝑛 value is less than 1, and it shows that the sorption ofmetal ions is favorable on UMRS. As the Freundlich modelstates that the adsorption is exclusively physical one [18, 37],so it can be inferred that Cd(II) ions are sorbed physically onthe heterogeneous surface (ofUMRS) forming a layers havingnonzero interactions between them.

Temkin model provides information about the heat ofadsorption and the adsorbent-adsorbate interaction on thesurfaces. Harkin-Jura model indicates the multilayer adsorp-tion. The parameters for the both equilibrium models areshown in Table 3, and the fitting with the experimental datais represented in Figure 4. It can be observed that both themodels do not fit with the experimental data, although dataseemed to follow Temkin model at the later ends (𝐶

𝑒

>

14mgL−1). Although certain information about the nature ofsorption are achieved yet due to disagreement of the experi-mental data with the models, these information/models arenot suitable to explain the process, and further studies arerequired to establish the facts. However, 𝐵

𝑇

value (less than8) indicated low heat of adsorption and very weak interactionsuggesting the process to be physisorption [38, 39].

The physical or chemical nature of biosorption of Cd(II)onto UMRS can be assessed by determining the energy ofsorption (𝐸) using the following equation:

𝐸 =1

√2𝛽, (3h)

where 𝛽 is a coefficient related to the mean free energy ofadsorption (mol2 J−2).The value of𝛽 can be determined usingthe Dubinin-Radushkevich (D-R) model. The adsorptionprocess will be a physical adsorption for𝐸 < 8 kJmol−1, and itwill be chemisorption for 8 < 𝐸 < 16 kJmol−1 [40].The valueof energy (𝐸) was found to be less than 0.71 kJmol−1, andthus the sorption of Cd(II) by UMRS was physical in nature.This is in accordance with the inference obtained from theTemkin model. Figure 4 shows that D-R model is not beingfollowed by the Cd(II)-UMRS biosorption system. 𝑅2 valueis quite less than 0.98. Hence, the values of Polanyi potential(𝜀),𝛽 and the energy of sorption (𝐸) were determinedwithoutsignificant accuracy. As the systems did not follow thismodel,the inference about the nature of adsorption process seemednot to be reliable.

3.4. Effect of Temperature: Feasibility of the Process. The effectof change in temperature on the Cd(II)-UMRS sorption

Table 4: Values of thermodynamic parameters for Cd(II)-UMRSsystem.

Temperature(K)

ΔG∘(kJmol−1)

ΔH∘(kJmol−1)

ΔS∘(J K−1 mol−1)

293.16 −1126.58 117.81 409.08303.16 −7979.85313.16 −9308.26

systemwas studied to resolve the thermodynamic parametersand to investigate the nature/feasibility of the process. It wasobserved that the sorption capacity increased with increasein temperature. The 𝑞

𝑒

values increased during the studiedtemperature range (293–313 K) from 1.9175 to 3.0400mg g−1of UMRS (figure not shown).This indicated that the sorptionof Cd(II) by UMRS was an endothermic process and thatUMRS could be effectively used for the biosorptive removalof Cd(II) ions from aqueous solutions at relatively highertemperatures.

The experimental data were used to determine the ther-modynamic parameters like changes in standard free energy(Δ𝐺∘), enthalpy (Δ𝐻∘), and entropy (Δ𝑆∘) using the followingequations:

Δ𝐺∘

= −𝑅𝑇 ln𝐾𝐷

, (4a)

Δ𝐺∘

= Δ𝐻∘

− 𝑇Δ𝑆∘

, (4b)

where 𝑇 is the absolute temperature (K) and 𝐾𝐷

[(𝐶0

−

𝐶𝑒

)/𝐶𝑒

] is the distribution coefficient.The thermodynamic parameters given in Table 4 were

determined from the plot of Δ𝐺∘ versus 𝑇 (Figure 5(a)).The negative values of Δ𝐺∘ at the studied temperaturerange indicated that the sorption of Cd(II) by UMRS wasthermodynamically feasible and spontaneous. The decreasein the value of Δ𝐺∘ with temperature further showed theincrease in feasibility of sorption at elevated temperatures.In other words, the sorption was endothermic in nature.The positive value of Δ𝐻∘ also supported this statement. Thepositive value ofΔ𝑆∘ showed the increased randomness at thesolid-solution interface during the sorption ofmetal ions, andit also reflected the affinity of UMRS for metal ions [41].

3.4.1. Activation Energy. Activation energy (𝐸𝑎

) is an impor-tant parameter related to the strength and type of forcespresent between Cd(II) ions and UMRS. It was determinedusing the following linear Arrhenius equation [26]:

ln 𝑘2

= ln𝐴1

−𝐸𝑎

𝑅𝑇, (4c)

where 𝑘2

is known as the pseudo-second-order rate constant,𝐴1

is the Arrhenius constant,𝐸𝑎

refers to energy of activation(kJ mol−1), 𝑅 is the ideal gas constant (8.3134 Jmol−1 K−1),and 𝑇 is the temperature of the medium (K). A graph wasplotted between ln𝑘

2

and 1/𝑇. The values of energy of activa-tion (𝐸

𝑎

) and Arrhenius contact (𝐴) were determined from


0

−2000

−4000

−6000

−8000

−10000

−12000290 295 300 305 310 315

T (K)

y = −409.08x + 117814

R2 = 0.8681

ΔG0

(a)

0

0.5

1

1.5

2

2.5

3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45(1/T) ×10−3

lnk2

y = −4.1419x + 14.99

R2 = 0.9668

(b)

Figure 5: (a) Thermodynamic modeling and (b) activation energy for the Cd(II) onto UMRS under optimum conditions.

the slope and intercept of the linear plot (Figure 5(b)).The 𝐸𝑎

value is found to be 34.42 kJmol−1 indicating relatively weakforces to be involved in the biosorption process indicating theprocess to be physisorption.

3.5. Dose of UMRS. Dose of biomass is very important indetermining the minimum amount required to treat a solu-tion of given metal concentration. By increasing the amountof biomass, the number of available sites is also increased.Theeffect of dose of UMRS on percentage adsorption of Cd(II)ions was studied at an initial concentration of 50mg L−1 byvarying the amount from 0.2 to 1.4 g per 50mL of Cd(II)solution. At optimum conditions, it was observed that byincreasing the dose, the 𝑞

𝑒

values also increased initially(figure not shown). After increasing the dose to 0.8 g per50mL, the 𝑞

𝑒

values decreased. The increase in 𝑞𝑒

can beattributed to the increase in the number of sites at UMRS.After maximum Cd(II) ions are attached at 0.8 g per 50mL,the increase in dose caused 𝑞

𝑒

values to decrease. As 𝑞𝑒

valuesare calculated by dividing 𝐶

0

− 𝐶𝑒

by mass of UMRS, so 𝑞𝑒

values decreased, although the removal (𝑅%) increased. Inaddition, the decrease may be attributed to overlapping oraggregating of sites at UMRS resulting in decrease in totalUMRS surface area available to Cd(II) ions. It can be inferredthat 0.8 g of UMRS was sufficient to detoxify a solutioncontaining 50mg Cd(II) per liter of solution.

3.6. Effect of Agitation Speed. Agitation speed is considered asone of the important process parameter which significantlyaffects the biosorption of Cd(II) onto UMRS. When UMRSis made to come into contact with Cd(II) bearing solution,the metal ions present close to it are readily attached. Thisgenerates a concentration gradient in the metal-biomasssystem. By agitating the metal-biomass system, the effect ofsuch a concentration gradient is minimized, and the metalions present in the solution are distributed evenly in thesolution. Moreover, agitation distributes the biomass in thesolution more evenly as compared to the situation whenthere is no agitation. On the other hand, agitation also causesdesorption of the loosely bound metal ions from the surfaceof biomass. So an optimum speed of agitation is very much

Table 5: Characterization of UMRS.

Elemental analysisC (%)H (%)N (%)S (%)

49.986.867.080.82

FTIR analysis (cm−1) 3819.1; 3287.4; 2349.4; 1697.9; 1429.1; 1048.8;768.5; 674.9

essential for the efficient removal of metal ions from thesolution.

The effect of agitation speed was monitored on thebiosorption of Cd(II) by UMRS by varying the speed from50 to 250 rpm at optimum conditions (figure not shown). Asthe agitation speed was increased, 𝑞

𝑒

value initially increasedand reached a maximum at 125 rpm. After that, the increasein speed caused the biosorption of Cd(II) to decrease. Ifagitation speed is low UMRS accumulates in the solutioninstead of distributing in the solution. Various active sites areburied forming layers of UMRS on one another and thus donot participate in the biosorption process. So sorption occursonly at the top surface layer resulting in lesser 𝑞

𝑒

values.On the other hand, at higher agitation speeds, desorptionof bound Cd(II) ions increases, and the sorption-desorptionequilibrium is shifted towards the desorption and the 𝑞

𝑒

values decreases. On the basis of the previous discussion, anagitation speed of 125 rpm was selected as an optimum speedfor the Cd(II) biosorption by UMRS.

3.7. Characterization of UMRS. Elemental analysis and FTIRanalysis were carried out using powdered, dried urea modi-fied rice straw.The characterization revealed the informationregarding adsorption sites in terms of functional groups.Simple rice straw consists of cellulose (32.24%), hemicellulose(21.34%), lignin (21.44%), and mineral ash (15.05%) [37].The elemental analysis of urea modified rice straw showedrelatively higher percentage of carbon, hydrogen, oxygen, andnitrogen (Table 5). The percentage of nitrogen, that is, 7.08%is of special concern. The high content of nitrogen in UMRSpointed to the fact that after modification urea was attachedto already present functional groups present in the rice straw.


3819.12

3736.433612.40

3287.46

2113.69

2349.46

1697.78

1651.29

1429.15

1048.81

768.53674.90

T (%

)

4000 2000 1000 600.0(cm−1)

157.1156155154153152151150149148147146145144143142141140139138

136.7

Figure 6: FTIR of UMRS.

O

CUMRS

Cd(II)

UMRS CO-N

Cd(II)

Figure 7: Proposed attachment/binding sites of Cd(II) ions onto theUMRS biosorbent (𝐶

0

= 50mg/L).

The FTIR is an important technique to identify potentialfunctional groups thatmay participate in the binding ofmetalions. The characteristic FTIR bands for UMRS (Figure 6) aregiven in Table 5. A number of peaks/bands can be recognized.The broadband in 3000–2800 cm−1 region is mainly due toO–H stretching vibrations. The N–H and C–H bands alsoarise in this region, and these are buried under the broadO–H band, although some very weak peaks may be seen.The presence of C≡C and C≡N can be observed by the peakaround 2300 and 2100 cm−1. The sharp peak at 1697.78 cm−1may be attributed to the carbonyl groups of aldehydes andketones present in the large molecules in the cell wall of thebiosorbent material. The presence of a band near 1650 cm−1pointed to the presence of amide group. The C–H bendingand C–O–C stretching may be observed around 1430 and1050 cm−1, respectively. Thus it can be inferred that UMRSis polyfunctional in nature. Cd(II) has been found to attachwith the oxygen and nitrogen containing functional groupspresent in the biomass. As UMRS is rich in these functionalgroups, it can be used for the binding of Cd(II) ions in asignificant amount. The binding may be proposed as shownin Figure 7.

4. Conclusion

The present study was based on the efficiency evaluation ofa low cost urea modified agricultural waste material for theadsorption of Cd(II) ions from water. Characterization ofthe modified adsorbent using FTIR and elemental analysisaffirmed the urea modification by showing peaks of amidegroup and a high nitrogen content, respectively. Process

parameters were optimized for equilibrium study. Accordingto the results maximum adsorption was observed when 0.8 gper 50mL of modified adsorbent remained in contact with50mg L−1 Cd(II) ions solution for 10min at pH 6 keepingagitation speed 125 rpm at temperature 303K. Five differ-ent adsorption isotherms (Langmuir, Freundlich, Temkin,Harkin-Jura, and Dubinin-Radushkevich) were used for theadsorption modeling and equilibrium study. It was observedthat Langmuir model better fitted to the equilibrium data,the maximum uptake capacity was found to be 20.70mg g−1,and 𝑅

𝐿

factor showed the favorability of the adsorptionprocess. Temkin isotherm indicated physisorption to be theoperating process for the uptake of Cd(II) ions by urea mod-ified adsorbent due to weak interactions and lower heat ofadsorption. 𝐸 value obtained from D-R model corroboratedthis statement. Kinetics of the process was investigated bysix different kinetic models. The kinetic study indicated thatadsorption mechanism obeyed pseudo-second-order kineticmodel. Intraparticle diffusion model proposed boundarylayer diffusion as rate determining step for the process.Thermodynamic parameters such as Δ𝐺∘, Δ𝐻∘, and Δ𝑆∘showed the exothermic nature of the process along with itsfeasibility and spontaneity. Increase in entropy Δ𝑆∘ indicatedthe favorability of adsorption of Cd(II) ions by urea modifiedrice straw affirming its effectiveness and application forwaste water treatment. Thus urea modified rice straw canbe considered a cost effective and benign adsorbent for theremoval of heavymetal ions such asCd(II) ions from aqueoussolutions.

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research article insight into equilibrium and kinetics of ... · adsorption process is considered...

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