Journal of environmental chemical engineering volume issue 2014 doi 10 1016 j jece 2014 06 015 pandey, ruchi prasad, ram lakhan ansari, nasreen ghazi murthy utilization of NaOH modified desmostachya bi

Abstract Abstract A fundamental investigation for the CdII ions removal from aqueous solutions by NaOH modified Desmostachya bipinnata Kush Grass Leaves MDBL and Bambusa arundinacea Ba

Accepted Manuscript

Title: Utilization of NaOH modified Desmostachya bipinnata

(Kush Grass) Leaves & Bambusa arundinacea (Bamboo)

Leaves for Cd(II) removal from aqueous solution

Author: Ruchi Pandey Ram Lakhan Prasad Nasreen Ghazi

Ansari Ramesh Chandra Murthy

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Utilization of NaOH modified Desmostachya bipinnata

(Kush Grass) Leaves & Bambusa arundinacea

(Bamboo) Leaves for Cd(II) removal from aqueous

solution

solution

a CSIR, Indian Institute of Toxicology Research, Analytical Chemistry Section,

Post Box-80, M.G Marg, Lucknow-226001, India

b Department of Chemistry, Faculty of Science, Banaras Hindu University,

Varanasi, 221005, India

* Corresponding Author

Abstract

Abstract

A fundamental investigation for the Cd(II) ions removal from aqueous solutions by NaOH

modified Desmostachya bipinnata (Kush Grass) Leaves (MDBL) and Bambusa arundinacea

(Bamboo) Leaves (MBAL) was conducted in batch experiments The influence of different

experimental parameters such as pH, contact time, initial Cd(II) ion concentration, adsorbent

dosage, on the Cd(II) adsorption was studied The Cd(II) uptake by MDBL and MBAL was

quantitatively evaluated using sorption isotherms Freundlich and Langmuir isotherm models

were used to fit the equilibria data, of which Langmuir model is considered better in correlation

and the maximum adsorption capacity was found to be 15.22 mg g −1 for MDBL and 19.70 mgg

-1 for MBAL at room temperature The kinetic data were found to follow closely the pseudo

second order kinetic model by both adsorbents FTIR and SEM were recorded, before and after

adsorption, to explore number and position of the functional groups available for Cd(II) binding

on to studied adsorbents and changes in surface morphology of adsorbent Desorption studies

show 94.18% and 92.08% recovery for adsorbed Cd(II) ions from MDBL and MBAL,

respectively using 0.1 N HNO 3 Thermodynamic studies indicated that the adsorption reaction

was a spontaneous and exothermic process It can be concluded that MDBL and MBAL are

Accepted Manuscript

low-cost biosorbent alternatives for wastewater treatment, since both have a considerable high

adsorption capacity

Hig

Highlights hlights hlights

• • A Novel and efficient biosorbent is developed from NaOH modification of

Desmostachya bipinnata (Kush Grass) Leaves (MDBL) & Bambusa arundinacea

(Bamboo) Leaves (MBAL) for removal of Cd(II) ions from aqueous solution

• • Apparent high adsorption capacity of 19.84 and 19.71 mg g−1 was shown by

MDBL and MBAL at pH = 6.5, respectively with a fast adsorption rate

• • 94.18 percent and 92.08% desorption of adsorbed Cd(II) ions from MDBL and

MBAL, respectively was observed using 0.1 N HNO3.

1111 Introduction Introduction Introduction

An increased flux of heavy metals in the aquatic environment due to their swift use [1] in

industries, has led severe threat to human being Beyond permitted concentration, they

can cause grave health disorders; therefore, considerable attention has been paid to

wastewater treatment prior to its discharge in the environment Among these metallic

pollutants, Cd(II), an extremely toxic heavy metal causes a potential risk to

environmental and human health because it is incorporated into the food chain, mainly

by plant uptake [2] The main anthropogenic pathway through which Cd(II) enters the

water bodies is via wastes from industrial processes such as electroplating, plastic

manufacturing, metallurgical processes, and Cd/Ni batteries Over exposures may befall

even in conditions where a little amount of Cd(II) found because of its low permissible

limit (0.005 mg L−1) in drinking water [3] So many surface chemistry practices for

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wastewater treatment such as precipitation, adsorption, membrane processes, ionic

exchange, floatation, and others [4,5] have been studied However, because of inherent

limitation of such techniques as less competent, perceptive operating settings, and

production of sludge, they further require costly disposal [6], whereas, adsorption is by

far the most versatile and widely used method, and activated carbon is the furthermost

commonly used adsorbent [7] Conversely, the use of activated carbon is expensive, so

considerable interest has been shown towards the use of other efficient sorbent

materials, particularly biosorbents [8] In recent years, agricultural by-products have

been widely considered for metal sorption studies including peat, banana pith, pine bark,

peanut, shells, hazelnut shell, rice husk, wood, sawdust, wool, soybean and cottonseed

hulls, orange peel, leaves and compost [9-13] In a previous study we have carried out

Cd(II) removal using Cucumber peel and obtained a maximum adsorption capacity of

7.142 mg g−1[14] In the adsorption process, various metal-binding mechanisms are

thought to be involved, including ion exchange, surface adsorption, chemisorption,

complexation, and adsorption–complexation [15-18]

In the present study, Cd(II) sorption using Desmostachya bipinnata (Kush Grass)

Leaves (DBL) and Bambusa arundinacea (Bamboo) Leaves (BAL), members of true

grass family: Poaceae, had been studied and as both adsorbents correspond to same

family, their major constituents must be same These materials are the major organic

components of the solid waste, comprising about 14.6% of total municipal solid waste

(MSW) and about 50% of the organic fraction of the MSW [19] However, in the entire

world, India has the huge rate of biomass production, including organic wastes, such as

grass, leaves and flowers Therefore, it is essential to search for a better use of these

abundant agricultural wastes such as, remediation of heavy metal from contaminated

aqueous solutions Both materials found abundantly throughout the year, and these

kinds of materials exhibit strong potential due to their high content of lignin and cellulose

[20] that abide numerous polar functional groups, including phenolic and carboxylic acid

groups, which may be involved in metal binding [21,22] Due to the low cost, DBL and

BAL are an attractive and inexpensive option for the adsorption of Cd(II) ion from

aqueous solution Further NaOH was used in the modification process because it can

enhance surface characteristics of DBL and BAL with increased adsorption capacity

[23] The adsorption capacity of modified DBL and BAL (MDBL & MBAL, respectively)

was investigated by batch experiments The influences of parameters such as pH,

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adsorbent dosage, contact time, initial ion concentration were investigated and the

experimental data obtained were evaluated and fitted using adsorption equilibrium and

kinetic models

2222 Materials and methods Materials and methods Materials and methods

2.1 Adsorbent & chemicals

DBL and BAL were obtained from the Indian Institute of Toxicology Research, Gheru

Campus (Lucknow, India) Both were dried under the sunlight for 2 days then, ground,

washed several times with double distilled water (DDW) and afterwards screened to

obtain 80 µm sized particles These samples were modified using 0.5 M NaOH solution

with 1:20 for 30 min (solid–liquid ratio) [24] The MDBL and MBAL were again dried at

100 °C for 24 h and stored in an airtight container The physical characteristics of the

MDBL & MBAL are presented in Table 1

The stock solution of Cd(II) (1000 mg L−1) was prepared in DDW using Cd

(NO3)2.4H2O salt (Merck); all working solutions were prepared by diluting the stock

solution with DDW

2.2 Biosorption experiments

Batch experiments were executed for adsorption studies A Pre-weighted sample of the

adsorbents (MDBL & MBAL) with a measured volume of Cd(II) solution were taken in

100 mL Erlenmeyer flask and stirred in an incubator shaker (250 rpm) at a steady

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temperature (25 ± 2 °C), for 240 min to ensure equilibrium After shaking the flasks for

regular intervals, samples were withdrawn, filtered and the filtrates were analyzed by

Atomic absorption Spectrophotometer (PerkinElmer AAnalyst 300, USA) for the

concentration of Cd(II) A first series of sorption experiments was carried out with an

initial concentration of 20 mg L−1 In these experiments the most favorable pH of

biosorption was determined Subsequently, the influence of adsorbent dosage, contact

time, initial ion concentration was also evaluated Percentage metal removal was

calculated using the following formula:

(1)

where C0 is initial and Ct is the final concentration of Cd(II) The morphological

characteristics of adsorbents were evaluated by using a scanning electron microscope

(SEM) and disposition of the functional group present on the adsorbent surface were

studied before and after biosorption using Fourier Transform Infrared (FTIR)

spectrophotometer

2.3 Adsorption isotherms studies

Isotherm studies were recorded by varying the initial concentration of Cd(II) solutions

from 10–150 mg L−1 with MDBL and MBAL separately A known amount of adsorbents

was then added into solutions in different flasks followed by agitating them at 250 rpm till

equilibrium The metal ion concentrations, retained in the adsorbent phase qe (mgg−1)

which is defined as adsorption capacity, was calculated by using the following mass

balance equation for the process at equilibrium condition:

(2)

where V is the volume of solution (L) and W is the mass of adsorbate (g)

2.4 Desorption study

Desorption experiments were performed to consider the practical usefulness of the

biosorbents After the biosorption studies, 0.2 g of metal loaded sorbent were agitated in

100 mL of 0.1 M HCl and 0.1 M HNO3 same as described by Witek-Krowiak [25] After

60 min of contact time the metal concentration in the solution was determined To check

the applicability as the best eluent the sorption desorption steps were repeated five

times

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3333 Results and discussion Results and discussion Results and discussion

3.1 Characteristic of MDBL & MBAL before and after adsorption

Presence of functional groups on MDBL & MBAL powder were analyzed using FTIR, as

shown in Fig 1(A) and (B) Occurrences of diverse type of functional groups are

confirmed by the peaks, and their detailed illustration is shown in Table 2 Mainly, metal

ions were bonded by functional groups such as carboxylic groups (pectin, hemicellulose

and lignin), phenolic groups (lignin and extractives) and a little amount may also

adsorbed by hydroxyl (cellulose, lignin, extractives, and pectin) and carbonyl groups

(lignin) [26] After sorption, several functional groups which were initially present

disappear, while some other had their position altered and thus confirming the active

participation of bonded OH groups, secondary amine group, carboxyl groups, C−O

stretching of ether groups and −C−C− group [3,27] as shown in Fig 1(A) and (B) and

Table 2 Hence, the good sorption properties of both MDBL and MBAL towards Cd(II)

ions can be ascribed to the presence of these functional groups on their surfaces

Fig 2(A) and (B) shows SEM images for MDBL and MBAL before and after the

adsorption process, respectively From Fig 2(A & B), it is clearly visible that before

adsorption, both the adsorbents have rough heterogeneous porous surface and a large

number of steps and kinks on the adsorbent surface, with wrecked edges [28] The

change in the morphology of the adsorbent after adsorption indicates that there is a

good possibility for Cd(II) ions to be trapped and adsorbed onto the surface

Fig

Fig 1111FTIR spectra of MDBL (A) and MBAL (B)

before and after Cd(II) adsorption

Table

Table 2222 Functional groups and mode of vibration from the FTIR spectrum of

MDBL and MBAL before and after adsorption

Functional group

Functional group

MDBL MDBL (cm −1 ))))

Cd (II) loaded MDBL MDBL (cm −1 ))))

MBAL MBAL (cm −1 ))))

Cd (II) loaded MDBL MDBL (cm −1 ))))

Stretching vibration of bonded −OH 3418.35 3415.14 3402.93 3431.08

Strong stretching vibration of C−O from

carboxylic acid in presence of

intermolecular H bonding

1631.46 1639.24 1634.58 1644.39

Generation of this peak after adsorption

is outcome of an overlapped band of

bending vibration of N−H and stretching

vibration of C−N

− 1516.20 − −

C−C stretching of aromatic ring 1380.35 1377.07 1376.61 1514.93

Bending vibration oh OH and stretching

vibration of C−O−C in lignin structure

1059.02 1103.39

C−O stretching of carboxylic acid 1246.16 1252.85 − −

Bending vibration of OH and stretching

vibration of C−O−C in lignin structure

1049.61 1073.96 − −

605.21 664.60 563.42 602.96 Fingerprint region: adsorption cannot be

clearly assigned to any particular

vibration because they correspond to

complex interacting vibration systems

466.37 469.10

Fig

Fig 2222Scanning electron micrograph showing morphology of MDBL (A) and MBAL

(B) before and after Cd(II) adsorption

3.2 Effect of pH

The pH plays a very significant role in the sorption of heavy metals by affecting the

surface charge of adsorbent, the degree of ionization, and speciation of adsorbate

Thus, the effects of initial pH of the solution on the Cd(II) removal efficiency were studied

at different pH ranging from 3.5 to 8.5 A sharp increase in the Cd(II) removal was

observed from 64.4% to 77.6% and 60.88%–75.2% at pH 6.5 (Fig 3) for MDBL and

MBAL, respectively and after that with a slight decrease, the value became constant

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because of saturation of active sites on the adsorbents surface So the pH 6.5 was

selected as the best pH to study the overall adsorption process Precipitation of Cd(II)

ions was observed at pH 8 [29] At low pH, the little removal efficiency is due to

occurrence of higher concentration of protons in the solution which compete with the

Cd(II) ions for the adsorption sites of the adsorbents As the pH increases, the H+

concentration decreases, leading to enhanced Cd(II) uptake The effect of pH can be

explained in terms of pHzpc of the adsorbent The pH at, which the charge of the whole

surface is zero is referred as the zero point of charge (pHzpc) and above which the

surface become negatively charged The obtained pHzpc of MDBL and MBAL is 5 and 5.5

respectively by using the batch equilibration technique [30] Positively charged Cd(II)

species are soft acids and as a rule the interaction of Cd2+ and Cd (OH)+ with the

negatively charged adsorbent surface containing carboxyl and hydroxyl groups are

responsible for the sorption of Cd(II) ions and also supported by FTIR studies At low pH,

particularly below pHzpc the Cd2+ and Cd (OH) + species present in the solution may

exchange with H+ from peripheral Apparently, at very low pH (≤ 3), the presence of

higher concentrations of H+ ions in the mixture, owes electrostatic repulsion between

both positively charged adsorbent surface and metal ion A decreasing trend in

adsorption was also observed at very high pH also, and this may be due to the formation

of soluble hydroxy complexes [31] Dissociation of the –COOH groups (pKa = 3 8-5.0) is

the plausible reason for becoming the surface of MDBL and MBAL negatively charged at

optimum pH 6.5 and thus, favorable to the adsorption of Cd(II) at this pH [32] Cd(II) may

most likely be bound on the MDBL and MBAL surface via an ion exchange mechanism

as following equation:

(3)

Fig

Fig 3333Effect of pH on Cd(II) adsorption by MDBL and MBAL at 25 °C

(condition: 20 mg L−1 of Cd(II) solution, 250 rpm, 1 g L−1 adsorbent dosage,

60 min)

(where -R represents the matrix of the adsorbents)

3.3 Effect of biosorbent dosage

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One of another important parameter that strongly affects the sorption capacity is the

biosorbent dosage As shown in Fig 4 with the increasing adsorbent dosage from 2 to

12 g L−1, it can easily be inferred that the percent removal of metal ions boosts from

63.95% to 85.50% and 65.50%–82.05% for MDBL and MBAL, respectively, whereas the

amount adsorbed per unit mass decreases It is apparent that the percent removal of

heavy metals increases rapidly with an increase in the dosage of the adsorbents due to

the greater availability of the exchangeable sites or surface area [3], whereas the

decrease in Cd(II) uptake with increasing adsorbent dosage is mainly due to

unsaturation of adsorption sites through the adsorption reaction and the similar results

were obtained in a study performed by Chen et al [33]

Fig

Fig 4444Effect of adsorbent dosage for Cd(II) adsorption by MDBL and MBAL at

25 °C (condition: 20 mg L−1 of Cd(II) solution, 250 rpm, 60 min)

3.4 Effect of initial ion concentration

The Cd(II) ion uptake is particularly reliant on the initial Cd(II) concentration At the lower

range, Cd(II) is adsorbed by specific active sites, while at higher sides; decreased

adsorption is due to the saturation of adsorption sites and also because of lack of

sufficient surface area to accumulate further available ions This is due to the

competition for the available active sites on the surface The influence of the initial Cd(II)

concentration on its removal with MDBL and MBAL shown in Fig 5, where a decrease in

removal percent from 74.2–66.60% and 77.90-62.61% (for Co = 10 − 150 mg L−1) could

be observed respectively

Fig

Fig 5555Effect of initial Cd(II) ion concentration for adsorption process by MDBL and

MBAL at 25 °C (condition: 1 g L−1 adsorbent dosage, 250 rpm, 60 min)

3.5 Effect of contact time

The effect of contact time on adsorption was studied up to 240 min It appeared from

Fig 6 that the metal uptake is very rapid up to 90 and 180 min of equilibrium for MBAL

and MDBL respectively, after that Cd(II) uptake does not significantly change with time

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Therefore, 240 min of contact time is chosen to achieve the equilibrium [34] For 20 mg

L−1 of Cd(II) concentration with an increase in contact time from 10–180 min for MDBL

and 10–150 min for MBAL, the percentage Cd(II) sorption increased from 36.80–89.55%

and 29.15–90.05%, respectively and after which a plateau is obtained, which showed

saturation of the active points (Fig 6) The subsequent increase in contact time had no

effect on Cd(II) adsorption

Fig

Fig 6666Effect of contact time on Cd(II) adsorption by MDBL and MBAL, at 25 °C,

(condition: 20 mg L−1 of Cd(II) solution, 250 rpm, 1 g L−1 adsorbent dosage)

3.6 Adsorption isotherms

The equilibrium isotherm study is very essential for designing adsorption systems and

also facilitate the comprehensive of the interaction involved between adsorbate and

adsorbent The adsorption data were analyzed Freundlich (1906) and Langmuir (1918)

isotherm models The linearized forms of the Freundlich isotherm can be expressed as

in Eq 4[35] and of Langmuir isotherm as in Eq 5[35] below:

(4) (5)

where Ce (mg L−1) is the equilibrium Cd(II) concentration in solution, qe and qmax

(mgg−1) are the equilibrium and maximum adsorption capacities (mgg−1), 1/n is the

heterogeneity factor, kF (Lg−1) and kL(Lg−1) are Freundlich and Langmuir constant

respectively The values of Freundlich and Langmuir parameters were obtained based

on the linear correlation between the values of (i) log qe vs log Ce and (ii) (Ce/qe) vs Ce,

respectively

Fig 7(a) and (b) shows the adsorption isotherms of Cd(II) using MDBL and MBAL,

respectively Freundlich isotherm is the most basic relationship describing non-ideal and

reversible adsorption, valid for heterogeneous surface having non uniform energy

distribution and is not restricted to monolayer formation [36] In the present study

(Table 3) at increasing temperature, higher kF values and n > 1 indicated, favorable

process [37] and are classified as L type isotherm reflecting a high affinity between

qqqq max (mg g −1 ))))

kkkk LLLL (min −1 )))) R2222

298 1.251 ± 0.057 1.505 ± 0.002 0.9770.977 15.222 ± 0.162 0.077 ± 0.006 0.997 0.997

308 1.460 ± 0.067 1.522 ± 0.045 0.9790.979 19.843 ± 0.570 0.090 ± 0.006 0.992 0.992MDBL

318 1.552 ± 0.028 1.501 ± 0.006 0.9550.955 15.540 ± 0.085 0.104 ± 0.003 0.995 0.995

298 1.424 ± 0.025 1.472 ± 0.016 0.9770.977 19.705 ± 0.097 0.064 ± 0.001 0.987 0.987

308 1.961 ± 0.020 1.596 ± 0.011 0.9490.949 18.483 ± 0.151 0.107 ± 0.001 0.986 0.986MBAL

318 2.024 ± 0.061 1.602 ± 0.018 0.9330.933 17.331 ± 0.425 0.135 ± 0.008 0.993 0.993The Langmuir model is based on the hypothesis that all binding sites are, likewise,

active on the energetically homogeneous surface, and monolayer surface formation

takes place without any interaction between the adsorbed molecules [12] As it can be

seen from Table 3, the coefficients of determination (R2 > 0.980) at varying temperatures

proved that the linear fit of the Langmuir model agrees well for both the adsorbents

Initially, on increasing temperature from 298 to 308K adsorption capacity increases

for MDBL and decreasing values were noticed from 308–318 K On the other side

continuous decrease in adsorption capacity is observed for MBAL up to 318 K, and it

may be due to either the damage of adsorbent’s active binding sites [39] or increasing

propensity of desorption of Cd(II) ions from the interface to the solution [40] This is

because at the higher temperature the thickness of the boundary layer decreases and

thus tendency of the metal ion to escape from the biomass surface to the solution phase

increases [41] This trend in the result indicates the exothermic nature of Cd(II)

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adsorption The admirable qmax data as shown in the Table 3, it is obvious that MDBL

and MBAL exhibited a favorable combination of a high number of accessible adsorption

sites, and evidenced excellent promising adsorbents for Cd(II) ions from aqueous

solution

Freundlich and Langmuir equations have their own limitations in describing the

equilibrium data adequately Both are based on utterly unlike principles, and the fact that

the experimental fallouts fit one or another equation, imitates a purely mathematical apt

[13] So here it may be concluded from Table 3, that the Cd(II) adsorption isotherm

exhibit Langmuir behavior with comparative high coefficients of determination

(R2 > 0.980) than Freundlich isotherm

3.7 Thermodynamic study

Langmuir isotherm constant kL for varying temperatures, i.e 298, 308 and 318 K has

been used to evaluate the thermodynamic parameters, the change in Gibbs free energy

(ΔGo), enthalpy (ΔHo) and entropy (ΔSo) for the adsorption process [38], and the

following equations are used for the calculations for these parameter:

(6) (7)

where R is the universal gas constant (1.987 cal mol−1) and T is the temperature (K)

The entropy (ΔSo) and enthalpy (ΔHo) change were obtained from the slope and

intercept of the plot of log kL against 1 T−1 (Fig 8), respectively All the thermodynamic

parameters of the adsorption process are shown in Table 4 As expected, the negative

ΔG° value indicates feasibility and spontaneity of the adsorption process The change of

the standard free energy decreases with increasing temperature regardless the nature of

adsorbent, indicate that a better adsorption is actually obtained at the higher

temperature [42] Moreover, the standard free energy change for multilayer adsorption

was more than −20 kJ mol−1 and less than zero It should be noted that the magnitude of

ΔG◦ values is in the range of multilayer adsorption [43] Each metal ion has to displace

more than one ion of the solvent The net result corresponds to the endothermic process

[44] Moreover, the positive ΔS° value corresponds to the increased randomness at the

solid/liquid interface during the adsorption process, which suggests that Cd(II) ions

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replace some water molecules from the solution previously adsorbed on the surface of

adsorbent Positive value of ΔH◦ shows the endothermic nature of reaction [45] The

values of adsorption (ΔH°) obtained in this study (< 20 kJ mol−1) are consistent with

hydrogen bond and dipole bond forces for both adsorbents [46]

Fig

Fig 8888Plot of ln KL versus 1/T for Cd(II)

sorption onto MDBL and MBAL

Table

Table 4444 Thermodynamic parameters at different temperatures for Cd(II)

adsorption using MDBL and MBAL

Thermodynamic parameters Thermodynamic parameters

Temp (K) Temp (K) ΔΔΔΔGoooo

(kJ (kJ mol mol −1 )))) ΔΔ Δ ΔHoooo (kJ mol −1 ))))

Δ ΔΔ

2.051 ± 0.085

3.8 Kinetic study

Adsorption kinetic studies are important as they provide valuable information about the

mechanism of the adsorption process [47] such as mass transfer and chemical reaction

In the present study, applicability of the first-order [Eq 8] [48] and

pseudo-Accepted Manuscript

second-order kinetics [Eq 9] [37] based on solid capacity and solid phase sorption has

been evaluated, respectively

(8)

(9)

qe and qt are adsorption capacities (mgg−1) at equilibrium and at time t respectively

k1ad is the first order rate constant (Lmin.−1) and k2ad is the rate equilibrium constant (g mg−1

min−1) for pseudo second order kinetics

(10)

where h is the initial sorption rate (mg g−1 min−1) [49]

In the present study, at different initial ion Cd(II) concentration all the kinetic data

were considered up to 240 min for pseudo-second-order rate model, but for

pseudo-first-order rate model, only initial kinetic data up to 150 and 120 min for MDBL and MBAL

respectively have been used, because using a whole range of contact time, the

calculated values of qe (mgg−1) from pseudo-first-order rate model is physically

unacceptable The pseudo-first-order rate model does not fit well to the whole range of

contact time in several other cases also and is generally applicable to the initial phase of

the adsorption processes [49,50]

The equilibrium adsorption capacity and the second-order rate constant were

calculated from the slope and the intercept of the plot t/qt against t (Fig 9a & b) A

competitively high adsorption capacity (qe) was obtained for MBAL than MDBL The data

illustrated good compliance with the pseudo-second order rate law based on sorption

capacity because the coefficients of determination, R2 from Table 5 were higher than

0.990 for both MDBL and MBAL It can also be perceived by values of Table 5 that, for

both the adsorbents, with an increase in initial metal concentration, the adsorption

capacity increases while the rate constant of adsorption (k2ad) decreases A similar

observation was also reported by some earlier researchers [49] The observed decrease

in the rate constants with an increase in initial metal ion concentration may be because,

at higher concentration, the average distance between the adsorbed ion is contracted to

a point where each affect the charge distribution of its adjacent ions The second-order

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rate constant decreases with an increase in initial Cd(II) concentration, while the initial

sorption rate, h, generally increases with an increasing initial Cd(II) concentration at all

temperatures This interaction can alter the ability of the ions to adsorb onto adsorbent,

hence higher concentration of metal ions may limit the ability of the biomass to adsorb

metal ions and consequently sorption process may require several cycles in order to

meet regulatory standards [31]

Fig

Fig 9999Kinetic parameters for the

adsorption of Cd(II) on MDBL and

MBAL at various initial Cd(II) ion

kkkk 1ad (min −1 )))) R2222

qqqq e, cal (mg g −1 ))))

kkkk 2ad (g mg 1111 min −1 )))) hhhh

The kinetic data were analyzed by intraparticle diffusion model [Eq 11] to elucidate the

key steps involved during the adsorption process

(11)