MECHANISM OF ADSORPTION OF FERROUS IONS FROM WASTE …

www.wjpps.com Vol 4, Issue 07, 2015.

767

Satish et al. World Journal of Pharmacy and Pharmaceutical Sciences

MECHANISM OF ADSORPTION OF FERROUS IONS FROM WASTE

WATER ON NATURAL ADSORBENTS

*1Satish Patil,

2Jayesh Patil,

3Sameer Renukdas and

4Naseema Patel

*1,2

Department of Chemistry, K. E. S. A. P. Science College, Nagothane - (MS), India.

3Department of Chemistry, Vaidyanath College, Parli Vaijnath – (MS), India.

4Department of Chemistry, Yeshwant Mahavidyalaya, Nanded - (MS), India.

ABSTRACT

Adsorption studies of Fe (II) ions on six different natural adsorbents

were carried out by batch experiments. Different parameters such as

initial dye concentration, adsorbent dose, pH, agitation time, agitation

speed, particle size of adsorbent and temperature were studied.

Freundlich and Langmuir isotherm models were used to test the

equilibrium data. Both the isotherm models were found to be best

fitted. The monolayer (maximum) adsorption capacities (qm) were

found between 5.181 to 10.526 mg/g for adsorbents under study. The

correlation coefficient R2 for second order adsorption model has very

high values for all adsorbents (R2 = 0.999) and qe(the) values are

consistent with qe(exp) showed that pseudo second order adsorption

equation of Lagergren fit well with whole range of contact time. Intra particle diffusion plot

showed boundary layer effect and larger intercepts indicates greater contribution of surface

sorption in rate determining step. Adsorption was found to increase on increasing pH,

increasing temperature, increasing agitation speed and decreasing particle size.

Thermodynamic analysis showed negative values of ∆G indicating adsorption was favourable

and spontaneous, small positive values of ∆H below 40 KJ/mole indicating endothermic

physical adsorption and positive values of ∆S indicating increased disorder and randomness

at the solid- solution interface of ferrous ions with biosorbents. Desorption studies showed

that adsorption of ferrous ions on biosorbents involved an ion exchange mechanism.

Adsorption capacity of Cinnamon plant leaf powder towards ferrous ions was found to be

more than other adsorbents under study.

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Article Received on

22 April 2015,

Revised on 11 May 2015,

Accepted on 05 June 2015

*Correspondence for

Author

Satish Patil

Department of Chemistry,

K. E. S. A. P. Science

College, Nagothane -

(MS), India.


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KEY WORDS: Adsorption, mechanism, ferrous ions, isotherms, kinetic, thermodynamic.

INTRODUCTION

Many industries such as coating, car, aeronautics and steel generate large quantities of

wastewater containing various concentrations of iron.[1-2]

Water Percolating through soil and

rock can dissolve minerals containing iron and hold it in solution. Iron pipes also may be a

source of iron in water. Ferrous iron (Fe2+

) is a highly soluble species that exists in the

aqueous phase and is easily absorbed into biological tissues. Therefore, it is considered to be

the most acutely toxic form of iorn. Fe2+

creates oxidative stress by inducing the formation of

oxygen based radicals that cause membrane and DNA damage. Common techniques used to

remove heavy metal ions from industrial wastewater have been reported in literature which

includes ion-exchange, reverse-osmosis, electro-coagulation, chemical precipitation,

neutralization, and adsorption.[3]

Adsorption on low cost-adsorbent for removal of toxic

metals from wastewater has been investigated extensively. These materials include

thioglycolic acid modified oil-palm[4]

, wild cocoyam biomass[5]

, brewery Biomass[6]

, sodium

hydroxide modified Lalang (Imperata cylindrica) leaf powder[7]

,apple residue[8]

, Modified

soybean hull[9]

, kaolinite[10]

, sugar beet pulp[11]

, wollastonite[12]

, baggage fly ash.[13]

In the present study, we attempt to use six plant residues as biosorbents for the removal of

ferrous ions from wastewater.

MATERIALS AND METHODS

Adsorbents: Adsorbents used in the present study are:

1) Mangrove plant (Sonneratia apetala) leaf powder ( MPLP)

2) Jackfruit plant (Artocarpus heterophyllus) leaf powder (JPLP)

3) Teak tree (Tectona grandis) bark powder (TTBP)

4) Almond tree (Terminialia cattapa) bark powder (ATBP)

5) Chiku (Manikara zopata ) leaf powder (CLP)

6) Cinnamon Plant (Cinnamonum zeylanicum) leaf powder (CPLP)

FTIR spectra of adsorbent JPLP was taken for the identification of functional groups on the

surface of adsorbents and to identify the possible adsorption mechanism.

Adsorbate (Ferrous ions): Ferrous ammonium sulphate with molecular weight 392.12

supplied by S.D. Fine Chemicals, Mumbai, is used for generation of Fe (II) ions in aqueous

solution.


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Synthetic Preparation of Fe (II) Solution: An aqueous stock solution (1000mg/l) of Fe (II)

ions is prepared by dissolving 7.022 g of ferrous ammonium sulphate is in 500 ml of double

distilled water and 50 ml of 1:1 sulphuric acid is added. The solution is diluted and made up

to one litre.

Fresh dilutions are used for each study.

Spectrophotometric determination of Fe (II) by 1, 10- phenanthroline method

Fe (II) ions reacts with 1, 10- phenanthroline to form an orange red complex (i.e. ferroin

indicator) [(C12H8N2)3Fe]2+

,Triorthophenanthroline complex. The colour intensity is

dependent of the acidity in the pH range 2 to 9 and it is stable for longer time. Concentration

of Ferrous ions can be determined spectrophotometrically. The reddish orange complex of Fe

(II) absorbs at 515nm at pH = 4. The solution slightly acidic is treated with 1, 10-

phenanthroline buffered to pH = 4 with potassium hydrogen phthalate. If any Fe (II) oxidized

to Fe (III) by dissolved oxygen again reduced to Fe (II) by treating it with hydroxylamine

hydrochloride.

Concentration of Fe (II) in the supernatant after batch mode adsorption studies is determined

by treating 10 ml of supernatant with 0.05M potassium hydrogen phthalate, 0.25% 1, 10 –

Phenanthroline and hydroxylamine hydrochloride in the same way as followed during

construction of calibration curve in 100 ml volumetric flasks. The concentration of Fe (II)

ions in supernatant is determined from calibration curve.

Calculation of concentration of Supernatant from calibration curve

Concentration of 10 ml of treated supernatant solution = ‘X’ mg/l of Fe (II) ions.

Therefore, Concentration of supernatant = ‘10X’ mg/l of Fe (II) ions.

Batch Mode Adsorption Studies

Effect of Contact Time: 25 mg of adsorbent of ≥ 120 mesh size with 25 ml of Fe (II) ion

solution is kept constant for batch experiments with an initial Fe (II) ion concentration of 10

mg/l are performed at nearly 303K on a oscillator at 230 rpm for 5, 10, 15, 20, 30, 40, 50

and 60 minutes at pH = 2.5. Then optimum contact time is identified for further batch

experimental study.

Effect of Adsorbent Dosage: Initial Fe (II) ion concentration of 25 mg/l are used in

conjunction with adsorbent dose of 1, 2, 3, 4, 5, and 6 g/l. Contact time, agitation speed,


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temperature and particle size of 40 minutes, 230 rpm, 303K and ≥ 120 mesh respectively are

kept constant. pH of solution maintained at 2.5.

Effect of Initial Metal ion Concentration: Initial Fe (II) ion concentration of 4, 6, 8, 10, 12

and 14 mg/l are used in conjunction with adsorbent dose of 1 g/l . Contact time, agitation

speed, temperature and particle size of 40 minutes, 230 rpm, 303K and ≥ 120 mesh

respectively are kept constant. pH of solution maintained at 2.5.

Effect of pH: Initial pH of Fe (II) ion solution is adjusted to 1, 1.5, 2, 2.5 and 3 for 10 mg/l

concentration. Contact time, adsorbent dose, agitation speed, temperature and particle size of

30 minutes, 1 g/l, 230 rpm, 303K and ≥ 120 mesh respectively are kept constant.

Effect of Particle Size: Three different sized particles of ≥ 120, 120 ≤ 85 and 85 ≤ 60 meshes

are used in conjunction with 6 mg/l Fe (II) ion concentration. Contact time, adsorbent dose,

agitation speed and temperature of 40 minutes, 1 g/l, 230 rpm and 303K respectively are kept

constant. pH of solution maintained at 2.5.

Effect of Temperature: 303K, 313K and 323K temperatures are used in conjunction with 8

mg/l Fe (II) ion concentration. Contact time, adsorbent dose, agitation speed and particle size

of 40 minutes, 1 g/l, 230 rpm and ≥ 120 mesh respectively are kept constant. pH of solution

maintained at 2.5.

Effect of Agitation Speed: 100, 170 and 230 rpm agitation speeds are used in conjunction

with initial Fe (II) ion concentration of 10 mg/l. Adsorbent dose, temperature, contact time

and particle size of 1 g/l, 303K, 40 minutes and ≥ 120 mesh respectively are kept constant.

pH of solution maintained at 2.5.

Desorption Studies

After adsorption, the Fe (II) loaded adsorbent are separated from the solution by

centrifugation and the supernatant is drained out. The adsorbent is gently washed with water

to remove any unadsorbed Fe (II) ions. Regeneration of Fe (II) from the Fe (II) laden

adsorbent is carried out using the desorbing media – distilled water at pH 1, 2 and 3 using

dilute solutions of HCl. Then they are agitated for the equilibrium time of respective Fe (II).

The desorbed Fe (II) in the solution is separated and analyzed for the residual Fe (II) ions

concentration.


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For the desorption studies, metal ion loaded adsorbents from 10 mg/l initial Fe (II) ion

concentrations are used.

RESULTS AND DISCUSSION

Infrared spectroscopic studies

The FTIR spectrum of JPLP (Fig. 1) showed bands for bonded – OH group, C=O group of

unionized carboxylate stretching of carboxylic acid (-COOH) or pectin ester (-COO-),

secondary amino (-NH-) group, – SO3, C-O stretching in ether group. The adsorbent

containing these functional groups are likely to participate in metal binding.

Fig. 1: FTIR Spectra of JPLP

Mechanism

Electrostatic attraction between oxyanion (―O–) or carboxylate ions (―COO

–) from

adsorbent surface and cationic metal ion results into either chemisorption (strongest among

all types of forces) or electrostatic physisorption between two oppositely charged ions

(strongest among the physical bonds).

2 S―O– + Fe

+2 → (S―O―)2 Fe

2 S―COO– + Fe

+2 → (S―COO―)2 Fe

Electrostatic physisorption between ions and dipole (second strongest among the physical

bonds). Here, Electrostatic attraction between negative poles of partial polar bonds of OH,

NH and CO groups of adsorbent and cationic metal ion takes place.


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Adsorption of metal ion also takes place due to exchange of ions between –OH or –COOH

groups from the surface of adsorbents and cationic metal ions. Thus, protons (H+ ions) are

released into the solution.

2S―OH + Fe+2

→ (S―O―)2 Fe + 2H+

(Ion exchange)

2S―COOH + Fe+ 2

→ (S―COO―)2 Fe + 2H+ (Ion exchange)

Nature of metal ions is also found to be responsible for adsorption process.

Batch Mode Adsorption Studies

Parameters which influence the extent of adsorption such as adsorbate concentration,

agitation time, temperature, adsorbent dosage and agitation speed are investigated. In addition

to the above parameters, effect of pH on adsorption of metal ions is investigated. The use of

the adsorbent for continuous use is also determined by regeneration studies.

Effect of Contact Time: Effect of contact time on adsorption of Fe (II) from 10 mg/l initial

Fe (II) ion concentration solutions is presented in Fig. 2. Uptake of Fe (II) is rapid in first 5

minutes and after 20-40 minutes amount adsorbed is almost constant. Therefore, further batch

experiments are carried out at 40 minutes optimum contact time.

About 15 to 68 % for Fe (II) of total metal ion uptake is observed in first 5 minutes duration

and at equilibrium it reached up to 32 to 86 % depending upon the adsorption ability of

different adsorbents.

The rapid uptake at initial stage of adsorption is because of more number of active sites on

the surface of adsorbent. After that, slower adsorption was due to intra particle diffusion. The

uptakes of ferrous ions (qt) against time curves are single, smooth and continuous leading to

saturation, suggesting the possible monolayer coverage of the Fe (II) on the surface of the

adsorbent.


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The adsorption capacity of adsorbents is found to be of the order of CPLP > CLP > ATBP >

TTBP > MPLP > JPLP.

Fig. 2: Effect of contact time on adsorption of Fe (II).

To investigate the mechanism of adsorption, pseudo - first order, pseudo- second order,

intraparticle diffusion and Elovich kinetic models are used.

The Lagergren pseudo- first order rate expression is given as

log (qe- qt) = log qe – (k1 / 2.303) t (1)

Where, qe and qt are amounts of ferrous ions adsorbed (mg/g) on adsorbent at equilibrium and

at time t, respectively and k1 is rate constant of pseudo first order adsorption (min-1

). The

slope and intercept values of plot log (qe- qt) against t, used to determine pseudo first order

rate constant (k1) and theoretical amount of ferrous ions adsorbed per unit mass of adsorbent

qe(the), respectively.

Results of Lagergren pseudo first order kinetic model are shown in Fig. 3 and Table 1.

Correlation coefficients (R2) values are appeared linear at initial stage of adsorption but qe(exp)

values differ from corresponding qe(the) calculated from the linear plot of log (qe-qt) against t

showed that adsorption of Fe (II) ions on these adsorbents is not a first order kinetics.

The Lagergren pseudo- second order kinetic model is given as

t/qt = 1/(k2qe2) + t/qe (2)

Where k2 is rate constant of second order adsorption (g/mg/min). Slope and intercept of plot

of t/qt against t, gives values of qe(the) and k2 respectively.


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Results of Lagergren pseudo second order kinetic model are shown in Fig. 4 and Table 1.

The correlation coefficients (R2) for second order adsorption model lie between 0.995 to

0.999 and qe(the) values are consistent with qe(the) showed that pseudo second order adsorption

equation of Lagergren fit well with whole range of contact time and metal ion adsorption

process appears to be controlled by chemisorption. This shows that the adsorptions of the Fe

(II) ions onto adsorbents under study follow second order kinetics.

Fig. 3: Pseudo first order plot of effect of

contact time on adsorption of Fe (II).

Fig. 4: Pseudo second order plot of effect

of contact time on adsorption of Fe (II).

Table 1: Effect of contact time on adsorption of Fe (II) ions

Adsorbent

Initial Fe (II)

ion Conc.

(mg/l)

Pseudo -first order model Pseudo -second order model

qe(exp)

(mg/g)

K1

(min-1

)

qe(the)

(mg/g) R

2

qe(exp)

(mg/g)

K2

(g/mg/min)

qe(the)

(mg/g)

H (mg/g

.min) R

2

MPLP 10 4.7 0.06679 2.01372 0.944 4.7 0.06216 4.92611 1.5083 0.999

JPLP 10 3.5 0.0737 2.71019 0.99 3.5 0.03236 4.01606 0.52192 0.999

TTBP 10 6.6 0.0783 2.86418 0.976 6.6 0.04997 6.94444 2.40964 0.999

ATBP 10 7.3 0.07139 3.34965 0.987 7.3 0.03782 7.75194 2.27273 0.999

CLP 10 7.35 0.11976 6.26614 0.994 7.35 0.03194 7.93651 2.01207 0.999

CPLP 10 8.1 0.08751 2.35505 0.955 8.1 0.07539 8.33333 5.2356 0.999

According to Weber and Morris, the intra particle diffusion rate constant (Ki) is given by the

following equation

qt = Ki t 1/2

+ A (3)

Ki (mg/g/ min1/2

) intraparticle diffusion constant value can be determined from the slope of

the plot qt against t ½.

The intra particle diffusion rate constant Ki (mg g-1

min-1/2

) values are determined from the

slope of the plot qt against t 1/2

and are shown in Fig. 5 and Table 2. The lines of the plot do


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not pass through origin due to boundary layer effect .Higher values of Ki and intercepts

illustrate an enhancement in the rate of adsorption. The larger intercepts showed the greater

contribution of surface sorption in rate determining step. Initial portion is attributed to the

liquid film mass transfer and linear portion to the intra particle diffusion.

The linearized form of Elovich kinetic equation is presented as

qt = 1/ β[ln (αβ)] + ln t /β (4)

Where α and β are the constants calculated, from the intercept and slope of plot qt against ln t.

Elovich kinetic model constants α and β are calculated, from the intercept and slope of plot qt

against ln t and are shown in Fig. 6 and Table 2. Elovich kinetic model gave quiet linear

relationship with respect to R2. The simple Elovich model used to describe second-order

kinetic, assuming that the actual solid surface is energetically heterogeneous. Constant α

depended upon initial rate of adsorption which is found to be high for an adsorbent which has

high adsorption capacity but constant β which is desorption constant has the low value for

the same adsorbent and vice versa. This indicates Elovich kinetic model fit reasonably well

for adsorbents under study.

Fig. 5: Intra particle diffusion plot of effect

of contact time on adsorption of Fe (II).

Fig. 6: Elovich plot of effect of contact time

on adsorption of Fe (II).


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Table 2: Effect of contact time on adsorption of Fe (II) ions

Adsorbent

Initial

Fe (II)

ion

Conc.

(mg/l)

Intra particle diffusion model Elovich Model

Ki

(mg/g/min1/2

)

A

(mg/g) R

2

α

(mg/g/min)

β

(g.mg-1

) R

2

MPLP 10 0.429 2.238 0.893 1.2579 1.243 0.963

JPLP 10 0.519 0.514 0.945 2.68 * 1024

1.044 0.989

TTBP 10 0.567 3.407 0.982 1.5167 0.952 0.981

ATBP 10 0.652 3.542 0.946 1.8173 0.831 0.99

CLP 10 0.928 2.412 0.948 5.2581 0.584 0.991

CPLP 10 0.478 5.491 0.902 1.0614 1.124 0.96

Effect of Adsorbent Dosage: The adsorption of Fe (II) ions is studied by varying the

adsorbent dosage. It is found that % removal of metal ion increased Fig. 7 but amount of

metal ion adsorbed per unit mass of adsorbent decreased Fig. 8 with increase in adsorbent

dose from 1 to 6 g/l. As amount of adsorbent increases, number of active sites available for

adsorption also increases thus % removal also increases but as all active sides may not be

available during adsorption due to overlapping between the active sites themselves and thus

amount adsorbed mg/g of adsorbent decreases. Thus, the adsorption of metal ion increased

with the sorbent dosage and reached an equilibrium value after certain sorbent dosage.

Fig. 7: Effect of adsorbent dosage on %

Removal of Fe (II).

Fig. 8: Effect of adsorbent dosage on

amount of Fe (II) adsorbed in mg/g of

adsorbent.

Adsorbent doses of 4 to 5 g/l are required to attend equilibrium. It is observed that the

adsorbent which has adsorption capacity required lowest adsorbent dose to attained the

equilibrium and vice versa. But amount of Fe (II) adsorbed after equilibrium per unit mass of


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adsorbent is different. For above 90% removal of metal ions about 4 g/l of CPLP, 4g/l of CLP

and 6 g/l of ATBP doses is required. But for other adsorbents even 6 g/l dose also is not

enough to remove 90% of metal ions. Thus CPLP is found to be the excellent adsorbent for

removal of Fe (II) from the aqueous solutions.

Fig. 7: Effect of adsorbent dosage on %

Removal of Fe (II).

Fig. 8: Effect of adsorbent dosage on

amount of Fe (II) adsorbed in mg/g of

adsorbent

Effect of Initial Metal Ion Concentration: Amount of metal ions adsorbed per unit mass of

adsorbent increased (Fig. 9) but % removal decreased (Fig. 10) with increase in initial metal

ion concentration from 4 to 14 mg/ l.

With increase in metal ions concentration from 4 to 14 mg/l, the percentage sorption

decreased from

1) 75 to 38.21 % Fe (II) for MPLP

2) 67.5 to 32.14 % Fe (II) for JPLP

3) 95 to 54.29 % Fe (II) for TTBP

4) 95 to 62.5 % Fe (II) for ATBP

5) 93.75 to 61.43 % Fe (II) for CLP

6) 95 to 69.29 % Fe (II) for CPLP

but amount of metal ions adsorbed per unit mass of adsorbent increased from

1) 3 to 5.35 mg/g Fe (II) for MPLP


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2) 2.7 to 4.5 mg/g Fe (II) for JPLP

3) 3.8 to 7.6 mg/g Fe (II) for TTBP

4) 3.8 to 8.75 mg/g Fe (II) for ATBP

5) 3.75 to 8.6 mg/g Fe (II) for CLP

6) 3.8 to 9.7 mg/g Fe (II) for CPLP.

Fig. 9: Effect of initial metal ion

concentration on adsorption of Fe (II).

Fig. 10: Effect of initial metal ion

concentration on % removal of Fe (II).

At any contact time, with increase in initial Fe (II) ion concentration, the percent adsorption

is decreased and the amount of adsorbate uptake (qt) per unit weight of the adsorbent is

increased. It is seen that for the low initial concentrations, the percent uptake of the Fe (II)

ions was high. Even though the percent uptake of the adsorbate is smaller at high initial

concentrations, the actual amount of the metals adsorbed (qt) increased with increase in the

initial Fe (II) ion concentration in the solution. The uptake (qt) against time curves is single,

smooth and continuous leading to saturation, suggesting the possible monolayer coverage of

the adsorbate Fe (II) on the surface of the adsorbent.

Freundlich and Langmuir adsorption isotherms were used to study the adsorption behaviour

of ferrous ions on adsorbents.

The linear form of Freundlich isotherm equation was employed for the adsorption of ferrous

ions onto the adsorbents was represented by

logqe = log Kf + 1/n log Ce (5)

Where qe is amount of ferrous ions adsorbed at equilibrium (mg/g), Ce is the equilibrium

concentration of ferrous ions in solution (mg/l), A plot of log qe against log Ce gives a straight


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line, Kf and n are constant incorporating factors affecting the adsorption capacity and

intensity of adsorption calculated from the intercept and slope of the plot respectively.

Freundlich plot of log qe against log Ce showed good linearity (R2 = 0.974 to 0.997) except

JPLP (R2 = 0.927) indicating the adsorption of metal ions obeyed the Freundlich adsorption

isotherm, Fig. 11. The values of Kf and n are given in the Table 3. It is found that the

adsorbent which has greater adsorbent capacity which has higher value of Kf and vice versa.

Values of n lie in between 1 to 10 indicate an effective adsorption while higher values of Kf

represent an easy uptake of Fe (II) ions from the solution. Thus, Freundlich adsorption

isotherm fit well to these adsorption studies.

The linear form of Langmuir isotherm was represented by the following equation

Ce / qe = 1/ (qm b) + Ce /qm (6)

Where qm is monolayer (maximum) adsorption capacity (mg/g) and b is Langmuir constant

related to energy of adsorption (1/mg) obtained from the slope and intercept values of the plot

Ce/qe against Ce respectively.

The essential features of the Langmuir isotherm can be expressed in terms of dimensionless

constant separation factor, RL, which is defined by the following relation given by,

RL = 1/ (1+bCo) (7)

Where, Co is initial ferrous ions concentration (mg/l).

If, RL> 1, Unfavorable adsorption RL = 1, Linear adsorption

RL = 0, Irreversible adsorption 0 < RL< 1,Favourable adsorption.

A linear plot of Ce / qe against Ce suggest the applicability of the Langmuir isotherms Fig.

12. The values of qm and b are determined slope and intercepts of the plot, Table 3.

Monolayer (maximum) adsorption capacities (qm) obtained from Langmuir plots gives the

idea of effectiveness of adsorbents towards metal ion. These qm values vary from adsorbent to

adsorbent depending upon available active sites on the surface of adsorbent. RL values lie in

between 0 to 1 indicates favourable adsorption Table 4. Correlation coefficient values (R2=

0.969 to 0.994) showed very good linearity to almost all sorbents and sorbates systems. Thus,

Langmuir adsorption isotherm fit well to these adsorption studies.

qm values are found to be order of CPLP > CLP = ATBP > TTBP > MPLP > JPLP.


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Both adsorption isotherms fit well for adsorption of Fe (II) ions on the adsorbents under

study.

Fig. 11: Freundlich isotherm plot of effect

of initial metal ion concentration on

adsorption of Fe (II).

Fig. 12: Langmuir isotherm plot of effect

of initial metal ion concentration on

adsorption of Fe (II).

Table 3: Effect of initial metal ion concentration on adsorption of Fe (II) ions

Adsorbent Freundlich isotherm parameters Langmuir isotherm parameters

Kf n R2 qm b R

2

MPLP 2.97167 3.57143 0.993 6.17284 0.72646 0.993

JPLP 2.38781 3.84615 0.927 5.18135 0.52446 0.969

TTBP 5.15229 4.92611 0.993 8.06452 1.87879 0.991

ATBP 5.66239 3.92157 0.997 9.43396 1.70968 0.988

CLP 5.50808 3.61011 0.996 9.43396 1.63077 0.994

CPLP 6.59174 3.38983 0.974 10.5263 2.2619 0.987

Table 4: Dimensionless separation factor (RL) calculated from Langmuir constant (b)

for Fe (II) ions

Initial Fe (II)

ion Conc.

(mg/l)

MPLP JPLP TTBP ATBP CLP CPLP

4 0.25603 0.32281 0.11744 0.12757 0.13292 0.09953

6 0.18661 0.24115 0.08148 0.08883 0.09272 0.06863

8 0.14681 0.19247 0.06238 0.06813 0.07119 0.05237

10 0.121 0.16014 0.05054 0.05526 0.05778 0.04234

12 0.10291 0.13711 0.04247 0.04648 0.04862 0.03553

14 0.08952 0.11987 0.03663 0.0401 0.04196 0.03061


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Effect of pH: pH is an important factor in controlling the adsorption of Fe (II) ions onto

adsorbent. The adsorption of Fe (II) from 5 mg/l concentration on different adsorbents is

studied by varying the pH from 1 to 3. The amount of Fe (II) ions adsorbed per unit mass of

adsorbent at equilibrium (qe) increased with increase in pH (Fig. 13). Thus, adsorption batch

mode studies are carried out at pH 2.5.

Fig. 13: Effect of pH on adsorption of Fe (II) from initial concentration of 5 mg/l.

At low pH (high H+ ion concentration), the H+ ions compete with metal cation for the

exchange sites in the system thereby partially releasing the metal cations14

. pH affects both

cell surface metal binding sites and metal chemistry in water. At low pH values, the

functional groups of the biosorbent are closely associated with the hydronium ions and

repulsive forces limit the approach of the metal ions.

S-OH + H+

→ S-OH2+ (protonation) [+ Fe

+2 (Repulsion)]

S-C= O + H+

→ S-C=OH+ (protonation) [+ Fe

+2 (Repulsion)]

S-HN + H+

→ S -H2N+ (protonation) [+ Fe

+2 (Repulsion)]

With increasing pH (decreasing H+ ion concentration), more functional groups such as amino

and carbonyl groups, would be exposed leading to attraction between these negative charges

and the metals and hence increases in biosorption on to the surface of adsorbent.[15]

The lower

uptake at higher pH value is probably due to the formation of anionic hydroxide complexes.

Also the –OH, C=O, -HN- of the surface of sorbent set free due to deprotonation by base and

are then available for adsorption.

S- OH + OH- → H2O + S-O

- (Anion formation) + [Fe

+ 2(Attraction)]

S -COOH + OH- → H2O + S-COO

- (Anion formation) + [Fe

+2 (Attraction)]


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The optimum pH for biosorption of ferrous ion on to the adsorbents is observed at pH 2.5. To

avoid precipitation of ferric ion as their hydroxides, all the experiments are carried below pH

3.5. The solution pH influences both the metal binding sites as well as metal chemistry in

solution. The initial adsorption rates increased with increasing initial pH up to optimum pH

values. At higher pH values, Fe (III) precipitated because of the high concentration of OH

ions in the adsorption medium[16, 17]

and so adsorption experiments at pH > 3 cannot be

performed.

Iron typically enters water bodies in the form of ferrous iron, which can be oxidized to ferric

iron by the oxygen dissolved in water. The rate of oxidation reaction depends primarily on

the pH and on the level of dissolved oxygen in water (DO). At pH < 4 and relatively low

dissolved oxygen, the oxidation process to ferric iron is very slow. At pH>4, however Fe (II)

ions oxidize quickly to Fe (II) ions which then react with water producing ferric hydroxide

precipitate and acidity.

Fe (II) + ¼ O2 + H+ → Fe (III) + ½ H2O

Fe (III) + 3H2O → Fe (OH)3 + 3 H+

If the pH drops below 3, the ferric ions cease to precipitate and remain in water in partially

hydrolyzed forms.

At lower pH, surface of the adsorbents are positively charged, precluding the electrostatic

attraction of cationic metal ions. On the other hand, at higher pH, surface of the adsorbents

becomes negatively charged, facilitating the electrostatic attraction of cationic Fe (II) ions

and thus more adsorption takes place.

Effect of Particle Size: Adsorption o Fe (II) on three sized particles ≥ 120, 120 ≤ 85 and 85

≤ 60 mesh of adsorbent is studied for 4 mg/l Fe (II) ion concentrations. The results of

variation of these particle sizes on metal ion adsorption are shown in Fig. 14. It is observed

that as the particle size increases the adsorption of metal ion decreases and hence the

percentage removal of Fe (II) ions also decreases. This is because the smaller particles have

more surface area and access to the particle pores is facilitated when their size is small. For

larger particles, the diffusion resistance to mass transfer is higher and most of the internal

surface of the particle may not be utilized for adsorption and consequently amount of metal

ion adsorbed is small. It is also believed that the breaking up of large particles to form smaller


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ones opens some tiny sealed channels, which might then become available for adsorption,

and so the sorption by smaller particles is higher than that by larger particles. These

observations indicate that metal ion sorption occurs by a surface mechanism. Thus batch

mode studies are carried out with smaller particle size (≥ 120 mesh).

Fig. 14: Effect of particle size on % removal of Fe (II).

Effect of Agitation Speed: The sorption is influenced by mass transfer parameters. Fig. 15

illustrates the sorption kinetics of Fe (II) from 4 mg/l initial concentrations by biosorbents for

different agitation speeds. The amounts adsorbed at equilibrium (qe) in mg/g are found to

increase with increase in agitation speed from 100 to 230 rpm of an oscillator. Maximum

adsorption is observed at 230 rpm agitation speed. With increasing the agitation speed , the

rate of diffusion of metal ion molecules from bulk liquid to the liquid boundary layer

surrounding the particle become higher because of an enhancement of turbulence and a

decrease of thickness of the liquid boundary layer. Thus, uptake of Fe (II) ions increased with

increase in agitation rate. Increasing agitation rate decreases the film resistance to mass

transfer surrounding the sorbent particles thus increasing sorption of metal ion molecules.

Thus batch mode studies are carried out at 230 rpm agitation speed.

Fig. 15: Effect of agitation speed on adsorption of Fe (II).


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Effect of Temperature: Temperature has important effects on adsorption process.

Adsorption of Fe (II) at three different temperatures (303K, 313K and 323K) onto different

biosorbents is studied for 8 mg/l initial metal ion concentration. The results variations in

temperatures on metal ion adsorption are shown in Fig. 16. It is observed that as the

experimental temperature increased from 303K to 323K, the Fe (II) ions adsorption also

increased. This is probably due to the fact that at higher temperature, an increase in free

volume occurred and leading to an increment in the mobility of the solute. Alternatively, the

enlargement of the pore sizes of the adsorbent particle at elevated temperatures can also be

beneficial towards the metal ion sorption. Adsorption increased with temperature is probably

due to the increase of intra particle diffusion rate of the sorbate into the interior sites of the

sorbent since diffusion is an endothermic reaction.

Fig. 16: Effect of temperature on adsorption of Fe (II)

Thermodynamic analysis

Thermodynamic parameters such as change in free energy (∆G) (kJ/mole), enthalpy (∆H)

(kJ/mole) and entropy (∆S) (J/K/mole) were determined using following equations

Ko = Csolid /Cliquid (08)

∆G = - RT lnKo (09)

∆G = ∆H - T∆S

lnKo = ∆S/R - ∆H/RT (10)

Where Ko is equilibrium constant, Csolid is solid phase concentration at equilibrium (mg/l),

Csolid is liquid phase concentration at equilibrium (mg/l), T is absolute temperature in Kelvin

and R is gas constant. ∆G values obtained from equation (09), ∆H and ∆S values obtained


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from the slope and intercept of plot ln Ko against 1/T (Fig. 17) presented in Table 5.

Negative ∆G, low positive ∆H (< 40 KJ/mole) and positive ∆S values are obtained for these

adsorbent – adsorbate systems indicates favourable, spontaneous, endothermic physisorption.

Fig. 17: Von’t Hoff plot of effect of temperature on adsorption of Fe (II)

Table 5: Equilibrium constants and thermodynamic parameters for the adsorption of

Fe (II) ions

Adsorbent Ko ∆G (J/mole) ∆H

(J/mole)

∆S

(J/K/mole) 303K 313K 323K 303K 313K 323K

MPLP 1.1333 1.2039 1.2535 -315.3 -482.8 -606.79 4107.95 14.616

JPLP 0.7391 0.7778 0.8824 761.488 653.991 336.116 7170.83 21.043

TTBP 2.8095 3.0609 3.4444 -2602.3 -2911.2 -3321.2 8272.43 35.833

ATBP 3.7059 4.0315 4.5172 -3299.9 -3627.9 -4049.4 8038.81 37.371

CLP 4 4.1613 4.5172 -3492.3 -3710.4 -4049.4 4926.88 27.719

CPLP 7 9 11.308 -4902 -5717.8 -6513.4 19513 80.588

Desorption Studies

Fig. 20 represents desorption of Fe (II) respectively from metal ion loaded adsorbents at

initial pH 1, 2 and 3. For the desorption studies, dye loaded adsorbents from 10mg/l initial

metal ion concentration are used.


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Fig.20: Effect of pH on desorption of Fe (II).

Among the adsorbents at pH 1, least desorption of Fe (II) is observed for CLP (74.83%) and

maximum for MLP (94.17%).

At pH 2, least desorption of Fe (II) is observed for TFSP (44.52%) and maximum for ATBP

(73.77%). At pH 3, least desorption of Fe (II) is observed for PPP (35.71%) and maximum

for TTBP (61.54%). Desorption studies showed reverse mechanism. Desorption of Fe (II)

ions from the surface of adsorbents increased with decrease in pH of solution. Because the

electronegativity value of H+

ions (EN. H+ is 2.20) are able to replace Fe (II) ions [EN. Fe (II)

is 1.6]. As the concentration of H+ ions increases % desorption also increases and also about

70 to 95 % desorption values are observed at pH = 1. This showed that adsorption of ferrous

ions on biosorbents involves an ion exchange mechanism.

CONCLUSION

Bio sorption technology, utilizing natural materials to passively remove metal ions from

aqueous solutions, offers an efficient and cost effective alternative compared to traditional

chemical and physical remediation and decontamination techniques.

Sorption amount increased with but percentage removal decreased with increase in initial

metal ion concentration. Uptake of Fe (II) is rapid in first 5 minutes and after 20-40 minutes

amount of metal ion adsorbed is almost constant.

Freundlich and Langmuir isotherm models are used to test the equilibrium data. Both the

models were found to be best fitting with adsorption studies.


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Lagergren pseudo -second order model found to be best fitted the kinetics of adsorption.

Charges on surface of adsorbent and metal ion are found to be responsible for adsorption.

Intra particle diffusion plot showed boundary layer effect and larger intercepts indicates

greater contribution of surface sorption in rate determining step. Elovich second order model

also fit reasonably well. The initial rate of adsorption constant and desorption constant are

found to be in consistent with the adsorption studies.

Adsorption is found to increase with increasing pH, increasing temperature and decreasing

particle size. Especially pH plays an important role in these studies because functional groups

like –COOH, -OH, -NH-, =CO from the adsorbent surface are major active sites for

adsorption of cationic metal ions. These active sites decreased with decrease in pH and thus

adsorption of Fe (II) ions also decreased and vice versa.

Thermodynamic analysis showed negative values of ∆G indicating adsorption is favourable

and spontaneous, positive values of ∆H indicating endothermic physisorption and positive

values of ∆S indicating increased disorder and randomness at the solid- solution interface of

metal ion with the adsorbents. Desorption of Fe (II) ions from the surface of adsorbents was

found to increase with decrease in pH of solution. Percentage desorption increased with

increase in H+ ions concentration and about 70 to 95 % desorption values are observed at pH

= 1 indicating ferrous ion adsorption on biosorbents involved an ion exchange mechanism.

This showed that regeneration of adsorbent is possible at lower pH. In conclusion, physical

adsorption and ion exchange mechanism were found to be main mechanisms of adsorption of

Fe (II) on natural adsorbents under study.

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