2.6Sorption of Hexavalent Chromium From Aqueous Solution Using Marine Green Algae

Journal of Environmental Chemical Engineering 2 (2014) 12611274

Contents lists available at ScienceDirect

Journal of Environmental

w . e l

que

im

tud

niver

ting v

meth

VI) by

e annin

r, pseu

RSM

Thermodynamics

Kinetics

Green algae

models. The results showed that the sorption process of Cr(VI) ions followed pseudo second order and power

function kinetics. The sorption data of Cr(VI) ions are tted to Langmuir, Freundlich, DubininRadushkevich

Temkin, Sips and Toth isotherms. Sorption of Cr(VI) onto H. gracilis biomass followed the Langmuir isotherm

model ( R 2 = 0.997) with the maximum sorption capacity of 55.55 mg / g. The calculated thermodynamicparameters such as G , H and S showed that the sorption of Cr(VI) ions onto H. gracilis biomass wasfeasible, spontaneous and endothermic. Desorption study shows that the sorbent could be regenerated using

0.2 M HCl solution, with up to 80% recovery. c 2014 Elsevier Ltd. All rights reserved.

Introduction

Cr(VI) is one of the pollutants introduced into natural waters from

a variety of industrial wastewaters. Chromium is a highly toxic metal,

considered as a precedence pollutant because of its mutagenic and

carcinogenic properties [ 1 ]. Cr(VI) is both carcinogenic and muta-

genic [ 2 ], and it may cause damages to the kidney, lungs and ulcer-

ations to the skin [ 3 ]. According to the World Health Organization

(WHO) drinking water guidelines, the maximum allowable limit for

total chromium is 0.05 mg / L [ 4 ]. Sources of chromium pollution are electroplating, leather tanning, textile dyeing, and metal nishing

industries. In metal cleaning, plating and metal processing indus-

tries, chromium concentration can approach 20,00075,000, 15,000

52,000 and 100,000270,000 mg / L, respectively [ 5 ]. Chromium exists in several oxidation states ( 2 to + 6), the most stable and common forms are the hexavalent Cr(VI) and trivalent Cr(III). The chemistry

of Cr(VI) is greatly dependent upon the pH and concentration of

the solution and it normally exists in the anionic form, as Cr 2 O 7 2

(dichromate), HCrO 4 (hydrogen chromate) or CrO 4 2 (chromate)

forms depending on pH and concentration. At pH value below 1, the

predominant species is H 2 CrO 4 (chromic acid). In acidic media around

* Corresponding author.

E-mail address: [email protected] (R. Jayakumar).

2, Cr(VI) exists mostly in the form of dichromate (Cr 2 O 7 2 ) ions. At pH

between 2 and 6, Cr 2 O 7 2 and HCrO 4 ions exist in equilibrium and

under alkaline conditions (pH > 8) it exists predominantly as chro-mate anion [ 6 ]. Several international environmental agencies have

introduced strict policy with regard to metal expulsion, especially

from industrial activities. According to USEPA, the permissible limit

for the discharge of Cr(VI) into surface water is 0.5 mg / L, while total Crincluding Cr(III), Cr(VI) and its other forms is synchronized to below

2 mg / L [ 7 ]. Many conventional techniques, including chemical pre-cipitation, membrane separation, ion exchange, reverse osmosis and

solvent extraction have been employed for the treatment of metal

bearing industrial efuents [ 8 10 ]. However, the disadvantages of

these methods such as secondary pollution, high chemical or energy

requirements, or high cost have recently shifted a large number of

studies to develop more efcient removal processes for heavy metal

control.

Sorption is an emerging and innovative technology using different

biomass to remove pollutants from wastewater, especially those that

are not easily biodegradable such as heavy metals [ 11 14 ]. Among

the biological materials, algae have been found to be potentially more

suitable sorbents because of their cheap availability both in fresh and

saltwater, relatively high surface area and high binding afnity [ 15 ].

Research in the eld of sorption has mostly anxious itself with green

algae [ 16 18 ]. Green algae are mainly cellulose, and a high percentage

of the cell wall containing proteins bonded to polysaccharides to formj o u r n a l h o m e p a g e : w w

Sorption of hexavalent chromium from a

marine green algae Halimeda gracilis : Opt

kinetic, thermodynamic and desorption s

R. Jayakumar * , M. Rajasimman, C. Karthikeyan Environmental Engineering Laboratory, Department of Chemical Engineering, Annamalai U

a r t i c l e i n f o

Article history:

Received 13 November 2013

Accepted 12 May 2014

Keywords:

Adsorption

Chromium(VI)

a b s t r a c t

In this work, effect of opera

studied. Response surface

percentage removal of Cr(

136 rpm and contact timspectroscopy (FTIR) and sc

terms of pseudo rst orde2213-3437/ $ - see front matter c 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jece.2014.05.007 Chemical Engineering

s e v i e r . c o m / l o c a t e / j e c e

ous solution using

ization, equilibrium,

ies

sity, Annamalai Nagar 608002, Tamilnadu, India

ariables on Cr(VI) uptake capacity of marine green algae Halimeda gracilis was

odology (RSM) was applied to optimize the operating variables. A maximum

H. gracilis occurs when, pH 4.9, sorbent dosage 2.2 g / L, agitation speed47 min. The sorbent was characterized by using Fourier transform infrared

g electron microscope (SEM) analysis. Experimental data were analyzed in

do second order intra particle diffusion, power function and Elovich kinetic

1262 R. Jayakumar et al. / Journal of Environmental Chemical Engineering 2 (2014) 12611274

g

(

t

s

v

g

T

e

aNomenclature

RSM response surface methodology

FTIR Fourier transform infrared spectroscopy

SEM scanning electron microscope

USEPA United States of Environmental Protection Agency

WHO World Health Organization

CCD central composite design

AAS atomic absorption spectrophotometer

EDS energy dispersive spectrum

X i uncoded value of the i th test variable

X o uncoded value of the i th test variable at center point

G Gibbs free energy change H enthalpy change S entropy change K equilibrium constant

q e amount of adsorbed chromium per unit mass of

adsorbent (mg / g) q t amount of adsorbed chromium per unit mass of

adsorbent at time t (mg / g) C o initial concentration of chromium metal ion (mg / L) C e equilibrium chromium metal ion concentration

(mg / L) C t concentration of metal ion at time t (mg / L) V volume of solution treated (L)

M amount of biomass (g m)

q m maximum sorption capacity of the sorbent (mg / g) b Langmuir sorption constant (L / mg) K f Freundlich constant relating the sorption capacity

1 / n empirical parameter relating the sorption intensity activity coefcient related to sorption mean energy

(mol 2 / kJ 2 ) Polanyi potential

R gas constant (8.314 10 3 kJ / mol K) T temperature (kelvin)

E mean free energy of sorption per molecule of sorbate

B heat of sorption

t time (min)

K T equilibrium binding constant (L / mg) a s , K s , s Sips parameter b T , n T Toth parameter

K 1 rate constant for the pseudo rst order equation

(min 1 ) K 2 rate constant for the pseudo second order equation

(g / mg min) K id intra particle diffusion rate constant (mg / g min

0.5 ) K , v power function constant

initial adsorption rate (mg / g min) desorption constant (g / mg) K R separation factor (dimensionless)

K a isotherm constant

lycoproteins. These compounds contain several functional groups

amino, carboxyl, sulphate, hydroxyl) which could play a vital role in

he sorption process.

In this work, marine macro green algae H. gracilis was used as a

orbent for removing Cr (VI) from aqueous solution. The inuence of

arious operating variables on the sorption of Cr(VI) onto Halimeda

racilis was studied using a central composite design (CCD) method.

he experimental data are analyzed by thermodynamic, kinetic and

quilibrium isotherm. FTIR spectroscopy and SEM were used to char- cterize the sorbent. Materials and methods

Chemicals and equipment

All chemicals used were of analytical reagent grade. Deionized

double distilled water was used throughout the experimental studies.

Analytical grade HCl, NaOH and buffer solutions (E. Merk) were used

to adjust the solution pH. Elico (L1-129) make pH meter was used

for pH measurements. The metal concentrations in the samples were

determined using atomic absorption spectrophotometer (AAS) (Elico

SL-176).

Biomass preparation

The marine green algae H. gracilis were collected from the Man-

dapam coast, Ramanadhapuram district, Tamilnadu, India. They were

washed several times using deionized water to remove extraneous

materials and salts. The washing process was continued till the wash

water contained no dirt. The washed algae were completely dried in

sunlight for 10 days. The dried samples were cut into small pieces

and powdered using domestic mixer. The structure of the marine

algae was modied by adding 0.1 M HCl. The content was stirred at

200 rpm for 8.0 h at room temperature. The acid treated algal biomass

was then centrifuged and washed with the physiological saline solu-

tion and dried in an oven at 333.15 K. The dried sorbent was ground

on an agate stone pestle mortar and sieved. In this work, the pow-

dered raw and acid treated algae of 100 mesh particle size were used

as sorbents for sorption process.

Preparation of metal ion solution

Metal ion solution was prepared from analytical grade K 2 Cr 2 O 7 supplied by (Merck Ltd.) India. Stock solution of 1000 mg / L of Cr(VI) was prepared from K 2 Cr 2 O 7 using deionized water. The working so-

lutions were prepared from the stock solutions by diluting it to ap-

propriate volumes.

Electroplating wastewater was collected from a small scale indus-

try located at Chennai, Tamilnadu, India. The wastewater was charac-

terized according to APHA methods [ 19 ] and it was given in Table 1 .

Batch adsorption experiment

All the batch experiments were carried out according to the CCD.

The sorbentsorbate mixtures were taken in a 250 mL conical ask

and agitated in an incubator shaker (LARK). The samples were cen-

trifuged in the research centrifuge (REMI) at 10,000 rpm and the supernatant was used for analysis of metal concentrations by using

AAS. Experimental analysis was repeated three times and the results

were statistically analyzed. The amount of adsorbed chromium per

unit mass of adsorbent ( q e , mg / g) was obtained using the following expression:

q e = ( C o C e ) V m

(1)

The amount of adsorbed chromium per unit mass of adsorbent at time

t ( q t , mg / g) was obtained by using following expression:

q t = ( C o C t ) V m

(2)

where V is the volume of solution treated in liter, C o is the initial

concentration of chromium metal ion in mg / L, C e is the equilibrium Chromium metal ion concentration in mg / L, C t (mg / L) is the concen-

tration of adsorbents at time t , and m is the biomass in gram.

R. Jayakumar et al. / Journal of Environmental Chemical Engineering 2 (2014) 12611274 1263

C

D

1

1

3

9

3

0

1

0

1

5

2

Table 1

Characteristics of the electroplating wastewater.

Parameters

Color

pH

Total dissolved solids

BOD

COD

Sulphate

Phosphate

Cr(VI)

Copper

Zinc

Iron

Nickel

Experimental design by RSM

RSM is a statistical tool designed to nd the optimal response

within specic ranges of pre-established factors, through a second-

order equation. In industrial applications, RSM designs involve a small

number of factors, because the required number of experimental runs

increases dramatically with the number of factors [ 20 ]. CCD was cho-

sen to study the effects of pH, sorbent dosage (g / L), agitation speed(rpm) and contact time (min) on Cr(VI) sorption. In order to describe

the effects of these variables on percentage removal of chromium,

batch experiments were conducted. The coded values of the process

parameters were determined by the following equation.

X i = ( X i X 0 ) X

(3)

where X i coded value of the i th variable, X i uncoded value of the i th test

variable and X o uncoded value of the i th test variable at center point.

The range and levels of individual variables were given in Table 2 . The

experimental design was given in Table 3 . The regression analysis

was performed to estimate the response function as a second order

polynomial.

A second-order polynomial equation is:

Y = 0 + k

i= 1 i X i +

k i= 1

ii X 2 i +

k= 1 i = 1 , i < j

k j= 2

ij X i X j (4)

where Y is the predicted response b i , b j , b ij are coefcients estimated

from regression, they represent the linear, quadratic and cross prod-

ucts of x 1 , x 2 , x 3 on response.

A statistical program package Design expert 7.1.5 was used for re-

gression analysis of the data obtained and to estimate the coefcient

of the regression equation. The equation was validated by the statis-

tical test called ANOVA analysis. After sorption, the contents of the

beakers were centrifuged at 10,000 rpm for 3 min. and the sorbent

was successfully separated from aqueous solution. The supernatantswere analyzed for residual Cr(VI) concentration using AAS. All the

experiments were performed in triplicate and average value was re-

ported.

SEM and EDS analysis

The micrographs were recorded using JEOL scanning electron

microscope model, JSM 5610 L V, with an accelerating voltage of 20 kV, at high vacuum (HV) mode and secondary electron image (SEI),

an energy dispersive spectrum analyzer (EDS) of oxford instrument

is attached with the SEM for elemental analysis.

FTIR measurements

FTIR spectra for both fresh and Cr(VI) treated H. gracilis were ob-

tained by KBr pellets methods operated on FTIR spectrophotometer haracteristics

ark brown

.80

9,820 mg / L

46 mg / L

12 mg / L

74 mg / L

.31 mg / L

12 mg / L

.56 mg / L

53 mg / L

.21 mg / L

8 mg / L

(Thermo Scientic Nicolet iS 5 FTIR, USA) was used for the IR spectral

studies (4000400 cm 1 ) of sorbent. For IR spectral studies, 10 mg ofsample was mixed and ground with 100 mg of KBr and made into

pellet to investigate the functional groups present in the H. gracilis

and to look into possible Cr(VI) binding sites.

Desorption / reuse procedure

The recycling of sorbent is a most important aspect from the eco-

nomical point. Hence sorptiondesorption experiments were carried

out up to ten cycles using 10 mL of 0.2 M HCl. A single cycle se-

quence consists of sorption followed by desorption. In order to use

the biomass for the next stage of cycle, the biomass was washed with

excess of 0.2M HCl solution and distilled water, sequentially.

Desorption efciency = Amount of metal ions desorbed Amount of metal ions adsorbed

100 (5)

Results and discussion

Characteristics of sorbent

The physical and chemical properties of the green algae Helimeda

gracilis were determined by the standard methods. The elemental

analysis depicted the composition of sorbents as C, 20.3%; N, 5.02%;

S, 1.63%. The apparent density of the sorbent was determined to be

1.1 g / cm 3 . EDX analysis of sorbent before and after Cr(VI) sorptionconrmed this observation. The humidity and the zeta potential were

calculated to be 1.26% and 0.051 V for the sorbent. The cell wallof green algae contains cellulose, hydroxyproline, glucosides, xylans

(polysaccharides made from units of xylose) and mannan (polymer of

sugar mannose). The major functional groups that took part in adsorp-

tion were OH, C O, C O, C H and COOH. The functional groupSO 2 was additionally involved in adsorption with H. gracilis . The3 FTIR results obtained give an idea about the presence of functional

groups on the algal cell surfaces and also the mechanism of adsorp-

tion, which is dependent on functional groups especially hydroxyl,

carboxyl, and carbonyl groups.

Effect of sorbent size

Before optimization, the effect of sorbent size (36, 60, 100 and

150 mesh) on Cr(VI) sorption by Helimeda gracilis were carried out.

The sorbent was transferred to 250 mL Erlenmeyer ask containing

100 mL of Cr(VI) solutions and agitated at 120 rpm for a desired

contact time. Then the sorbents were separated and the Cr(VI) con-

centration in the supernatant was analyzed by AAS. From Fig. 1 , it

was inferred that the Cr(VI) removal efciency increases as the mesh

size increases from 36 to 150 mesh. This is because; smaller particles

provide larger surface area and results in higher removal efciency.


T

E

T

C

F

c

T

B

s

sable 2

xperimental range and levels of independent process variables.

Independent variable Coded levels Code 2 1

pH A 3 4

Sorbent dosage

(g / L)

B 1 1.5

Agitation speed

(rpm)

C 40 80

Contact time (min) D 20 30

able 3

CD based experimental design and its response for chromium removal.

Run order (A) pH

(B) Sorbent dosage

(g / L)

(C) Agitation

(rpm)

1 1 (6) 1 (1.5) 1 (80) 2 1 (4) 1 (2.5) 1 (160)

3 0 (5) 0 (2) 0 (120)

4 0 (5) 2 (1) 0 (120)

5 0 (5) 0 (2) 0 (120)

6 0 (5) 0 (2) 2 (200)

7 1 (6) 1 (1.5) 1 (160) 8 2 (7) 0 (2) 0 (120)

9 0 (5) 0 (2) 0 (120)

10 0 (5) 0 (2) 0 (120)

11 0 (5) 0 (2) 0 (120)

12 0 (5) 0 (2) 0 (120)

13 1 (4) 1 (1.5) 1 (80) 14 1 (4) 1 (2.5) 1 (160)

15 0 (5) 0 (2) 2 (40)

16 1 (6) 1 (1.5) 1 (80) 17 0 (5) 0 (2) 0 (120)

18 1 (4) 1 (2.5) 1 (80)

19 0 (5) 0 (2) 0 (120)

20 1 (4) 1 (2.5) 1 (80)

21 1 (4) 1 (1.5) 1 (80) 22 1 (6) 1 (2.5) 1 (80)

23 1 (6) 1 (2.5) 1 (160)

24 0 (5) 2 (3) 0 (120)

25 1 (4) 1 (1.5) 1 (160) 26 1 (4) 1 (1.5) 1 (160) 27 1 (6) 1 (2.5) 1 (80)

28 1 (6) 1 (1.5) 1 (160) 29 2 (3) 0 (2) 0 (120)

30 0 (5) 0 (2) 0 (120)

31 1 (6) 1 (2.5) 1 (160)

ig. 1. Effect of sorbent size on sorption of Cr(VI) on Helimeda gracilis . Initial Cr(VI)

onc. = 50 mg / L, sorbent dosage = 1 g / L, contact time = 60 min, and pH = 5.

he maximum removal efciency was attained for a mesh size of 150.

ut for regeneration process, the smaller size particles will not with

tand the extreme conditions [ 21 ]. Hence 100 mesh particle size was

elected for further studies. 0 + 1 + 2 5 6 7

2 2.5 3

120 160 200

40 50 60

speed (D) Contact time

(min) Percentage Cr(VI) removal

Experimental Predicted

1 (30) 34.41 27.435

1 (30) 62.11 68.791

0 (40) 79.22 79.220

0 (40) 25.76 32.483 0 (40) 79.22 79.220

0 (40) 77.8 74.578

1 (50) 44.5 48.785

0 (40) 25.75 31.926

0 (40) 79.22 79.220

0 (40) 79.22 79.220

2 (20) 42.56 47.883

0 (40) 79.22 79.220

1 (50) 43.07 47.548

1 (50) 66.43 74.953

0 (40) 63.2 62.380

1 (50) 47.43 43.244

2 (60) 79.22 69.855

1 (30) 64.87 62.133

0 (40) 79.22 79.220

1 (50) 61.9 70.418

1 (30) 49.13 49.911

1 (50) 71.5 72.876

1 (50) 72.5 74.214

0 (40) 80.9 70.135

1 (50) 58.22 56.286

1 (30) 60.6 60.771

1 (30) 41.99 46.419

1 (30) 41.12 35.097

0 (40) 65.36 55.141

0 (40) 79.22 79.220

1 (30) 52.81 49.880

Fitting models

Sorption of Cr(VI) was carried out according to the CCD and the

results obtained were given in Table 3 . The results of theoretically pre-

dicted response were given in Table 3 . The mathematical expression

of relationship to the response with variables is:

Y = 79 . 2200 5 . 80375 A + 9 . 41292 B + 3 . 04958 C + 5 . 49292 D 8 . 92156 A 2 6 . 97781 B 2 2 . 68531 C 2 5 . 08781 D 2

+ 1 . 69063 AB 0 . 799375 AC + 4 . 54312 AD 1 . 05063 BC + 2 . 66188 BD 0 . 530625 C D (6)

where Y is the percentage removal of Cr(VI) and A, B, C and D are the

coded values of pH, sorbent dosage(g / L), agitation speed (rpm) and contact time (min) respectively.

The ANOVA results for Cr(VI) sorption onto green alga were given

in Table 4 . F value of 11.51 implies that the model was signicant.

The sher F -test with a very low probability value ( P model > 0.0001) reveals a very high signicance for the regression model. The good-

ness of t of the model was checked by coefcient of determination


Fig. 2. Interactive effect of pH and sorbent dosage on Cr(VI) removal by Halimeda

gracilis .

( R 2 ). For a good statistical model, R 2 value should be close to 1.0. The

R 2 was found to be 0.9097, which implies that more than 90.97% of

experimental data was compatible with the data predicted by the

model. A low CV value (11.66), indicate that the deviations between

experimental and predicted values were low. Adeq precision mea-

sures the signal to noise ratio. A ratio greater than 4 is desirable. In

this work, the ratio is found to be 10.487, which indicates an adequate

signal. Values of P less than 0.05 indicates the signicance of model

terms. In this case, A, B, C, D, A 2 , B 2 , D 2 and AD were signicant model

terms for the sorption of Cr(VI). This implies that the linear and square

effects of pH, sorbent dosage and contact time were more signicant

factors. The linear effect of agitation speed was more signicant factor

and the interactive effect of pH and contact time was also signicant.

Effect of variables on Cr(VI) removal

The sorption efciency depends on several parameters, like pH,

structural properties of both sorbate and sorbent, sorbent dosage,

contact time, agitation speed, initial concentration of metal ions, etc.

[ 20 ]. Effect of pH on sorption

The effect of pH on the sorption of chromium onto raw H. gra-

cilis algae biomass were studied by changing the pH from 3.0 to 7.0.

The result obtained was shown in Fig. 2 . It was observed from the

plot that the sorption was favored by acidic pH range of 3.05.0 and

maximum adsorption by the algae biomass was observed at pH 4.9.

Further increase in pH decreased the adsorption of chromium by the

algae. Maximum metal adsorption at pH 4.9 seems to be due to a

net positive charge on algae surface at low pH. Similar results were

reported by Srinivasa Popuri et al. [ 22 ] and Izabela Michalak et al.

[ 23 ]. Chromium, which may exist as HCrO 4, Cr 2 O 7 , etc. in solution at

optimum sorption pH has a tendency to bind the protonated active

sites of the sorbent [ 24 ]. But as pH of the solution increases, algae cell

wall becomes more and more negatively charged due to functional

groups, which repulse the negatively charged chromate ions thereby

affecting Cr(VI) sorption on the algae surface.

Effect of sorbent dose

The effect of sorbent dose on the removal of Cr(VI) was shown in

Fig. 3 . The amount of sorbent signicantly inuenced the extent of

Cr(VI) sorption, i.e., the sorption of metal ions increases with increase

in biomass dosage and almost constant at dose higher than 2.2 g / L.

Fig. 3. Interactive effect of agitation speed and contact time on Cr(VI) removal by

Halimeda gracilis .

This trend could be explained as a consequence of partial aggregation

of biomass at higher biomass concentration, which results in the de-

crease in effective surface area for the sorption [ 25 ]. Therefore, the

optimum algae biomass dose selected was 2.2 g / L for the rest of theexperimental studies.

Effect of agitation speed

The effect of agitation speeds on adsorption for Cr(VI) was studied

in the range of 40200 rpm. The results were presented in Fig. 3 . From

the results, the maximum sorption of Cr(VI) occurred at 136 rpm for

H. gracilis sorbent. At low agitation speed, the sorbent do not spread

in the sample but accumulated. This may cover the active sites of the

lower layer adsorbent and only the upper layer adsorbent active sites

adsorb the metal ion. Therefore agitation rate should be sufcient

to assure that all the surface binding sites were readily available for

metal uptake. But at higher agitation speed, the percentage removal

decrease. This may be attributed to an increase desorption tendency

of adsorbate molecules [ 26 ].

Effect of contact time

Fig. 3 shows the interactive effect of agitation speed and contact

time on the sorption of Cr(VI) on to the green algae. It has been ob-

served that the Cr(VI) removal efciency was high at the beginning

stages and then decreases slowly till it reaches the saturation level(47 min). The initial phase may involve physical adsorption or ion

exchange at cell surface and the subsequent slower phase may in-

volve other mechanisms such as complexation, micro-precipitation

or saturation of binding sites [ 27 ].

The perturbation plot ( Fig. 4 ) shows the comparative effects of

the variables on the sorption of Cr(VI). A steep curvature in sorbent

dosage, B curve, shows that the sorption is very sensitive to sorbent

dosage. The comparatively semi-at C curve show less sensitivity of

the sorption to alter with respect to a change in agitation speed. It is

clear from the perturbation plot that the most signicant factor on

the response is sorbent dosage followed by pH and contact time.

Second order polynomial models obtained in this study was uti-

lized to determine the optimum conditions. The optimum condi-

tions were: initial pH 4.9, sorbent dosage 2.2 g / L, agitation speed 136 rpm and contact time 47 min.

Effect of temperature and thermodynamic study

It is well known that, temperature inuences the sorption process

rate. An increase in temperature from 293.15 to 308.15K increases the

specic uptake to 43.5 mg / g of Cr(VI) by H. gracilis . Further increasing


T

A

M

5

8

2

2

7

4

1

3

1

1

4

2

1

2

7

5

8

0

S 1.66

p

t

f

i

t

s

f

(

t

G

c

e

w

t

t

F

a

r

lable 4

NOVA for chromium removal using Halimeda gracilis .

Source Sum of squares df

Model 8121.96 14

A 808.40 1

B 2126.47 1

C 223.20 1

D 724.13 1

AB 45.73 1

AC 10.22 1

AD 330.24 1

BC 17.66 1

BD 113.37 1

CD 4.51 1

A 2 2276.06 1

B 2 1392.32 1

C 2 206.20 1

D 2 740.22 1

Residual 806.35 16

Lack of t 806.35 10

Pure error 0.000 6

Cor total 8928.31 30

td. dev. 7.10; R -squared 0.9097; mean 60.89; adj. R -squared 0.8307; C.V. % 1

red R -squared 0.7798; PRESS 4644.56; adeq precision 10.487. Fig. 4. Perturbution plot on Cr(VI) removal by Halimeda gracilis .

he temperature from 308.15 to 313.15 K decreases the specic uptake

rom 43.5 to 42.84 mg / g of Cr(VI). This is probably caused by a change n the texture of the sorbent and a loss in the sorption capacity due

o material deterioration [ 28 ].

Thermodynamic parameters were calculated to conrm the ad-

orption nature of the investigation. The thermodynamic constants,

ree energy change ( G ), enthalpy change ( H ) and entropy change S ) were calculated to evaluate the thermodynamic feasibility of he process and to conrm the nature of the adsorption process. The

ibbs adsorption process, free energy, as well as, the enthalpy pro-

ess were calculated from experimental results using the following

quations:

G = RT ln K (7)

G = H T S (8) here R is the universal gas constant (8.314 10 3 kJ / mol K), T is the emperature in Kelvin and K is the equilibrium constant, calculated as

he surface and solution metal distribution ratio (K = q e / C e ) [ 29 ]. From ig. 5 , the values of H and S were calculated from the intercept

nd slope of a plot of G versus T according to Eq. (8) by linear egression analysis. The calculated thermodynamic parameters were

isted in Table 5 . Positive values of H suggest the endothermic ean square F -value P -value

80.14 11.51 < 0.0001

08.40 16.04 0.0010

126.47 42.19 < 0.0001

23.20 4.43 0.0515

24.13 14.37 0.0016

5.73 0.91 0.3550

0.22 0.20 0.6585

30.24 6.55 0.0210

7.66 0.35 0.5621

13.37 2.25 0.1531

.51 0.089 0.7688

276.06 45.16 < 0.0001

392.32 27.63 < 0.0001

06.20 4.09 0.0601

40.22 14.69 0.0015

0.40

0.63

.000 Fig. 5. Plot of G versus T for the estimation of thermodynamic parameters for sorption of Cr(VI) by Halimeda gracilis .

nature of the sorption and the negative values of G indicate the spontaneous nature of the sorption process. However, the negative

value of G decreases with an increase in temperature, indicating that the spontaneous nature of sorption is inversely proportional to

the temperature. Similar endothermic nature of the sorption process

has been reported for other sorbent systems [ 24 , 30 ]. The increase in

sorption with temperature may be attributed to either increase in the

number of active surface sites available for sorption on the adsorbent

or due to the decrease in the boundary layer thickness surrounding

the sorbent, so that the mass transfer resistance of sorbate in the

boundary layer decreased [ 31 ]. The positive values of S showed the increasing the randomness at the solid / solution interface during the sorption process.

Equilibrium isotherm study

In order to determine the mechanism of Cr(VI) sorption onto

H. gracilis , the batch experimental data was applied to the lin-

ear isotherms namely, Langmuir, Freundlich, DubininRadushkevich,

Temkin and three parameter isotherm namely Sips and Toth.

Langmuir isotherm

Langmuir [ 32 ] proposed a theory to describe the sorption of gas

molecules onto metal surfaces. The linear form of Langmuir isotherm


)

and is the Polanyi potential described as Table 5

Thermodynamic parameters for the sorption of Cr(VI) on to Halimeda gracilis .

Metal ion Temp . (K) G (kJ / mol

Cr(VI) 293.15 2.469

Cr(VI) 298.15 3.309

Cr(VI) 303.15 4.149

Cr(VI) 308.15 5.102

Cr(VI) 313.15 5.07

Fig. 6. Langmuir isotherm plots for the sorption of Cr(VI) onto H. gracilis at 308.15 K

temperature.

is given by,

1

q e = 1

q m bC e + 1

q m (9)

where q m is the monolayer sorption capacity of the sorbent (mg / g),q e is the equilibrium metal ion concentration on the sorbent (mg / g),C e is the equilibrium metal ion concentration in the solution (mg / L),and b is the Langmuir sorption constant (L / mg) related to the freeenergy of sorption. Fig. 6 shows the Langmuir plots of Cr(VI) sorption

isotherms for H. gracilis at different initial metal concentration. The

constants q m and b are tabulated in Table 6 . Table 7 represents thecomparison of sorption capacity ( q m ; mg / g) of H. gracilis biomass for Cr(VI) with various sorbents. Based on this table, it can be concluded

that the H. gracilis has very good potential for the removal of Cr(VI)

from aqueous solution. Afnity between sorbent and sorbate was

represented by the constant b . In general good sorbents have a high

q max and a high R 2 (0.997). H. gracilis have high saturation ( q max ) for

different initial metal concentration of Cr(VI).

Freundlich isotherm

The Freundlich [ 36 ] isotherm is an empirical equation used to de-

scribe heterogeneous systems. The linear form of Freundlich isotherm

is represented by the equation:

log q e = log K f + 1 n

log C e (10)

where K f is a constant relating the sorption capacity and 1 / n is an empirical parameter relating the sorption intensity, which varies with

the heterogeneity of the material. From the graphs, K f value was found

to be 19.67 and 1 / n value was found as 0.206. Usually, 1 / n values between 0 and 1 indicate good sorption. In this work, a value of 0.206

indicates that the sorption of Cr(VI) onto the H. gracilis was favorable.

Fig. 7 shows the Freundlich plots of Cr(VI) sorption isotherms for H.

gracilis at different initial metal concentration and the constants K f and 1 / n were tabulated in Table 6 . The values of K f and 1 / n were calculated from the intercept and slope of the plot between log q e versus log C e . K f , for all cases, the Langmuir equation ts the experi-

mental data better than the Freundlich equation. This isotherm does

not predict any saturation of the adsorbent by the sorbate. Instead, S (kJ / mol k) H (kJ / mol)

0.139 38.38

0.139 38.38

0.139 38.38

0.139 38.38

0.139 38.38

Fig. 7. Freundlich isotherm plots for the sorption of Cr(VI) onto Halimeda gracilis at

308.15 K temperature.

innite surface coverage is predicted, indicating multilayer sorption

on the surface.

Dubinin Radushkevich isotherm The linear form of the D R isotherm equation [ 37 ] is:

ln q e = ln q m 2 (11)where q e is the amount of metal ions adsorbed on per unit weight

of biomass (mg / g), q m is the maximum sorption capacity (mg / g), is the activity coefcient related to sorption mean energy (mol 2 / kJ 2 ) = RT ln (1 + 1

C e

)(12)

where R is the gas constant 8.314 10 3 kJ / mol K , T is the temperature in Kelvin and C e is the equilibrium concentration of the Cr(VI)

in solution (mg / L). The mean free energy of sorption per molecule of sorbate required to transfer 1 mol of ion from the innity in the solu-

tion to the surface of biomass and can be determined by the following

Eq. (13) :

E = 1 2 (13)

The DubininRadushkevich constants and q m were calculated in from the slope and the intercept of the plot of ln q e versus 2

as shown in Fig. 8 and the results are given in Table 6 . The energy

value obtained ( Table 6 ) have E < 8 kJ / mol, which indicate that all metal cation adsorptions were physical processes, since a chemical

adsorption process has an E > 8 kJ / mol [ 38 , 39 ]. The sorption capacity was lower than the Langmuir model, which may be attributed to

different assumptions taken into consideration. From R 2 values, it

was concluded that the sorption of Cr(VI) onto H. gracilis followed the

Langmuir model.

Temkin isotherm

The Temkin isotherm [ 40 ] has been used in the following form:

q e = RT b

ln K T + RT b

ln C e (14)


T

L sorpt

P

q max 55.55 mg / g

B 0.162

R 2 0.997

T

C

F

g

q

w

i

i

b

eable 6

angmuir, Freundlich, DubininRadushkevich, Temkin, Sips and Toth constants for the

Sl. no. Isotherm model

1. Langmuir 2. Freundlich K

1

R

3. Dubinin Redushkevich q

E

R

4. Temkin B

K

R

5. Sips a

K

R

6. Toth q

n

b

R

able 7

omparison of adsorption capacity of different sorbents for Cr(VI) removal.

Metal ion Biosorbent S

Cr(III) Spirulina (sp.) 3

Spirogyra spp. 3

Cr(VI) Sphaeroplea 2

Oedogonium hatei 3

Padina boergesenli 4

Ulva lactuca 1

Chlorella valgaris 0

Scenedesmus obliquus 1

Sargassum sp. 1

Padina (brown algae) 5

Sargassum (brown algae) 3

Sargassum sp. 6

Sargassum sp. 6

Sargassum siliquosum 6

Cysteria indica 2

Turbaneria ornate 6

Halimeda gracilis 5

ig. 8. DubininRadushkevich isotherm plots for the sorption of Cr(VI) onto Halimeda

racilis at 308.15 K temperature.

e = B ln K T + B ln C e (15) here constant B = RT / b , which is related to the heat of sorption, R

s the universal gas constant (kJ / mol K ), T is the temperature ( K ), b s the variation of sorption energy (J / mol) and K T is the equilibrium inding constant (L / mg) corresponding to the maximum binding en- rgy. From the plot, q e versus ln C e ( Fig. 9 ) the isotherm were found ion of Cr(VI) on to Halimeda gracilis .

arameters Cr(VI) sorption at temperature 308.15 K f 19.67

/ n 0.206

2 0.927

max 49.25

4.259

0.342

2 0.975

7.881

T 6.22

2 0.937

s 0.2335

s 13.09

s 0.8616

2 0.578

max 55.69

T 1.145

T 0.228

2 0.577

orption capacity Reference

4.6 (mg / g) [ 22 ]

0.21 (mg / g) [ 33 ]

9.8 (mg / g) [ 21 ]

1 (mg / g) [ 26 ]

9 (mg / g) [ 34 ]

0.61 (mg / g) [ 35 ]

.5341.52 (mmol / g) [ 53 ]

.131 (mmol / g) [ 53 ]

.301.3257 (mmol / g) [ 53 ]

4.6 (mg / g) [ 54 ]

1.79 (mg / g) [ 54 ]

8.94 (mg / g) [ 55 ]

5 (mg / g) [ 56 ]

6.4 (mg / g) [ 57 ]

0.927.9 (mmol / g) [ 58 ]

5% [ 59 ]

5.55 Present study

Fig. 9. Temkin isotherm plots for the sorption of Cr(VI) onto Halimeda gracilis at

308.15 K temperature.

and given in Table 6 . The correlation factors show that the Langmuir

model approximation to the experimental results was better than the

Temkin model. Consequently, among the four isotherm models used,

the Langmuir model offers the best correlation factors.


equilibrium parameter K R ( Table 8 ), which is dened by the following

relationship:

K R = 1 1 + K a C 0

(18)

where K R is a dimensionless separation factor, C o is initial metal con-

centration (mg / L) and K a is isotherm constant (L / mg). From the K R values in Table 9 , it was found that, all isotherms were favorable.

High value of K R , shows that the Langmuir isotherm ts better than

all other isotherm. Fig. 10. Sips isotherm plots for the sorption of Cr(VI) onto Halimeda gracilis at 308.15 K

temperature.

Sips isotherm

To circumvent the problem of continuing increase in the adsorbed

amount with a rising concentration as observed for Freundlich model;

an expression was proposed by Sips in 1948 which has a similar form

to the Freundlich isotherm, differs only on the nite limit of adsorbed

amount at sufciently high concentration.

q e = K s C e s

1 + a s C e s (16)

The parameter s is regarded as the parameter characterizing thesystem s heterogeneity. Moreover, the heterogeneity could system

from the sorbent or the heavy metal, or a combination of both. As

a rule, all of the Sips parameters a s , K s and s were governed byoperating conditions such as pH, temperature, etc. The model shown

in Fig. 10 and the results are given in Table 6 . In the adsorption of Cr(VI)

on H. gracilis , the parameter s stays close to unity [ 41 ]. Howeverthe correlation coefcient was very low, the Langmuir isotherm is

considered more appropriate. Toth isotherm

Another empirical equation that is popularly used and satises the

two end limits is the Toth equation. This isotherm was derived from

the potential theory. Toth equation has been proved as a valuable

tool in describing sorption for heterogeneous systems. It assumes an

asymmetrical quasi-Gaussian energy distribution with its left-hand

side form widened, i.e., most sites have sorption energy less than the

mean value [ 42 ].

q e = q max b T C e ( 1 + ( b T C e ) n T ) 1 /n T

(17)

Toth equation posses the correct Henry law type limit besides a pa-

rameter to describe the heterogeneities of the system. The model

shown in Fig. 11 and the results are given in Table 6 .

However, this equation is still unable to predict the isotherm in

a particular heterogeneous system as illustrated in the sorption of

Cr(VI) into Helimeda gracilis.

Separation factor to nd the feasibility and type of isotherm

The effect of isotherm shape can be used to predict whether an

adsorption system is favorable or unfavorable [ 43 ]. According to

Hall et al. [ 44 ], the essential features of the Langmuir isotherm can be

expressed in terms of a dimensionless constant separation factor or Fig. 11. Toth isotherm plots for the sorption of Cr(VI) onto Halimeda gracilis at 308.15 K

temperature.

Fig. 12. Pseudo rst order plots for the sorption of Cr(VI) onto Halimeda gracilis . Sorption kinetics

Equilibrium analysis is fundamental in order to evaluate the afn-

ity or capacity of a sorbent. However, it is important to assess how

sorption rates vary with aqueous free metal concentrations, and how

rates are affected by sorption capacity or by the sorbent character

in terms of kinetics. Three kinetic models, pseudo rst order, pseudo

second order and intra particle diffusion, power function and Elovich

model were applied in order to interpret the experimental results.

Pseudo rst order model

Lagergren [ 45 ] suggested a pseudo rst order equation for the

sorption of a liquid / solid system based on the solid capacity. The linear form of the pseudo rst-order rate equation is given as follows:

log ( q e q t ) = log q e K 1 2 . 303

t (19)

where q t and q e (mg / g) were the amounts of the Cr(VI) ions sorbed at equilibrium (mg / g) and t (min), respectively and K 1 is the rate constant of the equation (min 1 ). The sorption rate constants ( K 1 ) is determined and shown in Fig. 12 and the values were reported in

Table 10 .


T

S

T

U

L

F

Ir

T

S

T

K

P

o

w

t

a

s

s

e

o

able 8

eperation factor ( K R ) values.

Values of K R

K R > 1

K R = 1 0 < K R < 1

K R = 0

able 9

eparation factor and their condition to nd the feasibility and type of isotherm.

Type of isotherm KR

Langmuir 0.058

Freundlich 0.00050

Dubinin Redushkevich 0.0023

Temkin 0.0016

Sips 0.0007

Toth 0.0086

able 10

inetic parameters obtained from various kinetic models for Cr(VI) on to Halimeda gracilis.

Sl. no. Kinetic model P

1. Pseudo rst order K

R

2. Pseudo second order K

R

3. Intra particle diffusion K

R

4. Power function K

V

R

5. Elovich

R

Fig. 13. Pseudo second order plots for the sorption of Cr(VI) onto Halimeda gracilis .

seudo second order model

The pseudo second order model predicts the sorption behavior

ver the whole time adsorption [ 46 ].

t

q t = 1

K 2 q 2 e + 1

q e t (20)

here K 2 (g / mg min) is the rate constant of the second-order equa- ion, q t (mg / g) is the amount of sorption time t (min) and q e is the mount of sorption equilibrium (mg / g). In Fig. 13 , sorption rate con- tants ( K 2 ) can be determined experimentally by plotting of t / q t ver- us t . The rate constants and R 2 values were given in Table 10 . How-

ver, the correlation coefcients, R 2 , showed that the pseudo second

rder model ts better with the experimental data than the pseudo-

rst order model [ 39 ]. ype of isotherm

nfavorable

inear

avorable

reversible

Condition

Favorable

Favorable

Favorable

Favorable

Favorable

Favorable

arameters Cr(VI) sorption at temperature 308.15 K

1 (min 1 ) 0.073

2 0.972

2 ((g / mg)min) 0.00081

2 0.999

id ((mg / g)min 0.5 ) 6.579

2 0.978

7.175

0.4782

2 0.999

8.8657

0.0833

2 0.978 Intra particle diffusion model

Weber s intra particle diffusion model [ 47 ] is dened by the fol-

lowing equation:

q t = K id t 0 . 5 + C (21) where K id is the intraparticle diffusion rate constant (mg / (g min

0.5 )) and C is the intercept. It was observed that all the plots have an initial

curved portion, followed by a linear portion and a plateau region. The

initial curve of the plot was due to the diffusion of metal ion through

the solution to the external surface of H. gracilis . The linear portion of

curves describes the gradual sorption stage, where intraparticle dif-

fusion of metal ion on H. gracilis takes place and nal plateau region

indicates equilibrium uptake. Based on the results it may be con-

cluded that intraparticle diffusion is not only the rate determining

factor. The rate constants of intra particle diffusion were calculated

from Fig. 14 . The values for all the kinetic models were calculated

and summarized in Table 10 . Pseudo second order model has higher

correlation coefcient values indicating that the sorption of Cr(VI) on

the sorbent follows pseudo second order kinetic model. Higher values

of R 2 show a better tness of the sorption data [ 37 , 38 ].

Power function model

The power function can be expressed as

q = K t v (22) where q is amount of sorbate per unit mass of sorbent at time t , K and

v are constants and v is positive and < 1. Eq. (22) is empirical, except for the case where v = 0.5, when it is similar to the parabolic diffusion equation. Eq. (22) and various modied forms have been used by a

number of researchers to describe the kinetics of reactions on natural

materials [ 48 ]. The constants of power function were calculated from

Fig. 15 . The values for all the kinetic models were calculated and


Fig. 14. Intra particle diffusion model plots for the sorption of Cr(VI) onto Halimeda

gracilis .

Fig. 15. Power function model plots for the sorption of Cr(VI) onto Halimeda gracilis . summarized in Table 10 . The model has higher correlation coefcient

values indicating that the sorption of Cr(VI) on the sorbent follows

the power function kinetic model.

Elovich model

The Elovich equation (23) [ 49 ] incorporates as the initial adsorption rate (mg / g min) and (g / mg) as the desorption constant. This relates the extent of the surface coverage and activation energy for

chemisorptions.

dq

dt = e q t (23)

Eq. (23) can be simplied to Eq. (24) by considering t and by applying the boundary conditions q t = 0 at t = 0 and q t = q t at t = t

q t = 1

ln ( ) + 1

ln ( t ) (24)

where q t is the amount of gas chemisorbed at time t . From the results

( Table 10 ) it was found that the Cr(VI) adsorption on green algae ts

the Elovich model [ 39 ]. A plot of q t versus ln( t ) ( Fig. 16 ) should give

a linear relationship with a slope of (1 /) and an intercept of (1 /) ln( ).

Desorption and regeneration studies

In sorption process, to decrease the processing cost and to open the

possibility of recovering the metal extracted from the liquid phase,

it is desirable to regenerate the sorbent material. In order to investi-

gate desorption of metal ion from metal loaded H. gracilis , the metal

loaded sorbent was treated with HCl [ 50 52 ]. Desorption studies were

performed with different HCl concentrations and the results were

shown in Fig. 17 . From the results of this study, with the increasing

of hydrochloric acid concentration, the desorption rate also increased Fig. 16. Elovich model plots for the sorption of Cr(VI) onto Halimeda gracilis . Fig. 17. Desorption efciency with different concentration of HCl (biomass concentra-

tion: 2.2 g / L; contact time: 45 min; temperature: 308.15 K).

initially, and then become almost stable. The maximum percentage

recovery of Cr(VI) was 98.02% with 0.2 M HCl solution.

The regenerated sorbent was reused for up to ten sorption

desorption cycles and the results were illustrated in Fig. 18 . A maxi-

mum efciency of 95.02% recovery of Cr(VI) were obtained with 0.2

M HCl in the rst cycle and is therefore suitable for regeneration of

sorbent. There was a gradual decrease in Cr(VI) sorption with an in-

crease in the number of cycles. After a sequence of ten cycles, the

Cr(VI) uptake capacity of the sorbent was reduced from 92.01% to

73.61%. The lost in the sorption capacity of the biomass for metal

ions was found to be 8%. This might be due to the ignorable amount

of biomass lost during the sorptiondesorption process. These results

indicate that the H. gracilis could be used repeatedly in Cr(VI) sorption

studies without any detectable loss in the total sorption capacity.

SEM with EDS

SEM images were used for the surface analysis of H. gracilis as

shown in Fig. 19 . These gures demonstrate the brous supercial

structure of the algal biomass surface where the metal cations could

be adsorbed. The EDS images for the seaweed before and after adsorp-

tion were presented in Fig. 20 and show the metal cations adsorbed

on the algae surface.


F

2

F

a

F

w

d

r

t

c

gig. 18. Biosorptiondesorption efciency with cycle number (biomass concentration:

.2 g / L; contact time: 45 min; temperature: 308.15 K).

ig. 19. SEM images for Cr(VI) on Halimeda gracilis : (A) before adsorption and (B) after

dsorption.

TIR Study

The infrared (IR) spectrum obtained from FTIR of the H. gracilis

as shown in Fig. 21 . It displays a number of absorption peaks, in-

icating the complex nature of the examined biomass. The results

evealed sorbent heterogeneity, evidenced by different characteris-

ic peaks. The infrared absorption wavelengths of each peak and the

orresponding functional groups were presented in Table 11 for H.

racilis . As seen in Table 11 , the major functional groups that took Fig. 20. EDS images for Cr(VI) on Halimeda gracilis : (A) before adsorption and (B) after

adsorption. Fig. 21. FTIR Images for Cr(VI) on Halimeda gracilis : (A) before adsorption and (B) after

adsorption.

part in adsorption were OH, C O, C O, C H and COOH [ 60 , 61 ]. The functional group SO 3

2 was additionally involved in adsorption with H. gracilis.

Sorption of Cr(VI) from electroplating wastewater

The results obtained from the sorption of electroplating industry

using raw and acid treated H. gracilis was shown in Fig. 22 . From the

gure it was inferred that a maximum of 85.21% and 83.69% Cr re-

moval was achieved by the acid treated and raw algae, respectively

for electroplating wastewater. From the gure it was also found that,

the Cr removal was found to be higher for aqueous solution, than the

electroplating wastewater. The decrease in Cr removal in electroplat-

ing wastewater may be due to the presence of other metal ions like Cu,

Zn, Fe and Ni in wastewater as indicated in Table 1 , which occupies

the sorption sites. In the aqueous sample of potassium dichromate

only Cr(VI) metal ions were present, so the binding sites on adsorbent

surface were occupied by the single metal ions, where as in a com-

plex industrial wastewater where more than one metal ion species

are present. Hence Cr removal is higher in aqueous solution [ 62 , 63 ].

Conclusion

In this study, the feasibility of sorption of chromium(VI) onto a

green algae, H. gracilis , which is abundant and cheaply available, was

studied. RSM is utilized to optimize the operating conditions and


of Cr

Table 11

FTIR spectral characteristics of Halimeda gracilis before and after sorption of Cr(VI).

Wavelength range

(cm 1 ) Halimeda gracilis

Before loading of Cr(VI) After loading

3500 3000 3423.88 3421.57 2900 2800 2961.91 2922.94

2852.33 2855.07

2500 2300 2545.53 2522.98 2493.48 2323.64

17401680 1786.82 1786.76

1500 1400 1490.00 1489.59 1280 1240 1261.87 1260.77 1150 950 1030.66 1022.45 650 480 713.04 713.08

Fig. 22. Removal percentage of Cr (VI) from Aqueous and Electroplating wastewater

using raw and acid treated algae. maximize the chromium(VI) removal. Analysis of variance showed a

high coefcient of determination value ( R 2 = 0.9097), thus ensuring a satisfactory adjustment of the second order regression model with

the experimental data. The initial pH signicantly inuenced metal

uptake. Sorption kinetics follows a pseudo-second-order and power

function model. Experimental data were analyzed using Langmuir,

Freundlich, DubininRadushkevich, Temkin, Sips and Toth isotherm

models and it was found that the Langmuir model presented a better

t. SEMEDS conrmed the presence of Cr(VI) ions on the biomass

surface. Temperature affects the sorption process and the thermody-

namic parameters show the spontaneous character of the sorption

reaction. The ndings of the present study indicates that H. gracilis

can be successfully used for separation of Cr(VI) from aqueous and

industrial waste water solutions.

Conict of interest

None.

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Sorption of hexavalent chromium from aqueous solution using marine green algae Halimeda gracilis: Optimization, equilibrium, kinetic, thermodynamic and desorption studiesIntroductionMaterials and methodsChemicals and equipmentBiomass preparationPreparation of metal ion solutionBatch adsorption experimentExperimental design by RSMSEM and EDS analysisFTIR measurementsDesorptionreuse procedure

Results and discussionCharacteristics of sorbentEffect of sorbent sizeFitting modelsEffect of variables on Cr(VI) removalEffect of temperature and thermodynamic studyEquilibrium isotherm studySorption kineticsDesorption and regeneration studiesSEM with EDSFTIR StudySorption of Cr(VI) from electroplating wastewater

ConclusionConflict of interestReferences

2.6Sorption of Hexavalent Chromium From Aqueous Solution Using Marine Green Algae

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