ORIGINAL ARTICLE

Batch sorption–desorption of As(III) from waste waterby magnetic palm kernel shell activated carbon using optimizedBox–Behnken design

Chinedum Anyika2• Nur Asilayana Mohd Asri1 • Zaiton Abdul Majid1

•

Jafariah Jaafar1• Adibah Yahya2

Received: 25 March 2017 / Accepted: 29 August 2017 / Published online: 8 September 2017

� The Author(s) 2017. This article is an open access publication

Abstract In this study, we converted activated carbon

(AC) into magnetic activated carbon (MAC), which was

established to have removed arsenic (III) from wastewater.

Arsenic (III) is a toxic heavy metal which is readily soluble

in water and can be detrimental to human health. The MAC

was prepared by incorporating Fe3O4 into the AC by using

Fe3O4 extracted from a ferrous sulfate solution, designated:

magnetic palm kernel shell from iron suspension

(MPKSF). Batch experiments were conducted using two

methods: (1) one-factor-at-a-time and (2) Box–Behnken

statistical analysis. Results showed that the optimum con-

ditions resulted in 95% of As(III) removal in the wastew-

ater sample. The adsorption data were best fitted to the

Langmuir isotherm. The adsorption of As(III) onto the

MPKSF was confirmed by energy dispersive X-ray spec-

trometry analysis which detected the presence of As(III) of

0.52% on the surface of the MPKSF. The Fourier transform

infrared spectroscopy analysis of the MPKSF–As presented

a peak at 573 cm-1, which was assigned to M–O (metal–

oxygen) bending, indicating the coordination of As(III)

with oxygen through the formation of inner-sphere com-

plexation, thereby indicating a covalent bonding between

the MPKSF functional groups and As(III). The findings

suggested that the MPKSF exhibited a strong capacity to

efficiently remove As(III) from wastewater, while the

desorption studies showed that the As(III) was rigidly

bound to the MPKSF thereby eliminating the possibility of

secondary pollution.

Keywords As(III) � Sorption � Desorption � Waste water

treatment � Magnetic palm kernel shell activated carbon

Introduction

Removal of contaminants such as arsenic (As) from

wastewater by adsorption mechanisms remains the most

effective method (Elizalde-Gonzalez et al. 2001). Most

studies in the field of adsorption for the removal of heavy

metals from water have mainly focused on the use of AC,

activated alumina, sand impregnated with iron, polymer

resins, hydrous ferric oxide and natural ores (Addo Ntim

and Mitra 2011). Although AC has been found to be more

effective relative to the other adsorbents mentioned above,

especially for the removal of heavy metals from aqueous

solutions, with percentage removal ranging from 82 to 96%

(Ribeiro et al. 2006), this technique may not be adequate

when it comes to a heavy metal like As(III) which is known

to be highly soluble in water. Arsenic exists in two dif-

ferent oxidation states (1) arsenite, As(III) and (2) arsenate,

As(V). As(III) is different from As(V) in a number of ways.

Firstly, difficulties arise when it comes to the removal of

As(III) from wastewater compared to arsenate As(V) due to

its high solubility in the aqueous environment, hence

As(III) which is the most toxic is also the most difficult to

remove from water (Pattanayak et al. 2000). Secondly,

whereas As(V) is mostly removed by outer-sphere

& Zaiton Abdul Majid

zaiton@kimia.fs.utm.my; zaitonmajid@utm.my

1 Department of Chemistry, Faculty of Science, Universiti

Teknologi, Malaysia, 81310 Johor Bahru, Malaysia

2 Environmental Biotechnology Laboratory, Faculty of

Biosciences and Medical Engineering, Universiti Teknologi,

Malaysia, 81310 Johor Bahru, Malaysia

123

Appl Water Sci (2017) 7:4573–4591

https://doi.org/10.1007/s13201-017-0610-9

complexation, i.e., by electrostatic attraction (Cheng et al.

2016), As(III) can be removed by inner-sphere complexa-

tion, which is a covalent bonding as demonstrated in this

study.

On the other hand, the long-term effects of drinking

water contaminated with arsenic include cancer of the skin,

lung, bladder, and kidney, skin thickening, neurological

disorders, muscular weakness and nausea (Jain and Ali

2000; WHO 1981). This has led to numerous studies

regarding the improvement of AC by magnetic modifica-

tion to increase its capacity to remove heavy metal con-

taminants. The MAC adsorbents exhibited magnetic

properties with greater efficiency for the adsorption of

contaminants from aqueous solution (Xu et al. 2010).

Whereas palm kernel shells were used in the production of

AC due to its high carbon content and low organic content

(Daud and Ali 2004), while at the same time high quality

of AC can be synthesized from PKS waste (Adhoum and

Monser 2002; Budinova et al. 2006; Hussein et al. 1996).

One major advantage of the MAC is that it exhibits

magnetic characteristics, in addition to having demon-

strated to be effective for adsorption in dilute solutions,

while its high specific surface area due to the presence of

microporous structure results in greater capacity for the

adsorption of heavy metals (Nakahira et al. 2006). So far,

magnetization has been identified as being potentially

important in improving the sorption characteristics of

organic adsorbents with well-developed structures, i.e., of

woody feedstock such as PKS (Trakal et al. 2016). How-

ever, previous studies on the removal of As(III) from

wastewater employed the use of biochar made from empty

fruit bunch and rice husks (Samsuri et al. 2013), which are

fibrous in nature, unlike the PKS feedstock used in this

study, which is a woody feedstock. Moreover, they used

biochar which is a different material. Another problem with

their study was that they did not carry out a desorption test

to ascertain the stability of the Fe-coated organic adsor-

bents. Further, their study was on competitive adsorption

between As(III) and As(V). Similarly, in the paper by

Payne and Abdel-Fattah (2005) using Fe-coated AC for the

removal of As(III) and As(V) from water, they relied on

commercially procured AC, which had a poorly developed

structure and hence was only able to remove 60% As(III)

from waste water unlike in this study where 95% removal

was achieved by using MPKS which has a well-developed

structure. In addition, they did not look at the magnetic

properties of the Fe-coated AC.

Therefore, the influence of the MAC on the removal of

As(III) from waste water has not been completely eluci-

dated. The objectives of this study are: (1) to prepare

magnetic activated carbon for the removal of arsenic from

water; (2) to optimize the parameters for As(III) sorption

using Box–Behnken design; (3) to study the

sorption–desorption of As(III) onto the prepared magnetic

activated carbon.

Experimental

Chemicals and reagents

Phosphoric acid (H3PO4, 85 wt%, Merck, Germany) was

used to pretreat and impregnate the raw materials. The

chemical reagents for the preparation of magnetic activated

carbon were of three kinds: (1) iron (III) chloride ([96%):

Sigma-Aldrich; (2) iron (II) sulfate (99.5%), Qrec; (3)

sodium hydroxide, Merck Germany. A 1000 ppm stock

solution of arsenite, As(III), was prepared in double dis-

tilled water from 0.05 M sodium arsenite (NaAsO2) pre-

pared by Fluka. Hydrochloric acid (HCl) and sodium

hydroxide (NaOH) were used to adjust the solution pH of

As(III).

Experimental methods

Material development and characterization

The raw PKSs (100 g) used in this study were obtained

from a palm oil estate located at Jalan Sawah, Pekan

Nenas, Johor Bahru, Malaysia. Sample pretreatment was

carried out by weighing out a 50-g portion of the raw PKS,

which was ground and sieved to particle sizes in the range

of 75–250 lm, then soaked and impregnated with 10 mL

of 30 wt% of phosphoric acid, H3PO4, at room temperature

with PKS-to-acid ratio of 1:1. The sample was left

impregnated for 24 h, and afterwards it was washed with

distilled water and dried at room temperature.

A 10-g portion of the pre-treated PKSs was transferred

into five conical flasks containing different concentrations

of H3PO4, i.e., 10, 20, 30, 40 and 50% wt/wt, respec-

tively. Similarly, the pre-treated PKS was treated again

using dilute acid (10, 20 and 30% wt/wt H3PO4). This

was done to determine the appropriate surface area at

which if the surface area obtained by using dilute acid

was high, then the use of concentrated acid can be

reduced and the experimental costs will be less. This ratio

(1:1) implied that 10 g of the raw PKSs was soaked in

10 g of H3PO4 for 24 h. The excess acid was then filtered

and the soaked PKSs were placed in a muffle furnace and

heated at 200 �C for 30 min to initiate the carbonization

process.

Subsequently, the temperature of the furnace was

increased to the range of 400–550 �C and held for 2 h

followed by cooling to room temperature. Afterwards, the

samples were thoroughly washed and rinsed using vacuum

filtration with hot distilled water to remove all the excess

4574 Appl Water Sci (2017) 7:4573–4591

123

acid until the pH of the filtrate was approximately 7. The

samples were then dried in the oven at a temperature of

110 �C for 24 h. The AC samples were then stored in

desiccators for further characterization and adsorption

studies. The preparation of the MPKSF was achieved by

utilizing a suspension of ferric chloride/ferrous sulfate. The

characterization experiments were conducted on the

MPKSF by Fourier transform infrared spectroscopy

(FTIR), X-ray diffraction (XRD), particle size analysis,

nitrogen adsorption analysis, scanning electron microscopy

(SEM), field emission scanning electron microscopy

(FESEM), energy dispersive X-ray spectrometry (EDX)

and the point of zero charge (pHpzc) was also determined

as part of the characterization. The magnetic saturation of

the MAC sample was characterized using vibrating sample

magnetometer (VSM).

Batch experimental procedure and testing methods

Batch experiments were carried out by grinding the

MPKSF into fine powder of 75 lm particle sizes. The stock

solution was prepared by diluting 0.05 M of sodium

arsenite (NaAsO2) with distilled water up to a concentra-

tion of 1000 ppm to give a 1000 ppm arsenite (As(III))

stock solution. The pH of the solution was then adjusted

using hydrochloric acid (HCl) until it reached a pH of 7. A

0.05–0.30 g portion of the powdered MPKSF was placed

into a conical flask together with 200 mL arsenite solution.

The solution was then shaken for 24 h at different tem-

peratures ranging from 10 to 40 �C. The contact time in the

shaker was also varied ranging from 5 to 720 min. The

initial concentration of the arsenite solution was varied

ranging from 5 to 100 lg L-1. This was done to mimic the

drinking water standard. The preliminary experiment was

conducted at an equilibrium time of 180 min. After that,

the suspension was filtered through 0.45-lm pore size

membrane filter.

The preliminary study on the adsorption of As(III) on

MPKSF, was divided into 6 factors and are described

briefly namely (1) effect of contact time, which was found

to be 180 min and established to be the time taken to reach

adsorption–desorption equilibrium; (2) effect of initial

As(III) concentration, which was found to be 5–70 lg L-1

with percentage removal of 87.58–89.42%; (3) effect of

adsorbent dosage, was found to be 0.3 g, which resulted in

an increase in percentage removal of As(III) from 42.17 to

96.58%. This was due to a higher dose of adsorbent utilized

which provides greater proportion of adsorption sites for

As(III) to bind on the MPKSF surface (Yao et al. 2014); (4)

effect of pH, in this case, the effect of the initial solution

pH in the range of 6–8 on the adsorption of As(III) onto the

MPKSF, indicated that the removal of As(III) was better

under pH 6 and 7, (5) effect of temperature, in this case, a

temperature of 30 �C was considered preferable for the

adsorption of the As(III) onto the MPKSF.

The residual arsenic solution was analyzed using gra-

phite furnace atomic absorption spectrometry (GFAAS).

And the same procedures were repeated using a real water

waste sample from Skudai River, Johor Bahru Malaysia,

spiked with arsenite to analyze the percentage of arsenite

removal using MPKSF.

Determination of pHpzc (point of zero charge) of the

samples The pHpzc of both PKSAC and MPKSF was

determined. A 50-mL solution of 0.01 M NaCl was placed

in a closed Erlenmeyer flask. The pH of the solution was

adjusted to achieve a suspension pH of between 2 and 12

by adding 0.1 M HCl or 0.1 M NaOH solutions in ten

conical flasks. Approximately 0.15 g each of the PKSAC

and MPKSF were added and the final pH was measured

after 48 h. The pH of each solution recorded was plotted.

The intersection of pHinitial and pHfinal of the solution was

then taken as the pHpzc.

Desorption procedure of magnetic activated carbon

(MPKSF) The optimum amount of MPKSF loaded with

arsenite (48.08 lg g-1) obtained after the adsorption pro-

cess was then added into a 50 mL of distilled water in a

centrifuge tube. The solution was shaken at 150 rpm and

then agitated at specific time intervals for up to 48 h. The

solution was then centrifuged and the supernatant was

collected for further analysis to examine the concentration

of As(III) desorbed from the MPKSF. The desorption

procedure was repeated three times and the MPKSF

adsorbent was reserved for further analysis.

Graphite furnace atomic absorption spectrometry

(GFAAS) The detection and the concentration of arsen-

ite were conducted by a graphite furnace atomic absorp-

tion spectrometry (GFAAS). The samples were analyzed

in triplicates, to obtain the optimum result of

10–40 lg L-1. A small amount of the sample which was

in the range of 20–100 lL was placed into the graphite

tube manually. Further, the arsenite samples were acidi-

fied with nitric acid to a pH of less than 2. Upon injection

of the samples into the graphite tube, they are vaporized.

Subsequently, the amount of light energy absorbed in the

vapor was considered to be proportional to the atomic

concentration.

Box–Behnken design

In this study, the Box–Behnken design was used to opti-

mize the number of experiments to be conducted with the

Appl Water Sci (2017) 7:4573–4591 4575

123

aim of determining the probable interactions between the

parameters under study and their influence on the adsorp-

tion of As(III) onto the MPKSF. Further characteristics of

the Box–Behnken design has been described elsewhere

(Kumar et al. 2008).

In Table 1, the experimental parameters and a

3-level 5-factorial Box–Behnken experimental design

are presented. This was applied to examine and validate

the adsorption system parameters influencing the

adsorption of As(III) onto the MPKSF. Contact time

(5–720 min), pH (6–8), adsorbent dose (0.05–0.30 g),

initial As(III) concentration (5–100 lg L-1), tempera-

ture (20–40 �C) represent the variable input parameters.

The factor levels (3) were coded as (-1 = low,

0 = medium level or central point, and ?1 = high

level). Response surface methodology (RSM) was

applied to the experimental data using the Design

Expert statistical software version 7.1.6 by Stat-Ease,

Inc., Minneapolis, USA.

The regression equation of the designed experiment was

obtained by applying four models namely linear, interac-

tive, quadratic and cubic models which were fitted to the

experimental data obtained from the design system.

To select the best model, i.e., after the responses have

been recorded, the data were analyzed using three different

tests in order to decide the adequacy of the models stated

above to represent the adsorption process of MPKSF–As.

These validation tests are the sequential model sum of

squares (F test), lack-of-fit test and the model summary

statistics. Further, a quadratic polynomial was used to

explain the relationship between the parameters and As(III)

residual concentration (%).

The second-order polynomial is represented by Eq. (1):

� ¼ b0 þXk

i�1

bixi þXk

i�1

biix2i þ

Xk

1sisj

bijxixj þ e; ð1Þ

where the terms have their established meanings (Ku-

namneni and Singh 2005; Shehzad et al. 2016). A design of

46 tests was formulated.

Results and discussion

Characterization of adsorbents

The characterization results (detailed characterization of

the adsorbent (s) has been described elsewhere, Anyika

et al. 2017) revealed that the MPKSF presented better

characteristics and was therefore selected as the sole

adsorbent for the adsorption studies. It presented a higher

BET surface area of 257 m2 g-1, higher pore volume of

0.1124 cc g-1 and higher magnetic properties with a

magnetic saturation of 49.55 emu g-1. The FTIR spectrum

of the MPKSF exhibited intense OH bending at 1629 cm-1

which can be attributed to the presence of oxygen in the

samples, while the absorption bands at 1093 and 579 cm-1

indicated the presence of C–O stretching and metal–oxy-

gen (M–O) bands due to the interaction of iron and oxygen.

To illustrate that the MPKSF acquired magnetic properties,

the XRD data were analyzed. The MPKSF exhibited the

presence of Fe3O4 at 2h 30.75�, 35.95�, 57.35�, 63.20�from the XRD diffractogram. Similarly, three peaks at 2h19.30�, 43.45�, 54.10� and a peak at 2h 24.40� which can

be assigned to c-Fe2O3 and a-Fe2O3, respectively, were

detected.

The point of zero charge of the MPKSF was determined

to explain the surface charge phenomena as well as the

magnetic properties of the MPKSF. With respect to the

surface charge, the point of zero charge of MPKSF

occurred at pH 5.94. Since As(III) is positively charged, it

is critical for the surface of the MPKSF to be negatively

charged in order for the adsorption process to occur. As the

solution pH is higher than 5.94, the surface of the MPKSF

exhibits greater formation of hydroxide ions. However, at

pH 6 and 7, the MPKSF demonstrated greater adsorption

compared to pH 8. This was presumed to have resulted

from the formation of As(III) precipitate at a higher pH

hence reducing the adsorption efficiency. Additionally, at

higher pH, As(III) has the potential to be oxidized to

As(V), which may significantly reduce the adsorption of

As(III) onto the MPKSF (Vance 2002). In Table 2, the

Table 1 Independent parameters and their levels used for Box–Behnken design

Parameters, unit Factors Levels

-1 0 1

Contact time (min) A 5 362.5 720

pH B 6 7 8

Adsorbent dosage (g) C 0.05 0.17 0.30

Initial As concentration (lg L-1) D 5 52.5 100

Temperature (�C) E 20 30 40

4576 Appl Water Sci (2017) 7:4573–4591

123

initial pH and final pH of the reaction solutions for both

adsorbent samples, PKSAC and MPKSF are presented.

In Fig. 1a, b, a plot of pHfinal versus pHinitial for adsorbent

PKSAC and MPKSF is presented. From the graph, the point

of zero charge (pHpzc) of the sample represents the point

where the plot of final pH versus initial pH intersects with

the line at which the final pH equals to the initial pH. The

blue line in both graphs indicates the line of pHfinal = -

pHinitial, while the red and green curves indicate the plots of

pHfinal against pHinitial for PKSAC and MPKSF, respec-

tively. Figure 1a shows that the pHpzc of PKSAC adsorbent

was 3.94, which indicated that the sample was acidic, due to

the impregnation of PKSAC using phosphoric acid

(H3PO4). A suitable acid activation results in the production

of high quality and high surface area AC (Yakout and

Sharaf El-Deen 2016). In the adsorption process of As(III)

onto MPKSF, at a pH above the point of zero charge of the

MPKSF, i.e., pHpzc = 5.94, its surface becomes negatively

charged, hence the protonated As(III) will have a greater

affinity towards the MPKSF surface. Based on the Box–

Behnken optimization, the optimum adsorption occurred at

pH 6.55, which conformed to the aquatic environmental

range of pH of 5–9 (Zou et al. 2016).

As seen in Fig. 1b, the pHpzc value of MPKSF

decreased its acidity to attain a pH of 5.94. This may be

due to the iron oxide extracted from the ferric chlo-

ride/ferrous sulfate solution (FeOF) which was used in the

production of MPKSF. To further illustrate that the

MPKSF acquired magnetic properties, the hydration of

Fe3O4 in aqueous solution resulted in the formation of a-

Fe2O3 in an acidic condition as depicted by the reaction in

Eq. (2). To illustrate that a-Fe2O3 was formed, previous

studies have reported that the pHpzc for the untreated

Fe3O4 was 6.5 while for c-Fe2O3, it was at pH 5.9

(Milonjic et al. 1983) which indicate that the value of pH

for both was nearly acidic even in the untreated condition.

Further, Schwertmann and Murad (1983) had reported that

a-Fe2O3 is predominantly formed at pH 7–8. In this study,

it was demonstrated that upon impregnation of the PKSAC

with FeOF, the acidity of the modified PKSAC (MPKSF)

was reduced from 3.94 to 5.94 due to the formation of a-

Fe2O3 from the reaction of Fe3O4 with water as represented

by the Eq. (2) below:

2Fe3O4 þ H2O � 3Fe2O3 þ 2Hþ þ 2e�: ð2Þ

Box–Behnken statistical analysis

In Table 3, the most important parameters influencing the

efficiency of adsorption of As(III) onto the MPKSF are

represented by the letters: A, B, C, D and E, which repre-

sents the coded symbols for the respective factors: contact

time, pH, adsorbent dosage, initial As(III) concentration

and the temperature parameters. The combined effects of

these factors were evaluated by performing experiments on

the different combinations of these parameters.

The applied Box–Behnken model can be expressed as

Eq. (3):

Y ¼ X0 þ X1Aþ X2Bþ X3C þ X4Dþ X5E þ X6AB

þ X7AC þ X8ADþ X9AE þ X10BC þ X11BD

þ X12BE þ X13CDþ X14CE þ X15DE þ X16A2

þ X17B2 þ X18C

2 þ X19D2 þ X20E

2; ð3Þ

where Y is the response, X0 and Xi depicted the global mean

and other regression coefficients, respectively, while A, B,

C, D and E are the coded symbols for the respective fac-

tors: contact time, pH, adsorbent dosage, initial As(III)

concentration and the temperature parameters.

In Table 4, the statistical significance of the ratio of

mean square variation due to regression and mean square

Fig. 1 a Graph of pHfinal versus pHinitial for the adsorbent PKSAC

suspension. b Graph of pHfinal versus pHinitial graph for the adsorbent

MPKSF suspension

Table 2 Initial and final pH value of PKSAC and MPKSF reaction

solutions

Initial pH Final pH

PKSAC MPKSF

4.00 3.94a 4.26

6.00 4.22 5.94a

8.00 4.42 5.31

10.00 4.29 6.54

12.00 11.41 11.59

a Point of zero charge

Appl Water Sci (2017) 7:4573–4591 4577

123

Table 3 Experimental, actual and predicted values of Y for As(III) onto MPKSF

Standard run order A B C D E Actual value Predicted value

1 5 6 0.17 52.5 30 86.93 86.91

2 720 6 0.17 52.5 30 99.81 99.80

3 5 8 0.17 52.5 30 81.67 81.65

4 720 8 0.17 52.5 30 93.96 93.94

5 362.50 7 0.05 5 30 57.41 57.43

6 362.50 7 0.3 5 30 51.48 51.50

7 362.50 7 0.05 100 30 18.24 18.22

8 362.50 7 0.3 100 30 92.46 92.44

9 362.50 6 0.17 52.5 20 91.63 91.61

10 362.50 8 0.17 52.5 20 88.46 88.47

11 362.50 6 0.17 52.5 40 96.80 96.78

12 362.50 8 0.17 52.5 40 88.80 88.81

13 5 7 0.05 52.5 30 28.03 28.05

14 720 7 0.05 52.5 30 70.81 70.81

15 5 7 0.3 52.5 30 92.36 92.37

16 720 7 0.3 52.5 30 74.79 74.78

17 362.50 7 0.17 5 20 79.93 79.91

18 362.50 7 0.17 100 20 83.05 83.05

19 362.50 7 0.17 5 40 84.96 84.94

20 362.50 7 0.17 100 40 83.52 83.53

21 362.50 6 0.05 52.5 30 63.35 63.36

22 362.50 8 0.05 52.5 30 29.41 29.41

23 362.50 6 0.3 52.5 30 69.10 69.11

24 362.50 8 0.3 52.5 30 91.95 91.95

25 5 7 0.17 5 30 78.30 78.28

26 720 7 0.17 5 30 84.89 84.89

27 5 7 0.17 100 30 73.18 73.17

28 720 7 0.17 100 30 91.72 91.73

29 362.50 7 0.05 52.5 20 35.69 35.68

30 362.50 7 0.3 52.5 20 96.26 96.26

31 362.50 7 0.05 52.5 40 64.88 64.86

32 362.50 7 0.3 52.5 40 72.58 72.57

33 5 7 0.17 52.5 20 87.86 87.88

34 720 7 0.17 52.5 20 98.26 98.28

35 5 7 0.17 52.5 40 88.42 88.45

36 720 7 0.17 52.5 40 103.20 103.22

37 362.50 6 0.17 5 30 81.57 81.59

38 362.50 8 0.17 5 30 75.49 75.50

39 362.50 6 0.17 100 30 81.90 81.92

40 362.50 8 0.17 100 30 76.89 76.90

41 362.50 7 0.17 52.5 30 85.53 85.53

42 362.50 7 0.17 52.5 30 85.52 85.53

43 362.50 7 0.17 52.5 30 85.51 85.53

44 362.50 7 0.17 52.5 30 85.54 85.53

45 362.50 7 0.17 52.5 30 85.52 85.53

46 362.50 7 0.17 52.5 30 85.53 85.53

4578 Appl Water Sci (2017) 7:4573–4591

123

residual error was tested using ANOVA. The results of the

ANOVA indicated that the F values obtained for all the

regressions were higher, which indicated that that majority

of the variation in the response can be explained by the

regression equation (Kumar et al. 2008). To determine

whether the F is large enough to result in a statistical

significance, the p value is examined. In this case, the

model is considered to be statistically significant if the

values under the column p[F value is \0.05 (Table 4)

(Segurola et al. 1999). The ANOVA result for the MPKSF–

As design system shows that the F value of 99.63 and its

p value of \0.05 imply that the model was significant

towards the response. Hence in this analysis, A, B, C, D,

AC, AD, BC, CD, CE, C2 and D2 were the significant terms

(Table 4). Besides, the ANOVA results for the MPKSF–As

adsorption system showed that the F value is 99.63

(Table 4), indicating that the terms on the model are having

a significant effect on the response.

In Table 5, the adequacy of the model for the adsorption

of As(III) onto the MPKSF was determined by three tests:

(1) sequential model sum of squares; (2) lack-of-fit tests;

(3) model summary statistics. The results showed that the

p value for majority of the regression were \0.05. This

implied that one of the terms in the regression equation was

significantly correlated to the response variable. Further,

the quadratic model was found to yield the best fit of R2,

Adjusted R2 and predicted R2 values of 0.9876, 0.9777 and

0.9505, respectively (Table 5). This also implied that the

model does not explain 1% of the experimental results.

Again, the high R2 values in Table 5 and the p value of

\0.0001 in Table 4, indicate that the quadratic polynomial

was highly significant in explaining the relationship

between the parameters and As(III) residual concentration

(%).

In Table 5, since the cubic model was established to be

aliased, the quadratic model was therefore, chosen to be

used for further analysis. Further, under the lack of fit

(Table 5) the F value is not significant, with an F value of

4.79 and the p value of 0.0687. This shows that the lack of

fit was not significant relative to the pure error. However,

the lack-of-fit value indicates that there is 6.87% possibility

that the error resulted from noise. Thus, the non-significant

Table 4 ANOVA for response surface quadratic model (Y)

Source Sum of squares Df Mean square F value p value (p[F)

Model 6726.26 20 335.36 99.63 \0.0001

A: contact time 218.30 1 218.30 64.85 \0.0001

B: pH 32.86 1 32.86 9.76 0.0045

C: adsorbent dosage 2084.38 1 2084.38 619.20 \0.0001

D: initial conc. As 2242.50 1 2242.50 666.17 \0.0001

E: temperature 5.87 1 5.87 1.74 0.1987

AB 0.03 1 0.03 9.098E-003 0.9248

AC 248.38 1 248.38 73.78 \0.0001

AD 40.51 1 40.51 12.04 0.0019

AE 2.89 1 2.89 0.86 0.3630

BC 221.71 1 221.71 65.86 \0.0001

BD 4.49 1 4.49 1.34 0.2588

BE 1.59 1 1.59 0.47 0.4986

CD 732.51 1 732.51 217.61 \0.0001

CE 190.85 1 190.85 56.70 \0.0001

DE 0.26 1 0.26 0.08 0.7833

A2 0.34 1 0.34 0.10 0.7516

B2 1.85 1 1.85 0.55 0.4654

C2 493.91 1 493.91 146.73 \0.0001

D2 120.20 1 120.20 35.71 \0.0001

E2 6.70 1 6.70 1.99 0.1706

Residual 84.16 25 3.37

Lack of fit 84.11 20 48.21 4.79 0.0687

Pure error 0.04 5

Corr. total 7154.40 45

Appl Water Sci (2017) 7:4573–4591 4579

123

value for the lack of fit showed that the model was valid for

further analysis. The final mathematical equation given by

the Box–Behnken design in terms of actual values deter-

mined by Design Expert software is presented in Eq. (4).

Removal of As(III) (%) is given by:

Y ¼ 85:53 þ 6:29 � A� 2:78 � Bþ 17:07 � C

þ 0:43 � Dþ 1:38 � E � 0:15 � A� B

� 15:09 � A� C þ 2:99 � A� Dþ 1:09 � A� E

þ 14:20 � B� C þ 0:27 � B� D� 1:21 � B� E

þ 20:04 � C � D� 13:22 � C � E � 1:14 � D� E

þ 4:05 � A2 þ 1:01 � B2 � 23:07 � C2 � 7:56 � D2

þ 4:89 � E2: ð4Þ

The equation implied that the constant with a value of

85.53 (see Eq. (4)) which is independent of any factor or

interaction between factors suggests that the average

removal of As(III) by MPKSF was 85.53%. Although

this average removal is independent of the factors in the

experimental setup (Kumar et al. 2008). In addition,

Eq. (4) shows that pH and initial As(III) concentration

had a positive effect while contact time, temperature and

adsorbent dosage had a negative effect on the adsorption

percentage of As(III) by MPKSF. In general, the positive

sign represents the synergistic effect and the negative sign

shows the antagonistic effect on the adsorption process

(Tan et al. 2008). Further, the positive value of the model

term indicates the effect that favors the optimization

process while the negative model term value represents the

inverse interaction between the parameters with the

response.

Adsorption studies

In this study, the Box–Behnken design was used for the

optimization of the selected parameters. Thus, the effects

of these parameters on the MPKSF–As adsorption was

shown by the response surface plots using two parameters

simultaneously while the remaining parameters were set at

their center points. However, it should be noted that since

two parameters were used simultaneously, hence, only one

set of the figures were presented per variable, while the rest

were referred to accordingly under the affected variables,

i.e., from ‘‘Effect of temperature’’ to ‘‘Effect of contact

time’’.

Effect of selected variables and response surface plots

Effect of pH As seen in Fig. 2a–d, the combined effects

of pH with contact time, initial As(III) concentration,

adsorbent dosage and temperature, respectively, were

Table 5 Adequacy of the model tested for MPKSF–As design system

Source Sum of squares Df Mean square F p value (p[F) Remark

Sequential model sum of squares

Mean vs total 5863.8 1 5863.8 – – –

Linear vs mean 4583.91 5 916.78 16.61 \0.0001 –

2FI vs linear 1443.23 10 144.32 5.67 \0.0001

Quadratic vs 2FI 680.13 5 136.03 40.41 \0.0001 Suggested

Cubic vs quadratic 59.05 15 3.94 1.57 0.2386 Aliased

Residual 25.10 10 2.51 – – –

Total 12,655.22 46 275.11 – – –

Lack-of-fit tests

Linear 2207.47 35 63.07 7345.18 \0.0001 –

2FI 764.24 25 30.57 3560.12 \0.0001 –

Quadratic 84.11 20 48.21 4.79 0.0687 Suggested

Cubic 25.06 5 5.01 583.72 \0.0001 Aliased

Pure error 0.043 5 8.587E-003 – –

Source Std. dev. R2 Adjusted R2 Predicted R2 PRESS Remark

Model summary statistics

Linear 7.43 0.6750 0.6343 0.5575 3005.16 –

2FI 5.05 0.8875 0.8312 0.7113 1960.64 –

Quadratic 1.83 0.9876 0.9777 0.9505 336.51 Suggested

Cubic 1.58 0.9963 0.9834 0.7638 1603.97 Aliased

4580 Appl Water Sci (2017) 7:4573–4591

123

presented. From the responses, the lower value of As(III)

residual indicates a higher percentage of removal as given

by Eq. (5):

% Removal ¼ Cinitial � Cfinal

Cinitial

� �� 100%: ð5Þ

Figure 2a shows the interaction between the effect of pH

and the contact time of As(III) solution with the MPKSF

adsorbent. The optimum removal of As occurred at pH 6.55

and at a contact time of 44.73 min with a removal percentage

of 94.76% as the other parameters were set to their center

points. Figure 2b illustrates the interaction of initial As(III)

concentration with pH with the optimum removal of As(III)

achieved at pH 6.55 at an initial As(III) concentration of

5 lg L-1 with a removal percentage of 99.10%.

Further, Fig. 2c, d shows the relationships between pH

with adsorbent dosage and temperature, respectively. The

excluded parameters for the respective response surface

plots remain at their center points. Figure 2c shows that the

removal of As(III) attained a maximum of 100% at pH 8

with an adsorbent dose of 0.3 g, while the other parameters

were at their center points. In Fig. 2d, the relationship

between pH and temperature gives an optimum removal of

91.89% at pH 6 and a temperature of 40 �C.

As reported previously, the pHpzc value for MPKSF

was 5.94. At a pH above the pHpzc value, the As(III)

cations are attracted to the negatively charged surface of

MPKSF, which resulted from the deprotonation of MPKSF

hydroxyl groups. Meanwhile, at pH lower than pHpzc, the

adsorption of As(III) cations occurs only slightly because

the surface of MPKSF is positively charged due to the

protonation of MPKSF hydroxyl group. Thus, a solution

pH of 6.55 provides the optimum condition for adsorption

of MPKSF–As.

Effect of temperature

The experiment was conducted within a temperature range

of 20–40 �C. As seen in Fig. 2d (refer to Fig. 2d in ‘‘Effect

of pH’’ above for the effect of temperature and pH) and 3a–

c, the relationships between temperature and pH, contact

time, adsorbent dose and initial concentration, respectively,

are presented. All the four response surface plots show that

the removal of As(III) increases with temperature. Thus,

the adsorption process could be described as an endother-

mic process, as the removal of As(III) is optimum at higher

temperatures with efficiency of 91–100% removal.

5.00 183.75

362.50 541.25

720.00

6.00

6.50

7.00

7.50

8.00

81

85.75

90.5

95.25

100

Rem

oval

%

A: contact time

B: pH

6.00

6.50

7.00

7.50

8.00 5.00 28.75

52.50 76.25

100.00

75

78.75

82.5

86.25

90

Rem

oval

%

B: pH

D: initial conc As

6.00

6.50

7.00

7.50

8.00

0.05 0.11

0.17 0.24

0.30

29

45.5

62

78.5

95

Rem

oval

%

B: pH C: adsorbent dosage

6.00

6.50

7.00

7.50

8.00 20.00 25.00

30.00 35.00

40.00

83

86.5

90

93.5

97

Rem

oval

% B: pH

E: temperature

ab

cd

Fig. 2 3D response surface plots for As(III) removal vs a contact

time (5–720 min) and the pH of the adsorbent MPKSF suspension

(6–8); b initial As concentration (5–100 lg L-1) and pH (6–8);

c adsorbent dose (0.05–0.3 g) and the pH of the adsorbent MPKSF

suspension (6–8); d temperature (20–40 �C) and the pH of the

adsorbent MPKSF suspension (6–8)

Appl Water Sci (2017) 7:4573–4591 4581

123

Effect of adsorbent dosage

Refer to Fig. 2c under ‘‘Effect of pH’’ and Fig. 3b under

‘‘Effect of temperature’’ above and Fig. 4a, b under ‘‘Effect

of contact time’’ below, for the combined effect of adsor-

bent (MPKSF) dose with pH, temperature, contact time and

initial As(III) concentration, respectively. The results

showed that the percentage removal increases as the

adsorbent dose increases. The increase in the amount of

adsorbent used can be attributed to the formation of a

greater surface area and provides more available vacant

sites for the adsorption of As(III) on MPKSF surface.

Effect of initial As(III) concentration

Adsorption of MPKSF–As was carried out with different

initial concentrations ranging from 5 to 100 lg L-1. Refer

to Fig. 2b under ‘‘Effect of pH’’ and 3c under ‘‘Effect of

temperature’’ above, and Fig. 4b, c, under ‘‘Effect of

contact time’’ below. The results illustrates the combined

effect of initial As(III) concentration with pH of the

adsorbent MPKSF suspension, temperature, adsorbent dose

and contact time, respectively, while two parameters are

kept constant. As shown by the response surface plots, it

revealed that as the initial concentration increases, some of

the percentage removal data showed increasing pattern

while other percentages removal data became slightly

decreased. In this case, other parameters need to be con-

sidered especially the adsorbent dosage which provides the

information on the available vacant adsorption sites for

adsorption of As(III) at higher concentrations.

Effect of contact time

Refer to Fig. 2a (under ‘‘Effect of pH’’, and Fig. 3a under

‘‘Effect of temperature’’) above, and Fig. 4a, c below. The

results illustrate the combined response surface plots of

contact time with pH of the adsorbent MPKSF suspension,

temperature, adsorbent dosage and initial As concentration,

respectively. It shows that the increase in contact time

duration causes the removal percentages of As(III) to

increase. As the contact time increased, the adsorbate had

enough time to disperse and be adsorbed onto the surface

and into the pores of MPKSF until the adsorption reached

equilibrium at an optimum contact time.

Optimization using desirability function

A desirable value for each input parameter and the

response can be selected. The multiple response methods

5.00 183.75

362.50 541.25

720.00

20.00 25.00 30.00 35.00 40.00

83

88.25

93.5

98.75

104

Rem

oval

%

A: contact time E: temperature

0.05

0.11

0.17

0.24

0.30

20.00 25.00

30.00 35.00

40.00

35

51

67

83

99

Rem

oval

%

C: adsorbent dosage E: temperature

Rem

oval

%

5.00

28.75

52.50

76.25

100.00 20.00

25.00

30.00

35.00

40.00

77

80.75

84.5

88.25

92

D: initial conc As E: temperature

ab

c

Fig. 3 3D response surface plots for As(III) removal vs a temperature (20–40 �C) and contact time (5–720 min); b temperature (20–40 �C) and

adsorbent dosage (0.05–0.3 g); c temperature (20–40 �C) and initial As(III) concentration (5–100 lg L-1)

4582 Appl Water Sci (2017) 7:4573–4591

123

was applied to obtain the optimum condition for the five

parameters used including contact time, pH, adsorbent

dose, initial As(III) concentration and temperature. The

numerical optimization reveals the points that maximize

the desirability function. In this study, the maximum level

for As(III) removal was set for desirability at a minimum

level of contact time (5 min), minimum adsorbent dosage

(0.05 g), the maximum level of initial As(III) concentration

(100 lg L-1), the level of solution pH in the range of 6–8

and the level of temperature within a range of 20–40 �C(data not shown for the desirability ramp).

Using Eq. (5) reported previously to determine the

removal percentage, 96% removal of As(III) was achieved

using the optimal conditions at contact time of 45 min,

initial As(III) concentration maximized to 100 lg L-1,

adsorbent dosage of 0.29 g, initial solution pH of 6.55 and

the temperature at 26 �C as shown in Table 6. Experi-

mentally, these optimum values are applied in a verifica-

tion experiment and resulted in 95% removal of As(III) by

MPKSF adsorbent with a percentage error of 1% as com-

pared to the predicted removal of As(III). Thus, this indi-

cated that the Box–Behnken design model is reliable for

the optimization of various parameters used in an adsorp-

tion process.

Adsorption equilibrium studies of As(III)

onto MPKSF adsorbent

Adsorption isotherm models

In this study, several isotherm models were applied to

determine the type of adsorption that occurred on the

magnetic activated carbon surface, by fitting the data to the

Langmuir, Freundlich, and Temkin isotherm models. The

correlation coefficient, R2 obtained from the isothermal

5.00

183.75

362.50

541.25

720.00

0.05 0.11

0.17 0.24

0.30

28

45

62

79

96

Rem

oval

%

A: contact time

C: adsorbent dosage 0.05 0.11

0.17 0.24

0.30

5.00

28.75

52.50

76.25

100.00

18

37

56

75

94

C: adsorbent dosage D: initial conc As

Rem

oval

%

5.00 183.75

362.50 541.25

720.00 5.00 28.75

52.50 76.25

100.00

73

79

85

91

97

Rem

oval

%

A: contact time D: initial conc As

ab

c

Fig. 4 3D response surface plots for As(III) removal vs a contact time (5–720 min) and adsorbent dose (0.05–0.3 g); b initial As(III)

concentration (5–100 lg L-1) and adsorbent dose (0.05–0.3 g); c initial As(III) concentration (5–100 lg L-1) and contact time (5–720 min)

Table 6 Optimal condition and model validation for As(III) removal

by MPKSF

Parameters Optimal conditions

Contact time (min) 45

pH 6.55

Adsorbent dosage (g) 0.29

Initial As(III) concentration (lg L-1) 100.00

Temperature (�C) 26

Removal (%)

Predicted 96

Experimental 95

Error (%) 1

Appl Water Sci (2017) 7:4573–4591 4583

123

plots was used to identify the isotherm model that best

described the adsorption of As(III) onto MPKSF.

The Langmuir adsorption isotherm model

The Langmuir is represented by the following equations;

(1) Eq. (6) which was formulated based on the kinetic

theory; (2) Eq. (7) depicts the equation for the value of

adsorbate adsorbed on the adsorbent; (3) the linear form of

the Langmuir equation is given by Eq. (8):

x

m¼ qe ¼

qmaxKLCe

1 þ KLCe

; ð6Þ

qe ¼Co � Ceð ÞV

m; ð7Þ

Ce

qe

¼ Ce

qmax

þ 1

qmaxKL

; ð8Þ

where x = mass of adsorbate adsorbed (lg), m = mass of

adsorbent (g), V = volume of solution used for adsorption

process (L), Ce = equilibrium concentration (lg L-1),

Co = initial concentration (lg L-1), qe = amount of

adsorbate adsorbed at equilibrium (lg g-1), qmax = max-

imum adsorption at monolayer coverage, KL = Langmuir

isotherm constant (L lg-1).

To assess the Langmuir isotherm model, a graph of Ce/

qe against Ce is plotted, a straight line is obtained and the

values of KL and qmax are computed from the values of the

slope and intercept of the graph. The fitted isotherm in this

study was illustrated by a plot of equilibrium concentration

of adsorbate Ce on the adsorbent, qe, i.e., Ce/qe against the

equilibrium concentration of adsorbate, Ce. Figure 5

depicts the fitted As(III) adsorption isotherm which indi-

cated the presence of a linear part of plotted curve at low

Ce followed by a slight curvature around Ce = 2.56 -

lg L-1 towards the completion of coverage of the mono-

layer with the R2 value of 0.9599. Hence, this indicated that

adsorption of arsenite onto MPKSF is L-type (having no

strict plateau). A similar observation was reported by El-

Said et al. (2009) whose study demonstrated the adsorption

of As(III) and As(V) using Nigella sativa L.

Further, the inherent feature of this isotherm namely a

dimensionless constant separation factor, RL can be

expressed as an equilibrium parameter. The parameter is

calculated using Eq. (9). The effect of the separation factor

on the adsorption nature of the Langmuir isotherm is

summarized in Table 7.

RL ¼ 1

1 þ KLCo

; ð9Þ

where Co = initial concentration of surfactant (mg dm-3),

KL = Langmuir isotherm constant.

To illustrate, that the homogeneous adsorption process

predicted by the Langmuir isotherm assumes a monolayer

adsorption of molecules onto the surface of the adsorbent.

The Langmuir linear plot of the specific adsorption (Ce/qe)

against the equilibrium concentration (Ce) for adsorption of

As(III) onto MPKSF is depicted in Fig. 6. The Ce was

obtained after 180 min. The correlation coefficient (R2)

obtained from the Langmuir isotherm is 0.9973, which

indicates that the adsorption data of the As(III) onto the

MPKSF surface was well fitted to the Langmuir isotherm

model. The monolayer adsorption capacity (qmax) of

As(III) onto MPKSF was found to be 48.08 lg g-1.

A dimensionless equilibrium parameter, RL was used to

express the nature of adsorption of the Langmuir isotherm.

The RL value calculated from the adsorption data is 0.0765

indicating that the adsorption of As(III) on MPKSF was a

favorable process as the RL value lies between 0 to 1

Table 7 Effect of separation factor on Langmuir isotherm

RL Adsorption nature of isotherm

RL[ 1 Unfavorable

RL = 1 Linear

0\RL\ 1 Favorable

RL = 0 Irreversible

Fig. 6 Langmuir isotherm representing the variation of specific

adsorption (Ce/qe) against the equilibrium concentration (Ce) for

adsorption of As(III) onto MPKSF

Fig. 5 Langmuir equilibrium isotherm for the adsorption of As(III)

onto MPKSF

4584 Appl Water Sci (2017) 7:4573–4591

123

(0\RL\ 1) at an equilibrium temperature of 30 �C(Table 7).

The Freundlich isotherm model

This isotherm was applied as an empirical model which

considers the data often fit to the empirical equation stated

in Eq. (10):

x

m¼ qe ¼ KFC

1ne; ð10Þ

where x = weight of solute adsorbed, m = weight of

adsorbent, Ce = equilibrium concentration of adsorbate

(mg L-1), qe = amount of solute adsorbed at equilibrium

(mg g-1), KF and n = Freundlich isotherm constant

(mg g-1)

From the isotherm, the n value reveals the nature of the

adsorption, i.e., how favorable the adsorption process was.

The value of n was used as the linearity parameter implying

that if the n value lies between one and ten, this indicates a

favorable sorption process of the adsorbent (Freundlich

1906). The effect of the n value on the nature of adsorption

as represented by the Freundlich isotherm is shown in

Table 4. The linear form of the equation is given by

Eq. (11). A graph of log (x/m) against log Ce results in a

straight line at which the value slope and intercept are the

value of 1/n and log KF, respectively.

logx

m¼ log qe ¼ logKF þ

1

nlogCe: ð11Þ

A multilayer adsorption for the heterogeneous surface is

indicated by the Freundlich isotherm model. The

heterogeneous system of adsorption assumes that there is

no formation of monolayer adsorption of As(III) on

MPKSF. Figure 7 illustrates the plot of log qe against log

Ce. The correlation coefficient, R2 obtained from the graph

illustrates that the Freundlich isotherm model is not well

fitted to the adsorption data with an R2 value of 0.9744. The

Freundlich constants, KF and the n value are shown in

Table 8.

The slope of the Freundlich isotherm (Fig. 7) shows that

the 1/n value is less than 1 and this has shown that the

process is a favorable physical adsorption process. The

smaller 1/n value indicates that a strong bond is present

between the adsorbent and adsorbate molecules (Okeola

and Odebunmi 2010).

In the paper by Mayo et al. (2007), it was demonstrated

that the adsorption of As(III) and As(V) onto Fe3O4

nanoparticles exhibits the surface complexation reaction by

forming either inner-sphere monodentate or bidentate-bin-

uclear complex with iron oxide (Fe3O4). A similar reaction

of magnesium with iron oxide, i.e., Fe3O4 and c-Fe2O3 was

reported by Jolstera et al. (2012). The proposed adsorption

of As(III) and Fe3O4 in MPKSF is depicted in Fig. 8 and

supported by the FTIR analysis of the MPKSF–As.

It is well known that sodium arsenite can be represented

as sodium ortho-arsenite (Na3AsO3) and sodium meta-

arsenite (NaAsO2), the latter was used in this study for

adsorption. Based on Fig. 9, MPKSF reacted with arsenous

acid (H3AsO3) which is produced according to the chem-

ical reaction (Eq. 11). Firstly, sodium meta-arsenite reacts

with water producing arsenic trioxide with sodium ions and

hydroxide ions followed by the slow hydrolysis of arsenic

trioxide. The reaction proceeds in a basic condition, finally

producing arsenous acid:

2NaAsO2 þ H2O ! As2O3 þ 2Naþ

þ 2OH��������!slow hydrolysis2H3AsO3: ð12Þ

Liu et al. (2015) reported that magnetite (Fe3O4) is

known as a mixture of two iron oxides which are composed

of 67% of Fe(III) and 33% of Fe(II). In the adsorption

study, the major Fe(III) species was reported to interact

with As(III) to form either inner-sphere monodentate or

bidentate-binuclear complex. This can be attributed to the

oxidation reaction which takes place in the presence of

oxygen under atmospheric experimental conditions hence

suggesting the oxidation of Fe(II) to Fe(III).

Temkin isotherm

The Temkin isotherm model was applied to consider the

effect of the interactions between the adsorbent and the

adsorbate on an adsorption isotherm. The model assumes

Table 8 Effect of n values on Freundlich isotherm

n value Adsorption nature of isotherm

n = 1 Linear

n\ 1 Chemical process

n[ 1 Physical process

Fig. 7 Freundlich isotherm indicating the variation of log qe with

respect to log Ce for adsorption of As(III) onto MPKSF

Appl Water Sci (2017) 7:4573–4591 4585

123

that the heat of adsorption of the molecules present in the

adsorbed layer is reducing linearly with the coverage of the

molecules instead of in logarithmic pattern due to this

interaction (Temkin 1941). This means that as the coverage

of adsorbed layer increased, the heat of adsorption

decreased.

Temkin isotherm is given by Eq. (13):

qe ¼RT

bT

� �lnðACeÞ: ð13Þ

The linear form of Eq. 13 is given by:

qe ¼RT

blnAþ RT

blnCe: ð14Þ

Substituting RT/b with B and hence,

qe ¼ B lnAþ B lnCe; ð15Þ

where b = Temkin isotherm constant, A = equilibrium

binding constant correspond to maximum binding energy,

R = gas constant (8.314 J mol-1 K-1), T = absolute

temperature, K.

The values of constant A and B are obtained from a plot

of qe against ln Ce.

Figure 9 shows the plot of qe against ln Ce whereby the

slope and intercept values obtained from the graph plot are

used to calculate Temkin constant A, and the heat of

sorption constant B. The R2 value obtained from the linear

plot of Temkin isotherm model is 0.9816 indicating that the

adsorption data are applicable to this model. Similar

observations have been reported by Itodo and Itodo (2010)

on the adsorption of atrazine onto sheanut shell. Similarly

by Hamdaoui and Naffrechoux (2007) whose study

demonstrated the adsorption of phenol and chlorophenol

onto granular activated carbon. Maurya and Mittal (2006)

had also established the linear Temkin plot for the

adsorption of methylene blue and Rhodamine B onto

activated carbon.

Comparing the three correlation coefficients; R2 for the

three isotherms, the Langmuir isotherm gave the best fit of

adsorption isotherm with highest correlation coefficient, R2

value of 0.9973 followed by Temkin (0.9816) and Fre-

undlich (0.9744) isotherm models (Table 9). Langmuir

isotherm implies that adsorption of As(III) onto MPKSF

adsorbent occurs in a monolayer adsorption at which when

the available sorption sites of MPKSF are fully occupied,

no further adsorption process can take place at those sites.

It is corroborated by the formation of inner-sphere com-

plexes between iron oxide and As(III) molecules on

MPKSF surface.

Fig. 8 Proposed adsorption of

As(III) onto MPKSF. (adapted

from Ciuro Juncosa 2008;

O’Reilly et al. 2001)

Fig. 9 Temkin isotherm showing the variation of qe against ln Ce for

adsorption of As(III) onto MPKSF

4586 Appl Water Sci (2017) 7:4573–4591

123

The R2 value for Langmuir and Freundlich are 0.9973

and 0.9744, respectively (Table 9). The Temkin isotherm

shows that the heat of adsorption is low (9.72 J mol-1)

indicating physical adsorption. Furthermore, the calculated

Freundlich (qmax) (54.48 lg g-1) was higher than the

adsorption capacity (qmax = 48.08 lg g-1) determined

from the Langmuir isotherm. Based on the qmax value

obtained, the adsorption of As(III) is more of a physical

adsorption as described by the Freundlich isotherm. Studies

by Liu et al. (2015) reported that the presence of chemical

interaction between As(III) and iron oxide forming inner-

sphere surface complex can be best explained by Langmuir

isotherm, suggesting a monolayer As(III) adsorption onto

the Fe3O4 surface. However, they also reported that the

adsorption of As(III) onto AC showed the best fit with

Freundlich isotherm. Based on the experimental data, it is

suggested that both chemisorption, involving the formation

of inner-sphere complex and physisorption on activated

carbon occurred in the adsorption of As(III) onto MPKSF.

Desorption of As(III) from MPKSF

Desorption experiment was conducted to examine the

reusability of the MPKSF adsorbent and the reversibility of

the adsorption process. Figure 10 illustrates the percentage

desorption of As(III) from MPKSF. During the first stage

of desorption using distilled water, the percentage of

As(III) desorbed in the solution was 0.72% followed by

0.78 and 0.97% at second and third stages of desorption,

respectively. The small amount of As(III) detected after the

first desorption (Des1) was presumably due to the com-

plexation reaction between As(III) ions with the iron in

MPKSF adsorbent (CiuroJuncosa 2008) which prevents the

dissociation of As(III) from iron oxide on the surface of

MPKSF. This is also consistent with the results of the

Langmuir adsorption isotherm, thus explaining the forma-

tion of chemical bonds between As(III) and the surface of

MPKSF which prevent the As(III) from desorbing easily

from MPKSF.

Identification of the proposed mechanism

of the adsorption of As(III) onto the MPKSF

Fourier transform infrared spectroscopy (FTIR) analysis

As stated previously a mechanism was proposed for the

adsorption of As(III) onto the MPKSF, in this section, this

mechanism was identified using the results of the spec-

troscopy studies and supported by the results of the char-

acterization of the MPKSF after the adsorption. Figure 11

shows the FTIR spectrum of MPKSF after adsorption of

As(III) (MPKSF–As). Five peaks could be observed in the

spectrum identified at wavenumber of 3430, 1625, 1387,

1078 and 573 cm-1. These are assigned to the functional

groups OH, C=O, C–C, C–O, and M–O, respectively.

Both spectra of MPKSF before and after adsorption

show the presence of peaks at a wavenumber of 3430, 1625

and 573 cm-1. Additionally, in MPKSF–As, two new

peaks were detected at 1387 cm-1 and around 800 cm-1

which are assigned to C–C bending (Mayo et al. 2007) and

As–O interaction (Ito et al. 1995), respectively, on MPKSF

surface after adsorption. However, the spectrum showed a

less significant band at 800 cm-1. Thus the EDX analysis

data are used to validate the presence of As(III). According

to Sayle (2000), at a pH lower than the pKa value, the

molecules will be mostly protonated. Thus, the negatively

Table 9 Langmuir, Freundlich and Temkin isotherm model param-

eters and correlation coefficient for adsorption of As(III) on MPKSF

Isotherm Parameters

Langmuir qmax (lg g-1) 48.08

KL (L lg-1) 0.1208

R2 0.9973

Freundlich KF 10.86

1/n 0.36

qmax (lg g-1) (calculated) 54.48

R2 0.9744

Temkin A (L g-1) 1.436

B (J mol-1) 9.72

R2 0.9816

Fig. 10 Desorption of As(III)

from MPKSF. Data obtained:

n = 3 for each desorption

experiment

Appl Water Sci (2017) 7:4573–4591 4587

123

charged MPKSF provides greater affinity towards the

protonated As(III) ions at pH lower than 9.2 which result in

the formation of inner-sphere complexes between As(III)

and Fe3O4 in MPKSF. Additionally, Bundschuh et al.

(2005) reported that as the pH increases, the dominant

negative charges are present on adsorbent surface hence

interference with the adsorption of As(III) and As(V) are

significantly governed by the surface charge of adsorbent.

X-ray diffraction (XRD) analysis

Figure 12 illustrates the diffractogram pattern of MPKSF

after adsorption (MPKSF–As). The pattern obtained is

quite similar to the diffractogram of MPKSF before the

adsorption process. There are four intense peaks assigned

to Fe3O4 at 2h 30.45�, 35.75�, 57.85� and 63.00�, mean-

while one peak at 2h 43.35� is assigned to c-Fe2O3,

respectively. The increment in crystallite size of MPKSF–

As is probably due to crystal defect after the adsorption of

As(III) onto the MPKSF. This is consistent with the work

of Mandal et al. (2013) on the adsorption of As(III) by

zirconium polyacrylamide hybrid material. However, no

crystalline As(III) was detected using this analysis, prob-

ably due to the small amount of As(III) adsorbed on

MPKSF.

Scanning electron microscopy (SEM) analysis

The morphology of MPKSF–As under magnification of

15509 is depicted in Fig. 13a illustrating that MPKSF–As

surface experienced a distinct change as compared to

MPKSF. In addition, the presence of a layer of coating on

the surface is presumably assigned to the adsorbed layer of

As(III) complexes on the MPKSF. The entire coverage of

As(III) in the pores as depicted in Fig. 13b under higher

magnification (62009) shows that the adsorption of As(III)

occurs evenly over the surface and in the pores of MPKSF.

Further, it can be seen that As(III) particles do not block

the external pores as it is still observed clearly even after

adsorbing As(III). With respect to both figures, it shows

4000 4003500 3000 2500 2000 1500 1000 500

30

0

2

4

6

8

10

12

14

16

18

20

22

24

26

28

cm-1

%T

3430.14cm-1

1625.71cm-1

635.83cm-1573.35cm-1

3780.52cm-1

3698.94cm-1

1387.99cm-1

1078.32cm-13914.78cm-1

3435.28cm-1

636.19cm-1

579.58cm-1

1629.52cm-1

1093.92cm-1445.06cm-1

MPKS

MPKSF-As

OH C=O C-O M-O

1625.71

1387.99

C-C

1078.32

3430.14

573.35

As-O

Fig. 11 FTIR spectrum of MPKSF and MPKSF–As

Fig. 12 XRD diffractogram of

MPKSF–As

4588 Appl Water Sci (2017) 7:4573–4591

123

that the pore size of MPKSF was affected after the

adsorption process. The size of the pores reduced compared

to the pore size of MPKSF before adsorption. This is

possibly due to the formation of the inner-sphere com-

plexes of As(III) ion with Fe3O4 in MPKSF on the walls of

the external pores.

Energy dispersive X-ray analysis (EDX) analysis

The energy dispersive X-ray analysis is used to detect the

presence of arsenic after adsorption by MPKSF. Figure 14

illustrates EDX spectrum of MPKSF–As. The spectrum

showed the peaks similar to the untreated MPKSF for

carbon, oxygen, iron and phosphorus. However, after the

adsorption process, the presence of As(III) was detected

with a composition of 0.52% as illustrated in Table 10

below. The amount of arsenic detected is small due to the

low concentration of As used in the adsorption process

which is in the range of 5–100 lg L-1. The composition of

other elements present in the sample was unaffected by the

adsorption process as the composition of these elements are

quite similar to the composition of the untreated MPKSF.

Conclusions

This study demonstrated that the MPKSF effectively

removed As(III) from waste water by the formation of

inner-sphere complexes between the Fe and the As(III).

Further, desorption of the sorbed As(III) was found to be in

very minute concentrations, thereby suggesting that the

sorbed As(III) was rigidly bound by inner-sphere com-

plexation mechanism. An efficient adsorption process was

revealed to take place at pH 6 and 7 and at a longer contact

time. The initial As(III) concentration and adsorbent dose

Table 10 Elemental analysis for sample MPKSF and MPKSF–As

Sample MPKSF MPKSF–As

Weight (%) Weight (%)

Carbon (C) 17.14 18.16

Oxygen (O) 31.63 30.03

Phosphorus (P) 1.78 3.24

Iron (Fe) 49.45 48.05

Arsenic (As) – 0.52

Fig. 13 Micrograph of

MPKSF–As under

magnification a 91550 and

b 96200

Fig. 14 EDX spectrum for

MPKSF–As sample

Appl Water Sci (2017) 7:4573–4591 4589

123

were concluded to be dependent on each other due to the

availability of adsorption binding sites for As(III) presented

by greater dose of the MPKSF. Further, the higher reaction

temperature was shown to generate more residual As(III) in

the solution hence reducing the removal efficiency of

As(III) by MPKSF.

The predicted results from the designed experiment on

the adsorption of As(III) onto MPKSF using Box–Behnken

statistical data revealed that 96% of As(III) removal was

achieved utilizing the optimal conditions at contact time of

44.73 min, initial As(III) concentration maximized to

100 lg L-1, adsorbent dosage of 0.29 g, initial solution pH

of 6.55 and the temperature at 26.38 �C. A verification

experiment conducted using a real waste water sample

resulted in a 95% As(III) removal, which indicated that

0.96% error had occurred.

The adsorption studies suggest that the adsorption of

As(III) onto the MPKSF was highly dependent on pH and

contact time relative to the initial As(III) concentration,

adsorbent dose and temperature. The adsorption data of the

As(III) onto the MPKSF was well described by the Lang-

muir isotherm followed by Temkin and Freundlich

isotherms.

Acknowledgements This work was funded by the Universiti

Teknologi Malaysia research Grant no. 12H29.

Open Access This article is distributed under the terms of the

Creative Commons Attribution 4.0 International License (http://

creativecommons.org/licenses/by/4.0/), which permits unrestricted

use, distribution, and reproduction in any medium, provided you give

appropriate credit to the original author(s) and the source, provide a

link to the Creative Commons license, and indicate if changes were

made.

Publisher’s Note Springer Nature remains neutral with regard to

jurisdictional claims in published maps and institutional affiliations.

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