EVALUATION OF THE EFFICIENCY OF SULFONATED POLYSTYRENE …jestec.taylors.edu.my/Special Issue 2_SOMCHE_2014/SOMCHE 2014_2_2015... · efficiency of the adsorbent sulfonated polystyrene

Journal of Engineering Science and Technology Special Issue on SOMCHE 2014 & RSCE 2014 Conference, January (2015) 24 - 35 © School of Engineering, Taylor’s University

24

EVALUATION OF THE EFFICIENCY OF SULFONATED POLYSTYRENE IN THE REMOVAL OF CD2+ FROM

GROUNDWATER

M. R. G. OMAPAS, A. C. N. PUEBLA, M. J. K. G. RIVERA, J. E. D. TOCALDO, R. R. VILLAGRACIA, E. C. ROQUE

1*, A. ARCEGA

Department of Chemical Engineering, Adamson University,

900 San Marcelino Street, Ermita, Manila, 1000 Philippines

*Corresponding Author: [email protected]

Abstract

This research study focused on the evaluation of sulfonated polystyrene in the

adsorption of Cadmium metal from groundwater. The researchers studied the

effect of the following parameters: pH of 2, 4 and 6, initial concentration of 100,

200 and 300 ppm, contact time of 60, 120 and 180 minutes, in evaluating the

efficiency of the adsorbent sulfonated polystyrene in removing metal ion from

synthetic and actual groundwater. The optimum conditions for each parameter

were determined using Box–Behnken design. The experiments showed that the

only parameter which affects the adsorption capability of sulfonated polystyrene

was the initial concentration while the other two parameters were insignificant.

Further, experiments showed that the order of the adsorption mechanism was

pseudo–second order kinetics while obeying Langmuir monolayer curve. Also the

predicted percent removal obtained using Box–Behnken design was 52.88%.

Under the same optimal parameters, the percent removal obtained for the

synthetic groundwater was 51.27% while a value of 47.6% was obtained for the

actual groundwater. The percentage removal for synthetic groundwater is almost

the same to the predicted value. On the other hand, the removal for the actual

groundwater obtained was lower than that of the synthetic groundwater which can

be accounted to other species present in groundwater.

Keywords: Sulfonated polystyrene, Adsorption mechanism, Monolayer curve,

Pseudo-second order, Synthetic groundwater

1. Introduction

Cadmium is one of the heavy metals released into the soil by application of

phosphate fertilizers and other agrochemicals and which, at about half, is released

Evaluation of the Efficiency of Sulfonated Polystyrene in the Removal of Cd2+ . . . 25

Journal of Engineering Science and Technology Special Issue 2 1/2015

Nomenclatures

A Concentration

K Equilibrium constant; rate constant

M Molar

P Pressure

p p-value

pH Power of hydrogen

Ppm Unit of concentration; parts per million

Q amount of metal ion adsorbed

t Time, minute

x/m adsorption per gram of adsorbent

Greek Symbols

∝ Level or standard for how extreme the data must be before we can reject the null hypothesis.

Θ number of sites of the surface which are covered with gaseous

molecule

Abbreviations

ANOVA Analysis of Variance

into rivers through weathering of rocks, air through forest fires and volcanoes,

while the rest is released through human activities, such as manufacturing [1].

The need to develop new approaches and innovative techniques for heavy metal

removal from groundwater grows as the restrictive regulations of allowable levels

of heavy metal discharge and requirements for the remediation of contaminated

sites increase [2]. Hence, sulfonated polystyrene suggests an effective removal of

groundwater contaminants particularly the heavy metals and is a practical way for

groundwater remediation.

Investigations have adopted chemical recycling of waste polystyrene into the

corresponding monomers or hydrocarbons. However, the process is cost-

inefficient since the cost of the hydrocarbons and monomers is lower compared to

that retrieved by recycling. [3] Sulfonation is a new reclamation technology

reported in converting waste plastic to functional polymer. It is popularly known

that the sulfonation of polystyrene could be used to create products such as

cationic exchanger resin, polyelectrolyte and fuel cell membranes. The research

was based on the sulfonation of waste polystyrene in producing flocculant. This

was then utilized to eliminate the turbidity of wastewater.

Sulfonated polystyrene, being one of the mounting solid wastes was

used in a study in the reduction of lead in wastewater from electroplating

industry to propose a useful remedy for its disposal. It was found to

significantly lessen the lead content in electroplating industry wastewater.

Moreover, it was established that the variation in amounts of volume and

mass of sulfonated polystyrene has a considerable effect in the percentage

reduction of the heavy metal [4].

26 E. C. Roque et al.


2. Adsorption

Analytical methods and design regarding the evaluation of the efficiency of

sulfonated polystyrene in the removal of Cd+2

from groundwater are presented

in this section. The analytical methods are based on theory and thus

adsorption only occurs when a gas or liquid solute collects on the surface of a

solid or a liquid (adsorbent), forming a molecular or atomic film (adsorbate).

The design charts are replicated with the analytical methods to improve the

accuracy of the results. These design charts used for the effect of contact

time, initial concentration and pH are adapted from Box Behnken Design

through various experiments.

2.1. Adsorption mechanism

The fluid is passed through the bed and the solid particles adsorb components

from the fluid. When the bed is almost saturated, the flow in this bed is

stopped and the bed is regenerated thermally or by other methods, so

desorption occurs.

2.2. Adsorption Isotherm

Freundlich Adsorption Isotherm is an empirical equation for representing the

isothermal variation of adsorption of a quantity of gas adsorbed by unit mass of

solid adsorbent with pressure. It is described by:

𝑥

𝑚= 𝑘𝑃

1

𝑛 (1)

Langmuir Isotherm is a semi-empirical isotherm derived from a proposed

kinetic mechanism. Langmuir Equation is described by:

𝜃 =𝐾𝑃

1+𝐾𝑃 (2)

A linear plot obtained proves the applicability of the isotherm.

2.3. Adsorption Kinetics

The determination of a rate law is simplified by the isolation method in which the

concentrations of all the reactants except one are in large excess. If B is in large

excess, for example, then to a good approximation, its concentration is constant

throughout the reaction. Although the true rate law follows a second-order

reaction, it can be forced into first-order form by assuming that the concentration

of B is constant. It is called a pseudo-first-order rate law [5].

The linear form of pseudo-first order model is shown in the equation:

ln(𝑞𝑒−𝑞𝑡) = 𝑙𝑛(𝑞𝑒) − 𝐾1𝑡 (3)

The linear form of pseudo-second order model is given by:

𝑡

𝑞𝑡=

1

𝐾2𝑞𝑒2 +

1

𝑞𝑒𝑡 (4)



3. Materials and Methods 3.1. Chemicals

An amount of 1M Cadmium Nitrate (Cd(NO3)2) was used as a source of Cd+2

.

Sodium Chloride anhydrous as a source of Na+ and concentrated Sulfuric Acid

(H2SO4) were obtained in the highest purity and used as received. 0.1 M of NaOH

and HCl solutions were used for the pH adjustments. All chemical solutions were

prepared using distilled water.

3.2. Preparation of sulfonated polystyrene

Collected polystyrene in the form of disposable Styrofoam cups were washed

with liquid soap and rinsed thoroughly with water. These were then dried using

convection oven. Dried polystyrene materials were treated with 600 ml of

concentrated sulfuric acid with complete mixing for 2 hours at room temperature.

Treated Polystyrene materials were filtered using ash-less filter paper, washed

with distilled water and were dried in convection oven at 30˚C until constant

weight was obtained. Dried sulfonated polystyrene was treated with 500 mL of

1.0 M NaCl and agitated for 1 hour in order to convert the sulfonated polystyrene

into a cation-exchanger resin [4].

3.3. Adsorption of cadmium

An amount of 0.5 gram of sulfonated polystyrene was used in adsorbing cadmium

ion present in the following samples: cadmium-spiked water and actual

groundwater. Using serial dilution, the cadmium-spiked water solutions with the

concentrations 100 mg/L, 200 mg/L and 300 mg/L were prepared from 1M

Cd(NO3)2 stock solution. The solutions were adjusted to desired pH values by

adding 0.1 M of either NaOH or HCl solution. Adsorption of cadmium from these

solutions was done under the desired contact time parameters. The adsorbing

capacity of the treated sulfonated polystyrene with cadmium ion Cd2+

was

assessed using the following procedures:

For Cadmium Spiked Water, 250 ml of Cadmium-Spiked water was treated

with 0.5 g of sulfonated polystyrene at different time interval and varying

concentration and pH. After each interval, samples were filtered using filter

paper. Filtrate was analyzed for the final cadmium concentration using Flame

Atomic Adsorption Spectroscopy (FAAS).

For the Actual Groundwater, 250 mL groundwater sample spiked with

Cadmium ion at the determined optimum conditions was treated with 0.5 g of

sulfonated polystyrene. Then it was filtered and was analyzed for the final

cadmium concentration using Flame Atomic Absorption Spectroscopy (AAS).

Actual groundwater sample was spiked with Cadmium ion at the determined

optimum condition in terms of initial Cd+2

concentrations and was adjusted to the

optimum pH value achieved. This sample was treated with 0.5 gram of the

sulfonated polystyrene for which adsorption proceeded under the optimum

contact time.



3.4. Instrumental analysis

The pH of solutions was adjusted using of pH meter model 500 series benchtop-

Beckman Coulter (calibrated by AccuLab).

Another factor in this study was the contact time monitored using Ming Jong

Multi Segment Plug-In Timer. After the adsorption processes, sulfonated

polystyrene materials were filtered out of the solutions using filter paper. The

final cadmium concentration from each filtered solution was measured using

Flame Atomic Adsorption Spectroscopy (FAAS) [6].

3.5. Statistical treatment data

The data that were obtained from the experimental part were evaluated by Box-

Behnken Design to establish the relationship of the adsorbing capacity of the

sulfonated polystyrene with cadmium ion (Cd2+

) with respect to contact time, pH

and concentration variations. Design Expert was used as the program in the

software. In the ANOVA analysis of Box Benhken Design, the probability value (p

value) declares the significance of each factor to the said experiment. Whenever the

p value is greater than α = 0.05, it is considered insignificant in the study.

4. Results and Discussion

The evaluation of the efficiency of sulfonated polystyrene in the adsorption of

Cadmium (Cd+2

) in actual groundwater shown in this research was carried using

the methods and the software described above. The effects of pH, initial

concentration and contact time are analysed in this paper. Also, kinetics and

isotherm curves have been determined.

4.1. Effect of pH The pH range was set from 2.0 to 6.0. Based on the Fig. 1, the graph showed that pH

has insignificant effect on the adsorption capacity of sulfonated polystyrene. ANOVA

analysis confirmed the adequacy of the quadratic model, with p=0.0519, which means

that this variable has insignificant effect on the adsorption of cadmium. The pH can be

set at any level, except on pH where cadmium will precipitate, in order to achieve the

optimum condition within the selected range. Based on the range selected for the

design, the peak performance was achieved at pH value of 6 at which the highest

percentage removal of Cd2+

in the solution, 51.27%, was retrieved. This can be

explained by formation of hydroxyl ions for solutions at low pH.

Comparing to other studies, it was found that the adsorption of cobalt is optimum

at the pH range 5-6. This is due to the structural hydroxyl groups of the clay or

hydroxyl - aluminum compounds [7]. The adsorption of Cd2+

on montmorillonite in

water is slightly dependent on pH, to which no other electrolyte had been added [8]. It

was also reported that as the pH increases, metal uptake increases. This is due to the

fact that at lower pH values, excess concentration of H+ ions competes with the

cadmium ions on the active sites of the adsorbent. The optimum solution pH was

found to be 5.5 for both adsorbents. At higher pH values, precipitation of the heavy



metal may occur. In this study, the peak performance was achieved at the highest pH

level (based on the range) [9].It was also found that Cd+2

removal increases as pH

increases on the basis of the decrease in competition between proton and Cd+2

for the

surface sites and by the decrease in positive surface charge, which results in a lower

coulombic repulsion of the sorbing Cd+2

[10].

Fig. 1. Percentage Adsorption vs. pH.

Contrary to the results retrieved, a study showed that the removal of metal ions

from aqueous solution by adsorption is highly dependent on the pH of the

solution which affects the surface charge of the adsorbent and the degree of

ionization and speciation of the adsorbate [11]. In general, most of the related

studies conformed to the result that adsorption is pH dependent, which is opposite

the result obtained in this study. However, the optimum pH level found in the

experiment based on the range was consistent with the theories cited from

different researches that adsorption is low at lower pH due to presence of

hydroxyl ions that compete with Cd2+

on the adsorption sites.

4.2. Effect of initial concentration

The initial concentration range was set from 100ppm to 300ppm. Based on Fig. 2,

the graph showed that as initial concentration of cadmium ion increases, the

percentage removal of the metal ion also increases. ANOVA analysis confirmed

the adequacy of the quadratic model, the probability value for initial

concentration term p=0.0008. The optimum concentration based on the range set

in the design was 300 ppm. It is because at higher initial concentration, the ratio

of the initial number of moles of cadmium to the available surface area is high.

Thus, the fractional adsorption becomes dependent on the initial concentration.

This result conformed to the study that made use of activated and non-

activated date pits in adsorption of Cd2+

[9]. It also confirmed the significance of

the effect of initial concentration to the percentage of adsorption. They studied the

adsorption of Cd2+

on China clay and found that the percentage removal increased

by decreasing the concentration of Cd2+

in solution. The process is highly

dependent on initial concentration of cadmium.Another study concluded that the



equilibrium was attained within 60 min in the adsorption of dye (MG) using egg

shell powdered adsorbent [12]. It was revealed that the percent adsorption was

highly dependent on initial concentration and that the actual amount of dye

adsorbed per unit mass of adsorbent increased with increase in the initial

concentration. It is because at lower concentration, the ratio of the initial number

of dye molecules to the available surface area is low. But at high concentration,

the available sites of adsorption become fewer. Hence, the percentage removal of

dye is dependent on the initial concentration.

Fig. 2. Percentage Adsorption vs. Initial Concentration.

Another study regarding the effect of initial Cd2+

concentration on the

adsorption using rice husk concludes that the initial concentration provided the

necessary driving force to overcome the resistances to the mass transfer of

Cadmium between the aqueous phase and the solid phase [13]. The increase in

initial concentration also enhanced the interaction between Cd2+

and rice husk. An

increase in the initial concentration of cadmium enhanced the adsorption uptake

of cadmium. This is due to the increase in the driving force of the concentration

gradient produced by the increase in the initial cadmium concentration.

Contrary to the results obtained in this study, the adsorption of Cu(II) and

Cd(II) on ACRH and found that adsorption decreased by increasing the Cu(II)

and Cd(II) concentrations[11]. Still, this result established that the removal of

copper and cadmium are highly concentration dependent. Most of the related

literature reported that adsorption is dependent on the initial concentration of

cadmium ions, as the initial Cd2+

concentration increases the equilibrium

concentration and the uptake of Cd2+

increases. This result is consistent with the

result obtained from this study.

4.3. Effect of contact time

The adsorption rates were determined at pH of 6 and initial concentration of 300

ppm in aqueous media. The adsorption kinetics consisted of two phases: an initial

rapid phase where adsorption was fast, and a second slower phase where metal

ion equilibrium uptake was achieved.



The contact time range was set from 60 minutes to 180 minutes. Based on Fig.

3, the graph showed that the contact time had no significant effect on the

adsorption of cadmium ion using sulfonated polystyrene. ANOVA analysis

confirmed the adequacy of the quadratic model, with p=0.5247. The contact time

can be set at any level in order to achieve the optimum condition within our range.

Based on the range selected for the design, the peak performance was achieved at

contact time value of 120 minutes at which the highest percentage removal of Cd2+

in the solution, 51.27%, was retrieved. Based on our given range, this was the

optimum contact time that was used in the adsorption kinetics experiment.

Comparing to the study which investigated the potential of egg shell powder

as a low cost adsorbent for malachite green removal from its solution, it was

found that contact time had significant effect on the adsorption, and the

equilibrium time was 60 min [12]. It is also found that the percent cadmium

removal is higher at the beginning, due to a larger surface area of the rice husk

being available at the beginning for the adsorption of cadmium [7]. As the surface

adsorption sites become exhausted, the uptake rate is controlled by the rate at

which the adsorbate is transported from the exterior to the interior sites of the

adsorbent particles. Most of the maximum percent cadmium removal was attained

after about 90 min of shaking time at different concentrations. As the contact time

increased, the cadmium adsorption and it remained constant after equilibrium was

reached in 60 min for different initial concentrations. [9] It showed that the

adsorption rate of cadmium on either the raw date pits or activated carbon was

found to be rapid in the initial period of contact time and then became slower until

the rate approached zero, where equilibrium is reached. The ultimate adsorption

for cadmium ions occurs within the first 45 min in the case of date pits and within

the first 60 min in the case of activated carbon.

Contrary to the results retrieved, another study showed that the equilibrium

was attained after shaking for 60 min. for Cu2+

and 80 min. for Cd2+

[11]. It

observed that the adsorption is governed by fast kinetics as the equilibrium is

reached within a short period of 30 min. In general, most of the related studies

showed that the contact time has significantly affected the percentage adsorption

of metal ions. This is contrary to the result obtained in this study.

Fig. 3. Percentage Adsorption vs. Contact Time.



4.4. Effect of three parameters

As shown in Fig. 4, both the contact time and pH have no significant effect on the

adsorption of Cd2+

. It means that the optimum condition for pH and contact time

in the determination of the kinetics and isotherm in this study can be set at any

level. Only the initial concentration has a significant effect on the percentage

adsorption. As initial Cd2+

concentration increases, adsorption increases. The time

of equilibrium is independent of the initial solute concentration.

Fig. 4. Percentage Adsorption vs. pH,

Initial Concentration and Contact Time.

4.5. Analysis of kinetics

The adsorption mechanism was evaluated using the pseudo-first-order and pseudo-

second-order equations. Figure 5 shows the kinetics for pseudo-second order

reaction. It is found that the plot of pseudo second order (1/qt versus t) gave a

straight line and a high correlation coefficient value with R2=0.968 compared to the

correlation coefficient value of the pseudo-first order with R2=0.906. The result

conformed to the assumption that the rate-limiting step of cadmium adsorption may

be chemisorption, in which the metal ions stick to the adsorbent surface by forming

a covalent bond and tend to find sites that maximize their coordination number with

the surface [14]. Using the values of slope and y-intercept from the best-fit

equation, the values of qe and k1were determined as 285.7143 ppm and 2.3904x105

min-1

. The established pseudo second order equation is:

1

𝑞𝑡=

1

2.3904𝑥105(285.71432) + (

1

285.7143𝑡) (5)

4.6. Isotherm curve

It is found that the plot of Langmuir isotherm curve (1/q versus 1/c) gave a

straight line and a high correlation coefficient value with R2=0.994 compared to

the correlation coefficient value of the Freundlich Isotherm Curve (ln q versus ln

c) with R2=0.953. Based on the R

2 values, Langmuir curve represented the



adsorption isotherm. By linearization, the values of the qo and K are 0.3198 mg

adsorbate/mg solid and 366.17 ppm, respectively, as shown in Fig. 6. The

established isotherm equation is:

𝑞 =0.3198𝑐

366.17+𝑐 (6)

Fig. 5. Kinetics for Pseudo - Second Order Reaction.

Fig. 6. Langmuir Isotherm Curve.

It means that the adsorption cannot proceed beyond monolayer coverage. All

the sites are equivalent and the surface is uniform, the surface is perfectly flat on a

microscopic scale. The ability of a molecule to adsorb at a given site is

independent of the occupation of neighboring site; there are no interactions

between adsorbed molecules [15].

4.7. Actual groundwater treatment

The optimum condition (based on the range) was used in the actual groundwater,

spiked with Cd2+

. The adsorption capacity was determined as 47.6%, proving the



efficiency of the experimental setup for Cadmium ion removal in the groundwater

sample. Relating this value to the maximum adsorption obtained in the initial

experiment, the percentage error computed is 9.99%. This can be accounted to the

physico-chemical and microbiological constituents of the groundwater.

Based on the study that used the same adsorbent in Pb2+

adsorption [4], the

percentage reduction was higher compared with the percentage reduction obtained

from this study. It can be concluded that the sulfonated polystyrene was more

efficient in Lead adsorption than Cadmium adsorption, but it was still effective in

groundwater treatment.

5. Conclusions

The purpose of this research is to evaluate the efficiency of sulfonated polystyrene

in the adsorption of Cadmium (Cd+2

) in actual groundwater. Using the Box-

Behnken Design and various experiments, the following statements have been

concluded:

The results showed that: as initial concentration increases, the percentage removal also increases, while the two other parameters, pH and contact time,

were insignificant in the adsorption process.

It was also found that the only parameter affecting the adsorption treatment was initial concentration with the optimum value (based from the range of

initial concentration that was set) of 300 ppm.The optimum condition for pH

and contact time can be set at any level within the given range.

The reaction mechanism followed pseudo-second order kinetics which means that the rate-limiting step may be chemical sorption.

The adsorption mechanism obeys Langmuir monolayer Isotherm model. It proved that it only occurred on localized sites on the surface, not with other

adsorbates at maximum adsorption.

The adsorption capacity was determined as 47.6%, proving the efficiency of the experimental setup for Cadmium ion removal in the groundwater sample.

Acknowledgement

This work was sponsored by the Center for Research and Continuing Education

(CRECE) of Adamson University.

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EVALUATION OF THE EFFICIENCY OF SULFONATED POLYSTYRENE …jestec.taylors.edu.my/Special Issue 2_SOMCHE_2014/SOMCHE 2014_2_2015... · efficiency of the adsorbent sulfonated polystyrene