Study of ZrIV-loaded Orange Waste Gel for Selenate Adsorption

ABSTRACT Adsorptive removal of selenate from aqueous phase using ZrIV-loaded orange waste gel was investigated. Adsorption kinetics showed that equilibrium was attained within 2 and 4 h for 12 and 23 mg/L of SeVI, respectively. A pseudo-second-order kinetic model was found to characterize the adsorption kinetics for all the initial selenium concentrations tested. Adsorption isotherms at various pH values were examined and the data fitted well with Langmuir model. The maximum sorption capacity was found to be 25 mg/g at optimum pH. The adsorption system was found to be favorable with separation factors between 0 and 1. Competitive adsorption by coexisting anions showed that chloride barely affected SeVI adsorption while phosphate and sulfate impeded SeVI adsorption. A ligand exchange mechanism was inferred for SeVI adsorption.

Study of ZrIV-loaded Orange Waste Gel for Selenate Adsorption

Biplob Kumar BISWAS * , Katsutoshi INOUE * , Hidetaka KAWAKITA * , Hiroyuki

HARADA * , Keisuke OHTO * , Shafiq ALAM **

*Department of Applied Chemistry, Saga University, Honjo 1, Saga 840-8502, Japan

**Department of Engineering and Applied Science, Memorial University, Canada

ABSTRACT

Adsorptive removal of selenate from aqueous phase using Zr IV -loaded orange waste gel was investigated Adsorption kinetics showed that equilibrium was attained within 2 and 4 h for 12 and 23 mg/L of Se VI , respectively A pseudo-second-order kinetic model was found to characterize the adsorption kinetics for all the initial selenium concentrations tested Adsorption isotherms at various pH values were examined and the data fitted well with Langmuir model The maximum sorption capacity was found to be 25 mg/g at optimum pH The adsorption system was found to be favorable with separation factors between 0 and 1 Competitive adsorption by coexisting anions showed that chloride barely affected Se VI adsorption while phosphate and sulfate impeded Se VI adsorption A ligand exchange mechanism was inferred for

Se VI adsorption

Keywords: adsorption, ligand exchange, orange waste, removal, selenate

INTRODUCTION

Selenium (Se) is a naturally occurring element, which is one of the inorganic contaminants that have become environmental concerns lately It is introduced in the environment from different sources, both natural and anthropogenic, such as from the activities related to thermal power stations, combustion of fossil fuels, roasting and refining of sulfide ores Selenium has an ambivalent behavior ranging from being essential to highly toxic, depending on species, oxidation state and concentration Selenium deficiency has been reported to be linked to certain endemic diseases in China (Tan and Huang, 1991), whereas selenium enrichment in soil and water has been implied as a major factor resulting in severe teratogenic symptoms in wildlife at Kesterson National Wildlife Refuge, California (Presser and Ohlendorf, 1987) Thus, an important feature of selenium is the very narrow margin between nutritionally optimal and potentially toxic dietary exposures for animals, which necessitates a clear knowledge of the processes affecting selenium distribution in the environment Selenium can be present in the aqueous environment in various forms such as elemental selenium (Se0), selenite (SeO32-), selenide (Se2-), selenate (SeO42-) and organic selenium Hydrogen selenide and selenium oxides have been known to be public health hazards (Lo and Chen, 1997) Selenite is present in reducing environments while selenate in oxidizing environments Selenite is difficult to be dissolved in water and easily adsorbed by soil colloids As a result, selenate becomes the major species of selenium in water

Numerous treatment technologies such as ion exchange, reverse osmosis, nanofiltration, solar ponds, chemical reduction with iron and microalgal-bacterial treatment have been reported for selenium removal from contaminated waters One of the important

processes to regulate the concentration of selenium in aqueous system is the adsorption

on solid surfaces Although much research has been devoted to study the effect of reaction time, pH and competitive anions on selenium adsorption on layered double hydroxides (LDH), activated carbon, activated alumina and magnetite, only a few literatures are reported on adsorption using natural waste materials Therefore, the present study aims to investigate the adsorption behavior of chemically modified orange waste, an available agro-industrial waste material, in aqueous system as a function of various adsorption parameters such as pH, time and initial selenate concentration

MATERIALS AND METHODS

Materials

Selenate and zirconium solutions were prepared by dissolving sodium selenate decahydrate (Na2SeO4·10H2O, Aldrich, USA) in deionized water and zirconium oxychloride octahydrate (ZrOCl2·8H2O, Wako, Japan) in 0.1 M hydrochloric acid, respectively However, the preparation of the adsorption gel (ZrIV-loaded saponified

orange waste (SOW)) has been described in detail elsewhere (Biswas et al., 2007)

About 100 g of orange waste was mixed along with 8 g of Ca(OH)2 and crushed to make small particles The mixture was transferred into a beaker and a substantial amount of deionized water was added, which was then stirred for 24 h at about 200 rpm

at room temperature in order to facilitate the saponification The pH of this suspension was maintained constant at around 12.5 by adding sodium hydroxide solution After stirring, the suspension was repeatedly washed with deionized water until neutral pH by means of decantation and finally filtered to obtain a wet gel, which was dried in a convection oven for about 48 hours at 70ºC to produce a dry gel The specific surface area of this gel was measured as 7.25 m2/g by using Belsorp 18PLUS-SP (BEL JAPAN INC.) according to the BET method The SOW gel was further modified by loading with ZrIV to facilitate ligand exchange adsorption of selenate Approximately 3 g of SOW gel was equilibrated with 500 mL of a 0.1 M zirconium solution at pH 2.11 for 24

h The gel was then filtered and washed with deionized water until neutral pH, followed

by drying in vacuum and finally sieved to produce a particle size fraction between 75 and 150 μm for the adsorption tests The presence of ZrIV ion in the gel was confirmed

by taking energy dispersive X-ray spectroscopy (EDS) (Biswas et al., 2009) and the

amount of zirconium loaded was calculated from the difference in the metal concentration in the solution before and after loading However, due to such chemical modification, the gel became aqueous-insoluble, which is a distinct benefit to operate

this gel for a prolonged time (Dhakal et al., 2005)

Selenate sorption studies

Batch adsorption tests were carried out to determine the adsorption behavior for selenate

on ZrIV-loaded SOW gel The kinetic studies of SeVI adsorption were carried out by first combining a 400 mg gel together with 240 mL of measured concentration of SeVI solutions into a 300 mL Erlenmeyer flask The suspensions (pH 3) were stirred at 150 rpm and the temperature was kept constant at 30˚C Reduction in selenium concentration was measured at varying time Adsorption isotherms were measured at different pH These tests were carried out by mixing20 mg of gel together with 10 mL

of selenate solution of different concentrations in 50 mL conical flasks The flasks were then shaken at 150 rpm for 24 h in a thermostated shaker maintained at 30ºC after which

0 5 10 15 20

VI /g )

Time (min)

23 mg/l

12 mg/l

Fig 1 - Adsorption kinetics of different initial selenium concentrations at pH 3

the suspensions were filtered and analyzed Earlier, using a similar procedure, the effect

of pH on SeVI adsorption was examined in a series of experiments where the initial SeVI concentration was maintained constant at different pH

Analysis

The pH and concentration of selenate in the test solutions were measured by using DKK-TOA model HM-25G pH meter, calibrated with buffers of pH 4.0 and 7.0 on a regular basis, and a Shimadzu ICPS-8100 ICP/AES spectrometer, respectively The linearity of calibration of ICP/AES reaches from 2 μg/L up to 1000 μg/L with a detection limit of about 1 μg/L for measured species Reproducibility of all adsorption experiments was confirmed by repeating the same adsorption test

RESULTS AND DISCUSSION

Kinetic studies

Sorption kinetics of SeVI is depicted in Fig 1 where it is shown that the sorption capacity of SeVI has been increased from 7.3 to 13.1 mg/g with the increase in the initial

SeVI concentration from 12 to 23 mg/L It is also shown that the initial selenium adsorption is very fast and then the sorption capacity does not appreciably change with time, which finally reaches plateau at 2 and 4 h for 12 and 23 mg/L, respectively, under the operating conditions

The time-dependent adsorption results (up to 120 min) of SeVI were analyzed using pseudo-first-order and pseudo-second-order models (Ho and McKay, 1998) The simplified kinetic equations of pseudo-first-order and pseudo-second-order reactions are

as follows:

t

k q

q

q e t e

303 2 log

)

1

2 2 2

2

1

e e

t

q

where k 1 and k 2 are the rate constants for pseudo-first-order (1/min) and

pseudo-second-order adsorption (g/mg/min), respectively, while q t and q e are the

amounts of selenate adsorbed (mg/g) at time t and at equilibrium, respectively The

mg/L mg/L

y = 0.137x + 0.61 R² = 0.99

y = 0.076x + 0.56 R² = 0.99 0

5

10

15

20

Time (min)

12 mg/l

23 mg/l

y = 1.27x + 2.4443

y = 0.57x + 2.6596

0 3 6 9 12 15

q t

t 0.5 (min 0.5 )

23 mg/l

12 mg/l

Fig 2 (a) - Second-order kinetic plot for

the adsorption of selenate on

the gel

Fig 2 (b) - Determination of intra-particle

diffusion rate constant

Table 1 - Pseudo-second-order kinetic parameters for the adsorption of SeVI

C in (mg/L) q e2(cal) (mg/g) k 2 (g/mg/min) R 2 q e(exp) (mg/g)

kinetic parameters can be evaluated from the linear plots of log(q e1 q t) vs t and t/q t

vs t It was found that the data fitted well with the pseudo-second-order equation (Fig 2

(a)) with respect to adsorption capacity and correlation coefficients (Table 1) At the same time, the rate was found to be a function of the initial selenium concentration However, this analysis does not provide any information regarding the transport of solutes inside adsorbents Figure 1 clearly shows that there are different mechanisms involved in the sorption process: film diffusion in the first step and intra-particle diffusion in the second step, which eventually leads to final equilibrium In order to identify the diffusion mechanisms during the sorption process, the experimental kinetic data (up to 60 min) were tested against the intra-particle diffusion model (Weber and Morris, 1963) as shown in Fig 2 (b) The initial rate of intra-particle diffusion can be defined as follows (Eq 3):

5

.

0

t

k

where k i is the intra-particle diffusion rate constant (mg/g/min0.5), which can be

determined from the slope of the straight line portion of q t vs t0.5 However from Fig

2(b), k i was found to increase with the increase in the initial SeVI concentration and was evaluated to be 0.57 and 1.27 mg/g/min0.5 for 12 and 23 mg/L of selenate, respectively

The increased slope value (k i) can be explained by the growing effect of concentration gradient as driving force Moreover, the slopes did not pass through the origin, which indicated that the adsorption mechanism is complex and both the surface adsorption and intra-particle diffusion contribute to the rate-determining step A similar observation

was reported by Teng et al (2009) for fluoride removal by hydrous manganese

oxide-coated alumina

Effect of pH on selenate adsorption

Since pH, in general, is considered as an important parameter that controls the

mg/L mg/L

mg/L mg/L

-

adsorption, the adsorption edges at two different concentrations of selenate were studied (Fig 3) at different pH values ranging from 0.5 to 8 At low pH, the adsorption of selenate increases with the increase in pH and reaches the maximum (from pH 1 to 2), which finally decreases with further increase in pH Selenate can exist in different ionic species depending on the solution pH The dominant species in the above-mentioned pH range are HSeO4- and SeO42- ions which can be adsorbed on the gel by substituting hydroxyl ions from the coordination sphere of the immobilized zirconium ions Since zirconium tends to be extensively hydrolyzed, a number of hydroxyl ions are therefore

available for ligand exchange with selenate anions (Cotton et al., 1999) Such

adsorption can be explained by ligand exchange mechanism as shown in Fig 4 This mechanism has been further confirmed by the increase in pH after adsorption However, leaking of zirconium was found to be very insignificant at pH > 1 Hence, an optimum

pH for the system operation was selected to be 1.5 as it yields the highest selenate removal

When the initial concentration of SeVI increased, the adsorption curve shifted to the left because the number of adsorption sites remained fixed If the gel provides a sufficient number of adsorption sites, SeVI adsorption density will increase with higher surface loading, but this phenomenon will be constrained by the concentration of adsorption sites Therefore, the adsorption density on the gel in relatively high concentration solution will not be greater than that in a more dilute solution This is in good agreement

with the phenomenon reported by Kuan et al (1998) for selenium removal using

aluminum-oxide-coated sand However, in both cases the sorption of SeVI decreases with the increase in pH, which could be due to the increasing amount of OH- in the solution resulting to an intermolecular competition between OH- and selenate

0.0 1.0 2.0 3.0 4.0 5.0

0 20 40 60 80 100

Equilibrium pH

0.11 mM (1) 5.5 mM (2) Zr(IV) leakage (1) Zr(IV) leakage (2)

Fig 3 - Effect of pH and initial concentration on the adsorption of SeVI

O

OH OH

C

O O

- A

n

A

-OH

-O

OH OH C

O O

- HO

n

Fig 4 - Inferred mechanism for SeVI adsorption on the gel via ligand-substitution

5

10

15

20

25

30

C e (mg/l)

pH 1.5

pH 2.2

pH 3

R² = 0.99

pH 2.2

y = 0.05x + 0.10 R² = 0.99

pH 1.5

y = 0.04x + 0.22 R² = 0.99 0

5 10 15

0 40 80 120 160 200

C e

C e (mg/l)

pH = 3

pH = 2.2

pH = 1.5

(b)

Fig 5 - (a) Adsorption isotherms at different pH values and (b) corresponding

Langmuir plots

Isotherm study

The experimental data obtained for different initial selenate concentrations at constant temperature and pH were taken for isotherm study The distribution between the adsorption gel and selenate at equilibrium was described by the Langmuir isotherm model as depicted in Figs 5 (a) and (b) This model is widely used under the assumption that maximum adsorption occurs when the surface of the adsorbent is covered (monolayer) by adsorbate The Langmuir equation applied for this study is given below (Eq 4):

m

e m

e

e

q

C

b

q

q

where C e is the equilibrium concentration (mg/L), q e is the amount adsorbed at

equilibrium (mg/g), q m is the adsorption capacity for Langmuir isotherms and b is the

Langmuir isotherm constant (l/mg) The adsorption capacity of the gel was evaluated to

be 25, 20 and 14 mg-SeVI/g at pH 1.5, 2.2 and 3.0, respectively

The essential feature of the Langmuir isotherm can be expressed in terms of a

dimensionless constant termed separation factor (R L) In order to predict the adsorption efficiency, the separation factor was calculated by using the following equation (Viswanathan and Meenakshi, 2008):

0

1

1

bC

R L

+

where b is the Langmuir isotherm constant (l/mg) and C 0 is the initial concentration

(mg/L) of selenate The significance of separation factor is that if R L > 1, the adsorption

is unfavorable; if 0 < R L < 1, the adsorption is favorable; and if R L = 0, the adsorption is

irreversible The value of R L for an initial concentration of 10 mg/L was found to be 0.355, 0.474 and 0.611 at pH 1.5, 2.2 and 3.0, respectively, which signifies that the adsorption module is favorable at all pH valuestested

Competitive adsorption of Se VI with other anions onto the Zr IV -loaded SOW gel

To assess the competing effects of other anions on SeVI removal by the ZrIV-loaded

0 20 40 60 80 100

IMR

Selenate and chloride Selenate and sulfate Selenate and phosphate

Fig 6 - Effect of Cl-, SO42- and PO43- on the adsorption of SeVI

SOW gel, batch adsorption tests were carried out by adding Cl-, SO42- and PO43- at various initial molar ratios (IMR), where IMR is the ratio of molarity of other anions to that of selenium

The effect of the presence of Cl-, SO42- and PO43- on the adsorption of SeVI at pH 2 (Fig 6) shows that Cl- does not interfere with selenium adsorption while SO42- and PO4

3-reduce the extent of selenium removal The difference in the reduction of selenium removal in the presence of such anions can be related to their different complex formation characteristics with the gel Since SeVI exists in solution as SeO42- whose chemical properties resemble those of SO42-, its adsorption onto the gel was affected by non-specific adsorption between the gel and sulfate (Lo and Chen, 1997) However, anion such as phosphate is bound relatively strongly with the adsorption sites via

complex formation on inner-sphere, which is barely affected by ionic strength (Zhang et

al., 2008) This is the reason why the removal efficiency of SeVI decreases significantly with the presence of sulfate and phosphate

CONCLUSIONS

From the kinetic study, it was observed that adsorption kinetics is governed not only by surface adsorption but also by intraparticle diffusion The adsorption isotherms fitted well withthe linear form of Langmuir equation and a maximum sorption capacity of 24 mg-SeVI/g was observed The trend of higher selenium sorption at acidic pH is due to the predominance of SeO42- and HSeO4- in the aqueous phase Although, the ZrIV-loaded SOW gel exhibited a good selenate removal at lower pH range, the reduction in sorption capacity at higher pH suggests a prospect for adsorbent regeneration The competitive adsorption of SO42- and PO43- with respect to SeVI is more evident than that of Cl-

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