Removal behavior of Babool bark (Acacia nilotica) for submicro concentrations of Hg2+ from aqueous solutions: a radiotracer study

Separation and Purification Technology 41 (2005) 21–28

Removal behavior of Babool bark (Acacia nilotica) for submicro concentrations of Hg2+ from aqueous solutions: a radiotracer study S.S. Dubey, R.K. Gupta∗ Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India Received in revised form 19 March 2004; accepted 29 March 2004

Abstract The uptake behavior of Babool bark (Acacia nilotica) for the micro to trace levels of Hg2+ from aqueous solutions has been carried out in batch equilibrium experiments, employing a ‘radiotracer technique’. A high level of uptake of metal ions on the solid surface occurs within ca. 2 h of contact time. The increase of sorptive concentration (10−7 to 10−2 M), temperature (303–333 K), and pH (3–10) favored the removal process of ions. The percentage adsorption increases from 54.3 to 91.3% with an increase in dilution of sorptive solution from 10−2 to 10−7 M. First order uptake of Hg2+ followed the Freundlich and Dubinin–Radushkevich (D–R) isotherms for the entire range of adsorptive concentration studied. Temperature dependence data show that this process is endothermic in nature. Desorption experiments further confirm the irreversibility of the sorption process as no significant desorption took place in the bulk concentration of the adsorptive. © 2004 Elsevier B.V. All rights reserved. Keywords: Sorption; Hg(II); Radiotracer; Freundlich; Babool bark; Desorption

1. Introduction In an environmental context, accelerating pollution by toxic metals radionuclides has provided an impetus for the research to look into the bio-technological potential of utilizing several low cost biomolecules/agricultural byproducts to replace existing expensive technology. Unlike organic pollutants, which are biodegradable, these metallic contaminants tend to persist rather indefinitely in the environment, and are eventually accumulated through the food chain thus posing serious threat to plants, animals, and men. A number of biosorbents (living or dead) have been successfully employed in the removal of these toxic metal ions. The bioaccumulation may proceed mainly through surface complexation with surface-active groups, exchange of metal cations, physical adsorption, transport of metal ions into the energy barrier and/or metabolic-mediated processes. Heavy metals toxic ions such as Hg2+ , Zn2+ , and Cd2+ are known as non-essential trace ions. The maximum permissible limit of Hg2+ ions in to drinking water is 0.001 mg/l [1]. In the removal of toxic heavy metals ions, the role of ∗ Corresponding author. Tel.: +91-542-2372886; fax: +91-542-2368174. E-mail address: [email protected] (R.K. Gupta).

1383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2004.03.012

dead biomass have long been utilized [2]. Scott et al. [3] reported that the sorption of metal ions increased on activated carbon by coating it with biofilms. The conventional methods often used in the removal of these ions are sometimes found to be expensive and are not able to provide the required permissible trace limits required by our water bodies. Attempts for removal of various toxic heavy metal ions from aqueous solutions employing several low cost waste products, i.e. rice hulls [4], soyabean hulls, cottonseed hulls, rice straw, and sugarcane bagasse [5,6] have been made. Recently it has been observed that the eucalyptus bark was used for removal of heavy metal ions [7]. Peat is found to be quite relevant as a natural sorbent in the removal of these toxic ions as it shows a high exchange capacity for cations [8]. Sawdust of Redfir was used for Hg2+ removal and it is found that the uptake of Hg2+ increases with increasing temperature and pH [9]. The uptake of some heavy metal toxic ions (Zn2+ , Hg2+ , and Cd2+ ) at micro to tracer level concentrations by a dead biomass (rice hulls, Neem, and Mango (tree bark)) has been reported successfully using the radiotracer technique [10]. The adsorption process has been found to be useful and popular due to its low maintenance costs, high efficiency, and ease of operation. The effects of various parameters such as concentration, temperature, and pH have been examined.

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The kinetics and adsorption isotherms during the process have also been discussed and thermodynamic parameters deduced to help in understanding the uptake process.

(g); ‘A2 ’ is the radioactivity of desorptive solution; ‘A1 ’ and ‘ae ’ are the radioactivity of sorptive solution and amount adsorbed at equilibrium, respectively. 2.3. Irradiation of sorbents

2. Experimental 2.1. Materials Tree bark of Babool (Acacia nilotica) was obtained from the Babool tree from the Banaras Hindu University Campus. Bark was crushed giving varying size (BS 40–70) particles, which was washed repeatedly by double distilled water and dried at room temperature before being employed as adsorbents. The density of the bark was 0.67 g cm−3 . Mercury(II) as its nitrate salt was used and stock solution (1.0 M) of metal ion was prepared in double distilled water. The solution was further standardized via the standard method of Flascha [11] and then diluted to obtain desired experimental concentrations (10−2 to 10−7 M). The radioactive mercury (203 Hg, t1/2 = 47 days) as its respective nitrate in dilute HNO3 (ca. 166.5 × 106 Bq) was obtained from the Board of Radiation and Isotope Technology (BRIT), Mumbai (India). A very small amount of this radionuclide was used to label the Hg(II) adsorptive solution to obtain a measurable radioactivity of minute aliquots of withdrawn samples from bulk. 2.2. Sorption measurements The sorption experiments were performed by stirring, at regular intervals, and equilibrating 0.1000 g Babool bark with 10.0 ml of labeled sorptive solution (specific activity of solution 6.02 mCi g−1 ) of desired concentration. The equilibrated solution was centrifuged for phase separation and then supernatant solution was analyzed for its ␤-activity measurements using an end-window GM-counter (Nucleonix, Hyderabad, India). Radioactivities of some samples were also checked for their ␥-activity using a Multi Channel Analyzer (Nucleonix, Hyderabad, India). The amount sorbed (at ) at any time ‘t’ and percentage adsorption (P) were estimated from the following equations: A0 − A t V at = mol g−1 c (1) A0 m P=

A0 − At × 100 A0

(2)

The percentage desorption was calculated by the relationship: A2 1 V % Desorption = (3) c × 100 A 0 − A 1 ae m where ‘A0 ’ and ‘At ’ are radioactivity of sorptive solutions at time zero and time ‘t’, respectively; ‘c’ is the initial concentration of sorptive solutions (mol l−1 ); ‘V’ is the total volume (l) of sorptive solutions; ‘m’ is the mass of adsorbent

Tree bark was irradiated with neutrons and ␥-rays from a 11.1 × 109 Bq (Ra–Be) neutron source having an integral neutron flux of 3.9 × 106 n cm−2 s−1 , associated with a nominal ␥-dose of ca. 1.72 Gy h−1 (total dose: from 41.28 to 123.84 Gy for 24–72 h of irradiation). Irradiated sorbents were then employed along with unirradiated materials for the uptake of Hg(II) ions from aqueous solutions.

3. Results and discussion 3.1. Effect of metal ion concentration The effect of concentration (10−2 to 10−7 M) of mercury ions on the uptake behavior of Babool bark have been studied and shown in Fig. 1. The results are plotted for amount sorbed (mol g−1 ) versus time. It is evident from the graph that initially the uptake of metal ion is quite rapid becoming slower with lapse of time and then an apparent equilibrium between the two phases was achieved within ca. 2 h of contact time. No further change in sorption was observed even after 24 h of contact of the bulk solution with solid sorbent. The present results show that the amount sorbed at equilibrium increases from 0.918 × 10−8 to 0.545 × 10−3 mol g−1 for Hg2+ with the rise in sorption concentration from 10−7 to 10−2 M. However, the relative change of sorption, i.e. percentage of sorption varies from 54.5 to 91.8% with an increase in dilution of sorptive solution from 10−2 to 10−7 M at pH 5.1. This increase in percentage sorption is due to the availability of large number of surface sites of the adsorbent for a relatively smaller number of adsorbing species at higher dilutions [12]. The sorption capacity of the Babool bark was comparable to other biosorbents reported in the literature [4,10]. 3.2. Equilibrium studies The results on the steady state values of sorption of Hg(II) ions on the Babool bark at various sorptive concentrations (10−2 to 10−7 M) were further analyzed using the Freundlich adsorption isotherm in its usual logarithmic form: log ae =

1 log Ce + log Kf n

(4)

where ae and Ce are the amount sorbed (mol g−1 ) and bulk concentration (mol dm−3 ) at equilibrium, respectively. Kf and 1/n are the Freundlich constants, referring to sorption capacity and adsorption intensity, respectively. The linear variation of sorption (viz., log ae versus log Ce ), is shown in Fig. 2. The linear plot confirm about the monolayer coverage

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23

1.0 -7

10 M -6 10 M -5 10 M

0.8

Amount sorbed (mol g-1 )

-4

10 M -3

10 M

0.6

-2

10 M Multiplying factor (y-axis)

0.4 3

10 4 10 5 10 6 10 7 10 8 10

0.2

0.0 0

40

80

120

160

200

240

Time (min) Fig. 1. Time variation of sorption of Hg(II) ions on Babool bark at various concentrations of sorptive (temperature: 303 K; pH 5.1).

of Hg(II) ions at the surface of sorbent [13]. The Freundlich constants, i.e. Kf and 1/n for the systems were obtained by intercept and slope of the line. The numerical value of Kf and 1/n are (5.37 ± 0.07) × 10−2 mol g−1 and 0.798 ± 0.004, respectively. The fractional value of (1/n) indicates that the surface of sorbent is of the heterogeneous type with an exponential distribution of energy sites [14]. The higher numerical values of Kf confirm the significant affinity of Hg(II) ions for Babool bark. The Langmuir adsorption isotherm [15] based on the assumption that there is small discrete equivalent energy sites

accommodating one molecule each. This has also been tested in the form:

 1 1 1 1 = (5) + ae mb Ce m where Ce is the equilibrium ion concentration is solution (mol dm−3 ), ae amount sorbed (mol g−1 ), m and b are the constants. Plots of 1/ae versus 1/Ce are linear (Fig. 3). The numerical value of monolayer sorption capacity ‘m’ and the binding energy related to the heat of sorption ‘b’ are evaluated from the slope and intercept and are 6.25 × 10−7 and

10

-1

-log ae (mol g )

8

6

4

2

0 0

2

4

6

8

-1

-log Ce (mol L ) Fig. 2. Freundlich isotherm of Hg(II) ions on Babool barks at 303 K.

10

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1.4x10

8

1.2x10

8

1/ Ce (mol -1 L)

1.0x10

7

8.0x10

7

6.0x10

7

4.0x10

7

2.0x10

0.0 0.0

2.0x10

7

4.0x10

7

6.0x10

7

8.0x10

7

1.0x10

8

1.2x10

8

-1

1/ ae (mol g) Fig. 3. Langmuir isotherm of Hg(II) ions on Babool barks at 303 K.

1.80 × 106 , respectively. The higher values of ‘m’ and ‘b’ constants for bark indicate high sorption affinity for mercury [15]. In order to study the nature of the sorption phenomena either physical or chemical sorption, the Dubinin–Radushkevich (D–R) isotherm [16] is verified in the form: ln ae = ln Xm − β2

(6)

where Polanyi potential () = RT ln{1 + (1/Ce )} and Xm is the sorption capacity and β is the constant related to the

sub monolayer region characterized by D–R isotherm. This is related to the sorption free energy (E) if β < 0 as: E=

1 (−2β)1/2

(7)

The plot of ln ae versus 2 for Hg2+ is shown in Fig. 4. The value of β and Xm constants have been evaluated from the slope and intercept and are equal to −5.29×10−9 and 3.36× 10−3 , respectively. The numerical value of sorption free energy (E = 9.72 kJ mol−1 ) corresponds to chemisorption

20

16

-ln ae

12

8

4

0 0

4

8

12

16

20

2

ε

Fig. 4. D–R plot for Hg(II) ions on Babool barks at 303 K.

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25

type uptake based on ion-exchange as also reported by other workers [17].

Table 1 Temperature dependence of equilibrium sorption/desorption of Hg(II) on Babool bark at equilibrium

3.3. Kinetic studies

Temperature (K)

Amount of Hg(II) sorbed

303 313 323 333

0.839 0.858 0.870 0.886

Boyd et al. [18] have proposed a model for the study of sorption behavior, which is in accordance with the observations of Reichenberg [19] as given by the following equations: α   61 2 F =1− 2 exp −n Bt (8) π n2 n=1

and B=

πDi r2

(9)

where F is fractional attainment of equilibrium at time t, Di the effective diffusion coefficient of sorbates in the sorbent phase, r the radius of adsorbent particle assumed to be spherical, and n an integer. The fractional attainment of the equilibrium can be determined by the following equation: F=

Qt Qα

(10)

where Qt and Qα are the amounts sorbed after time t and infinite time (24 h), respectively. For every calculated value of F, corresponding values of Bt are calculated from Eq. (8). The linearity test of Bt versus time plots (Fig. 5) is employed to find out the particle diffusion control mechanism. At the lower concentration, the Bt versus t plots pass through the origin indicating the sorption to be a particle diffusion

mol g−1 ± ± ± ±

(×106 )

Sorption %

0.004 0.003 0.003 0.005

83.9 85.8 87.0 88.6

Desorption % (at equlibrium) 2.4 3.7 4.1 4.4

Initial concentration of sorptive: 1.0 × 10−5 M; pH 5.1.

in nature, but at higher concentrations the line does not passes through origin signifying the adsorption to be film diffusion. 3.4. Effect of temperature The effect of solution temperature varying from 303 to 333 K in steps of 10 K on sorption of Hg(II) ions by Babool bark has been investigated, the initial concentration of Hg(II) ions being kept at 1.0 × 10−5 mol dm−3 at pH 5.1. It was observed that with the increase in temperature the uptake of metal ions increased from 0.839 × 10−6 to 0.886 × 10−6 mol g−1 at the equilibrium (Table 1). This increase in adsorption of metal ions may either be due to acceleration of slow sorption steps, to creation of some new active sites [20], to transport against concentration gradient/or diffusion controlled transport across the energy barrier [21]. Al-Asheh and Duvnjak [22] assumed that the increase biosorption with temperature is due, likely to the acceleration of the movement of metal ions from the bulk solution

4.0 3.5 3.0

Bt

2.5 2.0 1.5 -2

1 x 10 M -5 1 x 10 M -7 1 x 10 M

1.0 0.5 0.0 0

20

40

60

80

Time (min) Fig. 5. Bt vs. t plots for mercury nitrate solution of: (䊏) −1.0 × 10−2 M; (䊉) −1.0 × 10−5 M; and (䉱) −1.0 × 10−7 M concentrations.

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Table 2 Kinetic and thermodynamic parameters for uptake of Hg(II) ions on Babool bark as a function of temperature Temperature (K)

Rate constant (min−1 (×102 ))

303 313 323 333

3.38 3.55 3.69 3.85

± ± ± ±

0.05 0.02 0.04 0.01

Energy of activation (kJ mol−1 )

Enthalpy change (kJ mol−1 )

3.57 ± 0.04

11.17 ± 0.03

Initial concentration of sorptive: 1.0 × 10−5 M; pH 5.1.

to the surface of biosorbent or due to the enhanced temperature dependent hydrolysis of some components available at the surface of sorbent, which respond to metal sorption. A kinetic study for the uptake of Hg(II) ions over Babool bark has also been worked out, which follows the first order rate law obeying Lagergren equation: t log (ae − at ) = log ae − k1 (11) 2.303 where ae and at are the amounts sorbed at the equilibrium and at the contact time intervals t and k1 is the sorption rate constant. The values of sorption rate constants at different temperatures have been estimated from the slopes of straight lines obtained from log (ae − at ) versus time and the values are listed in Table 2. These values increase with the increase in the temperature. This is in conformity to expectations as adsorption increases with an increase in temperature for biosorption. The Arrhenius plot (log k1 versus 1/T) gave the activation energies for the sorption process of Hg(II) on Babool barks as 3.57 ± 0.04 kJ mol−1 . The low value of activation energy for the system indicate that the process of uptake can occur even under normal conditions of temperature and pressure as well as the strong force of attraction operating during the sorption.

A change in the standard enthalpy of sorption H0 has been evaluated using the van’t Hoff equation: log KD = −

H 0 + Constant 2.303 RT

(12)

where KD , H0 , R, and T have their usual meaning. 0 found from the slope of straight The value of H303K line (Fig. 6) obtained by plotting log KD versus 1/T is 11.17 ± 0.03 kJ mol−1 . The numerical value indicates an ion-exchange type mechanism [17] for the uptake of metal ions. 3.5. Desorption study Preadsorbed Hg(II) ion on Babool bark was washed with double distilled water to ensure the removal of adhering species and subsequently the sorbent was dried in an electric oven at 383 K. Desorption of preadsorbed Hg(II) ions on the bark was examined in Hg(II) solution (1.0 × 10−5 M) at different temperatures (i.e. 303–333 K). The very low values of percentage desorption at different temperatures (Table 1) indicates that desorption process is almost independent of the temperature. Thus a low value of desorption, unaffected by an increase of temperature, shows that the process of

2.92 1.42 2.88

1.44

-log k1

log K D

2.84

2.80 1.46 2.76 1.48 2.72

3.0

3.1

3.2 3

3.3

-1

1/ T x 10 ( K ) Fig. 6. Variation of log k1 and log KD with 1/T for sorption of Hg(II) ions on Babool bark (initial concentration of Hg(II) solution: 1.0 × 10−5 M; pH 5.1).

-1

Amount sorbed at equilibrium (mol g ) x 10

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27

1.0

0.8

0.6

0.4

0.2

0.0 0

2

4

6

8

10

12

pH Fig. 7. Amount of metal ions sorbed at equilibrium by Babool bark as a function of pH at 303 K (initial concentration of sorptive: 1.0 × 10−5 M; temperature: 303 K).

sorption for Hg(II) is irreversible in nature. It also indicates that a major part of Hg(II) ions was probably bound to the surface through strong interaction and converted to a final stable adsorption phase. 3.6. Effect of pH The effect of pH on the sorption of Hg(II) ions on Babool bark has been studied in the pH range 3–10 at 293 K at fixed initial sorption concentration (1.0×10−5 M) and shown in Fig. 7. It is evident that the increase of solution pH, the uptake of metal ions increases. However, below pH 3 no significant uptake of metal ions was observed. The low value of sorption at lower pH is explicable on the basis of the fact that the Babool bark is composition of various proteins/amino acids along with fats, carbohydrates, crude fibers, and ash contents, which would obviously be active at the surface, becoming protonated at lower pH and thus bearing positively charged surface sites which hinders the uptake of cations [23]. NH + H+
NH2 + –NH2 + H+
–NH3 + –MOH + H+
–MOH2 + –Cx OH + H+
–Cx OH2 + Galun et al. [24] have also observed that at lower pH, H+ ions compete with metal ions for solid surface, which cause a decrease in metal ion sorption. With the increase of pH the sorption of metal ion increases and a sharp rise in the uptake occurred after pH 3.8. It is well reported that amino acids or fiber (carbaneous surface) or ash content can un-

dergo acidic dissociation with an increase of pH, depending on their respective protein dissociation constant values. In general, on increasing pH, the following equilibrium may be involved for the surface-active groups: NH
N− + H+ –NH2
–NH− + H+ –MOH
–M–O− + H+ –Cx OH + H+
–Cx –O− + H+ and the surface are expected to be negatively charged. Hence these negatively charged surfaces might facilitate the gradual increase in metal ion sorption. With an increase in the pH of the sorptive solution, Hg2+ becomes hydrolyzed resulting in the formation of cationic, neutral, and anionic species, which are favorable species for the uptake of trace metal ions. Thus, the enhanced uptake of Hg2+ ions on Babool bark with pH is best described by mechanism of hydrolytic adsorption: –OH + Hg(OH)
–O–Hg+ + H2 O Hence, a mixed effect of bioaccumulation is likely to occur during the variation of pH for metal ion sorption. In general, in the absence of apparent precipitation, a mixed effect of ‘ion-exchange’ and ‘surface complexation’ is to be suggested for the sorption of these ions on chosen bark sample [10]. 3.7. Effect of irradiation The effect of irradiation on bark towards removal of Hg(II) ions has been assessed by experiments using a 11.1×109 Bq

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Table 3 Effect of irradiation on Babool bark by neutrons (Ra–Be) source on sorption of Hg(II) ions Time of irradiation (h)

Amount of Hg(II) sorbed (mol g−1 (×106 ))

Nil 24 48 72

0.838 0.821 0.801 0.783

± ± ± ±

0.005 0.004 0.007 0.005

Acknowledgements Authors thank Banaras Hindu University, Varanasi, India for awarding research fellowship.

Sorption % 83.8 82.1 80.1 78.3

Initial concentration of sorptive: 1.0×10−5 M; pH 5.1; temperature 303 K.

(Ra–Be) neutron source having the neutron flux of 3.9 × 106 cm−2 s−1 and the ␥-dose rate of 1.72 Gy h−1 (total dose: from 41.28 to 123.84 Gy for 24–72 h of irradiation). It is evident from the results (Table 3) that the uptakes at equilibrium were slightly decreased by the different length of exposure of radiation. This decrease in metal ion sorption by irradiated bark samples is attributed to: (a) decrease in the surface area of sorbent, (b) sintering of surface-active sites of solid materials, and (c) destruction of some surface functional groups of bark samples responsible for the metal uptake [22].

4. Conclusions Babool bark is found to be efficient in the removal of Hg2+ from aqueous solution. Increased adsorptive concentration, temperature, and pH favor the removal process. The uptake process follows first order rate law and obeys Freundlich isotherm. Deductions from thermodynamic and desorption data concluded that the uptake of the metal ions was irreversible and endothermic in nature. It may be concluded that the sorbent is very useful and economic for removal of mercury ions.

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