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Journal of Environmental Management 90 (2009) 634e643 www.elsevier.com/locate/jenvman Removal of mercury from aqueous solutions using activated carbo...

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Journal of Environmental Management 90 (2009) 634e643 www.elsevier.com/locate/jenvman

Removal of mercury from aqueous solutions using activated carbon prepared from agricultural by-product/waste M. Madhava Rao, D.H.K. Kumar Reddy, Padala Venkateswarlu, K. Seshaiah* Analytical and Environmental Chemistry Division, Department of Chemistry, Sri Venkateswara University, Tirupati 517 502, AP, India Received 19 April 2007; received in revised form 7 December 2007; accepted 23 December 2007 Available online 3 March 2008

Abstract Removal of mercury from aqueous solutions using activated carbon prepared from Ceiba pentandra hulls, Phaseolus aureus hulls and Cicer arietinum waste was investigated. The influence of various parameters such as effect of pH, contact time, initial metal ion concentration and adsorbent dose for the removal of mercury was studied using a batch process. The experiments demonstrated that the adsorption process corresponds to the pseudo-second-order-kinetic models and the equilibrium adsorption data fit the Freundlich isotherm model well. The prepared adsorbents ACCPH, ACPAH and ACCAW had removal capacities of 25.88 mg/g, 23.66 mg/g and 22.88 mg/g, respectively, at an initial Hg(II) concentration of 40 mg/L. The order of Hg(II) removal capacities of these three adsorbents was ACCPH > ACPAH > ACCAW. The adsorption behavior of the activated carbon is explained on the basis of its chemical nature. The feasibility of regeneration of spent activated carbon adsorbents for recovery of Hg(II) and reuse of the adsorbent was determined using HCl solution. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Mercury; Removal; Ceiba pentandra hulls; Phaseolus aureus hulls; Cicer arietinum waste

1. Introduction Water pollution by the disposal of effluents containing heavy metals has become worldwide concern for the past few decades. It is well known that some heavy metals are harmful and cause toxic effects to human beings and disturb sound ecological environments. Mercury is one of the first metals known and its compounds have been under use throughout human history. Mercury and its compounds with unusual chemical and physical properties are global pollutants. Even at very low concentration, mercury causes potential hazards due to its accumulation in food chain. A special characteristic of mercury is its strong absorption into biological tissues and slow elimination from them. The major sources of mercury pollution are effluents from chloralkali, pulp paper, oil refining, electrical, rubber processing and fertilizer industries (Baeyens et al., 1996). Other major source of mercury emission into the atmosphere is flue

* Corresponding author. Tel.: þ91 877 2242770. E-mail address: [email protected] (K. Seshaiah). 0301-4797/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2007.12.019

gases from coal combustors used in electricity generation (Morimoto et al., 2005; Li et al., 2003). This reveals that more than half of the mercury released into the environment today is from anthropogenic sources. Acute exposure to elemental mercury may cause chest pain, labored breathing, vomiting, diarrhea, fever, metallic taste in the mouth, and a skin rash. Chronic exposure may lead to tremors, limb weakness, loss of appetite, excessive shyness, irritability, headache and memory loss. Mercury also has adverse effects on the central nervous system, pulmonary kidney functions and the chromosomes (Yardim et al., 2003). It is reported that mercury exposure is more harmful for developing fetuses and children under the age of four because it interferes with normal brain development. This necessitates the removal of mercury from wastewater before its recycle transport and discharge into the environment. Commercial activated carbon is a well-known adsorbent for the removal of heavy metals from natural water and wastewaters. But its high cost limits its use as an adsorbent. Hence, there is a growing need to develop low cost activated carbon adsorbent materials from cheaper and locally available

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agricultural waste materials. Namasivayam and Kadirvelu (1999), Mohan et al. (2001), Krishnan and Anirudhan (2002) and Kannan and Malar (2005) studied the removal of mercury from water and wastewater using low cost activated carbon adsorbents prepared from different agricultural materials. Ceiba pentandra tree is widely distributed in the deciduous forests of western and eastern India especially in high temperature areas. The plant parts, root, bark, gum and leaf except hulls have medicinal applications. Cotton obtained from its fruits is used in stuffing pillows and mattresses. But hulls obtained from the fruits are of no economic importance and are considered as an agricultural waste. Phaseolus aureus plant is a native of India. Its small oval seeds are highly nutritious and sprouted seeds are one of the important components of Indian diet. The hulls generated after removing seeds are disposed as a solid waste in rural India. Cicer arietinum is another important pulse crop in India. The flour of its seeds is used for the preparation of unleavened bread, sweets and variety of food products. Germinated C. arietinum seeds are used as prophylactic against vitamin C deficiency disease, scurvy. After harvesting the crop and collecting the seeds the remaining parts of the plant are considered as a waste and burnt in the field. In the present work, removal of mercury from aqueous solutions using activated carbon prepared from C. pentandra hulls (ACCPH), P. aureus hulls (ACPAH) and C. arietinum waste (ACCAW) is reported. The effect of parameters such as pH, adsorbent dose and its initial concentration, adsorption isotherms and contact time and adsorption kinetics is also reported. The experimental equilibrium adsorption data are analyzed by both Freundlich and Langmuir isotherm models. The desorption studies for the regeneration of adsorbent were also carried out using hydrochloric acid solution. The concentration of mercury in solution was determined by inductively coupled plasma atomic emission spectroscopy. 2. Materials and methods 2.1. Procedure for adsorbent preparation and activation Hulls of C. pentandra, P. aureus and C. arietinum waste were collected from local fields of Rudrakota village in the Nellore district in the state of Andhra Pradesh (India), during the months of April to June. The hulls/waste was cut into small pieces, washed several times with deionized double distilled water and dried. The carbonization and steam activation of hulls/waste was performed following the reported procedure reported elsewhere (Rao et al., 2006). The resulting activated carbon powder was ground in a mill, washed and dried. The sample was sieved to 100 mesh size and stored in a desiccator. 2.2. Chemicals All the chemicals used were of analytical reagent grade. Deionized double distilled water was used throughout the experimental studies. Stock mercury solution (1000 ppm) was prepared by dissolving 1.354 g of HgCl2, in about 700 mL of deionized double distilled water and 1.5 mL of

635

conc. HNO3 was added and made up to 1000 mL with deionized double distilled water. Working standards were prepared by progressive dilution of stock mercury solution. ACS reagent grade HCl, NaOH and buffer solutions (E. Merck) were used to adjust the solution pH. 2.3. Instrumentation Inductively coupled plasma atomic emission spectrometry (Varian Liberty Series (II), Australia) with Wipro Acer computer was used for determining Hg(II) concentration. The instrumental operating parameters used were photomultiplier tube voltage 700 V, incident power 1.1 kW, plasma gas flow 15.0 L min1, auxillary gas flow 1.5 L min1, observation height 14.0 mm, pump rate 15.0 rpm, sample uptake 25 times and wave length used 253.652 nm. An Elico (LI-129) pH meter was used for pH measurements. The pH meter was standardized using buffer solutions of pH values 4.0, 7.0 and 9.2. A mechanical shaker (Macro scientific works, Delhi, India) was used for agitating the samples. Fourier transform infrared spectrometry (Thermo-Nicolet FT-IR, Nicolet IR-200, USA) was used to analyze the organic functional groups of the adsorbent. Vario EL, Elementar, Germany was used for elemental analysis of the prepared adsorbents, ACCPH, ACPAH and ACCAW. Muffle furnace (Tempo, Bombay) was used for carbonization and activation of the adsorbents. 2.4. Batch mode adsorption studies Removal of Hg(II) using activated carbon prepared from the hulls of C. pentandra (ACCPH), P. aureus (ACPAH) and C. arietinum waste (ACCAW) was carried out by batch method and the influence of various parameters such as effect of pH, contact time, activated carbon dose and initial sorbate concentration was studied. For each experimental run, 50 mL of metal solution of known concentration was taken in 100 mL stoppered reagent bottles and pH was adjusted to the desired value and a known amount of the activated carbon was introduced. The solutions in bottles were agitated at room temperature (30  1  C) using a mechanical shaker for a prescribed time to attain equilibrium. At the end of the predetermined time intervals, the samples were taken out and the supernatant solution was separated from the activated carbon (ACCPH, ACPAH, ACCAW) by centrifugation at 20,000 rpm for 20 min and analyzed the concentration of mercury by ICP-AES method. Blank solutions were treated similarly (without adsorbent) and the concentration was taken as initial concentration. Effect of pH was studied in the range of 2.0e9.0, by adjusting pH with the addition of dilute aqueous solution of HCl or NaOH and buffer solutions. Effect of adsorbent dose was studied in the range of 25e300 mg. Kinetics and effect of contact time on adsorption was determined at different time intervals of 5e120 min. Adsorption isotherms and effect of initial concentration were studied by varying the initial metal ion concentration from 10 mg/L to 140 mg/L. All the batch experiments were carried out in duplicate and the values reported

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are average of two readings. The experimental conditions for these studies are shown below. Initial concentration of mercury taken in each reagent bottle (mg/L) Amount of activated carbon added to each reagent bottle (mg) pH Agitation time (min)

can be The Freundlich isotherm equation qe ¼ kfC1/n e written in the linear form as given below: 1 log qe ¼ log kf þ log Ce n

10, 20, 30, 40, 50, 60, 80, 100, 120, 140

25, 50, 75, 100, 125, 150, 175, 200, 225, 250, 300

2.0e9.0 5, 15, 30, 40, 50, 60, 80, 100, 120

ð3Þ

where qe and Ce are the equilibrium concentrations of mercury in the adsorbed and liquid phases in mmol/g and mmol/L, respectively. kf and n are the Freundlich constants that are related to the sorption capacity and intensity, respectively. The Freundlich constants kf and n can be calculated from the slope and intercept of the linear plot, with log qe vs log Ce.

2.5. Desorption studies 2.7. Kinetic models The reversibility of adsorption was investigated by carrying out desorption experiments. Once equilibrium was reached, adsorbent saturated with mercury was removed from solution and transferred to stoppered reagent bottle (250 mL capacity), containing 100 mL of (0.05e0.15 M) concentrated HCl solution and the bottles were shaken at 150 rpm for 4 h at room temperature (30  1  C) using a mechanical shaker. The sorbent was then removed by centrifugation. The concentrations of mercury in the aqueous solution were determined by ICP-AES method. 2.6. Adsorption isotherm models The experimental adsorption equilibrium data were analyzed in terms of Langmuir and Freundlich isotherm models (Purna Chandra Rao et al., 2006) for the purpose of interpolation and limited extrapolation. The relative coefficients of these models were calculated using linear least-squares fitting. The Langmuir sorption isotherm equation qe ¼ QmbCe/ (1 þ bCe) on linearization becomes Ce Ce 1 ¼ þ qe Q m Q m b

ð1Þ

The sorption kinetic data of mercury measured on activated carbon was analyzed in terms of pseudo-first-order and pseudo-second-order sorption equations (Purna Chandra Rao et al., 2006). The pseudo-first-order equation is shown below: dqt ¼ k1 ðqe  qt Þ dt

where k1 (min1) is the rate constant of the pseudo-first-order sorption, qt (mg/g) denotes the amount of sorption at time t (min), and qe (mg/g) is the amount of sorption at equilibrium. After definite integration by application of the conditions qt ¼ 0 at t ¼ 0 and qt ¼ qt at t ¼ t, Eq. (4) becomes k1 t ð5Þ 2:303 The sorption rate constant, k1, can be calculated by plotting log(qe  qt) vs t. The pseudo-second-order equation can be written as log ðqe  qt Þ ¼ log qe 

dqt 2 ¼ k2 ðqe  qt Þ dt

where qe and Ce are the equilibrium concentrations of mercury in the adsorbed and liquid phases in mmol/g and mmol/L, respectively. Qm and b are Langmuir constants, which are related to sorption capacity and energy of sorption, respectively, and can be calculated from the intercept and slope of the linear plot, Ce/qe vs Ce. The essential characteristics of the Langmuir isotherm can also be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, RL, which is defined as

1 1 ¼ þ k2 t qe  q t q e

RL ¼ 1=ð1 þ bCo Þ

t 1 1 ¼ þ t 2 qt k 2 qe q e

ð2Þ

where b is the Langmuir constant and Co is the initial concentration of mercury. The RL value indicates the type of the isotherm. RL value RL > 1 RL ¼ 1 0 < RL < 1 RL ¼ 0

Type of isotherm Unfavorable Linear Favorable Irreversible

According to Mckay et al. (1982) RL values between 0 and 1 indicate favorable adsorption.

ð4Þ

ð6Þ

where k2 (g/mg/min) is the rate constant. Integration of Eq. (6) and application of the conditions qt ¼ 0 at t ¼ 0 and qt ¼ qt at t ¼ t, give ð7Þ

The following equation can be obtained on rearranging Eq. (7) into a linear form ð8Þ

k2 and qe can be obtained from the intercept and slope of plotting t/qt vs t. 3. Results and discussion 3.1. Characteristics of the adsorbent The physical characteristics along with the percentage of carbon, hydrogen, nitrogen and sulphur of the adsorbents

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500

400

Intensity

ACCPH, ACPAH and ACCAW are given in Table 1. X-ray diffraction spectra of the three adsorbents (Figs. 1e3) did not show any crystalline peaks indicating that the ACCPH, ACPAH and ACCAW are amorphous in nature, which is an advantageous property for well-defined adsorbents (Mohan and Singh, 2002). Surface area is the most important property of activated carbon adsorbents for its adsorption capacity. Generally, the higher the surface area, the larger is its adsorptive capacity (Noll et al., 1992). It was observed that the adsorbent ACCPH has the highest surface area and lowest ash content among the three adsorbents. The presence of ash inhibits surface area development because inorganics may fill or block some portions of existing micropore volume. This may be the reason for the lower surface area observed in carbons with high ash content (Bernardo et al., 1997; Pendyal et al., 1999). The IR absorption spectra of ACCPH, ACPAH and ACCAW adsorbents are shown in Figs. 4e6. The IR absorption spectra of three activated carbons show a broad band in the 3300e3500 cm1 region, which could be assigned to OH stretching mode from hydroxyl and phenolic groups, involved in hydrogen bonding may be due to adsorbed water (Vinke et al., 1994; Chen et al., 2002). A weak but sharp absorption band at 3735, 3790 cm1 appeared for the adsorbents ACPAH and ACCAW may be ascribed to isolated OH groups (Puziy et al., 2002). The doublet peak at 2920e2888 cm1 appeared in the IR spectra of ACCPH may be due to the CeH stretching vibrations. The absorption peaks at 2354, 2355 cm1 appeared in the spectra of ACPAH and ACCAW adsorbent could be attributed to C^N stretching (Jia et al., 2002). In ACCPH adsorbent spectra, the bands appearing at 1595 and 1380 cm1 are ascribed to the formation of oxygen functional groups like a highly conjugated C]O stretching in carboxylic groups and carboxylic moieties (Jia and Thomas, 2006). The IR spectra of ACPAH and ACCAW adsorbents show absorbance peaks at 1567 and 1595 cm1, which are due to the presence of C]O in quinine structure (Tsai et al., 2001). The absorption peak at 1118 cm1 of ACCPH adsorbent is due to S]O group and this assignment is supported by the elemental analysis studies (Krishnan and Anirudhan, 2003). The broad bands appeared at 1300e1000 cm1 for all adsorbents have been assigned to CeO stretching in alcohols and phenols confirming the OH groups in all the three

637

300

200

100

0 10

20

30

40

50

60

70

80

90

CuKα (2θ) Fig. 1. X-ray diffraction spectra of ACCPH.

adsorbents, ACCPH, ACPAH and ACCAW. The IR spectral studies thus revealed that the adsorbent ACCPH contains eOH, C]O and S]O whereas ACPAH and ACCAW contain eOH, C]O functional groups which are mainly responsible for the sorption of mercury through chemical bonding. 3.2. Effect of pH on removal of mercury The effect of pH on removal of Hg(II) by the adsorbents ACCPH, ACPAH and ACCAW was carried out in the pH range of 2.0e9.0 and the results are shown in Fig. 7. It can be noticed that the removal of Hg(II) increased with increasing pH of aqueous solution and reached maximum value at pH 6.0 for the adsorbent ACCPH. However, in the case of adsorbents, ACPAH and ACCAW the maximum Hg(II) removal was observed at pH 7.0. It is reported (Zhang et al., 2005) that Hg2þ exists as the dominant species in the solution at pH < 3.0 and Hg(OH)2 at pH > 5.0 whereas both the species exist in the pH range 3.0 and 5.0. In the presence of Cl, the species such as HgCl2, (HgCl2)2, Hg(OH)þ and HgOHCl

500

Parameter

ACCPH

ACPAH

ACCAW

Specific surface area (m2/g) pHpzc Conductivity (ms/cm) Ash (%) Moisture (%) Carbon (%) Hydrogen (%) Nitrogen (%) Oxygen (%) Sulphur (%)

521

325

280

6.2 45 9.1 5.0 20.63 0.35 48.90 30.12 e

6.5 40 11.4 7.0 21.24 0.43 48.23 30.11 e

5.7 52 6.7 10.0 56.25 2.60 0.44 39.22 1.49

Intensity

400

Table 1 Characteristics of the ACCPH, ACPAH and ACCAW

300

200

100

0 10

20

30

40

50

60

70

CuKα (2θ) Fig. 2. X-ray diffraction spectra of ACPAH.

80

90

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the optimum pH for removal of Hg(II) was fixed as 6.0 for ACCPH and 7.0 for ACPAH and ACCAW.

400 350

3.3. Effect of contact time on removal of mercury

300

Intensity

250

The results of the removal of Hg(II) by ACCPH, ACPAH and ACCAW as a function of contact time are graphically shown in Fig. 8. More than 50% of Hg(II) was adsorbed within 10 min and equilibrium adsorption was attained in 90, 100 and 110 min for the adsorbents ACCPH, ACPAH and ACCAW, respectively. Therefore, the optimum time for attaining adsorption equilibrium is 120 min.

200 150 100 50 0 10

20

30

40

50

60

70

80

90

3.4. Effect of initial metal ion concentration

CuKα (2θ) Fig. 3. X-ray diffraction spectra of ACCAW.

are also present in small amounts between pH 4.0 and 6.0. The lower removal of Hg(II) by the adsorbent ACCPH when pH was 6.0 and by ACPAH and ACCAW when pH was 7.0 can be explained on the basis that the point of zero charge (PZC) of the ACCPH, ACPAH and ACCAW which was found to be 5.7, 6.2 and 6.5, respectively. The surfaces of the three activated carbons were positively charged when the pH of the mercury solution was less than pHpzc of the respective adsorbents, which was unfavorable for the removal of cationic species such as Hg2þ and Hg(OH)þ thus decreasing the removal of Hg(II). By taking above considerations into account

The effect of initial mercury concentration in the aqueous solution on the removal of Hg(II) by the three adsorbents ACCPH, ACPAH and ACCAW is shown in Fig. 9. As expected the removal capacity of all the adsorbents increased with increasing initial concentration. The removal of Hg(II) increased from 2.5 mg/g to 29.20 mg/g for ACCPH, from 2.5 mg/g to 23.37 mg/g for ACPAH and from 2.5 mg/g to 21.49 mg/g for ACCAW when initial concentration of Hg(II) was increased from 10 mg/L to 150 mg/L, respectively. At lower concentration (below 20 mg/L), total removal of Hg(II) was observed. This suggests that the adsorbents ACCPH, ACPAH and ACCAW are effective for complete removal of Hg(II) from water if its initial concentration is below 20 ppm.

Fig. 4. FT-IR spectra of ACCPH.

M.M. Rao et al. / Journal of Environmental Management 90 (2009) 634e643

Fig. 5. FT-IR Spectra of ACPAH.

Fig. 6. FT-IR Spectra of ACCAW.

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640

30

9

25

qe, mg/g

10

qe, mg/g

8 7

20 15 10

6

ACCPH ACCPH

ACPAH

5

ACPAH

5

ACCAW

ACCAW

0 0

4 1

2

3

4

5

6

7

8

9

20

40

60

80

100

120

140

160

Co, mg/L

10

pH

Fig. 9. Effect of initial concentration on removal of mercury.

Fig. 7. Effect of pH on removal of mercury.

3.5. Effect of adsorbent dosage on removal of mercury Adsorbent dosage is an important parameter in adsorption studies because it determines the capacity of adsorbent for a given initial concentration of metal ion solution. It was observed that with increasing adsorbent dosage from 25 mg to 300 mg the percent removal of Hg(II) increased up to 99.7% for the adsorbent ACCPH, 98.0% for the adsorbent ACPAH and 96.29% for the adsorbent ACCAW. Further increasing dosage has no effect on Hg(II) removal. The increase in percent removal of Hg(II) is expected with increase in adsorbent dosage as the number of active sites increases with dosage. Hence higher dosage of adsorbent has positive effect on the initial rate of metal ion removal. However, there was decrease in adsorption density (adsorption capacity for unit mass of the adsorbent) from 32.75 mg/g to 6.65 mg/g for ACCPH adsorbent, from 26.82 mg/g to 6.60 mg/g for ACPAH adsorbent and from 22.90 mg/g to 6.50 mg/g for ACCAW adsorbent with increase in the dosage of adsorbent. This is basically

due to the fact that the availability of Hg(II) in solution phase decreases with increase in adsorbent dosage per unit adsorbent and adsorption sites remain unsaturated during the adsorption reaction. The results are presented graphically in Figs. 10e12. Similar type of trend was reported for the removal of Hg(II) from aqueous solution by sorption on polymerized saw dust (Raji and Anirudhan, 1996). 3.6. Adsorption isotherms The distribution of metal ions between the activated carbon and the metal ion solution for the system at equilibrium is of importance in determining the maximum adsorption capacity of the activated carbon. The experimental data relating to the adsorption of Hg(II) onto ACCPH, ACPAH and ACCAW adsorbents are interpreted by Langmuir and Freundlich adsorption isotherm models and the isotherms are graphically shown in Figs. 13 and 14. It illustrates a comparative adsorption of Hg(II) on three activated carbon adsorbents. The 35

10 100

% Removal

qe, mg/g

9

8

7

Adsorption density 60

15 10

ACPAH

5

ACCAW

20 0

5 0

20

40

60

80

100

120

140

t, min Fig. 8. Effect of contact time on removal of mercury.

20

% Removal

40

ACCPH 6

25

80

Adsorption density (mg/g)

30

160

50

100

150

200

250

300

Adsorbent dose, mg Fig. 10. Effect of adsorbent dose for the removal of mercury onto ACCPH (initial concentration 40 mg/L; pH 6.0; contact time 90 min).

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30

30

100

20 Adsorption density % Removal

60

15

10

40

25

qe, mg/g

% Removal

80

Adsorption density (mg/g)

25

20

15

10 ACCPH

5

5

20 0

50

100

150

200

250

ACPAH

300

ACCAW

Adsorbent dose, mg

0 0

Fig. 11. Effect of adsorbent dose for the removal of mercury onto ACPAH (initial concentration 40 mg/L; pH 7.0; Contact time 100 min).

10

20

30

40

50

Ce, mg/L Fig. 13. Langmuir sorption isotherms of mercury.

parameters of adsorption isotherms are calculated by linear least-squares method for each activated carbon and the regression coefficient values are presented in Table 2. As seen from the results, Q0 value of ACCPH adsorbent for removal of Hg(II) is higher than that of ACPAH and ACCAW adsorbents, respectively. This demonstrates the higher adsorption capacity of ACCPH adsorbent compared to other two adsorbents. Higher adsorption of Hg(II) onto ACCPH adsorbent can be attributed to the presence of surface sulphur groups on ACCPH, which are zero on ACPAH and ACCAW adsorbents. Maximum removal of Hg(II) obtained with ACCPH adsorbent can be explained on the basis of Pearson theory. According to Pearson theory, hard acids prefer to co-ordinate with hard bases and soft acids to soft bases. Hg(II) ions are soft Lewis acids and as a rule, the interaction of Hg(II) species such as HgCl2, (HgCl2)2, Hg(OH)2 and HgOHCl with surface sulphur groups (soft bases) would be favoured. Krishnan and Anirudhan (2002) reported enhanced adsorption of Hg(II) on activated carbon containing sulphur due to the formation of Hg(HS)2 and

Hg2(HS)2 species and their retention in the pores of the carbon particles by the following possible redox reaction  2 2þ 2Hg2þ þ SO2 3 þ 2OH /Hg2 þ SO4 þ H2 O

Further, Morimoto et al. (2005) discussed that SO3 present on the carbon reacts with water leading to the formation of H2SO4 on the surface of activated carbon. The maximum adsorption capacity of ACCPH adsorbent was 25.88 mg/g. The observed order of the adsorption capacity of three activated carbon adsorbents was ACCAW
30

25 100

Adsorption density % Removal

15

60

10

40

25

qe, mg/g

% Removal

80

Adsorption density (mg/g)

20

20

15

10 ACCPH 5

20

ACPAH ACCAW

5 0

50

100

150

200

250

300

Adsorbent dose, mg Fig. 12. Effect of adsorbent dose for the removal of mercury onto ACCAW (initial concentration 40 mg/L; pH 7.0; Contact time 110 min).

0 0

10

20

30

40

Ce, mg/L Fig. 14. Freundlich sorption isotherms of mercury.

50

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Table 2 Langmuir and Freundlich constants for mercury

12

Adsorbent

Freundlich Kf (mg11/n L1/n g1) n R2

ACCPH

ACPAH

ACCAW

25.88 0.45 0.8167

23.66 0.51 0.9016

22.88 0.36 0.9273

11.24 4.11 0.9686

9.51 3.73 0.9661

8.36 3.64 0.9660

10

t/qt, g/mg.min

Langmuir Q0 (mg/g) b (L/mg) R2

8

6

4 ACCPH 2

Freundlich coefficient, Kf, which represents the adsorption capacity, was 11.24 for ACCPH, 9.51 for ACPAH and 8.36 for ACCAW. The other Freundlich coefficient ‘n’ values were 4.11 for ACCPH, 3.73 for ACPAH and 3.64 for ACCAW fulfilling the condition of 1 < n < 10 for favorable adsorption. It is observed from isotherms and regression coefficients that the fit is better with Freundlich model than with Langmuir model. The Freundlich type adsorption isotherm is an indication of surface heterogeneity of the adsorbent. This leads to the conclusion that the surfaces of ACCPH, ACPAH and ACCAW adsorbents were made of small heterogeneous patches, which were very much similar to each other in respect to adsorption phenomenon. For comparison, adsorption capacities of the different activated carbon adsorbents are presented in Table 3. 3.7. Adsorption kinetics The adsorption kinetic study is quite significant in water treatment as it describes the solute uptake rate, which in turn controls the residence time of adsorbate uptake at the solidesolution interface. In order to study the kinetics of Hg(II) adsorption by ACCPH, ACPAH and ACCAW adsorbents, pseudo-first- and pseudo-second-order-kinetic models were employed. The kinetic plots were drawn for three activated carbons adsorbents and the adsorption rate constants (k1, k2) and adsorption capacity (qe) were calculated from the slope and intercepts of the plots of log(qe  qt) against ‘t’ (figure not shown) for the first order, and t/qt against ‘t’ (Fig. 15) for the second-order. The results are reported in Table 4. It can be seen from Fig. 15 that plots for all the three activated carbon adsorbents are linear with correlation coefficients of more than 0.98. This shows that pseudo-second-order model describes the Table 3 Comparison of adsorption capacity of Hg(II) with other activated carbons Adsorbent

Q0 (mg/g) References

PHC-peanut carbon Sago waste carbon ACCPH ACPAH ACCAW Commercial activated carbon Dates nut carbon (DC) Granular activated carbon

110 55.60 25.88 23.66 22.88 12.38 1.16 0.8

Namasivayam and Periasamy, 1993 Kadirvelu et al., 2004 Present work Present work Present work Namasivayam and Periasamy, 1993 Kannan and Malar, 2005 Ma et al., 1992

ACPAH ACCAW

0 0

20

40

60

80

100

120

t, min Fig. 15. Pseudo-second-order-kinetic plots for mercury removal.

adsorption kinetics of the systems studied more appropriately than the pseudo-first-order equation. In this study, rapid equilibration is attained not only by the affinity of activated carbon to mercury species but also by the hydrophilic character. 3.8. Desorption studies Desorption of the adsorbed Hg(II) from the spent adsorbent was carried out using different concentrations of HCl (0.05e 0.15 M).The percent recovery of Hg(II) increased with increase in concentration of HCl from 0.05 M to 1.10 M and then remained constant (Table 5). Desorption of Hg(II) by HCl can be attributed to the disruption of coordination with mercury ions. These results indicate that the removal of Hg(II) from water by the three activated carbon adsorbents is mainly through affinity adsorption. 4. Conclusions Removal of mercury from aqueous solution is shown to be influenced by the characteristics of the activated carbon prepared from C. pentandra hulls (ACCPH), P. aureus hulls (ACCPAH) and C. arietinum waste (ACCAW). Higher removal of mercury by ACCPH adsorbent may be due to presence of sulphur functional group. The optimum conditions for the removal of mercury is at pH 6.0, contact time 90 min and adsorbent dosage 200 mg/50 mL for ACCPH adsorbent; pH 7.0, contact time 100 min and adsorbent dosage of 225 mg/

Table 4 Adsorption kinetics for mercury Adsorbent

Pseudo-first-orderkinetics k1 (min1)

ACCPH ACPAH ACCAW

2

6.4  10 6.31  102 5.3  102

qe (mg/g)

R2 0.9284 0.9011 0.9131

Pseudo-second-orderkinetics k2 (g/mg min)

10.51 10.61 10.21

2

1.12  10 9.21  103 7.01  103

R2 0.9966 0.9968 0.9897

M.M. Rao et al. / Journal of Environmental Management 90 (2009) 634e643 Table 5 Desorption data corresponding to mercury Adsorbent

Initial concentration Hg(II) (mg/L)

Desorptiona with 0.05 M HCl

Desorptiona with 0.10 M HCl

Desorptiona with 0.15 M (HCl)

ACCPH

40 60

61.9 58.1

79.3 76.1

79.1 76.1

ACPAH

40 60

50.1 51.0

68.6 62.4

68.7 61.9

ACCAW

40 60

51.0 50.6

60.5 56.0

59.8 55.4

a

All values are percent recovery of mercury.

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