Mercury removal from water using activated carbons derived from organic sewage sludge

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Water Research 39 (2005) 389–395 www.elsevier.com/locate/watres

Mercury removal from water using activated carbons derived from organic sewage sludge Fu-Shen Zhanga,, Jerome O. Nriagua, Hideaki Itohb b

a Department of Environmental Health Sciences, School of Public Health, The University of Michigan, Ann Arbor, MI 48109, USA Division of Environmental Research, EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Received 2 February 2004; received in revised form 16 July 2004; accepted 21 September 2004

Abstract Various types of activated carbons were developed from organic sewage sludge (SS) using H2SO4, H3PO4 and ZnCl2 as chemical activation reagents, and the removal of Hg(II) from aqueous solution by these carbons was effectively demonstrated. The quality of the activated carbons was dramatically improved owing to chemical activation. ZnCl2 activated carbon had the highest capability for Hg(II) adsorption, followed by H3PO4 and H2SO4 activated carbons. The adsorption was greatly affected by Hg(II) concentration, solution pH and carbon dosage, and followed Lagergren first order rate equation and Freundlich isotherm model. Desorption results indicated that around 60% to 80% of the adsorbed Hg(II) could be recovered from the carbons to 0.1 M HNO3 solution by sonication treatment. Accordingly, it is believed that the activated carbons developed in this study are effective and practical for utilization in industrial wastewater treatment for mercury removal. r 2004 Elsevier Ltd. All rights reserved. Keywords: Mercury; Activated carbon; Sewage sludge; Adsorption; Wastewater; Desorption

1. Introduction Mercury, which is included in the list of priority pollutants of the US EPA, has been paid great attention for many years. The permitted discharge EPA limit of wastewater for total mercury is 10 mg/L, and the limit for drinking water is 2 mg/L (Nam et al., 2003; USEPA, 2001). Related limits established by the Ministry of the Environment of Japan are, however, 5 and 0.5 mg/L Takahashi et al., 2001, respectively. Meanwhile, the Word Health Organization (WHO) recommends a Corresponding author. Division of Environmental Research, EcoTopia Science Institute, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-8603, Japan. Tel.: +81 52 789 5848; fax: +81 52 789 5849. E-mail address: [email protected] (F.-S. Zhang).

maximum uptake of 0.3 mg per week and 1 mg/L as the maximum acceptable concentration in drinking water (Forster and Wase, 1997). Mercury is carcinogenic, mutagenic, teratogenic and promotes tyrosinemia. High-concentration of mercury causes impairment of pulmonary and kidney function, chest pain and dyspnousea (Berglund and Bertin, 1969). Several important ecological accidents caused by mercury, notably between 1953 and 1956 in Minamata’s bay in Japan, are very well known (Bockris, 1997; Rio and Delebarre, 2003). Numerous physical and chemical separation processes, such as solvent extraction, ion-exchange, precipitation, membrane separation, reverse osmosis, coagulation and photoreduction (Chiarle et al., 2000; Patterson and Passono, 1990; Larson, 1992; Skubal and Meshkov, 2002), have been applied for effective redu-

0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.09.027

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cing of mercury concentrations from various aqueous solutions. However, most of these methods require either high-energy or large quantities of chemicals. Adsorption, on the other hand, is an effective technique for mercury removal from wastewater. Activated carbons are found to be very effective, but it is expensive for large-scale application. In recent years, much of attention was paid to the carbonization of municipal sewage sludge (SS), and the carbonaceous products have been investigated for the adsorption of gaseous pollutants such as toluene, hydrogen sulfide, nitrogen dioxide, sulfur dioxide and liquid pollutants such as phenol, dye, etc. (Chiang and You, 1987; Lu and Lau, 1996; Martin et al., 2003). However, little report is available regarding their adsorption properties on the removal of mercury from aqueous solution. The emphasis of this study was to identify the properties of mercury removal from aqueous solution by activated carbons developed from organic SS. Three kinds of chemical reagents, i.e., H2SO4, H3PO4 and ZnCl2 were employed as activation reagents. The porous properties of the SS carbons, optimum mercury removal conditions and adsorption kinetics and isotherms were extensively examined.

Table 1 Chemical composition of the sewage sludge used for this study Element

Content (%)

Element

Content (mg/kg)

H C N Si P S Cl Na Mg Al K Ca Fe Zn Ba

6.10 39.4 4.46 2.48 1.61 1.93 0.04 0.42 0.27 0.15 0.49 1.10 1.00 0.18 0.11

Ti V Cr Mn Co Ni Cu Sr Cd Pb LOIa (%)

703 5.99 136 157 1.99 92.8 212 84.8 3.99 54.9 77.1

a

Loss on ignition at 850 1C.

pH value above 6. The activated carbons were then vacuum dried at 105 1C for 24 h, ground and sieved too2 mm for use. 2.2. Characterization of the activated carbons

2. Experimental details 2.1. Activated carbon preparation SS was obtained from the Yamazaki Sewage Sludge Disposal Plant in Nagoya of Japan. The chemical composition of the sludge was listed in Table 1. Since the sludge contains about 40% of carbon on a dry weight basis, and the loss on ignition (LOI) value is 77.1%, we therefore call it organic SS. Sludge sample was firstly dried at 105 1C for 24 h in an oven, air cooled, and crushed into uniform size. A portion of 100 g of the sample was impregnated into 250 ml of 3 M H2SO4, 3 M H3PO4 or 5 M ZnCl2 solutions, then stirred thoroughly until well mixed, and left to stand for 24 h. After the supernatant liquid was removed, the samples were subjected to vacuum drying at 105 1C or 24 h. The resulting chemical loaded samples were then pyrolyzed in a quartz tube (42 mm i.d.) in N2 atmosphere. The heating rate was 10 1C/min, and the N2 gas flow rate was 300 ml/min. The samples were held at 650 1C for 60 min. After cooling the carbonized products that have been activated with H2SO4 and H3PO4 were washed with 1 M NaOH solution, followed by filtration. The samples were then washed by distilled water for several times until the pH value of the leachates were o7. For the sample treated with ZnCl2, a 1 M HCl solution was used instead of NaOH solution in the washing procedure and the sample was then washed by distilled water to reach a

The BET surface areas of the SS carbons were obtained from nitrogen adsorption isotherms at 77 K using an ASAP 2010 analyzer (Micromeritics, Norcross, GA, USA). The isotherms were also used to calculate the micropore volume (Vmic), total pore volume (Vt) and pore size distribution. All the parameters were based on Dubinin–Radushkevich and Brunauer–emmet–Teller methods (Dubinin, 1966). The samples were degassed at 120 1C for 24 h in a vacuum oven before the experiment. The pH of the zero point charge (pHZPC) was measured according to the procedure proposed by Noh and Schawz (Trawczynski et al., 2002). Specifically, three solutions with different initial pH (pH ¼ 3,6 and 11) were prepared using 0.1 M solutions of HNO3 and NaOH. For each initial pH, different amount of SS carbon sample (1%, 5%, 10% and 20% in weight) was mixed with 100 ml of the solution. The equilibrium pH was measured after 24 h. NaNO3 was used as the background electrolyte. The plot of pH versus mass fraction exhibits a plateau and the pHZPC was taken as the average of three asymptotic pH values. 2.3. Chemical analyses The C, H, N elemental composition of the activated carbons and the SS sample was measured by a CHN corder (Yanako, MT-6). Si, P, S, Cl contents were

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determined using a scanning electron microscope (SEM, JSM-6330F) coupled with an energy-dispersive X-ray spectrometer (EDS, JED-2140). Metallic elements were analyzed with inductively coupled plasma atomic emission spectroscopy (ICP-AES, Perkin-Elmer). The pH value of the adsorption solutions were adjusted with diluted HCl or NaOH solutions and determined by an Orion 710A pH meter.

391

2.5. Desorption study The SS carbons used for removal of 120 mg/L of Hg(II) solution were separated by filtration. The Hg(II)loaded carbons were then transferred into triangle flasks containing 200 ml 0.1 M HNO3 solution, the flasks were capped immediately and then sonicated for 30 min at 60 1C. Mercury in the solution was determined as above.

2.4. Adsorption experiments An adsorbate stock solution of 1000 mg/L Hg(II) was prepared from Hg(NO3)2 (Aldrich). This solution was diluted to 10–120 mg/L for adsorption experiments. Two series of experiments were carried out, i.e., a Hgadsorption kinetics series and a Hg-adsorption isotherms series. The former one was aimed to determine the time necessary to reach the steady state and the Hg(II) removal efficiency by the activated carbons. In this section, optimum vibration time, pH range, effects of carbon dosage and Hg(II) concentration on the adsorption were examined. The studies were conducted in batch systems at 25 1C, and capped 250 ml polyethylene bottles were used. The SS carbon doses varied from 0.1 to 10 g/L and Hg(II) concentrations varied from 10 to 200 mg/L. The amount of Hg(II) removal was calculated from the differences between the concentrations of Hg(II) before and after adsorption. Hg(II) concentrations in the solutions were determined using a Tekran CVAFS mercury detector (Model 2500) connected with a Hewlett-Packard printer (HP 3396A integrator). Method detection limit (MDL) for this instrument is 0.2 ng/L. Carbon-free controls were run concurrently in all experiments. The later adsorption series was carried out to draw the adsorption isotherms. In this section, a series of 250 ml capped polyethylene bottles was employed and each bottle was filled with 100 ml of 120 mg/L Hg(II) solution. A known carbon dose varied from 0.01 to 1.0 g was added to each bottle, and the pH was adjusted to 5.0 using diluted HCl or NaOH solutions. The temperature was 25 1C and the contact time was 7 h to assure the equilibration of the adsorption according to the preliminary experiments.

3. Results and discussion 3.1. Porous properties The surface porous properties of the SS carbons are presented in Table 2. Chemical activation dramatically improved the quality of the carbonaceous products. For example, the BET surface areas of the three types of activated carbons (H2SO4, H3PO4 and ZnCl2 activated carbons were abbreviated as SS-S, SS-P and SS-Z, respectively) increased by 111–305% compared to that of SS-C (no activation treatment), and moreover, the total pore volumes (Vt) and the micro pore volumes (Vmic) increased by 92–231% and 63–394%, respectively. The average pore diameters (Dp) of the activated carbons are within 2.26–5.21 nm, indicating that the chemical activation mainly developed meso pores (2 nmoDpo50 nm) in the activated carbons. 3.2. Adsorption properties 3.2.1. Adsorption kinetics Results of the adsorption kinetic experiments are presented in Fig. 1. Hg(II) adsorption by the activated carbons increased sharply at a short contact time and slowed down gradually with approaching equilibrium. The times to attain equilibrium were different according to SS carbon types. Equilibriums were attained at 180 min for SS-C, 300 min for SS-S and SS-P, and 420 min for SS-Z, respectively. Of the four types of SS carbons, SS-Z has the highest Hg(II) adsorption capability, followed by SS-S, SS-P and SS-C. The high-adsorption capacity for SS-Z could attribute to

Table 2 Surface properties of the SS carbons and adsorption constants for Hg(II) uptake by the carbons Surface properties

SS-C SS-S SS-P SS-Z

pHZPC

SBET (m2/g)

Vmic (cm3/g)

Vt (cm3/g)

Dp (nm)

137 408 289 555

0.016 0.026 0.053 0.079

0.227 0.523 0.436 0.752

6.65 5.21 2.65 2.26

5.38 4.26 4.12 4.89

Lagergren constants

Freundlich constants

Kad (min1)

R2

1=n

KF (mg/g)

R2

0.018 0.014 0.013 0.008

0.9918 0.9893 0.9894 0.9905

0.96 0.91 0.86 0.32

10.1 13.6 16.0 42.6

0.9972 0.9901 0.9869 0.9926

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392

SS-C SS-S SS-P SS-Z

2.5

SS-S SS-Z

SS-C SS-P

120

2.0

90

log (q e-q)

Hg (II) adsorbed (g/kg)

150

60 30

1.5

1.0

0.5

0 0

100

200

300

400

500

600

700

Time (min)

0

50

100

150

200

250

300

350

Time (min)

Fig. 1. Hg(II) removal by various SS carbons as a function of contact time. Initial Hg(II) concentration ¼ 80 mg/L, pH ¼ 5.0, temperature ¼ 25 1C. SS-C: inactivated sewage sludge carbon; SS-S: H2SO4 activated SS carbon; SS-P: H3PO4 activated SS carbon; SS-Z: ZnCl2 activated SS carbon.

its high BET surface area and micro pore volume (Table 2). The Lagergren first-order rate equation (Mohan et al., 2001; Yardim et al., 2003) was applied so as to describe the adsorption kinetics for Hg(II) adsorption on the SS carbons. The equation is written as logðqe  qÞ ¼ log qe  ðK ad =2:303Þt;

0.0

(1)

where q is the amount of Hg(II) adsorbed at any time (t) in the unit of mg/g; qe is the amount of Hg(II) adsorbed at equilibrium (mg/g); Kad is the adsorption rate constant (min1). Plotting log (qeq) vs. t gives straight lines as can be seen in Fig. 2. The qe calculated from the plots were 44.8, 58.4, 96.9, 130 mg/g for SS-C, SS-S, SS-P, SS-Z, respectively. These values have a good agreement with the experimental ones (43.9, 57.6, 95.8, 128 mg/g, respectively), indicating that Hg(II) adsorption on the SS carbons can be approximated to the first order rate expression. The adsorption parameter Kad was calculated from the slope of the plot and was presented in Table 2. Kad values for the four types of SS carbons were within 0.008–0.018 min1, highest for SS-C and lowest for SS-Z. Therefore, among the four types of SS carbons, although SS-Z has a higher adsorption capacity for Hg(II), its adsorption rate is relatively lower, thus needs longer time to reach saturation. Furthermore, it is found that the Kad values obtained in this study are consistent with the values obtained from Hg(II) adsorption on activated carbon derived from fertilizer waste (Mohan et al., 2001), but lower than the values obtained from Hg(II) adsorption on activated carbon manufactured from furfural (Yardim et al., 2003), suggesting that the Hg(II) adsorption mechanism of SS carbon is similar to that of fertilizer waste carbon.

Fig. 2. Lagergren plots for Hg (II) adsorption onto various SS carbons.

3.2.2. Effects of pH, Hg(II) concentration and carbon dosage The effect of pH on Hg(II) adsorption was studied with the pH range adjusted to around 1–12 (Fig. 3). It is noticed that the adsorption increased with the increase of pH value, and reached a plateau value at the pH range of 5–12. In order to identify the mechanism of Hg(II) adsorption onto the SS carbons, the distribution diagrams of mercury species at different pH values were evaluated using the software PSEQUAD (Zekany and Nagyral, 1985). Fig. 4 indicates that the dominant mercury species in the solution is Hg 2+ at pHo3.0, and Hg(OH)2 at pH45 and both of these species between pH 3 and pH 5. In addition, small amount of HgOH+ was also detected between pH 2 and pH 6, with a percentage of around 1% to 13% of the total Hg(II). Moreover, the pH values of zero point charge (pHZPC) for the SS carbons were found to be within 4.12–5.38 (Table 2). The surfaces of the SS carbons are positively charged when the pH of the mercury solution is less than pHZPC, which is unfavorable for the adsorption of cationic mercury species such as Hg2+ and HgOH+, thus lower adsorption rates at PHo5 were obtained in Fig. 3. The predominant species of mercury is Hg(OH)2 at pH45 (Fig. 4), hence it is of concern if the mercury precipitates at a higher pH range. Fig. 5 depicts the solubility of Hg(II) vs. solution pH in the absence of the adsorbent. No significant change of dissolved Hg(II) was found at a pH range of 1–12 and a initial concentration of o120 mg/L, implying that Hg(OH)2 dissolves in the solution. Accordingly, it is impossible to precipitate the mercury only by adjusting the solution pH when the Hg(II) concentration is o120 mg/L. Similar results were also reported by other researchers (Linke, 1958; Spence et al., 2003).

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Hg (II) adsorbed (g/kg)

150 120 90 60 30

SS-C SS-P

SS-S SS-Z

0 0

2

4

6 pH

8

10

12

Fig. 3. Effect of pH on the removal of Hg(II) by various SS adsorbents. Initial Hg(II) concentration ¼ 80 mg/L, contact time ¼ 7 h, temperature ¼ 25 1C.

Furthermore, the high affinity of the SS carbons with Hg(OH)2 in the pH range of 5–12 could be well explained by the Pearson rule (Pearson, 1988), because SS carbon and mercury are soft base and soft acid, respectively. According to Pearson theory, during acid–base reaction, hard acid prefer to co-ordinate with hard base and soft acid to soft base, and neutral molecule is softer acid than metal cation (Manohar et al., 2002). Hg(II) removal by the SS carbons increased sharply along with the increase of Hg(II) concentration, e.g., a removal range of 73.5–151.3 mg/kg at 120 mg/L was obtained compared to 1.71–6.63 mg/kg at 10 mg/L (Fig. 6). Moreover, the effect of SS carbon dosage on the adsorption was also studied with SS carbon doses varying from 0.1 to 10 g/L at a fixed initial Hg(II) concentration of 200 mg/L ( Fig. 7). The initial Hg(II) concentration used in this study was relatively higher

100

Hg (II ) adsorbed (g/kg)

160

80

% Hg (II)

393

Hg(OH)2

Hg 2+

60 40

HgOH +

20 0

SS-C SS-S SS-P SS-Z

120

80

40

0 0

2

4

6

8

10

12

0

20

pH

40

60

80

100

120

Hg Concentration (mg/L)

Fig. 4. Distribution diagrams of mercury species established with PSEQUAD software for an initial concentration of Hg(II) ¼ 80 mg/L. Only major species are shown, those being less concentrated than 1% of the total concentration are omitted.

Fig. 6. Hg(II) removal by various SS carbons as a function of different Hg(II) concentrations. pH ¼ 5.0, contact time ¼ 7 h, temperature ¼ 25 1C.

100

Dissolved Hg (II) (mg/L)

200

Hg ( II) removal (%)

Initial Hg (II) 200 mg/L

Initial Hg (II) 120 mg/L

150

100

80 60 SS-C SS-S SS-P SS-Z

40 20

50

Initial Hg (II) 80 mg/L

0 0

2

4

6

8

10

Adsorbent dosage (g/L)

0 0

2

4

6 pH

8

10

12

Fig. 5. Dissolved Hg (II) concentration as a function of solution pH in the absence of the adsorbent.

Fig. 7. Percentage of Hg(II) removal from water as a function of various SS carbon doses. Initial Hg(II) concentration ¼ 200 mg/L, pH ¼ 5.0, contact time ¼ 7 h, temperature ¼ 25 1C.

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because according to our preliminary study, a minimum SS carbon dose of 1 g/L could remove 70–99% of Hg(II) from an initial concentration range of 10–60 mg/L. Fig. 7 shows in order to remove 200 mg Hg(II) from 1 L aqueous solution, at least 4 g of SS-Z, 6 g of SS-P or 8 g of SS-S were needed, but 10 g of SS-C could only remove 83.4% of the Hg(II) in the solution. Additional experiment indicated that at least 14 g of SS-C was necessary so as to remove the total Hg(II) in the solution. 3.2.3. Adsorption isotherms The adsorption isotherms of Hg(II) onto various SS carbons were studied at 25 1C with initial Hg(II) concentration of 120 mg/L. The data were fitted to Freundlich isotherm. The Freundlich expression is an empirical equation based on a heterogeneous surface, which is given as follows: log qe ¼ log K F þ ð1=nÞ log C e

(2)

where qe is the amount adsorbed (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/L), and KF and n are the Freundlich constants related to adsorption capacity and adsorption intensity, respectively. Linear plots of log qe vs. log Ce show that the adsorption follows Freundlich isotherm model (Fig. 8). The Freundlich constants 1=n and KF were calculated using the slopes and intercepts of the lines and were listed in Table 2. High values of R2 indicate that the adsorption follows Freundlich isotherm model perfectly. The values of 1=n; between 0 and 1, indicate the heterogeneity of the SS carbons (Mishra et al., 1998). Furthermore, the smaller 1=n and larger KF values for SS-Z indicate that SS-Z carbon has higher adsorption capacity, intensity and affinity for mercury than the other types of SS carbons (Zhang and Itoh, 2003; Vazquez et al., 2002).

3.3. Desorption study Desorption of the Hg(II) from the SS carbon was examined so as to better understand the adsorption mechanisms and elucidate the feasibility of recovering both the carbons and Hg(II). The recoveries for SS-C, SS-S, SS-P and SS-Z were found to be 68.8%, 63.5%, 77.2% and 81.6%, respectively. These results indicate that the removal of Hg(II) from water by the SS carbons was mainly through affinity adsorption.

4. Conclusion Carbonation of waste materials resulted from municipal wastewater treatment plant has been extensively conducted by previous researchers. However, less information is available regarding mercury removal properties by the carbonaceous solids from aqueous solution. In this study, various types of activated carbons were developed from organic sewage sludge, and the removal of Hg(II) from aqueous solution by these carbons was effectively demonstrated.

 The





2.5

2.0

log q e

 1.5 SS-C SS-S SS-P SS-Z

1.0

0.5



quality of activated carbons was dramatically improved due to chemical activation. Compared to inactivated SS-C, the BET surface areas of SS-S, SS-P and SS-Z increased by 111–305%, and the total pore volumes (Vt) and the micro pore volumes (Vmic) increased by 92–231% and 63–394%, respectively. Of the different types of SS carbons, SS-Z has the highest adsorption capability, followed by SS-S, SS-P and SS-C, and the adsorption capacities increased along with the increase of Hg(II) concentration. 4 g of SS-Z, 6 g of SS-P or 8 g of SS-S was sufficient for removal of 200 mg Hg(II) from 1 L water solution, but 10 g of SS-C could only remove 83.4% of the Hg(II) in the solution. Hg(II) removal by the SS carbons was pH dependent. The adsorption increased with the increase of pH value at pH 1–5, and reached a plateau value at the pH range of 5–12. The time to attain equilibrium on Hg(II) adsorption was different according to SS carbon types. Equilibrium was attained at 180 min for SS-C, 300 min for SS-S and SS-P, and 420 min for SS-Z, respectively. Desorption study indicated that around 60% to 80% of the adsorbed Hg(II) could be recovered from the SS carbons.

0.0 0.0

0.5

1.0

1.5

2.0

log Ce Fig. 8. Freundlich isotherm of Hg(II) adsorption onto various SS carbons. Initial Hg (II) concentration ¼ 120 mg/L, pH ¼ 5.0, contact time ¼ 7 h, temperature ¼ 25 1C.

Acknowledgements This work was supported in part by the Japan Society for the Promotion of Sciences (JSPS). Acknowledge-

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ment should also go to Mr. K. Sano for providing sewage sludge sample.

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