Adsorptive Removal of Pb(II) Ions from Aqueous Solution by (NH4

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Jan 24, 2014 - by 35%.14 In this study, ammonium persulfate was used as an oxidant to ... overnight with pure water to remove decomposed yellow and.

Adsorptive Removal of Pb(II) Ions from Aqueous Solution by (NH4)2S2O8 Oxidized Activated Carbons Motoi Machida,*1,2 Shimeng Chensun,2 Yoshimasa Amano,1,2 and Fumio Imazeki1 1

Safety and Health Organization, Chiba University, Inage-ku, Chiba 263-8522

2

School of Engineering, Chiba University, Inage-ku, Chiba 263-8522

E-mail: [email protected] Received: April 30, 2014; Accepted: September 8, 2014; Web Released: September 12, 2014

A commercially available activated carbon (AC) was oxidized using 1 M H2SO4 solution containing 2 M (NH4)2S2O8 oxidant with the solution to carbon ratio of 50 cm3 g¹1 by changing oxidation period ranging 1 to 10 days at ambient temperatures of 20 or 30 °C to introduce carboxy groups onto the carbon surface to capture Pb(II) ions. Adsorptive removal of Pb(II) ions from aqueous solution was also examined with the oxidized ACs. The Pb(II) ion adsorption progressed via ion exchange with protons of carboxy groups bound to graphite and 2.2 mmol g¹1 of Pb(II) could be accommodated at equilibrium pH above 4.0 for ACs oxidized for at least 10 days at 20 °C and 4 days at 30 °C. Decrease in specific surface area and yield and increase in oxygen and hydrogen content in ACs also observed during the (NH4)2S2O8 treatment implied that the oxidation could convert graphite sheets to smaller size by hydrolysis while many carboxy groups would be introduced to the peripherals of the graphite sheets to scavenge Pb(II) ions.

Activated carbons (ACs) have long been widely applied for water purification to principally remove organic pollutants such as phenol and nitrobenzene utilizing their hydrophobic nature on the carbon surface.1­3 Currently ACs have been extensively utilized and modified by numerous methods to change their structure and surface nature to remove such other contaminants as bulky and polar dye molecules,4 heavy metal ions,5,6 and anionic nitrate ions.7 For the removal of cationic heavy metal ions, a negatively charged surface is required; one is a basic site generated by introducing basic nitrogen and oxygen groups on the carbon surface and π-electrons in hydrophobic graphite layers, on the contrary the other is an acidic site such as carboxy groups in which cationic heavy metals are effectively adsorbed via ion-exchange with protons of carboxy groups on the hydrophilic carbon surface, and the ion-exchange mechanism has been supported in the literature.8­11 Hydroxy, lactone, and other oxygen functional groups, however, did not contribute to heavy metals adsorption very much particularly in acidic and neutral aqueous solution in our previous study.9 Comparing to basic sites such as amino-groups and π-electrons, acidic carboxy sites adsorb heavy metal cations over a wide pH range from 4 to above 7 although basic sites work in basic solutions above pH 8, thereby increasing carboxy groups is one strategy to enhance the uptake of heavy metal cations.10,12 Kasnejad et al. found that Cu(II) adsorption was increased as much as 3.1 mmol g¹1 at equilibrium aqueous solution pH about 6 for a commercial activated carbon modified by NH3 after preoxidation with HNO3.13 Rangel-Mendez and Streat examined the effect of electrochemically oxidized activated carbon cloth made from polyacrylonitrile on Cd(II) adsorption and adsorption of 1.2 mmol g¹1 could be achieved in solution pH above 5 at the expense of the reduction of specific surface area of AC by 35%.14 In this study, ammonium persulfate was used as an Bull. Chem. Soc. Jpn. 2015, 88, 127–132 | doi:10.1246/bcsj.20140124

oxidant to examine conditions for increasing surface oxygen functional groups to effectively remove Pb(II) cations from aqueous solutions.15 Li et al. studied the maximization of acidic oxygen functional groups of carboxy and lactone groups contributing to the adsorption of heavy metal cations with 2 M ammonium persulfate ((NH4)2S2O8, APS) in 1 M H2SO4 solution and found that oxygen functions could be maximized on the activated carbon (AC) with an APS/AC weight ratio of 9.1 at 60 °C for 3 h.16 In our experiments, 2 M APS in 1 M H2SO4 solution was also used and influence of oxidation temperature and time with a higher APS-solution/AC ratio of 50 cm3 g¹1 on the adsorption of Pb(II) ions onto oxidized ACs was investigated, because the higher APS-solution/AC ratio at ambient temperature for oxidizing AC was demonstrated to significantly increase adsorptive removal of Cd(II) ions in our previous study.17,18 In the present study, effect of equilibrium solution pH on the Pb(II) ions adsorption and changes in textural and chemical properties of oxidized ACs were also examined. Oxidation mechanism as well as enhancement of Pb(II) uptake by the oxidized ACs was discussed to elucidate the relationship between the adsorption amounts of Pb(II) ions and oxidation conditions of AC. Experimental Oxidation of Activated Carbon. Commercially available activated carbon (AC), namely A-BAC LP, purchased from Kureha Corporation, Japan, was used in this study. Although AC was bead shaped and ash free, it was repeatedly washed with pure water to remove all the fines and then oven dried at 110 °C before using it in all experiments in this study. The washed AC was oxidized employing 2 M (NH4)2S2O8 (APS) in 1 M H2SO4 solution with APS-solution/AC ratio of 50 cm3 g¹1 at either 20 or 30 °C. The APS solution to AC ratio of © 2014 The Chemical Society of Japan | 127

50 cm3 g¹1 has been already confirmed to be sufficient to prepare the oxidized AC in our previous study ranging 10 to 100 cm3 g¹1 for APS-solution/AC ratio at 25 °C.17 Influence of oxidation time from 1 to 12 days was mainly investigated in this study at ambient temperatures. The oxidized AC was separated from APS solution by filtration, washed with boiled pure water several times followed by Soxhlet extraction overnight with pure water to remove decomposed yellow and brown colored and soluble low molecular impurities and finally oven dried at 110 °C. The yield of oxidized AC was calculated from AC weights before and after oxidation on 110 °C dry base using the equation, ½yield of oxidized AC, % ¼ ð½weight of oxidized ACÞ=ð½weight of ACÞ  100

ð1Þ

The oxidized activated carbon is referred to as Ox-AC and in addition attaching oxidation temperature and time, e.g. Ox-AC(20-10d) represents AC oxidized at 20 °C for 10 days. Characterization of Oxidized Activated Carbons. Specific surface area was calculated by B.E.T. method using N2 adsorption and desorption isotherms at ¹196 °C with a Beckman Coulter SA3100 surface analyzer. Elemental composition of carbon, hydrogen, and nitrogen was measured with a Perkin-Elmer PE2400 microanalyzer. Sulfur content before and after oxidation of ACs was determined with a Rigaku RIX2000 X-ray fluorescence analyzer. Oxygen composition was determined by balance assuming that all other elements would be oxygen. Changes in specific surface area and elemental composition of Ox-ACs were represented as functions of oxidation temperature and time. The Ox-AC samples were dried in an oven at 110 °C in air for 2 h for the elemental analysis and drying at 110 °C in vacuum for 3 h was additionally carried out in the case of measurements of the N2 adsorption and desorption isotherms. Adsorption of Pb(II) onto Prepared Activated Carbons. Adsorption of Pb(II) ions onto Ox-ACs was conducted by a batch technique; 50 mg Ox-AC dosed into 25 mL of Pb(II) solution in a conical flask was agitated at 100 rpm for 24 h. The Pb(II) solution was prepared dissolving Pb(NO3)2 into pure water adjusting to 4.8 mmol-Pb L¹1 (ca. 1000 mg-Pb L¹1) and supplied to the experiments without any further dilution. Equilibrium Pb(II) concentration was measured by atomic absorption spectroscopy (AAS) model Rigaku novAA 300. The amount of Pb(II) adsorption at equilibrium (Qe) was calculated assuming that concentration change before and after dosing AC could be converted to adsorption amount of AC using the equation, v ð2Þ Qe ¼ ðC0  Ce Þ  w ¹1 where Qe is Pb(II) amount on Ox-AC in mmol g , C0 and Ce are the initial and the equilibrium concentration of Pb(II) in mmol L¹1, v is volume of Pb(II) aqueous solution of 25 mL, and w is weight of Ox-AC of 50 mg. Influence of pH on the amount of Pb(II) adsorption was also examined comparing with AC before oxidation by adjusting NaHCO3 solution in which the equilibrium state could be attained one day later after adding NaHCO3. For acidic regions pH was adjusted as well using HNO3 solution. Solution pH 128 | Bull. Chem. Soc. Jpn. 2015, 88, 127–132 | doi:10.1246/bcsj.20140124

was measured with a portable pH meter (HORIBA, D-51). As mentioned in detail in the latter section, equilibrium solution pHs were adjusted above 4.0 when determining the maximum Pb(II) adsorption amounts on Ox-ACs. The maximum Pb(II) adsorption amounts were plotted as functions of both oxidation temperature and oxidation time of ACs. The relationship between the amount of Pb(II) adsorption and that of protons released to aqueous solution was also checked to confirm ionexchange mechanism would be operative in the system using pH changes according to procedures described elsewhere.9,10 In addition, the pH of the point of zero charge (pHPZC) was measured using hydrochloric acid (HCl) and sodium hydroxide (NaOH) to determine surface charge of Ox-ACs comparing to AC before oxidation. The effect of ionic strength on the Pb(II) uptake by Ox-ACs was also inspected by using sodium chloride (NaCl) at the concentration of 2.0 g L¹1 (ca. 34 mmol L¹1) without any pH adjustment for aqueous solutions. Results and Discussion Characterization of Oxidized Activated Carbon. In Table 1 are tabulated textural properties and bulky elemental composition of the starting AC and the oxidized ACs. Not shown in Table 1, sulfur content of the original AC was 0.02 wt % and that after oxidation decreased to 0.01 and 0.003 wt % for Ox-ACs(20-10d) and (30-4d), respectively, even though abundant sulfur containing medium of 2 M (NH4)2S2O8 in 1 M H2SO4 solution was used for the oxidation. Thereby introduction of sulfur to AC as sulfonic groups (­SO3H), for example, could have hardly taken place in the system. Figure 1 shows the plots of specific surface area (SBET) of oxidized activated carbons (Ox-ACs) against oxidation time in days at ambient oxidation temperatures of 20 and 30 °C. The specific surface area (SBET) linearly decreased from 1380 to 20 m2 g¹1 or less until 7 days for 20 °C and 4 days for 30 °C accompanied by losing the hardness of activated carbon beads; part of them were powdered. Interestingly, however, pore diameter (Dtotal) shown in Table 1 kept constant narrow values ranging 1.8 to 2.1 nm, besides both specific surface area (SBET) and pore volume (Vtotal) were considerably decreased with oxidation period. Li et al. also reported significant decrease in specific surface area from 2311 to 469 m2 g¹1 for 24 h in the oxidation of activated carbon using an APS/AC weight ratio of 9.1 at 60 °C.16 Santiago et al. examined oxidation of two types of activated carbons by H2O2, HNO3, and (NH4)2S2O8 and confirmed that the (NH4)2S2O8 treatment significantly reduced specific surface area.19 Additionally sulfuric acid would also play an important role in the oxidation and the decline in specific surface area. The nitric and sulfuric acids mixture was reported to intercalate complete graphite layers and significantly exfoliate the graphite.20,21 Inagaki et al. also showed that only sulfuric acid in aqueous solution effectively exfoliated carbon fiber by either heating at above 500 °C or electrolysis after immersing the carbon fiber into sulfuric acid.22,23 For OxAC(20-10d) and Ox-AC(30-4d) outgassing in helium flow was additionally conducted at 900 °C, and then SBET was regained from below 20 m2 g¹1 to beyond 500 m2 g¹1. These observations revealed that introduction of oxygen functional groups would destroy the AC graphite structure and at the same time much of the functional groups, mainly carboxy groups, could © 2014 The Chemical Society of Japan

Table 1. Properties of AC without Oxidation, Oxidized Activated Carbons (Ox-ACs) and Outgassed AC (Ox-ACs 900OG) Sample name AC

Textural properties Oxidation Oxidation Pore volume, Pore diameter, Surface area temp./°C period/day SBET/m2 g¹1 Vtotal/cm3 g¹1 Dtotal/nma) ® 0 1380 0.61 1.8

Bulk elemental composition Carbon, Hydrogen, Nitrogen, Oxygen, /wt % /wt % /wt % /wt %b) 94 0.66 0.35 5.0

Ox-AC(20-1d) Ox-AC(20-2d) Ox-AC(20-3d) Ox-AC(20-4d) Ox-AC(20-5d) Ox-AC(20-6d) Ox-AC(20-7d) Ox-AC(20-8d) Ox-AC(20-9d) Ox-AC(20-10d)

20 20 20 20 20 20 20 20 20 20

1 2 3 4 5 6 7 8 9 10

1190 1110 840 650 490 290 20 ® ® 6.5

0.54 0.52 0.52 0.31 0.24 0.15 ® ® ® ®

1.8 1.9 1.9 1.9 2.0 2.1 ® ® ® ®

71 66 63 62 63 60 62 61 61 60

0.44 0.54 0.70 0.88 0.94 0.90 0.99 1.0 1.1 1.3

0.04 0.10 0.20 0.33 0.26 0.31 0.39 0.42 0.44 0.51

29 33 36 37 36 39 37 38 37 38

Ox-AC(30-1d) Ox-AC(30-2d) Ox-AC(30-3d) Ox-AC(30-4d) Ox-AC(30-7d) Ox-AC(30-10d)

30 30 30 30 30 30

1 2 3 4 7 10

1030 570 58 5.5 ® ®

0.48 0.27 ® ® ® ®

1.9 1.9 ® ® ® ®

60 61 61 63 60 53

1.8 1.9 1.7 1.7 1.3 1.1

0.29 0.45 0.58 0.84 0.61 0.67

38 37 37 34 38 45

537

0.24

1.8

92

0.37

1.2

6.9

561

0.25

1.8

90

0.22

1.1

8.9

Ox-AC(20-10d) 900OGc) Ox-AC(30-4d) 900OG

a) Calculated from SBET and Vtotal assuming cylindrical shaped pore. b) Calculated by balance. c) Outgassing at 900 °C under He flow for a hour.

Yield of oxidized activated carbon / wt %

130 120 110 100 90 80 0

2

4

6

8

10

12

Oxidation time / day

Figure 1. Decrease in B.E.T. specific surface area (SBET) of activated carbons (ACs) against oxidation period at oxidation temperatures of 20 °C ( ) and 30 °C ( ). Oxidation conditions; 1 M H2SO4 solution containing 2 M (NH4)2S2O8 to AC ratio of 50 mL g¹1.

fill up the space of AC micro pores,24 thereby nitrogen adsorption at ¹196 °C did not efficiently progress very much due to the blocked pores. And then nitrogen gas could enter the pore again after the reduction of the oxygen functional groups from the pore of AC10 due to lowering oxygen content from 40% to below 10% and hydrogen from 1.3­1.7% to less than 0.5% by outgassing at 900 °C, consequently the outgassed AC could Bull. Chem. Soc. Jpn. 2015, 88, 127–132 | doi:10.1246/bcsj.20140124

Figure 2. Changes in yield of oxidized activated carbons (Ox-ACs) against oxidation period at oxidation temperatures of 20 °C ( ) and 30 °C ( ). Oxidation conditions; 1 M H2SO4 solution containing 2 M (NH4)2S2O8 to AC ratio of 50 mL g¹1.

accommodate nitrogen molecules at ¹196 °C again resulting in the increase of SBET more than 500 m2 g¹1 (Table 1). Figure 2 shows changes in yield as a function of oxidation time at 20 and 30 °C. The yield went up at first and attained the maximum at about 110% and 125% within a few days for 20 °C and particularly 30 °C oxidation, respectively, and it was gradually decreased toward around 90% or less. The yields beyond 100% © 2014 The Chemical Society of Japan | 129

2.5

Pb(II) adsorption (Qe) / mmol g-1

Carbon and oxygen / wt %

100 80 60 40 20

2.0 1.5 1.0 0.5 0.0

0 0

2

4

6

8

10

Oxidation time / day Figure 3. Changes in carbon and oxygen composition of oxidized activated carbons (Ox-ACs) against oxidation period at oxidation temperatures of 20 °C ( ; carbon, ; oxygen) and 30 °C ( , ). Oxidation conditions; 1 M H2SO4 solution containing 2 M (NH4)2S2O8 to AC ratio of 50 mL g¹1.

indicated that introduction of oxygen to AC would be predominant compared with destruction of incomplete AC graphite structure that would cause decrease in yield. This is also supported by the fact that turbidity and darkness of the APS solution increased by dissolving decomposed and oxidized graphite from AC to the APS solutions after passing the maximum yields. Figure 3 displays the carbon and oxygen composition variation as a function of oxidation time. Oxygen composition rapidly increased from 5% to 40%, whereas carbon decreased from 95% and 60% in contrast. Considering the results together with decline in specific surface area and yield variation in Figures 1 and 2, oxygenation of incomplete AC graphite structures progressed at first followed by the decomposition of the AC structures, this was more pronounced at 30 than 20 °C. AC would be gradually destroyed by oxidation and hydrolysis while keeping total oxygen content after 1 or 2 days oxidation implying that oxygen might be removed as CO2 and H2O from some oxygen functional groups such as epoxy, carbonyl and hydroxy groups while the other oxygencontaining groups as carboxy would be predominantly introduced by the oxidation to compensate the removed oxygen because of the increase in Pb(II) adsorption as shown in the latter section. Liu et al. examined the oxidation of complete graphite and found that the formation and development of carboxy groups at their saturation level occurred by severe oxidation of graphite, whereas epoxy groups together with a few hydroxy and carbonyl groups were dominant in case of oxidant amount below a level of critical oxidation conditions.25 As can be seen in Table 1, hydrogen composition gradually increased from 0.66% to 1.3% for the 20 °C oxidation and rapidly increased to 1.9% in maximum for the 30 °C oxidation in Ox-ACs with increasing oxidation period. Rise in hydrogen composition from 0.66% to 1.9% in weight is equivalent to increasing from 7.5% to 20% in the molar composition which is 2.7 times more hydrogen introduced by the oxidation for the 130 | Bull. Chem. Soc. Jpn. 2015, 88, 127–132 | doi:10.1246/bcsj.20140124

1

2

3

4

5

6

7

Equilibrium pH (pHe) Figure 4. Increase in Pb(II) ions adsorption onto OxAC(20-10d) of 20 °C oxidation for 10 days ( ) and OxAC(30-4d) of 30 °C oxidation for 4 days ( ) as a function of equilibrium solution pH (pHe) and AC without oxidation ( ) for comparison. Initial Pb(II) ions concentration; 4.8 mmol L¹1. Oxidation conditions; 1 M H2SO4 solution containing 2 M (NH4)2S2O8 to AC ratio of 50 mL g¹1.

Ox-AC(30-2d) case. The increase in hydrogen composition implied that carboxy and hydroxy groups were introduced to the AC surface. Nitrogen composition decreased from 0.35% and then increased to 0.51% or more with oxidation time, implying weakly bound nitrogen groups particularly at the peripheral of graphite sheets would be removed as nitric acid (HNO3) at first and then nitration of AC would progress to introduce nitro groups (­NO2) to AC by released of HNO3 in H2SO4 solution leading to increase in nitrogen content again more than the original AC as listed in Table 1, whereas sulfonation could hardly occur even though AC was in H2SO4 solution with (NH4)2S2O8 oxidant as described previously. Some strongly bound nitrogen might persistently remain in the graphite structure in spite of progressing decomposition of graphite structure of AC as well. Increase in nitrogen content was also observed in the oxidation with air at 425 °C26 and also (NH4)2S2O827 without any nitrogen source in introducing oxygen functional groups. Influence of Equilibrium Solution pH on Pb(II) Adsorption. Solution pH is one of the most important factors affecting adsorption amounts of heavy metal ions, because the carbon surface is switched from positively to negatively charged when solution pH is shifted from acidic to basic; the switching point is commonly called the point of zero charge (pHPZC). The pHpzc values were found to be 7.0 and 2.0 for AC and two Ox-ACs, respectively, in which surface would be negatively charged above pHpzc. Figure 4 shows the influence of equilibrium solution pH (pHe) on the Pb(II) ions adsorption at 25 °C for Ox-AC(20-10d), Ox-AC(30-4d), and AC without oxidation. Adsorption amount of Pb(II) gradually increased when equilibrium solution pH rose from 2 to 4 consistent with pHpzc of Ox-ACs and no further significant increase in Pb(II) adsorption was observed at pH above 4.0 for Ox-ACs. In © 2014 The Chemical Society of Japan

2½­COOH þ ½Pb2þ  ! ½­COO 2 ½Pb2þ  þ 2½Hþ  ð3Þ where [­COOH] and [­COO¹] are carboxy groups formed at graphite structure of Ox-AC. These ion-exchanges were also confirmed by the heavy metals ion adsorption to other oxidized ACs for the Pb(II), Cd(II), and Zn(II) immobilization.9,10,12 The influence of coexisting sodium cations was observed for Ox-ACs, decline of adsorption amounts by only 5% to 7% in the presence of 2.0 g L¹1 (ca. 87 mmol L¹1) NaCl solution at ca. pH 2.3 indicating that Pb2+ ion could be predominantly adsorbed onto carboxy sites compared to Na+ ion. Changes in Pb(II) Adsorption Amounts as a Function of Oxidation Time of AC. Figure 5 represents the variation of Pb(II) uptake at the equilibrium pH (pHe) above 4.0 for the maximum adsorption amount as a function of oxidation time at oxidation temperature of 20 and 30 °C for the initial Pb(II) concentration of 4.8 mmol L¹1, the same Pb(II) concentration in Figure 4. Four days for the 30 °C oxidation (Ox-AC(30-4d)) was sufficient for the maximum amount of Pb(II) adsorption of 2.2 mmol g¹1 while at 20 °C oxidation it took as long as 10 days (Ox-AC(20-10d)) to attain the same capacity. On the basis of maximizing the number of oxygen-containing functional groups on activated carbon by Li et al.,16 oxidation at 60 °C was accessed with oxidation time ranging from 5 to 16 h and adsorption of Pb(II) reached 2.0 mmol g¹1 maximum at 9 h of oxidation which is a little less than 2.2 mmol g¹1 observed by ACs oxidized at 20 and 30 °C. As could be seen in Figure 2, oxidation yield remained around 100% for Ox-AC(30-4d) whereas it decreased to 90% for Ox-AC(20-10d) indicating that the decomposition of incomplete AC graphite was more pronounced for the 10 days oxidation at 20 °C than for 4 days Bull. Chem. Soc. Jpn. 2015, 88, 127–132 | doi:10.1246/bcsj.20140124

2.5 Pb(II) adsorption (Qe) / mmol g-1

contrast, no significant adsorption was observed for AC without oxidation, for which pHpzc was 7.0 indicating the AC surface was always positively charged in this experimental pH region; repulsive force was working between positive Pb2+ and the positively charged AC surface. The saturated adsorption of about 2.2 mmol g¹1 for Ox-ACs represented in Figure 4 is equivalent to the decrease in Pb(II) solution concentration from 4.8 to 0.4 mmol L¹1, which is more than 90% adsorptive removal of Pb(II) from the aqueous solution. In our previous study, the adsorption saturation was also observed at a similar equilibrium pH (pHe) above 4 in the case of the combination of lower initial Pb(II) concentration and fewer carboxy functional groups on AC.12,28 Then hereafter pHe was adjusted between 4.0 and 6.0 to obtain the maximum Pb(II) adsorption capacity for the prepared oxidized activated carbons (Ox-ACs). The speciation diagram of Pb(II) reveals that Pb2+ is the dominant species for the solution pH up to 6.029,30 supporting that Pb2+ was bound to Ox-AC via ion-exchange with two protons (H+) of carboxy groups as well. Since the severely oxidized AC prepared in this study was partially decomposed in the basic region accompanied by changing solution color from clear white to light to dark brown, Pb(II) adsorption was not examined at solution pHe above 7. As described previously, ion-exchange has been also proven to be operative for the Pb(II) adsorption in this study, because approximately 2.0 moles of protons (H+) were confirmed to be always released to aqueous solution per mole of Pb(II) ions removed from the solution in this study as schematically represented below,

2.0 1.5 1.0 0.5 0.0 0

2

4

6

8

10

12

Oxidation time / day Figure 5. Increase in Pb(II) ion adsorption onto Ox-AC of 20 °C oxidation ( ) and Ox-AC of 30 °C oxidation ( ) against oxidation period at the equilibrium solution pH above 4. Initial Pb(II) ions concentration; 4.8 mmol L¹1. Oxidation conditions; 1 M H2SO4 solution containing 2 M (NH4)2S2O8 to AC ratio of 50 mL g¹1.

oxidation at 30 °C in spite of attaining the same Pb(II) adsorption capacity. From the changes in specific surface area (SBET) as displayed in Figure 1, the more SBET decreased as oxidation progressed, the more the adsorption amount of Pb(II) ions increased. Although the increase in adsorption amount in Figure 5 seems to have nothing to do with oxygen composition shown in Figure 3, using the severe oxidation conditions particularly higher APS-solution/AC ratio of 50 mL g¹1, acidic oxygen functional groups could be effectively introduced to carbon surface at the sacrifice of rigid structure of activated carbon, because SBET was already decreased as low as 20 m2 g¹1 or less until attaining the maximum Pb(II) adsorption of 2.2 mmol g¹1. Based on these results, the incomplete AC graphite was presumed to be destroyed by the oxidation with increasing acidic oxygen groups together with the destruction of the graphite under the conditions; the size of graphite sheets would be decreased by hydrolysis of graphite and as a result the edges of graphite sheets would be increased causing the sites to form more acidic oxygen groups, particularly carboxy groups.25 Conclusion Examining oxidation of activated carbon (AC) with (NH4)2S2O8 (APS) in sulfuric acid to increase the adsorption capacity of Pb(II) ions, the following conclusions could be deduced on the basis of the experimental results. 1) Using higher APS solution to AC ratio, nearly 40% oxygen could be introduced to resultant oxidized activated carbons (Ox-ACs), whereas the significant decline in specific surface area (SBET) and decrease in AC yield was observed. 2) Adsorption amount of Pb(II) ions on Ox-AC attained 2.2 mmol g¹1 at equilibrium pH (pHe) above 4.0 for at least 4 days oxidation of AC at 30 °C and for 10 days at 20 °C in which the Pb(II) ions adsorption was estimated to progress via ion-exchange with protons of carboxy groups introduced onto Ox-AC. © 2014 The Chemical Society of Japan | 131

3) Introduction of oxygen functional groups to AC for the Pb(II) uptake is relatively faster than destruction of AC structure for the 30 °C oxidation compared to the 20 °C oxidation. This study was funded in part by the Japan Society for the Promotion of Science (JSPS) under Grants-in-aid for Scientific Research (C) (No. 23510091 and No. 26340058). Gratitude is greatly extended to Ms. Shizuka Ishibashi, Safety and Health Organization, Chiba University, for her dedicated support in the experiments. The authors also acknowledge Dr. Keiichi Nagao, Emeritus Prof. of Chiba University, and Masami Aikawa, Kisarazu National College of Technology, for their encouragement of this study. References 1 A. Dąbrowski, P. Podkościelny, Z. Hubicki, M. Barczak, Chemosphere 2005, 58, 1049. 2 S. Liu, R. Wang, J. Porous Mater. 2011, 18, 99. 3 Y. Kato, M. Machida, H. Tatsumoto, J. Colloid Interface Sci. 2008, 322, 394. 4 S. Oishi, Y. Amano, M. Machida, J. Chem. Eng. Jpn. 2013, 46, 134. 5 X. Zhuang, Y. Wan, C. Feng, Y. Shen, D. Zhao, Chem. Mater. 2009, 21, 706. 6 A. Bhatnagar, W. Hogland, M. Marques, M. Sillanpää, Chem. Eng. J. 2013, 219, 499. 7 T. Iida, Y. Amano, M. Machida, F. Imazeki, Chem. Pharm. Bull. 2013, 61, 1173. 8 H.-H. Cho, K. Wepasnick, B. A. Smith, F. K. Bangash, D. H. Fairbrother, W. P. Ball, Langmuir 2010, 26, 967. 9 M. Machida, T. Mochimaru, H. Tatsumoto, Carbon 2006, 44, 2681. 10 S. Sato, K. Yoshihara, K. Moriyama, M. Machida, H. Tatsumoto, Appl. Surf. Sci. 2007, 253, 8554.

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