Available online at www.sciencedirect.com
JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X
Journal of Environmental Sciences 2013, 25(Suppl.) S101–S105
www.jesc.ac.cn
Effect of calcium on adsorption capacity of powdered activated carbon Gang Li1,2,3, ∗, Junteng Shang1 , Ying Wang1,3 , Yansheng Li1,3 , Hong Gao2 1. College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028, China 2. School of Materials Science and Engineering of Dalian Jiaotong University, Dalian 116028, China 3. Liaoning Institution of Higher Learning Key Laboratory for Environmental Science & Technology, Dalian 116028, China
Abstract We investigated the effect of calcium ion on the adsorption of humic acid (HA) (as a target pollutant) by powered activated carbon. The HA adsorption isotherms at different pH and kinetics of two different solutions including HA alone and HA doped Ca2+ , were performed. It was showed that the adsorption capacity of powdered activated carbon (PAC) for HA was markedly enhanced when Ca2+ was doped into HA. Also, HA and Ca2+ taken as nitrate were tested on the uptake of each other respectively and it was showed that the adsorbed amounts of both of them were significantly promoted when HA and calcium co-existed. Furthermore, the adsorbed amount of HA slightly decreased with the increasing of Ca2+ concentration, whereas the amount of calcium increased with the increasing of HA concentration, but all above the amounts without addition. Finally, the change of pH before and after adsorption process is studied. In the two different solutions including HA alone and HA doped Ca2+ , pH had a small rise, but the extent of pH of later solution was bigger. Key words: powered activated carbon; calcium; adsorption; humic acids
Introduction Activated carbons with the highly developed porous structures and abundant chemical groups are being used more than ever as an adsorbent in many applications. These cover a wide spectrum of systems including water and wastewater treatment, separations, and hazardous waste treatment (Franz et al., 2000; wang et al., 2005; Terzyk and Rychlicki, 2000). Its high adsorption capacity is mainly due to special physical structures and chemical properties (Bacaoui et al., 2002; Fletcher et al., 2006). Depending on the form, activated carbons are grouped into two categories: granular activated carbon (GAC) and powered activated carbon (PAC). Compared with GAC, PAC having bigger specific surface area and more rich pores thus becomes effective adsorbent for organic micro-pollutants in water treatment (Quinlivan et al., 2005; Jin et al., 2008). Meanwhile, natural organic matter (NOM), either in public water treatment or in emergency drinking water treatment, is the commonest micro-pollutant in various bodies of water. With complex components (Newcombe and Drikas, 1997; Newcombe et al., 2002), NOM produced the toxic disinfection byproducts in the process of halogenated water disinfection treatment, causing secondary pollution (Guang and Reckhow, 2007). Therefore, the study of the * Corresponding author. E-mail: [email protected]
adsorption of NOM by PAC has been the hot topic for a long time. Qi and Schideman (2008) has studied the sorption mechanism of NOM on activated carbon using an overall isotherm method. Their results demonstrated the physical adsorption mechanism of NOM on activated carbon and effect of pore size on adsorption capacity. Schideman et al. (2007) has investigated competitive adsorption of NOM and other organic pollutants on granular activated carbon. Also Newcombe et al. (2002) has studied competitive adsorption of 2-methylisoborneol (MIB) and NOM on activated carbon. The group found that low-molecular-weight NOM compounds were the most competitive, participating in direct competition with MIB for adsorption sites. In that study, some evidence of pore blockage and/or restriction was also seen, with microporous carbons being the most affected by low-molecular-weight NOM and mesoporous carbons impacted by the higher-molecular-weight compounds. However, in practice, water components are more complex, which not only contains organic matter but also large amount of inorganic matter such as metal ions. Some researchers found inorganic materials such as calcium, iron, aluminum and manganese were accumulated on GAC treating natural water and the accumulation of calcium, 10 to 68 mg/g, was the largest among all of these materials (Valix et al., 2006; Baccar et al., 2009; Li et al., 2009;
S102
Journal of Environmental Sciences 2013, 25(Suppl.) S101–S105 / Gang Li et al.
Lee et al., 2003). Frederick et al. (2001) has studied the effect of calcium chelating agent on adsorption captivity of granular activated carbon in the presence of salicylate or phthalate. Mazyck et al. (2005) also found that pH of effluent water increased after calcium accumulated on the activated carbon. Moreover, the studies on regeneration of activated carbon, mainly thermal regeneration technology (Bagreev et al., 2001; Liu et al., 2004), has found that calcium accumulated on activated carbon produced the catalytic effect, destroying the micropore distributed on activated carbon by enlarging the pore size and thus reducing the adsorption capacity for low-molecular-weight compounds (Mazyck and Cannon, 2000). Hence, it is of vital importance to study the effect of calcium accumulation on the adsorption captivity of activated carbon. This study, based on the analysis of adsorption of calcium on PAC, investigated the effect of calcium ion on adsorption of HA by PAC. The interaction of calcium and HA in the process of adsorption was also studied.
1 Experimental 1.1 Analytical method Calcium nitrate was served as the source of calcium ion. The water quality was high with less than 0.5 µS/cm conductivity. All chemicals used were analytical reagent grade. The concentration of solute remaining in the solution was quantified. The HA concentration was measured with a UV-2102C UV/Vis Spectrophotometer (China Unico (Shanghai) Instrument Ltd.) at 254 nm. The calcium ion concentration was measured by method of EDTA titration. Solution pH was determined by a PHS-3C digital pH-meter (China) using a glass electrode. For powdered activated carbon, pore characteristics were measured on Tristar3020, ASAP2020 Automated surface area and pore size analyzer (USA Micromeritics Instrument Corporation). Zero charge was determined by gravimetric titrimetry (Liu et al., 2007) and functional groups were measured by Boehm method (Salame and Bandosz, 2003). Physical properties of powdered activated carbon: surface area 978.4269 m2 /g, pore volume 0.5417 cm3 /g, average pore size 2.2146 nm. Chemical properties of powdered activated carbon: acid group 0.39 mmol/g, base group 0.935 mmol/g, carboxyl group 0.365 mmol/g, phenolic group 0.16 mmol/g, zero point charge pHPZC 9.97.
Vol. 25
1.2.1 Isothermal adsorption experiments at different pH For the humic acid adsorption, adsorption isotherms at three different pH including neutral (pH 7.0), acidic (pH 4.0), and basic (pH 10.0) were conducted. Every six 100 mL aliquots containing 100 mg/L HA was added with 0.1– 0.6 g PAC as a series of solutions. The pH of each series was then adjusted to 4.0, 7.0, and 10.0, respectively. The final solutions were shaken for 12 hr to approach equilibrium and then absorbed amount of HA were measured. 1.2.2 Adsorption kinetics experiments Adsorption kinetics of humic acid in the absence and presence of calcium was carried out in stirred stoppered flasks. One series of 100 mg/L HA solutions was added 0.1 g PAC and 100 mg/L calcium ion, another was only added 0.1 g PAC. The samples were taken at appropriate time intervals, and then filtered and analyzed to obtain the HA concentration. 1.2.3 Simultaneous adsorption of calcium ions and humic acid To further investigate the mutual influence of HA and Ca2+ on the adsorption process, the simultaneous calciumHA adsorption experiments were conducted. Firstly, 0–100 mg/L calcium ion were added into 100 mL aliquots each containing 100 mg/L HA and 0.1 g PAC, and then the suspension was kept under stirring for 12 hr to reach equilibrium. Secondly, 0–25 mg HA was added into 100 mL aliquots containing 400 mg/L calcium nitrate and 0.1 g PAC to react. Then adsorbed amount of HA and Ca2+ was measured to investigate the interaction between them. 1.2.4 Change of pH before and after adsorption Before reaction, pH of different HA solutions varying from 50 to 250 mg/L in the absence and presence of calcium nitrate was measured. Each solution was then added 0.1 g PAC and shaken for 12 hr to reach equilibrium. After adsorption, the pH of each above solution was measured again to observe the change of pH. 1.2.5 Effect of pH on calcium adsorption To determine the effect of solution pH on the calcium adsorption, 1 g PAC was added to 100 mL of Ca2+ solution (100 mg/L) with different pH from 1 to 12 and then the mixed solution was shaken at 25°C and 125 r/min until the equilibrium was achieved. Finally, the adsorbed amount of Ca2+ was measured to determine the effect of pH.
1.2 Sorption experiments In all following cases, the pH of solutions were adjusted by adding small amounts of NaOH or HCl and stoppered flasks were shaken for 12 hr at 25°C and 125 r/min using a thermostat shaker bath (Aohua THZ-82A, China). Each mixture was filtered through 0.45 µm filter papers before measurement.
2 Results and discussion 2.1 Effect of pH on the adsorption of humic acid Many researches have found that adsorption of HA on activated carbon is mainly attributed to the abundant developed pores that well distributed on the activated carbon to
Effect of calcium on adsorption capacity of powdered activated carbon
cut off the HA molecular, the mechanism of which belongs to physical adsorption. In addition, the electrostatic forces also contributed to HA adsorption on PAC. As shown in Fig. 1, adsorption amount of HA decreased with pH increasing and this tendency was more significant at the higher concentration. On one hand, this is because at lower pH, more of the weakly acidic groups existing in HA are in an uncharged state, which makes them more adsorbable. On the other hand, at higher pH (during alkaline range) the high concentration of OH− groups may cover the surface of PAC, which can increase the electrostatic repulsion between HA and surface of PAC and can provide a better solvent for the humic matter, and then to induce desorption. 2.2 Effect of pH on calcium adsorption It has been found that enormous amount of functional groups, such as carboxyl, phenolic and aliphatic hydroxyl, exist on the surface of activated carbon. The adsorption of calcium on the activated carbon is accomplished mainly through the ion exchange and electrostatic attraction on these functional groups. When pH of solutions is less than pH of adsorbent, the surface of adsorbent would be positively charged, while in opposite situation the surface would be negatively charged. As shown in Fig. 2, the amount of calcium adsorbed on PAC increased with increasing of pH generally. Under pH of less than 3, the solution pH was far less than pH of PAC, the positively charged surface generated the electrostatic repulsion between calcium ion and PAC. At that time, the adsorption of calcium on activated carbon should mainly be attributed to ion exchange, but calcium adsorption nearly did not occur. Firstly, this is because at very low pH surface of PAC would take up a large amount of H+ , which competed with Ca2+ and then reduced its adsorption. Secondly, activated carbon with high hydrophilicity would 450 400 350
8
0
8 10 12 pH Fig. 2 Effect of pH on the adsorption of calcium by PAC at the initial Ca2+ concentration of 100 mg/L and 293 K.
100 50 0
50
100
150 Ce (mg/L)
200
250
Fig. 1 Adsorption isotherms of HA by PAC at different pH. Qe : adsorbing capacity; Ce : equilibrium concentration.
2
4
6
adsorb large amount of H2 O molecular, which would occupy the adsorption site and obstruct the calcium ions adsorption. When pH increased from 3 to 10, the amount of calcium adsorption steadily increased due to decreasing of repulsion forces. When the pH value was from 3 to 10, the adsorption of calcium ion was mainly completed through ion exchange with the positively charged functional groups on the surface of PAC, but the total amount was small. As the medium pH turned into strong alkaline range (that is above 10), the surface of PAC was negatively charged, which induced increasing greatly of electrostatic attraction between PAC and calcium. As a result, the adsorption capacity increased significantly (Fig. 2). 2.3 Kinetics of HA adsorption The data from the kinetic behavior of the HA adsorption by PAC from HA solution and one of HA doped Ca2+ solution were presented in Fig. 3. It was observed that the adsorption reaction concentrated in forepart of the whole reaction course, especially before 300 min. The reason was that adsorbate occupied the pores spaces of
Qe (mg/g)
Qe (mg/g)
150
4
0
300
200
6
2
pH 4 pH 7 pH 10
250
S103
10
Qe (mg/g)
Suppl.
760 740 720 700 680 660 640 620 600 580 560 540 520 500 480 460 440 420
HA + Ca2+ HA 0
100
200
300
400 500 600 700 800 Time (min) Fig. 3 Kinetics of HA adsorption from two different solutions.
Journal of Environmental Sciences 2013, 25(Suppl.) S101–S105 / Gang Li et al.
S104
700
Vol. 25
120
600 100 80
Qe (mg/g)
Qe (mg/g)
500 400 300
60 40
200
20
100 0
0 0 50 100 150 200 250 300 20 40 60 80 100 2+ Ca concentration (mg/L) HA concentration (mg/L) Fig. 4 Influence on HA adsorption by PAC at different Ca2+ concentrations (a) and Ca2+ adsorption at different HA concentrations (b) at pH 7 and 293 K. 0
PAC as reaction proceeded, which reduced adsorption capacity and rate of activated carbon. Figure 3 also showed the amount of HA adsorbed from the HA doped Ca2+ solution, comparing with HA alone, was higher regardless of reaching equilibrium, which proved the presence of calcium ion enhanced the overall adsorption capacity of PAC for HA.
Figure 4a showed adsorption of HA by PAC at different calcium concentrations, and Fig. 4b showed adsorption of calcium by PAC at different HA concentrations. It could be found from Fig. 4a that uptaken amount of HA increased in the presence of calcium. However with the increasing of calcium concentration, adsorbed amount reduced but all were more than the amount without calcium. Meanwhile, the presence of HA also enhanced the adsorption of calcium, just as observed from Fig. 4b, which indicated HA would chelate calcium ion and exist as complexion CaHA adsorbed then by PAC. 2.5 Change of pH values before and after adsorption As shown in Fig. 5, pH values of two different test solutions slightly increased. The reason was that after adsorption of HA reduced its concentration in solutions and then induced the increasing of solution pH. Moreover, the increase extend of pH was bigger in the solution of HA doped Ca2+ than HA alone, because the adsorption of HA was promoted in the presence of Ca2+ , which leaded to the lower HA concentration and higher pH.
HA+Ca before HA+Ca after
7 6 5 pH
2.4 Effect of calcium on HA adsorption and effect of HA on calcium adsorption
HA before HA after
8
4 3 2 1 0 20
40
60
80
100 120 140 160 180 200 220 HA concentration (mg/L) Fig. 5 pH values of test solutions before and after adsorption.
with the increase of pH. (3) The co-existence of calcium and humic acid promoted both HA and Ca adsorption comparing to each component existing alone. (4) The medium pH increased slightly after adsorption and this increase extend was bigger in the solution of HA doped Ca2+ than in the one of HA alone. Acknowledgments This work was supported by the Liaoning Institution of Higher Learning Key Laboratory for Environmental Science & Technology Dalian Jiaotong University.
3 Conclusions
References
(1) The adsorption of HA increased with the decrease of pH and the tendency was more obvious at the higher HA concentration. (2) The amount of calcium uptaken by PAC increased
Franz M, Arafat A H, Pinto G N, 2000. Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon. Carbon, 38: 1807–1819. Wang S B, Zhu Z H, Coomes A, Haghseresht F, Lu G Q,
Suppl.
Effect of calcium on adsorption capacity of powdered activated carbon
2005. The physical and surface chemical characteristics of activated carbons and the adsorption of Methylene Blue from wastewater. Journal of Colloid and Interface Science, 284: 440–446. Terzyk P A, Rychlicki G, 2000. The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro the temperature dependence of adsorption at the neutral pH. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 163: 135–150. Bacaoui A, Dahbi A, Yaacoubi A, Bennouna C, MaldonadoHodar F J, Rivera-Utrilla J et al., 2002. Experimental design to optimize preparation of activated carbons for use in water treatment. Environmental Science and Technology, 36: 3844–3849. ¨ Fletcher A J, YUzak Y, Thomas K M, 2006. Adsorption and desorption kinetics for hydrophilic and hydrophobic vapors on activated carbon. Carbon, 44: 989–1004. Quinlivan P A, Li L, Knappe D R U, 2005. Effects of activated carbon characteristics on the simultaneous adsorption of aqueous organic micropollutants and natural organic matter. Water Research, 39: 1663–1673. Jin W, Xu Z X, Cao D W, 2008. Study on removal of dissolved organic compounds in raw water by powdered activated carbon. Journal of Tongji University (Natural Science), 36: 1670–1674 Newcombe G, Drikas M, 1997. Adsorption of NOM onto activated caebon: electrostatic and non-electrostatic effects. Carbon, 35: 1239–1250. Newcombe G, Morrison J, Hepplewhite C, 2002. Simultaneous adsorption of MIB and NOM onto activated carbon. I. Characterisation of the system and NOM adsorption. Carbon, 40: 2135–2146. Guang H H, Reckhow D A, 2007. Characterization of disinfection byproduct precursors based on hydrophobicity and molecular size. Environmental Science and Technology, 41: 3309–3315. Qi S Y, Schideman L C, 2008. An overall isotherm for activated carbon adsorption of dissolved natural organic matter in water. Water Research, 42: 3353–3360. Schideman L C, Snoeyinka V L, Marinasa B J, Ding L, Campos C, 2007. Application of a three-component competitive adsorption model to evaluate and optimize granular activated carbon systems. Water Research, 41: 3289–3298.
S105
Newcombe G, Morrison J, Hepplewhite C, Knappe D R U, 2002. Simultaneous adsorption of MIB and NOM onto activated Carbon II. Competitive effects. Carbon, 40: 2147–2156. Valix M, Cheung W H, Zhang K, 2006. Role of heteroatoms in activated carbon for removal of hexavalent chromium from wastewaters. Journal of Hazardous Materials B, 135: 395– 405. Baccar R, Bouzid J, Feki M, Montiel A, 2009. Preparation of activated carbon from Tunisian olive-waste cakes and its application for adsorption of heavy metal ions. Journal of Hazardous Materials, 162: 1522–1529. Li Z L, Ma S F, Wang D, Wang C Y, Wei M Y, 2009. Adsorption of metal ions and organics by activated carbon during water treatment isotherms and mechanisms: A review. Environmental Science and Management, 34: 88–93. Lee S H, Nishijima W, Lee C H, Okada M, 2003. Calcium accumulation on activated carbon deteriorates synthetic organic chemicals adsorption. Water Research, 37: 4631– 4636. Frederick H T, Cannon F S, Dempsey B A, 2001. Calcium loading onto granular activated carbon with salicylate or phthalate. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 177: 157–168. Mazyck D W, Cannon F S, Bach M T, Radovic L R, 2005. The role of calcium in high pH excursions for reactivated GAC. Carbon, 43: 511–518. Bagreev A, Rahman H, Bandosz T J, 2001. Thermal regeneration of a spent activated carbon previously used as hydrogen sulfide adsorbent. Carbon, 39: 1319–1326. Liu X T, Quan X, Bo L L, Chen S, Zhao Y Z, 2004. Simultaneous pentachlorophenol decomposition and granular activated carbon regeneration assisted by microwave irradiation. Carbon, 42: 415–422. Mazyck D W, Cannon F S, 2000. Overcoming calcium catalysis during the thermal reactivation of granular activated carbon Part I. Steam-curing plus ramped-temperature N treatment. Carbon, 38: 1785–1799. Liu S J, Li Y G, She Y Q, 2007. Effect of pore structure and surface chemical characteristics of a activated carbon on the adsorption of aqueous nitrobenzene. Environmental Science and Management, 32: 108–112. Salame I I, Bandosz T J, 2003. Role of surface chemistry in adsorption of phenol on activated carbons. Journal of Colloid and Interface Science, 264: 307–312.
JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X
Journal of Environmental Sciences 2013, 25(Suppl.) S101–S105
www.jesc.ac.cn
Effect of calcium on adsorption capacity of powdered activated carbon Gang Li1,2,3, ∗, Junteng Shang1 , Ying Wang1,3 , Yansheng Li1,3 , Hong Gao2 1. College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028, China 2. School of Materials Science and Engineering of Dalian Jiaotong University, Dalian 116028, China 3. Liaoning Institution of Higher Learning Key Laboratory for Environmental Science & Technology, Dalian 116028, China
Abstract We investigated the effect of calcium ion on the adsorption of humic acid (HA) (as a target pollutant) by powered activated carbon. The HA adsorption isotherms at different pH and kinetics of two different solutions including HA alone and HA doped Ca2+ , were performed. It was showed that the adsorption capacity of powdered activated carbon (PAC) for HA was markedly enhanced when Ca2+ was doped into HA. Also, HA and Ca2+ taken as nitrate were tested on the uptake of each other respectively and it was showed that the adsorbed amounts of both of them were significantly promoted when HA and calcium co-existed. Furthermore, the adsorbed amount of HA slightly decreased with the increasing of Ca2+ concentration, whereas the amount of calcium increased with the increasing of HA concentration, but all above the amounts without addition. Finally, the change of pH before and after adsorption process is studied. In the two different solutions including HA alone and HA doped Ca2+ , pH had a small rise, but the extent of pH of later solution was bigger. Key words: powered activated carbon; calcium; adsorption; humic acids
Introduction Activated carbons with the highly developed porous structures and abundant chemical groups are being used more than ever as an adsorbent in many applications. These cover a wide spectrum of systems including water and wastewater treatment, separations, and hazardous waste treatment (Franz et al., 2000; wang et al., 2005; Terzyk and Rychlicki, 2000). Its high adsorption capacity is mainly due to special physical structures and chemical properties (Bacaoui et al., 2002; Fletcher et al., 2006). Depending on the form, activated carbons are grouped into two categories: granular activated carbon (GAC) and powered activated carbon (PAC). Compared with GAC, PAC having bigger specific surface area and more rich pores thus becomes effective adsorbent for organic micro-pollutants in water treatment (Quinlivan et al., 2005; Jin et al., 2008). Meanwhile, natural organic matter (NOM), either in public water treatment or in emergency drinking water treatment, is the commonest micro-pollutant in various bodies of water. With complex components (Newcombe and Drikas, 1997; Newcombe et al., 2002), NOM produced the toxic disinfection byproducts in the process of halogenated water disinfection treatment, causing secondary pollution (Guang and Reckhow, 2007). Therefore, the study of the * Corresponding author. E-mail: [email protected]
adsorption of NOM by PAC has been the hot topic for a long time. Qi and Schideman (2008) has studied the sorption mechanism of NOM on activated carbon using an overall isotherm method. Their results demonstrated the physical adsorption mechanism of NOM on activated carbon and effect of pore size on adsorption capacity. Schideman et al. (2007) has investigated competitive adsorption of NOM and other organic pollutants on granular activated carbon. Also Newcombe et al. (2002) has studied competitive adsorption of 2-methylisoborneol (MIB) and NOM on activated carbon. The group found that low-molecular-weight NOM compounds were the most competitive, participating in direct competition with MIB for adsorption sites. In that study, some evidence of pore blockage and/or restriction was also seen, with microporous carbons being the most affected by low-molecular-weight NOM and mesoporous carbons impacted by the higher-molecular-weight compounds. However, in practice, water components are more complex, which not only contains organic matter but also large amount of inorganic matter such as metal ions. Some researchers found inorganic materials such as calcium, iron, aluminum and manganese were accumulated on GAC treating natural water and the accumulation of calcium, 10 to 68 mg/g, was the largest among all of these materials (Valix et al., 2006; Baccar et al., 2009; Li et al., 2009;
S102
Journal of Environmental Sciences 2013, 25(Suppl.) S101–S105 / Gang Li et al.
Lee et al., 2003). Frederick et al. (2001) has studied the effect of calcium chelating agent on adsorption captivity of granular activated carbon in the presence of salicylate or phthalate. Mazyck et al. (2005) also found that pH of effluent water increased after calcium accumulated on the activated carbon. Moreover, the studies on regeneration of activated carbon, mainly thermal regeneration technology (Bagreev et al., 2001; Liu et al., 2004), has found that calcium accumulated on activated carbon produced the catalytic effect, destroying the micropore distributed on activated carbon by enlarging the pore size and thus reducing the adsorption capacity for low-molecular-weight compounds (Mazyck and Cannon, 2000). Hence, it is of vital importance to study the effect of calcium accumulation on the adsorption captivity of activated carbon. This study, based on the analysis of adsorption of calcium on PAC, investigated the effect of calcium ion on adsorption of HA by PAC. The interaction of calcium and HA in the process of adsorption was also studied.
1 Experimental 1.1 Analytical method Calcium nitrate was served as the source of calcium ion. The water quality was high with less than 0.5 µS/cm conductivity. All chemicals used were analytical reagent grade. The concentration of solute remaining in the solution was quantified. The HA concentration was measured with a UV-2102C UV/Vis Spectrophotometer (China Unico (Shanghai) Instrument Ltd.) at 254 nm. The calcium ion concentration was measured by method of EDTA titration. Solution pH was determined by a PHS-3C digital pH-meter (China) using a glass electrode. For powdered activated carbon, pore characteristics were measured on Tristar3020, ASAP2020 Automated surface area and pore size analyzer (USA Micromeritics Instrument Corporation). Zero charge was determined by gravimetric titrimetry (Liu et al., 2007) and functional groups were measured by Boehm method (Salame and Bandosz, 2003). Physical properties of powdered activated carbon: surface area 978.4269 m2 /g, pore volume 0.5417 cm3 /g, average pore size 2.2146 nm. Chemical properties of powdered activated carbon: acid group 0.39 mmol/g, base group 0.935 mmol/g, carboxyl group 0.365 mmol/g, phenolic group 0.16 mmol/g, zero point charge pHPZC 9.97.
Vol. 25
1.2.1 Isothermal adsorption experiments at different pH For the humic acid adsorption, adsorption isotherms at three different pH including neutral (pH 7.0), acidic (pH 4.0), and basic (pH 10.0) were conducted. Every six 100 mL aliquots containing 100 mg/L HA was added with 0.1– 0.6 g PAC as a series of solutions. The pH of each series was then adjusted to 4.0, 7.0, and 10.0, respectively. The final solutions were shaken for 12 hr to approach equilibrium and then absorbed amount of HA were measured. 1.2.2 Adsorption kinetics experiments Adsorption kinetics of humic acid in the absence and presence of calcium was carried out in stirred stoppered flasks. One series of 100 mg/L HA solutions was added 0.1 g PAC and 100 mg/L calcium ion, another was only added 0.1 g PAC. The samples were taken at appropriate time intervals, and then filtered and analyzed to obtain the HA concentration. 1.2.3 Simultaneous adsorption of calcium ions and humic acid To further investigate the mutual influence of HA and Ca2+ on the adsorption process, the simultaneous calciumHA adsorption experiments were conducted. Firstly, 0–100 mg/L calcium ion were added into 100 mL aliquots each containing 100 mg/L HA and 0.1 g PAC, and then the suspension was kept under stirring for 12 hr to reach equilibrium. Secondly, 0–25 mg HA was added into 100 mL aliquots containing 400 mg/L calcium nitrate and 0.1 g PAC to react. Then adsorbed amount of HA and Ca2+ was measured to investigate the interaction between them. 1.2.4 Change of pH before and after adsorption Before reaction, pH of different HA solutions varying from 50 to 250 mg/L in the absence and presence of calcium nitrate was measured. Each solution was then added 0.1 g PAC and shaken for 12 hr to reach equilibrium. After adsorption, the pH of each above solution was measured again to observe the change of pH. 1.2.5 Effect of pH on calcium adsorption To determine the effect of solution pH on the calcium adsorption, 1 g PAC was added to 100 mL of Ca2+ solution (100 mg/L) with different pH from 1 to 12 and then the mixed solution was shaken at 25°C and 125 r/min until the equilibrium was achieved. Finally, the adsorbed amount of Ca2+ was measured to determine the effect of pH.
1.2 Sorption experiments In all following cases, the pH of solutions were adjusted by adding small amounts of NaOH or HCl and stoppered flasks were shaken for 12 hr at 25°C and 125 r/min using a thermostat shaker bath (Aohua THZ-82A, China). Each mixture was filtered through 0.45 µm filter papers before measurement.
2 Results and discussion 2.1 Effect of pH on the adsorption of humic acid Many researches have found that adsorption of HA on activated carbon is mainly attributed to the abundant developed pores that well distributed on the activated carbon to
Effect of calcium on adsorption capacity of powdered activated carbon
cut off the HA molecular, the mechanism of which belongs to physical adsorption. In addition, the electrostatic forces also contributed to HA adsorption on PAC. As shown in Fig. 1, adsorption amount of HA decreased with pH increasing and this tendency was more significant at the higher concentration. On one hand, this is because at lower pH, more of the weakly acidic groups existing in HA are in an uncharged state, which makes them more adsorbable. On the other hand, at higher pH (during alkaline range) the high concentration of OH− groups may cover the surface of PAC, which can increase the electrostatic repulsion between HA and surface of PAC and can provide a better solvent for the humic matter, and then to induce desorption. 2.2 Effect of pH on calcium adsorption It has been found that enormous amount of functional groups, such as carboxyl, phenolic and aliphatic hydroxyl, exist on the surface of activated carbon. The adsorption of calcium on the activated carbon is accomplished mainly through the ion exchange and electrostatic attraction on these functional groups. When pH of solutions is less than pH of adsorbent, the surface of adsorbent would be positively charged, while in opposite situation the surface would be negatively charged. As shown in Fig. 2, the amount of calcium adsorbed on PAC increased with increasing of pH generally. Under pH of less than 3, the solution pH was far less than pH of PAC, the positively charged surface generated the electrostatic repulsion between calcium ion and PAC. At that time, the adsorption of calcium on activated carbon should mainly be attributed to ion exchange, but calcium adsorption nearly did not occur. Firstly, this is because at very low pH surface of PAC would take up a large amount of H+ , which competed with Ca2+ and then reduced its adsorption. Secondly, activated carbon with high hydrophilicity would 450 400 350
8
0
8 10 12 pH Fig. 2 Effect of pH on the adsorption of calcium by PAC at the initial Ca2+ concentration of 100 mg/L and 293 K.
100 50 0
50
100
150 Ce (mg/L)
200
250
Fig. 1 Adsorption isotherms of HA by PAC at different pH. Qe : adsorbing capacity; Ce : equilibrium concentration.
2
4
6
adsorb large amount of H2 O molecular, which would occupy the adsorption site and obstruct the calcium ions adsorption. When pH increased from 3 to 10, the amount of calcium adsorption steadily increased due to decreasing of repulsion forces. When the pH value was from 3 to 10, the adsorption of calcium ion was mainly completed through ion exchange with the positively charged functional groups on the surface of PAC, but the total amount was small. As the medium pH turned into strong alkaline range (that is above 10), the surface of PAC was negatively charged, which induced increasing greatly of electrostatic attraction between PAC and calcium. As a result, the adsorption capacity increased significantly (Fig. 2). 2.3 Kinetics of HA adsorption The data from the kinetic behavior of the HA adsorption by PAC from HA solution and one of HA doped Ca2+ solution were presented in Fig. 3. It was observed that the adsorption reaction concentrated in forepart of the whole reaction course, especially before 300 min. The reason was that adsorbate occupied the pores spaces of
Qe (mg/g)
Qe (mg/g)
150
4
0
300
200
6
2
pH 4 pH 7 pH 10
250
S103
10
Qe (mg/g)
Suppl.
760 740 720 700 680 660 640 620 600 580 560 540 520 500 480 460 440 420
HA + Ca2+ HA 0
100
200
300
400 500 600 700 800 Time (min) Fig. 3 Kinetics of HA adsorption from two different solutions.
Journal of Environmental Sciences 2013, 25(Suppl.) S101–S105 / Gang Li et al.
S104
700
Vol. 25
120
600 100 80
Qe (mg/g)
Qe (mg/g)
500 400 300
60 40
200
20
100 0
0 0 50 100 150 200 250 300 20 40 60 80 100 2+ Ca concentration (mg/L) HA concentration (mg/L) Fig. 4 Influence on HA adsorption by PAC at different Ca2+ concentrations (a) and Ca2+ adsorption at different HA concentrations (b) at pH 7 and 293 K. 0
PAC as reaction proceeded, which reduced adsorption capacity and rate of activated carbon. Figure 3 also showed the amount of HA adsorbed from the HA doped Ca2+ solution, comparing with HA alone, was higher regardless of reaching equilibrium, which proved the presence of calcium ion enhanced the overall adsorption capacity of PAC for HA.
Figure 4a showed adsorption of HA by PAC at different calcium concentrations, and Fig. 4b showed adsorption of calcium by PAC at different HA concentrations. It could be found from Fig. 4a that uptaken amount of HA increased in the presence of calcium. However with the increasing of calcium concentration, adsorbed amount reduced but all were more than the amount without calcium. Meanwhile, the presence of HA also enhanced the adsorption of calcium, just as observed from Fig. 4b, which indicated HA would chelate calcium ion and exist as complexion CaHA adsorbed then by PAC. 2.5 Change of pH values before and after adsorption As shown in Fig. 5, pH values of two different test solutions slightly increased. The reason was that after adsorption of HA reduced its concentration in solutions and then induced the increasing of solution pH. Moreover, the increase extend of pH was bigger in the solution of HA doped Ca2+ than HA alone, because the adsorption of HA was promoted in the presence of Ca2+ , which leaded to the lower HA concentration and higher pH.
HA+Ca before HA+Ca after
7 6 5 pH
2.4 Effect of calcium on HA adsorption and effect of HA on calcium adsorption
HA before HA after
8
4 3 2 1 0 20
40
60
80
100 120 140 160 180 200 220 HA concentration (mg/L) Fig. 5 pH values of test solutions before and after adsorption.
with the increase of pH. (3) The co-existence of calcium and humic acid promoted both HA and Ca adsorption comparing to each component existing alone. (4) The medium pH increased slightly after adsorption and this increase extend was bigger in the solution of HA doped Ca2+ than in the one of HA alone. Acknowledgments This work was supported by the Liaoning Institution of Higher Learning Key Laboratory for Environmental Science & Technology Dalian Jiaotong University.
3 Conclusions
References
(1) The adsorption of HA increased with the decrease of pH and the tendency was more obvious at the higher HA concentration. (2) The amount of calcium uptaken by PAC increased
Franz M, Arafat A H, Pinto G N, 2000. Effect of chemical surface heterogeneity on the adsorption mechanism of dissolved aromatics on activated carbon. Carbon, 38: 1807–1819. Wang S B, Zhu Z H, Coomes A, Haghseresht F, Lu G Q,
Suppl.
Effect of calcium on adsorption capacity of powdered activated carbon
2005. The physical and surface chemical characteristics of activated carbons and the adsorption of Methylene Blue from wastewater. Journal of Colloid and Interface Science, 284: 440–446. Terzyk P A, Rychlicki G, 2000. The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro the temperature dependence of adsorption at the neutral pH. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 163: 135–150. Bacaoui A, Dahbi A, Yaacoubi A, Bennouna C, MaldonadoHodar F J, Rivera-Utrilla J et al., 2002. Experimental design to optimize preparation of activated carbons for use in water treatment. Environmental Science and Technology, 36: 3844–3849. ¨ Fletcher A J, YUzak Y, Thomas K M, 2006. Adsorption and desorption kinetics for hydrophilic and hydrophobic vapors on activated carbon. Carbon, 44: 989–1004. Quinlivan P A, Li L, Knappe D R U, 2005. Effects of activated carbon characteristics on the simultaneous adsorption of aqueous organic micropollutants and natural organic matter. Water Research, 39: 1663–1673. Jin W, Xu Z X, Cao D W, 2008. Study on removal of dissolved organic compounds in raw water by powdered activated carbon. Journal of Tongji University (Natural Science), 36: 1670–1674 Newcombe G, Drikas M, 1997. Adsorption of NOM onto activated caebon: electrostatic and non-electrostatic effects. Carbon, 35: 1239–1250. Newcombe G, Morrison J, Hepplewhite C, 2002. Simultaneous adsorption of MIB and NOM onto activated carbon. I. Characterisation of the system and NOM adsorption. Carbon, 40: 2135–2146. Guang H H, Reckhow D A, 2007. Characterization of disinfection byproduct precursors based on hydrophobicity and molecular size. Environmental Science and Technology, 41: 3309–3315. Qi S Y, Schideman L C, 2008. An overall isotherm for activated carbon adsorption of dissolved natural organic matter in water. Water Research, 42: 3353–3360. Schideman L C, Snoeyinka V L, Marinasa B J, Ding L, Campos C, 2007. Application of a three-component competitive adsorption model to evaluate and optimize granular activated carbon systems. Water Research, 41: 3289–3298.
S105
Newcombe G, Morrison J, Hepplewhite C, Knappe D R U, 2002. Simultaneous adsorption of MIB and NOM onto activated Carbon II. Competitive effects. Carbon, 40: 2147–2156. Valix M, Cheung W H, Zhang K, 2006. Role of heteroatoms in activated carbon for removal of hexavalent chromium from wastewaters. Journal of Hazardous Materials B, 135: 395– 405. Baccar R, Bouzid J, Feki M, Montiel A, 2009. Preparation of activated carbon from Tunisian olive-waste cakes and its application for adsorption of heavy metal ions. Journal of Hazardous Materials, 162: 1522–1529. Li Z L, Ma S F, Wang D, Wang C Y, Wei M Y, 2009. Adsorption of metal ions and organics by activated carbon during water treatment isotherms and mechanisms: A review. Environmental Science and Management, 34: 88–93. Lee S H, Nishijima W, Lee C H, Okada M, 2003. Calcium accumulation on activated carbon deteriorates synthetic organic chemicals adsorption. Water Research, 37: 4631– 4636. Frederick H T, Cannon F S, Dempsey B A, 2001. Calcium loading onto granular activated carbon with salicylate or phthalate. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 177: 157–168. Mazyck D W, Cannon F S, Bach M T, Radovic L R, 2005. The role of calcium in high pH excursions for reactivated GAC. Carbon, 43: 511–518. Bagreev A, Rahman H, Bandosz T J, 2001. Thermal regeneration of a spent activated carbon previously used as hydrogen sulfide adsorbent. Carbon, 39: 1319–1326. Liu X T, Quan X, Bo L L, Chen S, Zhao Y Z, 2004. Simultaneous pentachlorophenol decomposition and granular activated carbon regeneration assisted by microwave irradiation. Carbon, 42: 415–422. Mazyck D W, Cannon F S, 2000. Overcoming calcium catalysis during the thermal reactivation of granular activated carbon Part I. Steam-curing plus ramped-temperature N treatment. Carbon, 38: 1785–1799. Liu S J, Li Y G, She Y Q, 2007. Effect of pore structure and surface chemical characteristics of a activated carbon on the adsorption of aqueous nitrobenzene. Environmental Science and Management, 32: 108–112. Salame I I, Bandosz T J, 2003. Role of surface chemistry in adsorption of phenol on activated carbons. Journal of Colloid and Interface Science, 264: 307–312.
