Study on palm shell activated carbon adsorption ... - UM Repository

Desalination 262 (2010) 94–98

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Study on palm shell activated carbon adsorption capacity to remove copper ions from aqueous solutions Gulnaziya Issabayeva a,⁎, Mohamed Kheireddine Aroua b, Nik Meriam Sulaiman b a b

Faculty of Science and Engineering, University Tunku Abdul Rahman (UTAR), 53300 Setapak, Kuala Lumpur, Malaysia Chemical Engineering Department, Faculty of Engineering, University Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e

i n f o

Article history: Received 4 February 2010 Received in revised form 24 May 2010 Accepted 25 May 2010 Available online 1 July 2010 Keywords: Adsorption Copper Lead Malonic acid Boric acid

a b s t r a c t Commercially produced in Malaysia palm shell activated carbon (PSAC) was evaluated in terms of adsorption capacity to remove copper ions from aqueous solutions. The results of batch and continuous adsorption experiments showed high adsorption capacity of the untreated PSAC to adsorb copper ions at pH 3 and 5. Higher pH of aqueous solution showed higher uptake of copper. Presence of complexing agents, boric and malonic acids, did not improve copper uptake. Moreover, lower adsorption capacity was observed in the presence of malonic acid that is probably due to the complex formations between the agent and investigated metal. The observed trends for continuous adsorption of copper are in line with the results obtained for batch mode adsorption. Also, changes of the solutions' initial pHs were measured and they are likely to be associated with the adsorbent's composition and characteristics. In addition, removal of copper ions from the solutions containing lead ions showed that adsorption capacity of copper was not significantly different compared to the single copper ion system. Whereas, the uptake of lead ions onto activated carbon was substantially reduced in the presence of copper ions, especially at pH 5. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Numerous important technological processes involve heavy metal applications: metal purification, metal smelting and electroplating are the major ones. Wastewaters of these processes bear substantial amounts of heavy metals, concentration of which should be reduced in accordance with the practiced environmental standards. It is well established that the presence of higher concentrations of heavy metals in the environment is associated with serious health effects including neurotoxicity, nephrotoxicity and various cancer types. Most of the health disorders are linked with the specific tendency of heavy metals to bioaccumulation in living tissues and their disruptive integration into normal biochemical processes. The economic considerations are also important, as the technologies keep improving and stricter environmental, health and safety regulations are being enforced, the demand and cost for many metals has increased, especially since 2000. Copper, in particular, is used in all electronic items and its recycling technology is well recognized and promoted [1]. Although, a few effective technologies are currently used to reduce and recover metals from wastewater streams e.g. chemical precipitation, electrochemical recovery, their application has some limitations. Firstly, these technologies are quite expensive; secondly they ⁎ Corresponding author. Tel.: +60 12 9230516; fax: +60 3 41079803. E-mail addresses: [email protected] (G. Issabayeva), [email protected] (M.K. Aroua). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.05.051

are not employed for all processes that generate heavy metal bearing wastewaters. The latter comment is especially important for the developing countries, where environmental quality control monitoring is limited mainly due to the insufficient finance, technical and human resource allocations. In Malaysia, medium and small enterprises of plating, polishing and metal-coating operations; paint, motor vehicle parts and accessories manufacturing significantly contribute to heavy metal water pollution. However precise qualitative estimations are hard to derive mostly due to poor environmental control provisions and difficulties to carry out technical monitoring for small, often family run enterprises. In view of such scenario, technological development and optimization should consider existing and growing demand for affordable, maintainable and yet effective means to curb continuous heavy metal release into the environment. In this sense, adsorption is one of the most common processes since the ancient times. The process is governed by a number of parameters that determine the efficiency level of an adsorbent. However, appropriate properties and cost of adsorbent materials are the key aspects for practical applications when dealing with heavy metal waste streams reduction. It was demonstrated that agricultural byproducts and various biosorbent materials have promising capacities to remove a variety of pollutants. Especially important are research observations that these materials exhibit higher affinity towards heavy metal ion bonding [2–16]. Since Malaysia is a leading producer of palm oil, this sector of economy generates great amount of wastes, namely palm shells. Palm shells are partially used as fuel material (within the industry) but

G. Issabayeva et al. / Desalination 262 (2010) 94–98

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mostly it is discarded. In the recent decade, a few local manufacturers started using palm shell materials to produce commercial activated carbon. Our research group collaborates with one of the commercial PSAC manufacturer in Malaysia. The main objective of our research is investigation of the adsorption capacity of palm shell activated carbon in removal of various heavy metal ions. This paper presents a part of our research that focuses on the adsorption of copper ions on palm shell activate carbon in batch and continuous modes. 2. Materials and methods The details of the experimental procedures for batch and continuous adsorption, results of the adsorbent characterization and adsorption capacity estimation are presented elsewhere [17,18]. The palm shell activated carbon used in the study was commercially produced under conditions of steam physical activation process by the Pacific activated carbon manufacturer in Malaysia. 3. Results and discussion 3.1. Batch adsorption of copper Overall, three systems were investigated: single copper, copper with malonic acid, and copper with boric acid at two pH values, 3 and 5. The Langmuir isotherm model [19] was used to estimate the capacity of the adsorption. Table 1 summarizes adsorption results for all investigated systems. High R2 values for most of the tested adsorption systems indicate that copper ion adsorption on palm shell activated carbon fit well into the Langmuir model. 3.1.1. Single copper system Figs. 1 and 2 show the adsorption isotherms for the tested systems, at pH 5 and 3, respectively. Uptake of copper was noticeably higher at pH 5 than at pH 3. The effect of solution's pH on adsorption process is indicated in many adsorption studies, and it is primarily explained by the tougher competition between protons and metal ions for the same adsorption sites on activated carbon surface at lower pH [20–24]. 3.1.2. Copper with malonic acid The uptake of copper ions in the solution with malonic acid was the lowest. However, the pH effect was similar to the single copper system: adsorption at pH 5 was relatively higher than at pH 3. Such negative effect of malonic acid may be associated with carbon pore blockage with the complexing agent that results in the decreased number of adsorption sites available for copper ions [25,26]. It was also noted that the presence of organic compounds decreased adsorption rate due to the effect of diffusion control regime [27,28]. In addition, the speciation diagram (Fig. 3) indicates formation of N60% copper-malonate (aq) and N40% of copper-malonate2(−2) complexes in pH range 2–10. Probably the complex formation process between copper and malonic acid dominates over the adsorption of single copper ions. In addition, these complexes are negatively charged and their accumulation on the carbon surface may form conditions of charge repulsion [29]. It was shown [30] that low molecular weight organic acids may inhibit varying complexation

Fig. 1. Adsorption of copper, pH 5. ◊ single copper; △ copper with malonic acid; ● copper with boric acid.

capacity towards metal ion mobilization in a solution, and their concentrations above 0.05 M stimulate desorption of inorganic structural ions into solution. Also, the increase in pH is usually associated with higher complexation affinity between organic acid and metal ions. However, quite insignificant difference in the copper uptake at pH 3 and 5 could be attributed to the moderate complexation capacity of malonic acid towards copper ions, and the agent's low concentration in the solution. However, additional information on stability constants of copper-malonic acid complexes is required to explain the observed behavior of the system components. 3.1.3. Copper with boric acid The uptake of copper ions in this system was slightly lower relatively to single copper system. The ineffectiveness of boric acid can be explained by its weak electrolytic properties and slow dissociation especially in the acidic aqueous solutions. It was shown [31] that sorption affinity of activated carbon towards monomeric, univalent ions is higher compared to polyvalent structures of boric acid present in aqueous solutions. Also, partial adsorption of boric acid on activated carbon is likely, which would explain a relative decrease in copper ion uptake at pH 5 relatively to the single copper ion system. In addition, the speciation diagrams showed no formation of complexes between copper ions and boric acid. 3.1.4. Copper and lead adsorption Adsorption of copper ions on palm shell activated carbon in solutions containing lead ions was studied. Table 2 presents adsorption data for metals' uptake in binary system at pH 3 and pH 5. The effect of pH was similar to the single copper system. However, the results showed that copper ions compete more successfully for the available adsorption sites thus suppressing uptake of lead ions. Such domination of copper over lead ions in adsorption process is probably associated with copper's electronegativity (2.0) and smaller ionic

Table 1 Langmuir parameters for copper adsorption. pH

Metal

a

b

qm (mmol/g)

SSE

R2

5 5 5 3 3 3

Cu Cu + MA Cu + BA Cu Cu + BA Cu + MA

6.89 0.78 10.67 25.9 2.33 0.55

0.22 0.03 0.48 1.39 0.12 0.06

0.48 0.13 0.35 0.29 0.30 0.37

0.42 0.13 0.47 0.43 0.66 0.18

0.99 0.97 0.99 0.99 0.99 0.95

Fig. 2. Adsorption of copper, pH 3. ◊ single copper; △ copper with malonic acid; ● copper with boric acid.

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Fig. 3. Speciation of copper with malonic acid; initial concentration of copper 50 mg/L.

radius. Metal ions with higher electronegativity and/or higher first hydrolysis equilibrium constants demonstrate higher affinities for adsorption sites on the carbonaceous materials [32–36]. The uptake of copper from binary system was in a comparable range with the single copper system at both pHs. However, a proportion of lead adsorbed was almost three times lower relatively to single lead systems [17]. It was reported [37] that lead is completely replaced by copper at sufficiently high concentration, confirming hypothesis that two metals compete for the same adsorption sites. Similarly, no change was observed in adsorption capacity of copper when zinc and lead were present in the solution [38,39]. In addition, a multi-component adsorption study [40] showed prevailing adsorption of copper ions compared to other metal ions present in the solution, which is attributed to the strong charge of copper ion.

3.2. Continuous adsorption of copper 3.2.1. Single copper system Fig. 4 shows breakthrough adsorption curves and pH changes in single copper system. Similarly to batch mode adsorption, pH 5 was more favorable for continuous adsorption. Significantly longer breakthrough period is also in line with the batch mode adsorption results. During continuous adsorption, the pH changes mainly occurred in the first 15–20 min of the experiment. Fig. 5 shows breakthrough curves of copper at pH 5 at different flow rates. As was anticipated the higher flow rate resulted in a shorter period to reach breakthrough point due to higher mass transfer rate through the carbon bed and shorter contact time of solution with carbon. Regardless of initial pH value, solution's pH sharply increased up to pH 10 in the first 20 min and then it gradually stabilized. However, the measured effluent' pH values were higher than the initial, around pH 6. The time period necessary for pH stabilization was slightly shorter for pH 3 solution. Observation on pH increase at the beginning of continuous adsorption of metals was reported earlier [41]. Two possible explanations for pH increase over initial value were proposed: adsorption of hydrogen ions from the solution, and dissolution of some impurities from the activated carbon [38]. It was suggested [42] that due to hydrolysis reaction, a release of hydroxyl ions from adsorbent into aqueous phase takes place. Furthermore, when influent pH carries more hydrogen ions (lower pH) through the bed, the hydrolysis reaction is accompanied by an

Fig. 4. Breakthrough curves (◊ copper pH 3; ▲ copper pH 5) and pH changes (♦ pH 3 solution; △ pH 5 solution). Flow rate 0.8 L/h, initial concentration of copper 50 mg/L.

exchange reaction between substances from adsorbent and hydrogen ions from solution. 3.2.2. Copper with complexing agents Fig. 6 shows comparison of three systems: single copper, copper with malonic acid, and copper with boric acid at pH 5 in continuous adsorption at 1.0 L/h flow rate. The presence of malonic acid at pH 5 resulted in higher effluent concentration compared to the other systems. It shows that complexing agents did not have a significant effect on copper adsorption. This observation is in good agreement with the results of batch mode adsorption described above. Fig. 7 shows pH stabilization in systems of copper with malonic and boric acids, at pH 3 and 5, and 1.0 L/h flow rate. In system of copper with malonic acid solution, pH increased at the beginning and then stabilized towards the initial pH. Buffering effect of malonic acid is associated with its pK1 = 2.85 and pK2 = 6.10 values, which are in close range with the tested solution's initial pHs. The effect of boric acid on pH stabilization was less apparent compared to malonic acid. This can probably be explained by quite low concentration of boric acid in copper solution and its high pK value (9.24). Similar to single copper adsorption, effluent's pH stabilized around pH 6.3. The pH stabilization process was very similar for both complexing agents, with boric acid showing slightly higher pH values from the beginning. The breakthrough in both pH systems was observed at already stabilized pH. As in the adsorption of copper with boric acid in batch mode experiments, no significant changes in removal of copper were observed. 3.2.3. Comparison of adsorption capacity Table 3 presents adsorption capacities of various adsorbent materials of agricultural origin. Comparison indicates that certain

Table 2 Langmuir parameters for adsorption of copper and lead ions. pH

Metal

a

b

qm (mmol/g)

SSE

R2

3 5 3 5

Cu Cu Pb Pb

22.22 3.26 4.1 1.45

0.94 0.2 0.22 0.1

0.23 0.26 0.09 0.11

0.46 0.78 0.13 0.29

0.99 0.99 0.99 0.99

Fig. 5. Breakthrough curves for continuous adsorption of copper at pH 5. Flow rates: ◊ 0.3 L/h; □ 0.5 L/h; △ 0.8 L/h; ◊ 1.0 L/h.


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Table 3 Maximum adsorption capacities (mg/g) for removal of copper by various adsorbents. Adsorbent

pH

Copper (II)

Reference

Sago waste AC cloths AC of Pecan shells Prawn shell Peanut shells Lignite Spend activated clay Chestnut shell

2–5.5 2–10 3.6 6 4.8 3.8–5.5 5–6 5 4.8 2 and 5 5 6 6–10 6 3, 5 and 7 6–10 6 6–10 2 and 5 6–10 6–10 3 and 5

12.42 11.05 95.0 17.15 12.9–38.13 17.8 10.9–13.2 12.56 4.8 38.76 and 98.04 10.34 6.74 94.1% 6.65 5.8, 6.99 and 10.4 94.1% 3.62 90.3% 31.84 and 32.15 91.6% 95.6% 18.6 and 30.8

[43] [44] [45] [46] [47] [9] [48] [49] [10] [50] [2] [3] [4] [3] [6] [4] [3] [4] [50] [4] [4] This study

Wheat shell Walnut shell Fig. 6. Comparison on continuous adsorption of single copper and copper with complexing agents at 1.0 L/h flow rate and pH 5: ◊ copper with malonic acid; ○ single copper; △ copper with boric acid.

Hazelnut shell

Almond shell

materials exhibit higher or lower affinity towards metal adsorption that is determined by adsorbents' nature and adsorption experimental parameters. Palm shell showed promising results in terms of metal adsorption. It is anticipated that optimization of the experimental conditions and modification of the adsorbent material would result in the substantial improvement of the adsorption process efficiency.

Grape seed AC Apricot stone Pistachio shell Palm shell activated carbon

References 4. Conclusion Our study results showed that untreated commercial palm shell activated carbon has good potential to adsorb heavy metal ions from wastewater. The results showed that higher pH is more favorable for the removal of copper ions from the solution. However, the presence of complexing agents did not improve copper ion uptake. The results of batch and continuous mode adsorption experiments are in good agreement. Further research on the characterization of palm shell carbon and optimization of adsorption process are planned to be conducted in the near future. Also investigation on competitive adsorption of metal ions from mixture solutions onto palm shell carbon is challenging and promising. Acknowledgments We would like to thank the Ministry of Science, Technology and Innovation, Malaysia for the IRPA Research Grant; the Institute for Postgraduate Studies and Research (IPSP), University Malaya for financing VotF 159-2003A research project; and the Pacific Activated Carbon company, Johor Bahru, Malaysia for the generous provisions of palm shell activated carbon for our research.

Fig. 7. pH changes in copper with malonic and boric acid solutions at 1.0 L/h flow rate: ○ copper with malonic acid, pH 5; ● copper with malonic acid, pH 3; ▲ copper with boric acid, pH 5; △ copper with boric acid, pH 3.

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