UTILIZATION of KAPOK [Ceiba Pentandra (L) Gaertn
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Philippine Journal of Crop Science 2002, 27(2): 37-42 Copyright 2004, Crop Science Society of the Philippines Released December 2004 FIBER OF KAPOK (CEIBA PENTANDRA) AS COMPONENT OF A METAL SENSOR FOR LEAD IN WATER SAMPLES ELMER-RICO E MOJICA, FLORINIA E MERCA & JOSE RENE L MICOR Institute of Chemistry, College of Arts & Sciences, University of the Philippines Los Baños, College, Laguna 4031 Kapok fibers were utilized as component of a metal sensor. The kapok fiber, which contains lignocellulosic materials, was used as the metal-binding substance in a carbon paste electrode and then used in the voltammetric analysis of heavy metals. Such analysis indicates the capacity to detect lead ions. Optimization of different parameters was done for possible application of the kapok metal sensor as a rapid method for lead analysis of water samples. This has immediate application in testing for purity of domestic and industrial waters. Keywords heavy metals, kapok, kapok metal sensor, lead analysis, rapid lead analysis, voltammetric analysis, water pollution INTRODUCTION lignocellulosic materials (Hori et al 2000), this study was aimed at looking into the possible Kapok [Ceiba pentandra (L) Gaertn., use of the fiber as a component of a metal Family Bombacaceae], the silk cotton tree, is sensor to electrochemically detect the native to the New World and Africa, found presence of heavy metals such as lead. widespread in the tropics and in several plantations in Southeast Asia, and is known MATERIALS & METHODS for its white silky floss. The Java kapok, Bombax malabarica, is native to tropical Kapok fibers were obtained from kapok India, has brownish-yellow floss and is less fruits, which were abundant around the moisture-resistant. Physical Science building in the summer of The kapok fibers attached to the seeds 2003. That is because less than 10 m away are utilized as packing materials for grows a giant kapok tree on the streambank upholstery, mattresses, pillows, quilts and of Molawin Creek at the UP Los Baños soft toys (Bates 2004). The kapok fiber is campus. The fiber was separated from the excellent material for these because it is soft, seeds and fruit cover, air-dried at room flossy and moisture-resistant. The leaves and temperature, and later cut into smaller pieces bark of kapok are used by the Chinese for using a pair of scissors. their medicinal value while the trunk has The fiber was mixed with carbon powder been tested as sources of pulp for and mineral oil to form a paste. A portion of papermaking (Peralta 1982). Kapok seeds the paste was packed into the end of a hard contain oil which is similar to cotton oil. plastic tube (with a thickness of 2 mm Lignocellulosic materials in plants have diameter) where a copper rod was inserted to been evaluated for their effectiveness in establish electrical contact. sorption of heavy metal ions and even non- The fabricated kapok modified carbon viable plant biomass have been found to paste electrode (CPE) was then used as the effectively bind toxic metals (Seki et al 1998). working electrode in a 3-electrode cell Since kapok fiber was found to contain connected to a Metrohm 693 VA processor where all voltammetric measurements were whose reduction potential are within this carried out. The processor was interfaced to a region can be detected if peak signal is personal computer via an RS232 connection, generated after accumulation of electrode which converts the generated data into ASCII with metal solutions. A distinct peak at about format. The data obtained was processed -500 mV appeared when the kapok-modified using Microcal Origin version 5. A platinum electrode was accumulated with lead (II) auxiliary electrode and Ag/AgCl reference solution in 0.1 M HCl. A peak near the -650 electrodes were used. All potentials were mV region was also observed when 0.1 M measured against the Ag/AgCl electrodes. NaOH was used. A standard solution containing 1000 Optimization of parameters using both mg/L of lead (II) was obtained from JT Baker. 0.1 M HCl and 0.1 M NaOH was done for lead Working solutions of lead were prepared just (II) analysis using DPASV. Accumulation before use by dilution with deionized distilled time, which is synonymous with time of water obtained from system. All chemicals preconcentration step, was the first were analytical grade reagents. parameter to be optimized. Peak current The electrode was immersed in 10 mL of increases with accumulation time because of lead (II) standard solution at a precon- the increase in the metal ion concentration at centration cell with constant stirring at open the electrode surface (Figure 3). However, it is circuit. The electrode was then taken out of expected that the increase in current will the preconcentration solution, rinsed with level off when all the binding sites have been water and transferred to the voltammetric cell occupied after continuous immersion. This containing a supporting electrolyte that saturation point is the maximum metal ion includes 0.1 M sodium hydroxide and 0.1 M concentration at the electrode surface. hydrochloric acid solution. The next parameter optimized was the Cyclic voltammetry was done using the effect of pH on the current signal generated experimental CPE. A potential range of –1000 by the electrode. The resulting pH profile to 1000mV and a scan rate of 100mV/s were (Figure 4) showed that pH 4-6 is the optimum employed. Optimization of parameters like pH pH range for maximum current response. The of preconcentration solution, accumulation pH obtained for the optimum response of the time, deposition time and deposition potential kapok modified CPE agrees with the result of was done using differential pulse adsorptive an experiment by Gardea-Torresday (1988), stripping voltammetry. which classified lead as among the metals that are tightly and rapidly bound at pH > 5 RESULTS & DISCUSSION and stripped at pH< 2. The voltammograms also showed that at pH higher than 6, the The electrochemistry of the kapok fiber peak current decreases. The decrease in the metal sensor prior to metal accumulation was current response of the electrode at higher examined by cyclic voltammetry. This pH is due to the formation of the hydroxylic technique is usually employed as a lead complexes that inhibit lead preconditioning mechanism to effectively accumulation on the electrode. On the other eliminate any extraneous matter incorporated hand, the low peak current at low pH values onto the electrode surface. In this instance, can be explained by the protonation of the the surface area of the binding sites for the weakly basic coordinating groups at the analyte increases. The potential window of surface of the modified electrode (Ramos et al the fabricated metal sensor was determined 1993). In addition to this, an acidic media is to be within the range of -1000 mV to +1000 not a good accumulating solution if the mV (Figure 1). From this potential range, the mechanism of accumulation of the lead onto sensor is inert and may be used to detect the electrode surface is through ion electroactive species. exchange. In acidic media, protons compete Aside from CV, differential pulse anodic with lead for the negative charges of the stripping voltammetry (DPASV), another carboxylic groups. As a result of this, less electrochemical method was used. A flat metal cation will be adsorbed onto the region from -1000 mV to 1000 mV was surface of the electrode, resulting in a observed (Figure 2). In this case, any metals decreased peak current response. 38 ER E Mojica, FE Merca & JR L Micor Figure 2. CV of kapok modified CPE before and after accumulation with lead ions in different supporting electrolytes 0.1M HCl 100 0.1M NaOH 90 80 70 60 50 Current, uA Current, 40 30 20 10 0 00.511.522.533.5 Accumulation time, min Figure 2. DPASV of kapok modified CPE before and after accumulation with lead ions in different supporting electrolytes 100 50 0 Curent, uA -50 kapok mCPE before kapok mCPE after(NaOH) kapok mCPE after(HCl) -100 -1000 -500 0 500 1000 Potential, mV Kapok In Metal Sensor For Lead 39 Figure 4. Current generated by different accumulation times of kapok modified CPE in 0.1 M HCl and 0.1 M NaOH 80 70 60 before after (NaOH) 50 after (HCl) 40 30 Current, uA 20 10 0 -10 -1000 -500 0 500 1000 Potential, mV Figure 3. Current generated at different pH of kapok modified CPE in 0.1 M HCl and 0.1 M NaOH 50 45 0.1M HCl 0.1M NaOH 40 35 30 25 Current, uA 20 15 10 5 0 012345678910 pH 40 ER E Mojica, FE Merca & JR L Micor Deposition time and deposition potential were low as 1.0 mg/L the other parameters optimized. The The ability of the fabricated metal sensor deposition time is referred to as the time to detect lead is probably due to the affinity of required for the reduction of mercury (II) to the lead ions with the functional groups its metallic state which happens on the present in kapok fibers. Kapok from the surface of the electrode. For the analysis Philippines has been found to contain 21% done using 0.1 M NaOH, peak current lignin (Hori 2000). Lignins and their increases as the deposition time increases. In derivatives are well known for their ability to comparison with analysis done in 0.1 M HCl, bind heavy metal ions (Varma et al 1990, deposition time of 60 sec gave the maximum Volchek et al 2000). Lignins are a group of response. Lastly, the different deposition phenolic polymers (Lignin Institute 2001) and potential applied in the analysis gave their derivatives contain an abundance of contrasting results in the two supporting oxygen-containing functional groups such as electrolytes used.