Carbon Paste Electrode Hexadecyltrimethylammonium bromida modified Natural Zeolite for Chromium(VI) Detection.

Abstract

The extensive use of chromium in metallurgic, leather tanning, electroplating, lumber, electricity generating and other industries have promoted enormous ecological impact in numerous sites that are being contaminated by chromium. Chromium has two dominant species, namely Cr(III) and Cr(VI). Among them, attention concerning the possible hazards arising from the use of Cr(VI) than Cr(III) because hexavalent chromium is around 1000 times more toxic than trivalent one. Therefore, methods for differentiation and quantification of these species are very important in water analysis. Research on chemical modified carbon paste electrode have been carried out starting from 1990 especially in electroanalysis field caused by better improvement than conventional electrode. Continuous research on chemical modified carbon paste electrode has been increasing use of inorganic materials as a modifiers of electrode material. Among inorganic materials, zeolites have been widely used as an ion exchanger due to size, shape, and unique charge selectivity, high capacity ion exchangers, high thermal stability, low cost resistance to extreme conditions. Zeolites have a permanent negative charge on its crystal structure making it suitable as a cation exchanger. This negative charge is also possible to do modifications on the zeolite surface using cationic surfactant of hexadecyltrimethylammonium (HDTMA) ions. Cation exchange capacity value of zeolite origin from Cikembar (23.7 meq/100 g), Cikalong (47.6 meq/100 g), Bayah (39.7 meq/100 g), and Lampung (42.4 meq/100 g). After activation with NaOH, all cation exchange capacity values showed increasing were Cikembar (60.4 meq/100 g), Cikalong (60.4 meq/100 g), Bayah (42.5 meq/100 g), and Lampung (107.5 meq/100 g). Therefore, Lampung zeolite was selected to do modification with surfactant and was obtained highest adsorption amount of 200 mM HDTMABr as 3.9923 mmol/g zeolit. Cr(VI) adsorption test using diphenylcarbazide (DPC) method toward Lampung zeolite, 100 mM and 200 mM HDTMABr modified NaOH activated Lampung zeolite showed increasing adsorption capacity (Q). Cr(VI) adsorption capacity (Q) at steady state were obtained toward Lampung zeolite, 100 mM and 200 mM zeolite-NaOH-HDTMABr as 58.0457, 134.0522, 148.5602 mg Cr(VI)/g zeolite, respectively. This showed an increase of Cr(VI) amount bound on the zeolite surface. XRD characterization showed 3 significant peaks were obtained on Lampung zeolite, Lampung zeolite activated NaOH, and HDTMABr modified zeolite-NaOH. This showed no structural change on natural and modified zeolite. The degree of crystallinity decreased with the order of natural zeolite, zeolite-NaOH, and HDTMABr modified zeolite-NaOH showed partial loss of impurities contained in natural zeolite and adsorption of HDTMABr molecules on zeolite surface. Zeolite-NaOH-HDTMABr showed incorporation of absorption peaks originating from bending δH-O-H (1637.65 cm-1), vibration vT-O (T= Si,Al) symmetric (1055.56 cm-1), vT-O asymmetric (793.17 cm-1), δSi-O-Al (608.86 cm-1 and 525.67 cm-1), and δO-T-O (468.41 cm-1) also peak absorption originating from HDTMABr were stretching vC-H methylene asymmetric (2920.05 cm-1) and bending δC-H methylene (2851.01 cm-1, 1489.21 cm-1). Characterization by thermal analysis (TG-DTA) showed percentage of loss mass was quite large aat 15.1% in temperature range of 200-300 oC. This result indicates surfactant double layer formed by hydrophobic-hydrophobic interaction of nonpolar groups on the surface of HDTMABr modified Lampung zeolite. The existence of electrostatic interactions between HDTMABr molecules on zeolite surface forming s single layer were shown at 300-400 oC with percentage of mass loss 1.7%. This due to higher energy was required to break electrostatic interactions between HDTMA and zeolite surface than hydrophobic-hydrophobic interactions among HDTMA molecules. Zeolite-NaOH-HDTMABr was used as modifiers in carbon paste electrode. Optimum composition of HDTMABr modified zeolite in mixture of graphite and paraffin oil was obtained 2.5% with Cr(VI) analyte measurement by cyclic voltammetry technique. Supporting electrolyte solution for Cr(VI) measurement was 0.3 M HNO3 with pH 0.5. Further measurement of analyte in composite electrode were based on principle of Cr(VI) into Cr(III) or irreversible redox system thus investigated a simpler voltammetric technique. Measurement of 13 mM Cr(VI) with Linear Sweep Voltammetry (LSV) provided highest sensitivity by reduction peak current as 31.45 μA compared with Square Wave Voltammetry (SWV) as 12.80 μA and Differential Pulse Voltammetry (DPV) as 1.98 μA. Performance of composite electrode was investigated using LSV technique provided linear response range at 0.2-1.0 mM with determination coefficient (r2) as 0.9669. Sensitivity, detection and quantification limit, also precision of Cr(VI) measurements were obtained 0.4294 μA mM, 3.63 x 10-4 M, 1.197 x 10-3 M, 4.49%, respectively.