Collection of Trace Heavy Metals on Dithizone-Impregnated Admicelles for Water Analysis
Ionic or nonionic surfactant molecules form selfaggregates called "micelles" above the critical micelle concentrations. It is well known that the hydrocarbon cores of the micelles solubilize many hydrophobic compounds in aqueous solutions. Similarly, surfactants can form their aggregates on solid surfaces, which are called "admicelles" or "hemimicelles". The admicelles also have a solubilization property of water-immiscible organic compounds. For example, sparingly soluble organic chemicals
... ganic chemicals (e.g., benzene, dichlorobenzene, pentachlorophenol and toluene) were incorporated into admicelles prepared on hydrated iron(III) oxide 1 or alumina. 2, 3 The use of admicelles in trace analysis has only recently been reported. 4,5 A typical chelating agent, dithizone, was incorporated into the inner hydrophobic part of admicelles by acidifying ammoniacal mixtures of dithizone, sodium dodecyl sulfate (SDS) and alumina particles. The dithizone-impregnated admicelles were used to separate traces of copper from a lead matrix. 5 Commercial SDS, however, contained appreciable amounts of metal impurities, which entered into the admicelles and influenced the detection limits. In addition, although the conditions required for adsorption from water have already been studied, much less is understood about collection from seawater. In the present work, the most effective purification method of admicelles was found to be washing with 4 mol dm -3 nitric acid. The adsorption and desorption of trace metals on purified admicelles were examined using water and seawater, compared with conventional liquid-liquid extraction. The influence of humic and fulvic acids was studied as well as reuse of the same admicelle column. The accuracy and precision were evaluated by an analysis of a certified reference seawater sample. Experimental Apparatus A Seiko SPQ-6500 inductively coupled plasma (ICP)-mass spectrometer was used for determining 59 Co + , 58 Ni + , 63 Cu + , 114 Cd + and 208 Pb + under the following plasma conditions: RF power of 1.2 kW; argon flow rates (dm 3 min -1 ) of 16 for outer, 0.7 for intermediate and 1.0 for carrier. A Seiko I & E SAS-715 graphitefurnace atomizer was used in conjunction with an SAS-760 atomic absorption spectrometer. The wavelengths (nm) used were 240.7 (Co), 232.0 (Ni), 324.8 (Cu), 228.8 (Cd) and 217.0 (Pb). The graphite tube was heated for 20 s to 150°C, held for 15 s and then heated for 5 s to 900°C (except for Cd, 500°C) and held for 10 s. The applied atomization temperatures were 2300 (Co, Ni, Cu) and 2100°C (Cd, Pb) for 3 s. A Branson ultrasonic cleaning bath (47 kHz, 120 W) was used to purify alumina particles. A Tokyo Rika NTS-1300S shaker was employed to prepare dithizoneimpregnated admicelles at a shaking rate of 160 min -1 . Separation procedures were mostly carried out in a Hitachi ECV-843 BY clean bench. Reagents Dithizone-SDS solution. After dissolving 15 mg of Admicelles formed on alumina were studied from the viewpoint of separation vehicles in trace analysis. Gamma-alumina (1.5 g) was suspended in 40 cm 3 of water and mixed with 10 cm 3 of ammoniacal solution containing 1.5 mg of dithizone and 100 mg of sodium dodecyl sulfate. The mixture was acidified to pH 2, where the anionic surfactant was strongly adsorbed on the positively charged alumina surface to form dithizone-impregnated admicelles. Metal contaminants in the admicelles were thoroughly removed by washing with 4 mol dm -3 nitric acid. The quantitative adsorption from water was performed at pH 2 -9 for Cu, pH 3 -9 for Pb and pH 4 -9 for Co, Ni and Cd. For seawater, simultaneous collection required adjusting the pH to ca. 8. Humic and fulvic acids did not interfere with the separation. The admicelle column was substantially stable during adsorption and desorption, which allowed the same column to be used at least three times. The proposed method was combined with inductively coupled plasma-mass spectrometry for the analysis of seawater.