ANALYTICAL SCIENCES JUNE 2003, VOL. 19 835 2003 © The Japan Society for Analytical Chemistry Enrichment Adsorption of a Labile Substance to the Surface of Particular Mineral Particles in River Water as Investigated by SEM-EDX and Dilute-Acid Extraction/ICP-MS Tomohiro KYOTANI*,**† and Satoshi KOSHIMIZU*** *Bio Nanotec Research Institute Inc. (BNRI), Mitsui & Co., Ltd. Nanotech Park, 2-1, Koyadai, Tsukuba, Ibaraki 305–0074, Japan **Japan Science and Technology Corporation (JST), 4-1-8, Honmachi, Kawaguchi, Saitama 332–0012, Japan ***Earth Science Division, Yamanashi Institute of Environmental Sciences, 5597-1, Kenmarubi Kamiyoshida, Fujiyoshida, Yamanashi 403–0005, Japan The selective enrichment behavior of a labile substance, such as hydroxides, to the surface of particular mineral particles in river water was clarified by scanning electron microscopy/energy dispersive X-ray microanalysis (SEM-EDX). Individual particles other than diatom collected on a 0.45 µm filter from the Fuji and Sagami rivers, central Japan, were analyzed by SEM-EDX and classified into seventeen groups according to the chemical composition and shape. Phosphorus, sulfur, chlorine, manganese and copper detected in each particle collected on the 0.45 µm filter could be successfully used as effective indicators of labile substance secondarily formed and adsorbed afresh in river water, because the detection frequencies of such elements are quite low, or negligible, in fresh mineral particles derived from igneous rocks. The labile substance adsorbed on mineral particles collected on the 0.45 µm filter was also evaluated by dilute-acid leaching, followed by inductively coupled plasma mass spectrometry (ICP-MS). Almost all parts of the manganese detected in individual particles were those adsorbed afresh as hydroxides together with iron and aluminum. Also, anionic elements, such as phosphorus, sulfur and chlorine, formed complexes with the hydroxides and/or were incorporated in them. Mg and/or Ca-rich aluminosilicate groups were the most effective adsorbers of such labile species. However, Si-rich and Na-, K- and Na-Ca rich aluminosilicates did not significantly adsorb the labile substance. Consequently, the remarkable selectivity was clarified in the adsorption process of labile substance to individual mineral particles in river water. (Received October 4, 2002; Accepted March 26, 2003) One of the processes for determining the dissolved elemental EPMA studies. concentration in river water is the interaction between the liquid The aim of this study was to clarify the factors affecting the and solid phases, that is, the elemental fractionation between dissolved elemental concentrations in river water in terms of the dissolved species and suspended particles.1 However, the chemical composition of insoluble individual particles. In this chemistry of river water has been long discussed concentrating study, the chemical composition of individual particles collected on the dissolved species only obtained from 0.45 µm filtrate.2,3 on a 0.45 µm filter from river water was determined by Consequently, information4–7 on the chemical composition of scanning electron microscopy/energy dispersive X-ray suspended particles in river water is not very much. The main microanalysis (SEM-EDX). Phosphorus, sulfur, chlorine, removal processes of dissolved elements in natural water are the manganese and copper detected in individual particles could be formation of metal-fumic complexes and coprecipitation with used as effective indicators of a labile substance secondarily hydroxides.8–10 Recently, the former has been intensively formed in river water. Consequently, the selective enrichment discussed based on speciation studies of trace elements by many behavior of labile substance to the surface of particular mineral researchers.8–10 Although the latter has been mainly discussed particles in river water was first clarified. using iron in a colloidal fraction from 0.45 µm filtrate,10 the action of the particles collected on the 0.45 µm filter has generally been neglected. While, electron probe X-ray Experimental microanalysis (EPMA) provides useful information on the chemical composition of individual particles collected on a 0.45 Geological characteristics of the study area µm filter.11–19 However, the role of individual mineral particles Water sampling was carried out at each of four sites in Fuji in the removal process of dissolved elements in river or lake and Sagami rivers, Yamanashi prefecture, central Japan. The water has not yet been clarified sufficiently by conventional details of the sampling location are shown in Fig. 1. Fuji river starts from areas around Shosenkyo and Mt. Yatsugatake, which † To whom correspondence should be addressed. consist of mainly granitic and andesitic rocks, respectively,3 E-mail: [email protected] runs through Kofu basin and flows into Suruga bay. Sites F1 836 ANALYTICAL SCIENCES JUNE 2003, VOL. 19 Fig. 2 Analytical procedures of labile substance adsorbed on suspended particles in river water. Dilute-acid soluble fraction from the particles collected on the 0.45 µm filter was evaluated by B – A. Fig. 1 Sampling locations. the residual 500 ml portion of the river-water sample, and the and F4 in tributaries of Fuji river reflect approximately the acidified sample was kept as a 0.02 M nitric acid solution for 4 effects from andesitic rocks and sediments, respectively.3 Sites h. Thereafter, the acidified sample was filtered through a 0.45 F2 and F3 in the main stream of Fuji river reflect complex µm membrane filter and the filtrate was subjected to ICP-MS. effects from granitic, andesitic and sedimentary rocks. Sagami The residue on the filter was not analyzed in this study. The river starts from the foothills on the northern area of Mt. Fuji, elemental concentrations (B) obtained mean the total sum of the for example, “Oshinohakkai”, turns east the direction at Otsuki original dissolved components and those extracted from all of and flows into Sagami bay. The foot of Mt. Fuji consists of the particles. Therefore, the dilute-acid soluble fraction from mainly basaltic rocks.2,3,20 Mt. Fuji is the largest basaltic the particles collected on the 0.45 µm filter was evaluated by stratovolcano from the quaternary period in Japan, which B – A. spewed tremendous amounts of volcanics since the prehistorical age.20 Sites S1, S2 and S3 in the main stream of Sagami river Single-particle analysis by SEM-EDX reflect the effects from basaltic rocks.3 Site S4 reflects the The particles on the filter were detached by both sides- effects from andesitic rocks.3 Although both rivers are fed by a adhesive carbon tape, and were fixed on an aluminum sample number of tributaries, it can be considered approximately that stand for SEM-EDX.19 Thereafter, the particles were carbon the Fuji and Sagami rivers show the effects from granitic and coated (∼200 Å) and the specimens were subjected to an SEM andesitic, and basaltic rocks, respectively. These sampling (Hitachi S-3000N)-EDX (Horiba EMAX550) analysis. About locations provide a good situation for geochemical studies of 200 particles other than diatoms in each specimen (F1 – F4 and river water, because this area covers extremely different S2) were analyzed under the condition of an accelerating geologies in spite of the narrow area. voltage of 20 kV for an X-ray intensity measurement and 25 kV for an SEM image observation, using a probe current of 0.3 nA River water sampling and pretreatment and a dead time of 20 – 30%. In the X-ray intensity The nitric acid used was of ultrapure grade. High-purity water measurements, an area analysis of individual particles was (Millipore, Milli-Q, Bedford, MA, USA) was used throughout. carried out as follows. The electron beam was usually Figure 2 shows the analytical scheme. River-water samples irradiated at the central part of an individual particle to obtain were collected in polypropylene containers (1 L) at each four the average composition, and the data were accumulated for 100 site in the Fuji (F1 – F4) and Sagami (S1 – S4) rivers on March s. The characteristic intensities of the detected elements were 1, 2000. The samples were shaken thoroughly prior to use. normalized to the total sum of the characteristic X-ray Firstly, a 500 ml portion was immediately filtered through a intensities of elements with an atomic number over 11 to avoid membrane filter (Millipore, Omunipore JH, Hydrophilized the particle-size effect.13,15,16,18 Obvious single particles with PTFE, 0.45 µm in pore size and 47 mm in diameter). The sizes approximately between 1 and 10 µm were classified into particles on the filter were used for an SEM-EDX analysis. sixteen groups according to the relative X-ray intensities of the Next, a leaching experiment was also carried out in order to major elements detected, as shown in Table 1. Aggregates or estimate the easily soluble components from the particles biomineral-like substances21 with sizes of approximately collected on the 0.45 µm filter, as follows. Above 0.45 µm between 20 and 30 µm were firstly discriminated from other filtrate was acidified (as 0.02 M nitric acid) with ultra-pure particles by their peculiar shapes, and then re-classified using nitric acid, and subjected to inductively coupled plasma mass the relative X-ray intensities of the constituents. The method of spectrometry (ICP-MS). The elemental concentrations (A) particle-type classification proposed in this study is not affected obtained mean the total sum of the original dissolved by minor