Speciation of Iron in Rain Water by Size Fractionation, Acid Decomposition, Acid Extraction and Catalytic Determination

Abstract

The bioavailability and toxicity of metal species in aquatic systems may depend on the lability, i.e., reactivity, which is different depending on their chemical forms 1,2 Especially, the association of iron with coexisting humic substances (HSs) changes the lability of iron. 3 The aggregation of iron and HSs probably influences the lability of other trace constituents by their association with iron-HSs aggregates. 4,5 Therefore, the speciation of labile and non-labile iron species is important in environmental and biological studies.Iron can be sensitively determined at pH values close to those 6 of natural rain water by a catalytic spectrophotometric method using an iron-catalyzed oxidation reaction of ophenylenediamine (OPDA) with hydrogen peroxide. 3 The determination of iron is based on a measurement of the formation rate of oxidized OPDA in proportion to the concentration of iron forming an active intermediate, probably Fe III (H2O2)(OPDA), in the catalytic reaction process. Such a labile form of iron was well characterized as Fe 3+ and Fe III Li (3-in)+ , where i = 1 or 2 for unidentate and i = 1 for bidentate ligand of L n-, in a study of the reactivity of hydroxo, fluoro and oxalato complexes of iron.3 Therefore, the interpretation of the lability is very simple compared with electrochemical methods, 7,8 which are affected by the complicated adsorption property of the organic matter on a working electrode. In this paper, the catalytic method was combined with the size fractionation and acid decomposition, and was applied to the speciation of labile and non-labile iron in rain water. Acid extraction of the particle fraction was also carried out for the speciation. The analytical results of natural rain-water samples were compared with those of synthetic samples. The proposed method was simpler than the speciation method 9 based on the sorption separation with four types of ion-exchangers and adsorbents. ExperimentalReagents and samples Unless stated otherwise, all chemicals were of analyticalreagent grade. High-purity NaOH and HNO3 solutions (Kanto Chemical Co., Inc., Ultrapur) and de-ionized and distilled water were used throughout. An iron stock standard solution (1.00 g Fe l -1 ) was prepared by dissolving FeNH4(SO4)2·12H2O (purity ≥ 99.0%) in 0.1 M HCl. Working iron standard solutions were prepared just before use by diluting the stock standard solution with water or 1 mM HCl for a spectrophotometric determination or with 0.1 M HCl for an atomic-absorption spectrometric determination. A humic acid (HA) solution was prepared by dissolving the reagent of humic acid (Wako Pure Chemical Industries, Ltd., isolated from soil) in 0.01 M NaOH and was neutralized with 0.2 M HCl.In three rainfall events on 1/19/1999, 1/24/1999 and 9/16/1999, rain water was collected on the roof of a building in Yamanashi University, Kofu, as the initial rainfall water from the beginning to 2 mm of rain precipitation, and as the successive rainfall water between 5 mm and 7 mm. The total precipitations on 1/19/1999, 1/24/1999 an...

Catalytic Flow-Injection Determination of Reactive Iron in Non-Acidified Water Samples

“…The concentration of reactive iron in the high-MW and colloidal fraction (MW 10 3 -0.45 µm) was calculated by A -C. The concentration of humic substances was determined as HA based on the fluorometric method. [3][4][5] For all of the acidified samples, the analytical results obtained by the proposed FIA method agreed with those by the AAS method within an error of 5%. These results indicate that the total concentration of iron is obtainable by the FIA method when the sample is acidified.…”

“…From the equilibrium of hydroxo Fe(III) species at pH 4.5 or 4.7, most of the iron adsorbed was characterized as iron(III) hydroxide(s). Iron(III) hydroxide(s), which is coagulated (particle size > 0.45 µm) 4 and probably charged positively, 10 may be adsorbed at the anion site (hydroxy group introduced by the substitution of fluorine) 8 of the PTFE tube. Peak profiles in catalytic spectrophotometry.…”

“…For example, the amount of iron adsorbed on the surface of a polystyrene bottle was different between iron(III) hydroxide(s) and its humic aggregates. 4 Therefore, the validity of the measurement of the peak height should be checked by using NAR for the type of analyzed sample. When the measurement of the peak height (the maximum Pt) at peak top is inappropriate for the determination, the Pt value before achieving the maximum may be usable.…”

“…However, acidification should be avoided for speciation, because it changes the chemical forms of iron, e.g., the dissociation of unreactive iron(III) hydroxide(s) and complexed iron species. 4,5 Flow injection analysis (FIA) is suitable for the catalytic method, because of easy and precise control of the kinetic processes of a micro volume of the sample. PTFE tubes are most commonly used for the manifold of the FIA system.…”

“…Significant, but low, concentrations (A -C) indicate the presence of reactive iron in the high-MW and colloidal fraction. This type of iron may be active amorphous Fe(III) hydroxide 4,12 or unknown iron species. Fluoride ion (0.07 mg L -1 ) was found in a sample collected on a separate day.…”