Fundamental studies for chemical speciation of iron in seawater with an improved analytical method

Abstract

We improved the analytical methods for determining iron in seawater, based on our established procedure, and carried out fundamental studies for chemical speciation of iron. The blank value of the system used was ∼ 0.05 n M and the detection limit (3 SD) was 0.01 n M. To examine the dissolution of iron from suspended particles in seawater, four types of particles (aged iron colloid, biogenic particles, sediment particles and freshly deposited iron) were used. At a pH of ∼ 3, dissolution of iron from the easily leachable fraction of suspended particles was prompt, and the fraction of iron [total dissolvable iron, TD(Fe)] was thought to be the sum of labile particulate and dissolved iron. The iron fractions dissolved from suspended particles in the solutions were constant at pHs < 1.5 when solutions were heated in a microwave oven. The fraction of iron dissolved by this procedure was defined as the “leachable iron, L(Fe)”. Some known organic complexing agents were also studied as a model group to examine the possible effects of naturally occurring organic ligands on the recovery of iron with chelating resin preconcentration. Established methods were applied to analyses of seawater samples obtained from the western Southern Indian Ocean (SIO) and the East China Sea. Surface seawater samples from the SIO showed very low iron concentrations, which may be due to a lack of aeolian transport of mineral dust.