Budgets and sources of dissolved platinum in the inland seas of Japan

Platinum (Pt) is used in a variety of industrial and medical applications, including automobile catalysis and anticancer drugs. At present, anthropogenic Pt is released and dispersed in the environment globally. Previously reported, vertical profiles of dissolved Pt in coastal seawaters indicated an increase in the Pt concentrations from the surface to the bottom, suggesting that Pt in seawater may be supplied from seafloor sediments, although this benthic supply process remains unresolved. Therefore, in this study, we sought to reveal the geochemical cycles of Pt between the seawater and sediments in inland seas, such as Japan's Ariake Sea, which is a semi-enclosed sea, thus allowing the examination of the interactions between seawater, river water, and sediments. Seawater and sediment samples were collected from this sea and the nearby Tachibana Bay. The former was analyzed for relevant hydrographic properties (temperature, salinity, and dissolved oxygen) along with Pt concentration, while the latter underwent fractional dissolution to determine dissolved Pt using isotope-dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) after column preconcentration with an anion exchange resin. The observed Pt concentrations in seawater ranged from 0.41 to 10.2 pmol/L with the highest value occurring at the deepest layer of the Ariake Sea's mouth. The Pt concentrations increased with depth, with decreasing concentrations toward the inner central Ariake Sea. Detectable Pt contents in the sediment were found in the exchangeable, Fe and Mn oxide, and residual phases. Loosely adsorbed Pt onto the sediment might be desorbed by interactions with chloride in seawater. A model for the Pt budget in the Ariake Sea estimated that most Pt was supplied from sediments, and the Pt residence time was longer than that of seawater.