A method was developed for separating several fine particles in suspension by controlling the solution environment in terms of pH and the sodium chloride concentration. The sedimentation test of the suspension, which included three kinds of the sample materials, α-iron oxide (III), silicon dioxide, zirconium oxide, was conducted under various solution environments and then the time variations of the concentration of each particle were measured at the upper and lower part of the sedimentation tube. The experimental data showed that pH control without the addition of sodium chloride did not lead to effective particle fractionation, although the sedimentation behaviors of three particles varied dramatically with the solution pH. Furthermore, the effect of the addition of the electrolyte (sodium chloride) was explored at pH 9, where all the particles are negatively charged. At the sodium chloride concentration of 0.04 mol/l, silicon dioxide dispersed, while both iron oxide and zirconium oxide flocculated severely due to the reduction of the zeta potentials and thus settled down. As a result, silicon dioxide was successfully separated from iron oxide and zirconium oxide. In conclusion, when the electrolyte concentration is adjusted appropriately under the pH condition where all the particles disperse due to a repulsive force, the effective fractionation associated with selective flocculation is attained.

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Dissolved organic carbon (DOC) in the northwest Pacific Ocean was measured by using a high-temperature catalytic oxidation (HTCO) method. The properties and behavior of dissolved organic matter (DOM) were identified by three dimensional excitation-emission matrix (3DEEM) spectroscopy. Seawater samples were collected in 1995 and 1996 during the NOPACCS (Northwest Pacific Ocean Carbon Cycle Study) cruise of R/V Hakurei-Maru. The mean concentrations of DOC at the surface in 1995 and 1996 were defined as 78.1 ± 8.9 and 67.4 ± 4.4 μmol/l, respectively. Below a depth of 500 m, the concentrations for 1995 and 1996 were 41.8 ± 5.3 and 41.9 ± 5.2 μ mol/l, respectively. High DOC concentration water mass was confirmed in the subtropical region (175° E, 28° N). The maximum depth of DOC concentration was different from that of chlorophyll a. This indicates that phytoplankton activity was not directly related with the production of DOC, and it is thought that the major source of DOC is degradation of dying phytoplankton. The fluorescence maximum peaks, which originated from marine humic like substances, were detected at excitation 230-240 nm /emission 370-440 nm (Peak A) and excitation 300-320 nm/emission 390-410 nm (Peak B). These fluorescence intensities are related to the regeneration of nutrients and the consumption of dissolved oxygen. It is suggested that marine humus is regenerated by oxidation and remineralization of organic substances.

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