Dissolved silicates are easily adsorbed onto Fe(OH)3
under an oxidative environment and are semi-permanently sedimented. However, these silicates are released from Fe(OH)3
under an anoxic environment. Therefore, we comparatively studied the geochemical processes and vertical distributions of dissolved monomeric (MSi), polymeric (PSi), and total (TSi) silicate species in the pore waters of some freshwater and saline areas in Japan. The PSi concentration was calculated by the difference between TSi and MSi concentrations. PSi was only detected in anoxic pore waters of freshwater areas such as Lake Biwa (Fig. 1A). The TSi concentrations in the pore waters of saline areas, such as Lake Suigetsu (Fig. 1B), were much higher than those of freshwater areas, but PSi was not detected in any saline areas. Therefore, we assumed that NaCl is a key factor for the formation and decomposition of PSi because it is the most abundant component in saline water. Thus, the effect of NaCl on the geochemical behavior of silicate species was further investigated.
Laboratory experiments were performed to examine the effect of NaCl on the formation and decomposition of PSi. The NaCl concentrations were adjusted to 0 or 0.6 M in test solutions containing 0.6 mM MSi and 0.3 mM Fe3+ at pH 7. After standing for one month, all test solutions were filtered, and precipitates were reacted with a Na2S solution (pH 7). After reacting for one day, the solution was filtered again to measure the concentrations of MSi, TSi, and PSi in filtrates. PSi was detected in the filtrate from 0 M NaCl solution but not in a 0.6 M NaCl solution. Furthermore, when NaCl was added to the pore water of Lake Biwa, most PSi decomposed to MSi after eight days.
Therefore, PSi is only formed in anoxic sediments of freshwater areas and is decomposed to MSi in saline areas. The formation of PSi is inhibited by the presence of large amounts of NaCl.