Abstract
Highly concentrated dissolved silicate was detected in pore water from anoxic-reducing sediment in Lake Nakaumi, a brackish lake. Silicate concentration also simultaneously increased with total hydrogen sulfide concentration during the summer. Generally, dissolved silicate is readily adsorbed onto ferric hydroxide and precipitates in an oxidative environment. In this study, we focused on the behavior of ferric hydroxide adsorbing silicate in sediment and determined that hydrogen sulfide was the main cause of dissolved silicate elution from ferric hydroxide adsorbing silicate because the hydrogen sulfide produced via microbiological processes in the anoxic-reducing environment was reducible for other metal oxides. According to laboratory experiments, silicate was released from ferric hydroxide by reacting with sodium sulfide, causing increasing elution of dissolved silicate from anoxic-reducing sediments with increasing concentration of sodium sulfide in the solutions. This result shows that hydrogen sulfide is very crucial for silicate release under a reducing environment. Therefore, in Lake Nakaumi, silicate would be released from the bottom after ferric hydroxide adsorbing silicate reacted with hydrogen sulfide in a summer reductive environment.
Highlights
Silicon is the second most abundant element in Earth’s crust and typically exists as silica (SiO2) or in silicate minerals
At Lake Nakaumi, a brackish lake located in western Japan, concentrations of dissolved silicate increase seasonally with total hydrogen sulfide in the anoxic hypolimnion and pore water in bottom sediment, which is characterized by elevated values
We investigated the mechanisms of silicate adsorbed on ferric hydroxide and silicate elution in the presence of hydrogen sulfide in the laboratory experiments
Summary
Silicon is the second most abundant element in Earth’s crust and typically exists as silica (SiO2) or in silicate minerals. Dissolved silicate in a water environment is generally found as monomeric silicate (H4SiO4) and is characterized by a non-ionic form in neutral and weakly acidic solutions but is ionized in an alkaline solution (Philippini et al 2006). The ionized ratio of dissolved silicate is approximately 0.016% at a pH of 7 and increases with increasing pH. This ratio is Handling Editor: Masahiro Maruo
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