Dissolved aluminum and silica release on the interaction of Okinawan subtropical red soil and seawater at different salinities: Experimental and field observations

抄録

The experiment was conducted to investigate the dissolution of Al and silica when two red soils interact with seawater at different salinities. Different soil weights (3, 10 and 20 g) of bulk soil, coarse sand and silt + clay were mixed with 100 mL of natural seawater, 10, 100, and 1000 times diluted seawaters and shaken for 4 hours to obtain the extracts. Two field surveys were also carried out to observe the impact of red soil in four red soil-impacted estuaries in Okinawa Island during September (summer) and January (winter). Our results revealed that the red soils were acidic (pH ∼5), dominated by SiO₂ and Al₂O₃ contents. During the interaction of red soils with the solutions, dissolved Al and silica increased with an increasing soil/solution ratio while the pH decreased to the lowest value of 3.96 in seawater extracts. The highest Al concentration ranged from 5.03 to 40.3 mgL^-1 in seawater extracts and differed with diluted extracts by 1 to 3 fold, 6 to 24 fold, and 12 to 45 fold for 10, 100 and 1000 times dilution, respectively. The highest silica concentrations of about 1 to ∼11 mgL^-1 were observed in seawater extracts and in 10 times dilution, and they varied with 100 and 1000 times diluted extracts by about 1 to ∼3 fold only. Salinity is more sensitive to Al release than silica but the pH significantly correlated to both logarithmic concentrations of Al (P<0.01) and silica (P<0.001) suggesting the exchange process and H⁺ promoted weathering of silicates, respectively. The field observation also revealed Al increase with salinity from <0.170 mgL^-1 in the river waters to the highest value of 0.474 mgL^-1 in seawater, suggesting an increasing Al solubility by exchange process. The trend of silica in the field differed from that of the laboratory with the highest concentration of 17.8 mgL^-1 in the river and the lowest value of 0.128 mgL^-1 in the seawater, which could be largely attributed to the diatom utilization in summer and dilution in seawater, particularly during winter.