Arsenic Adsorption in Soil with Increasing Levels of Monosilicic Acid in Solution: A Laboratory Incubation Study.

Frequent consumption of foods containing arsenic (As) can pose health problems to human. One of the ways by which As enters the food chain is through uptake by plants. Rice (Oryza sativa) is a staple food for more than half of the world’s population and is commonly cultivated under anaerobic condition in which As is present as a more soluble and plant-available form, arsenite. Studies have shown that high levels of plant-available silicon (Si; monosilicic acid) in the soil solution impair absorption of arsenite in rice. A laboratory experiment was conducted to examine As sorption in soil added with increasing levels of monosilicic acid. Sorption isotherms were evaluated at six As rates (0, 10, 20, 40, 80 and 160 µg mL^-1) and five levels of Si (0, 10, 20, 40, and 80 µg mL^-1) with three replicates. Silicon and As in the solution were measured at 0, 1, 4, 7, 30 and 60 days after incubation. There was greater adsorption of As in the presence of 40 and 80 µg Si mL^-1 at the beginning and one day after incubation, followed by a reduction by 30 µg mL^-1 after 4 days, but only in the presence of 80 µg Si mL^-1. Arsenic adsorption slightly increased with increased Si levels at 7 days. After 30 days, high levels of Si enhanced As adsorption, but adsorbed As was quickly reduced when As measured in solution exceeded 55 µg mL^-1. This was possibly due to precipitation of As with dissolved sulfides which was also found at elevated concentration in solution. The outcomes of this study suggest that high level of Si may enhance adsorption of As on exchange sites and potentially reduce plant uptake; however other ligands such as sulfide should be accounted as they also interfere with As sorption in the soil.