A mesocosm study of stranded Sargassum shows considerable volatilization and arsenic dominating in inorganic forms

Pelagic Sargassum seaweed is known to bioaccumulate arsenic, yet limited information exists about the fate of this arsenic once Sargassum strands on beaches. The objective of this study was to evaluate the transfer of arsenic from Sargassum to sand, leachate, and the atmosphere, as well as arsenic speciation, during a simulated stranding scenario. Two sets of mesocosms were used to measure the transfer: one containing Sargassum and sand (SS), and a control containing sand only (SO). Initial arsenic concentrations in Sargassum (34 mg/kg) increased during drying (peaking at 66.7 mg/kg), then declined to a steady level of 7.7 mg/kg by day 70. Of the arsenic lost from Sargassum in the SS mesocosms, 41% was detected in leachate, 5% was absorbed by the underlying sand, and 54% was volatilized. The sand, which was split for each mesocosm, had statistically different arsenic concentrations by the end of the experiment (p = 0.003) with sand beneath the Sargassum in the SS mesocosms showing a slight increase in arsenic concentrations, while the sand in the SO mesocosms experienced a slight decrease. The leachates from both mesocosms were compared to the mean rainfall arsenic concentration (2.2 μg/L). Only the SS mesocosm leachates were significantly elevated (p = 0.0067, maximum at 322 μg/L), while the SO mesocosm leachates were not significantly different (p = 0.97, maximum at 4.1 μg/L) from rainfall. Across all compartments (Sargassum, sand, and leachate), the dominant arsenic species was inorganic As(V), followed by dimethylarsinic acid, DMA(V). Overall, these findings demonstrate that Sargassum acts as a source of arsenic, primarily as inorganic As(V), to coastal environments, with most being lost through volatilization and leaching, and a smaller portion retained in sand. These results highlight the complex role of Sargassum in coastal arsenic cycling and its potential to serve as a source of arsenic exposures for beachgoers.