Temporal change of arsenic in leachates from Sargassum spp. biomass deposit: mass balance, speciation and microbial biotransformation pathways.

The decomposition of washed ashore Sargassum biomass is a major health and environmental concern due to arsenic (As) release. A 120-day mesocosm experiment without replication, simulating natural conditions at stockpiled Sargassum sites (temperature, sunlight, rain), was conducted to study the fate of As. In fresh Sargassum, total As concentrations were 85 mg/kg (dw). AsIII was the most abundant species, followed by TMAsO, AsV, AsB and DMAsV. In leachates, the As concentration was initially 10 mg/L and then decreased to 1.5 mg/L after 90 days of experimental monitoring. AsV was more abundant than AsIII, and varying amounts of AsB and DMAsV were also detected. High arsine content, i.e. mainly trimethylarsine ((CH3)3As), was released during the first weeks. After 120 days, 50 % of the initial total Sargassum As content was released in the leachate, while 51 μg volatilised in the atmosphere (0.06 ‰). As remaining in this biomass was still high (55 mg/kg dw) due to the substantial mass loss during its degradation. Bacterial diversity increased over time in the leachate and decomposed biomass, suggesting that bacteria had a major role in the As fate and speciation. According to the French legislation, the discharge limits in terms of As concentration in leachates of the decaying Sargassum biomass from hazardous waste treatment facilities would often be exceeded.