Arsenic volatilization in model anaerobic biogas digesters

• Published in: Applied geochemistry 2013-06, Vol.33, p.294-297
• Main Authors: Mestrot, Adrien, Xie, Wan-Ying, Xue, Ximei, Zhu, Yong-Guan
• Format: Article
• Language: English
• Subjects: anaerobic conditions; anaerobic digestion; animal manures; Animals; Arsenic; Biogas; carcinogens; chemical speciation; China; Digesters; drugs; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; feed conversion; Geochemistry; headspace analysis; methane production; Methylation; Oryza sativa; parasites; Pollution, environment geology; Rice; rice straw; risk; roxarsone; toxicity; Transformations; volatilization
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2013.02.023

•Arsenic is volatilized form all model anaerobic digesters, including the non-treated ones.•Volatile As species can be identified and quantified in all digesters.•Non-arsenic treated digesters volatilization rates are higher than Roxarsone treated ones. Arsenic is a class 1 non-threshold carcinogen which is highly ubiquitous. Arsenic undergoes many different transformations (biotic or abiotic) between and within environmental compartments, leading to a number of different chemical species possessing different properties and toxicities. One specific transformation is As biotic volatilization which is coupled with As biomethylation and has been scarcely studied due to inherent sampling issues. Arsenic methylation/volatilization is also linked with methanogenesis and occurs in anaerobic environments. In China, rice straw and animal manure are very often used to produce biogas and both can contain high amounts of As, especially if the rice is grown in areas with heavy mining or smelting industries and if Roxarsone is fed to the animals. Roxarsone is an As-containing drug which is widely used in China to control coccidian intestinal parasites, to improve feed efficiency and to promote rapid growth. Previous work has shown that this compound degrades to inorganic As under anaerobic conditions. In this study the focus is on biotic transformations of As in small microcosms designed as biogas digester models (BDMs) using recently validated As traps, thus, enabling direct quantification and identification of volatile As species. It is shown that although there was a loss of soluble As in the BDMs, their conditions favored biomethylation. All reactors produced volatile As, especially the monomethylarsonic acid spiked ones with 413±148ng As (mean±SD, n=3) which suggest that the first methylation step, from inorganic As, is a limiting factor. The most abundant species was trimethylarsine, but the toxic arsine was present in the headspace of most of the BDMs. The results suggest that volatile As species should be monitored in biogas digesters in order to assess risks to humans working in biogas plants and those utilizing the biogas.