Presence of oxygen in diffusive equilibrium in thin films (DET) probes: Effect on phosphate and iron porewater profiles and advice for correct deoxygenation and handling procedures
Anna-Marie Klamt, Theis Kragh, Ronnie N. Glud, Cecilie M. Wagner, Kasper Reitzel
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Abstract
Diffusive equilibrium in thin films (DET) probes are passive samplers that are designed to reflect in situ porewater concentrations. In this study, we show that the gel and the plastic housing of DET probes store a substantial amount of oxygen (O2) that affects the chemical composition of porewater. To ensure complete deoxygenation, DET probes need to be treated for 7 d with continuous nitrogen flow. Such fully deoxygenated probes can be handled in the air (exposure time: < 90 s) and deployed to sediments through oxic water (exposure time: < 2 s) without significant ad(b)sorption of O2. Furthermore, we deployed a set of untreated (i.e., in equilibrium with atmospheric O2) and a set of fully deoxygenated DET probes to lake sediments. The O2 present in untreated DET probes altered iron (Fe) and phosphate (P) porewater profiles significantly. This is caused by the oxidation, immobilization, and accumulation of redox-sensitive Fe (oxyhydr)oxides in the probe over time. Since P has a high binding affinity to Fe (oxyhydr)oxides, it is not in equilibrium with the porewater and is overestimated as well. Our results highlight the importance of thorough deoxygenation of DET probes before deployment in sediments, especially when addressing redox-sensitive porewater species.
薄膜中的扩散平衡（DET）探头是一种被动采样器，旨在反映原位孔隙水浓度。在这项研究中，我们发现 DET 探针的凝胶体和塑料外壳会储存大量氧气 (O2)，从而影响孔隙水的化学成分。为确保完全脱氧，DET 探针需要在氮气持续流动的情况下处理 7 天。这种完全脱氧的探针可以在空气中处理（暴露时间：90 秒），并通过含氧水（暴露时间：2 秒）部署到沉积物中，而不会产生大量的氧气吸附。此外，我们还在湖泊沉积物中布放了一组未经处理（即与大气中的 O2 处于平衡状态）的 DET 探针和一组完全脱氧的 DET 探针。未经处理的 DET 探针中存在的氧气显著改变了铁（Fe）和磷酸盐（P）的孔隙水剖面。这是由于对氧化还原敏感的铁（氧氢）氧化物在探针中随着时间的推移发生氧化、固定和积累而造成的。由于 P 与铁氧水氧化物有很高的结合亲和力，它与孔隙水不平衡，因此也被高估了。我们的研究结果突显了在沉积物中部署 DET 探针前彻底脱氧的重要性，尤其是在处理对氧化还原反应敏感的孔隙水物种时。
期刊介绍：
Limnology and Oceanography: Methods (ISSN 1541-5856) is a companion to ASLO''s top-rated journal Limnology and Oceanography, and articles are held to the same high standards. In order to provide the most rapid publication consistent with high standards, Limnology and Oceanography: Methods appears in electronic format only, and the entire submission and review system is online. Articles are posted as soon as they are accepted and formatted for publication.
Limnology and Oceanography: Methods will consider manuscripts whose primary focus is methodological, and that deal with problems in the aquatic sciences. Manuscripts may present new measurement equipment, techniques for analyzing observations or samples, methods for understanding and interpreting information, analyses of metadata to examine the effectiveness of approaches, invited and contributed reviews and syntheses, and techniques for communicating and teaching in the aquatic sciences.
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