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

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.