Characterization of iron diagenesis in marine sediments using refined iron speciation and quantized iron(Ⅲ)-oxide reactivity:a case study in the Jiaozhou Bay,China

As a case study, refined iron（Fe） speciation and quantitative characterization of the reductive reactivity of Fe（Ⅲ）oxides are combined to investigate Fe diagenetic processes in a core sediment from the eutrophic Jiaozhou Bay.The results show that a combination of the two methods can trace Fe transformation in more detail and offer nuanced information on Fe diagenesis from multiple perspectives. This methodology may be used to enhance our understanding of the complex biogeochemical cycling of Fe and sulfur in other studies. Microbial iron reduction（MIR） plays an important role in Fe（Ⅲ） reduction over the upper sediments, while a chemical reduction by reaction with dissolved sulfide is the main process at a deeper（〉 12 cm） layer. The most bioavailable amorphous Fe（Ⅲ） oxides [Fe（Ⅲ）am] are the main source of the MIR, followed by poorly crystalline Fe（Ⅲ） oxides [Fe（Ⅲ）pc）]and magnetite. Well crystalline Fe（Ⅲ） oxides [Fe（Ⅲ）wc] have barely participated in Fe diagenesis. The importance of the MIR over the upper layer may be a combined result of the high availability of highly reactive Fe oxides and low availability of labile organic matter, and the latter is also the ultimate factor limiting sulfate reduction and sulfide accumulation in the sediments. Microbially reducible Fe（Ⅲ） [MR-Fe（Ⅲ）], which is quantified by kinetics of Fe（II）-oxide reduction, mainly consists of the most reactive Fe（Ⅲ）am and less reactive Fe（Ⅲ）pc. The bulk reactivity of the MR-Fe（Ⅲ） pool is equivalent to aged ferrihydrite, and shows down-core decrease due to preferential reduction of highly reactive phases of Fe oxides.

Variation of iron species in sliding mud

Results of Mossbauer spectroscopy and ICP-AES measurement revealed a special distribution pattern of iron species in four crossing profiles of the Tertiary Formation Type of Landslides in Japan. In comparison with host rocks, the sliding mud in the landslide slip zone contains the higher ferrous iron content and/or trace pyrite, which indicates that the forming environment was relatively reducing. The total iron content also increased with an increase in ferric iron and pyrite, in contrast, decreased with an increase in ferrous iron content. The deep-gray and black mud layers formed and accumulated in various volumes within landslide slip zones are treated as the basic criteria for the slipping activity, because the soft mud saturated with groundwater is easy to create a plastic deformation.

Photooxidation of arsenic(Ⅲ) to arsenic(Ⅴ) on the surface of kaolinite clay

As one of the most toxic heavy metals, the oxidation of inorganic arsenic has drawn great attention among environmental scientists. However, little has been reported on the solar photochemical behavior of arsenic species on top-soil. In the present work, the influencing factors（p H, relative humidity（RH）, humic acid（HA）, trisodium citrate, and additional iron ions） and the contributions of reactive oxygen species（ROS, mainly HO^- and HO2^-/O2^-） to photooxidation of As（Ⅲ） to As（Ⅴ） on kaolinite surfaces under UV irradiation（λ = 365 nm）were investigated. Results showed that lower p H facilitated photooxidation, and the photooxidation efficiency increased with the increase of RH and trisodium citrate.Promotion or inhibition of As（Ⅲ） photooxidation by HA was observed at low or high dosages, respectively. Additional iron ions greatly promoted the photooxidation, but excessive amounts of Fe^2＋competed with As（Ⅲ） for oxidation by ROS. Experiments on scavengers indicated that the HOUradical was the predominant oxidant in this system.Experiments on actual soil surfaces proved the occurrence of As（Ⅲ） photooxidation in real topsoil. This work demonstrates that the photooxidation process of As（Ⅲ） on the soil surface should be taken into account when studying the fate of arsenic in natural soil newly polluted with acidic wastewater containing As（Ⅲ）.