Study On Factors Influencing Sulfate Reduction And Microbial Iron Reduction In Jiaozhou Bay Sediments

Sulfate reduction and microbial iron reduction are two main pathways coupled toorganic matter (OM) mineralization in marine sediments, and play an important rolein regulating geochemical cycling of C, S, Fe, P, and a number of trace elements. Inthis paper, Jiaozhou Bay sediments are investigated to reveal microbial iron reductionand sulfate reduction, which is important for understanding benthic geochemistry ofsemi-enclosed bays and eutrophic water in other areas of the world. By addingglucose, reactive iron (III) oxide to change the contents of labile iron (III)oxide(electron acceptor), labile OM (electron donor) and by adding molybdate toinhibit sulfate reduction, various Fe and S pools in the solid-phase sediments, i.e.,Fe(II), Fe (III), AVS and Spywere determined. The relative importance of sulfatereduction and microbial iron reduction in Jiaozhou bay sediments were quantitativelyevaluated. Main results are as follows:1. Sediments were amended with reactive iron (III) oxides and then incubated for63days. In comparison with origin sediments, AVS accumulation decreases by30%in amended sediments, Spyconsumption decreases by10%, and Fe(III) consumptionincreases by6.5times. The more the contents of labile iron(III) oxides were added inthe sediments, the less the AVS and Spywere produced. The results indicate that anincrease in labile iron(III) oxides can promote microbial iron reduction and inhibitsulfate reduction in anaerobic sediments, and the inhibition increases with the increasein the amount of added iron(III) oxide.2. The sediments were amended with labile OM and incubated for63days. Incomparison with origin sediments, AVS accumulation increases by89%in organicamended sediments, Spyconsumption increases by66%, and Fe(III) consumptionincreases by22%. The more the contents of labile OM were added, the more the AVS and Spywere produced. So does the consumption of Fe(III). These findings suggestthat an increase in labile OM can promote microbial iron reduction and sulfatereduction in anaerobic sediment. The promotion on sulfate reduction seems to bemore obvious. OM enrichment in shelf sediments and lower-latitude marinesediments may promote sulfate reduction, which results in accumulation of H2S thatare toxic to marine ecosystem.3. Labile iron(III) oxides or labile OM are added into the sediments and thenincubate for63days. Compared with sediment rich in OM, the results of adding labileiron(III) oxides are as follows: AVS accumulation decreases by1.4times; Spyconsumption decreases by28%; Fe(III) consumption increases by1.4times in thisexperiment. Adding labile iron(III) oxides to the sediment with rich OM can inhibitsulfate reduction and deter the transforming from FeS to FeS2. In eutrophic waters orculture zones where the concentration of labile OM is high in sediments, adding labileiron(III) oxides may contribute to toxic elements’ activation and migration, andeffectively weaken the accumulation of sulfide in water, and reduce its toxicity toorganisms.4. The sediments were amended with both labile OM or reactive iron(III) oxides,and then incubated for63days. In comparison with sediments amended with reactiveiron(III) oxides, AVS accumulation increases by2.1times in organic-amendedsediments, Spyconsumption increases by77%, and Fe(III) consumption increases by18%. In the sediments amended with reactive iron(III) oxides, an increase in labileOM can promote sulfate reduction and accelerate the FeS transformation to FeS2. It isspeculated that increase in the content of labile OM can promote sulfate reduction andFeS2formation in iron-rich salt marshes sediments, mangroves sediments, shelf andestuarine sediments.5. Labile OM or reactive iron(III) oxides can promote microbial iron reduction insediments, where sulfate reduction is inhibited. The effect of reactive iron(III) oxidesis more obvious. Microbial iron reduction is the main pathway of iron reduction inJiaozhou Bay sediments, sulfate reduction may be inhabited by dissimilatroy ironreduction. Without precipitation of iron sulfides, Fe(II) in sediments will diffuse upward together with phosphorus and heavy metal contaminants. Fe(II) can bereoxidized and enriched in the redox interface, which becomes geochemical barrierfor scavenging of phosphorus and heavy metal contaminants.