Manganese and iron oxide-coated redox bars as a tool to in situ study the element sorption in wet soils

When studying redox conditions in soils with manganese (Mn) and iron (Fe) oxide-coated redox bars, we observed the formation of Fe oxides along the Mn oxide coating and assumed sorption of other elements from soil solution to oxide surface. The objective of this study was to investigate the formation of Fe oxides along Mn redox bars and to analyze element sorption from soil solution to either Mn or Fe oxide along redox bar coatings. We protruded Mn redox bars into solutions with defined Fe2+ concentrations and removed the bars at distinct time intervals. The Mn oxide coating and potential Fe oxides were extracted using dithionite-citrate-bicarbonate (DCB). To investigate in situ element sorption behavior, we used previously field-installed redox bars, protruding these Mn redox bars into acidified hydroxylamine hydrochloride (AAH) to selectively extract Mn oxide and afterwards into DCB to dissolve the remaining Fe oxide coating. This two-step extraction procedure enabled the differentiation of elements bonded to either Mn or Fe oxide. Additionally, we analyzed the redox bar coatings at a very small scale ( Pb > Zn, Fe oxide preferentially sorbed oxyanions with As > P > Mo > V, respectively. Field-Fe oxides precipitating along the Mn redox bars sorbed elevated levels of As and P compared with the action of synthesized lab-Fe oxides along Fe redox bars, a finding which we attribute to short-range-ordered Fe phases with elevated sorption capacity. Besides providing information regarding the monitoring of soil redox status, the developed sequential two-step extraction procedure enables the differentiation of the selective sorption of elements in the soil solution to the coating of Mn and Fe redox bars. The collection of Fe oxides formed naturally along the Mn redox bar coatings further enables the investigation of temporally and spatially diverse Fe oxide-forming processes