ASSESSING THE ROLE OF WEATHERING IN CO-CYCLING OF VANADIUM, IRON, AND MANGANESE WITHIN OREGON COAST RANGE SOILS

Weathering of parent rock materials is essential to the cycling of major and trace elements as well as deleterious redox-sensitive metals in the critical zone. For example, vanadium (V) is a possible carcinogen, depending on its speciation, and weathering of parent rock has been posited as an important driver of V in soil systems. Soil mineral weathering can also concomitantly enhance the formation and transformation of secondary mineral phases such as Fe-oxides and Mn-oxides, which are unevenly distributed in soil profiles and are important drivers of V speciation and mobility. The heterogeneity in chemistry and distribution of secondary oxides creates challenges in quantifying spatial profiles of redox-sensitive elements such as Fe, Mn, and V in weathering systems.

The objective of this study is to improve our mechanistic understanding of co-cycling of redox sensitive metals in soils. To achieve this objective, a suite of field, wet-chemistry, spectroscopic and data-analysis techniques are being used. Soil, saprolite and bedrock samples have been collected along four soil profiles in the coast range of Oregon. X-ray absorption spectroscopy (XAS) has been conducted to quantify the speciation of Fe, Mn, and V. To obtain total elemental concentrations, elemental composition analysis has been carried out through acid digestions. Using a combination of data from acid digestions and XAS analysis, concentration profiles of Fe, Mn, and V will be quantified as a function of both parent material and secondary mineral oxides across depth.

Preliminary results from acid digestions have revealed a strong correlation (r = 0.76) between V and Fe. However, a very weak correlation (r = 0.07) was obtained between V and Mn across all sites. This suggests Mn may not directly influence V sorption in soil but may however be responsible for its oxidation. The good correlation between Fe and V indicates Fe is directly involved in V retention in these soils. This finding agrees with the mechanistic understanding of V mobility. Ongoing work is seeking to quantify proportions and speciation of Fe, Mn, and V from parent material and secondary oxides through different depth profiles.

Overall, this study will help improve our understanding of pedogenic-driven transformation and fluxes of V, Fe and Mn within active weathering soil systems. These results can be used for developing more robust predictive models of controls on availability of V and its deleterious species in active weathering systems.