Transformation of triclinic birnessite induced by vermicompost-enriched soil organic matter

Birnessite commonly co-occurs with other manganese oxy(hydr)oxides and may serve as a precursor to their formation in natural environments. Here, we assess the structural stability of triclinic birnessite (TcBi) in vermicompost-enriched soil systems under alkaline conditions. Triclinic birnessite was introduced to the soil at Mn:C ratios of 1:2 and 2:1, and reacted for 35 d. Mineral speciation and transformation in the mineral-associated and particulate organic carbon (POC) fractions, collected on days 1, 15, and 35, were assessed at the Mn K-edge using X-ray Absorption spectroscopy (XAS). Based on XAS, in the POC fraction, Mn(III) increased by 39 % by day 35, while in the mineral-associated organic carbon (MAOC) fraction, Mn(III) increased by ∼21 % and 46 % at Mn:C 1:2 and Mn:C 2:1, respectively. Triclinic birnessite was more stable at Mn:C 2:1, with the minimal formation of other mineral phases. At Mn:C 1:2, TcBi underwent increased transformation to hexagonal birnessite, with the formation of Mn(III) phases. By day 35, XAS indicated TcBi decreased by up to ∼31 % in both POC and MAOC fractions, with the emergence of additional Mn phases, including manganite, lithiophorite, and Mn(III) phosphate, particularly in the MAOC fraction. Notably, manganite was observed in both soil fractions, while bixbyite and lithiophorite were only observed in the POC and MAOC fractions, respectively. Using XAS, our findings show that organic C interactions with TcBi under alkaline conditions drive redox cycling of Mn, leading to the recrystallisation of diverse Mn phases, highlighting TcBi's role in Mn mineral transformations in natural environments.