Amorphous silica and short ranged ordered aluminosilicates as key player for CO2 binding during mineral weathering?

• Applicants: Dr. Rainer Remus; Professor Dr. Jörg Schaller
• Subject Area: Soil Sciences
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 576181788

The importance of silicate weathering for nutrient mobilization from silicate minerals, the concomitant increase in soil pH, as well as weathering related CO2 uptake has received recent attention in the context of both soil health and climate mitigation strategies. The CO2 binding, part of the weathering process, is related to the Ca or Mg atoms of the silicates being released from binding and increasing the availability of these cations due to silicic acid mobilization. The binding of e.g. Ca or Mg ions to CO2 leads to a long lasting fixation of CO2. However, the mobilized silicic acid may precipitate as amorphous silica (ASi) on the surface of the minerals. It was shown that ASi plays an important role in the context of silicate weathering. However, ASi layers formed at the surfaces of soil minerals through chemical weathering and re-precipitation of ASi may reduce the CO2 consumption by weathering due to a potential reduction of the transport of CO2 to the mineral surface as suggested. The same effect of a reduced CO2 consumption by mineral weathering was for short range ordered aluminosilicate (SROAS) layer forming on the mineral surface if aluminium is present in solution. We did a preliminary weathering experiment using five different minerals (kaolin, montmorillonite, olivine, biotite, and vermiculite) with and without addition of 3% ASi to test for the negative effects of ASi on weathering described in literature (see above). To be able to precisely measure the CO2 binding to the different minerals during weathering we used an atmosphere with 14C-labeled CO2. After the 3 month of incubation, we found a significant (ANOVA) effect of ASi increasing 14C-CO2 binding during mineral weathering (kaolin, montmorillonite, olivine, biotite, and vermiculite) which is in contrast to existing literature (see above). TukeyHSD revealed for the montmorillonite and biotite samples a significant increase in 14C-CO2 binding for the samples incubated with 3% ASi as compared to the control. While for kaolin, olivine and vermiculite no significant effect of ASi on 14C-CO2 binding was found. For olivine the strong positive effect was pronounced but the large standard deviation prevented significance. These preliminary results suggest that effects of ASi addition on CO2 binding during weathering processes may depend on the chemical composition of the minerals and if an ASi or a SRAOS layer is forming. In the proposed project we want to quantify the effect of ASi (and soil born ASi) as well as SRAOS on mineral weathering of pure minerals and for different samples soil collected along a chronosequence of soils from Hawaii. The latter were chosen since those soils have the same parent material but exhibit strong differences in amorphous silica and other non-crystalline phases (SROAS) due to the different time of weathering. In the proposed project we want to disentangle the importance of ASi and SROAS for CO2 binding during mineral weathering.

• DFG Programme: Research Grants