Data_Sheet_2_Enhanced weathering potentials—the role of in situ CO2 and grain size distribution.PDF

The application of rock powder on agricultural land to ameliorate soils and remove carbon dioxide (CO2) from the air by chemical weathering is still subject to many uncertainties. To elucidate the effects of grain size distribution and soil partial pressure of carbon dioxide (pCO2) levels on CO2 uptake rates, two simple column experiments were designed and filled nearly daily with an amount of water that simulates humid tropical conditions, which prevail in areas known for being hotspots of weathering. Multiple materials (dunite, basanite, agricultural oxisol, a combination of the latter two, and loess) were compared under ambient and 100% CO2 atmosphere. In a second series, single material columns (dunite) were filled with three different grain size distributions. Total alkalinity, pH, major ions, and dissolved silica were determined in the outflow water of the columns for about 300 days. Under ambient atmospheric conditions, the CO2 consumption was the lowest in the oxisol column, with 100 t CO2 km−2 year−1, while dunite and basanite showed similar consumption rates (around 220 t CO2 km−2 year−1). The values are comparable to high literature values for ultramafic lithologies. Interestingly, the mixture of basanite and oxisol has a much higher consumption rate (around 430 t CO2 km−2 year−1) than the basanite alone. The weathering fluxes under saturated CO2 conditions are about four times higher in all columns, except the dunite column, where fluxes are increased by a factor of more than eleven. Grain size distribution differences also play a role, with the highest grain surface area normalized weathering rates observed in the columns with coarser grains, which at first seems counterintuitive. Our findings point to some important issues to be considered in future experiments and a potential rollout of EW as a carbon dioxide removal method. Only in theory do small grain sizes of the spread-material yield higher CO2 drawdown potentials than coarser material. The hydrologic conditions, which determine the residence times in the pore space, i.e., the time available for weathering reactions, can be more important than small grain size. Saturated-CO2 column results provide an upper limit for weathering rates under elevated CO2.

将岩石粉末（rock powder）施用于农田以改良土壤，并通过化学风化（chemical weathering）作用从大气中去除二氧化碳（carbon dioxide, CO₂）的技术，仍存在诸多不确定性。为阐明粒度分布（grain size distribution）与土壤二氧化碳分压（soil partial pressure of carbon dioxide, pCO₂）对CO₂吸收速率的影响，本研究设计了两组简易柱实验（column experiments）：实验组每日补充模拟湿热热带条件（humid tropical conditions）的水量，该气候常见于全球风化热点区（weathering hotspots）。第一组实验对比了纯橄榄岩（dunite）、碧玄岩（basanite）、农业用氧化土（agricultural oxisol）、二者混合体系以及黄土（loess）五种材料在大气环境（ambient atmosphere）与100%CO₂气氛下的反应；第二组实验仅采用纯橄榄岩作为填充材料，设置了三种不同的粒度分布梯度。在约300天的实验周期内，研究人员定期测定柱体渗出液（outflow water）的总碱度（total alkalinity）、pH值、主要离子（major ions）浓度与溶解态硅（dissolved silica）含量。在大气环境条件下，氧化土柱的CO₂消耗量最低，仅为100 t CO₂ km⁻² year⁻¹；纯橄榄岩与碧玄岩的消耗速率相近，约为220 t CO₂ km⁻² year⁻¹，该数值与文献报道的超基性岩岩性（ultramafic lithologies）风化通量上限相当。值得注意的是，碧玄岩与氧化土的混合体系消耗速率（约430 t CO₂ km⁻² year⁻¹）远高于单一碧玄岩体系。在100%CO₂饱和气氛下，除纯橄榄岩柱的风化通量（weathering fluxes）提升超11倍外，其余所有柱体的风化通量均提升约4倍。粒度分布差异同样对结果存在显著影响：粗颗粒柱体的比表面积归一化风化速率（grain surface area normalized weathering rates）最高，这一现象乍看之下与直觉相悖。本研究结果揭示了未来实验需重点关注的若干关键问题，以及强化风化（enhanced weathering, EW）作为CO₂移除技术的潜在推广前景。仅在理论层面，粒径更小的施用材料才具备比粗颗粒更高的CO₂固碳潜力（drawdown potentials）；实际水文条件——其决定了反应物在孔隙空间（pore space）中的停留时间（residence times），即风化反应的可用反应时长——或许比细微粒度差异更为关键。饱和CO₂柱实验结果可为高CO₂分压（elevated CO₂）下的风化速率提供上限参考值。