Assessing the surface reactivity of metal (hydr)oxides in soils is essential for quantifying ion adsorption phenomena that control the availability of nutrients and pollutants. Despite the high natural abundance of Fe and Al (hydr)oxides in intensively weathered environments, the surface reactivity of these pedogenic materials has not been consistently characterized for weathered tropical soils. Here, we used a novel probe-ion methodology combined with state-of-the-art surface complexation modelling (SCM) to derive the reactive surface area (RSA) of the soils, as well as the amount of phosphate (PO₄) that can be potentially desorbed (R-PO₄) from the natural fraction of metal (hydr)oxides and thereby controlling the solid-solution partitioning of PO₄. We applied this methodology to a series of weathered topsoils from the sub-Saharan region. The results showed that nanocrystalline ferrihydrite (Fh) is a better proxy than well-crystallized goethite for describing with SCM the reactivity of the natural metal (hydr)oxides, even though well-crystallized materials dominate the mass fraction of metal (hydr)oxides of these weathered tropical soils. Using Fh as a proxy in the SCM, the RSA ranged from ∼2 to 40 m² g⁻¹ soil. Nanoparticles with a mean diameter of ∼1.5–5.0 nm dominate the reactive fraction of metal (hydr)oxides in these tropical topsoils. Our SCM in conjunction with soil extractions indicates that only a fraction of the total PO₄ associated with the metal (hydr)oxides is reversibly adsorbed, whereas the majority of the total PO₄ pool, on average ∼64%, is occluded in the crystalline Fe and Al (hydr)oxide fraction. Only this smaller reversibly adsorbed fraction is thus available for participating in sorption reactions that determine the solid-solution partitioning of PO₄. Overall, this research provides new insights into the reactivity of the metal (hydr)oxide fraction in weathered tropical soils and highlights the relevance of these pedogenic materials in determining the speciation and availability of PO₄.

评估土壤中金属（氢）氧化物的表面反应性对于量化控制养分和污染物可用性的离子吸附现象至关重要。尽管在强烈风化的环境中铁和铝（氢）氧化物的天然丰度很高，但这些成土材料的表面反应性并未在风化热带土壤中得到一致表征。在这里，我们使用了一种新的探针离子方法结合最先进的表面络合模型 (SCM) 来推导出土壤的反应表面积 (RSA) 以及磷酸盐 (PO ₄ )的含量。可以潜在地从金属（氢）氧化物的天然部分解吸（R-PO ₄），从而控制 PO ₄的固溶体分配. 我们将此方法应用于一系列来自撒哈拉以南地区的风化表土。结果表明，纳米晶水铁矿 (Fh) 比结晶良好的针铁矿更能用 SCM 描述天然金属（氢）氧化物的反应性，尽管结晶良好的材料支配着金属（氢）氧化物的质量分数。这些风化的热带土壤。使用 Fh 作为 SCM 中的代理，RSA 的范围从 ∼2 到 40 m ² g ^-1土壤。平均直径约为 1.5-5.0 nm 的纳米颗粒在这些热带表土中金属（氢）氧化物的反应部分占主导地位。我们的 SCM 与土壤提取相结合表明，PO ₄总量中只有一小部分与金属（氢）氧化物相关的铁被可逆地吸附，而总 PO ₄池的大部分（平均约 64%）被晶体 Fe 和 Al（氢）氧化物部分吸留。因此，只有这个较小的可逆吸附部分可用于参与决定 PO ₄固溶体分配的吸附反应。总体而言，这项研究为风化热带土壤中金属（氢）氧化物部分的反应性提供了新的见解，并突出了这些土壤材料在确定 PO ₄的形态和可用性方面的相关性。