Excessive inputs of phosphorus (P) to aquatic environments can lead to eutrophication. Adsorption of phosphate on Fe oxides is one of the main process that can limit P availability. Oxidized forms of Mn have also been suggested to play a role in P trapping. However, although a considerable number of studies have shown the relationships between the geochemistry of Fe and that of phosphates, few studies have attempted to show the direct links between Mn and P. In the present study, we studied the adsorption of phosphate on synthetized Mn(III) and Mn(IV) oxides placed under natural conditions. The aim was to compare the role of Fe and Mn oxides in phosphate adsorption. Two muddy sediments were collected in a river bed at the edge of a large agricultural area. A sandy sediment was collected downstream. A muddy and a sandy sample were taken in a coastal environment. The experiments on phosphate adsorption by sediment and Mn oxides were carried out in slurries containing in situ waters spiked with 10 μM phosphate. Control experiments without Mn-oxide addition showed that the natural sediments tested still had the capacity to adsorb phosphate, in particular due to the presence of reactive Fe(III) oxides, extractable by an ascorbate solution. The addition of Mn(III) and Mn(IV) oxides in much larger quantities than the initial quantity of Fe oxides had little impact on the rate of phosphate adsorption. For both Mn(III) and Mn(IV) oxides, the Mn/P ratio between added particulate Mn and adsorbed P was very high, with values between 260 and 1000. The Fe/P ratio of the iron oxides contained in the slurries was between 6 and 20. On average, the P adsorption capacity of the Fe oxides was 50 times greater than that of the Mn oxides. Manganese oxides are generally less abundant than iron oxides in natural environments. Mn oxides therefore play a minor role in P sequestration compared with Fe oxides. Even if there are environments where Mn oxides can concentrate, the reduction of Mn oxides and subsequent liberation of adsorbed P does not represent a major risk for eutrophication of aquatic ecosystems.

中文翻译：

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锰氧化物和羟基氧化物在天然水生环境中磷酸盐封存中的作用


水生环境中磷（P）的过量输入会导致富营养化。磷酸盐在铁氧化物上的吸附是限制磷利用率的主要过程之一。 Mn 的氧化形式也被认为在磷捕获中发挥作用。然而，尽管大量研究表明了铁的地球化学与磷酸盐的地球化学之间的关系，但很少有研究试图表明锰和磷之间的直接联系。在本研究中，我们研究了合成锰上磷酸盐的吸附(III) 和 Mn(IV) 氧化物置于自然条件下。目的是比较 Fe 和 Mn 氧化物在磷酸盐吸附中的作用。在一大片农业区边缘的河床上收集了两块泥质沉积物。下游收集了沙质沉积物。泥泞和沙质样本是在沿海环境中采集的。沉积物和锰氧化物吸附磷酸盐的实验是在含有 10 μM 磷酸盐的原位水中的浆液中进行的。未添加氧化锰的对照实验表明，所测试的天然沉积物仍然具有吸附磷酸盐的能力，特别是由于存在可通过抗坏血酸盐溶液提取的反应性 Fe(III) 氧化物。添加比初始 Fe 氧化物数量大得多的 Mn(III) 和 Mn(IV) 氧化物对磷酸盐吸附速率几乎没有影响。对于 Mn(III) 和 Mn(IV) 氧化物，添加的颗粒状 Mn 和吸附的 P 之间的 Mn/P 比率非常高，值在 260 到 1000 之间。浆料中所含铁氧化物的 Fe/P 比率为6 至 20 之间。平均而言，Fe 氧化物的 P 吸附能力比 Mn 氧化物大 50 倍。 在自然环境中，锰氧化物的含量通常低于铁氧化物。因此，与铁氧化物相比，锰氧化物在磷封存中发挥的作用较小。即使存在锰氧化物可以富集的环境，锰氧化物的减少和随后吸附的磷的释放并不代表水生生态系统富营养化的主要风险。