Extraction of aluminium and iron from organic associations in soil - developing a new selective method

Al~(3+)、Fe~(2+)/3+等金属离子在土壤发育过程中被释放，并与土壤有机质发生反应。金属阳离子与SOM之间的相互作用几十年来一直是已知的，导致SOM稳定，即SOM分解的减速。在酸性土壤中，这些相互作用包括Al和Fe离子在SOM官能团上的吸附，以及离子与SOM的共沉淀，导致金属之间的有机缔合。草酸盐以有机结合的形式溶解金属，并以矿物形式溶解金属。最近的研究表明，草酸盐萃取态Al的含量是土壤有机碳含量的预测因子之一。尽管金属在有机缔合中很重要，但目前还没有选择性提取金属的方法。拟议的项目旨在制定一种方案，以有机缔合方式完全和选择性地提取铝和铁，并对与铝有关的有机质进行定量和定性表征。为此目的，将开发一种以氟化物为基础的提取(pH 6.5)，并将其纳入顺序提取计划。该方案将分别从有机组合、结晶度较差的铁氧化物和短程有序铝硅酸盐中提取铝和铁。在将该方法转移到土壤之前，将使用有机缔合模型和参比矿物模型来测试基于氟化物的提取的选择性和完整性。然后，根据该方案提取预期有机结合中Al和Fe含量增加的土壤样品。样品将来自WRB分类的参考土组的土层，其中金属-有机组合已被输送到其中(Podzol，Luvisol)，其中的组合已在现场形成(Andosol，Cambiol)，并在其中因氧化还原过程而形成(Gley-sols，Stagnosols)。根据成壤环境的不同，有机组合中的金属和有机质的含量是不同的，有机组合中的金属对草酸盐可提取金属含量的贡献也是一个总和参数。从铝的缔合中提取的SOM将用光谱方法和热方法进行表征。这些结果预计也会有所不同，这取决于成壤环境，例如，与具有更多芳香SOM的安多索相比，Podzol中低分子质量有机酸的比例更大。该项目不仅是为了开发和应用选择性提取土壤有机结合中的铝和铁，而且还为了表征尚未有选择地获得的土壤有机质组分。因此，除了推进土壤化学分析外，该项目还将提供基本的土壤发生数据，如果将研究更大的样本集，这些数据可能会被应用到土壤有机碳储存的控制模型中。

Metal ions such as Al3+ and Fe2+/3+ are released during soil development, and they may react with soil organic matter (SOM). Interactions between metal cations and SOM are known for decades, inducing SOM stabilization, that is, the deceleration of SOM decomposition. In acidic soil, these interactions comprise adsorption of Al and Fe ions on functional groups of SOM, and co-precipitation of the ions with SOM, resulting in organic associations of the metals. Oxalate dissolves metals in organic association, together with metals in mineral forms. Recent studies have shown that particularly the contents of oxalate-extractable Al are one of the predictors of the contents of soil organic carbon. Despite the importance of metals in organic association, there is no method for their selective extraction available. The proposed project aims to develop a scheme for the complete and selective extraction of Al and Fe in organic association and for the quantitative and qualitative characterization of the SOM associated with Al. A fluoride-based extraction (pH 6.5) will be developed and integrated into a sequential extraction scheme for this purpose. The scheme will separately extract Al and Fe from organic associations, poorly crystalline Fe oxides, and short-range ordered aluminosilicates. Selectivity and completeness of the fluoride-based extraction will be tested with models of organic associations and reference minerals before transferring the method to soils. Then, soil samples, in which enhanced contents of Al and Fe in organic association are expected, will be extracted according to the scheme. The samples will originate from horizons of reference soil groups of the WRB classification, into which metal-organic associations have been transported (Podzols, Luvisols), in which the associations have formed in situ (Andosols, Cambisols), and in which they have formed because of redox processes (Gleysols, Stagnosols). Depending on the pedogenic milieu, variable contents of metals and SOM in organic association are expected, together with variable contributions of metals in organic association to the contents of oxalate-extractable metals as a sum parameter. SOM extracted from associations with Al will be characterized with spectroscopic methods and a thermal one. These results are expected to differ as well, depending on the pedogenic milieu, for instance, larger proportions of low-molecular-mass organic acids in Podzols compared to Andosols with more aromatic SOM. The project is not only for the development and for the application of a selective extraction of Al and Fe in organic association in soil, but also for the characterization of a SOM fraction, which was not selectively accessible yet. Thus, apart from advancing soil-chemical analytics, the project will provide fundamental pedogenetic data that may be implemented into models on the controls of organic-carbon storage in soil, if larger sample sets will be studied.