Cancès B, Juillot F, Morin G, Laperche V, Polya D, Vaughan D J, Hazemann J-L, Proux O, Brown G E, Calas G
Institut de Minéralogie et de Physique des Milieux Condensés, UMR CNRS 7590, Université Pierre et Marie Curie, Université Denis Diderot, IPGP, 140 rue de Lourmel, 75015 Paris, France.
Sci Total Environ. 2008 Jul 1;397(1-3):178-89. doi: 10.1016/j.scitotenv.2008.02.023. Epub 2008 Apr 11.
An impacted soil located near an industrial waste site in the Massif Central near Auzon, France, where arsenical pesticides were manufactured, has been studied in order to determine the speciation (chemical forms) of arsenic as a function of soil depth. Bulk As concentrations range from 8780 mg kg(-1) in the topsoil horizon to 150 mg kg(-1) at 60 cm depth. As ores (orpiment As2S3, realgar AsS, arsenopyrite FeAsS) and former Pb- and Al-arsenate pesticides have been identified by XRD at the site and are suspected to be the sources of As contamination for this soil. As speciation was found to vary with depth, based on XRD, SEM-EDS, EPMA measurements and selective chemical extractions. Based on oxalate extraction, As is mainly associated with amorphous Fe oxides through the soil profile, except in the topsoil horizons where As is hosted by another phase. SEM-EDS and EPMA analyses led to the identification of arseniosiderite (Ca2Fe3+3(AsVO4)3O2.3H2O), a secondary mineral that forms upon oxidation of primary As-bearing minerals like arsenopyrite, in these topsoil horizons. These mineralogical and chemical results were confirmed by synchrotron-based X-ray absorption spectroscopy. XANES spectra of soil samples indicate that As occurs exclusively as As(V), and EXAFS results yield direct evidence of changes in As speciation with depth. Linear combination fits of EXAFS spectra of soil samples with those of various model compounds indicate that As occurs mainly As-bearing Fe(III)-(hydr)oxides (65%) and arseniosiderite (35%) in the topsoil horizon (0-5 cm depth). Similar analyses also revealed that there is very little arseniosiderite below 15 cm depth and that As(V) is associated primarily with amorphous Fe oxides below this depth. This vertical change of As speciation likely reflects a series of chemical reactions downward in the soil profile. Arseniosiderite, formed most likely by oxidation of arsenopyrite, is progressively dissolved and replaced by less soluble As-bearing poorly ordered Fe oxides, which are the main hosts for As in well aerated soils.
法国奥宗附近中央地块的一个工业废料场附近有一块受污染土壤，该场地曾生产含砷农药。为确定砷的形态（化学形态）随土壤深度的变化情况，对该土壤进行了研究。土壤中总砷浓度范围从表层土壤层的8780毫克/千克到60厘米深度处的150毫克/千克。通过X射线衍射（XRD）在该场地已鉴定出砷矿石（雌黄As₂S₃、雄黄AsS、毒砂FeAsS）以及以前的铅和铝砷酸盐农药，怀疑它们是该土壤砷污染的来源。基于XRD、扫描电子显微镜-能谱仪（SEM-EDS）、电子探针微区分析（EPMA）测量以及选择性化学提取，发现砷的形态随深度变化。基于草酸盐提取，除了表层土壤层中砷由另一相承载外，在整个土壤剖面中，砷主要通过无定形铁氧化物结合。扫描电子显微镜-能谱仪和电子探针微区分析导致在这些表层土壤层中鉴定出砷铁石（Ca₂Fe³⁺₃(AsVO₄)₃O₂·3H₂O），这是一种次生矿物，由毒砂等原生含砷矿物氧化形成。这些矿物学和化学结果通过基于同步加速器的X射线吸收光谱得到证实。土壤样品的X射线吸收近边结构（XANES）光谱表明，砷仅以五价砷（As(V)）形式存在，扩展X射线吸收精细结构（EXAFS）结果提供了砷形态随深度变化的直接证据。土壤样品的EXAFS光谱与各种模型化合物的光谱进行线性组合拟合表明，在表层土壤层（0 - 5厘米深度）中，砷主要以含铁（III）的（氢）氧化物结合态（65%）和砷铁石（35%）形式存在。类似分析还表明，在15厘米深度以下砷铁石很少，在此深度以下五价砷主要与无定形铁氧化物结合。砷形态的这种垂直变化可能反映了土壤剖面中向下的一系列化学反应。砷铁石很可能由毒砂氧化形成，逐渐溶解并被溶解性较差的含砷无序铁氧化物取代，而这些铁氧化物是通气良好土壤中砷的主要载体。
