Abstract:

Arsenic (As) is widely present in soils of geogenic high-background areas, and its elevated concentrations and diverse chemical forms may pose potential ecological and human health risks. This study aims to elucidate the spatial distribution characteristics and enrichment mechanisms of As in a representative geogenic high-background area. A total of 2336 soil samples were systematically collected from depths of 0-12 m in a high-background site located in Guangzhou, China. Stratified statistical analysis was applied to examine the vertical variation trends of As concentrations. Based on regional geological survey data, the study area was classified into three lithological units. From lithologies representing high, medium, and low As concentrations, nine representative samples were selected to conduct sequential extraction experiments for As speciation analysis. Meanwhile, Spearman correlation analysis was performed to explore the relationship between As and Fe, and mineralogical characterization techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) were employed to identify the mineral associations and binding states of As. The results show that As concentrations in soils systematically increase with depth, indicating a pronounced enrichment in deeper horizons. Arsenic primarily occurs in the residual and Fe oxide-bound fractions, while the proportion of labile or bioavailable forms remains low, reflecting a contamination pattern characterized by “high total content-low mobility”. A significant positive correlation was observed between As and Fe concentrations (r=0.733, p<0.001). Mineralogical analyses further confirm that As mainly resides in mineral aggregates dominated by Fe and Al oxides. Overall, this study reveals the vertical distribution patterns and geochemical mechanisms of As in soils from geogenic high-background regions, providing a scientific basis for environmental risk assessment and pollution prevention in naturally As-enriched areas.

Key words: geogenic background, arsenic, arsenic speciation, contamination origin, heavy metals

摘要：

砷（As）作为广泛分布于地质高背景区土壤中的潜在有毒元素，其高含量与特定赋存形态可能引发显著的生态与健康风险。为揭示典型地质高背景区土壤中砷的空间分布规律及其富集机制，以广州某高背景地块为研究对象，系统采集0－12 m深度范围内2336组土壤样品，利用分层统计方法分析砷含量的垂向变化特征，并结合区域地质调查资料将研究区划分为三类岩性单元；从高、中、低砷含量岩性中各选取3组代表性样品，开展连续提取实验以解析砷的赋存形态；进一步运用Spearman相关分析探讨砷与铁（Fe）之间的关系，并借助XRD、XPS和SEM-EDS等表征手段，揭示砷的矿物结合机制与赋存状态。结果表明，研究区土壤砷含量整体随深度增加而升高，深层土壤富集特征明显；砷形态以残渣态和铁氧化物结合态为主，活性形态占比较低，呈现出“总量高－活性低”的污染特征。砷与铁含量之间呈显著正相关（r=0.733，p<0.001）。微观表征结果进一步证实砷主要赋存于以铁、铝氧化物为主的矿物团聚体中。该研究阐明了地质高背景区土壤砷的分布规律与地球化学过程，为地质高背景土壤砷的环境风险评估与污染防控提供科学依据。

关键词: 地质高背景, 砷, 形态分析, 污染成因, 重金属