U,S, Geological Survey, 345 Middlefield Rd,, MS 901 Menlo Park, CA, 94025, USA.
Geochem Trans. 2011 Jan 24;12(1):1. doi: 10.1186/1467-4866-12-1.
A realistic estimation of the health risk of human exposure to solid-phase arsenic (As) derived from historic mining operations is a major challenge to redevelopment of California's famed "Mother Lode" region. Arsenic, a known carcinogen, occurs in multiple solid forms that vary in bioaccessibility. X-ray absorption fine-structure spectroscopy (XAFS) was used to identify and quantify the forms of As in mine wastes and biogenic solids at the Lava Cap Mine Superfund (LCMS) site, a historic "Mother Lode" gold mine. Principal component analysis (PCA) was used to assess variance within water chemistry, solids chemistry, and XAFS spectral datasets. Linear combination, least-squares fits constrained in part by PCA results were then used to quantify arsenic speciation in XAFS spectra of tailings and biogenic solids.
The highest dissolved arsenic concentrations were found in Lost Lake porewater and in a groundwater-fed pond in the tailings deposition area. Iron, dissolved oxygen, alkalinity, specific conductivity, and As were the major variables in the water chemistry PCA. Arsenic was, on average, 14 times more concentrated in biologically-produced iron (hydr)oxide than in mine tailings. Phosphorous, manganese, calcium, aluminum, and As were the major variables in the solids chemistry PCA. Linear combination fits to XAFS spectra indicate that arsenopyrite (FeAsS), the dominant form of As in ore material, remains abundant (average: 65%) in minimally-weathered ore samples and water-saturated tailings at the bottom of Lost Lake. However, tailings that underwent drying and wetting cycles contain an average of only 30% arsenopyrite. The predominant products of arsenopyrite weathering were identified by XAFS to be As-bearing Fe (hydr)oxide and arseniosiderite (Ca2Fe(AsO4)3O3•3H2O). Existence of the former species is not in question, but the presence of the latter species was not confirmed by additional measurements, so its identification is less certain. The linear combination, least-squares fits totals of several samples deviate by more than ± 20% from 100%, suggesting that additional phases may be present that were not identified or evaluated in this study.
Sub- to anoxic conditions minimize dissolution of arsenopyrite at the LCMS site, but may accelerate the dissolution of As-bearing secondary iron phases such as Fe3+-oxyhydroxides and arseniosiderite, if sufficient organic matter is present to spur anaerobic microbial activity. Oxidizing, dry conditions favor the stabilization of secondary phases, while promoting oxidative breakdown of the primary sulfides. The stability of both primary and secondary As phases is likely to be at a minimum under cyclic wet-dry conditions. Biogenic iron (hydr)oxide flocs can sequester significant amounts of arsenic; this property may be useful for treatment of perpetual sources of As such as mine adit water, but the fate of As associated with natural accumulations of floc material needs to be assessed.
对人类接触历史采矿作业产生的固相砷（As）的健康风险进行现实评估，是加利福尼亚著名的“母矿脉”地区重新开发的主要挑战。砷是一种已知的致癌物质，以多种生物利用度不同的固相形式存在。X 射线吸收精细结构光谱（XAFS）用于确定和量化拉瓦卡普矿（LCMS）遗址、历史悠久的“母矿脉”金矿中的矿渣和生物固体中的砷形态。主成分分析（PCA）用于评估水化学、固体化学和 XAFS 光谱数据集内的差异。然后，使用线性组合、最小二乘拟合，部分受 PCA 结果的约束，以量化尾矿和生物固体的 XAFS 光谱中的砷形态。
在洛斯湖孔隙水和尾矿沉积区的一个地下水补给池塘中，发现了最高的溶解砷浓度。铁、溶解氧、碱度、比电导率和砷是水化学 PCA 的主要变量。与矿渣相比，生物产生的铁（氢）氧化物中砷的浓度平均高 14 倍。磷、锰、钙、铝和砷是固体化学 PCA 的主要变量。XAFS 光谱的线性组合拟合表明，黄铁矿（FeAsS），矿石中砷的主要形式，在最小风化的矿石样品和洛斯湖底部的水饱和尾矿中仍然大量存在（平均：65%）。然而，经历了干燥和润湿循环的尾矿仅含有平均 30%的黄铁矿。通过 XAFS 确定了黄铁矿风化的主要产物是含砷的铁（氢）氧化物和砷铁硅石（Ca2Fe（AsO4）3O3•3H2O）。前一种物质的存在是毋庸置疑的，但后一种物质的存在并未通过其他测量方法得到证实，因此其鉴定不太确定。几个样品的线性组合、最小二乘拟合总量与 100%相差超过±20%，表明可能存在未在此研究中识别或评估的其他相。
在 LCMS 遗址，亚缺氧条件会使黄铁矿的溶解最小化，但如果存在足够的有机物来刺激厌氧微生物活动，可能会加速含砷的次生铁相（如 Fe3+-氢氧化物和砷铁硅石）的溶解。氧化、干燥条件有利于次生相的稳定，同时促进原生硫化物的氧化分解。在干湿循环条件下，原生和次生砷相的稳定性可能最小。生物产生的铁（氢）氧化物絮体可以固定大量的砷；这一特性可能对处理矿脉水等永久性砷源有用，但需要评估与天然絮体物质积累相关的砷的归宿。
