Arsenic in Soils Affected by Mining: Microscopic Studies vs. Sequential Chemical Extraction

Abstract

Soil samples from three inactive mines, corresponding to different Arsenic-bearing mineralization types, were collected and studied. The aim was to determine the influence of mine wastes mineralogy/geochemistry and texture in As mobility and to compare results from sequential chemical extraction and microscopic techniques (optical and electron) at a grain scale. Arsenic in soils is found mainly associated to the residual fraction, indicating that mechanical As dispersion is mainly responsible for As soil pollution. The use of objective microscopic techniques (i.e., Scanning Electron Microscopy-Energy Dispersive Spectroscopy -SEM-EDS-, High Resolution Transmission Electron Microscopy -HR-TEM) has pointed out that the selected sequential extraction method overestimates the role of Mn amorphous oxy-hydroxides and organic matter in As retention while underestimating the mechanism of As adsorption onto clay particle surfaces.

Surface Microanalysis and Sequential Chemical Extraction as Tools for Reliable Environmental Mobility Assessment of Sb and Other Metals

“…The use of ammonium acetate seemed to be suitable for carbonate dissolution and for cation exchange capacity (CEC) determination in clays but did not seem to achieve complete solubilization of the As bound to clay surfaces. Some previous works have already indicated the underestimation of sequential extraction procedures in the role played by clays in As retention in soils [ 38 ].…”

“…All of the above influence the quantification of the bioavailable fraction and, thus, the results of the risk assessment methods. The authors’ experience with soils polluted by historical mining activities [ 38 ] also points to a misinterpretation of the modes of occurrence of various metals according to the different steps in SEPs. In the specialized literature, some recent research addresses this problem from the perspective of As [ 39 , 40 ], Pb [ 41 ], Cr-Ni [ 42 ] and, Zn-Cd [ 43 ].…”

“…The goal of this work, by means of a case study, was to carry out a critical comparison between chemical (sequential extraction and acid digestion) and microscopic methods to evaluate the mobility of Sb and other pollutants from mine wastes. In the specialized literature there are similar studies on the behavior of As [ 38 , 44 ], but none related to Sb, which in this case is the pollutant of greatest interest.…”

Features of Arsenic Distribution in the Soils of Potash Mines

“…In different soil horizons up to the depth of 45 cm within the area affected by tailing dumps of the mining plant, arsenic concentrations are 317.27-377.86 mg/kg [1], while the established approximate allowable concentrations are 2-10 mg/kg, depending on the grain size composition and acidity of soils. The arsenic content in tin ore processing dust ranges from 16.04 to 28.3 mg/kg [14]. In soils near an arsenic-containing ore (As 4 S 4 ) mining facility that closed in 2011 (Hunan Province, China), arsenic is the main pollutant, with average concentrations of 394 mg/kg, exceeding background values by 23 times.…”

“…According to [16], the majority of the As found in soils at abandoned mine sites (Rita Mine, Tres Amigos Mine, Las Viescas Mine (northern part of Castilla-León, Spain))is in the so-called "residual fraction", i.e., in grains of specific As minerals that come from wastes and are later integrated within the soil mineral fraction. Mechanical dispersion is thus quantitatively greater than chemical dispersion.…”