Evolution of the Speciation and Mobility of Pb, Zn and Cd in Relation to Transport Processes in a Mining Environment

Abstract

<jats:p>Elements in mining extracts can be potentially toxic if they are incorporated into soils, sediments or biota. Numerous approaches have been used to assess this problem, and these include sequential extractions and selective extractions. These two methods have limitations and advantages, and their combined use usually provides a rough estimate of the availability or (bio)availability of potentially toxic elements and, therefore, of their real potential as toxicants in food chains. These indirect speciation data are interesting in absolute terms, but in the work described here, this aspect was developed further by assessing the evolution of availability-related speciation in relation to the transport processes from the emission source, which are mainly fluvial- and wind-driven. This objective was achieved by characterizing tailings samples as the source of elements in soils and sediments at increasing distances to investigate the evolution of certain elements. The standard procedures employed included a sequential five-step extraction and a selective extraction with ammonium acetate. The results show that the highest percentages of Zn and Pb in tailings, soils and sediment samples are associated with oxyhydroxides, along with a significant presence of resistant mineralogical forms. In the case of Cd, its association with organic matter is the second-most important trapping mechanism in the area. The physicochemical mechanisms of transport did not transform the main mineralogical associations (oxyhydroxides and resistant mineralogical forms) along the transects, but they produced a chaotic evolution pattern for the other minor matrix associations for Zn and a decrease in exchangeable and carbonate-bound forms for Pb in soils. Interestingly, in sediments, these mobile forms showed a decrease in Zn and a chaotic evolution for Pb. The most probable reason for these observations is that Zn2+ can form smithsonite (ZnCO3) or hydrozincite (Zn5(CO3)2(OH)6), which explains the retention of a carbonate-bound form for Zn in the soil transect. In contrast, Pb and Cd can appear as different mineral phases. The order of (bio)availability was Pb &gt; Zn &gt; Cd in tailings but Cd &gt; Pb &gt; Zn in soils. The physicochemical processes involved in transport from tailings to soils produce an increase in Cd (bio)availability. The trend is a decrease in bioavailability on moving away from the source (tailings), with maximum values obtained for Cd near to the source area (200–400 m).</jats:p>

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

“…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 ].…”

Progress, Challenges, and Perspectives of Bioleaching for Recovering Heavy Metals from Mine Tailings

“…Cadmium in tailing soil is mainly bound to organic matter and appears in different mineral phases. The bioavailability sequence of lead and cadmium in soil is cadmium > lead > zinc , with the highest content of cadmium in the soil at around 200–400 m of tailings [105]. Cadmium inhibits the decomposition of leaf litter by affecting the activity of earthworms and leads to the degradation of soil fertility [106].…”

Fluvial Morphology as a Driver of Lead and Zinc Geochemical Dispersion at a Catchment Scale

“…This paper contributes to this knowledge gap by combining the study of fluvial morphological and geochemical processes and the categorisation of fluvial zones across a river with defined mineralogical paragenesis and metal release capability. In particular, it will focus on the geochemical behaviour of the mining-related metals Zn and Pb, whose (1) toxicities have been investigated extensively [19][20][21][22]; (2) impacts on mining catchments worldwide are well-known [23][24][25][26][27]; and (3) mobility poses management challenges in circum-neutral rivers [14,[28][29][30][31]. The specific objectives are to (i) map the spatial fluvial morphological variability along the study catchment; (ii) characterise the geochemistry of the metal-enriched sediment; and (iii) demonstrate the fluvial morphological controls on the Zn-and Pb-bearing mineral distribution along the catchment.…”