Effect of weathering on abundance and release of potentially toxic elements in soils developed on Lower Cambrian black shales, P. R. China

Abstract

This paper examines the geochemical features of 8 soil profiles developed on metalliferous black shales distributed in the central parts of the South China black shale horizon. The concentrations of 21 trace elements and 8 major elements were determined using ICP-MS and XRF, respectively, and weathering intensity (W) was calculated according to a new technique recently proposed in the literature. The data showed that the black shale soils inherited a heterogeneous geochemical character from their parent materials. A partial least square regression model and EF(bedrock) (enrichment factor normalized to underlying bedrock) indicated that W was not a major control in the redistribution of trace metals. Barium, Sn, Cu, V, and U tended to be leached in the upper soil horizons and trapped by Al and Fe oxides, whereas Sb, Cd, and Mo with negative EF values across the whole profiles may have been leached out during the first stage of pedogenesis (mainly weathering of black shale). Compared with the Chinese average soils, the soils were strongly enriched in the potentially toxic metals Mo, Cd, Sb, Sn, U, V, Cu, and Ba, among which the 5 first listed were enriched to the highest degrees. Elevated concentrations of these toxic metals can have a long-term negative effect on human health, in particular, the soils in mining areas dominated by strongly acidic conditions. As a whole, the black shale soils have much in common with acid sulfate soils. Therefore, black shale soils together with acid sulfate soils deserve more attention in the context of metal exposure and human health.

Geochemistry of major and trace elements and Pb–Sr isotopes of a weathering profile developed on the Lower Cambrian black shales in central Hunan, China

“…The above elemental mobility during black shale weathering is similar to those concluded by many previous studies (e.g., Nesbitt and Young, 1982;Peng et al, 2004Peng et al, , 2009bFalk et al, 2006;Lavergren et al, 2009aLavergren et al, , 2009bYu et al, 2012;Xu et al, 2013). However, the most important we find in this study is that the major elements SiO 2 , Al 2 O 3 , MgO, FeO, CaO, K 2 O, Na 2 O, LOI, and trace elements Ba, Cs, Rb, Sr, U, Th, Sn, Sc, Ge and REE (Y), have the highest amount of leaching (with the lowest s values) occurring for the soil samples (D1-D4) of the DS horizon (Fig.…”

“…In contrast, the later stage of chemical homogenisation was accompanied by the further transformation of the weathered materials, which was very important for soil formation. The soils derived from black shales (Peng et al, 2005(Peng et al, , 2009bYu et al, 2009Yu et al, , 2012Tang et al, 2009;Xu et al, 2013) distributed in South China (Fig. 1a), in which the present climate is subtropical and controlled by the East Asian monsoon, might result from the above two geochemical processes.…”

“…The weathering of black shale has therefore received substantial research attention, especially the chemical changes during weathering (e.g., Littke et al, 1991;Jaffe et al, 2002;Xu et al, 2013). Such chemical changes are generally explained by mineral dissolution, secondary mineral formation, cation exchange and loss and gain of amorphous oxide phases ; and references therein), during which most metals, including Cd, Co, Mn, Mo, Ni, U, Zn, and PGE, are lost from shales (Peucker-Ehrenbrink and Hannigan, 2000;Jaffe et al, 2002;Peng et al, 2004;) and others are enriched in soils derived from the shales (e.g., Chon et al, 1996;Lee et al, 1998a;Pašava et al, 2003;Poň avič et al, 2006;Peng et al, 2009b;Tang et al, 2009;Yu et al, 2012;Xu et al, 2013). The enrichment of metals in the soils may represent the geopollution of heavy metals.…”

Geochemistry of trace and rare earth elements during weathering of black shale profiles in Northeast Chongqing, Southwestern China: Their mobilization, redistribution, and fractionation

“…The behavior of trace elements and REEs during chemical weathering has been investigated previously in several protoliths (Nesbitt, 1979;Middelburg et al, 1988;Braun et al, 1993Braun et al, , 2005van der Weijden and van der Weijden, 1995;Nesbitt and Markovics, 1997;Aiuppa et al, 2000;Panahi et al, 2000;Ji et al, 2004;Brantley et al, 2007;Ma et al, 2007;Ndjigui et al, 2008;Beyala et al, 2009;Feng, 2010Feng, , 2011Gong et al, 2011;Sanematsu et al, 2013;Nguetnkam et al, 2014), including weathering profiles from black shales (Peng et al, 2004;Tuttle et al, 2009;Ma et al, 2011a,b;Jin and Brantley, 2011;Yu et al, 2012). Trace elements and REEs in weathered material can be easily mobilized out of the regolith zone and retained in secondary minerals or colloids in weathering profiles as weathering proceeds (e.g., Sholkovitz, 1992Sholkovitz, , 1995Faimon, 2003;Patino et al, 2003).…”

“…As a result of its sulfide and organic matter content, black shale is especially susceptible to chemical weathering processes, and thus serves to accelerate the decomposition of primary minerals and mobilize trace elements and REEs (Jin et al, 2010;Peng et al, 2007Peng et al, , 2014. Several field and laboratory studies have evaluated the mobility of trace elements in black shale during exposure to acidic groundwater (Lee et al, 1998;Tuttle et al, 2009;Yu et al, 2012) and reported on REE mobility and fractionation (Ma et al, 2011a;Peng et al, 2014). Although these investigations have contributed to the understanding of trace and REEs mobility, the geochemical behavior and factors controlling the mobility of these elements during chemical weathering processes require further investigation.…”