| Abstract |
Smelting activity produced tons of slags with large quantities of highly toxic metals, resulting in contamination in adjacent soils and sediments as well. This study investigated the fractionation and sources of metals Pb, Zn, and Cd in polluted soils and sediments in a region with once prosperous Zn smelting activities in southwestern China. Soils with varying land uses were of a special concern due to their connection to the food chain. Obtained data would offer a valuable reference to the development of land-use management strategy in this region.In total, 130 soils and 22 stream sediments were sampled in the studied region. After air-dried and passed through a 2 mm sieve, soils and sediments were subjected to a three-step sequential extraction for the fractionation of Pb, Zn, and Cd. Besides, 66 slags were sampled, and acid-digested for the determination of total Pb, Zn, and Cd. Soils/sediments with extremely high Pb, Zn, and Cd concentrations were selected for observation and analysis using a scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Stable lead and sulphur isotope techniques were applied for source tracing of metals in soils and sediments. Data were pooled for analysis of variance together with a post-hoc multiple comparison procedure.High concentrations of Pb (∼46,219 mg kg$^{−1}$ with medians of 846 mg kg$^{−1}$ in soil, 7,415 mg kg$^{−1}$ in sediment, and 8,543 mg kg$^{−1}$ in slag), Zn (∼57, 178 mg kg$^{−1}$ with medians of 1,085 mg kg$^{−1}$ in soil, 15,678 mg kg$^{−1}$ in sediment, and 14,548 mg kg$^{−1}$ in slag), and Cd (∼312 mg kg$^{−1}$ with medians of 29.6 mg kg$^{−1}$ in soil, 47.1 mg kg$^{−1}$ in sediment, and 47.9 mg kg$^{−1}$ in slag) were measured. Soils with no cultivation had greater concentrations of Pb (16,686 mg kg$^{−1}$ in median), Zn (13,587 mg kg$^{−1}$ in median), and Cd (44.1 mg kg$^{−1}$ in median) than those with cultivation. Al-normalized enrichment factors for Pb, Zn, and Cd were in 40.6–1,066, 11.6–382, and 128–647 in sediments, and in 58.0–1,771, 10.1–450, and 88.6–1,035 in soils. Greater Pb/Cd and Zn/Cd ratios in slags (241 and 386 in median) than in soils (uncultivated (187 and 267 in median) > cultivated) or sediments (106 and 260 in median) were observed, indicating extra atmospheric borne Cd in soils and sediments from smelting dust emission. Pb, Zn, and Cd were mainly associated with Fe/Mn oxides/hydroxides in soils and sediments, with their mobile fractions varying with land use in soils that had a close relation to pH (r = −0.569, p < 0.01). In $^{206}$Pb/$^{207}$Pb∼ $^{208}$Pb/$^{206}$Pb diagram, plots of soils and sediments constructed a satisfactory linear model (p < 0.01) with wall rock, coal, and ore samples. Four different domains can be identified in the δ$^{34}$S–$^{206}$Pb/$^{207}$Pb diagram, in which soils and sediments formed a domain with middle δ$^{34}$S (−2.79 ∼ +10.3‰) and $^{206}$Pb/$^{207}$Pb ratios (1.1823 ∼ 1.1853).Smelting activities result in great enrichments of Pb, Zn, and Cd in surveyed soils and sediment. Mobile fraction of these metals was controlled by land use type as well as soil pH, which is informative for land use management to prevent toxic substance from potentially menacing human health. Bean cultivation and low pH can result in more Cd of mobile fraction in soils, therefore, alternative cultivation and pH elevation are recommended for the studied region. This would help reduce the remobilization and uptake of Cd from the polluted soils. |
