Abstract
Aging process of exogenous heavy metals in soil is significant for reducing their environmental risk due to the redistribution of species of soil heavy metals. A red soil (ultisol) and a brown soil (alfisol) were selected to investigate the aging process of cadmium (Cd), copper (Cu), and lead (Pb) under different regimes of temperature and water content. Most introduced heavy metals were all transformed from dissolved fraction to more stable fractions within 5 days of incubation. During incubation, most Pb existed in the fraction bound to Fe/Mn oxides, while exchangeable and carbonate-associated fraction was the dominant portion for Cd and Cu, suggesting that the transformation rate followed the order: Pb > Cu > Cd. The exchangeable and carbonate-associated fraction in red soil, which was characterized with higher pH and Fe/Al/Mn oxides and lower organic matter (OM), was significantly higher than that in brown soil, implying that soil OM was the important factor affecting the aging process of soil heavy metals in the present study. In addition, increases of temperature and soil water content can accelerate the transformation of most introduced Cd, Cu, and Pb to more stable forms in the soils. The results indicated that soil properties, environmental factors (i.e., temperature and water content), types of heavy metals, and pollution time can significantly affect the aging process of exogenous heavy metals.
Highlights
Soil pollution by heavy metals has led to great concerns in the past few decades [1,2,3]
Heavy Metal Aging under Different Incubation Temperatures. e time-dependent fraction transformations of Cd, Cu, and Pb under different incubation temperatures were shown in Figures 2–4, respectively. e concentration of F1 fraction was extremely increased when exogenous heavy metals were added in the soils. us, the proportion of F1 fraction was predominant for Cd and Cu during all incubation experiments, and the proportion of F1 fraction of Pb was relatively high as well. e proportions of F1 fraction of Cd and Pb slightly reduced for 5–45 days of incubation for all cases and changed less after 45 days
A rapid reduction of F1 fraction of Cd, Cu, and Pb was not observed, indicating that most spiked heavy metals had transformed into more stable fractions within 5 days of incubation in the tested soils. e result was not consistent with other studies, in which rapid reduction of bioavailable Cd, Cu, and Pb was found within 10–20 days after adding heavy metals [20,21,22]. e concentrations of heavy metals in other aging experiments were 30 mg/kg for Cd, 500 mg/kg for Cu and Pb, and higher than heavy metal concentrations in the present study, which might be the reason for the rapid reduction of bioavailable Cd, Cu, and Pb within 5 days
Summary
Soil pollution by heavy metals has led to great concerns in the past few decades [1,2,3]. In acid mine drainage, concentrations of heavy metals can be up to tens of mg per liter, even hundreds of mg per liter [16], which often leads to severe heavy metal pollution of soils surrounding mines. When these exogenous heavy metals, especially soluble form, were discharged into the soil, they can be fixed via incorporation into crystal lattices, diffusion into micropores, or formation of metal precipitates on the surfaces of soil minerals [17] and present relatively low bioavailability and mobility. When these exogenous heavy metals, especially soluble form, were discharged into the soil, they can be fixed via incorporation into crystal lattices, diffusion into micropores, or formation of metal precipitates on the surfaces of soil minerals [17] and present relatively low bioavailability and mobility. is progress terms the aging of heavy metals and is crucial for reducing the environmental risk of soil heavy metals [18]. e aging process of soil heavy metals is time dependent [19]. e
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