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Lead (Pb) pollution in soil has attracted public attention because of its serious impacts on human health through food chain. However, the chemical and biological behaviors of exogenous Pb in soil remain unknown, specifically in the passivated soil with phosphate (P)-based materials. By spiking soil with stable enriched isotopes of 206Pb (> 99%), the exogenous Pb2 + in soil was simulated in the present study; the distribution, transformation, and bioavailability of exogenous Pb in soil passivated with the P-based compound were quantitatively investigated using an isotopic labeling method coupled with a Tessier approach of sequential extraction procedures. The results showed that exogenous Pb in water-soluble fraction (F1) accounted for 0.58% of the total spiked isotopes, which was slightly higher than that of the intrinsic Pb (0.34%) in soil. Nevertheless, the exogenous Pb percentages in exchangeable fraction (F2), carbonate bound fraction (F3), Fe/Mn oxide fraction (F4), and organic matter bound fraction (F5) were about twice more than those of the intrinsic Pb in the four fractions. But exogenous Pb in residual fraction (F6) was only 45.6%, while intrinsic Pb was 70%. After the soil was passivated with P-based compound consisting of diammonium phosphate, bone meal, and hydroxyapatite, more than 90% of exogenous Pb in F2 could be immobilized, but small Pb percentage in F1 was immobilized. Additionally, Pb species in F2, F3, and F4 might be converted into different forms in F5 and F6, but the immobilized Pb in F5 and F6 was labile because more than 50% of the exogenous Pb in the two fractions was lost after planting. The results demonstrated that the isotopic labeling technique coupled with Tessier approach was a novel method that can evaluate the distribution, transformation, and bioavailability of exogenous Pb in soil. These results are helpful for further immobilization of exogenous Pb in agricultural soil.

The distribution and bioavailability of Hg in vegetable-growing soils collected from the estuary areas of Jiulong River, China, were studied. Concentrations of Hg in top-soils, soil profiles and plant samples were measured with the method of hydride generation atomic fluorescence spectrometry after microwave digestion. Mercury species in soils were determined with the sequential extraction procedures based on Kingston method. Results showed that Hg concentrations in top-soils ranged from 49.8 to 1,685 ng g−1, with an average of 510 ng g−1 which was more than twice higher than the mercury limit (250 ng g−1 at pH < 6.5) of soil quality set for edible agricultural products in China (HJ 332-2006). High levels of Hg were found to mainly distribute in the top-soils from the northern, western and southern part of the estuary areas. Hg concentrations decreased with the increases of profile depths, except for one sample (S15) in which Hg level in the depth of 0–20 cm was found lower than that in the 20–40 cm. Hg in most of soil samples in non-mobile forms accounted for 46–82 % of total Hg in soils, while Hg in the mobile forms only 0.6–8.7 %. No significant correlation of Hg concentrations between the vegetables and the soils was observed in the studied areas, which indicates that the transfer factors could only reflect the abilities of Hg uptake and accumulation in a specific plant, but they are unsuitable to be used as the general indexes for the mobility and bioavailability of Hg in soils.

The species distribution of exogenous Hg (II) simulated to contaminate vegetable-growing soil has been investigated by a modified Tessier scheme of sequential extraction procedures (SEPs) detected with a quantitative analytical method and an isotopic labeling method, respectively. The results showed that the summed value of Hg concentrations in all eight species extracted with the SEPs protocol was lower than the total Hg concentration directly measured in soil. The exogenous Hg (II) was found to rarely distribute in the residual species (the maximum fraction in soil with more than 70% of intrinsic Hg), which indicated that Hg species in residual fraction might be inert for plants. However, the exogenous Hg (II) was found to largely distribute in humus substances, in which humic acid could effectively immobilize and weaken the bioactivity of Hg in soil, while fulvic acid bound Hg might be readily assimilated by plants. The exogenous Hg (II) was also found to readily distribute in water-soluble fractions and exchangeable fractions of soil which would result in serious impacts on plants. Little exogenous Hg (II) was observed to distribute in the fractions of carbonate, Fe/Mn oxides and organically bound species, which meant that the toxicity and bioavailability of these Hg species might be slight for plants. The results suggest that the isotopic labeling technique coupled with the modified Tessier scheme provides a novel method which can distinctly indicate the distribution, mobility and potential bioavailability of exogenous Hg (II) in contaminated soil.

The contamination of Pb in agricultural soils is one of the most important ecological problems, which potentially results in serious health risk on human health through food chain. Hence, the fate of exogenous Pb contaminated in agricultural soils is needed to be deeply explored. By spiking soils with the stable enriched isotopes of (206)Pb, the contamination of exogenous Pb(2+) ions in three agricultural soils sampled from the estuary areas of Jiulong River, China was simulated in the present study, and the distribution, mobility and bioavailability of exogenous Pb in the soils were investigated using the isotopic labeling method coupled with a four-stage BCR (European Community Bureau of Reference) sequential extraction procedure. Results showed that about 60-85% of exogenous Pb was found to distribute in reducible fractions, while the exogenous Pb in acid-extractable fractions was less than 1.0%. After planting, the amounts of exogenous Pb presenting in acid-extractable, reducible and oxidizable fractions in rhizospheric soils decreased by 60-66%, in which partial exogenous Pb was assimilated by plants while most of the metal might transfer downward due to daily watering and applying fertilizer. The results show that the isotopic labeling technique coupled with sequential extraction procedures enables us to explore the distribution, mobility and bioavailability of exogenous Pb contaminated in soils, which may be useful for the further soil remediation.

Pollution of Pb in the surface of agricultural soils is of increasing concern due to its serious impact on the plant growth and the human health through the food chain. However, the mobility, activity and bioavailability of Pb rely mainly on its various chemical species in soils. In the present study, E and L values, the labile pools of isotopically exchangeable Pb, were estimated using the method of isotope dilution in three vegetable-growing soils. The experiments involved adding a stable enriched isotope ((206)Pb96%) to a soil suspension and to soils in which plants are subsequently grown, the labile pools of Pb were then estimated by measuring the isotopic composition of Pb in soil solutions and in the plant tissues, respectively. In addition, the correlation of E values and soil pH was investigated at the ranges of pH 4.5-7.0. The amount of labile Pb in soils was also estimated using different single chemical extractants and a modified BCR approach. The results showed that after spiking the enriched isotopes of (206)Pb (96%) for 24 hours an equilibration of isotopic exchanges in soil suspensions was achieved, and the isotope ratios of (208)Pb/(206)Pb measured at that time was used for calculating the E(24 h) values. The labile pools of Pb by %E(24 h) values, ranging from 53.2% to 61.7% with an average 57%, were found to be significantly higher (p0.05) than the values estimated with L values, single chemical extractants and the Σ(BCR) values obtained with the BCR approach, respectively. A strong negative correlation (R(2) = 0.984) between E(24 h) values and soil pH was found in the tested soil sample. The results indicate that the %E(24 h) value can more rapidly and easily predict the labile pools of Pb in soils compared with L values, but it might be readily overestimated because of the artificial soil acidity derived from the spiked isotopic tracer and the excess of spiked enriched isotopes. The results also suggest that the amounts of Pb extracted with EDTA and the Σ(BCR) values extracted with the modified BCR approach are helpful to detect the labile pools of Pb in soils. In addition, the negative correlation between soil pH and the labile pools of Pb in soils may be useful for further remediation to reduce the bioavailability of Pb in contaminated soils.