Yasuo Nakamaru

Speciation and bioavailability of selenium and antimony in non-flooded and wetland soils: A review

Studies on the sorption behaviors of selenium (Se) and antimony (Sb) are reviewed. Both Se and Sb chemical speciation can be controlled by pH and redox potential, and both of them are likely to be sorbed onto oxy-hydroxides of aluminum, iron or manganese in soils. For agricultural soils especially, there are important physico-chemical and biological differences between non-flooded and wetland soils. Se forms Se(VI), Se(IV), Se(0), Se(-II), and organic Se species at soil pH and redox conditions. Under non-flooded conditions Se solubility is governed by an adsorption mechanism onto metal oxy-hydroxides rather than by precipitation and dissolution reactions; however, for the conditions of wetland soils, it can be expected that Se(0) and organic matter-bound Se play an important role. For Sb, in the soil environment, the dominant Sb forms are Sb(III) and Sb(V). Under aerobic soil conditions, Sb(III) is likely to be oxidized to Sb(V), and the dominant sorbed Sb species should be Sb(V). Under reducing conditions Sb mobility should be lower than under oxidizing conditions due to the lower mobility of Sb(III); however, reduction of Fe and Mn oxides could lead to dissolution of Fe and Mn-bound Sb. This indicates that the risk of Sb contamination to the food chain could be increased in wetland systems.

Sorption behavior of selenium and antimony in soils as a function of phosphate ion concentration

Abstract Both selenium (Se) and antimony (Sb) are major soil and water pollutants. Their sorption behavior in a soil–plant system was studied. Soil–soil solution distribution coefficients (K ds) for Se and Sb were measured, using a radiotracer, as an indicator of their sorption levels. Both Se and Sb behave as oxoanions (SeO2− 4, H2PO− 4 and SO2− 4) in soil; thus, the effects of concentrations of two major oxoanions (SeO2− 4 and SeO2− 3) on Se and Sb sorption were also examined. The K d values for Se for Japanese soils significantly correlated with the K d values for Sb (n = 141). The K ds of both Se and Sb similarly decreased with increasing SbO− 3 concentration. These results indicated that the sorption of Se and Sb was similarly controlled by a ligand-exchange mechanism such as phosphate sorption in soil. However, an increase in the concentration of SeO2− 3 did not decrease the K ds of Se and Sb. Furthermore, the ligand-exchangeable fractions of stable Se and Sb in major Japanese soils were determined by extraction with 0.1 mol L−1 Na2HPO4 solution. For both Se and Sb, the phosphate-extractable fractions were 10-fold higher for Se and fivefold higher for Sb than their water-soluble fractions. Although the total Se and Sb amounts in soils were the same, their ligand-exchangeable fractions were different. Approximately 0.9–12% of total Se and 0.2–1.3% of total Sb were extracted by the phosphate solution. These findings suggested that Se was more likely to be mobilized by the addition of phosphate than Sb. The effect of plant-available phosphate in the soil and the phosphate sorption capacity of soil on Se and Sb availabilities for plants were also examined using a pot experiment with soybean plants. The experimental results suggested that a high content of available phosphate and/or low phosphate sorption capacity of soil increased both Se and Sb availabilities to the plant. However, the results also suggested that the soil Se availability to the plant was higher than that of Sb even though the soil total Se and Sb amounts were the same.

Distribution coefficients of tin in Japanese agricultural soils and the factors affecting tin sorption behavior

Sorption behavior of tin (Sn) in Japanese agricultural soils was studied. Soil-soil solution distribution coefficient (K(d)) of Sn (K(d)-Sn) for 142 soil samples ranged between 128 and 1,590,000 L kg(-1) with the geometric mean of 12 400 L kg(-1). The K(d)-Sn values for Andosol tended to be higher than those of the other soil groups. Among the relationships between K(d)-Sn values and soil properties, a high correlation was observed for soil active-Al (Al-(hydr)oxide and Al-humus complex) amount and K(d)-Sn. The pH effect on Sn sorption was also investigated. The results suggested that the low pH condition enhanced the Sn sorption in soils. The soil-sorbed Sn fractions in each type of soil material were also evaluated with selective extraction methods. The results showed that most of the soil-sorbed Sn was as organic matter bound or Al/Fe-(hydr)oxide-bound forms.

Effect of long-term phosphorus fertilization on soil Se and transfer of soil Se to crops in northern Japan

Phosphorus (P) fertilizer can potentially serve as a source for Se accumulation in croplands. Furthermore, it has been reported that the addition of P fertilizer to soil may enhance Se availability. Japanese agricultural soils are typically enriched in P as a result of long-term, excessive P fertilization. Therefore, we conducted a three-year field experiment in order to evaluate the effect of P fertilization on the Se content of soils and crops. Potato, wheat and barley were cultivated with and without P fertilization at two field sites in Hokkaido (northern Japan) with different levels of historical P accumulation. The first field site consisted of an Andosol soil with low available P and the second site, a Cambisol soil with high available P. The three years of continuous P fertilization over the course of the experiment did not result in a significant increase in the Se content of soils or plants. The Se content of soils and plants, however, was higher in soil samples from the Cambisol field site than from the Andosol field site, and total soil Se was significantly correlated with available soil P. Soluble soil Se and the soil-plant transfer factor for Se were not affected by P fertilization. Thus, we concluded that the higher plant Se content at the Cambisol field site was primarily due to the higher levels of accumulated Se in the soil at the site and that historical excess P fertilization typical of agricultural soils in Japan contributes to increased Se uptake by crops.

Effect of soil organic matter on antimony bioavailability after the remediation process

We evaluated the long-term (18 year) and short-term (4 weeks) changes of Sb in contaminated soil with SOM increase under remediation process. In the Aznalcóllar mine accident (1998) contaminated area, the remediation measurement implemented the Guadiamar Green Corridor, where residual pollution is still detected. Soils of the re-vegetated area (O2) with high pH and high SOM content, moderately re-vegetated area (O1) and unvegetated area (C) were sampled. Soil pH, CEC, SOM amount and soil Sb forms were evaluated. Soil Sb was measured as total, soluble, exchangeable, EDTA extractable, acid oxalate extractable, and pyro-phosphate extractable fractions. Further, the short-term effect of artificial organic matter addition was also evaluated with incubation study by adding compost to the sampled soil from C, O1 and O2 areas. After 4 weeks of incubation, soil chemical properties and Sb forms were evaluated. In re-vegetated area (O2), soil total Sb was two times lower than in unvegetated area (C); however, soluble, exchangeable, and EDTA extractable Sb were 2-8 times higher. The mobile/bioavailable Sb increase was also observed after 4 weeks of incubation with the addition of compost. Soluble, exchangeable, and EDTA extractable Sb was increased 2-4 times by compost addition. By the linear regression analysis, the significantly related factors for soluble, exchangeable, and EDTA extractable Sb values were pH, CEC, and SOM, respectively. Soluble Sb increase was mainly related to pH rise. Exchangeable Sb should be bound by SOM-metal complex and increased with CEC. EDTA extractable fraction should be increased with increase of SOM as SOM-Fe associated Sb complex. From these results, it was shown that increase of SOM under natural conditions or application of organic amendment under remediation process should increase availability of Sb to plants.