Die Bodenkultur - Journal for Land Management, Food and Environment
Vertical mobility of Selenium, Arsenic and Sulfur in Model Soil Columns
In Europe, Se occurrence in soils, crops and groundwater is rather low, but it may be enriched from fertilization with organic amendments or selenium containing mineral fertilizers. For reasons of comparison, Se was treated together with arsenic and sulfur, which may exert similar properties, and compete each other. In order to predict mobilities and possible losses to the groundwater, chernozem topsoils (haplic, gleyic, and calcic) from the East of Austria were packed into 4 columns each, in order to investigate the maximum mobility of Se, As and S, added as fertilizer components, due to elution with natural rainfall. The columns contained about 5 kg soil within 30 cm column length, and received the annual precipitation within 2 months. They were kept in the dark at room temperature without green plants and algae to achieve maximum vertical mobility. Potentially anionic but nevertheless metabolizable elements sulphur (as sulfate), selenium (as selenite) and arsenic (as arsenate) were added to an NPK fertilizer matrix. The eluates from the columns were analyzed for total contents of dissolved species. For total selenium, aliquots of the eluate had to be ashed with magnesium nitrate. Within a pre-run, the effective pore volume of each column was determined from the passage of chloride, added as KCl. After 1 pore volume, steep peaks of dissolved sulfur appeared in all eluates which were 2–3 times larger than the added amounts, possibly due to ion-exchange and sulfur mineralization. Only in the haplic chernozem, some selenium passed the columns at one pore volume, similar to sulfur. Selenium was strongly retarded and rather constantly eluted in a non-quadrivalent form. Total arsenic started to elute after 2 pore volumes and remained at constant low levels in amounts negligible with respect to the total contents in the column.
After a drying period of 6 weeks, and re-wetting the columns, sulfur release was enhanced in all cases, but selenium released only from the gleyic chernozem. Within the soil profile, the added selenite preferably was seen in the top layers, both in aqua regia, oxalate and NaOH leachable fractions, and migrated down at different speeds depending on the soil type.
With respect to arsenic, the addition-elution experiment led to an increase in NaOH-releasable arsenic from the haplic and gleyic samples. For the gleyic chernozem, a maximum of oxalate soluble As appeared in the middle of the columns.
Speciation changes during the experiment seemed to be larger than the added amounts.
The significant washout of sulfur, which occurred in all cases, was clearly visible in the NaOH soluble fraction only for the calcic chernozem, whereas the release from the other samples derived from other soil fractions.
Key words: Selenium, arsenic, sulfur, fertilization, elution.