Immobilization of Lead in contaminated soil by silicon compounds (Case Study of Lead Mine Paji Miana)

Soil contamination with heavy metals is one of the main important environmental issues. Soils around mines have a high concentration of heavy metals that can affect the health of humans and animals. A cheap method to remediate soils contaminated with heavy metals is the, immobilization of heavy metals in soils. The use of environmentally friendly compounds that have the ability to immobilize of heavy metals in the soil environment has taken attention of many researchers. For this purpose, silicon compounds were used in the study to immobilize lead (Pb) in the contaminated soil.

In this study, four types of compound such as calcium silicate, potassium silicate, sodium silicate and pure silicon were used at five concentrations (0, 50, 100, 200 and 400 mg / kg soil) in terms of Si content. The Silicon compounds were added to the pots containing 700 grams of Pb contaminated soil, and maintained for one month at ambient temperature and soil water content equivalent to field capacity. The available Pb was then extracted with DTPA and the Pb concentration was measured by atomic absorption. The distribution of chemical forms of Pb was determined before and after of incubation time with sequential extraction method. This experiment was carried out in a factorial arrangement in a completely randomized design with three replications.

The results showed that among the 4 silicon compounds, calcium silicate at a concentration of 400 mg / kg soil had the highest rate (54%) of Pb stabilization. In general, all of four silicon compounds, at the concentration of 400 mg / kg soil, had the most Pb stabilization capability among the all of applied rates. The results of sequential extraction showed that by using silicon compounds in Pb contaminated soil and increasing the concentrations rates from 50 to 400 mg / kg the soluble and exchangeable form, the form attached to carbonates and the iron and manganese oxides reduced but the forms of Pb co-exist with organic matter and the residual form of Pb increased. The highest reduction in the form of exchangeable, carbonate and bounded to iron and manganese oxide, Conversely, the highest increase occurs in the form of residual and was belong to calcium silicate treatment.

In fact, by the increase of silicon compound rates application, more amount of these compounds are available for Pb immobilization. The Silicon can form a complex with heavy metals and thereby reduce the available concentration of them. The Calcium silicate has a higher specific surface area than potassium and sodium silicates, hence, it can initiate more complex with Pb ions. However, Potassium and sodium silicates had a greater ability to stabilize Pb ions than pure silicon. Therefore, pure silicon had less ability to decrease the available Pb concentration among the all treatments. In general, it can be concluded that silicon compounds are able to reduce the mobility of Pb ions in soil environment and these compound environmentally friendly compounds.