A study of immobilization of four heavy metals by solidification/stabilization with portland cement

Abstract

Two sites were established in Harris County, Texas, to relate seasonal saturation to soil morphology, micromorphology, soil reduction potential (Eh), and iron phases; and to test a,a'-dipyridyl dye for field identification of iron-reduction. The Wockley soil is a Plinthic Paleudalf and the Katy soil is an Aquic Paleudalf. Both soils have shallow fine sandy loam surface soils, and clay loam subsoils, overlying buried clayey soils. Field evidence of saturation consisted of low-chroma soil colors, mottling, and iron and manganese segregations. Ironstone nodules are plentiful in upper Bt horizons of the Wockley soil, and plinthite occurs. The Katy soil has distinct sand-silt coats on ped faces of Btg horizons. Micromorphology shows iron being actively reduced in the upper profiles of both soils, through destruction of nodules, the presence of cuffent pore and void linings; and some fine soft agglomerations in the Katy soil. Some iron nodules are not being reduced, having sharp boundaries to the matrix, or clay coatings or plugs covering the nodules. Active and inactive features can occur in the same horizons. Pore and void linings were the most indicative field characteristics of saturation in the Wockley soil, while the sand-silt coats on ped faces were convincing evidence of saturation and reduction in the Katy soil. Micromorphology showed that field evidence in these soils must be interpreted cautiously. The incidence of high rainfalls over the study period exceeded the expected probabilities, implying that rare high rainfall events or sequences of events promote saturation and reduction, but continuing high rainfalls suggest that climatic probabilities need to be more thoroughly documented. Manganese was reduced when soils were at or above field capacity. Iron reduction could not be checked when the Wockley soil was saturated. The E horizon of the Katy soil seems prone to reduction of iron when water tables are present. Poorly crystalline free iron was very low in both soils. Highly crystalline free iron was more prevalent in high-chroma segments of the soils. The cl,a'-dipyridyl dye could not be fully tested; however it did not give false positive readings.