Movement of selenite and selenate by saturated and unsaturated flow in East Texas overburden

Abstract

The objective of this research was to examine the potential mobility of selenite (SeO(,3)('-2)) and selenate (SeO(,4)('-2)) in a composite of east Texas overburden materials. Portions of deep (20-60 m) cores from along the primary lignite outcrop were used. Subsamples of the pH 7.2 composite were equilibrated with CaO until the pH was stable at 9 or with H(,2)SO(,4) until the pH had stabilized at 2, 3 or 5. Sorption of selenite and selenate from soil solution and movement by unsaturated water flow were determined as a function of overburden pH and selenium concentration by soil thin-layer chromatography. Movement of selenium by saturated water flow was determined as a function of pH and pore volumes of 0.01 M CaCl(,2). The self-diffusion coefficients were calculated as a function of pH and soil moisture. The greatest sorption of selenite in all experiments occurred at pH 3, near the pK(,1) of selenite (2.75). Almost all (497 (mu)g/g) of the applied 500 (mu)g selenite/g of overburden was sorbed from soil solution at pH 3 while 1,160 (mu)g/g was retained against movement by unsaturated flow. Less than 1.2 (mu)g selenite/g of overburden was retained against movement by unsaturated flow at pH 2 and 9. These pH values are below the pK(,1) (2.75) or above the pK(,2) (8.5) of selenite. Selenite self-diffusion coefficients showed similar trends. The self-diffusion coefficient at pH 2 and 100% of available water was 53.2 x 10('-9) cm('2)sec('-1), 100 times that at pH 3 (4.9 x 10('-10)) and three times that at pH 9 (15.7 x 10('-9)). Selenate moved with the wetting front under unsaturated flow conditions at pH 7 and 9 (Rf = 0.96) while the Rf value at pH 3 was 0.76. All selenate was removed from the leaching columns by saturated flow before 50 pore volumes of 0.01 M CaCl(,2) had passed. The self-diffusion coefficients of selenate were high (near 1 x 10('-6) cm('2)sec('-1)) and dependent on both pH and moisture. Sorption trends for selenite indicate two mechanisms; an instantaneous precipitation process and a ligand exchange type reaction. Precipitation dominates at pH 2 and 3 where free iron is present, while ligand exchange reactions become more important at higher pH values. Selenate is non-specifically adsorbed at pH values below 7 and is not retained by the overburden at higher pH values.