Effects of ionic strength and pore water velocity on cadmium mobility through contaminated soil in Mae Sot district, Tak province

Abstract

Contamination of cadmium (Cd) was reported to be found at agricultural areas nearby Mae Tao and Mae Ku creek in the Mae Sot District, Tak Province, Thailand. Cadmium is an extremely toxic substance and harmful to human health. Therefore, Cd levels in soils and mechanisms of Cd movement into soils are needed to be investigated systematically. The objective of this research was to investigate the effects of ionic strength (IS) and pore water velocity on the Cd movement through contaminated soil. There were three laboratory parts in this study as follows: 1) analysis of Cd contaminated in soils and properties of the selected soil, 2) batch sorption-desorption experiments, and 3) column experiments. The concentrations of Cd in soils collected from Mae Tao and Mae Ku creek were in the ranges of 0.71-62.04 mg/kg, some of them are higher than the European Union Maximum Permissible level (3 mg/kg). Based on hydraulic conductivity and level of contaminated Cd in soils, the contaminated agricultural soil was selected for further experiments, which is a sandy loam soil with 26.5 mg/kg Cd and hydraulic conductivity of 9.14 cm/h.

Both batch adsorption experiments and column experiments showed inverse relationship between sorption coefficients (Kd, KL, Kf) with increasing of IS, whereas desorption experiments revealed higher concentrations of desorbed Cd with increasing IS. For batch experiments, the Freundlich isotherm is the best adsorption isotherm, explaining Cd adsorption (R2 >0.93). Regardless of pore water velocity, the dispersivity (λ) of low and high flow columns was approx.1.73 cm. However, the Langmuir non-equilibrium model (or two site sorption model) is well-fitted with observed BTCs from column experiments (R2>0.95, RMSE<0.05) with KL ranging from 0.09-4.03 l/g. Ionic strength appeared to significantly affect Cd sorption and transport in a range of IS from 10-100 mM. According to modeling results of column tests, higher IS resulted in increasing ƒ and α, because the competition between Cd2+ and Ca2+. Peclet numbers showed that both diffusion and advection process is dominated in the column experiments. Based on pore water velocity effect, KL and ƒ is inversely related with pore water velocity, while α is positively related with pore water velocity, which is responsible for the rate-limited process.