Transport of cadmium, nickel, and zinc in Taoyuan red soil using one-dimensional convective–dispersive model

Pollution of agricultural lands by heavy metals has become more serious with increasing pollutants emitted from electroplate plants of northern Taiwan. This study aimed to assess the retardation factor (R) and the dispersion coefficient (D) using one-dimensional convective–dispersive model by comparing the breakthrough curve (BTC) and least squares methods. An acrylic column, with an inner diameter of 9 cm and a length of 15 cm, was uniformly packed with soil for metal leaching experiments. An unpolluted surface (0–20 cm) of the Taoyuan red soil was collected for this study. In miscible displacement experiments, a mixture of 4 mg L− 1 CdCl2, 20 mg L− 1 NiCl2, and 60 mg L− 1 ZnCl2 was leached from top of the column and the effluent was collected in increments with the aid of a fraction collector. The retardation factor (R) calculated from the least squares method was close to the R value observed from breakthrough curves. The R value showed the trend: Cd>Zn>Ni, indicating the rate of metals transport in red soils: Ni>Zn>Cd. Their reactivity was assessed within the framework of the hard–soft acid–base principle (HSAB). From sequential extractions, most of the Cd remained in exchangeable fractions, and Ni and Zn were found to exist in residual Fe- or Mn-oxides, and organic matter fractions.