Nickel (Ni) Transport in Soils: Competitive Effect of Cadmium (Cd).

The competitive effect influenced the transport behavior of Nickel (Ni) and Cadmium (Cd) in soils, as illustrated by the experimental findings obtained from miscible-displacement experiments and sequential extraction tests. Ni or Cd pulses were introduced consecutively or simultaneously into water-saturated soil columns. The results from consecutive application of Cd demonstrated that the rates of Ni sorption are increased by addition of Cd representing by significant concentration drop observed at sorption site of Ni breakthrough curves (BTCs) in tested soils. The results from sequential extraction following transport experiment provided evidence that the presence of Cd forced Ni sorption at higher affinity sites (Ni combined with oxide or/and organic matter). The results of simultaneous presence of Ni and Cd in soil column showed that, due to competitive effect at common reaction sites, the transport behavior of Ni and Cd was dominated by equilibrium behavior other than non-equilibrium or kinetic behavior when only Ni or Cd present in soil columns. We concluded that the competitive effects between Ni and Cd occurred at low affinity sites enhanced the mobility of Ni or Cd. Moreover, such competition at low affinity sites forced Ni or Cd sorption at higher affinity sites (more specific sites), which may cause metal stronger bonding with soil at higher affinity sites. The latter effect highly depends on the availability of higher affinity sites. At certain surface coverage, if there are specific sites available, the competition at low affinity sites forces the sorption to higher affinity sites but competition can only happen at low affinity sites when no specific sites are available.