Displacement of Zn through acidic light-textured soils

In order to gain understanding of the movement of pollutant metals in soil, the chemical mechanisms involved in the transport of zinc were studied. The displacement of zinc through mixtures of sand and cation exchange resin was measured to validate the methods used for soil. With cation exchange capacities of 2.5 and 5.0 cmol_c kg⁻¹, 5.6 and 8.4 pore volumes of 10 mM CaCl₂, respectively, were required to displace a pulse of ZnCl₂ . A simple Burns-type model (Wineglass) using an adsorption coefficient (K_d) determined by fitting a straight line relationship to an adsorption isotherm gave a good fit to the data (K_d=0.73 and 1.29 ml g⁻¹, respectively).Surface and subsurface samples of an acidic sandy loam (organic matter 4.7 and 1.0%, cation exchange capacity (CEC) 11.8 and 6.1 cmol_c kg⁻¹, respectively) were leached with 10 mM calcium chloride, nitrate and perchlorate. With chloride, the zinc pulse was displaced after 25 and 5 pore volumes, respectively. The K_d values were 6.1 and 2.0 ml g⁻¹, but are based on linear relationships fitted to isotherms which are both curved and show hysteresis. Thus, a simple model has limited value although it does give a general indication of rate of displacement. Leaching with chloride and perchlorate gave similar displacement and K_d values, but slower movement occurred with nitrate in both soil samples (35 and 7 pore volumes, respectively) which reflected higher K_d values when the isotherms were measured using this anion (7.7 and 2.8 ml g⁻¹, respectively). Although pH values were a little higher with nitrate in the leachates, the differences were insufficient to suggest that this increased the CEC enough to cause the delay. No increases in pH occurred with nitrate in the isotherm experiments. Geochem was used to calculate the proportions of Zn complexed with the three anions and with fulvic acid determined from measurements of dissolved organic matter. In all cases, more than 91% of the Zn was present as Zn²⁺ and there were only minor differences between the anions. Thus, there is an unexplained factor associated with the greater adsorption of Zn in the presence of nitrate.Because as little as five pore volumes of solution displaced Zn through the subsurface soil, contamination of ground waters may be a hazard where Zn is entering a light-textured soil, particularly where soil salinity is increased. Reductions in organic matter content due to cultivation will increase the hazard.