Cadmium and copper release kinetics in relation to afforestation of cultivated soil

Abstract

Afforestation of cultivated soils causes soil acidification and elevated concentrations of dissolved organic matter (DOC) in the soil solution, and hence, aggravate the risk of heavy metal leaching. The kinetics of cadmium and copper release from an unpolluted arable soil applied with forest floor soil solution was investigated in the laboratory, and the release rates correlated to pH and DOC in solution through log–log equations. The soil solution was isolated from Norway spruce ( Picea abies (L.) Karst.) by centrifugation, and the solution passed a cation-exchange column to remove metal cations and to protonate the DOC. Soil samples from an arable Ap horizon were placed in completely mixed flow cells, and influent solutions with 0 to 5 mM DOC were applied. The solution pH was adjusted to achieve effluent pH values in the range 3.6 to 6.9 in the flow cells at steady-state conditions. Cadmium release rates were very low at pH > 5 and increased exponentially as pH decreased to <5. The release rate was correlated to solution pH in a simple model: log(cadmium release rate) = −0.21 pH − 15.28 ( R 2 = 0.48), and no significant effect of DOC was observed. The kinetics of copper release from the soil was more complicated with effects of both pH and DOC. In experiments without DOC, the release rate of copper was slightly lower at high pH than at low pH. In experiments above pH 5, the presence of 5 mM DOC in the solution increased the release rate of copper. However, the copper release was retarded by DOC in the range pH 3.8 to 5.0, which coincided with a maximum retention of DOC in the flow cells. The release rate of copper was correlated to solution pH and concentration of DOC, including an interaction of pH and DOC: log(copper release rate) = 0.86 pH − 1.26 logDOC + 0.24 pH · logDOC − 19.26 ( R 2 = 0.60). If the changes in soil chemical conditions after afforestation influence the cadmium and copper release rates in a similar way as observed in the flow cell experiments, then the release rate of cadmium will increase exponentially at soil solution pH < 4.5. The inhibition of copper release by DOC observed at pH 3.8 to 5.0 indicates that copper is retained in the soil by interactions with adsorbed organic matter.