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

Afforestation of cultivated soils causes soil acidification and elevated co ncentrations of dissolved organic matter (DOC) in the soil solution, and he nce, aggravate the risk of heavy metal leaching. The kinetics of cadmium an d copper release from an unpolluted arable soil applied with forest floor s oil solution was investigated in the laboratory, and the release rates corr elated to pH and DOC in solution through log-log equations. The soil soluti on was isolated from Norway spruce (Picea abies (L.) Karst.) by centrifugat ion, and the solution passed a cation-exchange column to remove metal catio ns and to protonate the DOG. Soil samples from an arable Ap horizon were pl aced 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 value s in the range 3.6 to 6.9 in the Row cells at steady-state conditions. Cadm ium 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 simpl e 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 DOG. In experime nts without DOG. the release rate of copper was slightly lower at high pH t han at low pH. In experiments above pH 5. the presence of 5 mM DOC in the s olution increased the release rate of copper. However. the copper release w as retarded by DOC in the range pH 3.8 to 5.0, which coincided with a maxim um retention of DOC in the how cells. The release rate of copper was correl ated to solution pH and concentration of DOG. including an interaction of p H and DOG: log(copper release rate) = 0.86 pH - 1.26 logDOC + 0.23 pH . log DOC - 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 w ay as observed in the Row cell experiments, then the release rate of cadmiu m 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 re tained in the soil by interactions with adsorbed organic matter. Copyright (C) 2000 Elsevier Science Ltd.