Sorption of cadmium in soil treated by limed sewage sludges

The aim of the trials was to determine changes in sorption of Cd in soil after application of limed and untreated sewage sludges. Three different sewage sludges incubated in advance for eight months at the temperature 20 degrees C were used in the trials. The rate was 180 g per 1 kg of sludge dry matter. After application of CaCO3 pH of sludges increased to almost neutral value (Tab. II). In untreated sludges the following Cd concentration was determined: sludge 1 = 5.10 mg Cd.kg(-1) sludge 2 = 7.08 mg Cd.kg(-1), sludge 3 = 1.96 mg Cd.kg(-1). When CaCO3 was applied, the total Cd content was diluted and after analysis the following values were found: sludge 1 + v = 5.09 mg Cd.kg(-1), sludge 2 + v = 6.56 mg Cd.kg(-1), sludge 3 + v = 1.59 mg Cd.kg(-1) (Tab. III). Liming of sludges resulted in significant decrease of water-soluble and exchangeable forms of Cd in sludges (Fig. 1). The greatest share of exchangeable Cd 10.3% was in sludge 3 and decreased to 4.7% by liming. The same trend was also recorded in sludge 2 (decrease from 6.3 to 4.5%) and sludge 1 (decrease from 9.1 to 7.5%). In further stage of experiments new incubation trials were established with sludges treated in this way with three different soils (Chernozem, Luvisol and Fluvisol) (Tab. I). The rate of the fresh sludge here amounted to 1.665 g of dry matter per 30 g of fine soil. These trials lasted 240 days and extractable amount of Cd was determined five times (0, 14, 30, 60 and 240 days). Three different extract agents were used: 1 mol.l(-1) NH4NO3, 0.025 mol.l(-1) NH(4)EDTA (pH 4.6) and 2 mol.l(-1) HNO3. With respect to the fact that in limed treatments total Cd content was lower the changes induced are given in relative expression (in percentage to the total Cd content in soil + sludge). It is evident from all presented results that liming resulted in significantly lower Cd mobility as in sludges only, as in soil after application of limed sludges (Figs. 3 and 4). Gradual immobilization of Cd has been found during incubation of soil with sludges, as well as in control treatments what is in correlation with processes in incubated soils, i.e. with the process of gradual mineralization of organic matter of sludges and with subsequent sorption of Cd into more stable humus substances in soil acid with the Cd sorption to minerals. Mild extraction agent showed higher relative differences between studied treatments (NH4NO3). The incubation decreased differences between untreated and limed treatments, when HNO3 and NH,EDTA were used during incubations (Figs. 3 and 4). Lime application decreased Cd mobility in all studied sludges (Figs. 5 and 6). Explanation for reduced Cd mobility of limed sludges can be derived from pH changes of soils (Tab. IV). The most pronounced differences were induced after application of sewage sludges on Fluvisols corresponding to the total low sorption capacity of this soil. Average pH value of unlimed treatments amounted to 5.66 and was increased to 6.52 by liming.