In situ stabilization of metals (Cu, Cd, and Zn) in contaminated soils in the region of Bolnisi, Georgia

The topsoils of the Mashavera valley in SE Georgia are highly contaminated with Cu, Zn and Cd from mining activities in the upper reaches of the Mashavera River. Previous laboratory experiments have shown that an in situ immobilization of these metals (M) by Fe-oxides would be an adequate remediation technique for the study area. A field experiment was thus initiated in 2008 in order to assess such a treatment under natural conditions. To verify the results a pot experiment was conducted in a growth chamber. Two experimental fields with differing degrees of M contamination were chosen close to the town of Bolnisi, Georgia, and divided into 8 (Field I, weakly contaminated) and 16 (Field II, heavily contaminated) plots. Half of the plots at each field were chosen for remediation. A 1% (w/w) zerovalent iron grit was applied homogeneously and mixed in the topsoils (to a depth of 0-20 cm) of the treatment plots. Composite samples of all plots were taken directly after remediation in March, June and October 2008 and again in March and October 2009. Spinacia oleracea L. was cultivated from March to June 2008 and 2009. Mobile fractions of Cu, Zn and Cd in topsoil, M concentration of Spinacia oleracea L., contents of amorphous Fe-oxides, changes of soil pH and plant available inorganic P were then determined in 2008 and 2009. In the pot experiment topsoil of Field II was mixed with 1% (w/w) zerovalent iron grit, incubated for 4 weeks and subsequently Spinacia oleracea L. was cultivated for 4 weeks. Mobile fractions of Cu, Zn and Cd in soil, M concentration of Spinacia oleracea L., changes of soil pH and plant available inorganic P were also determined. The mobile fraction of Cu decreased significantly to 48% from March 2008 to October 2009 in the treated soils of Field I. In the control soils, mobile Cu also decreased significantly to 52%. In soils treated with iron grit in Field II, mobile Cu and Cd decreased significantly to 20% and 51%, respectively, while in the control soils no significant changes were found. These results were confirmed by data from the pot experiment. The significant decrease of mobile Cu and Cd concentrations is due to the formation of amorphous Fe-oxides after application of iron grit and fixation of M by innerspheric complexes. Nevertheless, plant uptake of M was not affected by the Fe treatment in the field experiment. In the pot experiment, plant growth increased in Fe treatment. In addition, Cd uptake of Spinacia oleracea L. increased while Cu uptake decreased in the treatment of the pot experiment. No influence of remediation on soil pH was detected. In the treatments, soil-P availability was reduced in the field as well as in the pot experiment. This could be traced back to P-fixation by freshly-formed amorphous Fe-oxides. Nevertheless, levels were still sufficiently high according to German regulations. The application of iron grit as an in situ remediation is suitable only to a limited extent for topsoils of the study area. The efficiency to reduce mobile M by application of 1% (w/w) zerovalent iron grit decreased in the sequence Cu > Cd > Zn. The decrease was higher if the mobile fraction had a higher portion of the total content. Soil pH was not affected and plant available inorganic P was only marginally affected by the remediation but plant uptake of Cd, the most important criteria, was not reduced.