A holistic approach to phosphate stabilization treatment of metal contaminated soil

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, SLOVENIA

^a marija.zupancic@fkkt.uni-lj.si
^b peter.bukovec@fkkt.uni-lj.si

Abstract

In our study we showed the importance of holistic approach to evaluation of chemical stabilization using phosphate amendments, where all aspects of chemical treatments were observed. An extensive evaluation of metal stabilization in contaminated soil and an evaluation of the leaching of phosphorus induced after treatment were performed. The soil from the former zinc smelter area in the Celje region, used in this study, was heavily polluted with Zn (34 400 ± 1500 mg kg⁻¹), Pb (20 400 ± 1500 mg kg⁻¹), As (950 ± 10 mg kg⁻¹), Cu (549 ± 7 mg kg⁻¹) and Cd (158 ± 4 mg kg⁻¹). The results of Toxicity Characteristic Leaching Procedure showed high mobility of Zn (540 ± 25 mg L−1), Pb (12.7 ± 0.5 mg L−1) and Cd (2.9 ± 0.1 mg L−1). To immobilize metals in the soil, mixtures of soil with phosphate (variable molar ratio of hydroxyapatite and phosphoric acid) were prepared with a constant molar Pb:P ratio of 1:10. Hydroxyapatite as the only source of phosphate showed a high stabilization efficiency, above all for Pb with over 97 % of primary acetic acid leachable Pb immobilized. The addition of phosphoric acid as a source of phosphate resulted in an increase in leaching of phosphorus up to 50 mg L⁻¹ in water extracts. Lime that was also added in some stabilization mixtures increased the stabilization efficiency of phosphate and also decreased the treatment-induced phosphate leaching. To evaluate the long-term stability of immobilization, leaching of metals and phosphorus was assessed in a column experiment with synthetic precipitation that in general showed steady decrease in leachability of metals and phosphorus with quite high cumulative Zn and Cd concentrations in leachates of soil sample and extremely high (30% of total added P concentration) cumulative P concentrations in leachates of mixture with highest amount of added phosphoric acid and no addition of lime.