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Methodology for Zinc Fate Assessment in Saturated Soil Material: Comparison between Batch and Continuous Laboratory Experiments

Petra Hlavackova1, Radu Barna2, and Maria Aurora Fernandez2. (1) Chemical Engineeering Department of Particulate Solids, Ecole des Mines d'Albi-Carmaux, Campus Jarlard - Route de Teillet, Albi, France, (2) LGPSD, Ecole des Mines d'Albi-Carmaux, Campus Jarlard - Route de Teillet, Albi, France

Polluted sites assessment and remediation process has to be performed according to the principles of the sustainable development. In the last period a significant progress is under accomplishing in this direction by many public administrations, in term of regulation and standardization at ISO and CEN level. Many scientific fields, like risk assessment, toxicology, agronomy, chemical engineering or hydrology are concerned by the fate of contaminants in soils. Their common interest is the short or long term prediction of pollutant mobility in a geo-physicochemical scenario using specific tests and models. In this context, we are studying methodologies for the analysis and prediction of short and long-term pollutant behavior in soils, based on chemical engineering principles. Metal mobility in a soil is strongly dependent on the soil composition, which determines, for example, soil pH and ion exchange capacity. A synthetic soil, composed of sand, calcium carbonate, kaolin, Fe oxide and peat was constituted based on the European classification of soils (Kuhnt, 1990) but also by considering the ISO guidelines for elaboration of biological test supports (ISO 11268-1, 1993). The main physical and chemical characterization was carried out in order to determine, for example, buffer capacity, ion exchange capacity or phenol and carboxylic groups repartition in peat. In order to evaluate metal mobility in soils, two methodologies are usually employed: laboratory column and batch reactor. Batch reactor experiments/tests can be considered as simpler, faster and more reproducible, but the results are just based on the solid/liquid equilibrium relationships, and the control of some operating conditions (S/L ratio, pH, temperature…) all along the test can be difficult. In addition, the long-term soil evolution would not be correctly evaluated by these discontinuous methods, and the hydraulic parameters can not be considered. The target pollutant was Zinc. Its retention was studied at constant input pH (between 5.5 and 6.8) and ionic strength (0,01 M), in continuous stirred semi batch reactor and up-flow percolation column (continuous) experiments. pH plays an important role in the mechanisms responsible for Zn retention: the ionic exchange controls its retention for input pH less than 6 and respectively carbonates precipitation for higher pH values. During column experiments, the chemical composition of the solid phase can change in time, as a function of the pH of the feed solution, and the equilibrium does not correspond to the initial solid phase (kinetics of calcite decomposition). For bimetallic input, competition was observed between Zn and Cu retention. In conclusion, the two approaches, close and open reactors, are complementary. Operating conditions influence the pollutant transport mechanisms in the laboratory experiments.

G. Kuhnt, The euro-soil concept as a basis for chemicals testing and pesticide research. 3rd workshop "Study and Prediction of Pesticides Behaviour in Soils, Plants and Aquatic Systems", GSF Munich, May, 30 – June, 1 (1990). ISO 11268. Qualité de Sols – Effets des polluants vis-à-vis des vers de terre (Eisenia fetida). Partie 1: Détermination de la toxicité aiguë en utilisants des substrat de sol artificiel. (1993).