Optimisation of the chemical immobilisation by limestone filler of heavy metals and metalloids in contaminated soils via response surface methodology (RSM)

Five soils contaminated with potentially toxic elements (Cu, Pb, Zn, As, Cd) from mining waste with different mineralogical and physico-chemical properties were amended with a limestone filler. In order to accelerate any alteration in the samples, the five soils mixed with filler were placed in a climatic chamber for a 1000-hour cycle at 20 °C and 80% of relative humidity. The amount of amendment agent was optimised by experimental design using response surface methodology (RSM) with two factors, the acid potential of these soils which varies between 29.2 and 5.3 mm mol H+/kg and the percentage of filler added which varies in the experimental design between 10% and 58%. The second order models obtained with the RSM showed a good adjustment for all elements except for As. The optimum filler value obtained through RSM was 40% for the five elements studied. This percentage of filler was validated by means of an experimental test with this content of filler. The results showed that Cu, Pb, Cd and Zn were immobilised over 99% due to the high carbonate content of the limestone filler and the precipitation process. However, As showed a 22% lower of immobilization. Therefore, alternative processes must occur for these elements as the adsorption.