Mobilization of Cu and Pb from multi-metal contaminated soils by dissolved humic substances extracted from leonardite and factors affecting the process

Purpose The aim of the research was to determine the potential of dissolved humic substances extracted from leonardite to mobilize Cu and Pb from multi-metal contaminated soils with the scope of finding an efficient washing solution as a possible substitute for conventional washing agents used to extract heavy metals from polluted soils around mining areas. Materials and methods The efficiency of dissolved humic substances extracted from leonardite to extract Cu and Pb from multi-metal contaminated soil collected from “Larga de Sus” mine (Romania) was determined under batch conditions by single-step extraction. All extraction experiments were conducted in a stirrer with orbital rotation-oscillation and thermostat cupola at 100 oscillations/min and oscillation amplitude of 32 mm. The influence of stirring time (120, 240, 360, 720, 1440, 2400 min), concentration of humic solution (2% and 5%), and soil:solution ratio (mass:volume (m/V)) of 1:8, 1:10, and 1:16 on the Cu and Pb removal efficiency was investigated. The heavy metal concentration from soil and extractant solution was determined through atomic absorption spectrometry. Results and discussion The Cu and Pb removal process from soil using dissolved humic substances proceeded relatively quickly. In investigated experimental conditions, the best Cu and Pb removal efficiencies were observed after 360 min of stirring when 60.3% of Cu and 48% of Pb were extracted from soil (2% humic washing solution). The removal efficiency of Cu increased with increasing the soil:solution ratio and humic solution concentration, at shorter stirring times. In the case of Pb, an important improvement of the mobilization with an increasing soil:solution ratio or humic solution concentration was not observed, along with investigated stirring time. A soil:solution ratio of 1:8 was sufficient to reduce the Cu concentration in the studied soil below the intervention threshold established by Romanian legislation, after 360 min of stirring. Conclusions The mobilization of Cu and Pb from multi-metal contaminated soil using dissolved humic substances is strongly dependent on stirring time, soil:solution ratio, and concentration of humic washing solution. Nevertheless, the results of the present study demonstrate that dissolved humic substances extracted from leonardite are fairly effective washing agents for soils polluted with Cu, Pb, multiple heavy metals, and other pollutants in high concentrations.