Abstract
Sorption and desorption experiments were performed by the batch method on the B horizons of five natural soils: Umbric Cambisol, Endoleptic Luvisol, Mollic Umbrisol, Dystric Umbrisol, and Dystric Fluvisol. Individual and competitive sorption and desorption capacity and hysteresis were determined. The results showed that Pb2+ was sorbed and retained in a greater quantity than Cd2+ and that the hysteresis of the first was greater than that of the second. The most influential characteristics of the sorption and retention of Pb2+ were pH, ECEC, Fe and Mn oxides and clay contents. For Cd2+ they were mainly pH and, to a lesser extent, Mn oxides and clay content. The combined use of TOF-SIMS, FE-SEM/EDS and sorption and desorption analyses was suitable for achieving a better understanding of the interaction between soil components and the two heavy metals. They show the preferential association of Pb2+ with vermiculite, chlorite, Fe and Mn oxides, and of Cd2+ with the same components, although to a much lesser extent and intensity. This was due to the latter’s higher mobility as it competed unfavourably with the Pb2+ sorption sites. TOF-SIMS and FE-SEM/EDS techniques confirmed the results of the sorption experiments, and also provided valuable information on whether the soil components (individually or in association) retain Cd2+ and / or Pb2+; this could help to propose effective measures for the remediation of contaminated soils.
Highlights
Increased concentrations of heavy metals of an anthropic origin in soils cause serious environmental pollution problems
The mineralogical analysis of the soil fraction < 2 μm showed that, except in the Endoleptic Luvisol (EL).Bt, Dystric Fluvisol (DF).Bw and Dystric Umbrisol (DU).Bw horizons, with a predominance of vermiculite, gibbsite, and quartz respectively, the most abundant mineral was kaolinite (S1 Fig). These results showed that selected soils were suitable for comparing the competitive and individual sorption and desorption capacity of Pb2+ and Cd2+
The specific surface area, which had a great influence on the sorption and fixation of the studied metal ions, varied widely among the different soils (13 to 78 m2 g-1), and the highest corresponded to the soil with the greatest clay content (S2 Fig)
Summary
Increased concentrations of heavy metals of an anthropic origin in soils cause serious environmental pollution problems. These metals are present mainly due to factors such as industrial activities, mining, or solid urban waste [1]. Soils can act as sinks for heavy metals, contamination problems can arise once their sorption capacity has been exceeded [2]. The mobility and bioavailability of heavy metals and the possibility of being transferred to other compartments of the ecosystem mainly depends on the sorption and desorption capacity of the different components of the soil [3]. The term sorption includes processes of adsorption, surface precipitation, and fixation, while desorption implies the release of sorbed species into the environment surrounding the soil particles [4]. The distribution of the metals amongst the soil components depends on the intrinsic properties of the types of metal involved, the soil properties, and the amount of metal added to it [5]
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