Using time of flight secondary ion mass spectrometry and field emission scanning electron microscopy with energy dispersive X-ray spectroscopy to determine the role of soil components in competitive copper and cadmium migration and fixation in soils
Introduction
The availability of heavy metals for plant uptake and the risk of these metals finding their way into surface or underground waters both depend on their sorption and desorption in soil (Vega et al., 2006, Loganathan et al., 2012). Copper and cadmium enter the soil not only through urban solid wastes but also via sewage sludge. Furthermore, Cd and Cu can reach soils via fertilizers, pesticides, liming agents, acid water drainage from mining activities, sewage sludge and urban and industrial waste (Yanqun et al., 2005, Nagajyoti et al., 2010, Selim, 2013).
Studies on the behaviour of Cu2 + and Cd2 + in soil have largely focused on interactions with solutions containing either metal, and have therefore ignored the potential effects of their mutual competition.
The migration of a heavy metal through a soil profile is a complex process that depends on multiple factors, namely soil pH and other soil characteristics, such as Fe and Mn oxide contents, clay and carbonate contents, clay mineral and the amount of organic matter types, textures and structures, as well as the intrinsic properties of the metal, its concentration in the soil solution and the presence of other metals (Simpson et al., 2004, Vega et al., 2011).
The heterogeneity of soil makes it very difficult to predict the potential mobility and distribution of even single metals, so experimental data are usually essential for this purpose. Consequently, sorption tests on soil should invariably be followed by desorption tests. Sorption and desorption isotherms reveal whether sorption is reversible or wholly or partially irreversible (hysteretic).
This paper examines the influence of soil characteristics on competitive Cu2 + and Cd2 + sorption, desorption and migration in five different soils. To this end, the capacities of soil horizons jointly polluted by Cu2 + and Cd2 + were comprehensively assessed and compared; this involved determining not only two general indicators of sorption and retention capacity (adimensional parameters) Kr,s and Kr,r but also the hysteresis and migration indices (HI, MI) defined by Vega et al., 2008, Vega et al., 2009a, Vega et al., 2011; as a result, it was possible to make an accurate comparison of the soil horizons in terms of Cu2 + and Cd2 + competitive sorption, retention, hysteresis, and migration.
In recent years, sorption processes have been studied (Sastre et al., 2006, Lair et al., 2007, Covelo et al., 2008b, Rees et al., 2014) in order to determine the influence of soil components on heavy metal fixation, mainly using statistical analysis such as linear regression and regression tree analyses (Covelo et al., 2008a, Vega et al., 2009b). However, little attention has been focused on providing information about the interactions between heavy metals and soil components using various techniques such as field emission scanning electron microscopy with energy dispersive X-ray spectroscopy (FE-SEM/EDS) and time of flight secondary ion mass spectrometry (TOF-SIMS) (Sipos et al., 2009, Cerqueira et al., 2011b). The starting hypothesis of this study is that the use of both techniques will provide very precise information on the nature of soil components as well as their interactions with heavy metals.
Therefore, the aims of the present study were to assess and compare the competitive sorption and desorption capacity and the sorption hysteresis of Cu2 + and Cd2 +, as well as the migration of both cations through the horizons of each soil. Another objective was to determine the interactions between the main soil components and Cu2 + and Cd2 + by TOF-SIMS and FE-SEM/EDS. An understanding of how Cu2 + and Cd2 + migrate through soil profiles will make it possible to propose measures to control their mobility and prevent pollution problems, given that the mobilization of Cu2 + and Cd2 + leads to plant uptake and leaching into groundwater; but this can be minimized by reducing the bioavailability of these metals.
Section snippets
Soil sampling
To carry out this work five natural soils developed on different parent matter were selected in Galicia (NW Spain). The soils studied were an Umbric Cambisol (UC), an Endoleptic Luvisol (EL), a Mollic Umbrisol (MU), a Dystric Umbrisol (DU) and a Dystric Fluvisol (DF) (IUSS Working Group WRB, 2014) developed on quartzite, amphibolites, slates, schists and amphibolites respectively. Samples from the horizons (UC.A, UC.Bw, MU.A, MU.Bw, EL.A, EL.Bt, DU.A, DU.Bw, DF.A and DF.Bw) were analyzed and
Soil characteristics
Statistical analysis shows that there are significant differences in the components and properties (Table 1) that most influence the sorption capacity, mobility and therefore the retention of metals by the soils. The pH of the soils varies between 4.6 (UC.A) and 6.4 (EL.Bt) and the total organic carbon content varies between 8.7 (EL.Bt) and 144.8 g kg− 1 (DF.A). The two horizons from soil EL have the highest proportion of Fe and Mn oxides (Table 1). In general, the Mn oxide content is low in all
Conclusions
The competitive sorption and desorption experiments showed, first of all, that the A horizons sorb and retain more Cu2 + than B, secondly, that the B horizons sorb and retain more Cd2 + than A, and also that the competitive sorption of Cd2 + is more irreversible than that of Cu2 +.
Cu2 + sorption and retention capacities are positively correlated with the ECEC and organic carbon content, while Cd2 + sorption and retention capacities are strongly correlated with the pH, the ECEC and the Fe and Mn oxide
Acknowledgements
This research was supported by Projects EM2013/18 (Xunta de Galicia) and MICIN- CGL2013-45494-R (Ministerio de Economía y Competitividad - Spain). F.A. Vega is hired under a Ramón y Cajal contract at the University of Vigo. D. Arenas-Lago is grateful to the Spanish Ministry of Science and Innovation and the University of Vigo for the FPI-MICINN.
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