Sorption of lead in soil as a function of pH: a study case in México
Introduction
Lead is a heavy metal that can occur naturally in soils; its occurrence is strongly related to the composition of the bedrock. Lead primary form in the natural state is galena (PbS). Lead occurs mainly as Pb2+, although its oxidation state, +4, is also known, and it forms other several minerals which are quite insoluble in natural waters. During weathering, Pb sulfides slowly oxidize and have the ability to form carbonates and also to be incorporated in clay minerals, in Fe and Mn oxides, and in organic matter. Lead has the ability to replace K, Ba, Sr, and even Ca, both in minerals and in sorption sites (Kabata-Pendias and Pendias, 2001). In addition to the natural occurrence of lead in soil from bedrock, lead can enter to soil via anthropogenic contributions; enriching and polluting the soil. For example, the accumulation of mine-waste material in valley-filling piles can suffer weathering and produce acid mine drainage. If mine waste material contains lead, it will be liberated to the environment during acid mine drainage reactions occur. Once in the environment, lead can interact with soil constituents and participate in the following processes: sorption processes between solid surfaces as minerals, Fe and Mn oxides, and organic matter, precipitation, nucleation, solid solutions formation as well as redox reactions. The interactions to be carried out mostly depend on lead properties such as charge, coordination chemistry, solubility and precipitation, and redox reactions. However, the mobility and the bioavailability of lead in soils are mainly governed by soil properties as pH, organic matter content, cation exchange capacity, texture, mineral species; specially those constituting the clay fraction. Among these, pH plays an important role in lead sorption on soils since it affects lead speciation and the charge of soil surface groups.
Because of the excess concentrations of a number of trace elements in the environment as a consequence of anthropogenic emissions and their consequences to human health, a significant effort has been expended over the last years to understand interactions between trace elements and soil. Also, the fate of anthropogenic lead in soils has recently received much attention because lead is hazardous to man and animals from two sources: the food chain and soil dust inhalation (Kabata-Pendias and Pendias, 2001). It has been reported that lead is accumulated in the surface soil horizon because of its low mobility and strong association to soil constituents (Bañuelos and Ajwa, 1999) as organic matter, minerals of the clay fraction, and oxides of iron and manganese (Walton and Conway, 1988, Kabata-Pendias and Pendias, 2001). The influence of factors as pH and ionic strength has been studied. It has been shown that lead adsorption on soils increases as pH increases (McKenzie, 1980, Basta and Tabatabai, 1992, Pierangeli et al., 2001a) and that ionic strength affects lead mobility and bioavailability in soils (Pierangeli et al., 2001b). Also, the interaction between these two factors has been studied in some specific soil fractions. It has been showed that at low ionic strength (on the order of 0.006 M) lead adsorption on montmorillonite is pH-independent (Strawn and Sparks, 1999); suggesting an outer-sphere complexation mechanism at that conditions. The lead sorption on soil has been modeled. Lead sorption by model associations of soil colloids can be described by the Langmuir equation (Cruz-Guzmán et al., 2003). Other equations, presented as pH functions, have been proposed to describe lead sorption by pedogenic oxides, ferrihydrite and leaf compost (Sauvé et al., 2000). However, the effect of the pH on the interactions between lead and soils will be different due intrinsic heterogeneous nature of soil. Francisco I. Madero in Zacatecas, Mexico is a region that can be impacted soon with mining waste because a new mine, located in this area, will start extraction processes of lead; Francisco I. Madero (FIM) mineralized district is located in a belt of mines that occur in a semiarid region, Terreno Guerrero (north-west central Mexico), where two main types of ore can be distinguished: one is formed by Pb–Zn sulfides and another is formed by Ag–Zn sulfides (Garcı́a-Fons et al., 1996). Because of the future presence of mining waste, the FIM soils can possibly be acidified causing, at the same time, the modification of several physical, chemical, and biological processes occurring in soil; the main driving force to cause an acidification of FIM soils is the generation of acid mine drainage. It is our interest to get information about the interaction between heavy metals and FIM soils and, in this study, particularly about lead. This information can help us to evaluate the future impact of lead onto FIM soils and to provide a background about the lead sorption capacity. The purpose of this study is to evaluate the lead sorption capacity of soil from FIM at different pH values.
Section snippets
Soil sample
Bulk soil samples were collected in Francisco I. Madero, Zacatecas, Mexico at depth of 0–55 cm. The soil samples were air-dried, homogenized, and sifted through a 2.0 mm sieve. The following characteristics of soil were determined: particle-size distribution (dry-sieving and sedimentation method), pH (soil–solution-ratio of 1:2.5; 0.1 M KCl), total carbon (dry-ashing at 1200 °C), amorphous oxides of iron (extracted by ammonium oxalate (Shuman, 1985) and measured by atomic absorption analysis in a
Results and discussion
The samples studied were classified as sandy-clay-loam soils. The samples had a pH closed to neutral and low content of organic matter. The minerals of the clay fraction were quartz, kaolinite, halloysite, and metahalloysite (see Fig. 2). The content of iron oxides extracted corresponds to 17–25% of the total iron.
The results obtained show a strongly non-linear relation between C and q, and a convergence to a maximum adsorption appeared (see Fig. 3). We can observe that the quantity of lead
Acknowledgments
Nadia Martı́nez Villegas thanks to CONACyT for financial support under the scholarship number 161791. Special gratitude to Roberto Leyva Ramos, Ph.D. and Norma Gascón Orta, M.Sc. for their comments to this study. This work was supported by CONACyT under the grant number 25151A and by FAI/UASLP under the grant number CO1-FAI-4-2229.
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