Saturday, 15 July 2006

Lead Behavior in a Contaminated Calcic Cambisol Profile.

Marie CECCHI1, Camille DUMAT2, Philippe PRADERE3, Ricardo BIDEGAIN4, Eric PINELLI1, and Maritxu GUIRESSE1. (1) INP-ENSAT AEE, BP 32607, castanet tolosane, 31326, France, (2) Pierre et Marie Curie Univ, UFR 928 4 place Jussieu, Paris cedex 05, 75251, France, (3) STCM, 11 route de Pithiviers, Bazoches les Gallerandes, 45480, France, (4) LAra Europe Analyses, 1 impasse de Lisieux BP 82553, Toulouse, 31025, France

Due to its persistence and numerous uses, lead is one of the most common pollutants in the environment (Alkorta et al., 2004) and it can induce negative effects on health (Henry, 2000). More than its total concentration, it's the association of lead with the soil components (speciation in the large signification of the term or compartimentation) that governs its mobility and availability in the soil. In the context of risk assessment due to the industrial activities, we studied the associations of lead with majors and others trace elements (inorganic and organic) in a calcic cambic soil near a recycling batteries plant. The studied soil is localized in the vicinity of a lead recycling plant (in activity since 1950) near Bazoches (45, France) and present a main lead contamination but others secondary Zn, Cu, Ni, As, Cr were also determined. A pedological global approach had been used to understand the behavior of lead inside the soil profile. Physical (texture, clay minerals,..) and chemical (CEC, pH, SOM) soil properties. Total major (Al, Ca, Fe, Mn, Mg, P) and trace (As, Cr, Cu, Ni, Pb, Zn) elements had been determined by ICP-OES analysis after aqua regia digestion. In order to study the distribution of lead through the various soil fractions and in particular the available fraction, sequential chemical extraction had been performed. According to Leleyter & Probst (1998), the following fractions were separated: (1) soluble with water; (2) exchangeable with magnesium nitrate, (3) bound to carbonates (sodium acetate); (4) bound to the manganese oxides (hydroxylammonium chloride); (5) bound to amorphous iron oxides (ammonium oxalate and oxalate acid); (6) bound to crystalline iron oxides (ammonium oxalate acid and ascorbic acid) and (7) bound to organic matter (nitric acid hydrogen peroxide and ammonium acetate). The calcic cambic soil studied presents an alkaline pH (between 7.2 and 8.8 from the top to the depth), a constant carbonate level (2.1%) and high clay fraction content (30 to 40 %). These factors could limit lead migration towards underground water. SOM and P (probably due to past amendments) present high values in the top soil and therefore could be the major carry phases for the lead. In particular the formation of lead-phosphates could reduce the availability. Concerning major elements, Ca follows the carbonate profile, as a result, that soil contents calcium carbonates. On the opposite, Al, Fe, Mg and Mn follow clay profile with little influence of organic matter on top soil. High lead concentration occurs in the top soil (1932 mg soil in the 0-10 cm horizon). These lead amounts decreased quickly with depth and the background concentration is reached at 50 cm. In the first 50 cm anthropogenic lead is probably associated with organic matter (R2= 0.96). According to Gavalda (2001), after 60 cm depth, the native lead is mainly associated with iron oxides (R2= 0.7). Moreover, Mg(NO3)2 extraction performed on the top soil demonstrated a relatively high availability of the lead (4 mg soil) which underlines a transfer risk into the trophic chain. For the others trace elements, As, Cu and Zn have a behaviour close to that of lead. They are bound to iron only from 40 cm depth (R2= 0.94 for As, 0.96 for Cu and 0.99 for Zn). These trace elements undergo a soil organic matter influence, as a consequence, an anthropogenic contamination in the first 40 cm can be deduced. Contrary to this data, Ni and Cr are bound to iron oxides along profile (R2= 0.99 for Cr and 0.93 for Ni) so, no contamination in these elements can be highlighted. The continuation of work must specify the bioavailability of lead for the ecosystems by using some plants (tomato, broad been, salad and pelargonium). Experiments of microculture in controlled condition (Guivarch and al, 1999) will be carried out in growth chambers on soil of the factory like in fields. They make it possible to evaluate the risks of transfer of the lead of this soil towards the plants. Combining EXAFS results with complementary analyses by Micro-SXRF and non-synchrotron-based techniques (XRD, SEM-EDS, toxicity tests, chemical extractions and DGT) will provide to determine the mechanisms of lead transport and stocking in the plants. The primary benefit of our work will be improvement of the scientific basis for risk assessment and remediation design for this toxic element.

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