Speciation and mobility of Zn, Cu and Pb in a truck farming soil contaminated by sewage irrigation

Abstract

The forms and mobility of Cu (290 mg/kg), Zn (1103 mg/kg), and Pb (535 mg/kg) in a soil used for intensive truck farming from the Region Ile-de-France, and which has been irrigated for more than a century with sewage water from the city of Paris, have been studied by chemical treatments and advanced x-ray techniques. Elemental associations and forms of metals were determined qualitatively and quantitatively by a combination of physical techniques, including μPIXE (Cu, Zn, Pb), μSXRF (Cu, Zn), μEXAFS (Zn) and powder EXAFS (Zn). The nature, number, and relative proportion of Zn species were obtained by principal component analysis and decomposition of Zn K-edge EXAFS spectra. These data collectively indicate that Cu is associated mainly with organic matter, and that Zn and Pb are bound essentially to mineral constituents. Four Zn-containing species were positively identified: Zn-phosphate and phyllosilicate as dominant species, and Zn-Fe oxyhydroxide and willemite (Zn 2 SiO 4 ) as minor species. Studies are underway to determine the structural forms of Cu and Pb. Depending on the chemical treatment, up to 70% Cu, ∼50% Zn, and ∼85% Pb can be removed in a few days with citrate and EDTA. Citrate is generally at least as efficient as EDTA, and it has the advantage of being easily biodegradable and, hence, less harmful to ecosystems.