ORIGINAL PAPER Fluorescence characterization of metal ion–humic acid interactions in soils amended with composted municipal solid wastes César Plaza & Gennaro Brunetti & Nicola Senesi & Alfredo Polo Received: 3 July 2006 / Revised: 5 September 2006 / Accepted: 7 September 2006 / Published online: 17 October 2006 # Springer-Verlag 2006 Abstract Fluorescence spectroscopy has been used to probe the structural properties and Cu(II), Zn(II), Cd(II), and Pb(II)-binding behavior of humic acid (HA)-like fractions isolated from a municipal solid waste compost (MSWC) and HAs from unamended and MSWC-amended soils. The main feature of the fluorescence spectra, in the form of emission-excitation matrix (EEM) plots, was a broad peak with the maximum centered at an excitation/ emission wavelength pair that was much shorter (340/ 437 nm) for MSWC-HA than for unamended and MSWC- amended soil HAs (455/513 and 455/512 nm, respectively). Fluorescence intensity for MSWC-amended soil HA was less than that for unamended soil HA. These results were indicative of more aromatic ring polycondensation and humification of soil HAs, and of partial incorporation of simple and low-humified components of MSWC-HA into native soil HA, as a result of MSWC amendment. Titrations of HAs with Cu(II), Zn(II), Cd(II), and Pb(II) ions at pH 6 and ionic strength 0.1 mol L -1 resulted in a marked decrease of the fluorescence intensities of untreated HAs. By successfully fitting a single-site fluorescence-quenching model to titration data, the metal ion complexing capacities of each HA and the stability constants of metal ion-HA complexes were obtained. The binding capacities and stability constants of MSWC-HA were smaller than those of the unamended soil HA. Application of MSWC to soil slightly reduced the metal-ion-binding capacities and affinities of soil HAs. Keywords Composted municipal solid wastes . Soil amendment . Humic acids . Metal-complexing capacities and stability constants . Fluorescence spectroscopy Introduction The increasing accumulation in soil of potentially toxic trace metals, including Cu, Zn, Pb, and Cd, because of industrial, mining, and agricultural practices, is a severe hazard to animal and human health [1–4]. Consequently, viable means of treatment of metal-contaminated soils have become a major focus of recent research. Because metals cannot be degraded, they must be either immobilized or removed [5, 6]. Immobilization and removal are, however, complex processes that require understanding of metal behavior in soil. The complexity arises mostly from the dependence of metal behavior on a variety of reactions that may occur in soil, including complexation with organic and inorganic ligands, ion exchange, adsorption and desorption processes, precipitation and dissolution of solids, and acid- base equilibria [7, 8]. Among these, binding reactions to soil organic matter and, especially, to its humified fractions, for example humic acids (HAs), are known to play a key role [9–11]. Land application of municipal solid waste compost (MSWC) is an important alternative to other municipal waste disposal options, for example incineration and land- filling, which cause increasing environmental and econom- Anal Bioanal Chem (2006) 386:2133–2140 DOI 10.1007/s00216-006-0844-0 C. Plaza (*) : A. Polo Centro de Ciencias Medioambientales, Consejo Superior de Investigaciones Científicas, Serrano 115 dpdo., 28006 Madrid, Spain e-mail: c.plaza@ccma.csic.es G. Brunetti : N. Senesi Dipartimento di Biologia e Chimica Agroforestale ed Ambientale, University of Bari, Via Amendola 165/A, 70126 Bari, Italy