Interactions of Three s-Triazines with Humic Acids of Different Structure GIUSEPPE CELANO, † DANIELA ˇ SMEJKALOV ´ A, ‡ RICCARDO SPACCINI, ‡ AND ALESSANDRO PICCOLO* ,‡,§ Dipartimento di Produzione Vegetale, Universita ` degli Studi della Basilicata, Via Nazario Sauro 85, 85100 Potenza, Italy and Dipartimento di Scienze del Suolo, della Pianta, dell’Ambiente e delle Produzioni Animali, and Centro Interdipartimentale di Risonanza Magnetica Nucleare (CERMANU), Universita ` di Napoli Federico II, Via Universita ` 100, 80055 Portici, Italy The adsorption of three s-triazines (atrazine, terbutylazine, and simazine) on three different humic acids (HAs), before and after acidic hydrolysis, was studied at four diverse pH values from 3 to 4.5. The Freundlich sorption affinity (K f ) and intensity (1/n) of s-triazines were related to the chemical and structural composition of HA and used in a multivariate statistical analysis. At low pH, the sorption values for s-triazines were not directly justified by the content of carboxyl groups in original HA, while only an increase of phenolic carbons in hydrolyzed HA supported the increased s-triazine adsorption. The structural composition of both s-triazines and humic samples explained 86% of the significance in multivariate analyses, whereas the role of pH remained hidden in only 14% of the statistical significance. Adsorption of s-triazines was mainly related to carbon content, hydrophobicity, and aromaticity of HA, thereby implying a predominant binding role of weak dispersive forces. Steric properties explained the larger adsorption affinity of atrazine and terbutylazine with respect to simazine. The occurrence of charge-transfer interactions between atrazine and mainly hydrolyzed HA was further suggested by the increased number of free radicals detected in atrazine-HA complexes at different pH values. Charge-transfer interactions were postulated to occur when HA conformations became progressively controlled by aromatic components. This work indicates that humic matter rich in hydrophobic and aromatic constituents is more likely to adsorb s-triazines and reduce their environmental mobility. KEYWORDS: Humic substances; adsorption isotherms; s-triazines; hydrophobic and charge-transfer mechanisms INTRODUCTION s-Triazines are selective persistent herbicides that are widely investigated due to their still large applications in forestry and pre- and postemergence in agricultural soils (1, 2). Even though these herbicides are now forbidden in some countries, the recalcitrance of s-triazines against chemical and biological degradation has led to their accumulation in the environment (3). As a result of their massive use, s-triazines and their metabolites have been detected at alarmingly high concentrations in soils, groundwaters, rivers, and lakes (4–6). The transport, bioavailability, and fate of herbicides in soils are recognized to be controlled from the major part by soil sorption processes. A fundamental understanding of sorption behavior is therefore important for accurate predictions of environmental load of released organic pollutants and the effective implementation of remedial strategies (7). For this reason, the mechanism of s-triazine sorption by soil organic matter, and in particular by humic substances, is a topic of great interest (8). Despite the number of studies related to the interaction between s-triazines and humic substances (8–11 and references herein), the binding mechanism is still the subject of contro- versial discussions. (i) Ionic interactions, (ii) proton-transfer, (iii) electron-transfer, and (iv) hydrophobic interactions belong to the most discussed interactions attributed to the adsorption capacity of humic substances toward s-triazines. Summaries and contradictions related to these binding mechanisms are reported elsewhere (10–12). Furthermore, it has been suggested that the prevalent type of binding depends on chemical and conforma- tional properties of humic substances because of the differences in their stereochemical nature and thus varying accessibility of specific functional groups on their surface (8, 12). * To whom correspondence should be addressed. E-mail: alpiccol@ unina.it. † Universita ` degli Studi della Basilicata. ‡ Dipartimento di Scienze del Suolo, della Pianta, dell’Ambiente e delle Produzioni Animali, Universita ` di Napoli Federico II. § Centro Interdipartimentale di Risonanza Magnetica Nucleare (CER- MANU), Universita ` di Napoli Federico II. 7360 J. Agric. Food Chem. 2008, 56, 7360–7366 10.1021/jf8008074 CCC: $40.75  2008 American Chemical Society Published on Web 07/26/2008