Elemental and spectral properties of peat and soil samples and their respective humic substances Andreia Neves Fernandes a, * , Marcelo Giovanela a , Valdemar Inocêncio Esteves b , Maria Marta de Souza Sierra c a Centro de Ciências Exatas e Tecnologia, Universidade de Caxias do Sul, 95070-560 Caxias do Sul, RS, Brazil b Centro de Estudos do Ambiente e do Mar & Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal c Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, SC, Brazil article info Article history: Received 15 December 2009 Received in revised form 24 February 2010 Accepted 25 February 2010 Available online 3 March 2010 Keywords: Organic matter Humic substances Peat and soil abstract In this study, peat and soil samples, as well as their respective humic substances (HS), were investigated through the techniques of elemental analysis, Fourier transform infrared/attenuated total reflectance (FTIR/ATR) spectroscopy and solid state 13 C nuclear magnetic resonance with cross-polarization/magic angle spinning (CP/MAS 13 C NMR) techniques. The data showed that, particularly in the case of peat, almost all of the signals relating to the presence of typical chemical groups of natural organic material (OM) were found in both the raw samples and in the HS extracted from them, being, in general, better defined in the latter. The combined information reinforces the role of HS as major representatives in nat- ural OM. The differences observed before and after extraction, which were more evident for the soils, were attributed to the strong presence of mineral material and, consequently, low concentration of organic material in the raw samples, weakening and masking the signals usually emitted by OM. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Natural organic material (OM) is at the center of most physical and chemical processes occurring in the environment. In soils, sed- iments and peats, for example, it contributes to the retention of heat, thus stimulating the germination of seeds and the develop- ment of roots. Its high water retention capacity helps to avoid drainage and erosion [1]. Also, OM interacts with metal ions and organic, modifying its fate in natural systems [2–4]. Humic substances (HS) represent around 80% of natural OM [5,6], and thus a better understanding of the structural and func- tional properties of HS is essential in the study of the reactivity of OM as a whole. These materials originate from the degradation and subsequent polymerization of organic residues derived from animals and plants, being a heterogeneous mixture of polydisperse molecules with distinct functional groups, the elemental composi- tion and properties of which vary according to the source of OM and the environment in which it is formed. The high degree of ali- phaticity of marine humus, for example, is the result of the contri- bution of lipids of microbial origin, particularly algae [7]. On the other hand, the abundance of carboxylic and phenolic substituents is correlated with terrestrial inputs, indicating that lignins and, probably, tannins play an important role in the formation of humus in continental environments [8]. Different models have been proposed in the literature in rela- tion to the structure of HS [9–11]. According to Stevenson [8], HS consist of heterogeneous organic macromolecules with molecular structures with characteristics similar to the original material. More recent studies, however, have indicated that HS are formed of aggregates of smaller molecules presenting a type of universal average formula unit [12,13]. In this context, humic acids (HA) would be formed by the association of predominantly hydrophobic compounds, stabilized at neutral pH by dispersive hydrophobic forces, while fulvic acids (FA) would be formed through the associ- ation of small hydrophilic molecules with a sufficient number of functional acid groups to maintain the aggregates disperse at any pH value [12]. Thus, the ideal extractor must completely remove the HS of the matrix without changing their characteristics, in or- der to provide samples representative of those present in the origi- nal material [14], and also be appropriate for use in any matrix [8]. Despite the large variety of HS extraction procedures, such as extraction using chelating agents, cationic exchange resins, organic solvents and aqueous saline solution, extraction using alkaline sol- vents has been the most widely used. Considering these aspects, the International Humic Substances Society (IHSS) has established an extraction procedure based on sample treatment with a strong base (generally 0.1 or 0.5 mol L 1 NaOH) under N 2 atmosphere. This procedure has been contested by some researchers, due to 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.02.069 * Corresponding author. E-mail address: anfernandes@ucs.br (A.N. Fernandes). Journal of Molecular Structure 971 (2010) 33–38 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc