Molecular behaviour of humic acid-like substances during co-composting of olive mill waste and the organic part of municipal solid waste Farid Barje a , Loubna El Fels a , Houda El Hajjouji a , Soumia Amir b , Peter Winterton c , Mohamed Hafidi a, * a Laboratoire d’Ecologie et Environnement, Unité associée au CNRST, URAC 32, Unité associée au CNERS, Faculté des Sciences Semlalia, Université Cadi Ayyad, Boulevard Moulay abdellah, BP 2390 Marrakech, Morocco b Département de Biologie, Faculté Polydisciplinaire, Beni Mellal, Morocco c Université Paul Sabatier-Toulouse III,118 route de Narbonne, 31062 Toulouse cedex, France article info Article history: Received 2 May 2012 Received in revised form 22 June 2012 Accepted 5 July 2012 Available online 9 August 2012 Keywords: Composting Humification Humic acids Aromaticity abstract Humic acids extracted at different stages of co-composting, of olive mill waste with municipal solid waste, were analysed spectroscopically. The findings highlight the contrasting variations in the proportions of C-aromatic and C-carbonyl ( 13 C-NMR) as the H/C atomic ratio changes. The decrease in the infrared FT-IR absorption bands of aliphatic bonds eCHe, eCH 2 e, carboxylic acids, esters, amides C]O, the CeOe bonds of polysaccharides and alcohols with respect to ether bonds CeOeC, was linearly correlated with the H/C ratio. Moreover, the gradual reduction of UV-Visible ratios E212/E280 and E465/ E665 and the increase in the C HA /C FA ratio with increasing humic acid aromaticity, is representative of a remarkable aspect of the restructuring brought, as discussed with respect to the mechanisms of polymerization and rearrangement of humic acid structures found to occur during composting. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The olive mill waste leads to environmental problems in olive oil producing zones. In addition, household refuse also presents a potential source of pollution. Failure to recycle and lack of monitoring of rejection into the environment exacerbates the problem. In order to overcome their negative impact, recycling the waste by composting represents an interesting alternative to transform the wastes into a stabilized product, rich in humic substances. The close correlation between the humified organic matter content of a soil and its fertility stresses the utility of increasing the levels of humic substances and, to know more about them, inves- tigating their chemical properties as the quantity and the structure of humic acids (HA) determines the degree of stabilisation and maturity of organic matter in composted waste (Chai et al., 2007). Many reports indicate that HA are composed of structures produced through physical, chemical and biological trans- formations of organic matter; their origins being quite diverse e plant cover, animal and microbial residues (Gonzalez et al., 2003). They are highly heterogeneous compounds composed of amino acids, hexosamines, aliphatics, ethers, esters, alcohols, poly- saccharides and aromatic compounds such as mono-, di- and tri- methoxy benzene and benzoic acids, related to structures found in lignins such as the degradation products of p-hydroxyphenyl, guaiacyl and syringyl residues (Gonzalez et al., 2003; Amir et al., 2006). All these different structures are interlinked by covalent bonds following polymerisation and condensation reactions. In fact, various authors propose that aromatic monomers bind oxidatively to other structural units in solution, polymerised through free radical mechanisms while being influenced by the physical, chemical and biological properties of the medium (Shu-Yen et al., 1981). Aryl and alkyl ethers and amides have been identified that participate in covalent bonding which leads to greater stabilisation and higher molecular weights for the HA structures (Piccolo et al., 2000; Pang-Hung and Hatcher, 2006). Studies have also shown molecular weight increases following polymerisation of smaller HA subunits (Ouatmane et al., 2000; Ait Baddi et al., 2004; Amir et al., 2006). Numerous studies on the chemical nature of humic substances have put forward various hypotheses on the mechanisms of their formation although the models proposed and the results obtained have not led to a consensus about the final products of the chemical reactions occurring. The aim of the present work was to determine, using spectro- scopic techniques (UV-Visible, 13 C-NMR and FT-IR but also * Corresponding author. Tel./fax: þ212 5 24 43 76 65. E-mail addresses: hafidi.ucam@gmail.com, hafidi@ucam.ac.ma (M. Hafidi). Contents lists available at SciVerse ScienceDirect International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod 0964-8305/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ibiod.2012.07.004 International Biodeterioration & Biodegradation 74 (2012) 17e23