EPR characteristics of heat treated complexes of metals with demineralised humic brown coal in air and ammonia atmospheres

Abstract

Investigations were carried out on complexes of metal ions with solvent pre-extracted and demineralised Lubstow humic brown coal (HC) ( Eocene, Midlands Poland; content of humic acids above 80%). Metal–humic brown coal complexes (HC–M) were obtained at pH 5.1 by ion exchange of solid HC with the following metal ions: Mg 2+, Ca 2+, Ba 2+, Ti 3+, Cr 3+, Mn 2+, Fe 3+, Co 2+, Ni 2+, Cu 2+, Ag +1, Zn 2+, Cd 2+, Al 3+, Ga 3+, Tl 3+, Sn 2+, Pb 2+, and Bi 3+. Metal uptake by the HC is dependent on the radius of the metal ion and its susceptibility to complexation in polyfunctionalised humic matrix. The complexes were heat treated to different final temperatures in a range up to 650 °C. Raw HC–M complexes and resultant chars were characterised by elemental composition, IR, and quantitative electron paramagnetic resonance spectroscopy (QEPR). The binding sites of the metal cations are oxygen functional groups (mainly –COOH, –OH phenolic). EPR parameters (i.e., concentration of stable free radicals and g-value) were determined in air and gaseous ammonia atmosphere. The nature of the observed free radicals in raw HC–M complexes is mainly of semiquinone type ( g=2.0036 in air and 2.0045 in gaseous ammonia atmosphere). Heat treatment of the complexes causes gradual defunctionalisation, resulting in elimination of quinone and hydroquinone structural units and generation of polyaromatic sheets containing aromatic type free radicals. Practically complete disappearance of semiquinone radicals occurs at 400 °C heat treatment, and intensive development of aromatic type free radicals occurs above that temperature. Paramagnetic metal ions cause strong decrease of the observed free radical concentration in HC–M complexes due to antiferromagnetic interaction with electrons of semiquinone type radicals (Cu 2+>Fe 3+>Mn 2+>Cr 3+>Ti 3+). The observed concentration of free radicals in the HC–M complexes with alkaline metal ions of s and p block metals as well as d 10 metal ions is higher compared to the value of uncomplexed HC. Gaseous ammonia strongly shifts hydroquinone–semiquinone–quinone equilibria towards generation of semiquinone anion radicals, which is manifested by elevated concentrations of free radicals and increases of g-value in chars of heat treated samples in the region below 400 °C. The control of free radical types and concentrations in environment allows to: (i) predict the state of temporal alkalinity (due to use of ammonia and urea based fertilisers), (ii) binding of heavy metal cations, and (iii) degree of humic matter maturation.