Characterization of the Redox Behaviour of Spruce-Wood Lignin and Humic Acid

The redox behaviour of spruce lignin and humic acid was characterized by their redox potential and redox capacity. Such polyelectrolytes are formed in natural degradation processes including wood decomposition. They influence the redox situation in water of flooded uranium mines.

The first approach to the standard potential using a platinum electrode in combination with a silver-siver chloride electrode (3 M KCl) in a cell with liquid junction [1] was done in two ways: (1) by direct redox potential measurements [2] and (2) by redox titration with potassium hexacyanoferrate(III) [3], both in dependence on the pH. The redox potential of spruce lignin at pH 0 determined in that way was found to be 580±20 mV. That represents a rough E⁰ which can be arranged in the series of redox couples with decreasing E⁰. The reduction capacity determined from the titration at pH 9 amounts to 15±0.5 meq/g. Iron(III) in non-complexing solution (0.1 M KCl) was completely reduced by lignin with a maximum capacity of 6.5±0.8 meq/g in the pH range 2 to 2.5. Humic acids of different origin are characterized in the same way.

In addition, the iron(III) reduction by lignin and humic acid was examined in the pH range 3 to 5 by sqare-wave voltammetric analysis after a four weeks equilibration under inert gas at 25 °C. The iron(III) waves at -1.40 V and -1.60 V disappeared and the typical iron(II) signal was found at -1.33 V (all potentials were measured vs. Ag/AgCl, 3 M KCl). Changing the iron(III)-lignin ratio at pH 4.7±0.2 the maximum iron(III) quantity that can be reduced by a given lignin amount could be determined to 4.0 meq/g. At higher ratios the wave of the unreduced iron(III) was detected at -1.6 V (pH 4). At ratios below 4 meq/g iron(III) is quantitatively reduced and there is a linear relationship between the introduced iron(III) concentration and the intensity of the found iron(II) signal.

The analytical proof of the formed iron(II) was carried out by spectrophotometric determination with o-phenantroline. This sensitive method with a detection limit of 0.01 ppm iron(II) was adapted in such a way that iron(II) could be determined in the presence of iron(III).