Effet du pH et de la Force Ionique sur la Liberation d'Ions Aluminium par des Sols Bruns Acides Limousins - Consequences Sur La Pollution Des Eaux Naturelles pH and Ionic Strength Effect on Release of Aluminium by Limousin Acidic Brown Earth Soils - Impact on Natural Water Pollution

Abstract

The objective of this work was to study the effect of pH and ionic strength on the ability of a forest soil to release aluminium in order to give information on the risk of contamination of natural water neighbouring forests of conifers. The horizons A, B and C, of an acid brown soil, planted with three different forests (Chestnuts (Castanea sativa), young Douglas fir (Pseudotsuga menzesii) and old Douglas fir) were selected and characterised. The effect of the pH (pH 0.5 to 9.0) and of the ionic strength (setting by NaCl or CaCl2, 0 to 1.71 M) on the release of aluminium monomer ions was investigated using batch reactor experiments. For moderately acid pH the released aluminium concentrations are related to the buffer value of the horizon, at very acid pH in addition to the mobile form of aluminium a part of the aluminium of the soil crystal structure particles is released. With the weak ionic forces, the solutions of CaCl2 involve released aluminium concentration more significantly than do the solutions of NaCl in the majority of the studied concentrations. For the horizons B and C, starting from 0.2 M this difference attenuates and could be reversed for strong ionic strength (1.7 M). The aluminium concentrations released under the effect of the ionic strength are related to the speciation of aluminium in the soils: released aluminium comes from the fractions of exchangeable aluminium for the horizons A, B, and C. But for A horizons a part comes from the fraction bound to the organic matter. With the weak ionic strength (0.017 M), the released aluminium rates are a function of the content of exchangeable cations of the horizon. A model according to the results of the literature, based on a reaction of exchange between protons, Na+ ions or Ca2+ ions in solution and the aluminium ions of the soil was developed, in order to obtain a conditional constant of desorption and an apparent number of exchanged ions. The conditional constants of desorption represents the effect of the two physicochemical factors well, but for high values: pH acid (from 3) and strong ionic strength (starting from 0.2 M). This study shows the importance of the ionic composition of water, which is in contact with soil. Acid or mineral-bearing water can involve strong aluminium releases and thus pollute the rivers neighbouring conifers forest or not very deep underground water by aluminium.