DETERMINATION OF POINT OF ZERO CHARGE AND ACID-BASE SUPERFICIAL COAL GROUPS IN WATER

Adsorption isotherms of protons (N(H)) and hydroxyl ions (N(OH)) of different water-dispersed coals, as a function of pH were determined by potentiometry. The points of zero charge (p.z.c.) for the various coals were given; these p.z.c. are located between pH 3.4 (for washed Menouna coal) and pH 9.5 (Chemviron coal). The acid and base surface groups of the different coals were dosed using Boehm's method. The results permitted calculation of the acidity (K(a)) and basicity (K(b)) constants of the different coals and therefore the free energy variations DELTA-G(a)-degrees and DELTA-G(b)-degrees of acid-base surface dissociation. The p.z.c. of the different coals displayed a linear dependency with DELTA-G(A)-degrees and DELTA-G(b)-degrees, which themselves exhibit a linear dependency versus the (SiO2/Al2O3) molar ratio of coal impurities. The effect of these impurities on the surface charge was once again demonstrated.