Some Aspects of Phosphate Adsorption by Charcoal

As a basis for eventually evaluating the effect of charcoal, formed during forest fires, on the availability of phosphate ions in the soil, the thermodynamics of aqueous phosphate adsorption on six typical synthetic and natural charcoal materials is investigated. A standard adsorption process is defined under the justifiable assumptions that primary orthophosphate is specifically adsorbed, probably through hydrogen bonding; and that the counter‐ion, K + , is nonspecifically adsorbed or oriented in the diffuse double layer. Adsorption isotherm data at 25°, 35° and 45° C. and surface area measurements allow determinations of the changes in free energy, ΔF 0 θ , enthalpy, ΔH 0 θ and entropy, ΔS 0 θ for the standard adsorption process. This process is endothermic for the four most active charcoals. The shapes of the isotherms indicate that at least two types of adsorption sites or processes are involved, at high phosphate concentrations, in all cases. The four most active charcoals show an increase in adsorptive capacity with increasing temperature, associated with large positive values of ΔS 0 θ . X‐ray diffraction patterns show that the four most active charcoals have graphite‐like structures, whereas the other two seem to have, largely, a three‐dimensionally cross‐linked structure. The adsorption of phosphate per unit area increases in the order soil surface, lodgepole pine, duff, B & A Code 1551, Darco S51 and Darco G60 charcoals. Theoretical interpretation of the thermodynamic data indicates that, at least for the four most active charcoals, an extensive dehydration of the phosphate and/or charcoal surface is a distinctive part of the adsorption mechanism.