Determination of points of zero charge of natural and treated adsorbents

Although particle size and its measurement are intuitively familiar to particle technologists, the concept of point of zero charge ( pzc) is less widely understood and applied. This is unfortunate since it is at least as fundamentally important as particle size in determining the behavior of particulate materials, especially those with sizes in the colloidal range below a micrometer. pzc is related to the charge on the surface of the particle and strongly depends on the pH of the material; so it influences a wide range of properties of colloidal materials, such as their stability, interaction with electrolytes, suspension rheology, and ion exchange capacity. The pH dependence of surface charges was quantified for four different adsorbent - aqueous solution interfaces. The points of zero charge were determined for activated charcoal, granite sand, lakhra coal, and ground corn cob materials using three methods: ( 1) the pH drift method, measuring pH where the adsorbent behaves as a neutral specie; ( 2) potentiometric titration, measuring the adsorption of H+ and OH- on surfaces in solutions of varying ionic strengths; ( 3) direct assessment of the surface charge via nonspecific ion adsorption as a function of pH. The intrinsic acidity constants for acid and base equilibria, pK(a1)(s) and pK(a2)(s), were also calculated. Lakhra coal was found to have the lowest pzc value among all other adsorbents studied owing to the presence of a large amount of humus material. The results were used to explain general connections among points of zero charges, cation exchange capacity, and base saturation % of adsorbents.