Polyacrylamide adsorption and aggregate stability

Abstract

The adsorption characteristics of synthetic organic polymers play a major role in their effects on the stabilization of aggregates. We hypothesized that high molecular weight polymers adsorb on both outer and inner surfaces of large aggregates (>1.0 mm), and thus enhance the resistance of aggregates to external forces (e.g., water drop impact). Adsorption of an anionic polyacrylamide (PAM) on aggregates of different sizes from a Worsham (Typic Ochraquults), and a Cecil (Typic Hapludults), Georgia, USA, and the resultant stability of the PAM-treated aggregates to wet sieving, were studied. Aggregates were packed in a small chamber, saturated with a polymer solution, and after 24 h leached with distilled water. PAM adsorption was calculated from carbon content in the leachate. Amount of adsorbed PAM from a 200 g/m 3 polymer solution ranged between 72 and 81 mg/kg in the Cecil; lower amounts (37–56 mg/kg) were obtained in the Worsham. Adsorption efficiency (ratio of the amount of PAM adsorbed to that added) remained unchanged in the Worsham, and decreased moderately in a linear fashion in the Cecil, with increases in aggregate size. Conversely, computed external surface area (ESA) of aggregates of the different size classes decreased hyperbolically with an increase in aggregate size. The significant difference in the pattern of the adsorption and ESA curves may serve as an indication that PAM adsorption could have taken place on both outer and inner aggregate surfaces. Aggregate stability results showed that mean weight diameter (MWD) of PAM-treated intact as well as broken aggregates was higher than that of untreated aggregates. This observation supported the conclusion drawn from the adsorption study, namely that PAM adsorption may not be restricted to outer aggregate surfaces only. It further indicated that the favorable effect of PAM on stabilizing aggregates lasts also after the initial aggregates were broken at plains of weakness.