Soil Factors Influencing Suspended Sediment Flocculation by Polyacrylamide

Abstract

Turbidity in stormwater runoff may be treated with polyacrylamide (PAM) to flocculate suspended sediment, but the relationships between PAM properties and those of the suspended sediment have not been widely studied. Our objective was to determine how soil physical and chemical properties affected flocculation by PAMs with a variety of characteristics. Subsoil materials were collected from 13 active construction sites around North Carolina. These were tested for flocculation using PAMs with charge densities of 0 to 50% and molecular weights of 14 to 28 Mg mol−1, at concentrations of 0.025 to 10 mg L−1 Soil was added to solutions of single and mixed PAMs with various molecular weights and charge densities and the turbidity was measured 30 s after mixing. Five kaolinitic subsoils had linear responses to PAM regardless of molecular weight or charge density, with an optimal dose of 1 to 2 mg L−1 to obtain >96% reduction in turbidity. Increased turbidity, indicating stabilization, occurred for two additional soils with anionic PAM concentrations >0.5 mg L−1, but a neutral or a mixed anionic PAM product reduced turbidity at those concentrations. The remaining six suboils had widely differing patterns of response, including little or no turbidity reduction with any single anionic PAM. Increasing smectite and vermiculite content (>20%) in suboils from the Coastal Plain reduced the effectiveness of single, anionic PAMs for flocculation. The mixed anionic PAM, however, reduced turbidity in most subsoils and did not have a stabilization reaction at higher concentrations (1–5 mg L−1). Texture, mineralogy, and extractable Fe were highly correlated with reductions in turbidity with PAM, but most of the differences in flocculation occurred in subsoils with 20% or more smectite or vermiculite.