Interactive effects of clay and polyacrylamide properties on flocculation of pure and subsoil clays

Abstract

Turbidity reduction is essential for improving surface water quality. Eroded fine sediments not only increase surface water turbidity, they also carry pollutants such as heavy metals and pathogens. The objectives of this study were to investigate the mechanisms of adsorption and desorption of polyacrylamide (PAM) by pure kaolinite and montmorillonite, and to test the ability of PAM to flocculate the two pure clays as well as four subsoils having distinct clay mineralogy. PAMs with different charge type, density, and molecular weights were used, and a standard flocculation series test and an isotherm adsorption method were employed. The results showed that the flocculating power of PAM for the two sodium (Na)-saturated clays followed the order cationic > non-ionic > anionic. Cationic PAM was the most effective flocculent and anionic PAM the least. The magnitude of PAM adsorption by pure clays was strongly affected by PAM charge, clay charge, and clay surface areas. Once adsorbed, desorption was almost impossible. The flocculation series test with natural subsoils showed that all six PAMs with different charge properties and molecular weights were more effective in flocculating the two kaolinitic loam soils than the two smectitic sand soils. The optimal flocculation concentration range was 1–3 mg L–1. The results also indicated that the presence of cation bridges was essential for anionic PAMs to flocculate montmorillonite-dominant soils effectively. The overall results showed that clay–PAM interactions were complex and dynamic, and such complexity necessitates that the optimal flocculation concentration be varied with clay mineralogy and PAM type.