Physicochemical properties of cross-linked poly-γ-glutamic acid and its flocculating activity against kaolin suspension

Cross-linked poly-γ-glutamic acid (C-L γ-PGA) was prepared with γ-PGA irradiated with γ-ray at various kGy values. The physicochemical properties including viscosity and water absorption capacity were compared between C-L γ-PGA and several typical flocculating agents. The viscosity of C-L γ-PGA increased with the dose of γ-irradiation, although the water absorption capacity of C-L γ-PGA did not, which showed a maximum of 1005.6 ml/g at 20 kGy. Flocculating activity against kaolin suspension was not observed for any of the test compounds when used singly. However, the activity of C-L γ-PGA markedly increased following the addition of polyaluminum chloride. The activity increased with temperature up to 80°C and remained at 80°C of heat pretreatment for 1 h, but did not at more than 50°C of heat pretreatment for 24 h. The activity was also observed within a pH range of 4.5–10.0. Both the water absorption capacity and flocculating activity of C-L γ-PGA decreased in parallel with increasing NaCl concentration, suggesting that the flocculating activity of C-L γ-PGA was associated with its water absorption capacity, rather than viscosity. An investigation of the effects of various cations on the flocculating activity of C-L γ-PGA showed that only trivalent cations had a synergistic effect. The mechanism of C-L γ-PGA flocculating activity is discussed based on the results of preliminary experiments.