The removal of Cu (II), Cd (II), Ni (II) and Pb (II) from dilute aqueous solution by a poly(acrylic acid) flocculant and its cross-linked analogue

The possible removal of copper (II), cadmium (II), nickel (II) and lead (II) from dilute aqueous solution by complexation with poly(acrylic acids) was investigated. The binding properties of two hydrosoluble poly(acrylic acids) of high molecular weight (which make them representative of synthetic flocculants used in the water treatment field; Mw = 2.5 × 10⁵ and 3 × 10⁶ g mol^-1) and of an insoluble cross-linked analogue were examined. Potentiometric titrations were carried out, first, to determine the acid-base properties of both polymers and, secondly, to determine the stability constants of the MA and MA₂ complex species formed. Metal complexation was also studied using differential pulse polarography. The complexing capacity of each polyacid for the metal ions under study was determined at pH 6.0, 5.0 and 4.0 and the ionic strength was 0.1 or 1.0 mol l^-1. The conditional stability constants were estimated assuming that only 1:1 complex species were formed. The results obtained by both methods agree and are complementary. It appeared that stability constants increased in the following order, depending on the metal ion : Ni (II) << Cd (II) < Cu (II) < Pb (II). The hydrosoluble polyacid was able to fix up to 6.3 mmol Pb (II)/g while the insoluble cross-linked analogue was able to fix up to 4.8 mmol Pb (II)/g.