Preparation of Pb-doped tailings-based nano-TiO₂ and study on photocatalytic degradation of PAM wastewater

As an efficient flocculant, polyacrylamide (PAM) is widely used in water treatment process. The acrylamide monomer (AM) produced by depolymerization of residual PAM has been listed as a class IIA carcinogen, and its treatment is imminent. Flotation tailing (TC) was used as raw material to prepare a modified tailing matrix by alkali excitation. Nano-TiO₂ was loaded onto the modified tailing matrix by hydrothermal method to prepare tailing-based nano-TiO₂ composite (TiO₂/TC) and Pb-doped tailing-based nano-TiO₂ composite (Pb-TiO₂/TC). The photocatalytic degradation properties of TC, modified coal matrix, TiO₂/TC and Pb-TiO₂/TC were analyzed with PAM as the target pollutant. The samples were characterized by XRD, SEM, UV-vis and BET, and the degradation products were analyzed and studied by high performance liquid chromatography-mass spectrometry. The degradation mechanism of PAM was explored. The results show that TC is irregular and the surface is rough; the surface of TiO₂/TC is loose, the pore structure is obvious, and the fine tubular TiO₂ is uniformly coated on the surface of modified tailings. The structure of Pb-TiO₂/TC is more fluffy, and the modified coal matrix is loaded with more tubular TiO₂. The band gap of TiO₂/TC was 3.16 eV, and Pb²⁺ doping reduced the band gap of Pb-TiO₂/TC to 3.08 eV. The specific surface area of TiO₂ is 286.66 m²/g, the specific surface area of TiO₂/TC and Pb-TiO₂/TC are 360.33 m²/g and 358.54 m²/g. The higher specific surface area provides more active sites for the reaction, and the adsorption-catalytic synergistic effect significantly improves the photocatalytic efficiency. The degradation rates of PAM by TC and modified tailing matrix were 3.39% and 4.68%. The degradation rates of polyacrylamide by TiO₂/TC and Pb-TiO₂/TC loaded with TiO₂ increased to 38.92% and 63.87%. After five cycles, the photocatalytic properties of TiO₂/TC and Pb-TiO₂/TC remained basically unchanged. In the degradation process of PAM, the macromolecular PAM breaks into small molecular weight PAM molecules, and further decomposes into NO₃⁻, acrylic acid, acetamide and acetic acid.