Free-Radical Induced Chain Degradation of High-Molecular-Weight Polyacrylamide in a Heterogeneous Electro-Fenton System

A heterogeneous electro-Fenton (EF) process is applied for degrading high-molecular-weight polyacrylamides (PAMs). Degradation efficiencies of the PAM compound with molecular weight of 15 000 kDa at different cathodic potentials are compared in terms of solution viscosity, NH4+–N concentration, chemical oxygen demand (COD), and total organic carbon (TOC). The degradation products of PAM are identified, and anaerobic biodegradability of the products is evaluated. Partial mineralization of PAM is observed in the heterogeneous EF process, producing polymeric products with newly generated ether and ketone groups. As the cathodic potential changes from −0.3 to −1.1 V (vs SCE), the mineralization efficiency of PAM increases from 20.8% to 32.9%. The PAM degradation in the heterogeneous EF process is proposed to follow a novel pathway starting with the cleavage of polymer backbone at the head-to-head linkage and ending with a termination reaction between the carbon-centered radical and oxygen-centered radical. The polymeric products from PAM degradation demonstrate favorable anaerobic biodegradability. Thus, the heterogeneous EF process has great potential as a pretreatment unit before anaerobic digestion for the treatment of nonbiodegradable high-molecular-weight PAMs.