Physicochemical degradation technology of polyacrylamide

Polyacrylamide can cause long-term damage to soil and aquatic environments through migration and ground adsorption. Furthermore, acrylamide, a degradation product of polyacrylamide, also poses a significant health risk. Therefore, the degradation of polyacrylamide is a critical issue facing humanity, and research into its degradation is essential for protecting the ecological environment and human health.
The degradation technologies and methods of polyacrylamide (PAM) have been extensively studied by scholars both domestically and internationally. Current research findings are mainly divided into two categories: physicochemical degradation methods and biodegradation methods. Among these, physicochemical methods are more widely studied, including chemical oxidation degradation, photocatalytic degradation, photochemical oxidation, mechanical degradation, and thermal degradation. These methods can cause PAM molecules to break down, oxidize, be removed from, or degrade through different pathways.

Physicochemical degradation technology:

(1) Oxidative degradation technology

Oxidative degradation is a free radical transport reaction process that requires the introduction of strong oxidants or other energy sources for catalysis to generate free radicals and initiate a free radical chain reaction. When oxygen is abundant in the solution, oxidative degradation proceeds smoothly, leading to the breakage of the polyacrylamide backbone and a reduction in molecular size. However, a lack of oxygen in the solution can cause the chain degradation process to terminate and make the polyacrylamide molecular chains more susceptible to coupling reactions, forming cross-linked structures. Therefore, the presence of oxygen is one of the key factors in oxidative degradation.
Currently, the most studied oxidants include Fenton’s reagent and potassium permanganate. Through experiments, Gu Xuefan found that the concentration of H₂O₂ affects the degradation efficiency of polyacrylamide, with the optimal ratio of H₂O₂ to polyacrylamide being 1:5. Han Bing used Fenton’s reagent to oxidize excess polyacrylamide in water, finding that pH, the H₂O/COD ratio, the H₂O₂/Fe²⁺ ratio, and oxidation time all affect the degradation effect. Experiments showed that the optimal degradation effect was achieved under the following oxidation conditions: pH=4, H₂O₂/COD=2.5, Fe²⁺/H₂O₂=1/10, and an action time of 90 minutes. However, this method requires significant investment, has high operating costs, and is prone to causing re-pollution.

(2) Photocatalytic degradation and photochemical oxidation

In recent years, photochemical oxidation and photocatalytic degradation have become research hotspots. Yang Haifeng et al. studied the catalytic degradation of polyacrylamide by nano-TiO₂ and found that different concentrations and crystal forms of nano-TiO₂, as well as different concentrations of polymer, affected the photocatalytic effect. Furthermore, Huang Yu et al. investigated the effect of nano-TiO₂ preparation conditions on the degradation effect of polyacrylamide and showed through experiments that under the conditions of calcination temperature of 600℃, pH value of 2, Ti⁴⁺ concentration of 0.2 mol/L, and m(DBS)/m(Ti⁴⁺) = 1:10, the degradation efficiency of polyacrylamide can exceed 80%. It should be noted that although this treatment method has many advantages, its conditions are relatively harsh, requiring specialized equipment and human resources, thus resulting in high costs. Currently, it is still in the research stage.

(3) Mechanical degradation

Mechanical degradation refers to the process of chemically reacting and breaking down polymer chains through the application of mechanical force, thereby achieving degradation. Rho T. et al. experimentally discovered that under high flow rates, polyacrylamide molecular chains break down, generating free radicals that further enhance the degradation effect through free radical transport. The addition of free radical scavengers to the solution demonstrated the generation of free radicals. Furthermore, factors such as polymer concentration, solution oxygen content, solution viscosity, and other impurities in the solution also affect the results of mechanical degradation. Liberatore W et al. experimentally studied the effect of dragging mechanical force on PAM degradation, conducting experiments in flat and wavy flow channels of circulating solutions, utilizing the flowability of polyacrylamide to alter polymer molecules. Within a certain shear range, under turbulent conditions, the degradation rate of PAM can reach 58%. The degradation of polyacrylamide depends on its molecular structure and size, with molecular structure having a greater impact.

(4) Thermal degradation

Under thermal stress, polymer molecular chains break down. Currently, the main methods for studying the thermal degradation of polyacrylamide (PAM) are differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). By observing the weight loss of PAM at different heating rates, its degradation effect and mechanism can be studied. Experiments show that PAM undergoes two degradation processes at temperatures of 326 °C and 410 °C. The first degradation occurs between adjacent amide groups, through the removal of amino groups to form imides. The second degradation occurs through dehydrogenation to produce carbon dioxide.
    Thermogravimetric analysis at different temperatures revealed activation energies of 137.1 kJ/mol and 190.6 kJ/mol for the two PAM cleavage processes. Adding transition metal ions to polyacrylamide strengthened the electrostatic interaction between the metal ions and the polyacrylamide, thus protecting the amide groups and enhancing the stability of PAM. Furthermore, the strength of the electrostatic interaction increased with decreasing ionic radius.