Comparison of initiation methods in the structure of CPAM and sludge flocs properties

Cationic polyacrylamides (CPAMs) synthesized by thermal, ultrasonic, microwave, and UV initiation were characterized through magnetic resonance hydrogen spectroscopy (¹H NMR), Fourier transform infrared spectra, scanning electron microscopy, and thermal gravimetric analysis. The CPAMs for flocculation and dewatering of alum sludge produced through drinking water treatment were evaluated based on the residual turbidity of the supernatant, dry solid content, mean volume diameter and floc size distribution, fractal dimension of the flocs, and zeta potential as a function of flocculant dosage. Comparisons of the characteristics and performance of CPAMs synthesized through different initiation methods were systematically conducted. Flocculation and dewatering test results demonstrated that CPAMs synthesized through microwave and UV initiation had better flocculation performance and dewatering capability than those synthesized through thermal and ultrasonic initiation. All four CPAMs exhibited a similar final floc size distribution but different mean volume diameters and floc structures. The fractal dimension of the flocs and the zeta potential were in the following order: CPAM3 (microwave initiation) > CPAM4 (UV initiation) > CPAM1 (thermal initiation) > CPAM2 (ultrasonic initiation). Discussions on fractal dimension and zeta potential indicated that the electrostatic patches model and adsorption/bridging effect mechanisms played the main role in the formation of sludge flocs. Lastly, microwave and UV initiation were found to be alternative and recommendable initiation methods for the synthesis of CPAMs with improved flocculation performance and sludge dewatering capability.