Performance and mechanism of phycocyanin removal from water by low-frequency ultrasound treatment

Ultrasonication pretreatment of raw water with high content of algal cells might cause an increase in dissolved organic nitrogen (DON) and proteins, which must be removed effectively before coagulation. In this study, the efficiency of sonication treatment in removing typical proteins derived from algal cells was investigated by applying ultrasonic waves at 20, 40, 60, 80, and 100 kHz, and the influencing factors and removal mechanism were discussed. The results showed that low-frequency sonication could degrade phycocyanin to some extent, achieving about 95% removal rate after 150 min of sonication. However, excitation emission matrix analysis indicated that ultrasonication could not entirely degrade phycocyanin into inorganic nitrogen, and many proteins and nitrogen-containing organics were found in the samples after sonication. While the total nitrogen concentration remained consistent during the entire sonication process (240 min), the total inorganic nitrogen concentration increased from 0.6 to 1.3 mg/L, indicating that only 33.3% of DON was oxidized into inorganic nitrogen. Nevertheless, sonication could significantly attenuate the interference of phycocyanin in coagulation and enhance coagulation. The mechanical effects and free-radical oxidation resulting from cavitation collapse could be responsible for the degradation of phycocyanin and proteins following sonication. In all, the use of ultrasonic treatment as a posttreatment following sonication to remove algal cells from raw water may not be beneficial.