Efficient removal of tetracycline by boosting persulfate activation with ball milling biochar derived from iron-rich paper sludge

Excess iron-rich sludge in the paper industry is a type of hazardous solid waste that requires urgent disposal for environmental protection and resource utilization. In this study, the ball-milling iron-rich sludge biochar (Fe-MSDBC) synthesized from paper sludge was used to activate persulfate (PDS) for tetracycline (TC) removal. The results showed that the Fe-MSDBC/PDS system exhibited excellent removal efficiency against various concentrations of TC. Under the lower (20 mg/L) and higher (200 mg/L) initial TC concentration, the TC removal rates were 96.4 % and 86.1 %, respectively. The Fe(II) species and the O-C=O functional group were the main active sites of biochar. TC removal in the Fe-MSDBC/PDS system was mainly contributed by the synergistic effect of adsorption by biochar itself and oxidative degradation by reactive oxidative species, the contributions of which were in the following order: center dot OH > O-1(2) > electron transfer > others. Finally, the degradation pathways of TC were proposed, mainly including demethylation, deamination, hydroxylation, and ring opening. The toxicity assessment demonstrated that most of the intermediates were less toxic than TC. In addition, coexisting ions (CO32-, HCO3-, and PO43-) had an inhibitory effect on the degradation of TC. This research will provide a new recycling route for paper sludge.