The effect of persistent free radicals in sludge derived biochar on p-chlorophenol removal

Sewage sludge pyrolysis can effectively dispose of sludge and obtain sludge-derived biochar (SDBC) as an adsorbent for pollutant removal . Recently, persistent free radicals (PFRs), which have also been detected in many types of biochar, have attracted considerable attention for organic pollutant degradation . Sludge collected from a sewage treatment plant was pyrolyzed into SDBC, which contained a large amount of PFRs, and the resulting SDBC was then applied for the removal of p-chlorophenol. An SDBC dosage of 5 g L −1 was applied for treating 5 mg L −1 of p-chlorophenol; the highest removal efficiency of 90% was achieved at pH 3, and 22% of the initial p-chlorophenol was degraded by the SDBC. Hydroxyl free radicals were observed and contributed to the degradation of p-chlorophenol. The spent SDBC was reused five times after regeneration through the desorption of adsorbed p-chlorophenol. The p-chlorophenol removal efficiency remained constant, but the degradation decreased with increasing reuse cycles, suggesting that the p-chlorophenol degradation efficiency was positively correlated with the intensity of PFRs on SDBC. Further modification of the SDBC sample in HNO 3 or NaOH increased the amount of PFRs, and consequently, the degradation of p-chlorophenol under low oxygen conditions, further confirming the crucial role of PFRs in p-chlorophenol degradation. This study provides insights into the application of SDBC, a promising material, for contaminant abatement.