Efficient photocatalyst with multi-heterostructures and appropriate structural defects simply from paper mill sludge for photodegradation of bisphenol-S

Exploring photocatalysts with the merits of simple and economical preparation, excellent efficiency of charge separation and ease of scaling up holds paramount importance for photocatalytic degradation of emerging contaminants (ECs). In this pursuit, a biochar-based photocatalyst was fabricated from paper mill sludge (PMS) via a straightforward one-step pyrolysis. Notably, the pyrolysis temperature emerged as a pivotal factor that modulated the phase composition and structural defects (including surface oxygen vacancy and carbon defects) of materials, thus further influencing the band structure, the migration of interfacial photoexcited charges and the efficaciousness of the separation of e⁻/h⁺ pairs. Specifically, the PMS-derived biochar pyrolyzed at 550 °C (SBC550) exhibited excellent photocatalytic degradation efficiency towards bisphenol S (BPS) with 86.5 % of BPS being degraded within 180 min. The optimum performance of SBC550 was attributed to the ingenious and fitting construction of a multi-heterostructure comprising Ca₂Fe₂O₅/Fe₃O₄ embedded within S-doped graphitic carbon (SDC), enhanced electron transfer stemmed from appropriate structural defects as well as the facilitative electron shuttle effect endowed by the porous carbonaceous substrate. Besides, ¹O₂ and h⁺ were confirmed to be the main ROS attributed to the elimination of BPS and its degradation routes of phenoxy radical and electrophilic cycloaddition of ¹O₂ were reasonably deduced based on the HR-MS tests and DFT calculations. Collectively, this work provided crucial and new insights into the utilization of PMS in the domain of photocatalytic strategy for tackling the challenge of emerging pollutants.