Shape stability control of a 3D-printed clay/biochar-based monolith to support Fe catalyst for continuous levofloxacin degradation with low metal leaching

Abstract

Disposing of wet solid waste sludge has become a prominent issue because it has high environmental risks. Herein, a clay/biochar-based monolith (CBM) is prepared using a three-dimensional (3D) printing design method. The CBM is loaded with iron (Fe) and used as a catalyst (Fe/CBM) to activate peroxymonosulfate (PMS) for degrading levofloxacin (LVF) in an aqueous solution. The monolith preparation process is inexpensive, and the monolith has designable flow paths that facilitate improved mass transport and material recycling. ZnCl2, as a chemical activator, is added to the 3D-printed slurry to improve the catalytic performance of Fe/CBM. Oscillatory stress sweep and frequency sweep tests prove that incorporating ZnCl2-containing sludge in the slurry effectively increases the elastic modulus and critical stress, improving structural stability and constructability. The slurry containing 10% sludge (S10%-Zn with 1 M ZnCl2), exhibits 30% higher static yield stress than the slurry containing only clay and water (S0%). Fe/CBM is prepared by loading Fe active sites onto the surface of CBM via an impregnation method, and it leaches very low amounts of Fe (<0.2 mg/L) and other heavy metals (e.g., Cd, and Pd, etc). Fe/CBM-S10%-Zn/PMS with 10% mass fraction of sludge exhibits much higher (∼80% at 20 min) degradation efficiency than Fe/CBM-S0%/PMS (∼15% at 20 min) and Fe/CBM-S10%/PMS (∼43% at 20 min). Combining with XPS results, when ZnCl2 is added to the slurry, the content of Fe2+ in the catalytic material increases, improving the catalytic performance. The PMS activation mechanism of Fe/CBM-S10%-Zn for LVF degradation is also discussed, which is ascribed to the radical and nonradical pathways. This provides new insights on how to recycle solid waste and make high-performance clay/biochar-based monolith catalysts for the continuous wastewater treatment.