2,4,5-trichlorophenol degradation using a novel TiO ₂-coated biofilm carrier: Roles of adsorption, photocatalysis, and biodegradation

Intimate coupling of photocatalysis and biodegradation (ICPB) offers potential for degrading biorecalcitrant and toxic organic compounds. This study reports on a novel sponge-type, TiO ₂-coated biofilm carrier that showed significant adherence of TiO ₂ and ability to accumulate biomass in its interior. This carrier was tested for ICPB in a continuous-flow photocatalytic circulating-bed biofilm reactor (PCBBR) to mineralize 2,4,5-trichlorophenol (TCP), which is biorecalcitrant. Four mechanisms possibly acting in ICPB were tested separately: TCP adsorption to the carrier, UV photolysis, UV photocatalysis, and biodegradation by biofilm inside the carrier. The carrier exhibited strong TCP adsorption that followed a Freundlich isotherm with an exponent near 2. Whereas UV photolysis was negligible, photocatalysis produced TCP-degradation products that could be mineralized, and the strong adsorption of TCP to the carrier enhanced biodegradation by relieving toxicity. Validating the ICPB concept, biofilm was protected inside the carriers, although biomass originally on the outer surface of the carriers was eliminated. ICPB significantly lowered the diversity of the bacterial community, but five genera known to biodegrade chlorinated phenols (Ralstonia, Bradyrhizobium, Methylobacterium, Cupriavidus, and Pandoraea) were markedly enriched.