Selective Photoelectrochemical Response of TiO2 to Wastewater-Associated Organic Molecules: From Model Compounds to Real Effluent Matrices

The detection of organic carbon in wastewater is essential for process monitoring and regulatory assessment. Yet conventional chemical oxygen demand (COD) and total organic carbon (TOC) methods remain reagent-dependent, slow, and unsuitable for inline operation. Photoelectrochemical (PEC) sensing based on TiO2 offers a reagent-free alternative, but its response to wastewater-relevant dissolved organic matter (DOM) and real effluent matrices is still poorly understood. In this study, a TiO2-based PEC system was systematically evaluated using four representative model compounds—glucose, potassium hydrogen phthalate, L-tryptophan, and urea—covering major fractions typically present in municipal wastewater. For the first time, representative wastewater-associated organic compound classes, conductivity effects, and the transferability of the PEC response to real wastewater effluent were systematically investigated. The photocurrent response showed distinct, highly linear concentration–signal relationships for each substance, suggesting a dominant contribution of surface-associated electronic effects. Conductivity variations across a relevant range had no measurable influence on sensitivity or photocurrent magnitude, indicating that the PEC response is not governed by bulk ionic transport but primarily is an interfacial process at the site of TiO2. When applied to real wastewater effluent, the sensor exhibited an excellent linear correlation with dilution level (R2 = 0.9954), demonstrating a linear response within a defined matrix and an LOD of 1.12 mg L−1 COD. For the investigated model compounds, LOD values ranged from 1.06 to 3.00 mg L−1 COD, while a linear response was maintained up to approximately 80–100 mg L−1 COD. These findings establish TiO2-based PEC sensing as a promising platform for the reagent-free, online monitoring of organic loads in wastewater treatment.