Photoelectrocatalytic degradation of the antibiotic sulfamethoxazole using TiO2/Ti photoanode

Photoelectrocatalysis (PEC) of sulfamethoxazole (SMZ) was investigated using TiO2/Ti (UV-A) photoanode. Complete photoelectrocatalytic degradation of SMZ was achieved after 70 min at 0.5 V, while SMZ resisted to photolysis and electrolysis. Langmuir-Hinshelwood analysis yielded an adsorption constant (K) of 0.519 L/mg for the studied PEC-TiO2 system, with K being 3–4 times of those from photocatalysis. PEC degradation of SMZ accelerated with increasing anodic potential (0–0.5 V), declining pH (from 10.1 to 2.7), and increasing NaCl concentration (1–100 mM). These trends were consistent with the understanding that higher potential gradient reduces recombination of holes and electrons, lower pH favors photoadsorption through modulation of SMZ speciation and TiO2 surface charge, and higher Cl− level encourages formation of chlorine radicals. Molar ratios of HO and chlorine radicals showed that the effects of pH and Cl− levels were coupled, with the level of PEC activity linked to the relative dominance of [Cl2−] over [HO]. In the presence of radical scavengers, SMZ photoelectrocatalysis was lowered by 30% at merely 10 mg/L of humic acid, and was reduced to <5% of the original rate at 10% methanol. The inhibitory effect of humic acid and methanol on PEC was consistent with the view that HO and Cl2−/active chlorine species were the main oxidants in a Cl−-present environment. A 3-times above-average photocurrent was observed in 10% methanol, whose unusual photocurrent increase was attributed to sequential current doubling – additional electron flow due to reactive adsorbate-hole interaction – through a series of proposed reactions: conversion of hVB+-bound methanol through formaldehyde and formic acid to CO2. Implications of the findings with respect to practical application of PEC are presented.