Photoelectrocatalytic chemical oxygen demand analysis using a TiO₂ nanotube array photoanode

Abstract

The chemical oxygen demand (COD) is a widely used parameter to evaluate the quality of water for industrial applications. Currently, the standardized method for COD analysis employs expensive and harmful reagents that require a special treatment for disposal upon use. The photoelectrocatalytic COD detection, based on the photocatalytic activity of a reduced TiO₂ nanotube array photoanode (Ti|NT-TiO₂) under supply of a low bias potential, represents a fast, cheap and eco-friendly alternative to the standard COD method (COD_STD). Here, Ti|NT-TiO₂ was synthesized by the anodization method followed by heat treatment and electrochemical reduction. Potassium hydrogen phthalate, glucose and acetic acid were used as model organic compounds. The photoelectrocatalytic detection of COD (COD_PEC) is based on the photoelectrocatalytic oxidation of target compounds on the surface of the reduced Ti|NT-TiO₂ under UV illumination. Photocurrent transients were recorded using chronocoulometry, and the net charge (Δ<em>q</em>) was plotted as a function of the theoretical COD (COD_TH). A linear relationship was found between these two parameters regardless of the model compound. That relationship was used to determine the COD_PEC for acetylsalicylic acid and Terasil Blue dye solutions at concentrations within the range of 0–15 mg L^-1. A good agreement between COD_PEC and COD_STD was achieved. The limit of detection of the method was 3.6 mg L^-1 COD, with the linear range established from 0 to 50 mg L^-1.