Monitoring of ORP, pH and DO in heterogeneous Fenton oxidation using nZVI as a catalyst for the treatment of azo-dye textile wastewater

• The monitored ORP, pH, and DO in the nZVI-Fenton showed direct and meaningful correlations to the removal of color and COD.
• The monitored pH can identify the ends of color and COD removals.
• The variation of monitoring DO indicated a proper H2O2 dose.
• The BPN model with monitored ORP, pH, and DO provided very precise prediction results for COD and color removals with the R2 values of 0.96–0.99.

The nanoscale zero valent iron-Fenton (nZVI-Fenton) process combines the advantages of nZVI reduction and Fenton oxidation, was regarded as a very effective process for the treatment of azo-dye/textile wastewater. In this paper, we present the results of the investigation of the on-line monitoring of oxidation–reduction potential (ORP), pH, and dissolved oxygen (DO) to evaluate the effectiveness of the nZVI-Fenton process for the removal of color and chemical oxygen demand (COD) from azo-dyes textile wastewater. The experimental results indicated that the optimal doses of nZVI and H2O2 for color removal were 60–80 mg/L and 300–400 mg/L, respectively. The nZVI reduction removed around 80–90% of the color; however, less than 15% of the COD was removed. To attain the desired removal of COD, much higher doses of nZVI and H2O2 were required, i.e., 200–225 mg/L and 1000–1125 mg/L, respectively. ORP, pH, and DO had direct and meaningful correlations with the removal efficiencies of both color and COD. Thus, regression models and artificial neural networks (ANN) models were used to predict the color and COD removal efficiencies using the monitoring data acquired for ORP, pH, and DO. The ANN model provided very precise prediction results with R2 values in the range of 0.96–0.99. The predictions provided by the regression model had R2 values in the range of 0.92–0.95. These results indicated that on-line ORP, DO, and pH monitoring data can be used as input to either model to obtain a reliable evaluation of the effectiveness of the nZVI-Fenton process for the removal of color and COD from azo-dyes textile wastewater.