Tripolyphosphate-assisted electro-Fenton process for coking wastewater treatment at neutral pH

The first application of a novel electro-Fenton (EF) for coking wastewater (CW) treatment at the original pH (6.80) by using tripolyphosphate (TPP) ligand was proposed. Total organic carbon (TOC) decay of CW followed a pseudo-first kinetic rate constant with an apparent rate constant (k_app) of 1.07 × 10^-2 min^-1 for the EF in the presence of TPP (EF/TPP), which was 2.10 times higher than that of conventional EF (k_app = 5.10 × 10^-3 min^-1) working at pH 3. The high efficiency of EF/TPP at neutral pH was mainly attributed to the newly formed Fe-O-P coordination in the iron-ligand compound (Fe²⁺-TPP) supported by UV-absorption spectra results, activating oxygen to produce ^•OH and hence enhancing the oxidation capacity. Key operating parameters of CW mineralization by EF/TPP including Fe²⁺ concentration and pH value were systematically investigated. Excitation-emission matrix (EEM) spectra technique was used to assess the variance of dissolved organic matters during the EF/TPP process. Results showed an 81% mineralization of CW after 3 h electrolysis coupled with a low energy consumption (0.129 kWh g^-1 TOC) which were obtained by the EF/TPP process. Microtox toxicity demonstrated that TPP could reduce the toxicity of raw CW and importantly, it showed that EF/TPP was effective for detoxification. Mechanism study via simulated matrix with similar components as CW revealed that ^•OH produced both from Fenton and Fe²⁺-TPP activation together with the generated active chlorine was responsible for CW mineralization. In summary, the TPP-assisted EF process was presented as a promising technique for extending coking wastewater treatment at near-neutral pH with a high mineralization.