Dongsheng Zhao* , Wanli Fan, Zonghua Wang, Fuxiang Tian, Kang Xie, Guicai Liu,* ,  Jiajun Liu, Yuying Li , B. Larry Li. Sustainable cycling and mechanism of Co(II)/Co(IV)/Co(III) with S(IV)  highly triggered by Co(IV) in E/Co(II)/S(IV) for enhanced removal of  reactive brilliant red X-3B. 

Separation and Purification Technology，https://doi.org/10.1016/j.seppur.2024.126997.

Abstract: The massive discharge and inadequate treatment of dye wastewater have led to the extensive accumulation of  azo dyes in natural water bodies, which is a severe threat to human health. In this study, a novel homogeneous  electrochemical system, denoted E/Co(II)/S(IV), was developed by the incorporation of an electric field and  cobalt/sulfite. Notably, the decontamination efficiency of the E/Co(II)/S(IV) system for reactive brilliant red X-  3B (X-3B) was 95.14 % within 10 min, which was accomplished by utilizing only a trace amount of Co(II). The  entire process exhibited an exceptionally low energy consumption of only 0.331 kWh/m3 . Single-factor experiments were conducted to explore the influence of various operational parameters and water matrix components  on the system decontamination efficiency. By combining the chelator EDTA and chemical probe PMSO, the  presence of Co(III) and Co(IV) was detected. The results showed that Co(IV) mainly triggered the chain reaction.  The activation products of S(IV), including SO3•− , SO4•− , SO5•− and HO•, played a dominant role in the degradation  of X-3B, with SO4•− having the most substantial contribution. Under anodic oxidation, Co(II) underwent double electron transfer, leading to the conversion of Co(II) into Co(IV). Subsequently, Co(IV) activated S(IV) via a  single-electron transfer mechanism, initiating a free radical chain reaction. During the reaction, Co(IV) transformed into Co(III) and Co(II), thus achieving the Co(II)/Co(IV)/Co(III) cycle. The generation of oxygen as a  result of the electrochemical process offers a viable solution for addressing the issue of hypoxia stemming from  the rapid activation of S(IV). The active sites in the X-3B molecule were elucidated through density functional  theory predictions, and three degradation pathways for X-3B were identified via mass spectrometry analysis. The  toxicological characteristics of both X-3B and its degradation products were comprehensively assessed using  toxicity estimation software tool and algal culture experiments. This research offers valuable insights into the  refinement of cobalt/sulfite systems and introduces innovative concepts for electrochemical applications, which have broad-reaching implications for dye wastewater purification.

Keywords:  Azo dyes  Sulfites  High-valent cobalt species  Electrochemical oxidation  Active site prediction