Rapid catalytic degradation of refractory textile dyes in Fe(II)/chlorine system at near neutral pH: Radical mechanism involving dichlorine radical anion (Cl2●−)-mediated transformation pathways and impact of environmental matrices

Abstract Owing to the marked stability and resistance to biodegradation of synthetic dyes, their removal from industrial wastewater is a very difficult process. Herein, we investigated the feasibility of Fe(II)/chlorine system to remove synthetic dyes from water. It was found that the presence of chlorine in solution containing 25 µM of Fe(II) at pH 5 accelerated the removal of dyes by more than 60% within 30 s of treatment. Radical scavengers studies using nitrobenzene, benzoic acid, t-butanol and sodium azide as specific probes revealed that Cl 2 ● − plays the dominant role in the oxidative process involving Fe(II)/chlorine system, although other reactive radicals such as ● OH, Cl ● and ClO ● may also be involved in the process. The best performance was achieved at pH 5 and 6, although higher degradation degrees were also obtained at pH 3 and 4. At pH 5 and 6, hydroxyl radicals contributed to about 11-22% in the overall removal process, whereas negligible contribution was noticed at pH 3. The degradation efficiency increased with the increase in Fe(II) and chlorine dosages up to an optimal level and detrimental effects were recorded at high dosages. Chloride addition enhanced the process efficiency whereas sulfate and bromide ions reduced the degradation rate. Besides, drastic inhibition was observed with nitrite and sulfite even at low concentrations. Organic competitors such as humic acids and surfactants didn’t affect the process efficiency. The one exception is that of Adogen 464 surfactant, which showed potential scavenging of Cl 2 ●− radicals. More interestingly, the process efficiency was not affected greatly in real matrices. A reduction of only 2% was remarked in a natural mineral water, 13% in seawater and ∼ 20% in river water. Therefore, the Fe(II)/chlorine process is highly efficient in removing dyes from real matrices in very short time ( i.e. , 30 s).