Metal ion/surfactant coupled system for activation of persulfate: Sulfate and hydroxy radical based in-situ oxidation of Congo red

The role of metal ions and surfactant in the thermally activated persulfate has attracted extensive attention due to its potential application in the generation of reactive radical species. The presence of cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride (CPC), sodium dodecyl sulfate (SDS), and sodium dodecylbenzenesulfonate (SDBS) inhibited the metal ions assisted activation of S₂O₈²⁻ and decreased Congo red (CR) degradation. The efficiency of heat and metal ions (Ag⁺, Mn²⁺, Fe²⁺, Ce³⁺, and Ru³⁺) for activation was studied to determine the role of activators on the degradation of CR. The oxidative degradation of CR is accelerated by temperature, dose of S₂O₈²⁻, and nature of metal ions. The degradation efficiency was found to be 95.2, 85.5, 80.5, 76.2, 73.5, and 69.1, respectively, for Ag⁺, Fe²⁺, Ru³⁺, Mn²⁺, Ce³⁺, and heat/S₂O₈²⁻ at 30 °C. The standard reduction potentials of Ag⁺/Ag⁰ = 0.799 V, Fe²⁺/Fe⁰ = −0.447 V, Ru³⁺/Ru²⁺ = 0.248 V, Mn²⁺/Mn⁰ = −1.185 V, and Ce³⁺/Ce⁰ = −2.336 V for the redox couples. The degradation rate of CR decreases in the order: Ag⁺ ˃ Fe²⁺ ˃ Ru³⁺ ˃ Mn²⁺ ˃ Ce³⁺ ˃ heat/S₂O₈²⁻ under similar experimental conditions. Triton X-100 (TX-100) has no effect on the S₂O₈²⁻/CR redox system. The polar and nonpolar head group of surfactants was responsible for the incorporation, complex formation, and/or solubilization of S₂O₈²⁻ as well as CR into the reaction site. In summary, metal ions/S₂O₈²⁻ and surfactants/S₂O₈²⁻systems might provide a potentially useful technique for remediation of water contaminants.