Effect of co-Pollutants thiosulfate and ammonia onto tio2-mediated photocatalytic removal of thiocyanate from synthetic wastewater

Effects of the co-pollutants thiosulfate/S₂O₃ ^2- and NH₄₊/NH₃ (ammonium/ammonia) onto photocatalytic removal of aqueous phase thiocyanate (SCN-) species were investigated employing titanium dioxide (TiO₂)-mediated photocatalytic degradation (PCD) process. Generally, presence of thiosulfate caused reduced thiocyanate PCD as compared to thiocyanate-only systems. Nevertheless, significant thiocyanate degradation was still observed for the respective mixed systems at lower thiosulfate amounts and 6-h reaction time. The thiocyanate/thiosulfate studies at pH 4-10 showed increasing thiocyanate degradation with pH. Furthermore, any notable thiocyanate degradation happened only after thiosulfate removal. Also a significant portion of sulfur originally introduced in the thiocyanate and thiosulfate forms was converted to sulfate. However, the accumulation of cyanide species during pH 4-10 studies, and its insignificant conversion to cyanate, posed a concern. Nevertheless, thiocyanate/thiosulfate systems at pH 12 showed not only significant thiocyanate removal but also cyanate and sulfate as the major reaction byproducts. This trend was noted, even after doubling the concentrations of thiocyanate and thiosulfate. For the thiocyanate/ammonia mixed studies at pH 4-10, significant thiocyanate degradation was observed, but ammonia removal was minimal. However, at pH 12, both thiocyanate and ammonia degradation transpired though ammonia showed a slower and gradual removal trend. Also, cyanate and sulfate were noted to be the dominant intermediates. In summary, when comparing results from both mixed systems, the optimum pH was noted to be 12, which resulted in degradation of thiocyanate, along with conversion of the toxic reaction intermediate cyanide to cyanate.