Photosensitized oxidation of cyanide and sulfide species in aqueous solution

Two known photosensitizers, methylene blue (MB) and riboflavin (RF), were studied for application in the treatment of cyanide and sulfide bearing wastewaters. Experimental variables considered in the study included sensitizer concentration, initial substrate concentration, reaction pH, light intensity and dissolved oxygen concentration.; Both RF and MB were found to be effective in sensitizing the oxidation of dissolved sulfide species {dollar}rm(Hsb2S, HSsp-, Ssp{lcub}2-{rcub}).{dollar} The primary oxidation product was observed to be sulfate for both RF- and MB-sensitized reactions. Sulfite was observed as an intermediate product that was quickly oxidized to sulfate. MB was approximately three times faster than RF in providing 95% removal of dissolved sulfide {dollar}rm(Ssb{lcub}o{rcub}=100{dollar} mg/L). Experiments conducted to build a mass balance on sulfur in the photooxidation reaction provided 90-95% closure by measuring the amount of sulfate produced. Experiments performed under artificial lighting within the laboratory ({dollar}sim{dollar}30 W/m{dollar}sp2){dollar} exhibited the same sulfide removal rates as outdoor experiments with impinging irradiation on the order of 300-800 W/m{dollar}sp2,{dollar} thus exhibiting minimal dependence on light intensity once a threshold intensity had been attained. A Michaelis-Menten type rate equation was derived based on experimental data and used to predict sulfide removal rates during subsequent experiments.; Dye sensitized photooxidation processes involving RF and MB as sensitizers were less effective on free cyanide ion. Both RF and MB were studied at varying initial concentrations over the pH range 7-12 on solutions containing initial cyanide concentrations of 5 mg/L. MB was observed to be more effective than RF, with removal increasing with increasing MB dosage. First-order rate constants reached a maximum at pH 9.9 with MB dosed (8 mg/L) reactors, which exhibited greater than 95% degradation of free cyanide in 8-12 hours, while cyanide degradation was minimal in RF dosed reactors.