Abstract:

An automated system for the determination of total cyanides and sulfides in wastewater has been developed using flow injection, ultraviolet (UV) photo-dissociation, gas-membrane diffusion, column trapping, ion chromatography separation and pulsed amperometric detection. When the sample was mixed with sulfuric acid and hypophosphorous acid medium containing the appropriate amount of sulfamic acid, ascorbic acid, EDTA and citric acid, metal-cyanide complexes such as Fe(CN)₆^3- can be photo-dissociated by 312 nm UV light, which results in hydrogen cyanide (HCN) and similarly, sulfides release hydrogen sulfide (H₂S). These products were diffused through a 0.45 μm hydrophobic porous polypropylene membrane and were then absorbed in the dilute NaOH solution, concentrated with a Metrosep A PCC 1 HC/4.0 column, separated on an IonPac AS7 column, and finally detected by the pulsed amperometric detector with Ag electrode. The total cyanides and sulfides were good linear in the range of 0.5-1000 μg/L with correlation coefficients of 0.9989 and 0.9997 respectively. The recoveries were 93%-102% and the limits of detection were 0.5 μg/L for total cyanides and 1.0 μg/L for sulfides under the conditions of the sample volume of 100 μL and the signal to noise ratio of 5. The sample throughput of the system was six samples per hour. The results from this new method have been compared with the ones obtained with the standard method, which shows a good agreement.

Key words: flow injection, gas-membrane diffusion, ion chromatography (IC), sulfides, total cyanides, ultraviolet photo-dissociation, wastewaters