Preservation and simultaneous analysis of relevant soluble sulfur species in sewage samples

An ion chromatographic method for the simultaneous determination of sulfide, sulfite, sulfate and thiosulfate in wastewater samples has been developed. The samples are preserved with a special sulfide anti-oxidant buffer (SAOB) that allows storing the samples for at least four days at 4 DC without significant deterioration of the different sulfur species. The study demonstrates the extreme importance of a careful sample handling and preservation procedure for these samples as oxidation with air and volatilisation of hydrogen sulfide can severely impact upon the accuracy of the analytical results. The benefits of accurately analysing all of the relevant soluble sulfur species in a single, fully automated method outweigh the increased costs of the method compared to traditional spectrophotometric methods. Introduction The wastewater industry worldwide is faced with a major issue due to the corrosion of concrete structures, particularly sewer pipes. In certain areas of the sewer network losses of concrete of up to 2cm per year have been observed. The main source of the corrosion seems to be hydrogen sulfide present or generated in domestic and industrial wastewaters. Under anaerobic conditions, sulfides (ie. a combination of H2S, HSand S2-) are formed from sulfate and organic sulfur compounds by bacterial and chemical reduction. Hydrogen sulfide (H2S) can then be transferred to the gas phase and is oxidised to sulfuric acid in the presence of oxygen by a biofilm growing in the top part of the sewer pipes. The formation of sulfuric acid can reduce the pH to less than 1 in localised areas of the concrete pipe. As concrete is not very stable under low pH conditions, it starts to dissolve, eventually leading to holes in the pipe. The costs to replace damaged concrete pipes is massive, and the disruption to traffic and other aspects of public life in the cities is considerable given the large size of the sewer networks existing in most developed cities worldwide. In order to better understand the different chemical and microbiological processes leading to the sulfide generation in sewers the analysis of the different sulfur species in wastewater has become an important issue. To date, however, aqueous mixtures of oxidised and reduced sulfur species have been difficult to analyse for a variety of reasons. First of all, there are many different sulfur species that can be present in water (Table 1) [1]. Quite a number of them are unstable and can readily react with each other or get oxidised by oxygen present in the water. The rapid oxidation of celiain sulfur species (particularly H2S) is another reason for the difficulties in analysing these species in water samples and extreme care must be taken to avoid contact with air during sampling and to prevent oxidation during sample storage. Furthermore, even though hydrogen sulfide dissolves quite well in water, small concentrations can be transferred readily into the gas phase, particularly if the pH of the solution is low (H2S ~ H+ + HSpKa=6.99). Therefore, correct sampling and sample preservation are critically impoliant for any analysis of sulfur-containing samples (except when only sulfate is present). Caution must be taken at every step to make sure that the composition of the relevant species is not altered. Preservation of these samples is essential if they can not be analysed immediately, with sample deterioration possible within minutes if not stored correctly. Table 1 Sulfur species occurring in aqueous media. The most common species are printed in bold. [1]. 5ulfur species (Y) Name Oxidation pK Oxidation products state Sx0 6 2· X >= 3 polythionates 0, V S20 a 2peroxodisulfate VII 0,0.9 S04 2S20 7 2disulfate VI S04 2504 2sulfate VI 1.98, -3 very stable S20 6 2dithionate V s20 ldisulfite IV S03 2S04 2S03 2sulfite IV 1.89,7.21 S04 2S20 42dithionite III 0.35,2.45 S20 3 2-,S03 2,S04 2S40 6 2tetrathionate 111/2 S04 2Mmx+ (S20 3)y (mx-2y) metal thiosulfate complexes II 520 3 2thiosulfate II 0.6, 1.72 504 25° & 5a elemental sulfur 0 CH3SxCH3 dimethylpoly-sulfide (DMPS) 0,1 RSH sulfhydryl thiols ° SCN thiocyanate ° -1.8 Sx 2X >= 2 polysulfides 0,1So, S20 32H5sulfide, hydrogen sulfide 116.99,12.9 520 3 2-, 503 -,504 2Sulfide is one of the species that is rapidly oxidised by oxygen/air, especially in the presence of heavy metals or when exposed to light with the main product being thiosulfate. The oxidation of sulfite to sulfate seems to be even faster, esp. in acidic conditions andlor in the presence of transition metal ions (e.g. Fe(III) and Cu(lI)) [3]. Therefore any sample preservation method has to either minimise oxidation by eliminating any air or oxygen input or transform the unstable sulfur species to a stable compound by the addition of chemicals. Sulfide, sulfate, thiosulfate and elemental sulfur seem to be the major components in aqueous systems [4]. As elemental sulfur is not soluble in water and remains with the solid fraction present in wastewater is therefore difficult to quantify. On the other hand, there are only very limited conditions that lead to the formation of elemental sulfur, as is further discussed in the results section. Therefore the aim of this study was to determine a suitable sample preservation and analysis method for the concurrent analysis of sulfide, sulfate and thiosulfate in raw wastewater (sewage) samples. Existing Techniques A summary of a range of different analytical techniques for soluble sulfur species is provided in Table 2. The most common method for the analysis of sulfide is the methylene blue test. It is an easy and fast analysis that can be performed in the field using a visual colour comparison. Sulfide reacts with N,N-dimethylphenyl-I ,4-diamine in the presence of catalytic amounts of ferric ions to methylene blue, which absorbs strongly at 66Snm. The main drawbacks of this method are that (1) it only determines one sulfur species and (2) the measurement range is relatively small (O.03-2ppm) and the correlation between absorbance and sulfide concentration is non-linear [5]. The concentrations of the chemicals that can interfere with this method are generally too low in most wastewaters to have a negative effect on the results. The Australian Standard for sulfide analysis [6] describes a spectrophotometric method where sulfide reacts with Bis(DMP)copper(Il) (DMP=2,9-dimethyl-l, 1 O-phenanthroline). The absorbance of the formed dye is measured at 454nm. This method was tested extensively in this study but did not generate any satisfactory results as the slope of the calibration curve was changing over time indicating that the reagents are not stable for very long (less than 24 hours). In addition, spiked wastewater samples did not give the expected results, possibly indicating the presence of interferences from other compounds. In commercial laboratories sulfate is often measured with the turbidimetric method since it is fast and with low-cost [7]. Barium chloride is added to the sample and the turbidity caused by the precipitated barium sulfate is determined. Unfortunately this method is subject to many errors, which could be either positive or negative, leading unreliable measurement of sulfate. In wastewater samples the suspended solids, nitrate, carbonate, sulfide and iron can interfere with the turbidity measurement. The most reliable results for sulfate analysis are obtained by ion chromatography (IC) [8]. This method has the least interferences and has a wide measurement range with a relatively high sensitivity. However, as some of the sulfur species can rapidly convert to sulfate it is most important that the solutions are preserved in order to avoid any compositional changes after the sample collection. Table 2 Companson of different analytica methods. Analyte Method Advantages I Disadvantages Sulfide Thiosulfate Sulfite Sulfate