Presence of inorganic sulfoxyanions in natural waters
Sulfur is predominantly present as sulfate in aerated waters and as sulfidic sulfur (H2S and HS-) in anaerobic waters undergoing sulfate reduction. However, in addition to sulfate and sulfidic sulfur, natural waters may also contain some combination of the following: bisulfite (HSO3-), sulfite (SO32-), polysulfides (H2-xS
-x), polythionates (S
O62-) and thiosulfate (S2O32-). These species are sometimes collectively referred to as intermediate sulfur species (ISS) because the average oxidation state of sulfur in these species is between that of sulfidic-sulfur (- II) and that of sulfate-sulfur (VI).[1, 2] Except for the polysulfide species all other ISS are sulfoxyanions. On the basis of equilibrium speciation calculations, the concentration of none of the sulfoxyanions is expected to be higher than 0.01% of the total dissolved sulfur concentration, Stot. Hence, if 10-2 molals are taken as a reasonable upper limit for the concentration of total dissolved sulfur in most fresh waters and hydrothermal waters, none of the sulfoxyanions are expected to have concentrations over 1 μM.
However, several studies have reported sulfoxyanion. concentrations well in excess of 1 μM. For example, thiosulfate in three brines collected from the French Dogger Formation ranged in concentration from 100 to 200 μM (Stot ranged from 6.88 to 7.3 mM). Thiosulfate concentrations of 705 to 875 μM were reported for Champagne Pool, New Zealand (Stot = 2.5 × 10-3 M).[5, 6] A survey of twenty-seven Bulgarian hydrothermal waters found thiosulfate concentrations ranging from 5 to 38 μM along with sulfite concentrations ranging from 5 to 20 μM for waters with Stot less than 3100 μM. Thiosulfate concentrations up to 36 μmol L-1 were found in several Italian hot springs with sulfide-bearing waters with a Stot of around 12 mmol L-1. In an extensive survey of the hot springs of Yellowstone National Park, Allen and Day[9, 10] reported thiosulfate concentrations for several alkaline hot-spring waters. For example, a thiosulfate concentration of 45 μM for Ojo Caliente which has a Stot of about 250 μM was reported. Xu et al.[11, 12] determined thiosulfate in about 40 hot-spring waters in Yellowstone National Park. They found elevated sulfoxyanion concentrations in several pools, including a thiosulfate concentration in Azure Spring at about 20 mol% of Stot and tens of μmolar concentrations of polythionate in Cinder Pool.[11, 12]
High polythionate concentrations are often found in acid crater lakes associated with active volcanoes and some acid hot springs. A high total polythionate concentration of 113 μM (average n = 5.5, Stot = 3.1 × 10-3 M) was found in a sample taken from Ketetahi Cauldron, Tongariro National Park, New Zealand. For Ruapehu Crater Lake, New Zealand, Takano et al. reported an extensive survey of polythionate concentrations. Some of the samples contained considerable amounts of polythionates. For example, sample R18F collected at Ruapehu. Crater Lake contained 1.95 mM S4O62-, 2.1 mM S5O62-, and 0.82 mM S6O62-. The total amount of S represented by these three polythionates accounts for 12% of the total dissolved sulfur in this water. There are more studies that report sulfoxyanion concentrations than summarized here, but none of these other studies provide enough data to evaluate the abundance of sulfoxyanions in relation to the total sulfur in these waters.[14, 15]
Hence, there are a number of studies that suggest sulfoxyanions persist at higher concentrations in various types of natural waters than expected based on equilibrium thermodynamics. The occurrence of non-equilibrium concentrations of sulfoxyanions in natural waters is likely to result from sluggish and often incomplete redox reactions involving hydrogen sulfide, sulfur dioxide, or sulfate. The two most important redox processes in which sulfoxyanions form are the oxidation of hydrogen sulfide and the reduction of sulfate. Oxidation of hydrogen sulfide in O2-bearing waters proceeds via the formation of thiosulfate and elemental sulfur as demonstrated in several laboratory studies [6–18] and field studies.[12, 18–21] Oxidation of thiosulfate in the presence of pyrite or other metal sulfides leads to the formation of tetrathionate.[22, 23] This process has been documented to occur in Cinder Pool, Yellowstone National Park. Reduction of sulfate proceeds via microbial pathways [24–27]via poorly understood mechanisms. [28–31] Sulfidic sulfur is typically the main product of microbial sulfate reduction but thiosulfate is also formed in sediments and acts as a link between sulfate reduction and hydrogen sulfide oxidation in sediments.[32, 33] Thermochemical sulfate reduction yields elemental sulfur as well as hydrogen sulfide. Interaction of sulfidic water with elemental sulfur can lead to waters with a high thiosulfate concentration. Incomplete oxidation of SO2 may play a role in the ISS distribution in crater lakes on active volcanoes. [34–36] Releases of magmatic SO2 gas in a crater lake can lead to complex sulfur speciation dominated by sulfate with polythionates as a minor species. Many acid crater lakes are underlain by a pool of molten sulfur which interacts with the crater-lake water to produce a complex sulfur speciation that includes polythionates. Thus, monitoring of ISS in crater lakes may be a viable method to characterize changes in SO2 release, an important indicator of potential volcanic activity.[35, 37]
The presence and persistence of sulfoxyanions in natural waters is potentially of importance in several other processes. Sulfoxyanions may play a role in the transport of metals. Experimental studies suggest that sulfite and thiosulfate form stable complexes with gold and silver. [38–42] Hence, if the concentration of these oxyanions is significant they may account for some of the mobility of gold and silver in natural waters. Sulfoxyanions are also thought to play an important role in the isotopic equilibration of sulfate-sulfur and sulfidic-sulfur. The isotopic equilibration of sulfate-sulfur and sulfidic-sulfur in a hydrothermal system must proceed via a sequence of reactions where a sulfur(VI) species is reduced to a sulfur(- II) species and vice versa. One such scheme involves the formation of thiosulfate. Finally, many microorganisms can use thiosulfate and polythionates as an energy source during oxidation or reduction.[33, 45–47]
Sampling and analysis of dissolved S species in natural waters
Given the geochemical importance of dissolved sulfide, sulfate, and the potential importance of sulfoxyanions in natural water, it is important to be able to determine the sulfur speciation in a wide range of natural solutions. In the last two decades, several studies have ben published presenting analytical methods for the determination of sulfidic sulfur,[48, 49] sulfur oxyanions,[1, 50–52] sulfur oxyanions and sulfide species,[6, 7, 53–56] as well as inorganic S-species and thiols in natural waters and waste waters. Analytical schemes for the determination of complete or nearly complete sulfur speciation in aliquots withdrawn from H2S oxidation experiments have been presented in several studies. [16–18] In addition, several papers have discussed the general problems in sampling and preserving unstable (i.e., volatile or reactive) constituents in natural waters.[58, 59]
The sampling, preservation and analysis of sulfur species in spring waters and hot spring waters presents a major challenge because the discharging water is often far from equilibrium with the atmosphere. Upon discharge, processes such as degassing (e.g., CO2, H2S), mineral precipitation (e.g., BaSO4, CaCO3), and oxidation (H2S) may drastically change the chemical composition, including the sulfur speciation, of the water. Most of these processes proceed even after a sample has been collected and sealed in a bottle. Hence, either the analyses have to be conducted immediately after collection of the sample, which is often impractical, or the sample must be preserved so that later analysis is meaningful. Several studies have evaluated the effect of prolonged storage on preserved, sealed samples.[5, 13] The results by Webster indicate that the thiosulfate concentration obtained after about one hour is the same as for duplicate sealed samples analyzed after 2, 3, 4, 9 and 13 days. However, the H2S content of the duplicates decreased with storage; after 13 days only a small fraction of the initial H2S was recovered. Takano et al. analyzed several duplicate samples for polythionates in which one duplicate had been opened previously for analysis and resealed and the other sample had never been opened. Except for one pair of samples, the result showed that there was no significant difference in polythionate composition and concentration between the duplicates. However, for logistical reasons, neither of the studies were able to evaluate changes on a time scale of minutes to an hour after sampling.
Previous studies using resins to determine dissolved sulfur species
Resins have been used before in the field and laboratory to concentrate dissolved ionic constituents [61–63] and sequester redox species (e.g., As(III)). Resins capable of retaining anions have been used in the past to separate sulfoxyanions, sulfide and sulfate from waste waters and ground waters. [65–69] In all these prior studies, the resin was used to sequester the dissolved species, followed by a sequential elution to allow for the analyses of the different types of sulfur species. In one study an anion exchange resin was used to remove dissolved sulfide from an alkaline groundwater.
Present study objectives
The primary objective in this study was to evaluate the use of anionic resins to sequester sulfur oxyanions from hydrothermal solutions and crater lakes. Field tests and laboratory experiments were conducted to evaluate the technique. In addition the technique was used in the determination of the sulfur species of a stratified lake with high dissolved sulfide and sulfate concentrations. Subsequent field studies in Yellowstone National Park and a field study of the hydrothermal springs in Lassen Volcanic National Park using the technique presented in this paper will be published separately.