Environ. Sci. Technol. 2008, 42, 2637–2642
The Effect of pH on Thiosulfate Formation in a Biotechnological Process for the Removal of Hydrogen Sulfide from Gas Streams P I M L . F . V A N D E N B O S C H , * ,† D I M I T R Y Y . S O R O K I N , ‡,§ CEES J. N. BUISMAN,† AND A L B E R T J . H . J A N S S E N †,4 Sub-department of Environmental Technology, Wageningen University, Bomenweg 2, P.O. Box 8129, 6700 EV Wageningen, The Netherlands, Winogradsky Institute of Microbiology RAS, Moscow, Russia, Department of Biotechnology, TU Delft, Delft, The Netherlands, and Shell Global Solutions International. B.V. P.O. Box 38000, 1030 BN Amsterdam, The Netherlands
Received September 28, 2007. Revised manuscript received January 3, 2008. Accepted January 3, 2008.
In a biotechnological process for hydrogen sulfide (H2S) removal from gas streams, operating at natronophilic conditions, formation of thiosulfate (S2O32-) is unfavorable, as it leads to a reduced sulfur production. Thiosulfate formation was studied in gas-lift bioreactors, using natronophilic biomass at [Na+]+[K+] ) 2 mol L-1. The results show that at sulfur producing conditions, selectivity for S2O32- formation mainly depends on the equilibrium between free sulfide (HS-) and polysulfide (Sx2-), which can be controlled via the pH. At pH 8.6, 21% of the total dissolved sulfide is present as Sx2and selectivity for S2O32- formation is 3.9–5.5%. At pH 10, 87% of the total dissolved sulfide is present as Sx2- and 20–22% of the supplied H2S is converted to S2O32-, independent of the H2S loading rate. Based on results of bioreactor experiments and biomass activity tests, a mechanistic model is proposed to describe the relation between S2O32- formation and pH.
Introduction A biotechnological process is developed for hydrogen sulfide (H2S) removal from high pressure natural gas and sour gas streams produced in the petrochemical industry. The process is based on an earlier described technology (1–3), but operates at much higher salt concentrations and alkalinity, using natronophilic sulfide-oxidizing bacteria (SOB). The high carbonate concentration makes it possible to treat gases at high pressure (up to 100 bar) with high partial CO2 concentrations (4). In the process, H2S is removed from the gas stream by an absorber using an alkaline carbonate solution (eqs 1 and 2A,B). The dissolved sulfide (HS-) is subsequently oxidized to elemental sulfur (S0) at oxygen-limiting conditions by natronophilic SOB in a bioreactor, thereby regenerating the hydroxyl- (OH-, eq 3) and carbonate ions (CO32-, eq 4) that are consumed by the absorption of H2S. The produced * Corresponding author e-mail:
[email protected]. † Wageningen University. ‡ Winogradsky Institute of Microbiology RAS. § Department of Biotechnology, TU Delft. 4 Shell Global Solutions International B.V. 10.1021/es7024438 CCC: $40.75
Published on Web 02/27/2008
2008 American Chemical Society
S0 particles are separated from the process in a gravity settler, whereas the regenerated alkaline solution is recycled over the gas absorber. A more detailed description of the process can be found elsewhere (4). Formation of sulfate (SO42-) and thiosulfate (S2O32-) ions decreases the fraction of H2S that is converted to reusable S0 particles, i.e., lowers the selectivity for S0 formation. Other disadvantages of SO42- and S2O32– formation are (I) no OHregeneration takes place, leading to a demand for caustic to control the pH; (II) the O2 demand increases, leading to higher energy consumption; (III) SO42- and S2O32– need to be removed from the process by means of a bleed stream. With this stream also (bi)carbonate ions are removed, leading to an additional caustic demand. In previous papers on biological removal of H2S (2, 5), a decreasing selectivity for S0 formation was attributed to SO42production as a result of complete biological oxidation of HS- (eq 5). However, a recently presented study using natronophilic SOB from soda lake sediments (4), demonstrates that also S2O32- formation is an important cause for a decreased selectivity for S0 formation. It was shown that at pH 10, SO42- formation gradually decreases with increasing total dissolved sulfide (S2-tot) concentrations and even approaches zero at S2-tot of 0.25 mmol L-1. Thiosulfate formation, on the other hand, ranges from 15.3 to 31.7% at O2 limiting conditions. In order to maximize selectivity for S0 formation, more research is needed into processes that lead to S2O32- formation. Laboratory studies show that abiotic oxidation of HS- can result in the formation of S2O32- (eq 6) (6, 7). The rate of HS- oxidation depends on several factors, such as HS- and O2 concentrations, type of buffer used, and the presence of metal ions, which may act as a catalyst (6, 8). Besides abiotic HS- oxidation, abiotic oxidation of polysulfide ions (Sx2-) can result in S2O32- formation (eq 7) (9). Polysulfide ions are formed when biologically produced S0 particles react with HS- (eq 8) (10, 11). For Sx2- originating from biologically produced S0 particles, an equilibrium constant of pK x ) 9.17 ( 0.09 (eq 9) is reported at 35 °C, with an average Sx2- chain length of x ) 4.59 ( 0.31 (12). Previous research (4, 13) has shown that at moderate alkaline conditions (pH 8.5, 0.26 mol L-1 Na+) as well as at natronophilic conditions (pH 10, [Na+]+[K+] ) 2 mol L-1), the Sx2- concentration is close to equilibrium with HS-. It is assumed that S2O32- mainly originates from abiotic Sx2- oxidation because the abiotic Sx2- oxidation rate is higher than that of HS-, even at very low concentrations and in absence of a catalyst (14, 15). In addition to HS- and Sx2- oxidation, also nonoxidative processes result in S2O32- formation. For instance, disproportionation of S0 particles leads HS- and S2O32- formation (eq 10) at elevated temperatures and alkaline conditions (15, 16). This may be especially important when colloidal S0 particles are formed due to abiotic oxidation of Sx2-. It was found that disproportionation of this, so-called, “nascent sulfur” already takes place at 30 °C at pH values above 9 (14). H2S(g) a H2S(aq)
(1)
H2S(aq) + OH- a HS- + H2O 2-
H2S(aq) + CO3 HS- +
-
(2A) -
a HS + HCO3
(2B)
1 O f S0 + OH2 2
(3)
OH- + HCO3- a CO32- + H2O -
HS + 2O2 f SO4
2-
(4)
+
+ H
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(5) 9
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HS- + O2 f
(6)
HS- + (x - 1)S0 a Sx2- + H+
(8)
Values for a and b were obtained by calibration in the bioreactors at each experimental pH value. The calculated S2-tot was used to control the O2 supply rate, using a proportional control law, according to the following equation:
(9)
O2 supply ) (1 + P) · ([S2-tot] - [S2-tot]set) · 0.5H2S supply (12)
+
[Sx ][H ] [HS-]
×
γSx2-γH+ γHS-
with pKx ) 9.17
(10)
Depending on the process conditions, all of the abovementioned reactions can play a role in the formation of S2O32in a process for the removal of H2S from gas streams at natronophilic conditions. It is however not known what the contribution of each individual reaction is. This paper therefore focuses on the effect of pH and Sx2- concentration on S2O32- formation in a H2S oxidizing bioreactor operating at natronophilic conditions, with the overall goal to maximize the selectivity for S0 formation.
Experimental Section Materials. Reactor experiments were conducted in two identical gas-lift bioreactors with a wet volume of 4.8 L each, as described elsewhere (4). The same analytical and side equipment (pH-, redox-, and dissolved oxygen (DO) electrodes, water baths, H2S, O2, and N2 gas and mass flow controllers) was used. The gas flow (300 L h-1) was completely recycled to prevent any release of H2S gas and to reach low O2 concentrations. Nitrogen gas was added to the gas flow when the pressure dropped below atmospheric pressure. In case of pressure build-up, excess gas was discharged via a water-lock. The reactors were operated at 35 ( 1 °C. No biomass support material was applied. Medium. The mineral medium consisted of a mixture of a bicarbonate (pH 8.3) and a carbonate (pH 12.3) buffer. Both buffers contained 0.66 mol L–1 Na+ and 1.34 mol L–1 K+, as carbonates. The final pH of the medium ranged from 8.5 to 10.5. Furthermore, the medium contained (in g L-1 demineralised water): K2HPO4, 1.0; urea, 0.6; NaCl, 6.0; MgCl2 · 6 H2O, 0.20. Trace elements solution was added as described in ref 17. Inoculum. Reactors were inoculated with centrifuged biomass from a S0 producing gas-lift reactor. The original inoculum of this reactor consisted of a mixture of sediments from hypersaline soda lakes in Mongolia, southwestern Siberia and Kenya, obtained from Delft University of Technology. An overview of the physiology of the SOB present in the inoculum is given elsewhere (18, 19). Each fed-batch experiment was started with a biomass fraction taken from a previous experiment. Reactor Operation. Reactors were first filled with medium, whereafter biomass was added. The total reactor liquid volume after inoculation was 4.7 L at 35 °C. After temperature stabilization, addition of H2S was started at a load of 10 mmol h-1, unless stated otherwise. A start-up phase (0–20 h) was applied to allow the biomass to activate and to increase the biomass concentration. During this phase, the reactor was operated with an excess O2 supply, i.e. the DO concentration remained above 70% saturation. After completion of the startup phase, the O2 supply rate was reduced to obtain O2 limiting conditions, while the H2S supply was kept at a constant rate. Selectivity for product formation was determined over a period of at least 48 h of stable reactor operation in fedbatch mode. Fresh medium was only supplied to make up for the sample volume (20–50 mL d-1). O2 Supply Strategy. As shown previously (4), the measured redox potential (ORP) in the reactor can be related to the logarithm of [S2-tot] by the following equation: 9
(11)
(7)
4S0 + 4 OH- f S2O32- + 2HS- + H2O
2638
ORP ) - a · log [S2-tot] - b
1 Sx2- + 1 O2 f S2O32- + (x - 2)S0 2
2-
Kx )
1 1 S O 2- + H2O 2 2 3 2
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With P ) proportional control factor; [S2-tot] ) total sulfide concentration, based on ORP; [S2-tot]set ) total sulfide concentration setpoint. For [S2-tot] e [S2-tot]set, the O2 supply was set to 0.5 times the H2S supply. The value of P was manually set to values between 0.5 and 4. This control strategy turned the reactor into a “sulfido-stat”, maintaining constant S2-tot concentrations slightly above the desired setpoint. Respiration Measurements. Bacterial cells present in reactor samples were separated from extracellular S0 by subsequent steps of low-speed (500 rpm) centrifugation, washed and resuspended in a pure (bi)carbonate buffer (pH 9.5, 2 mol L-1 Na+), at a cell protein density of 30 g L-1. The final protein content in the experiments was 0.065 g L-1 with an initial O2 concentration of 0.150 mmol L-1. Respiration rates with different sulfur substrates (HS-, S2O32-, Sx2-, and S0) were measured at 30 °C in a 5 mL glass chamber mounted on a magnetic stirrer and fitted with a DO electrode (Yellow Springs Instruments, OH). The buffers contained 2 mol L-1 of Na+ and consisted of HEPES + NaCl (pH 7–8); NaHCO3/Na2CO3 + NaCl (pH 8.5); and NaHCO3/Na2CO3 (pH 9–11). Total Sulfide Concentration. Sulfide was measured as total sulfide (S2-tot), being the sum of concentrations of H2S, HS-, S2–, H2Sx, HSx-, and Sx2-. Of these, only HS- and Sx2- are taken into account as the sum of the other species is less than 3.7% of [S2-tot] at pH > 8.5, calculated according to disproportionation constants of inorganic polysulfides as determined by ref 20. Because HS- and Sx2- comprise at least 96.3% of the measured S2-tot concentration, the following equation applies: [S2-tot] ) [HS-] + [Sx2-]
(13)
The method used was based on a modified methylene blue method as described in ref 4. Polysulfide Concentration. The concentration of Sx2– was determined spectrophotometrically as described by ref 12, at a wavelength of 285 nm (Perkin-Elmer, Lambda 2, Norwalk, CT). With this method, the total concentration of zerovalent sulfur atoms in Sx2– is determined: [Sx2--S0] (11, 21). When the average chain length, x, is known, [Sx2–] can be calculated according to (x - 1) · [Sx2-] ) [Sx2- - S0]
with x ) 4.59
(14)
The average chain length of Sx2- produced with biological sulfur, was found to be x ) 4.59 ( 0.31 at 35 °C (12). Before analysis, samples were filtrated over a 0.2 µm cellulose acetate membrane filter (Schleicher & Schuell OE 66, Dassel, Germany). The molar extinction coefficient (1300 L mol–1 cm–1) was determined as described by ref 12, using the high salt medium. In some occasions, the analysis of [Sx2--S0] was affected by unknown compounds with an absorbance at 285 nm. This was found by detection of a residual absorbance after removal of Sx2- by, respectively, oxidation and acidification. These data were however not used in this study. Sulfoxyanions and Sulfur Concentration. Concentrations of SO42– and S2O32– were determined by ion chromatography as described elsewhere (4). Because S0 partly attached to the reactor wall, the S0 concentration in the reactor was not
TABLE 1. Selectivity for Product Formation during Fed-Batch Operation at Different Operational Conditions reactor experiment Nr. (period)
sulfide supply (mmol h-1)
pH
[S2-tot] (mmol L-1) ((0.05)
1, (I, SO42-) 1, (II) 1, (III) 2, (I) 2, (II) 3 4 5 6 7
10 10 100 10 20 10 10 10 10 10
10.1 10 9.9 8.6 8.6 8.7 8.9 9.0 9.5 10.2
