Abstract

Hydrogen sulfide is a toxic and usually undesirable by-product of the anaerobic treatment of sulfate-containing wastewater. It can be removed through microaeration, a simple and cost-effective method involving the application of oxygen-limiting conditions (i.e., dissolved oxygen below 0.1 mg L−1). However, the exact transformation pathways of sulfide under microaerobic conditions are still unclear. In this paper, batch experiments were performed to study biochemical and chemical sulfide oxidation under microaerobic conditions. The biochemical experiments were conducted using a strain of Sulfuricurvum kujiense. Under microaerobic conditions, the biochemical sulfide oxidation rate (in mg S L−1 d−1) was approximately 2.5 times faster than the chemical sulfide oxidation rate. Elemental sulfur was the major end-product of both biochemical and chemical sulfide oxidation. During biochemical sulfide oxidation elemental sulfur was in the form of white flakes, while during chemical sulfide oxidation elemental sulfur created a white suspension. Moreover, a mathematical model describing biochemical and chemical sulfide oxidation was developed and calibrated by the experimental results.

INTRODUCTION

Anaerobic treatment of wastewater to convert organic material to biogas, mainly consisting of methane, leads to the simultaneous reduction of sulfate compounds to liquid and gaseous hydrogen sulfide (Ramos et al. 2013). Sulfide in the dissolved form can inhibit methanogenic and acetogenic organisms, may lead to the accumulation of inert material in the sludge (e.g. metal sulfides) and to the deterioration of aerobic post-treatment systems (activated sludge bulking; excessive growth of phototrophs) (Sarti & Zaiat 2011). Gaseous sulfide is toxic, corrosive and flammable and its presence in biogas results in the emission of sulfur dioxide upon combustion (Tang et al. 2009).

Biochemical desulfurization processes are considered to be attractive alternatives to the physical-chemical techniques, because of their lower requirements for energy and chemicals, easy and automated operation, the long life expectancy of system elements, the potential for elemental sulfur recovery and the absence of a solid waste stream (Tang et al. 2009; Díaz et al. 2011; Ramos et al. 2013; Ramos et al. 2014a, 2014b).

Microaeration is a biochemical desulfurization method that is based on the introduction of a small (limited) amount of oxygen into an anaerobic system. This simple sulfide removal technique has already been applied at full scale (Jeníček et al. 2017). Oxygen or air can be dosed directly into the reactor to oxidize sulfide to elemental sulfur, so no additional process units are required (van der Zee et al. 2007; Krayzelova et al. 2014; Krayzelova et al. 2015).

The oxygen availability is the main factor determining the final sulfur products (Janssen et al. 1995). Under oxygen limiting (microaerobic) conditions, i.e. at oxygen concentration below 0.1 mg L−1, sulfur is expected to be the main end product of biological sulfide oxidation (Roosta et al. 2011), with a partial biological oxidation to thiosulfate (van den Ende & van Gemerden 1993). On the other hand, sulfate is the dominant end-product under higher oxygen availability (Roosta et al. 2011). Chemical oxidation gains importance especially in the systems with higher sulfide concentration (Janssen et al. 1995). Under those conditions, biochemical activity may be limited and sulfide is oxidized chemically, mainly to thiosulfate (Janssen et al. 1995; van der Zee et al. 2007).However, information on the relative importance of biochemical and chemical sulfide oxidation under microaerobic conditions (at oxygen concentration below 0.1 mg L−1) and on their final products was not found in literature.

Mathematical models are a helpful tool for process understanding and for the simulation of process performance. Pokorna-Krayzelova et al. (2017) presented a model for microaeration in UASB reactor, including several biochemical pathways for sulfate reduction to sulfide and oxidation of sulfide to elemental sulfur. Further biochemical oxidation of elemental sulfur to sulfate was neglected and so was chemical sulfide oxidation. Roosta et al. (2011) estimated kinetics for biochemical sulfide oxidation in a fed batch reactor at dissolved oxygen (DO) concentrations 0.5–6 mg L−1 and Xu et al. (2013) described the kinetics of biochemical sulfide oxidation under DO concentrations from 0.03 to 0.3 mg L−1. However, in all of these studies chemical sulfide oxidation was neglected. A mathematical model describing combined biochemical and chemical sulfide oxidation under microaerobic conditions has not yet been developed.

This study compared chemical and biochemical oxidation of sulfide under microaerobic conditions (DO below 0.1 mg L−1). Biochemical sulfide oxidation experiments were conducted with a pure culture of Sulfuricurvum kujiense, which under microaerobic condition utilizes sulfide and thiosulfate as an electron donor and oxygen as an electron acceptor (Díaz et al. 2011; Ramos et al. 2014a). Kinetic expressions to describe sulfide oxidation under microaerobic conditions were proposed based on the experimental results.

MATERIALS AND METHODS

Experimental set-up

A batch reactor with a total volume of 2 L was used to study the kinetics of chemical and biochemical sulfide oxidation under microaerobic conditions (Figure 1). The reactor temperature was kept at 35 °C.

Figure 1

The scheme of experimental reactor: 1 - stirring plate, 2 - reactor, 3 - magnetic stirrer, 4 - DO probe, 5 - ORP probe, 6 - nitrogen reservoir, 7 - sampling point, 8 - air pump.

Figure 1

The scheme of experimental reactor: 1 - stirring plate, 2 - reactor, 3 - magnetic stirrer, 4 - DO probe, 5 - ORP probe, 6 - nitrogen reservoir, 7 - sampling point, 8 - air pump.

The experiments were conducted in the cultivation medium MBM 1020 (Kodama & Watanabe 2004). The MBM medium consists of (per 1,000 mL): 0.2 g KH2PO4, 0.2 g NH4Cl, 0.4 g MgCl2.6H2O, 0.2 g KCl, 0.1 g CaCl2.2H2O, 2.5 g Na2S2O3.5H2O, 0.1 mg EDTA, 0.2 mg NaNO3, 0.4 mg FeSO4.7H2O, 0.02 mg ZnSO4.7H2O, 0.006 mg MnCl2.4H2O, 0.06 mg H3BO3, 0.04 mg CoCl2.6H2O, 0.002 mg CuCl2.2H2O, 0.004 mg NiCl2.6H2O, and 0.006 mg Na2MoO4.2H2O.

Prior to each experiment, the medium was sparged with nitrogen gas to decrease the oxygen concentration to less than 0.1 mg L−1. The headspace of the reactor was flushed with nitrogen gas from a nitrogen reservoir to remove oxygen traces. The reactor was sealed and samples for initial sulfide, sulfate and thiosulfate concentrations were taken.

The experiments on biochemical and chemical sulfide oxidation (both in triplicates) were initiated by injecting a 10–15 mL of mixed sulfide and thiosulfate stock solution to obtain the initial sulfide concentration from 8 to 10 mg L−1. This concentration was chosen because it is relevant for anaerobic digesters (Krayzelova et al. 2014; Pokorna-Krayzelova et al. 2018). It was nearly impossible to prepare a sulfide stock solution without thiosulfate being present. Thanks to the instability of sulfide and thiosulfate stock solution, the initial concentration of sulfide and thiosulfate slightly varied. The value of pH was kept at 7 ± 1 using 2 M HCl and 0.1 M NaOH solutions. Oxygen concentration was kept below 0.1 mg L−1 with nitrogen gas and the experiments were stopped when the concentration of oxygen reached that value.

DO concentration, oxidation reduction potential (ORP), pH, and the concentration of sulfide, sulfate and thiosulfate were measured hourly. Sulfide and thiosulfate removal rates were determined as the difference between the initial and final concentration over the measured period of time. The concentration of elemental sulfur formed, (mg S L−1), was calculated as the difference between the initial and final concentrations of sulfide, thiosulfate and sulfate (Equation (1)). All concentrations are in mg S L−1. 
formula
(1)

Biochemical sulfide oxidation

Biochemical sulfide oxidation experiments were conducted with the type strain of Sulfuricurvum kujiense (DSM 16994). This strain was obtained from the German Collection of Microorganisms and Cell Cultures and was cultivated according to provided instructions. S. kujiense is a facultative anaerobic, chemolithotrophic, sulfur oxidizing bacterium, which under microaerobic conditions utilizes sulfide as an electron donor and oxygen as an electron acceptor (Kodama & Watanabe 2004). The experiments were conducted in triplicates.

Chemical sulfide oxidation

Chemical sulfide oxidation was carried out in the absence of bacteria. To prevent the biological activity during chemical sulfide oxidation, MDM 1020 medium solution was autoclaved prior to use and the batch reactor was washed with ethanol and distilled water. Atmospheric oxygen was used for chemical sulfide oxidation. The experiments were conducted in triplicates.

Analytical methods

The DO concentration and the oxidation reduction potential (ORP) were measured by LD0101 probe (Hach Lange Company, Germany); pH was measured with a SensoLyt probe (WTW s.r.o., Czech Republic). The concentration of sulfide, sulfate and thiosulfate were measured with spectrophotometer DR 3900 (Hach Lange Company, Germany) applying the following protocols: APHA, Standard Methods for the Examination of Water and Wastewater (APHA 2012) for sulfide, sulfate was measured based on the barium sulfate method (Horáková 2007) and thiosulfate concentration as in Nor & Tabatabai (1975). The quantification (concentration in μg mL−1) of Sulfuricurvum kujiense was measured by Lowry's method (Waterborg & Matthews 1984).

Sulfur conversion stoichiometry and kinetics

Four main sulfur conversion processes were assumed to take place during biochemical and chemical sulfur oxidation measured in the batch assays: biochemical oxidation of hydrogen sulfide to elemental sulfur by sulfide oxidizing bacteria (SOB) (Table 1, Process 1), biochemical oxidation of elemental sulfur to sulfate by SOB (Table 1, Process 2), chemical oxidation of hydrogen sulfide to thiosulfate (Table 1, Process 3), and thiosulfate disproportionation to elemental sulfur and sulfur dioxide (Table 1, Process 4). The decay of SOB was also incorporated in the model (Table 1, Process 5). The hydrogen sulfide acid-base reaction was considered for reasons of completeness (Table 1, Process 6), even though pH was kept constant in the simulations performed in this study. For each process, the stoichiometric coefficients were calculated from COD and sulfur balances (Table 1). Monod-type equations were used to describe the biological oxidation rates.

Table 1

Stoichiometric matrix Aij and composition matrix for chemical and biochemical sulfide oxidation

Aij Components i → Process rate (ρi.g COD L−1d−1
Processes j ↓          
Uptake of H2S by XSOB   −1     YSOB  
Uptake of S0 by XSOB    −1    YSOB  
Chemical H2S oxidation   −1 0.5   − 1    
S2O32− disproportionation    −1     
Decay of XSOB        −1  
A1 H2S acid-base reaction  −1        
Composition matrix 
g COD per unit 64 64 64 48 16 −32  
mole S per unit  
  Sulfate (mole S L−1Hydrogen sulfide ion (mole S L−1Hydrogen sulfide (mole S L−1Thiosulfate (mole S L−1Elemental sulfur (mole S L−1Sulfur dioxide (mole S L−1Oxygen (mole O2 L−1Composites (g COD L−1SOB degraders (g COD L−1 
Aij Components i → Process rate (ρi.g COD L−1d−1
Processes j ↓          
Uptake of H2S by XSOB   −1     YSOB  
Uptake of S0 by XSOB    −1    YSOB  
Chemical H2S oxidation   −1 0.5   − 1    
S2O32− disproportionation    −1     
Decay of XSOB        −1  
A1 H2S acid-base reaction  −1        
Composition matrix 
g COD per unit 64 64 64 48 16 −32  
mole S per unit  
  Sulfate (mole S L−1Hydrogen sulfide ion (mole S L−1Hydrogen sulfide (mole S L−1Thiosulfate (mole S L−1Elemental sulfur (mole S L−1Sulfur dioxide (mole S L−1Oxygen (mole O2 L−1Composites (g COD L−1SOB degraders (g COD L−1 

The biological conversions were taken up in a model describing the batch reactors, which was implemented in Aquasim 2.0 (Reichert 1998).

Parameter estimation

The maximum H2S uptake rate (km,H2S,SOB), yield (YSOB) and decay rate (kdec) of SOB were determined separately from batch experiments with a pure culture of SOB Sulfuricurvum kujiense (see Supporting information, available with the online version of this paper).

The maximum uptake rate was determined based on the maximum uptake of sulfide by SOB over a period of time. The cultivation media with the excess of sulfide was prepared. Both the concentration of sulfide and SOB were measured regularly during 4 hours (Supporting information, section S.2). The decay rate constant was calculated based on SOB concentration decrease over time. The cultivation media without sulfide was prepared. The decrease of SOB concentration was measured during 6 hours (Supporting information, section S.3). The biomass yield was determined by relating the growth of SOB to the decrease of sulfide concentration over time. The cultivation media with the excess of sulfide was prepared and during 4 hours the increase of SOB and the decrease of sulfide concentrations were measured (Supporting information, section S.4).

All other kinetic parameters were estimated during model calibration (based on the experimental data gained in the presence of biomass) by the least squares method, minimizing the sum of squared errors for all compounds simultaneously.

RESULTS

Biochemical sulfide oxidation

In the presence of biomass, sulfide was oxidized at an average rate of 29.86 mg S L−1 d−1 over the time period of 7 hours. The removal of thiosulfate was 5.25 mg S L−1 d−1 (Figure 2). The concentration of sulfate was stable during the experiments. The oxidation rate of sulfur to sulfate was 0.56 mg S L−1 d−1. During the experiments, slightly yellowish flakes appeared in the medium.

Figure 2

Evolution of sulfur species concentrations in the presence of biomass. The error bars show the standard deviation of the triplicates from the average.

Figure 2

Evolution of sulfur species concentrations in the presence of biomass. The error bars show the standard deviation of the triplicates from the average.

Chemical sulfide oxidation

In the absence of biomass, sulfide was oxidized at an average rate of 12.06 mg S L−1 d−1 over a 7-hour time period. The removal of thiosulfate was 6.83 mg S L−1 d−1 (Figure 3). The concentration of sulfate was stable (7.48 ± 0.44 mg S L−1) during the experiments. The oxidation rate of elemental sulfur to sulfate was only 0.03 mg S L−1 d−1. During the experiments, the colour of the medium changed from colourless to slightly yellowish.

Figure 3

Evolution of sulfur species concentrations in the absence of biomass. The error bars show the standard deviation of the triplicates from the average.

Figure 3

Evolution of sulfur species concentrations in the absence of biomass. The error bars show the standard deviation of the triplicates from the average.

Kinetic parameter estimation

The maximum H2S uptake rate, decay rate, and the yield coefficient were determined by the experiments with the pure culture of Sulfuricurvum kujiense (data in Supporting information, available with the online version of this paper). The maximum H2S uptake rate, km,H2S,SOB, was 482 mmol S mg−1 COD h−1, the decay rate, kdec, was 0.24 h−1 and the yield coefficient, YSOB, was 10.37 mg COD mmol−1 S.

The remaining parameters were estimated by fitting simulated data to the experimental results for biochemical oxidation (Figure 2). Table 2 summarizes the kinetic parameter values obtained.

Table 2

Model parameters and their values as calculated (1) (Supporting information, available with the online version of this paper) or obtained through model calibration (2) (Figure 4)

Parameter Description Unit Value 
α Reaction order with respect to H2– 1.1 (2) 
β Reaction order with respect to O2 – 0.9 (2) 
γ Reaction order with respect to S2O32− – 0.5 (2) 
kdec.xSOB Decay rate h−1 0.24 (1) 
km.H2S.SOB Maximum H2S uptake rate mmol S mg COD−1 h−1 482 (1) 
km.S.SOB Maximum S0 uptake rate mmol S mg COD−1 h−1 0.001 (2) 
Ks.H2S Half saturation constant for H2mmol S L−1 0.001 (2) 
Ks.O2 Half saturation constant for O2 mmol O2 L−1 0.1 (2) 
Ks.S Half saturation constant for S0 mmol S L−1 0.1 (2) 
kH2S.chemox Chemical H2S oxidation rate h−1 0.001 (2) 
kS2O3.disp. S2O32− disproportionation rate h−1 10 (2) 
YSOB Biomass yield mg COD mmol S−1 10.37 (1) 
Parameter Description Unit Value 
α Reaction order with respect to H2– 1.1 (2) 
β Reaction order with respect to O2 – 0.9 (2) 
γ Reaction order with respect to S2O32− – 0.5 (2) 
kdec.xSOB Decay rate h−1 0.24 (1) 
km.H2S.SOB Maximum H2S uptake rate mmol S mg COD−1 h−1 482 (1) 
km.S.SOB Maximum S0 uptake rate mmol S mg COD−1 h−1 0.001 (2) 
Ks.H2S Half saturation constant for H2mmol S L−1 0.001 (2) 
Ks.O2 Half saturation constant for O2 mmol O2 L−1 0.1 (2) 
Ks.S Half saturation constant for S0 mmol S L−1 0.1 (2) 
kH2S.chemox Chemical H2S oxidation rate h−1 0.001 (2) 
kS2O3.disp. S2O32− disproportionation rate h−1 10 (2) 
YSOB Biomass yield mg COD mmol S−1 10.37 (1) 

The simulated concentrations of sulfide, thiosulfate, and DO showed a good fit with the experimentally measured data (Figure 4), corresponding with a root-mean-square error of 0.065 mg S L−1 for DO concentration, 0.192 mg S L−1 for thiosulfate concentration, and 0.738 mg S L−1 for sulfide concentration. The root-mean-square error of sulfate was 1.034 mg S L−1. The total sulfur concentration in the model compare to the experiment was almost the same (21.33 mg S L−1 for experiments compared to 21.34 mg S L−1 for the model at the end).

Figure 4

Model fit to the experimental results for sulfide oxidation in the presence of biomass. Full lines represent simulation results; markers indicate experimental values.

Figure 4

Model fit to the experimental results for sulfide oxidation in the presence of biomass. Full lines represent simulation results; markers indicate experimental values.

DISCUSSION

Biochemical versus chemical sulfide oxidation

Under oxygen limiting (microaerobic) conditions, at oxygen concentrations below 0.1 mg L−1, elemental sulfur was the major end product of both chemical and biochemical sulfide oxidation. The biochemical and chemical sulfide oxidation rates were 29.9 and 12.0 mg S L−1 d−1, respectively. That is, sulfide oxidation in the presence of biomass was about 2.5 times faster. Assuming that the rate of chemical sulfide oxidation was independent of the presence of bacteria, approximately 60% of the elemental sulfur was formed through biochemical oxidation of sulfide and 40% through the chemical pathway. Alcántara et al. (2004) reported that the activity of SOB severely decreased at oxygen to sulfide ratios of 0.15 mmoL O2 mmoL S2− or less. In this study the SOB were active even below the O2/S2− ratio of 0.011 mmoL O2 mmoL S2−.

During biochemical sulfide oxidation 98.4% of elemental sulfur and 1.6% of sulfate was formed; during chemical sulfide oxidation 99.8% of elemental sulfur and 0.2% of sulfate was observed. These differences are likely to fall within the measurement uncertainty range. Munz et al. (2009) observed a slightly lower elemental sulfur formation, namely 91%, for a slightly higher O2/S2− molar ratio, of 0.015.

The thiosulfate disproportionation rate was about the same in the presence or absence of biomass (namely 5.3 and 6.8 mg S L−1 d−1, respectively).

During the experiments, the colour of the medium changed from colourless to slightly yellowish, indicating the formation of elemental sulfur (Chen & Morris 1972). While during the chemical experiments, the sulfur was in the form of a yellowish suspension, in biochemical experiments, yellow flakes appeared in the reactor. This is in accordance with the findings of Janssen et al. (2009) and Kleinjan et al. (2003), who observed the same difference in the properties of biologically produced compared to chemically produced sulfur.

Kinetic parameter estimation

The predictions of the model presented in this paper correlated well with the experimental data. The concentration of DO, sulfide and thiosulfate showed a good fit (Figure 4). The concentration of sulfate was little overestimated in the simulations. However, the lower concentration of sulfate obtained during experiments could be caused by the uncertainties accompanied with the measurements.

The kinetic parameters of chemical sulfide oxidation obtained in this study were compared with literature (Table 3 ). The chemical H2S oxidation rate, kH2S.chemox = 0.06 min−1, was about the same as determined by Wilmot et al. (1988) (0.055 min−1). However, the reaction orders, α and β, were different. The concentration of sulfide was comparable, but the concentration of oxygen was 53 to 207 times lower in the present study. The reaction orders, α and β, in this study (1.1 and 0.9, respectively) were similar to the results of O'Brien & Birkner (1977) (1.02 and 0.8, respectively). However, the chemical H2S oxidation rate was different. Again, it could be caused by the different oxygen concentration (70–367 times lower in this study).

Table 3

The kinetic parameters of chemical sulfide oxidation

kH2S.chemox   c (S2c (O2Reference 
min−1 α β mg S2 L−1 mg O2 L−1 
0.06 1.1 0.9 8.64 <0.096 This paper 
0.57 0.41 0.39 5.12–300 0.1–8.5 Buisman et al. (1990)  
0.055 0.38 0.21 2.88–9.6 5.1–19.8 Wilmot et al. (1988)  
67.6 1.15 0.69 1.6–6.4 19.2 Jolley & Forster (1985)  
1.44 1.02 0.80 0.64–38.7 6.7–35.2 O'Brien & Birkner (1977)  
– 0.81–0.99 0.19–0.16 0–8 0–4.2 Nielsen et al. (2004)  
a 1.34 0.56 1.6–6.4 2.7–5.1 Chen & Morris (1972)  
kH2S.chemox   c (S2c (O2Reference 
min−1 α β mg S2 L−1 mg O2 L−1 
0.06 1.1 0.9 8.64 <0.096 This paper 
0.57 0.41 0.39 5.12–300 0.1–8.5 Buisman et al. (1990)  
0.055 0.38 0.21 2.88–9.6 5.1–19.8 Wilmot et al. (1988)  
67.6 1.15 0.69 1.6–6.4 19.2 Jolley & Forster (1985)  
1.44 1.02 0.80 0.64–38.7 6.7–35.2 O'Brien & Birkner (1977)  
– 0.81–0.99 0.19–0.16 0–8 0–4.2 Nielsen et al. (2004)  
a 1.34 0.56 1.6–6.4 2.7–5.1 Chen & Morris (1972)  

akH2S.chemox depended on the pH value and varied from 11.8 to 16.38 M−1 h−1.

As a direct comparison of kinetic parameter values available in the literature with the ones obtained in this study (Table 3) is hampered by varying experimental conditions, the resulting sulfide oxidation rates were compared instead (Figure 5), for a range of sulfide (1–19 mg S2− L−1, x-axis) and oxygen (0.01–0.20 mg O2 L−1, Figure 5 (a)-(f), mind different units) concentrations.

Figure 5

The comparison of various chemical sulfide oxidation rates for the different concentration of sulfide (1–19 mg S2− L−1) under fixed oxygen concentration (0.01–0.20 mg O2 L−1). (a) for 0.01 mg O2 L−1, (b) for 0.04 mg O2 L−1, (c) for 0.08 mg O2 L−1, (d) for 0.12 mg O2 L−1, (e) for 0.16 mg O2 L−1, (f) for 0.20 mg O2 L−1. Note the different scales.

Figure 5

The comparison of various chemical sulfide oxidation rates for the different concentration of sulfide (1–19 mg S2− L−1) under fixed oxygen concentration (0.01–0.20 mg O2 L−1). (a) for 0.01 mg O2 L−1, (b) for 0.04 mg O2 L−1, (c) for 0.08 mg O2 L−1, (d) for 0.12 mg O2 L−1, (e) for 0.16 mg O2 L−1, (f) for 0.20 mg O2 L−1. Note the different scales.

The dependency of chemical sulfide oxidation rate on the sulfide concentration differs across various literature sources (Figure 5). While for Wilmot et al. (1988) and the present paper the rate was almost independent of the sulfide concentration, for Jolley & Forster (1985) and O'Brien & Birkner (1977) chemical sulfide oxidation rate strongly depended on the concentration of sulfide. In the study of Buisman et al. (1990) it depended on the concentration of oxygen. Higher oxygen concentration implied a lower dependency of the sulfide concentration on the chemical sulfide oxidation rate.

For oxygen concentration, the trend was the same for all sources: the higher the concentration of oxygen, the higher the chemical sulfide oxidation rate. However, the actual values were different. In the present paper, the chemical sulfide oxidation rate ranged between 0.001 and 0.36 mg L−1 min−1. The closest values were obtained by Wilmot et al. (1988) (0.02–0.12 mg L−1 min−1) and Buisman et al. (1990) (0.09–1.02 mg L−1 min−1). Buisman et al. (1990) estimated the kinetic parameters for low oxygen concentration (starting at 0.1 mg O2 L−1). However, the concentration of sulfide was very high (up to 300 mg S2− L−1) compared to the present study. In the case of Wilmot et al. (1988), it was the other way round: the sulfide concentration was similar (up to 9.6 mg S2− L−1), while oxygen was too high (up to 19.8 mg O2 L−1). The experiments of Jolley & Forster (1985) and O'Brien & Birkner (1977) were made with too high oxygen concentration (19.2 and up to 35.2 mg O2 L−1, respectively), resulting in too high chemical sulfide oxidation rate (2.8–658.0 mg L−1 min−1 and 0.04–8.0 mg L−1 min−1, respectively).

Table 4 summarizes the results of the kinetics of biochemical sulfide oxidation to elemental sulfur found in the literature and compares it with this study. It clearly shows that few authors have quantified the kinetics of chemical and biochemical sulfide oxidation in one oxygen-limited system. Moreover, the public data so far are not consistent and more dedicated measurements should still be performed to allow independent calibration of mathematical models.

Table 4

The kinetic parameters of biochemical oxidation of sulfide to elemental sulfur

μSOB [d−1Ks,H2S [mg S2 L−1Ks,O2 [mg O2 L−1YSOB [mg x mg−1 S2Reference 
204.9 0.032 3.2 0.32 (COD) This study 
0.67 11.00 0.0002 0.0900 (x = VSS) Xu et al. (2013)  
8.64 63.68 n.a. 0.0006 (x = ATP) Gadekar et al. (2006)  
n.a. 8.96 n.a. 0.0891 (x = protein) Alcántara et al. (2004)  
7.20 0.32 n.a. 0.0969 (x = protein) De Zwart et al. (1997)  
μSOB [d−1Ks,H2S [mg S2 L−1Ks,O2 [mg O2 L−1YSOB [mg x mg−1 S2Reference 
204.9 0.032 3.2 0.32 (COD) This study 
0.67 11.00 0.0002 0.0900 (x = VSS) Xu et al. (2013)  
8.64 63.68 n.a. 0.0006 (x = ATP) Gadekar et al. (2006)  
n.a. 8.96 n.a. 0.0891 (x = protein) Alcántara et al. (2004)  
7.20 0.32 n.a. 0.0969 (x = protein) De Zwart et al. (1997)  

n.a. – not available.

CONCLUSIONS

  • Under microaerobic conditions (DO below 0.1 mg L−1), elemental sulfur was the major end-product of both biochemical and chemical sulfide oxidation.

  • In the presence of bacteria, approximately 60% of the elemental sulfur was formed through biochemical oxidation of sulfide and 40% through the chemical pathway.

  • The volumetric biochemical sulfide oxidation rate was approximately 2.5 times faster than the chemical sulfide oxidation rate.

  • Flakes of elemental sulfur appeared during biochemical oxidation, while suspended elemental sulfur was formed during chemical oxidation.

ACKNOWLEDGEMENT

This research was financially supported by the specific university research (MSMT No. 20/2017). Lucie Pokorna-Krayzelova received funding for a joint doctorate from Ghent University's Special Research Fund (BOF-01SF2012). Lucie Pokorna-Krayzelova received the Martina Roeselová Memorial Fellowship from Martina Roeselová Foundation.

REFERENCES

REFERENCES
Alcántara
S.
,
Velasco
A.
,
Muñoz
A.
,
Cid
J.
,
Revah
S.
&
Razo-Flores
E.
2004
Hydrogen sulfide oxidation by a microbial consortium in a recirculation reactor system: sulfur formation under oxygen limitation and removal of phenols
.
Environmental Science & Technology
38
(
3
),
918
923
.
APHA
2012
Standard Methods for the Examination of Water and Wastewater
,
22nd edn
.
American Public Health Association (APHA), American Water Works Association (AWWA) & Water Environment Federation (WEF)
,
Washington, DC, USA
.
Buisman
C. J. N.
,
Geraats
B. G.
,
Ijspeert
P.
&
Lettinga
G.
1990
Optimization of sulphur production in a biotechnological sulphide-removing reactor
.
Biotechnology and Bioengineering
35
(
1
),
50
56
.
Chen
K. Y.
&
Morris
J. C.
1972
Kinetics of oxidation of aqueous sulfide by oxygen
.
Environmental Science & Technology
6
(
6
),
529
537
.
De Zwart
J.
,
Sluis
J.
&
Kuenen
J. G.
1997
Competition for dimethyl sulfide and hydrogen sulfide by Methylophaga sulfidovorans and Thiobacillus thioparus T5 in continuous cultures
.
Applied and Environmental Microbiology
63
(
8
),
3318
3322
.
Horáková
M.
2007
Analytika vody (Water Anaytics) University of Chemical Technology Prague, Prague, Czech Republic (in Czech)
.
Janssen
A. J. H.
,
Sleyster
R.
,
Van der Kaa
C.
,
Jochemsen
A.
,
Bontsema
J.
&
Lettinga
G.
1995
Biological sulphide oxidation in a fed-batch reactor
.
Biotechnology and Bioengineering
47
(
3
),
327
333
.
Janssen
A. J.
,
Lens
P. N.
,
Stams
A. J.
,
Plugge
C. M.
,
Sorokin
D. Y.
,
Muyzer
G.
,
Dijkman
H.
,
Van Zessen
E.
,
Luimes
P.
&
Buisman
C. J.
2009
Application of bacteria involved in the biological sulfur cycle for paper mill effluent purification
.
Science of the Total Environment
407
(
4
),
1333
1343
.
Jeníček
P.
,
Horejš
J.
,
Pokorná-Krayzelová
L.
,
Bindzar
J.
&
Bartáček
J.
2017
Simple biogas desulfurization by microaeration – full scale experience
.
Anaerobe
46
,
41
45
.
Jolley
R. A.
&
Forster
C. F.
1985
The kinetics of sulphide oxidation
.
Environmental Technology Letters
6
(
1–11
),
1
10
.
Kleinjan
W.
,
Keizer
A.
&
Janssen
A. H.
2003
Biologically produced sulfur
. In:
Elemental Sulfur and Sulfur-Rich Compounds I
(
Steudel
R.
, ed.).
Springer Berlin Heidelberg
,
Berlin, Germany
,
230
, pp.
167
188
.
Krayzelova
L.
,
Bartacek
J.
,
Kolesarova
N.
&
Jenicek
P.
2014
Microaeration for hydrogen sulfide removal in UASB reactor
.
Bioresource Technology
172
(
0
),
297
302
.
Krayzelova
L.
,
Bartacek
J.
,
Díaz
I.
,
Jeison
D.
,
Volcke
E. P.
&
Jenicek
P.
2015
Microaeration for hydrogen sulfide removal during anaerobic treatment: a review
.
Reviews in Environmental Science and Bio/Technology
14
(
4
),
703
725
.
Nielsen
A. H.
,
Vollertsen
J.
&
Hvitved-Jacobsen
T.
2004
Chemical sulfide oxidation of wastewater–effects of pH and temperature
.
Water Science and Technology
50
(
4
),
185
192
.
O'Brien
D. J.
&
Birkner
F. B.
1977
Kinetics of oxygenation of reduced sulfur species in aqueous solution
.
Environmental Science & Technology
11
(
12
),
1114
1120
.
Pokorna-Krayzelova
L.
,
Mampaey
K. E.
,
Vannecke
T. P. W.
,
Bartacek
J.
,
Jenicek
P.
&
Volcke
E. I. P.
2017
Model-based optimization of microaeration for biogas desulfurization in UASB reactors
.
Biochemical Engineering Journal
125
,
171
179
.
Pokorna-Krayzelova
L.
,
Bartacek
J.
,
Theuri
S. N.
,
Segura Gonzalez
C. A.
,
Prochazka
J.
,
Volcke
E. I. P.
&
Jenicek
P.
2018
Microaeration through a biomembrane for biogas desulfurization: lab-scale and pilot-scale experiences
.
Environmental Science: Water Research & Technology
4
(
8
),
1190
1200
.
Ramos
I.
,
Peña
M.
&
Fdz-Polanco
M.
2014a
Where does the removal of H2S from biogas occur in microaerobic reactors?
Bioresource Technology
166
,
151
157
.
Reichert
P.
1998
AQUASIM 2.0 - User Manual. Computer Program for the Identification and Simulation of Aquatic Systems
.
Swiss Federal Institute for Environmental Science and Technology (EAWAG)
,
Dubendorf
,
Switzerland
.
Roosta
A.
,
Jahanmiri
A.
,
Mowla
D.
&
Niazi
A.
2011
Mathematical modeling of biological sulfide removal in a fed batch bioreactor
.
Biochemical Engineering Journal
58–59
(
0
),
50
56
.
van den Ende
F. P.
&
van Gemerden
H.
1993
Sulfide oxidation under oxygen limitation by a Thiobacillus thioparus isolated from a marine microbial mat
.
FEMS Microbiology Ecology
13
(
1
),
69
77
.
van der Zee
F. P.
,
Villaverde
S.
,
García
P. A.
&
Fdz.-Polanco
F.
2007
Sulfide removal by moderate oxygenation of anaerobic sludge environments
.
Bioresource Technology
98
(
3
),
518
524
.
Waterborg
J. H.
&
Matthews
H. R.
1984
The Lowry method for protein quantitation
.
Methods in Molecular Biology
1
,
1
3
.
Wilmot
P. D.
,
Cadee
K.
,
Katinic
J. J.
&
Kavanagh
B. V.
1988
Kinetics of sulfide oxidation by dissolved oxygen
.
Water Pollution Control Federation
60
(
7
),
1264
1270
.
Xu
X.
,
Chen
C.
,
Lee
D. J.
,
Wang
A.
,
Guo
W.
,
Zhou
X.
,
Guo
H.
,
Yuan
Y.
,
Ren
N.
&
Chang
J. S.
2013
Sulfate-reduction, sulfide-oxidation and elemental sulfur bioreduction process: modeling and experimental validation
.
Bioresource Technology
147
,
202
211
.

Supplementary data