Treatment of Odorous Sulphur Compounds by Chemical Scrubbing

but nowadays, chemical scrubbing in packed towers remains one of the most used ... Scrubbing solutions employed are an acid solution in the first stag...
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Environ. Sci. Technol. 2006, 40, 7881-7885

Treatment of Odorous Sulphur Compounds by Chemical Scrubbing with Hydrogen Peroxide Stabilisation of the Scrubbing Solution ISABELLE CHARRON,† A N N A B E L L E C O U V E R T , * ,‡ ALAIN LAPLANCHE,‡ CHRISTOPHE RENNER,† L U C I E P A T R I A , † A N D B E N O ˆI T R E Q U I E M E § Anjou RecherchesVeolia Environnement Chemin de la digue BP 76 - 78 603 Maisons Laffitte, France, Sciences Chimiques de RennessUMR CNRS 6226 URI/ENSCR/INSAsEquipe Chimie et Inge´nierie des Proce´de´ssENSCR, Avenue du Ge´ne´ral Leclerc - 35 700 Rennes, France, and AtofinasCentre de recherche Rhoˆne-Alpes - 69 493 Pierre-Be´nite, France

To slow down the hydrogen peroxide decomposition in basic aqueous conditions, the addition of stabilizers and costabilizers in the scrubbing solution was investigated. Results found with sodium silicate (Na2SiO3) were quite promising but several problems still remained. Based on these observations, this study focused on the research of a better stabilizer. Several ways were investigated: the use of silicate solutions employed in pulp industries, the addition of co-stabilizers to sodium silicate, or the use of an another stabilizer (the poly-R-hydroxyacrylic acid). Experiments revealed that the poly-R-hydroxyacrylic acid is the best stabilizing compound.

Introduction Industrial development has led some plants to come closer to living areas, or people to come closer to industrial sites. Some of these plants represent a real problem because of the odors that they generate, and public authorities have received more and more complaints from the neighboring population in recent years. When located near residential areas, wastewater treatment plants (WWTP) can be responsible for such unpleasant odor emissions. Thus, odor treatment has become essential. Several technologies have been designed in order to remove odorous pollutants from air (thermal oxidation, activated carbon adsorption, chemical or biological absorption/oxidation, etc.), but nowadays, chemical scrubbing in packed towers remains one of the most used and effective process in the case of wastewater treatment plants. Conventional processes are performed in a two or three stage scrubber with the purpose to remove the odorous compounds which are mainly nitrogenous and sulfurous compounds. Scrubbing solutions employed are an acid solution in the first stage and an alkaline hypochlorite sodium solution in the second and third stages. Up to now, this process has * Corresponding author phone: 33.2.23.23.80.48; fax: 33.2.23.23.81.20; e-mail: [email protected]. † Anjou RecherchesVeolia Environnement Chemin de la digue. ‡ Sciences Chimiques de Rennes. § AtofinasCentre de recherche Rho ˆ ne-Alpes. 10.1021/es060414d CCC: $33.50 Published on Web 10/21/2006

 2006 American Chemical Society

FIGURE 1. Hydrogen peroxide radical decomposition mechanism. proved its efficiency. Unfortunately, the use of chlorinate solutions is today known to lead to the formation of chlorinated byproducts that can be both odorous and potentially harmful for human health. One solution to this problem could be to replace chlorine with another oxidant compound, which would not generate secondary pollution. This oxidizing agent could be ozone, which is well-known for its high oxidation potential, but also the hydrogen peroxide (H2O2) which has been shown to be able to oxidize the sulfurous odorous pollutants without producing unwanted oxidation byproducts (1). The combination of both oxidants has also been tested (2), just as the combination of hydrogen peroxide with a catalyst (3). Experiments conducted on a pilot unit (4) clearly demonstrated the H2O2 ability to treat the hydrogen sulfide (H2S) pollution in spite of an important consumption of reactants due to high decomposition rates of H2O2 (more than 40 mol‚L-1 for pH 9.5-11, without any stabilizer). H2O2 is, in fact, well-known to be unstable in basic aqueous solution and to change into oxygen and water. Several mechanisms have been discussed in the literature to explain this decomposition according to the purity of the solutions. In the total absence of metal and impurity, two molecular mechanisms are commonly proposed (5): one (1) based on a reaction between a perhydroxyle ion and an hydrogen peroxide molecule, which presents both properties of an oxidizing and a reducing agent, and a second one (2) based on the association of an hydrogen peroxide molecule with a perhydroxyle ion to form a transitory six atom-cycle.

H2O2 + HO2- f H2O + O2 + OH-

(1)

In the presence of non-purified solutions, the hydrogen peroxide decomposition can also be explained by a radical mechanism initiated by traces of metals present in the solution. Reactions involved in this mechanism, after initiation, are described by Buxton et al. (6) and summarized in Figure 1. In 2001, Ni et al. (7) also the manganese-induced decomposition of hydrogen peroxide. Experiments proved that the molecular H2O2 decomposition can be neglected compared to its radical decomposition. Indeed, lots of physical and chemical parameters promote this last one: UV light and transition metals as initiators of radical mechanisms, a high temperature by increasing the VOL. 40, NO. 24, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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reaction kinetic constants, a basic pH (8, 9) and the presence of hydrogeno-carbonates and carbonates ions which react in free radical reactions according to the following equations (10):

HCO3- + OH• f CO3•- + H2O

k3 ) 1.5 × 10 7 L‚mol-1‚s-1 (3)

CO32- + OH• f CO3•- + OH-

k4 ) 2.0 × 10 8 L‚mol-1‚s-1 (4)

•-

CO3

-

•

+ H2O2 f HCO3 + HO2

k5 ) 8.0 × 10 5 L‚mol-1‚s-1 (5)

CO3•- + CO3•- f CO42- + CO2

k6 ) 7.5 × 10 6 L‚mol-1‚s-1 (6)

During a basic oxidant scrubbing, most of these parameters are unfortunately combined: a basic pH in order to enhance the acid pollutant transfer, a high carbonate concentration due to carbon dioxide absorption, and some traces of transition metal coming from various parameters (dust, water, chemical reactants, etc.). Temperature can also vary, according to operating conditions. Therefore, the aim of this work was to stabilize hydrogen peroxide, in order to maintain an acceptable concentration in the scrubbing solution. Several studies have been carried out in pulp domain (these factories commonly use H2O2 in their bleaching processes) and the efficiency of many compounds has been investigated (11, 12, 13) in order to reduce the hydrogen peroxide decomposition without inhibiting its reactivity. According to Kutney (12), stabilizers and co-stabilizers can have different actions: they can either disrupt the chain reaction decomposition, or deactivate the decomposition catalysts by chemical modification, or form complex. Among these compounds, sodium silicate (Na2SiO3) proved to be one of the most efficient. Several hypotheses have been proposed to explain the hydrogen peroxide stabilization mechanism: interaction between sodium silicate and hydrogen peroxide, formation of stable peroxide compounds, ending of the chain reactions by destruction of the free radicals, and formation of complexes from metals. However, according to Colodette et al. (14), the hydrogen peroxide stabilization should be explained by the formation of stable complexes between metals and sodium silicate. Their studies, as well as the ones carried out by Brown et al. (15), demonstrate more specifically that sodium silicate reduces the hydrogen peroxide decomposition in the presence of iron and manganese, but not in the presence of copper. Another drawback of sodium silicate is its use at high concentrations, which drives it to a jellification, unsuitable in a packed tower. Other compounds were tried in order to slow down the hydrogen peroxide decomposition (directly or indirectly): organic chelating agents like DTPA (dimethylenetriaminepentaacetic acid) or others (16), but also magnesium sulfate, which is cited by Liden et al. (17) or Qiu et al. (18) because it seems to precipitate with ferrous and manganese ions that are responsible for the hydrogen peroxide decomposition. The aim of this study was to slow down the H2O2 decomposition by using various stabilizing products. Some pure compounds, as well as commercial solutions, were tested, in order to choose the best product to implement during the scrubbing.

Materials and Methods Lab-assays were conducted in batch conditions, in dark glass bottles of 0.125 L volume, placed in a thermostatic bath, in the dark to avoid photolytic reactions. Samplings of 5 mL 7882

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FIGURE 2. Influence of the sodium silicate concentration on the hydrogen peroxide decomposition (tap water; [H2O2]0 ) 5.5 g‚L-1; [CO32-] ) 1.2 g‚L-1; pH 10.5; T ) 20 °C). were done to follow the hydrogen peroxide decomposition. To reproduce the scrubbing conditions, the hydrogen peroxide solution was made of tap water and a 35 wt/wt % hydrogen peroxide initial solution. Metals (Fe, Mn, Cu, Zn) and sodium carbonate (Na2CO3) were also added at the concentrations of 50 µg‚L-1 for each metals and 1.2 g‚L-1 for the carbonate ions. The different stabilizers tested were mainly made of silica (Na2SiO3, V141, V268, and V287) except the poly-R-hydroxyacrylic acid. V268 and V287 contained EDTA, stabilizer well-known (19). According to the information found in the literature, co-stabilizers used with Na2SiO3 were urea, magnesium sulfate (MgSO4, 7H2O), dimethylenetriaminepentaacetic acid (DTPA), and sodium nitrate (NaNO3). All these co-stabilizers were organic and inorganic compounds tested by Kutney (12). Their influence was observed versus time and pH at two constant temperatures (20 °C and 40 °C). Temperature and pH were measured thanks to specific probes. Hydrogen peroxide concentration was measured by iodometric titration according to reactions 7 and 8: in a first step, H2O2 oxidizes iodide ions into iodine in the presence of acid and molybdate catalyst and then, the iodine formed is titrated with a thiosulphate solution in the presence of a starch indicator.

H2O2 + 2 I- + 2 H+ f I2 + 2 H2O

(7)

I2 + 2 S2O32- f S4O62- + 2 I-

(8)

Results on the hydrogen peroxide decomposition are presented in figures or tables after 24 h and/or 48 h of contact time.

Results and Discussion Evaluation of the Sodium Silicate Stabilization Capacities. Figure 2 shows the residual hydrogen peroxide when various sodium silicate concentrations are applied, after 24 and 48 h, for T ) 20 °C and pH 10.5. It can be observed that there is an optimal concentration for which H2O2 is not much decomposed. Indeed, for [SiO2] ) 0.6 g.L-1, more than 99% remains in solution. The results are similar whatever the temperature, 20 or 40 °C. As a consequence, further experiments were conducted with this concentration. The other trials (Figure 3) revealed that the percentage of hydrogen peroxide decomposition increases with the pH and the temperature. A maximal decomposition is in fact observed for a pH equal to 11.5. This result is in agreement with the theory and the literature. Indeed, if the hydrogen peroxide degradation is based on the molecular mechanism, the maximal decomposition should occur for a pH equal to the pKa of the couple H2O2/HO2-, which is 11.7. If the hydrogen peroxide degradation is based on the radical mechanism (reaction with OH•), it has been observed that the reaction kinetic constant is maximal when the pH ranges between 11 and 12 (8). Nevertheless, the hydrogen peroxide decomposition rate seems too high to be considered in an industrial

FIGURE 3. Decomposition after 48 h of a hydrogen peroxide solution stabilized with sodium silicate ([H2O2]0 ) 5.5 g‚L-1; [SiO2] ) 0.6 g‚L-1).

FIGURE 4. Chelating action of DTPA on metal ions.

TABLE 1. Hydrogen Peroxide Decomposition in Presence of a Co-Stabilizer of Sodium Silicatea H2O2 decomposition at T ) 20 °C (%)

H2O2 decomposition at T ) 40 °C (%)

co-stabiliser

after 24 h

after 48h

after 24 h

after 48h

/ urea MgSO4, 7H2O DTPA NaNO3

1.0 1.2 1.5 0.3 0

3.9 2.7 3.6 0.6 1.8

9.9 9.3 9.0 6.3 9.6

15.6 15.6 18.9 8.4 16.8

a Tap water; [H O ] ) 5.5 g‚L-1; [co-stabilizer] ) 0.2 g‚L-1; [CO 2-] ) 2 2 0 3 1.2 g‚L-1; Metals: Fe, Mn, Cu, Zn at respective concentrations of 50 -1; pH 10.5. µg‚L

application. So, other compounds, called co-stabilizers, were added to sodium silicate to improve the stabilization effect. Improvements of Sodium Silicate Stabilization Capacities by the Addition of Co-Stabilizers. Tests of Co-Stabilizers. Based on several studies carried out in the pulp industries (11, 12, 14), four compounds were tested in addition to sodium silicate in order to improve hydrogen peroxide stabilization: urea, magnesium sulfate, DTPA, and sodium nitrate. Experiments were realized with the same concentration (0.2 g‚L-1) of co-stabilizer at the temperature of 20 and 40 °C. The results are reported in Table 1. Whatever the temperature, DTPA (dimethylenetriaminepentaacetic acid) constitutes the best inhibitor to the hydrogen peroxide decomposition. As an example, for T ) 40 °C and after 48 h, only 8.4% of hydrogen peroxide is degraded in the presence of DTPA compared to 15.6% or more when no co-stabilizer or another co-stabilizer is used. The DTPA molecular structure permits to understand why this type of compound can be efficient (Figure 4): in solution, DTPA has a global negative charge (due to acetate functions), which can trap a cation, acting like a chelating agent.

TABLE 2. Influence of DTPA Concentration on the Peroxide Decompositiona [DTPA] (mg‚L-1) 0 1 5 10 50 100 200 300

H2O2 decomposition at T ) 20 °C (%) after 24 h after 48h 1.0 0 0 0 0 0.3 0.3 3.6

3.9 0 0 0 0 0.6 0.6 6.2

H2O2 decomposition at T ) 40 °C (%) after 24 h after 48h 9.9 3.6 3.6 3.7 2.7 2.1 6.3 13.9

15.6 5.1 8.0 5.2 3.0 3.6 8.4 23.9

a Tap water; [H O ] ) 5.5 g‚L-1; [SiO ] ) 0.6 g‚L-1; [CO 2-] ) 1.2 2 2 0 2 3 g‚L-1; Metals: Fe, Mn, Cu, Zn at respective concentrations of 50 µg‚L-1; pH 10.5.

Consequently, the metallic ions responsible for the hydrogen peroxide decomposition are likely to be maintained inside the DTPA structure, and not to react with hydrogen peroxide (11). Addition of DTPA to Sodium Silicate. Then, the DTPA efficiency was studied according to its concentration. The results are reported in Table 2. For DTPA concentrations lower than 200 mg‚L-1, a diminution of the hydrogen peroxide decomposition is clearly observed for both temperatures (20 and 40 °C). The analyses realized after 24 h and after 48 h also seem to show that DTPA keeps its stabilization effect in time. However, when the DTPA concentration becomes too high (over 200 mg‚L-1), the effect of this compound on the hydrogen peroxide decomposition is reversed: decompositions higher than the ones observed with sodium silicate alone are observed. This result has already been reported in the literature by Colodette et al. (20) and can be explained by the reaction between the hydrogen peroxide and the tertiary amines. VOL. 40, NO. 24, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Decomposition after 48 h of a solution consisting of hydrogen peroxide, sodium silicate and DTPA ([H2O2]0 ) 5.5 g‚L-1; [SiO2] ) 0.6 g‚L-1; [CO32-] ) 1.2 g‚L-1; Metals: Fe, Mn, Cu, Zn at respective concentrations of 50 µg‚L-1).

FIGURE 6. Decomposition after 48 h of an hydrogen peroxide solution in presence of different stabilizers made of silicate versus the pH and the temperature ([SiO2] ) 0.6 g‚L-1; [H2O2]0 ) 5.5 g‚L-1; [CO32-] ) 1.2 g‚L-1; Metals: Fe, Mn, Cu, Zn at respective concentrations of 50 µg‚L-1). Finally, a DTPA concentration of 50 mg‚m-3 appears to be one of the most efficient to improve hydrogen peroxide stabilization in the presence of sodium silicate. This concentration was consequently chosen to pursue the experiments and to evaluate the gain of stabilization obtained by the addition of DTPA. As an example, Figure 5 shows the evolution of the hydrogen peroxide decomposition versus the pH and the temperature in the presence and in the absence of DTPA. Experimental results clearly demonstrate that the addition of DTPA improves the hydrogen peroxide stabilization whatever the pH and the temperature of the solution. Indeed, very good decreases of the hydrogen peroxide decomposition are observed, especially when the temperature is equal to 40 °C. Nevertheless, the saving observed at pH 12 is less significant and the decomposition rate still remains important for this pH. Test of Commercial Solutions of Silicates. Figure 6 presents the hydrogen peroxide decomposition rates measured after 48 h in the presence of various commercial stabilizing solutions based on sodium silicate and employed in pulp industries (V141, V268, V287). The SiO2 concentration chosen for the study was equal to 0.6 g‚L-1 according to the previous experiments. The pH and temperature were changed and the decomposition rates were compared to the ones obtained with classic sodium silicate (Figure 6). The results show that, for a temperature of 20 °C, the efficiency of the four stabilizers 7884

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is almost quite equivalent and decreases with the pH. For a temperature of 40 °C, V268 appears to be a bit better than the others when pH < 11.5, whereas V287 is the worst. One could explain the difference between these two products by the fact that V287 contains ten times less magnesium than V268 (information provided by the manufacturer). As a conclusion, it can be underlined that the use of such commercial solutions during a basic oxidant scrubbing does not present a real interest. Use of the Poly-R-Hydroxyacrylic Acid as a Stabilizer. The poly-R-hydroxyacrylic acid is an organic polymer used in Japan in the place of sodium silicate to stabilize hydrogen peroxide during the bleaching process. Its chemical formula is as follows:

R1 and R2 are hydrogen atoms or alkyl groups (1-3 carbon atoms), and n is equal or greater than 3. Due to its potentiality to stabilize hydrogen peroxide, this polymer was tested in batch in the conditions of the basic oxidant scrubbing. In Table 3, values of the hydrogen peroxide decomposition are reported versus the poly-R-hydroxyacrylic acid concen-

To confirm these results, this stabilizer will have to be implemented in a scrubbing tower.

TABLE 3. Influence of the Poly-r-Hydroxyacrylic Acid Concentration on the Hydrogen Peroxide Decompositiona stabilizer concentration (mg‚L-1)

Literature Cited 0

10

50

100

130

H2O2 decomposition at 20 °C (%) after 24 h 1 0.9 0.3 0.3 0 after 48 h 3.9 2.1 0.3 0.3 0

150

200

250

380

500

0.3 0 0.9 1.2 0.9 0.3 0.3 0.9 1.2 1.2

H2O2 decomposition at 40 °C (%) after 24 h 9.9 7.6 3.2 2.1 0.9 0.9 0.9 1.8 2.4 1.8 after 48 h 15.6 22 5.6 4.1 1.5 1.8 1.5 1.8 2.4 2.4 a Tap water; [H O ] ) 5.5 g‚L-1; [CO 2-] ) 1.2 g‚L-1; Metals: Fe, Mn, 2 2 0 3 Cu, Zn, at respective concentrations of 50 µg‚L-1; pH 10.5.

FIGURE 7. Decomposition after 48 h of a hydrogen peroxide solution stabilized by the poly-r-hydroxyacrylic acid and comparison to sodium silicate ([H2O2]0 ) 5.5 g‚L-1; [CO32-] ) 1.2 g‚L-1; Metals: Fe, Mn, Cu, Zn at respective concentrations of 50 µg‚.L-1). tration, at pH 10.5. It clearly appears that the poly-Rhydroxyacrylic acid is a very efficient stabilizer, particularly when concentrations range between 130 and 200 mg‚L-1. Under 130 mg‚L-1, the poly-R-hydroxyacrylic acid reduces the hydrogen peroxide decomposition, but values are not optimal. For concentrations higher than 200 mg‚L-1, the hydrogen peroxide decomposition seems to increase again. As a consequence, the concentration of 150 mg‚L-1 was chosen as one of the most efficient to slow down the hydrogen peroxide degradation. Figure 7 shows the results obtained with this concentration when the pH ranges between 9.5 and 12, and for two different temperatures. Whatever the pH and the temperature, the poly-R-hydroxyacrylic acid is more efficient than sodium silicate. Indeed, the hydrogen peroxide decomposition does not overcome 5.5%. Thus, the poly-Rhydroxyacrylic acid can be considered as a suitable stabilizer to improve the hydrogen peroxide scrubbing process. Several hypotheses about the mechanism used by this stabilizer to slow down the hydrogen peroxide decomposition could be made: •the presence of ionic carboxylic functions in the polymer could lead to the trapping of metals by a mechanism similar to the one described by Bambrick (11), that is charge neutralization; •the hydroxide function could cause hydrogen bonds with the ionic carboxylic groups; •the formation of interactions between oxygen (of the hydroxide group for example) and metals could limit the presence of these species in the solution. All these hypotheses converge toward the idea that the poly-R-hydroxyacrylic acid could be a chelating agent permitting to trap the metals responsible for the hydrogen peroxide decomposition.

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Received for review February 21, 2006. Revised manuscript received June 12, 2006. Accepted September 12, 2006. ES060414D

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