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Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using UV/H2O2



Sabtanti Harimurti, Anisa Ur Rahmah, Abdul A. Omar and Thanapalan Murugesan
 
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ABSTRACT

The presence of bicarbonate affects the degradation efficiency of effluents containing aqueous methyldiethanolamine (MDEA) solution leaving the CO2 absorption/regeneration unit of natural gas processing units. In the present study the effect of bicarbonate at three different pH conditions of (acidic, neutral and alkaline) simulated MDEA solution were conducted, by the addition of six different concentration of NaHCO3 (0.025, 0.05, 0.075, 0.1, 0.125 and 0.15 M). The presence of bicarbonate increased the mineralization of MDEA when the reaction was conducted at neutral initial pH conditions, where as the MDEA mineralization was reduced when the reaction was conducted at alkaline pH condition.

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Sabtanti Harimurti, Anisa Ur Rahmah, Abdul A. Omar and Thanapalan Murugesan, 2014. Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using UV/H2O2. Journal of Applied Sciences, 14: 1147-1153.

DOI: 10.3923/jas.2014.1147.1153

URL: https://scialert.net/abstract/?doi=jas.2014.1147.1153
 
Received: November 25, 2013; Accepted: January 05, 2014; Published: March 24, 2014



INTRODUCTION

Methyldiethanolamine (MDEA) is one of the alkanolamines that is commonly used for the removal of acidic gases (such as H2S and CO2) from natural gas (Kohl and Nielsen, 1997). Removal of acidic gas from the natural gas is necessary since the acidic gases cause corrosion in pipeline and processing equipment, also reduce the heating value which has an effect on the price of natural gas. MDEA has two ethanol functional groups and one methyl group attached to a nitrogen atom. Due to the existence of nitrogen atom with a pair of free electrons, MDEA forms weak base with water, hence MDEA is often used for scrubbing/sweetening of acidic gases (CO2 and H2S) from raw natural gas. Aqueous MDEA solution chemically binds with the acidic gases and when heated it releases the absorbed gases (Kohl and Nielsen, 1997).

During shutdown and maintenance of the processing equipments, high concentrations of residual MDEA will be carried over into the effluent. Conventional biological oxidation process is not effective for the treatment of effluent containing MDEA. Furhacker et al. (2003) reported that MDEA was not biodegradable in the bioreactor for a test period of 28 days. Therefore, an Advanced Oxidation Process (AOP) i.e., UV/H2O2 was used to treat the aqueous MDEA solution, by which approximately 86 % of TOC was removed (Harimurti et al., 2012). During the absorption/scrubbing process, the bicarbonate is also generated and present along with MDEA in the effluent stream. A number of researchers have reported that the presence of bicarbonate during the AOP’s reduces the degradation efficiency. Mehrvar et al. (2001) reported that the degradation rate of tetrahydrofuran (THF) and 1,4-dioxane (DIOX) were affected/reduced due to the presence of bicarbonate in the system. During the UV/H2O2 process, the presence of bicarbonate was reported to be strongly inhibit the degradation of organophosphorus pesticides namely, malathian and diazinon (Fadaei et al., 2012). The addition of inorganic ion such as bicarbonate in the Procion H-exl dyes solution gave an adverse effect on the decolorization rate of dye using Fenton’s process, as reported by Riga et al. (2007). Daneshvar et al. (2007) concluded that the presence of bicarbonate during the photooxidative degradation (UV/H2O2) reduced the degradation rate of 4-nitrophenol (4-NP) (Daneshvar et al., 2007). Muruganandham and Swaminathan (2004) studied the effect of bicarbonate during the photodecolorization of reactive azo dye (Reactive orange 4) and concluded that only 3.58% of decolorization was achieved (Muruganandham and Swaminathan, 2004). Klamerth et al. (2010) reported that bicarbonate competes with the organic contaminant for hydroxyl radical during the degradation of municipal wastewater using photo-Fenton and hence the presence of bicarbonate reduced the degradation efficiency of the effluent. Degradation of Atrazine by manganese-catalysed ozonation was also inhibited by the presence of bicarbonate (Ma and Graham, 2000). Based on the fore going observation from the available literature, the present study will focus on the effect of bicarbonate on the mineralization of effluents containing MDEA using UV/H2O2 process.

MATERIALS AND METHODS

Methyldiethanolamine (MDEA), potassium permangate (KMnO4), sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) were obtained from Merck (Germany). Sodium hydroxide (NaOH) and sodium bicarbonate (NaHCO3) were obtained from RM Chemicals (Malaysia). Simulated MDEA solution was prepared by dissolving a desired amount of MDEA in distilled water.

All the experiments were conducted in 700 mL cylindrical stirred jacketed glass reactor to monitor the progress of mineralization. The photoreactor was equipped with 8 Watt low pressure Hg vapor lamp GPH295T5L (which produces UV light at 254 nm was made in USA with serial no. EC90277), a current-voltage control unit and an opening at the top for sample collection. Intensity of UV lamp was measured by using UV radiometer (Cole-Parmer model: 97651-10 with sensor UV 254 nm model: 97651-20). The pH value of the solution was measured using pH meter (HACH-senion1) and the adjustment was carried out using NaOH or H2SO4 accordingly. The temperature of reaction was adjusted/maintained by circulating cooling water through the jacket. To study the effect of bicarbonate on the degradation of MDEA, NaHCO3 with known concentrations were added into the mixture and allowed to dissolve before the start of experiments. Liquid samples were withdrawn at specific time intervals and the TOC of the samples were measured using TOC analyzer (Shimadzu TOC-VCSH). H2O2 concentration in the solution during the photochemical oxidation process was monitored by titrating the samples using standard KMnO4 solution (Mendham et al., 2000).

RESULTS AND DISCUSSION

The present research includes the preliminary studies on the individual effect of UV and H2O2 as well as the combination of UV/H2O2 on the photochemical mineralization of aqueous MDEA solution and later extended to study the effect of the presence of bicarbonate on the photochemical oxidation system. The experiments were conducted using the following optimum conditions: Intensity of UV lamp = 12.06 mW/cm2, irradiation time = 3 hours, oxidation temperature = 30°C, [MDEA]0 = 2000 ppm, [H2O2]0 = 0.22 M (Behnajady et al., 2008).

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
Fig. 1: Individual effect of UV, H2O2 and the combination of UV/H2O2 on the MDEA mineralization

To study the bicarbonate effect, six different concentrations of NaHCO3 (0.025, 0.05, 0.075, 0.1, 0,125, and 0.15 M) were used. The initial pH of reaction was = 7, since at the acidic conditions, the bicarbonate will be neutralized to form CO2 and hence for the present work two different initial pH conditions (7 and 10.18) were used.

UV light and H2O2 process are well-known for the degradation of many organic compounds in aqueous solution. The capability of UV light to degrade the organic compound follows photolysis mechanism. The organic compound absorbs UV spectrum and then results in an excited (organic) compound which later decomposed to form a product (Massachelein, 2002; Oppenlander, 2003; Lester et al., 2010; Seraghni et al., 2012). Based on the preliminary experiments it was observed that, UV spectrum at 254 nm (used in the present experiments ) was not capable to remove the total organic carbon from the system (Fig. 1). The reason could be attributed to the fact that MDEA did not absorb the UV light at 254 nm, since the spectrum absorbed by MDEA was at 200 nm region, therefore, the direct photolysis did not occur (Harimurti et al., 2013). The capability of H2O2 to degrade organic compound is mainly due to the high reduction potential of H2O2 i.e., +1.8 V. This reduction potential indicates the high tendency of H2O2 to act as an oxidant which refers to direct electron-transfer reaction between organic compound and H2O2 (Petri et al., 2011). The results of the present experiments showed no degradation when the H2O2 alone was used, indicating that H2O2 alone was not capable to remove the total organic carbon in the aqueous MDEA solution (Fig. 1). This might be due to the reduction potential of H2O2 which is not sufficient for the oxidation process. The photolysis resistance of MDEA toward UV light and H2O2 was in agreement with the observation of (Xu et al., 2009), based on their studies on the photolysis resistance of dimethyl phthalate against UV photolysis and H2O2. However, the reduction of total organic carbon was found when the UV and H2O2 were applied in combination. The total organic carbon was reduced to a certain level (Fig. 1) which was due to the hydroxyl radical generated from H2O2 photolysis. It is well known that H2O2 strongly absorbs UV spectrum at 254 nm (Massachelein, 2002). Therefore the probability of H2O2 photolysis to generate hydroxyl radicals is very high. In other words, the combination of UV and H2O2 will generate hydroxyl radical which plays an important role during the degradation of many recalcitrant organic contaminants (Daneshvar et al., 2007; Lester et al., 2010; Behnajady et al., 2008; Abramovic et al., 2010).

Absorption/scrubbing of CO2 by aqueous MDEA solution occurs according to the following reactions:

Ionization of water:

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(1)

Hydrolysis and ionization of dissolve CO2:

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(2)

Protonation of MDEA:

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(3)

Acid-basic reaction with the amine:

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(4)

During the scheduled shut down of scrubbing unit in the natural gas plant, bicarbonate (HCO3¯) is expected to present in the effluents leaving the gas processing unit. Generally, the presence of bicarbonate in the AOP’s will act as a scavenger for hydroxyl radical. Bicarbonate (HCO3¯) reacts with hydroxyl radical (HO•) to form bicarbonate radical (HCO3•). This radical is also a well-known oxidant, but much less reactive compared to hydroxyl radical (Riga et al., 2007; Daneshvar et al., 2007; Jones, 1999; Andreozzi et al., 1999; Chiang et al., 2006). Consequently, the degree of oxidation is expected to be less.

In order to study the effect of bicarbonate on the mineralization of aqueous MDEA solution using the combination of UV/H2O2 process, experiments were conducted at two different initial pH (7 and 10.18) conditions and six different concentrations of NaHCO3 in the aqueous MDEA solution.

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
Fig. 2:
Total organic carbon profile during the degradation of MDEA in the presence of NaHCO3 using UV/H2O2, Inital pH = 7

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
Fig. 3:
Percentage TOC removal achieved at initial pH reaction = 7

At acidic pH conditions (pH <7) the bicarbonate will get neutralized (Eq. 5) and the product i.e., CO2 will be released from the system, hence there will be no effect on the mineralization process.

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(5)

When the initial pH of the process was approximately 7, the presence of bicarbonate in the synthetic aq. MDEA solution increased the mineralization of MDEA (Fig. 2). The degree of MDEA mineralization was increased by increasing of NaHCO3 concentration in the system and complete mineralization was achieved when the concentration of NaHCO3 was = 0.125 M (Fig. 3). This trend can be explained as: The capability of bicarbonate that can act as a good buffer was found at this condition. The pH during the mineralization process was maintained at 7 (Fig. 4).

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
Fig. 4: pH profile during the degradation of MDEA using UV/H2O2 at initial pH = 7

Bicarbonate is an amphoteric ion that can act either as an acid which can donate its H+ to form CO32¯ or as a base which is capable to accept an H+ to form H2CO3. Formation of organic acid during the degradation process, reduce the pH of the system. At this pH condition (pH = 7), the bicarbonate reacts with the hydroxyl radical (which is available in the system) to form bicarbonate radical, however, the formation rate of bicarbonate radical is less (8.5x106 M-1 s-1). Equation 6-7 shows the formation of bicarbonate radical in the system (Oppenlander, 2003; Andreozzi et al., 1999; Tang, 2003).

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(6)

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(7)

Klare et al., (2000) reported that free electron pair of nitrogen atom of amine compounds are in un-protonated form for pH = 7 and under these conditions, more active sites for oxidation by hydroxyl radical are available.

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
Fig. 5: Active sites of MDEA at (a) Acidic and (b) High pH (pH≥7) for the oxidation by hydroxyl radical

Thus, the presence of HCO3¯ in the solution maintained the free electron pair of nitrogen atom of MDEA in un-protonated condition and hence more active sites for reaction are always provided (Fig. 5) which in turn leads to higher degradation of MDEA.

The effect of the presence of bicarbonate during mineralization of aq. MDEA solution was also studied at alkaline pH conditions. Based on the preliminary studies, the optimum pH for mineralization process of aqueous MDEA solution was found to be 10.18, hence this pH was chosen for the present study at alkaline pH condition (Harimurti et al., 2012). The presence of bicarbonate (HCO3¯) during the UV/H2O2 mineralization of simulated aq. MDEA solution at an initial pH of 10.18 was found to reduce the mineralization efficiency (Fig. 6). Approximately 25% of TOC removal was reduced when the concentration of NaHCO3 was = 0.050 M (Fig. 7). This might be due to the presence of bicarbonate in the system which act as a good buffer and maintain the pH at constant and not allowed to drop to lower pH levels (Fig. 8). At alkaline pH condition, the bicarbonate (HCO3¯) was converted into carbonate (CO32¯) and then react with hydroxyl radical to form carbonate radical. Reaction between carbonate and hydroxyl radical is shown below:

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(8)

Since the reaction rate of carbonate radical formation is high (i.e., 3.9x108 M-1 s-1) and the scavenger reaction was significant enough to reduce the concentration of hydroxyl radical in the system that act as an important oxidant in the UV/H2O2 process, hence the reduction in MDEA mineralization.

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
Fig. 6: Total organic carbon profile during the degradation of MDEA in the presence of NaHCO3 using UV/H2O2 at initial pH = 10.18

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
Fig. 7: Percentage TOC removal achieved at initial pH reaction = 10.18

Moreover, H2O2 tend to ionize to form hydroperoxide anion (HO2¯) at high pH with pKa equals to 11.6 (Xu et al., 2009; Ren et al., 2010). Hydroperoxide anion is well-known to be a strong scavenger to hydroxyl radical (Eq. 9 and 10).

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(9)

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
(10)

Reaction between hydroperoxide anion (HO2¯) and hydroxyl radical (HO•) generates a less reactive radical species i.e., hydroperoxyl radical (HO2•) which is very effective to reduce the hydroxyl radical in the system and hence, the mineralization efficiency decreased.

Image for - Effect of Bicarbonate on the Mineralization of Methyldiethanolamine by using 
  UV/H2O2
Fig. 8: TOC and pH profile during the degradation of MDEA using UV/H2O2 at initial pH = 10.18 at (a) 0 M NaHCO3 and (b) 0.15 M NaHCO3

Even though bicarbonate reacts with the hydroxyl radical to form bicarbonate radical in the neutral pH (initial pH ≈7), the presence of bicarbonate radical did not interfere in the photodegradation process, since the reaction rate of bicarbonate and hydroxyl radical was less. Based on these analysis, it can be concluded that at neutral pH, due to the enhancement of active sites of MDEA for oxidation the bicarbonate increases the TOC removal. However, at alkaline pH condition of reaction, the bicarbonate acts as a strong scavenger to hydroxyl radical by converting to carbonate which further reacts with hydroxyl radical and reduce the TOC removal.

CONCLUSION

Based on the present experiments it can be concluded that the presence of bicarbonate increase the mineralization rate of aqueous MDEA solution when the reaction was conducted at neutral pH conditions (pH = 7) but slows down the mineralization rate of MDEA when the reaction was conducted at pH = 10.18. The enhancement of mineralization efficiency is attributed to the capability of bicarbonate to maintain the pH at 7, during the reaction and hence the active sites for oxidation by hydroxyl radical are always available. This information will be of useful for the design and scale up of the UV/H2O2 oxidation process for the photochemical degradation of effluents from CO2 absorber/scrubbing units which normally contains bicarbonate along with MDEA.

ACKNOWLEDGMENT

The scholarship to Sabtanti Harimurti, under the Graduate Assistant Scheme from Universiti Teknologi PETRONAS, is highly acknowledged.

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