Materials and methods
4.4 Biodegradation experiments in soil
To study the effect of small concentrations of amine in a plant-soil system, six plant pots of equal size (cubical, 8∙8∙8 cm) were used for each treatment (treatment = concentration/amount of amine). This study had a total of 36 pots, with some reserve pots in case any system looked different from the other at the time of treatment start, so that these may be replaced. Each pot was filled with approximately 400 mL of soil, its surface wetted and grass seeds were sowed. The grass was allowed to grow until a height of approximately 5-8 cm at conditions of growth light and regular watering (every 2nd day), to make sure that a solid system of roots had developed in each pot.
This took ~1.5 months.
The pots were systematically numbered for the sake of logging the results throughout the experiment. When the grass was deemed healthy and strong, treatments with
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MEA was conducted by giving each plant-soil system a one-one treatment. The pots were divided into sets of 6 (because there were 6 individual treatments), resulting in 6 sets of 6 pots. Prior to the treatment, an objective observer assessed the health of each plant-soil systems, by giving each individual system a score between 0 and 5, where 0 represented 0% brown leaves, 1: 1-10%, 2: 11-20%, 3: 31-60%, 4: 61-90%, and 5 91-100% brown leaves. Every plant-soil system was then given the same volume of treatment, but each treatment contained different concentrations of MEA.
Within each set of six, every pot then received a different treatment, in a randomized order. The number of the pot and the treatment given was systematically logged.
After the treatment, the plant-soil systems were all kept at conditions of growth light and regular watering, as before the treatment, but every 3-4 days the score was logged anew by the same objective observer. The observer was unaware of which treatment each plant-soil system had received. Scoring was continued for three weeks in this work, but for future studies and deeper insight it would be recommended to continue for a while longer. Longer intervals between scorings can be used.
The statistical relevance of the results was assessed, both to verify significant differences between treatments over all scoring times by using a Friedman test, and at any given time, by using a Kruskal-Wallis test. The effect size was determined using Kendall’s W and Cohen’s interpretation. P-values were adjusted with Bonferroni correction method.
4.2. Titration
Titration methods were used both for quantification of alkalinity, heat stable salts (HSSs) and dissolved oxygen, and each methodology is found described in the following sections.
4.4.1 Amine quantification
Amine concentrations were primarily quantified indirectly, by titration with sulfuric acid (H2SO4) to find total alkalinity of the solutions. Total alkalinity does not necessarily describe the total concentration of one given amine species but will indiscriminately quantify all alkaline species. Alkaline degradation products will therefore be included in the total amine concentration in this way. The method bases on the description found in Ma'mun et al. (2006).
0.2 g of the sample is added to 50 mL deionized water and its mass exactly noted down. The sample is then titrated using a Metrohm 702 SM Titrino automatic titrator and 0.1 M H2SO4 until the end point around pH 4-5. The concentration of alkaline species is calculated from the amount of acid used to titrate.
119 𝑐amine=2 ∙ 𝑐H2SO4∙ 𝑉H2SO4
𝑚sample Eq. 4.1
Analysis of samples of known concentrations gave maximum deviations of ± 2%. All samples were analysed in 2 parallels and the average of these are given as the result.
The maximum deviation between the two parallel analyses should not exceed 2%.
4.4.2 Heat stable salt analysis
Heat stable salt (HSS) is a collective term for all ionic species, which can be found in the amine solution, that can withstand elevated temperatures over time. The temperature elevation removes the reversibly formed carbonate, carbamate and bicarbonate species and effectively strips the solution of CO2. The method described in this section is based on that described in Reynolds et al. (2015) as well as method developed by SINTEF Industry.
Dowex 50W-X8 anion exchange resin (CAS: 69011-20-7) was used in this procedure, which was activated with hydrochloric acid (HCl) prior to use. The activation was performed by magnetically stirring two volumetric parts resin with one-part HCl (10%) for 10 min before letting the solution settle and then decanting off the acidic supernatant. The resin is then rinsed with deionized water by filling the container, stirring the solution for a few minutes, letting it settle and discarding the supernatant.
The rinsing step is repeated until the supernatant has the pH of deionized water. The resin is now activated and should not dry out. The activated resin needs to be stored in deionized water until it is in use.
The HSS analysis is performed by adding 2 g of the sample to 40 mL of activated resin and 40 mL deionized water, noting the exact mass of the sample. The beaker is partly covered with i.e. parafilm or a watch glass and is magnetically stirred at 70 °C for 1 h. After the solution has cooled down and the resin settled at the bottom of the glass, the supernatant is carefully poured through a frit, to avoid resin particles in the liquid, and into another beaker. 40 mL deionized water is added to the resin and stirred for a couple of minutes before allowing the resin to settle again. This supernatant is also gently decanted through the frit and into the same beaker as the previous supernatant. This rinsing step is repeated until the supernatant has the pH of deionized water, normally 3-4 times, combining all the supernatants. The combined supernatants are then titrated with 0.05 M sodium hydroxide (NaOH) using a Metrohm 702 SM Titrino automatic titrator until the end point of pH 5-6. The concentration of HSS is calculated from the amount of NaOH used to reach end point, using Eq. 4.2. All samples were analysed in 2 parallels and the average of these are given as the result. The maximum deviation between the two parallel analyses should not exceed 5%. Blank samples should be analysed regularly.
120 𝑐HSS=𝑉NaOH∙ 𝑐NaOH
𝑚sample Eq. 4.2
The anion exchange resin can be regenerated by covering it with HCl (10%) and stirring it, partly covered, at 70 °C for 1 h. The solution is allowed to settle and cool before the acid is gently decanted and the resin repeatedly rinsed with deionized water until the liquid shows the pH of deionized water.
To validate the HSS analysis, “artificially degraded” samples of MEA were analysed as unknowns. These were made by adding known concentrations of formic, acetic, oxalic and/or glycolic acid to a 30wt% (aq.) solution of MEA. The analysis of these artificially degraded samples gave a maximum deviation of ± 0.007 mol kg-1 or ± 7%.
4.4.3 Winkler titration
The Winkler method is a colorimetric titration technique for the determination of dissolved oxygen in aqueous solutions (Winkler, 1888). The dissolved oxygen is bound by divalent manganese (Mn2+) in a manganese chloride (MnCl2) or sulphate (MnSO4) solution, forming a white manganese(IV)oxide (MnO2) precipitate. Upon hydration, brown MnO(OH)2 is formed.
2Mn2++ O2+ 2OH−→ 2MnO2+ 2H2O → 2MnO(OH)2 Eq. 4.3 Potassium iodide (KI) and sulfuric acid (H2SO4) are then added to the solution, forming iodine (I2) in equal proportion to the amount of dissolved oxygen in the initial solution.
2MnO(OH)2+ 2I−+ 4H+→ 2Mn2++ I2+ 3H2O Eq. 4.4 A starch indicator is then used to recognize the equivalence point in a titration with thiosulfate solution (Na2S2O3), which gives a characteristic dark purple colouration of the solution with I3- (formed from I- + I2), but not with I-, giving a loss of colour when all I2 is consumed.
I2+ 2S2O32−→ 2S4O62−+ 2I− Eq. 4.5
It is of upmost importance to maintain the bottle where the reactions take place out of contact with air during the titration, as oxygen from the air influences the result of the titration. All Winkler titrations in this work were performed using a commercially available test kit purchased from Hanna Instruments (HI-3810).
Since the second step of the Winkler method (Eq. 4.4) depends on acidification with a strong acid to form highly volatile I2, the direct Winkler titration method is not suitable for determination of dissolved oxygen in alkaline solutions. Wang et al.
(2013) therefore developed an indirect Winkler titration method for alkaline solutions, which includes adding thiosulfate before the acidification, and titration with a potassium iodate (KIO3) solution to determine the amount of excess thiosulfate. This method was not used in this thesis.
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4.5 Chromatography
Chromatography methods allow the separation of components from a mixture where they are dissolved in a mobile phase (liquid or gas), and passed through a solid phase, where they are separated based on their properties. Chromatography coupled with a detection method therefore allows for single components to be identified and quantified. One gas chromatography (GC) method and two liquid chromatography (LC) techniques were used in this work.
4.5.1 Gas chromatography
Gas chromatography (GC) was performed on an Agilent 7890A GC-MS system according to the description found in chapter 5.
4.5.2 Liquid chromatography
Quantification of ethanolamine (MEA) and some degradation products was performed by LC-MS/MS as SINTEF using the method shortly described in chapter 6.
4.5.3 Ion chromatography
Ion chromatography (IC) is a subcategory of liquid chromatography, where ionic components, dissolved in water, are separated based on their affinity to a solid phase.
Anion and cation chromatography with conductivity detection were used for quantifying both single amines and anionic degradation compounds. The ion chromatograph is an instrument that is very sensitive to change and requires a lot of maintenance. Ideally, the instrument should be in continuous operation, with a constant solvent flow through the column. Vevelstad et al. (2012) discussed the challenges with using IC as an analytical tool for degraded amine solvents, addressing matrix effects, and separation challenges. These challenges were also observed in the scope of this work.
4.5.3.1 Cation chromatography
Single amine compounds can be separated by cation chromatography, due to their ability to get protonated in presence of an acidic eluent. The method used is based on that developed by Fytianos et al. (2015). The eluent used for separating the amines was in this case a 15 mM methanesulfonic acid (MSA), prepared freshly once a week with water from an ICW-3000 Millipore purification system. The instrument used was a Thermo Scientific™ Dionex™ ICS-5000 system, with a Thermo Scientific Dionex IonPacTM CS19 analytical column (2 mm ∙ 250 mm) and a CG19 guard column (2 ꞏ 50 mm). An eluent flow of 0.300 mL min-1 was used, the temperature of the column compartment was 30 °C and the cell compartment 25 °C. A 20 min method sufficed to elute all the studied amines, although for smaller amines like MEA
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15 min sufficed. A suppressor current of 11 mA was used to suppress the background noise of the conductivity detection.
Not all amines were quantified in this manner due to a series of problems with the instrument, not allowing further analysis on the cationic system despite of replacing almost every replaceable component of it.
4.5.3.2 Anion chromatography
In an opposite manner to cation chromatography, anion chromatography is based on the detection of negatively charged species, that i.e. get deprotonated in the alkaline eluent. Acetate, formate and oxalate were quantified using a Thermo Scientific™
Dionex™ ICS-5000 system located at USN Porsgrunn, with a Dionex™ AG11-HC RFIC™ analytical (4 ꞏ 250 mm) and guard column (2 ∙ 50 mm) and conductivity detection. The column compartment was kept at 35 °C and the cell temperature at 30 °C. A gradient of potassium hydroxide (KOH), generated by an eluent generation (EG) system, was used as the eluent, with the program given in Table 4.1. Standards of the organic acids were prepared in the concentration range from 1 to 30 ppm and the degraded amine samples were diluted between 1:100 and 1:350 with deionised water, depending on their known total content of heat stable salts (HSS). All standards and samples were filtered from any remaining particulate matter before analysis and peak areas were used for calculating the concentrations of the anions.
Table 4.3: KOH gradient used in the anion IC analysis of formate, acetate and oxalate in degraded amine samples
Time CKOH,start [mM] CKOH,stop [mM]
0-30 3 3
30-32 3 30
32-52 30 30
52-54 30 60
54-64 60 60
64-66 60 3
66-74 3 3
The anion chromatographic system at NTNU, Thermo Scientific™ Dionex™ ICS-5000 IC system, connected to an ICW-3000 Millipore water purification system and equipped with an ASRS300 suppressor (2mm), a carbonate removal device and conductivity detection was used to quantify iodide in the oxidatively degraded KI SAS solutions. The column was a 15IonPac 2×250mm with an AG15 guard column 2×50mm and column temperature 30 °C. An eluent generator provided the gradient given in Table 4.2. Quantification of iodide concentrations were performed based on calibration in a concentration range of 0–116 ppm of iodide in the form of KI and
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dilution of the fresh and degraded samples to the corresponding concentration range.
Peak areas were used for calculating iodide concentration.
Table 4.4: KOH gradient used in the anion IC analysis of iodide in the KI SAS solutions.
Time CKOH,start [mM] CKOH,stop [mM]
0-10 13 13
10-15 13 45
15-49 45 45
49-60 13 13