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Surfactants Facilitating Carbonic Anhydrase EnzymeMediated CO2 Absorption into a Carbonate Solution Shihan Zhang, and Yongqi Lu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b00711 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017
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Surfactants Facilitating Carbonic Anhydrase Enzyme-Mediated CO2
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Absorption into a Carbonate Solution
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Shihan Zhang†,‡ and Yongqi Lu*,‡ †
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Key Laboratory of Microbial Technology for Industrial Pollution Control of Zhejiang
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Province, College of Environment, Zhejiang University of Technology, Hangzhou, 310014,
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China
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‡
Illinois State Geological Survey, Prairie Research Institute, University of Illinois at UrbanaChampaign, Champaign, Illinois, 61820, United States
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ABSTRACT: Carbonic anhydrase (CA) enzyme-mediated absorption processes are regarded as
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promising alternatives to the conventional amine-based process for CO2 capture because of their
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low energy penalty and low risk of causing secondary pollution. The activity and stability of the
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CA enzyme are crucial to reducing the equipment and operating costs of the enzyme-mediated
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process. This work investigated three cationic and nonionic surfactants to improve the activity
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and stability of a technical-grade CA enzyme in a 20 wt % potassium carbonate solution.
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Experimental results revealed that the impact of the surfactants on the CA enzyme depended on
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their properties. For example, the cationic surfactant significantly enhanced the activity of CA
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enzyme but adversely affected enzyme stability. However, in the presence of the cationic
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surfactant after 30 days at 50 °C, the activity of CA enzyme still outperformed that of CA
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without added surfactant. The nonionic surfactant significantly improved enzyme stability.
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Furthermore, the addition of surfactants within a critical micelle concentration of 1.0 did not
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distinctly influence the gas–liquid mass transfer, indicating that surfactant–enzyme interaction
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was responsible for the observed variations in the activity and stability of the tested enzyme. 1
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1. INTRODUCTION
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Carbon dioxide (CO2) emissions are the driving force behind climate change. On November 4,
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2016, the Paris Agreement, which was signed by nearly 200 countries, went into effect to curtail
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CO2 emissions in a bid to keep the global average increase in temperature below 2 °C. The best
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option for reducing CO2 emissions in the near term is postcombustion carbon capture process
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(PCCP) because it requires a minimal retrofitting of existing facilities.1
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Among the available PCCP, the monoethanolamine (MEA)-based process is costly because of its
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high energy penalty.2-4 This energy penalty is mainly due to the steam used for solvent
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regeneration and the required CO2 compression work, which accounts for 60% to 70% of the
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total cost.5,6 Thus, it is crucial to develop new solvents and tailored absorption processes for CO2
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capture.
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The enzyme carbonic anhydrase (CA) is attractive as a biocatalyst for CO2 absorption because
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solvent regeneration can be accomplished by using solvents that potentially incur a low energy
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use but which are restricted from practical use because of their slow absorption kinetics in the
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absence of a rate promoter.7–10 One CA enzyme-enabled technology is the Integrated Vacuum
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Carbonate Absorption Process (IVCAP), which uses a potassium carbonate (PC, K2CO3)
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aqueous solution promoted by CA as a solvent for CO2 capture.11,12 A thermodynamic analysis
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of the IVCAP showed an energy saving of up to 30% compared with the conventional 5 M MEA
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process for CO2 capture when integrated into a 528 MWe subcritical pulverized coal-fired power
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plant.11 A kinetic analysis also established the technical feasibility of the IVCAP by showing that
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the effective packing volume of the absorber with a 20 wt % PC solution in the presence of 10 g
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L−1 of CA was only slightly greater than that with the benchmark 5 M MEA.13
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For processes in which CA enzyme is restricted to the CO2 absorption operation, the primary
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concern related to the enzyme-enabled process is the activity and stability of CA at the typical
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flue gas temperature (40–60 °C). The activity and stability of the enzyme determine not only the
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footprint and thus the capital cost of the absorber, but also the operating cost associated with
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enzyme degradation. In our previous work, CA enzymes were immobilized on various supports,
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including nonporous nanoparticles and porous materials.14–16 The immobilized CA enzymes
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exhibited improved thermal and chemical stability compared with their free counterparts.
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However, the activity of all the immobilized enzymes was reduced by covalent bonding between
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the enzyme and supports as well as significant intraparticle diffusion resistance to the porous
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supports. Therefore, the desired tradeoff between the activity and stability of CA should be
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determined if the immobilized enzyme is to be utilized for CO2 capture.
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Recently, surfactant–enzyme interactions (e.g., hydrophobic and electrostatic interactions) have
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gained increasing attention because they tend to improve both enzyme activity and stability.17–19
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Literature data indicate that the activity and stability of an enzyme depend on the nature and
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concentration of the surfactant. Primarily, a surfactant concentration higher than the critical
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micelle concentration (CMC) is not beneficial for enzyme activity because the surfactant
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molecules aggregate to form micelles with a typically hydrophilic exterior and a hydrophobic
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interior.20,21 At concentrations below the CMC, the surfactant is present as a monomer and binds
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to the amino acid groups of the enzyme, eventually resulting in a conformational change in the
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enzyme structure. Once the binding sites of the enzyme are saturated, adding more surfactant
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results in the formation of clusters, leading to protein unfolding.20 The activity and stability of
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enzymes (e.g., tyrosinase, lipases, and laccase) have been reported to improve in the presence of
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the surfactant under CMC.21–24 Note that these researches have focused on homogeneous enzyme
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solutions. Little information is available from the literature for immobilized enzyme-surfactant
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interactions and any synergistic effect on the activity and stability of the immobilized enzyme.
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Enzyme immobilization together with beneficial immobilized enzyme-surfactant interactions
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could be another option to improve enzyme stability and activity although it is not a focus of this
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study.
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In this study, experiments were performed on CO2 absorption in a homogeneous CA–PC solvent
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mixture, facilitated by adding one of three surfactants, either a cationic surfactant
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(cetyltrimethylammonium bromide [CTAB] or dodecyltrimethylammonium bromide [DTAB])
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or a nonionic surfactant (Tween-80), to evaluate the impact of the surfactant on CA enzyme
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activity. The thermal stability of CA enzyme with an optimized dosage of surfactant was
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investigated under typical IVCAP operating conditions over a 30-day period at 50 °C. Moreover,
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the effect of the surfactant on the liquid-side mass transfer coefficient was examined
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experimentally. The overall goal of this study was to improve the activity and stability of
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homogeneous CA enzyme by adding a suitable surfactant, thus ameliorating the use of CA
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enzyme for CO2 capture.
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2. EXPERIMENTAL METHODOLOGY
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2.1 Materials
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Potassium carbonate (≥99.0%), potassium bicarbonate (≥99.5%), CTAB (≥98.0%), DTAB
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(≥99.0%), and Tween-80 (10%, wt/wt) were purchased from Sigma-Aldrich (St. Louis, MO).
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Carbon dioxide gas (99.99% purity) was purchased from S.J. Smith (Urbana, IL). A
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developmental technical-grade CA enzyme of microbial origin was provided by Novozymes A/S
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(Bagsvaerd, Denmark). All materials were used as received without further purification. Table 1
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shows the molecular structures and CMC levels of the CTAB, DTAB, and Tween-80.25-26 The
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referenced CMC data was measured in water at room temperature. It has been reported that the
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addition of a salt such as NaCl or NaBr induced a sphere-to-rod transition of cationic surfactant
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micelles.27 Furthermore, depending upon its type and concentration, the presence of a counter ion
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could reduce the electrostatic repulsion between the charged headgroups of the ionic surfactant,
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resulting in a decrease in the CMC of the surfactant.28 Compared with the ionic surfactant, the
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non-ionic surfactant without the charged headgroups possesses much less significant dependence
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on the ionic strength. It should be noted that the surfactant concentration was presented as a
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fraction of the CMC in this work for comparison purposes.
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2.2 Experimental Methods
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The liquid-side mass transfer coefficient of the CA–PC solvent mixture in the presence of
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CTAB, DTAB, or Tween-80, kL, was determined by measuring the rate of CO2 absorption into
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pure water with a pH below 3.0 in a stirred tank reactor (STR). This was done to minimize the
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chemical reaction between CO2 and OH− in the STR, which was equipped with an overhead
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mechanical stirrer (300 rpm) in the gas phase and a magnetic stirrer (450 rpm) in the liquid
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phase. A detailed description of the STR setup is provided in a previous work.12 In a typical
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experiment, the STR was charged with 800 mL of acidified pure water and degassed by a
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vacuum pump to strip off residual air. Pure CO2 gas was then injected into the STR until the
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required initial pressure was reached. The absorption process was operated at 50 °C and
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measured by monitoring the pressure change in the STR over time. The absorbed CO2 primarily
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exists in a physically dissolved molecular form in the liquid phase. An initial CO2 partial
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pressure in the STR of up to approximately 68 kPa was used to achieve a more rapid absorption
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rate and to minimize the gas-phase mass transfer resistance.
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The activity of CA enzyme (50–300 mg L−1) in promoting the absorption of CO2 into a 20 wt %
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PC solution with a 20% carbonate-to-bicarbonate conversion (PC20-20, pH 10.5) was measured
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in the STR in the presence of CTAB, DTAB, or Tween-80 with concentrations varying from
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0.05 to 1.0 CMC. This procedure was similar to the physical absorption of CO2 into pure water,
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as described above. In a typical experiment, 800 mL of the desired solvent was used for the
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absorption of pure CO2 gas under a pressure of 12 kPa at 50 °C. In the enzyme stability
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experiment, the desired solvents were prepared by adding optimal doses of individual surfactants
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to the PC20-20 solution with 100 mg L−1 of CA enzyme, and then storing the mixture in an
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incubator at 50 °C for 30 days. The solvents were sampled at a certain time interval during the 30
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days, and the activity of the CA enzyme was then determined for each sample.
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2.3 Determination of the Liquid-Side Mass Transfer Coefficient (kL)
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For gas absorption taking place in a STR, the following equations can be derived based on the
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conservation principle, the ideal gas law, and Henry’s law:29
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R=−
dPCO2 VG = kL E × ( C * − C b ) A × RgasT dt
(1)
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and C* =
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PCO2 He
,
(2)
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where VG is the volume of the gas phase, m3; A is the interfacial area between the gas and liquid
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phases, m2; Rgas is the gas constant, kPa m3 kmol−1 K−1; T is the temperature, K; PCO2 is the partial
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pressure of CO2, kPa; t is the time, s; He is the Henry’s law coefficient, kPa m3 kmol−1; C* and
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Cb are the physical solubility of CO2 under the pressure prevailing at the interface and the
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concentration of CO2 in the bulk solution, respectively, kmol m−3; kL is the individual liquid-side
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mass transfer coefficient, m s−1; and E is the enhancement factor, dimensionless.
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For physical absorption, the absorbed CO2 primarily exists in a physically dissolved molecular
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form and E is equal to 1. Therefore, the following equations can be obtained: R=−
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dPCO2 PCO2 VG = kL × − Cb A × RgasT dt He
(3)
and
C b ×VL =
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(P
CO2 ,0
)
− PCO2 × VG RgasT
,
(4)
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where VL is the volume of the liquid phase, m3, and PCO2 ,0 is the initial partial pressure of CO2,
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kPa.
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By arranging eq. (3) and eq. (4) and integrating them, we obtain
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C1 PCO 2 ,0 + C2 ln C1 PCO + C2 2
= − k L C 1 ∆t ,
(5)
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where
C1 = −
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V RT × A 1 + G VG He V L RT
(6)
and C2 =
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PCO2 ,0 × A VL
.
(7)
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The values of kL can then be obtained from the slopes of the straight trend lines of
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ln C1 PCO 2 ,0 + C 2 C1 PCO 2 + C2
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pressure profiles.
(
)
versus −C 1 ∆t based on measurement of the changing CO2
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2.4 Determination of the Overall Rate Constant (kov)
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For chemical absorption with a pseudo first-order reaction, the enhancement factor can be
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determined as30
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Dk E = 1 + 2ov kL
,
(8)
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where D is the diffusion coefficient of CO2 in the liquid phase, m2 s−1, and kov is the overall first-
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order rate constant, s−1.
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The absorption reaction can be considered pseudo first order with respect to dissolved CO2 when
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the following criterion is satisfied:31
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1 2 Dk C * + 1 + 2ov − 1
