45 Figure 2.2: Dissolution rates of labeled material at 21°C in Nuclepore mesh packs (triangles) and pre-assembled Niskin reactors (circles) match dissolution rates of dispersed calcite (diamonds) and coccolith (stars) in Supelco bags. Symbols indicate surface species present in Eq. c) the relative difference between species concentrations in 0 mM and 28 mM SO4T seawater versus Ω.
Methods
No change in dissolution rate was observed at higher agitation speeds, but the rate decreased significantly when agitated below 60 rpm (Subhas et al., 2015). Experiments in supersaturated conditions (Ω=1.3) using the same methods saw no enrichment over nine days after an initial 1-3‰ increase in δ13C (Subhas et al., 2015).
Results
Our data show that calcite dissolution rates increase by four orders of magnitude as Ω decreases from 1 to 0. Dissolution rates appear less sensitive to temperature for Ω>0.9, but they transition into a regime where temperature sensitivity increases many from 0.9>Ω> 0.75.
Discussion
Analysis within the 1-Ω framework
The temperature dependence of the far-from-equilibrium ks can still be used to gain insight into the decomposition mechanism. Plotting the far-from-equilibrium rate constants in Arrhenius space (Figure 1.7) yields a value for Ea/R that corresponds to an apparent activation energy of 25±2 kJ/mol.
Identification of changes in dissolution mechanism
At low driving forces, dissolution is limited to retraction of pre-existing steps (1), kinks (a) and adatoms (b). At colder temperatures relevant to the deep ocean, resolution will be set by the density of already existing steps for 1>Ω>0.75.
Using temperature dependence to extract physical and energetic parameters of calcite
The fitted slopes and intercepts (Figure 1.11) have been resolved for both homogeneous and defect-assisted etch pit formation (Table 1.3); the cut-off value of each fit is set to í9ìí=3.5 (Ω=0.75) to remain consistent with our analysis in the 1-Ω framework. We therefore assume that the same βs we calculated for homogeneous solution also hold for defect-supported solution.
Role of Solution Chemistry
Since back-precipitation on our calcite grains can be identified by areas of elevated 12C (Subhas et al., 2017), we will be able to quantify the role of back-precipitation in the future by resolving calcite surfaces near equilibrium at low temperatures. The role of solution chemistry on the dissolution rate of calcite in seawater has been supported by recent work by Subhas et al. 2017) using carbonic anhydrase (CA) to increase the re-equilibration rate of H2CO3 in seawater.
Conclusions
Given that the rate constant for the hydration of CO2(aq) to H2CO3 increases exponentially with temperature in dilute solutions (Wang et al., 2010), it is possible that the behavior we observed for Ω>0.75 can be partly explained explained by an increase in the formation rate of H2CO3. Future work evaluating the temperature dependence of calcite dissolution in the presence of carbonic anhydrase will help to further dissect the effects of solution chemistry and surface processes on the overall dissolution rate.
Introduction
Naviaux et al. 2019) show more generally that n and k are variable functions of Ω and temperature, so attempts to describe marine calcite dissolution rates with a single fit to Eq. Advances in measurement techniques (Dickson, 1993; Liu et al., 2011) have revealed that pH on the total hydrogen ion scale (pHT) measured spectrophotometrically is offset from pHT calculated from combinations of alkalinity (Alk), total dissolved inorganic carbon (DIC ). ), and/or pCO2. Without a way to evaluate the “true” in-situ Ω, the position of the “true” saturation horizon remains unknown (Carter et al., 2018).
Methods
- Description of Materials
- Laboratory Measurements of Dissolution
- In-situ Reactor Design and Lab Verification
- Deployment of Reactors in the Field
- Field Sampling Methods
- Quality Checking Reactors
The pump runs continuously and pushes water over the marked material in the direction of the blue arrows. We used the box model of Subhas et al. 2017) to our Nuclepore packet data to understand the linearization of the δ13C-DIC versus time signal. A full description of the model can be found in the appendix of Subhas et al.
Results
Discrepancy in Ω Calculations
If these changes were large, they would change the temperature and pressure experienced by the reactor and thus the calculated in situ saturation state. At Station 3 (151°W / 35.265°N) we set up a reactor where the waters were supersaturated according to our own Alk-DIC measurements, but undersaturated according to Alk-pH. When measuring Dickson seawater alkalinity, DIC and pH under laboratory conditions, no deviation was observed between Ω calculations.
In-situ Dissolution Results
Dissolution rates gradually increase with undersaturation until Ω, after which calcite dissolves more rapidly in response to changes in Ω. While in-situ dissolution rates measured before the storm (diamonds at 1 - Ω = 0.25) followed the rate vs . The Ω trend established at earlier stations, data collected immediately after the storm (diamonds closer to equilibrium than 1 - Ω = 0.2) did not.
Laboratory Results
Despite being stored in the dark in gas permeable bags without headspace, the DIC of the archived seawater was found to increase by 152 μmol kg-1 after 3 months. Experiments conducted in the altered, archived seawater produced dissolution rates consistent with the rates measured in Dickson seawater (Figure 2.8b). Spraying Dickson seawater with different potential inhibitors had variable effects on dissolution, with the addition of 100 μmol kg-1 D-glucose slowing rates to comparable values to those measured in the N.
Discussion
- Implications for Ocean Saturation State
- Laboratory versus In-situ Dissolution Rates
- Role of Inhibitors
- Implications for In-situ Calcite Dissolution Rates
2018) documented a pressure-dependent increase in calcite dissolution rates in the laboratory, but we are unable to assess this effect in-situ. Therefore, our results only prove that DOC, as a class of compounds, can explain why dissolution rates in situ are slower than in the laboratory. Given the issues with Ω(Alk, DIC) discussed in the text, Peterson's data were plotted against Ω(Alk, pH) measured at the same latitude on a 2015 P16 cruise. b) The P16 line was 20 ° east of the Peterson zone, but the profile is similar in shape to the original Berner & Wilde estimate.
Conclusion
The Dickson seawater used in the lab was filtered, poisoned and UV treated, and represents the upper limit for the rate of dissolution in water with a low DOC. Their ground calcite was exposed only to low DOC soil water, and the rate of recovery matched the upper limit of our laboratory measurements. For modeling the dissolution of calcite in the water column, we recommend using the lower limit, because natural carbonates form in surface waters with a high DOC content and dissolve when they sink.
Introduction
With these results in mind, Van Cappellen et al. 1993) developed a constant capacity model (CCM) of calcite surface complexation. Naviaux et al also provided the first estimates in seawater of the step kinetic coefficient (β), density of active nucleation sites (ns), step-edge free energy (α), activation energy of kink/step detachment (klmno), and activation energy of etch pit initiation (kpqpm ). However, the bottom-up approach is more accurate in low to medium ionic strength conditions than in seawater (Hain et al., 2015).
Experimental Methods
Song et al, but they have not yet been coupled with dissolution/precipitation rate data. Reference material (Dickson, 2010) or in phosphate-free, “Aquil”, artificial seawater (Morel et al., 1979) with different sulfate concentrations (Table 3.1) The ionic strength of the Aquil solutions was kept constant by compensating for differences in sulfate with KCl. Dickson seawater Ω was calculated with the carbonic acid system dissociation constants K1′ and K2′ from Lueker et al.
Background and Modification of MyAMI Code
Solution saturation conditions were calculated using pairs of alkalinity (Alk) and dissolved inorganic carbon (DIC) measurements as input parameters in either CO2SYS v1.1 (van Heuven et al., 2011), or a modified version of CO2SYS discussed below. Pierrot, 1998) use the Pitzer equations (Pitzer, 1973) to calculate the activity coefficients and equilibrium constants for seawater of arbitrary composition. MyAMI” model, Hain et al. The first modification, as recommended by Hain et al., (2016), was to update the Pitzer model calcium bicarbonate coefficients from those in Table 1 of Harvie et al.
Implementation into PHREEQC
Comparison with CO2SYS
Under conditions where ΩCO2SYS = 1, the default PHREEQC database greatly underestimates the saturation of the solution and calculates ΩPHREEQC = 0.68. The equilibrium between each species is shifted towards higher pH values when calculated by the default PHREEQC database. Even with the updated Ksp*, the default PHREEQC database is more than 0.15 Ω units off from ΩCO2SYS.
![Figure 3.2: (a) Log 10 Concentration versus pH T in seawater at T=25°C, S=35, total DIC = 2 mM, for [H 2 CO 3 *], [HCO 3 - ] T , and [CO 3 2- ] T calculated using the default PHREEQC database (red), and an updated database using the constants in Table](https://thumb-ap.123doks.com/thumbv2/123dok/10400901.0/103.918.175.804.127.721/figure-concentration-seawater-calculated-phreeqc-database-database-constants.webp)
Choice of Surface Speciation Model
Note that the pKs in Table 3.2 are based on concentrations, so the updated PHREEQC database now calculates concentrations of solution species, rather than activities. The updated database also no longer calculates the CaHCO3+ and CaCO30 ion pairs, both of which are important in the A&M model. In seawater over the full Ω range, the standard PHREEQC database calculates that CaHCO3+ will be 2.2% of HCO3-.
Results and Discussion
- Dissolution Experiments with Variable [SO 4T ]
- Surface and Solution Speciation Calculations
- Proposed Kinetic Model
- Comparison of Model Fits in Seawater versus Freshwater
The dissolution rate of calcite in 0 mM SO4T Aquil exhibits a similar curve in slope at Log(1-Ω) = - 0.6, but the behavior on either side of this Ω differs from the other solutions. The calculated Ω's were used to match the experimental dissolution rate data with the corresponding speciation calculations. Examination of the fitted terms (k2-k5) suggests that H2CO3* attack is an extremely small contributor to the overall dissolution rate in seawater.
Summary and Conclusions
Differences between dissolution in freshwater and seawater are also seen in the rate constants for the reverse reactions. These findings have important implications for calcite dissolution rates in ancient oceans where sulfate concentrations were much lower than in modern ones (Canfield and Farquhar, 2009; Fakhraee et al., 2019; Luo et al., 2010). We find that sulfate removal simultaneously decreases the gross forward and backward rates, with the overall effect being to decrease the net dissolution rate.
Introduction
While the mechanism of CO2 hydration by CA is known, the mechanism by which the enzyme increases the carbonate dissolution rate is not. A detailed study is needed to understand the solution catalysis of CA, and the results will inform the design of any reactor that hopes to exploit the enzyme's catalytic effects. Finally, we combine our solid CA products with our reactors and use the results to understand the mechanism by which the enzyme improves the dissolution rate of calcite.
Methods
Reactor Designs
We implement methods for retention of CA in hydrogels and on the surface of CPG beads, and we evaluate the resulting effects on the enzyme's activity and effective lifetime. Initial experiments utilized this larger rock grain size, but the high flow rate discharged a full tank of compressed CO2 gas within 12 hours, requiring 10 hours for the reactor alkalinity to reach a steady state . The course of the reaction of the experiment was monitored by measuring the alkalinity of the effluent.
CA Immobilization Strategies
1 ml samples of the enzyme solution were taken at 0, 3, 10 and 60 minutes for later testing of the reaction progress using protein UV absorbance spectroscopy. The final step of the process was to block unreacted GA sites on the CPG beads and remove loosely bound CA. The amount of CPG-bound CA was estimated from the decrease in UV absorbance at 280 nm of the 1 mL enzyme solution samples.
MIMS Method for CA Activity and Lifetime Measurements
A concentrated enzyme solution was created by adding 12 mg of CA to 0.1 M pH 7 phosphate buffer for each gram of CPG beads to be reacted. The coupled CA-CPG beads were mixed together with a 0.1 M glycine pH 7 solution for 1 hour and 45 minutes at room temperature. Finally, CA-CPG beads were filtered and stored at 5 °C in 50 mM phosphate buffer pH 7.
Results and Discussion
Fluidized Bed Reactors
The added alkalinity (defined as the alkalinity in excess of the initial solution alkalinity) for each pair of reactors was proportional to the % CO2 gas feed and inversely proportional to the water feed rate. CA appears to have the largest relative effect when dissolution rates are high, either due to low solution Ω due to rapid flows or dissolution of fine-grained material. Adding free CA to reactors can increase carbon capture rates by 20+%, but CA is expensive and the marginal cost of captured CO2 is several hundred dollars per gram (Table 4.2).
Physical Immobilization of Hydrogels
Despite the aforementioned downsides, pure CA hydrogels were introduced into a fluidized bed reactor in an attempt to test whether MIMS activity corresponded to dissolution catalysis. No change in the alkalinity of the reactor effluent was observed over a period of 3 hours. Although this greatly improved the mechanical stability of the hydrogels, the best mg-1 activity that was achieved was still only 7.2% of free CA.
Coupling CA to CPG Beads
The activity of CA-CPG beads was batch-dependent, with the highest mg-1 activity being 80% of free CA. After an initial equilibration period, 1 g of CA-CPG beads were mixed into the test reactor. After an initial equilibration period, 1 g of CA-CPG beads were mixed into reactor 2 (pink color in the reactor on the right panel).
Summary and Conclusions
One of the goals of CDisK-IV was to quantify the contribution of inorganic solute to "Alk*", the amount of alkalinity beyond that expected from pure transport processes (Feely et al., 2002). Reply to comment by Zeebe and Tyrrell on "The effects of secular calcium and magnesium concentration changes on the thermodynamics of seawater acid/base chemistry: Implications for Eocene and Cretaceous ocean carbon chemistry and buffering." Global Biogeochem. An atomic force microscopy study of the dissolution of calcite in the presence of phosphate ions.
