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Article

Cooperative and Inhibited Adsorption of DRibose onto Brucite [Mg(OH)] with Divalent Cations 2

Charlene Fae Estrada, Alyssa K Adcock, Dimitri A. Sverjensky, and Robert M Hazen ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00095 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 28, 2017

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ACS Earth and Space Chemistry

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To be submitted to ACS Earth and Space Chemistry

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Cooperative and Inhibited Adsorption of D-

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Ribose onto Brucite [Mg(OH)2] with Divalent

4

Cations

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Charlene F. Estrada*,1,2,†, Alyssa K. Adcock3,⊥, Dimitri A. Sverjensky1

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and Robert M. Hazen2

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Johns Hopkins University, Department of Earth & Planetary Sciences,

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3400 N Charles St, Baltimore MD, 21218 USA †

Present Address: School of Earth and Space Exploration, Arizona State University, 781 E Terrace Mall, Tempe,

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AZ 85287, USA. 2

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5251 Broad Branch Rd NW, Washington DC 20015 USA 3

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Geophysical Laboratory, Carnegie Institution for Science,

⊥Present

Jacobs University, Campus Ring 1, 28759 Bremen, Germany

Address: Department of Chemistry, Georgetown University, 3700 O St NW, Washington DC 20057, USA *Corresponding Author email address: [email protected] (C. Estrada).

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ACS Paragon Plus Environment

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Abstract

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The adsorption and concentration of sugars onto mineral surfaces in geochemical environments, such as hydrothermal systems, may have influenced the evolution of early life on Earth. We conducted batch adsorption experiments between D-ribose and brucite [Mg(OH)2], a mineral produced from serpentinite-hosted hydrothermal systems, over variable initial ribose concentrations at four ionic strengths resulting from different Mg2+ and Ca2+ ion concentrations in the aqueous phase. Ribose adsorption generally increased with greater initial concentration, and up to 0.3 µmol•m-2 ribose attached onto brucite with 0.6 mM Mg2+ present. Ribose adsorption decreased over six-fold (4.9x10-2 µmol•m-2) when the total Mg2+ ion concentration increased to 5.8 mM. Ribose adsorption increased to 0.4 µmol•m-2 when 4.2 mM CaCl2 was added to the system. Substantial amounts (over 21 µmol•m-2) of dissolved Ca also attached to the brucite surface independent of ribose concentration. We characterized the interactions between ribose, Ca, and the brucite surface by fitting a surface complexation model to adsorption data. We propose three types of surface reactions that were consistent with the experimental data and involve 1) a bidentate outer-sphere or a “standing” ribose surface species, 2) a monodentate Ca-ribose outer-sphere species, and 3) a monodentate Ca outer-sphere species. Our model predicts brucite particle surface charge is negative at low Mg2+ concentrations and further decreases upon the addition of MgCl2, which may hinder our proposed surface complexation of the ribose species, Rib-. We predict that brucite becomes positivelycharged with CaCl2 addition, which may be a consequence of the significant extent of Ca adsorption. The increase in ribose adsorption with CaCl2 is likely driven by Ca attachment and the formation of a positively-charged, cooperative Ca-ribose species that our model predicts will predominate over the “standing” ribose species on brucite. Our model of the ribose-brucite system, established by a combination of batch adsorption experiments and surface complexation modeling, has enabled predictions of ribose adsorption over a wide range of pH and complex environmental conditions.

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Keywords: Ribose; Brucite; Surface Complexation; Mineral Surface Chemistry; Calcium;

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RNA Evolution

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1. Introduction

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The interactions between organic molecules and the mineral-water interface may further

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our current understanding of fundamental geological, biological, and atmospheric processes. For

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instance, the reaction of organic matter, water, and minerals at the soil profile significantly

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affects chemical weathering, the global carbon cycle, and atmospheric concentrations of CO21,2.


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Complex suites of biomolecules have condensed from inorganic precursors among gas, aqueous

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fluids, and a limited assemblage of minerals within carbonaceous chondrites traveling through

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space3. Microbes are closely associated with minerals of a wide variety of crystallographic habits

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following biomineralization4-6. Filamentous microfossils cemented by iron oxides and silica have

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been preserved at timescales exceeding one billion years7. Biomolecule interactions at mineral

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surfaces have also been proposed to contribute to hypotheses on the origins of life on early Earth,

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in which prebiotic molecules may have concentrated at the mineral-water interface prior to

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assembling into more biochemically complex macromolecules such as RNA, DNA, and

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proteins8,9. The emergence of RNA and DNA from nucleic acids and sugars is a fundamental

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process in such an origins of life scenario8,10,11. Clay surfaces, such as montmorillonite, catalyze

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the formation of ribonucleotide oligomer chains11,12. The mineral-water interface is a potentially

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important local environment in the formation of more complex biomolecules because the

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adsorbing organic species may typically form in water under the pH conditions or salinity of

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interest11-13. Thus, the mineral-water interface may increase the availability of a particular

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organic species that could contain more polarized or charged functional groups, and the

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concentration of such groups may facilitate reactions among other sorbed biomolecules.

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The monosaccharide

D-ribose

(C5H10O5) forms the backbone of RNA and DNA

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molecules, and in water it forms an equilibrium mixture of five neutral isomers, among which

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the β-ribopyranose predominates14. Researchers have previously synthesized ribose and other

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sugars under plausible early Earth conditions15-17. LaRowe and Regnier18 calculated that abiotic

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ribose formation under elevated pressures and temperatures up to 150 ºC and a bulk water

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composition similar to that of ultramafic hydrothermal systems19,20 is thermodynamically


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favorable18. Such a determination may hold implications for the evolution of more complex

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organic compounds that contain ribose within hydrothermal environments.

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A number of studies have focused on the adsorption of ribonucleotides onto mineral

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surfaces, such as nontronite21, montmorillonite11,12,21-23, kaolinite23,24, α-Al2O325, SiO226,

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Al(OH)324,27, manganese oxides28, allophane29, and TiO230. In the latter study, ribonucleotides

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attached onto TiO2 to a greater extent compared with deoxyribonucelotides, and the authors

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suggested that the hydroxyl groups of the ribose moiety might have enhanced ribonucleotide

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adsorption at the mineral-water interface. It is possible that this observation holds implications

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for the selection of nucleic acids with a ribose base. However, in comparison to investigations on

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ribonucleic acids, ribose attachment at mineral surfaces has received relatively little attention,

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presumably due to the low (< 5 %) extent of adsorption that is difficult to characterize as a

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function of different environmental conditions. A notable exception is the work of Hashizume

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and coworkers23,29, which reported ribose attachment onto the surfaces of montmorillonite and

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allophane using a total organic carbon detector in the ppb range. However, in both studies, very

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little (10

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mM) of Ca or organic compounds might affect the dissolution of brucite in aqueous solution, and

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accordingly, we studied relatively low (SOH2+>SOH_Rib-

(4b)

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resulting in the overall reaction

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2>SOH + HRib = >SOH2+>SOH_Rib–

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and two surface sites are protonated in the second reaction:

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HRib = Rib- + H+

(5a)

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2>SOH + Rib- + 2H+ = 2(>SOH2+)_Rib-

(5b)

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2>SOH + H+ + HRib = 2(>SOH2+)_Rib–

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in which “>SOH” represents a single brucite site at the solid-water interface (see Supporting

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Information). In Figure 1a, both of these reactions resulted in close fits to the adsorption data

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within experimental uncertainty, which provided support for the use of either eq 4 or 5 to

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describe ribose adsorption onto brucite. The two surface reactions might result in a bidentate

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outer-sphere complex, or a “standing” complex, at the brucite surface. Assuming that an outer-

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sphere complex requires hydrogen bonding between a hydroxyl functional group of ribose and

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the brucite surface, we illustrated a schematic representation of both surface complexes in Figure

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2a,b.

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(4c)

(5c)

If we express the neutral ribose molecule as H2Rib, we can rewrite eq 4 as H2Rib = Rib- + 2H+

(6a)


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2>SOH + 2H+ + Rib- = 2(>SOH2+)_Rib-

(6b)

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2(>SOH2+)_Rib- = >SOH2+>SRib- + H2O

(6c)

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2>SOH + H2Rib = >SOH2+>SRib– + H2O

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in which the ribose molecule might adsorb as a bidentate complex with one inner-sphere and one

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outer-sphere attachment. The ETLM cannot distinguish between these two surface reactions.

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However, inner-sphere complexation might be less likely given the relatively short time for

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ribose to attain a steady state and its relatively small amount of adsorption onto brucite in

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comparison to aspartate, which we have predicted to form an inner-sphere complex on brucite35.

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(6d)

The reactions in eqs 4 and 5 correspond with the equilibrium constants ∗ $ K _ !"#

=

%&' %&'_()*#

%&' ()*

-./0,4

10 .4 4(5

(7)

and ∗ $ K 6()_ !"#

(%&' )_()*#

=

%&' ()*

-./0,5

10 .4 4(5

(8)

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The superscript “*” indicates that these reactios are expressed relative to the neutral >SOH

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surface and “0” refers to a hypothetical 1.0 molal standard state55. The values ∆ψr,4 and ∆ψr,5

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represent the electrical work involved in eqs 4 and 5, respectively. The electrical work includes a

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contribution from the movement of water dipoles off the brucite surface according to ∆ψr = -

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nH2O(ψ0- ψβ), where nH2O is the number of water molecules on the right-hand side of the reaction.

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In eqs 4-5, nH2O = 0, which results in ∆ψr,4 = ψ0-ψβ and ∆ψr,5 = 2ψ0 - ψβ.

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3.2. The Low-Ca2+ and High-Ca2+ Experiments

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Ribose adsorption isotherms for the low-Ca2+ and high-Ca2+ experiments are shown in

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Figure 1b. Γads was between 1.0x10-2 and 0.3 µmol•m-2 for the low-Ca2+ experiment. This

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amount generally corresponded with 4.4 to 2.5 % ribose adsorption and generally decreased with


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increasing Γads. Ribose adsorption was significantly greater in the high-Ca2+ compared with the

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Mg2+ experiments. Γads was between 4.8x10-2 and 0.4 µmol•m-2, and this range generally

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corresponded with 11.4 to 3.8 % ribose adsorption. We also observed substantial amounts of Ca

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adsorption onto brucite independent of ribose concentration (Figure 3). The amount of Ca

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adsorption averaged 21.3 ± 1.3 µmol•m-2 (39.9 ± 2.8 %) for the low-Ca2+ experiment and 22.7 ±

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3.5 µmol•m-2 (10.4 ± 1.6 %) for the high-Ca2+ experiment. All Ca adsorption data is reported in

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Table S2.

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We proposed that two surface reactions in combination with the “standing” ribose

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complex represented by eqs 4 and 5 characterized the adsorption of Ca and ribose on brucite.

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The first surface reaction involves the formation of a Ca-ribose surface complex:

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HRib + Ca2+ = CaRib+ + H+

(9a)

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>SOH + CaRib+ = >SOH_CaRib+

(9b)

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>SOH + HRib + Ca2+ = >SOH_CaRib+ + H+

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We can interpret this reaction to involve the adsorption of a monodentate outer-sphere species on

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the brucite surface (Figure 4a). The second reaction involves the adsorption of a hydrated Ca

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complex:

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Ca2+ + 2H2O = Ca(OH)2 + 2H+

(10a)

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>SOH + H+ + Ca(OH)2 = >SOH2+_Ca(OH)2

(10b)

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>SOH + Ca2+ + 2H2O = >SOH2+_ Ca(OH)2 + H+

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This reaction also involves a monodentate outer-sphere surface species shown in Figure 4b.

(9c)

(10c)


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In Figure 4a, we speculated that the Ca-ribose outer-sphere species might attach through

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a hydrogen bond between a deprotonated ribose hydroxyl group and the neutral brucite surface.

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The Ca2+ ion might attach to 3 oxygen atoms of a single α-furanose ribose molecule, as

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suggested by previous studies of Ca-ribose aqueous complexes51,56. The equilibrium constant of

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the surface reaction in eq 9 can be expressed as

310 311 312 313

∗ $ K _( !")

=

%&'_:;(()*) %&' ()* :;

-./0,9

10 .4 4(5

(10)

where ∆ψr,9 = +ψβ. We expressed the equilibrium constant of the surface reaction proposed in eq 10 as ∗ $ K _()

=

%&' _:;(') %&' :; '

-./0,10

10 .4 4(5

(11)

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where ∆ψr,10 = +ψ0. We proposed the reaction in eq 10 in a previous study of the brucite-

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aspartate system35. Because the same batch of brucite was used in this study, we have adopted

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the same equilibrium constant recalculated for the decreased site density of Ns = 31 sites•nm-2

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(see Table 2).

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We combined the surface reactions in eqs 4 and 5 and eqs 9 and 10 to fit the ribose

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adsorption data obtained from the low-Ca2+ and high-Ca2+ experiments in Figure 1b. The

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calculated curves are consistent with all the adsorption data collected from both Ca2+

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experiments within analytical uncertainty. The proposed surface reactions also predict Ca

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adsorption percentage data within error, as demonstrated in Figure 3. Therefore, we considered

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the models predicted by the surface reactions in eqs 4 and 5 and eqs 9 and 10 to sufficiently

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characterize ribose and Ca adsorption at the brucite-water interface at different ionic strengths,

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pH conditions, and ribose concentrations.

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3.4. Prediction of Brucite Particle Surface Charge


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The decrease in ribose surface adsorption in the high-Mg2+ experiment and its increased

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adsorption in the high-Ca2+ experiment might be related to the electrostatic environment at the

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brucite surface. We used the ETLM to predict brucite particle surface charge (ζ-potential, in mV),

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which we assumed was equal to ΨD, otherwise known as the potential beginning at the diffuse

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layer of the brucite-water interface. This prediction is shown in Figure 5 for the four conditions

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investigated in this study, and it revealed that the ζ-potential is slightly negative following the

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low-Mg2+ experiment. However, the particle surface charge decreased by 20 mV in the high-

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Mg2+ experiment. This shift in ζ-potential might be a result of the decrease in pH that was

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required to maintain equilibrium between the brucite and aqueous solution when MgCl2 was

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added, which is expressed by eq 2. In both Mg2+ experiments, the ζ-potential indicated that the

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negatively-charged brucite surface was electrostatically inhibited from adsorbing ribose, which

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formed a surface complex as a negatively-charged species, Rib–. As the brucite surface became

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more negative with increasing Mg2+ ion concentration, ribose adsorption further decreased,

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which we have experimentally observed in the high-Mg2+ experiment.

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In contrast to the Mg2+ experiments, we predicted that the ζ-potential significantly

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increases toward positive values of 55 mV and 65 mV for the low- and high-Ca2+ experiments,

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respectively. For both Ca2+ experiments, we observed a large amount of Ca adsorption, which

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may have caused this predicted increase in the ζ-potential. Additionally, we observed a small

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increase in ribose adsorption, which may have been promoted by the formation of the Ca-ribose

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complex at the surface. This conclusion was supported by our predicted distribution of the ribose

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species on brucite at low-Ca2+ and high-Ca2+ ion concentrations as shown by Figure 6 a,b. At

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both Ca2+ ion concentrations, we predicted that the Ca-ribose outer-sphere species predominates

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over the “standing” species on the brucite surface.


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In our previous investigation of the brucite-aspartate system, we observed a significantly

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greater increase in aspartate adsorption onto brucite under similar Ca2+ ion concentrations in

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comparison to ribose35. The difference in the amount of adsorption between aspartate and ribose

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with Ca highlights how a Ca-biomolecule surface complex is affected by different species. It is

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possible that ribose forms a weaker attachment to Ca than the aspartate molecule, as is indicated

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by the multiple bonding sites required to form the Ca-ribose complex and the low association

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energy from the aqueous Ca-ribose complex51(Table 2). Should a positively-charged molecule

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adsorb onto the brucite surface along with added CaCl2, it is possible that the molecule will

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compete with the Ca2+ ion for adsorption onto the brucite surface. Such observations have been

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previously reported in the case of lysine attachment with Ca2+ on rutile57 and montmorillonite58.

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3.5. Prediction of Ribose Adsorption as a Function of pH

361

Once we were able to accurately model the dependence of ribose adsorption with ionic

362

strength and initial ribose concentration, we could also use the ETLM in a predictive manner to

363

extrapolate our results as a function of pH. Such a prediction is possible because in the course of

364

establishing a model for the brucite surface, we determined equilibrium constants for pH-

365

dependent surface protonation and deprotonation reactions (Table 2), as well as pH-dependent

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surface reactions in eqs 4 and 5 and eqs 9 and 10. Because shifts in pH are accompanied by Mg2+

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ion dissolution at the brucite surface, which maintains equilibrium with the aqueous phase (eq 2),

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it is unlikely that these ionic strengths would naturally persist at this pH range. Nevertheless, this

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prediction may provide a basic estimate of the magnitude of ribose adsorption in different

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aqueous environments until further experiments can be done to better constrain the adsorption

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predictions. As illustrated by Figure 7a,b, there is a significant amount of ribose surface

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adsorption at low- and high- Ca2+ ion concentrations at pH 9 (Γads= 0.9 and 1.5 µmol•m-2 at


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[Rib]0 = 75 and 150 µM respectively). Additionally, we predict a similar extent of ribose

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adsorption at both low- and high-Mg2+ ion concentrations under alkaline pH conditions.

375

Our predictions may indicate that ribose adsorption is favorable on brucite within alkaline

376

environments containing elevated Ca2+ ion concentrations. It may be possible that a

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hyperalkaline aqueous system would act as a suitable environment for ribose adsorption,

378

although it is not likely that such an environment would be globally widespread. Nonetheless, the

379

condensation of ribose from simpler precursor molecules such as formaldehyde may favor

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alkaline environments in addition to clay surfaces, including brucite15,33,59. Both brucite and

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montmorillonite have been demonstrated to catalyze the formation of amino sugars and

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pyrophosphate33,60,61, which are essential polymers for the formation of nucleic acids and the

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evolution of RNA. Experimental observations of phosphorylation of nucleic acids on the mineral

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schreibersite (Fe,Ni)3P have also indicated a prerequisite for alkaline conditions62. Our

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predictions of increased ribose adsorption on brucite at elevated pH together with these previous

386

investigations may indicate that alkaline natural environments, such as serpentinizing

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hydrothermal systems, may potentially favor the formation of ribose and the emergence of

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ribonucleic acids at mineral surfaces.

389 390

4. Conclusions

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We conducted low-Mg2+, high-Mg2+, low-Ca2+, and high-Ca2 batch adsorption

392

experiments to characterize the attachment of ribose onto brucite. Our results revealed that ribose

393

adsorption increased when we added CaCl2 to the system, whereas it decreased when we added

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MgCl2. We characterized the brucite-ribose and brucite-Ca system with the ETLM and proposed

395

three types of surface reactions involving 1) a bidentate outer-sphere, or “standing” ribose


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Page 18 of 33

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species, 2) a Ca-ribose complex attaching as a monodentate outer-sphere species, and 3) a

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hydrated Ca complex attaching as a monodentate outer-sphere species. We tested our surface

398

complexation model against the data collected at four different ionic strengths, variable initial

399

ribose concentrations, and different pH conditions dependent on Mg2+ ion concentration. In each

400

case, the three surface reactions were consistent with the experimental data within analytical

401

uncertainty. Our success in representing the data with the ETLM has provided support that our

402

proposed surface reactions accurately depict ribose and Ca adsorption onto brucite.

403

We determined that ribose adsorption significantly decreases when MgCl2 is added, and

404

we predicted that the brucite particle surface charge becomes more negative with added MgCl2,

405

which creates an unfavorable electrostatic environment for the negatively-charged Rib- species to

406

adsorb as a “standing” complex. In contrast, we calculated a reversal in brucite particle surface

407

charge with the addition and subsequent adsorption of Ca, and the observed increase in ribose

408

adsorption may be due to the formation of a cooperative Ca-ribose complex. We predicted that

409

the Ca-ribose species predominates over the “standing” species at both Ca2+ ion concentrations.

410

However, we observed only a small increase in ribose surface adsorption when CaCl2 was added,

411

and this minor effect might be a consequence of the weak attachment of ribose to Ca.

412

We further used the ETLM to predict ribose adsorption over the pH range 5 to 12. The

413

model revealed that ribose surface adsorption might significantly increase at highly alkaline

414

conditions (Figure 7a,b), which might hold implications for ribose adsorption and concentration

415

at serpentinite-hosted hydrothermal environments. Arrhenius and coworkers50 have previously

416

observed that phosphate and polyphosphate intercalate into mixed-valence double layer

417

hydroxides (DLHs), in which trivalent cations Fe3+ or Al3+ have substituted into the brucite

418

structure. Given that the lateral brucite surface at which ribose sorbs is also located at the edge of


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the interlayer space of the brucite structure where the intercalation of phosphate is thought to

420

occur, it is possible that ribose might phosphorylate at a DLH surface containing phosphate.

421

Furthermore, a previous investigation50,51 has demonstrated that intercalated nucleic acids such

422

as AMP and even DNA can hybridize within the interlayer space of DLHs. The concentration of

423

ribose at the brucite (or DLH) surface at alkaline, CaCl2-rich conditions, as those found in

424

modern serpentinizing fluids, together with phosphate and nucleobases may influence the

425

assembly of ribonucleic acids, and possibly the emergence of RNA, through the process of

426

hybridization within the interlayer space. Our current model provides us with a fundamental

427

understanding of ribose adsorption behavior and the importance, as well as limitations, of

428

divalent ions in cooperatively enhancing its attachment onto the surface in the context of a

429

potentially more complex geochemical environment.

430 431

Supporting Information

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Ribose polymerization blank experiment, surface complexation modeling considerations, SEM

433

image of brucite, equilibrium constant vs. pH of brucite, ribose and Ca adsorption data.

434 435

Acknowledgements

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We thank Manuel Pelletier, Angélina Razafitianamaharavo, Cécile Feuillie, Namhey Lee,

437

George Cody, Timothy Strobel, Dionysis Foustoukos, Paul Goldey, John Armstrong, Adrian

438

Villegas-Jimenez, Stephen Hodge and Steven Coley for their invaluable expertise and advice

439

throughout this project. We also thank the three anonymous reviewers of this manuscript for

440

their useful comments and suggestions. This research was supported by the NASA Astrobiology

441

Institute, the Deep Carbon Observatory, Johns Hopkins University, the Carnegie Institute for


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Page 20 of 33

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Science and funded by grants from the National Science Foundation EAR-1023865 (DAS) and

443

EAR-1023889 (RMH) and Department of Energy DE-FG02-96ER-14616.

444

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Figure 1. Adsorption of ribose on brucite as function of the concentration remaining in water for the (a) low-Mg2+ (red) and high-Mg2+ (blue) experiments and the (b) low-Ca2+ (purple) and high-Ca2+ (green) experiments. Predictions of ribose adsorption from the surface reactions in eqs 4 and 5 are indicated by dashed and dotted lines, respectively. Symbols represent experimental data following a triplicate run and vertical error bars represent one standard deviation.


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Figure 2. Two potential visual representations of the ribose surface species predicted from eqs 4 and 5 as a bidentate outer-sphere, or “standing” species using the ETLM and the parameters in Table 2. Ribose may attach onto (a) one neutral and one positively-charged surface site or (b) two positively-charged surface sites. The idealized, lateral (100) brucite growth surface is shown with red spheres as O, yellow spheres as Mg, light pink spheres as H, and black spheres as C atoms.


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Figure 3. Adsorption of Ca onto brucite as a function of initial ribose concentration added for the low-Ca2+ experiment (purple) and the high-Ca2+ experiment (green). Predictions of Ca adsorption from the surface reactions in eqs 9 and 10 are indicated by dashed curves. Symbols represent experimental data following a triplicate run and vertical error bars represent one standard deviation.


26

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Figure 4. A visual representation of the (a) cooperative Ca-ribose and (b) a hydrated Ca surface outer-sphere species predicted by eqs 9 and 10 using the ETLM and the parameters in Table 2. The idealized, lateral (100) brucite surface is shown with red spheres as O, yellow spheres as Mg, light pink spheres as H, black spheres as C atoms, and light blue spheres as Ca.


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Figure 5. ζ-potential (mv), or particle surface charge, predicted with the ETLM and the parameters in Table 2 as a function of initial ribose concentration and the four experimental conditions examined in this study. Predictions from eqs 4 and 5 are represented with dashed and dotted curves, respectively, and overlay one another for the much of studied initial ribose conditions.


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Figure 6. Predicted surface speciation of ribose on brucite for the (a) low-Ca2+ and (b) high-Ca2+ batch adsorption experiments. The “standing” (blue) and Ca outer-sphere (orange) species refer to those proposed in eqs 4 and 5, and eq 9, respectively. Predictions from eq 4 (dashed curve) and eq 5 (dotted curve) overlay one another over the entire initial ribose concentration range. Symbols represent experimental data following a triplicate run and vertical error bars represent one standard deviation.


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Figure 7. Predictions of ribose adsorption on brucite as a function of pH at the low-Mg2+ (red), high-Mg2+ (blue), low-Ca2+ (purple), and high-Ca2+ (green) conditions at (a) 75 µM and (b) 150 µM ribose. Predictions are based on existing adsorption data (symbols), ETLM parameters in Table 2, and eqs 4 and 5, which are indicated by the dashed and dotted curves, respectively.


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Table 1. Description of the low-Mg2+, high-Mg2+, low-Ca2+, and high-Ca2+ experiments. Dissolved Mg2+ concentrations from brucite are present in each experiment in addition to added MgCl2 and CaCl2. All concentrations are expressed in mmol/L.

Experiment

Dissolved Mg2+

Added MgCl2

Added CaCl2

Ionic Strength

pH

Low-Mg2+

0.6

--

--

2.1

10.4

High-Mg2+

0.6

5.2

--

17.4

9.8

Low-Ca2+

0.9

--

1.0

5.8

10.2

High-Ca2+

0.6

--

4.2

15.2

10.3


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Table 2. Aqueous ribose properties, characteristics of brucite [Mg(OH2)], and extended triple-layer model parameters for proton electrolyte and ribose adsorption onto brucite. Reaction Type Aqueous ribose equilibria Surface equilibria

Reaction Rib– + H+ = HRib0

log K 12.00

HRib0 + Ca2+ = HRibCa2+

0.3051

Hypothetical 1.0 m standard state

logK=$

>SOH + H+ = >SOH2+

9.69

logK $6

>SO- + H+ = >SOH

11.31

log ∗ K $>

>SOH + Na+ = >SO-_Na+ + H+

-8.91

log ∗ K $?#

>SOH + Cl- + H+ = >SOH2+_Cl-

11.89

2>SOH + HRib0 = >SOH>SOH2+_Rib–

4.97

log ∗ K $_

log ∗ K $6_

log ∗ K $_

!"#

!"#

2+

!"

log ∗ K $__()

2>SOH + H+ + HRib0 = 2>SOH2+_Rib– 2+

logK @_

+

-3.43 -5.11

2>SOH2 + HRib0 = >SOH>SOH2+_Rib– + 2H+

7.51

>SOH + Ca + 2H2O = >SOH2 _Ca(OH)2 + H Site-occupancy standard states

logK @_

15.45 + +

Surface equilibria

logK @6_

+

>SOH + Ca + HRib = >SOH_CaRib + H

a

0

+

!"#

+

!"#

0

logK @__()

–

+

2>SOH2 + HRib = 2>SOH2 _Rib + H 2+

!"

+

0

+

17.99 +

>SOH + Ca + HRib = >SOH_CaRib + H +

2+

+

>SOH2 + Ca + 2H2O = >SOH2 _Ca(OH2) + H

-3.66 +

-5.34

a

Equilibrium constants relative to site occupancy standard states can be written relative to charged surface sites calculated using the equations: logK @ _ !"# = log ∗ K $ _ !"# + log(NsAsCs)/100 ; logK @6 _ !"# = log ∗ K $6 _ !"# + log(NsAsCs)/100; logK @_ !" =

log ∗ K $_ !" + log(NsAs)/100 ; logK @ __() = log ∗ K $ __() + log(NsAs)/100, where Ns is site density, As is BET edge surface area m2•g-1, and Cs is the mineral loading g•L-1.


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Graphical Abstract


33

Ribose onto Brucite - ACS Publications

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