Cooperative and Competitive Adsorption of Amino Acids with Ca2+ on

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Cooperative and competitive adsorption of amino acids with Ca2+ on rutile (#-TiO2) Namhey Lee, Dimitri A. Sverjensky, and Robert M Hazen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es501980y • Publication Date (Web): 10 Jul 2014 Downloaded from http://pubs.acs.org on July 13, 2014

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Environmental Science & Technology

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Cooperative and competitive adsorption of amino acids with Ca2+ on rutile (α-TiO2)

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Namhey Leea,b*, Dimitri A. Sverjenskya,b, and Robert M. Hazenb a

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Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA.

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b

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Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, 20015, USA. *Correspondence to: [email protected] Abstract The interactions of biomolecules such as amino acids with mineral surfaces in the near-

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surface environment are an important part of the short and long-term carbon cycles.

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Amino acid-mineral surface interactions also play an important role in biomineralization,

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biomedicine, and in assembling the building blocks of life in the prebiotic era. Although

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the pH effects during adsorption of amino acids onto mineral surfaces have been studied,

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little is known about the effects of environmentally important divalent cations. In this

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study, we investigated the adsorption of the oppositely charged amino acids glutamate

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and lysine with and without the addition of divalent calcium. Without calcium, glutamate

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shows a maximum in adsorption at a pH of ~4 and lysine shows a maximum in

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adsorption at a pH of ~9.4. In comparison, with calcium present, glutamate showed

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maxima in adsorption at both low and high pH, whereas lysine showed no adsorption at

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all. These dramatic effects can be described as cooperative adsorption between glutamate

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and Ca2+ and as competitive adsorption between lysine and Ca2+. The origin of these

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effects can be attributed to electrostatic phenomena. Adsorption of Ca2+ at high pH makes

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the rutile surface more positive, which attracts glutamate and repels lysine. Our results

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indicate that the interactions of biomolecules with mineral surfaces in the environment

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will be strongly affected by the major dissolved species in natural waters.

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Keyword: glutamate, lysine, cation, rutile, cooperative/competitive adsorption

1. Introduction The interactions between mineral surfaces and organic molecules are ubiquitous,

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ranging from the fate of organic matter during weathering and transport to the oceans, the

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transport of nutrients in soil, and the fate of contaminants, to medical issues including

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biotechnology, pharmaceuticals and the viability of metal implants in the human body1-9.

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Recent dramatic increases of interest in nanoparticles have also brought our attention to

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how organic molecules alter mineral growth at surfaces10-12. Furthermore, mineral

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surfaces and organic molecule interactions can provide vital clues for the evolution of

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abiotically-formed organic molecules in prebiotic times.

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By definition, all amino acids have both positive and negatively charged -NH3+, -COOH

-COO-). As a result, the net

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functional groups on them (-NH2

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charge of the molecules varies with pH, leading to a variety of interesting adsorption

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behavior. Previous studies of amino acid adsorption have been restricted to simple

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systems typically containing one amino acid and one mineral in a 1:1 background

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electrolyte13-16. However, near-surface natural waters are more complex: they are most

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commonly Ca2+-Na+-HCO3--SiO2 waters. As a first step to begin addressing this

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complexity, we investigated the potential competitive or cooperative adsorption of amino

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acids and Ca2+ on the rutile surface. We compared how the addition of Ca2+ changes the

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adsorption characteristics of glutamate and lysine on rutile. The results illustrate the

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responses of two very different amino acids to the presence of Ca2+ under a range of

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environmental conditions. While it is known that cations can form bridging complexes

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with organics by forming tertiary complexes17, few studies have been done involving

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cations and amino acids18. Studies involving metal-humic acid interactions show that in

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the presence of divalent cations, the adsorption of organics is enhanced at high pH

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values19, 20. However due to the complex structure of NOM and its massive size, it is not

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easy to pinpoint what is causing this enhanced adsorption. In the present study, we built

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on our previous studies of glutamate adsorption on rutile13, 21 together with our surface

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complexation modeling studies of calcium on rutile22, to enable an experimental and

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surface complexation modeling study of the adsorption of amino acids on rutile in the

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presence of calcium.

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2. Materials and Methods

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All solutions were made from milli-Q water (Millipore resistance 18.2 Mega

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ohm).

L-glutamic acid (Acros Organics, 99%), L-lysine monohydrochloride (Acros

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Organics, 99%) and calcium chloride standard solution of 0.5 M (Fluka analytical) were

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used without any further purification. The solutions were sonicated for more than 15

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minutes and visually checked prior to use to ensure complete dissolution. The pH was

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adjusted by adding precise volumes of standardized NaOH and HCl. Measurement of pH

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was carried out using a combination electrode (Thermo-Electron, Orion 8103 BNUWP)

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that was previously calibrated with standardized pH buffers.

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The rutile powder used in this study was obtained from Oak Ridge National

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Laboratory (courtesy of J. Rosenqvist, D. Wesolowski, and M. Machesky). At Oak Ridge

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National Laboratory, rutile powder from Tioxide Specialties Ltd. (Cleveland, UK) was 3



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pretreated using the procedure developed by Machesky et al.23 that includes numerous

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washing-boiling-decanting cycles in Milli-Q water until the supernatant had a pH>4. The

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suspension was then thermally treated at 200 °C for two weeks in a Teflon-lined

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autoclave. The acid released during the thermal treatment was removed by repeated

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washing-decanting cycles, until the pH of the supernatant was above 5. Then the powder

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was dried in a vacuum oven at 60 °C. The BET surface area was determined to be 18.1 ±

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0.1 m2.g-1 by N2 adsorption. X-ray powder diffraction (XRD) confirmed that the particles

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were rutile. SEM images showed needled-shaped particles that are approximately 400-

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500 nm long and 50-100 nm wide. The predominant face is (110). Additional (101) and

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(111) faces were present near both ends of the particles as well as on (110) face in the

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form of steps and kinks

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(pHpzc) for this rutile is 5.413.

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. Previous titration data showed that the point of zero charge

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2.1 Batch adsorption experiments

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Batch adsorption experiments were conducted with a solid concentration of 3.0

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g.L-1 and amino acid concentration of 10 µM. Precise volume of calcium chloride

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standard solution was added to amino acid solutions to give desired concentrations. In a

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50 mL falcon tube, 0.09 g of rutile powder was mixed with 30 mL of each amino acid

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solution. The pH was adjusted ranging from 3 to 11 while constantly purged with argon

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gas to avoid contamination by CO2. Then the suspensions were sealed, put on the rotator

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for about 18 hours and allowed to reach steady state. After that, the pH was measured, the

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suspensions were centrifuged for 15 min at a relative centrifugal force of 1073×g (Fisher

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Scientific accuSpin 400), and the supernatant was collected by filtering through 0.2 mM

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filter syringes (Waters). The supernatants were then analyzed using ion chromatography

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to determine the amino acids remaining in the solution. For the detection and

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measurement of amino acids a Dionex ICS-5000 AAA-Direct ion chromatograph was

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used. The chromatograph was equipped with a 2-250 AminoPac PA10 analytical column

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and an integrated pulsed amperometry (IPAD) electrochemical detector25. Unlike other

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traditional methods for analyzing amino acids, the AminoPac column allows the accurate

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detection of amino acids in water without the complications of pre- or post-column

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derivitization. The method consists of high-pH anion exchange separation of the analyte

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solution in the AminoPAC column followed by integrated pulsed amperometry

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electrochemical detection. Compounds containing aliphatic amine functional groups are

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oxidized at the gold electrode in high pH solutions and are therefore amenable to

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electrochemical detection. Direct analysis of the amino acid used in this method not only

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minimizes the uncertainties of incomplete derivitization of conventional amino acid

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detection methods, but also allows monitoring for any additional amine-bearing

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molecules in the system. In this study, the amino acids did not show any sign of

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degradation after being exposed to the rutile.

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2.2 Surface complexation modeling We applied the extended triple-layer model (ETLM) of surface complexation22, 26,

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glutamate adsorption in the presence of calcium, the model used was completely

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predictive, without fitting parameters. Amongst other features, the ETLM specifically

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accounts for the electrical work associated with desorption of chemisorbed water

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molecules during inner-sphere surface complexation of ligands. As a consequence, it

to our adsorption data. We wish to emphasize that for the calculations modeling

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indicates the number of inner-sphere linkages (e.g. >Ti–O–C) for an adsorbate ligand, as

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well as the number of Ti surface sites involved in the reaction stoichiometry. These

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results can significantly constrain the likely mode of surface attachment. The calculations

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reported below were carried out with the computer code GEOSURF described

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previously28. We used the same surface protonation and electrolyte adsorption parameters

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established in our previous study of the rutile–NaCl system13 (Table 1). The glutamate

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adsorption reactions and equilibrium constants used in the present study were also

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consistent with those established in our previous study. For calcium adsorption we used

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the previously published ETLM for calcium adsorption on rutile22. For both the glutamate

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and the calcium surface reactions, the log K values previously published were adjusted

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for the tetranuclear complexes to account for the lower solid concentrations in the present

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study following the standard state theory previously published29. In this way, the surface

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complexation model for glutamate with calcium used in the present study was completely

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predictive. For lysine on rutile, we carried out iterative application of the surface

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complexation modeling to our experimental adsorption data over a wide range of pH

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values. Attachments of lysine were adopted based on surface species suggested in the

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previous ATR–FTIR studies to establish the most appropriate reaction stoichiometries for

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lysine on rutile5, 30-32. The detailed attachment mode of lysine is discussed below.

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3. Results and discussion

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3.1 Glutamate adsorption with and without calcium present

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The previous glutamate adsorption study on rutile established the adsorption as a

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function of salt concentration and surface coverage over a wide range of pH with a rutile

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concentration of 20 g/L13. The results indicate that glutamate has its maximum adsorption 6


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near a pH of about 4 where the rutile surface is positively charged and the glutamate is

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negatively-charged (HGlu-). However, at high pH values in the range of 7 to about 10,

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there is essentially no adsorption as the rutile surface is negatively charged which

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repulses the negatively-charged glutamate. Detailed studies of the adsorption mechanism

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of glutamate on rutile using the ETLM and ATR-FTIR spectroscopy combined with

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quantum chemical calculations revealed that there are at least two surface species present

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for glutamate attachment on rutile surfaces13,

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species where glutamate attaches "lying-down" on the surface and binding occurs through

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inner-sphere coordination of both α-and γ-carboxyl groups. The other species is a

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chelating-monodentate species in which glutamate binds through inner-sphere

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coordination with the γ-carboxyl group in a "standing-up" configuration (with or without

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protonation of the α-carboxyl). This binding conformation is illustrated in Fig. 1 from

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Jonsson et al. (2009)13. The predicted surface speciation shows that the bridging-

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bidentate species is dominant at low pH values and low glutamate concentrations13. The

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chelating species becomes dominant at high pH values and high glutamate

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concentrations.

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. One species is a bridging-bidentate

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In the present study, batch adsorption experiments were carried out in the same

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manner, but with a much lower solid concentration of 3.0 g.L-1, no added NaCl and with

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about two orders of magnitude lower glutamate concentrations (10 µM). These conditions

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were chosen to amplify the percent adsorption of amino acids and to increase the surface

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coverage by calcium. The adsorption data for glutamate alone are depicted in Fig. 2a.

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Overall, the results showed comparable pH-dependent adsorption behavior to that

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previously observed, with a maximum at a pH value of about 4 (Fig. 2a). The maximum

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adsorption density was about 0.13 µM.m-2.

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In Figs. 2c and d, the adsorption of glutamate is shown in the presence of calcium.

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With 1 and 3 mM of calcium ion present, the glutamate adsorption properties changed

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substantially. Glutamate still has a maximum in its adsorption near a pH of 4 (about 0.1

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µM/m2), albeit slightly lower than when only glutamate is present. However, an

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additional adsorption maximum is now also observed at high pH values, from about 8 to

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11. With 1 mM of calcium present, glutamate was adsorbed up to 0.05 µM/m2 and with 3

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mM of Ca2+ the adsorption density increased up to 0.075 µM.m-2 at a pH of about 9.6.

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Glutamate adsorption at a pH of about 4 did not vary significantly with the calcium

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concentration. However, near a pH of about 9.6, the glutamate adsorption shows a strong

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dependence on the Ca2+ concentrations. This suggests that the adsorption near a pH of 4

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originates solely from the rutile and glutamate interactions whereas near a pH of 9.6, the

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adsorption is clearly facilitated by Ca2+.

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It has long been known that strong adsorption of Ca2+, and other divalent cations,

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at high pH converts the surface charge on rutile from negative to positive33. Therefore, it

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can be expected that the positively charged rutile surface at high pH values will

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electrostatically attract the negatively-charged glutamate. Effectively, this should result in

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a cooperative adsorption of calcium and glutamate at high pH values. Although in

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principle the cooperative adsorption of calcium and glutamate could result from the

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adsorption of an aqueous Ca-glutamate complex, aqueous speciation calculations indicate

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that this is a very weak complex in the aqueous phase, present at low concentrations

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(TiOH functional group, the other attachment point occurs

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through the ε-NH3+ group. This outer-sphere species is predicted to predominate at pH

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values greater than about 9.

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Application of the same surface complexation model to the data shown in Fig. 4c

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was made in a predictive mode. With Ca2+ present, the model predicted no significant

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amount of lysine adsorption, in agreement with the experimental data. These results are

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well explained by the change in surface charge of the rutile with added calcium (Fig. 3).

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The more positive surface charge of the rutile in the presence of calcium repels the

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positively-charged lysine, resulting in no adsorption

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These observations can be generalized to other amino acids as well as other

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simple organic acids. Acidic amino acids that are largely in negatively-charged forms

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will probably show cooperative adsorption with all divalent cations at high pH values

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given that the substrate becomes positively charged at those pH values. On the other

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hand, basic amino acids that are mainly in a positively charged form will be outcompeted

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by divalent cations at high pH conditions where the surface is deprotonated. This is a

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simple yet very interesting observation. It may have wide application because Ca2+ is a

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major ion in near-surface natural waters. Our study illustrates that the interactions of

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amino acids with mineral surfaces in 2:1 electrolytes can differ greatly from the results of

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laboratory experiments in 1:1 electrolytes. The cooperative or competitive adsorption of

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divalent cations and amino acids shows that biomolecules could participate in complex

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adsorption behavior in natural systems. Further experiments are needed to generate a

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better understanding of and provide potential for prediction in natural systems.

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Furthermore, it can be expected that a better understanding of cooperative or competitive

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adsorption could provide insights into such diverse areas as the chemical evolution of

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biomolecules in the origin of life, and the design of biosensors and biodetection methods.

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Acknowledgements

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The authors are extremely grateful for the specially cleaned rutile powder sample

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provided to them by J. Rosenqvist and D. Wesolowski of Oak Ridge National

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Laboratory, as well as M. Machesky.

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assistance in the laboratory from C. M. Jonsson, C. L. Jonsson, C. F. Estrada and C.

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Feuillie. N. Lee and D. A. Sverjensky greatly appreciate the support of R. J. Hemley

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during their stay as visiting researchers at the Geophysical Laboratory. This research was

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conducted with support from the NSF EAR-1023865 (DAS), DOE DE-FG02-96ER-

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14616 (DAS), NSF EAR-1023889 (RMH), NASA Astrobiology Institute, and the

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Carnegie Institution of Washington.

We greatly appreciate discussion with and

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Figure 1. Previously proposed diagrammatic representation of surface species of glutamate: at left is the “lying-down” species, bridging-bidentate species with four points of attachements involving one inner-sphere Ti-O-C bond and one Ti-OH…O=C hydrogen bond for each carboxylate , on the right is the “standing-up” species, chelating with two points of attachment involving one inner-sphere Ti-O-C bond and one TiOH2+…O=C to a single titanium (Jonsson et al., 2009)13).

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Figure 2. Adsorption of glutamate on rutile as a function of pH: (a) Glutamate adsorption without Ca2+; (c) Glutamate adsorption with 1 mM Ca2+; (e) Glutamate adsorption with 3 mM Ca2+. The solid curves were predicted using previously published, separate glutamate and Ca+2+ adsorption models (Table 1); (b), (d), (f) Predicted glutamate surface speciation. Blue dotted line indicates the sum of species. (a) (b)

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(c)

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(e)

(d)

(f)

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Figure 3. Predicted ζ potential vs. pH. Without any ligands on the surface, the particle charge continues to decrease with increasing pH. Glutamate adsorption does not make a noticeable difference. In contrast, with1 and 3 mM Ca2+ present, the surface charge is converted from negative to positive at high pH values.

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Figure 4. Adsorption of lysine on rutile as a function of pH. (a) Percent adsorbed lysine without added Ca2+. The solid curve was calculated using the lysine adsorption model with parameters in Table 1 in order to fit the data shown; (b) Predicted surface speciation of lysine on rutile; (c) Percent adsorbed lysine with 3 mM added Ca2+. (a)

(b)

(c)

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Figure 5. Diagrammatic representation of surface species of lysine based on surface complexation modeling and published ATR-FTIR studies (see text): at left is the “lyingdown” species where lysine attaches via the ε- NH3+ group and the α-carboxylic group forming an hydrogen bond. On the right is the “standing-up” species where lysine is adsorbed to the surface via the ε- NH3+ only. Both species are weakly bound outer-sphere species.

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Table 1. Aqueous glutamate, lysine properties, rutile characteristics, and Extended Triple Layer Model (ETLM) parameters for proton, electrolyte and glutamate, lysine and calcium adsorption on rutile. All the rutile surface protonation and electrolyte adsorption equilibrium constants, and the glutamate adsorption equilibrium constants with a superscript of theta refer to the site-occupancy standard state29. The numerical values of these were taken from previous studies on the same rutile sample13. The equilibrium constants with a superscript of zero refer to the hypothetical 1.0 M standard state and have numerical values consistent with the site densities, the BET surface area, and the solid concentration used in the present study29 Similar considerations apply to the calcium adsorption equilibrium constants which were derived in a previous study22.

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Protonation constants from Smith and Martell (2004)34, electrolyte ion pair constants given by De Stefano et al (2000) 35 3535. Rutile properties are Ns= 3.0. sites/nm2 and 12.5 sites/nm2, As = 18.1 m2g-1, C1= 120 µFcm-2, C2= 120 µFcm-2, pHppzc= 5.4, ∆pKnθ= 6.3, log θ K1θ= 5.25, log K2θ= 8.50, log K θNa+ = 2.68, log KCl − = 2.48.

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(α-TiO2) in NaCl Solutions. Environ. Sci. Technol. 2011, 45, 3959-3966. 16. Vlasova, N. N.; Golovkova, L. P., The adsorption of amino acids on the surface of highly dispersed silica. Colloid J. 2004, 66, (6), 657-662. 17. Schindler, P. W., Co-adsorption of metal ions and organic ligands; formation of ternary surface complexes. Rev. Mineral. Geochem. 1990, 23, 281-307. 18. Fitts, J.; Persson, P.; Brown Jr., G. E.; Parks, G. A., Structure and Bonding of Cu(II)–Glutamate Complexes at the γ-Al2O3–Water Interface. J. Colloid Interface Sci. 1999, 220, (1), 133-147. 19. Weng, L.; Riemsdijk, W. H.; Hiemstra, T., Cu2+ and Ca2+adsorption to goethite in the presence of fulvic acids. Geochim. Cosmochim. Acta 2008, 72, 5857-5870. 20. Zhao, Y.; Geng, J.; Wang, X.; Gu, X.; Gao, S., Adsorption of tetracycline onto goethite in the presence of metal cations and humic substances. J. Colloid Interface Sci. 2011, 361, 247-251. 21. Parikh, S. J.; Kubicki, J. D.; Jonsson, C. M.; Jonsson, C. L.; Hazen, R. M.; Sverjensky, D. A.; Sparks, D. L., Evaluating Glutamate and Aspartate Binding Mechanisms to Rutile (α-TiO2) via ATR-FTIR Spectroscopy and Quantum Chemical Calculations. Langmuir 2011, 27, (5), 1778-1787. 22. Sverjensky, D. A., Prediction of the speciation of alkaline earths adsorbed on mineral surfaces in salt solutions. Geochim. Cosmochim. Acta 2006, 70, (10), 2427-2453. 23. Machesky, M. L.; Wesolowski, D. J.; Palmer, D. A.; Ichiro-Hiyashi, K., Potentiometric titrations of rutile suspensions to 250 C. J. Colloid Interface Sci. 1998, 200, (2), 298-309. 24. Livi, K. S., B; Azzolini, D; Seabourne, CR;Hardcastle, TP; Scott, AJ; Scott, AJ; Erlebacher, JD ;Brydson, R;Sverjensky, DA, Atomic-Scale Surface Roughness of Rutile and Implications for Organic Molecule Adsorption. Langmuir 2013, 29, (23), 6876-6883. 25. Clarke, A. P.; Jandik, P.; Rocklin, R. D.; Liu, Y.; Avdalovic, N., An integrated amperometry waveform for the direct, sensitive detection of amino acids and amino sugars following anion-exchange chromatography. Anal. Chem. 1999, 71, (14), 27742781. 26. Sverjensky, D. A., Prediction of surface charge on oxides in salt solutions: Revisions for 1 : 1 (M+L-) electrolytes. Geochim. Cosmochim. Acta 2005, 69, (2), 225257. 27. Sverjensky, D. A.; Fukushi, K., Anion adsorption on oxide surfaces: Inclusion of the water dipole in modeling the electrostatics of ligand exchange. Environ. Sci. Technol. 2006, 40, (1), 263-271. 28. Sahai, N.; Sverjensky, D. A., GEOSURF: A computer program for modeling adsorption on mineral surfaces from aqueous solution. Comput. Geosci. 1998, 24, (9), 853-873. 29. Sverjensky, D. A., Standard states for the activities of mineral surface sites and species. Geochim. Cosmochim. Acta 2003, 67, (1), 17-28. 30. Kitadai, N.; Yokoyama, T.; Nakashima, S., ATR-IR spectroscopic study of Llysine adsorption on amorphous silica. J. Colloid Interface Sci. 2009, 329, (1), 31-37. 31. Kitadai, N.; Yokoyama, T.; Nakashima, S., In situ ATR-IR investigation of Llysine adsorption on montmorillonite. J. Colloid Interface Sci. 2009, 338, (2), 395-401.

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32. Roddick-Lanzilotta, A. D.; Connor, P. A.; McQuillan, A. J., An In Situ Infrared Spectroscopic Study of the Adsorption of Lysine to TiO2 from an Aqueous Solution. Langmuir 1998, 14, (22), 6479-6484. 33. Jang, H. F., DW, The specific adsorption of alkaline-earth cations at the rutile/water interfae. Colloids surfaces 1986, 21, 235-257. 34. Smith, R. M.; Martell, A. E., NIST Critically Selected Stability Constants of Metal Complexes Database. In Administration, T., Ed. U. S. Department of Commerce: Washington, DC, 2004. 35. De Stefano, C.; Foti, C.; Gianguzza, A.; Sammartano, S., The interaction of amino acids with the major constituents of natural waters at different ionic strengths. Marine Chemistry 2000, 72, (1), 61-76.

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No Ca2+

With Ca2+


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Cooperative and Competitive Adsorption of Amino Acids with Ca2+ on

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