Article pubs.acs.org/Langmuir
Adsorption of Lysine on Na-Montmorillonite and Competition with Ca2+: A Combined XRD and ATR-FTIR Study Yanli Yang,†,‡ Shengrui Wang,*,†,‡ Jingyang Liu,§ Yisheng Xu,† and Xiaoyun Zhou§ †
State Key Laboratory of Environmental Criteria and Risk Assessment, ‡State Environmental Protection Key Laboratory for Lake Pollution Control, Research Center of Lake Eco-Environment, and §State Key Laboratory of Environmental Protection Ecology Industry, Chinese Research Academy of Environmental Sciences, Beijing 100012, China S Supporting Information *
ABSTRACT: Lysine adsorption at clay/aqueous interfaces plays an important role in the mobility, bioavailability, and degradation of amino acids in the environment. Knowledge of these interfacial interactions facilitates our full understanding of the fate and transport of amino acids. Here, X-ray diffraction (XRD) and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) measurements were used to explore the dynamic process of lysine adsorption on montmorillonite and the competition with Ca2+ at the molecular level. Density functional theory (DFT) calculations were employed to determine the peak assignments of dissolved lysine in the solution phase. Three surface complexes, including dicationic, cationic, and zwitterionic structures, were observed to attach to the clay edge sites and penetrate the interlayer space. The increased surface coverage and Ca2+ competition did not affect the interfacial lysine structures at a certain pH, whereas an elevated lysine concentration contributed to zwitterionic-type coordination at pH 10. Moreover, clay dissolution at pH 4 could be inhibited at a higher surface coverage with 5 and 10 mM lysine, whereas the inhibition effect was inconspicuous or undetected at pH 7 and 10. The presence of Ca2+ not only could remove a part of the adsorbed lysine but also could facilitate the readsorption of dissolved Si4+ and Al3+ and surface protonation. Our results provide new insights into the process of lysine adsorption and its effects on montmorillonite surface sites.
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INTRODUCTION Dissolved free amino acids are widely spread in soils, sediments, and natural waters. The interactions between minerals and amino acids are of fundamental interest in a variety of natural processes ranging from biomineralization and biomedicine to the chemical evolution of life on earth.1−3 Lysine has two NH3+ groups and one COOH group in its molecular structure. This type of basic amino acid can easily adsorb onto clay minerals, which facilitates the removal of bioavailable lysine from the soil solution, overlying water, and sediment pore water.2 Moreover, the adsorption reactions can retard lysine migration and alter its bioavailability and stability in the environment.4,5 Insight into the molecular-level adsorption process is critical to revealing the fate and transport of amino acids and the geochemical cycling of nitrogen. The interactions between lysine and mineral surfaces are strongly affected by the abundant coexisting substances in natural waters, i.e., metal ions, oxyanions, dissolved organic matter, and so forth. It is of great importance to explore competitive adsorption (or coadsorption) with the major dissolved species in the environment.6−9 However, there have been only a few studies examining single lysine adsorption on TiO2, SiO2, and montmorillonite.3,10−13 Little is known about © XXXX American Chemical Society
the molecular-level process of lysine adsorption in competing systems as well as its effect on mineral dissolution. For example, Lee et al. studied the adsorption characteristics of lysine on TiO2 in the presence of Ca2+ via the pH adsorption edge and surface complexation modeling.1 Lysine adsorption mainly occurred under basic conditions, whereas Ca2+ competition led to an unfavorable capacity within the pH values of 3−11. Montmorillonite is composed of units with two silica tetrahedral sheets sandwiching a central alumina octahedral sheet.14 Adsorption interactions can occur at both the clay edge sites and interlayer sites (cation exchange),15 which are different from those on metal oxide surfaces. Notably, the bonding mechanisms of lysine on montmorillonite are inconsistent in previous reports. Parbhakar et al. found that lysine adsorption was first dominated by cation exchange and then by electrostatic adsorption in a zwitterionic structure.11 The intercalated lysine was changed from a cationic to a zwitterionic form with increasing lysine concentration. Kitadai suggested that the adsorbed lysine was predominantly present Received: February 12, 2016 Revised: April 21, 2016
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DOI: 10.1021/acs.langmuir.6b00563 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
peak numbers and locations. Overlapping peaks were analyzed via the curve-fitting procedure with Gaussian line shapes.5,6,19,20 The spectra of dissolved lysine were obtained by subtracting the background spectrum (0.01 M NaCl) from the sample spectrum at the same pH. Interfacial lysine was measured with a montmorillonite film, which was coated on the ATR crystal by applying 1 mL of the clay suspension (3 g/L) and drying at 50 °C for 1 h. The background electrolyte (0.01 M NaCl) was passed through the flow cell at a rate of 0.25 mL/min. The background spectrum was collected after the clay film was equilibrated for 30 min. Then, the solution was changed to the sample with the same pH. Interfacial spectra were recorded as a function of time for 60 min. In single-adsorbate systems, a lysine concentration ranging from 1 to 10 mM was selected to obtain highquality spectra. In competing systems, lysine and Ca2+ were analyzed at a concentration of 1 mM in 0.01 M NaCl. The dynamic process herein was investigated by flowing single lysine first (25 min) and then with the competing Ca2+. DFT Calculations. Geometry optimization and IR frequency calculations of the lysine dication, cation, and zwitterion were performed with the Gaussian 03 program21 at the DFT/B3LYP level. The modeled structures were energy minimized without symmetry or other constraints during the optimization procedure. The frequencies based on the energy-minimized structures were calculated with the 6-31+G (d, p) basis set on C, H, O, and N (scale factor = 0.964).22 The solvation effect was considered by adding explicit H2O molecules around the functional groups in the gas phase.23,24 Ten H2O molecules were necessary for simulating the solvation sphere in order to ensure at least one H-bond to each O and H atom in the carboxyl and amino groups.
in the cationic state within pH values of 4.9−9.7, which interacted with the montmorillonite surface through the protonated NH3+ side chain.12 Inconsistent and insufficient studies have motivated our present research to reveal the adsorption behavior of lysine on montmorillonite in single and competing systems at the molecular level. The objective of this study was to explore the dynamic process of lysine adsorption on montmorillonite and the spectral changes of ν(Si−O) and δ(Al−OH) peaks under different pH values, lysine concentrations, and Ca2+ competition. X-ray diffraction (XRD) analysis and attenuated total reflectance Fourier-transform infrared spectroscopy (ATRFTIR) flow-cell measurements were used to determine the bonding mechanisms, interfacial structures, and clay dissolution. Density functional theory (DFT) calculations were employed to identify peak assignments of the lysine dication, cation, and zwitterion in solution. The dynamic process was analyzed on the basis of absorbance variations in the adsorbed lysine and in the kinetic model simulation. Our research provides new and complementary insights for understanding lysine adsorption and can be used to predict and describe the behaviors of amino acids in the environment.
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EXPERIMENTAL SECTION
Materials. L-Lysine and CaCl2 were obtained from Sigma-Aldrich. All chemicals were of analytical or guaranteed reagent grade and were used as received. The samples were prepared with ultrapure water (18.2 MΩ·cm, 25 °C) that was boiled for 60 min to remove CO2. SWy-2 montmorillonite (Crook County, Wyoming, USA) was purchased from the Source Clays Repository of the Clay Minerals Society. The Brunauer−Emmett−Teller (BET) surface area, cation exchange capacity (CEC), and point of zero charge (PZC) were 32 m2/g, 76 molc/kg, and 4.2, respectively.16,17 Na-montmorillonite was prepared according to a modified method reported by Ghayaza et al.18 Five grams of SWy-2 was mixed with 50 mL of 1 M NaCl solution in a centrifuge tube and was shaken on an end-over-end rotator for 24 h. The mixture was centrifuged, the supernatant was discarded, and the tube was then refilled with 1 M NaCl solution. This exchange procedure was repeated six times. The suspension containing the fine clay fraction (
