Influence of Biomacromolecules on the Stability of Colloidal

Chapter 10

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 23, 2016 | http://pubs.acs.org Publication Date (Web): November 17, 2015 | doi: 10.1021/bk-2015-1197.ch010

Influence of Biomacromolecules on the Stability of Colloidal Manganese Dioxide Xiaoliu Huangfu and Jun Ma* State Key Laboratory of Urban Water Resource and Environment, School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China *E-mail: [email protected]

The stability of common manganese dioxide (MnO2) colloids has great impact on their surface reactivity and therefore on the fates of these particles as well as the associated organism communities. In this work, MnO2 stability was uncovered by the early stage aggregation kinetics of MnO2 colloids in aqueous solution and the effects of biomacromolecules (i.e., alginate and bovine serum albumin).Markalbe aggregation was obtained in the presence of both mono- and divalent cations, and the the critical coagulation concentration concentrations were 28 mM NaNO3 and 0.45 mM Ca(NO3)2. The Hamaker constant of MnO2 colloids calculated by classical DLVO theory based on the experimental data in aqueous solution was about 7.84×10-20 J. Both macromolecules tested enhanced MnO2 colloidal mobility greatly, and the stabilizing effects might mainly result from the steric repulsive forces which were introduced by organic layers adsorbed on MnO2 colloidal surfaces. Nevertheless, the complexes formed by alginate and Ca2+ (>5 mM) enhanced MnO2 colloidal aggregation due to a bridging role.

Introduction It is well documented that the occurrence of manganese dioxide (MnO2) colloids in aquatic systems (1–3). MnO2 colloidal surface reactions, such as adsorption/desorption and redox etc., may impact the fat and transport of both © 2015 American Chemical Society In Advances in the Environmental Biogeochemistry of Manganese Oxides; Feng, Xionghan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


natural and synthetic organic matter in the aquatic environment and related organism community once they are formed (3–5). However, MnO2 colloids can also undergo aggregation in aqueous solution, and thus their surface reactivity may be changed (6–8). Thus, data on the MnO2 colloidal stability and aggregation kinetics in aquatic environments are critical for understanding the fate and transport of themselves as well as associated contaminants. Numerous studies have been conducted to examine the aggregation behavior of various nanomaterials, such as silica nanoparticles (NPs), carbon nanotubes, and TiO2 NPs in synthetic and/or natural waters. Numerous publications reported the aggregation behavior of various nanomaterials, such as silica nanoparticles (NPs) and TiO2 NPs in natural waters (9–11). According to classical Derjaguin-LandauVerwey-Overbeek (DLVO) theory, the energy barrier between colloids, strongly dependent upon solution chemistry (e.g., pH, ionic strength, and electrolyte ion valence), affected their aggregation (11). However, when macromolecular organic matter (e.g., humic substances (HS), polysaccharides, and proteins) was present, their interactions were much more complicated. These macromolecules could stabilize colloidal particles due to the combining effects of the electrostatic, steric, and bridging forces induced by their adsorption on particle surfaces. Alginate stabilize NPs (e.g., SWNTs and hematite NPs) in the presence of Na+ or low concentrations of Ca2+ (10, 12), while in the presence of high concentrations of Ca2+, alginate could increase their aggregation rates instead through increasing the measured particle diameters due to the bridging effect of alginate-Ca2+ complexes (10, 12). Moreover, as an important biomacromolecule, protein could also stabilize colloids by increasing the steric repulsion between particles. For instance, bovine serum albumin (BSA) could enhance the stability of SWNTs colloids remarkably in the natural aquatic environments (10). A similar steric stabilization was also observed for TiO2 colloids in the presence of fetal serum albumin and human serum albumin (13). Theory DLVO Interactions The aggregation kinetics data of MnO2 NPs in the presence of NaNO3 were compared with DLVO theory. Accordingly, the attachment efficiency (α, obtained from experimental data) of aggregating nanoparticles which accounts for colloidal and hydrodynamic interactions is given by (14, 21):

186 In Advances in the Environmental Biogeochemistry of Manganese Oxides; Feng, Xionghan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


where h is the surface-to-surface distance between two particles, a is the intensityweighted radius of MnO2 NPs measured by DLS (27.93 nm), kB is Boltzmann constant, and T is the absolute temperature (298.15 K). The total interaction energy between the two particles (VT(h)) is the sum of the van der Waals attraction (VA(h)) and electrical double layer interaction (VEDL(h)). The expression proposed by Gregory was used to calculate VA(h) (15):

VEDL(h) was computed by the linear superposition approximation (LSA) expression (16):

where A121 is the Hamaker constant of the particle-water-particle, λ is the characteristic wavelength of the dielectric (assumed to be 100 nm), ε0 is the vacuum dielectric permittivity, εr is the relative dielectric permittivity of solution for water (εr=78.5), z is the valence of bulk ions, 1 for sodium ions, e is the electron charge, Γ is the dimensionless surface potential for particles, and κ-1 is the Debye length. Γ function is given by:

where ψ is the surface potential. The dimensionless function β(h) corrects for the hydrodynamic interaction between two approaching particles (17):

Ohshima’s Soft Particle Theory Theoretical analysis of Electrophoretic mobility (EPM) data by Ohshima’s soft particle theory could define the characteristics of the adsorbed organic layers and further our understanding on the interactions between colloids. Because both biomacromolecules (Alginate and BSA) and MnO2 colloids carry charges at various aqueous conditions, the EPM data can be interpreted according 187 In Advances in the Environmental Biogeochemistry of Manganese Oxides; Feng, Xionghan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


to Ohshima’s soft theory for a charged bare particle with a charged layer. The characteristics of adsorbed organic matter layers (layer thickness, d, soft parameter, λf and charged density, ZN) was obtained by fitting EPM data (μe) of MnO2 NPs in the presence of biomacromolecules with Ohshima’s soft theory:

where η is the viscosity of water, ζ is the apparent Zeta potential of the bare particles calculated from EPM measurements based on Smoluchowski’s formula, Z is the valance of the charged functional groups in the adsorbed layer, N is the number density of the charged groups, f(d/a) is the function, varies according to layer thickness: 1 for a thin adsorbed layer relative to a core particle size and 2/3 for a thick layer, ψDON is the Donnan potential, κm is the effective Debye-Hückel parameter, and calculated as follows (18, 19):

where ψ0 is the surface potential at the boundary between the charged adsorbed layers and the surrounding solution, and calculated as follows (20):

where n is the concentration of bulk ions, Eq. 10 is valid when λfd>1 and κd>1. 188 In Advances in the Environmental Biogeochemistry of Manganese Oxides; Feng, Xionghan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Impacts of Biomacromolecules


Characteristics of Aqueous MnO2 Colloids MnO2 colloids were synthesized by reduction of KMnO4 by Na2S2O3 method. KMnO4 solution was rapidly stirred with a magnetic stirrer, and the solution was purged with N2 at the same time. 30 min late, the stoichiometric amount of Na2S2O3 solution was added drop-wise. The brown MnO2 colloids were formed and continuously stirred over 12 hours. Maximum absorption peaks was obtained at the wavelengths of 215 and 365 nm through the UV-vis spectra (Figure 1) of colloidal suspensions. The MnO2 colloidal sizes exhibited a narrow distribution with the peak width at half-maximum of 11.12 nm (Figure 2). The diameters of 24-105 nm with an average value of 55.86±0.26 nm (n=30) was observed in the DLS measurements. The diameters obtained by in situ DLS measurements were slightly smaller than that of TEM result. of freshly prepared MnO2 colloids determined by were slightly greater than the one (Figure 3). MnO2 aggregates showed an irregular structure. The absolute Zeta potential (ζ potential) of these colloids was about -42mV in the presence of 10 mM NaNO3 at pH 6 and 25°C.

Figure 1. UV-vis spectra of MnO2 colloids dispersed in Milli-Q water (0.1mM MnO2, pH 6, and 25°C).

189 In Advances in the Environmental Biogeochemistry of Manganese Oxides; Feng, Xionghan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Figure 2. Representative number-weighted hydrodynamic diameter distribution of MnO2 colloids (0.1mM MnO2, pH 6, and 25°C).

Figure 3. Representative TEM micrograph of MnO2 aggregates (pH 6 and 25°C). 190 In Advances in the Environmental Biogeochemistry of Manganese Oxides; Feng, Xionghan, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

MnO2 Colloidal Stability in the Presence of Monovalent and Divalent Cations


DLVO-type interactions were observed in the presence of monovalent and divalent cations (Figure 4). As can be seen in Figure 4a, the increase of the electrolyte concentration resulted to a corresponding increase in the aggregation rate at relatively low concentration regime (i.e., < 20 mM), in the presence of NaNO3. However, no change was detected for MnO2 colloidal aggregation rate at higher concentration regime (30 and 500 mM). The similar observation was also obtained for aggregation behavior of MnO2 colloids in the presence of Ca(NO3)2 (Figure 4b).

Figure 4. Aggregation profiles of MnO2 colloids (0.03mM) in various electrolyte solutions: a) NaNO3; b) Ca( NO3)2 at pH 6 and 25°C (21). Representative α values as a function of electrolyte concentrations are presented in Figure 5. At low concentration regime of NaNO3, defined as reaction-limited (slow) aggregation regime (α

Influence of Biomacromolecules on the Stability of Colloidal

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