Unresolved Questions

9.4.1 How Does the Presence of Metals Affect the Properties of Fibrils?

Metal oxide precipitates have been found coating fibrillar material in aquatic systems5-7 and Ca ions are known to form "egg-box" structures with alginic acids.42 Whether metal ions are preferentially scavenged from the water column by fibrils, and how metal binding influences fibril conformation and interaction both with inorganic colloids and other fibrils is generally not known. It is likely that metal-fibril and fibril-inorganic-colloids interactions will render the electrical and chemical properties of the fibrils similar to those of the metal oxides. However, the association of metal ions with fibrils is expected to alter their aggregation characteristics and stickiness factors. In addition, the presence of inorganic colloids would, in general, accelerate floc formation.

Currently, there are no experimental values for stickiness between natural inorganic colloids and organic chains, and no quantitative information about interactions between metals and natural biopolymers. Reported values for stickiness factors of inorganic colloids in freshwater are as low as 10-3 to 10-2.117 Fibrils will have different stickiness properties depending on their chemical composition and nanoscale structure. Stickiness will also almost certainly vary according to the concentration of metals such as Fe and Mn oxide colloidal particles as these will bind to the fibrils and affect the surface charge distribution. Metals are likely to bind at specific sites on the fibrils and can consequently alter the fibril conformation.

9.4.2 How Does the Presence or Absence of Fibrils Affect Particle Formation and Particle Aggregation Rates?

Colloid formation and particle aggregation rates are determined by the rates at which components are brought together (e.g., by Brownian diffusion, turbulent shear) and the probability that they will adhere once they have collided. In almost all models of aggregation in aquatic environments, interparticle adhesion is represented by a single parameter, the stickiness coefficient. It is likely that interparticle interactions depend significantly on the physical and chemical properties of fibrils and thus are crucial for predicting rates of particle aggregation. One example is the presence of covalently bound proteins in acid polysaccharidic hydrocolloids,65 or their general hydrophobicity,69 which determine the degree of stickiness. Furthermore, the presence of metal nucleation sites on fibrils likely alters the dynamics of inorganic colloid formation and, hence, the structure and function of the inorganic colloid fraction.

9.4.3 What Role Do Nanoscale Fibrils Play in Determining the Structure of Larger Scale Aggregates?

Typically, models of particle aggregation in aquatic systems use only a single monomer, which is regarded as being a spherical particle. Recent simulations by Stoll and Buffle112,113 have incorporated a mixture of polymer chains and spherical monomer particles. Different proportions of polymer chains and spherical monomers result in different fractal dimensions for the resulting aggregates. These simulations used constant stickiness factors for different interactions, whereas, in reality, the stickiness will change with the environment. A combination of computer simulations and Small Angle X-ray Spectroscopy (SAXS) experiments would be needed to examine how fibril properties influence the properties (such as the fractal nature of the assemblage) of aggregates.

Theoretical models of fibrils need to be improved. While the heterogeneous composition of fibrils and their high molecular weight poses challenges for detailed molecular modeling, techniques for simulating sections of polysaccharide models are available118; models of solvated proteins have also been constructed.119 Fibrils and their dynamics can be represented using relatively simple models, such as the "pearls-on-a-necklace" (Figure 9.3 and Figure 9.4) construction112,113 or using the optimized Rouse-Zimm theory.120 Simulations need to be performed using both rigid and flexible chains and charge distributions estimated through a combination of experimental data and modeling — for example, using the Macro Model modeling system.121 Similar approaches have been used elsewhere.120,122,123 These models will make use of persistence-length measurements from AFM,75,76 the coordination environment of metals attached to the fibrils from the x-ray Absorption Near Edge Structure (XANES) measurements, as well as charge and structural characteristics from scattering experiments.

Fibril properties (e.g., hydrodynamic radii, adhesion forces) change with the pH and ionic strength of the bulk medium.124 In these simulations, the bulk medium can be represented as a dielectric medium entering the model through its dielectric permittivity.125 Changes in salinity, pH, and ionic strength could be modeled by changes to the dielectric medium used in the simulation, and predictions of aggregation and scavenging will be made for freshwater, estuarine, and coastal environments.

What will be needed is to assess potential changes at the molecular scale of metal oxide nanoparticles entrapped by the fibrils and also to probe the coordinative environment of the metals that are interacting with the fibrils. It is likely that binding of either Fe or Mn nanoparticles by fibrils affect their chemical identity. Nanoparticles of MnOx when precipitated using various trace metals show different features in spectra of XANES measurements (Figure 9.6). These differences in the XANES features show that the Mn local environment is responsive to the type of metal sorbed. In the case of the metals attaching to the fibrils, the average coordination environment of the metal will hold the answer to this question. To follow the "aggregation" process

Energy (keV)

FIGURE 9.6 Mn K-edge XANES spectroscopy of colloidal MnO2 particles prepared according to the method of Perez-Benito et al.126 that were precipitated/aggregated by addition of either protons or various metals. The sorption of the different metals leads to shifts in the characteristic energy of the white line and changes in the pre-edge features illustrating that the coordination of Mn in the nanoparticles is affected.127

Energy (keV)

FIGURE 9.6 Mn K-edge XANES spectroscopy of colloidal MnO2 particles prepared according to the method of Perez-Benito et al.126 that were precipitated/aggregated by addition of either protons or various metals. The sorption of the different metals leads to shifts in the characteristic energy of the white line and changes in the pre-edge features illustrating that the coordination of Mn in the nanoparticles is affected.127

at a molecular scale, time resolved analyses to determine how various metals develop chemical bonds with the fibrils, as well as the formation of metal clusters on the fibrils as a function of time need to be studied.

A better understanding of fibril-metal interactions by theoretical methods is thus necessary. The adsorption of metals onto the fibril affects the charge density and hence the physico-chemical properties of the fibril. In particular, the aggregation (e.g., stickiness) and chemical (e.g., scavenging) properties will change depending on the form of interaction between the fibril and the metal ion. Different forms of interaction can occur: interactions may involve the sharing of electrons (inner-sphere bonds) or electrostatic interactions (outer-sphere bonds). Models need to utilize a combination of standard particle simulation techniques128 and molecular dynamics techniques.129-131 In particular, simulations to examine how fibril interactions change with changing environmental conditions (e.g., changing pH) and metal content would need to be carried out.

Finally, a better understanding of the role of natural organic matter, both humics and fibrils, is required, as they support the self-cleansing capacity of fresh, estuarine, and marine waters, and regulate the export of production of organic matter in open water systems. Knowledge of the detailed mechanisms of trace metal removal, pollutant transport, and formation of sedimentary deposits in aquatic environments hinges on an improved knowledge of the nano-science of natural organic matter.

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