Adsorption of Heterogeneously Charged Nanoparticles on a Variably Charged Surface by the Extended Surface Complexation Approach: Charge Regulation, Chemical Heterogeneity, and Surface Complexation

A new consistent modeling framework for the competitive adsorption of humic nanoparticles and oxyanions to metal (hydr)oxides: Multiple modes of heterogeneity, fractionation, and conformational change

HYPOTHESIS

The competitive interaction of oxyanions and humic nanoparticles (HNPs) with metal (hydr)oxide surfaces can be used to trace the ligand and charge distribution of adsorbed HNPs in relation to heterogeneity, fractionation, and conformational change.

EXPERIMENTS

Batch adsorption experiments of HNPs on goethite were performed in the absence and presence of phosphate. The size of HNPs was measured with size exclusion chromatography. The Ligand and Charge Distribution (LCD) model framework was further developed to describe the simultaneous interaction of HNPs and phosphate with goethite.

FINDINGS

Preferential adsorption decreases the mean molar mass of adsorbed HNPs, independent of the phosphate presence, showing a linear dependency on the adsorbed HNPs fraction. Phosphate ion can be used as a probe to trace the distribution of functional groups and the variation in affinity of HNPs. The spatial distribution of adsorbed HNPs is driven by the potential gradients in the electrical double layer, which changes the conformation of the adsorbed HNPs. At the particle level, the adsorption of heterogeneous HNPs has an affinity distribution, which can be explained by the variation in molar mass (kDa) and density of the functional groups (mol kg-1) of the HNPs. The presented model can simultaneously describe the competitive adsorption of HNPs and phosphate in a consistent manner.

Proton binding to humic nano particles: electrostatic interaction and the condensation approximation

Proton binding to "carboxylic" and "phenolic" sites of humic nano particles (HNPs) is determined by the total proton affinity that is due to a specific and an electrostatic affinity. Both affinities are accounted for in the bi-modal Langmuir-Freundlich (bi-LF)-equation extended with a Boltzmann factor that includes the electrostatic site potential(s), y. For y → 0 the equation reduces to the bi-LF Master Curve (MC). Commonly, an electrical double layer model is used to obtain y, e.g., the bi-LF-Donnan-V_app (monocomponent NICA-Donnan) model and bi-LF-soft-particle-Poison-Boltzmann-Theory (SPBT). A new method is presented that combines the "condensation approximation" (CA) with the MC concept (CA-MC). With the CA, the proton binding curve and MC can be transformed in, respectively, the total and specific affinity distribution. The difference at a given charge density provides the electrostatic affinity and CA-potentials vs. charge density. The MC can be obtained theoretically or by using the convention that the electrostatic interaction is negligible at 1 M salt concentration. For five HNPs CA-potentials corresponding with the bi-LF-SPBT are compared with results of the bi-LF-Donnan-V_app model using the MC(SPBT). The bi-LF-Donnan-V_app model fails when the Debye length > hydrated particle radius. The CA-MC(1M) method does not require characteristics of the HNPs. Combination of the bi-LF-eq. with the CA-MC(1 M) method gives the bi-LF-CA-MC(1 M) model. The CA-MC(1 M) differs from the MC(SPBT); therefore, resulting parameters can only be compared when the same method is used.

Theoretical Modeling of Chemical Equilibrium in Weak Polyelectrolyte Layers on Curved Nanosystems

Surface functionalization with end-tethered weak polyelectrolytes (PE) is a versatile way to modify and control surface properties, given their ability to alter their degree of charge depending on external cues like pH and salt concentration. Weak PEs find usage in a wide range of applications, from colloidal stabilization, lubrication, adhesion, wetting to biomedical applications such as drug delivery and theranostics applications. They are also ubiquitous in many biological systems. Here, we present an overview of some of the main theoretical methods that we consider key in the field of weak PE at interfaces. Several applications involving engineered nanoparticles, synthetic and biological nanopores, as well as biological macromolecules are discussed to illustrate the salient features of systems involving weak PE near an interface or under (nano)confinement. The key feature is that by confining weak PEs near an interface the degree of charge is different from what would be expected in solution. This is the result of the strong coupling between structural organization of weak PE and its chemical state. The responsiveness of engineered and biological nanomaterials comprising weak PE combined with an adequate level of modeling can provide the keys to a rational design of smart nanosystems.

Magnetization of active inclusion bodies: comparison with centrifugation in repetitive biotransformations

Background

Physiological aggregation of a recombinant enzyme into enzymatically active inclusion bodies could be an excellent strategy to obtain immobilized enzymes for industrial biotransformation processes. However, it is not convenient to recycle “gelatinous masses” of protein inclusion bodies from one reaction cycle to another, as high centrifugation forces are needed in large volumes. The magnetization of inclusion bodies is a smart solution for large-scale applications, enabling an easier separation process using a magnetic field.

Results

Magnetically modified inclusion bodies of UDP–glucose pyrophosphorylase were recycled 50 times, in comparison, inclusion bodies of the same enzyme were inactivated during ten reaction cycles if they were recycled by centrifugation. Inclusion bodies of sialic acid aldolase also showed good performance and operational stability after the magnetization procedure.

Conclusions

It is demonstrated here that inclusion bodies can be easily magnetically modified by magnetic iron oxide particles prepared by microwave-assisted synthesis from ferrous sulphate. The magnetic particles stabilize the repetitive use of the inclusion bodies .

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0987-7) contains supplementary material, which is available to authorized users.

Effect of Soil Fulvic and Humic Acids on Pb Binding to the Goethite/Solution Interface: Ligand Charge Distribution Modeling and Speciation Distribution of Pb

The effect of adsorbed soil fulvic (JGFA) and humic acid (JGHA) on Pb binding to goethite was studied with the ligand charge distribution (LCD) model and X-ray absorption fine structure (XAFS) spectroscopy analysis. In the LCD model, the adsorbed small JGFA particles were evenly located in the Stern layer, but the large JGHA particles were distributed over the Stern layer and the diffuse layer, which mainly depended on the JGHA diameter and concentrations. Specific interactions of humic substances (HS) with goethite were modeled by inner-sphere complexes between the -FeOH₂^0.5+ of goethite and the -COO^- of HS and by Pb bridges between surface sites and COO^- groups of HS. At low Pb levels, nearly 100% of Pb was bound as Pb bridges for both JGFA and JGHA. At high Pb levels and low HS loading, Pb-goethite almost dominated over the entire studied pH range, but at high HS loading, the primary species was goethite-HS-Pb at acidic pH and goethite-Pb at alkaline pH. Compared with JGFA, there was a constant contribution of Pb bridges of about 10% for JGHA. The linear combination fit of EXAFS, using Pb-HS and Pb-goethite as references, indicated that with increased HS loading more Pb was bound to adsorbed HS and less to goethite, which supported the LCD calculations.

Magnetically modified biochar for organic xenobiotics removal

Large amounts of biochar are produced worldwide for potential agricultural applications. However, this material can also be used as an efficient biosorbent for xenobiotics removal. In this work, biochar was magnetically modified using microwave-synthesized magnetic iron oxide particles. This new type of a magnetically responsive biocomposite material can be easily separated by means of strong permanent magnets. Magnetic biochar has been used as an inexpensive magnetic adsorbent for the removal of water-soluble dyes. Five dyes (malachite green, methyl green, Bismarck brown Y, acridine orange and Nile blue A) were used to study the adsorption process. The dyes adsorption could be usually described with the Langmuir isotherm. The maximum adsorption capacities reached the value 137 mg of dye per g of dried magnetically modified biochar for Bismarck brown Y. The adsorption processes followed the pseudo-second-order kinetic model and the thermodynamic studies indicated spontaneous and endothermic adsorption. Extremely simple magnetic modification of biochar resulted in the formation of a new, promising adsorbent suggested for selected xenobiotics removal.

Molecular simulation of bovine beta-lactoglobulin adsorbed onto a positively charged solid surface

To obtain detailed insight into the mechanism of beta-lactoglobulin (beta-Lg) adsorption to a stainless steel surface at acidic pH, the adsorption of positively charged beta-Lg to a positively charged surface (Au (100) surface with virtual positive charge) was simulated using classical molecular dynamics. The initial orientation and position of beta-Lg on the surface were determined using Monte Carlo simulation using the implicit water system. Molecular dynamics simulation with the explicit water system was conducted for a 5 ns simulation time to monitor beta-Lg adsorption. To investigate surface charge density effects on adsorption of beta-Lg, the positive charge number per Au atom on the (100) surface, C, was varied from 0 to +0.0250|e|. Stable adsorption occurred in MD simulations when C was equal to or less than +0.0200|e|. Among these surface Au charge conditions, no large difference was observed in the vertical separation distance between the surface and the protein's center of mass, and the orientation angle. This fact indicates that the main interactions contributing to the adsorption were van der Waals interactions. The protein domain contacting the surface was near Thr125, agreeing with previous experimental studies. Considering simulation results and those previous experimental studies suggests a detailed adsorption mechanism of beta-Lg at acidic pH: beta-Lg molecule is adsorbed initially with the specific part of 125-135th residues close to the surface by van der Waals interactions. Simultaneously or subsequently, side carboxylic groups of acidic amino acid residues near the surface in 125-135th residues dissociate, leading to firmer adsorption by attractive electrostatic residue-surface interaction.