Characterization of humic substances and polyacrylic acid: A high precision conductimetry study

Influence of natural organic matter on membrane capacitive deionization performance

Membrane capacitive deionization (MCDI) is a prospective desalination technology that removes ions using an electric potential difference across charged porous carbon electrodes. Natural organic matter (NOM) in feed water could influence the electrochemical process by leading to pore-blockages or forming a cake layer on the ion-exchange membrane-coated porous carbon electrode, thereby hindering ion removal. In this study, we explored the influence of different types of NOM, namely, humic acid (HA) and tannic acid (TA), on the MCDI desalination process for feed waters with inorganic salts (NaCl and CaCl₂). HA significantly interfered with the adsorption process and reduced the salt removal rate by up to 68% in the case of NaCl-based feed water. However, the influence of HA on salt removal in the case of CaCl₂-based feed water was marginal owing to the formation of a charge-neutralized complex, which was caused by the egg-box effect between Ca²⁺ and HA. TA reduced removal rates of salts (NaCl and CaCl₂) by 37% and 60%, respectively. This is because of the lower molecular weight and smaller hydrodynamic diameter of TA relative to that of HA, owing to which TA exhibits a stronger adhesion to the electrode pore structure. Furthermore, as TA substantially reduces MCDI performance with regard to the adsorption of inorganic salts, its presence in feed water results in higher electrical resistance and energy consumption.

Effect of pH or Metal Ions on the Oil/Water Interfacial Behavior of Humic Acid Based Surfactant

Humic acid, a kind of widespread organic macromolecule on earth, is naturally formed through the microbial biodegradation of plant, animal, and microorganism residues. Because of the large number of active functional groups (phenolic hydroxyl and carboxyl), humic acid has been considered as a biocompatible, green, and low-cost biosurfactant recently. In this work, based on the sensitivity of humic acid to the external chemical environment, the oil/water interfacial behavior of sodium humate at different pH or in the presence of metal ions is closely investigated. Sodium humate is significantly enriched toward the oil/water interface at either low pH or high metal-ion concentration to adjust the properties of the prepared emulsion, but the mechanisms are proved to be different when considering the influence of pH and metal ions. Besides, to the best of our knowledge, humic acid based surfactant is proposed as a Pickering emulsifier for the first time, known as solid surfactant. This work promises the great potential of humic acid as a natural environment-responsive surfactant and has important implications for the application of humic acid based surfactant in industry and understanding of the role of humic acid in the natural environment.

Inhibition of Cationic Polymer-Induced Colloid Flocculation by Polyacrylic Acid

Although the dosage of cationic flocculants used for water treatment is well known to increase in the presence of natural organic matter (NOM), the underlying reasons for this increase are not properly understood. Herein, we studied the flocculation behavior of polystyrene latex (PSL) particles in the presence and absence of polyacrylic acid (PAA5K) as an NOM analogue using an end-over-end rotation apparatus for standardized flow mixing. In the absence of PAA5K, the initial rate of cationic flocculant (PAM5M)-induced flocculation was enhanced, as reflected by the size of flocculant in solution. Additionally, flocculation experiments were performed in the presence of 0.5 mg/L PAA5K for five concentrations of PAM5M and two ionic strengths, and the increase of the initial rate of PAM5M-induced flocculation was suppressed by PAA5K immediately after mixing, with the most pronounced suppression obtained at a PAM5M concentration similar to PAA5K. Based on the above insights and the results of viscosity measurements, the inhibitory effect of PAA on flocculation was ascribed to (1) the reduction of PAM charge and the concomitant shrinkage via electrostatic association with PAA and (2) the termination of polycation adsorption on PSL caused by polyion complex formation when the charge ratio of PAM:PAA is close to unity.

Chunks of charge: effects at play in the assembly of macromolecules at fluid surfaces

Large macromolecules with hydrophobic backbones are known to assemble at the interface between immiscible liquids. This assembly is often unpredictable because of the subtle interplay among hydrophobic interactions, hydrophilic solvation, structural constraints, and the thermodynamics of adsorption. In these studies, we employ vibrational sum frequency spectroscopy and interfacial tension measurements to study the assembly of a simple polyelectrolyte, poly(methacrylic acid), as it assembles at the interface between two immiscible liquids, specifically, carbon tetrachloride and water. By adjusting the polyelectrolyte charge through pH studies and the polymer size through molecular weight studies, we demonstrate that charge accumulation in segments of the polymer chains is a critical factor in macromolecular interfacial adsorption and desorption. The results have implications for related charged macromolecules whose ability to assemble between two immiscible fluid media is essential for many biological processes, water remediation efforts, and enhanced oil recovery.

Molecular models of natural organic matter and its colloidal aggregation in aqueous solutions: Challenges and opportunities for computer simulations

Natural organic matter (NOM) is ubiquitous in soil and groundwater, and its aqueous complexation with various inorganic and organic species can strongly affect the speciation, solubility, and toxicity of many elements in the environment. Despite significant geochemical, environmental, and industrial interest, the molecular-scale mechanisms of the physical and chemical processes involving NOM are not yet fully understood. Recent molecular dynamics (MD) simulations using relatively simple models of NOM fragments are used here to illustrate the challenges and opportunities for the application of computational molecular modeling techniques to the structural, dynamic, and energetic characterization of metal–NOM complexation and colloidal aggregation in aqueous solutions. The predictions from large-scale MD simulations are in good qualitative agreement with available experimental observations, but also point out the need for simulations at much larger time- and length-scales with more complex NOM models in order to fully capture the diversity of molecular processes involving NOM.

Review of Pb availability and toxicity to plants in relation with metal speciation; role of synthetic and natural organic ligands

Biogeochemical behavior of lead (Pb), a persistent hazardous pollutant of environmental concern, strongly depends on its chemical speciation. Therefore, in this review, link between Pb speciation: presence of organic ligands and its environmental behavior has been developed. Both, biogeochemical and ecotoxicological data are discussed in environmental risk assessment context and phytoremediation studies. Three kinds of organic ligands selected for this review include: (1) ethylene diamine tetra-acetic acid (EDTA), (2) low molecular weight organic acids (LMWOAs) and (3) humic substances (HSs). The review highlights the effect of Pb speciation on: (i) Pb fate and behavior in soil; (ii) Pb plant uptake and accumulation in different plant parts; and (iii) Pb-induced phyto-toxicity. Effects of organic ligands on Pb speciation are compared: how they can change Pb speciation modifying accordingly its fate and biogeochemistry in soil-plant system? EDTA forms soluble, stable and phytoavailable Pb-chelates due to high binding Pb affinity. LMWOAs can solubilize Pb in soil by decreasing soil pH or increasing soil organic contents, but have little effect on its translocation. Due to heterogeneous structure, HSs role is complex. In consequence Pb speciation knowledge is needed to discuss phyto-toxicity data and improved soil phytoremediation techniques.

Enhanced transport of 2,2',5,5'-polychlorinated biphenyl by natural organic matter (NOM) and surfactant-modified fullerene nanoparticles (nC60)

Stable colloidal suspensions of buckminsterfullerene (nC(60)) in aqueous environments can significantly affect the fate and transport of hydrophobic organic contaminants by serving as a contaminant carrier. In this study, we examined enhanced transport of 2,2',5,5'-polychlorinated biphenyl (PCB) in saturated sandy soil columns by a variety of nC(60) samples, including an nC(60) sample prepared by the typical solvent exchange method, as well as eight natural organic matter (NOM) or surfactant-modified nC(60) samples, prepared by phase-transferring C(60) from toluene to an NOM or a surfactant solution. Whereas the NOM- and surfactant-modified nC(60) samples have mobility similar to the unmodified nC(60), their contaminant-mobilizing capabilities are significantly greater: breakthrough of PCB increases by 47.2 to 227% with the surfactant-modified nC(60) samples and by 233 to 370% with the NOM-modified nC(60) samples. The significantly enhanced contaminant-mobilizing capability of the modified nC(60) is likely due to a combined effect of increased adsorption affinities and increased tendency of desorption nonequilibrium, likely caused by the changes of nC(60) aggregation properties induced by the presence of NOM or surfactant. Findings in this study indicate that nC(60) formed in different processes might have vastly different effects on contaminant fate and transport.

Probing Ag nanoparticle surface oxidation in contact with (in)organics: an X-ray scattering and fluorescence yield approach

Characterizing interfacial reactions is a crucial part of understanding the behavior of nanoparticles in nature and for unlocking their functional potential. Here, an advanced nanostructure characterization approach to study the corrosion processes of silver nanoparticles (Ag-Nps), currently the most highly produced nanoparticle for nanotechnology, is presented. Corrosion of Ag-Nps under aqueous conditions, in particular in the presence of organic matter and halide species common to many natural environments, is of particular importance because the release of toxic Ag(+) from oxidation/dissolution of Ag-Nps may strongly impact ecosystems. In this context, Ag-Nps capped with polyvinolpyrrolidone (PVP) in contact with a simple proxy of organic matter in natural waters [polyacrylic acid (PAA) and Cl(-) in solution] has been investigated. A combination of synchrotron-based X-ray standing-wave fluorescence yield- and X-ray diffraction-based experiments on a sample consisting of an approximately single-particle layer of Ag-Nps deposited on a silicon substrate and coated by a thin film of PAA containing Cl revealed the formation of a stable AgCl corrosion product despite the presence of potential surface stabilizers (PVP and PAA). Diffusion and precipitation processes at the Ag-Nps-PAA interface were characterized with a high spatial resolution using this new approach.