Critical Coagulation Concentration Research Articles

Brownian Dynamics Simulation of Colloidal Gels as Matrix for Controlled Release Application

The form of crystallization colloidal gels is important as matrix for controlled release application. In this work, we use the Brownian Dynamics simulation to study the formation of gels by varying the inverse Debye length. We choose a fixed volume fraction, ϕ = 0.1 and a fixed quenched temperature at room temperature, while the inverse Debye length, κ, is varied. To ensures that the simulations cover the fluid-phase region down to the unstable phase region above the critical coagulation concentration, the inverse Debye length is varied between κ = 120σ−1 to 250σ−1. It shows that at the inverse Debye length κ = 250σ−1 the gel forms by colloidal particles that can support the active ingredient by forming long range network.

Effects of salt ions and heating on the behaviour of whey protein particle dispersions

We investigated the aggregation stability of whey protein isolate (WPI) particles in the presence of different salts (NaCl, MgCl2, and AlCl3) and the heat stability of dispersions in the presence of protein (native WPI) and polysaccharides (dextran and gum Arabic, GA). WPI particles coagulated either at pH values close to the iso-electric point of proteins or at high salt concentrations. Critical coagulation concentration (CCC) for different salts decreased with increasing valence of ions. CCC values for colloidal WPI particle dispersions closely followed the Schulze-Hardy rule, which is derived from the DLVO theory; however, deviations from the theory, particularly in the case of multivalent ions, showed that the specific ion adsorption also played a role. To investigate the heat stability of WPI particles, particle size and viscosities of dispersions containing different biopolymers were measured. Using dynamic viscosity of dilute dispersions, with the assumption of no particle aggregation after heating, swelling ratio (SR) of particles was estimated. Increasing concentrations of biopolymers decreased the SR of particles, and the presence of GA totally inhibited the swelling. Shear viscosity of concentrated dispersions showed an increase and shear-thickening behaviour after heating, when native WPI or dextran was present, whereas viscosity did not change when GA was present in the continuous phase. At low volume fractions, swelling was more likely to occur; whereas at high volume fractions, possibly both swelling and aggregation occurred. In addition, concentration and type of biopolymer were found to be important for the heat stability of dispersions.

Facile fabrication of GO/Al composites with improved dispersion of graphene and enhanced mechanical properties by Cu doping and powder metallurgy

In recent years, graphene has attracted a lot of attention as a reinforcement to enhance aluminum (Al) matrix composites due to its two dimensional structure and excellent mechanical properties. However, there are still some obstacles that limit the performance of the composites, such as uniform distribution of graphene oxide (GO) and the interfacial bonding between GO and Al matrix. In this work, Cu2+ ions acting as a bonding bridge were added into GO/Al powder suspension to improve the distribution of GO sheets on Al powder via a simple electrostatic adsorption method. Then, vacuum hot-pressing sintering was applied to fabricate bulk composites, and their mechanical properties were investigated. The results showed that doping 3 wt% Cu2+ ions illustrated the best dispersion of GO on the surface of Al powder, meanwhile GO was maximally reduced. Compared with unreinforced Al, the hardness, compressive strength and flexural strength of the 3 wt% Cu-doped bulk composite were increased by 121%, 120% and 28%, respectively. However, the mechanical properties were deteriorated after doping 5 wt% Cu2+ ions, due to the aggregation of GO on the surface of Al powder. This was attributed that Cu2+ ions had a critical coagulation concentration in aqueous GO suspensions. It was expected this work could offer some useful information for preparing graphene reinforced metal composites.

Specific ion effects on the aggregation behavior of aquatic natural organic matter

Specific ion effects on the aggregation behavior of a reference aquatic natural organic matter (NOM), Suwannee River NOM (SRNOM), were investigated using kinetic, titration, calorimetric, and surface tension methods. Monovalent cations induced structural compacting of SRNOM, but not its aggregation. Their ability to induce structural compacting follows the order: Cs+ > Rb+ > K+ > Na+ > Li+. Divalent cations except Mg2+ can readily induce SRNOM aggregation. Their critical coagulation concentrations (CCC) follow the order: CCCSr > CCCCa > CCCBa. Electrokinetic, titration, and calorimetric data suggest that monovalent cations have weak interactions with SRNOM, while divalent cations strongly interact with SRNOM. Overall, the cation specificity in aggregation is determined by cation-NOM interactions and their ability to modulate surface tension. Specific ion effects of monovalent cations correlate to their hydration free energy, while that of divalent cations correlate to the ratio of the hydration entropy of cation to the enthalpy change of cation-NOM interactions. The cation specificity is consistent with the extended Derjaguin-Landau-Verwey-Overbeek theory, and the intermolecular interaction energy is dominated by the Lewis acid-base interactions. Our results suggest that specific cations should be targeted to predict or manipulate intermolecular interactions of aquatic NOM in natural and engineered settings.

Colloidal properties and stability of UV-transformed graphene oxide in aqueous solutions: The role of disorder degree

Motivated by the use of ultraviolet (UV) radiation in the disinfection processes of drinking and waste water treatment plants, this study explores the colloidal properties and stability of UV irradiated graphene oxide (GO) by using the batch technique and time-resolved dynamic light scattering over a wide range of salt types (NaCl, MgCl2, and CaCl2) and ionic strength relevant to engineered and natural systems. The results show that the UV irradiation time has an important impact on the physicochemical properties of GO and consequently on its colloidal properties and stability. The aggregation kinetics, attachment efficiency and critical coagulation concentration (CCC) values of UV irradiated GO are obtained for the first time. By correlating CCC values with physicochemical properties, we find that the disorder degree plays a more important role in colloidal properties and stability of UV irradiated GO than oxygen containing functional groups. The findings are valuable for environmental fate assessments on various families of functionalized GO.

Effects of free patchy ends in ssDNA and dsDNA on gold nanoparticles in a colorimetric gene sensor for Hepatitis C virus RNA.

The presence or absence and nature of the free patchy ends in DNA sequences has a decisive effect on the performance of colorimetric sensors based on the use of gold nanoparticles (Au NPs). The authors have designed two unmodified gene probes (probe 1: a 19-mer; probe 2: an 18-mer). They are complementary to either half of a 37-mer target derived from the conserved region of Hepatitis C Virus (HCV) RNA. Each probe has further been modified with 10-mer poly(A) and thiol-functionalized 10-mer poly(A) at the 5' positions. Nine combinations of probe and HCV RNA were then designed to investigate the effect of free patchy ends on the stability of citrate-modified Au NPs against salt-induced aggregation which lead to color change from red to blue. The aggregation of Au NPs can be monitored by ratiometric spectroscopy at wavelengths of 520 and 700nm. The differentiation between HCV RNA and control has also been studied by varying the concentration of probe and analyte. The particle size and zeta potentials were determined before and after aggregation. It is demonstrated that the change in surface charge density of the Au NPs governs the critical coagulation concentration of NaCl. The method presented here can be used to quantify HCV RNA in the 370nM to 3μM concentration range, and the detection limit is 500nM. The results obtained with Au NPs that are chemically non-conjugated with the oligonucleotides have been found to be valuable in rationally devising the design rules for rapid and efficient colorimetric sensing of oligonucleotides. Graphical abstract Schematic representation of the nine combinatorial pairs of oligonucleotides that vary in the length of patchy ends and their position to unearth their effect in rapid gold nanoparticle-based colorimetric gene sensing without time-consuming and expensive thiol-conjugation step.

Dispersion, sedimentation and aggregation of multi-walled carbon nanotubes as affected by single and binary mixed surfactants.

Two commonly used dispersants, octyl phenol ethoxylate (Triton X-100) and sodium dodecyl sulfate (SDS), were employed to explore the effects of single or mixed surfactants on the dispersion, sedimentation and aggregation of multi-walled carbon nanotubes (MWCNTs). Non-ionic surfactant TX100 showed much superior capability to anionic surfactant SDS in dispersing MWCNTs due to the benzene ring structure in its tail group. The addition of SDS reduced the adsorption of TX100 on the surface of MWCNTs and the consequent suspension of MWCNTs. The dispersing ability of TX100–SDS binary mixture was between those of individual SDS and TX100. The introduction of SDS greatly retarded the sedimentation and aggregation of suspended MWCNTs. The critical coagulation concentration (CCC) values of suspended MWCNTs dispersed by TX100 (2000 mg l−1), SDS (2000 mg l−1) and TX100–SDS (2000 mg l−1 of each component) were 48.6, 398 and 324 mM, respectively, for Na+ treatments. The CCC values were much lower for Ca2+ treatments, which were 30.4 and 32.1 mM, respectively, for MWCNTs dispersed by TX100 and TX100–SDS mixture. Overall, these results demonstrated that although the introduction of SDS did not improve the ability of TX100 in suspending MWCNTs, the suspensions exhibited more stable properties than those dispersed by TX100 alone. Our findings have important implications for the design of surfactant mixtures and the prediction of the behaviour and fate of MWCNTs in the water environment.

Specific ion effects of Cu2+, Ca2+ and Mg2+ on montmorillonite aggregation

Differences in ion-surface reactions among various divalent cations may result in large differences in clay particle interaction behaviors. Specific ion effects of Cu2+, Ca2+ and Mg2+ in montmorillonite (Mt) aggregation were studied by laser light scattering. The aggregation rate in the presence of Cu2+ was much higher than that in the presence of Ca2+ and Mg2+. The critical coagulation concentrations (CCCs) were Cu2+ (0.650 mM)<<Ca2+ (2.38 mM)<Mg2+ (7.99 mM). The differences in aggregation rates and CCC values were attributed to non-electrostatic interaction forces between the cations and the clay mineral surface. These non-electrostatic forces generated from (i) non-classical polarization of cations in the strong electric field near the clay mineral surface; and (ii) the quantum-mechanical interaction between the strongly polarized cations, especially for the d-type orbital electronic configuration of Cu2+, and the strongly polarized surface oxygen atoms. It was assumed that a new, polarization-induced covalent bond formed between Cu2+ and the surface O.

Chemical Aging Changed Aggregation Kinetics and Transport of Biochar Colloids.

Little is known about aggregation and transport behaviors of aged biochar colloids in the terrestrial environment. This study investigated aggregation kinetics and transport of biochar colloids from aged (HNO3 treatment) and pristine pinewood biochars pyrolyzed at 300 and 600 °C (PB300 and PB600) in NaCl and CaCl2 solutions. In NaCl solutions, critical coagulation concentrations (CCCs) of aged PB300 and PB600 colloids (540 mM and 327 mM) were much greater than the CCCs of pristine biochar colloids (300 mM and 182 mM). This is likely due to substantial increase of negatively charged oxygen-containing functional groups (primarily carboxyl) on aged biochar surfaces. Intriguingly, in CaCl2 solutions the CCCs of the aged PB300 and PB600 colloids decreased to 25.2 mM and 32.1 mM from 58.6 mM and 41.7 mM for the pristine colloids, respectively. This probably resulted from greater surface charge neutralization and Ca2+ bridging for the aged biochar colloids. In salt solutions (e.g., 10 and 50 mM NaCl and 1 and 10 mM CaCl2), the aged biochar colloids showed higher mobility in porous media than the pristine biochar colloids. This study demonstrated that pristine and aged biochar colloids were stable in the solutions with environmentally relevant ionic strength, and the aging process might substantially increase their mobility in the subsurface.

Spectroscopic investigation of aggregation kinetics and catalytic activity of gluconate and N-(1-naphthyl)ethylenediammonium stabilized silver nanoparticles

Gluconate anions (Gluc−) and N-(1-naphthly)ethylenediammonium cations (NEDA2+) capped Ag-NPs (Gluc-Ag-NPs and NEDA-Ag-NPs) were employed to study interactions with macrocyclic polyammonium cations (MCPACs), which are reputed anion recognizers and as to mimic biopolyamines. The studies were done on UV–Vis spectroscopy and Transmission Electron Microscope (TEM). Surface adsorbed Gluc− ions being anionic in nature, interact with MCPACs kinetically, whereas positively charged NEDA2+ ions do not show any such interactions. The resulting associated complex, Gluc−…MCPACs for Me6[14]ane-(NH2+)4 (Tet-A/Tet-B) or [32]ane-(NH2+)8 (32-MCPAC) is existed with the particles up to certain concentration of MCPACs, termed as critical coagulation concentration (CCC). The CCC value of 32-MCPAC (0.021 μM) is quite less than that of Tet-A (21.06 μM) and Tet-B (20.90 μM); as a consequence, the association constant (Kasso) of Gluc—32-MCPAC (1.38×107 M−1) complex is much greater than that of Gluc—Tet-A/Tet-B (∼4.5×104 M−1) complex. Reduction of 4-nitrophenol (4-NP) (anionic in presence of BH4−) to 4-aminophenol (4-AP) using the anionic Gluc-Ag-NPs as a potential catalyst occurs more effectively and rapidly, whereas using the cationic NEDA-Ag-NPs the reaction does not occur. Again the surface charges of the particles play an important role to explore their employability as a potential catalyst.

The effect of monovalent anion species on the aggregation and charging of allophane clay nanoparticles

Allophane is a kind of clay mineral which has amorphous hollow spherical structure. To clarify the dispersion, aggregation and charging behaviour of allophane in the presence of various monovalent anions, F−, Cl−, Br−, I−, BrO3−, IO3−, and SCN−, the stability ratio and electrophoretic mobility of allophane were investigated. The stability ratio was obtained from the temporal change in the average hydrodynamic diameter of allophane measured by dynamic light scattering. The charging behaviour was evaluated from the measurement of electrophoretic mobility (EPM) of allophane. These experiments were performed as a function of electrolyte concentration at pH 5, where the net charge of allophane is positive. The experimental results demonstrated that the stability ratio decreased with increasing the salt concentration and finally it became unity independent of the salt concentration. Consequently, we observed slow aggregation regime, fast aggregation regime, and critical coagulation concentration (CCC). Therefore, the stability ratio follow the classical theory of Derjaguin, Landau, Verwey, Overbeek (DLVO) at least qualitatively. However, the CCC of allophane showed difference for each anion species; the CCC followed the order F− < IO3− < Cl− < SCN− < BrO3− < Br− < I−. We also confirmed the difference in EPM among anion species; the reduction of EPM magnitude was significant for well-hydrated ions. We presume that the CCC and EPM of allophane depend on the affinity of anions to the allophane surface characterized by the degree of hydration of each ion. That is, well-hydrated anions can adsorb to the hydrophilic surface of allophane and effectively decrease the positive charge of allophane. Besides this, the fluoride ion induced much lower CCC and charge reversal. These results are due to the strong affinity of fluoride ion for the allophane surface. Meanwhile the dependence of CCC on the zeta potential followed the DLVO prediction. Although the zeta potential of allophane is affected by anion species, the CCC behaviors can be understood by the DLVO theory through the evaluation of zeta potential of allophane. This means that the measurement of stability ratio and electrophoretic mobility accompanied with the DLVO theory are valuable tools for the understanding of the aggregation-dispersion of charge-stabilized natural clays.

Aggregation Kinetics of Manganese Oxides Formed from permanganate activated by (Bi)sulfite: Dual Role of Ca2+ and MnII/III

Aqueous aggregation kinetics of manganese oxides, the solid products formed during water treatment and subsurface remediation with permanganate, are crucial for its application. In this study, manganese oxides nanoparticles were in situ formed in a permanganate/(bi)sulfite system, which was found to have excellent oxidation ability. Aggregation kinetics of such manganese oxides (i.e., MnOx-1.5, MnOx-2.5 and MnOx-5; the number represents the molar ratio of (bi)sulfite to permanganate) were evaluated by employing time-resolved dynamic light scattering under various aquatic conditions. In NaNO3 solution, the stability of manganese oxides decreased in the order of MnOx-1.5 > MnOx-2.5 > MnOx-5, indicated by their critical coagulation concentrations (CCCs). X-ray photoelectron spectroscopy (XPS) and zeta potential measurements indicated that MnII/III were responsible for the decreased stability due to their charge neutralization effects. However, in Ca(NO3)2 solution, three manganese oxides had similar CCCs, probably due to the relatively great charge neutralization ability of Ca2+. Suwannee River fulvic acid (SRFA), through electrosteric interaction, suppressed the aggregation of MnOx-1.5 in Ca(NO3)2 solution, but had no such effect in NaNO3 solution. Comparatively, the stability of MnOx-5 was markedly enhanced with SRFA in NaNO3 solutions. It was proposed that Ca2+ and MnII/III could increase the adsorption of SRFA through charge neutralization and cation bridging. This study highlights the dual role, dependent on either presence or absence of SRFA, of Ca2+ and MnII/III in controlling the aggregation of manganese oxides nanoparticles.

Stability of Titania Nanomaterials Dispersed in Aqueous Solutions of Ionic Liquids of Different Alkyl Chain Lengths

Charging and aggregation of titania nanosheets (TNS) and spherical titania nanoparticles (TNP) were studied in aqueous solutions of ionic liquids. The pH and the length of the alkyl chain of the IL cations [1-methylimidazolium (MIM+), 1-ethyl-3-methylimidazolium (EMIM+), and 1-butyl-3-methylimidazolium (BMIM+)] were systematically varied in the experiments. No detectable interaction was observed between the IL cations and the positively charged TNS or TNP surfaces at low pH, where the imidazolium derivatives are the co-ions. For the negatively charged titania nano-objects, significant adsorption of MIM+ and EMIM+ took place, leading to charge neutralization and overcharging at appropriate concentrations. The BMIM+ behaved like a simple salt constituent causing charge screening. For both TNS and TNP, the MIM+ < EMIM+ < BMIM+ counterion order was obtained in the critical coagulation concentrations, indicating that MIM+ was the most effective in destabilization of the dispersions. The major interparticle for...

Deposition of protein-coated multi-walled carbon nanotubes on oxide surfaces and the retention in a silicon micromodel.

The aggregation, deposition and porous retention of bovine serum albumin treated multi-walled carbon nanotubes (BSA-MWCNTs) are investigated using dynamic light scattering (DLS), quartz crystal microbalance with dissipation (QCM-D) and 2-dimensional silicon micromodel, respectively. The aggregation of BSA-MWCNTs is consistent with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The critical coagulation concentration (CCC) is 175 mM NaCl and 2.7 mM CaCl2, suggesting that Ca2+ causes stronger aggregation. The BSA-MWCNT deposition on SiO2 surface is unfavorable with critical deposition concentration (CDC) of 100 mM in NaCl and 0.9 mM in CaCl2. The deposition on the Al2O3 surface is favorable. Deposition rate is dominated by electrostatic forces at low ionic strength (IS), but electrostatic interaction is eliminated when IS is above CDC. Therefore the deposition rate on SiO2 or Al2O3 surface starts decreasing at the CDC point due to the reduced particle diffusion. In micromodel, the amount and position of attached BSA-MWCNTs in pore space can be observed by a microscope. The retention attachment efficiency increases at higher IS. The suspended BSA-MWCNTs approach to the collector through either diffusion or interception. The attached BSA-MWCNTs narrow the pore space and then clog the pore throats. The straining process happens on the clogged pore throats.

The interaction of the colloidal species in hydrosols of nanodiamond with inorganic and organic electrolytes

This article is aimed to disclose the properties of diluted (≤100 mg dm−3) hydrosols of detonation nanodiamond (DND). Examining of the behavior of such highly diluted systems is significant for understanding their fate in biological systems. The disperse system consists of the particles with the size of ca. 30–100 nm, which are positively charged: ζ ≈ +(50–70) mV. This favors adsorption of an anionic dye dicyano‑[3‑(dicyanomethylene)cyclohex‑1‑en‑1‑yl]methanide. Even more so, despite similar electrical charge a hydrophobic enough cationic dye 1,3,3‑trimethyl‑2‑[7‑(1,3,3‑trimethylindolin‑2‑ylidene)hepta‑1,3,5‑trien‑1‑yl]‑3H‑indoliu also interacts with the DND nanoparticles in aqueous media. The surface structure and morphology are examined after evaporation using high-resolution transmission electron microscopy. These data show that the aggregates consist of nanometer-sized well-rounded crystallites of cubic diamond phase and allow suggesting a huge surface area of the colloidal species. The coagulation is studied using inorganic electrolytes via spectrophotometric procedure and dynamic light scattering measurements, for both freshly prepared and aged hydrosols. The determination of the coagulation rates allows calculating the Fuchs function and thus estimating the critical coagulation concentration, CCC. The experimentally determined CCC value of NaCl is in line with the Hamaker diagram constructed using the DLVO theory. The ratio of the reciprocal CCC values is as follows: 1 (NaCl):48 (Na2SO4):228 (K3Fe(CN)6), in agreement with the Schulze–Hardy rule. On the other hand, another ratio of CCC−1, 1 (NaCl):0.8 (HCl):16 (NaOH), gives support to the acidic nature of the positive charge of the colloidal particles of DND. The peculiar of the interactions with inorganic electrolytes and organic dyes of the system under study are compared with those of the C60 hydrosol.

Aggregation Behavior of Inorganic 2D Nanomaterials Beyond Graphene: Insights from Molecular Modeling and Modified DLVO Theory.

We report the comparative aggregation behavior of three emerging inorganic 2D nanomaterials (NMs): MoS2, WS2, and h-BN in aquatic media. Their aqueous dispersions were subjected to aggregation under varying concentrations of monovalent (NaCl) and divalent (CaCl2) electrolytes. Moreover, Suwanee River Natural Organic Matter (SRNOM) has been used to analyze the effect of natural macromolecules on 2D NM aggregation. An increase in electrolyte concentration resulted in electrical double-layer compression of the negatively charged 2D NMs, thus displaying classical Derjaguin-Landau-Verwey-Overbeek (DLVO)-type interaction. The critical coagulation concentrations (CCC) have been estimated as 37, 60, and 19 mM NaCl and 3, 7.2, and 1.3 mM CaCl2 for MoS2, WS2, and h-BN, respectively. Theoretical predictions of CCC by modified DLVO theory have been found comparable to the experimental values when dimensionality of the materials is taken into account and a molecular modeling approach was used for calculating molecular level interaction energies between individual 2D NM nanosheets. Electrostatic repulsion has been found to govern colloidal stability of MoS2 and WS2 while the van der Waals attraction has been found to govern that of h-BN. SRNOM stabilizes the 2D NMs significantly possibly by electrosteric repulsion. The presence of SRNOM completely stabilized MoS2 and WS2 at both low and high ionic strengths. While h-BN still showed appreciable aggregation in the presence of SRNOM, the aggregation rates were decreased by 2.6- and 3.7-fold at low and high ionic strengths, respectively. Overall, h-BN nanosheets will have higher aggregation potential and thus limited mobility in the natural aquatic environment when compared to MoS2 and WS2. These results can also be used to mechanistically explain fate, transport, transformation, organismal uptake, and toxicity of inorganic 2D NMs in the natural ecosystems.

Elucidating the Role of Sulfide on the Stability of Ferrihydrite Colloids under Anoxic Conditions.

While the reaction mechanisms between ferrihydrite and sulfide are well-documented, the role of redox reactions on the particle-particle stability of ferrihydrite colloids is largely overlooked. Such reactions are critical for a number of (bio)geochemical processes governing ferrihydrite-based colloid processing and their associated role in nutrient and contaminant subsurface dynamics. Taking a fundamental colloid chemistry approach, along with a complementary suite of characterization techniques, here, we explore the stability mechanisms of ferrihydrite colloids over a wide range of environmentally relevant sulfide concentrations at pH 6.0. Results show that sulfide lowered the stability of both ferrihydrite colloids in a concentration-dependent fashion. At lower sulfide concentrations (15.6-62.5 μM), ferrihydrite colloids are apparently stable, but their critical coagulation concentration (CCC) in NaCl linearly decreases with increasing sulfide concentration. This is attributed to the formation of negatively charged elemental sulfur (S(0)) nanoparticles on the surfaces of positively charged ferrihydrite, intensifying the electrostatic attractions between oppositely charged regions on adjacent ferrihydrite surfaces. Further increasing sulfide concentration generates more S(0) attaching to the ferrihydrite surface. This results in effective surface charge neutralization and then subsequent charge reversal, leading to extensive aggregation of ferrihydrite (core) colloids. Interestingly, for the ferrihydrite colloids with higher hydrodynamic diameter, aggregation rates linearly decreases with increasing sulfide concentration from 156.3 to 312.5 μM, which is likely due to the formation of substantial amounts of negatively charged S(0) and FeS. Findings highlight the significance of sulfidation products in controlling the stability of ferrihydrite colloids in sulfidic environments.

Aggregation of ferrihydrite nanoparticles: Effects of pH, electrolytes,and organics

To better understand the fate and transport of ferrihydrite nanoparticles (FNPs), which carry many contaminants in natural and engineered aquatic environments, the aggregation of FNPs was systematically investigated in this study. The pH isoelectric point (pHIEP), surface zeta potential, and particle size evolutions of FNPs were measured under varied aqueous conditions using dynamic light scattering (DLS). The influence of pH (5.0 ± 0.1 and 7.0 ± 0.1), ionic strength (IS), electrolytes (NaCl, CaCl2 and Na2SO4), and organics (humic acid, fulvic acid and CH3COONa) on the aggregation behaviors of FNPs were explored. Meanwhile, Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was employed to better understand the controlling mechanisms of FNP aggregation. In the presence of sulfate, the surface charge of FNPs was neutralized under varied pH and ionic strength due to ion adsorption and FNPs phase transformation to schwertmannite based on FT-IR results. This phase transformation resulted in rapid aggregation in all water chemistries tested, whereas other salt species affected the aggregation primarily by ion adsorption and charge screening. Presence of increasing concentrations of the organic acids significantly shifted the pHIEP of FNPs (7.0 ± 0.2) to lower pH (< 4.0) due to adsorption of organics on FNPs surfaces making them negatively charged. The adsorption of HA/FA inhibited FNP aggregation significantly while CH3COONa did not, due to different effects on steric and/or electrosteric interactions among FNPs by organics with varied pKa values and molecular weights. After accounting for the important effects of pH, electrolytes, and organics in modifying FNPs’ surface charge, DLVO calculations agreed well with measured critical coagulation concentrations (CCC) values of FNPs at both pH 5.0 ± 0.1 and 7.0 ± 0.1 in the presence of NaCl. This study will hence be useful to better predict and control the fate and transport of FNPs in the presence of electrolytes and organics with different molecular weights, as well as the fate of the associated contaminants in natural and engineered systems.

Effects of interactions between humic acid and heavy metal ions on the aggregation of TiO2 nanoparticles in water environment

Nanoparticles (NPs), heavy metal and natural organic matter (NOM) may simultaneously exist in the aquatic environment, where they will affect the behavior of each other and may enhance their toxicities. Studies on the influences of interactions between NOM and heavy metal ions on the behavior of NPs are scarce. In this study, combined effects of Pb2+ and HA on the aggregation behavior of TiO2 NPs in water environment were investigated by Dynamic light scattering (DLS) and Nanoparticle tracking analysis (NTA). The results illustrated that interactions between Pb2+ and HA could case the aggregation of TiO2 NPs obviously. The concurrence of Pb2+ and HA resulted in decreased critical coagulation concentration (CCC) and increased attachment efficiencies. Meanwhile, we found that the addition sequences of HA and heavy metal clearly influenced the aggregation kinetics of TiO2 NPs. At different addition sequences, the complex reaction between Pb2+ and HA changed the surface charge of TiO2 NPs, and caused the different aggregation behavior which depended on the complex locations and complex sites. Furthermore, the excitation-emission-matrix (EEM) fluorescence spectra was used to verify the significant effects of the complex interactions between Pb2+ and HA on the aggregation of TiO2 NPs. Our results would be significant for interpreting TiO2 behavior in the complicated water system.The complexation between Pb2+ and HA promoted the aggregation of TiO2 NPs, meanwhile, complex locations and complex sites played an important role.

Predictions of Pair Interaction Potentials between Kraft Lignin Macromolecules in Black Liquors by Utilization of a Modified Poisson–Boltzmann Approach

In this investigation, a modified Poisson–Boltzmann approach has been used to estimate the mean ion concentration distributions of ionic species present in black liquors around charged kraft lignin macromolecules at industrially relevant conditions. The distributions were utilized to predict the double layer repulsion and, further, predict the overall pair interaction potentials between two kraft lignin macromolecules or particles immersed in the black liquor electrolyte. The properties of softwood black liquors were used as input data to the model. The numerical results predicted the pair interaction potentials to remain repulsive up to salt concentrations of about 2.5 M at pOH 1. The critical coagulation concentration of salt was predicted to decrease as the pOH of the black liquor was increased. The predictions at 65 °C and moderate levels of pOH (3.5) and salt concentration (1.9 M) were found to be in good agreement with previous observations reported in the literature.