Increase In Negative Surface Charge Research Articles

Beyond the Charge: Interplay of Nanogels' Functional Group and Zeta-Potential for Antifungal Drug Delivery to Human Pathogenic Fungus Aspergillus Fumigatus.

The ubiquitous mold Aspergillus fumigatus (A. fumigatus) is one of the main fungal pathogens causing invasive infections in immunocompromised humans. Conventional antifungal agents exhibit limited efficacy and often cause severe side effects. Nanoparticle-based antifungal delivery provides a promising alternative, which can increase local drug concentration; while, mitigating toxicity, thereby enhancing treatment efficacy. Previous research underscores the potential of poly(glycidol)-based nanogels (NG) with negative surface charge as carriers for delivering antifungals to A. fumigatus hyphae. In this study, NG is tailored with 2-carboxyethyl acrylate (CEA) or with phosphoric acid 2-hydroxyethyl acrylate (PHA). It is discovered that quenching with PHA clearly improves the adhesion of NG to hyphal surface and the internalization of NG into the hyphae under protein-rich conditions, surpassing the outcomes of non-quenched and CEA-quenched NG. This enhancement cannot be solely attributed to an increase in negative surface charge but appears to be contingent on the functional group of the quencher. Further, it is demonstrated that itraconazole-loaded, PHA-functionalized nanogels (NGxPHA-ITZ) show lower MIC in vitro and superior therapeutic effect in vivo against A. fumigatus compared to pure itraconazole. This confirms NGxPHA as a promising antifungal delivery system.

Two-Dimensional Modeling of Nano Zero-Valent Iron Transport and Retention before and after Phosphate Adsorption.

The mobility of nano zero-valent iron (nZVI) will greatly affect its practical application as a remediation material for contaminated groundwater. One-dimensional (1D) column tests are commonly used in previous work to study its migration behavior, but the two-dimensional (2D) test is still very limited. This study reports a novel research system to study the 2D transport and retention behavior of colloids and solutes, which includes a 2D model test setup and the corresponding image analysis method. The transport behaviors of methyl orange (MO), nZVI, and phosphate-loaded nZVI (PnZVI) are studied using this system. The results show that the research system can reasonably describe the tempo-spatial concentration distribution of colloids and solutes. After phosphate adsorption, the mobility of nZVI is enhanced due to the increase in negative surface charge, which implies a potential environmental risk of nZVI to facilitate contaminant transport. The migration of PnZVI is not significantly influenced by its density, which is faster than MO in the longitudinal direction. The range of the plume of PnZVI in the longitudinal direction is larger than that of MO, which implies that PnZVI has a stronger longitudinal dispersion than MO.

The Sensing Mechanism of InAlN/GaN HEMT

The sensing mechanism of InAlN/GaN high electron mobility transistors (HEMTs) is investigated systematically by numerical simulation and theoretical analysis. In detail, the influence of additional surface charge on device performance and the dependence of surface sensing properties on InAlN barrier thickness are studied. The results indicate that the saturation output drain current Idsat and two-dimensional electron gas (2DEG) concentration increase with the increase of positive surface charge density, which decrease with the increase of negative surface charge. The influence of negative surface charge on device performance is more remarkable than that of positive surface charge. Additionally, the modulation ability of surface charge on device performance increases with the decrease ofInAlN barrier thickness. The modulation of surface charge on device performance and the influence of barrier thickness on surface sensing sensitivity are mainly attributed to the variation of the energy band structure, surface potential and 2DEG concentration in the HEMT heterostructure. This work provides important support for structural optimization design of GaN-based HEMT sensors.

New insight into filamentous sludge bulking: Potential role of AHL-mediated quorum sensing in deteriorating sludge floc stability and structure

The microcosmic mechanisms underlying filamentous bulking remain unclear. The role of extracellular polymeric substances (EPS) governed by quorum sensing (QS) in deteriorating sludge floc stability and structure during filamentous bulking and the feasibility of using quorum quenching (QQ) to maintain sludge floc stability and structure and sludge settling were investigated in this study. The results indicated that the concentration of C6HSL increased from 22.08±3.22 ng/g VSS to 81.42±5.98 ng/g VSS during filamentous bulking. The filamentous bacteria gradually evolved the hdtS gene related to the synthesis of C6HSL with increases in the population density. Triggered QS by filamentous bacteria proliferation induced variation in the composition and structure of EPS within the sludge flocs. The proteins (PN) content of the EPS increased evidently from 40.06 ± 2.41 mg/g VSS to 110.32 ± 4.32 mg/g VSS, and the polysaccharides (PS) content slightly increased during filamentous bulking. The upregulated proteins in the EPS led to a decrease in the relative hydrophobicity of the sludge and an increase in negative surface charge. The α-helix/(β-sheet+random coil) ratio evidently increased from 0.76 to 0.99 during filamentous bulking, revealing that the proteins were tightly structured, which prevented the exposure of inner hydrophobic groups. The total energy of the interaction (WT) between bacteria increased during sludge bulking, which resulted in the weakening of sludge aggregation. Variation in the physicochemical properties of EPS induced by QS in the filamentous bacteria markedly restrained adhesion between the filamentous bacteria and floc-forming bacteria. The production of PN in the EPS and the expression of the hdtS gene were inhibited by vanillin, which served as a QS inhibitor. The WT between bacteria with 50 mg/L of vanillin basically did not change. Filamentous bulking was significantly inhibited by the addition of vanillin. Therefore, QQ is a potential strategy for the prevention and control of filamentous bulking. This study provides new information regarding the microcosmic mechanisms of filamentous bulking.

Introducing Negatively Charged Residues on the Surface of Fetal Hemoglobin Improves Yields in Escherichia coli.

Fetal hemoglobin (HbF) has been developed into an important alternative protein for oxygen therapeutics. Such applications require extensive amounts of proteins, which only can be achieved via recombinant means. However, the expression of vertebrate hemoglobins in heterologous hosts is far from trivial. There are several issues that need to be dealt with. These include, among others, the solubility of the globin chains, equimolar expression of the globin chains, and access to high levels of free heme. In this study, we examined the impact of introducing negative charges on the surface of HbF. Three different HbF mutants were examined, carrying four additional negative charges on the α-subunit (rHbFα4), two additional negative charges on the γ-subunit (rHbFγ2) or a combination of these (rHbFα4/γ2). The increase in negative surface charge in these HbF mutants required the development of an alternate initial capture step in the downstream purification procedures. For the rHbFα4 mutant, we achieved a significantly enhanced yield of purified HbF with no apparent adverse effects on Hb functionality. However, the presence of non-functional Hb portions in the rHbFγ2 and rHbFα4/γ2 samples reduced the yields significantly for those mutants and indicated an imbalanced expression/association of globin chains. Furthermore, the autoxidation studies indicated that the rHbFγ2 and rHbFα4/γ2 mutants also were less oxidatively stable than rHbFα4 and wt rHbF. The study further verified the need for an improved flask culture protocol by optimizing cultivation parameters to enable yield-improving qualities of surface-located mutations.

Effect of carbamylation on protein structure and adsorption to self-assembled monolayer surfaces

Protein adsorption research has primarily focused upon the effects of surface chemistry, with almost no emphasis on how changes to proteins that occur in various disease states may influence their adsorption. One such situation occurs with chronic kidney disease where, despite hemodialysis treatment, the retention of urea within the blood compartment leads to protein carbamylation. Protein carbamylation has been shown to alter the function and structure of proteins. This work is focused on understanding how different degrees of carbamylation affect the physicochemical properties (structure, charge, water interactions) of single proteins (α-lactalbumin, albumin, and fibrinogen) and their adsorption to self-assembled monolayers. It was found that, unlike its secondary structure, the protein’s tertiary structure was significantly altered upon carbamylation. Also, compared to native proteins, an increase in carbamylation lead to an increase in the negative surface charge of the protein and a weaker hydration state of the protein. In order to study the effects of different types of neutral surfaces, of different surface-water properties, on protein adsorption both bare and alkanethiol modified (−CH3 or −OH end-groups) Au surfaces with were used as model surfaces. A significant decrease in adsorbed amounts of carbamylated fibrinogen and carbamylated α-lactalbumin, but not for carbamylated albumin, relative to native proteins was observed for both surfaces; suggesting that the increase in negative surface charge is more influential on adsorption than the change in hydration that occurs throughout the protein upon carbamylation. This data suggests that protein alterations that occur due to disease states have a significant effect on the overall protein structure and these changes affect their adsorption to surfaces.

The influence of ligand charge and length on the assembly of Brome mosaic virus derived virus-like particles with magnetic core

Virus-like particles (VLPs) have sparked a great interest in the field of nanobiotechnology and nanomedicine. The introduction of superparamagnetic nanoparticles (SPIONs) as a core, provides potential use of VLPs in the hyperthermia therapy, MRI contrast agents and magnetically-powered delivery agents. Magnetite NPs also provide a significant improvement in terms of VLPs stability. Moreover employing viral structural proteins as self-assembling units has opened a new paths for targeted therapy, drug delivery systems, vaccines design, and many more. In many cases, the self-assembly of a virus strongly depends on electrostatic interactions between positively charged groups of the capsid proteins and negatively charged nucleic acid. This phenomenon imposes the negative net charge as a key requirement for the core nanoparticle. In our experiments, Brome mosaic virus (BMV) capsid proteins isolated from infected plants Hordeum vulgare were used. Superparamagnetic iron oxide nanoparticles (Fe3O4) with 15 nm in diameter were synthesized by thermal decomposition and functionalized with COOH-PEG-PL polymer or dihexadecylphosphate (DHP) in order to provide water solubility and negative charge required for the assembly. Nanoparticles were characterized by Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS), Zeta Potential, Fourier Transformed Infrared Spectroscopy (FTIR) and Superconducting Quantum Interference Device (SQUID) magnetometry. TEM and DLS study were conducted to verify VLPs creation. This study demonstrates that the increase of negative surface charge is not a sufficient factor determining successful assembly. Additional steric interactions provided by longer ligands are crucial for the assembly of BMV SPION VLPs and may enhance the colloidal stability.

Degradation and biocompatibility of photoembossed PLGA-acrylate blend for improved cell adhesion.

We have shown previously that PMMA-acrylate photopolymers are biocomopatible and can exhibit improved cell adhesion compared to PMMA, due to an increase in negative surface charge caused by UV radiation PLGA has been used widely in soft tissue regeneration due to its high biocompatibility and cell adhesion. This polymer is also biodegradable and can be utilised in the field of vascular regeneration. In this study, PLGA is blended with a triacrylate monomer (TPETA) to create a degradable photopolymer blend. Surface relief structures are formed on this PLGA-TPETA by photoembossing. An optimum height of 950 nm was achieved for a 10 µm pitch with the height of these relief structures being controlled by changing UV intensity, processing temperature and time. Degradation studies of this blend revealed a bulk degradation mechanism with PLGA-TPETA degrading slower compared to pure PLGA. We also evaluated the adhesion of human umbilical vein endothelial cells (HUVECs) on both smooth and textured PLGA-TPETA films. Embossed PLGA-TPETA films showed improved cell adhesion compared to smooth substrates. Furthermore, HUVECs proliferated faster on the embossed surface compared to their smooth counterparts. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 163-171, 2018.

Determination of the activity of maleimide-functionalized phospholipids during preparation of liposomes

Numerous examples exist in the literature for the use of maleimide-thiol-reactions in the area of functionalized nanoparticles. Although the hydrolysis tendency of maleimides is well-known, qualitative and quantitative information on the stability and reactivity of maleimide groups during preparation and in final formulations are missing. This is surprising, since hydrolysis of maleimides prevents nanoparticle functionalization and results in an increase of negative surface charge due to the hydrolysis product maleic acid. In this study we investigated the stability of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[maleimide-2000] (DSPE-PEG2000-Mal) during the preparation of liposomes via two common preparation methods, which can be distinguished by the insertion of DSPE-PEG2000-Mal during or after the liposome formation process (pre-insertion and post-insertion process). The liposomes prepared by the pre-insertion method had 63% active maleimide groups remaining on their surface. The activity decreased dramatically during the purification process down to 32%. The preparation by post-insertion showed minimal effects with regard to maleimide activity. 76% of maleimide groups were active and therefore available for coupling reaction. By identifying active maleimide groups on the surface of the final formulations, the presented work revealed the dramatic impact of preparation methods on the activity of maleimide groups.

Surface Properties of Wild-Type Rhizobium leguminosarum bv. trifolii Strain 24.2 and Its Derivatives with Different Extracellular Polysaccharide Content.

Rhizobium leguminosarum bv. trifolii is a soil bacterium able to establish symbiosis with agriculturally important legumes, i.e., clover plants (Trifolium spp.). Cell surface properties of rhizobia play an essential role in their interaction with both biotic and abiotic surfaces. Physicochemical properties of bacterial cells are underpinned by the chemical composition of their envelope surrounding the cells, and depend on various environmental conditions. In this study, we performed a comprehensive characterization of cell surface properties of a wild-type R. leguminosarum bv. trifolii strain 24.2 and its derivatives producing various levels of exopolysaccharide (EPS), namely, pssA mutant Rt5819 deficient in EPS synthesis, rosR mutant Rt2472 producing diminished amounts of this polysaccharide, and two EPS-overproducing strains, Rt24.2(pBA1) and Rt24.2(pBR1), under different growth conditions (medium type, bacterial culture age, cell viability, and pH). We established that EPS plays an essential role in the electrophoretic mobility of rhizobial cells, and that higher amounts of EPS produced resulted in greater negative electrophoretic mobility and higher acidity (lower pKapp,av) of the bacterial cell surface. From the tested strains, the electrophoretic mobility was lowest in EPS-deficient pssA mutant. Moreover, EPS produced by rhizobial strains resulted not only in an increase of negative surface charge but also in increased hydrophobicity of bacterial cell surface. This was determined by measurements of water contact angle, surface free energy, and free energy of bacterial surface–water–bacterial surface interaction. Electrophoretic mobility of the studied strains was also affected by the structure of the bacterial population (i.e., live/dead cell ratio), medium composition (ionic strength and mono- and divalent cation concentrations), and pH.

Quasi-Spherical Cell Clusters Induced by a Polyelectrolyte Multilayer.

Fibroblasts cultured on polyelectrolyte multilayers, PEMUs, made from poly(diallyldimethylammonium), PDADMA, and poly(styrene sulfonate), PSS, showed a variety of attachment modes, depending on the charge of the last layer and deposition conditions. PEMUs terminated with PDADMA (cationic) were cytotoxic when built in 1.0 M NaCl but cytophilic when built in 0.15 M NaCl. Cells adhered poorly to all PSS-capped (anionic) films. PEMUs built in 0.15 M NaCl but terminated with a layer of PSS in 1.0 M NaCl induced most cells to form spherical clusters after about 48 h of culture. These clusters still interrogated the surface, and when they were replated on control tissue culture plastic, cells emerged with close to 100% viability. Differences between the various surfaces were probed in an effort to identify the mechanism responsible for this unusual behavior, which did not follow accepted correlations between substrate stiffness and cell adhesion. No significant differences in roughness or wetting were observed between cluster-inducing PSS-capped multilayers and those that did not produce clusters. When the surface charge was assayed with radiolabeled ions a strong increase in negative surface charge was revealed. Viewing the multilayer as a zwitterionic solid and comparing its surface charge density to that of a cell membrane yields similarities that suggest a mechanism for preventing protein adhesion to the surface, a necessary step in the integrin-mediated mechanotransduction properties of a cell.

Nickel Oxide Grafted Andic Soil for Efficient Cesium Removal from Aqueous Solution: Adsorption Behavior and Mechanisms

An andic soil, akadama clay, was modified with nickel oxide and tested for its potential application in the removal of cesium from aqueous solution. Scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), and powder X-ray diffraction (XRD) results revealed the nickel oxide was successfully grafted into akadama clay. N2 adsorption-desorption isotherms indicated the surface area decreased remarkably after modification while the portion of mesopores increased greatly. Thermogravimetric-differential thermal analysis (TG-DTA) showed the modified akadama clay had better thermostability than the pristine akadama clay. Decreases in cation exchange capacity (CEC) and ζ-potential were also detected after the modification. Adsorption kinetic and isotherm studies indicated the adsorption of Cs+ on the modified akadama clay was a monolayer adsorption process. Adsorption capacity was greatly enhanced for the modified akadama clay probably due to the increase in negative surface charge caused by the modification. The adsorption of Cs+ on the modified akadama clay was dominated by an electrostatic adsorption process. Results of this work are of great significance for the application of akadama clay as a promising adsorbent material for cesium removal from aqueous solutions.

The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis

A series of reduced graphene oxide (RGO) colloids with different amounts of surface charges was prepared. The change in surface charge at different pH values was detected by zeta potential measurement, and the evolution of oxygen-containing functional groups attached to the RGOs was analysed by Fourier-transform infrared spectroscopy and ultraviolet-visible absorption spectroscopy. Results showed that the edge phenolic hydroxyl and carboxyl groups made more contributions to the negative surface charge compared with the basal-plane hydroxyl and epoxy groups. Electrical impedance spectroscopy results proved that the surface charge of RGOs significantly affected their electrochemical properties. Furthermore, GO and RGOs (graphene oxide materials) were also used to construct electrochemical sensors for quantitive measurement of Cu2+ by differential pulse anodic stripping voltammetry. The results revealed that the increase in negative surface charge on RGO enhanced its electrocatalytic activity for Cu2+ reduction. Thus, considering that studies on the properties of graphene oxide materials can be simplified by the surface charge, its analysis is an important means of material characterisation.

Photoadsorption and photodesorption for GaN

The effect of an ambient environment on the surface photovoltage and photoluminescence observed for GaN is studied. In air ambient the upward band bending gradually increases under UV illumination and is explained by the photoinduced chemisorption of surface adsorbates. Specifically, the increase in negative surface charge is consistent with the transfer of electrons from surface states or bulk to oxygen species physisorbed at the GaN surface. In contrast, the upward band bending gradually decreases in vacuum under UV illumination and can be explained by the photoinduced desorption of these species. The photoadsorption and photodesorption of negatively charged species cause the surface depletion region to increase and decrease, respectively. This change in depletion region width is consistent with the observed decrease in photoluminescence intensity in air ambient and its significant increase in vacuum for a sample with low free electron concentration.

Effect of arsenate on adsorption of Zn(II) by three variable charge soils

The effect of arsenate on adsorption of Zn(II) in 3 variable charge soils (Hyper-Rhodic Ferralsol, Rhodic Ferralsol, and Haplic Acrisol) and the desorption of pre-adsorbed Zn(II) in the presence of arsenate were investigated in this study. Results showed that the presence of arsenate led to an increase in both the adsorption and desorption of Zn(II) in these variable charge soils. It was also suggested that the enhanced Zn(II) adsorption by arsenate was mainly due to the increase in negative surface charge of the soils induced by the specific adsorption of arsenate, and the increase in electrostatically adsorbed Zn(II) was responsible for the increase in the desorption of Zn(II). The effect of arsenate on Zn(II) adsorption primarily depends on the initial concentration of arsenate and Zn(II), the system pH, and the nature of soils. The enhanced adsorption of Zn(II) increased with the increase in the initial concentration of arsenate and the amount of arsenate adsorbed by the soils. The presence of arsenate decreased the zeta potential of soil suspensions and soil IEP and thus shifted the adsorption edge of Zn(II) to a lower pH region. The effect of arsenate on Zn(II) adsorption in these 3 soils followed the order Hyper-Rhodic Ferralsol > Rhodic Ferralsol > Haplic Acrisol, which was consistent to the contents of iron oxides in these soils and the amount of arsenate adsorbed by the soils.

Regularities of the Two-Layer Adsorption of Anionic Surfactant and Cationic Polyelectrolyte on the Fused Quartz Surface

The adsorption of sodium dodecyl sulfate (SDS) on the quartz capillary surface that was preliminarily covered with a layer of cationic polyelectrolyte (CPE) is studied. The charge sign change of the modified quartz surface and subsequent increase of negative surface charge observed upon the adsorption of SDS indicate the formation of the second adsorption layer composed of surfactant molecules. It is shown that the surfactant layer is stronger attached to the surface at a higher charge of the CPE adsorption layer. Upon the formation of the surfactant layer on a looser CPE adsorption layer, the desorption of SDS molecules decreases, which can be associated with the partial penetration of these molecules into the CPE layer.

Effect of Low Molecular Weight Organic Anions on Adsorption of Potassium by Variable Charge Soils

Low‐molecular‐weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes. In the present paper, the effect of the anions of four LMW organic acids on the adsorption of potassium (K) by two variable charge soils was investigated. The results showed that the presence of organic anions led to an increase in K adsorption. The ability of organic anions to induce K adsorption followed the order: citrate>malate>malonate≈oxalate>acetate. The adsorption of K induced by organic anions increased with the increases in pH and the concentration of the anions. Both Langmuir and Freundlish equations can be used to describe the adsorption of K by variable charge soils in the presence of organic anions. Adsorption of ions is a direct consequence of carrying of surface charge for soils. Recent works have shown that the presence of organic anions led to an increase in negative surface charge and a decrease in positive surface charge of variable charge soils. The results indicated that the extents of the effect of different organic anions on adsorption of K and surface charge were generally of the same order. Therefore, it can be considered that the main reason for the adsorption of K induced by organic anions is the increase in net negative surface charge caused by the adsorption of the anions on the soils.

Effect of low-molecular-weight organic anions on exchangeable aluminum capacity of variable charge soils

Low-molecular-weight (LMW) organic acids exist widely in soils and have been implicated in many soil processes, such as nutrient availability, translocation of metals, fate of heavy metals, and mineral weathering. In this paper, the effect of the LMW organic anions on the exchangeable aluminum of two variable-charge soils was examined. The results showed that the organic anions induced an increase or a decrease in the exchangeable Al, and the extent and direction of the effect depended on the nature of organic anions, surface chemical properties of soils, and pH. For example, at pH 4.5, the quantity of exchangeable Al of Oxisol in the control system was 2.65 mmol kg −1, whereas the values in the citrate, oxalate, malonate, malate, tartarate, salicylate, and lactate systems increased by 3.25, 1.93, 1.95, 1.82, 1.28, 0.88, and 0.45 times, respectively. In contrast, the quantity of the exchangeable Al of Ultisol at pH 4.5 in the oxalate and the citrate systems decreased by 8.8 and 19.6%, respectively. The increase in the exchangeable Al was caused mainly by the increase in negative surface charge of the soils due to the specific adsorption of organic anions. The ability of organic anions at low concentrations to increase exchangeable Al for Oxisol followed the order citrate > oxalate and malonate > malate > tartarate > salicylate > maleate > lactate. This order is consistent with that of the effect of the adsorption of anions on the increase in the negative surface charge and/or the decrease in the positive surface charge of the soil. On the other hand, the organic anions could depress the exchangeable Al through the formation of soluble Al–organic anion complexes under certain conditions. The anions with small stability constants of Al–organic anion complexes, such as lactate, caused an increase in exchangeable Al with the change in surface charge of the soils, while those with large stability constants, such as citrate and oxalate, caused an increase in the exchangeable Al at low concentration and a decrease at high concentration.

Characterization of the early stages of programmed cell death in maize root cells by using comet assay and the combination of cell electrophoresis with annexin binding.

An emerging topic in plant biology is whether plant cells display similar elements of programmed cell death (PCD) as animal cells do. We have studied cell death in maize roots exposed to cold stress by using fluorescence microscopy, terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL), DNA gel electrophoresis, single cell gel electrophoresis (SCGE), cell electrophoresis, and annexin binding techniques. The results showed that cell death in maize root cells triggered by cold stress was accompanied by a subset of features characteristic of animal PCD such as nuclear condensation and fragmentation, and oligonucleosomal DNA fragmentation. In addition to DNA laddering and TUNEL positivity, a "comet" pattern indicative of DNA breakage appeared as short as after one day of treatment. The maize root cell PCD process was also accompanied by an increase in negative surface charge of the dying cells due to exposure of phosphatiolylserine (PS) from inner to outer membrane. After annexin binding, however, the enhanced electrophoretic mobility (EPM) of the dying cells decreased nearly to normal values. This result suggests that the combination between cell electrophoresis and annexin binding provides a quantitative method for monitoring PS exposure during plant PCD.

Quinoline Sorption on Kaolinite–Humic Acid Complexes

Adsorption of quinoline and background electrolyte (LiCl) onto specimen kaolinite was measured as a function of surface‐bound humic acid (HA) concentration , pH (3–10), and ionic strength (1–10 mM). Complexation of HA on the kaolinite surface (4.5 mg C kg−1) reduced the point of zero net charge (pznc) for kaolinite by more than one pH unit and resulted in a significant increase in negative surface charge. Maximum sorption of quinoline occurred near its pKa for all sorbents. Below the pKa, quinoline sorption increases with increasing pH and decreasing proton competition. Above the pKa, sorption is reduced in parallel with (but offset to a higher pH from) the ionized fraction. Competition with Li+ for surface sites is apparent from diminished quinoline adsorption with increasing ionic strength, but sorption of the ionized form of quinoline is always favored and kaolinite exhibits selectivity for cationic quinoline over However, increasing fOC diminishes quinoline sorption and selectivity and increases sorption reversibility relative to uncoated kaolinite. Humic acid alone exhibits lower selectivity for quinoline The results indicate that mineral‐sorbed humic substances can diminish retention of cationic quinoline despite an increase in overall cation‐exchange capacity.