Natural Colloids Research Articles

Influence of goethite on the fate of antibiotic (tetracycline) in the aqueous environment: Effect of cationic and anionic surfactants.

Over the last decades, the release and occurrence of organic pollutants in aquatic systems have become a major global concern due to their bioaccumulation, toxicity, and adverse effects on the ecosystem. Tetracycline (TC), a widely used antibiotic, is often found at high concentrations in the aqueous environment and tends to bind with the natural colloids. Post-COVID-19 pandemic, the release of surfactants in the environment has increased due to the excessive use of washing and cleaning products. This study systematically investigated the interaction of goethite with TC in the absence and presence of anionic (sodium dodecyl sulfate, SDS) and cationic (cetyltrimethylammonium bromide, CTAB) surfactants. The impact of various environmental parameters like pH, ionic strength, temperature, and organic matter was also studied. It was observed that SDS has drastically increased TC sorption onto goethite from 11mg/g to 19mg/g, while CTAB had the opposite effect. To delineate the goethite-TC interaction mechanisms, FTIR with two-dimensional correlation analysis (2D-COS) was performed. The pH of the solution was crucial in the presence of SDS, while ionic strength did not affect the interaction process. The sorption process was endothermic, as evidenced by the increase in sorption capacity with the rise in the temperature. The presence of organic matter hinders the sorption of TC onto goethite, which is also observed in river water where the organic content is very high. Overall, our study helps to predict the fate of organic pollutants like antibiotics in aqueous environments in the coexistence of surfactants and iron oxyhydroxides.

Effects of colloids with different compositions on benzophenone-3 biotoxicity in zebrafish embryos.

The fate of the pollutants in aquatic environment is closely related to colloids, and the carrier effect of colloids on pollutants not only affects their bioaccumulation, but may also affect their toxicity. In this study, the effects of natural colloid with different components on the biological toxicity of benzophenone-3 (BP3) to zebrafish larvae (Diano rerio) were studied. BP3 caused oxidative stress damage, thyroid system disorders and neurotoxicity in zebrafish larvae. And in the co-exposure groups, the organic and black carbon mineral (BCM) colloids enhanced the organism's antioxidant system by regulating the activities of superoxide dismutase (SOD) and glutathione peroxidase (GPx), reducing the lipid peroxidation damage in larvae. BCM colloids caused the thyroid system disorders in organisms, while organic colloids exacerbated the thyroid toxicity by transporting more BP3 into organisms, inducing severe abnormal heartbeats. The BCM and organic colloids regulated the acetylcholinesterase (AChE) activity and/or 5-hydroxytryptamine (5-ht) contents by affecting the neuroactive ligand receptor interaction pathway in zebrafish larvae, significantly increasing their swimming speed in co-exposure groups under the light condition. In addition, the effects of colloid-bound and freely dissolved BP3 absorbed by organisms on their physiological and biochemical activities were different. By analyzing the relative expression of the significant differential metabolites affected by BP3 in all experimental groups, it was found that colloid-bound and freely dissolved BP3 had a synergistic effect on most of these metabolites and pathways. However, the freely dissolved BP3 interfered with the purine metabolic pathway by mediating 2-(amidino)-n1-(5-phospho-d-ribosyl)acetamidine, and the tyrosine metabolic pathway by mediating choline and uranylacetic acid, while the colloid-bound BP3 has no or inverse regulatory effects on these three metabolites. This study provided a new perspective for the biotoxicity study of the pollutants in aquatic environment, necessitating a reconsideration of the real ecological risks of emerging pollutants in the presence of natural colloids.

Nanoplastics as competitors of natural colloids in the environment: the case of gadolinium complexes

This study investigates whether nanoplastics are significant contributor of rare earth elements (Gd) transportation under environmental conditions. Important effects of nanoplastic concentration, Gd concentration, complexation with organic ligands, pH, ionic strength and occurrence of natural colloids competitor were studied thanks to an experimental design combining incubation follow by ultrafiltration and ICPMS analyses. In particular, we observed that even in sea water, about half of the gadolinium can be adsorbed on nanoplastics. Surprisingly, the adsorption is more pronounced at low concentration of nanoplastic (10μgL-1) probably because it prevents homoaggregation of nanoparticles. These are the first experimental clues that adsorption on nanoplastics is governed by fundamentally different processes than adsorption on microplastics.

Developing machine learning algorithms to predict the dissolution of zinc oxide nanoparticles in aqueous environment

Engineered nanoparticles (ENPs) are of particular concern due to their ubiquitous occurrence and potential to cause adverse effects on aquatic biota. Consequently, a comprehensive understanding of ENP interactions and the mechanisms that underpin their fate and behaviour in the aquatic system is important to support their long-term applications and protection of ecology. However, due to a wide range of physicochemical parameters, as well as possible dynamic interactions with natural colloid particles, it is not practical to undertake experimental testing for each variation of ENPs using different aquatic permutations. This study describes machine learning (ML) algorithms for prediction of nZnO dissolution in aquatic systems using experimental data. The input parameters with the highest correlation were size and pH. On the contrary, categorical input variables such as coating, coating type, salt, and NOM type had a low correlation. The random forest regression and the extreme gradient boost algorithms performed remarkably well, with coefficients of determination (R2) of 0.85 and 0.92, respectively. The least effective method was multiple linear regression, which had a root mean square error of 0.15 and an R2 of 0.31. ML offers a convenient and low-cost approach for screening nZnO dissolution in aquatic systems.

Multifaceted Aquatic Environmental Differences between Nanoplastics and Microplastics: Behavior and Fate.

Microplastics and nanoplastics are emerging pollutants of concern in the aquatic environment that are causing increasing global environmental and human health problems. Although there has been extensive research on microplastics and nanoplastics, little has been said about the differences in their behavior in the aquatic environment, and many studies have considered them as the same class of hazardous materials; but in fact, microplastics and nanoplastics should be considered as two different types of environmentally hazardous materials. In this review, we propose that microplastics and nanoplastics behave in the aquatic environment in a size-dependent manner and should be distinguished. And we systematically analyzed the differences in the behavior of microplastics and nanoplastics in the aquatic environment in terms of five aspects: 1) distribution behavior; 2) adsorption behavior; 3) reaction with natural colloids; 4) aging and leaching behavior; 5) interaction with organisms. This paper has been written to draw academic attention to the different behaviors of microplastics and nanoplastics in the aquatic environment in order to distinguish between their effects on humans and the environment.

The formation of multi-metal(loid)s contaminated groundwater at smelting site: Critical role of natural colloids

The occurrence and migration of colloids at smelting sites are crucial for the formation of multi-metal(loid)s pollution in groundwater. In this study, the behavior of natural colloids (1 nm-0.45 µm) at an abandoned smelting site was investigated by analyzing groundwater samples filtered through progressively decreasing pore sizes. Smelting activities in this site had negatively impacted the groundwater quality, leading to elevated concentrations of zinc (Zn), lead (Pb), arsenic (As), and cadmium (Cd). The results showed that heavy metal(loid)-bearing colloids were ubiquitous in the groundwater with the larger colloidal fractions (∼75 −450 nm) containing higher abundances of pollutants. It was also observed that the predominant colloids consisted of Zn-Al layered double hydroxide (LDH), sphalerite, kaolinite, and hematite. By employing multiple analytical techniques, including leaching experiments, soil colloid characterization, and Pb stable isotope measurements, the origin of groundwater colloids was successfully traced to the topsoil colloids. Most notably, our findings highlighted the increased risk of heavy metal(loid)s migration from polluted soils into adjacent sites through the groundwater because of colloid-mediated transport of contaminants. This field-scale investigation provides valuable insights into the geochemical processes governing heavy metal(loid) behavior as well as offering pollution remediation strategies specifically tailored for contaminated groundwater.

Co-transport behavior of Am(III) and natural colloids in the vadose zone sediments

Americium (III) (Am(III)) in the natural environment is considered immobile due to its low solubility, strong adsorption, and high affinity to solid surfaces. However, the presence of natural colloids may carry Am(III) transport for long distance. The individual and co-transport behaviors of Am(III) and natural colloids through the unsaturated packed columns were investigated under the influence of pH, electrolyte concentration, velocity, Am(III) concentration and natural colloids concentration. Under all experimental conditions, Am(III) individual transport construct sight breakthrough curves (BTCs, CAm/C0 < 3%), but the presence of natural colloids increased the BTCs plateau of Am(III) significantly (30% < CAm/C0 < 80%), indicating that the colloids were able to promote Am(III) transport in the unsaturated porous media. DLVO theoretical calculations reveal that the increased pH and decreased electrolyte concentration lead to a rase in electrostatic repulsion, and the natural colloids tend to be dispersed and stabilized, which facilitates elution. In addition to this, the increase of velocity and colloids concentration will lead to greater breakthrough of natural colloids. The non-equilibrium two-site model and the two-site kinetic retention model well-described the BTCs of Am(III) and natural colloids, respectively. This study provide new insights into the behavior of natural colloids carrying the Am(III) into aquifers through the vadose zone sediments.

Influence of Natural Colloids on the Kinetic Adsorption Behavior of Am(III) on Vadose Zone Sediments

Influence of Natural Colloids on the Kinetic Adsorption Behavior of Am(III) on Vadose Zone Sediments

Influence of Soil Colloids on the Transport of Cd2+ and Pb2+ under Different pH and Ionic Strength Conditions

The co-transport of contaminants by soil colloids can generate substantial environmental risk, and this behavior is greatly affected by environmental conditions. In this study, AF4-ICP-MS was used to investigate the size distribution and composition of Cd/Pb-bearing colloids; saturated sand column experiments were used to investigate the impact of soil colloids on the transport of Cd/Pb under different pH and ionic strength conditions. AF4-ICP-MS characterization showed that natural colloids were primarily associated with two sizes ranges: 0.3–35 KDa (F1, fine nanoparticles) and 280 KDa–450 nm (F2, larger nanoparticles), which mainly consisted of organic matter (OM), iron (Fe), and manganese (Mn) (oxy)hydroxides and clay minerals. Fine nanoparticles could strongly adsorb Cd and Pb under all environmental conditions. Mn and Fe (oxy)hydroxides generally formed under neutral to alkaline conditions and exhibited adsorption capabilities for Cd and Pb, respectively. Transport experiments were conducted under different pH and ionic strength conditions. At pH 3.0, soil colloids had little effect on the transport of Cd2+ and Pb2+. At pH 5.0, soil colloids inhibited the transport of Cd2+ by 16.1%, and Pb2+ recovery was still 0.0%. At pH 7.0 and 9.0, soil colloids facilitated the transport of Cd2+ by 15.6% and 29.6%, facilitated Pb2+ by 1.3% and 6.4%. At an ionic strength of 0, 0.005, and 0.01 mol L−1 NaNO3, soil colloids facilitated the transport of Cd2+ by 77.7%, 45.8%, and 15.6%, only facilitated the transport of Pb2+ by 46.2% at an ionic strength of 0 mol L−1 NaNO3. At an ionic strength of 0.05 mol L−1 NaNO3, soil colloids inhibited the transport of Cd2+ and Pb2+ by 33.1% and 21.0%, respectively. The transport of Cd2+ and Pb2+ facilitated by soil colloids was clearly observed under low ionic strength and non-acidic conditions, which can generate a potential environmental risk.

Multi-metal contaminant mobilizations by natural colloids and nanoparticles in paddy soils during reduction and reoxidation

Naturally-occurring colloids and nanoparticles are crucial in transporting heavy metal contaminants in soil-water systems. However, information on particle-bound metals’ size distribution and elemental composition in paddy soils under redox-fluctuation is scarce. Here, we investigated the mobilization of Cu, Cd, and Pb-containing nanoparticles and colloids in four contaminated soils with distinctive geochemical properties during reduction and subsequent re-oxidation. Using AF4-UV-ICP-MS and STEM-EDS, we observed that particle-bound metals were primarily associated with two sizes ranges: 0.3−40 kDa (F1) and 130 kDa−450 nm (F2), which mainly consisted of organic matter (OM), iron hydroxide and clay minerals. Cu and Pb were more likely bound to colloid than Cd. Colloidal Cu, Pb and Cd accounted for averages of 83.2%, 72.4% and 19.8% of their total concentration in solution (<0.45 µm) during soil reduction, and decreased during soil re-oxidation. This proportion was also positively correlated with aqueous pH and DOC but negatively correlated with Eh. Further quantitative analysis demonstrated that Cu/Cd positively correlated with OM at nanometric scale (F1). This study provides quantitative insights into the size, composition and abundance of polymetallic pollutant-carrying particles in paddy soils during redox fluctuation, and highlights the importance of nanometric interactions between OM and toxic cationic metals for their release.

Natural organic matter and ionic strength (CaCl2) affect transport, retention and remobilization of silica encapsulated DNA colloids (DNAcol) in saturated sand columns

In the terrestrial environment, interactions between natural organic matter (NOM) and colloids can lead to the formation of an environmental corona around colloids, influencing their transport behaviour and, ultimately, their ecotoxicity. We used a synthetically designed colloid tagged with DNA (DNAcol) as a surrogate for natural colloids and investigated its transport in saturated sand columns. We varied the concentrations of NOM and ionic strength (CaCl2), to better understand the transport and release of DNAcol in porous media under both steady and transient porewater chemistry conditions. In addition, we aimed to understand the main factors that control deposition and release of DNAcol under tested conditions. To induce transient chemistry, we replaced the injection solution containing NOM and/or CaCl2 with Milli-Q water. The results showed that the deposition rate of DNAcol was inversely proportional to the concentration of NOM. The deposition rate increased significantly even under low ionic strength (CaCl2) conditions of tested conditions. Notably, the influence of NOM on the transport of DNAcol was most pronounced at the lowest range of [Ca2+]/DOC ratios, and the attachment of DNAcol to the sand grains was negligible. Moreover, the results showed while the DLVO theory captured the general trend of experimental results, it significantly underestimated the deposition of DNAcol in the presence of CaCl2. Under transient porewater chemistry conditions, colloid remobilization was observed upon flushing the column with Milli-Q water, leading to a secondary peak in the breakthrough curves. We observed that under transient porewater chemistry conditions, when the ionic strength of the solution was 10 mM, the magnitude of the remobilization peak was more significant compared to conditions with 1 mM ionic strength. Our work emphasized the complex interplay between water quality on the one hand and deposition and release of colloidal matter in saturated porous media on the other hand.

Triboelectric probes for investigating charge transfer at the colloid-solid interface

The charge transfer between the colloid and solid interface is a crucial factor for predicting colloid behavior and rational utilization of colloids. Despite its significance, analyzing the characteristics and subsequent effects of charge transfer at the colloid-solid interface remains challenging due to the complex multiphase nature of colloids. In this study, a liquid-solid triboelectric nanogenerator was used as a probe to detect the interfacial charge transfer during the contact electrification process of colloid-solid. The impact of colloid type, colloidal nanoparticle residue on the solid surface, and colloidal nanoparticle surface charge were investigated. The results indicate that in cases where the colloidal dispersion medium contains an excessive number of ions, ion adsorption dominates the interfacial charge transfer. The generation of hydrated protons after electron transfer between the colloid and solid leads to the deposition of colloidal nanoparticles on the solid surface, inhibiting charge transfer. When the dispersion medium contains fewer free ions and the colloidal nanoparticles possess a hydrophilic surface, the colloidal electric double layer significantly influences the interfacial charge transfer. The findings of this study provide valuable insights and open up new possibilities for further research in understanding the interactions between colloids and solids.

Extraction of Common Small Microplastics and Nanoplastics Embedded in Environmental Solid Matrices by Tetramethylammonium Hydroxide Digestion and Dichloromethane Dissolution for Py-GC-MS Determination.

Determination of microplastics and nanoplastics (MNPs), especially small MPs and NPs (<150 μm), in solid environmental matrices is a challenging task due to the formation of stable aggregates between MNPs and natural colloids. Herein, a novel method for extracting small MPs and NPs embedded in soils/sediments/sludges has been developed by combining tetramethylammonium hydroxide (TMAH) digestion with dichloromethane (DCM) dissolution. The solid samples were digested with TMAH, and the collected precipitate was washed with anhydrous ethanol to eliminate the natural organic matter. Then, the MNPs in precipitate were extracted by dissolving in DCM under ultrasonic conditions. Under the optimized digestion and extraction conditions, the factors including sizes and concentrations of MNPs showed insignificant effects on the extraction process. The feasibility of this sample preparation method was verified by the satisfactory spiked recoveries (79.6-91.4%) of polystyrene, polyethylene, polypropylene, poly(methyl methacrylate), polyvinyl chloride, and polyethylene terephthalate MNPs in soil/sediment/sludge samples. The proposed sample preparation method was coupled with pyrolysis gas chromatography-mass spectrometry to determine trace small MPs and NPs with a relatively low detection limit of 2.3-29.2 μg/g. Notably, commonly used MNPs were successfully detected at levels of 4.6-51.4 μg/g in 6 soil/sediment/sludge samples. This proposed method is promising for evaluating small solid-embedded MNP pollution.

Influence of temperature, ion type, and ionic strength on dynamic viscoelastic, steady‐state, and dilute‐solution behavior of Melissa officinalis seed gum

AbstractIn this study, the rheological properties of Melissa officinalis seed gum (MOSG) as a function of temperature (25, 35, 45, 55, and 65°C) and salt concentrations (10, 50, 100, and 200 ppm NaCl and CaCl2) were investigated. The power law and Herschel‐Bulkley models could describe the rheological behavior of gum dispersions. The results showed that the intrinsic viscosity of the samples significantly decreased with increasing temperature and salt concentration. Steady‐state analysis of the samples revealed a non‐Newtonian, shear‐thinning behavior at tested temperatures and concentrations. The apparent viscosity increased with increasing salt concentrations and decreasing temperature. The results obtained from the frequency sweep test showed that the loss modulus (G″) values were higher than storage modulus (G′) in the concentrations of 0.25% to 0.5%, but beyond these concentrations, G′ values were greater than G″. The elasticity and viscosity of the gum samples increased with increasing concentration. The results of this study confirmed that MOSG has good rheological properties and can be used in food and pharmaceutical systems.Practical ApplicationsThe rheological behavior is the most important functional property of natural colloids. This property is related to the capacity of hydrocolloids in improving viscosity and the formation of gel structure at low concentrations. The thickening property of hydrocolloids has a determinant role in consumer acceptance. Furthermore, the consumer acceptance of different food products such as fruit juices, sauces, ready jells, and desserts is mainly dependent on rheological behavior. Therefore, investigation of rheological properties of the new source of food hydrocolloids is interested in food industries.

The role of silver nanoparticles effects in the homeostasis of metals in soybean cultivation through qualitative and quantitative laser ablation bioimaging

BackgroundNanoparticles (NPs) are currently found in the world in the form of natural colloids and volcanic ash, as well as in anthropogenic sources, such as nanofertilizers; however, in the literature, there is still a lack of toxicological evidence, risk assessment, and regulations about the use and environmental impact of NPs in the agroindustrial system. Therefore, the aim of this work was to evaluate alterations caused by the presence of AgNPs during the development of soybean plants. MethodsThe BRS232 non-transgenic (NT) soybean plant and 8473RR (TRR) and INTACTA RR2 PRO (TIntacta) transgenic soybean plants were irrigated for 18 days under controlled conditions with deionized water (control), AgNPs, and AgNO3. The isotopes 107Ag+, 55Mn+, 57Fe+, 63Cu+, and 64Zn+ were mapped in leaves, using 13C+ as an internal standard (IS), and carried out using a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique with a Nd:YAG (213 nm) laser source in the imagagin mode using the LA-iMageS software and also Mathlab. ResultsLeaf images showed a low Ag translocation, indicated by the basal signal of this ion. Additionally, the presence of Ag in the ionic form and as NPs altered the homeostasis of 112Cd+, 64Zn+, 55Mn+, 63Cu+, and 57Fe+ in different ways. Quantitative image analysis was performed for Cu. ConclusionThe behavior of TRR and TIntacta plants was different in the presence of ionic silver or AgNPs, confirming that the metabolism of these two plants, despite both being transgenic, are different. Through the images, it was observed that the response of plants was different in the face of the same stress conditions during their development.

Adsorption behaviors and influencing factors of antibiotic norfloxacin on natural kaolinite-humic composite colloids in aquatic environment

Particles are ubiquitous and abundant in natural waters and play a crucial role in the fate and bioavailability of organic pollution. In the present study, natural mineral (kaolinites, KL), organic (humic/fulvic acid, HA/FA) and their composite particles were further separated into particles fractions (PFs, >1 μm) and colloidal fractions (CFs, 1 kDa-1 μm) by cross-flow ultrafiltration (CFUF). This research demonstrated the role of kaolinite-humic composite colloids on the adsorption of fluoroquinolone norfloxacin (NOR). The Freundlich model satisfactory described adsorption curves, showing strong affinity of NOR to CFs, with sorption capacity (KF) between 8975.50 and 16638.13 for NOR. The adsorption capacities of NOR decreased with the particle size increasing from CFs to PFs. In addition, composite CFs showed excellent adsorption capacity, which was mainly attributed to the larger specific surface area of composite CFs and electronegativity and numerous oxygen-containing functional groups on the surfaces of the complexes, and electrostatic attraction, hydrogen bond and cation exchange could dominate the NOR adsorption onto the composite CFs. The best pH value under adsorption condition of composite CFs varied from weakly acidic to neutral with the increase of load amount of humic and fulvic acids on the surface of inorganic particles. The adsorption decreased with higher cation strength, larger cation radius and higher cation valence, which depended on the surface charge of colloids and the molecular shape of NOR. These results provided insight into the interfacial behaviors of NOR on the surfaces of natural colloids and promoted the understanding of the migration and transport of antibiotics in environmental systems.

A multimedia model to estimate the environmental fate of microplastic particles

Nano- and microplastic (NMP) is a diverse and challenging contaminant and data on NMP concentrations are therefore not fully available for all environmental compartments. For environmental assessments of NMP, screening-level multimedia models can fill this gap, but such models are not available. Here, we present SimpleBox4Plastic (SB4P) as the first multimedia ‘unit world’ model capable of addressing the full NMP continuum, explore its validity, and evaluate it based on a case study for microbeads and by comparisons with (limited) concentration data. SB4P links NMP transport and concentrations in and across air, surface water, sediment, and soil, taking into account processes such as attachment, aggregation, and fragmentation, by solving mass balance equations using matrix algebra. These link all concentrations and processes known to be relevant for NMP using first-order rate constants, which are obtained from the literature. The SB4P model, as applied to microbeads, provided mass or number concentrations of NMP as the total of ‘free’ particles, heteroaggregates with natural colloids, and larger natural particles in each compartment at steady state. Processes most relevant in explaining observed Predicted Exposure Concentrations (PECs) were determined using rank correlation analysis. Although the predicted PECs remained uncertain due to the propagating uncertainty, inferences regarding these processes and relative distribution across compartments can be considered robust.

Electroanalytical Trace Metal Cations Quantification and Speciation in Freshwaters: Historical Overview, Critical Review of the Last Five Years and Road Map for Developing Dynamic Speciation Field Measurements.

An historical overview covering the field of electroanalytical metal cations speciation in freshwaters is presented here, detailing both the notable experimental and theoretical developments. Then, a critical review of the progress in the last five years is given, underlining in particular the improvements in electrochemical setups and methodologies dedicated to field surveys. Given these recent achievements, a road map to carry out on-site dynamic metal speciation measurements is then proposed, and the key future developments are discussed. This review shows that electroanalytical stripping techniques provide a unique framework for quantitatively assessing metals at trace levels while offering access to both thermodynamic and dynamic features of metal complexation with natural colloidal and particulate ligands.

Generation and properties of organic colloids extracted by water from the organic horizon of a boreal forest soil

Organic colloids are an important part of dissolved organic matter (DOM) yet many of their properties remain elusive. The main aims of this study were to assess how the colloidal properties of DOM extracted with water from an organic boreal soil horizon varied with the extraction protocol, and thereby provide insight into the nature of the DOM colloids and develop a mechanistic understanding of how the colloids were generated from the parent soil aggregates. This was accomplished by systematic variations of extraction temperature (4 °C–100 °C), time, mechanical agitation, and pH, together with a combination of chemical analyses, and light and X-ray scattering. Our results agreed with the previous identification of two main colloidal DOM species, one fractal cluster and a second, smaller colloidal DOM species described as chains or coils. Fractal clusters completely dominated the colloidal DOM in extracts from our soil at room temperature and below. Colloidal coils only existed in DOM extracted above room temperature, and their amount increased significantly between 50 °C–100 °C. Moreover, the temperature variation indicated that the fractal clusters partly dissolved into colloidal coils at elevated temperatures. Mechanical agitation at 4 °C significantly increased the amount of DOM extracted, increasing the concentrations of both fractal clusters and low-molecular weight organic compounds. While the clusters were extracted from agitated and non-agitated soil suspensions, the low molecular weight organics were mainly released by agitation. Based on the experimental observations, we propose a conceptual model where parent soil aggregates contain the fractal clusters in mobile and occluded forms, and that the occluded clusters co-exist with occluded low molecular weight organics. These occluded forms may be released by mechanical forces, increasing pH and temperature. At higher temperatures, the soil aggregates and the fractal clusters start to break up, and subsequently individual colloidal coils, presumably carbohydrates, disperse in the water phase. The model explains the origin and properties of the fractal clusters that completely dominate the colloidal DOM extracted from our soil at room temperature and below.

Coupled effects of pH and kaolinite colloids on antibiotic transport in porous media

Antibiotics can interact with natural colloids and the surrounding media upon entry into soil and groundwater systems, which significantly alters their dynamic behavior and complicates our understanding of antibiotic fate and transport in porous media. In this study, co-transport of antibiotics and kaolinite colloids was systematically investigated using combined column experiments and numerical simulation under different pH conditions. Sulfadiazine (SDZ) transport was enhanced by kaolinite colloids under neutral and alkaline conditions, which was attributed to the higher mobility of colloids as SDZ carriers, as well as competitive sorption. However, most injected SDZ was transported in a dissolved form owing to the low sorption capacity of SDZ to kaolinite colloids and quartz sand. The colloid-facilitated transport model provided a good description of total SDZ transport, but underestimated colloidal SDZ transport using parameters from kinetic sorption experiments. Kaolinite colloids significantly promoted ciprofloxacin (CIP) transport at pH 4.0, but inhibited it at pH 7.0 and 9.0. Interestingly, enhanced CIP transport was due to the decreased number of effective sorption sites on quartz sand and the increased desorption of CIP from kaolinite colloids. Under neutral and alkaline conditions, deposited colloids provided additional sorption sites for CIP, which contributed to CIP retention. Moreover, CIP significantly inhibited the transport of kaolinite colloids owing to the increases in colloidal aggregate size and zeta potential. Overall, our results highlighted the different effects of mobile and immobile colloids on antibiotic transport, in addition to the implications of antibiotic speciation and clay colloids when predicting the transport behavior of these compounds.