Colloidal aggregation and structural assembly of aspect ratio variant goethite (α-FeOOH) with nC60 fullerene in environmental media

The aggregation of natural inorganic colloids in aqueous environment: A review

Natural inorganic colloids (NICs) are the most common and dominant existence in the ecosystem, with high concentration and wide variety. In spite of the low toxicity, they can alter activity and mobility of hazardous engineered nanoparticles (ENPs) through different interactions, which warrants the necessity to understand and predict the fate and transport of NICs in aquatic ecosystems. Here, this review summarized NICs properties and behaviors, interaction mechanisms and environmental factors at the first time. Various representative NICs and their physicochemical properties were introduced across the board. Then, the aggregation and sedimentation behaviors were discussed systematically, mainly concerning the heteroaggregation between NICs and ENPs. To speculate their fate and elucidate the corresponding mechanisms, the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) and extended DLVO (X-DLVO) theories were focused. Furthermore, a range of intrinsic and extrinsic factors was presented in different perspective. Last but not the least, this paper pointed out theoretical and analytical gaps in current researches, and put forward suggestions for further research, aiming to provide a more comprehensive and original perspective in the fields of natural occurring colloids.

Soil colloids affect the aggregation and stability of biochar colloids

The stability of biochar colloids plays an important role in the transport and fate of contaminants and nutrients in soil. This study aimed to investigate the effects of main soil components, kaolin (Kao), goethite (Goe), and humic acid (HA) colloids on the aggregation kinetics of biochar colloids derived from dairy manure (DM), sewage sludge (SS), and wheat straw (WS). The WS biochar colloid had the highest critical coagulation concentration (CCC) (624 mM) than that of SS (200 mM) and DM (75 mM) due to its richest hydroxyl and carboxyl groups, showing the highest stability. Kao markedly improved the stability of DM and SS biochar colloids with 171% and 52.5% increase of CCC, respectively, by increasing the electrostatic repulsion of the system. However, the WS biochar colloid became more aggregated in the presence of Kao since the hydroxyl and carboxyl functional groups in WS biochar colloid could complex with Kao, generating electrostatic shielding. Goe could rapidly combine with biochar colloids via electrostatic attraction, resulting in the aggregation of SS and WS, while the aggregation rate of DM/Goe mixed colloids was inhibited. The HA increased the electrostatic repulsion of all biochar colloids through adsorbed on the surface of biochar colloids, resulting in the increased steric hindrance and stability of biochar colloids, with the CCC increased from 75 to 624 mM to 827-1012 mM. Our findings reveal that soil kaolin, goethite, and humic acid colloids have remarkable effects on the stability and aggregation of biochar colloid, which will advance understanding of the potential environmental fate and behaviors of biochar colloids.

Nanomaterials in the environment, human exposure pathway, and health effects: A review

Nanomaterials (NMs), both natural and synthetic, are produced, transformed, and exported into our environment daily. Natural NMs annual flux to the environment is around 97% of the total and is significantly higher than synthetic NMs. However, synthetic NMs are considered to have a detrimental effect on the environment. The extensive usage of synthetic NMs in different fields, including chemical, engineering, electronics, and medicine, makes them susceptible to be discharged into the atmosphere, various water sources, soil, and landfill waste. As ever-larger quantities of NMs end up in our environment and start interacting with the biota, it is crucial to understand their behavior under various environmental conditions, their exposure pathway, and their health effects on human beings. This review paper comprises a large portion of the latest research on NMs and the environment. The article describes the natural and synthetic NMs, covering both incidental and engineered NMs and their behavior in the natural environment. The review includes a brief discussion on sampling strategies and various analytical tools to study NMs in complex environmental matrices. The interaction of NMs in natural environments and their pathway to human exposure has been summarized. The potential of NMs to impact human health has been elaborated. The nanotoxicological effect of NMs based on their inherent properties concerning to human health is also reviewed. The knowledge gaps and future research needs on NMs are reported. The findings in this paper will be a resource for researchers working on NMs all over the world to understand better the challenges associated with NMs in the natural environment and their human health effects.

Suspended state heteroaggregation kinetics of kaolinite and fullerene (nC60) in the presence of tannic acid: Effect of π-π interactions

The colloidal heteroassociation between natural mineral colloids and engineered nanoparticles (ENPs) can reduce the bioavailability and toxicity of the ENPs. However, the efficacy of this heteroassociation-based entrapment of ENPs depends on the intrinsic material properties of the particles and the physicochemical parameters of the aquatic environment. Natural organic matter (NOM)-induced surface modifications of clay colloids, functionalization of ENPs, and efficiency of counterions as effective coagulants profoundly affect the effectiveness of heteroaggregation-based attenuation of anthropogenic colloids. In this study, tannic acid (TA), a surrogate of NOM, prevented the edge-to-face self-association of sodium-saturated kaolinite (Na-kaolinite) at acidic pH, as evaluated from the transverse proton spin-spin relaxation data (T₂). Likewise, fullerene water suspension (FWS) adhesion to Na-kaolinite prevented the self-association of Na-kaolinite and enhanced the colloidal stability. At pH 4 and diffusion-limited aggregation regime salt concentrations, the Na-kaolinite and FWS heteroaggregation rates were lower than the Na-kaolinite homoaggregation rates, and eventually reached a plateau. The higher colloidal stability of the Na-kaolinite and FWS binary mixture than that of Na-kaolinite, regardless of stronger charge screening by Ca²⁺_, reflects steric stabilization. However, at pH 7, the increased electrostatic barrier reduces the feasibility of colloidal heteroassociation between Na-kaolinite and FWS; thus, higher salt concentrations are required to initiate aggregation. Weak adsorption of TA on Na-kaolinite at pH 7 facilitated stronger π-π interactions with FWS. All suspensions exhibited faster aggregate growth at pH 7 than pH 4, possibly due to the stronger cation response at pH 7. In situ atomic force microscopy imaging and line profile plots of Na-kaolinite, TA, and FWS mixture in CaCl₂ further corroborated the difference in the heteroaggregation rates observed at the two different pH values. Thus, TA-induced surface functionalization of FWS and the consequent increased electrostatic barrier to heteroassociation with Na-kaolinite may facilitate the environmental mobility of FWS in aquatic media.