Aggregation kinetics of biochar nanoparticles in aqueous environment: Interplays of anion type and bovine serum albumin

Influence of natural organic matter on the aggregation dynamics of biochar colloids derived from various feedstocks

Abundant biochar colloids (BCs) produced from a wide range of feedstocks, resulting from forest fires, agricultural production, and environmental restoration, exhibit varying aggregation behaviors influenced by feedstock type and natural organic matter. However, the impact of natural organic matter on the colloidal stability of BCs derived from different feedstocks remains poorly understood. In this study, six selected biochars were derived from various feedstocks as follows: sewage sludge (SS), rice husk (RH), oil seed rape straw pellets (OSR), wheat straw pellets (WS), miscanthus straw pellets (MS) and softwood pellets (SW). The colloidal stability of BCs, with the exogenous addition of organic matter, was further determined. The order of critical coagulation concentrations (CCCs) of BCs with the presence of humic acid (HA) was as follows: RH (989.48 mM) < MS (1084.69 mM) < SS (1149.76 mM) < WS (1338.99 mM) < OSR (2402.98 mM) < SW (3151.32 mM). This order was significantly positively correlated with the specific surface area and negatively correlated with the ash content of the bulk biochar. Compared to HA, bovine serum albumin (BSA) more effectively inhibited the aggregation behavior of BCs due to steric hindrance. The initial aggregation rate constant (k) of BCs at 3000 mM NaCl was as follows: MS (0.238 nm/s) > OSR (0.142 nm/s) > WS (0.128 nm/s) > SS (0.126 nm/s) > RH (0.118 nm/s) > SW (0.112 nm/s). The stabilizing effects of BSA on biochar colloids were independent of the physicochemical properties of bulk biochar. In the presence of BSA, a thin layer of protein corona significantly enhanced the stability of biochar colloids, particularly the BCs derived from MS. Our results underscore the importance of considering feedstock resources and natural organic matter type when assessing the aggregation and potential risks of BCs in aquatic systems.

The colloidal stability of molybdenum disulfide nanosheets in different natural surface waters: Combined effects of water chemistry and light irradiation

With the increasing production and application, more molybdenum disulfide (MoS2) nanosheets could be released into environment. The aggregation and dispersion of MoS2 nanosheets profoundly impact their transport and transformation in the aquatic environment. However, the colloidal stability of MoS2 remains largely unknown in natural surface waters. This study investigated the colloidal stability of MoS2 nanosheets in six natural surface waters affected by both light irradiation and water chemistry. Compared to that of the pristine MoS2 nanosheets, the colloidal stability of MoS2 photoaged in ultrapure water declined. Light irradiation induced the formation of Mo-O bonds, the release of SO42- species, and the decrease in 1T/2H ratio, which reduced negative charge and enhanced hydrophobicity. However, the colloidal stability of MoS2 photoaged in natural surface waters was increased relative to that in ultrapure water not only for the smaller extent of photochemical transformation but more importantly the surface modification by water chemistry. Furthermore, the colloidal stability of MoS2 photoaged in natural surface waters followed the order of sea water > lake water > river water. The abundant cations (e.g., Ca2+ and Mg2+) in sea water facilitated the covalent grafting (S-C bonds) of more dissolved organic matter (DOM) on MoS2 via charge screening and cation bridging, thus inducing stronger electrostatic repulsion and steric effect to stabilize nanosheets. The crucial role of the covalent grafting of DOM was further confirmed by the positive correlation between the critical coagulation concentration values and S-C ratios (R2 = 0.82, p < 0.05). Our results highlighted the dominant role of water chemistry than light irradiation in dictating the colloidal stability of MoS2 photoaged in natural surface waters, which provided new insight into the environmental behavior of MoS2 in aquatic environment.

New Insights into Aggregation Behaviors of the UV-Irradiated Dissolved Biochars (DBioCs) in Aqueous Environments: Effects of Water Chemistries and Variation in the Hamaker Constant

The aggregation behavior of ubiquitous dissolved black carbon (DBC) largely affects the fate and transport of its own contaminants and the attached contaminants. However, the photoaging processes and resulting effects on its colloidal stability remain yet unknown. Herein, dissolved biochars (DBioCs) were extracted from common wheat straw biochar as a proxy for an anthropogenic DBC. The influences of UV radiation on their aggregation kinetics were systematically investigated under various water chemistries (pH, electrolytes, and protein). The environmental stability of the DBioCs before and after radiation was further verified in two natural water samples. Hamaker constants of pristine and photoaged DBioCs were derived according to Derjaguin-Landau-Verwey-Overbeek (DLVO) prediction, and its attenuation (3.19 ± 0.15 × 10-21 J to 1.55 ± 0.07 × 10-21 J after 7 days of radiation) was described with decay kinetic models. Pearson correlation analysis revealed that the surface properties and aggregation behaviors of DBioCs were significantly correlated with radiation time (p < 0.05), indicating its profound effects. Based on characterization and experimental results, we proposed a three-stage mechanism (contended by photodecarboxylation, photo-oxidation, and mineral exposure) that DBioCs might experience under UV radiation. These findings would provide an important reference for potential phototransformation processes and relevant behavioral changes that DBC may encounter.

Effect of water-soluble polymers on the transport of functional group-modified polystyrene nanoplastics in goethite-coated saturated porous media

The research on the impact of water-soluble polymers (WSPs) on the migration and fate of plastic particles is extremely limited. This article explored the effects of polyacrylic acid (PAA, a common WSP) and physicochemical factors on the transport of polystyrene nanoparticles (PSNPs-NH2/COOH) with different functional groups in QS (quartz sand) and FOS (goethite-modified quartz sand, simulates mineral colloids). Research has shown that PAA can selectively adsorb onto the surface of PSNPs-NH2, forming ecological corona heterogeneous aggregates. This process increased the spatial hindrance and elastic repulsion, resulting in the recovery of PSNPs-NH2 always exceeding that of PSNPs-COOH. Overall, PAA can hinder the migration of PSNPs in QS but can promote their migration in FOS. When multivalent cations coexist with PAA, the transport of PSNPs in the media is primarily affected by cation bridging and CH-cation-π interaction. The presence of oxyanions and PAA prevents PSNPs from following the Hofmeister rule and promotes their migration (PO43-: 82.34 ± 0.16% to 94.63 ± 2.82%>SO42-: 81.38 ± 2.73% to 91.15 ± 0.93%>NO3-: 55.85 ± 0.70%-87.16 ± 3.80%). The findings of this study contribute significantly to a better understanding of the migration of WSPs and group-modified NPs in complex saturated porous media.

Properties of biochar colloids and behaviors in the soil environment: Influencing the migration of heavy metals

Biochar pyrolyzed by biomass shows excellent application prospects for heavy metal (HM) remediation, but a part of biochar can be inevitably broken into micro- and nano-sized biochar colloids (BCs) under biological and physicochemical actions in soil. BCs derived in the process of remediation have rough surface, rich elemental species and contents, and multiple functional groups, which are similar to biochar. However, BCs have some unique colloidal properties because of their micro and nano scale size. Due to these properties, BCs exhibit strong mobilities in the soil environment, and the mobilities may be influenced by a combination of colloidal properties of BCs and environmental factors including soil colloids and other soil environmental conditions. In addition, BCs may have affinity effects on HMs through electrostatic adsorption, ion exchange, surface complexation, precipitation/co-precipitation, and redox because of the properties such as large specific surface area, and rich oxygen-containing functional groups and minerals on the surface. This review summarizes the physicochemical and migratory properties of BCs, and the internal and external factors affecting the migration of BCs in the soil environment, and the possible effects of BCs on HMs are high-lighted. This review provides a theoretical basis for the optimization of soil contaminated with HMs after remediation using biochar. Notably, the innovative idea that BCs may influence the presence of HMs in soil needs to be further confirmed by more targeted detection and analysis methods in future studies to prevent the possible environmental toxicities of the lateral and vertical diffusion of HM caused by BCs in soil.

Aggregation behavior of polystyrene nanoplastics: Role of surface functional groups and protein and electrolyte variation

Aggregation kinetics of plastics are affected by the surface functional groups and exposure orders (electrolyte and protein) with kinds of mechanisms in aquatic environment. This study investigates the aggregation of polystyrene nanoplastics (PSNPs) with varying surface functional groups in the presence of common electrolytes (NaCl, CaCl2, Na2SO4) and bovine serum albumin (BSA). It also examines the impact of different exposure orders, namely BSA + NaCl (adding them together), BSA → NaCl (adding BSA firstly and then NaCl), and NaCl → BSA (adding NaCl firstly and then BSA), on PSNPs aggregation. The presence of BSA decreased the critical coagulation concentration in NaCl (CCCNa+) of the non-modified PS-Bare from 222.17 to 142.81 mM (35.72%), but increased that of the carboxyl-modified PS-COOH from 157.34 to 160.03 mM (1.71%). This might be ascribed to the thicker absorbed layer of BSA onto the PS-Bare surface, known from Ohshima's soft particle theory. Their aggregation in CaCl2 was both increased because of Ca2+ bridging. Different from the monotonous effects of BSA on PS-Bare and PS-COOH, BSA initially facilitated PS-NH2 aggregation via patch-charge attraction, then inhibited it at higher salt levels through steric repulsion. Furthermore, exposure orders had no significant effect on PS-Bare and PS-COOH, but had a NaCl concentration-dependent impact on PS-NH2. At the low NaCl concentrations (10 and 100 mM), no obvious influence could be observed. While, at 300 mM NaCl, the high concentrations of BSA could not totally stabilize the salt-induced aggregates in NaCl → BSA, but could achieve it in the other two orders. These might be attributed to the electrical double layer compression by NaCl, "patch-charge" force and steric hindrance by BSA. These experimental findings shed light on the potential fate and transport of nanoparticles in aquatic environments.

Effect of reduced inherent organic matter on stability and transport behaviors of black soil colloids

Soil organic matter plays an important role in the stability, transport, and fate of soil colloids. At present, studies have mostly focused on the effects of adding exogenous organic matter on soil colloidal properties, while there is very limited research on the effect of reduced inherent soil organic matter on the environmental behavior of soil colloids. This study investigated the stability and transport behavior of black soil colloids (BSC) and black soil colloids with reduced inherent organic matter (BSC-ROM) under different ionic strength (5, 50 mM) and background solution pH (4.0, 7.0, and 9.0) conditions. Meanwhile, the release behavior of two soil colloids in the saturated sand column under transient ionic strength conditions was also studied. The results showed that both ionic strength reduction and pH increase increased the negative charges of BSC and BSC-ROM, and improved the electrostatic repulsion between soil colloids and grain surface, thereby promoting the stability and mobility of soil colloids. The decrease in inherent organic matter had little effect on the surface charge of soil colloids, suggesting that the electrostatic repulsive force was not the main force affecting the stability and mobility of BSC and BSC-ROM, and reducing inherent organic matter might significantly reduce the stability and mobility of soil colloids by weakening the steric hindrance interaction. The decrease of transient ionic strength reduced the depth of the energy minimum and activated the soil colloids retained on the surface of the grain at three pH conditions. This study is helpful to predict the potential impact of soil organic matter degradation on the fate of black soil colloids in natural environment system.

Aggregation of biochar nanoparticles and the impact on bisphenol A sorption: Experiments and molecular dynamics simulations

The unique properties and environmental implications of biochar nanoparticles (BNPs) have attracted increasing attention. The abundant functional groups and aromatic structures in BNPs may promote the aggregation of BNPs, but the mechanism and implications of this aggregation process remain unclear. Thus, this study investigated the aggregation of BNPs and the sorption of bisphenol A (BPA) on BNPs by combining experimental investigations with molecular dynamics simulations. As the concentration of BNP increased from 100 mg/L to 500 mg/L, the particle size increased from approximately 200 nm to 500 nm, and the exposed surface area ratio in the aqueous phase decreased from 0.46 to 0.05, which confirmed the aggregation of BNPs. The sorption of BPA on BNPs decreased with increasing BNP concentration in both the experiments and molecular dynamics simulations because of BNP aggregation. According to a detailed analysis of the BPA molecules adsorbed on BNP aggregates, the sorption mechanisms were hydrogen bonding, hydrophobic effect, and π-π interactions, which were driven by aromatic rings and O- and N-containing functional groups. The aggregation of BNPs embedded some functional groups in the aggregates and thus inhibited sorption. Interestingly, the steady configuration of the BNP aggregates in the molecular dynamics simulations (2000 ps relaxation) also determined the apparent BPA sorption. BPA molecules were adsorbed in the V-shaped interlayers of the BNP aggregates that acted as semi-closed pores, but could not be adsorbed in the parallel interlayers because of their small layer spacing. This study can provide theoretical guidance for the application of BNPs in pollution control and remediation.

Carbon content determines the aggregation of biochar colloids from various feedstocks

The aggregation kinetics of biochar colloids (BCs) play a crucial role in the fate and transport of contaminants, as well as the carbon (C) cycle in the environment. However, the colloidal stability of BCs from various feedstocks is very limited. In this study, the critical coagulation concentration (CCC) of twelve standard biochars pyrolyzed from various feedstocks (municipal source, agricultural waste, herbaceous residue, and woody feedstock) at 550 °C and 700 °C were investigated, and the relationship between the physicochemical characteristics of biochar and the colloidal stability of BCs was further analyzed. The CCC of BCs in the NaCl solution followed the trend of municipal source < agricultural waste < herbaceous residue < woody feedstock, which was similar to the order of C content in biochar. The CCC of BCs showed a strong positive correlation with the C content of various biochars, especially pyrolyzed at a higher temperature of 700 °C. The BCs derived from lignin-rich feedstock (e.g., woody feedstock) had the highest colloidal stability, followed by cellulose-rich feedstock (e.g., agricultural waste and herbaceous residue). The BCs derived from organic matter-rich feedstock (municipal source) were easy to aggregate in the aqueous environment. This study quantitatively provides new insights into the relationship between BCs stability and biochar characteristics from various feedstocks, which is critical to assess biochar environmental behavior in aqueous environments.