Climate-zone-dependent effect mechanism of humic acid and fulvic acid extracted from river sediments on aggregation behavior of graphene oxide

Humic acid changes effect of naturally occurring oxidants on the environmental transformation of molybdenum disulfide nanosheets

2H-phase molybdenum disulfide (2H-MoS2) has been considered to be a chemically stable two-dimensional (2D) nanomaterial. Nonetheless, the persistence of 2H-MoS2 in the presence of environmental redox-active matrices, such as naturally occurring oxidants (e.g., manganese dioxide (MnO2)) and natural organic matter (NOM), remains largely unknown. Herein, we examined the interplay between 2H-MoS2, MnO2 (a common natural oxidant), and NOM species (i.e., Aldrich humic acid (ALHA) and Suwannee River natural organic matter (SRNOM)). The results show that MnO2 accelerates the oxidative dissolution of 2H-MoS2, regardless of the presence of dissolved oxygen. The effect of NOM on the MnO2-induced fate of 2H-MoS2 was found to depend on its affinity for 2H-MoS2 and the functionality of NOM. ALHA preferentially adsorbed on hydrophobic 2H-MoS2 nanosheets due to the enrichment of reductive polycyclic aromatics and polyphenolic constituents. The preferential ALHA adsorption counteracted the MnO2-triggered oxidative transformation of 2H-MoS2, as revealed by the cathodic response of 2H-MoS2 (i.e., decreased the open circuit potential by 0.0338 V) and the emergence of reductive Mo‒C bonds at 228.8 and 231.9 eV upon the addition of ALHA. This work evaluated the persistence of 2H-MoS2, illustrating its susceptibility to decomposition by naturally occurring oxidants and the influence of NOM on it. These findings are crucial for revealing the fate and transport of MoS2 in aquatic environments and provide guidelines for related applications in natural or engineered systems for MoS2 and potentially other 2D materials.

Effects of dissolved organic matter on the environmental behavior and toxicity of metal nanomaterials: A review

Metal nanomaterials (MNMs) have been released into the environment during their usage in various products, and their environmental behaviors directly impact their toxicity. Numerous environmental factors potentially affect the behaviors and toxicity of MNMs with dissolved organic matter (DOM) playing the most essential role. Abundant facts showing contradictory results about the effects of DOM on MNMs, herein the occurrence of DOM on the environmental process change of MNMs such as dissolution, dispersion, aggregation, and surface transformation were summarized. We also reviewed the effects of MNMs on organisms and their mechanisms in the environment such as acute toxicity, oxidative stress, oxidative damage, growth inhibition, photosynthesis, reproductive toxicity, and malformation. The presence of DOM had the potential to reduce or enhance the toxicity of MNMs by altering the reactive oxygen species (ROS) generation, dissolution, stability, and electrostatic repulsion of MNMs. Furthermore, we summarized the factors that affected different toxicity including specific organisms, DOM concentration, DOM types, light conditions, detection time, and production methods of MNMs. However, the more detailed mechanism of interaction between DOM and MNMs needs further investigation.

Anaerobic reduction of graphene oxide induces the release of sorbed organic contaminants and enhances environmental risk

Graphene oxide (GO) is an oxidized form of graphene-based materials with abundant hydrophilic oxygen-containing functional groups, forming well-dispersed suspensions and serving as pollution carriers. The natural anaerobic environment might alter the sorption behavior of GO, which in turn affects the fate and bioavailability of GO-sorbed organic contaminants. In this study, GO can be reduced by diverse environmental reductants, including sodium sulfide, DL-1,4-dithiothretiol, and L-cysteine, forming aggregates. Meanwhile, the GO-sorbed organic contaminants were released during the reduction process owing to the decreasing oxygen content and sorption sites. The effect of solution chemistry conditions (dissolved humic acid/HA and ionic strength) on the reduction release process was also investigated. HA reduced the release rate of organic contaminants due to its stabilization effect. Adding NaCl did not alter the release rate, while CaCl2 markedly enhanced the release rate. Toxicity tests with Bacillus subtilis indicated that releasing the pre-sorbed organic compound on GO led to a lower survival ratio and enhanced the superoxide dismutase activity. The findings of this study imply that the anaerobic environment could alter the dispersion/aggregation status of GO, affecting the sorption interaction between GO and the organic compounds and consequently influencing the toxicity and risk of pollution in the environment.

Remediation of the microecological environment of heavy metal-contaminated soil with fulvic acid, improves the quality and yield of apple

The excessive application of chemical fertilizers and pesticides in apple orchards is responsible for high levels of manganese and copper in soil, and this poses a serious threat to soil health. We conducted a three-year field experiment to study the remediation effect and mechanism of fulvic acid on soil with excess manganese and copper. The exogenous application of fulvic acid significantly reduced the content of manganese and copper in soil and plants; increased the content of calcium; promoted the growth of apple plants; improved the fruit quality and yield of apple; increased the content of chlorophyll; increased the activity of superoxide dismutase, peroxidase, and catalase; and reduced the content of malondialdehyde. The number of soil culturable microorganisms, soil enzyme activity, soil microbial community diversity, and relative abundance of functional bacteria were increased, and the detoxification of the glutathione metabolism function was enhanced. The results of this study provide new insights that will aid the remediation of soil with excess manganese and copper using fulvic acid.

Componential and molecular-weight-dependent effects of natural organic matter on the colloidal behavior, transformation, and toxicity of MoS2 nanoflakes

The potential widespread applications in water processing have rendered the necessity for investigations of the fate and hazard of molybdenum disulfide (MoS2) nanosheets. Herein, it was found that humic acid (HA) had better performances toward stabilizing pure 2H phase MoS2 and chemical-exfoliated MoS2 (ce-MoS2) in electrolyte solutions than fulvic acid (FA), and molecular weight (MW)-dependent manners were disclosed due to steric repulsions. Compared with darkness, the extent to which the aggregation and sedimentation of ce-MoS2 facilitated by visible light irradiation was greater in the presence of HA and FA fractions, likely due to the introduction of stronger plasmonic dipole-dipole interaction and Van der Waals attraction forces. HA-triggered structural disintegration of nanosheets was performed after irradiation and it was observed to be more significant with the increase in MWs, whereas the MW-dependent dissolution of MoS2 caused by FA was much quicker than that by HA owing to the higher generation of singlet oxygen. Moreover, FA lowered the bioavailability of MoS2 and relieved its toxicity to zebrafish more effectively than HA. Our findings boost the insights into the effects of organic molecules on the fates and hazards of MoS2, providing guidance for the MoS2-based nanotechnological development on environment.

Promotion effect of ultraviolet light on graphene oxide aggregation in the presence of different climatic zone's humic and fulvic acid

Aggregation of graphene oxide (GO) is significantly affected by dissolved organic matter (DOM) in natural waters, while DOM's climate zone and light irradiation is seldom considered. This study investigated the effect of humic/fulvic acid (HA/FA) from various climate zones of China on aggregation of small (200 nm) and large (500 nm) GO under 120-h UV irradiation. GO aggregation was promoted by HA/FA because UV irradiation decreased hydrophilicity of GO and steric forces among particles. GO generated electron and hole pair under UV irradiation, which reduce GO with more hydrophilic oxygen-containing functional group (C-O) to rGO with high hydrophobicity and oxidize DOM into organic matter with smaller molecular weight. Most severe GO aggregation was observed with Makou HA from Subtropical Monsoon climate zone and Maqin FA from Plateau and Mountain climate zone, which was primarily because HA/FA's high molecular weight and aromaticity dispersed GO initially that facilitated UV penetration. GO aggregation ratio was positively correlated with graphitic fraction content (R2 = 0.82-0.99) and negatively correlated with C-O group content (R2 = 0.61-0.98) in the presence of DOM under UV irradiation. This work highlights different dispersity of GO during photochemical reactions in various climate zones, providing new insight into the environmental implications of nanomaterial release.

Enhanced degradation of few-layer black phosphorus by fulvic acid: Processes and mechanisms

The oxidation of the emerging nanomaterial black phosphorus (BP) affected by pH and oxygen has been carefully documented. However, in natural waters, there is a large amount of chemically reactive organic matters like fulvic acid (FA), whose impacts on degradation and stability of few-layer BP or BP nanosheets (BPNS) are scarcely disclosed. Hence, we investigated the kinetics of BPNS degradation products (H₂PO₂^-, HPO₃^2-, and PO₄^3-) in the presence of FA. The results showed that the apparent reaction rate constants of BPNS were 0.026, 0.050, and 0.060 d^-1 under oxygen-and-light condition and 0.005, 0.016, and 0.023 d^-1 under hypoxia-and-darkness condition at FA gradients of 0, 2.5, and 5 mgC/L, respectively. Microscopic observations, simple molecular simulation experiment, and density functional theory computation explained that FA significantly enhanced the degradation of P atoms on the BPNS surface through the indirect pathway of reducing the energy barrier of O₂ dissociative adsorption and the direct pathway of chemical adsorption, which caused the P-P bond on the BPNS surface to break down and formed P-O bonds or C-P bonds. This study revealed for the first time the degradation mechanism of BPNS in the presence of FA, which is a chemical mechanism of the BPNS transformation behavior. It helps to make a more scientific risk assessment of BP in natural waters.

Biodegradation of organic compounds in the coal gangue by Bacillus sp. into humic acid

Coal gangue (CG), one of the world's largest industrial solid wastes produced during coal mining, is extremely difficult to be used owing to its combined contents of clay minerals and organic macromolecules. This study explored a novel process of degrading the harmful organic compounds in the CG into humic acid using a biological method characterized by scanning electron microscope-energy dispersive spectrometer, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and elemental analyzer. The results reveal that adding selected Bacillus sp. to the CG for 40 days can increase the humic acid content by ~ 17 times, reaching 17338.17 mg/kg, which is also the best level for promoting plant growth. FTIR and XPS spectra show that the organic compounds in the CG transforms primarily from C=C to C=O, COOH, and O-H groups, indicating that the organic compounds are gradually oxidized and activated, improving the humic acid concentration of soil. In addition, Bacillus sp. decreases pH and benzo[a]pyrene contents, and increases the content of available nutrients. After microbial degradation, coal gangue can be turned into ecological restoration materials.

Influence of fulvic acid sub-fractions on aggregation kinetics of graphene oxide in aqueous environments

Fulvic acid (FA) can affect the dispersion of graphene oxide (GO) in aquatic environments, however, the possible mechanisms remain unclear. Dynamic light scattering techniques combined with a multiple regression model were applied to explore the influence of FA sub-fractions (FA_pH3 - FA_pH13) on the aggregation kinetics of GO in aqueous environments. The ratios of critical coagulation concentration (CCC) values were CCC_Na: CCC_Mg: CCC_La: CCC_Ce = 1:2^-5.15:3^-7.31:3^-7.35, which were consistent with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and Schulze-Hardy rules. The GO remained stable at pH 3-10 and aggregated at pH < 3 or pH > 10, and its critical coagulation pH values were 1.44 and 12.25 with 10 mM NaCl as background. The CCC values of GO in the presence of FA_pH3 - FA_pH13 were greater than those in the absence of FA sub-fractions. The ratios of CCC values of GO (r) increased in the presence of FA sub-fractions in the order of FA_pH13 > FA_pH9 > FA_pH7 > FA_pH5 > FA_pH3 and ranged from 1.01 to 2.15 for certain metal ions including Na⁺, Mg²⁺, La³⁺, and Ce³⁺. The CCC values of GO were significantly related to C, H, O, N, S, H/C, O/C, carboxylic C, and carbonyl C of FA sub-fractions (P < 0.05), respectively, and could be predicted using the multiple linear regression eq. CCC = Z^-n (98.959- 60.911 ∗ O/C + 4.799 ∗ O-alkyl C - 0.845 ∗ aromatic C - 6.237 ∗ carbonyl C). The predicted CCC values for GO were within 90 % prediction intervals, and the average error of the CCC values was 3.3 % and R² = 0.986. This investigation is expected to provide a scientific basis for the transport and ecotoxicity of GO in environments.

Modified biochar/humic substance/fertiliser compound soil conditioner for highly efficient improvement of soil fertility and heavy metals remediation in acidic soils

Fertile and uncontaminated soil with appropriate pH is crucial in terms of the agricultural sustainable development. Herein, a compound soil conditioner containing chitosan modified straw biochar (CBC), kitchen waste compost product-derived humic substance (HS), NPK compound fertiliser (NPK-CF) was prepared to simultaneously adjust acidic soil pH, improve fertility, and immobilize heavy metal. The results exhibited that the best Pb and NH₄⁺ adsorption performance was obtained in CBC with chitosan:biochar of 1:5. Then, the acid soil pH was improved from 5.03 to 6.66 in the presence of CBC/HS (5:5) with 3% addition weight (the mass ratio of conditioner to soil). Meanwhile, compared with the control, the contents of organic matter, available nitrogen, and available phosphorus significantly increased by 52.4%, 92.6%, and 136.3%, respectively. Moreover, Pb was highly efficient immobilised by CBC, and the concentration of Pb in the soil was decreased by 55.2%. The optimal growth trend of ryegrass was obtained in the presence of 3% addition weight (the mass ratio of conditioner to soil) CBC/HS (CBC:HS = 5:5) combined with 60% of the recommended NPK-CF application weight, which was mainly contributed by the improvement of the soil microbial abundance and community structure diversity. The addition of CBC/HS could effectively reduce the addition of NPK-CF and contribute to simultaneous controlling nitrogen loss, releasing phosphorus, immobilising Pb, adjusting pH, improving soil quality and controlling nonpoint pollution.

Every coin has two sides: Continuous and substantial reduction of ammonia volatilization under the coexistence of microplastics and biochar in an annual observation of rice-wheat rotation system

Microplastics (MPs) are verified to affect the fate of ammonia (NH₃) in agricultural soils. However, the impacts and mechanisms of MPs coupled with biochar (BC), a widely used agricultural conditioner, on NH₃ losses are mostly untapped. The aim of this study was to investigate the mechanisms of common MPs (i.e., polyethylene, polyester, and polyacrylonitrile) and straw-derived BC on NH₃ volatilization in rice-wheat rotation soils. Results showed that BC alone and MPs with BC (MPs + BC) reduced 5.5 % and 11.2-26.6 % cumulative NH₃ volatilization than the control (CK), respectively, in the rice season. The increased nitrate concentration and soil cation exchange capacity were dominant contributors to the reduced soil NH₃ volatilization in the rice season. BC and MPs + BC persistently reduced 44.5 % and 60.0-62.6 % NH₃ losses than CK in the wheat season as influenced by pH and nitrate concentration. Moreover, BC and MPs + BC increased humic acid-like substances in soil dissolved organic matter by an average of 159.1 % and 179.6 % than CK, respectively, in rice and wheat seasons. The increased adsorption of soil NH₄⁺ and the promotion of crop root growth were the main mechanisms of NH₃ reduction. Our findings partially revealed the mechanisms of the coexistence of MPs and BC on NH₃ mitigation in rice-wheat rotational ecosystems.

Differential responses of the properties of soil humic acid and fulvic acid to nitrogen addition in the North China Plain

Humus (HS) is an important component of soil organic matter. Humic acid (HA) and fulvic acid (FA) are two of the most important components of HS, as they substantially affect biogeochemical processes and the migration and transformation of pollutants in soil. Long-term nitrogen (N) addition can lead to changes in soil physical and chemical properties, affect the structural characteristics of soil HS (HA and FA), cause changes in the adsorption and migration of pollutants, and ultimately result in the continuous deterioration of the soil ecological environment. However, few studies have examined the effects of N addition on the structural characteristics of soil HS, including the responses of soil HA and FA to N addition. Here, we conducted a long-term positioning experiment with different levels of N addition (CK: 0 kg N ha^-1 yr^-1, LN: 100 kg N ha^-1 yr^-1, and HN: 300 kg N ha^-1 yr^-1) in typical farmland soils of the North China Plain to study the response of soil HA and FA to N addition. N addition altered the physical and chemical properties of soil (e.g., pH, SOC, TN, and enzyme activity), which affected the responses of the chemical structure, quality indexes, and composition distribution of soil HA and FA to N addition. Differences in the response to N addition between HA and FA were observed. The structural characteristics of FA were stronger in response to HN compared with those of soil HA. As the level of N added increased, soil FA degradation increased, the composition distribution changed, the aromatization degree and molecular weight decreased, and the molecular structure became simpler. The properties of soil HA did not significantly respond to N addition. Given increases in the global N input (N addition and N deposition), our results have implications for agricultural fertilization, soil management, and other activities.

Insight into complexation of Cd(II) and Cu(II) to fulvic acid based on feature recognition of PARAFAC combined with 2DCOS

Fulvic acid which could govern the environmental geochemistry behavior of heavy metals is considered as the eco-friendly substances for controlling heavy metal pollutants in environment. Knowledge on the individual fulvic acid ligand is crucial to characterize the effect of fulvic acid on the migration and toxicity of metal pollutants. Herein, fulvic acid substances were analyzed by fluorescence quenching associated with parallel factor analysis (PARAFAC). Three components were identified based on PARAFAC. Furthermore, two-dimensional correlation spectroscopy (2DCOS) associated with complexation model were used to elucidate the Cd(II)- and Cu(II)-binding characteristics of the individual fulvic acid ligand. The Cd(II)- and Cu(II)-binding capability and speed of different fulvic acid ligands were revealed and theoretical guidance and technical support were provided for the practical application. The Cd(II) contaminated soil could be amended with high fulvic acid ligands A1 and Y2 containing composting products and the Cu(II) contaminated soil could be amended with high fulvic acid ligands Y1, T1 and A1 containing composting products to control the pollution and improve the soil condition. Based on these excellent results, the different fulvic acid ligands-contaminants-binding properties was characterized for the theoretical supporting of environmental pollution control.

A comprehensive estimate of the aggregation and transport of nSiO2 in static and dynamic aqueous systems

Silicon dioxide nanoparticles (nSiO₂) are extensively used in diverse fields and are inevitably released into the natural environment. Their overall aggregation behaviour in the environmental matrix can determine their fate and ecotoxicological effect on terrestrial and aquatic life. The current study systematically evaluates multiple parameters that can influence the stability of colloidal nSiO₂ (47 nm) in the natural aquatic environment. At first, the influence of several hydrochemical parameters such as pH (5, 7, and 9), ionic strength (IS) (10, 50, and 100 mM), and humic acid (HA) (0.1, 1, and 10 mg L^-1) was examined to understand the overall aggregation process of nSiO₂. Furthermore, the synergistic and antagonistic effects of ionic strength and humic acid on the transport of nSiO₂ in the aqueous environment were examined. Our experimental findings indicate that pH, ionic strength, and humic acid all had a profound influence on the sedimentation process of nSiO₂. The experimental observations were corroborated by calculating the DLVO interaction energy profile, which was shown to be congruent with the transport patterns. The present study also highlights the influence of high and low shear forces on the sedimentation process of nSiO₂ in the aqueous medium. The presence of shear force altered the collision efficiency and other interactive forces between the nanoparticles in the colloidal suspension. Under the experimental stirring conditions, a higher abundance of dispersed nSiO₂ in the upper layer of the aqueous medium was noted. Additionally, the transport behaviour of nSiO₂ was studied in a variety of natural water systems, including rivers, lakes, ground, and tap water. The study significantly contributes to our understanding of the different physical, chemical, and environmental aspects that can critically impact the sedimentation and spatial distribution of nSiO₂ in static and dynamic aquatic ecosystems.

Long-Term Forest Conversion Affects Soil Stability and Humic Substances in Aggregate Fractions in Subtropical China

Soil aggregates are the basic structural components of soil, which are important factors that can predict erosion resistance. However, few researchers have investigated the effects of forest conversion on the stability of soil aggregates, particularly in subtropical forests. In this study, soils from various depths (0 to 30 cm) were collected from four forest types (transformed from broadleaved forests (BMF) to combined coniferous broadleaved (CBMF), Chinese fir (FF), and bamboo forests (BF)) to determine the impacts of forest conversion on the physical and chemical properties of soil, water-stable soil aggregates, and aggregate-associated humic substances. The results showed that forest conversion had no effects on the soil’s physical properties, or the humic substances in bulk soil, but had significant effects on soil aggregates. In addition, the conversion of broadleaved forest to Chinese fir forest increased the soil stability, and to bamboo forest, decreased the soil stability. Finally, the soil’s physicochemical properties were closely related to aggregate-associated humic substances. In summary, specific forest management measures should be applied to strengthen the positive impacts and reduce the negative impacts associated with forest conversion.

Colloidal stability of nanosized activated carbon in aquatic systems: Effects of pH, electrolytes, and macromolecules

Nanosized activated carbon (NAC) is a novel adsorbent with great potential for water reclamation. However, its transport and reactivity in aqueous environments may be greatly affected by its stability against aggregation. This study investigated the colloidal stability of NAC in model aqueous systems with broad background solution chemistries including 7 electrolytes (NaCl, NaNO₃, Na₂SO₄, KCl, CaCl₂, MgCl₂, and BaCl₂), pH 4-9, and 6 macromolecules (humic acid (HA), fulvic acid (FA), cellulose (CEL), bovine serum albumin (BSA), alginate (ALG), and extracellular polymeric substance (EPS)), along with natural water samples collected from pristine to polluted rivers. The results showed that higher solution pH stabilized NAC by raising the critical coagulation concentration from 28 to 590 mM NaCl. Increased cation concentration destabilized NAC by charge screening, with the cationic influence following Ba²⁺ > Ca²⁺ > Mg²⁺ >> Na⁺ > K⁺. Its aggregation behavior could be predicted with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with a Hamaker constant (A_CWC) of 4.3 × 10^-20 J. The presence of macromolecules stabilized NAC in NaCl solution and most CaCl₂ solution following EPS > BSA > CEL > HA > FA > ALG, due largely to enhanced electrical repulsion and steric hindrance originated from adsorbed macromolecules. However, ALG and HA strongly destabilized NAC via cation bridging at high Ca²⁺ concentrations. Approximately half of NAC particles remained stably suspended for ∼10 d in neutral freshwater samples. The results demonstrated the complex effects of water chemistry on fate and transport of NAC in aquatic environments.

New insights into the colloidal stability of graphene oxide in aquatic environment: Interplays of photoaging and proteins

Wide application leads to release of graphene oxide (GO) in aquatic environment, where it is subjected to photoaging and changes in physicochemical properties. As important component of natural organic matters, proteins may greatly affect the aggregation behaviors of photoaged GO. The effects of a typical model protein (bovine serum albumin, BSA) on the colloidal stability of photoaged GO were firstly investigated. Photoaging reduced the lateral size and oxygen-containing groups of GO, while the graphene domains and hydrophobicity increased as a function of irradiation time (0-24 h). Consequently, the photoaged GO became less stable than the pristine one in electrolyte solutions. Adsorption of BSA on the surface of the photoaged GO decreased as well, leading to thinner BSA coating on the photoaged GO. In the solutions with low concentrations of electrolytes, the aggregation rate constants (k) of all the photoaged GO firstly increased to the maximum agglomeration rate constants (k_fast, regime I), maintained at k_fast (regime Ⅱ) and then decreased to zero (regime Ⅲ) as the BSA concentration increased. In both regime I and III, the photoaged GO were less stable at the same BSA concentrations, and the impacts of BSA on the colloidal stability of the photoaged GO were less than the pristine one, which was attributed to the weaker interactions between the photoaged GO and BSA. This study provided new insights into the colloidal stability and fate of GO nanomaterials, which are subjected to extensive light irradiation, in wastewater and protein-rich aquatic environment.

Ionic-strength-dependent effect of suspended sediment on the aggregation, dissolution and settling of silver nanoparticles

Suspended sediment (SS) is ubiquitous in natural waters and plays a key role in the fate of engineered nanomaterials. In this study, the effect of SS on the aggregation, settling, and dissolution of polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) was investigated under environmentally relevant conditions. The heteroaggregation of AgNPs with SS was not observed at low ionic strength (≤0.01 M) due to high electrostatic repulsion and steric forces. At higher NaCl concentrations (0.1 and 0.3 M), PVP-AgNPs were found to attach onto the SS surface, and the formation of AgNP-SS heteroaggregates strongly promoted settling of PVP-AgNPs due to the overwhelming gravity force. PVP-AgNP dissolution was reduced after the addition of sediment to ultrapure water because the presence of sediment-associated dissolved organic matter (SS-DOM). The formation of an AgCl layer on PVP-AgNP surface in 0.01 M NaCl solution resulted in the minor effect of SS on AgNP dissolution. After addition of SS, the dissolved silver concentrations of PVP-AgNP increased in 0.1 and 0.3 M NaCl solution. The interactions of SS-DOM with AgNPs under different NaCl concentrations interfered the dissolution of AgNPs in sediment-laden water. This study provides new insight into the fate of AgNPs in sediment-laden water under various environmental conditions.

Effects of different types of humic acid isolated from coal on soil NH3 volatilization and CO2 emissions

Humic acid can improve soil nutrients and promote plant growth. Weathered coal and lignite can be used as agricultural resources due to high humic acid content, but their impact on soil NH₃ volatilization and CO₂ emissions are yet to be determined. In this study, a field experiment was carried out to compare the effects of four types of humic acid isolated from coal (pulverized weathered coal (HC), pulverized lignite (HL), alkalized weathered coal (AC) and alkalized lignite (AL)) on NH₃ volatilization, CO₂ emissions, pH, the C/N ratio and enzyme activities in soil cultivated with maize. The effect of biotechnology humic acids (BHA) was also examined for comparison. HL, AC, AL and BHA all increased cumulative NH₃ losses by 147.7, 278.5, 113.9, and 355.3%, respectively, compared with the control (chemical fertilizer only), and notably, BHA caused an increase of 90.71% compared with the humic acids isolated from coal. A significant increase in cumulative CO₂ losses was observed only under AL treatment, by 14.44-24.90% compared with all other treatments. Soil urease activity was positively correlated with cumulative NH₃ losses (P < 0.001), while the soil C/N ratio (P < 0.001) and soil sucrase activity (P < 0.05) were positively correlated with cumulative CO₂ losses. Since humic acid from pulverized weathered coal caused no increase in NH₃ volatilization or CO₂ emissions, it is therefore thought to be the most suitable humic acid for field application.

Mn2+ effect on manganese oxides (MnOx) nanoparticles aggregation in solution: Chemical adsorption and cation bridging

Manganese oxides (MnO_x) and Mn²⁺ usually co-exist in the natural environment, as well as in water treatments for Mn²⁺ removal. Therefore, it is necessary to investigate the influence of Mn²⁺ on the stability of MnO_x nanoparticles, as it is vital to their fate and reactivity. In this study, we used the time-resolved dynamic light scattering technique to study the influence of Mn²⁺ on the initial aggregation kinetics of MnO_x nanoparticles. The results show that Mn²⁺ was highly efficient in destabilizing MnO_x nanoparticles. The critical coagulation concentration ratio of Mn²⁺ (0.3 mM) to Na⁺ (30 mM) was 2^-6.64, which is beyond the ratio range indicated by the Schulze-Hardy rule. This is due to the coordination bond formed between Mn²⁺ and the surface O of MnO_x, which could efficiently decrease the negative surface charge of MnO_x. As a result, in the co-presence of Mn²⁺ and Na⁺, a small amount of Mn²⁺ (5 μM) could efficiently neutralize the negative charge of MnO_x, thereby decreasing the amount of Na⁺, which mainly destabilized nanoparticles through electric double-layer compression, required to initiate aggregation. Further, Mn²⁺ behaved as a cation bridge linking both the negatively charged MnO_x and humic acid, thereby increasing the stability of the MnO_x nanoparticles as a result of the steric repulsion of the adsorbed humic acid. The results of this study enhance the understanding of the stability of the MnO_x nanoparticles in the natural environment, as well as in water treatments.

The aggregation kinetics of manganese oxides nanoparticles in Al(III) electrolyte solutions: Roles of distinct Al(III) species and natural organic matters

This study explored the aggregation kinetics of manganese oxides (MnO_x) nanoparticles in Al(III) electrolyte solutions. This is a common process in both water treatments and the natural environment. The results show that aggregation kinetics are Al(III) species-dependent. Without natural organic matters (NOM), ferron Al_a (monomeric Al(III)) and ferron Al_b (polymeric Al(III)) are the main species controlling the Derjaguin-Landau-Verwey-Overbeek (DLVO) type aggregation behavior of MnO_x at pH 5.0 and 7.2, respectively. Al_a and Al_b can neutralize and reverse the negative charge of MnO_x. Correspondingly, the attachment efficiency as a function of Al(III) concentrations contains three stages: destabilization, diffusion-limited, and re-stabilization stage. Interestingly, due to the tiny size of Al_b nanoclusters, they behave similar to free ions and do not induce heteroaggregation at pH 7.2. The influence of some model NOM (i.e., bovine serum albumin (BSA), Sigma humic acid (HA), and alginate) was also studied. At pH 5.0, alginate polymers, while Sigma HA and BSA cannot be, are linked by Al(III) to form alginate gel clusters which bridge MnO_x nanoparticles, and thus induce bridging flocculation. At pH 7.2, NOM induce the aggregation of Al_b nanoclusters to form NOM-Al(III) aggregates through charge neutralization effects. Consequently, highly enhanced aggregation rate, due to the heteroaggregation between these aggregates and MnO_x, was observed.