Influence of natural organic matter in porous media on fine particle transport

Nanocolloids in the soil environment: Transformation, transport and ecological effects

Nanocolloids (Ncs) are ubiquitous in natural systems and play a critical role in the biogeochemical cycling of trace metals and the mobility of organic pollutants. However, the environmental behavior and ecological effects of Ncs in the soil remain largely unknown. The accumulation of Ncs may have detrimental or beneficial effects on different compartments of the soil environment. This review discusses the major transformation processes (e.g., agglomeration/aggregation, absorption, deposition, dissolution, and redox reactions), transport, bioavailability of Ncs, and their roles in element cycles in soil systems. Notably, Ncs can act as effective carriers for other pollutants and contribute to environmental pollution by spreading pathogens, nutrients, heavy metals, and organic contaminants to adjacent water bodies or groundwater. Finally, the key knowledge gaps are highlighted to better predict their potential risks, and important new directions include exploring the geochemical process and mechanism of Ncs's formation; elucidating the transformation, transport, and ultimate fate of Ncs, and their long-term effect on contaminants, organisms, and elemental cycling; and identifying the impact on the growth and quality of important crops, evaluating its dominant effect on agro-ecosystems in the soil environment.

The impact of biofilms and dissolved organic matter on the transport of nanoparticles in field-scale streams

The fate and transport of nanoparticles (NPs) in streams is critical for understanding their overall environmental impact. Using a unique field-scale stream at the Notre Dame-Linked Experimental Ecosystem Facility, we investigated the impact of biofilms and the presence of dissolved organic matter (DOM) on the transport of titanium dioxide (TiO₂) NPs. Experimental breakthrough curves were analyzed using temporal moments and fit using a mobile-immobile model. The presence of biofilms in the stream severely reduced the transport of the TiO₂ NPs, but this was mitigated by the presence of DOM. Under minimal biofilm conditions, the presence of DOM increased the mass recovery of TiO₂ from 4.2% to 32% for samples taken 50 m downstream. For thriving biofilm conditions only 0.5% of the TiO₂ mass was recovered (50 m), but the presence of DOM improved the mass recovery TiO₂ to 36%. The model was suitable for predicting early, peak, tail, and truncation time portions of the breakthrough curves, which attests to its ability to capture a range of processes in the mobile and immobile domains of the stream. The model outcomes supported the hypothesis that DOM changed the interaction of NP-biofilm from an irreversible to a reversible process. Collectively, these outcomes stress the importance of considering biogeological complexity when predicting the transport of NPs in streams.

Enhancing effects of dissolved and media surface-bound organic matter on titanium dioxide nanoparticles transport in iron oxide-coated porous media under acidic conditions

Natural organic matter (NOM) and iron oxides have been proved to be crucial factors controlling the behaviors of nanoparticles in heterogenous environment. Here, we conducted experimental and modeling study on the transport of titanium dioxide nanoparticles (TiO₂ NPs) in iron oxide-coated quartz in the presence of NOM under acidic conditions. Results showed the antagonistic effects of iron oxides and NOM on TiO₂ NPs mobility. The inhibition of iron oxides coated on quartz was crystal form-dependent other than quantity-dependent. Amorphous ferric oxyhydroxide with higher specific surface area brought more positive charge and favorable deposition sites onto quartz, and induced more retention of nanoparticles than two crystalline iron oxides, goethite and hematite. Dissolved organic matter (DOM) facilitated TiO₂ NPs transport in iron oxide-coated quartz. In comparation with the limited enhancing effects of DOM, the NOM coatings on media surface partially or largely offset the inhibition of goethite on nanoparticles mobility through direct occupation of attachment sites and sites screening due to the steric repulsion of the macromolecules. Owing to the higher steric hindrance, humic acid, both in dissolved and media surface-bound states, exerted stronger facilitating effects on TiO₂ NPs mobility relative to fulvic acid.

The evolution of stable nanohybrids to complex heteroaggregates between nZVI and soil nanoparticles: The influence of ionic strength and soil components

The heteroaggregation mechanism of nZVI with four types of natural soil nanoparticles (SNPs) extracted from representative soils in northern and southern China was investigated. Heteroaggregation rates between nZVI and SNPs were quantified by dynamic light scattering and evaluated as a function of ionic strength at pH 7. The nZVI-SNPs heteroaggregates were stable with hydrodynamic diameters (D_h) ranging from 400 to 600 nm in 0.1 mM solution. Based on the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, nZVI underwent heteroaggregation with SNPs to form stable nZVI-SNPs nanohybrid due to the attachment of nZVI on the SNPs. However, with enhanced ionic strength, SNPs accelerated the aggregation of nZVI and formed large heteroaggregates with D_h in the range from 1200 to 2000 nm, owing to insignificant electrostatic repulsions and oppositely charged patches. In addition, the differences in the heteroaggregation rates of nZVI with four SNPs were negligible, caused by the negligible impacts of SNPs components such as soil organic matter and Fe/Al oxyhydroxides on the heteroaggregation of nZVI in the 10 mM NaCl solution. These findings are helpful for understanding the interaction between nZVI and SNPs and of significance to groundwater remediation using nZVI.

Groundwater antibiotic pollution and its relationship with dissolved organic matter: Identification and environmental implications

The occurrence of veterinary antibiotics and hydro-chemical parameters in eleven natural springs in a livestock production area is evaluated, jointly with the characterization of their DOM fingerprint by Orbitrap HRMS. Tetracycline and sulfonamide antibiotics were ubiquitous in all sites, and they were detected at low ng L^-1 concentrations, except for doxycycline, that was present at μg L^-1 in one location. DOM analysis revealed that most molecular formulas were CHO compounds (49 %-68 %), with a remarkable percentage containing nitrogen and sulphur (16 %-23 % and 11 %-24 %, respectively). Major DOM components were phenolic and highly unsaturated compounds (~90 %), typical for soil-derived organic matter, while approximately 11 % were unsaturated aliphatic, suggesting that springs may be susceptible to anthropogenic contamination sources. Comparing the DOM fingerprint among sites, the spring showing the most different profile was the one with surface water interaction and characterized by having lower CHO and higher CHOS formulas and aliphatic compounds. Correlations between antibiotics and DOM showed that tetracyclines positively correlate with unsaturated oxygen-rich substances, while sulfonamides relate with aliphatic and unsaturated oxygen-poor compounds. This indicates that the fate of different antibiotics will be controlled by the type of DOM present in groundwater.

Co-transport of biogenic nano-hydroxyapatite and Pb(II) in saturated sand columns: Controlling factors and stochastic modeling

Biogenic nano-hydroxyapatite (bio-nHAP) has recently gained great interest in many domains, especially in the remediation of heavy metal-contaminated soil, due to its high reactivity, low cost, and eco-friendly nature. The co-transport and reaction of bio-nHAP with Pb(II) in saturated porous media, however, are not well understood. This work investigated the effects of ionic strength (IS), ionic composition (IC), dissolved organic matter (DOM), and flow velocity on transport-reaction dynamics of Pb(II) and bio-nHAP by combining column breakthrough experiments and model simulations. Results showed that the mobility of Pb(II) was significantly enhanced with increasing IS/IC but less affected by flow velocity during the transport-reaction process of bio-nHAP and Pb(II) in the saturated sand column; while the transport of bio-nHAP was restricted by increasing IS/IC but facilitated by increasing velocity. IC, IS, and velocity only slightly affected the reaction kinetics between Pb(II) and bio-nHAP, likely due to the fast reaction rate between Pb(II) and bio-nHAP and precipitation of pyromorphite. The transport dynamics of bio-nHAP and Pb(II) were significantly changed by DOM, and this effect depended strongly on the type of DOM with different molecular weights. Breakthrough curves of Pb(II) and bio-nHAP exhibited apparent "anomalous", sub-diffusive transport behaviors, which could be well quantified by a novel tempered fractional derivative bimolecular reaction equation (T-FBRE). Our findings highlighted the accurate simulation of the co-transport and reaction of bio-nHAP with Pb(II) using T-FBRE and had a great benefit for risk assessment and remediation strategy development for Pb(II) contaminated soil.

Co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions

To investigate co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions, NH₄⁺ was selected as the target contaminant, and silicon and humic acid (HA) were selected as typical organic and inorganic colloids in groundwater. Column experiments were then conducted to investigate the transport of NH₄⁺ colloids under various hydrochemical conditions. The results showed that because of the different properties of colloidal silicon and HA after combining with NH₄⁺, the co-transport mechanism became significantly different. During transport by the NH₄⁺-colloid system, colloidal silicon occupied the adsorption sites on the medium surface to promote the transport of NH₄⁺, while humic acid (HA) increased the number of adsorption sites of the medium to hinder the transport of NH₄⁺. The co-transport of NH₄⁺ and colloids is closely related to hydrochemical conditions. In the presence of HA, competitive adsorption and morphological changes of HA caused NH₄⁺ to be more likely to be transported at a higher ionic strength (IS = 0.05 m, CaCl₂) and alkalinity (pH = 9.3). In the presence of colloidal silicon, blocking action caused the facilitated transport to be dependent on higher ionic strength and acidity (pH = 4.5), causing the recovery of NH₄⁺ to improve by 7.99%, 222.25% (stage 1), and 8.63%, respectively. Moreover, transport increases with the colloidal silicon concentrations of 20 mg/L then declines at 40 mg/L, demonstrating that increased concentrations will lead to blocking and particle aggregation, resulting in delayed release in the leaching stage. Graphical abstract.

The role of Fe oxyhydroxide coating, illite clay, and peat moss in nanoscale titanium dioxide (nTiO2) retention and transport in geochemically heterogeneous media

Natural media such as soil and sediment contain mineralogical and organic components with distinct chemical, surface, and electrostatic properties. To better understand the role of various soil and sediment components on particle transport, columns were packed with quartz sand and natural sediment with added Fe oxyhydroxide coating, illite clay, and peat moss to investigate how these added components influence nTiO₂ retention and transport in geochemically heterogeneous medium. Results showed that nTiO₂ transport was low at pH 5, attributable to the electrostatic attraction between positively-charged nTiO₂ and negatively-charged medium. While illite did not notably affect nTiO₂ transport at pH 5, Fe oxyhydroxide coating increased nTiO₂ transport due to electrostatic repulsion between Fe oxyhydroxide and nTiO₂. Peat moss also increased nTiO₂ transport at pH 5, attributable to the increased DOC concentration, which resulted in higher DOC adsorption to nTiO₂ and intensified electrostatic repulsion between nTiO₂ and the medium. At pH 9, nTiO₂ transport was high due to the electrostatic repulsion between negatively-charged nTiO₂ and medium surfaces. Fe oxyhydroxide coating at pH 9 slightly delayed nTiO₂ transport due to electrostatic attraction, while illite clay and peat moss substantially inhibited nTiO₂ transport via straining/entrapment or electrostatic attraction. Overall, this study demonstrated that pH has a considerable effect on how minerals and organic components of a medium influence nTiO₂ transport. At low pH, electrostatic attraction was the dominant mechanism, therefore, nTiO₂ mobility was low regardless of the differences in mineralogical and organic components. Conversely, nTiO₂ mobility was high at high pH and nTiO₂ retention was dominated by straining/entrapment and sensitive to the mineralogical and organic composition of the medium.

Effect of bovine serum albumin on stability and transport of kaolinite colloid

The stability and transport of clay colloids in groundwater are strongly influenced by colloid interactions with dissolved organic matter (DOM). Protein is an important DOM component that is ubiquitous in natural water, reclaimed water, and soil solutions. To date, the interactions between clay colloids and proteins have not been fully studied. The objective of this study was to examine the effect of bovine serum albumin (BSA), a representative protein, on the stability, aggregation, and transport of kaolinite colloids under neutral pH conditions. Hydrodynamic diameter and ζ-potential measurements, stability tests, and column transport experiments were performed in salt solutions with a range of ionic strengths and different BSA concentrations at pH 7. Additionally, BSA-kaolinite colloid interactions were studied using TEM and batch adsorption experiments. The experimental results showed that BSA prevented colloid aggregation and increased the stability and transport of colloids, especially at high ionic strength, even though the charges of kaolinite colloids were less negative in the presence of BSA. Theoretical calculation of the interaction energies indicated that XDLVO theory, in which the steric force is considered due to BSA adsorption, could correctly quantify the interaction energies in the presence of BSA. This study demonstrated that the role of protein needs to be determined in order to better predict the overall effect of DOM on particle aggregation and transport in the soil environment.

Organic contaminated sediments remediation with active caps: Nonlinear adsorption unveiled by combined isotherm and column transportation studies

Bench-scale column studies were performed with four cap materials (sand, apatite, organoclay and granular activated carbon) for five target compounds (toluene, naphthalene, phenanthrene, pyrene and dichlorobiphenyl) to represent a range of cap materials and pollutants (volatile & semi-volatile compounds) commonly found in sediments. Two moment-derived methods were used to model cap performance. Rough agreement was observed between the column experiments and modeling data with the fronting and tailing effects identified from certain breakthrough curves indicating a high potential of non-linear adsorption. Distribution coefficients (k_d) were experimentally determined with isotherm studies together with measurements of surface area and microporosity of the cap materials via nitrogen adsorption porisimetry. These studies unveiled the occurrence of nonlinear adsorption by Freundlich simulation. The effects of nonlinear adsorption of the cap were further explored via modeling. Results suggested better prediction of cap performance assuming nonlinear adsorption instead of linear adsorption results based upon the risk of release for a 30-year period.

Fine particle attachment to quartz sand in the presence of multiple interacting dissolved components

In natural aquatic systems water chemistry is complicated and fine particles encounter multiple water components simultaneously, yet the combined effects of some multiple components on the fate and transport of these particles have not been elucidated. In this study nTiO₂ and illite colloid attachment to quartz sand was investigated in 1 mM NaCl and 0.5 mM CaCl₂ background solutions using a range of phosphate concentrations (0 to 10 mg/L) at pH 5 and 9. The results obtained from the batch experiments indicated that without using phosphate, nTiO₂ aggregation and attachment was strongly influenced by pH and Ca²⁺, both of which modified nTiO₂ surface charges. nTiO₂ attachment was high in CaCl₂ solution at pH 9 due to attractive forces between nTiO₂ and sand, as well as ripening. Furthermore, phosphate adsorption to nTiO₂ was higher in CaCl₂ solution at pH 9 than that at pH 5 due to attractive forces between nTiO₂ and phosphate anions, and also potential surface precipitation of Ca-P minerals at pH 9. Phosphate adsorption to illite was low owing to strong repulsive forces between illite and phosphate. The effect of phosphate on nTiO₂ and illite attachment to sand was influenced by pH and cation valency. A decreasing trend in nTiO₂ attachment with phosphate addition was observed in NaCl solution at pH 5 and 9, and in CaCl₂ solution at pH 5; however, in CaCl₂ solution at pH 9, the surface charge of nTiO₂ reversed from negative to positive and a substantial amount of nTiO₂ attached to sand. Moreover, illite attachment to sand was much lower than that of nTiO₂ under all the conditions tested in this study. These findings are important for understanding of the fate and transport of nTiO₂ and illite colloids in natural aquatic systems where various anions and cations co-exist.