Comparative study on the micelle properties of synthetic and dissolved organic matters

Electrokinetically-delivered persulfate and pulsed direct current electric field induced transport, mixing and thermally activated in situ for remediation of PAHs contaminated soil

Herein, we proposed and proved a novel strategy that enhanced the delivery of persulfate (PS) to soil by electrokinetics (EK), and then applying a pulsed direct current (DC) electric field thermally activated the PS in situ, and synchronously promoted PS plume mixing, contaminants-free radicals reaction and continued to replenish PS to the soil, to achieve efficient degradation of contaminants in low permeability zones. Results showed that transport rate of PS in tested soil by EK was approximately 12.3 times than diffusion. Applying an irregular pulsed DC field maintained the targeted temperature (30-50 ℃) during activation phase, and generated two oxidative radicals (SO₄^∙-/∙OH). Concurrently, in the case, electromigration and electroosmosis have promoted the PS transport and the reactive mixing of PS/free radicals with polycyclic aromatic hydrocarbons (PAHs) contaminated soil and enhance the PAHs degradation. PS concentrations in pore fluid was characterized by an increase accompanied by continuous fluctuations. Eventually, in case of the long-term low-temperature activation (i.e., 30-40 ℃), a significant decreases (nearly 60%) in average concentration of PAHs in the whole soil cell was observed over 10 days. These results demonstrates that the novel strategy has great potentiality in the remediation of low permeability contaminated soil.

Adsorption of benzalkonium chlorides onto polyethylene microplastics: Mechanism and toxicity evaluation

Usage of disposable plastic products and disinfectants has been skyrocketing due to the COVID-19 pandemic. The random disposal of plastic products may result in greater microplastic pollution. Benzalkonium chloride is known as one of the most common ingredients of disinfectants. In this study, the adsorption behavior of benzalkonium chlorides (BAC₁₂, BAC₁₄, BAC₁₆) on polyethylene microplastics (PE-MPs) and the combined toxic effects were investigated using batch adsorption experiment and Daphnia magna. The results showed that PE-MPs had strong adsorption capacity for BACs and the adsorption capacity increased (11.03-22.77 mg g^-1) with their octanol-water distribution coefficients. The effect of pH was negligible while dissolved organic matter inhibited the adsorption. A slightly inverse relationship between particle size of PE-MPs and adsorption was observed. Additionally, the MP aging with UV/H₂O₂ increased the adsorption of BAC₁₂ but decreased that of relatively hydrophobic BAC₁₄ and BAC₁₆. The survival rate of Daphnia magna increased up to 100% in the presence of PE-MPs depending upon their adsorption capacities, suggesting that PE-MPs do not act as a carrier but rather as a scavenger for BACs. This study provides important information necessary for environmental risk assessment with regard to the combined pollution of MPs and toxic chemicals.

Soil water solutes reduce the critical micelle concentration of quaternary ammonium compounds

Quaternary alkyl ammonium compounds (QAACs) are produced in large quantities for use as surfactants and disinfectants and also found in soils, sediments, and surface waters, where they are potentially involved in the selection of antibiotic resistance genes. Micelle formation influences fate and effects of QAACs. The critical micelle concentration (CMC) of six homologs of benzylalkylammonium chlorides (BAC) was determined in deionized water, 0.01 M CaCl₂ solution, and aqueous soil extracts, using both spectrofluorometric and tensiometric methods. Additionally, eight organic model compounds were employed at concentrations of 15 mg C L^-1 as background solutes in order to test the effect of dissolved organic carbon (DOC) on CMCs. Results found CMCs decreased with an increasing length of the alkyl chain from 188 mM for BAC-C8 to 0.1 mM for BAC-C18. Both methods yielded similar results for measurements in water and CaCl₂ solution; however, the spectrofluorescence method did not work for soil extracts due to fluorescence quenching phenomena. In soil extracts, CMCs of BAC-C12 were reduced below 3.7 mM, while the CMC reduction in soil extracts was less pronounced for BAC-C16. Besides ionic strength, molecular structures of BACs and dissolved organic compounds also affected the CMC. The number of carboxyl groups and small molecular weights of the DOC model compounds reduced the CMCs of BAC-C12 and BAC-C16 at pH 6. This study highlights that CMCs can be surpassed in soil solution, pore waters of sediments, or other natural waters even at (small) concentrations of QAACs typically found in the environment.

Rapidly probing the interaction between sulfamethazine antibiotics and fulvic acids

Antibiotics residuals in the environments receive wide concerns due to the high risk of generating antibiotic resistance. Natural organic matters (NOM) existed in the environments are considered to have the capacity of binding with organic contaminants, consequently influencing their speciation and transformation in the natural environments. To assess the migration of antibiotics in the environments, it is crucial to understand the binding mechanisms between NOM and antibiotics, which is still unclear due to the limit of available research methods. In this study, the interaction between fulvic acids (FA), one of the main components of NOM, and sulfamethazine (SMZ) was characterized by nuclear magnetic resonance (NMR) combined with surface plasmon resonance (SPR) and isothermal titration calorimetry (ITC) technology. The parameters related to kinetics and thermodynamics of the interaction were determined, and the possible mechanisms driving the interaction were also proposed. In addition, density functional theory (DFT) was used to predict the binding mode between FA and SMZ to reveal the interaction mechanism. Results indicate that FA can effectively bound with SMZ to form a stable complex with a binding constant at the level of 10³ L/mol. The kinetic parameters including association and dissociation constants were 29.4 L/mol/s and 6.64 × 10^-3 1/s, respectively. Hydrophobic interaction might play significant roles in the binding interaction with ancillary contribution of π-π conjunction arising from the aromatic rings stacking of FA and SMZ.

Effects of single and combined exposures to copper and benzotriazole on Eisenia fetida

Benzotriazole (BTR), an emerging class of environmental pollutant, is widely used in industrial applications and household dishwashing agents. Despite the reported toxicity of BTR to aquatic organisms, little is known about its effects on terrestrial invertebrates. Copper (Cu) accumulates in agricultural soils receiving urban waste products, fertilizers, fungicides, and urban sewage. In this study, two different types of bioassays (acute toxicity test and behavioral toxicity test) were performed to evaluate the toxicity of Cu and BTR, both singly and together, on the earthworm (Eisenia fetida) in artificial soil. The results of avoidance behaviour tests showed that the EC_50,48 h values for Cu and BTR were 1.47 and 0.46 mmol kg^-1, respectively. The results of the acute toxicity tests showed that the LC_50,7 d and LC_50,14 d of Cu in earthworms were 9.19 and 5.28 mmol kg^-1, respectively, and the LC_50,7 d and LC_50,14 d of BTR were 2.43 and 1.76 mmol kg^-1, respectively. Toxicity analysis demonstrated that the binary BTR and Cu mixture had predominantly antagonistic effects on the avoidance behaviour and survival of earthworms. The Cu²⁺ activities and mortality of earthworms decreased significantly with increasing concentrations of BTR, while the solid-liquid distribution coefficient of Cu increased. These results indicated that the presence of BTR can reduce the toxicity as well as the bioavailability of Cu in soil with both BTR and Cu.

Kinetics and thermodynamics of interaction between sulfonamide antibiotics and humic acids: Surface plasmon resonance and isothermal titration microcalorimetry analysis

The presence of sulfonamide antibiotics in the environments has been recognized as a crucial issue. Their migration and transformation in the environment is determined by natural organic matters that widely exist in natural water and soil. In this study, the kinetics and thermodynamics of interactions between humic acids (HA) and sulfamethazine (SMZ) were investigated by employing surface plasmon resonance (SPR) combined with isothermal titration microcalorimetry (ITC) technologies. Results show that SMZ could be effectively bound with HA. The binding strength could be enhanced by increasing ionic strength and decreasing temperature. High pH was not favorable for the interaction. Hydrogen bond and electrostatic interaction may play important roles in driving the binding process, with auxiliary contribution from hydrophobic interaction. The results implied that HA existed in the environment may have a significant influence on the migration and transformation of organic pollutants through the binding process.

Effect of humic acid on the sulfamethazine adsorption by functionalized multi-walled carbon nanotubes in aqueous solution: mechanistic study

The presence of humic acid (HA) inhibited sulfamethazine (SMZ) adsorption by three types of multi-walled carbon nanotubesviacompetitive interactions, molecular sieving, and pore blockage in solution (pH < 9).

Amplified solubilization effects of inherent dissolved organic matter releasing from less-humified sediment on phenanthrene sorption

Soil/sediment organic matter (SOM) releasing with inherent dissolved organic matter (DOM) formed in solution was confirmed both in rhizosphere sediment (S) and uncultivated sediment (P) water systems, and correlations between SOM characteristics subject to sediment's humification degree and its releasing effects on phenanthrene sorption were emphasized. The sequential SOM releasing evidenced by fluorescence and (1)H-NMR profiles coupled with aqueous DOM solubilization was found to make sorption kinetics atypical and sorption capacity reduced, by comparing sorption results among sediments of different pretreatments. More importantly, the tested S was proved less humified with inherent DOM rich in microbial sources than P, and DOM affinity to phenanthrene was thus weakened (K doc values of 2.02-3.63 × 10(4) L kg(-1)), while the inhibitive effects of SOM releasing on sorption were strengthened, ascribing to the enlarged alterations of sediment characters, and particularly the amplified solubilization effects resulted from the larger proportion of soluble SOM and lower critical micelle concentration (5.66 mg L(-1)) of DOM. Moreover, relative contribution of DOM solubilization to the releasing effects enhanced from 0.67 for P to 0.78 for S relative to alterations of sediment characters. Consequently, mobility and exposure risk of polycyclic aromatic hydrocarbons would be enhanced in a plant-soil/sediment-water system.

Combined effects of DOM and biosurfactant enhanced biodegradation of polycylic armotic hydrocarbons (PAHs) in soil-water systems

This study systematically investigated the interactive effects of dissolved organic matter (DOM) and biosurfactant (rhamnolipid) on the biodegradation of phenanthrene (PHE) and pyrene (PYR) in soil-water systems. The degradations of two polycyclic aromatic hydrocarbons (PAHs) were fitted well with first order kinetic model and the degradation rates were in proportion to the concentration of biosurfactant. In addition, the degradation enhancement of PHE was higher than that of PYR. The addition of soil DOM itself at an environmental level would inhibit the biodegradation of PAHs. However, in the system with co-existence of DOM and biosurfactant, the degradation of PAHs was higher than that in only biosurfactant addition system, which may be attributed to the formation of DOM-biosurfactant complex micelles. Furthermore, under the combined conditions, the degradation of PAH increased with the biosurfactant concentration, and the soil DOM added system showed slightly higher degradation than the compost DOM added system, indicating that the chemical structure and composition of DOM would also affect the bioavailability of PAHs. The study result may broaden knowledge of biosurfactant enhanced bioremediation of PAHs contaminated soil and groundwater.

Responses of kinetics and capacity of phenanthrene sorption on sediments to soil organic matter releasing

Soil organic matter (SOM) releasing with dissolved organic matter (DOM) formed in solution was confirmed in a sediment/water system, and the effects of SOM releasing on the sorption of phenanthrene on sediments were investigated. Inorganic salt (0-0.1 mol L(-1) NaCl) was used to adjust SOM releasing, and two sediments were prepared, the raw sediment (S1) from Weihe River, Shann'xi, China, and the eluted sediments with and without DOM supernatant remained, termed as S2a and S2b, respectively. The FTIR and (1)H NMR analysis indicate that the low molecular weight hydrophilic SOM fraction released prior to the high molecular weight hydrophobic fraction. As a response, phenanthrene sorption kinetics on S1 showed atypical and expressed as three stages: rapid sorption, pseudo sorption with partial desorption, and slow sorption, thus a defined "sorption valley" occurred in kinetic curve. In all cases, partition dominates the sorption, and sorption capacity (Kd) ranked as S2b > S1 > S2a. Compared with the alterations of sediment characters, DOM solubilization produced by SOM releasing exhibited a greater inhibitory effect on sorption with a relative contribution of 0.67. Distribution coefficients (K(doc)) of PHE into DOM clusters were 2.10 × 10(4)-4.18 × 10(4) L kg(-1), however a threshold concentration of 6.83 mg L(-1) existed in DOM solubilization. The study results will help to clarify PAHs transport and their biological fate in a sediment/water system.

Characterizing the interactions between polycyclic aromatic hydrocarbons and fulvic acids in water

Polycyclic aromatic hydrocarbons (PAHs) are persistent, bioaccumulative, and toxic chemicals and are listed as priority pollutants by the US EPA. Although they are sparsely soluble in water, their solubility can be increased by binding to dissolved organic matter in natural waters, which will further increase their environmental risk as toxic pollutants. In this study, the interaction between PAHs, exemplified by fluorene and anthracene, and fulvic acid (FA) was studied using fluorescence quenching titration method with fluorescence emission spectra, respectively. The association of FA with the mixture of fluorene and anthracene was also evaluated by excitation-emission matrix (EEM) fluorescence spectrometry combined with parallel factor (PARAFAC) analysis. Results demonstrate that EEM fluorescence spectrometry with PARAFAC analysis was sensitive and reliable to determine the binding properties of PAHs with FA in a mixed aqueous solution. The conditional stability constants and binding capacities show that both PAHs bind to FA tightly.

Combined effects of DOM extracted from site soil/compost and biosurfactant on the sorption and desorption of PAHs in a soil-water system

The combined effects of DOM and biosurfactant on the sorption/desorption behavior of phenanthrene (PHE) and pyrene (PYR) in soil water systems were systematically investigated. Two origins of DOMs (extracted from soil and extracted from food waste compost) and an anionic biosurfactant (rhamnolipid) were introduced. The presence of DOM in the aqueous phase could decrease the sorption of PAHs, thus influence their mobility. Desorption enhancement for both PHE and PYR in the system with compost DOM was greater than that in the soil DOM system. This is due to the differences in specific molecular structures and functional groups of two DOMs. With the co-existence of biosurfactant and DOM, partitioning is the predominant process and the desorption extent was much higher than the system with DOM or biosurfactant individually. For PHE, the desorption enhancement of combined DOM and biosurfactant was larger than the sum of DOM or biosurfactant; however desorption enhancement for PYR in the combined system was less than the additive enhancement in two individual system under low PAH concentration. This could be explained as the competition sorption among PAHs, DOM and biosurfactant. The results of this study will help to clarify the transport of petroleum pollutants in the remediation of HOCs-contaminated soils.