Bioavailability Of Contaminants Research Articles

Tiered ecological risk assessment combined with ecological scenarios for soil in abandoned industrial contaminated sites

Redeveloping abandoned contaminated industrial sites can effectively relieve urban land shortages. An economical and accurate ecological risk assessment (ERA) approach that guides remediation measures for enormous sites is therefore needed. A tiered ERA approach that combined with ecological scenarios is proposed in this study. At first step, prospective land use of site and the bioavailability of contaminants were integrated to create six ecological scenarios and different protection goals assigned to each ecological scenario. Industrial information and soil parameters of specific sites were then obtained to identify major contaminants and matching scenario. With collected toxicity data, tiered ERA combining a hazard quotient (HQ) and joint probability curve (JPC) was used to assess the ecological risk, under corresponding scenario. Recommendations for regulatory management were given based on the ERA and the tolerance of decision makers to ecological risks. This approach was validated using case studies at four abandoned industrial contaminated sites. The results indicated that the ecological scenarios defined as II and V matched with each site, respectively, and that Cd, Cu, Ni, and Pb were the major contaminants and had different ecological risk values under different ecological scenarios. Different remediation measures should be adopted to address different levels of ecological risk. The results demonstrated that the combination of a tiered ERA with matching ecological scenarios effectively reduced workloads and improved ERA accuracies for abandoned industrial contaminated sites with redevelopment potential.

Does pH variation influence the toxicity of organic contaminants in estuarine sediments? Effects of Irgarol on nematode assemblages

Natural pH values in coastal waters vary largely among locations, ecosystems, and time periods; still, there is an ongoing acidification trend. In this scenario, more acidic pH values can alter bioavailability of organic contaminants, to organisms. Despite this, interactive effects between pH and chemical substances are not usually considered in Ecological Risk Assessment protocols. This study investigated the effects of pH on the toxicity of a hydrophobic organic compound on a benthic community using a microcosm experiment setup to assess the response of nematode assemblages exposed to environmentally relevant concentrations of Irgarol at two natural pH conditions. Estuarine nematode assemblages were exposed to two concentrations of Irgarol at pH 7.0 and 8.0 for periods of 7 and 35 days. Lower diversity of nematode genera was observed at the highest tested Irgarol concentration (1281 ± 65 ng.g−1). The results showed that the effects of Irgarol contamination were independent of pH variation, indicating no influence of acidification within this range on the toxicity of Irgarol to benthic meiofauna. However, the results showed that estuarine nematode assemblages are impacted by long-term exposure to low (but naturally occurring) pHs. This indicates that estuarine organisms may be under naturally high physiological pressure and that permanent changes in the ecosystem's environmental factors, such as future coastal ocean acidification, may drive organisms closer to the edges of their tolerance windows.

Improving the assessment of polluted sites using an integrated bio-physico-chemical monitoring framework

Soil – water pollution resulting from anthropogenic activities is a growing concern internationally. Effective monitoring techniques play a crucial role in the detection, prevention, and remediation of polluted sites. Current pollution monitoring practices in many geographical locations are primarily based on physico–chemical assessments which do not always reflect the potential toxicity of contaminant ‘cocktails’ and harmful chemicals not screened for routinely. Biomonitoring provides a range of sensitive techniques to characterise the eco–toxicological effects of chemical contamination. The bioavailability of contaminants, in addition to their effects on organisms at the molecular, cellular, individual, and community level allows the characterisation of the overall health status of polluted sites and ecosystems. Quantifying bioaccumulation, changes to community structure, faunal morphology, behavioural, and biochemical responses are standard procedures employed in biomonitoring studies in many High-Income Countries (HICs). This review highlights the need to integrate biomonitoring tools alongside physico–chemical monitoring techniques by using ‘effect–based’ tools to provide more holistic information on the ecological impairment of soil-water systems. This paper considers the wider implementation of biomonitoring methods in Low to Middle Income Countries (LMICs) and their significance in pollution investigations and proposes an integrated monitoring framework that can identify toxicity drivers by utilising ‘effect–based’ and ‘risk–based’ monitoring approaches.

Dual role of titanium dioxide nanoparticles in the accumulation of inorganic and methyl mercury by crustacean Daphnia magna through waterborne and dietary exposure

Titanium dioxide nanoparticles (nTiO2) are widely used in numerous products, yet their role in the accumulation and transfer of other contaminants in the aquatic food webs is not well understood. The influence of nTiO2 on inorganic (IHg) and monomethyl mercury (MeHg) accumulation in invertebrate Daphnia magna through waterborne and dietary exposure was thus thoroughly investigated. The results showed that nTiO2 led to a substantial decrease of the total mercury body burden (THg) in D. magna in direct waterborne exposure to IHg/MeHg. However, exposure to nTiO2 pre-treated with IHg/MeHg resulted in an increase of the THg body burden in daphnids. The presence of nTiO2 led to a substantial decrease of the THg in D. magna when exposed to IHg/MeHg via algal food. These effects were more pronounced for IHg than that for MeHg due to the higher adsorption capabilities of nTiO2 for IHg. In addition, high concentrations of nTiO2 favored the trophic transfer of IHg/MeHg through feeding on nTiO2 pre-treated with Hg, however lessened it when D. magna were fed on alga pre-treated with IHg/MeHg. Comparable assimilation efficiency (AE), determined as Hg retained in daphnids after depuration, was observed in D. magna when exposed to IHg/MeHg via algal food regardless the absence or presence of 20 mgL−1 nTiO2. By contrast, an increase of the AE of MeHg through feeding on nTiO2 and alga was found in the presence of higher concentration of 200 mgL−1 nTiO2. The present results will help to better understand the role of nTiO2 on bioavailability and trophic transfer of global contaminants, such as mercury, known to bioaccumulate and biomagnify in the aquatic environment.

Elucidating the effect and mechanism of the brown coal-based amendment on plant availability of zinc, lead and cadmium in a Haplic Luvisols

Plants are a key link in the trophic chain and therefore may determine the global circulation of pollutants, including heavy metals (HMs). In the context of sustaining soil functions associated with food safety, the bioavailability of HMs should be reduced to a minimum needed for adequate plant nutrition. The objective of the study was to analyse the bioavailability of zinc, lead and cadmium in phacelia (Phacelia tanacetifolia Benth.) under conditions of varied soil pH and doses of brown coal-based organo-mineral amendment so-called the Rekulter. The experiment was carried out on Haplic Luvisols in field stone pots that sank into the ground, with the following HM content (in mg kg−1 of soil): 90.0 (Zn), 60.4 (Pb) and 0.80 (Cd). The Rekulter was applied to the soil in the amounts of 180, 360 and 720 g per pot. The bio-accumulation index (BI) was calculated as a ratio of a HM content in a plant to its total content in a soil sample, and it was used to evaluate bioavailability. The application of the Rekulter reduced the bioavailability of the studied heavy metals: the lowest BI values were found in the case of Pb. The uptake of HMs by phacelia was the smallest for the highest applied Rekulter dose at a soil pH of approximately 6.0. The bioavailability of Zn, Pb and Cd was influenced by soil pH and organic matter content, reducing their mobility and possible environmental risks. The Rekulter reduced HM bioavailability: the lowest bio-accumulation index (BI) values were found in the case of Pb. The application of the Rekulter into soil improved the physical, chemical and biological properties of soil, including the reduction of contaminant bioavailability.

An approach to remediation of a tropical aquifer contaminated with hydrocarbons

This paper deals about an approach to remediation for treating soils and groundwater contaminated with hydrocarbons in a tropical aquifer caused because of leaks from underground tanks in a gas station of Venezuela. Ex situ and in situ remediation treatment options were designed, recommending the ex situ remediation due to microbial inhibition produced by the presence of heavy metals and multiple strata with different soil materials in the interface between the unsaturated and saturated zones, where the most significant hydrocarbon concentrations were found. Under these conditions, ex situ bioremediation option based on two coupled bioreactors can overcome environmental limiting factors by optimizing temperature, oxygen supply, nutrient status, contaminant bioavailability, and microbial population. Ex situ remediation included for soil treatment, a Soil-Slurry Sequencing Batch Reactor (SS-SBR) and Upflow Anaerobic Filter separated in three phases (UAF-3SS). For groundwater treatment SBR-UAF-3SS sequence was established. The first phase was represented by SS-SBR for ex situ bioremediation of soils, configuring COD removal efficiency of 80%. It was set that SS-SBR performs functions for groundwater treatment operating as SBR, removing 60% COD. For the second phase, an upflow anaerobic filter separate in three phases (UAF-3SS) designed to ensure 99% COD removal efficiency.

Transport of chemotactic bacteria in granular media with randomly distributed chemoattractant-containing NAPL ganglia: Modeling and simulation

Chemotaxis is the biased movement of bacteria upon sensing chemical gradients, which can facilitate bioremediation of nonaqueous phase liquids (NAPLs) by transporting oil-degrading bacteria more efficiently to contaminants in groundwater aquifers. Transport phenomena of chemotactic bacteria in porous media can be described within the context of a convection-dispersion equation (CDE) that includes an additional convection-like chemotactic velocity. In this paper, we present a novel modification of a previously reported CDE by including a single first-order kinetic term which can differentiate bacterial sorption on porous matrix and chemoattractant-containing NAPL ganglia. Our model was solved numerically using a finite-element scheme to compute bacterial transport in granular media at a continuum level and also captured chemotaxis to discrete, randomly distributed NAPL ganglia of chemoattractant sources. Simulation results revealed the influence of oil ganglia on bacterial transport. It was found that chemotactic bacteria exhibited localized hotspots near NAPL ganglia because of chemotaxis toward and sorption on oil surfaces. The presence of oil ganglia reduced recovery of chemotactic bacteria by 71% and nonchemotactic bacteria by 44%, which accompanied enhanced retention inside the column. Model results suggested that chemotactic bacteria can locate NAPL and thereby sorb on surfaces more efficiently, which may overcome the limited bioavailability of contaminants dissolved in NAPLs. Simulated outputs demonstrated good agreement with experimental data in literature. Our model provides insight into the impact of chemotaxis and interaction with oil-phase chemoattractant sources on bacterial transport in porous media.

On-site evaluation of the effects of carbonaceous amendments on the bioavailability of aged organochlorine pesticide residues in soil

Reducing the bioavailability of persistent organic contaminants in soil by incorporating carbonaceous material (CM) has been investigated and confirmed by numerous laboratory studies. However, the efficacy of these methods under more complex field conditions needs exploration. We conducted an 18-month, small-scale plot study (n = 7) to evaluate the ability of two CMs, a compost aged for four months and a compost aged for two years, to reduce the bioavailability of the highly aged organochlorine pesticides (OCPs) dichlorodiphenyltrichloroethane (DDT), its metabolites (together as DDx), and dieldrin in soil. The study was carried out in a former orchard where OCPs were routinely applied until they were banned in the early 1970s. Soil and earthworms were collected, contaminant concentrations were measured, and bioaccumulation factors (BAF) of the OCPs were calculated for the 28 subplots at multiple timepoints. A decrease in the uptake and bioaccumulation of the OCPs was observed with increasing soil concentration, i.e., the relationship between soil and worm concentrations was non-linear. In addition, substantial spatial variability in soil and earthworm concentrations were observed, which resulted in variable BAFs across the site. The soil treated with CMs, 14 subplots in total, showed varying effects on bioaccumulation: for DDx 8 showed a reduction, 4 showed no difference, and 2 showed an increase; and for dieldrin 6 showed a reduction, 5 showed no difference, and 3 showed an increase. Although this study showed some evidence that CM amendments may reduce bioaccumulation over time, the results were not statistically significant due to the spatial and temporal variability. This work illustrates the challenges of extrapolating laboratory results to onsite determinations and suggests that more robust methods are needed in calculating ecological risk assessments for indigenous animals, particularly for legacy sites with substantial spatial variability.

Earthworm Depuration: Analysis of Coprophagy and Light Impacts

Earthworms are used as biomarkers to determine the bioavailability of contaminants. As such, their uptake of contaminants has been studied extensively. Protocols have been established to ensure that laboratory-obtained data are valid and comparable. However, the method of removing the organism’s gut content (depuration) before assessing the contaminant in the tissue is not standardized. The aim of this research project is to investigate some parameters for earthworm depuration: light conditions and coprophagy prevention. Eisenia fetida were depurated for 48 hours in two separate studies according to guidelines ASTM-E1676 and OECD Test No. 317. In one study, 2 frequencies of egesta removal were employed during depuration to prevent coprophagy and compared to the control (egesta and worms removed after 48 hours). In another study, the subjects and material egested were assessed under conditions of continuous darkness and the control (continuous light). The depuration methods that included egesta removal every 12 and 24 hours resulted in 62% and 10% more egested material per mg of earthworm than the control (filter paper disc change after 48 hours), respectively. The earthworms depurated in continuous darkness egested 94% more material per mg of earthworm than the control. The results indicate that depuration would be more total under continuous darkness and employing a coprophagy prevention method. These findings could lead to more efficient depuration methods.

Structure and Thermodynamics of Eu(III) and Cm(III) Complexes with Glucuronic Acid.

Complexation by small organic ligands controls the bioavailability of contaminants and influences their mobility in the geosphere. We have studied the interactions of Cm3+, as a representative of the trivalent actinides, and Eu3+, as an inactive homologue, with glucuronic acid (GlcA) a simple sugar acid. Time-resolved laser-induced luminescence spectroscopy (TRLFS) shows that complexation at pH 5.0 occurs only at high ligand to metal ratios in the form of 1:1 complexes with standard formation constants log β0 = 1.84 ± 0.22 for Eu3+ and log β0 = 2.39 ± 0.19 for Cm3+. A combination of NMR, QMMM, and TRLFS reveals the structure of the complex to be a half-sandwich structure wherein the ligand binds through its carboxylic group, the ring oxygen, and a hydroxyl group in addition to five to six water molecules. Surprisingly, Y3+, which was used as a diamagnetic reference in NMR, prefers a different coordination geometry with bonding through at least two hydroxyl groups on the opposite side of a distorted GlcA molecule. QMMM simulations indicate that the differences in stability among Cm, Eu, and Y are related to ring strain induced by smaller cations. At higher pH a stronger complex was detected, most likely due to deprotonation of a coordinating OH group.

Current Insights on Wastewater Treatment and Application of Spirulina platensis in Improving the Water Quality

Excessive generation of wastewater is one of the major reasons for pollution in natural reservoirs. Given the normal circumstances, natural water bodies revive and rejuvenate themselves; but upon increased waste load, the self-revival system of the ecosystem slows down, causing water pollution. Hazardous waste, especially heavy metals and organic pollutants, have affected the ecology to the detriment of humans. Thus, the need arises for wastewater treatment, before its discharge. Current methods undertaken include the use of physical settling of solid waste, filtration, aerobic and anaerobic microbes, and chemical treatments. Low removal of pathogens, dependence on the uninterrupted power supply, high maintenance cost, generation of explosive biogas and bioaccumulation of chemicals are some disadvantages of activated sludge technology, one of the modern technologies used. Hence, the focus has been shifted on organisms capable of metabolizing, immobilizing or absorbing toxic compounds from their environment, making it both environment-friendly and cost-effective. This review provides perspicacity about the generation of sewage and the various methods available for its treatment. Emphasis is made on bioremediation using Spirulina platensis. Since the organism assimilates the bioavailable contaminants of sewage water photosynthetically; it can overcome the demerits of conventional methods. It also discusses possibilities of using Spirulina grown on the sewage as a food supplement, animal fodder or source of bioactive compounds.

Application of a Multi-Metal Stable-Isotope-Enriched Bioassay to Assess Changes to Metal Bioavailability in Suspended Sediments.

The direct measurement of particulate contaminant bioavailability is a challenging aspect for the environmental risk assessment of contaminated sites. Here, we demonstrated a multi-metal stable-isotope-enriched bioassay to simultaneously measure the bioavailability of Cd, Cu, and Zn in naturally contaminated sediments following differing periods of resuspension treatment. Freshwater filter-feeding clams were pre-labeled with the isotopes 114Cd, 65Cu, and 68Zn to elevate isotope abundances in their tissues and then exposed to metal-contaminated suspended sediments. The assimilation of sediment-associated metals by clams would decrease the isotope ratios (Cd114/111, Cu65/63, and Zn68/64) in tissues, providing a direct measurement of metal bioavailability. For the sediments tested here, the method revealed bioavailable cadmium and non-bioavailable copper in sediments but was inconclusive for zinc. With a longer resuspension time, the bioavailability of particulate cadmium increased, but that of copper was unaffected. Metal bioavailability predicted using traditional wet-chemical extraction methods was inconsistent with these findings. The study indicated that multi-metal stable-isotope-enriched bioassay provides a new tool for directly assessing metal bioavailability in sediments, and this method is amenable for use in in situ assessments.

Determining the bioavailability of benzo(a)pyrene through standardized desorption extraction in a certified reference contaminated soil

There is a strong need for certified reference materials in the quality assurance of nonionic soil contaminant bioavailability estimations through physicochemical methods. We applied desorption extraction, a method recently standardized as ISO16751, to determine the bioavailable concentration of the most commonly regulated polycyclic aromatic hydrocarbon (PAH), benzo(a)pyrene (BaP), in the reference industrial soil BCR-524 with a certified BaP total concentration of 8.60 mg kg−1. This concentration represented BaP levels found in many PAH-polluted soils. The method, based on single-point extraction of the analyte desorbed into the aqueous phase by a receiving phase (Tenax or cyclodextrin), was applied ten times. The data fulfilled highly demanding quality criteria based on recovery and repeatability. The bioavailable BaP concentration detected through Tenax extraction, 1.82 mg kg−1, was comparable to bioavailable concentrations determined in field-contaminated soils and to environmental quality standards based on previously observed total BaP concentrations. There was good agreement (Student's t-test, P ≤ 0.05) with the bioavailable BaP concentration determined by cyclodextrin extraction (1.53 mg kg−1). The methods were extended to four other certified 4- and 5-ringed PAHs for comparative purposes. We suggest ways of improving of the ISO16751 standard related to further systematic assessment of the Tenax-to-soil ratio and incorporation of mass balances. Furthermore, BCR-524 is suitable for quality-assurance protocols with these methods when used in site-specific risk assessments of PAH-polluted environments.

Quantification of multi-scale links of anthropogenic pressures with PAH and PCB bioavailable contamination in French freshwaters

Aquatic ecosystems are exposed to multiple environmental pressures including chemical contamination. Polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) are persistent organic pollutants (POPs) known as preoccupying substances for the environment. Active biomonitoring (ABM) is a surveillance method for polluted aquatic ecosystems measuring bioavailable contamination. In this work, the aim was to quantify the total links between environmental pressures and bioavailable contamination (for PAHs and PCBs) at the French national scale. Based on 245 sites experimented by ABM from 2017 to 2019, environmental pressures (anthropogenic pressures and environmental parameters) were defined (point source landfill density, point source urban density, point source industry density, point source road density, nonpoint source industry density, nonpoint source road density, nonpoint source urban density, nutrients and organic matter, slope, dams, straightness, coarse sediment, summer precipitation, hydrographic network density and watershed size) and characterized by one or a combination of measures called stressor indicators. The links between environmental pressures and bioavailable POPs contamination (ABM measure) at a large spatial scale were defined and quantified via structural equation modeling. Point source urban density, nutrients and organic matter, summer precipitation, straightness and point source industry density are correlated positively with PAH bioavailable contamination. In contrast, nonpoint source urban density, nonpoint source industry density, nonpoint source road density and watershed size are positively correlated with PCB bioavailable contamination. The dominant pressures linked to PAHs and PCBs were different, respectively local and large-scale pressures were linked to PAH bioavailable contamination, and only large-scale pressures were linked to PCB bioavailable contamination.

Coupling polydimethylsiloxane vials with a physiologically based extraction test to predict bioavailability of hydrophobic organic contaminants in soils

As alternatives to in vivo assays, physiologically based in vitro methods have been developed to measure bioaccessibility of hydrophobic organic contaminants (HOCs) in soils. However, bioaccessibility can usually be underestimated since in vitro tests fail to provide sufficient affinity for HOCs. Sorption sink was therefore included to simulate intestinal cell absorption and to promote the mobilization of HOCs from soils. In this study, polydimethylsiloxane (PDMS) vials, widely used as passive dosing, were introduced as a sorption sink to improve the performance of physiologically based extraction test (PBET). The bioaccessibility of PCBs (representatives of HOCs) in 13 lab-spiked soils measured by PBET coupled with PDMS vials ranged from 56.5 ± 2.7% to 109.3 ± 1.5%. Correlation was conducted between the bioaccessibility and relative bioavailability (RBA) of PCBs assessed using an in vivo mouse model. A significant correlation (p < 0.001, R2 = 0.72, slope = 0.85 ± 0.16) was observed between in vitro and in vivo data, indicating that the proposed method here can be a robust in vitro method to predict PCB RBA in soils. The accuracy of this novel method was further shown by extracting one field contaminated soil with environmental relevant levels of PCBs. The relative standard deviation of bioaccessibility measured by PBET with PDMS vials was 1.2–9.8%, and much lower than those by PBET alone with values of 17.1–63.6%. In addition, the PDMS vials can be reusable as sorption sink, and no significant variation (p = 0.44) in PCB bioaccessibility was observed among 5 cycles of extracting soils with PBET coupled with PDMS vials. Due to the high sorption capacity of PDMS and flexibility of PDMS mass used for vials, the novel method here is expected to be applicable in soils with a wide range of contamination levels.

Bioavailability quantification and uptake mechanisms of pyrene associated with different-sized microplastics to Daphnia magna

Microplastics (MPs) are the significant environmental factor for bioavailability of hydrophobic organic contaminants (HOCs) in aquatic environments. Nevertheless, the bioavailability of microplastic-associated HOCs remains unclear. In this research, the freely dissolved pyrene concentrations were kept stable with passive dosing devices, and the pyrene content in D. magna tissues as well as D. magna immobilization were analyzed to quantify bioavailability of pyrene (a representative HOC) associated with naturally-aged polystyrene (PS) MPs. Furthermore, the uptake mechanisms of pyrene associated with MPs of different sizes were explored by investigating the distribution of MPs in D. magna tissues with scanning electron microscopy. Especially, a new schematic model of bioavailability process was established. The results demonstrated that a part of pyrene associated with 0–1.5 μm MPs could directly cross cell membrane through endocytosis from intestine and exposure solutions to D. magna tissues except the 10–60 and 60–230 μm MPs. The bioavailability of microplastic-associated pyrene was ordered as 0–1.5 μm (20.0–21.6%) > 10–60 μm (10.7–13.8%) > 60–230 μm MPs (6.0–9.8%), which were essentially resulted from the difference in uptake mechanisms of pyrene associated with MPs of different sizes. This work suggests that the bioavailability of microplastic-associated HOCs should be considered when assessing water quality and environmental risk of HOCs in natural waters.

Research Progress of in Vitro Models for Bioaccessibility and Bioavailability and Their Applications in Food Contaminants Exposure Assessment

With the new generation of toxicity test method technology feasible in different fields, its application in food safety risk assessment further improves the accuracy of exposure assessment. The key correction factors introduced in the process of food assessment, such as bioaccessibility and bioavailability of contaminants, can provide more precise essential data for food safety management, including food safety control and food safety standard establishment and revision. Accordingly, this paper reviews and compares various in vitro alternative models and their applications for bioaccessibility and bioavailability in food risk assessment of chemical contaminants, and presents an outlook on the future of new methods and technologies, to provide a methodological reference for accurate assessment of the exposure of chemical contaminants via dietary.

Effects of carbonaceous materials and particle size on oral and inhalation bioaccessibility of PAHs and OPEs in airborne particles.

Bioavailability of environmental contaminants is attracting considerable scientific attention due to growing awareness of its importance for risk assessment. In this study, size-segregated airborne particles were collected from six point-source sites, an urban residential site, and a sub-urban site. Potential factors governing bioaccessibility of the particle-bound polycyclic aromatic hydrocarbons (PAHs) and organophosphorus esters (OPEs) in stimulated gastrointestinal and respiratory tracts were elucidated. Particle concentrations of PAHs and OPEs at the eight sites were 2.4-32.3 ng/m3 and 1.6-19.9 ng/m3, respectively. In fine particles (with aerodynamic diameter less than 2.5 μm), 4- to 6-ring PAHs were more strongly correlated with organic carbon (OC) than elemental carbon (EC); while 3- and 4-ring PAHs in coarse particles (2.5-10 μm) tended to associate with EC. OPEs mostly showed significant correlations with EC in both fine and coarse particles. OC and EC exerted a significantly restraining effect on the oral and inhalation bioaccessibility of most hydrophobic organic contaminants (HOCs) in fine particles due to sorption of HOC molecules to these components. Furthermore, the effects varied, which could depend either on the emission sources (for oral bioaccessibility of PAHs) or the physicochemical properties of HOCs (for bioaccessibility of OPEs and inhalation bioaccessibility of PAHs). Linear regression between OC/EC contents and HOC bioaccessibility indicated that EC should play a more important role in the inhalation bioaccessibility than the oral bioaccessibility. Particle size of airborne particles is a relatively less significant factor determining the bioaccessibility.

Time-dependent desorption of anilines, phenols, and nitrobenzenes from biochar produced at 700 °C: Insight into desorption hysteresis

Biochar, once amended or released into the soil environment, may alter the transport, fate, and bioavailability of organic contaminants in the environment by adsorption and desorption. In this study, adsorption–desorption of aniline, 4-nitroaniline, phenol, 4-nitrophenol, nitrobenzene, and trinitrobenzene on biochars produced at 700 °C from 5 biomass including wood chips, chitin, cellulose, lignin, and rice straw (i.e., W700, Ch700, Ce700, Li700, and S700, respectively) was investigated to examine the underlying mechanisms for desorption hysteresis of organic contaminants. The desorption hysteresis was dependent on the adsorption–desorption time and should be attributed to the nonequilibrium adsorption–desorption, i.e., it was apparent and an artifact. Moreover, within 2-day equilibration, the desorption hysteresis was observed for trinitrobenzene (with the maximum cross-sectional area of 0.890 nm2) but not for the other organic contaminants with smaller sizes (the maximum cross-sectional area ≤ 0.611 nm2). Similarly, trinitrobenzene had a shorter equilibrium time on Ch700 (15 days) than on Ce700, Li700, and S700 (>30 days) because of the larger pores of Ch700. Diffusion rates (kip2) of trinitrobenzene on biochars were positively correlated with the Raman spectra ID/IG values of biochars (R2 = 0.928) because higher ID/IG values are possibly associated with the looser structure of biochars which having faster diffusion of molecules in pores. The time-dependent desorption hysteresis suggests that the adsorbed organic contaminants by biochars can be re-released into the soil environment for a long period. Thus, the long-term releasing of adsorbed organic contaminants on biochars should be investigated more for assessing the environmental risks of organic contaminants.

Photochemical Reactivity of Humic Substances in an Aquatic System Revealed by Excitation-Emission Matrix Fluorescence.

The photochemical reactivity of humic substances plays a critical role in the global carbon cycle, and influences the toxicity, mobility, and bioavailability of contaminants by altering their molecular structure and the mineralization of organic carbon to CO2. Here, we examined the simulated irradiation process of Chinese standard fulvic acid (FA) and humic acid (HA) by using excitation-emission matrix fluorescence combined with fluorescence regional integration (FRI), parallel factor (PARAFAC) analysis, and kinetic models. Humic-like and fulvic-like materials were the main materials (constituting more than 90%) of both FA and HA, according to the FRI analysis. Four components were identified by the PARAFAC analysis: fulvic-like components composed of both carboxylic-like and phenolic-like chromophores (C1), terrestrial humic-like components primarily composed of carboxylic-like chromophores (C2), microbial humic-like overwhelming composed of phenolic-like fluorophores (C3), and protein-like components (C4). After irradiation for 72 h, the maximum fluorescence intensity (F max) of C1 and C2 of FA was reduced to 36.01–58.34%, while the F max of C3 of both FA and HA also decreased to 0–9.63%. By contrast, for HA, the F max of its C1 and C2 increased to 236.18–294.77% when irradiated for 72 h due to greater aromaticity and photorefractive tendencies. The first-order kinetic model (R 2 = 0.908–0.990) fitted better than zero-order kinetic model (R 2 = 0–0.754) for the C1, C2, and C3, of both FA and HA, during their photochemical reactivity. The photodegradation rate constant (k 1) of C1 had values (0.105 for FA; 0.154 for HA) that surpassed those of C2 (0.059 for FA, 0.079 for HA) and C3 (0.079 for both FA and HA) based on the first-order kinetic model. The half-life times of C1, C2, and C3 ranged from 6.61–11.77 h to 4.50–8.81 h for FA and HA, respectively. Combining an excitation-emission matrix with FRI and PARAFAC analyses is a powerful approach for elucidating changes to humic substances during their irradiation, which is helpful for predicting the environmental toxicity of contaminants in natural ecosystems.