Predicting apparent adsorption capacity of sediment-amended activated carbon for hydrophobic organic contaminants using machine learning

In-situ stabilization of hydrophobic organic compounds (HOCs) using activated carbon (AC) is a promising sediment remediation approach. However, predicting HOC adsorption capacity of sediment-amended AC remains a challenge because a prediction model is currently unavailable. Thus, the objective of this study was to develop machine learning models that could predict the apparent adsorption capacity of sediment-amended AC (KAC,apparent) for HOCs. These models were trained using 186 sets of experimental data obtained from the literature. The best-performing model among those employing various model frameworks, machine learning algorithms, and combination of candidate input features excellently predicted logKAC,apparent with a coefficient of determination of 0.94 on the test dataset. Its prediction results and experimental data for KAC,apparent agreed within 0.5 log units with few exceptions. Analysis of feature importance for the machine learning model revealed that KAC,apparent was strongly correlated with the hydrophobicity of HOCs and the particle size of AC, which agreed well with the current knowledge obtained from experimental and mechanistic assessments. On the other hand, correlation of KAC,apparent to sediment characteristics, duration of AC-sediment contact, and AC dose identified in the model disagreed with relevant arguments made in the literature, calling for further assessment in this subject. This study highlights the promising capability of machine learning in predicting adsorption capacity of AC in complex systems. It offers unique insights into the influence of model parameters on KAC,apparent.

Filtration of biopolymer PHB particles loaded with synthetic musks does not cause significant bioaccumulation in marine mussels

The role of the biopolymer polyhydroxybutyrate (PHB, <250 µm) as a vehicle of a synthetic musks mixture (celestolide, galaxolide, tonalide, musk xylene, musk moskene and musk ketone) to Mytilus galloprovincialis was investigated. For 30 days, virgin PHB, virgin PHB+musks (6.82 µg g-1) and weathered PHB+musks, were daily spiked into tanks containing mussels, followed by a 10-day depuration period. Water and tissues samples were collected to measure exposure concentrations and accumulation in tissues. Mussels were able to actively filter microplastics in suspension but the concentration of the musks found in tissues (celestolide, galaxolide, tonalide) were markedly lower than the spiked concentration. Estimated Trophic Transfer Factors suggest that PHB will only play a minor role on musks accumulation in marine mussels, even if our results suggest a slightly extended persistence in tissues of musks loaded to weathered PHB.

Evaluation of the use of biochar to stabilize polycyclic aromatic hydrocarbons and phthalates in sediment

Three types of biochar (BC) (mulberry biochar (MB), wheat straw biochar, and pine tree sawdust biochar) were prepared and used to stabilize hydrophobic organic compounds (HOCs) in contaminated sediment. The kinetics of HOC adsorption to the BCs had two distinct stages. The second stage adsorption process was longer for MB than the other BCs, presumably because MB contained large pores, mesopores, and micropores. The adsorption isotherms for the three BCs were described well by the Freundlich model. The adsorption capacities of MB, WS and PT for HOCs ranged between 106.7 and 1202 μg/g, 135.1 and 1002 μg/g, and 255.6 and 909 μg/g, respectively. The apparent HOC adsorption coefficients (K_BC-w) for the three BCs were determined from the isotherm data and were similar. The HOC logK_OW values correlated well with the logK_BC-w values. In sediment slurry experiments, HOCs were much more effectively stabilized by MB than wheat straw and pine tree sawdust biochar. This was probably because of the MB pore characteristics that favored adsorption of HOCs of various molecular sizes. The Fourier-transform infrared and Raman spectra indicated that the main binding mechanisms were hydrogen boding, hydrophobic interactions, and π-π interactions. MB was found to be a possible agent for stabilizing HOCs in contaminated sediment. HOCs in sediment slurry continued to become adsorbed to MB for a long time, indicating that relatively long reaction times should be allowed for in situ remediation using MB.

Promotion of the biodegradation of phenanthrene adsorbed on microplastics by the functional bacterial consortium QY1 in the presence of humic acid: Bioavailability and toxicity evaluation

The adsorption of hydrophobic organic compounds (HOCs) by microplastics (MPs) has attracted great attention in recent years. However, the ultimate environmental fate of the HOCs sorbed on MPs (HOCs-MPs) is poorly understood. In this work, we investigated the potential influence of the biotransformation process on the environmental fate of phenanthrene (PHE, a model HOC) sorbed on MPs (PHE-MPs) under the existence of humic acid (HA, the main ingredient of dissolved organic matter (DOM)) in the aquatic environment. The results indicated that the adsorption behavior of PHE on MPs decreased its bioavailability and thus inhibited its biotransformation efficiency. However, HA significantly promoted the biodegradation rate and percentage of PHE-MPs. This was probably because HA improved the desorption of PHE from MPs, which promoted the acquisition of PHE by bacteria from the aqueous phase. Further, HA dramatically increased the bacterial community diversity and richness and altered the community composition. The richness of some PHE-degrading bacteria, such as Methylobacillus and Sphingomonas, significantly increased, which may also be an important factor for promoting PHE biodegradation. Molecular ecological network analysis implied that HA enhanced the modularity and complexity of bacterial interaction networks, which was beneficial to maintaining the functional stability of the consortium QY1. Besides, HA decreased the cytotoxicity of functional microbes induced by HOCs-MPs. This work broadens our knowledge of the environmental fate of HOCs-MPs and interactions of MPs, HOCs, DOMs and functional microbial consortiums in aqueous environments.

Microplastics and associated organic pollutants in beach sediments from the Gulf of Guinea (SE Atlantic) coastal ecosystems

Microplastics (MPs) are emerging pollutants of global concern due to their pervasiveness, sorptive capacity for organic and inorganic pollutants, and direct and indirect toxicity to organisms and ecosystems. This study aimed to assess the concentration and the statistical difference in the concentration of microplastic-sorbed organic pollutants from two ecosystems, the marine and estuarine lagoon. Surface sediment from the estuarine lagoon and marine ecosystems were sampled for microplastics (1-5 mm). A total of 3680 MP particles were collected. The plastics were analyzed for polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs). ∑PAHs, ∑PCBs and ∑OCPs were between 0.00 and 0.32 mg/kg, 0.00-0.53 mg/kg and 0.04-2.02 mg/kg, respectively. The results showed a correlation of -0.2, 0.8 and 0.2 between the number of MPs and the concentration of PAHs, PCBs and OCPs, respectively, suggesting that the potential risk of accumulation of plastic-sorbed PCBs is higher than those of OCPs and PAHs. Mann Whitney U test (at 95% confidence level) indicated no statistical difference in the concentration of organic pollutants in lagoon and beach MPs. In addition, we found no significant difference in the accumulation of organic pollutants in MPs from beach drift and high waterlines. The result suggests that the concentration of microplastics-sorbed organic pollutants in both ecosystems is comparable and likely to pose similar potential risks. We recommend that plastic pollution in all ecosystems require attention.

Performance comparison of silicone and low-density polyethylene as passive samplers in a global monitoring network for aquatic organic contaminants

Contamination with hydrophobic organic compounds (HOCs) such as persistent organic pollutants negatively affects global water quality. Accurate and globally comparable monitoring data are required to understand better the HOCs distribution and environmental fate. We present the first results of a proof-of-concept global monitoring campaign, the Aquatic Global Passive Sampling initiative (AQUA-GAPS), performed between 2016 and 2020, for assessing trends of freely dissolved HOC concentrations in global surface waters. One of the pilot campaign aims was to compare performance characteristics of silicone (SSP) and low-density polyethylene (PE) sheets co-deployed in parallel under identical conditions, i.e. at the same site, using the same deployment design, and for an equal period. Individual exposures lasted between 36 and 400 days, and samples were collected from 22 freshwater and 40 marine locations. The sampler inter-comparability is based on a rationale of common underlying principles, i.e. HOC diffusion through a water boundary layer (WBL) and absorption by the polymer. In the integrative uptake phase, equal surface-specific uptake in both samplers was observed for HOCs with a molecular volume less than 300 Å³. For those HOCs, transport in the WBL controls the uptake as mass transfer in the polymer is over 20-times faster. In such a case, sampled HOC mass can be converted into aqueous concentrations using available models derived for WBL-controlled sampling using performance reference compounds. In contrast, for larger molecules, surface-specific uptake to PE was lower than to SSP. Diffusion in PE is slower than in SSP, and it is likely that for large molecules, diffusion in PE limits the transport from water to the sampler, complicating the interpretation. Although both samplers provided mostly well comparable results, we recommend, based on simpler practical handling, simpler data interpretation, and better availability of reliable polymer-water partition coefficients, silicone-based samplers for future operation in the worldwide monitoring programme.

Prediction of hydrophobic organic compound partition to algal organic matter through the growth cycle of Microcystis aeruginosa

Algal organic matter (AOM) is an important source for the dissolved organic matter (DOM) pool in aquatic systems, particularly in eutrophic waters. In this study, we reported the dynamic pattern of AOM hydrophobicity during the growth cycle of Microcystis aeruginosa using the partition coefficients of AOM in the aqueous two-phase system (K_ATPS) as a simple quantitative measure. AOM hydrophobicity had significant and non-monotonic changes during the growth cycle. It increased in the lag and early exponential phases, then decreased in the late exponential and stationary phases, and rebounded in the decline phase. AOM hydrophobicity determined using the resin fractionation, SUVA₂₅₄, and nuclear magnetic resonance methods shared similar non-monotonic pattern. Nevertheless, the correlations among these indicators were poor. The partition behavior of polycyclic aromatic hydrocarbons and chlorobenzenes to AOM was assessed based the K_ATPS dataset and the two-phase system (TPS) model. The TPS model showed good prediction power for the partition behavior of AOM with an RMSE of 0.23, suggesting that it was applicable to AOM from Microcystis aeruginosa. Our results indicate that algae activity will influence the overall hydrophobicity of the DOM pool depending on the growth phase, resulting in changes in the bioavailability of hydrophobic organic compounds in aquatic systems.

Toxicokinetics of hydrophobic organic compounds in oligochaeta: A critical review

Toxicokinetic studies appertain to the fundamental research of soil bioavailability. However, the research outcomes of aspects influencing uptake and elimination of hydrophobic organic compounds have not been summarized so far. In our review, a recapitulation of available toxicokinetic data (i.e. experimental conditions, if the steady state was reached, uptake and elimination rate constants, and bioaccumulation factors) is presented in well-arranged tables. Further, toxicokinetic models are overviewed in the schematic form. In the review, the required information could be quickly found and/or the experimental gaps easily identified. Generally a little is known about the effects of soil properties other than soil organic matter. Limited or no data are available about soil treatment, food supply during laboratory exposure, and metabolization in oligochaeta. The impact of these factors might be important especially for arable soils with typically low organic matter content but high consequences on humans. Besides these circumstances, other uncertainties between published studies have been found. Firstly, the scientific results are provided in heterogenous units: bioaccumulation factors as well as the rate constants are reported in dry or wet weight of soil and earthworms. The steady state is another critical factor because the time to reach the equilibrium is influenced not only by soil and compound characteristics but for example also by aging. Nevertheless, toxicokinetic studies bring irreplaceable information about the real situation in soil and our review help to define missing knowledge and estimate the scientific priorities.

Can the hydrophobic organic contaminants in the filtrate passing through 0.45 μm filter membranes reflect the water quality?

In the traditional water quality assessment, the concentration of total dissolved hydrophobic organic compounds (HOCs) passing through 0.45 μm filter membranes is usually used to evaluate the influence of HOCs on water quality. However, the bioavailability of dissolved organic matter (DOM)-associated and particle-associated HOCs is not considered. In the present work, pyrene, fulvic acid, and natural suspended particles (SPS) were used to simulate natural water (raw water). The immobilization and pyrene content in the tissues of D. magna caused by total pyrene in the raw water and those caused by freely dissolved pyrene with the concentration equal to the total dissolved pyrene in the filtrate of raw water were compared to determine whether the total dissolved pyrene concentration can reflect the water quality. The results indicated that when the DOM concentration was 5 mg C L^-1 and the SPS concentration was higher than 0.2-0.4 g L^-1, the bioavailability of pyrene was underestimated by the traditional water quality assessment because of the SPS-associated pyrene, and it was underestimated by 23.6-63.9% when SPS concentration was higher than 0.6 g L^-1 due to the neglection of SPS-associated pyrene. Furthermore, the threshold value of SPS concentration was related to the SPS size and composition, and the effects of SPS and DOM on water quality were influenced by the concentration, size, and composition of SPS as well as the molecular weight of DOM. This study suggests that the traditional water quality assessment should be improved by comprehensively considering concentrations and characteristics of SPS and DOM.

Application of equilibrium passive sampling to profile pore water and accessible concentrations of hydrophobic organic contaminants in Danube sediments

Total concentrations of hydrophobic organic contaminants (HOCs) in sediment present a poor quality assessment parameter for aquatic organism exposure and environmental risk because they do not reflect contaminant bioavailability. The bioavailability issue of HOCs in sediments can be addressed by application of multi-ratio equilibrium passive sampling (EPS). In this study, riverbed sediment samples were collected during the Joint Danube Survey at 9 locations along the Danube River in 2013. Samples were ex-situ equilibrated with silicone passive samplers. Desorption isotherms were constructed, yielding two endpoints: pore water (C_W:0) and accessible (C_AS:0) concentration of polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polycyclic aromatic hydrocarbons (PAHs) and polybrominated diphenyl ethers in sediment. C_W:0 concentrations of DDT and its breakdown products exhibited elevated levels in the low Danube, with the maximum in the river delta. Other investigated HOCs did not show any clear spatial trends along the river, and only a moderate C_W:0 variability. C_AS:0 in sediment ranged from 10 to 90% of the total concentration in sediment. C_W:0 was compared with freely dissolved concentration in the overlaying surface water, measured likewise by passive sampling. The comparison indicated potential compound release from sediment to the water phase for PAHs with less than four aromatic rings, and for remaining HOCs either equilibrium between sediment and water, or potential compound deposition in sediment. Sorption partition coefficients of HOC to organic carbon correlated well with octanol-water partition coefficients (K_OW), showing stronger sorption of PAHs to sediment than that of PCBs and OCPs having equal logK_OW. Comparison of C_W:0 values with European environmental quality standards indicated potential exceedance for hexachlorobenzene, fluoranthene and benzo[a]pyrene at several sites. The study demonstrates the utility of passive sampling as an innovative approach for risk-oriented monitoring of HOCs in river catchments.

Impact of passive sampler protection apparatus on sediment porewater profiles of hydrophobic organic compounds

Passive sampling techniques have been widely used to determine the dissolved concentration profiles of hydrophobic organic compounds (HOCs) in sediment porewater. However, the effects of having a protection for the passive sampler on profiling HOCs concentrations in sediment porewater, especially in deep sediment, have remained unclear. To address this issue, low density polyethylene passive samplers with and without protectors, which consisted of glass fiber filter and porous stainless steel shield, were simultaneously deployed in sediment of the Dongjiang River, South China. The results showed that the protectors retarded the dissipation of performance reference compounds (PRCs) from the sampler by a factor of 2-9. The protectors seemed to exert a negligible effect on the measured concentrations of PAHs, BDE-47, and BDE-99 in surficial sediment porewater (0-14 cm depth) from both samplers. However, the sediment porewater concentration profiles of PAHs and BDE-47 from the sampler with protectors were in agreement with those normalized by dry weight in deep sediment (16-34 cm depth), indicating that a diffusion layer established by the protectors may minimize the probability of local depletion of the target analytes in deep sediment. In addition, the log K_oc values of PAHs, BDE-47, and BDE-99 exhibited a slight increasing trend with sediment depth. This finding suggested that in situ passive sampling techniques could be a feasible tool in determining the site-specific log K_oc values of HOCs at different sediment depths.

Effect of suspended particles with different grain sizes on the bioaccumulation of PAHs by zebrafish (Danio rerio)

The physicochemical characteristics are different for suspended particles (SPS) with different sizes in rivers. Here, we studied the effect of SPS (1 g L^-1) with three different sizes (63-106 μm, 20-63 μm, and <20 μm) on the bioaccumulation of deuterated polycyclic aromatic hydrocarbons (phenanthrene-d10, anthracene-d10, fluoranthene-d10 and pyrene-d10) in zebrafish using passive dosing devices to maintain the freely dissolved concentrations of PAHs-d10 constant in water. The results showed that all the three grain size SPS could be ingested by zebrafish, and there was no significant difference in the amount of ingested SPS among the three grain sizes. The concentrations (lipid-normalized or not) of PAHs-d10 in zebrafish were promoted in the presence of the three different size SPS, and the PAH-d10 concentrations in zebrafish increased with decreasing particle size. Compared with the systems without SPS, the lipid-normalized concentrations of PAHs-d10 increased by 12%-72%, 34%-130%, and 60%-196%, respectively in zebrafish in systems with 63-106 μm, 20-63 μm, and <20 μm of SPS after exposure for 20 h. The stronger effect of SPS with smaller grain sizes was probably due to their lower organic carbon content, lower ratio of black carbon to organic carbon content, smaller particle size, and higher dissolved organic matter contents, which could promote the desorption of PAHs-d10 from ingested SPS and bioaccessibility of PAHs-d10 to zebrafish. This study suggests that in addition to SPS concentration, the suspended particle size should be considered in ecological risk assessment for hydrophobic organic compounds in aquatic environment.

Heterogeneous Fenton-like catalyst for treatment of rhamnolipid-solubilized hexadecane wastewater

The treatment of wastewater containing hydrophobic organic pollutants solubilized by surfactants is of great environmental importance. In this work, the removal of rhamnolipid-solubilized hexadecane via a salicylic acid-methanol-acetone modified steel converter slag (SMA-SCS) catalyzed Fenton-like process was studied. First, we investigated the adsorption of rhamnolipid and hexadecane onto SCS and SMA-modified SCS. Compared to that of SCS, SMA-SCS exhibited better adsorption performance with maximum adsorption capacities of 0.23 and 0.28 mg/g for hexadecane and rhamnolipid, respectively. Degradation experiments showed that hexadecane was more readily degraded by the Fenton-like process than rhamnolipid. Up to 81.1% of hexadecane removal was achieved over 20 g/L of SMA-SCS within 24 h, whereas only 36% of rhamnolipid was degraded. On the other hand, the results indicated that increased rhamnolipid concentration had a negative effect on the degradation of hexadecane. During the oxidation reaction, the pH value of solution remained between 6.0 and 6.72. All these results demonstrated that the SMA-SCS/H₂O₂ Fenton-like process could be a cost-effective and promising approach for the treatment of surfactant-solubilized hydrophobic organic compounds.

Evaluation of dimethyl sulfoxide (DMSO) as a co-solvent for toxicity testing of hydrophobic organic compounds

Toxicity testing of hydrophobic compounds with low aqueous solubility remains challenging. Dimethyl sulfoxide (DMSO) is widely used as a co-solvent for toxicity testing of hydrophobic chemicals, but it may modulate chemical toxicity patterns. In this study, we critically evaluated the suitability of DMSO as a co-solvent for toxicity testing of hydrophobic organic compounds in aqueous solutions. As the toxicity measure, we used growth inhibition of a natural bacterial community, and the test toxicants included phenol, BTEX (benzene, toluene, ethylbenzene and xylene) and transformation products of polycyclic aromatic hydrocarbons (PAHs). We found that dose-response curves for phenol were unaffected by DMSO concentrations up to 10% (v/v) and that DMSO (5% v/v) did not affect the degree of bacterial growth inhibition for any of the other test compounds in short-term experiments (3.5 h). By contrast, marked co-solvent effects of DMSO were observed in the long-term assay (25 and 27 h). We therefore conclude that DMSO has excellent co-solvent properties for short-term (≤3.5 h) toxicity testing of sparingly water-soluble compounds and its application provides a simple, inexpensive approach for screening of various environmentally relevant hydrophobic chemicals. Importantly, the use of DMSO allows for generation of full dose-responses that may otherwise not be attained.

Microplastic contamination and pollutant levels in mussels and cockles collected along the channel coasts

Nowadays, environmental pollution by microplastics (<5 mm; MP) is a major issue. MP are contaminating marine organisms consumed by humans. This work studied MP contamination in two bivalve species of commercial interest: blue mussel (Mytilus edulis) and common cockle (Cerastoderma edule) sampled on the Channel coastlines (France). In parallel, 13 plastic additives and 27 hydrophobic organic compounds (HOC) were quantified in bivalves flesh using SBSE-TD-GS-MS/MS to explore a possible relationship between their concentrations and MP contamination levels. MP were extracted using a 10% potassium hydroxide digestion method then identified by μ-Raman spectroscopy. The proportion of contaminated bivalves by MP ranged from 34 to 58%. Blue mussels and common cockles exhibited 0.76 ± 0.40 and 2.46 ± 1.16 MP/individual and between 0.15 ± 0.06 and 0.74 ± 0.35 MP/g of tissue wet weight. Some HOC and plastic additives were detected in bivalves. However, no significant Pearson or Spearman correlation was found between MP loads and plastic additives or HOC concentrations in bivalve tissues for the two species.

SSP silicone-, lipid- and SPMD-water partition coefficients of seventy hydrophobic organic contaminants and evaluation of the water concentration calculator for SPMD

Passive sampling is increasingly applied for monitoring neutral hydrophobic compounds (HOC) in various environmental media like water, sediment, air and also soft biota tissue. Passive samplers for HOC are often constructed from permeable polymers like silicone and polyethylene (PE), while also SPMD are often applied. Their HOC uptake can be converted to freely dissolved or equivalent lipid-based concentrations using appropriate partition coefficients with or without the use of kinetic uptake models to adjust for non-equilibrium. To facilitate such conversions for seventy HOC partition coefficients are derived by combining polymer-water for Altesil™ silicone and PE, with new and earlier published polymer-polymer, polymer-lipid partition coefficients. Derived SSP silicone-water, lipid-water (K_lip/w), and SPMD-water (K_spmd/w) partition coefficients demonstrate good agreement with literature data, except for K_spmd/w. For SPMD, this work demonstrates a linear K_spmd/w - K_ow relationship (R² = 0.99) in contrast to the parabolic K_spmd/w - K_ow relationship utilized in the USGS "SPMD Water Concentrations Calculator". Following a thorough evaluation of this Calculator it is recommended that in combination with revised K_spmd/w, a radical different model approach should be used for obtaining accurate water concentrations from passive sampling with SPMD.

Spatial distribution and source identification of hydrophobic organic compounds (HOCs) on sedimentary microplastic in Hong Kong

The spatial distribution, composition and source of hydrophobic organic compounds (HOCs) including polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorinated pesticides (OCPs) of the sedimentary microplastics (0.25-5 mm) in Hong Kong were investigated. The concentration of ΣPAHs ranged between 70.8 and 1509 ng g^-1 with inter-site differences although the regional difference was insignificant, indicating localised pyrolytic and petrogenic input of PAHs. The concentration of ΣPCBs (13-1083 ng g^-1) varied with both study sites and regions with higher concentrations obtained in the western waters, possibly due to the input from Pearl River. Significantly higher concentrations of OCPs on eastern shores highlighted fishing and aquaculture activities in South China Sea a potential major source of OCPs. DDT and its metabolites (DDX, ranged from 1.96 to 626 ng g^-1) were the dominant forms of OCPs (45%-80%). Since most of the DDX existed as DDT, this suggested that there was a fresh input of DDT into the microplastics. As microplastics and HOCs cannot be removed effectively from the environment, reduction of potential ecotoxicological risks should rely on minimizing the use of plastics and HOCs.

Silicone-water partition coefficients determined by cosolvent method for chlorinated pesticides, musks, organo phosphates, phthalates and more

To further support implementation of monitoring by passive sampling, robust sampler-water partition coefficients (K_pw) are required to convert data from passive sampler into aqueous phase concentrations. In this work silicone-water partition coefficients were determined for ∼80 hydrophobic organic contaminants using the cosolvent method. Partition coefficients (K_pm) were measured in pure water and water-methanol mixtures up to a methanol mole fraction of 0.3 (50% v/v). Subsequently, logK_pw in pure water was determined as the intercept of linear regression of the logK_pm with the corresponding methanol mole fractions. LogK_pw were determined for phthalates, musks, organo phosphorus flame-retardants, chlorobenzenes, pesticides, some PCBs and a number of miscellaneous compounds. The median standard error and 95% confidence interval of the measured logK_pw was 0.06 and 0.13, respectively. The overall relationship between K_pw and K_ow seems insufficient to predict K_pw for unknown compounds. Prediction may work within a group of compounds with similar nature, e.g. homologues but HCH isomers having the same K_ow exhibit K_pw ranging over an order of magnitude. Long alkyl-chain phthalates and tris(2-ethylhexyl) phosphate; all having a molecular volume >400 ų, deviated the most from the K_pw-K_ow relationship.

Mechanisms for rhamnolipids-mediated biodegradation of hydrophobic organic compounds

The widespread existence of hydrophobic organic compounds (HOCs) in soil and water poses a potential health hazard to human, such as skin diseases, heart diseases, carcinogenesis, etc. Surfactant-enhanced bioremediation has been regarded as one of the most viable technologies to treat HOCs contaminated soil and groundwater. As a biosurfactant that has been intensively studied, rhamnolipids have shown to enhance biodegradation of HOCs in the environment, however, the underlying mechanisms are not fully disclosed. In this paper, properties and production of rhamnolipids are summarized. Then effects of rhamnolipids on the biodegradation of HOCs, including solubilization, altering cell affinity to HOCs, and facilitating microbial uptake are reviewed in detail. Special attention is paid to how rhamnolipids change the bioavailability of HOCs, which are crucial for understanding the mechanism of rhamnolipids-mediated biodegradation. The biodegradation and toxicity of rhamnolipids are also discussed. Finally, perspectives and future research directions are proposed. This review adds insight to rhamnolipids-enhanced biodegradation process, and helps in application of rhamnolipids in bioremediation.

Ecological Risk Assessment of HCH and DDT Residues in a Sediment Core from the Küçükçekmece Lagoon, Turkey

The residues of organochlorine pesticides (HCHs and DDTs) in a dated (¹³⁷Cs and ²¹⁰Pb) sediment core from the Küçükçekmece Lagoon, an urban lake at the southern entrance of the future Canal Istanbul Project, an artificial watercourse, were analyzed with the gas chromatograph equipped with electron capture detector (GC ⁶³Ni-ECD). Concentrations of ∑HCH and ∑DDT along the core varied between 0.4 and 469.4 (average 51.4) ng g^-1 dw, and between 0.5 and 72.0 (average 7.4) ng g^-1 dw, respectively. α-HCH was the predominant isomer (98%) and followed by γ-isomer (2%). The highest concentrations of technical HCH were found in the sediments dated 1963-1972, whilst the DDT concentrations had their maximum around 1945. The highest concentrations of γ-HCH and p,p'-DDT exceeded probable-effect level values which can lead to frequently cause adverse effects on aquatic biota.

Passive sampling for spatial and temporal monitoring of organic pollutants in surface water of a rural-urban river in Kenya

Passive sampling is an emerging monitoring strategy for surface waters and can be applied in a range of environments including remote locations. Silicone rubber (SR) as a robust single-phase passive sampler was applied to characterize the spatial and temporal variability of organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs) and three phthalates, namely, dibutyl phthalate (DBP), benzyl butyl phthalate (BBP) and bis(2-ethylhexyl)phthalate (DEHP) in a tropical river traversing rural and urban catchments. OCPs and PCBs were not detected. Up to 31.8ng/L of freely dissolved concentrations of PAHs were quantified and were dominated by the lower molecular weight members. Mean concentrations of DBP, BBP and DEHP were 72.6ng/L, 3.9ng/L and 7.1ng/L respectively. However, in sampling for phthalates using SR, quality control and assurance remains the key challenge and must always be ensured. Spatial variability in concentrations was evident and could be related to land use. Temporal variability was not significant.

Parameterizing sorption isotherms using a hybrid global-local fitting procedure

Predictive modeling of the transport and remediation of groundwater contaminants requires an accurate description of the sorption process, which is usually provided by fitting an isotherm model to site-specific laboratory data. Commonly used calibration procedures, listed in order of increasing sophistication, include: trial-and-error, linearization, non-linear regression, global search, and hybrid global-local search. Given the considerable variability in fitting procedures applied in published isotherm studies, we investigated the importance of algorithm selection through a series of numerical experiments involving 13 previously published sorption datasets. These datasets, considered representative of state-of-the-art for isotherm experiments, had been previously analyzed using trial-and-error, linearization, or non-linear regression methods. The isotherm expressions were re-fit using a 3-stage hybrid global-local search procedure (i.e. global search using particle swarm optimization followed by Powell's derivative free local search method and Gauss-Marquardt-Levenberg non-linear regression). The re-fitted expressions were then compared to previously published fits in terms of the optimized weighted sum of squared residuals (WSSR) fitness function, the final estimated parameters, and the influence on contaminant transport predictions - where easily computed concentration-dependent contaminant retardation factors served as a surrogate measure of likely transport behavior. Results suggest that many of the previously published calibrated isotherm parameter sets were local minima. In some cases, the updated hybrid global-local search yielded order-of-magnitude reductions in the fitness function. In particular, of the candidate isotherms, the Polanyi-type models were most likely to benefit from the use of the hybrid fitting procedure. In some cases, improvements in fitness function were associated with slight (<10%) changes in parameter values, but in other cases significant (>50%) changes in parameter values were noted. Despite these differences, the influence of isotherm misspecification on contaminant transport predictions was quite variable and difficult to predict from inspection of the isotherms.

Isotopic exchange on solid-phase micro extraction fiber in sediment under stagnant conditions: Implications for field application of performance reference compound calibration

An overlooked issue for field application of in situ performance reference compound (PRC) calibration methods is the validity of the assumption that both the sorption of a target compound and desorption of its corresponding PRC follow the first-order kinetics with the same rate constants under stagnant conditions. In the present study, disposable polydimethylsiloxane fibers of 2 sizes (7 and 35 µm) impregnated with 8 (13) C-labeled or deuterated PRCs were statically deployed into different marine sediments, from which the kinetics for sorption of the target compounds and desorption of the PRCs were characterized. Nonsymmetrical profiles were observed for exchange of the target analytes and their corresponding PRCs in sediment under stagnant conditions. The hysteretic desorption of PRCs in the kinetic regime may be ascribed to the low chemical potential between the fiber and sediment porewater, which reflects the inability of water molecules to rapidly diffuse through sediment to solvate the PRCs in the aqueous layer around the fiber surface. A moderate correlation (r = 0.77 and r = 0.57, p < 0.05 for both regressions) between the PRC-calibrated equilibrium concentrations of 1,1-dichloro-2,2-bis-(chlorophenyl) ethylene (p,p'-DDE) and polychlorinated biphenyl (PCB)-153 and the lipid normalized levels in worms (Neanthes arenaceodentata) was obtained in co-exposure tests under simulating field conditions, probably resulting from slightly overestimated bioavailability because of the hysteretic desorption of PRCs and toxic effects. Environ Toxicol Chem 2016;35:1978-1985. © 2015 SETAC.

Sorption of non-polar organic compounds by micro-sized plastic particles in aqueous solution

The presence of microscale polymer particles (i.e., microplastics) in the environment has become a major concern in recent years. Sorption of organic compounds by microplastics may affect the phase distribution within both sediments and aqueous phases. To investigate this process, isotherms were determined for the sorption of seven aliphatic and aromatic organic probe sorbates by four polymers with different physico-chemical properties. Sorption increased in the order polyamide < polyethylene < polyvinylchloride < polystyrene. This order does not reflect the particle sizes of the investigated microplastics within the aqueous dispersions, indicating the influence of additional factors (e.g., π-π-interactions) on the sorption of aromatic compounds by polystyrene. Linear isotherms by polyethylene suggested that sorbate uptake was due to absorption into the bulk polymer. In contrast, non-linear isotherms for sorption by PS, PA, and PVC suggest a predominance of adsorption onto the polymer surface, which is supported by the best fit of these isotherms using the Polanyi-Manes model. A strong relationship between the sorption coefficients of the microplastics and the hydrophobicity of the sorbates suggests that hydrophobic interactions are of major importance.

Understanding the rates of nonpolar organic chemical accumulation into passive samplers deployed in the environment: Guidance for passive sampler deployments

Polymeric passive samplers have become a common method for estimating freely dissolved concentrations in environmental media. However, this approach has not yet been adopted by investigators conducting remedial investigations of contaminated environmental sites. Successful adoption of this sampling methodology relies on an understanding of how passive samplers accumulate chemical mass as well as developing guidance for the design and deployment of passive samplers. Herein, we outline the development of a simple mathematical relationship of the environmental, polymer, and chemical properties that control the uptake rate. This relationship, called a timescale, is then used to illustrate how each property controls the rate of equilibration in samplers deployed in the water or in the sediment. Guidance is also given on how to use the timescales to select an appropriate polymer, deployment time, and suite of performance reference compounds. Integr Environ Assess Manag 2016;12:486-492. © 2015 SETAC.

Sorption of hydrophobic organic compounds to a diverse suite of carbonaceous materials with emphasis on biochar

Carbonaceous materials like biochars are increasingly recognized as effective sorbent materials for sequestering organic pollutants. Here, we study sorption behavior of two common hydrophobic organic contaminants 2,2',5,5'-tetrachlorobiphenyl (CB52) and phenanthrene (PHE), on biochars and other carbonaceous materials (CM) produced at a wide range of conditions and temperatures from various feedstocks. The primary aim was to establish structure-reactivity relationships responsible for the observed variation in CM and biochar sorption characteristics. CM were characterized for their elemental composition, surface area, pore size distribution, aromaticity and thermal stability. Freundlich sorption coefficients for CB52 and PHE (i.e. LogK(F,CB52) and K(F,PHE), respectively) to CM showed a variation of two to three orders of magnitude, with LogK(F,CB52) ranging from 5.12 ± 0.38 to 8.01 ± 0.18 and LogK(F,PHE) from 5.18 ± 0.09 to 7.42 ± 1.09. The highest LogK(F) values were observed for the activated CM, however, non-activated biochars produced at high temperatures (>700 °C) sorbed almost as strongly (within 0.2-0.5 Log units) as the activated ones. Sorption coefficients significantly increased with pyrolysis temperature, CM surface area and pore volume, aromaticity, and thermal stability, and decreased with H/C, O/C, (O + N)/C content. The results of our study contribute to the understanding of processes underlying HOC sorption to CM and explore the potential of CM as engineered sorbents for environmental applications.

Measurement of Setschenow constants for six hydrophobic compounds in simulated brines and use in predictive modeling for oil and gas systems

Treatment and reuse of brines, produced from energy extraction activities, requires aqueous solubility data for organic compounds in saline solutions. The presence of salts decreases the aqueous solubility of organic compounds (i.e. salting-out effect) and can be modeled using the Setschenow Equation, the validity of which has not been assessed in high salt concentrations. In this study, we used solid-phase microextraction to determine Setschenow constants for selected organic compounds in aqueous solutions up to 2-5 M NaCl, 1.5-2 M CaCl2, and in Na-Ca binary electrolyte solutions to assess additivity of the constants. These compounds exhibited log-linear behavior up to these high NaCl concentrations. Log-linear decreases in solubility with increasing salt concentration were observed up to 1.5-2 M CaCl2 for all compounds, and added to a sparse database of CaCl2 Setschenow constants. Setschenow constants were additive in binary electrolyte mixtures. New models to predict CaCl2 and KCl Setschenow constants from NaCl Setschenow constants were developed, which successfully predicted the solubility of the compounds measured in this study. Overall, data show that the Setschenow Equation is valid for a wide range of salinity conditions typically found in energy-related technologies.

Influence of feeding and earthworm density on compound bioaccumulation in earthworms Eisenia andrei

Earthworm density and feeding during exposure to contaminated soil have been used inconsistently in bioaccumulation studies, which may lead to possible errors in risk assessment and modeling. Hydrophobic organic pollutants with a wide range of environmental properties (phenanthrene, pyrene, lindane, p,p'-DDT, and PCB 153) were used to study the effect of different earthworm densities in combination with the presence or absence of feeding on bioaccumulation factors (BAFs). Similar BAFs were found at various soil-to-worm ratios, with the exception of phenanthrene. We recommend using at least 15 gsoil dw per earthworm. The absence of feeding doubled the BAFs and, thus, using no food ration can be considered as "the worst case scenario". Whenever food is to be applied (i.e. to ensure the validity of the test in earthworm mass loss), we suggest feeding depending on the organic carbon content of the studied soil.

Passive dosing versus solvent spiking for controlling and maintaining hydrophobic organic compound exposure in the Microtox® assay

Microbial toxicity bioassays such as the Microtox® test are ubiquitously applied to measure the toxicity of chemicals and environmental samples. In many ways their operation is conducive to the testing of organic chemicals. They are of short duration, use glass cuvettes and take place at reduced temperatures in medium lacking sorbing components. All of these are expected to reduce sorptive and volatile losses, but particularly for hydrophobic organics the role of such losses in determining the bioassay response remains unclear. This study determined the response of the Microtox® test when using solvent spiking compared to passive dosing for introducing the model hydrophobic compounds acenaphthene, phenanthrene, fluoranthene and benzo(a)pyrene. Compared to solvent spiking, the apparent sensitivity of the Microtox® test with passive dosing was 3.4 and 12.4 times higher for acenaphthene and phenanthrene, respectively. Furthermore, fluoranthene only gave a consistent response with passive dosing. Benzo(a)pyrene did not result in a response with either spiking or passive dosing even at aqueous solubility. Such differences in the apparent sensitivity of the Microtox® test can be traced back to the precise definition of the dissolved exposure concentrations and the buffering of losses with passive dosing. This highlights the importance of exposure control even in simple and short-term microbial bioassays such as the Microtox® test.

Polyparameter linear free energy relationship for wood char-water sorption coefficients of organic sorbates

Black carbons, including soots, chars, activated carbons, and engineered nanocarbons, have different surface properties, but the extent to which these affect their sorbent properties is not known. To evaluate this for an environmentally ubiquitous form of black carbon, biomass char, the surface of a well-studied wood char was probed using 14 sorbates exhibiting diverse functional groups, and the data were fit with a polyparameter linear free energy relationship to assess the importance of the various possible sorbate-char surface interactions. Sorption from water to water-wet char evolved with the sorbate's degree of surface saturation and depended on only a few sorbate parameters: log K(d)L/kg) = [(4.03 ± 0.14) + (-0.15 ± 0.04) log a(i)] V + [(-0.28 ± 0.04) log a(i)] S + (-5.20 ± 0.21) B, where a(i) is the aqueous saturation of the sorbate i, V is McGowan's characteristic volume, S reflects polarity, and B represents the electron-donation basicity. As is generally observed for activated carbon, the sorbate's size encouraged sorption from water to the char, whereas its electron donation and proton acceptance discouraged sorption from water. The magnitude and saturation dependence differed significantly from what has been seen for activated carbons, presumably reflecting the unique surface chemistries of these 2 black carbon materials and suggesting that black carbon-specific sorption coefficients will yield more accurate assessments of contaminant mobility and bioavailability, as well as evaluation of a site's response to remediation.

Isotherm ranking and selection using thirteen literature datasets involving hydrophobic organic compounds

Numerous isotherm expressions have been developed for describing sorption of hydrophobic organic compounds (HOCs), including "dual-mode" approaches that combine nonlinear behavior with a linear partitioning component. Choosing among these alternative expressions for describing a given dataset is an important task that can significantly influence subsequent transport modeling and/or mechanistic interpretation. In this study, a series of numerical experiments were undertaken to identify "best-in-class" isotherms by refitting 10 alternative models to a suite of 13 previously published literature datasets. The corrected Akaike Information Criterion (AICc) was used for ranking these alternative fits and distinguishing between plausible and implausible isotherms for each dataset. The occurrence of multiple plausible isotherms was inversely correlated with dataset "richness", such that datasets with fewer observations and/or a narrow range of aqueous concentrations resulted in a greater number of plausible isotherms. Overall, only the Polanyi-partition dual-mode isotherm was classified as "plausible" across all 13 of the considered datasets, indicating substantial statistical support consistent with current advances in sorption theory. However, these findings are predicated on the use of the AICc measure as an unbiased ranking metric and the adoption of a subjective, but defensible, threshold for separating plausible and implausible isotherms.

Effects of humic acids on the aggregation and sorption of nano-TiO2

In this study, humic acids (HAs) from three sources, peat, sediment and straw, used to coat nano-TiO2 were investigated. The results indicated that HAs isolated from peat were aromatic-rich, whereas those isolated from sediment and straw were aliphatic-rich. The nano-TiO2 sedimentation experiments indicated that the presence of aromatic-rich HAs was more capable of stabilizing nano-TiO2 particles than was the presence of aliphatic-rich HAs. This result is because the deionized phenolic groups in the HAs were preferentially adsorbed on the nano-TiO2 surfaces, which generated a higher charge density on the nano-TiO2 surfaces and caused stronger repulsive forces among particles. Furthermore, the aromatic-rich TiO2-HA complexes exhibited a greater sorption capacity than the aliphatic-rich TiO2-HAs complexes and nonlinear phenanthrene sorption because of their higher affinity and the condensed state of aromatic fractions. Note that natural organic matters, such as humic acids, in aquatic environments can not only increase the stability of nanoparticles but can also influence the mobility of hydrophobic organic compounds (HOCs).

Characterization of a biosurfactant produced by Pseudomonas cepacia CCT6659 in the presence of industrial wastes and its application in the biodegradation of hydrophobic compounds in soil

The bacterium Pseudomonas cepacia CCT6659 cultivated with 2% soybean waste frying oil and 2% corn steep liquor as substrates produced a biosurfactant with potential application in the bioremediation of soils. The biosurfactant was classified as an anionic biomolecule composed of 75% lipids and 25% carbohydrates. Characterization by proton nuclear magnetic resonance ((1)H and (13)C NMR) revealed the presence of carbonyl, olefinic and aliphatic groups, with typical spectra of lipids. Four sets of biodegradation experiments were carried out with soil contaminated by hydrophobic organic compounds amended with molasses in the presence of an indigenous consortium, as follows: Set 1-soil+bacterial cells; Set 2-soil+biosurfactant; Set 3-soil+bacterial cells+biosurfactant; and Set 4-soil without bacterial cells or biosurfactant (control). Significant oil biodegradation activity (83%) occurred in the first 10 days of the experiments when the biosurfactant and bacterial cells were used together (Set 3), while maximum degradation of the organic compounds (above 95%) was found in Sets 1-3 between 35 and 60 days. It is evident from the results that the biosurfactant alone and its producer species are both capable of promoting biodegradation to a large extent.

Enhanced desorption of persistent organic pollutants from microplastics under simulated physiological conditions

Microplastics have the potential to uptake and release persistent organic pollutants (POPs); however, subsequent transfer to marine organisms is poorly understood. Some models estimating transfer of sorbed contaminants to organisms neglect the role of gut surfactants under differing physiological conditions in the gut (varying pH and temperature), examined here. We investigated the potential for polyvinylchloride (PVC) and polyethylene (PE) to sorb and desorb (14)C-DDT, (14)C-phenanthrene (Phe), (14)C-perfluorooctanoic acid (PFOA) and (14)C-di-2-ethylhexyl phthalate (DEHP). Desorption rates of POPs were quantified in seawater and under simulated gut conditions. Influence of pH and temperature was examined in order to represent cold and warm blooded organisms. Desorption rates were faster with gut surfactant, with a further substantial increase under conditions simulating warm blooded organisms. Desorption under gut conditions could be up to 30 times greater than in seawater alone. Of the POP/plastic combinations examined Phe with PE gave the highest potential for transport to organisms.