Bacterial community assembly and antibiotic resistance genes in soils exposed to antibiotics at environmentally relevant concentrations.

Understanding how bacterial community assembly and antibiotic resistance genes (ARGs) respond to antibiotic exposure is essential to deciphering the ecological risk of anthropogenic antibiotic pollution in soils. In this study, three loam soils with different land management (unmanured golf course, dairy-manured pasture, and swine-manured cornfield) were spiked with a mixture of 11 antibiotics at the initial concentration of 100 and 1000 µg kg-1 for each antibiotic and incubated over 132 days, mimicking a scenario of pulse disturbance and recovery in soils, with unspiked soil samples as the control treatment. The Infer Community Assembly Mechanisms by Phylogenetic-bin-based null model (iCAMP) analysis demonstrated that drift and dispersal limitation contributed to 57%-65% and 16%-25%, and homogeneous selection 12%-16% of soil bacterial community assembly. Interestingly, antibiotic exposure to 1000 µg kg-1 level significantly increased the contribution of drift to community assembly, largely due to the positive response from Acidobacteria-6 in the golf course and pasture soils and from Chthoniobacteraceae in the cornfield soil to the antibiotic exposure. However, ARG abundance and diversity in the three soils exhibited antibiotics-independent temporal fluctuations, but were associated with the changes in soil bacterial communities over time. This study provides the first insight into the relative contributions of different bacterial community assembly processes in soils upon antibiotic exposure at environmentally relevant concentrations.

Interfacial Structure and Interaction of Kaolinite Intercalated with N-methylformamide Insight from Molecular Dynamics Modeling.

The evolution of basal spacing and interfacial structure of kaolinite-N-methylformamide (NMF) complexes during the intercalation process were difficult to obtain using experimental methods. In present study, a series of kaolinite-NMF complex models with various numbers of NMF molecules in the interlayer space were constructed to mimic the progressive stage of the intercalation process of kaolinite intercalated by NMF. The MD simulations were performed on these models to explore the evolution of basal spacing and interfacial structure of kaolinite-NMF complexes during the intercalation process. It was found that the basal spacing of complex was stabilized at 11 Å during the intercalation process, where the molecular plane of NMF oriented at small angles with respect to the interlayer surface with the C=O groups and N-H bonds pointing toward the octahedral and tetrahedral surfaces, respectively, due to the hydrogen bonding interactions. The basal spacing can be enlarged to larger values with the prerequisite of overcoming the energy barrier. With the increase of basal spacing during the intercalation process, the NMF were rearranged as a pillar with the molecular planes orienting at higher angles with respect to the interlayer surface, and then developed to disordered bilayer structure. For the interfacial interaction of kaolinite-NMF complex, both the octahedral surface and tetrahedral surface showed binding affinity to the NMF, which is the driving force for the intercalation of NMF in kaolinite. The octahedral surface displays stronger binding affinity to the NMF in terms of the H-bonds and energetics compared to the tetrahedral surface partially due to the highly active surface hydroxyl groups. The present study provides insight into the basal spacing evolution, and interfacial structure and interaction of kaolinite-NMF complexes, which can enhance the understanding of kaolinite intercalated by small molecules.

Mechanism Responsible for Intercalation of Dimethyl Sulfoxide in Kaolinite: Molecular Dynamics Simulations.

Intercalation is the promising strategy to expand the interlayer region of kaolinite for their further applications. Herein, the adaptive biasing force (ABF) accelerated molecular dynamics simulations were performed to calculate the free energies involved in the kaolinite intercalation by dimethyl sulfoxide (DMSO). Additionally, the classical all atom molecular dynamics simulations were carried out to calculate the interfacial interactions between kaolinite interlayer surfaces and DMSO with the aim at exploring the underlying force that drives the DMSO to enter the interlayer space. The results showed that the favorable interaction of DMSO with both kaolinite interlayer octahedral surface and tetrahedral surface can help in introducing DMSO enter kaolinite interlayer. The hydroxyl groups on octahedral surface functioned as H-donors attracting the S=O groups of DMSO through hydrogen bonding interaction. The tetrahedral surface featuring hydrophobic property attracted the methyl groups of DMSO through hydrophobic interaction. The results provided a detailed picture of the energetics and interlayer structure of kaolinite-DMSO intercalate.

Bioavailability of Soil-Sorbed Tetracycline to Escherichia coli under Unsaturated Conditions.

Increasing concentrations of anthropogenic antibiotics in soils are partly responsible for the proliferation of bacterial antibiotic resistance. However, little is known about how soil-sorbed antibiotics exert selective pressure on bacteria in unsaturated soils. This study investigated the bioavailability of tetracycline sorbed on three soils (Webster clay loam, Capac sandy clay loam, and Oshtemo loamy sand) to a fluorescent Escherichia coli bioreporter under unsaturated conditions using agar diffusion assay, microscopic visualization, and model simulation. Tetracycline sorbed on the soils could be desorbed and become bioavailable to the E. coli cells at matric water potentials of -2.95 to -13.75 kPa. Bright fluorescent rings were formed around the tetracycline-loaded soils on the unsaturated agar surfaces, likely due to radial diffusion of tetracycline desorbed from the soils, tetracycline uptake by the E. coli cells, and its inhibition on E. coli growth, which was supported by the model simulation. The bioavailability of soil-sorbed tetracycline was much higher for the Oshtemo soil, probably due to faster diffusion of tetracycline in coarse-textured soils. Decreased bioavailability of soil-sorbed tetracycline at lower soil water potential likely resulted from reduced tetracycline diffusion in soil pore water at smaller matric potential and/or suppressed tetracycline uptake by E. coli at lower osmotic potential. Therefore, soil-sorbed tetracycline could still exert selective pressure on the exposed bacteria, which was influenced by soil physical processes controlled by soil texture and soil water potential.

TCDD administered on activated carbon eliminates bioavailability and subsequent shifts to a key murine gut commensal.

Activated carbon (AC) is an increasingly attractive remediation alternative for the sequestration of dioxins at contaminated sites globally. However, the potential for AC to reduce the bioavailability of dioxins in mammals and the residing gut microbiota has received less attention. This question was partially answered in a recent study examining 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced hallmark toxic responses in mice administered with TCDD sequestered by AC or freely available in corn oil by oral gavage. Results from that study support the use of AC to significantly reduce the bioavailability of TCDD to the host. Herein, we examined the bioavailability of TCDD sequestered to AC on a key murine gut commensal and the influence of AC on the community structure of the gut microbiota. The analysis included qPCR to quantify the expression of segmented filamentous bacteria (SFB) in the mouse ileum, which has responded to TCDD-induced host toxicity in previous studies and community structure via sequencing the 16S ribosomal RNA (rRNA) gene. The expression of SFB 16S rRNA gene and functional genes significantly increased with TCDD administered with corn oil vehicle. Such a response was absent when TCDD was sequestered by AC. In addition, AC appeared to have a minimal influence on murine gut community structure and diversity, affecting only the relative abundance of Lactobacillaceae and two other groups. Results of this study further support the remedial use of AC for eliminating bioavailability of TCDD to host and subsequent influence on the gut microbiome.

Sorption of Lincomycin by Manure-Derived Biochars from Water.

The presence of antibiotics in agroecosystems raises concerns about the proliferation of antibiotic-resistant bacteria and adverse effects to human health. Soil amendment with biochars pyrolized from manures may be a win-win strategy for novel manure management and antibiotics abatement. In this study, lincomycin sorption by manure-derived biochars was examined using batch sorption experiments. Lincomycin sorption was characterized by two-stage kinetics with fast sorption reaching quasi-equilibrium in the first 2 d, followed by slow sorption over 180 d. The fast sorption was primarily attributed to surface adsorption, whereas the long-term slow sorption was controlled by slow diffusion of lincomycin into biochar pore structures. Two-day sorption experiments were performed to explore effects of biochar particle size, solid/water ratio, solution pH, and ionic strength. Lincomycin sorption to biochars was greater at solution pH 6.0 to 7.5 below the dissociation constant of lincomycin (7.6) than at pH 9.9 to 10.4 above its dissociation constant. The enhanced lincomycin sorption at lower pH likely resulted from electrostatic attraction between the positively charged lincomycin and the negatively charged biochar surfaces. This was corroborated by the observation that lincomycin sorption decreased with increasing ionic strength at lower pH (6.7) but remained constant at higher pH (10). The long-term lincomycin sequestration by biochars was largely due to pore diffusion plausibly independent of solution pH and ionic composition. Therefore, manure-derived biochars had lasting lincomycin sequestration capacity, implying that biochar soil amendment could significantly affect the distribution, transport, and bioavailability of lincomycin in agroecosystems.

Integrating structural and thermodynamic mechanisms for sorption of PCBs by montmorillonite.

Strong sorption of planar nonionic organic chemicals by clay minerals has been observed for important classes of organic contaminants including polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and dioxins, and such affinity was hypothesized to relate to the interlayer hydrophobicity of smectite clays. In batch sorption experiments of two trichlorobiphenyls on homoionic Na-, K-, Cs-montmorillonites, considerably greater sorption coefficient (Kw) was observed for coplanar 3,3',5-trichlorobiphenyl (PCB 36); log Kw for Na-, K-, and Cs-montmorillonite were 3.69, 3.72, and 4.53 for coplanar PCB 36 vs 1.21, 1.46, and 0.87 for the nonplanar 2,2',6-trichlorobiphenyl (PCB 19). MD simulations were conducted utilizing X-ray diffraction determined clay interlayer distances (d-spacing). The trajectory, density distribution, and radial distribution function of interlayer cation, water, and PCBs collectively indicated that the hydrophobic nature of the interlayer regions was determined by the hydration status of exchangeable cations and the associated d-spacing. The sorption free energies calculated for both coplanar and nonplanar PCB molecules by adaptive biasing force (ABF) method with an extended interlayer-micropore two-phase model consisting of cleaved clay hydrates and "bulk water" are consistent with the Gibbs free energies derived from the measured log Kw, manifesting enhanced sorption of coplanar PCBs was attributed to shape selectivity and hydrophobic interactions.

Role of tetracycline speciation in the bioavailability to Escherichia coli for uptake and expression of antibiotic resistance.

Tetracycline contains ionizable functional groups that manifest several species with charges at different locales and differing net charge; the fractional distribution of each species depends on pH-pKa relationship in the aqueous phase. In nature, these species interact with naturally abundant cations (e.g., Ca(2+) and Mg(2+)) to form metal-tetracycline complexes in water. In this study, we used Escherichia coli MC4100/pTGM whole-cell bioreporter to investigate tetracycline uptake from solution under varying conditions of pH, salt composition and concentration by quantifying the corresponding expression of antibiotic resistance gene. The expression of antibiotic resistance gene in the E. coli bioreporter responded linearly to intracellular tetracycline concentration. Less tetracycline entered E. coli cells at solution pH of 8.0 than at pH 6.0 or 7.0 indicating reduced bioavailability of the antibiotic at higher pH. Both Mg(2+) and Ca(2+) in solution formed metal-tetracycline complexes which reduced uptake of tetracycline by E. coli hence diminishing the bioresponse. Among the various tetracycline species present in solution, including both metal-complexed and free (noncomplexed) species, zwitterionic tetracycline was identified as the predominant species that most readily passed through the cell membrane eliciting activation of the antibiotic resistance gene in E. coli. The results indicate that the same total concentration of tetracycline in ambient solution can evoke very different expression of antibiotic resistance gene in the exposed bacteria due to differential antibiotic uptake. Accordingly, geochemical factors such as pH and metal cations can modulate the selective pressure exerted by tetracycline for development and enrichment of antibiotic resistant bacteria. We suggest that tetracycline speciation analysis should be incorporated into the risk assessment framework for evaluating environmental exposure and the corresponding development of antibiotic resistance.

Reduction of carbadox mediated by reaction of Mn(III) with oxalic acid.

Manganese(III) geocomponents are commonly found in the soil environment, yet their roles in many biogeochemical processes remain unknown. In this study, we demonstrated that Mn(III) generated from the reaction of MnO(2) and oxalic acid caused rapid and extensive decompositions of a quinoxaline-di-N-oxide antibiotics, viz carbadox. The reaction occurred primarily at the quinoxaline-di-N-oxide moiety resulting in the removal of one -O from N1-oxide and formation of desoxycarbadox. The reaction rate was accelerated by increasing amounts of Mn(III), carbadox and oxalate. The critical step in the overall reaction was the formation of a quinoxaline-di-N-oxide/Mn(III)/oxalate ternary complex in which Mn(III) functioned as the central complexing cation and electron conduit in which the arrangement of ligands facilitated electron transfer from oxalate to carbadox. In the complex, the C-C bond in oxalate was cleaved to create CO(2)(-•) radicals, followed by electron transfer to carbadox through the Mn(III) center. This proposed reaction mechanism is supported by the reaction products formed, reaction kinetics, and quantum mechanical calculations. The results obtained from this study suggest that naturally occurring Mn(III)-oxalic acid complexes could reductively decompose certain organic compounds in the environment such as the antibiotic quinoxaline-di-N-oxide.

Reaction of decabromodiphenyl ether with organo-modified clay-templated zero-valent iron in water-tetrahydrofuran solution: Nano- to micrometric scale effects.

Smectite clay-templated nanoscale zero-valent iron (CZVI) was modified with tetramethylammonium (TMA), trimethylphenylammonium (TMPA) and hexadecyltrimethylammonium (HDTMA) to achieve organoclay-templated ZVI (OCZVI). The reactivity of various OCZVIs was evaluated on the basis of degradation of decabromodiphenyl ether (DBDE) in tetrahydrofuran (THF)-water binary solution. Characterization of OCZVI interlayer at nanometric scale indicated that the clay particles had the domains with three basal spacings in the THF/water solution. In the 50 % THF solution TMPA modification promoted the formation of the domains with a basal spacing at 1.56 nm, which could promote the degradation of DBDE. At the micrometric scale, in the 90 % THF solution TMA and TMPA modification tended to enhance the aggregation of OCZVI particles, while the HDTMA modification reduced the aggregation, and high percentage of modification yielded viscous gel structures. The relatively rapid sedimentation processes in 90 % THF solution (compared to that in 50 % THF solution) and formation gel structures could reduce the access of DBDE to the interlayer reactive nZVIs, and lead to the significant reduction in reaction rate. These results provide important insights to the organo-modification on clays which could alter their orientations and dispersion in organic-water binary solution to achieve the desired reactivity on confined clay surfaces.

Rapid and extensive debromination of decabromodiphenyl ether by smectite clay-templated subnanoscale zero-valent iron.

Subnanoscale zerovalent iron (ZVI) synthesized using smectite clay as a template was utilized to investigate reduction of decabromodiphenyl ether (DBDE). The results revealed that DBDE was rapidly debrominated by the prepared smectite-templated ZVI with a reaction rate 10 times greater than that by conventionally prepared nanoscale ZVI. This enhanced reduction is plausibly attributed to the smaller-sized smectite-templated ZVI clusters (∼0.5 nm) vs that of the conventional nanoscale ZVI (∼40 nm). The degradation of DBDE occurred in a stepwise debromination manner. Pentabromodiphenyl ethers were the terminal products in an alkaline suspension (pH 9.6) of smectite-templated ZVI, whereas di-, tri-, and tetrabromodiphenyl ethers formed at the neutral pH. The presence of tetrahydrofuran (THF) as a cosolvent at large volume fractions (e.g., >70%) in water reduced the debromination rates due to enhanced aggregation of clay particles and/or diminished adsorption of DBDE to smectite surfaces. Modification of clay surfaces with tetramethylammonium (TMA) attenuated the colsovent effect on the aggregation of clay particles, resulting in enhanced debromination rates. Smectite clay provides an ideal template to form subnanoscale ZVI, which demonstrated superior debromination reactivity with DBDE compared with other known forms of ZVIs. The ability to modify the nature of smectite clay surface by cation exchange reaction utilizing organic cations can be harnessed to create surface properties compatible with various contaminated sites.

Implication of cation-bridging interaction contribution to sorption of perfluoroalkyl carboxylic acids by soils.

Sorption of four perfluoroalkyl carboxylic acids (PFCAs) including perfluoropentanoic acid, perfluoroheptanoic acid, perfluorodecanoic acid, and perfluorododecanoic acid by three soils with cation exchange sites occupied by K+, Ca2+, or Fe3+ was measured using the batch equilibration method. We hypothesize that partitioning in soil organic matters (SOM) is the primarily operative mechanism for PFCA sorption by K+-soils, and sorption by Ca2+- or Fe3+-soils could be enhanced via cation-bridging interaction. The measured sorption isotherms for all four PFCAs by soils were linear within the aqueous concentration between 0 and 60 µg/L, and the distribution coefficients ranged between 14.8 and 173 L/kg. Long-chain PFCAs manifested greater sorption by the soils with higher SOM content. Compared to sorption by K+-soils, sorption of PFCAs by Ca2+- and Fe3+-soils increased by 19.9-90.2% and 38.5-219%, respectively. The relative contributions of cation-bridging interaction to the overall PFCA sorption were estimated to be 16.6-48.7% for Ca2+-soils and 27.8-67.7% for Fe3+-soils. These results demonstrate that multivalent exchangeable cations could play an important role, yet previously ignored, in controlling sorption and transport of PFCAs in soils.

Pentachlorophenol radical cations generated on Fe(III)-montmorillonite initiate octachlorodibenzo-p-dioxin formation in clays: density functional theory and fourier transform infrared studies.

Octachlorodibenzodioxin (OCDD) forms spontaneously from pentachlorophenol (PCP) on the surfaces of Fe(III)-saturated smectite clay. (1) Here, we used in situ Fourier transform infrared (FTIR) methods and quantum mechanical calculations to determine the mechanism by which this reaction is initiated. As the clay was dehydrated, vibrational spectra showed new peaks that grew and then reversibly disappeared as the clay rehydrated. First-principle density functional theory calculations of hydrated Fe/PCP clusters reproduced these transient FTIR peaks when inner-sphere complexation and concomitant electron transfer produced Fe(II) and PCP radical cations. Thus, our experimental (FTIR) and theoretical (quantum mechanical) results mutually support the hypothesis that OCDD formation on Fe-smectite surfaces is initiated by the reversible formation of metastable PCP radical cations via single-electron transfer from PCP to Fe(III). The negatively charged clay surface apparently selects for this reaction mechanism by stabilizing PCP radical cations.

Clay mediated route to natural formation of Polychlorodibenzo-p-dioxins.

Recent studies have documented the ubiquitous occurrence of polychlorodibenzo-p-dioxins and dibenzofurans (PCDD/Fs) of unknown origin in soils and clay deposits. Interestingly, the PCDD/F congener profiles do not match any known natural or anthropogenic source, and global PCDD/F budgets fail to account for the observed levels in soils. To reconcile these observations, clay minerals had been hypothesized to play a central role in the natural in situ synthesis of PCDD/Fs. We recently demonstrated the clay-mediated formation of the most prevalent PCDD congener in soils, octachlorodibenzo-p-dioxin (OCDD), supporting this hypothesis. Here we report the formation of the direct precursors ("predioxins") of the most toxic PCDD congener, 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD), and of 1,2,4,7,8-pentachlorodizenzodioxin (1,2,4,7,8-PeCDD), and two additional dimers, from the reaction of 2,4,5-trichlorophenol (2,4,5-TCP) with Fe(III)-montmorillonite clay. We propose plausible reaction pathways, each initiated by single electron transfer from 2,4,5-TCP to Fe(III)-montmorillonite forming the 2,4,5-TCP radical cation. The operative reaction mechanisms, inferred from experimental results, are supported by quantum mechanical calculations. The key role of montmorillonite is apparently to stabilize the reactive radical cation intermediate. Fortuitously, PCDD formation reactions on clay surfaces are more facile for less toxic higher chlorinated congeners like OCDD, as predicted by the proposed reaction mechanism and consistent with the observed PCDD congener distributions in soils. Importantly, increasing the toxicity equivalency factor of OCDD would immediately cause many soils to exceed PCDD regulatory levels due to the predominance of this congener.

Synthesis and evaluation of Fe3O4-impregnated activated carbon for dioxin removal.

Polychlorinated dibenzo-p-dioxins and -furans (PCDD/PCDFs) are highly toxic organic pollutants in soils and sediments which persist over timescales that extend from decades to centuries. There is a growing need to develop effective technologies for remediating PCDD/Fs-contaminated soils and sediments to protect human and ecosystem health. The use of sorbent amendments to sequester PCDD/Fs has emerged as one promising technology. A synthesis method is described here to create a magnetic activated carbon composite (AC-Fe3O4) for dioxin removal and sampling that could be recovered from soils using magnetic separation. Six AC-Fe3O4 composites were evaluated (five granular ACs (GACs) and one fine-textured powder AC(PAC)) for their magnetization and ability to sequester dibenzo-p-dioxin (DD). Both GAC/PAC and GAC/PAC-Fe3O4 composites effectively removed DD from aqueous solution. The sorption affinity of DD for GAC-Fe3O4 was slightly reduced compared to GAC alone, which is attributed to the blocking of sorption sites. The magnetization of a GAC-Fe3O4 composite reached 5.38 emu/g based on SQUID results, allowing the adsorbent to be easily separated from aqueous solution using an external magnetic field. Similarly, a fine-textured PAC-Fe3O4 composite was synthesized with a magnetization of 9.3 emu/g.

Natural organic matter does not diminish the mammalian bioavailability of 2,3,7,8-tetrachlorodibenzo-p-dioxin.

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a toxic and persistent organic pollutant found in soils and sediments. It has been linked to several adverse health outcomes in humans and wildlife, including suppression of the immune system. TCDD is strongly sorbed to soils/sediments due to its extremely low water solubility. Presently, the bioavailability of soil/sediment-sorbed TCDD to mammals is not completely understood. Our previous studies demonstrated that TCDD adsorbed to representative inorganic geosorbents (i.e. porous silica and smectite clay) exhibited the same bioavailability to mice as TCDD dissolved in corn oil, whereas sequestration by activated carbons eliminated TCDD bioavailability. In this study, we evaluated the effects of amorphous natural organic matter (NOM), primarily in the form of aquatic humic and fulvic acids, on the mouse bioavailability of TCDD. An aqueous suspension of TCDD mixed with NOM was administered to mice via oral gavage. The relative bioavailability of TCDD was assessed by two sensitive aryl hydrocarbon receptor-mediated responses in mice: 1) hepatic induction of cyp1A1 mRNA; and 2) suppression of immunoglobulin M (IgM) antibody-forming cell (AFC) response which is an indicator of immunotoxicity. Hepatic induction of cyp1A1 mRNA and suppression of IgM AFC induced by TCDD were similar in the NOM-sorbed form and dissolved in corn oil, revealing no loss of bioavailability when associated with NOM. Hence, NOM-associated TCDD is as capable of suppressing humoral immunity in mice as TCDD dissolved in corn oil, indicating that NOM-sorbed TCDD is likely to fully retain its bioavailability to mammals and, by inference, humans.

Bioavailability of clay-adsorbed dioxin to Sphingomonas wittichii RW1 and its associated genome-wide shifts in gene expression.

Polychlorinated dibenzo-p-dioxins and dibenzofurans are a group of chemically-related pollutants categorically known as dioxins. Some of their chlorinated congeners are among the most hazardous pollutants that persist in the environment. This persistence is due in part to the limited number of bacteria capable of metabolizing these compounds, but also to their limited bioavailability in soil. We used Sphingomonas wittichii strain RW1 (RW1), one of the few strains able to grow on dioxin, to characterize its ability to respond to and degrade clay-bound dioxin. We found that RW1 grew on and completely degraded dibenzo-p-dioxin (DD) intercalated into the smectite clay saponite (SAP). To characterize the effects of DD sorption on RW1 gene expression, we compared transcriptomes of RW1 grown with either free crystalline DD or DD intercalated clay, i.e. sandwiched between the clay interlayers (DDSAP). Free crystalline DD appeared to cause greater expression of toxicity and stress related functions. Genes coding for heat shock proteins, chaperones, as well as genes involved in DNA repair, and efflux were up-regulated during growth on crystalline dioxin compared to growth on intercalated dioxin. In contrast, growth on intercalated dioxin up-regulated genes that might be important in recognition and uptake mechanisms, as well as surface interaction/attachment/biofilm formation such as extracellular solute-binding protein and LuxR. These differences in gene expression may reflect the underlying adaptive mechanisms by which RW1 cells sense and deploy pathways to access dioxin intercalated into clay. These data show that intercalated DD remains bioavailable to the degrading bacterium with implications for bioremediation alternatives.

Activated carbons of varying pore structure eliminate the bioavailability of 2,3,7,8-tetrachlorodibenzo-p-dioxin to a mammalian (mouse) model.

The use of activated carbon (AC) as an in situ sorbent amendment to sequester polychlorinated-dibenzo-p-dioxins and furans (PCDD/Fs) present in contaminated soils and sediments has recently gained attention as a novel remedial approach. This remedy could be implemented at much lower cost while minimizing habitat destruction as compared to traditional remediation technologies that rely on dredging/excavation and landfilling. Several prior studies have demonstrated the ability of AC amendments to reduce pore water concentrations and hence bioaccumulation of PCDD/Fs in invertebrate species. However, our recent study was the first to show that AC had the ability to sequester 2,3,7,8­tetrachlorodibenzo­p­dioxin (TCDD) in a form that eliminated bioavailability to a mammalian (mouse) model. Here we show that three commercially available ACs, representing a wide range of pore size distributions, were equally effective in eliminating the bioavailability of TCDD based upon two sensitive bioassays, hepatic induction of cyp1A1 mRNA and immunoglobulin M antibody-forming cell response. These results provide direct evidence that a wide range of structurally diverse commercially available ACs may be suitable for use as in situ sorbent amendments to provide a cost-effective remedy for PCDD/F contaminated soils and sediments. Potentially, adaption of this technology would minimize habitat destruction and be protective of ecosystem and human health.

Long-term sorption of lincomycin to biochars: The intertwined roles of pore diffusion and dissolved organic carbon.

Sequestration of anthropogenic antibiotics by biochars from waters may be a promising strategy to minimize environmental and human health risks of antibiotic resistance. This study investigated the long-term sequestration of lincomycin by 17 slow-pyrolysis biochars using batch sorption experiments during 365 days. Sorption kinetics were well fitted to the Weber-Morris intraparticle diffusion model for all tested biochars with the intraparticle diffusion rate constant (kid) of 25.3-166 µg g-1 day-0.5 and intercept constant (Cid) of 39.0-339 µg g-1, suggesting that the sorption kinetics were controlled by fast initial sorption and slow pore diffusion. The quasi-equilibrium sorption isotherms became more nonlinear with increasing equilibration time at 1, 7, 30, and 365 days, likely due to increasing abundance of heterogeneous sorption sites in biochars over time. Intriguingly, low-temperature (300 °C) and high-temperature (600 °C) biochars had faster sorption kinetics than intermediate-temperature (400-500 °C) biochars at the long term, which was attributed to greater specific surface area and pore volume of high-temperature biochars and the substantial and continuous release of dissolved organic carbon (DOC) from low-temperature biochars, respectively. DOC release enhanced lincomycin sorption by decreasing biochar particle size and/or increasing the accessibility of sorption sites and pores initially blocked by DOC. Additionally, a large fraction (>75%) of sorbed lincomycin in biochars after a 240-day equilibration could not be extracted by the acetonitrile/methanol extractant. The strong sorption and low extraction recovery demonstrated the great potential of biochars as soil amendments for long-term sequestration of antibiotics in-situ.

Molecular Dynamics Simulation of Basal Spacing, Energetics, and Structure Evolution of a Kaolinite-Formamide Intercalation Complex and Their Interfacial Interaction.

Molecular dynamics simulations were performed on kaolinite-formamide complex models with various numbers of formamide molecules loaded in the kaolinite interlayer to explore the basal spacing, energetics, and structure evolution of the kaolinite-formamide complex during the intercalation process. Additionally, the interfacial interactions of formamide with kaolinite interlayer surfaces were calculated. The calculation revealed that the basal spacing of kaolinite was enlarged to 9.6 Å at the beginning of intercalation. Formamide was arranged as a monolayer structure in the kaolinite interlayer with the molecular plane oriented at small angles with respect to the interlayer surface. With continuous intercalation, the basal spacing readily reached a stable stage at 10.6 Å, where formamide rearranged its structure by rotating the molecule plane along the C-N bond that was parallel to the interlayer surface, which resulted in the molecular plane orienting at higher angles with respect to the interlayer surface. During this process, the C═O groups oriented toward the hydroxyl groups on the interlayer octahedral surface, and one of N-H bonds progressively pointed toward the basal oxygens on the opposing interlayer tetrahedral surface. Continuous intercalation can enlarge the basal spacing to more than 14 Å with the prerequisite of overcoming the energy barrier, and then formamide evolved to a disordered bilayer structure in the kaolinite interlayer. The affinity of kaolinite interlayer surfaces for formamide motivated the intercalation process. The octahedral surface displayed a relatively larger affinity toward formamide compared to the tetrahedral surface partially due to the presence of hydroxyl groups that are more active in the intermolecular interactions with formamide.