Evidence of free tropospheric and long-range transport of microplastic at Pic du Midi Observatory.

The emerging threat of atmospheric microplastic pollution has prompted researchers to study areas previously considered beyond the reach of plastic. Investigating the range of atmospheric microplastic transport is key to understanding the global extent of this problem. While atmospheric microplastics have been discovered in the planetary boundary layer, their occurrence in the free troposphere is relatively unexplored. Confronting this is important because their presence in the free troposphere would facilitate transport over greater distances and thus the potential to reach more distal and remote parts of the planet. Here we show evidence of 0.09-0.66 microplastics particles/m3 over 4 summer months from the Pic du Midi Observatory at 2877 meters above sea level. These results exhibit true free tropospheric transport of microplastic, and high altitude microplastic particles <50 µm (aerodynamic diameter). Analysis of air/particle history modelling shows intercontinental and trans-oceanic transport of microplastics illustrating the potential for global aerosol microplastic transport.

Temporal Archive of Atmospheric Microplastic Deposition Presented in Ombrotrophic Peat.

Ombrotrophic peatland-fed solely from atmospheric deposition of nutrients and precipitation-provide unique archives of atmospheric pollution and have been used to illustrate trends and changes in atmospheric trace element composition from the recent decadal to the Holocene period. With the acknowledgment of atmosphere plastic pollution, analysis of ombrotrophic peat presents an opportunity to characterize the historical atmospheric microplastic pollution prevalence. Ombrotrophic peatland is often located in comparatively pristine mountainous and boreal areas, acting as sentinels of environmental change. In this paired site study, a Sphagnum ombrotrophic peat record is used for the first time to identify the trend of atmospheric microplastic pollution. This high altitude, remote location ombrotrophic peat archive pilot study identifies microplastic presence in the atmospheric pollution record, increasing from <5(±1) particles/m2/day in the 1960s to 178(±72) particles/m2/day in 2015-2020 in a trend similar to the European plastic production and waste management. Compared to this catchment's lake sediment archive, the ombrotrophic peat core appears to be effective in collecting and representing atmospheric microplastic deposition in this remote catchment, collecting microplastic particles that are predominantly ≤20 µm. This study suggests that peat records may be a useful tool in assessing the past quantities and trends of atmospheric microplastic.

Treatment of heavy metals by iron oxide coated and natural gravel media in Sustainable urban Drainage Systems.

Sustainable urban Drainage Systems (SuDS) filter drains are simple, low-cost systems utilized as a first defence to treat road runoff by employing biogeochemical processes to reduce pollutants. However, the mechanisms involved in pollution attenuation are poorly understood. This work aims to develop a better understanding of these mechanisms to facilitate improved SuDS design. Since heavy metals are a large fraction of pollution in road runoff, this study aimed to enhance heavy metal removal of filter drain gravel with an iron oxide mineral amendment to increase surface area for heavy metal scavenging. Experiments showed that amendment-coated and uncoated (control) gravel removed similar quantities of heavy metals. Moreover, when normalized to surface area, iron oxide coated gravels (IOCGs) showed poorer metal removal capacities than uncoated gravel. Inspection of the uncoated microgabbro gravel indicated that clay particulates on the surface (a natural product of weathering of this material) augmented heavy metal removal, generating metal sequestration capacities that were competitive compared with IOCGs. Furthermore, when the weathered surface was scrubbed and removed, metal removal capacities were reduced by 20%. When compared with other lithologies, adsorption of heavy metals by microgabbro was 10-70% higher, indicating that both the lithology of the gravel, and the presence of a weathered surface, considerably influence its ability to immobilize heavy metals. These results contradict previous assumptions which suggest that gravel lithology is not a significant factor in SuDS design. Based upon these results, weathered microgabbro is suggested to be an ideal lithology for use in SuDS.

Monitoring bacterially induced calcite precipitation in porous media using magnetic resonance imaging and flow measurements.

A range of nuclear magnetic resonance (NMR) techniques are employed to provide novel, non-invasive measurements of both the structure and transport properties of porous media following a biologically mediated calcite precipitation reaction. Both a model glass bead pack and a sandstone rock core were considered. Structure was probed using magnetic resonance imaging (MRI) via a combination of quantitative one-dimensional profiles and three-dimensional images, applied before and after the formation of calcite in order to characterise the spatial distribution of the precipitate. It was shown through modification and variations of the calcite precipitation treatment that differences in the calcite fill would occur but all methods were successful in partially blocking the different porous media. Precipitation was seen to occur predominantly at the inlet of the bead pack, whereas precipitation occurred almost uniformly along the sandstone core. Transport properties are quantified using pulse field gradient (PFG) NMR measurements which provide probability distributions of molecular displacement over a set observation time (propagators), supplementing conventional permeability measurements. Propagators quantify the local effect of calcite formation on system hydrodynamics and the extent of stagnant region formation. Collectively, the combination of NMR measurements utilised here provides a toolkit for determining the efficacy of a biological-precipitation reaction for partially blocking porous materials.

Investigation of nanoparticle transport inside coarse-grained geological media using magnetic resonance imaging.

Quantifying nanoparticle (NP) transport inside saturated porous geological media is imperative for understanding their fate in a range of natural and engineered water systems. While most studies focus upon finer grained systems representative of soils and aquifers, very few examine coarse-grained systems representative of riverbeds and gravel based sustainable urban drainage systems. In this study, we investigated the potential of magnetic resonance imaging (MRI) to image transport behaviors of nanoparticles (NPs) through a saturated coarse-grained system. MRI successfully imaged the transport of superparamagnetic NPs, inside a porous column composed of quartz gravel using T(2)-weighted images. A calibration protocol was then used to convert T(2)-weighted images into spatially resolved quantitative concentration maps of NPs at different time intervals. Averaged concentration profiles of NPs clearly illustrates that transport of a positively charged amine-functionalized NP within the column was slower compared to that of a negatively charged carboxyl-functionalized NP, due to electrostatic attraction between positively charged NP and negatively charged quartz grains. Concentration profiles of NPs were then compared with those of a convection-dispersion model to estimate coefficients of dispersivity and retardation. For the amine functionalized NPs (which exhibited inhibited transport), a better model fit was obtained when permanent attachment (deposition) was incorporated into the model as opposed to nonpermanent attachment (retardation). This technology can be used to further explore transport processes of NPs inside coarse-grained porous media, either by using the wide range of commercially available (super)paramagnetically tagged NPs or by using custom-made tagged NPs.

Application of paramagnetically tagged molecules for magnetic resonance imaging of biofilm mass transport processes.

Molecules become readily visible by magnetic resonance imaging (MRI) when labeled with a paramagnetic tag. Consequently, MRI can be used to image their transport through porous media. In this study, we demonstrated that this method could be applied to image mass transport processes in biofilms. The transport of a complex of gadolinium and diethylenetriamine pentaacetic acid (Gd-DTPA), a commercially available paramagnetic molecule, was imaged both in agar (as a homogeneous test system) and in a phototrophic biofilm. The images collected were T(1) weighted, where T(1) is an MRI property of the biofilm and is dependent on Gd-DTPA concentration. A calibration protocol was applied to convert T(1) parameter maps into concentration maps, thus revealing the spatially resolved concentrations of this tracer at different time intervals. Comparing the data obtained from the agar experiment with data from a one-dimensional diffusion model revealed that transport of Gd-DTPA in agar was purely via diffusion, with a diffusion coefficient of 7.2 x 10(-10) m(2) s(-1). In contrast, comparison of data from the phototrophic biofilm experiment with data from a two-dimensional diffusion model revealed that transport of Gd-DTPA inside the biofilm was by both diffusion and advection, equivalent to a diffusion coefficient of 1.04 x 10(-9) m(2) s(-1). This technology can be used to further explore mass transport processes in biofilms, either by using the wide range of commercially available paramagnetically tagged molecules and nanoparticles or by using bespoke tagged molecules.

Magnetic resonance imaging of structure, diffusivity, and copper immobilization in a phototrophic biofilm.

Magnetic resonance imaging (MRI) was used to spatially resolve structure, water diffusion, and copper transport and fate in a phototrophic biofilm [corrected]. MRI was able to resolve considerable structural heterogeneity, ranging from classical laminations approximately 500 mum thick to structures with no apparent ordering. Pulsed-field gradient (PFG) analysis spatially resolved water diffusion coefficients which exhibited relatively little or no attenuation (diffusion coefficients ranged from 1.7 x 10(-9) m(2) s(-1) to 2.2 x 10(-9) m(2) s(-1)). The biofilm was then reacted with a 10-mg liter(-1) Cu(2+) solution, and transverse relaxation time parameter maps [corrected].were used to spatially and temporally map copper immobilization within the biofilm. Significantly, a calibration protocol similar to that used in biomedical research successfully quantified copper concentrations throughout the biofilm. Variations in Cu concentrations were controlled by the biofilm structure. Copper immobilization was most rapid (approximately 5 mg Cu liter(-1) h(-1)) over the first 20 to 30 h and then much slower for the remaining 60 h of the experiment. The transport of metal within the biofilm is controlled by both diffusion and immobilization. This was explored using a Bartlett and Gardner model which examined both diffusion and adsorption through a hypothetical film exhibiting properties similar to those of the phototrophic biofilm. Higher adsorption constants (K) resulted in longer lag times until the onset of immobilization at depth but higher actual adsorption rates. MRI and reaction transport models are versatile tools which can significantly improve our understanding of heavy metal immobilization in naturally occurring biofilms.

Characterization and implications of the cell surface reactivity of Calothrix sp. strain KC97.

The cell surface reactivity of the cyanobacterium Calothrix sp. strain KC97, an isolate from the Krisuvik hot spring, Iceland, was investigated in terms of its proton binding behavior and charge characteristics by using acid-base titrations, electrophoretic mobility analysis, and transmission electron microscopy. Analysis of titration data with the linear programming optimization method showed that intact filaments were dominated by surface proton binding sites inferred to be carboxyl groups (acid dissociation constants [pK(a)] between 5.0 and 6.2) and amine groups (mean pK(a) of 8.9). Sheath material isolated by using lysozyme and sodium dodecyl sulfate generated pK(a) spectra similarly dominated by carboxyls (pK(a) of 4.6 to 6.1) and amines (pK(a) of 8.1 to 9.2). In both intact filaments and isolated sheath material, the lower ligand concentrations at mid-pK(a) values were ascribed to phosphoryl groups. Whole filaments and isolated sheath material displayed total reactive-site densities of 80.3 x 10(-5) and 12.3 x 10(-5) mol/g (dry mass) of cyanobacteria, respectively, implying that much of the surface reactivity of this microorganism is located on the cell wall and not the sheath. This is corroborated by electrophoretic mobility measurements that showed that the sheath has a net neutral charge at mid-pHs. In contrast, unsheathed cells exhibited a stronger negative-charge characteristic. Additionally, transmission electron microscopy analysis of ultrathin sections stained with heavy metals further demonstrated that most of the reactive binding sites are located upon the cell wall. Thus, the cell surface reactivity of Calothrix sp. strain KC97 can be described as a dual layer composed of a highly reactive cell wall enclosed within a poorly reactive sheath.