Environmental Science: Processes & Impacts blog

Humans are continuing to release increasing amounts of greenhouse gases to the atmosphere that have been implicated as key contributors to climate change. One major greenhouse gas is CO₂. It is estimated that geological storage could result in a 17% reduction in atmospheric CO₂ by 2050. Such projects aim to use sedimentary basins as they are believed to be the safest option for long-term storage of CO₂ and other greenhouse gases. However, sedimentary basins are also valued for a range of extractable natural resources such as groundwater, oil and gas, and geothermal energy. Thus, there is a need to understand the potential interactions between these resources and subsurface storage of CO₂ when targeting a particular site for CO₂ storage.

A recently published study involving researchers from various institutes in Western Australia explain the potential risks involved with geological CO₂ storage. Considering all the potential interactions between basin resource use and CO₂ storage, the authors outline a Framework for Basin Resource Management Strategy (FBRMS) for optimizing interactions that may occur during the management of sedimentary basins.

Potential impacts of CO₂ geological storage on other basin resources (reproduced from K. Michael
et al., EAGE Third Sustainable Earth Sciences 2015 Conference, DOI: 10.3997/2214-4609.201414262)

The initial stage of the FBRMS is an assessment of potential basins for CO₂ storage based on containment potential of the subsurface. Optimal conditions include thick layers of low permeability rocks such as shales and anhydrites that can effectively cap the CO₂. Lateral containment should be provided by lateral decreases in permeability, low permeability faults, or, in the absence of physical barriers, residual trapping, dissolution into water and mineral formation via reactions with the subsurface matrix. In addition to geologic characterization, modelling, monitoring and risk assessment are crucial for verifying long-term CO₂ containment.

Also considered in the FBRMS are the two general means by which CO₂ storage can affect other basin resources: migration and increasing basin pressure. Vertical or horizontal migration may result in contamination of natural gas or another currently used or potentially extractable resource.

Once CO₂ is present in the subsurface, the future potential use of the injected area is instantly limited. An important concern associated with using shale basins is that shale may be used as an unconventional gas resource in future, and the methods for utilizing this resource negate its effectiveness as a long-term cap for CO₂ storage. Though the probability is extremely low, increasing subsurface pressure from CO₂ injection could also force saline water upwards along a wellbore or through existing fractures into groundwater resources, or may even cause fractures. Regardless of the likelihood, identifying all of these risks is an important facet of the FBRMS, presumably leading to relevant monitoring activities. On the other hand, increased pressure could also be beneficial in counteracting reduced pressure in mature oil or gas fields, low groundwater levels, or subsidence.

The FBRMS integrates all of these concerns to evaluate the likelihood of various basin resource-storage interactions, how beneficial or detrimental the interactions would be, and to determine how best to manage these interactions. It is intended to be used by various project stakeholders throughout the lifetime of a project, and may require intensive data collection or expert risk assessment depending on the individual project being assessed.

To read the full paper for free*, click the link below:

Framework for the assessment of interaction between CO₂ geological storage and other sedimentary basin resources
K. Michael, S. Whittaker, S. Varma, E. Bekele, L. Langhi, J. Hodgkinson, and B. Harris
Environ. Sci.: Processes Impacts, 2016, 18, 164-175
DOI: 10.1039/C5EM00539F

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About the webwriter

Abha Parajulee is a Ph.D. student at the University of Toronto Scarborough. She is interested in water resources and the behavior of organic contaminants in urban environments.

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* Access is free until 01/06/2016 through a registered RSC account.

Passive air samplers (PAS) have found great utility in monitoring environmental concentrations of semivolatile organic contaminants (SVOCs) all over the world. They provide a picture of longer-term average air concentrations of SVOCs while being relatively portable, low-cost and extremely low-maintenance. Knowing the deployment time, the amount of chemical accumulated in the passive sampling medium (PSM), and the sampling rate derived when a PAS is first calibrated before widespread use, a time-averaged volumetric air concentration can be calculated.

Graphical Abstract

A key assumption underlying the calculation of PAS-derived air concentrations is that the passive sampling medium takes up chemicals uniformly. But this assumption has not been thoroughly tested so far and studies to date have indicated that the sampling rates of some commonly used PSM can differ with position inside a sampler housing. For example, sampling rates decreased with increasing distance from the opening at the bottom of a cylindrical sampler housing for the commonly used styrene-divinylbenzene copolymer or “XAD” resin.

In a study recently published in Environmental Science: Processes & Impacts, Zhang and co-workers at the University of Toronto Scarborough have put their XAD PAS to the test once more to determine if exposure to sunlight, wind, and the presence of multiple units of XAD-filled mesh cylinders in one PAS housing caused differential chemical uptake across the length of a single cylinder.

The chemicals of interest in this series of experiments, polychlorinated biphenyls (PCBs), were chosen because their environmental partitioning properties are inclusive of a range of SVOCs commonly measured in the environment. One indoor experiment included axially segmented PAS at four indoor locations, one of which also used fans to simulate the effect of wind. At one of the indoor locations, a similar experiment was conducted outdoors, where the effect of heat conduction resulting from sunshine was also tested. This involved using PAS with regular housings, housings painted black to enhance heat absorption, and housings shaded by a steel cover.

Two additional experiments varied the number of mesh cylinders inside each housing. One experiment deployed a pair of PAS containing one and four mesh cylinders at one outdoor and one indoor location. A final outdoor experiment attempted to incorporate a variety of temperatures and wind speeds by deploying PAS at nine locations on the Big Island of Hawaii. Each site had one PAS containing one XAD-filled mesh cylinder and another containing two.

Environmental Science: Processes & Impacts front cover image highlighting the article

In the first indoor experiment, the total amount of PCBs accumulated in all segments was not significantly different from the amount accumulated in a mesh cylinder that had not been segmented. In those cylinders that were axially segmented, the amount of PCBs accumulated in the bottom segments was significantly higher than in the upper two segments in office and storage areas, and assumed to have little activity and therefore air turbulence. But this difference was not significant in the mesh cylinder placed in a cargo-loading area, presumably because of the relatively higher level of activity and therefore air turbulence. Similarly, gradients within PAS deployed outdoors were also not as strong, and the samplers exposed to fans indoors showed no significant gradients – strong indications that increased air turbulence allows for more uniform uptake across the length of the sampler.

The effect of heat convection on total accumulation and axial distribution of PCBs was determined to be minor, as was the presence of multiple mesh cylinders within one housing, but only outdoors. Indoors, the amount of PCB accumulated per sampler was significantly lower in those PAS with four mesh cylinders, and the gradient was also steeper.

The final outdoor deployment across varying temperature and wind conditions in Hawaii, which measured accumulation of PCBs, pesticides, polycyclic aromatic hydrocarbons, and polybrominated diphenyl ethers, showed no significant difference in chemical accumulation in PAS with one versus two XAD-filled mesh cylinders. The finding that uptake of SVOCs by XAD PAS is affected very little by the presence of multiple mesh cylinders in one housing in a variety of outdoor conditions means that fewer housings can be used during a given sampling campaign that uses XAD PAS. This augments the low-maintenance nature of this monitoring method, and thus the value of this particular PAS as a tool for monitoring SVOCs in the environment.

To read the full Open Access article, click the link below:

Exploring the role of the sampler housing in limiting uptake of semivolatile organic compounds in passive air sampler
Xianming Zhang, Michelle Hoang, Ying D. Lei, and Frank Wania
Environ. Sci.: Processes Impacts, 2015, 17, 2006-2012
DOI: 10.1039/C5EM00447K

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About the webwriter

Abha Parajulee is a Ph.D. student at the University of Toronto Scarborough. She is interested in water resources and the behavior of organic contaminants in urban environments.

Environmental Science: Processes & Impacts cover highlights this research (issue 9)

Monomethylmercury (MeHg) is a chemical of great concern due to its neurotoxic nature and its efficient bioaccumulation in aquatic systems, eventually reaching humans through fish consumption.

MeHg is produced by the action of bacteria that transform the most commonly found species of mercury in aquatic environments. Remediation of mercury-contaminated sites requires insight into factors that facilitate the action of these mercury methylators. For example, anaerobic conditions and relatively large quantities of total and dissolved organic carbon both enhance production of MeHg.

The original speciation of mercury is also important, as some forms of mercure are more bioavailable to mercury methylators than others. Past work has traditionally focused on the influence of these factors individually; however, under environmental conditions these factors likely work in concert to affect mercury methylation.

Kucharzyk and co-workers at Duke University take the next step forward with their recent study published in Environmental Science: Processes & Impacts which aims to assess the relative influence of microbial productivity and mercury speciation on MeHg production. The researchers enriched cultures of mercury methylating bacteria found in two different marine sediments containing similar, elevated mercury concentrations. The cultures were determined to contain mostly one type of anaerobic bacteria known to methylate mercury.

For each of the two cultures, microbial growth was varied by adding different amounts of carbon substrate, and mercury speciation was varied with the addition of either dissolved or nanoparticulate mercury. The cultures were then incubated for 64 hours, during which two or three replicates were analyzed for various chemical and biological parameters at several time points across the incubation period.

In both cultures, mercury methylation increased with increasing concentrations of carbon substrate for a given type of mercury. When carbon substrate concentration was kept constant, the percentage of mercury that was methylated was 3 to 4 times lower in cultures amended with nanoparticulate mercury relative to those containing dissolved mercury instead. This could not have been due to differences in bacterial growth rates as the observed cell growth was the same across both types of added mercury, implying that the differences are probably a result of lower bioavailability of nanoparticulate mercury versus dissolved mercury.

The differences in microbial productivity between cultures spiked with the two different types of mercury became smaller with decreasing levels of carbon substrate. Interestingly, this data suggest there may be a threshold in the activity of mercury methylating bacteria, below which net MeHg production is controlled by the availability of carbon substrate, and above which the bioavailability of mercury becomes more important. However, further study including lower levels of carbon substrate is required to better confirm the existence of this threshold in microbial methylation activity.

Click on the link below to read the full article for free*:
Relative contributions of mercury bioavailability and microbial growth rate on net methylmercury production by anaerobic mixed cultures
Katarzyna H. Kucharzyk, Marc A. Deshusses, Kaitlyn A. Porter and Heileen Hsu-Kim
Environ. Sci.: Processes Impacts, 2015, 17, 1568-1577
DOI: 10.1039/C5EM00174A

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About the webwriter

Abha Parajulee is a Ph.D. student at the University of Toronto Scarborough. She is interested in water resources and the behavior of organic contaminants in urban environments.

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* Access is free until 18/11/2015 through a registered RSC account.

Bisphenol A, phtalates, PCBs and other organic pollutants often embedded in plastics have been under the spotlight for a while. However, very few people have studied the degradation pathways of oceanic plastic debris. An article recently published in Environmental Science: Processes & Impacts critically reviews this topic.

Plastics have been widely used since the 1940s. In 2013, global plastic production reached 300 mega tonnes (50 times the Great Pyramid of Giza). We discard most of the plastic right away, within 3 years of its production. Yet, it is engineered to last for hundreds or even thousands of years. Thus, tonnes of plastic debris accumulate in the environment.

Scientists are especially concerned about plastic debris accumulating in the oceans. By 2050, 99% of all seabirds will have ingested, at least, one small piece of plastic. Nowadays, plastics represent more than 60% of all the floating debris in the oceans. All these pieces of plastic may release organic pollutants to the sea, either additives or adsorbed substance. Despite their high stability, plastics might also decompose by the action of different factors, releasing potentially dangerous chemicals.

Polymer degradation is, technically, a decline of its original properties. Usually it can be easily spotted by observing the colour changes or cracking of the surface. Small plastic particles decompose faster, due to their higher surface to volume ratio. Depending on the structure of their backbone, plastics can degrade in very different ways. Plastic with carbon-carbon backbones (i.e. PE, PP, PS, PVC) often suffer abiotic decomposition, usually initiated by UV radiation or thermal processes. Plastics with heteroatoms in their backbones (i.e. PU, PET) may also suffer abiotic decomposition, but in this case hydrolysis is the most common process. Furthermore, enzymes may also break amide and ester bonds, therefore these plastics are also exposed to biodegradation.

If you want to better understand the different degradation pathways, I strongly recommend that you go over this Environmental Science: Processes & Impacts Critical Review where all the mechanisms are carefully examined and explained.

Click on the link below to read the full article for free*:

Pathways for degradation of plastic polymers floating in the marine environment
Berit Gewert, Merle M. Plassmann and Matthew MacLeod
Environ. Sci: Processes Impacts, 2015, 17, 1513-1521
DOI: 10.1039/C5EM00207A

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About the webwriter

Fernando Gomollón-Bel is a PhD Student at the ISQCH (CSIC–University of Zaragoza). His research focuses on asymmetric organic synthesis using sugars as chiral-pool starting materials towards the production of fungical transglycosidase inhibitors.

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* Access is free through a registered RSC account.