Environmental Science: Processes & Impacts blog

Determining the fate of compounds once they’re released into the environment is a complex issue. However such study is vital in order to assess the persistence of a compound as well as its bioavailability.  

It is well-established that polyaromatic hydrocarbons (PAHs) bind to organic complexes through hydrophobic interactions and that this can occur within soils to matter such as humus and soot etc.  So how does the presence of multi-walled carbon nanotubes (MWNTs) impact this?  Particularly given they are organic particles with relatively high surface areas available for adsorption.  Given the increasing interest in MWNTs there is now an elevated risk of ‘traditional’ organic pollution being released into an environment where MWNTs are already present, co-released with them or for MWNTs to be released onto existing polluted soil.

Li et al. used batch equilibrium experiments to assess the relationship between soil types, MWNTs and PAHs.  The MWNTs used were 1-3 µm long, 11 nm diameter with relatively little bundling providing a high surface area.  The group assessed three soil types: sand, sandy loam and silt loam with 2 mg g^-1 of MWNTs.  The compounds and concentrations of PAHs assessed were naphthalene 0.18–7.94 mg L-¹, fluorine 0.16–1.62 mg L^-1 and phenanthrene 0.1–0.91 mg L^-1.  Sorption tests (HPLC-fluorescence) were carried out after 5 days, at which point the aqueous solution was removed and replaced with fresh and left for 24hrs under the same conditions to assess desorption.

The results confirmed that the sorption of these compounds increased with increasing organic content of the soil. However, the presence of MWNTs did not appear to influence this adsorption, even at these relatively high concentrations of MWNTs.  Desorption was minimal in all cases.

In addition, the group derived equations based on the ‘rule of mixtures’ capable of predicting the sorption coefficients of composite sorbents.  They found good correlations between predicted and experimental data using these equations despite limitations of the model, such as assuming no void spaces. 

This HOT article would be of interest to anyone carrying out batch equilibrium experiments, or is interested in the sorption of PAH and/or the fate of MWNTs.  You can access it from the website for free for the next 4 weeks*!

Polyaromatic hydrocarbons (PAHs) sorption behavior unaffected by the presence of multi-walled carbon nanotubes (MWNTs) in a natural soil system.
Shibin Li, Todd A. Anderson, Micah J. Green, Jonathan D. Maul and Jaclyn E. Cañas-Carrell
DOI: 10.1039/C3EM00099K

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In this HOT article, collaborators in Australia and California devise methods of assessing contributions of landfill and natural gas methane to mixtures in soil gas and groundwater. They use half-lives and concentration ratios to evaluate the age of release.

The group at URS Australia Pty Ltd, Geosyntec Consultants and Redwood Waste Management California, focus on the problem of methane from landfill gas migrating away underneath the surface of landfill sites. It is a particular problem due to methane’s flammable nature. Landfill gas is not the only source of migrating methane. Sources include natural organic matter decomposition, natural gas in supply lines, degradation of petroleum products and underground reservoirs of natural gas. There are many indicators used to determine the source of natural gas, such as the presence of CO₂ being a marker for biodegradation. Carbon and hydrogen isotope composition is also used.

These researchers argue that using multiples of these indicators is the most reliable way to understand sources and migration pathways. This paper introduces a methodology to assess all of these different indicators at a complex site with multiple methane sources. Knowing the age of landfill gas using VOCs concentrations is a helpful parameter when assessing migration distance and time. This methodology using methane radioisotope data can distinguish on-going release from an inactive source and determine relative contributions of landfill gas and thermogenic methane to the environment. The theoretical basis for estimating landfill gas age is described in detail and applied to a case study at a municipal solid waste disposal facility in California.

As always, we’ve made this fascinating HOT research free to access for 4 weeks*!

Evaluation of the age of landfill gas methane in landfill gas–natural gas mixtures using co-occurring constituents
Henry B. Kerfoot, Benjamin Hagedorn and Mark Verwiel 
DOI: 10.1039/C3EM30971A

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J. Abell and colleagues at the University of Waikato and National Institute of Water and Atmospheric Research, New Zealand, have quantified nutrient and sediment inputs to a large eutrophic lake. They draw conclusions on how hydrological and landscape factors interact to produce pollutant flux.

Pollutant concentrations after a rain event such as a storm vary greatly and therefore high-frequency sampling is needed.  The amount of pollutant must be measured as a function of the stream discharge. The relationship between the two parameters can provide insights into sources and transport mechanisms in a catchment area. A catchment area can act as a filter, regulating downstream transport.  Therefore changes over time that occur after a rain event give insights into upstream hydrological and landscape factors.

The team looked at two streams flowing into a large eutrophic lake, focusing on levels of suspended sediment, nitrogen and phosphorus over a range of discharge values. One stream flowed through a mainly forested area, the other through pasture land. They conducted high frequency sampling over two years for both streams, taking over 900 samples. Comparison of two streams allowed spatial conclusions to be drawn.

This article includes an in-depth discussion of the factors related to suspended sediment and nutrient levels of the two streams, and how all of these factors contribute to the eutrophic nature of the lake. The researchers examine how these results could to lead to improved management of lake water quality.

Free to access for 4 weeks*, read the detailed discussion here:

Quantifying temporal and spatial variations in sediment, nitrogen and phosphorus transport in stream inflows to a large eutrophic lake
J. M. Abell, D. P. Hamilton and J. C. Rutherford
DOI: 10.1039/C3EM00083D

*Free access to individuals is provided through an RSC Publishing personal account. Registration is quick, free and simple