‘Frozen in or chilled out?’ Clay mineral-herbicide reactions in freeze–thaw cycles.

Biogeochemical Cycles



Following a rapid rise in the world population beginning in the 19th century, the use of artificial chemicals to aid in crop growth has become a non-negotiable component of modern agriculture and sustaining human population levels. Herbicides, substances that control and prevent the growth of detrimental competitor plants, are one such group, with global production nearly doubling in the first two decades of the new millennium (from 1.3 to 2.2 million tonnes) [1]. The role they perform is essential to crop growth, however the harmful environmental effects pesticides can have on the environment has been well established (notably including the use of DDT as an insecticide from the mid-1940’s).

A plethora of research has been carried out into the processes that result in the bioaccumulation and subsequent effects of herbicides, however, there has been relatively little work done in investigating the effect variable environmental conditions have on the uptake of herbicides. [2] This poorly understood area may be vitally important to continued efforts to make pesticide use environmentally conscious and reduce long-term damage to ecosystems, as well as enabling predictions related to the impact of climate change on agriculture. In particular, freezing results in concentration of ions and dissolved species into concentrated brines as water molecules are removed into the ice lattice. This happens from the surface downwards and may both drive herbicides into the lower soil and act to concentrate them, as well as amplify any soil water pH.

Clay minerals make up large proportions of soil (up to 25%) and are known to adsorb many different types of molecules (including the small organic cations found in common herbicides) which is responsible for their long-term presence in ecosystems. [2]. The absorption of molecules by clay minerals depends on the initial concentration present, hence freezing processes may drastically increase the herbicide concentrations near the clay mineral. How this adsorption changes with variable climatic conditions is presently unclear, however initial studies by the Greenwell group, via an IAPETUS Research Experience Placement, suggests that the adsorption isotherm of phenylalanine (an amino acid precursor to the phenoxy-herbicides) to the clay sodium montmorillonite is impacted when exposed to freeze-thaw cycles (Figure 1). [3].

This proposed project will build on this previous work to more firmly establish the relationship between climate and clay-herbicide interactions, then identify the interactive mechanisms responsible for the altered adsorption characteristics observed under different conditions. A number of key research questions will be addresses by the project;

• What is the relationship between molecular adsorption and freeze-thaw processes?
• How do other factors (pH, repeated dosing of herbicide, repeated environmental cycles etc…) affect the observed relationship and what chemical reasoning can be established for this?
• Can similar phenomena be observed for other climatic processes, for example seasonal heat and dryness?
• What is the potential impact of these processes on the use of herbicides for agricultural purposes?

This fundamental science that will be developed during this PhD studentship also has relevance to a wide range of other science disciplines where mineral-organic molecule interactions during freeze-thaw cycling may be important. For example, freeze-thaw concentration processes can be important as a control on rates of biogeochemical cycling of carbon and nutrients in the cryosphere [4]; and could impact the rates of persistence of anthropogenic organic pollutants such as pesticides in fragile Arctic ecosystems [5]. This research may also gain insight into the potential survival of organic molecules Mars, where freeze thaw cycles within near-surface brines will have been active in the deep past, and may even continue to a limited extent to the present day [6].


Representative clay minerals and herbicide molecules will be placed in suitable vessels (centrifuge tubes) with model soil pore waters (0.01 M CaCl2). These will be subjected to unidirectional freezing using specific methods developed by the Greenwell Group. The frozen ice core is then sectioned along its vertical profile and each section analysed for the herbicide using UV-vis spectroscopy. The clay mineral-herbicide complex will be analysed by X-ray diffraction, thermogravimetric analysis, infrared, and using a novel stable isotope mixing method developed by the Durham investigators.

Project Timeline

Year 1

Month 1-3. Detailed literature review on clay mineral/soil herbicide interactions, with emphasis on effect of freezing. Search of wider literature on freezing and mineral interactions.

Month 4-6. Training on laboratory techniques, analytical methods and set-up of control experiments (concentration effects of freezing (no clay). Plan first experimental conditions.

Months 6-12. Undertake experiments of simple herbicide molecules/analogues on common soil clay minerals (kaolinite, montmorillonite) with water and comparison with controls.

Year 2

Months 13-18. Experiments of simple herbicide molecules/analogues on common soil clay minerals (kaolinite, montmorillonite) with model pore water and comparison with controls and previous experiments. Produce first paper.

Months 19-24. Experiments of simple herbicide molecules/analogues on common soil clay minerals (kaolinite, montmorillonite) with model pore water and pH effects, comparison with controls and previous experiments. Produce second paper.

Year 3

Months 25-30. Experiments of simple herbicide molecules/analogues on common soil clay minerals (kaolinite, montmorillonite) with model pore water and pH effects, now with added model soil organic matter, comparison with controls and previous experiments. Produce third paper.

Months 31-36. Understand fundamentals of interactions. Compare one set of experiments with similar experiments performed using heating to concentrate the herbicides. Identify mechanisms of absorbtion and effect of ice at mineral surface.

Year 3.5

Completing writing up of thesis and final journal articles.

& Skills

Training will be given in clay mineralogy, absorption isotherms, soil structure and composition, understanding dissolved organic species transport and fate in environment, use of spectroscopy and isotopes in environmental science.
Specific skills will be gained in laboratory experiment design and implementation, spectroscopy, use of stable isotopes, mineralogy and surface chemistry.

References & further reading

[1] Food and Agricultural Organization of the United Nations, http://www.fao .org/faostat/en/#data/RP/visualize, (Accessed July 2021).[2] Yariv, S. & Cross H, (2002) Organo-Clay Complexes and Interactions, Marcel Dekker, New York, 1st Edn.[3] Wagstaff, O.J, (2021), ‘Frozen in or chilled out?’ Clay mineral-herbicide reactions in freeze–thaw cycles., unpublished (Available on Request).[4] Telling J, Anesio AM, Tranter M, Fountain A, Nylen T, Hawkings J, Singh VB, Kaur P, Musilova M, Wadham JL (2014) Spring thaw ionic pulses boost nutrient availability and microbial growth in entombed Antarctic Dry Valley cryoconite holes. Frontiers in Microbiology 5, 694.[5] Pawlak, F., Koziol, K., Polkowska, Z. (2021) Chemical hazard in glacial melt? The glacial system as a secondary source of POPs (in the Northern Hemisphere). A systematic review. Science of The Total Environment 778, 145244.[6] Rivera-Valentín, E.G., Chevrier, V.F., Soto, A., and Martínez, G. (2020) Distribution and habitability of (meta)stable brines on present-day Mars. Nat Astron. 2020 Aug; 4: 756–761. doi: 10.1038/s41550-020-1080-9.

Further Information

For further information or an informal discussion, please contact Prof. Chris Greenwell (Tel: +44 1913 342324; E-mail: chris.greenwell@durham.ac.uk).

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