Threats
Threats to aquatic systems.
When fundamental research is involved.
Aquatic systems are essential in the everyday life. Our health as well as a large number of our activities is related to their good conditions. Globally, they are more and more threaded and altered by economic activities and world demographic increase. Recently in the Hunan province, in the south east of China, more that 500 people have been contaminated by an accidental release of Cadmium in the Xiangjiang river, which is also a fresh water reserve for more than 6 million people. Climatic changes will certainly have non negligible impacts on the aquatic systems. Indeed, global warming will deserve fresh water quality and, as a result, new techniques will be needed to produce potable water. The decrease of the quality of aquatic systems will also affect Switzerland and the Office Federal for the Environment is expected to impose new limits and rules for micro pollutant concentrations for a better protection of fresh waters.
In aquatic systems, the accumulation, circulation, transport and pollutant toxicity, are still ill understood because of their complexity and chemical heterogeneity. However, water is not the only important component and the suspended natural material is also playing an important role. As a result colloids which represent the major fraction of the suspended material are playing a key role in the physical and chemical reactions and processes found in aquatic systems. Colloids are composed of inorganic particles (clays, silica, iron hydroxides,…), organic compounds (polysaccharides, biopolymers,…) and supramolecular compounds (fulvic acids).
Colloids are directly participating to the stability of the aquatic systems, and, depending on the physico-chemical conditions (pH, ionic strength, redox potential), they undergo transformation and alteration processes. They can also associate to each other to form aggregates which will be eliminated from the water column by sedimentation process. Moreover, owing to their high specific surfaces, colloids exhibit strong adsorbing properties for dissolved species such as pollutants. Thus, all these processes are controlling the transport and mobility of many pollutants, and largely modifying their chemical reactivities. Understanding the exact role and dynamics of colloids is hence of prime importance so as to better understand the evolution of aquatic systems and to better protect them.
The research of the environmental physical chemistry group at the F-A. Forel Institute is related to the identification and understanding of the key processes which are controlling the pollutant circulation and the overall behaviour of colloids in aquatic systems. Research is made using computer modelling, as well as laboratory studies, and is situated at the interface between environmental chemistry, soft matter science, analytical chemistry and computational chemistry. It involves many physical and mathematical aspects dealing with adsorption and aggregation processes, sedimentation and supramolecular assemblies through, for example, fractal concepts.
The behaviour of iron hydroxyde particles was investigated in presence of fulvic compounds (natural colloids). Manufactured iron hydroxide nanoparticles are part of a new class of polluting and emerging compounds which are potentially dangerous for the human health1. Unfortunately, the industrial and massive production of nanoparticules has already started in parallel to the development of nanotechnologies, despite the fact that their possible impact and toxicity on the aquatic systems is still not understood.
As shown in the above picture, the nanoparticle reactivity (red spheres) is directly related to the fulvic acid quantity (yellow spheres) which is adsorbed at the nanoparticle surface. The number of adsorbed nanoparticles and the nature of the interface (total charge for example) are expected to control the production of reactive species such as free radicals. In one of our articles2 in the Environmental Sciences and Technology journal, we clearly demonstrate that the total amount of adsorbed fulvics, structure of the interface, and possible nanoparticle aggregation are directly related to the solution ionic strength, pH, nanoparticle surface charge, thus allowing to discuss some hypothesis related to the toxicity of nanoparticles in aquatic systems.
S. Stoll
1. Nanoparticules, Effets on Human and Environment, D. Palomino, Gas Wasser Abwasser, 12 (2009), 979-990.
2. Modeling the Adsorption and Coagulation of Fulvic Acids on Colloids by Brownian Dynamics Simulations, Seijo M., Ulrich S., Filella M., Buffle J., and Stoll S., Environmental Science and Technology (2009), 7265-7269.