Transport mechanism aquatic systems

Transport in solution or aqueous suspension is the major mechanism for metal movement from the land to the oceans and ultimately to burial in ocean sediments. In solution, the hydrated metal ion and inorganic and organic complexes can all account for major portions of the total metal load. Relatively pure metal ores exist in many places, and metals from these ores may enter an aquatic system as a result of weathering. For most metals a more common sequence is for a small amount of the ore to dissolve, for the metal ions to adsorb onto other particulate matter suspended in flowing water, and for the metal to be carried as part of the particulate load of a stream in this fashion. The very insoluble oxides of Fe, Si, and A1 (including clays), and particulate organic matter, are the most important solid adsorbents on which metals are "carried."... 

By no means is this book intended to provide a detailed account of all progress made in the past 25 years by aquatic chemists. Rather, chapters provide examples of recent developments in the field and contribute toward a better understanding of the mechanisms regulating the chemical composition of natural waters. Also, the transformation and transport of species (abiotic and biotic or soluble and insoluble) in aquatic systems (lakes, rivers, estuaries,... 

The physical transport of dissolved xenobiotics within aquatic systems may obviously take place under the influence of currents and tides. It has also been shown, however, that particulate matter and river sediment may be important vehicles, and that the dissemination of xenobiotics may also take place by subsequent diffusion from these transported sediments into the water column. An attempt will be made to illustrate the operation of these mechanisms. 

It has been suggested that bacteria may play a role in the transport of hydrophobic compounds in soils (Lindqvist and Enfield 1992), and it has been shown in batch experiments with Bacillus subtilis that sorption of 2,4,6-trichlorophenol may involve both neutral and anionic species (Daughney and Fein 1998). This mechanism could potentially apply also to aquatic systems where such processes could reasonably be included under particulate transport. There are, however, obvious unresolved issues concerning the subsequent desorption and bioavailability of these sorbed compounds. 

The wide use of nitro organic based energetic chemicals (NOCs), such as the aromatic TNT, and the nonaromatic cyclic nitramines RDX and HMX has resulted in the contamination of terrestrial and aquatic systems. Several reports (see Chapters 3-5 and 7-9 of this book) described the toxic and carcinogenic effects of explosives and their degradation products to various terrestrial, aquatic, and avian receptors. However, to determine the true identity of the chemicals that cause toxicity, the transport and transformation mechanisms of these chemicals must be understood. 

The DLVO-Lifshitz theory should be regarded as a principal mechanism governing the adsorption of viruses on various inorganic surfaces. This finding has direct application to problems concerning transport of viruses in aquatic systems and soils. It is possible that it could lead to the design and optimization of adsorption-filtration processes for removing viruses and other particulates from contaminated water. 

Distribution pathways are summarized in Figure 1.1. First of all, there can be movement within a compartment for example, any chemical introduced into an aquatic compartment can move to the extent that the water moves, whether or not the chemical is in solution or sorbed on a particle. This movement would be defined by the appropriate hydrological parameters. A chemical may find its way into the atmosphere where it may be transported in atmospheric currents In this situation the appropriate meteorological phenomena will determine the rate and direction of movement. Distribution in a plant or animal wifi be controlled by the transport mechanisms in that organism either the vascular system in an animal or the phloem in a plant. In a much broader context, the transport of a chemical in an ecosystem must have some relation to the overall mass flow in the system since the chemical moves with the food constituents of the various components in the ecosystem. 

The removal of heavy metal ions from both natural water supplies and industrial wastewater streams is becoming increasingly important as awareness of the environmental impact of such pollutants is fiilly realized. In particular, the likelihood of such metal ions precipitating out of solution and/or coating other materials can have a profound effect on both aqueous and nonaqueous environments. There is considerable evidence in the literature that the primary mechanism for transportation of metal contaminants in aquatic systems is the movement of suspended particulate material containing the adsorbed pollutant metals [1,2]. It is also known that a strong correlation exists between the concentration of trace metals in the (aquatic) environment and the extent to which those metal ions adsorb onto colloidal substrates present in the environment [2,3], A similar correlation between the concentration of trace metals in the (aquatic) environment and their precipitation behavior is not so clear. There is, then, a well-founded need to study adsorption-related phenomena in order to understand and predict the behavior of toxic metals in the environment. 

Chemical transport across the water-sediment interface takes place through numerous chemical, biological, and physical mechanisms (Reible et al., 1991 Thibodeaux and Mackay, 2007). These reflect in large part the characteristics of the particular aquatic system, which include flowing freshwater streams, lakes, estuaries, and the marine, both near shore and beyond. This chapter is focused on so-called diffusive type processes on either side of the sediment-water interface. Specifically, it covers the water-side mass transfer coefficient in the fluid boundary layer above the bed and diffusion within the interparticle pore spaces in the near-surface bed sediment layers. 

Fig. 2.6 Molecular mechanisms hold to explain accumulation of transition metal ions by and in plants. Letters (a) to (e) are to be taken in the same vertical arrangement in both plant and this picture, e.g. a = mobilization around the root, c = transport within the xylem. (a) metal ions get mobilized by secretion of chelators which in addition acidify the rhizosphere. (b) uptake of hydrated metal ions or (rather) their chelate complexes is augmented by various systems bound to the plasma membrane, (c) transport of transition metals from roots to shoot occurs via the xylem. Presumably the larger share is transported by means of the root symplast an apoplastic passage in the root tips is also conceivable. After exchange (oxidative destruction) of the original ligands metals which made it into the xylem are other kinds of chelator complexes or else aquated ions, (d) After getting into the leaf apoplast several metals are bound to the...

This topic will be developed by first outlining the stmctural features of a biological membrane and discussing the different mechanisms involved in transporting compounds across this barrier. The extent to which absorption is influenced by the properties of the compound will be illustrated by considering both animal and plant systems. Some compounds can absorb and accumulate, particularly in aquatic organisms, and the requirements for this to occur will be considered. 

---