Cadmium transport

Van Duyn-Henderson, J. and D.C. Lasenby. 1986. Zinc and cadmium transport by the vertically migrating opossum shrimp, Mysis relicta. Canad. Jour. Fish. Aquat. Sci. 43 1726-1732. 

Each country is not only a receptor but also a source of the trans-boundary transport. As much as 153 t (60% of anthropogenic emission in Europe) of cadmium, emitted in Europe, leaves the territory of the counties and is involved in the long-range transport. The highest absolute value—30 t/yr—of cadmium transported across national borders was obtained for Poland. The significant exporters of cadmium are Spain, the Russian Federation, Romania and Italy. Nearly 40 t of cadmium is transported outside the European Union. Besides, only nine countries control more than 75% of cadmium trans-boundary transport. 

Martin J., Bruland K. W., and Broenkow W. (1976) Cadmium transport in the CaUfomia current. In Marine Pollutant Transfer (eds. H. L. Windom and R. A. Duce). Lexington Books, Toronto, pp. 84-159. 

Martin, J. H., Bruland, K. W., and Broenkow, W. W. (1976) Cadmium Transport in the California Current, in Windom, H. and Duce, R., Eds., Marine Pollutant Transfer, Heath, Toronto. 

Variations of the feed and strip flow rates have little effect on the cadmium transport performance the values of individual cadmium mass-transfer coefficients are similar at carrier or strip flow rates variations. Thus, diffusion of cadmium species through the feed and strip aqueous boundary layers does not control the transport rate. The ratecontrolling steps could act as resistances to diffusion of the cadmium species in the carrier solution layers, especially in the membrane pores or the interfacial backward-extraction reaction kinetics. 

Resistance to diffusion in the LM solution layers and membrane pores is not a rate-controlling step, since the overall mass-transfer coefficients on the LM-strip interface of the system are two orders less than that on the feed-LM side. Thus, we can conclude that the interfacial backward-extraction reaction rate is a rate-controlling step of cadmium transport in the system. 

Table 2.2 Individual and overall mass-transfer coefficients, accounting for coupling effects ofthe cadmium transport (external driving force). Combined overall mass-transfer coefficients...

Figure 2.8 shows the dependence of the cadmium transport internal driving force coefficients, external driving force coefficients, K, and overall mass-transfer coefficients, K, on feed flow rate (Fig. 2.8A) or strip flow rate (Fig. 2.8B) variations. It is clearly seen that the resistivity to the diffusion of protons is much lower than that of cadmium species themselves. 

Figure 2.9 Cadmium transport concentration profiles. Comparison of the calculated (continuous lines) and experimentally obtained (dotted curves) data.

External, coupling driving force motivated cadmium transport perhaps is not a high factor in the system studied, because there is no a large proton concentration gradient between feed and strip aqueous phases. 

Many researchers, proposing the LM processes for application, are based on the steady state of the system. Experimental and model simulation data show much higher mass-transfer rates through the HLM (for details see Chapter 5), with cadmium concentration in the carrier solution, reaching its maximum. At this stage, the both internal (extraction-backward-extraction distribution ratio) and external (coupling) driving forces motivate the cadmium transport in an optimal way. 

At steady-state cadmium transport permeation is motivated mostly by an external driving force and the fluxes are about an order lower. A much more effective HLM module, with continuously flowing feed (open system), can be designed if the feed side membrane area Sp and the feed flow rate Up enable us to obtain a fixed cadmium feed outlet concentration (e.g., 1 ppm) at a contact time, less than that at the maximum on the simulated concentration profile of the carrier solution. 

Buchter, N., C. Hinz, M. Gfeller, and H. Hinz. 1996. Cadmium transport in an unsaturated stony subsoil monolith. Soil Sci. Soc. Am. J. 60 716-721. 

An interesting example of the application of SEC is the separation of free Cd (II) and the complex with fulvic acid (FA), which is the soil organic acid considered to be responsible for metal ion transport in the environment. Cadmium is of increasing concern as a heavy metal environmental pollutant and the understanding of cadmium transport requires knowledge of the equilibrium constant for... 

Cook and Morrow 1995, Anthropogenic Sources of Cadmium in Canada, National Workshop on Cadmium Transport Into Plants, Canadian Network of Toxicology Centres, Ottawa, Ontario, Canada, June 20-21,1995. 

Total copper and cadmium concentrations increased from Sites 1-4, ranging from 1.2 to 6.8 /xg/L for copper, and 0.2 to 1.0 /xg/L for cadmium. Copper transported in the dissolved phase ranged from 56% at Site 4 to 68% at Sites 1 and 3, and 80% at Site 2. Dissolved cadmium accounted for more than 50% of the total concentrations at all sites. Thus, copper and cadmium transport in the Mississippi River was dominated by apparent solution-phase transport. Total mean lead concentrations ranged from 2.3 to 5.7 /xg/L at Sites 1-4, with dissolved metal accounting for 16 to 38% of the total. The unexpectedly large contribution of dissolved lead indicates the potential role played by organic matter in maintaining lead in solution. 

Figure 13. Cadmium transport phases in the upper Mississippi River,...

Metal oxide coatings are most important for manganese, iron, and cadmium transport at Sites 3 and 4, but represent less than 25% of the total metal transport in all cases. Crystalline-bound aluminum (> 80% ) and iron (> 50% ) represented the dominant transport mode, which was unimportant for copper, cadmium, and manganese, and crystalline lead contributed about 20 and 6% at Sites 3 and 4, respectively, to total lead transport. The decrease in crystalline lead at Site 4 may result from dilution of lattice-bound lead by solid organic lead emanating from the sewage effluent. 

Cadmium transport within plant and cadmium phytotoxicity... 

Pigman, E.A., J. Blanchard, and H.E. Laird 11. 1997. A study of cadmium transport pathways using the Caco-2 cell model. Toxicol. Appl. Pharmacol. 142 243-247. 

Tsai K-J, Linet AL (1993) Formation of a phosphorylated enzyme intermediate by the cadA Cd -ATPase. Arch Biochem Biophys 305 267-270 Tsai K-J, Yoon KP, Lynn AR (1992) ATP-dependent cadmium transport by the cadA cadmium resistance determinant in everted membrane vesicles of Bacillus subtilis. J Bacteriol 174 116-121... 

Divalent metal transporter 1 was identified in the enterocytes of the small intestine and in other cells as a proton symporter, favoring the transport from more acidic media, and it was assigned a major role in the assimilation of iron from the animal diet [54]. But its lack of selectivity in reconstituted systems soon became apparent, showing that Cd " is efficiently transported by DMTl in vitro [54,55]. This observation was extended to include a correlation of DMTl expression and Cd " transport, both in cellular models [56-58] and in animals with iron-manipulated diets [59,60]. Furthermore, animal [61], and human [62] studies comparing iron status and cadmium burden fully corroborated the association of Cd " transport with the plasma membrane form(s) of DMTl. Hence, there is no doubt that DMTl transports Cd " in cells that produce this transporter. This conclusion is relevant to an individual s susceptibility to cadmium toxicity since DMTl is strongly iron-regulated and cadmium transport thus depends on the nutritional and (patho)physiological iron status, in particular for intestinal absorption. 

The fact that metal cation importers mediate cadmium entry into cells whereas Cd " does not appear to be a substrate for exporters of essential transition metals points at an important issue of cadmium speciation in biological systems. Upon exposure of cells, particularly those protecting the body from its envirmunent, such as epithelial cells, Cd " is presented in relatively weak complexes allowing Cd " to easily dissociate and bind to cadmium transporters, and it may cross membranes as a free ion. In contrast, once inside cells, strong Ugands of Cd " are generally... 

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