Colloidal bound metals

SOME IMPLICATIONS OF A RATE-LIMITING DIFFUSION 7.5.1 Bioavailability of Colloidal Bound Metal... 

F igu re 15.3 Percentages of Cd, Cr, and Zn in the cytoplasm of the diatom, Thalassiosira pseudonana, and the dinoflagellate, Procentrum minimum, after 1 day exposure to low molecular (LMW) bound metals and colloidal bound metals (COC). (Modified from Wang and Guo, 2000.)... 

Ruzic [278 ] considered the theoretical aspects of the direct titration of copper in seawaters and the information this technique provides regarding copper speciation. The method is based on a graph of the ratio between the free and bound metal concentration versus the free metal concentration. The application of this method, which is based on a 1 1 complex formation model, is discussed with respect to trace metal speciation in natural waters. Procedures for interpretation of experimental results are proposed for those cases in which two types of complexes with different conditional stability constants are formed, or om which the metal is adsorbed on colloidal particles. The advantages of the method in comparison with earlier methods are presented theoretically and illustrated with some experiments on copper (II) in seawater. The limitations of the method are also discussed. 

Colloid concentrations in the eluent were determined with a Bio-Tek multichannel (optical densitometer with fiber-optics technology Bio-Tek Instruments, Winooski, VT) microplate reader, precalibrated with known concentrations of each colloid at 540 nm. Total metal concentration in the eluents was allocated to solution phase and colloidal phase (colloid-bound contaminant). The eluent samples were centrifuged for 30 min at 3500 rpm (x 2750 g) to separate the soluble contaminant fraction from the colloid-bound contaminant fraction. The absence of colloidal material in the supernatant solution was verified by filtration through a 0.2-pm membrane filter. The soluble metal (Cu, Zn, Pb) fractions were analyzed by atomic absorption (concentrations >0.5 mg/H) or inductively coupled plasma (ICP) spectrometry (concentrations <0.5 mg/H). The colloid fraction was extracted with 1 M HNO3-HCI [32] solution and analyzed with the same methodology used for the soluble fraction. The results for the duplicate soil monoliths and for the two soils were combined for practical purposes, because the reproducibility between soil monoliths was within 15%. 

Other examples of ultrafiltration as a separation operation on a laboratory scale employing continuously operated membrane reactors (vide infra) have been reported [1, 29, 30]. These examples will be discussed throughout Section 7.4. Continuous ultrafiltration of polymer-stabihzed metal colloids, in addition to polymer-bound metal complexes, has also been studied [31]. 

Colloid-bound Cd is taken up by phytoplankton either through dissolution of Cd or, possibly, through direct internalization of lipophihc colloid-bound Cd [66]. As is the case for other metals, Cd complexes with low molecular weight lipophilic organic ligands enter phytoplankton by passive diffusion across the plasma membrane [67]. 

Colloidal and particulate metal species also contribute to maintaining low free metal ion concentrations. The behaviour of colloidal species of similar size may however be very different, depending on their chemical speciation and on their structure. Metals bound to HA of large size may exchange with the... 

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