Mechanisms of metal removal

Stephenson T, Lawson PS, Rudd T, et al. 1987. Mechanism of metal removal in activated sludge. Journal of Environmental Engineering 113 1074-1087. 

Other proposed mechanisms of metal removal from sewage include physical capture by microbial floes, cellular accumulation and volatilization by such organisms as Klebsiella, Pseudomonas, Zoogloea and Penicillium spp. (Brown Lester, 1979). In laboratory and pilot-scale studies, up to 98% metal removal by these mechanisms combined has been documented (Lester, 1987). 

Beyond the biological process of sullate reduction with subsequent metal precipitation as sulfides, other mechanisms of metal removal can be present during the runs, particularly in the inoculated column (A) precipitation as metals carbonates using the bicarbonate and/or carbonates formed during the reduction of sulfate by the SRB cells or by fermentation from other microorganisms, precipitation as metal hydroxides, complexing with substances excreted by the cells, and accumulation on the surface of cells, through reactions between metal ions and cell wall components [10]. 

In Section IV, the kinetics and mechanisms of catalytic HDM reactions are presented. Reaction pathways and the interplay of kinetic rate processes and molecular diffusion processes are discussed and compared for demetallation of nickel and vanadium species. Model compound HDM studies are reviewed first to provide fundamental insight into the complex processes occurring with petroleum residua. The effects of feed composition, competitive reactions, and reaction conditions are discussed. Since development of an understanding of the kinetics of metal removal is important from the standpoint of catalyst lifetime, the effect of catalyst properties on reaction kinetics and on the resulting metal deposition profiles in hydroprocessing catalysts are discussed. 

Similar electropolishing experiments were carried out using different grades of stainless steel (410, 302, 304, 316 or 347) and it was found that the mechanism of metal dissolution and the oxidation potentials for the metals were very similar. The slight exception was the 410 series steel (which has no Ni, unlike the 300 series steels which have 8-14%). The 410 steel required a more positive oxidation potential to break down the oxide in the ionic liquid whereas once the oxide was removed the... 

Pamukcu and Wittle [133] investigated the feasibility of electrokinetic treatment at 30 V of different clay mixtures containing a number of heavy metals including Cd, Co, Ni, and Sr. The metal removal success ranged between 85-95% and appeared to depend on the soil matrix, the metal, and the pore fluid composition. At low initial metal concentrations, electroosmosis appeared to be the dominant mechanism for metal removal. At higher concentrations, electrolytic migration of the ionic species played a more dominant role. Of the three soil types tested, kaolinite had the highest electroosmotic efficiency. 

Improving the efficiency of metals removal using wetlands has proved to be difficult. Detailed assessments of long-term performance of wetland systems with a focus on metal-retention mechanisms, eutrophication, and flushing rates, and their effects on downgradient ecosystems are required if wetland treatment is to be applied in temperate regions. 

A reference vanadium deposition experiment is carried out in order to assess the influence of quinoline and HjS. Quinoline showed to decrease the rate of metal removal, the amount of vanadium deposited is lower as compared to the reference experiment. The shape of the vanadium deposition profiles is similar in both cases. A deposition maximum is observed in the centre of the pellet, indicating that the vanadium deposition process is not diffusion limited and that a sequential reaction mechanism applies for VO-TPP HDM. Low H2S partial pressure resulted in different vanadium deposition profiles as a function of the axial position in the reactor. At the inlet of the reactor, similar shaped profiles as the reference experiment were found, however, at the outlet of the reactor a shift towards M-shaped profiles was found indicating a diffusion limited vanadium deposition proeess. This shift in vanadium deposition profiles is explained by the build-up of the last intermediate resulting in a higher metal deposition rate. 

The vanadium deposition process showed profiles with deposition maxima in the centre of the pellet indicating the absence of diffusion limitations and supporting a sequential reaction mechanism for VO-TPP HDM. Quinoline addition showed to have an decreasing effect on the rate of metal removal and showed similar shaped deposition profiles. The low HjS partial pressure caused a change of the vanadium deposition profiles into M-shaped profiles due to the build-up of the last intermediate and an increasing metal deposition rate. 

Mechanism and the transport rates were studied for the Am(III) permeation through the SLM with octyl (phenyl)-N,N-diisobutylcarbamoyl-methylphosphine oxide as the carrier and diethylbenzene as the diluent. At low metal concentrations, the rate of metal diffusion of through the stagnant layer of the feed phase, adjacent to the feed phase/LM, was the rate-limiting step of metal removal from the wastewater [129]. At high metal concentration, the rate-limiting step of the removal/permeation process was the diffusion of the Am(IIl)-carrier complex through the LM phase [129]. The scope of the study was further widened to include aU actinides and lanthanides [64]. This study also serves as an example of more complex stoichiometries of the extraction reactions. 

In this study, the investigations were concentrated on the polishing of the Ai alloys and the adhesion/barrier metal (Ti) in the slurries containing A1,03 abrasive, phosphoric acid, citric acid and hydrogen peroxide. The electrochemical analyses, including the potentiodynamic polarization and galvanic corrosion measurements, have been used to elucidate the electrochemical mechanism of the removal selectivity in Al CMP. 

Unfortunately, higher amoimts of some minerals in seaweed have been the result of pollution of the seawater or natural environment. Thus, many studies were conducted with respect to the contamination of seaweed by heavy metals. Because of their high sorption capacity, they were also probed for their utilization as biosorbents to remove heavy metals from the environment and to elucidate mechanisms of metal biosorption by seaweeds (Davis et al., 2003 Murphy et al., 2008 Suzuki et al., 2005). Further, these conclusions could be utilized for the understanding of the uptake mechanisms by seaweed. Finally, endogenous and exogenous factors have participated on the variability of seaweed mineral composition. 

The mechanism of conversion of an achiral molecule into a chiral one was found by Kuhn in circular dichroism spectra. The type of natural quartz used, amethyst crystals, contained colloidal inclusions of Fe. The circular dichroism spectra revealed bonds involving Fe. It would be of importance from that point of view if the films of metals removed from surfaces of quartz crystal would show circular dichroism spectra, too. 

The biphasic kinetic pattern described for the removal of iron from transferrin by pyrophosphate can be ascribed to the two different iron-containing sites in transferrin/ Various anions and acids can assist such removal/ Details of iron removal have been probed by studying the kinetics of metal removal from transferrin derivatives containing iron and cobalt variously distributed between the two inequivalent binding sites, and from transferrins containing iron in only one of the two sites. The kinetics of iron removal from the Fee sites show a first-order dependence on pyrophosphate concentration, from the FeN sites show saturation kinetics. The current situation with respect to mechanisms of iron removal from, and incorporation into, transferrin have been reviewed. ... 

Taking into account the strong dependence of the rate of extraction of metal ions with the HCl concentration, when using the IL [omim+][PFg ], and the strong interaction observed by several authors [63,64] between metal ions and anions that are part of ILs, de los Rfos et al. [28] proposed that one of the predominant mechanisms of metal ion removal for imidazolium-based ILs might be the formation of ion pairs with the IL mediated by HCl in the case of Zn(II), Fe(III), and Cd(II). 

Hambright et al. (1988) have also studied the kinetics of displacement of the Gd " ion from the gadolinium(III) complex of TSPP by ethylenediaminetetraacetate (EDTA) giving Gd(EDTA) and H2(TSPP) as the main products. This represents the first example of metal removal from a metalloporphyrin by a chelating ligand. A mechanism has been proposed to account for the kinetic data. The water-soluble lanthanide complexes of TMPyP also undergo demetallation in the presence of EDTA (Haye and Hambright 1991). Similar to the acid solvolysis reactions, a linear relationship between log k and the ionic radius of the metal center can be established, and complexes with smaller metal center are more stable toward demetallation by EDTA. 

Fig. 4. Potential mechanism for metal removal by the rhamnolipid biosurfactant. (1) Adsorption of the biosurfactant on the soil surface and interaction via electrostatic attraction and solubilization of soil fractions containing the metal. (2) Removal of the metal from the soil surface. To maintain electrostatic neutrality, two carboxylic groups are needed per divalent metal. (3) Incorporation of the metal-biosurfactant complex into micelles (adapted from Mulligan et al. [68]).

Regardless of its function as an alternate corrosion indicator of Ico , the LPR serves as a useful standalone probe of the chemical component of CMP. Since the faradaic activity of a metal surface proportionally decreases with increasing values of Rp, it is usually possible to establish a direct correlation between the MRRs and Rp as long as the mechanism of material removal is dictated by electrochemical mechanisms. The quality of this correlation can then be used as an empirical measure of the chemical component of CMP. An example of this analysis is shown in Figure 3.10, where the polish rates of TaN measured by CMP in different solutions (Janjam et al., 2010a) are... 

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