Removal of heavy metals

Heavy metals such as copper, cadmium, mercury, and lead are found in wastewaters from a number of industrial processes. Because of the toxicity of many heavy metals, their concentrations must be reduced to very low levels before release of the wastewater. Several approaches are used in heavy metals removal. 

The form of the heavy metal has a strong effect on the efficiency of metal removal. For instance, generally soluble chromium(VI) is much more difficult to remove than chromium(HI). Chelation may prevent metal removal by solubilizing metals. 

Even when not specifically designed for the removal of heavy metals, some wastewater treatment processes remove appreciable quantities of the more troublesome heavy metals encountered in waste-water. Biological waste treatment effectively removes metals from water. These metals accumulate in the sludge from biological treatment, so sludge disposal must be given careful consideration. 

Lime treatment, discussed earlier in this section for calcium removal, precipitates heavy metals as insoluble hydroxides, basic salts, or coprecipitated with calcium carbonate or iron(III) hydroxide. This process does not completely remove mercury, cadmium, or lead, so their removal is aided by addition of sulfide (most heavy metals are sulfide-seekers)  

Heavy chlorination is frequently necessary to break down metal-solubilizing organic ligands. Lime precipitation does not normally permit recovery of metals and is sometimes undesirable from the economic viewpoint. 

Removal of Heavy Metals. Federal, state and local regulations place strict limits on the quantities of heavy metals which may be released to the environment. The controlled metals include Ag, As, Cd, Cr, Cu, Pb, Hg, Ni, Sb and Zn. U.S. electroplaters, metal finishers, and printed-circuit-board manufacturers are under mounting pressure to clean up their waste waters. 

MF may be used to remove these heavy metals provided pretreatment chemicals are added to precipitate the metals to particles of filterable size. The chemical pretreatment step is crucial since it will affect the performance of the membrane and the resultant sludge volume as well as the contaminant removal efficiency. Reduction/oxidation, absorption/oxidation, and/or catalytic reactions are utilized along with pH adjustment to provide the optimum precipitation. Although conventional methods of waste water treatment may use a similar pretreatment chemistry, the final solid/liquid separation by gravity settling is usually not as effective as membrane filtration. 

If the metals are of high value, the metal precipitate may be redissolved in concentrated acid to recover the metals in solution.28 2  

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A number of papers have appeared on the removal of heavy metals in the effluents of dyestuff and textile mill plants. The methods used were coagulation (320—324), polymeric adsorption (325), ultrafiltration (326,327), carbon adsorption (328,329), electrochemical (330), and incineration and landfiU (331). Of interest is the removal of these heavy metals, especiaUy copper by chelation using trimercaptotria2ine (332) and reactive dyed jute or sawdust (333). 

Sodium trimetaphosphate was used as an eluting agent for the removal of heavy metals such as Pb, Cd, Co, Cu, Fe, Ni, Zn and Cr from aqueous solutions. Distribution coefficients of these elements have been determined regarding five different concentrations of sodium trimeta phosphate (3T0 M 5T0 M 0.01 M 0.05 M 0.1 M) on this resin. By considering these distribution coefficients, the separation of heavy metals has been performed using a concentration gradient of 3T0 - 5T0 M sodium trimetaphosphate. Qualitative and quantitative determinations were realized by ICP-AES. 

Polyelectrolytes such as the ion exchange plastics form an interesting group of materials because of their ability to interact with water solutions. They have been used in medical applications involving the removal of heavy metal ions from the human body. They can be used to interact with external electric fields and change their physical properties drastically as is illustrated by the fact that some electrically active liquid crystals are polyelectrolytes of low molecular weight. 

Although the natural zeolites are widely used (around 4 million tpa) they are not particularly valuable as commercial catalysts. This is due to a number of factors including natural variations in crystal size and porosity as well as the actual small pore size, which limits their synthetic usefulness. Natural zeolites do, however, find widespread use in applications such as removal of heavy metals from water, odour removal and building materials e.g. cavity grouting and sprayed concrete). 

Removal of Heavy Metals and Sulfate from Coil Coating Wastewater by Ion Exchange Process... 

Oliveira, W.E., Franca, A.S., Oliveira, L.S., and Rocha, S.D., Untreated coffee husks as biosorbents for the removal of heavy metals from aqueous solutions, Journal of Hazardous Materials, 152, 1073-1081, 2008. 

Miretzky, P., Saralegui, A., and Fernandez Cirelli, A., Aquatic macrophytes potential for the simultaneous removal of heavy metals (Buenos Aires, Argentina), Chemosphere, 57 (8), 997-1005, 2004. 

Maine, M.A., Sufie, N., Hadad, H., Sanchez, G., and Bonetto, C., Influence of vegetation on the removal of heavy metals and nutrients in a constructed wetland, Journal of Environmental Management, 90 (1), 355-363, 2009. 

Gardea-Torresdey, J.L., de la Rosa, G., and Peralta-Videa., J.R., Use of phytofiltration technologies in the removal of heavy metals A review, Pure and Applied Chemistry, 76 (4), 801-813, 2004. 

Gavrilescu, M., Removal of heavy metals from the environment by biosorption, Engineering Life Sciences, 4 (3), 219-232, 2004. 

Ahluwalia, S.S. and Goyal, D., Microbial and plant derived biomass for removal of heavy metals from wastewater, Bioresource Technology, 98 (12), 2243-2257, 2007. 

Ngah, W.S.W. and Hanafiah, M.A.K.M., Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents A review, Bioresource Technology, 99, 3935-3948, 2008. 

Amuda, O.S., Giwa, A.A., and Bello, I.A., Removal of heavy metal from industrial wastewater using modified activated coconut shell carbon, Biochemical Engineering Journal, 36, 174-181, 2007. 

Leung, W.C., Wong, M.F., Chua, H., Lo, W., Yu, P.H.F., and Leung, C.K., Removal of heavy metals by bacterial isolated from activated sludge treating industrial effluents and municipal wastewater, Water Science Technology, 41 (12), 233-240, 2000. 

Barbaras, D., Brozio, J., Johannsen, I. and Allmendinger, T. (2009) Removal of heavy metals from organic reaction mixtures preparation and application of functionalized resins. Organic Process Research SI Development, 13 (6), 1068-1079. 

The reason for lack of microbial conversion of these molecules may be the difficulty in transporting them across the cell membrane. However, the possibility of an extracellular conversion exists. The enzymatic treatment of asphaltenes can be seen as an interesting alternative for the removal of heavy metals to reduce catalyst poisoning in hydrotreatment and cracking processes, for instance. 

Ligand exchange Equilibrium Chromatographic separation of glucose-fructose mixtures with Ca-form resins Removal of heavy metals with chelating resins Affinity chromatography... 

TETRA HDS [High density solids] A process for aiding the removal of heavy metals from wastewaters. It is a physical process which controls the characteristics of heavy metal hydroxide precipitates so that they settle quicker. The precipitates have a hydrophobic surface, so they are easy to de-water. Developed and licensed by Tetra Technologies, Houston, TX. Widely used by the iron and steel industry in the United States. Not to be confused with hydrodesulfurization, often abbreviated to HDS. 

Net uptake of heavy metals is due to the removal of heavy metals in crops or trees in the catchment and/or in aquatic plants and fish in a lake. Weathering relates to the... 

The advection scheme of the regional model is improved to take into account the surface orography. Terrain following vertical structure of the model domain with higher resolution was incorporated. Wet removal of heavy metals from the atmosphere was enhanced by developing newparameterizations of precipitation scavenging. Both in-cloud and sub-cloud wet removal were modified on the basis of the up-to-date scientific literature data. 

Chemical precipitation has traditionally been a popular technique for the removal of heavy metals and other inorganics from wastewater streams. However, a wide variety of other techniques also exist. For example, ion-exchange, reverse osmosis, evaporation, freeze crystallization, electrodialysis, cementation, catalysis, distillation, and activated carbon have all been used for removal of inorganics. 

Hermann, E., DulUes, F., Griebel, I., and KieBig, G., 2001, Filter material and process for the removal of heavy metals, arsenic, uranium, and radium from contaminated waters, Patents Pending DE 101 16 951, DE 101 16 953 (2001). 

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