Chromium electrochemical treatment

The process can be used to treat dissolved metals and is commonly used in groundwater treatment for the reduction and precipitation of hexavalent chromium, as well as in the oxidation of cyanide wastes (at concentrations up to 10%). Other potential applications of electrochemical treatment include remediation of arsenic, cadmium, molybdenum, aluminum, zinc,... 

Electrochemical treatment has been used for many years in the mining and utility industries and is a proven technology for removing hexavalent chromium from wastewater. 

Chromate conversion coatings are formed by a chemical or an electrochemical treatment of metals or metallic coatings in solutions containing hexavalent chromium (Cr " ") and, usually, other components. The process results in the formation of an amorphous protective coating composed of the substrate, complex chromium compounds, and other components of the processing bath. 

The selected treatment option involves the reduction of hexavalent chromium to trivalent chromium either chemically or electrochemically. The reduced chromium can then be removed using a conventional precipitation-solids removal system. Alternative hexavalent chromium treatment techniques include chromium regeneration, electrodialysis, evaporation, and ion exchange.16... 

Andco Environmental Processes, Inc., has developed an electrochemical iron generation process to remove hexavalent chromium and other metals from gronndwater and aqneons wastes. As contaminated water flows through a treatment cell, electrical cnrrent passes between electrodes, releasing ferrons and hydroxyl ions. The small gap between electrodes allows almost instantaneons rednction of chromium ions. Depending on the pH, varions solids may form. 

As discussed, the aqueous waste with hexavalent chromium requires reduction of chromium to the trivalent state prior to metal removal because hexavalent chromium does not form a precipitate. Demonstrated reducing agents are sodium metabisulfite (Na2S205), sulfur dioxide (S02), ferrous sulfide (FeS), and other ferrous ion (ferrous sulfate, ferrous chloride, or electrochemically generated ferrous ion). The treatment processes using these are described below. 

An ultrathin layer of plasma polymer of trimethylsilane (TMS) has been utilized in the corrosion protection of aluminum alloys by means of system approach interface engineering (SAIE) [1 ]. SAIE by means of low-temperature plasmas utilizes low-temperature plasma treatment and the deposition of a nanolilm by luminous chemical vapor deposition (LCVD). This approach does not rely on the electrochemical corrosion-protective agents such as six-valence chromium, and hence the process is totally environmentally benign. 

An alternative, and very successful, use of electrochemical methods for metal clean-up is to oxidise metals in effluents so that they can be recycled. One example is the treatment of chromium(III)-containing effluents. The largest use of chromium is as an oxidising agent (Cr04 , Cr207 ) where chromium is in its +VI oxidation state. The by-product from the... 

The strong interactions with metal ions extend to the use of metal-modified electrodes in electrocatalysis. Catalysis has been demonstrated with four systems. Chromium treatment results in as much as a 200 mV positive shift in the reduction peak for lO in acetate buffer This has been compared to the necessity for prior oxidation of the platinum electrode surface Ruthenium pretreatment of (SN), electrodes results in a catalytic current for the I / couple in phosphate buffer, pH 7.6. These electrodes also photoelectrochemically reduce protons to hydrogen at —0.05 V versus SCE in dilute sulfuric acid solution Molybdate treated electrodes have been used to electrochemically reduce acetylene at potentials of 1.5 V versus SCE in borate and hydroxide solutions. Iron treated electrodes show some ability to facilitate this reaction, but the rate is slower than with the molybdate treated electrodes... 

Abda M, Gavra Z, Oren Y (1991) Rtanoval of chromium from aqueous solutions by treatment with fibrous carbon electrodes column effects. J Appl Electrochem 21 734—739... 

Almeida, E., Fedrizzi, L., Diamantino, T.C. (1998). Oxidizing alternative species to chromium VI in zinc - galvanized steel surface treatment Part 2-An electrochemical study. Surface and Coating Technology, Vol. 105, pp. 97-101, ISSN 0257-8972... 

Electrochemical corrosion resistance of a NC surface of 316 stainless steel developed by surface mechanical attrition treatment was found to be considerably inferior to the MC unmodified bulk. This behaviour is attributed to the considerable increase in the fast diffusion channels for ions, i.e. grain boimdaries and triple junctions in the NC material. In another study, the grain refinement of stainless steels to a nanometric level is reported to improve the corrosion resistance, as suggested by the considerably extended passivation for NC in the polarisation plots. The authors have attributed this behaviour to the greater chromium diffusion in the NC structure, which may be argued given the very low diffusivity (i.e. 10 -10 s ) at ambient temperatures. 

Another electrochemical way to separate the analyte from the matrix that has been described is electrolytic deposition of the matrix elements on a mercury cathode in the presence of low sulfuric acid concentrations. Once the matrix is separated, the solution is carried to the spectrometer and elements are determined. This procedure is especially useful for the analysis of alloys, because elements such as rare earth elements can be easily separated from iron, chromium, nickel, copper, etc. For the analysis of steel samples, this method provides limits of quantification 10 times better than direct analysis, without the need for matrix matching. Recently, an FI electrolytic dissolution procedure has been reported for the treatment of metallic (i.e. high-purity copper) samples, the specimen acting as the anode of an electrodissolution ceU. ... 

Chromate conversion coatings are produced on various metals (Al, Cd, Cu, Mg, Ag, Zn) by chemical treatment (sometimes electrochemical) with hexavalent chromium solutions with... 

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