Hazardous wastes heavy metal ions

Two major divisions of biochemical metabolism that operate on hazardous waste species are oxic processes, which use molecular O2 as an oxygen somce, and anoxic processes, which make use of another oxidant. For example, when sulfate ion acts as an oxidant (electron receptor), the transformation sol" H2S occurs. (This has the benefit of providing sulfide, which precipitates insoluble metal sulfides in the presence of hazardous waste heavy metals.) Because molecular oxygen does not penetrate to such depths, anoxic processes predominate in the deep sediments, as shown in Figure 15.6. 

Solidification with cement generally is accomplished with a Portland cement and other additives. The quantity of cement can be varied according to the amount of moisture in the waste. Heavy metal cations in the waste form insoluble carbonates and hydroxides at the high pH of the mixture. The surface of the hardened mass can be coated with asphalt or other material to reduce leaching of hazardous components. If the waste is mixed with anhydrous cement and water there is the possibility of ions incorporation in the cement structure during the hydrolysis process. Heavy metal ions could bind with the cement by the process of chemisorption, precipitation, surface adsorption,... 

Decomposition of a wide variety of hazardous organic and inorganic chemical wastes (e.g., phenols, pesticides, detergents, cyanides) Sterilization of surfaces, potable water, municipal wastes, and specialty waters Precipitation of some undesirable heavy metal ions as oxides and hydroxides (e.g., iron, manganese)... 

In the section entitled Cleaning Up at the end of each experiment the goal is to reduce the volume of hazardous waste, to convert hazardous waste to less hazardous waste, or to convert it to nonhazardous waste. The simplest example is concentrated sulfuric acid. As a by-product from a reaction, it is obviously hazardous. But after careful dilution with water and neutralization with sodium carbonate, the sulfuric acid becomes a dilute solution of sodium sulfate, which in almost every locale can be flushed down the drain with a large excess of water. Anything flushed down the drain must be accompanied by a large excess of water. Similarly, concentrated base can be neutralized, oxidants such as Cr " can be reduced, and reductants such as hydrosulfite can be oxidized (by hypochlorite— household bleach). Dilute solutions of heavy metal ions can be precipitated as their insoluble sulfides or hydroxides. The precipitate may still be a hazardous waste, but it will have a much smaller volume. 

Other important toxicological con tarn in ants that can be found in waste-waters are metals. Toxic heavy metal ions are introduced to aquatic streams by means of various industrial activities viz. nfrning, refining ores, fertilizer industries, tanneries, batteries, paper industries, pesticides, etc., and possess a serious threat to the environment. The major toxic metal ions hazardous to humans as well as other forms of hfe are Cr, Fe, Se, V, Cu, Co, Ni, Cd, Hg, As, Pb, Zn, etc. These heavy metals are of specific concern due to their toxicity, bioaccumulation tendency, and persistency in nature [190]. The SLM technique has been widely apphed for the transport and recovery of almost ah important metals from various matrices an exceUent review of ah aspect of metal permeation through SLM (covering both theoretical and practical considerations) is available [191]. Here, only some selected recent examples of the use of SLM for metal separation whl be presented. 

The use of electricity in reactions is clean and, at least in some cases, can produce no waste. Toxic heavy metal ions need not be involved in the reaction. Hazardous or expensive reagents, if needed, can be generated in situ where contact with them will not occur. The actual oxidant is used in catalytic amounts, with its reduced form being reoxidized continuously by the electricity. In this way, 1 mol% of ruthenium(III) chloride can be used in aqueous sodium chloride to oxidize benzyl alcohol to benzaldehyde at 25°C in 80% yield. The benzaldehyde can, in turn, be oxidized to benzoic acid by the same system in 90% yield.289 The actual oxidant is ruthenium tetroxide. Naphthalene can be oxidized to naphthoquinone with 98% selectivity using a small amount of cerium salt in aqueous methanesulfonic acid when the cerium(III) that forms is reoxidized to cerium(IV) electrically.290 Substituted aromatic compounds can be oxidized to the corresponding phenols electrically with a platinum electrode in trifluoroacetic acid, tri-ethylamine, and methylene chloride.291 With ethyl benzoate, the product is a mixture of 44 34 22 o/m/fhhy-... 

Chemical precipitation is used in hazardous waste treatment primarily for the removal of heavy metal ions from water. The most widely used means of precipitating metal ions is by the addition of base (Ca(OH)2), NaOH, or Na2C03), leading to the formation of hydroxides such as chromium(III) hydroxide... 

Heavy metal ions in soil contaminated by hazardous wastes may be present in a coprecipitated form with insoluble iron(lll) and manganese(IV) oxides, Fc203 and Mn02, respectively. These oxides can be dissolved by reducing agents, such as solutions of sodium dithionate/citrate or hydrox-ylamine. This results in the production of soluble Fe " and Mn and the release of heavy metal ions, such as Cd " or Ni ", which are removed with water. 

Ion exchange is a means of removing cations or anions from solution onto a solid resin, which can be regenerated by treatment with acids, bases, or salts. The greatest use of ion exchange in hazardous waste treatment is for the removal of low levels of heavy metal ions from wastewater ... 

Chemical precipitation is used in hazardous-waste treatment primarily for the removal of heavy-metal ions from water as shown below for the chemical precipitation of cadmium ... 

Enzyme extracts collected from microbial cultures and purified have been considered for in-situ detoxification. One cell-free enzyme that has been used for detoxification of organophosphate insecticides is parathion hydrolase. The hostile environment of a chemical-waste landfill, including the presence of enzyme-inhibiting heavy-metal ions, is detrimental to many biochemical approaches to in-situ treatment. Furthermore, most sites contain a mixture of hazardous constituents, which might require several different enzymes for their detoxification. 

When antifreeze becomes unsuitable for use, either because of depletion of inhibitors, presence of corrosion products or corrosive ions, or degradation of the fluid, recycling and reuse of the antifreeze, rather than disposal, may be considered. Although ethylene glycol is readily biodegraded in typical municipal waste treatment faciHties, antifreeze disposal becomes problematic because the coolant may contain hazardous quantities of heavy metals picked up from the cooling system. Recycling may be economically preferred over coolant disposal and reduces the concern for environmental impact. 

Cleaning Up The filtrate should have pH < 3 and then be treated with a 50% excess of sodium bisulfite to reduce the orange dichromate ion to the green chromic ion. This solution should then be made basic with ammonium hydroxide to precipitate chromium as the hydroxide. This precipitate is collected on a filter paper, which is placed in the hazardous solid waste container for heavy metals. The filtrate from this latter treatment can go down the drain. 

Cleaning Up Add 3 M sulfuric acid until the pH is 1. Complete the reduction of any remaining chromic ion by adding solid sodium thiosulfate until the solution becomes cloudy and blue colored. Neutralize with sodium carbonate and filter the flocculent precipitate of Cr(OH)3 through Celite in a Buchner funnel. The filtrate can be diluted with water and flushed down the drain, while the precipitate and Celite should be placed in the heavy metals hazardous waste container. 

The complexation/ultrafiltration process is efficient for retention of non-active hazardous components of the wastes, e.g., heavy metals, which concentration has to be reduced according to national regulations. In the process described above, the retention of heavy metals was high, and reached 98% for manganese, 98.6% for iron, 92.4% for copper, 91.5% for nickel, and 94.7% for zinc. Some part of bivalent ions was also retained, for instance, Mg in 67%, Ca in 58%, in 22.6%. 

Sengupta, S. Sengupta, A.K. Solid phase heavy metal separation using composite ion exchange membranes. Hazard. Waste Hazard. Mater. 1996, 13 (2), 245. 

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