Professor: 莊順興 Presenter: 李謙詳 2016/01/04

* Unlike physical separation processes that merely concentrate or change the phase of hazardous wastes. * chemical treatment processes can reduce the toxicity. ignitability, reactivity, or corrosivity of wastes. * Chemical treatment is commonly applied on-site. * Applications include metal-bearing wastes, inorganic contaminants, and dissolved organics.

* Acid/base neutralization is a common hazardous waste treatment method and is sometimes the only treatment required to declassify a waste as hazardous. * Virgin chemicals (acids or bases) can be added to the waste to achieve neutralization, or acidic waste can be mixed with basic waste.

* Commonly used neutralization agents for acidic wastes include limestone (caco3), quicklime (CaO), hydrated lime (or slaked lime, Ca(OH)2), soda ash (Na2CO3), and sodium hydroxide. * Limestone is generally very inexpensive, but it dissolves more slowly than lime and produces a larger quantity of residual sludge for treatment. * Use of a weak base such as soda ash instead of a strong base such as sodium hydroxide can be advantageous, since it avoids the potential production of a strongly basic waste due to overdosing.

* A measure of the amount of base that requires neutralization in a basic waste is the alkalinity (or basicity) of the waste. * Potential sources of alkalinity in wastes include strong bases such as sodium hydroxide (NaOH) and weak bases such as ammonia (NH3), carbonates, and heavy- metal salts.

* Precipitation is a chemical treatment process * Precipitation is soluble contaminants from solution that forms an insoluble product be removed by settling or filtration. * Precipitation is the most commonly applied process(EX: contain arsenic, barium, cadmium, chromium, copper, lead, mercury, nickel, selenium, or zinc) * In these cases, precipitation is a concentration technique rather than a destruction method * The most common processes involve hydroxide, sulfide, or carbonate reactions

* Hydroxide precipitation using lime is by far the most commonly applied precipitation reaction. * It is effective for heavy-metal removal as well as for removing hardness and phosphorus. * Hydroxide precipitation is performed by adding a strong or weak base to the wastewater of interest to raise the pH and cause formation of insoluble hydroxide species. * A precipitation reaction involving a generic dissolved metal (M2+) and lime is given as follows:

* Chromium removal is a common application for hydroxide precipitation. * chromium can be precipitated only in the trivalent form (Cr(III)). The highly toxic hexavalent chromium (Cr(VI)) form cannot be treated using hydroxide or sulfide. * Hexavalent chromium must be reduced to the trivalent state, first by reducing the pH to 2–3 and then by adding a reducing agent such as sulfur dioxide (SO2), sodium bisulfite (NaHSOs), or ferrous sulfate (FeSO4). After the chromium is reduced to Cr3+.

* There are several advantages to using sulfide precipitation over hydroxide precipitation for the treatment of metal-bearing aqueous waste. * 1. First, most metals have much lower solubilities when combined with sulfide then with hydroxide, * 2. A second advantage is the monotonically decreasing metal sulfide solubility with increasing pH * 3. A third advantage to sulfide precipitation is that sulfide precipitates are more stable and easier to dewater than hydroxide precipitates

* Sulfide precipitation can reduce hexavalent chromium to the trivalent form and precipitate it in a single step. This is illustrated in the following reacting, in which dichromate ions react with ferrous sulfide to produce insoluble chromium hydroxide:

* Carbonate precipitation is sometimes used instead of hydroxide precipitation to remove certain metals ( Cd, Ni, Pb) because it operates at lower pH values and generates a denser precipitate that is easier to dewater. * Carbonate is typically added in the form of soda ash (Na2CO3). A generic carbonate precipitation reaction is

* Solidification and stabilization were originally developed as processes to treat industrial, hazardous, and certain radioactive wastes * The goal of solidification and stabilization is not to transform or destroy wastes, but rather to contain and minimize the release of wastes into the environment. * These processes are applied to heavy-metal wastes, contaminated soils * Solidification and stabilization may reduce the volume and mass of the waste.

* Although the definitions of the terms solidification and stabilization overlap significantly and are sometimes used interchangeably, it is useful to distinguish them. * Stabilization is a fixation process that converts the contaminant to an insoluble form by means of binding additives. * Solidification is a process in which the waste is encapsulated or otherwise encased in a solid of high structural integrity.

* Glassification is an expensive solidification/stabilization process in which the waste material is mixed with sand, borax, and nitric acid before being subjected to high temperatures (> 1600 °C) to fuse the mixture into glass. * because of the high costs, it is used mainly to treat radioactive waste.

* Glassification is an expensive solidification/stabilization process in which the waste material is mixed with sand, borax, and nitric acid before being subjected to high temperatures (> 1600 °C) to fuse the mixture into glass. * because of the high costs, it is used mainly to treat radioactive waste.

* Ozone, an unstable, slightly water-soluble gas is a strong general oxidizing agent that is commonly used to treat hazardous wastes that contain organics, cyanide, sulfides, The instability of ozone necessitates its generation on site. * ozonation benefit is that it causes some organics in water to be transformed to more water-soluble, less volatile, more degradable compounds such as organic acids, aldehydes, and ketones. Disadvantages include high capital and operating costs and the potential for generating trihalomethanes.

* Ozone can react directly with a contaminant or indirectly by means of generating hydroxyl radicals, as shown in Figure 8.C.10. Ozone reacts with hydroxide ions in water to form hydroxyl radicals, which are very powerful nonselective oxidizing agents.

* Chemical reduction is a treatment process that is applied to decrease nature of contaminants by adding electrons. Electron addition, or reduction, decreases the valence charge of the species. * The most common application of chemical reduction for hazardous waste treatment is the reduction of highly toxic hexavalent chromium (Cr(VI)) to the less toxic trivalent chromium (Cr(III))

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