Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – Including organic component
Reexamination Certificate
1998-04-22
2001-07-17
Dunn, Tom (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
Including organic component
C502S060000, C502S064000, C502S066000, C502S074000
Reexamination Certificate
active
06261986
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of producing an article ,and to the article itself, for example in pellet shape, which can simultaneously retain and destroy several classes of water pollutants.
Chlorinated compounds and inorganic oxyanions such as chromate and arsenate are major water pollutants at industrial sites, DOE facilities, and military installations. Once introduced into water, these toxic compounds tend to be only slowly degraded/transformed by natural processes. Improved methods and products are needed to trap and destroy these pollutants. Among these products are chemical filters.
Current chemical filters can be classified as either degradation filters or adsorption/precipitation filters. In degradation filters, the pollutant is converted to an innocuous form as water passes through. In adsorption/precipitation filters, pollutants are immobilized within the filter until the retention capacity is exceeded. Each of these filter types has certain disadvantages. Degradation filters require a minimum residence time to allow complete pollutant destruction. In systems with rapid water flow and/or recalcitrant pollutants, prohibitively thick filters may be required to provide the needed residence time. Adsorption/precipitation filters have a limited capacity for pollutant retention; once this capacity is exceeded, the filter needs to be regenerated or replaced. Clearly, a filter material which combines the properties of pollutant adsorption (to increase residence time) and pollutant destruction or transformation (to obviate the need for regeneration/replacement) would be very desirable.
It is therefore an object of the present invention to provide a filter material that combines the properties of water pollutant adsorption and destruction, thus overcoming the drawbacks of heretofore known filter materials used for removing pollutants from water. This new material should result in a less expensive and more efficient treatment of water polluted with chlorinated hydrocarbons and/or inorganic oxyanions.
This object, and other objects and advantages of the present invention, will appear more clearly from the following specification and examples.
SUMMARY OF THE INVENTION
The object of the present invention is realized by a method of intimate mixing and binding of materials to create a combined pollutant adsorption/degradation filter material and includes the steps of: providing at least one adsorbent (for example, natural zeolite or clay), providing at least one pollutant destruction or transformation material (for example, iron or other reactive metal), possibly providing a binder (for example, an aluminosilicate binder), mixing the components together to form a mixture, and processing the mixture to form porous, highly permeable pellets, blocks, or other shaped forms to produce a durable filter product which is stable in water. The adsorbent is modified with surfactant prior to or after the processing. Other components may be added or the processing modified to vary the density, porosity, or permeability of the filter product or to improve the adsorption and pollutant destruction properties of the product.
The most common adsorbents for polluted water treatment are granular activated carbon and synthetic ion exchange resins. Both of these adsorbents are relatively expensive. Inexpensive surfactant-modified clay minerals have been shown excellent for removal of nonpolar organics from water (Boyd et al. 1988), but their low hydraulic conductivities and their tendency to shrink or swell limit their use in filters. Surfactant-modified zeolite (SMZ), in contrast to surfactant-modified clay minerals, has excellent hydraulic properties. In addition to effectively adsorbing nonpolar organic contaminants, SMZ has great affinity for inorganic oxyanions such as chromate, arsenate, sulfate, selenate (Bowman etal., 1995) and heavy metal cations such as lead (Bowman et al., 1997). Thus SMZ can be used to remove multiple contaminants simultaneously and its economical cost makes it attractive for adsorption filters. Many different surfactants may be used, including hexadecyltrimethylammonium, octadecyltrlmethylammonium, and 4-methylpyridinium.
The most commonly used destruction filter is zero-valent iron (Fe) for treatment of water contaminated with chlorinated organic compounds or inorganic compounds in higher oxidation states (Gillham and O'Hannesin, 1994; Blowes et al. 1998). The advantage of iron and other metal filters is the complete destruction of chlorinated organic compounds and immobilization of oxidized inorganic compounds. Iron and some other metals or metal combinations chemically reduce chlorinated hydrocarbons to innocuous organic compounds and chloride. These same metals also reduce oxidized inorganic compounds, which generally have high water solubilities and are mobile in subsurface environments, to less soluble and less mobile forms. The major drawback of metal filters is the residence time required for complete pollutant destruction/immobilization. Depending upon reaction kinetics, very long or very large filters may be necessary.
The advantage of using SMZ as a filter is its low cost as well as the instantaneous adsorption of multiple types of contaminants. However, the SMZ filter will cease to function when the amounts of pollutants adsorbed exceed the filter adsorption capacity. We have found that a combination of the particular assets of Fe and SMZ allows optimization of filter performance. Adsorption of pollutants in a combined Fe/SMZ filter increases pollutant residence time and reduces the required filter length, lowering costs of filter materials and operation.
A number of advantages are realized in the preparation of the surfactant modified iron/zeolite using the present technique, including the selection of raw materials in terms of size, shape and composition, as well as in the ease of manufacturing. It is well known that available surface area is the key to the reactivity of adsorptive materials. Following this logic, the selection of finely ground raw materials is superior to larger sizes. However, finer or smaller aggregates result in reduced hydraulic conductivity or permeability of the product. To overcome this limitation, the finely ground material can be agglomerated or pelletized. In this fashion, not only can the permeability of the product be greatly improved, but so can the strength of the individual pellets. Strength is important for increased resistance to mechanical abrasion of the material in service.
While the pelletization of iron and zeolite materials can be accomplished by a number of conventional techniques, a major concern is the reduction in the overall reactivity or adsorptive ability of the agglomerated pellets due to the particular techniques and/or binder system used to form the pellets. In order to fully utilize the benefits afforded by the combination of ingredients in the proposed invention (surfactant, iron and zeolite) the diffusion characteristics of the contaminants in question should not be limited or unduly interfered with. Consequently, an ideal binder system, if needed, would be one in which a pellet with maximum adsorption capacity is achieved with maximum macroporosity for minimum diffusion resistance to the critical ingredients while at the same time providing sufficient strength to withstand the normal handling and abrasion associated with its use in service. If the binder used in the pelletizing process seals the surface of the pellet thereby preventing access to the surrounding contaminated environment, the effectiveness of the adsorption properties is also severely limited. In addition, if the binder used acts as an inert filler to bond the reactive/adsorption materials together, then the overall useable content is also reduced. For example, if 20% by volume of an inert binder phase is required to bond the reactive/adsorption material, then the overall availability of the reactive/adsorptive material is reduced by this same 20%. In the present technique, all of these shortcom
Bowman Robert S.
Helferich Richard L.
Dunn Tom
New Mexico Tech Research Foundation
Robert W. Becker & Associates
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