SCIENCE/TECHNOLOGY

"This is the first mitochondrial super­oxide dismutase that has been done, however."

Tainer lists three of the most interest­ing structural features discovered in the mitochondrial Mn-SOD: First, at the lev­el of the individual subunits, the N-ter-minal domain—amino acid residues 1 through 84—is "formed primarily by two long antiparallel α-helices separated by a tight turn, forming a helical hairpin structure." The C-terminal domain—res­idues 85-198—contains five α-helices and a three-stranded β sheet. The active-site manganese joins the two domains and is positioned between the N-termi-nal α-helices and C-terminal β sheet, Tainer says.

Second, at the level of the dimers that make up the tetrameric enzyme, Tainer and his collaborators discovered that each active site is made up of residues from both subunits of the dimer. That is, al­though each manganese ion is bound within a subunit, residues from both sub-units contribute to the metal binding site.

And third, the tetrameric structure of Mn-SOD is stabilized by interactions between the helical hairpins. The heli­cal hairpins of two subunits assemble to form a "four-helix bundle" with a left-handed twist, and the helical hair­pins of the other two subunits assem­ble to form another four-helix bundle on the opposite side of the enzyme.

Tainer points out that the four-helix bundle interfaces in Mn-SOD appear to be important not only for the enzyme's assembly, but also for its stability and activity. In the subunit, if the helical hairpin is not tethered by formation of the four-helix bundle, the two ligands contributed from the hairpin helices to the metal binding site are destabilized, adversely affecting the manganese ac­tive-site geometry and the enzyme's ac­tivity, Tainer says.

It is the existence of Mn-SODs contain­ing destabilized four-helix bundles that caught the attention of Tainer and his co­workers. Other researchers had shown that the sequence of the helical hairpin is highly conserved in mitochondrial Mn-SODs from different species. Yet human Mn-SOD is polymorphic, Tainer says, and one naturally CHzajirring variant, in which a hydrophobic isoleucine in the helical hairpin is replaced with a hydrophilic threonine, has been found in two of six complementary DNA libraries examined by the Scripps researchers.

That single amino acid change is far

from insignificant, Tainer stresses. ''Within the four-helix bundle interface, that isoleucine is the largest contributor to buried surface area," he says. Replace­ment of this isoleucine with threonine likely would destabilize the bundle, leading to the observed decrease in en­zyme activity. Together with Tainer, Hickey, and Hallewell, Scripps research­ers Maurice Boissinot and Michael J. Johnson are working with Diane E. Ca-belli of Brookhaven National Laboratory and James R. Lepock of the University of Waterloo, Canada, to complete a com­prehensive characterization of the stabil­ity and assembly of this mutant.

The mystery is: Why should such a change be preserved in humans? Mito­chondria consume more than 90% of the cell's oxygen, and the mitochondrial res­piratory chain is the source of a large flux of oxygen radicals. Mitochondrial DNA is particularly susceptible to oxida­tive damage because of this exposure to oxygen as well as relatively inefficient DNA repair mechanisms and the lack of histone proteins, Tainer points out.

The solution may lie in the immune system's apparent use of oxygen radi­cals to eliminate cells infected with pathogens. Other researchers have shown, for example, that the ability of tumor necrosis factor (TNF) to kill vi­rus-infected cells is inhibited by SOD. And stimulated neutrophils and mac­rophages, two types of immune cells responsible for eliminating pathogens, exhibit a characteristic "respiratory burst" in which superoxide is generat­ed at a membrane.

Thus, Tainer suggests, Mn-SOD vari­ants may exhibit a balanced polymor­phism analogous to that of human he­moglobin, in which a structurally and functionally defective single-site mu­tant—sickle cell hemoglobin—is main­tained in the population owing to a com­pensatory advantage—malarial resis­tance—conferred on the hétérozygote. The advantage conferred by destabilized Mn-SOD could be greater resistance to infections in general. The downside, a severe one, is greater susceptibility to degenerative diseases, many associated with old age, that appear to be caused by oxidative damage to cells.

Tainer says a great deal of work needs to be done to test this hypothesis, and he hopes other chemists will become in­volved. "There are a lot of mysteries that deserve attention," he says.

Rudy Baum

Facility recovers metals from spent catalyst The tightening of environmental laws and practices has meant that chemical companies and oil refiners are finding it increasingly difficult to get rid of "spent" catalysts, mainly because of the potentially harmful metals they contain. But now that Metrex is operating a plant in Heerlen, the Netherlands, that breaks down catalysts into their components, hope is at hand for some alleviation of the problem.

"The company name is coined from the process, which entails metals recov­ery by extraction," notes Metrex director Wladimir Kesber. "It says what we're doing, and can be pronounced fairly eas­ily in most languages. Ours is the only operation of its kind anywhere. It could be the forerunner of similar units as de­mand for our service grows. In a sense, we are using the Heerlen operation as a demonstration plant."

Adds Hans S. M. Jocker, Metrex gen­eral manager, who cofounded the com­pany with Kesber: "We recover the met­als and the alumina usually used as car­rier, and sell the products to companies that can use them. We don't regenerate the catalysts as such."

The two partners' work experience makes them ideally suited for the task at hand. Jocker has had close dealings with refinery and petrochemical operations through his service company, Reakt, headquartered near Cambridge, En­gland. Kesber, who has been associated with a number of international chemical companies over the years, now has his own chemicals recovery business called Chemconserve, based at Rijswijk, on the outskirts of The Hague. To raise the cash needed to build the plant and start the business, the Metrex shareholding has been extended to include four financial concerns.

The $15 million facility is located in the southeast corner of the Netherlands close to the Belgian and German bor­ders. It can process up to 15 million lb a year of hydrodesulfurization catalysts that have reached the end of their useful life. But it is designed to take double that quantity without much difficulty, or even more as demand requires. Cobalt, molybdenum, and nickel contained in the original catalyst formulation, along with vanadium adsorbed during the

20 OCTOBER 26,1992 C&EN

processing of the oil fractions, are recov­ered. Other products stemming from the operation are aluminum oxide, which goes to make ceramic and refractory bricks and tiles, and gypsum, from the flue gas.

"We can justifiably claim that we fully recover the materials for reuse," Jocker maintains. "We have the unique capabil­ity of being able to achieve a full metals extraction." Although the operation cur­rently centers on hydrodesulfurization catalysts, he and Kesber foresee its even­tual extension into use with other spent catalyst systems.

During the 1960s, the need to remove sulfur from crude oil and from refinery fractions spurred the development of hydrodesulfurization catalysts. But dur­ing use, deposits of carbon, hydrocar­bons, and sulfur build up on the cata­lysts7 surface, clogging the pores of the aluminum oxide carrier and thereby re­ducing the catalysts7 ability to function. Although they can be revitalized by sub­jection to various processes, there is a limit to the number of times regenera­tion is practicable before the structure becomes permanently destroyed or the catalysts become poisoned.

Some 25 years ago, when the cata­lysts reached the end of their useful life, it was the practice for oil refining companies to burn off the entrained carbon, sulfur, and hydrocarbons be­fore selling the material to iron and steel producers as a relatively inexpen­sive source of cobalt, molybdenum, and nickel, Jocker recalls. But this out­let tailed off in the 1970s when refiners stopped the stripping step because of the increasing costs involved.

"Steel makers don't like sulfur be­cause of the problems it causes," Jocker observes. 'They do like carbon, but not

in the form in which it occurs in the spent catalysts. Yet another problem that arose in the late 1970s was that in some cases phosphorus was added to enhance catalytic capability. And steel and phos­phorus don't go well together because phosphorus makes steel brittle."

As a result, the untreated spent cata­lysts were no longer of value to the smelters. So instead of being usable wastes, they ended up being dumped in landfills. That practice, however, was almost universally banned by the mid-1980s.

"Some are looking into the possibili­ty of encapsulating spent catalysts be­fore sending them to landfills," Jocker remarks. "We see this as a short-term solution."

There is no shortage of material. As a rule of thumb, Jocker points out, some 10 million lb of hydrodesulfurization catalysts are in use annually through­out Western Europe. The same amount is regenerated by service companies, while a similar quantity builds up each year as unusable waste.

Following a thorough review, Jocker and Kesber came to realize that the di­lemma the oil refiners faced presented a potentially attractive business oppor­tunity provided a viable method for ex­tracting and recovering the metals could be developed. Toward this end, they sought the help of TNO, the Neth­erlands state-sponsored organization for applied scientific research.

"There is nothing particularly new in what we eventually came up with," says Ruud Gerritsen, who directed the devel­opment of the process at TNO before join­ing Metrex as technology manager. "For instance, the solvent extraction method for separating molybdenum is similar to that used for recovering the element from ura­

nium ores. What makes the process unique is the way the different steps of the system have been linked together." Following successful pilot plant trials at TNCs Apeldoorn facilities, some of the equipment was transported and refitted at the Heerlen site.

The composition of used catalyst that arrives at the plant varies somewhat from one delivery to another. So the catalysts are analyzed and graded by their makeup.

When a batch has been selected for putting through the system, it goes first to a vibrating sieve to separate unwant­ed debris. It then passes to a fluidized-bed combustion chamber and on to a ro­tary tube furnace, both maintained at temperatures up to 850 °C. The spent catalyst charge is pulverized in the fluid-ized bed to a particle size of less than a millimeter. Air injection during the heat­ing process converts entrained carbon, hydrocarbons, and sulfur mainly to carbon dioxide and sulfur dioxide, that then move out with the flue gas. At the same time, metal sulfides become oxi­dized to their corresponding sulfates. After cooling, these are dissolved in a di­lute sulfuric acid leaching step, leaving behind most of the aluminum oxide and silica, if present in the original catalyst charge, as insoluble residue.

The acid solution of the metal sulfates goes through a series of extraction steps with various organic solvents. From the fractions, cobalt and nickel are recovered as sulfates, vanadium is converted to ammonium vanadate, and molybdenum is extracted as ammonium molybdate or recovered as molybdenum oxide from the flue gas. Meanwhile, the alumina residue, recovered by vacuum filtration, is washed and dried, and the flue gas passes through lime, which converts the sulfur oxides to calcium sulfate, or gyp­sum. The sulfuric acid is regenerated and recycled.

The metal content of hydrodesulfur­ization catalysts typically is 2 to 3% by weight cobalt, about 4% nickel, and up to 8% molybdenum. In broad terms, about half the weight of spent catalyst consists of alumina. The combined carbon, hydrocarbons, and sulfur ac­count for some 35% of the weight. The remaining 15% or so represents cobalt, molybdenum, nickel, and vanadium sulfides. The Metrex process results in a metals recovery efficiency that ex­ceeds 90%.

Dermot O'Sullivan Metrex plant in Heerlen, the Netherlands

OCTOBER 26,1992 C&EN 21