Publications
For Just One Drink of Water...
by Glen E. Fryxell, Ph.D.
Water is the principal requirement for human survival. Food and shelter rank highly, but we can live without those for much longer than we can live without water. Only the air that we breathe equals water for importance, but air is generally all around us, all the time. Clean drinking water is harder to come by. Wars have been fought (and continue to be fought) over access to clean water.
Here in the eastern Washington and eastern Oregon, we live in a desert. Our communities, and our agricultural economy, would not exist if not for a steady supply of water from the Columbia, Snake and Yakima Rivers.
We are blessed in modern-day America to be able to turn most any tap we find and expect safe, potable drinking water to erupt from within. Sure, sometimes we encounter a "sour" well or hear in the news about an occasional bacterial outbreak in some city's water supply, but when was the last time you thought to yourself as you bent over a water-fountain, "Will this make me sick?" Science and technology have indeed made our lives better and safer.
Other societies are not so blessed. An example in the news recently is the country of Bangladesh, where it has been found that most of the drinking water wells are contaminated with small amounts of arsenic. Not enough to cause acute arsenic poisoning in the short-term, but enough to induce chronic arsenicosis over time, with continued ingestion. The people of Bangladesh, just like us, must drink water everyday. Tens of thousands of people in Bangladesh are slowly, and painfully, dying of chronic arsenicosis because they have no choice. They have no other water supply.
Other examples are found in Japan, the Philippines, and parts of India where mercury has been introduced to the groundwater (and foodchain) by industrial or mining activity. These local populations have no choice but to drink, cook and wash from this mercury laden water supply. The human toll, generation after generation, in birth defects, disease and death, is staggering.
Toxic heavy metals are a global concern and we have our share here in America as well. Lead is known to cause serious developmental problems in children as they grow. In the 1960s and 1970s, blood lead levels in American children were climbing to unacceptable levels. In this case, the primary sources of exposure were coming from lead-based pigments in household paints and from the combustion of leaded gasoline. Both of these practices have been phased out and children's blood lead levels have dropped precipitously in recent years; a clear victory. The downside of this is that we burned huge quantities of leaded gas in this country and that lead was widely dispersed throughout the environment. It is slowly, but surely, creeping into the country's water supply.
Another environmentally problematic metal here in the States is chromium. The hexavalent state (chromic acid or chromate) is commonly used in the electroplating industry to chrome plate everything from car bumpers to rifle barrels. Unfortunately, this oxidized form of chromium is the most readily absorbed by the body and also the most toxic.
Not all toxic metals are introduced into the environment by human activity, some occur there naturally (like the arsenic in Bangladesh). Selenate (an oxidized form of selenium) is common to the geology of central California. As a result of the agricultural development of the region, heavy irrigation has caused significant amounts of selenate to be extracted from the soils and mobilized in the run-off from these fields. This water-borne selenate has accumulated in the wetlands of central California, causing massive die-offs in the local fish populations, as well as to the migratory waterfowl that find "shelter" in these poisonous swamps.
How can we deal with these toxic metals in our drinking water? Ideally, we would make some kind of filter matrix that we could filter our water supply through to selectively remove all of the toxic heavy metals, leaving only clear, clean water. Many different filters and sorbent materials are available today (look at the booming business in household water filters). These vary from simple fiber filters to remove sediments, to ceramic oxides to remove rust and other metal salts, to activated carbon to remove organic solvents. While these filters are usually modestly effective at achieving their desired goals, there are some inherent limitations due to the materials chosen for their construction. First off, these are usually low surface area materials. Since the goal is to bind a toxic metal to the surface of the sorbent, the more surface area it has, the more metal it can bind. Secondly, these filters use materials that usually just physisorb the problematic species to the surface instead of binding it fast with covalent chemical bonds (think of it this way, a child's muddy hand-print is physisorbed to the wall and can be wiped off with a wet sponge, the enamel that coats the wall is bound to the wall and is not removed so easily). A stronger bond is desirable to prevent the toxic metal from leaching off the sorbent matrix and into tomorrow's glass of water. Thirdly, these materials are non-specific and can remove things like calcium, magnesium and zinc that pose no health risk at all, and are in fact beneficial nutrients. Thus, a significant portion of their already limited capacity is wasted removing common materials that we don't need, or even want, to remove.
At Pacific Northwest National Laboratory (PNNL), we are focused on solving these problems. In 1992, a novel ceramic morphology was reported by researchers from Mobil. They called these new materials "mesoporous ceramics" since the pore diameter was "in-between" that known for macroporous ceramics (e.g. the multi-micron sized pores of glass frits) and microporous ceramics (e.g. the 5-10 Angstrom sized pores of zeolites). To envision what mesoporous ceramic look like, think of a glass honeycomb in which the holes in the honeycomb are very uniform and are only 60 Angstroms across (roughly twice the width of a double-helix strand of DNA), and the walls are only 10 Angstrom thick (about the size of a "typical" amino acid molecule). These parallel, open-ended pores result in a material that has extremely high surface area, all of which is accessible to solution-borne heavy metals.
There has been an on-going research effort at PNNL in the area of mesoporous ceramics which has been spear-headed by the gifted intellect of my co-worker Dr. Jun Liu. In this work, a detailed understanding of the roles ceramic composition and pore diameter play in the surface chemistry and properties of these materials has been obtained. In the Materials Synthesis Group, we have also had a long-standing research effort in the area of self-assembled monolayers, in which we have studied the chemistry of special classes of molecules that coat surfaces in ordered, uniform layers, a single molecule thick. Self-assembled monolayers have proven to be powerful tools for the synthetic chemist in the manipulation of interfacial chemistry. By joining these two efforts, PNNL has created a novel, and powerful, class of hybrid materials -- Self-Assembled Monolayers on Mesoporous Supports (SAMMS™).
The beauty of these materials is that we can take a single foundation (in this case, mesoporous silica, SiO2) and by coating it with different "flavors" of monolayers we can dictate the chemical properties of the final, high surface-area SAMMS™. For example, by installing a thiol-terminated monolayer on mesoporous silica, a sorbent material is created that has an extremely high affinity for binding mercury, and in fact is significantly faster and more efficient than any other method for scavenging this highly toxic heavy metal. A mercury concentration of 10,000 ppb constitutes a severe environmental threat. Treatment of this 10,000 ppb solution of mercury with the existing state-of-the-art technology will eventually reduce the mercury concentration to 1,000 ppb over the course of 8 hours. A single treatment with thiol-SAMMS™ will reduce the mercury concentration to less than 1 ppb in under 10 minutes. This is very powerful stuff.
Not surprisingly, thiol-SAMMS™ have also demonstrated an affinity for binding other "soft" heavy metals as well. Included in this list are other "bad actors" like lead and cadmium (a toxic component of "nicad" batteries), as well as gold, silver and copper.
Anions like arsenate, chromate and selenate form a significant environmental problem. Existing anion-exchange methods don't work very well. A superior method for sequestering these anions is to incorporate cationic metal complexes into the SAMMS™ superstructure. This also provides a very subtle way of using the stereochemistry of the metal complex as a template to attract certain anions based on their shape and to selectively remove only those of environmental concern (such arsenate and chromate). Thus, by installing a specific copper complex in the pores of SAMMS™, we have shown that it is possible to remove more than 99% of either chromate or arsenate, even in the presence of competing tetrahedral anions like sulfate. This technology can save lives today.
As a result of 40 years of weapons-grade plutonium production at Hanford, there are trace levels of various long-lived actinide isotopes (plutonium, americium, etc.) in the Hanford tank wastes. These need to be carefully analyzed and have all of the issues associated with their chemistry and radiolytic decay fully understood so responsible decisions can be made regarding the long-term storage of the waste. These actinides are present at very low levels, making accurate analysis difficult. We are currently evaluating several different SAMMS™ for their abilities to selectively bind actinides in order to increase both the speed and accuracy of these analyses.
These are but a few of the many significant environmental problems that we are faced with today. However, because these all have a common theme, it is possible that they can all be solved with a single solution. By creating SAMMS™, we hope to help industry operate more cleanly and efficiently, we hope to help clean up existing problems arising from toxic metals in the environment and we hope to make drinking water cleaner and safe from future contamination. To achieve this vision, however, a broad range of technical and business issues need to be addressed, such as how the SAMMS™ will be deployed in the field, the range of acceptable conditions for use, cost-effective manufacturing, and marketing, sales and distribution of the materials. Realization of our vision will come only when each of these obstacles are overcome. Each day brings new challenges for our multi-faceted team of scientists, engineers, lawyers and business leaders!
The next time you go to pour yourself a glass of water, stop and think for a moment about how valuable that water really is. Cheers!
Dr. Fryxell is a Senior Research Scientist in the Materials Synthesis Group at Pacific Northwest National Laboratory. His research focuses on surface chemistry, molecular self-assembly and organic synthesis.