A comprehensive methodology has been created to quantify the degree of criticality of the metals of the periodic table. In this paper, we present and discuss the methodology, which is comprised of three dimensions: supply risk, environmental implications, and vulnerability to supply restriction. Supply risk differs with the time scale (medium or long), and at its more complex involves several components, themselves composed of a number of distinct indicators drawn from readily available peer-reviewed indexes and public information. Vulnerability to supply restriction differs with the organizational level (i.e., global, national, and corporate). The criticality methodology, an enhancement of a United States National Research Council template, is designed to help corporate, national, and global stakeholders conduct risk evaluation and to inform resource utilization and strategic decision-making. Although we believe our methodological choices lead to the most robust results, the framework has been constructed to permit flexibility by the user. Specific indicators can be deleted or added as desired and weighted as the user deems appropriate. The value of each indicator will evolve over time, and our future research will focus on this evolution. The methodology has proven to be sufficiently robust as to make it applicable across the entire spectrum of metals and organizational levels and provides a structural approach that reflects the multifaceted factors influencing the availability of metals in the 21st century.

Keywords:end-of-life recycling rate (EOL-RR) industrial ecology old scrap ratio (OSR) recycled content (RC) recycling input rate (RIR) recycling metrics Supporting information is available on the JIE Web site SummaryThe recycling of metals is widely viewed as a fruitful sustainability strategy, but little information is available on the degree to which recycling is actually taking place. This article provides an overview on the current knowledge of recycling rates for 60 metals. We propose various recycling metrics, discuss relevant aspects of recycling processes, and present current estimates on global end-of-life recycling rates (EOL-RR; i.e., the percentage of a metal in discards that is actually recycled), recycled content (RC), and old scrap ratios (OSRs; i.e., the share of old scrap in the total scrap flow). Because of increases in metal use over time and long metal in-use lifetimes, many RC values are low and will remain so for the foreseeable future. Because of relatively low efficiencies in the collection and processing of most discarded products, inherent limitations in recycling processes, and the fact that primary material is often relatively abundant and low-cost (which thereby keeps down the price of scrap), many EOL-RRs are very low: Only for 18 metals (silver, aluminum, gold, cobalt, chromium, copper, iron, manganese, niobium, nickel, lead, palladium, platinum, rhenium, rhodium, tin, titanium, and zinc) is the EOL-RR above 50% at present. Only for niobium, lead, and ruthenium is the RC above 50%, although 16 metals are in the 25% to 50% range. Thirteen metals have an OSR greater than 50%. These estimates may be used in considerations of whether recycling efficiencies can be improved; which metric could best encourage improved effectiveness in recycling; and an improved understanding of the dependence of recycling on economics, technology, and other factors.

The relative proportions of metal residing in ore in the lithosphere, in use in products providing services, and in waste deposits measure our progress from exclusive use of virgin ore toward full dependence on sustained use of recycled metal. In the U.S. at present, the copper contents of these three repositories are roughly equivalent, but metal in service continues to increase. Providing today's developed-country level of services for copper worldwide (as well as for zinc and, perhaps, platinum) would appear to require conversion of essentially all of the ore in the lithosphere to stock-in-use plus near-complete recycling of the metals from that point forward.copper ͉ material flow analysis ͉ metal services F or at least three decades, scientists and economists have debated whether humanity is rapidly depleting the resources on which it depends (1-4). Unlike oil, which is irremediably consumed when used, metals have the potential for almost infinite recovery and reuse. Nonetheless, the rate of extraction of many geochemically scarce metals from the lithosphere has increased in excess of 3% per year through the last half century or longer and continues to do so. Because these are finite resources, it is instructive to ponder how long these extraction rates can be sustained.As goods in use are increased and replenished, metal is transferred from the stock of ore in the lithosphere to a stock of metal-in-use providing services, and some of the metal in the original ore is transferred to wastes during mining, milling, and smelting. Over time, some of the metal-in-use recycles from old to new products, some is dissipated through corrosion and wear, and some enters waste repositories such as landfills in the end-of-life products that are not recycled. The relative sizes of the remaining stock in the lithosphere, the stock-in-use, and the stock transferred to wastes at any given time are measures of how far we have progressed toward the need for total reliance on recycling rather than on virgin ore to provide material for new products.The demand for metal resides in the services that people receive from metal and metal-containing products, e.g., housing, transportation, and electrical power. The amount of metal in use therefore depends on the level of services and the efficiency with which metal is used in providing those services. For example, attaining a specified level of illumination in a home depends on a stock of copper in power station equipment and in transmission lines; this stock can increase if more illumination is wanted and decrease if new techniques permit the same amount of power to be generated and transmitted with less copper. Eventually the equipment and transmission lines reach the end of their service lives and are replaced. This end-of-life copper may be recycled or landfilled; if the latter, dilution with other wastes will make future recovery unlikely unless considerably improved technologies to separate metals from mixed, primarily organic, materials are implemented.Few, if any, metals have u...

Surface‐active organic molecules are common constituents of atmospheric aerosol particles, raindrops, and snowflakes. If these compounds are present as surface films, transfer of gases into the atmospheric water systems could be impeded, evaporation could be slowed, and the aqueous chemical reactions could be influenced. To investigate these possibilities, we have reviewed the chemical literature pertaining to organic films on aqueous surfaces: their composition, structure, properties, and effects. We then review the surface‐active organic compounds in atmospheric water. We report the results of new measurements of surface tension of aqueous solutions of common atmospheric organic compounds (β‐pinene, n‐hexanol, eugenol, and anethole) and demonstrate that the compounds produce films with properties similar to those of the more well known surfactants. We conclude that organic films are probably common on atmospheric aerosol particles and that they may occur under certain circumstances on fog droplets, cloud droplets, and snowflakes. If present, they will increase the lifetimes of aerosol particles, fog droplets, and cloud droplets, both by inhibiting water vapor evaporation and by reducing the efficiency with which these atmospheric components are scavenged. The presence of the films will not cause a significant reduction of solar radiation within the aqueous solution. It appears likely, however, that the transport of gaseous molecules into and out of the aqueous solution will be impeded by factors of several hundred or more when organic films are present. Since incorporated gas molecules provide much of the oxidizing potential of atmospheric water droplets, the organic films will play a major role in droplet chemistry by strongly inhibiting solution oxidation.