ABSTRACT
For well over a century, metal-contaminated industrial wastewaters have been released into the environment by industry, agriculture, sewage treatment, and mining operations worldwide. Since the WWII era, the nuclear fuel cycle has contributed an additional and unique waste burden of uranium and other radioactive metals. Metal ions, unlike most organic chemicals, can persist in the environment indefinitely, posing threats to organisms which are exposed to them (Volesky and Holan 1995). Governmental control of such discharges has only been energetically regulated in the past two or three decades. Many toxic inorganic chemicals have, over time, accumulated in soils, sediments, and impoundments throughout the world. Metal-bearing liquid wastes may be of known and predictable composition if generated by a single industry, e.g., electroplating wastewaters, or in other cases may be a heterogeneous mix of many dissolved metal ions and organic compounds at various pH values and ionic strengths, with colloidal and particulate matter present as well. Governments are now regulating this problem by mandating preventative actions and forcing industries and laboratories and other waste generators to intercept toxic metals before they are discharged. Most heavy metal containing wastewaters are treated using remediation technologies that have been borrowed primarily from the unit operations of the chemical industry which rely on a mixture of physical and chemical processes (Table 1) to render the metal ion contaminants less toxic or more easily handled. Unfortunately the chemical form of the converted metal (e.g., a gelatinous precipitate) is itself often in need of careful and expensive disposal, and conventional treatment becomes less efficient and more expensive when metal ion concentrations fall into the 1−10 mg/1 range. Table 2 provides a listing of discharge limits of metal finishing wastewaters in the United States, and as such represents goals that must be met by any new technology or existing technologies. Discharge limits for municipal wastewater treatment plants in the United States are much stricter than those listed in Table 2. Influent levels of Cu2+ in wastewaters arriving at municipal sewage treatment plants range from 100 to 250 µg/1, but effluent levels of 625 µg/1 are expected to be attained under new U.S. Environmental Protection Agency guidelines (Amer 1998). Regulations governing aqueous metal discharges in the United Kingdom have been reviewed by Forster and Wase (1997) who also discussed the toxic biological effects of several of the important heavy metals. A genuine need now exists for new and certainly more costeffective technologies to replace or supplement the physicochemical approaches currently in use for removing metal contamination at existing sites and for preventing future contamination of natural waters by heavy metals.
