Heavy Metal Remediation of Wastewaters by Microbial Biotraps

Abstract

Industry, agriculture, sewage treatment and mining operations all combine to produce enormous amounts of metal (and other inorganic) ion contaminated wastewaters. Nuclear fuel cycle waste adds an additional and unique burden of radioactive metals. Unlike organic chemicals, metals persists in the environment indefinitely, posing threats to all living organisms which are exposed to them (Volesky and Holan, 1995). Control of such discharges has only been energetically regulated by governments in the past two or three decades. As a result, many toxic inorganics have accumulated in soils, sediments and impoundments throughout the world. Metal bearing wastewaters may be of known and predictable composition if derived from a single industry, e.g., electroplating wastewaters, or in other cases are a heterogeneous mix of many dissolved metal ions and organics at various pHs, with salts, colloidal and particulate matter present as well. The approach by many governmental regulators is now to prevent further deterioration of the environment by intercepting toxic metals before they are discharged. It is recognized that new, hopefully more cost-effective, technologies are needed to replace those borrowed primarily from the unit operations of the chemical industry which rely on a mixture of physical and chemical processes (Table I) to render the contaminants less toxic or more easily handled (Crusberg et al., 1994; Gretsky, 1994). Unfortunately, the form of the converted metal is itself often in need of careful and expensive disposal, and conventional treatment becomes less efficient and more expensive when metal ion concentrations fall in the 1–100 mg/l range. Table II provides a listing of discharge limits of metal finishing wastewaters in the U.S., and as such represent goals that must be met by any new technology or existing technologies.