This issue of Environmental Practice (June 2004) focuses on mercury (Hg) pollution and remedial procedures. Six of the seven full-length articles in this special issue were presented at the 6th International Conference on Mercury as a Global Pollutant, which was held in Minamata, Japan, from October 15 to 19, 2001. These articles describe mercury emissions from artisanal gold mines in two important countries (China and Brazil), show a critical review of the use of bioaccumulation factors in stream monitoring, and demonstrate how different procedures can be applied to remove mercury from contaminated soils/sediments.

As this issue of Environmental Practice goes to press, the United States Environmental Protection Agency (USEPA) has put mercury from coal-fired plants into front-page news. The battle seems simple on the surface: USEPA seeks to reverse a decision made in December 2000 that Section 112 of the Clean Air Act would govern these emissions.

Over the last couple of years, the toxic metals spotlight in the United States has shifted from lead to mercury. Today it seems that mercury is in everything, from contact lens solution to lab chemicals, medical equipment, fluorescent light bulbs, cars, McDonald's toys, and fish. The latter finding has now taken this industrial chemical closer to home. Are we poisoning our children and ourselves with a powerful neurotoxin by eating fish, breathing air, and perhaps even getting our cavities filled at the dentist's office? Is mercury a truly serious problem?

“Don't worry, this won't hurt me a bit.” I'll never forget that clever “joke” as my dentist proceeded to drill a hole in my tooth to replace a cavity with “amalgam.” Another tooth, filled with amalgam decades ago, had to be refilled because the amalgam had worn due to eating and other abuse over the years.

In December 2000, the United States Environmental Protection Agency (USEPA) determined that mercury emitted to the air during coal-fired electric generation posed a public health concern. This finding was coupled with an announcement that the agency intended to establish stringent limits on stack emissions of the pollutant no later than December 2004.

Throughout the world, the growth of artisanal or small-scale mining has resulted in serious mercury pollution issues. While these problems are well studied in much of the world, little has been written about the state of affairs in the People's Republic of China. It is vital to understand how the situation has evolved in China to determine if China's methods of dealing with these environmental and health problems could be applicable to other parts of the world, or if techniques and solutions developed outside of China could be used by the Chinese. Artisanal mining employs about 6 million people in China, who often work in deadly conditions, and contributes to environmental degradation on a huge scale. Chinese artisanal mercury, coal, and gold miners emit hundreds of tonnes of mercury into the environment annually. Artisanal gold miners often use muller mills, which amalgamate whole ores and thus increase mercury contamination in the tailings; more than 14 parts mercury can be lost per part of gold recovered. Many operations do not use retorts, leading to mercury vapor contamination. In 1997, artisanal and small-scale mining emitted approximately 381 tonnes of mercury, of which 226 tonnes were due to gold mining. The implementation of simple measures, such as providing communities with accessible education regarding environmental issues, encouraging the use of retorts, and discouraging the amalgamation of whole ores, would help to mitigate these problems.

An estimated 50 tons of mercury (Hg) have been emitted by gold miners in the Bento Gomes river basin, in the municipality of Poconé, Brazil, since the 1980s. Since the mid-l990s, the state agency for environmental protection, FEMA (Fundação Estadual do Meio Ambiente de Mato Grosso), has enforced regulations to reduce Hg emissions to air and water and has also implemented an environmental assessment program. The objectives of this study were to evaluate efforts to reduce emissions of Hg to air and water from nine improved amalgamation centers, and to assess the pollution level in sediment at 25 sites around Poconé. In spite of the fact that retorts were used, results showed large emissions of Hg when burning amalgam, resulting in Hg air concentrations above the limit for occupational air (50 μg/m3) at all centers except one. Keeping washing water in closed systems and dumping residues in specially prepared sites reduced Hg emissions to watercourses. The average Hg concentration of fine sediments (<74 μm) in the Bento Gomes river basin was 104 ng Hg/g dry weight, three to four times higher than the background level; large amounts of Hg-contaminated sediments are re-suspended during the rainy season. In conclusion, present emissions to local watercourses have been efficiently reduced, but the use of retorts in improved amalgamation centers has not adequately reduced Hg emissions to air, which is why the use of Hg remains an occupational and environmental problem.

The bioaccumulation of methylmercury in fish is a complex process affected by many site-specific environmental factors. The US Environmental Protection Agency (USEPA) recently recommended changing the basis for expressing the ambient water quality criterion for mercury from an aqueous concentration to a measure of the methylmercury concentration in fish. This change would make the regulation of mercury in surface waters a site-specific exercise in which fish-based bioaccumulation factors (BAF; the ratio of mercury concentration in fish to the concentration of mercury in water) are used to calculate aqueous concentration limits for total mercury. These limits would then be used to allocate mercury loading among various point and nonpoint sources and guide regulatory actions. In order for this approach to succeed, it is critical that the site-specific BAFs and methylmercury:total mercury conversion factors be independent of aqueous total mercury concentration (HgT). We investigated this relationship by measuring aqueous methylmercury and HgTs and mercury in fish in ecologically similar warm-water streams in the southeastern United States. Bioaccumulation factors based on HgT in water were found to decrease with increasing HgT, primarily as a consequence of the reduction in the ratio of aqueous methylmercury to total mercury with increasing HgT. Methylmercury-based BAFs did not vary as a function of HgT. The implication of this relationship is that site-specific determination of aqueous HgT limits at contaminated sites may use BAFs that would be underestimates of the appropriate BAFs to describe mercury bioaccumulation in the system after mercury inputs have been reduced. In such cases, regulatory limits set using site-specific BAFs might not achieve their intended purpose of reducing mercury contamination in fish to acceptable concentrations.

Overall sampling of the sediments in the Adriatic Sea at the mouth of the Isonzo (Soça) River in Italy has permitted updating of the extent and level of mercury contamination. The lsonzo River transports mercury-bearing residues from the ldrija mine (Slovenia); that area is contaminated due to the mining of cinnabar (HgS). The mercury mine started operations in the 15th century and was finally shut down in the 1980s because of the decreased demand for mercury. An attempt was made to remove mercury from the contaminated sediments by thermal desorption, with the aim of ascertaining whether low temperature and short residence time could be suitable parameters for sediment cleanup if future needs should so require. To date, no studies have been done on the health of the Italian population living in the Gulf of Trieste area; hence there is no known correlation between the anomalous mercury content of the sediments and symptoms attributable to the ingestion of even small quantities of mercury. Desorption times of about 20 to 30 minutes, at temperatures ranging from 325° C to 350° C, yielded residues with a mercury content below the limit imposed by Italian regulations for contaminated soils and sediments (5 ppm). The air and the mercury vapors driven off during roasting were treated before being released to the atmosphere.

The extension of surfactant-based technologies for use in remediation of mercuric ion [Hg(II)]-impacted soils and ground-waters was explored. In concept, a target metal ion can be selectively sequestered and mobilized from the subsurface by a ligand solubilized in surfactant solution. The selected ligand, 1-decyl-2-thiourea (DTU), was used in this study due to its extremely high selectivity for Hg2+ and its compatibility with micellar solubilization. In batch semiequilibrium dialysis studies using a mixture of 0.3 mM DTU and 30 mM cetylpyridinium nitrate (a cationic surfactant), 99.8% of applied Hg(II) (0.1 mM) was retained, thus demonstrating the effectiveness of this ligand-surfactant system for separating the mobile contaminant from the waste stream. Isolation of the target metal ion from the complex is desirable to allow for ligand and surfactant reuse. As a function of the ligand type, this can be achieved by precipitation, pH stripping, or ligand-ligand exchange. In theory, for DTU, Hg(ll) removal can be done at an elevated pH by formation of a soluble mercury-hydroxide complex, which would pass a secondary ultrafiltration stage, allowing retention and reuse of the ligand-surfactant colloid. While only batch studies were conducted for this feasibility study, the possibility of utilizing flow-through ultrafiltration units coupled with contaminant isolation steps in pump-and-treat field applications is discussed.

In most contaminated soils and mine tailings, mercury (Hg) is not readily available for plant uptake. A strategy for inducing Hg mobilization in soils to increase accumulation potential in plants was investigated to enhance Hg phytoremediation. Accumulation of Hg in the nickel hyperaccumulator Berkheya coddii, the salt-tolerant Atriplex canescens, and the nonaccumulators Brassica juncea and Lupinus sp. was studied by pot trials containing mine tailings treated with either soluble Hg or sulfur-containing ligands. Accumulation of Hg in shoots of B. coddii and A. canescens after addition of soluble Hg was lower than 10 mg/kg dry weight. The addition of ammonium thiosulfate (NH4S2O3) to tailings mobilized Hg in substrates, as indicated by the elevated Hg concentrations in leachates from the pots of both species. Ammonium thiosulfate caused a significant increase in the Hg concentration in shoots of B. juncea. Conversely, Hg translocation to Lupinus sp. shoots was significantly reduced in the presence of this ligand. Mass balance calculations revealed a significant fraction of Hg was lost from the system. This unaccounted-for Hg may indicate Hg volatilization. The results suggest that there is potential for induced plant Hg accumulation for phytoremediation of Hg-contaminated sites. Issues of Hg leaching and volatilization, however, need to be addressed before this technology can be implemented in the field.

Bacterial reduction of mercury ions to elemental mercury by the mer operon-encoded microbial resistance mechanism has recently been shown to be a promising approach in the bio-remediation of mercury-contaminated wastewater. Mercury resistance is widespread among environmental bacteria and several isolates have proven to be adaptable catalyzers for mercury reduction in bioreactors. To accomplish high, stable, and predictable performance, however, the genetically engineered microorganism (GEM) Pseudomonas putida KT2442::mer73 has been constructed, which constitutively expresses the mercury resistance operon at a high level, is nonpathogenic, and does not contain plasmids. To assess the safety of this GEM in an open environmental application, gene transfer was investigated in stream and estuarine microcosms containing sediments from the Elbe River and Roskilde Fjord, Denmark. In P. putida KT2442::mer73, the merTPAB genes have been stably integrated into the chromosome to reduce the chance of horizontal transfer. Consequently gene transfer to an isogenic recipient strain, P. putida KT2442::Tc, could not be detected, although parameters such as recipient cell density, cell shock, continuous addition of cells, or application of mercury selective pressure were adjusted with respect to increasing the probability of gene exchange. On the basis of these experiments, the strain P. putida KT2442::mer73 can be regarded as safe.