Chemical Speciation Of Trace Metals Research Articles

Chemical Speciation and Leaching of Trace Metals in Groundwater from the Depleted Landfills, India

Abstract The poor groundwater quality through the leaching of contaminants from depleted landfill is a concern to the scientific community. Therefore, the role of landfills on groundwater quality cannot be neglected in an urban area. The factors influencing the leaching of trace metals in groundwater are soil profile, geochemical and environmental condition of disposed of refuse materials, groundwater-table depth, and climatic factors. This research work delineates landfill role in contaminating groundwater through the chemical speciation of trace metals. Analyzed groundwater quality data indicate most of the samples were classified under Ca2+-Na+ type cation and Cl- type of anion hydrogeochemical facies. The investigations of the mineral equilibrium indicate equilibrium with silicate minerals, which favors kaolinite formation. Saturation index indicates that hematite, goethite, chrysotile, dolomite, ferric-hydroxide, hydro-xyapatite, jarosite-K, cerussite, vivianite, and willemite are reactive minerals in the aquifer water and control their hydrogeochemistry. The study of chemical speciation of trace metals indicates the high possibility of oxidation-reduction, ion-exchange, and chemical-weathering reaction mechanism, which causes the release of trace metal ions and further contaminated aquifer water through leaching. It also justifies through study of contaminant movement in vertical profile of the soil. The chemical speciation of trace metals indicates a reducing atmosphere in the aquifer due to the dominance of Fe2+, Mn2+, Zn2+, Pb2+, and Cu2+ ions in aquifer water. Mn2+ and Zn2+ concentration decreases with depth, while Fe2+ and Pb2+ ion concentration low in the middle layer of the aquifer indicate the contribution through anthropogenic input since it is not available in geology of study area.

Potential pollution assessment of labile trace metals in Xixi River estuary sediments in Xiamen, China

The release of trace metals caused by industrial effluents and anthropogenic activities has been recorded in the Xixi River estuary, southern China. However, a thorough understanding of the behavior of trace heavy metals in Xixi River sediments is lacking. A total of 12 sediment cores were collected in June and December in the upper estuary section and mouth of the estuary. Here, an in situ high-resolution sampling technique, namely, diffusive gradients in thin films (DGT), was employed to acquire profiles of trace element concentrations and the release of bioavailable metals from sediments in different seasons. A three-step Community Bureau of Reference (BCR) sequential extraction method was used to explore the chemical speciation of trace metals in different seasons and to thereby assess the release potential of trace elements in sediments. The BCR sequential extraction results showed that the trace metals Fe, Mn, Co and Pb were mainly in the residual fraction, which rarely influences living organisms. The total mobile fractions (F1 + F2 + F3) of all trace metals were higher in winter than in summer, suggesting that accumulation occurred from summer to winter. DGT measurements showed that the intensity of sulfate reduction was higher in summer than in winter because of the high temperatures and high organic matter in summer. The intensity of sulfate and Mn(III/IV) reduction increased from the upper estuary section to the lower estuary. Fe(III) reduction decreased in summer but increased slowly in winter. The Pearson correlation results showed that the release of DGT-labile Co in pore water was related to Mn(III/IV) reduction, while the release of DGT-labile Pb was basically not controlled by the Fe-Mn-S redox transition. Abnormally high DGT-labile Pb concentrations were observed at the sampling station (XR3) closest to the estuary in winter, which might have been caused by the high Pb content in the local micro-sediments.

Chemical speciation of trace metals in atmospheric deposition and impacts on soil geochemistry and vegetable bioaccumulation near a large copper smelter in China

Atmospheric deposition is an important source of trace metals to surface environments, but knowledge about plant bioavailability of recently deposited metals and their fate in the soil-plant system is limited. We performed a fully factorial soil and atmosphere exposure experiment with three vegetables (radish, lettuce, and soybean). Treatments included soil profiles collected from three sites located along a strong gradient of atmospheric deposition with each soil type deployed across the three sites for one year, which allowed to effectively distinguish impacts of recently deposited metals (<1 year) from longer-term trace metal exposures in soils. Results showed that recently deposited copper (Cu), cadmium (Cd), and lead (Pb) accounted for 0.5–15.2% of total soil Cu, Cd, and Pb pools at the site most heavily impacted by atmospheric deposition, while recent deposition contributed 15–76% of Cu, Cd, and Pb concentrations in edible parts of vegetables. In addition, soil geochemical extractions showed that bioavailable fractions of trace metals from recent deposition (52–73%) were higher compared to metals previously present in soils (7–42%). These findings highlight a preferential uptake and high rates of bioaccumulation of deposited metals in vegetables and suggest a high potential of environmental risks of food pollution under high atmospheric metal deposition.

Chemical speciation of trace metals in seawater

Chemical speciation of trace metals in seawater

Effects of Planting Patterns on Trace Metals in Soils Following Wetland Restoration

To understand the effects of planting patterns on trace metal fractions in degraded wetland soils in the Jinjiang Estuary, the chemical speciation of trace metals (Cu, Cr, Zn, and Pb) before and after wetland restoration is analyzed based on a measurement and testing program. The results show that wetland restoration, in general, increases the ratios of Cu, Cr, Zn, and Pb in the acid‐soluble fraction and the ratios of Cr, Zn, and Pb in the residual fraction and decreases the ratios of Cr, Zn, and Pb in the reducible fraction and the ratios of Cu, Cr, Zn, and Pb in the oxidizable fraction; thus, the mobility of Cu increases, and the mobility of Cr, Zn, and Pb decreases. Moreover, the planting patterns influence the levels of Cu, Cr, Zn, and Pb to some extent. After wetland restoration, the levels of Cu show an upward trend, and the Pb levels exhibit a downward trend. However, the potential ecological risk of Cu, Cr, Zn, and Pb fail to decrease. The restoration time may have been too short. The results of this study indicate that the planting patterns influence the transformation of trace metal fractions in soil and affect the pollution degree and potential ecological risk, contributing to a better understanding of the effects of planting patterns on trace metals following wetland restoration. The results will be helpful for subsequent scientific research on wetland restoration.

Chemical speciation of trace metals in the industrial sludge of Dhaka City, Bangladesh

The objective of this study was to assess total concentration and chemical fractionation of trace metals in the industrial wastewater and sludge collected from seven different types of industries in Dhaka City, Bangladesh. The sludge from industries is either dumped on landfills or reused as secondary resources in order to preserve natural resources. Metals were analyzed using inductively coupled plasma mass spectrometry (ICP-MS). The ranges of Cr, Ni, Cu, As, Cd, and Pb in the sludges were 1.4-9,470, 4.8-994, 12.8-444, 2.2-224, 1.9-46.0 and 1.3-87.0 mg/kg, respectively. As a whole, the average concentrations of trace metals in samples were in the decreasing order of Cr > Ni > Cu > As > Pb > Cd. The results of the Community Bureau of Reference (BCR) sequential extraction showed that the studied metals were predominantly associated with the residual fraction followed by the oxidizable fraction. The study revealed that the mobile fractions of trace metals are poorly predictable from the total content, and bioavailability of all fractions of elements tends to decrease.

Influence of Ocean Acidification on the Organic Complexation of Iron and Copper in Northwest European Shelf Seas; a Combined Observational and Model Study

The pH of aqueous solutions is known to impact the chemical speciation of trace metals. In this study we conducted titrations of coastal seawaters with iron and copper at pH 7.91, 7.37 and 6.99 (expressed on the total pH scale). Changes in the concentration of iron and copper that complexed with the added ligands 1-nitroso-2-napthol and salicylaldoxime respectively were determined by adsorptive cathodic stripping voltammetry - competitive ligand equilibrium (AdCSV-CLE). Interpretation of the results, assuming complexation by a low concentration of discrete ligands, showed that conditional stability constants for iron complexes increased relative to inorganic iron complexation as pH decreased by approximately 1 log unit per pH unit, whilst those for copper did not change. No trend was observed for concentrations of iron and copper complexing ligands over the pH range examined. We also interpreted our titration data by describing chemical binding and polyelectrolytic effects using non-ideal competitive adsorption in Donnan-like gels (NICA-Donnan model) in a proof of concept study. The NICA-Donnan approach allows for the development of a set of model parameters that are independent of ionic strength and pH, and thus calculation of metal speciation can be undertaken at ambient sample pH or the pH of a future, more acidic ocean. There is currently a lack of basic NICA-Donnan parameters applicable to marine dissolved organic matter (DOM) so we assumed that the measured marine dissolved organic carbon could be characterized as terrestrial fulvic acids. Generic NICA-Donnan parameters were applied within the framework of the software program visual MINTEQ and the metal –added ligand concentrations [MeAL] calculated for the AdCSV-CLE conditions. For copper, calculated [MeAL] using the NICA-Donnan model for DOM were consistent with measured [MeAL], but for iron an inert fraction with kinetically inhibited dissolution was required in addition to the NICA-Donnan model in order to approximate the trends observed in measured [MeAL]. We calculated iron and copper speciation in Northwest European shelf water samples at ambient alkalinity and projected increased pCO2 concentrations as a demonstration of the potential of the approach.

Influence of ocean acidification on the complexation of iron and copper by organic ligands in estuarine waters

The uptake of anthropogenic atmospheric CO2 by the oceans causes a shift in the carbonate chemistry system which includes a lowering of pH; this process has been termed ocean acidification. Our understanding of the specific effects of ocean acidification on chemical speciation of trace metals, in particular on organic–metal interactions, is limited. In this study we have experimentally determined the effects of changing pH from 8.3 to 6.8 (on the NBS scale) on the speciation of iron and copper in estuarine waters. Our experimental results indicated that complexation of iron and copper would decrease and inorganic iron and copper concentrations increase, as pH decreased, although it was not possible to confidently quantify changes in speciation at lower pH due to constraints of the analytical technique. In addition to our experimental approach, we used a non-ideal competitive adsorption (NICA)–Donnan model to determine the chemical speciation of iron and copper as a function of pH. The NICA–Donnan model was optimised in order to produce similar metal binding characteristics to those observed in our sample across the pH range examined in our study. The model allowed for simultaneous modelling of solubility and organic complexation. Model results indicated that a decrease in iron and copper binding by organic matter at lower pH, coupled with increased solubility (for iron), resulted in a 3 fold increase in inorganic iron concentration and a 6 fold increase in inorganic copper concentration at pH of 7.41 compared to a pH of 8.18 (expressed on the total scale). This compared to a 10 fold increase in inorganic iron concentration, and a 5 fold increase in inorganic copper concentration, obtained at pH8.18, when the dissolved organic carbon (DOC) concentration was halved. Variability in DOC might thus be expected to have a greater impact on metal speciation in seawater, than projected variability in pH resulting from increases in atmospheric CO2. Our study therefore suggests that increases in the concentrations of the more bioavailable inorganic iron and copper species in estuarine waters resulting from increased pCO2 are likely to occur, but that such changes will be moderate in magnitude.

Assessment of contamination, distribution and chemical speciation of trace metals in water column in the Dakar coast and the Saint Louis estuary from Senegal, West Africa

The water column from Dakar coast and Saint Louis estuary in Senegal, West Africa, was sampled in order to measure the contamination level by trace metals. The speciation of metals in water allowed performing a distribution between dissolved and particulate trace metals. For the dissolved metals, the metallic concentration and repartition between the organic fraction and the inorganic fraction were performed. The results show that the pollution of the estuary was more serious than in Dakar coast for Co, Cr, Ni, Pb and Zn; while, Cd and Cu were higher in Dakar coast. A strong affinity between metals and suspended particles has been revealed. Dissolved metals that have a tendency to form organic metal complexes are in decreasing order: Cd, Zn, Pb, Co=Cr=Mn, Cu and Ni. The results showed that the mobility of trace metals in estuary is controlled by dissolved organic carbon, while in coast it depends on chlorides.

마산만 표층퇴적물에서 미량금속의 화학적 존재형태 및 생태계 위해도 평가

Total concentration and chemical speciation of trace metals (Cu, Pb, Zn, Cd, and Ni) were determined to evaluate pollution level and potential ecological risk in surface sediments of the Masan Bay. The results showed that the trace metal concentrations, except for Ni, were high in the inner Masan Bay. Based on the chemical speciation of metals in sediments, the percentage of total concentrations of Cd and Pb in non-residual fraction was 92% and 88%, respectively, indicating that these metals originated mainly from anthropogenic sources. However, Ni (70%) was dominant in residual fraction. Pollution load index (PLI) and ecological risk index (ERI) values in the inner bay indicate the presence of anthropogenic pollution and considerable-moderate ecological risk, respectively. Ecological index (Ei) value for Cd was high at most stations in the Masan Bay, and Cd content was the highest in acid soluble fraction, which presents the highest ecological risk. The results obtained in this study indicate that Cd presents a high potential ecological risk to benthic biota in the Masan Bay.

Chemical speciation of trace metals emitted from Indonesian peat fires for health risk assessment

Regional smoke-induced haze in Southeast Asia, caused by uncontrolled forest and peat fires in Indonesia, is of major environmental and health concern. In this study, we estimated carcinogenic and non-carcinogenic health risk due to exposure to fine particles (PM2.5) as emitted from peat fires at Kalimantan, Indonesia. For the health risk analysis, chemical speciation (exchangeable, reducible, oxidizable, and residual fractions) of 12 trace metals (Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Ti, V and Zn) in PM2.5 was studied. Results indicate that Al, Fe and Ti together accounted for a major fraction of total metal concentrations (~83%) in PM2.5 emissions in the immediate vicinity of peat fires. Chemical speciation reveals that a major proportion of most of the metals, with the exception of Cr, Mn, Fe, Ni and Cd, was present in the residual fraction. The exchangeable fraction of metals, which represents their bioavailability, could play a major role in inducing human health effects of PM2.5. This fraction contained carcinogenic metals such as Cd (39.2ngm−3) and Ni (249.3ngm−3) that exceeded their WHO guideline values by several factors. Health risk estimates suggest that exposure to PM2.5 emissions in the vicinity of peat fires poses serious health threats.

Trace metal speciation in marine sediments from Sulaibikhat Bay, Kuwait

The chemical speciation of trace metals (Cd, Co, Cr, Cu, Ni, Pb and Zn) in marine sediments from Sulaibikhat Bay, Kuwait was determined using a three-step sequential extraction procedure. To obtain a mass balance, a fourth step, i.e. digestion and analysis of the residue was undertaken using a microwave-assisted acid digestion procedure. The sum of the 4 steps (acid-soluble + reducible + oxidizable + residual) was in good agreement with the total content (71–116%), suggesting that the microwave extraction procedure is efficient. The results showed that all metals except for Pb and Zn were present at higher percentages in the residual fraction. The reducible fraction was the next followed by the oxidizable fraction. The exchangeable fraction was least important as a host for most metals. The mobility order of exchangeable fraction of the elements in surface sediments decreased in the order Cd &amp;gt; Zn &amp;gt; Cu &amp;gt; Co &amp;gt; Pb &amp;gt; Ni &amp;gt; Cr; the reducible fraction in the order Pb &amp;gt; Zn &amp;gt; Cu &amp;gt; Cd &amp;gt; Co &amp;gt; Ni &amp;gt; Cr; the oxidizable fraction in the order Pb &amp;gt; Cu &amp;gt; Cd &amp;gt; Ni &amp;gt; Co &amp;gt; Zn &amp;gt; Cr, and the residual fraction in the order Cr &amp;gt; Ni &amp;gt; Co &amp;gt; Cu &amp;gt; Cd &amp;gt; Zn &amp;gt; Pb. The data showed that the speciation of trace elements in sediments close to a sewage outfall was different from that of sediment from other parts of Sulaibikhat Bay. This suggests that the contribution of the sewage outfall to metal pollution in adjacent marine area is positive and is associated with fine-grained sediments with high level of organic content, which are major controlling factors for the distribution of trace metals in this part of the Bay.

Chemical speciation and bioavailability of Cd, Cu, Pb and Zn in Western Balkan soils

Three thermal power plants in Serbia, Croatia and Bosnia of the Western Balkan region were expected to be metal polluting sources, and this study was performed to investigate the bioavailability and chemical speciation of trace metals in soils and soil water extracts, respectively. Surface (0–15 cm) soil samples along with maize and grass samples were collected at a gradient from the pollution source. The chemical speciation of metals was conducted using the Windereme Humic Aqueous Model (WHAM)/Model VI for water, whereas the Diffusion Gradient in Thin Films (DGT) technique was used to estimate plant availability. The chemical speciation indicated that more than 99% of all four metals in soil water extracts were complexed to fulvic acid. This is connected to relatively high soil pH (> 6.5) and high contents of soil organic matter in these soils. The accumulation of trace metals by DGT was not correlated to plant uptake. This is connected to the very low partitioning of free ions in solution, but also to the low variation in metal solubility and metal concentration in plant tissue between sites. In spite of active thermal power plants located in the areas, hardly any differences in concentration of soil metals between sites were seen and the partition of metals in soil waters was insignificant. The latter indicates that these soils have a large metal-retaining capacity. The only significant soil chemical variable affecting the variation in metal solubility was the soil pH. In a time with large infrastructure and industrial expansion in these areas, this investigation indicates the importance of protecting these high-quality soils from industrial use and degradation. High industrial activity has so far had insignificant effect on soil quality with respect to bioavailability of trace metals in these soils.

Combined Application of QEM‐SEM and Hard X‐ray Microscopy to Determine Mineralogical Associations and Chemical Speciation of Trace Metals

We describe the application of quantitative evaluation of mineralogy by scanning electron microscopy in combination with techniques commonly available at hard X-ray microprobes to define the mineralogical environment of a bauxite residue core segment with the more specific aim of determining the speciation of trace metals (e.g., Ti, V, Cr, and Mn) within the mineral matrix. Successful trace metal speciation in heterogeneous matrices, such as those encountered in soils or mineral residues, relies on a combination of techniques including spectroscopy, microscopy, diffraction, and wet chemical and physical experiments. Of substantial interest is the ability to define the mineralogy of a sample to infer redox behavior, pH buffering, and mineral-water interfaces that are likely to interact with trace metals through adsorption, coprecipitation, dissolution, or electron transfer reactions. Quantitative evaluation of mineralogy by scanning electron microscopy coupled with micro-focused X-ray diffraction, micro-X-ray fluorescence, and micro-X-ray absorption near edge structure (mXANES) spectroscopy provided detailed insights into the composition of mineral assemblages and their effect on trace metal speciation during this investigation. In the sample investigated, titanium occurs as poorly ordered ilmenite, as rutile, and is substituted in iron oxides. Manganese's spatial correlation to Ti is closely linked to ilmenite, where it appears to substitute for Fe and Ti in the ilmenite structure based on its mXANES signature. Vanadium is associated with ilmenite and goethite but always assumes the +4 oxidation state, whereas chromium is predominantly in the +3 oxidation state and solely associated with iron oxides (goethite and hematite) and appears to substitute for Fe in the goethite structure.

Investigation of DGT as a metal speciation technique for municipal wastes and aqueous mine effluents

This paper reports the results of an investigation on the performance of the Diffusive Gradient in Thin Films (DGT) Technique in speciation of metals in aqueous samples of municipal wastes and mine effluents. The DGT was assessed regarding its suitability for in situ determination of metal speciation in municipal wastes and aqueous mine effluents. As the thickness of the diffusive gel layer of the DGT was increased to 0.40, 0.80, and 1.60 mm, a decrease in the amount of accumulated metal mass was observed for most of the metals studied in all the effluent samples. However, the results were different from one field-study site to another. Effect of kinetics also was observed in the amount of accumulated metal mass by the DGT. The computer speciation code, Windermere Humic Aqueous Model (WHAM VI), was used to predict the metal speciation of Cd, Cu, Ni, Pb, Co, and Zn, and WHAM predictions were compared with those of the experimentally determined metal speciation by the DGT technique (free and labile metal ions). This comparison showed good similarities between the theoretically predicted WHAM VI values and the experimentally measured values by DGT. The DGT technique was found to be simple and useful for investigating chemical speciation of trace metals in aqueous samples of municipal wastes and aqueous mine effluents.

Chemical speciation of trace metals in fine airborne particles: advances in operational performance of a new sequential extraction scheme

A speciation method of inorganic metal compounds in airborne particulate matter based on selective sequential extractions is described. The sequential leaching procedure was perfected for improvement of performance at different stages of scheme. The loss of fine particles during the leaching procedure was the main source of error that was assessed and corrected. Loss of sample volume due to the heating steps and due to the absorption on filter material was also a difficulty to solve. New experimental conditions were optimized for the operational scheme to remove the different sources of loss. Microwave assistance with an optimum programme as a heating system, filtration of extractable solutions, and the use of internal standards were the main contributions.

Chemical Speciation of Trace Metals in Seawater: a Review

The recent development of the chemical speciation of trace metals in seawater is described. The speciation studies reveal that metal ion complexation is one of the most important processes in seawater; especially, most bioactive trace metals, such as Fe(III) and Cu, exist as complexes with ligands in dissolved organic matter. The organic ligands in seawater are characterized with metal ions selected by the HSAB concept. A strong organic ligand, which originates from marine microorganisms, is classified as a hard base including carboxylates. The free organic ligand concentrations in seawater are buffered by complexation with excess amounts of Ca and Mg in seawater. The chemical equilibrium model suggested that the concentrations of bioactive free metal ions are at an optimal level to activities of marine microorganisms. For chemical speciation, it is important to have a better understanding of the ecological roles of trace metals in seawater.

Sequential extractions on mine tailings samples after and before bioassays: implications on the speciation of metals during microbial re-colonization

Mine tailings may be remediated using metal tolerant microorganisms as they may solve the limiting conditions for healthy development of plants (i.e., low organic mater content and poor physical conditions). The aim of this study was to investigate the consequences of microbial colonization on the chemical speciation of trace metals. Surface samples from the Valenciana mine tailings (Guanajuato, Mexico) were used for long-term bioassays (BA), which consisted in the promotion of microorganisms, development on tailings material under stable laboratory conditions (humidity, temperature, and light exposure). A five-step sequential extraction method (exchangeable, carbonate/specifically adsorbed, Fe–Mn oxides, organic matter (OM)/sulfide, and residual fractions) was performed before and after BA. Extraction solutions and leachates were analyzed by inductively coupled plasma-mass spectrometry. OM content, cationic exchange capacity, and pH values were also assessed before and after BA. The results indicate that trace elements are generally present in nonresidual fractions, mainly in the Fe–Mn oxides fraction. The concentration of total Zn, As, Se, Pb, and exchangeable Cu and Pb is above the recommendable limits for soils. Despite the high bioavailability of the former elements, biofilms successfully colonized the tailing samples during the BA. Cyanobacteria and green algae, heterotrophic fungi, aerobic bacteria, and anaerobic bacteria composed the developed biofilms. Chemical controls of trace elements could be attributed to absorption onto inorganic complexes (carbonates, metal oxides), while biofilm occurrence seems to enhance complexation and immobilization of Cr, Ni, Cu, Zn, As, and Pb. The biofilm developed does not increase the bioavailable forms and the leaching of the trace elements, but significantly improves the OM contents (natural fertilization). The results suggest that biofilms are useful during the first steps of the mine tailings remediation.

Equilibrium Speciation of Cadmium, Copper, and Lead in Soil Solutions

Determination of chemical speciation of trace metals may help us to better understand the bioavailability and toxicity to living organisms in terrestrial and aquatic environments. This paper reports the speciation data of cadmium (Cd), copper (Cu), and lead (Pb) in soil solutions sampled from areas around metal smelters. A column ion exchange technique (IET) using a cation exchange resin was applied to determine the concentrations of Cd2+, Cu2+, and Pb2+. It was shown that the pH and the amount of calcium (Ca) in solutions affected the distribution of metal ions to the resin. In the synthetic solutions involving Cl, SO4, and citrate, the metal‐ligand complexes did not affect the measurement of free metal ions by the IET. In soil solutions, the concentrations of free ions were affected by solution pH, varying within 10−9–10−7 M for Cd, 10−9–10−6 M for Cu, and 10−9–10−7 M for Pb. The IET results were comparable to those measured by Cu‐ion selective electrode (ISE) and anodic stripping voltammetry (ASV). These findings suggest that the IET can be applied to soil solutions for metal speciation measurements.

Predicting the Bioavailability of Metals and Metal Complexes: Critical Review of the Biotic Ligand Model

Environmental Context. The chemical speciation of trace metals greatly influences their biological effects. Nonetheless, no clear consensus currently exists as to when metal complexes are bioavailable, especially for field conditions. Recently, the USA EPA has incorporated the biotic ligand model (BLM) into their regulatory framework and many other countries are now examining the implications of following suit. This review examines the fundamental basis of the BLM in order to provide the reader with an understanding of its potential uses and limitations. Abstract. The biotic ligand model is a useful construct both for predicting the effects of metals to aquatic biota and for increasing our mechanistic understanding of their interactions with biological surfaces. Since biological effects due to metals are always initiated by metal bioaccumulation, the fundamental processes underlying bio-uptake are examined in this review. The model assumes that the metal of interest, its complexes, and metal bound to sensitive sites on the biological surface are in chemical equilibrium. Therefore, many of the equilibrium constants required for the model have been compiled and their methods of determination evaluated. The underlying equilibrium assumption of the BLM is also examined critically. In an attempt to identify which conditions are appropriate for its application, several documented examples of failures of the BLM are discussed. Finally, the review is concluded by identifying some important future research directions.