Chlor-alkali Industry Research Articles

Heavy Metal Pollution in Lentic Ecosystem of Sub-Tropical Industrial Region and its Phytoremediation

Metals and several physicochemical parameters, from four sampling sites in a tropical lake receiving the discharges from a thermal power plant, a coal mine and a chlor-alkali industry, were studied from 2004–2005. Pertaining to metal pollution, the site most polluted with heavy metals was Belwadah, i.e., waters and sediments had the highest concentration of all the metals examined. The reference site was characterized by the presence of low concentrations of metals in waters and in sediments. Further, several wetland plants were harvested from different sites, and simultaneously, these were assessed for their metal concentration efficiency. Following the water quality monitoring and metal concentration efficiency, two-month field phytoremediation experiments were conducted using large enclosures at the discharge point of different polluted sites of the lake. Eichhornia crassipes, Lemna minor, and Azolla pinnata were frontier metal accumulators hence selected for previously mentioned field phytoremediation experiments. During field phytoremediation experiments using aquatic macrophytes, marked percentage reduction in metals concentrations were recorded. The percentage decrease for different metals was in the range of 25–67.90% at Belwadah (with Eichhornia crassipes and Lemna minor), 25–77.14% at Dongia nala (with Eichhornia crassipes, Lemna minor, and Azolla pinnata) and 25–71.42% at Ash pond site of G.B. Pant Sagar (with Lemna minor and Azolla pinnata). Preliminary studies of polluted sites are therefore useful for improved microcosm design and for the systematic extrapolation of information from experimental ecosystems to natural ecosystems.

Blood biomarkers and contaminant levels in feathers and eggs to assess environmental hazards in heron nestlings from impacted sites in Ebro basin (NE Spain)

Blood biomarkers and levels of major pollutants in eggs and feathers were used to determine pollution effects in nestlings of the Purple Heron Ardea purpurea and the Little Egret Egretta garzetta, sampled on three Ebro River (NE Spain) areas: a reference site, a site affected by the effluents of a chlor-alkali industry and the river Delta. The two impacted heron populations showed mutually different pollutant and response patterns, suggesting different sources of contamination. In the population nesting near the chlor-alkali plant, elevated levels of hexachlorobenzene (HCB) and polychlorobiphenyls (PCBs) in eggs, and mercury in feathers in A. purpurea chicks were related with reduced blood antioxidant defenses and increased levels of micronuclei. In Ebro Delta, high levels of plasmatic lactate dehydrogenase in A. purpurea chicks and high frequency of micronuclei in blood of both species were tentatively associated with intensive agricultural activities taking place in the area. These results provide the first evidence of a biological response in heron chicks to the release of pollutants at a chlor-alkali plant.

Reactive Polymers in Mercury Removal from Electrolytic Brine

The adsorption of mercury from concentrated aqueous solutions and chlor-alkali industry brine on the reactive polymers has been studied. The investigations were performed in the batch and column systems. The batch experiments were carried out to assess mercury ion exchange adsorption isotherms and adsorptive capacity. The effects of agitation time and pH were investigated. On the basis of dynamic experiment results, mercury concentration in the effluent obtained from the column against the flow rate of brine through the bed was deduced and the effective and total capacities were calculated. The regeneration of the spent resins was also investigated.

Stability and morphology of (Ti0·1Ru0·2Sn0·7)O2 coating on Ti in chloralkali medium

Today's mixed metal oxide anodes are widely used in the chloralkali industry. Stability is one of the main properties of these anodes because of their high cost. Morphology of mixed metal oxide coating is a determining factor in electrochemical stability of mentioned anodes. In this article the effect of temperature and times of heat treatment on the morphology and electrochemical stability of Ti/(Ti0·1Ru0·2Sn0·7)O2 anode has been investigated. The anodes were prepared by brushing and thermal decomposition method. Mud cracked morphology appeared on the sample by this process. Increasing the temperature of calcination causes the density of cracks on the surface and electrochemical stability of coating to decrease. Increasing the time had different effects in the first and final calcination times. The anode which was prepared at 450°C, for 10 min in the first calcination time and 1 h in the final calcination time had the highest stability and optimum morphology.

Identification of water soluble and particle bound compounds causing sublethal toxic effects. A field study on sediments affected by a chlor-alkali industry

A combination of cost effective sublethal Daphnia magna feeding tests, yeast- and cell culture-based bioassays and Toxicity Identification Evaluation (TIE) procedures was used to characterize toxic compounds within sediments collected in a river area under the influence of the effluents from a chlor-alkali industry (Ebro River, NE Spain). Tests were designed to measure and identify toxic compounds in the particulate and filtered water fractions of sediment elutriates. The combined use of bioassays responding to elutriates and dioxin-like compounds evidenced the existence of three major groups of hazardous contaminants in the most contaminated site: (A) metals such as cadmium and mercury bound to sediment fine particles that could be easily resuspended and moved downstream, (B) soluble compounds (presumably, lye) able to alkalinize water to toxic levels, and (C) organochlorine compounds with high dioxin-like activity. These results provided evidence that elutriate D. magna feeding responses can be used as surrogate assays for more tedious chronic whole sediment tests, and that the incorporation of such tests in sediment TIE procedures may improve the ability to identify the toxicity of particle-bound and water-soluble contaminants in sediments.

Stock Investment Value Study Based on Fuzzy Comprehensive Evaluation

This paper constructs 3-level evaluation model based on index system for the listed corporations and confirm the weights by the factor analysis, then compute the investment value of listed companies in chlor-alkali industry making use of fuzzy comprehensive evaluation method and illustrate the method. The objective and reasonable basis for making investment decision is provided.

Phytoremediation of heavy metals in a tropical impoundment of industrial region

Aquatic pollution poses a serious challenge to the scientific community worldwide, since lakes or reservoirs find multifarious use and most often their water is used for drinking, bathing, irrigation, and aquaculture. Nine metals and several physicochemical parameters, from four sampling sites in a tropical lake receiving the discharges from a thermal power plant, a coal mine, and a chlor-alkali industry, were studied from 2004 to 2005. Pertaining to metal pollution, the site most polluted with heavy metals was Belwadah, i.e., waters and sediments had the highest concentration of all the metals examined. The reference site was characterized by the presence of low concentrations of metals in waters and sediments. Following the water quality monitoring, 2-month field phytoremediation experiments were conducted using large enclosures at the discharge point of different polluted sites of the lake. During field phytoremediation experiments using aquatic macrophytes, marked percentage reduction in metals concentrations were recorded. The percentage decrease for different metals was in the range of 25% to 67.90% at Belwadah (with Eichhornia crassipes and Lemna minor), 25% to 77.14% at Dongia nala (with E. crassipes, L. minor and Azolla pinnata), and 25% to 71.42% at Ash pond site of G.B. Pant Sagar (with L. minor and A. pinnata). Preliminary studies of polluted sites are useful for improved microcosm design and for the systematic extrapolation of information from experimental ecosystems to natural ecosystems.

Seasonal monitoring of heavy metals and physicochemical characteristics in a lentic ecosystem of subtropical industrial region, India

Assessment of seasonal changes in surface water quality is an important aspect for evaluating temporal variations of lentic ecosystem (lakes and reservoirs) pollution due to industrial effluent discharge. In this study, nine metals and 15 physicochemical parameters, collected from four sampling sites in a tropical lake receiving the discharge from thermal power plant, coal mine, and chloralkali industry, during the years from 2004 to 2005, were analyzed. For greater efficacy in monitoring of heavy metals, particle-induced X-ray emission has been used during present investigation. Different statistical techniques like analysis of variance, Pearson correlation, principal component analysis, and factor analysis were employed to evaluate the seasonal correlations of physicochemical parameters. Most of the metals and physicochemical parameters monitored in the present study exhibited high spatial and temporal variability. Pertaining to metal pollution, the most polluted site was Belwadah, i.e., waters and sediments had the highest concentration of all the relevant metals. The reference site was characterized by the presence of low concentrations of metals in waters and in sediments. Based on the high metal concentration recorded in lake ambient, drinking, bathing, and irrigation water should not be used by the local people at the effluent discharge points.

Historical record of mercury contamination in sediments from the Babeni Reservoir in the Olt River, Romania

Mercury (Hg) is a ubiquitous and hazardous contaminant in the aquatic environment showing a strong biomagnification effect along the food chain. The most common transfer path of Hg to humans is contaminated fish consumption. In severely exposed humans, Hg poisoning may lead to damage in the central nervous system. Thus, it is important to examine current and past contamination levels of Hg in aquatic milieu. The Olt River is the largest Romanian tributary of the Danube River. The use of Hg as an electrode in a chlor-alkali plant contributed to the contamination of the aquatic environment in the Rm Valcea region. The purpose of this study was to compare the current state of Hg contamination with the past contamination using a historical record obtained from a dated sediment core from one of the Olt River reservoirs (Babeni) located downstream from the chlor-alkali plant. To our knowledge, no published data on Hg contamination in this region are available. The Babeni Reservoir was selected for this study because it is situated downstream from the chlor-alkali plant, whilst the other reservoirs only retain the pollutants coming from the upstream part of the watershed. Preliminary analyses (unpublished) showed high Hg concentrations in the surface sediment of the Babeni Reservoir. One core was taken in the upstream Valcea Reservoir to provide a local background level of Hg concentrations in sediments. Sediment texture was uniform in the cores from both reservoirs. Laminated sediment structure, without any obvious discontinuities, was observed. Hg concentrations in the sediment core from the Valcea Reservoir were low and constant (0.01-0.08 mg/kg). In Babeni Reservoir sediments, Hg concentrations were very high in the deeper core section (up to 45 mg/kg in the longest core) and decreased to lower concentrations toward the top of the cores (1.3-2.4 mg/kg). This decrease probably reflects technological progress in control of emissions from the Hg-cell-based chlor-alkali industry. Two strong peaks could be distinguished in older sediments. The mean rate of sedimentation (5.9 cm/year) was calculated from the depth of the (137)Cs Chernobyl peak. This was in good agreement with the sedimentation rate estimated at this site from a bathymetric study. Assuming a constant sedimentation rate, the two Hg peaks would reflect two contamination events in 1987 and 1991, respectively. However, it is also possible that the two peaks belong to the same contamination event in 1987 but were separated by a sediment layer richer in sand and silt. This layer had a low Hg concentration, which can be interpreted as a mass deposition event related to a major flood bringing Hg-free sediments. Whilst the chlor-alkali plant partly switched to a cleaner technology in 1999, no obvious decrease of Hg concentrations was observed in recent decade. Results from the sediment core reflected the historical trend of Hg release from the chlor-alkali plant, revealed important contamination episodes and confirmed a legacy of contamination of Hg in recent sediments even if the concentrations of Hg decreased toward the surface due to a more efficient emission control. Although the Hg concentrations in Babeni Reservoir sediments were extremely high in the late eighties and they remain one order of magnitude higher in the surface sediments than in sediments from the upstream reservoir, little is known about the transfer of Hg to the biota and human population. Our initial measurements indicate the presence of monomethyl-Hg (MMHg) in pore water, but further studies are necessary to evaluate fluxes of MMHg at the sediment-water interface. Samples of fish and hair from various groups of the local population were recently collected to evaluate the potential hazard of Hg contamination to human health in the Rm Valcea region.

Adsorptive Removal of Mercury(II) Ions from Water and Wastewater by Polymerized Tamarind Fruit Shell

A novel adsorbent, formaldehyde polymerized tamarind fruit shell (FPTFS) containing sulphonic acid functional groups was prepared and its utility for Hg(II) adsorption from water and wastewater was investigated. The kinetic and isotherm data, obtained at optimum pH value 6.0 for different concentrations and temperatures, could be fitted with the Ritchie modified second-order equation and Sips isotherm model respectively and the coefficients indicated favorable adsorption of Hg(II) on the FPTFS. The complete removal of 23.86 mg/L Hg(II) from chlor-alkali industry wastewater was achieved by 4 g/L FPTFS. The reusability of the FPTFS for several cycles was also demonstrated using 0.1 M HCl solution.

A 130 year record of pollution in the Suances estuary (southern Bay of Biscay): Implications for environmental management

Geochemical composition (Al, Zn, Pb, Cd, Cu, Ni, Cr and As) and foraminiferal assemblages in surface and core sediments were determined to assess the current situation and the recent environmental transformation of the Suances estuary (southern Bay of Biscay, Spain). Dating of the historical record has been achieved using isotopic analysis ( 210Pb, 137Cs) and two benchmark events such as the beginning of the mineral exploitation in the Reocín Pb–Zn deposits and the evolution of the chlor-alkali industry (inputs of Hg). Concentrations of Zn, Pb and Cd in both surface and core samples are remarkably higher than background values, reflecting the existence of significant amounts of polluted materials. The dramatic environmental impact of this pollution is clearly recorded by the change of the foraminiferal assemblages that even reach an afaunal stage during recent decades. Application of two different sets of Sediment Quality Guidelines confirm that they exert potential risk to the environment, and therefore if dredged they should need specific management measures. The results provide a reference database to monitor future environmental changes in the Suances estuary, particularly as regards the contaminated sediment storage and the re-colonization by autochtonous meiofauna.

Adsorptive Potential of 2-Mercaptobenzimidazole-Immobilized Organophilic Hydrotalcite for Mercury(II) Ions from Aqueous Phase and Its Kinetic and Equilibrium Profiles

An organophilic calcined hydrotalcite (OHTC) was prepared by treating calcined hydrotalcite (HTC) with sodium dodecylbenzene sulphonate (an anionic surfactant) to achieve a high loading of thiol functionality through the immobilization of 2-mercaptobenzimidazole (MBI) as a chelating agent. The adsorbent (MBI-OHTC) obtained was characterized using XRD, FTIR, SEM, TG/DTG, surface area analysis and potentiometric titration. The adsorption of MBI-OHTC to remove Hg(II) ions from aqueous solutions was studied as a function of pH, contact time, metal ion concentration, ionic strength and adsorbent dose. The optimum pH range for the maximum removal of Hg(II) was 6.0–8.0. The maximum value of Hg(II) adsorption was found to be 11.63 and 21.52 mg g−1 for an initial concentration of 25 and 50 mg l−1, respectively at pH 8.0. The equilibrium conditions were achieved within 3 h under the mixing conditions employed. A reversible pseudo-first-order used to test the adsorption kinetics. The adsorption mechanism consisted of external diffusion and intraparticle diffusion and the intraparticle mass transfer diffusion was predominated after 20 min of experiment. Extent of adsorption decreased with increase of ionic strength. The experimental isotherm was analyzed with two parameters (Langmuir and Freundlich) and three parameters (Redlich–Peterson) equations. The isotherm data were best modeled by the Freundlich isotherm equation. Complete removal (≈100%) of Hg(II) from 1.0 l of chlor-alkali industry wastewater containing 9.86 mg Hg(II) ions, was possible with 3 g of the adsorbent dose at pH 8.0. About 95.0% of Hg(II) can be recovered from the spent adsorbent using 0.1 M HCl.

Inputs from a Mercury-Contaminated Lagoon: Impact on the Nearshore Waters of the Atlantic Ocean

Ria de Aveiro, a Portuguese coastal lagoon that exchanges water with the Atlantic Ocean, received the effluent from a chloralkali industry for over 50 years; consequently several tons of mercury had been buried in the sediments of an inner basin. To assess the importance (and seasonal variation) of the lagoon waters as carriers of mercury to the nearby coastal area, we measured total mercury levels in several compartments: in surface sediments, in surface and deep waters (including dissolved and particulate matter), and in biota. Dissolved (reactive and total) mercury concentrations both in surface and deep waters were low (<1 to 15 ng L−1). Mean mercury values in suspended particulate matter varied between 0.2 and 0.6 μg g−1 and in sediments between 1 and 9 ng g−1. Aquatic organisms displayed levels below regulatory limits but exhibited some bioaccumulation of mercury, with concentrations ranging from 0.05 to 0.8 μg g−1 [dry weight (dw)]. No seasonal pattern was found in this study for mercury-related determinations. Levels found in the estuary mouth during ebb tide provide evidence for the transport of mercury to the coastal zone. No significant changes in the partition of mercury between dissolved and particulate phases were found in the coastal waters in comparison with the values found in the estuary mouth. In spite of the high levels of mercury found inside some areas of the lagoon, the wide web of islands and channels allows some spreading of contaminants before they reach the coastal waters. Moreover, the low efficiency of local marine sediments in trapping mercury contributes to a dilution of mercury transported in suspended particulate matter over a broader area, reducing the impact in the nearby marine coastal zone.

Mercury(II) removal from aqueous solutions and wastewaters using a novel cation exchanger derived from coconut coir pith and its recovery

A new adsorbent (PGCP-COOH) having carboxylate functional group at the chain end was synthesized by grafting poly(hydroxyethylmethacrylate) onto coconut coir pith, CP (a coir industry-based lignocellulosic residue), using potassium peroxydisulphate as an initiator and in the presence of N, N′-methylenebisacrylamide as a cross-linking agent. The adsorbent was characterized with the help of infrared spectroscopy, powder X-ray diffraction, scanning electron microscopy, and potentiometric titrations. The ability of PGCP-COOH to remove Hg(II) from aqueous solutions was assessed using batch adsorption technique under kinetic and equilibrium conditions. Adsorbent exhibits very high adsorption potential for Hg(II) and more than 99.0% removal was achieved in the pH range 5.5–8.0. Adsorption process was found to follow first-order-reversible kinetics. An increase of ionic strength of the medium caused a decrease in metal removal, indicating the occurrence of outer-sphere surface complex mechanism. The equilibrium data were fitted well by the Freundlich isotherm model ( R 2 = 0.99; χ 2 = 1.81). The removal efficiency was tested using chlor-alkali industry wastewater. Adsorption isotherm experiments were also conducted for comparison using a commercial carboxylate-functionalized ion exchanger, Ceralite IRC-50. Regeneration experiments were tried for four cycles and results indicate a capacity loss of <9.0%.

Long-term effects of mercury in a salt marsh: Hysteresis in the distribution of vegetation following recovery from contamination

During four decades, the Ria de Aveiro was subjected to the loading of mercury from a chlor-alkali industry, resulting in the deposition of several tons of mercury in the sediments. The present study evaluates the impact of this disturbance and the recovery processes, temporally and spatially, by means of examining the richness of the species of salt marsh plants and mercury concentrations in sediments over the last fifty years. The temporal assessment showed that the mercury loading induced a shift in the species composition of the salt marsh from a non-disturbed salt marsh with higher species richness to an alternative state dominated by Phragmites australis. The horizontal assessment, through a mercury gradient, presents the same trend, indicating that P. australis is the species most tolerant to higher mercury concentrations, comparative to Halimione portulacoides, Arthrocnemum fruticosum, Triglochin maritima, Juncus maritimus and Scirpus maritimus. After the reduction of mercury discharges in 1994, the salt marsh shows a slowly return path recovery response. The hysteresis in the response results in the temporal gap between the reduction in mercury concentrations in the sediment and the salt marsh species richness response, comparatively to the existing diversity in the local reference marsh.

Seasonal and spatial character of PCBs in a chemical industrial zone of Shanghai, China

As one of China's great metropolises, Shanghai is suffering from the impact of manufacture and the use of chemical industrial products, and it faces serious pollution from polychlorinated biphenyls (PCBs). Therefore, in this study, in order to assess the seasonal and spatial character of contamination from chemical industrial zones, the concentrations of PCBs have been measured in various environmental media, including soil, leaves, and atmospheric particulate samples collected in a chemical industrial zone of Shanghai and compared with samples from presumably unpolluted sites of rural areas. In soils, the PCB concentrations ranged from 0.5 ng g(-1) (unpolluted site) to 586.85 ng g(-1) (chlor-alkali industry site). The concentrations of PCBs in evergreen leaves ranged from 0.3 ng g(-1) to 32.46 ng g(-1), and more chlorinated biphenyls congeners, such as penta-biphenyls and hexa-biphenyls, were the dominant contributors in winter and spring. Seasonal differences and the constitution patterns of congeners might be affected by the temperature and industrial activities. The PCB concentrations in the leaves of deciduous trees increase over time as the leaves grow. The PCB concentration in atmospheric particulates was in the range of 9.22-14.15 x 10(3)pg m(-3), which might be the result of influence from climate and industrial activities. The relativity of PCB contents among the environmental media was discussed. The results in this paper provide an important profile of the current contamination status of a key chemical industrial zone in China.

Catalytic generation of chlorine with slight overpotential by micellar ferrocene

Ferrocene solubilized with poly(vinylpyrrolidone) in aqueous KCl solution exhibited a well-defined voltammetric peak at 1.33 V vs. Ag∣AgCl at a platinum electrode. The wave was attributed to the oxidation of chloride to chlorine, demonstrated by smell of chlorine, by a view of formation of gas bubbles, by coloration through the reaction with diethyl- p-phenylene diamine, and by the increase in the anodic current with the concentration of chloride. Since no wave was observed in the ferrocene-free solution or KCl-free solution in this potential domain, the reaction mechanism was suggested to be the oxidation of chloride into chlorine catalyzed by micellar ferrocene. The potential at the foot of the wave (1.08 V) was less positive that the standard potential of Cl 2/Cl −, and hence the reaction may be useful for enhancing the energetic efficiency at chlor-alkali industry. The value of the peak current was one-sixth the theoretical diffusion-controlled current, and was proportional to the square-root of the potential scan rate.

New opportunities for osmotic membrane distillation

This article discusses a potential large-scale application for osmotic membrane distillation — a relatively new membrane technology. More specifically, commercial opportunities for the technology are discussed using an example of NaCl brine re-concentration in the chlor-alkali industry. Some general considerations concerning the possibility of maintaining a water balance between different technological loops at large chemical plants, by means of osmotic membrane distillation, are also discussed, as is the selection of suitable membranes for an osmotic membrane distillation application in the chlor-alkali industry. In addition, this article covers research activities at Aker Kvaerner Chemetics that aim to commercialize the membrane contactor technology. A relevant patent application cited in the article has been filed recently.

Environmental costs of mercury pollution

Mercury (Hg) has been used for millennia in many applications, primarily in artisanal mining and as an electrode in the chlor–alkali industry. It is anthropogenically emitted as a pollutant from coal fired power plants and naturally emitted, primarily from volcanoes. Its unique chemical characteristics enable global atmospheric transport and it is deposited after various processes, ultimately ending up in one of its final sinks, such as incorporated into deep sediment or bioaccumulated, primarily in the marine environment. All forms of Hg have been established as toxic, and there have been no noted biological benefits from the metal. Throughout time, there have been notable incidents of Hg intoxication documented, and the negative health effects have been documented to those chronically or acutely exposed. Today, exposure to Hg is largely diet or occupationally dependent, however, many are exposed to Hg from their amalgam fillings. This paper puts a tentative monetary value on Hg polluted food sources in the Arctic, where local, significant pollution sources are limited, and relates this to costs for strategies avoiding Hg pollution and to remediation costs of contaminated sites in Sweden and Japan. The case studies are compiled to help policy makers and the public to evaluate whether the benefits to the global environment from banning Hg and limiting its initial emission outweigh the benefits from its continued use or lack of control of Hg emissions. The cases we studied are relevant for point pollution sources globally and their remediation costs ranged between 2500 and 1.1 million US$ kg − 1 Hg isolated from the biosphere. Therefore, regulations discontinuing mercury uses combined with extensive flue gas cleaning for all power plants and waste incinerators is cost effective.

Distribution of mercury in the upper sediments from a polluted area (Ria de aveiro, Portugal)

It is well known that chlor-alkali industries using mercury as a cathode are an important source of this heavy metal to aquatic environments. Once discharged into rivers, lakes or coastal waters, mercury accumulates in sediments and in the aquatic food web (Gonzalez, 1991). Both elemental mercury and its compounds are noxious to all living organisms, and are bioaccumulative and persistent. Metal concentrations in sediments usually exceed those of the overlying water column. As a consequence, metals originating from human activities can often be identified more readily by analysis of sediments than by the quantification of metal concentrations present in water (Haynes et al., 1995). Sediments also integrate the temporal variability that characterizes metals originating from human sources. However, metals may be remobilized during the early stages of post-depositional transformations of the sediment (Gobeil and Cossa, 1993). Ria de Aveiro is a long, narrow, coastal lagoon, of 43 km wet area with a single, artificially maintained connection to the sea (Fig. 1). The coastal plain around the lagoon supports an intensive, diversified agriculture, a variety of heavy and light industries and a population of about half a million people, a part of which discharge their untreated or partially treated sewage into the lagoon (Lucas et al., 1986). An extensive web of islands and channels make water circulation inside the lagoon difficult and complex, thus allowing the spread of any conservative contaminants before they are discharged into the costal waters through the single sea mouth. One such contaminant is the mercury, released by a chlor-alkali plant into one of the remotest branches of the lagoon—Estarreja Channel (Fig. 1) (Hall et al., 1988). The Channel ends in a basin (Laranjo Basin) where most of the discharged mercury settles. Laranjo Basin has an area of about 1.5 km and is totally emptied in most ebb tides. The average volume of water exported with each tidal cycle may be around 2 km (Lucas et al., 1986). This work reports levels of Hg, Fe and Mn and sulphides in the upper sediments of the Laranjo Basin, and examines the influence of these elements on the distribution of mercury. Short sediment cores (10 cm) were collected by hand at sites A, B and C of Ria de Aveiro (Fig. 1). Cores were sampled in March, May, June 1996 and April 1997. The sediments were sliced in situ at the following depths: 0– 2, 2–4, 8–10, 14–16, 24–26, 34–36, 44–46, 54–56, 64–66 and 94–96 mm. The samples were rapidly transferred to vials which were placed under nitrogen atmosphere and frozen. In the laboratory, part of the sediments samples Edited by Bruce J. Richardson