Treatment of Process Water Containing Heavy Metals with a Two‐Stage Electrolysis Procedure in a Membrane Electrolysis Cell

The capability of a two‐stage electrochemical treatment for the regeneration of acidic heavy‐metal containing process water was examined. The process water came from sediment bioleaching and was characterized by a wide spectrum of dissolved metals, a high sulfate content, and a pH of about 3. In the modular laboratory model cell used, the anode chamber and the cathode chamber were separated by a central chamber fitted with an ion exchanger membrane on either side. The experiments were carried out applying a platinum anode and a graphite cathode at a current density of 0.1 A/cm². The circulation flow of the process water in the batch process amounted to 35 L/h, the electrolysis duration was 5.5 h at maximum and the total electrolysis current was about 1 A. In the first stage, the acidic process water containing metals passed through the cathode chamber. In the second stage, the cathodically pretreated process water was electrolyzed anodically. In the cathode chamber the main load of dissolved Cu, Zn, Cr and Pb was eliminated. The sulfuric acid surplus of 3–4 g/L decreased to about 1 g/L, the pH rose from initially 3.0 to 4–5, but the desired pH of 9–10 was not achieved. Precipitation in the proximity to the cathode evidently takes place at a higher pH than farther away. The dominant process in the anode chamber was the precipitation of amorphous MnO₂ owing to the oxidation of dissolved Mn(II). The further depletion of the remaining heavy metals in the cathodically pretreated process water by subsequent anodic treatment was nearly exhaustive, more than 99 % of Cd, Cr, Cu, Mn, Ni, Pb, and Zn were removed from the leachate. The high depletion of heavy metals might be due to both the sorption on MnO₂ precipitates and/or basic ferrous sulfate formed anodically, and the migration of metal ions through the cation exchanger membrane via the middle chamber into the cathode chamber. In the anode chamber, the sulfuric acid content increased to 6–7 g/L and the pH sank to 1.7. All heavy metals contained, with the exception of Zn, were removed to levels below the German limits for discharging industrial wastewaters into the receiving water. Moreover, the metal‐depleted and acid‐enriched process waters could be returned to the leaching process, hence reducing the output of wastewater. The results indicated that heavy metals could be removed from acidic process waters by two‐stage electrochemical treatment to a large extent. However, to improve the efficiency of metal removal and to establish the electrochemical treatment in practice, further work is necessary to optimize the operation of the process with respect to current density, energy consumption, discharging of metal precipitates deposited in the electrode chambers and preventing membrane clogging.     

Environmental impact of former gold mining on the Orangi river,Serengeti N.P., Tanzania

The Orangi river is an important all-year source of water for wildlife in the northern part of the Serengeti National Park. At two points along the river in the Banagi area, tributaries draining the adit and tailings of the Kilimafeza mine impact the Orangi. The former Au-Cu mine is subject to occasional wet season flooding leading to the release of iron ochres from the adit and physical as well as chemical mobilization of tailings material. The unpolluted river chemistry is essentially Na-Ca-HCO ₃ ^– and well-buffered. Drainage water; from the tailings are characterized by low pH (2.3) and high concentration of sulphate (up to 3280 mg/l), aluminium (275 mg/l), arsenic (324 mg/l), copper (125 mg/l), iron (622 mg/l), lead (21 mg/l), manganese (65 mg/l), and zinc (126 mg/l). Adit-drainage waters are typically of a lower pH (4.6) and have a lower concentration of sulphate (up to 1840 mg/l) and metals (up to 25 mg/l Al, 92 mg/l As, 42 mg/l Cu, 258 mg/l Fe, 9.6 mg/l Pb, 53 mg/l Mn, and 102 mg/l Zn). Mixing of these acidic waters with the alkaline river leads to rapid metal precipitation as Fe-ochre coatings on clastic sediment. This effect is more noticeable in the dry season. Consequently, although the tributaries draining the two sources are heavily contaminated, the effective buffering of the mine drainage waters restricts any potential pollution to within 1 km of the mine workings. Faecal coliforms show an antipathetic relationship to low pH and high metal conditions. The only mobile metals in the water outside this area are Mn and Zn and their contamination can be biologically monitored using a protozoan-based bioassay.     

Behavior of Enzyme Activities Exposed to Contamination by Heavy Metals and Dissolved Organic Carbon in Calcareous Agricultural Soils

To investigate the relevance of biochemical parameters in biogeochemical mechanisms of the soil, it is important to gather data related to different soil types under different pedogeoclimatic conditions. In this study, we investigated on the calcareous agricultural soils in the Saiss plain (North Morocco). Four agricultural soils exposed to multi-metal (Cr, Cu, Zn, and Ni) and organic matter (OM) contamination as a result of irrigation with Oued Fez and Oued Sebou waters that are affected by urban and industrial activities around the city of Fez were studied and compared to a reference site irrigated with uncontaminated water. The study concerned soil physicochemical properties and the activity of a range of enzymes [phosphatase (PHOS), arylsulfatase (SULF), urease (UREA), arylamidase (AMID), β-galactosidase (GALA), glucosidase (GLUC), and laccase (LACA)] related to nutrients cycles. Pearson's correlations between these parameters showed that soil enzymatic activities (PHOS, SULF, UREA, GALA, GLUC, and LACA) were correlated positively with heavy metals (Cu, Zn, and Cr) concentrations in the soil and also with dissolved organic carbon (DOC), and negatively with the aromaticity (AROM) of these compounds. Interestingly, analysis of intra-site correlations showed strong relationships among enzyme activities in the reference soil, while in contaminated soils, these activities were largely unrelated to each other. It was concluded that soil irrigation with heavy-metal- and OM-contaminated watercourses over decades has resulted in soils with high enzymatic activities function and nutrient turnover but altered relationships among geochemical cycles.     

Seasonal fluctuation of the Oomycetes in a polluted environment: Santigo River and affluents (Buenos Aires, Argentina).

Oomycetes are ubiquitous filamentous water molds. They occur as saprotrophs on bits of substrata of both plant and animal origin, principally in fresh water. This paper results from an integral research on the Oomycetous flora of Santiago River and several associated shallow streams (Buenos Aires, Argentina). Information is provided concerning this polluted habitat and seasonal distribution of the zoosporic fungi. The principal physico-chemical characteristics of the aquatic environmental samples are described: temperature, pH, dissolved oxygen, nitrate concentration, sulphate concentration, phosphate concentration and heavy metals.     

Cleaning up of heavy metals-polluted water by a terrestrial hyperaccumulator Sedum alfredii Hance

Sedum alfredii Hance is a terrestrial zinc/cadmium （Zn/Cd）-hyperaccumulating and lead （Pb）-accumulating plant. Previous studies on S. alfredii were mostly focused on its physiological mechanism of heavy metal uptake and the application in phytoextraction of metals from contaminated soils. In this study, we evaluated the application potential of S. alfredii in the cleanup of heavy metals from contaminated lake water. Our research revealed that changing pH in lake water would not make particular difference on the final accumulation amount of heavy metals, because the acidic water environment negatively affected plant growth compared with the neutral and alkaline environments, but was more conducive for heavy metal absorption and accumulation. In addition, S. alfredii showed an increase of approximately 2.2-fold in dry weight （DW） when cultured with lake water for 25 d. At the same time, it accumulated approximately 5.0 mg/kg DW of Cd and 41.4 mg/kg DW of Pb. The absorption of heavy metals was highly effective during the first 10 d of culture. Also, the quality of lake water was greatly improved after only 2-d cleanup by S. aifredii. In general, this hyperaccumulator exhibits great potential for application in the cleanup of heavy metals-polluted waters.     

Impact of acid mine-drainage from abandoned spoils on the chemistry of an intermittent stream in the arid Southwest

Drainage from an orphaned copper mine (Sheldon Mine Complex) contributes highly mineralized, acidic waters to Lynx Creek, a small intermittent, arid-climate stream. This results in localized elevation of major cations, silica, sulfate, and heavy metals (Cu, Fe, Mn, Zn), depression in pH, and complete neutralization of bicarbonate alkalinity. Levels of chloride, nitrogen and phosphorus are unaffected by mine-drainage. During stable flow, water quality of the creek improves downstream through precipitation of metal salts, dilution by less mineralized tributaries, and additional buffering from the creek channel and tributaries. Climatic aridity, via high evaporation, concentrates and precipitates metal salts during summertime periods of low flow. Creek sediments thus contain large amounts of heavy metals and phosphorus that are transported downstream in suspended particulates during spates. This reflects an annual cycle of temporary creek-bed storage followed by episodic mobilization toward Lynx Lake, a downstream reservoir. Although the influence of mine drainage is not overtly apparent in the dissolved chemistry of the Lake, high concentrations of metals and phosphorus occur in lake sediments.Represents a portion of a thesis by A. J. L. III submitted in partial fulfillment of the requirements of a M.S. Degree in Botany, Arizona State University.     

How does storage affect the quality and quantity of organic carbon in sewage for use in the bioremediation of acidic mine waters?

Pit lakes (abandoned flooded mine pits) represent a potentially valuable water resource. However, acid mine drainage (AMD) generation due to mining activities often results in pit lake waters with low pH, high sulphate and dissolved metal concentrations. Sulphate reduction-based bioremediation offers tremendous scope for removal of acidity and metals from pit lake water. In this study, the effect of storing sewage on its carbon quality for bioremediation of acidic pit lake water was studied. In addition, the effectiveness of labile organic carbon (lactic acid and ethanol) on SRB activity was tested. Bioremediation experiments were performed in controlled and replicated microcosms with acidic (pH 2.2) water from a pit lake by addition of stored (3 years at 4 °C) sewage for stimulation of sulphate reducing bacteria (SRB) activity. This sewage had been previously used successfully in remediating to pH 7 water from this pit lake. The initial aim was to test the sewage at lower doses (18 and 28 g/L) and in a pulsed addition (over 5 weeks). Bioremediation efficacy was evaluated by measuring pit lake water pH increase, redox potential decrease, and acidity and sulphate removal. Though the stored sewage had retained a very similar high total organic carbon (TOC) equivalent to prior to storage, it failed to increase dissolved organic carbon (DOC) levels in pit lake water. Microcosms amended with doubled doses of sewage and an extended remediation time still failed to demonstrate any substantial improvement in water quality, other than a small amount of sulphate reduction and direct neutralisation by the sewage. In order to determine if low DOC concentrations in sewage were the cause of the bioremediation failure, labile organic carbon (LOC), consisting of 50:50 (w/w) lactic acid and ethanol, was added to all microcosm treatments at concentrations of 3000, 6000 and 9000 mg/L. After LOC addition, water quality improved with effective removal of acidity, sulphate and metals in the lowest carbon concentration (3000 mg/L). However, 6000 and 9000 mg/L LOC concentrations showed a delay in response due to the increased acidity associated with the lactic acid addition. The experiments showed that pulsed dosing of carbon simply slowed the commencement of remediation but it was ultimately able to reach the same effectiveness as the equivalent quantity added all at once. Prolonged storage of sewage leads to loss of LOC. In situ pit lake remediations which aim to make use of sewage as the main carbon source will need to factor in the storage time required to obtain sufficient sewage for the treatment into the design. Pulsing may help reduce issues with storage or supplementation with LOC may need to be considered. Results highlight that LOC is a more useful indicator of material effectiveness compared to a simple measures of TOC.     

Elimination of Heavy Metals from Leachates by Membrane Electrolysis

The elimination of heavy metals from bioleaching process waters (leachates) by electrolysis was studied in the anode and cathode region of a membrane electrolysis cell at current densities of 5–20 mA/cm² using various electrode materials. The leaching waters containing a wide range of dissolved heavy metals, were high in sulfate, and had pH values of approx. 3. In preliminary tests using a rotating disc electrode the current density‐potential curve (CPK) was recorded at a rotation velocity of 0, 1000 and 2000 rpm and a scan rate of 10 mV/s in order to collect information on the influence of transport processes on the electrochemical processes taking place at the electrodes. The electrochemical deposition‐dissolution processes at the cathode are strongly dependent on the hydrodynamics. Detailed examination of the anodic oxidation of dissolved Mn(II) indicated that the manganese dioxide which formed adhered well to the electrode surface but in the cathodic return run it was again reduced. Electrode pairs of high‐grade steel, lead and coal as well as material combinations were used to investigate heavy metal elimination in a membrane electrolysis cell. Using high‐grade steel, lead and carbon electrode pairs, the reduction and deposition of Cu, Zn, Cr, Ni and some Cd in metallic or hydroxide form were observed in an order of 10–40 % in the cathode chamber. The dominant process in the anode chamber was the precipitation of manganese dioxide owing to the oxidation of dissolved Mn(II). Large amounts of heavy metals were co‐precipitated by adsorption onto the insoluble MnO₂. High‐grade steel and to some extent lead anodes were dissolved and hence were proven unsuitable as an anode material. These findings were largely confirmed by experiments using combination electrodes of coal and platinized titanium as an anode material and steel as a cathode material. With both electrode combinations and current densities of 5 or 10 mA/cm², in the cathode region low depositions of 10–20 % Cd, 2–10% Mn, 5–20 % Zn, 1–20 % Co and 5–15 % Ni were measured. By contrast, the elimination of other metals was substantially larger: Fe 40 –60 %, Cu 20–40 %, and Cr 40–60 %. In the anode region the removal of heavy metals was in the order of 30–50%, with Mn being as high as 80 %. The anode materials exhibit good resistance at the current densities tested. The precipitates deposited in both electrode regions contained as main components Al with 10–20 %, Mg with approximately 10 %, and SO₄ with 5–20 %. The solid material in the cathode chamber consisted of relatively high proportions of Zn and Mn. Calcium in the solids indicated the co‐precipitation of calcium sulfate. The main components in the solids of the anode chamber were Mn in the form of pyrolusite, Al as basic sulfate, and Mg. The results indicate that electrochemical metal separation in the membrane electrolysis cell can represent a practical alternative to the metal separation by alkalization. Regarding the main heavy metals Zn, Mn and Ni in the process water, combination electrodes using steel as a cathode material and coal or platinized titanium as an anode material proved to be suitable for eliminating the heavy metals from the aqueous phase. However, for practical application, further work is necessary to improve the efficiency, applicability and costs of the process.     

Application of in situ cage cultures of phytoplankton for monitoring heavy metal pollution in two Norwegian fjords

A simple apparatus for in situ use of the cage culture technique for growing planktonic algae has been developed and used to follow growth and metal uptake of three diatoms in two Norwegian fjords polluted by heavy metals. Continuous pumping of sea water through a chelating resin and continuous water sampling was used to obtain average values for dissolved and particulate heavy metal content at the various test sites. The metals investigated (zinc, copper, lead, cadmium, and mercury) showed differences in the proportion of dissolved to particulate fractions.The three species of diatoms tested gave systematic growth responses to heavy metal pollution in the moderately polluted fjord; one alga died, one showed reduced growth rate at the more contaminated site compared with those at less polluted sites, while the most tolerant alga was apparently not affected. In the heavily polluted fjord only the most tolerant alga survived, showing decreasing growth rate with increasing pollution.Uptake of heavy metals increased generally with increasing heavy metal content in the sea water. The contents in the algae grown in the most polluted fjord were much higher than those obtained in the less polluted fjord, which were higher than the contents reported for algae from non-contaminated areas.     

Drought-induced release of metals from peatlands in watersheds recovering from historical metal and sulphur deposition

Climate change is predicted to cause an increase in frequency and severity of droughts in the boreal ecozone, which can result in the lowering of water tables and subsequent release of acidic, metal-contaminated waters from wetlands. We believe that in areas where historical deposition of metals and sulphur was severe, these episodic pulses of metals could reach concentrations sufficiently high to severely affect aquatic communities in receiving waters and cause a delay in biological recovery. The objective of this study is to evaluate the impact of drought on the chemistry of water draining from two Sudbury peatlands with widely contrasting peat organic matter content to determine the response of stream water chemistry to drought from peatland types in the region. Stream samples were collected using ISCO? automated water collectors from June to November 2011. Following a period of drought, there was a decline in pH and a large increase in concentrations of sulphate and metal ions (Al, Co, Cu, Fe, Mn, Ni, and Zn) in water draining both peatlands, with extreme concentrations occurring over a period of about two weeks. At the site with the higher peat organic matter content there was an increase in metals that have a high affinity to bind to DOC (Al, Cu, and Fe) during the onset of drought. This study demonstrates a dramatic response to drought at two sites that differ in metal and nutrient pool sizes, hydrology, and topography, suggesting the potential for a majority of peatlands in the region to experience this response. Efforts to restore aquatic ecosystems and protect freshwater resources must take into account these processes, as disruptions to biogeochemical cycles are likely to become more prevalent in a changing climate.     

Pollution,sources, and risks of heavy metals in coastal waters of China

Abstract

Comprehensive information on heavy metals in coastal waters at national scale of China is limited. Therefore, this study investigated the distribution, pollution, and ecological-health risks of heavy metals in coastal waters along 18,000?km coastline of China. Total 13 target heavy metals in coastal waters along coastline of China showed drastic spatial variations with average concentrations ranging from .14 (Cd) to 136.26 (Cu) μg/L. Cu was the dominant heavy metal with the maximal concentration of 1485.92?μg/L. Three methods including heavy metal pollution index (HPI), Nemerow index (NI), and contamination degree (CD) were adopted to explore heavy metal pollution. HPI obtained the worst-case evaluation results to illustrate that heavy pollution occurred at over 50% of sampling sites. Anthropogenic sources were the main sources of heavy metals in the coastal waters. Approximately 28.13% and 9.38% of sampling sites illustrated considerable and very high ecological risks, respectively. Metals including Cu, As, and Hg were the main pollution and risk contributors. Heavy metals in coastal waters posed high cancer risks and unacceptable non-cancer risks to both adults and children. Therefore, effective control of heavy metals is necessary for regional sustainability and well-beings of residents in coastal regions of China.     

Microbial leaching of zinc concentrate in fresh‐water microcosms: Comparison between aerobic and oxygen‐limited conditions

Microbially mediated leaching and solubilization of zinc ore concentrate by native aquatic microbial communities incubated under aerobic and oxygen‐limited conditions were examined in static microcosms consisting of stream sediment and water. Sterile controls provided information on abiotic sulfide oxidation and leaching of zinc, lead, cadmium, and copper. The flux of these heavy metals from the sediments to the water column was greatest under biotic oxygen‐limited conditions. When calculated as the percentage of total metal available in zinc concentrate‐amended microcosms, the order of metal solubilization under oxygen‐limited conditions was lead, copper, zinc, and cadmium. Under biotic aerobic conditions, the order of solubilization was zinc, lead, cadmium, and copper. This study indicates that aquatic heterotrophs are capable of leaching and solubilizing metallic sulfides under conditions of neutral to slightly acidic pH and are effective in releasing heavy metals to the water column under oxygen‐limited conditions.     

Microbial Diversity and Its Relationship to Physicochemical Characteristics of the Water in Two Extreme Acidic Pit Lakes from the Iberian Pyrite Belt (SW Spain)

The Iberian Pyrite Belt (IPB) hosts one of the world’s largest accumulations of acidic mine wastes and pit lakes. The mineralogical and textural characteristics of the IPB ores have favored the oxidation and dissolution of metallic sulfides, mainly pyrite, and the subsequent formation of acidic mining drainages. This work reports the physical properties, hydrogeochemical characteristics, and microbial diversity of two pit lakes located in the IPB. Both pit lakes are acidic and showed high concentrations of sulfate and dissolved metals. Concentrations of sulfate and heavy metals were higher in the Nuestra Señora del Carmen lake (NSC) by one order of magnitude than in the Concepción (CN) lake. The hydrochemical characteristics of NSC were typical of acid mine waters and can be compared with other acidic environments. When compared to other IPB acidic pit lakes, the superficial water of CN is more diluted than that of any of the others due, probably, to the strong influence of runoff water. Both pit lakes showed chemical and thermal stratification with well defined chemoclines. One particular characteristic of NSC is that it has developed a chemocline very close to the surface (2 m depth). Microbial community composition of the water column was analyzed by 16S and 18S rRNA gene cloning and sequencing. The microorganisms detected in NSC were characteristic of acid mine drainage (AMD), including iron oxidizing bacteria (Leptospirillum, Acidithiobacillus ferrooxidans) and facultative iron reducing bacteria and archaea (Acidithiobacillus ferrooxidans, Acidiphilium, Actinobacteria, Acidimicrobiales, Ferroplasma) detected in the bottom layer. Diversity in CN was higher than in NSC. Microorganisms known from AMD systems (Acidiphilium, Acidobacteria and Ferrovum) and microorganisms never reported from AMD systems were identified. Taking into consideration the hydrochemical characteristics of these pit lakes and the spatial distribution of the identified microorganisms, a model explaining their geomicrobiology is advanced.     

The degradation of lignocellulose in a chemically and biologically generated sulphidic environment

Acid mine drainage waters are characterised by a low pH, high concentrations of heavy metals, high levels of sulphate salts and low concentrations of organic material. The biological treatment of these waters has been a subject of increasing focus as an alternative to physico-chemical treatment. The utilisation of lignocellulose as a carbon source has been restricted by the amount of reducing equivalents available within the lignocellulose matrix. This present study demonstrated that lignocellulose could be utilised as a carbon source for sulphate reduction. It was shown that the initial reduction of sulphate observed using lignocellulose as a carbon source was due to the easily extractable components. This degradation resulted in the production of sulphide ( approximately 500 mg/l), which further aided in the degradation of lignin (observed as a release of aromatic compounds), allowing greater access to cellulose (and release of reducing sugars).     

Breeding birds from reedbeds to alpine meadows

The main processes that determine the behaviour ofheavy metals in the Scheldt estuary are tidalhydrodynamics, sediment transport, and sorption ofheavy metals on suspended matter. The water qualitymodel WASP is applied to simulate the spatialdistribution of five heavy metals in the estuary,under average hydrodynamic and suspended sedimenttransport regimes. First, the hydrodynamical part ofthe model is constructed and the results are verifiedby comparison with measured water levels and flowvelocities. Secondly, a salt transport model is set upin order to evaluate the hydrodynamical dispersivemixing characteristics. Thirdly, a suspended sedimenttransport model is constructed and finally a transportmodel for heavy metals. The simulated distributions of the sorbedamounts of heavy metals, suspended sediment andsalinity in the estuary agree well with observations.The calculated profiles of dissolved and sorbedconcentrations of heavy metals in the water columnindicate an accumulation of heavy metals in the zoneof the turbidity maximum, while closer to the sea theconcentrations diminish due to mixing of the pollutedfluvial sediments with unpolluted marine sediments andbecause of sediment deposition in the estuary. It canbe concluded that only a small part of the heavymetals reaches the sea. This revised version was published online in July 2006 with corrections to the Cover Date.     

Floristic changes in shallow soft waters i n relation to underlying environmental factors

SUMMARY 1. Historical and recent data on the occurrence of macrophytes in twenty-eight lentic soft waters in The Netherlands are summarized. These waters were, and a few still are, characterized by a submerged vegetation of isoetid plants. Changes in the species composition of macrophytes are visualized by means of multivariate analysis and by shifts in species-spectra.
2. Ordination of the available data shows that the pH, alkalinity, acidity, contents of heavy metals, dissolved organic matter and some important salts and nutrients in water and interstitial water are strongly related to the recent distribution of aquatic plants in waters, which were originally of low alkalinity. In addition, the available inorganic carbon and the redox potential in the sediment are also important environmental parameters in explaining differences in aquatic vegetation.
3. The recorded changes in the macrophyte species composition can be attributed to the effects of acidification and eutrophication. The most important, overall change is a reduction of the number of species.
4. Hydrology proves to be important in controlling the sensitivity of a body of water for acidifying deposition.     

Variation in acid tolerance of certain freshwater crustaceans in different natural waters

On the island of Rhum (Inner Hebrides: Western Scotland) several taxonomically diverse species of small crustaceans live in water that is more acidic and of lower ionic content than that in which they have ever been found in Yorkshire (England). Physiological difficulties appear to be experienced by these species in Yorkshire in waters that would evidently be suitable in Rhum. This may be due to the presence of heavy metals and other substances derived from atmospheric pollution, of which Rhum is largely free, that act synergistically with other stressful factors. Evidence from other areas is in agreement with this suggestion.The few species that are specialised for life in highly acidic water can frequent more acidic conditions in Yorkshire than any encountered on Rhum. Nevertheless certain species that are common in the Northern Pennines have not been found in the Southern Pennines where pollution has been most intense. One species that is common in the Northern, but has not been found in the Southern Pennines, formerly occurred there as shown by abundant remains in the peat.     

Heavy Metal Concentration in the Urban Environment of Addis Ababa,Ethiopia

This work assesses the issue of whether the measured concentrations of heavy metals in soil, rocks, surface and ground waters in Addis Ababa can be related to anthropogenic contamination or natural weathering of rocks. Heavy metal analyses of rocks, soils, streams, springs and boreholes have been carried out to identify the presence of potentially harmful solutes. The maximum concentration of total chromium measured is 269 ppm in the northern, industry-free zone of Addis Ababa in the B ₂-horizon of soil profile (cambisol). The Ni/Cr ratio in the rocks is higher than soils, which could indicate the presence of high concentrations of Cr in soils is from weathering processes. A comparative study of different samples collected from various parts of the city indicates that the chemical composition of the hydrothermally affected volcanic rocks plays an important role in increasing heavy metal concentration in the study area. The fresh country rocks contain relatively low concentrations of heavy metals, as shown by background values. The statistical evaluation indicates that the hydrothermally altered rocks contain far higher mean heavy metal concentrations than the fresh acidic rocks (background values). Consequently, soils derived from altered rocks are enriched with respect to heavy metals. From this study it was possible to observe that the rock and soil outcrops of Addis Ababa are anomalously rich in heavy metals derived from hydrothermal activity. Therefore, heavy metal concentrations in the surrounding rocks and soils are related to geogenic sources whereas anthropogenic contribution as a cause of these concentrations is minor.     

Microbial Community Composition and Ecology of an Acidic Aquatic Environment: The Tinto River, Spain

We studied the correlation between physicochemical and biological characteristics of an acidic river, the Tinto River, in Southwestern Spain. The Tinto River is an extreme environment characterized by its low pH (mean of 2.2) and high concentrations of heavy metals (Fe 2.3 g/L, Zn 0.22 g/L, Cu 0.11 g/L). These extreme conditions are the product of the metabolic activity of chemolithotrophic microorganisms, including iron- and sulfur-oxidizing bacteria, that can be found in high concentrations in its waters. The food chain in the river is very constrained and exclusively microbial. Primary productivity in the Tinto River is the sum of photosynthetic and chemolithotrophic activity. Heterotrophic bacteria and fungi are the major decomposers and protists are the major predators. A correlation analysis including the physicochemical and biological variables suggested a close relationship between the acidic pH values and abundance of both chemolithotrophic bacteria and filamentous fungi. Chemolithotrophic bacteria correlated with the heavy metals found in the river. A principal component analysis of the biotic and abiotic variables suggested that the Tinto River ecosystem can be described as a function of three main groups of variables: pH values, metal concentrations, and biological productivity.     

Anaerobic treatment of sulphate-rich wastewaters

Until recently, biological treatment of sulphate-rich wastewater was rather unpopular because of the production of H₂S under anaerobic conditions. Gaseous and dissolved sulphides cause physical-chemical (corrosion, odour, increased effluent chemical oxygen demand) or biological (toxicity) constraints, which may lead to process failure. Anaerobic treatment of sulphate-rich wastewater can nevertheless be applied successfully provided a proper treatment strategy is selected. The strategies currently available are discussed in relation to the aim of the treatment: i) removal of organic matter, ii) removal of sulphate or iii) removal of both. Also a whole spectrum of new biotechnological applications (removal of organic chemical oxygen demand, sulphur, nitrogen and heavy metals), recently developed based on a better insight in sulphur transformations, are discussed.