Metal bioaccumulation in spontaneously grown aquatic macrophytes in Fe-rich substrates of a passive treatment plant for acid mine drainage.

Some plants may thrive in polluted environments, accumulating high concentrations of metal/loids in their organs. This study investigates for the first time the bioaccumulation and translocation of metal/loids in Typha domingensis spontaneously grown in extremely Fe-rich substrates (38-44% of Fe2O3) from different components of an acid mine drainage disperse alkaline substrate passive treatment. Most metal/loids were predominantly accumulated in the roots over the aerial parts of the plant, with concentrations of 0.66-9.5% of Fe, 0.02%-0.18% of Al, 55-2589 mg kg-1 of Mg, 51-116 mg kg-1 of Zn, 17-173 mg kg-1 of Cu, and 5.2-50 mg kg-1 of Pb. Bioconcentration factors were mostly below 1 for metal/loids in the studied aneas (e.g. 0.03-0.47 for Cu, 0.10-0.73 for Zn, 0.04-0.28 for As, 0.07-0.55 for Pb, 0.27-055 for Cd, 0.24-0.80 for Ni), which evidences that T. domingensis behaves as an excluder species in these substrates. Translocation factors were below 1 for most elements (e.g. 0.01-0.42 for As, 0.06-0.50 for Pb, 0.24-0.65 for Cd, and 0.10-0.56 for Sb), except for Mn, Ni and in some cases for Tl, Cu and Zn, which indicates limited transfer of metals between plant tissues. Mineralogical and geochemical substrate properties are pointed out as the main factors responsible for the lower bioconcentration and translocation of potentially toxic elements. In addition, the oxidizing conditions existent in the pore water-root system may also limit the mobility of metals from Fe oxides and hydroxysulfates, the main component of the substrate. The formation of a Fe plaque inside the roots may also limit the transfer of metals to the aerial parts. The spontaneous occurrence of T. domingensis in the substrates of the acid mine drainage passive treatments is an environmental indicator of the efficiency of the system and could be used as a complementary polishing step, given the strong tolerance of this plants to high concentrations of metal/loids.

Wildfire effects on the hydrogeochemistry of a river severely polluted by acid mine drainage.

This study evaluates for the first time the impact of a large wildfire on the hydrogeochemistry of a deeply AMD-affected river at the beginning of the wet season. To accomplish this, a high-resolution water monitoring campaign was performed within the basin coinciding with the first rainfalls after summer. Unlike similar events recorded in AMD-affected areas, where dramatic increases in most dissolved element concentrations, and decreases in pH values are observed as a result of evaporitic salts flushing and the transport of sulfide oxidation products from mine sites, a slight increase in pH values (from 2.32 to 2.88) and decrease in element concentrations (e.g.; Fe: 443 to 205 mg/L; Al: 1805 to 1059 mg/L; sulfate: 22.8 to 13.3 g/L) was observed with the first rainfalls after the fire. The washout of wildfire-ash deposited in the riverbanks and the drainage area, constituted by alkaline mineral phases, seems to have counterbalanced the usual behavior and patterns of the river hydrogeochemistry during autumn. Geochemical results indicate that a preferential dissolution occurs during ash washout (K > Ca > Na), with a quick release of K followed by an intense dissolution of Ca and Na. On the other hand, in unburnt zones parameters and concentrations vary to a lesser extent than burnt areas, being the washout of evaporitic salts the dominant process. With subsequent rainfalls ash plays a minor role on the river hydrochemistry. Elemental ratios (Fe/SO4 and Ca/Mg) and geochemical tracers in both ash (K, Ca and Na) and AMD (S) were used to prove the importance of ash washout as the dominant geochemical process during the study period. Geochemical and mineralogical evidences point to intense schwertmannite precipitation as the main driver of reduction in metal pollution. The results of this study shed light on the response of AMD-polluted rivers to certain climate change effects, since climate models predict an increase in the number and intensity of wildfires and torrential rain events, especially in Mediterranean climates.

Effects of estuarine water mixing on the mobility of trace elements in acid mine drainage leachates.

This research reports the effects of pH increase on contaminant mobility in acid mine drainage from the Iberian Pyrite Belt by seawater mixing in the laboratory, simulating the processes occurring in the Estuary of Huelva (SW Iberian Peninsula). Concentrations of Al, Fe, As, Cu and REY in mixing solutions significantly decreased with increasing pH. Schwertmannite precipitation at pH 2.5-4.0 led to the total removal of Fe(III) and As. Subsequently, iron-depleted solutions began to be controlled by precipitation of basaluminite at pH 4.5-6.0, which acted as a sink for Al, Cu and REY. Nevertheless, as the pH rises, schwertmannite becomes unstable and releases back to solution the previously retained As. Moreover, other elements (S, Zn, Cd, Ni and Co) behaved conservatively in mixing solutions with no participation in precipitation processes. Some toxic elements finally end up to the Atlantic Ocean contributing to the total pollutant loads and environmentally threatening the coastal areas.

Stream-pit lake interactions in an abandoned mining area affected by acid drainage (Iberian Pyrite Belt).

Opencast mining of sulfide ore deposits may lead to the formation of anthropogenic acidic lakes with highly polluted waters. In these systems, it is crucial to understand the hydrological connections between surface and groundwater and their contribution to the pollutant load delivered to the downgradient streams. This study characterizes the interactions between surface and groundwater in an acidic pit lake using different geochemical tracers (i.e., REE and other trace metals). The San Telmo pit lake, located in one of the most pollutant sources of the Iberian Pyrite Belt (IPB), can be considered as a flow-through pit lake except during dry periods, when it behaves as a terminal lake due to lower inputs by surface waters and higher outputs by evaporation. Results based on geochemical tracers indicate that the main inputs to the pit lake come from surface waters, with minor groundwater inputs rich in As, Cr, Cu, Fe and Pb. The contaminant load released from the mining area is very high (e.g., median values of 520 kg/day of Fe and 38 kg/day of Zn), causing the degradation of the fluvial network downstream. Most of released pollutants come from waste dumps located at the W of the mining zone (~50-70% of Al, Cd, Mg, Mn, Ni, SO4 and Zn and > 70% for Cu, Cr, Fe and, V), while the contribution of the water coming out the pit lake and other dumps is much lower. Thus, remediation efforts to improve the area and fluvial courses downstream must focus on the W waste dumps.

Environmental management and potential valorization of wastes generated in passive treatments of fertilizer industry effluents.

A phosphogypsum stack located in SW Spain releases highly acidic and contaminated leachates to the surrounding estuarine environment. Column experiments, based on a mixture of an alkaline reagent (i.e., MgO or Ca(OH)2) dispersed in an inert matrix (dispersed alkaline substrate (DAS) technology), have shown high effectiveness for the treatment of phosphogypsum leachates. MgO-DAS and Ca(OH)2-DAS treatment systems achieved near total removal of PO4, F, Fe, Zn, Al, Cr, Cd, U, and As, with initial reactive mass:volume of leachate treated ratios of 3.98 g/L and 6.35 g/L, respectively. The precipitation of phosphate (i.e., brushite, cattiite, fluorapatite, struvite and Mn3Zn(PO4)2·2H2O) and sulfate (i.e., despujolsite and gypsum) minerals could control the solubility of contaminants during the treatments. Therefore, the hazardousness of these wastes must be accurately assessed in order to be properly managed, avoiding potential environmental impacts. For this purpose, two standardized leaching tests (EN-12457-2 from the European Union and TCLP from the United States) were performed. According to European Union (EN-12457-2) regulation, some wastes recovered from DAS treatments should be classified as hazardous wastes because of the high concentrations of SO4 or Sb that are leached. However, according to United States (US EPA-TCLP) legislation, all DAS wastes are designated as non-hazardous wastes. Moreover, the solids generated in the DAS systems could constitute a promising secondary source of calcite and/or P. This research could contribute to worldwide suitable waste management for the fertilizer industry.

Eco-sustainable passive treatment for mine waters: Full-scale and long-term demonstration.

This paper tries to analyse the technical and economic performance of a full-scale passive Disperse Alkaline Substrate (DAS) treatment plant steadily operating for 28 months (840 days) to treat extremely acidic and metal rich mine waters in the Iberian Pyrite Belt (SW Spain). For the first time, an economic evaluation of this technology and its comparison with other passive treatments is reported. During this period, around 56,000 m3 of mine waters have been treated, without significant clogging or exhaustion of the alkaline substrate. The efficiency of the system is demonstrated by a significant decrease in the average net acidity (from 2005 to -43 mg/L as CaCO3 equivalent) and the total elimination of Al, Cu, REY, Zn, As, Cr, Mo, V, Cd, Pb, Co and other trace metals. Water quality of the treated output discharge meets the threshold values for irrigation and drinking standards, except for Fe, Mn and sulphate. The accumulation of elements of economic interest in the waste (e.g., 32 t of Fe, 6.1 t of Al, 0.8 t of Cu, 0.8 t of Zn, 39.4 kg of REE, 20 kg of Co or 1 kg of Sc), easily extractable with diluted acids, may turn a hazardous waste into a valuable resource. The benefits associated with the revalorization of this metal-rich waste could reach a total of 27478 USD, but is more reliably estimated to be around 8243 USD due to technologic limitations. This benefit would help to defray the maintenance costs (8428 €) and make DAS an economically self-sustainable treatment. The annual treatment cost for DAS was 0.27 €/m3, being the lowest value found among other reported conventional passive schemes, and from 8 to 12 times lower compared to active technologies. The results obtained prove that the DAS technology is the most technically and economically sustainable way to decontaminate acid and metal-rich mine waters in abandoned mines.

Design and optimization of sustainable passive treatment systems for phosphogypsum leachates in an orphan disposal site.

The optimization of the dispersed alkaline substrate (DAS) technology was investigated to achieve the treatment of highly acidic and polluted effluents from a phosphogypsum pile in an orphan site of SW Spain. This phosphogypsum disposal area is located on the Tinto river marsh soils, where it acts as a source of pollution for the estuarine environment, releasing high concentrations of metal(loid)s and radionuclides, which degrade the surrounding waters. The methodology consists of flowing the leachates through columns loaded with a combination of a fine-grained alkaline reagent scattered in a non-reactive matrix to raise the water pH while decreasing the solubility of dissolved contaminants. Seven columns were built, one for each of the alkaline reagent used: limestone, barium carbonate, biomass ash, fly ash, MgO, Mg(OH)2, and Ca(OH)2. The Ca(OH)2-DAS and MgO-DAS treatment systems showed the highest effectiveness, reaching near-total removal for PO4, F, Fe, Zn, Cu, Al, Cr, and U with initial reagent mass:treated volume ratios of 36.3 g/L and 7.57 g/L, respectively. Total As removal was only achieved in the Ca(OH)2-DAS treatment. Phosphate precipitation was the main mechanism responsible for pollutants removal. Geochemical modeling using PHREEQC code and mineralogical evidence confirmed the precipitation of these minerals. This study forms the basis of an effective and environmentally sustainable treatment system for phosphogypsum leachates to reduce the impact of the fertilizer industry worldwide.

Trace metal mobility in sub-seabed sediments by CO2 seepage under high-pressure conditions.

Carbon capture and storage (CCS) is the third contributor to cumulative carbon emission reductions required by the second half of this century. Although this is a promising technology for reducing atmospheric CO2, it is only affordable if the confinement of the gas is guaranteed for hundreds of years. Hence, it is of paramount importance to figure out and predict the chemical and biological effects associated with potential CO2 leakage, to provide decision makers with a good basis for choosing technology and potential storage sites. To this end, a titanium reactor (1.4 m3) was used to study CO2 seepage under realistic sub-seabed conditions (30 bar pressure and 7 °C). The injection of CO2 was calibrated to decrease the pH value from 8.1 to 7.3, which may be the pH found near a leakage point. This pH value also coincides with predictions for near-future ocean pH under current CO2 emissions worldwide. The results from this study demonstrate that there are some elements, i.e., Fe, Co, Pb, Ce, Zn and Cu, present in deep marine sediments, that are strongly affected by the reduced pH levels related to CO2 addition. The dissolved concentrations of Fe, Pb and, to a lesser extent, Cr increased, due probably to weakening of the Fe/Mn shuttle by increased dissolved concentrations of CO2. Desorption processes from oxyhydroxide surfaces due to acidification may explain the release of Co, Ni and Ce observed during the experiment. The increased CO2 concentration also led to increased metal bioavailability, suggested by higher values for labile metal species. Conversely, Cd mobility seems not to be affected by CO2-associated acidification. It is concluded that the determination of those elements most affected by CO2-related acidification in a sub-seabed CO2 storage perimeter (i.e., sediment, sediment-water interface and water column) would be a simple and effective technique to verify suspected leakage.

Assessment of metals mobility during the alkaline treatment of highly acid phosphogypsum leachates.

This research evaluates the feasibility of an alkaline treatment system for highly acid leachates from a phosphogypsum stack located in an estuarine environment degraded by such pollution. The presented methodology consists of the addition of a Ca(OH)2 solution to the different types of phosphogypsum-related acidic leachates with the aim to increase their pH and subsequently, to provoke the precipitation and immobilization of the dissolved contaminants. In fact, phosphates and fluorides reached removal of 100% and 90%, respectively. As regards metals, removal values close to 100% were reached for Fe, Al, Cr, Cd, U and Zn, whereas it did not seem to be totally effective for other elements such as As (removal of 57-82%) and Sb (4-36%). The decrease of contaminant concentrations was caused probably by co-precipitation and/or adsorption to phosphate phases, together with by fluoride precipitation. The solid phases formed during the treatment were subjected to two standard leaching tests (EN 12457-2 from the EU and TCLP from the US) in order to conduct a risk and management assessment. In this context, some of the precipitates formed during the treatment would be classified as hazardous wastes, due to the high concentration of As leached. Moreover, the potential economic costs of a convectional active treatment system were also explored. This study sets the basis for a new research line with the aim to minimise the impact of the phosphogypsum stacks worldwide to their adjacent environment.

Sulfate reduction processes in salt marshes affected by phosphogypsum: Geochemical influences on contaminant mobility.

Sulfate reduction and its associated contaminant immobilization in marsh soils supporting a phosphogypsum stack was examined by pore-water and solid analysis, selective extractions, microscopy and sulfur K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The negative impact of this stack on estuarine environments is a concerning problem. In the weathering profile, total concentrations of most pollutants increase with depth; instead, dissolved contents in pore-waters increase to middle of the saturated zone but then decrease drastically down to reach the marsh due to sulfide precipitation. Excess of acid-volatile sulfide plus pyritic sulfur over metals bound to the oxidizable fraction indicates that sulfide precipitation is the main mechanism responsible for metal removal in the marsh. Thus, abundant pyrite occurred as framboidal grains, in addition to other minor sulfides of As, Zn and Cu as isolated particles. Moreover, high contents of elemental sulfur were found, which suggest partial sulfide oxidation, but marsh may have capacity to buffer potential release of contaminants. The importance of sulfur species was quantitatively confirmed by XANES, which also supports the accuracy of selective extraction schemes. Accordingly, managing pore-water quality through organic carbon-rich amendments over phosphogypsum stacks could lead to a decrease in contaminant loading of leakages resulting from weathering.

An anomalous metal-rich phosphogypsum: Characterization and classification according to international regulations.

Phosphogypsum is the main waste generated by the phosphate fertilizer industry. Despite the high level of pollutants found in phosphogypsum and the proximity of stacks to cities, there are no specific regulations for the management of this waste. This study addresses this issue by applying to phosphogypsum, from a fertilizer plant in Huelva (SW Spain), the leaching tests proposed by the current European and US environmental regulations for wastes management and classification. Two main conclusions were obtained: 1) the anomalous metal and metalloid concentrations (e.g. As, Fe, Pb, Sb, Mn, V and Cu) and higher mobility observed in the Huelva phosphogypsum compared to other stacks worldwide, and 2) the discrepancies observed between EU and US regulations dealing with hazardousness classification of these materials. This latter finding suggests the need to use complementary assessment protocols to obtain a better characterization and classification of these wastes. An evaluation of the potential risk to the aquatic life according to the US EPA regulation is proposed in this study. The results warn about the acute and chronic effects on the aquatic life of this waste and suggest the adoption of more strict measures for a safe disposal of phosphogypsum stacks.

A geochemical approach to the restoration plans for the Odiel River basin (SW Spain), a watershed deeply polluted by acid mine drainage.

The Odiel River Basin (SW Spain) drains the central part of the Iberian Pyrite Belt (IPB), a world-class example of sulfide mining district and concomitantly of acid mine drainage (AMD) pollution. The severe AMD pollution and the incipient state of remediation strategies implemented in this region, coupled with the proximity of the deadline for compliance with the European Water Framework Directive (WFD), urge to develop a restoration and water resources management strategy. Furthermore, despite the presence of some reservoirs with acid waters in the Odiel basin, the construction of the Alcolea water reservoir has already started. On the basis of the positive results obtained after more than 10 years of developing a specific passive remediation technology (dispersed alkaline substrate (DAS)) for the highly polluted AMD of this region, a restoration strategy is proposed. The implementation of 13 DAS treatment plants in selected acid discharges along the Odiel and Oraque sub-basins and other restoration measurements of two acidic creeks is proposed as essential to obtain a good water quality in the future Alcolea reservoir. This restoration strategy is also suggested as an economically and environmentally sustainable approach to the extreme metal pollution affecting the waters of the region and could be considered the starting point for the future compliance with the WFD in the Odiel River Basin.

Acid mine drainage in the Iberian Pyrite Belt: 1. Hydrochemical characteristics and pollutant load of the Tinto and Odiel rivers.

Acid mine drainage in the Iberian Pyrite Belt is probably the worst case in the world of surface water pollution associated with mining of sulphide mineral deposits. The Iberian Pyrite Belt is located in SW Iberian Peninsula, and it has been mined during the last 4,500 years. The central and eastern part of the Iberian Pyrite Belt is drained by the Tinto and Odiel rivers, which receive most of the acidic leachates from the mining areas. As a result, the main channels of the Tinto and Odiel rivers are very rich in metals and highly acidic until reaching the Atlantic Ocean. A significant amount of the pollutant load transported by these two rivers is delivered during the rainy season, as is usual in rivers of Mediterranean climate regions. Therefore, in order to have an accurate estimation of the pollutant loads transported by the Tinto and Odiel rivers, a systematic sampling on a weekly basis and a high temporal resolution sampling of floods events were both performed. Results obtained show that metal fluxes are strongly dependent on the study period, highlighting the importance of inter-annual studies involving dry and wet years.

Natural attenuation processes in two water reservoirs receiving acid mine drainage.

Characteristics of water profiles and sulphide formation processes in sediments were studied in two water reservoirs affected by acid mine drainage in order to investigate the mechanisms controlling the physical and chemical processes that, under favourable conditions, act to reduce the toxicity, mobility and concentration of metals and metalloids in the water column. Water columns and pore-waters from sediments were analysed for Fe species, trace elements (As, Cd, Co, Cu, Mn, Ni, Pb, Zn, Cr), sulphide, sulphate and bicarbonate. Inorganic reduced sulphur compounds (acid volatile sulphur, pyrite sulphur and elemental sulphur) and reactive Fe were determined in the sediments. A sequential extraction was also performed. Both reservoirs behave like holomictic and monomictic lakes, with a summer thermal stratification that disappears during winter. pH values between 4 and 7 can be observed along the water columns. Pore-water concentrations of up to 25 mg/l of Fe, 4 mg/l of Al, 1.3 mg/l of Zn, 170 microg/l of Pb, 11 microg/l of As, etc. have been found. The results suggest that toxic elements such as Cu, Zn, Co, Pb, Cr, As, etc. are mainly found in the bioavailable fraction which is the most hazardous for the environment. The calculated degree of sulphidization (DOS) and degree of pyritization (DOP) values indicates that removal of trace elements from anoxic pore-waters occurs by coprecipitation and/or adsorption on newly formed Fe sulphides (framboidal pyrite), attenuating the contamination. However oxidation of the sediments during turnover periods also occurs, which releases toxic elements back into the water column.

Acid mine drainage pollution in the Tinto and Odiel rivers (Iberian Pyrite Belt, SW Spain) and bioavailability of the transported metals to the Huelva Estuary.

The Tinto and Odiel rivers are seriously affected by acid mine drainage (AMD) from the long-term mining activities in Iberian Pyrite Belt (IPB). As a consequence, the Huelva estuary is heavily contaminated by metals and metalloids. This study presents an estimation of the seasonal variation, and the dissolved contaminant load transported by both rivers from February 2002 to September 2004. Besides, toxicity and bioaccumulation tests with the sediments of the estuary have been conducted in order to measure the mobility of the toxic metals. Results show that the Tinto and Odiel rivers transport enormous quantities of dissolved metals to the estuary: 7900 t yr(-1) of Iron (Fe), 5800 t yr(-1) Aluminium (Al), 3500 t yr(-1) Zinc (Zn), 1700 t yr(-1) Copper (Cu), 1600 t yr(-1) Manganese (Mn) and minor quantities of other metals and metalloids. These values represent 37% of the global gross flux of dissolved Zn transported by rivers in to the ocean, and 15% of the global gross flux of dissolved Cu. These metals and metalloids usually sink in the estuarine sediments due to pH and salinity changes. The increase of salinity in the estuary favours the adsorption and trapping of metals. For this reason, the mobility and bioavailability of metals such as Zn, Cd and Cu is higher in sediments located in the area of fresh water influence that in sediments located in the marine influenced area of the estuary, showing a higher percentage of fractionation and bioaccumulation of these metals in the station influenced by the fresh water environment.