Prediction of the long-term performance of abandoned lead zinc mine tailings in a Welsh catchment

In this study we investigated the sulphidic mine tailings from Frongoch and Grogwynion, two abandoned lead zinc mines in mid-Wales, UK. Despite falling within the same ore field the mine waste characterisation has identified differences in the tailings from the two sites. Bulk concentrations range from 10 to 52 g kg^− 1 for Pb, 1.1 to 2.9 g kg^− 1 for Zn in Grogwynion and from 1.0 to 130 g kg^− 1 for Pb, 11 to 110 g kg^− 1 for Zn in Frongoch. An experimental (European standard leaching tests TS 14429 and TS 14405) and geochemical modelling approach was used to study the leaching composition as a function of pH and liquid/solid ratio. There was little correlation between the tailings bulk metal concentrations and the leachate composition, but variations in Pb and Zn concentrations were found to be consistent with control of dissolved Pb and Zn by secondary minerals and the mechanisms of dissolution/precipitation/sorption involving them. Specifically, the Grogwynion mine tailings with near-neutral pH have predominantly lead and zinc carbonates controlling Pb and Zn solubility in the leachates, whereas the Pb and Zn concentrations in Frongoch leachates are best modelled with a surface complexation model for metal sorption to oxyhydroxides. The different speciation results in a greater sensitivity of Grogwynion tailings to acidification with a potential release of Pb in solution up to 10 times higher than in Frongoch, despite similar bulk Pb concentrations. At acid pH, Zn is similarly dissolved to a greater extent in Grogwynion than in Frongoch tailings. There was no evidence of sulphide oxidation during the batch and column leaching tests and the suitability of using these European leaching standards for the characterisation of sulphidic mine waste materials for waste management purposes has been considered.     

Leaching behaviour of slag and fly ash from laterite nickel ore smelting (Niquelândia,Brazil)

The laterite nickel (Ni) ore smelting operation in Niquelândia (Goiás state, Brazil) produced large amounts of smelting wastes, stockpiled on dumps (slags) and in settling ponds (fly ash). In this study we present data on the chemistry, mineralogy and pH-dependent leaching behaviours of these two waste materials.Bulk chemical analyses indicated that both wastes contained significant amounts of potentially toxic elements (PTEs), with substantially higher concentrations in the case of the fly ash (up to 2.51 wt% Ni, 1870 mg/kg Cr and 488 mg/kg Co). The mineralogical investigations carried out using X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and electron microprobe (EPMA) indicated that the slag was mainly composed of silicate glass, olivine and pyroxene. In contrast, the fly ash was composed of Ni-bearing serpentine-like phases (originating from the furnace feed), Ni-bearing glass, olivine, pyroxene and spinel. The pH-dependent leaching behaviour was performed according the EU standard experimental protocol (CEN/TS 14997) in the pH range of 3–12. The leaching was highly pH-dependent for both materials, and the highest releases of PTEs occurred at pH 3. The slag generally exhibited an U-shaped leaching behaviour of the PTEs as a function of pH, and was found to release up to 48.0 mg/kg Ni, 25.6 mg/kg Cr, and 1.42 mg/kg Co. The fly ash was significantly more reactive, and exhibited its highest leaching level of PTEs between pH 3 and 7. The maximum observed release corresponded to 5750 mg/kg Ni, 4.35 mg/kg Cr, and 112 mg/kg Co. The leached Ni concentrations after 24 h of leaching in deionized water exceeded the limit for hazardous waste by more than 100x according to the EU legislation (40 mg/kg Ni). X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structures (EXAFS) spectra indicated that Ni in the fly ash is predominantly bound in a serpentine-like phase, and during the fly ash experimental alteration it was mainly released from the second shell (corresponding to the atomic distances between Ni and Si, Mg, Fe in high-temperature silicates, glass, and partially dissolved serpentine). This study shows that disposal sites for the fly ash can represent a significant source of local pollution, and direct recycling of the fly ash in the smelting technology (as currently adopted at the Barro Alto new smelter and since few years also at the Niquelândia smelter) is the best environment-friendly option for handling of fly ash in the future.     

Multivariate analysis of contamination in the mining district of Linares (Jaén, Spain)

Historically, a significant level of mining activity has taken place in the batholite-related metalogenic enclave of Linares (Jaén province, Spain), associated with Pb–Ag, Cu, Zn and Fe sulphides and Ba sulphate mineralization, though mining here has now been abandoned. Additionally, the area features a significant amount of urban, industrial and agricultural activities. These considerations, taken together, explain the need to assess the levels of concentration of trace elements and to determine their relationship with geogenic and anthropogenic factors. For geochemical characterisation of the soil, the region has been divided into 126 grid squares with an area of 1 km². For each grid square, 32 trace elements have been analysed. Elemental concentrations of Cu, Pb, Zn, As and Mn have been included in statistical analyses. According to the reference levels established by the Regional Government (Junta de Andalucía), soils in a large part of the study area require amendment applications. The comparison of the mean content for each grid square with the reference levels reveals a significant degree of contamination of the soil by Cu (719 mg kg⁻¹), Pb (22,964 mg kg⁻¹) and As (100 mg kg⁻¹) in those grid squares affected by metallurgic activities. By means of factor analysis, four scores have been identified which together account for 80% of the variance observed. The first score is highly correlated with the logarithms of the variables Fe, Th, La, Ti, Al, Na, K, Zr, Y, Nb, Be and Sc. It is a “natural” factor that indicates the type of soil matrix (fundamentally granites and, to a lesser degree, Triassic materials). The second score shows high correlation with the logarithms of the variables Mo, Cu, Pb, Zn, Ag, Co, Mn, As, Cd, Sb, Ba, W and Sn, and is the “metallization” factor related to the mineralization that has been exploited. The third score is mainly determined by the logarithms of the variables Sr, Ca and Mg. This is a “natural” factor that indicates a type of carbonate soil matrix (Miocene). Finally, the fourth factor groups the logarithms of the variables Ni, V and Cr, elements that are associated with the combustion of fossil fuels. Analysis of the patterns of each of the factors identified enabled achieving a global characterisation of the study area. Cluster analysis of the observations showed there to be five clusters relating to the grid squares, differentiated by lithologies and degrees of contamination. These clusters are used to determine the background of granite and to calculate the anomalous load.     

Geomorphic controls on mercury accumulation in soils from a historically mined watershed, Central California Coast Range, USA

Historic Hg mining in the Cache Creek watershed in the Central California Coast Range has contributed to the downstream transport of Hg to the San Francisco Bay-Delta. Different aspects of Hg mobilization in soils, including pedogenesis, fluvial redistribution of sediment, volatilization and eolian transport were considered. The greatest soil concentrations (>30 mg Hg kg⁻¹) in Cache Creek are associated with mineralized serpentinite, the host rock for Hg deposits. Upland soils with non-mineralized serpentine and sedimentary parent material also had elevated concentrations (0.9–3.7 mg Hg kg⁻¹) relative to the average concentration in the region and throughout the conterminous United States (0.06 mg kg⁻¹). Erosion of soil and destabilized rock and mobilization of tailings and calcines into surrounding streams have contributed to Hg-rich alluvial soil forming in wetlands and floodplains. The concentration of Hg in floodplain sediment shows sediment dispersion from low-order catchments (5.6–9.6 mg Hg kg⁻¹ in Sulphur Creek; 0.5–61 mg Hg kg⁻¹ in Davis Creek) to Cache Creek (0.1–0.4 mg Hg kg⁻¹). These sediments, deposited onto the floodplain during high-flow storm events, yield elevated Hg concentrations (0.2–55 mg Hg kg⁻¹) in alluvial soils in upland watersheds. Alluvial soils within the Cache Creek watershed accumulate Hg from upstream mining areas, with concentrations between 0.06 and 0.22 mg Hg kg⁻¹ measured in soils 90 km downstream from Hg mining areas. Alluvial soils have accumulated Hg released through historic mining activities, remobilizing this Hg to streams as the soils erode.     

ANC,BNC and mobilization of Cr from polluted sediments in function of pH changes

During the manufacturing of chromate salts (1972–1992) large quantities of Chromite Ore Processing Residue (COPR) were released into a decantation pond east of the former chemical plant of Porto-Romano (Durres, Albania), giving rise to yellow colored pond sediments. These Cr(VI) bearing sediments were deposited upon Quaternary silty-clay lagoonal sediments rich in iron oxides and organic matter. The pH values in these lagoonal sediments vary around 6.6, while in the pond sediments, it is mainly acidic (due to the presence of the sulfur stock piles in the area and the release of the H₂SO₄ from the activity of the former chemical plant), varying between 1.4 and 3.8. Continuous leaching of the COPR waste resulted in yellow-colored surface water runoff. The prediction of pH changes in the different types of sediments based upon acid/base neutralizing capacity (ANC/BNC) jointly with the quantitative data on release of heavy metals and especially Cr is considered an important advantage of the pH_stat leaching test if compared to conventional leaching procedures. Thus, factors controlling the leaching of Cr(VI), Cr(III), Ca, Al, Fe, Mg from the COPR were investigated by means of pH_stat batch leaching tests and mineralogical analysis. Moreover, mathematical and geochemical modeling complemented the study. The COPR in the area contain very high concentrations of chromium 24,409 mg/kg, which mainly occurs as Cr(III) (75–90%) as well as Cr(VI) (25–10%). The leaching of Cr(VI) occurs in all the range (2–10) of the tested pH values, however, it decreases under acidic conditions. Beside some reduction of Cr(VI) to Cr(III), the Cr(VI) content of the leachtes remains relatively high in the acidic environment, while the limning of Cr(VI) pond sediments will increase the release of the latter specie. The leaching of the Cr(III) occurs strictly under acidic conditions, whereby limning of these sediments will give rise to the lower solubility of Cr(III). The key mineral phases responsible for the fast release of the Cr(VI) are: the chromate salts (i.e. sodium chromate and sodium dichromate), while sparingly soluble chromatite (CaCrO₄) and hashemite (BaCrO₄) release Cr(VI) very slowly. Thus, pH and mineral solubility have been identified as key factors in the retention and the release of the hexavalent CrO₄ ²⁻ and Cr₂O₇ ⁻ from the COPR-rich pond sediments.     

Radiocaesium dispersion and fixation in the lagoon systems of “Ria de Aveiro”, Portugal

Radiocaesium (¹³⁷Cs) dispersion and Cs⁺ fixation were studied in the sediments collected from the lagoon systems of “Ria de Aveiro”. The Cs⁺ sorption was tested for the fine mica grains and for the < 2 μm clay fractions extracted from silty clays. The Cs⁺ exchange is found strongly onto mica-rich fractions than smectite-rich fractions. The distribution coefficient increases if the silty material is constituted by rich-mica clay fractions or if the non-clay minerals are removed from the silty-clay material. The samples studied behave as multisite ion exchange, where Cs⁺ engages in ion-exchange reactions with hydrated cations on planar sites on expansible layer silicates. Higher concentrations of the ¹³⁷Cs were found associated with mica-rich silty clays. The ¹³⁷Cs ranges from 3.2 to 3.9 Bq kg^− 1 in the < 38 μm fractions and from 2.9 to 3.3 Bq kg^− 1 in the < 64 μm fractions.     

High-sensitive measurement of uranium LIII-edge X-ray absorption near-edge structure (XANES) for the determination of the oxidation states of uranium in crustal materials

The uranium L_III-edge XANES spectra for natural rocks at the concentration range of 0.96–124 mg kg⁻¹ were measured using a log spiral bent crystal Laue analyzer (BCLA) combined with a multi-element Ge detector. It was found that the quality of the XANES spectra using the BCLA was greatly improved due to a reduction of interfering fluorescence from major components such as Rb and Sr. The ratio of signal to background intensities in the U L_III-edge XANES spectra increased by a factor of 2.9–17 with the use of the BCLA, which greatly enhanced the detection limit for the speciation of the oxidation states of U or the U(IV)/U(VI) ratio of natural samples. In addition, it was demonstrated that the fluorescence XANES method coupled with the BCLA enable determination of the speciation of U for various natural samples such as acidic igneous rocks, ferromanganese nodules, sediments, and some sedimentary rocks such as shale and limestone.     

Total mercury and monomethylmercury in water, sediments, and hydrophytes from the rivers, estuary, and bay along the Bohai Sea coast, northeastern China

Mercury contamination in aquatic environments is of worldwide concern because of its high biomagnification factor in food chains and long-range transport. The rivers, estuary and the bay along the northwestern Bohai Sea coast, northeastern China have been heavily contaminated by Hg due to long-term Zn smelting and chlor-alkali production. This work investigated the distributions of total Hg (THg) and monomethylmercury (MMHg) in the water, sediment and hydrophytes from this area. Concentrations of THg in sediment (0.5–64 mg kg⁻¹) and water (39–2700 ng L⁻¹) were elevated by 1–3 orders of magnitude compared to background concentrations, which induced high concentrations of MMHg in these media. The highest concentration of MMHg in sediment reached 35 μg kg⁻¹, which was comparable to that in the Hg mining area, Wanshan, China, and the highest MMHg concentration of 3.0 ng L⁻¹ in the water sample exceeded the MMHg Chinese drinking water guideline of 1.0 ng L⁻¹. Concentrations of THg in a sediment profile from Jinzhou Bay were found to be consistent with annual Hg emission flux from a local Zn smelter (r = 0.74, p < 0.01), indicating that Hg contamination was mainly caused by Zn smelting locally. For some freshwater hydrophytes, concentrations of THg and MMHg ranged from 5.2 to 100 μg kg⁻¹ and 0.15 to 12 μg kg⁻¹, respectively. Compared to sediment, concentrations of THg in hydrophytes were 2–3 orders of magnitude lower but MMHg was comparable or higher, indicating that the bioaccumulation in plants was distinct for the two Hg species studied. The data suggest that a significant load of Hg has been released into the northwestern coastal region of the Bohai Sea.     

Comparison of methods for the estimation of pyrite oxidation rate in a waste rock pile at Mine Doyon site, Quebec, Canada

Several methods were evaluated and compared for the estimation of pyrite oxidation rates (POR) in waste rock at Mine Doyon, Quebec, Canada. Methods based on data collected in situ, such as the interpretation of temperature and oxygen concentration profiles (TOP) measured in the waste rock pile and pyrite mass balance (PMB) on solid phase samples were compared with the oxygen consumption measurements (OCM) in closed chamber in the laboratory. A 1-D analytical solution to a gas and heat transport equation used temperature and oxygen profiles (TOP) measured in the pile for the preliminary POR estimates at a site close to the slope of the pile (Site 6) and in the core of the pile (Site 7). Resulting POR values were 1.1 × 10^− 9 mol(O₂) kg^− 1 s^− 1 and 1.0 × 10^− 10 mol(O₂) kg^− 1 s^− 1 for the slope site and the core site, respectively. Oxidation rates based on pyrite mass balance (PMB) calculations for solid samples were 2.21 × 10^− 9 mol(O₂) kg^− 1 s^− 1 and 2.03 × 10^− 9 mol(O₂) kg^− 1 s^− 1, respectively, for the same slope and core sites, but the difference between sites was within the error margin. The OCM measurements in the laboratory on fresh waste rock samples yielded higher POR values than field methods, with average oxidation rate of 6.7 × 10^− 8 mol(O₂) kg^− 1 s^− 1. However, the OCM results on weathered and decomposed material from the rock stockpile (average oxidation rate 3.4 × 10^− 9 mol(O₂) kg^− 1 s^− 1) were consistent with results from the field-based estimates. When POR values based on fresh material are excluded, the remaining POR values for all methods range from 1.0 × 10^− 10 to 3.4 × 10^− 9 mol(O₂) kg^− 1 s^− 1. The lowest estimated value (1.0 × 10^− 10 mol(O₂) kg^− 1 s^− 1) was based on TOP estimates in the interior of the pile where oxygen transport was limited by diffusion from the surface. These results suggest that small-scale OCM laboratory experiments may provide relatively representative values of POR in the zones of waste rock piles in which oxygen transport is not dominated by diffusion.     

Correlation between local structure and molar ratio of Au (III) complexes in aqueous solution: An XAS investigation

The local and geometrical structure around gold (III) e.g., Au³⁺ ions in aqueous solution with different OH⁻/Cl⁻ molar ratios, has been investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure (XANES) spectra of [AuCl_n(OH)_4−n]⁻ solutions have been calculated and the multiple-scattering spectral features have been attributed to Cl d-states, axial water molecules and the replacement of Cl⁻ ligands by OH⁻ ligands. A square–planar geometry for [AuCl_n(OH)_4−n]⁻ with two axial water molecules has been identified. Moreover, a spectral correlation between XANES features and the type of planar atoms has been identified. By extended X-ray absorption fine structure spectra (EXAFS), the planar Au bond distances in the solutions have also been determined, e.g., 2.28 Å for Au–Cl and 1.98 Å for Au–O, respectively. The same EXAFS analysis provides evidence that the peak at about 4.0 Å in solutions with the lowest OH⁻/Cl⁻ molar ratio arises from collinear Cl–Au–Cl multiple-scattering contributions. For the first time, a complete detailed reconstruction of the hydration structure of an Au ion at different pH values has been achieved.     

Comparison of Cu,Zn and Fe bioleaching from Cu-metallurgical slags in the presence of Pseudomonas fluorescens and Acidithiobacillus thiooxidans

Metal leaching from metallurgical wastes (slags) by means of environmentally friendly approaches is promising for practical applications. The goal of this study was to compare the feasibility of metal bioleaching from Cu slags by means of Pseudomonas fluorescens and Acidithiobacillus thiooxidans. Two size particles (<0.3 mm and 1–2 mm) of two types of Cu slags (massive crystalline slag and granulated amorphous slag) were used to study metal (Cu, Zn and Fe) bioleaching. The 40-days bioleaching experiments with P. fluorescens began at circumneutral pH (7.0), whereas the experiments with A. thiooxidans were started under acidic (pH 2.5) conditions. The results demonstrated that A. thiooxidans catalyzes metal leaching from both slag types investigated. After 21 days of incubation, optimal leaching was achieved and up to 79% Cu, 76% Zn and 45% Fe could be extracted from crystalline slag under conditions of 1 wt.% pulp density and particle size <0.3 mm. The optimal efficiency achieved with amorphous slag was 81% Cu, 79% Zn and 22% Fe when 1% pulp density and 1–2 mm particle size were used. The use of P. fluorescens resulted in poor leaching efficiencies as compared to the performance of A. thiooxidans, presumably due to the higher pH conditions maintained during the P. fluorescens incubations. The maximum metal leaching efficiencies with P. fluorescens were achieved at 1% pulp density and particle size <0.3 mm and did not exceed 10% Cu, 4% Zn, 0.3% Fe for crystalline slag and 4% Cu, 3% Zn, 0.7% Fe for amorphous slag. Both slags exhibited a good potential for bioleaching with A. thiooxidans, however; further optimization of the process parameters (e.g. pulp density, particle size and pH) is needed to improve the efficiency.     

Modelling ion composition in seepage water from a column experiment with an open cut coal mine sediment

Secondary reactions occurring in pyrite-containing sediments from open cut coal mines are complex and not fully understood. In this study, the changes in seepage water composition in a column experiment with a sediment containing pyrite (5.6 g kg⁻¹) were evaluated using a chemical equilibrium model. A column experiment with artificial irrigation (730 mm water yr⁻¹) was carried out for 2 yr with a sediment from the open pit mine Garzweiler, Germany, at the Institute of Applied Geology. Tracer (LiCl) was added to the sediment. Seepage water composition at 52 cm depth was sampled weekly. Redox potential and the water potential were also recorded weekly. Sulphate and Fe(II) were the dominant ions in the seepage water with concentration maxima of 500 and 350 mmol l⁻¹ after 50 days (0.7 pore volumes (PV)). Minimum pH values were around 0.8 after 100 days (1.4 PV), but increased subsequently and reached 2.4 after 700 days (9.5 PV). Ion activity product calculations indicated the intermediate formation of gypsum (19th–480th day of the experiment). Solutions were undersaturated with respect to alunite, jarosite, jurbanite, schwertmannite, melanterite, gibbsite and goethite during the whole experiment. The model of coupled equilibria which included inorganic complexation, precipitation/dissolution of gypsum and multiple cation exchange was tested. Pyrite oxidation and pH-dependent silicate weathering were considered using simple input functions. Transport was modelled using a field capacity cascade submodel. Model results showed satisfactory agreement with measured values for pH and concentrations of SO₄, Fe, Mg, Ca and Al. Correlation coefficients lay between 0.7 and 0.9 and linear regression coefficients (modelled against measured) were 1.5 (Ca), 1.0 (Fe, SO₄), 0.8 (Mg), 0.7 (pH) and 0.6 (Al). The results showed that the protons produced during pyrite oxidation (94 mmol_c H⁺ kg⁻¹) were mainly released into seepage water (as HSO₄⁻ and H⁺). Cation exchange reactions buffered 20 mmol_c of H⁺ kg⁻¹ sediment, and Al released by silicate weathering accounted for 3.6 mmol_c H⁺ kg⁻¹. Modelling was useful to further understand the significance of different pH buffer reactions.     

Zero-valent iron and organic carbon mixtures for remediation of acid mine drainage: Batch experiments

A series of laboratory batch experiments was conducted to evaluate the potential for treatment of acid mine drainage (AMD) using organic C (OC) mixtures amended by zero-valent Fe (Fe⁰). Modest increases in SO₄ reduction rates (SRRs) of up to 15% were achieved by augmenting OC materials with 5 and 10 dry wt% Fe⁰. However, OC was essential for supporting SO₄ reducing bacteria (SRB) and therefore SO₄ reduction. This observation suggests a general absence of autotrophic SRB which can utilize H₂ as an electron donor. Sulfate reduction rates (SRRs), calculated using a mass-based approach, ranged from −12.9 to −14.9 nmol L⁻¹ d⁻¹  g⁻¹ OC. Elevated populations of SRB, iron reducing bacteria (IRB), and acid producing (fermentative) bacteria (APB) were present in all mixtures containing OC. Effective removal of Fe (91.6–97.6%), Zn (>99.9%), Cd (>99.9%), Ni (>99.9%), Co (>99.9%), and Pb (>95%) was observed in all reactive mixtures containing OC. Abiotic metal removal was achieved with Fe⁰ only, however Fe, Co and Mn removal was less effective in the absence of OC. Secondary disordered mackinawite [Fe_1+xS] was observed in field-emission scanning electron microscopy (FE-SEM) backscatter electron micrographs of mixtures that generated SO₄ reduction. Energy dispersive X-ray (EDX) spectroscopy revealed that Fe–S precipitates were Fe-rich for mixtures containing OC and Fe⁰, and S-rich in the absence of Fe⁰ amendment. Sulfur K-edges determined by synchrotron-radiation based bulk X-ray absorption near-edge structure (XANES) spectroscopy indicate solid-phase S was in a reduced form in all mixtures containing OC. Pre-edge peaks on XANES spectra suggest tetragonal S coordination, which is consistent with the presence of an Fe–S phase such as mackinawite. The addition of Fe⁰ enhanced AMD remediation over the duration of these experiments, however long-term evaluation is required to identify optimal Fe⁰ and OC mixtures.     

Lower Eocene carbonate-cemented “chimney” structures (Varna, Bulgaria) — Control of seepage rates on their formation and stable isotopic signature

The genesis of Lower Eocene calcite-cemented columns, “pisoid”-covered structures and horizontal interbeds, clustered in dispersed outcrops in the Pobiti Kamani area (Varna, Bulgaria) is related to fossil processes of hydrocarbon migration. Field observations, petrography and stable isotope geochemistry of the cemented structures and associated early-diagenetic veins, revealed that varying seepage rates of a single, warm hydrocarbon-bearing fluid, probably ascending along active faults, controlled the type of structure formed and its geochemical signature. Slow seepage allowed methane to oxidize within the sediment under ambient seafloor conditions (δ¹⁸O = − 1 ± 0.5‰ V-PDB), explaining columns' depleted δ¹³C ratios of − 43‰. Increasing seepage rates caused methane to emanate into the water column (δ¹³C = − 8‰) and raised precipitation temperatures (δ¹⁸O = − 8‰). Calcite-cemented conduits formed and upward migrating fluids also affected interbed cementation. Even higher-energy fluid flow and temperatures likely controlled the formation of “pisoids”, whereby sediment was whirled up and cemented.     

Kinetics of leaching of oxidized and reduced ilmenite in dilute hydrochloric acid solutions

Ilmenite separated from beach sands of Bangladesh was oxidized for 1 h at 950 °C and then reduced in charcoal for 4 h at 1050 °C. This was followed by leaching in 5% to 15% hydrochloric acid solution in temperature range of 30 to 75 °C for periods of up to 2 h. The results were compared with those obtained by leaching of ilmenite reduced without oxidizing. Oxidation prior to reduction of the ilmenite was found to increase both the extent and the rate of leaching. The residual iron contents after leaching were also found to be lower than that obtained for non-oxidized samples. The kinetic data of leaching of ilmenite reduced after oxidation was found to follow first order reaction model, i.e., G(α) = − ln(1 − α) up to an α value of 0.5 (i.e. up to 50% reduction) and then changed to spherical model, i.e., G(α) = [1 − (1 − α)]³. On the other hand, leaching of ilmenite reduced without oxidizing was found to follow the Ginstling-Brounshtein reaction, i.e., G(α) = 1 − (2/3)α − (1 − α)^2/3 throughout the leaching process. Oxidation of ilmenite prior to reduction was also found to have decreased the activation energy of leaching from 43 kJ/mol, found for samples leached after reduction without oxidizing, to 30 kJ/mol.     

A comparative study on the dissolution and solubility of hydroxylapatite and fluorapatite at 25 °C and 45 °C

Dissolution of the synthetic hydroxylapatite (HAP) and fluorapatite (FAP) in pure water was studied at 25 °C and 45 °C in a series of batch experiments. The XRD, FT-IR and SEM analyses indicated that the synthetic, microcrystalline HAP and FAP with apatite structure used in the experiments were found to have no obvious variation after dissolution except that the existence of OH groups in FT-IR spectra for FAP after 2880 h dissolution was observed. During the HAP dissolution (0–4320 h), the aqueous calcium and phosphate concentrations reached the maxima after 120 h and then decreased slowly with time. For the FAP dissolution in pure water, after a transient time of 1440 h (< 60 d), element concentrations and pH became constant suggesting attainment of a steady-state between the solution and solid. During early stages of the FAP dissolution reaction (< 72–120 h), mineral components were released in non-stoichiometric ratios with reacted solution ratios of dissolved Ca:P, Ca:F and P:F being lower than mineral stoichiometric ratios of Ca₅(PO₄)₃F, i.e., 1.67, 5.0 and 3.0, respectively. This indicated that F were preferentially released compared to Ca from the mineral structure. The mean K_sp values were calculated by using PHREEQC for HAP of 10^− 53.28 (10^− 53.02–10^− 53.51) and for FAP of 10^− 55.71 (10^− 55.18–10^− 56.13) at 25 °C, the free energies of formation ΔG_f^o[HAP] and ΔG_f^o[FAP] were calculated to be − 6282.82 kJ/mol and − 6415.87 kJ/mol, respectively.     

Treatment of Acid Mine Drainage (AMD) in South Brazil: Comparative active processes and water reuse

This work describes AMD techniques of neutralization, with lime, flocculation of the precipitates and comparative flocs/liquid separation by flotation with microbubbles or by lamellar settling (LS). The AMD treated water was characterized by its quality for recycling in terms of inorganic or organic elements, suspended or dissolved solids, among others. Two types of flocs were formed, “aerated” or not, in a special flocculation reactor, patented by this research group (RGF^®). Aerated flocs formed (within seconds) entered into contact with microbubbles under high shearing and raised-up at rates > 120 mh^− 1 allowing a rapid solid–liquid separation by flotation (HR-high rate), at about 13–15 m³m^− 2 h^− 1 loading capacity. Conversely, the non-aerated flocs settled at about 5–6 m h^− 1 in a lamella settler. Both AMD treatment techniques showed similar efficiencies (removal of ions > 90%) but the separation by lamella settling presented advantages, namely less reagents (no flotation collector required), lower power requirements and easier to operate. The operating costs (approximate values) of the AMD treatment by LS at pH 9 reaches about 0.3 US$ m^− 3 against 0.6 US$ m^− 3 for the HR-flotation process. Results found were proved to be similar to those found in recent ADM treatment installations in South Brazil. The quality of the treated water is fairly good, nearly free of heavy metals ion, low BOD (biological oxygen demand) and TOC (total organic content), low solids content and may be readily reused for irrigation, industrial processes and as wash water (among others, streets, vehicles, dust control). However, there is a need to extend the use of this treated water resource, but this, at least in Brazil, has not been legislated properly. It is concluded that this research will contribute in the discussion of this old and complex problem in acid mining effluents worldwide.     

Characterization of the reactivity of riverine heterogeneous sediments using a facies-based approach; the Rhine–Meuse delta (The Netherlands)

Large groundwater resources are found in densely populated lowland areas, which consist often of young unconsolidated and reduced sediments. When anthropogenic activities lead to oxygenation of the aquifer, breakdown of the main reduced fractions, i.e. sedimentary organic matter (SOM) and pyrite, could lead to severe groundwater deterioration such as acidification, heavy metal mobilization, and increased hardness. The characterization of the reactive properties of these sediments is important in predicting groundwater deterioration, but is often complicated by the high degree of heterogeneity of these sediments. In this study, the potential reduction capacity (PRC, based on SOM and pyrite content), the potential buffer capacity (PBC, based on carbonate content), potential acidification capacity (PAC, based on the potential acid production by sulfide oxidation), and the measured reduction capacity (MRC) of five facies, which are typical of the riverine sediments in the Rhine–Meuse delta (The Netherlands) were determined. A universal facies-classification model was used to classify the deposits into more homogeneous sub-units based on lithologic and geogenic properties, with a further sub-division into oxic or anoxic redox environment based upon groundwater data and field observations. The bulk chemical data show strong variation across facies for the median values of PRC (186–9093 mmol O₂ kg⁻¹), PBC (17–132 mmol O₂ kg⁻¹), and PAC (36–1530 mmol H⁺ kg⁻¹). The MRC was measured as reactivity to molecular O₂ exposure and was 0.5–567.3 mmol O₂ kg⁻¹. Steady-state oxidation rates were in the wide range of 0.001–10.355 mmol O₂ kg⁻¹ day⁻¹ but were typically about 3–8 times faster in fine facies than in coarse facies. Both the PRC and MRC depend strongly on grain size, but also on the syn/post-depositional environment and redox conditions. The main part of the PRC consists of SOM, but pyrite reactivity is higher than SOM reactivity as shown by the relative depletion of pyrite in oxic subfacies and the preferential oxidation during the oxidation experiments. Some facies are very prone to acidification because the PAC is higher than the PBC, but the oxidation experiments also show that acidification could already start before the PRC is fully exhausted. This study, is one of the few that combines bulk chemical data, groundwater data, and reactivity measurements and shows that a facies-based approach is a practical tool in characterizing the reactivity of heterogeneous deposits.     

The impact of acid rain on phosphorus leaching from a sandy loam calcareous soil of western Iran

Simulated acidic precipitation (1:1 equivalent basis H₂SO₄:HNO₃) at pH values of 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 and 7.0 were conducted using column leaching to determine impacts of simulated acid rain on phosphorus (P) leaching from a calcareous sandy loam soil over a 40-day period. Soil columns were irrigated every day to make a total of 1,061 mm, equivalent to 3.5 years of rainfall (based on average annual rainfall). Leachates were collected and analyzed for anions and cations. There was significant nonlinear correlation between the amount of P leached and the simulated acid rain (R ² = 0.61). Losses of P from the pH 2.5 and 7.0 treatments were 1.23, and 1.32 mg kg⁻¹, respectively. The results showed that the amount of P leached from pH 4 (1.46 mg kg⁻¹) and 5 treatments (1.52 mg⁻¹ kg⁻¹) were significantly larger than other treatments. Linear equation adequately described leaching of P in different treatments. The slope (mg kg⁻¹ day⁻¹) in the linear equation was defined as the leaching rate and for the pH 2.5 was 0.0354, and 0.0382 and 0.0406 for pH 4.5 and 7.0, respectively. The geochemical code Visual MINTEQ was used to calculate saturation indices. Leaching of P in different treatments was controlled by rate-limited dissolution of hydroxyapatite, β-tricalcium phosphate and to some extent octacalcium phosphate. The results indicate that acid rain in calcareous sandy loam soils may pose a risk in terms of groundwater contamination with P.     

Control of As and Ni releases from a uranium mill tailings neutralization circuit: Solution chemistry, mineralogy and geochemical modeling of laboratory study results

Processing U ores in the JEB Mill of the McClean Lake Operation in northern Saskatchewan produces spent leaching solutions (raffinates) with pH  1.5, and As and Ni concentrations up to 6800 and 5200 mg L⁻¹, respectively. Bench-scale neutralization experiments (pH 2–8) were performed to help optimize the design of mill processes for reducing As and Ni concentrations in tailings and raffinates to 1 mg L⁻¹ prior to their disposal. Precipitate mineralogy determined by chemical analysis, XRD, SEM, EM, XM and EXAFS methods, included gypsum (the dominant precipitate), poorly crystalline scorodite (precipitated esp. from pH 2–4), annabergite, hydrobasaluminite, ferrihydrite, green rust II and theophrastite. The As was mostly in scorodite with smaller amounts in annabergite and trace As adsorbed and/or co-precipitated, probably by ferrihydrite. Geochemical modeling indicated that above pH 2, the ion activity product (IAP) of scorodite lies between the solubility products of amorphous and crystalline phases (log K_sp = −23.0 and −25.83, respectively). The IAP decreases with increasing pH, suggesting that the crystallinity of the scorodite increases with pH. Forward geochemical models support the assumption that during neutralization, particles of added base produce sharp local pH gradients and disequilibrium with bulk solutions, facilitating annabergite and theophrastite precipitation.