Towards greener lixiviants in value recovery from mine wastes: Efficacy of organic acids for the dissolution of copper and arsenic from legacy mine tailings

This is the author accepted manuscript. The final version is freely available from MDPI via the DOI in this recordIn many cases, it may be possible to recover value (e.g. metals, land) from legacy mine wastes and tailings when applying leaching-based remediation such as dump/heap leaching or in-vessel soil washing. However, if the lixiviant used has the potential to cause environmental damage upon leakage, then this approach will have limited practicability due to actual or perceived risk. This study focused on comparing the efficacy of organic acids, namely methanesulfonic (CH3SO3H) and citric (C6H8O7) acid, with mineral acids, namely sulfuric (H2SO4) and hydrochloric (HCl) acid, for the dissolution of Cu and As from mine tailings. The advantage of the former acid type is the fact that its conjungate base is readily biodegradable which should thereby limit the environmental impact of accidental spill/leakage (particularly in non-carbonate terrain) and might also be directly useful in capture/recovery systems coupled with percolation leaching (e.g., as an electron donor in sulphate-reducing bioreactors). The operational factors acid concentration, leaching time, mixing intensity and solid–liquid ratio, were tested in order to determine the optimum conditions for metal dissolution. HCl, H2SO4, and CH3SO3H typically exhibited a relatively similar leaching ability for As despite their different pKa values, with dissolutions of 58%, 56%, 55%, and 44% recorded for H2SO4, HCl, CH3SO3H, and C6H8O7, respectively, after 48 h when using 1 M concentrations and a 10:1 L:S ratio. For the same conditions, H2SO4 was generally the most effective acid type for Cu removal with 38% compared to 32%, 29% and 22% for HCl, CH3SO3H and C6H8O7. As such, CH3SO3H and C6H8O7 demonstrated similar performances to strong mineral acids and, as such, hold great promise as environmentally compatible alternatives to conventional mineral acids for metal recovery from ores and waste.The work was financially supported by the Natural Environment Research Council (grant number: NE/L013908/1)

Selective formation of copper nanoparticles from acid mine drainage using nanoscale zerovalent iron particles

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.Nanoscale zerovalent iron (nZVI) has been investigated for the selective formation of Cu nanoparticles from acid mine drainage (AMD) taken from a legacy mine site in the UK. Batch experiments were conducted containing unbuffered (pH 2.67 at t=0) and pH buffered (pH <3.1) AMD which were exposed to nZVI at 0.1-2.0 g/L. Results demonstrate that nZVI is selective for Cu, Cd and Al removal (>99.9% removal of all metals within 1 h when nZVI ≥1.0 g/L) from unbuffered AMD despite the coexistent of numerous other metals in the AMD, namely: Na, Ca, Mg, K, Mn and Zn. An acidic pH buffer enabled similarly high Cu removal but maximum removal of only <1.5% and <0.5% Cd and Al respectively. HRTEM-EDS confirmed the formation of discrete spherical nanoparticles comprised of up to 68% wt. Cu, with a relatively narrow size distribution (typically 20-100 nm diameter). XPS confirmed such nanoparticles as containing Cu0, with the Cu removal mechanism therefore likely via cementation with Fe0. Overall the results demonstrate nZVI as effective for the one-pot and selective formation of Cu0-bearing nanoparticles from acidic wastewater, with the technique therefore potentially highly useful for the selective upcycling of dissolved Cu in wastewater into high value nanomaterials.This work was financially supported by the Natural Environmental Research Council (grant number: NE/L013908/1) and the Camborne School of Mines Trust

Nanoscale zero-valent iron particles for the remediation of plutonium and uranium contaminated solutions

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.In the current work the uptake of plutonium onto nanoscale zero-valent iron nanoparticles (nZVI) under anoxic conditions has been investigated. A uranyl solution was also studied under similar geochemical conditions to provide a comparative dataset. Following nZVI addition, a rapid and significant decrease in aqueous actinide concentration was recorded for both systems. The removal rate recorded for plutonium was slower, with 77% removal recorded after 1 h of reaction, compared to 99% recorded for uranium. Low aqueous contaminant concentrations (<25%) were then recorded for both systems until the end of the 7 day reaction period. XPS confirmed contaminant uptake onto the nZVI. For the plutonium system, the recorded photoelectron spectra exhibited Pu 4f lines centred at ∼439 and ∼427 eV, characteristic of Pu4+ and implying that chemical reduction of the sorbed plutonium had occurred, ascribed to the formation of PuO2. Similarly, with the U-system, the recorded U 4f photoelectron peaks were centred at energies of ∼380 and ∼391 eV, characteristic of U4+ in UO2. Results provide clear evidence that nZVI may be used as an effective material for the removal of plutonium from contaminated waters.AWE plcEngineering and Physical Sciences Research Counci

The influence of calcium, sodium and bicarbonate on the uptake of uranium onto nanoscale zero-valent iron particles

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.This work investigates the influence of calcium (Ca), sodium (Na) and bicarbonate (HCO3−) on the uptake of uranium (U) onto nanoscale zero-valent iron particles (nZVI). Solutions tested contained U at 1 mg/L, NaHCO3 ranging from 0 to 100 mg/L and CaCl2 or NaCl ranging from 625 to 719 mg/L to normalise the ionic strength. Mine water containing a similar concentration of U (1.03 mg/L) and HCO3− at 845 mg/L was also tested as a natural analogue. All solutions were stored in sealed glass jars in the open laboratory with headspace comprising ambient air. Results demonstrate Ca, Na and HCO3− as having no significant inhibitive influence on the efficacy of nZVI for initial U removal from solution, with ⩾95.7% uptake recorded for all systems studied after 0.5 h reaction. Similar U retention (>97.1%) was recorded throughout the entire 672 h experiment for all solutions with Ca absent. In contrast, partial U desorption in the latter stages of the experiment was recorded for all solutions with Ca present, with 87.3%, 85.2% and 84.7% removal recorded after 672 h for solutions containing 0, 10 and 100 mg/L HCO3−, respectively, and 10.9% removal recorded for the mine water. Maximum U removal onto nZVI was recorded as directly proportional to HCO3− concentration for solutions with Ca absent, however, no trend was identified for the Ca-bearing solutions. Overall results demonstrate Ca as having a significant inhibitive influence on the long-term retention (e.g. >48 h) of U on nZVI, which is independent of HCO3− concentration when also present at <100 mg/L.Engineering and Physical Sciences Research CouncilNAT

Technical Note: The use of an interrupted-flow centrifugation method to characterise preferential flow in low permeability media

This is the final version. Available from European Geosciences Union (EGU) via the DOI in this record. We present an interrupted-flow centrifugation technique to characterise preferential flow in low permeability media. The method entails a minimum of three phases: centrifuge-induced flow, no flow and centrifuge-induced flow, which may be repeated several times in order to most effectively characterise multi-rate mass transfer behaviour. In addition, the method enables accurate simulation of relevant in situ total stress conditions during flow by selecting an appropriate centrifugal force. We demonstrate the utility of the technique for characterising the hydraulic properties of smectite-clay-dominated core samples. All core samples exhibited a non-Fickian tracer breakthrough (early tracer arrival), combined with a decrease in tracer concentration immediately after each period of interrupted flow. This is indicative of dual (or multi-)porosity behaviour, with solute migration predominately via advection during induced flow, and via molecular diffusion (between the preferential flow network(s) and the low hydraulic conductivity domain) during interrupted flow. Tracer breakthrough curves were simulated using a bespoke dual porosity model with excellent agreement between the data and model output (Nash–Sutcliffe model efficiency coefficient was > 0.97 for all samples). In combination, interrupted-flow centrifuge experiments and dual porosity transport modelling are shown to be a powerful method to characterise preferential flow in low permeability media.Australian Research CouncilNational Water CommissionGary Johnson TrustEuropean Community’s Seventh Framework Programm

Selective leaching of ecotoxic metals from lime dosing plant metalliferous ochre using acid mine drainage and organic acids

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordLime dosing in a high-density sludge (LD-HDS) process is amongst the most widely applied acid mine drainage (AMD) treatment processes worldwide. It is effective and reliable, however, an intrinsic issue for metalliferous mining derived AMD is the formation of an ecotoxic metal(loid)- bearing (e.g. As, Pb, Ni, Cd and Sb) ochre waste product. Herein the efficacy and selectivity of AMD (pH 3.5; 1-500 g/L), citric and oxalic acid (0.01-1M) for ecotoxic metal(loid) dissolution from such ochre has been investigated. When AMD is applied at a low solid-liquid ratio (namely 1 g/L) or citric and oxalic acids are at low concentrations (namely ≤0.1M) such lixiviants are selective for certain ecotoxic metals which have been designated by BCR 4-step sequential extraction as “exchangeable” (namely: Sb, Cd, Cu and Ni) in preference to Fe. This suggests that they could potentially be integrated into the LD-HDS process in order to lower the ecotoxicity of the final ochre product and thereby valorise it for use in a wide range of commercial applications. Conversely when AMD is applied at a higher solid-liquid ratio (namely ≥100 g/L) ecotoxic metal precipitation occurred (due to the alkaline pH of the ochre). This suggests that the ochre could also be reused for AMD remediation.Interreg I

Environmental behaviour of iron and steel slags in coastal settings

\ua9 The Author(s) 2024. Iron and steel slags have a long history of both disposal and beneficial use in the coastal zone. Despite the large volumes of slag deposited, comprehensive assessments of potential risks associated with metal(loid) leaching from iron and steel by-products are rare for coastal systems. This study provides a national-scale overview of the 14 known slag deposits in the coastal environment of Great Britain (those within 100 m of the mean high-water mark), comprising geochemical characterisation and leaching test data (using both low and high ionic strength waters) to assess potential leaching risks. The seaward facing length of slag deposits totalled at least 76 km, and are predominantly composed of blast furnace (iron-making) slags from the early to mid-20th Century. Some of these form tidal barriers and formal coastal defence structures, but larger deposits are associated with historical coastal disposal in many former areas of iron and steel production, notably the Cumbrian coast of England. Slag deposits are dominated by melilite phases (e.g. gehlenite), with evidence of secondary mineral formation (e.g. gypsum, calcite) indicative of weathering. Leaching tests typically show lower element (e.g. Ba, V, Cr, Fe) release under seawater leaching scenarios compared to deionised water, largely ascribable to the pH buffering provided by the former. Only Mn and Mo showed elevated leaching concentrations in seawater treatments, though at modest levels (&lt;3 mg/L and 0.01 mg/L, respectively). No significant leaching of potentially ecotoxic elements such as Cr and V (mean leachate concentrations &lt;0.006 mg/L for both) were apparent in seawater, which micro-X-Ray Absorption Near Edge Structure (μXANES) analysis show are both present in slags in low valence (and low toxicity) forms. Although there may be physical hazards posed by extensive erosion of deposits in high-energy coastlines, the data suggest seawater leaching of coastal iron and steel slags in the UK is likely to pose minimal environmental risk

A Multi-Scale Feasibility Study into Acid Mine Drainage (AMD) Monitoring Using Same-Day Observations

This is the final version. Available on open access from MDPI via the DOI in this recordGlobally, many mines emit acid mine drainage (AMD) during and after their operational life cycle. AMD can affect large and often inaccessible areas. This leads to expensive monitoring via conventional ground-based sampling. Recent advances in remote sensing which are both non-intrusive and less time-consuming hold the potential to unlock a new paradigm of automated AMD analysis. Herein, we test the feasibility of remote sensing as a standalone tool to map AMD at various spatial resolutions and altitudes in water-impacted mining environments. This was achieved through the same-day collection of satellite-based simulated Sentinel-2 (S2) and PlanetScope (PS2.SD) imagery and drone-based UAV Nano-Hyperspec (UAV) imagery, in tandem with ground-based visible and short-wave infrared analysis. The study site was a historic tin and copper mine in Cornwall, UK. The ground-based data collection took place on the 30 July 2020. Ferric (Fe(III) iron) band ratio (665/560 nm wavelength) was used as an AMD proxy to map AMD pixel distribution. The relationship between remote-sensed Fe(III) iron reflectance values and ground-based Fe(III) iron reflectance values deteriorated as sensor spatial resolution decreased from high-resolution UAV imagery (<50 mm2 per pixel; r2 = 0.78) to medium-resolution PlanetScope Dove-R (3 m2 per pixel; r2 = 0.51) and low-resolution simulated Sentinel-2 (10 m2 per pixel; r2 = 0.23). A fractioned water pixel (FWP) analysis was used to identify mixed pixels between land and the nearby waterbody, which lowered spectral reflectance. Increases in total mixed pixels were observed as the spatial resolution of sensors decreased (UAV: 2.4%, PS: 3.7%, S2: 8.5%). This study demonstrates that remote sensing is a non-intrusive AMD surveying tool with varying degrees of effectiveness relative to sensor spatial resolution. This was achieved by identifying and successfully mapping a cross-sensor Fe(III) iron band ratio whilst recognizing water bodies as reflectance inhibitors for passive sensors.European Space AgencyUniversity of Exete

Ecto-5′-nucleotidase and intestinal ion secretion by enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) triggers a large release of adenosine triphosphate (ATP) from host intestinal cells and the extracellular ATP is broken down to adenosine diphosphate (ADP), AMP, and adenosine. Adenosine is a potent secretagogue in the small and large intestine. We suspected that ecto-5′-nucleotidase (CD73, an intestinal enzyme) was a critical enzyme involved in the conversion of AMP to adenosine and in the pathogenesis of EPEC diarrhea. We developed a nonradioactive method for measuring ecto-5′-nucleotidase in cultured T84 cell monolayers based on the detection of phosphate release from 5′-AMP. EPEC infection triggered a release of ecto-5′-nucleotidase from the cell surface into the supernatant medium. EPEC-induced 5′-nucleotidase release was not correlated with host cell death but instead with activation of phosphatidylinositol-specific phospholipase C (PI-PLC). Ecto-5′-nucleotidase was susceptible to inhibition by zinc acetate and by α,β-methylene-adenosine diphosphate (α,β-methylene-ADP). In the Ussing chamber, these inhibitors could reverse the chloride secretory responses triggered by 5′-AMP. In addition, α,β-methylene-ADP and zinc blocked the ability of 5′-AMP to stimulate EPEC growth under nutrient-limited conditions in vitro. Ecto-5′-nucleotidase appears to be the major enzyme responsible for generation of adenosine from adenine nucleotides in the T84 cell line, and inhibitors of ecto-5′-nucleotidase, such as α,β-methylene-ADP and zinc, might be useful for treatment of the watery diarrhea produced by EPEC infection