Acid Generation Rate Research Articles

Are there any session-to-session changes in ventilation during a weekly hemodialysis cycle?

Significant changes in pre-dialytic partial pressure of CO2 (pCO2) during a week-long cycle of hemodialysis (HD) can be an effect of the intermittent supplementation of bicarbonate to correct chronic acidosis in patients. Mathematical modeling efforts carried out using the same parameters before each HD session might fail to produce accurate predictions of pCO2 and plasma bicarbonate concentration (CBic) because of this variability. A numerical model describing acid-base equilibrium changes during HD was applied to predict pCO2, pH, and CBic in 24 chronic HD patients, using both fixed parameters for the whole week and estimating a new value of minute ventilation (VE) and net acid generation rate (GH) for each interdialytic interval. Dialysances of bicarbonate and dissolved CO2 were also estimated independently for each HD session. The error of the model compared to the pre-dialytic data of CBic and pCO2 significantly decreased when VE and GH were estimated piecewise throughout the week. To fit the data, VE changed from 3.9 ± 1.0 mL/min before HD1, to 3.8e1 mL/min after HD1, 3.6 ± 1.0 mL/min after HD2, and 3.9 ± 1.1 mL/min after HD3 (p < 0.05). GH changes after each session were not statistically significant. VE values strongly correlated with pre-dialytic pCO2 (Spearman's ρ = -0.97), but GH only weakly correlated with pre-dialytic CBic (ρ = -0.30). Acid-base equilibrium is extremely sensitive to respiratory regulation. When attempting to predict the evolution of pCO2 a CBic during the HD cycle, changes in the respiration parameters must be accounted for by the model, at the risk of a significant loss of prediction accuracy.

Built-in electric field in NiO-CuO heterostructures to regulate the hydroxide adsorption sites for 5-hydroxymethylfurfural electrooxidation assisted hydrogen production

The development of catalysts with suitable adsorption behavior for the reaction molecules and the elucidation of their internal structure-adsorption-catalytic activity relationships are crucial for the electrooxidation of 5-hydroxymethylfurfural (HMF). In this work, NiO-CuO heterostructures with a spontaneous built-in electric field (BEF) are specifically designed and used to regulate the OH− adsorption site for freeing up the active site of HMF for the HMF oxidation reaction (HMFOR). The mechanism driving electron pumping/accumulation of the BEF is examined by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Electrochemical data and theoretical calculations show that BEF modulates the adsorption energy and adsorption site of substrate molecules, thereby enhancing the performance of HMFOR and hydrogen evolution reaction (HER). Notably, the NiO-CuO electrode demonstrates high 2,5-Furandicarboxylic acid (FDCA) selectivity (99.76 %) and generation rate (13.79 mmol gcat−1 h−1). It only requires 1.33 V to obtain a current density of 10 mA cm−2 for HMFOR-coupled H2 evolution. This research introduces a novel approach by regulating the adsorption of reactive molecules for HMFOR-assisted H2 evolution.

Electricity generation and acid and alkaline recovery from pickled waters/wastewaters through anaerobic digestion, bipolar membrane electrodialysis and solid oxide fuel cell hybrid system

Pickled industrial wastewater (PIWW) containing concentrated organics, ammonia, phosphorus and NaCl is a bioenergy and renewable resource. This study developed a hybrid system of anaerobic digestion (AD), bipolar membrane electrodialysis (BMED) and solid oxide fuel cells (SOFC) to recover biogas and ammonia as renewable fuels, and generate acid and alkaline from four pickle waters/wastewaters. The results demonstrated that anaerobic sludge particles were granulated in hypersaline circumstances and halophilic Bacteroidetes and Firmicutes were two dominant phyla. AD converted 70% of chemical oxygen demand (COD) to biogas and methane yield was 0.051 L-CH4 g-COD−1 on average. The maximum desalination, acid and alkaline generation rates achieved by BMED were 0.304 mol h−1, 0.114 mol h−1, and 0.136 mol h−1, respectively. SOFC used recovered biogas and NH3/H2 and output 500 mW cm−2 and 530 mW cm−2 of peak power densities, respectively. AD-BMED-SOFC hybrid system finally gained 55% of maximum net energy efficiency for the 3rd pickled water. These results signify that AD-BMED-SOFC hybrid system is more feasible and potential when pickled waters are independently treated from pickled wastewaters.

Photoacid Generators Activated through Sequential Two-Photon Excitation: 1-Sulfonatoxy-2-alkoxyanthraquinone Derivatives.

Two sulfonate ester derivatives of anthraquinone, 1-tosyloxy-2-methoxy-9,10-anthraquinone (1a) and 1-trifluoromethylsulfonoxy-2-methoxy-9,10-anthraquinone (1b), were prepared and their ability to produce strong acids upon photoexcitation was examined. It is shown that these compounds generate acid with a yield that increases with light intensity when the applied photon dose is held constant. Additional experiments show that the rate of acid generation increases fourfold when visible light (532 nm) laser pulses are combined with ultraviolet (355 nm) compared with ultraviolet alone. Continuous wave diode laser photolysis also affects acid generation with a rate that depends quadratically on the light intensity. Density functional theory calculations, laser flash photolysis, and chemical trapping experiments support a mechanism, whereby an initially formed triplet state (T1) is excited to a higher triplet state which in turn undergoes homolysis of the RS(O2)-OAr bond. Secondary reactions of the initially formed sulfonyl radicals produce strong acids. It is demonstrated that high-intensity photolysis of either 1a or 1b can initiate cationic polymerization of ethyl vinyl ether.

Effects of Liquid Smoking on the Microbiological and Physicochemical Characteristics of Suan Yu, a Traditional Chinese Fermented Fish Product

ABSTRACT This work studied the changes in microbiological and physiochemical properties during the fermentation of Suan yu by liquid smoking and traditional smoking (batches B, C, and A). Lactobacillus plantarum, Pediococcus pentosaceus, Saccharomyces cerevisiae, and Hansenula polymorpha were the dominant microorganisms in the three samples. Only the acid-resistant Staphylococcus xylosus and Staphylococcus carnosus survived in the acid environment of the final fermented products. The amounts of Enterobacteriaceae, Enterococci, and Pseudomonades in the final fermented products were under the detection limit. Batch B and C samples had higher rate of reduction in pH (4.24 ± 0.17–4.39 ± 0.18 from 6.21 ± 0.24, respectively) and lower acetic acid generation rate (0.06%±0. 00% and 0.05%±0. 00%, respectively) than batch A samples. Large amounts of free amino acids were generated during the fermentation of the samples. Batches B and C had thiobarbituric acid reactive substances (TBARS) value between 0.88 ± 0.03 and 0.89 ± 0.03, total volatile basic nitrogen (TVB-N) value between 18.9 ± 0.34 and 18.8 ± 0.33 mg/kg, and benzo(a)pyrene (BaP) content of 0.1 ± 0.00 μg/kg, which was lower than those of fermented Suan yu prepared by traditional smoking. Sensory analysis indicates that these sample groups have unique smoking and fermentation flavor. Consequently, the fermented Suan yu produced through liquid smoking is a kind of fermented aquatic product with full-bodied smoking flavor, palatable acid flavor, abundant nutrition, and high safety.

Oxidative Dissolution of Sulfide Minerals in Single and Mixed Sulfide Systems under Simulated Acid and Metalliferous Drainage Conditions.

Chalcopyrite, galena, and sphalerite commonly coexist with pyrite in sulfidic waste rocks. The aim of this work was to investigate their impact, potentially by galvanic interaction, on pyrite oxidation and acid generation rates under simulated acid and metalliferous drainage conditions. Kinetic leach column experiments using single-minerals and pyrite with one or two of the other sulfide minerals were carried out at realistic sulfide contents (total sulfide <5.2 wt % for mixed sulfide experiments), mimicking sulfidic waste rock conditions. Chalcopyrite was found to be most effective in limiting pyrite oxidation and acid generation with 77-95% reduction in pyrite oxidation over 72 weeks, delaying decrease in leachate pH. Sphalerite had the least impact with reduction of pyrite dissolution by 26% over 72 weeks, likely because of the large band gap and poor conductivity of sphalerite. Galena had a smaller impact than chalcopyrite on pyrite oxidation, despite their similar band gaps, possibly because of the greater extent of oxidation and the significantly reduced surface areas of galena (area reductions of >47% for galena vs <1.5% for chalcopyrite) over 72 weeks. The results are directly relevant to mine waste storage and confirm that the galvanic interaction plays a role in controlling acid generation in multisulfide waste even at low sulfide contents (several wt %) with small probabilities (≤0.23%) of direct contact between sulfide minerals in mixed sulfide experiments.

Dechlorination of 2,4‐dichlorophenoxyacetic acid using biochar‐supported nano‐palladium/iron: Preparation, characterization, and influencing factors

In the present study, peanut shell, a green waste raw material, was used to prepare biochar (BC) and to obtain BC‐supported nano‐palladium/iron (BC‐nPd/Fe) composites for removing 2,4‐dichlorophenoxyacetic acid (2,4‐D) from water. Characterization analysis demonstrated that nPd/Fe particles were well dispersed on the BC surface with weakened magnetic properties. The average particle diameter and specific surface area of nPd/Fe were 101.3 nm and 6.7 m2 g−1, whereas the corresponding values of the BC‐nPd/Fe materials were 88.8 nm and 14.8 m2 g−1, respectively. Several factors were found to influence the dechlorination of 2,4‐D, including the weight ratio of BC to Fe, Pd loading ratio, initial solution pH, 2,4‐D concentration, and reaction temperature. Dechlorination results indicated that the 2,4‐D removal and phenoxyacetic acid (PA) generation rates were 44.1% and 20.1%, respectively, in the nPd/Fe system, and 100.0% and 92.1%, respectively, in the BC‐nPd/Fe system. The dechlorination of 2,4‐D was well described by the pseudo‐first‐order kinetic model (R2 &gt; 0.97), and the observed rate constants kobs were 0.0042 min (nPd/Fe) and 0.0578 min (BC‐nPd/Fe), respectively. The reaction mechanism indicated that the dechlorination hydrogenation was the main process to remove 2,4‐D from water in the BC‐nPd/Fe system. In addition, BC inhibited the formation of a passivation layer on the particle surface during the reaction, thus maintaining the high reactivity of BC‐nPd/Fe. The easy preparation technique, high 2,4‐D dechlorination capacity, and mild reaction conditions suggest that BC‐nPd/Fe may be a promising alternative composite to remove 2,4‐D from water.

Optimizing serum total carbon dioxide concentration during short and nocturnal frequent hemodialysis using lactate as dialysate buffer base.

Definitive clinical studies to determine the optimal dialysate lactate concentration to prescribe during frequent hemodialysis when using the NxStage System One dialysis delivery system at low dialysate flow rates have not been reported. We used clinical data from patients who transferred from in-center thrice-weekly hemodialysis (ICHD) to daily home hemodialysis using the NxStage System One and the H+ mobilization model to calculate acid generation rates in patient sub-groups during the FREEDOM study. Assuming those acid generation rates were representative, we then predicted using the H+ mobilization model the effect of using dialysate lactate concentrations of 40 and 45 mEq/L on predialysis serum total carbon dioxide (tCO2 ) concentrations in patients who transfer from ICHD to short and nocturnal frequent hemodialysis prescriptions used in current clinical practice; the prescriptions evaluated varied by treatment frequency, dialysate volume per treatment, and treatment times. With frequencies of four to six treatments per week and treatment times of 170 to 210 minutes per treatment, the effect of dialysate lactate concentration was primarily dependent on weekly dialysate volume. For weekly dialysate volumes of 150 to 160 L per week, use of dialysate lactate concentrations of 45 mEq/L, but not 40 mEq/L, resulted in an increase of predialysis serum tCO2 concentration. When longer treatment times typical of nocturnal frequent hemodialysis were evaluated, model predictions showed that the use of dialysate lactate concentration of 45 mEq/L may not be appropriate for many patients because of excessive increases in predialysis serum tCO2 concentration. Reducing dialysate volume from 60 to 30 L may limit the increase in predialysis serum tCO2 concentration when patients transfer from ICHD to nocturnal frequent hemodialysis. Predictions from the H+ mobilization model show that dialysate lactate concentration and weekly dialysate volume are the primary prescription parameters for optimizing predialysis serum tCO2 concentration during short and nocturnal frequent hemodialysis.

The evaluation of acid mine drainage by kinetic procedures and empirical models for field scale behaviour

Acid mine drainage is an important environmental hazard due to its serious chemical contamination to the surface and groundwater resources. To provide enough and representative data for developing restoration techniques, this time-dependent geochemical process should be investigated based on kinetic principles. Thus, the kinetic column test generally utilized in the earlier studies without any accepted procedures related to the column dimension and properties of materials such as particle size and mass of samples to be tested for this purpose. To overcome the dissimilarities between the mass release rate of contaminants specified in the laboratory and in the field, and to upscale laboratory-based measurements to the field, kinetic column tests were performed by using different columns filled with crushed coarse and fine ore samples. The fluctuations of pH values and the concentration of various constituents were determined during kinetic column tests. The effluents of columns turned to acid after a lag time of between 21 and 65 weeks depending on the column dimensions and particle size. Statistically significant predictive models for upscaling geochemical behaviour of AMD processes were presented based on simple and multiple regression analyses among column dimensions and main parameters controlling the rate of acid generation.

Bile acid kinetic modeling in end-stage liver support patients

Background & AimsSolute generation rates, distribution volumes and compartment effects control the in vivo efficiency of any extracorporeal therapy such as extracorporeal liver support (ELS) used to remove bile acids accumulating in acute-on-chronic liver patients. The aim of this study was to identify and to examine kinetic parameters of two major bile acids using mathematical modeling. MethodsThe kinetics of cholic (CA) and chenodeoxycholic acid (CDCA) were described by one- and two-compartment models with central elimination by decreasing or constant extracorporeal clearance, constant bile acid generation rate, and constant apparent distribution volume. Concentration profiles collected in 13 ELS sessions done in 8 patients were included for model calculations ResultsFor the one-compartment model, the average volumes and generation rates were 30 ± 6 [l], 0.19 ± 0.06 [μmol/min] for CA and 22 ± 5 [l], 0.29 ± 0.08 [μmol/min] for CDCA, respectively. For the one-compartment model and average normalized concentrations, the volumes and generation rates were 25 [l], 0.28 [μmol/min] for CA and 18 [l], 0.37 [μmol/min] for CDCA, respectively. For the two-compartment model, average normalized concentrations, the same initial concentration in both compartments, and assuming a 10% post-treatment rebound, the volume and generation rates were 25 [l], 0.27 [μmol/min] for CA and 19 [l], 0.32 [μmol/min] for CDCA, respectively. ConclusionsThe generation rate for CDCA is higher when compared to that of CA and independent of the number of compartments. Assuming a constant extracorporeal clearance overestimates generation rate and distribution volume. The kinetic parameters of one- and two-compartment models are comparable for the same bile acid.

Acid-base kinetics during hemodialysis using bicarbonate and lactate as dialysate buffer bases based on the H+ mobilization model.

The H+ mobilization model has been recently reported to accurately describe intradialytic kinetics of plasma bicarbonate concentration; however, the ability of this model to predict changing bicarbonate kinetics after altering the hemodialysis treatment prescription is unclear. We considered the H+ mobilization model as a pseudo-one-compartment model and showed theoretically that it can be used to determine the acid generation (or production) rate for hemodialysis patients at steady state. It was then demonstrated how changes in predialytic, intradialytic, and immediate postdialytic plasma bicarbonate (or total carbon dioxide) concentrations can be calculated after altering the hemodialysis treatment prescription. Example calculations showed that the H+ mobilization model when considered as a pseudo-one-compartment model predicted increases or decreases in plasma total carbon dioxide concentrations throughout the entire treatment when the dialysate bicarbonate concentration is increased or decreased, respectively, during conventional thrice weekly hemodialysis treatments. It was further shown that this model allowed prediction of the change in plasma total carbon dioxide concentration after transfer of patients from conventional thrice weekly to daily hemodialysis using both bicarbonate and lactate as dialysate buffer bases. The H+ mobilization model can predict changes in plasma bicarbonate or total carbon dioxide concentration during hemodialysis after altering the hemodialysis treatment prescription.

Non-Steady State and Steady State Silicate Dissolution: Non- Carbonate Acid Neutralisation for Long-Term Acid and Metalliferous Drainage Control

The dissolution of silicate minerals has been largely examined under steady state conditions. The primary aim of this study was to understand the potential of the non-steady state dissolution of silicate minerals in treatment of acid and metalliferous drainage (AMD) resulting predominantly from pyrite oxidation. To this end, flow-through dissolution cell experiments were carried out using selected silicate minerals (biotite, chlorite, olivine and K-feldspar), all commonly found in AMD environments, under various pH and flow rate conditions, for comparison to pyrite dissolution carried out under the same conditions. Both acid generation rate (pyrite) and steady-state and non-steady state acid neutralisation rates (silicates) were calculated and compared. Results showed that the non-steady state acid neutralisation rates due to silicate dissolution were greater than the steady-state neutralisation rates and that all silicate minerals investigated in this study, except K-feldspar, can provide acid neutralisation rates to match the acid generation rate due to pyrite dissolution under certain conditions.

Unexpected Non-acid Drainage from Sulfidic Rock Waste

Most rock extraction sites, including mine sites and building construction sites, require a plan to assess, and mitigate if present, the risk of acid mine drainage (AMD). AMD is typically the major environmental concern where sulfide minerals are present in the excavated material and AMD prediction and remediation is based on internationally-accepted acid-base accounting (ABA) tests of representative field samples. This paper demonstrates that standardized ABA tests may not always be provide the correct AMD classification for commonly occurring waste rocks containing low-pyrite and -carbonate due to mineralogic assumptions inherent in their design. The application of these standard ABA tests at a copper mine site in South Australia resulted in the classification of a portion of its waste material as potentially acid forming in apparent contradiction to long term field measurements. Full definition of the sulfide and silicate minerals enabled re-evaluation of the weathering reactions occurring. The overall rate of neutralisation due to silicate dissolution was found to always exceed the rate of acid generation, in agreement with field observations. Consequently, the waste rock was redefined as non-acid forming. The methods developed represent a significant advance in AMD prediction and more strategic, cost-effective environmental planning, with potential for reclassification of wastes with similar characteristics.

Evaluation of the rate of dissolution of secondary sulfate minerals for effective acid and metalliferous drainage mitigation

The presence of latent acidity in the form of the jarosite-alunite mineral group complicates assessment of the rates of acid and metalliferous drainage (AMD) generation from potentially acid-forming waste rocks and tailings. To enable better prediction of the contributions from these secondary sulfate minerals, the relationships between pH and the rate of dissolution for natrojarosite and alunite, for comparison with pyrite, have been determined.In single mineral unstirred systems, the dissolution rates of natrojarosite and alunite in the pH range 2–7 were in the range of 10−14 to 10−13 mol m−2 s−1 and were found to be slowest at pH 3.4 and 5.5, respectively. At pH above these respective minima, alunite and natrojarosite dissolution results in acid release. The surface area-normalised dissolution rates of both minerals were 2–3 orders of magnitude slower than that of pyrite under the same conditions, and were 2–4 orders of magnitude slower than the rates of stirred dissolution of synthetic jarosite and alunite reported in the literature. In mixed mineral mini-column dissolution tests, pyrite oxidation proceeded at almost the same rate in the longer term (>50 days), regardless of the presence or absence of natrojarosite or alunite. As the presence of these secondary sulfate minerals does not impact significantly on pyrite oxidation, their contributions to AMD can be taken into account separately enabling accurate calculation of the neutralisation rates needed to effectively manage AMD.

Ultrafast fabrication of nanostructure WO3 photoanodes by hybrid microwave annealing with enhanced photoelectrochemical and photoelectrocatalytic activities

Tungsten oxide (WO3) nanoplate films with a relatively rough surface were successfully fabricated via a simple hydrothermal method, followed by the hybrid microwave annealing (HMA) treatment only a few minutes for the first time. The microscopic morphology and phase were characterized by scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM), Raman spectrum and X-ray diffraction (XRD). Photoelectrochemical measurements demonstrated that the obtained WO3 film of microwave processing for 11 min exhibited the photocurrent density of 1.60 mA/cm2 at 1.2 V (vs. Ag/AgCl) and the IPCE value of 55% at 355 nm under an applied voltage of 1.0 V (vs. Ag/AgCl), which were about 3 and 2.5 times compared with the WO3 film prepared by the conventional annealing method, respectively. Moreover, the WO3-HMA films were applied to the versatile photoanode-driven photoelectrochemical system for CO2 reduction into formic acid. The maximum formic acid generation rate and faradaic efficiency of the WO3-HMA films were 9.21 μmol h−1 cm−2 and 45.45%, respectively. This study provided a facial and rapid method to synthesize the high-performance WO3 photoanodes with better photoelectrochemical and photoelectrocatalytic activities.

The Effects of Galvanic Interactions with Pyrite on the Generation of Acid and Metalliferous Drainage.

Although the acid generating properties of pyrite (FeS2) have been studied extensively, the impact of galvanic interaction on pyrite oxidation, and the implications for acid and metalliferous drainage, remain largely unexplored. The relative galvanic effects on pyrite dissolution were found to be consistent with relative sulfide mineral surface area ratios with sphalerite (ZnS) having greater negative impact in batch leach tests (sulfide minerals only, controlled pH) and galena (PbS) having greater negative impact in kinetic leach column tests (KLCs, uncontrolled pH, >85 wt% silicate minerals). In contrast the presence of pyrite resulted consistently in greater increase in galena than sphalerite leaching suggesting that increased anodic leaching is dependent on the difference in anodic and cathodic sulfide mineral rest potentials. Acidity increases occurred after 44, 20, and 12 weeks in the pyrite-galena, pyrite-sphalerite, and the pyrite containing KLCs. Thereafter acid generation rates were similar with the Eh consistently above the rest potential of pyrite (660 mV, SHE). This suggests that treatment of waste rocks or tailings, to establish and maintain low Eh conditions, may help to sustain protective galvanic interactions and that monitoring of Eh of leachates is potentially a useful indicator for predicting changes in acid generation behavior.

The Formation of Silicate-Stabilized Passivating Layers on Pyrite for Reduced Acid Rock Drainage.

Acid and metalliferous release occurring when sulfide (principally pyrite)-containing rock from mining activities and from natural environments is exposed to the elements is acknowledged as a major environmental problem. Acid rock drainage (ARD) management is both challenging and costly for operating and legacy mine sites. Current technological solutions are expensive and focused on treating ARD on release rather than preventing it at source. We describe here a viable, practical mechanism for reduced ARD through the formation of silicate-stabilized iron oxyhydroxide surface layers. Without silicate, oxidized pyrite particles form an overlayer of crystalline goethite or lepidocrocite with porous structure. With silicate addition, a smooth, continuous, coherent and apparently amorphous iron oxyhydroxide surface layer is observed, with consequent pyrite dissolution rates reduced by more than 90% at neutral pH. Silicate is structurally incorporated within this layer and inhibits the phase transformation from amorphous iron (oxy)hydroxide to goethite, resulting in pyrite surface passivation. This is confirmed by computational simulation, suggesting that silicate-doping of a pseudoamorphous iron oxyhydroxide (ferrihydrite structure) is thermodynamically more stable than the equivalent undoped structure. This mechanism and its controlling factors are described. As a consequence of the greatly reduced acid generation rate, neutralization from on-site available reactive silicate minerals may be used to maintain neutral pH, after initial limestone addition to achieve neutral pH, thus maintaining the integrity of these layers for effective ARD management.

Strategies for Reduced Acid and Metalliferous Drainage by Pyrite Surface Passivation

Acid and metalliferous drainage (AMD) is broadly accepted to be a major global environmental problem facing the mining industry, requiring expensive management and mitigation. A series of laboratory-scale kinetic leach column (KLC) experiments, using both synthetic and natural mine wastes, were carried out to test the efficacy of our pyrite passivation strategy (developed from previous research) for robust and sustainable AMD management. For the synthetic waste KLC tests, initial treatment with lime-saturated water was found to be of paramount importance for maintaining long-term circum-neutral pH, favourable for the formation and preservation of the pyrite surface passivating layer and reduced acid generation rate. Following the initial lime-saturated water treatment, minimal additional alkalinity (calcite-saturated water) was required to maintain circum-neutral pH for the maintenance of pyrite surface passivation. KLC tests examining natural potentially acid forming (PAF) waste, with much greater peak acidity than that of the synthetic waste, blended with lime (≈2 wt %) with and without natural non-acid-forming (NAF) waste covers, were carried out. The addition of lime and use of NAF covers maintained circum-neutral leachate pH up to 24 weeks. During this time, the net acidity generated was found to be significantly reduced by the overlying NAF cover. If the reduced rate of acidity production from the natural PAF waste is sustained, the addition of smaller (more economically-feasible) amounts of lime, together with application of NAF wastes as covers, could be trialled as a potential cost-effective AMD mitigation strategy.

Kinetics of sulfide mineral oxidation in seawater: Implications for acid generation during in situ mining of seafloor hydrothermal vent deposits

Growth in global metal demand has fostered a new age of unconventional mining on the seafloor. In situ pulverization and extraction of seafloor massive sulfide (SMS) deposits is economically attractive due to minimal overburden and high ore grades. However, important environmental questions remain on the significance of localized acid generation via irreversible sulfide mineral oxidation. Data on the reaction kinetics are necessary to estimate anthropogenic acid production during seafloor mining.Laboratory experiments were performed to evaluate the effects of pH, temperature, dissolved oxygen, and surface area on the oxidation rate of pyrrhotite and chalcopyrite in seawater. These minerals were chosen to constrain the range of reaction rates because pyrrhotite oxidizes relatively quickly while chalcopyrite is kinetically slow. The rate laws for the abiotic oxidation of pyrrhotite and chalcopyrite in seawater at 22 °C are given in the form:Rsp=K(mO2)a(mH+)bwhere Rsp is the specific rate (moles m−2 sec−1), k is the rate constant, oxygen and proton concentrations are expressed in molalities (m), and their reaction orders as a and b, respectively. The specific rate laws obtained for each sulfide studied are:Rsp(pyrrhotite)=−10−7.27(mO2(aq))0.51±0.08(mH+)0.08±0.03Rsp(chalcopyrite)=−10−9.38(mO2(aq))1.16±0.03(mH+)0.36±0.09When used to quantitatively predict maximum acid generation rates, these rate laws indicate that acid production from in situ SMS mining is insufficient to exceed the buffering capacity of advecting seawater. We also calculated the residence times of crushed sulfides in seawater with low PO2 (0.10 atm, pH of 8, 23 °C) and find that, depending on grain size, mining waste may persist near the seafloor for years. The implications are positive in terms of slow acid production, but potentially problematic considering the potential ecological effects of an unnatural influx of particulates.

Assessing Environmental Risks Associated with Ultrafine Coal Wastes Using Laboratory-Scale Tests

Characterisation of the risk of acid rock drainage is typically achieved through the quantification of acid-generating and acid-consuming components present within a sample using initial laboratory-scale, chemical static tests. Such tests, however, consider ARD generation under chemical conditions and do not account for the role of micro-organisms. Their focus is exclusively on the net potential for acid generation, with no account of metal deportment or the relative rate of acid generation and consumption. The present study investigates the ARD potential of two ultrafine coal wastes samples using the standard static tests as well as the UCT biokinetic test to account for microbial ARD generation. The deportment of metal species under each test condition was also considered. The UCT biokinetic test results supported the static test classification, adding provided preliminary kinetic data on the ARD generation. Sequential chemical extraction tests allowed for differentiation of the host minerals according to their leaching potentials, providing supporting evidence for the deportment of metal species under the characterisation tests, thereby improving the knowledge base on which to classify coal wastes as benign or otherwise.