Extent Of Sulfide Oxidation Research Articles

Effect of Stoichiometry on Nanomagnetite Sulfidation.

Magnetite (Mt) has long been regarded as a stable phase with a low reactivity toward dissolved sulfide, but natural Mt with varying stoichiometries (the structural Fe(II)/Fe(III) ratio, xstru) might exhibit distinct reactivities in sulfidation. How Mt stoichiometry affects its sulfidation processes and products remains unknown. Here, we demonstrate that xstru is a master variable controlling the rates and extents of sulfide oxidation by magnetite nanoparticles (11 ± 2 nm). At pH = 7.0-8.0 and the initial Fe/S molar ratio of 10-50, the partially oxidized magnetite (xstru = 0.19-0.43) can oxidize dissolved sulfide to elemental sulfur (S0), but only surface adsorption of sulfide, without interfacial electron transfer (IET), occurs on the nearly stoichiometric magnetite (xstru = 0.47). The higher initial rate and extent of sulfide oxidation and S0 production are observed with the more oxidized magnetite that has the higher electron-accepting capability from surface-complexed sulfide (S(-II)(s)). The FeS clusters formed from magnetite sulfidation can be oxidized by the most oxidized magnetite with xstru = 0.19 but not by other magnetite particles. A linear relationship between the Gibbs free energy of reaction and the surface area-normalized initial rate of sulfide oxidation is observed in all experiments under the different conditions, suggesting the S(-II)(s)-magnetite IET dominates magnetite sulfidation at high Fe/S molar ratios and near-neutral pH.

Remoción de sulfuros presentes en el agua residual del proceso de curtido mediante cavitación hidrodinámica

El sulfuro presente en el agua residual de la industria del curtido de cueros proviene de la operación de pelambre (depilado de la piel) en concentraciones que afectan significativamente la calidad del agua del cuerpo receptor debido a la toxicidad del vertimiento produciendo la mortandad de flora y fauna, responsable de la presencia de olores desagradables en el cauce y la notable disminución del oxígeno disuelto en el agua del afluente. En la presente investigación, la remoción de sulfuro es evaluada empleando la técnica cavitación hidrodinámica (CH), un proceso de oxidación avanzada analizado desde diferentes parámetros de operación como pH inicial del agua residual y presión de entrada para un tiempo de reacción fijo de 90 minutos; parámetros de diseño fueron establecidos mediante el uso de dos prototipos, variando el número y diámetro de orificios en el punto de estrangulamiento con el objetivo de determinar las condiciones óptimas del reactor para el tratamiento de este tipo de contaminante en las aguas residuales. Se obtuvo una remoción máxima del 32,6 % de la concentración inicial de sulfuro sin aplicar algún tipo de catalizador o reactivo químico; de igual modo se realizó un análisis costobeneficio debido a la eficiencia de la tecnología aplicada en comparación con la energía requerida por el reactor empleado, donde se observa una disminución del 198 % del costo total del tratamiento actual. Finalmente, la cavitación hidrodinámica es una tecnología sustentable para la industria del curtido de cueros.

Arsenic release from chlorine-promoted alteration of a sulfide cement horizon: Evidence from batch studies on the St. Peter Sandstone, Wisconsin, USA

► We conducted laboratory experiments to examine chlorine-promoted sulfide oxidation. ► Chlorine increases oxidation of sulfides, releasing As. ► Chlorine increases ferrous iron oxidation, resulting in iron oxide formation to which As sorbs. ► Iron oxides can be sinks for As but can also release As if biogeochemical conditions change. ► Results have implications for aquifer storage and recovery in sulfide-bearing aquifers. Elevated As concentrations have been measured in wells in the St. Peter Sandstone aquifer of eastern Wisconsin, USA. The primary source is As-bearing sulfide minerals (pyrite and marcasite) within the aquifer. There is concern that well disinfection by chlorination may facilitate As release to groundwater by increasing the rate and extent of sulfide oxidation. The objective of this study was to examine the abiotic processes that mobilize As from the aquifer solids during controlled exposure to chlorinated solutions. Thin sections made from sulfidic aquifer material were characterized by quantitative electron probe micro-analysis before and after 24 h exposure to solutions of different Cl 2 concentrations. Batch experiments using crushed aquifer solids were also conducted to examine changes in solution chemistry over 24 h. Results of the combined experiments indicate that Cl 2 addition affects As release and uptake in two ways. First, Cl 2 increases oxidation of sulfide minerals, releasing more As from the mineral structure. Chlorine addition also increases the rate of Fe(II) oxidation and subsequent hydrous ferric oxide (HFO) precipitation, allowing for increased uptake of As onto the mineral surface. Although HFOs can act as sinks for As, they can release As if biogeochemical conditions (e.g. redox, pH) change. These results have implications not only for disinfection of drinking water wells in the study area, but also suggest that introduction of oxidants may adversely affect water quality during aquifer storage and recovery programs in aquifers containing As-bearing minerals.

Modeling pyrite bioleaching in isothermal test columns with the HeapSim model

The HeapSim model developed to design heap leach processes was employed herein to evaluate unknown parameters and to identify the rate-controlling steps governing a simple leach system consisting of only pyrite under isothermal conditions. The temperature at which the column tests were performed encompassed the range of the mesophilic cells (15–40 °C), moderate thermophilic cells (30–55 °C), and extreme thermophilic cells (50–80 °C). The ore-, geometry-, and hydrology-related parameters characteristic of the column tests were known from previous experiments. This left only the biological parameters of iron- and sulfur-oxidizing cells and the oxygen gas–liquid mass transfer rate to be found by trial and error from simultaneous best fits of five important leach data sets: extent of sulfide oxidation, effluent solution potential, iron concentration, cell numbers, and sulfur grade. The challenge was to find a unique value of the oxygen mass transfer rate common to all temperatures. Good to excellent fits of the leach indicators were obtained, while the values of the parameters were largely within the ranges expected. The model revealed the rate-limiting step to shift from particle kinetics to oxygen gas–liquid mass transfer with increasing temperature, increasing proportion of fine pyrite grains, and higher pyrite head grades. Competition for oxygen between sulfur- and iron-oxidizing microorganisms lowered potentials and retarded pyrite oxidation.

Patterns of water chemistry and discharge in the glacier-fed Kennicott River, Alaska: evidence for subglacial water storage cycles

Runoff from the temperate Kennicott Glacier in the Wrangell Mountains, Alaska, displays distinctive oscillations in discharge, electrical conductivity, and solute concentrations during the summer melt season. In addition to daily fluctuations, the Kennicott River undergoes quasi-cyclic changes in discharge over 12- to 18-day periods probably linked to weather-induced variations in melt. Solute variations are lagged from discharge by several days; solute concentrations peak during falling discharge, and reach minima during rising discharge. We explore probable weathering reactions through mass balance analysis. The reactions responsible for most solute fluctuations are halite dissolution and H 2CO 3-driven calcite dissolution, while sulfide oxidation and associated calcite dissolution are significant but steady components of the solute flux. Halite dissolution is probably limited by the glacial abrasion rate. Calcite dissolution by carbonation co-varies directly with the calculated P CO 2 of the runoff. This result is not expected in an environment that is poorly connected to the atmosphere; a possible explanation is that respiration by subglacial microbes produces CO 2 at the glacier bed. The extent of sulfide oxidation is limited by availability of O 2, and the lack of temporal variation in this reaction suggests that this limit operates in both the distributed system and subglacial conduits. All solute variations are consistent with changes in subglacial water storage and are therefore indicative of changing efficiency of subglacial drainage through time.

Effects of Nickel Mining Activities on Water Quality

Acid Mine Drainage (AMD) originates from the sulphide mineral oxidation in the presence of water and oxygen, and to some extent bacteria, yielding sulphuric acid. The acid generated mobilizes some metals resulting in their increased bioavailabilities, creating a hazardous environment for aquatic animals and plants. The groundwater quality emanating from adjacent mine sites is a useful tool for measuring AMD. The purpose of this study was to determine extent of mine drainage from an active nickel mine and to develop mitigation. Major mining pollution sources were identified for constant monitoring. Recommendations for possible remediation and abatement of AMD were made. Several liquid samples were taken from selected sites along two streams, which flow out of the mining complex. Eight water samples were collected once a month from June to October. pH and conductivity were immediately measured followed by determination of Cu, Fe, Ni, sulphates, Total Suspended Solids (TSS) and Total Dissolved Solids (TDS). Elevated concentrations of sulphate, high conductivity and TDS were observed indicating the presence of AMD. An increase in sulphate concentration, TDS and Conductivity levels for the Community Rivers was observed. This was caused by the water flowing from the nickel mine. The quality of the natural river water was grossly affected. A passive treatment strategy would need to be developed. For the treatment process to succeed, a complete characterisation of the contaminated mine drainage is needed. Topsoiling and revegetation are other cheaper methods that can be used to reduce the extent of sulphide oxidation.

Batch biooxidation of a gold-bearing pyrite-arsenopyrite concentrate

Batch biooxidation data were obtained for a gold-bearing arsenopyrite-pyrite concentrate at 12% solids, pH of 1.6 and temperature of 40°C. The specific rates of biooxidation of arsenopyrite and pyrite were found to be very similar at 0,15 d −1 or as mass rates, for arsenopyrite 2,1 kg m −3 d −1 and pyrite 6,5 kg m −3 d −1. However, it was found that the biooxidation of arsenopyrite preceded that of pyrite and was almost complete before the oxidation of pyrite commenced. Data were also obtained for four narrow size fractions prepared from the bulk concentrate. Sequential oxidation of arsenopyrite and pyrite was observed for all size fractions. The oxidation rate obtained from the linear portion of the curve was found to have a linear dependence on surface area concentration suggesting that area-based rates are more appropriate for characterizing biooxidation kinetics. The logistic equation was found to be a good fit to all the data obtained. It was found that gold liberation was linearly dependent upon the extent of arsenic oxidation. However, sulphide oxidation appeared to be preferential in the gold-rich regions, leading to a non-linear dependence of gold liberation on extent of sulphide oxidation.

Patterns of sulfate reduction and the sulfur cycle in a South Carolina salt marsh1,2

Rates of sulfate reduction and the magnitude of various sediment parameters related to the sulfur cycle were measured at about quarterly intervals for a 3‐yr period at sites in a South Carolina salt marsh containing tall‐ or short‐form Spartina alterniflora. Sulfate reduction rates were greater at all times in the short‐form sites and decreased markedly with depth. Seasonal changes were highly correlated with ambient temperature except in May and August when interactions with plants may have affected rates directly. In the short‐form site, the annual integrated sulfate reduction rate accounted for about 47.5–67.0% of the estimated belowground plant production. Dissolved sulfides and pH varied seasonally in the short‐ but not tall‐form sites; seasonal minima in the short‐form site occurred during May and were probably due to oxidation of the sediment with the onset of plant production. Mechanisms resulting in oxidation were not evident however. Chromium‐reducible sulfur, which is primarily pyrite, was substantially greater in the tall‐ than short‐form site but apparently stable temporally at both sites. The differences in pool sizes of pyrite were probably due to differences in iron dynamics and the extent of sulfide oxidation. Microprofiles of dissolved oxygen were consistent with greater oxidation at the short‐form site since the penetration of oxygen was deeper. Differences in sediment texture and tidal inundation may have contributed to greater oxygen inputs at the short‐form site.