Acid-producing Reactions Research Articles

Chemo-hydro-mechanical effects of CO2 injection into a permeable limestone

CO2 storage in carbonate aquifers is a way to mitigate atmospheric CO2 emissions, but it leads to acid-producing reactions that may induce alterations in the rock pore structure and hydromechanical properties. To gain a better understanding of these changes in rock properties, percolation experiments under constant flow-rate conditions with (i) CO2-saturated water (PCO2 = 100 bar and T = 60 °C) and (ii) HCl solutions (P = 1 bar and T = 60 °C) were performed on centimetric cores of the grain-supported Pont du Gard Limestone (permeability about 10−14–10−13 m2). Effluent chemistry analyses, X-ray imaging, and measurements of the hydromechanical properties of intact and altered specimens were used to quantify acid-induced changes in the two acid-rock systems. Experimental results show that the rock dissolution patterns highly depend on the acid type and pore space heterogeneity. Under the flow conditions of these experiments, complete HCl dissociation (strong acid) and rapid HCl-rock reaction result in a compact dissolution pattern that only affects the hydromechanical properties of the core inlet. Conversely, partial dissociation of H2CO3 (weak acid) produces chemical reactions all along the core. Initial structural heterogeneities localize chemical reactions along preferential flow pathways, causing instabilities in the reaction front and wormhole formation, which markedly enhances permeability. A powerlaw relationship with a very large exponent (as high as 24) accounts for the variation in permeability as a function of porosity. Altered cores render both a significant attenuation in mechanical rock properties and ultrasonic velocities, which is reproduced using a Differential Effective Medium (DEM) homogenization approach. The high-pressure percolation experiments of this study using CO2-saturated water represent an extreme scenario of CO2-brine-rock interactions in carbonate reservoirs, which needs to be considered to improve the prediction and monitoring of the storage performance.

Functions of membrane-bound alcohol dehydrogenase and aldehyde dehydrogenase in the bio-oxidation of alcohols in Gluconobacter oxydans DSM 2003

In this study a new insight was provided to understand the functions of membrane-bound alcohol dehydrogenase (mADH) and aldehyde dehydrogenase (mALDH) in the bio-oxidation of primary alcohols, diols and poly alcohols using the resting cells of Gluconobacter oxydans DSM 2003 and its mutant strains as catalyst. The results demonstrated that though both mADH and mALDH participated in most of the oxidation of alcohols to their corresponding acid, the exact roles of these enzymes in each reaction might be different. For example, mADH played a key role in the oxidation of diols to its corresponding organic acid in G. oxydans, but it was dispensable when the primary alcohols were used as substrates. In contrast to mADH, mALDH appears to play a relatively minor role in organic acid-producing reactions because of the possible presence of other isoenzymes. Aldehydes were, however, found to be accumulated in the mALDH-deficient strain during the oxidation of alcohols.

Biokinetic test for the characterisation of AMD generation potential of sulfide mineral wastes

Acid mine drainage (AMD) is formed by the microbially catalysed oxidation of sulfide minerals on exposure to moisture and air. It results in the ongoing contamination of water streams with acidity, sulfate and metal ions in solution, limiting subsequent use of the water without its remediation. AMD prevention is a key requirement for meeting mine closure standards and AMD prediction plays an integral role in waste management and AMD prevention. However, both the static and kinetic tests used currently have shortfalls, including only providing a worst case scenario, providing limited kinetic data, particularly with respect to microbial catalysis or requiring an excessive time frame for the provision of useful data. In this study, we review biological tests reported to predict AMD generation potential and propose an extension to these tests in the form of a biokinetic test. The proposed test provides information on the potential and likelihood of acidification upon microbial colonisation as well as the relative kinetics of the acid-consuming and acid-producing reactions. This provides more meaningful data than static tests, within a reasonable timeframe. Experiments performed to evaluate the biokinetic test, using copper sulfide flotation tailings, show results consistent with those of traditional static tests. However, these also provide additional kinetic information that could help to inform management strategies.

Causes of changes in pH in acidic mineral soils

Seasonal variation in soil pH and long-term decreases in pH have been observed for the soils of the south-western slopes of New South Wales. We have determined and measured some processes which are likely to contribute to pH changes within these acidic, mineral soils. The studies were undertaken on closed systems in the absence of plants. Changes in soil pH, in the absence of plants, were controlled by the relative rates of alkali-producing (acid-consuming) reactions and of acid-producing (alkali-consuming) reactions. The alkali-producing reactions studied were ammonification, reduction of Mn-oxides, oxidation of organic anions and SO 4 2− adsorption. The acid-producing reactions were nitrification, oxidation of Mn 2+ and oxidation of organic S. Measurements of changes in the concentration of the chemical species involved in these reactions were converted to the corresponding changes in μmol H + or OH − g −1 soil. The sum of the H + budget for each soil was transformed to change in pH by using the soil's titration curve. With this procedure 96% of the variance of the observed changes in soil pH was explained. On the basis of our results we propose that changes in temperature and water potential determine changes in microbiological activity which in turn determine changes in the H + budget and its physicochemical consequences.

Secondary Porosity in Sandstones and Acid-Producing Reactions in Adjacent Shales, Oligocene, South Texas: ABSTRACT

Frio and Vicksburg sandstones in the McAllen Ranch and Hinde fields in south Texas show two depth-related zones of secondary porosity. Anomalously high porosity in the upper zone, from 4,000 to 6,500 ft (1,220 to 1,980 m), is related to the dissolution of analcime cement by acidic pore fluids released from adjacent shales. Much of this acid may be produced by the reduction of octahedral iron in smectite clay. Sodium released by analcime dissolution may be responsible for abnormally high salinities calculated for pore fluids in these sandstones. Calcite cementation below the upper zone may be related to fluids rich in calcium carbonate that have migrated up fault zones from great depth. The lower zone of secondary porosity extends from 8,400 to 10,000 ft (2,560 to 3,050 m) at Hinde and from 8,400 to 14,000 ft (2,560 to 4,270 m) at McAllen Ranch. The top of this zone also corresponds to the clay dehydration depth and the top of the abnormally pressured zone. At this depth and temperature, randomly interstratified illite and/or smectite clay becomes ordered and a large amount of water is expelled from the clay structure by potassium fixation. Smectite illitization produces minor amounts of kaolinite and chloritemore » and, more importantly, releases silica and acidic pore water. The acidic pore water migrates into adjacent sandstones and enhances the dissolution of feldspar grains and calcite, creating secondary porosity. The shales at McAllen Ranch have undergone more extensive illitization than those at Hinde, and more secondary porosity is observed in the McAllen Ranch sandstones. Data from detailed chemical and mineralogical analyses were used in mass-balance calculations to confirm that the amounts of acid produced by these reactions can account for the observed secondary porosity in these sandstones.« less

Detectability of acid producing reactions in natural clouds

Oxidation reactions of SO 2 and NO 2 in clouds are considered important atmospheric acid formation processes. This paper evaluates the feasibility of detecting the occurrence of these oxidation reactions in natural clouds by means of field measurements. This evaluation is performed by calculating the changes expected in reagent and product concentrations resulting from these reactions in representative cloud types, and comparing those changes with concentration differences detectable by available analytical methods in the context of typical atmospheric variability. Four in-cloud oxidation reactions are considered: aqueous-phase reactions of SO 2 with O 3 and with H 2O 2, and gas-phase reactions of NO 2 with OH radical and with O 3, the latter leading to acid formation by reaction of N 2O 5 with cloud liquid water. The cloud types considered are fog, stratus, cumulus and mountain lee wave. This evaluation indicates that oxidation of SO 2 by H 2O 2 should be detectable in a wide variety of cloud conditions, but that oxidation of SO 2 by O 3 is unlikely to be detected by field measurements. Oxidation of NO 2 may be detectable in fog and stratus clouds, which provide long in-cloud residence times. The paper includes discussion of factors which favor or hinder detection of acid producing reactions in clouds, and reviews evidence from published field studies on the occurrence of these reactions.

Inhibition of bacterial activity in acid mine drainage

Acid mine drainage water give rise to rapid growth and activity of an iron- and sulphur- oxidizing bacteriumThiobacillus ferrooxidians which greatly accelerate acid producing reactions by oxidation of pyrite material associated with coal and adjoining strata. The role of this bacterium in production of acid mine drainage is described.