My Account: Log In | Join | Renew
Table of Contents
Select All Chapters
Potentially acid sedimentary material contains pyrite in excess of acid-neutralizing substances. Formation of such material requires (1) ingredients for pyrite formation (sulfate, sulfate reducers, organic matter, iron, and anaeroby alternating with limited aeration), (2) low contents of acid-neutralizing substances, and (3) removal of dissolved alkalinity formed during sulfate reduction. Intertidal environments with mangroves or reeds are particularly favorable for pyritization of ferric iron. Highest pyrite contents build up where tidal flushing is strong. Rapid rises in relative sea level, as after the last glaciation, caused deposition of extensive, thick and highly pyritic sediments (examples: interior parts of the Chao Phraya, Mekong and Orinoco deltas, parts of Sumatra, old sea clay of Holland). After stabilization of the sea level, some 5,000 years B.P., pyrite contents remained low where high rates of sedimentation and coastal accretion caused a rapid shift of the intertidal zone (Irrawaddy and Mekong deltas, Guyana coast). High pyrite contents in the most recent sediments are associated with low sedimentation rates (e.g., along the Saigon, Niger, and Gambia rivers), or with a high density of tidal creeks. In humid climates very low sedimentation rates result in the formation of pyritic peaty material on top of older pyritic clay (Niger delta, western Netherlands).
A brief review is given of the process of bacterial sulfate reduction in respect to other processes in ocean sediments. In particular, rates of sulfate reduction are discussed in context of control mechanisms and geochemical consequences. It is concluded that besides temperature and pressure, which are cosmopolitan parameters influencing most biological processes, the rate of sulfate reduction is dependent on 1) total organic carbon preserved in sediment and 2) state of complexing of the organic matter and its availability for biogenic degradation. These two parameters are in turn influenced by 3) the environment of deposition and 4) the rate of sediment accumulation. Correlations are presented showing a direct relationship between rate of sulfate reduction and rate of sediment accumulation. The consequences of different rates of sulfate reduction on pyrite formation and isotope fractionation are discussed.
The oxidation of pyrite in aqueous systems is a complex biogeochemical process involving several redox reactions and microbial catalysis. This paper reviews the kinetic data on pyrite oxidation, compares available data on the inorganic vs. microbial oxidative mechanisms and describes the occurrence of mineral products resulting from pyrite oxidation. Although oxygen is the overall oxidant, kinetic data suggests that ferric iron is the direct oxidant in acid systems and that temperature, pH, surface area, and the presence of iron and sulfur-oxidizing bacteria can greatly affect the rate of reaction. The vast amount of literature on the microbial and geochemical investigations on this subject have limited usefulness for understanding natural systems. Additional research is needed on the hydrologic, geologic and microbiologic characteristics of field sites where oxidation occurs. The acid water resulting from pyrite oxidation may precipitate a large suite of soluble and insoluble iron minerals depending on pH, degree of oxidation, moisture content, and solution composition.
Laboratory experiments show that the acidophilic iron-oxidizing bacterium Thiobacillus ferrooxidans is invariably isolated from acidic environments (pH 1.9 to 3.4) containing pyrite and basic ferric sulfates. When solutions of FeSO4 (pH 2.9) containing either K+ NH4+, or Na+ are inoculated with the bacterium, Fe2+ oxidation, and formation of basic ferric sulfates begins within a few days. Their rates of formation are in accord with analyses of acid sulfate soils. Thus it is likely that the iron-oxidizing bacterium takes part in the formation of basic ferric sulfates in situ and plays a major role in the genesis of acid sulfate soils. In tidal marsh areas where some acid sulfate soils are subjected to prolonged submergence, Desulfovibrio desulfuricans (a sulfate-reducing bacterium) aids in the pyritization of the basic ferric sulfates. Hence in such areas there appears to be a generic relationship between pyrite and basic ferric sulfates and the above two microbes help to maintain this relationship by cycling sulfur and iron between the two minerals.
Microbial formation of basic ferric sulfates in laboratory systems and in soils was investigated. It was shown that in laboratory systems incubated at 28 C and containing Thiobacillus ferrooxidans, ferrous sulfate, feldspars, micas, and montmorillonites, the alkali cations required for basic ferric sulfate formation were supplied by the minerals. During this process feldspars and illite dissolved congruently and released Na and K non-preferentially but glauconite released K preferentially producing a nontronite phase. In the presence of feldspars and micas the rate of basic ferric sulfate formation depended on the weather ability of these minerals; in the presence of K, NH4, and Na-saturated montmorillonite the rate followed the order jarosite > ammoniojarosite > natrojarosite. Hydronium jarosite formed slowly in the presence of Li-saturated montmorillonite. In systems containing dissolved salts of K + NH4, K + Na, and NH4 + Na, solid solutions of basic ferric sulfates containing these cation pairs were formed. The rapid formation of jarosite as compared with other forms of basic ferric sulfates agrees with its reported more common occurrence in acid sulfate soils. However, in four out of six acid sulfate soils from widely separated areas of Canada amounts of natrojarosite were dominant or equal to amounts of jarosite.
Acid sulfate soils form when potentially acid pyritic marshes are drained and tidal influence decreases, either naturally or by man. During oxidation of pyrite to ferrous sulfate, ferric oxide, or jarosite and sulfuric acid, the supply of O2 is ratelimiting. Upon slow oxidation of pyritic soil in situ, buffering by clay minerals, jarosite, and ferric oxide, keeps pace with acid formation, and the pH remains between 3 and 4. Lower pH values may develop with rapid oxidation as in excavated soil and aerated pyritic soil samples. Compared to non-acid marine soils, acid sulfate soils exhibit retarded physical development due to lower evapotranspiration by a less luxuriant vegetation. A well-developed acid sulfate soil shows, from bottom to top, an unoxidized pyritic substratum, a jarositic horizon due to oxidation of ferrous sulfate diffusing upward from oxidizing pyrite, and a horizon high in ferric oxide from hydrolysis of jarosite. As these soils become older and better drained, the jarositic horizon and the pyritic substratum are found at progressively greater depth. The distinction between Sulfaquents and Sulfic subgroups in Soil Taxonomy is practical but sulfidic material and sulfuric horizon need to be redefined.
Soils and geologic materials exhibiting properties related to sulfide oxidation in upland positions in the Coastal Plain and Appalachian surface mine areas of Maryland were investigated. Features attributable to acid sulfate processes were observed both in recently excavated sulfide-bearing materials as well as in soils on undisturbed land surfaces. Relative to underlying unoxidized strata, near-surface zones of active or recently active sulfide oxidation are characterized by higher contents of free iron oxides, lower contents of total and sulfide-S, and higher contents of sulfate-S. In a profile developed in recently exposed sulfidic sediments, ratios of sulfate-S to total S range from almost 1 near the surface to less than 0.2 below 80 cm. Sulfate minerals associated with recent sulfide oxidation include jarosite, gypsum, copiapite, rozenite, and szomolnokite. These minerals were observed as mottles or efflorescences along ped faces, in soil pores, and on rock surfaces. Jarosite and gypsum were the only sulfate minerals identified under natural land surfaces. The subsoil and substrata of an undisturbed Ultisol developed from glauconitic sediments were found to contain up to 0.3 % S as jarosite.
Jarosite has been identified in soils classified as Ultisols (e.g., Aubrey soil series) and/or Alfisols (e.g., Lufkin soil series) developed on the Woodbine, Cata-houla, and Yegua geologic formations in Texas. The Woodbine is Upper Cretaceous, the Catahoula is Oligocene, and the Yegua is Eocene. The extent of these geologic formations suggests that there may be many other upland soils that contain jarosite. Jarosite has been identified in very acid soil horizons and in other soil horizons with near neutral pH. Some horizons containing jarosite have as much as 8.1 meq/100 g exchangeable aluminum (Al) and 23.8 meq/100 g nonexchangeable titratable acidity. Appreciable amounts of acidity determined in soils with a near neutral pH may reflect an earlier acid sulfate weathering regime. None of these upland soils have pH values low enough to have any horizons that qualify as sulfuric horizons today, but the presence of jarosite is interpreted as indicating that these soils underwent active sulfuricization at some time in the past.
Jarosite has been identified in these soils by noting mottles with yellow (5Y) hues and with verification in the laboratory by X-ray diffraction or differential thermal analysis on relatively pure hand picked specimens. Barite, gypsum, and calcite have been identified in some horizons that contain jarosite by employing a similar combination of field and laboratory techniques. The mineralogy of the matrix soil materials associated with the presence of jarosite is usually mixed in the Woodbine derived soils and smectitic in the Yegua and Catahoula derived soils. Pyrite weathering has apparently been the source of sulfate ions and acidity necessary for jarosite formation. Jarosite seems to be preserved in some Claypan soils due in part to their dense clayey subsoil.
Gypsum (CaSO4·2H2O) is the only pedogenic calcium sulfate mineral that has been found in soils with ustic, xeric, and aridic moisture regimes. It has been found in soils in 14 of the 17 conterminous western states by the National Soil Survey Laboratory and likely will be found in the other three. In these arid to subhumid soils, parent material differences in large part control the occurrence of gypsum. But gypsum in soils can be from other sources too. For example, drainage of coastal wetlands oxidizes sulfides to acid sulfates, and in their reclamation, if not before, the acid sulfates are neutralized by carbonate to form gypsum. Gypsum is also formed in minesoils by neutralization of the acid sulfates released by oxidation of sulfides. Pedogenic gypsum, in contrast to allogenic gypsum, accumulates in subsurface horizons relative to surface and underlying horizons mostly as euhedral to subhedral spindle-shaped crystals in pores and veins. As the s-matrix in a developing gypsic horizon becomes plugged, the pore volume decreases and the restricted hydraulic conductivity keeps the soil moist longer allowing the growing gypsum crystals to interlock and indurate the horizon. Subsidence of soils through solution and removal of gypsum can crack building foundations, break irrigation canals, and make roads uneven. Concrete in slab structures, irrigation canals, and building foundations deteriorates and cracks as extremely high pressures develop during formation of highly hydrated ettringite [Ca6·Al2(SO4)3(OH)12·26H2O] or during conversion of thenardite (Na2SO4) to mirabilite (Na2SO4·10H2O) if the temperature in the concrete and the soil drops low enough.
The mineralogical properties of lignite overburdens differ significantly between the oxidized zone and the reduced zone. Reclamation of lignite mine spoil generally involves earthy materials that overlie the lignite. Hard rock types are exceptional. These overburden materials are sometimes capped by good soils that need to be reinstalled as illustrated by the loessial soils of West Germany. On the other hand reclamation may utilize mixed overburdens as is being done in the coastal plain deposits of eastern Texas where the soils may have poor physical and chemical properties and topsoiling is not practiced. In East Texas fresh overburden materials are left on the surface thus exposing carbonates, sulfides, and labile silicates to immediate weathering. The thickness of the weathered zone is about 5 to 20 m depending on the texture of the material and the depth to impermeable strata. Clayey or lignite strata impede penetration of air and water and thus the weathering front.
Iron sulfides and Fe-chlorite are generally absent from the oxidized zone. Labile iron compounds have weathered to iron oxides. Chlorite usually occurs in the unoxidized overburdens analyzed in eastern Texas. Pyrite is present in these deposits but occurs in local concentrations in varying amounts. Feldspars appear to have partially weathered out of the upper oxidized zone based on preliminary data. Feldspars occur mostly in the silt and sand fractions. Mica of the muscovite type is present in modest amounts throughout the overburden section. It is relatively resistant to weathering and persists to some extent to the soil surface. Smectite and kaolinite are abundant in the clay fraction throughout the overburden section except near the soil surface where much of the smectite has been removed. These conclusions are based on data from only a few locations in East Texas and require reevaluation throughout the lignite belt to account for possible variation along the strike of the beds and under different climatic conditions.
Carbonates occur infrequently in materials that overlie Wilcox lignite of East Texas. Thus they are not a reliable indicator of weathering. Calcite has been identified in a few layers. Siderite (FeCO3) is important in local concentrations and weathers to goethite coated rocks that persist in exposed land surfaces.
Jarosite and gypsum are the most common sulfate minerals associated with lignite overburden of East Texas. Both minerals form on the exterior of overburden cores when they are allowed to oxidize and dry in storage. These localized fresh efflorescences suggest the rapid weathering of pyrite. Also, gypsum forms as a widespread white powder on dry shale surfaces suggesting that it formed as the interstitial solution evaporated in the absence of localized pyrite. Melanterite and szmolnokite also form on samples of lignite overburden from East Texas.
The discussions of mineralogical properties of lignite overburden presented here and the experience with reclaiming lignite overburdens in the United States and abroad reflect a favorable outlook for the continuation of such practices. However, it must be recognized that the extent of these experiences has been limited to a few locations and a short interval of time. In the East Texas lignite mining area many of the native upland soils, presumably formed from similar parent material to lignite overburdens, are infertile and difficult to manage. Thus continued investigations are needed of the overburden composition at different locations. Appropriate revegetation practices and long term management practices are needed to assure successful reclamation and return of the land to agricultural use.
Mahoning sandstone rock strata above surface mineable coal was compared chemically and mineralogically with minesoils developed there—from and with adjacent soils to evaluate weathering and soil development. The weathered zone of rock was found to be about 6 m deep and was acidic, high in Al and free Fe, but low in S and in exchangeable bases. This weathered (high chroma) sandstone contained quartz, kaolinite, and minor amounts of mica and vermiculite. An unweathered zone of rock below 6 m was basic to slightly acid, low in exchangeable Al and free Fe, but higher in S and exchangeable bases. This unweathered (low chroma) sandstone contained quartz, kaolinite, and mica along with authigenic pyrite and carbonates. Weathering resulted in vermiculitization of the highly crystalline mica of the Mahoning sandstone. Soil development was directly related to the underlying rock material as determined by chemical and mineralogical analyses. The data show that proper placement of overburden rock resulted in edaphologically desirable minesoils, whereas haphazard overburden replacement enhanced pyritic oxidation and acidity which inhibited successful revegetation.
Studies were initiated to provide sound geologic and pedologic bases for improved mined land reclamation and pollution abatement. Geologic features of multi-county regions of coal surface mining activity, examined from the viewpoint of sediment depositional and compositional trends, demonstrated variations in overburden composition associated with the local geographic position within the coal depositional swamp. Data, which are exemplary of more extensive arrays cited and published elsewhere, are presented showing the trends in composition of rock strata of the upper Pennsylvanian System in northern West Virginia and southwestern Pennsylvania. Chemical analyses for pyritic sulfur and soluble carbonates in these coal overburden materials verify the relative base-rich quality of the younger part of the section (Conemaugh Formation and above) and the both base-poor and low-pyrite composition of the older rocks in the region for which data are presented. These data also quantitate the penetration of the zone of oxidative weathering to depths ranging from 6 to 12 m, varying with rock type and whether pyrite or carbonate minerals are considered. The balance, or Acid-Base Account, of net acid-producing potential calculated from pyritic sulfur content and intrinsic calcium carbonate equivalent permits evaluation of the rock with respect to the ultimate acidity or basicity that might be expected in a new soil developing in excavated rock materials. Comparison of recognized properties of native soils on an area with projections from analyses of properties of rocks from which they were believed to have been formed shows agreement sufficient to demonstrate the value of studying potential soil parent materials prior to their placement into a pedologic setting. This aggregation of geologic, pedologic, and chemical information has re-suited in development of a basis for predicting the nature of soils that form in specified rock environments. Knowledge of minesoil properties is being used by land reclamation and pollution abatement planners, and soil scientists who are involved in both agronomic studies of minesoil capabilities and genetic studies of new soil development.
Several factors have been identified, any one of which may seriously limit the establishment of vegetative cover on acid sulfate coal mine spoils. The addition of liming agents such as calcium carbonate as required by sample testing will reduce the acidity, but spoils in which large amounts of lime are required often have other factors limiting plant growth. Acid sulfate coal mine spoils frequently tend to have very low levels of available phosphorus, occasionally low levels of potassium, and these spoils are usually droughty.
Several experiments have been established in Kentucky to learn which successful combinations of soil fertility amendments and which soil test for lime requirement (i.e., pH, buffer, or total acidity) may be best used in reclamation practices on acid sulfate coal mine spoils. Success in revegetation of these spoils was achieved with the addition(s) of lime and plant nutrients in combination with appropriate steps to reduce runoff by providing a rough micro-relief and with the selection of adapted species and/or varieties.