Lignite Mine Spoil Research Articles

Probability distributions of geotechnical properties for heterogeneous lignite mine spoils

ABSTRACT During surface coal mining, large amounts of overburden waste materials, known as spoils, are excavated and dumped, creating massive heaps. The sustainable management of these areas is a global priority, but a comprehensive geotechnical characterization has not yet been achieved due to limited research on the spoils’ geotechnical properties. This study conducts an advanced statistical analysis of lignite mine spoil materials from a large heap in Greece to determine suitable probability distributions. Five distributions - normal, lognormal, Weibull (2 and 3 parameters), and Gamma - were evaluated, considering the high variability of spoil material properties. The lognormal and Weibull distributions were found to best fit most geotechnical parameters, except for cohesion, which showed an uncommon distribution. In some cases, the normal distribution can be used for a simplified analysis. The findings can guide the analysis of spoil materials, as advanced numerical and stochastic methods are essential for robust geotechnical design.

Stability and characterisation of spoil heaps in European surface lignite mines: a state-of-the-art review in light of new data

The large amount of spoil material produced during the mining process imposes a significant economic and environmental liability on lignite producers. In this context, the present paper provides an overview of the geotechnical characteristics of European lignite mine spoil heaps and discusses their significance to the stability of the heaps. In order to achieve this, samples collected from spoil heaps of Polish, Czech and Greek mines are analysed and the results are compiled with data from the literature. A major conclusion drawn is that both physical and engineering properties of spoil heaps indicate a noteworthy variability, which is larger than typical in situ ground material. This is because of the additional factors affecting spoil heap deposition, such as the transportation and dumping method. Furthermore, failure mechanisms and case histories of large instabilities in lignite spoil heaps are critically discussed in order to better understand triggering failure mechanisms. It is concluded that classical assumptions made for natural soil slopes and relevant limit equilibrium models should be cautiously applied to spoil heaps. The challenges associated with numerical and probabilistic modelling of spoil heap stability, such as the inherent spatial variability of spoils and the time-dependent changes in their geotechnical properties, are also critically discussed. Finally, important research gaps in design and analysis of spoil heap stability, such as the absence of appropriate constitutive models developed specifically for spoil materials, are summarised.

A general framework for coupled hydro-mechanical modelling of rainfall-induced instability in unsaturated slopes with multivariate random fields

An accurate estimation of rainfall-induced instability of slopes for extremely nonhomogeneous materials such as lignite mine spoils is a major challenge. This paper investigates the stability of nonhomogeneous soil slopes with respect to slip surface development, size of sliding volume, and determination of safety factor. Specified dependent random variables are cross-correlated using a multivariate Gaussian copula, the use of which provides a faster and more accurate representation of the inter-dependent properties of randomly-distributed soil. A Monte-Carlo simulation is used to generate a series of multivariate random fields for slopes. These are then implemented in Abaqus and analysed under constant rainfall conditions using a fully coupled hydro-elasto-plastic model. The resulting stress, strain, pore pressure, and displacement data are further processed in MATLAB to evaluate critical slip surfaces and safety factors. Results indicate that the factor of safety in a homogenous case is overestimated compared to the nonhomogeneous condition, while the sliding volume is underestimated. Moreover, the factor of safety decreases as the rainfall simulation continues and the probability of failure increases to nearly 100% after 10 days of rainfall. The framework developed in this paper can provide guidance for conducting relatively inexpensive probabilistic analyses.

Tree species effects on bryophyte guilds on a reclaimed post-mining site

Tree species effects in afforestation of post-industrial lands have important impacts on biodiversity of restored sites. We aimed to assess tree species effects on bryophytes in a novel ecosystem – a reclaimed lignite mine spoil heap. We investigated bryophyte species pools in tree stands of six species: Alnus glutinosa, Betula pendula, Pinus sylvestris, Quercus robur, Q. rubra and Robinia pseudoacacia in two substratum groups: epigeic and epiphytic species. We assessed beta-diversity among tree stand types and bryophyte guilds. We also analyzed impacts of light availability, pH and C/N ratios of bark and soil, annual litterfall and bark water capacity of the main tree species, on bryophyte species pools among tree stands and species groups, using canonical correspondence analysis. Our study revealed tree species effects on bryophyte species richness, beta-diversity and composition. Main mechanisms connected with tree species effects were light availability and substratum C/N ratio, as well as substratum pH and bark water capacity. We confirmed that tree species traits connected with C/N ratios and light availability affect bryophyte species composition. Guilds of bryophytes responded differently to tree species effects on ecosystem properties and their turnover differed between tree stands. Influence of the factors studied on species pools was similar to those reported from mature woodlands. Presence of many woodland specific bryophytes has shown restoration success ca. 30 years after afforestation of post-industrial land. Different tree species provided different habitats for bryophytes, and therefore decisions regarding what tree species to plant affect restoration success and the future bryophyte species pool.

Tree species effects on litter decomposition in pure stands on afforested post-mining sites

Abstract Tree litter decomposition on disturbed post-mining sites has been mainly studied within successional gradients, whereas almost no results were shown from afforested spoil heaps. Litterfall and its decomposition rate are considered the most important ecological processes for soil restoration during stand development on such initial forest habitats. These processes allow development of a functional ecosystem and productive forest stands. Moreover, the pedogenesis process on such “soilless”’ habitats can be significantly improved and accelerated by tree species selection during afforestation. The main aim of the study was to determine litter decomposition rates of nine tree species used for afforestation of a lignite mine spoil heap. We assumed that leaf litter decomposition rates would differ among tree species studied and that the site conditions would significantly influence this process. Our study was conducted on the spoil heap of the lignite open cast mine in Belchatow, central Poland. We studied leaf litter decomposition of Alnus glutinosa, Betula pendula, Pinus sylvestris, Quercus robur, Q. rubra and Robinia pseudoacacia in pure stands of these species (home stands), and litter decomposition of Acer pseudoplatanus, A. glutinosa, Fagus sylvatica, Prunus serotina, Q. rubra, and R. pseudoacacia in Scots pine stands. We used the litterbag method. The experiments lasted for three years and the samples were collected every three months. Leaf litter decomposition calculated for home stands after three years of decomposition was 94.4% of the initial leaf mass for A. glutinosa, 70.9% for R. pseudoacacia, 70.1% for P. sylvestris, 68.3% for B. pendula, 66.9% for Q. rubra and 61.5% for Q. robur. In Scots pine stands, after three years of the experiment, 92.3% of the initial leaf mass decomposed for P. serotina, 85.7% for A. glutinosa, 83.5% for A. pseudoplatanus, 65.2% for R. pseudoacacia, 50.9% for Q. rubra and 40.1% for F. sylvatica. A. glutinosa, R. pseudoacacia and Q. rubra leaves decomposed significantly faster in home stands than in Scots pine stands. Site aspect significantly influenced litter decomposition of the species studied, with higher rates mostly on the western slope. Our study revealed that the decision on tree species used for afforestation might shorten the period needed for soil restoration and achievement of sustainability of novel ecosystems. Proper selection of main and admixture tree species for afforestation of the post-mining sites might reduce the renewal period of the soilless and newly created habitats, which may provide noticeable ecological and economical effects during stand management.

Assessment of a sulfidogenic system utilizing microalgal biomass of Chlorella pyrenoidosa as an electron donor: Taguchi based grey relational analysis

This study assessed remediation of sulfate-rich wastewater using microalgal biomass of Chlorella pyrenoidosa as the sole electron donor for sulfidogenic enrichment culture obtained from lignite mine spoils (NLC India Ltd.). Phylogenetic analysis of 16S rRNA gene showed that the microbial culture used, mainly composed of bacteria closely related to Desulfovibrio sp.. An L9 orthogonal array was selected and designed for three parameters varied through three levels by applying Taguchi's design of experiments. The influence of inoculum size (% v/v), initial COD (mg/L) and incubation time (days) on sulfate reduction (%) and COD removal (%) was experimentally investigated. A maximum of 70.29% of COD removal was achieved and the corresponding sulfate reduction was 60.81%. The EDX qualitative analysis of sulfide precipitate confirmed the removal of S and metals such as Fe, Na and Mg from wastewater. Grey relational analysis was coupled to obtain a grey relational grade for evaluating multiple outputs. The percentage contribution of each parameter on grey relational grade was assessed using ANOVA. The results showed that initial COD contributes much towards sulfate reduction (%) followed by inoculum size and incubation time.

Biodiversity of ectomycorrhizal fungi in surface mine spoil restoration stands in Poland – first time recorded, rare, and red-listed species

Results of mycological research conducted in the years 2001–2013 in the restoration stands growing on reclaimed mine spoils are presented. Four opencast lignite mine spoil heaps in Poland were examined: Pątnów-Jóźwin, Adamów, Mt Kamieńsk, and Turów. The paper focuses on 71 species of ectomycorrhizal fungi: recorded for the first time in the country (16 taxa), currently red-listed (23 taxa), known from few localities only (32 taxa). Notes on their ecology and habitats are provided, as well as their distribution in Europe and in Poland. Restoration tree stands, established as part of the reclamation process of mine spoils, form a unique habitat for many rare and interesting fungal taxa. Among them are pioneer species, species known mainly from Northern Europe or mountainous locations, highly specialized and narrow-niche taxa, and many threatened species. Afforested mine spoils contribute significantly to the fungal biodiversity, both at a local and at a larger scale.

Properties and possible ceramic uses of clays from lignite mine spoils of NW Spain

Clay spoils from lignite mines are studied to know their industrial possibilities. The clays come from a coal mining area in NW Spain. In relation to their mineralogical composition, clays can be classified into three main groups. Group 1 was characterized by the presence of kaolinite with an average value of 39%, illite with an average value of 28% and quartz with an average value of 30%. Group 2 was characterized to have the lowest content of kaolinite (30%), 12% of illite content, the highest quartz content (47%), an absence of dolomite and calcite. Group 3 shows the highest kaolinite content (72%), illite (10%) and lower quartz content (15%). The chemical and mineralogical composition allows the evaluation of the applicability of the clays studied. The heated samples were composed of bulky particles, predominantly of quartz and hematite, which are agglomerated in a vitreous matrix. An intermediate phase between spinel and hercynite was formed at 950°C in the Groups 1 and 3, and remains in this series up to a temperature of 1150°C. At higher temperatures other phases such as mullite was formed in Group 3 series together an abundant amorphous phase. Imported clays (ball-clays) from the UK and France by Spanish ceramic industry have similar characteristics, behaviour and uses that the clays from lignite deposits studied, (i.e. Groups 1 and 3).

Aboveground biomass allocation and accumulation in a chronosequence of young Pinus sylvestris stands growing on a lignite mine spoil heap

It is not clear to what extent trees growing on anthropogenic sites change their growth and biomass allocation to different organs. We assessed the aboveground biomass in a chronosequence of six Scots pine monocultures (between 6 and 20 years old) to examine how precisely the site-specific or con - trol allometric equations may reflect the value of biomass accumulation and allocation in stands growing in harsh site conditions on the overlayer spoil heap made as a result of opencast brown coal mining. The site is characterized by poor edaphic and water conditions and nutrient deficiency. The control equations were developed from Scots pine stands growing on post-agricultural lands in the close vicinity of the spoil heap. We found that equation type significantly influenced results of predicted biomass accumulation for all biomass components studied (although results were only marginally significant for total aboveground bio - mass, P=0.08). Total aboveground biomass in younger stands (6-9 years old) estimated using site-specific equations was >40% higher and for older stands (17-20 years old) from 7 to 27% lower than estimated using equations developed for the control sites. Our study revealed that under harsh environmental con- ditions in spoil heaps, biomass of young Scots pine stands significantly differ from values calculated based on control equations developed for more fertile soils with better water conditions in the same region. The control biomass equations may not be suitable to estimate biomass accumulation in stands growing on infertile habitats with poor water conditions, if the control equations are developed for nearby stands but growing under better site conditions.

Improving Soil Fertility to Support Grass–Legume Revegetation on Lignite Mine Spoils

The short-term sequential effects of different treatments on soil fertility and revegetation of mine spoils were examined in a lignite mine in northwestern Spain. Experimental plots were established both on old and recent spoils after tillage and treated with compost or nitrogen (N), phosphorus (P), potassium (K), + magnesium limestone before seeding with a grass–legume species mixture. Compost improved plant production and, contrary to NPK, maintained soil N levels and supplied enough P for the establishment and early growth of the vegetation. Severe magnesium (Mg), calcium (Ca), and K limitations in recent spoils were only alleviated by compost + magnesium limestone, allowing the rapid growth and coating of the soil surface. The amendment based on NPK + magnesium limestone improved plant production in the short term but caused proliferation of weeds. Results suggest that revegetation in combination with the appropriate amendments is a key issue for the reclamation of lignite mine spoils.

Phosphatase activity of lignite mine spoils

A talaj foszforforgalmában meghatározó szerepet játszó foszfatázenzimek forrásai az élő szervezetek, ezért az enzimaktivitás a külszíni meddőhányók felszínén az élő szervezetek megtelepedésének és aktivitásának, végső soron a talajfejlődésnek a jellemzője. Észak-Magyarország legnagyobb lignitbányáinak (Bükkábrány, Ecséd, Visonta) meddőhányóin vizsgáltuk a talajban végbemenő változások és fejlődés jelzőjeként a különböző korú és rekultivációs hasznosítású fedőrétegek foszfatázaktivitását. Célunk annak megállapítása volt, hogy a meddőhányók talajfejlődése a foszfatáz-aktivitáson keresztül mennyiben követhető. Vizsgálatunk a fedőréteg tulajdonságainak, a kor, a rétegmélység, a művelés és egyes talajtulajdonságok foszfatázaktivitásra gyakorolt hatásának megismerésére, elkülönítésére irányult. A meddőhányót 20 cm mélységig mintáztuk 3 rétegben (0–2, 2–7 és 7–20 cm). A meddőhányók igen nagy heterogenitását mind a mintázás, mind az értékelés során figyelembe vettük. A meddőhányó foszfatázaktivitása a környező bolygatatlan területekénél jóval alacsonyabb volt. A foszfatázaktivitás a meddőhányó korával nőtt, míg a rétegmélységgel általánosságban csökkent. A rétegmélységgel csökkenő foszfatáz-aktivitás a régi (≥ 5 év) hányófelszínek általános jellemzője, azonban a szántó művelésbe vont részeken jóval kisebb volt a csökkenés mértéke, mint a mechanikai művelés nélküli területeken. Az általános tendencia nem érvényesült valamennyi, új építésű (0–4 éves) meddőrészen, ahol esetenként a mélyebb réteg foszfatáz-aktivitása magasabb volt, mint a felszíni rétegé, jelezve a hányóépítés során felszínről elkerült réteg „megőrzött” foszfatázaktivitását. Az enzimaktivitást alakító talajtulajdonságok hatását a kor és rétegmélység és mintavételi idő szerint csoportosított adatokon vizsgáltuk. A humusztartalom valamennyi vizsgálati helyen és rétegben pozitív kapcsolatot mutatott a foszfatáz-aktivitással. Az Arany-féle kötöttség hatása elsősorban a régi meddőhányó részeken volt jelentős és pozitív. A CaCO3-tartalom magas szintje néhány esetben az enzimaktivitást gátolta. A tápanyagtartalmak összefüggését a foszfatázaktivitással nem tudtuk kimutatni, ugyanakkor a gombaszám pozitív hatása a foszfatázaktivitásra a szántott talajban egyértelműen megállapítható volt. Vizsgálatsorozatunk igazolta, hogy a fedőréteg foszfatázaktivitása jelzi a talajfejlődést, azonban megmutatta azt is, hogy az egyéb, enzimaktivitást alakító tényezők ismerete az elemzésekhez nélkülözhetetlen.

Effects of stand conversion by thinning and underplanting on water and element fluxes of a pine ecosystem ( P. sylvestris L.) on lignite mine spoil

Not site conform and overstocked Scots pine monocultures ( Pinus sylvestris L.) on mine spoils in the Lusatian Lignite District (Germany) should be transformed into mixed deciduous forests by thinning and subsequent underplanting. But the effects of stand conversion on ecosystem dynamics continue to be poorly understood. Therefore, in a case study of a 66-year-old Scots pine stand ( Pinus sylvestris L.) on Quaternary sand, the ecosystem water and element fluxes of a conversion and a moderately thinned reference plot were measured. Reduction of stand basal area from 34 to 20 m 2 ha −1 decreased interception of annual bulk precipitation (729 mm) from 38 to 30%. In contrast, evapotranspiration remained almost constant, due to additional transpiration of the underplanted trees and ground vegetation. Thus, deep percolation increased from 121 to 164 mm year −1. Both measurement plots were characterized by considerable Ca and SO 4–S leaching (up to 1.5 and 2.0 kmol ha −1 year −1, respectively). However, stand conversion had no major effect on the element budgets; despite higher soil water fluxes, element output by seepage water was quite unchanged. The stand still acted as almost complete sink for growth-limiting nutrients such as N and P with an annual storage of 1.7–2.1 and 0.023–0.026 kmol ha −1, respectively. Nutrient cycling was not decoupled as the minimal N fluxes in 20 cm soil depth indicate.

Organic Nitrogen Mineralization Rate in Sewage Sludge‐Amended Mine Soil

This study was carried out on lignite mine tailings at Meirama (north‐western Spain). A laboratory experiment was conducted to investigate the effects of amending lignite mine spoil with municipal sewage sludge at rates of 5, 25, and 50 Mg ha−1 triplicate samples of soil, and each treatment was incubated from 0 to 90 days at constant moisture and temperature (85% air relative humidity and 25°C). Each sewage sludge dose was packed into PVC columns and inserted vertically into the recipient containing the upper layer of mine soil. A set of the incubated columns was removed at 15, 45, and 90 days, and the soil underlying each column was sampled for analysis. Nitrogen (N) mineralization rate and the evolution of soil pH, C/N ratio, organic matter, total N, organic‐N, NO3 −‐N, and NH4 +‐N content were measured. Sewage sludge application produced an immediate increase of the inorganic N at all dosage levels, mainly in the ammonium form. The nitrogen mineralization in the amended soils was higher than in the control soil, mainly in the case of those treated with 25 Mg ha−1. The net mineralized organic N was always positive, except at the end of the period of incubation in the 5 Mg ha−1sewage sludge treated soil. The nitrogen mineralization rate, calculated as a decrease in the organic nitrogen content, overestimates the amount of inorganic nitrogen that is available. The measurement of the cumulative amount of NH4 +‐N and NO3 −‐N provided a much lower estimation of N mineralization under incubation conditions.

Lignite Mine Spoil Characterization and Approaches for Its Rehabilitation

Open cast mining of lignite leaves behind stockpiles of excavated materials (dumps) and refilled mining pits (spoils). Dumps and spoils remain barren and their surface materials are deposited to adjoining fields through wind and water erosion deteriorating their productivity. Physicochemical and biochemical properties of both kinds of sites were estimated to identify the reasons for their barrenness. Subsequently, surface modifications were attempted, first in a greenhouse and later in field to develop a suitable approach for their rehabilitation. Dumps had low pH (4.8) and high Na+ (2.5 mg g−1), spoils high pH (8.7) and high Na+ (1.59 mg g−1 soil). Both sites had low available nitrogen and phosphorus and showed very low dehydrogenase and phosphatases activity but no nitrification. The extreme physicochemical conditions and inert nature of dumps and spoils explained their barrenness. In the greenhouse experiment, 14 plant species sown in surface materials of dumps and spoils after spreading a 0.15 m thick layer of dune sand, germinated ( > 85%), and their seedlings survived for two months. This technique was followed at a spoil site (modified spoil site). After three years of stabilization the modified spoil site had only one-fifth Na+ of that in spoil surface in the beginning and also showed higher dehydrogenase and phosphatase activity and nitrification. Pearl millet and Cenchrus ciliaris grown in modified spoil produced 128 to 394 kg and 2.25 to 3.50 Mg dry matter ha−1. Addition of farmyard manure with N and P fertilizers increased pearl millet yields.

Spatial distributions of lignite mine spoil properties for simulating 2-D variably saturated flow and transport

In open–cast brown coal mine spoils, the effect of heterogeneity on water flow and solute transport is largely unknown. The objective of this study is to analyze the effects of differently generated hydraulic property distributions on transport processes in mine spoil heaps originating from open–cast lignite mining. Spatial distributions of hydraulic properties are generated with two different approaches. A deterministic approach is based on the description of mixing, segregation, and compaction during dumping. Using information on the mining and dumping technology along with geological properties of overburden sediments, we generate spatial distributions of the spoil bulk density and texture for a representative 2-D-vertical cross-section of 22 m depth and 29 m width. Water retention is calculated with the Arya and Paris approach and hydraulic conductivity with the Kozeny–Carman equation. The second approach uses direct geostatistical simulation of relative hydraulic conductivity. Water flow and solute transport is simulated with the numerical finite element code SWMS_2 D. The deterministic approach leads to a two-peaked concentration front with tailing. A significant fraction of the solutes is transported faster as compared with a homogeneous profile. Both approaches induce a spreading of the concentration front. Utilizing available information on the dumping technology and on the overburden geology may help to improve predictions of water flow and solute transport in spoil heaps.

Acidification-neutralization processes in a lignite mine spoil amended with fly ash or limestone.

A laboratory experiment was conducted to investigate the longterm effects of amending sulfide-rich lignite mine spoil with fly ash (originating from a coal-fired power station and largely comprised of aluminosilicates) and/or agricultural limestone. The experiment was carried out with soil moisture maintained at field capacity or alternate cycles of wetting and drying. Results obtained suggest that the principal acidification processes were oxidation of sulfide and formation of hydroxysulfate (FeOHSO4), whereas the main neutralization processes were weathering of aluminosilicates in fly ash-treated samples and dissolution of calcium carbonate in limestone-treated samples. The highest dose of limestone rapidly raised the pH of the spoil, but this increase was not maintained throughout the one-year experiment. In contrast, fly ash-treated samples showed a more sustained increase in pH, attributable to the gradual weathering of aluminosilicates. The best results (i.e., good short- and long-term neutralization) were obtained in samples treated with both fly ash and limestone. The low liming capacity of the fly ash (47.85 cmol kg(-1)) means that it must be used in large quantities, an advantage in achieving the further aim of disposing of the fly ash.

Weathering processes in a lignite mine spoil treated with a CaCO3-rich waste slurry under two moisture programmes.

Chemical processes affecting the acidity of asulphide-rich lignite mine spoil treated with high orlow doses of a CaCO3-rich industrial waste slurrywere studied in the laboratory under two moistureprogrammes (cycles of alternate waterlogging anddrying, and percolation) so as to determine the slurrydose neutralizing spoil acidity and the possibledetrimental effects of high doses. The most importantacidity-reducing process was the dissolution ofapplied calcium carbonate, and CaCO3 consumptionwas greater under percolation conditions than underwaterlogging and drying conditions. The most importantacidity-increasing process was the oxidation ofsulphides, which was again more intense underpercolation. Under waterlogging and drying conditions,the formation of hydroxysulphates may also havecontributed to acidification.

Soil microcosm experiments to study the effects of waste material application on nitrogen and carbon turnover of lignite mine spoils in Lusatia (Germany)

The lignite and pyrite containing spoil substrates of the Lusatian mining district are marked by very high acidity and salt concentrations due to pyrite oxidation and by a very low content of pedogenic organic matter and nutrients. The effects of fly ash application to neutralize the produced acid and of organic waste material application to improve the ecological soil functions were studied considering the carbon and nitrogen cycling. Nineteen, 38 and 57 t ha−1 sewage sludge and 22, 44 and 66 t ha−1 compost were applied to ameliorated lignite and pyrite containing substrate. An automated soil microcosm system was used to analyse the solid, gaseous and liquid phases. Almost 9% of total N applied with sewage sludge (620, 1240 and 1860 t N ha−1 applied) were lost over a period of 150 days mainly as NO3-N. The total N losses from compost treatments were three times lower (2.8–3.1% of applied Nt) and occurred in similar quantities as NH4-N and NO3-N. Only sewage sludge treatments showed slightly increased N2O emissions at the beginning of the experiment. CO2 emissions determined the carbon losses of all treatments. The C losses amounted to 3.2–4.7% and 1.5–2.7% of Ct applied with sewage sludge and with compost, respectively.

EFFECT OF ORGANIC AND INORGANIC AMENDMENTS ON BHENDI IN LIGNITE MINE SPOIL

Reclamation of mine spoil with various types of amendments like coir pith, gypsum, farm yard manure and pressmud and the effect on the performance of bhendi Abelmoschus esculentus (L.) Moench) was studied in Neyveli lignite mine spoil. The results indicated that incorporation of 12.5 tha each of coir pith and gypsum along with 25 t/ha each of farm yard manure and pressmud resulted in significant increase in the yield of bhendi. A minimum of 12.5 tha each of coir pith, gypsum, farm yard manure and pressinud are recommended for any reclamation and revegetation programme.

Weathering processes under various moisture conditions in a lignite mine spoil from as pontes (N.W. Spain)

Processes contributing to acid release/consumption during weathering of a lignite mine spoil (2.3% w/w S as sulfides) from As Pontes (N.W. Spain) were studied under three moisture conditions (at field capacity or under alternate wetting-drying or forced percolation), which were simulated in laboratory experiments. Oxidation of sulfides to sulfates was favoured under all three moisture conditions, releasing most acid in spoil kept at field capacity. Hydroxysulfates formed in spoil kept at field capacity or under alternate wetting-drying conditions, thereby contributing to acid release. Acid consumption by dissolution of clay minerals, especially micas, was favoured under all three moisture conditions, but was particularly intense in spoil at field capacity. Dissolution of aluminium oxides was also favoured under all the moisture conditions studied.