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  • br Hydrogen production by chemical looping reforming

    2018-10-26


    Hydrogen production by chemical looping reforming of SWCO
    Results and discussion
    Conclusions The main conclusions that can be drawn from the present study are the following:
    Introduction As reported by Coetzee et al. (2007) endothelin receptor antagonist mine drainage (AMD) occurs in sulphide-bearing mine waste due to the oxidation of iron sulphides, especially pyrite (FeS2), pyrrhotite (FeS) and marcasite (FeS2) leading to the creation of several soluble hydrous iron sulphates, the generation of acidity and subsequent leaching of metals. AMD have several negative human and environmental impacts (Gray, 1997; Manzano et al., 1999; Wates and Rykaart, 1999; Vermaak et al., 2004) which may be alleviated by lining possible acid-generating waste rock or mine tailings impoundments with clayey or composite clayey liners. For the competent performance of these liners, it is important that their properties are not compromised over time when exposed to AMD. As such, assessment of the chemical interaction and compatibility of clay/clayey liners with aggressive AMD is a pertinent consideration in the proper management of potentially acid generating mine waste (Hobbs and Cobbing, 2007). These AMD-soil interactive/compatibility studies may involve percolation/hydraulic conductivity testing, contaminant migration analysis and determination of soil chemical alterations. Although, some studies on the interaction of soils from temperate regions with AMD have been conducted (e.g., Yanful et al., 1995; Kashir and Yanful, 2000, 2001) there still remains however, insufficient data on the interactive outcomes/compatibility of soils from tropical and subtropical regions with AMD. More to this, sufficient outcomes of chemical studies were not reported in most of these works which nonetheless, paved way for this study to bridge that gap in knowledge. Notwithstanding, it is known that the chemical composition, texture and crystal structure of a mineral may influence its stability, the type and severity of the environment to which it is exposed could also accelerate its alteration (Coetzee et al., 2002). AMD usually has high acidity and is rich in sulphate and potentially toxic elements and heavy metals. For instance, acid attack of clay/clayey soils can lead to chemical alteration and the release of cations and toxic elements, thereby influencing several physical, biological and chemical processes occurring in the geological media. These include pH variation in natural waters and metal mobility (Kalinowski and Schweda, 1996; Lin and Hansen, 2010), ground water flow and contaminant migration mechanisms (Welch and Ullman, 1996; Coetzee et al., 2006) and the availability of essential soil micronutrients (Hamer et al., 2003; International Network for Acid Prevention-INAP, 2009). In the long run, aquifer porosity and permeability, ground water and soil quality may be adversely affected. Similarly, AMD could lead to soil acidification and potentially minimise soil fertility through the depletion of vital micronutrients. These have consequential impacts for most of the agrarian economies in the tropics and subtropics as it has been recorded by Hartemink (2004) endothelin receptor antagonist that one-third of the soils in the world occur in the tropics which supports more than 75% of the world\'s population. It is therefore imperative that the soil chemical alteration from the interactions between AMD and these clayey soils is understood towards the efficient management of AMD and related contaminated soils. AMD attack on clays could also affect the soil-electrolyte system; as a decrease in ion concentration or valency of the medium forming the absorbed layer (counter ion of clay) could trigger an increase in electric repulsive forces between the clay particles (Mitchell, 1993; Iwata, 1995). These upsurges in repulsive forces can result in swelling and dispersion of the clay particles, which in turn may increase hydraulic conductivity if adequate confining pressures are not imposed on the compacted clay/clayey lining system. Consequently, clay surface charge characteristics have an impact on hydraulic conductivity. As most tropical soils contain kaolinite, iron and aluminium hydroxides that have variable (pH-dependent) charge characteristics (Mendoza et al., 2002), the pH of the permeating fluid can have an impact on their hydraulic conductivity. In this study however, the physicochemical compositions as well as the mechanical properties of three subtropical clayey soils as relates to the buffering efficacy of the soils were determined. A bespoke constant flow rigid wall permeameter device was used to percolate compacted samples of the respective clayey soils with several pore volumes of AMD. The chemical alterations of the soils after AMD permeation were assessed using x-ray diffraction and chemical analyses in order to ascertain their contaminant buffering efficacy or potential value for use as clay liners in aggressive chemical waste containment facilities. In view of the overall objective of evaluating the contaminant buffering ability of the soils for use as liners in mine tailings dams and effluent ponds, the impact of the permeant on the hydraulic conductivity of the clayey soils after interaction with AMD were also evaluated.