PRODUCTION OF MANUFACTURED SOIL (TURBA) FROM BAUXITE RESIDUE – AN OVERVIEW OF ITS DEVELOPMENT

Research funded by EGA has been aimed at the production of a manufactured soil product for landscaping purposes. The procedures used were aimed at rapidly (within 48 h) producing a product similar to that produced by 10-20 years of weathering and leaching under high rainfall conditions followed by the addition of gypsum and organic waste and further leaching as is commonly produced prior to revegetation of bauxite residue. A major consideration of the method is that rainfall in the UAE is very sparse as is the fresh water supply so water use needs to be minimized. The residue was treated by a method patented by EGA which involves acidification and removal of excess salts to produce Optimized Bauxite Residue (OBxR). Composted green waste was added to the wet OBxR at a rate of 10% and the product was then dried to cause irreversible solidification, crushed and sieved to give a requisite aggregate size range. This resulted in the added organic matter being sequestered inside aggregates as it occurs in natural soils. The procedure resulted in a product with an Electrical Conductibity (ECwater) of about 1 mS cm-1, a pHwater of about 8.0 and an Exchangeable Sodium Percentage (ESP) of 35-45%. Over time the exchangeable Na concentration and ESP declined further as Na was released from Na-containing minerals (e.g. sodalite) and was then removed by leaching and/or plant uptake. Concentrations of extractable Cu, Zn and Mn in Turba were low and additions of a micronutrient fertilizer will be required. Turba had a much greater microporosity, mesoporosity, plant available water and water held at field capacity than dune sand (the major growth medium in UAE). The addition of compost during Turba production resulted in an increase in total and soluble organic carbon and production of a large active soil microbial community. The Toxicity Characteristic Leaching Procedure (TCLP) showed that extractable metal concentrations were an order of magnitude below legislated imits and the material can be considered as non-hazardous substance. Extensive greenhouse experiments have been carried out which have shown grass yields consistently greater in Turba than dune sand. There will be major challenges in translating the present greenhouse scale research into large scale implementation in the UAE natural environment due to the different scale, climate and the requirement for continual irrigation to ensure plant survival.