How Gypsum Works as a Carbon Trap in Alkaline Wastes.

Cation exchange leading to physical improvement motivates the practice of applying gypsum to sodic soils. In highly alkaline soils and mineral processing wastes, however, Ca from gypsum serves to sequester strong alkalinity as calcite, converting toxic, sterile material into an acceptable medium for plants after leaching. One prerequisite is carbon dioxide supply. This is best provided by biological respiration which will be limited by availability of alkali-tolerant plants and microbes and by the diffusion rate of oxygen. Red mud from the Bayer process is especially challenging because of negligible porosity and extreme alkalinity in the form of sodalite. Carbonation with waste carbon dioxide is a known method for beneficiating red mud but is inefficient. There is a slow hydrolysis of sodalite and conversion into dawsonite and sodium carbonate but it is difficult to achieve a stable pH below about 9-10 which is still too agressive for plant growth. It was found that pretreating red mud with gypsum enhances carbonation rate significantly. Thereafter the introduction of a small concentration of hydrogen peroxide generates porosity which facilitates both the leaching of excess sodium sulfate and the diffusion of oxygen into the mud. This is expected to facilitate the restoration of a plant cover and ultimately the conversion of a toxic waste into high quality soil. Of special interest in this system is the potential role of ettringite (Ca, Al hydroxy sulfate). Thermodynamic calculations together with empirical evidence from cement research suggest that gypsum addition to red mud will form ettringite which buffers the system around pH 10 after which carbonation converts the ettringite to calcite which buffers the system at about pH 8. Quite likely the benefit of amending red mud with gypsum is also to minimise sodium toxicity to plants. The relative contributions of alkalinity, sodium toxicity, nutrient deficiency, salinity and aeration to creating a suitable plant growth medium from red mud are not easily quantified and have probably not been adequately considered in previous rehabilitation studies. A method has been devised for rapid determination of the residual alkalinity in red mud and other alkaline wastes which involves hot reaction of a sample with an acetate buffer, similar to those used in determination of the lime requirement of soils, followed by pH measurement. The change in buffer pH correlates with the amount of strong acid required to neutralise the mud by serial titration. A further consequence of gypsum addition to a high-pH growing medium is that calcite formation acts as a sink for carbon dioxide and Le Chatelier's principle therefore suggests that this should enhance the rate of respiration by plant roots. We have devised an experimental set-up to demonstrate such enhancement by pH-stat titration of roots in a nutrient solution comparing calcium vs. potassium hydroxides as titrants.