Enhancing chemical and physical properties of bauxite residue: A one-year assessment using Açaí Waste and Gypsum in in-situ rehabilitation

Bauxite residue (BR) from alumina production poses significant environmental challenges, including establishment of a vegetation cover, due to its high alkalinity, salinity, and trace element content. The practice of using topsoil as a cover in bauxite residue storage facilities (BRSF) is unsustainable due to mounting demands on natural topsoil reserves. As an alternative, blending BR with gypsum and organic waste may offer a cost-effective method to improve its chemical and physical properties important to foster better conditions for plant growth. Contrary to previous investigations, which have focused primarily on weathered BR, this study was done on fresh, filter-pressed BR, resulting from more modern, and now commonly used, technologies in alumina production. Here, we assess the in-situ impact of a mixture of gypsum and açaí berry waste one-year after its application to a BRSF of non-weathered, filter-pressed BR, in the tropical climate of Northern Brazil. The site has an average annual precipitation of 2085 mm, and the study tested the effect of additions of various mixtures of gypsum (0 %, 5 %, 8 %, and 13 % by weight) and açaí waste (0 % and 8 % by weight). Results showed that the treatments with gypsum and açaí waste successfully reduced pH levels from 12.0 to 7.7–8.1. Also, Electrical conductivity (EC) reduced significantly and achieved the rehabilitation goal (4mS/cm) after month 4 of the experiment. The exchangeable sodium percentage (ESP) values decreased from 98.8 % in fresh BR to 18.0 % in BR with 8 % gypsum, and to 8.4 % in treatment with 8 % gypsum and 8 % acaí waste. Gypsum treatment also decreased the fraction of unstable aggregates of BR from ∼70 % to ∼40 %. Despite precipitation playing an important role in lowering pH, EC and alkalinity, we found that the combined use of gypsum and açaí waste was the most effective in enhancing BR properties by reducing pH and ESP and increasing aggregate stability. As expected, organic material like açaí seed waste adds both organic matter and major plant nutrients to BR, thus playing a key role in enhancing soil quality, and in establishing a vegetation cover. This study offers insights into the initial transformations in amended BR and provides a solid foundation for developing closure strategies using gypsum and available and low-cost organic wastes as soil enhancers.