Açaí waste and gypsum rapidly enhance ecological functioning in non-weathered, filter press bauxite residue: A one-year field experiment in the humid tropics of Northern Brazil

In our study, the average annual precipitation is approximately 2085 mm, with an average temperature around 26.6°C. In such climate, organic amendments, like açaí seed waste, decompose relatively fast, thus promptly supplying nutrients, and humified organic matter, while promoting microbial activity essential for initiating soil formation processes. Although, the speed and direction of these processes depend on climatic conditions (e.g. rainfall intensity and distribution) their fluctuations are not expected to be such that they will greatly affect the outcome. Also, frequency and abundance of rainfall may vary in the humid tropics thereby affecting the leaching rate of sodium and other soluble contaminants from the BR following gypsum application. This leaching process is important for the rehabilitation of BR as it facilitates the reduction of alkalinity and salinity—critical factors limiting plant growth in untreated BR (Courtney and Kirwan, 2012; Bray et al., 2018; Miura et al., 2024). Intense rainfall events may cause erosion or loss of amendments applied superficially, thus potentially reducing their effectiveness if appropriate erosion controls are not implemented. In the experimental set up the amendments were well mixed into the soil and the test plots were situated in horizontal terrain to limit surface water runoff and erosion. Important in this respect is that, in some experimental plots, where neither açaí seed waste nor gypsum were applied, a slow infiltration of water into the soil was noted (associated with poor aggregation and structure), commonly leading to water ponding in the plots. Bauxite residue (BR), remaining after alumina production, presents high alkalinity and salinity and to its low fertility hindering ecological restoration. This experiment occurred in Northern Brazil's humid tropics, between August 2021 to August 2022, where we evaluated the effects of adding açaí seed waste (8 % w/w) and gypsum (5 %, 8 %, and 13 % w/w) on plant growth, contaminants uptake and microbial activity in BR. Results showed significant aboveground net primary production of Urochloa brizantha up to 1.46 kg/m², within 6 months while no growth was observed on unamended BR or on BR amended with gypsum alone. Furthermore, the amendments significantly enhanced microbial and enzyme activities as indicated by increases in dehydrogenase, β-glucosidase, and average well color development (AWCD) values, indicating improved microbial metabolic functions essential for ecological restoration. Our study is among the first to show that non-weathered filter-press BR in the humid tropics can be remediated effectively and within a year by a combination of açaí seed waste and gypsum. Our results provide a promising approach for large-scale remediation and ecological restoration of bauxite residue storage facilities (BRSF) in tropical regions, contributing to sustainable land management practices with significant biomass production thereby supporting the broader goals of environmental rehabilitation and sustainable development.