Nickel dynamics and immobilization in soil-bauxite residue systems: insights from sequential extraction and FTIR analysis

Bauxite residue (BR) or "red mud" is an industrial by-product that its cumulative total reached 4 billion tonnes by 2022. Developing alternative management strategies is a matter of crucial significance on a worldwide scale. According to many references, BR has been proven an efficient barrier of metal mobility in soil environments. In this study, the results of BR addition 20% w/w in six soils with different properties on Ni mobility are presented. The results indicate that BR could be used in some cases as a Ni barrier. More specifically, BR increased pH values of all soils by approximately 2 units, leading to pH values > 9 for alkaline soils while for the acid soil, pH < 8.5 was recorded. Ni sorption on soils and soil-BR mixtures was tested for initial concentrations of 1 to 90 mg Ni L^-1. Soil properties governed Ni sorption on soils that was much higher for alkaline or neutral soils. On the contrary, BR impressively increased Ni sorption on soil-BR mixtures, up to 24.5, 71.8, and 204%, for alkaline, neutral, and acid soils, respectively, clearly emphasizing the dominant role of BR on Ni mobility. Immediately after Ni sorption, soil and soil-BR-mixture samples were subjected to sequential extraction following the Tessier protocol for Ni fractionation. In alkaline soils, almost 50% of sorbed Ni was found in the residual fraction at the lowest initial Ni concentration that was gradually reduced as initial Ni concentration increased leading to an increase of the exchangeable fraction up to 40%. In acidic soil, the exchangeable fraction included over 50% of sorbed Ni in the widest range of tested initial Ni concentration. The addition of BR in soils reversed the picture since regardless of soil properties and initial Ni concentrations, over 80% of retained Ni extracted mainly from the residual and the oxidizable fractions suggesting Ni immobilization. Possible mechanisms involved could be surface precipitation as Ni(OH)₂ and Ni-Al layered double hydroxides and adsorption on the negatively charged surface of cancrinite strongly supported by the FTIR analysis performed on samples of BR and BR amended with 5, 40, and 90 mg Ni L^-1.