Low-Rate Bauxite Residue Application Controls Nickel Adsorption, Fractionation, and Mobility in Soils of Different Physicochemical Properties

Soils in industrially influenced areas are often exposed to elevated nickel (Ni) levels due to metallurgical and alumina production activities. In this context, this study evaluated bauxite residue (BR) as an amendment to mitigate Ni availability and mobility in five agricultural soils from the Attica region, Greece, selected according to their pH values. Apart from the pH, soil properties were greatly varied. A very small amount of 1% BR (w/w) was incorporated into soils and batch adsorption experiments with eight Ni concentrations ranging between 1 and 90 mg Ni L−1 were performed, followed by the direct application of the Tessier sequential fractionation scheme. BR addition increased the Ni adsorption capacity of soils, particularly those of low and neutral pH. BR increased the pH of acid soils, thus increasing the negatively charged sites on soil colloids. The Langmuir bL constant provided indications of advanced Ni surface precipitation in the presence of BR. However, the desorption results suggested that, in addition to pH, Fe-Mn free oxides, noticeably those of amorphous form, controlled Ni fractionation in the studied soils. The mobility factor (MF) showed that the availability of Ni was restricted in all soil–BR mixtures. Yet, the distribution of Ni among the chemically active phases was different depending mainly on Fe-Mn free oxide content. Due to its high content in iron oxides, BR assisted the retention of Ni in soils with low Fe-Mn oxide concentration and increased significantly the Ni proportion extracted from the reducible phase. However, in soils richer in Fe-Mn oxides, BR incorporation resulted in enhanced oxidizable and residual fractions, suggesting stronger Ni binding. The results demonstrate that even a low BR application effectively enhances Ni immobilization by increasing adsorption capacity, shifting Ni toward more stable geochemical fractions, and significantly reducing its mobility, highlighting its potential as a sustainable soil amendment for Ni-contaminated soils.