Evaluation of clay adsorption and plant-assisted nickel removal in ultramafic soils

Abstract

Nickel (Ni)-rich ultramafic soils present significant environmental and agricultural challenges due to the high bioavailability and toxicity of Ni. This study evaluated whether natural clay amendments could reduce Ni availability in ultramafic agricultural soils and consequently limit Ni uptake by the hyperaccumulator Odontarrhena chalcidica. Natural clays collected from Rubjekë (Durrës, Albania) were characterized by X-ray diffraction, showing a composition dominated by smectite (30%), illite (30%), and chlorite (34%). In batch adsorption experiments, Ni extracted from ultramafic soil of Tropoja was brought into contact with the natural clay from Rubjekë, where clays immobilized up to 95% of Ni with an adsorption capacity of 6.6 mg g-1 (R²=0.9993), following the pseudo-second-order model. An ex-situ pot experiment using Tropoja soil tested four clay doses (0-64 g kg-1). Clay amendment significantly reduced Ni availability, lowering Ni concentration in the soil solution by up to 80.4% and the bioavailable fraction to 15% of the initial value at the highest application rate. In the presence of Odontarrhena chalcidica and with soil amended with 1.07%-3.2% clay, Ni concentration in the soil solution decreased by 25.4-86.5%, while the bioavailable fraction declined by 12.3-19.4% in the first harvest. Furthermore, clay application markedly reduced Ni bioaccumulation in O. chalcidica while enhancing plant biomass production. Mineralogically active aluminosilicate clays effectively reduced Ni mobility and plant uptake while enhancing growth, demonstrating their potential as a sustainable solution for managing Ni-contaminated ultramafic soils.