<p>The main goal of this study was to investigate the geochemical speciation of Cu (II) and Pb (II) by sequential extraction, as well as the mechanism of their adsorption in sediments from the Benue River in Lokoja (Nigeria). Geochemical phases of Cu and Pb in the sediments by five metal fractional methods and adsorption by the batch method were investigated. X-ray diffraction (XRD), scanning electron microscope and an energy-dispersive spectrometer (SEM/EDS), and FTIR analysis were used to analyse the mineral composition, morphology/mass percentage of elements, and functional groups in the sediment. According to metal speciation data, both metals were identified in non-residual fractions, with 25.85% of Pb (II) in carbonate form and 33.06% of Cu in Fe–Mn oxide, with distribution coefficients (Kd) of 2.64 and 2.72, respectively, showing bioavailability of metals. The adsorption of Cu (II) and Pb (II) by sediments at various pH levels rises with increasing pH, with an optimal pH above 4. The kinetics of adsorption can be better explained using the pseudo-second-order model (R² = 0.90). The linear form of Langmuir's equation fared better in modelling Pb (II) adsorption isotherms, whereas Cu (II) fitted to the linear form of Freundlich's equation. According to thermodynamic parameter estimations, the adsorption of both heavy metal ions on sediments was spontaneous, albeit exothermic for Cu (II) and endothermic for Pb (II). The findings suggested that metal mobility and bioavailability might considerably reduce water environmental quality in the river, necessitating the development of more effective metal absorption and remediation technologies.</p>