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• Lateritic geopolymer & Grewia biopolymer were explored for adsorption of Co & Ni. • Negatively charged groups in Grewia biopolymer makes it more efficient adsorbent. • Adsorption process follows second-order kinetic model. • Sorption on geopolymer is endothermic and on biosorbent is exothermic in nature. • Combined use of natural adsorbents offered better removal (∼80%) efficiency. Two environmentally friendly materials (lateritic clay-based geopolymer; LGP and Grewia biopolymer; GBP) were successfully processed from the locally available resources in Cameroon and their applicability for removal of Ni(II) and Co(II) from aqueous solution was explored. The materials were characterized by FTIR, BET, XRD, Zeta potential and SEM. Metal ion concentrations were quantified using ICP-OES. Crystalline LGP showed higher specific surface area (73.32 m2/g) than GBP (22.73 m2/g). Removal of Ni (19.62%) and Co (22.22%) (initial metal ion concentrations 5 mg/L) achieved using LGP (2 g/L) were lesser than that achieved using GBP (0.6 g/L) (60.15%, and 57.68%, respectively) at the identical conditions. Higher adsorptive capacity of GBP was ascribed to the presence of negatively charged groups (glucuronic, galacturonic) in its structure. Langmuir and Dubinin-Radushkevic adsorption isotherm models fit well to Co(II) and Ni(II) removal data on LGP (R 2 = 0.989 and 0.949, respectively). However, Freundlich isotherm renders best fit for both ions (R 2 = 0.969 and 0.989, respectively) using GBP. Combined use of the adsorbents showed much higher removal efficiency (∼80%) for both the metal ions (pH 5) indicating that LGP and GBP can be used together to achieve better removal of heavy metals from contaminated water. [Display omitted] [ABSTRACT FROM AUTHOR]