Aloe vera leaves (AVL), a by-product of agricultural waste, have been applied as a biosorbent for reducing Ni(II) ions in aqueous solutions. The biosorption capability of AVL powder was enhanced through chemical treatment with 0.10 M citric acid solution. Fourier-transform infrared (FTIR) spectrophotometer, scanning electron microscope coupled with energy dispersive X-ray (SEM-EDX), pH of point-zero-charge (pHPZC), and pHslurry analyses were used to study the surface and chemical properties of citric acid-treated Aloe vera leaf powder (CAAVLP). The setting for experiments such as pH solution, CAAVLP dose, initial concentration, and biosorption time were investigated. Maximum Ni(II) ion biosorption capability was determined to be 48.65 mg/g based on the Langmuir model at pH 6, a CAAVLP dose of 0.02 g, initial Ni(II) concentrations of 5 to 50 mg/L and biosorption time of 120 min. The data for the isotherm and kinetics were well matched with the Freundlich and pseudo-second-order models, respectively, with high regression correlation (R2) and low chi-square (χ2) values. The presence of more -COOH groups after treating AVL with citric acid resulted in more Ni(II) ions being able to be removed.