In this research, activated carbon (AC) was prepared from date seed (DS) biomass using a chemical activation method for the removal of the Bismarck Brown R (BBR) dye and zinc metal ions from water. As-prepared AC was characterized using thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) analysis for understanding the porous carbon surface and pore structure, which are essential properties for removing organic and inorganic pollutants. DSs are complex and were selected to prepare AC as they can yield hard activated carbon and perform better in packed-bed and fluidized-bed adsorption columns. AC samples were prepared at different soaking temperatures, specifically at 45 °C, 55 °C, and 65 °C, and subsequently tested for the removal of both the BBR dye and Zn ions. Various parameters were studied to complete the batch adsorption process, including solution pH, initial concentration (BBR: 100–500 mg L⁻¹; Zn ions: 10–50 mg L⁻¹), contact time (0–240 min), and temperature (30–60 °C). The maximum monolayer adsorption capacity for BBR and Zn metal ions were found to be 192.31 mg g⁻¹ and 15.55 mg g⁻¹, respectively. The data was most accurately described by the pseudo-second-order and Elovich kinetics models. Analysis using the particle diffusion model indicated that both film-diffusion and pore-diffusion mechanisms governed adsorption. Thermodynamic assessments revealed the endothermic behavior of BBR dye adsorption and the exothermic behavior of Zn metal ion adsorption.