In order to achieve the recovery and utilization of gelatin (Gel) extracted from leather solid wastes, a renewable, eco-friendly and low-cost honeycomb biomass adsorbent based on oxidized corn starch-gelatin (OCS-G) was synthesized by one-pot reaction equipped with sustainable electrochemical synthesis system. The prepared biomass adsorbent was characterized using BET, FTIR, SEM, XRD, TGA, EDS, XPS, and other analysis technologies. The results demonstrated that the OCS-G has been successfully prepared, and the OCS-G’s maximum decomposition temperatures and specific surface area reached 366.4 °C and 30.48 m² g⁻¹, respectively. Then, the prepared OCS-G was used as a biomass absorbent to remove potassium dichromate (Cr(VI)), methylene blue (MB) and methyl orange (MO) from aqueous solutions. The effects of initial concentration (C₀), adsorption time, and pH on the Cr(VI), MB and MO adsorption were investigated. The results revealed that the OCS-G showed ultrahigh adsorption properties for Cr(VI) (581.4 mg g⁻¹), MB (1551.5 mg g⁻¹) and MO (1107.2 mg g⁻¹). The adsorption process of the OCS-G on Cr(VI), MB and MO followed the pseudo-second-order and the Langmuir adsorption model. Meanwhile, according to the desorption/regeneration experiments, the OCS-G with high specific surface area had good adsorption–desorption capacity and reusability. As a renewable, eco-friendly and low-cost biomass adsorbent with ultrahigh adsorption capacity, the OCS-G can be effectively used for wastewater treatment.