Excess accumulation of chromium and its compounds in ecosystems poses serious risks to the environment, living organisms, and human health. As a result, there is a growing need for efficient and environmentally friendly methods to remove these contaminants. One promising approach is biosorption using marine algae. To enhance the biosorption capacity of algae, green algal biomass was chemically activated with a 1 M solution of nitric acid (HNO₃, denoted as H1-AB) and calcium nitrate [Ca(NO₃)₂, denoted as Ca1-AB]. These modified biosorbents were then applied to remove dichromate anions from aqueous solutions. Experiments were conducted at pH 1 and room temperature (20 ± 2 °C), using dichromate concentrations ranging from 21.6 mg/L to 302.4 mg/L and contact times between 5 and 180 min. Surface characterization of the biosorbents was performed using Scanning Electron Microscopy (SEM). Biosorption studies aimed to evaluate the retention efficiency of dichromate anions using untreated green algae (AB) compared to the two chemically activated variants (H1-AB and Ca1-AB). While activation with HNO₃ and Ca(NO₃)₂ previously proved effective for phosphate anion removal, it resulted in lower biosorption capacities for dichromate anions: 73.23 mg/g for AB, 58.55 mg/g for H1-AB, and 49.33 mg/g for Ca1-AB.