In the present study, adsorption of Cr(VI) ions from aqueous solutions was investigated and compared under various conditions using two different nano-structured adsorbents, i.e., carbon nanotubes (CNTs) and low-cost activated carbons (AC). Walnut shell, an agricultural solid waste, was used as a raw material for the preparation of ACs. Multi-walled CNTs (MWCNTs) with average diameter of 20 nm and length of about 2 mm were also used for comparison purposes. Adsorption studies were carried out by varying the parameters such as: treatment time, metal ion concentration, adsorbent amount and pH. The adsorption capacities of AC and MWCNT for Cr(VI) ions were measured as 35 and 24 mg g−1, respectively. The efficiency was observed fairly high at pH = 2–3 for AC and pH = 5–6 for MWCNTs. The adsorption was significantly enhanced by increasing the adsorbent dose up to 0.4 and 0.3 g for AC and MWCNTs, respectively. It was also determined that Cr(VI) adsorption behavior follows both Langmuir and Freundlich isotherms. The content of functional groups, which was obtained by applying the Boehm’s method, revealed that phenolic groups are mostly present on the surface of MWCNTs, while basic groups are predominant on the walnut shell AC structures.

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