Influence of inorganic acid modification on Cr(VI) adsorption performance and the physicochemical properties of activated carbon

Macadamia activated carbon (MAC) was impregnated with different concentrations of sulphuric or phosphoric or nitric acid ranging from 20 to 60% v/v and heated in a muffle furnace to improve the structural characteristics of the adsorbents for enhanced Cr(VI) removal. The %S content ranging between 2.27 and 3.23% in H2SO4 treated MAC corroborated the incorporation of sulphonate groups during modification. Fourier transform infrared spectroscopy displayed presence of CN, NO, sulphur and phosphorus peaks at 1213, 1531, 1204, and 1214 cm−1, respectively, proving efficacious attachment of the functional groups because of acid treatments. The surface area of pristine MAC increased from 545 to 824 m2/g following acid treatment. The optimum performance for the adsorption of Cr(VI) was 98% at pH 1 and 93% at pH 4 after 120 min of contact time. Compared to pristine carbons (22.3 mg/g), there was an improvement in adsorption capacity to 40.99 mg/g when 20% (v/v) HNO3 activating agent was used, but the adsorption capacity dropped to 9.66 mg/g when 20% (v/v) H2SO4 was used. Materials modified with HNO3 and H3PO4 exhibited superior performances to H2SO4 modified adsorbents indicating the importance of evaluating various concentrations and type of acid as these had positive and negative effects on Cr(VI) removal. The removal mechanism was better explained by both the Langmuir monolayer and Freundlich multilayer isotherms. •Chemical modification of activated carbons influenced their physicochemical properties and adsorption performance.•Sulphuric acid produced carbons with inferior performance while nitric acid modification displayed superior Cr(VI) removal.•The adsorption process varied between Langmuir and Freundlich as well as pseudo first and pseudo second order.•The different mechanisms observed were attributed to chemical impact of activating agents.