Functionalized agricultural biomass as a low-cost adsorbent: Utilization of rice straw incorporated with amine groups for the adsorption of Cr(VI) and Ni(II) from single and binary systems

•Agricultural residues were used as low cost adsorbents.•The amine groups were grafted onto the surface of rice straw successfully.•The competitive adsorption of Cr(VI) and Ni(II) on adsorbent was investigated.•The modified rice straw showed great affinity for heavy metal ions.

The adsorption of Cr(VI) and Ni(II) applying amine-functionalized modified rice straw (MRS) as an adsorbent in single and binary systems was investigated. The MRS was characterized by Fourier transform infrared spectroscopy (FTIR), Thermo-gravimetric Analysis (TG), Brunauer–Emmett–Teller (BET) analysis, Scanning Electron Microscope (SEM) and the Energy Dispersive Spectrometer (EDS) analysis. The adsorption study was conducted systematically by varying adsorbent dosage, initial pH value, temperature and initial metal ion concentration. The results showed that the maximum adsorption capacity for Cr(VI) was 15.82 mg/g with MRS dosage 10 g/L at pH 2.0, temperature 318 K and initial metal ion concentration 200 mg/L, and 3.95 mg/g for Ni(II) at pH 7.0, temperature 318 K and initial metal ion concentration 80 mg/L with MRS dosage 10 g/L, respectively. The adsorption data were suitable for Langmuir isotherm model better both in single and binary systems. The pseudo-second-order model could better describe the adsorption process in single and binary systems. In binary systems, the existence of Ni(II) hindered the adsorption of Cr(VI) but the removal of Ni(II) was enhanced with the existence of Cr(VI) ions. Thermodynamic constant values (ΔG0 < 0, ΔH0 > 0, ΔS0 > 0) illustrated that the adsorption of Cr(VI) and Ni(II) onto MRS were spontaneous and endothermic.

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