Exhaustive studies on toxic Cr(VI) removal mechanism from aqueous solution using activated carbon of Aloe vera waste leaves

Abstract

Abstract The study was aimed to synthesize activated carbon from waste Aloe vera leaves powder by H2SO4 and HNO3, as activators followed by pyrolysis. The prepared activated carbon AL-SAT-AC (Aloe vera leaves treated with H2SO4 activated carbon) and AL-NAT-AC (Aloe vera leaves treated with HNO3 activated carbon) were used for hazardous Cr(VI) ions removal from aqueous solution. For getting information about chemical, physical, and surface properties, the synthesized AL-SAT-AC, AL-NAT-AC were characterized by several techniques such as elemental analysis, BET, XRD, FTIR, SEM, FE-SEM, EDS, TEM and HR-TEM. The change in its properties after Cr(VI) adsorption, the AL-SAT-AC-Cr(VI), and AL-NAT-AC-Cr (VI were also examined by FTIR, SEM, FE-SEM and EDS. The optimum condition for maximum Cr(VI) removal of 98.66% by AL-SAT-AC was initial pH 1.23, equilibrium time 150 min, adsorbent dose 2 g/L, initial Cr(VI) concentration 50 mg/L, agitation speed 150 rpm and temperature 25 ± 1 °C. While for AL-NAT-AC, the maximum of 98.89% removal occurred at initial pH 1.21 and equilibrium time 90 min, whereas other experimental conditions remained same as AL-SAT-AC adsorbent. The Cr(VI) removal efficiency was rapid for AL-NAT-AC as compared with AL-SAT-AC, due to the presence of amino or nitro groups. According to R2 values of fitted models, the pseudo-second-order kinetic and Langmuir isotherm model described the adsorption process very well for AL-SAT-AC and AL-SAT-AC. Intraparticle diffusion and Bangham's model were applied to the experimental data to check the mass transfer mechanism for adsorption phenomena of Cr(VI) onto AL-SAT-AC, and AL-NAT-AC. The enthalpy change for this adsorption process was found to be negative for both adsorbents, indicating that the process was exothermic nature and highly feasible at low temperatures. Regeneration studies were also done for both the adsorbents using desorbing reagents such as KOH, Na2CO3, NaHCO3, and NaOH.