Aqueous Removal Of U(?),Pb(?),and Eriochrome Black T By Using Chemical And Low-Cost Green Adsorbents

Industrialization plays a major role in polluting the environment with heavy metal ions,radionuclides,and organic dyes.The environmental pollution in general and water pollution,in particular,is a challenging issue for the modern world.Heavy metals,radionuclides,and organic dyes are used on the bulk scale and our environment is becoming more exposed to them.The removal of such unwanted species is of global interest.Schiff bases and their derivatives are applied as efficient adsorbents for the removal of heavy metal ions and radionuclides.Schiff bases are synthesized by the condensation reaction of primary amines and aldehydes.The azo groups in their structures are electron donating in nature and have strong complexation ability with heavy metal ions and radionuclides.Silicon-Schiff base complex(Si-SBC)and magnetite@SiO2-Si-SBC(M/SiO2-Si-SBC)materials were applied for the immobilization of lead(Pb(?))and uranium(U(?))whereas microwave-assisted spent black tea leaves(MASTL)was used for the adsorption of Eriochrome black T(EBT)from aqueous solutions.The adsorbent materials were characterized by powder X-ray diffraction(XRD),Fourier-transform infrared spectroscopy(FT-IR),scanning electron microscopy(SEM),transmission electron microscopy(TEM),vibrating sample magnetometry(VSM),thermogravimetric analysis(TGA),and nuclear magnetic resonance spectroscopy(NMR).The BET specific surface areas(SSA)of Si-SBC,M/SiO2-Si-SBC,and MASTL were 16.4,148.9,and 42.3 m2/g,respectively.The zeta-potential values for Si-SBC and M/SiO2-Si-SBC were 3.4 and 4.3,respectively.The point of zero charge(pHpzc)of MASTL was 4.6.Under different experimental conditions,the affinity of Si-SBC and M/SiO2-Si-SBC towards U(?)and Pb(?)and that of MASTL towards EBT was determined with macroscopic,spectroscopic,and modelling techniques.The high concentration of U(?)and Pb(?)were moved down the solution at pH=5.00±0.05 while suitable pH for maximum adsorption of EBT was 2.0.The macroscopic study showed that the adsorption of U(?)and Pb(?)was independent of the ionic strength of the solution.It indicated that the adsorption was mainly due to the inner-sphere surface complexation.The adsorption of both U(?)and Pb(?)was endothermic and well simulated by Sips isotherm model.However,the monolayer maximum adsorption capacity of Si-SBC obtained from Langmuir model at 25? and pH=5.00±0.05,was 5.18×10-4 mol/g for Pb(II)and 3.70×10-4 mol/g forU(VI).Similarly,the maximum adsorption capacity of M/SiO2-Si-SBC was 6.45 ×10-4 mol/g for Pb(?)and 4.82×10-4 mol/g for U(?).The adsorption capacity of M/SiO2-Si-SBC,for both U(?)and Pb(?),was higher than that of Si-SBC.The adsorption data of EBT onto MASTL followed the Langmuir adsorption isotherm and the maximum adsorption capacity at 25? was 242.72 mg/g.The adsorption kinetic data of U(?)and Pb(?)best followed the pseudo-second-order kinetic model and thus was chemical in nature.Whereas,the adsorption kinetic data of EBT followed both the pseudo-first-order and pseudo-second-order kinetic models and was physicochemical in nature.The thermodynamic study showed that the adsorption of U(?)and Pb(?)on Si-SBC and M/SiO2-Si-SBC,and that of EBT on MASTL was spontaneous,endothermic,and random at solid-solution interface.At pH<pHpzc(4.6),the surface of MASTL was positively charged,whereas that of EBT was negatively charged and the sorption process was promoted due to electrostatic attractions between the two counter ions.The density functional theory(DFT)calculations revealed that the adsorption energies of 7.61 kcal/mol for Pb2+-(Si-SBC)and 2.72 kcal/mol for UO22+-(Si-SBC)were mainly attributed to stronger electrostatic interactions.The DFT calculations further revealed that these attractive forces operated between N and O atoms of the adsorbents,and U(?)and Pb(?)ions of the pollutants.The DFT calculation results were consistent with the results of the batch sorption experiments.The results revealed that the Si-SBC and M/SiO2-Si-SBC can be used as efficient adsorbents for treating U(?)and Pb(?)polluted water.Similarly,MASTL can be used for the purification of EBT contaminated water.Moreover,the U(VI)and Pb(?)can be desorbed by lowering the pH of the solution and both the adsorbent materials(Si-SBC and M/SiO2-Si-SBC)can be recycled for at least five times without any considerable loss in their adsorption capacities,which was necessary for their practical applications in environmental pollution clean-up.