Qualitative and Quantitative Characterization of Adsorption Mechanisms for Pb2+by Rice-straw Biochar
The adsorption characteristics of Pb2+in water by rice-straw biochar were elucidated by adsorption isotherm and kinetics,and the adsorption mechanism of biochar was qualitatively and quantitatively revealed by means of elemental analysis,BET(Brunauer-Emmet-Teller)-N2 specific surface area analysis,Zeta potential,scanning electron microscope-energy dispersive X-ray spectroscopy(SEM-EDS),Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS),along with the calculation for relative distribution of each adsorption mechanism.The results indicated that the maximum adsorption capacity of Pb2+by biochar were 233.48 mg/g,236.00 mg/g and 237.50 mg/g under the temperatures of 20 ℃、30 ℃ and 40 ℃,respectively.The experimental data fitted the Langmuir(R2＞0.96)better than the Freundlich isotherm,which suggested monolayer adsorption and a homogeneous distribution of active surface sites.Under the initial concentrations of 50 mg/L,100 mg/L and 300 mg/L,the equilibrium time of biochar were 30 min,90 min and 360 min,respectively,which followed the pseudo-second-order mechanism and corresponded to a chemisorption process.The main mechanisms responsible for Pb2+adsorption by rice-straw biochar included chemical precipitation,cation exchange,Cπ-electron coordination,and functional group complexation,in which the chemical precipitates could be mainly Pb4(CO3)2(SO4)(OH)2 and PbCO3,accounting for 47.15％～50.81％of total adsorption.Cation exchange also played an important role in the adsorption mechanisms due to high contribution proportions to total adsorption(32.82％～37.77％).Among the adsorption mechanisms,both the Cπ-electron coordination and functional group complexation could contribute 15.03％to 16.37％to the total adsorption,but these were overshadowed by the chemical precipitation and cation exchange,accounting for 83.63％to 84.92％in the total adsorption.These suggested rice-straw biochar could be an efficient adsorbent for the removal of Pb2+from aqueous solution.
straw biocharheavy metalschemical precipitationion exchangemechanism ratio
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