Adsorption Performance And Mechanism Of Modified Sludge-based Biochar On Methyl Orange And Hexavalent Chromium

In this study,sludge-based biochar was prepared by slow thermal cracking method using sewage sludge from wastewater treatment plant,and the sludge-based biochar was modified by adding the biomass materials hybrid liriodendron leaves（SLAC）,loaded Fe³⁺（SLAC-Fe）and secondary calcination（SLAC-Fe-600）.Meanwhile,the optimal modification conditions of the three modified materials were determined by single factor analysis using the removal rates of methyl orange and Cr（VI）as the target response values,and the modified materials were characterized by SEM and XRD.The removal performance of the three modified sludge-based biochars for methyl orange and Cr（VI）was examined,and the effects of different influencing factors on the removal of methyl orange and Cr（VI）were investigated by single factor batch experiment,and the experimental data were fitted with adsorption kinetics and isothermal adsorption models to explore the types of adsorption of methyl orange and Cr（VI）by the modified sludge based biochar materials.The interaction between methyl orange and Cr（VI）was investigated in a binary adsorption system,the adsorption data were fitted with an isothermal adsorption model,and the adsorption and removal mechanism of methyl orange and Cr（VI）by the modified sludge based biochar SLAC-Fe-600 was proposed in combination with FTIR and XPS analysis techniques.This thesis provides a theoretical basis for the treatment of dyeing wastewater,and the following research results were obtained.（1）The highest removal efficiency of methyl orange and Cr（VI）was achieved when the mass ratio of sludge and hybrid liriodendron leaves was 5:2 and the concentration of modifier Fe Cl₃ was0.002 and 0.05 mol/L,respectively.The maximum removal efficiency for Cr（VI）was obtained at the recalcination temperature of 600°C.SLAC-Fe-600 has the largest specific surface area of243.823 m²/g,which is much larger than that before secondary calcination.The loading of Fe³⁺promoted the growth of total pore volume and the increase of pore size,but the pore size became smaller after the second calcination due to the blockage of pore size by Fe₂O₃.The functional groups contained on the surface of the modified sludge-based biochar SLAC-Fe included-OH,N-H and-NH₂.After loading with Fe ions,the samples were recalcined and crystalline diffraction peaks of Si O₂ and Fe₂O₃ were found in the XRD spectra of the material SLAC-Fe-600.SLAC-Fe-600 was not easily decomposed at a high temperature of 400°C and had good thermal stability.（2）Modified sludge-based biochar SLAC-Fe is a more efficient adsorbent for MO adsorption than SLAC.The pseudo-second-order kinetic models of both materials fitted well with the experimental data,and the correlation fit coefficients were both greater than 0.99.The removal rate of MO by the modified sludge-based biochar SLAC-Fe was maintained above 70%after six cycles,and the recovery rate of the adsorbent material was over 80%.The modified sludge-based biochar SLAC-Fe-600 is a more efficient adsorbent for Cr（VI）than SLAC.the experimental data on the adsorption equilibrium of Cr（VI）by SLAC-Fe-600 matched well with the Langmuir isotherm model,with a maximum adsorption capacity of 100.55 mg/g and a high fitting coefficient with the pseudo-second-order adsorption kinetic model The results showed that the adsorption of Cr（VI）on SLAC-Fe-600 was a chemisorption of a single molecular layer.The removal rate of Cr（VI）by modified sludge-based biochar SLAC-Fe-600 was maintained above 70%after six cycles,and the recovery rate of adsorbed material by magnetic separation was also over 95%,and the material recovery efficiency was greater than that of SLAC-Fe.（3）In the binary system in the presence of Cr（VI）,the maximum removal efficiency of methyl orange was 96.5%,95.8%and 94.2%when the Cr（VI）concentrations were 0 mmol/L,0.2 mmol/L and 0.8 mmol/L,respectively,and decreased gradually with the increase of Cr（VI）concentration.On the other hand,the maximum adsorption capacity of methyl orange decreased with the increase of Cr（VI）concentration,and the maximum adsorption capacity Q₀ for methyl orange was 0.26 and0.22 mmol/g when the Cr（VI）concentration was 0.2 and 0.8 mmol/L,which decreased by 10.3%and 24.1%,respectively.Meanwhile,the adsorption experimental data of methyl orange were in good agreement with the Freundlich isothermal adsorption model.The R_L values of both the single and binary systems were less than 1,indicating that the adsorption process of methyl orange was favorable.The removal mechanism of methyl orange mainly includes electrostatic attraction force between SO₃^-and positively charged functional groups;nitrogen atoms in methyl orange form hydrogen bonds with-OH or-NH₂.The removal mechanism of Cr（VI）mainly includes electrostatic attraction force between HCr O₄^-and Cr₂O₇^2-and positively charged functional groups on the surface;the adsorbed Cr（VI）is partially reduced by Si-O-Si,-OH and Fe³⁺and other functional groups were partially reduced to Cr（III）;ion exchange between Cl^-and HCr O₄^-/Cr₂O₇^2-.The affinity between Cr（VI）and SLAC-Fe was greater than that between methyl orange and SLAC-Fe,so Cr（VI）was more easily adsorbed,resulting in a lower removal efficiency of methyl orange.（4）The presence of methyl orange in the aqueous solution enhanced the adsorption potential of Cr（VI）,and the maximum adsorption uptake（Q₀）of Cr（VI）was 0.35 mmol/g when the concentration of methyl orange was 0.2 mmol/L,and 0.46 mmol/g when the concentration of methyl orange was 0.8 mmol/L,compared with the single-component system.The maximum adsorption capacity increased by 6.1%and 39.4%,respectively.The Langmuir-Freundlich model fitted the Cr（VI）adsorption data well（0.969）.The Langmuir-Freundlich model assumes that the surface of the adsorbent is homogeneous and that the adsorption is a cooperative process due to the adsorbate-adsorbate interaction,which confirms that the presence of methyl orange promotes the adsorption of Cr（VI）.The adsorption mechanism of Cr（VI）on the material SLAC-Fe-600mainly includes:protonated imines and amines can bind to Cr（VI）by electrostatic attraction;protonated quinone imines exhibit chelation of Cr（III）and doping adsorption of Cr（VI）;redox effect of Fe³⁺on Cr（VI）.When methyl orange was adsorbed on the surface of SLAC-Fe-600,there was a positively charged N-CH₃⁺on the methyl orange molecule,which promoted the electrostatic attraction between HCr O₄^-and Cr O₄^2-.（5）The interference experiments of anions and cations found that the presence of cation Na⁺promoted the adsorption of the material on methyl orange,and the removal efficiency of methyl orange increased by 3.5%when the concentration of Na⁺was 10 mg/L,and by 9.7%when the concentration of Na⁺was 50 mg/L.SLAC-Fe had higher selective adsorption on methyl orange.In the experiment of interference of anions on Cr（VI）adsorption performance,the presence of SO₄^2-reduced the adsorption of SLAC-Fe-600 on Cr（VI）,and the adsorption capacity of Cr（VI）was reduced by 8.0-16.0%when the concentration of SO₄^2-was increased from 10 mg/L to 50mg/L,which was due to the different electronegativity between SO₄^2-and CO₃^2-,NO₃^-,PO₄^3-.The metal cations Zn²⁺,Mg²⁺,Na⁺and K⁺all showed a facilitative effect on the removal efficiency of Cr（VI）.Both modified sludge-based biochar showed high engineering utility.