Phosphoric Acid Plus Hydrogen Peroxide Modified Biochar To Improve The Removal Performance Of Pollutants

Biochar（BC）was a carbon-rich solid produced by the pyrolysis of biomass in a limited oxygen environment.The performance of BC was largely related to its physical and chemical properties,which were affected by the type of biomass,pyrolysis process and treatment method.In this work,the wheat straw（WS）,oak sawdust（OS）and birch sawdust（BS）were employed as precursors that pyrolyzed at the different pyrolysis temperature.BC modification was attempted by the phosphoric acid（H₃PO₄）plus hydrogen peroxide（H₂O₂）（PHP）,and the modification concentration,temperature and duration were regulated.The adsorption capacity of methylene blue（MB）was used to reflect the effect of PHP modification.The effects of precursors,pyrolysis temperature and modification conditions on the physicochemical properties of BC were investigated,and the modification mechanism of BC modified by PHP and potential adsorption mechanism for MB were discussed.Results as follows:（1）To study the feasibility of PHP-modification to harvest an efficient BC adsorbent.BC from different pyrolysis temperatures（300-700℃）were modified with PHP to calculate the adsorption capacity of MB,and the similar process was also performed using H₃PO₄ and H₂O₂ alone to harvest BC as the compassions.The results showed that the PHP-modified BC significantly promoted the adsorption of MB,while the BC modified by H₃PO₄ and H₂O₂ alone did not significantly improve the adsorption performance of MB.With the increase of pyrolysis temperature,the adsorption capacity of PHP-modified BC to MB did not increase,and the adsorption capacity of PHP-modified BC at 300℃ to MB increased significantly.Correspondingly,approximately 21.5-fold MB adsorption capacity was achieved by PHP-modified BC comparing with its parent BC.To evaluate the compatibility of PHP-modification biochar from different precursors,the coefficient of variation based on MB adsorption capacity by the BC from various precursors was calculated,in which the coefficient of variation of PHP-modified BCs was 0.0038comparing to 0.64 of the corresponding parent BCs.These results suggested that the PHP method displayed the excellent feedstock compatibility on BC modification.（2）To study the influence of modification conditions on the effect of BC modified by PHP,OS was used as precursor to modify.The modification was carried out under the control of a single factor（H₃PO₄-H₂O₂ ratio,modification temperature and time）.The results showed that the BC modified by H₃PO₄ and H₂O₂ alone could not obtain the desired modification performance,but the addition of H₃PO₄ or H₂O₂ could promote the PHP modification effect,suggesting that H₃PO₄ and H₂O₂ had synergistic effect of PHP modification.When the temperature of PHP modification was 30-60℃ and the modification time was 1-5 h,the adsorption capacity of the modified BC to MB increased significantly.The maximum MB adsorption capacity was 454.1 mg/g and the removal rate was 90.8%when the H₃PO₄ and H₂O₂ fraction in PHP were 65.2%and 7.0%;the modification was further intensified by promoting temperature and duration.（3）To study the physicochemical properties of PHP-modified BC,elemental analysis,Fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）and solid-state nuclear magnetism（NMR）were used to characterize the modified BC.The content of C in PHP-modified BC was significantly reduced to 38.06%,while the content of O was increased to 56.46%.These results indicated that PHP modification improved the polarity of BC and reduced the aromatics.FT-IR,XPS and NMR spectra showed that the vibration peaks of carboxyl and ester corresponding to-C=O-O(1704 cm^-1)were significantly enhanced on BC after modified by PHP,and the contents of-C=O-O and carboxyl-C were increased to 8.28%and 8.78%,respectively,while the parent BC contents were 1.90%and 5.63%,respectively.It was indicated that the oxidation modification of PHP could increase the oxygen-containing functional groups on the BC surface.The disappearance of PHP-modified BC at 1514 cm^-1（aliphatic carbon-C=C）and the weakening of vibration at 1603 cm^-1（aromatic carbon-C=C）also supported the oxidation modification of PHP.（4）To study the adsorption process of MB by PHP-modified BC,the effects of BC dose and initial pH of MB on MB removal were investigated.When the adsorption dose ratio was 1.0 g/L,the removal rate reached 90.8%.The adsorption capacity of MB increased with the increase of solution pH,and the solution pH decreased after adsorption of MB.FT-IR,XPS and NMR spectra before and after adsorption showed that the characteristic peaks C-N and C-H of MB appeared at 1332 cm^-1 and 885 cm^-1,while the intensity and content of-C=O-O peak decreased.It showed that the oxygen-containing functional groups on PHP-modified BC participate in the adsorption reaction of MB.These results indicated that the potential adsorption mechanisms of PHP-modified BC on MB were cation exchange,complexation,electrostatic attraction,hydrogen bonding,n-πandπ-πinteraction dominated by the oxygen-containing functional groups.The adsorption of MB by PHP-modified BC was in accordance with the second-order kinetic model（R²=0.99）and Langmuir isothermal adsorption model（R²=0.98）.The results showed that the adsorption of MB by PHP-modified BC was a monolayer homogeneous chemical adsorption process.Thermodynamic parameters（ΔG°、ΔH°andΔS°）indicated that MB adsorption on PHP-modified BC was spontaneous endothermic reaction.（5）To study the practical application feasibility of PHP-modified BC,the recycling of H₃PO₄,the regeneration of PHP-modified BC and the feasibility of removing other pollutants were discussed.The average 94.5%H₃PO₄ was recovered after 10-batch modification,implying 1.0 kg H₃PO₄（85%）in PHP can maximally modify 2.37 kg BC.Regeneration experiments showed that PHP-modified BC had a strong ability to fix pollutants.The adsorption capacity of PHP-modified BC for removing tetracycline hydrochloride and heavy metal Pb（Ⅱ）were 20.9 and 4.5 times of parent BC,respectively.These results indicated that PHP-modified BC had a good application prospect.