Research On Multiple Modifications Based On Calcium-rich Biochar And Its Structure And Adsorption Performance

Heavy metal pollution in water is still one of the key problems facing the world today.It is particularly important to choose treatment technologies with high efficiency,economy and environmentally friendliness for heavy metal pollution.In recent years,biochar adsorption has become one of the most promising technologies due to its low cost,high removal rate,and wide sources of raw materials.Crustacean biological shells such as shrimp shells are excellent precursors of calcium-rich biochar.However,the adsorption performance of the original calcium-rich biochar for heavy metals remains to be improved.Therefore,multiple modification methods were used to promote its application and development in the field of heavy metal adsorption.1.In this paper,shrimp shell was used as raw material to prepare calcium-rich biochar.The calcium-rich biochar loading Si/Mn binary oxide after NaOH activation（A-Si/Mn-CRBC）was prepared by chemical modification method.The main mechanism of the modification was studied.Adsorption performance and mechanism of A-Si/Mn-CRBC for Cu（Ⅱ）were investigated and comprehensively analyzed.The main conclusions were as follows:（1）The main modification mechanism of NaOH pre-activation and Si/Mn binary oxide loading was explored by using SEM+EDS,FTIR,XRD and BET.The results showed that Si and Mn were successfully supported on the surface of calcium-rich biochar in the form of oxides.NaOH pre-activation was beneficial to improve the uniformity and crystallinity of Si/Mn oxide loading,and made the surface of A-Si/Mn-CRBC smoother and denser.（2）The adsorption performance of A-Si/Mn-CRBC was investigated by the batch adsorption experiment of Cu（Ⅱ）.The results showed that the A-Si/Mn-CRBC with the strongest Cu（Ⅱ）adsorption capacity was prepared under the conditions of pyrolysis temperature of 800℃ and Si:Mn molar ratio of 2:1.When the dosage was 1 g/L and the initial p H=3-6,the removal rate of 50 mg/L Cu（Ⅱ）solution by A-Si/Mn-CRBC was close to 100%.The adsorption process of Cu（Ⅱ）on A-Si/Mn-CRBC can be well described by pseudo-second-order kinetic model and Langmuir isotherm model,indicating that the process was spontaneous and endothermic monolayer chemisorption combined with thermodynamic analysis.The Langmuir maximum theoretical adsorption capacity of A-Si/Mn-CRBC obtained at 30℃ was 141.76 mg/g.The results fitted by Webber-Morris model showed that the adsorption rate of 100 mg/L Cu（Ⅱ）on A-Si/Mn-CRBC was controlled by both surface and pore diffusion.After 5 cycles of adsorption experiments,the removal rate of Cu（Ⅱ）by A-Si/Mn-CRBC can still reach76.60%,which was much higher than the original calcium-rich biochar（35.02%）.（3）A-Si/Mn-CRBC before and after Cu（Ⅱ）adsorption were characterized by FTIR and XPS.The adsorption mechanism of Cu（Ⅱ）was comprehensively discussed combined with the ion leaching and demineralization experiments of A-Si/Mn-CRBC.The results showed that the adsorption mechanism involved surface complexation,precipitation,ion exchange and pore filling.Among them,the contribution rate of complexation and pore filling was 34.46%,ion exchange was 9.63%,and precipitation was 55.91%.Therefore,precipitation was the main mechanism for the adsorption of Cu（Ⅱ）by A-Si/Mn-CRBC.2.The co-pyrolysis calcium-rich biochar was prepared by a more economical and environmentally friendly modification method in which a certain proportion of straw（CS）was doped into shrimp shells（SS）for co-pyrolysis.The co-pyrolysis synergistic effects were comprehensively analyzed from the three aspects of pyrolysis gas,bio-oil and biochar.The adsorption performance and mechanism of co-pyrolysis calcium-rich biochar on Cu（Ⅱ）were explored.The main conclusions were as follows:（1）The interaction between SS and CS in the co-pyrolysis process and the morphology of co-pyrolysis calcium-rich biochar were investigated by TG-FTIR,pyrolysis kinetics,Py-GC/MS,SEM+EDS,FTIR,XRD and BET.The results showed that the doping of CS can not only improve the thermal degradation characteristics and pyrolysis reactivity,but also reduce the reaction activation energy and promote the continuous pyrolysis of biomass,but did not change the main pyrolysis mechanism.When the CS doping ratio was 25%（75SS+25CS）,the synergistic effects between SS and CS were the strongest,which led to the increase of CH₄,CO₂,CO and NH₃ release in cracked gas,while the relative content of N-heterocyclics and oxygenates in bio-oil decreased.75SS+25CS₈₀₀ had a more developed microporous and aromatic structure,and a more thorough mineral decomposition degree.（2）The adsorption performance of co-pyrolysis calcium-rich biochar was analyzed by Cu（Ⅱ）batch adsorption experiment.The results showed that75SS+25CS₈₀₀ can efficiently remove Cu（Ⅱ）at dosage of 1 g/L and p H=3-6.The adsorption of Cu（Ⅱ）on 75SS+25CS₈₀₀ was well fitted by the pseudo-second-order kinetic model and Langmuir isothermal model,and the maximum theoretical adsorption capacity of Langmuir was 79.77 mg/g.Combined with the fitting results of Webber-Morris,the adsorption process was summarized as a diffusion monolayer chemical adsorption.After 5 adsorption-desorption experiments,the removal rate of Cu（Ⅱ）by 75SS+25CS₈₀₀ can reach 68.94%.（3）FTIR and XPS spectrum before and after the adsorption of Cu（Ⅱ）combined with leaching and demineralization experiments of 75SS+25CS₈₀₀ were analyzed to comprehensively explore the adsorption mechanism.The results showed that the efficient adsorption of Cu（Ⅱ）by 75SS+25CS₈₀₀ was mainly through surface complexation,precipitation,ion exchange,cation-πinteraction and pore filling.Among them,the contribution rate of ion exchange reached 48.01%,which was the main mechanism of Cu（Ⅱ）adsorption on 75SS+25CS₈₀₀.Complexation,cationic-πinteraction and pore filling jointly contributed 37.95%,and precipitation contributed14.04%.