Study On The Preparation Of Modified Straw Biochar And Its Adsorption Performance And Mechanism Of Low Concentration Heavy Metals In Water

Heavy metals have cumulative effects in the ecosystem,even if they have low toxicity,they will be transformed into highly toxic compounds under the change of the surrounding environment and accumulate to toxic concentrations in the environment or organisms.In this paper,the original biochar（BC）was made from corn stalk,HBC was made by hydrochloric acid activation,Fe³⁺@HBC was made by ferric chloride（Fe Cl₃·6H₂O）modification,and TEPA-Fe³⁺@HBC was made by chitosan（CS）and TEPA-TEPA composite modification.Taking Cr（Ⅵ）and Ni（Ⅱ）as the research objects,the adsorption performance and recycling performance of single heavy metal solution and binary heavy metal mixed solution were studied by static and dynamic adsorption experiments,and the effects of p H,dosage,adsorption time,initial concentration and adsorption temperature on the adsorption performance were studied.The adsorption mechanism was discussed and analyzed by SEM,EDS,BET,FTIR,XRD and XPS.The effects of TEPA-Fe³⁺@HBC-CS gel spheres on the dynamic adsorption of low-concentration Cr（Ⅵ）and Ni（Ⅱ）were studied and analyzed under the conditions of different packing height,influent concentration and influent flow rate,which provided theoretical data for the application of this adsorbent.（1）The optimum conditions for preparing BC and Fe³⁺@HBC were determined by single factor and orthogonal experiments:corn stalk was pyrolyzed at 400℃for 70min to prepare BC;Fe³⁺@HBC was prepared with hydrochloric acid concentration of 5mol/L and the mass ratio of HBc:Fe Cl ₃6H₂O of 1:0.4.The prepared Fe³⁺@HBC has rough surface,rich network structure and amorphous structure.Static experiments show that the optimal adsorption p H of Fe³⁺@HBC for Cr（Ⅵ）is 2 and Ni（Ⅱ）is 9.The adsorption of Cr（Ⅵ）and Ni（Ⅱ）conforms to the pseudo-second-order kinetic model,mainly chemical adsorption,which is an endothermic entropy-increasing reaction.The adsorption is spontaneous and disorderly,and the adsorption reaction is easy to occur.The Freundlich model of Cr（Ⅵ）has a high degree of fitting,which is heterogeneous adsorption.Ni（Ⅱ）accords with Langmuir model,and single-layer adsorption is dominant,while Freundlich model R²>0.9000,and heterogeneous adsorption also exists.（2）Preparation and characterization of TEPA-Fe³⁺@HBC-CS:The effects of CS mass,TEPA concentration,ECH mass fraction and Fe³⁺@HBC dosage on pelletization and adsorption were studied by single factor experiment.The optimum ratio was determined by orthogonal experiment.The adsorption capacity of Cr（Ⅵ）was increased by 10.8 times,and the adsorption capacity of Ni（Ⅱ）was increased by 1.7 times.The surface was rough,the layered thickness became thinner,the pores were evenly distributed and the pores were through.The increase of n element content and the appearance of Fe element peak;TEPA successfully crosslinked and destroyed the original hydrogen bond between amino group and hydroxyl group,which made the material structure amorphous.（3）In a single system,the optimal p H of TEPA-Fe³⁺@HBC-CS gel spheres for adsorption of Cr（Ⅵ）and Ni（Ⅱ）were 3 and 8,respectively,and the removal rate increased with the increase of dosage.The fitting degree between adsorption process and pseudo-second-order kinetic model is high,and chemical adsorption is the main one.The adsorption process accords with Freundlich model,and there are multiple layers of adsorption.Increasing the temperature is beneficial to the adsorption,which is spontaneous and disorderly,and the adsorption reaction is easy to occur.After five times of adsorption-desorption,the adsorption rate of TEPA-Fe³⁺@HBC-CS gel ball for Cr（Ⅵ）and Ni（Ⅱ）remained at 80.93%and 71.71%.（4）In binary system,TEPA-Fe³⁺@HBC-CS gel ball has the best adsorption p H of 4.7,and the adsorption efficiency of Cr（Ⅵ）is obviously higher than that of Ni（Ⅱ）,indicating that the material preferentially adsorbs Cr（Ⅵ）.Cr（Ⅵ）and Ni（Ⅱ）are mainly chemisorbed.When the initial concentrations of Cr（Ⅵ）and Ni（Ⅱ）increase,the equilibrium adsorption capacity is significantly improved,and the single factor in the binary system has limited adsorption or the interaction between the low concentrations is not obvious.In the binary system influenced by coexisting ions,the inhibitory effect on Cr（Ⅵ）decreases with the increase of the concentration of coexisting ions.CO₃^2-has the strongest inhibitory effect on Cr（Ⅵ）,and Cu²⁺has the greatest influence on the adsorption inhibition of Ni（Ⅱ）.（5）The dynamic adsorption experiment shows that the dynamic breakthrough curve of TEPA-Fe³⁺@HBC-CS gel ball for adsorbing Cr（Ⅵ）and Ni（Ⅱ）is related to the adsorbent filling height（H）,influent concentration（C₀）and influent flow rate（Q）.Penetration time and depletion time are directly proportional to H,and inversely proportional to C₀and Q.（6）Analysis of adsorption mechanism:XRD analysis after adsorption showed that the amorphous crystallinity of TEPA-Fe³⁺@HBC-CS gel ball structure decreased,which confirmed that ECH successfully crosslinked TEPA.EDS analysis showed that BC was successfully modified.The modified TEPA-Fe³⁺@HBC-CS gel sphere reacts with heavy metal ions by complexation,ion exchange,redox and electrostatic adsorption,and the metal ions are fixed on its surface.FTIR and XPS analysis show that-OH,-COOH,-NH₃⁺,-NH₂and Fe³⁺may be involved in the removal of Cr（Ⅵ）.Oxygenates of-OH,-NH₂,-COOH and Fe can coordinate with water to form hydroxylated surfaces,and can exchange with Ni（Ⅱ）.