Preparation Of Water Chestnut Shell-based Biochar And Sodium Alginate/gelatin @ Biochar Composites And Their Adsorption Behavior Of Cd²⁺

The past few decades have seen rapid urbanization and industrialization,heavy metal pollution from mining,metallurgy,machinery manufacturing and smelting industries has become a serious threat to human health.Adsorption method has been widely used in the treatment of cadmium ion wastewater due to its advantages of easy operation,rapid,high efficiency,low cost,and variety of adsorbents biochar is a promising adsorbent due to its wide range of raw materials and easy preparation.However,pyrolytic biochar was not satisfactory in the removal of cadmium wastewater due to its small specific surface area and low surface functional group content.Therefore,further modification of biochar is needed to improve its adsorption performance.Conventional acid and alkali modification not only has the disadvantage of high cost,but also causes corrosion of equipment.Therefore,green environmental protection,low cost and high efficiency activators need to be developed.Based on the above problems being solved,the second problem follows.In practical production and application,dust was easily formed after biochar application due to its low density,resulting in air pollution and low utilization rate of biochar.Moreover,biochar after use is difficult to separate solid-liquid or recycle.In addition,due to its low hydraulic conductivity,biochar in powder form is not suitable for column filtration,which further limits its application as an adsorbent.Therefore,it is urgent to develop new adsorbents to overcome these problems.Sodium alginate（SA）is a linear polysaccharide mainly extracted from kelp or seaweed.The large number of carboxyl and hydroxyl groups distributed along the molecular chain can serve as excellent sites for capturing heavy metals.Sodium alginate will become gelatinous in the presence of cationic crosslinking agent.Based on this characteristic,this study considers to prepare a new adsorbent material by combining it with Water chestnut shell-based biochar.To sum up,this study is mainly divided into two parts:In the first part,HPO₄^2-,PO₄^3-,CO₃^2-and CH₃COO^-,which have been proved to have potential activation ability,are used as acidic groups;The Na⁺,which can be used as the in-situ activator,is used as the basic group.Modified biochar SCBC,SPBC,DSPBC and SABC were prepared using strong base and weak acid salts Na₃PO₄,Na₂CO₃,Na₂HPO₄ and CH₃COONa as activators,and pyrolytic biochar BC without activator was also prepared.The Cd²⁺batch experiments of SCBC,SPBC,DSPBC,SABC and BC were carried out simultaneously to study the effects of adsorption dose,initial p H of solution,adsorption time and initial concentration of solution on Cd²⁺.FT-IR,SEM,XRD and XPS were used to analyze the biochars before and after adsorption,and the adsorption mechanism of biochars was explored by modern analysis methods.Based on previous studies,each adsorption mechanism was quantitatively analyzed.The second part:the natural macromolecular material gelatin,which is also rich in hydroxyl and carboxyl groups,was blended with sodium alginate.At the same time,the pyrolytic biochar BC was added to prepare multiple composite gel materials SA/GE@BC.The adsorption properties of SA/GE@BC to Cd²⁺under different p H,adsorbent dosage,adsorption time,adsorption temperature and other conditions were investigated by static adsorption experiments.Modern analytical instruments such as FT-IR,SEM,XRD and XPS were used to analyze the adsorbent before and after adsorption,and the adsorption mechanism of Cd²⁺was discussed.Moreover,the adsorption experiment under the condition of coexistence of humic acid and repeated regeneration experiment of SA/GE@BC were carried out to explore its adsorption effect of Cd²⁺and reusability under the condition of exogenous influence.The main research results obtained in this study are as follows:（1）The adsorption capacity of biochar to Cd²⁺was stable when the p H value was4-7.The adsorption behavior of biochars for Cd²⁺can be well fitted by pseudo-second-order kinetic model and Langmuir model,and the maximum adsorption capacity of SCBC,SPBC,DSPBC,SABC and BC fitted by Langmuir model were 91.47,86.52,74.16,63.27 and 60.08 mg/g,respectively.The adsorption of Cd²⁺by SCBC,SPBC,DSPBC,SABC and BC were spontaneous endothermic processes.Characterization analysis of SCBC,SPBC,DSPBC,SABC and BC before and after adsorption and quantitative calculation of adsorption mechanism can be concluded that biochar adsorption mechanism of Cd²⁺includes oxygen-containing functional group complexation,Cd²⁺-π,cation exchange,and mineral precipitation.Compared with other biochars,the biochars prepared in this study has the characteristics of low cost,high efficiency,and green environmental protection,so it has good development and application potential.（2）The prepared multicomponent composite gel spheres have the best adsorption performance when the SA concentration in organic matrix was 0.5%and the mass ratio of BC,GE and SA was 1:1:1.The adsorption performance of SA-GE@BC gel spheres with three components was better than that of two-component composite gel spheres and single-component biochar.Compared with SA-GE@BC,BC has a higher specific surface area and a more developed pore structure,while SA-GE@BC has a richer number of oxygen-containing functional groups than BC.The adsorption capacity of SA-GE@BC to Cd²⁺is positively correlated with the p H value from 4 to 7,and then reaches a maximum of 12.36 mg/g when the solution p H reached to 7.Pseudo-second-order kinetic model and Langmuir model can explain the adsorption behavior of SA-GE@BC on Cd²⁺,and the maximum adsorption capacity of SA-GE@BC on Cd²⁺calculated by Langmuir model was 86.25 mg/g.The adsorption mechanisms of SA-GE@BC for Cd²⁺include oxygen-containing functional group complexation,Cd²⁺-πcoordination,cation exchange and electrostatic attraction.High concentration of humic acid promoted the adsorption of SA-GE@BC on Cd²⁺.After five regeneration experiments,the adsorption capacity of SA-GE@BC to Cd²⁺can still reached 70%of the original adsorption capacity.Therefore,as a new adsorbent,SA-GE@BC has a good practical application potential for the treatment of cadmium ion polluted wastewater.