Study On The Adsorption Behaviors Of Cadmium And Nickel By Porous Media

Recharge of groundwater with reclaimed water is helpful to solve the problem of water shortage,but heavy metals such as Cd and Ni contained in the water may also be imported into the soil and groundwater environment,resulting in pollution risk.Adding montmorillonite or biochar to the actual soil can reduce the migration risk of heavy metals through adsorption.In this study,experiments were carried out by batch equilibrium method,and the adsorption capacity and mechanisms of Cd and Ni on montmorillonite,river sand and biochar were explored by correlation analysis.The findings of this study have a certain significance to evaluate the environmental behavior and migration risk of Cd and Ni in different media.The main conclusions are as follows:（1）Montmorillonite has strong adsorption capacity for heavy metals Cd and Ni.The maximum adsorption capacity(Q_max)of Cd and Ni can reach 214 and 252μmol/g respectively in the solution with Cd and Ni alone,and 128 and 188μmol/g respectively in the solution with both Cd and Ni.Cd and Ni exchange mainly with Ca²⁺and Mg²⁺in montmorillonite.In addition,the surface of montmorillonite is negatively charged,and there is electrostatic attraction between montmorillonite and two cationic heavy metals.The adsorption capacity of montmorillonite to Ni is slightly stronger than that of Cd,which is mainly caused by the difference in ion species.In the solution with both Cd and Ni,Cd mainly exists in the form of CdCl⁺,while Ni almost exists in the free state of Ni²⁺,therefore,Ni is more likely to have electrostatic adsorption and ion exchange with montmorillonite.（2）The adsorption capacity of river sands is 2.56 and 13.2μmol/g for Cd and Ni,respectively.The main reason is that there are a small amount of montmorillonite and calcite in the sands.Cd is mainly adsorbed on the surface of river sediment by cation exchange.Under the same experimental conditions,the cation exchangeable capacity of montmorillonite in river sands is lower than that of commercial montmorillonite,which makes its adsorption capacity of Cd lower.The main adsorption mechanisms of Ni on river sediment include not only cation exchange,but also coprecipitation,consequently,the adsorption capacity of Ni on river sediment is higher than Cd under the same conditions.（3）In the solution with both Cd and Ni,the adsorption capacities of biochars with carbonization temperature of 300,500 and 700℃for Cd（15.6,33.8 and 95.2μmol/g,respectively）are significantly higher than those of Ni（12.0,25.4,57.7μmol/g,respectively）.The main adsorption mechanisms of biochar with carbonization temperature of 300℃are cation exchange and surface complexation with oxygen-containing functional groups;the main adsorption mechanisms of biochar with carbonization temperature of 500℃and 700℃are electrostatic（including combination with aromatic ring）and coprecipitation.