Study On Adsorption Characteristics And Mechanism Of Pb(?),Cd(?),and Cr(?) By Bentonite-Steel Slag Composite Particles

Mining area pollution caused by acid mine drainage(AMD)is a major environmental problem in the world.In particular,a large amount of AMD containing Pb(?),Cd(?),and Cr(?)produced by lead-zinc ore,copper-lead-zinc ore,cadmium ore,and chromium ore during mining and smelting has serious harm to the mining environment and human body.Aiming at the treatment of Pb(?),Cd(?),and Cr(?),this paper makes a systematic and in-depth study on the adsorption characteristics and mechanism of Pb(?),Cd(?),and Cr(?)by using bentonite-steel slag composite particles developed by our research group,which not only release alkalinity to neutralize acid,but also have the properties of adsorption,flocculation,and co-precipitation of heavy metal ions.It provides a theoretical basis for the application of the new multi-functional mineral adsorption material in the treatment of heavy metals such as Pb(?),Cd(?),Cr(?)and other acid mine drainage.It is important to enrich the adsorption and precipitation theory of aqueous solution.Through static experiments,the effects of different factors such as dosage,reaction time,initial ion concentration,reaction temperature,and pH on the adsorption and removal of Pb(?),Cd(?),and,Cr(?)of composite particles were determined.The adsorption rate was fast at the initial stage of the reaction.After the reaction for 300 min,the adsorption of Pb(?)and Cd(?)reached equilibrium.After 420 min of reaction,Cr(?)adsorption reached equilibrium.Moreover,the total adsorption rate of composite particles to heavy metal ions is determined by both membrane diffusion and intra-particle diffusion,adsorption characteristics are consistent with the pseudo-second order kinetics model.The adsorption process of Pb(?),Cd(?),and Cr(?)onto composite particles is multi-layer adsorption,which meets the BET model.Furthermore,Thermodynamic analysis shows that ?G0 becomes negative,?H0 and ?S0 become positive,the adsorption process is spontaneous and endothermic,and that the system degree of freedom increases with time.The composite particles have excellent adsorption properties for Pb(?),Cd(?),and Cr(?).Through ion exchange experiment,ion strength effect experiment,composite particles,dealkalization particles,and NaOH removal effect experiment,combined with SEM and XRD microscopic test analysis,it is revealed that the removal mechanism of composite particles on Pb(?),Cd(?),Cr(?)has electrostatic adsorption,ion exchange,external surface complexation,precipitation effect.In particular,the adsorption-coagulation,that is,a large number of tiny pores formed after the precipitation formed by the adsorption of heavy metal ions on the particle surface continue to play the mechanism of adsorption and precipitation removal.The mineral phase after the reaction mainly existed in stable forms of PbO,Pb2SiO4,Pb5Si4O8(OH)10,Pb3(CO3)2(OH)2,Cd(OH)2,Cd2SiO4,Cd5Si2O8(OH)2,Cr2O3,and Cr3SiO4(OH)2.In view of the coexistence of a variety of typical metal ions in AMD,the binary competitive adsorption characteristics and mechanism of Pb(?),Cd(?),Cr(?),Fe(?),Mn(?),Cu(?),and Zn(?)onto composite particles were studied.The adsorption quantity of Pb(?),Cd(?),and Cr(?)in binary system are decreased,but no matter single or binary competitive adsorption,adsorption quantity meets the Pb(?)>Cd(?)>Cr(?).According to the sum of the distribution coefficients,the selection sequence of heavy metal ions is Pb(?)>Cd(?)>Fe(?)>Cu(?)>Cr(?)>Zn(?)>Mn(?).The selection adsorption sequence is determined by the metal ion hydrolysis constant,the hydration ion radius,the molar mass,and the solubility product constant of metal ion hydroxide.With low cost and excellent performance,bentonite-steel slag composite particles have excellent neutralization-adsorption-coagulation properties for AMD containing Pb(?),Cd(?),and Cr(?).The research results in this paper have enriched and improved the adsorption chemistry theory and precipitation theory of aqueous solution,and provided theoretical basis for practical engineering applications.There are 80 figures,23 tables,and 178 references in this study.