Adsorption And Desorption Process And Mechanism Of Cadmium And Arsenic At The Interface Of Ferrihydrite-Aqueous Solution

Cadmium(Cd(Ⅱ))and arsenic(As(Ⅴ))are both trace carcinogens,which are widely distributed in contaminated paddy soil and water.Due to the difference in chemical properties and acid-base characteristics,the environmental behavior of Cd(Ⅱ)and As(Ⅴ)under a single presence is different from coexisting systems,which makes it difficult to predict the interface behavior and environmental fate of Cd(Ⅱ)and As(Ⅴ)under co-presence systems using the binding mechanism gained from a single system.Ferrihydrite is an important iron mineral that is widely distributed in paddy soil and rice rhizosphere.It usually exists in the form of nanoparticles,with a huge reaction-specific surface area that can significantly control the bioavailability and fate of nutrients and heavy metals in the soil.Therefore,research on the molecular binding mechanism and environmental processes of Cd(Ⅱ)and As(Ⅴ)at the ferrihydrite-water solution interface will help to predict the fate of Cd(Ⅱ)and As(Ⅴ)in polluted soil environments,and provide a theoretical basis for the remediation of contaminated soil.In the present study,we combined the wet chemistry,in-situ spectroscopy,surface complexation,modern analytical instruments(XPS,XRD,HR-TEM,etc.)to study the coexistence behavior and adsorption-desorption process of Cd(Ⅱ)and As(Ⅴ)at the ferrihydrite-solution interface,as well as the solid-liquid distribution of Cd(Ⅱ)and As(Ⅴ)on the surface of ferrihydrite during the aging process.The main results and conclusions are as follows:(1)The retention mechanism of Cd(Ⅱ)-As(Ⅴ)under the ternary coexistence system of Cd(Ⅱ)-As(Ⅴ)-ferrihydrite are clarified.The increased Cd(Ⅱ)sequestration on the surface of Fh in the presence of As(Ⅴ)involved multiple processes,including electrostatic interactions,surface ternary complexation,and precipitation.In addition,it revealed the concentration-dependent interactions between As(Ⅴ)and Cd(Ⅱ)on each other’s binding behavior on Fh.Both adsorption and precipitation contributed to the immobilization of Cd(Ⅱ)in the presence of As(Ⅴ).Cd(Ⅱ)-As(Ⅴ)tended to co-precipitate at high concentrations,while the adsorption process controlled the fate of As(Ⅴ)and Cd(Ⅱ)at lower element ratios.Besides,the results of in-situ spectroscopy experiments proved the existence of Cd(Ⅱ)-As(Ⅴ)ternary surface complex.(2)The interaction and adsorption characteristics of Cd(Ⅱ)-As(Ⅴ)at the ferrihydritesolution interface are revealed.As(Ⅴ)and Cd(Ⅱ)have a mutually beneficial cooperation process on the surface of ferrihydrite,that is,the preload of As(Ⅴ)on the surface of ferrihydrite can promote the adsorption of Cd(Ⅱ),while the preloaded Cd(Ⅱ)can also enhance the adsorption of As(Ⅴ)on the surface of ferrihydrite.p H is the key factor to control the promotion effect.At low p H,As(Ⅴ)preload promotes the adsorption of Cd(Ⅱ);while at high p H,Cd(Ⅱ)preload promotes the adsorption of As(Ⅴ).The results of in-situ ATR-FTIR spectra analysis and the second derivative peak shape fitting reveal the complex surface chemical processes of Cd(Ⅱ)and As(Ⅴ)mediated by ferrihydrite under the Cd(Ⅱ)and As(Ⅴ)ternary adsorption system.Combining the macroscopic adsorption trend and the results of spectral analysis,the interaction process of Cd(Ⅱ)and As(Ⅴ)on the surface of ferrihydrite involves two types of surface ternary complexes,namely metal-ligand complex and ligand-metal Complex.These surface complexes may be the precursors of arsenate-cadmium surface precipitation,which can change with the change of reactant concentration and p H,but in most systems,multiple complexes may coexist on the surface of ferrihydrite.(3)The desorption process of Cd(Ⅱ)and As(Ⅴ)at the ferrihydrite-water solution interface under the ternary system is clearly analyzed.Cd(Ⅱ)and As(Ⅴ)under the ternary system are more likely to be detached from the surface of ferrihydrite under the influence of competing ions,and there is the possibility of mutual promotion between Cd(Ⅱ)and As(Ⅴ).The co-adsorption of Cd(Ⅱ)on the surface of ferrihydrite can promote the desorption of As(Ⅴ),and the co-adsorption of As(Ⅴ)on the surface of ferrihydrite can also promote the desorption of Cd(Ⅱ).In all systems,the retention of Cd(Ⅱ)on the surface of ferrihydrite was lower than that of As(Ⅴ)on its surface,which was due to the difference in the adsorption affinity of Cd(Ⅱ)and As(Ⅴ)on the surface of ferrihydrite.The results of in-situ infrared experiments show that the reason for the enhanced mobility of As(Ⅴ)-Cd(Ⅱ)in the ternary system may be due to the cooperative desorption of the ternary surface complex and the single ions complex species.(4)The influence of the aging process on the stability of Cd(Ⅱ)and As(Ⅴ)on the surface of ferrihydrite is clarified.The aging process has significantly changed the distribution of elements on the surface of ferrihydrite,and this effect is related to the adsorption affinity of Cd(Ⅱ)and As(Ⅴ)on the surface of ferrihydrite.The mobility of As(Ⅴ)decreases with the extension of aging time.This process may be due to the transformation of minerals and the diffusion of surface adsorbed elements into the pores structure of Fh.In addition,the effect of the aging process on the surface properties of minerals changes the stability of Cd(Ⅱ)on the mineral surface,making it easier to re-release into the environment with the introduction of competing ions.