Adsorption Behavior Of Ferricyanide On Goethite Surface And Exploration Of Adsorption Mechanism

At present,the soil pollution situation in China is very serious,and the publishing of the Action Plan for Soil Pollution Prevention and Control has aroused the public's attention to soil pollution further.The cyanide in the soil mainly comes from industrial activities such as gold mining,electroplating and coking.The leakage of cyanide during use or the emission of cyanide-containing waste residue and waste liquid make the soil environment around enterprises seriously polluted by cyanide.Cyanide entering the soil environment can migrate to groundwater with rainwater,or volatilized into air,thus becoming the source of secondary pollution.Related researchs have shown that ferricyanide complex is the major state in cyanide contaminated soil.The ferricyanide complex is relatively stable itself,but once exposed to sunlight,it can be rapidly decomposed to highly toxic free cyanide,which has a great threat to organisms and human health.However,compared with organic pollutants and heavy metal inorganic pollutants,related awareness on cyanide pollution control is still not enough,and lacking relevant basic research too.Therefore,it is necessary to carry out basic research on the migration and transformation behavior of cyanide in the soil environment.The research results are expected to provide some valuable information for the accurate assessment of the environmental risk of cyanide and the source pollution control of cyanide.In this paper,the interaction relationship between active minerals of ferricyanide complex in soil is taken as the pointcut,and the influence mechanism of soil active components and environmental factors on cyanide migration and transformation are intensive studied.Ferricyanide was selected as the pollutant,and the experimentally synthesized goethite was used as the adsorbent.The adsorption of ferricyanide on goethite was investigated by isothermal adsorption experiment and pH dependence experiment.Phosphate widely present in soil is selected as competitive anionic.The effect of phosphate on the adsorption of ferricyanide by goethite is investigated by the amount and the order of phosphate added.The surface structures of goethite after adsorption of ferricyanide were characterized by FTIR.The surface behavior of ferricyanide in goethite was further analyzed by Zeta potential.And provide adsorption mechanism for the retention of ferricyanide.The main results of this paper are as follows:?1?In this study,the synthesized goethite was characterized by SEM,BET,XRD and FTIR.The goethite powder is yellowish brown or dark brown when forming block structure.The morphological analysis of the synthetic goethite showed that the goethite has a regular strip structure with a length of about 200 nm,which is a nanometer-sized particle;the specific surface area by BET analytical is 90.57 m²/g;XRD results show that the synthesized goethite being good crystallinity,and the characteristic diffraction peak of goethite indicating that the goethite synthesized successfully;FTIR characterization results show that the apparent absorption peaks of goethite appeared at3407 cm^-1,3125 cm^-1,890 cm^-1,indicating that the synthesized material was pure goethite.In addition,the Zeta potential curve of pure goethite indicates that the isoelectric point of goethite is 9.17 in this experiment.?2?The adsorption isotherms of ferricyanide show that under the two pH conditions?4 and 7?,the adsorption amount of ferricyanide on goethite increases with the equilibrium concentration of ferricyanide.When the pH=4,the saturation adsorption is reached when the equilibrium concentration of ferricyanide is about 200?M.When the pH=7,the saturation adsorption is reached when the equilibrium concentration of ferricyanide is about 150?M,and the saturated adsorption amount at pH=4 is higher than pH=7.The results of isothermal adsorption experiments were fitted by three adsorption isotherms:Langmuir,Freundlich and Barrow.The best fitting effect at pH=4was Freundlich?R²=0.943?,so the adsorption of ferricyanide at pH=4 is a multi-molecular layer.When the pH=7,the Langmuir isothermal fitting effect?R²=0.923?is best,so the ferricyanide adsorption process is a monolayer under this condition.The maximum theoretical adsorption capacity obtained by Langmuir is 16.15?mol/g at pH=4,and 6.87?mol/g at pH=7,which is closer to the actual maximum adsorption amount.?3?Exploring the effect of environmental factors on the adsorption of ferricyanide,the results show that pH and ionic strength have a great influence on the adsorption,with the increase of pH,the adsorption amount decreases significantly.The adsorption rate of ferricyanide ions can reach more than 80%under the condition of pH less than 3.7,so the acidic conditions are favorable for the adsorption of ferricyanide ions.When the pH of the system exceeds the isoelectric point of goethite,there is still a certain amount of adsorption,which indicates that the adsorption of ferricyanide on the surface of goethite is chemical adsorption.In this experiment,three ionic strengths?I=0.001,I=0.01 and I=0.1?were selected.When the ionic strength of the system was increased,the adsorption amount of ferricyanide decreased,and the ionic strength had a greater influence on the adsorption of ferricyanide under acidic conditions.The ionic strength had a negative effect on the adsorption of ferricyanide,indicating that electrostatic adsorption was mainly adsorption mechanism of ferricyanide.?4?In addition to exploring the adsorption law of ferricyanide in a single system,this paper also carried out a series of competitive adsorption experiments of ferricyanide ion and phosphate ion.Observation of effects of addition amount of phosphate,ionic strength,pH and order of addition of phosphate in the study on adsorption has been conducted.The results show that the amount of phosphate added had a great effect on the adsorption of ferricyanide.In this study,two phosphate additions were selected.It was found that the larger the amount of phosphate added,the lower adsorption ferricyanide ions compared with the single system.Competitive adsorption experiments with different ionic strengths showed that the higher ionic strength,competitive adsorption is more obvious of phosphate.As the pH increases,the variation of adsorption of ferricyanide ions in the competitive system is just like that of the single system.In this study,the effects of two phosphate addition sequences?adsorption of ferricyanide before phosphate,adsorption of ferricyanide and phosphate simultaneously?on the adsorption of ferricyanide were also investigated,and found that the adsorption amount of ferricyanide ions did not change significantly.Compared with the single system,it is found that the saturated adsorption capacity of ferricyanide in the competive system reduces about 26%,which indicates that the adsorption affinity of ferricyanide ion on the surface of goethite is weaker than phosphate.?5?In order to investigate the adsorption mechanism of ferricyanide on the surface of goethite futher,FTIR characterization is also carried out.Under the conditions of pH 4 and 7,FTIR characterization was conducted on goethite after adsorption ferricyanide reaching saturation.Compare with the characteristic absorption peak of pure ferricyanide potassium(2117 cm^-1 and 2044 cm^-1),absorption peak of2117 cm^-1 is not observed,only the absorption peak of 2044 cm^-1 existed,and new absorption peak is found at 2026 cm^-1.The disappearance of the characteristic peak and the appearance of new peaks indicate the occurrence of chemical reactions,so there must be chemical reaction between ferricyanide and goethite.At pH 4 and 7,the intensity of the absorption peak of goethite at 2044 cm^-1 is different,the higher of the pH,the weaker the peak intensity,which indicates that the adsorption under acidic conditions is stronger than the neutral condition.?6?The Zeta potential of goethite in the single system is mainly influenced by ferricyanide adsorption,pH,and ionic strength.The rising of pH leads to the decrease of Zeta potential,which is related to the chargeability of the mineral surface.The higher the adsorption amount of ferricyanide ions,the lower the Zeta potential.The Zeta potential decreases as the ionic strength increases.The main reason is that the increased ionic strength changes the thickness of the electric double layer,so the potential of shear plane decreases.On the other hand,the Zeta potential is rising caused by the decrease in the adsorption amount of ferricyanide,so the decrease of Zeta potential caused by ionic strength is dominant.Shifting downward of the Zeta potential curve confirms the existence of specific adsorption,but the degree of migration is small,indicating that the adsorption of ferricyanide ions is dominated by non-specific adsorption,and existing specific adsorption.?7?In order to further understand the adsorption mechanism of ferricyanide,this paper also discusses the variation of Zeta potential on the surface of goethite on competitive system.Exploring the effects of different amounts of phosphate added,pH and ionic strength on Zeta potential,it was found that the higher the amount of phosphate added,the lower the Zeta potential as the equilibrium concentration of ferricyanide increased.As the increasesing of pH,Zeta potential of the mineral in the competition system decreases.With the increase of ionic strength,the Zeta potential of the competitive system decreases,which is consistent with the variation of single system.This is because the adsorption amount of phosphate is constant under different ionic strengths.Therefore,the variation of Zeta potential with ionic strength is mainly affected by ionic strength.The change of Zeta potential is caused by ionic strength itself and the ferricyanide adsorption,so the Zeta potential of competitive system changes with the ionic strength is consistent with the single system.