Study On The Promotion And Interfacial Dynamics Of Cr(?) Sequestration By Zero Valent Iron Under Non-uniform Magnetic Field

Chromium is a typical heavy metal pollutant in waste water.Due to human factors such as industrial production,The discharge and migration of chromium in the environment cause chromium pollution in water.Chromium mainly exists in the forms of hexavalent chromium(Cr(VI))and trivalent chromium(Cr(III))compounds in the environment,in which(Cr(VI))can exist in the environment for a long time,The ability to migrate or enrich organisms in a watery environment will ultimately affect human health and the ecological environment.Zero valent iron(ZVI)has been widely used in the field of water treatment due to its advantages such as being rich in earth,easy to obtain,non-toxic,environmental friendly and strong reduction capacity.However,there are some limitations in the research and application of ZVI.Since iron is an active metal,the passivation inert oxide or hydroxide passivation layer of iron is commonly found in the process of synthesis and application of ZVI,affecting the reaction of inner layer Fe0 with the pollutants.In this study,Cr(VI)was taken as the target pollutant,and the removal of Cr(VI)in water was improved by applying a non-uniform magnetic field.In this paper,microscale ZVI was used as water treatment material.A non-uniform magnetic field was applied during the experiments and the influence of the superposition mode of non-uniform magnetic field and the intensity of non-uniform magnetic field on the removal of Cr(VI)was studied.The results suggested that the non-uniform magnetic field can increase the rate of Cr(VI)removal in water by ZVI.The superposition mode of magnetic field has no obvious difference in magnetic field promotion.When there was no magnetic field applied,the removal process of Cr(VI)has a delay period,that is,the initial reaction rate is slow.When a non-uniform magnetic field is applied,with the increasing of non-uniform magnetic field,the lag period in the removal of Cr(VI)is gradually shortened.The average removal rate of Cr(VI)at each stage increased gradually.In this paper,under different experimental conditions(ZVI dosage,initial Cr(VI)concentration,pH),the effect of non-uniform magnetic field on the removal of Cr(VI)was analyzed by comparing the average rate.The instantaneous rate was calculated under the conditions of different ZVI dosage and different initial Cr(VI)concentration,and the effects of ZVI on the removal of Cr(VI)at each stage in the presence/absence of non-uniform magnetic field were analyzed.The results showed that,without the non-uniform magnetic field,the removal rate of Cr(?)gradually increased,and the removal of Cr(?)was a process of ZVI reactivity continuous recovery,until Cr(?)was completely removed,ZVI reactivity reaches its maximum.In the presence of non-uniform magnetic field,the removal of Cr(?)has a rapid phase and a constant velocity phase.In the early stage of the experiment,the instantaneous rate is very fast;after the addition of ZVI,40%of Cr(?)is quickly removed;subsequently,the removal rate of Cr(?)decreases and remains stable until the complete removal of Cr(?).The instantaneous rate in the constant speed stage was positively correlated with the dosage of ZVI,and the removal rate of Cr(?)by ZVI per unit mass was 0.455 mg/(L·min·gZVI).The experiments with different initial Cr(?)concentrations,it can also be concluded that the instantaneous rate in the constant velocity phase is hardly affected by the initial and residual concentration of Cr(?),indicating that the magnetic field can weaken the passivation of Cr(?)on ZVI in the reaction and overcome the inhibition of Cr(VI)on the reaction by high concentration.The mechanism of Cr(?)removal by ZVI in the presence or absence of magnetic field was analyzed by means of SEM and XPS characterization.The surface of ZVI used in this study is covered with a layer of iron oxide and hydroxide,forming a passivation layer,which is the key factor hindering the reaction of ZVI with pollutants.Compared with the non-magnetic field,ZVI particles are more fracture and breakage after ZVI reacts with Cr(?)in the presence of non-uniform magnetic field.The main removal method of Cr(?)is to reduce Cr(?)to Cr(?).Part of Cr(?)forms the precipitation of Cr2O3 and Cr(OH)3 from the aqueous solution,so as to remove it from the water.Through experiment to examine non-uniform magnetic field quenching agent on the Cr(?)removal process,the role of Fe2+ and other reducing agents,research shows that under the action of inhomogeneous magnetic field,can promote the ZVI in the early years of the corrosion reaction,generate more types also Fe2+,magnetic field accelerated ZVI on the removal of Cr(VI)is,have improved the ZVI surface adsorption state reductant(Fe0,Fe2+)removal of Cr(?)in water.