Adsorption And Coprecipitation Research Of Ferrihydrite On Cadmium

An increasing seriousness of heavy metal contamination,causing a profoundthreat to human health, and it causes people’s attention more and more. Theremediation methods of heavy metal pollution have physical method, chemicalmethod and biological method.Different adsorption material has different effect toremove the different heavy metals. Ferrihydrite is a widespread mineral in the naturalenvironment.It has high specific surface area and high surface activity. Ferrihydrite isan excellent adsorption material and can affect the migration, distribution andtransformation of many harmful heavy metals by adsorption and coprecipitation.Theessay is based on the project “Study of abnormal isotope tracer of cadmium in soil oftypical mining district in Yangtze River”supported by NSFC. The essay focuses ondifference between ferrihydrite’s absorption on cadmium and coprecipitation offerrihydrite and cadmium by their comparison experiments. The essay also shows thatthe key factors to that ferrihydrite absorb cadmium by HRTEM and XRD. The mainachievements as below:1. WhileCd2+and Fe3+have a coprecipitation, an amount of amorphous2LFhprecipitation is formed. Cadmium ions are precipitated as well.2. PH valueis one of the significant factors that affect ferrihydrite adsorbcadmiumboth inadsorption experiments andcoprecipitation experiment.While low pHvalue varies from3to5.4, adsorption capacity and removal rate of ferrihydrite oncadmium is low. As pH value increases, its adsorption capacity and removal rategrows indistinctly. While pH value varies from5.4to8.5, its adsorption capacity andremoval rate rises sharply as pH value increases.While pH value varies from8.8to11,cadmium ions combine with hydroxide ions and Cd(OH)2precipitate is formed.Itsadsorption capacity and removal ratereach climax point and removal rate approachesone hundred percent. As pH value increases, its adsorption capacity and removal rateremain nearly unchanged and turns to stable.3. Initial concentration of cadmium is also one of key factors to ferrihydrite’sabsorbing cadmium and ferrihydrite’s coprecipitation with cadmium.In the essay,absorption is not complete and does not reach a balanced point both in absorption experiments and coprecipitation experiments due to low initial concentration ofcadmium.Within the same pH value,as the concentration of cadmium begins toincrease, the removal rate of ferrihydrite on cadmium decreasing insteadofrising.While pH value varies from3to8, cadmium ions are only absorbed andprecipitated. Initial concentration of cadmium rising from0.3mM to0.6mM,theincreased amount of adsorptioncapacityis more than doublewithin the same pH value.In the desorption experiments, within the same EDTA-2Na concentration anddifferent initial concentration of cadmium, the difference of their desorption rates issmall. It shows that initial concentration of cadmium has a small effect on desorptionrate.4. Concentration of EDTA-2Na is also one of significant factors to ferrihydrite’sdesorbing ability of cadmium. Concentration of EDTA-2Na has a big effect ondesorption and sorption experiments while a small effect on coprecipitationdesorption experiments. When concentration of desorbent EDTA-2Na increases, itsdesorption rate rises as well. Within different concentration of EDTA-2Na, thedesorption rate of cadmium varies from44.23%to54.14%in desorption and sorptionexperiments and30.63%to34.29%in coprecipitation desorption experiments.5. When pH value varies from3to8.8, cadmium ions are only absorbed andprecipitated. When the initial concentration of cadmium is at a low level, absorption isnot complete. In absorption and coprecipitation experiments, when pH value reaches8.5and cadmium’s initial concentration is0.3mM, adsorption capacityof ferrihydriteon cadmium is35.399mg/g and37.148mg/g respectively;adsorption capacity offerrihydrite on cadmium is69.562mg/g and73.106mg/g when cadmium’s initialconcentration is0.6mM and pH value reaches8.5. It is more than the absorptioncapacity of goethite.thus indicating that ferrihydrite has a high ability to removecadmium.6. Within the same pH value and initial concentration of cadmium, adsorptioncapacity and removal rate of ferrihydrite on cadmium are larger than absorptionexperiments in coprecipitationexperiments. In desorption experiments, desorption rateof adsorption experiments is much larger than coprecipitationexperiments’ and thediscrepant range is13.60%～20.17%. The results indicate that the ability of removingcadmium in coprecipitation experiments is higher than that in adsorptionexperiments.coprecipitationthat ferrihydrite on cadmium is more stable than ferrihydrite’sabsorption on cadmium, less likely to be desorbed. It shows that they have differentmechanisms.