Application Of Spirulina Biosorbent In Rare Earth Wastewater Treatment

The exploitation and utilization of rare earth resources are of great significance to the economic development and strategic safety of our country.However,the pollution of rare earth wastewater has become a serious environmental problem in the exploitation and application of rare earth.According to incomplete statistics,the annual discharge of ion-based rare earth wastewater in China is about 2 million tons,which causes serious waste of rare earth resources and serious environmental pollution.In addition,the ion-based rare earth wastewater usually contains a certain amount of heavy metal ions,which is harmful to the environment.Therefore,it is urgent to develop effective methods to treat ion-based rare earth wastewater and to enrich and recover valuable elements.In recent years,biosorption has attracted much attention because of its low operating cost,high selectivity,strong universality,good treatment efficiency,fast adsorption and desorption rate and friendly environment.In this work,the adsorption of typical rare earth ions and heavy metal ions in rare earth wastewater by Spirulina was carried out.The adsorption behavior of rare earth Y³⁺and heavy metals Cu²⁺,Pb²⁺by Spirulina was studied.The effects of pH,Spirulina dosage,initial concentration of metal ions,temperature and adsorption time on adsorption performance of Spirulina for various metal ions were studied.The kinetic behavior and adsorption isotherms of the adsorption process were analyzed.Combined with SEM-EDS,FT-IR and XPS analysis on the Spirulina before and after the adsorption of Y³⁺,the mechanism of the adsorption process was revealed.The main conclusions are as follows:The effects of solution pH,Spirulina dosage,initial concentration of metal ions,adsorption temperature and time on the adsorption performance of spirulina for Y³⁺,Cu²⁺and Pb²⁺were studied.The results showed that the adsorption capacity and removal rate of Spirulina for various ions could be improved by appropriately increasing solution pH,adsorption temperature and time.In addition,low dosage of Spirulina and high initial ion concentration will help to obtain high adsorption capacity,but on the other hand,it will reduce the ion removal rate.Therefore,the optimal adsorption conditions need to be balanced between the adsorption capacity and removal rate.For example,under the fixed conditions of pH=5,adsorption temperature T=40℃,solution volume of 100ml and adsorption time t=720min,the adsorption capacity of spirulina for Y³⁺reached 75.6mg/g when the Spirulina dosage is 0.6g and rare earth concentration is 1600mg/L,while the removal rate of Y³⁺could be about 78.5%when the dosage of Spirulina is 0.7g and rare earth concentration is 200mg/L.The results of orthogonal experiment showed that the optimal process parameters of Y³⁺adsorption by Spirulina were as follows:pH=5.5,adsorbent dosage of1.6g,initial Y³⁺concentration of 600 mg/L,and the adsorption time of 120 min.The adsorption process of rare earth Y³⁺ and heavy metal ions of Cu²⁺and Pb²⁺by Spirulina was in accordance with the pseudo-second-order-kinetic model and Langmuir adsorption isotherm,indicating that the adsorption of various metal ions by Spirulina was mainly monolayer chemical adsorption.Combined with the analysis of SEM-EDS,FT-IR and XPS on Spirulina before and after adsorption of Y³⁺ions,the mechanism of adsorption of Y³⁺by Spirulina was proposed:the surface of Spirulina is negatively charged in a wide range of pH,and Y³⁺ions can rapidly diffuse onto the surface of Spirulina through electrostatic attraction,and then ion exchange with Na⁺,K⁺,Mg²⁺and Ca²⁺on the surface of Spirulina.In addition,the Spirulina cell wall contains a large number of active groups such as carboxyl,carbonyl,hydroxyl and phosphate groups,whih can coordinate with the Y³⁺ions to produce strong adsorption.The desorption behavior of Y³⁺ ions adsorbed on Spirulina by HCl,（NH₄）₂SO₄ and Na NO₃solution was studied.The results showed that the desorption rate of Y³⁺ions by HCl solution could reach 95.5%,while that by（NH₄）₂SO₄ and Na NO₃ was not expected.（NH₄）₂SO₄ is an effective extractant of ion-based rare earth in the south of China.However,in this experiment,the desorption rate of Y³⁺ions by（NH₄）₂SO₄ solution is not high,indicating that the adsorption state of rare earth ions on the surface of Spirulina is very different from that on clay mineral surface for the ion-type rare earth minerals.By comparing the adsorption performance of Spirulina for rare earth Y³⁺and heavy metal ions of Cu²⁺and Pb²⁺,it was shown that the adsorption performance of Spirulina for three metal ions was in the order of Y³⁺>>Pb²⁺>Cu²⁺.This might attribute to that on the one hand,Y³⁺has higher valence and will have stronger electrostatic attraction with Spirulina;on the other hand,Y³⁺probably has stronger coordination ability with the carboxyl,amino and other functional groups in Spirulina comparing to the other two metal ions.