The Interaction And Mechanism Between Heavy Metals(Cu²⁺, Cd²⁺and Pb²⁺) And Extracellular Polymeric Substances Produced By Aspergillus Fumigatus

Extacellular polymeric substances (EPS), produced by many microorganisms, such asalgae, bacteria, and fungi, play a crucial role in the biosorption of heavy metals. A number ofstudies showed that fungi were very effective for heavy metal adsorption. Moreover, fungican be easily cultured and obtained from a number of industrial by-products. Heavy metalsare accumulated mainly in cell wall in the process of biosrption. About75%of thecomponents of fungi cell wall are EPS. In recent years, some researchers have focused onEPS extracted from activated sludge and pure bacterial strains and investigated differentcombinations of metal ions, microorganisms and experimental conditions. But the biosorptionproperties of EPS from fungi remain less explored. The components of EPS are differentfrom microorganisms and results in different biosorption capacity and mechanism. In thepresent study, Aspergillus fumigatus, an isolated fungus in our lab, was used for EPSextraction. This study aims to investigate the sorption behaviour of the EPS for Cu²⁺and Cd²⁺in aqueous solutions. The equilibrium isotherms and the effect of various factors, such as pHand initial metal concentration, on binding of these metals were studied. The presence offunctional groups in the EPS which may play a role in sorption process was confirmed byFTIR. And EDX analysis was used for further confirmation of the mechanism of the sorptionof heavy metals by the EPS.The efficacies of extracting EPS from Aspergillus fumigatus using H2SO4, NaOH, EDTAand CER (cation exchange resin) under various conditions were compared. Results showedthat CER was most effective in extracting EPS from Aspergillus fumigatus. The content ofpolysaccharide in EPS extracted by CER was very high. While, nucleic acids were very low,suggesting the EPS extracted were not contaminated by intracellular substances. So, CERwas chosen as extracting reagent and the optimal conditions were as follows: CER dosage70g g^-1, oscillating speed200r min^-1, extracting time2h and pH6-8. The contents of thecarbohydrate, protein nucleic acid and lipids in EPS were392±8,17.6±2,18.3±2,25.7±2.8mg g^-1DWEPS, respectively.Biosorption of Cu²⁺, Cd²⁺and Pb²⁺of EPS was investigated as a function of pH usingdifferential pulse polarography (DPP) and the Ruzic model. Results showed that the EPSbiosorption capacity determined using either the direct titration curves i=f (CM) or the methodproposed by Ruzic were coincident. Cu²⁺had the highest affinity with EPS followed by Pb²⁺and Cd²⁺. The total number of binding sites for Cu²⁺and Cd²⁺increased with pH in the range of4.0-7.0. Similar trend was observed for Pb²⁺at pH4.0-5.0, while precipitates wereobserved at pH6.0and7.0.Sorption of Cu²⁺and Cd²⁺onto the extracellular polymeric substances (EPS) producedby Aspergillus fumigatus was investigated for the initial pH of the solution, EPSconcentrations, contact time, NaCl concentration, initial metal ion concentration and thepresence of other ions in the solution. The results showed that the adsorption of metal ionswas significantly affected by pH, EPS concentrations, initial metal concentration, NaClconcentration and co-ions. When pH was7.0, temperature was25℃, the initial Cu²⁺andCd²⁺concentration was5mg L^-1and7mg L^-1, EPS dosage was135mg L^-1, the removalefficiency of Cu²⁺and Cd²⁺reached90.6%and96.7%, respectively. The sorption processesof Cu²⁺and Cd²⁺with EPS both have two steps: fast sorption and slow sorption, sorptionequilibrium time was about3hours. Both the Langmuir and Freundlich adsorption modelscould describe the thermodynamic sorption processes of Cu²⁺and Cd²⁺. The maximumsorption capacities of A. fumigatus EPS were40mg g^-1EPS and85.5mg g^-1EPS. Values of ncalculated from the Freundlich model were4.04and3.59for Cu²⁺and Cd²⁺, respectively.The sorption was a spontaneous endothermic process. The sorption kinetics fitted well topseudo-second-order kinetic model.The desoption of Cu²⁺and Cd²⁺absorbed in EPS was investigated. Results revealed thatdesorption of Cu²⁺and Cd²⁺from EPS was significantly affected by pH. The desorptionpercentage was increased with decrease of pH. The maximum desorption percentage washigher than90%for both Cu²⁺and Cd²⁺. The optimal desorption pH was1.0. However, thedesorption percentage for both Cu²⁺and Cd²⁺was low when using NaCl as desorptionreagent. When concentration of NaCl was2.0mM, the desorption percentage for Cu²⁺andCd²⁺was50.3%and53.8%, respectively.Analysis of FTIR spectra demonstrated that hydroxyl, carboxyl and C-O-C ofpolysaccharide were the main functional groups in the reaction between metal and EPS.Energy dispersive X-ray (EDX) system analysis revealed that C and O were the dominantelements in EPS, and P, S, K, Ca, Mg, Al and Na were also included in. The EDX pattern forthe Cu²⁺-loaded and Cd²⁺-loaded EPS revealed distinctive peaks for the respective metals.However, Ca and Mg were not detected in the EDX spectrum of Cu²⁺-loaded EPS.Furthermore, for the Cd²⁺-loaded EPS, the Ca content was found to be lower and Mg was notdetected. The results showed that the ion-exchange was an important mechanism involved inthe Cu²⁺and Cd²⁺sorption process taking place on EPS. Complexation and electrostaticinteractions were also involved in. There were several mechanisms responsible for the sorption of Cu²⁺and Cd²⁺together. And complexation was the main mechanism.