Coarsening Of Extracellularly Biosynthesized Heavy Metals Crystal And The Study Of Its Coarsening Mechanism

White rot fungi is a class of microorganisms that can handle both organic andinorganic contaminated wastewater. Besides handling organic pollutants, such as dyes,hormones, pesticides and et al., they are capable to handle heavy metals, such as Cd,Pb, Hg, Zn and et al., because they could secrete many biomolecules like proteins andenzymes. As a result, white rot fungi have broad application prospects in wastewatertreatment.With the deepening of the study, various research groups have shown thatmetallic crystals can be extracellularly biosynthesized by many microorganisms undersuitable conditions in toxic metal–contaminated water. Extracellular biosynthesis hasbeen recognized as a defense mechanism against toxicity of microbes. Commonly,these formed crystal particles are in the nanometer range, which make them stillhighly mobile and have the ability to redissolve if the conditions change. Theseproperties of nanocrystals may lead to recontamination of the water and will increasethe separation cost of the wastewater treatment process. An earlier study has reportedthat coarsening could restrict nanoparticle transport by inducing settling, drivingcrystal growth, and result in decreased solubility. Therefore, to decrease the solubilityand mobility of metal nanocrystals in the solution and to lower the separation costduring wastewater treatment, there is an imperative need for methods that facilitatenanocrystal coarsening.In the present study, C. versicolor was used to biosynthesize cadmium crystalparticles and nanocrystal coarsening was facilitated by the addition of thioacetamide(TAA). We report here our findings that the cadmium nanocrystals could beextracellularly biosynthesized using C. versicolor and their coarsening could beinduced by TAA under certain conditions. Scanning electron microscopy(SEM) andenergy-dispersive x-ray spectra(EDX) were employed to describle the characteristicof the coarsening process. The physico-chemical property of the cadmium crystalparticles were studied by FTIR, TEM, XRD and UV-vis. Based on the results above, apossible mechanism for the formation and coarsening of cadmium crystal particle isalso discussed.Results show that the coarsening of cadmium nanocrystals was successfullyinduced by additional TAA. Meanwhile, when the coarsening was completed, cadmium ion removal efficiency greatly improved from17%to87%(correspondingto the sorption capacity of biomass from4to24mg/g) and the dissolved proteincontent in the aqueous solution increased from20to61μg/mL. From the SEM imageswe can see that the size of the cadmium crystals increased from about100nm to2-3μm and the shape of the enlarged particles were square. The EDX data indicated thepresence of amino acids, peptide bond, or even proteins in the enlarged particles. TheTEM image reveals that the enlarged particles are actually clusters of several smallparticles with different sizes. Characterization of the coarsened particles by XRDtechniques confirmed the formation of cubic crystalline cadmium sulfide particles. Inaddition, FTIR analysis revealed the important role of proteins in the formation andcoarsening of cadmium nanocrystals.