Preparation Of Baker's Yeast-based Adsorbents And Their Adsorption Characteristics For Heavy Metals And Dyes

The rapid development of modern industry and agriculture brings people a good material basis and living conditions,while a large amount of wastewater containing heavy metals and dyes was discharged into the environment.These two kinds of pollutants are generally toxic,difficult to degrade,easy to enrich and other characteristics.Once discharged into the environment,they will cause serious water pollution,which ultimately endangers the health of organisms and human beings.Therefore,it is extremely urgent to fundamentally solve the problem of treatment of heavy metals and dye wastewater.In the existing wastewater treatment methods,microbial adsorption is widely used in the treatment of wastewater containing heavy metals and dyes because of its simple operation,high adsorption efficiency and no secondary pollution.Among them,baker's yeast,as a cheap,widely used and environmentally friendly adsorbent,has been widely concerned by many environmental protection workers.However,as a biosorbent,baker's yeast also faces many problems such as poor mechanical strength and low adsorption efficiency,which limits its application in wastewater treatment to some extent.In view of this,three kinds of baker's yeast-based adsorptive materials were prepared by various chemical modification methods on the basis of full literature investigation on the structure,physicochemical properties,related modification research status and application of baker's yeast in the treatment of heavy metals and dye wastewater.Firstly,the physical and chemical properties of three baker's yeast-based adsorbents were characterized by Zeta potentiometer,Scanning electron microscopy?SEM?,Energy dispersive spectroscopy?EDS?,Fourier transform infrared spectroscopy?FTIR?,X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?.Secondly,taking Pb?II?,Cd?II?and methylene blue?MB?in aqueous solution as the target pollutants,the effects of various reaction conditions,such as pH value of solution,initial concentration of pollutants,reaction temperature and other factors on the adsorption effect were systematically investigated,and the optimal adsorption conditions were determined.The adsorption and removal mechanisms of Pb?II?,Cd?II?and MB by three adsorbents were studied by means of adsorption kinetics,adsorption isotherm,adsorption thermodynamics,and SEM+EDS,FTIR,XRD,XPS analysis.Finally,according to the fact that some specific wastewater contains heavy metals and dyes at the same time,a kind of adsorbent with better adsorption performance was selected and its adsorption characteristics in Cd?II?-MB binary system are discussed.The specific research contents and achievements of this dissertation can be divided into four aspects:?1?Using cheap and readily available baker's yeast as substrate,epichlorohydrin?EPI?was used as crosslinking agent to improve the mechanical properties and stability of baker's yeast under acidic conditions.Then,in order to introduce xanthate group which has strong chelating and adsorbing effect on heavy metal ions to the surface of baker's yeast,xanthate modified baker's yeast adsorbent?XCBY?was prepared by single factor and response surface methodology.The results of adsorption kinetics and isotherms showed that the adsorption process of Pb?II?,Cd?II?and MB by XCBY fitted with pseudo-second-order kinetics and Langmuir adsorption isotherm model.That is,the adsorption processes with Pb?II?,Cd?II?and MB were chemical adsorption occurring on the monolayer,and the adsorption efficiency was faster and accompanied by a certain degree of intra-particle diffusion.The calculated adsorption capacity values from Langmuir isotherm showed that the maximum adsorption capacities of XCBY to Pb?II?,Cd?II?and MB at 25?were 308.95,231.36 and 63.10mg/g,respectively.Compared with other similar xanthate adsorption materials,the adsorption properties of Pb?II?,Cd?II?and MB were obviously improved.?2?Different from the traditional crosslinking agent,thiomalic acid was selected as a cross-linking agent,a simple and environmentally friendly synthesis process is used to graft?-cyclodextrin with a special cavity structure onto the surface of baker's yeast to prepare a low-cost,high-efficiency cross-linked yeast-?-cyclodextrin polymer adsorbent?Y-?-CDP?.The adsorption kinetics,isotherms and thermodynamics of Pb?II?,Cd?II?and MB on Y-?-CDP were studied.The results showed that the adsorption of Pb?II?,Cd?II?and MB on Y-?-CDP conformed to the pseudo-second-order kinetics and Langmuir isotherm adsorption model.The adsorption process was a spontaneous endothermic reaction and the intraparticle diffusion also plays a certain role.The adsorption capacity of Y-?-CDP for Pb?II?,Cd?II?and MB reached 150.08,102.80 and 93.63 mg/g respectively at 25?for 25and 240 min,which was also higher than that of similar adsorbents.SEM-EDS,FTIR,XRD and XPS analysis showed that the adsorption process of Pb?II?and Cd?II?on Y-?-CDP was mainly dependent on the adsorption of-OH,the coordination of N-lone electron pairs on-NH₂ and the chelation of-SH on thiomalic acid.While the adsorption of Y-?-CDP on MB was dependent on the inclusion of the cavity structure of?-cyclodextrin.Besides,the-OH on the Y-?-CDP will also form a hydrogen bond between-OH and MB to adsorb MB.?3?Based on the study of Y-?-CDP,it was used as a raw material for further modification to preparing the xanthate-crosslinked yeast-?-cyclodextrin polymer adsorbent?X-Y-?-CDP?.In the process of preparation,the hydroxyl groups on the six positions of?-cyclodextrin also participate in xanthation reaction,which could further improve its ability to remove pollutants.X-Y-?-CDP combines the xanthate group with high efficient heavy metal chelating property and the excellent organic dye adsorption characteristics of?-cyclodextrin,and the"1+1>2"adsorption characteristics was achieved.At room temperature,the adsorption capacity of Pb?II?,Cd?II?and MB reached 420.8,284 and 123.76 mg/g,respectively.The experimental results show that the peseudo-second-order kinetic and Langmuir isotherm model could well describe the adsorption process of X-Y-?-CDP on Pb?II?,Cd?II?and MB.The standard Gibbs free energy??G⁰?of the three pollutants were negative,and the enthalpy change??H⁰?were greater than 0,indicating that the adsorption of X-Y-?-CDP on Pb?II?,Cd?II?and MB were a spontaneous chemical endothermic process occurring on the monolayer.?4?By comparing the adsorption performance of three adsorbents,and considering the characteristics of actual wastewater,X-Y-?-CDP with better adsorption performance was selected to further study the adsorption properties and adsorption mechanism of Cd?II?and MB in binary system.The results showed that the adsorption capacity ratio(R_q,MB)of Cd?II?to MB was between 0.783-1.027 in the binary system,indicating that the presence of Cd?II?had no significant effect on the adsorption of MB.This is because the adsorption of MB and Cd???on X-Y-?-CDP adsorbent depends mainly on their adsorption sites.However,in the process of studying the effect of MB on the adsorption of Cd?II?in binary system,it was found that the adsorption capacity ratio R_q,Cd?II?values were between 0.996-1.708,which proved that the existence of MB in binary system was beneficial to the adsorption of Cd?II?.Moreover,the adsorption process could be well described by the Freundlich adsorption isotherm model,and there is a multi-molecular layer adsorption process.This can be explained by the fact that Cd?II?in the solution could not only interact with specific groups on the X-Y-?-CDP,but also chelate with the extra nitrogen groups provided by MB that has been adsorbed in the cavities of?-cyclodextrin.Research shows that X-Y-?-CDP has excellent performance for wastewater treatment,which contains both dyes and heavy metals,and it has certain application prospects.