Removal Efficiency And Mechanism Of Heavy Metal Ions In Water By Protein Type Bioflocculant

Heavy metal contamination of water bodies presents a severe hazard to public health and environment owing to their accumulation in the food chain as toxic pollutants and persistence in nature.Bioflocculants have gained tremendous significance in the detoxification of heavy metals contaminated water due to their biocompatible and non-toxicity to environment.However,most of bioflocculants only remove heavy metals by adsorption and they could not reduce the highly toxic heavy metals to low toxic ions.Bioflocculants also suffered from its poor separability after heavy metals removal,which greatly limits its subsequent disposal processing.The research on mechanism was superficial,lacking of direct evidence,which resulted in the selection of bioflocculants was not supported.Therefore,this project chose the protein type bioflocculants from Klebsiella sp.J1(MFX)as research object,and magnetic bioflocculants(Fe3O4@MFX)was synthesized under chemical reaction,overcoming the difficulty of separation.Based on these,the development of bioflocculants will be improved.The removal mechanism and competition mechanism of heavy metals in aquatic environments was explored and directly proven by qualitative and quantitative methods in order t o offer the guidance for the selection and modification of bioflocculants.Composition and properties of MFX were analyzed by qualitative and quantitative methods.The results showed that the main components of MFX were protein(80.65%)and polysaccharide(14.86%),thus MFX belong to protein type bioflocculants.Proteins were made up of 16 kinds of amino acids,such as asparagine,and the polysaccharides were composed of mice,lilacs,lithosaccharides,mannose,glucose and galactose.An abundance of functional groups in protein type MFX with reduction potential could effectively react with heavy metals.In order to investigate the removal efficiency of magnetic MFX for heavy metals,eight metals were selected for adsorption tests,and the results showed adsorption capacity of MFX for each matals was significant different.Magnetic bioflocculants(Fe3O4@MFX)was synthesized under chemical reaction,and its structure and properties were analyzed.Fe3O4@MFX exhibited the well-defined core-shell structure with a size of around 50 nm.With a high magnetic,Fe3O4@MFX can be readily separated from solution.The adsorption results of Fe3O4@MFX for eight heavy metals showed that the coating of MFX on Fe3O4 did not affect the adsorption capacity of MFX for heavy metal ions in water.Fe3O4 played their part in magnetic separation rather than heavy metal ions removal.Moreover,Fe3O4@MFX had a good recycle performance.Based on these,the development of bioflocculants will be improved.It has certain scientific significance and promotion value.Pb(II)and Cr(VI)were used as the typical representatives of common and toxic heavy metal ions.The removal efficiency and mechanism of heavy metals was explored by qualitative and quantitative methods,such as Zeta potential,FTIR,EDS and the mass balance of the elements.At room temperature(22oC),the maximum removal pencentages of MFX for Pb(II)and Cr(VI)were 99.47% and 80.02%,and the maximum adsorption capacities were 99.5 mg/g and 53.3 mg/g.The chelation and ion-exchange were identified as the major mechanism of adsorption Pb(II)by MFX,while the surface physical sorption only plays an auxiliary role.For Cr(VI),the adsorbed Cr(VI)(82.3%)on MFX were reduced to Cr(III)by benzenoid amine(–NH–)on MFX.Redox was the major mechanisms of Cr(VI)onto MFX.The surface physical sorption,the ion-exchange and chelation play auxiliary role.Compared to others biomaterials,the removal capacity of MFX for Pb(II)and Cr(VI)was higher and it can be generally used.The analysis of the mechanism provides theoretical basis and guidance for the directional selection and modification of biological adsorption materials.The real water environment often contains more than one heavy metal ion.For example,the rapid development of battery and feed processing industries resulted in a large number of Cu(II)and Zn(II)coexist in water.Therefore,to further investigate the adsorption of heavy metal ions onto bioflocculants,Cu(II)and Zn(II)coming from the same water source was used as the typical representatives of coexisting metal ions.The competition adsorption of Cu(II)and Zn(II)on MFX in the coexistence system was investigated.The results showed the presence of Cu(II)had a relevant negative effect on the Zn(II)sorption,while the interference of Zn(II)on the sorption of Cu(II)was less intense.The competitive Langmuir and Langmuir-Freundlich isotherm models were proven to be effective in describing the experimental data of binary component system,indicating that one binding site was only available for one metal ion,and Cu(II)was preferentially adsorbed on EPS over Zn(II).The competition mechanism revealed that the adsorption sites of Cu(II)entirely overlapped with those of Zn(II),and Cu(II)exhibited stronger binding ability onto tyrosine and tryptophan groups in MFX.The conditional stability constant(K)and the proportion of organic ligands(f)of tryptophan and tyrosine-like substances for Cu(II)were higher than those for Zn(II),indicating that the combined ability of MFX for Cu(II)was higher than that for Zn(II).Therefore,when Cu(II)and Zn(II)coexist in water,Cu(II)was preferentially adsorbed.Base on the structure and properties of MFX,magnetic Fe3O4 nanoparticles were introduced to improve the separability of bioflocculants.The removal and conversion mechanisms of common and toxic heavy metals(i.e.Pb(II)and Cr(VI))were analyzed.The multi-parameter models were established to describe the competitive adsorption and to reveal the mechanism of competitive ad sorption.They provide theoretical basis and guidance for the directional selection and modification of MFX,improve the development of bioflocculants and lay the foundation for the development of biosorption toward a more efficient and economically feasible direction.