| A large amount of wastewater containing heavy metal ions is discharged into theenvironment, which has polluted the water environment badly. The new methods, for thetreatment of heavy metal ions from industrial effluents, have been the hot spot of researchcurrently. Therefore, it is of great significance to develop a novel reagent which canefficiently remove heavy metal ions from the wastewater. In this paper, a strong ligand ofheavy metal ions, dithiocarboxylic acid groups, is introduced to a macromolecular flocculant(polyethyleneimine). The novel flocculant synthesized in this way is calledpolyethyleneimine-sodium xanthogenate (PEX), and it has the function for trapping heavymetal ions and removing turbidity from wastewater. Because PEX has the two functions, it ispossible to reduce some treatment units in the treatment of wastewater containing heavy metalions, and the wastewater treatment system would be greatly simplified.The flocculant PEX is synthesized by using polyethyleneimine (PEI), carbon disulfide(CS2), and sodium hydroxide (NaOH). The orthogonal experiments are employed for theoptimum synthetic condition of PEX, and three kinds of products (PEX–1、PEX–6、PEX–60)are obtained in the experiments. The structures of PEX are characterized by elementalanalysis, ultraviolet analysis and infrared analysis. The PEX performances for removal ofturbidity and heavy metal ions are mainly investigated by flocculation experiments. Thesolution property and storage stability of PEX are also studied. Furthermore, the treatmentefficiency for actual wastewater containing heavy metal ions, the recovery for heavy metal,the stability and fractal of flocs are discussed. The main results are given as follows.The main influencing factors for optimum synthetic condition of PEX are the moleculeweight of PEI, dose of CS2, reaction temperature and reaction time. The optimum syntheticcondition of PEX products is different for the different molecule weight of PEI. For productPEX–1: PEI, CS2and NaOH are put into the reactor in accordance with molar ratio1:2:2. Themixture of reactants is stirred magnetically at35℃for30min, and then the temperature israised to40℃for150min under constant stirring. For product PEX–6: PEI, CS2and NaOHare put into the reactor with molar ratio1:3:3. The mixture of reactants is stirred magneticallyat20℃for30min, and then the temperature is raised to40℃for60min under constantstirring. For product PEX–60: PEI, CS2and NaOH are put into the reactor with molar ratio1:1:2. The mixture of reactants is stirred magnetically at25℃for30min, and then thetemperature was raised to60℃for60min under constant stirring.PEX is an amphoteric macromolecular flocculant, and it exists isoelectric point. The zetapotential for PEX solution is equal to zero, and the solubility is minimum at its isoelectric point. The isoelectric point (pHiep) for PEX–1、PEX–6、PEX–60is4.6、5.0、8.4in the systemof distilled water, respectively, and3.8、5.0、8.7in the system of tap water, respectively. Theperformances for removal of turbidity by using PEX are better than by using PEI, and theremoval rate of turbidity is maximum at isoelectric point of PEX. The coexistence of heavymetal ions may notably promote the removal of turbidity.PEX can effectively remove the divalent heavy metal ions in the single system ormixture system of heavy metal. The treatment efficiency for the heavy metal ions is high atthe acidity condition by using PEX, and the demand of pH value is more flexible. For productPEX, the trapping order for heavy metal ions is that PEX–1> PEX–6> PEX–60. For heavymetal ions, the trapping order is that Cu(II)> Pb(II)> Cd(II)> Ni(II)> Zn(II). Moreover,Cr(VI) ions can be efficiently removed in a strongly acidic solution, but the removalefficiency for Cr(III) ions is not ideal at the acidic condition.The presence of inorganic cations, inorganic anions and turbidity coexisting with heavymetal ions affects slightly the removal of heavy metal ions. However, the coexisting organiccomplexing agent will inhibit or promote the removal of heavy metal ions, and these effectsare not absolutely same. The inhibition influence may be minished or eliminated throughadding the dosage of PEX. When the substances causing turbidity exist in the system, heavymetal ions and turbidity will be both removed simultaneously. The removal rate of heavymetal ions increases with the increase in the fractal dimensions of chelating flocs. That is, thefractal dimensions of chelating flocs are higher, and the removal efficiency for heavy metalions is better.FTIR spectra reveals that the dithiocarboxylic acid groups have been successfully graftedto polyethyleneimine molecule, and dithiocarboxylic acid groups and amino groups on themacromolecular chains of PEX play a major role in the removal of heavy metal ions. PEXmay be well applied to the practical wastewater treatment for some advantages. For example,the storage stability of PEX and the stability of chelating flocs are good; the treatmentefficiency is high for the actual wastewater by using PEX; and heavy metal ions may beremoved from the chelating flocs.In conclusion, the novel macromolecular flocculant PEX can effectively remove heavymetal ions from wastewater under a certain condition. Therefore, PEX may be as a newtreatment agent for the wastewater containing heavy metal ions, and it has a favorableapplication prospect. |
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