Preparations Of Novel Chelating Polymers And Their Performances For Trapping Heavy Metal Ions

With the rapid development of modern industry,environmental pollution has intensified,and water pollution caused by heavy metals has become a problem of globalization.Due to the toxicity,persistence,non-biodegradability and bioaccumulation of heavy metals,the treatment of heavy metal wastewater has become the focus of environmental protection work all over the world.It has become a research hotspot to find new treatment methods for the treatment of heavy metal ions from industrial effluents.This paper aims to develop an efficient,inexpensive and practical heavy metal wastewater treatment agent to make the treatment of heavy metal wastewater easier and more effective.Chitosan has attracted more and more attention in the field of water treatment due to its advantages of good environmental compatibility,renewable,abundant resources and high degradability.In this paper,on the basis of supplementary studies on the performance of the novel heavy metal capturing agent O-xanthogenated chitosan?XCTS?previously synthesized by our research group,a novel chelating polymer O-xanthogenated-N-tolyl chitosan?RXCTS?is synthesized.The performance of RXCTS to capture heavy metal ions is better than that of XCTS.Moreover,acrylamide is also used as the main raw material in this paper.A strong ligand of dithiocarboxy groups for heavy metal ions,is introduced into acrylamide monomer through a chemical synthesis method to prepare a small molecule chelating agent.They are then polymerized to the higher graft rate novel chelating polymer,Poly?dithiocarboxyl-acrylamide??PDTAM?.As both RXCTS and PDTAM have the double abilities of chelating precipitation towards heavy metal ions and flocculation,they show excellent capture performance for heavy metals in wastewater,while simultaneously also removing turbidity in wastewater.The optimum preparation conditions of RXCTS and PDTAM are determined by orthogonal experiments.They are characterized with elemental analysis,infrared analysis and NMR.The heavy metal water sample prepared in the laboratory is used as the treatment object,and the performances of RXCTS and PDTAM for the removal of heavy metal ions and turbidity are investigated by flocculation experiments.Moreover,the solution property and storage stability of RXCTS or PDTAM,morphology and stability of flocs,recovery of heavy metals and the treatment efficiency for actual wastewater containing heavy metal ions are discussed.The main research results are as follows:The optimum preparation conditions for RXCTS are a molar ratio of NaBH4 to CTS is1.5:1,a reaction temperature of 30?,and a reaction time of 0.5 h.PDTAM is prepared under the following conditions:In the grafting reaction stage the mass concentration of AM is3%,the molar ratio of reactants[n?NaOH?:n?CS2?:n?AM?]is 2:2:1,the pre-reaction temperature?T₁?is 35?,the pre-reaction time?t₁?is 60 min,the main reaction temperature?T₂?is 50?,and the main reaction time?t₂?is 150 min.During the polymerization stage the dosage of initiator?AIBI?is 0.24 g,the reaction temperature is 60?,and the reaction time is18 h.The infrared spectrum analysis shows that the molecular chain of RXCTS contains not only xanthogenate groups and secondary amine groups,which can chelate or coordinate with heavy metal ions.The infrared spectrum and nuclear magnetic analysis for PDTAM also show that the dithiocarboxy group has been successfully introduced onto the macromolecular chain.The weight-average molecular weight?Mw?of PDTAM measured by the laser light scattering method is about 3.5×10⁴,indicating that small molecule salt was successfully polymerized.Both RXCTS and PDTAM belong to amphoteric polyelectrolytes,and the isoelectric point(pH_iep)of RXCTS in distilled water and tap water is 3.3 and 3.0 respectively;the isoelectric point(pH_iep)of PDTAM in distilled water and tap water is 4.0 and 1.6,respectively.The removal rate of heavy metal ions usually decreases with the decrease of the pH value of the system at the same dosage of RXCTS/PDTAM.However,at a lower pH value,the ideal removal effect can also be obtained by increasing the dosage of RXCTS/PDTAM.The coexisting turbidity may inhibit or promote the removal of heavy metal ions,and the effects are not absolutely the same at different conditions.However,in most cases,metal ions can promote the removal of turbidity.The removal of metal ions can also be inhibited or promoted by coexisting organic complexing agents,complexing agents.This mainly depends on the coordination competition or the symbiosis between the two ligands in the coexistence system.RXCTS are not only used to remove non-oxidizing heavy metal ions,but are also used to remove oxidizing heavy metal ions in wastewater,such as Mn?VII?and Cr?VI?.For water samples with initial concentration of 25 mg/L,the removal rate of total Mn can reach 100%when dosage of RXCTS is 210 mg/L and pH value is 6.0;when the dosage of RXCTS is 480mg/L and the pH value is 5.0,the removal rates of Cr?VI?and total Cr reach 98.8%and96.6%,respectively.PDTAM can effectively remove heavy metal ions from single or mixed metal ions system.In the mixed system,PDTAM is selective for the chelation removal of the metal ions in the following order:Cu?II??29?Pb?II??29?Cd?II??29?Mn?II??29?Ni?II?.Moreover,the chelation stability constants of PDTAM and the above metal ions are measured as 6.76×10¹¹?5.64×10¹¹?3.38×10¹¹?8.55×10¹⁰?5.96×10¹⁰,indicating that the stability of chelates is in the following sequence PDTAM-Cu?29?PDTAM-Pb?29?PDTAM-Cd?29?PDTAM-Mn?29?PDTAM-Ni.It is also found that PDTAM can achieve an ideal effect on the actual wastewater treatment,and the cost is much lower than the traditional methods of neutralization and sedimentation.SEM images of chelated flocs show that the flocs are different in shape and appearance due to different composition and flocculation mechanism.However,the surface of all flocs is very rough,which is caused by the accumulation and adhesion of tiny flocs.This indicates that the adsorption and bridging of polymer chains play an important role in the flocculation process.The stability of flocs is not only closely related to the stability of chelate and the acidity of leaching solution,but also to the shape and appearance of flocs.In conclusion,RXCTS and PDTAM can effectively remove heavy metal ions from wastewater under certain conditions,and most of the heavy metals can be effectively recovered from flocs.They have a good applicability as a high removal rate of heavy metal ions can also be achieved under acidic and high turbidity conditions.Therefore,both RXCTS and PDTAM have very broad application prospects.