| Chitosan is the alkaline natural polysaccharide polymer in nature, which contains a large number of functional groups along the molecular chain, such as: amino,hydroxy and N-acetyl reactive groups. Especially the lone pair electrons on the nitrogen atom in amino groups can enter into the empty orbital of heavy metal ions to from stable chelates by coordination bonds. Chemical modification of chitosan and its application in the field of water treatment has become a research hotspot in recent years. In this paper, a novel macromolecule chelating-flocculant dithiocarboxyl chitosan(DTCTS) was synthesized by grafting a dithiocarboxylic group onto chitosan.The flocculant DTCTS was synthesized by using chitosan(CTS), carbon disulfide(CS2) and sodium hydroxide(NaOH). The single-factor experiment,orthogonal experiment and response surface methodology were employed for the determination of optimum synthetic conditions of DTCTS. The structures of DTCTS are characterized by scanning electron microscopy,ultraviolet analysis, infrared analysis and elemental analysis. The removal of heavy metal ions and turbidity of DTCTS were investigated by flocculant experiments. The characteristics of DTCTS’s solution property and storage stability were also studied. Furthermore, the fractal of flocs, the stability of cheating flocs and the recovery for heavy metal were discussed. The main results are given as follows:(1)The molar ration of CTS, CS2 and NaOH, the molecular weight and the deacetylation degree of chitosan, the concentration of chitosan, per-reaction time and main-reaction time, per-reaction temperature and main-reaction temperature are the main factors affecting the synthesis of DTCTS by single-factor experiment.(2)The molar ration of reactant, reaction time and recation temperature were choosed for orthogonal experiment, with regard to the removal efficiency of Cu2+ and Cd2+ from aqueous solution at the dosage of 300mg/L and 360mg/L. According to the orthogonal experiment,four different optimum synthetic conditions can be obtained which show that the optimal synthesis condition varies with the species of heavy metals ions and the dosage of DTCTS.(3)The response surface experiment was conducted by three main factors: the molar ration of reactant, per-recation time(t1) and main-recation temperature(T2)and the optimum condtions for preparation of DTCTS are as follows:the molar ratio of CTS/CS2/NaOH was 1:1.5:2, per-reaction temperature is 25℃, per-reaction time is 30 min, main-reaction temperature was 60℃and main-reaction time is 120 min. Compared with orthogonal experiment this synthesis condition will not change with ions species.(4)DTCTS is an amphoteric polymer flocculant, its isoelectric point(pHiep) in distilled water system is 6.8, while in tap water system the pHiep is 6.0.(5)DTCTS can effectively remove the single heavy metal ions in wastewater such as Cu2+,Cd2+,Pb2+very well, and the demand of pH value is more flexible. When the pH value is 6, the dosage of DTCTS are suitable, the remove efficiency of these three heavy metals can reach 100% respectively. While for wastewater containing Ni2+, DTCTS has poor efficiency, when the dosage is 720mg/L, the remove rate can only reach 86.18%, more DTCTS dosage needs to be achieve better removal efficiency. For the mixture system of heavy metal, the overall trapping order is Cu2+>Pb2+>Cd2+>Ni2+.(6) The coexisted turbidity and EDTA have different effects on the trapping ability of DTCTS for heavy metals. The organic complexing agent EDTA have inhibit effect for DTCTS remove Cu2+or Cd2+ ions, But the inhibition influence can be minished through adding the dosage of DTCTS. The presence of turbidity has slightly effects on DTCTS remove heavy metals, but in the coexisting system, the removal efficiency of turbidity can be increased significantly.(7)There exists a certain relationship between the removal efficiency of heavy metal ions of DTCTS and floc size.The bigger the fractal dimensions of chelating flocs are, the higher the removal rate is. The chelating flocs are stable and easy recovery, which is good for DTCTS application.In short, the novel the macromolecular flocculant DTCTS can effectively remove heavy metal ions from wastewater under a certain condition, and the flocs can set well, making heavy metal water treatment become simply, and it has a favorable application prospect. |
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