Study On Synthesis Of Amphoteric Chelating Polymer Flocculant And Its Chelation-flocculant Performance

The heavy metal ions, such as Hg²⁺, Cd²⁺, Pb²⁺, Cr6+, As³⁺, Cu²⁺ and Ni²⁺, can produce kinds of significant toxic effects due to their toxicities and nonbiodegradabilities. Therefore, the wastewater containing heavy metal ions will do great harm to the environment and the health of human beings. Chelating flocculation can make heavy metal ions become into insoluble chelating precipitates separate directly by using chelating flocculant in the treatment for the heavy metal wastewater, which can reduce cost, simplify processes, and enhance flocculation efficiency. In this paper, a novel amphoteric chelating polymer flocculant (ACPF) was designed and synthesized, the properties of chelating flocculant were investigated systematically, and the mechanisms were discussed preliminarily, the applicability and the ecological safety of ACPF also were studied generally.Because the polymer chelant exists steric hindrance and spatial mismatch in chelating process, some of chelation group fail to combine with heavy metal ions completely, the flocs taking the surplus negative charge, which will hinder the formation and furthergrowth of them. For this reason, the cationic monomer dimethyldiallylammonium chloride (DMDAAC) was brought into the polymer chains to prepare ACPF. A series of cationic degrees of P(DM-AM) were synthesized by using DMDAAC and acrylamide (AM) as raw materials; then triethylene tetramine (TETA) was introduced into the chains of copolymer P(DM-AM) to react with methanal by Mannich reaction under alkaline conditions; finally, the amphoteric chelating polymer flocculant (ACPF) was prepared for using the above products reacting with carbon disulfide (CS2) and sodium hydroxide (NaOH) by xanthation reaction. The optimal condition of Mannich reaction were as follows: the pH value was 10.5, the reaction temperature was 45℃, n(AM)∶n(HCHO)∶n(TETA)=1∶0.95∶1.05, reacting which lasts for 30min after adding HCHO, and reacting sequentially 2h after adding TETA. In the xanthation, n(CS2)∶n(TETA)=3.5∶1 and n(NaOH)∶n(CS₂)=1.1∶1, when the temperature was 55℃and the reaction time was 4h in the first reaction process; while in the second process the room temperature was enough and its react time was 2h.The results showed that the applicabilities of ACPF for treating the simulative heavy metal wastewater were investigated, among a quite wide pH value, ACPF not only can remove the free heavy metal ions easily, but also was applied effectively in the simulative waste water containing complexing heavy metal ions and turbidity. The residual concentration of heavy metal ions and turbidity can meet the frist grade criteria of China's Integrated Wastewater Discharge Standard (GB8978-1996); and the properties of chelating flocculant were investigated systematically, and the mechanisms were discussed preliminarily.By studying the ultraviolet spectra of ACPF and its chelating precipitates, the maximum absorption peaks of ACPF were in 204.05nm, 251.12nm and 285.25nm. However, the maximum absorption peaks of the chelates were in 318.45nm, 326.72nm, 325.72nm and 312.86nm, respectively, which presented significant redshift compared with ACPF's. The complexing ratios of -CSS- and the bivalent heavy metal ions were 2∶1, the stability constants of these chelates were 4.86×10¹¹, 2.54×10¹⁰, 2.47×10¹¹ and 2.25×10¹¹, respectively, and the steady sequence was ACPF-Cu>ACPF-Pb>ACPF-Cd>ACPF-Ni.The structures, the components and the stability of the four chelates were studied by EMS, SEM, EDS, and the leaching-out experiments. The experimental results showed that, the aqueous flocs of the chelating sediments were loose and retiform, but appearing irregularly, rough and dense granules after having been dried; The atoms number ratios and the mass ratios of S and Cu, Ni, Pb, Cd were close to the theoretical results. Under the neutral and alkalescence conditions, the chelating precipitates all had favourable stability, the leaching-out concentration of heavy metal ions all were below the frist grade criteria of China's Integrated Wastewater Discharge Standard; In this paper the biology toxicity of ACPF was also investigated. The study focused on the growth inhibition ratio of the mixed bacteria by using ACPF, which showed that ACPF can be utilized safely and the biotoxicity will not be brought to the environment.The heavy metal wastewater, which was took from the copper plating process, was treated effectively by Chelating precipitation that make use of the combination of CaCl₂ and ACPF. We found that, under a certain conditions which in the neutral and alkalescence, it was treated effectively by using ACPF, when the dosages of ACPF and CaCl₂ were 4.0g/L, 100mg/L, respectively, the reaction time was 30min, the removal rate of COD and Cu²⁺ were 98.26% and 99.58%, meanwhile, the residual concentration were 24mg/L and 0.185mg/L, respectively. The indicators of COD and Cu²⁺ can reach the Emission standard of Pollutants for Electroplating (GB21900-2008).In a word, the amphoteric chelating polymer flocculant(ACPF) can treat the heavy metal wastewater in an effective way under a certain condition. It had a good ecological safety and a well setteability, its chelating sediments having a better stability, and undoubtedly it had a favourable application prospect in water treatment without bringing about the secondary pollution.