Synthesis of poly(dimethyldiallylammonium chloride-co-acrylamide)-graft-triethylenetetramine–dithiocarbamate and its removal performance and mechanism of action towards heavy metal ions

Poly(dimethyldiallylammonium chloride-co-acrylamide)-graft-triethylenetetramine–dithiocarbamate (PDCATD) was synthesized using dimethyldiallylammonium chloride, acrylamide, triethylenetetramine, formaldehyde, carbon bisulfide and sodium hydroxide. The resulting compound was characterized by FTIR, UV–Vis spectroscopy, and elementary analysis. The removal performances of PDCATD towards Ni2+, Cu2+, Cd2+, Pb2+ and Zn2+, as well as the heavy metal recovery from the flocs were systematically investigated. The zeta potential change of PDCATD-Ni flocs, and its images and compositions were investigated using SEM and energy spectrum analysis, respectively. The mechanism of chelation–flocculation was also studied. The CSS− group of PDCATD reacts with Ni2+, Cu2+, Cd2+, Pb2+ or Zn2+ to form chelates according to a molar ratio of 2:1 in the order of Cd2+ > Cu2+ > Pb2+ > Ni2+ > Zn2+. PDCATD can effectively remove the above mentioned heavy metal ions when the molar ratio of CSS− to the metal ions is close to 2:1. The flocs (PDCATD-Cu, PDCATD-Ni, PDCATD-Pb, PDCATD-Zn and PDCATD-Cd) are all very stable in a low-concentration acid solution, and the heavy metals can be efficiently recovered by refluxing in 1 mol/L nitric acid or hydrochloric acid for 30 min at boiling. The zeta potential of flocs treated with PDCATD is higher than that of flocs treated with sodium triethylenetetramine–multidithiocarbamate at the same sulfur dosage, indicating that the positive charges on the polymer chains can effectively neutralize the excess negative charges on the flocs. This is beneficial for promoting the formation and growth of flocs. PDCATD possesses both strong chelating abilities and excellent flocculation properties.

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► A solution to the problem of excess negative charges in flocs is proposed.
► Synthesis of a novel amphoteric chelating polymer flocculant.
► The positive charges on polymer chain can neutralize excess negative floc charges.
► The positive charges on polymer chain promote the formation and growth of flocs.
► Exploration of the process and mechanism of chelation-flocculation.