Flocculation performance and evaluation of a sulfur-containing tannin flocculant for Cu2+ removal

Abstract

To introduce sulfur compounds into tannin-based flocculants, thiosemicarbazide (TC) was selected as a raw material to react with black wattle bark tannin and formaldehyde through the Mannich reaction in order to synthesize a sulfur-containing tannin flocculant (TFA) with the functions of chelation, flocculation and precipitation. The physicochemical characteristics of TFA were characterized by elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR), 13C solid-state nuclear magnetic resonance (13C NMR) and zeta potential to identify the successful synthesis. The flocculation performance of TFA was evaluated by measuring the Cu2+ removal rate. The effects of pH, initial Cu2+ concentration, TFA dosage, coexisting inorganic compounds and organic complexing agents on the Cu2+ removal rate were investigated through flocculation experiments. The results showed that the Cu2+ removal rate increased with increasing pH and the maximum Cu2+ removal rate for 5, 10 and 25 mg/L initial Cu2+ reached 100 % by raising the TFA dosage. Coexisting inorganic compounds, such as KCl, NaCl, CaCl2, MgCl2, NaNO3, and Na2SO4, had a negligible impact on Cu2+ removal, while the presence of organic complexing agents (humic acid, disodium ethylenediamine tetraacetate, citric acid and aminoacetic acid) had a significant influence on Cu2+ removal. The flocculation mechanisms of Cu2+ by TFA mainly involved a combination of bridge-aggregation and chelating precipitation, accompanied by a sweeping effect.