Heavy metals removal by flocculation/precipitation using N-(2-carboxyethyl)chitosans

Here we report on solution properties and flocculation performance of polyampholyte chitosan derivatives – N-carboxyethylated chitosans (CEC) – synthesized “in gel” by treatment of chitosan with acrylic acid. We show that carboxyethylation with the substitution degrees (DS) from 0.7 to 1.6 yields water-soluble derivatives, whose isoelectric points (IEP) range from 6.30 to 3.55. At pH > 7.5 interaction of negatively charged CEC derivatives with positively charged colloids of the heavy metal hydroxides (Zn2+, Cu2+, Ni2+), serving as a model of postgalvanic wastewaters, results in lowering of electrokinetic potential of the hydroxide colloids and their precipitation at appropriate flocculant dose. Investigations of CEC flocculation performance depending on pH and DS show that the efficiency of metal removal decreases in the row Cu2+ > Zn2+ > Ni2+ corresponding to the row of the hydroxocomplex stability for these metals. The higher was pH of the system and DS of CEC derivative, the narrower was the flocculation window and the stronger was negative effect of flocculant overdosing on the efficiency of heavy metal removal. Based on the estimation of floc settling rates and the residual metal concentrations, we have concluded that the optimal DS of CEC derivatives for the precipitation of metal hydroxides is 0.7–1.0.