Bottom-Up Synthesis of an Oligo-dithiocarbamate Chelating Precipitant and Its Direct Removal of Complexed Heavy Metals from Acidic Wastewater

Abstract

Complexed heavy metal (HM) in acidic wastewater is one of the toughest issues for environmental pollution control. Aimed at directly eliminating complexed HMs from acidic wastewater, a novel cost-effective oligo-dithiocarbamate (oligo-DTC) chelating precipitant was synthesized by a bottom-up method from ammonia and 1,2-dichloroethane (DCE). Synthesis conditions were systematically optimized based on Ni removal performance at pH 2 and pH 7. An NH3/DCE ratio of 3:1 or above at temperatures between 100 and 160 °C were suitable for amination, while a NaOH/DCE ratio of 0.5–1.0:1 and a CS2/DCE ratio of 1.0:1 were favorable for dithiocarbamylation. The theoretical S/Ni mole ratio of oligo-DTC, representing the theoretically required oligo-DTC dose for complete Ni removal, was 8.2–9.4 at pH 2 and 4.0–4.6 at pH 7. Oligo-DTC showed high removal efficiencies for various HMs and had advantages over similar DTCs with high synthesis costs. Oligo-DTC successfully removed complexed HMs from two typical acidic wastewaters with residual HMs below the stringent discharge limits. Oligo-DTC had an average molecular weight of 1150 Da based on LCHRMS. Characterizations (scanning electron microscopy, Fourier transform infrared, and X-ray photoelectron spectroscopy) showed that oligo-DTC removed HMs by chelation with its NCS2– functional groups and also indicated that oligo-DTC suffered nonnegligible self-aggregation and decomposition in HM removal under strongly acidic conditions.