Alternative physicochemical processes for removal of metals from wastewater

Experiments were conducted to test alternative treatment processes to remove selected heavy metals (Cu, Ni, Zn, and U) from wastewater treated at the West End Treatment Facility (WETF) located at the Department of Energy's Y-12 Gaseous Diffusion plant in Oak Ridge, TN. The treatment processes evaluated included centrifugation, iron-oxide coated sand, insoluble starch xanthate (ISX), insoluble starch thiocarbamate (ISTC), alkyl dithiocarbamate (DTC) and ion exchange resins. Batch tests were used to evaluate all processes except ion exchange, which was evaluated using a continuous flow column. The Cu (II), Ni (II), and Zn (II) concentrations of wastewater samples were determined by flame atomic adsorption spectroscopy (AAS) (Jared-Ash) while uranium was measured using the kinetic fluorescence technique (Chemchek Uranium Analyzer). The processes were initially screened using simulated wastewater to provide a reproducible basis for comparison. Promising processes were further tested using actual wastewater. Metals removal using ISX increased with increasing pH, increasing ISX dosage, and decreasing carbonate concentration. Only the removal of Zn (II) was the pH significant. It was found that in addition to adsorption of metals, an important role of ISX is to increase the pH of the wastewater, thereby facilitating metals removal by precipitation. Carbonate interferes with metals removal by ISX in two distinct ways: at low pH the buffering capacity of carbonate prevents an increase in pH necessary for good metals removal and at high pH it complexes the metals, increasing their aqueous solubility. ISTC was moderately effective in the removal of Cu (II) from wastewater. No other metals were removed using ISTC. The effectiveness of metal removal using iron-oxide coated sand increased with increasing sand dosages. The maximum metals removal occurred at pH 7. At lower values of pH, metals removal was hindered by a positive surface charge on the iron oxide surface. At higher values of pH, metals removal was hindered by carbonate complexation. Removal of Cu (II) and Ni (II) using DTC was effective over a wide range of pH in simulated wastewater. Zn (II) removal increased with increasing pH in simulated wastewater. U (IV) removal decreased with increasing pH because of strong carbonate complexation. Metals removal in wastewater containing strong chelating agents using DTC is very poor. No significant removal of metals occurred during centrifugation. During ion exchange, treatment of simulated wastewater using Chelex 200, a chelating ion exchange resin, metals removal was poor. Precipitation of metals interfered with the evaluation of the effectiveness of the ion exchange resin. Effluent concentrations of Ni (II) and Zn (II) varied erratically during the run. The experiment was terminated after 640 hours of operation.