The Simultaneous Removal of Oil and Heavy Metals from Industrial Wastewaters Using Hydroxide or Sulfide Precipitation Coupled with Air Flotation

Abstract

A combined chemical-precipitation/air-flotation system was studied to simultaneously remove heavy metals and oil from a synthetic wastewater typically containing 25 mg/L each of copper, lead, and zinc, plus 1000 mg/L of oil. The wastewaters were subjected to hydroxide treatment or sulfide treatment and collectors simultaneously with air flotation. The precipitants were mixed rapidly into solution for 2 min, followed by a flotation time of 4 min. The effects of various collectors and their dosages, as well as the type of precipitant, were examined in terms of removals of oil and heavy metals. Five different chemical collectors were investigated -- sodium lauryl sulfate (NaLS), Atlasep 2A2, Nalco-7182, Nalco-7734, and TFL-365)^‡; applied collector dosages ranged from 0 to 1442 mg/L. Removals of heavy metals and oils of at least 93.85% can be attained using this innovative technology. The residual concentrations are sensitive functions of pH, collector type and dosage, precipitant type, and air-injection rate. The presence of oil had little, if any, effect on the removal of Cu and Pb by sulfide precipitation, while the removal of Zn decreased by about 38% due to the presence of oil. No single chemical collector or precipitant type provided the optimal removal of all constituents studied (Cu, Pb, Zn, and oil). Hydroxide precipitation resulted in lower concentrations than sulfide precipitation using the NaLS and Atlasep 2A2 collectors, while sulfide precipitation resulted in lower residual concentrations using the Nalco-7182 and Nalco-7734 collectors. The TFL-365 collector had lower Pb and oil concentrations employing hydroxide precipitation and lower Zn and Cu concentrations employing sulfide precipitation. By proper choice of precipitant type and collector dosage, residual metal concentrations lower than 0.2 mg/L can easily be obtained using the combined chemical precipitation-air flotation technology.