Enhanced removal of hexavalent chromium and nitrate in aquifers by alkali-modified emulsified vegetable oil

Abstract

The cocontamination of hexavalent chromium (Cr(VI)) and nitrate is common in groundwater, particularly in plains region. Injecting emulsified vegetable oil (EVO) into aquifer is an effective method for the simultaneous removal of Cr(VI) and nitrate from groundwater. In this study, EVO and alkali-modified emulsified vegetable oil (AM-EVO) were prepared from soybean oil and magnesium hydroxide. Characterization methods such as atomic force microscope (AFM), inverted fluorescence microscope (IFM), laser particle size analysis, and Fourier transform infrared spectroscopy (FTIR) were used to examine the microscopic morphology, particle size distribution, and surface functional groups. The remediation efficiency of EVO and AM-EVO were investigated and compared for Cr(VI) and nitrate removal in groundwater. Influencing factors on the removal of Cr(VI) and nitrate such as initial dissolved oxygen, alkaline substance type, alkaline substance existing form, and reagent dosage were studied. AM-EVO with good stability, good dispersity, and high removal efficiency was obtained with the concentrations of soybean oil, magnesium hydroxide, polyoxyethylene sorbitan monooleate (Tween-80), and sodium dodecyl sulfate (SDS) were respectively 45, 25, 9.3, and 1.75 g/L. The AM-EVO could effectively accelerate the removal of contaminants at initial pH 10 and 30 °C, 10 mg/L Cr(VI) and 350 mg/L nitrate both could be completely removed within five days. Cr(VI) was reduced by microbial secondary ferrous iron, and the apparent rate constant was increased from 0.07 day−1 to 0.41 day−1. Nitrate was reduced by denitrifying bacteria, and the degradation rate constant was increased from 2.01 day−1 to 3.65 day−1.