Assessing the harmfulness of high-salinity oilfield-produced water related to trace metals using vortex-assisted dispersive liquid-liquid microextraction combined with inductively coupled plasma optical emission spectrometry

A method for quantifying trace levels of cadmium, lead, nickel and cobalt in high-salinity produced water (salinities up to 168) using vortex-assisted dispersive liquid-liquid microextraction (VA DLLME) combined with inductively coupled plasma optical emission spectrometry (ICP OES) was developed. Diethyldithiocarbamate (DDTC) was used as chelating agent, and complexes were extracted adding methanol (dispersive solvent) and carbon tetrachloride (extraction solvent) to aqueous samples. The organic (CCl4) sedimented phase was separated and evaporated, and the solubilized residue was analysed by ICP OES. The extraction and quantification parameters were optimized by univariate and Doehlert strategies. Detection limits of 0.006, 0.15, 0.036 and 0.003 mu g L-1 for cadmium, lead, nickel and cobalt, respectively, with relative standard deviations (ten replicates) of 10 mu g L-1 between 1.02% and 1.85% were obtained. Accuracy was verified by CRM (NASS-5) and by ICP-MS analysis. The proposed method was applied to high-salinity oilfield-produced water (from 116 to 158 salinity) with concentration ranges of 0.58-0.95, 1.57-11.70, 0.24-0.37 and 0.71-7.48 mu g L-1 for Co, Ni, Cd and Pb, respectively. The simplicity and low cost of the developed method allow it to be readily applied to the characterization of high-salinity produced water to assess its harmfulness before ocean disposal or reuse.