Microwave-assisted ultraviolet digestion: an efficient method for the digestion of produced water from crude oil extraction and further metal determination

Abstract

A sample preparation method based on microwave-assisted ultraviolet digestion (MW-UV) was evaluated for the first time to treat produced waters from crude oil extraction containing variable salinity and organic carbon content for the subsequent determination of Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, Sr, V and Zn by inductively coupled plasma optical emission spectrometry (ICP-OES). Some parameters of the proposed MW-UV method were evaluated, such as the concentrations of HNO₃ and H₂O₂, microwave power and heating time in order to obtain high efficiency of digestion, which was verified by the determination of total organic carbon content (TOC) in the final solutions. Digestion was performed using about 8 g of produced water, 0.8 mol L⁻¹ of HNO₃ and 0.4 mL of H₂O₂ in high-pressure quartz vessels at 700 W for 15 min (5 min of ramp). Under these conditions, the TOC in solution was reduced by 98, 80, 78 and 48% for samples A, B, C and D, respectively, and residual acidity was lower than 0.41 mol L⁻¹ for all samples evaluated. Conventional microwave-assisted acid digestion (MW-AD) was also performed under the same conditions as MW-UV for comparison, and it was observed that some samples were not completely digested. For analyte determination by ICP-OES, the use of an internal standard was investigated, and better results were obtained using Sc. The accuracy of the proposed method was evaluated using a spiked sample and certified reference material (CRM). Using MW-UV, recoveries for the spiked sample were better than 90% for all analytes, and the results obtained for the CRM were in agreement, better than 94%, with the certified reference values (t-test, 95% confidence level). It was possible to use diluted reagents to treat complex samples, like produced waters, in only 15 min for further determination of sixteen metals by ICP-OES.