BACKGROUNDDiethylenetriaminepentaacetic acid (DTPA) dissolution and reprecipitation (DDARP) method is used frequently to eliminate encapsulated nitrate during barite precipitation. However, the effect of DDARP on barite precipitated from different kinds of natural water is rarely reported, particularly it is not clear whether the DDARP method plays a role in eliminating the effect of organic matter on barite from organic-rich water samples.

OBJECTIVESTo obtain more accurate sulfate oxygen isotope values, using the modified DDARP method and identify the factors affecting the oxygen isotope values of barite from natural water samples.

METHODSElement Analyzer (EA) coupled with Isotope Ratio Mass Spectrometry (IRMS) was used to determine the oxygen isotope values of original and purified barite.

RESULTSAfter purification, oxygen isotope values of high grade barite reagent show little variations, indicating that purification of high grade barite reagent is not needed and it can be directly used as the laboratory standard. However, oxygen isotope values of barite from Yellow River water have large variations from 0.6 to 1.0 with an average of 0.8‰, indicating that it is necessary to purify natural water barite. The differences in oxygen isotope values between original and purified barite from natural water samples vary from -0.4‰ to +1.9‰, and the average differences of oxygen isotope values are 0.7‰, 0.2‰, 0.3‰, -0.3‰, 0.1‰ and 1.4‰ for dissolved detergent, groundwater, river water, sewage, dissolved fertilizer, and rainwater, respectively. The large positive differences of oxygen isotope values (from +0.5‰ to +1.9‰) between original and purified barite is found in rainwater samples due to nitrate encapsulated in barite, which is related to the high molar ratio of nitrate to sulfate in rainwater (from 0.2 to 1.9 with an average of 0.7). The negative differences of oxygen isotope values (from -0.2‰ to 0.4‰) are observed in sewage with respect to the organic matter encapsulated in barite from organic-rich sewage, suggesting that the purification method can remove organic interference.

CONCLUSIONSThe modified DDARP method works well in eliminating nitrate and organic matter encapsulated in barite. It can be used to purify barite to obtain more accurate oxygen isotope values of sulfate in natural water samples. The proposed method can be used as a standard purification method for water body sulfate oxygen isotope determination.