Preparation of Reference Material for Deep-Sea Rare Earth-Enriched Sediments in the Pacific Ocean

(1) Surface sediments from three representative Pacific Ocean sites were selected as candidates and pulverized by fluidized bed flow to ensure that the particle size of the reference material candidates met the requirements of the national code.

(2) A total of 62 components of major, minor and trace components in deep-sea rare earth-enriched sediments were determined, which covered the test items required for chemical composition analysis of marine sediments, and the total rare earth content was 2103μg/g, demonstrating that a standard basis for testing samples with high rare earth content can be provided.

(3) The uncertainty of a certified value was synthesized by the uncertainties caused by homogeneity, short- and long-term stability, and values.

Significance: Deep-sea rare earth is the fourth kind of deep-sea mineral resource discovered after polymetallic nodules, cobalt-enriched crusts and polymetallic sulfides. It is characterized by rich medium-heavy rare earth elements. Rare earth elements are widely used in military, aerospace, electronics, petrochemical, metallurgy, agriculture and other fields. However, according to the International Database for Certified Reference Materials (COMAR), there are no REY sediment reference materials worldwide. A deep-sea sediment certified material rich in rare earth elements has been developed to meet the analysis needs. The successful development of the reference material has further enriched the categories of deep-sea sediment reference material worldwide, and provided scientific and technological support for scientific research, deep-sea resource development evaluation and geochemical analysis and testing.

Methods: The candidate was collected from the Pacific Ocean. The collected samples were left to stand for several days, then the free seawater was removed in a clean place, and they were placed on a clean plastic cloth and dried in a clean and ventilated place. In this process, a wooden tool was used to rotate the sample, remove the fragments and mix thoroughly. The sediment was put in the sample pan and then into an oven at 80℃ for 48h. A fluidized-bed jet mill was used to finely crush the sediment to −74μm with a cooled high-pressure airflow and pass it through a −200 mesh sieve to ensure that the sieving rate reached 99.9%. The treated samples were temporarily stored in polyethylene plastic drums under constant temperature and clean conditions, and then packaged in 50g bottles for testing. Random samples were selected for uniformity and stability tests. More targeted evaluation methods such as gravimetry, atomic fluorescence spectrometry, inductively coupled plasma-mass spectrometry and inductively coupled plasma-optical emission spectrometry were used to ensure the accuracy of rare earth-enriched sediments in deep sea through eleven collaborating laboratories.

Data and Results: According to the newly issued National Metrology Technical Specification “The Production of Reference Materials for Geoanalysis” (JJF 1646—2017), 33 bottles of samples were randomly selected from the smallest packing unit, and each sample was tested twice. 56 items were tested for uniformity and stability and are SiO₂, Al₂O₃, TFe₂O₃, MgO, CaO, Na₂O, K₂O, TiO₂, MnO, P₂O₅, As, Ba, Be, Bi, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Ga, Gd, Hf, Hg, Ho, La, Li, Lu, Mo, Nb, Nd, Ni, Pb, Pr, Rb, S, Sb, Sc, Se, Sm, Sr, Ta, Tb, Th, Tl, Tm, U, V, W, Y, Yb, Zn, Zr. The F value of the variance test was less than the critical value [F_0.05(32,33)=1.76] (Table 2), indicating that the homogeneity of the reference material was good. During the stability inspection period, there was no significant difference in the content of the 56 components, indicating that the reference material was stable (Table 5). Data were processed according to General and Statistical Principles for Characterization of Reference Materials (JJF 1343—2012) and outlier tests were performed using the Grubbs and Dixon Method. According to statistics, there were 2975 original data of deep-sea rare earth-enriched sediments, and 32 outliers were eliminated. The elimination rate was 1.08%. Standard values and uncertainties of reference materials were determined by statistical processing of constant value test data. The Shapiro-Wilk method was used to test the normal distribution of the mean data set, and the test results were all normal distribution. The final value comprised 62 components, covering major, trace, and all rare earth elements. The total content of rare earth was 2103μg/g (Table 7).