Investigation of metal mobility in gold and silver mine tailings by single-step and sequential extractions.

Metal leachate from mine tailings has the potential to release toxic metals into the surrounding environment. A single-step extraction procedure mimicking rainwater and a three-step BCR sequential extraction procedure (acid, reducing and oxidizing conditions) were applied to gold (GMT) and silver (SMT) mine tailings. Major (Al, Ca, Fe, Mg, and Mn) and trace metals were monitored to better understand the mobility and geochemistry of these metals when exposed to various environmental leaching conditions. Rainwater extraction released only small quantities of metals, while the three-step BCR extraction was more effective in mobilizing metals from the tailings. Under the acidic conditions of BCR step 1, Ca, Mg, Cd, Cu, and Mn were released in high concentrations. The dissolution of Fe, Ca, and Mg were dominant along with Pb in step 2 (reducing conditions). In step 3 (oxidizing conditions), Fe was the most dominant species together with Co, Cu, Ni, and Se. A high fraction of Al, Be, Cr, Li, Mo, Sb, Tl, and V remained in the residue. From SMT, larger quantities of As, Ca, Cd, and Zn were released compared to GMT. The BCR extraction could be applied to tailings to predict the potential release of toxic metals from mine wastes; however, excessive amounts of Ca and Fe in the tailings could cause carry-overs and incomplete extraction and carry-overs, resulting in a misinterpretation of results.

Tessier sequential extraction on 17 elements from three marine sediment certified reference materials (HISS-1, MESS-4, and PACS-3).

The four-step Tessier sequential extraction procedure is a well-known approach used for environmental and geochemical studies in soil and sediments. However, a lack of reference materials limits its use making implementation and quality control cumbersome. This study applied Tessier sequential extraction to three globally used marine sediment certified reference materials (CRMs) including HISS-1, MESS-4, and PACS-3 with varying levels of contamination. The study analyzed the distribution of 17 elements throughout the extraction phases. Overall, the percent recovery (sum of steps vs total metal concentration) of all analyzed elements in Tessier extraction was 92% + 40% in HISS-1, 101% + 12% in MESS-4, and 102% + 10% in PACS-3. The observed uncertainty of the individual elemental concentrations averaged at 13%, which compares favorably with the 16% target uncertainty derived from the Horwitz equation. The reference data set produced here using the Tessier sequential extraction procedure will serve as a quality control and method development tool for laboratories. Graphical abstract.

Determination of 27 metals in HISS-1, MESS-4 and PACS-3 marine sediment certified reference materials by the BCR sequential extraction.

The BCR sequential extraction procedure developed by the European Community Bureau of Reference was applied to the three marine sediment certified reference materials (CRMs). These CRMs are designated as HISS-1, MESS-4, and PACS-3 and comprise respectively pristine, moderately-contaminated, and highly-contaminated sediments, respectively. The study aimed to provide values of extractable elements in reference materials of varied geological origin to support method development and quality control efforts. Concentrations of 27 elements extracted in the three sequential extraction steps and in the residue were determined. The extraction steps consisted of: Step 1 - acetic acid extraction (targeting the exchangeable, water- and acid-soluble fraction); Step 2 - hydroxylammonium chloride extraction (targeting the reducible fraction); and Step 3 - hydrogen peroxide extraction (targeting the oxidizable fraction). The results from two independent laboratories using the sequential extraction procedure for the 27 elements were combined using the DerSimonian-Laird method of analysis implemented in the NIST Consensus Builder software. The percent recovery (sum steps vs total metal content) of 27 elements ranged from 68% to 125% in HISS-1, 76%-119% in MESS-4, and 70%-125% in PACS-3, based on the certified values. The combined uncertainty (k = 2) was from 1% to 39% for HISS-1, 3%-45% for MESS-4, and 3%-21% for PACS-3. Comparing the three extraction steps, the uncertainty of Step 3 was the highest for all the three CRMs. The agglomerate sediments in MESS-4 resulted in high uncertainty when compared to HISS-1 and PACS-3. The method validation showed the BCR sequential extraction procedure can apply to other elements including As, Co, Li, Mn, and V.