Statistical Techniques for Leveraging Geochemical Data in Ore and Non-Ore Characterization for Mining and Environmental Stewardship

Digestion methods for trace element measurements in shales: Paleoredox proxies examined

It is important to evaluate different digestion methods to accurately determine elemental concentrations in soils. Three commonly used digestion procedures, hotplate aqua regia, microwave aqua regia, and microwave aqua regia + HF, were compared for digestion of three standard reference materials (SRMs 2704, 2709, and 2711) and 20 Florida soils (three groups of Quartzipsammemts, one group of Alaquods, and one group of Paleudults). Sixteen elements (Al, Ba, Ca, Fe, K, Mg, Mn, P, Zn, As, Cd, Cr, Cu, Ni, Pb, and Se) were analyzed using either an inductively coupled plasma optical emission spectrometer (ICP‐OES) or a graphite furnace atomic absorption spectrophotometer (GFAAS). Precise analysis was achieved for all elements except As (1.0–25%), Cd (1.8–22%), and Se (4.1–22%) in the SRMs using all three procedures, with the microwave aqua regia + HF procedure having slightly better precision (3.7%) averaged across all elements. Compared with the elemental recoveries in SRMs by the microwave aqua regia digestion (80%), microwave aqua regia + HF digestion achieved greater accuracy (94%), whereas the hotplate aqua regia digestion achieved less accuracy (74%). In general, the microwave aqua regia + HF digestion was overall the best procedure for determining concentrations of most metals in SRMs and Florida soils, followed by the microwave aqua regia and the hotplate aqua regia digestion. However, this so‐called total–total digestion procedure (microwave aqua regia + HF) may underestimate total Pb concentration in Florida soils and total Al, Ca, Fe, Mg, and Ba concentrations in the Florida Paleudults.

ICP-OES assessment of heavy metal contamination in tropical marine sediments: A comparative study of two digestion techniques

Ecological and human health risk assessments at active and abandoned metal mines require accurate geochemical data for earth materials, including soils, sediments, dusts, and mine wastes. Reliable data on metal(loid) concentrations are also important for establishing geochemical baselines and to support environmental monitoring activities. However, the reported concentration for an element in a given sample can be affected by many factors, including mineralogy, grain size, laboratory digestion conditions (reagents, temperature, time), and the instrument(s) used for elemental analysis. In particular, aggressive multi-acid digestion methods used to provide 'near-total' data for some metals of environmental interest (e.g. chromium (Cr), uranium (U)) may result in the loss of other elements (e.g. antimony (Sb), arsenic (As), sulfur (S)) through volatilization prior to analysis. When comparing data from different studies or sampling periods, it is crucial to ensure that the analytical protocols used are consistent and that changes in element concentrations over time are not simply due to variations in sample processing or laboratory procedures. The purpose of this study is to compare common digestion protocols used in the analysis of geological materials and provide recommendations on the most appropriate techniques for use in environmental risk assessments. We analyzed samples of lake sediments, stream sediments, soils, and mine tailings collected at metal mine sites across Canada. These samples were digested using two of the most commonly employed procedures in exploration and environmental geochemistry: a modified aqua regia digestion and a 4-acid digestion. All solutions were analyzed using ICP-ES/MS, and certified reference materials (CRMs) and duplicate samples were used to monitor analytical accuracy and precision. The concentration of elements (e.g. Cr) hosted in relatively insoluble mineral phases are consistently higher following 4-acid digestions as compared to digestions using aqua regia, but generally lower than total values measured using direct methods of analysis (e.g. Instrumental Neutron Activation Analysis). In contrast, the concentrations of As are often higher in samples following aqua regia versus 4-acid digestions and generally in good agreement with total values in CRMs. The behaviour of other elements (e.g. Sb, copper (Cu), lead (Pb), S, zinc (Zn)) was more variable and for many samples, the results following 4-acid and aqua regia digestions were statistically indistinguishable. Variations in the mineralogy of different samples play a key role in determining the fraction of different metal(loid)s released by these digestion techniques. This poster highlights some of the advantages and disadvantages of using each of these digestion protocols for risk assessment and environmental monitoring purposes, and provides recommendations for using geochemical data to help guide environmental decision-making at both active and abandoned metal mines.

Determination of gold and silver in geological samples by focused infrared digestion: A re-investigation of aqua regia digestion

Integrating temporal decomposition and data-driven approaches for predicting coastal harmful algal blooms

Land use regression model established using Light Gradient Boosting Machine incorporating the WRF/CMAQ model for highly accurate spatiotemporal PM2.5 estimation in the central region of Thailand

Quantification of gold in geological samples using fire-assay technique is a well-established and recommended method worldwide. However, the method is laborious, complicated, sluggish, and health hazardous. Only a well-trained chemist can perform gold analysis through fire assay routinely. Although classical aqua regia digestion for determination of gold yields high throughput, this method too has disadvantages of sample nature dependency, thus brings down the recovery levels. Herein, we report a modified aqua regia digestion method, followed by solvent extraction using methyl isobutyl ketone for the accurate and precise estimation of Au in variety of geological samples of different genre in broad concentration range. Roasting of sample was avoided and excess of aqua regia (6 times or higher than of sample weight) was used to bring all gold in aqueous phase. This method was applied to soil and rock samples and the results obtained were compared with conventional fire-assay technique. The analytical results for international reference materials of gold (SRM/CRM) measured by the proposed method were in close agreement with those obtained by fire-assay method and recommended values. Very low detection limits were achieved in the proposed method (1 ng/g by GF-AAS and 100 ng/g in F-AAS). This method also works satisfactorily at moderately higher concentration of gold in rock samples, at least up to 15 ppm. Thus fire-assay technique may be substituted by the proposed method for detection of Au in 10 ppb–15 ppm concentration range in the geological samples mentioned under present study.

Saudi Arabian soil samples from different locations have been collected and analyzed for traces of barium (Ba), cobalt (Co), nickel (Ni), titanium (Ti), vanadium (V), silver (Ag), gold (Au), copper (Cu), lithium (Li), and lead (Pb). Inductively coupled plasma mass spectrometry (ICP/MS) has been found to be useful for soil analysis. Two commonly used digestion methods, one employing nitric acid and the other aqua regia, are employed for sample pretreatment. Percentage recovery of added element quantities are found to be within the 97.4 to 101.2% range for Ba, Co, Ni, Ti, and V using aqua regia digestion and within the 95.0 to 05.0% range for Ag, Au, Cu, Li, and Pb when using the nitric acid digestion method. The percentage relative standard deviation (% RSD) for five replicate samples for the two digestion procedures is less than 5% for the analyzed elements.

Aqua regia digestion cannot completely extract Hg from biochar: A synchrotron-based study

The geochemistry of sedimentary rocks is increasingly being used in palaeoenvironmental studies, in the identification of marine versus continental stratigraphy and in chemostratigraphic correlation. The selection of an appropriate research methodology, particularly in terms of sample digestion, can have a significant impact on the accuracy of the results obtained. Depending on the type of rock being studied and the aim of the analysis, a suitable mixture of acids should be used. The most commonly used sample digestion methods are based on a mixture of four acids (multi-acid), aqua regia and inverse aqua regia. As opposed to multi-acid whole-rock digestion, the use of aqua regia and inverse aqua regia result in only the partial digestion of sedimentary rocks. Geochemical analyses using these two different methods were carried out on Carboniferous sedimentary rocks from the Lublin Coal Basin from Poland.The elemental concentrations obtained showed essentially different results for some of the elements. A comparison of the elemental concentrations allowed the distinction of three groups of elements:  - those that showed small differences between the results from the preparation methods (Co, Mn, Bi, Cu, Zn and Fe), - those where the elemental concentrations were 20–50% lower using aqua regia digestion (i.e. Ni, P, Pb, Mg, Cd, Th, Mo, Sr),  - elemental concentrations that were significantly lower (by up to 80%) following aqua regia digestion (U, Cr, Ba, Na, V, Al, Rb, K, Zr).

The method of digestion as a part of sample preparation is very important to determine the accuracy of the analysis result. In this study, the methods of wet and dry digestions were applied to determine of Fe content in several kinds of spinach obtained from the traditional market in Denpasar Bali. This research aimed to compare the result of Fe analysis by AAS method using both of digestion methods. This research was divided into several steps starting from sampling, determination of the samples species, sample preparation, digestion by wet digestion using aqua regia and dry digestion in the furnace, and Fe analysis by AAS. The result showed that the concentrations of Fe in root spinach, red spinach, cut spinach, and tricolor spinach through wet digestion method varied between 68.08–105.45 mg/kg, while the concentrations of Fe by dry digestion obtained between 27.52–42.03 mg/kg, which was over the accepted value. Based on the one-way ANOVA statistical test with a significance level of 5%, there was a significant differences of Fe concentration in spinaches by wet and dry digestions.

Determination of silica coating efficiency on metal particles using multiple digestion methods

The determination of thallium (T1) content in environmental samples is important because of the high toxicity of this metal. Fast and accurate analytical methods are required. Therefore, various open system digestion methods were investigated for the rapid determination of T1 in plant and soil samples. For soils, all procedures were tested by analysis of two National Institute of Standards and Technology (NIST) standard reference materials. For plant samples, the accuracy of the methods was evaluated using three digested materials, with and without a standard addition of 1 μg T1 on each one before digestion. Thallium was determined directly on digested solutions, without separation or preconcentration procedures and with two electrothermal atomic absorption spectrophotometers (ETAAS). Results showed that only one procedure was suitable. The method is based on the solubuization of sample by heating a solution made from 30% hydrogen peroxide (H2O2) and 65% nitric acid (HNO3). The procedure gave good agreement with the NIST certified concentration for soils and adequate recoveries for plant samples. No further hydrofluoric acid (HF) digestion was required on soils samples. Thallium concentrations numerically were higher but the increase was not significant. Volatility of T1 compounds was a drawback with the other digestion methods (aqua regia digestion, HNO3‐H2SO4 digestion and sample dry‐ashing at 480°C) where Tl losses occurred. After volumation in flasks, changes in Tl concentration in digested solutions were not detected during 10 days. The detection limit for the wet digestion method in 10% HNO3 is 60 ng TI/L which is adequate with other investigations.

Acid‐base accounting (ABA) is a common procedure to predict the alkaline or acid‐producing potential of overburdens. Neutralization potential (NP) as currently written in ABA overestimates alkalinity when siderite (FeCO3) is present in the overburden. Siderite initially yields alkalinity upon digestion, but with time the alkalinity is neutralized by acidity from ferric iron (Fe3+) hydrolysis and precipitation. Thirty‐one overburden samples containing varying amounts of siderite, calcite, pyrite, and quartz were analyzed by four NP digestion methods and titrated either by hand or by autotitration. The NP methods were: (i) standard Sobek method (Sobek); (ii) a method that boils the sample for 5 min (BOIL); (iii) a method similar to BOIL but it includes filtering and treating the sample with hydrogen peroxide before back‐titrating (H2O2); and (iv) a modified Sobek method that adds H2O2 after the first hand titration (SobPer). For samples containing primarily calcite, quartz, or clays, the NP values for a particular sample were similar among digestion methods. For samples containing pyrite, the SobPer method (no filtering) produced the lowest NP values. Siderite‐containing samples showed wide variation in NP values among methods. The H2O2 method decreased NP values of siderite samples compared to Sobek and BOIL methods. Lower NP values were generally obtained with autotitration vs. hand titration because autotitration added the base slowly, which allowed concurrent oxidation and hydrolysis of iron. Hand‐titration of siderite samples requires H2O2 treatment to accelerate iron oxidation. Variation in NP values for a particular sample was high among three laboratories using the Sobek hand titration method, but the average variation in NP values among labs decreased by 66% when using the H2O2 hand method. Variation in NP values among labs was also due to the same samples being assigned different fizz ratings by laboratory technicians, which changed the concentration of acid added in the digestion procedure. With more acid, NP values generally increased, especially for siderite samples. A more quantitative approach is needed to determine the amount of acid to add for NP digestion, and the percent insoluble residue of the sample used in this study may be a good alternative but requires more testing and multilaboratory screening. The ABA values (using %S and NP from the various methods) were compared with soxhlet leachate pH and cumulative alkalinity. The ABA values with H2O2 digestion were consistent with soxhlet leachate quality in 13 out of 13 samples. It is suggested that laboratories conducting NP in the ABA procedure use the H2O2 method.