Comparison of alkaline fusion and acid digestion methods for the determination of rhenium in rock and soil samples by ICP-MS

Improved borate fusion technique for determination of rare earth elements in electronic waste components

The information regarding the abundance of rare earth elements (REEs) in electronic waste components (EWC) helps the recycling industry. However, after the end of use, improper disposal may be detrimental to the environment by releasing toxic substances. An optimised alkaline borate fusion method for REEs determination in electronic waste (e-waste) was developed. It is divided into three phases. Firstly, the sample preparation stages were investigated. Secondly, the optimisation of experimental particulars comprises the choice of oxidiser, flux composition, non-wetting agents, fusion time, and sample to flux ratio. The third phase consists of spectroscopic determination by inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray fluorescence (XRF) spectroscopy. The highest REEs recovery (mg/g of e-waste) were found more in fine fractions of less than 0.09 mm compared to gross fractions. The optimum and safe fusion conditions for e-waste were achieved after slow thermal decomposition up to 550°C, pulverisation to 90% of -53 µm, flux composition (90%LiBO₂ + 10% Li₂B₄O₇), 3:1 oxidant ratio of Na₂CO₃: NaNO₃, LiBr as the non-wetting agent. Also, a sample to flux ratio of 1:15 and a total fusion time of 10 min was optimised. The newly improved alkaline fusion results compared better to those obtained from classical mineral acid dissolution with at most 5% RSD on REEs studied. The alkaline borate fusion results in smartphones e-waste were at least 15% and 25% higher than in four acid digest and microwave-assisted digest techniques, respectively. The results indicated enrichment of REEs in smartphones followed by non-smartphones and computer waste.

Substituting HF by HBF4- an optimized digestion method for multi-elemental sediment analysis via ICP-MS/MS

Determination of elemental mass fractions in sediments plays a major role in evaluating the environmental status of aquatic ecosystems. Herewith, the optimization of a new total digestion protocol and the subsequent analysis of 48 elements in different sediment reference materials (NIST SRM 2702, GBW 07313, GBW 07311 and JMC-2) based on ICP-MS/MS detection is presented. The developed method applies microwave acid digestion and utilizes HBF₄ as fluoride source for silicate decomposition. Similar to established protocols based on HF, HBF₄ ensures the dissolution of the silicate matrix, as well as other refractory oxides. As HBF₄ is not acutely toxic; no special precautions have to be made and digests can be directly measured via ICP-MS without specific sample inlet systems, evaporation steps or the addition of e.g. H₃BO₃, in order to mask excess HF. Different acid mixtures with and without HBF₄ were evaluated in terms of digestion efficiency based on the trace metal recovery. The optimized protocol (5 mL HNO₃, 2 mL HCL, 1 mL HBF₄) allows a complete dissolution of the analyzed reference materials, as well as quantitative recoveries for a wide variety of certified analytes. Low recoveries for e.g. Sr, Ba and rare earth elements due to fluoride precipitation of HF-based digestions protocols, can be avoided by the usage of HBF₄ instead. Based on the usage of high purity HBF₄ all relevant trace, as well as matrix elements can be analyzed with sufficiently low LOQs (0.002 μg L^-1 for U up to 6.7 μg L^-1 for Al). In total, 34 elements were within a recovery range of 80%-120% for all three analyzed reference materials GBW 07313, GBW 07311 and JMC-2. 14 elements were outside a recovery range of 80%-120% for at least one of the analyzed reference materials.

Direct determination of total mercury in phosphate rock using alkaline fusion digestion

The aim of this work was to develop a new method to determine the mercury (Hg) concentrations in phosphate rock using a dedicated analytical instrument (the DMA80 Tricell by Milestone) that employs an integrated sequence of thermal decomposition followed by catalyst conversion, amalgamation and atomic absorption spectrophotometry. However, this instrument underestimates Hg concentrations when phosphorite and apatite rocks are investigated with a classic thermal decomposition treatment that complies with US EPA method 7473. Therefore, to improve the recovery of total Hg, we performed alkaline fusion digestion (AFD) directly inside the furnace of the instrument, using BCR(32) as a certified reference material (Moroccan phosphate rock--phosphorite). The salts used for the AFD were a mixture of Na2CO3, K2CO3 and Li2CO3, which melt at about 400°C, due to their ability to form a ternary eutectic and to decompose the phosphorite matrices at 700°C. By adopting this analytical approach, the Hg recovery in BCR(32) was about 100%, compared to 40% when the reference material was analysed without using the alkaline fusion salt. We suggest that the AFD allowed the decomposition of the sample matrix and that some Hg compounds linked with other functional groups may be transformed in carbonates that sublimate at lower temperatures than other Hg compounds. This original method was tested on a number of different geological samples to compare the differences between the AFD method and the thermal treatment in order to verify the working range and to check the robustness of the new approach.

Examining the efficiency of muffle furnace-induced alkaline hydrolysis in determining the titanium content of environmental samples containing engineered titanium dioxide particles

A novel muffle furnace (MF)-based potassium hydroxide (KOH) fusion digestion technique was developed and evaluated for different titanium dioxide materials in various solid matrices. Digestion of different environmental samples containing sediments, clay minerals and humic acid with and without TiO(2) particles was first performed utilizing the MF-based KOH fusion technique and its dissolution efficacy was compared to a Bunsen burner (BB)-based KOH fusion method. The three types of TiO(2) particles (anatase, brookite and rutile) were then digested with the KOH fusion techniques and microwave (MW)-based nitric (HNO3)–hydrofluoric (HF) mixed acid digestion methods. Statistical analysis of the results revealed that Ti recoveries were comparable for the KOH fusion methods (BB and MF). For pure TiO(2) particles, the measured Ti recoveries compared to calculated values were 96%, 85% and 87% for anatase, brookite and rutile TiO(2) materials, respectively, by the MF-based fusion technique. These recoveries were consistent and less variable than the BB-based fusion technique recoveries of 104%, 97% and 72% and MW-based HNO3–HF mixed acids digestion recoveries of 80%, 81% and 14%, respectively, for anatase, brookite and rutile. Ti percent recoveries and measurement precision decreased for both the BB and MF methods when TiO(2) was spiked into sediment, clay minerals, and humic acid. This drop in efficacy was counteracted by more thorough homogenization of the spiked mixtures and by increasing the mass of KOH in the MF fusion process from 1.6 g to 10.0 g. The MF-based fusion technique is consistently superior in digestion efficiency for all three TiO(2) polymorphs. The MF-based fusion technique required 20 minutes for digestion of 25 samples (based on in-house Lindberg MF capacity) compared to 8 hours for the same number of samples using the BB-based fusion technique. Thus, the MF-based fusion technique can be used to dissolve a large number of samples in a shorter time (e.g., 500 samples per 8 hours) while conserving energy and eliminating health and safety risks from methods involving HF.