During the National Geochemical Survey of Australia over 1300 top (0–10cm depth) and bottom (~60–80cm depth) sediment samples (including ~10% field duplicates) were collected from the outlet of 1186 ...catchments covering 81% of the continent at an average sample density of 1 site/5200km2. The <2mm fraction of these samples was analysed for 59 elements by ICP-MS following an aqua regia digestion. Results are used here to establish the geochemical background variation of these elements, including potentially toxic elements (PTEs), in Australian surface soil. Different methods of obtaining geochemical threshold values, which differentiate between background and those samples with unusually high element concentrations and requiring attention, are presented and compared to Western Australia's ‘ecological investigation levels’ (EILs) established for 14 PTEs. For Mn and V these EILs are so low that an unrealistically large proportion (~24%) of the sampled sites would need investigation in Australia. For the 12 remaining elements (As, Ba, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, Sn and Zn) few sample sites require investigation and as most of these are located far from human activity centres, they potentially suggest either minor local contamination or mineral exploration potential rather than pollution. No major diffuse source of contamination by PTEs affects Australian soil at the continental scale. Of the statistical methods used to establish geochemical threshold values, the most pertinent results come from identifying breaks in cumulative probability distributions, the Tukey inner fence and the 98th percentile. Geochemical threshold values for 59 elements, including emerging ‘high-tech’ critical elements such as lanthanides, Be, Ga or Ge, for which no EILs currently exist, are presented.
•Aqua regia concentrations for 59 elements in Australian surface soil presented.•Terminology and methods to determine background and threshold discussed.•Very few samples exceed Western Australian ‘ecological investigation levels’.•No major diffuse contamination of Australian soil by PTEs at continental scale•Few outliers mostly reflect geology, minor contamination or mineral potential.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Large variability in the spatial distribution and content of metals is generally recognised in anthropogenically-polluted soils, hence, a detailed site investigation implying the collection and ...analysis of a large number of soil samples is often necessary. To this regard, the selection of a rapid, cost-effective and accurate analytical technique to assess the concentration of metals in soil is of paramount importance. The overall objective of this work was to evaluate the possibility of assessing the aqua regia-extractable (AR) content of metals in soil from the multi-element profile of the soil obtained by a portable X-ray fluorescence analyser (pXRF). To this objective, we attempted: (i) to establish, by simple linear regressions, the relations occurring between the metal contents measured by pXRF and AR in laboratory setting on air-dried and 2 mm-sieved soil samples from two case studies (A-agricultural and B-industrial sites); (ii) to define metal-based linear models predicting metal AR contents from pXRF measurements; (iii) to assess the influence of metal properties and sources on relations found between the two analytical methods. Very satisfying correlations (R2 > 0.90) were observed between the AR and pXRF contents of Ca, Cu, Cr, Ni, Pb and Zn in the site A, and of Cd, Cu, Pb and Zn in the site B. For the majority of metals, lower AR than pXRF contents were measured, as result of the AR incomplete dissolution of metal-bearing silicates. This was not observed when metals - of anthropogenic origin - occurred in soil in very high concentrations (i.e., Cr for A and Pb for B). In both sites, the comparison among different regression parameters revealed a strong metal-dependence. Moreover, for most of the metals, the parameters of each metal-regression line significantly differed between the two case studies, indicating site-dependence of regression fits.
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•Assessment of heavy metal pollution in soil by two different analytical approaches•Linear regressions relating AR-extractable and pXRF metal contents in 2 case studies•Comparison of regression parameters reveals metal-dependence and site-specificity•Lower AR than pXRF contents were measured due to AR partial dissolution of silicates.•Predicting models allow the use of pXRF to reduce costs of soil-pollution assessment.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Simultaneous solvent extraction using tributyl phosphate (TBP) was investigated.•Gold ion and HNO3 from gold-bearing aqua regia solution successfully extracted.•Loaded gold and HNO3 can be ...selectively recovered by stepwise stripping.
To develop a process for simultaneous extraction and recovery of nitric acid and gold from gold-bearing aqua regia solutions, solvent extraction study was conducted by using tributyl phosphate (TBP) as the extractant. The extraction behaviors of acid from the pure aqua regia, nitric acid, and hydrochloric acid were investigated with varying concentration of extractant, and the concentration ratio of HNO3 and HCl. The simultaneous recovery of HNO3 and Au with the recycling of HCl from the gold-bearing aqua regia solutions was also examined. Selective stripping was then carried out for co-extracted HNO3 and Au. Equilibrium conditions for HNO3 and Au recovery optimized from the experimental results have been found to be: extraction using 80 vol% TBP at O/A ratio of 1, four contacts of stripping for HNO3 at O/A ratio of 1 using distilled water, and two contacts of stripping for Au at an O/A ratio of 1 using a mixed solution at 1:1 (vol.) ratio of 0.1 M sodium thiosulfate (Na2O2S3) and 0.1 M ammonium hydroxide (NH4OH). It was thus possible to simultaneously recover HNO3 and Au from the gold-bearing aqua regia solution through co-extraction and selective stripping of HNO3 and Au leaving HCl in the raffinate for recycling.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Biohydrometallurgy utilizing microorganisms capable of reducing precious metal ions to zero-valent precious metals is attracting attention as an environmentally benign process, but challenges in ...terms of recovery costs have hindered its practical application. In this study, we focused on addressing one of the challenges in biohydrometallurgy through the exploration of anaerobic conditions for bioreduction under the atmosphere. Interestingly, we revealed that Shewanella algae cells possess the capability to catalyze formate oxidation and reduce dissolved oxygen by pre-mixing the cell suspension with low concentrations of cytotoxic formate under atmospheric conditions. These findings indicate that bioreduction conditions can be attained without the need for anaerobic manipulation. Furthermore, we demonstrated that a 50% aqua regia leachate containing Pd(II) can be bioreduced under atmospheric condition when neutralized to pH 7 or higher. Finally, the Pd concentration following a 1-h bioreduction was determined to be 7.2 wt% for wet cells and 33 wt% for dry cells, meaning the remarkable ability of bioreduction to achieve significant Pd concentration.
•Pre-mixing Shewanella algae and formate could create bioreduction condition in air.•Neutralized Pd(II) in aqua regia leachate was bioreduced by Shewanella algae in air.•Pd concentration after bioreduction was 7.2 wt% for wet cells and 33 wt% for dry cells.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Focused infrared radiation-based digestions, for the determination of gold and silver, can be achieved in a timeframe as short as 10–15min, making it an attractive candidate technology for the mining ...industry, where very large numbers of samples are analyzed on a daily basis. An investigation was carried out into gold and silver dissolution chemistry from geological samples using this novel digestion technique. This study investigated in-depth the issue of low recoveries of gold from aqua regia (AR) digestions, reported by a number of researchers. Conventional AR digestions consistently delivered gold recoveries in a range of 69–80% of the certified values for the four certified reference materials (CRM) employed (CCU-1d, SN26, OREAS 62c, and AMiS 0274), while silver recoveries were satisfactory. By gradually shifting the HCl:HNO3 ratio (v/v) from 3:1 to a reversed 1:3 ratio, recoveries of gold and silver exhibited inverse trends. At a HCl:HNO3 ratio of 1:3, complete recovery of gold was achieved with excellent reproducibility in all CRMs. Meanwhile, silver recoveries plunged significantly at this ratio in samples with higher silver concentrations. Silver values were recovered, however, when the silver was re-solubilized by adding a small volume of concentrated HCl to the cooled reverse aqua regia digests. Recoveries of base metals, such as Fe and Cu, were satisfactory throughout and were much less sensitive to changes in the digestion medium. Using four CRMs and five real-world gold/silver containing samples, the utility of the proposed reverse aqua regia was systematically studied. The uncomplicated nature of the digestion methods reported here, that are fast, effective and inexpensive, may be useful to analysts developing/optimizing their methods for the rapid determination of Au and Ag in a variety of mineral phases, particularly where rapid results are desirable, such as in prospecting and mine development.
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•Determination of silver and gold with aqua regia and variants.•Full recovery of gold achieved by reverse aqua regia.•Full recovery of silver achieved for reverse aqua regia, by chloride addition.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•Aqua regia assessed for precious metal recovery from secondary sources.•Aqua regia-based hybrid methods: Sorption, extraction, biohydrometallurgy, etc., overviewed.•Sorption for precious metal ...recovery: Materials & key aspects assessed.•Sustainable precious metal recovery: Challenges and future perspectives highlighted.
The increasing scarcity of precious metals necessitates the development of sustainable recycling strategies to fulfill industrial requirements while mitigating environmental impact. Aqua regia has emerged as a key leaching agent for precious metal recovery from secondary raw materials due to its economic viability, swift leaching kinetics, and straightforward implementation. Due to synergistic benefits such as enhanced recovery rates, improved selectivity, and increased environmental sustainability, aqua regia leaching integrated with sorption, solvent extraction, and biohydrometallurgical techniques. This review article provides a comprehensive overview of aqua regia-based hybrid approaches for recovering Au, Pt, and Pd from diverse secondary sources, including waste electrical and electronic equipment, spent catalysts, industrial effluents, and others. The study evaluates a wide range of functional sorbent and extractant materials for their effectiveness in recovering precious metals from acidic environments, and aqua regia settings. Evaluating various factors that affect precious metal sorption, this study emphasizes the need for acid-resistant and highly selective sorbents. The review also addresses the challenges associated with aqua regia and evaluates potential solutions like selective leaching and diluted aqua regia applications. Finally, the review outlines future directions, focusing on advancements in hybrid methods to achieve superior precious metal recovery with minimal environmental impact.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In the present paper, the authors attempt to explain the importance of pH in soil environment studies and show what mistakes to avoid when measuring pH and interpreting the results obtained. The ...tests conducted (i.e., extraction in aqua regia, buffer capacity determination, and the impact of acidification on the amount of heavy metals extracted from soils) demonstrated how soil pollution and buffer capacity affect the pace of extracting cadmium (Cd), lead (Pb) and zinc (Zn) cations from heavily polluted and unpolluted soils following gradual acidification. It was shown that soil acidification caused a significant increase in metal mobility in the following order Cd > Zn > Pb and that the highest decrease in pH was observed after adding the first portion of acid. Further addition of acid caused a gradually lower decrease in pH. Soils from the polluted area presented a high buffer capacity. The control samples displayed a distinctly poorer resistance to pH changes in the soil environment. Special focus was placed on cadmium due to its high mobility in soils, even with neutral and slightly alkaline pH. The analyses revealed that in areas heavily polluted by long‐term industrial activity (Igeo > 5 for Zn, Pb and Cd), it is very important to conduct extensive geochemical studies related to the presence and circulation of particularly toxic elements. This is because every environmental factor, especially pH, may significantly affect their mobility, causing metal ions to become more or less active or increasing or decreasing environmental risk related to their presence.
Highlights
Soil acidification caused a significant increase in metal mobility in the following order: Cd > Zn > Pb.
A special focus needs to be placed on Cd due to its high mobility in soils (up to 70% of total concentration), even with neutral and slightly alkaline pH.
A higher pH buffering capacity would lead to a lower increase in percentage of bioavailable pollutants with an increase in acidity.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Traditional mechanical pretreatment and hydrometallurgy is the mainstream technology to recycle gold from waste printed circuit boards (PCBs). However, this method is cumbersome and often employs ...toxic/harsh chemicals. In the present study, gold was easily recycled from PCBs gold-plated layer of waste mobile phones (WMPs) in DMF-CuCl2-CaCl2 (DMF: dimethyl formamide) system what we call “mild aqua regia” without pretreatments or enrichment process, and the reaction system could be cyclic utilized. In such reaction system, gold could be recycled through leaching, precipitation and filtration. The reaction parameter optimization and reaction mechanisms were studied. Under the optimized conditions, the gold leaching rate and precipitation rate could reach over 99%. DMF plays a significant role in the leaching system. As a typical polar aprotic solvent, DMF cannot solvate Cl−. Thus, “Cl−” exists in a “naked state”, and it can easily react with Au. Besides, Au (III) could generate a complex through the complexation with DMF which could result in the redox potentials reduce and move the equilibrium to the Au leaching side. In summary, this study provided a fundamental basis as well as practical guidance for recovering precious metals from waste electrical and electronic equipment in an efficient manner.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A fluorine-free and time-saving process was developed to prepare superhydrophobic (SHP) surface of 304 stainless steel (SLS). Dilute aqua regia and hydrogen peroxide solution were used to construct ...rough micro-nanostructure on the 304 SLS surface, followed by modification with stearic acid. The effects of various etching and modification parameters on the surface wettability of SLS were examined in order to achieve high surface roughness and low surface energy. The results indicated that the molar ratio of H+/H2O2 showed the most significant influence on the water contact angle (WCA) of SLS. The optimal SHP with a maximum WCA of 158.3° and a sliding angle (SA) of 2.3° could be achieved at the etching time of 30 min, the molar ratio of H+/H2O2 = 1/0.4, the stearic acid concentration of 0.03 mol/L, the modification time of 3 h, and the modification temperature of 70 °C. The long-term stability, wearing durability, thermal stability, and corrosion resistance of the optimal SHP sample were examined, and the results indicated that there was only a slight change in WCA and SA after exposing the sample in air for 75 days, that the sample could keep SHP property after abrasion for 300 cm under a load of 1000 g, that the sample showed good thermal stability at temperature lower than 200 °C, and that the corrosion current density of the SHP SLS decreased by about 2 orders of magnitude compared to the raw SLS, with a corrosion inhibition rate of 98.6 %. The good performance of the SHP SLS in water repellency, durability, abrasion resistance, and corrosion resistance could greatly extend the application potential of SLS.
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•A fluorine-free, fast method of fabricating superhydrophobic steel surface was developed.•The maximum contact angle of 158.3° and sliding angle of 2.3° were achieved.•H+:H2O2 ratio showed the greatest influence on the water contact angle of stainless steel.•There was excellent durability, abrasion and corrosion resistance for the prepared sample.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZRSKP
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