²¹⁰Pb and ²¹⁰Po in Geological and Related Anthropogenic Materials: Implications for Their Mineralogical Distribution in Base Metal Ores

Abstract

The distributions of ²¹⁰Pb and ²¹⁰Po, short half-life products of ²³⁸U decay, in geological and related anthropogenic materials are reviewed, with emphasis on their geochemical behaviours and likely mineral hosts. Concentrations of natural ²¹⁰Pb and ²¹⁰Po in igneous and related hydrothermal environments are governed by release from crustal reservoirs. ²¹⁰Po may undergo volatilisation, inducing disequilibrium in magmatic systems. In sedimentary environments (marine, lacustrine, deltaic and fluvial), as in soils, concentrations of ²¹⁰Pb and ²¹⁰Po are commonly derived from a combination of natural and anthropogenic sources. Enhanced concentrations of both radionuclides are reported in media from a variety of industrial operations, including uranium mill tailings, waste from phosphoric acid production, oil and gas exploitation and energy production from coals, as well as in residues from the mining and smelting of uranium-bearing copper ores. Although the mineral hosts of the two radionuclides in most solid media are readily defined as those containing parent ²³⁸U and ²²⁶Ra, their distributions in some hydrothermal U-bearing ores and the products of processing those ores are much less well constrained. Much of the present understanding of these radionuclides is based on indirect data rather than direct observation and potential hosts are likely to be diverse, with deportments depending on the local geochemical environment. Some predictions can nevertheless be made based on the geochemical properties of ²¹⁰Pb and ²¹⁰Po and those of the intermediate products of ²³⁸U decay, including isotopes of Ra and Rn. Alongside all U-bearing minerals, the potential hosts of ²¹⁰Pb and ²¹⁰Po may include Pb-bearing chalcogenides such as galena, as well as a range of sulphates, carbonates, and Fe-oxides. ²¹⁰Pb and ²¹⁰Po are also likely to occur as nanoparticles adsorbed onto the surface of other minerals, such as clays, Fe-(hydr)oxides and possibly also carbonates. In rocks, unsupported ²¹⁰Pb- and/or ²¹⁰Po-bearing nanoparticles may also be present within micro-fractures in minerals and at the interfaces of mineral grains. Despite forming under very limited and special conditions, the local-scale isotopic disequilibrium they infer is highly relevant for understanding their distributions in mineralized rocks and processing products.

1. Introduction

²¹⁰Pb and ²¹⁰Po are intermediate isotopes within the ²³⁸U decay-series (Figure 1) and occur in minute amounts in nature [1]. Details of the uranium (²³⁸U, ²³⁵U) and thorium (²³²Th) decay-series radionuclides are concisely provided in the review by Cowart and Burnett (1994) [2].

²¹⁰Pb has a half-life of 22.2 years and is generated via: (i) the decay of atmospheric ²²²Rn gas (“unsupported” ²¹⁰Pb); and (ii) via the continuous production of ²²²Rn from natural ²²⁶Ra contained in crustal materials (“supported” ²¹⁰Pb). ²¹⁰Pb decays to ²¹⁰Bi by emission of a beta particle. ²¹⁰Pb is useful for determining the age of a recent sediment in that, provided that the atmospheric flux is constant, the decay profile relates directly to sedimentation rate. Hence, ²¹⁰Pb is useful for dating sediments up to a century or so old.

Polonium has no stable isotope [3,4]. ²¹⁰Po is by far the longest-lived of the 7 naturally-occurring Po isotopes in the U and Th decay-series (half-life = 138.376 days). ²¹⁰Po is generated via beta decay from ²¹⁰Bi (half-life 5.01 days) and decays to ²⁰⁶Pb by emission of an alpha particle. It has a high specific activity (1.66 × 10¹⁴ Bq/g) and is highly toxic [5] and one of the most radioactive natural radioisotopes; 1 mg of ²¹⁰Po emits as many alpha particles per second as 5 g ²²⁶Ra. Its toxicity in nature is, however, limited by its vanishingly small mass concentration, even compared to ²²⁶Ra. As an energy-generating alpha emitter, ²¹⁰Po has been used as a lightweight heat source to power thermoelectric cells, for example in the Russian Lunokhod lunar rovers to keep their internal components warm during the lunar nights. The principal source of both ²¹⁰Pb and ²¹⁰Po in the environment is natural ²²²Rn gas which escapes to the atmosphere and undergoes radioactive decay. Airborne particles containing sorbed amounts of these highly particle-reactive decay products of ²²²Rn fall to the land or water surface and either dissolve, are deposited onto soils, or become subject to sedimentation.

Both radionuclides are, however, found in trace amounts in a range of solid media, both natural and anthropogenic in origin. ²¹⁰Pb and ²¹⁰Po are present in higher activity concentrations relative to crustal averages in uranium-bearing ores, and in the products of mining and processing from such deposits, including concentrates and wastes. While polonium concentrations are extremely low (< 0.1 mg/tonne even in high-grade uranium ores [6]), the presence of short half-life radionuclides (RN) can impact on the treatment, processing and transport of ore and resulting concentrates. In mining operations in which uranium is present, whether exploited or not, a knowledge of the geochemical behaviour of ²¹⁰Pb and ²¹⁰Po during mineral processing is critical to ensure that produced concentrates are as clean as possible. These radionuclides, whether occurring together with parent uranium in the mineralized rocks, or spatially decoupled from it as U-bearing minerals begin to break down, may represent a non-target element that could attract a financial penalty when sold on the open market. If present at high enough concentrations, saleability of that product may be prevented altogether (e.g., [7]). More extensive, and expensive, safety protocols and transport measures may also be required [8].

A comprehensive understanding of the geochemical behaviour and mineralogical distribution of ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po during ore processing is a pre-requisite for the development of methods to remove or reduce their concentration in products from minerals processing, and provides the motivation behind the present study. The following review of the distributions of ²¹⁰Pb and ²¹⁰Po (and of parent ²²⁶Ra) in a broad range of geological materials builds on existing reviews of various length [9,10,11,12], many of which have emphasised the environmental and health risks that ²¹⁰Pb and ²¹⁰Po present. Our emphasis is on the concentrations and distributions of ²¹⁰Pb and ²¹⁰Po in solid media, with focus on mineralized rocks and ores, and in anthropogenic materials resulting from the exploitation of natural resources. An assessment of the likely mineral hosts for both ²¹⁰Pb and ²¹⁰Po in critical geological environments, including ores and in non-nuclear industrial sources (technologically enhanced naturally occurring radionuclide material (TENORM; [11]) follows.

2. Crustal Distribution of ²¹⁰Pb and ²¹⁰Po

²¹⁰Pb and ²¹⁰Po are widely dispersed in a large variety of natural media because they often mimic the distributions of parental ²³⁸U, ²²⁶Ra, or ²²²Rn. Much of the literature on ²¹⁰Pb and ²¹⁰Po distributions in nature is focused on their concentrations in the atmosphere (e.g., [13]), in oceans (e.g., [14,15,16,17]), rivers (e.g., [18]), lakes (e.g., [19]), groundwaters (e.g., [20,21,22,23]), drinking waters (e.g., [24,25,26]), and soils [27,28,29]. Attention has also been given to mosses and lichens, which efficiently capture atmospheric ²¹⁰Pb and ²¹⁰Po, peat bogs (also anomalous with respect to ²¹⁰Pb and ²¹⁰Po), and in the animal and human food chains, e.g., milk or berries, and particularly seafood (e.g., [12,30,31,32]). A smaller body of data is available for some natural environments, notably hydrothermal ore deposits or active volcanic fumaroles, where there is evidence for the selective fractionation of ²¹⁰Pb and ²¹⁰Po from other ²³⁸U decay products, and where ²¹⁰Pb and ²¹⁰Po are concentrated in “new” precipitates. These particle-sensitive decay product isotopes are always fractionated from gaseous parent ²²²Rn that has extreme dispersion and mobility in the environment, particularly in the atmosphere.

There is a large body of data addressing the distribution and behaviour of ²¹⁰Pb and ²¹⁰Po in both magmatic and marine/lacustrine sedimentary environments, as well as in relation to the mining, processing and smelting of uranium-bearing ores, processing of phosphate ores for phosphoric acid production and other human activity. Somewhat less well documented are concentrations of ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po in other anthropogenic materials such as coal ash (e.g., [33,34]), and as scales and sludges associated with oil and gas production [35,36,37]. Not infrequently, observed distributions are the product of a complex interplay between natural and anthropogenic ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po from different mining and non-mining sources, the effects of which can only be elucidated by high-quality analysis and a good understanding of the physical and historical context of the samples in question (e.g., [38,39]). There is generally a strong link between the distributions of ²¹⁰Pb and ²¹⁰Po and that of parent ²²⁶Ra in many industrial wastes (uranium mill tailings, phosphogypsum, coal fly ash, oilfield scales and sludges) such that understanding the mode-of-occurrence of ²²⁶Ra will enable prediction of ²¹⁰Pb and ²¹⁰Po behaviour. There are, however, some exceptions in which selective fractionation and concentration of ²¹⁰Pb and ²¹⁰Po takes place, as will be shown below.

A detailed treatment of analytical techniques for the determination of short-lived isotopes at concentrations of small fractions of parts-per-billion, and quantification of ²¹⁰Pb and ²¹⁰Po in rocks, minerals, concentrates and leachates, lie beyond the scope of this contribution. The reader is referred to References [1,40,41], in which comprehensive reviews of methodologies used for the determination of ²¹⁰Po in environmental materials are provided, building on earlier studies [42] and others. Clayton and Bradley (1995) [43] describe their methodology to measure ²¹⁰Pb and ²¹⁰Po in a range of environmental materials. In a series of papers, Jia and co-authors [44,45,46] have put forward procedures for analysis of ²¹⁰Pb and ²¹⁰Po in mineral, biological and soil samples. Particularly relevant to our focus on ²¹⁰Pb and ²¹⁰Po distributions in ores and ore processing residues are separation techniques outlined by Prud’homme et al. (1999) [47] for fine-grained multi-phase materials.

2.1. Magmatic Rocks and Related Hydrothermal Systems

Volcanoes represent the largest single contributor of atmospheric ²¹⁰Pb and ²¹⁰Po. For example, Allard et al. [48] document extremely high fluxes from the Ambrym basaltic volcano, Vanuatu Island Arc, in the South Pacific Ocean. Based on direct measurements, these authors maintain that this volcano is among the most powerful volcanic gas emitters on Earth, producing between 5% and 9% of current estimates for global subaerial volcanic emissions of ²¹⁰Pb and ²¹⁰Po. Data for both dissolved and emitted magmatic volatiles are used to estimate the depth, size and degassing rate of the basaltic magma reservoir that sustains the eruptive activity [48]. In the aforementioned paper, Allard et al. note radioactive disequilibrium of ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po in the volcanic gas phase and use this to constrain the renewal rate and dynamic time scales of the magma reservoir. Extensive radioactive disequilibrium between the three radionuclides reported in Ambrym volcanic gas is concordant with observations from other basaltic volcanoes [49,50]. The radioisotopic disequilibrium is attributed by Allard et al. [48] to the very different volatilisation rates of the three radionuclides during high-temperature basalt degassing (²¹⁰Po > ²¹⁰Bi > ²¹⁰Pb). All ²¹⁰Po is volatilised, whereas the emanation rate is two orders of magnitude lower. ²¹⁰Po–²¹⁰Pb geochronology is routinely used to date recent volcanic eruptions (e.g., [51]). Measurable activity of ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po is not restricted to basaltic volcanoes. The volatility of all three radionuclides have been studied in andesitic gases from Merapi Volcano, Indonesia [52], although the authors note that the emanation coefficients are significantly lower than observed at basaltic volcanoes, a feature attributed to lower magma temperatures. The same authors state that the radionuclides are mainly transported in the volcanic gases as Pb-chlorides, and as “Bi- and Po-metallic species”.

The radionuclide systematics of igneous activity nevertheless differ considerably with respect to tectonic environments [53]. Enrichment of ²¹⁰Po and ²²⁶Ra relative to ²³⁰Th is noted to be more common and greater in island arcs than in continental margin subduction environments. Levels of enrichment tend to decrease with differentiation. Differences were attributed [53] to variations in the process of melt extraction, changes in bulk partition coefficients within the mantle wedge, or preferential addition of U from subducted lithosphere.

Interest in the activity of ²¹⁰Pb and relationships with parent radionuclides in young volcanic rocks centres on the useful geochronological information the radioisotope distributions can provide. The literature reveals substantial debate about the possible causes of observed isotopic disequilibria in many young volcanic rocks (e.g., [54]). The observed ²¹⁰Pb deficits relative to ²²⁶Ra are attributed to magma degassing over decades rather than partial melting or interaction with cumulates [55].

Most igneous rocks contain both U and Th, with concentrations increasing as silica content increases. Granites are thus the rock type with the highest concentration of all daughter radionuclides, which remain in secular equilibrium until weathered. Uranium, Th and daughter radionuclides are important heat producers in granitic rocks [56].

Radionuclide concentrations have been examined in volcanic fumaroles from La Fossa cone, Vulcano Island, Italy [57,58]. Sulphur and sulphide incrustations show relative but variable enrichments in ²¹⁰Po (as high as 500 Bq/g), and in ²¹⁰Pb relative to ²²⁶Ra, which are related to degassing of the fumarolic fluids. The published data record mobility of sublimates within the fumaroles. ²¹⁰Po is almost fully volatilised due to the relatively high velocity of the gas, even though temperatures did not exceed 280–350 °C. Much ²¹⁰Po may therefore be present in gaseous form within the fumarole. Sublimates at La Fossa Crater, Aeolian Islands, Italy [59] contain an abundance of rare Pb–Bi-sulphosalt mineral species (e.g., wittite, cannizzarite, mozgovaite, etc. [60,61]). Several of sulphosalts, including Cl- and Br-bearing species (e.g., vurroite [62]) have been first described from the locality.

The unusually high sulphur-reducing environments offered by deep-sea hydrothermal vents display ²¹⁰Pb and ²¹⁰Po enrichment relative to ²²⁶Ra. Boisson et al. (2001) [63], for example, describe the relative enrichment in naturally-occurring ²¹⁰Po and ²¹⁰Pb associated with the high particle fluxes brought about by hydrothermal venting off the island of Milos, Aegean Sea. ²¹⁰Po levels in organisms living on or near the microbial mat in the vent zone were higher than from non-vent areas. It was, however, stressed [63] that input of ²¹⁰Po and ²¹⁰Pb to oceans through venting activity is probably not significant compared to that of atmospheric origin. High levels of natural radioactivity, including ²¹⁰Po–²¹⁰Pb, in vent biota from both the East Pacific Rise and Mid-Atlantic Ridge have been confirmed [64].

2.2. Sedimentary Environments

Measurable concentrations of ²¹⁰Pb and ²¹⁰Po in sedimentary environments, whether marine, lacustrine, deltaic or fluvial have proven invaluable for understanding age relationships of sediments on the decade-scale and for calculation of rates of sedimentation. Many dozens of case studies, e.g., [65,66,67,68], document the spatial distributions of ²¹⁰Pb and ²¹⁰Po, and successfully separate natural from anthropogenic sources. Activities of ²¹⁰Pb and ²¹⁰Po have also proven valuable for studies of glaciation and the accumulation and melting rates of ice sheets [69]. Remarkably few of these studies have addressed the mineralogy of the sediments, and the likely host(s) of ²¹⁰Pb and ²¹⁰Po.

A key feature of many studies of sediments and water columns in marine or lake waters is the recognition of disequilibrium between ²¹⁰Pb and ²¹⁰Po that is linked to differential cycling patterns, rates of sequestration by sediments, as well as the contributions of atmospheric deposition, particularly for ²¹⁰Po.

Some sedimentary rocks contain anomalous radionuclide concentrations. Of particular note are restricted marine and estuary environments supplied by organic- and clay-rich sediment. Under reducing conditions, uranium is readily adsorbed onto the organics and/or clay particles.

Sedimentary rocks are also the host for many of the World’s largest and richest uranium deposits, formed via migration of dissolved U⁶⁺ in oxidising waters along paleoaquifers and deposited in reduced rocks. The genesis of such deposits has been amply described elsewhere [70].

3. ²¹⁰Pb and ²¹⁰Po from Anthropogenic Sources

The mining of uranium, smelting of copper and polymetallic ores, phosphoric acid and oil/gas production and combustion of coal (and peat) are the main extractive activities leading to generation of materials with high contents of ²¹⁰Pb, ²¹⁰Po, and other RN. Surveys of the generation of Naturally Occurring Radioactive Materials (NORM) from industrial operations [71,72,73,74,75] also cover other industrial sectors, including manufacture of zirconia, titanium dioxide pigment production, cement production and alumina and iron and steel production.

3.1. ²¹⁰Pb and ²¹⁰Po in Uranium Mill Tailings

The radiological risks associated with management of uranium mill tailings have been discussed by [76,77,78,79], among many others. Several authors have suggested that ²²⁶Ra occurs in radium-bearing sulphate minerals in uranium mill tailings [80,81,82]. Landa et al. [83] examined uranium mill tailings from Monticello, Utah. ²²⁶Ra was found to be associated with particles and colloids of alkaline-earth sulphates, alkaline-earth carbonates, and surfaces of quartz, clay, and feldspar. Landa [77] inferred from the leaching behaviour of uranium mill tailings that ²²⁶Ra occurs with hydrous oxides of iron and manganese. In one study specially aimed at understanding the mineralogy of ²¹⁰Po [84], material from three uranium mill sites in the USA were examined to establish where contaminants reside as a prerequisite for modelling contaminant mobilisation. Four mineral hosts were suggested using a combination of electron probe microanalysis, thin-section petrography, α-emission mapping, and selective chemical extractions (although none of these techniques can identify mineral hosts for specific radioactive decay products): uranium minerals, authigenic siliceous material, Ba–Sr-sulphates, and Fe–Ti–V-oxides. These themes are explored further by Landa and Bush [79], who recognised a redistribution of radium by particle size during milling but also of the components in the tailings onto which radionuclides are adsorbed. The following potential sorbents were identified: clay minerals, Fe- and Al-oxides, feldspars, fluorite, barite, jarosite, coal, and volcanic glass. Both ²²⁶Ra and ²¹⁰Pb showed both higher degrees of adsorption than either ²³⁸U or ²³⁰Th, attributable to either selective adsorption or ingrowth of ²¹⁰Pb daughter isotopes in minerals containing substituted radium (e.g., Ra²⁺ for Ca²⁺).

In a review of the mineralogical controls on radionuclide mobility in uranium mill tailings [78], the importance of amorphous silica, carbonates and phosphates, and microbial reduction processes is noted. The same publication also examines radionuclide behaviour (although not mentioning ²¹⁰Pb and ²¹⁰Po in this context) during in-situ leach (ISL) recovery operations. Jarosite [KFe₃(SO₄)₂(OH)₆], which may precipitate and severely restrict permeability along ISL aquifers, is said to be a significant host for radium. The presence of sulphides and Fe-(hydr)oxides will also impact on recovery dependent on the extracting agent used. These ideas are expanded in a more recent study of uranium mill tailings [85] that also stresses the potential role of sulphates and secondary galena as hosts for ²¹⁰Pb. It is reasonable to infer that these minerals scavenge ²¹⁰Pb dissolved within pore fluids.

Radionuclide distributions, including ²¹⁰Pb deposition rates and inventories, have been examined in and around the Ranger Uranium Mine, N.T., Australia [86]. Natural redistributions of ²²²Rn and ²¹⁰Pb occur via atmospheric dispersion of ²²²Rn, (seasonal) deposition of ²¹⁰Pb on surfaces, and eventual migration creating ²¹⁰Pb depth profiles prior to decay to ²⁰⁶Pb. A net loss of ²¹⁰Pb from the region occurs during the dry season by attachment to aerosols.

3.2. ²¹⁰Pb and ²¹⁰Po in Copper and Polymetallic Ores and Products of Their Mining and Processing

Some copper ores contain anomalously high concentrations of RN, meaning that daughter radionuclides are present in products of mining and smelting, and within wastes resulting from those activities. Due to selective volatilization/condensation of ²¹⁰Pb and ²¹⁰Po, smelter dusts tend to be enriched in ²¹⁰Pb and ²¹⁰Po compared to ²²⁶Ra, compared to their activity concentrations in the original ore feed, and compared to their activity concentrations in other solids produced during processing. Such a scenario is documented for the Olympic Dam copper mining and smelting operation, South Australia [87]. The behaviour of ²¹⁰Pb and ²¹⁰Po during concentration and smelting of copper ores, and their preferential partitioning into smelter flue dusts where they may accumulate, has been documented in several studies. One of the best studied examples is the contamination generated through centuries of exploitation of copper-bearing bituminous shales (Kupferschiefer) in the Mansfeld district of eastern Germany. A characterisation of scrubber dust slurries (Theisenschlamm) containing ²¹⁰Pb and ²¹⁰Po, which were produced as a by-product of the Mansfeld smelting operations, is given in [88]. Around 220,000 tonnes of these sludges are deposited at several sites and continue to represent a serious environmental risk [89,90,91,92]. The material contains: 18% Zn (as sphalerite and wurtzite); 14% Pb (as galena and secondary anglesite); a wide variety of hazardous (As, Tl, Cd) elements and rare metals (Ag, Ge, Re); polycyclic aromatic hydrocarbons and other organic compounds, as well as radionuclides. ²¹⁰Pb and ²¹⁰Po are concentrated in the finest size fractions, typically of sub-micron size and combined as aggregates with mean diameter of 1.25 µm [88]. Additional characterisation of the Theisenschlamm is provided by Morency et al. [93], with experimental evidence in support of oxidative processes designed to immobilize elements and isotopes of concern. Of relevance to the overarching aim of the present study is the observation that almost all ²¹⁰Pb and ²¹⁰Po can be concentrated into a lead sulphate phase. Ores of comparable age and origin, albeit with generally lower associated radioactivity, are currently mined in the Lubin region, Poland [94]. The highly selective enrichment of ²¹⁰Pb and ²¹⁰Po compared to ²²⁶Ra in certain wastes from copper smelting highlights the importance of understanding the distribution of these isotopes when designing plant operations or planning waste clean-up or handling.

Hypogene tin ores are generally genetically related to granites and often contain anomalous concentrations of Cu, Pb, Bi, U and Te relative to crustal averages. They may contain measurable concentrations of ²¹⁰Pb, ²¹⁰Po and ²¹⁰Bi. The smelter process, for which feedstock may not only comprise tin ores/concentrates but also tin-rich residues from other processes, involves a molten metal stream and separation into tin, lead and lead-bismuth alloys. ²¹⁰Po will rapidly volatilise, and according to Martin et al. [71], can be highly enriched in smelter fumes (200,000 Bq/kg). Slags will contain the non-volatile radionuclides but also some ²¹⁰Pb and ²¹⁰Po (10,000 Bq/kg). Within the bismuth metal, short-lived ²¹⁰Bi rapidly decays to ²¹⁰Pb, which may have activity concentrations up to 100,000 Bq/kg. Hipkin and Paynter (1991) [95] address activity concentrations of materials and the radiation exposures of workers in the tin industries of Bolivia and SE Asia, while background data on ²¹⁰Pb and ²¹⁰Po behaviour during the tin smelting and electro-refining process are given by Harvey et al. [96]. ²¹⁰Pb and ²¹⁰Po activities in and around a large, now-closed, tin smelter in northern England are discussed by Baxter et al. [97]. Here, over the course of ca. 55 years of tin production, about 30% of the ²¹⁰Po was isolated in tellurium dross, 48% decayed within the refinery, 19% went to waste slag, and 2% was released into the atmosphere.

3.3. ²¹⁰Pb and ²¹⁰Po in Mining and Processing of Phosphates

Many phosphate formations exploited for the fertiliser industry contain concentrations of naturally occurring radionuclides of the uranium and thorium decay series that exceed those in other rocks. The risks associated with mining, milling and manufacturing of phosphoric acid and phosphate fertilizers have been widely documented at different sites around the world. Solid waste products of the phosphate industry, notably gypsum (CaSO₄·2H₂O), termed phosphogypsum, but also dusts generated during milling, can carry particularly high concentrations of ²²⁶Rn, ²¹⁰Pb and ²¹⁰Po (e.g., [74,98,99,100,101,102,103,104,105,106,107,108,109]. Although waste from the phosphate industry has, in some cases, been disposed of in the marine environment (e.g., [101]), elsewhere, most spectacularly in Florida, where the World’s largest phosphoric acid industry is centred, huge waste piles (gypstacks) have been generated, creating serious waste management issues.

In the manufacture of phosphoric acid, an important industrial chemical, phosphate rock (which typically contains U within apatite or other phosphates) is treated with sulphuric acid, resulting in by-product gypsum in volumes three times greater than the phosphoric acid (Figure 2). The reaction involved in phosphoric acid production can be simplified, after Burnett et al. [103], as:

Ca₁₀(PO₄)₆F₂ + 10H₂SO₄ + 20H₂O → 10CaSO₄·2H₂O +6H₃PO₄ + 2HF

The above reaction creates disequilibrium between U, Th and Ra. The majority of U if found in the phosphoric acid, ca. 90% of the ²²⁶Ra, and effectively all the ²¹⁰Pb and ²¹⁰Po, will be preferentially concentrated within the phosphogypsum (e.g., [110]), with ²¹⁰Pb and ²¹⁰Po in secular equilibrium [111]. The ²¹⁰Pb and ²¹⁰Po activities in phosphogypsum are typically a few hundreds of Bq·kg⁻¹. For example, Brasilian phosphogypsum described by Mazzilli et al. [106] shows concentration ranges of 47–894 Bq·kg⁻¹ and 53–677 Bq·kg⁻¹ for ²¹⁰Pb and ²¹⁰Po, respectively. These activities depend on the uranium concentrations in the phosphate ore, which can vary over as much as an order of magnitude. For comparison, phosphate rocks in Florida, Morocco and Jordan, three important producing areas, contain 1300–1850 Bq·kg⁻¹ U [112]. In the phosphate rocks, U (and Th) is present mostly within the mineral apatite. U⁴⁺ has a similar ionic radius to Ca²⁺ (0.97 and 0.99 Å, respectively) and readily substitutes into the apatite lattice, resulting in less abundant uraninite in some ores.

The physical distribution of ²²⁶Ra and ²¹⁰Pb in phosphogypsum waste piles is addressed by Rutherford et al. [100], who note heterogeneity in which ²¹⁰Pb is concentrated in the finest fraction (no more than a few microns). In studies of Florida phosphogypsum, ²¹⁰Pb/²¹⁰Po disequilibrium in mature phosphogypsum samples has been demonstrated [104], suggesting that ²¹⁰Pb was more mobile than either ²¹⁰Po or ²²⁶Ra. Although none of the phosphogypsum research has demonstrated that the mineral hosts for each radionuclide differ (phosphogypsum is, in any case essentially monomineralic with around 1% impurities), these observations are indirect evidence to the suggestion that ²¹⁰Pb may be less well bound within the crystal lattice of gypsum.

3.4. ²¹⁰Pb and ²¹⁰Po Associated with Oil and Gas Production

Radioactive scales and sludges associated with offshore oil and gas production often contain anomalously high concentrations of ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po, and have been well documented [35,36,37,113,114]. After the mixture of oil, gas and water is brought to the surface, and the gas and formation waters are separated from the oil, hard scales will build up on the internal surfaces of the oil field extraction and production equipment. Well fluids (saline formation waters that are co-produced with oil and gas and require separate handling) are characteristically enriched in Ca, Sr, Ba and associated Ra. Pipes and tanks that come into contact with these waters are subject to scale and sludge build-up. The relative amounts of solid waste (sludges and sands) vary with the production area due to the different geological and fluid characteristics of the reservoirs. ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po-enriched scales precipitate as solids directly from the formation waters following changes in temperature, pressure and salinity.

The volumes of these materials are, however, generally rather small and typically comprise a mixture of carbonate and sulphate compounds of calcium, strontium and barium. Generation of ²¹⁰Pb and ²¹⁰Po is due to the presence of parent U and Th in the oil/gas source rocks, which decays to Ra that dissolves within the saline formation waters that also contain P, Sr, Ba and Ca. The radium isotopes, ²²⁶Ra and ²²⁸Ra, co-precipitate with the salts of these elements, enriching the sludges and, particularly, scales in radium and products of radium decay, including radon isotopes, ²¹⁰Pb and ²¹⁰Po. The relatively longer-lived decay products such as ²¹⁰Pb, will accumulate as very thin films and deposits in gas handling equipment and storage tanks.

3.5. ²¹⁰Pb and ²¹⁰Po from Combustion of Coal and Other Solid Fuels

The combustion of solid fuels such as coal for heat and power applications is an important source of atmospheric ²¹⁰Pb and ²¹⁰Po and has been studied in detail in various parts of the World. During the combustion of solid fuels, trace elements, including ²¹⁰Pb and ²¹⁰Po, partially volatilise along with organic constituents in the fuel matrix. The remaining trace elements remain in the fuel bed and are eventually collected as bottom ash. The volatilised fraction generally condenses on fly ash particles in the flue gases as the flue gases are cooled [115]. Fly ash samples collected from coal-fired power stations have been reported to contain elevated concentrations of ²¹⁰Pb and ²¹⁰Po, when compared with the original feedstocks [116], indicating that these two radionuclides volatilise to a large extent during the combustion of solid fuels. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) provide further evidence for the enrichment of ²¹⁰Pb and ²¹⁰Po relative to precursor RN in fly ashes during solid fuel combustion in their survey on the activity concentrations of radionuclides in coal samples sourced from a wide range of geographical locations and in fly ashes sourced from various coal-fired power stations [117]. Country-averaged activity concentrations for coal were all within 50 Bq·kg⁻¹ for both ²¹⁰Pb and ²¹⁰Po. Average concentrations of ²¹⁰Pb and ²¹⁰Po in the fly ash samples were much higher by comparison (930 Bq·kg⁻¹ for ²¹⁰Pb and 1700 Bq·kg⁻¹ for ²¹⁰Po). Studies on the size distribution of radionuclides in fly ash show ²¹⁰Pb and ²¹⁰Po to preferentially condense on fine particles below 10 microns in size [118,119]. This makes the capture of ²¹⁰Pb and ²¹⁰Po difficult, since conventional particulate control devices (e.g., electrostatic precipitators and bag filters) generally become less effective with diminishing particle size [119]. Consequently, a small but nevertheless not insignificant fraction of the ²¹⁰Pb and ²¹⁰Po in solid fuels is emitted from the stack into the atmosphere. According to Roeck et al. [120], in old coal-fired power plants, ca. 3% of the initial radioactivity will be discharged from the stack but this proportion is no more than 0.5% in modern plants. UNSCEAR [121] state that annual emissions from a “typical” 600 MW coal fired power station was 0.4 GBq for ²¹⁰Pb and 0.8 GBq for ²¹⁰Pb. ²¹⁰Pb and ²¹⁰Po may also accumulate in deposits on furnace walls and on the fireside of boiler tubes.

The amounts of ²¹⁰Pb and ²¹⁰Po released into the different product streams during solid fuel combustion not only depend on processing parameters (e.g., combustion temperature and gas atmosphere) but also on the properties of the feedstock, particularly the activity concentrations of ²¹⁰Pb and ²¹⁰Po. Most coals contain small amounts of parent U and Th but their concentrations can vary over several orders of magnitude from deposit to deposit. There is a substantial volume of literature on naturally occurring radionuclide distributions in coals and fly ash [116,122,123,124,125]. Coal is formed via reduction of organic material, in which uranium is trapped or adsorbed onto clay particles, carbonaceous matter, pyrite and organic matter. In contrast, Th occurs within minerals such as monazite or apatite. Since organic matter is an effective reductant, coal horizons may accumulate additional uranium over time by extracting dissolved uranium from circulating groundwaters [126]. Lower rank sub-bituminous coals, brown coals, and lignites may contain higher concentrations of parent U and Th ([125] and references therein). Unconsolidated analogues, including peats, may also contain anomalous RN concentrations.

Enhanced atmospheric ²¹⁰Po in urban areas may be attributed to coal-fired power stations [127,128]. In a comparison of emissions from power plants fuelled by different hydrocarbon fuels, Häsänen et al. [129] point out that the greatest emissions of ²¹⁰Po per burnt volume of fuel were from combustion of peat.

3.6. ²¹⁰Pb and ²¹⁰Po from Exploitation of Mineral Sands

Anomalous concentrations of ²¹⁰Pb and ²¹⁰Po are associated with exploitation of mineral sands for production of zircon and zirconia, titanium dioxide and rare earth elements. Radiological risks associated with such ores (and corresponding products and wastes) are generally low (around 10,000 Bq/kg [71]). This could, however, be an area in which ²¹⁰Pb and ²¹⁰Po release is set to increase, since demand for these commodities, especially REE, is booming, and new mineral sands operations are being established around the globe.

Mineral sands are of particular interest because of the indirect information they provide on potential mineral hosts for ²¹⁰Pb and ²¹⁰Po. The minerals within such sands (zircon, baddeleyite, monazite, xenotime, ilmenite, rutile, etc.) all host trace to minor amounts of uranium and thorium (varying up to as much as 1 wt % depending on primary source) and are all highly refractory. Despite this, there are relatively few published studies detailing ²¹⁰Pb and ²¹⁰Po geochemistry in mineral sands enabling an understanding of whether daughter radionuclides are retained in the crystal structures. The manufacture of zirconia for glazes and ceramics involves production of small volumes (~1% of feed) of highly RN-enriched waste, as well as volatilisation of ²¹⁰Pb and ²¹⁰Po.

Titanium oxide pigment production, from rutile, a mineral that also commonly contains primary minor U and Th, also results in a RN-enriched solid waste residue.

3.7. ²¹⁰Pb and ²¹⁰Po in Other Anthropogenic Materials

Most iron ores contain only low concentrations of ²¹⁰Pb and ²¹⁰Po and thus accumulation of radioactivity in waste materials produced by iron and steel production are mostly attributable to other feed materials (coal/coke and limestone). ²¹⁰Pb and ²¹⁰Po tend to accumulate in the sinter plant in dust collected from the gas cleaning systems and are generally very low or absent in saleable products. According to Martin et al. [71], for every million tonnes of steel produced, 2000 tonnes of contaminated dust will be generated. In a study of sinter plant radioactivity in the Port Kembla foundry, NSW, Australia, Brown et al., in a report cited by Martin et al. [71], report activities of 18,900 and 15,600 Bq·kg⁻¹ for ²¹⁰Pb and ²¹⁰Po, respectively, in ductwork dust.

Production of the elements niobium and tantalum is also associated with generation of RN-rich residues. Like tin ores, Nb–Ta ores are granite-related and commonly contain minor amounts of other elements including uranium (and thorium). Pyrochlore, a primary ore mineral of niobium and tantalum, will often contain actinides at measurable concentrations. ²¹⁰Pb activities as high as 16,700 Bq·kg⁻¹ in slags from a Brasilian niobium processing facility are reported [11].

Martin et al. (1997) [71] compiled information on ²¹⁰Pb and ²¹⁰Po in cement, bricks and other building materials, and provide activity concentrations for ²²⁶Ra and ²³²Th. They note that activities are only of concern if substantial amounts of waste materials in which ²¹⁰Pb and ²¹⁰Po are concentrated are added into the materials. Examples include the common addition of copper slag in concrete (in the former East Germany), phosphogypsum waste in wallboard and road construction, and fly-ash in bricks and some cements. Lightweight building blocks may contain both slag and fly-ash. Several publications detail representative ²¹⁰Pb and ²¹⁰Po activity concentration data for building materials, focussing on eastern European countries where the practice of adding smelter slags and fly-ash was commonplace [130,131,132]. The environmental impact of radionuclide release during processing of granite rock for ornamental stone has also been explored [133]. In this paper, Guillén et al. suggest that even basic mechanical processing of granites can lead to increased levels of ²¹⁰Po and ²¹⁰Pb in the surrounding environment, as dusts, solid waste and slurries.

Numerous studies have sought to demonstrate the impact of historical human industrial activity by monitoring the levels of lead isotopes in peat bogs, salt marsh, lakes and estuarine sediments [38,134,135,136,137,138,139,140].

4. Mineral Repositories for ²¹⁰Pb and ²¹⁰Po

The literature contains extensive reference to the potential mineral repositories for ²¹⁰Pb and ²¹⁰Po, although much of this evidence has been obtained indirectly. Very few studies to date have been able to provide direct confirmation that a given phase contains these RN. This is largely due to those RN with relatively short half-lives being present at minute concentrations well below minimum detection limits of conventional microanalysis.

Polonium has no non-radioactive isotope and does not occur naturally as a metal or essential component of naturally occurring compounds, although the compound PbPo has been reported [141] and has been attributed to natural alpha decay of polonium to form lead. Polonium is readily vaporised, forming Po₂ molecules even well below the melting and boiling points (254 and 962 °C, respectively) via small clusters of polonium atoms spalled off by alpha decay. These particles are readily adsorbed. Chemically, polonium displays similar behaviour to that of tellurium and bismuth [141,142]. More than 50 polonium compounds have been synthesized including metal polonides, polonium hydride, the two oxides PoO₂ and PoO₃, halides and sulphates. Various oxidation states, including ²⁺, ⁴⁺ and ²⁻, are known.

Valuable insights into the behaviour of ²¹⁰Pb and ²¹⁰Po in polymetallic ores are provided by Golubev et al. [143]. By comparative analysis of different volumes of ore within a single vein-type uranium orebody, these authors provide critical evidence for migration of ²³⁸U and, critically, of intermediate decay products within an effectively open system, creating disturbances of U–Pb systematics, expressed as local enrichments or depletions in ²⁰⁶Pb, giving rise to discrepancies between ages based on ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ratios. The authors note the presence of zones with low U contents but with excess ²⁰⁶Pb. These contain pyrite/marcasite onto which migrating longer-lived radionuclides, including ²¹⁰Po and ²¹⁰Pb, are considered to accumulate, effectively playing the role of geochemical barriers within the system.

In a study attempting to identify the solid-phase partitioning of ²¹⁰Po and ²¹⁰Pb in anoxic marine sediments [144], the influence of early diagenetic processes on distribution patterns is documented. In sediment, ²¹⁰Po was found to be either bound to organic matter, sulphides such as pyrite, clay minerals or refractory oxides. ²¹⁰Po was found not to be significantly bound to acid volatile sulphides in sediment, even if the authors considered that ²¹⁰Po initially bound to acid volatile sulphides may have been redistributed by bioturbation.

Because of the extremely low concentrations and the difficulty of separating fine-grained minerals, there is a paucity of published concentration data for ²¹⁰Pb and ²¹⁰Po in individual minerals. Identification of the potential mineral repositories for either radionuclide, except for post-decay reincorporation of daughter RN into parent U-minerals is therefore reliant on indirect observation. Good indications as to potential hosts can nevertheless be made based on the literature we have summarised above (

Table 1. Summary of potential mineral hosts for ²¹⁰Pb and ²¹⁰Po.

Mineral Group	Mineral	Formula (e)	Host for
Uranium minerals	Uraninite	UO2 (ideally)	U, radiogenic Pb, minor Th
	Coffinite	U(SiO4)1−x(OH)4x	ditto -
	Brannerite	(U,Ca,REE)(Ti,Fe)2O6	ditto -
	Uranothorite	(Th,U)SiO4	Th, U, radiogenic Pb
	Carnotite	K2(UO2)2(VO4)2·3H2O	U, (Th and Pb?)
REE-, Zr- and Nb–Ta-minerals	Monazite	(REE)PO4	Minor U, Th radiogenic Pb?)
	Bastnäsite	REE(CO3)F	ditto -
	Synchysite	Ca(REE)(CO3)2F	ditto -
	Xenotime	(Y,REE)PO4	ditto -
	Alunite Supergroup Minerals (especially crandallite and beudantite groups)	(Various minerals)	ditto -
	Baddeleyite	ZrO2	ditto -
	Zircon	ZrSiO4	ditto -
	Tantalite-(Fe)–tantalite-(Mn) series	(Fe,Mn,Mg)(Nb,Ta)2O6	Trace U,Th,Pb (?)
	Euxinite	(Y,Ca,Ce,U,Th)(Nb,Ta,Ti)2O6	ditto -
Sulphides/selenides/tellurides	Galena	PbS	Radiogenic Pb
	Clausthalite	PbSe	ditto -
	Altaite	PbTe	ditto -
	Bi-chalcogenides	Bix(Te,Se,S)y	?
	Pb-Bi-sulphosalts	(various)	?
	Pyrite	FeS2	as a sorbent (?)
Carbonates	Calcite, dolomite, ankerite	CaCO3, CaMg(CO3)2, Ca(Fe,Mg)(CO3)2	Ra, minor Pb
	Strontianite	SrCO3	ditto -
	Rhodocrosite	MnCO3	ditto -
	Magnesite	MgCO3	ditto -
	Witherite	PbCO3	Pb
Sulphates	Barite	BaSO4	Ra, Pb
	Celestite	SrSO4	ditto -
	Anglesite	PbSO4	Pb
	Gypsum	CaSO4·2H2O	Pb (?)
Fe-oxides, hydroxides	Hematite	α-Fe2O3	Minor/trace U, Pb, and as sorbent
	Goethite	FeO(OH)	As sorbent
Fe–Ti-oxides, Ti-oxides	Ilmenite	FeTiO3	Minor U and Th
	Rutile	TiO2	ditto -
Jarosite sub-group	Jarosite	KFe(SO4)2(OH)6	Pb?
Other potential hosts	Apatite group	Ca5(PO4)3(F,Cl,OH)	Minor U and Th
	Fluorite	CaF2	?
	Feldspar group	-	Pb (replacing Ca?)

).

4.1. Re-Incorporation of Radionuclides into Parent Minerals

All minerals originally containing uranium will host daughter RN if those products do not migrate from the parent. Plausible ²¹⁰Pb and ²¹⁰Po carriers thus include the more common uranium minerals, such as uraninite, coffinite, uranothorite and brannerite, as well as the large number of minerals which carry trace to minor amounts of uranium. The latter include REE-fluorocarbonates and phosphates (monazite, bastnäsite, synchysite, florencite, xenotime, etc.), and common accessory minerals in rocks and ores such as apatite, allanite, zircon, titanite and rutile). The assumption that ²¹⁰Pb and ²¹⁰Po can be found in these minerals, however, infers that all decay chain products, including ²³⁴Th, ²²⁶Ra and ²²²Rn, are retained within the parent mineral, either within the crystal lattice, or as inclusions.

In the case of the mineral uraninite, strong supporting evidence for this emerges from our own recent research on the Olympic Dam Cu–U–Au–Ag ore deposit, South Australia, where (re-)incorporation of radiogenic lead within the crystal lattice of uraninite is recognised [145,146,147]. Lead concentrations in Olympic Dam uraninite can, locally exceed 10 wt % in solid solution within the uraninite structure. This contradicts the findings of Janeczek and Ewing [148], where it is maintained that Pb²⁺ is incompatible within the fluorite-type uraninite structure at concentrations greater than a few wt %. Other U-minerals will also contain Pb, albeit at lower concentrations, e.g., coffinite and brannerite from Olympic Dam [149]. Lower Pb concentrations relative to contained U suggest these structures accommodate daughter RN less well than uraninite.

The presence of radiogenic lead is also well known in minerals such as monazite and zircon where it directly substitutes into the crystal structure (e.g., [150,151]). The ratios between parent uranium, thorium and radiogenic lead underpin U–Pb geochronology. In the Olympic Dam deposit, both U and Pb (²⁰⁶Pb) are also noted in a characteristic oscillatory-zoned textural type of hematite [152,153]. Hematite is the most abundant gangue mineral in the deposit. Within this hematite, uranium and lead isotopes, both in solid solution and as nanoparticle inclusions [154] are in apparent secular equilibrium, indicating a closed system and providing a basis for U–Pb geochronology using hematite [152,155,156].

4.2. Migration and Precipitation as New Minerals

During radioactive decay of U-bearing minerals, metamictisation will take place. This is a natural process occurring over geological time (millions to billions of years) in which the crystal structure of the parent mineral is gradually, and ultimately completely, destroyed, rendering that mineral amorphous [157]. During that process, any impurities may be expelled from the metamict phase. Even if a portion of the ²¹⁰Pb and ²¹⁰Po is retained within the U-bearing parent mineral, migration of daughter isotopes and other trace elements initially incorporated within the parent mineral (Th, REE, Nb, etc.) is widely observed to take place following metamictisation, alteration and recrystallization. The radionuclides are either precipitated as new minerals, or alternatively, are incorporated into other existing minerals at distances ranging from microns to metres from the parent phase. Such phenomena are particularly common in hydrothermal ores, in which transport is assisted by permeability and the presence of fluids. There thus exist several potential mineral hosts for daughter RN. Obvious products resulting from ²¹⁰Pb and ²¹⁰Po migration include the common lead mineral, galena, which is often observed within, or immediately adjacent to parent U-minerals ([158,159], and many others). For example, nanometre-scale galena is documented within uraninite in parts of grains in which Cu-sulphides and fluorite fill sub-micron-scale fractures [145]. Finch and Murakami [160] have outlined how galena will form in close association with uraninite if the sulphur activity is high enough. Direct evidence for the presence of ²¹⁰Pb within galena is limited but has been shown within recently formed galena from burning heaps associated with coal mining in the Lower Silesian basin, Czech Republic, [161]. Migrating radiogenic lead may also combine with Se or Te, either within existing minerals or from fluid, to form clausthalite (PbSe) or altaite (PbTe). Owen et al. [162] have recently documented the formation of ²⁰⁶Pb-enriched nanoscale inclusions of clausthalite in Cu–(Fe)-sulphides, which formed via interaction between migrating Pb in fluids with Se initially hosted in solid solution within the sulphides. These may be as small as 1–2 nm in size but display coarsening.

Rollog et al. [163] have used nanoSIMS mapping to directly observe the sub-micron-scale distributions of ²¹⁰RN in copper ores and flotation concentrates from Olympic Dam; measurements are the sum of ²¹⁰Po + ²¹⁰Bi + ²¹⁰Pb but are overwhelmingly dominated by ²¹⁰Pb. Although concentrated within U-bearing minerals, migration of ²¹⁰RN away from the parent is observed on the scale of microns, with formation of nanoinclusions of “new” phases at sulphide grain boundaries, within microfractures, and within micropores in a range of host minerals. This phenomenon leads to daughter ²¹⁰RN becoming readily trapped within their host phases and accompanying those hosts through processing. Figure 3 shows an example of this innovative method to visualise RN distributions within individual mineral grains.

4.3. Sulphates, Carbonates, and Other Potential Hosts

Sulphates, notably barite (BaSO₄), celestine (SrSO₄), anglesite (PbSO₄) and gypsum are commonly considered as hosts for radium and subsequent daughter radionuclides including ²¹⁰Pb and ²¹⁰Po. Prieto et al. [164] provide compelling experimental evidence for (Ba,Pb)SO₄ solid solution. We are nevertheless unaware of empirical proof for the presence of ²¹⁰Pb and ²¹⁰Po in these sulphates.

The similarity in chemical behaviour between Ra and Ba, and to some extent also Pb, makes sulphates a good host for ²¹⁰Pb and, potentially, for ²¹⁰Po. For example, Al Attar et al. [114] report barite-strontianite solid solution and hokutolite [(Ba,Pb)SO₄] as the main mineral components of radionuclide-containing scales associated with oil production. Extensive solid solution between isostructural Ba- and Ra-sulphates (so called radiobarite, (Ba,Ra)SO₄ [165,166,167,168,169,170,171,172,173,174] has been modelled, and a number of natural occurrences have been documented e.g., [175,176]. In a detailed mineralogical insight into radionuclide host phases, Landa and Bush [79] document intense alpha particle activity associated with the presence of ²¹⁰Pb within micron-scale inclusions of anglesite within laths of gypsum.

Other possible candidates as hosts for ²¹⁰Pb and ²¹⁰Po in natural samples include common carbonates, notably the Pb-carbonate cerussite (PbCO₃). Reactions of dissolved Ra and Ba onto the surfaces of different carbonate minerals were examined by Jones et al. [177]. Calcite, dolomite, strontianite, rhodochrosite, ankerite and witherite all showed evidence of a co-precipitation reaction (increased uptake with increasing Ra concentration), siderite, magnesite and ankerite demonstrated a behaviour suggesting simple sorption. Magnesite showed a particularly high sorption capacity. An extensive treatment of the principles and mechanisms of co-precipitation with application to radionuclide incorporation within, and onto carbonate substrates, is given by Curti [178].

The role played by tellurium-bearing minerals, notably altaite (PbTe) and bismuth tellurides, as hosts for ²¹⁰Pb and/or ²¹⁰Po is unknown at the present time. These phases, as well as the selenide analogue of galena, clausthalite (PbSe) are minor yet persistent components of many hydrothermal ores, could potentially be important carriers of either radionuclide, especially given the similar chemistry of Po and Te. The environmental geochemistry of tellurium itself is only recently beginning to become better understood [179]. There may be a number of less obvious candidate hosts for ²¹⁰Pb and ²¹⁰Po. We can reasonably speculate that these will include those minerals capable of trapping gaseous radon within pore spaces during, or subsequent to growth.

4.4. Clay Minerals, Iron-Manganese-Oxides and Organics

Clay minerals, Fe–Mn oxy-hydroxides, and organic matter are well known sorbants for dissolved uranium and radium. ²¹⁰Pb and ²¹⁰Po sorption may not be primary and abundances may relate to in-situ decay of the sorbed U and Ra. As an alternative, sorption of ²²²Rn daughter isotopes, including ²¹⁰Po and ²¹⁰Pb may follow their generation via decay of dissolved ²²²Rn.

In a study of a range of sites on the River Danube, preferential accumulation of ²¹⁰Po in sediments that are rich in clay minerals has been shown [180]. These authors invoked the ion exchange and adsorption characteristics of different types of clay minerals, suggesting that they may represent both a sink or a source for ²¹⁰Pb and ²¹⁰Po and other contaminants, as element mobilities are influenced by evolving physical, chemical and biological conditions.

A strong association between ²¹⁰Po and ²¹⁰Pb and iron oxide minerals is demonstrated in beach sands [181], even if other, far more voluminous hosts contained the majority of these and other radionuclides. Yang et al. [182] report on the adsorption properties of ²¹⁰Po and ²¹⁰Pb onto micro-particles and reported that Fe- and Mn-oxides were stronger sorbents of ²¹⁰Po and ²¹⁰Pb than SiO₂ and CaCO₃. They did, however, note preferential adsorption of ²¹⁰Po over ²¹⁰Pb onto both SiO₂ and CaCO₃. In the presence of the protein BSA; acid polysaccharides appeared to produce the opposite effect, enhancing ²¹⁰Pb adsorption.

Interaction, by both adsorption and incorporation, between Pb and Fe(III) (oxyhydr) oxide minerals, has been demonstrated by Yang et al. [183], who considered that the presence or absence of these minerals plays a major influence on the partitioning and transport of lead. They have shown how Pb is both surface-adsorbed and incorporated within ferrihydrite during crystallisation to hematite and goethite, depending on pH conditions.

Further evidence for the importance of both organic compounds and nanoparticles is given by Yang et al. [184], who have documented adsorption and fractionation of ²¹⁰Po and ²¹⁰Pb onto chemically simple oxide and carbonate nanoparticles in the presence or absence of various macromolecular organic compounds (MOCs) in natural seawater. MOCs were found to enhance sorption of selected nuclides on most nanoparticles (partition coefficients for ²¹⁰Po and ²¹⁰Pb increasing 2.9- and 5-fold, respectively), even if adsorption was largely dependent on particle composition.

In soils, ²¹⁰Po is adsorbed onto clay particles and organic material [9,185]. Sequential leaching techniques enable insights into the speciation of ²¹⁰Pb and ²¹⁰Po in soils to be gained [46]. Of the five fractions into which ²¹⁰Pb and ²¹⁰Po were fractionated, they found the majority of both ²¹⁰Pb (67.2%) and ²¹⁰Po (77.4%) bound to the insoluble residue. Small, but still significant fractions of the total ²¹⁰Pb (14.3%) and ²¹⁰Po (21.0%) were extracted with NH₂OH·HCl in 25% v/v acetic acid, which may indicate partial association of the radionuclides with Fe–Mn-oxides [186]. The bio-reactivity of ²¹⁰Pb and ²¹⁰Po was demonstrated by Kim and Kim [187], who asserted that colloids play a major role in their cycling within oceans.

Further supporting evidence for the affinity of ²¹⁰Pb and ²¹⁰Po for Fe-oxides comes from the Talvivaara mine, Eastern Finland, where microbe-induced heap leaching is used to recover Ni and by-product Zn, Cu, Co from a black schist [188,189]. Non-target metals in the deposit include uranium (as uraninite) and its daughter isotopes. The behaviour of ²²⁶Ra, ²¹⁰Pb and ²¹⁰Po were studied in the mining process. It was found that they mostly remain in the heaps during leaching, where they are associated with jarosite, goethite and gypsum.

5. Discussion

5.1. Geochemical Behaviour of Daughter Radionuclides

Considerations of the likely mineral hosts for ²¹⁰Pb and ²¹⁰Po needs to include not only the geochemistry of each specific radionuclide, but the geochemical behaviour of the entire ²³⁸U decay chain in the context of half-lives that range from fractions of a seconds (²¹⁴Po) to billions of years (²³⁸U 4.5 × 10⁹ years). The geochemical behaviours of each daughter radionuclide differ fundamentally from those of parent uranium, and thus, if released from the parent mineral and able to migrate, even if only at the sub-micron-scale, they will be readily incorporated, by virtue of ionic size and/or charge, into quite different minerals. They may potentially even undergo several “metamorphoses” before accumulation of stable radiogenic lead within minerals such as galena or clausthalite that which do not necessarily coexist with the parent phases. Geochemical differences are particularly pronounced between U and Th, between Th and Ra, between Ra and Rn, and between Rn and Pb [2]. Figure 4 is a schematic diagram that attempts to illustrate how RN behaviour in different minerals may be viewed, also indicating how decoupling of RN within the decay chain might be achieved.

Uranium can form a great variety of stable phases with different ligands. No less than 262 different uranium minerals are currently recognised, one of the largest numbers for any element in the periodic table [190]. Uranium thus belongs among a small group of elements whose mineralogical diversity is anomalous compared to its low crustal abundance. The unusual mineralogical behaviour of uranium is attributable to the unique combination of relatively large-size, high-valence and unusual coordination geometry displayed by the U⁶⁺ ion [191]. The hexavalent oxidation state is dominant in most U-minerals although U occurs in the tetravalent state in the most common minerals, including uraninite. Uranium mineral diversity has been amplified by Earth evolution over time with respect to oxidation state [192]. Although U⁴⁺ is not especially mobile, U⁶⁺ is highly mobile under oxidising conditions when it will readily dissolve, re-precipitating upon contact with reductants such as sulphides or graphite. Certain minerals containing uranium undergo a process of structural breakdown (metamictisation), during which daughter radionuclides are not normally captured within the host lattice but rather migrate within or outside the mineral (e.g., [157]). In the case of uraninite, rapid annealing kinetics (e.g., [145,193]), do, however, allow for repair of radiation damage, allowing for sustained inclusion of decay product radionuclides in the mineral.

A good example of how daughter products differ geochemically from their parents is the ability for radium (²²⁶Ra), unlike U or Th, to be readily enriched in the common Ba-sulphate mineral, barite. This, as examined above, takes place via direct ionic substitution (Ra²⁺ for Ba²⁺) due to its comparable geochemical behaviour to barium with respect to charge, electronegativity and ionic size (0.143 and 0.134 nm for Ra and Ba, respectively). Although less well constrained, the potential incorporation of ²¹⁰Pb into galena and other Pb-bearing minerals, and of ²¹⁰Bi and ²¹⁰Po into bismuth minerals (including tellurides and other chalcogenides) may be viewed in the same way.

Decay of ²²⁶Ra involves production of ²²²Rn, a gas, which will seek to escape from the host mineral. Malczewski and Malczewski [194] have provided empirical data on ²²²Rn and ²²⁰Rn emanations from a range of metamict oxides, phosphates and silicates. These data demonstrate exponential differences in emanation rates among common minerals. Moreover, these rates vary as a function of many factors [195] closely linked to crystal structure and placing important controls on the decoupling of post-²²²Rn decay products from parent ²³⁸U. Significantly, minerals with the highest ²³⁸U concentrations, notably uraninite and brannerite, showed some of the lowest ²²²Rn and ²²⁰Rn emanation coefficients. We believe that these rates—which are closely linked to crystal structure—place important controls on the decoupling of post-²²²Rn daughter isotopes, including ²¹⁰Po and ²¹⁰Pb, from parent ²³⁸U. This decoupling is expressed as secular disequilibrium on the small scale yet may not be noticed in bulk samples, or on the scale of a mineral deposit. The importance of the “radon stage” is illustrated in detailed mineralogical-isotopic study on a weathered aplite dyke [196], which showed significant (>40%) release of radon, leading to significant differences in the absolute concentrations of distributions of pre-Ra and post-Ra radionuclides.

Several studies cited in this review have given compelling evidence that particles of, or containing, ²¹⁰Pb and ²¹⁰Po are readily adsorbed onto the surfaces of clay minerals, Fe-oxides, and potentially also other minerals that can readily adsorb heavy atoms. This raises the possibility that the two radionuclides may not necessarily occur within the crystal structures of mineral phases. Evidence from disparate sources suggests that any study addressing the mineralogical deportment of ²¹⁰Po and ²¹⁰Pb must also consider that these radionuclides, or at least a significant part thereof, may not be hosted within minerals at all, but largely as nanoparticles of unknown speciation, which are adsorbed onto the surface of other minerals, such as clays, and probably, Fe-(hydr)oxides. In solid rocks, such nanoparticles may be present within micro-fractures in minerals and/or at the interfaces of mineral grains. We also believe that the affinity of ²¹⁰Po and ²¹⁰Pb for organic matter, and the potential role of organic complexes in the transport and sequestration of ²¹⁰Pb and ²¹⁰Po (e.g., biopolymers [197], should also be taken into consideration in any effort to derive a quantitative mineralogical balance for ²¹⁰Pb and ²¹⁰Po.

The physical process involved in radionuclide decay has profound consequences for understanding the mineralogical location of each daughter radionuclide. An alpha particle comprises two protons and two neutrons, the nucleus of a helium atom. When alpha decay takes place, the energy of the nucleus recoiling from alpha decay is sufficient to break chemical bonds [198], and the newly-formed nucleus (with new geochemical behaviour) will be deposited at a different site in the damaged crystal. This new position and matrix damage may make the nucleus more vulnerable to mobilisation or leaching [2]. A newly-formed radionuclide may thus migrate and be incorporated into another mineral, although this may be limited if the half-life is short, e.g., for ²²²Rn (3.83 days) or ²¹⁰Bi (5 days). Cowart and Burnett [2], citing Reference [199], also make the valuable point that, if sited close to a grain boundary, the resulting nucleus may be recoiled across the grain boundary and end up in an entirely different matrix.

Roudil et al. [200] explored the creation of “bubbles” in uranium minerals from production of radiogenic helium as a product of alpha decay. This He may be concentrated in the mineral grains, or trapped at grain boundaries, also generating matrix damage. Their measurements of He outgassing from a uranium ore showed that less than 5% of the He produced was conserved, and that one-third of the residual He is occluded in the matrix and vacancy defects, and two-thirds occurred as bubbles observable by high-resolution transmission electron microscopy.

5.2. Research Trends and Future Directions

A large part of the existing literature on ²¹⁰Pb and ²¹⁰Po and their distributions in rocks and minerals concerns radioactive contamination resulting from nuclear weapons testing, uranium mining, energy generation and industrial production of fertilisers and other products from materials containing RN. These issues remain serious but have attracted markedly less research attention in the 21st century literature as industrial practices have improved. This has been compounded no doubt by the relatively short half-lives of ²¹⁰Pb and ²¹⁰Po such that their concentrations, in at least some materials produced by human activity in the past, have declined to safer levels. The contemporary literature also reflects the many ways in which radioactive waste can be efficiently treated and immobilised ([201] and references therein). Indeed, the “benefits” of man-made contamination have been highlighted in the more recent literature, notably the potential of short-lived anthropogenic radionuclides, including ²¹⁰Pb and ²¹⁰Po, as geochemical tracers for understanding processes and rates of sedimentation (e.g., [67]).

The toxicological risks from exposure to ²¹⁰Pb and ²¹⁰Po in materials from a wide range of terrestrial and marine environments are likely to remain a research focus. As older anthropogenic sources of air- and water-borne radionuclides (e.g., nuclear testing, energy generation) begin to diminish in significance, others emerge, e.g., ²¹⁰Po release from large-scale burning of forest biomass (e.g., [202]), or even ingestion of ²¹⁰Pb from calcium dietary supplements [203]. It should, however, be borne in mind that natural sources of ²¹⁰Pb and ²¹⁰Po are more significant in scale than man-made sources, e.g., from volcanic eruption. Indeed, a study of radionuclide hazards in seafood from the NW Pacific fishing area contaminated following the 2011 Fukushima nuclear accident [204] showed that despite elevated Fukushima-derived ⁹⁰Sr, ¹³⁴Cs and ¹³⁷Cs, these were exponentially subordinate in dose terms to natural ²¹⁰Pb and ²¹⁰Po in the same ocean area.

Despite the relative slowdown in the construction of nuclear power stations in the aftermath of the Fukushima nuclear accident, production of uranium for power generation continues to increase. Nuclear power is currently advanced as a possible low-carbon emission “green” energy alternative among the conservationist movement (e.g., [205]). In the absence of available technology to process thorium (a potential alternative fuel; [206]), increased demand necessitates a continued supply of uranium, and inevitably, as for other commodities, a need to exploit lower-grade resources, and to optimise extraction from deposits in which uranium occurs alongside other metals, notably copper, as in giant iron-oxide copper gold deposits such as Olympic Dam, South Australia.

6. Conclusions

This review has highlighted some of the many advances made in understanding the distribution of radionuclides in the environment during the past three decades. It also highlights that although the deportment and behaviour of ²¹⁰Pb and ²¹⁰Po at the scale of individual minerals is reasonably well understood, or at least predictable to some degree, there remains a paucity of direct observational data at the nano- to micron-scales to support these models. This is particularly true for metal ores and the products of their processing, for which there is an outstanding gap in knowledge. Bridging this gap is essential for generation of clean concentrates from a range of uranium-bearing ores.

Characterisation of short half-life radionuclides down to the atomic scale is now possible via use of a combination of nanoscale techniques: nanoSIMS isotope mapping [163]; scanning transmission electron microscopy with electron energy-loss spectroscopy (e.g., [207]); and high angle annular dark-field scanning transmission electron microscopy on foils prepared in-situ using focused ion beam methods (e.g., [162,208,209]). Valuable additional constraints on RN deportment may also come from the application of nanoscale analysis techniques to radioisotope dating of minerals within hydrothermal mineral deposits forming on the present-day seafloor [210,211]. Despite these crucial advances, a fully quantitative understanding of the physical form of ²¹⁰Pb and ²¹⁰Po, and the quantitative mineral deportment of these RN in solid media, remains elusive. Researchers can, however, expect to respond to these outstanding challenges by capitalising on micro-/nanoanalytical technology, which is rapidly advancing in terms of both spatial resolution and analytical sensitivity. This will enable reliable, predictive information on the physical state of ²¹⁰Pb, ²¹⁰Po and other RN to be communicated to stakeholders, including mining companies and environmental authorities.