•Primary sources of gallium were estimated.•Various resources and corresponding extraction methods of gallium were documented.•Commercial gallium recovery process employed in Guizhou branch of Chalco was summarized.•Promising resources of gallium and environmental extraction method were proposed.Gallium (Ga) is extensively employed in integrated circuits and advanced electronic devices as it provides the benefits of low energy consumption and high computation speeds. However, the Ga-bearing host minerals are scarce in nature. Ga occurs in combination with several minerals, mainly including aluminum, zinc, iron ores and coals, of which bauxite, zinc ores and coals are the primary original sources of Ga currently. Mining minerals for the sole extraction of Ga is not economical due to the low concentration of Ga. Accordingly, Ga is mainly recovered as a by-product from the processing of minerals. The current main commercial resources of Ga are Bayer liquor and zinc residue, which contribute to nearly all of the worldwide Ga production. The production of low-grade (99.99% pure) Ga has been increasing at an average rate of 7.4% p.a. for the past four decades and amounted to 375 tons in 2016. It is estimated to increase by 20-fold by the year 2030 compared to the yield of 275 tons in 2012. The mounting worldwide demand for Ga necessitates the search for additional resources and recovery technologies for this particular element. Apart from the Bayer liquor and the zinc residue, there are several other Ga-resources, which include red mud, coal fly ash, Ga-bearing electronics industrial waste, and flue dust from electric furnaces at phosphorus factories. Based on the chemical properties of Ga, it is evident that both strong acidic/basic conditions and high temperatures favor the efficient extraction of Ga from its corresponding minerals. Several hydrometallurgical processes based mainly on acid/alkaline leaching along with solution purification and recovery (e.g. ion exchange, solvent extraction and precipitation) have been proposed for Ga extraction from these resources. In this paper, the current status of Ga recovery was reviewed and specific examples were utilized for each resource to discuss the extraction methods, the recoveries and the optimum Ga-recovery conditions for each resource. Additional research appears to be necessary to establish a highly efficient and environmentally friendly process to recover Ga from these resources.

•Distribution and occurrence of toxic elements in coals and coal wastes in a combustion fluorosis area were illustrated.•Mobility features of toxic elements in coals during weathering and leaching processes were elucidated.•Potential health risks of toxic elements in the study area were estimated.Fluorine (F) is a topic of great interest in coal-combustion related endemic fluorosis areas. However, little extent research exists regarding the environmental geochemistry of toxic elements that are enriched in coals and coal wastes in traditional endemic fluorosis areas, particularly focusing on their occurrences and mobilities during the weathering-leaching processes of coals and coal wastes in the surface environment. This paper addressed the issue of toxic elements in coals and coal wastes in the Three Gorges Region, Southwest (SW) China, where endemic fluorosis has historically prevailed, and investigated the distribution, occurrence, mobility features, and associated potential health risks. For this purpose, a modified experiment combined with long-term humidity cell test and column leaching trial was applied to elucidate the mobility of toxic elements in coals and coal wastes. In addition, sequential chemical extraction (SCE) was used to ascertain the modes of occurrence of toxic elements. The results demonstrated that the contents of toxic elements in the study area followed the order: stone coals > gangues > coal balls > coals. Furthermore, modes of occurrence of toxic elements were obviously different in coals and coal wastes. For example, cadmium (Cd) was mainly associated with monosulfide fraction in coals, molybdenum (Mo) and arsenic (As) were mainly associated with carbonate and silicate in coal gangues and stone coals, chromium (Cr) mainly existed in silicate and insoluble matter in coal gangues and coal balls, thallium (Tl) mainly occurred in organic matter in stone coals and sulfide in coals, and the occurrence of antimony (Sb) varied with different kinds of samples. Moreover, a large amount of toxic elements released to the leachates during the weathering and leaching process, which might pollute the environment and threaten human health. Based on the geo-accumulation index (Igeo), single factor index (Pi) and Nemerow index (PN), soils in the study area were mainly polluted by Cd, which constituted a potential risk to locally planted crops.

•Elevated levels of trace metals and arsenic were observed in drinking water of Tehsil Mailsi, Pakistan.•Spatial distribution of aqueous As and trace metals were examined.•Anthropogenic activities mainly contributed to arsenic and trace metals pollution in drinking water.•Trace metals, particularly As and Cd exhibited high health risks to humans.Trace metal contamination in drinking water poses severe threat to human health through long-term exposure. The present study highlighted the elevated arsenic (As) and trace metal concentrations in drinking water and associated potential health risk to local residents of Tehsil Mailsi (Punjab), Pakistan. Our results showed that concentrations of As, Cd, Fe, Cr and Pb exceeded the WHO limits in drinking water, whereas Cu, Mn, Co, Ni and Zn concentrations were below the safe limits. The calculated estimated daily intake (EDI) of metals from local drinking water had the order of Zn > As > Cu > Pb > Cd > Ni > Mn > Cr, and the consequent target hazard quotient (THQ) above 1 was observed for As and Cd, which employed high potential health risk to local residents. Spatial distribution of As and trace metals in drinking water were related to the local anthropogenic sources, due to intensive application of agrochemicals. The study area presents high potential health risk associated with As and trace metals pollution in drinking water. The local wells have never been tested for metal concentrations prior to use, and necessary processes should be taken to test the wells with respect to As and trace metals contamination.

•High concentrations of Cd in soils of a rural area in the Three Gorges region, China, are reported.•Lognormal distribution plots and enrichment factors were applied to identify Cd sources.•Sedimentary rocks are main source of Cd in the Cd-enriched soils.•Results of sequential extractions show that a considerable fraction of the Cd in soils is in a labile form.This study investigated the distribution and sources of Cd in soils from a Cd-rich area in the Three Gorges region, China. The results showed that in the study area arable soils contain 0.42–42 mg kg−1 Cd with 0.12–8.5 mg kg−1 in the natural soils, corresponding to high amounts of Cd (0.22–42 mg kg−1) in outcropping sedimentary rocks in the area. Both lognormal distribution and enrichment factor (EF) plots were applied in an attempt to distinguish between geogenic and anthropogenic origins of Cd in the local soils. The lognormal distribution plots illustrated that geogenic sources dominated in soils with low and moderate Cd concentrations (<8.5 mg kg−1), whereas anthropogenic sources (agricultural activities, coal mining) significantly elevated Cd contents in some arable soils (>8.5 mg kg−1). The enrichment factor plots illustrated that the majority of the soil samples had EF values of <5, pointing to a geogenic origin of Cd in the soils, whereas some arable soils had EF values >5, pointing to an additional anthropogenic input of Cd to the soils. Sequential extraction results showed that Cd soluble in water and weak acid (water-soluble, exchangeable and carbonate fraction of the soil) accounts for an average of 31% of the total soil Cd, which indicates high potential for Cd mobility and bioavailability. The findings point to a potential health risk from Cd in areas with high geogenic background concentrations of this metal.

•Geogenic cadmium sources and environmental health risks were reviewed.•Occurrence of Cd in rocks was discussed with emphasis on black shale.•Cadmium pollution in the environment also existed in areas with high Cd geochemical background.•Potential health risks related to naturally occurring Cd were discussed.Cadmium (Cd) is a non-essential trace element that is toxic to humans. Previous studies of Cd in the environment have primarily focused on pollution resulted from anthropogenic sources, but little is known on naturally occurring sources of Cd. This paper aims to review the geochemical distribution of geogenic Cd and associated environmental risk. The source, accumulation, mobility, transportation, and health risk of Cd are discussed in a geo-environmental perspective, with an emphasis on black shale soils. Cadmium generally occurs in sulfides in black shale, and is easily released when exposed to oxygen and water. Leaching of these rocks tends to elevate Cd concentrations in aquatic systems, and may pose the potential to produce acid rock drainage (ARD) as well. Weathering of Cd-rich rocks also elevates soil Cd concentrations, and influence the geochemical species of Cd. Crops grown in these soils tend to accumulate higher Cd and threaten the food safety. Local inhabitant exposed to high geogenic Cd via food chains may experience Cd-related health risk. High Cd concentrations are observed in urine, and renal damage is also detected in Cd naturally enriched area based on low molecular weight proteins in urine. Overall, the findings in literature have provided with insights for potential health risk of Cd in areas with high Cd geochemical background levels, particular for the black shale exposed areas, more attentions should be paid on the geogenic Cd pollution, and suitable strategies of remediation and geo-environmental management for geogenic Cd pollution need further research.

There is a lack of information in the literature regarding Tl exposure from naturally occurring Tl enrichment. This paper draws attention to the potential health risk posed by high concentrations of naturally occurring Tl in the environment. The inhabitants of a rural area in SW Guizhou Province, China, reside within a natural Tl accumulated environment resulting from Tl-rich sulfide mineralization, and they face ongoing severe Tl exposure. High Tl concentrations were detected in urine of the local residents. Urinary Tl concentrations are as high as 2668 μg/L, with most subjects surpassing the accepted world urinary Tl concentration at <1 μg/L for “non-exposed” humans. The urinary Tl concentrations show significant differences among three communities (n = 21, p = 0.001), but no significant difference in either sex or age groups (n = 21, p = 0.7806). However, there is a positive statistical relationship between the extent of Tl exposure from Tl concentrations in soil and crops in the immediate environment and the concentrations of Tl detected in urine. A majority of the volunteer subjects from the communities have urinary Tl concentrations above 4.5–6 μg/L, implying early adverse health effects, and some of them have over 500 μg/L urinary Tl, considered to be at/about the level of clinical intoxication. This study has been able to identify that the elevated urinary Tl concentrations are mainly attributable to Tl accumulation in locally grown vegetables, which acquire Tl from the soil. This study also shows that Tl in urine of the local population represents a steady-state condition with long-term exposure, and that urinary Tl concentrations can be taken as a bio-marker of total dose based upon total daily dietary intake. This study demonstrates that natural sources of elevated Tl pose a potential health risk to the population, and that monitoring the urinary Tl concentration is a reliable and accurate way of bio-marking Tl exposure.

Environmental contamination with cadmium (Cd) and fluorine (F) and the associated health impacts on humans have raised significant concerns in the literature, but the additional health risks created by Cd have not been investigated in areas with endemic fluorine intoxication (fluorosis). Here, we report for the first time that naturally occurring Cd in areas where endemic fluorosis is related to coal combustion is a serious hidden toxin. The high Cd levels in rocks and soils of these areas may increase health risks to epidemiological level, irrespective of fluorine levels. We implemented a pilot study in a fluorosis-affected rural area within China’s Three Gorges region, and revealed enrichment of Cd in local bedrock (4.48–187 mg kg−1), coal (11.5–53.4 mg kg−1), and arable soils (1.01–59.7 mg kg−1). Cadmium was also observed to concentrate in local food crops (0.58–14.9 mg kg−1) and in the urine of local residents (1.7–13.4 μg L−1). A routine epidemiological investigation revealed that the two major Cd exposure pathways were through crop consumption and inhalation of emissions from coal combustion. Therefore, the naturally occurring Cd in areas with endemic fluorosis related to coal combustion represents a previously unrecognized toxin that must be addressed as part of efforts to control the endemic problem. The biogeochemical processes of Cd and the associated environmental effects will require additional in-depth study.

•Two contrasting soil vertical profiles were selected.•All vertical profiles were contaminated by antimony and arsenic.•The contamination fractions shaped the innate soil microbiota.•Metabolic potentials in soil microbiota indicated arsenic metabolism.Investigation of microbial communities of soils contaminated by antimony (Sb) and arsenic (As) is necessary to obtain knowledge for their bioremediation. However, little is known about the depth profiles of microbial community composition and structure in Sb and As contaminated soils. Our previous studies have suggested that historical factors (i.e., soil and sediment) play important roles in governing microbial community structure and composition. Here, we selected two different types of soil (flooded paddy soil versus dry corn field soil) with co-contamination of Sb and As to study interactions between these metalloids, geochemical parameters and the soil microbiota as well as microbial metabolism in response to Sb and As contamination. Comprehensive geochemical analyses and 16S rRNA amplicon sequencing were used to shed light on the interactions of the microbial communities with their environments. A wide diversity of taxonomical groups was present in both soil cores, and many were significantly correlated with geochemical parameters. Canonical correspondence analysis (CCA) and co-occurrence networks further elucidated the impact of geochemical parameters (including Sb and As contamination fractions and sulfate, TOC, Eh, and pH) on vertical distribution of soil microbial communities. Metagenomes predicted from the 16S data using PICRUSt included arsenic metabolism genes such as arsenate reductase (ArsC), arsenite oxidase small subunit (AoxA and AoxB), and arsenite transporter (ArsA and ACR3). In addition, predicted abundances of arsenate reductase (ArsC) and arsenite oxidase (AoxA and AoxB) genes were significantly correlated with Sb contamination fractions, These results suggest potential As biogeochemical cycling in both soil cores and potentially dynamic Sb biogeochemical cycling as well.Download high-res image (532KB)Download full-size image