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Arsenic (As) is of widespread concern, as its elevated contents in soil and water have a serious impact on the ecological environment and human health. Soils in karst regions are characterized by a high geochemical background of As. However, the bioavailability of As in paddy soils and the potential risk of As transfer from the soil to rice remain unclear. In this study, 305 paired soil–rice samples were collected from karst regions in Guangxi, China, in order to examine the controlling variables and As bioavailability in the soil–rice system. According to this study, the karst region's paddy soil had higher As concentrations than the non-karst region's paddy soil. The As concentration in the rice grains was low, with only 0.62% of the rice samples exceeding the permissible value of inorganic As (0.2 mg/kg). Arsenic in the karstic paddy soils existed mainly in the residual fraction, while the water-soluble and exchangeable fractions, which are readily absorbed by rice, accounted for a relatively small proportion. The high content but low bioavailability of As in the karstic paddy soil was mostly attributed to the abundant Fe–Mn nodules, which contributed 64.45% of the As content in the soil. Within the Fe–Mn nodules, As was primarily bound to Fe-(oxyhydr)oxides, which could be released into the paddy soil under certain reduction conditions via the reductive dissolution of Fe-(oxyhydr)oxides. Under the natural pH conditions of the karstic paddy soil (pH 4.9–8.38), the leaching of As was almost negligible, and As could be steadily retained within the Fe–Mn nodules. However, extremely acidic or alkaline conditions promoted the release of As from the Fe–Mn nodules. [ABSTRACT FROM AUTHOR]

Selenium (Se) is essential to human health, anti-cancer, possessing antioxidant, and antiviral properties. In this study, the spatial patterns of rice Se and their varying relationship with soil Se on a regional scale were studied using hot spot analysis for the agricultural soils in Guangxi. According to the hot and cold spot maps, rice Se correlates positively with soil Se in Guangxi agricultural soils. High rice Se accompanies high soil Se in the central part of Guangxi (e.g., Liuzhou, Laibin), and low rice Se is in line with low soil Se in the western part (e.g., Baise). However, the hot spot analysis maps indicate that southwestern Guangxi exhibits a special characteristic of low rice Se with high soil Se (e.g., Chongzuo). This special pattern is strongly associated with the high concentrations of Fe2O3 (ferromanganese nodules) in the carbonate rock area. The hot spot analysis proves useful in revealing the spatial patterns of rice Se in Guangxi and identifying the hidden patterns. [ABSTRACT FROM AUTHOR]

The rational development of mineral resources provides necessary materials for economic development, but environmental pollution caused by mining activities is an inevitable consequence. Here, we present a case study of Chehe Town in Guangxi, an area with integrated metals mining and smelting. The geochemical distribution, migration, and transformation behaviors of Cd and other heavy metals were studied in detail by systematically collecting surface media such as atmospheric dust, surface water and stream sediments, ores, tailings, mine drainage, soil, and crops in and around the mining area. We used these data to explore the geochemical response of the surface environment to mining and smelting of metal sulfide deposits. The annual flux of Cd and other heavy metals near the mining and smelting sites was high. Due to the topography, heavy metals in the atmosphere are mainly transported via vertical deposition, influencing areas downwind for 25 km. The mine drainage exceeded As and Zn standards but had little impact on the surface water. The surface water quality was good, without acidification. Risks due to ore were much higher than that for tailings. Heavy metals buffered by surrounding carbonate rocks and secondary minerals mainly migrated as solid particles, resulting in the contamination of stream sediment by heavy metals. In mountainous areas, rivers are mainly affected by topography, flowing fast and dominated by downcutting, which caused heavy metal pollution in the sediment have a limited effect on the soil near the river. Heavy metal concentrations in the cultivated soil were greatly influenced by external input such as substantial atmospheric dust. However, only Cd accumulated in the crops, with very high concentrations in rice, but safe and edible levels in corn. Thus, in the mining area, the most sensitive to heavy metals was the atmospheric environment. High concentrations of heavy metals beyond the ore district are mainly concentrated in the sediment, with distant impacts. Therefore, it is necessary to monitor and control risks associated with sediment transport, conduct treatment, and adjust crop planting. The soil, river, and agriculture respond differently to mining activities, but the risk is low and can be managed as needed. [ABSTRACT FROM AUTHOR]

The anomalous enrichment of the rare earth elements and yttrium (REY), U, Mo, As, Se, and V in the coal-bearing intervals intercalated within the carbonate successions in South China has attracted much attention due to the highly promising recovery potential for these elements. This study investigates the mineralogical and geochemical characteristics of the late Permian coal-bearing intervals (layers A–F) intercalated in marine carbonate strata in the Xian'an Coalfield in Guangxi Province to elucidate the mode of occurrence and enrichment process of highly elevated elements. There are two mineralogical assemblages, including quartz-albite-kaolinite-carbonates assemblage in layers D–F and quartz-illite-kaolinite-carbonates assemblage in layers A–C. Compared to the upper continental crust composition (UCC), the REY, U, Mo, As, Se, and V are predominantly enriched in layers A and B, of which layer A displays the REY–V–Se–As assemblage while layer B shows the Mo–U–V assemblage. The elevated REY contents in layer B are primarily hosted by clay minerals, zircon, and monazite; Mo, U, and V show organic association; and As and Se primarily display Fe-sulfide association. Three geological factors are most likely responsible for geochemical anomaly: (1) the more intensive seawater invasion gives rise to higher sulfur, Co, Ni, As, and Se contents, as well as higher Sr/Ba ratio in layers A–C than in layers D–F; (2) both the input of alkaline pyroclastic materials and the solution/rock interaction jointly govern the anomalous enrichment of REY; and (3) the influx of syngenetic or early diagenetic hydrothermal fluids is the predominant source of U, Mo, V, Se, and As. [ABSTRACT FROM AUTHOR]

In recent years, the biogeochemical behavior and environmental impact of Selenium (Se) on soil-plant systems have received widespread attention, and traditional statistical methods reveal generally positive correlations between rice Se and soil Se. However, that initial positive relationship may have been obscured by local external factors. Using local scale data from the geochemical evaluation of land quality project, this work employed geographically weighted regression (GWR) to examine the spatial variation of rice Se (as the dependent variable) and soil Se (as the independent variable) in Guangxi. Strong and weak correlation coefficients occur between rice Se and soil Se, thereby indicating that their relationships are spatially varying. Guangxi is characterized by significantly positive correlations in most areas, with weak correlations mostly found in the south-western and central-eastern regions. Areas with weak correlation can be divided into two patterns: high soil Se with low rice Se and high rice Se with low soil Se. The unique patterns are correlated with distinct natural factors, particularly the abundance of Fe-rich soils in the carbonate area; by contrast, sandstone areas in central Guangxi may have been affected by anthropogenic activities. To reveal the spatially varying relationships at the local scale, we employed GWR, an effective tool that allowed us to identify the association between environmental variables and influencing factors and explore spatially varying relationships between them. This study breaks through the existing understanding that soil Se is completely positively correlated with rice Se for the first time, and concludes that their correlation is spatially variable, providing an effective approach for the study of complex relationships. [Display omitted] • Spatially varying relationships between rice Se and soil Se are revealed by GWR. • Weak correlations are observed in north-western and central-eastern Guangxi. • Fe-enriched soil is the main influencing factor of the weak relationship. • The utility of Se-enriched land is proposed based on the spatial relationships. [ABSTRACT FROM AUTHOR]

Carbonate rocks are closely related to the genesis and spatial distribution of polymetallic sulfide deposits. The natural buffering of carbonate rocks can reduce the ecological impact of heavy metals produced by mining and smelting. Ignoring the buffering effect of carbonate rocks on the heavy metals in the mine environment leads to inaccurate ecological risk assessment, wasting land resources and funds. This study investigates Cd, Zn, and Pb distribution and speciation in the water and soil-rice system in the polymetallic sulfide deposit at Daxin, Guangxi. The study aims to reveal the effects of the natural buffering of carbonate rocks on the migration and transformation of heavy metals. The results show that the water Zn and Cd concentrations decreased from 1857.0 to 0.9 mg L−1 to 0.16 and 0.001 mg L−1, respectively, from the mining area to 4 km downstream. The natural buffering of carbonate increases the water pH from 2.80 to 7.64, resulting in a tendency for Cd, Zn, and Pb to separate from the aqueous phase and enrich the sediments. Soil Cd content in the mining area reached 110.0 mg kg−1 (mean value 55.88 mg kg−1), and rice Cd seriously exceeded the maximum limit. However, the weathering of carbonate reduces the migration ability and bioavailability of Cd. Soil Cd is mainly in the Fe–Mn bound and carbonate-bound fractions, resulting in lower Cd content in downstream soils (mean value 2.73 mg kg−1). Soil CaO, tFe 2 O 3 , and Mn hindered the uptake of soil Cd by rice rendering a lower exceedance of Cd in downstream rice. Therefore, this study recommends a farmland management plan under the premise of rice Cd content and integrated soil Cd content, which ensures food safety and fully utilizes farmland resources. This result provides a scientific basis for ecological risk assessment, mine environmental protection, and management in the carbonatite sulfide mine environment. [Display omitted] • Carbonate' buffering reduces the ecological risk of heavy metal in sulfide deposits. • The heavy metal of water tended to enrich in sediment. • Soil Cd is mainly in the Fe–Mn bound and carbonate bound. • Soil CaO and tFe 2 O 3 restrict the uptake of Cd by rice. [ABSTRACT FROM AUTHOR]

Soils developed in karst regions have naturally high background values of molybdenum (Mo) due to geological factors. However, the enrichment mechanism of Mo in these soils are not fully understood, making it challenging to assess their ecological risk and utilize Mo-rich land resources. To shed light on this issue, this study collected and analyzed data from the 1:50,000 geochemical survey in Guangxi, including 536,503 sets of soil data and 3043 sets of rock data, as well as 40 sets of carbonate rock-soil from typical karst regions. The results showed that soil Mo enrichment is highly correlated with the distribution of carbonate rocks in karst regions. The carbonate rocks in these regions contain Mo ranging from 0.03 to 1.06 mg·kg−1 (with a mean of 0.22 mg·kg−1). In comparison, the soil Mo derived from carbonate rocks can reach up to 6.00 mg·kg−1 (with a mean of 2.75 mg·kg−1), representing an average enrichment of soil Mo that is 24 times higher compared to the carbonate parent rock. The enrichment of soil Mo in karst regions is primarily controlled by secondary enrichment during the weathering process of carbonate. During the insoluble residue accumulation process, the dissolution of carbonate leads to a dramatic reduction in bedrock volume, and the adsorption of clay minerals and Fe minerals in insoluble residues plays an essential role in Mo enrichment during these stages. During the soil-forming stage of the insoluble residue, most Mo leaches into the water body due to the mineral transformation of insoluble residue. Consequently, as Fe–Mn nodules in soils become more enriched with increasing weathering intensity, some Mo is absorbed and passivated by iron and manganese oxides (hydroxides). Accordingly, the contribution of Fe–Mn nodules and the degree of leaching were closely related to the enrichment of soil Mo in karst regions. This study provides insights into the enrichment mechanisms of Mo in soils developed in karst regions, which will help to evaluate their ecological risk in these environments. [Display omitted] • Soil Mo is correlated with the distribution of carbonate rocks in karst area. • Soil Mo higher in karst regions (mean 2.32 mg kg−1) than in non-karst regions. • Bedrocks volume reduction and mineral composition affect soil Mo enrichment. • Adsorption of Mo by Fe-Mn nodules contributes to the enrichment. [ABSTRACT FROM AUTHOR]

Although the accumulation of potentially toxic elements in soil and crops has attracted widespread attention, the characteristics of the transfer and accumulation of potentially toxic elements in soil–crop systems with different soil parent materials are still not clear. Soil and crop samples were collected from agricultural regions with different soil parent materials in Guangxi, China. This study analyzed the concentrations of Cd, Zn, and Fe in the roots, straws, and seeds of rice (Oryza sativa L.) and soils with Quaternary sediments and clastic rocks as the parent materials. The concentration of several potentially toxic elements in rice tissue from the two areas followed the order of C root > C straw > C seed. The transport capability of Cd and Zn from roots to straws is higher than straws to seeds, and Fe showed a strong capability for transport from straws to seeds. In general, the transfer capacity of potentially toxic elements in the soil–rice system in the Quaternary sediments area was stronger than that in the soil–rice system in the clastic rocks area. Soil pH and minerals, which were represented by major elements, were the main factors affecting the transfer of metals from soil to seeds. This approach could help to evaluate the bioaccumulation risk of potentially toxic elements in crops in different areas quantitatively. [Display omitted] • The rice tissues with the highest Cd content are roots, followed by straws and seeds. • The use of element pairs is more meaningful than a single element content. • The mineral composition is critical to affecting the accumulation of the metal in seeds. [ABSTRACT FROM AUTHOR]