Accumulation of atmospheric deposition of As, Cd and Pb by bush bean plants

Environmental cadmium pollution and health risk assessment in rice-wheat rotation area around a smelter

Cadmium (Cd) pollution induced by smelting process is of great concern worldwide. However, the comprehensive risk assessment of Cd exposures in smelting areas with farming coexist is lacking. In this study, atmospheric deposition, soil, surface and drinking water, rice, wheat, vegetable, fish, pork, and human hair samples were collected in rice-wheat rotation area near nonferrous smelter to investigate smelting effect on environmental Cd pollution and human health. Results showed high Cd deposition (0.88-2.61 mg m-2 year-1) combined with high bioavailability (37-42% totality) in study area. Moreover, 90%, 83%, 57%, and 3% of sampled soil, wheat, rice, and vegetable of Cd were higher than national allowable limits of China, respectively, indicating smelting induced serious environmental Cd pollution. Especially, higher Cd accumulation occurred in wheat compared to rice by factors of 1.5-2.0. However, as for Cd exposure to local residents, due to rice as staple food, rice intake ranked as main route and accounted for 49-53% of total intake, followed by wheat and vegetable. Cd exposure showed high potential noncarcinogenic risks with hazard quotient (HQ) of 0.63-4.99 using Monte Carlo probabilistic simulation, mainly from crop food consumption (mean 94% totality). Further, residents' hair Cd was significant correlated with HQ of wheat and rice ingestion, highlighting negative impact of cereal pollution to resident health. Therefore, smelting process should not coexist with cereal cultivating.

Differences in particulate matter retention and leaf microstructures of 10 plants in different urban environments in Lanzhou City

Particulate matter (PM) is a major primary environmental air pollutant and poses a threat to human health. Differences in the environment and leaf microstructures of plants will result in varying abilities to retain PM, but the effects of changes in these factors on PM retention are not yet well understood. This study selected 10 plant species in four urban areas (sports field, park, residential green space, and greenway) as the study objects. The amount of retained PM by the different species was measured, and the leaf microstructures were observed. It was found that the environment significantly affected both PM retention and leaf microstructure. The ranking of PM retention in the 10 species in four areas was greenway > residential green space > park > sports field. The ranking of average stomatal width and length was park > sports field > residential green space > greenway, while that of average stomatal density was greenway > residential green space > park > sports field. Different environments affected the length and density of trichomes in the leaves. These changes represented the adaptation of plant species to the growth environment. The stomata and grooves of the leaf surface significantly affected the ability of plants to retain PM. The amount of PM retained by different species varied. In all four urban areas, Prunus × cistena N. E. Hansen ex Koehne (purple leaf sand cherry), Prunus cerasifera Ehrhart f. atropurpurea (Jacq.) Rehd. (cherry plum), Buxus sinica var. parvifolia M. Cheng (common boxwood), and Ligustrum × vicaryi Rehder (golden privet) showed strong PM retention. The results of this study will provide information for planners and urban managers for the selection of plant species.

Tracking cadmium pollution from source to receptor: A health-risk focused transfer continuum approach

Cadmium (Cd) exposure poses a substantial risk to human health. Despite this, the multi-stage process through which Cd is released to the environment before being taken up and impacting human receptors has rarely been investigated. Here we utilized an integrated model involving Cd emissions, atmospheric transport, deposition, uptake by rice, receptor ingestion and metabolic processing in quantifying the critical emission sources and human health risks of Cd. Atmospheric Cd emissions in the study area in southeastern China were estimated at 147 kg (2016), with >53 % of emissions from non-ferrous metals (NFM) smelting activities. Atmospheric Cd depositions caused elevated Cd content in soil and rice, accounting for 3-79 % and 50-85 % of, respectively, soil and rice Cd. Cumulative frequency analysis showed that an estimated 1.3 % of predicted urine Cd through the consumption of Cd-contaminated rice and exceeded existing safety standards (1 μg g^-1), thus highlighting the risks posed to health from high levels of Cd pollution. Applying stricter industrial emission standards to the NFM sector in particular and effective soil management practices could substantially reduce exposure to Cd pollution. The results contribute to understanding of the Cd transfer process and draw attention to the relative health benefits of interventions aimed at mitigating Cd levels and exposure risks at different stages along the Cd transfer continuum from source to receptor.

The mechanism of enhanced lignin regulating foliar Cd absorption and yield in rice (Oryza sativa L.)

The impact of atmospheric deposition of cadmium (Cd) in cereal crops has become a global concern. Enhanced lignin content was expected to benefit the plant performance against Cd exposure. To date, however, the underlying mechanisms of lignin regulating foliar Cd absorption in rice (Oryza sativa L.) and its effect on grain yield remains unclear. In present study, the effect and mechanism of rice in response to leaf Cd exposure were investigated using 113Cd stable isotope and a lignin-increased rice mutant. The highest Cd uptake efficiency and uptake amount was observed in wild type (WT) plant grown in the maturity period, which were 3-fold higher than in mutant plant. Compared to WT, the mutant exhibited 14.75% and 25.43% higher contents in G- and S-unit of lignin monomers. Lignin biosynthesis and polymerization related genes (OsPAL/OsCOMT/Os4CL3/OsLAC5/OsLAC15) were significantly up-regulated in mutants. In addition, the enzyme activities involved in the above process were also significantly increased by 1.24-1.49-fold. The increased Cd retention in cell wall and decreased gene expression levels of OsNRAMP5, OsHMA3 and OsIRT1 in mutant indicated that lignin effectively inhibited Cd transportion in plant tissues. Moreover, the antioxidant capacity and photosynthesis efficiency in mutant plant were obviously improved, leading to higher Cd tolerance and increased grain yield. Our results revealed the molecular and physiological mechanisms of enhanced lignin regulating foliar Cd absorption and yield in rice, and provided the valuable rice genotype to ensure food safety.

Effects of the Foliar Application of Water-soluble Chitosan or Na2SiO3 Fertilizer on the Pb Accumulation by a Low-Pb Accumulator Brassica napus Grown on Farmland Surrounding a Working Smelter

Field experiments were conducted to investigate the effects of two foliar fertilizers, water-soluble chitosan (WSC) and Na₂SiO₃ (Si), on the accumulation of Pb by a low-Pb accumulator Brassica napus cultivar (QY-1) grown at two mildly Pb-contaminated farmland sites surrounding working smelters in Jiyuan city, Henan province, China. Regardless of the frequency of the fertilizer treatments, the foliar application of WSC (0.01%) or Si (0.15%) significantly increased the QY-1 biomass and decreased the grain Pb concentrations. Compared with the control treatment, spraying plants once with WSC or Si during the flowering period achieved the best effect in the two soils with different pollution, which may be because inhibiting the accumulation of Pb in grains by decreasing the husk-to-grain transfer coefficient. Thus, the foliar application of WSC or Si combined with the cultivation of a low-Pb accumulator is a promising approach for optimizing the utility of Pb-contaminated farmland affected by atmospheric deposition.

Future food contaminants: An assessment of the plant uptake of Technology-critical elements versus traditional metal contaminants

Technology-critical elements (TCEs) include most rare earth elements (REEs), the platinum group elements (PGEs), and Ga, Ge, In, Nb, Ta, Te, and Tl. Despite increasing recognition of their prolific release into the environment, their soil to plant transfer remains largely unknown. This paper provides an approximation of the potential for plant uptake by calculating bioconcentration factors (BCFs), defined as the concentration in edible vegetable tissues relative to that in cultivation soil. Here data were obtained from an indoor cultivation experiment growing lettuce, chard, and carrot on 22 different European urban soils. Values of BCFs were determined from concentrations of TCEs in vegetable samples after digestion with concentrated HNO3, and from concentrations in soil determined after 1) Aqua Regia digestion and, 2) diluted (0.1 M) HNO3 leaching. For comparison, BCFs were also determined for 5 traditional metal contaminants (TMCs; As, Cd, Cu, Pb, and Zn). The main conclusions of the study were that: 1)BCF values for the REEs were consistently low in the studied vegetables;2)the BCFs for Ga and Nb were low as well;3) the BCFs for Tl were high relative to the other measured TCEs and the traditional metal contaminants; and 4) mean BCF values for the investigated TCEs were generally highest in chard and lowest in carrot. These findings provide initial evidence that there are likely to be real and present soil-plant transfer of TCEs, especially in the case of Tl. Improvements in analytical methods and detection limits will allow this to be further investigated in a wider variety of edible plants so that a risk profile may be developed.

Dynamic Behaviors of Newly Deposited Atmospheric Heavy Metals in the Soil-Pak Choi System

Dynamic behaviors of the newly deposited atmospheric heavy metals in the soil-pak choi (Brassica chinensis L.) system are investigated by a fully factorial atmospheric exposure experiment using soils exposed to 0.5-year and 1.5-year atmospheric depositions. The results showed approximately 17-87%, 19-64%, and 43-84% of the Cu, Cd, and Pb in pak choi edible parts were contributed from the new depositions, respectively. For the newly deposited metals, foliar uptake was the key pathway of shoot bioaccumulation rather than from root uptake of the deposited metals in soils, resulting in no significant soil contribution differences between pak chois growing in 0.5-year and 1.5-year exposed soils. Indeed, highly bioavailable metals in atmospheric deposition significantly increased the soil plant-bioavailable Cu, Cd, and Pb fractions; however, soil aging resulted in similar percentages of the plant-bioavailable fractions in 0.5-year and 1.5-year exposed soils, which indicated the bioavailability of metals deposited into soils rapidly decreased with aging. The soil aging process of the deposited metals was well fitted with the first-order exponential decay model, and soil organic matter and clay were the major driving factors. Our findings highlight high plant bioaccumulation rates and the rapid soil aging process of newly deposited metals during the plant growth period.

Soil contamination with permissible levels of lead negatively affects the community of plant-associated insects: A case of study with kale

This study investigated whether lead (Pb), at concentrations allowed for soil, affects the community of insects that live in the aerial part of plants. We evaluated the effect of Pb concentrations on accumulated species richness, composition, and abundance of different functional groups of insects. Kale plants were grown in soil experimentally contaminated with four concentrations of lead nitrate: 0 (control), 144 (T1), 360 (T2), and 600 (T3) mg/kg of soil. The experiment was conducted in an open greenhouse for the natural colonization of insects. Insects were collected twice using trap bags attached to the plant leaf and by direct removal. The concentration of Pb in the stem and leaf samples increased with the increased soil contamination, even showing values above the limit allowed by the legislation for this plant species. Control plants showed a higher richness of accumulated insect species. In addition, the treatments had an effect on the community composition, in which Diaeretiella rapae (primary parasitoid) was found as an indicator of the control + T1 treatments and the top species Pachyneuron sp. (parasitoid of predators) was associated with the control. The abundance of chewing and sucking herbivores, their respective parasitoids, predators, and parasitoids of predators were negatively affected. Hyperparasitoid abundance was not affected, but their accumulated species richness was. This study was innovative in demonstrating that soil contamination by different concentrations of a heavy metal (Pb) can negatively affect the community of plant-associated insects, even at concentrations allowed for soil, reflecting possible damage to the ecosystem.

Predicting the spatial pollution of soil heavy metals by using the distance determination coefficient method

Identifying the spatial distribution of heavy metal concentrations is a prerequisite for soil contamination assessment and control. In this study, soil surface samples (0-20 cm) were collected in Wuwei, China, and heavy metal concentrations were determined. The LUR (land use regression) model was used to simulate the spatial distribution of seven heavy metal concentrations in the study area, considering various factors, and the results were compared with ordinary kriging (OK) interpolation. Based on A Distance Decay REgression Selection Strategy (ADDRESS), the distance-coefficient of determination (DCD) was proposed to more easily and accurately select the optimal buffer of the relevant covariates. The simulation results showed that the adj R² values of the LUR models of the remaining heavy metals were all above 0.6, and the empirical comparison showed that LUR was better than OK. The variables that showed a more significant impact on soil heavy metal concentrations in the research area included road length, normalized difference vegetation index (NDVI), and soil sample nutrients around the sample site. In the research area, the concentration of heavy metals in the soil was greatly affected by motorway, primary roads, and secondary roads in the range of 1.2-2.1 km (r > 0.5), while building and As, Cu, Pb and Zn in the range of 3.6-4.8 km had a significant correlation (r > 0.5). This study provides scientific evidence and basic information for the prevention and control of heavy metal contamination and human health risk assessment management in arid zones.

Concentrations and transportation of metal and organochlorine pollutants in vegetables and risk assessment of human exposure in rural, urban and industrial environments (Bouches-du-Rhône, France)

The bioaccumulation of metals (As, Cd, Co, Cr, Cu, Ni, Pb, Sb, V, Zn, Al, Fe) and organochlorine compounds (PCDD-Fs and PCBs) was assessed in soils and vegetables of 3 sites of contrasted anthropogenic influence (rural and industrial-urban areas). Cultivated soils in industrial areas exhibited diffuse pollution in organochlorine pollutants (PCBs and PCDD-Fs). The pollutant levels encountered in vegetables were always lower than the EU regulatory or recommended values. However, the contents measured in vegetables cultivated near industrialized areas were significantly higher than those observed in rural areas. This was notably the case for Co, Cd, Cr, Ni, Pb, V, NDL- and DL-PCB, PCDD, and PCDF. The leaf pathway appeared as the main absorption pathway for many contaminants. The results suggested that population exposure to pollutants was mainly caused by vegetable ingestion. In the vegetables and soils, the toxicity was mainly caused by the V, Co, Cd, and Pb contents to which can be added As and PCDD-Fs for soils. Therefore, the proximity of vegetable crops to highly anthropised areas has led to long-term exposure of vegetables and soils to air pollutants, leading to an accumulation in the food chain and thus a risk for human health.

Speciation, transportation, and pathways of cadmium in soil-rice systems: A review on the environmental implications and remediation approaches for food safety

Cadmium (Cd) contamination in paddy fields is a serious health concern because of its high toxicity and widespread pollution. Recently, much progress has been made in elucidating the mechanisms involved in Cd uptake, transport, and transformation from paddy soils to rice grains, aiming to mitigate the associated health risk; however, these topics have not been critically reviewed to date. Here, we summarized and reviewed the (1) geochemical distribution and speciation of Cd in soil-rice systems, (2) mobilization, uptake, and transport of Cd from soil to rice grains and the associated health risks, (3) pathways and transformation mechanisms of Cd from soil to rice grains, (4) transporters involved in reducing Cd uptake, transport, and accumulation in rice plants, (5) factors governing Cd bioavailability in paddy, and (6) comparison of remediation approaches for mitigating the environmental and health risks of Cd contamination in paddy fields. Briefly, this review presents the state of the art about the fate of Cd in paddy fields and its transport from soil to grains, contributing to a better understanding of the environmental hazards of Cd in rice ecosystems. Challenges and perspectives for controlling Cd risks in rice are thus raised. The summarized findings in this review may help to develop innovative and applicable methods for controlling Cd accumulation in rice grains and sustainably manage Cd-contaminated paddy fields.

Temporal and spatial distributions and sources of heavy metals in atmospheric deposition in western Taihu Lake, China

Heavy metals in atmospheric dust can directly pollute the soil, water and sediment, causing serious harm to human health. In this study, the temporal and spatial distribution characteristics of heavy metals in atmospheric deposition in western Taihu Lake were studied. We established 10 sampling sites to collect atmospheric deposition for two years in different seasons. The atmospheric deposition flux follows the order urban area (95.6 g m^-2·a^-1) > suburban area (80.2 g m^-2·a^-1) > forestland (56.8 g m^-2·a^-1). The concentrations of heavy metals in atmospheric deposition show trends of high values in the winter and low values in the summer and are significantly negatively correlated with distance from the city. The pollution level of I_geo-Cd is 6, which is very high, and that of E-Cd is 219, which means high risk. Heavy metals in atmospheric deposition are mainly taken up via hand-mouth intake, and the harm to children is significantly higher than the harm to adults. The highest health risk assessment values for the four analyzed heavy metals in atmospheric deposition are located near the city and in suburbs (within 5 km of the city center), that is, in areas where human activities are concentrated. The health risk assessment values in areas outside the suburbs are low; these areas are less affected by human activities. The health risk assessment values of heavy metals in the winter and spring are higher than those in the summer and autumn. The Pb isotope ratios show that the main sources of heavy metals in atmospheric deposition and local soil are human activities, such as industry and coal combustion, with less input from natural sources. Heavy metals in atmospheric deposition in the western part of Taihu Lake not only directly threaten local human health but also enter Taihu Lake, posing a serious threat to the Taihu Lake ecosystem.

Chemical speciation of trace metals in atmospheric deposition and impacts on soil geochemistry and vegetable bioaccumulation near a large copper smelter in China

Atmospheric deposition is an important source of trace metals to surface environments, but knowledge about plant bioavailability of recently deposited metals and their fate in the soil-plant system is limited. We performed a fully factorial soil and atmosphere exposure experiment with three vegetables (radish, lettuce, and soybean). Treatments included soil profiles collected from three sites located along a strong gradient of atmospheric deposition with each soil type deployed across the three sites for one year, which allowed to effectively distinguish impacts of recently deposited metals (<1 year) from longer-term trace metal exposures in soils. Results showed that recently deposited copper (Cu), cadmium (Cd), and lead (Pb) accounted for 0.5-15.2% of total soil Cu, Cd, and Pb pools at the site most heavily impacted by atmospheric deposition, while recent deposition contributed 15-76% of Cu, Cd, and Pb concentrations in edible parts of vegetables. In addition, soil geochemical extractions showed that bioavailable fractions of trace metals from recent deposition (52-73%) were higher compared to metals previously present in soils (7-42%). These findings highlight a preferential uptake and high rates of bioaccumulation of deposited metals in vegetables and suggest a high potential of environmental risks of food pollution under high atmospheric metal deposition.

Contribution of PM2.5-Pb in atmospheric fallout to Pb accumulation in Chinese cabbage leaves via stomata

Foliar uptake of Pb is especially important when Chinese cabbage (Brassica rapa spp. pekinensis), having a large leaf surface area, is cultivated in North China during seasons with heavy haze. However, the mechanisms of foliar Pb uptake via stomata by Chinese cabbage exposed to atmospheric fallout are unclear. A field experiment was conducted to explore the impacts of Pb in particulate matter with sizes ≤ 2.5 µm (PM_2.5-Pb) from atmospheric fallout to Pb accumulation in cabbage leaves through stomata. Cabbage varieties with low-Pb-accumulation (LPA) and high-Pb-accumulation (HPA) were examined using inductively coupled plasma-mass spectrometry and scanning electron microscopy/energy-dispersive X-ray analysis. The ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios of PM_2.5, plants, and soil demonstrated that the major source of Pb in cabbage leaves was PM_2.5. The average width and length of the stomatal apertures were 7.14 and 15.61 µm for LPA cabbage and 8.10 and 16.64 µm for HPA cabbage, which are large enough for PM_2.5-Pb to enter the leaves. The HPA cabbage had significantly higher stomatal width-to-length ratios than the LPA cabbage, indicating that the former trapped much more PM_2.5-Pb and accumulated more Pb. These results clarify the contributions of the stomatal characteristics to PM_2.5-Pb accumulation in the edible parts of Chinese cabbage.

Urban kitchen gardens: Effect of the soil contamination and parameters on the trace element accumulation in vegetables - A review

Trace element contaminants in kitchen garden soils can contribute to human exposure through the consumption of homegrown vegetables. In urban areas, these soils can be contaminated to various degrees by trace element (TE). They are characterized by a great variability in their physicochemical parameters due to the high anthropization level, the wide variety and combination of disturbance sources, as well as the diversity of cultivation practices and the large range of contamination levels. Pollutants can be taken up by vegetables cultivated in these soils and be concentrated in their edible parts. In this review, the behavior of vegetables cultivated in contaminated kitchen gardens is assessed through six examples of the most widely cultivated vegetables (lettuce, tomato, bean, carrot, radish, potato). The role of soil parameters that could influence the uptake of As, Cd, Cr, Ni, Pb, and Zn by these vegetables is also discussed.

The limits of lead (Pb) phytoextraction and possibilities of phytostabilization in contaminated soil: a critical review

This review article focuses on lead (Pb), one of the most ubiquitous and harmful toxicants found in soil. Our objective is to address misconceptions regarding the ability of plants to uptake Pb through their roots and translocate it to above-ground tissues, and their ability to act as hyperaccumulators and thereby phytoextract Pb. In accordance with a number of cited definitions, we suggest that species capable of Pb phytoextraction can be rated with the following three criteria: (1) root uptake above a nominal threshold of 1,000 mg Pb/kg, (2) bioconcentration factor (BCF or shoot/soil concentration) >1, and (3) translocation factor (TF or shoot/root concentration) > 1. We review the literature in the updated USDA Phytoremediation Database and conclude that without amendments: no plant has met all three criteria; no plant has been confirmed as a Pb hyperaccumulator. Our analysis suggests that Pb phytoextraction is not a viable remediation option. Pb phytostabilization, however, may be an effective remediation tool in a variety of settings. Planting some of the many species capable of tolerating soil Pb exposure and sequestering it in or around the root zone will limit Pb movement into other ecosystems, prevent resuspended dusts, and mitigate Pb exposure.

Seasonal disparities in airborne lead (Pb) and associated foliar uptake by ryegrass (Lolium perenne L.): A Pb isotopic approach

Foliar uptake of airborne lead (Pb) may be particularly important for Pb accumulation in plant organs. However, the aerosol bioconcentration factor (BCF) in different seasons has seldom been reported. In the present study, we collected ryegrass (Lolium perenne L.) and size-segregated aerosols (SSA) during the corresponding growing seasons, and analyzed these for both Pb concentrations and isotopic ratios. Airborne Pb showed a seasonally varying concentration that was approximately 20% higher in winter than in spring. The bioavailability index, however, was higher in spring. Coupling the stable isotope technique with the bioavailable Pb of aerosol was more reliable in identifying airborne Pb accumulation in leaves than the total determination, suggesting that the hydrophilic absorption pathway was probably dominant for the foliar uptake of Pb in ryegrass. Contributions of airborne Pb accumulation were 88%-92% for washed ryegrass growing outdoors, indicating that the foliar uptake of Pb in the field was mainly from atmospheric deposition. The aerosol BCF of Pb for ryegrass was 6.4-11.4 m³/g in winter and 22.9-31.5 m³/g in spring. The increased aerosol BCF in spring was due to the suitable temperature, abundant rainfall, and increased Pb solubility of the aerosol. Therefore, our results indicate that, for the foliar uptake of Pb, both the aerosol Pb concentration, composition, and seasonal influence should be considered.

The bioavailability and contribution of the newly deposited heavy metals (copper and lead) from atmosphere to rice (Oryza sativa L.)

Despite the global importance of atmospheric heavy metal input into agricultural soils, research has primarily focused on the amount of the depositions with limited attention given to the risk of the newly deposited heavy metals. To understand the remobilization of the newly deposited copper (Cu) and lead (Pb) from the atmosphere and explore the metals' mobility and bioavailability to rice (Oryza sativa L.), a soil transplant experiment was conducted in three areas along a gradient of atmospheric depositions. Approximately 61% of the Cu and 76% of the Pb depositions tended to be present in potentially mobile fractions. The soil retention of newly deposited Cu and Pb presented as higher mobile fractions than these in the original soil. The newly deposited Cu and Pb in soils only accounted for 0.34-8.7% and 0.07-0.29% of the total soil Cu and Pb pools, but they contributed 30-84% and 6-41% in rice tissues, respectively. A major implication of these findings is that once the heavy metal is deposited, it may be reactivated in soils and transported to aerial parts or foliar uptake into plant tissues, emphasizing the important role of the newly deposited Cu and Pb in contributing to the edible parts of crops.

Edible weeds: Are urban environments fit for foraging?

Foraging wild-growing edible plants (WEPs) is a re-emerging practice with increasing popularity worldwide, including in urban areas. However, in cities, this practice raises questions about the safety of foraging these plants for human consumption, due to the potential exposure of plants to higher levels of pollutants. In this study, the concentration of 12 elements (Al, V, Cr, Mn, Co, Ni, Zn, As, Rb, Cd, Ba and Pb) in three different WEPs (Amaranthus spp., Plantago tomentosa and Taraxacum officinale) were determined according to different traffic categories in the municipality of São Paulo. Additionally, plants were sampled within the inner areas of three municipal parks in the same study region. Different gradients of elemental concentrations were obtained according to the traffic categories. Freeways presented higher concentrations of several elements than local roads or parks. For the WEPs collected along freeways and some plants along arterial roads, the concentrations of Pb exceeded safety levels for human consumption. Our data suggest that foraging in large urban centres should be performed preferentially in low-traffic areas.

Impacts of atmospheric particulate matter pollution on environmental biogeochemistry of trace metals in soil-plant system: A review

Atmospheric particulate matter (PM) pollution and soil trace metal (TM) contamination are binary environmental issues harming ecosystems and human health, especially in the developing China with rapid urbanization and industrialization. Since PMs contain TMs, the air-soil nexus should be investigated synthetically. Although the PMs and airborne TMs are mainly emitted from urban or industrial areas, they can reach the rural and remote mountain areas owing to the ability of long-range transport. After dry or wet deposition, they will participate in the terrestrial biogeochemical cycles of TMs in various soil-plant systems, including urban soil-greening trees, agricultural soil-food crops, and mountain soil-natural forest systems. Besides the well-known root uptake, the pathway of leaf deposition and foliar absorption contribute significantly to the plant TM accumulation. Moreover, the aerosols can also exert climatic effects by absorption and scattering of solar radiation and by the cloud condensation nuclei activity, thereby indirectly impact plant growth and probably crop TM accumulation through photosynthesis, and then threat health. In particular, this systematic review summarizes the interactions of PMs-TMs in soil-plant systems including the deposition, transfer, accumulation, toxicity, and mechanisms among them. Finally, current knowledge gaps and prospective are proposed for future research agendas. These analyses would be conducive to improving urban air quality and managing the agricultural and ecological risks of airborne metals.

Atmospheric deposition as a source of cadmium and lead to soil-rice system and associated risk assessment

Atmospheric deposition of heavy metals is widely documented and has been connected to adverse ecological and health impacts. The influence of atmospheric deposition on the soil-rice system in a typical urban agglomeration region was studied continuously through a field contrast experiment for two years. The results showed that the Cd and Pb in rice grains is mainly from soil, but Cd and Pb from the atmospheric deposition should be a focus of attention. The bioavailable content of heavy metals in atmospheric deposition is higher than that in corresponding surface soil. Atmospheric deposition contributed 10.8-47.7% of the Cd and Pb in brown rice, and 13.7-60.3% of the Cd and Pb in rice leaves was from atmospheric deposition. In the traffic area, a high deposition site, the contributions of atmospheric depositions to heavy metals in rice plants were higher than those from abandoned mine area and suburban area. Atmospheric deposition also consistently decreased the pH (0.17-0.66) and increased the exchangeable Cd (27.1-62.1%) and Pb (3.3-26.1%) in surface soil. In addition, the health risk index (HRI) of rice consumption was also increased as a result of the different atmospheric depositions of heavy metals, which accounted for 40.0% and 35.5% of Cd and Pb at the high deposition site, respectively. These findings demonstrate the potential influences of atmospheric deposition on the soil-crop system and human health, and can also provide a useful reference for developing the emission control strategies.

Differences in quantity and composition of leaf particulate matter and morphological structures in three evergreen trees and their association in Harbin, China

Urban trees have been assumed to effectively clean air particulate matter (PM), while the inter-species differences are not yet well defined, especially the PM chemical composition. In this study, PM from leaf surface and wax layer of 3 evergreen tree species (Juniper: Juniperus rigida; Black pine: Pinus tabuliformis var. mukdeais; Spruce: Picea koraiensis) were used for finding differences in PM adsorption and its compositional traits (characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrum and Inductively coupled plasma-optical emission spectrometry). Possible improvement in PM removal was also evaluated by a detail whole city tree census and different scenarios of species adjustment data. We found that: 1) the amount of PM on juniper leaves was 5.73 g m^-2, 2-2.5-fold higher than black pine and spruce (p < 0.05). Of them, 38.73%, 38.22%, and 23.11% were in the wax layer. 2) Compared with the explicit interspecies differences in PM quantity, more complex interspecies difference showed different patterns for different compositional traits. In general, leaf surface PM had higher O, Si, Al, Fe, N, Pb, Cu, Ni, Cr, and Cd, while the wax PM had higher C and Na contents (p < 0.05). 3) Association ordination found that the smaller leaf size, lower leaf water content, higher leaf area per unit mass, higher wax content, and larger stomatal openness aligned with the more PM adsorption by leaf, together with the higher amounts of CO stretching, O, Si, Al, N, heavy metals of Pb, Cu, Ni, Cr, and Cd in PM. 4) Compared with the other 2 species, increase of juniper percentage in urban forests is more effective for maximizing PM removal from air, accompanying more heavy metal removal but less crystalized minerals in PM. Our findings highlight that proper species configuration in urban afforestation could maximize the air PM removal capacity.

Potential of indigenous plant species for phytoremediation of metal(loid)-contaminated soil in the Baoshan mining area, China

A field survey was conducted to investigate metal(loid) concentration in soils and native plants in the Baoshan mining area for potential application in phytoremediation. Total concentrations of arsenic (As), cadmium (Cd), lead (Pb), and zinc (Zn) in soil varied from 125 to 6656, 5.10 to 1061, 568 to 49294, and 241 to 17296 mg kg^-1, respectively, showing severe contamination. Among 20 species native to this area, Pteris ensiformis accumulated 1091 mg kg^-1 As in the shoot, and its translocation factor (TF) was greater than 1, suggesting potential capacity for As phytoextraction. Boehmeria nivea, Aster prorerus, and Hydrocotyle sibthorpioides showed potential for phytoextraction of Cd due to their high accumulation of Cd in shoots (490.3, 175.4, and 128.5 mg kg^-1, respectively) and high TFs (92.0, 22.1, and 6.7, respectively). Eleusine indica and P. ensiformis were found to contain high concentrations of Pb (7474 mg kg^-1) and Zn (1662 mg kg^-1) in roots, but with low TFs for Pb (0.4) and Zn (0.2), suggesting potential capability for phytostabilization. There was a positive correlation (p < 0.01, N = 25) of TFs between the metal(loid)s, indicating a synergic interaction in the uptake of metal(loid)s by these plants. According to metal(loid) concentrations in shoots, bioconcentration factors (BFs), and TFs, as well as the botanical features such as wide occurrence, high biomass yield, and rapid growth of the plants, the five native species identified above have the potential for phytoremediation in the Baoshan mining area.

Airborne foliar transfer of particular metals in Lactuca sativa L.: translocation, phytotoxicity, and bioaccessibility

The uptake, translocation, and human bioaccessibility of metals originating from atmospheric fine particulate matters (PM) after foliar exposure is not well understood. Lettuce (Lactuca sativa L.) plants were exposed to micronic PbO, CuO, and CdO particulate matters (PMs) by the foliar pathway and mature plants (6 weeks old) were analyzed in terms of: (1) metal accumulation and localization on plant leaf surface, and metal translocation factor (TF) and global enrichment factor (GEF) in the plants; (2) shoot growth, plant dry weight (DW), net photosynthesis (Pn), stomatal conductance (Gs), and fatty acid ratio; (3) metal bioaccessibility in the plants and soil; and (4) the hazard quotient (HQ) associated with consumption of contaminated plants. Substantial levels of metals were observed in the directly exposed edible leaves and newly formed leaves of lettuce, highlighting both the possible metal transfers throughout the plant and the potential for human exposure after plant ingestion. No significant changes were observed in plant biomass after exposure to PbO, CuO, and CdO-PMs. The Gs and fatty acid ratio were increased in leaves after metal exposure. A dilution effect after foliar uptake was suggested which could alleviate metal phytotoxicity to some degree. However, plant shoot growth and Pn were inhibited when the plants are exposed to PbO, and necrosis enriched with Cd was observed on the leaf surface. Gastric bioaccessibility of plant leaves is ranked: Cd > Cu > Pb. Our results highlight a serious health risk of PbO, CuO, and CdO-PMs associated with consumption of vegetables exposed to these metals, even in newly formed leaves in the case of PbO and CdO exposure. Finally, the study highlights the fate and toxicity of metal rich-PMs, especially in the highly populated urban areas which are increasingly cultivated to promote local food.

Study of the bioavailability of heavy metals from atmospheric deposition on the soil-pakchoi (Brassica chinensis L.) system

The objective of this study was to investigate the bioavailability of heavy metals from atmospheric deposition on the soil-pakchoi (Brassica chinensis L.) system near a smelter. Soil reciprocal translocation experiment was conducted with seven groups of pot culture (filled with soils of gradient levels of heavy metals) in three sites of gradient atmospheric heavy metal depositions. Results showed that the newly deposited heavy metals (Cu and Cd) were preferential retention in topsoil (0-4 cm) and presented as higher bioavailable fractions compared to those in original soils. Atmospheric depositions contributed to 20-85% of shoot Cu and Cd in high deposition site, which were likely resulted not only from the direct transfer of contaminants from atmosphere to foliar but also from the atmosphere-soil-root transfer. However, the 52-62% of Pb in shoot from atmospheric depositions was mainly resulted from foliar direct uptake. The increasing atmospheric heavy metal depositions significantly decreased the photosynthetic parameters of pakchoi. Additionally, the potential health risks associated with the consumption of pakchoi were elevated in high deposition site and the bioaccessibility values were observed up to 56-81%. This study will provide useful reference information for the newly deposited heavy metal dynamics in the surface environment.

Atmospheric deposition of mercury and cadmium impacts on topsoil in a typical coal mine city, Lianyuan, China

Mercury (Hg) and cadmium (Cd) in the atmosphere from coal combustion emissions play an important role in soil pollution. Therefore, the purposes of this study were to quantitatively evaluate the atmospheric Hg and Cd deposition and to determine the influence of atmospheric deposition on Hg and Cd contents in surface soil in a typical coal mine city. Atmospheric deposition samples were collected from May 2015 to May 2016 at 17 sites located in industrial, agricultural and forest areas in the Lianyuan city. Atmospheric Hg and Cd deposition fluxes in the different land use types showed high variability. Curvilinear regression analysis suggested that the atmospheric Hg deposition fluxes were positively related with Hg contents in soils (R² = 0.86359, P < 0.001). In addition, atmospheric Cd deposition fluxes were also positively correlated with Cd contents in soils when the site LY02, LY04 and LY05 (all belong to agricultural land) were not included in the fitting (R² = 0.82458, P < 0.001). When they were included, there was no significant relationship between them (R² = 0.2039, P = 0.05). The accumulation of Hg and Cd concentration in topsoil due to the influence of atmospheric deposition will increase rapidly in the next 30 years, and the mean value of the increment will reach 2.6007 and 33.344 mg kg^-1. After 30 years, the Hg and Cd concentration will increase slowly. The present study advocates that much attention should be paid to the potential ecological hazards in soil resulting from the atmospheric Hg and Cd deposition.

Cadmium bioaccumulation and gastric bioaccessibility in cacao: A field study in areas impacted by oil activities in Ecuador

Cacao from South America is especially used to produce premium quality chocolate. Although the European Food Safety Authority has not established a limit for cadmium (Cd) in chocolate raw material, recent studies demonstrate that Cd concentrations in cacao beans can reach levels higher than the legal limits for dark chocolate (0.8 mg kg^-1, effective January 1st, 2019). Despite the fact that the presence of Cd in agricultural soils is related to contamination by fertilizers, other potential sources must be considered in Ecuador. This field study was conducted to investigate Cd content in soils and cacao cultivated on Ecuadorian farms in areas impacted by oil activities. Soils, cacao leaves, and pod husks were collected from 31 farms in the northern Amazon and Pacific coastal regions exposed to oil production and refining and compared to two control areas. Human gastric bioaccessibility was determined in raw cacao beans and cacao liquor samples in order to assess potential health risks involved. Our results show that topsoils (0-20 cm) have higher Cd concentrations than deeper layers, exceeding the Ecuadorian legislation limit in 39% of the sampling sites. Cacao leaves accumulate more Cd than pod husks or beans but, nevertheless, 50% of the sampled beans have Cd contents above 0.8 mg kg^-1. Root-to-cacao transfer seems to be the main pathway of Cd uptake, which is not only regulated by physico-chemical soil properties but also agricultural practices. Additionally, natural Cd enrichment by volcanic inputs must not be neglected. Finally, Cd in cacao trees cannot be considered as a tracer of oil activities. Assuming that total Cd content and its bioaccessible fraction (up to 90%) in cacao beans and liquor is directly linked to those in chocolate, the health risk associated with Cd exposure varies from low to moderate.

Biomonitoring of genotoxic effects and elemental accumulation derived from air pollution in community urban gardens

Urban gardening is a growing global phenomenon with a positive impact on society. Despite several associated benefits, growing vegetables in urban gardens that are localized in highly polluted areas poses questions about the safety of the produced food. Therefore, the identification of risk factors that result in possible deleterious effects to human health is important for realizing all of the benefits to society. We evaluated the use of two biomonitoring methods in ten urban gardens of Sao Paulo city and one control site: the micronuclei frequencies for early tetrads of Tradescantia pallida (Rose) Hunt. cv. "Purpurea" Boom (hereafter, Trad-MCN) as a short-term indicator of genotoxic response and tree barks to quantify the accumulation of traffic-related chemical elements as a long-term biomarker of air pollution in urban gardens. Mature plants of Tradescantia pallida were exposed in each garden, and their inflorescences were sampled over three months. A random set of 300 early tetrads in 13 to 21 slides per garden were evaluated for micronuclei frequencies. Elemental concentrations in 428 tree barks samples from 107 different trees in the areas surrounding urban gardens were quantified using an energy dispersive X-ray fluorescence spectrometer. The frequency of Trad-MCN has a significant correlation with traffic variables and chemical elements related to road dust and tailpipe emissions deposited in tree barks. Negative associations between Trad-MCN and both the distance through traffic and the presence of vertical obstacles were observed in the community gardens. The Mn/Zn concentrations in tree barks were associated with increased Trad-MCN.

The influence of atmospheric particles on the elemental content of vegetables in urban gardens of Sao Paulo, Brazil

Although urban horticulture provides multiple benefits to society, the extent to which these vegetables are contaminated by the absorption of chemical elements derived from atmospheric deposition is unclear. This study was designed to evaluate the influence of air pollution on leafy vegetables in community gardens of Sao Paulo, Brazil. Vegetable seedlings of Brassica oleracea var. acephala (collard greens) and Spinacia oleracea (spinach) obtained in a non-polluted rural area and growing in vessels containing standard uncontaminated soil were exposed for three consecutive periods of 30, 60 and 90 days in 10 community gardens in Sao Paulo and in one control site. The concentrations of 17 chemical elements (traffic-related elements and those essential to plant biology) were quantified by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Tillandsia usneoides L. specimens were used as air plant biomonitors. The concentrations of As, Cd, Cr and Pb found in vegetables were compared to the recommended values for consumption. Principal Component Analysis (PCA) was used to cluster the elemental concentrations, and Generalized Linear Models (GLMs) were employed to evaluate the association of the factor scores from each PCA component with variables such as local weather, traffic burden and vertical barriers adjacent to the gardens. We found significant differences in the elemental concentrations of the vegetables in the different community gardens. These differences were related to the overall traffic burden, vertical obstacles and local weather. The Pb and Cd concentrations in both vegetables exceeded the limit values for consumption after 60 days of exposure. A strong correlation was observed between the concentration of traffic-related elements in vegetables and in Tillandsia usneoides L. An exposure response was observed between traffic burden and traffic-derived particles absorbed in the vegetables. Traffic-derived air pollution directly influences the absorption of chemical elements in leafy vegetables, and the levels of these elements may exceed the recommended values for consumption.

Potential of Opuntia ficus-indica for air pollution biomonitoring: a lead isotopic study

Opuntia ficus-indica (Ofi) is a long-domesticated cactus that is widespread throughout arid and semiarid regions. Ofi is grown for both its fruits and edible cladodes, which are flattened photosynthetic stems. Young cladodes develop from mother cladodes, thus forming series of cladodes of different ages. Therefore, successive cladodes may hold some potential for biomonitoring over several years the local atmospheric pollution. In this study, cladodes, roots, dust deposited onto the cladodes, and soil samples were collected in the vicinity of three heavily polluted sites, i.e., a fertilizer industry, the road side of a highway, and mine tailings. The lead content was analyzed using atomic absorption spectroscopy (AAS) and inductively coupled plasma-mass spectrometry (ICP-MS). Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) was used to characterize the cladode surfaces and the nature of dust deposit, and the lead isotopes were analyzed to identify the origin of Pb. The results show that (i) Ofi readily bioaccumulates Pb, (ii) the lead isotopic composition of cladodes evidences a foliar pathway of lead into Ofi and identifies the relative contributions of local Pb sources, and (iii) an evolution of air quality is recorded with successive cladodes, which makes Ofi a potential biomonitor to be used in environmental and health studies.