Riparian trees in mercury contaminated riverbanks: An important resource for sustainable remediation management.

Mining operations generate sediment erosion rates above those of natural landscapes, causing persistent contamination of floodplains. Riparian vegetation in mine-impacted river catchments plays a key role in the storage/remobilization of metal contaminants. Mercury (Hg) pollution from mining is a global environmental challenge. This study provides an integrative assessment of Hg storage in riparian trees and soils along the Paglia River (Italy) which drains the abandoned Monte Amiata Hg mining district, the 3rd former Hg producer worldwide, to characterize their role as potential secondary Hg source to the atmosphere in case of wildfire or upon anthropic utilization as biomass. In riparian trees and nearby soils Hg ranged between 0.7 and 59.9 µg/kg and 2.2 and 52.8 mg/kg respectively. In trees Hg concentrations were below 100 µg/kg, a recommended Hg limit for the quality of solid biofuels. Commercially, Hg contents in trees have little impact on the value of the locally harvested biomass and pose no risk to human health, although higher values (195-738 µg/kg) were occasionally found. In case of wildfire, up to 1.4*10-3 kg Hg/ha could be released from trees and 27 kg Hg/ha from soil in the area, resulting in an environmentally significant Hg pollution source. Data constrained the contribution of riparian trees to the biogeochemical cycling of Hg highlighting their role in management and restoration plans of river catchments affected by not-remediable Hg contamination. In polluted river catchments worldwide riparian trees represent potential sustainable resources for the mitigation of dispersion of Hg in the ecosystem, considering i) their Hg storage capacity, ii) their potential to be used for local energy production (e.g. wood-chips) through the cultivation and harvesting of biomasses and, iii) their role in limiting soil erosion from riparian polluted riverbanks, probably representing the best pragmatic choice to minimize the transport of toxic elements to the sea.

Pinus nigra bark from a mercury mining district studied with high resolution XANES spectroscopy.

Tree bark near former mercury (Hg) mines and roasting plants is known to have exceptionally high (up to several mg kg-1) Hg concentrations. This study explores the change of Hg speciation with depth (down to 25-30 mm from the outermost surface) in black pine (Pinus nigra) bark by means of high-resolution X-ray absorption near edge structure (HR-XANES) spectroscopy at the Hg LIII-edge. Principal component analysis and linear combination fitting applied to the HR-XANES spectra suggested that in the outermost layer (∼0-2 mm from the surface), roughly 50% of Hg is in the form of nanoparticulate metacinnabar (nano-ß-HgS). A progressive increase in Hg-organic species (Hg bound to thiol groups) is found in deeper bark layers, while nano-ß-HgS may decrease below the detection limit in the deepest layers. Notably, bark layers did not contain cinnabar (α-HgS), which was found in the nearby soils along with ß-HgS (bulk), nor Hg0, which is the main Hg species in the atmosphere surrounding the sampled trees. These observations suggested that nano-ß-HgS, at least in part, does not originate from mechanically trapped wind-blown particulates from the surrounding soil, but may be the product of biochemical reactions between gaseous elemental Hg and the bark tissue.

Total Mercury Mass Load from the Paglia-Tiber River System: The Contribution to Mediterranean Sea Hg Budget.

The Mediterranean Sea is characterized by a marked mercury (Hg) geochemical anomaly, arising in part from large Hg deposits. Mercury mass loads discharged from the Monte Amiata mining district (Central Italy) to the Mediterranean Sea through the Paglia-Tiber River system were estimated. Data from two seasons showed that up to 40 kg year-1 of Hg are drained to Tiber River and finally to the Mediterranean Sea. The mercury mass loads varied in different seasons, from 3 mg day-1 in the upper section of Paglia River in November to 42 g day-1 before the confluence with Tiber River in June. Along Tiber River, up to 15 ng L-1 of the total Hg found at a site after Rome showed that Hg can be discharged to the sea. The Alviano reservoir along Tiber River acts as a temporary trap for Hg-rich particulate, while dam operations may promote Hg release (up to 223 g day-1). The combination of hydrologic factors controlling Hg transport, the torrential regime in the upper catchment of Paglia River, the waterway steepness, together with Hg-contaminated legacy sediments in the Paglia River floodplain, make the Paglia-Tiber River system a long-lasting intermittent source of Hg to Tiber River and the Mediterranean Sea.

Occurrence and Quantification of Natural and Microplastic Items in Urban Streams: The Case of Mugnone Creek (Florence, Italy).

The terrestrial environment is an important contributor of microplastics (MPs) to the oceans. Urban streams, strictly interwoven in the city network and to the MPs' terrestrial source, have a relevant impact on the MP budget of large rivers and, in turn, marine areas. We investigated the fluxes (items/day) of MPs and natural fibers of Mugnone Creek, a small stream crossing the highly urbanized landscape of Florence (Italy) and ending in the Arno River (and eventually to the Tyrrhenian Sea). Measurements were done in dry and wet seasons for two years (2019-2020); stream sediments were also collected in 2019. The highest loads of anthropogenic particles were observed in the 2019 wet season (109 items/day) at the creek outlet. The number of items in sediments increased from upstream (500 items/kg) to urban sites (1540 items/kg). Fibers were the dominant shape class; they were mostly cellulosic in composition. Among synthetic items, fragments of butadiene-styrene (SBR), indicative of tire wear, were observed. Domestic wastewater discharge and vehicular traffic are important sources of pollution for Mugnone Creek, especially during rain events. The study of small creeks is of pivotal importance to limit the availability of MPs in the environment.

Monitoring of Airborne Mercury: Comparison of Different Techniques in the Monte Amiata District, Southern Tuscany, Italy.

In the present study, mercury (Hg) concentrations were investigated in lichens (Flavoparmelia caperata (L.) Hale, Parmelia saxatilis (L.) Ach., and Xanthoria parietina (L.) Th.Fr.) collected in the surrounding of the dismissed Abbadia San Salvatore Hg mine (Monte Amiata district, Italy). Results were integrated with Hg concentrations in tree barks and literature data of gaseous Hg levels determined by passive air samplers (PASs) in the same area. The ultimate goal was to compare results obtained by the three monitoring techniques to evaluate potential mismatches. Lichens displayed 180-3600 ng/g Hg, and Hg concentrations decreased exponentially with distance from the mine. Mercury concentration was lower than in Pinus nigra barks at the same site. There was a moderate correlation between Hg in lichen and Hg in bark, suggesting similar mechanisms of Hg uptake and residence times. However, correlation with published gaseous Hg concentrations (PASs) was moderate at best (Kendall Tau = 0.4-0.5, p > 0.05). The differences occurred because a) PASs collected gaseous Hg, whereas lichens and barks also picked up particulate Hg, and b) lichens and bark had a dynamic exchange with the atmosphere. Lichen, bark, and PAS outline different and complementary aspects of airborne Hg content and efficient monitoring programs in contaminated areas would benefit from the integration of data from different techniques.

Evidence for the natural origins of anomalously high chromium levels in soils of the Cecina Valley (Italy).

The problem of high levels of chromium is one of the most important issues in soils of the Mediterranean area, in particular those deriving from ophiolitic parent materials. Very often the chromium concentration is greater than the threshold values of legislation on soil pollution and the knowledge of the origin of contamination (natural or anthropogenic) is important to formulate risk characterization. This study evaluated the soils from three coastal areas of the Cecina Valley (Tuscany, Italy) to understand the origin of chromium in the soils, where high levels of hexavalent chromium were found in well and spring waters of the areas. The main soil characteristics and the correlations among the values of chromium and nickel were determined. Chromium speciation was evaluated by synchrotron radiation X-ray absorption spectroscopy. The results showed the presence of only trivalent chromium in soil and a positive linear correlation between chromium and nickel (e.g. r = 0.76 for the Marina di Bibbona-Bolgheri area), corroborating the hypothesis of a geogenic origin of contamination. This hypothesis was also supported by the low CRI index for the soils with high total Cr content, indicating a higher presence of refractory minerals in the Marina di Bibbona-Bolgheri area than Cecina and Collemezzano areas. The refractory material found in soils was attributed to the presence of ophiolite outcrops in the surroundings and their sedimentary remnants. The weathering of ultramafic-derived constituents and their regional-scale transport are believed to be responsible for the enrichment of chromium and nickel in the investigated soils.

Mercury speciation in Pinus nigra barks from Monte Amiata (Italy): An X-ray absorption spectroscopy study.

This study determined, by means of X-ray absorption near-edge structure (XANES) spectroscopy, the speciation of mercury (Hg) in black pine (Pinus nigra) barks from Monte Amiata, that were previously shown to contain exceptionally high (up to some mg kg-1) Hg contents because of the proximity to the former Hg mines and roasting plants. Linear fit combination (LCF) analysis of the experimental spectra compared to a large set of reference compounds showed that all spectra can be fitted by only four species: ß-HgS (metacinnabar), Hg-cysteine, Hg bound to tannic acid, and Hg0. The first two are more widespread, whereas the last two occur in one sample only; the contribution of organic species is higher in deeper layers of barks than in the outermost ones. We interpret these results to suggest that, during interaction of barks with airborne Hg, the metal is initially mechanically captured at the bark surface as particulate, or physically adsorbed as gaseous species, but eventually a stable chemical bond is established with organic ligands of the substrate. As a consequence, we suggest that deep bark Hg may be a good proxy for long term time-integrated exposure, while surface bark Hg is more important for recording short term events near Hg point sources.

Black pine (Pinus nigra) barks as biomonitors of airborne mercury pollution.

Tree barks are relevant interfaces between plants and the external environment, and can effectively retain airborne particles and elements at their surface. In this paper we have studied the distribution of mercury (Hg) in soils and in black pine (Pinus nigra) barks from the Mt. Amiata Hg district in southern Tuscany (Italy), where past Hg mining and present-day geothermal power plants affect local atmospheric Hg concentration, posing serious environmental concerns. Barks collected in heavily Hg-polluted areas of the district display the highest Hg concentration ever reported in literature (8.6mg/kg). In comparison, barks of the same species collected in local reference areas and near geothermal power plants show much lower (range 19-803µg/kg) concentrations; even lower concentrations are observed at a "blank" site near the city of Florence (5-98µg/kg). Results show a general decrease of Hg concentration from bark surface inwards, in accordance with a deposition of airborne Hg, with minor contribution from systemic uptake from soils. Preliminary results indicate that bark Hg concentrations are comparable with values reported for lichens in the same areas, suggesting that tree barks may represent an additional useful tool for biomonitoring of airborne Hg.

Binding of bis-(2-ethylhexyl) phthalate at the surface of hydrozincite nanocrystals: An example of organic molecules absorption onto nanocrystalline minerals.

As a contribution to understand the interactions between mineral surfaces and organic molecules, this study reports an accurate characterization of the bis-(2-ethylhexyl) phthalate (DEHP)-Hydrozincite (DEHP-HY), that has been conduced combining the following techniques: FTIR, NMR, XAS spectroscopies and XRD. XRD patterns indicate that the HY is made of nanocrystals whose size is not influenced by the presence of DEHP. The (1)H NMR analysis of DEHP-HY samples points out the presence of interactions of DEHP with HY. CPMAS NMR analysis suggests that the interaction is operated by ester carbonyl groups while the aliphatic chain, as expected, is not involved. MAS and CPMAS NMR measurements, performed on (13)C ester carbonyl enriched DEHP, allow to demonstrate that there are two ester carbonyl linkage sites interacting at the HY surface: an acid site with a strong link and a second one with weak chemical interactions. Zn K-edge XAS spectroscopy demonstrates that the local atomic structure around Zn in DEHP-HY sample remains essentially unchanged with respect to that of HY. Such a weak structural effect suggests that HY interaction with DEHP is limited to the nanoparticle surface.

Microscopic processes ruling the bioavailability of Zn to roots of Euphorbia pithyusa L. pioneer plant.

Euphorbia pithyusa L. was used in a plant growth-promoting assisted field trial experiment. To unravel the microscopic processes at the interface, thin slices of E. pithyusa roots were investigated by micro-X-ray fluorescence mapping. Roots and rhizosphere materials were examined by X-ray absorption spectroscopy at the Zn K-edge, X-ray diffraction, and scanning electron microscopy. Results indicate some features common to all the investigated samples. (i) In the rhizosphere of E. pithyusa, Zn was found to exist in different phases. (ii) Si and Al are mainly concentrated in a rim at the epidermis of the roots. (iii) Zn is mostly stored in root epidermis and does not appear to be coordinated to organic molecules but mainly occurs in mineral phases such as Zn silicates. We interpreted that roots of E. pithyusa significantly promote mineral evolution in the rhizosphere. Concomitantly, the plant uses Si and Al extracted by soil minerals to build a biomineralization rim, which can capture Zn. This Zn silicate biomineralization has relevant implications for phytoremediation techniques and for further biotechnology development, which can be better designed and developed after specific knowledge of molecular processes ruling mineral evolution and biomineralization processes has been gained.

Long-distance transport of Hg, Sb, and As from a mined area, conversion of Hg to methyl-Hg, and uptake of Hg by fish on the Tiber River basin, west-central Italy.

Stream sediment, stream water, and fish were collected from a broad region to evaluate downstream transport and dispersion of mercury (Hg) from inactive mines in the Monte Amiata Hg District (MAMD), Tuscany, Italy. Stream sediment samples ranged in Hg concentration from 20 to 1,900 ng/g, and only 5 of the 17 collected samples exceeded the probable effect concentration for Hg of 1,060 ng/g, above which harmful effects are likely to be observed in sediment-dwelling organisms. Concentrations of methyl-Hg in Tiber River sediment varied from 0.12 to 0.52 ng/g, and although there is no established guideline for sediment methyl-Hg, these concentrations exceeded methyl-Hg in a regional baseline site (<0.02 ng/g). Concentrations of Hg in stream water varied from 1.2 to 320 ng/L, all of which were below the 1,000 ng/L Italian drinking water Hg guideline and the 770 ng/L U.S. Environmental Protection Agency (USEPA) guideline recommended to protect against chronic effects to aquatic wildlife. Methyl-Hg concentrations in stream water varied from <0.02 to 0.53 ng/L and were generally elevated compared to the baseline site (<0.02 ng/L). All stream water samples contained concentrations of As (<1.0-6.2 µg/L) and Sb (<0.20-0.37 µg/L) below international drinking water guidelines to protect human health (10 µg/L for As and 20 µg/L for Sb) and for protection against chronic effects to aquatic wildlife (150 µg/L for As and 5.6 µg/L for Sb). Concentrations of Hg in freshwater fish muscle ranged from 0.052-0.56 µg/g (wet weight), mean of 0.17 µg/g, but only 17 % (9 of 54) exceeded the 0.30 µg/g (wet weight) USEPA fish muscle guideline recommended to protect human health. Concentrations of Hg in freshwater fish in this region generally decreased with increasing distance from the MAMD, where fish with the highest Hg concentrations were collected more proximal to the MAMD, whereas all fish collected most distal from Hg mines contained Hg below the 0.30 µg/g fish muscle guideline. Data in this study indicate some conversion of inorganic Hg to methyl-Hg and uptake of Hg in fish on the Paglia River, but less methylation of Hg and Hg uptake by freshwater fish in the larger Tiber River.

Arsenic-bearing calcite in natural travertines: evidence from sequential extraction, µXAS, and µXRF.

Recent studies demonstrated that synthetic calcite may host considerable amounts of arsenic (As). In this paper, the concentration of As in natural calcite was determined using two novel, specifically designed, sequential extraction procedures. In addition, the oxidation state of As and its distribution between calcite and coexisting Fe-oxyhydroxides was unravelled by µXRF elemental mapping and As K-edge µXAS spectroscopy. Our results conclusively demonstrate that arsenic can be found in natural calcite up to 2 orders of magnitude over the normal crustal As abundances. Because of the large diffusion of calcite in the environment, this phase may exert an important control on As geochemistry, mobility, and bioavailability.

Concentration, distribution, and translocation of mercury and methylmercury in mine-waste, sediment, soil, water, and fish collected near the Abbadia San Salvatore mercury mine, Monte Amiata district, Italy.

The distribution and translocation of mercury (Hg) was studied in the Paglia River ecosystem, located downstream from the inactive Abbadia San Salvatore mine (ASSM). The ASSM is part of the Monte Amiata Hg district, Southern Tuscany, Italy, which was one of the world's largest Hg districts. Concentrations of Hg and methyl-Hg were determined in mine-waste calcine (retorted ore), sediment, water, soil, and freshwater fish collected from the ASSM and the downstream Paglia River. Concentrations of Hg in calcine samples ranged from 25 to 1500 µg/g, all of which exceeded the industrial soil contamination level for Hg of 5 µg/g used in Italy. Stream and lake sediment samples collected downstream from the ASSM ranged in Hg concentration from 0.26 to 15 µg/g, of which more than 50% exceeded the probable effect concentration for Hg of 1.06 µg/g, the concentration above which harmful effects are likely to be observed in sediment-dwelling organisms. Stream and lake sediment methyl-Hg concentrations showed a significant correlation with TOC indicating considerable methylation and potential bioavailability of Hg. Stream water contained Hg as high as 1400 ng/L, but only one water sample exceeded the 1000 ng/L drinking water Hg standard used in Italy. Concentrations of Hg were elevated in freshwater fish muscle samples and ranged from 0.16 to 1.2 µg/g (wet weight), averaged 0.84 µg/g, and 96% of these exceeded the 0.3 µg/g (methyl-Hg, wet weight) USEPA fish muscle standard recommended to protect human health. Analysis of fish muscle for methyl-Hg confirmed that >90% of the Hg in these fish is methyl-Hg. Such highly elevated Hg concentrations in fish indicated active methylation, significant bioavailability, and uptake of Hg by fish in the Paglia River ecosystem. Methyl-Hg is highly toxic and the high Hg concentrations in these fish represent a potential pathway of Hg to the human food chain.

Uptake of Pb by hydrozincite, Zn5(CO3)2(OH)6--implications for remediation.

Hydrozincite, Zn(5)(CO(3))(2)(OH)(6), periodically precipitates from heavy metal contaminated waters of the Rio Naracauli stream, Sardinia, in association with a biological photosynthetic community. The precipitation removes not only zinc from the waters, but also other toxic "heavy metals", such as Cd, Cu, Pb. The phenomenon is therefore of potential interest for "soft" remediation of contaminated waters. Previous cation exchange experiments suggested that binding of Pb to hydrozincite is fairly strong. This suggestion is in agreement with new release tests in deionized water and X-ray absorption spectroscopy (XAS) spectra collected at the Pb L(III) edge for natural hydrozincites from Naracauli, and synthetic Pb-doped hydrozincites. The results suggest that, up to bulk concentration of 1.5 wt.% Pb, uptake of this metal occurs in two distinct ways: (1) as a substitution for Zn in the tetrahedral site of the hydrozincite structure, possibly via formation of a surface mononuclear tridentate inner sphere complex; (2) as an ill-defined, presumably amorphous, phase with a local atomic structure similar to cerussite. These data support the concept that Pb binding to hydrozincite is strong enough to make this mineral a potential sink for the metal.