Bioavailability Assessment of Heavy Metals Using Various Multi-Element Extractants in an Indigenous Zinc Smelting Contaminated Site, Southwestern China

Despite recent studies have investigated the strong influences of smelting activities on heavy metal contamination in the soil environment, little studies have been conducted on the current information about the potential environmental risks posed by toxic heavy metals in smelting contaminated sites. In the present study, a combination of the bioavailability, speciation, and release kinetics of toxic heavy metals in the indigenous zinc smelting contaminated soil were reliably used as an effective tool to support site risk assessment. The bioavailability results revealed that the bioavailable metal concentrations were intrinsically dependent on the types of chemical extractants. Interestingly, 0.02 mol/L EDTA + 0.5 mol/L CH3COONH4 was found to be the best extractant, which extracted 30.21% of Cu, 31.54% of Mn, 2.39% of Ni and 28.89% of Zn, respectively. The sequential extraction results suggested that Cd, Pb, and Zn were the most mobile elements, which would pose the potential risks to the environment. The correlation of metal bioavailability with their fractionation implied that the exchangeable metal fractions were easily extracted by CaCl2 and Mehlich 1, while the carbonate and organic bound metal fractions could be extracted by EDTA and DTPA with stronger chelating ability. Moreover, the kinetic modeling results suggested that the chemical desorption mechanism might be the major factor controlling heavy metal release. These results could provide some valuable references for the risk assessment and management of heavy metals in the smelting contaminated sites.

Biochemical Responses of Medicinal Plant Tussilago farfara L. to Elevated Heavy Metal Concentrations in Soils of Urban Areas

This study was conducted in Tyumen (Russian Federation) to establish the effects of heavy metals’ (Cu, Zn, Fe, Mn, Pb, and Cd) accumulation in soil and coltsfoot, as well as plants’ biochemical responses to such an accumulation. The mobile and acid-soluble heavy metal fractions in soils, and the heavy metal contents in plants, were determined by atomic absorption spectrophotometry. The Cu, Zn, Fe, Mn, and Pb concentrations in soils exceeded background values. Pb content at the battery manufacturing plant was above the maximum permitted concentration. The percentages of the mobile heavy metal fractions decreased in the following order: Mn > Zn > Cu > Fe. The greatest heavy metal accumulation in soils and plants was found at the battery manufacturing and metallurgical plants examined in our study. Heavy metals’ accumulation in the aboveground part of Tussilago farfara decreased in the following order: Fe > Zn > Cu > Mn > Pb > Cd. The accumulation of heavy metals stimulated the synthesis of photosynthetic pigments by 6–30%. Heavy metals provoked oxidative stress in cells, increasing the concentration of lipid peroxidation in products by up to 80%. Plant phenolics and flavonoids in the urban area of our study decreased compared to those in the control by 1.05, reaching up to 6.5 times. The change in coltsfoot catalase activity both increased and declined. Biochemical responses and heavy metal accumulation in coltsfoot from urban areas limit its use for medicinal purposes.

Heat-treated serpentine-reached materials: application for sorption of heavy metals and remediation of industrially polluted peat soil

<p>Serpentine minerals are widely distributed in the Earth’s crust, forming in some provinces with specific vegetation. Like clay minerals, serpentine minerals can be referred to as eco-friendly materials and can be used for the sorption of heavy metals in contaminated soil. The sorption of metals by serpentine minerals can occur by adsorption on the surface, entering into the mineral’s structure, and the precipitation of low-soluble compounds in an alkaline environment. It is possible to intensify these processes by modifying serpentines, namely by heat treatment. Our study used two types of serpentine-reached materials from mining wastes: ortho-chrysotile from overburden rocks of Khalilovsky magnesite deposit (Cht) and lizardite from host rocks of Khabozersky olivine deposit (Lt) (Russia), thermally activated in a tube furnace at 650-750 ºC.</p><p>The process of hydration occurs in the field conditions when serpentine interacts with soil solutions. Therefore, the process of nickel sorption by Cht and hydrated Cht was studied. Results indicated the formation of magnesium silicates during hydration. These chemical compounds were found to be more stable than components of initial Cht (test for leaching in 1N ammonium acetate solution, pH 4.68). Hydration of Cht reduced the activity of nickel sorption processes in the initial period of interaction. However, the nickel sorption value of hydrated Cht eventually was similar to the initial Cht when reactive phases’ contact increased up to 30 days.</p><p>In the field experiment, the topsoil (0-5 cm) of industrially polluted peat near the active Cu/Ni plant (Murmansk region, Russia) was mixed with Cht and Lt in 3:1 proportion. Initial polluted peat contained more than 500 mg/kg of exchangeable Ni and 6300 mg/kg of Cu. After eight years of the experiment in conditions of continuing aerial metal emissions, the concentration of exchangeable metal fractions in soil mixtures was lower than in peat soil by 3-5 times for Cu and by 1.3 times for Ni. Simultaneously, the concentration of immobile metal fractions (bound by organic matter, Fe/Mn (hydr)oxides, and included in other insoluble compounds) was 1.5 times higher than in peat soil. The lack of nutrients (mostly Mg and Ca) in the polluted soil causes vegetation degradation in the smelter’s impact zone. Soil mixed with heat-treated serpentine minerals led to increased plant-available Mg compounds (by 11-42 times) and Ca (by 2.6-4.4 times). These findings indicate the fixation of metal pollutants by heat-treated serpentine minerals and soil enrichment in essential elements. The use of the heat-treated serpentine-reached materials is promising for the long-term decrease of metal mobility and remediation of industrially polluted soils.</p><p>The research was conducted with the support of the Russian Science Foundation grant 19-77-00077.</p>

Heavy metal translocation in soil-plant system in conditions of urban anthropogenic pollution (Tyumen, Russian Federation)

<p>Soil contamination by heavy metals causes metal accumulation by plants, which leads to the degradation of plants communities and migration of toxicants with food chains to man. Therefore, the investigation of heavy metal concentration in soils of urban areas is an urgent scientific task. This study aims to examine the translocation of heavy metals from urban soils to herbs in Tyumen (Russian Federation).  Soil surface layer was collected at control site, near the highway as well as from areas with metallurgical, motor building, oil refinery and battery manufactory plants in Tyumen. Meadow grass, red clover, wild vetch, chamomile and coltsfoot were collected at all examined sites.  The mobile and acid-soluble heavy metal fractions in soils, as well as the heavy metal contents in plants, were determined by atomic absorption spectrophotometry. The bioconcentration factor was estimated as the ratio of the amount of heavy metals in soils to that in plants. The study was performed during three-year period from 2017 to 2019. Heavy metal concentrations in urban soils were higher than those at the control site by 20% and by up to 10 times. The greatest heavy metal accumulation in both soils and plants was found at the battery manufacturing and metallurgical plants, exceeding the control levels of Pb and Fe by 2-17 times. The Cu, Fe and Mn contents in soil were positively correlated with those in plants. Heavy metal translocation by the plants was species-specific. The percentages of the mobile heavy metal fractions decreased in the following order: Mn>Zn>Cu>Fe. Heavy metal accumulation in plants in the urban sites compared to that at the control site decreased in the following order: Fe>Zn>Cu>Mn>Pb>Cd. Coltsfoot exhibited the highest Fe, Mn, and Zn accumulation, which exceeded the control levels by 17, 5, and 3.5 times, respectively. The heavy metal bioconcentration factors, indicators of translocation, decreased in the following order: Cu>Zn>Cd>Pb>Mn>Fe. The heavy metal translocation suggests the need to relocate industrial facilities to outside the city. Future monitoring of the study area is needed to ensure its long-term ecological safety.</p>

Elimination of casting defects using epoxy resin

The article considers a method for eliminating casting defects using Permabond ES558 metal-filled epoxy resin. This method refers to modern and progressive methods of restoring the surfaces of metal castings. The Permabond ES558 epoxy resin is a silver-gray paste with inclusions of metal fractions to increase the hardness after temperature curing. When heated, the resin spreads evenly filling the cavity of the shell. After the composition has fully cured, the restored surface can be machined by grinding.