Effect of soil additives on biogeochemistry of ultramafic soils-an experimental approach with Brassica napus L.

Ultramafic soils are characterized by low productivity due to the deficiency of macroelements and high content of Ni, Cr, and Co. Incorporation of ultramafic soils for agricultural and food production involves the use of fertilizers. Therefore, this study aims to find the soil additive that decreases the metallic elements uptake by plant using Brassica napus as an example. In this study, we evaluate the effect of manure (0.5 g N/kg of soil), humic acids (1 g of Rosahumus/1 dm3 H2O; 44% C), KNO3 (0.13 g K/kg of soil), lime (12.5 g/kg of soil), (NH4)2SO4 (0.15 g N/kg of soil), and Ca(H2PO4)2) (0.07 g P/kg of soil) on the phytoavailability of metallic elements. The effect of soil additives on metallic elements uptake by Brassica napus was studied in a pot experiment executed in triplicates. Statistical analysis was applied to compare the effects of additives in ultramafic soil on plant chemical composition relative to control unfertilized ultramafic soil (one-way ANOVA and Kruskal-Wallis test). The study shows that in almost all treatments, metallic elements content (Ni, Cr, Co, Al, Fe, Mn) is higher in roots compared to the aboveground parts of Brassica napus except for (NH4)2SO4, in which the mechanism of Mn accumulation is opposite. The main differences between the treatments are observed for the buffer properties of soil and the accumulation of specific metals by studied plants. The soils with the addition of lime and manure have the highest buffer properties in acidic conditions (4.9-fold and 2.1-fold increase relative to control soil, respectively), whereas the soil with (NH4)2SO4 has the lowest effect (0.8-fold decrease relative to control soil). Also, the addition of manure increases the biomass of aboveground parts of B. napus (3.4-fold increase) and decreases the accumulation of Ni (0.6-fold decrease) compared to plants cultivated in the control soil. On the contrary, the addition of (NH4)2SO4 noticeably increases the accumulation of Ni, Co, Mn, and Al in aboveground parts of B. napus (3.2-fold, 18.2-fold, 11.2-fold, and 1.6-fold, respectively) compared to plant grown in control soil, whereas the humic acids increase the accumulation of Cr in roots (1.6-fold increase). Therefore, this study shows that manure is a promising fertilizer in agricultural practices in ultramafic soil, whereas (NH4)2SO4 and humic acids must not be used in ultramafic areas.

Bacterial leaching of critical metal values from Polish copper metallurgical slags using Acidithiobacillus thiooxidans.

Global economy faces an increasing problem of the supply risk of critical raw materials, therefore the search for secondary resources has become an urgent issue. Copper orebodies in Poland contain substantial amounts of metals deemed critical (e.g. Co, Mo, rare earth elements (REE) or V), which are not recovered during processing. The metals of interest are concentrated in metallurgical waste residues that should be classified as a secondary resource rather than as a waste. Bioleaching is a frontier technology promising for environment-friendly treatment of slags. Therefore, the objective of this work was to study the feasibility of metal (Co, Mo, REE, V) bioleaching from copper metallurgical wastes employing Acidithiobacillus thiooxidans bacterial strain as the leaching agent. The effect of particle size (fractions <0.25 mm and 0.25-0.5 mm) and pulp density (1%, 2%) was studied using three different slag samples (lead slag - LS, shaft furnace slag - SFS and granulated slag - GS). The bioleaching experiment was set up for 28 days under acidic conditions (pH t0 = 2.5). The results revealed that the microorganisms can catalyze metal extraction from slags as compared to leaching achieved under abiotic conditions. The optimal bioleaching yield was achieved under conditions at 0.25-0.5 mm particle size and 1% pulp density, regardless of used type of slag. After 28 days, the extracted amounts of metals were: 88% Co, 40% Mo, 83% REE and 55% V from LS, 100% Co, 44% Mo, 70% REE and 70% V from SFS and 95% Co, 70% Mo, 99% REE and 93% V from GS. All examined slags showed good potential for bioleaching of valuable elements. However, optimization of initial parameters is still needed for further efficiency improvement, especially in terms of the process duration.

Perspectives regarding the use of metallurgical slags as secondary metal resources - A review of bioleaching approaches.

Smelting activity by its very nature produces large amounts of metal-bearing waste, often called metallurgical slag(s). In the past, industry used to dispose of these waste products at dumping sites without the appropriate environmental oversight. Once there, ongoing biogeochemical processes affect the stability of the slags and cause the release of metallic contaminants. Rather than viewing metallurgical slags as waste, however, such deposits should be viewed as secondary metal resources. Metal bioleaching is a "green" treatment route for metallurgical slags, currently being studied under laboratory conditions. Metal-laden leachates obtained at the bioleaching stage have to be subjected to further recovery operations in order to obtain metal(s) of interest to achieve the highest levels of purity possible. This perspective paper considers the feasibility of the reuse of base-metal slags as secondary metal resources. Special focus is given to current laboratory bioleaching approaches and associated processing obstacles. Further directions of research for development of more efficient methods for waste slag treatment are also highlighted. The optimized procedure for slag treatment is defined as the result of this review and should include following steps: i) slag characterization (chemical and phase composition and buffering capacity) following the choice of initial pH, ii) the choice of particle size, iii) the choice of the liquid-to-solid ratio, iv) the choice of microorganisms, v) the choice of optimal nutrient supply (growth medium composition). An optimal combination of all these parameters will lead to efficient extraction and generation of metal-free solid residue.

Weathering of historical copper slags in dynamic experimental system with rhizosphere-like organic acids.

This study was undertaken to simulate experimentally the weathering of slags disposed nearby soil rhizosphere. The aim of the research was to differentiate the effect of pH and organics on slags dissolution as well as to indicate weathering sequence of phase components. The studied slags are mainly composed of Fe (34.5 wt%) and Si (17.9 wt%) and contain up to 3761 mg kg-1 of Cu and 3628 mg kg-1 of Zn. The main identified phases are fayalite and glass, whereas sulfides and metallic Cu are volumetrically minor. A 30 days long slag weathering experiment was carried out with artificial root exudates (43.7 mM) and demineralized water at initial pH = 3.5 and pH = 6.7. The highest metal release (up to 10.9% of Zn and 4.6% of Cu) was observed in ARE solution at initial pH 3.5. Dissolution of sulfides and fayalite was mainly driven by pH. Artificial root exudates enhance glass dissolution as compared to demineralized water regardless of initially fixed pH. Based on this study following weathering sequences are delineated: i) under ARE 3.5 conditions: silicates > glass > sulfides, ii) under DW 3.5 conditions: sulfides > silicates > glass, iii) under near-neutral conditions: sulfides > glass > silicates.

Microbial influence and dynamics of metallurgical waste dissolution in a landfill and recovery context: A multi-phase experimental approach and geochemical model.

The effects of abiotic and biotic dissolution on metallurgical wastes (slag and matte) were examined combining a multi-phase experimental approach with a solution renewal and geochemical model. The conditions studied included the exposure of solid wastes to water, a growth medium, and heterotrophic siderophore-producing bacterium Pseudomonas fluorescens. Quantification of the dissolution was performed by tracking the element release, whereas immobilization processes were predicted by applying a geochemical model and scanning electron microscopic observations. The results demonstrated that the phase composition of these wastes subjected to (bio)dissolution was linked to the behavior of Pseudomonas fluorescens, with the deteriorative dissolution effects varying between slag and matte. The contribution level of the bacteria to the mobilization of the elements exhibited the following order: Cu > Pb > Zn; however, the entrapment of these elements in biomass affected the amount of metals released. Copper-bearing sulfides and metallic lead-bearing phases were specifically found to be highly susceptible to (bio)dissolution. Moreover, the bacteria contribution to the Zn release from sphalerite played less of an important role compared to its abiotic mobilization. Overall, this study underlines the important role of bacteria at the interface where wastes are exposed to weathering, resulting in accelerated dissolution. The application of bacteria to Co and Mo recovery is an important approach to the detoxification of landfilled wastes.

Drivers of Pb, Sb and As release from spent gunshot in wetlands: Enhancement by organic matter and native microorganisms.

In many countries the use of lead-based ammunition is prevalent, and results in exposure and poisoning of waterfowl and other species of birds. In waterfowl hunting areas large quantities of spent shot may be deposited in wetland and terrestrial habitats. These pellets can undergo transformations, which are influenced by various abiotic and biotic factors. In addition to lead (Pb), other elements like antimony (Sb) and arsenic (As) can be leached from Pb shot into the environment. In vitro simulations that included organic matter and microorganisms were utilized to examine elemental leaching from gunshot. We found that leaching efficiency was the greatest in solutions rich in organic matter derived from artificial root exudates (2.69 % for Pb, 1.16 % for Sb, 1.83 % for As), while leaching efficiency was considerably lower in river water (0.04 %). In vitro simulations containing native microorganisms also exhibited greater leaching efficiency (0.49 % for Pb, 0.52 % for Sb, 1.32 % for As) than in ultrapure deionized water and river water. Surface alterations in gunshot included the formation of a weathering crust and secondary phases dominated by carbonates. Spent gunshot is a source of Pb, Sb and As in wetlands that could affect aquatic ecosystems.

Combustion wastes from thermal power stations and household stoves: A comparison of properties, mineralogical and chemical composition, and element mobilization by water and fertilizers.

Wood and coal combustion generate wastes, which may negatively influence the environment. However, studies concerning coal combustion products serving as soil additives are currently in progress. Hence, this study was conducted to compare properties (mineralogy, metallic element content, and rare earth element content) of combustion wastes of different genesis (ash and soot after wood and coal combustion in households, ash from thermal power stations) and to assess possible risk posed to the soil environment when used as soil additive. This study demonstrated the diversity of chemical and mineralogical features of ashes of household genesis originating from thermal power stations. Ash from household stoves showed a higher concentration of metallic elements (i.e., Zn) compared to those originating from thermal power stations. Antimony (Sb) content in household ash can serve as an indicator of plastic (polyethylene) combustion, which is legally prohibited. Leaching tests using water and common mineral fertilizers showed that ammonium sulfate mobilizes metallic elements (Cu, Zn, Pb) more significantly than potassium nitrate or deionized water. The leaching of metallic elements from household stove's ash certainly excludes the possibility of applying the ash as a soil additive even when the ash contains a source of beneficial elements for plants (i.e., Ca).

Spent sulfuric acid plant catalyst: valuable resource of vanadium or risky residue? Process comparison for environmental implications.

The enormous amount of spent catalysts generated worldwide may pose a risk to the environment because of their high load of metals, including vanadium. The latter may be mobilized and released to the environment if managed improperly. Moreover, the catalysts could be considered as secondary resources rather than waste. This study aimed at the efficient extraction of vanadium from spent desulfurization catalyst (SDC) from a sulfuric acid production plant. The raw SDC and the post-extraction residues were characterized in terms of their chemical and phase composition. The metal mobility from the materials was examined with both single-step and multi-step extractions. The environmental risk assessment was performed using sequential extraction. The study revealed that both tested methods (citric acid leaching and bioleaching with Acidithiobacillus thiooxidans) enable the extraction of nearly 96% of V from SDC with a simultaneous reduction of metal mobility. However, the bacterial treatment was found more suitable. The leached residue was mostly (> 90%) composed of SiO2, which makes it a potential candidate for application in construction (e.g., concrete mixtures) after additional examinations. The study highlights the need to develop a metal extraction process for SDC in a way that metal-free residue could be a final product.

Metal mobilization from metallurgical wastes by soil organic acids.

Three types of Cu-slags differing in chemical and mineralogical composition (historical, shaft furnace, and granulated slags) and a matte from a lead recovery process were studied with respect to their susceptibility to release Cu, Zn and Pb upon exposure to organic acids commonly encountered in soil environments. Leaching experiments (24-960 h) were conducted with: i) humic acid (20 mg/L) at pH t0 = 4.4, ii) fulvic acid (20 mg/L) at pH t0 = 4.4, iii) an artificial root exudates (ARE) (17.4 g/L) solution at pH t0 = 4.4, iv) ARE solution at pH t0 = 2.9 and v) ultrapure water (pH t0 = 5.6). The results demonstrated that the ARE contribute the most to the mobilization of metals from all the wastes analyzed, regardless of the initial pH of the solution. For example, up to 14%, 30%, 24% and 5% of Cu is released within 960 h from historical, shaft furnace, granulated slags and lead matte, respectively, when exposed to the artificial root exudates solution (pH 2.9). Humic and fulvic acids were found to have a higher impact on granulated and shaft furnace slags as compared to the ultrapure water control and increased the release of metals by a factor up to 37.5 (Pb) and 20.5 (Cu) for granulated and shaft furnace slags, respectively. Humic and fulvic acids amplified the mobilization of metals by a maximal factor of 13.6 (Pb) and 12.1 (Pb) for historical slag and lead matte, respectively. The studied organic compounds contributed to different release rates of metallic contaminants from individual metallurgical wastes under the conditions tested.