In Situ Control of Thermal Activation Conditions by Color for Serpentines with a High Iron Content

Serpentine heat treatment at temperatures of 650–750 °C yields magnesium–silicate reagent with high chemical activity. Precise and express control of roasting conditions in laboratory kilns and industrial aggregates is needed to derive thermally activated serpentines on a large scale. Color change in serpentines with a high iron content during roasting might be used to indicate the changes in chemical activity in the technological process. This study gives a scientific basis for the express control of roasting of such serpentines by comparing the colors of the obtained material and the reference sample. Serpentines with different chemical activity were studied by X-ray diffraction, Mössbauer spectroscopy, and optical spectroscopy. The color parameters were determined using RGB (red, green, blue), CIELAB (International Commission on Illumination 1976 L*a*b), and HSB (hue, brightness, saturation) color models. The color of heat-treated samples was found to be affected by changes in the crystallochemical characteristics of iron included in the structure of the serpentine minerals. The color characteristics given by the CIELAB model were in good coherence with the acid-neutralizing ability and optical spectra of heat-treated serpentines. Thus, in contrast to the long-term analysis by these methods, the control by color palette provides an express assessment of the quality of the resulting product.

Near-Infrared Spectroscopy Study of Serpentine Minerals and Assignment of the OH Group

Three different kinds of serpentine mineral samples were investigated using Fourier transform near-infrared spectroscopy (FTNIR). The results show that there are obvious differences in the characteristic infrared spectra of the three serpentine group minerals (lizardite, chrysotile, and antigorite), which can easily be used to identify these serpentine minerals. The combination of weak and strong peaks in the spectrum of lizardite appears at 3650 and 3690 cm−1, while the intensities of the peaks at 4281 and 4301 cm−1 (at 7233 and 7241 cm−1, respectively) are similar. A combination of weak and strong peaks in chrysotile appears at 3648 and 3689 cm−1 and at 4279 and 4302 cm−1, and a single strong peak appears at 7233 cm−1. In antigorite, there are strong single peaks at 3674, 4301, and 7231 cm−1, and the remaining peaks are shoulder peaks or are not obvious. The structural OH mainly appears as characteristic peaks in four regions, 500–720, 3600–3750, 4000–4600, and 7000–7600 cm−1, corresponding to the OH bending vibration, the OH stretching vibration, the OH secondary combination vibration, and the OH overtone vibration, respectively. In the combined frequency vibration region, the characteristic peak near 4300 cm−1 is formed by the combination of the internal and external stretching vibrations and bending vibrations of the structural OH group. The overtone vibrations of the OH stretching vibration appear near 7200 cm−1, and the practical factor is about 1.965. The near-infrared spectra of serpentine minerals are closely related to their structural differences and isomorphous substitutions. Therefore, near-infrared spectroscopy can be used to identify serpentine species and provides a basis for studies on the genesis and metallogenic environment of these minerals.

Separate deposition of metals from highly concentrated solutions with granulated magnesia-silicate reagent

Multi-stage deposition of metals from a sulfate solution with a high concentration of iron, aluminum, copper, zinc, and nickel has been studied. The concentrations of the components correspond to the composition of the sub-basement waters of the Gaisky GOK. Granular magnesia-silicate reagent based on serpentinite (Khalilovsk magnesite deposit, the Orenburg region, Russia) has been used as an alkaline agent. The magnesia-silicate reagent's ability to reduce the acidity of solutions is due to the presence of products of destruction of the original serpentine mineral, mainly magnesium oxide. The results of the solutions multi-stage purification from metals simulation have been presented. It has been found that the reagent did not wholly exhaust its activity during a single contact with the solution. Therefore, the possibility of its repeated use for the 2nd and 3rd time has been studied. As the solution is neutralized according to the known pH range of the beginning and complete deposition of metal compounds, first iron, and then aluminum are deposited. For copper and nickel, the effect of co-precipitation is observed until the pH of precipitation of poorly soluble compounds is reached. Iron is the main component of precipitations at the 1st, 2nd, and 3rd stages, which corresponds to pH = 2.4-3.7. At the 4th stage (pH = 4.0), the precipitations consisted mainly of aluminum compounds. The copper and nickel content in precipitations increase due to decreased concentration of major components (aluminum and iron) and a pH increase. The deposition of zinc from the solution occurs not to the precipitations, but on the granules surfaces. Precipitations enriched in aluminum and iron have been obtained. Sorption and co-precipitation processes have been observed for copper, zinc, and nickel, which prevents individual precipitation by these metals. Thermally activated serpentine minerals can be considered a promising alkaline reagent for technogenic solutions neutralization and purification.

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>

Serpentinization as a route to liberating phosphorus on habitable worlds

Planetary habitability is in part governed by nutrient availability, including the availability of the element phosphorus. The nutrient phosphorus plays roles in various necessary biochemical functions, and its biogeochemical cycling has been proposed to be extremely slow due to a strong coupling to the rock cycle via mineral weathering. Here we show a route to P liberation from water-rock reactions that are thought to be common throughout the Solar System. We report the speciation of phosphorus in serpentinite rocks to include the ion phosphite (HPO32- with P3+) and show that reduction of phosphate to phosphite is predicted from thermodynamic models of serpentinization. As a result, as olivine in ultramafic rocks alters to serpentine minerals, phosphorus as soluble phosphite should be released under low redox conditions, liberating this key nutrient for life. Thus, this element may be accessible to developing life where water is in direct contact with ultramafic rock, providing a source of this nutrient to potentially habitable worlds.

Serpentine-reached mining wastes as a geochemical barrier for the soil remediation under the ongoing Cu-Ni pollution in the Russian Arctic

<p>The main factors for the degradation of the ecosystems in the metal-polluted territories are soil toxicity, organic matter degradation and violation of macro-element cycles. Heavily contaminated soils lose their ability to maintain sustainable vegetation, which leads to the formation of industrial barrens as the final stage of plant cover digression, where the vegetation cover is less than 10%. The deposition of metal mobile compounds into an insoluble form by alkaline sorbents is one of the most effective remediation techniques in situ. Technosol engineering is a trigger for the beginning of plant and soil cover development and the recovery succession under high pollution with metals compounds.</p><p>Field experiment of remediation using three types of serpentine mining wastes, expanded vermiculite and grass seeds mixture was laid down in 2010-2013 in the impact zone of the copper-nickel ore processing enterprise on the Kola peninsula (northern Europe) beyond the Arctic Circle at two sites with podzol and peat soil. The results obtained in 2019 showed that the immobilization effect was strengthened by high pH inherited from the alkaline wastes making Technosols a geochemical barrier. For the first 5-8 years of the experiment, the Technosol upper layers primary consisted of serpentine minerals, accumulated more than 1 g·kg-1 Ni and 0.1 g·kg<sup>-1</sup> Cu which are constantly deposited from the atmosphere as a result of the Cu-Ni enterprise activity. They also affected the underlying soil and neutralized the most toxic water-soluble and exchangeable fractions of Cu and Ni. Grass growing and litter deposition (in total 4.5-6 kg·m<sup>-2</sup>) during the experiment term led to the accumulation of organic carbon by serpentine minerals about 1.5%. Organic matter accumulation also played a significant role in metal binding by upper Technosol layers. Summarily, the remediation technology through the use of serpentine-reached mining wastes bound metals emitted by smelter into insoluble forms, reduced the toxicity of water-soluble and exchangeable fractions of heavy metals and promoted the sustainable development of plant cover.</p><p>Research was carried out with the support of the Russian Science Foundation grant 19-77-00077.</p><p> </p>

Asbestos determination in ophiolitic rocks by Image Analysis coupled with Raman Spectroscopy

<p>The determination of the asbestos content in ophiolitic rocks is carried out by well-known and standardized analytical techniques (SEM-EDS according to Italian regulation on environmental parameters on spoils, waste and rock and soil). Despite the high resolution and the possibility to obtain elemental information, SEM-EDS is not always able to discriminate serpentine minerals, including chrysotile and non-regulated fibrous antigorite, lizardite, and possibly polygonal serpentine.</p><p>Moreover, the analytical procedures using electron microscopies are time-consuming and show an intrinsic lack of statistical representativeness, due to the low portion of the analytical sample that is effectively analyzed. Conversely, optical microscopy delivers fast results affected by a lower resolution and unreliable mineral fibre identification. Many sectors related to the realization of geo-engineering projects would take enormous advantages from a more efficient and statistically-sound approach.</p><p>To evaluate the results obtained from a state-of-the-art optical microscope with automatic image analysis in-line with micro-Raman spectrometer, we designed a study to comparatively determine the asbestos content from a large set of samples deriving from asbestos-bearing rock of the ophiolitic domain. The performance of a Malvern G3 Morphology microscope equipped with a 850 nm laser Raman spectrometer was tested on 40 samples. The same samples, prepared from ophiolitic rocks from the Ligurian Alps comminuted down to top-size = 100 μm, were parallelly analyzed and results compared with SEM-EDS quantitative method described by Italian regulation (Ministerial Decree 6 September 1994, All 1B).</p>