Sensitivity of microbial bioindicators in assessing metal immobilization success in smelter-impacted soils.

While plant toxicity reduction remains the primary metric for judging the success of metal immobilization in soil, the suitability of microorganisms as universal indicators of its effectiveness in various contaminated soils remains a point of contention. This study assessed the sensitivity of microbial bioindicators in monitoring metal immobilization success in smelter-impacted soils. It compared plants and microorganisms as indicators of the efficiency of natural Fe-Mn nodules from the Gulf of Finland in immobilizing metals in soils contaminated by a Ni/Cu smelter, on the Kola Peninsula, Murmansk region, Russia. Perennial ryegrass (Lolium perenne) was grown on nodule-amended and control soils. Plant responses in the smelter-impacted soils proved to be sensitive and robust indicators of successful metal immobilization. However, microbial responses exhibited a more complex story. Despite the observed reductions in soluble metal concentrations, shoot metal contents in ryegrass, and significant improvements in plant growth, certain microbial bioindicators were unresponsive to metal immobilization success brought about by the addition of Fe-Mn nodules. Among microbial bioindicators studied, community-level physiological profiling, microbial biomass carbon, and basal respiration were sensitive indicators of metal immobilization success, whereas the number of saprotrophic, oligotrophic, and Fe-oxidizing bacteria and fungi, the biomass of bacteria and fungi, and enzymatic activity were less robust indicators. Interestingly, the correlations between different microbial responses measured were weak or even negative. Some microbial responses also exhibited negative correlations with plant biomass. These findings underscore the need for further research on comparative evaluations of plants and microorganisms as reliable indicators of metal immobilization efficacy in polluted environments.

Serpentine Overburden Products-Nature-Inspired Materials for Metal Detoxification in Industrially Polluted Soil.

The possibility of plants growing on serpentine soils and the ability of serpentine minerals to accumulate significant amounts of metals was the basis for developing a method for using serpentine-containing materials to restore vegetation in areas with a high level of metal pollution. Serpentine-containing products obtained from phlogopite mining overburden (Kovdor, Murmansk region, Russia) with and without thermal activation were used in a field experiment on the remediation of industrially polluted peat soil. According to the geochemical mobility of the components, one of four fractions was allocated depending on the acidic (HCl) concentration of the solution used for the material treatment: readily mobile (0.001 mol/L), mobile (0.01 mol/L), potentially mobile (0.1 mol/L), and acid-soluble (1.0 mol/L). This study showed that the addition of serpentinites to peat soil changed the fraction composition. The most significant changes were noted for serpentinite components such as Ca and Mg: their concentrations increased 2-3 times even in the smallest portion of serpentine material. On the contrary, the contents of metals in the readily mobile fraction decreased 3-18, 3-23, 5-26, and 2-42 times for Cu, Ni, Fe, and Al, respectively. The main factor causing the decrease in metal mobility was the pH rise due to the release of Ca and Mg compounds into the soil solution. This study showed that the addition of serpentine-containing material at 25 vol.% to peat soil was sufficient to create a geochemical barrier with a stable-functioning vegetation cover. All serpentine-containing materials are recommended for the remediation of large industrially polluted areas.

Thermally activated serpentine materials as soil additives for copper and nickel immobilization in highly polluted peat.

Heat-treated serpentine products from mining wastes have been examined to remediate highly contaminated soil with total concentration of Cu 10470 mg/kg and Ni 5300 mg/kg. The series of laboratory and field experiments (for 10 years) were conducted. The modified Tessier method was used to assess the metals geochemical mobility. The effect of hydration on the chemical stability of the components and sorption properties of thermally activated serpentine were studied. The hydration of heat-treated serpentine decreased the leaching of the main components (Mg and Si) that indicates their partial binding in a newly formed compound-magnesium silicate. Hydration of heat-treated serpentine did not lead to the changes in the phase composition and the geochemical mobility of the precipitated Ni and Cu compounds. The hydration affected the sorption value at the 1 day of the interaction but after 30 days this difference partially leveled. A laboratory experiment showed that thermally activated serpentine was effective for the Cu and Ni sorption from sulfate solutions. The substantial changes in chemical properties of soil mixtures after ten years of the field experiment were found. In the first year of the field experiment, the pH values of soil mixtures were alkaline (9.4-9.9) and were significantly higher compared to the pH 4.0 of the initial peat soil. Over 10 years, the soil pH at the experimental sites gradually decreased and reached values of 7.2-8.6. The introduction of thermoactivated serpentines led to a decrease in the share of the most mobile exchangeable fraction. The most noticeable effect of thermoactivated serpentines on metal mobility in the polluted peat soil revealed for Cu; its migration coefficient decreased from 1.8 in the peat soil to 0.7 in the mixtures with heat-treated serpentines. The sum of Cu mobile fractions in the experimental variants became lower compared with initial peat by 50-70%, while Fe was lower by 30%, and Zn-by 80%. The increase in the proportion of the most strongly bound fraction was observed for all metals in the experimental variants compared with initial soil. The coefficient of metal accumulation for Ni and Cu was significantly lower than 1, indicating protective mechanisms in plants. The high content of mobile Mg and Ca compounds seems to be the determining factor in this process. The grass communities forming in the 10-years experiment showed high productivity and stability even under constant airborne industrial pollution. The thermally activated serpentine minerals can be recommended for the in situ remediation of landscapes with completely lost vegetation during the long-term impact of industrial emissions.

Magnesium Silicate Binding Materials Formed from Heat-Treated Serpentine-Group Minerals and Aqueous Solutions: Structural Features, Acid-Neutralizing Capacity, and Strength Properties.

The influence of structural features of three serpentine-group minerals (antigorite, chrysotile, and lizardite) on the hydration of heat-treated materials and the formation of magnesium silicate binder has been studied. Initial serpentine samples have been fired in the interval 550-800 °C with a step of 50 °C; acid neutralization capacity (ANC) values have been determined for all samples. Antigorite samples (SAP) have exhibited a maximum reactivity at a temperature of 700 °C (ANC 7.7 meq/g). We have established that the acid-neutralizing capacity of chrysotile and lizardite samples in the temperature range of 650-700 °C differ slightly; the capacity varied in the interval of 19.6-19.7 meq/g and 19.6-19.7 meq/g, respectively. The samples obtained at optimal temperatures (antigorite-700 °C, lizardite, and chrysotile-650 °C) have been studied. Heat-treated serpentines have interacted with water vapor for a year; serpentine hydration has been investigated. The strength characteristics of the resulting binder agents were studied after 7, 28, 180, and 360 days. Upon hardening within 7 days, the strengths of the SAP and SCH samples have been almost the same (2.2 MPa), whereas this indicator for the SLH and SLK samples has been significantly lower (0.5 MPa). After hardening for over a year, the chrysotile sample SCH had the highest strength (about 8 MPa), whereas the strength of antigorite SAP was 3 MPa. The samples of initial, heat-treated, and hydrated heat-treated serpentines have been studied using XRD, differential scanning calorimetry, and surface texture analysis. The serpentine structure is crucial in destroying the mineral crystal lattice during heat treatment. In contrast to heat-treated chrysotile and lizardite, antigorite did not adsorb water. Structural features of chrysotile provided the highest compressive strength of the binding agent compared with antigorite and lizardite. The acid-neutralizing ability of lizardite was noticeably higher than antigorite, whereas its compressive strength was lower due to the layered mineral structure and impurities. We have established that the minerals' structural features are crucial for the hydration of heat-treated serpentines; the structure determines material utilization in various environmental technologies.

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.