Darya Ilieva

The evaluation of the environmental impact arising from mine tailing toxicity can be accomplished by chemical and biological assays. Several research groups and environmental agencies established working protocols for heavy metal determination and risk assessment. Nevertheless, there is no standardized methodology that integrates chemical and biological approaches for the toxicity assessment of soils and waters contaminated with heavy metals and metalloids. This study presents a comprehensive algorithm designed to assess the environmental toxicity of mine tailings and surrounding soils by applying a germination assay. Therefore, wheat germination bioassays andevaluation of bioconcentration of As, Pb, Cu, Zn, Cd, Ag, Fe, Mn, Ni, and Cr in contaminated environmental samplesare carried out. The germination index showed results greater than 80 % classified as “excellent” in soil samplesP1 and P2, and less than 40 % in soil sample P3 due to high Pb toxicity. The capability of plant species to remediatesoils was evaluated by the Bioconcentration factor (BCF) and the Translocation factor (TF). The average results forBCF, based on Pb, As, Zn, and Cu, were 0.25, and a low TF was obtained, with values ranging between 0.05 and 0.2.

Various chemical, biological, and biochemical assays are currently employed to evaluate the state of mine tailing dumps. However, some chemical approaches could overestimate the environmental footprint. This study presents an assessment of the current state of the inactive mine tailing dump area, employing a sequential chemical extraction procedure (SEP) to study heavy metal fractionation and contamination indices, thereby assessing the mobility and potential bioavailability. The studied samples contained elevated concentrations of As, Cu, Pb, and Zn, of which Pb was the most abundant element. The results showed that although high total heavy metal content in soil and mine tailing was observed, the mobility factors rarely exceeded 10 %. Moreover, the values of contamination factors, individual and global, were below 1 and 6, respectively. Thus, the mine tailings and surrounding soils could be classified as sources with low contamination potential. The applied approach could benefit the ongoing efforts toward the sustainable management and conservation of post-mining landscapes.

Alkaline reactivity characterizes the dissolution and reactive capacity of a solid material in alkaline media. The results are of significance in the development of technology for geopolymers obtaining based on the studied raw materials. However, no validated analytical procedure for estimation of alkaline reactivity of coal combustion by-products and mine tailing exists. It imposed the development of specific procedure for analysis and its verification. This report presents a procedure based on ICP-OES measurement of the concentration of Al, Si and Ca in alkali solution. Some problems of the analysis were successfully addressed, as a high salt content of leachates, and the procedure was verified applying a reference laboratory approach. The alkaline reactivity was studied as a function of time at 6.5 M NaOH during 48 h. Additionally, to reactive silica determination, the study was extended to reactive alumina and calcium, as these components contribute to the development of geopolymer matrix. To estimate the percentage of dissolved Al, Si and Ca detailed precise data about chemical composition of raw materials were required. To achieve it, samples components were determined by ICP-OES after digestion in acid oxidizing medium, combined with alkali fusion for silica determination, as well as loss of ignition and humidity determination. In contrast to the approach, usually reported in the literature, presenting the alkaline reactivity as a concentration of dissolved ions in the alkaline solution, in this study we proposed a new procedure. Alkaline reactivity was estimated: by a percentage of dissolved component in mg per kg dry sample. This approach allowed comparability of the data between various laboratories, procedures or raw materials. Moreover, the approach allowed the relative ratio between reactive components to be calculated.

Geopolymers present a modern low CO2 footprint alternative of traditional building materials. The reuse of industrial waste or by-products as raw materials could address important ecological problems. The aim of this study is to evaluate the characteristics of mine tailings and coal combustion by-products from Bulgaria as raw materials for the preparation of geopolymers. Three mine tailings and two coal combustion by-products from thermal power stations were characterized. Appropriate validated analytical methods were combined to retrieve detailed information. The studied samples contented relatively high percentage of Al2O3 ­(17-21%) and SiO2 (68-53%), and low concentration of sulfur (0.02-3.5% as SO3). The alumosilicates, proved by XRD, are expected to show high reactivity in alkaline media, however some less reactive mineral phases were also observed. The aqueous slurry was alkaline pH 8-11, the fly ash contained the highest concentration of components ionized in water solution. Heavy metals content was found to be at micro or trace levels. The copper content was 56-818 mg/kg, Pb 127-2 mg/kg, Cr 1-71 mg/kg. The results showed that the studied mine tailings and fly ash could be used as precursors for geopolymer obtaining after fine adjustment of the technology to account for the specific characteristics of raw materials.

The most used raw materials to produce geopolymers are metakaolin, fly ash, mine tailings, and granulated blast furnace slag. The geochemical composition of the starting materials is decisive for the structure and physical properties of the newly synthesized geopolymer. The present research is aimed at validating methods to determine pH, electrical conductivity of initial raw materials for the preparation of geopolymers. Methods trueness and precision were estimated by bias, z-score and repeatability determination. The results from certified reference material clay soil for pH and EC CRM498-100G, Lot LRAC5544 tests showed that the bias was 0.18% for pH and –1.7% for EC. The repeatability was 0.18% and 2.38% for pH and EC, respectively. The z-score was below 2 and the analytical behavior of both studied methods was evaluated as satisfactory. The validated procedures were applied to mine tailings and coal combustion by-products from four sources from Bulgaria, Romania, and Portugal. The aqueous slurry of industrial wastes was with pH 6-12, all studied fly ashes contained high concentration of components ionized in water solution. The results showed that the studied raw materials could be used as precursors for geopolymer.

This paper discusses approaches for verification of methods of measurements of chemical and physical characteristics of specific samples. The limitations of well-known approaches are discussed. Some examples of alternatives are given to demonstrate specific issues encountered in the research laboratory analyzing new materials or characterizing new properties of materials. Application of sequential procedure using lower quantities of samples and reagents is presented. A standard addition to solid samples is discussed. The approach of control charts for estimation of method uncertainty for determination of plant available phosphorus is presented. The method comparison is applied as an approach to verification of alkaline reactivity by inductively coupled plasma–optical emission spectroscopy (ICP-OES) measurement, as well as density of newly synthesized chalcogenide glass materials. The presented examples demonstrated that alternative approaches are needed in order to verify the methods applied due to the great variety of activities and corresponding tasks in a research laboratory.

This comparative study was aimed at estimating analytical behavior of methods for determination of plant available potassium applied to Bulgarian arable soils and to reveal the relationship between the amount of extractable K. Twenty-four samples from two traditional agricultural regions in Bulgaria were studied. Soil potassium was extracted by NH4OAc/HOAc pH 4.5 (AA), diluted double acid (Mehlich 1), CaCl2, BaCl2 and a modified acetate/lactate method (ALM) and determined by Flame AES. The factors influencing the methods accuracy were identified and uncertainty was estimated. The expanded uncertainty was (in mg K2O (100 g dry soil)-1): 0.10 (ALM), 0.64 (Mehlich 1), 0.17 (CaCl2) and 1.1 (AA). The study revealed that the factor which mainly influence the uncertainty of the applied analytical methods for plant available potassium in soil was the calibration of Flame AES determination. The obtained results showed that extractable potassium lowered in the following order . Soil potassium extracted by ALM procedure correlated with AA, BaCl2-K, CaCl2 –K and Mehlich 1 - K at 0.05 level of significance. ALM extracted between 1.2 to 5.8 times more soil K than other methods did. The obtained results provided a base for further study on correlation between extractable K and soil fertility indices for particular soil types and climatic regions in Bulgaria.