REPOSITORY > RESULTS

Abstract

Recycling and reuse of waste by-products from steel industry is a common practice worldwide, which is in the last decade becoming more and more established also in Slovenia. Wastes from steel-making industry can be reused as alternative materials to successfully replace natural raw materials in asphalt mixes and in numerous applications in civil engineering. Re-use of waste materials leads to preservation of natural resources, substantial reduction of landfills load and protection of the environment. The use of materials which contain waste by-products is possible only when such materials possess appropriate technical characteristics, durability and are environmentally acceptable. Waste materials from steel industry contain high amounts of metals which can be potentially leached from new materials.
The long-term environmental impacts of new materials are estimated by different leaching tests. Among them, leaching tests based on diffusion, are commonly applied. It should be considered that the toxicity of element does not depend only on its total concentration, but also on the chemical form in which it is present in the environment. Chromium, which is an important constituent of steel waste by-products, is, depending on conditions, present in the environment in trivalent form (Cr(III)) or as hexavalent chromium (Cr(VI)). The latter species is very toxic. Therefore, it is important to determine not only the total chromium concentration, but also the concentration of leached Cr(VI).
In the frame of the doctoral thesis, the long-term environmental impacts of cement composites to which 1.5 % by mass of electric arc furnace dust has been added were investigated. For this purpose, the concentrations of total Cr and Cr(VI) as well as some selected metals were determined in leachates of compact and ground cement composites and cement composites with addition of filter dust over a time period of 175 days. Ground composites simulated decomposition in the environment with time, while salt water as leaching solution, simulated the seawater environment and release of salt due to salting of roads during the winter time. Concentrations of total chromium were determinated by electrothermal atomic absorption spectrometry (ETAAS) and Cr(VI) by anion exchange fast protein liquid chromatography (FPLC) with ETAAS detection. At the end of the experiment, the concentrations of some other metals that are present in steel waste were also determined in leachates. The results indicated that chromium was leached almost solely in its hexavalent form and only from ground composites of cement and cement composites with addition of filter dust. Leaching of Cr(VI) originated primarily from cement and in much lesser extent from filter dust. The extent of leaching was higher in salt water than in water, but the concentrations of Cr(VI) did not exceed 100 µg L-1 and did not represent environmental burden. Nevertheless, it is important to ensure the conditions that would prevent Cr(VI) release into ground water and drinking water reservoirs. Leaching form other metals was negligible.
Environmental impacts of cement composites with addition of filter dust were also compared to those of asphalt mixes with filter dust and electric arc furnace steel slag.
Accurate determination of chromium concentrations in samples from the environment is important when environmental impacts of chromium are investigated. Complex matrices of samples with high ionic strength, like for instance salt water and seawater, represent analytical challenge in determination of chromium by inductively coupled plasma mass spectrometry (ICP-MS).
The analytical procedure was developed for the determination of total chromium content in seawater, after matrix removal by the use of chelating ion exchange resin Chelex-100, followed by determination of chromium by ICP-MS. Cr(III) present as cation (Cr3+) is quantitatively retained by the column resin in its NH4+ form, while Cr(VI), which exists as an anion, is not retained by the resin. Parameters that influence the binding of Cr3+ on the resin Chelex-100 were investigated. It was experimentally found that MgCl2 and KCl, which are constituents of seawater, partially change the resin form, and therefore inhibit the binding of Cr3+. The influence of seawater matrix was reduced by the dilution of samples. In order to allow determination of total chromium content in seawater, Cr(VI) was before loading on the column resin, reduced to Cr3+ by iron(II) sulphate. This original and simple approach, based on the reduction of Cr(VI) and dilution of sample prior to its application onto the chelating ion exchange column, followed by elution of bound Cr3+ with HNO3, enables quantitative and reliable determination of total chromium in seawater by ICP-MS. Limit of detection of the developed procedure was 0.3 ng Cr mL-1.