HEXAVALENT CHROMIUM LEACHING FROM CEMENTS AND CEMENT COMPOSITES
Chromium is one of the 25 most widespread elements in the Earth’s crust and occurs as an indelible non-volatile trace element in raw materials used in cement clinker production (clay, limestone and iron additives in particular). Chromium in raw materials mentioned above is mainly in form of trivalent chromium. Naturally occurring chromium (III) is not initially harmful, since it is chemically stable. Only at high temperatures found in cement rotary kilns, inert trivalent chromium oxidizes to form more dangerous hexavalent chromium. The paper presents the results of the experimental investigation of the hexavalant chromium releasing from cement samples and cement composites to the water environment under various conditions. The samples of Portland cement as well as the cement composites with different composition were used for the experiments. The chemical composition of cement samples and cement composites were determined by using X-ray fluorescence analysis (XRF); the soluble hexavalent chromium concentrations in cement leachates and cement composites leachates were analysed by colorimetric analysis. The determination of chromium (VI) by spectrometry was based on the reaction of hexavalent chromium with diphenylcarbazide by forming the purple coloured compound. The content of water soluble chromium was measured to be lower in cement composites leachates than in cement leachates. The soluble hexavalent chromium concentrations in cement composites were measured from 0.13 to 0.22 mg/kg and 0.07 to 0.15 mg/kg, when studied powdered and compact cement composites leachates, respectively. The concentration of Cr (VI) in cement leachates reached the average value of 2.85 mg/kg.
Determination of hexavalent chromium in South African cements and cement-related materials with electrothermal atomic absorption spectrometry
This paper seeks to develop a method to determine the total chromium (VI) in cement and cement-related materials, with the particular aim of ensuring that no changes take place in the original state of chromium owing to the sample preparation procedure. The selective extraction of chromium (VI) in a cement matrix, based on treatment with 0.1 M sodium carbonate (Na2CO3) solution and subsequent determination of chromium by electrothermal atomic absorption spectrometry has proposed and applied to South African cement clinkers, natural gypsum, limestone and certified reference materials. Between 30% and 80% of the total chromium in the cement clinkers are Cr(VI) compounds and 8-26% of the total amount of Cr(VI) species is water soluble. This disproves the general perception that all Cr(VI) in cement and cement-related materials is water soluble. Results from an analysis of a sulphate-resisting Portland cement compared with the certified values were used to verify the analytical performance of the proposed method.
Speciation of chromium in steelmaking solid wastes by selective retention on ion-exchange media and determination by isotope dilution inductively coupled plasma mass spectrometry
The separation and determination of CrVI and CrIII were carried out by using a combination of ion-exchange resins and inductively coupled plasma mass spectrometry (ICP-MS). Once in solution the species were retained on Dowex 1-X8 and Dowex 50W-X8 resins, for CrVI and CrIII, respectively. Following elution of the CrVI with HNO3 and of the CrIII with HCl, and after elimination of chloride ions, chromium contents were determined by ICP-MS. The sample dissolution and preparation method was developed using CrO3 (CrVI) and Cr2O3 (CrIII) mixtures. A general method was established, consisting in the leaching of the soluble chromium species (CrVI and any soluble CrIII compound) with a NaOH–Na2CO3 solution, and in digesting the residue (insoluble CrIII compounds and Cr metal) by fusion with NaHSO4 and extracting the melt with HCl. The obtained solutions were submitted to the ion-exchange speciation processes. The scope of the method was investigated by analysing two electric arc furnace flue dust samples: MS-2 (carbon steel flue dust) and MS-3 (stainless-steel flue dust). The determination limits (10s), considering test samples of 10 mg, were 0.1 µg g−1 for both CrVI and CrIII. Validation of the proposed method was carried out by analysis of BCR CRM 544 and by standard additions of CRM 544 to CrO3–Cr2O3 mixtures; recoveries of both chromium species (CrVI and CrIII) close to 100%, with precision values better than 3%, were achieved. External calibration and isotope dilution (ID) were applied for ICP-MS measurements; the latter system (ID) was developed for application to Cr measurements in steelmaking solid wastes, to overcome any possible interference from the complex sample matrix.