E. Paterson

Identification and geochemical modeling of processes controlling leaching of Cr(VI) and other major elements from chromite ore processing residue

Abstract Chromite ore processing residue (COPR) contains very high levels of chromium as Cr(III) and Cr(VI) and has a pH of ∼11.5 to 12. Millions of tonnes of COPR have in the past been deposited in urban areas. We have studied the factors that control leaching of Cr(VI), Ca, Al, Si, and Mg from COPR by means of batch experiments, mineralogical characterization of COPR via X-ray powder diffraction and scanning electron microscopy, and chemical equilibrium modeling. Batch experiments at a range of pH values and two liquid:solid ratios showed that mineral solubility control exists for aqueous concentrations of Cr(VI) above pH 10. Calculations indicate that the solid phases that control the solubility of Cr(VI) at pH values above 11 are Cr(VI)-substituted hydrogarnet (Ca3Al2(H4O4,CrO4)3) and Cr(VI)-hydrocalumite (Ca4Al2(OH)12CrO4·6 H2O), a layered double-hydroxide clay with chromate anions held in the interlayers. In the pH range 9.5 to 11, the description of the Cr(VI) concentration in solution was strongly improved by the incorporation in the model of Cr(VI)-ettringite (Ca6Al2(OH)12(CrO4)3·26 H2O), which precipitates as a secondary phase when hydrocalumite dissolves. The proposed model for leaching of COPR at high pH includes Cr(VI)-bearing hydrogarnet, Cr(VI)-hydrocalumite, Cr(VI)-ettringite, brucite, calcite, Ca2Al2(OH)10·3 H2O, CaH2SiO4, and gehlenite hydrate (Ca2Al2(OH)6SiO8H8·H2O). The model accurately predicts the concentrations of Cr(VI), Ca, Al, Si, and Mg in solution in the pH range 10 to 12 as well as the pH-buffering behavior. Below pH 8, a decrease in the Cr(VI) concentration in solution is observed, which may be attributed to sorption of chromate onto freshly precipitated Al and Fe hydroxide surfaces. Sulfate and carbonate show the same type of behavior as chromate. The chemistry of COPR shows similarities with cement and high-pH municipal waste incinerator bottom ash.

Role of quantitative mineralogical analysis in the investigation of sites contaminated by chromite ore processing residue

A range of techniques, normally associated with mineralogical studies of soils and sediments, has been used to characterise the solid materials found on sites contaminated with chromite ore processing residue (COPR). The results show that a wide range of minerals are present, many of which are found extensively in high-temperature synthetic systems such as cements and clinkers and their low temperature hydration products. Thus, the minerals in COPR can be divided into three main categories: unreacted feedstock ore (chromite); high temperature phases produced during chromium extraction (brownmillerite, periclase and larnite); and finally, minerals formed under ambient weathering conditions on the disposal sites (brucite, calcite, aragonite, ettringite, hydrocalumite, hydrogarnet). Apart from chromite, chromium occurs in brownmillerite, ettringite, hydrocalumite and hydrogarnet. Detailed study of the chemistry and stoichiometry of chromium-bearing phases in conjunction with phase abundance provides a quantitative description of the solid state speciation of Cr(III) and Cr(VI) in and amongst these minerals and in the COPR as a whole. Of the total chromium present in the samples most, approximately 60-70% is present as Cr(III) in chromite, whilst brownmillerite also represents a significant reservoir of Cr(III) which is approximately 15% of the total. The remaining chromium, between 20 and 25%, is present as Cr(VI) and resides mainly in hydrogarnet, and to a slightly lesser extent in hydrocalumite. In the latter, it is present principally in an exchangeable anionic form. Chromium (VI) is also present in ettringite, but quantitatively ettringite is a much less important reservoir of Cr(VI), accounting for approximately 3% of total chromium in one sample, but less than 1% in the other two. This description provides insight into the processes likely to control the retention and release of Cr(VI) from COPR-contaminated sites. Such information is of particular value in chemical modelling of the system, in risk assessment and in the development of methods of informed remediation.

Use of solid phase characterisation and chemical modelling for assessing the behaviour of arsenic in contaminated soils

A soil, containing waste material from an industrially contaminated site, was found to be heavily contaminated with several heavy metals and As. A risk assessment for As leaching from this material has been carried out in several stages, collation and examination of historical records, solid-phase characterization and chemical modelling. The historical record indicates that the most probable source of As was arsenopyrite. However, the solid phase characterization of the soil, using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive microanalysis (EDAX), did not yield any direct evidence for pyritic phases, although there was clear evidence of known pyrite-weathering products, such as jarosite. The relative stability of pyrite and arsenopyrite have been modelled for the range of acidity and redox potentials likely to be encountered on the site. For adsorption modelling, a surface complexation model was used to predict arsenate desorption as a function of pH. It was assumed that the principal reactive adsorbent for As was hydrous ferric oxide (HFO) and this assumption was supported by the results of direct and indirect measurements and by the mineral stability calculations. This approach was successful at predicting the increased mobility of As at increasingly alkaline conditions. The modelling predictions were supported by results from batch equilibration experiments. Thus, it was possible to link direct observations of mineralogy, mineral stability calculations and adsorption models in order to predict the mobility of As. The success of this approach was dependent on identifying the reactive phase in this particular soil and having the appropriate data required for the adsorption modelling.

Chromium speciation and fractionation in ground and surface waters in the vicinity of chromite ore processing residue disposal sites

Chromium concentrations of up to 91 mg l(-1) were found by ICP-OES for ground water from nine boreholes at four landfill sites in an area of S.E. Glasgow/S. Lanarkshire where high-lime chromite ore processing residue (COPR) from a local chemical works had been deposited from 1830 to 1968. Surface water concentrations of up to 6.7 mg l(-1) in a local tributary stream fell to 0.11 mg l(-1) in the River Clyde. Two independent techniques of complexation/colorimetry and speciated isotope dilution mass spectrometry (SIDMS) showed that Cr was predominantly (>90%) in hexavalent form (CrVI) as CrO4(2-), as anticipated at the high pH (7.5-12.5) of the sites. Some differences between the implied and directly determined concentrations of dissolved CrIII, however, appeared related to the total organic carbon (TOC) content. This was most significant for the ground water from one borehole that had the highest TOC concentration of 300 mg l(-1) and at which < 3% of Cr was in the form of CrVI. Subsequent ultrafiltration produced significant decreases in Cr concentration with decreasing size fractions, e.g. <0.45 microm, < 100 kDa, <30 kDa and < 1 kDa by the tangential-flow method. As this appeared related more to concentrations of humic substances than of TOC per se, horizontal bed gel electrophoresis of freeze-dried ultrafilter retentates was carried out to further characterise the CrIII-organic complex. This showed for the main Cr-containing fraction, 100 kDa-0.45 microm, that the Cr was associated with a dark brown band characteristic of organic (humic) matter. Comparison of gel electrophoresis and FTIR results for ultrafilter retentates of ground water from this borehole with those for a borehole at another site where CrVI predominated suggested the influence of carboxylate groups, both in reducing CrVI and in forming soluble CrIII-humic complexes. The implications of this for remediation strategies (especially those based on the addition of organic matter) designed to reduce highly mobile and carcinogenic Cr(VI)O4(2-) to the much less harmful CrIII as insoluble Cr(OH)3 are discussed.

Methods for the study of interrelationships between micro-organisms and soil structure

Abstract Common methods of assessing soil structure that could be used by microbiologists are reviewed and involve size fractionation of soil aggregates, water release characteristics, intrusion porosimetry, gas adsorption and microscopic methods with image analysis. These methods could also be used for studying aspects of the interactions between soil structure and larger fauna or plant roots. Some recent refinements of these methods are described and the use of artificial soil aggregates in microbial studies is discussed.

The mineralogy and morphology of iron and manganese oxides in an imperfectly-drained Scottish soil

Abstract The distribution, mineralogy and morphology of the iron and manganese oxides in an imperfectly-drained Scottish soil, with a diffuse Fe/Mn pan, have been examined using electron microscopy, X-ray diffraction and chemical dissolution. The Fe oxide mineralogy above and within the Fe-pan is dominated by lepidocrocite, with subsidiary amounts of ferrihydrite. In the Mn-pan itself, feroxyhyte is present and below this, goethite predominates. Vernadite is the main manganese oxide mineral within the Mn-pan. The conditions necessary for the synthesis of these minerals in the laboratory are broadly consistent with the soil conditions implied from their location within the soil profile.

BASAL SOIL MONITORING SCHEME IN THE PROTECTED AREAS OF THE CZECH REPUBLIC

The purpose and aims of environmental monitoring are discussed, particularly in the context of soils as part of the environment, and examples of such schemes currently in operation within Europe are outlined. These are compared with the Basal Soil Monitoring Scheme (BSMS) in the Czech Republic which is the product of collaboration between the Ministries of Agriculture and the Environment and five agricultural, forestry and environmental research institutes. The BSMS consists of over 250 monitoring plots covering three distinct land uses: agriculture, forestry and environmentally protected areas. Each land use class has its own subsystem of monitoring plots which together comprise the whole system. The main principles and methodologies employed in the environmentally protected areas of the Czech Republic are described in some detail.