Mechanistic insights into Pb and sulfates retention in ordinary Portland cement and aluminous cement: Assessing the contributions from binders and solid waste.

Identifying immobilization mechanisms of potentially toxic elements (PTEs) is of paramount importance in the field application of solidification/stabilization. Traditionally, demanding and extensive experiments are required to better access the underlying retention mechanisms, which are usually challenging to quantify and clarify precisely. Herein, we present a geochemical model with parametric fitting techniques to reveal the solidification/stabilization of Pb-rich pyrite ash through conventional (ordinary Portland cement) and alternative (calcium aluminate cement) binders. We found that ettringite and calcium silicate hydrates exhibit strong affinities for Pb at alkaline conditions. When the hydration products are unable to stabilize all the soluble Pb in the system, part of the soluble Pb may be immobilized as Pb(OH)2. At acidic and neutral conditions, hematite from pyrite ash and newly-formed ferrihydrite are the main controlling factors of Pb, coupled with anglesite and cerussite precipitation. Thus, this work provides a much-needed complement to this widely-applied solid waste remediation technique for the development of more sustainable mixture formulations.

Arsenic release from pyrite ash waste over an active hydrogeological system and its effects on water quality.

Arsenic is a potentially toxic element of concern for environmental compartments, and it is a frequent pollutant in many abandoned industrial sites. In this study, geochemical and hydrogeological tools were used to determine the long-term effects of As-rich pyrite ash disposal (83,000 m3 as estimated by geostatistical tools) in a brownfield located over a quaternary alluvial aquifer. Throughout the site, soil pollution and water table oscillation led to leachates in the form of both run-off and infiltration waters, thereby reducing (ground)water quality (e.g. pH, electrical conductivity) and, in particular, increasing the concentration of arsenic (average approx. 4000 µg/l for one hydrological year). By means of laboratory and in situ measurements, the main mechanisms through which the sulphide remaining in the pyrite ash leaches were identified. In addition, to evaluate the effects of the polluted groundwater on the nearby main river, a mathematical approach using the Domenico analytical groundwater transport model revealed potential concentrations of 49 µg/l of arsenic in the junction between the study aquifer and the river, equivalent to an annual quantity of 49 kg of this element.

Composition and risk assessment of roasted pyrite ash from fertiliser production.

Pyrite ash is a residue from the roasting of pyrite ores to obtain sulphuric acid used in the fertiliser industry and its production is widely extended worldwide. The mismanagement of this waste may result in environmental and health damages due to its physico-chemical characteristics. The main objective of this study was to examine the physico-chemical and mineralogical composition of roasted pyrite ash from an abandoned fertiliser company, and to evaluate the environmental risk caused by the wind and water dispersion of metals posed by this waste. In order to achieve these objectives, a sequential extraction procedure and a physical fractionation into six size fractions: >100, 100-50, 50-20, 20-10, 10-2.5 and < 2.5 µm were applied. Results showed that pyrite ash is composed mainly of iron-oxides such as hematite (46%) and secondary minerals as anglesite and shows high concentrations of Pb (7464 mg kg-1), Zn (2663 mg kg-1) and Cu (585 mg kg-1). The highest Risk Assessment Code (RAC) values were found for Cd, Pb and Zn, bound to the more labile fractions. Conversely, Pb showed the lowest water solubility due to the covering effect provided by a coating of anglesite in the pyrite ash surface. Most of the metals were associated to both the coarsest (>100 µm) and the finest (2.5-10 µm) fractions, although none represented an environmental risk according to the ecological risk index results. However, 30% of the metals were bound to the respirable fraction (≤100 µm) posing a potential risk for human health and a high potential dispersion by wind to the surrounding areas.

Evaluation of strategies for mitigating risks associated with metals in pyrite ash.

The main objective was to evaluate and optimise strategies for the immobilisation or mobilisation of Cd, Cu, Cr, Ni, Pb, and Zn from pyrite ash. Alkaline amendments were used for the immobilisation test: cement, sandstone, marl, marble waste and calcareous crust. The amendments were mixed with pyrite ash at a 1:2 rate, incubated for 28 days, and leachates analysed at the beginning of the experiment (day 0) and after 2, 7, 14 and 28 days. The mobilisation experiment tested metal release from pyrite ash by four concentrations of H2SO4 (0.25 M, 0.5 M, 1 M and 2 M) and contact times (60, 120, 180 and 240 min). Results for the immobilisation/mobilisation tests for Cr and Ni are not presented due to the low concentration in pyrite ash. In the immobilisation test, optimum results across metals and amendments were obtained after two days with percentages of retention being about 90% compared to leachates from pyrite ash only. The release success (in % of total content) using sulphuric acid followed the order: Cd (75%) > Zn (62%) > Cu (37%) > Pb (7%). The concentration of acid was more important than contact time (release enhanced at higher concentrations) except for Zn. The two strategies tested were successful to reduce the risk posed by metals. In terms of optimization, all alkaline materials showed high efficiency for metal retention after a short contact time; for mobilisation, treatment with sulphuric acid at high concentration (up to 2 M tested) resulted to be the optimum with contact time having limited influence.

A novel bioreactor system for simultaneous mutli-metal leaching from industrial pyrite ash: Effect of agitation and sulphur dosage.

Simultaneous multi-metal leaching from industrial pyrite ash is reported for the first time using a novel bioreactor system that allows natural diffusion of atmospheric O2 and CO2 along with the required temperature maintenance. The waste containing economically important metals (Cu, Co, Zn & As) was leached using an adapted consortium of meso-acidophilic Fe2+ and S oxidising bacteria. The unique property of the sample supported adequate growth and activity of the acidophiles, thereby, driving the (bio) chemical reactions. Oxido-reductive potentials were seen to improve with time and the system's pH lowered as a result of active S oxidation. Increase in sulphur dosage (>1g/L) and agitation speed (>150rpm) did not bear any significant effect on metal dissolution. The consortium was able to leach 94.01% Cu (11.75% dissolution/d), 98.54% Co (12.3% dissolution/d), 75.95% Zn (9.49% dissolution/d) and 60.80% As (7.6% dissolution/d) at 150rpm, 1g/L sulphur, 30°C in 8days.

Synergistic effect of biogenic Fe3+ coupled to S° oxidation on simultaneous bioleaching of Cu, Co, Zn and As from hazardous Pyrite Ash Waste.

Pyrite ash, a waste by-product formed during roasting of pyrite ores, is a good source of valuable metals. The waste is associated with several environmental issues due to its dumping in sea and/or land filling. Although several other management practices are available for its utilization, the waste still awaits and calls for an eco-friendly biotechnological application for metal recovery. In the present study, chemolithotrophic meso-acidophilic iron and sulphur oxidisers were evaluated for the first time towards simultaneous mutli-metal recovery from pyrite ash. XRD and XRF analysis indicated higher amount of Hematite (Fe2O3) in the sample. ICP-OES analysis indicated concentrations of Cu>Zn>Co>As that were considered for bioleaching. Optimization studies indicated Cu - 95%, Co - 97%, Zn - 78% and As - 60% recovery within 8days at 10% pulp density, pH - 1.75, 10% (v/v) inoculum and 9g/L Fe2+. The productivity of the bioleaching system was found to be Cu - 1696ppm/d (12% dissolution/d), Co - 338ppm/d (12.2% dissolution/d), Zn k 576ppm/d (9.8% dissolution/d) and As - 75ppm/d (7.5% dissolution/d). Synergistic actions for Fe2+ - S° oxidation by iron and sulphur oxidisers were identified as the key drivers for enhanced metal dissolution from pyrite ash sample.

Copper and cobalt recovery from pyrite ashes of a sulphuric acid plant.

The pyrite ashes formed as waste material during the calcination of concentrated pyrite ore used for producing sulphuric acid not only has a high iron content but also contains economically valuable metals. These wastes, which are currently landfilled or dumped into the sea, cause serious land and environmental pollution problems owing to the release of acids and toxic substances. In this study, physical (sulphation roasting) and hydrometallurgical methods were evaluated for their efficacy to recover non-iron metals with a high content in the pyrite ashes and to prevent pollution thereby. The preliminary enrichment tests performed via sulphation roasting were conducted at different roasting temperatures and with different acid amounts. The leaching tests investigated the impact of the variables, including different solvents, acid concentrations and leach temperatures on the copper and cobalt leaching efficiency. The experimental studies indicated that the pre-enrichment via sulphation roasting method has an effect on the leaching efficiencies of copper and cobalt, and that approximate recoveries of 80% copper and 70% cobalt were achieved in the H2O2-added H2SO4 leaching tests.

Insights into a 20-ha multi-contaminated brownfield megasite: An environmental forensics approach.

Here we addressed the contamination of soils in an abandoned brownfield located in an industrial area. Detailed soil and waste characterisation guided by historical information about the site revealed pyrite ashes (a residue derived from the roasting of pyrite ores) as the main environmental risk. In fact, the disposal of pyrite ashes and the mixing of these ashes with soils have affected a large area of the site, thereby causing heavy metal(loid) pollution (As and Pb levels reaching several thousands of ppm). A full characterisation of the pyrite ashes was thus performed. In this regard, we determined the bioavailable metal species present and their implications, grain-size distribution, mineralogy, and Pb isotopic signature in order to obtain an accurate conceptual model of the site. We also detected significant concentrations of pyrogenic benzo(a)pyrene and other PAHs, and studied the relation of these compounds with the pyrite ashes. In addition, we examined other waste and spills of minor importance within the study site. The information gathered offered an insight into pollution sources, unravelled evidence from the industrial processes that took place decades ago, and identified the co-occurrence of contaminants by means of multivariate statistics. The environmental forensics study carried out provided greater information than conventional analyses for risk assessment purposes and for the selection of clean-up strategies adapted to future land use.

Modeling the kinetics of pyrite ash biodesulfurization by Saccharomyces cerevisiae and Acetobacter aceti in liquid state bioreactors

Background: Modeling the kinetics of the biodesulphurization bioprocess for the refining of pyrite ash by Saccharomyces cerevisiae and Acetobacter aceti have been studied in batch-type liquid- state bioreactors. Results: The biodesulphurization experiments were performed at varying temperatures of 25ºC, 30ºC and 35ºC for eight weeks. Glucose, acetic acid and ethyl alcohol were used in the incubation media as substrates and acid sources. pH and oxidation reduction potential (ORP) observations have been determined weekly and the dissolved sulphur was measured at the end of the eight weeks trials. An equation calculating pH was derived from the iron oxidation reaction containing the ferric to ferrous iron [Fe+3/Fe+2] ratio as a variable. The Michaelis-Menten predictive specific growth rates (qFe+2), which were estimated from pH and ORP observations, were compared by plotting [qFe+²]pH vs. [qFe+2]mV. The highest ratio of dissolved sulphur over total sulphur (Sd/St) was found to be 0.5 in the biodesulphurization processes. Conclusions: The model provides predictions of ferric to ferrous iron rates and specific growth rates [qFe+²]pH vs. [qFe+2]mV and can be used for the determination of oxidized and reduced ions. The ratios of dissolved sulphur to total sulphur (Sd/St) have shown some promising results for S. cerevisiae to be used as a biodesulphurization and refining microorganism for pyrite ash and the other sulphide minerals.