Anthimos Xenidis

Potential use of lignite fly ash for the control of acid generation from sulphidic wastes

In the present paper, the potential use of lignite fly ash in the control of acid generation from sulphidic tailings disposed of at Lavrion, Greece was studied. Long-term laboratory column kinetic tests were performed on tailings containing 27% S, which were homogeneously mixed with various amounts of fly ash, ranging from 10 to 63% w/w. The drainage quality of the columns was monitored over a test period of 600 days. Chemical and mineralogical characterisation of column solid residues was performed after a 270-day test period. The hydraulic conductivity of the mixtures was also measured to evaluate the potential of fly ash to form a low permeability layer. Based on the results, the addition of fly ash to sulphidic tailings, even at the lower amount, increased the pH of the drainage at values of 8.6-10.0 and decreased the dissolved concentrations of contaminants, mainly Zn and Mn, to values that meet the European regulatory limits for potable water. Higher fly ash addition to tailings, at amounts of 31 and 63% w/w also reduced the water permeability of material from 1.2 x 10(-5) cm/sec to 3 x 10(-7) and 2.5 x 10(-8) m/s, respectively.

The use of municipal sewage sludge for the stabilization of soil contaminated by mining activities

The ability of municipal sewage sludge to immobilize Pb, Zn and Cd contained in contaminated soil originating from a former mining area in Lavrion, Greece was investigated. The soil was cured with sewage sludge in various proportions. The stabilization was evaluated primarily by applying chemical tests and complemented by the performance of additional biological tests. Application of the U.S. EPA Toxicity Characteristic Leaching Procedure (TCLP) on the stabilized mixtures proved that Pb, Zn and Cd solubility was reduced by 84%, 64% and 76%, respectively, at 15% w/w sludge addition, while a 10% w/w addition was sufficient to reduce Pb solubility below the U.S. EPA TCLP regulatory limit. The results of the extraction using EDTA solution showed the same trend, resulting in 26%, 36% and 53% reduction in the Pb, Zn and Cd extractable fractions, respectively. Speciation analysis of the treated soils revealed a significant decrease in the mobile fractions of heavy metals, which was attributed to their retention in sewage sludge by adsorption and organic complexation mechanisms. For the assessment of possible phytotoxicity, experiments including growing dwarf beans in the treated soil was carried out. It was found that sewage sludge addition had a positive effect on plant growth. Furthermore, the Pb and Zn uptake of plant leaves and roots was reduced, while Cd uptake was unaffected by the sludge treatment. The results of this study support the hypothesis that municipal sewage sludge is a potential effective stabilizing agent for contaminated soil containing Pb, Zn and Cd.

Evaluation of monobasic calcium phosphate for the immobilization of heavy metals in contaminated soils from Lavrion

The objective of this work was to evaluate the efficiency of monobasic calcium phosphate for the stabilization of heavy metals in contaminated soils. The treatment was applied on a soil sample from the Lavrion mining area, Greece, heavily contaminated with Pb, Zn, Cd and As and characterized as toxic in respect to Pb according to the US EPA toxicity characteristics leaching procedure (TCLP). The efficiency of stabilization was evaluated based on two criteria: (a) the reduction of metals mobility below the TCLP regulatory limits; (b) the reduction of phytoaccumulation. Phytoaccumulation was evaluated both indirectly by applying leaching tests using EDTA, DTPA and NaHCO(3) solutions and directly by carrying out pot experiments with Phaseolus vulgaris as plant indicator. This treatment was found to immobilize Pb and Cd, whereas As and Zn were slightly mobilized. No effect on phytoaccumulation was observed. Moreover, the treatment had a negative effect on plants growth, which was combined with a strong deficiency of Ca in the tissue of leaves.

Stabilization of Pb and As in soils by applying combined treatment with phosphates and ferrous iron.

The chemical immobilization of Pb and As in contaminated soil from Lavrion, Greece, using monocalcium phosphate and ferrous sulfate as stabilizing agents was investigated. Monocalcium phosphate was added to contaminated soil at PO(4) to Pb molar ratios equal to 0, 0.5, 1, 1.5 and 2.5, whereas ferrous sulfate was added at Fe to As molar ratios equal to 0, 2.5, 5, 10 and 20. Phosphates addition to contaminated soil decreased Pb leachability, but resulted in significant mobilization of As. Simultaneous immobilization of Pb and As was obtained only when soil was treated with mixtures of phosphates and ferrous sulfate. Arsenic uptake by plants was also seen to increase when soil was treated only with phosphates, but co-addition of ferrous sulfate was efficient in maintaining As phytoaccumulation at low levels. The addition of at least 1.5M/M phosphates and 10M/M iron sulfate to soil reduced the dissolved levels of Pb and As in the water extracts to values in compliance with the EU drinking water standards. However, both additives contributed in the acidification of soil, decreasing pH from 7.8 to values as low as 5.6 and induced the mobilization of pH sensitive elements, such as Zn and Cd.

Environmental characterisation of the sulphidic tailings in Lavrion

Abstract Intensive mining and metallurgical activities in Lavrion over a time span of over 2700 years resulted in the formation of huge spoils of mining, milling and metallurgical wastes and tailings, most of them characterised as toxic and hazardous. Toxic elements are released by a number of mechanisms and migrate to the surrounding area, contributing to the widespread soil contamination. Three main types of spoils exist: sulphidic flotation tailings, oxidic-carbonaceous-silicate beneficiation tailings and metallurgical slags. This paper deals with the sulphidic flotation tailings. For the estimation of the risk that these tailings pose to the environment and for the conceptual development of a remediation strategy, complete characterisation was done according to the following methodology: Drillholes were placed within the spoils and undisturbed core samples were taken and characterised chemically and mineralogically, their net neutralisation potential (NNP) was determined using static tests and their toxicity using the EPA TCLP test, speciation of the toxic metals was determined in some cases using the sequential extraction technique and the bioavailable fraction determined by EDTA extraction. Some geotechnical characteristics of the spoils, as density and permeability were determined in-situ. Piezometers were installed within the boreholes and the pore water quality and level was monitored. The above information is being critically assessed and employed for the selection and design of the optimum rehabilitation scheme.

Carbonate-rich mining tailings in Lavrion: risk assessment and proposed rehabilitation schemes☆

Abstract In Lavrion, Greece, the mining and metallurgical activities that were particularly intensive during the last century resulted in the generation of huge amounts of wastes, including acid-generating sulfidic tailings, carbonaceous tailings and slags. Their improper management in the past resulted in the migration of heavy metals to the surroundings, contributing to widespread soil contamination. As a result, the local population is exposed to multiple hazardous pollution sources. The present study aims to combine the available geochemical characterisation techniques and risk assessment tools in order to identify the specific risks associated with the carbonate wastes and determine the remedial measures required accordingly. The risk assessment study of the carbonate tailings resulted in the identification of three high-risk exposure routes for humans: direct contact, ingestion and inhalation of contaminated material. Plant uptake and secondary contamination of soils are rated as high-risk exposure routes only for ‘Savoura’ tailings. Quantification of the human health risks indicated that direct ingestion of contaminated particles is the most important exposure route for the intake of contaminants by humans. The intake of Pb, As and Cd by adults and children living in the vicinity of carbonaceous tailings was found to exceed the maximum tolerable risk. Based on the risk assessment results, several remediation options were examined, aimed at either removing the source or breaking the significant source–pathway–target relationships.

Inhibition of acid generation from sulphidic wastes by the addition of small amounts of limestone

Abstract Limestone addition is a commonly applied technique to prevent acid generation from sulphidic wastes containing 1–10% S. In the present paper, the effectiveness of small amounts, lower than the stoichiometric requirement, of this alkaline additive in inhibiting acid generation from a pyrite concentrate material is studied. Long term laboratory column tests were conducted on a partially oxidised pyrite concentrate, where limestone was added by thoroughly mixing. The amount of alkaline additive ranged from 6.4 to 29% wt. corresponding to 5–30% of the stoichiometric quantity. The performance of the pyrite-limestone mixtures was evaluated by monitoring the drainage quality of the columns. Furthermore, a detailed geochemical characterisation of the column solid residues was performed after a monitoring period of 270 days. The effect of secondary oxidation-neutralisation products on the hydraulic conductivity of material was also examined. Dissolution of previously formed oxidation products occurred in the control column during the monitoring period, resulting in the release of a significant amount of Fe, Zn, Mn, Cd, As and SO4 and to a lesser extent Pb. However, due to the presence of secondary products, further oxidation of pyrite particles was delayed. The experimental results showed that homogeneous mixing of pyrite with limestone amounting to only a fraction of the contained acidity inhibited the generation of acidic drainage and significantly reduced the dissolved amount of metals and sulphates for a test period of 270 days. Under the alkaline conditions prevailing in the limestone amended columns, secondary precipitation of ferric hydroxides and gypsum occurred. A ten fold decrease of hydraulic conductivity was observed for the material amended with 207 kg CaCO3/t, corresponding to 15% of the contained acidity.

Incineration of tannery sludge under oxic and anoxic conditions: study of chromium speciation.

A tannery sludge, produced from physico-chemical treatment of tannery wastewaters, was incinerated without any pre-treatment process under oxic and anoxic conditions, by controlling the abundance of oxygen. Incineration in oxic conditions was performed at the temperature range from 300°C to 1200°C for duration of 2h, while in anoxic conditions at the temperature range from 400°C to 600°C and varying durations. Incineration under oxic conditions at 500°C resulted in almost total oxidation of Cr(III) to Cr(VI), with CaCrO4 to be the crystalline phase containing Cr(VI). At higher temperatures a part of Cr(VI) was reduced, mainly due to the formation of MgCr2O4. At 1200°C approximately 30% of Cr(VI) was reduced to Cr(III). Incineration under anoxic conditions substantially reduced the extent of oxidation of Cr(III) to Cr(VI). Increase of temperature and duration of incineration lead to increase of Cr(VI) content, while no chromium containing crystalline phase was detected.

Oxidation of pyrite and arsenopyrite in sulphidic spoils in Lavrion

This paper aims to present the aspects of oxidation of pyrite and arsenopyrite in the sulphidic spoils in Lavrion and to assess the environmental impact of this process. Sulphidic spoils, under the combined action of atmospheric oxygen, water and bacteria are oxidised and toxic elements are released and migrate to surrounding areas contributing to widespread soil contamination and severe deterioration in the quality of surface and underground waters. The critical factors which control the generation of acidic waters are availability of oxygen and water, temperature, bacterial activity and rate of oxygen diffusion through the pores in the upper zones of spoils. Special emphasis will be given in the oxidation of sulphidic spoils at Lavrion, Greece, where the generation of acidic waters causes widespread contamination. This study is focused in the first part on the preferential oxidation of arsenopyrite. Flotation products containing pyrite and arsenopyrite were subjected to bacterial leaching tests at 3% w/v pulp density and the degrees of oxidation were calculated and compared to with previous experimental studies. These results provided the basis to assess the behaviour of the above minerals in waste stockpiles. Moreover, and in order to assess the oxidation and precipitation mechanisms, several samples were collected up to a depth of 20 cm from the surface from the same spoils profile, where successive layers of oxidation products were present and characterised chemically and mineralogically. The above information is critically evaluated for the determination of the environmental impact of the oxidation of sulphide phases in spoils.

Origin and concentration profile of chromium in a Greek aquifer

In this paper the origin and concentration of chromium (Cr) in an ophiolitic aquifer in Vergina, northern Greece were investigated. The study area has only agricultural activity so that industrial Cr contamination was precluded. Soil sampling included topsoil and drillcore samples collected down to 98 m depth. Groundwater samples were collected from three existing wells and a spring at the area and from different depths of the soil boring using the discrete sampling method. Mineralogical analysis of soils confirmed the presence of ultramafic minerals, including chrysotile and chromite. Soil elemental analysis showed significant concentration of total chromium (Crtot; max 12,000 mg/kg) and hexavalent chromium (Cr(VI); max 7.5mg/kg). Significant Crtot (91 μg/L) and Cr(VI) (64 μg/L) concentrations exceeding the drinking water limit of 50 μg/L were also detected in groundwater. In both the discrete soil and groundwater samples a decreasing trend of Cr(VI) concentration was observed with increasing depth, while Crtot increased. The increasing trend in Crtot is attributed to the increasing contribution of unweathered ultramafic minerals with depth, while the decreasing Cr(VI) may be related to the increasing soil pH that does not favor Cr(III) oxidation by Mn-oxides.