Mineralogical study and leaching behavior of a stabilized harbor sediment with hydraulic binder

Lead Mobilization and Speciation in Mining Waste: Experiments and Modeling

Mining produces significant amounts of solid mineral waste. Mine waste storage facilities are often challenging to manage and may cause environmental problems. Mining waste is often linked to contaminated mine drainage, including acidic waters with more or less elevated concentrations of trace metals such as lead. This work presents a study on the mobilization of lead from waste from two typical mining sites: Zeida and Mibladen, two now-closed former Pb–Zn mines in the Moulouya region of Morocco. Our research investigates the mobilization potential of Pb from the waste of these mines. The study involved acid–base neutralization capacity tests (ANC–BNC) combined with geochemical modeling. Experimental data allowed for the quantification of the buffering capacity of the samples and the mobilization rates of lead as a function of pH. The geochemical model was fitted to experimental results with thermodynamic considerations. The geochemical model allowed for the identification of the mineral phases involved in providing the buffering capacity of carbonated mining waste (Mibladen) and the meager buffering capacity of the silicate mining waste (Zeida). These cases are representative of contaminated neutral drainage (CND) and acid mine drainage (AMD), respectively. The results highlight the consistency between the ANC–BNC experimental data and the associated modeling in terms of geochemical behavior, validating the approach and identifying the main mechanisms involved. The modeling approach identifies the dissolution of the main solid phases, which impact the pH and the speciation of lead as a function of the pH. This innovative approach, combining ANC–BNC experiments and geochemical modeling, allowed for the accurate identification of mineral phases and surface complexation phenomena, which control the release of lead and its speciation in drainage solutions, as well as within solid phases, as a function of pH.

ANC–BNC Titrations and Geochemical Modeling for Characterizing Calcareous and Siliceous Mining Waste

Pyrite and calcite are mineral phases that play a major role in acid and neutral mine drainage processes. However, the prediction of acid mine drainage (AMD) or contaminated neutral drainage (CND) requires knowledge of the mineral composition of mining waste and the related potential for element release. This paper studies the combination of acid–base neutralizing capacity (ANC–BNC) with geochemical modeling for the characterization of mining waste and prediction of AMD and CND. The proposed approach is validated with three synthetic mineral assemblages: (1) siliceous sand with pyrite only, representing mining waste responsible for AMD, (2) siliceous sand with calcite and pyrite, representing calcareous waste responsible for CND, and (3) siliceous sand with calcite only, simulating calcareous matrices without any pyrite. The geochemical modeling approach using PHREEQC software was used to model pH evolution and main element release as a function of the added amount of acid or base over the entire pH range: 1 < pH < 13. For calcareous matrices (sand with calcite), the results are typical of a carbonated environment, the geochemistry of which is well known. For matrices containing pyrite, the results identify different pH values favoring the dissolution of pyrite: pH = 2 in a pyrite-only environment and pH = 6 where pyrite coexists with calcite. The neutral conditions can be explained by the buffering capacity of calcite, which allows iron oxyhydroxide precipitation. Major element release is then related to the dissolution and precipitation of the mineral assemblages. The geochemical modeling allows the prediction of element speciation in the solid and liquid phases. Our findings clearly prove the potential of combined ANC–BNC experiments along with geochemical modeling for the characterization of mining waste and the assessment of risk of AMD and CND.

Stabilization/solidification and recycling of sediment from Taihu Lake in China: Engineering behavior and environmental impact

Investigations of stabilized/solidified sediment (S/S sediment) by simulated field-construction processes (crushing and filling) are fundamental to evaluating the potential reuse as fill materials. A series of tests were conducted on the samples prepared from S/S sediment grains (SG), which was obtained by crushing the cement treated sediment. By sampling the SG with different field-curing durations (t1: 28, 35, 56 and 98 days) and measuring them by unconfined compressive strength (UCS) tests, the effect of t1 on the UCS was investigated. By continually curing the samples prepared from the SG with 28 field-curing days in laboratory for an additional 7, 28, 35 and 70 days (t2) and subjecting them to UCS and tank leaching tests with different ambient (leachate) pH values (1, 4, 7, 10 and 14), the effect of t2 and ambient pH was evaluated. Increasing t1 and t2 was found to significantly influence the strength of SG, which highlights the importance of an appropriate curing period. The releases of the metals (As, Cr, Cu, Zn, Pb, Ni, and Hg) in the SG exhibited a strongly pH-dependence but less correlation with t2. Neutral conditions (pH = 7) offered the best immobilization capacity for Cu; As, Cr, Ni and Zn exhibited the lowest release at pH = 10; the release of Pb decreased moderately with increasing pH. The S/S sediment complied with the acceptance criteria in terms of metal release and can be regarded as an environmentally friendly fill material. The results highlight the technical feasibility of stabilized sediment recycling in aquatic environment projects.

Use of hydraulic binders for reducing sulphate leaching: application to gypsiferous soil sampled in Ile-de-France region (France)

Polluted soils are a serious environmental risk worldwide and consist of millions of tons of mineral waste to be treated. In order to ensure their sustainable management, various remediation options must be considered. Hydraulic binder treatment is one option that may allow a stabilisation of pollution and thus offer a valorisation as secondary raw materials rather than considering them as waste. In this study, we focused on sulphate-polluted soil and tested the effectiveness of several experimental hydraulic binders. The aim was to transform gypsum into ettringite, a much less soluble sulphate, and therefore to restrict the potential for sulphate pollutant release. The environmental assessment of five formulations using hydraulic binders was compared to the gypsiferous soil before treatment (contaminated in sulphate). The approach was to combine leaching tests with mineralogical quantifications using among others thermogravimetric and XRD methods. In the original soil and in the five formulations, leaching tests indicate sulphate release above environmental standards. However, hydraulic binders promote ettringite formation, as well as a gypsum content reduction as observed by SEM. The stabilisation of sulphates is, however, insufficient, probably as a result of the very high content of gypsum in the unusual soil used. The mineralogical reactions highlighted during the hydration of hydraulic binders are promising; they could pave the way for the development of new industrial mixtures that would have a positive environmental impact by allowing reuse of soils that would otherwise be classified as waste.

Geotechnical and mineralogical characterisations of marine-dredged sediments before and after stabilisation to optimise their use as a road material

Dredging activities to extend, deepen and maintain access to harbours generate significant volumes of waste dredged material. Some ways are investigated to add value to these sediments. One solution described here is their use in road construction following treatment with hydraulic binders. This paper presents the characterisation of four sediments, in their raw state and after 90 days of curing following stabilisation treatment with lime and cement, using a combination of novel and established analytical techniques to investigate subsequent changes in mineralogy. These sediments are classified as fine, moderately to highly organic and highly plastic and their behaviour is linked to the presence of smectite clays. The main minerals found in the sediments using X-ray diffraction (XRD) and automated mineralogy are quartz, calcite, feldspars, aluminium silicates, pyrite and halite. Stabilisation was found to improve the mechanical performances of all the sediments. The formation of cementitious hydrates was not specifically detected using automated mineralogy or XRD. However, a decrease in the percentage volume of aluminium silicates and aluminium-iron silicates and an increase of the percentage volume of feldspars and carbonates was observed.

Effects of mineral amendments on trace elements leaching from pre-treated marine sediment after simulated rainfall events

Bauxite extraction by-products (red mud) were used to evaluate their potential ability to stabilize trace elements from dredged and aerated/humidified marine sediment. The investigated by-products were: bauxaline^®(BX) that is a press-filtered red mud; bauxsol™(BS) that is a press-filtered red mud previously washed with excess of seawater, and gypsum neutralized bauxaline^® (GBX). These materials were separately mixed to dredged composted sediment sample considering 5% and 20% sediment: stabilizer ratios. For pilot experiments, rainfall events were regularly simulated for 3 months. Concentrations of As, Mo, Cd, Cr, Zn, Cu, and Ni were analyzed in collected leachates as well as toxicity. Results showed that Cd, Mo, Zn, and Cu were efficiently stabilized in the solid matrix when 20% of BX, BS, and GBX was applied. Consequently, toxicity of leachates was lower than for the untreated sediment, meaning that contaminants mobility was reduced. A 5% GBX was also efficient for Mo, Zn and Cu stabilization. In all scenarios, As stabilization was not improved. Compared to all other monitored elements, Mo mobility seemed to depend upon temperature-humidity conditions during pilot experiments suggesting the need of further investigations.

Influence of enhancing electrolytes on the removal efficiency of heavy metals from Gabes marine sediments (Tunisia)

This study focused on the feasibility of the treatment of heavy metals-contaminated sediments from Gabes harbor (Tunisia) using enhanced electrokinetic process. It presented a laboratory short-time electrokinetic experiment. The enhancing agents, as citric, acetic acids and sodium dodecyl sulfate (SDS) were used regarding their low environmental hazard. The electrokinetic cell was specially designed in order to elaborate two experiments at the same time. This paper is composed of three parts. The first part introduces the characterization of Gabes sediments. The second part describes the design of laboratory electrokinetic cell and the followed methods. The third part is dedicated to the results analysis. Treatment efficiency revealed that more than 80% of lead was removed from Gabes marine sediments. The reduction of cooper concentration, in sediments after treatment, ranged from 74 to 87%. Despite, the high removal of cadmium that ranged from 58 to 79%, treated sediments presented Cd concentration above the threshold limit.

Environmental evaluation of dredged sediment submitted to a solidification stabilization process using hydraulic binders

PURPOSE

Dredging of sediments, a requirement for harbor maintenance, removes millions of tons of mineral wastes, contaminated at varying degrees with trace metals, from the water. In previous investigations, Cu and Zn have been identified as highly concentrated trace metals associated to sulfides, mineral phases sensitive to oxidation. In order to ensure their sustainable management, the solidification/stabilization (S/S) and/or the valorization of contaminated sediments as secondary raw materials is a way to be promoted. Indeed, their reuse as a substitute of sand in cemented mortar formulation would allow combining both treatment and valorization of such wastes.

METHODS

In the present study, the environmental assessment of mortars formulated with raw and weathered marine sediments (in particular contaminated with Cu, Pb and Zn), compared to sand reference mortars, was conducted through two kinetic leaching tests: weathering cell tests (WCTs), in which mortars were crushed and leached twice a week, and a tank monolith leaching test (MLT), in which leaching was performed on monolithic mortars with increasing leachate renewal time.

RESULTS

In both leaching tests, calcium and sulfur were released continuously from sediment mortars, showing the oxidation-neutralization processes of sulfides and carbonates. In the MLT, Cu was released by sediment mortars through diffusion, particularly by weathered mortars, at low concentrations during 60 days of the test duration. With the more aggressive WCT, Cu concentrations were higher at the beginning but became negligible after 7 days of testing. Pb was released through diffusion mechanisms until depletion in both tests, whereas Zn was particularly well immobilized in the cemented matrices.

CONCLUSIONS

The S/S process applied using hydraulic binders proved to be efficient in the stabilization of Cu, Pb, and Zn highly presents in studied sediments, and further valorization in civilian engineering applications could be considered.

Geochemical behavior and environmental risks related to the use of abandoned base-metal tailings as construction material in the upper-Moulouya district, Morocco

In some developing countries, base-metal residues that were abandoned in tailing ponds or impoundments are increasingly used as construction material without any control, engineering basis, or environmental concern. This uncontrolled reuse of mine tailings may constitute a new form of pollution risks for humans and ecosystems through metal leaching. Therefore, the aim of the current study is to assess mine drainage, metal mobility, and geochemical behavior of two abandoned mine tailings commonly used in the upper-Moulouya region (eastern Morocco) as fine aggregates for mortar preparation. Their detailed physical, chemical, and mineralogical properties were subsequently evaluated in the context of developing appropriate alternative reuses to replace their conventional disposal and limit their weathering exposure. The obtained results showed that both tailings contain relatively high quantities of residual metals and metalloids with lead (ranging between 3610 and 5940 mg/kg) being the major pollutant. However, the mineralogical investigations revealed the presence of abundant neutralizing minerals and low sulfide content which influence mine drainage geochemistry and subsequently lower metals mobility. In fact, leachate analyses from weathering cell kinetic tests showed neutral conditions and low sulfide oxidation rates. According to these results, the tailings used as construction material in the upper-Moulouya region have very low generating potential of contaminated effluents and their reuse as aggregates may constitute a sustainable alternative method for efficient tailing management.

Solidified structure and leaching properties of metallurgical wastewater treatment sludge after solidification/stabilization process

The presented study investigates solidification/stabilization process of hazardous heavy metals/arsenic sludge, generated after the treatment of the wastewater from a primary copper smelter. Fly ash and fly ash with addition of hydrated lime and Portland composite cement were studied as potential binders. The effectiveness of the process was evaluated by unconfined compressive strength (UCS) testing, leaching tests (EN 12457-4 and TCLP) and acid neutralization capacity (ANC) test. It was found that introduction of cement into the systems increased the UCS, led to reduced leaching of Cu, Ni and Zn, but had a negative effect on the ANC. Gradual addition of lime resulted in decreased UCS, significant reduction of metals leaching and high ANC, due to the excess of lime that remained unreacted in pozzolanic reaction. Stabilization of more than 99% of heavy metals and 90% of arsenic has been achieved. All the samples had UCS above required value for safe disposal. In addition to standard leaching tests, solidificates were exposed to atmospheric conditions during one year in order to determine the actual leaching level of metals in real environment. It can be concluded that the EN 12457-4 test is more similar to the real environmental conditions, while the TCLP test highly exaggerates the leaching of metals. The paper also presents results of differential acid neutralization (d-AN) analysis compared with mineralogical study done by scanning electron microscopy and X-ray diffraction analysis. The d-AN coupled with Eh-pH (Pourbaix) diagrams were proven to be a new effective method for analysis of amorphous solidified structure.

An innovative coupling between column leaching and oxygen consumption tests to assess behavior of contaminated marine dredged sediments

Contaminated dredged sediments are often considered hazardous wastes, so they have to be adequately managed to avoid leaching of pollutants. The mobility of inorganic contaminants is a major concern. Metal sulfides (mainly framboïdal pyrite, copper, and zinc sulfides) have been investigated in this study as an important reactive metal-bearing phase sensitive to atmospheric oxygen action. An oxygen consumption test (OC-Test) has been adapted to assess the reactivity of dredged sediments when exposed to atmospheric oxygen. An experimental column set-up has been developed allowing the coupling between leaching and oxygen consumption test to investigate the reactivity of the sediment. This reactivity, which consisted of sulfide oxidation, was found to occur for saturation degree between 60 and 90 % and until the 20th testing week, through significant sulfates releases. These latter were assumed to come from sulfide oxidation in the first step of the test, then probably from gypsum dissolution. Confrontation results of OC-Test and leachate quality shows that Cu was well correlated to sulfates releases, which in turn, leads to Ca and Mg dissolution (buffer effect). Cu, and mostly Zn, was associated to organic matter, phyllosilicates, and other minerals through organo-clay complexes. This research confirmed that the OC-Test, originally developed for mine tailings, could be a useful tool in the dredged sediment field which can allow for intrinsic characterization of reactivity of a material suspected to readily reacting with oxygen and for better understanding of geochemical processes that affect pollutants behavior, conversion, and transfer in the environment.