Trace metal and mineral speciation of remediated wastes using electron microscopy

Electron microscopic techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalyses (EPMA), were used to evaluate metal species and mineralogical phases associated with metal-bearing contaminated soil and industrial wastes that have been solidified and stabilized with Portland cement. Metals present in the wastes included arsenic, barium, cadmium, chromium, copper, lead, nickel, and zinc. In addition, mineral alterations and weathering features that affect the durability and containment of metals in aged remediated wastes were analyzed microscopically. Physical and chemical alteration processes identified included: freeze-thaw cracking; cracking caused by the formation of expansive minerals, such as ettringite and thaumasite; carbonation; and the movement of metals from waste aggregates into the surrounding cement matrix. Preliminary results show that although the extent of degradation after 6 years is considered slight to moderate, evaluations of durability and permanence of metals containment cannot be based on leaching and bulk chemistry analyses alone. The use of electron microscopic analyses is vital in studies that evaluate trace metal and mineral species and that attempt to predict the long-term performance of metal containment in solidified and stabilized wastes.     

The Effect of Different Bentonites on Cement Hydration During Solidification/Stabilization of Tannery Wastes

In the present work, a Portland cement blended with calcium carbonate is being used to study the solidification/stabilization (S/S) of a Brazilian tanning waste arising from leather production. Chromium is the element of greatest concern in this waste, but the waste also contains a residual organic material. Using thermogravimetry (TG) and derivative thermogravimetry (DTG) to identify and quantify the main hydrated phases present in the pastes, this paper presents a comparative study between the effects of Wyoming and Organophilic bentonites (B and OB) on cement hydration. Samples containing combinations of cement, B, OB and waste have been subjected to thermal analysis after different setting times during the first 28 days of the waste S/S process. Both bentonites affect the cement hydration, with no significant differences in hydration degree after 1 week. This work shows further examples of the great utility of thermal analysis techniques in the study of very complex systems containing both crystalline and amorphous mineral materials as well as organics. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tannery waste solidification and stabilization

The stabilization/solidification of tannery waste containing chromium was studied as an option for its treatment and final disposal, by using a Portland cement type II and two different commercial bentonites (sodium and organophilic) as additives. Different compositions were evaluated by compressive strength analysis, porosity measurement, leaching tests and thermal analysis. The effect on the compressive strength is directly related to the resulting effect of the components present in the original paste on the hydration degree of the cement, which can be evaluated by thermogravimetric analysis from the dehydration steps of tobermorite and ettringite phases of the pastes. The results show that this process is suitable for the treatment of the tanning waste and that the best conditions of stabilization are obtained when both additives are used. This revised version was published online in August 2006 with corrections to the Cover Date.     

城市生活垃圾焚烧飞灰重金属的浸出特性

对用流化床焚烧炉混烧垃圾和煤的布袋飞灰进行了重金属的TCLP(Toxicity Characteristic Leaching Procedure)浸出特性实验，探讨了液固比、初始pH值及浸出时间对飞灰中重金属Pb、Cr、Cd、Ni、Cu、Zn的浸出影响。结果表明，重金属的浸出量都随着液固比的增加而增加，其中Cr、Cu一般呈上升趋势，Cd、Ni、Zn在液固比大于20时曲线变化较平缓，Pb的浸出规律比较特殊，有一个明显的波峰和波谷。重金属在浸取液的pH≤2.90时的浸出浓度远远大于pH≥4.03时的浸出浓度。Pb、Cr、Zn随着浸出时间的增加，浸出浓度下降，而Cd、Ni上升，Cu是先上升后下降。在液固比、初始pH值及浸出时间这三个影响因素中，pH值对重金属的浸出影响较大，重金属在酸性环境下较易浸出。     

Measurement and modeling of the surface potential evolution of hydrated cement pastes as a function of degradation

Hydrated cement pastes (HCP) have a high affinity with a lot of (radio)toxic products and can be used as waste confining materials. In cementitious media, elements are removed from solution via (co)precipitation reactions or via sorption/diffusion mechanisms as surface complexation equilibria. In this study, to improve the knowledge of the surface charge evolution vs the degradation of the HCP particles, two cements have been studied: CEM-I (ordinary Portland cement, OPC) and CEM-V (blast furnace slag and fly ash added to OPC). Zeta potential measurements showed that two isoelectric points exist vs HCP leaching, i.e., pH. Zeta potential increases from -17 to +20 mV for pH 13.3 to pH 12.65 (fresh HCP states) and decreases from 20 to -8 mV for pH 12.65 to 11 (degraded HCP states). The use of a simple surface complexation model of C-S-H, limited in comparison with the structural modeling of C-S-H in literature, allows a good prediction of the surface potential evolution of both HCP. Using this operational modeling, the surface charge is controlled by the deprotonation of surface sites (>SO(-)) and by the sorption of calcium (>SOCa(+)), which brings in addition a positive charge. The calcium concentration is controlled by portlandite or calcium silicate hydrate (C-S-H) solubilities.     

Application of isothermal and isoperibolic calorimetry to assess the effect of zinc on cement hydration

The amount of zinc in the clinker or in the secondary raw materials has been increasing in recent years. Zinc can get to Portland cement from solid waste or tires which are widely used as a fuel for burning in a rotary kiln. The aim of this work was to determine the effect of zinc on Portland cement hydration. This effect was studied by isothermal and isoperibolic calorimetry. Both calorimetry methods are suitable for measurements during the first days of hydration. Isoperibolic calorimetry monitors hydration process in real-life conditions, while isothermal calorimetry does it at a defined chosen temperature. Zinc was added to the cement in the form of two soluble salts of Zn(NO₃)₂, ZnCl₂ and a poorly soluble compound ZnO. The concentration of zinc added was chosen as 0.05, 0.1, 0.5 and 1 mass%. The results show that increasing amounts of zinc ions in cement pastes lead to hydration retardation and reduce both the maximum temperature and the maximum heat flow due to the retarding effect of zinc. The newly formed compounds during hydration were identified by X-ray diffraction method.     

Analytical application of organic reagents in hydrophobic gel media-I General principle and the use of dithizone gel

Two types of hydrophobic gel particles containing dithizone were prepared, either by the swelling of low cross-linkage polystyrene beads with a dithizone solution in chlorobenzene, or by the gelatinization of a dithizone solution with dibenzalsorbitol. The extraction capacity of the gel particles for metal ions such as mercury, cadmium, zinc and lead from dilute aqueous solution, was investigated at various dithizone concentration and pH values. A column packed with such gel particles is found to be useful for the selective preconcentration or the selective trapping of specific metal ions at low concentration levels.     

Adsorption of polyelectrolytes and its influence on the rheology, zeta potential, and microstructure of various cement and hydrate phases

In this study the influence of polycarboxylate-based polyelectrolytes on the particle interaction among tricalcium silicate (C(3)S, main clinker phase), calcium silicate hydrates (CSH), and calcium aluminate sulfate hydrates (ettringite) (main hydration phases) has been examined. These phases are the constituents of major concern during early hydration of cement suspensions. The results of zeta potential measurements on single mineral phase experiments show that the phases C(3)S and CSH are positively charged in synthetic pore solution (liquid phase of hydrating cement suspension), whereas the ettringite is negatively charged. Due to these opposite charges, ettringite crystals should coagulate with CSH phases and/or deposit on surfaces of the much larger C(3)S clinker particles. This behavior was proven by cryo-microscopic analysis of high-pressure frozen cement suspensions, which illustrates the consequences of colloidal mechanisms on the microstructure of early cement suspensions. Furthermore, it is shown that the polyelectrolytes have a much higher adsorption affinity to ettringite surfaces (hydrate phase) compared to silicate surfaces. However, the results from rheology experiments reveal that the presence of polyelectrolytes has a strong impact on the suspension properties of all investigated mineral phases by decreasing yield stress and plastic viscosity. From the results it can be concluded that the ettringite is the dominant mineral phase in terms of the state of dispersion which includes particle-particle and particle-polyelectrolyte interaction in the bulk cement system.     

Synthesis of Pure Cementitious Phases by Sol‐Gel Process as Precursor

Calcium silicates and aluminates are the main constituents of ordinary Portland cement (OPC) and calcium aluminate cements (CAC) and therefore the pure phases are of great importance for the investigation of interactions between binder and additives or admixtures. Additionally, investigations on clinker phases doped with foreign ions enable the improvement of the performance of cements. For this purpose great amounts of pure phases are needed. These phases are hard to synthesize via a solid state reaction of solid educts. Thus there is a need for a new, more efficient route to synthesize these phases. The sol‐gel process as precursor provides an alternative to the conventional method. In this paper experimental evidence is presented for an improved synthesis of calcium silicates and aluminates via sol‐gel processes, the characterisation of these clinker phases and their hydration behaviour.     

Ultrasonic-assisted leaching of trace metals from sediments as a function of pH

The effects of ultrasonication on the leaching of trace metals form sediments as a function of pH have been investigated using atomic absorption spectrometry. Cu, Pb, Ni, Zn and Mn were leached from sediments by ultrasonic effect using phthalate buffers at pH values of 2.2-6.0. Parameters influencing leaching, such as leaching time, solution matrix, wet or dry sample and final pH were evaluated. Results from ultrasonic leaching experiments were comparable to those of conventional procedure. In addition, this ultrasonic-assisted leaching method reduces the time required for conventional method approximately from 12 h to 25 min. Depending on the metal and sample type, metal removal increased linearly or exponentially with decreasing pH. The accuracy of the method was tested by comparing obtained results with this of conventional method. The average relative standard deviation (R.S.D.) of ultrasonic-assisted leaching method (ULM) varied between 1.71 and 3.00% for N = 36, depending on the analyte. This technique shows promise for studying chemical and biological availability and uptake/release processes for metals in sediment and soil as a function of pH.     

"Acid extractable" metal concentrations in solid matrices: a comparison and evaluation of operationally defined extraction procedures and leaching tests

Different frequently used methods to determine the influence of acid conditions on heavy metal release from soils, sediments and waste materials, namely pH_stat leaching tests and acid extractions with acetic acid (HOAc) (0.11 M and 0.43 M) and sodium acetate (NaOAc) (1 M) were compared for 30 samples (soils, sediments and waste materials) with different physico-chemical properties and a different degree of contamination. However, no distinct relationship was found between physico-chemical sample characteristics, total element concentrations and acid-extractable metal concentrations in the presented dataset.

pH played an important role in explaining the release of metals from the contaminated soils, sediments and waste materials. The pH-shift after extraction with the different acetic acid solutions (0.11 M and 0.43 M) was both explained by the initial pH of the sample and its acid neutralizing capacity. The pH of the NaOAc extract was well buffered and the release of elements from solid matrices by NaOAc was both the result of the complexation with acetate and pH (pH 5). Generally, a linear correlation was found between the amount of Zn and Cd extracted by 0.11 M HOAc, 0.43 M HOAc and 1 M NaOAc. The amounts of Zn and Cd extracted with HOAc (0.11 M and 0.43 M) were comparable with amounts of respectively Zn and Cd released during pH_stat leaching at pH 4. However, for Cu, Pb and As, it was often not possible to relate the results of a pH_stat leaching test to the results of single extractions with acetic acid solutions.     

The effects of solvents on C–S–H as determined by thermal analysis

Un-hydrated Portland cement consists of several anhydrous and reactive phases, that when mixed with water react to form hydrates. The main hydration product of Portland cement is calcium silicate hydrate (C–S–H). It is the main binding phase in a concrete system, hence is important to construction chemists. The concrete engineer measures the compressive strength of concrete after prescribed hydration periods, typically 1, 3, 7, 28 days. It is often convenient to mimic these intervals by stopping the hydration reaction at the same times. Several techniques can be employed to stop this hydration reaction. One of which is solvent-based and involves mixing a polar solvent such as acetone or isopropyl alcohol, with the hydrated cement. This mixing should be vigorous enough to blend the free water, in the partially hydrated cement system, with the polar solvent without altering the cement system’s matrix. The solvent-water mixture has a much lower boiling point and the mixture quickly evaporates out of the system. This achieves two goals. It stops the hydration reaction at the moment of solvent mixing, and it removes free water to prevent further hydration from occurring. This procedure theoretically leaves behind a dry, chemically unaltered, partially hydrated cement paste. In this way, pastes can be analyzed after the prescribed 1, 3, 7 or 28 days of hydration. This paper uses thermogravimetric analysis (TG) results to investigate the assumption that solvents have no thermodynamic or chemical effect on the hydrated cement paste phases.     

Swelling effects in cross-linked polymers by thermogravimetry

A Brazilian coal power plant generates a waste composed by the fly and bottom ashes produced from coal combustion and by a spent sulfated lime generated after SO₂ capture from combustion gases. This work presents a study of the early stages of the hydration of composites formed by this waste and a type II Portland cement, which will be used for CO₂ capture. The cement substitution degrees in the evaluated composites were 10, 20, 30 and 40%, and the effect of the coal power unit waste on the hydration reaction was analyzed on real time by NCDTA, during the first 40 h of hydration. The results show that the higher is the substitution degree, the higher is the retarding effect on the cement hydration process. Actually, by respective thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis on initial cement mass basis, this effect is caused by double exchange reactions among Ca and Mg components of the waste, during the first 4 h of hydration, which promote a much higher exothermic effect in the NCDTA curve, simultaneously to respective induction periods. The pozzolanic reactions, due to the presence of the waste silica and alumina containing amorphous phases, consume part of the original Ca(OH)₂ content existent in the waste in the case of 30 and 40% substituted pastes, and also from part of the Ca(OH)₂ produced in cement hydration reactions, in the case of the 10 and 20% substituted pastes.     

Simultaneous determination of traces of iron, cobalt, nickel, copper, mercury and lead in water by energy-dispersive x-ray fluorescence spectrometry after preconcentration as their piperazino-1,4-bis(dithiocarbamate) complexes

A bidentate chelating agent has been proposed to preconcentrate seven metal ions dissolved in an aqueous sample for their simultaneous determination using energy-dispersive x-ray fluorescence spectrometry. The metal ions are precipitated as their polymeric piperazino-1,4-bis(dithiocarbamate) chelates, which are then collected by vacuum filtration on a Millipore membrane filter for direct examination by x-ray fluorescence analysis. Iron, cobalt, nickel, copper and zinc are determined by means of their K x-rays and mercury and lead by means of their L x-rays. A detection limit in the μg1⁻¹ range can be achieved for all metals tested in 250-ml water samples with a counting time of 600 s. Effective precipitation of all metals occurs at pH 6–7. The recoveries of eight analyses of the metals in a multielement standard using the proposed method ranged from 97 to 105% and the precision ranged from 2.3 to 3.1%. High concentrations of calcium and magnesium do not interfere with the method. The method is simple, sensitive and accurate, and has been used for the simultaneous determination of the seven metals under study in environmental samples and synthetic mixtures.     

Simultaneous solidification of two catalyst wastes and their effect on the early stages of cement hydration

Two catalyst wastes (RNi and RAI) from polyol production were considered as hazardous, due to their respective high concentration of nickel and aluminum contents. This article presents the study, done to avoid environmental impacts, of the simultaneous solidification/stabilization of both catalyst wastes with type II Portland cement (CP) by non-conventional differential thermal analysis (NCDTA). This technique allows one to monitor the initial stages of cement hydration to evaluate the accelerating and/or retarding effects on the process due to the presence of the wastes and to identify the steps where the changes occur. Pastes with water/cement ratio equal to 0.5 were prepared, into which different amounts of each waste were added. NCDTA has the same basic principle of Differential Thermal Analysis (DTA), but differs in the fact that there is no external heating or cooling system as in the case of DTA. The thermal effects of the cement paste hydration with and without waste presence were evaluated from the energy released during the process in real time by acquiring the temperature data of the sample and reference using thermistors with 0.03 °C resolution, coupled to an analog–digital interface. In the early stages of cement hydration retarding and accelerating effects occur, respectively due to RNi and RAl presence, with significant thermal effects. During the simultaneous use of the two waste catalysts for their stabilization process by solidification in cement, there is a synergic resulting effect, which allows better hydration operating conditions than when each waste is solidified separately. Thermogravimetric (TG) and derivative thermogravimetric analysis (DTG) of 4 and 24 h pastes allow a quantitative information about the main cement hydrated phases and confirm the same accelerating or retarding effects due to the presence of wastes indicated from respective NCDTA curves.     

Effect of Hydrated Sodium Silicates on Cement Paste Hardening

The hydration of cement clinker minerals occurs at a high rate in the presence of sodium silicates owing to a decrease in the initial pH of pore solutions of cement pastes and to intensive formation of calcium hydrosilicates.     

Pollutants leaching behaviour from solidified wastes: a selection of adapted various models

Leaching tests are essential in the environmental assessment of stabilized wastes. Research programmes were conducted on their interpretation in order to develop tools for the evaluation of long term release of pollutants contained in solidified wastes. Models for the leaching of porous materials are discussed in this paper according to the specificity of the chemical species (i.e. transport model with total dissolution of species-diffusional model; transport model with progressive dissolution of species due to limitation of solubility-shrinking core model; and the model coupling transport and chemical phenomena). The leaching behaviour of pollutants (i.e. lead) solidified in a cement matrix was studied under different chemical conditions. Results have shown that the release of species whose solubilities depend on the physico-chemical conditions, and especially the pH (e.g. amphoteric metals), is governed by the solubility of the species in the pore water at local conditions and by the pH evolution within the matrix. A coupled dissolution/diffusion model was developed to describe the release of chemically complex species contained in a porous medium in contact with water. Leaching tests of cement matrices and artificial porous matrices containing calcium hydroxide and pollutants were conducted in order to validate the coupled dissolution/diffusion model. A good assessment of the retention of some pollutants contained in cement matrices could then be obtained by the association of two tests: solubilization of the pollutants related to the chemical context (pH) under steady state conditions and monolithic long term dynamic leaching tests in order to characterize the evolution of the chemical context (pH) and consequently the release of pollutants. The objective is to integrate this approach in the standardization process (CEN TC 292- WG 6, in progress).     

Thermogravimetrical analysis for monitoring carbonation of cementitious materials

Cement paste carbonation, i.e., the reaction between CO₂ and the hydrated cement phases, mainly calcium hydroxide or portlandite, can lead to a pH decrease, which in turn can give rise to steel corrosion in reinforced concrete. At the same time, the carbonation reaction contributes to combine CO₂ and fix it as calcium carbonate. It is a crucial phenomenon from the point of view of structure durability and also for cement-based materials sustainability. Cement paste specimens with two w/c ratios and eight types of cements were submitted to different environmental conditions for 4?years and the evolution of calcium carbonate formed or carbon dioxide bound was followed by TG performed in inert atmosphere. The amounts of calcium hydroxide, evaporable and C?CS?CH gel water were also measured. The CO₂ bound follows the same trend in all samples and environments: at the beginning there is a sharp increase followed by a very slow stretch and reaching a maximum after less than 2?years in most cases. The calcium hydroxide amounts evolve very differently in each environment. While outside it is almost consumed after 1?year, inside there is a decrease in the first year, but an increase in the next 3?years. The behavior of the C?CS?CH water in both environments is similar to that of the portlandite inside. The evaporable water diminishes in all cases to 1?%. From the data obtained by TG, the quantification of the C?CS?CH gel as well as the calculation of the Ca/Si ratio and the hydration of the gel formed by different type of binders has been possible.     

Voltammetric methods for the determination of heavy metals in domestic waste and compost produced from it

Methods of sampling an preparation of waste and compost samples for analysis are described. A voltammetric method has been used for the determination of the contents of toxic heavy metals (one of the most important criterion for compost quality evaluation) in domestic waste and in compost produced from it. A novel method was proposed for the UV mineralisation of water extracts obtained during leaching of waste and compost samples. Copper, lead, zinc and cadmium were determined in wet-digested samples of domestic waste, compost and its extracts by anodic stripping voltammetry. Nickel and cobalt were determined by adsorption voltammetry. The determination of five metals in one sample over a wide range of concentrations and the low cost of the apparatus used are the main advantages of the analytical method described. This has been shown by the determination of the metals in fractions of domestic waste, in compost produced of that waste and in compost mixed with sewage sludge. Special attention has been paid to investigations of the mobilisation of metals from waste and compost during the leaching test.     

Study by thermogravimetry of the evolution of ettringite phase during type II Portland cement hydration

Thermogravimetry (TG) and derivative thermogravimetry (DTG) have been used by the authors as very effective tools to study hydration steps of cements used for solidification/stabilization of tanning wastes. The present paper presents a method which was applied to separate the peaks shown by DTG curves of type II Portland cement pastes, analyzed at different times during the first 4 weeks of setting. Through a specific software a more detailed study of the evolution of the cement hydration may be done, which allows the measurement of the amount of hydrated water present in tobermorite gel as well as in ettringite, which are the main phases formed from the original components of the cement. The number of moles of water present in the ettringite phase calculated by the method is in very good agreement with the values found in the literature, validating the method to calculate the same parameter in tobermorite gel. In the latter case the water content decreases significantly during the first day of hydration, then remains at a constant value over the rest of the analyzed period.This revised version was published online in November 2005 with corrections to the Cover Date.