Immobilisation of heavy metal in cement-based solidification/stabilisation: a review

Heavy metal-bearing waste usually needs solidification/stabilization (s/s) prior to landfill to lower the leaching rate. Cement is the most adaptable binder currently available for the immobilisation of heavy metals. The selection of cements and operating parameters depends upon an understanding of chemistry of the system. This paper discusses interactions of heavy metals and cement phases in the solidification/stabilisation process. It provides a clarification of heavy metal effects on cement hydration. According to the decomposition rate of minerals, heavy metals accelerate the hydration of tricalcium silicate (C3S) and Portland cement, although they retard the precipitation of portlandite due to the reduction of pH resulted from hydrolyses of heavy metal ions. The chemical mechanism relevant to the accelerating effect of heavy metals is considered to be H+ attacks on cement phases and the precipitation of calcium heavy metal double hydroxides, which consumes calcium ions and then promotes the decomposition of C3S. In this work, molecular models of calcium silicate hydrate gel are presented based on the examination of 29Si solid-state magic angle spinning/nuclear magnetic resonance (MAS/NMR). This paper also reviews immobilisation mechanisms of heavy metals in hydrated cement matrices, focusing on the sorption, precipitation and chemical incorporation of cement hydration products. It is concluded that further research on the phase development during cement hydration in the presence of heavy metals and thermodynamic modelling is needed to improve effectiveness of cement-based s/s and extend this waste management technique.     

Modelling the effects of waste components on cement hydration

Ordinary Portland Cement (OPC) is often used for the solidification/stabilization (S/S) of waste containing heavy metals and salts. These waste components will precipitate in the form of insoluble compounds on to unreacted cement clinker grains preventing further hydration. In this study the long term effects of the presence of contaminants in solidified waste is examined by numerically simulating cement hydration after precipitation of metal salts on the surface of cement grains. A cement hydration model was extended in order to describe pore water composition and the effects of cement grain coating. Calculations were made and the strength development predicted by the model was found to agree qualitatively with experimental results found in literature. The complete model is useful in predicting the strength and leaching resistance of solidified products and developing solidification recipes based on cement.     

A thermodynamic approach to the hydration of sulphate-resisting Portland cement

A thermodynamic approach is used to model changes in the hydrate assemblage and the composition of the pore solution during the hydration of calcite-free and calcite-containing sulphate-resisting Portland cement CEM I 52.5 N HTS. Modelling is based on thermodynamic data for the hydration products and calculated hydration rates for the individual clinker phases, which are used as time-dependent input parameters. Model predictions compare well with the composition of the hydrate assemblage as observed by TGA and semi-quantitative XRD and with the experimentally determined compositions of the pore solutions. The calculations show that in the presence of small amounts of calcite typically associated with Portland cement, C-S-H, portlandite, ettringite and calcium monocarbonate are the main hydration products. In the absence of calcite in the cement, however, siliceous hydrogarnet instead of calcium monocarbonate is observed to precipitate. The use of a higher water-to-cement ratio for the preparation of a calcite-containing cement paste has a minor effect on the composition of the hydrate assemblage, while it significantly changes the composition of the pore solution. In particular, lower pH value and higher Ca concentrations appear that could potentially influence the solubility and uptake of heavy metals and anions by cementitious materials.     

Solidification/stabilisation of air pollution control residues using Portland cement: Physical properties and chloride leaching

Portland cement (CEMI) was used to solidify air pollution control (APC) residues from an energy-from-waste plant burning municipal solid waste. APC residue/CEMI mixes were prepared with CEMI additions ranging from 0 to 50 weight% (wt%) of total dry mass and water/solids ratios between 0.40 and 0.80. Isothermal conduction calorimetry was used to assess the effect of APC residues on the hydration of CEMI. Although up to 30wt% additions of APC residues accelerated CEMI hydration, the total heat of hydration during the initial 98h was significantly reduced. Higher levels of APC residues severely inhibited CEMI hydration. The consistence, setting time, compressive strength, porosity and chloride leaching characteristics of the solidified products were determined. As might be expected, increasing the CEMI addition and reducing the water content resulted in increased compressive strengths. All mixes achieved compressive strengths greater than 1MPa at 7 and 28days but only 50wt% samples did not show significant strength reduction when tested after immersion in water. Monolithic leaching tests indicated low physical immobilisation of chloride in the CEMI solidified APC residues, with chloride leaching in excess of relevant UK landfill waste acceptance criteria (WAC). The results of this study show that greater than 50% CEMI additions would be required to effectively treat APC residues to meet current WAC limits.     

Microstructure of Portland cement pastes containing metal nitrate salts

In recent years, Backscattered Scanning Electron microscopy techniques (BSE), coupled with an image analysis system have been recognised as a powerful tool for quantitative analysis. This paper investigates the effect of metal additions (Ba, Cu, Ni, Zn, Cr(III), Pb and Cd) to Portland cement to produce a solidified product which meets the durability criteria quantified by the ratio of hydrated products and porosity. In addition, other indicators of the progress of cement hydration such as the bulk density and evaporable water of the solidified products were also measured. Metal concentrations of 0.1 and 1% per weight of cement at a constant water/cement ratio of 0.4 were examined. The same measurements were conducted on control samples of different water/ cement ratio. The results have shown that the control samples at different W/C ratio showed consistent trend in residual cement porosity, density and evaporable water content. It also showed that low dosage of metal nitrate additions can reduce cement hydration by up to 50% and at the same time reduce the observable porosity. Overall, this work has shown that Scanning Electron Microscopy (SEM) and image analysis are powerful tools and could be used to quantify the observable porosity and cement hydration in solidified systems.     

Stabilization of chloro-organics using organophilic bentonite in a cement-blast furnace slag matrix.

The application of cement-based stabilisation/solidification treatment to organic-containing wastes is made difficult by the adverse effect of organics on cement hydration. The use of organophilic clays as pre-solidification adsorbents of the organic compounds can reduce this problem because of the high adsorption power of these clays and their compatibility with the cementitious matrix. This work presents an investigation of the effect on hydration kinetics, physico-mechanical properties and leaching behaviour of cement-based solidified waste forms containing 2-chlorophenol and 1-chloronapthalene adsorbed on organophilic bentonites. These were prepared by cation exchange with benzyldimethyloctadecylammonium chloride and trimethyloctadecylammonium chloride. The binder was a 30% pozzolanic cement, 70% granulated blast furnace slag mixture. Several binder-to-bentonite ratios and different concentrations of the organics on the bentonite were used. Kinetics of hydration were studied by measurement of chemically bound water and by means of thermal and calorimetric analyses. Microstructure and other physico-mechanical properties of the solidified forms were studied by means of mercury intrusion porosimetry, scanning electron microscopy and unconfined compressive strength measurement. Leaching was checked by two different leaching tests: one dynamic, on monolithic samples, and the other static, on powdered samples. This study indicates that the incorporation of the organic-loaded bentonite in the binder matrix causes modifications in the hardened samples by altering cement hydration. The effects of the two organic contaminants are differentiated.     

The influence of mix parameters and binder choice on the carbonation of cement solidified wastes

This paper explores the kinetics of carbonation of cement-based solidified hazardous waste. This study is part of a wide investigation into the effects of carbonation on solidified waste forms. Two commercially produced heavy metal wastes were solidified with three different types of Portland cement and two mineral admixtures and carbonated under controlled conditions. Measurements of the uptake of carbon dioxide were made for the different mixes and areas showing the degree of carbonation for each cement system were defined. The effects of water/binder ratio, waste and binder type on both total uptake of carbon dioxide and rate of carbonation were investigated and are discussed.     

Municipal incineration bottom ash treatment using hydrothermal solidification

Solidification of municipal incineration bottom ash (MIBA) has been carried out using a hydrothermal processing method, in which the MIBA was first compacted in a mold at 5-20 MPa, and then hydrothermally cured in an autoclave under saturated steam pressure at 150-250 degrees C for 10-72 h. Experimental results showed that the tensile strength of the solidified body was greatly influenced by the addition of NaOH solution and fresh cement in the MIBA. The hydrothermal curing temperature and time exerted a significant influence on the development of tensile strength of solidified body. The strength development is speculated to be due primarily to the formation of 1.1 nm tobermorite. Laboratory leaching tests were conducted to determine the amount of heavy metals dissolved from the solidified bodies and the results showed that under the hydrothermal conditions of this study the leaching of heavy metals was very low. As such, the hydrothermal processing method may have a high potential for recycling MIBA.     

Hydration and leaching characteristics of cement pastes made from electroplating sludge

The purpose of this study was to investigate the hydration and leaching characteristics of the pastes of belite-rich cements made from electroplating sludge. The compressive strength of the pastes cured for 1, 3, 7, 28, and 90 days was determined, and the condensation of silicate anions in hydrates was examined with the ²⁹Si nuclear magnetic resonance (NMR) technology. The leachabilities of the electroplating sludge and the hardened pastes were studied with the multiple toxicity characteristic leaching procedure (MTCLP) and the tank leaching test (NEN 7345), respectively. The results showed that the electroplating sludge continued to leach heavy metals, including nickel, copper, and zinc, and posed a serious threat to the environment. The belite-rich cement made from the electroplating sludge was abundant in hydraulic β-dicalcium silicate, and it performed well with regard to compressive-strength development when properly blended with ordinary Portland cements. The blended cement containing up to 40% the belite-rich cement can still satisfy the compressive-strength requirements of ASTM standards, and the pastes cured for 90 days had comparable compressive strength to an ordinary Portland cement paste. It was also found that the later hydration reaction of the blended cements was relatively more active, and high fractions of belite-rich cement increased the chain length of silicate hydrates. In addition, by converting the sludge into belite-rich cements, the heavy metals became stable in the hardened cement pastes. This study thus indicates a viable alternative approach to dealing with heavy metal bearing wastes, and the resulting products show good compressive strength and heavy-metal stability.     

Evaluation of leaching characteristics and environmental compatibility of solidified/stabilized industrial waste

An attempt was made to judge the environmental compatibility, the governing leaching mechanism, and mobility patterns in a waste–cement solidified/stabilized matrix after 21 days of curing. The mixed sludge from a steel pickling facility was stabilized by ordinary Portland cement. Iron was the major leachable metal, along with Pb, Zn, and Mn. To characterize the leaching behavior, the mobility and mechanism of leaching were determined by diffusion leaching tests. In the solidified/stabilized matrix, the mobility of all the elements studied was reduced, and average to low mobility was observed. Values for the negative log of the effective diffusion coefficient of more than 12.5 were obtained for the metals. The dominant leaching mechanisms were surface wash off in the initial stage, followed by diffusion. The environmental performance of the solidified/stabilized product when considered in terms of mass leached over 64 days was found to be satisfactory for consideration for the safe disposal and reuse of waste.     

Physical containment of municipal solid waste incineration bottom ash by accelerated carbonation

Accelerated carbonation of municipal solid waste incineration residues is effective for immobilizing heavy metals. In this study, the contribution of the physical containment by carbonation to immobilization of some heavy metals was examined by some leaching tests and SEM–EDS analysis of untreated, carbonated, and milled bottom ash after carbonation that was crushed with a mortar to a mean particle size of approximately 1 μm. The surface of carbonated bottom ash particles on SEM images seemed mostly coated, while there were uneven micro-spaces on the surface of the untreated bottom ash. Results of Japan Leaching Test No. 18 (JLT18) for soil pollution showed that milling carbonated bottom ash increased the pH and EC. The leaching concentration of each element tended to be high for untreated samples, and was decreased by carbonation. However, after the milling of carbonated samples, the leaching concentration became high again. The immobilization effect of each element was weakened by milling. The ratio of physical containment effect to immobilization effects by accelerated carbonation was calculated using the results of JLT18. The ratio for each element was as follows: Pb: 13.9–69.0 %, Cu: 12.0–49.1 %, Cr: 24.1–99.7 %, Zn: 20.0–33.3 %, and Ca: 28.9–63.4 %.     

Study of the treatment effectiveness of a solidification/stabilization process for waste bearing heavy metals

The sludge from a steel processing unit bearing zinc, lead, iron, and manganese was solidified with ordinary Portland cement. The waste was stabilized in the specimens with a waste/binder ratio range of 0.16–4.0. On the basis of the available leaching and unconfined compressive strength, the performance of the solidified/stabilized waste was compared for different numbers of curing days. It was found that curing up to 28 days resulted in a performance improvement, as shown by less leaching of heavy metals and the increased unconfined compressive strength of the specimen. The treatment effectiveness of the solidification/stabilization process was assessed for the metals Pb, Zn, Fe, and Mn, and was found to be 89%, 95%, 74%, and 90%, respectively, for an optimum ratio of 4.0 after 28 days of curing.     

Microstructural study of carbonated cement-solidified synthetic heavy metal waste

Synthetic wastes have been widely employed to help elucidate the complex interactions between real wastes and hydraulic binders during solidification. In this work, a laboratory produced metal waste mixed with Portland cement and immediately carbonated it using an accelerated method. The microstructures of carbonated and non-carbonated control samples were distinct despite both being dominated by unusually large phenograins derived from the waste. In the carbonated sample waste phenograins remained unaltered, whereas cement grains were largely decalcified. As a consequence of decalcification, observable porosity was significantly reduced by the formation of precipitated carbonates.     

Hybrid cement-assisted dewatering,solidification and stabilization of sewage sludge with high organic content

Sewage sludge with high organic content is particularly difficult to dewater before disposal in landfill. In this study, different hybrid cement binders were investigated to evaluate their ability to dewater the sewage sludge with high organic content. After 7 days of stabilization, the CASC (Mayenite/Sulfoaluminate cement) hybrid binder showed an excellent efficiency on both water content reduction and strength development; the water content and unconfined compressive strength value of solidified sludge reached 52.43 % and 109.55 kPa, respectively, at 8 % binder/sludge mass rate. The horizontal vibration leaching test (HJ 557-2009) indicated that leachability of heavy metals of the CASC-solidified sludge was far lower than that of non-solidified sludge and CAPC-solidified sludge. Furthermore, SEM and XRD analyses suggested that certain hydrates formed in the solidification process might have accelerated the depletion of interstitial water and strength development in the CASC-solidified sludge.     

Solidification and recycling of incinerator bottom ash through the addition of colloidal silica (SiO2) solution

The possibility of using incinerator bottom ash as a substitute for natural aggregates was investigated. Rough, porous surface of bottom ash, which diminishes the strength of solidified products, was improved by colloidal silica solution. As a result, a significant increase of mechanical strength was accomplished by a slight amount of silica (<1 wt% to total). Moreover, pozzolanic reaction was induced in initial cement hydration due to the nano-particle size of about 20 nm in colloidal silica solution. Cylindrical specimens and bricks were prepared from bottom ash added to a colloidal silica (SiO2) solution and cement, and then their compressive strengths were evaluated. Cylindrical specimens showed an increase of approximately 60% in compressive strength when colloidal solution containing 4 wt% silica particles was sprayed onto the bottom ash. The strength of bricks containing colloidal silica was in excess of 20 MPa, which meets the requirement of construction materials. Results of leaching tests based on Toxicity Characteristic Leaching Procedure (TCLP) proved that the solidified bottom ash possessed good chemical stability.     

Leaching of heavy metals from solidified waste using Portland cement and zeolite as a binder

This study investigated the properties of solidified waste using ordinary Portland cement (OPC) containing synthesized zeolite (SZ) and natural zeolite (NZ) as a binder. Natural and synthesized zeolites were used to partially replace the OPC at rates of 0%, 20%, and 40% by weight of the binder. Plating sludge was used as contaminated waste to replace the binder at rates of 40%, 50% and 60% by weight. A water to binder (w/b) ratio of 0.40 was used for all of the mixtures. The setting time and compressive strength of the solidified waste were investigated, while the leachability of the heavy metals was determined by TCLP. Additionally, XRD, XRF, and SEM were performed to investigate the fracture surface, while the pore size distribution was analyzed with MIP. The results indicated that the setting time of the binders marginally increased as the amount of SZ and NZ increased in the mix. The compressive strengths of the pastes containing 20 and 40wt.% of NZ were higher than those containing SZ. The compressive strengths at 28 days of the SZ solidified waste mixes were 1.2-31.1MPa and those of NZ solidified waste mixes were 26.0-62.4MPa as compared to 72.9MPa of the control mix at the same age. The quality of the solidified waste containing zeolites was better than that with OPC alone in terms of the effectiveness in reducing the leachability. The concentrations of heavy metals in the leachates were within the limits specified by the US EPA. SEM and MIP revealed that the replacement of Portland cement by zeolites increased the total porosity but decreased the average pore size and resulted in the better containment of heavy ions from the solidified waste.     

Stabilization/solidification of MSW incineration residues from facilities with different air pollution control systems. Durability of matrices versus carbonation

This paper discusses the stabilisation/solidification process with Portland cement applied to municipal solid waste incineration residues. Two types of residues were considered: fly ash (FA) produced in an electrostatic precipitator, and air pollution control (APC) residues from a semi-dry scrubber process. Cement pastes with different percentages of FA and APC residues were characterised according to their physical properties, the effect of the hydration products and their leaching behaviour. Portland pastes prepared with APC residues showed a rapid setting velocity in comparison with setting time for those pastes substituted with FA residues. Portland cement hydration was retarded in FA pastes. Leaching test results showed that heavy metals (such as Zn, Pb and Cd) and sulphates are immobilised within the paste, whereas chlorides are only partially retained. The carbonation process increases the leachability of S04(2-) and heavy metals such as Zn and Cr.     

Heavy metal leaching from hydroxide, sulphide and silicate stabilised/solidified wastes

A synthetic, mixed-metal solution has been stabilised by treatment with sodium hydroxide, sodium sulphide, and sodium silicate, respectively. The three stabilised filter cakes have subsequently been solidified using additions of ordinary Portland cement and pulverised fuel ash (PFA) which are typically used in UK solidification operations. Both the stabilised filter cakes and the solidified wastes have been subjected to an equilibrium extraction test, a modified TCLP test, and a series of single-extraction, batch leach tests using an increasingly acidic leachant. Metal release was found to be primarily dependent on the pH of the leachate. Under mildly acidic conditions, the percentages leached from the stabilised and the stabilised/solidified wastes were comparable for most metals. A high-volume fraction of these solidified wastes is occupied by the stabilised filter cake. When they are broken up and tested in single-extraction leach tests, the primary effect of the cementitious additives is to increase the pH of the leachate so that most heavy metals remain insoluble. When tested under acidic leachate conditions, copper, lead, and mercury were found to be particularly well retained within sodium sulphide stabilised wastes. Under similar test conditions, cadmium was leached at very low levels from the sodium silicate stabilised waste.     

Cement-clay pastes for stabilization/solidification of 2-chloroaniline

Immobilization of a model liquid organic pollutant, i.e. the 2-chloroaniline (2-CA), into a cement matrix using organoclays as pre-sorbent agents was investigated. Five cement-clay pastes were prepared with different nominal water-to-cement ratios (w/c=0.40, 0.25 and 0.15 wt/wt) and various amounts of waste (waste-to-cement o/c=0.20, 0.60 and 1.00 wt/wt); for comparison, a neat cement paste was also prepared. Dynamic leach tests were performed on solidified monoliths in order to assess the successful immobilization of the 2-CA. In monoliths at constant w/c ratio (0.40) the total amount of pollutant released increases with its initial content, and ranges from 15 to 35% with respect to it. By lowering w/c from 0.40 to 0.15 at constant o/c, the performances improved (<25% released). The microstructure of the hardened cement-clay pastes was characterized by quantitative X-ray diffraction (QXRD) and electronic microscopy (SEM-EDS) techniques; hydration degree was estimated by means of thermogravimetric analysis (TGA) in addition to QXRD. No evidence of any chemical reaction between 2-CA and cement phases was found. Moreover, it was shown that the most important factors affecting the cement hydration process were the total water content, i.e. the one taking also into account the water contained in the wet polluted clay, and the amount of 2-CA not firmly sorbed by the organoclay, and then freely dispersed in the paste.     

Strength, leachability and microstructure characterisation of Na2SiO3-activated ground granulated blast-furnace slag solidified MSWI fly ash.

The chemical composition and the leachability of heavy metals in municipal solid waste incinerator (MSWI) fly ash were measured and analysed. For the leachability of unstabilized MSWI fly ash it was found that the concentrations of Pb and Cr exceeded the leaching toxicity standard. Cementitious solidification of the MSWI fly ash by Na2SiO3-activated ground granulated blast-furnace slag (NS) was investigated. Results show that all solidified MSWI fly ash can meet the landfill standards after 28 days of curing. The heavy metals were immobilized within the hydration products such as C-S-H gel and ettringite through physical encapsulation, substitution, precipitation or adsorption mechanisms.