Pilot-scale field investigation of ex situ solidification/stabilization of soils with inorganic contaminants using two novel binders

Recently, two novel binders, one by-product-based binder named as GM and one phosphate-based binder named as KMP, have emerged that can stabilize soils spiked with mixed lead and zinc contaminants. However, field evaluations of the stabilization of actual soils that contaminated with mixed zinc (Zn) and inorganic chloride (Cl^?) using GM and KMP have not been performed yet. This study presents a pilot-scale field test to evaluate the performance of GM and KMP to stabilize these inorganic contaminants in soils at two locations in an abandoned industrial plating plant site. The field soils were stabilized and cured for 1, 3, 7, and 28 days and tested for dry density, dynamic cone penetration, soil pH, and leachability. Laboratory unconfined compression tests were performed, and the relationship between unconfined compressive strength and dynamic cone penetrometer index was assessed. The results showed that the strength of both the GM- and KMP-stabilized soils after 28-day curing improved significantly, and the leached Zn and Cl^? concentrations were well below the corresponding remediation limits. In general, the KMP-stabilized soil demonstrated superior performance in terms of higher dry density, unconfined compressive strength, average dynamic cone penetration resistance, lower dynamic cone penetrometer index in the early curing stage (7 days), and lower leached Zn concentration under all curing times. In contrast, the GM exhibited superior immobilization of Cl^? in the contaminated soil irrespective of the curing time. The results demonstrate that GM and KMP are promising binders for treating Zn- and Cl^?-contaminated soils at plating and other industry sites with similar contaminants.

     

Experimental Evaluation of Construction Waste and Ground Granulated Blast Furnace Slag as Alternative Soil Stabilisers

In this study, the effect of ground granulated blast furnace slag (GGBFS) and recycled construction waste (CW) on bentonite clay stabilisation were investigated. The unconfined compressive strength (UCS) of specimens was evaluated with different combinations of GGBFS and CW over various curing periods. A series of micro analysis tests consisting of scanning electron microscope, energy dispersive spectrometer and X-ray diffraction were also conducted to determine the microstructural arrangement and mineralogical effect of the stabilisation treatment. The UCS results showed an increment in strength after introduction of GGBFS and CW and the longer curing period produced more pronounced results. The optimum additive ratio was calculated as 5 % of slag and 20 % of construction waste under all curing conditions. The micro analytical results also indicated formation of structural bonds between admixtures and bentonite in stabilised specimens, as slag crystals and bentonite particles were observed to occupy the cavities and vesicles on the construction waste grains. However, the experimental data shows that the strength improvement is not significant with the addition of only construction waste.     

Cement–fly ash stabilisation/solidification of contaminated soil: Performance properties and initiation of operating envelopes

This study was aimed at evaluating the mechanical and pH-dependent leaching performance of a mixed contaminated soil treated with a mixture of Portland cement (CEMI) and pulverised fuel ash (PFA). It also sought to develop operating envelopes, which define the range(s) of operating variables that result in acceptable performance. A real site soil with low contaminant concentrations, spiked with 3000 mg/kg each of Cd, Cu, Pb, Ni and Zn, and 10,000 mg/kg of diesel, was treated with one part CEMI and four parts PFA (CEMI:PFA = 1:4) using different binder and water contents. The performance was assessed over time using unconfined compressive strength (UCS), hydraulic conductivity, acid neutralisation capacity (ANC) and pH-dependent leachability of contaminants. With binder dosages ranging from 5% to 20% and water contents ranging from 14% to 21% dry weight, the 28-day UCS was up to 500 kPa and hydraulic conductivity was around 10⁻⁸ m/s. With leachant pH extremes of 7.2 and 0.85, leachability of the contaminants was in the range: 0.02–3500 mg/kg for Cd, 0.35–1550 mg/kg for Cu, 0.03–92 mg/kg for Pb, 0.01–3300 mg/kg for Ni, 0.02–4010 mg/kg for Zn, and 7–4884 mg/kg for total petroleum hydrocarbons (TPHs), over time. Design charts were produced from the results of the study, which show the water and/or binder proportions that could be used to achieve relevant performance criteria. The charts would be useful for the scale-up and design of stabilisation/solidification (S/S) treatment of similar soil types impacted with the same types of contaminants.     

Can fly-ash extend bentonite binder for iron ore agglomeration?

There is great incentive to reduce bentonite use in iron ore pelletization by improving its effectiveness. In order to make bentonite more effective, it is necessary to understand the actual binding mechanisms so that they can be properly taken advantage of. Bentonite use could also be reduced by replacing bentonite with even lower-cost binders, such as high-carbon fly-ash based binder (FBB). While FBBs can be used alone as binders, it was considered possible that mixtures of FBB and bentonite could exhibit superior binding properties. In this study, it was found that bentonite bonds by a physical mechanism, while FBB bonds by a chemical mechanism. These mechanisms were determined to be incompatible. Mixtures of the two binders resulted in reduced dry magnetite concentrate pellet compressive strengths below the industrially acceptable value of 22 N (5 lbf). Activators and accelerators, which were necessary components of the FBB, deactivated the bentonite. The compatibilities and mechanisms of the two binders are explained in this paper. The classical theory of the binding mechanism of bentonite binder is challenged by the bentonite fiber mechanism that was recently identified by the authors.     

Evaluation of chemical immobilization treatments for reducing arsenic transport in red mud

Red mud (RM) was produced during alumina production from bauxite known as the Bayer process. Arsenic was detected in the solid phase of RM (RMsf) which was disposed in the disposal area. This study investigates the effectiveness of using Zero-valent iron (ZVI), ferrihydrite, ferrous sulfate (FeSO₄), waste acid (WA) or CO₂ for immobilization of arsenic in the RMsf. To test the effect of the amendments on the arsenic leachability, the RMsf samples were amended with the iron-based materials or acidifiers at various w/w (weight/weight) ratios (1–10 %) for 30 days. The leachability of arsenic in the RMsf was evaluated by a 4-step water elusion process. After 30-day treatment of the RMsf, the leachability of As decreased from an initial (12.7 %) to (7.0 %) with a w/w ratio of 5 % ZVI (0 %) with 5 % FeSO₄·7H₂O, (3.4 %) with 5 % ferryhydrite, (2.0 %) with 6 % WA and (11.8 %) with 6 % CO₂. FeSO₄·7H₂O and WA showed more effectively than other amendments for immobilizing arsenic. Arsenic fractionation with a sequential extraction procedure was used to evaluate the arsenic migration potential in the RMsf. FeSO₄ and WA were effective in increasing the hydrous oxide combined arsenic in the RMsf. The leachable Cl^? and SO₄ ^2? in the RMsf increased from 2.9 to 14.1 mg/g and 19.9–44.4 mg/g with 6 % WA and 5 % FeSO₄·7H₂O added, respectively. The estimated cost of the FeSO₄ and WA treatment was 0.47 and 0.49 USD per ton, respectively.     

Electrokinetic remediation of perchloroethylene-contaminated soil

One large group of persistent and toxic contaminants is the hydrophobic organic contaminants. Among them, perchloroethylene (PCE) has been recognized as a representative group of these pollutants with low solubility. This study reports on the effects of electrokinetic remediation with non-ionic surfactant on PCE-contaminated soil. The performance of electrokinetic process was investigated in the treatment of clay soil that artificially contaminated with two levels: 10,000 and 30,000 mg/kg PCE and 0.33 g/kg Triton X-100. A DC power supply with electric voltage (1 V/cm) was used for 8–16 days. A negatively charged soil surface resulted in a more negative zeta potential and greater electroosmotic flow toward the cathode. The PCE was measured after extraction using n-hexane and analyzed by Fourier transform infrared spectroscopy instrument. The water content of soil was kept 25 % (w/w). Results were shown that PCE removal efficiency achieved was 74 and 89 % for 10,000 and 30,000 mg/kg PCE, respectively, for 16 days. Therefore, in this study, the integration of electrokinetic with non-ionic surfactant as a hybrid method was most effective for the remediation of PCE-contaminated soils.     

污染物的固化/安定化处理的种类和研究应用现状

污染物的固化／安定化技术处理可归纳为6大类,即基于水泥的固化技术、基于石灰的固化技术、矿物吸附技术、热固技术、热塑技术和玻璃化技术等。其中有机聚合物固化剂（热固、热塑材料）一般用于处理固态有机污染物;玻璃化技术一般用于处理污染土、城市垃圾焚烧物和放射性物质。因无机胶结剂操作方便、成本低、能处理的无机污染物范围很广,因此使用频率较高（90％以上）,而有机胶结剂固化技术和废物玻璃化处理技术成本高,仅被用于特殊污染物的处理。     

Solidification of Tank Bottom Sludge

Tank bottom oily sludge (S) is collected from tank bottoms during cleaning operations and contaminated soil is collected after spills and leakages. Disposal of tank sludge is a significant item of tank maintenance for producers, refiners and transporters of petroleum materials. The objective of this paper is to investigate the use of various additives in the solidification of tank bottom sludge. The sludge was solidified using various combinations of additives including ordinary Portland cement (OPC), cement by-pass dust (CBPD) and quarry fines (QF). Geotechnical and leachability properties of the mixtures were determined. The use of OPC alone as a solidifying agent yielded the best results. This was followed by blends S:OPC:QF of 1:0.5:1.5 and S:CBPD of 1:2. Economically, the latter two mixtures would be considered more cost-effective in solidifying the sludge as the additives are waste by-product materials. The Toxicity Characteristic Leaching Procedure (TCLP) results indicated that no extracts exceeded the threshold TCLP limits established by U.S. Environmental Protection Agency (USEPA). The solidified material can be used in construction of roads, embankments and landfill layers.     

Immobilization of As and Pb in contaminated sediments using waste resources

As and Pb-contaminated sediment obtained from the Nakdong Lake and Yeongsan River in the Republic of Korea was stabilized using a combination of calcined oyster shell (COS), waste cow bone (WCB) and coal mine drainage sludge (CMDS). The effectiveness of the stabilization treatment was evaluated using the Korean Standard Test (KST). The KST tests were performed using 1 N HCl extraction fluid for As and 0.1 N HCl extraction fluid for Pb. Scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDX) analyses were performed to investigate the mechanism responsible for As and Pb immobilization upon treatment. The treatment results showed that effective stabilization of As and Pb-contaminated sediment was obtained. Specifically, 10 wt% COS–5 wt% CMDS was the best treatment for As immobilization and 5 wt% COS–5 wt% WCB was the best treatment for Pb immobilization. The COS–WCB treatment outperformed the COS–CMDS treatment in immobilizing Pb in the contaminated sediment. SEM–EDX results indicated that Pb immobilization was strongly associated with Ca, Si, Al and P while As immobilization was strongly associated with Fe and O. Therefore, utilization of COS, WCB and CMDS is beneficial for the stabilization of contaminated sediment.     

Leaching evaluation of cement stabilisation / solidification treated kaolin clay

Sustainable land management often requires redevelopment of existing and often contaminated Brownfield sites over use of Greenfield or agricultural land. Stabilisation / solidification (S/S) offer a viable remediation option with particular suitability in treating heavy metal contaminants. However, uncertainties over long-term durability and previously cheap landfill disposal costs resulted in limited use in many countries. There is a need to characterise treated material and assess components containment and release, to improve S/S confidence and inform remedial design, using the evaluation of the leaching controlling mechanisms. To improve understanding of key features that will enable improvements to such designs, this paper presents an evaluation of leaching for CEM II stabilised kaolin, using the leaching behaviour of 4 main components—aluminium, silicon, calcium and sulphur. Results show no detrimental implications on chemical durability due to kaolin degradation with hydration under induced alkalinity, with increased formation of stable cementitious minerals. Availabilities for components show that aluminium and silicon had minimal leachability compared to total content, whilst calcium and sulphur show almost total leaching availability. Decreasing calcium and silicon leachability under natural chemical conditions with increased hydration duration supports the increased formation of stable cementitious products with time, using the products of kaolin dissociation.     

Monitoring and modeling concentration and dry deposition of ambient air particulates and metallic elements Mn, Fe, Zn, Cr and Cu in central Taiwan

Experiments were carried out between September 2009 and August 2010 at Bei-shi and Gao-mei in Taichung, and at Quan-xing, Chang-Hua and He-mei in Changhua. Dry-deposition flux models of metallic elements Mn, Fe, Zn, Cu and Cr bound to air particulates at five characteristic sampling sites were developed. Results concerning metallic elements Mn, Zn, Cr and Cu, revealed that Zhang’s model yielded the best results concerning the dry-deposition flux of particles of size 10 μm at Bei-shi, Chang-Hua, He-mei, Quan-xing and Gao-mei sampling sites. For the metallic element Cr, the model of Noll and Fang exhibited the best dry-deposition flux results for particles of size 10 μm at Bei-shi, He-mei and Gao-mei sampling sites. For metallic element Fe, the model of Noll and Fang yielded the best results for the dry-deposition flux of particles of size 5.6 μm at Bei-shi, Chang-Hua, He-mei and Quan-xing sampling sites. For metallic element Fe, Zhang’s model yielded the best dry-deposition flux results for particles of size 5.6 μm at the Gao-mei sampling site. For ambient air particles, Zhang’s model yielded the best average calculated/modeled ratios for particles of size 3 μm, and the model of Noll and Fang yielded the best average calculated/modeled for particles of size 10 μm. Finally, the models of both Zhang and Noll and Fang yielded more accurate predictions of the dry-deposition of metallic elements in ambient air when the particles were larger than 5.6 μm.     

Microscopic morphology and elemental composition of size distributed atmospheric particulate matter in Urumqi, China

Microscopic morphology and elemental composition of atmospheric particulate matter (PM) in 13 different size fractions from 0.01 to 10 μm were studied using a Field Emission Scanning Electron Microscope with Energy-Dispersive Spectrometer (FESEM–EDX). The relative mass fractions exhibited a bimodal distribution with a major mode in the fine range (0.18–1 μm) and a minor mode in the coarse range (>1 μm), suggesting that the major pollution of PM is fine particles in this area of Urumqi atmosphere. The PM could be classified as follows: aluminosilicate/silica mineral, Si–Al rich fly ash, Fe oxide particle, Ti dominant particle, sulfate/carbonate crystal, carbonaceous aerosols (including soot, organic carbon, tar ball and irregularly shaped carbon). The soot and organic carbon with anthropogenic sources are dominant types in fine range samples (<1 μm). The natural source minerals and secondary synthesized sulfate/carbonate crystals were accumulated in the coarse range (>1 μm). Elemental composition of various types of particles (0.056–5.6 μm) was also analyzed by EDX. C, S, O, N, Si, Al, Fe, Ca, Na, K, Mg, Cl, F, Hg were detected in most samples. Si, Al and Ca accumulated in coarse fractions, while S and Hg mainly accumulated in fine fractions. Concentrations of 15 metallic elements in size range from 0.1 μm to 5.6 μm were divided into three groups based on their possible sources. (1) The crustal elements (Al, Mg, Fe, Mn and V), mainly present in coarse particles (>1 μm); and (2) the anthropogenic source elements (Ca, Ni, As, Cu, Pb, Cd and Hg). The concentrations of Ca and Ni increased with increasing particle size, while As, Cu, Pb, Cd and Hg showed opposite trends. As, Cu, Pb, Cd and Hg accumulated mainly in fine fraction (<1 μm). (3) The multi sources elements (Cr, Co and Se) possibly come from both natural and anthropogenic sources. High levels of heavy metals, especially Hg in nanosize particles, may pose great risk to human health.     

Effect of alkaline solutions on bentonite properties

The influence of alkaline aqueous solutions on the properties of bentonite was investigated to evaluate the performance of bentonitic engineered barriers when contacted with alkaline groundwater. Batch and hydraulic conductivity tests were conducted on Na-bentonite using six different alkaline aqueous solutions. For the batch tests, almost no change in the montmorillonite fraction of the bentonite was observed after reacting with alkaline solutions (pH = 8.4–13.1), regardless of the solution type. On the other hand, aluminosilicate minerals (e.g., albite) were dissolved and secondary minerals (e.g., anorthite) were formed in alkaline NaOH solutions (pH > 13). The cation (Ca or Na) concentration primarily affected the swelling properties of bentonite rather than the pH of the solution, which was comparable to the results of the hydraulic conductivity tests. For the Ca solutions, the hydraulic conductivity of the bentonite specimen to the 0.02 mol/L Ca(OH)₂ solution (6.5 × 10^?9 cm/s) was approximately an order of magnitude lower than that of the bentonite specimen to the 0.02 mol/L Ca(OH)₂ + 1 mol/L CaCl₂ solution (5.0 × 10^?8 cm/s), whereas the hydraulic conductivity to the 0.02 mol/L Ca(OH)₂ + 1 mol/L CaCl₂ solution (pH = 11.3) (5.0 × 10^?8 cm/s) was slightly higher than that to the 1 mol/L CaCl₂ solution (pH_i = 8.4) (4.4 × 10^?8 cm/s). For the NaOH solutions with pH > 13, the hydraulic conductivity of the bentonite specimen decreased with increasing Na concentration, suggesting that the effect of Na concentration was more dominant than that of permeant pH.     

Carcinogenic risk evaluation for human health risk assessment from soils contaminated with heavy metals

Human activities have progressively increased in recent years. Consequently, significant environment deterioration resulted. Soils have a particularly varied vulnerability to heavy metal pollution, especially in the vicinity of industrial areas. Heavy metal contamination of soil may induce risks and hazards to humans and the ecosystem, while toxic metals in soil can severely inhibit the biodegradation of organic contaminants. This paper is focused on human health risk assessment from extremely contaminated soil with heavy metals, mainly with carcinogenic elements. The study refers to an agricultural area in the vicinity of an old metallurgical processing industrial facility. The contaminants evaluated in the present paper are beryllium (Be), cadmium (Cd), chromium (Cr), nickel (Ni) and lead (Pb). Contamination level is pointed out through laboratory analysis results of soil samples taken from 0–0.2 m, 0.2–0.4 m soil layers and up to 2.1 m soil depth. Some heavy metal concentrations (Cd, Cr and Pb) exceed the intervention thresholds for sensitive areas, as they are stipulated in the national regulation in Romania. The identified average concentration levels of Cd, Cr^VI and Pb in the first layer of the investigated land are 23.83, 7.71 and 704.22 mg/kg_d.w, respectively. The results show that the potential risk of human health is relevant (higher than the acceptable one after World Health Organization) and a possible solution for the remediation should become a major concern for the investigated area.     

Effect of Water and Leachate on Hydraulic Behavior of Compacted Bentonite,Calcareous Sand and Tuff Mixtures for Use as Landfill Liners

Compacted soil–bentonite liners, consisting of a sandy soil mixed with bentonite as backfill, are used extensively as engineered barriers for contaminant containment. This paper studies the valorization of local materials containing calcareous sand, tuff obtained from Laghouat region (in the South Algeria), to associate with bentonite in order to improve their hydraulic characteristics for use as landfill liner material. Firstly, a geotechnical characterization of mixtures chooses from a fixed percentage to 10% bentonite and different percentages of calcareous sand and tuff so that they are complementary to 90% by not 10%. Thereafter, the determination of saturated hydraulic conductivity at falling-head permeability (K_v) and oedometer (K_id, indirect Measure) tests of all compacted mixtures at Optimum Normal Proctor have been carried out using both permeates by tap water and a landfill leachate in order to simulate long-term conditions. The results showed that the saturated hydraulic conductivity of tap water is relatively lower than the one saturated by leachate in the falling-head test, unlike the oedometer test. The B₁₀CS₂₀T₇₀ mixture has satisfied the hydraulic conductivity criterion of bottom barriers (i.e. water permeated: k_v20° = 1.97 × 10^?9 and k_id from 7 × 10^?9 to 1.83 × 10^?10 < 10^?9m/s; leachate permeated: k_v20° = 2.91 × 10^?9 and k_id from 7 × 10^?9 at 1.44 × 10^?10 < 10^?9 m/s). Finally, a comparison between direct measurements of the saturated hydraulic conductivity by triaxial (K_d) test and oedometer test (K_id) in the range of effective stress applied 100–800 kPa led to propose equations of correlations between these two methods. In conclusion, adopted formulation B₁₀CS₂₀T₇₀ perfectly meets the regulatory requirements in force and constitutes an economic product based on available local materials for engineers barriers.     

Remediation technologies for metal-contaminated soils and groundwater: an evaluation

Metals including lead, chromium, arsenic, zinc, cadmium, copper and mercury can cause significant damage to the environment and human health as a result of their mobilities and solubilities. The selection of the most appropriate soil and sediment remediation method depends on the site characteristics, concentration, types of pollutants to be removed, and the end use of the contaminated medium. The approaches include isolation, immobilization, toxicity reduction, physical separation and extraction. Many of these technologies have been used full-scale. This paper will review both the full-scale and developing technologies that are available. Contaminants can be isolated and contained to minimize further movement, to reduce the permeability of the waste to less than 1×10⁻⁷ m/s (according to U.S. guidelines) and to increase the strength or bearing capacity of the waste. Physical barriers made of steel, cement, bentonite and grout walls can be used for isolation and minimization of metal mobility. Another method is solidification /stabilization, which contains the contaminants in an area by mixing or injecting agents. Solidification encapsulates contaminants in a solid matrix while stabilization involves formation of chemical bonds to reduce contaminant mobility. Another approach is size selection processes for removal of the larger, cleaner particles from the smaller more polluted ones. To accomplish this, several processes are used. They include: hydrocyclones, fluidized bed separation and flotation. Addition of special chemicals and aeration in the latter case causes these contaminated particles to float. Electrokinetic processes involve passing a low intensity electric current between a cathode and an anode imbedded in the contaminated soil. Ions and small charged particles, in addition to water, are transported between the electrodes. This technology have been demonstrated in the U.S. full-scale, in a limited manner but in Europe, it is used for copper, zinc, lead, arsenic, cadmium, chromium and nickel. The duration of time that the electrode remains in the soil, and spacing is site-specific. Techniques for the extraction of metals by biological means have been not extensively applied up to this point. The main methods include bioleaching and phytoremediation. Bioleaching involves Thiobacillus sp. bacteria which can reduce sulphur compounds under aerobic and acidic conditions (pH 4) at temperatures between 15 and 55°C. Plants such as Thlaspi, Urtica, Chenopodium, Polygonum sachalase and Alyssim have the capability to accumulate cadmium, copper, lead, nickel and zinc and can therefore be considered as an indirect method of treating contaminated soils. This method is limited to shallow depths of contamination. Soil washing and in situ flushing involve the addition of water with or without additives including organic and inorganic acids, sodium hydroxide which can dissolve organic soil matter, water soluble solvents such as methanol, nontoxic cations, complexing agents such as ethylenediaminetetraacetic acid (EDTA), acids in combination with complexation agents or oxidizing/reducing agents. Our research has indicated that biosurfactants, biologically produced surfactants, may also be promising agents for enhancing removal of metals from contaminated soils and sediments.

In summary, the main techniques that have been used for metal removal are solidification/stabilization, electrokinetics, and in situ extraction. Site characteristics are of paramount importance in choosing the most appropriate remediation method. Phytoremediation and bioleaching can also be used but are not as well developed.     

Effects of humic acid on heavy metal uptake by herbaceous plants in soils simultaneously contaminated by petroleum hydrocarbons

The effects of humic acid (HA) on heavy metal uptake by herbaceous plants in soil simultaneously contaminated with heavy metals and petroleum hydrocarbons were investigated. The results showed that HA reduced readily soluble and exchangeable forms of heavy metals in the contaminated soil but increased their plant-available forms. Potential bioavailability and leachability factors became larger than 1 after adding HA to the soil, except for those of Ni, suggesting that more heavy metals could be potentially phytoavailable for plant uptake. Furthermore, HA increased the accumulation of Pb, Cu, Cd, and Ni in the shoots and roots of selected plants. The greatest increase in the accumulation of heavy metals was 264.7 % in the shoot of Festuca arundinacea, with the bioconcentration factor (BCF) increasing from 0.30 to 1.10. Humic acid also increased the BCFs of the roots of Brassica campestris for Ni and Pb. These results suggest that HA amendment could enhance plant uptake of heavy metals, while concurrently reducing heavy metal leachability and preventing subsurface contamination, even in soils simultaneously contaminated with petroleum hydrocarbons.     

Characterization and low-cost treatment of an industrial arid soil polluted with lead sulfide in northern Chile

Lead (Pb) dust exposure can have detrimental environmental and human health effects. Improperly enclosed stockpiles of Pb concentrates can cause dust emissions, subsequent pollution of the soil and environmental risk. The aim of this work was to study Pb form, distribution and immobilization (by using eggshell and seashell) in an industrial arid soil near a storage area of Pb mineral concentrates in northern Chile. High amounts of sulfur (S; 9900 mg kg^?1) and Pb (6530 mg kg^?1) were found in the polluted soil. The energy-dispersive X-ray spectroscopy analysis revealed a lead sulfide (PbS: galena). Metallic Pb particles, which were between 41 and 46 µm, were identified in the soil. After eggshell and seashell (20%) were applied, the soil pH increased from 6.0 to 7.84 and 8.07, respectively. In the studied soil, the leaching test showed a 59 mg L^?1 average Pb extractable concentration. After 240 days, extractable Pb by toxicity characteristics leaching procedure decreased to 4.79 mg L^?1 (93.3%) with the application of seashell at 20% compared with a decrease of 33.33 mg L^?1 (53.6%) using eggshell. Pb in the polluted soil was mainly found in the exchangeable fraction (66%), followed by the reducible (24%), residual (7%) and oxidizable (6%) fractions. According to the risk assessment code, the contaminated soil before treatment was classified as very high risk. Adding eggshell (20%) and seashell (20%) decreased the exchangeable fractions to 39 and 35%, respectively. Applying these liming materials achieved Pb immobilization in the soil, but the soil remained in the high environmental risk category. We conclude that the application of seashell waste, resulting from high aquaculture activity, opens an interesting window to the treatment of contaminated arid soils.     

Minimal alteration of montmorillonite following long-term interaction with natural alkaline groundwater: Implications for geological disposal of radioactive waste

Bentonite is one of the more safety-critical components of the engineered barrier system in the disposal concepts developed for many types of radioactive waste. Bentonite is utilised because of its favourable properties which include plasticity, swelling capacity, colloid filtration, low hydraulic conductivity, high retardation of key radionuclides and stability in geological environments of relevance to waste disposal. However, bentonite is unstable under the highly alkaline conditions induced by Ordinary Portland Cement (OPC: initial porewater pH > 13) and this has driven interest in using low alkali cements (initial porewater pH9-11) as an alternative to OPC. To build a robust safety case for a repository for radioactive wastes, it is important to have supporting natural analogue data to confirm understanding of the likely long-term performance of bentonite in these lower alkali conditions. In Cyprus, the presence of natural bentonite in association with natural alkaline groundwater permits the zones of potential bentonite/alkaline water reaction to be studied as an analogy of the potential reaction between low alkali cement leachates and the bentonite buffer in the repository. Here, the results indicate that a cation diffusion front has moved some metres into the bentonite whereas the bentonite reaction front is restricted to a few millimetres into the clay. This reaction front shows minimal reaction of the bentonite (volumetrically, less than 1% of the bentonite), with production of a palygorskite secondary phase following reaction of the primary smectites over time periods of 10⁵–10⁶ years.     

Utilization of fly ash for stabilization/solidification of heavy metal contaminated soils

Pozzolanic-based stabilization/solidification (S/S) is an effective, yet economic remediation technology to immobilize heavy metals in contaminated soils and sludges. In the present study, fly ash waste materials were used along with quicklime (CaO) to immobilize lead, trivalent and hexavalent chromium present in artificially contaminated clayey sand soils. The degree of heavy metal immobilization was evaluated using the Toxicity Characteristic Leaching Procedure (TCLP) as well as controlled extraction experiments. These leaching test results along with X-ray diffraction (XRD), scanning electron microscope and energy dispersive X-ray (SEM–EDX) analyses were also implemented to elucidate the mechanisms responsible for immobilization of the heavy metals under study. Finally, the reusability of the stabilized waste forms in construction applications was also investigated by performing unconfined compressive strength and swell tests. The experimental results suggest that the controlling mechanism for both lead and hexavalent chromium immobilization is surface adsorption, whereas for trivalent chromium it is hydroxide precipitation. Addition of quicklime and fly ash to the contaminated soils effectively reduced heavy metal leachability well below the nonhazardous regulatory limits. Overall, fly ash addition increases the immobilization pH region for all heavy metals tested, and significantly improves the stress-strain properties of the treated solids, thus allowing their reuse as readily available construction materials. The only potential problem associated with this quicklime–fly ash treatment is the excessive formation of the pozzolanic product ettringite in the presence of sulfates. Ettringite, when brought in contact with water, may cause significant swelling and subsequent deterioration of the stabilized matrix. Addition of minimum amounts of barium hydroxide was shown to effectively eliminate ettringite formation. Overall, due to the presence of very high levels of heavy metal contamination along with sulfates in the solid matrices under study, the results presented herein can be applied to the management of incinerator and coal fly ash, boiler slag and flue gas desulfurization wastes.