Leaching of elements from cement activated fly ash and slag amended soils

Reduction, stabilization, and solidification of Cr(VI) in contaminated soils with a sustainable by-product-based binder

This study evaluated the use of a sustainable GFD binder for the stabilization/solidification (S/S) of chromium VI (Cr(VI))-contaminated soil. The GFD binder was composed of ground granulated blast furnace slag (GGBFS), fly ash and desulfurization ash, named after the initials of the three materials. The effects of curing time and binder dosage on soil unconfined compressive strength (UCS), Cr leachability, soil pH, and reduction ratio of Cr (VI) were tested. The immobilization mechanisms of Cr(VI) in contaminated soil were further explored using X-ray diffraction (XRD), scanning electron microscopy (SEM), and sequential extraction procedure (SEP). The results showed that the UCS and pH of the soil increased substantially after the GFD binder was added. After 28 days of curing with a 20% binder dosage, the leached total Cr concentration decreased from 34.4 mg/L in the contaminated soil to 1.44 mg/L in the treated soil, and the leached Cr(VI) concentration decreased from 28.0 mg/L to 0.45 mg/L. A Cr(VI) reduction ratio of 96.2% was achieved, indicating the strong reducibility of GGBFS. XRD revealed that the main hydration products of the GFD binder were hydrated calcium silicate (C-S-H) and ettringite. SEM results showed that the formation of hydration products and Cr-bearing precipitates filled the soil pores, resulting in a dense soil structure. The SEP results demonstrated that the levels of the unstable fraction F1 decreased considerably, and that the levels of the stable fractions F3 and F5 increased after treatment. Encapsulation by C-S-H, reduction by sulfides, adsorption of C-S-H, and precipitation of Cr-bearing hydroxides were the main mechanisms involved in Cr immobilization using the GFD binder.

Effect of calcium oxide on chromium solidification during the melting of hazardous waste incineration fly ash

Thermal treatment technology considerably affects the harmlessness of fly ash (FA), but highly toxic heavy metals, such as Cr, attract considerable attention. In this study, we investigated the influence of CaO dosage at 600°C-1200 °C on the curing effect of Cr during FA thermal treatment based on the combination effect of CaO. Static, dynamic, and continuous sequential leachings were performed for the sintered products. Results showed that the leaching concentration of Cr decreased by approximately 91% when CaO dosage was 8.57%, and the difference in the residual state was the main reason for the difference in the leaching behavior of Cr. The proportion of the residual state in the sintered products increased from 35.16% to 64.01%. The transition between Cr₂O₃, Cr₅O₁₂, and CaCr₂O₄ is the fundamental reason for the leaching behavior of Cr and the change in the residual state. This study provides a scientific basis for preventing and controlling heavy metal pollution and optimizing environmental supervision in the FA thermal treatment process.

The mechanistic understanding of potential bioaccessibility of toxic heavy metals in the indigenous zinc smelting slags with multidisciplinary characterization

Smelting slags is a well-known industrial solid waste, while there were limited studies on the key factors controlling the potential health risks caused by these smelting slags. In this work, the metal bioaccessibility in the size fractionated-zinc smelting slags was examined using various In vitro assays, in combination with multidisciplinary methods. The results indicated that the bioaccessible fractions of heavy metals showed a significant difference, but no statistical difference among different particle sizes of the zinc smelting slags. The bioaccessible metal fractions in the gastric (GP) and gastrointestinal (GIP) phases were 0 (Cr) - 91.39% (Cd)) and 0 (Cr) - 47.80% (Ni). Among the studied metals, Cd, Cu, Mn, Pb and Zn were the most bioaccessible to human. The Pearson correlation analysis showed that the carbonate bound phases of heavy metals were responsible for their bioaccessibility in GP and GIP. Moreover, the combined results of multidisciplinary characterization also further implied that the solubility behaviors of toxic elements in the smelting slags were dominated by soluble metal bearing- mineral phases and absorbable Fe, Mn and Al-rich minerals and metal bearing-precipitates during SBRC extractions. Therefore, these study results provide a insight into the potential controls of metal bioaccessibility in the zinc smelting slags, which was of great significance from the aspects of their resource recycling and risk management.

Mechanical Properties, Curing Mechanism, and Microscopic Experimental Study of Polypropylene Fiber Coordinated Fly Ash Modified Cement-Silty Soil

Silty soil has the characteristics of low natural moisture content and poor viscosity, and the strength and deformation required for foundation engineering can be satisfied by reinforcing and improving the silt. In order to study the reinforcement and improvement effects of polypropylene (PP) fiber and fly ash (FA) on cement–silty soil, an unconfined compressive strength (UCS) test, scanning electron microscope (SEM) test, and X-ray diffraction (XRD) analysis test were carried out. Cement (mixed amounts are 4%, 8%, 12%, and 16% of dry soil mass) was used as the basic modifier, and PP fiber (mixed amounts are 0%, 0.15%, 0.3%, and 0.45% of dry soil mass) compounded with FA (adding amounts of 0%, 5%, 10%, and 15% of dry soil mass) were used as an external admixture of cement–silty soil to study the mechanical properties, curing mechanism, and microstructure of the modified soil in different ages of 7 d, 14 d, 28 d, and 60 d. The test results show that with the increase in cement and curing age, the UCS of the modified soil increases, and with the increase in the PP fiber and FA, the UCS of the modified soil first increases and then decreases; there is an optimal content of FA and PP fiber, which are 10 and 0.15%, respectively. A large amount of C-S-H and AFt substances are produced inside the modified soil to cover the surface of soil particles or fill in the pores between soil particles, forming a tight spatial network structure and improving the mechanical properties of the cement–soil. The intensity of the diffraction peaks of the mineral components within the modified soils is more influenced by the cement and age, and the effect of FA is weaker. The stress–strain curve of the modified soil is divided into elastic stage, plastic deformation stage, and strain-softening stage, and the specimens in each stage have corresponding deformation characteristics. By analyzing the behavioral characteristics and curing improvement mechanism of modified soil from the duo perspective of macro-mechanical properties and microstructural composition, it can provide some basis for the engineering application of silty soil.

Assessment of geochemical modeling applications and research hot spots-a year in review

Geochemical modeling has been employed in several fields of science and engineering in recent years. This review seeks to provide an overview of case studies that applied geochemical modeling in the 2019 year, which includes over 250 articles. This review is intended to inform new users on the possibilities that geochemical modeling brings, while also informing existing and past users on its latest developments. The survey of studies was conducted with an emphasis on the modeling techniques, the objective of studies, the prevalent simulated variables and the use of specific software packages. The analysis showed that geochemical modeling is still predominantly employed in experimental projects and in the form of equilibrium modeling. PHREEQC and Visual MINTEQ were recognized as the most popular software packages for simulating a wide range of processes, using equilibrium or other geochemical modeling forms. The study of fluid-rock interactions and pollution and remediation processes can be regarded as the principal geochemical modeling objectives, constituting 37% and 36% of the reviewed studies, respectively. Focusing on fluid-rock interactions, hydrogeochemical processes, carbon capture and storage and enhanced oil recovery have been the main topics examined with geochemical modeling. Assessments of the toxicity of metals in terms of leachate and mobilization, as well as their removal from soil and water systems, have been major topics investigated with the aid of geochemical modeling in terms of pollution and remediation research. It was found that the scholars benefit from geochemical modeling in their research both as a main technique and as an accessory tool. Saturation index, elemental concentration and speciation, mineral mass and composition and pH were among the most common variables modeled in reviewed studies. Geochemical modeling has gained a wider user base in recent years, and many research groups have used it in consecutive studies to deepen knowledge. However, much potential for further dissemination still remains.

Assessment of the long-term leaching characteristics of cement-slag stabilized/solidified contaminated sediment

The use of stabilized/solidified (S/S) soils and sediments as sustainable construction materials is a global concern due to the potential risk of contaminant leaching including potentially toxic elements. The long-term leachability of four metals (Zn, Pb, Cd and As) in sediments mixed with OPC (Ordinary Portland Cement) and OPC/GGBS (Ground Granulated Blast Furnace Slag) binders were investigated through a combination of tank leaching tests and kinetic leaching models, with varying ranges of curing ages and ambient pH. The leaching data revealed that both binder compositions offer an excellent immobilization capacity for the four metals, while their releases are strongly pH-dependent and are a complex function of curing time. The partial replacement of OPC by GGBS is more effective for fixing Zn and As at pH of 1, Pb at pH of 7, Zn and Pb at pH of 10. Controlling leaching mechanisms and leachability indices were determined using nonlinear regression analysis and kinetic leaching models. The first-order diffusion model (FRDM) was the most applicable model for describing the leaching characteristics of these metals under the investigated cases, the leaching rate is controlled by surface wash-off initially and then by diffusion. The leachability indices indicate that the cement-slag S/S sediment can be regarded as an environmentally sustainable material with potential beneficial uses in construction.

Mineralogical and morphological factors affecting the separation of copper and arsenic in flash copper smelting slag flotation beneficiation process

Separating copper and arsenic has always been a major problem in the copper slag flotation process, which influences copper slag utilization and the environmental safety. A comparative study of flash smelting furnace (FSF) slag and its flotation products (concentrate and tailing) reveals the factors affecting the separation of copper and arsenic in the beneficiation process from the perspective of mineralogy and morphology. The elemental fractionation in the process shows a positive correlation of As, Cu and Cd and an obvious correlation between speciation transformation of copper and arsenic was observed. The occurrence of arsenic and copper in FSF slag correlate the key phases of arsenic copper alloys, accounted for 88.91 % of total arsenic-bearing phases and 32.28% of copper-bearing phases. Closely-embeded matte and copper-arsenic alloys incerease the difficulty of the separation suggesting the finer grinding is needed for slag. Arsenic is liberated and oxidized into arsenate compounds while the recombination of As-O and Cu-S happened in the process affecting the selectivity of copper and arsenic. Arsenic fixed in silicate minerals is discharged into tailing which suggested to induce and fix arsenic into silicate minerals can facilitate arsenic removal from concentrate. FSF slag and its flotation concnetrate show risks of some of some of HMs which should be cautiously transported, disposed, and utilized.

Partitioning, leachability, and speciation of chromium in the size-fractions of soil contaminated by chromate production

Soil particle size significantly affects the distribution and migration of chromium (Cr) in soil. Limited studies have investigated the impact of soil particle size on Cr partitioning at chromate contaminated sites. In this study, the physicochemical properties of coarse sand, medium sand, fine sand, and silt-clay were analyzed. And the particle size effects on partitioning, leachability, and bioaccessibility of total Cr and Cr(VI) were determined. The results showed the distribution factor (DF) of Cr(VI) in the coarse sand, medium sand, fine sand, and silt-clay fractions were 0.70, 0.79, 1.35, and 1.60, respectively. The total Cr DF values also had the similar result. The leached concentrations of total Cr and Cr(VI) in silt-clay (562.89 mg/L and 551.71 mg/L) was higher than in coarse sand (238.55 mg/L and 228.68 mg/L) fraction. The bioaccessibility of total Cr and Cr(VI) in silt-clay (77.72% and 88.58%) was higher than in fine sand (60.72% and 79.55%) fraction. The total Cr proportion of the exchangeable fraction (45.92%-73.67%) was relatively high in the four soil particle size fractions and gradually increased as soil particle size decreased. These implied that finer soil particles are more capable of enriching, mobilizing, and bioaccessibility of Cr and Cr(VI) than the coarse particles, which was related to the higher organic matter, cation exchange capacity, specific surface area, and clay components in smaller particles. The results suggested that higher environmental risk occurred in the finer fraction than in the coarser fraction for the chromate production contaminated soil.

Dredged marine sediments stabilized/solidified with cement and GGBS: Factors affecting mechanical behaviour and leachability

Management strategies for the safe disposal of contaminated dredged marine sediment constitute a global-scale environmental issue. The stabilization/solidification method was investigated as a sustainable approach to the recycling of the sediment as a construction material. A systematic study of the factors affecting the mechanical performance and contaminant release was performed. The physico-chemical variables selected to assess the potential re-use of the sediment treated with Ordinary Portland cement (OPC) and Ground Granulated Blast Furnace Slag (GGBS) in an aquatic environment were: curing duration (7, 28, 56 and 98 days), curing temperature (5, 20 and 40 °C) and ambient (leachate) pH (1, 4, 7 and 10). Unconfined compressive strength (UCS) tests were conducted and extended-duration tank leaching tests were used to characterize the long-term leaching of Al, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba, Pb. The results showed that S/S methods provide excellent immobilization of metals in marine sediment at a pH range of 4 to 10. Immobilization efficiencies of >99.9% for Mn, Fe, Zn, As, Ba, Pb and >97.8% for Al, Cu and Zn are reported over 100 days. GGBS replacement is an effective way to further improve sediment properties by enhancing strength, mitigating sediment alkalization and offering a better immobilization capacity for Fe, Ni and Zn. The release of metals (Al, Mn, Cu, As, Ba and Pb) was strongly associated with a coupling effect of the physico-chemical factors, with metal-specific responses to curing temperature, curing duration and pH. Mn mobility showed a dramatic sensitivity to ambient pH while Ba was less pH-dependent. Al release is related to strength and leached out by dissolution in all situations considered. Considering that dredged marine sediments may contain multiple metal contaminants which exhibit individual responses to remediation, treatment with GGBS may be considered a potentially suitable management option.

Carbonation based leaching assessment of recycled concrete aggregates

The leaching characteristics of seven different recycled concrete aggregates (RCA) samples derived from building demolition waste, concrete pavement, stockpiled, and freshly crushed concrete were evaluated focusing on the effects of carbonation, liquid-to-solid ratio (L/S), and particle sizes. Batch water leach test (WLT), toxicity characteristic leaching procedure (TCLP), and synthetic precipitation leaching procedure (SPLP) were performed to assess the pH, electrical conductivity (EC), alkalinity, and the leached concentrations of Ba, Ca, Cr, Mg, and SO₄ in RCA effluent. The leaching efficiency of the test methods at different RCA carbonation levels was also evaluated. Results indicated that the effluent pH, EC, and alkalinity decreased, while the leached fractions of elements increased with an increase in L/S ratio. An increase in calcium carbonate content tended to increase the leaching of Mg, Cr, and SO₄. For highly carbonated RCA samples, effluent pH, EC, alkalinity, and Ca concentrations were higher for particle sizes of 1.19 mm-0.149 mm, while fresher RCA samples resulted in higher values from particles finer than 0.149 mm. Carbonated RCA samples leached higher Ca, Mg, and Ba in TCLP, whereas the maximum concentrations of Cr and SO₄ were found in WLT effluent. For less carbonated RCA samples, Ca concentrations in WLT and TCLP effluents were comparable, SPLP leached higher amounts of Mg, Ba concentrations were maximum in WLT, and TCLP concentrations of Cr and SO₄ were the most critical ones. TCLP alkalinity increased, whereas WLT and SPLP alkalinity decreased with an increase in calcium carbonate content of the RCA.

Effect of cement incorporation on the leaching characteristics of elements from fly ash and slag treated soils

Inclusion of cement in fly ash and slag mixed soils could potentially alter the leaching behavior of elements. This study investigated the leaching characteristics of calcium (Ca), magnesium (Mg), sulfur (S), manganese (Mn), barium (Ba) and chromium (Cr) from cement activated soil-fly ash, soil-slag mixtures and soil, fly ash, steel slag and cement alone. Batch water leach tests, acid neutralization capacity and pH-dependent leach tests were performed. Test results indicated that, effluent concentrations of Ca and Ba increased, while Mg concentrations decreased with cement additions. No consistent trend was observed between S concentrations and cement content. The leaching of Cr and Mn remained unaffected by cement incorporation. Results of this study showed that the solution pH had the greatest influence on the leaching behaviors of the elements. Ca, Mg, S and Mn followed cationic leaching patterns, whereas Ba showed both cationic and amphoteric leaching patterns. The highest concentrations of Cr were observed at extreme acidic conditions, followed by a concentration plateau at the pH range of 5.5-10, and subsequent decrease and increase in concentrations at pH of 11.5 and 13, respectively. Geochemical modeling results suggested that except for Cr, the leaching mechanisms of the elements were controlled by their sulfate and (hydr)oxide minerals. The leaching of Cr was possibly controlled by BaCrO₄ and CaCrO₄. It was observed that the presence of carbonate minerals did not play a significant role on the leaching mechanisms of the elements, when cement was used as an activator.