Removal of B, Cr, Mo, and Se from wastewater by incorporation into hydrocalumite and ettringite

Study of microscopic properties and heavy metal solidification mechanism of electrolytic manganese residue-based cementitious materials

Electrolytic manganese residue (EMR) is a solid waste that contains a significant amount of soluble manganese and ammonia nitrogen, which can pose risks to human health if improperly disposed of. This study aimed to prepare cementitious materials containing abundant ettringite crystals by mixing EMR with various proportions of granulated blast furnace slag (GBFS) and alkaline activators (CaO, Ca(OH)2, clinker, NaOH). The resulting cementitious material not only utilized a substantial amount of EMR but also exhibited comparable strength to ordinary Portland cement. The optimal ratios were determined through mechanical testing. Additionally, the leaching toxicity of cementitious materials was assessed using ICP-MS (inductively coupled plasma mass spectrometer) tests. The microscopic properties, hydration, and mechanism of heavy metal solidification in the cementitious materials were evaluated using XRD (X-ray diffraction), SEM (scanning electron microscope), EDS (energy-dispersive spectrometer), FTIR (Fourier transform infrared spectroscopy), and TG (thermogravimetric) techniques. The results showed that the optimal ratio for the cementitious materials was 60% EMR, 36% GBFS, and 4% Ca(OH)2. The hardened mortar exhibited compressive strengths of 34.43 MPa, 41.3 MPa, and 50.89 MPa at 3 days, 7 days, and 28 days, respectively, with an EMR utilization rate of 60%. The hydration products of EMR-based cementitious materials were C-(A)-S-H, AFt, and ferromanganese compounds, which contribute to the mechanical strength. The Mn2+ and NH4+-N contents of raw EMR were 1220 and 149 mg/L, respectively. Nonetheless, the leaching of Mn2+ and NH4+-N in the alkali-EMR-GBFS system was significantly below the limits set by the Chinese emission standard GB8978-1996. Within this system, C-(A)-S-H and AFt could physically adsorb and displace heavy metals, Ca6Mn2(SO4)2(SO3)2(OH)12·24H2O could replace Al ions with Mn ions, and ferromanganese compounds Fe2Mn(PO4)2(OH)2·(H2O)8 and MnFe2O4 could chemically precipitate Mn2+.

Efficient aqueous molybdenum removal using commercial Douglas fir biochar and its iron oxide hybrids

Molybdenum (Mo) is a naturally-occurring trace element in drinking water. Most commonly, molybdate anions (MoO₄^2-) are in well water and breast milk. In addition, it is used in medical image testing. Recently, the EPA classified Mo as a potential contaminant, as exposure can lead to health effects such as gout, hyperuricemia, and even lung cancer. We have assessed the sorptive removal of aqueous molybdate using Douglas fir biochar (DFBC) and a hybrid DFBC/Fe₃O₄ composite containing chemically-coprecipitated iron oxide (Fe₃O₄). Adsorption was studied at various: pH values, equilibrium times (5 min-24 h), initial Mo concentrations (2.5-1000 mg/L), and temperatures (5, 25, and 40 °C) using batch sorption and fixed-bed column equilibrium methods. Langmuir capacities for DFBC and DFBC/Fe₃O₄ (at pH 3, 2 hrs equilibrium) were within 459.3-487.9 mg/g and 288-572 mg/g, respectively. These adsorbents and their Mo-laden counterparts were characterized by elemental analysis, BET, PZC, SEM, TEM, EDS, XRD, and XPS. MoO₄^2- adsorption on DFBC is thought to be governed primarily via electrostatic attraction. Adsorption by DFBC/Fe₃O₄ is primarily governed by chemisorption onto magnetite surface hydroxyl groups, while electrostatics prevail in the DFBC-exposed phase. Stoichiometric precipitation of iron molybdates triggered by iron dissolution was also considered. The data suggest that DFBC and DFBC/Fe₃O₄ are promising candidates for molybdate sorption.

Application and uncertainty of a geochemical speciation model for predicting oxyanion leaching from coal fly ash under different controlling mechanisms

Three primary mechanisms (adsorption to iron oxides or analogous surfaces, co-precipitation with Ca, and substitution in ettringite) controlling oxyanion retention in coal fly ashes (CFAs) were identified by differentiating the leaching behavior of As, B, Cr, Mo, Se, and V from 30 CFAs. Fidelity evaluation of geochemical speciation modeling focused on six reference CFAs representing a range of CFA compositions, whereby different leaching-controlling mechanisms of oxyanions were systematically considered. For three reference CFAs with low Ca and S content, calibration of adsorption reactions for the diffuse double-layer model for hydrous ferric oxides improved the simultaneous prediction of oxyanion leaching, which reduced uncertainties in Se and V predictions caused by nonideal adsorption surfaces and competitive adsorption effects. For two reference CFAs with intermediate Ca content, the solubility constants for Ca-arsenates from literature and postulated phases of B, Cr, Se, and V were used to describe co-precipitation of oxyanions with Ca-bearing minerals under alkaline conditions. For the reference CFA with high Ca and S content, an ettringite solid solution was used to capture the simultaneous retention of all oxyanions at pH> 9.5. Overall, the simultaneous leaching predictions of oxyanions from a wide range of CFAs were improved by calibration of adsorption reactions and controlling solid phases.

An Overview to Technical Solutions for Molybdenum Removal: Perspective from the Analysis of the Scientific Literature on Molybdenum and Drinking Water (1990–2019)

A bibliometric analysis using the Scopus database was performed to investigate the research documents published from 1990 to 2019 in scientific sources related to molybdenum in drinking water and determine the quantitative characteristics of the research in this period. The results from the analysis revealed that the number of publications was maintained at a regular production of around 5 papers per year until 2009, followed by a fast linear increase in the production in the period from 2010 to 2016 (29 papers in 2016), but the scientific production regarding this topic was reduced in 2017 and 2018 to recover the production obtained in 2016 once again in 2019. The total contribution of the three most productive countries (USA, China and India, respectively) accounted for around 50% of the total number of publications. Environmental Science was the most common subject (51.4% contribution), followed by Chemistry (26.7% contribution). The research efforts targeted toward the search for technical solutions for molybdenum removal from water are not as important as the ones focused on the identification of molybdenum-polluted water bodies and the analysis of the health effects of the intake of molybdenum. Nevertheless, examples of technological treatments to remove molybdenum from the aqueous solution include the use of adsorption and ion exchange; coagulation, flocculation and precipitation followed by filtration; membrane technologies and biological treatments.

A Bibliometric Analysis of Research on Selenium in Drinking Water during the 1990-2021 Period: Treatment Options for Selenium Removal

A bibliometric analysis based on the Scopus database was carried out to summarize the global research related to selenium in drinking water from 1990 to 2021 and identify the quantitative characteristics of the research in this period. The results from the analysis revealed that the number of accumulated publications followed a quadratic growth, which confirmed the relevance this research topic is gaining during the last years. High research efforts have been invested to define safe selenium content in drinking water, since the insufficient or excessive intake of selenium and the corresponding effects on human health are only separated by a narrow margin. Some important research features of the four main technologies most frequently used to remove selenium from drinking water (coagulation, flocculation and precipitation followed by filtration; adsorption and ion exchange; membrane-based processes and biological treatments) were compiled in this work. Although the search of technological options to remove selenium from drinking water is less intensive than the search of solutions to reduce and eliminate the presence of other pollutants, adsorption was the alternative that has received the most attention according to the research trends during the studied period, followed by membrane technologies, while biological methods require further research efforts to promote their implementation.

Behavior of potentially toxic elements from stoker-boiler fly ash in Interior Alaska: paired batch leaching and solid-phase characterization

Despite significant investigation of fly ash spills and mineralogical controls on the release of potentially toxic elements (PTEs) from fly ash, interactions with the surficial environment remain relatively poorly understood. We conducted 90-day batch leaching studies with paired analysis of supernatant and solid-phase mineralogy to assess the elemental release and transformation of fly ash upon reaction with aquatic media (18 MΩ cm^-1 water and simulated rainwater). The fly ash in this study, collected from the University of Alaska Fairbanks stoker-boiler power plant, is high in unburned carbon (~20% LOI) and highly enriched in several PTEs relative to the upper continental crust. Supernatant concentrations of oxyanion-forming elements (e.g., As, Se, Mo, Sb) remained relatively low and constant, suggesting equilibrium with the solid phase, possibly ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂•26H₂O], which is known to incorporate and sorb oxyanion-forming PTEs and was identified by X-ray diffraction. Synthetic precipitation leaching procedure (SPLP) results failed to capture important temporal trends. Lead and Ba supernatant concentrations consistently exceeded drinking water standards, as well as others upon exposure to simulated physiological solutions. Seven-day experiments with dissolved organic matter-isolate solutions indicated that for certain elements, liberation was influenced by carbon concentration and/or the identity of the isolate. Overall, this paired approach can serve as a model for future studies, bridging existing gaps between batch leaching and single-element mineralogical, sorption, or speciation studies.

Characterization of Monochromate and Hemichromate AFm Phases and Chromate-Containing Ettringite by 1H, 27Al, and 53Cr MAS NMR Spectroscopy

The calcium aluminate hydrate AFm and AFt phases formed upon hydration of Portland cement have an important role in the stabilization and solidification of hazardous chromate ions in hardened cement. AFm monochromate (Ca4[Al(OH)6]2(CrO4)·12H2O), AFm hemichromate (Ca4[Al(OH)6]2(CrO4)0.5(OH)·12H2O) and the chromate-containing AFt phase, Ca6[Al(OH)6]2-(CrO4)3·24H2O, were synthesized and investigated by 1H, 27Al, and 53Cr MAS NMR spectroscopy. 27Al quadrupolar coupling parameters (CQ, ηQ) and isotropic chemical shifts (δiso) were determined for the three phases, including two distinct Al sites in chromate-AFt, as observed by 27Al MAS and MQMAS NMR. Two dominant peaks are apparent in the 1H MAS NMR spectra of each of the phases. For the AFm phases, these resonances are assigned to framework hydroxyl groups (1.7–2.0 ppm) and water molecules/hydroxyls (5.0–5.5 ppm) in the interlayer. For chromate-AFt, the peaks are ascribed to framework hydroxyl groups in the [Ca6Al2(OH)12]6+ columns (~1.4 ppm) and water molecules (~4.8 ppm) associated with the Ca ions. 53Cr MAS NMR spectra acquired at 22.3 T for the samples show a narrow resonance for both chromate AFm phases, whereas indications of three distinct Cr resonances are apparent for the chromate AFt. The absence of any second-order quadrupolar effects in the 53Cr NMR spectra strongly suggests that the chromate ions are highly mobile in the anionic sites of the AFm and AFt structures. The NMR data reported in this work are in agreement with the reported crystal structures for the chromate AFm and AFt phases and may be useful for identification and characterization of chromate fixation in cementitious systems, complementing information gained from conventional powder X-ray diffraction studies.

Leaching and Geochemical Modelling of an Electric Arc Furnace (EAF) and Ladle Slag Heap

Old metallurgical dumps across Europe represent a loss of valuable land and a potential threat to the environment, especially to groundwater (GW). The Javornik electric arc furnace (EAF) and ladle slag heap, situated in Slovenia, was investigated in this study. The environmental impact of the slag heap was evaluated by combining leaching characterization tests of landfill samples and geochemical modelling. It was shown that throughout the landfill the same minerals and sorptive phases control the leaching of elements of potential concern, despite variations in chemical composition. Although carbonation of the disposed steel slags occurred (molar ratio CO₃/(Ca+Mg) = 0.53) relative to fresh slag, it had a limited effect on the leaching behaviour of elements of potential concern. The leaching from the slag heaps had also a limited effect on the quality of the GW. A site-specific case, however, was that leachates from the slag heap were strongly diluted, since a rapid flow of GW fed from the nearby Sava River was observed in the landfill area. The sampling and testing approach applied provides a basis for assessing the long-term impact of release and is a good starting point for evaluating future management options, including beneficial uses for this type of slag.

Environmental impact of amino acids on selenate-bearing hydrocalumite: Experimental and DFT studies

Selenium (Se) radioactive wastes can be disposed through stabilization/solidification (S/S) based on the cementitious matrix on hydration products, where hydrocalumite (Ca₂Al-LDH) is expected to play an important role in the retention of SeO₄^2-. Natural organic matters (NOMs) are known to be a risk to affect the transportation and mobility of undesirable chemical species in the pedosphere which receives the low level radioactive wastes (LLW). In the present work, five amino acids were selected as the simplified models of NOMs in the pedosphere to explore their effects on the stability of Ca₂Al-LDH after immobilized SeO₄^2- under alkaline conditions. As the loading amount of amino acids on Ca₂Al-LDH increasing, release of SeO₄^2- was enhanced in HGly, H₂Asp, and H₂Cys series, while no enhancement was observed in HPhe and HTrp series. Density functional theory (DFT) calculation predicted ion-exchange of amino acids and CO₃^2- with SeO₄^2- in a unit cell of LDH model. The intercalation of Asp^2- and CO₃^2- caused 003 peaks in XRD sharper and d₀₀₃ decreased from 8.15 Å to 7.70 Å which is assigned to Ca₂Al-LDH(Asp, CO₃). In H₂Cys series, the 003 peaks were kept broad and SeO₄^2- was still relatively maintained in LDH which was caused by the lower amounts of intercalated CO₃^2- in the presence of H₂Cys. Amino acids in the interlayer of Ca₂Al-LDH have several possible configurations, where the most stable one is prone to be in a horizontal direction through hydrogen bonds and Ca-O chemical bonds. This provides an insight on the stability of selenate immobilized in hydrocalumite, which can be produced in cement disposing in the pedosphere for a long term of burying. Not only carbonate but also small molecular organic matters like amino acids possibly give environmental impact on the mobility of low level anionic radionuclides in LDH.

Synthesis and Structural Analysis of Ternary Ca–Al–Fe Layered Double Hydroxides with Different Iron Contents

Hydrocalumite structured layered double hydroxides (LDHs) with various Fe3+ ratios were prepared through a coprecipitation method. In order to control the Fe3+ content in LDH, binary Ca–Fe LDHs were first synthesized with various Ca/Fe ratios. The X-ray diffraction pattern showed that only a limited Ca/Fe ratio resulted in LDH formation. The Fe3+ content in LDH was controlled by applying Al3+ while the divalent and trivalent metal ratio was set to 2. Through X-ray diffraction patterns, ternary LDHs with Ca–Al–Fe composition were successfully synthesized without significant impurities, with the Al increasing crystallinity. Quantification showed that Al moiety participated in the formation of the LDH framework more than Ca and Fe, implying a structural stabilization in the presence of Al. In order to investigate the global and local structure of Fe moiety in the LDH, both solid state UV-vis and X-ray absorption spectroscopies were carried out. Both spectroscopies revealed that the existence of Al induced slight local distortion in coordination but global crystal stabilization.

Immobilization of molybdenum by alternative cementitious binders and synthetic C-S-H: An experimental and numerical study

Excavation operations during construction produce millions of tons of soil sometimes with high leachable molybdenum (Mo) contents, that can lead to risks for both human health and the environment. It is therefore necessary to immobilize the Mo in excavated soils to reduce pollution and lower the costs of soil disposal. This paper studies the immobilization of Mo by three cementitious binders. To this end, one Ordinary Portland cement (OPC), one binder composed of 90% ground granulated blast furnace slag (GGBS) and 10% OPC, and one supersulfated GGBS binder were spiked with sodium molybdate at six different Mo concentrations from 0.005 wt% to 10 wt% before curing. In addition, to gain mechanistic insights, the capacity of synthetic calcium silicate hydrates (C-S-H) to immobilize Mo was studied. This study was completed by thermodynamic modeling to predict the immobilization of Mo at low Mo concentrations (<0.005 wt%). Paste leaching tests results showed that more than 74% of the initial Mo spike was immobilized by the three binders. The supersulfated GGBS binder consistently showed the highest retention levels (92.0 to 99.7%). The precipitation of powellite (CaMoO4) was the dominant mechanism of Mo retention in all binders and most leaching solutions were oversaturated with respect to powellite. Also, in C-S-H syntheses, Mo was largely immobilized (>95%) by the coprecipitation of powellite. Thermodynamic modeling was in good agreement with measured values when the equilibrium constant of powellite was modified to LogK = -7.2. This suggested that powellite is less stable in cementitious environments than would be expected from thermodynamic databases. Moreover, modeling showed that, for a solution at equilibrium with portlandite or C-S-H, the Mo concentration is limited to 1.7 mg/L by powellite precipitation. In contrast, for a solution saturated with respect to ettringite, the threshold concentration for powellite precipitation is 6.5 mg/L.

Unraveling the Underlying Heavy Metal Detoxification Mechanisms of Bacillus Species

The rise of anthropogenic activities has resulted in the increasing release of various contaminants into the environment, jeopardizing fragile ecosystems in the process. Heavy metals are one of the major pollutants that contribute to the escalating problem of environmental pollution, being primarily introduced in sensitive ecological habitats through industrial effluents, wastewater, as well as sewage of various industries. Where heavy metals like zinc, copper, manganese, and nickel serve key roles in regulating different biological processes in living systems, many heavy metals can be toxic even at low concentrations, such as mercury, arsenic, cadmium, chromium, and lead, and can accumulate in intricate food chains resulting in health concerns. Over the years, many physical and chemical methods of heavy metal removal have essentially been investigated, but their disadvantages like the generation of chemical waste, complex downstream processing, and the uneconomical cost of both methods, have rendered them inefficient,. Since then, microbial bioremediation, particularly the use of bacteria, has gained attention due to the feasibility and efficiency of using them in removing heavy metals from contaminated environments. Bacteria have several methods of processing heavy metals through general resistance mechanisms, biosorption, adsorption, and efflux mechanisms. Bacillus spp. are model Gram-positive bacteria that have been studied extensively for their biosorption abilities and molecular mechanisms that enable their survival as well as their ability to remove and detoxify heavy metals. This review aims to highlight the molecular methods of Bacillus spp. in removing various heavy metals ions from contaminated environments.

Toxic metals distribution, seasonal variations and environmental risk assessment in surficial sediment and mangrove plants (A. marina), Gulf of Kachchh (India)

Toxic metal pollution in the coastal ecosystem is becoming a serious problem, particularly in developing countries as a result of the industrial revolution. In recent years, mangroves are continuously contaminating with toxic metals and receiving global attention due to its toxicity, non-degradability, abundance, subsequent bioaccumulation, and biomagnification through successive trophic levels. This study aims to investigate the toxic metal content and pollution status in mangroves surface sediment and plants. Results showed that toxic metals in sediments were higher than natural background levels indicate anthropogenic sources. Fe, Mn, Sb, Ti found higher in concentration among all toxic metals, and site 9, 15, 18, 19, 21, 31 found the highest total metal load. Contamination indices like enrichment and contamination factor, geo-accumulation index, suggest minimal to extremely high level of contamination, and sediments have found extremely contaminated with Sb and As. Contamination degree and modified contamination degree suggest very high degree of contamination at all sites. Pollution load index indicates significant deterioration of sediment quality. Ecological risk and potential ecological risk index also indicate about 72% of sites come under higher ecological risk. Toxic metal in Avicennia marina was found higher in root than leaf. High bioconcentration factor has observed for Pb, Cu, Mo, Zn. Translocation factor for Cu and Zn at all sites, and As, Ni, Pb, Fe, Sr, Mn at some sites indicate high-efficiency in plants for toxic metal translocation.

Ultrastrong Anion Affinity of Anionic Clay Induced by Its Inherent Nanoconfinement

Layered double hydroxide (LDH), the only anionic clay in the environment, plays a key role in natural ion transportation. The ion retention effect of LDHs was traditionally attributed to ion exchange with low affinity. Here, we demonstrated an ultrastrong interaction between anions and LDHs induced by their inherent nanoconfinement using chromium ore processing residue (COPR) that contained several Cr(VI)-bonded LDHs as a probe. Hydrocalumite (Ca/Al-Cl LDH) was verified as the primary phase for Cr(VI) retention through two types of interactions such as ion exchange and Cr-Ca coordination. More significantly, the confined spacing between two layers of hydrocalumite provided spatial restriction and shielding effects to the intercalated Cr(VI), which enhanced Cr-Ca coordination by shortening the bonding distance and modulating the binding angle to achieve the lowest bonding energy. Such enhancement boosted Cr(VI) affinity up to 3.2 × 10⁵ mL/g, which was 1-3 orders of magnitudes higher than ion exchange. The universality of this mechanism was verified using another Mg/Al-Cl LDH and various anions. This study broke the traditional awareness of low ion affinities of LDHs limited by single ion exchange and disclosed an essential mechanism for unexpected ion retention effects of anionic clays in nature.

Competitive TcO4-, IO3-, and CrO42- Incorporation into Ettringite

Ettringite is a naturally occurring mineral found in cementitious matrices that is known for its ability to incorporate environmentally mobile oxyanion contaminants. To better assess this immobilization mechanism for contaminants within cementitious waste forms intended for nuclear waste storage, this work explores how mixed oxyanion contaminants compete for ettringite incorporation and influence the evolving mineralogy. Ettringite was precipitated in the presence of TcO₄^-, IO₃^-, and/or CrO₄^2-, known contaminants of concern to nuclear waste treatment, over pre-determined precipitation periods. Solution analyses quantified contaminant removal, and the collected solid was characterized using bulk and microprobe X-ray diffraction coupled with pair distribution function and microprobe X-ray fluorescence analyses. Results suggest that ≥96% IO₃^- is removed from solution, regardless of ettringite precipitation time or the presence of TcO₄^- or CrO₄^2-. However, TcO₄^- removal remained <20%, was not significantly improved with longer ettringite precipitation times, and decreased to zero in the presence of IO₃^-. When IO₃^- is co-mingled with CrO₄^2-, calcite and gypsum are formed as secondary mineral phases, which allows for oxyanion partitioning, e.g., IO₃^- incorporation into ettringite, and CrO₄^2- incorporation into calcite. Results from this work exemplify the importance of competitive immobilization when assessing waste form performance and environmental risk of contaminant release.

Evaluation of Trace Elements from Used Industrial Waste in Soil improvement

The use of industrial waste as an additive in soil improvement has many advantages, including recycling of waste, reducing the need for waste storage, and obtaining an economical material. With the use of these wastes, desired positive results are obtained in some geotechnical properties of soil. However, the wastes can create trace element contamination in soil and groundwater. In this study, trace elements originating from industrial wastes contaminating groundwater are investigated. The industrial wastes were mixed at different proportions with the soil. These mixtures were compacted into a permeameter cells, and a seepage tests were performed. The leachates obtained from seepage tests were analyzed with inductively coupled plasma mass spectrometry (ICP-MS) to determine trace elements. The measured trace element quantities were compared with the allowable values in the relevant standards (EPA 822, WHO, TS266). The results reveal that quantitative values of the trace elements from the leachates were within the allowable limits, except for arsenic and chromium. Furthermore, when fly ash is used As and Cr can be combined with ettringite and be immobilized. Boron and silica fume are hazardous substances caused by trace elements. However, considering its long-term effect, they can be used with fly ash.

Stabilization of borate by hot isostatic pressing after co-precipitation with hydroxyapatite using MAP

Boric acid is one of the most mobile inorganic contaminant species in nature due to its pK_a of 9.23. Co-precipitation of borate with hydroxyapatite (HAp: Ca₅(PO₄)₃OH) facilitates the simultaneous removal of borate with co-existing oxoanions in natural waters. The cost of phosphate is an impediment to industrialize the co-precipitation of borate with HAp for treatment of geothermal waters. In the present work, an inexpensive industrial by-product of magnesium ammonium phosphate (MAP) derived from sewage sludge, was examined as a phosphate source. MAP includes 89% pure magnesium ammonium phosphate, resulting in better performance than the pure chemical form of NH₄H₂PO₄, because Mg²⁺ and Al³⁺ (trace elements in MAP product) play roles in enhancing the removal rate of borate and lowering the equilibrium borate concentration. These ions have a good affinity with phosphate to nucleate crystal seeds independently of powdery Ca sources. To reduce the bulky volume of solid residues, hot isostatic pressing (HIP) was applied. There is structural water in HAp; therefore, the greatest volume reduction was achieved with 78.3 ± 2.0% (n = 3). Additionally, a synergic effect to suppress the released borate, greater than the sequential combination of calcination and cold isostatic pressing was accomplished in the toxicity contents leaching procedure (TCLP) test. This is not due to larger crystal sizes alone, but it is derived from boron stabilization in HAp at an atomic level by the synergic effect of heating and pressing simultaneously.

Effect of Additive Material on Controlling Chromium (Cr) Leaching from Coal Fly Ash

Coal fly ash contains a considerable number of toxic elements that can be leached into the environment, such as chromium (Cr), thereby quickly leading to severe contaminations. In this research, the leaching behaviors of Cr were analyzed from 14 kinds of coal fly ash samples collected from the electrostatic precipitators of coal-fired thermal power plants in Japan. The level of Cr concentration found in the samples varied from 0.00 to 82.93 μg/L. However, Cr toxicity depends on its valence state; Cr6+ is more toxic than Cr3+. Additive materials containing high calcium content were used to control the leaching concentration of Cr, such as Ca(OH)2, paper sludge ash, and blast furnace cement. This research used several instruments. An X-ray fluorescence was adopted to measure the major chemical composition of the fly ash samples and the additive materials. A thermogravimetric analyzer was used to examine the calcium compounds in the additive materials. Inductively coupled plasma was used to determine the Cr leaching concentrations from the fly ash samples. Findings showed that the three-additive mixture had a promising effect on controlling the Cr leaching concentrations. These results were also supported by FactSage 7.2 simulation.

Spectroscopic and first-principles investigations of iodine species incorporation into ettringite: Implications for iodine migration in cement waste forms

Low-level radioactive wastes are commonly immobilized in cementitious materials, where cement-based material can incorporate radionuclides into their crystal structure. Specifically, ettringite (Ca₆Al₂(OH)₁₂(SO₄)₃∙26H₂O) is known to stabilize anionic species, which is appealing for waste streams with radioactive iodine (¹²⁹I) that persists as iodide (I-) and iodate (IO₃-) in the cementitious nuclear waste repository. However, the structural information and immobilization mechanisms of iodine species in ettringite remain unclear. The present results suggested minimal I- incorporation into ettringite (0.05 %), whereas IO₃- exhibited a high affinity for ettringite via anion substitution for SO₄^2- (96 %). The combined iodine K-edge extended X-ray absorption fine structure (EXAFS) spectra and first-principles calculations using density functional theory (DFT) suggested that IO₃- was stabilized in ettringite by hydrogen bonding and electrostatic forces. Substituting IO₃- for SO₄^2- was energetically favorable by -0.41 eV, whereas unfavorable substitution energy of 4.21 eV was observed for I- substitution. Moreover, the bonding charge density analysis of the substituted IO₃- and I- anions into the ettringite structure revealed the interaction between intercalated ions with the structural water molecules. These results provided valuable insight into the long-term stabilization of anionic iodine species and their migration in cementitious nuclear waste repository or alkaline environments.

Sequestration of Selenite and Selenate in Gypsum (CaSO4·2H2O): Insights from the Single-Crystal Electron Paramagnetic Resonance Spectroscopy and Synchrotron X-ray Absorption Spectroscopy Study

Gypsum is the most common sulfate mineral on Earth's surface and is the dominant solid byproduct in a wide variety of mining and industrial processes, thus representing a major source for heavy metal(loid) contamination, including selenium. Gypsum crystals grown from the gel diffusion technique in 0.02 M Na₂SeO₄ solution at pH 7.5 and 0.02 M Na₂SeO₃ solutions at pH 7.5 and 9.0 contain 828, 5198, and 5955 ppm Se, respectively. Synchrotron Se K-edge X-ray absorption spectroscopic analyses show that selenite and selenate are the dominant species in Se⁴⁺- and Se⁶⁺-doped gypsum, respectively. The single-crystal EPR spectra of Se⁴⁺- and Se⁶⁺-doped gypsum after gamma-ray irradiation reveal five selenium-centered oxyradicals: SeO₂^-(I), SeO₂^-(II), SeO₂^-(III), SeO₃^-, and HSeO₄^2-. The former three radicals provide unequivocal evidence for the substitution of their paramagnetic precursor SeO₃^2- for SO₄^2- in the gypsum structure, while the latter two confirm the replacement of SeO₄^2- for SO₄^2-. These results demonstrate that gypsum has a significant capacity for sequestrating both selenite and selenate in the structure but has a marked preference for the former, thus confirming important controls on the mobility and bioavailability of selenium oxyanions and pointing to optimal applications of gypsum for remediating selenium contamination under neutral to alkaline conditions.

Solidification/stabilization of flue gas desulfurization brine and coal fly ash for heavy metals and chloride immobilization: Effects of S/S conditions and zero-valent-iron pretreatment

Effective management of flue-gas-desulfurization (FGD) wastewater and coal-combustion-residues (CCRs) are major challenges in the coal-fired power industry. The zero-liquid-discharge (ZLD) method of combining FGD brines and CCRs in solidification/stabilization (S/S) is promising due to its potential of treating both wastes in the same process. This study evaluated the performance of such a ZLD method for immobilizing heavy metals (Se, As, Cd and Cr) and chloride in FGD wastewater and/or CCRs. Effects of different coal fly ash (bituminous (BCFA) and sub-bituminous (SCFA)), activating agent (Portland cement (PC) and lime) and pretreatment of brines by zero valent iron (ZVI) on the S/S process were evaluated. Short-term and long-term leaching tests were conducted to evaluate performance of the S/S solids in pollutant retainment. The pre-treatment of FGD brine by ZVI enhanced the retainment of heavy metals when BCFA was used, but not when SCFA was used since it already performed quite well without ZVI pretreatment. Quantitative X-ray diffraction and scanning electron microscopy analyses strongly indicated the formation of Friedel's salt, Ca₂Al(OH)₆(Cl,OH)·2H₂O, is critical in the retainment of heavy metals and chloride. SCFA contained higher lime and reactive aluminate contents than BCFA; thus, S/S solids made with SCFA contained higher amounts of Friedel's salt.

Low-Carbon Binder for Cemented Paste Backfill: Flowability, Strength and Leaching Characteristics

Blast furnace slag was used as the main raw material to prepare the alkali activated slag (AAS), a low-carbon binder, for cemented paste backfill (CPB). The optimum parameters for preparing the AAS binders using an orthogonal experiment were obtained. Under the optimum conditions (NaOH content was 3 wt. %, Ordinary Portland cement (OPC) content was 7 wt. %, and gypsum dosage was 4 wt. %), the 28 days compressive strength of the binder was 29.55 MPa. The flow ability of the fresh CPB slurry decreased with solid content due to the increased yield stress, while the flow ability increased when rising the binder dosage. A predictive model for the compressive strength of CPB samples was reached through multivariate analysis and the R2 values were higher than 0.9. Sensitivity analysis showed that the solid content is the most important parameter which influences on the development of the CPB strength with a correlation coefficient of 0.826. From the Toxicity Characteristic Leaching Procedure (TCLP) tests, the leaching concentrations of Pb and Cd were below the threshold. As a result, the AAS has potential application as an alternative binder and cemented paste backfill.

Mechanistic Insights into a Sustainable Mechanochemical Synthesis of Ettringite

Mechanochemistry offers an environmentally benign and facile synthesis method for a variety of cement paste constituents. In addition, these methods can be used to selectively tune the properties of cement components. The mineral ettringite is an important component of cementitious materials and has additional technological potential due to its ion exchange properties. Synthesis of ettringite via mechanochemistry is an environmentally friendly alternative to conventional wet-chemical synthesis established in industry. This contribution explores the mechanism of a two-step mechanochemical synthesis of ettringite, which was previously found to greatly improve the reaction conversion as compared with one-pot synthesis. The crystallinity of Al(OH)3 was found to decrease during the first stage of this mechanochemical synthesis. This was correlated to a significant decrease in the particle size of Al(OH)3 in this stage. No other significant changes were found for the other components, suggesting that mechanochemical activation of Al(OH)3 is responsible for the enhanced formation of ettringite by the two-step approach. The environmentally friendly approach developed for ettringite synthesis offers a versatile synthetic strategy, which can be applied to synthesise further cementitious materials.

Suppression processes of anionic pollutants released from fly ash by various Ca additives

Harmful trace elements, which are initially included in the coal fly ash, have the potential to be leached when coal fly ash comes in contact with water. This causes a risk of pollutant species being released, considering the long lifetime of building structures where coal fly ash was applied. Some Ca additives effectively function to suppress the release of anionic pollutants; however, the detailed suppression processes remains unclear. In this work, the influences of various Ca additives on the released anionic pollutants (B, F, S, As, and Cr) was systematically investigated. According to the comprehensive results of solution data with the solid characterization, the 60% hydroxylated calcined dolomite (HCD 60) was the best Ca additive for the suppression of different anionic pollutants since this Ca source not only simply provides an alkaline reagent but also supplies MgO and Mg(OH)₂, which affect the phase transformation that accompanies with hydration. The phase transformation occurs from Ca(OH)₂ to ettringite via hydrocalumite, which is the most important suppression processes of released pollutants. The precipitation of Ca salts is another pathway to immobilize these pollutants. In this scheme, MgO and Mg(OH)₂ were proven to enhance the formation of ettringite and hydrocalumite, respectively.

Ettringite via Mechanochemistry: A Green and Rapid Approach for Industrial Application

Here, we report on a first mechanochemical synthesis of ettringite, an important cement hydrate phase. The mineral compound ettringite ([Ca₃Al(OH)₆]₂·(SO₄)₃·26H₂O) occurs rarely in nature, but is common for cement-based materials. Ettringite has wide technical application in the ceramic and paper industry. However, its typical wet-chemical synthesis is cumbersome and produces waste water and CO₂ emissions. Here, we investigate the first mechanochemical synthesis of ettringite for developing an easy and sustainable alternative for industrial application. The mechanosynthesis was monitored in situ by coupled synchrotron X-ray diffraction (XRD) and infrared thermography (IRT). The consumption of the reactants and the formation of the reaction product were monitored with time-resolved XRD. IRT showed the temperature increase based on the exothermic reaction. The reaction conversion was significantly improved changing the strategy of the mechanosynthesis from a one- to a two-step process. The latter included neat pregrinding of solid reactants followed by a delayed addition of the stoichiometric amount of water. Thus, an increase of reaction conversion from 34 to 94% of ettringite could be achieved.

Reduction of undesirable element leaching from fly ash by adding hydroxylated calcined dolomite

Fly ash always contains many toxic elements which can be released into environment, thereby easily leading to environmental contaminations. In order to dispose fly ash safely, related strategies are needed. In this investigation, two kinds of hydroxylated calcined dolomites (HCD60 and HCD100) were used as the additives and compared with lime on the leachabilities of anionic species from fly ash. Both additives were found effective in reducing the leaching concentrations of these elements, which was better than that of only lime addition. Mg(OH)₂ and MgO were believed to play important roles in the hydration reaction of fly ash. In the presence of Mg(OH)₂ and MgO, there were more hydration products including calcium silicate hydrate, ettringite, hydrocalumite and other Layered double hydroxides (LDHs) generated which were effective candidates for anion removal. Thus, the final leaching results were controlled by these newly formed phases through adsorption, incorporation or encapsulation. On the other hand, compared with Mg(OH)₂, MgO can promote the formation of hydration products in a larger extent because of the hydration process of MgO into Mg(OH)₂. There was no systematic trend in the promotion of fly ash hydration by Mg(OH)₂ or MgO because it had a close relationship with the properties of original fly ash. Objectively, hydroxylated calcined dolomites can be promising candidate additives for reduction of toxic elements leaching from fly ash.

Selenium Nanoparticle Synthesized by Proteus mirabilis YC801: An Efficacious Pathway for Selenite Biotransformation and Detoxification

Selenite is extremely biotoxic, and as a result of this, exploitation of microorganisms able to reduce selenite to non-toxic elemental selenium (Se⁰) has attracted great interest. In this study, a bacterial strain exhibiting extreme tolerance to selenite (up to 100 mM) was isolated from the gut of adult Monochamus alternatus and identified as Proteus mirabilis YC801. This strain demonstrated efficient transformation of selenite into red selenium nanoparticles (SeNPs) by reducing nearly 100% of 1.0 and 5.0 mM selenite within 42 and 48 h, respectively. Electron microscopy and energy dispersive X-ray analysis demonstrated that the SeNPs were spherical and primarily localized extracellularly, with an average hydrodynamic diameter of 178.3 ± 11.5 nm. In vitro selenite reduction activity assays and real-time PCR indicated that thioredoxin reductase and similar proteins present in the cytoplasm were likely to be involved in selenite reduction, and that NADPH or NADH served as electron donors. Finally, Fourier-transform infrared spectral analysis confirmed the presence of protein and lipid residues on the surfaces of SeNPs. This is the first report on the capability of P. mirabilis to reduce selenite to SeNPs. P. mirabilis YC801 might provide an eco-friendly approach to bioremediate selenium-contaminated soil/water, as well as a bacterial catalyst for the biogenesis of SeNPs.

Preliminary Study on Additives for Controlling As, Se, B, and F Leaching from Coal Fly Ash

The application of paper sludge ash as an additive in controlling the leaching of trace elements has been satisfactorily effective to date. Previous studies have found that paper sludge ash has a promising effect in controlling the leaching of arsenic, selenium and boron. The content of calcium oxide in paper sludge ash is believed to be one of the important factors in decreasing the concentration of trace elements in leachate. Therefore, this study aimed to verify the effect of paper sludge ash in the leaching process and to propose an effective and applicable suppressing material that can control the leaching of As, Se, B and F simultaneously. In light of this aim, Ca(OH)2, PS ash 8 and blast furnace cement (BF cement) were tested as single and mixed additives in two different coal fly ashes (FA C and FA H). The results indicate that the application of a mixture of additives is necessary to control the leaching of trace elements. A mixture of PS ash 8, Ca(OH)2 and blast furnace cement (BF cement) was proposed to be an applicable and suitable additive that could suppress arsenic, selenium, boron, and fluorine leaching simultaneously.

Dried powder of corn stalk as a potential biosorbent for the removal of iodate from aqueous solution

Removal of IO₃^- from environmental samples with low-cost methods and materials is very useful approach for especially large-scale applications. Corn stalk is highly abundant agriculture residual, which is employed as useful biosorbent in many studies. In the present work, dried powder of corn stalk is applied for the removal of IO₃^- under various conditions. The results indicate that the K_d is 49.73 ml g^-1 under general conditions (m/V = 8 g L^-1, t = 5 day, equilibrium pH = 7 ± 0.3, T = 298 K and C₀ = 15 mg L^-1). The sorption kinetics follows the pseudo-second-order equation, and the isotherm is well described by the Langmuir model. The sorption reaction was non-spontaneous and endothermic. Hydroxyl and carbonyl groups of the corn stalk contribute to IO₃^- sorption by ion-exchange, electrostatic attraction and redox reactions. Spectroscopic analyses and the effect of equilibrium pH prove that corn stalk was not only removed IO₃^- from aqueous solution but also reduced IO₃^- into I₂ and I^-. These results demonstrate that corn stalk is a promising biosorbent for the environmental remediation of radioactive iodine pollution.

Recycling ground MSWI bottom ash in cement composites: Long-term environmental impacts

In the present study, the long-term leaching behaviors of heavy metals in the cement composites prepared by the municipal solid wastes incineration (MSWI) bottom ash are evaluated based on the modified NEN 7375 protocol. The leaching test of the compact and ground cement composites were performed in both the deionized water and saline water for 180 days. The results showed that the heavy metals investigated could be classified into three categories according to their leaching behaviors. In the first category, the concentrations of Cu, Cd, Pb, As, V and Ba in the leachate increased with the leaching time. Zn and Sn can be included into the second category, because a decline in their leaching concentrations was observed after the initial increase. In the third category, the concentration of Ni in the leachate decreased initially, but increased afterward. The results revealed that the concentrations of most heavy metals were within the corresponding regulation, except for As in saline water. The kinetic study revealed that, for most heavy metals, the leaching kinetic is controlled by diffusion in the deionized water, while by the surface wash-off in the saline water. Finally, the mechanical tests confirmed that the cement composites prepared by MSWI bottom ash were durable in the saline water. The overall results demonstrate that the MSWI bottom ash can be a promising alternative as the cementitious component applied in cements or concretes for civil engineering.

Leaching mechanisms of heavy metals from fly ash stabilised soils

Fly ash is an industrial waste material that is repurposed as a soil stabiliser worldwide. In Thailand, many ground improvement projects utilise mixtures of cement and fly ash to stabilise weak soils. In this study, leaching mechanisms of arsenic, chromium, lead, and zinc from cement and fly ash stabilised soils were investigated in the laboratory. Leaching tests were performed, with different leachants and pH conditions, on cement and fly ash stabilised soils used for soil improvement in road embankment construction projects in Northern Thailand. The results suggested that chemical compounds (CaO and MgO) on fly ash surfaces can control the pH of the fly ash and soil leachant. The dissolution of chromium and zinc was found to be amphoteric and controlled by oxide minerals at a high or low pH. Arsenic leaching was found to be oxyanionic where AsO₄^3- prevented the adsorption of arsenic onto the negatively charged fly ash surface. Different types of leachant also leached out in different amounts of heavy metals.

Thermodynamic and crystallographic model for anion uptake by hydrated calcium aluminate (AFm): an example of molybdenum

Amongst all cement phases, hydrated calcium aluminate (AFm) plays a major role in the retention of anionic species. Molybdenum (Mo), whose ⁹³Mo isotope is considered a major steel activation product, will be released mainly under the form of MoO₄²⁻ in a radioactive waste repository. Understanding its fate is of primary importance in a safety analysis of such disposal. This necessitates models that can both predict quantitatively the sorption of Mo by AFm and determine the nature of the sorption process (i.e., reversible adsorption or incorporation). This study investigated the Cl⁻/MoO₄²⁻ exchange processes occurring in an AFm initially containing interlayer Cl in alkaline conditions using flow-through experiments. The evolution of the solid phase was characterized using an electron probe microanalyzer and synchrotron high-energy X-ray scattering. All data, together with their quantitative modeling, coherently indicated that Mo replaced Cl in the AFm interlayer. The structure of the interlayer is described with unprecedented atomic-scale detail based on a combination of real- and reciprocal-space analyses of total X-ray scattering data. In addition, modeling of several independent chemical experiments elucidated that Cl⁻/OH⁻ exchange processes occur together with Cl⁻/MoO₄²⁻ exchange. This competitive effect must be considered when determining the Cl⁻/MoO₄²⁻ selectivity constant.

Dynamics of metals in backfill of a phosphate mine of guiyang, China using a three-step sequential extraction technique

Phosphate rock in Guiyang (Southwest of China) is used for the phosphate production, and hence generating a by-product phosphogypsum (PG). From 2007, part of the PG was used as main raw material for cemented backfill. The main objective of this paper is to investigate the geochemical evolution of metals before and after the PG inclusion into the backfill matrix. A sequential extraction procedure was selected to determine the chemical speciation of metals in phosphate rock, PG, binder and field backfill samples. Dynamics of metals going from phosphate rock and PG to backfill have been evaluated. The results showed that almost all the metals in the PG and binder had been effectively transferred to the backfill. Furthermore, compared to metals taken out along with phosphate rock exploitation, PG-based cemented backfill might bring some metals back but with only little metals in mobile fraction. Additionally, in order to determine the long-term behavior of metals in PG-based cemented backfill, the field samples which were backfilled from 2007 to 2016 were collected and analyzed. The results showed that total amounts of metals in backfill were all within similar range, indicating that the cemented PG backfill could be an effective method to solidify/stabilize metals in PG. Nevertheless, Due to the high water-soluble fractions detected, the concentrations of As, Mn and Zn should be continuously monitored.

Hydrothermal synthesis of hydrocalumite assisted biopolymeric hybrid composites for efficient Cr(vi) removal from water

Hydrocalumite (HC) incorporated biopolymer (alginate and chitosan) based hybrid composite materials were developed for the selective removal of chromium.

Retention of selenium by calcium aluminate hydrate (AFm) phases under strongly-reducing radioactive waste repository conditions

Strong Se(-ii) sorption mainly in the interlayer of hemicarbonate (AFm-HC). Weak Se(-ii) sorption restricted to sorption sites on the surface of monocarbonate (AFm-MC).

Surface glycopolymer-modified functional macroporous polyHIPE obtained by ATRP for the removal of boron in water

Macroporous polymeric monoliths PHIPE-PGAMA were obtained from polyHIPE with surface modification of PGAMA and used for boron removal.

Long-term leaching behaviours of cement composites prepared by hazardous wastes

In order to evaluate the long-term environmental impact of Eco-Ordinary Portland Cement (EOPC) prepared by municipal solid wastes (MSS) and hazardous wastes (HW), consecutive leaching tests with a time span of 180 days were conducted on the EOPC composites in the compact and ground forms under deionized and saline water conditions. The results show that the heavy metals investigated can be classified into three groups according to their leaching behaviours. The concentrations of V, Pb, Ni, Ba, Cd and Zn in the leachate increase with the leaching time, which can be classified into the first group. Cu and Sn are in the second group, and their concentrations increase initially, and decline afterward. Cr and As are in the third group, and their concentrations decline firstly, followed by a clear increase. Besides, a kinetic study was also conducted in the present study, revealing that the leaching behaviours of heavy metals follow a second-order model. Furthermore, our results suggest that the EOPC is resistant to the saline water, but the application of such materials in marine conditions should be paid attention to due to the pollution of arsenic.

In order to evaluate the long-term environmental impact of Eco-Ordinary Portland Cement (EOPC) prepared by hazardous wastes, long-term leaching tests were conducted on the EOPC composites under deionized and saline water conditions.

Synthesis and thermodynamic properties of arsenate and sulfate-arsenate ettringite structure phases

Arsenic is a toxic and carcinogenic contaminant of potential concern. Ettringite [Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O] has the ability to incorporate oxyanions as a solid solution with SO₄²⁻, which could lower the soluble oxyanion concentrations. Therefore, ettringite containing SO₄²⁻ and AsO₄³⁻ has been synthesized. Results indicated that AsO₄³⁻ could substitute for SO₄²⁻ inside the channels of ettringite in the form of HAsO₄²⁻, and a linear correlation existed between X_{initial solution} and X_solid. The thermodynamic characterization of the solid samples was investigated by means of Visual MINTEQ, a freeware chemical equilibrium model, and the solubility product logK of -48.4 ± 0.4 was calculated for HAsO₄–ettringite at 25°C. The Lippmann phase diagram and X_HAsO4–X_HAsO4,aq plot showed that the solid solution series containing arsenate has HAsO₄-poor aqueous solutions in equilibrium. These findings can be helpful to arsenate solidification and arsenate leaching modeling projects.

The mechanisms of heavy metal immobilization by cementitious material treatments and thermal treatments: A review

Safe disposal of solid wastes containing heavy metals is a significant task for environment protection. Immobilization treatment is an effective technology to achieve this task. Cementitious material treatments and thermal treatments are two types of attractive immobilization treatments due to that the heavy metals could be encapsulated in their dense and durable wasteforms. This paper discusses the heavy metal immobilization mechanisms of these methods in detail. Physical encapsulation and chemical stabilization are two fundamental mechanisms that occur simultaneously during the immobilization processes. After immobilization treatments, the wasteforms build up a low permeable barrier for the contaminations. This reduces the exposed surface of wastes. Chemical stabilization occurs when the heavy metals transform into more stable and less soluble metal bearing phases. The heavy metal bearing phases in the wasteforms are also reviewed in this paper. If the heavy metals are incorporated into more stable and less soluble metal bearing phases, the potential hazards of heavy metals will be lower. Thus, converting heavy metals into more stable phases during immobilization processes should be a common way to enhance the immobilization effect of these immobilization methods.

Leaching mechanisms of constituents from fly ash under the influence of humic acid

As a low-cost material for adsorption, FA is one of the most efficient adsorbents of HA. However, the leaching of elements from FA is problematic during utilization in water treatment. In this investigation, the potential leaching behaviors of Calcium, Arsenic, Born, Chromium, and other elements from FA in HA solution were studied via batch test. The data show that HA had an effect on the leaching of each element of FA, depending on the pH, the initial concentration of HA and the addition of calcium oxide (CaO). The Langmuir isotherm could better fit the equilibrium data in different initial concentrations of HA from 10 to 100mg/L. Because of the interaction between HA and the FA leaching elements, multi-layer adsorption occurred when the initial concentration of HA was more than 100mg/L. The pH and free CaO content played major roles in HA adsorption and FA leaching. Using SEM and XRD to characterize the solid of FA being mixed with CaO treated in solution, the results demonstrated that the reaction between FA and CaO could generate crystal minerals, such as portlandite, gismondine, ettringite (AFt) and calcite, which effectively restrained the leaching of elements, reduced secondary pollution.

Structural transformation of selenate ettringite: a hint for exfoliation chemistry

Selenate ettringite could be destructuralized by sulfate and could be exfoliated into nanoscale columnar parts in strong polar solvent.

Heavy metal removal capacity of individual components of permeable reactive concrete

Permeable reactive barriers (PRBs) are a well-known technique for groundwater remediation using industrialized reactive media such as zero-valent iron and activated carbon. Permeable reactive concrete (PRC) is an alternative reactive medium composed of relatively inexpensive materials such as cement and aggregate. A variety of multimodal, simultaneous processes drive remediation of metals from contaminated groundwater within PRC systems due to the complex heterogeneous matrix formed during cement hydration. This research investigated the influence coarse aggregate, portland cement, fly ash, and various combinations had on the removal of lead, cadmium, and zinc in solution. Absorption, adsorption, precipitation, co-precipitation, and internal diffusion of the metals are common mechanisms of removal in the hydrated cement matrix and independent of the aggregate. Local aggregates can be used as the permeable structure also possessing high metal removal capabilities, however calcareous sources of aggregate are preferred due to improved removal with low leachability. Individual adsorption isotherms were linear or curvilinear up, indicating a preferred removal process. For PRC samples, metal saturation was not reached over the range of concentrations tested. Results were then used to compare removal against activated carbon and aggregate-based PRBs by estimating material costs for the remediation of an example heavy metal contaminated Superfund site located in the Midwestern United States, Joplin, Missouri.

Study of the leaching behaviour of ladle slags by means of leaching tests combined with geochemical modelling and mineralogical investigations

In this study, the leachability of freshly produced ladle slag derived from both austenitic and ferritic stainless steel production, and from electrical and structural steel production, was investigated, in order to determine whether variations in the chemical and mineralogical composition of these slags affect their leaching behaviour. The effect of the method used for slag cooling was also studied. The results obtained by using the single batch test were combined with those obtained by means of more sophisticated characterisation leaching tests, which, in combination with geochemical speciation modelling, helped to better identify the release mechanisms and phases that control the release of individual elements. It was found that, although variations in the chemical composition of the slag can affect the slag's minerology, neither such variations, nor the choice of the slag cooling treatment, have a significant effect on the leachability of individual elements, since the leaching is governed by surface phenomena. In fact, the mineral transformations on the slag surface, rather than the bulk mineral composition, dictate the release of these elements from the ladle slag. The solubility-controlling phases were predicted by multi-element modelling, and verified to the extent made possible by the performed mineralogical investigations.

Moisture content-affected electrokinetic remediation of Cr(VI)-contaminated clay by a hydrocalumite barrier

An electrokinetic-permeable reaction barrier (EK-PRB) system was introduced in this study with hydrocalumite as the barrier material. The combined system effectively remediated the Cr(VI)-contaminated clay after a 72-h treatment, and the Cr(VI) removal efficiency increased with the initial soil moisture content. Further evidence was found that the changing soil pH value and current density were highly associated with the initial moisture content, showing its important roles in the Cr(VI) removal process. Additionally, the total Cr removal efficiency was much lower than that of Cr(VI) owing to the partial conversion of Cr(VI) to Cr(III) in the electrokinetic remediation process. Under high soil moisture conditions (40%), the removal efficiency of Cr(VI) and total Cr was 96.6 and 67.3%, respectively. Further analysis also revealed the new mineral phase, chromate hydrocalumite, for Cr fixation in the hydrocalumite barrier, which was significantly affected by the initial soil moisture content. Our results showed that the EK-PRB system with a hydrocalumite barrier is highly promising with great potential for the effective remediation of Cr(VI)-contaminated clay and engineering implementation.