Previous studies of cadmium and mercury immobilization in geopolymers have produced inconsistent results due to their different pozzolans, metal concentrations, and mixing procedures. Understanding the effects of these parameters on heavy metal immobilization is key to predicting their long-term stability. In this study, cadmium and mercury were incorporated into a metakaolin-based K-activated geopolymer by three mixing procedures and concentrations of 0.02–1.00 wt.%. The samples were then immersed in water for 90 d to determine their stability. The results show that mercury is readily leached from the geopolymer, but cadmium is retained. Adding the heavy metals in salt form converts the metals into cadmium hydroxide and mercury oxide that reside at the bottom of the geopolymer. Mixing the salts with water forms soluble heavy metals prior to geopolymerization. This procedure produces more-homogeneous geopolymers. Cadmium is associated with silicate and aluminate, giving a better stability, whereas mercury forms mercury oxide. Different cadmium and mercury concentrations do not change the metal speciation as mercury is affected by relativistic contribution.

Enhanced rock weathering (ERW) involving the spreading of basaltic rock powder on cropland is gaining interest as a promising carbon dioxide removal (CDR) technique. To optimise CDR efficiency and address the limitations of mineral dissolution associated with cropland, this study explored the potential of deploying basalt-based ERW in acid mine drainage (AMD), utilizing its strong acidity and flow-through nature. One-dimensional reactive transport modeling was undertaken by simulating various AMD scenarios with pH levels of 2–4 and flow rates of 0.005–0.5 m³ s–1, with a range of particle sizes. Results indicate that a single AMD system could dissolve up to 10,500 tonnes (t) of basalt annually, potentially removing 3,660 t CO₂. AMD ERW may thus contribute to CDR with a co-benefit of arsenic removal. In the absence of direct basalt-based ERW applications to AMD settings, this study provides guidance for designing alternative ERW protocols alongside traditional cropland-ERW applications.

Magnesium silicate hydrate (M-S-H) is a poorly crystalline Mg-silicate phase formed under alkaline conditions at low temperatures (T < 100 degrees C). Its formation has been studied in closed systems but not in open-flow systems, which better represent natural surface/subsurface environments. Here, MgO powder was used in reactions at pH similar to 10 and 50 degrees C in flow-through experiments to study the formation of M-S-H as a function of aqueous Si con-centration (1.5, 0.15, and 0 mM). Consumption of aqueous Si during precipitation of M-S-H resulted in an increase in the dissolution rate of the primary material (Mg hydroxide). Steady-state Si concentrations were used to calculate the dissolution rate of Mg hydroxide and the precipitation rate of M-S-H. Analyses of retrieved solids by electron microscopy and nuclear magnetic resonance spectroscopy confirmed the formation of M-S-H, although X-ray diffraction patterns pro-vided no clear evidence of the presence of M-S-H because of the small amount precipitated and its nano -crystallinity. The chemical composition (Mg/Si ratio) of the M-S-H varied with the aqueous Si concentration of the injected solution. Mg/Si ratios of 1.00 +/- 0.09 and 1.59 +/- 0.15 were obtained with Si concentrations of 1.5 and 0.15 mM, respectively. Results indicate that the formation of M-S-H is feasible under Earth surface condi-tions, with dissolved silica coexisting with Mg-bearing minerals at alkaline pH.

Montmorillonite (Mt) expansion and swelling are key factors determining the barrier performance of bentonite in trans-uranic (TRU) and high-level radioactive waste disposal. In the case of co-located geological disposal of TRU and high-level waste, NH4+ ions formed from NO3− ions leached from TRU waste may contact bentonite, exchanging with interlayer cations of Mt. to form NH4-Mt, with a reduction in barrier performance. Because of the similar hydration energies of NH4+ and K+, NH4-Mt may have less expandability or even change to a non-expandable mineral such as K-Mt. An understanding of the expansion and alteration behavior of NH4-Mt, especially in comparison with K-Mt, is thus necessary in waste-disposal safety assessment. Here, the hydration behavior of NH4-Mt was investigated by X-ray diffraction (XRD) analysis and molecular dynamics (MD) simulation and compared with that of K-Mt. XRD profiles under relative humidity (RH) control indicate that expansion of NH4-Mt is similar to that of K-Mt at >40% RH with slightly different d-values. However, NH4-Mt expansion is kept at ∼20% RH, while K-Mt tends to dehydrate. MD simulations indicate that hydrogen bonding with NH4+ ions causes the differences in hydration behaviors of NH4- and K-Mt, increasing basal spacing in dehydrated states and promoting hydration. This “hydration gap” may be attributed to differences in alteration to non-expandable minerals by dehydration, with NH4+ having less tendency for alternation and less of an effect on the barrier performance of bentonite than K+.

In Mondulkiri province, Cambodia, artisanal gold miners dump tailings and wastewater from gold processing into a tributary of the Prek Te River. In the rainy season, heavy metal concentrations in the tributary decrease below the WHO drinking water standard levels through natural attenuation; however, this does not occur in the dry season. To further understand the natural attenuation mechanism, detailed analyses of the wastewater from tailing and tributary water, tributary sediments, waste rock, and ore minerals were undertaken in both seasons. The high concentration of dissolved Fe in the contaminated tributary plays a significant role in As removal during the rainy season, whereas other elements such as Ni, Se, and Cu concentration decrease due to dilution. Schwertmannite formation, controlled by iron-oxidizing bacteria, was only found at the bottom of the tributary during the rainy season. In the dry season, As, Ni, Se, and Cu concentrations remained at their original levels because there was no formation of schwertmannite or dilution by rainwater. The existing schwertmannite also starts to dissolve as the pH decreases. Seasonal dynamics cause the failure of natural attenuation; thus, methods for maintaining its effectiveness in the dry season are needed. In addition, geochemical modeling was conducted to determine the significant roles of schwertmannite formation and dilution of rainwater in the tributary. Schwertmannite is a potential adsorbent for As removal from drainage. However, dilution provided indirect and direct impacts on the tributary, such as increasing the pH and diluting the concentration of toxic elements.

Disposal of cesium-137 (Cs-137)-loaded chabazite generated from decontaminating cooling water of the damaged reactor at the Fukushima Daiichi Nuclear Power Station (FDNPS) has become a crucial concern. The potassium aluminosilicate-based alkali activated material (K-AAM) matrix is one of the candidate encapsulation matrices proposed for encapsulating cesium-137. In this study, chabazite loaded with a low Cs concentration (1 mg/g of Cs), embedded into a K-AAM matrix (K-AAM-C), was analysed to determine its capability to immobilise Cs, which was investigated by batch leaching experiments, field emission-electron probe microscopy analysis (FE-EPMA), X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. The leaching experiments revealed that K-AAM-C efficiently immobilised Cs, with only 3 % of the Cs leached out after 360 days of leaching in deionised water. Characterisation using XRD, TEM, and Raman analysis confirmed that the alkali-activator was responsible for the phase transformation of chabazite. FE-EPMA demonstrated that K entered the chabazite structure. This phenomenon resulted in the breakdown and subsequent reconstruction of the chabazite structure. TEM observation showed that the Cs was concentrated into the aggregates of pre-cipitates, heterogeneously forming a pollucite-like structure in the chabazite after the fabrication process. Thermodynamic calculations indicated that pollucite was preferably stable in an AAM environment. When immersed in water, the amount of nano-pollucite increased over time, leading to the structural re-arrangement of aluminosilicate rings of chabazite according to TEM and Raman analysis. Pollucite is well known as a Cs-bearing natural zeolite, which can encapsulate Cs in its structure. Therefore, Cs retention was achieved in the spent chabazite adsorbent embedded into the K-AAM due to the resultant pollucite structure formed during AAM fabrication.

Abstract Silicate glasses are durable materials in our daily life, but corrosion rate accelerates under alkaline aqueous environment. Such situation has raised concerns, for example, in nuclear waste disposal where vitrified wastes encounter to alkaline leachate from surrounding concrete materials. Here we report volcanic glass example surviving with a hyperalkaline groundwater (pH > 11) and high flow rate for about 4000 years. The tiny glass fragments were extracted from the volcanic ash layer sandwiched between ultramafic sediments using microanalytical techniques. Sharp elemental distributions at the glass surface, where amorphous-like smectite precursors and crystalline smectites coexist, suggest the corrosion by an interface-coupled dissolution-precipitation mechanism rather than inter-diffusion. The secondary minerals acted as a protective film for the glass by limiting water accessibility. The corrosion rate was maintained at, the minimum, 2.5 orders of magnitude less than the rate observed for fresh glass, even in the presence of Fe and Mg that might have consumed Si through the silicate precipitation.

Understanding the behavior of secondary minerals under alkaline conditions is important for predicting the potential alteration of the constituent minerals in radioactive-waste disposal facilities. A previous study reported the formation of uncommon Fe- and Mg-bearing clays under natural alkaline conditions in the Philippines; these were referred to as iron-magnesium-silicate-hydrates (F-M-S-H) and nontronite-like minerals. The current study aimed to investigate the structural and chemical characteristics and to understand the formation pathways of these clays by performing a detailed characterization. F-M-S-H comprised tetrahedral–octahedral–tetrahedral (TOT) layers, imperfect interlayer hydroxide sheets, and interlayer Ca ions. The systematic changes in the characteristics of F-M-S-H at different sampling depths, such as the gradual decrease of the interlayer hydroxide sheets to form smectitic domains, were caused by the differing interaction periods between each sediment at different sampling depths and alkaline seepage. Furthermore, F-M-S-H was ferrous in form prior to oxidation. In contrast, a nontronite-like mineral comprised nontronite and part of an interlayer hydroxide sheet. This mineral was inferred to be formed under chemically different conditions from F-M-S-H, and probably formed in the presence of aqueous Fe 3+ and Mg ions.

The Tagaung Taung deposit is the only Ni laterite deposit that is currently mined in Myanmar.Although Myanmar has a drier sub-tropical climate than countries that typically host Ni laterite deposits, the Tagaung Taung Ni laterite deposit produces highgrade Ni ores.Weathering products formed under the drier climatic conditions may play important roles in the enrichment of Ni in the saprolite layers.In this study, geochemical and mineralogical evolution of a weathering profile in the Tagaung Taung deposit was investigated to identify key factors controlling the Ni enrichment during chemical weathering of ultramafic rocks in Myanmar and possibly under similar climatic conditions.The whole-rock geochemical data indicate that Si was retained relative to Fe and Al in the weathering profiles [1], which shows a distinct chemical weathering trend from that observed in weathering profiles of Ni laterite deposits in Indonesia [2].The bulk NiO contents were as high as 4.89 wt.% in the saprolite layers.The XRD analysis indicates that smectite was abundant in the saprolite layers.The EPMA analysis suggest that smectite is an important host mineral in the saprolite layer.The results of sequential extraction modified for Ni laterite samples support the idea, showing that most Ni in the saprolite layers is present in the residual fraction (e.g., crystalline silicate phases), although some fraction (up to ~20%) of Ni was also likely hosted by low crystalline Fe oxides.Microscopic observations indicate that Ni-rich smectite (>10 wt.% NiO) formed as a replacement product of orthopyroxene.Chemical weathering of pyroxene under oxidized and Si-rich conditions may have caused the formation of the Ni-rich smectite.These results imply that high-grade Ni laterite deposits may develop on unaltered or partly serpentinized harzburgite under the climatic conditions typical of Myanmar.

Soil organic matters may inhibit the pozzolanic reaction, and thus influence the strength development of soil-employed construction materials. To understand their interaction, the effect of lignosulfonate, here used as model soil organic matter, on the pozzolanic reaction was investigated through batch experiments. Lignosulfonate inhibited the pozzolanic reaction, suppressing calcium silicate hydrate (C-S-H) formation. The suppression did not take place in a continuous way with the addition of lignosulfonate but was triggered at a certain dosage of lignosulfonate. We propose that the inhibition was primarily due to formation of Si-(Ca)-lignosulfonate complex. Such interaction may illustrate the inhibition of the pozzolanic reaction by organic matters in soils at alkaline activation. Below the threshold, lignosulfonate allowed C-S-H formation though modified its structure, which also suggested the possibility of soil organic matters to influence the strength development of construction materials in coexistence of C-S-H formation.

Silicate glasses are durable materials in our daily life, but corrosion rate accelerates under alkaline aqueous environment. Such situation has raised concerns, for example, in nuclear waste disposal where vitrified wastes encounter to alkaline leachate from surrounding concrete materials. Here we report volcanic glass example surviving with a hyperalkaline groundwater (pH > 11) and high flow rate for about 4000 years. The tiny glass fragments were extracted from the volcanic ash layer sandwiched between ultramafic sediments using microanalytical techniques. Sharp elemental distributions at the glass surface, where amorphous-like smectite precursors and crystalline smectites coexist, suggest the corrosion by an interface-coupled dissolution–precipitation mechanism rather than inter-diffusion. The corrosion rate was maintained at, the minimum, 2.5 orders of magnitude less than the rate observed for fresh glass, even in the presence of Fe and Mg that might have consumed Si through the silicate precipitation.

The radionuclide selenium-79 (Se-79) is predicted to be a key contributor to the long-term radiologic hazards associated with geological high-level waste (HLW) repositories; hence its release is of pertinent concern in the safety assessment of repositories. However, interactions of reduced Se species with aqueous Fe(II) species and solid phases arising from the corrosion of a steel overpack could play a role in mitigating its migration to the surrounding environment. In this study, we examined the immobilization mechanisms of Se(-II) during its interaction with aqueous Fe(II) and freshly precipitated Fe(OH)2 at circumneutral and alkaline conditions, respectively, its response to changes in pH, and its behavior during aging at 90 °C. Using microscopic and spectroscopic techniques, we observed β-FeSe precipitation, regardless of whether Se(-II) reacts with aqueous species or solid phases, and that modifying the pH following initial immobilization did not remobilize Se(-II). These observations indicate that Se(-II) migration beyond the overpack can be effectively and rapidly retarded via interactions with Fe(II) species arising from overpack corrosion. Thermodynamic calculations, however, showed that iron selenides became metastable at alkaline conditions and will dissolve in the long term. Aging experiments at 90 °C showed that Se(-II) can be completely retained via the crystallization of ferroselite at circumneutral conditions, while it will be largely remobilized at alkaline conditions. Our results show that Se(-II) mobility can be significantly influenced by its interactions with the corrosion products of the steel overpack and that these behaviors will have to be considered in repository safety assessments.

Myanmar has a drier sub-tropical climate than countries that typically contain Ni laterite deposits, but hosts a Ni laterite deposit at Tagaung Taung. Given that Ni enrichment processes in the Tagaung Taung deposit are poorly understood, we investigated the geochemical and mineralogical evolution of two weathering profiles developed on different bedrocks in the central part of Myanmar: a partly serpentinized harzburgite at Tagaung and an almost completely serpentinized peridotite at Budaung. The whole-rock geochemical data indicate that Si was retained relative to Fe and Al in the weathering profiles. Nickel has been enriched to contents as high as 4.89 wt.% NiO in the saprolite layers at Tagaung, whereas the saprolite layers at Budaung contain ≤ 1.55 wt.% NiO. Smectite is the main mineral that formed in the saprolite layers at Tagaung, whereas secondary serpentine dominates the saprolite layers at Budaung. Microscopic observations indicate that Ni-smectite (> 10 wt.% NiO), which is only observed at Tagaung, formed as a replacement product of orthopyroxene. In addition to the high Ni fixation capacity of smectite relative to secondary serpentine, Ni-rich pore water derived from the dissolution of olivine likely contributed to the high Ni contents of smectite. Our results imply that high-grade Ni laterite deposits may develop on unaltered or partly serpentinized harzburgite under the climatic conditions typical of Myanmar.

From an abandoned sulfur and limonite mine in northern Japan, acid mine drainage (AMD) rich in dissolved Fe, As, Pb, and Cd come to mix in the nearby rivers. Chemistry in two nearby rivers, hereafter referred to river A and river B, are affected by the acid mine drainage from two neighboring abandoned mines. However, river A is self-remediated, and the concentration of the above contaminants decreases to that below the environmental regulation standard at the monitoring point. In contrast, there is no self-remediation in the river B. If the geochemical process of natural remediation in river A is well understood, the lesson learned at river A would be quite valuable for geochemical passive-treatment in river B. Thus, this study aims to understand the natural remediation and its implication of river A by geochemical modeling. The acid mine drainage contains high ferric iron concentration from the source with pH 2.6 and toxic elements such as As, Pb, and Cd. After the AMD mixes with the rivers, river A's pH dropped from 6.9 to 3.1 while river B’s pH decreased from 7.1 to 2.9. In river A, schwertmannite formation was observed just after mixing with wastewater. On the other hand, schwertmannite formed in river B at the midstream after the

Magnesium silicate hydrate (M-S-H) is a low-crystalline phase formed at low temperature (< 100 ºC) in Mg-SiO 2 -H 2 O systems. In previous studies related to cement chemistry, M-S-H formation was investigated in a closed system (e.g. batch experiments) under conditions of low water/solid ratio and high solution saturation states with respect to M-S-H [1] . However, there are no experimental studies for the formation of this compound in open systems (e.g. flow-through experiments) that mimic the geochemical reactions under Earth surface conditions. This work aims to study the formation of M-S-H in an aqueous open system. To this end, MgO powder reacted at pH 10 and 50 ºC in two flow-through experiments, in which two solutions with different concentration of dissolved silica (0.15 mM and 1.5 mM) were injected. The aqueous chemistry of the outflow was monitored over time and the reacted solids were analyzed by XRD, SEM, TEM and 29 Si MAS NMR. The steady-state output solutions were undersaturated with respect to Mg-hydroxide and supersaturated with respect to M-S-H. XRD patterns of the reacted solids did not reveal the presence of M-S-H. However, SEM images showed secondary silicate phases on the surface of the MgO particles. TEM and 29 Si MAS NMR analyses of the newly formed phases exhibited crystallographic characteristics similar to those of M-S-H

Sludge effluents and solid deposits generated from the conventional lime treatment processes on the Zambian Copperbelt have led to reports of copper (Cu) and cobalt (Co) contamination into the nearby water bodies. To better understand the behaviour of the metals; partitioning, adsorption and their specific binding forms were studied through sequential extraction, batch adsorption experiments and surface complexation modeling (SCM). Results of mineral composition analyses indicated that micas, kaolinite, quartz and feldspar are abundant with hydrous ferric oxide (HFO) precipitates that formed as a result of the weathering of biotite grains existing as grain surface coating. Sequential extractionrevealed that Cu and Co metals are partitioned in the order of: exchangeable (F1: 600-1500 mg/kg Cu; 100-200 mg/kg Co), acid-soluble (F2: 2200-5500 mg/kg Cu; 190-220 mg/kg Co) and reducible fraction (F3: 2200-5500 mg/kg Cu; 260-300 mg/kg Co). Metals in F1 are hosted by kaolinite, F2 by both kaolinite and HFO whereas in F3 by dominantly HFO. Equal Cu concentration between F2 and F3 is due to both the limited amount of HFO (i.e. 5-10 g/kg) and desorption of loosely adsorbed Cu and Co metals to HFO surfaces. Batch adsorption experiments revealed adsorption as the dominant metal retention mechanism. According to modeling predictions, HFO sites are the dominant metal adsorption sites. At HFO site; >(s)FeOCo+, Co showed adsorption decrease from 40% in single system to 25% in binary system between pH 7 - 7.5 due to metal competition for adsorption sites. The high Cu concentration (i.e. 0.5-1.1% Cu) displaced low Co (i.e. 0.03-0.07% Co) concentration from the adsorption sites present in sludge, thus rendering Co mobile into the environment. To keep the adsorbed metals stable from release, optimal pH of 7.5 is suggested during treatment with lime. At this optimal pH, metals are decreased to below the regulation standard values and with less generation of voluminous sludge. Adsorbed Cu and Co can be recoverable from sludge through acid treatment at pH <3 based on sequential extraction results. The resultant metal-free sludge material has potential of been used as aggregate in construction. (C) 2020 The Authors. Published by Elsevier B.V.

In radioactive waste disposal facilities, low-permeability engineered barrier materials are important for inhibiting radionuclide migration. However, dissolution–precipitation reactions under alkaline conditions change the permeability of engineered barriers. To understand long-term dissolution–precipitation reactions under alkaline conditions in chemically complex systems, trenches and drill holes were excavated at Narra in Palawan, where alkaline fluids (pH > 11) have been naturally produced, seeping into clastic sediments derived from serpentinized ultramafic rocks and gabbro of Palawan ophiolite. Interaction between the alkaline seepage and clastic sediments, which have been deposited since 15,000 radiocarbon years before present (14C yr BP), led to dissolution of minerals and the precipitation of Si-bearing phases which were divided into two main categories: Fe-Mg-Si infillings and Ca-Si infillings. The former category was composed of iron-magnesium-silicate-hydrate (F-M-S-H) and a nontronite-like mineral and was widely recognized in the clastic sediments. The nontronite-like mineral likely formed by interaction between silicates and alkaline seepage mixed with infiltrated seawater, whereas F-M-S-H formed by the reaction of silicates with alkaline seepage in the absence of seawater infiltration. Ca-Si infillings included 14 Å tobermorite and were precipitated from alkaline seepage combined with the Ca and Si supplied by the dissolution of calcite and silicates in the clastic sediments.