Rafael M. Santos

Strategic selection of an optimal sorbent mixture for in-situ remediation of heavy metal contaminated sediments: Framework and case study

Aquatic sediments contaminated with heavy metals originating from mining and metallurgical activities pose significant risk to the environment and human health. These sediments not only act as a sink for heavy metals, but can also constitute a secondary source of heavy metal contamination. A variety of sorbent materials has demonstrated the potential to immobilize heavy metals. However, the complexity of multi-element contamination makes choosing the appropriate sorbent mixture and application dosage highly challenging. In this paper, a strategic framework is designed to systematically address the development of an in-situ sediment remediation solution through Assessment, Feasibility and Performance studies. The decision making tools and the experimental procedures needed to identify optimum sorbent mixtures are detailed. Particular emphasis is given to the utilization and combination of commercially available and waste-derived sorbents to enhance the sustainability of the solution. A specific case study for a contaminated sediment site in Northern Belgium with high levels of As, Cd, Pb and Zn originating from historical non-ferrous smelting is presented. The proposed framework is utilized to achieve the required remediation targets and to meet the imposed regulations on material application in natural environments.

Recent developments and perspectives on the treatment of industrial wastes by mineral carbonation — a review

Besides producing a substantial portion of anthropogenic CO2 emissions, the industrial sector also generates significant quantities of solid residues. Mineral carbonation of alkaline wastes enables the combination of these two by-products, increasing the sustainability of industrial activities. On top of sequestering CO2 in geochemically stable form, mineral carbonation of waste materials also brings benefits such as stabilization of leaching, basicity and structural integrity, enabling further valorization of the residues, either via reduced waste treatment or landfilling costs, or via the production of marketable products. This paper reviews the current state-of-the-art of this technology and the latest developments in this field. Focus is given to the beneficial effects of mineral carbonation when applied to metallurgical slags, incineration ashes, mining tailings, asbestos containing materials, red mud, and oil shale processing residues. Efforts to intensify the carbonation reaction rate and improve the mineral conversion via process intensification routes, such as the application of ultrasound, hot-stage processing and integrated reactor technologies, are described. Valorization opportunities closest to making the transition from laboratory research to commercial reality, particularly in the form of shaped construction materials and precipitated calcium carbonate, are highlighted. Lastly, the context of mineral carbonation among the range of CCS options is discussed.

Susceptibility of mineral phases of steel slags towards carbonation: mineralogical, morphological and chemical assessment

Process limitations have thus far prevented mineral carbonation of alkaline wastes from been widely applied. These barriers are caused by inefficient processing, but also by mineralogical aspects inherent to the materials. Better understanding and predictability of the effects of mineral carbonation on alkaline materials could be obtained by studying the carbonation susceptibility of constituent minerals separately, allowing for detailed and accurate analysis of their reaction kinetics and maximal conversions and of the carbonate products formed. For this purpose, this paper presents the synthesis and carbonation of the seven most abundant alkaline minerals found in AOD, CC and BOF slags, namely: akermanite (Ca 2 MgSi 2 O 7 ), bredigite (Ca 7 Mg(SiO 4 ) 4 ), cuspidine (Ca 4 Si 2 O 7 F 2 ), β– and γ–C 2 S (Ca 2 SiO 4 ), merwinite (Ca 3 Mg(SiO 4 ) 2 ), and srebrodolskite (Ca 2 Fe 2 O 5 ). Two experimental approaches to mineral carbonation of increasing levels of process severity are utilized: (mild) incubator carbonation, and (accelerated) pressurized slurry carbonation. In addition, the slags and two free oxides (CaO and MgO) are equally carbonated and evaluated. Data regarding CO 2 uptake, mineral conversion and formed carbonate and non-carbonate products in the samples were obtained through TGA, QXRD (Rietveld refinement) and SEM techniques. Reduction in material basicity and evolution of particle morphology were also assessed. The synthesized mineral purities (>70 wt% target mineral phase) were found sufficient for more accurate assessment of carbonation behaviour of the individual minerals. Bredigite was found to be the most reactive mineral under all processing conditions; C 2 S and wollastonite were more reactive under slurry carbonation, while srebrodolskite and calcium monoferrite were found to be more reactive under moist carbonation. Merwinite and diopside had the slowest carbonation conversions. Calcite and aragonite were the dominant carbonate products formed, whereby aragonite formation was promoted in Mg-containing materials. The morphology of aragonite crystals and the packing density of its product layer were found to vary depending on the parent mineral. Characteristic slag carbonation products, not observed as extensively from synthetic mineral samples, were magnesian calcite from slurry carbonation, and monohydrocalcite and vaterite from moist carbonation. Wollastonite was the main crystalline non-carbonate product, occurring predominantly from slag carbonation, while silica-rich amorphous matter formed in all samples proportionally to CO 2 uptake. Free lime, when present, controlled material basicity above pH 12, while silicates were found to typically possess pH in the range of 11.3–11.9, and Ca-carbonates eventually controlled the pH of well carbonated samples to values under 10.

Atom-Efficient Route for Converting Incineration Ashes into Heavy Metal Sorbents

Bottom ashes produced from municipal solid-waste incineration are suitable for sorbent synthesis because of their inherent composition, high alkalinity, metastable mineralogy, and residual heat. This work shows that bottom ashes can be atom-efficiently converted into valuable sorbents without the need for costly and hazardous chemicals. The ashes were hydrothermally treated in rotary autoclaves at autogenic pH conditions to promote the conversion of precursor mineral phases into zeolites and layered silicate hydrates. Two main mineral phases were formed: katoite and sodium aluminum phosphate silicate hydrate. These mineral alterations are accompanied by a tenfold increase in specific surface area and a twofold reduction in average particle size. Performance evaluation of the new sorbents for Cd(2+), Zn(2+), and Pb(2+) adsorption at pH5 indicates sorption capacities of 0.06, 0.08, and 0.22 mmol g(-1), respectively, which are similar to those of natural adsorbents and synthetic materials obtained from more demanding synthesis conditions.

Impacts of Nickel Nanoparticles on Mineral Carbonation

This work presents experimental results regarding the use of pure nickel nanoparticles (NiNP) as a mineral carbonation additive. The aim was to confirm if the catalytic effect of NiNP, which has been reported to increase the dissolution of CO2 and the dissociation of carbonic acid in water, is capable of accelerating mineral carbonation processes. The impacts of NiNP on the CO2 mineralization by four alkaline materials (pure CaO and MgO, and AOD and CC steelmaking slags), on the product mineralogy, on the particle size distribution, and on the morphology of resulting materials were investigated. NiNP-containing solution was found to reach more acidic pH values upon CO2 bubbling, confirming a higher quantity of bicarbonate ions. This effect resulted in acceleration of mineral carbonation in the first fifteen minutes of reaction time when NiNP was present. After this initial stage, however, no benefit of NiNP addition was seen, resulting in very similar carbonation extents after one hour of reaction time. It was also found that increasing solids content decreased the benefit of NiNP, even in the early stages. These results suggest that NiNP has little contribution to mineral carbonation processes when the dissolution of alkaline earth metals is rate limiting.

DEVELOPMENTS ON WETTING EFFECTS IN MICROFLUIDIC SLUG FLOW

Wetting effects form a dimension of fluid dynamics that becomes predominant, precisely controllable, and possibly useful at the micro-scale. Microfluidic multiphase flow patterns, including size, shape, and velocity of fluidic particles, and mass and heat transfer rates are affected by wetting properties of microchannel walls and surface tension forces between fluid phases. The novelty of this field, coupled to difficulties in experimental design and measurements, means that literature results are scarce and scientific understanding is incomplete. Numerical methods developed recently have enabled a shortcut in obtaining results that can be perceived as realistic and that offer insight otherwise not possible. In this work the effect of the contact angle on gas-liquid two-phase flow slug formation in a microchannel T-junction was studied by numerical simulation. The contact angle, varied from 0 to 140 degrees, influenced the interaction of the gas and liquid phases with the channel wall, affecting the shape, size, and velocity of the slugs. The visualisation of the cross-sectional area of gas slugs allowed insight into the existence of liquid flow along rectangular microchannel corners, which was affected by the contact angle and determined the occurrence of velocity slip. The velocity profile within the gas slugs was also found to change as a function of contact angle, with hydrophilic channels inducing greater internal circulation, compared to greater channel wall contact in the case of hydrophobic channels. These effects play a role in heat and mass transfer from channel walls and highlight the value of numeral simulation in microfluidic design. Supplementary materials are available for this article. Go to the publishers online edition of Journal of Chemical Engineering Communications to view the supplemental file.

CO2 Energy Reactor – Integrated Mineral Carbonation: Perspectives on Lab-Scale Investigation and Products Valorization

To overcome the challenges of mineral CO2 sequestration, Innovation Concepts B.V. is developing a unique proprietary Gravity Pressure Vessel (GPV) reactor technology, and has focussed on generating reaction products of high economic value. The GPV provides intense process conditions through hydrostatic pressurization and heat exchange integration that harvests exothermic reaction energy, thereby reducing energy demand of conventional reactor designs, in addition to offering other benefits. In this paper, a perspective on the status of this technology and outlook for the future is provided. To date, laboratory-scale tests of the envisioned process have been performed in a tubular “rocking autoclave” reactor. The mineral of choice has been olivine (~Mg1.6Fe2+0.4(SiO4) + ppm Ni/Cr), although asbestos, steel slags and oil shale residues are also under investigation. The effect of several process parameters on reaction extent and product properties have been tested: CO2 pressure, temperature, residence time, additives (buffers, lixiviants, chelators, oxidizers), solids loading, and mixing rate. The products (carbonates, amorphous silica and chromite) have been physically separated (based on size, density and magnetic properties), characterized (for chemistry, mineralogy and morphology) and tested in intended applications (as pozzolanic carbon-negative building material). Economically, it is found that product value is the main driver for mineral carbonation, rather than, or in addition to, the sequestered CO2. The approach of using a GPV and focusing on valuable reaction products could thus make CO2 mineralization a feasible and sustainable industrial process.

Gas-Liquid Slug Formation at a Rectangular Microchannel T-Junction: A CFD Benchmark Case

Computational fluid dynamics (CFD) is an important tool for development of microfluidic systems based on gasliquid two-phase flow. The formation of Taylor slugs at microchannel T-junctions has been studied both experimentally and numerically, however discrepancies still exist because of difficulties in correctly representing experimental conditions and uncertainties in the physics controlling slug flow, such as contact line and velocity slip. In this paper detailed methods and results are described for the study of Santos and Kawaji [1] on the comparison of experimental results and numerical modeling. The system studied consisted of a rectangular microchannel Tjunction nominally 100 μm in hydraulic diameter, used to generate Taylor slugs from air-water perpendicular flow. The effect of flow rates on parameters such as slug length, velocity slip, void fraction and two-phase frictional pressure drop were studied. Numerical simulation was performed using FLUENT volume-of-fluid (VOF) model. It is proposed in this paper that this microfluidic problem be taken up by researchers in the field as a benchmark case to test other numeric codes in comparison to FLUENT on the prediction of micro-scale multiphase flow, and also to model in more detail the experimental system described to obtain greater accuracy in prediction of microfluidic slug formation.

Two-way Valorization of Blast Furnace Slag: Synthesis of Precipitated Calcium Carbonate and Zeolitic Heavy Metal Adsorbent

The aim of this work is to present a zero-waste process for storing CO2 in a stable and benign mineral form while producing zeolitic minerals with sufficient heavy metal adsorption capacity. To this end, blast furnace slag, a residue from iron-making, is utilized as the starting material. Calcium is selectively extracted from the slag by leaching with acetic acid (2 M CH3COOH) as the extraction agent. The filtered leachate is subsequently physico-chemically purified and then carbonated to form precipitated calcium carbonate (PCC) of high purity (<2 wt% non-calcium impurities, according to ICP-MS analysis). Sodium hydroxide is added to neutralize the regenerated acetate. The morphological properties of the resulting calcitic PCC are tuned for its potential application as a filler in papermaking. In parallel, the residual solids from the extraction stage are subjected to hydrothermal conversion in a caustic solution (2 M NaOH) that leads to the predominant formation of a particular zeolitic mineral phase (detected by XRD), namely analcime (NaAlSi2O6∙H2O). Based on its ability to adsorb Ni2+, as reported from batch adsorption experiments and ICP-OES analysis, this product can potentially be used in wastewater treatment or for environmental remediation applications.

Improving the Yield of Sonochemical Precipitated Aragonite Synthesis by Scaling up Intensified Conditions

Precipitated aragonite can be synthesized at relatively low temperatures by combining the application of low-frequency sonication with the use of magnesium chloride additive, as demonstrated by our prior study. In the present study, new process conditions were found that promote aragonite formation while accelerating and increasing the reaction yield. It was found that Mg-to-Ca molar ratio of 3:1, together with higher slurry concentration (74 g/L Ca(OH)2) and higher power-to-volume ratio (800 W/L gross, achieved by reducing slurry volume), promoted the aragonite formation while working at a higher CO2 flow rate (2.0 NL/min), and consequently higher precipitated calcium carbonate production rate (1 g/(L · min) CaCO3). The yield was thus improved while conserving the desired product properties as follows: high polymorph purity (95.7 wt%), small and narrow particle size distribution (D[3,2] = 0.74 µm), and unique shape (hubbard squash-like).