Microwave-assisted Acid Treatment Research Articles

Reversed Mg-Based Smectites: A New Approach for CO2 Adsorption

Addressing climate change requires transitioning to cleaner energy sources and adopting advanced CO2 capture techniques. Clay minerals are effective in CO2 adsorption due to their regenerative properties. Recent advancements in nanotechnology further improve their efficiency and potential for use in carbon capture and storage. This study examines the CO2 adsorption properties of montmorillonite and saponite, which are subjected to a novel microwave-assisted acid treatment to enhance their adsorption capacity. While montmorillonite shows minimal changes, saponite undergoes significant alterations. Furthermore, the addition of silica pillars to smectites results in a new nanomaterial with a higher surface area (653 m2 g−1), denoted as reversed smectite, with enhanced CO2 adsorption capabilities, potentially useful for electrochemical devices for converting captured CO2 into value-added products.

Microwave-assisted acid treatment for the mineral transformation of chrysotile as an alternative for asbestos waste management

In this work, the effect of microwave-assisted acid treatments on the morphological and crystallochemical characteristics of chrysotile fibers is investigated. A low concentration of nitric acid (0.2 N) is used to remove Mg2+-species located in the octahedral sheet of its structure, thereby causing a crystallo-chemical change forming a skeleton of non-crystalline amorphous silica. This skeleton maintains an elongated morphology but characterized by rounded -not sharp-edges and porous surfaces whose physical resistance under stress is reduced when compared with the initial fibers of chrysotile, favoring a lower pathogenicity of the fibers. Thus, microwave-assisted acid treatment rise as a low-cost, fast and effective option in avoiding the dangerousness associated with asbestos waste management.

Sources and distribution of trace elements in surface snow from coastal Zhongshan Station to Dome A (East Antarctica)

The spatial distribution and possible sources of Al, As, Cd, Fe, Mn, Pb, Sb, Ti, V, and Zn were investigated in 120 samples of surface snow collected along a ∼1260 km long traverse from coastal Zhongshan Station to the Antarctic ice sheet summit (Dome A or Dome Argus, at 4093 m a.s.l.). The chemical composition of snow samples was analyzed using microwave-assisted acid treatment with HNO3/HF to determine element concentrations, reflecting their abundance in the Earth's crust. The observed relative concentration pattern of elements in snow suggests a predominant contribution from soil and rock dust particles. The spatial distribution of total element concentrations along the traverse revealed higher values in inland areas compared to coastal areas. The annual deposition flux of elements in each sampling site indicates a significant decreasing trend in Al, Ti, Zn, Fe, Cd, As, and Pb from the coast to inland areas. Significant variations in element concentrations and fluxes observed along the Antarctic traverse suggest a notable influence of distance from the coast on snow deposition patterns. Soil dust from coastal ice-free areas and human activities in scientific stations are the primary sources of analyzed elements in coastal snow, with marine biogenic emissions playing a marginal role, likely contributing to As and Cd. The contribution of sea salt spray to surface snow element concentrations in the East Antarctic traverse is negligible. An analysis of backward trajectories confirmed that long-distance transport from the South American continent and local Antarctic research stations potentially contributes to Antarctic snow elements.

Modification of the textural properties of palygorskite through microwave assisted acid treatment. Influence of the octahedral sheet composition

Five palygorskite samples from different geological environments including lacustrine and perimarine deposits and hydrothermal fillings were treated by a microwaveassisted acid leaching process inaqueous HNO3 solution (0.2 M). In all cases, the samples have been characterized by X-ray fluorescence (XRF), X-ray diffraction (FTIR), Fourier transform infrared spectroscopy (FTIR), N2 adsorption-desorption at −196 °C and Scanning electron microscope (SEM). The mineralogical study indicates that palygorskite is the predominant mineral although the presence of other associated minerals has been observed including: quartz, calcite, dolomite, feldspars, smectite, illite-mica and traces of chlorite and sepiolite. After the acid treatment the main mineralogical change is the disappearance of the carbonates, not being observed in any case the occurrence of amorphous silica. Palygorskite remains virtually unchanged from acid attack in four of the samples studied. Only in the sample with the highest Mg(II) content (Mace), mineralogical and chemical data evidence the partial solution of the octahedral layer in the fibrous clay mineral due to the release of magnesium. The textural study indicates significant differences in the specific surface area of the samples ranging from 75 to 225 m2 g−1, this variation being related mostly to the purity of the samples. Under similar experimental conditions, palygorskite is more resistant to acid attack than magnesian clay minerals (saponite or sepiolite). In addition, a relationship has been observed between the magnesium content of palygorskite and the susceptibility to microwave-assisted acid attack. In all cases, an increase in the specific surface area value has been observed, reaching a highest SBET value of 354 m2 g−1 for Mace sample after 16 min of microwave-assisted acid treatment as a consequence of the partial leaching of its octahedral sheet, although its fibrous structure is maintained yet.

Microwave assisted acid treatment of kerolitic clays from the Neogene Madrid Basin (Spain) and its use in CO2 capture processes

Abstract The effect of the microwave assisted acid treatment on five kerolite-Mg-stevensite samples from the Neogene Madrid Basin was studied. Samples with lower amount of kerolite are more prone to suffer chemical modifications thought the microwave assisted acid treatment than the kerolite-rich clay ones by the partial leaching of Mg2+-species located in the octahedral sheet. This fact supposes that two structures with similar chemical compositions (kerolite and stevensite) display a different response to the microwave assisted acid treatment. Thus, Mg-smectite with interlayer cations, such as stevensite, are more labile to the acid treatment than kerolite, which does not have interlayer cations in such a way that acid digestion of stevensite occurs simultaneously at the edges of particles and within the interlayer space, whereas in the case of kerolite, it occurs exclusively at the particle edges. The acid treatment implies an increase of the specific surface area in all cases, reaching a highest SBET value of 435 m2/g. In the case of samples with higher stevensite content, the increase in the SBET is mainly due to increases of the microporosity, whereas in the sample with higher kerolite content the increase in the SBET is mainly ascribed to increase of the external surface area of the particles. Then, these materials were evaluated in CO2 capture processes reaching a maximum value of 1.98 mmol/g at 65 °C and 760 mm Hg.

Microwave-Assisted Acid Activation of Clays Composed of 2:1 Clay Minerals: A Comparative Study

The effect of the microwave-assisted acid treatment (MAT) on the structure and texture of dioctahedral (montmorillonite) and trioctahedral (saponite) smectites, kerolitic clays and sepiolites were studied by scanning electron microscopy, nitrogen adsorption, X-ray fluorescence, X-ray diffraction and Fourier transform infrared spectroscopy. This study shows that in the smectite group of minerals the effectiveness of MAT is notably influenced by the chemical composition of the octahedral sheet, whereas in kerolitic clays it is influenced by the proportion of the expandable phase in the mixed layer sequence. On the other hand, the reactivity of sepiolites depends on the width of the natural fibres. With these treatments, extremely high specific surface area (SSA) increments are achieved in just 16 min. The SSA increments reach values up to 231 m2/g (%ΔSBET = 80) in sepiolites, 198 m2/g (%ΔSBET = 155) in Mg-smectites, and 161 m2/g (%ΔSBET = 61) in kerolitic clays.

Influence of the Structure and Experimental Surfaces Modifications of 2:1 Clay Minerals on the Adsorption Properties of Methylene Blue

In this work the adsorption capacities of methylene blue on Mg-smectite and sepiolite and its derived material obtained after acid treatment and/or the addition of Fe at its surfaces are studied. Natural smectite with high Mg-content in its octahedral sheet (Mg-smectite) displays higher adsorption capacity than the sepiolite due to the ability of Mg-smectite to expand the basal spacing to accommodate methylene blue cations between adjacent layers and the inability of sepiolite to adsorb these cations within the structural channels. The acid-activation of Mg-smectite causes a clear decrease in the adsorption capacity attributed to the partial loss of the interlayer positions by the loss of the octahedral sheet and subsequent formation of amorphous silica. Moreover, the adsorption of the Mg-smectite decreases even more when iron oxohydroxides species are incorporated in its interlayer spacing making inaccessible the interlayer active sites for the adsorption of methylene blue cations. On the contrary, the microwave-assisted acid treatment of sepiolite causes a slight increase in the adsorption capacity related with the dispersion of fibers. Nevertheless, contrary to that observed with Mg-smectite, higher increasing of adsorption capacities are obtained after the formation of new adsorption centers due to the incorporation of iron oxohydroxides species at the external surfaces of sepiolite.

Evaluation of two fibrous clay minerals (sepiolite and palygorskite) for CO2 Capture

Two fibrous clay minerals (sepiolite and palygorskite) have been employed as adsorbents to assess their adsorption capacity of pure CO2 in a volumetric setup. The raw sepiolite reached a CO2 uptake of 1.48 mmol g−1 due to the presence of nanocavities acting as a molecular sieve. Both sepiolite and palygorskite were modified by microwave-assisted acid treatment, which led to an increase in specific surface area and pore volume due to Mg2+ leaching, particularly in the case of sepiolite. However, the partial digestion of these fibrous structures does not improve CO2 adsorption uptake due to the progressive increase of the size of nanocavities. In a next step, both fibrous clay minerals were functionalized with amine species by different procedures (grafting with (3-Aminopropyl) triethoxysilane (APTES), impregnation with polyethyleneimine (PEI) and double functionalization by grafting with APTES and then impregnation with PEI. In all cases, it can be observed that the incorporation of amine species favors the chemical interaction between the amines species and the CO2 molecules, although it also produces obstruction of the nanochannels so the adsorption takes place mainly on the outer surface of the fibers. Finally, the incorporation of amine species by double functionalization led to the highest CO2 adsorbed concentration of 2.07 mmol g−1 at 760 mm Hg and 65 °C due to a larger proportion of available amines sites as well as the use of higher adsorption temperature, which favored the diffusion of CO2 molecules within the adsorbent.

Functionalization of multiwalled carbon nanotubes by microwave irradiation for lysozyme attachment: comparison of covalent and adsorption methods by kinetics of thermal inactivation

Proteins suffer changes in their tertiary structure when they are immobilized, and enzymatic activity is affected due to the low biocompatibility of some supporting materials. In this work immobilization of lysozyme on carbon nanotubes previously functionalized by microwave irradiation was studied. The effectiveness of the microwave-assisted acid treatment of carbon nanotubes was evaluated by XPS, TEM, Raman and FTIR spectroscopy. The carboxylic modification of nanotube surfaces by this fast, simple and feasible method allowed the physical adsorption and covalent linking of active lysozyme onto the carbonaceous material. Thermal inactivation kinetics, thermodynamic parameters and storage stability were studied for adsorbed and covalent enzyme complexes. A major stability was found for lysozyme immobilized by the covalent method, the activation energy for inactivation of the enzyme was higher for the covalent method and it was stable after 50 d of storage at 4 °C. The current study highlights the effect of protein immobilization method on the biotechnological potential of nanostructured biocatalysts.

Physicochemical treatment for improving bioconversion of cassava industrial residues

In the present study, a comparison of two types of microwave pretreatment such as microwave assisted alkali and microwave assisted acid were studied using cassava pulp as the substrate. The effect of pretreatment agent concentration, microwave power and pretreatment time on enzymatic saccharification was studied using a response surface method according to Central Composite design. Under optimized conditions, microwave assisted alkali pretreatment of cassava pulp with 2.27% NaOH at 125 W for 18 min followed by enzymatic hydrolysis gave reducing sugar yield of 0.472 g g−1 dry biomass, while microwave assisted acid treatment with 1.95% H2SO4 at 135 W for 25 min showed reducing sugar yield of 0.306 g g−1 dry biomass. The yield from microwave assisted alkali was higher and was 2.7 times higher than that obtained with raw cassava pulp. Microwave assisted liquefaction of pretreated cassava pulp at 100 W for 30 min using amylase lowered the process time. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 577–583, 2018

Purification and defibering of a Chinese sepiolite

To extend the utilization of the low-grade Human (China) sepiolite, the purification of the ore is the prerequisite for all applications. Sepiolite accompanied by quartz and calcite was effectively purified via sedimentation method and the microwave assisted acid treatment. The influence of treatment time on sepiolite purity and structure was qualitatively studied based on XRD and FESEM. Sepiolite had been successfully purified and the content was elevated from 42wt.% to more than 90wt.%. Microwave assisted acid treatment demonstrated to be an effective purification process by removing carbonates without affecting sepiolite length. Furthermore, both physical method of lyophilization and chemical method of surface modification were proposed as simple alternatives to defibering acid-treated sepiolite crystal bundles. The effects of freezing time and surfactant chain length on the disaggregation state, micro-structure and properties of treated sepiolite were analyzed comparatively using FESEM, TEM and BET. Results of FESEM and TEM indicated that the crystal structure of sepiolite was not changed after freezing at −50°C for 12h or modification with hexadecyl trimethyl ammonium bromide (HDTMA), but the aggregates were effectively disaggregated. The BET specific surface areas of the dispersed sepiolite rods obtained by these two methods increased to 144.3 and 93.1m2/g, respectively, showing good agreement with the perfect dispersion of sepiolite fibers.

The saccharification of destarched wheat bran with microwave-assisted acid treatment

ABSTRACTDestarched wheat bran was hydrolyzed for saccharification using a microwave-assisted acid treatment. The result of the wheat bran with treatment obtained the maximum reducing sugar content of 13.2 g/L with the glucose content of 1.5 g/L, xylose content of 4.8 g/L, and arabinose content of 3.2 g/L under the optimized conditions with microwave power of 400 w, 30 min, and 2% concentration of H2SO4. The scanning electron microscope and infrared spectroscopy showed that the wheat bran could be greatly degraded using the treatment of microwave-assisted acid.

Effectiveness of microwave assisted acid treatment on dioctahedral and trioctahedral smectites. The influence of octahedral composition

The effect of the microwave assisted acid treatment (MAT) on the structure and texture of dioctahedral (montmorillonite and nontronite) and trioctahedral (saponite and stevensite) smectites were studied by SEM, nitrogen adsorption, XRF, XRD and FTIR. The effectiveness of this treatment is notably influenced by the chemical composition of the octahedral sheet. Thus, trioctahedral smectites seem to be very reactive to the microwave assisted acid treatment. In saponite and stevensite this treatment caused in the first 20min the progressive Mg2+ depletion of the octahedral sheet, the gradual destruction of the smectite structure and the formation of an amorphous silica phase which contributes to a noticeable increase of the BET specific surface area. After 20min of microwave assisted acid treatment the increase in SSA is of 315m2/g in saponite and 244m2/g in stevensite. The assistance of microwaves during the acid treatment allows us to get, in 20min, specific surface areas equivalent to those obtained after 48h with conventional heating methods. On the contrary, dioctahedral smectites seems to be less reactive to the microwave assisted acid treatment.

Microwave assisted acid treatment of sepiolite: The role of composition and “crystallinity”

Three sepiolites with differences in composition and structural ordering were treated with 0.2N HCl and 0.2N HNO3 solutions assisted with microwave radiation. This treatment caused in the firsts 16min the progressive Mg2+ depletion of the octahedral sheet, the gradual degradation of the sepiolite structure and the formation of an amorphous silica phase which contributes to a noticeable increase of the BET specific surface area. The assistance of microwaves during the acid treatment allows us to get specific surface areas equivalent to those obtained after 48h with conventional heating methods. The influence of mineralogical impurities, “crystallinity” and chemical composition on the reactivity of sepiolite were also studied.

Improvement of interaction in and properties of PMMA-MWNT nanocomposites through microwave assisted acid treatment of MWNT

Soluble derivatives of multi-walled carbon nanotubes (MWNT) embedded in a poly(methylmethacrylate) (PMMA) matrix forming thick, homogeneous and transparent nanocomposites, were prepared and characterized. A new photo-assisted method using microwaves, to purify the MWNTs from amorphous carbon and synthesis catalyst clusters, was tested in a sulphonitric mix. This method shortened the processing time compared to other methods. Pristine and functionalized MWNTs were introduced into the MMA, then in situ photo-polymerized. Transmission electron microscopy (TEM) and X-ray diffractometry (XRD), as well as Fourier-transform infrared (FTIR), Raman and nuclear magnetic resonance (NMR) spectroscopy were used to monitor the effects of the treatment on the different components. The thermal properties of the composites were determined through differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The short, microwave treated and polyethylene glycol (PEG) functionalized MWNTs showed the best dispersion in and interaction with PMMA, and had the most significant influence on the thermal properties of this polymer.

Synthesis and purification of single-walled carbon nanotubes in the cottonlike soot

The cottonlike soot with single-walled carbon nanotubes (SWNTs) is synthesized in large quantity under an improved arc discharge condition. It is found that the soot obtained under this condition contains much fewer multi-shell graphite particles compared to the clothlike soot produced earlier by our group and is much easier to be purified. High-purity SWNTs were obtained by combining oxidation in air and microwave-assisted acid treatment without multi-step filtration. SWNT purity was evaluated by SEM, TEM, thermogravimetric analysis, and Raman spectroscopy. Furthermore, the formation mechanism of the cottonlike soot is also discussed.