Silica powder has been extracted from rice husk employing a simple extraction procedure for preparing poly(lactic acid) bio-composites with differing filler contents (namely, 5, 10, 20 and 30 wt%). These materials have been obtained through a semi-industrial process of extrusion. The thermal, mechanical and morphological properties have been investigated. Despite a broad distribution of silica particle size, the prepared bio-composites have exhibited improved storage modulus and reduced oxygen permeabilities. The collected values at 10 and 30 wt% have been compared with those of homologous samples prepared using a commercial silica. Two micro-mechanical models (Voigt and Halpin–Tsai) have been used to also fit the mechanical data. Finally, an economic analysis of material and energetic costs has been performed on samples containing silica from rice husk in order to establish if the proposed process is sustainable or advantageous.

Rice husk as bio-source of silica: preparation and characterization of silica bio-composites

Abstract In the present research, magnetic rhamnolipid-Co/Al layered double hydroxide (MR-LDH) was synthesized to uptake methylene blue (MB) and reactive orange 16 (RO16) from aqueous solution. The main parameters, including pH, adsorbent dosage, contact time, and initial analyte concentration, were optimized to achieve the best adsorption efficiency. Accordingly, the elimination of MB on MR-LDH is improved in the basic medium due to the electrostatic interactions between the negative charge of MR-LDH and the positive charge of MB dye. In contrast, the acidic medium (pH = 3) was favored for RO16 adsorption because of hydrogen bonding between the protonated form of azo dye and protonated hydroxyl groups at the surface of MR-LDH. The calculated maximum adsorption capacities for MB and RO16 were 54.01 and 53.04 mg/g at 313 K, respectively. The Langmuir model, which assumes monolayer adsorption on the adsorbent surface, provides the best explanation for the adsorption of both dyes (R 2 = 0.9991 for MB and R 2 = 0.9969 for RO16). Moreover, the pseudo-second-order kinetic model best described the adsorption process for MB (R 2 = 0.9970) and RO16 (R 2 = 0.9941). The proposed adsorbent maintains stable adsorption performance for four consecutive cycles. After each adsorption process, MR-LDH is easily separated by an external magnet. The findings show that MR-LDH was found to be an excellent adsorbent for the removal of both cationic and anionic organic dyes from aqueous solutions. 

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Adsorption behavior of rhamnolipid modified magnetic Co/Al layered double hydroxide for the removal of cationic and anionic dyes

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In recent years, the amount of agricultural and industrial wastes being generated is increasing rapidly. This leads to numerous disposals, governance, and environmental issues. Therefore, the re-utilisation of these wastes into value-added products such as mesoporous silica nanoparticles (MSNs) has attracted great attention. MSNs have garnered immense attention in various applications owing to their tuneable pore dimensions, high surface areas, and tailorable structure, suitable for different post-functionalisation. In this review, recent progress on the synthesis of MSNs from waste using a chemical synthesis route is presented. This route offers the possibility to control silica structure, phase, morphology, and sizes by fine tuning the reaction parameters/conditions. The characteristics and applications of the MSNs produced are also analysed to examine the potential of using agricultural and industrial waste as the silica precursor. Although the use of inexpensive waste-derived materials seems promising for both waste reduction and value-added products synthesis, further research is still needed to boost the production of silica especially at a larger scale. 

Agricultural and industrial waste-derived mesoporous silica nanoparticles: A review on chemical synthesis route

Silica nanoparticles (SNPs) have shown great applicability potential in a number of fields like chemical, biomedical, biotechnology, agriculture, environmental remediation and even wastewater purification. With remarkably instinctive properties like mesoporous structure, high surface area, tunable pore size/diameter, biocompatibility, modifiability and polymeric hybridizability, the SNPs are growing in their applicable potential even further. These particles are shown to be non-toxic in nature, hence safe to be used in biomedical research. Moreover, the molecular mobilizability onto the internal and external surface of the particles makes them excellent carriers for biotic and non-biotic compounds. In this respect, the present study comprehensively reviews the most important and recent applications of SNPs in a number of fields along with synthetic approaches. Moreover, despite versatile contributions, the applicable potential of SNPs is still a tip of the iceberg waiting to be exploited more, hence, the last section of the review presents the future prospects containing only few of the many gaps/research extensions regarding SNPs that need to be addressed in future work. 

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A Comprehensive Review of Synthesis, Applications and Future Prospects for Silica Nanoparticles (SNPs)

Mesoporous silica nanoparticles (MSN) were synthesized using rice husk (RH) as the raw material via sol-gel pathway using cetyltrimethylammonium bromide (CTAB) as the structure directing agent. Silica nanoparticles were successfully functionalized with 3-aminopropyl triethoxysilane (APTES) via in-situ and post functionalization methods. Synthesized nanoparticles were characterized by X-Ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller (BET) surface area analysis. The amorphous silica nanoparticles were of 50–60 ​nm in diameter with a surface area of 150 ​m2/g, pore volume of 0.237 ​cm3/g and average pore size of 3.62 ​nm. Morphology and textural parameters were changed upon functionalization. The equilibrium adsorption capacity of MSN-A (4.94 ​mg/g) to adsorb 10 ​mg/L methylene blue (MB), was higher than in amine functionalized silica nanoparticles. The influence of experimental factors such as pH, adsorbent dosage, and initial MB concentration on adsorption of MB to MSN-A were studied. The equilibrium data for MB adsorption on mesoporous silica nanoparticles well fitted to Langmuir equation, with a maximum monolayer capacity of 19.26 ​mg/g. The adsorption of MB could be best described by the pseudo-second order model. The results indicate that MSN-A is a potential mesoporous material fabricated by cheap natural resources to remove MB from aqueous solutions.

Synthesis of mesoporous silica nanoparticles derived from rice husk and surface-controlled amine functionalization for efficient adsorption of methylene blue from aqueous solution

Heterogeneous photocatalysis is an attractive alternative route to enhance the degradation of environmental pollutants. In this work, we have fabricated Fe2TiO5/TiO2 binary nanocomposites using natural ilmenite via bottom up approach. Synthesized nanocomposites were characterized by X-ray diffractometry, X-ray fluorescence, transmission electron microscopy, Raman spectroscopy, diffuse reflectance UV–Visible spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. These nanoparticles are in the range of 40–70 ​nm and are of type I heterostructure with a band gap of 2.02 ​eV. They are sensitive to visible light and show higher photocatalytic activity under direct solar energy. Photocatalytic activity of Fe2TiO5/TiO2 was assessed using a model textile dye, methylene blue. Over a period of 2 ​h, 76% of methylene blue was photodegraded at a rate of 0.0084 min−1 in the presence of Fe2TiO5/TiO2.

Fabrication of Fe2TiO5/TiO2 binary nanocomposite from natural ilmenite and their photocatalytic activity under solar energy

The photocatalytic activity of single transition metal-doped TiO2 nanoparticles is well established. This article reports the synthesis of Fe and Cu co-doped TiO2 nanoparticles with varying Fe and Cu concentrations by the sol–gel method and their photocatalytic activity towards photodegradation of methylene blue under visible light. Nanoparticles were characterized by X-ray diffractometry (XRD), Raman spectroscopy, Transmission electron microscopy (TEM), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence spectroscopy (XRF), and Diffuse reflectance UV–Visible spectroscopy. XRD patterns revealed the existence of both anatase and rutile phases which was confirmed by Raman and TEM analysis. Both XRD and Raman analysis confirmed the successful doping of Fe and Cu without causing any significant lattice distortions. Nanoparticles were aggregated as shown in TEM and SEM images. XPS analysis revealed the presence of the only Ti4+ in pure TiO2 while both Ti4+ and Ti3+ were present in doped TiO2 in addition to Fe3+, Cu+, and Cu2+. XRF analysis showed the presence of only Ti, Fe, and Cu in the co-doped nanoparticles. According to the diffuse reflectance spectroscopic analysis, the visible light sensitivity of TiO2 has increased upon doping with Fe and Cu. Single metal-doped nanoparticles were efficient than the co-doped nanoparticles for the degradation of methylene blue under visible light. Among the single doped nanoparticles, 0.05 Cu/TiO2 showed the highest rate constant (0.0195 min−1) while the maximum activity from the co-doped nanoparticles resulted in 0.05 Cu + 0.05 Fe/TiO2 (0.0098 min−1). The photocatalytic activity was decreased upon increasing the dopant (Fe/Cu) concentration due to the recombination of photogenerated electron-hole pairs, while due to the shielding effect, low photocatalytic activity resulted in co-doped nanoparticles with varying Fe and Cu loadings.

Photocatalytic activity of Fe and Cu co-doped TiO2 nanoparticles under visible light

Photocatalytic activity of N doped with one transition metal is well known. This article reports the synthesis of TiO2 nanoparticles doped with only N, N co-doped with Fe, N co-doped with Cu and N co-doped with both Fe and Cu by sol-gel method. Their photocatalytic activity was evaluated for the degradation of methylene blue under sunlight. Nanoparticles were characterized by X-ray diffractometry (XRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and diffuse reflectance UV–visible spectroscopy (DRS). XRD analysis revealed the presence of anatase phase TiO2 nanoparticles which was confirmed by Raman spectroscopic and TEM analysis. The crystal structure has not been disturbed by doping with N, Fe and Cu as shown by both XRD and Raman analysis. TEM and SEM images exhibit the nanoparticles which are interconnected due to sintering. XPS analysis revealed the presence of the only Ti4+ in undoped TiO2 but both Ti4+ and Ti3+ are present in doped TiO2 nanoparticles. According to the DRS analysis, the band gap of all doped TiO2 is lower than that of the undoped TiO2, where N, Fe and Cu co-doped TiO2 showed the lowest band gap (2.51 ​eV) proving that the visible light sensitivity of TiO2 increase with metal and non-metal doping. The rate of photodegradation of methylene blue in the presence of undoped TiO2 (0.016 min−1) is higher than all the doped TiO2 nanoparticles. N doped TiO2 show the highest activity among the doped TiO2 nanoparticles (0.006 min−1). Doped TiO2 nanoparticles showed a lower photocatalytic activity due to the electron hole pair recombination and the shielding effect of the dopants.

Photocatalytic activity of N, Fe and Cu co-doped TiO2 nanoparticles under sunlight

Wastewater generation due to anthropogenic activities has become a tremendous problem that the world is struggling to solve. Dyes release to normal water reservoirs badly impacts the environment causing severe issues. Removal of dyes from wastewater streams is important. The advanced oxidation process is advantageous as the dye molecules are degraded into harmless species. TiO2 is the most promising semiconductor that has been researched. However, the use of it in the visible range is restricted due to its high band gap (3.0 eV). TiO2 has been modified in order to enhance visible light sensitivity. This review mainly focused on the effects of doping TiO2 with metals and non-metals and coupling with metal and non-metal oxides to improve its efficiency in photodegrading dyes. TiO2 doped with Fe, Cu and Ag as the main metal species, N, S, and C as the main non-metals are summarized. Further, the effect of doping with multi non-metals and co-doping of metals and non-metals are also discussed. Moreover, coupling TiO2 with metal oxides and graphene oxide for enhanced photocatalytic activity is also summarized in this review.

Charitha Thambiliyagodage

Papers

Synthesis of mesoporous silica nanoparticles derived from rice husk and surface-controlled amine functionalization for efficient adsorption of methylene blue from aqueous solution

Fabrication of Fe2TiO5/TiO2 binary nanocomposite from natural ilmenite and their photocatalytic activity under solar energy

Photocatalytic activity of Fe and Cu co-doped TiO2 nanoparticles under visible light

Photocatalytic activity of N, Fe and Cu co-doped TiO2 nanoparticles under sunlight

Activity enhanced TiO2 nanomaterials for photodegradation of dyes - A review