I certify that this project report entitled "ANODISC AND HYDROTHERMAL GROWTH OF IRON (III) OXIDE NANOPOROUS-NANOROD FILM FOR CHROMIUM REMOVAL" prepared by CHUAH CHUNG ZHIH has met the required standard of submission for partial fulfillment of the requirements for the award of the Master of Engineering (Mechanical) at Universiti Tunku Abdul Rahman. Due acknowledgment must always be given to the use of material contained in or derived from this report. To improve the Cr (VI) removal properties, α-Fe2O3 nanoporous-nanorod structure with high surface-to-volume ratio was fabricated on iron foil substrate.
Research background
Problem statement
Objectives
Report outline
Introduction
Chromium
Cr (III) is an essential dietary nutrient for maintaining the efficient metabolism of glucose, lipids and proteins in the human body and for the prevention of diabetes, infertility and cardiovascular diseases. On the other hand, Cr (VI) is known as a Group A carcinogen and is extremely harmful to the human body, causing various adverse effects such as skin irritation, respiratory tract irritation and obstruction, and genetic mutation (Shanker, 2019; DesMarias and Costa ). , 2019). As such, environmental pollution with Cr (VI) via industrial wastewater is a growing problem in the current society at an alarming rate, causing major adverse effects on the general population through contamination of food sources.
Hexavalent chromium removal methods
Adsorption
Adsorption is a surface-based technique that induces an attachment process through surface contact between an adsorbent and the target compound known as an adsorbate in aqueous solution. The adsorption process can be divided into two types of interaction which are physisorption and chemisorption. Physisorption works by using weak and reversible van der Waals forces to bind the adsorbate to the adsorbent while chemisorption involves the transfer of electrons between the exposed surface of the adsorbent and the adsorbate in solution to effectively bind the adsorbent to the adsorbate.
Comparison of adsorbents in Cr (VI) removal capacities
In general, an adsorbent, in order to have high adsorption capacity for effective removal of hazardous pollutants for water treatment purposes, requires a large active site surface area, chemical and thermal stability, unique morphology and good mechanical strength. While the performance of adsorbents is generally judged by their adsorption capacity, the wide range of Cr dissolution conditions used in the different studies adds complexity to the comparison. Variables such as pH, Cr dosage and density in solution, temperature, adsorption time and adsorption percentage cause problems when comparing adsorbent performance.
Iron oxide
- Fundamentals of iron (III) oxide
- Iron oxide nanostructures
- Nanoporous iron oxide
- Iron oxide nanorods and nanowires
- Synthesis methods of iron oxide nanostructures
- Anodization
- Hydrothermal process
- Annealing for post-treatment
- Effect of pH value in chromium removal efficiency
Various studies are also conducted on the synthesis of 1-D nanostructures and sometimes 2-D nanostructures on their base materials to further discover compatibility for new applications in various fields (Rafique et al., 2020). Also, an insufficient supply of tension will cause the surface of the pores to be covered and non-homogeneous (Martín-González et al., 2020). Comparing hydrothermal and thermal oxidation methods based on nanorod sizes, it was reported that the hydrothermal method produces nanorods with diameters 13 nm to 250 nm as shown in Figure 2.7 and Figure 2.8 (Carvalho-Jr et al., 2019; Chew et al. ., 2019).
Similarly, the thermal oxidation method has been reported to produce nanowires or nanosheets with a thickness of 50 nm to 70 nm and a width of 50 nm to 150 nm (Budiman et al., 2016). Anodization of iron foil to produce self-assembled nanoporous iron(III) oxide was first synthesized by Prakasam et al. This dimple formation produces an amorphous FeF3 shell, which then requires the presence of water to remove the amorphous shell from the nanopores (Martín-González et al., 2020; Rozana et al., 2016).
This process first attracted attention for its application in the growth of quartz crystals in a hydrothermal solution in steel autoclaves placed under a certain temperature and pressure condition (Byrappa et al., 2015). These hydrothermal solutions aim to provide 𝐹𝑒3+ and OH− ions to produce precipitation of iron (III) hydroxide (Fe(OH)3), which will then form 𝛼FeOOH nanorods (as shown in Figure 2.11) on the substrate through the conditions assisted with pressure above 1. atmospheric pressure and heating between 100°C to 120°C (Ou et al., 2008; Chew et al., 2019). The chemical reactions from the preparation of the hydrothermal solution to the formation of FeOOH nanorods are shown in Equations 2.1 and 2.2 (Iwasaki et al., 2012).
It contains contamination from a small amount of impurities caused by the electrolyte solution (Rozana et al., 2016; Martín-González et al., 2020). This deformation of the nanopores is not desirable due to the reduction of the active surface area of the iron oxide, potentially reducing its adsorption properties, defeating the purpose of forming nanoporous structures on the Fe foil substrate (Rozana et al. , 2016). As such, it can be concluded that an acidic medium is necessary to ensure optimal effectiveness of the Cr(VI) removal method, especially for adsorbents related to activated carbon and metal oxide (Li et al., 2019; Mortazavian et al., 2018; Chew et al., 2019).
Summary of literature review
Introduction
Experimental details
- Anodization process on iron foil substrate
- Annealing post-treatment for anodization
- Hydrothermal process on nanoporous iron oxide substrate
- Second stage annealing post-treatment process
For the preparation of the nanoporous Fe2O3, iron foil with 99.5% purity by Nilaco Corporation was used as the base material. Prior to the anodizing process, the iron foil was ultrasonically cleaned with acetone, ethanol and distilled water for 5 minutes each. The sample was also polished with 2000 grit sandpaper to remove the oxide layer on the iron foil before the anodizing process.
Anodization was carried out using a conventional two-electrode setup, where iron foil was used as the anode and platinum foil as the cathode. After the anodization process, the sample was sonicated in acetone for 20 seconds and dried to remove any remaining electrolyte on the anodic iron sheet sample. After the anodization process was completed, a post-treatment annealing process was performed to transform the amorphous nanopores into a crystalline state.
The annealing process was performed in nitrogen to prevent thickening of the pore walls and oxide barrier. After the anodization and the first annealing after the treatment process, the hydrothermal process was performed to grow FeOOH nanorods on the samples. The hydrothermal solution and the prepared sample were then placed in a Teflon-lined autoclave and heated to 120 °C for 4 h using an oven as shown in Figure 3.4 and Figure 3.5.
The second annealing was performed to transform the amorphous FeOOH nanorods into crystalline Fe2O3 nanorods (Chew et al., 2019).
Sample characterization
Morphological analysis
Structural analysis
Hexavalent chromium adsorption efficiency
- Preparation of chromium (VI) solution
- Chromium adsorption test and analysis
The Cr (VI) stock solution was first prepared to perform the Cr (VI) adsorption test. However, since 0.02829 g K2Cr2O7 is difficult to weigh, the Cr (VI) solution was first prepared in higher concentration as shown in Figure 3.7. A pH indicator paper was used to verify the pH level of the solution as shown in Figure 3.8.
Before the Cr (VI) adsorption test, 2.5 ml of aliquot solution from 100 ml of 10 mg/L Cr (VI) solution was first collected with a micropipette and placed in a centrifuge tube. Then, the prepared samples of 1 cm x 1 cm size were placed in an Erlenmeyer flask containing a Cr (VI) solution. The Cr (VI) removal efficiency test was also performed on bare iron foil as a reference point.
After the adsorption test, the colorimetric method with diphenylcarbazide was used to analyze the adsorption efficiency of the produced samples. The 1,5-diphenylcarbazide solution consisted of 0.05 g of diphenylcarbazide powder dissolved in 10 ml of acetone. The sample was then shaken manually to ensure complete mixing of the solution as shown in Figure 3.10.
The solutions were then transferred to plastic cuvettes for UV-vis testing as shown in Figure 3.11.
Morphological Analysis
- Anodization process on iron foil substrate
- Annealing post-treatment for anodization
- Hydrothermal process on nanoporous iron oxide substrate
- Second stage annealing post-treatment process
After anodization, the nanoporous iron oxide film was baked at and 500 °C for 0.5 h in nitrogen to increase the crystallinity of the nanoporous structure and prevent soluble amorphous structures from dissolving in the hydrothermal solution during the hydrothermal process. The surface morphologies of the annealed nanoporous iron oxide samples are shown at high (30kx) and low (5kx) magnification in Figure 4.3 and Figure 4.4, respectively. Also, a slight thickening of the barrier layer caused by the presence and reaction with air can be observed.
After preparing the annealed nanoporous iron oxide samples, hydrothermal process was performed to grow FeOOH nanorods on them. The morphologies of FeOOH nanorods grown on nanoporous iron oxide via hydrothermal process are shown in Figure 4.5 (5kx magnification) and Figure 4.6 (10kx magnification). An-HT shows collapse and deformation of the nanoporous structure with nano/microplate structures with thickness 80-234 nm as shown in Figure 4.6(b).
However, An-200-HT no longer shows the presence of microplate structures and deformation of the sample surface. On the other hand, An-300-HT, An-400-HT and An-500-HT show increasing amount of nanoporous structure that can be retained after the hydrothermal process as shown in Figure 4.6f-j. The retention of the nanoporous structure layer allows the formation of nanorods on top of it, thus forming nanoporous-nanorod structure.
After the hydrothermal process, the second phase annealing post-treatment process was performed at temperature 300 °C for 2 hours to transform the nanorods into Fe2O3 phase.
Structural analysis
Cr (VI) removal test
In addition, some oxide layers peeled off from An-HT and An-200-HT samples during the Cr (VI) removal test. This is probably caused by the presence of FeO or FeOOH on the samples (as seen in Figure 4.10) which reacted with the Cr (VI) solution. This exfoliated powder can also increase the total surface area of iron oxide within the chromium solution.
This led to An-200-HT having a higher Cr(VI) removal efficiency than An-300-HT in the early stage between 0 and 20 minutes, as shown in figure. However, peeled iron oxide powder is not recommended as it requires a further filtration process to remove the chromium adsorbed iron oxide powder from wastewater. Based on section 2.4.3.3, the best chromium removal efficiency was initially expected to come from the An-400-HT sample.
However, by reducing the annealing time from 2 h to 0.5 h, the annealing time allowed enough time to induce crystallinity without deforming the nanoporous α-Fe2O3 structure. It is likely that upon further annealing at 600 °C, deformation of the nanoporous structure will begin to occur, causing less α-Fe2O3 surface area for the chromium adsorption process, thus reducing the chromium removal efficiency. Also, An-300-HT and An-400-HT show a more linear adsorption rate compared to An-500-HT, which shows a high initial adsorption with a decreasing rate trend.
This is possibly caused by the crystalline growth orientation of α-Fe2O3 in the (110) rather than (104) direction that affected the uptake kinematics (Hirai et al., 2014).
Summary
Conclusions
Recommendations for future work
Rapid formation of nanoplates and nanowires by thermal oxidation of iron in water vapor and their applications as Cr(VI) adsorbents. Adsorption of Cr(VI) on Al-substituted hematites and its reduction and retention in the presence of Fe2+ under conditions similar to subsurface soil environments. Facile modification of activated carbon with highly dispersed nano-sized Α-Fe2O3 for enhanced removal of hexavalent chromium from aqueous solutions.
Nanoparticle-impregnated zero-valent iron (AC/nZVI) activated carbon optimized for simultaneous adsorption and reduction of aqueous hexavalent chromium: Material characterizations and kinetic studies.
