Preparation of activated redmud and its application for removal of hydrogen sulfide in air

Use your smartphone to scan this QR code and download this article ABSTRACT Red mud is a highly alkaline solid waste from the Bayer process for aluminum production. Red mud reservoirs are usually considered as a potential environmental risk. The treatment of red mud is costly due to the lack of an effective and economical treatment technology. On the other hand, the main components of red mud are Fe2O3 , Al2O3 , SiO2 , and Na2O, which could be employed as a promising precursor for the preparation of various nanomaterials. In this study, we prepared activated red mud by thermal and acid treatment method and applied it for adsorption of H2S in air. The red mud was activated under different temperatures (i.e., 200, 400, 600, and 800 oC for 4 h), types of acid (i.e., H2SO4 and HCl), and acid concentrations (i.e., 0.5, 1.5, and 2.5 M). The produced materials were then applied for H2S removal in air with concentration of 90 – 110 mg/m3 using a fix-bed adsorption column test. Results showed that red mud activated at 800 oC and with 1.5 M H2SO4 solution had the highest adsorption capacity of 29.38 mg/g with an average removal efficiency of 80.2%. The effects of gas flow rate and initial H2S concentration were also investigated, and the highest removal capacity was achieved at an inlet concentration of 100 mg/m3 and flow rate of 1 L/min. Both Langmuir and Freundlich adsorption isotherms were employed for modelling the H2S adsorption by this material and the experimental result was more fitted with the Langmuir isotherm. The thermal desorption and recyclability test were also conducted for evaluating the practical application of activated red mud material and 200 oC was the suggested desorption temperature with 81.7% adsorption capacity recovery.


INTRODUCTION
Hydrogen sulfide (H 2 S) is a toxic and colorless gas with a very unpleasant odor that originated from both nature and human activities. It greatly affects the air quality and also causes the corrosion of equipment and pipes 1 . H 2 S is a common pollution gas in industry, biogas, coal storage, and in the processes that release odor such as sewage systems, wastewater treatment, and solid waste composting 2 . Air pollution due to H 2 S gas is a problem that has been mentioned in lots of documents and research works 3 . For H 2 S treatment, many methods were studied and applied such as absorption, oxidation, and biofiltration 4 . Among them, adsorption is considered as a simple but effective method. Therefore, finding a new, effective, and inexpensive adsorbent for H 2 S removal is of interest.
On the other hand, red mud is a highly alkaline solid waste with pH from 10 -12 from the Bayer process for aluminum production 5,6 , which requires a large amount of NaOH 7 . It comprises very fine-grained particles with a size of < 10 µm and a specific surface area of about 10 -30 m 2 /g 8 . The main components of red mud are Fe 2 O 3 , Al 2 O 3 , SiO 2 , and Na 2 O. Many studies showed that red mud has a good adsorption capacity, particularly when activated by acid, heat, or combining activation with other metal oxides [9][10][11][12][13] . Currently, the research of using red mud to adsorb H 2 S emission is still limited 14 . Therefore, in this study, we aimed to collect red mud from Tan Rai bauxite plant and then activate it by acid and thermal treatment for H 2 S adsorption. Besides, other factors were also investigated such as flow rate and input concentration as well as the absorption and reuse of the adsorbent.

MATERIALS AND METHODS
According to the study of Minh 15 , the pH of raw red mud from Tan Rai bauxite plant was very high at pH 11.5. Their X-ray diffraction analysis showed that the phase composition of raw red mud is mainly gibbsite (Gi) γ-Al(OH) 3 16,17 . The produced materials were denoted as RMXC-Y (activated by HCl) and RMXS-Y (activated by H 2 SO 4 ) where X represents the calcined temperature (e.g., X = 4 for 400 o C) and Y is the concentration of acid. In this study, commercial activated carbon (AC) with a size of 0.097 -0.45 mm was also prepared and employed as reference material. The schematic for the H 2 S adsorption test is illustrated in Figure 1.

Adsorption test
The adsorption tests were conducted with 29 different materials, including activated carbon, thermal activated red mud, and acid activated red mud. The H 2 S concentration was in range of 90 -110 mg/m 3 and 3 g of adsorbent was used. The results are presented in Figure 2. As seen in Figure 2, the adsorption capacity of most adsorbents derived from red mud was higher than that of AC except for RM2, RM2C-0.5, and RM4 materials. It is also obvious that the adsorption capacity of the thermally treated materials is proportional to their activation temperature. Under high temperature, there was a phase transformation of red mud component (e.g., goethite to hematite) and the join of aluminum into the material lattice to form Alhematite 15 , which acts as internal adsorption sites. In addition, since water is removed from the material at the high temperatures, the pore system is enhanced, and the material surface area could be improved. For acid-activated red mud, it is reported that the specific surface area of material increases while the particle size tends to decrease with the acid concentration 15 . Therefore, the adsorption capacity also increases with the increases of acid concentration in a certain range but then decreases due to the material structure disruption under high acidic treatment condition. Besides, H 2 SO 4 was proved to be more effective than HCl for activating of red mud in terms of H 2 S adsorption, possibly due to the higher volatility of HCl than H 2 SO 4 . Among all materials, RM8S-1.5 had the highest H 2 S adsorption capacity of 29.38 mg/g, which was about 1.4 times better than that reported by Sahu et al. 14 .

Isotherm study
RM8S-1.5 material was then chosen for isotherm study with input H 2 S concentration from 40 to 120 mg/m 3 . As seen in Figure 3, the adsorption capacity increases when input H 2 S concentration increases from 40 to 100 mg/m 3 but then decreased with a further increase of input concentrations from 100 to 120 mg/m 3 . Langmuir and Freundlich adsorption isotherm models were established to determine the parameters of H 2 S adsorption by RM8S-1.5. As summarized in Table 1, the adsorption of H 2 S on RM8S-1.5 is more fitted with Langmuir (R 2 = 0.906) than with the Freundlich isotherm adsorption model (R 2 = 0.781). This implied that the adsorption of H 2 S on RM8S-1.5 not only physical adsorption by electrostatic attraction but also chemical interaction of H 2 S and oxides of iron and aluminum formed after calcined at a high temperature of 800 o C. The maximum adsorption capacity was calculated to be 36.68 mg/g. To evaluate if an adsorption process is fitted with the single-layer adsorption model described by Langmuir equation, it is required to be evaluated through equilibrium parameter R L 17 , as expressed in Equation (1). Results from Table 2 with R L < 1 confirmed the suitability of the Langmuir isotherm model for H 2 S adsorption by RM8S-1.5 in this input concentration range.

Influence of input flow rate
This experiment was carried out with the flow rate in a range of 1.0 -3.0 L/min and an input concentration of 100 -110 mg/m 3 . Obviously, the adsorption capacity continuously decreased from 30.49 mg/g to 16.58 mg/g with an increase of flow rate from 1.0 to 3.0 L/min (Figure 4). This is because of the decrease of contact time between H 2 S and adsorbent with the increase of gas flow rate, which leads to the low H 2 S adsorption on the surface of RM8S-1.5 material.

Regeneration of adsorbent
The recycle test was also conducted to investigate the effect of the desorption process on the sorption capacity of RM8S-1.5 material. The desorption process was carried out by drying saturated RM8S-1.5 samples at 200 and 400 o C for 20 min. After desorption, the material was cooled and then reused for adsorption. As presented in Figure 5, the capacity of the regenerated materials was lower than the original one although still at high levels. The adsorption capacity of material regenerated at 400 o C was higher than that at 200 o C. However, the difference was not much since capacity increased only from 24.0 to 26.9 mg/g as compared to double temperature with higher energy consumption.

CONCLUSION
Adsorbents from red mud were successfully synthesized and applied for H 2 S adsorption. Results showed that adsorption capacity increased with the increase of calcination temperature and H 2 SO 4 was better than HCl for red mud activation. The highest adsorption capacity of 30.49 mg/g was achieved at input concentration of 100 mg/m 3 and flow rate of 1 L/min using red mud calcined at 800 o C and activated with 1.5

CONFLICT OF INTEREST
There is no conflict of interest regarding this manuscript.