Evaluation of rice husk ash in adsorption of Remazol Red dye from aqueous media
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The rice husk ash (RHA) extracted from the rice husk (RH) was measured on the removal of the Remazol Red dye. Adsorption variables such as solution pH, dye concentration, adsorbent amount, contact time, and reuse of the RHA were measured. The adsorption equilibrium was achieving in approximately 30 min, due to the rapid electrostatic attraction between the OH2+ groups of the RHA surface and the sulfonic groups of the dye, which occurred mainly at a pH = 2 value. The removal efficiency of the RHA decreased when the initial dye concentrations were higher, due to pore-filling within of the RHA structure with the migration dye molecules. However, the removal percentage increased with increasing adsorbent amount. The adsorption data were suitable with the Freundlich isotherm and pseudo-second order models.
KeywordsRice husk Rice husk ash Adsorption Dye Remazol red
The production of waste from industrial activities is enormous, so the reduction of waste generation needs to be optimized; however this is a limited technology [1, 2]. Therefore, the recycling of these agroindustrial wastes is a plausible alternative to minimize their possible impacts on the environment [1, 3]. Rice is very consumed by the Brazilian population, since this cereal is among the highlights of the agricultural crops of this country . The processing of the rice leads to the generation of the rice husk (RH) as the main residue, and this can represent about 20% of the production [5, 6, 7]. The production of the rice husk as a residue in Brazil was approximately 2.3 million tons in 2018, but the world production of the RH was much higher (around 154 million tons) .
The amorphous silica from RH may have its chemical purity and particle size modified by the thermal treatment temperature . The rice husk ash (RHA) obtained from RH can be contains > 99% (w/w) silica and some amount of impurities [1, 9, 10], however, in order to obtain such a high silica content, the RH should be subjected to a water washing process in order to remove unwanted materials and then be pretreated with an acidic or basic solution, in order to remove impurities . Thus, the RHA can be used as an alternative sorbent because the adsorption technology is an efficient and green process for the removal of different toxic and persistent pollutants present in the environment.
Disposal of untreated organic waste in the aquatic ecosystem is a global issue, mainly because these pollutants can cause serious damage to aquatic organisms as well as to humans [11, 12]. Among the main organic pollutants are dyes, which are classified as toxic and carcinogenic. However, despite its proven toxicity, synthetic dyes are still widely used in the textile industry .
Different methods of treatment of wastewaters with physical separation , chemical oxidation [15, 16], and biological degradation  have been reported in the removal of the dye. In view of the different methods for remediation of the dye, the adsorption has a great prominence due to its operational facilities and for being considered a green technology. Some studies found in the literature have reported the removal of dye from aqueous media using a variety of adsorbent materials. Huang et al.  employed modified bentonite for remove Rhodamine B and Acid Red 1, while Liu et al.  used mesoporous carbons on the adsorption of Acid Red 73 and Reactive Black 5. Zeolitic imidazolate framework was used by Li et al.  to remove Rhodamine B, Methyl Orange, and Methylene Blue.
In the literature, some studies report the removal of dye from the use of RHA as adsorbent material, for example, Sumanjit and Prasad  used the rice husk ash for the removal of the acid dyes, Lakshmi et al.  performed the adsorption tests of the carmine dye, Sharma et al.  removed the methylene blue from aqueous waste, and Bhowmick et al.  used the RHA for the removal of the Amaranthus gangeticus pigments as dye, while that Setthaya et al.  and Akshaya et al.  performed the removal of the methylene blue and reactive yellow dyes, respectively, on the other hand, Peres et al.  used a bio-nanosilica obtained from rice husk for the adsorption of crystal violet.
Thus, in this investigation, the RHA used for the adsorption of the Remazol Red dye was obtained from the optimization of the calcination time and temperature of the RH of the agulhinha variety from Brazil. In addition to the biogeographic importance of the RH used, we highlight the importance of the thermal treatment performed to obtain a rice husk ash with excellent structural and texture properties. On the other hand, up to the present moment we have not found any investigation that has used a Brazilian RHA to remove Remazol Red dye, in that way, the adsorption process was evaluated by means of kinetic and isotherms batch experiments.
2.1 Collection of the RH and obtain of the RHA
The rice husk of the agulhinha (Indian origin) variety was given by the Brazilian Agricultural Research Corporation (Embrapa), São Carlos, São Paulo, Brazil. Thermal treatment performed to obtain RHA from RH and characterizations of the RH and RHA are reported previously .
2.2 Characterization of the RH and RHA
The characterization of the RH and RHA was complemented by scanning electron microscopy (SEM) analysis. SEM analysis was performed in a Phillips FEG-XL 30 microscopy in a secondary electron detector and with an accelerator power of 3 kV .
2.3 Batch adsorption experiments
2.3.1 Effect of solution pH
The removal process of the dye by RHA was optimized at pH values in the range 2–12 to determine the ideal pH of the adsorption process. Solutions of 0.10 mol L−1 HCl or 0.10 mol L−1 NaOH were used for pH adjustments.
2.3.2 Effect of initial dye concentration
Concentrations of the dye in the range of 10–150 mg L−1 were used on the adsorption assays at a fixed pH value of 2 and with duration of 12 h.
2.3.3 Effect of sorbent amount
Values of the mass of the RHA in the range of 10–100 mg were used at a solution pH of 2, with time of 12 h, and concentration of 10 mg L−1 of dye.
2.3.4 Effect of contact time
Contact times in the range 0–300 min were studied using 100 mg of the RHA, solution pH equal to 2, and a concentration of 10 mg L−1.
2.3.5 Regeneration of the RHA
At the end of the adsorption the saturated RHA was separated by filtration, and then regenerated by shaking in a solution of 0.1 mol L−1 NaOH, followed with centrifugation, washing, and drying at 70 °C. The regenerated RHA was reused in the next run under the same conditions.
3 Results and discussion
3.1 Characterization of the RH and the RHA
The characterizations of the RH and RHA are reported previously , the same were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), thermogravimetric analysis (TGA), X-ray diffractometry (XRD), digital photographic images, and nitrogen adsorption–desorption isotherms. Thus, the FTIR and TGA results showed that the cellulose, hemicellulose, and lignin are the main components of the RH, as well as the silica bonded to these fibers, however the XRD analysis showed that RH has a crystalline structure. On the other hand, the presence of silica in RH was confirmed by the characterization performed in the RHA (A2-700), which presented a high SiO2 content, an amorphous silica pattern, absence of organic matter, and high surface area (SBET = 293.89 m2 g−1) and total pore volume (VT = 0.36 cm3 g−1) according to with the XRF, XRD, digital photographic image, and N2 adsorption results, respectively .
3.2 Batch adsorption experiments
3.2.1 Effect of solution pH
As shown in Fig. 2, it can be seen that the decrease in pH value caused an increase in adsorption efficiency of the RHA. This increase can be attributed by protonation of O–H groups present in the pores of the RHA, as well as the anionic properties of the Remazol Red dye. Therefore, the adsorption of dye by RHA is attributed to the electrostatic attraction between the OH2+ groups and the sulfonic groups of the dye at the adsorbent/adsorbate interface . On the other hand, at pH > 7, there is a large amount of OH− ions that promote the deprotonation of the O–H groups, and thus, these OH− ions compete and/or interact with the dye molecules on the active sites of RHA, thereby providing a reduction of the adsorption efficiency of the RHA. So, the other experiments were performed at pH 2.
3.2.2 Effect of initial dye concentration
3.2.3 Effect of sorbent amount
3.2.4 Effect of contact time
3.3 Kinetics of adsorption
Comparison of the pseudo-first order and pseudo-second order kinetic models on the removal of the Remazol Red dye by RHA
qe (exp.) (mg g−1)
qe (calc.) (mg g−1)
hi [(mg g−1)2 min−1]
qe (calc.) (mg g−1)
k2 (g mg−1 min−1)
hi (mg g−1 min−1)
3.4 Adsorption isotherms
Comparison of equilibrium data obtained using the Freundlich and Langmuir isotherm models applied on the removal of the Remazol Red dye by RHA
KF (L g−1)
r F 2
Qmax (mg g−1)
b (L mg−1)
r L 2
As can be seen in Table 2, the Freundlich model is more representative than Langmuir model for the experimental data of equilibrium adsorption of the Remazol Red by the RHA, according to the r2 values obtained by the two models. According to Santos et al.  the n > 1 value for the removal of the dye is an indicative of the positive cooperation between Remazol Red molecules, so the removal process is not restricted to the formation of the monolayer, as well as the system has a heterogeneous active site energy distribution and a possible interaction between the adsorbed molecules of the Remazol Red dye within of the pores of the RHA. In addition, this result can be due to the textural and structural properties of the RHA promoting the diffusion and transport of the dye molecules onto the adsorbent.
3.5 Regeneration of the adsorbent
The adsorption equilibrium of the Remazol Red dye by RHA was achieving in approximately 30 min. A higher sorbent amount, as well as low pH medium favored the adsorption process, however, the adsorption efficiency of the RHA decreased with increase of the initial dye concentration. The pseudo-second order and Freundlich models are more representative than the pseudo-first order and Langmuir models for the kinetics and equilibrium experimental data of adsorption of the Remazol Red by the RHA, respectively. The adsorption results obtained suggested that the RHA is a strong candidate as an adsorbent material for the removal of dyes from textile industry wastewater.
The authors thank FAPESP (Research Support Foundation of the State of São Paulo) (Grants 2014/05679-4, 2017/06775-5, and 2018/18894-1), CAPES (Coordination for the Improvement of Higher Education Personnel) (Grant 309342/2010-4), and CDMF (Center for the Development of Functional Materials) (Grant 2013/07296-2) for the financial support.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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