Alternative treatments to improve the potential of rice husk as adsorbent for methylene blue

Nowadays, the presence of synthetic dyes in effluents has become one of the major concerns to the scientific community. It is well known that the reduction of light penetration and photosynthetic activity, toxicity, mutagenicity and carcinogenicity can be effects caused by the incorrect discharge of the dye-containing effluents into the environment (Gupta & Suhas 2009). Adsorption is a commonly used technique for removal of dyes, due its low cost, low energy requirements, high efficiency, simplicity of implementation and operation (Dotto et al. 2015a).

One problem of the adsorption process is related to the adsorbent. Generally, activated carbon is the main choice; nonetheless, the high cost is a major drawback (Weng et al. 2009). One way to reduce the costs is the use of waste materials, especially from agro-industrial sources, due their low cost and high availability (Ali 2012; Weber et al. 2014; Wang et al. 2016; Silva et al. 2016). Rice husk fit well in the concept of low-cost adsorbents, due its high annual generation and presence of organic compounds (Soltani et al. 2015). However, its low surface area and non-functionalized/accessible sites prejudice its efficiency (Thakur 2014). With the objective of improving the adsorbent performance, alternative surface modifications can be implemented, increasing the surface area and providing new accessible sites.

For this purpose, some modifications were performed on the rice husk surface, including acid or alkaline treatments. The treatment with NaOH, for example, can attack the cellulose chains, generating new accessible sites (Soltani et al. 2015). Other modifications such as ultrasound assisted (UA) and supercritical CO₂ (SCO₂) have been used as viable methods for surface modification of organic materials. It was concluded that these technologies can provide new adsorption sites and an additional porosity (Dotto et al. 2015b).

In this work, rice husk was modified with NaOH, UA and SCO₂ treatments, in order to improve its adsorption potential regarding the methylene blue (MB) dye. All adsorbents (raw and treated rice husks) were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). For all materials, experimental equilibrium curves were constructed at different temperatures (from 298 to 328 K). Langmuir and Hill models were used to interpret the equilibrium curves. Also, thermodynamic parameters, such as standard Gibbs free energy change (ΔG⁰), standard enthalpy change (ΔH⁰) and standard entropy change (ΔS⁰) were estimated.

Raw rice husk was obtained from a local processing industry (Santa Maria RS, Brazil). First, rice husk was washed several times with distilled water, for cleaning and removing any undesirable particles. After, it was dried at 313 K for 24 h and stored for further use. Rice husk was used without milling, with particle size of 5 mm.

Sodium hydroxide treatment: a sodium hydroxide solution (700 mL and 2.5 mol L⁻¹) was poured into a beaker with 20 g of raw rice husk. The suspension was agitated for 2 h and the rice husk was removed. The NaOH treated rice husk was washed with distilled water until neutral pH was achieved, and oven dried for 24 h at 313 K. This adsorbent was named NaOH-rice husk.

UA treatment: for the UA treatment, 20 g of raw rice husk were mixed with 700 L of distilled water. Then the mixture was treated by an ultrasonic processor (UP400S, Hielscher, Germany) of 400 W equipped with a titanium sonotrode for 2 h. The treatment was performed with cycle of 1 and amplitude of 90% at 313 K. The ultrasound treated rice husk (UA-rice husk) was dried at 313 K for 24 h and stored for further experiments.

SCO₂ treatment: initially, 20 g of rice husk were put in a sealed jacketed stainless steel 304 reactor at 313 K (the temperature was controlled by a thermostat bath). The CO₂ (White Martins, 99.5%) was pumped and pressurized by a high pressure syringe pump (500D, Teledyne Isco, USA). When CO₂ reached 20 MPa, it was fed into the reactor; then rice husk was exposed to the treatment for 2 h. Finally the reactor was depressurized and the supercritical treated rice husk (SCO₂-rice husk) was collected and stored.

All the abovementioned treatments and their conditions were determined by preliminary tests and literature (Soltani et al. 2015; Dotto et al. 2015a, 2015b). The treatments were performed in replicate (n = 3).

Raw rice husk and the respective modifications (NaOH-rice husk, UA-rice husk and SCO₂-rice husk) were characterized by FT-IR, XRD, SEM and EDS. FT-IR (Shimadzu, Prestige 21, Japan) was carried out in the range of 500–3,500 cm⁻¹ to identify the functional groups and the changes caused by the treatments (Silverstein et al. 2007). XRD (Rigaku, Miniflex 300, Japan) was performed to verify the possible alterations in the amorphous/crystalline structure of the samples (Waseda et al. 2011). SEM and EDS were performed (Tescan, Vega3 SB, Czech Republic) to visualize the surface and composition of the samples (Goldstein et al. 2003).

Methylene blue (color index 52015, molar weight of 319.8 g mol⁻¹, λ_max = 664 nm, solubility of 43.6 g L⁻¹ at 25 °C) was purchased from Vetec, Brazil. All other reagents utilized were of analytical grade, purchased from Sigma-Aldrich. To ensure the experimental accuracy, reproducibility and reliability of the collected data, the adsorption experiments were performed in triplicate; blanks were run in parallel and all dye solutions were stored in glass flasks, which were cleaned by soaking in HNO₃ (1.5 mol L⁻¹) for 24 h.

For all samples, the major intense bands were found at 3,450 cm⁻¹ (OH stretching vibrations), 2,910 cm⁻¹ (C-H alkane stretch), 1,650 cm⁻¹ (C=O), 1,429 (C-C, C-O ring), 1,065 cm⁻¹ (Si-O-Si asymmetric stretch), 450 cm⁻¹ (Si-O-Si bend). These groups are common for lignocellulosic materials, which contain cellulose, hemicellulose and lignin (Thakur 2014). In particular the NaOH-rice husk has demonstrated a lower intensity in the 1,650 cm⁻¹ region. These groups can be potential sites for dye adsorption. For the other samples, the spectra were similar. This indicates that only physical modifications occurred in the rice husk.

From the EDS analysis (figure not shown), it was found that the raw rice husk surface is composed mainly of C, O and Si. For the others samples (NaOH-rice husk, UA-rice husk and SCO₂-rice husk), Si was not detected on the surface. These results corroborate the XRD and SEM analyses. Therefore, it is possible to affirm that the organic fraction of rice husk surface was exposed by the treatments.

Regarding to the effect of surface treatments, it was found that, in general, the adsorption capacity followed the order: NAOH-rice husk > UA-rice husk > SCO₂-rice husk > raw rice husk. This shows that all modifications promoted improvements in the rice husk performance as adsorbent. The NaOH-rice husk presented a better adsorption capacity in comparison with the other surface modifications. This can be attributed to the exposure of the endocarp structure (tube cells). This exposure created new adsorption sites and new diffusion paths, which can help the adsorption process (Dotto et al. 2015a).

Table 1 shows the Langmuir parameters for the MB adsorption onto raw rice husk, NaOH-rice husk, UA-rice husk and SCO₂-rice husk. The Langmuir model presented a satisfactory fit with the experimental data (R² > 0.95 and ARE < 10%). It was found that, for all adsorbents, the temperature increase caused an increase in the q_m values, with the maximum values attained at 328 K. The q_m values were in the following order: NaOH-rice husk > UA-rice husk > SCO₂-rice husk. This behavior indicated that the MB adsorption capacity is favored by the use of NaOH-rice husk at 328 K. In general, the same trend was found for the K_L parameter. This indicated that the adsorbent–adsorbate affinity is higher at 328 K. Also, the higher affinity was found using NaOH-rice husk, confirming that this adsorbent is the most adequate for MB.

Table 1

Langmuir parameters for the adsorption of MB onto raw rice husk, NaOH-rice husk, UA-rice husk and SCO₂-rice husk

.	.	T (K) .
Adsorbent .	Parameters .	298 .	308 .	318 .	328 .
Raw rice husk		17.0 ± 1.5	18.7 ± 1.5	33.5 ± 2.6	52.2 ± 1.3
		0.089 ± 0.009	0.118 ± 0.004	0.107 ± 0.005	0.069 ± 0.007
		0.987	0.986	0.959	0.956
NaOH-rice husk		39.2 ± 2.7	46.5 ± 2.4	49.7 ± 0.6	65.0 ± 0.4
		0.122 ± 0.007	0.158 ± 0.005	0.17 ± 0.001	0.209 ± 0.003
		0.979	0.958	0.959	0.954
UA-rice husk		23.5 ± 2.0	27.4 ± 1.2	52.6 ± 1.4	58.7 ± 2.1
		0.085 ± 0.001	0.131 ± 0.012	0.114 ± 0.008	0.138 ± 0.005
		0.979	0.958	0.959	0.954
SCO2-rice husk		18.4 ± 1.4	19.7 ± 2.2	42.0 ± 3.8	56.4 ± 0.6
		0.07 ± 0.001	0.101 ± 0.002	0.105 ± 0.005	0.093 ± 0.005
		0.979	0.958	0.959	0.954

.	.	T (K) .
Adsorbent .	Parameters .	298 .	308 .	318 .	328 .
Raw rice husk		17.0 ± 1.5	18.7 ± 1.5	33.5 ± 2.6	52.2 ± 1.3
		0.089 ± 0.009	0.118 ± 0.004	0.107 ± 0.005	0.069 ± 0.007
		0.987	0.986	0.959	0.956
NaOH-rice husk		39.2 ± 2.7	46.5 ± 2.4	49.7 ± 0.6	65.0 ± 0.4
		0.122 ± 0.007	0.158 ± 0.005	0.17 ± 0.001	0.209 ± 0.003
		0.979	0.958	0.959	0.954
UA-rice husk		23.5 ± 2.0	27.4 ± 1.2	52.6 ± 1.4	58.7 ± 2.1
		0.085 ± 0.001	0.131 ± 0.012	0.114 ± 0.008	0.138 ± 0.005
		0.979	0.958	0.959	0.954
SCO2-rice husk		18.4 ± 1.4	19.7 ± 2.2	42.0 ± 3.8	56.4 ± 0.6
		0.07 ± 0.001	0.101 ± 0.002	0.105 ± 0.005	0.093 ± 0.005
		0.979	0.958	0.959	0.954

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From the perspective of the statistical physical Hill model (R² > 0.97 and ARE < 10%), it is possible to describe and understand the influence of temperature and surface modifications on the MB adsorption. The Hill parameters are presented in Table 2. According to the literature (Khalfaoui et al. 2003; Franco et al. 2015; Dotto et al. 2016), the stoichiometric coefficient n describes the adsorption reaction and is a parameter which estimates the number of molecules per site during the adsorption process and the adsorbate position on adsorbent surface. This number can be less than, greater than or equal to 1. If this value is less than 1, 1/n represents the anchorage number of one molecule on several different receptor sites, i.e. the molecules adsorb in a parallel position (multi-anchorage) on the adsorbent. If the parameter is greater than to 1, a certain number of molecules are adsorbed on one receptor site. If the number of molecules per site is equal to 1, the molecule interacts by an inclined position (mono-anchorage) with the adsorbent surface. In Table 2, it can be noticed that n increased with the temperature. The higher n values were found for NaOH-rice husk. These results indicate that higher temperatures favored the MB aggregation into the adsorption sites. Also, they suggest that the modification with NaOH was efficient, since the surface of NaOH-rice husk is able to interact with more than one molecule per site.

Table 2

Hill parameters for the adsorption of MB onto raw rice husk, NaOH-rice husk, UA-rice husk and SCO₂-rice husk

.	.	T(K) .
Adsorbent .	Parameters .	298 .	308 .	318 .	328 .
Raw rice husk		18.3 ± 1.6	9.7 ± 1.8	3.0 ± 0.1	2.8 ± 0.2
		21.0 ± 1.4	19.8 ± 6.6	17.9 ± 2.0	19.2 ± 1.2
		0.85 ± 0.25	0.94 ± 0.27	1.9 ± 0.44	2.15 ± 0.26
		0.983	0.983	0.978	0.994
NaOH-rice husk		2.9 ± 0.3	1.5 ± 0.1	1.5 ± 0.1	1.4 ± 0.6
		19.4 ± 1.7	20.4 ± 1.3	21.8 ± 1.8	26.2 ± 1.3
		1.64 ± 0.16	2.31 ± 0.37	2.16 ± 0.39	2.58 ± 0.59
		0.996	0.989	0.987	0.976
UA-rice husk		16.5 ± 1.2	6.8 ± 1.9	1.9 ± 0.5	1.3 ± 0.1
		27.5 ± 1.3	26.2 ± 1.3	21.5 ± 2.1	20.1 ± 1.4
		0.89 ± 0.21	1.04 ± 0.11	1.82 ± 0.50	2.72 ± 0.52
		0.989	0.996	0.973	0.983
SCO2-rice husk		20.6 ± 0.2	11.2 ± 0.9	2.6 ± 0.4	1.9 ± 0.1
		21.8 ± 1.9	20.8 ± 1.3	20.2 ± 1.1	19.3 ± 1.0
		0.89 ± 0.20	0.94 ± 0.33	1.76 ± 0.28	2.37 ± 0.25
		0.990	0.972	0.991	0.995

.	.	T(K) .
Adsorbent .	Parameters .	298 .	308 .	318 .	328 .
Raw rice husk		18.3 ± 1.6	9.7 ± 1.8	3.0 ± 0.1	2.8 ± 0.2
		21.0 ± 1.4	19.8 ± 6.6	17.9 ± 2.0	19.2 ± 1.2
		0.85 ± 0.25	0.94 ± 0.27	1.9 ± 0.44	2.15 ± 0.26
		0.983	0.983	0.978	0.994
NaOH-rice husk		2.9 ± 0.3	1.5 ± 0.1	1.5 ± 0.1	1.4 ± 0.6
		19.4 ± 1.7	20.4 ± 1.3	21.8 ± 1.8	26.2 ± 1.3
		1.64 ± 0.16	2.31 ± 0.37	2.16 ± 0.39	2.58 ± 0.59
		0.996	0.989	0.987	0.976
UA-rice husk		16.5 ± 1.2	6.8 ± 1.9	1.9 ± 0.5	1.3 ± 0.1
		27.5 ± 1.3	26.2 ± 1.3	21.5 ± 2.1	20.1 ± 1.4
		0.89 ± 0.21	1.04 ± 0.11	1.82 ± 0.50	2.72 ± 0.52
		0.989	0.996	0.973	0.983
SCO2-rice husk		20.6 ± 0.2	11.2 ± 0.9	2.6 ± 0.4	1.9 ± 0.1
		21.8 ± 1.9	20.8 ± 1.3	20.2 ± 1.1	19.3 ± 1.0
		0.89 ± 0.20	0.94 ± 0.33	1.76 ± 0.28	2.37 ± 0.25
		0.990	0.972	0.991	0.995

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The concentration at half saturation (C_1/2) decreased with the temperature increase (Table 2), for all used adsorbents. This shows that at higher temperatures, the MB remaining in the solution is low, when the half saturation is attained. One major observation is that, for the NaOH-rice husk, the C_1/2 value is low, even for the lowest temperatures. This shows that, even at low temperatures, the NaOH-rice husk is a good adsorbent. The last Hill parameter is the adsorbed quantity at saturation (nN_M). In general this parameter presented no clear tendency. However, for NaOH-rice husk, the adsorbed quantity at saturation increased with the temperature, corroborating the abovementioned parameters.

The standard Gibbs free energy (ΔG⁰), enthalpy (ΔH⁰) and entropy (ΔS⁰) changes are shown in Table 3.

For all adsorbents, the ΔG⁰ values were negative, confirming that the adsorption was a spontaneous and favorable operation. Also, the temperature increase provided more-negative ΔG⁰ values, indicating that the adsorption was most favorable at higher temperatures. More-negative ΔG⁰ values were found for NaOH-rice husk, showing that the use of this adsorbent was the most favorable. The ΔH⁰ values were positive, indicating that the adsorption was endothermic in nature. The magnitude of ΔH⁰ is typical of physisorption (Khalfaoui et al. 2003; Franco et al. 2015). TΔS⁰ contributed more than ΔH⁰ to provide negative ΔG⁰ values. This shows that the MB adsorption was an entropy-controlled process.

In this work, techniques for surface modification were applied on the rice husk, in order to improve its adsorption potential regarding MB dye. FT-IR, XRD, SEM, and EDS analyses showed that the silica was dragged from the rice husk to the solution and the organic fraction was exposed, creating new adsorption sites. The treatment with NaOH provided the most pronounced surface modification. Regarding to the MB adsorption, Langmuir and Hill models were adequate to represent the equilibrium data. The MB adsorption was favored by the temperature increase. The maximum adsorption capacities for the MB solutions, estimated from the Langmuir model at 328 K, were in the following order: NaOH rice-husk (65.0 mg g⁻¹) > UA-rice husk (58.7 mg g⁻¹) > SCO₂-rice husk (56.4 mg g⁻¹) > raw rice husk (52.2 mg g⁻¹). The thermodynamic parameters demonstrated that the MB adsorption was a spontaneous, favorable, endothermic and entropy-controlled process. The magnitude of ΔH⁰ is in agreement with physisorption. Based on these results, it is possible to affirm that mainly the NaOH treatment is an alternative to improve the potential of rice husk as adsorbent.

The authors would like to thank CAPES (Brazilian Agency for Improvement of Graduate Personnel), CNPq (National Council of Science and Technological Development) and FAPERGS for the financial support.