Assessing the Adsorptive and Photodegradative Efficiencies of ZSM-11 Synthesized from Rice Husk Ash

University of Cape Coast, College of Agriculture and Natural Sciences, School of Physical Sciences, Department of Chemistry, Industrial Chemistry Unit, Cape Coast, Ghana University of Cape Coast, College of Agriculture and Natural Sciences, School of Physical Sciences, Department of Chemistry, Cape Coast, Ghana University of Cape Coast, College of Agriculture and Natural Sciences, School of Physical Sciences, Department of Physics, Cape Coast, Ghana


Introduction
Zeolites are crystalline aluminosilicate group materials that have pores and frames in molecular dimensions (less than 2 nm), composed of tetrahedral units SiO 4 4− and AlO 4 4− interconnected with each other [1]. ey are also described as crystalline microporous aluminosilicates built up of a 3dimensional framework of [SO 4 ] 4− and [AlO 4 ] 5− tetrahedral linked by strong oxygen atoms [2]. ey are a group of hydrated aluminosilicates of alkali or alkali Earth metals: principally sodium, potassium, magnesium, barium, lithium, and calcium [3,4] containing pores and channels of molecular dimensions that are used widely in industry as ion-exchange resins, [5], adsorbents [6], and catalysts. ZSM-11 is a shape-selective catalyst that has unique channel structures with 10-membered ring openings and rather unusual catalytic properties [7,8] having a similar structure of ZSM-5 [8] and therefore having important values in the petrochemical industry.
ZSM-11 is a MEL type zeolite that has high silica content and has similarities with zeolite type MFI (ZSM-5) [9]. Tetrabutylammonium hydroxide (TBAOH) is an organic template commonly used for the synthesis of ZSM-11 but the silica source is relatively expensive and environmentally unfriendly [10]. Heavy metals are generally considered to be those whose density exceeds 5 g cm − 1 [11]. Presently, the heavy metals contamination in drinking water is being investigated by many scientists in both the natural and anthropogenic sources [12][13][14].
ey are known to be very toxic and can accumulate in the living organism and are nonbiodegradable as well. Lead and zinc are two important heavy metals that are well-known toxics, used in metallurgy, tannery manufacturing, pulp and paper industries, metal products, paints and varnishes, battery manufacturing industries, galvanizing plants, and so forth. [15]. Zinc can travel through the food chain via bioaccumulation and becomes harmful and can cause anemia, pancreatic damage, protein metabolism disturbance, arteriosclerosis, and respiratory disorders. Synthetic dyes are mostly used by textile, food, plastic, paper, printing, pharmaceutical, and cosmetic industries.
ey are water-soluble and as such become a common organic pollutant in industrial wastewater. Large volumes of these cationic dyes are consumed by the textile industries and most of them usually discharge their effluents into drains. ese effluents end up in water bodies which cause devastating effects on the environment and human well-being. ere is, therefore, the need to research into dyes removal from wastewaters [16]. e conventional methods for treating wastewater include chemical precipitation, coagulation-flocculation, floatation, filtration, ion-exchange, reverse osmosis, membrane filtration, evaporation recovery, and electrochemical technologies [17,18]. ese conventional techniques have their inherent limitations such as less efficiency, sensitive operating conditions, and production of secondary sludge and further the disposal is a costly affair [19] and although these processes are commonly used, they do not eliminate the waste, involve high operating costs, and can generate toxic secondary pollution [20]. Adsorption is universally accepted and the most widely used method of water treatment as it involves the least sophisticated instrumentation with easy operations and can be operated with a wide range of adsorbents [21] as it involves simple design, low-cost adsorbents, and treatment cost, and mild conditions are criteria for selection of best mode and adsorbent [22]. Photodegradation is an emerging branch of advanced oxidation technologies used for water and wastewater treatment that has been widely documented for its ability to degrade indiscriminately almost all water pollutants such as dyes [20] and have been proven to be of interest due to its efficiency in degrading recalcitrant organic compounds [23]. It consists of processes such as thermic plasma [24], Fenton radiation [25], photocatalysis, sonolysis, and ozonization [26] and combinations of these are increasingly adopted in the destruction of organic contaminants, due to their high efficiency, simplicity, good reproducibility, and easy handling [27]. Synthetic zeolites obtained from industrial, municipal, or agricultural waste materials especially rice husk ash (RHA) and coal fly ash (CFA) have huge potential as a cost-effective environmentally friendly solution that can improve the efficiency of wastewater treatment [28]. Polyaniline/CoFeC 6 N 6 nanocomposite has been used for the sorption of Ni (II) and Pb (II) as investigated by Moazeai et al., 2018 [29]. Cu (II), Cd (II), and Pb (II) present in industrial effluents have been removed using Moringa stenopetala seed husk by Temesgen et al., 2018 [30]. In this research work, Zeolite (ZSM-11) was synthesized from RHA and was used for the treatment of groundwater samples from some communities within the Cape Coast metropolis in Ghana. It was also used for the removal of zinc (Zn) and lead (Pb) from a laboratory prepared solution. e synthesized ZSM-11 was also used to explore the degradation characteristics of rhodamine blue and methyl orange. Figure 1 shows the chemical structures of the dyes used in this research work.

Preparation of Rice Husk into Rice Husk Ash.
e pretreatment process of the rice husk can be considered in stages involving mechanical, thermal, and chemical. Dry raw rice husk (RH) was sieved to eliminate residual rice and clay particles [31]. It was washed with water to remove dirt, rinsed with distilled water, and dried in an oven at a temperature of 100°C overnight. e metallic impurities were removed with hydrochloric acid [32]. Rice husk was acid leached with 3 M HCl for 24 hours, acidified, washed with distilled water, and oven-dried at 100°C. It was then calcined in a furnace (Nabertherm model) at 750°C for 6 hours to eliminate incorporated organic matter to obtain the rice husk ash (RHA).

Synthesis of Zeolite from Rice Husk Ash.
e RHA was mixed with aqueous NaOH, heated at 100°C for 1 hour, and filtered to produce sodium silicate (Na 2 SiO 3 ) solution. Bauxite was pretreated and an appropriate amount was weighed and digested in 4 M NaOH solution and heated at 150°C for 4 hours in an electrical oven to obtain sodium aluminate solution. A calculated portion of the sodium aluminate was mixed and stirred vigorously for 1 hour. e resulting gel was homogenized and transferred into a Teflon autoclave and crystallized at 110°C for 24 hours without stirring or agitation for the crystals to precipitate, be filtered, and be dried.

Zero Point Charge of Synthesized Zeolite.
e pH at which the sorbent surface charge takes a zero value is defined as point of zero charge (pHpzc) where the charge of the positive surface sites is equal to that of the negative ones.
is information allows the researchers to evaluate the ionization of functional groups and their interaction with metal species in solution. If the pH of the solution is higher than pHpzc, the sorbent surface is negatively charged and could interact with the metal positive species and when the pH is lower than pHpzc, the solid surface is positively charged and could interact with negative species. e point of zero charge of the synthesized zeolite was determined using the potentiometric mass titration process as applied by Bourikas et al., 2003 [33].

Groundwater Sampling.
Groundwater samples were taken from six communities within the Cape Coast Metropolis, in the central region of Ghana, namely, Abura, Kwaprow, Apewosika, Ammamoma, Ebubonko, and Amoyaw. Physicochemical parameters of the samples such as turbidity, total dissolved solids, total hardness, conductivity, nitrate, phosphate, chloride, and sulfate were determined.

Sample Preparation.
Groundwater samples were taken from the bottom of the wells, which had average depths between 15 m and 20 m into 500 ml bottles which have been washed and rinsed with deionized water. It was kept in a cool box with a temperature below 4°C before transporting to the laboratory for analysis. On-site analyses of temperature, electrical conductivity, total dissolved solids (TDS), and pH were conducted using portable HACH conductivity meter and Metrolin model 691-pH meter; alkalinity was also determined on-site using HACH Digital Multisampler Titrator Model 1690. e concentrations of major ions nitrate NO 3− and phosphate PO 3− 4 were determined spectrophotometrically in the laboratory using portable HACH DR/890.

Laboratory Analysis
(1) Sample Treatment. 1 g mass of the synthesized zeolite was added to 100 ml of the groundwater samples and stirred for two (2) hours using the magnetic stirrer. e physicochemical parameters were measured after the treatment period and recorded. Laboratory prepared concentrations of lead (Pb 2+ ) and zinc (Zn 2+ ) solutions were prepared and a mass of the zeolite was added. It was also placed on a magnetic stirrer and, under batch conditions with consistent time intervals of 30 min, 10 ml was taken and centrifuged. e final concentration of the metal solution was determined using the Atomic Absorption Spectrophotometer (AAS).

(2) Photodegradation of Rhodamine Blue and Methyl Orange.
Methyl orange and rhodamine blue were chosen as representative dye contaminants to help evaluate the photodegradation activity of ZSM-11 with absorption band at 464 nm and 530 nm under sunlight. 1 g of the synthesized ZSM-11 was dissolved in 100 ml of the dye solution and stirred for almost 30 min in the dark to enable the establishment of an adsorption-desorption equilibrium between ZSM-11 and the MO or RhB. It was, then, placed under sunlight and after some 30 min intervals, the samples were taken on a batch basis and centrifuged to separate the zeolite before analyzing with the UV-Vis 1600 Spectrophotometer e efficiency of the degradation process was calculated using the formulae [34] %η where %η � degradation efficiency, C o � initial dye concentration, and C f � final dye concentration.

Determination of Percentage
Removal/Reduction. e percentage reduction of the concentrations of the groundwater sample was calculated using the formulae where R% � percentage removal/reduction of parameters/ metal ion from solution, C o � initial concentration (mg/L) of metal ions/in solutions, and C t � final concentration (mg/L) of metal ion in solution.

Kinetic Studies
e sorption kinetics of the process was determined using the equations which were used by Anang et al. [35] in their research work on Chrysophyllum albidum. e pseudo-first-order kinetic model equation is given as After this equation has been integrated using the boundary conditions t � 0: q t � 0, and t � t, q t � q t , the equation becomes From the pseudo-first-order model equation (3) above, q e and q t become the mass ratios of lead (Pb) and zinc (Zn) Journal of Chemistry adsorbed at equilibrium at time t (mg/g), respectively, t is the contact time (min), and K 1 is the pseudo-first-order rate constant (/min). A plot of equation (4) would give a straightline of log (q e − q t ) against t with log (q e ) as the intercept and the slope as − (K 1 /2.303). e pseudo-second-order equation is also given as Using the same boundary conditions as in (2), the integration will give this equation is will also give a straight-line plot of (t/q t ) in (mg/g) against t (time.) with (1/q e ) as the slope and (1/K 2 qe 2 ) as the intercept and k 2 is the second-order rate constant for the adsorption process.
Determination of the adsorption capacity of the zeolite for the ions in solution at various equilibrium concentrations (C e ) was also achieved by evaluating the adsorption isotherm, commonly described by the Langmuir and/or Freundlich models which are represented by equations (7) and (8), respectively.
Q e � K F C e 1/n .

Fourier Transform Infrared Analysis of the Synthesized
Zeolite. e FTIR analysis of the synthesized zeolite is shown in Figure 2. It has zeolitic vibrational bands [37,38] with zeolite functional group which is present as a result of the bending vibrational mode of zeolitic water [39] and a very weak peak at 1630 cm − 1 . e strong peak at 1052 cm − 1 shows asymmetric vibrations of the internal T-O tetrahedron with a weak symmetric T-O stretching vibration recorded at 797 cm − 1 similar to that observed by Katrick et al. and Zhang et al. [40,41]. However, there was a very strong peak at 445 cm − 1 as a result of the O-T-O bending vibration which confirms the presence of a five-membered structure as discussed by Sundraramurthy et al., 2000 [42]. Figure 3 shows that it has some dominant peaks which were observed at 2θ � 8.09°, 18.06°, 19.78°, 26.96°, 21.56°, 16.56°, 26.96°, and 50.91°, respectively, which is similar to that synthesized by Anang et al. [43], a product identified as Zeolite Socony Mobil-11 (ZSM-11), which has a quartz phase [44] shown in blue at 2θ � 21.56°, 21.56°, 26.96°, and 50.91°. e ZSM-11 phase was also designated as green was dominant at 8.09°as also reported by [45].

Point of Zero Charge.
e mechanism of adsorption depends on the pH of the solution and the pH PZC of the zeolite [46]. e surface is positively charged at pH < pHpzc favoring the adsorption of anionic species and negatively charged at pH > pHpzc, which will favor the adsorption of cationic species [47]. e pHpzc of the adsorbent was found to be around 7 as shown in Figure 4 which is similar to the activated carbon used by Belhachemi et al., 2011.

Analysis of Water Samples.
e water samples taken from the various communities were analyzed and the result is presented in Table 1.
From Table 1, the result showed that most of the parameters measured were within the values for GSA except sulfate and phosphate. After two (2) hours of treatment with the synthesized zeolite, the change in values for the two parameters is presented in Table 2 and their percentage removal/reduction was calculated and graphically presented in Figures 5 and 6. e percentage reduction in the nitrate and phosphate concentration was presented graphically in Figures 5 and 6. e percentage reduction in nitrate concentration as shown in Figure 4 was very high for Amoyaw followed by Apewosika, Amamoma, Kwaprow, Ebubonko, and Abura with values of 76.2%, 63.2%, 60.5%, 60.4%, 40%, and 36.7%, respectively. e phosphate concentration also showed a reduction as shown in Figure 5 with the highest percentage reduction with Ebubonko recording a 96.1% reduction followed by Abura (95.7%), Amoyaw (93%), Amamoma (93%), and Apewosika (92.5%). 2+ and Zn 2+ Removal/Reduction. e effect of the mass of adsorbent with time on the percentage removal of Pb 2+ and Zn 2+ concentrations is shown in Figures 7 and 8. In Figure 7, the mass of adsorbent used was 1.0 g with a treatment time of 5 hours. e maximum percentage removal/reduction of Zn 2+ and Pb 2+ achieved was 90.57% and 86.61%, respectively, after 4 hours of treatment.

Analysis of Pb
Considering an increase in the mass of adsorbent (1.5 g), the percentage reduction calculated for Zn 2+ and Pb 2+ was 93.26% and 89.36%, respectively, as shown in Figure 8 and the maximum reduction was also recorded at after 4 hours of treatment.
It can, therefore, be concluded that an increase in the mass of the adsorbent caused an increase in the percentage removal of the ions in solution.

Kinetics of the Reduction Process.
e pseudo-firstorder kinetics was determined to assess the feasibility of the reaction. e pseudo-first-order reaction kinetics is graphically presented in Figure 9. e R 2 values for the reaction were found to be 0.9929 and 0.8503, respectively, for Zn 2+ and Pb 2+ with their pseudo-first-order adsorption rate constants K 1 (min − 1 ) values of 0.006909 min − 1 and 0.010594 min − 1 as shown in Table 3.
is implies that the reduction in the concentration of Zn 2+ favors the first-order reaction compared to Pb 2+ . e pseudo-second-order kinetics of the process was also determined and the R 2 values were 0.9662 and 0.6912, respectively, for Zn 2+ and Pb 2+ as depicted in Table 4. eir pseudo-second-order adsorption reaction constants K 2 (g·mg − 1 ·min − 1 ) were 0.003255 and 0.000446 for Zn 2+ and Pb 2+ . e values for the pseudo-second-order reaction are also graphically presented in Figure 10. e adsorption isotherms for Langmuir and Freundlich models for zinc and lead were also evaluated to assess the adsorption capacity of the zeolite. is can be seen in Table 5 which shows the summary of the adsorption isotherm parameters. Table 5 gives a summary of the adsorption isotherms of zinc and lead to the surface of the synthesized zeolite. Considering the Freundlich model as represented in Figure 11    e adsorption isotherm becomes favorable for (1/n) < 1 (or n > 1), unfavorable for (1/n) > 1 (or < 1), and linear or n � 1 [36]. is implies that the adsorption of zinc and lead follows the Freundlich model and not the Langmuir model and the low values of R L and R F indicate high and favorable adsorption of adsorbate onto adsorbates as determined by Belhachemi et al., 2011 [46].

Mechanism of Degradation.
e mechanism for the photodegradation of rhodamine blue and methyl orange dyes was studied and observed to have occurred under two different mechanisms. e first process starts with the illumination of the zeolite which results in the excitation of a photon with energy greater than the bandgap between the valance and conduction bands as shown by [48] MO RB e excited electron leaves a corresponding hole which is highly oxidative. e photoinduced hole (h+) and the formed hydroxyl radical (OH− ) oxidize the dye e second step is the sensitization where the dyes undergo a radicalization process to give mineralized products [49,50]. ese two mechanisms can be shown in Figure 13. e drastic change in concentration of dye with time was very sharp for the first 30 min of the degradation time for RhB compared to MO as seen in Figures 14 and 15. Figure 14 shows that there was a sharp decrease in the concentration of rhodamine blue compared to methyl orange within the first 30 min by 61% to 38.3%, respectively, as shown in Figure 15. e changes in the concentrations continued with a further decrease from 75% to 90% after the 60 min to the 180 min for rhodamine blue and 47.73% to 67.3%, respectively.

Kinetics of Degradation.
e kinetics of the dye degradation process was monitored by their pseudo-firstand second-order reactions.
From Figure 16, it was realized that the R 2 values were 0.9376 and 0.9737, respectively, for methyl orange and rhodamine blue.
e pseudo-first-order degradation constant (K 1 ) was observed to be 0.03 and 0.02 with the q e calculated values to be 1.303 and 1.13, respectively, for rhodamine blue and methyl orange and q e experimental values of 1.38 and 1.06, respectively, as shown in Table 6.  Similarly, the pseudo-second-order also showed degradation constant of 0.035 and 0.021 for rhodamine blue and methyl orange, respectively. e mass ratio (q e ) of the dyes at equilibrium was 1.50 and 1.23 as shown in Table 7.        e pseudo-second-order reaction constant (K 2 ) was found to be 0.035 and 0.021, respectively, for rhodamine blue and methyl orange with R 2 values of 0.9986 and 0.9997 for rhodamine blue and methyl orange as shown in Figure 17.
is implies the photodegradation of rhodamine blue and methyl orange using laboratory synthesized ZSM-11 under sunlight favors the pseudo-second-order kinetic process.

Conclusion
ZSM-11 was synthesized using rice husk and was characterized using the FTIR and the X-ray diffraction. e point of zero charge of the zeolite was also determined. It was used to treat underground water samples from some communities within the Cape Coast metropolis in the central region of Ghana. It was also used to remove some heavy metals, namely, Pb 2+ and Zn 2+ , from laboratory prepared aqueous solutions. e synthesized zeolite was also used to photodegrade rhodamine blue and methyl orange. In all these applications, it was found to be very effective and can be used industrially when scaled-up.

Data Availability
All the data concerning this research work are with the authors.

Conflicts of Interest
ere are no conflicts of interest regarding this research work. Rhodamine blue (mg/L) Methyl orange (mg/L) Figure 17: Pseudo-second-order degradation reaction of rhodamine blue and methyl orange.