Assessing the Adsorptive and Photodegradative Efficiencies of ZSM-11 Synthesized from Rice Husk Ash
Rice husk was used to synthesize zeolite (ZSM-11). FTIR and X-ray diffraction methods were used to characterize the product. The synthesized zeolite was used to treat underground water from some communities in Cape Coast considering parameters such as total dissolved solids, total hardness, conductivity, nitrate, and phosphate. The percentage reduction in was 96.1% in Ebubonko and 92.5% in Apewosika. Similarly, the levels also decreased significantly in Kwaprow. The adsorption capability was also determined by using it to remove Pb2+ and Zn2+ from laboratory prepared solutions with varying masses. The percentage reduction recorded 90.57% and 86.61% for the 1.0 g whilst the 1.5 g showed 93.26% and 89.36%, respectively. It was also realized that the adsorption process followed a pseudo-first-order rather than the pseudo-second-order process with their R2 values of 0.9929 and 0.8503 for the pseudo-first-order and 0.9662 and 0.6912 for the second-order for Pb2+ and Zn2+, respectively. The adsorption capacity also favored the Freundlich isotherm with R2 values of 0.7578 and 0.642 rather than Langmuir isotherm with R2 values of 0.1742 and 0.3856 for Pb2+ and Zn2+, respectively. The photodegradation ability of the synthesized zeolite was analyzed using rhodamine blue (RhB) and methyl orange (MO). The process was realized to favor the pseudo-second-order with R2 values of 0.9986 and 0.0007 and a constant K2 of 0.035 and 0.021 for RhB and MO, respectively, whereas the pseudo-first-order showed an R2 value of 0.9376 and 0.9757 with K1 values of 0.03 and 0.02.
Zeolites are crystalline aluminosilicate group materials that have pores and frames in molecular dimensions (less than 2 nm), composed of tetrahedral units SiO44− and AlO44− interconnected with each other . They are also described as crystalline microporous aluminosilicates built up of a 3-dimensional framework of [SO4]4− and [AlO4]5− tetrahedral linked by strong oxygen atoms . They 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, , adsorbents , 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  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) . 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 . Heavy metals are generally considered to be those whose density exceeds 5 g cm−1 . Presently, the heavy metals contamination in drinking water is being investigated by many scientists in both the natural and anthropogenic sources [12–14]. They 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. . 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. They 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. These effluents end up in water bodies which cause devastating effects on the environment and human well-being. There is, therefore, the need to research into dyes removal from wastewaters . The 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]. These 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  and although these processes are commonly used, they do not eliminate the waste, involve high operating costs, and can generate toxic secondary pollution . 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  as it involves simple design, low-cost adsorbents, and treatment cost, and mild conditions are criteria for selection of best mode and adsorbent . 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  and have been proven to be of interest due to its efficiency in degrading recalcitrant organic compounds . It consists of processes such as thermic plasma , Fenton radiation , photocatalysis, sonolysis, and ozonization  and combinations of these are increasingly adopted in the destruction of organic contaminants, due to their high efficiency, simplicity, good reproducibility, and easy handling . 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 . Polyaniline/CoFeC6N6 nanocomposite has been used for the sorption of Ni (II) and Pb (II) as investigated by Moazeai et al., 2018 . Cu (II), Cd (II), and Pb (II) present in industrial effluents have been removed using Moringa stenopetala seed husk by Temesgen et al., 2018 . 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. The 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.
2. Materials and Methods
2.1. Preparation of Rice Husk into Rice Husk Ash
The 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 . It was washed with water to remove dirt, rinsed with distilled water, and dried in an oven at a temperature of 100°C overnight. The metallic impurities were removed with hydrochloric acid . 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).
2.2. Synthesis of Zeolite from Rice Husk Ash
The RHA was mixed with aqueous NaOH, heated at 100°C for 1 hour, and filtered to produce sodium silicate (Na2 SiO3) 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. The 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.
2.3. Zero Point Charge of Synthesized Zeolite
The 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. This 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. The point of zero charge of the synthesized zeolite was determined using the potentiometric mass titration process as applied by Bourikas et al., 2003 .
2.4. 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.
2.4.1. 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. The concentrations of major ions nitrate and phosphate were determined spectrophotometrically in the laboratory using portable HACH DR/890.
2.4.2. 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. The physicochemical parameters were measured after the treatment period and recorded. Laboratory prepared concentrations of lead (Pb2+) and zinc (Zn2+) 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. The 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 The efficiency of the degradation process was calculated using the formulae where = degradation efficiency, = initial dye concentration, and = final dye concentration.
2.5. Determination of Percentage Removal/Reduction
The percentage reduction of the concentrations of the groundwater sample was calculated using the formulaewhere = percentage removal/reduction of parameters/metal ion from solution, = initial concentration (mg/L) of metal ions/in solutions, and = final concentration (mg/L) of metal ion in solution.
2.6. Kinetic Studies
2.6.1. Sorption Kinetics
The sorption kinetics of the process was determined using the equations which were used by Anang et al.  in their research work on Chrysophyllum albidum. The pseudo-first-order kinetic model equation is given as
After this equation has been integrated using the boundary conditions t = 0: = 0, and t = t, = , the equation becomes
From the pseudo-first-order model equation (3) above, and become the mass ratios of lead (Pb) and zinc (Zn) adsorbed at equilibrium at time t (mg/g), respectively, t is the contact time (min), and K1 is the pseudo-first-order rate constant (/min). A plot of equation (4) would give a straight-line of log against t with log as the intercept and the slope as −.
The pseudo-second-order equation is also given as
Using the same boundary conditions as in (2), the integration will give this equation
This will also give a straight-line plot of in (mg/g) against t (time.) with as the slope and as the intercept and k2 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 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.
Equations (7) and (8) can be linearized to give the following equations:where the Langmuir parameters are given by (mg Zn2+ g−1 or mg Pb2+ g−1) and KL (Lmg−1 Zn2+ or L mg−1 Pb2+) is the maximum capacity adsorption at high equilibrium concentrations and the equilibrium constant. The Freundlich parameters KF (mg g−1 Zn2+ or (mg g−1 Pb2+ and are the Freundlich capacity factor and the Freundlich intensity parameter, respectively .
3. Results and Discussion
3.1. Fourier Transform Infrared Analysis of the Synthesized Zeolite
The 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  and a very weak peak at 1630 cm−1. The 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 .
3.2. X-Ray Diffraction Analysis
Analysis of Figure 3 shows that it has some dominant peaks which were observed at = 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. , a product identified as Zeolite Socony Mobil-11 (ZSM-11), which has a quartz phase  shown in blue at = 21.56°, 21.56°, 26.96°, and 50.91°. The ZSM-11 phase was also designated as green was dominant at 8.09° as also reported by .
3.3. Point of Zero Charge
The mechanism of adsorption depends on the pH of the solution and the pHPZC of the zeolite . The 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 . The 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.
3.4. Percentage Removal/Reduction
3.4.1. Analysis of Water Samples
The 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.
The percentage reduction in the nitrate and phosphate concentration was presented graphically in Figures 5 and 6. The 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. The 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%).
3.4.2. Analysis of Pb2+ and Zn2+ Removal/Reduction
The effect of the mass of adsorbent with time on the percentage removal of Pb2+ and Zn2+ 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. The maximum percentage removal/reduction of Zn2+ and Pb2+ achieved was 90.57% and 86.61%, respectively, after 4 hours of treatment.
Considering an increase in the mass of adsorbent (1.5 g), the percentage reduction calculated for Zn2+ and Pb2+ 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.
3.4.3. Kinetics of the Reduction Process
The pseudo-first-order kinetics was determined to assess the feasibility of the reaction.
The pseudo-first-order reaction kinetics is graphically presented in Figure 9. The R2 values for the reaction were found to be 0.9929 and 0.8503, respectively, for Zn2+ and Pb2+ with their pseudo-first-order adsorption rate constants K1 (min−1) values of 0.006909 min−1 and 0.010594 min−1 as shown in Table 3. This implies that the reduction in the concentration of Zn2+ favors the first-order reaction compared to Pb2+.
The pseudo-second-order kinetics of the process was also determined and the R2 values were 0.9662 and 0.6912, respectively, for Zn2+ and Pb2+ as depicted in Table 4. Their pseudo-second-order adsorption reaction constants K2 (g·mg−1·min−1) were 0.003255 and 0.000446 for Zn2+ and Pb2+.
The values for the pseudo-second-order reaction are also graphically presented in Figure 10.
The adsorption isotherms for Langmuir and Freundlich models for zinc and lead were also evaluated to assess the adsorption capacity of the zeolite. This 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, the R2 values for Zn2+ and Pb2+ were derived to be 0.7576 and 0.7183. It was realized that the values for Zn2+ and Pb2+ are 0.795 and 0.642, respectively, and these values are less than one (1/n < 1).
The Langmuir isotherm model for the adsorption process gave R2 values of 0.1742 and 0.3350 for Zn2+ and Pb2+, respectively, as seen in Figure 12. These R2-values are very low and together with other factors such as the average of the triplicate data and the sorption mechanism may have contributed to the negative values of .
The adsorption isotherm becomes favorable for < 1 (or n > 1), unfavorable for > 1 (or < 1), and linear or n = 1 . This implies that the adsorption of zinc and lead follows the Freundlich model and not the Langmuir model and the low values of RL and RF indicate high and favorable adsorption of adsorbate onto adsorbates as determined by Belhachemi et al., 2011 .
3.5. Photodegradation Process
3.5.1. Mechanism of Degradation
The mechanism for the photodegradation of rhodamine blue and methyl orange dyes was studied and observed to have occurred under two different mechanisms. The 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 
The excited electron leaves a corresponding hole which is highly oxidative. The photoinduced hole (h+) and the formed hydroxyl radical (OH−) oxidize the dye
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. The 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.
3.5.2. Kinetics of Degradation
The kinetics of the dye degradation process was monitored by their pseudo-first- and second-order reactions.
From Figure 16, it was realized that the R2 values were 0.9376 and 0.9737, respectively, for methyl orange and rhodamine blue. The pseudo-first-order degradation constant (K1) was observed to be 0.03 and 0.02 with the calculated values to be 1.303 and 1.13, respectively, for rhodamine blue and methyl orange and experimental values of 1.38 and 1.06, respectively, as shown in Table 6. This shows a good relationship between the calculated and experimental values. It also indicates that the adsorption process was very effective.
Similarly, the pseudo-second-order also showed degradation constant of 0.035 and 0.021 for rhodamine blue and methyl orange, respectively. The mass ratio of the dyes at equilibrium was 1.50 and 1.23 as shown in Table 7.
The pseudo-second-order reaction constant (K2) was found to be 0.035 and 0.021, respectively, for rhodamine blue and methyl orange with R2 values of 0.9986 and 0.9997 for rhodamine blue and methyl orange as shown in Figure 17.
This implies the photodegradation of rhodamine blue and methyl orange using laboratory synthesized ZSM-11 under sunlight favors the pseudo-second-order kinetic process.
ZSM-11 was synthesized using rice husk and was characterized using the FTIR and the X-ray diffraction. The 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, Pb2+ and Zn2+, from laboratory prepared aqueous solutions. The 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.
All the data concerning this research work are with the authors.
Conflicts of Interest
There are no conflicts of interest regarding this research work.
The authors want to acknowledge the role of the technicians at the Chemistry Laboratory of the Department of Chemistry at the University of Cape Coast. This research work was funded by the authors involved.
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