Novel Wastewater Treatment by Using Newly Prepared Green Seaweed-Zeolite Nanocomposite.

A dependent step-by-step study that included experimental and field study was applied to explore the simplest and most effective system that could be applied for adsorption of Congo Red (CR) dye from the effluent of wastewater that comes out from different industries. Zeolite (Z) surface and pores were subjected to a modification process using green seaweed (GS) algae. Thereafter, each Z, GS, and composite from both were evaluated based on the adsorption efficacy to clean up CR dyes from aqueous solutions. A wet impregnation method was followed to fabricate the zeolite/algae (ZGS) nanocomposite which was characterized using the most appropriate characterization techniques. Batch experiments were selected to be the method of choice in order to follow up the performance of the adsorption process versus different practical variables. Moreover, dye adsorption kinetics and isotherms were investigated as well. At lowered concentrations of CR, the novel nanocomposite ZGS revealed more efficacy than its counterparts, Z and GS, in terms of the adsorption capacity. The maximum adsorption capacities were found to be 8.10, 10.30, and 19.70 mg/g for Z, GS, and ZGS, respectively. Laboratory tests confirmed that the novel nanocomposite ZGS could be introduced as a new and economical nanoadsorbent to capture and remove negatively charged dyes from wastewater effluents that come out from industries at lower concentrations of CR dye and analogous compounds. The dye adsorption on GS, Z, and ZGS coincide with the pseudo-first, Langmuir isotherm, and second-order models. Evaluation for the sorption mechanism was conducted using a diffusion model known as Weber's intraparticle. Depending on the last findings, field experiments on removing dyes from industrial wastewater revealed optimistic findings as the efficiency of our modern and eco-friendly nanoadsorbent reached 91.11%, which helps in the reuse of industrial wastewater.

Introduction

According to the latest environmental estimates, the production of dyes all over the world exceeded 1.3 million tons annually. Approximately, 10 000 different types of these dyes are used in more than one industrial product, which in turn resulted in a continuous discharge of these pollutants into effluent wastewater coming out from these industries.[1] According to an estimate, about 40–65 L of wastewater is generated to produce 1 kg fabric. Thus, a considerable amount of wastewater including dye is directly discharged into streams and rivers through aqueous effluent.[2] Small quantities of some dyes are toxic and have to be either eliminated or controlled at a definite concentration. Generally, such dyes could be classified into three main species: anionic, acid, direct, and reactive dyes; cationic, basic dyes; and nonionic, dispersed dyes.[3] The anionic type of Azo-dye contains several compounds from different classes of dyes. Upon ingestion it possesses high toxicity, moreover, could cause irritation in the eye, mucous membrane, skin, and upper respiratory tract; in addition to nausea, severe headaches, and waterborne diseases.[1,4,5] A category of dyes that are widely used in different dyeing processes is known as azo dyes. Because of their higher activity, they generally represent nearly 50 to 70% of the total quantities of dyes directed toward each of plastic, textile, and paper industries.[6] One of those dyes is the anionic type of diazo-benzidine known as Congo red. The latter is very familiar with its high resistance to being biodegraded because of its complex aromatic structure. Congo red (CR) is famous because of its metabolized product, which is known as benzidine. This is well known to have certain effects such as allergic symptoms as well as human carcinogenicity that is potentially dangerous upon bioaccumulation.[7] Not only humans but also many organisms are affected by Congo red dye, where both mutagen and carcinogen are detected,[8] indicating strongly the need to remove such kind of dyes. Different approaches, such as electrochemical, reverse osmosis, coagulation, membrane separation process, dilution, flotation, filtration, photocatalysis, softening, and adsorption technologies have been used for this purpose.[1,2,9−22] In comparison with the above-mentioned techniques, the most convenient technique is the adsorption technique because it is simple, inexpensive, easy to handle, requires less maintenance, and the amounts of sediment are smaller than that produced from other methods.[23−29] In the last decades, agricultural wastes, biomass waste, algae, fly ash, functionalized mesoporous silica materials, and clay minerals have been used as efficient, less expensive, adsorbents for the elimination of dye,[30−38] as well as heavy metals,[39−41] from wastewater. A novel candidate between these sorbents that is strongly recommended by several researchers is aquatic plant biomass or fern. The latter category is introduced as biosorbents to remove hazardous contaminants that pollute wastewater. Enhanced morphological properties, availability in more than sufficient quantities in the aquatic regions, affordable preparation prices, wide safety margin nature, and efficiency are all properties that make this kind of sorbents preferable to others.[42] Furthermore, the presence of multifunction groups such as carboxylic, amino, hydroxyl, carbonyl, phosphate, and sulfonic groups inspire contaminants to attach to the wall of the biomaterial.[43] Zeolite also has several and vital applications: wastewater treatment,[44] as a catalyst in different chemical processes, potent antimicrobial agent, separation process involving membranes, paint, pulp, construction, refractories, and ceramic purposes, and industries involving paper, plastic, in addition to the traditional application of zeolite for water softening aspects.[44−47]

In our work, a dependent step-by-step study including experimental and field study was applied to examine and explore the most adequate catalysis system that cleans-up industrial wastewater effectively from waste dyes, especially CR dye. An estimation was applied for the capability of Z, GS, and ZGS nanocomposite in terms of adsorption performance. The last-mentioned estimation was applied for CR dye removal from wastewater under various experimental conditions to explore the performance of Z after being doped with GS in terms of adsorption capability. Although, Z and GS are reported many times in the literature and are not considered a new type of adsorbents, the innovation in this paper lies in the impact of using natural algae, particularly GS as a dopant for Z in terms of adsorption performance, as well as the simple mixing and stirring without the use of a chemical process or traditional nanostructuring methods to prepare a nanostructured ZGS composite. GS and Z were chosen for this study because they are low-cost natural adsorbents. Moreover, the regeneration costs of each of Z, GS, and ZGS for reusability are much lower than that of many other reported methods, which could be a significant indicator for industrial applications. The batch mode experiment was selected as a model to investigate the effect of different variables such as starting nanoadsorbent doses, CR concentrations, reaction times and temperatures, and pH values on CR dye elimination. Moreover, isotherms and adsorption kinetics were investigated.

Results and Discussion

Adsorbent Characterizations

Morphological Characterization

Figure reveals the SEM graphs of Z, GS, and ZGS adsorbent. In Figure A, the SEM image of zeolite revealed a rough surface, agglomerated regular rounded shaped particles, and different particle sizes, and the surface reveals pores and cavities. In Figure B the SEM image of GS exhibits a surface with less pores, showing the surface area for GS and adsorption capacity. Finally, Figure C reflects the morphological effects of the treatment of Z with GS. The SEM image of the nanocomposite reveals that the pores in the zeolite surfaces are nearly covered with the GS particles, and as a result agglomerated particles appear. The generation of ZGS nanocomposite may be established from the alterations in the morphological topographies of the nanocomposite as compared to those of Z and GS.

Figure 1

SEM images of (A) Z, (B) GS, and (C) ZGS adsorbents.

Data obtained from the DLS analysis (Figure S1(Supporting Information)) are used to calculate the particle size distribution and hydrodynamic diameter of the Z, GS, and ZGS particles. They have 82.5, 92.8, and 91.1 nm hydrodynamic diameters, in the same order. In addition, the Z, PG, and ZPG PET surface areas are found to be 91.6, 120.9, and 116.5 m2/g, in the same order.

XRD Characterizations

XRD charts of Z, GS, and ZGS adsorbents are illustrated in Figure A. Zeolite mineral’s main XRD peaks are revealed at 2θ of ∼9.85°, 22.41°, 26.15°, 26.84°, 28.12°, 30.075°, and 32.04° which are in line with the data reported by other researchers.[48,49] Also, the values of d-spacing in the main peaks of zeolite at 22.41° and 28.12° are 3.96809 and 3.17323 Å. The same figure reveals the main XRD peaks of GS which appeared at nearly 12.51°, 22.94°, 23.66°, 25.00°, and 29.30 o, and finally, the main peaks that characterize ZGS appeared at about 22.58°, 26.61°, 30.14°, and 68.29°. The newly synthesized composite exhibits an average crystallite size of about 19.5 nm. The latter was calculated using the Scherer equation, which confirms that the newly synthesized composite is of a nanostructure nature.

Figure 2

(A)XRD and (B) FTIR charts of Z, GS, and ZGS adsorbents.

FT-IR Analyses

Z, GS, and ZGS adsorbents exhibit FT-IR charts in Figure B. The FT-IR spectrum shows broad bands which displayed from 3452 to 3432 cm–1 and correspond to the stretching hydroxyl (OH) groups.[50−52] In the case of zeolite, Si–O vibration mode is revealed at 1029 cm–1 which is shifted to 1039 cm–1 in the case of ZGS.[53] While Si–O–Al and octahedral aluminum (Al–OH) showed peaks at 603 and 919 cm–1,[54] the peak at 464 cm–1 corresponds to the Si–O–Si bending of zeolite, which also shifted to 461 cm–1 in the case of ZGS.[54] Nearly, all peaks in the region from 400 to 800 cm–1 correspond to the metal oxides.[55]

GS exhibits FTIR bands appearing at 3787 and 3432 cm–1. These bands correspond to the stretching amine (−NH) group and the hydroxyl group (−OH) found in phenolic groups. Also, the presence of an alkyl (CH) group stretching mode is confirmed by the presence of the characteristic peak at 2915 cm–1, whereas the −C=O mode is characterized by the peak located at 1627 cm–1. Also, the peak located at 1425 cm–1 corresponds to the presence of the C–H mode.[32,56] Another peak is located at 1019 cm–1 which refers to either the sulfate group or the C–O bond.[57] Whereas, all modes located between 3300 and 3500 cm–1 are characterized for the presence of the N–H stretching mode of amines. The O–H stretching mode of a carboxylic group exhibits a band around 2915 cm–1.[58] Almost peak disappearance and peak shift coincide with data obtained from other characterization techniques in the same study, which confirms that a new compound has been formed. The wavelengths of the characteristic FT-IR bands for Z, GS, and ZGS adsorbents are listed in Table S1.

Factors Influencing the Adsorption Process

Influence of Initial CR Concentration

It is well-known that the percent of dye removed by the adsorption process is strongly affected by the starting concentration of the adsorbate. Figure panels A–C and D–F reveal, respectively, the variations in the removal percent, as well as the amount of CR, adsorbed regarding the time using nanoadsorbents Z, GS, and ZGS at different initial concentrations of CR dye. Elevated results were recorded during the first stage of the adsorption process for the amount of dye removed as well as the dye removal percent. Then after, there was a gradual decrease until equilibrium. Afterward, contact time had no distinguishable influence on the adsorption process. The existence of uncountable reachable active adsorption spots on the adsorbent’s surfaces is the main reason behind the elevated results recorded during the first stage of the adsorption process. During progress in contact time between adsorbent and adsorbate, these hot spots were subjected to a gradual and complete covering process by CR molecules. And hence, great and increased repulsions were raised between different CR molecules that adsorbed on adsorbent surfaces and others in the bulk liquid phase.[59] As the initial concentration of CR dye increased, the dye removal percent decreased. Contrary to that, the strong driving force that causes mass transferring at an increased concentration of CR dye is considered the main reason for the rising capacity of the adsorbent in acquiring more quantity of CR dye.

Figure 3

Influence of concentration of CR dye and contacting time on the removal percent and the quantity of CR dye adsorbed at pH 7 and 25 °C by 20 mg of mass of (A and D) Z, (B and E) GS, and (C and F) ZGS.

A much higher efficiency for CR adsorption at all concentrations was observed for ZGS nanocomposite and the sequence of CR removal percent was ZGS > GS > Z in order. The quantities of CR adsorbed increased as the starting CR concentration increased as shown in Figure (D–F) due to the growth of the concentration gradient with an increase in the starting concentration of CR. Hence, reasonable growth in the driving forces was created to break down the resistance in mass transfer between Z, GS, and ZGS nanoadsorbents and the CR adsorbate.[13,60] ZGS revealed maximum adsorption capacities at 4.55, 8.68, 12.60, 16.00, and 19.70 mg/g for concentrations of CR dye solutions at 5, 10, 15, 20, and 25 mg/L, in order, at neutral pH and nearly room temperature (25 °C). Within the same frame, GS revealed adsorption capacities at 4.0, 6.53, 9.0, 9.15, and 10.3 mg/g. Furthermore, Z revealed maximum adsorption capacities at 3.1, 5.9, 7.25, 7.6, and 8.1 mg/g at the same starting concentrations. These results confirm that using GS for the modification of Z is a promising approach to enhance the performance of CR removal by Z at decreased concentrations.

Influence of Nanoadsorbent Dose

The nanoadsorbent different doses were subjected to an evaluation process in terms of CR removal percent to determine the best nanoadsorbent dose that reveals the most adequate performance for cost evaluation in real field experiments in the near future. The correlation between CR dye removal percent and nanoadsorbent dosage is illustrated in Figure A. Doses of the nanoadsorbent were adjusted to be varied from 0.01 to 0.05 g. A direct proportion relationship was noticed between CR dye removal percent and nanoadsorbents Z, GS, and ZGS dosage, an increase in the removal percent was recorded as the nanoadsorbent dosage increased from 0.01 to 0.05 g. In the case of Z nanoadsorbent, the removal percent showed an increase from 46.0% to 64.6%, while for the GS nanoadsorbent the recorded increase was raised from 68% to 88%. Finally, the ZGS nanoadsorbent recorded an increase from 70% to 91%. The presence of an increased number of hot spots on the surface of all nanoadsorbents may be the driving force behind the recorded increase in removal percent.[13,14] The major increase in removal percent in the case of ZGS was recorded after increasing the nanoadsorbent dosage from 0.01 to 0.02 g and from 0.01 to 0.03 g in the case of Z adsorbent. Any increase in the nanoadsorbent dose over 0.03 g revealed a slight increase in the removal percent. The screening effect phenomenon which is usually noticed at elevated adsorbent weights is believed to be the main reason behind the decrease in removal percent at higher nanoadsorbent doses. At these kinds of phenomena, a thick screening layer of the adsorbent accumulated on the surface which in turn results in a reduction in the distance between molecules available for adsorption. Moreover, a certain type of competition between CR molecules for less available binding sites resulted as a result of Z and ZGS overlapping. Agglomerations and/or aggregations at greater doses of ZGS and Z lead to an increase in the diffusion path length for adsorption of CR leading in turn to a noticeable decrease in the rate of adsorption[61−64] as well as a general reduction in the sorbent–sorbate contact.[6,42]

Figure 4

Influence of different adsorption conditions on the removal percent of CR dye (20 mL solution and 10 mg/L) by ZGS, Z, and GS: (A) effect of adsorbent dose, (B) effect of initial pH of the solution, (C) effect of adsorption temperature, (D) reusability test, and (E) the zeta potential as a function of the pH value of the solution.

Influence of pH

The values of pH play a crucial role in monitoring the way that the nanoadsorbent works in wastewater treatment. These variations in pH values result in changes in the properties of the surface of the adsorbent as well as the degree of ionization of adsorptive molecules. As it is clear from Figure B pH variations affect strongly the removal percent of CR dye from the solution. A pH range between 3 and 10 was studied using an initial CR concentration of 10 mg/L and a nanoadsorbent dose of 0.02 g. Nanoadsorbent Z revealed removal percent of ∼51%, 38%, 53%, and 49% for CR dye at pH 2, 5, 7, and 10; in the same order. While nanoadsorbent GS showed removal percent of ∼67.8%, 60.7%, 69.6%, and 69.6%, and finally nanoadsorbent ZGS showed removal percent of 81.1%, 74.5%, 87.7%, and 67.9% at pH2, 5, 7, and 10, in the same order and at the same mentioned conditions. It is clear from Figure B that when the pH of the solution becomes 7, the removal percent of CR dye achieved by nanoadsorbents ZGS, Z, and GS reaches the maximum values. This can occur because the interactions resulting between nanoadsorbents ZGS, Z, and GS and CR dye molecules are more dominant over those with OH– ions found in the medium.[65] To investigate the effects on Z, GS, and ZGS, the zeta potential of composites in the solution was determined. The effect of pH on the zeta potential of Z, GS, and ZGS in an aqueous solution is shown in Figure E. In this figure, Z, GS, and ZGS acquired a negative surface charge in the pH range of 2–10. The surface charge of Z, GS, and ZGS shifted from lower to higher values with increasing the pH value from 2 to 10, as a result of the gradual increase in the electrostatic attraction between the negatively charged CR and nanoadsorbent. The large change in values obtained from the zeta potential was expected due to large variations in adsorption capacity values at a pH ranging from 2 to 7. Generally, the more the zeta potential value shifted to the positive values, the greater was the removal percent. Lower values of zeta potential point to the nanoadsorbent surfaces possessing a partially negative charge at a pH of 2 to 10. Moreover, there is repulsive electrostatic force between nanoadsorbents ZGS, Z, and GS and CR through the sulfonic acid group (SO3–) during the experiment.[66,67] In other words, when the pH of the system increases, the gathering of sites that have negative charges on the surface of the nanoadsorbent decreases, and vice versa for positively charged ones. So at higher pH values, the surface of the nanoadsorbent is positively charged and hence there is considerable high electrostatic interaction with anionic dyes which in turn increase the adsorption process. In addition, when the pH of the solution reaches 5, the negatively charged surface and the anionic dye molecules suffer from ionic repulsion which decreases adsorption.[8] The pH values at which the adsorbent has a zero-point charge (pHzpc) were not detected in the case of Z, GS, and ZGS under the investigated pH ranges.

Influence of Temperature

Because of the variations caused by temperature on the adsorption capability of adsorbent, and since it is considered a physico-chemical processing factor, temperature changes were studied during CR dye adsorption.[68] The following temperatures were selected to study the effect of temperature on the adsorption process: 25, 40, 50, 60, 70, 80, and 90 °C. The data are illustrated in Figure C. A reverse proportional relationship can be seen between CR removal percent and increasing temperature. This may be explained by the fact that the desorption characteristic results from the destroying forces created by the action of temperature between hot binding spots of the nanoadsorbent and the CR adsorbate moieties.[13,69−71] And hence, nearly room temperature (25 °C) was selected to be the best temperature for adsorption of CR onto all tested adsorbents. Therefore, our conclusion that the adsorption process is exothermic comes out from the observed results between removal percent and the temperature.

Reusability Test

The reusability tests for nanoadsorbents ZGS, GS, and Z for the elimination of CR dye were applied four times with the same adsorbent and dose and illustrated in Figure D. The power of removal of all each nanoadsorbents showed considerable variations throughout the reusability test. For nanoadsorbent Z, the obtained dye removal percent was 53.06% for the first cycle, 40.82% for the second cycle, 34.69% for the third cycle, and 28.57% for the last cycle. While GS nanoadsorbent revealed a reduction in dye removal percent from 69.64% in the first cycle to 37.50% in the last cycle. Also, nanoadsorbent ZGS revealed a reduction in the dye removal percent from 87.72% in the first cycle to 37.28% in the last cycle. This reduction in the CR removal percent could be explained by the agglomeration of the CR molecules over the surface of nanoadsorbents Z, GS, and ZGS, which in turn hide the adsorbent surface as well as pores from the dissolved CR molecules and hence, a reduction in adsorption capacity recorded.[72]

Adsorption Isotherm

The nonlinear fitting of Ce against qe was selected to fit the experimental data obtained from this study with the individual isotherms of Temkin, Freundlich, and Langmuir. The values of KL, KF, KT, Qo, n, B, and R2 were obtained from the nonlinear plots and illustrated in Figure and also recorded in detail in Table . Table demonstrates the CR adsorption on ZGS, Z, and GS nanoadsorbents following the Langmuir isotherm models which offer the highest correlation coefficient (R2 value). Hence, the adsorption of CR dye molecules by a multilayer mechanism is achieved at the surface of the adsorbent active sites with unequal different adsorption energies, available heterogeneous sites, and interactions between adsorbed molecules. At room temperature (25 °C), the R2 values recorded for the Langmuir isotherms of ZGS, Z, and GS adsorbents were 0.99203, 0.9878, and, 0.98867, in the same order. The value of RL is lower than 1, pointing to the adsorption of CR being the most favorable in our case.[73]

Figure 5

Nonlinear plots of (a) Langmuir, (b) Freundlich, and (c) Temkin adsorption isotherms for the adsorption of CR dye by 50 mg of Z, GS, and ZGS at 25 °C and initial pH 7.

Table 1

Isotherm Parameters for CR Adsorption on Z, GS, and ZGS

Langmuir Isotherm
	Qo(mg/g)	KL(L/mg)	RL	R2
ZGS	29.4	0.327	0.109	0.99203
GS	11.5	0.468	0.042	0.98867
Z	9.8	0.306	0.116	0.98780

The calculated values of (1/n) in the Freundlich isotherm model are smaller than unity, which in turn indicates that the adsorption process is favorable, heterogeneity of the surface, and also fewer interactions between the adsorbed ions. Furthermore, multimolecular and multianchorage adsorption mechanisms occur through CR adsorption.[74,75] Depending on the Langmuir isotherm model, the maximum adsorption capacities are calculated to be 9.8, 11.5, and 29.4 (mg/g) for Z, GS, and ZGS nanoadsorbents, respectively.

Adsorption Kinetic Models

For the best selection for the adsorption kinetics model, CR adsorption over ZGS, GS, and Z under altered starting concentrations of CR was addressed. Kinetics of first-order, second-order, and Elovich nonlinear plotting graphs were introduced in Figure by plotting q against time. Parameters of adsorption kinetics represented in k1, k2, q, β, and α of the evaluation model besides R2 were calculated theoretically using the nonlinear fitting and were well presented in Table .

Figure 6

(A–C) Nonlinear pseudo-first-order, (D–F) nonlinear pseudo-second-order, and (G–I) nonlinear Elovich sorption kinetics of CR dye at 25 °C and pH 7 by 20 mg of Z, GS, and ZGS, respectively.

Table 2

Parameters of the Nonlinear Kinetic Models for CR Dye Adsorption on Z, GS, and ZGS

	first-order			second-order					Elovich kinetic model
concn, ppm	qe exp	qe calc	k1	R2	qe exp	qe calc	k2	R2	β (g/mg)	α (mg/min)	R2
GS
25	10.3	14.6	0.00298	0.96304	10.3	23.8	0.00008	0.95886	0.1050	0.0456	0.95504
20	9.15	10.3	0.00555	0.97828	9.15	14.4	0.00031	0.96871	0.2151	0.0731	0.95854
15	9.0	12.7	0.00305	0.97107	9.0	20.3	0.00010	0.96716	0.1253	0.0411	0.96358
10	6.53	7.0	0.00680	0.98940	6.53	9.3	0.00065	0.97924	0.3713	0.0717	0.96734
5	4.0	4.0	0.01486	0.99517	4.0	4.5	0.00474	0.99721	1.3401	0.4187	0.98999
Z
25	8.1	8.6	0.00684	0.99560	8.1	11.4	0.00054	0.99279	0.3039	0.0896	0.98714
20	7.6	7.9	0.00753	0.97889	7.6	9.9	0.00077	0.98650	0.3838	0.1097	0.99003
15	7.25	7.6	0.00731	0.99483	7.25	9.8	0.00072	0.99345	0.3652	0.0893	0.98851
10	5.9	5.8	0.01074	0.96541	5.9	6.9	0.00186	0.98071	0.6470	0.1662	0.98673
5	3.1	3.1	0.00797	0.95182	3.1	3.8	0.00234	0.97413	1.0662	0.0522	0.98926
ZGS
25	19.7	30.3	0.00251	0.97288	19.7	50.5	0.00003	0.97067	0.0482	0.0791	0.96868
20	16	24.0	0.00266	0.97672	16	39.5	0.00004	0.97439	0.0626	0.0669	0.97227
15	12.6	16.1	0.00380	0.97482	12.6	24.6	0.00011	0.97025	0.1105	0.0681	0.96598
10	8.68	9.8	0.00537	0.98390	8.68	13.7	0.00031	0.97877	0.2273	0.0685	0.97310
5	4.55	5.1	0.00557	0.98449	4.55	7.1	0.00064	0.97906	0.4458	0.0375	0.97293

For all studied kinetic models, regression coefficient and nonlinear fit values are reported in Table . The values confirmed that adsorption of CR over nanoadsorbent ZGS is well-defined with the pseudo-first-order model in all studied concentrations of CR, which is in good agreement with experimental qexp. The adsorption of CR dye over the GS nanoadsorbent almost looked like that of ZGS in the kinetic model except for the 5 ppm concentration. It almost matches with pseudo-second order which is very clear from the higher R2 values and the nearly matched values of both calculated and experimental qe. Finally, adsorption over the Z nanoadsorbent matches with two different kinetics adsorption models. Depending on the CR dye concentration, a first-order kinetic model appears at concentrations of 25 and 15 ppm, while the Elovich kinetic model appears at concentrations of 5, 10, and 20 ppm.

Sorption and Adsorption Mechanisms

Sorption Mechanism

Weber’s intraparticle diffusion was selected to fit the practical data for better understanding the process of adsorption kinetics as well as rate-controlling steps. The intraparticle diffusion model is selected to be the model of application via plotting q against time (t) and q against t1/2, the plots of which are illustrated in Figure S2 and Figure S3. The supplementary data figures reflect both nonlinear and linear plots of Weber’s intraparticle diffusion. Table reflects theoretical calculations including the slope as well as the intercept of the fitted curve in Figure S3. These were utilized for calculating the values of both intraparticle propagation model rate constant (K3) as well as boundary thickness constant (I). Regarding the value of the intercept, I which was found to be unequal to zero is considered a strong indicator for including the adsorption process in an intraparticle diffusion model. However, the intraparticle diffusion model is not necessarily the sole rate-controlling factor in the determination of the kinetics of the adsorption process.[76] The greater is the intercept calculated from Figure S3, the greater is the contribution of the adsorption process on the surface in the rate control step which refers to the boundary layer effect.[76]

Table 3

Intraparticle Diffusion Constants for Different Initial CR Concentrations at 25 °C

		intraparticle diffusion kinetic model
catalyst	concn ppm	I	k3 (mg/g min1/2)	R2
GS	25	–1.38943	0.56606	0.92189
	20	–0.32155	0.47782	0.93675
	15	–1.08322	0.48958	0.93343
	10	0.0927	0.32948	0.94293
	5	0.76516	0.17478	0.86479
Z	25	0.00692	0.41202	0.96974
	20	0.33788	0.36642	0.98080
	15	0.13098	0.36091	0.97034
	10	0.69336	0.26853	0.95085
	5	0.19761	0.14176	0.98893
ZGS	25	–3.05001	1.08458	0.93812
	20	–2.39436	0.88394	0.94358
	15	–1.39252	0.68844	0.94494
	10	–0.38778	0.45451	0.95849
	5	–0.18073	0.23807	0.9577

Figures S2 and S3 clearly reveal that the kinetics adsorption of CR dye over Z and GS is composed of two consecutive phases while that for CR dye over ZGS consists of three consecutive phases, bulk, film, and pore diffusion. For more clarification, phase number one reflects the surface and intraparticle diffusion processes, while the second phase reflects liquid film diffusion, and the third reflects the diffusion of CR dye molecules through pores to the active sites of ZGS; then, equilibrium conditions are achieved.[3,6]

Adsorption Mechanism

We hypothesize that there are specific interactions between sorbent molecules and surfaces and dye structure. These interactions play a very important role in the adsorption process. We hypothesize that free electrons found in different active sites in the sorbates which may be a sulfur atom, oxygen atom, nitrogen atom, and electron clouds found in aromatic rings formed hydrogen bonding with several active sites found on the surface and inside the pore structure of the sorbents. Moreover, electrostatic interactions resulted from different charges in both sorbates and sorbents.

Field Experiments and Comparison with Analogous Reported Catalysts

The novel ZGS nanoadsorbent synthesized from mixing Z with GS was subjected to a field experiment with optimized parameters of 0.02 g as a catalyst mass, near room temperature, and a 420 min contact time, with keeping the pH of the wastewater containing the waste dye unchanged. The industrial wastewater contained several types of dyes which were confirmed by optical scanning of the as-received industrial wastewater that recorded different wavelengths. After the time of the reaction was completed, the absorbance was measured at different wavelengths to determine the dye removal percent from the industrial wastewater sample. Promising results appeared from the data recorded from the field study for the newly synthesized nanoadsorbent. The removal percent reached 91.1%, indicating the creation of a novel affordable eco-friendly nanoadsorbent that will help much in the reusability of industrial wastewater.

Data recorded in Table reveal a comparison for different optimized conditions such as adsorption capacity, and removal percent of nanoadsorbents under investigation in this study: Z, GS, and ZGS for the removal of CR dye to other analogous adsorbents that have been reported previously.[36,84−90] The data confirm that our novel ZGS nanoadsorbent at the selected optimized conditions possesses a superior efficiency over analogous adsorbents.[36,84−90]

Table 4

Comparison of the Optimized Conditions, Removal %, and Adsorption Capacity of Different CR Adsorbents Relative to Our Z, GS, and ZGS Nanoadsorbents

adsorbent	conditions	adsorption capacity (qm) (mg/g)	% removal	ref
natural clinoptilolite (NC)	contact time: 240 min	16.92		(44)
	pH: 7
	concn: 250 mg/L
	temp: 25 °C
kaolin	concn: 150 mg/L	5.44	100	(77)
	dosage: 5g/100 mL
	pH: 3
	temp: 25 °C
zeolite	concn: 200 mg/L	4.3	95
	dosage: 10g/100 mL
	pH: 3
	temp: 25 °C
cashew nut shell	adsorbent dosage: 30 g/L	5.18	99.34	(78)
	concn: 50 mg/L
	time: 120 min
	pH: 3
	temp: 25 °C
acid-activated red mud	adsorbent dose: 125 mg	4.05	85	(79)
	concn: 25 mg/L
	pH: 7
	temp: 25 °C
tamarind fruit shell (TFS)	adsorbent dosage: 8 g/L	10.48	87	(80)
	concn: 20 mg/L
	time: 90 min
	pH: 7
	temp: 25 °C
bagasse fly ash (BFA)	adsorbent dosage: 1 g/L	11.88		(81)
	concn: 10 mg/L
	time: 240 min
	pH: 3, 13
	temp: 30 °C
activated coir pith carbon	adsorbent dosage: 200 mg/L	6.72		(82)
	concn: 20 mg/L
	time: 40 min
	pH: 7.7
	temp: 35 °C
peanut shell	adsorbent dose: 1 g	15.09	85.94	(83)
	time: 40 min
	concn: (3–15) mg/L
	pH: 8
	temp: 40 °C
ZGS	contact time:480 min	19.7	91.11	this work
GS	catalyst dose: 0.02 g	10.3	78.947
Z	concn: 20 mg/L	8.1	65.00
	pH: 7.0
	temp: 25 °C

Conclusion

A novel alga/zeolite was obtained from the simple treatment of Z and GS. The nanocomposite ZGS was applied as a novel adsorbent for CR from aqueous solutions. Both morphologies and structures of all used adsorbents Z, GS, and ZGS have been demonstrated, and the data revealed that GS nanoparticles and Z nanopores aggregate to form crystallite nanocomposites with particle size 40.5 nm. The removal percent obtained from experimental results indicated that the adsorption of CR dye from aqueous solutions is generally enhanced by reducing each starting CR concentration and reaction temperature. A noticeable increase in the removal percent was recorded by increasing Z, GS, and ZGS doses from 0.01 g to 0.05 g to reach 64.62%, 88.82%, and 91.84%, in the same order. Also an increase in the CR removal percent with an increase in the initial pH of solution for all adsorbents, until neutral pH, at which the removal percent reaches its maximum values was recorded. By studying the adsorption isotherms of CR dye over Z, GS, and ZGS, the data revealed that Z and GS are best fitted with the Langmuir isotherm model, while ZGS was found to be matched with the Freundlich isotherm model. Moreover, the adsorption of CR dye over ZGS is well handled with a first-order diffusion model, while second-order kinetics are matched with Z, and finally, GS follows two different kinetics adsorption models according to the CR concentration. Field experiments confirmed that the newly synthesized ZGS nanoadsorbent possesses a removal percent of 91.1% during its application in removing dyes from industrial wastewater. Therefore, the production of a novel and affordable eco-friendly nanoadsorbent was successfully achieved that will aid in the best recycling of industrial wastewater.

Experimental Details

Raw Materials, Dyes, And Reagents

Zeolite raw materials were purchased from A&O Co. for mining, and employed without further alteration. GS was taken from the Mediterranean Sea coasts between Ras elbar and Baltim, Egypt. The CR dye, which was dissolved in deionized water, was obtained from Sigma-Aldrich and had a purity of 97.0%. Also, NaOH with a purity of 99.99% and HCl of a 36% concentration were obtained from Sigma-Aldrich and used to alter the pH value.

Preparation of Zeolite/Green Seaweed (ZGS) Nanocomposite

The ZGS nanocomposite was fabricated by following the steps of the wet impregnation technique.[84,85] First, 1 g of zeolite and 1 g of GS were mixed in 20 mL of DI H2O and magnetically stirred at 500 rpm for 60 min. The mixture was ultrasonically treated for 60 min. This treatment was repeated three times. The resulting ZGS nanocomposite was filtered, washed with DI H2O several times, and finally dried in an oven at 60 °C for 1 day. The Z, GS, and ZGS nanoadsorbents were characterized using XRD, SEM, and FTIR analyses. The optical absorbance was characterized using a UV/vis/IR spectrophotometer (PerkinElmer Lambada 950).

Preparation of the Adsorbate

The adsorbate in this investigation was the anionic CR dye. CR’s molecular formula is C32H22N6Na2O6S2 (Figure S4). A 1 g sample of CR dye was dissolved in 1 L of DI H2O to make a 1000 mg/L stock solution. To obtain dye solutions of certain concentrations, the stock was diluted with DI H2O. Using either a 0.1 M HCl or NaOH solution, the pH values of the dye solutions were adjusted to 3, 5, 7, and 10.

Samples Characterizations

The XRD charts were obtained in the range 20°–70° with scan step of 0.02° utilizing a PANalytical diffractometer (Empyrean) with Cu Kα source of λα = 0.154045 nm operated at 40 kV and 35 mA. The average crystallites size, Ds, of the adsorbents was obtained using the Scherer equation, Ds = 0.94 λα/βw cos ϕ; where βw and ϕ are the corrected full width at half-maximum and the diffraction angle.[86] SEM images were measured using a Quanta FEG 250 microscope (Switzerland). A Bruker VERTEX 70 FTIR spectrophotometer was utilized to obtain the FTIR spectra using the dry KBr pellet technique.

Adsorption Studies

As shown in Table S2 (Supplementary Data), four adsorption testing series were carried out on Z, GS, and ZGS nanoadsorbents under various adsorption conditions, including beginning CR concentration, reaction temperature, nanoadsorbent dose, and the pH value. All CR adsorption tests were carried out on a batch mode scale with continuous shaking and varied experimental parameters such as starting CR concentration (5–25 mg/L), contact duration (0–480 min), nanoadsorbent dose (0.01–0.05 g/20 mL of CR solution), pH (3–10), and temperature (25–90 °C). In all trials, the adsorption period was 480 min and the CR solution volume was 20 mL. By tracking the adsorption peak using a UV–vis spectrophotometer, the variation in CR concentration was determined. The reusability of Z, GS, and ZGS nanoadsorbents was tested five times with 0.02 g of each, 20 mL of 10 mg/L beginning CR concentration, and 480 min contact time at 25 °C and pH 7. After each run, the Z, GS, and ZGS nanoadsorbents were removed from the solution, rinsed with DI H2O, and made ready for the next run.

The quantity of CR uptake by the nanoadsorbents at equilibrium (qe(mg/g) and time t (q), as well as the CR dye removal percent, were calculated using eqs and 2.[59,87]Here Co, C, and Ce are the CR concentrations (mg/L) at the beginning, after time t, and at equilibrium, respectively. V is the CR volume (mL) and m is the Z, GS, and ZGS masses (mg). The findings provided were the averages of three separate trials.

The reaction isotherms of the developed Z, GS, and ZGS nanoadsorbents for the tested CR have been clarified using Langmuir, Freundlich, and Tempkin isotherms.[34−36] In the supplemental data, all linear isotherm equations and their parameters are introduced. Equation may be used to determine the degree of favorability of the Langmuir isotherm given equilibrium data using the value of the dimensionless separation factor (RL).[88]where Cmax refers to the maximum initial CR concentration.

Adsorption Kinetics and Mechanism

To determine the adsorption processes and kinetics models that best match the adsorption of CR onto Z, GS, and ZGS adsorbents, several adsorption mechanisms and kinetics models such as intraparticle diffusion, pseudo-first-order, pseudo-second-order, and basic Elovich kinetic model are utilized.[14,64,89−93] In the Supporting Information, all linear kinetics equations and their parameters are introduced. The average values of all adsorption findings were assessed in triplicate. Origin Pro 2018’s statistical functions were used to calculate regression coefficients (R2) for several kinetic and isotherm models.

Field Experiments

The newly produced nanoadsorbent system was put to the test as an efficient eco-friendly adsorbent that could be utilized to extract industrial waste dyes from vast amounts of industrial effluent. Wastewater containing dyes was provided for this purpose by a clothes dying plant in Beni-Suef. The wastewater containing dyes was utilized untreated and undiluted. The optimal adsorbent system was chosen based on our modified trial findings.