Effective Sequestration of Phosphate and Ammonium Ions by the Bentonite/Zeolite Na-P Composite as a Simple Technique to Control the Eutrophication Phenomenon: Realistic Studies.

A bentonite/Zeolite-P (BE/ZP) composite was synthesized by controlled alkaline hydrothermal treatment of bentonite at 150 °C for 4 h for effective sequestration of phosphate and ammonium pollutants. The composite is of 512 m2/g surface area, 387 meq/100 g ion-exchange capacity, and 5.8 nm average pore diameter. The experimental investigation reflected the strong effect of the pH value in directing the uptake behavior and the best results were attained at pH 6. The kinetic properties showed an excellent agreement for phosphate and ammonium adsorption results with the pseudo-second-order model showing equilibrium intervals of 600 and 360 min, respectively, and maximum experimental capacities of 170 and 155 mg/g, respectively. Additionally, their equilibrium modeling confirmed excellent fitness with the Langmuir hypothesis, signifying homogeneous and monolayer uptake processes with a theoretical q max of 179.4 and 166 mg/g for phosphate and ammonium, respectively. Moreover, the calculated Gaussian adsorption energies of phosphate (0.8 kJ/mol) and ammonium (0.72 kJ/mol) suggested physisorption for them with mechanisms close to the zeolitic ion-exchange process or the coulumbic attractive forces. This was supported by the assessed thermodynamic parameters which also suggested spontaneous uptake by endothermic reaction for phosphate and exothermic reaction for ammonium. The BE/ZP composite is of excellent reusability and used for eight recyclability runs achieving removal percentages of 61.5 and 74.5% for phosphate and ammonium, respectively, in run 8. Finally, the composite was applied in the purification of sewage water and groundwater, achieving complete removal for phosphate from sewage water and ammonium from groundwater and reduction of the ammonium ions in the sewage water to 2.3 mg/L.

Introduction

Contamination of the water resources by different types of contaminants resulted in several environmental and health hazards.[1−3] The eutrophication process is a common phenomenon associated with the extensive and uncontrolled growth of algae and other phytoplankton biomass in the present water bodies. This related mainly to the overloading of the water resources by huge quantities of nutrients, especially nitrogen- and phosphorus-bearing compounds.[4,5] The occurrence of eutrophication commonly causes considerable depletion in the dissolved oxygen, which reduces the quality of the water resources and negatively affects the present aquatic ecosystems.[6]

However, phosphorous is a vital element in several industries especially in fertilizers, and its discharge as drainage water from agricultural activities can increase its abundance in water resources above the accepted limit (0.03 mg/L), which induces rapid and random growth of algae and microorganisms.[5,7] This was also observed for the saturation of the water resources by ammonium ions with a concentration higher than 3 mg/L.[8] Additionally, the over concentrations of ammonium cause disorder effects on the tissues of fish and other aquatic organisms and physiological parameters, especially disease resistance and the growth rate.[9−11] Thus, developing effective and advanced techniques to reduce the ammonium and phosphate ions from sewage water and drainage water is a critical challenge for modern civilization.

Utilizing novel materials of significant ion exchange and adsorption properties in the elimination of phosphate and ammonium ions from water was evaluated as a promising technique that can be applied at low cost with environmentally friendly properties.[3,12,13] Synthetic adsorbents and ion exchangers based on natural resources such as natural clays, bentonite, muscovite, activated carbon, synthetic zeolite, biochars, struvite, cellulose-based adsorbents, and magnetite were applied commonly for this target.[12−17] Zeolite-based materials either of natural or synthetic forms were studied extensively in the elimination of phosphate and ammonium from sewage water and polluted lakes.[18] This controlled mainly by their unique microporous structures, superior surface areas, and ion-exchange properties as well as their environmental values.[16] This was also recorded for natural layered aluminosilicate structures (clays) that were applied extensively in their natural and modified phases as favorable adsorbents for different types of pollutants including phosphate and ammonium.[4,19] Bentonite is one of the most studied clays in water remediation technologies for its exceptional physicochemical features, flexible crystalline and chemical structure, superior surface area, environmental value, high ion replacement, high uptake capacities, and high natural availability.[10,20]

The integration between zeolite and clay minerals was inspected recently as an innovative methodology to produce a hybrid product of improved physicochemical properties.[4] Such materials were synthesized by controlled hydrothermal alkaline alteration of clays and were inspected as potential adsorbents for dyes and some chemical ions.[4,10,21] In our previous studies, we investigate the synthesis of bentonite/zeolite-P and muscovite/phillipsite composites as novel materials with enhanced textural and physicochemical properties as compared to the integrated phases.[4] Unfortunately, the investigation of the bentonite/zeolite-P composite as an adsorbent for phosphate and ammonium and its application in the purification of sewage and groundwater has not been carried out until now.

The aim of the introduced study is to investigate the synthetic bentonite/zeolite-P composite as a promising adsorbent for phosphate and ammonium ions with effective adsorption capacity, low preparation cost, and high recyclability properties. The adsorption tests were accomplished considering prepared aqueous solutions to investigate the optimized conditions and the capacity of the material, and then, the composite was applied in the purification of realistic groundwater and sewage water samples to avoid their effects in the eutrophication phenomenon. The expected nature of the uptake processes was evaluated considering the commonly studied kinetic and equilibrium theoretical models.

Results and Discussion

Characterization

Structural and Chemical Properties

The X-ray diffraction (XRD) pattern of bentonite confirmed the presence of montmorillonite as the main clay mineral. The principal peaks of the montmorillonite as a crystalline phase were observed at 6.55, 19.85, 25.1, and 28.35° as the characteristic 2Theta angles showing a crystallite size of 12.9 nm (card nos: 00-003-0010 and 00-058-2010) (Figure AA). The crystallize size was calculated considering the commonly used Scherrer equation (D = 0.9λ/W cos θ), where D, W, θ, and λ symbols refer to the crystallite size, the full width of the mean peak at the half maximum (radians), the Bragg’s angle, and the wavelength of the used X-ray (CuKα = 0.15405 nm), respectively. After the activation of the sample by heating, the obtained pattern declared a significant reduction in the intensities of the detected montmorillonite peaks reflecting partial destruction and dehydration for its crystalline structure (Figure AB). Under the alkaline transformation process, the obtained pattern confirmed the formation of hybrid material between bentonite represented by the montmorillonite peaks and the synthetic zeolite represented by zeolite Na–P (Figure AC). The main peak of montmorillonite mineral appeared as a strongly reduced peak with a noticeable deviation from its position to be detected at 6.73° (Figure AC). Synthetic zeolite Na–P showed several characteristic peaks at 12.46, 16.6, 21.66, 28.1, 33.38, and 38°[10] (Figure AC).

Figure 1

XRD patterns of bentonite, thermally activated bentonite, and synthetic BE/ZP composite (A) and FTIR spectra of bentonite and the synthetic BE/ZP composite (B).

This was also supported by the chemical properties of both BE and BE/ZP from the FT-IR spectra (Figure BA). The original bentonite showed its main groups of crystalline OH, interlayer water molecules, Si–O, and Al–O at the absorption bands of 3400, 1640, 1000, and 918 cm–1, respectively.[10,22] Also, the characteristic bands of Si–O–Al, Si–O–Mg, and Mg–Fe–OH were identified by the detected minor bands within the area from about 400 to 1000 cm–1[22] (Figure BA). The synthetic BE/ZP showed the same bands but with significant deviation for their positions and a noticeable increase in the intensities of the characteristic bands of structural OH and bounded water (Figure BB). This might be related to the role of the alkaline modification processes in causing etching for the surficial siloxane groups of the present montmorillonite minerals which induce the exposure of Si–OH groups as well as the Al–OH group[10] (Figure BB). Additionally, the formation of zeolite-P is associated with an increase in the water content because of the entrapment of the water molecules within the interchannels of zeolite. This was supported by the distinguished band at 1429 cm–1 which signifies the coordinated water molecules within the pores of the synthetic zeolite[20] (Figure BB). The observed disappearance for some bentonite-related bands confirmed the partial destruction of the montmorillonite structure and the hydration of the interunit water layers by the alkaline transformation.

This supported by the obtained full chemical analysis for both bentonite and BE/ZP composite. The used bentonite as a precursor is of 54.82% SiO2, 9.5% Fe2O3, 17.56% Al2O3, 2.4% CaO, 2.5% MgO, 1.45% TiO2, 2.6% Na2O, and 9.2% LOI. The synthetic BE/ZP is composed mainly of 51.46% SiO2, 14.52% Al2O3, 7.5% Fe2O3, 10.3% Na2O, 1.52% MgO, 1.62% CaO, 0.77% TiO2, and 12.6% LOI. The observed declination in the Si and Al content reflected the role of the alkaline treatment process in leaching of such structural elements from the units of bentonite. This was also confirmed by the significant increase in the Na2O content in the composite as compared to the used raw sample.

Morphological and Textural Features

The surface features of the bentonite precursors (BEs) and BE/ZP composite were evaluated considering their scanning electron microscopy (SEM) images. The bentonite particles were identified as stacked layers compacted with each other in agglomerated clusters and their surfaces showed tiny chunks from the present nonclay impurities (Figure A). For the synthetic BE/ZP, the bentonite grains appeared as a substrate of numerous zeolite cubes (Zeolite Na–P) that randomly distributed on them (Figure B–D). Such a morphology confirms the formation of the composite without extensive destruction for montmorillonite as the main mineral of bentonite. This is of strong influence in the textural properties as compared to both bentonite and commercial synthetic zeolite Na–P (Table ). The surface area increased to 512 m2/g as compared to 91 and 123 m2/g for bentonite and zeolite-P, respectively. Also, the determined average pore diameter demonstrated changes in the value for BE/ZP (5.8 nm) as compared to bentonite (10.4 nm) and synthetic zeolite Na–P (1.2 nm). Moreover, the composite showed considerable enhancement in the ion-exchange capacity to be 387 meq/100 g which is of very high value considering the measured values for both bentonite and synthetic zeolite Na–P (Table ).

Figure 2

SEM images of bentonite (A) and the synthetic BE/ZP composite showing the distribution of zeolite-P cubes on the surface of bentonite particles (B–D).

Table 1

Textual and Physical Properties of the B/ZP Catalyst in Comparison With Bentonite and Zeolite-P

sample	cation exchange capacity (meq/100 g)	swelling capacity	specific surface area (m2/g)	micropores volume (mL/g)	mesopores volume (mL/g)	total volume (mL/g)	average pore size (nm)
bentonite	104	8.37	91	0.013	0.262	0.312	10.4
zeolite Na–P	320	2.54	123	0.36	0.01	0.37	1.2
bentonite/zeolite Na–P	387	5.62	512	0.244	0.175	0.422	5.8

Adsorption Results

Effect of the Main Parameters

pH Value

The solution pH is the most important factor in any studied adsorption process for its influence in regulating the H+/OH– competition within the present receptor sites as well as its vital effect in direct the speciations of both phosphate and ammonium ions.[4,13] Generally, phosphate ions were identified in four species at the different pH values including H3PO4, H2PO4–, HPO42–, and PO43–.[15] This reflected in the observed trend for the uptake of phosphate by BE/ZP at different pH values (Figure A). The achieved adsorption quantity increased from 10 to 37 mg/g with the regular expansion in the adjusted pH from pH 2 to pH 6 (Figure A). Beyond pH 6, the BE/ZP composite showed noticeable declination in its uptake capacity until pH 8 (Figure A). This behavior related to the existence of phosphate as H2PO4– within the pH range from pH 3 to pH 6 which induced the electrostatic attraction between them and the BE/ZP surface which is positively charged within this range.[5] Below pH 3, the phosphate ions were detected in their neutral form and of significant difficulty to be adsorbed by BE/ZP.[19] Beyond pH 6, the dominance of HPO42– and PO43– as the phosphate species resulted in highly repulsive forces with BE/ZP functional groups which are of negative charges within this pH range under the deprotonation processes.[13]

Figure 3

Effect of the main adsorption parameters on the uptake capacity of phosphate and ammonium by the BE/ZP composite including the pH value (A), time intervals (B), initial concentrations (C), and BE/ZP dosages (D).

The uptake trend of ammonium displayed continuous enhancement in the adsorbed quantity from 5.2 to 36.5 mg/g with the regular expansion in the adjusted pH from pH 2 to pH 8 (Figure A). This was assigned to the continuous deprotonation of BE/ZP characteristic groups with the augmentation in the studied pH values to be of higher attractive forces for the positively charged ammonium ions as compared to the acidic conditions.[9,23] The increment in the pH from pH 6 to pH 8 is of slight impact on the BE/ZP capacity for ammonium ions which might be credited to the transformation of NH4+ ions into NH3 as well as the reported reduction in the ion-exchange properties of the natural silicate minerals in the high alkaline environments.[24] The saturation of the system by the hydronium ions at the acidic environments induced the stability of ammonium ions in their normal forms and the depletion of the system in the hydronium ions in the alkaline environments accelerates the transformation of ammonium into ammonia gas which might be released out of the system.[4] This was supported by the determined value of pH of the zero point charge (pH (PZC)) which is 6.74. That is, the BE/ZP composite is of negative charge above this value.

Time Intervals

Investigation of the adsorption properties of BE/ZP for both phosphate and ammonium within different intervals is of critical importance in the detection of the equilibration time to achieve the best uptake capacities. The recognized curves for the both ions highlighted two segments related to low different adsorption rates (Figure B). The first segment exhibited a rapid change in the uptake rates which appeared in the experimental adsorbed quantities. This was signified from 30 to 600 min for the phosphate ions and from 30 to 360 min for the ammonium ions (Figure B). The second segment was identified from 600 and 360 min for phosphate and ammonium, respectively, to 1320 min showing slight or fixed adsorption rates indicating the attendance of their equilibration states (Figure B). The recorded experimental uptake capacities at their equilibrium time intervals are 82 and 71 mg/g for phosphate and ammonium, respectively (Figure B). This was reported extensively in the literature and explained as a considerable result for the regular occupation of the BE/ZP active receptor sites by phosphate and ammonium ions with expanding the tested time intervals until achieving the full occupation of all the present sites demonstrating the saturation or the equilibration stage.[10,25]

Initial Concentration

The investigation of the adsorption properties of BE/ZP as a function of the pollutant concentrations is of vital role to detect the experimental maximum uptake capacities as well as the uptake behaviors. The plotted results showed an increase in the adsorbed quantities with expanding the studied concentrations from 25 to 300 mg/L for phosphate and from 25 to 250 m/L for ammonium (Figure C). 300 and 250 mg/L are the equilibrium concentrations for phosphate and ammonium, respectively. The fixed adsorption capacities at such equilibration concentrations reflected the complete saturation of the BE/ZP surface by the adsorbed ions achieving maximum experimental capacities of 170 and 155 mg/g for phosphate and ammonium, respectively (Figure C). The recounted increment in the adsorbed quantities at the studied higher concentrations might be related to the effect of such concentrations in promoting the driving forces of both phosphate and ammonium ions in the direction of the BE/ZP receptor sites.[26]

BE/ZP Dosages

The role of BE/ZP dosages in inducing the removal percentages of both phosphate and ammonium ions was studied within the range from 0.02 to 0.14 g (Figure D). The results revealed an excellent increase in the reported removal percentages with the regular incorporation of higher dosages of BE/ZP from 0.02 to 0.14 g for the two ions achieving complete removal for them using the highest dosage (0.14 g) (Figure D). This enhancement in the recognized removal percentages might be related to the predicted increase in the BE/ZP receptor sites, ion-exchange sites, and the total surface area with the incorporation of higher masses of it in the addressed system.[17]

Kinetic and Equilibrium Studies

Kinetic Properties

The kinetic properties of BE/ZP adsorption systems for phosphate- and ammonium-dissolved ions were evaluated according to the commonly investigated models of pseudo-first-order, pseudo-second-order, Elovich, and intraparticle-diffusion models, and their illustrative equations are documented in Table S1. Considering the acquired nonlinear fitting results, the uptake of both phosphate and ammonium is of higher fitting with the pseudo-second-order than pseudo-first-order model based on the correlation coefficient and the Chi-square values (Figure A,B; Table ). However, the pseudo-first-model suggested physical uptake reactions, and the pseudo-second-order model suggested reactions of more chemical affinity which might include the electron-sharing process, electron exchange, surface complexation, and internal diffusion.[27,28] The reported good fitness with the Elovich model gives indication about the energetic heterogeneity of the BE/ZP surface during the uptake reactions and support the estimated conclusion from the pseudo-second-order model.[29] The reported fitness reflected complex adsorption process or the dominance of surface complexation and/or ion exchange (Coulombic attractive forces) as the main mechanisms without the formation or destruction of ionic or covalent bonds. The isotherm models will give support and more explanation for the nature of the adsorption reactions.

Figure 4

Nonlinear fitting of phosphate (A) and ammonium (B) adsorption results with pseudo-first-order, pseudo-second-order, and Elovich models and the intraparticle-diffusion model (C).

Table 2

Theoretical Parameters of Kinetic, Isothermal, and Thermodynamic Studies

model	parameters	phosphate	ammonium
Kinetic Models
pseudo-first order	K1 (min–1)	0.015	0.002
	qe(Cal) (mg/g)	92.3	80.35
	R2	0.88	0.77
	X2	0.66	1.19
pseudo-second order	k2 (g mg–1 min–1)	0.0028	0.0013
	qe(Cal)(mg/g)	89.26	77.95
	R2	0.99	0.96
	X2	0.04	0.17
Elovich	β (g/mg)	0.05	0.056
	α (mg/g min)	1.47	1.27
	R2	0.93	0.88
	X2	0.41	0.57
Isotherm Models
Langmuir	qmax (mg/g)	179.4	161.72
	b (L/mg)	5.13 × 10–5	2.54 × 10–4
	R2	0.99	0.99
	X2	0.13	0.24
	RL	0.98–0.99	0.998–0.99
Freundlich	1/n	0.66	0.5
	kF (mg/g)	195	170.3
	R2	0.98	0.98
	X2	0.45	0.55
D–R model	β (mol2/kJ2)	0.77	0.95
	qm (mg/g)	174.6	166.2
	R2	0.97	0.985
	X2	0.86	0.51
	E (kJ/mol)	0.8	0.72
Thermodynamic Parameters
ΔGo (kJ mol–1)	303 K	–15.09	–14.93
	308 K	–15.59	–15.05
	313 K	–16.16	–15
	318 K	–16.85	–14.73
	323 K	–17.43	–14.42
	328 K	–17.93	–14.02
	333 K	–18.36	–13.27
	338 K	–18.75	–12.49
	343 K	–19.06	–11.85
ΔH° (kJ mol–1)		16.2	–39.6
ΔS° (J K–1 mol–1)		103.49	–79.34

The intraparticle-diffusion kinetic model was assessed to provide more information about suggested mechanisms that can control the uptake reactions of phosphate and ammonium by BE/ZP. The recognized intraparticle diffusion curves displayed three observable segments without intersection between them and the original points, signifying the operation of different types of adsorption mechanisms in addition to the diffusion of dissolved phosphate and ammonium ions[10] (Figure C). The adsorption process first involved the uptake of the ions by the surficial or the external receptors forming the first segment which appears as the dominant mechanism and characterizes the initial stages of the reactions. After the saturation of the external receptors, the process appeared to be controlled by the layered adsorption mechanism with restricted diffusion for the phosphate and ammonium ions forming the second adsorption segments in the curves. After attending the equilibration or the complete saturation, the third segment of the curves was identified which refers to the formation of thick layers from the adsorbed phosphate and ammonium on the BE/ZP surface. This stage characterized by an interionic attraction and/or molecular association as the principal uptake mechanisms.[30]

Isotherm Properties

The isotherm properties of BE/ZP during the uptake of both phosphate and ammonium pollutants were assessed by the nonlinear fitting of the results with Langmuir, Freundlich, and Dubinin–Radushkevich models (Figure A,B). The representative theoretical equations of the inspected models are listed in Table S1 and the isotherm parameters appear in Table . The supposition of the Langmuir model suggested homogeneous and monolayer uptake of both phosphate and ammonium by BE/ZP while the Freundlich model suggested multilayer and heterogeneous uptake processes for them.[3,31] Considering the values of Chi-squared (χ2) and the correlation coefficient, the adsorption results of phosphate and ammonium by BE/ZP are of strong agreement with Langmuir as well as Freundlich model with a preference for the Langmuir model as the fitting results are of low Chi-squared (χ2) values (Figure A,B; Table ). The calculated theoretical values of the RL parameter are less than one for both phosphate and ammonium reflecting favorable reactions (Table ). Also, the values of the heterogeneity factor of the Freundlich model give an indication about the heterogeneous properties of the BE/ZP surface during the uptake of phosphate and ammonium with minimum interaction expectations.[32] Moreover, the appraised theoretical maximum phosphate and ammonium uptake capacities are 179.4 and 161.7 mg/g, respectively (Table ).

Figure 5

Nonlinear fitting of the phosphate (A) and ammonium (B) adsorption results with Langmuir, Freundlich, and D–R isotherm models.

The Dubinin–Radushkevich model (D–R) was studied to identify the nature of the effective mechanisms during the uptake of phosphate and ammonium contaminants by BE/ZP (physical or chemical) depending on the role of its related parameters in calculating the Gaussian adsorption energy.[33] The fitted adsorption results of phosphate and ammonium are of excellent agreement with the D–R model, and the theoretical parameters of the model showed 174.6 and 166 mg/g as the theoretical values for the maximum uptake of phosphate and ammonium by BE/ZP, respectively (Figure A,B; Table ). Additionally, the theoretical adsorption energies of phosphate and ammonium are 0.8 and 0.72 kJ/mol, respectively (Table ). Such values imply physisorption uptake of both phosphate and ammonium by BE/ZP which might be occurred by coulumbic attractive forces. Such types of adsorption refer to the ion-exchange process that involves weak electrostatic attractions without construction or destruction of any type of chemical bonds (ionic or covalent).[4,34] This is supported by the high agreement between the presented results and the reported results by Inglezakis and Zorpas,[35] as they stated that the detection of the adsorption energies within the range from 0.6 to 25 kJ/mol related mainly to ion-exchange mechanisms which were identified for zeolite-based adsorbents.

Thermodynamic Studies

The thermodynamic investigation of BE/ZP adsorption systems for phosphate and ammonium was accomplished based on the determined theoretical values for the Gibbs free energy (ΔG°), the enthalpy (ΔH°), and the entropy (ΔS°). The ΔG° values during the uptake reactions of phosphate as well as the ammonium ions are calculated from eq . The obtained values of ΔS° and ΔH° were appraised as the theoretical parameters for the performed linear fitting of the phosphate and ammonium adsorption results with the commonly used Van’t Hoff equation (eq )[9,36] (Figure ).

Figure 6

Fitting of the adsorption results of phosphate and ammonium ions by BE/ZP with the Van’t Hoff equation.

The recognition of ΔGo values with negative signs at all the studied temperature values for both phosphate and ammonium ions suggested spontaneous uptake of them by the BE/ZP composite[16,37] (Table ). The increase in the calculated adsorption ΔGo values for phosphate related to the decrease in the favorability of the reaction at the tested higher temperature values and the reverse behavior was observed for the uptake of ammonium by BE/ZP. For ΔH° values, the observed positive sign for phosphate and negative sign for ammonium reflected endothermic and exothermic uptake reactions, respectively (Table ). The positive sign of ΔS° during the uptake of phosphate reflected increase in the randomness degree of the reaction with the tested temperature values and suggested high affinity of BE/ZP for phosphate ions, and the negative sign of ΔS° for the ammonium ions reflected a significant decrease in the reaction randomness[10,34] (Table ). Finally, the values of the Gibbs free energies and enthalpies for the uptake of both phosphate and ammonium within the range of physisorption mechanisms support the previously estimated conclusion from the isotherm studies (Table ).

Reusability Properties

The evaluation of the BE/ZP composite as a sustainable adsorbent for phosphate and ammonium which can be reused effectively is of commercial and economic value. The reusability properties of BE/ZP were assessed eight times and the experimental results proved excellent stability and reusability for the BE/ZP composite (Figure ). For the phosphate removal reusability tests, the results showed successful decontamination of phosphate with percentages of 100, 98, 93.5, 88.2, 82, 75.8, 69, and 61.5% with repeating the decontamination reusing runs by run 1, run 2, run 3, run 4, run 5, run 6, run 7, and run 8, respectively. This also was reported for the ammonium decontamination reusability tests, and the accomplished percentages for the same evaluated runs are 100, 99, 96.5, 93.2, 90.8, 86.5, 81.2, and 74.5% in order. The observed higher stability of BE/ZP during the uptake of ammonium as compared to the uptake of phosphate might be related to the tendency of the adsorbed phosphate ions to form a strong complex with the external functional groups of both the synthetic zeolite Na–P and the bentonite substrate which will be discussed further in the mechanism section.

Figure 7

Reusability properties of the BE/ZP composite in the removal of the phosphate and ammonium ions.

Suggested Mechanism

The investigated adsorbent in this study is a novel composite from two types of silicate minerals. The composite is advanced integration between layered silicate montmorillonite and tectosilicate microporous zeolite Na–P which was formed by the controlled alkaline transformation of the raw bentonite sample (Figure ). Such an integration process resulted in a hybrid product of higher porous properties, higher surface area, and higher ion-exchange properties than the single phases of bentonite and zeolite. Additionally, the alkaline treatment affected greatly the crystalline and surficial properties of montmorillonite layers. The montmorillonite became enriched in the sodium ions as substituted ions within the tetrahedral and octahedral units, as trapped ions in molecular cavities of the tetrahedral sheets, and as free ions within the interunit water layer[10] (Figure ). Moreover, alkaline modification is of strong etching effect in the silicate and aluminosilicate minerals as it causes more exposure for the surficial siloxane functional groups.[10] This is of positive impact on the ion exchange and the adsorption properties under the reported modifications in the structural and surficial properties.[38]

Figure 8

Schematic diagram for the adsorption mechanism of phosphate and ammonium by the BE/ZP composite.

The adsorption mechanisms of phosphate and ammonium by such heterogeneous structures can be explained for each structure as a separate phase. The uptake of them by the synthetic zeolite Na–P was controlled mainly by both the ion exchange and adsorption mechanisms in addition to minor effects for other assist processes.[39,40] The ion-exchange process in the zeolite adsorption system is known as the outer surface complexation mechanism and involves the replacement of either the free or the structural cations by the dissolved ions in the surrounding environments.[41] The adsorption mechanism which is known as the inner surface complexation process involves the formation of chemical bonds between the adsorbed ions and the main functional groups of zeolite Na–P and/or the electrostatic attractions for the ions by these groups.[42,43] For the adsorption mechanisms of montmorillonite, the adsorption properties were signified to the ion exchange between the free ions in the interunit water layer and the tested dissolved ions in addition to the electrostatic attractions with its chemical groups.[4]

Generally, the phosphate adsorption by silicate minerals occurred by both the chemical interaction and the electrostatic attraction.[24] The electrostatic process was represented mainly by the Coulombic attractive forces between the present phosphate ions and the main binding sites of BE/ZP (Figure ). This mechanism involved the formation of monodentate and bidentate complexes within the pH range from the values lower than the point of zero charge (PZC) to the value higher than the PZC according to eqs and 4.[24,44]

The adsorption of ammonium either by montmorillonite or by zeolite Na–P was controlled mostly by the ion-exchange processes, as summarized in eqs –7. Other studies demonstrated the controlling of the ammonium adsorption by (A) the successful replacement of the zeolitic interchannel water and the montmorillonite interlayer water by the dissolved ammonium ions creating coordination complexes with the existing replaceable ions, (B) the occurrence of physical trapping for the ammonium ions within the interchannel of the zeolite structure or between the montmorillonite units, and (C) the normal electrostatic attraction processes[45,46] (Figure ).

Comparison Study

The adsorption capacities of BE/ZP for phosphate and ammonium were compared with different types of addressed adsorbents in the literature considering the estimated theoretical qmax (Table ). As compared to the presented natural and synthetic adsorbents, BE/ZP exhibited higher uptake capacities for both phosphate and ammonium pollutants. It achieved better results than natural and synthetic zeolite as well as their modified products, natural clay minerals and their modified products, and carbonaceous adsorbents from different resources (Table ). Also, the composite displayed higher results than some advanced adsorbents as graphene and graphene oxide-based adsorbents, nanometal oxide-based adsorbents, different types of modified mesoporous silica, and layered double hydroxide (Table ). Considering the obtained results, the synthetic BE/ZP is an advanced adsorbent for phosphate and ammonium which can be used to capture them effectively from real water samples and can be reused again as a fertilizer carrier.

Table 3

Adsorption Capacities of BE/ZP for Phosphate and Ammonium in Comparison with Different Types of Synthetic and Natural Adsorbents

phosphate			ammonium
adsorbents	qmax (mg/g)	ref.,	adsorbents	qmax (mg/g)	ref.,
MCM-41/rice husk	21	(15)	zeolites	10.72	(63)
Mg(OH)2/ZrO2	87.2	(47)	rice husk biochar	39.8	(64)
lanthanum hydroxides	107.5	(48)	AC treated with nitric acid	28.43	(65)
titanium modified zeolite	37.60	(24)	mesolite	49	(66)
La100SBA-15	45.6	(49)	turkey clinoptilolite	30	(67)
calcined Mg–Al-LDHs	40.78	(50)	natural bentonite	26.63	(68)
ZrO2 nanoparticles	99	(51)	carbon-zeolite composite	27.47	(69)
smectic clay	42.19	(52)	modified corncob-biochar	22.6	(8)
zirconia/graphite oxide	149.3	(53)	vermiculite	50.06	(17)
titania/GO	33.11	(54)	Mg-biochar	51.48	(70)
La doping magnetic graphene	116.28	(55)	DODMAB/montmorillonite	76.92	(71)
hydrous zirconium oxide	51.8	(56)	modified purified bentonit	46.9	(68)
milled furnace slag	43.1	(57)	HDTMAB/clinoptilolite	125	(71)
kaolintic clay	38.46	(52)	natural zeolite	43.47	(72)
Fe–Mn binary oxide	36	(58)	Na-zeolite from coal fly ash	109	(73)
zeolite A	52.91	(52)	palygorskite	237.6	(74)
biochar	133	(59)	HDTMAB/montmorillonite	80.65	(71)
MgO	75.13	(60)	MgO/diatomite	121.07	(11)
Fe-based MOFs of MIL-101	106.67	(61)
NH2–Fe-based MOFs of MIL-101	121.67	(61)
halloysite	3.56	(62)
Fe2O3 doped halloysite	5.13	(62)
MgO/diatomite	63.3	(11)
BE/ZP	179.4	this study	BE/ZP	161.7	this study

Realistic Study

The appropriateness of BE/ZP to be used directly in realistic remediation of sewage water and groundwater samples from the present phosphate and ammonium contaminants was evaluated by mixing 0.14 g of BE/ZP with 100 mL of the tested water samples for 600 min. After that, the solid adsorbents were separated and the treated water samples were evaluated depending on the full chemical analysis of the samples. The chemical analysis of the studied sewage water sample demonstrated the existence of phosphate, ammonium, and nitrate in concentrations higher than the permitted limits. Additionally, other types of metal ions as iron, zinc, and copper in addition to COD were detected but at concentrations lower than the limits (Table ). After the real remediation of the sewage water by BE/ZP, the chemical analysis reflected the reduction of phosphate, ammonium, and nitrate to concentrations lower than the permitted limits for purified sewage water (Table ). Moreover, the results reflected a significant reduction in the COD content and complete decontamination of the detected metal ions and sulfide (Table ).

Table 4

Chemical Analysis of Sewage Water and Groundwater Sample before and after Treating Them with the BE/ZP Composite

sewage water				groundwater
analyses	sample	after treatment	limit (max)	analyses	sample	after treatment	limit (max)
pH	7.8	8.4	6.5–9	turbidity (NTU)	0.45	0.35	1
TSS (mg/L)	18	16	50	pH	7.49	8.1	6.5–8.5
TDS (mg/L)	634	612	2000	alkalinity (mg/L)	189	124
BOD (mg/L)	6	3.8	60	conductivity (Us/cm)	1315	578
COD (mg/L)	30	9.8	80	total hardness (mg/L)	191	80.6	500
oil & grease (mg/L)	7	1.77	10	Ca hardness (mg/L)	112	47.2	350
sulphide (mg/L)	0.01	nil	1	Mg hardness (mg/L)	79	33.4	150
ammonium (mg/L)	12.3	2.3	10	chloride (mg/L)	103	95.4	250
nitrate (mg/L)	27	6.2	30	sulfate (mg/L)	150	112.3	250
phosphate (mg/L)	4	nil	2	ammonia (mg/L)	0.62	nil	0.45
iron (mg/L)	0.5	nil	3.5	nitrite (mg/L)	0.009	nil	0.9
Cu (mg/L)	0.01	nil	0.5	nitrate (mg/L)	7.05	nil	45
Zn (mg/L)	0.011	nil	0.2	iron (mg/L)	0.38	nil	0.3
				manganese (mg/L)	0.91	nil	0.4
				Cu (mg/L)	0.009	nil	2
				Zn (mg/L)	0.02	nil	3

For the evaluated groundwater sample, it showed significant concentrations of the total hardness elements and the sulfate, ammonia, and nitrate pollutants without detection of phosphate ions (Table ). Also, the iron (Fe) and manganese (Mn) contents were identified at concentrations above the acceptable limits for groundwater (Table ). The results after the remediation process accomplished complete decontamination of the existing ammonia, nitrate, iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn) pollutants (Table ). Also, the application of BE/ZP resulted in an excellent reduction in the content of the total hardness and sulfate pollutants from the water sample. Such results either for the sewage water or the groundwater support the suitability of BE/ZP materials to be used effectively in the realistic water remediation applications.

Conclusions

The bentonite/zeolite-P composite (BE/ZP) was synthesized as a potential adsorbent of enhanced capacity for phosphate and ammonium pollutants. The composite is of 512 m2/g surface area, 387 meq/100 g ion-exchange capacity, and 5.8 nm average pore diameter. The adsorption behaviors were controlled strongly by the pH value, and the best results were achieved at pH 6. The kinetic studies indicated the agreement of phosphate and ammonium uptake results with the pseudo-second-order behavior, reflecting equilibration time intervals of 600 and 360 min, respectively, and maximum experimental capacity of 170 and 155 mg/g, respectively. The equilibrium properties of both phosphate and ammonium fitted with the Langmuir hypothesis suggesting homogeneous and monolayer uptake of them by BE/ZP with a theoretical qmax of 179.4 and 166 mg/g for phosphate and ammonium, respectively. Moreover, the calculated Gaussian adsorption energies [0.8 kJ/mol (phosphate) and 0.72 kJ/mol (ammonium)] suggested physisorption for them with the mechanism close to the zeolitic ion-exchange process or coulumbic attractive forces. This was supported by the estimated thermodynamic parameters which imply spontaneous uptake by an endothermic reaction for phosphate and an exothermic reaction for ammonium. The BE/ZP composite is of excellent reusability and used for eight recyclability runs, achieving removal percentages of 61.5 and 74.5% for phosphate and ammonium, respectively, at the final run. Finally, the composite was applied in the purification of sewage water and groundwater, achieving complete removal of phosphate from sewage water and ammonium from ground water and reduction of the ammonium ions in the sewage water to 2.3 mg/L.

Experimental Section

Materials

Natural bentonite samples collected from the Western desert bentonite quarry and NaOH pellets (97%, Sigma-Aldrich) were used in the preparation of the composite. The studied aqueous solutions of phosphate and ammonium pollutants were prepared by dilution processes for standard solutions of them (1000 mg/L, Sigma-Aldrich).

Preparation of the Bentonite/Zeolite Composite (BE/ZP)

The composite was prepared according to Abukhadra et al.[20] The bentonite sample was ground to fine fractions of size less than 70 μm and heated to about 750 °C for 4 h as a thermal activation step to produce a chemically reactive aluminosilicate product of a more flexible chemical structure. The activated bentonite (6 g) was treated by NaOH solution (12 g in 100 mL of distilled water) under vigorous stirring (500 rpm) for 2 h at 70 °C. Then, the mixture was set into a conversion reactor consisting of a Teflon autoclave and lined by stainless as a closed alteration system after fixing the conditions at 150 °C for 4 h. By the end of the alteration time and after cooling the system, the solid fractions were separated by centrifugation, washed extensively, and dried at 65 °C for 24 h.

Characterization Techniques

The structural properties of bentonite (BE) and the synthetic BE/ZP composite were examined based on the XRD patterns which were obtained using a PANalytical X-ray diffractometer (Empyrean). A scanning electron microscope (Gemini, Zeiss-Ultra 55) was used to study the changes in the morphological features. Additionally, the chemical functional groups were investigated using the FTIR-FT Raman spectrometer (Vertex 70). The changes in the textural properties especially the surface area, as well as the pore-size distribution, were studied based on the plotted nitrogen adsorption/desorption curves depending on Brunauer–Emmett–Teller and Barrett–Joyner–Halenda, respectively.

Phosphate and Ammonium Decontamination

The adsorption tests were conducted as triplicate tests and the mentioned results are the average values with the standard deviation less than 4.2 and 3.5% for the uptake of phosphate and ammonium, respectively. The BE/ZP fractions which were used in the adsorption tests are in the size range from 10 to 23 μm (50% of the sample with size less than 12 μm and 95% of the sample with size less than 20 μm).

Effect of the pH Value

The pH value is the main controlling parameter in the adsorption systems as it controls the surface properties of BE/ZP and the dissolved ion speciation. The influences of pH values were studied within a range from pH 2 to pH 8 after fixing the other factors at 0.02 g as BE/ZP dosage, 100 mg/L as ammonium and phosphate concentrations, 100 mL as aqueous solution volumes, 120 min as contact time, and 30 °C as reaction temperature. By the end of each test, the residual concentrations were determined using the ion chromatography technique.

Kinetics

The influence of the uptake time and the kinetic behaviors was studied for different selected time intervals within a systematic range from about 30 min to about 1320 min. The other reacting factors were adjusted at 0.02 g of BE/ZP as dosage, 100 mL as volume, 100 mg/L as inspected concentration, pH 6, and reaction temperature of 30 °C. The kinetic behavior of both phosphate and ammonium was examined by mathematical fitness between the obtained results and four theoretical kinetic models (pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models).

Effect of Adsorbent Dose

The role of the composite masses on enhancing the decontamination percentages was studied for different dosages from 0.02 g to about 0.14 g. This was accomplished after adjusting the conditions at pH 6, 30 °C as temperature, 100 mL as volume, 1320 min as time interval, and 100 mg/L as concentration.

Equilibrium Studies

The equilibrium behavior and the isotherm studies were assessed to investigate the adsorption mechanism of the BE/ZP composite. The experiment tests were designed to be accomplished at a fixed time of 1320 min, a fixed BE/ZP dose of 0.02 g, a fixed volume of 100 mL, pH 6, and the concentration within a range from 25 mg/L to about 400 mg/L. The investigated equilibrium models involved Langmuir, Freundlich, and Dubinin–Radushkevich models.

Thermodynamic Studies

The adsorption behavior of ammonium and phosphate at different temperature values is a vital factor in controlling the adsorption capacity and detecting the nature of the reactions. The thermodynamic tests were accomplished after fixing the conditions at 120 min as uptake time, 0.02 g as BE/ZP mass, 100 mL as volume, and 100 mg/L as the studied concentrations, and the inspected temperature was adjusted within a range from about 303 to 343 K.

Reusability of the Adsorbent

Applicability of reusing the BE/ZP in several cycles of adsorption removal for the ammonium and phosphate is of critical value in the commercial sale. The reusability experiments were conducted for eight cycles by mixing 0.14 g of BE/ZP with 100 mL of distilled water contaminated with ammonium and phosphate ions (100 mg/L) for 1320 min at pH 6. After each experiment, the solid fractions were separated, washed, dried at 80 °C, and reused again in the next cycle.

Realistic Application

After the experimental detection of the best adsorption parameters, the synthetic BE/ZP composite was used directly in the elimination of ammonium and phosphate ions from real water samples (groundwater and sewage water). The inspected groundwater samples were collected as representative samples for groundwater from different wells in Beni-Suef Governorate, Egypt, and the sewage samples were collected from some sewage water stations as final treated products. The water samples were preserved within polypropylene bottles and acidified using diluted nitric acid. Finally, the samples were kept in a refrigerator at 2 °C for further studies.