Sonochemical immobilization of MnO2 nanoparticles on NaP-zeolite for enhanced Hg (II) adsorption from water

Journal Pre-proof Sonochemical immobilization of MnO2 nanoparticles on NaP-zeolite for enhanced Hg (II) adsorption from water Ayoob Bahiraei, Jamshid Behin

PII:

S2213-3437(20)30138-X

DOI:

https://doi.org/10.1016/j.jece.2020.103790

Reference:

JECE 103790

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

29 October 2019

Revised Date:

11 February 2020

Accepted Date:

15 February 2020

Please cite this article as: Bahiraei A, Behin J, Sonochemical immobilization of MnO2 nanoparticles on NaP-zeolite for enhanced Hg (II) adsorption from water, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103790

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Sonochemical immobilization of MnO2 nanoparticles on NaP-zeolite for enhanced Hg (II) adsorption from water

Ayoob Bahiraei, Jamshid Behin* Advanced Chemical Engineering Research Center, Faculty of Petroleum and Chemical Engineering, Razi

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University, Kermanshah, Iran

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lP

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Graphical Abstract

Research highlights: 

MnO2 nanoparticles were loaded sonochemically on NaP-zeolitic support



Homogeneous distribution of nanoparticles was achieved in sonochemical method compared to

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conventional one.



Ultrasound irradiation provides thermal and mechanical energies required for reaction of precursors

 *

The sonochemical loading improves the BET surface area of adsorbent significantly

Corresponding author E-mail: [email protected]

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

The synthesized MnO2/NaP-zeolite was applied efficiently for mercury removal from drinking water

Abstract MnO2 nanoparticles were loaded on NaP-zeolite sonochemically to assess their efficiency for

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mercury removal from water, and the substrate was synthesized via hydrothermal conversion of

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clinoptilolite. The ultrasonic waves supplied the reaction temperature and provided loading homogeneity. The influence of nanoparticle loading, concentration of potassium permanganate

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solution, and concentration of hydrochloric acid were investigated on the surface area and adsorption capacity of the product. The optimal synthesis conditions were obtained with

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ultrasonic waves, 15 wt% MnO2 loading, 0.05 mol.L-1 KMnO4 solution, and 36 wt% HCl

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concentration. The microscopic images showed that the ultrasonic-assisted method resulted in nano-sized (<10 nm) and uniformly distributed MnO2 particles on the surface of the substrate.

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Compared with conventional loading, the ultrasonic one resulted in 24.4% and 8.5% improvement in BET surface area and removal efficiency, respectively. The dynamics of adsorption from water at a very low initial concentration of mercury (10 µg.L−1) and the influence of competitive copper ions were investigated. The dual nature of ultrasonic waves

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helped synthesize a cost-effective adsorbent that fulfills the need for high quality drinking water at a large scale.

Keywords: MnO2 immobilization; Sonochemical heating; Nanoparticle loading; Trace mercury adsorption; Water decontamination. 2

1. Introduction Nowadays, water quality standards are becoming more stringent globally. The existence of heavy metal pollutants, especially mercury (Hg), in the environment is an important issue for human health leading to different disorders and neurological and renal dysfunctions [1]. Hg is listed as a bio-accumulative toxic material in many technical reports of international regulatory

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organizations [2]. Various countries have executed strict regulations for reducing Hg emissions

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in water. Almost all Hg in freshwater (>0.5 μg.L−1) is in the form of highly reactive Hg(II) which is bio-converted to methylmercury [3]. The governments of many countries have established

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recommended safety limits of aquatic food consumption to protect their people from Hg

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poisoning (highest intake is 20 μg.day−1 per person) [4]. Accordingly, Hg levels must be controlled in underground and surface waters to meet the recommended levels. The highest

Protection Agency [5].

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content of Hg ions in potable water was determined to be 2 µg.L−1 by the US Environmental

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Removing Hg2+ from water is always a technical challenge because of the current accepted regulations. Conventional methods including synthetic ion-exchange resins, membrane filtration, photoreduction, and precipitation procedures generally require a significant amount of energy as well as chemicals. Among the various methods, adsorption is an economical, simple, effective,

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and reliable alternative procedure that has been successfully employed [6-8]. The removal of Hg2+ from aqueous media has been studied using different types of adsorbents initiated with activated carbon and its modified forms [9]. Carbon has also been replaced with some cost-effective materials such as zeolites [10], fly ashes [11], coal fly ash [12], chitosan [13], biomass, animal bone charcoal, and raw natural materials like coconut pith, bamboo leaf, 3

and rice straw [14-16], among them natural zeolites are the most promising materials. Surface modification of natural zeolite (clinoptilolite) improves its mechanical resistance to physicochemical conditions and its adsorption capacity [17, 18]. Among the different types of modified clinoptilolites, NaP is one of the most appropriate structured zeolites with a small pore size that has been successfully applied for heavy metal removal [19]. In the last two decades, nanoadsorbents have been proposed for the removal of water pollutants.

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Commonly used nanoparticles (NPs) for water treatment are fabricated from alumina, silica,

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cadmium sulfide, cobalt ferrite, zinc sulfide, zinc oxide, copper oxide, iron oxide, iron hydroxide, nickel oxide, tin oxide, titanium oxide, and manganese oxide.

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Nano-sized manganese oxide (MnO2) has a negative surface charge with high affinity for metal

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ions [20-22] and is the most important scavenger of aqueous trace metals [23]. Nano-sized MnO2 particles can be immobilized on the zeolitic surface, thus improving the reactive materials to

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enhance heavy metal decontamination from aqueous media. They can overcome practical limitations such as weak thermo-mechanical resistance and pressure drop for continuous

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adsorption processes [24-26]. Simultaneous synthesis and loading of NPs on structured solid substrates, such as NaP-zeolite, is a convenient method to control particle size distribution, enhance reactivity, and improve mechanical resistance [27]. Hydrothermal, chemical, biological, microwave, sonochemical and hybrid methods are different

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ways to synthesize and load NPs. New technologies have been focusing on improving surface characteristics, using low-cost sorbents and minimizing energy consumption of such materials. Application of new sources of energy such as ultrasound for rapid manufacture of metal oxides in the nanometer dimensions can lead to homogenous nucleation, uniform size distribution, improved phase purity, and high surface area [19, 28, 29]. 4

The goal of this study was to use the naturally occurring zeolite with surface modification to enhance its mechanical properties as well as its retention of Hg(II). Compared with chemical precursors, clinoptilolite is a cost-effective material for large-scale applications. The preparation of NaP-zeolite from clinoptilolite and loading nanostructured MnO2 on it were carried out by a simple route. Although some investigations have been conducted on the employment of MnO2 for heavy metal removal especially Hg(II) from water [30,31], no study has focused on the

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sonochemical synthesis of nanoscale MnO2/NaP-zeolite as well as its application for the ultra-

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purification of Hg(II) contaminated water. The dual effect of ultrasound cavitation, i.e., simultaneous shocking and heating, was served to propose an economic way for preparation of a

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superior adsorbent. It is expected that the developed adsorbent will bring the Hg concentration of

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the treated water to the standard levels.

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2. Materials and methods

Clinoptilolite is an abundant low-cost natural zeolite resource with huge sedimentary deposits in

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most regions of the world. Iranian clinoptilolite (Semnan Negin Powder Co., Iran) was used as the starting material for fabricating NaP-zeolite. The chemical composition of the clinoptilolite is shown in Table S1 (supplementary information). Sodium hydroxide and sodium chloride (Kian Kaveh Azma, Iran) were used for NaP-zeolite preparation. Potassium permanganate (KMnO4),

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hydrochloric acid (HCl) and anhydrous mercury chloride (HgCl2) of analytical grade were obtained from Merck (Germany) and were used for nanoparticle synthesis and adsorption analyses. Double distilled water (conductivity ~2 μS·cm−1) was used to make all solutions. An ultrasonic apparatus (UP400S Hielscher, Germany) including a sonotrode H22 with the highest processor power of 400 W and a frequency of 24 kHz was used for the sonication process. 5

2.1. Preparation of NaP-zeolite Clinoptilolite has relatively weak mechanical toughness and low hydrothermal stability, and forms feeble chemical bonds with metals [32]. Sieved-powder-raw clinoptilolite was firstly soaked in tap water for almost 8 h and rinsed with sufficient deionized water to reduce alkalescence and other contamination. Pre-washed zeolite was subjected to drying at an ambient

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temperature for 24 h. To prepare the Na-form, the obtained clinoptilolite was suspended in 1

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mol.L−1 NaCl at a 1:6 mass ratio and shaken using a magnetic homogenizer for 24 h. The suspension was filtered, rinsed twice with deionized water, and then, dried in an oven at 90 °C

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overnight. The zeolite was then hydrothermally converted to the NaP-form within a Teflon

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reactor where the desired amount of the natural precursor was added to a 1.5 mol.L−1 solution of NaOH (0.07 g.mL−1). The stirred reactor was immersed in a water bath at 80 °C for a period of

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24 h under atmospheric pressure. The suspended solid was separated and washed with sufficient deionized water until the pH value of the filtrate reached approximately 7. The prepared NaP

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samples were dried at ambient temperature (25±3 °C) overnight and kept for further anlysis. Notably, a rapid procedure for NaP-zeolite synthesis, based on ultrasonic irradiation, has also been presented in literature [19].

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2.2. Nanoparticle loading

Conventional reduction and ultrasonic-assisted methods were used for the synthesis of MnO2 nanoparticles supported on NaP-zeolite [33]. In both methods, a reductive chemical reaction produced MnO2 followed by precipitation of colloids of metal oxides onto the NaP-zeolite surface that can be expressed as follows: 6

3 KMnO4 + 4HCl → MnO2 + KCl + Cl2 + 2H2 O 2

(1)

In each experimental run, 5 g NaP-zeolite was added into a jacket batch reactor containing preheated 1 mol.L−1 potassium permanganate at 80 °C. The reaction temperature was kept

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constant at 80 °C under reflux condensing. The synthesis procedure begun by pouring hydrochloric acid drop-wise into the solution, and the reaction continued under vigorous

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agitation to obtain a well-homogenized mixture. After completing the reaction for 1 h, the

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suspension was subjected to a filtration process. Then, the recovered particles were washed thoroughly with deionized water several times to eliminate free potassium and chloride ions,

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until a neutral filtrate (pH: 7) was achieved. The solid sample was dried at 90 °C for 24 h in an oven and maintained in a sealed polypropylene bag for characterization and further experiments.

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The chemical reduction experiments were conducted in two modes: i) under 550 rpm magnetic agitation and using a water bath (conventional) and ii) under ultrasound radiation without

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external heating and mixing. In the latter mode, the suspension was sonicated with 20% of the highest power (equivalent to 80 W). Upon preliminary experiments, a working cycle period of 0.2 (apparatus off) and 0.8 (apparatus on) was selected for all runs. The reasons for choosing

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these conditions were i) the solution temperature was raised to 80 °C in less than 5 min and remained almost constant over the entire experiment, and ii) a relatively low power consumption was required and appropriate mixing was achieved.

2.3. Characterizations 7

The X-ray diffraction patterns (XRD) of the raw and prepared samples were obtained using an AXS D8 Bruker diffractometer (Germany) equipped with Cu-Kα irradiation (5-80°). A PerkinElmer spectrometer (USA) was used to achieve the Fourier transform infrared (FTIR) spectra. Photo-micrography and elemental analyses of the surface were carried out using a scanning electron microscope (SEM) at 20 keV and SD (Sirius, UK) energy-dispersive X-ray spectroscopy (EDS) system of Tescan Vega3 (Czech Republic). Field emission electron scanning

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microscopy (FESEM) was performed for the synthesized samples using an S-4160 Hitachi

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instrument (Japan). An 851e Mettler Toledo TGA analyzer (Switzerland) accompanying Stare software (v 6.1) was used to carry out thermo-gravimetric analyses. Measurements were

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performed with a dynamic temperature program at a heating rate of 10 °C.min−1 from 25 to 600

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°C under N2 purge. The particle size distribution (PSD) was determined by applying Malvern Mastersizer 2000 (England) where the liquid medium was deionized water without any

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dispersant.

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2.4. Adsorption/desorption experiments

Appropriate care should be taken in all sampling and measuring procedures because it is crucial to analyze low concentrations of Hg. Therefore, all glassware were immersed in nitric acid (25%) and washed with deionized water prior to use.

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A stock solution containing 1000 mg.L−1 Hg(II) was obtained by dissolving anhydrous HgCl2 in double-distilled water and was used to prepare dilute solutions with the desired concentration of Hg(II). Ion-exchange batch-mode experiments were conducted in a 50 mL highly resistant polypropylene container. To determine the adsorption capacity of the synthesized adsorbent, 0.5 g of sample was poured into a 15 mL solution containing an initial Hg2+ concentration of 20 8

ppm. The suspension was shaken for 2 h at room temperature. At the end of the experiment, the suspension was filtered through a microporous membrane of 0.45 µm and the two phases were separated completely. Two droplets of 1 mol.g−1 potassium-dichromate (1:1 nitric acid/distilled water solution) were added to stabilize the residual Hg concentration in the supernatant liquid. The amount of adsorbed Hg2+ was obtained from the difference between the initial and final detected metal concentrations. An Atomic Absorption Spectrometer (AAnalyst 300, Perkin-

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Elmer) at 253.7 nm was used to analyze the Hg2+ concentration. Calibration solutions of Hg2+

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were prepared by dissolving different amounts of HgCl2 in the stabilized solution. The different amounts of Tin (II) chloride in the HCl solution, depending on Hg concentration, were also

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prepared. After adding SnCl2 as a reducing agent, the atomic absorbance of mercury was

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immediately measured. A blank using stabilized solution as a sample solution was also measured. The measured mercury content was subtracted from that obtained for each sample

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solution. The absorbance was converted to the concentration using the calibrated curve. To regenerate the spent adsorbent and investigate the reusability of MnO2/NaP-zeolite,

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successive adsorption/desorption cycles were performed five times. Regeneration experiments were carried out using a 5% NaCl solution (20 ml.g−1 metal-saturated adsorbent) in batch mode at two temperatures of 25 and 50 °C for 12 h under moderate shaking (20 rpm). Then, the solid sample was subjected to filtration, washing, and drying (100 °C) operations for further use. The

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subsequent processes were similar to the above cycle.

2.5. Design of experiment To quantify the impact of operating factors on the characteristics of loaded samples, response surface methodology (RSM) was employed using a two-level fractional factorial design (FFD). 9

One categorical factor, i.e., the loading method (conventional or ultrasound-assisted), as well as three numerical ones along two center points were adopted for the optimization goal. Loading of NPs (5-15 wt%), concentration of potassium permanganate solution (0.05-0.15 mol.L-1), and concentration of hydrochloric acid (12.5-37.5 wt%) were considered as numerical factors. At higher loading, the agglomeration of NPs may result in blockage of the substrate's pores and a loss in the accessible surface area. The design proposed 12 runs, as shown in Table 1. The BET

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surface area of the synthesized product, determined using the N2 adsorption-desorption data, and

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Hg removal efficiency were selected as the response factors.

The concentration of Hg2+ (mg.L−1) in the aqueous solution was measured before (Co ) and after

(2)

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Ct ) × 100 Co

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η(%) = (1 −

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(Ct ) adsorption. The Hg removal efficiency (η) was determined using the following equation:

The adsorption capacity (q t ) was defined as the amount of Hg cations adsorbed by the adsorbent

qt =

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(mg.g−1) at time "t" and was determined according to the following equation:

v(C0 − Ct ) m

(3)

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where v(L) is the volume of the Hg aqueous solution and m(g) is the dry mass of the adsorbent used in the experiment. Under equilibrium conditions, Ct and q t were denoted as Ce and q e , respectively. Each experimental run was performed in duplicate, and the average removal efficiency was reported.

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2.6. Adsorption kinetics and thermodynamics Two of the most widely used kinetics models, i.e., pseudo-first-order and pseudo-second-order models were used to evaluate the kinetic of the Hg2+ adsorption onto MnO2/NaP-zeolite. Pseudofirst-order model is defined as: Ln (q e − q t ) = Ln q e − k1 . t

(4)

Pseudo-second-order model:

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t 1 t = + q t k 2 q2e q e

(5)

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where k1 (min−1) and k2 (g mg−1 min−1) are the rate constants of the adsorption.

Two adsorption isotherm models were also considered including linear form of Langmuir (eq. 6)

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1 Ln q e = ( ) lnCe + ln K F n

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Ce 1 Ce = + q e bq m q m

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and Freundlich isotherm (eq.7) expressed as:

(6)

(7)

where q m (mg.g−1) and b (L. mg−1) represent the maximum adsorption capacity and energy of the adsorption (equilibrium constant) in Langmuir model, respectively; and KF (Ln mg1/n g−1) and

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"n" are the Freundlich constants indicating adsorption capacity and favorability of adsorption, respectively.

Thermodynamic consideration of adsorption process can imply whether the process is spontaneous or not. The thermodynamic parameters, namely Gibbs free energy (∆G°), enthalpy (∆H°), and entropy (∆S°) differences should be used to determine spontaneity and heat change of 11

the adsorption process. The change in Gibbs free energy of the process is related to the equilibrium constant by the following equation [23]: ∆G° = −RT lnK D

(8)

where KD (qe/Ce) is the distribution constant. ∆H° and ∆S° can be calculated from the slope and intercept of the Van’t Hoff’s plot of ln KD versus 1/T:

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∆S° ∆H° 1 − ( ) R R T

(9)

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ln K D =

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3. Results and discussion

Figure S1 (supplementary information) represents the X-ray diffractograms of raw clinoptilolite,

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NaP-zeolite, and conventional and sonochemical loaded zeolites prepared at a mean level of operation variables, i.e., loading of NPs: 10 wt%, concentration of potassium permanganate

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solution: 0.1 mol.L−1, and concentration of hydrochloric acid: 25 wt%. The 2-theta peaks observed at 6.4°, 35.6°, 36.9°, 48.4° and 50.8° in the clinoptilolite pattern were attributed to the

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Mordenite phase and the remaining ones belonged to the monoclinic structure of natural clinoptilolite. The NaP-zeolite pattern with three intense 2-theta peaks at 12.4°, 28.1° and 21.6° showed the successful conversion of clinoptilolite to high-silica zeolite P.

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The loaded NaP-zeolite pattern showed a slight difference from the non-loaded one. The peaks belonging to manganese oxide NPs were not intense and mainly represented Vernadite. During the loading process, cation exchange of Mg2+ with Na+ of zeolite P led to a structural change and a decrease in peak intensities. The peaks achieved by the conventional method were similar to the ones obtained in previous studies [34], and the few overlapped diffraction lines with the 12

support’s XRD patterns showed no intensive peaks of MnO2. As expected, the sonication process could not assist the formation of more crystalline NPs accompanying the more intensive peak of MnO2 NPs because of acoustic cavitation, and smaller NPs were produced. Nucleation centers were formed in each collapsing bubble, and the non-volatility of the precursors facilitated the crystallization of NPs. However, compared with conventional loading, the smaller crystals that were formed were associated with limited growth because of very quick bubble collapse. The

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peaks corresponding to NaP-zeolite were less intensive in the sonochemically prepared samples

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compared with conventionally loaded samples because of a similar cavitation phenomenon.

The full width at half maximum (FWHM) intensity value of the XRD peak was determined at

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36.4°. It was 0.63° for the sample obtained conventionally, and 1.26° for the sample obtained

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sonochemically. An augmentation in the FWHM of peaks of the latter sample was also evident for the other peaks. Consequently, ultrasonic waves were concluded to reduce the particle size of

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the synthesized sample as well as the crystallinity.

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Figure 1 illustrates the TGA/DTA curves of clinoptilolite, NaP, and MnO2/NaP-zeolites. The weight loss of all selected samples under 150 °C was due to the evaporation of water from zeolite’s capillary pores and cavities (drying). The temperature corresponding to the maximum water release of clinoptilolite and NaP-zeolite was approximately ~100 and ~150 °C,

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respectively. The curves of the loaded zeolites prepared by both methods exhibited weight loss peaks at 140 °C (conventional) and 145 °C (sonochemical). These peaks may be ascribed to the evaporation of water molecules in the zeolitic micro-pores which were not obstructed by loaded NPs. No significant difference was observed between the curves belonging to conventionally and sonochemically synthesized samples, although the water contents of both loaded samples were 13

more than zeolite P. The obstruction of micro-pores by agglomerated NPs in the conventionally loaded samples may have caused the slight difference between the peaks of the two loaded samples (~5°C). At higher temperatures (>150 °C), water molecules were unbounded from the framework cations. The similar shape of the curves for both loaded NaP-zeolites at higher temperatures (>400°C) indicates the thermal decomposition of oxygen atoms from the complexes framework associated with the partial reduction of Mn4+ to Mn3+ as well as hydroxyl

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bonds being destroyed at higher temperatures.

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Figure 1.

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Particle size distribution (PSD) analyses of the natural precursor, NaP and MnO2/NaP-zeolites samples are demonstrated in Figure 2. The particle sizes of clinoptilolite ranged between 0.40

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and 92 μm with a mean (volume average) diameter of 15.81 μm. Hydrothermal conversion of clinoptilolite to NaP-zeolite had a significant influence on the mean diameters of particles toward

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a higher value of 24.74 µm. The ultrasonic-assisted MnO2 synthesis process resulted in finer zeolitic particles than the conventional method, and the mean diameters appeared were found to be 9.86 and 20 µm, respectively. A shift of approximatly 50.7% toward a smaller size (10.14 μm) was observed in the sonicated sample. De-agglomeration or collapse of the solid structure

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during sonication is a well-known phenomenon causing a decrease in the particle size of solids. Thus, sonication can even crack the zeolitic substrate particles to smaller pieces, which has an efficient influence on the enhancement of the specific surface area [35].

Figure 2. 14

Macroscopic specific surface area (S), surface-weighted-mean diameter (dmean,

surf),

volume-

weighted-mean diameter (dmean, vol) and uniformity for both MnO2/NaP-zeolite adsorbents are summarized in Table S1 (supplementary information). The specific surface area of the ultrasonic-assisted MnO2/NaP-zeolite was 36.5% larger than that of the conventional one. This could be because of both finer zeolitic grains and smaller homogenously distributed MnO2 NPs

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obtained by the sonication process. In comparison to the conventional method, ultrasound

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irradiation also resulted in higher uniformity (21%) of the species of samples.

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Figure 3 demonstrates the energy-dispersive X-ray spectroscopy (EDS) mapping of NaP-

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zeolites. Besides the uniform distribution of elements on the surface, the maps identified the existence of newly formed manganese particles. Furthermore, considerable quantities of oxygen

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were detected along the structure because of the formation of MnO2.

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Figure 3.

Figure 4 illustrates the micrographs of the samples synthesized under conventional and sonochemical loadings at the mean level of variables. The morphology of natural clinoptilolite

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was changed from amorphous to aggregated spherical shapes when it was converted to NaPzeolite (Figure 4a,b). SEM/FESEM micrographs of conventionally and sonochemically synthesized NPs showed that the NPs were obviously visible and almost uniformly dispersed on the surface of the zeolitic support (Figure 4c). Although there were some larger agglomerated NPs on the surface of the conventionally loaded support, sonication led to smaller 15

deagglomerated NPs with a more narrow size distribution. The average particle sizes of the conventionally and sonochemically loaded samples were determined to be 15.1 and 8.7 nm, respectively. The concentration (wt%) of metallic elements confirmed the presence of manganese, which agreed with the expected chemical compositions. The produced MnO2 flakes formed during the loading process appeared to be growing together to make some bigger clusters which were not favorable for the synthesis and adsorption efficiency. Bigger and more

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amorphous shapes of MnO2 particles were observed in the samples loaded by the hydrothermal

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method, whereas very fine and hard to observe almost spherical dots appeared in the samples loaded by the ultrasonic-assisted method. FESEM images clearly showed that the latter synthesis

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method yielded much favorable results, although the hydrothermal synthesis and coating of

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MnO2 particles on the surface of NaP-zeolite could result in nanometer-sized (<100 nm) dimensions. However, the use of sonication in the synthesis process could produce much finer

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MnO2 NPs (<10 nm) which were distributed more uniformly on the surface. Thus, the ultrasonicassisted method for loading MnO2 nanoparticles on the surface of the NaP-zeolite substrate was a

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simple and less consuming process and could yield adsorbents with much finer grain sizes as well more uniform and smaller-distributed coating agents.

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Figure 4.

BET surface areas of natural, NaP, and conventionally and sonochemically loaded MnO2/NaPzeolites were 52.6, 38.8, 64.2 and 69.5 m2.g−1, respectively (Table S1, supplementary information). Sonication augmented the surface area by 9% in comparison to the conventional method. Loading nano-sized MnO2 into microporous cavities of zeolite increased the number of 16

surface adsorptive sites. In such nano-MnO2/NaP-zeolite synthesis, changes occur in the microporosity of the composite. Therefore, the exact control of both the dispersed state of MnO2 and the porosity of the support is essential to enhance the adsorption activity.

Figure S2a (supplementary information) presents the adsorption-desorption isotherms for NaP-

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zeolite and loaded MnO2/NaP-zeolites. Their characteristic feature is the hysteresis loop associated with a sharp increase at a high relative pressure, indicating capillary condensation in

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mesopores [36]. The central flat region in the isotherm corresponds to monolayer formation. This could be attributed to MnO2 occupying the zeolitic pores that restrict the adsorption of N2 inside

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the pores. The hysteresis loops were almost identical and showed no significant differences. The

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pore size distribution of all samples was almost similar. Inter-crystalline voids of the modified zeolites can give such type of hysteresis. Therefore, it corresponded to inter-crystalline

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mesopores formed by the accumulation of nano-crystallites. The sonochemical technique produced a sample with pore volume almost 20-25% higher than that of sample obtained by the

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conventional procedure.

Figure S2b shows the pore size distribution (BJH desorption curve) for synthesized samples. The mean pore diameter of NaP-zeolite shifted to a lower value for the loaded ones. No significant difference was observed between pore distributions of zeolites loaded by the two synthesis

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methods.

3.1. Optimization Fractional

factorial

design

(FFD)

of

the

experiment,

which

is

an

adequate

mathematical/statistical tool, was used to optimize the operating conditions in terms of the 17

interactions between the variables. An optimal response can be achieved using this method with the least number of experimental runs. Four operating variables were considered employing Design Expert Software (version 10.0) including one categorical factor (loading method: US) and three numerical ones (MnO2 loading: LMnO2, KMnO4 concentration: CKMnO4, HCl concentration: CHCl). A total of 12 runs with two replicates at the central points were considered to assess the influence

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of the operating variables on the BET surface area and Hg removal efficiency (Table 1). Each

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experimental run was performed twice, and the response factors ± standard deviation (Std. Dev.) were noted with a confidence limit of 95%. Clinoptilolite and NaP-zeolite were also tested as

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two control samples. They both showed relatively low removal efficiency, even though NaP-

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zeolite had higher efficiency than clinoptilolite. The superior efficiency of the NaP-zeolite is attributed to its greater cation exchange capacity.

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Compared with conventional synthesis, the NPs synthesized under ultrasonic waves resulted in 24.43% and 8.5% improvement in the BET surface area and removal efficiency, respectively.

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This may be attributed to the generation of more segregated and smaller NPs by ultrasonication. The aggregation/segregation degree of loaded NPs affects their potential for mercury removal. Surface transport of the Hg cations is more effective in well-dispersed NPs in comparison with the highly aggregated ones. Application of a porous support associated with ultrasound

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irradiation decreases both NP size as well as their agglomeration degree; therefore, the efficiency of the composite adsorbent for Hg removal will increase. The BET surface area, as well as removal efficiency, was reduced with increasing concentration of KMnO4 because of the formation larger NPs, whereas higher MnO2 loading increased the BET surface area and removal efficiency. 18

Table 1.

Table 2 displays the analysis of variance (ANOVA) of the FFD study. The F-values of independent factors within the correlated model showed the following order of impact of the

A(LMnO2) > B(CKMnO4) > D(method) > C(CHCl)

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The order of influence of variables on Hg removal efficiency was

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investigated variables on the BET surface area:

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B(CKMnO4) >A(LMnO2) > D(method) >C(CHCl)

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Table 2.

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A two-factor interaction polynomial model equation was proposed to assess the effect of the individual factors on each response considering significant terms. Analysis of regression yielded

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the equations for the BET surface area and Hg removal efficiency in coded units.

SBET (m2 ⁄g) = 117.75 + 30.38 A − 12.63 B + 1.12 C + 6.58 D − 4.37 A. B (10)

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+ 8.87 A. C + 13.88 A. D

η0.5 (%) = 9.32 + 0.18 A − 0.22 B + 0.046 C + 0.13 D + 0.037 A. B + 0.084 A. C (11)

+ 0.085 A. D

19

where "D" is the synthesis method which takes "−1" for conventional or "+1" for sonochemical loading. The interaction coefficient of the two variables showed the significance of the combination effect of variables on the response. The F-values of 357.12 and 20.62 implied both models were significant, and there were only 0.01% and 0.55% chances that it could happen because of noise. Moreover, the lack-of-fit Fvalues of 1.05 and 0.15 implied that it was not significant in comparison to the pure error for

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both models, and there were 48.87% and 86.92% chances that the lack-of-fit F-values this large

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could occur because of noise. The values of "Adequate Precision" of more than four for both responses confirmed that the model’s equations could handle the design space. Furthermore, the

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regression coefficients of 0.998 (SBET) and 0.973 (η) indicated that the predicted results were

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satisfactory.

The p-values of less than 0.05 showed the significance of the investigated variables and their

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interactions. A, B, D, AC, and AD were significant terms, whereas C, BC, BD, and CD were insignificant terms. AB interaction was significant for the BET surface area and insignificant for

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Hg removal efficiency.

Although the concentration of HCl in the loading procedure seemed to have less influence on the BET surface area of prepared zeolites, its interactions with other variables were significant. Increasing manganese content of NPs and sonication energy led to homogenous loading and de-

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agglomerated NPs accompanying greater MnO2 active sites compared to conventional loading, whereas a simultaneous increase of both mentioned factors had no significant effect on the surface area of samples. The relatively high cross-interaction between NP loading and metallic solution concentration was a more influencing factor on the chemical reduction from the reaction mechanism and kinetics point of view. 20

Figure 5 plots the three-dimensional response surface graphs achieved through the FFD investigation showing the significant interaction of variables while the other variables were kept constant at their mean level (A: 10%, B: 0.1 mol.L−1, C: 25%). Sonochemical treatment, higher loading, and the lower concentration of KMnO4 appeared to be favorable for response minimization.

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Figure 5.

Diagnostic and probability graphs of the residuals were applied to depict the desirability of the

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proposed models and conditions (Figure not included). An adequate agreement was observed

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between the experimental and predicted data for both response factors.

As the optimization methodology involved two response factors, various solutions could be

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collected equal to the investigated variables. A procedure based on the desirability function was applied because various responses have to be studied simultaneously to find the optimal

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agreement among all investigated responses. The goals were set to "exact value" for variables and "within range, target, maximize, minimize, and none" for responses. For each of the considered variables, the lowest and the highest level were defined. In this investigation, the "goal", "weight", and "importance" of the variables were set as "in range", "1", and "3",

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respectively. Moreover, two criteria were chosen: "maximizing BET surface area" and "maximizing removal efficiency". The optimization was implemented for a combination of goals.

The solution that met the criteria best was selected, and a verification experiment was conducted utilizing the optimized parameters. BET surface area and removal efficiency acquired from the 21

verification experiment satisfactorily confirmed the data acquired from the desirability optimization with FFD. Optimal levels of factors and response rates for both methods are presented in Table 3. Further synthesis of nanoparticles was performed at these optimal levels. Table 3.

Two experimental runs were performed under optimal conditions to synthesize NPs with both

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conventional and ultrasonic-assisted methods. Figure 6 illustrates the FESEM images of

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MnO2/NaP-zeolite under optimal synthesis conditions. Characterization analysis results confirmed that the ultrasonic-assisted method produced finer MnO2 NPs (<10 nm) that were

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more homogenously distributed on the substrate surface than the particles produced by the

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conventional method. The ultrasound causes collisions, vibration, and implosion of cavitation bubbles leading to the breakdown of the agglomerate crystals and creates more active and

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homogeneous dispersed sites at the surface. The synthesized samples were employed for

Figure 6.

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dynamic and equilibrium adsorption tests in the next step.

The equilibrium data were obtained in batch experiments and were used to determine the Hg

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removal efficiency and adsorption capacity of the samples. Each experiment was performed twice and the average result was reported (Table 4). The obtained results were compared with those obtained using other hybrid adsorbents. Adsorption capacities for both loaded NaP-zeolites were found to be more than 5.5 times greater than those of unloaded NaP-zeolite. Obviously, the removal efficiency of Hg2+ also showed considerable differences compared with substrates with 22

and without MnO2 coating. These results sufficiently declared the effective rule of added MnO2 particles on the surface of the zeolitic substrate for heavy metal cation adsorption. Hg2+ adsorption efficiency increased from 92.1% for conventional MnO2/NaP-zeolite to 96.7% for ultrasonically loaded zeolites. Obviously, the adsorption capacity of the latter was approximately 3.5% higher than that of the former. The finer size of both substrate grains and MnO2 particles resulted in higher specific surface area and higher chemical activity, especially when the

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dimensions of MnO2 particles were below 10 nm.

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Table 4

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The high affinity of the available nano-sized MnO2 at the zeolitic substrate surface was involved in the ion-exchange process with the solution containing Hg cations, in which MnO2 had surface

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hydroxyl groups with acidic and basic characteristics. This could have been the reason of this improvement in removal efficiencies according to the following ionization and hydrolysis

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reactions [43]:

(12)

≡ MnOH + H2 O ↔ MnO− + H3 O+

(13)

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≡ MnOH2+ + H2 O ↔ MnOH + H3 O+

There are two main mechanisms for such an adsorption process: physical and chemical adsorption. Regarding the high porosity of zeolitic materials, one of the most applicable mechanisms of adsorption of heavy metal cations is the way that cations are placed in holes and cavities at the surface and inside the zeolite grains, which is called physical adsorption. As more 23

pore volume per adsorbent unit mass is available, more cation species can be gathered from the solution into these pores and can be removed from the solution. Ion-exchange between cations placed on the zeolite surface and metal cations in the solution is another well-known mechanism of adsorption in this case. If there are more and stronger exchangeable cations on the surface, more metal cations can be replaced from the solution. A possible mechanism whereby the ionexchange process occurs in the zeolite structure is one where the Hg2+ cations replace Na+ ions

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[44]. It seems that the presence of nano-sized MnO2 particles, causing an increment in the total

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specific surface area of the adsorbent, with high chemical activities in solution, makes the chemical adsorption mechanism stronger than the physical mechanism, thus allowing the higher

3.2. Adsorption/regeneration test

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adsorption capacities and removal efficiencies for loaded NaP-zeolites.

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Figure 7 reveals the effects of five consecutive regeneration cycles of adsorbent and regeneration temperature on removal efficiency. At a regeneration temperature of 25 °C, the removal

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efficiencies reached 94.1% and 91.1% for the first and second cycles, respectively. The corresponding values were 95.6% and 92.5% at a higher regeneration temperature of 50 °C, respectively. This is because the slope of the equilibrium curve increases with regeneration temperature. The expected fall in removal efficiency of the regenerated adsorbent from 96.7% to

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81.4% was observed during five regeneration cycles at 25 °C. This may be due to the partial destruction of the structure of zeolitic adsorbent at regeneration conditions and slightly decreasing MnO2 content of the regenerated adsorbent. The ICP analysis of one selected sample of the regeneration solution after the first cycle showed nearly a 4% loss in the MnO2 content of adsorbent. 24

Figure 7.

3.3. Adsorption kinetics and isotherm models The effect of initial Hg2+concentration on dynamic of adsorption was investigated (Figure S3a). As expected, increasing the initial concentration of adsorbate leads to an increase in adsorption

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capacity and the curves follow a similar trend. The kinetic data were also fitted to first- and

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second-order kinetic models, where a poor fitting was observed between experimental data and first-order kinetic model (Figure not shown). According to the correlation coefficient for the

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second-order kinetic model (Figure S3b), it revealed that the adsorption system strongly follows

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a second-order kinetic model (qe: 0.072 mg.g−1, k2: 0.180 g.mg−1 min−1). Therefore, it can be concluded that Hg2+ adsorption appeared to be controlled by chemical process.

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From the adsorption kinetics point of view, the smaller particle size of adsorbent facilitates the Hg ions transfer within the pores and decreases the time required for ions to reach the adsorption

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active sites. Therefore, equilibrium was achieved in a shorter period for smaller size particles which can affect significantly the breakthrough curve of adsorbent to remove cations from solution (dynamic adsorption).

To find the best adsorption isotherm, the equilibrium data were fitted on the Langmuir and

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Freundlich isotherms (Figures S3c,d). The constants were determined as qm: 1.667 mg.g−1, and b: 84.5 L.mg−1 with a linear correlation coefficient of R2: 905 for Langmuir isotherm. The Langmuir model is based on the assumption that the maximum adsorption occurs when a saturated monolayer of solute molecules is present on the adsorbent surface, the energy of adsorption is constant and there is no migration of adsorbate molecules in the surface plane. The 25

corresponding constants were determined as: KF: 122.32, "1/n": 0.780 with a correlation coefficient of R2: 0.995 for Freundlich isotherm. The Freundlich isotherm is applicable to highly heterogeneous surfaces and the magnitude of "1/n" quantifies the favorability of adsorption and the degree of heterogeneity on the surface of sonochemical loaded MnO2/NaP-zeolite. As "1/n" value was obtained less than unity, the adsorption was favorable. As a conclusion, the Freundlich

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equation is the best one to describe equilibrium data.

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3.4. Effects of pH, contact time, and temperature

To evaluate the time-dependent removal efficiency (dynamic adsorption) from drinking water,

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extra experiments were conducted at different pH values and low initial concentrations of Hg2+,

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i.e. 10 μg.L−1. The modified zeolite could adsorb Hg2+ from water at a wide range of pH values (Figure 8a). However, acidic conditions led to lower adsorption because of strong competition

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between H3O+ and Hg2+ as well as electrostatic repulsion between Hg2+ and the protonated active group on the adsorbent surface. The highest efficiency was observed when the solution was

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neutral. The protonated H+ on the adsorbent surface was released, and a negatively charged surface provided supportive conditions for the attraction of Hg2+. The removal efficiency was relatively higher in a basic solution compared to an acidic one because of the electrostatic repulsion of the surface and negatively charged metallic complexes formed because of excess

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OH− ions [42, 45]. Under neutral pH and after 8 h of adsorption, the removal efficiency reached 98.1% and the mercury residual was 0.19 μg.L−1. The results for contact time (over a total tested period of 8 h) showed that Hg2+ was rapidly adsorbed during the first few hours because of the readily accessible sites in pores of zeolite [46]. To reach the equilibrium state for Hg2+ adsorption, a contact time of at least 8 h was necessary. 26

After almost 2 h, the adsorption reached a quasi-equilibrium state (more than 95 % of the final value), and the removal efficiency became almost constant till the end of the experiment. Indeed, an adsorption period of 2 h was enough for practical cases. The dynamics of adsorption of Hg2+ on MnO2/NaP-zeolite for a period of 8 h is illustrated in Figure 8b. An increase of 10 °C in the temperature led to an increase in adsorption capacity of as much as 0.4 µg.g−1. A quasi-equilibrium state was attained after ~2 h, and the higher temperature

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resulted in higher ion mobility and favored more removal by adsorption [47, 48]. It characterizes

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an endothermic process with chemical bonding being involved.

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Figure 8.

The thermodynamic parameters of adsorption were determined (Figure S4) utilizing the Van’t

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Hoff plot [23]. The negative values of ∆G° imply the spontaneity of the process. The values became more negative with increasing temperature. This means that increasing temperature

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favors adsorption. The positive values of ∆H° for Hg2+ adsorption indicate the sorption has an endothermic nature. The endothermic process showed that movement of solutes from the bulk solution to the adsorbent interface may require energy to overcome the interaction of dissolved ions with dissolution molecules. The positive value of ∆S° implies the increased disorder and

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randomness during the adsorption of Hg(II) species at the MnO2/NaP-zeolite solution interface. Finally, the influence of Cu2+ as a competitive metallic ion with Hg2+ was investigated at the identically low initial concentration (10 μg.L−1) and neutral pH. This ion was chosen because of its selective adsorption on MnO2 NPs and natural zeolite compared to other ions such as Ni2+,

27

Cd2+, and Zn2+. The Hg removal efficiency was decreased from 87% (single Hg cation in water) to 57.5% (binary Hg and Cu cations in water) after 1 h of adsorption experiment. Mercury adsorption onto the surface of MnO2 loaded zeolitic materials is a complicated phenomenon where the mechanism may consist of both physical (on zeolite) and chemical (on MnO2) adsorptions. As expected, the Hg2+ adsorption capacity of loaded adsorbent depends on the adsorption conditions as well as surface area. Theoretical investigations on the adsorption

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mechanism showed that the dominant mechanism for mercury adsorption on the MnO2 surface

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was chemisorption (strongly adsorbed).

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4. Conclusions

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MnO2 nanoparticles were immobilized on NaP-zeolite sonochemically and applied for Hg2+ removal from aqueous solution. This method produced finer and more homogenous distribution

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of NPs on the substrate surface than the conventional method. The adsorption capacity of NaPzeolite was improved by more than 550% using both conventional and ultrasonic-assisted

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loading. Although the improvement of characteristics of synthesized samples was not very remarkable by the proposed method because of the the lower impact of ultrasound waves at higher temperatures, it is a more economic synthesis process that does not require external heating and mixing. The use of natural clinoptilolite as an abundant mineral support associated

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with ultrasonic energy (at low power consumption) reduces the total cost of the process. Synthesized adsorbent is a convenient hybrid agent for Hg removal from drinking water because of the short contact time required, although it is a relatively larger period compared with that of costly adsorbents. Further investigations on sonochemical loading at lower temperatures are necessary to make the sonication process more efficient. 28

AUTHORSHIP STATEMENT Authorship contributions Please indicate the specific contributions made by each author (list the authors’ initials followed by their surnames, e.g., Y.L. Cheung). The name of each author must appear at least once in each of the three categories below. Category 1 Conception and design of study: J. Behin acquisition of data: J. Behin , A. Bahiraei analysis and/or interpretation of data: : J. Behin , A. Bahiraei

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Category 2 Drafting the manuscript: J. Behin , A. Bahiraei revising the manuscript critically for important intellectual content: J. Behin, A. Bahiraei

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Category 3 Approval of the version of the manuscript to be published (the names of all authors must be listed): J. Behin Declaration of interests

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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements All persons who have made substantial contributions to the work reported in the manuscript (e.g., technical help, writing and editing assistance, general support), but who do not meet the criteria for authorship, are named in the Acknowledgements and have given us their written permission to be named. If we have not included an Acknowledgements, then that indicates that we have not received substantial contributions from non-authors.

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removal

http://dx.doi.org/10.1016/j.jece.2016.11.015. [46] H.T. Fan, W.Sun, B. Jiang, Q.J. Wang, D.W. Li, C.C. Huang, K.J. Wang, Z.G. Zhang, W.X. Li, Adsorption of antimony(III) from aqueous solution by mercapto-functionalized silica-

Jo

supported organic–inorganic hybrid sorbent: Mechanism insights, Chem. Eng. J. 286 (2016) 128-138. https://doi.org/10.1016/j.cej.2015.10.048.

[47] N. You, Y.X. Song, H.R. Wang, L.X. Kang, H.T. Fan, H. Yao, Sol-Gel derived benzocrown ether-functionalized silica gel for selective adsorption of Ca2+ ions, J. Chem. Eng. Data 64 (2019) 1378-1384. https://doi.org/10.1021/acs.jced.8b00955. 35

[48] N. You, X. F. Wang, J.Y. Li, H.T. Fan, H. Shen, Q. Zhang, Synergistic removal of arsanilic acid using adsorption and magnetic separation technique based on Fe3O4@ graphene nanocomposite,

J.

Ind.

Eng.

Chem.

70

(2019)

346-354.

Jo

ur na

lP

re

-p

ro

of

https://doi.org/10.1016/j.jiec.2018.10.035.

36

Figures

98

ro

94 90

86

re

82 0.000

lP

-0.005 -0.010

-0.015

ur na

d(Weight loss )/dt (%.s-1)

of

Natural clinoptilolite NaP zeolite Conventionally loaded NPs Sonochemically loaded NPs

-p

Weight loss (%)

102

140 oC

-0.020

145 oC

150 oC

-0.025

Jo

0

100

200

300

400

500

600

Temperature (oC)

Figure 1. TGA/DTA curves of clinoptilolite, NaP zeolite and synthesized MnO2/NaP-zeolites

37

7

Sonochemical loaded MnO2/NaP-zeolite Conventional loaded MnO2/NaP-zeolite NaP zeolite Clinoptilolite

6

of

4

ro

Volume (%)

5

3

-p

2 1 0.1

1

re

0 10

100

1000

lP

Particle size (µm)

Jo

Si

ur na

Figure 2. Particle size distribution curves

Al

Na

O

Ca

Mn

(a) Conventional loading (O: 55.09, Na: 2.97, Al: 10.16, Si: 27.77, K: 2.12, Ca: 1.17, Mn: 0.23, Fe: 0.49)

38

Si

Al

Na

O

Mn

Ca

(b) Sonochemical loading (O: 49.54, Na: 2.65, Al: 10.22, Si: 32.17, K: 2.90, Ca: 1.23, Mn: 0.33, Fe: 0.97)

ur na

lP

re

-p

ro

of

Figure 3. EDS mapping of synthesized NPs (values in wt %)

Jo

5 μm

(a) Raw clinoptilolite

5 μm

(b) NaP-zeolite

39

Sonochemical

Conventional

5 μm

ro

of

5 μm

Sonochemical

Jo

ur na

500nm

lP

re

-p

Conventional

40

500nm

Sonochemical

Conventional

200nm

ro

of

200nm

-p

(c) MnO2/NaP-zeolites

Jo

ur na

lP

re

Figure 4. SEM/FESEM images of raw and synthesized materials

41

of ro -p re lP ur na

(a) Conventional treatment

(b) Sonochemical treatment

Jo

Figure 5. 3D response surface plots of factors against SBET and η

42

50 nm

of

50 nm

(b) Ultrasonic-assisted method

ro

(a) Conventional method

re

-p

Figure 6. Micrograph of MnO2 nanoparticles loaded on NaP-zeolite

90 85 80 75

lP

25 °C 50 °C

ur na

95

Jo

Removal efficiency (%)

100

70

0

1

2

3

4

5

Number of regeneration cycle (-) Figure 7. Comparison of fresh and regenerated adsorbents at two temperatures 43

90 80

60 50

pH: 3

pH: 5

pH: 9

pH: 11

pH: 7

40 2

4 Time (h)

8

re

a) Removal efficiency

6

-p

0

lP

3.5 3 2.5

ur na

qt (µg.g-1)

of

70

ro

Removal efficiency (%)

100

2

Jo

1.5

35 deg C 25 deg C 15 deg C

1

0

2

4 Time (h)

6

8

b) Sorption capacity (pH: 5) Figure 8. Effects of temperature and pH (solution volume: 15 mL, mass of adsorbent: 44

0.05 g, initial concentration of Hg: 10.14 μgL-1)

ro

of

Tables

Table 1. Experimental conditions of synthesis MnO2/NaP-zeolite accompanying with BET

-p

surface area (SBET) and Hg removal efficiency (η)

Run no.

re

Variables

Responses

CKMnO4 (mol.L−1)

CHCl (wt %)

US

SBET (m2.g-1)

η (%)

1

5

0.15

12.5

+

79

80.08

2

5

0.05

12.5

−

110

88.06

3

5

0.05

37.5

+

80

88.09

5 6

15

0.15

12.5

−

100

80.54

10

0.1

25

−

111

85.01

5

0.15

37.5

−

78

77.36

15

0.05

37.5

−

154

92.12

Jo

7

ur na

4

lP

LMnO2 (wt %)

8

15

0.15

37.5

+

161

93.13

9

10

0.1

25

−

114

84.03

10

15

0.05

12.5

+

175

95.21

11

10

0.1

25

+

127

92.05

45

12

10

0.1

25

+

124

88.00

Table 2. ANOVA test for response functions Mean square

SBET η

SBET

Model

11509.83

1

7

7

A: Loading

7381.13

0.27

1

1

B: CKMnO4

1275.13

0.39

C: CHCl

10.13

0.017

D: Ultrasound

520.08

0.21

A.B

153.13

A.C

η

SBET

η

SBET

η

1644.26 0.14

20.62

<0.0001 0.0055

7381.13 0.27

1603.14 38.67

˂0.0001 0.0034

357.12

1

1

1275.13 0.39

276.95

55.45

˂0.0001 0.0017

1

1

10.13

0.017

2.20

2.39

0.2122

0.1969

1

1

520.08

0.21

112.96

29.90

0.0004

0.0054

0.011

1

1

153.13

0.011

33.26

1.56

0.0045

0.2794

630.13

0.056

1

1

630.13

0.056

136.86

8.02

0.0003

0.0472

A.D

1540.13

0.058

1

1

1540.13 0.058

334.51

8.33

˂0.0001 0.0447

Residual

18.42

0.028

4

4

4.60

0.0070

Lack of fit

9.42

0.0036

2

2

4.71

0.0018

1.05

0.15

0.4887

Pure error

9.00

0.024

2

2

4.50

0.012

Total

11528.25

1.03

11

11

SBET

R2 :0.998, Adjusted R2 : 0.996, Adequate precision: 55.318

Jo

re

Η

p-value

ur na

SBET

F-ratio

-p

d.f.

lP

Sum of squares

ro

of

+: present, −: not present

46

0.8692

Η

R2 :0.973, Adjusted R2 : 0.936, Adequate precision: 14.262

Table 3. Optimum levels of factors and responses determined for both loading methods. Variable

Response

Method

Desirability

CKMnO4 (mol.L−1)

CHCl (wt %)

SBET (m2.g−1)

η (%)

Conventional

15.00

0.05

37.5

154.667

92.160

Ultrasound

14.96

0.05

36.0

192.450

SBET

Adsorbent

2 −1

(g. L )

lP

(m g )

Fe3O4/Chitosan

ro

Dose of adsorbent −1

99.995

0.810 1.000

-p

re

Table 4. Mercury removal of some hybrid adsorbents

of

LMnO2 (wt %)

pH

Co

T −1

η

qm −1

(−)

(mg.L )

(°C)

(mg.g )

(%)

Ref.

N.A.

6

3

50

25

10

84.5 [13]

110.4

1

6

50

25

58.8

90.4 [23]

169.9

0.16

4.6

5

25

157.9

88.0 [37]

Papain/Activated charcoal

1188

N.A.

7

20

35

0.2

99.4 [38]

Polyaniline/Humic acid

35.4

0.5

5

50

24

610

95.0 [39]

30.1

0.61

7

20

25

119

94.5 [40]

Dithizone/Natural zeolite

N.A.

2

5

5

N.A. 2.62

99.4 [41]

Fe3O4/SiO2

N.A

0.2

6.5

60

30

148.8

N.A

38.8

3.3

7

20

25

103.2

17.2

MnO2/Carbon nanotube

Jo

Fe3O4/Graphene oxide

ur na

CoFe2O4/Reduced graphene oxide

NaP-zeolite, Ce (mg.L−1): 16.561, qe (mg.g−1): 103.2

This

MnO2/NaP-zeolite (best sample) −1

[42]

work −1

Conventional, Ce (mg.L ): 1.332, qe (mg.g ): 560.1

154

3.3

47

7

20

25

560.1

93.3

185

3.3

7

20

25

580.5

Jo

ur na

lP

re

-p

ro

of

Sonochemical, Ce (mg.L−1): 0.651, qe (mg.g−1): 580.5

48

96.7