Silico-manganese fumes waste encapsulated cryogenic alginate beads for aqueous environment de-colorization

Journal Pre-proof Silico-manganese fumes waste encapsulated cryogenic alginate beads for aqueous environment de-colorization Moonis Ali Khan, Saikh Mohammad Wabaidur, Masoom Raza Siddiqui, Ayoub Abdullah Alqadami, Akhtar Hussain Khan PII:

S0959-6526(19)33737-0

DOI:

https://doi.org/10.1016/j.jclepro.2019.118867

Reference:

JCLP 118867

To appear in:

Journal of Cleaner Production

Received Date: 21 May 2019 Revised Date:

9 October 2019

Accepted Date: 12 October 2019

Please cite this article as: Khan MA, Wabaidur SM, Siddiqui MR, Alqadami AA, Khan AH, Silicomanganese fumes waste encapsulated cryogenic alginate beads for aqueous environment decolorization, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.118867. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Silico-Manganese Fumes Waste Encapsulated Cryogenic Alginate Beads for Aqueous Environment De-colorization Moonis Ali Khana,*, Saikh Mohammad Wabaidura, Masoom Raza Siddiquia, Ayoub Abdullah Alqadamia, Akhtar Hussain Khanb a

Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

b

National Environmental Preservation Company (BeeA'h), P.O. Box 10628, Madinat Al-

Jubail Al-Sinaiyah 31961, Saudi Arabia

______________________________________________________________________________ *Corresponding author. E-mail address: [email protected]; [email protected] (M.A. Khan).

SA

Stirring

Beads formation

H H H H

O

O O

H

H

O- +

Si O

O O

CaCl2 NaOH

OH

O H O

Si

Si

O O H

H O

M+

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Si

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Si

O O O

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Si O O

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O Si

H O

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Si O O

O H

H

H

H Si

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Si

H

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50

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+M

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+M

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20

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H O

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H

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O O

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H H

10

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CB CV MG MB

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+M

30

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Si

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40

O

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60

O +

O

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Adsorption Dyes

O

H O

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H +M

H

H

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O

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O O

O

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M+

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cSMFB

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Lyophilisation

Calcium bath

qt, mg/g

SMF

0 H

0

200

400

t, min

600

800

Total word count: 7152 (Text: 6570, Tables: 464, Figures: 118) Silico-Manganese Fumes Waste Encapsulated Cryogenic Alginate Beads for Aqueous Environment De-colorization Moonis Ali Khana,*, Saikh Mohammad Wabaidura, Masoom Raza Siddiquia, Ayoub Abdullah Alqadamia, Akhtar Hussain Khanb a

Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia

b

National Environmental Preservation Company (BeeA'h), P.O. Box 10628, Madinat Al-Jubail Al-

Sinaiyah 31961, Saudi Arabia

_______________________________________________________________________________ *Corresponding author. E-mail address: [email protected]; [email protected] (M.A. Khan). 1

Abstract Safe and economical disposal of ultrafine silico-manganese fumes (SMF), a waste, generated during steel production operation is a challenge for environmental chemists. Herein, SMF waste was encapsulated into alginate, lyophilized to form cryogenic SMF beads (cSMFB). The cSMFB was characterized and its adsorptive potential was tested in the removal of methylene blue (MB), methylene green (MG), crystal violet (CV) and celestine blue (CB) from aqueous environment. The cSMFB having respective Brunauer-Emmett-Teller (BET) surface area and Barrett, Joyner, and Halenda (BJH) pore diameter of 59.88 m2/g and 18.58 Å was developed. Fourier transform infra-red (FT-IR) analysis revealed bands at 1630 and 1412 cm-1, attributed to asymmetric and symmetric stretching vibrations of carboxylate ions, characteristic of alginate (used to encapsulate SMF). Also, the bands at 1092 and 1031 cm-1 corresponding to carbohydrate rings of alginate were present. A band ascribed to metal oxides was appeared at 467 cm-1. The encapsulation of SMF waste in alginate was further confirmed by X-ray photoelectron spectroscopy (XPS) (with peaks at binding energies 34, 284.7, and 532 eV ascribed to Na 2p, C1s, and O 1s), X-ray diffraction (XRD) (with 2θ peaks at 13.5 and 22.2° for sodium alginate), and energy-dispersive X-ray (EDX) analyses. The crosssectional image of pristine cSMFB showed highly porous rough and uneven surface. Thermogravimetric (TGA-DTA) analysis of cSMFB sample revealed the five stages total weight loss of ~17.5% between 40 and 800°C. Alkaline pH (pHi range: 6 – 8) favored the adsorption of dyes, while the equilibration time at Co: 50 mg/L was varied between 480 and 600 min. The adsorption of dyes was possibly governed through electrostatic interaction, coordinate bond formation, and cationic interchange.

Keywords: Steel industry waste; Alginate; Lyophilize; Cationic dyes; Adsorption. 2

Nomenclature Co – Initial concentration, mg/L Cf – Final concentration, mg/L V – Volume, L m – Mass, g t – Contact time, min λmax – Maximum wavelength, nm qe – Adsorption capacity at equilibrium, mg/g qt – Adsorption capacity at any time t, mg/g qm – Maximum monolayer adsorption capacity, mg/g qe,cal – Calculated adsorption capacity, mg/g qe,exp – Experimental adsorption capacity, mg/g pHi – Intial pH pHf – Final pH KL – Langmuir constant, L/mg KF – Freundlich constant, (mg/g) (L/mg)1/n ∆G°– Gibb’s free energy change, kJ/mol ∆H°– Standard enthalpy change, kJ/mol ∆S°– Standard entropy change, J/mol-K R2 – Regression coefficient RL – Separation factor n – Adsorption intensity coefficient PFO – Pseudo-first-order

3

PSO – Pseudo-second-order K1 – Pseudo-first-order rate constant, 1/min K2 – Pseudo-second order rate constant, g/mg-min h – Initial adsorption rate, mg/g-min 1. Introduction Natural and synthetic coloring agents have countless applications in everyday life. They are used in food industries to make eatable attractive, in cosmetic and textile industries to uplift human appearance, in paint industries to beautify houses, offices, and vehicles. Though, these coloring agents are brightening our lives, however, their excessive usage simultaneously acting as a curse towards the aquatic environment and human health. Among the coloring agents, synthetic dyes and their intermediate products are the issue of major concern. Synthetic dyes can change the natural salinity levels of water (Panic et al., 2013). Also, they can hinder the photosynthetic process of aquatic plants by reducing the penetration of sunlight through water. Therefore, detecting and reducing the concentration of synthetic dyes in wastewater to permissible levels by selecting an appropriate treatment process before there discharge to surface and sub-surface water is essential. Bio- and photo-degradation are the two most common technologies used for the abatement of dyes from contaminated water. However, the generation of secondary pollutants restricts their wide range usage. Also, during photo-degradation, the dyes impede the penetration of light into the treatment vessels, thus, upsetting the process (Al-Oodah, 2000). Additionally, most of the synthetic dyes are toxic to some micro-organisms, and may inhibit their catalytic activities during the biodegradation process. Adsorption is a versatile water treatment technology capable of removing even trace amount of pollutants from aqueous medium (Rao and Khan, 2007). Nevertheless, the higher monetary value restricts wide range usage of commercially available carbonaceous and noncarbonaceous adsorbents. 4

Industrial establishments, agricultural sector, and municipal activities generate tons of wastes annually. The generated wastes require large capita for their efficient treatment and safe disposal. Environmental scientists have successfully utilized industrial (Yusuf et al., 2014), agricultural (Guillossou et al., 2019), and municipal (Genuino et al., 2018) wastes for aqueous environment remediation, providing an alternate waste management and utilization approach. Silico-manganese (Si-Mn) fume (SMF) is a solid waste particulate matter having ultrafine particle size, ranged between nanometer and micrometer (Kero et al., 2015). It is generated during basic oxygen furnace steel production process. The basic oxygen furnace steel production process is used to develop low-carbon steel (Yang et al., 2019). During steel production operation lime, silica, and dolomite along with other essential reagents are added to the precursor materials for oxidizing the impurities (such as carbon and phosphorus) at high temperature, consequently generating SMF (Chen et al., 2019). It is estimated that one ton of steel produced can generate more than hundred kilograms of SMF. Prolong inhaling of SMF can be detrimental to human’s respiratory system (Geys et al., 2006). Moreover, SMF can influence the air quality of local and urban areas through diffusion emission (Marris et al., 2012). Thus, for sustainable environment and human health conservation it is essential to restrict SMF from entering into the atmosphere and water table. The conventional applications of SMF in road construction (Gao et al., 2017), cement production (Zhao et al., 2017), and as a sintering material (Gao et al., 2017) are well acknowledged. However, large scale waste generation cannot fulfill the SMF’s disposal demand by aforementioned applications. Therefore, alternate usage methods have been explored by researchers. The usage of steel manufacturing waste for the adsorptive removal of heavy metals ions under acidic conditions (Yang etal., 2019), from soil washing effluents (Gao et al., 2017), storm water and landfill leachate (Nehrenheim and Gustafsson, 2008), and acid mine drainage (Masindi et al., 2018) have been

5

reported previously. Moreover, the waste was efficiently used for the adsorption and decomposition of hazardous gases such as CO2 (Humbert and Castro-Gomes, 2019), H2S (Miguel et al., 2012), and for the SF6 (Zhang et al., 2014). To the best of our knowledge, studies are limited on utilization of steel manufacturing waste in water de-colorization. Herein, an ultrafine SMF solid waste was encapsulated into sodium alginate (SA), a hydrophilic, biodegradable, and abundantly available natural polymer to form beads (SMFB) for rectifying phase separation issue during batch adsorption experiments. Previous studies have reported the development of SA encapsulated bentonite (Belhouchat et al., 2017), gelatin and albumin (Kulkarni et al., 2002), and DTPA-chitosan micro gel (Huang et al., 2018) beads. However, the use of toxic cross-linkers such as chlorhydric acid and glutaraldehyde during beads development had questioned their environmental and economic feasibility. To circumvent the usage of crosslinking agent, developed SMFB was lyophilized (to withhold SMFB geometry) to cryogenic SMF encapsulated beads (cSMFB). Physico-chemical characteristics of cSMFB were assessed by state of art characterization tools. The application of cSMFB as an adsorbent for the removal of azo dyes viz., methylene blue (MB), methylene green (MG), crystal violet (CV), and an oxazine dye viz., celestine blue (CB) from aqueous environment was evaluated through batch-scale experiments.

2. Experimental 2.1. Chemicals and reagents Cationic dyes viz. malachite green (MG: C16H17ClN4O2S, Sigma-Aldrich, USA), crystal violet (CV: C25H30ClN3, Sigma-Aldrich, USA), celestine blue (CB: C17H18ClN3O4, Acros Organics, USA), and methylene blue (MB: C16H18ClN3S, CDH, India) were used as the model adsorbates. Sodium alginate (SA: C6H9NaO7, Sigma-Aldrich, USA), sodium hydroxide (NaOH: Sigma-Aldrich, USA), and calcium chloride (CaCl2: Sigma-Aldrich, USA) were utilized during the synthesis of cSMFB. 6

All reagents used throughout the research were of analytical reagent (A.R) grade or specified. Deionized (D.I) water was used throughout the experiments. 2.2. Development and characterization of beads The SMF waste was collected from industrial area, Jubail, Saudi Arabia. 25 g of SMF waste was taken in 250 mL beaker, washed several times with D.I water to remove unsettled dust particles. Thereafter, SMF waste was dried overnight at 333K. Sodium alginate (2%) was solubilized in 12.5 mL NaOH (0.1 M) solution. 2 g of SMF under continuous magnetic stirring at 120 rpm was added to a solution and a mixture was magnetically stirred for 2 h to ensure uniform mixing. 500 mL CaCl2 (0.1M) solution was prepared, cooled to 277K in a refrigerator. The homogenized SA/SMF mixture was loaded in a 5 mL clinical syringe. A syringe was inserted into a syringe pump and beads at 5 mL/min flow rate from 6 cm height were dropped into a calcium bath to form SMF encapsulated beads (SMFB). The respective flow rate and drop height were chosen to avoid sedimentation of SMF particles during beads formation process and to obtain spherical beads of uniform size (Rocher et al., 2008). The SMFB were aged for 48 h in a calcium bath. Thereafter, the beads were washed with D.I water until neutral pH value of washing solution was achieved. The developed beads samples were distributed into wet beads sample (S2: these beads were stored in D.I water), room dried beads sample (S3: these beads were dried at room temperature), oven dried beads sample (S4: these beads were dried overnight at 323K in an oven), cryogenic dried beads sample (S5: these beads were lyophilized). For comparative study cryogenic alginate beads (S1: these beads were lyophilized) were also prepared following aforesaid methodology. Detailed information regarding beads nomenclature is given in Table 1 and Scheme 1a illustrates the proposed development of cSMFB. The chemical composition of SMF waste was tested through X-ray fluorescence spectrometer (XRF: S4 Pioneer, Bruker, USA). The surface area and pore size of the beads were

7

determined by BET surface area analyzer (Micromeritics Gemini VII 2390, USA). The surface morphology and elemental content of the beads were explored by scanning electron microscopyenergy dispersive X-ray analysis (SEM-EDX: JOEL, JSM-6380LA, USA). The functional groups present on the beads surface were examined by Fourier transform infrared spectrometry (FT-IR: Nicolet 6700, Thermo Scientific, USA).The surface charge of cSMFB was determined by zeta potential analyzer (Nano Plus Series, Particulate Systems, USA). X-ray photoelectron spectroscopy (XPS: Joel JPS-9200) was used to determine the elemental composition on cSMFB. X-ray diffraction (XRD: Shimadzu model 6000, Japan) analysis was used to analyze the XRD pattern of beads samples. The thermal stability of cSMFB was tested through thermogravimetric (TGA-DTA: Q500 TGA, USA) analysis. 2.3. Dyes adsorption studies The adsorption of MG, MB CV, and CB on cSMFB was studied though batch experiments. In 100 mL Erlenmeyer flask, 0.02 g cSMFB was equilibrated with 25 mL dye solution of initial concentration (Co): 25 mg/L. At equilibrium, the supernatant was separated through filtration and residual dye concentration (concentration of dye at equilibrium, Ce) was quantitatively determined by UV-Vis Spectrophotometer (Thermo Scientific Evolution 600, USA). During the analysis residual concentrations of MG, MB, CV, and CB were determined at maximum wavelengths (λmax): 618, 665, 590, and 642 nm, respectively. The adsorption percentage (%) and uptake capacities of dyes at equilibrium (qe) and at any time t (qt) were calculated as: % =

× 100 1









, !/! = #$ − #

(,

!/! = #$ − #( ×

8

) *

×

&

2 3

where Co, Ce, and Ct are the respective initial, equilibrium, and at any time t concentrations of dyes, mg/L, V is the volume of dyes solution, L, and m is the mass of cSMFB, g. During pH study, the initial pH (pHi) of dyes solutions of Co: 25 mg/L was varied between 2 and 10. The effect of initial concentration (Co) on dyes adsorption was studied in range: 20 – 100 mg/L at temperatures varied between 298 and 328K. The contact time (t) studies for dyes adsorption at Co: 50 mg/L were carried out in time range: 5 – 1440 min. A flow sheet illustrating overall research plan steps is given in Scheme S1.

3. Results and discussion 3.1. Selectivity studies of adsorbent The adsorption of MG, MB, CV, and CB was tested on five different beads samples (Table 1). The respective adsorption of MB, CV, and CB on cryogenic alginate beads (cAB) was 17.4, 13.5, and 10.7 mg/g, while only 0.5 mg/g MG adsorption was observed on cAB. Minimal dyes uptake was observed on wet SMF encapsulated beads (wSMFB). This might be due to the occupation of active binding sites with excessive water molecules on wSMFB. As tabulated, comparatively better dyes adsorption performances were observed on room dried SMF encapsulated beads (rdSMFB) and oven dried SMF encapsulated beads (odSMFB). However, maximum dyes uptake was observed on cryogenic SMF encapsulated beads (cSMFB). The adsorption of MG on cSMFB was 29.3 mg/g followed by MB (24.3 mg/g) > CV (17.5 mg/g) > CB (16.3 mg/g). Therefore, cSMFB was selected for detailed dyes adsorption studies. 3.2. Characterization of beads and dyes adsorption mechanism XRF analysis showed the presence of Si (17.99%) and Mn (56.35%) as the major elements along with other trace amount elements in SMF waste (Table S1). Freshly developed and 48 h aged beads samples were weighed. The observed weight loss for wSMFB sample was negligible, while a drastic

9

(95.3%) weight loss was observed for cAB sample. The observed rdSMFB and odSMFB samples weight loss was 78.2 and 73.5%, respectively, while a cSMFB sample showed 70.6% weight loss (Table 1). Additionally, the fresh and 48 h aged beads samples diameter (in mm) was manually measured. 23 to 48% decrease in cAB, wSMFB, rdSMFB, and odSMFB samples diameter due to the shrinkage in beads structure was observed, while only 5.7% decrease in cSMFB diameter was found (Table 1). Here, it is noteworthy, that though 48 h aged cAB sample was lyophilized, but still a 23% loss in its diameter was observed. However, only 5.7% loss in diameter was observed on cSMFB sample. This shows that lyophilization process along with SMF waste encapsulation in SA played a synergic role to uphold the size and shape of cSMFB sample. An isoelectric point (IEP) of cSMFB, measured by zeta potential analyzer was 3.6 (Figure 1a). The observed BET surface area of cAB was 9.49 m2/g, while the respective BJH average pore diameter and volume were 19.04Å and 0.0040 cm3/g. For odSMFB, the BET surface area was 38.44 m2/g, while BJH average pore diameter and volume were 18.61Å and 0.0054 cm3/g, respectively. Among the developed beads, the BET surface area was highest (59.88 m2/g) for cSMFB. The respective BJH pore diameter and volume of cSMFB were 18.58Å and 0.0078 cm3/g. The homogenized distribution of SMF in SA suspension during the development of beads leads to the formation of SMFB with well dispersed SMF. However, odSMFB cannot withstand its preceding geometry, and therefore results in structural loss. Conversely, the lyophilic drying of cSMFB acts as an aid to withhold its structural integrity and thereby had a comparatively highest surface area. Figure 1b illustrates the FT-IR spectra of cAB and cSMFB. The cAB spectrum showed broad and strong band at 3430 cm-1, associated with hydroxyl (–OH) group stretching, characteristic of water molecules adsorbed on alginate and existence of hydrogen bonding in alginate matrix (Jung et al., 2017). The two adjacent bands at 2890 and 2950 cm-1 were assigned to the equatorial and axial

10

sp3 C – H stretching vibration (Bedin et al., 2016). The intense bands at 1630 and 1412 cm-1 were attributed to asymmetric and symmetric stretching vibrations of carboxylate (–COO-) ions present on SA (Elzinga et al., 2012), respectively. The respective bands at 1092 and 1031 cm-1 were corresponds to the vibrational modes of C–O, C–O–H, and C–C present on carbohydrate rings of alginate (Voo et al., 2015; Qiusheng et al., 2015). The spectrum of cSMFB revealed shifting of respective bands at 3430, 2950, 2890, and 1031 cm-1 to 3450, 2975, 2900, and 1020 cm-1, confirming the impregnation of SMF into SA. A new strong band, ascribed to metal oxides (present in SMF waste) appeared at 467 cm-1, while overshadowing bands between 618 and 947 cm-1. The absorption bands associated with asymmetric and symmetric stretching vibrations of –COO- ions of SA were shifted to 1452 and 1660 cm-1 in cSMFB spectrum. Also, a strong band corresponding to the characteristic silica (Si–O) stretching vibrations of SMF appeared at 1020 cm-1. Here, it is noteworthy that the bands present in cAB spectrum shifted to higher/lower wavenumbers in cSMFB spectrum, confirming strong molecular affinity between the functionalities present on SA and SMF waste. This demonstrates the successful impregnation of SMF waste in SA. The FT-IR spectra of dyes (CB, CV, MB, and MG) saturated cSMFB showed shift and/or change in band intensities at 3430, 1660, 1452, 1031, and 473 cm-1 (Figure S1). The possible interaction between – OH group and cationic dyes resulted a change in band intensity at 3430 cm-1. The attachment of dyes ions with –COO- moieties of cSMFB was confirmed by a change in peak intensities at 1660 and 1452 cm-1. A change in band position at 1020 to 1031 cm-1 indicating the possible interaction between Si–O with cationic species (N and S) of the target dyes ions. The interaction or replacement of dyes ions with metal ions of metal oxide was confirmed by a change in band intensity at 467 cm-1. X-ray photoelectron spectroscopy (XPS) analysis of cSMFB showed the respective peaks at 52, 109.9, 155, 202, 348, and 711 eV binding energies due to Mn 3p, Al 2s, Si 2p, Cl 2p, Ca 2p, Fe

11

2p present as oxides in SMF waste (Figure 1c) (Kong et al., 2015; Woo et al., 2017). Also, the peaks at binding energies 34, 284.7, and 532 eV were ascribed to Na 2p, C 1s, and O 1s, respectively (Khan et al., 2019), due to the encapsulation of SMF waste in SA. X-ray diffraction (XRD) pattern of cSMFB illustrates semi-crystalline nature (Figure 1d). The 2θ peaks (marked with red stars) at 13.5 and 22.2° were characteristic peaks of SA powder (Wang et al., 2010). The XRD peaks at 18.3, 26.9, 32.9, 35.2, 38, 42.7, and 62.5° were associated with metal oxides present in SMF waste. The cross-sectional SEM image of pristine cSMFB (Figure 2a) showed highly porous rough and uneven cSMFB surface, while smooth surface with only few pores were observed for dyes saturated cSMFB samples (Figure 2 b – e). The presence of Si and Mn along with the other metals in trace amounts during elemental (EDX) analysis confirmed that SMF waste was well impregnated into SA during cSMFB formation (Figure 2f). The thermogravimetric (TGA-DTA) analysis plot showed five stages weight loss of cSMFB sample (Figure S2). A first stage weight loss of ~2.5%, in between 40 and 100°C was due to the evaporation of moisture from cSMFB (Salisu et al., 2016). A ~7.5% weight loss in temperature range: 100 –290°C during second stage was due to the cleavage of C=O and C-C bonds (Fan et al., 2013) and water molecules loss from crystalline metal oxides like MnO (Naderi et al., 2016) present over cSMFB surface. A total weight loss of ~ 7.5% during third, fourth, and five stages in temperature range: 290 – 800°C was due to the degradation of methane and a release of carbon dioxide gas from the alginate beads skeleton (Fan et al., 2013). Mechanistically, the adsorption of dyes on cSMFB was possibly governed by electrostatic interaction, coordinate bond formation, and/or cationic interchange (Scheme 1b). The cSMFB surface was enriched with negatively charged –COO- and –OH functionalities. These functionalities are electrostatically capable of binding with positively charged heteroatoms (N+ and S+) of cationic

12

dyes. The cSMFB was developed by encapsulating SMF waste into SA and thereafter aged in a calcium bath. Thus, cSMFB was enriched with Mn and Ca ions (M+ ions, Scheme 1b). The interchange between M+ and dyes ions might be the possibility governing the adsorption of dyes on cSMFB. The coordinate bond formation might be the other possibility of binding dyes ions with cSMFB surface. The cSMFB surface contains M+ and Si ions. These ions have a tendency of accepting lone pair of electrons. Hence, during the adsorption a coordinate bond might form between oxygen and nitrogen atoms of the cationic dyes and M+ and Si ions of cSMFB surface. 3.3. Adsorption studies Figure 3a illustrates the effect of pH on dyes adsorption onto cSMFB. At pHi: 2.77, the adsorption of MG was 5.6 mg/g, drastically increased to 26.3 mg/g at pHi: 3.61, attaining maximum (29.41 mg/g) value at pHi: 7.39. The adsorption of MB at pHi: 2.57 was 24.04 mg/g, increased to 29.48 mg/g at pHi: 3.65, reaching maximum (30.71 mg/g) value at pHi: 6.15. The CV adsorption on cSMFB was 12.95 mg/g at pHi: 2.66, increased to 20.41 mg/g at pHi: 3.26, reaching 23.59 mg/g (maximum) at pHi: 6.39. For CB, the adsorption was 12.67 mg/g at pHi: 3.28, attaining maximum (23.27 mg/g) adsorption at pHi: 8.26. Thus, alkaline pH conditions (pHi in between 6 and 8) were favorable for the dyes adsorption on cSMFB, supported well with IEP value (~ 3.6) of cSMFB. At lower pHi, the cationic dyes solutions were highly protonated resulting in protonation of the carboxylate ions present on cSMFB surface. Also, lower pH results in a competition between protons (H+) and cationic dyes, hindering latter’s adsorption. Additionally, the protonation of cSMFB at lower pH results in an electrostatic repulsion with cationic dyes, which in turn diminishing the dyes adsorption. At higher pHi, the concentration of hydroxyl ions increases, consequently results in an increase in cationic dyes adsorption on cSMFB surface. In addition, alginate (used to encapsulate SMF) provides supplementary carboxylic sites for cationic dyes adsorption, consistent with previous studies on MB and CV adsorption onto activated 13

biomass/alginate beads (Aichour et al., 2018) and alginate/acid activated bentonite beads (Oladipo et al., 2014), respectively. The pHi versus pHf plot (Figure 3b) shows that pHf tending towards neutral pH at equilibrium. This might be due to the hydrolysis of silicates and oxides of calcium (present in SMF) in aqueous phase, resulting in discharge of hydroxyl ions, tending pH towards basicity, and therefore, neutralizing highly acidic aqueous phase pH (Yang et al., 2019). Correspondingly, pointing out the buffering effect of cSMFB under acid-base conditions (Khan et al., 2015). Figures 4 (a- d) presents the effect of initial dyes concentrations (Co: 10 – 100 mg/L) on adsorption onto cSMFB at varied temperatures. The amount of dyes (MB, MG, CV, and CB) adsorbed per unit mass of cSMFB was increased with an increase in Co of dyes. At a lower dyes concentration longer time duration was required to fully saturate cSMFB adsorption sites (Auta and Hameed, 2013). In addition, the driving force for the adsorption of dyes at lower concentrations contributes a slower transport of the dyes onto the cSMFB leading to lower adsorption compared to higher dyes concentrations, where the higher flux aids in to move large number of dyes molecules onto the cSMFB surface (Malkoc et al., 2006). As a function of temperature, the adsorption of CV (Figure 4a), MG (Figure 4c), and CB (Figure 4d) increases with increase in temperature from 298 to 328 K. Contrarily, a decreased in MB adsorption with an increase in temperature was observed (Figure 4b). Thus, the maximum adsorption of CV, MG, and CB was observed at 328K (highest studied temperature), while for MB at 298K (lowest studied temperature). The adsorption capacities of CV, MG, and CB for studied Co range at 328K varied between 10.8 and 57.05 mg/g; 24.6 and 120.9 mg/g; and 12.2 and 124.7 mg/g, respectively, while the uptake capacity of MB at 298K varied between 10.5 and 72.7 mg/g, respectively. Figure 5 displayed a contact time studies plot for the adsorption of dyes. A slow increase in dyes adsorption at Co: 50 mg/L was observed. The adsorption of CB after 5 min was 6.12 mg/g,

14

increased to 62.1 mg/g at equilibrium. For CV, the adsorption after 5 min was 0.54 mg/g increased to 49.57 mg/g at equilibrium. After 5 min of contact time, MG adsorption was 0.54 mg/g (similar to CV), increased to 57.42 mg/g at equilibrium. Comparatively lowest (0.43 mg/g) MB uptake was observed after 5 min of contact time, reaching to 46.32 mg/g at equilibrium. The equilibration time for CB, CV, and MG adsorption on cSMFB was 600 min, while for MB the equilibration time was 480 min. Variations in equilibration time indicated that the chemical and structural properties of dyes played a vital role during the dyes adsorption kinetics process (Li et al., 2016). In the previous studies, comparatively longer equilibration time (of ~1500 min) were observed for MB adsorption on SA/PAMPS10 (Shao et al., 2018) and ABA (Benhouria et al., 2015) beads, while on HPB-based Al-PILMt/CaCO3 beads the equilibration time for MG adsorption was ~ 360 min (Chabane et al., 2017), slightly shorter than a current study. 3.4. Adsorption modeling 3.4.1. Isotherm modeling Langmuir (Langmuir, 1918) and Freundlich (Freundlich, 1906) models were applied to isotherm data. Detailed information regarding the models is given in supplementary data (Text S1). The respective regression coefficient (R2) values for the CV, MB, and MG adsorption on cSMFB in temperature range: 298 – 328 K were higher (nearer to unity) for Langmuir model, confirming the fitting of a model to adsorption data. Thus, monolayer coverage of CV, MB, and MG could be assumed over cSMFB surface. Contrarily, the CB adsorption data was fitted better to Freundlich model, confirmed by higher R2 values (Table 2), hinting towards multilayer coverage of CB over cSMFB surface. The maximum monolayer adsorption capacities (qm) of MG (138.1 mg/g), CB (153.8 mg/g) and CV (65.36 mg/g) were found at temperature 328K (highest studied temperature) However, for MB the maximum qm value was 71.9 mg/g at 298K (lowest studied temperature). The separation factor (RL) values for dyes adsorption on cSMFB were well in range of favorable 15

adsorption (0 < RL < 1). Additionally, the adsorption intensity constant (n) values for the adsorption of dyes on cSMFB were in range of physical adsorption (> 1). Hence, both RL and n values have confirmed that cSMFB had a high affinity for the analyzed dyes molecules. Thus, the adsorption of dyes onto cSMFB was considered as a favorable adsorption process. 3.4.2. Kinetic modeling Pseudo-first-order (PFO) (Lagergren, 1898) and pseudo-second-order (PSO) (Ho and McKay, 1998) kinetic models were applied to dyes adsorption data. The detailed information regarding models is given in supplementary information (Text S2). Higher R2 values supporting the applicability of PFO kinetic model for dyes adsorption data on cSMFB (Table 3). The applicability of PFO kinetic model to dyes adsorption data was further confirmed by comparing the PFO’s calculated adsorption capacity (qe,cal) values with the experimental adsorption capacity qe,exp.values. The qe,exp values were found to be nearer to qe,cal. values of PFO than qe,cal. values of PSO. Hence, the higher R2, and nearer qe,cal and qe,exp values confirmed that the kinetics data for MG, CV, MB, and CB adsorption on cSMFB was fitted to PFO kinetic model, in line with previous study (Rezaei et al., 2017). The initial adsorption rate (h) value was highest for CB, followed by MG > CV > MB, consistent with contact time studies data (Figure 5). 3.4.3. Thermodynamic modeling The thermodynamic parameters viz. Gibb’s free energy (∆G°), standard enthalpy (∆H°), and standard entropy changes (∆S°) for the adsorption of MG, CV, MB, and CB at varied concentrations on cSMFB were evaluated by employing the expressions given in supplementary information (Text S3). The positive magnitudes of ∆H° and ∆S° for MG, CV, and CB ensured endothermic and random nature of adsorption, while negative ∆H° and ∆S° magnitudes for MB asserted towards exothermic and non-random adsorption process (Table 4). The negative ∆G° values for MG, CV,

16

CB, and MB adsorption confirms the spontaneity of adsorption process. A decrease in ∆G° values for MG, CV, and CB with an increase in temperature was observed, indicating the increase in favorability of adsorption process with temperature, while ∆G° values for MB adsorption increases with increase in temperature, suggesting less favorable MB adsorption at higher temperature, in line with MB adsorption on cross-linked succinyl chitosan (Huang et al., 2011). The adsorption of dyes on cSMFB was physical in nature confirmed by ∆H° (< 40 kJ/mol (Feng et al., 2011)) and ∆G° (in range: – 20 to 0 kJ/mol (Auta and Hameed, 2011)) values. 4. Conclusions and future research cSMFB was developed from SMF waste precursor through green and economical approach, utilized to sequester cationic dyes (MG, CV, CB, and MB) from aqueous environment by batch mode operation. The characterization of cSMFB revealed a BET surface area of 59.88 m2/g. FT-IR analysis confirmed strong molecular affinity between functionalities present on SA and SMF waste during beads formation, while the XPS analysis results confirmed successful encapsulation of SMF waste in SA. Alkaline pH (between 6 and 8) favored the adsorption of cationic dyes. The adsorption of CV, MG, and CB was endothermic, while exothermic MB adsorption was observed. The equilibration time was varied between 480 and 600 min. The dyes adsorption was possibly governed through electrostatic, coordinate interaction, and ion interchanges. In conclusion, the developed cSMFB displayed excellent dyes remediation performance from aqueous environment, well accomplishing a cleaner production concept. Generally, waste effluents are not only confined to one or two pollutants and the batch scale experiments are static, primarily used to obtain optimal adsorption efficiency of adsorbent in the presence of different influencing agents. Conversely, the column mode experiments are dynamic, essential to examine the practical applicability of adsorbent. Therefore, in future, laboratory scale 17

adsorption studies focusing multi-pollutants solutions in various matrices by both batch and column mode operations should be carried out to test the scale-up probabilities of cSMFB. Acknowledgements The authors acknowledge funding from the Research and Development (R&D) Program (Research Pooling Initiative), Ministry of Education, Riyadh, Saudi Arabia, (RPI-KSU). References Aichour, A., Zaghouane-Boudiaf, H., Iborra, C.V., Polo. M.S., 2018. Bioadsorbent beads prepared from activated biomass/alginate for enhanced removal of cationic dye from water medium: Kinetics, equilibrium and thermodynamic studies. J. Mol. Liq. 256, 533–540. Al-Qodah, Z., 2000. Adsorption of dyes using shale oil ash. Water Res. 34(17), 4295 – 4303. Auta, M., Hameed B.H., 2011. Optimized waste tea activated carbon for adsorption of methylene blue and acid blue 29 dyes using response surface methodology. Chem. Eng. J. 175, 233– 243. Auta, M., Hameed, B.H., 2013. Acid modified local clay beads as effective low-cost adsorbent for dynamic adsorption of methylene blue. J. Ind. Eng. Chem. 19(4), 1153–1161. Bedin, K.C., Martins, A.C., Cazetta, A.L., Pezoti, O., Almeida, V.C., 2016. KOH-activated carbon prepared from sucrose spherical carbon: adsorption equilibrium, kinetic, and thermodynamic studies for methylene blue removal. Chem. Eng. J. 286, 476–484. Benhouria, A., Islam, M.A., Zaghouane-Boudiaf, H., Boutahala, M., Hameed B.H., 2015. Calcium alginate–bentonite–activated carbon composite beads as highly effective adsorbent for methylene blue. Chem. Eng. J. 270, 621–630. Belhouchat, N., Zaghouane-Boudaif, H., Viseras, C., 2017. Removal of anionic and cationic dyes from aqueous solution withactivated organo-bentonite/sodium alginate encapsulated beads. Appl. Clay. Sc. 135, 9 – 15.

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Table 1 Beads nomenclature, physical characteristics, and dyes adsorption capacity Sample

Nomenclature

Average beads weight loss, %

Average beads diameter, mm

Adsorption capacity, mg/g

S1

Cryogenic alginate beads (cAB)

95.3

Before 48 h 3.0

After 48 h 2.3

MG

MB

CV

CB

0.50

17.41

13.55

10.70

S2

Wet SMF encapsulated beads (wSMFB)

0.03

3.5

3.5

4.33

1.79

1.02

0.87

S3

Room dried SMF encapsulated beads (rdSMFB)

78.2

3.5

2.0

24.26

22.56

16.40

15.78

S4

Oven dried SMF encapsulated beads (odSMFB)

73.5

3.5

1.8

25.91

20.45

16.10

13.03

S5

Cryogenic dried SMF encapsulated beads (cSMFB)

70.6

3.5

3.3

29.35

24.26

17.55

16.34

Experimental conditions: Drying time: 24 h; Drying temperature: 333 K; Concentration of dyes: 25 mg/L; Equilibration time: 24 h; Reaction temperature: 298 K.

Table 2 Isotherms data for the adsorption of dyes at varied temperatures on cSMFB. Isotherm models

Dyes MG

CV

MB

CB

298K

308K

318K

328K

298K

308K

318K

328K

298K

308K

318K

328K

298K

308K

318K

328K

Langmuir qm, mg/g

136.99

137.21

137.79

138.13

54.34

58.48

58.73

65.36

95.24

91.74

90.91

71.94

120.48

129.05

151.51

153.85

KL, L/mg

1.073

1.349

1.613

2.413

0.065

0.082

0.146

0.140

0.080

0.056

0.040

0.042

0.161

0.208

0.214

0.230

R2

0.9974

0.9977

0.9938

0.9955

0.9876

0.9946

0.9954

0.9974

0.9972

0.9921

0.9937

0.9948

0.9841

0.9785

0.9632

0.9817

RL

0.0092

0.0073

0.0061

0.0041

0.1333

0.1087

0.0641

0.0667

0.1111

0.1515

0.2000

0.1923

0.0585

0.0459

0.0446

0.0417

KF, (mg/g) (L/mg)1/n

67.34

72.68

77.53

89.12

5.29

6.88

11.16

10.93

9.44

6.77

5.22

4.15

18.46

22.52

28.82

29.28

n

2.752

2.853

2.975

2.983

1.85

1.94

2.43

2.25

1.70

1.59

1.53

1.54

1.78

1.78

2.01

1.75

R2

0.8944

0.8825

0.8062

0.8181

0.9188

0.9536

0.9711

0.9724

0.9737

0.9595

0.9872

0.9768

0.9917

0.9941

0.9951

0.9872

Freundlich

Table 3 Kinetic data for the adsorption of dyes on cSMFB. Kinetic models

Dyes MG

CV

CB

MB

qe,cal, mg/g

59.4

48.9

63.5

47.4

K1, 1/min

0.0074

0.0055

0.0064

0.0057

R2

0.9976

0.9980

0.9942

0.9943

78.7

45.7

74.1

65.3

K2, g/mg-min

0.0001

0.0001

0.0001

0.0001

h, mg/g-min

0.4442

0.3301

0.6794

0.2692

R2

0.9906

0.9610

0.9849

0.9714

57.4

49.6

62.1

46.3

PFO

PSO qe,cal, mg/g

qe,exp, mg/g

Table 4 Thermodynamic data for the adsorption of dyes on cSMFB. Dyes

Co, mg/L

Thermodynamic parameters ∆H°, kJ/mol

MG

CV

MB

CB

∆S°, J/mol-K

∆G°, kJ/mol 298K

308K

318K

328K

20

6.78

55.95

-9.78

-10.62

-11.01

-11.49

40

17.78

94.01

-10.33

-11.22

-11.73

-13.32

60

20.67

103.35

-10.22

-11.06

-12.06

-13.35

20

24.73

88.32

-1.63

-2.31

-3.55

-4.15

40

15.86

56.55

-1.15

-1.31

-2.12

-2.79

60

14.34

49.61

-0.54

-0.82

-1.37

-2.02

20

-33.34

-98.16

-4.06

-3.26

-2.13

-1.15

40

-23.58

-69.03

-2.95

-2.48

-1.51

-0.96

60

-22.16

-66.35

-2.41

-1.64

-1.17

-0.35

20

19.34

86.71

-6.14

-7.74

-8.77

-8.82

40

15.77

70.05

-5.16

-5.80

-6.33

-7.33

60

15.70

67.77

-4.56

-5.20

-5.51

-6.75

Scheme 1a Proposed scheme of cSMFB development

Figure 1 Zeta potential plot of cSMFB (a), FT-IR spectra (b), Over all survey XPS spectrum (c), and XRD pattern (d) of cSMFB

Figure 2 Cross-sectional SEM images of pristine (a), of pristine (a), CB saturated (b), MG saturated (c), MB saturated (d), CV saturated (e), and EDX plot of pristine (f) cSMFB.

Scheme 1b Proposed dyes adsorption mechanism on cSMFB

35

8.5

(a)

30

(b)

7.5

25

pHf

qe, mg/g

6.5

20

5.5

15 MG

10

MG CV MB CB

4.5

CV MB CB

5 2

4

6

pHi

8

10

3.5 2

4

Figure 3 pH studies (a), initial and final pH (b) plots for dyes adsorption on cSMFB.

6

pHi

8

10

60

80

(a)

70

50

(b)

60

qe, mg/g

qe, mg/g

40 30 20

298K 308K 318K 328K

10

50 40 30

10

0

0 0

120

298K 308K 318K 328K

20

10

20

30

40

50

Ce, mg/L

60

70

0

140

(c)

120

100

10

20

30

Ce, mg/L

40

50

60

(d)

100

qe, mg/g

qe, mg/g

80 80

60

60

40

298K 308K 318K 328K

20 0

298K 308K 318K 328K

40 20 0

0

1

2

3

Ce, mg/L

4

5

6

0

5

10

15

20

Ce, mg/L

Figure 4 Effect of concentration and temperature on CV (a), MB (b), MG (c), and CB (d) adsorption onto cSMFB.

25

60

qt, mg/g

50

40

30

20 CB 10

CV MG MB

0 0

100

200

300

400

500

t, min Figure 5 Contact time studies plot for the adsorption of dyes on cSMFB

600

700

Research Highlights Solid waste management approach converting SMF to cryogenic SMF beads was proposed. cSMFB was tested for the adsorptive removal of MB, MG, CV, and CB dyes from aqueous environment. FT-IR and XPS analyses supported the development of cSMFB. Experimental parameters influenced dyes adsorption. Electrostatic forces, coordinate bond, and cationic interchange governed adsorption.

Conflict of interest The authors declare no conflict of interest.