Sustainable Water Purification Using an Engineered Solvothermal

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Sustainable Water Purification Using Engineered Solvothermal Carbon Based Membrane Derived from a Eutectic System Manohara HM, Kanakaraj Aruchamy, Supratim Chakraborty, Radha N, Nidhi M R, Debasis Ghosh, Nataraj Sanna Kotrappanavar, and Dibyendu Mondal ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b01983 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Sustainable Chemistry & Engineering

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Sustainable Water Purification Using Engineered Solvothermal Carbon Based

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Membrane Derived from a Eutectic System

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Manohara H. M.,a Kanakaraj Aruchamy,a Supratim Chakraborty,a Radha N.,a Nidhi M. R.,a Debasis

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Ghosh,a Sanna Kotrappanavar Nataraj,a,b* Dibyendu Mondal a*

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aCentre

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Bangalore- 562112, India.

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bIMDEA

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de Alcalá. Alcalá de Henares. 28805 MADRID.

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*Corresponding authors: [email protected]; [email protected];

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for Nano & Material Sciences, JAIN (deemed to be University), Jain Global Campus,

Water Institute, Avenida Punto Com, 2. Parque Científico Tecnológico de la Universidad

[email protected]; [email protected]

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Abstract

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The surface water is widely adulterated by hazardous pollutants such as dyes, pharmaceutical

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wastes, surfactants, heavy metals, hormones, etc. Hence, there is a necessity to develop a water

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treatment technology which can overcome all the major water related problems. The

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conventional methods for water disinfection are very specific and expensive. The challenge is to

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devise a purification protocol without forming harmful byproducts, which opens up the

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opportunity for new technologies with efficient materials towards water treatment. The present

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work demonstrates a sustainable strategy for the robust water purification through powder

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based membrane fabricated from a highly oxygenated and Al-functionalized solvothermal carbon

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(Al-STC) composite. AlOOH/Al(OH)3 functionalized Al-STCs with improved surface acidity were

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prepared by low temperature solvothermal process from a eutectic system (ES) comprising

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ethylene glycol (EG), choline chloride (ChoCl), glucose (Glu) and aluminum salt. The ES acts as

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both carbon precursor and catalyst. Attributed to the unique properties such as high surface

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functionality, moderately high surface area, and caterpillar-like morphology, Al-STCs were

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employed for the fabrication of powder based membrane to purify water in dead-end filtration

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mode. Various pollutants for instance dyes such as malachite green (with rejection of >99.9% and

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flux 1522 LMH) and methylene blue (with rejection of >99.9% and flux 885 LMH), pharmaceutical

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drugs such as ciprofloxacin and (with rejection of >99.9% and flux 1011 LMH) and paracetamol

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(with rejection of 53% and flux 1010 LMH), oxytocin hormone (with rejection of 88.6% and flux

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955 LMH), surfactant CTAB (with rejection of 94.9% and flux 1436 LMH), and heavy metal [Cr(VI)

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with rejection of >99.9% and flux 932 LMH] were successfully removed from aqueous solution

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using Al-STCs based membrane. Moreover, membrane active surface was regenerated by simple

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ethanol washing and reused for five consecutive cycles without compromising the flux and

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rejection, thus demonstrating the utility of Al-STC based membrane as easy-to-use and

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ecofriendly membrane towards all kind of water purification in a sustainable and affordable

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manner.

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Keywords: Eutectic system; Solvothermal carbon; Easy-to-use membrane; Sustainable water

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purification; High flux; High rejection 2 ACS Paragon Plus Environment

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INTRODUCTION

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Due to the rapid growth in the world’s population and industrialization, the availability of

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portable water is declined day-by-day.1 The surface water is widely contaminated by hazardous

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chemical and biological pollutants such as dyes, pharmaceutical wastes, surfactants, heavy

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metals, hormones, etc.2 Due to large-scale production and extensive application in many

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industries such as dyestuffs, textile, paper, plastics, rubber, tannery, paints, and cosmetics, over

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50,000 tons of 10,000 commercially available dyes are entered industrial wastewaters annually

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and cause considerable environmental pollution and serious health-risk.3,4 Also, the world

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production of surfactants increased enormously. Several of these compounds are biologically not

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degradable and even a small quantity of surfactant can break the surface tension of water and

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present a threat to the environment.5 The Extensive research for sustainable technologies such

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as adsorption, decolorization by photocatalysis, chemical coagulants, membranes, hydrogels,

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etc. have been attempted to remove synthetic dyes and surfactants from wastewaters to

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decrease their impact on the environment.3 Further, heavy metal ions released from industrial

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wastewater attracted much concern in recent years. Among heavy metals, Cr (VI) is considered

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as harmful even in small intake quantity, since, it is bio-accumulating in human organs.6 It has

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been widely used or generated as byproducts in many industries such as metal plating, leather

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tanning, metal corrosion inhibition, and pigment production which contaminates surface and

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groundwater as well.7,8 On the other hand, the consumption of pharmaceutical compounds

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around the world has increased noticeably. Besides excessive uses, easy access and self-

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medication have made the pharmaceuticals as one of the emerging pollutant which are

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commonly discharged into the aquatic environment.8 Moreover, there are fewer reports about

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hormones as an emerging pollutant which contaminating surface water and directly affects eco-

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toxicological impacts even at low-level exposures.9 Hence, for the overall improvement of the

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public health, there is a need to develop state-of-the-art water and wastewater treatment

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technology which address all the major water related problems. The conventional methods for

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water disinfection are very specific and expensive while few are cost-effective but have

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operational limit under certain circumstances. Conventional chemical oxidation processes such

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as Fenton reaction are destructive techniques and require toxic chemicals such as hydrogen 3 ACS Paragon Plus Environment


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peroxide and ozone molecule.10 Further, activated carbons excessively used as a powerful

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adsorbents for many contaminants but, the preparation processes are relatively high cost

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capital.11 Another important conventional method is membranes purification, but, technology is

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not economical since, we have to supply external pressure.12 Therefore, numerous efforts have

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been made to develop technologies capable of facile water purification using oxides/hydroxides,

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silicates, carbonaceous materials, and etc.13 Among them, carbonaceous materials have gained

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remarkable attention considering their environmental friendly characteristics.14 Moreover,

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functionalized carbonaceous materials also found useful in many other applications, especially,

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doped carbon materials with heteroatoms such as S, N and etc., were widely used because of its

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potential in water purification and environmental significance.15,16 Nevertheless, development of

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simple synthetic methods with designed architectures and controlled chemical functionalities,

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which strongly affect the properties of carbonaceous material is still challenging.17

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Hydrothermal carbonization is an ecofriendly process for the conversion of sugars to oxygenated

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hydrothermal carbon (HTC) material at low temperature (150–300 °C) in water at high pressure

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at different timings.18 Such HTCs are extensively used as adsorbent for water purification besides

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several other applications.19 Nevertheless, the problem such as limited chemical functionalities,

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low surface charge, and less surface area associated with HTC led to an alternative carbonization

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process which is solvothermal carbonization in presence of semi-aqueous media.20 Solvothermal

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carbon (STC) has several advantages as compared to HTC in terms of both structure and

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functions.21 Many ionic liquids and eutectic mixture have shown to be excellent solvent and/or

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precursors due to their negligible volatility, high thermal stability, and molecular tenability, which

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reduce mass loss before the beginning of the decomposition process, and favor the carbonization

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processes by lowering process time and temperature.22 Ionic liquids (ILs) are molten salts having

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melting points below 100 °C and considered as substitutes of volatile organic solvents.23 Even

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though, tedious preparation methods, high cost and toxic nature limited usage of IL in industries.

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Deep eutectic solvents or eutectic mixtures considered as new class of IL analogues because they

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share many characteristics and properties with ILs and initiated potentially more cost-effective

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approaches in material synthesis. Deep eutectic solvent obtained by the complexation of a

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quaternary ammonium salt with a metal salt or hydrogen bond donor.22 Fechler et al. introduced 4 ACS Paragon Plus Environment

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‘salt-templated’ strategy to prepare nitrogen/boron-doped functional carbon using imidazolium

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based salts.24 Further, high surface area functional carbon was synthesized using different

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combination of alkali metal salts along with pyrrole based eutectic mixture.25

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From the exposed above, herein, a facile strategy has been demonstrated for the preparation of

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AlOOH and Al(OH)3 functionalized STCs from an engineered eutectic mixture combining choline

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chloride, glucose, ethylene glycol and aluminum nitrate. Al3+ was chosen because it possess large

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surface area, tailored pore sizes, high thermal stability,26,27 and aluminum hydroxide and

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oxyhydroxide based functional biomaterials are also emerged as the prospective materials for

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water purification. The as prepared AlOOH and Al(OH)3 functionalized STCs (Al-STC) have been

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employed as powder based membrane for the purification of water in dead-end filtration

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mode.28,29 The suitability of the Al-functionalized membrane has been demonstrated for the

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purification of water from several contaminants for instance oxytocin hormones, pharmaceutical

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drugs, dyes, surfactant, and heavy metal with significantly high permeate flux and adequate

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rejection of the each contaminant. To the best of our knowledge no such study has been reported

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wherein functionalized STC based membrane is used for different kind of water purification. The

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STCs were regenerated after filtration and was reused for several cycles without affecting the

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membrane performance in terms of flux and rejection of dye. Overall, the present study discloses

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the preparation of functionalized STC from an all-in-one eutectic system and demonstrated their

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utility as easy-to-use and ecofriendly membrane towards all kinds of water purification in a

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sustainable and affordable manner.

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EXPERIMENTAL SECTION

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Materials: Choline chloride, ethylene glycol and iso-propanol were acquired from SD fine

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chemicals. D-(+)-Glucose was purchased from Fisher scientific. Aluminum nitrate heptahydrate

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(Al(NO3)3.9H2O), Potassium dichromate and cetyltrimethylammonium bromide were purchased

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from Sigma Aldrich. Methylene blue, malachite green, and eriochrome black T were purchased

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from NICE Chemicals. All chemicals were analytical grade and used without further purification.

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Ciprofloxacin, paracetamol and oxytocin were procured from local medical store. Whatmann



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filter paper Grade 42 with pore size of 2.5 µm was procured from Merck. Deionized (DI) water

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was used in all the experimental processes.

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Preparation of Eutectic system (ES): In a typical experiment, 30 mL of eutectic mixture was

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obtained by mixing equimolar (0.2 M each) mixture of ethylene glycol, glucose and choline

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chloride and heated to 80 ˚C with constant stirring in the rate of 500 rpm for 4 h. Afterwards the

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solvent was degassed for 15 mins.

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Synthesis of Al-STCs: 0.5 M (4.16 g) of Al(NO3)3 .9H2O was dissolved in 15mL of DI water. The

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solution was sonicated for 5 minutes for complete solubility. Further, the solution was mixed

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with 15mL of ES and stirred till homogenous liquid formed. Then the solution was transferred to

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a 100mL Teflon-lined stainless steel autoclave which was then sealed and heated to temperature

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of 180-220 °C and time duration of 5-15 h using hot air oven. The resulting black monoliths were

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washed with excess DI water and isopropanol and dried at 60 °C for 6 h using hot air oven (Al-

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STC-1 to Al-STC-5). The control material was prepared using 0.5 M of glucose dissolved in 15 mL

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of DI water with 0.5 M Al(NO3)3 .9H2O for 5 h, 200 C (Al-STC-6). All the obtained STCs were

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ground to fine powders prior to further characterization and labelled as shown in the Table S1.

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Water purification using Al-STC based membrane filtration: In this study, removal of various

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pollutants such as dyes (malachite green (MG), methylene blue (MB) and eriochrome black T

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(EBT)), pharmaceutical waste (ciprofloxacin (Cpf) and Paracetamol (Pct)), hormones (oxytocin

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(Oxt)), surfactant (Cetyltrimethylammonium bromide (CTAB)), and a heavy metal, Cr(VI) ion were

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carried out using Al-STC based membrane under dead-end filtration mode. The chemical

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structure of above said organic contaminants are given in Figure 1. First, the thickness of the

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membrane was optimized to find out the best performance. For this purpose, membranes were

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fabricated by using aqueous dispersion of finely ground Al-STC-2 of concentration 10 mgmL-1.

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Exactly 1, 1.5, 2, 2.5 and 3 mL of Al-STC-2 dispersion (10 mgmL-1) was filtered using Whatman

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filter paper (area=7.8*10-4 m2) as a support which was fitted in a 10 mL stainless steel syringe

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filter holder and then, thickness of the membranes were recorded using digital screw gauge.

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Whatmann filter paper was used because of its easy availability, low cost and biodegradable

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nature. Moreover, Whatmann filter paper made of cellulose and facilitates less resistance to the

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flow of water. Membranes having thickness of 0.49±0.07 mm, 1.05±0.09 mm, 1.65±0.11 mm, 6 ACS Paragon Plus Environment

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2.32±0.17 mm, and 2.99±0.30 mm were obtained according to screw gauge thickness data.

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Further, the thickness of the membranes prepared from 1 and 1.5 mL dispersion of Al-STC-2 were

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analyzed by FESEM (Figure S1) images which showed membrane thickness of 0.67 mm and 1.29

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mm, respectively. Thus the thickness data recorded by screw gauge and FESEM are close to each

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other. Further, flux and rejection were recorded for 5 mL of MG dye and 5 mL of Cr (VI) with initial

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concentration to optimize the membranes having different thickness. Once the thickness was

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optimized, further removal of other pollutants was conducted with optimized thickness and then,

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%R and flux were calculated using Eq. (1) and (2).

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The removal efficiency (R) of MG from solutions was calculated by equation (1):30

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%𝑅 =

(𝐶𝑜 ― 𝐶𝑒) 𝐶𝑜

𝑋 100

Eq.1

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Where, Co and Ce (mgL-1) are the initial and residual concentration of MG solution, respectively.

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And the flux was calculated according to the equation (2).30

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𝐽=

𝑉 𝐴𝑡𝑝

Eq.2

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Where, V is the volume of the filtered water (L), A is the effective membrane filtration area (m2),

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t is the filtration time (h), and P is the applied pressure (bar) but, there is no any measurable

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pressure applied while using glass syringe for the experiment purpose. The UV-visible maximum

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wavelength for the individual contaminants are given in ESI, Table S2. Further, to optimize the

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performance of the different STCs, a study was conducted to remove dye (MG), drug (Cpf) and

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heavy metal anion (Cr (VI)) using different STCs at optimized thickness. Typically, powder based

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membranes were fabricated using 1.5 mL of finely ground Al-STCs dispersion (concentration 10

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mgmL-1 in DI water). The dispersion was passed through filtration bed fitted with Whatman filter

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paper of area=7.8*10-4 m2 using glass syringe. Then, 5 mL of MG solution with concentration 5

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mgmL-1 was passed through the membrane. For Cpf and Cr (VI), similar protocol was used,

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however, to fabricate membrane with optimised thickness of 2.99±0.30 mm for Cr (VI), 3 mL

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dispersion (of concentration 10 mgmL-1 in DI water) of Al-STC dispersions was utilised. There was



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no leaching of Al-STCs through Whatmann filter paper which is confirmed by absence of

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deposition of material even after centrifuging filtrate at 10,000 rpm.

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Dye removal experiment in bench scale: Large scale experiment conducted for 50 mL MG

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solution of initial concentration 10-100 mgL-1 using vacuum filtration method. Firstly, membrane

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of thickness 1.65±0.11 mm on the Whatman filter paper (Area = 1.25*10-3 m2) whereas, 6 mL

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dispersion (of concentration 10 mgmL-1 in DI water) of Al-STC dispersions were utilized while

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fabricating membrane, because the area of membrane support increases in lab-scale experiment.

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Then, through Vacuum filtration (attached with 0.75 HP powered vacuum pump) 50 mL of 10

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mgL-1, 50 mgL-1 and 100 mgL-1 concentrated solutions were passed through filtration membrane

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and the rejection and flux was calculated using eq. (1) and (2). Moreover, another experiment

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was carried out to demonstrate the use of Al-STC membrane in large scale using 50 mgmL-1

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concentrated MG dye aqueous solution. For this purpose Al-STC-2 based membrane with

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thickness 1.65±0.11 mm was fabricated by taking 6 mL dispersion (of concentration 10 mgmL-1 in

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DI water) of Al-STC-2 material. Further, the material recovery has been carried out by washing

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with 30 mL of ethyl alcohol once after the dye removal processed.

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Figure 1: Chemical structure of dyes, drugs and surfactants.


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Recovery and Reuse: The recovery experiment was carried out for 5mL MG solution of initial

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concentration 5 mgL-1 by using membrane of thickness 1.65±0.11 mm with effective filtration

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area of 7.8*10-4 m2 with 10 mL glass syringe. Once the dye adsorbed through membrane then, 3

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mL of ethyl alcohol has been used to wash the adsorbent and again utilized for the filtration

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process.

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Characterization: Characterization of Al-STCs were carried out using different analytical

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techniques. The surface morphology were studied using Field emission scanning electron

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microscopy (FESEM) with JEOL Model-JSM7100F instrument. FESEM micrographs were taken at

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accelerating voltage 5.0 kV using LED secondary electrons detected by 129 eV resolution silicon

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drift detector (30 mm2). The crystallinity were analyzed using powder X-ray diffraction (RIGAKU)

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operating with Cu–Kα1 radiation (l = 1.54 A˚) at scan rate of 3° min-1 and a 2 theta range of 5-80°.

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To quantitatively analyses the elemental composition of samples X-ray photoelectron spectra

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also were carried out by Thermo Scientific instrument model ESCALAB 250XI BASE SYSTEM WITH

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UPS AND XPS IMAGE MAPPING. Further, FTIR characterization was done using PerkinElmer

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instrument between 4000–600 cm-1. Al-STC materials were finely powdered and then 3 wt.% of

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material was mixed with KBr and made a pellet using palette maker then, FTIR analyzed. Raman

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spectra were recorded by instrument model LabRAM HR Evolution of HORIBA using CCD and

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InGaAs detectors with automated HeNe laser switching source of 514 nm. Surface areas of above

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mentioned samples were carried out by Nitrogen adsorption/desorption study. Nitrogen

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adsorption/desorption study was carried out on static volumetric gas adsorption analyzer

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(Micromeritics Inc. USA, Model ASAP) to determine the surface area and pore volume of the

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samples at 77.4 K up to 1.0 bar. In Brunauer-Emmett-Teller (BET) experiments, the specific

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surface area of a powder is determined by physical adsorption of a gas on the surface of the solid

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and by calculating the amount of adsorbate gas corresponding to a monomolecular layer on the

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surface. Determination of internal surface area is based on adsorption and condensation of N2 at

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liquid N2 temperature (77K) after degassing the samples at 150 °C under vacuum for 20h. The

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zeta potential of dispersed aqueous solutions (1 mg L-1) of the STCs was measured using a

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Zetasizer Nano ZS light scattering apparatus (Malvern Instruments, U.K.) with He–Ne laser (633

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nm, 4 mW) at 298.15 K. UV-visible absorption spectra were collected on a Shimadzu UV-Vis 9 ACS Paragon Plus Environment


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Spectrophotometer, UV-2600. Contact angle was measure by taking a close-up photo of the drop

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on the membrane surface and Image-J software was used to measure the angle manually from

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the picture. Boehm’s Titration was carried out using 50 mg sample. The material was immersed

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in 50 mL of 5 mM NaHCO3 solution, further, mixture was incubated at room temperature for 24

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h. Then, the solution was filtered using Whatman filter paper to separate Al-STCs from reaction

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mixture. Further, 10 mL filtrate was pipetted into the conical flask and the excessive base was

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titrated with 5 mM HCl taken in a burette, whereas, methyl orange was used as an indicator.31

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RESULT AND DISCUSSION

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Preparation and Characterization of the Al-STCs: Scheme 1 shows the methodology adopted to

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prepare Al-STCs. Glucose, a biomass-derived renewable monosaccharide is widely used as C-

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source in hydrothermal carbon.32 Therefore, glucose based ES was designed combing choline

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chloride as the H-bond acceptor, ethylene glycol as a structural directing agent, and with the

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Al(NO3)3 .9H2O as a metal ion source which induces active sites in carbon matrix for improved

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removal efficiency for pollutants. This “all-in-one” ES was reacted under solvothermal condition

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to obtain Al-STCs with abundant surface oxygen functional groups. Because of extensive inter

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and intra-molecular hydrogen bonding network, the mixture forms a low toxic eutectic system

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thereby making it a green synthesis.

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Scheme 1: Schematic for the synthesis of Al-STCs from glucose based eutectic system.

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Typical Field emission scanning electron micrographs (FESEM) of the resulting Al-STCs prepared

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at different reaction temperatures and duration are shown in Figure 2. Al-STCs showed variations

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in morphology, particle size and dispersion. Al-STC-1 is composed of both spheres and sheet like 10 ACS Paragon Plus Environment

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structure attributed to low temperature (180 °C) treatment (Figure 2a). However, with increase

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in reaction temperature, caterpillar like aggregated morphology was observed for Al-STC-2 and

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Al-STC-3, respectively (Figure 2b-c). SEM image of Al-STC-3 with more particles is provided is

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supporting information (Figure S2). From the figure, aggregated smaller size carbon spheres were

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observed which supports particle size development having smaller size. With increase in reaction

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duration from 5h to 10h, similar morphology was observed as Al-STC-3 but with smaller size

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(Figure 2d). Further, when reaction duration was increased to 15h, a mixture of sphere and flake

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like morphology with indefinite sizes are obtained (Figure 2e). Interestingly, the role of ES in

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functionalizing the carbonaceous material is further confirmed when comparing the morphology

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of Al-STCs with controlled material. Al-STC-6 (Glucose with Al-salt) showed isolated sphere like

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structure (Figure 2f). Overall, besides the role of eutectic systems and Al3+, reaction temperature

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and time also played important role towards the caterpillar-like morphology of STCs.

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Figure 2: FESEM images of all Al-STCs materials. (a) Al-STC-1, (b) Al-STC-2, (c) Al-STC-3, (d) Al-STC-

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4, (e) Al-STC-5, and (f) Al-STC-6.

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Further, the functionality of carbonaceous materials was initially probed by FTIR Spectroscopy.

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The oxygen containing functional group such as hydroxyl, ester, ether and carboxylate groups are

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obtained upon solvothermal treatment of eutectic system in presence of aluminium salt (Figure 11 ACS Paragon Plus Environment


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3a). However, it is observed that the intensity of the adsorption band at 1688 cm-1, corresponding

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to the carboxyl group increased in Al-STC-2 which indicated a higher degree of carboxylic groups

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in this STC material. The band at wavenumber ~1208 cm-1 and 1590 cm-1 relate to C‒O and C=C

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groups respectively and the peaks corresponds to wavenumber ~3350 cm-1 and 535 cm-1 confirms

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the presence of –OH and Al-O functionalities, respectively.33 FTIR also suggested the existence of

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small aromatic groups in the STCs materials, which further confirmed by the Raman spectra,

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where, signal obtained in the 1000–1900 cm-1 range (Figure 3b). The band at 1589 cm-1, known

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as the G band (Graphitic) is due sp2 carbon both in aromatic and olefin molecules.34 The band at

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1360 cm-1, known as the D band is assigned to sp3 carbon.35

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Figure 3: (a) FT-IR spectra, (b) Raman spectrum (c) XRD spectra for all the prepared materials and

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(d) XRD spectrum obtained for Al-STC-2 material.

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Further, powder X-ray diffraction patterns of prepared Al-STCs shows semi crystalline peaks and

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confirms the presence of AlOOH/Al(OH)3 composite embedded in it (Figure 3c-d). As shown in 12 ACS Paragon Plus Environment

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Figure 3d, peaks corresponding to (020), (120), (031), (131) and (200) can be indexed to

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orthorhombic AlOOH which match with the JCPDS number # 21-1307. Also, presence of Al(OH)3

3

is also confirmed from the pXRD data. The peaks corresponding to (001), (020), (021), (023) and

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(311) attributed to Al(OH)3 which match with the JCPDS number # 76-1782 and the peak present

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at 23.5° corresponds graphitic carbon. For the further confirmation of presence of Al, the Al-STC-

6

2 was calcined at 550 °C for 3h and characterized using pXRD. The pXRD graph confirms the

7

formation of aluminium oxide match with the JCPDS number # 31-0026 as shown in ESI, Figure

8

S3.

9 10

Figure 4: (a) Survey spectra for Al-STC-2 and deconvoluted XPS spectra of (b) Al 2p, (c) C 1s and

11

(d) O 1s obtained for Al-STC-2 material.

12

X-ray photoelectron spectroscopy (XPS) was used to characterize the oxygen functionalities and

13

the presence of Al3+ in Al-STCs materials. As a representative example, Figure 4 shows the Al 2p,

14

C 1s and O 1s core-level spectra of Al-STC-2 material and in the Figure 4a, the peaks at 283.95 eV,

15

531.21 eV and 73.46 eV are corresponding to C 1s (Atomic% = 58.39%), O 1s (Atomic% =32.61%) 13 ACS Paragon Plus Environment


Page 14 of 27

1

and Al 2p (Atomic% =7.4%), respectively.34 A representative Al 2p core level spectrum obtained

2

for the Al-STC-2 showed two peaks at 72.5 eV and 74.3 eV corresponds to Al 2p3/2 and Al 2p1/2

3

(Figure 4b). It indicated that the abundance of Al 2p3/2 is more than Al 2p1/2 confirms. Peak at

4

74.3 eV corresponds to aluminium hydroxide which confirm the stable aluminium composite.36

5

From C 1s spectrum in Figure 4c, three signals at 282.9, 285.1 and 287 eV were identified. They

6

are attributed, to carbon group (C=C, CHx, C-C), ethers (C-O), carbonyl groups (>C=O) and

7

carboxylic groups (-COOR). Further, in Figure 4d, the peak at 533.0 eV is mainly attributed to –

8

C=O groups, although a certain contribution from -COOR is also present, as shown in C 1s spectra

9

as well. The content of carboxylic groups were further determined by Boehm’s titration method

10

as given in experimental section and it is confirmed that Al-STC-2 have significantly high surface

11

acidic group than other STCs materials (Table 1). Remarkably, engineered materials using ES

12

contains more surface acidic groups than control material irrespective of Al salt treatment.

13

Table 1: BET and Boehm’s titration results for the prepared Al-STCs

Material

Surface area (m2g-1)

Total pore volume (cm3g-1)

Mean Pore diameter (nm)

Content of carboxyla (mmol/g)

Al-STC-1

9.6

0.015

6.37

35

Al-STC-2

22.3

0.032

5.77

50

Al-STC-3

76.8

0.095

4.99

14

Al-STC-4

34.8

0.033

3.78

8

Al-STC-5

10.8

0.018

6.82

5

Al-STC-6

13.5

0.021

6.31

2

aDetermined

using Boehm’s Titration32

14

This further confirms the importance of ES in engineering valuable carbonaceous materials.

15

Based on the adsorption isotherms, the surface areas were calculated and summarised in Table

16

1 and the graphs corresponding to BET isotherm given in ESI, Figure S4. The surface area and the

17

pore volume increases with increase in reaction temperature and shows maximum surface area

18

of 76.8 m2g-1 for the STC prepared at 220 °C (Al-STC-3). When reaction time increases to 10h, the 14 ACS Paragon Plus Environment

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1

surface area was increased to 34.8 m2g-1 (Al-STC-4), but with 15h treated sample (Al-STC-5),

2

surface area was decreased to 10.8 m2g-1. However the control material prepared using glucose

3

(Al-STC-6) shows less surface area than the Al3+ treated ES even though both of them treated at

4

same reaction condition (Al-STC-2), which implies the role of ES in increasing surface area.

5

Al-STCs Based Membrane for Sustainable Water Purification: Since, the abundant presence of

6

highly oxygenated functional groups with improved surface acidity on the surface of the Al-STCs

7

was confirmed (Table 1, Scheme 2), determination of materials efficiency towards universal

8

water purification was undertaken by fabricating materials as powder based membrane as

9

discussed in the experimental section and shown in scheme 2. As depicted in Scheme 2, the easy-

10

to-use Al-STCs membrane was fabricated for the removal of various pollutants such hormones,

11

pharmaceutical drugs, dyes, surfactants and heavy metals from aqueous solutions.

12 13

Scheme 2: Schematics showing the protocol followed in the whole process to purify the

14

contaminated water from Al-STC-2 material.

15

Figure 5 a1-a2 shows the membrane before materials deposition and after material deposition.

16

The uniform coating of Al-STCs originated an effective barrier for cationic pollutants and allowed

17

their robust removal. For instance, removal of cationic dyes such as MG and MB (pH = 6.5) with

18

an initial concentration of 5 mgL-1 were carried out by syringe filter and as can be evident from

19

Figure 5 a3-a4, complete removal of both dyes was observed. Membrane thickness is important 15 ACS Paragon Plus Environment


1

parameter when envisaging universal water purification having different varieties of pollutants

2

since the permeate flux and rejection are dependent on membrane thickness besides surface

3

functionalities. Therefore, a study was conducted to know the best membrane thickness for the

4

efficient rejection of dye and heavy metal with high flux. The membranes thickness was adjusted

5

by controlling the amount of the Al-STC-2 material dispersed in water during processing.

6 7

Figure 5: (a1) Whatman filter paper before Al-STC material deposition, (a2) after material

8

deposition, (a3-4) typical glass syringe filtration model used for MG and MB dye removal

9

respectively, (b-c) Graph of flux and rejection with respect to thickness of the membrane for MG

10

dye and hexavalent chromium, respectively. (d-e) Graph showing flux and rejection for MG

11

obtained using prepared Al-STC membranes and respective UV graph (f) Digital photographs of

12

permeate obtained with different materials (f1) Initial dye with concentration 10 mgL-1 and (f2-

13

f7) Permeates from different membranes Al-STC 1-6 membranes, figure (g-h) indicates UV plot 16 ACS Paragon Plus Environment

Page 16 of 27

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1

for graph showing flux and rejection for Cr(VI) obtained using different prepared Al-STC

2

membranes and respective UV graph (i) Graph showing flux and rejection for ciprofloxacin

3

obtained using prepared Al-STC membranes and inset showing UV graph obtained for optimized

4

material. All the experiments were carried out three times at least to calculate standard

5

deviation.

6

Figure 5b and 5c show the effect of membrane thickness on the flux and rejection of MG dye and

7

Cr(VI), respectively. Remarkably, MG dye was adsorbed completely (99.9% rejection) through a

8

single pass through the 1.65±0.11 mm thick membrane with the flux rate of 1152 Lm-2h-1 (Figure

9

5b). Further decrease in thickness resulted lower rejection and with increasing thickness more

10

than 1.65±0.11 mm caused gradual decrease in flux rate. The scenario was different in case of Cr

11

(VI), wherein the best rejection (99.9%) was recorded with 2.99±0.30 mm thick Al-STC-2

12

membrane with a flux rate of 892 Lm-2h-1 (Figure 5c). The corresponding UV-vis graphs for

13

thickness dependent study of MG and Cr (VI) are given in Figure S5. After optimizing the

14

membrane thickness, a study was thereafter undertaken to examine the efficiency of other Al-

15

STCS based membrane towards removal of MG dye (Figure 5d-f), Cr (VI) (Figure 5g-h), and Cpf

16

drug (Figure 5i), respectively with optimized thickness. UV-spectra for the performance of Al-

17

STCs for Cpf removal through membrane like filtration method showed in Figure S6 and all results

18

are summarized in Table S3. In general, all the Al-STCs material exhibit better flux and rejection

19

at some point depending on the pollutants, but in all cases, Al-STC-2 has shown outstanding

20

performance for adsorbing MG, Cr (VI), and Cpf drug. These phenomena can be attributed to the

21

highest surface acidity of Al-STC-2 as compared to the other Al-STCs (Table 1). Moreover, Al-STC-6

22

(control material prepared with Glucose) showed the lowest performance in terms of rejection

23

of all the pollutants confirming the advantage of engineered material with the ES in membrane

24

performance.

25

Since, Al-STC-2 showed excellent performance in all the cases, the study was extended in order

26

to remove emerging pollutants such as hormones, surfactants and pharmaceutical waste using

27

Al-STC-2 based membrane filtration. The results are summarized in Table 2 and UV-vis graphs for

28

the feed and permeate of all the contaminants are shown in Figure S7. Even though surface area 17 ACS Paragon Plus Environment


1

of others Al-STCs are higher than Al-STC-2 but, surface acidic group plays an important role.

2

Therefore, to understand the effect of surface charge on the membrane, a study was undertaken

3

for the removal of MG, MB and EBT dye with 1.65±0.11 mm thickness membrane, wherein, 27.7%

4

rejection was observed for EBT (negatively charged dye) and complete removal (99.9%) of MG

5

and MB was observed which confirmed charge specific nature of the Al-STC-2 based membrane.

6

From Table 2, it is observed that, apart from charge of the molecule, molecular weight of the

7

pollutants also play a crucial role where Cpf molecule having small molecular weight required

8

membrane with thickness of 2.99±0.300000000000 mm to give complete rejection, whereas,

9

paracetamol having less molecular weight and neutral charge in aqueous medium shows only

10

55% rejection even with thickness of 2.99±0.30 mm. However, Oxt and CTAB having high

11

molecular weight as well cationic charge exhibited the 88.6% and 94.9% rejection, respectively

12

with filtration membrane of thickness ~1.65±0.11 mm. Further, Cr (VI) in chromate ion having

13

small molecular size and anionic charge taken as a representative heavy metal ion shows >99%

14

rejection with membrane of thickness 2.99±0.30 mm may attributed to Gibbs-Donnan effect,39

15

since, the Al-STC-2 membranes has high negatively charged surface. Overall, any positive charged

16

contaminants can be removed robustly with minimum thickness of 1.65±0.11 mm without losing

17

much flux rate, whereas anionic pollutants such as EBT and chromate ion required minimum

18

2.99±0.30 mm thickness to get complete removal from aqueous medium. Further, the effect of

19

initial pH of the dye solution on the amount of dye removed was studied by varying the initial pH

20

under constant process parameters. The results are shown in Figure S8a-b. The increase in initial

21

pH increases the amount of dye adsorbed, this is may be because of increase in (-)ve charge on

22

the surface of the Al-STC as confirmed by zeta potential data (Fig. 6h). From temperature

23

dependent study as shown in the figure S8c-d, removal efficiency of MG dye remained similar in

24

the temperature range of 30-50 °C.

25

Table 2. Removal performance of Al-STC-2 material for model compounds. *Indicates

26

performance with filtration membrane of thickness ~1.65±0.11 mm. **Indicates performance

27

with filtration membrane of thickness ~2.99±0.30 mm. pH= 7±0.2 of the solution was maintained

28

for the all experiments.


Page 18 of 27

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Pollutants

Nature

Mw

Initial

Analyte

Flux

Rejection

(gmol-1)

conc.

Charge

(LMH)

(%)

Malachite green*

Dye

364.91 10 ppm

+

1522 ± 81

99.9

Eriochrome Black T*

Dye

461.38 10 ppm

-

1543 ± 23

27.6

Methylene Blue*

Dye

319.85 10 ppm

+

885 ± 18

99.9

Ciprofloxacin**

Drug

331.34 10 ppm

+

1011 ± 94

99.9

Paracetamol**

Drug

151.16 10 ppm

0

1010 ± 19

53

Oxytocin*

Hormone

10 ppm

+

955 ± 01

88.6

CTAB*

Surfactant

364.45 10 ppm

+

1436 ± 36

94.9

+

932 ± 72

99.9

Chromium (VI)**

1007

Heavy metal 51.99

10 ppm

1 2

In order check the compatibility of material in bulk process, an experiment was conducted by

3

vacuum filtration method using Al-STC-2 membrane of radius 2 cm with optimized thickness of

4

1.65±0.11 mm (Figure 6 a1-a3). Initially, effect of initial concentration was examined. 50 mL MG

5

solution of initial concentration 10 mgL-1 and 50 mgL-1 were allowed to pass through the

6

membrane which resulted complete rejection was confirmed through UV-vis spectra and optical

7

images as showed in Figure 6b and 6c, respectively. However, with the increase in dye

8

concentration up to 100 mgL-1 resulted declines in the dye rejection (Figure 6d). Figure 6e

9

demonstrates the Al-STC-2 based membrane fabricated using Whatman filter paper as a

10

substrate showed excellent stability even after bending. The important property of a membrane

11

for sustainable water purification is its reusability. In the present study as well, an attempt has

12

been made to regenerate the membrane active sites and reuse for further filtration purpose.

13

Ethanol was successfully utilized to regenerate the materials and separation of dye molecules. 19 ACS Paragon Plus Environment


1

Figure 6f documented the results obtained during the recyclability of the membrane for MG dye

2

rejection. From the Figure 6f, gradual decrease in rejection was observed from 99.9% to 88.5%

3

for consecutive five recycles. This is may be because of pore blocking or unrecovered MG from

4

the surface of the material. Nevertheless, >88% rejection after 5th cycle consider to be better

5

performance of a membrane. Further, to demonstrate the use of Al-STC membrane in large scale

6

an experiment was performed for removal of MG dye. For this purpose Al-STC-2 based

7

membrane with thickness 1.65±0.11 mm was utilized. As evident from Figure 6g, approximately

8

193 Litres water/m2 membrane can be purified even at high dye concentrated water (50 mgL-1)

9

with more than 90% rejection and can be reused for 5 cycles (Figure 6f). This high performance

10

may be attributed to improved surface charge and high hydrophilicity of the solvothermal carbon

11

based membrane. The surface charge of Al-STC-2 was calculated by zeta potential (Figure 6h) and

12

the wettability of Al-STC-2 based membrane was tested using contact angle measurement (Figure

13

6i). The results suggested the membrane has -18.5 mV surface charge (at pH 7) and water contact

14

angle of 70.8±1.4, confirming that the membrane is negatively charged with hydrophilic surface.

15

Hence, the present Al-STC based membrane can be envisaged as promising water purifier

16

platform in industries as well.


Page 20 of 27

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1 2

Figure 6: (a1-2) Al-STC-2 based membrane before (top) and after (bottom) material deposition

3

(a3) Image of the vacuum filtration method used to remove dye in bulk. (b-d) UV plot showing

4

removal of MG dye with different initial concentration on the same membrane and inset showing

5

images of initial (I) dye solution and final (F) permeate solution. (e) Image showing membrane

6

bending without rupture, and (f) plot showing % rejection of dye from Al-STC-2 membrane for a

7

number of cycles, (g) Treatment capacity of Al-STC-2 based membrane of thickness 1.65±0.11

8

mm for MG dye contaminated water (Co=50 mgL-1, pH=7.3±0.2 and T=30 °C), (h) Zeta potential

9

result obtained for Al-STC-2 material (i) Water contact angle measurement for Al-STC-2 based

10

membrane (Inset digital image showing water droplet on Al-STC-2 membrane).



1

CONCLUSION

2

In summary, a ES was designed to synthesize AlOOH/Al(OH)3 functionalized STCs with highly

3

oxygenated functionalities, improved surface acidity, moderately good surface area, and

4

caterpillar-like morphology via simple and cost-effective solvothermal method. Such

5

multifunctional Al-STCs were successfully used to fabricate stable and flexible powder based

6

membrane, which was employed for water purification in dead-end filtration mode. A number of

7

pollutants (with different molecular weights and analyte charges) such as dyes (MG, MB, and

8

EBT), pharmaceutical drugs (Cpf and Pct), oxytocin hormones, surfactant (CTAB), and heavy metal

9

[Cr(VI)] were successfully and efficiently removed from aqueous solution using Al-STCs based

10

membrane with extraordinarily high flux rate (885-1543 Lm-2h-1) and high rejection (53-99.9%).

11

Lowest rejection, 27.6% was recorded with EBT which is a negatively charged dye, and thus,

12

confirmed that Al-STCs based membrane is more selective for cationic pollutants. In general, the

13

surface functionalities and surface acidity of the membrane played major role than surface area

14

towards removal of varieties of cationic pollutants. Furthermore, the membrane active surface

15

was regenerated by simple washing with ethanol and reused for five cycles without surrendering

16

the flux and rejection. Overall, Al-STCs based membrane filtration methodology demonstrated

17

herein is highly scalable, easy-to-use and ecofriendly process towards all kind of water

18

purification in a sustainable and affordable manner. Considering that the synthesized Al-STCs

19

materials are enriched with surface acidic group, so that can also be applied as heterogeneous

20

catalyst, sensors and energy materials as well.

21

ASSOCIATED CONTENT

22

Supporting information: Table S1 described the code name of the samples; Table S2 provided the

23

details of absorption maxima for different pollutants; Table S3 detailed the flux and rejection

24

data; Figure S1 provides FESEM images of Al-STC-2 based membranes with different thickness;

25

Figure S3 provide the pXRD of ash obtained from Al-STC-2; Figure S4 showed BET adsorption plots

26

Figures S5-S7 provided the UV-vis spectra of different pollutants as obtained during the course

27

of filtration process. Figures S8 provided the UV-vis spectra showing removal efficiency for MG

28

dye at different pH and temperature. 22 ACS Paragon Plus Environment

Page 22 of 27

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1

AUTHOR INFORMATION

2

Corresponding Authors

3

*E-mails: [email protected]; [email protected];

4

[email protected]; [email protected].

5

Notes: The authors declare no competing financial interest.

6

ACKNOWLEDGEMENT

7

The authors acknowledge CNMS, Jain (Deemed to be University), Bangalore, India for providing

8

infrastructure to conduct this research work. DM thanks SERB-DST, India for the research Grant

9

(EEQ/2017/000417). SKN thanks DST-INSPIRE (IFA12-CH-84), Talent Attraction Programme

10

funded by the Community of Madrid (2017-T1/AMB5610) and NANOMISSION PROJECT

11

SR/NM/NT-1073/2016 for financial support. NANOMISSION PROJECT "SR/NM/NS-20/2014" is

12

acknowledged for using FESEM and pXRD characterization facility. The authors also acknowledge

13

Dr. D. Kalpana for extending the XPS facilities.

14

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17.

2 3 4

18.

5 6 7

19.

8 9 10

20.

11 12 13

21.

14 15 16 17

22.

18 19 20

23. 24.

21 22 23

25.

24 25 26 27

26.

28 29 30 31

27.

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Table of Content

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Synopsis: The present study disclosed a facile and affordable water purification technology

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using solvothermal carbon based membrane engineered from a eutectic system

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Sustainable Water Purification Using an Engineered Solvothermal

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