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Sustainable water reclamation from different feed streams by Forward Osmosis Process Using Deep Eutectic Solvents as Reusable Draw Solution Ashesh Mahto, Dibyendu Mondal, Veerababu Polisetti, Jitkumar Bhatt, Nidhi M. R., Kamalesh Prasad, and Nataraj Sanna Kotrappanavar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03046 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Industrial & Engineering Chemistry Research

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Sustainable water reclamation from different feed streams by Forward

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Osmosis Process Using Deep Eutectic Solvents as Reusable Draw

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Solution

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Ashesh Mahto,a,d Dibyendu Mondal,a Veerababu Polisetti,c Jitkumar Bhatt,b,d Nidhi M. R., Kamalesh Prasadb,d* and S.K. Nataraja*

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a

Sustainable Energy Materials and Processes group, Centre for Nano and Material Science, Jain University, JGI Global Campus, Kanakapura Road, Ramanagaram, Bangalore 562 112, India

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b

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c

Reverse Osmosis Membrane Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), G. B. Marg, Bhavnagar 364 002, India.

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d

Academy of Scientific and Innovative Research (AcSIR), Central Salt and Marine Chemicals Research Institute, G.B. Marg, Bhavnagar 364 002, India.

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*Correspondence:

Natural Products and Green Chemistry Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), G. B. Marg, Bhavnagar 364 002, India

Dr. S. K. Nataraj ([email protected]; [email protected]) and Dr. Kamalesh Prasad ([email protected]/[email protected]) Tel: +91-278-2567563

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ABSTRACT: Sustainable and low-cost reusable water recovery process from industrial

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wastewater may have potential positive impact towards potable water crisis in several

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industrial processes. Quite a number of approaches such as chemical processes, biological

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treatment and high-pressure membrane based processes have been developed to address this

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crucial issue but most of them are suffered from several discrepancies. Due to the suitability

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of deep eutectic solvents (DESs) as promising draw solution (DS) for forward osmosis (FO)

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processes, herein the potential of DESs for the recovery of reusable water from seawater, dye

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contaminated wastewater and tannery wastewater by continuous FO process is demonstrated.

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Under optimized process conditions, > 90% reusable water was recovered from different feed

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solutions with an average water flux of 5 L.m-1.h-1. Due to the high freezing point (Tf)

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difference between water & DES, phase separation of the DES and water mixture obtained at

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the end of the process was achieved at a temperature ca. -5 °C and DSs thus recovered were

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reused in subsequent batch experiments (three cycles), where minimal loss of water flux was

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observed. The recovered water was found to have low contamination making it suitable for

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different purposes.

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Key Words: Forward Osmosis; Tannery Wastewater, Draw Solution, Recovery; Deep

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Eutectic Solvent; Recycle

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INTRODUCTION

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Resource recovery from waste is a challenging task which has often been in focus for greater

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importance ahead of simply being one of the principles of ‘reduce, reuses or recycle’. Today,

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the world is facing serious problem due to declining sources for fresh water and the

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commitment to reclaim potable and/or reusable water from wastewater is the call of the day.

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As of today, reverse osmosis (RO) holds the largest share of market for sea water desalination

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[1] with well-defined and projected economical parameters, challenges and future prospects

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for the process [2,3]. On the other hand, nanofiltration (NF) and ultrafiltration (UF) processes

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have found widespread applications in waste water treatment, standalone [4,5] or sometimes

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in combination with other processes like membrane bioreactor (MBR) [6,7] and membrane

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distillation (MD)[8]. Microfiltration (MF) and UF processes are also employed as a

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pretreatment step for other membrane-based processes like RO and MBR [7,9,10].

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The market for forward osmosis is growing rapidly owing to the extraordinary

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progress being made in the fields of membrane development [11-14], draw solution[15-18]

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and process design[19, 20]. Albeit a high number of publications and patents being filed in

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the niche of FO, there are only a limited number of reports which deals with the overall

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process, i.e., recovery and the quality of final potable/reusable water [18, 19, 21]. With robust 2 ACS Paragon Plus Environment

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FO membranes developed by HTI, Albany, US being utilized in most of the FO research

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works and pilot plants [19, 20, 22]. Furher with the pioneering work by Modern Waters &

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Oasys Water Inc. in developing the process and technology at commercial scale [19], FO is

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now competing with other technologies in both sea water desalination and waste water

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treatment [23].

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Even though there are inherent advantages associated with FO, such as requirements of no

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or low-pressure for operation, low fouling propensity of membranes which in turn reduces

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the cleaning & replacement cost, high water flux and recovery rate[24, 25], it still face some

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challenges due to the need for a regeneration step to recover the draw solution (DS) or draw

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agent at the end of the operation [26]. There are many instances where FO process is

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integrated with another membrane based process viz. RO[27, 28], NF[16, 29] and MD[30,

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31]or other external stimuli like magnetic field [32, 33] or heat [19, 34] to recover the DS and

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product water. Recent research endeavours being carried out in this area suggest that the

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specific energy consumption for hybrid FO-NF cannot be lower than that of RO desalination

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[35] even with a membrane having infinite permeability and 100% rejection. Another report

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invariably points out the regeneration of DS as the most energetically unfavorable step

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compared to direct desalination [36] and for FO to be able to compete, the regeneration

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process must be significantly more efficient than RO.

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Following several estimations and assessments[25] it is projected that for FO to be

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economically viable, it should be employed for applications where RO fails, for example

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high-quality water recovery from highly contaminated wastewater. The FO process is viable

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where potential fouling risk for RO membranes are very high and pretreatment cost adds up,

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such as oil & gas, coal tailing wastewaters etc.[37, 38]. The FO process can be economically

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advantageous in applications where DS recovery is not essential such as intermediate process

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of dewatering [24, 39]. 3 ACS Paragon Plus Environment


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DESs are new class of solvents which have similar physicochemical properties to that

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of ionic liquids (ILs) but are more advantageous over the later due to their cost-effective

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preparation, complete atom efficiency during syntheses etc., and their sourcing from various

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bio-based resources make them bio-degradable as well as bio-compatible in number of

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instances [40], [41]. DES is generally prepared by the combination of a hydrogen bond

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acceptor (generally quaternary ammonium salts) and a hydrogen bond donor species, which

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may include glycols like ethylene glycol (EG) and glycerol (Gly), organic acids, amino acids

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and sugars [42]. In one of the reports, crude glycerol was used as a DS for FO application

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[43]. Apart from this there are no other reports available on the application of DES as DS for

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FO applications except our previous attempt to show the potential of DES as DS for delicate

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applications like concentration of DNA and protein by FO process [17]. However, there

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certain DESs are being used in separation and purification for different biomolecules by

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chromatographic technique and other solid-liquid extraction methods [44].

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The present work focuses on the utilization of potentials of DESs as DS for the

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recovery of fresh water from brackish water, seawater, dye contaminated wastewater and

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tannery wastewater while regenerating and reusing DS for successive runs. Unlike previous

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study [17], here emphasis has also been given on quality water recovery from highly

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contaminated feed streams.

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EXPERIMENTAL SECTION

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Materials: Choline chloride, Glycerol & Ethylene Glycol were purchased from SD Fine

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Chemicals, Mumbai. Polysulfone (MW=77,000-80,000) was purchased from Solvay. TMC,

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MPD and Reactive Black-5 dye were procured from Sigma Aldrich, USA. Sea water was

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collected from seashore (Ghogha beach 21.68˚N 72.27˚E), Gujarat, brackish water from a

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local bore well in Bhavnagar, Gujarat, India. Tannery effluent and its concentrated solutes


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were obtained as gift from CSIR-CLRI, Chennai, India. Further, concentrated solutes like

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polyphenol, phenol-HCHO, Red brown dye and NaCl (SD Fine Chemicals) were used to

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prepare simulated tannery wastewater. RO water was used for all control experiments and

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washing purpose.

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Preparation of Deep eutectic solvent (DES): The DESs were prepared following the

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method as described in the earlier report [17]. In a typical reaction, both the hydrogen bond

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acceptor (HBA) i.e., choline chloride (CC) and hydrogen bond donor (HBD) i.e., ethylene

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glycol (EG) or glycerol (Gly) were mixed in 1:2 molar ratio followed by heating at 80 °C

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with continuous stirring for 2 h until a clear homogenous liquid was formed(CC–EG 1:2 &

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CC-Gly 1:2 respectively) (Figure 1).

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Figure 1: Structure of two deep eutectic solvents used as draw solution in the present study.

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Membrane fabrication: Indigenous thin-film composite polyamide (TFC-PA) membrane

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comprised of an ultrafiltration (UF) support (polysulfone 15% w/w in DMF) layer on

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polyester fabric (~30+100µm), with the thin polyamide selective layer (~200 nm) prepared on

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UF-support by interfacial polymerization between trimesoyl chloride (TMC, 0.125 w/v% in

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hexane) and m-phenylenediamine (MPD, 5 w/v% in water) monomers. The resultant

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membrane was used as the semi-permeable barrier in all FO experiments. Post-treatments of 5 ACS Paragon Plus Environment


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the TFC membranes were carried out by treating with citric acid (2%) and deionized water,

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followed by glycerol coating (20%) [45,46]. Before using membranes in FO process,

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membrane was washed with deionized water to regenerate the original surface and the

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membrane was immersed in isopropyl alcohol (IPA) for 60-120 seconds and repeatedly

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washed with deionized water.

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Experimental Setup: FO experiments were carried out in lab-scale FO cells with an

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effective membrane area of 0.0057 m2 and 0.006325 m2. Both FS and DS were circulated at a

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flow rate of 1.8 L/min. In all experiments, the active layer of the membrane was facing the

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FS. All the experiments were carried out for at least 6 hours after 1 hour of stabilization to

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achieve steady state at 28±2°C. The reverse draw flux was estimated by conductivity meter

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and total organic carbon (TOC) analyzer using Ultra-Pure water as FS. For every experiment

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using recovered DS, fresh TFC-PA membrane was used.

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Feed and Draw Solutions (FS & DS): For pure water flux, deionized water was used as a

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feed and initial membrane characterization for flux and salt rejection were done using 2000

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ppm NaCl and 2000 ppm MgSO4 as model FS. Tannery waste effluent (TDS- 60,600 ppm),

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sea & brackish water, RB-5 dye in deionized water with different concentrations were used as

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FS. Detailed characterization of feed streams is provided in subsequent sections. All the

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experiments were carried with an initial FS:DS ratio of 10:1.

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Methods:

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Density was measured using Density & Sound Velocity Meter model DSA 500 M (Anton

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Paar). The osmotic pressure was measured using Vapro® vapor pressure Osmometer (Model

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5600 ELI Tech France). The viscosity was recorded on a Brookfield DV-II + Pro Viscometer

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at 25 °C employing spindle No 18 at 10 rpm. All the experiments were performed at different

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volume fraction of DES so as to replicate the dilution process during FO application. The 6 ACS Paragon Plus Environment

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morphology of the membrane surface was recorded using scanning electron microscopy

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(SEM, LEO 1430VP), UV-Visible Spectroscopy was used for estimating the concentration of

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color and organic contaminants (Varian CARY 500), ATR-FT-IR (Perkin Elmer Spectrum

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GX, FT-IR system, USA) spectra was employed for membrane surface characterization. The

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elemental quantification was carried out for both FS and DS using inductively coupled

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plasma- optical emission spectroscopy ICP-OES (ICP, Perkin Elmer, Optima 2000). Total

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dissolved salts (TDS) and conductivity analysis were done using CON700, EUTECH

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instrument. Further, the morphology of the membrane surface was studied using atomic force

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microscope (AFM), Ntegra Aura, Model: nt-mdt (Russia). Total organic carbon (TOC) of

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feed solution was measured using Elementar, Liqui TOC.

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Membrane characterization:

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Flux & rejection: As prepared TFC-PA membranes were tested for their performance in both

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RO and FO modes. Average flux (Jv), solute back diffusion flux (Jb) and percentage rejection

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were calculated by the following equations (1-3), respectively;

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=

..………………………………..(1)

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Jb = ∆TOC X

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% =

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Where, = Volumetric flux, Vt0 and Vtn are the volume levels of feed solution at time t0 and

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tn respectively. Jb= Solute back diffusion rate, ∆TOC is the change in total organic carbon on

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feed side in time “t”, A is the active membrane surface area and c is the concentration of

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draw solution. Percentage rejection (% R) as the ratio of difference in concentration of feed

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(Cf) and permeate (Cp) to the initial feed concentration.

..

………………………………..

∁ ∁! x100 ∁"

(2)

……………………………… (3)



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RESULTS AND DISCUSSION

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Properties of DES as draw solution (DS)

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In the present study CC was used as HBA for the preparation of the DESs with EG and Gly.

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This HBA is inexpensive as well as nontoxic and is produced in the megaton scale [47]. On

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the other hand, Gly and EG are industrially important chemicals [48-51]. When HBD (Gly or

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EG) is mixed in 2:1 molar ratio with CC (HBA), it effectively results formation of a solvent

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with very low water activity (aw) [42]. The properties of DES with addition of water were

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studied by monitoring changes in the thermodynamic (density variation, Figure 2a),

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colligative property (osmotic pressure, Figure 2b) and physicochemical properties (viscosity,

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Figure 2c) of CC-EG and water system at different ratios. Viability of a DS depends on many

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factors, out of which effective regeneration process of DS is of utmost importance as it

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directly influences the energy input. Evaluation of low concentration and low temperature

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(~5-10°C) phase separation studies will give significant information in designing an

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integrated FO system for simultaneous DS and water recovery. Understanding the variation

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in density data at different dilution with water will provide valuable insight into the behavior

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of DES components in aqueous mixtures and their effective recovery at low temperature.

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Figure 2a shows the variation in the density of CC-EG-water binary mixture. The density

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increased with the weight ratio of DES in the binary mixture owing to the higher density of

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DES as compared to water. The density also increases with the hydrogen bond acidity

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resulting in higher values for CC-Gly system as compared to CC-EG (Figure S1) [52].

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Osmolality is the most important property of a draw solution for osmotic gradient driven FO

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process. It is expected that dilution of DS for some extent may result in the dissipation of

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osmotic energy, which can be minimized by monitoring the required dilution level without

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comprising the osmotic gradient to maintain similar water flux. Therefore, it is important to

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know the effect of osmolality of DES at different dilution levels with H2O. Osmolality 8 ACS Paragon Plus Environment

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variation (Figure 2b) follows the same trend of a decay function as observed for density

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(Figure 2a). The highlighted portion corresponds to the variation in the ratio of DES: water

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from 1:2.6 to 1:5 (v/v). A steep drop in osmolality can be observed between 1:3.4 and 1:3.8

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(CC-EG:Water) which looks more like an exponential decay function. Chilling (-5˚C) a DES-

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water mixture at this composition did not result into segregation of ice (water) and DES

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(liquid) layers while on the other hand, 1:8, 1:9 and 1:10 ratio mixtures showed visible

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separation of ice and liquid DES. Although it might be a good option to stop the FO

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experiment at 1:4 dilutions in terms of osmotic pressure drop, a ̴10 dilution showed best

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results in terms of separation of DES from water. On the other hand, a highly viscous solution

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is a nightmare to run FO process through the recirculation mode wherein cost will add-up to

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run the pumps. Therefore, it is also important to know the change in viscosity of DES-water

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mixture when employed as DS. This is one of the bottlenecks to use CC-Gly (viscosity 222

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cP) as a DS in FO application [17]. CC-EG on the other hand is less viscous compared to CC-

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Gly and interestingly, a 1:1 CC-EG:water mixture showed viscosity of almost the same

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magnitude as that of water (Figure 2c). This resulted in ease of circulation and will also have

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a profound effect on the pump life.



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Figure 2: (a) Density (ρ) (inset- speed of sound in DES-Water mixture), (b) Osmolality

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(inset: zoomed-in region of exponential decay like trend in osmolality with increase in water

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concentration), and (c) Viscosity as a function of volumetric ratio of CC-EG and water at

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different ratio.

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Properties of the membrane used for FO process

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Firstly, TFC membrane used in this study has an asymmetric structure consisting of polyester

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fabric, ultrafiltration (UF) microporous support layer and polyamide selective layer as

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depicted in the schematic (Figure 3(a)).The membrane was prepared following previous

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procedure [45]. Figure 3(b) shows SEM image of the dense polyamide selective layer. The

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ATR FT-IR spectrum in Figure 3(c) corresponds to polyamide layer formation as on the

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asymmetric support. The characteristic Amide-I stretching can be seen at ~1650 cm-1 (C=O)

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which partially overlaps with the N-H bending vibration (amide-II) and can be seen around

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1545 cm-1 with the peak at 1250 cm-1 corresponding to N–C stretch. All membranes were


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conditioned and characterized for salt rejection experiments under applied pressure prior to

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use in FO.

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Figure 3: (a) Schematics depicting across-sectional view of asymmetric TFC-PA membrane,

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(b) Surface FE-SEM image of selective polyamide layer (top view). (c) ATR FT-IR spectra

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correspond to thin-film composite polyamide on polysulfone UF support, (d-f) AFM surface

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morphology and 3D images of pristine polyamide layer, height profile corresponding to the

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green horizontal line in (d), and (g&h) AFM image of IPA treated polyamide membrane with

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(i) height profile corresponding green line in (g).

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Before testing, each membrane was treated with isopropyl alcohol (IPA) to condition the

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selective layer. Figure 3(d & h) shows the AFM images of surface topography of pristine

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TFC membrane in 2D and 3D format. The average roughness value noted to be 16.708 nm.

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Polyamide thin film (~160 nm) selective layer results in a “splash” like finish during

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interfacial polymerization. On the other hand, IPA treated TFC shows an average roughness



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of 37.744 nm. Interfacial polymerization in case of TFC membrane preparation is optimized

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to be quick and imperfect so as to retain minimum thickness. Thus, the higher roughness of

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TFC membrane obtained after IPA treatment may be due to the removal of unreacted

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monomers of amine leftover during interfacial polymerization. Increase in roughness may

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also be due to removal of low molecular weight polymer chains from the selective layer

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during IPA washing. It can be concluded that IPA contact with polyamide layer results in

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membrane swelling and washing off the IPA removes unreacted monomers, debris and by-

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products resulting in a more open membrane surface [53, 54]. The physical swelling of

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aromatic polyamide chains in the presence of IPA can be attributed to similar Hildebrand

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solubility parameters (δ) which is 23.6 MPa1/2 for IPA and 23 MPa1/2 for the polyamide

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active layer[55]. Higher roughness in the IPA treated TFC membrane may cause fouling

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problem, however the fouling propensity increases with roughness (although not linearly)

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mostly in RO processes, wherein the composition of feed and applied pressure plays a major

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role in fouling. Whereas in FO, as there is no applied pressure the extent of fouling will be

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less.

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Further, membranes were characterized by measuring the flux and standard salt (NaCl and

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MgSO4) rejection at different applied pressure. At 4 bar and 26 bar NaCl rejection was >94%

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whereas MgSO4 rejection was >99%. In both the cases, specific flux varied between 1.3 L.m-

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2

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characterized with pure water feed followed by 2000 ppm NaCl and MgSO4 simulated

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solutions as FS and the two DESs as DS. In all cases, a feed tank carrying 1L FS and draw

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tank having 100 mL of DES (CC-EG or CC-Gly) were connected to circulating pumps. The

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volume of FS in the feed tank was observed to be decreasing gradually and permeate flux was

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volumetrically measured every hour for each feed solution. Once the experimental conditions

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were optimized, ~2000 ppm NaCl and ~2000 MgSO4 solutions were subjected to dewatering

.h-1.bar-1 and 1.8 L.m-2.h-1.bar-1. The membrane flux performance in FO mode was first


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process through TFC membrane. For a 10-hour FO run with pure water as FS and CC-EG as

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DS, an average flux of 3.6 L.m-2.h-1 (Figure S2). The FO performance of home-made TFC

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membrane using 3M NaCl as DS and DI water as feed solution has also been carried out. The

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results shows an average flux of 2.8 L.m-2.h-1, which is 28.5 % lower than the flux obtained

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with CC-EG (1:2 mol ratio) as DS.. To know the solute transport from DS to FS during FO

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process, an experiement was conducted using tap water as feed solution (Initial TOC = 31.89

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mg.L-1) and CC-EG as DS. After 10h of continous FO run the TOC value of the FS was

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58.93 mg.L-1 which results a reverse solute flux of 0.098 g.m-2.h-1.

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Recovery of reusable water from Seawater and brackish water using DES as Draw solution

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Initially, desalination experiments were conducted to evaluate the DESs (for their osmotic

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dewatering efficiency from feed solutions of MgSO4 and NaCl (~2000 ppm). With time, the

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volume of FS in the feed tank was observed to be decreasing gradually. All experiments were

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concluded after every 10 hours FO run to quantify FS and DS for their concentrations.

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Pristine DES solution CC-EG produced 5.2 L.m-2.h-1 fluxes for NaCl feed with ~42 % water

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recovery. On the other hand, CC-Gly osmotically dewatered at a rate of 7.54 L.m-2.h-1 and 4.5

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L.m-2.h-1 for NaCl and MgSO4 feeds with 33.33 % and 20.5 % water recovery, respectively

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(Figure 4(a-d)). For both DESs, NaCl concentration varied from 20-40% whereas for MgSO4

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feed, 30-60% concentration was achieved during the FO run. Interestingly, in seawater

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desalination, CC-Gly showed marginally higher initial fluxes compared to CC-EG. However,

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compared to pure water and simulated salt feed solutions, seawater and brackish water

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desalination flux was marginally higher, which according to the osmolality difference

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between the FS and DS seems improbable. Experiments were repeated thrice to corroborate

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the findings. One possible reason may be the presence of high concentration of electrolytes in

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the seawater which results in preferential adsorption and transport of water molecules through

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the polymeric membrane, nevertheless more detailed analysis need to be carried out to claim



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such possibility. Brackish water was desalinated at a higher rate compared to seawater

288

(Figure 4c & d), CC-EG produced higher fluxes for brackish water while for seawater, CC-

289

Gly exhibited higher flux. After about 5 hours of FO run, the flux data obtained by CC-EG

290

and CC-Gly for brackish water desalination coincided at about 7 LMH but in case of

291

seawater desalination, CC-Gly continued to produce higher fluxes than CC-EG. Although

292

higher flux was obtained using CC-Gly as DS as compared to CC-EG, the later was

293

considered as an optimum DS than former due to the higher viscosity of CC-Gly (222 cP, at

294

25 ˚C) for which continuous circulation will require more energy input when compared to

295

CC-EG (viscosity, 33 cP 25 ˚C). Another reason for employing CC-EG as DS for all the

296

experiments below is the ease of separation by chilling (discussed below).

297

Further, exploiting the colligative property of diluted draw solution, theoretical freezing point

298

depression of water upon dilution was calculated. Calculated Tf

299

mixing (Figure S3) shows that phase separation of water can be achieved from diluted DES at

300

-5 to -10 °C, assuming homogenous interactions between water and DES moiety throughout

301

the sample. A typical photograph of diluted CC-EG after freezing step for the recovery of

302

product water and draw solution is shown in Figure 4(e). There is a clear separation of

303

concentrated draw solution and icy product water, however close analysis through TOC and

304

conductivity measurements showed some trapped CC at the interfacial ice layer.


of water upon CC-EG

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306 307

Figure 4: (a&b) Water flux using CC-EG & CC-Gly as DS and simulated standard MgSO4

308

and NaCl (2000 ppm) solutions as FS, respectively and (c&d) give flux data for seawater

309

(collected form Gujarat coast) and locally sourced brackish water, respectively. (e)

310

Photograph of phase separated water and DES portion when chilled post FO brackish water

311

desalination and concentration of their constituents. (Pristine CC-EG 1:2 mol ratio was used

312

as DS)

313 314

Overall estimation of contaminants in feed, diluted draw solution, and recovered product

315

water shows good water quality. In-sets in Figure 4e and Table 1 shows the concentration of

316

various ions in feed and recovered product water from diluted CC-EG. Higher concentration

317

of chloride ions is attributed to entrapped CC in the segregated ice. The product water

318

recovered (after FO process using seawater as feed solution) from diluted draw solution

319

shows higher TDS making it unsuitable for drinking purpose. Hence, generating drinking

320

water from seawater by FO process may be not a viable option keeping in mind the high



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Page 16 of 33

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energy input. However, production of industrial grade water and irrigation water are the

322

feasible options in the desalination process of seaweed using FO.

323

Table 1. The composition of initial feed brackish water (BW) and sea water (SW) and final

324

desalinated product water recovered from draw solution (CC-EG) upon chilling. Feed Solution (mg/L)

Recovered Water from DS (mg/L)

Constituent +

Na K+ Mg2+ Ca2+ Cl-

Brackish Water 1179 31 477 903 5650

Sea Water 11800 900 1103 600 19600

Product water-BW 18 4.0 2.5 4.1 1843

Product water-SW 800 ND 100 50 10600

325 326

Recovery of reusable water from Tannery wastewater and dye contaminated water using

327

DES as Draw solution

328

Simulated dye wastewater comprising of 20 ppm RB5 was employed as a model feed

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solution to test the efficiency of CC-EG as DS. The results indicated stable flux of ~ 5 LMH

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upto 12 hours under no external pressure stimuli. The concentration of dye was quantified

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with respect to the run-time of FO experiment and shows an almost linear increase till 10 h

332

(Figure 5a). Water recovery of ca. 80% was achieved and the diluted DS was chilled to -5ºC

333

to phase separate waster and recovered DES (Figure 5d). The as recovered DES was

334

subjected to another FO run with the same feed solution and showed similar flux results

335

(Figure 5b). It can be noticed that during the second run, the flux became identical in value to

336

that of first cycle after 5 hours of operation. It is safe to say that the recovered DES has some

337

level of water as impurity and is not as effective as pristine DES in terms of water activity

338

which in turn affects the dewatering capacity of recovered DES.


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340 341

Figure 5: (a) Flux and concentration profiles with simulated dye solution as FS and CC-EG

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as DS (inset is the UV spectra for initial FS, final FS and diluted DS), (b) Flux recovery in

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the second cycle with regenerated DS, (c) Picture showing initial and final FS and DS, (d)

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Phase separation of water (ice) and DES (liquid) on cooling. (Pristine CC-EG 1:2 mol ratio

345

was used as DS)

346

Further, DES (CC-EG) was used as effective draw agent in FO for recovering high-

347

quality reusable water and simultaneously concentrating high-value products in tannery

348

wastewater. Generally, tannery wastewater can be termed as ‘dirtiest water’ because of the

349

nature of contaminants involved. Pressure-driven membrane processes have shown limited

350

potential to treat tannery wastewater so far. Moreover, pre-treatment is an unavoidable step to

351

protect the membrane from potential and drastic fouling under pressure. Unlike conventional

352

pressure-driven separation processes, FO offers greater benefits towards achieving zero waste

353

disposal and high-quality reusable product water. To the best of our knowledge, the FO study

354

presented herein is first of its kind effort towards obtaining high-quality reusable water and

355

achieving zero liquid disposals from real tannery wastewater. Direct concentration of tannery

356

feed wastewater may significantly contribute to the handling of toughest wastewater related 17 ACS Paragon Plus Environment


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issues. In this direction, tannery effluent was circulated on the active layer of TFC membrane

358

against draw solution (CC-EG). Two types of effluents were employed in the current study,

359

(1) pre-treated, organics-free liquid discharge with TDS as high as 60,000 ppm and (2)

360

simulated tannery wastewater comprising of concentrated powder extracts of real tannery

361

effluents namely polyphenol, phenol-HCHO, red brown dye and ~2000 ppm NaCl in 1000

362

mL water.

363

Figure 6(a1-a4&b) shows photographs of lab scale FO experiment depicting one complete

364

cycle of high-quality water recovery from simulated tannery wastewater. CC-EG dewatered

365

>90% reusable water at a rate of ~3.2 -1.8 L.m-2.h-1 from simulated tannery wastewater over

366

a period of 12 hours (Figure S4), leaving behind concentrated contaminants. In addition to

367

this, DES was recovered following similar protocol (Figure 6b) and was tested against

368

simulated tannery feed for the second cycle. The flux trend coincides with the first cycle’s

369

flux values after 6 h of FO run (Figure S4), corroborating the flux variations observed for dye

370

wastewater runs. Further, Figure 6(c) shows osmotic flux trend recorded for liquid tannery

371

wastewater feed and CC-EG as draw solution, in this case an average osmotic flux of~1.5

372

L.m-2.h-1 was recorded for all the runs with recycled DES (2nd- 4th runs). The initial flux

373

trend (1st run) for pristine DES varied between ~ 2 - 1.25 L.m-2.h-1 over a period of 12 h of

374

FO run showing that the recovered DES has the capacity to regenerate its water

375

activity/osmotic pressure for continuous FO cycles. A water recovery of ca. 50% was

376

achieved and the constituents of initial and final solutions were quantified using ICP-OES

377

(Figure 6d). It can be concluded from the results that apart from Cl-, none of the cationic

378

species showed any significant crossover to the draw side. It should also be emphasized that

379

the ionic contaminants in the diluted DS tends to partition in the DES phase rather than the

380

icy water after phase separation. This can be seen in the distribution of contaminants in

381

recovered water and recovered DES. In case of simulated effluent, after ~90% dewatering,


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concentrated feed and diluted draw were analyzed for concentration and possible crossover.

383

Figure 6e-f shows concentration distribution of different constituent in initial feed,

384

concentrated feed, recovered water and regenerated draw solution. The UV-visible spectral

385

trend near 275 nm and ~460 nm clearly indicates colorless diluted draw solution, whereas

386

estimation of recovered product water reveals insignificant crossover of NaCl. The feed

387

concentration of NaCl increased from 786 mg.mL-1 initial to 2732 mg.L-1 and diluted DS

388

showed~270 mg.L-1. But in all cases, no color was detected in diluted draw solution and

389

product water.

390

391

392 393

Figure 6: (a1-a4) Photographs of FO experiments of tannery wastewater treatment showing

394

initial feed, intermediate FO operation and product water recovery and concentrated draw

395

(CC-EG) recovery, respectively, (b) shows frozen diluted draw solution with separation

396

specifications, (c) flux trend using pristine CC-EG and performance of recovered draw

397

solution for three repeated fresh feed experiments. (d) The concentration of feed before and 19 ACS Paragon Plus Environment


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after reusable water recovery. (b) UV-Vis spectra of initial feed, concentrated feed, product

399

water and recycled CC-EG layer, and (f) overall performance from experiments using real

400

tannery waste constituents. (Pristine CC-EG 1:2 mol ratio was used as DS)

401

DES recovery after FO and role of excess thermodynamic functions

402

The colligative property of a solution depends on the number of solvent molecules accessible

403

in that solution. The properties of DESs with addition of water were studied by monitoring

404

changes in the thermodynamic and physicochemical properties of CC-EG-water and CC-Gly-

405

water mixtures as a result of dilution (Figure 2). Here we demonstrated potential applications

406

with case studies that DES can be used effectively and efficiently for dewatering and

407

concentration of waste purposes. In both desalination and wastewater treatment cases, we try

408

to demonstrate a viable recovery method following solid-liquid phase separation of diluted

409

draw solution by freezing. Energy benefits from freezing diluted DES to recover product

410

water and reusable DS compared to pressure driven processes and other FO systems are very

411

significant. Generally, diluted draw solutions are being heat treated to recover draw solute

412

and collect product water or to condense pure water [26]. Freezing 1 Kg of water around -5°C

413

takes ~0.8 MJ which is 3 times less than evaporating the same (See supporting information

414

for details). Freezing may thus be considered as an energy less step for recovery of draw

415

solution with additional benefit of no loss in product water due to undesirable evaporation, a

416

common problem where DS recovery is brought about by elevated temperatures [56]. With

417

appropriate process design and better membranes, DESs can be an economically viable draw

418

solution. As the solid-liquid phase separation on freezing DES-water mixtures have not been

419

reported earlier and bring many questions to counter regarding the change in thermodynamic

420

properties at lower temperatures. These thermodynamic understandings may well be the basis

421

for designing effective draw recovery system. Isobaric thermal expansion coefficients, the

422

fractional change in volume that accompanies a rise in temperature (αp) for CC-EG is

423

reported to vary from ~0.525*103/K-1-0.550*103/K-1 as the temperature is decreased from 20 ACS Paragon Plus Environment

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298.15K to 250 K and for CC-Gly it is even lower, ~0.450*103K-1[52]. A small value of αp

425

means that the volume of the sample responds weakly to changes in temperature. To the best

426

of our knowledge, there are no reported values of ρ and αp of DES at low temperatures. So,

427

we extrapolated these results (they all show linear dependency) to low temperature values to

428

understand the phase separation mechanism (Figure S5) [57-59]. At temperatures around -

429

7°C, the density of pure water is 0.998899 g.mL-1[61] and that of pure DES will be around

430

1.13 g.mL-1& 1.2 g.mL-1 for CC-EG and CC-Gly respectively [58-59]. When the diluted DES

431

(DS) was kept in a freezer at -7°C there is a sudden change in the surrounding temperature

432

(drop from 298 K to 266K) which results in entrapment of DES molecules in the ice crystal

433

structure. Gradual cooling was also attempted by first keeping the diluted DS at ~3-5 °C in a

434

normal refrigerator for 1 hour and then transferring it to deep freeze at -7 °C for 3 hours. The

435

final product water in later case was characterized by lower chloride ion concentration (500

436

ppm to 1000 ppm). A shift of negative excess functions to positive values for DES- Water

437

mixtures can be observed as the temperature decreases, which points towards a diminished

438

DES-Water interaction which facilitates the phase separation at sub-zero temperature [52].

439

Density difference is not the only property responsible for this separation; change in excess

440

molar volumes of the mixture also plays an important role. DES synthesis using HBD and

441

HBA dissolved in aqueous phase has been reported where freeze drying technique was

442

employed to remove water and obtain pure DES [62]. Although this study confirms the

443

possibility of extracting DES from aqueous systems, much work needs to be done in this

444

direction to minimize the loss of DS and production of high quality water from difficult FS.

445

In addition to regenerating potentials, CC-EG and CC-Gly solutions exhibit low toxicity and

446

effective multiple recovery cycles over other tested ionic liquids. Table 2 shows list of ionic

447

liquids used as draw solution in FO process in comparison to the present study. Retaining

448

high osmotic pressure which is evident from repeated cycle test are added advantages. 21 ACS Paragon Plus Environment


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Page 22 of 33

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However, energy efficiency in recycling diluted draw solution and cost comparison to present

450

competitive process, RO requires an extended technology evaluation. Nevertheless,

451

observation in this study highlights potentials of eutectic solvents as easily recyclable and

452

reusable draw solution for wide range of FO applications.

453

Table 2. Comparison with already reported Ionic Liquid Draw Solutions for FO applications Draw Solution

Properties

Flux

Recovery Experimental Membran Refn

(LMH) 3.2 M protonated betaine bis(trifluoromethylsulfon yl)imide ([Hbet][Tf2N]) in water

UCST-type IL immiscible with water at room temperature 1) Tetrabutylphosphoniu LCSTm 2,4immiscible dimethylbenzenesulfon with water ate (P4444DMBS) at elevated 2) Tetrabutylphosphoniu temperatures m mesitylenesulfonate (P4444TMBS) 3) Tributyloctylphosphonium bromide (P4448Br) 1) Choline ChlorideEthylene Glycol 2) Choline ChlorideGlycerol

Miscible with water in all ratios (25°C) Phase separation (ice and liquid DES) at ~-7°C

Conditions

0.50LMH with Cooling the Experiments were 0.6 M NaCl as diluted DS carried out at 60feed (sea water to 23 °C 80 °C concentration)

Feed-= 0.6 M NaCl solution P4444DMBS:

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