Silk fibres exhibiting biodegradability & superhydrophobicity for recovery of petroleum oils from oily wastewater

Journal Pre-proof Silk Fibres Exhibiting Biodegradability and Superhydrophobicity for Recovery of Petroleum Oils from Oily Wastewater Prakash M. Gore (Methodology) (Data curation) (Investigation) (Writing - review and editing), Minoo Naebe (Supervision) (Conceptualization), Xungai Wang (Supervision) (Conceptualization), Balasubramanian Kandasubramanian (Supervision) (Conceptualization)

PII:

S0304-3894(19)31777-7

DOI:

https://doi.org/10.1016/j.jhazmat.2019.121823

Reference:

HAZMAT 121823

To appear in:

Journal of Hazardous Materials

Received Date:

23 September 2019

Revised Date:

21 November 2019

Accepted Date:

3 December 2019

Please cite this article as: Gore PM, Naebe M, Wang X, Kandasubramanian B, Silk Fibres Exhibiting Biodegradability and Superhydrophobicity for Recovery of Petroleum Oils from Oily Wastewater, Journal of Hazardous Materials (2019), doi: https://doi.org/10.1016/j.jhazmat.2019.121823

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Silk Fibres Exhibiting Biodegradability & Superhydrophobicity for Recovery of Petroleum Oils from Oily Wastewater

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Prakash M. Gore,1,2 Minoo Naebe,1 Xungai Wang,1 Balasubramanian Kandasubramanian2,*

Institute for Frontier Materials, Deakin University, Warun Ponds Campus, Geelong - 3220,

Nano Surface Texturing Lab, Department of Metallurgical & Materials Engineering, Defence

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Victoria, Australia

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Institute of Advanced Technology (DU), Ministry of Defence, Girinagar, Pune - 411025, India

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Corresponding Author: Prof. (Dr.) Balasubramanian Kandasubramanian,

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Email: [email protected] ; Phone: +91-20-24304207

Graphical Abstract Recovery of petroleum oils from oily wastewater using biodegradable superhydrophobic-

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oleophilic degummed silk fibers.

Highlights Raw and Degummed Silk Fibres for Oily Wastewater Treatment

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Degummed Silk Fibres Exhibit Superhydrophobicity-Oleophilicity

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Degummed Silk Fibres Show Effective Oil Absorption & Oil-Water Separation

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Raw and Degummed Silk Fibres Show Biodegradability

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Silk Fibres Can Be Utilized For Oil-Spill Cleanup

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Abstract

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Present study reports superhydrophobic-oleophilic, environment-friendly, & biodegradable silk material derived from Bombyx mori silkworm, for practical oil-water separation and oil recovery

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applications. In this study, raw silk fibers were degummed using water and Na2CO3 (at 100oC), for removal of outer gummy sericin protein layer, which was confirmed using FTIR & FE-SEM analysis. The water & Na2CO3 degummed silk fibers showed superhydrophobicity with water contact angles (WCA) of 153o & 158o, respectively, demonstrating Wenzel & Cassi-Baxter

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states. Degummed silk fibers showed superoleophilicity (OCA~0o) towards petroleum oils like Petrol, Diesel, & Engine oil. The water & Na2CO3 degummed silk fibers showed oil-water separation efficiencies of 95% & 87.5%, respectively. Both degummed silk fibers showed more than 50% efficiency till 10 separation cycles. Further, raw & degummed silk fibers showed an environmental biocompatibility, by their biodegradation under in-house developed biotic decompost culture consisting of biodegrading micro-organisms. Their analysis showed that biotic

de-compost culture rendered biodegradation weight loss of 11% and 18%, respectively, in 35 days. Successive results showed that, degummed silk fibers can be effectively utilized for practical oil-water separation, and further, they can be environmentally biodegraded, thereby mitigating their waste generation and disposal problem. Keywords: Oil-water separation; Superhydrophobic; Silk fibers; Bombyx mori; Degumming.

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1. Introduction

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The increased demand of petroleum oils in recent decades for maintaining the ever-growing

energy requirements has accelerated the growth of associated industries [1–3]. Continuous transit

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of these petroleum oils many times lead to oil-spill incidents e.g. Gulf of Mexico in 2010, which causes severe hazards to aquatic animals, pollution of water services, and ecological imbalance

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[4–6].

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Water is the most essential component on mother Earth, for the survival of living animals. Thus, the contamination of water sources is obnoxious, since it causes the water scarcity, ecological disturbance, and carcinogenicity to living animals. It has been found that nearly 15% of

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population (of world) resides in water deficient zones, and the diarrheal illness arising due to water pollution leads to death of nearly 2~2.5 million people each year [4,7]. It has been reported that plastic waste weighing nearly ~2,50,000 tons (non-degradable) is littering in the seas, which

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is damaging its flora and fauna [4,5,8]. Considering this problem, scientific community has developed new synthetic materials and methods for oil-water segregation, but, their nondegradation and disposal further creates burden to environment [2,3,9]. Thus, there is critical requirement of naturally derived materials, for water treatments, which facilitate their biodegradation (after service life) in an environmental conditions, without causing any threat to surrounding environment (which is complicated with synthesized materials) [3,10,11]. In order

to address this problem, different biologically compatible and biodegradable materials like Polylactic acid (PLA), cotton, starch, silk, etc., have been investigated for oil-water separation [2,3,12–17], and adsorption toxic ions [18–24]. Silk which is a fully protein based biopolymer, is largely utilized in biomedical & textile applications, owing to its exemplary properties like excellent biological compatibility, non-toxicity, and mechanical stability [25,26]. Raw silk materials require degumming and functionalization, prior to their utilization in desired

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applications. In raw form, silk fibers possess core-shell structure [Figure 1 (a)], where it

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contains an outer shell layer of gummy sericin protein layer, which is generally allergic to

humans, and an inner part contains fibroin protein [Figure 1 (a)] [3]. Depending on the end

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application, the sericin layer is removed by various degumming treatments based on water

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boiling, Na2CO3 treatment, soap solution, alkali/acid treatment, etc. After degumming, the silk fibroin can be regenerated and functionalized with various additives and biomaterials for further

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requirements [3,27]. During regeneration, and functionalization, inherent properties of silk fibers get diminished e.g. decrease in molecular weight, reduction in functionality, etc., and also

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increases time and energy costs [3,27].

In of study, Patowary et al. reported surface functionalization of silk fibers, using Octadecylamine (ODA), for sequential oil-water segregation by utilizing solution dip-coating

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technique [28]. They claimed that ODA modified silk fibers demonstrated superhydrophobic/oleophilic attributes with WCA~150°±3° and OCA~0°, respectively. Further, these functionalized silk fibers revealed an absorption performance of 84.14 g g-1 & 46.83 g g-1 for motor oil & crude and oil, respectively, and maintained the stability till 05 absorption cycles [28].

In other study, Maleki et al. have reported regeneration & modification of silk fibroin into aerogels using polymethylsilsesquioxane (PMSQ), via sol-gel based condensation polymerization technique, for continuous oil-water separation [29]. These functionalized silk aerogels (bulk density~0.08-0.23 gcm-3) exhibited compressive strength & strain of 14 MPa, and ~80%, respectively. Further, these silk aerogels revealed superhydrophobicity-oleophilicity

g.g-1 selectively for organic solvents, pump oil, and vegetable oil [29].

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exhibiting WCA>150°, and OCA~0°, respectively, with an absorption performance of 500-2600

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In a similar study, Li et al. have reported regenerated silk nanofiber membrane possessing

diameter of ~106 nm. These silk nanofibers, which mimicked the Manta Ray fish gills, were

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fabricated using electrospinning method, for sequential oil-water separation. The generated silk

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nanofibers demonstrated superhydrophilic/Superoleophobic attributes, exhibiting WCA~0°, and oil contact angle (OCA)~154°, respectively. Further, these fibers showed an oil-water separation

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performance of 99.90% towards oil-water mixtures of Diesel and Engine oil [30]. Considering, above studies on silk fibers for oil-water treatment, it has been observed that, the

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regeneration, and functionalization are time and energy intensive methods, moreover, after these treatments, silk fibers lose their intrinsic properties [3,27]. Till now, researchers have not explored the applicability and direct utilization of degummed silk fibers for oil-water treatments, which is evident from the extensive literature search analysis [3,27–30].

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Considering the literature analysis, it has been found that, silk fibers show higher biodegradation rate under the protease XIV enzymes [31–33], further researchers have also performed silk biodegradation under soil based conditions [34,35], and gamma radiation [36]. However, literature analysis also shows that, there is no study on silk biodegradation using de-compost culture (using biotic setup), performed under normal environmental conditions. Generally, any

discarded waste of material frequently remains in an open environment for some time, and therefore, if such a material is able to degrade on its own, then it can reduce the generated waste, and will cause minimum damage to environment. Thus, considering the above-mentioned research gaps, the authors have demonstrated the direct utilization of silk fibers for sequential oil-water treatments. The reported degummed silk fibers

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demonstrate superhydrophobicity (WCA>150o) and superoleophilicity (OCA~0o), simultaneously. The degummed silk fibers reveal an oil-water separation efficiency of 87.5% to

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95%, along with an oil recovery performance of >50%, till 10 separation & absorption cycles.

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Further, the raw & degummed silk fibers showed environmental biocompatibility, through their biodegradation performance, under in-house developed biotic de-compost culture consisting of

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biodegrading micro-organisms. Biodegradation study showed weight loss of 11% and 18%, in 35 days, respectively, for raw & degummed silk fibers. All silk fibers were characterized using

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contact angle method, ATR-FTIR, FE-SEM analysis, and Thermogravimetric analysis method. The successive results showed that, degummed silk fibers can be effectively utilized for

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sequential oil-water separation, and oil recovery. Furthermore, degummed silk fibers can be effectively biodegraded in an environment to mitigate the waste generation and disposal problem.

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2. Experimental Section 2.1 Materials

Raw silk cocoons were procured from a government registered farmer from Chakan village in Pune city of Maharashtra state, India. De-ionized (DI) water (purity = 18.2 MΩ cm) was derived from a Barnstead Nanopure Water Purification System (Cole-Parmer, India), from organic chemistry synthesis lab. Sodium carbonate (Na2CO3) (anhydrous, purity~>99.5%) was procured

from Sigma-Aldrich Pvt Ltd., India. Petroleum oils i.e. Petrol, Diesel, & Engine oil, were procured from Hindustan Petroleum Corporation Ltd., India. Fresh cow dung was procured from a local farm, and the Decompost culture (bacterial content = 1 x 108 cells per g) was purchased from INORA India Pvt Ltd., Pune, India. 2.2 Silk Degumming: Water & Na2CO3 Based Solution

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Initially, raw silk fibers in the form of cocoons were chopped in small pieces, before using for

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degumming treatment. Then, the chopped silk fiber pieces were treated using water and 0.02 M Na2CO3 solution based degumming method [Figure 1 (b)]. In both degumming methods, silk

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fiber pieces weighing 5 gms are utilized for a 2 liter solution in beaker. In water based degumming, raw silk fibers were kept in a boiling DI water at a temperature of 100oC for 3 hours

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in a 2 liter beaker [3]. The beaker containing mixture of chopped silk fibres and DI water was

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placed on a hot plate stirrer at 200 rpm speed. After degumming, silk fiber samples were dried in hot air woven at a temperature of 60oC for overnight. In Na2CO3 solution based degumming, 0.02 molar solution is prepared using DI water in a 2 liter beaker [3]. Further, Na2CO3 solution is

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boiled at a temperature of 100oC on a magnetic hot plate stirrer with 200 rpm speed, then the chopped silk fiber pieces are added in 2 liter beaker, and degummed continuously for 1 hour. After drying, degummed fiber samples were analyzed for weight loss, and were utilized for

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further characterizations.

The sericin weight loss during degumming was calculated using following equation: 𝑾𝒆𝒊𝒈𝒉𝒕 𝑳𝒐𝒔𝒔 (%) =

(𝑾𝟐 −𝑾𝟏 ) 𝑾𝟐

. 𝟏𝟎𝟎

Where, W2 = Weight after degumming (gms); W1 = Weight before degumming (gms).

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2.3 Biodegradation Setup Considering the research gaps described in the Introduction section, the biodegradation study of silk fibers was performed under in-house prepared biotic de-compost culture for 35 days, at a room temperature [2,3,37,38] Figure 2. The study involved a biotic setup consisting of a fresh cow dung (procured from local farm) and de-compost culture (enriched with biodegrading microorganisms) procured from a market [2]. As per the manufacturer, the supplement of cow

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dung in the de-compost culture was prime requirement, as it helps in enhancing the development

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of biodegrading microorganisms, which further helps in improving the biodegradation rate [2].

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Therefore, based on the quantity of silk fibers, the de-compost culture formulation (in 500 mL beaker) involved, cow dung (~25 gms), de-compost culture (~25 gms), along with 1 Liter of tap

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water. Then, the prepared biotic de-compost solution placed in 100 mL beakers. The silk fiber samples of pre-measured dry weights were in the 100 mL beakers containing biotic de-compost

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solution. Then, these pre-weighed samples were immersed in the 100 mL beakers for the biodegradation analysis. After first 7 days, the silk fiber samples were removed from beaker,

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cleaned with DI water and ethanol, and were kept overnight in an air-oven at 40oC temperature for drying, then the silk fibers were taken for weight loss analysis [2,39]. Further, contact angle analysis was performed on biodegraded silk sample, for determining surface wettability after

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biodegradation.

2.4 Characterization Studies on Silk Fibers Degummed silk fibers were characterized using Fourier Transform Infrared (FTIR) microscopy instrument (ATR-FTIR, Bruker-Alpha 200630, Bruker GmbH, Germany). Morphological analysis was performed using Field Emission Scanning Electron Microscopy (FE-SEM) (Sigma 03-18, Carl-Zeiss GmbH, Germany). Weight analysis was performed using electronic balance

analyzer (Shimadzu Inc., Japan ). Surface wettability analysis was performed using Contact Angle Goniometer (DSA 25E, Kruss GmbH, Germany). Thermal stability study of all silk fibres was performed using Thermogravimetric Analysis (TGA) using STA 6000, Perkin-Elmer Inc., USA, instrument. During TGA experiment, the heating rate was maintained at 10oC/min, Nitrogen gas flow rate was maintained at 20 ml/min, and the silk fiber samples were heated from

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30oC to 800oC. 2.5 Theories on Surface Wettability

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The wetting behaviour of the utilized silk fibers i.e. hydrophobic, superhydrophobic, &

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oleophilic, were explained using Wenzel and Cassie-Baxter theories [Figure 9 (b)]. Wenzel’s theory helps in understanding the hydrophobic nature of the homogeneous surface exhibiting

( 𝜸𝒔𝒗 −𝜸𝒔𝒍 ) 𝜸𝒍𝒗

Where, 𝜽𝒘 = WCA of surface,

= 𝒓 𝐜𝐨𝐬 𝜽

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𝐜𝐨𝐬 𝜽𝒘 = 𝒓

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some roughness. This is elucidated using following Wenzel’s equation as [2,40,41]:

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 = Young’s contact angle,

r = surface roughness factor i.e. ratio of authentic & projected surface areas. Wenzel’s theory does not explain the superhydrophobicity exhibited by heterogeneous rough

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surfaces exhibiting porosity, however, it can be explained using Cassie-Baxter theory. Cassie-Baxter theory helps in explaining the superhydrophobic nature of heterogeneous rough surface possessing porous nature, which is elucidated using following equation [2,3,40]: 𝐜𝐨𝐬 𝜽𝒄 = 𝒇𝟏 𝐜𝐨𝐬 𝜽𝟏 − 𝒇𝟐

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Where, 𝜽𝒄 = contact angle (apparent), 𝒇𝟏 & 𝒇𝟐 = phase 1 & phase 2 representing respective surface fractions, 𝜽𝟏 = contact angle of phase 1.

2.6 Oil Absorption & Oil-Water Separation Study

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The maximum oil absorption capacities of raw & degummed silk fibers were evaluated by

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dipping method. The dry pre-weighed raw & degummed silk fiber samples were dipped (for

average 5 seconds) in a 50 ml volume beaker containing respective petroleum oils for up to 10

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cycles.

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The oil absorption capacities of silk fibers were calculated using following equation [2,13]: (𝑾𝟐 −𝑾𝟏 ) 𝑾𝟐

. 𝟏𝟎𝟎

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𝑶𝒊𝒍 𝑨𝒃𝒔𝒐𝒓𝒑𝒕𝒊𝒐𝒏 𝑪𝒂𝒑𝒂𝒄𝒊𝒕𝒚 (%) = Where, W2 = Weight after oil absorption (gms);

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W1 = Weight before oil absorption (gms).

During Oil Absorption study, the analytical dry weights of raw, water degummed, and Na2CO3 degummed silk fibers were measured before absorption cycles, and then sequentially wet weights after absorption were also measured. Absorption study was continued till 10 cycles for

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all silk fiber samples, for evaluating its durability on repeated absorption cycles. Before starting absorption study, dry weights of silk fibers were measured, then after every cycle, the wet weights were measured. In a typical single cycle, after every absorption, silk fibers were squeezed with a gentle hand pressure, in order to drain out (recover) maximum oil, then, same

fibers were again gently squeezed using cotton tissue papers, to remove any remaining traces of oil. After evaluating the oil absorption performance, raw & degummed silk fibers were utilized for practical oil-water separation study. In this study, an oil-in-water mixtures were prepared for simulating the oil-spill condition in the oceans. The amount of oil used in oil-in-water mixture

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was determined from the average absorption capacity of the respective silk fibers, which was calculated during oil absorption study. The weight of oil calculated by analyzing the average

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absorption capacity during oil absorption study was taken in a measuring cylinder (kept on

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weighing balance), and then the respective volume reading (in ml) of the filled oil was measured. Then, the amount of the oil in oil-in-water mixture was taken as per the respective absorption

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capacity of the utilized silk fibers.

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A typical oil-in-water mixture was prepared in a ratio of 1:5 i.e. oil:water, which was placed in a small sized beaker. Then, raw, and degummed silk fibers were fully immersed in the oil-water mixture in the beaker, for about average 5 seconds, in order to allow it to absorb the oil from the

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mixture [Figure 9 (a)]. After oil absorption, the silk fibers were removed from the beaker, and the remaining mixture was transferred to measuring cylinder, for determining the amount of oil left in the oil-water mixture. The oil-water separation efficiency of the used silk fibers was

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calculated using following equation [2,13]: 𝑾

𝑶𝒊𝒍/𝑾𝒂𝒕𝒆𝒓 𝑺𝒆𝒑𝒂𝒓𝒂𝒕𝒊𝒐𝒏 𝑷𝒆𝒓𝒇𝒐𝒓𝒎𝒂𝒏𝒄𝒆 (%) = 𝑾𝟐 . 𝟏𝟎𝟎 𝟏

Where, W1 = Amount of Oil Recovered (ml); W2 = Amount of Oil Initially taken in oil-water mixture (ml). 3. Results & Discussions

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3.1 Degumming Analysis Silk fibers were analyzed by measuring their weight loss before and after water degumming and Na2CO3 solution based degumming. In both methods, prior to degumming raw silk fiber sample weighing 05 gms were utilized. After water based degumming, the silk fibers showed average 18% weight loss, whereas Na2CO3 degummed silk fibers showed 28% weight loss, which corroborated to removal of sericin protein layer from raw silk fibers [3,27,42].

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Degumming of silk fibers was also evaluated using ATR-FTIR analysis, for confirming the

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sericin protein removal (Figure 3). ATR-FTIR analysis of raw, water degummed, and Na2CO3 degummed silk fibers showed common peaks at 3268 cm-1, 1224 cm-1, and 1038 cm-1 which

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corroborated to Amide A linkage, Amide-III linkage, and in-plane ‘=C-H’ vibrations,

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respectively, present in fibroin proteins [42–44]. Common strong peaks observed at 1615 cm-1, and 1507 cm-1 in raw and degummed silk fibers also confirm the existence of Amide-I linkage

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(C=O stretching), and Amide-III (C-N stretching, and N-H bending), respectively, which are usually present in silk fibroin proteins [42–44]. Peaks observed at 2922 cm-1, and 2850 cm-1

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correspond to aliphatic ‘C-H stretching’ generally present in sericin protein of silk fibers. Peak observed at 1743 cm-1, correspond to ‘C=O stretching’, which usually appears in carboxylic acid of the side chains, present in sericin protein [42–44]. Peak observed at 1382 cm-1 correspond to ‘C-N- stretching’, which typically present in amine linkages of the sericin protein of silk fibers

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[45]. Considering ATR-FTIR analysis of raw silk fiber samples, it has been clearly observed that peaks present at 2922 cm-1, 2850 cm-1, 1382 cm-1, 1741 cm-1, and 1382 cm-1 have diminished in water degummed and Na2CO3 degummed silk fiber samples. Considering the weight loss, and peak intensity difference between both degummed silk fibers, it can be concluded that, Na2CO3 based degumming has demonstrated high efficiency, as compared

to water based degumming. Further, it is also observed that, the intensity of peaks present in raw silk fibers has overall decreased, as compared to water and Na2CO3 degummed silk fiber samples [3,42]. Sericin protein removal performance of degumming treatment was also studied using morphological analysis, for raw, and degummed silk fibers. FE-SEM micrographs of raw, water

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degummed, and Na2CO3 degummed silk have been shown in Figure 4 & Figure 5. Figure 4 (a) shows the core-shell morphology of raw silk single microfiber, where sericin

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protein layer is seen to be present on the outer side of the microfiber, whereas the fibroin proteins

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attached together are present as core at the inner side of the microfiber [42,43]. Figure 4 (b) & (c) show morphology of single microfibers of water degummed, and Na2CO3 degummed single

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silk microfibers, where only half part of silk fibroin is observed, as compared to two attached silk fibroin proteins seen in Figure 4 (a). The absence of sericin protein in degummed silk single

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microfibers is attributed to the degumming action performed at 100oC (boiling), as it helps in loosening/breaking of the Hydrogen bonds present in serine amino acid, which renders the glue-

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like property to sericin protein, which helps in maintaining the intactness between sericin and fibroin [42,43]. The intact raw silk fibres attached together are observed in FE-SEM micrographs shown in Figure 5 (a). Figure 5 (b) & (c), show morphology of separated fibers of water and Na2CO3 degummed silk fibres (at a magnification of 10 μm), due to action of

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degumming treatment. Thus, it is confirmed that, water Na2CO3 degumming have facilitated the removal of gummy sericin protein layer from raw silk fibers. 3.2 Thermogravimetric Analysis: Thermal stability of all silk fibers was characterized using Thermogravimetric Analysis (TGA). TGA characterization plots of all silk fiber samples have been shown in Figure 6.

TGA analysis shows that, raw silk & water degummed silk have identical thermal stability, which show nearly 5% weight loss at ~100oC, 35% weight loss at 300 oC to 350oC, and nearly 50% weight loss from 350oC to 750 oC, followed by a residue of 10%. Whereas, Na2CO3 degummed silk fibres show initial weight loss of 22% till 120oC, followed by 8% weight loss from 120 oC to 300 oC, 38% weight loss from 300 oC to 371 oC, and final 32% weight loss from 363 oC to 588 oC, with no left residue. The higher weight loss in Na2CO3 degummed silk, as

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compared to raw and water degummed silk reveals that, alkali based degumming has effectively

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removed the sericin protein. The higher weight loss in Na2CO3 degummed silk is also attributed to the higher solution contact time i.e. 60 min, during degumming treatment. The literature

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analysis, shows that, alkali based degumming is generally performed between 30 min to 45 min,

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however, depending on the functionalization routes post degumming treatment, the solution contact time can be maintained up to 60 min.

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3.3 Surface Wettability Analysis

In order to establish the applicability of raw and degummed silk fibers for oil-water treatment,

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it’s surface wettability analysis is primarily required. Therefore, the surface wettability analysis of raw, water degummed, and Na2CO3 degummed silk fibers was performed at a room temperature using contact angle goniometer (Figure 7). As observed in Figure 7 (a), WCA of raw silk fibers is found to be 105o±3o (hydrophobic state),

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and this could be attributed to the presence of sericin protein layer, which exhibits gummy type nature [3]. The presence of outer gummy sericin protein layer is also confirmed from the FESEM analysis shown in Figure 4 (a). Figure 7 (b) shows WCA of 153o±3o (superhydrophobic state), for water degummed silk fibers, which is higher than WCA of raw silk fibers, whereas, Na2CO3 degummed silk fibers showed WCA of 158o±3o (superhydrophobic state) Figure 7 (c),

which is in turn greater than WCA of water degummed silk fibers. Furthermore, the wetting behaviour of raw, water degummed, and Na2CO3 degummed silk fibers towards petroleum oil was also analyzed, where it showed, superoleophilicity i.e. OCA~0o, for all fibers [Figure 7 (d)]. The hydrophobic surface of any material can be explained using Wenzel’s theory, which states that, the roughness on the surface of a material plays a prime role in rendering a hydrophobic characteristics to it [2,40,41]. Wenzel’s theory describes the hydrophobic nature of a material

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exhibiting homogeneous rough surface, but, it does not sufficiently explain the

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superhydrophobic nature of the heterogeneous surface exhibiting porous surface. Raw silk fibers in its cocoon shell form, contains closely infused fibers, held together by sericin protein [3,43]

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[Figure 4 (a) & 5 (a)], has most probably influenced its surface characteristic, which is evident

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from it WCA~105o±3o [Figure 7 (a)].

Considering the FE-SEM micrographs shown in Figure 4 (b & c), and Figure 5 (b & c), and

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WCA’s observed in Figure 7 (b) & (c), it is found that water degummed (WCA~153o), & Na2CO3 (WCA~158o) degummed samples have demonstrated superhydrophobic nature, which is

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attributed to their heterogeneous rough surface created due to randomly arranged microfibers, and the voids i.e. porosity, present between them [2,12,40]. The increase in superhydrophobicity of the Na2CO3 degummed silk fibers as compared water degummed silk fibers, could be

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attributed to the high degumming efficiency of Na2CO3 solution towards raw silk fibers, which helped in bringing the maximum availability of silk fibroin protein to the degummed fiber. By molecular composition, silk fibroin contains highly arranged heavy chains of amino proteins, which are composed of 12 hydrophobic units (repeated pattern) dispersed with 11 hydrophilic units (non-repeated pattern). These closely packed chains arrange themselves collectively via hydrogen bonding, intra/inter molecular forces, and Van der Walls force, and hydrophobic

interactions, which lead to formation of highly ordered β-sheets in the silk fibroin [3,27,43]. Therefore, it is most likely that, the collective effect of above-mentioned factors has made the Na2CO3 degummed silk fibers more superhydrophobic than the water degummed silk fibers [3,27,43]. It has been established by the researchers that, the superhydrophobic surfaces in general possess

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low surface energy, which resist the liquids with high surface tension liquid [2,40,46,47]. In present study, the solvents used for surface wettability analysis are DI Water, Petrol, Diesel,

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Engine oil, which have surface tensions γw~72.8 mN/m, γp~29 mN/m, γd~29.5 mN/m, and

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γe~28.0 mN/m respectively [2,12,40,46,48,49]. After considering the above factors, it has been observed that degummed superhydrophobic silk fibers have shown higher WCA for water owing

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to its higher surface tension, whereas, same fibers have showed superoleophilic nature (OCA~0o) towards Petrol, Diesel, and Engine oil. Thus, it is concluded that the degummed

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superhydrophobic silk fibers show superhydrophobicity (WCA>150o) and superoleophilicity (OCA~0o), thereby revealing its strong applicability for oil-water treat application.

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3.4 Oil Absorption & Oil-Water Separation Analysis The analysis of degummed superhydrophobic silk fibers for practical oil-water treatment application is important for establishing its performance against various petroleum oils.

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Considering, the superhydrophobic-oleophilic nature of the degummed silk fibers, its maximum absorption capacities towards petroleum oils were evaluated before oil-water separation study.. The oil absorption study was performed as per the procedure described in section 2.6 for all silk fiber samples [Figure 8 (a), (b), & (c)]. The oil absorption study of raw silk fibers showed it exhibited maximum average absorption performance towards Engine oil (absorption capacity~523%), whereas it showed average absorption performance of 251%, and 188%, for

Diesel, and Petrol, respectively. The oil absorption study of water degummed silk fibers showed that it possessed maximum absorption capacity for Engine oil, and Diesel, with an average performance efficiency of 559%, and 517%, respectively, whereas, for Petrol it showed lower efficiency i.e. 389%. The oil absorption study of Na2CO3 degummed silk fibers showed that it exhibited maximum absorption efficiency towards petroleum oils, as compared raw, and water degummed silk fibers, however, it showed a decreasing trend in absorption efficiency for Engine

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oil, with an average absorption capacity of 978%. Further, Na2CO3 degummed silk fibers showed

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nearly similar absorption performance for Diesel (~555%), and Petrol (~573%). It has been

clearly noticed that, all silk fibers have showed maximum absorption for Engine oil, which could

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be attributed to its high viscosity, and lower surface tension, as compared to Diesel, and Petrol

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[3,46,48–50]. Furthermore, Na2CO3 degummed silk fibers (exhibiting maximum silk fibroin content, achieved due to Na2CO3 based degumming) showed higher absorption for all petroleum

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oils, as compared to raw, and water degummed silk fibers, is attributed to its enhanced superhydrophobic nature (WCA~158o), which possesses hierarchical rough surface with porous

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morphology [3,50,51].

After establishing the average oil absorption capacities of the silk fibers, they were employed for practical oil-water separation study. Oil-water separation study was performed for raw, water degummed, and Na2CO3 degummed silk fibers using petroleum oils like Engine oil, Diesel, and

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Petrol [Figure 8 (d, e, & f)]. Oil-water separation study of raw silk fibers showed that it possessed maximum oil separation rate for Engine Oil, which was higher than 70% till 10 cycles, however, it showed a decreasing trend for Diesel, and Petrol till 10 cycles i.e. 50%, and <40%, respectively. Oil-water separation study of water degummed silk fibers showed that it exhibited nearly constant separation performance for Engine oil i.e. ~80% (till 10 cycles), however, it

showed a decreasing trend for Diesel, and Petrol till 10 cycles i.e. 50%. Considering the superhydrophobic & superoleophilic nature of Na2CO3 degummed silk fibers, it showed maximum oil-water separation performance for Petrol i.e. >90%, and for Engine oil >80%, till 10 separation cycles. Na2CO3 degummed silk fibers showed higher Diesel separation i.e. >90, till 6 cycles, after which it decreased to nearly 75% till 10 cycles [52–54].

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Considering the overall oil-water separation study, Na2CO3 degummed silk fibers have undoubtedly showed greater performance, as compared to raw, and water degummed silk fibers,

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which is attributed to its elevated superhydrophobicity (WCA~158o), and maximum silk fibroin

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content (which exhibits inherently hydrophobic nature, highly ordered structure, and higher mechanical stability, due to presence of β-sheet structure), and hierarchical porous structure

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morphology rendered by randomly oriented silk microfibers [3,27,30,50]. The proposed working

3.5 Biodegradation Analysis

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mechanism of silk fibers during oil-water separation has been shown pictorially in Figure 9 (b).

Different synthetic materials have been frequently investigated by scientific community for oil-

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water separation, however, their waste generation, & non-biodegradability (of its constituent elements) in an environment is a problem, as it promotes global warming. Therefore, in order to address this issue, researcher have started exploring of biomaterials [2,3,15,55], which facilitate

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environmental biocompatibility, and biodegradability in an environment. In this sense, in present study we have successfully demonstrated biodegradation of silk fibers under environmental conditions, using in-house prepared biotic setup consisting of de-compost culture containing biodegrading microorganisms. Commercially available de-compost culture was utilized which possessed microorganisms like Aspergillus awamori, Penicillium fusarium, Bacillus polymyxa, Trichoderma viride, and Bacillus megaterium [2].

The biodegradation study showed a weight loss of around 11%, and 18%, for raw, and water degummed silk fiber samples in 35 days [Figure 11 (d)], under the influence of biotic decompost culture [Figure 10 (a & b)], which was also confirmed with contact angle, ATR-FTIR, and FE-SEM analysis. Weight loss analysis showed that, raw & de-gummed silk fibres maintained biodegradation rate of 2.20%/week, & 3.63%/week, for raw, and water degummed silk fibers, respectively. FE-SEM micrographs of raw silk & water degummed silk fibres shows

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partially eroded surface, which is attributed to the biodegradation action rendered by

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microorganisms, as shown in Figure 11 (a) & (b) [2,3,33,34]. FE-SEM of raw biodegraded silk fibres [Figure 11 (a)] showed higher rough surface as compared to water degummed silk fibres,

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which is attributed to the presence of sericin protein layer, which suggests that it partially

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sustained the attack from biodegrading microorganisms, and protected the silk fibroin. FE-SEM micrograph of biodegraded water degummed silk fibers showed less surface erosion, as

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compared biodegraded raw silk fibers, is probably due to cleaning and dying action rendered on fiber, after it was removed biotic de-compost culture.

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ATR-FTIR analysis of biodegraded silk fibers showed a decrease in peak intensities at 3267 cm, 1615 cm-1, 1509 cm-1, 1438 cm-1, 1224 cm-1, and 1061 cm-1, thereby indicating the effect of

biodegradation action rendered by microorganisms [Figure 11 (c)]. Analysis of these ATR-FTIR peaks has been described in detail, in earlier section 3.1.

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In one of study, Sheik et al. have reported that biodegrading microorganisms attack on protein based structure (which exhibits amino acid based functional groups) of silk, and simultaneously the presence of water helps in loosening of delicate structures (protein chains) of silk fibers [34,35]. In other study, Balan & Sundaramoorthy have reported that, enzyme based microorganisms initially attack on silk-I structure i.e. amorphous region, then sequentially it

attacks on silk-II structure i.e. crystalline domains. Further, they reported that fiber diameter and porous structure also influence the biodegradation of silk fibers [31]. In one more study, Arai et al. have claimed that silk biodegradation is also attributed to abundant presence of Glycine (90%) in amino acid groups of silk fibroin, which contains about 11 active sites, available in silk amorphous phase, thereby promoting enzyme based interactions. Further, they claimed that biodegradation also occurs as a result of bond cleaving of other present

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proteins and Glycine, and/-or bond cleavage between Glycine, and large hydrophobic side chains

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[33].

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In other study, Lu at al. and Numata et al. have claimed that enzymatic conditions effectively biodegrades β sheets, the hydrophilic blocks (present in fibroin), and the anti-parallel β sheets i.e.

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i.e. non-crystalline domains [32,56]. A pictorial representation on biodegradation of β sheets, and

Conclusion

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the hydrophilic blocks (present in fibroin) has been shown in Figure 12.

Present study reported direct utilization of all environment-friendly biodegradable silk materials

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derived from Bombyx mori silkworm, for practical oil-water separation and oil recovery applications. The water & Na2CO3 degummed silk fibers showed superhydrophobicity with water contact angles (WCA) of 153o & 158o, respectively, demonstrating Wenzel & Cassi-Baxter states. The degummed silk fibers showed superoleophilicity (OCA~0o) towards petroleum oils

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like Petrol, Diesel, & Engine oil. The water & Na2CO3 degummed silk fibers showed oil-water separation efficiencies of 95% & 87.5%, respectively. Further, raw & degummed silk fibers showed environmental biocompatibility, by their biodegradation under in-house developed biotic de-compost culture consisting of biodegrading micro-organisms. Their analysis showed that biotic de-compost culture rendered biodegradation weight loss of 11% and 18%, respectively, in

35 days. The successive results depict that, degummed silk fibers can be effectively utilized for practical oil-water separation, and further they can be environmentally biodegraded, for mitigating the waste generation and disposal problem.

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Author Contributions

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Prakash M. Gore: Methodology, Data curation, Investigation, Writing- Reviewing and Editing.

Xungai Wang: Supervision, Conceptualization.

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Minoo Naebe: Supervision, Conceptualization.

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Balasubramanian Kandasubramanian: Supervision, Conceptualization.

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Declaration of Interest Statement

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The authors do not have any conflicts of interest.

Acknowledgement The authors are thankful to Dr. C. P. Ramanarayanan, Vice-Chancellor of DIAT (DU), India, and Prof. (Dr.) Jane den Hollander, Vice-Chancellor, Deakin University, Australia, for motivation and support. First author is thankful to Deakin University for research fellowship, and DIAT

(DU) for laboratory support and facilities. The authors are thankful to research group members at DIAT (DU), and IFM, Deakin University for technical discussions during preparation of the manuscript. The authors are also thankful to the Editor, and all anonymous reviewers, for

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improving the quality of the revised manuscript by their valuable suggestions and comments.

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Figure 1. (a) Structure of raw silk fiber, (b) silk degumming treatments.

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Figure 2. Biotic de-compost setup for biodegradation of silk fibers.

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Figure 3. ATR-FTIR analysis of raw and degummed silk fibers.

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Figure 4. FE-SEM micrographs of (a) Single raw silk microfiber (magnification~1 μm), (b)

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Water degummed silk single microfiber (magnification~1 μm), and (c) Na2CO3 degummed silk

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single microfiber (magnification~2 μm).

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Figure 5. FE-SEM micrographs of (a) Raw silk microfibers (magnification~10 μm), (b) Water

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(magnification~10 μm).

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degummed silk microfibers (magnification~10 μm), and (c) Na2CO3 degummed silk microfibers

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Figure 6. Thermogravimetric Analysis of silk fibres.

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Figure 7. Surface wettability analysis (a) WCA of raw silk fibers, (b) WCA of water degummed

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degummed silk fibers.

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silk fibers, (c) WCA of Na2CO3 degummed silk fibers, (d) Oleophilic state of Na2CO3

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Figure 8. (a) Absorption performance of raw silk fibers, (b) Absorption performance of water degummed silk fibers, (c) Absorption performance of Na2CO3 degummed silk fibers, (d) OilWater separation performance of raw silk fibers, (e) Oil-Water separation performance of water degummed silk fibers, (f) Oil-Water separation performance of Na2CO3 degummed silk fibers.

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Figure 9. (a) Practical oil-water separation study of Na2CO3 degummed silk fibers for Engine

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oil-water mixture, (b) pictorial representation of oil-water separation mechanism.

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Figure 10. Silk fibers (a) before biodegradation (0 days), and (b) after biodegradation (35 days).

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Figure 11. (a) FE-SEM micrograph of raw biodegraded silk fibers (after 35 days), (b) FE-SEM micrograph of water degummed biodegraded silk fibers (after 35 days), (c) ATR-FTIR of

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biodegraded silk fibers, (d) weight loss of biodegraded silk fibers.

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Figure 12. Possible biodegradation mechanism in silk fibers under enzymatic microorganisms.