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

https://doi.org/10.1016/j.jhazmat.2019.121823Get rights and content

Highlights

  • Raw and Degummed Silk Fibres for Oily Wastewater Treatment.

  • Degummed Silk Fibres Exhibit Superhydrophobicity-Oleophilicity.

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

  • Raw and Degummed Silk Fibres Show Biodegradability.

  • Silk Fibres Can Be Utilized For Oil-Spill Cleanup.

Abstract

Present study reports superhydrophobic-oleophilic, environment-friendly, & biodegradable silk material derived from Bombyx mori silkworm, for practical oil-water separation and oil recovery applications. In this study, raw silk fibers were degummed using water and Na2CO3 (at 100 °C), 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 153° & 158°, respectively, demonstrating Wenzel & Cassi-Baxter states. Degummed silk fibers showed superoleophilicity (OCA∼0°) 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 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. 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.

Graphical abstract

Recovery of petroleum oils from oily wastewater using biodegradable superhydrophobic-oleophilic degummed silk fibers.

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Introduction

The increased demand of petroleum oils in recent decades for maintaining the ever-growing energy requirements has accelerated the growth of associated industries (Fingas, 2015; Gore and Kandasubramanian, 2018; Gore et al., 2019a). Continuous transit 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 (van der Perk, 2014; Montagna et al., 2013; Zabbey and Olsson, 2017).

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 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 (van der Perk, 2014; Fenwick, 2006). It has been reported that plastic waste weighing nearly ∼250,000 tons (non-degradable) is littering in the seas, which is damaging its flora and fauna (van der Perk, 2014; Montagna et al., 2013; Eriksen et al., 2014). Considering this problem, scientific community has developed new synthetic materials and methods for oil-water segregation, but, their non-degradation and disposal further creates burden to environment (Gore and Kandasubramanian, 2018; Gore et al., 2019a; Cheng et al., 2017). 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) (Gore et al., 2019a; Shirvanimoghaddam et al., 2018; Rajkhowa et al., 2012). 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 (Gore and Kandasubramanian, 2018; Gore et al., 2019a, a; Gupta and Kandasubramanian, 2017; Arora and Balasubramanian, 2014; Gore et al., 2019b; Mishra and Balasubramanian, 2014; Saini and Kandasubramanian, 2018), and adsorption toxic ions (Amal Raj et al., 2017; Gore et al., 2018a, 2017; Khurana and Balasubramanian, 2016; Sharma et al., 2016; Gore et al., 2018b; Rajhans et al., 2019). 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 (Eisoldt et al., 2011; Gosline et al., 1986).

Raw silk materials require degumming and functionalization, prior to their utilization in desired applications. In raw form, silk fibers possess core-shell structure [Fig. 1(a)], where it contains an outer shell layer of gummy sericin protein layer, which is generally allergic to humans, and an inner part contains fibroin protein [Fig. 1(a)] (Gore et al., 2019a). Depending on the end application, the sericin layer is removed by various degumming treatments based on water 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 requirements (Gore et al., 2019a; Li et al., 2012). During regeneration, and functionalization, inherent properties of silk fibers get diminished e.g. decrease in molecular weight, reduction in functionality, etc., and also increases time and energy costs (Gore et al., 2019a; Li et al., 2012).

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 technique (Patowary et al., 2016). 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 (Patowary et al., 2016).

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 (Maleki et al., 2018). 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 exhibiting WCA>150°, and OCA∼0°, respectively, with an absorption performance of 500−2600 g.g-1 selectively for organic solvents, pump oil, and vegetable oil (Maleki et al., 2018).

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 fabricated using electrospinning method, for sequential oil-water separation. The generated silk nanofibers demonstrated superhydrophilic/Superoleophobic attributes, exhibiting WCA∼0°, and oil contact angle (OCA)∼154°, respectively. Further, these fibers showed an oil-water separation performance of 99.90 % towards oil-water mixtures of Diesel and Engine oil (Li et al., 2018).

Considering, above studies on silk fibers for oil-water treatment, it has been observed that, the regeneration, and functionalization are time and energy intensive methods, moreover, after these treatments, silk fibers lose their intrinsic properties (Gore et al., 2019a; Li et al., 2012). 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 (Gore et al., 2019a; Li et al., 2012; Patowary et al., 2016; Maleki et al., 2018; Li et al., 2018).

Considering the literature analysis, it has been found that, silk fibers show higher biodegradation rate under the protease XIV enzymes (Balan and Sundaramoorthy, 2019; Numata et al., 2010; Arai et al., 2004), further researchers have also performed silk biodegradation under soil based conditions (Sheik et al., 2018, 2017), and gamma radiation (Kojthung et al., 2008). 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 demonstrate superhydrophobicity (WCA>150°) and superoleophilicity (OCA∼0°), simultaneously. The degummed silk fibers reveal an oil-water separation efficiency of 87.5%–95%, along with an oil recovery performance of >50 %, till 10 separation & absorption cycles. Further, the raw & degummed silk fibers showed environmental biocompatibility, through their biodegradation performance, under in-house developed biotic de-compost culture consisting of 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 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 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.

Section snippets

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

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 (Gore et al., 2019a; Li et al., 2012; Kim et al., 2017).

Degumming of

Conclusion

Present study reported direct utilization of all environment-friendly biodegradable silk materials 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 153° & 158°, respectively, demonstrating Wenzel & Cassi-Baxter states. The degummed silk fibers showed superoleophilicity (OCA∼0°) towards petroleum oils like Petrol, Diesel, & Engine oil. The

CRediT authorship contribution statement

Prakash M. Gore: Methodology, Data curation, Investigation, Writing - review & editing. Minoo Naebe: Supervision, Conceptualization. Xungai Wang: Supervision, Conceptualization. Balasubramanian Kandasubramanian: Supervision, Conceptualization.

Declaration of Competing Interest

The authors do not have any conflicts of interest.

Acknowledgements

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

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