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Use of microbially desulfurized rubber to produce sustainable rubberized bitumen

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Abstract

This paper examines the merits of using microbially desulfurized rubber to enhance theperformance ofbituminous pavements and promote recycling of scrap tires. To prepare microbially desulfurized rubber, we incubated crumb rubber obtained from waste tiresin medium with microbes from waste activated sludge. The concentration of sulfate was monitored during the treatment process and desulfurization was estimated to be about34%.Microbially desulfurized crumb rubber (MDR) was then added to bitumen to produce rubberized bitumen. Performance of the rubberized bitumen was compared with those of conventional crumb rubber modified (CRM) bitumen. To do so, Fourier transform infrared spectroscopy (FTIR), inverse gas chromatography (IGC), and rheometry were utilized. Chemical analysis of rubber particles after desulfurization showed a significant reduction of the peak at 500-540 cm−1; this was attributed to the breakage of disulfide bonds after microbial desulfurization. Measurement of surface energy showed that the acid-base component of surface energy increased three times (increasing from 1.85mJ/m2 to 5.55mJ/m2) after desulfurization. Such enhancement could lead to increased interactions between rubber and bitumen reducing their separation. This was evidenced ina68% reduction inseparation indexin bitumencontaining microbially-treated (desulfurized) rubber compared to bitumen havingnon-treated rubber. In addition, study results showed a 6% increase in elastic recovery,a 27% increase in resistance to moisture diffusion,a 12.5% decrease in viscosity,a 10% increase in stiffness, and a 5% increase in stress relaxation capacity compared to bitumen's having non-treated rubber. The outcome of the study promotes resource conservation by offering a simultaneous solution for recycling of scrap tire via a low-cost bio-inspired approach while enhancing pavement performance.

Introduction

In 2018, almost 7% of the bitumen used worldwide was modified by polymers. Evaluation of the polymer-modified bitumen marketin 2018 was$5.14 billion and it is projected to reach $7.44 billion by 2026(Globenewswire, 2019).The pavement construction industry uses crumb rubber from scrap tires as an inexpensive source of polymer.In the US, the states of California, Arizona, Texas, and Florida have beenamong the first to implement crumb-rubber-modified bitumen in highway pavements(Way et al., 2012). Studies have shown that adding crumb rubber in bitumen improvesits thermo-mechanical properties and long-term performance (Huang et al., 2007; Liang et al., 2017; Xiaowei et al., 2017). Theuse of crumb rubber in construction has increased by 15% from 2015 to 2017 (UTMA, 2017). Despite the benefits of adding rubber to bitumen, the separation of crumb rubber from bitumen at high temperatures remains a challenge (Zani et al., 2017). The size of the rubber particles, the difference in density between rubber and bitumen, and the swelling of rubber particles are factors contributing to the separation of rubber from bitumen (Shu and Huang, 2014). Commonly used methods for applying crumb rubber to bitumen necessitate continuous agitation to preventseparation of rubber and bitumen, making this process costly. As a result, many road authorities and contractors are reluctant to incorporate crumb rubber in bitumen(Presti et al., 2018; Yu et al., 2017). A variety of physical and/or chemical methods have been tried in efforts to address the issues of phase separation and inadequate dispersion of rubber in bitumen (Liu et al., 2020; Xiang et al., 2019; Xie et al., 2019). For example, microwave-irradiated crumb rubber in combination with bio-oil resulted in partial surface desulfurization, which enhanced the rubber-bitumen interaction and reduced the rubber-bitumen separation tendency (Hosseinnezhad et al., 2019; Kabir et al., 2019). Previous studies have shown surface activation or partial desulfurization of rubber improves the interaction between rubber and bitumen (Hosseinnezhad et al., 2019; Li et al., 2020; Liang et al., 2017).However,the cost and scalability of those approaches remain an issue (Kabir et al., 2019). However, abovementioned approach involves high initial cost due to chemical and energy applied for treatment.

A potentially low-cost approach could be microbial desulfurization of rubber.Table 1 summarizes the pertinent studies on microbial desulfurization. A variety of naturally occurring aerobic or anerobic microorganisms at neutral or acidic pH have been shown to desulfurize tire rubber (Cui et al., 2016; Ghavipanjeh et al., 2018; Kim and Park, 1999; Tatangelo et al., 2019). It has even been reported that desulfurizationby Acidithiobacillusis more effective than chemical treatment (Kim and Park, 1999).Studies using Sphingomonous sp. reported that S–C and S–S bond scission occurred in thedesulfurized rubber (Jiang et al., 2011).The greatest extent of desulfurization (62.5%) was reported in a study withAlicyclobacillus sp.(Yao et al., 2013).

While several studies have been performed on desulfurizing rubber, to the best of our knowledge application of microbially desulfurized rubber in asphalt has not been done before. Wehypothesize that desulfurizing rubber via microbial treatment of scrap tire enhances interaction of rubber and bitumen to reduce theirseparation. We expected the broken di-sulfide bondsenhance interaction with bitumen molecules. This in turnreduce phase separation and therebyreduce the energy needed for continuous agitation of bitumen during application.

Section snippets

Materials and preparation of bitumen containing crumb rubber

The bitumen used in this study was PG 64-22 (Table 2), obtainedfrom Holy Frontier Company in Phoenix, Arizona. Crumb rubber of <0.25 mm was acquired from Crumb Rubber Manufacturers, Mesa, Arizona. Bitumen samples were prepared by introducing 15% treated (MDR) or un-treated (CR) rubber to85% bitumenat a reduced temperature of 165°C using a high shear mixture for 30 minutes and named microbially desulfurized rubber modifier (MDRM) and crumb rubber modifier (CRM) respectively. A crumb rubber (CR)

Results and discussion

Sulfate concentrations weremeasured at regular intervals to monitor the rates and extent of microbial desulfurization. Fig. 1A shows the time-course sulfate concentrations in the flasks containing 50 g crumb rubber (CR). As seen in Fig. 1A, sulfate concentrationssubstantiallyincreased during incubation in all three conditions. The rates of sulfate release were fastest in the flasks with 50 g CR + waste activated sludge; this condition also reached the highest final concentration of ~2000 mg/L

Conclusion

This paper investigatesthe merits of using microbially desulfurized rubber in bitumen to reduce the rubber-bitumen separation. Crumb rubberparticles were desulfurized by waste activated sludge microorganisms for 36 days.The percent of desulfurization in crumb rubberusing activated sludge was found to be nearly 34%. Chemical analysis of rubber particles showed a reduction of the peak between 500-540 cm−1, associated to disulfide bonds (S-S)and in the range of 2800 cm−1, associated to C-H

Supplemental information

The composition of solutions used for microbial treatment of scrap tire

Declaration of Competing Interest

We confirm that to the best of our knowledge no conflict of interest exists.

Acknowledgments

This research was sponsored by the National Science Foundation (Award Numbers 1935723 and 1928795) and startup funds to Anca Delgado from Arizona State University. The authors acknowledge Evelym M. Miranda for help with sulfate concentration measurements and Daniel Burnett withSurface Measurement Systems foranalyzing the surface energy of the crumb rubber.

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