Photocatalytic Degradation of Skim-Latex-Vapor Odor Using Iron-Doped Zinc Oxide

The photocatalytic degradation of the vapor odor of skim natural rubber latex, which is composed of volatile organic compounds (VOCs-NR), was conducted using iron-doped zinc oxide (Fe-ZnO) photocatalyst nanoparticles. The research objectives were to study the photocatalyst, to develop the photocatalytic reactor used for the photodegradation of VOCs-NR, and to analyze the odor reduction of the degraded VOCs-NR. Fe-ZnO was prepared by the sol-gel method and immobilized onto fiberglass cloth as the support material, followed by annealing. The Fe-ZnO nanoparticles were dispersed on the fiberglass surface. A photocatalytic reactor was developed for the gas phase. The research findings showed that gas chromatography-mass spectroscopy detected 24 dominant chemical components in the VOC-NR stock before photocatalytic degradation. The components decreased to 11 after 30 min, only 3 components remained after 40 min of the photocatalytic process, and no component was observed after 80 min of photoirradiation. An organoleptic test was conducted to validate that the odor degraded to a very low level.

Degradation; Fe-ZnO; Odor; Photocatalyst; Sol-gel

       According to statistics, the world’s natural rubber production in 2019 was 13.804 million tons, which will increase by 2.7% annually (Businesswire, 2020). During rubber processing, waste is produced in almost every step: cleaning, coagulant dipping, drying, latex dipping, leaching, beading, vulcanizing, post leaching, slurry dipping, and stripping. This waste contains volatile organic compounds (VOCs), a severe air pollutant that cannot be easily eliminated from waste treatment plants. The presence of VOCs produces bad odor, which is caused by the microorganism degradation of organic materials present in natural rubber, such as glucose, protein, and lipids (Juntarachat et al., 2013). This lousy odor in rubberfactories is disturbing and degrades the quality of life of the surrounding community, as reported by detik.com (Hidayat, 2017). Many similar cases occur at different locations, which indicates the serious problem in the rubber production globally).

Waste treatments in rubber factories usually focus on liquid waste (Mokhtar et al., 2015). A new smoke chamber prototype for rubber latex sheets was developed by Dejchanchaiwong et al. (2019). However, the smoke chamber prototype was not tested to eliminate the skim latex odor, especially during the rubber drying process. The odor treatment methods in industries usually involve materials such as activated carbon, biofilters, and biotrickling (Komala et al., 2013; Pan et al., 2018). Activated carbon only adsorbs and accumulates pollutants in the pores; however, it does not eliminate odor. Biopond and biotrickling use microorganisms to degrade the organic compounds; however, these compounds must be wet or under high humidity conditions in the gas phase. Thereby, odor treatment in the gas phase remains unresolved.

Zinc oxide (ZnO) photocatalysts have proven to degrade many VOCs effectively. Buazar et al. (2015) found that ZnO/hydroxyapatite successfully degrades 2-mercaptobenzoxazole, which is an unpleasant odorous organosulfur pollutant. Maurer and Koziel (2019) reported that the production of carbon dioxide (CO₂) increases significantly during the photodegradation process. Fotiou et al. (2016) studied the photodegradation of cyanotoxins in an aqueous phase and found the involvement of radical hydroxyl (·OH) and radical oxygen (·O^-) in the reaction, as indicated by the scavenger treatment. Unlike the microorganism treatment, photocatalysts are usually more compatible with higher temperature fluctuations and toxic environments. In several cases, the catalyst can also be recovered, and the pollutant is converted into harmless compounds, such as CO₂ and H₂O, through the reduction-oxidation process (Bechambi et al., 2016).

In this study, iron-doped ZnO(Fe-ZnO) was used to reduce the vapor odor of skim latex as a VOC in the gas phase, which is different from most ZnO photocatalytic tests that were conducted in the liquid phase using a dye solution as the substrate. Slamet et al. (2017) reported a gas-phase photocatalysis; however it was for methylcyclohexane, not for skim latex.  A common photocatalytic test has been conducted for the degradation of methylene blue and other dye solutions (Bousslama et al., 2017; Rahman et al., 2018).

In the previous study, instead of the powder form, Fe-ZnO was immobilized onto a fiberglass cloth to make it compatible in the gas phase (Omar et al., 2020). Fiberglass cloth, with a large surface area, is an ideal temperature-resistant support material to disperse the catalyst particles, as the catalyst preparation involves annealing at high temperature. The prototype of a new photocatalytic reactor for skim- latex odor degradation was designed and tested. This study focuses on the preparation, characterization, and immobilization of this photocatalyst. In addition, the photocatalytic activity and reactor prototype are explored to reduce the skim latex odor.

        Fe-ZnO prepared by the sol-gel method was successfully immobilized in fiberglass cloth. The Fe-ZnO particles were dispersed on the surface of the fiberglass cloth, and some particles were in the nanosize range. The Fe-ZnO photocatalyst prepared using chitosan, as the stabilizer, actively decomposed the VOCs-NR (the skim latex vapor) and reduced the odor level. Odor reduction progressively depended on the characteristics of the chemical components. The catalytic photodegradation progress was monitored by using GC-MS and validated with an organoleptic test. The polycyclic compounds in VOCs-NR took more time to degrade than the aliphatic compounds. Thus, VOC-NR photocatalytic degradation is an essential step for industrial application.

        This research was funded by a PRUU-PD research grant from Universitas Syiah Kuala, Darussalam Banda Aceh 23111, the Republic of Indonesia with the grant of “Program Riset Unggulan Unsyiah, Percepatan Doktor” Reg# 292/UN11/SPK/PNBP/2020. We appreciate Dr Syahidah Akmal Muhammad from the School of Technology Industry, USM for the discussions on the GC-MS.

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