Recycling polystyrene wastes into photo-responsive adsorbents for environmental remediation
Creators: name: Liu, Aotian
Publisher: name: The University of Edinburgh
Year: 2023
Subjects: Recycling polystyrene wastes
photo-responsive adsorbents
anthropogenic environmental issues
waste Styrofoam
industrial dye wastewater treatment

description: Global industrialization and urbanization exacerbate anthropogenic environmental issues, including plastic pollution, water pollution, and CO₂-driven global warming, imperiling Earth. Unchecked plastic consumption could add 33 billion tonnes by 2050. Coloured wastewater from industries contributes to 17 - 20% of global water pollution. To reduce CO₂ emissions and protect the atmosphere, the International Panel on Climate Change advocates halving CO₂ emissions by 2030 and achieving net-zero emissions by 2050. We must develop an efficient technology to address these issues. One promising approach uses solid adsorbents from plastic waste, while traditional regeneration methods are energy-intensive and wasteful. Photo-responsive adsorbents offer a sustainable alternative, using eco-friendly photo-modulation form adsorption and regeneration. The aim of this work is to create an innovative photo-responsive material (HCPs@Azo) from waste Styrofoam. This material serves as an effective adsorbent for tackling dye discharge and CO₂ emissions. Initially, we successfully synthesized HCPs@Azo by incorporating azobenzene photo-switches into hypercrosslinked polymers (HCPs) made from waste Styrofoam. HCPs@Azo stands out for its reversible dimensional changes driven by trans-cis photo-isomerization of azobenzene. When exposed to UV and visible light, HCPs@Azo displayed reversible changes in its BET surface area and pore volume. Under UV light, the CO₂ uptake capacity at 273 K increased by 22%, reaching 56.9 cm3 g-1 compared to the as-prepared HCPs. Computational simulations revealed that this increase was due to an enhanced accessible pore volume and a reversible positive dipole moment, addressing a common limitation of traditional photo responsive adsorbents with lower CO₂ adsorption capacities. Following visible light exposure, the CO₂ uptake capacity returned to the initial level of 33 cm³ g⁻¹. These findings highlight the potential of HCPs@Azo as photo-responsive adsorbents for further research. In this study, we deployed the light-triggered changes in texture (pore sizes and accessible pore volumes) of photo-responsive HCPs@Azo to cyclically remove dyes during water treatment. While many adsorbents are used in dye removal, regenerating them often requires energy-intensive thermal methods or chemical waste-producing solvents. Here, we employed light instead of solvents or heat to induce desorption and regenerate our adsorbents. Four types of dyes (Methyl Orange, Methylene Blue, Rhodamine B, Uniblue A) were tested at concentrations from 10 to 500 mg L-1. UV-induced photo-isomerization of azobenzene, shifting from trans to cis states, led to opening the pore gates and desorption of 1.7% to 20% of previously adsorbed dyes. Importantly, these desorption rates were 71% to 209% higher than water-only regeneration without UV. Notably, MO and MB dyes showed 50% higher desorption using photo-modulated regeneration compared to traditional solvent-based methods like methanol. We also demonstrated the reusability of HCPs@Azo through four cycles of photo-modulated adsorption and desorption. This approach has the potential to environmentally regenerate adsorbents in industrial dye wastewater treatment, eliminating the need for solvents or heat-intensive processes. We also assessed the potential of deploying photo-responsive HCPs@Azo for efficient CO₂ adsorption in real-world post-combustion carbon capture applications. Conventional methods for regenerating carbon capture adsorbents are highly effective but energy-intensive, involving temperature and pressure swing processes. We demonstrated that visible light can release adsorbed CO₂ from our photo-responsive adsorbents. Exploiting the reversible photo-modulated accessible pore volumes and dipole moment of HCPs@Azo, we used UV light to enhance CO₂ adsorption capacity and selectivity in mixed gas concentrations (1 – 40 v/v% CO₂ in N₂). Remarkably, 14% of adsorbed CO₂ on HCPs@Azo could be desorbed by visible light, a substantial over 350% increase in desorption capacity compared to processes without light. The photo-regeneration efficiency remained stable at around 50% after 5 cycles, indicating excellent reusability. Crucially, the cost and energy required for one cycle of photo-modulated adsorption and desorption were only one-five-hundredth of traditional temperature/vacuum swing methods. This suggests that photo-responsive HCPs@Azo has the potential to significantly reduce carbon capture costs and establish a more environmentally friendly, light-driven carbon capture process. In summary, the photo-responsive adsorbents developed in my PhD study have effectively tackled plastic pollution, industrial water pollution, and power plant CO₂ emissions. They introduce an environmentally friendly approach through photo-modulation for adsorption and regeneration, compared to conventional energy-intensive and solvent-based methods. This research marks a significant stride toward sustainable and eco-friendly solutions for pressing global environmental challenges.