Removal of malachite green dye from aqueous solution by adsorbents derived from polyurethane plastic waste

Highlights

• Comparative characterizations and MG adsorption evaluation of PUPW-ACs were implemented.

• The maximum adsorption capacity of MG on the champion PUPW-AC was up to 1428 mg g⁻¹, highly superior to other ACs.

• The adsorption drive forces were electrostatic attraction, oxygen-containing groups, and π-π interaction.

• The PUPW-AC could serve as a promising adsorbent for MG removal from aqueous systems.

Abstract

Upcycling polyurethane plastic waste (PUPW) into activated carbon (AC) for the removal of malachite green dye (MG) in wastewater via physical adsorption could provide abundant low-cost adsorbent and facilitate treatment of plastic waste and wastewater simultaneously. For this reason, four types of PUPW-ACs were prepared by various preparation procedures. The properties of PUPW-ACs were characterized using BET, FT-IR, XPS, Raman spectroscopy, and XRD; the adsorption performances of PUPW-ACs were also evaluated. It was found that the properties of PUPW-ACs varied in terms of porosity, microstructures, and surface chemistry. Intentional modulation of PUPW-AC would be possible via preparation procedure adjustment. Two-step preparation procedure (1. carbonization; 2. activation) was recommended to acquire PUPW-AC (i.e., PUPW-AC-C-A) with a high specific surface area of 1034 m² g⁻¹ and the maximum MG adsorption capacity of 1428 mg g^-1. Electrostatic attraction, oxygen-containing groups (especially carboxyl functional groups), and π–π interaction were believed to contribute to the adsorption driving force for the outstanding adsorption performance of MG on PUPW-AC-C-A. The PUPW-AC prepared from PUPW could serve as a promising adsorbent used in MG dye removal from aqueous systems.

Introduction

Enormous plastic wastes (approximately 6300 million metric tons, as of 2015 [1]) had been generated due to the massive production and consumption in modern society, and most of them were dumped in landfills or abandoned into the aquatic environment (e.g., the ocean) [2,3]. Presumably by 2050, around 12 billion metric tons of plastic wastes would end in landfills and the natural environment, if plastics are currently produced and applied currently in various fields, such as packaging, automobile industry, agriculture, and construction industry [2,4]. Universally used plastics, which originates from the petrochemical and coal chemical industries, are scarcely biodegradable. Consequently, generated plastic wastes accumulate instead of naturally decomposing; this severely threatens human health, animal survival (especially marine life), and ecological environment [5]. Thus, effective plastic waste management, including reusing and recycling, is of great necessity.

Recently, the practice of “Upcycling” concept has arisen and drawn much attention [6]. The term refers to the conversion of waste substance(s) into more valuable product(s), for instance, from plastic wastes to useful carbon materials. Derived from fossil hydrocarbons, plastics were composed largely of carbon constituent, which implied that the extant plastic wastes could be considered as an unimaginable amount of carbon source for manufacturing carbonaceous high additional value products [6,7]. Such products included carbon fiber [8], carbon nanotube [9], carbon microsphere [10], carbon black [11], and activated carbon (AC) [12]. As a highly porous material with an annual production rate exceeding 2 million metric tons, AC has found application in the environmental protection, including organic dye adsorption in aquatic systems [13], because of its operational simplicity, stability, and high efficiency [14]. Another important reason why the organic dye adsorption via AC attracted considerable attention was the relatively lower cost of absorbent, compared with other species of absorbents, e.g., zinc oxide carbon foam (Zn-CFst) material and Magnetic Ba(PO)/FeO-nanoparticle [15,16].

However, the cost of using AC for organic dye adsorption was strongly dependent on the precursor material and the carbonization/activation conditions. Currently, most of commercial AC in China is produced from either coal (79 $ t⁻¹, raw material cost was obtained from Chemical Materials Network) [17] or cost-uncompetitive biomass, such as bamboo and coconut shell (216 $ t⁻¹, raw material cost was obtained from Chemical Materials Network) [18,19]. In this situation, plastic wastes (very low cost) could be a promising candidate feedstock for manufacturing low-cost AC [20]. In the present study, polyurethane plastic waste (PUPW) was chosen as the feedstock because the share of PUPW among plastic wastes has been fast-expanding [21]. Hence, upcycling PUPW into high value-added PUPW-based AC (PUPW-AC) and applying it in organic dyes (such as malachite green, labeled as MG) adsorption, which can not only bring economic profits to waste management industry, but also environmentally friendly solutions to the wastewater treatment.

Furthermore, PUPW contained abundant heteroatom (O:20−35wt%, N:5−15 wt%), which can probably promote the adsorption capacity of carbon materials [22]. Researches preparing AC material using the PUPW as the feedstock for adsorption application have been implemented. For instance, Chen W, et al. prepared N-doped AC from PUPW using K₂CO₃ activating agentand carried out H₂S adsorption [23]. Similarly, PUPW-AC has been prepared with KOH activation and applied as CO₂ adsorbent [24]. But these chemical activation methods (such as KOH, K₂CO₃, ZnCl₂, KHCO₃) are common costly, time-consuming or have complex operations, Also, they cause secondary pollution to the environment. CO₂ activation is one of the physical activation methods, which can be used to prepared porous carbon due to its relatively low-cost, require simple procedure, and conservation of the environment. However, researches on the systematic investigation of organic dye adsorption performance of PUPW-AC using CO₂ activation are barely reported.

Therefore, the main objective of this work is to investigate and evaluate the application feasibility of PUPW-AC for MG removal from wastewater via adsorption. Various preparation methods of PUPW-AC were implemented for comparison because variation in the preparation process would be most likely inherited by resultant products [25]. The effect of the preparation process on the structure of resultant AC, the adsorption performance, and factors affecting MG adsorption, including contact time, solution concentration, equilibrium adsorption capacity, adsorption model, and solution pH were systematically studied. At last, theoretical calculations were employed to tentatively investigate the MG adsorption mechanism of PUPW-ACs.