Solid-phase photocatalytic degradation of PVC by Tungstophosphoric acid—a novel method for PVC plastic degradation
Introduction
A massive amount of polyvinyl chloride (PVC) is now produced globally (23 mt per year) and is consumed as one of the most extensive plastic materials in the world. Its products, such as PVC heat shrink membranes, transparent slices, one-off medical goods, emulsion gloves and so on, are all one-off products with exceedingly low recovery value. Disposal of such wasted PVC through abandoning or incineration causes serious environmental problems due to the formation of toxic by-products like dioxins and corrosive acids [1], [2], [3]. Therefore, studying and developing degradable PVC plastic have an important meaning. The attractive idea of degradable plastic was proposed by some researchers already [4], [5], [6]. The dispersed catalysts in these work were not photo-activated but thermally activated catalysts and the degradation temperature employed is rather higher that cannot achieve naturally. Some biodegradable PVC plastics have also been reported [7], [8], [9]. But these biodegradable PVC plastics have to include a great quantity of starch (about 30%) to achieve better degradation effect, which affects the transparency and mechanical performance and do not fit uses for transparent PVC material, and there are some defects such as overlong degradation cycle, halfway degradation and others in the degradation of biodegradable PVC plastics. All such disadvantages limit their expanded applications. Thus studying transparent PVC packing materials with photodegradable performance will overcome the above weaknesses and achieve significant theory and applications. However, far less study has been focused on photodegradable PVC materials. This study achieved quick photocatalytic degradation of PVC by adding the photocatalyst of a heteropoly compound into PVC. This supplies completely new ideas and means for waste PVC disposal.
A heteropoly compound is a multi-function catalyst that is amongst the most promising catalysts because of its special catalytic action and wonderful application foreground [10], [11], [12], [13], [14], [15], [16]. Its structure of heteropolyanion consists of several transition metal ions apt to transfer electrons. So it can easily accept many electrons and thus be reduced, and this kind of reduction condition is reversible. These characteristics endow a heteropoly compound with unique redox property. Under the light radiation, the activated heteropolyanion has much higher oxidation potential and is able to obtain the electrons in CH bond, thereby carrying out free radical reaction. The present study, guided by the above thought, luckily found that one heteropoly compound, tungstophosphoric acid, can solid-phase photocatalytically degrade PVC plastic with far higher photocatalytic activity. Irradiating PVC film doped with HPW for 250 h by visible light with wavelength above 300 nm reduced its weight up to 70%. Furthermore, most heteropoly compounds are nonpoisonous and colourless and accord with the conditions to be the auxiliaries of transparent PVC packing material. So the solid-phase photocatalytic degradation of PVC by heteropoly compound provides a fresh idea and means for waste PVC disposal.
In this study, the photocatalytic degradation of PVC by excellent semiconductor nanomaterials TiO2 and CdS was also investigated, when we studied PVC degradation by HPW in order to carry on the comparing. It was confirmed that the photodegradation efficiency of heteropolyacid was higher than that of both TiO2 and CdS under the irradiation of either UV or visible light through weight loss monitoring of PVC films in the photodegradation process. The SEM analysis of PVC composite films further confirmed the above conclusion. The photodegradation of PVC by boric acid and such acid catalysts implied that the high photocatalytic activity of HPW-doped PVC film was due to the unique redox property of HPW, rather than the acidity. FT–IR and UV–vis spectroscopic studies provided some results about the structure of HPW–PVC composite system and will lay some foundations for further studying the photocatalytic mechanism. At last the advantages for PVC photodegradation by heteropoly compound were assessed and we can show that this method possesses feasibility.
It's worthy of note that the current studies on the photocatalysis of a heteropoly compound mainly focus on liquid and gas phase [17]. The solid-phase photocatalytic degradation of PVC by heteropoly compound represents a special example of its solid-phase photocatalysis and pioneers another application field of heteropoly compound, which has not been reported so far.
Section snippets
Materials
Tungstophosphoric acid (HPW) (A.R.), purchased from Jiangyan Shi Di Er Huagong Chang, was used directly. TiO2 and CdS nanoparticles were prepared, according to references [18] and [19]; their particles had diameters in 3–6 and 7–10 nm ranges and their crystalline phases were anatase and hawleyite, respectively. The powder of polyvinyl chloride (PVC) [–(CH2CHCl)n–] was supplied by Tianjin chemical plant. The average degree of polymerization was 1000–1100. The PVC powder was free from additives
Comparison on photodegradation of different kinds of catalyst–PVC composite films
Fig. 1a displays the photoinduced weight losses of the pure PVC film and of the catalyst–PVC composite films under irradiation of λ >300 nm visible light. The weight loss rate was highest for the HPW–PVC film; it rapidly decreased with irradiation and led to the total reduction of 69.6% in 250 h, while TiO2–PVC and CdS–PVC films showed only 49.1 and 47.2% weight loss and H3BO3–PVC and pure PVC films had the two lowest weight losses of 14.9 and 15.7% respectively, under the identical experimental
The advantages and feasibility for PVC solid-phase photocatalytic degradation by heteropoly compound
This study not only put forward a completely new idea and means for waste PVC disposal, i.e., the solid-phase photocatalytic degradation of PVC by heteropoly compound, but also listed the advantages that a heteropoly compound possessed in photocatalytic degradation of waste PVC, as follows.
- (i)
Most heteropoly compounds are nonpoisonous and colourless and accord with the requirments to be the auxiliaries of transparent PVC packing material.
- (ii)
Heteropoly compounds and polymer PVC can be homogeneously
Conclusions
Solid-phase photocatalytic degradation of polyvinyl chloride (PVC) by heteropoly compounds is a novel and unique thesis put forward by this study. It promises important results in two aspects. Firstly, tungstophosphoric acid can solid-phase photocatalytically degrade PVC plastic with far higher photocatalytic activity. Irradiating PVC film doped with HPW for 250 h by visible light with a wavelength above 300 nm reduced its weight up to 70%. Secondly, the photocatalytic efficiency was higher for
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. 20076004).
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