Research articlePreparation, characterization and application of polystyrene based activated carbons for Ni(II) removal from aqueous solution
Introduction
Activated carbons (ACs) are highly effective adsorbents with wide range of applications that are generally produced through pyrolysis and subsequent physical or chemical activation of different materials with high carbon and low inorganic content. However, due to the high production cost, ACs tend to be more expensive than other adsorbents and their widespread application is somewhat limited. This instigated a growing interest into production of low cost activated carbons through the usage of low-cost raw materials that are economically attractive and at the same time show similar or even better performance than the conventional ones. Therefore, cheaper and common precursors as lignocellulosic biomasses have been widely tested for ACs preparation [1]. Recently, a large number of studies are dealing with the preparation of ACs from various polymeric wastes as well [2], [3], [4]. These materials are successfully used for the production of high yield of ACs characterized by low ash content, high adsorption capacity and considerable mechanical strength.
Thermoplastic polymers, i.e. polypropylene, polyethylene, polyvinylchloride, polystyrene, polyamide, etc., are the major constituents of municipal solid waste. More than 25 million tons of plastic waste is annually generated in the region of European countries [5]. This creates significant ecological concern since the degradation of plastic waste on a landfill is an extremely slow process, ongoing for centuries. Consequently, the use of these waste materials for higher-value products preparation such as fuels, carbon nanotubes, and porous carbons is very attractive to decrease the negative impact on the environment and the costs of waste disposal or treatment.
Polystyrene (PS) is a petroleum-based plastic which is available as a solid or foamed. Several papers have been published in recent years on ACs preparation from “pure” PS wastes or their blends with an additional carbon source [6], [7], [8], [9] as well as from polystyrene-based macroreticular ion-exchange resin spheres (copolymer of polystyrene and divinylbenzene) [10], [11]. Although these studies focus on physical and chemical characterization of obtained ACs, information concerning ACs adsorption efficiency towards heavy metals removal is rather scarce.
Water polluted by heavy metals can be problematic due to their stability, mobility and toxicity. A number of technologies have been used to remove heavy metals, i.e. chemical precipitation, ion exchange, membrane separation, flotation, electrocoagulation, etc. from wastewaters. However, most of them suffer from disadvantages such as incomplete removal, expensive equipment/reagent usage, production of toxic sludge requiring disposal, and long treatment time. [12]. An alternative and attractive choice for heavy metal removal from aqueous solutions appears to be their adsorption since it is considered as a simple, relatively low-cost and effective method. The main limitation for the industrial application of this process is the high cost of the commercial available adsorbents. Therefore, the production of low cost adsorbents with specific characteristics for heavy metals removal is promising as can been seen from the number of publications in the last decade.
Among the group of heavy metals, Ni(II) have widespread application and is released in waste waters of various industries like metallurgical, chemical and refractories [13]. In view of its toxic effects (highly mutagenic and carcinogenic) it is of extreme importance to treat Ni(II) polluted industrial effluents before discharging them in water bodies. Several papers examining Ni(II) adsorption by biomass based ACs have shown the effectiveness of these adsorbents towards Ni(II) removal from aqueous solutions [14], [15], [16], [17]. However, there are missing references about Ni(II) removal from aqueous solution by ACs prepared from solid polystyrene waste.
The aim of the present study is to produce ACs from solid PS waste suitable for heavy metals removal through appropriate procedure. To achieve this goal, pyrolysis of PS followed by different activation treatments, i.e. steam activation and air oxidation at low temperature, are tested. Additionally, the evaluation of physical and chemical properties as well as adsorption efficiencies and kinetic behavior of produced ACs with respect to Ni(II) removal from aqueous solutions is considered.
Section snippets
Preparation of activated carbons
Precursor material for ACs production is solid PS waste in the form of post-consumer solid plastics of disposable cutlery, cups, plates and containers for dairy products. A three-stage process, comprising i) modification of the raw PS (step 1), ii) carbonization or so called slow pyrolysis (step 2) and iii) physical activation (step 3), was carried out for ACs preparation. During the first step, initial PS material was cut into small pieces and heated up to the temperature of PS melting point (~
Results and discussion
In general, thermoplastics, similarly to coking coals, are not appropriate raw material for ACs production unless they are pretreated, i.e. low temperature pre-oxidized [38]. The pre-oxidation treatment is required since it stabilizes the thermoplastic by creating cross-links within its structure and thus prevents the formation of fluid or pseudo-fluid phase and creates solid skeleton during subsequent carbonization. In our study, thermal pre-oxidation treatment of PS with concentrated sulfuric
Conclusions
Our study demonstrated the production of ACs with sufficient yields and properties from solid PS waste through appropriate experimental strategy constituting precursor modification, pyrolysis and two different physical activation treatments. The produced ACs, using high temperature steam activation and low temperature air oxidation demonstrated different characteristics, i.e. surface chemistry, surface area and porous texture, and different adsorption efficiency towards Ni(II) removal. Steam
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