Preparation, characterization and application of polystyrene based activated carbons for Ni(II) removal from aqueous solution

Highlights

• Activated carbons with decent yield/properties are produced from polystyrene.

• Steam activated carbon performs superior in Ni(II) removal than air activated.

• Adsorption appears to be governed by an exchange and complexation process.

• Maximal reached adsorption capacity is 40.8 mg g^− 1.

Abstract

The production of activated carbon from polystyrene waste is tested in order to limit its negative environmental impact through conversion to value added products. For this purpose modification of the precursor, slow pyrolysis and subsequent activations, i.e. high temperature steam activation and low temperature air oxidation, are applied. The physical/chemical properties as well as adsorption capacities of obtained activated carbons (ACs) towards Ni(II) removal in aqueous solutions are explored.

Steam activated carbon S-ACMPS performs superior in Ni(II) removal at applied circumstances. Ni(II) adsorption by this AC has been investigated using different process parameters and occurs through cation exchange mechanism optimal at pH range of initial solution of 4–8. Several reaction based kinetic models, i.e. pseudo-first, pseudo-second and Elovich models, and intra-particle diffusion model, are applied on experimental data. The adsorption kinetics of Ni(II) is best approximated by a pseudo second-order model. The equilibrium adsorption data best fits the Langmuir adsorption isotherm. Calculated maximum adsorption capacity for S-ACMPS is 40.8 mg g^− 1.

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.