Automotive shredder residue (ASR) : a rapidly increasing waste stream waiting for a sustainable response

Recycling scrapped cars plays an important role in reducing pollution by decreasing the amount of waste that ends up in landfills. Directive 2000/53/EC regulates the management of ELVs. ELVs are collected and dismantled to remove the battery, tyres, fluids and any parts that can be re-used and the wreck is shredded. The metallic parts are separated by physical processes and recovered as ferrous scrap and nonferrous metals, all of which is recycled. The 25% remainder is the automotive shredder residue (ASR), which is composed mainly of plastics, contaminated with any metallic and other parts that could not be separated. This is often disposed of in landfills as solid urban waste and is not recycled. ASR generation in EU is approximately 2-2.5 million tonnes /year, constituting 10% of total hazardous waste in the EU. The study suggests that recovery rates for ELVs set in the EU Directive on end-of life vehicles will not be met until the volume of the ASR is further reduced. Treatment of the ASR focuses on recovering any useable materials, reducing the volume of the ASR to cut down on the quantity that will end up in landfill, and recovering the energy from the petrochemical content of the plastics. Up-to-date there are 8 post-shredder technologies (PST) used or potentially used for the treatment of auto shredder residues (ASR). The aim of this study is to give an overview of what problem the ASR presents to modern society and what the options are for processing this waste into recovered products or materials, or energy, with a minimum of useless by-products for which landfilling is the only route

Experimental study of zeolitic diffusion by use of a concentration-dependent surface diffusion model

Surface diffusivity in adsorption and ion exchange processes is probably the most important property studied expensively in the literature but some aspects, especially its dependence on solid phase concentration, is still an open subject to discussion. In this study a new concentration-dependent surface diffusion model, equipped with a flexible double selectivity equilibrium relationship is applied on the removal of Pb2+, Cr3+, Fe3+ and Cu2+ from aqueous solutions using a natural zeolite. The model incorporates the Chen-Yang surface diffusivity correlation able to deal with positive and negative dependence with surface coverage. The double selectivity equilibrium relationship successfully represents the experimental equilibrium data, which follow Langmurian isotherm type for Pb2+, sigmoidal for Cr3+ and Fe3+ and linear for Cu2+. The concentration-dependent surface diffusion model was compared with the constant diffusivity surface diffusion model and found to be moderately more accurate but considerably more useful as it provides more insights into the diffusion mechanism. The application of the model resulted in an average deviation of 8.56 ± 6.74% from the experimental data and an average solid phase diffusion coefficients between 10−9 and 10−10 cm2/s. The results showed that the diffusion of metal ions in the zeolite structure is unhindered following the surface diffusion mass transfer mechanism

Silver nanoparticles synthesised within the silica matrix in hyperstoichiometrical of mercury from aqueous solutions

Mercury adsorption of silver containing silica-based nanocomposites was evaluated. Maximum adsorption capacity of 0.4 mmol g-1 was achieved at silver loading of 0.5 mmol g-1. Nevertheless, if to calculate in respect to silver content the mercury adsorption capacity was generally elevated along with decreasing silver nanoparticle diameter. It has been demonstrated that silver particle diameters and loading should collectively be taken into consideration in designing the optimal mercury removal process. Further recommendations have been proposed with the aim of increasing the mercury removal efficiency using silver nanoparticles deposited on the surface of silica with lover silver loading, while achieving similar or even higher efficiencies due to observed hyperstoichiometry effect

Silver nanoparticles impregnated zeolites derived from coal fly ash : effect of the silver loading on adsorption of mercury (II)

Removal of mercury (II) from aqueous phase is of utmost importance, as it is highly toxic and hazardous to the environment and human health. A promising method for the removal of mercury (II) ions from aqueous solutions is by using adsorbents derived from coal fly ash (CFA), such as synthetic zeolites. In this work we present the hydrothermal production of synthetic zeolites from CFA followed by a modification for impregnation of silver nanoparticles, in solid concentrations from 0.15 to 4.71 wt.%. All produced zeolites and parent materials are characterized by XRD, XRF, BET and PSA to obtain morphological and microstructural data. Moreover, mercury (II) ions removal from aqueous solutions with initial concentration of 10 ppm is studied. According to results, zeolites and Ag-nanocomposites demonstrate much higher removal than parent CFA (up to 98%). In addition to this, we could observe a distinct adsorption behavior depending on the loading of Ag NPs in nanocomposites. A possible removal mechanism for both zeolites and Ag-nanocomposites is discussed

Variable diffusivity homogeneous surface diffusion model and analysis of merits and fallacies of simplified adsorption kinetics equations

Adsorption and ion exchange phenomena are encountered in several separation processes, which in turn, are of vital importance across various industries. Although the literature on adsorption kinetics modeling is rich, the majority of the models employed are empirical, based on chemical reaction kinetics or oversimplified versions of diffusion models. In this paper, the fifteen most popular simplified adsorption kinetics equations are presented and discussed. A new versatile variable-diffusivity two-phase homogeneous diffusion model is presented and used to evaluate the analytical adsorption models. Aspects of ion exchange kinetics are also addressed

A fractal-based correlation for time-dependent surface diffusivity in porous adsorbents

Fluid-solid adsorption processes are mostly governed by the adsorbate transport in the solid phase and surface diffusion is often the limiting step of the overall process in microporous materials such as zeolites. This work starts from a concise review of concepts and models for surface transport and variable surface diffusivity. It emerges that the phenomenon of hindered surface diffusion for monolayer adsorption, which is common in zeolites, and models able to fit a non-monotonic trend of surface diffusivity against adsorbate solid phase concentration, have received limited attention. This work contributes to the literature of hindered diffusion by formulating a time-dependent equation for surface diffusivity based on fractal dynamics concepts. The proposed equation takes into account the contributions of both fractal-like diffusion (a time-decreasing term) and hopping diffusion (a time-increasing term). The equation is discussed and numerically analyzed to testify its ability to reproduce the possible different patterns of surface diffusivity vs. time

Insights into the S-shaped sorption isotherms and their dimensionless forms

Isotherms are of paramount importance for the interpretation of adsorption and ion exchange mechanisms and the design of separation and catalytic processes. Although the literature on sorption isotherms is rich, most of experiments and models employed are limited to systems that obey simple equilibrium isotherms, such as linear, favorable (convex-upward) or unfavorable (concave-upward). This paper deals with the rather overlooked S-shaped isotherms and their dimensionless forms, which are of great importance for the modeling of separation processes and interpretation of equilibrium data. A review on the physical significance and applications of S-shaped isotherms in adsorption and ion exchange process is also presented

Optimized production of coal fly ash derived synthetic zeolites for mercury removal from wastewater

Coal fly ash (CFA) derived synthetic zeolites have become popular with recent advances and its ever-expanding range of applications, particularly as an adsorbent for water and gas purification and as a binder or additive in the construction industry and agriculture. Among these applications, perpetual interest has been in utilization of CFA derived synthetic zeolites for removal of heavy metals from wastewater. We herein focus on utilization of locally available CFA for efficient adsorption of mercury from wastewater. To this end, experimental conditions were investigated so that to produce synthetic zeolites from Kazakhstani CFAs with conversion into zeolite up to 78%, which has remarkably high magnetite content. In particular, the effect of synthesis reaction temperature, reaction time, and loading of adsorbent were systematically investigated and optimized. All produced synthetic zeolites and the respective CFAs were characterized using XRD, XRF, PSA and porosimetric instruments to obtain microstructural and mineralogical data. Furthermore, the synthesized zeolites were studied for the removal of mercury from aqueous solutions. A comparison of removal eficiency and its relationship to the physical and chemical properties of the synthetic zeolites were analyzed and interpreted

Two-phase homogeneous diffusion model for the fixed bed sorption of heavy metals on natural zeolites

In this work, the fixed bed removal kinetics of Pb2+, Zn2+, Mn2+, Cr3+, Fe3+ and Cu2+ from aqueous solutions on natural zeolites was studied. For this aim, a non-dimensional two-phase homogeneous solid diffusion model including axial dispersion and equipped with a universal double-selectivity equilibrium model is developed and applied. In total 9 isotherms, representing 128 experimental points and 25 breakthrough curves, representing 764 experimental points are used in modeling. The application of the model is satisfactory resulted in an average deviation from the experimental data of 11.19 ± 5.53%. The solid phase diffusion coefficients are between 10−7 and 10−9 cm2/s depending on the metal, flow rate and particle size in the decreasing order of Cu > Fe, Cr > Zn, Pb > Mn. The study is supplemented by an extended literature review on fixed bed models and experimentally derived solid phase diffusion coefficients in zeolites

Hydrothermal synthesis of zeolite production from coal fly ash : a heuristic approach and its optimization for system identification of conversion

Commercialization of synthetic zeolites has given considerable impetus to optimization of its production routes. The preferred production route involves hydrothermal treatment of coal fly ash in a strong alkali solution. The process involves several parameters, such as reaction temperature, time, the concentration and amount of alkali solution, and silica content in the fly ash, all of which strongly and non-monotonically affect the conversion. We herein perform several experiments with the Kazakhstani fly ash, and obtained a highest conversion of zeolites of 78% using 3 M NaOH at 110 °C. Further, we propose a conversion model using zero-order Takagi-Sugeno fuzzy system to analyze the effect of individual process parameters on conversion, and thereby, the reaction mechanism(s) of zeolite formation. The model is designed and developed, using the data, both from literature and our experiments on Kazakhstani fly ash. The obtained results clearly illustrate that the model accurately predict the conversion percentage of zeolite for a given set of reaction parameters. The model is further optimized to provide accurate inferences and an average deviation between the model predictions and experimental values for zeolite yield is observed to be less than 5%