Investigations of different kinetic models for zinc ions uptake by a natural zeolitic tuff

Abstract

The kinetics of uptake of zinc ions from aqueous solutions by natural zeolitic tuff has been investigated. Batch experiments at constant temperature and hydrodynamic conditions have been performed. A decrease in the initial zinc concentration in aqueous solutions prolongs the time needed for equilibrium. Various kinetic models including the film-diffusion model, the surface diffusion model and the heterogeneous diffusion model have been tested for the description of the experimental results of zinc concentration in the solution over time. Diffusion through the film and diffusion through the surface of the particle could be the rate limiting steps at the initial reaction time. However, the heterogeneous diffusion model seems to be the best model providing a satisfactory fitting of the experimental results from the beginning of the process to the point of equilibrium, particularly at lower initial zinc concentrations.

Introduction

Groundwater contamination with heavy metals represents a serious threat to human health and environmental resources. Heavy metals are not biodegradable and tend to accumulate as metalloorganic complexes in living organisms, which increase their concentrations in biological cycles. One of the most dangerous types of contamination is the continuous release of liquid effluents with relatively high concentrations, primarily of Cr, Pb, Ni, Zn, Cu and Cd ions. Many of these metals are essential for life in trace concentrations, but higher concentrations have various toxic effects. Zinc is also a trace element that is essential for human health. But oral contamination or exposure to an environment contaminated with a high concentration of zinc causes serious health problems such as stomach and skin irritations, respiratory disorders and disturbance of protein metabolism. The conventional methods of heavy metals removal, such as precipitation methods, usually do not meet the standard criteria for releasing heavy metal cations to the environment (Alvarez-Ayuso et al., 2003, Cerjan-Stefanović et al., 1996, Vaca Mier et al., 2001). These techniques produce a large amount of sludge, which is disposed of in landfills of solid waste and could cause contamination of the soil due to the leaching by rainwater. An alternative method for purification of contaminated waters and wastewaters with heavy metal cations is using low-cost sorbents such as natural zeolites. The porous three-dimensional structure of natural zeolites consisting of (Si, Al)O₄ tetrahedra is negatively charged. The negative charge in the zeolite framework is compensated by hydrated exchangeable cations such as Na, K, Ca, Mg, Ba, and Sr (Stolz et al., 2000). Zeolites have the ability to exchange most of their exchangeable cations as well as hydrate or dehydrate without major change of the structural framework. Besides their ion exchange capacity zeolites exhibit sorption behaviour that is based on the theory of acidic and basic sites in the framework structure. Sorption is the general term used to define the surface retention process without specifying the exact retention mechanism (Alvarez-Ayuso et al., 2003). Their structural properties give them a wide range of capabilities such as molecular sieves, e.g. uptaking of ammonium ions, radionuclides and heavy metal ions from wastewaters (Perić et al., 2004), adsorbents for odour and gas emissions control, as well as agricultural fertilizers. Furthermore, natural zeolites show high cation exchange capacity, good thermal and radiation resistance, and excellent compatibility with the environment.

The rate of heavy metals uptake in natural minerals (such as in clays and zeolites) is a critical parameter that could be used for determination of the rate of their contamination of the environment. This strongly depends on the hydrodynamics of the system, concentration of heavy metal ions, and porosity and heterogeneity of the soil. In the literature, the kinetic models that describe adsorption/ion exchange on porous particles with heterogeneous composition (such as zeolites) have been classified into three categories; staged equilibrium models, interference theory model, and rate equation models (Kithome et al., 1998, Robinson et al., 1994). In the literature, these models are applied to the experimental results of heavy metal ions removal, considering the possibilities of other resistances to the mass transfer process. The rate equation models consider diffusion as the mass transfer mechanism responsible for the overall rate of the processes in a zeolite–aqueous solution system. These models are the most applicable to the experimental results and will be discussed in this paper (Guibal et al., 1998, Jardine and Sparks, 1984, Liang and Tsai, 1995, Rengaraj and Moon, 2002).

The aim of this paper is to evaluate the kinetic models that can be used to describe the rate of zinc ions uptake within heterogeneous and porous zeolitic particles.