Interaction of the zeolitic tuff with Zn-containing simulated pollutant solutions

Abstract

The possibility of removing Zn²⁺ cations from wastewater by ion exchange using natural zeolites as exchangers has been investigated. The process of binding of zinc ions into zeolite structure has been established by several reaction mechanisms as a fast chemical reaction of ion exchange, accompanied by slower adsorption of different ionic species and possible precipitation or coprecipitation with the zeolite structure. The physicochemical phenomena such as hydrolysis and dissolution of surface layers are the result of interaction of zeolite with hydrogen or hydroxyl ions from the solution. Complexation of OH⁻ with Zn²⁺ to form the zinc–hydroxy species strongly depends on pH value and affect the uptake mechanism as to lower dissolution of surface aluminosilicate layers. Structure imperfections as a surface property of mineralogical nonhomogeneous zeolitic grains can lead to formation of sorption surface sites with different energy, which affects the nonuniform distribution of different zinc species adsorbed. It is particularly possible in zeolitic tuff samples with relatively high content of aluminosilicates as minor mineralogical components, which is characteristic of Croatian deposits.

Introduction

The application of a group of crystalline aluminosilicates—natural zeolites—has attracted increasing attention for the past two decades, both in scientific circles and in the chemical processing industry. More than 50 different species of this mineral group have been identified, clinoptilolite among them, as abundant natural zeolite materials found in igneous, sedimentary, and metamorphic deposits. Due to their specific structure and chemical and physical properties, natural zeolites have found applications in the field of pollution control, including control of hardness, heavy metals, ammonium nitrogen, air pollutants, and radioactive contaminants [1], [2], [3], [4], [5].

The framework structure of clinoptilolite consists of interlinked tetrahedral rings creating a layer. Between these layers there are 8- and 10-tetrahedral-ring channels open in which exchangeable cations (Na, K, Ca, and Mg) are located, coordinated with H₂O molecules [6], [7]. Structural properties and large natural deposits make attractive the potential use of clinoptilolite as inexpensive cation exchange material for removal of heavy metal ions from the industrial waste solutions.

This study aims to explain the fundamentals of ion exchange equilibria in zeolite–aqueous solution systems from the aspect of stoichiometry of ionic species exchanged, amount of metal ion uptake, and ion exchange capacity [7], [8]. The chemical behavior of this natural material in aquatic environment of different initial pH values helps to explain the mineral–water interactions. These interactions are associated with different physicochemical reactions as dissolution, ion exchange, sorption, and possibly surface precipitation. Investigations of these interactions are important for their application in environmental chemistry, especially in development of tertiary wastewater treatment processes with extreme pH values and saline environments.