Comparative sorption kinetic studies of ammonium onto zeolite

Abstract

The sorption kinetics of ammonium onto three types of zeolite, natural zeolite, natural zeolite covered by biofilm and ammonium-bearing zeolite covered by biofilm, at two particle sizes were studied. The pseudo-first order and pseudo-second order models were fitted to the results by a non-linear method. The batch sorption model, based on a pseudo-second order mechanism, was applied to predict the rate constant of sorption, the equilibrium capacity and the initial sorption rate. Ion exchange between NH₄⁺ and cations in the zeolite increased with decreasing particle size of the zeolite. Biofilm covered on the zeolite did not affect the ion exchange for the smaller particle size but decreased the ion exchange capacity by 22% for the larger particle size. In addition, bioregeneration should be considered for the recovery of ion exchange capacity of the bio-zeolite made from ammonium-bearing zeolite by the microorganisms in the biofilm, 78.0 and 63.9% regeneration for the smaller and larger particle size of zeolite, respectively.

Introduction

Nitrogen pollution in hydrosphere is attracting increasing attention for eutrophication of lakes and rivers all over the world. Ammonium is the inorganic ion form of nitrogen pollution contained in municipal sewage, industrial wastewater and agricultural wastes or decomposed from organic nitrogen compounds in those wastewater and wastes. Higher concentration of ammonium will cause a sharp decrease of dissolved oxygen and obvious toxicity on aquatic organisms [1]. Hence, removing ammonium from wastewater is of great importance to control nitrogen pollution. Considering from the technical efficiency and economic competition, zeolite, used as an ion exchanger, seems to be much more capable to remove ammonium from wastewater. Zeolite is natural porous mineral described as crystalline hydrated aluminosilicates. Inside the framework structure of zeolite, alkali or alkaline-earth cations are reversibly fixed in the cavities and can easily be exchanged by surrounding positive ions [2]. Early in 1970s, natural zeolite were tested for removing ammonium from wastewater [3]. In recent years, typical studies reported its use for removing ammonium from secondary effluent or sewage [4], [5] as well as from industrial wastewaters, such as tannery wastewater [6], aquaculture wastewater [7] and piggery wastewater [8], [9]. According to their composition, natural zeolite have different forms, of which clinoptilolite has the best performance for ammonium removal [10], [11].

Sorption kinetics was available for describing the ammonium removing process by zeolite. The process was largely governed by intraparticle pore diffusion [12], [13], and fitting first order kinetic model [14], [15]. Other kinetic models including parabolic diffusion, Elovich and heterogeneous diffusion were also evaluated and all adequately described the ammonium sorption process [15].

Biological decomposition has been considered in the treatment system using zeolite, since microorganisms would probably grow on the surface of the zeolite immerged in aqueous solution or wastewater. Zeolite covered by biofilm was called bio-zeolite, acting as the ion-exchanger and growth media for microorganisms [16], [17]. Compared to the virgin zeolite, the ion-exchange rate in the bio-zeolite was reduced about 25–30%, and the rate-controlling step for ion exchange shifted from pore diffusion in the virgin zeolite to film diffusion in the bio-zeolite [18].

In this study, the ammonium sorption by three zeolite, i.e. natural zeolite, natural zeolite covered by biofilm and ammonium-bearing zeolite covered by biofilm, at two zeolite particle sizes were studied. A non-linear method of pseudo-second order kinetic model was examined the sorption of ammonium by zeolite. A trial-and-error procedure was used for the non-linear method using the solver add-in with Microsoft's spreadsheet, Microsoft Excel.