Treatment of industrial liquid waste from the metal industry using zeolites
Postgraduate Thesis – Author: Irini Papadimitriou, Supervisor: Ch. Karagianni, Associate Professor, National Technical University of Athens, School of Chemical Engineering, 2010.
The problem of widespread pollution incidents involving heavy metals and toxic chemicals, due to the uncontrolled management of toxic and hazardous industrial waste, has arisen not only in Greece but also in other European countries, such as France, Italy, Ireland, etc.• It is recognised as a very serious and pressing issue that must be addressed as a European problem requiring immediate action.
On the one hand, the priority is the effective management of environmental risk, with the aim of safeguarding public health and ensuring immediate access to clean water; on the other, environmental restoration, where, for the first time, environmental liability legislation will be implemented, under which ‘the polluter pays’ for the cost of measures to prevent and remedy environmental damage.
The company ‘ALUMINIUM OF GREECE S.A.’ is one of the largest industrial production units in Greece, with an annual aluminium production capacity of 165,000 tonnes. It uses bauxite as its raw material, from which it isolates pure alumina by applying the well-known Bayer process (treatment of bauxite with a concentrated solution of caustic soda at high temperature and pressure). The solution resulting from the treatment of bauxite with the caustic soda solution contains sodium, potassium, calcium and iron ions, aluminium, titanium and dissolved silicate compounds and sulphate ions, which originate from the leaching of part of the ions from the solid mass of the bauxite into the aforementioned aqueous solution. The latter, which also contains traces of radioactive elements, is unsuitable for industrial use, as the presence of the aforementioned elements reduces or/inhibits the action of the initial solution on new batches of bauxite, meaning that its recycling remains a difficult problem for the aluminium industry. Furthermore, discharging it into the sea poses a risk to human health. It is therefore of critical importance that the waste bauxite leaching solution be completely purified of both the aforementioned trace elements and NaOH, so that it can be reused in the Bayer process.
Within this context, and with a view to proposing a solution to the problem, this Master’s thesis was undertaken. Specifically, an attempt was made to thoroughly investigate the role of the adsorption of the iron and aluminium ions present in the aforementioned waste solution from bauxite leaching (using the Bayer process) on Greek natural zeolite (which is a cheap, locally available and environmentally friendly adsorbent, given that there are abundant deposits of it in Greece, such as, for example, the sample under investigation from Northern Evros) and on synthetic zeolite. Kinetic experiments were carried out in a batch reactor at ambient temperature, from the results of which conclusions were drawn regarding the adsorption mechanism and the adsorption isotherms of the studied cation/zeolite systems. It was thus found that the adsorption of iron cations onto Greek Natural Zeolite (87% w/w clinoptilolite), as well as onto the synthetic zeolite (Baylith WE894: type 13X) follows pseudo-second-order kinetics (pseudo-second-order chemical reaction kinetics) and is described very satisfactorily by the Freundlich, Dubinin–Kaganer–Radushkevich and Temkin isotherms, whilst the kinetics of aluminium cation adsorption on Greek natural zeolite follow a mechanism which is described equally well by the simple first- and second-order kinetic models, the intraparticle diffusion model, as well as by the kinetic models of Elovich and Freundlich. These kinetic models satisfactorily describe the mechanism of aluminium ion adsorption on Greek Natural Zeolite, either over the same time interval (parallel mechanisms – coexistence), or over successive time intervals (mechanism change – participation). No chemisorption is observed for aluminium, at least over the total study period of two hours, although after approximately one and a half hours, the pseudo-second-order kinetics appear to describe the experimental data satisfactorily. This final experimental result may indicate the slow formation of chemical bonds between the adsorbed aluminium and constituent elements/groups on the zeolite surface (perhaps in its inner layers). However, given the co-existence of iron and aluminium ions in the waste solution and the fact that iron is chemisorbed almost immediately on the surface of the zeolite, the possibility that aluminium might also be chelated onto the zeolite, even over a prolonged period, is likely to remain a possibility that does not materialise, due to the ‘blocking’ of the pores on the surface of the solid adsorbent by the already formed iron(III) oxide precipitate. Finally, with regard to the isothermal model governing the adsorption of aluminium ions onto zeolite (Greek natural or synthetic), the isothermal adsorption models of Freundlich, Dubinin-Kaganer-Radushkevich and Temkin satisfactorily describe the natural adsorption of aluminium onto zeolite.
However, none of the metal ion/zeolite adsorption systems studied conforms to the Langmuir adsorption isotherm.