Heavy metals removal from electroplating wastewater by aminopropyl-Si MCM-41

Abstract

The potential of removing nickel and copper from industrial electroplating wastewaters by using mesoporous materials with MCM-41 type structure functionalised with different ratios of aminopropyl groups, namely Na₅₀, Na₂₅ and Na₅, were evaluated. The synthesised solids sorbents obtained were characterised by X-ray diffraction, elemental chemical analysis and IR spectroscopy. In preliminary experiments, studies were carried out to determine the optimal experimental conditions for the retention of heavy ions. Effects of concentration, optimal pH, interference with humic substances and other metals were studied for Na₅, which showed the best capacity of absorption determined by the corresponding isotherm. This material has a greater selectivity against sodium, indicating that ionic strength does not affect the extraction. Results of an application of this material to remove nickel and copper in synthetic and real industrial wastewater samples from an electrochemical industry area are shown with successful results. The lowest level of nickel and copper were observed when Na₅ was used. This observation suggests that reactive aminopropyl-Si MCM-41 and similar materials may be a promising and provide for alternative environmental technologies in the future.

Introduction

The increasing level of heavy metals in the environment represents a serious threat to human health, living resources and ecological systems. Although there are many sources of heavy metals, some industrial sectors are at present those which contribute the most to environmental pollution with these toxic metals. Among such industrial sectors, the metal finishing industry is an important one due to the large number of enterprises by which is integrated as well as their geographical dispersion. The main way of contamination by these industries is the emission of liquid effluents with relatively low, although harmful metal concentrations (up to some hundreds of mg l⁻¹), among these metals, Cr, Ni, Zn, Cu and Cd are usually the most abundant ones.

These contaminants must be removed from wastewaters before discharge as they are considered persistent, bioaccumulative and toxic substances (US EPA, 1998). Of special technical and economic importance is the selective removing of metals derived from the discharge from electrochemical activities into industrial wastewater. In comparison with other industries, the industrial electrochemistry uses less water, hence the volume of wastewater produced is smaller, and the wastewater is highly toxic in nature because of the presence of metals such as copper, nickel, zinc, cadmium and cyanides. It is well established that the presence of heavy metals in the environment, even in moderate concentration is responsible for producing a variety of illnesses related with the risk of dermal damage, respiratory problems and several kinds of cancer, as accurately reviewed in a recent papers (Barceloux, 1999a, Barceloux, 1999b).

Various treatment technologies have been developed for the removal of these metals from wastewater, depending on the concentration, such as precipitation of the metal hydroxide and filtration when higher concentration are treated (Aziz et al., 2001; Kim et al., 2001). Ion exchange using zeolites (Zamow and Murphy, 1992), clays (Reddy and Chinthamreddy, 2003) or ionic resins (Abollino et al., 2000; Yalcin et al., 2001) are generally employed when the range of contamination is low. These systems due to the degree of interaction, present a small selectivity when cationic interferences are also present. Electro-winning, is another way to remove heavy metals using electrochemical tools. The metal is recovered as pure metal, in one stage, without the necessity of sludge disposal and regeneration of saturated ion exchange resin (Njau et al., 2000). Bioremedial alternatives have been employed in contaminated industrial wastewater and groundwater, namely microalgal (Chong et al., 2000) and bacteria (Vainshtein et al., 2003). Materials such as activated carbon (Kadirvelu et al., 2000) or alumina (Cervera et al., 2003) give very good results but they have the inconvenience of high cost when pure sorbents are employed. There is an increasing trend for substituting these by unconventional metal cations removal as red mud or fly ash (Apak et al., 1998) or composting (Barker and Bryson, 2002). These are future alternatives which are starting to give good results, although their selectivity is low.

The use of materials with surface functional groups, such as functionalised MCM-41, shows improved selectivity for the removal heavy metals in wastewater. For this purpose we have developed, in this work, an effective, simple and low cost alternative employing chelating agent with aminopropyl groups incorporated on inorganic materials, which may prevent the problems associated with conventional methods. The synthesis of MCM-41 structures provide solids with several physico-chemical properties, which can be optimised for the removal process i.e. high specific area and a narrow uniform distribution of pores (Beck and Vartuli, 1996). They are synthesised with surfactants, used as template and can be functionalised later on with different organic groups such as thiol, as previously reported for the removal of mercury from contaminated effluent stream (Mercier and Pinnavaia, 1997). Hall has extended the method of template for the direct co-condensation to synthesize MCM-41 structures containing allyl, phenyl or amino groups (Hall et al., 1999), but no applications in the environmental field have been reported yet So far experiments were carried out at a lab scale employing aminopropyl used as solid phase extraction for the quantification of bismuth, lead, and nickel in seawater (Tokman et al., 2003).

Following the same line of research, this work introduces some alternatives in their synthesis, in order to improve their structure characteristics, and propose MCM-41 functionalised with aminopropyl groups to be employed as an alternative for the removal heavy metals in wastewater from electroplating effluent with a higher concentration of nickel and copper.