The adsorption of heavy metals by waste tea and coffee residues

Abstract

This thesis is concerned with the use of natural waste materials, specifically used tea leaves and coffee grounds, as adsorbents for the removal of trace metals from water, e.g. in waste water treatment. Trace metals such as lead, mercury, zinc, copper, nickel and cadmium are released to the environment in waste water as a result of human activities, and generate concern because of their potential toxicity, persistence and tendency to become concentrated in food chains. While there are many conventional methods for removing these metals from waste water, such as chemical precipitation, ion exchange, membrane technologies and electrochemical treatment, these processes can be expensive. Thus in recent years there has been increasing interest in low cost adsorbent materials as alternative adsorbents, particularly waste natural products such as rice hulls and spent coffee grounds. Most of the research reported in this thesis has been conducted with spent coffee grounds, both grounds produced by leaching of commercial ground coffee and spent grounds obtained from the manufacture of instant coffee. However, some preliminary work was also conducted using spent tea leaves. In the initial work, the adsorption of the metal ions Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ by these adsorbents was investigated using batch adsorption methods to determine the effects of pH, metal ion concentration, adsorbent concentration and other factors such as temperature and metal ion competition. The competitive effects of soluble material leached from the adsorbents that also bind metal ions were studied. The adsorption of the metal ions was found to follow the Langmuir adsorption isotherm. However, the maximum adsorption density was found to depend on the concentration of coffee adsorbent present. Further investigation indicated that this was a result of flocculation of the coffee solids, which acted to reduce the available surface area and thus the maximum density of adsorption sites. This was confirmed using a dispersant to break up the flocculated solids. Column adsorption studies showed that metal ions adsorbed by coffee grounds could be quantitatively leached in dilute acid solution without any loss of subsequent adsorption properties, thus promoting efficient re-use of the column for many adsorption cycles. The adsorbent was also found to be largely unaffected by biological degradation. A prototype waste water treatment column was used to treat tap water samples, with and without known additions of metal ions. The results showed that the grounds efficiently adsorbed trace metal contaminants at levels as low as [mu]g L⁻¹. The acid base chemistry of both tea leaves and coffee grounds, and the soluble materials leached from the fresh tea and coffee, were studied using potentiometric titration. In addition the stoichiometry of H⁺ released during metal ion adsorption was also investigated. The latter results indicated that the stoichiometry of metal ion adsorption is not simple, i.e. it probably involves more than one type of adsorption site. The results of this thesis suggest that the use of waste coffee grounds shows considerable promise for the treatment of waste water containing trace metals, and provides an alternative commercial use for such exhausted coffee materials.