Abstract
Heavy metals in drinking water are a serious global concern and efforts are underway to develop sustainable decontamination techniques. The present project was developed with an aim to prepare biosorbents for treating household drinking water. Six natural fruit and vegetable peels (FVPs), namely, apple peel (AP), banana peel (BP), cucumber peel (CP), kiwifruit peel (KP), orange peel (OP) and potato peel (PP) were immobilised on sodium alginate (SA) beads and used as biosorbents for the removal of one metalloid and six heavy metal ions, namely, arsenic (As V), cadmium (Cd II), chromium (Cr VI), copper (Cu II), mercury (Hg II), lead (Pb II) and nickel (Ni II) from drinking water. Batch experiments were performed to study some of the main parameters that influence the biosorption process, namely, contact time, pH, biosorbent concentration, initial ion concentration and temperature, using solutions prepared by spiking the ions in distilled deionised laboratory water. A comparison of the biosorption efficacies of the various FVP beads was made, and biosorption was studied in spiked real drinking water collected from a mine region in New Zealand. The surface of the beads was imaged using scanning electron microscopy coupled with energy dispersive spectroscopy, and the concentration of ions in solution was measured using inductively coupled plasma with a mass spectrometer detector. The applicability of kinetic models such as film diffusion, pore diffusion, pseudo-first order, pseudo-second order and Elovich equation was examined using the biosorption data. A novel effort to study isotherms and thermodynamics in non-equilibrium conditions was made using the Probability Isotherm theory proposed for biochemical reactions. The suitability of isotherm models such as Langmuir, Freundlich, Temkin and Dubinin-Radushkevich to non-equilibrium data was examined. The nature of the binding of the ions to the surface of the beads and the energy changes involved were determined from the parameters calculated in these models. The results showed that the roughness and heterogeneity of the surface was related to the biosorption efficacy of the beads. Therefore, FVP beads were better performing relative to the SA beads. CP and KP beads had the highest biosorption capacities, followed by AP and BP beads. The beads also showed different affinities for the ions. While 70-90% of the divalent metal ions were removed simultaneously from solution, the biosorption of As and Cr anions was less than 35%. Although KP bead biosorbed the highest percentage of Cr (~34%), it also showed the second-lowest Hg removal percentages (~53%) after AP bead (~50%). In contrast, CP bead removed ~90% Cd and Hg, but had the lowest Cu removal (~72%) from solution. BP bead showed the highest efficient in removing Cu (~87%) among all the beads. Biosorption of Pb was the lowest among all the divalent ions and OP bead had the lowest removal (~57%) among the beads. On the other hand, PP beads and BP beads biosorbed the highest percentage of Pb (~74%) from solution, although PP beads were the least effective for the simultaneous removal of heavy metal ions in general. All the beads biosorbed approximately 15%, 90% and 70-80% of As, Cd and Ni, respectively, from cocktail solution. The beads successfully removed metal ions in the drinking water pH range. The use of more number of beads in solution resulted in an increase in biosorption percentage but also a corresponding decrease in the biosorption capacity of the beads. In contrast, at higher ion concentrations (up to 15 mgL-1), there was a decrease in the biosorption percentage and a corresponding increase in the biosorption capacity of the beads. The temperature of the solution played a significant role in the biosorption process and affected the uptake of the ions to various degrees. Furthermore, the presence of co-ions suppressed the biosorption of individual ions and this was observed both in laboratory and real drinking water. In conclusion, CP and KP beads showed the most promising removal of heavy metals from drinking water, but no bead could significantly biosorb As. Appropriate modifications to the beads will be required to increase heavy metal ion biosorption when many competing natural ions are present in drinking water. If that is achievable, then this technology could revolutionise drinking water treatment processes especially in developing countries where low cost purification systems are needed.