Carbonaceous materials for the electrochemical clean-up of wastewater

Using carbon adsorbents is the most common method of removing waste organics from water. However, the question remains of what to do with the carbon after adsorption. Landfill or incineration come with the obvious environmental consequences, and thermal regeneration is energetically costly and can be wasteful. The wastewater management company Arvia have developed a process for using electrochemical regeneration as an alternative. This thesis has examined that process in detail, in particular the role of the specific granular activated carbon used (Nyex). Nyex was characterised using a range of microscopic and spectroscopic methods. It is a highly crystalline graphite material which has been intercalated with bisulphate ions and water. It has an accessible surface area of 2.04 m2/g and a classic platy morphology with oxygen surface functionality. Additionally, the intercalation process has not changed the inter-layer distance. It is known that graphite intercalation compounds can expand upon thermal treatment. Upon heating using either an oven or a domestic microwave, Nyex expanded rapidly, resulting in an expanded carbon with a very typical vermiform morphology. The maximum expansion volume recorded for Nyex was 394 mL g 1, for microwave expanded Nyex. Surface area analysis confirmed that this volume expansion had not introduced porosity into the structure. The structure was damaged by the expansion but the overall number of defects has decreased. Analytical electrochemistry has been carried out, using Nyex as a working electrode and four test materials. Phenol can be irreversibly oxidised at a Nyex working electrode, and the build-up of breakdown products on the surface can be prevented by the application of 1.85 V for 7 s between each cyclic voltammogram. The increased surface area of expanded Nyex led currents to flow at an order of magnitude higher (approximately 40 µA compared to 5 µA for unexpanded Nyex) meaning more phenol could be oxidised at the same potentials. Humic acid and hydroquinone both showed no electrochemical activity at Nyex. Additionally, repeated cycling of potentials in hydroquinone led to the build-up of carbonaceous material on the surface of the electrode which passivated it. The industry provided test material, Tellus oil, passivated both Nyex and platinum electrodes. Two extraction methods to analyse the quantity of oil left on the Nyex after the Arvia process were developed. Using supercritical CO2 (scCO2) an extraction efficiency of 63% was achieved using 1 hour of extraction. Using acetone extraction, efficiencies in the region of 80 – 90% could be achieved in 10 min. The scCO2 extraction was complex, and required specialist equipment and training. In contrast the acetone extraction was simple and gave more reproducible results. Samples provided by Arvia from their pilot plant trial were extracted using the acetone extraction method. The concentration of oil on the samples was approximately 50 wt.% and did not decrease after 1100 hours of regeneration. A trial was run at Nottingham using 25 wt.% of oil which also did not decrease after 8 hours of regeneration. In summary, the combination of analytical electrochemistry and solvent extraction has shown Nyex is able to absorb up to 50 wt.% of oil from water, but the Arvia process does not destroy complex oily wastes. It is effective at oxidising phenol from water, with cycles of regeneration ensuring the Nyex surface is not passivated by the build-up of breakdown products. Nyex expanded in either the microwave or an oven is capable of oxidising much higher quantities of phenol because of its higher surface area