Water Treatment By Adsorption with Electrochemical Regeneration

Abstract

There have been several studies of electrochemical regeneration of activated carbon adsorbents used for removal of organic contaminants [1], including both cathodic and anodic treatment. The process offers advantages over thermal regeneration including in-situ (on-site) regeneration and minimal adsorbent losses [2]. However, long regeneration times, high energy costs and in some cases poor regeneration efficiencies have constrained implementation [2]. An alternative approach has been developed using graphite intercalation compound (GIC) adsorbents [3], that have low adsorption capacities but are amenable to rapid electrochemical regeneration. The electrochemical regeneration using GIC adsorbents is achieved through the anodic oxidation of the adsorbed species whereby a packed bed of the loaded GIC adsorbent is used as the anode within an electrochemical cell. The regenerated adsorbent, including any water trapped in the bed of adsorbent, is transferred without additional treatment to the next adsorption cycle. This means that no secondary waste is generated in the process. However, partially oxidized organics could be released into the treated water causing contamination with potentially more toxic compounds. In addition, some studies have suggested that phenol can polymerise during electrochemical oxidation, especially on the surface of graphite electrodes. The nature and concentrations of the breakdown products generated during electrochemical oxidation determines the toxicity of the treated effluent. In this context, the breakdown products released in the liquid phase has been studied [4, 5]. Phenol was chosen as a model pollutant in these studies. A variety of oxidation intermediates including aromatics, aliphatic acids and chlorinated species have been observed during electrochemical regeneration of the GIC adsorbent in treating the aqueous solution of phenol. However, the concentrations of the breakdown products were fairly low compared to the initial concentration of phenol to be treated. The main mechanism responsible for the formation of breakdown products was found to be associated with indirect oxidation of phenol in solution i.e phenolic oxidation from solution as opposed to the oxidation of sorbed phenolics and therefore, the phenol adsorbed onto the surface of GIC adsorbent was not observed to contribute to the formation of these products [4]. The present study is concerned with the formation of gaseous breakdown products generated during electrochemical regeneration of GIC adsorbents. Carbon dioxide and carbon monoxide were detected as the main gaseous breakdown products formed in the during the regeneration process under a range of conditions. When electrochemical regeneration was carried out galvanostatically, both the volume and CO2 concentration of the gas evolved during regeneration of phenol loaded adsorbent were higher than the volume and CO2 concentration of the gas obtained when no phenol was present. In order to show that the CO2 formed is from the phenol, experiments using a 13C labelled phenol were carried out. These experiments confirm that a significant proportion of the adsorbed phenol was oxidized to CO2. The results suggest that about 50 to 60% of the adsorbed phenol is accounted for by the measured carbon dioxide in the evolved gas. This does not account for any carbon dioxide dissolved in the water during regeneration, and dynamic analysis suggests that this could be a significant fraction of CO2 in the evolved gases. Work is on-going to investigate the amount of carbon dioxide dissolved in the treated water. This study confirms that electrochemical mineralization of adsorbed organics on GIC adsorbents is achievable. This is an important finding for the development of practical water treatment processes. [1] R.M. Narbaitz, J. Cen, Electrochemical regeneration of granular activated carbon. Wat. Res, 28(1994),1771. [2] R.M. Narbaitz, A. Karimi‐Jashni, Electrochemical regeneration of granular activated carbons loaded with phenol and natural organic matter. Environ. Technol, 30 (2009), 27. [3] K.T. Eccleston, A.J. Eccleston, N.W. Brown, E.P.L. Roberts, Apparatus for the electrochemical regeneration of adsorbents. US Patent 7790024 B2, 2010. [4] S.N. Hussain, E.P.L. Roberts, H.M.A. Asghar, A.K. Campen, N.W. Brown, Oxidation of phenol and adsorption of breakdown products using a graphite adsorbent with electrochemical regeneration, Electrochim. Acta. 92 (2013) 20 [5] S.N. Hussain, H.M.A. Asghar, A.K. Campen, N.W. Brown, E.P.L. RobertS, Breakdown products formed due to oxidation of adsorbed phenol by electrochemical regeneration of a graphite adsorbent,Electrochim. Acta. 110 (2013) 550