Advances in greener separation processes - case study: recovery of chlorinated aromatic compounds

Effective waste management must address waste avoidance, waste reuse, waste recovery and, as the least progressive option, waste treatment. Separation methodologies fit well within a modern green processing ethos, serving to isolate, concentrate and recover valuable material from toxic waste (notably aqueous) streams. An overview is provided of the established separation techniques that focuses on organic contaminants and the associated remediation objectives. The Case Study chosen to illustrate the role separation has to play in environmental pollution control deals with the removal of toxic chloroarenes from wastewater. This is a topic of growing concern as governmental legislation focuses on a substantial reduction in the emission of a broad range of chlorinated compounds. The sources and environmental impact associated with chloro-emissions and the existing treatment technologies are presented and discussed. Adsorption represents the most widely applicable non-destructive control technology, offering the possibility of recovery/recycle. Taking adsorption on activated carbon as a standard, the performance of novel catalytically generated structured carbon is evaluated and shown to outperform activated carbon in batch and semi-batch operation. The level of uptake is markedly higher on carbon surfaces that have been functionalised by acid washing and partial oxidation. The carbon nanofilament adsorbent bed exhibits lower back pressures when treating a continuous contaminant flow, separation of the liquid from the solid in batch operation is more facile and a chloroarene recovery/concentration is possible in polar solvents, a combination of factors that facilitates cleaner separation/recycle.