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"A good scientist is a person with original ideas."
— Freeman Dyson
Almost all uses of activated carbon (AC) to clean polluted air or water result in used AC. This eventually will allow pollutants to break through into the media’s effluent when the media are exhausted, as illustrated in Figure 1.
In previous years, methods have been developed to evaluate used AC media to determine when they need to be re-bedded with new or reactivated AC. Methods such as a heat-of-immersion test method, which determines how much longer the working AC can be used before it should be re-bedded1,2, and software programs that can predict the AC adsorbate loadings at equilibrium3 have been created. Heat-of-immersion is useful in determining change-out decisions, and software programs are useful in determining marketplace applications for AC. Most used AC are reactivated thermally. New liquid phase-based processes to regenerate spent AC have been conducted. Two liquid processes, competitive adsorbate displacement and supercritical fluid (SCF) regeneration, will be discussed.
Competitive Displacement Regeneration
A list of reasons for commercializing the liquid regeneration process to complement existing thermal reactivation is provided in Table 1. It has long been known that you can chase off existing AC adsorbates by a stronger competitor for the adsorption space. This phenomenon has been referred to as rollover. Rollover literally means more of the rollover compound is in the effluent than in the influent to the AC media. Another way of interpreting this phenomenon is that the AC media accumulate target compounds present in the influent, and later stronger adsorbate(s) displaces it off the used AC into the effluent. The rollover phenomenon has been used to design a competitive displacement regeneration scheme.
Choosing displacer adsorbate(s) for liquid regeneration that allows its conversion to the ionic form of the competitive displacer by adjusting the pH of the bulk water matrix is key to the strategy. A competitive displacer that meets these critera (compounds with ionic and non-ionic forms) are selected. Changing the pH two units above the pKa of the acid (non-ionic) species provides complete ionization and desorption from the AC surface. The organic acid displacer adsorbate then will ionize at this pH and become less adsorbent because of increased water solubility. Thus, the AC adsorbed displacer can be displaced into the effluent and recovered from the water matrix for reuse of the displacer and the regenerated activated carbon column. This liquid-based strategy allows in-situ regeneration for additional uses of the AC media.
Supercritical Fluid Regeneration
Preliminary evaluations have been conducted of SCF as a new liquid process toregenerated spent AC that have been exhausted with hydrocarbons. Model hydrocarbon spent AC were prepared for these SCF evaluations. This SCF process offers the ultimate in "green chemistry" because SCF carbon dioxide is used to strip off the adsorbates to regenerate the AC. Spent AC was converted to fresh AC and adsorbed liquid hydrocarbons was recovered. The SCF regeneration experiments are summarized in Tables 2 and 3. This tabular data shows that the SCF process works equally well on granular and powdered AC. The SCF carbon dioxide could be recovered for multiple uses. The possibility of having no by-products in the SCF regeneration process is attractive.
Table 1: Comparisons of Thermal and New Liquid Regeneration Processes
Opportunities for Liquid Regeneration