On the reversibility of sorption to black carbon: distinguishing true hysteresis from artificial hysteresis caused by dilution of a competing adsorbate.
Sorption hysteresis in environmental sorbents has important implications for pollutant transport and bioavailability. We examined the reversibility of sorption of benzene, toluene, and nitrobenzene, both singly and in pairs, by wood charcoal. A previous study showed that these compounds compete for the same set of adsorption sites on the char. Single-solute sorption was weakly hysteretic at high concentrations. The finding of comparable irreversibility for these compounds was taken as evidence that hysteresis is true and caused by pore elasticity. Hysteresis in the presence of a competitor was weak at low cosolute concentration but became stronger as the cosolute concentration increased. We attribute the growing hysteresis with cosolute concentration to a thermodynamic "competitor dilution effect"--a heretofore-unrecognized cause of hysteresis in multi-solute systems when the competing solute is simultaneously diluted with the target solute in the desorption step. It arises because the target solute re-equilibrates from a sorption point where competition is relatively high, to a desorption point where competition is relatively low. Simulations based on Ideal Adsorbed Solution Theory, a thermodynamic competition model, support the hypothesis. The cosolute also causes an increase in the linearity of the target solute isotherm, also attributable to competition thermodynamics. The competitive dilution effect can play a role in pollutant behavior in real systems if competing substances, natural or anthropogenic, are diluted or degraded making the target less accessible with time.
Sander
Michael
M
Department of Chemical Engineering, Environmental Engineering Program, Yale University, New Haven, Connecticut 06511, USA.
Pignatello
Joseph J
JJ
D000327:Adsorption; D001554:Benzene / Q000737:chemistry; D002244:Carbon / Q000737:chemistry*; D004548:Elasticity; D004785:Environmental Pollutants / Q000737:chemistry* / Q000302:isolation & purification*; D007201:Indicator Dilution Techniques; D009578:Nitrobenzenes / Q000737:chemistry; D009930:Organic Chemicals / Q000737:chemistry*; D016062:Porosity; D013816:Thermodynamics; D013997:Time Factors; D014050:Toluene / Q000737:chemistry
D004785:Environmental Pollutants; D009578:Nitrobenzenes; D009930:Organic Chemicals; D014050:Toluene; D002244:Carbon; C036077:nitrobenzene; D001554:Benzene