Desorption Kinetics of Trace Organic Contaminants and Natural Organic Matter from Superfine Powdered Activated Carbon

Powdered activated carbon (PAC) particles that have been ground to a submicron size are being studied for use as an adsorbent during the water treatment process. These superfine powdered activated carbon (S-PAC) particles can be used for the removal of trace contaminants and natural organic matter (NOM) from water. It has been shown that these smaller particles have faster adsorption kinetics and an increased adsorption capacity for NOM due to their larger specific surface area and shorter internal diffusion rate into the particles. One thing that has not been studied is the desorption kinetics of trace contaminants on these S-PAC particles. Desorption is typically not an issue in real systems as long as the effluent concentration remains below regulatory limits; however, it could become a problem when the carbon residence time in an absorber is greater than the hydraulic residence time. If there are high transient loadings of strongly competing compounds, these compounds can displace the more weakly adsorbing contaminants. This would result in a shorter adsorbent life. The main objectives behind studying the desorption kinetics of trace contaminants on S-PAC were to improve our understanding of the characteristics of S-PAC by: (i) measuring the desorption kinetics of trace contaminants on S-PAC when in competition with a strongly competing compound and (ii) measuring the desorption kinetics of trace contaminants on S-PAC when in competition with a strongly competing compound and a pore blocking organic compound. A bituminous coal based activated carbon, WC800, and a wood based activated carbon, Aqua Nuchar, were used as the adsorbent materials in this study. Radiolabeled atrazine was used as the model trace contaminant, 1,4-dichlorobenzene (p-DCB) was used as the model strongly competing compound, and Suwanee River natural organic matter (SRNOM) was used as the model pore blocking organic matter. The desorption kinetics of atrazine were faster for S-PAC than PAC for WC800 and Aqua Nuchar activated carbon. This is because of the larger fraction of external surface area and shorter internal diffusion length on the smaller particles. However, after the 30 minute milling time, there appears to be no trend in desorption kinetics. A two-step time dependence showing a fast initial desorption of atrazine followed by a slower, diffusion limited step of internally absorbed material can be seen for the S-PAC milling times. It was also found that NOM does not block as many pores on the S-PAC due to the larger specific external surface area. This allowed p-DCB to displace the atrazine faster on the WC800 and Aqua Nuchar S-PAC particles. Desorption kinetics are faster without SRNOM in the water because there is less competition from pore blockage. There is also less SRNOM competition on the S-PAC particles than there is on the PAC, therefore the desorption kinetics on S-PAC are faster than on PAC when SRNOM is present