Direct observation of solid-phase adsorbate concentration profile in powdered activated carbon particle to elucidate mechanism of high adsorption capacity on super-powdered activated carbon.

Decreasing the particle size of powdered activated carbon (PAC) by pulverization increases its adsorption capacities for natural organic matter (NOM) and polystyrene sulfonate (PSS, which is used as a model adsorbate). A shell adsorption mechanism in which NOM and PSS molecules do not completely penetrate the adsorbent particle and instead preferentially adsorb near the outer surface of the particle has been proposed as an explanation for this adsorption capacity increase. In this report, we present direct evidence to support the shell adsorption mechanism. PAC particles containing adsorbed PSS were sectioned with a focused ion beam, and the solid-phase PSS concentration profiles of the particle cross-sections were directly observed by means of field emission-scanning electron microscopy/energy-dispersive X-ray spectrometry (FE-SEM/EDXS). X-ray emission from sulfur, an index of PSS concentration, was higher in the shell region than in the inner region of the particles. The X-ray emission profile observed by EDXS did not agree completely with the solid-phase PSS concentration profile predicted by shell adsorption model analysis of the PSS isotherm data, but the observed and predicted profiles were not inconsistent when the analytical errors were considered. These EDXS results provide the first direct evidence that PSS is adsorbed mainly in the vicinity of the external surface of the PAC particles, and thus the results support the proposition that the increase in NOM and PSS adsorption capacity with decreasing particle size is due to the increase in external surface area on which the molecules can be adsorbed.

Modeling high adsorption capacity and kinetics of organic macromolecules on super-powdered activated carbon.

The capacity to adsorb natural organic matter (NOM) and polystyrene sulfonates (PSSs) on small particle-size activated carbon (super-powdered activated carbon, SPAC) is higher than that on larger particle-size activated carbon (powdered-activated carbon, PAC). Increased adsorption capacity is likely attributable to the larger external surface area because the NOM and PSS molecules do not completely penetrate the adsorbent particle; they preferentially adsorb near the outer surface of the particle. In this study, we propose a new isotherm equation, the Shell Adsorption Model (SAM), to explain the higher adsorption capacity on smaller adsorbent particles and to describe quantitatively adsorption isotherms of activated carbons of different particle sizes: PAC and SPAC. The SAM was verified with the experimental data of PSS adsorption kinetics as well as equilibrium. SAM successfully characterized PSS adsorption isotherm data for SPACs and PAC simultaneously with the same model parameters. When SAM was incorporated into an adsorption kinetic model, kinetic decay curves for PSSs adsorbing onto activated carbons of different particle sizes could be simultaneously described with a single kinetics parameter value. On the other hand, when SAM was not incorporated into such an adsorption kinetic model and instead isotherms were described by the Freundlich model, the kinetic decay curves were not well described. The success of the SAM further supports the adsorption mechanism of PSSs preferentially adsorbing near the outer surface of activated carbon particles.

Comparison of natural organic matter adsorption capacities of super-powdered activated carbon and powdered activated Carbon.

We examined the natural organic matter (NOM) adsorption characteristics of super-powdered activated carbon (S-PAC) produced by pulverizing commercially available, normal PAC to a submicron particle size range. The adsorption capacities of S-PAC for NOM and polystyrene sulfonates (PSS) with molecular weights (MWs) of 1.1, 1.8, and 4.6 kDa, which we used as model compounds, were considerably higher than those of PAC. The adsorption capacity increases were observed for all five types of carbon tested (two wood-based, two coconut-based, and one coal-based carbon). The adsorption capacities of S-PAC and PAC for polyethylene glycols (PEGs) with MWs of 0.3 and 1.0 were the same. The adsorption capacities of S-PAC for PEGs with MWs of 3.0 and 8.0 kDa were slightly higher than the adsorption capacities of PAC, but the difference in adsorption capacity was not as large as that observed for NOM and the PSSs, even though the MW ranges of the adsorbates were similar. We concluded that the adsorption capacity differences between S-PAC and PAC observed for NOM and PSSs were due to the difference in particle size between the two carbons, rather than to differences in internal pore size or structure, to differences in activation, or to non-attainment of equilibrium that resulted from the change in particle size. The difference in adsorption capacity between S-PAC and PAC was larger for NOM with a high specific UV absorbance (SUVA) value than for low-SUVA NOM. The larger adsorption capacities of S-PAC compared with PAC were explained by the larger specific external surface area per unit mass. We hypothesize that a larger fraction of the internal pore volume is accessible with carbon of smaller particle size because the NOM and PSS molecules preferentially adsorb near the outer surface of the particle and therefore do not completely penetrate the adsorbent particle.

Branched pore kinetic model analysis of geosmin adsorption on super-powdered activated carbon.

Super-powdered activated carbon (S-PAC) is activated carbon of much finer particle size than powdered activated carbon (PAC). Geosmin is a naturally occurring taste and odor compound that impairs aesthetic quality in drinking water. Experiments on geosmin adsorption on S-PAC and PAC were conducted, and the results using adsorption kinetic models were analyzed. PAC pulverization, which produced the S-PAC, did not change geosmin adsorption capacity, and geosmin adsorption capacities did not differ between S-PAC and PAC. Geosmin adsorption kinetics, however, were much higher on S-PAC than on PAC. A solution to the branched pore kinetic model (BPKM) was developed, and experimental adsorption kinetic data were analyzed by BPKM and by a homogeneous surface diffusion model (HSDM). The HSDM describing the adsorption behavior of geosmin required different surface diffusivity values for S-PAC and PAC, which indicated a decrease in surface diffusivity apparently associated with activated carbon particle size. The BPKM, consisting of macropore diffusion followed by mass transfer from macropore to micropore, successfully described the batch adsorption kinetics on S-PAC and PAC with the same set of model parameter values, including surface diffusivity. The BPKM simulation clearly showed geosmin removal was improved as activated carbon particle size decreased. The simulation also implied that the rate-determining step in overall mass transfer shifted from intraparticle radial diffusion in macropores to local mass transfer from macropore to micropore. Sensitivity analysis showed that adsorptive removal of geosmin improved with decrease in activated carbon particle size down to 1microm, but further particle size reduction produced little improvement.