Last November, our paper titled “Effect of bead milling on chemical and physical characteristics of activated carbons pulverized to superfine sizes” was published in the journal Water Research. We presented our findings from this paper at the 2015 AWWA Water Quality Technology Conference and Exposition (WQTC) that was held in Salt Lake City, UT. In fact, we received the announcement that the paper was accepted for publication while we were in Utah! The paper was well received . Many of the leading researchers on S-PAC were in attendance since the program had included a special topics session specifically to discuss research on S-PAC.
Our motivation came from reading other papers on superfine powdered activated carbon (S-PAC) and not being satisfied with the analysis of material parameters. We know that the production method for S-PAC is highly intense and also highly variable. S-PACs are created through pulverization of parent PACs using a wet bead milling process, which is required to produce the desired product size of near or under one micrometer in diameter.
Aside from particle size reduction, we could not determine from literature what material parameters might change as a result of high intensity milling. Characteristics such as the point of zero charge, specific surface area, and pore volume distributions are known to affect the adsorption performance of activated carbons. We set out to see if milling had a deterministic effect on these material properties.
Size measurements confirmed that reduced particle size is a deterministic effect of milling. However, there were few other deterministic physical changes to the carbons. One carbon, made from a wood material, lost a significant amount of specific surface area, but other carbons were affected randomly. Pore size distributions also did not change much, except for the creation of a mesoporous fraction in the coconut shell based carbon, which starts as heavily microporous.
The most interesting findings came from the chemical measurements. We had measured the carbons for pH using two methods: one to find the point of zero charge via pH drift and the other to determine the isoelectric point via electrokinetic measurements. However, the two methods produced drastically different pH data. The isoelectric point showly highly acidic pH for all S-PACs, while the point of zero charge showed a sharply decreasing pH with increased milling energy.
The answer to this seeming conundrum lies in knowing what is measured in each method. The pH drift method requires two days to reach equilibrium and thus allows for reaction from charges on all surfaces, external and internal. However, electrokinetic measurements occur on the order of seconds and only charges in contact with the bulk fluid are considered. Thus, the external surface of the carbons must be highly acidic, most likely in the form of oxygen-containing groups. The decreasing point of zero charge is explained by a decreasing average pH since smaller particle sizes now have a higher proportion of external surface area to total surface area. The picture below shows a proposed rapid oxidation process occurring as a result of milling.
The next step in preparing S-PAC for application is to determine the method of separation from the treated water. Stay tuned for the results of our work in combining S-PAC with microfiltration membranes!
